Network Working Group CIP Working Group
Request for Comments: 1190 C. Topolcic, Editor
Obsoletes: IEN-119 October 1990
EXPerimental Internet Stream Protocol, Version 2 (ST-II)
Status of this Memo
This memo defines a revised version of the Internet Stream Protocol,
originally defined in IEN-119 [8], based on results from experiments
with the original version, and subsequent requests, discussion, and
suggestions for improvements. This is a Limited-Use Experimental
Protocol. Please refer to the current edition of the "IAB Official
Protocol Standards" for the standardization state and status of this
protocol. Distribution of this memo is unlimited.
1. Abstract
This memo defines the Internet Stream Protocol, Version 2 (ST-II), an
IP-layer protocol that provides end-to-end guaranteed service across
an internet. This specification obsoletes IEN 119 "ST - A Proposed
Internet Stream Protocol" written by Jim Forgie in 1979, the previous
specification of ST. ST-II is not compatible with Version 1 of the
protocol, but maintains mUCh of the architecture and philosophy of
that version. It is intended to fill in some of the areas left
unaddressed, to make it easier to implement, and to support a wider
range of applications.
1.1. Table of Contents
Status of this Memo . . . . . . . . . . . . 1
1. Abstract . . . . . . . . . . . . . . . 1
1.1. Table of Contents . . . . . . . . . . . 2
1.2. List of Figures . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . 7
2.1. Major Differences Between ST and ST-II . . . . 8
2.2. Concepts and Terminology . . . . . . . . . 9
2.3. Relationship Between Applications and ST . . . . 11
2.4. ST Control Message Protocol . . . . . . . . 12
2.5. Flow Specifications . . . . . . . . . . . 14
3. ST Control Message Protocol Functional Description . 17
3.1. Stream Setup . . . . . . . . . . . . . 18
3.1.1. Initial Setup at the Origin . . . . . . . 18
3.1.2. Invoking the Routing Function . . . . . . 19
3.1.3. Reserving Resources . . . . . . . . . . 19
3.1.4. Sending CONNECT Messages . . . . . . . . 20
3.1.5. CONNECT Processing by an Intermediate Agent . . 22
3.1.6. Setup at the Targets . . . . . . . . . 23
3.1.7. ACCEPT Processing by an Intermediate Agent . . 24
3.1.8. ACCEPT Processing by the Origin . . . . . . 26
3.1.9. Processing a REFUSE Message . . . . . . . 27
3.2. Data Transfer . . . . . . . . . . . . . 30
3.3. Modifying an Existing Stream . . . . . . . . 31
3.3.1. Adding a Target . . . . . . . . . . . 31
3.3.2. The Origin Removing a Target . . . . . . . 33
3.3.3. A Target Deleting Itself . . . . . . . . 35
3.3.4. Changing the FlowSpec . . . . . . . . . 36
3.4. Stream Tear Down . . . . . . . . . . . . 36
3.5. Exceptional Cases . . . . . . . . . . . 37
3.5.1. Setup Failure due to CONNECT Timeout . . . . 37
3.5.2. Problems due to Routing Inconsistency . . . . 38
3.5.3. Setup Failure due to a Routing Failure . . . 39
3.5.4. Problems in Reserving Resources . . . . . . 41
3.5.5. Setup Failure due to ACCEPT Timeout . . . . 41
3.5.6. Problems Caused by CHANGE Messages . . . . . 42
3.5.7. Notification of Changes Forced by Failures . . 42
3.6. Options . . . . . . . . . . . . . . . 44
3.6.1. HID Field Option . . . . . . . . . . . 44
3.6.2. PTP Option . . . . . . . . . . . . . 44
3.6.3. FDx Option . . . . . . . . . . . . . 45
3.6.4. NoRecovery Option . . . . . . . . . . 46
3.6.5. RevChrg Option . . . . . . . . . . . 46
3.6.6. Source Route Option . . . . . . . . . . 46
3.7. Ancillary Functions . . . . . . . . . . . 48
3.7.1. Failure Detection . . . . . . . . . . 48
3.7.1.1. Network Failures . . . . . . . . . . 48
3.7.1.2. Detecting ST Stream Failures . . . . . . 49
3.7.1.3. Subset . . . . . . . . . . . . . 51
3.7.2. Failure Recovery . . . . . . . . . . . 51
3.7.2.1. Subset . . . . . . . . . . . . . 55
3.7.3. A Group of Streams . . . . . . . . . . 56
3.7.3.1. Group Name Generator . . . . . . . . 57
3.7.3.2. Subset . . . . . . . . . . . . . 57
3.7.4. HID Negotiation . . . . . . . . . . . 58
3.7.4.1. Subset . . . . . . . . . . . . . 64
3.7.5. IP Encapsulation of ST . . . . . . . . . 64
3.7.5.1. IP Multicasting . . . . . . . . . . 65
3.7.6. Retransmission . . . . . . . . . . . 66
3.7.7. Routing . . . . . . . . . . . . . . 67
3.7.8. Security . . . . . . . . . . . . . 67
3.8. ST Service Interfaces . . . . . . . . . . 68
3.8.1. Access to Routing Information . . . . . . 69
3.8.2. Access to Network Layer Resource Reservation . 70
3.8.3. Network Layer Services Utilized . . . . . . 71
3.8.4. IP Services Utilized . . . . . . . . . 71
3.8.5. ST Layer Services Provided . . . . . . . 72
4. ST Protocol Data Unit Descriptions . . . . . . . 75
4.1. Data Packets . . . . . . . . . . . . . 76
4.2. ST Control Message Protocol Descriptions . . . . 77
4.2.1. ST Control Messages . . . . . . . . . . 79
4.2.2. Common SCMP Elements . . . . . . . . . 80
4.2.2.1. DetectorIPAddress . . . . . . . . . 80
4.2.2.2. ErroredPDU . . . . . . . . . . . . 80
4.2.2.3. FlowSpec & RFlowSpec . . . . . . . . 81
4.2.2.4. FreeHIDs . . . . . . . . . . . . 84
4.2.2.5. Group & RGroup . . . . . . . . . . 85
4.2.2.6. HID & RHID . . . . . . . . . . . . 86
4.2.2.7. MulticastAddress . . . . . . . . . . 86
4.2.2.8. Name & RName . . . . . . . . . . . 87
4.2.2.9. NextHopIPAddress . . . . . . . . . . 88
4.2.2.10. Origin . . . . . . . . . . . . . 88
4.2.2.11. OriginTimestamp . . . . . . . . . . 89
4.2.2.12. ReasonCode . . . . . . . . . . . . 89
4.2.2.13. RecordRoute . . . . . . . . . . . 94
4.2.2.14. SrcRoute . . . . . . . . . . . . 95
4.2.2.15. Target and TargetList . . . . . . . . 96
4.2.2.16. UserData . . . . . . . . . . . . 98
4.2.3. ST Control Message PDUs . . . . . . . . 99
4.2.3.1. ACCEPT . . . . . . . . . . . . . 100
4.2.3.2. ACK . . . . . . . . . . . . . . 102
4.2.3.3. CHANGE-REQUEST . . . . . . . . . . 103
4.2.3.4. CHANGE . . . . . . . . . . . . . 104
4.2.3.5. CONNECT . . . . . . . . . . . . . 105
4.2.3.6. DISCONNECT . . . . . . . . . . . . 110
4.2.3.7. ERROR-IN-REQUEST . . . . . . . . . . 111
4.2.3.8. ERROR-IN-RESPONSE . . . . . . . . . 112
4.2.3.9. HELLO . . . . . . . . . . . . . 113
4.2.3.10. HID-APPROVE . . . . . . . . . . . 114
4.2.3.11. HID-CHANGE-REQUEST . . . . . . . . . 115
4.2.3.12. HID-CHANGE . . . . . . . . . . . . 116
4.2.3.13. HID-REJECT . . . . . . . . . . . . 118
4.2.3.14. NOTIFY . . . . . . . . . . . . . 120
4.2.3.15. REFUSE . . . . . . . . . . . . . 122
4.2.3.16. STATUS . . . . . . . . . . . . . 124
4.2.3.17. STATUS-RESPONSE . . . . . . . . . . 126
4.3. Suggested Protocol Constants . . . . . . . . 127
5. Areas Not Addressed . . . . . . . . . . . . 131
6. Glossary . . . . . . . . . . . . . . . 135
7. References . . . . . . . . . . . . . . . 143
8. Security Considerations. . . . . . . . . . . 144
9. Authors' Addresses . . . . . . . . . . . . 145
Appendix 1. Data Notations . . . . . . . . . . 147
1.2. List of Figures
Figure 1. Protocol Relationships . . . . . . . . . 6
Figure 2. Topology Used in Protocol Exchange Diagrams . . 16
Figure 3. Virtual Link Identifiers for SCMP Messages . . 16
Figure 4. HIDs Assigned for ST User Packets . . . . . 18
Figure 5. Origin Sending CONNECT Message . . . . . . 21
Figure 6. CONNECT Processing by an Intermediate Agent . . 22
Figure 7. CONNECT Processing by the Target . . . . . . 24
Figure 8. ACCEPT Processing by an Intermediate Agent . . 25
Figure 9. ACCEPT Processing by the Origin . . . . . . 26
Figure 10. Sending REFUSE Message . . . . . . . . . 28
Figure 11. Routing Around a Failure . . . . . . . . 29
Figure 12. Addition of Another Target . . . . . . . . 32
Figure 13. Origin Removing a Target . . . . . . . . 34
Figure 14. Target Deleting Itself . . . . . . . . . 35
Figure 15. CONNECT Retransmission after a Timeout . . . . 38
Figure 16. Processing NOTIFY Messages . . . . . . . . 43
Figure 17. Source Routing Option . . . . . . . . . 47
Figure 18. Typical HID Negotiation (No Multicasting) . . . 60
Figure 19. Multicast HID Negotiation . . . . . . . . 61
Figure 20. Multicast HID Re-Negotiation . . . . 62
Figure 21. ST Header . . . . . . . . . . . . . 75
Figure 22. ST Control Message Format . . . . . . . . 77
Figure 23. ErroredPDU . . . . . . . . . . . . . 80
Figure 24. FlowSpec & RFlowSpec . . . . . . . . . . 81
Figure 25. FreeHIDs . . . . . . . . . . . . . . 85
Figure 26. Group & RGroup . . . . . . . . . . . . 85
Figure 27. HID & RHID . . . . . . . . . . . . . 86
Figure 28. MulticastAddress . . . . . . . . . . . 86
Figure 29. Name & RName . . . . . . . . . . . . 87
Figure 30. NextHopIPAddress . . . . . . . . . . . 88
Figure 31. Origin . . . . . . . . . . . . . . 88
Figure 32. OriginTimestamp . . . . . . . . . . . 89
Figure 33. ReasonCode . . . . . . . . . . . . . 89
Figure 34. RecordRoute . . . . . . . . . . . . . 94
Figure 35. SrcRoute . . . . . . . . . . . . . . 95
Figure 36. Target . . . . . . . . . . . . . . 97
Figure 37. TargetList . . . . . . . . . . . . . 97
Figure 38. UserData . . . . . . . . . . . . . . 98
Figure 39. ACCEPT Control Message . . . . . . . . . 101
Figure 40. ACK Control Message . . . . . . . . . . 102
Figure 41. CHANGE-REQUEST Control Message . . . . . . 103
Figure 42. CHANGE Control Message . . . . . . . . . 105
Figure 43. CONNECT Control Message . . . . . . . . . 109
Figure 44. DISCONNECT Control Message . . . . . . . . 110
Figure 45. ERROR-IN-REQUEST Control Message . . . . . . 111
Figure 46. ERROR-IN-RESPONSE Control Message . . . . . 112
Figure 47. HELLO Control Message . . . . . . . . . 113
Figure 48. HID-APPROVE Control Message . . . . . . . 114
Figure 49. HID-CHANGE-REQUEST Control Message . . . . . 115
Figure 50. HID-CHANGE Control Message . . . . . . . . 117
Figure 51. HID-REJECT Control Message . . . . . . . . 119
Figure 52. NOTIFY Control Message . . . . . . . . . 121
Figure 53. REFUSE Control Message . . . . . . . . . 123
Figure 54. STATUS Control Message . . . . . . . . . 125
Figure 55. STATUS-RESPONSE Control Message . . . . . . 126
Figure 56. Transmission Order of Bytes . . . . . . . 147
Figure 57. Significance of Bits . . . . . . . . . . 147
+--------------------+
Conference Control
+--------------------+
+-------+ +-------+
Video Voice +-----+ +------+ +-----+ +-----+ Application
Appl Appl SNMP Telnet FTP ... Layer
+-------+ +-------+ +-----+ +------+ +-----+ +-----+
V V ------------
+-----+ +-----+
PVP NVP
+-----+ +-----+ +
\ \ \
+-------+-----+
Appl.control V V V V V
ST data +-----+ +-------+ +-----+
& control UDP TCP ... Transport
+-----+ +-------+ +-----+ Layer
/ / \ / / / /
\ / +------+----\-----+-/----- ... -+ /
\ / \ / /
\ / \ +------- ... -+ -----------
\ / \ /
V V
+------+ +------+ +------+
SCMP ICMP IGMP Internet
+------+ +------+ +------+ Layer
V V V V V V V V V
+-----------------+ +-----------------------------------+
STream protocol -> Internet Protocol
+-----------------+ +-----------------------------------+
\ /
\ /
X ------------
/ \
/ \
VV VV
+----------------+ +----------------+
(Sub-) Network ... (Sub-) Network (Sub-)Network
Protocol Protocol Layer
+----------------+ +----------------+
Figure 1. Protocol Relationships
2. Introduction
ST has been developed to support efficient delivery of streams of
packets to either single or multiple destinations in applications
requiring guaranteed data rates and controlled delay characteristics.
The motivation for the original protocol was that IP [2] [15] did not
provide the delay and data rate characteristics necessary to support
voice applications.
ST is an internet protocol at the same layer as IP, see Figure 1. ST
differs from IP in that IP, as originally envisioned, did not require
routers (or intermediate systems) to maintain state information
describing the streams of packets flowing through them. ST
incorporates the concept of streams across an internet. Every
intervening ST entity maintains state information for each stream
that passes through it. The stream state includes forwarding
information, including multicast support for efficiency, and resource
information, which allows network or link bandwidth and queues to be
assigned to a specific stream. This pre-allocation of resources
allows data packets to be forwarded with low delay, low overhead, and
a low probability of loss due to congestion. The characteristics of
a stream, such as the number and location of the endpoints, and the
bandwidth required, may be modified during the lifetime of the
stream. This allows ST to give a real time application the
guaranteed and predictable communication characteristics it requires,
and is a good vehicle to support an application whose communications
requirements are relatively predictable.
ST proved quite useful in several early experiments that involved
voice conferences in the Internet. Since that time, ST has also been
used to support point-to-point streams that include both video and
voice. Recently, multimedia conferencing applications have been
developed that need to exchange real-time voice, video, and pointer
data in a multi-site conferencing environment. Multimedia
conferencing across an internet is an application for which ST
provides ideal support. Simulation and wargaming applications [14]
also place similar requirements on the communication system. Other
applications may include scientific visualization between a number of
workstations and one or more remote supercomputers, and the
collection and distribution of real-time sensor data from remote
sensor platforms. ST may also be useful to support activities that
are currently supported by IP, such as bulk file transfer using TCP.
Transport protocols above ST include the Packet Video Protocol (PVP)
[5] and the Network Voice Protocol (NVP) [4], which are end-to-end
protocols used directly by applications. Other transport layer
protocols that may be used over ST include TCP [16], VMTP [3], etc.
They provide the user interface, flow control, and packet ordering.
This specification does not describe these higher layer protocols.
2.1. Major Differences Between ST and ST-II
ST-II supports a wider variety of applications than did the
original ST. The differences between ST and ST-II are fairly
straight forward yet provide great improvements. Four of the more
notable differences are:
1 ST-II is decoupled from the Access Controller (AC). The
AC, as well as providing a rudimentary access control
function, also served as a centralized repository and
distributor of the conference information. If an AC is
necessary, it should be an entity in a higher layer
protocol. A large variety of applications such as
conferencing, distributed simulations, and wargaming can
be run without an explicit AC.
2 The basic stream construct of ST-II is a directed tree
carrying traffic away from a source to all the
destinations, rather than the original ST's omniplex
structure. For example, a conference is composed of a
number of such trees, one for traffic from each
participant. Although there are more (simplex) streams in
ST-II, each is much simpler to manage, so the aggregate is
much simpler. This change has a minimal impact on the
application.
3 ST-II defines a number of the robustness and recovery
mechanisms that were left undefined in the original ST
specification. In case of a network or ST Agent failure,
a stream may optionally be repaired automatically (i.e.,
without intervention from the user or the application)
using a pruned depth first search starting at the ST Agent
immediately preceding the failure.
4 ST-II does not make an inherent distinction between
streams connecting only two communicants and streams among
an arbitrary number of communicants.
This memo is the specification for the ST-II Protocol. Since
there should be no ambiguity between the original ST specification
and the specification herein, the protocol is simply called ST
hereafter.
ST is the protocol used by ST entities to exchange information.
The same protocol is used for communication among all ST entities,
whether they communicate with a higher layer protocol or forward
ST packets between attached networks.
The remainder of this section gives a brief overview of the ST
Protocol. Section 3 (page 17) provides a detailed description of
the operations required by the protocol. Section 4 (page 75)
provides descriptions of the ST Protocol Data Units exchanged
between ST entities. Issues that have not yet been fully
addressed are presented in Section 5 (page 131). A glossary and
list of references are in Sections 6 (page 135) and 7 (page 143),
respectively.
This memo also defines "subsets" of ST that can be implemented. A
subsetted implementation does not have full ST functionality, but
it can interoperate with other similarly subsetted
implementations, or with a full implementation, in a predictable
and consistent manner. This approach allows an implementation to
be built and provide service with minimum effort, and gives it an
immediate and well defined growth path.
2.2. Concepts and Terminology
The ST packet header is not constrained to be compatible with the
IP packet header, except for the IP Version Number (the first four
bits) that is used to distinguish ST packets (IP Version 5) from
IP packets (IP Version 4). The ST packets, or protocol data units
(PDUs), can be encapsulated in IP either to provide connectivity
(possibly with degraded service) across portions of an internet
that do not provide support for ST, or to allow access to services
such as security that are not provided directly by ST.
An internet entity that implements the ST Protocol is called an
"ST Agent". We refer to two kinds of ST agents: "host ST
agents", also called "host agents" and "intermediate ST agents",
also called "intermediate agents". The ST agents functioning as
hosts are sourcing or sinking data to a higher layer protocol or
application, while ST agents functioning as intermediate agents
are forwarding data between directly attached networks. This
distinction is not part of the protocol, but is used for
conceptual purposes only. Indeed, a given ST agent may be
simultaneously performing both host and intermediate roles. Every
ST agent should be capable of delivering packets to a higher layer
protocol. Every ST agent can replicate ST data packets as
necessary for multi-destination delivery, and is able to send
packets whether received from a network interface or a higher
layer protocol. There are no other kinds of ST agents.
ST provides applications with an end-to-end flow oriented service
across an internet. This service is implemented using objects
called "streams". ST data packets are not considered to be
totally independent as are IP data packets. They are transmitted
only as part of a point-to-point or point-to-multi- point stream.
ST creates a stream during a setup phase before data is
transmitted. During the setup phase, routes are selected and
internetwork resources are reserved. Except for explicit changes
to the stream, the routes remain in effect until the stream is
explicitly torn down.
An ST stream is:
o the set of paths that data generated by an application
entity traverses on its way to its peer application
entity(s) that receive it,
o the resources allocated to support that transmission of
data, and
o the state information that is maintained describing that
transmission of data.
Each stream is identified by a globally unique "Name"; see
Section 4.2.2.8 (page 87). The Name is specified in ST control
operations, but is not used in ST data packets. A set of streams
may be related as members of a larger aggregate called a "group".
A group is identified by a "Group Name"; see Section 3.7.3 (page
56).
The end-users of a stream are called the "participants" in the
stream. Data travels in a single direction through any given
stream. The host agent that transmits the data into the stream is
called the "origin", and the host agents that receive the data are
called the "targets". Thus, for any stream one participant is the
origin and the others are the targets.
A stream is "multi-destination simplex" since data travels across
it in only one direction: from the origin to the targets. A
stream can be viewed as a directed tree in which the origin is the
root, all the branches are directed away from the root toward the
targets, which are the leaves. A "hop" is an edge of that tree.
The ST agent that is on the end of an edge in the direction toward
the origin is called the "previous-hop ST agent", or the
"previous-hop". The ST agents that are one hop away from a
previous-hop ST agent in the direction toward the targets are
called the "next-hop ST agents", or the "next-hops". It is
possible that multiple edges between a previous-hop and several
next-hops are actually implemented by a network level multicast
group.
Packets travel across a hop for one of two purposes: data or
control. For ST data packet handling, hops are marked by "Hop
IDentifiers" (HIDs) used for efficient forwarding instead of the
stream's Name. A HID is negotiated among several agents so that
data forwarding can be done efficiently on both a point-to-point
and multicast basis. All control message exchange is done on a
point-to-point basis between a pair of agents. For control
message handling, Virtual Link Identifiers are used to quickly
dispatch the control messages to the proper stream's state
machine.
ST requires routing decisions to be made at several points in the
stream setup and management process. ST assumes that an
appropriate routing algorithm exists to which ST has access; see
Section 3.8.1 (page 69). However, routing is considered to be a
separate issue. Thus neither the routing algorithm nor its
implementation is specified here. A routing algorithm may attempt
to minimize the number of hops to the target(s), or it may be more
intelligent and attempt to minimize the total internet resources
consumed. ST operates equally well with any reasonable routing
algorithm. The availability of a source routing option does not
eliminate the need for an appropriate routing algorithm in ST
agents.
2.3. Relationship Between Applications and ST
It is the responsibility of an ST application entity to exchange
information among its peers, usually via IP, as necessary to
determine the structure of the communication before establishing
the ST stream. This includes:
o identifying the participants,
o determining which are targets for which origins,
o selecting the characteristics of the data flow between any
origin and its target(s),
o specifying the protocol that resides above ST,
o identifying the Service Access Point (SAP), port, or
socket relevant to that protocol at every participant, and
o ensuring security, if necessary.
The protocol layer above ST must pass such information down to the
ST protocol layer when creating a stream.
ST uses a flow specification, abbreviated herein as "FlowSpec", to
describe the required characteristics of a stream. Included are
bandwidth, delay, and reliability parameters. Additional
parameters may be included in the future in an extensible manner.
The FlowSpec describes both the desired values and their minimal
allowable values. The ST agents thus have some freedom in
allocating their resources. The ST agents accumulate information
that describes the characteristics of the chosen path and pass
that information to the origin and the targets of the stream.
ST stream setup control messages carry some information that is
not specifically relevant to ST, but is passed through the
interface to the protocol that resides above ST. The "next
protocol identifier" ("NextPcol") allows ST to demultiplex streams
to a number of possible higher layer protocols. The SAP
associated with each participant allows the higher layer protocol
to further demultiplex to a specific application entity. A
UserData parameter is provided; see Section 4.2.2.16 (page 98).
2.4. ST Control Message Protocol
ST agents create and manage a stream using the ST Control Message
Protocol (SCMP). Conceptually, SCMP resides immediately above ST
(as does ICMP above IP) but is an integral part of ST. Control
messages are used to:
o create streams,
o refuse creation of a stream,
o delete a stream in whole or in part,
o negotiate or change a stream's parameters,
o tear down parts of streams as a result of router or
network failures, or transient routing inconsistencies,
and
o reroute around network or component failures.
SCMP follows a request-response model. SCMP reliability is
ensured through use of retransmission after timeout; see Section
3.7.6 (page 66).
An ST application that will transmit data requests its local ST
agent, the origin, to create a stream. While only the origin
requests creation of a stream, all the ST agents from the origin
to the targets participate in its creation and management. Since
a stream is simplex, each participant that wishes to transmit data
must request that a stream be created.
An ST agent that receives an indication that a stream is being
created must:
1 negotiate a HID with the previous-hop identifying the
stream,
2 map the list of targets onto a set of next-hop ST agents
through the routing function,
3 reserve the local and network resources required to
support the stream,
4 update the FlowSpec, and
5 propagate the setup information and partitioned target
list to the next-hop ST agents.
When a target receives the setup message, it must inquire from the
specified application process whether or not it is willing to
accept the stream, and inform the origin accordingly.
Once a stream is established, the origin can safely send data. ST
and its implementations are optimized to allow fast and efficient
forwarding of data packets by the ST agents using the HIDs, even
at the cost of adding overhead to stream creation and management.
Specifically, the forwarding decisions, that is, determining the
set of next-hop ST agents to which a data packet belonging to a
particular stream will be sent, are made during the stream setup
phase. The shorthand HIDs are negotiated at that time, not only
to reduce the data packet header size, but to access efficiently
the stream's forwarding information. When possible, network-layer
multicast is used to forward a data packet to multiple next-hop ST
agents across a network. Note that when network-layer multicast
is used, all members of the multicast group must participate in
the negotiation of a common HID.
An established stream can be modified by adding or deleting
targets, or by changing the network resources allocated to it. A
stream may be torn down by either the origin or the targets. A
target can remove itself from a stream leaving the others
unaffected. The origin can similarly remove any subset of the
targets from its stream leaving the remainder unaffected. An
origin can also remove all the targets from the stream and
eliminate the stream in its entirety.
A stream is monitored by the involved ST agents. If they detect a
failure, they can attempt recovery. In general, this involves
tearing down part of the stream and rebuilding it to bypass the
failed component(s). The rebuilding always occurs from the origin
side of the failure. The origin can optionally specify whether
recovery is to be attempted automatically by intermediate ST
agents or whether a failure should immediately be reported to the
origin. If automatic recovery is selected but an intermediate
agent determines it cannot effect the repair, it propagates the
failure information backward until it reaches an agent that can
effect repair. If the failure information propagates back to the
origin, then the application can decide if it should abort or
reattempt the recovery operation.
Although ST supports an arbitrary connection structure, we
recognize that certain stream topologies will be common and
justify special features, or options, which allow for optimized
support. These include:
o streams with only a single target (see Section 3.6.2 (page
44)), and
o pairs of streams to support full duplex communication
between two points (see Section 3.6.3 (page 45)).
These features allow the most frequently occurring topologies to
be supported with less setup delay, with fewer control messages,
and with less overhead than the more general situations.
2.5. Flow Specifications
Real time data, such as voice and video, have predictable
characteristics and make specific demands of the networks that
must transfer it. Specifically, the data may be transmitted in
packets of a constant size that are produced at a constant rate.
Alternatively, the bandwidth may vary, due either to variable
packet size or rate, with a predefined maximum, and perhaps a
non-zero minimum. The variation may also be predictable based on
some model of how the data is generated. Depending on the
equipment used to generate the data, the packet size and rate may
be negotiable. Certain applications, such as voice, produce
packets at the given rate only some of the time. The networks
that support real time data must add minimal delay and delay
variance, but it is expected that they will be non-zero.
The FlowSpec is used for three purposes. First, it is used in the
setup message to specify the desired and minimal packet size and
rate required by the origin. This information is used by ST
agents when they attempt to reserve the resources in the
intervening networks. Second, when the setup message reaches the
target, the FlowSpec contains the packet size and rate that was
actually oBTained along the path from the origin, and the accrued
mean delay and delay variance expected for data packets along that
path. This information is used by the target to determine if it
wishes to accept the connection. The target may reduce reserved
resources if it wishes to do so and if the possibility is still
available. Third, if the target accepts the connection, it
returns the updated FlowSpec to the origin, so that the origin can
decide if it still wishes to participate in the stream with the
characteristics that were actually obtained.
When the data transmitted by stream users is generated at varying
rates, including bursts of varying rate and duration, there is an
opportunity to provide service to more subscribers by providing
guaranteed service for the average data rate of each stream, and
reserving additional network capacity, shared among all streams,
to service the bursts. This concept has been recognized by analog
voice network providers leading to the principle of time assigned
speech interpolation (TASI) in which only the talkspurts of a
speech conversation are transmitted, and, during silence periods,
the circuit can be used to send the talkspurts of other
conversations. The FlowSpec is intended to assist algorithms that
perform similar kinds of functions. We do not propose such
algorithms here, but rather expect that this will be an area for
experimentation. To allow for experiments, and a range of ways
that application traffic might be characterized, a "DutyFactor" is
included in the FlowSpec and we expect that a "burst descriptor"
will also be needed.
The FlowSpec will need to be revised as experience is gained with
connections involving numerous participants using multiple media
across heterogeneous internetworks. We feel a change of the
FlowSpec does not necessarily require a new version of ST, it only
requires the FlowSpec version number be updated and software to
manage the new FlowSpec to be distributed. We further suggest
that if the change to the FlowSpec involves additional information
for improved operation, such as a burst descriptor, that it be
added to the end of the FlowSpec and that the current parameters
be maintained so that obsolete software can be used to process the
current parameters with minimum modifications.
**** ****
* * ST Agent 1 * * +---+
* *------- o ---------* *-------+ B
* * * * +---+
* * ****
+---+ * *
* *
A +---------* * o ST Agent 3
* *
+---+ * *
* * ***
* * * * +---+
* * ST Agent 2 * *-------+ C
* *------- o --------* * +---+
* * * *
**** * *
* *
+---+ * * +---+
E +--------* *-------+ D
+---+ * * +---+
***
Figure 2. Topology Used in Protocol Exchange Diagrams
**** ST Agent 1 ****
* +--+---14--- o -----15--+----+--44---+---+
* +-+--11--- -----16--+-+ * B
* * * +-+--45---+---+
* * *++*
+---+ * * 34 32
+----4----+--+ *
A +----6----+----+ * o ST Agent 3
+----5----+---+ *
+---+ * * 33
* * ST *+*
* * Agent * *
* * 2 -----24-+--+ * +---+
* +--+--23--- o -----25-+-----+--54---+ C
* * -----26-+---+ * +---+
**** -----27-+-+ *
* *
+---+ * * +---+
E +---74---+-+ +-+--64---+ D
+---+ * * +---+
***
Figure 3. Virtual Link Identifiers for SCMP Messages
3. ST Control Message Protocol Functional Description
This section contains a functional description of the ST Control
Message Protocol (SCMP); Section 4 (page 75) specifies the formats of
the control message PDUs. We begin with a description of stream
setup. Mechanisms used to deal with the exceptional cases are then
presented. Complications due to options that an application or a ST
agent may select are then detailed. Once a stream has been
established, the data transfer phase is entered; it is described.
Once the data transfer phase has been completed, the stream must be
torn down and resources released; the control messages used to
perform this function are presented. The resources or participants
of a stream may be changed during the lifetime of the stream; the
procedures to make changes are described. Finally, the section
concludes with a description of some ancillary functions, such as
failure detection and recovery, HID negotiation, routing, security,
etc.
To help clarify the SCMP exchanges used to setup and maintain ST
streams, we have included a series of figures in this section. The
protocol interactions in the figures assume the topology shown in
Figure 2. The figures, taken together,
o Create a stream from an application at A to three peers at B,
C and D,
o Add a peer at E,
o Disconnect peers B and C, and
o D drops out of the stream.
Other figures illustrate exchanges related to failure recovery.
In order to make the dispatch function within SCMP more uniform and
efficient, each end of a hop is assigned, by the agent at that end, a
Virtual Link Identifier that uniquely (within that agent) identifies
the hop and associates it with a particular stream's state
machine(s). The identifier at the end of a link that is sending a
message is called the Sender Virtual Link Identifier (SVLId); that
at the receiving end is called the Receiver Virtual Link Identifier
(RVLId). Whenever one agent sends a control message for the other to
receive, the sender will place the receiver's identifier into the
RVLId field of the message and its own identifier in the SVLId field.
When a reply to the message is sent, the values in SVLId and RVLId
fields will be reversed, reflecting the fact the sender and receiver
roles are reversed. VLIds with values zero through three are
received and should not be assigned in response to CONNECT messages.
Figure 3 shows the hops that will be used in the examples and
summarizes the VLIds that will be assigned to them.
Similarly, Figure 4 summarizes the HIDs that will eventually be
negotiated as the stream is created.
**** ST Agent 1 ****
* +>+--1200-> o -------->+--->+-3600->+---+
* ^ * * * B
* * * +->+-6000->+---+
* * *+**
+---+ * * ^
+-------->+-->+ *
A * * o St Agent 3
+-------->+-->+ * ^
+---+ * * 4801
* * *+*
* V * ST Agent 2 * ^ * +---+
* +>+--2400-> o ------->+->+->+-4800->+ C
**** * * 4801 +---+
* *
+---+ * V * +---+
E +<-4800--+<-+->+-4800->+ D
+---+ * * 4801 +---+
***
Figure 4. HIDs Assigned for ST User Packets
Some of the diagrams that follow form a progression. For example,
the steps required initially to establish a connection are spread
across five figures. Within a progression, the actions on the first
diagram are numbered 1.1, 1.2, etc.; within the second diagram they
are numbered 2.1, 2.2, etc. Points where control leaves one diagram
to enter another are identified with a continuation arrow "-->>", and
are continued with "[a.b] >>-->" in the other diagram. The number in
brackets shows the label where control left the earlier diagram. The
reception of simple acknowledgments, e.g., ACKs, in one figure from
another is omitted for clarity.
3.1. Stream Setup
This section presents a description of stream setup assuming that
everything succeeds -- HIDs are approved, any required resources
are available, and the routing is correct.
3.1.1. Initial Setup at the Origin
As described in Section 2.3 (page 11), the application has
collected the information necessary to determine the
participants in the communication before passing it to the host
ST agent at the origin. The host ST agent will take this
information, allocate a Name for the stream (see Section
4.2.2.8 (page 87)), and create a stream.
3.1.2. Invoking the Routing Function
An ST agent that is setting up a stream invokes a routing
function to find a path to reach each of the targets specified
in the TargetList. This is similar to the routing decision in
IP. However, in this case the route is to a multitude of
targets rather than to a single destination.
The set of next-hops that an ST agent would select is not
necessarily the same as the set of next hops that IP would
select given a number of independent IP datagrams to the same
destinations. The routing algorithm may attempt to optimize
parameters other than the number of hops that the packets will
take, such as delay, local network bandwidth consumption, or
total internet bandwidth consumption.
The result of the routing function is a set of next-hop ST
agents and the parameters of the intervening network(s). The
latter permit the ST agent to determine whether the selected
network has the resources necessary to support the level of
service requested in the FlowSpec.
3.1.3. Reserving Resources
The intent of ST is to provide a guaranteed level of service by
reserving internet resources for a stream during a setup phase
rather than on a per packet basis. The relevant resources are
not only the forwarding information maintained by the ST
agents, but also packet switch processor bandwidth and buffer
space, and network bandwidth and multicast group identifiers.
Reservation of these resources can help to increase the
reliability and decrease the delay and delay variance with
which data packets are delivered. The FlowSpec contains all
the information needed by the ST agent to allocate the
necessary resources. When and how these resources are
allocated depends on the details of the networks involved, and
is not specified here.
If an ST agent must send data across a network to a single
next-hop ST agent, then only the point-to-point bandwidth needs
to be reserved. If the agent must send data to multiple next-
hop agents across one network and network layer multicasting is
not available, then bandwidth must be reserved for all of them.
This will allow the ST agent to
use replication to send a copy of the data packets to each
next-hop agent.
If multicast is supported, its use will decrease the effort
that the ST agent must expend when forwarding packets and also
reduces the bandwidth required since one copy can be received
by all next-hop agents. However, the setup phase is more
complicated. A network multicast address must be allocated
that contains all those next-hop agents, the sender must have
access to that address, the next-hop agents must be informed of
the address so they can join the multicast group identified by
it (see Section 4.2.2.7 (page 86)), and a common HID must be
negotiated.
The network should consider the bandwidth and multicast
requirements to determine the amount of packet switch
processing bandwidth and buffer space to reserve for the
stream. In addition, the membership of a stream in a Group may
affect the resources that have to be allocated; see Section
3.7.3 (page 56).
Few networks in the Internet currently offer resource
reservation, and none that we know of offer reservation of all
the resources specified here. Only the Terrestrial Wideband
Network (TWBNet) [7] and the Atlantic Satellite Network
(SATNET) [9] offer(ed) bandwidth reservation. Multicasting is
more widely supported. No network provides for the reservation
of packet switch processing bandwidth or buffer space. We hope
that future networks will be designed to better support
protocols like ST.
Effects similar to reservation of the necessary resources may
be obtained even when the network cannot provide direct support
for the reservation. Certainly if total reservations are a
small fraction of the overall resources, such as packet switch
processing bandwidth, buffer space, or network bandwidth, then
the desired performance can be honored if the degree of
confidence is consistent with the requirements as stated in the
FlowSpec. Other solutions can be designed for specific
networks.
3.1.4. Sending CONNECT Messages
A VLId and a proposed HID must be selected for each next-hop
agent. The control packets for the next-hop must carry the
VLId in the SVLId field. The data packets transmitted in the
stream to the next-hop must carry the HID in the ST Header.
The ST agent sends a CONNECT message to each of the ST agents
identified by the routing function. Each CONNECT message
contains the VLId, the proposed HID (the HID Field option bit
must be set, see Section 3.6.1 (page 44)), an updated FlowSpec,
and a TargetList. In general, the HID, FlowSpec, and
TargetList will depend on both the next-hop and the intervening
network. Each TargetList is a subset of the received (or
original) TargetList, identifying the targets that are to be
reached through the next-hop to which the CONNECT message is
being sent. Note that a CONNECT message to a single next-hop
might have to be fragmented into multiple CONNECTs if the
single CONNECT is too large for the intervening network's MTU;
fragmentation is performed by further dividing the TargetList.
If multiple next-hops are to be reached through a network that
supports network level multicast, a different CONNECT message
must nevertheless be sent to each next-hop since each will have
a different TargetList; see Section 4.2.3.5 (page 105).
However, since an identical copy of each ensuing data packet
will reach each member of the multicast group, all the CONNECT
messages must propose the same HID. See Section 3.7.4 (page
58) for a detailed discussion on HID selection.
In the example of Figure 2, the routing function might return
that B is reachable via Agent 1 and C and D are reachable via
Agent 2. Thus A would create two CONNECT messages, one each
for Agents 1 and 2, as illustrated in Figure 5. Assuming that
the proposed HIDs are available in the receiving agents, they
would each send a responding HID-APPROVE back to Agent A.
Application Agent A Agent 1 Agent 2
1.1. (open B,C,D)
V
1.2. +-> (routing to B,C,D)
V
1.3. +->(reserve resources from A to Agent 1)
V
1.4. +-> CONNECT B --------->>
<RVLId=0><SVLId=4>
<Ref=10><HID=1200>
V
1.5. +->(reserve resources from A to Agent 2)
V
1.6. +-> CONNECT C,D ------------------>>
<RVLId=0><SVLId=5>
<Ref=15><HID=2400>
Figure 5. Origin Sending CONNECT Message
3.1.5. CONNECT Processing by an Intermediate Agent
An ST agent receiving a CONNECT message should, assuming no
errors, quickly select a VLId and respond to the previous-hop
with either an ACK, a HID-REJECT, or a HID-APPROVE message, as
is appropriate. This message must identify the CONNECT to
which it corresponds by including the CONNECT's Reference
number in its Reference field. Note that the VLId that this
agent selects is placed in the SVLId of the response, and the
previous-hop's VLId (which is contained in the SVLId of the
CONNECT) is copied into the RVLId of the response. If the
agent is not a target, it must then invoke the routing
function, reserve resources, and send a CONNECT message(s) to
its next-hop(s), as described in Sections 3.1.2-4 (pages 19-
20).
Agent A Agent 1 Agent B
[1.4] >>-> CONNECT B -------->+--+
<RVLId=0><SVLId=4> V
2.1. <Ref=10><HID=1200> (routing to B)
V
2.2. V +->(reserve resources from 1 to B)
2.3. +<- HID-APPROVE <------+ V
2.4. <RVLId=4><SVLId=14> +-> CONNECT B ---------->>
<Ref=10><HID=1200> <RVLId=0><SVLId=15>
<Ref=110><HID=3600>
Agent A Agent 2 Agent C
[1.6] >>-> CONNECT C,D ------>+-+
<RVLId=0><SVLId=5> V
2.5. <Ref=15><HID=2400> (routing to C,D)
V
2.6. V +-->(reserve resources from 2 to C)
2.7. +<- HID-APPROVE <------+ V
2.8. <RVLId=5><SVLId=23> +-> CONNECT C ---------->>
<Ref=15><HID=2400> <RVLId=0><SVLId=25>
<Ref=210><HID=4800>
Agent D
V
2.9. +->(reserve resources from 2 to D)
V
2.10. +-> CONNECT D ---------->>
<RVLId=0><SVLId=26>
<Ref=215><HID=4800>
Figure 6. CONNECT Processing by an Intermediate Agent
The resources listed as Desired in a received FlowSpec may not
correspond to those actually reserved in either the ST agent
itself or in the network(s) used to reach the next-hop
agent(s). As long as the reserved resources are sufficient to
meet the specified Limits, the copy of the FlowSpec sent to a
next-hop must have the Desired resources updated to reflect the
resources that were actually obtained. For example, the
Desired bandwidth might be reduced because the network to the
next-hop could not provide all of the desired bandwidth. Also,
the delay and delay variance are appropriately increased, and
the link MTU may require that the DesPDUBytes field be reduced.
(The minimum requirements that the origin had entered into the
FlowSpec Limits fields cannot be altered by the intermediate or
target agents.)
3.1.6. Setup at the Targets
An ST agent that is the target of a CONNECT, whether from an
intermediate ST agent, or directly from the origin host ST
agent, must respond first (assuming no errors) with either a
HID-REJECT or HID-APPROVE. After inquiring from the specified
application process whether or not it is willing to accept the
connection, the agent must also respond with either an ACCEPT
or a REFUSE.
In particular, the application must be presented with
parameters from the CONNECT, such as the Name, FlowSpec,
Options, and Group, to be used as a basis for its decision.
The application is identified by a combination of the NextPcol
field and the SAP field in the (usually) single remaining
Target of the TargetList. The contents of the SAP field may
specify the "port" or other local identifier for use by the
protocol layer above the host ST layer. Subsequently received
data packets will carry a short hand identifier (the HID) that
can be mapped into this information and be used for their
delivery.
The responses to the CONNECT message are sent to the previous-
hop from which the CONNECT was received. An ACCEPT contains
the Name of the stream and the updated FlowSpec. Note that the
application might have reduced the desired level of service in
the received FlowSpec before accepting it. The target must not
send the ACCEPT until HID negotiation has been successfully
completed.
Since the ACCEPT or REFUSE message must be acknowledged by the
previous-hop, it is assigned a new Reference number that will
be returned in the ACK. The CONNECT to which the ACCEPT or
REFUSE is a reply is identified by placing the CONNECT's
Reference number in the LnkReference field of the ACCEPT or
REFUSE.
Agent 1 Agent B Application B
3.1. (proc B listening)
[2.4] >>-> CONNECT B ---------->+------------------+
<RVLId=0><SVLId=15>
3.2. <Ref=110><HID=3600> V (proc B accepts)
3.3. +<- HID-APPROVE <--------+
<RVLId=15><SVLId=44>
<Ref=110><HID=3600> V
3.4. (wait until HID negotiated) <---+
V
3.5. <<--+<- ACCEPT B <-----------+
<RVLId=15><SVLId=44>
<Ref=410><LnkRef=110>
Agent 2 Agent C Application C
3.6. (proc C listening)
[2.8] >>-> CONNECT C ---------->+------------------+
<RVLId=0><SVLId=25>
3.7. <Ref=210><HID=4800> V (proc C accepts)
3.8. +<- HID-APPROVE <--------+
<RVLId=25><SVLId=54>
<Ref=210><HID=4800> V
3.9. (wait until HID negotiated) <---+
V
3.10. <<--+<- ACCEPT C <-----------+
<RVLId=25><SVLId=54>
<Ref=510><LnkRef=210>
Agent 2 Agent D Application D
3.11. (proc D listening)
[2.10] >>-> CONNECT D ---------->+------------------+
<RVLId=0><SVLId=26>
3.12. <Ref=215><HID=4800> V (proc D accepts)
3.13. +<- HID-APPROVE <--------+
<RVLId=26><SVLId=64>
<Ref=215><HID=4800> V
3.14. (wait until HID negotiated) <---+
V
3.15. <<--+<- ACCEPT D <-----------+
<RVLId=26><SVLId=64>
<Ref=610><LnkRef=215>
Figure 7. CONNECT Processing by the Target
3.1.7. ACCEPT Processing by an Intermediate Agent
When an intermediate ST agent receives an ACCEPT, it first
verifies that the message is a response to an earlier CONNECT.
If not, it responds to the next-hop ST agent with an ERROR-IN-
REPLY (LnkRefUnknown) message. Otherwise, it responds to the
next-hop ST agent with an ACK, and propagates
the ACCEPT message to the previous-hop along the same path
traced by the CONNECT but in the reverse direction toward the
origin. The ACCEPT should not be propagated until all HID
negotiations with the next-hop agent(s) have been successfully
completed.
The FlowSpec is included in the ACCEPT message so that the
origin and intermediate ST agents can gain access to the
information that was accumulated as the CONNECT traversed the
internet. Note that the resources, as specified in the
FlowSpec in the ACCEPT message, may differ from the resources
that were reserved by the agent when the CONNECT was
Agent A Agent 1 Agent B
+<-+<- ACCEPT B <-------<< [3.5]
V <RVLId=15><SVLId=44>
4.1. (wait for ACCEPTS) V <Ref=410><LnkRef=110>
4.2. V +-> ACK --------------->+
4.3. (wait until HID negotiated)<-+ <RVLId=44><SVLId=15>
V <Ref=410>
4.4. <<--+<-- ACCEPT B <---------+
<RVLId=4><SVLId=14>
<Ref=115><LnkRef=10>
Agent A Agent 2 Agent C
+<-+<- ACCEPT C <------<< [3.10]
<RVLId=25><SVLId=54>
V <Ref=510><LnkRef=210>
4.5. +-> ACK --------------->+
<Ref=510>
<RVLId=54><SVLId=25>
Agent D
V
+<-+<- ACCEPT D <------<< [3.15]
V <RVLId=26><SVLId=64>
4.6. (wait for ACCEPTS) V <Ref=610><LnkRef=215>
4.7. V +-> ACK --------------->+
4.8. (wait until HID negotiated)<-+ <RVLId=64><SVLId=26>
V <Ref=610>
4.9. <<--+<- ACCEPT C <----------+
<RVLId=5><SVLId=23>
<Ref=220><LnkRef=15>
V
4.10. <<--+<- ACCEPT D <----------+
<RVLId=5><SVLId=23>
<Ref=225><LnkRef=15>
Figure 8. ACCEPT Processing by an Intermediate Agent
originally processed. However, the agent does not adjust the
reservation in response to the ACCEPT. It is expected that any
excess resource allocation will be released for use by other
stream or datagram traffic through an explicit CHANGE message
initiated by the application at the origin if it does not wish
to be charged for any excess resource allocations.
3.1.8. ACCEPT Processing by the Origin
The origin will eventually receive an ACCEPT (or REFUSE or
ERROR-IN-REQUEST) message from each of the targets. As each
ACCEPT is received, the application should be notified of the
target and the resources that were successfully allocated along
the path to it, as specified in the FlowSpec contained in the
ACCEPT message. The application may then use the information
to either adopt or terminate the portion of the stream to each
target. When ACCEPTs (or failures) from all targets have been
received at the origin, the application is notified that stream
setup is complete, and that data may be sent.
Application A Agent A Agent 1 Agent 2
+<-- ACCEPT B <--------<< [4.4]
<RVLId=4><SVLId=14>
V <Ref=115><LnkRef=10>
5.1. +--> ACK ----------------->+
<RVLId=14><SVLId=4>
V <Ref=115>
5.2. +<-- (inform A of B's FlowSpec)
+<-- ACCEPT C <----------------<< [4.9]
<RVLId=5><SVLId=23>
V <Ref=220><LnkRef=15>
5.3. +--> ACK ------------------------->+
<RVLId=23><SVLId=5>
V <Ref=220>
5.4. +<-- (inform A of C's FlowSpec)
+<-- ACCEPT D <----------------<< [4.10]
<RVLId=5><SVLId=23>
V <Ref=225><LnkRef=15>
5.5. +--> ACK ------------------------->+
<RVLId=23><SVLId=5>
V <Ref=225>
5.6. +<-- (inform A of D's FlowSpec)
V
5.7. (wait until HIDs negotiated)
V
5.8. (inform A open to B,C,D)
Figure 9. ACCEPT Processing by the Origin
There are several pieces of information contained in the
FlowSpec that the application must combine before sending data
through the stream. The PDU size should be computed from the
minimum value of the DesPDUBytes field from all ACCEPTs and the
protocol layers above ST should be informed of the limit. It
is expected that the next higher protocol layer above ST will
segment its PDUs accordingly. Note, however, that the MTU may
decrease over the life of the stream if new targets are
subsequently added. Whether the MTU should be increased as
targets are dropped from a stream is left for further study.
The available bandwidth and packet rate limits must also be
combined. In this case, however, it may not be possible to
select a pair of values that may be used for all paths, e.g.,
one path may have selected a low rate of large packets while
another selected a high rate of small packets. The application
may remedy the situation by either tearing down the stream,
dropping some participants, or creating a second stream.
After any differences have been resolved (or some targets have
been deleted by the application to permit resolution), the
application at the origin should send a CHANGE message to
release any excess resources along paths to those targets that
exceed the resolved parameters for the stream, thereby reducing
the costs that will be incurred by the stream.
3.1.9. Processing a REFUSE Message
REFUSE messages are used to indicate a failure to reach an
application at a target; they are propagated toward the origin
of a stream. They are used in three situations:
1 during stream setup or expansion to indicate that there
is no satisfactory path from an ST agent to a target,
2 when the application at the target either does not
exist does not wish to be a participant, or wants to
cease being a participant, and
3 when a failure has been detected and the agents are
trying to find a suitable path around the failure.
The cases are distinguished by the ReasonCode field and an
agent receiving a REFUSE message must examine that field in
order to determine the proper action to be taken. In
particular, if the ReasonCode indicates that the CONNECT
message reached the target then the REFUSE should be propagated
back to the origin, releasing resources as appropriate along
the way. If the ReasonCode indicates that
the CONNECT message did not reach the target then the
intermediate (origin) ST agent(s) should check for alternate
routes to the target before propagating the REFUSE back another
hop toward the origin. This implies that an agent must keep
track of the next-hops that it has tried, on a target by target
basis, in order not to get caught in a loop.
An ST agent that receives a REFUSE message must acknowledge it
by sending an ACK to the next-hop. The REFUSE must also be
propagated back to the previous-hop ST agent. Note that the ST
agent may not have any information about the target in
Appl. Agent A Agent 2 Agent E
(proc E NOT listening)
1. (add E)
2. +----->+-> CONNECT E ---------->+->+
<RVLId=23><SVLId=5>
<Ref=65> V
3. +<-- ACK <---------------+
<RVLId=5><SVLId=23> V
4. <Ref=65> (routing to E)
V
5. (reserve resources 2 to E)
V
6. +--> CONNECT E --------->+
<RVLId=0><SVLId=27>
<Ref=115><HID=4600>
V
7. +<-+<- REFUSE B <-----------+
<RVLId=27><SVLId=74>
<Ref=705><LnkRef=115>
V <RC=SAPUnknown>
8. +-> ACK ---------------->+
<RVLId=74><SVLId=27>
V <Ref=705>
9. (free link 27) V
10. V (free link 74)
11. +<- REFUSE B <-----------+
<RVLId=5><SVLId=23>
<Ref=550><LnkRef=65> V
12. <RC=SAPUnknown> (free resources 2 to E)
V
13. +-> ACK --------------->+
<RVLId=23><SVLId=5>
<Ref=550> V
14. V (keep link 23 for C,D)
15. (keep link 5 for C,D)
V
16. (inform application failed SAPUnknown)
Figure 10. Sending REFUSE Message
the TargetList. This may result from interacting DISCONNECT
and REFUSE messages and should be logged and silently ignored.
If, after deleting the specified target, the next-hop has no
remaining targets, then those resources associated with that
next-hop agent may be released. Note that network resources
may not actually be released if network multicasting is being
Appl. Agent A Agent 2 Agent 1 Agent 3 Agent B
1. (network from 1 to B fails)
2. (add B)
3. +-> CONNECT B ----------------->+
<RVLId=0><SVLId=6>
<Ref=35><HID=100>
3. +<- HID-APPROVE <---------------+
<RVLId=6><SVLId=11>
<Ref=35><HID=100> V
4. (routing to B: no route)
V
5. +<-+-- REFUSE B ----------------+
<RVLId=6><SVLId=11>
<Ref=155><LnkRef=35>
V <RC=NoRouteToDest>
6. +-> ACK -------------------->+
<RVLId=11><SVLId=6> V
7. V <Ref=155> (drop link 6)
8. V (drop link 11)
9. (find alternative route: via agent 2)
10. (resources from A to 2 already allocated:
V reuse control link & HID, no additional resources required)
11. +-> CONNECT B -------->+->+
<RVLId=23><SVLId=5>
<Ref=40> V
12. +<- ACK <--------------+
<RVLId=5><SVLId=23> V
13. <Ref=40> (routing to B: via agent 3)
V
14. +-> CONNECT B -->+
15. <RVLId=0><SVLId=24> +-> CONNECT B --------->+
<Ref=245><HID=4801> V <RVLId=0><SVLId=32>
16. +<- HID-APPROVE -+ <Ref=310><HID=6000>
<RVLId=24><SVLId=33>
<Ref=245><HID=4801> V
17. +<- HID-APPROVE --------+
<RVLId=32><SVLId=45>
<Ref=310><HID=6000> V
18. (ACCEPT handling follows normally to complete stream setup)
Figure 11. Routing Around a Failure
used since they may still be required for traffic to other
next-hops in the multicast group.
When the REFUSE reaches a origin, the origin sends an ACK and
notifies the application via the next higher layer protocol
that the target listed in the TargetList is no longer part of
the stream and also if the stream has no remaining targets. If
there are no remaining targets, the application may wish to
terminate the stream.
Figure 10 illustrates the protocol exchanges for processing a
REFUSE generated at the target, either because the target
application is not running or that the target application
rejects membership in the stream. Figure 11 illustrates the
case of rerouting around a failure by an intermediate agent
that detects a failure or receives a refuse. The protocol
exchanges used by an application at the target to delete itself
from the stream is discussed in Section 3.3.3 (page 35).
3.2. Data Transfer
At the end of the connection setup phase, the origin, each target,
and each intermediate ST agent has a database entry that allows it
to forward the data packets from the origin to the targets and to
recover from failures of the intermediate agents or networks. The
database should be optimized to make the packet forwarding task
most efficient. The time critical operation is an intermediate
agent receiving a packet from the previous-hop agent and
forwarding it to the next-hop agent(s). The database entry must
also contain the FlowSpec, utilization information, the address of
the origin and previous-hop, and the addresses of the targets and
next-hops, so it can perform enforcement and recover from
failures.
An ST agent receives data packets encapsulated by an ST header. A
data packet received by an ST agent contains the non-zero HID
assigned to the stream for the branch from the previous-hop to
itself. This HID was selected so that it is unique at the
receiving ST agent and thus can be used, e.g., as an index into
the database, to obtain quickly the necessary replication and
forwarding information.
The forwarding information will be network and implementation
specific, but must identify the next-hop agent or agents and their
respective HIDs. It is suggested that the cached information for
a next-hop agent include the local network address of the next-
hop. If the data packet must be forwarded to multiple next-hops
across a single network that supports multicast, the database may
specify a single HID and may identify the next-hops by a (local
network) multicast address.
If the network does not support multicast, or the next-hops are on
different networks, then the database must indicate multiple
(next-hop, HID) tuples. When multiple copies of the data packet
must be sent, it may be necessary to invoke a packet replicator.
Data packets should not require fragmentation as the next higher
protocol layer at the origin was informed of the minimum MTU over
all paths in the stream and is expected to segment its PDUs
accordingly. However, it may be the case that a data packet that
is being rerouted around a failed network component may be too
large for the MTU of an intervening network. This should be a
transient condition that will be corrected as soon as the new
minimum MTU has been propagated back to the origin. Disposition
by a mechanism other than dropping of the too large PDUs is left
for further study.
3.3. Modifying an Existing Stream
Some applications may wish to change the parameters of a stream
after it has been created. Possible changes include adding or
deleting targets and changing the FlowSpec. These are described
below.
3.3.1. Adding a Target
It is possible for an application to add a new target to an
existing stream any time after ST has incorporated information
about the stream into its database. At a high level, the
application entities exchanges whatever information is
necessary. Although the mechanism or protocol used to
accomplish this is not specified here, it is necessary for the
higher layer protocol to inform the host ST agent at the origin
of this event. The host ST agent at the target must also be
informed unless this had previously been done. Generally, the
transfer of a target list from an ST agent to another, or from
a higher layer protocol to a host ST agent, will occur
atomically when the CONNECT is received. Any information
concerning a new target received after this point can be viewed
as a stream expansion by the receiving ST agent. However, it
may be possible that an ST agent can utilize such information
if it is received before it makes the relevant routing
decisions. These implementation details are not specified
here, but implementations must be prepared to receive CONNECT
messages that represent expansions of streams that are still in
the process of being setup.
To expand an existing stream, the origin issues one or more
CONNECT messages that contain the Name, the VLId, the FlowSpec,
and the TargetList specifying the new target or targets. The
origin issues multiple CONNECT messages if
either the targets are to be reached through different next-hop
agents, or a single CONNECT message is too large for the
network MTU. The HID Field option is not set since the HID has
already been (or is being) negotiated for the hop;
consequently, the CONNECT is acknowledged with an ACK instead
of a HID-REJECT or HID-APPROVE.
Application Agent A Agent 2 Agent E
1. (open E)
2. V (proc E listening)
3. +->(routing to E)
V
4. +-> (check resources from A to Agent 2: already allocated,
V reuse control link & HID, no additional resources needed)
5. +-> CONNECT E --------->+->+
<RVLId=23><SVLId=5> V
6. <Ref=20> V (routing to E)
7. +<- ACK <---------------+ V
<RVLId=5><SVLId=23> +->(reserve resources 2 to E)
<Ref=20> V
8. +-> CONNECT E --------->+
<RVLId=0><SVLId=27>
<Ref=230><HID=4800>
9. +<- HID-APPROVE <-------+
<RVLId=27><SVLId=74>
<Ref=230><HID=4800> V
10. (proc E accepts)
11. (wait until HID negotiated)
V
12. +<-+<- ACCEPT E <----------+
V <RVLId=27><SVLId=74>
13. (wait for ACCEPTS) V <Ref=710><LnkRef=230>
14. V +-> ACK --------------->+
15. (wait until HID negotiated)<-+ <RVLId=74><SVLId=27>
V <Ref=710>
16. +<- ACCEPT E <-------+
<RVLId=5><SVLId=23>
V <Ref=235><LnkRef=20>
17. +-> ACK ------------>+
<RVLId=23><SVLId=5>
V <Ref=235>
18. +<-(inform A of E's FlowSpec)
V
19. +<-(wait for ACCEPTS)
V
20. +<-(wait until HID negotiated)
V
21. (inform A open to E)
Figure 12. Addition of Another Target
An ST agent that is already a node in the stream recognizes the
RVLId and verifies that the Name of the stream is the same. It
then checks if the intersection of the TargetList and the
targets of the established stream is empty. If this is not the
case, then the receiver responds with an ERROR-IN-REQUEST with
the appropriate reason code (RouteLoop) that contains a
TargetList of those targets that were duplicates; see Section
4.2.3.5 (page 106).
For each new target in the TargetList, processing is much the
same as for the original CONNECT; see Sections 3.1.2-4 (pages
19-20). The CONNECT must be acknowledged, propagated, and
network resources must be reserved. However, it may be
possible to route to the new targets using previously allocated
paths or an existing multicast group. In that case, additional
resources do not need to be reserved but more next-hop(s) might
have to be added to an existing multicast group.
Nevertheless, the origin, or any intermediate ST agent that
receives a CONNECT for an existing stream, can make a routing
decision that is independent of any it may have made
previously. Depending on the routing algorithm that is used,
the ST agent may decide to reach the new target by way of an
established branch, or it may decide to create a new branch.
The fact that a new target is being added to an existing stream
may result in a suboptimal overall routing for certain routing
algorithms. We take this problem to be unavoidable since it is
unlikely that the stream routing can be made optimal in
general, and the only way to avoid this loss of optimality is
to redefine the routing of potentially the entire stream, which
would be too expensive and time consuming.
3.3.2. The Origin Removing a Target
The application at the origin specifies a set of targets that
are to be removed from the stream and an appropriate reason
code (ApplDisconnect). The targets are partitioned into
multiple DISCONNECT messages based on the next-hop to the
individual targets. As with CONNECT messages, an ST agent that
is sending a DISCONNECT must make sure that the message fits
into the MTU for the intervening network. If the message is
too large, the TargetList must be further partitioned into
multiple DISCONNECT messages.
An ST agent that receives a DISCONNECT message must acknowledge
it by sending an ACK back to the previous-hop. The DISCONNECT
must also be propagated to the relevant next-hop ST agents.
Before propagating the message, however, the TargetList should
be partitioned based on next-hop ST
agent and MTU, as described above. Note that there may be
targets in the TargetList for which the ST agent has no
information. This may result from interacting DISCONNECT and
REFUSE messages and should be logged and silently ignored.
If, after deleting the specified targets, any next-hop has no
remaining targets, then those resources associated with that
next-hop agent may be released. Note that network resources
may not actually be released if network multicasting is being
used since they may still be required for traffic to other
next-hops in the multicast group.
Application Application
Agent A Agent 1 Agent 2 Agent B C
1. (close B,C ApplDisconnect)
V
2. +->+-+-> DISCONNECT B ----->+
3. <RVLId=14><SVLId=4>+-+-> DISCONNECT B ------>+
<Ref=25> <RVLId=44><SVLId=15>
V <RC=ApplDisconnect> <Ref=120>
4. (free A to 1 resrc.) V <RC=ApplDisconnect>
5. V (free 1 to B resrc.)
6. +<- ACK <--------------+ V
7. <RVLId=4><SVLId=14> +<- ACK <---------------+
V <Ref=25> <RVLId=15><SVLId=44>
8. (free link 4) V <Ref=120>
9. (free link 14) V
10. (free link 15) V
11. (inform B that stream closed ApplDisconnect)
12. (free link 44)
V
13. +<-+-+-> DISCONNECT C ---------->+
14. <RVLId=23><SVLId=5> +-+-> DISCONNECT C ------>+
<Ref=30> <RVLId=54><SVLId=25>
V <RC=ApplDisconnect> <Ref=240>
15. (keep A to 2 resrc for V <RC=ApplDisconnect>
16. data going to D,E) (free 2 to C resrc.)
V
17. +<- ACK <-------------------+ V
18. <RVLId=5><SVLId=23> +<- ACK <---------------+
V <Ref=30> <RVLId=25><SVLId=54>
19. (keep link 5 for D,E) V <Ref=240>
20. (keep link 23 for D,E) V
21. (free link 25) V
22. (inform C that stream closed ApplDisconnect>)
23. V (free link 54)
24. (inform A closed to B,C ApplDisconnect)
Figure 13. Origin Removing a Target
When the DISCONNECT reaches a target, the target sends an ACK
and notifies the application that it is no longer part of the
stream and the reason. The application should then inform ST
to terminate the stream, and ST should delete the stream from
its database after performing any necessary management and
accounting functions.
3.3.3. A Target Deleting Itself
The application at the target may inform ST that it wants to be
removed from the stream and the appropriate reason code
(ApplDisconnect). The agent then forms a REFUSE message with
itself as the only entry in the TargetList. The REFUSE is sent
back to the origin via the previous-hop. If a stream has
multiple targets and one target leaves the stream using this
REFUSE mechanism, the stream to the other targets is not
affected; the stream continues to exist.
An ST agent that receives such a REFUSE message must
acknowledge it by sending an ACK to the next-hop. The target
is deleted and, if the next-hop has no remaining targets, then
the those resources associated with that next-hop agent may be
released. Note that network resources may not actually be
released if network multicasting is being used since they may
still be required for traffic to other next-hops in the
multicast group. The REFUSE must also be propagated back to
the previous-hop ST agent.
Agent A Agent 2 Agent E
1. (close E ApplDisconnect)
V
2. +<- REFUSE E --+
<RVLId=27><SVLId=74>
<Ref=720>
V <RC=ApplDisconnect>
3. +<-+-> ACK ------>+
<RVLId=74><SVLId=27>
4. V V <Ref=720>
5. +<-+<- REFUSE E --+ (prune allocations)
<RVLId=5><SVLId=23>
<Ref=245>
V <RC=ApplDisconnect>
6. +-> ACK ------>+
<RVLId=23><SVLId=5>
V <Ref=245>
7. V (prune allocations)
8. (inform application closed E ApplDisconnect)
Figure 14. Target Deleting Itself
When the REFUSE reaches the origin, the origin sends an ACK and
notifies the application that the target listed in the
TargetList is no longer part of the stream. If the stream has
no remaining targets, the application may choose to terminate
the stream.
3.3.4. Changing the FlowSpec
An application may wish to change the FlowSpec of an
established stream. To do so, it informs ST of the new
FlowSpec and the list of targets that are to be changed. The
origin ST agent then issues one or more CHANGE messages with
the new FlowSpec and sends them to the relevant next-hop
agents. CHANGE messages are structured and processed similarly
to CONNECT messages. A next-hop agent that is an intermediate
agent and receives a CHANGE message similarly determines if it
can implement the new FlowSpec along the hop to each of its
next-hop agents, and if so, it propagates the CHANGE messages
along the established paths. If this process succeeds, the
CHANGE messages will eventually reach the targets, which will
each respond with an ACCEPT message that is propagated back to
the origin.
Note that since a CHANGE may be sent containing a FlowSpec with
a range of permissible values for bandwidth, delay, and/or
error rate, and the actual values returned in the ACCEPTs may
differ, then another CHANGE may be required to release excess
resources along some of the paths.
3.4. Stream Tear Down
A stream is usually terminated by the origin when it has no
further data to send, but may also be partially torn down by the
individual targets. These cases will not be further discussed
since they have already been described in Sections 3.3.2-3 (pages
33-35).
A stream is also torn down if the application should terminate
abnormally. Processing in this case is identical to the previous
descriptions except that the appropriate reason code is different
(ApplAbort).
When all targets have left a stream, the origin notifies the
application of that fact, and the application then is responsible
for terminating the stream. Note, however, that the application
may decide to add a target(s) to the stream instead of terminating
it.
3.5. Exceptional Cases
The previous descriptions covered the simple cases where
everything worked. We now discuss what happens when things do not
succeed. Included are situations where messages are lost, the
requested resources are not available, the routing fails or is
inconsistent.
In order for the ST Control Message Protocol to be reliable over
an unreliable internetwork, the problems of corruption,
duplication, loss, and ordering must be addressed. Corruption is
handled through use of checksumming, as described in Section 4
(page 76). Duplication of control messages is detected by
assigning a transaction number (Reference) to each control
message; duplicates are discarded. Loss is detected using a
timeout at the sender; messages that are not acknowledged before
the timeout expires are retransmitted; see Section 3.7.6 (page
66). If a message is not acknowledged after a few retransmissions
a fault is reported. The protocol does not have significant
ordering constraints. However, minor sequencing of control
messages for a stream is facilitated by the requirement that the
Reference numbers be monotonically increasing; see Section 4.2
(page 78).
3.5.1. Setup Failure due to CONNECT Timeout
If a response (an ERROR-IN-REQUEST, an ACK, a HID-REJECT, or a
HID-APPROVE) has not been received within time ToConnect, the
ST agent should retransmit the CONNECT message. If no response
has been received within NConnect retransmissions, then a fault
occurs and a REFUSE message with the appropriate reason code
(RetransTimeout) is sent back in the direction of the origin,
and, in place of the CONNECT, a DISCONNECT is sent to the
next-hop (in case the response to the CONNECT is the message
that was lost). The agent will expect an ACK for both the
REFUSE and the DISCONNECT messages. If it does not receive an
ACK after retransmission time ToRefuse and ToDisconnect
respectively, it will resend the REFUSE/DISCONNECT message. If
it does not receive ACKs after sending NRefuse/ NDisconnect
consecutive REFUSE/DISCONNECT messages, then it simply gives up
trying.
Sending Agent Receiving Agent
1. ->+----> CONNECT X ------>//// (message lost or garbled)
<RVLId=0><SVLId=99>
V <Ref=1278><HID=1234>
2. (timeout)
V
3. +----> CONNECT X ------------>+
4. <RVLId=0><SVLId=99> +----> CONNECT X ----------->+
<Ref=1278><HID=1234> V <RVLId=0><SVLId=1010>
5. //<- HID-APPROVE <----------+ <Ref=6666><HID=6666> V
6. <RVLId=99><SVLId=88> +<- HID-APPROVE <---------+
V <Ref=1278><HID=1234> <RVLId=1010><SVLId=1111>
7. (timeout) <Ref=6666><HID=6666>
V
8. +----> CONNECT X ------------>+
<RVLId=0><SVLId=99>
<Ref=1278><HID=1234> V
9. +<-+<- HID-APPROVE <----------+
<RVLId=99><SVLId=88>
V <Ref=1278><HID=1234>
(cancel timer)
Figure 15. CONNECT Retransmission after a Timeout
3.5.2. Problems due to Routing Inconsistency
When an intermediate agent receives a CONNECT, it selects the
next-hop agents based on the TargetList and the networks to
which it is connected. If the resulting next-hop to any of the
targets is across the same network from which it received the
CONNECT (but not the previous-hop itself), there may be a
routing problem. However, the routing algorithm at the
previous-hop may be optimizing differently than the local
algorithm would in the same situation. Since the local ST
agent cannot distinguish the two cases, it should permit the
setup but send back to the previous-hop agent an informative
NOTIFY message with the appropriate reason code (RouteBack),
pertinent TargetList, and in the NextHopIPAddress element the
address of the next-hop ST agent returned by its routing
algorithm.
The agent that receives such a NOTIFY should ACK it. If the
agent is using an algorithm that would produce such behavior,
no further action is taken; if not, the agent should send a
DISCONNECT to the next-hop agent to correct the problem.
Alternatively, if the next-hop returned by the routing function
is in fact the previous-hop, a routing inconsistency has been
detected. In this case, a REFUSE is sent back to
the previous-hop agent containing an appropriate reason code
(RouteInconsist), pertinent TargetList, and in the
NextHopIPAddress element the address of the previous-hop. When
the previous-hop receives the REFUSE, it will recompute the
next-hop for the affected targets. If there is a difference in
the routing databases in the two agents, they may exchange
CONNECT and REFUSE messages again. Since such routing errors
in the internet are assumed to be temporary, the situation
should eventually stabilize.
3.5.3. Setup Failure due to a Routing Failure
It is possible for an agent to receive a CONNECT message that
contains a known Name, but from an agent other than the
previous-hop agent of the stream with that Name. This may be:
1 that two branches of the tree forming the stream have
joined back together,
2 a deliberate source routing loop,
3 the result of an attempted recovery of a partially
failed stream, or
4 an erroneous routing loop.
The TargetList is used to distinguish the cases 1 and 2 (see
also Section 4.2.3.5 (page 107)) by comparing each newly
received target with those of the previously existing stream:
o if the IP address of the targets differ, it is case 1;
o if the IP address of the targets match but the source
route(s) are different, it is case 2;
o if the target (including any source route) matches a
target (including any source route) in the existing
stream, it may be case 3 or 4.
It is expected that the joining of branches will become more
common as routing decisions are based on policy issues and not
just simple connectivity. Unfortunately, there is no good way
to merge the two parts of the stream back into a single stream.
They must be treated independently with respect to processing
in the agent. In particular, a separate state machine is
required, the Virtual Link Identifiers and HIDs from the
previous-hops and to the next-hops must be different, and
duplicate resources must be reserved in both the agent and in
any next-hop networks. Processing is the same for a deliberate
source routing loop.
The remaining cases requiring recovery, a partially failed
stream and an erroneous routing loop, are not easily
distinguishable. In attempting recovery of a failed stream, an
agent may issue new CONNECT messages to the affected targets;
for a full explanation see also Section 3.7.2 (page 51),
Failure Recovery. Such a CONNECT may reach an agent downstream
of the failure before that agent has received a DISCONNECT from
the neighborhood of the failure. Until that agent receives the
DISCONNECT, it cannot distinguish between a failure recovery
and an erroneous routing loop. That agent must therefore
respond to the CONNECT with a REFUSE message with the affected
targets specified in the TargetList and an appropriate reason
code (StreamExists).
The agent immediately preceding that point, i.e., the latest
agent to send the CONNECT message, will receive the REFUSE
message. It must release any resources reserved exclusively
for traffic to the listed targets. If this agent was not the
one attempting the stream recovery, then it cannot distinguish
between a failure recovery and an erroneous routing loop. It
should repeat the CONNECT after a ToConnect timeout. If after
NConnect retransmissions it continues to receive REFUSE
messages, it should propagate the REFUSE message toward the
origin, with the TargetList that specifies the affected
targets, but with a different error code (RouteLoop).
The REFUSE message with this error code (RouteLoop) is
propagated by each ST agent without retransmitting any CONNECT
messages. At each agent, it causes any resources reserved
exclusively for the listed targets to be released. The REFUSE
will be propagated to the origin in the case of an erroneous
routing loop. In the case of stream recovery, it will be
propagated to the ST agent that is attempting the recovery,
which may be an intermediate agent or the origin itself. In
the case of a stream recovery, the agent attempting the
recovery may issue new CONNECT messages to the same or to
different next-hops.
If an agent receives both a REFUSE message and a DISCONNECT
message with a target in common then it can release the
relevant resources and propagate neither the REFUSE nor the
DISCONNECT (however, we feel that it is unlikely that most
implementations will be able to detect this situation).
If the origin receives such a REFUSE message, it should attempt
to send a new CONNECT to all the affected targets. Since
routing errors in an internet are assumed to be temporary, the
new CONNECTs will eventually find acceptable routes to the
targets, if one exists. If no further routes exist after
NRetryRoute tries, the application should be
informed so that it may take whatever action it deems
necessary.
3.5.4. Problems in Reserving Resources
If the network or ST agent resources are not available, an ST
agent may preempt one or more streams that have lower
precedence than the one being created. When it breaks a lower
precedence stream, it must issue REFUSE and DISCONNECT messages
as described in Sections 4.2.3.15 (page 122) and 4.2.3.6 (page
110). If there are no streams of lower precedence, or if
preempting them would not provide sufficient resources, then
the stream cannot be accepted by the ST agent.
If an intermediate agent detects that it cannot allocate the
necessary resources, then it sends a REFUSE that contains an
appropriate reason code (CantGetResrc) and the pertinent
TargetList to the previous-hop ST agent. For further study are
issues of reporting what resources are available, whether the
resource shortage is permanent or transitory, and in the latter
case, an estimate of how long before the requested resources
might be available.
3.5.5. Setup Failure due to ACCEPT Timeout
An ST agent that propagates an ACCEPT message backward toward
the origin expects an ACK from the previous-hop. If it does
not receive an ACK within a timeout, called ToAccept, it will
retransmit the ACCEPT. If it does not receive an ACK after
sending a number, called NAccept, of ACCEPT messages, then it
will replace the ACCEPT with a REFUSE, and will send a
DISCONNECT in the direction toward the target. Both the REFUSE
and DISCONNECT will identify the affected target(s) and specify
an appropriate reason code (AcceptTimeout). Both are also
retransmitted until ACKed with timeout ToRefuse/ ToDisconnect
and retransmit count NRefuse/NDisconnect. If they are not
ACKed, the agent simply gives up, letting the failure detection
mechanism described in Section 3.7.1 (page 48) take care of any
cleanup.
3.5.6. Problems Caused by CHANGE Messages
An application must exercise care when changing a FlowSpec to
prevent a failure. A CHANGE might fail for two reasons. The
request may be for a larger amount of network resources when
those resources are not available; this failure may be
prevented by requiring that the current level of service be
contained within the ranges of the FlowSpec in the CHANGE.
Alternatively, the local network might require all the former
resources to be released before the new ones are requested and,
due to unlucky timing, an unrelated request for network
resources might be processed between the time the resources are
released and the time the new resources are requested, so that
the former resources are no longer available. There is not
much that an application or ST can do to prevent such failures.
If the attempt to change the FlowSpec fails then the ST agent
where the failure occurs must intentionally break the stream
and invoke the stream recovery mechanism using REFUSE and
DISCONNECT messages; see Section 3.7.2 (page 51). Note that
the reserved resources after the failure of a CHANGE may not be
the same as before, i.e., the CHANGE may have been partially
completed. The application is responsible for any cleanup
(another CHANGE).
3.5.7. Notification of Changes Forced by Failures
NOTIFY is issued by a an ST Agent to inform upsteam agents and
the origin that resource allocation changes have occurred after
a stream was established. These changes occur when network
components fail and when competing streams preempt resources
previously reserved by a lower precedence stream. We also
anticipate that NOTIFY can be used in the future when
additional resources become available, as is the case when
network components recover or when higher precedence streams
are deleted.
NOTIFY is also used to inform upstream agents that a routing
anomaly has occurred. Such an example was cited in Section
3.5.2 (page 38), where an agent notices that the next-hop agent
is on the same network as the previous-hop agent; the anomaly
is that the previous-hop should have connected directly to the
next-hop without using an intermediate agent. Delays in
propagating host status and routing information can cause such
anomalies to occur. NOTIFY allows ST to correct automatically
such mistakes.
NOTIFY reports a FlowSpec that reflects that revised guarantee
that can be promised to the stream. NOTIFY also
identifies those targets affected by the change. In this way,
NOTIFY is similar to ACCEPT. NOTIFY includes a ReasonCode to
identify the event that triggered the notification. It also
includes a TargetList, rather than a single Target, since a
single event can affect a branch leading to several targets.
NOTIFY is relayed by the ST agents back toward the origin,
along the path established by the CONNECT but in the reverse
direction. NOTIFY must be acknowledged with an ACK at each
hop. If intermediate agent corrects the situation without
causing any disruption to the data flow or guarantees, it can
choose to drop the notification message before it reaches the
origin. If the originating agent receives a NOTIFY, it is then
expected to adjust its own processing and data rates, and to
submit any required CHANGE requests. As with ACCEPT, the
FlowSpec is not modified on this trip from the target back to
the origin. It is up to the origin to decide whether a CHANGE
should be submitted. (However, even though the FlowSpec has
not been modified, the situation reported in the
Application Agent A Agent 1 Agent B
1. (high precedence request preempts 10K of
the stream's original 30Kb bandwidth
allocated to the hop from 1 to B)
V
2. +<------+-- NOTIFY -------------+
<RVLId=4><SVLId=14>
<Ref=150>
V <FlowSpec=20Kb,...><TargList=B>
3. +-> ACK --------------->+
<RVLId=14><SVLId=4>
V <Ref=150>
4. (inform application)
....
5. change(FlowSpec=20Kb,...)
V
6. +---------> CHANGE B ---------->+
7. <RVLId=14><SVLId=4> +--> CHANGE B ------------>+->+
<Ref=60> <RVLId=44><SVLId=15>
<FlowSpec=20Kb,...> V <Ref=160>
8. +<- ACK ----------------+ <FlowSpec=20Kb,...>
<RVLId=4><SVLId=14> V
9. <Ref=60> +--- ACK ------------------+
<RVLId=15><SVLId=44>
<Ref=160> V
... perform normal ACCEPT processing ... <-----+
Figure 16. Processing NOTIFY Messages
notify may have prevented the ST agents from meeting the
original guarantees.)
3.6. Options
Several options are defined in the CONNECT message. The special
processing required to support each will be described in the
following sections. The options are independent, i.e., can be set
to one (1, TRUE) or zero (0, FALSE) in any combination. However,
the effect and implementation of the options is NOT necessarily
independent, and not all combinations are supported.
3.6.1. HID Field Option
The sender of a CONNECT message may or not specify an HID in
the HID field. If the HID Field option of the CONNECT message
is not set (the H bit is 0), then the HID field does not
contain relevant information and should be ignored.
If this option is set (the H bit is 1), then the HID field
contains a relevant value. If this option is set and the HID
field of the CONNECT contains a non-zero value, that value
represents a proposed HID that initiates the HID negotiation.
If the HID Field option is set but the HID field of the CONNECT
message contains a zero, this means that the sender of that
CONNECT message has chosen to defer selection of the HID to the
next-hop agent (the receiver of a CONNECT message). This
choice can allow a more efficient mechanism for selecting HIDs
and possibly a more efficient mechanism for forwarding data
packets in the case when the previous-hop does not need to
select the HID; see also Section 4.2.3.5 (page 105).
Upon receipt of a CONNECT message with the HID Field option set
and the HID field set to zero, a next-hop agent selects the HID
for the hop, enters it into its appropriate data structure, and
returns it in the HID field of the HID-APPROVE message. The
previous-hop takes the HID from the HID-APPROVE message and
enters it into its appropriate data structure.
3.6.2. PTP Option
The PTP option (Point-to-Point) is used to indicate that the
stream will never have more than a single target. It
consequently implies that the stream will never need to support
any form of multicasting. Use of the PTP option may thus allow
efficiencies in the way the stream is built or is
managed. Specifically, the ST agents do not need to request
that the intervening networks allocate multicast groups to
support this stream.
The PTP option can only be set to one (1) by the origin, and
must be the same for the entire stream (i.e., propagated by ST
agents). The details of what this option does are
implementation specific, and do not affect the protocol very
much.
If the application attempts to add a new target to an existing
stream that was created with the PTP option set to one (1), the
application should be informed of the error with an ERROR-IN-
REQUEST message with the appropriate reason code. If a CONNECT
is received whose TargetList contains more than a single entry,
an ERROR-IN-REQUEST message with the appropriate reason code
(PTPError) should be returned to the previous-hop agent (note
that such a CONNECT should never be received if the origin both
implements the PTP option and is functioning properly).
As implied in the last paragraph, a subsetted implementation
might choose not to implement the PTP option.
3.6.3. FDx Option
The FDx option is used to indicate that a second stream in the
reverse direction, from the target to the origin, should
automatically be created. This option is most likely to be
used when the TargetList has only a single entry. If used when
the TargetList has multiple entries, the resulting streams
would allow bi-directional communication between the origin and
the various targets, but not among the targets. The FDx option
can only be invoked by the origin, and must be propagated by
intermediate agents.
This option is specified by inclusion of both an RFlowSpec and
an RHID parameter in the CONNECT message (possibly with an
optional RGroup parameter).
Any ST agent that receives a CONNECT message with both an
RFlowSpec and an RHID parameter will create database entries
for streams in both directions and will allocate resources in
both directions for them. By this we mean that an ST agent
will reserve resources to the next-hop agent for the normal
stream and resources back to the previous-hop agent for the
reverse stream. This is necessary since it is expected that
network reservation interfaces will require the destination
address(es) in order to make reservations, and because all ST
agents must use the same reservation model.
The target agent will select a Name for the reverse stream and
return it (in the RName parameter) and the resulting FlowSpec
(in the RFlowSpec parameter) of the ACCEPT message. Each agent
that processes the ACCEPT will update its partial stream
database entry for the reverse stream with the Name contained
in the RName parameter. We assume that the next higher
protocol layer will use the same SAP for both streams.
3.6.4. NoRecovery Option
The NoRecovery option is used to indicate that ST agents should
not attempt recovery in case of network or component failure.
If a failure occurs, the origin will be notified via a REFUSE
message and the target(s) via a DISCONNECT, with an appropriate
reason code of "failure" (i.e., one of DropFailAgt,
DropFailHst, DropFailIfc, DropFailNet, IntfcFailure,
NetworkFailure, STAgentFailure, FailureRecovery). They can
then decide whether to wait for the failed component to be
fixed, or drop the target via DISCONNECT/REFUSE messages. The
NoRecovery option can only be set to one (1) by the origin, and
must be the same for the entire stream.
3.6.5. RevChrg Option
The RevChrg option bit in the FlowSpec is set to one (1) by the
origin to request that the target(s) pay any charges associated
with the stream (to the target(s)); see Section 4.2.2.3 (page
83). If the target is not willing to accept charges, the bit
should be set to zero (0) by the target before returning the
FlowSpec to the origin in an ACCEPT message.
If the FDx option is also specified, the target pays charges
for both streams.
3.6.6. Source Route Option
The Source Route Option may be used both for diagnostic
purposes, and, in those hopefully infrequent cases where the
standard routing mechanisms do not produce paths that satisfy
some policy constraint, to allow the origin to prespecify the
ST agents along the path to the target(s). The idea is that
the origin can explicitly specify the path to a target, either
strictly hop-by-hop or more loosely by specification of one or
more agents through which the path must pass.
The option is specified by including source routing information
in the Target structure. A target may contain zero or more
SrcRoute options; when multiple options are present, they are
processed in the order in which they occur. The parameter code
indicates whether the portion of the path contained in the
parameter is of the strict or loose variety.
Since portions of a path may pass through portions of an
internet that does not support ST agents, there are also forms
of the SrcRoute option that are converted into the
Application Agent A Agent 2 Agent 3 Agent B
1. (open B<SR=2,3>)
2. V (proc B listening)
3. (source routed to 2)
V
4. (check resources from A to Agent 2: already allocated,
V reuse control link & HID, no additional resources needed)
5. +-> CONNECT B<SR=2,3>->-+-+
<RVLId=23><SVLId=5>
6. <Ref=50> V
7. +<- ACK ----------------+
<RVLId=5><SVLId=23>
<Ref=50> V
8. (source routed to 3)
V
9. (reserve resources 2 to 3)
V
10. +-> CONNECT B<SR=3> ---->+
<RVLId=0><SVLId=24>
<Ref=280><HID=4801> V
11. +<- HID-APPROVE <--------+
<RVLId=24><SVLId=33>
<Ref=280><HID=4801>
V
(routing to B)
V
(reserve resources from 3 to B)
V
12. +-> CONNECT B ---------->+
<RVLId=0><SVLId=32>
<Ref=330><HID=6000> V
13. +<- HID-APPROVE <--------+
<RVLId=32><SVLId=45>
<Ref=330><HID=6000> V
14. (proc B accepts)
V
... perform normal ACCEPT processing ... <-----+
Figure 17. Source Routing Option
corresponding IP Source Routing options by the ST agent that
performs the encapsulation.
The SrcRoute option is usually selected by the origin, but may
be used by intermediate agents if specified as a result of the
routing function.
For example, in the topology of Figure 2, if A wants to add B
back into the stream, its routing function might decide that
the best path is via Agent 3. Since the data is already being
multicast across the network connected to C, D, and E, the
route via Agent 3 might cost less than having A replicate the
data packets and send them across A's network a second time.
3.7. Ancillary Functions
There are several functions and procedures that are required by
the ST Protocol. They are described in subsequent sections.
3.7.1. Failure Detection
The ST failure detection mechanism is based on two assumptions:
1 If a neighbor of an ST agent is up, and has been up
without a disruption, and has not notified the ST agent
of a problem with streams that pass through both, then
the ST agent can assume that there has not been any
problem with those streams.
2 A network through which an ST agent has routed a stream
will notify the ST agent if there is a problem that
affects the stream data packets but does not affect the
control packets.
The purpose of the robustness protocol defined here is for ST
agents to determine that the streams through a neighbor have
been broken by the failure of the neighbor or the intervening
network. This protocol should detect the overwhelming majority
of failures that can occur. Once a failure is detected,
recovery procedures are initiated.
3.7.1.1. Network Failures
In this memo, a network is defined to be the protocol
layer(s) below ST. This function can be implemented in a
hardware module separate from the ST agent, or as software
modules within the ST agent itself, or as a combination of
both. This specification and the robustness protocol do not
differentiate between these alternatives.
An ST agent can detect network failures by two mechanisms;
the network can report a failure, or the ST agent can
discover a failure by itself. They differ in the amount of
information that ST agent has available to it in order to
make a recovery decision. For example, a network may be
able to report that reserved bandwidth has been lost and the
reason for the loss and may also report that connectivity to
the neighboring ST agent remains intact. In this case, the
ST agent may request the network to allocate bandwidth anew.
On the other hand, an ST agent may discover that
communication with a neighboring ST agent has ceased because
it has not received any traffic from that neighbor in some
time period. If an ST agent detects a failure, it may not
be able to determine if the failure was in the network while
the neighbor remains available, or the neighbor has failed
while the network remains intact.
3.7.1.2. Detecting ST Stream Failures
Each ST agent periodically sends each neighbor with which it
shares a stream a HELLO message. A HELLO message is ACKed
if the Reference field is non-zero. This message exchange
is between ST agents, not entities representing streams or
applications (there is no Name field in a HELLO message).
That is, an ST agent need only send a single HELLO message
to a neighbor regardless of the number of streams that flow
between them. All ST agents (host as well as intermediate)
must participate in this exchange. However, only agents
that share active streams need to participate in this
exchange.
To facilitate processing of HELLO messages, an
implementation may either create a separate Virtual Link
Identifier for each neighbor having an active stream, or may
use the reserved identifier of one (1) for the SVLId field
in all its HELLO messages.
An implementation that wishes to send its HELLO messages via
a data path instead of the control path may setup a separate
stream to its neighbor agent for that purpose. The HELLO
message would contain a HID of zero, indicating a control
message, but would be identified to the next lower protocol
layer as being part of the separate stream.
As well as identifying the sender, the HELLO message has two
fields; a HelloTimer field that is in units of milliseconds
modulo the maximum for the field size, and a
Restarted bit specifying that the ST agent has been
restarted recently. The HelloTimer must appear to be
incremented every millisecond whether a HELLO message is
sent or not, but it is allowable for an ST agent to create a
new HelloTimer only when it sends a HELLO message. The
HelloTimer wraps around to zero after reaching the maximum
value. Whenever an ST agent suffers a catastrophic event
that may result in it losing ST state information, it must
reset its HelloTimer to zero and must set the Restarted bit
for the following HelloTimerHoldDown seconds.
An ST agent must send HELLO messages to its neighbor with a
period shorter than the smallest RecoveryTimeout parameter
of the FlowSpecs of all the active streams that pass between
the two agents, regardless of direction. This period must
be smaller by a factor, called HelloLossFactor, which is at
least as large as the greatest number of consecutive HELLO
messages that could credibly be lost while the communication
between the two ST agents is still viable.
An ST agent may send simultaneous HELLO messages to all its
neighbors at the rate necessary to support the smallest
RecoveryTimeout of any active stream. Alternately, it may
send HELLO messages to different neighbors independently at
different rates corresponding to RecoveryTimeouts of
individual streams.
The agent that receives a HELLO message expects to receive
at least one new HELLO message from a neighbor during the
RecoveryTimeout of every active stream through that
neighbor. It can detect duplicate or delayed HELLO messages
by saving the HelloTimer field of the most recent valid
HELLO message from that neighbor and comparing it with the
HelloTimer field of incoming HELLO messages. It will only
accept an incoming HELLO message from that neighbor if it
has a HelloTimer field that is greater than the most recent
valid HELLO message by the time elapsed since that message
was received plus twice the maximum likely delay variance
from that neighbor. If the ST agent does not receive a
valid HELLO message within the RecoveryTimeout of a stream,
it must assume that the neighboring ST agent or the
communication link between the two has failed and it must
initiate stream recovery activity.
Furthermore, if an ST agent receives a HELLO message that
contains the Restarted bit set, it must assume that the
sending ST agent has lost its ST state. If it shares
streams with that neighbor, it must initiate stream recovery
activity. If it does not share streams with that neighbor,
it should not attempt to create one until that
bit is no longer set. If an ST agent receives a CONNECT
message from a neighbor whose Restarted bit is still set, it
must respond with ERROR-IN-REQUEST with the appropriate
reason code (RemoteRestart). If it receives a CONNECT
message while its own Restarted bit is set, it must respond
with ERROR-IN-REQUEST with the appropriate reason code
(RestartLocal).
3.7.1.3. Subset
This failure detection mechanism subsets by reducing the
complexity of the timing and decisions. A subsetted ST
agent sends HELLO messages to all its ST neighbors
regardless of whether there is an active ST stream between
them or not. The RecoveryTimeout parameter of the FlowSpec
is ignored and is assumed to be the DefaultRecoveryTimeout.
Note that this implies that a REFUSE should be sent for all
CONNECT or CHANGE messages whose RecoveryTimeout is less
than DefaultRecoveryTimeout. An ST agent will accept an
incoming HELLO message if it has a HelloTimer field that is
greater than the most recent valid HELLO message by
DefaultHelloFactor times the time elapsed since that message
was received.
3.7.2. Failure Recovery
Streams can fail from various causes; an ST agent can break, a
network can break, or an ST agent can intentionally break a
stream in order to give the stream's resources to a higher
precedence stream. We can envision several approaches to
recovery of broken streams, and we consider the one described
here the simplest and therefore the most likely to be
implemented and work.
If an intermediate agent fails or a network or part of a
network fails, the previous-hop agent and the various next-hop
agents will discover the fact by the failure detection
mechanism described in Section 3.7.1 (page 48). An ST agent
that intentionally breaks a stream obviously knows of the
event.
The recovery of an ST stream is a relatively complex and time
consuming effort because it is designed in a general manner to
operate across a large number of networks with diverse
characteristics. Therefore, it may require information to be
distributed widely, and may require relatively long timers. On
the other hand, since a network is a homogeneous system,
failure recovery in the network may be a relatively faster and
simpler operation. Therefore an ST agent that detects a
failure should attempt to fix the network failure before
attempting recovery of the ST stream. If the stream that
existed between two ST agents before the failure cannot be
reconstructed by network recovery mechanisms alone, then the ST
stream recovery mechanism must be invoked.
If stream recovery is necessary, the different ST agents may
need to perform different functions, depending on their
relation to the failure.
An intermediate agent that breaks the stream intentionally
sends DISCONNECT messages with the appropriate reason code
(StreamPreempted) toward the affected targets. If the
NoRecovery option is selected, it sends a REFUSE message with
the appropriate reason code(StreamPreempted) toward the origin.
If the NoRecovery option is not selected, then this agent
attempts recovery of the stream, as described below.
A host agent that is a target of the broken stream or is itself
the next-hop of the failed component should release resources
that are allocated to the stream, but should maintain the
internal state information describing the stream. It should
inform any next higher protocol of the failure. It is
appropriate for that protocol to expect that the stream will be
fixed shortly by some alternate path and so maintain, for some
time period, whatever information in the ST layer, the next
higher layer, and the application is necessary to reactivate
quickly entries for the stream as the alternate path develops.
The agent should use a timeout to delete all the stream
information in case the stream cannot be fixed in a reasonable
time.
An intermediate agent that is a next-hop of a failure that was
not due to a preemption should first verify that there was a
failure. It can do this using STATUS messages to query its
upstream neighbor. If it cannot communicate with that
neighbor, then it should first send a REFUSE message with the
appropriate reason code of "failure" to the neighbor to speed
up the failure recovery in case the hop is unidirectional,
i.e., the neighbor can hear the agent but the agent cannot hear
the neighbor. The ST agent detecting the failure must then
send DISCONNECT messages with the same reason code toward the
targets. The intermediate agents process this DISCONNECT
message just like the DISCONNECT that tears down the stream.
However, a target ST agent that receives a DISCONNECT message
with the appropriate reason code (StreamPreempted, or
"failure") will maintain the stream state and notify the next
higher protocol of the failure. In effect, these DISCONNECT
messages tear down the stream from the point of the failure to
the targets, but inform the targets that the stream may be
fixed shortly.
An ST agent that is the previous-hop before the failed
component first verifies that there was a failure by querying
the downstream neighbor using STATUS messages. If the neighbor
has lost its state but is available, then the ST agent may
reconstruct the stream if the NoRecovery option is not
selected, as described below. If it cannot communicate with
the next-hop, then the agent detecting the failure releases any
resources that are dedicated exclusively to sending data on the
broken branch and sends a DISCONNECT message with the
appropriate reason code ("failure") toward the affected
targets. It does so to speed up failure recovery in case the
communication may be unidirectional and this message might be
delivered successfully.
If the NoRecovery option is selected, then the ST agent that
detects the failure sends a REFUSE message with the appropriate
reason code ("failure") to the previous-hop. If it is breaking
the stream intentionally, it sends a REFUSE message with the
appropriate reason code (StreamPreempted) to the previous-hop.
The TargetList in these messages contains all the targets that
were reached through the broken branch. Multiple REFUSE
messages may be required if the PDU is too long for the MTU of
the intervening network. The REFUSE message is propagated all
the way to the origin, which can attempt recovery of the stream
by sending a new CONNECT to the affected targets. The new
CONNECT will be treated by intermediate ST agents as an
addition of new targets into the established stream.
If the NoRecovery option is not selected, the ST agent that
breaks the stream intentionally or is the previous-hop before
the failed component can attempt recovery of the stream. It
does so by issuing a new CONNECT message to the affected
targets. If the ST agent cannot find new routes to some
targets, or if the only route to some targets is through the
previous-hop, then it sends one or more REFUSE messages to the
previous-hop with the appropriate reason code ("failure" or
StreamPreempted) specifying the affected targets in the
TargetList. The previous-hop can then attempt recovery of the
stream by issuing a CONNECT to those targets. If it cannot
find an appropriate route, it will propagate the REFUSE message
toward the origin.
Regardless of which agent attempts recovery of a damaged
stream, it will issue one or more CONNECT messages to the
affected targets. These CONNECT messages are treated by
intermediate ST agents as additions of new targets into the
established stream. The FlowSpecs of the new CONNECT messages
should be the same as the ones contained in the most recent
CONNECT or CHANGE messages that the ST agent had sent toward
the affected targets when the stream was operational.
The reconstruction of a broken stream may not proceed smoothly.
Since there may be some delay while the information concerning
the failure is propagated throughout an internet, routing
errors may occur for some time after a failure. As a result,
the ST agent attempting the recovery may receive REFUSE or
ERROR-IN-REQUEST messages for the new CONNECTs that are caused
by internet routing errors. The ST agent attempting the
recovery should be prepared to resend CONNECTs before it
succeeds in reconstructing the stream. If the failure
partitions the internet and a new set of routes cannot be found
to the targets, the REFUSE messages will eventually be
propagated to the origin, which can then inform the application
so it can decide whether to terminate or to continue to attempt
recovery of the stream.
The new CONNECT may at some point reach an ST agent downstream
of the failure before the DISCONNECT does. In this case, the
agent that receives the CONNECT is not yet aware that the
stream has suffered a failure, and will interpret the new
CONNECT as resulting from a routing failure. It will respond
with an ERROR-IN-REQUEST message with the appropriate reason
code (StreamExists). Since the timeout that the ST agents
immediately preceding the failure and immediately following the
failure are approximately the same, it is very likely that the
remnants of the broken stream will soon be torn down by a
DISCONNECT message with the appropriate reason code
("failure"). Therefore, the ST agent that receives the ERROR-
IN-REQUEST message with reason code (StreamExists) should
retransmit the CONNECT message after the ToConnect timeout
expires. If this fails again, the request will be retried for
NConnect times. Only if it still fails will the ST agent send
a REFUSE message with the appropriate reason code (RouteLoop)
to its previous-hop. This message will be propagated back to
the ST agent that is attempting recovery of the damaged stream.
That ST agent can issue a new CONNECT message if it so chooses.
The REFUSE is matched to a CONNECT message created by a
recovery operation through the LnkReference field in the
CONNECT.
ST agents that have propagated a CONNECT message and have
received a REFUSE message should maintain this information for
some period of time. If an agent receives a second CONNECT
message for a target that recently resulted in a REFUSE, that
agent may respond with a REFUSE immediately rather than
attempting to propagate the CONNECT. This has the effect of
pruning the tree that is formed by the propagation of CONNECT
messages to a target that is not reachable by the routes that
are selected first. The tree will pass through any given ST
agent only once, and the stream setup phase will be completed
faster.
The time period for which the failure information is maintained
must be consistent with the expected lifetime of that
information. Failures due to lack of reachability will remain
relevant for time periods large enough to allow for network
reconfigurations or repairs. Failures due to routing loops
will be valid only until the relevant routing information has
propagated, which can be a short time period. Lack of
bandwidth resulting from over-allocation will remain valid
until streams are terminated, which is an unpredictable time,
so the time that such information is maintained should also be
short.
If a CONNECT message reaches a target, the target should as
efficiently as possible use the state that it has saved from
before the stream failed during recovery of the stream. It
will then issue an ACCEPT message toward the origin. The
ACCEPT message will be intercepted by the ST agent that is
attempting recovery of the damaged stream, if not the origin.
If the FlowSpec contained in the ACCEPT specifies the same
selection of parameters as were in effect before the failure,
then the ST agent that is attempting recovery will not
propagate the ACCEPT. If the selections of the parameters are
different, then the agent that is attempting recovery will send
the origin a NOTIFY message with the appropriate reason code
(FailureRecovery) that contains a FlowSpec that specifies the
new parameter values. The origin may then have to change its
data generation characteristics and the stream's parameters
with a CHANGE message to use the newly recovered subtree.
3.7.2.1. Subset
Subsets of this mechanism may reduce the functionality in
the following ways. A host agent might not retain state
describing a stream that fails with a DISCONNECT message
with the appropriate reason code ("failure" or
StreamPreempted).
An agent might force the NoRecovery option always to be set.
In this case, it will allow the option to be propagated in
the CONNECT message, but will propagate the REFUSE message
with the appropriate reason code ("failure" or
StreamPreempted) without attempting recovery of the damaged
stream.
If an ST agent allows stream recovery and attempts recovery
of a stream, it might choose a FlowSpec to specify exactly
the current values of the parameters, with no ranges or
options.
3.7.3. A Group of Streams
There may be a need to associate related streams. The Group
mechanism is simply an association technique that allows ST
agents to identify the different streams that are to be
associated. Streams are in the same Group if they have the
same Group Name in the GroupName field of the (R)Group
parameter. At this time there are no ST control messages that
modify Groups. Group Names have the same format as stream
Names, and can share the same name space. A stream that is a
member of a Group can specify one or more (Subgroup Identifier,
Relation) tuples. The Relation specifies how the members of
the Subgroup of the Group are related. The Subgroups
Identifiers need only be unique within the Group.
Streams can be associated into Groups to support activities
that deal with a number of streams simultaneously. The
operation of Groups of streams is a matter for further study,
and this mechanism is provided to support that study. This
mechanism allows streams to be identified as belonging to a
given Group and Subgroup, but in order to have any effect, the
behavior that is expected of the Relation must be implemented
in the ST agents. Possible applications for this mechanism
include the following:
o Associating streams that are part of a floor-controlled
conference. In this case, only one origin can send data
through its stream at any given time. Therefore, at any
point where more than one stream passes through a branch
or network, only enough bandwidth for one stream needs
to be allocated.
o Associating streams that cannot exist independently. An
example of this may be the various streams that carry
the audio, video, and data components of a conference,
or the various streams that carry data from the
different participants in a conference. In this case,
if some ST agent must preempt more than a single stream,
and it has selected any one of the streams so
associated, then it should also preempt the rest of the
members of that Subgroup rather than preempting any
other streams.
o Associating streams that must not be completed
independently. This example is similar to the preceding
one, but relates to the stream setup phase. In this
example, any single member of a Subgroup of streams need
not be completed unless the rest are also completed.
Therefore, if one stream becomes blocked, all the others
will also be blocked. In this case, if there are not
enough resources to support all the conferences that are
attempted, some number of the conferences will complete
and other will be blocked, rather than all conferences
be partially completed and partially blocked.
This document assumes that the creation and membership of the
Group will be managed by the next protocol above ST, with the
assistance of ST. For example, the next higher protocol
would request ST to create a unique Group Name and a set of
Subgroups with specified characteristics. The next higher
protocol would distribute this information to the other
participants that were to be members of the Group. Each
would transfer the Group Name, Subgroups, and Relations to
the ST layer, which would simply include them in the stream
state.
3.7.3.1. Group Name Generator
This facility is provided so that an application or higher
layer protocol can obtain a unique Group Name from the ST
layer. This is a mechanism for the application to request
the allocation of a Group Name that is independent of the
request to create a stream. The Group Name is used by the
application or higher layer protocol when creating the
streams that are to be part of a group. All that is
required is a function of the form:
AllocateGroupName()
-> result, GroupName
A corresponding function to release a Group Name is also
desirable; its form is:
ReleaseGroupName( GroupName )
-> result
3.7.3.2. Subset
Since Groups are currently intended to support
experimentation, and it is not clear how best to use them,
it is appropriate for an implementation not to support
Groups. At this time, a subsetted ST agent may ignore the
Group parameter. It is expected that in the future, when
Groups transition from being an experimental concept to an
operational one, it may be the case that such subsetting
will no longer be acceptable. At that time, a new
subsetting option may be defined.
3.7.4. HID Negotiation
Each data packet must carry a value to identify the stream to
which it belongs, so that forwarding can be performed.
Conceptually, this value could be the Name of the stream. A
shorthand identifier is desirable for two reasons. First,
since each data packet must carry this identifier, network
bandwidth efficiency suggests that it be as small as
possible. This is particularly important for applications
that use small data packets, and that use low bandwidth
networks, such as voice across packet radio networks.
Second, the operation of mapping this identifier into a data
object that contains the forwarding information must be
performed at each intermediate ST agent in the stream. To
minimize delay and processing overhead, this operation should
be as efficient as possible. Most likely, this identifier
will be used to index into an internal table. To meet these
goals, ST has chosen to use a 16-bit hop-by-hop identifier
(HID). It is large enough to handle the foreseen number of
streams during the expected life of the protocol while small
enough not to preclude its use as a forwarding table index.
Note, however, that HID 0 is reserved for control messages,
and that HIDs 1-3 are also reserved for future use.
When ST makes use of multicast ability in networks that
provide it, a data packet multicast by an ST agent will be
received identically by several next-hop ST agents. In a
multicast environment, the HID must be selected either by
some network-wide mechanism that selects unique identifiers,
or it must be selected by the sender of the CONNECT message.
Since we feel any network-wide mechanism is outside the scope
of this protocol, we propose that the previous-hop agent
select the HID and send it in the CONNECT message (with the
HID Field option set, see Section 3.6.1 (page 44)) subject to
the approval of the next-hop agents. We call this "HID
negotiation".
As an origin ST agent is creating a stream or as an
intermediate agent is propagating a CONNECT message, it must
make a routing decision to determine which targets will be
reached through which next-hop ST agents. In some cases,
several next-hops can be reached through a network that
supports multicast delivery. If so, those next-hops will be
made members of a multicast group and data packets will be
sent to the group. Different CONNECT messages are sent to
the several next-hops even if the data packets will be sent
to the multicast group, because the CONNECT messages contain
different TargetLists and are acknowledged and accepted
separately. However, the HID contained by the different
CONNECT message must be identical. The ST agent selects a
16-bit quantity to be the HID and inserts it into each
CONNECT message that is then sent to the appropriate
next-hop.
The next-hop agents that receive the CONNECT messages must
propagate the CONNECT messages toward the targets, but must
also look at the HID and decide whether they can approve it.
An ST agent can only receive data packets with a given HID if
they belong to a single stream. If the ST agent already has
an established stream that uses the proposed HID, this is a
HID collision, and the agent cannot approve the HID for the
new stream. Otherwise the agent can approve the HID. If it
can approve the HID, then it must make note of that HID and
it must respond with a HID-APPROVE message (unless it can
immediately respond with an ERROR-IN-REQUEST or a REFUSE).
If it cannot approve the HID then it must respond with a
HID-REJECT message.
An agent that sends a CONNECT message with the H bit set
awaits its acknowledgment message (which could be a
HID-ACCEPT, HID-REJECT, or an ERROR-IN-REQUEST) from the
next-hops independently of receiving ACCEPT messages. If it
does not receive an acknowledgment within timeout ToConnect,
it will resend the CONNECT. If each next-hop agent responds
with a HID-ACCEPT, this implies that they have each approved
of the HID, so it can be used for all subsequent data
packets. If one or more next-hops respond with an
HID-REJECT, then the agent that selected the HID must select
another HID and send it to each next-hop in a set of
HID-CHANGE messages. The next-hop agents must respond to
(and thus acknowledge) these HID-CHANGE messages with either
a HID-ACCEPT or a HID-REJECT (or, in the case of an error, an
ERROR-IN-REQUEST, or a REFUSE if the next-hop agent wants to
abort the HID negotiation process after rejecting NHIDAbort
proposed HIDs). If the agent does not receive such a
response within timeout ToHIDChange, it will resend the
HID-CHANGE up to NHIDChange times. If any next-hop agents
respond with a REFUSE message that specifies all the targets
that were included in the corresponding CONNECT, then that
next-hop is removed from the negotiation. The overall
negotiation is complete only when the agent receives a
HID-ACCEPT to the same proposed HID from all the next-hops
that do not respond with an ERROR-IN-REQUEST or a REFUSE.
This negotiation may continue an indeterminate length of
time. In fact, the CONNECT messages could propagate to the
targets and their ACCEPT messages may potentially propagate
back to the origin before the negotiation is complete. If
this were permitted, the origin would not be aware of the
incomplete negotiation and could begin to send data packets.
Then the agent that is attempting to select a HID would have
to discard any data rather than sending it to the next-hops
since it might not have a valid HID to send with the data.
To prevent this situation, an ACCEPT should not be propagated
back to the previous-hop until the HID negotiation with the
next-hops has been completed.
Although it is possible that the negotiation extends for an
arbitrary length of time, we consider this to be very
unlikely. Since the HID is only relevant across a single
hop, we can estimate the probability that a randomly selected
HID will conflict with the HID of an established stream.
Consider a stream in which the hop from an ST agent to ten
next-hop agents is through the multicast facility of a given
network. Assume also that each of the next-hop agents
participates in 1000 other streams, and that each has been
created with a different HID. A randomly selected 16-bit HID
will have a probability of greater than 85.9% of succeeding
on the first try, 98.1% of succeeding on the second, and
99.8% of succeeding on the third. We therefore suggest that
a 16-bit HID space is sufficiently large to support ST until
better multicast HID selection procedures, e.g., HID servers,
can be deployed.
An obvious way to select the HID is for the ST agents to use
a random number generator as suggested above. An alternate
mechanism is for the intermediate agents to use the HID
contained in the incoming CONNECT message for all the
outgoing CONNECT messages, and generate a random number only
as a second choice. In this case, the origin ST agent would
Agent 3 Agent B
1. +-> CONNECT B -------------->+
<RVLId=0><SVLId=32>
<Ref=315><HID=5990> V
2. (Check HID Table, 5990 busy, 6000-11 unused)
V
3. +<- HID-REJECT --------------+
<RVLId=32><SVLId=45>
<Ref=315><HID=5990>
V <FreeHIDs=5990:0000FFF0>
4. +-> HID-CHANGE ------------>+
<RVLId=45><SVLId=32>
<Ref=320><HID=6000> V
5. (Check HID Table, 6000 (still) available)
V
6. +<- HID-APPROVE -------------+
<RVLId=32><SVLId=45>
<Ref=320><HID=6000>
7. (Both parties have now agreed to use HID 6000)
Figure 18. Typical HID Negotiation (No Multicasting)
be responsible for generating the HID, and the same HID could
be propagated for the entire stream. This approach has the
marginal advantage that the HID could be created by a higher
layer protocol that might have global knowledge and could
select small, globally unique HIDs for all the streams. While
this is possible, we leave it for further study.
Agent 2 Agent C Agent D
1. +->+-> CONNECT ---------------------------------->+
<RVLId=0><SVLId=26>
<Ref=250><HID=4824>
V <Mcast=224.1.18.216,01:00:5E:01:12:d8>
2. +-> CONNECT --------------------+
<RVLId=0><SVLId=25>
<Ref=252><HID=4824> V
3. <Mcast=224.1.18.216, V (Check HID Table)
4. 01:00:5E:01:12:d8> (Check HID Table) (4824 ok)
(4824 busy) (4800-4809 ok)
(4800-4820 ok)
V
5. +<- HID-REJECT -----------------+
<RVLId=25><SVLId=54>
<Ref=252><HID=4824>
V <FreeHIDs=4824:FFFFF800> V
6. +<-+<- HID-APPROVE -------------------------------+
<RVLId=26><SVLId=64>
<Ref=250><HID=4824>
V <FreeHIDs=4824:FFC00080>
(find common HID 4800)
V
7. +->+-> HID-CHANGE ------------------------------->+
<RVLId=64><SVLId=26>
V <Ref=253><HID=4800>
8. +-> HID-CHANGE ---------------->+
<RVLId=54><SVLId=25> V
9. <Ref=254><HID=4800> V (Check HID Table)
10. (Check HID Table) (4800 ok)
(4800-4820 ok) (4800-4809 ok)
V
11. +<- HID-APPROVE ----------------+
<RVLId=25><SVLId=54>
<Ref=254><HID=4800>
V <FreeHIDs=4800:7FFFF800> V
12. +<-+<- HID-APPROVE -------------------------------+
<RVLId=26><SVLId=64>
<Ref=253><HID=4800>
V <FreeHIDs=4800:7FC00080>
13. (all parties have now agreed to use HID 4800)
Figure 19. Multicast HID Negotiation
Agent 2 Agent C Agent D Agent 3
1. +----> CONNECT B ------------------------------------>+
<RVLId=0><SVLId=24> V
2. <Ref=260><HID=4800> (Check HID Table)
<Mcast=224.1.18.216, (4800 busy, 4801-4810 ok)
01:00:5E:01:12:d8> V
3. +<---- HID-REJECT <-----------------------------------+
<RVLId=24><SVLId=33>
<Ref=260><HID=4824>
V <FreeHIDs=4824:7FE00000>
4. (find common HID 4810)
V
5. +->+-> HID-CHANGE ----------------------------------->+
<RVLId=33><SVLId=24>
V <Ref=262><HID=4810>
6. +-> HID-CHANGE-ADD ------------------->+
<RVLId=64><SVLId=26> V
7. V <Ref=263><HID=4810> (Check HID Table)
8. +-> HID-CHANGE-ADD ---->+ (4801-4815 ok)
<RVLId=54><SVLId=25> V
9. <Ref=265><HID=4810> V (Check HID Table)
10. (Check HID Table) (4810 busy)
(4801-4812 ok) (4801-4807 ok)
V
11. +<- HID-APPROVE <-------+
<RVLId=25><SVLId=54>
<Ref=265><HID=4810>
V <FreeHIDs=4810:7FD8000> V
12. +<- HID-REJECT <-----------------------+
<RVLId=26><SVLId=64>
<Ref=263><HID=4810>
V <FreeHIDs=4810:7F000000> V
13. +<-+<- HID-APPROVE <----------------------------------+
<RVLId=24><SVLId=33>
<Ref=262><HID=4810>
V <FreeHIDs=4810:7FDF0000>
14. +->+-> HID-CHANGE-DELETE ---------------------------->+
<RVLId=33><SVLId=24>
V <Ref=266><HID=4810>
15. +-> HID-CHANGE-DELETE ->+
<RVLId=54><SVLId=25>
<Ref=268><HID=4810> V
16. +<- HID-APPROVE --------+
<RVLId=25><SVLId=54>
<Ref=268><HID=0> V
17. +<- HID-APPROVE -----------------------------------+
<RVLId=24><SVLId=33>
V <Ref=266><HID=0>
18. (find common HID 4801)
Figure 20. Multicast HID Re-Negotiation (part 1)
Agent 2 Agent C Agent D Agent 3
18. (find common HID 4801)
V
19. +->+-> HID-CHANGE ----------------------------------->+
<RVLId=33><SVLId=24>
V <Ref=270><HID=4801>
20. +-> HID-CHANGE-ADD ------------------->+
<RVLId=64><SVLId=26> V
21. V <Ref=273><HID=4801> (Check HID Table)
22. +-> HID-CHANGE-ADD ---->+ (4801-4815 ok)
<RVLId=54><SVLId=25> V
23. <Ref=274><HID=4801> V (Check HID Table)
24. (Check HID Table)(4801-4807 ok)
(4801-4812 ok)
V
25. +<- HID-APPROVE <-------+
<RVLId=25><SVLId=54>
<Ref=274><HID=4801>
V <FreeHIDs=4801:3FF80000> V
26. +<- HID-APPROVE <----------------------+
<RVLId=26><SVLId=64>
<Ref=273><HID=4801>
V <FreeHIDs=4801:3F000000> V
27. +<-+<- HID-APPROVE <----------------------------------+
<RVLId=24><SVLId=33>
<Ref=270><HID=4801>
V <FreeHIDs=4801:3FFF0000>
28. (switch data stream to HID 4801, drop 4800)
V
29. +->+-> HID-CHANGE-DELETE ---------------->+
<RVLId=64><SVLId=26>
V <Ref=275><HID=4800>
30. +-> HID-CHANGE-DELETE ->+
<RVLId=54><SVLId=25>
<Ref=277><HID=4800> V
31. +<-+<- HID-APPROVE --------+
<RVLId=25><SVLId=54>
V <Ref=277><HID=0> V
32. +<-+<- HID-APPROVE -----------------------+
<RVLId=26><SVLId=64>
V <Ref=275><HID=0>
(all parties have now agreed to use HID 4801)
Figure 20. Multicast HID Re-Negotiation (part 2)
3.7.4.1. Subset
The above mechanism can operate exactly as described even if
the ST agents do not all use the entire 16 bits of the HID.
A low capacity ST agent that cannot support a large number
of simultaneous streams may use only some of the bits in the
HID, say for example the low order byte. This may allow
this disadvantaged agent to use smaller internal data
structures at the expense of causing HID collisions to occur
more often. However, neither the disadvantaged agent's
previous-hop nor its next-hops need be aware of its
limitations. In the HID negotiation, the negotiators still
exchange a 16-bit quantity.
3.7.5. IP Encapsulation of ST
ST packets may be encapsulated in IP to allow them to pass
through routers that don't support the ST Protocol. Of course,
ST resource management is precluded over such a path, and
packet overhead is increased by encapsulation, but if the
performance is reasonably predictable this may be better than
not communicating at all. IP encapsulation may also be
required either for enhanced security (see Section 3.7.8 (page
67)) or for user-space implementations of ST in hosts that
don't allow demultiplexing on the IP Version Number field (see
Section 4 (page 75)), but do allow access to raw IP packets.
IP-encapsulated ST packets begin with a normal IP header. Most
fields of the IP header should be filled in according to the
same rules that apply to any other IP packet. Three fields of
special interest are:
o Protocol is 5 to indicate an ST packet is enclosed, as
opposed to TCP or UDP, for example. The assignment of
protocol 5 to ST is an arranged coincidence with the
assignment of IP Version 5 to ST [18].
o Destination Address is that of the next-hop ST agent.
This may or may not be the target of the ST stream.
There may be an intermediate ST agent to which the
packet should be routed to take advantage of service
guarantees on the path past that agent. Such an
intermediate agent would not be on a directly-connected
network (or else IP encapsulation wouldn't be needed),
so it would probably not be listed in the normal routing
table. Additional routing mechanisms, not defined here,
will be required to learn about such agents.
o Type-of-Service may be set to an appropriate value for
the service being requested (usually low delay, high
throughput, normal reliability). This feature is not
implemented uniformly in the Internet, so its use can't be
precisely defined here.
Since there can be no guarantees made about performance across
a normal IP network, the ST agent that will encapsulate should
modify the Desired FlowSpec parameters when the stream is being
established to indicate that performance is not guaranteed. In
particular, Reliability should be set to the minimum value
(1/256), and suitably large values should be added to the
Accumulated Mean Delay and Accumulated Delay Variance to
reflect the possibility that packets may be delayed up to the
point of discard when there is network congestion. A suitably
large value is 255 seconds, the maximum packet lifetime as
defined by the IP Time-to-Live field.
IP encapsulation adds little difficulty for the ST agent that
receives the packet. The IP header is simply removed, then the
ST header is processed as usual.
The more difficult part is during setup, when the ST agent must
decide whether or not to encapsulate. If the next-hop ST agent
is on a remote network and the route to that network is through
a router that supports IP but not ST, then encapsulation is
required. As mentioned in Section 3.8.1 (page 69), routing
table entries must be expanded to indicate whether the router
supports ST.
On forwarding, the (mostly constant) IP Header must be inserted
and the IP checksum appropriately updated.
On a directly connected network, though, one might want to
encapsulate only when sending to a particular destination host
that does not allow demultiplexing on the IP Version Number
field. This requires the routing table to include host-route
as well as network-route entries. Host-route entries might
require static definition if the hosts do not participate in
the routing protocols. If packet size is not a critical
performance factor, one solution is always to encapsulate on
the directly connected network whenever some hosts require
encapsulation. Those that don't require the encapsulation
should be able to remove it upon reception.
3.7.5.1. IP Multicasting
If an ST agent must use IP encapsulation to reach multiple
next-hops toward different targets, then either the packet
must be replicated for transmission to each next-hop, or IP
multicasting [6] may be used if it is implemented in the
next-hop ST agents and in the intervening IP routers.
This is analogous to using network-level service to
multicast to several next-hop agents on a directly connected
network.
When the stream is established, the collection of next-hop
ST agents must be set up as an IP multicast group. It may
be necessary for the ST agent that wishes to send the IP
multicast to allocate a transient multicast group address
and then tell the next-hop agents to join the group. Use of
the MulticastAddress parameter (see Section 4.2.2.7 (page
86)) provides one way that the information may be
communicated, but other techniques are possible. The
multicast group address in inserted in the Destination
Address field of the IP encapsulation when data packets are
transmitted.
A block of transient IP multicast addresses, 224.1.0.0 -
224.1.255.255, has been allocated for this purpose. There
are 2^16 addresses in this block, allowing a direct mapping
with 16-bit HIDs, if appropriate. The mechanisms for
allocating these addresses are not defined here.
In addition, two permanent IP multicast addresses have been
assigned to facilitate experimentation with exchange of
routing or other information among ST agents. Those
addresses are:
224.0.0.7 All ST routers
224.0.0.8 All ST hosts
An ST router is an ST agent that can pass traffic between
attached networks; an ST host is an ST agent that is
connected to a single network or is not permitted to pass
traffic between attached networks. Note that the range of
these multicasts is normally just the attached local
network, limited by setting the IP time-to-live field to 1
(see [6]).
3.7.6. Retransmission
The ST Control Message Protocol is made reliable through use of
retransmission when an expected acknowledgment is not received
in a timely manner. The problem of when to send a
retransmission has been studied for protocols such as TCP [2]
[10] [11]. The problem should be simpler for ST since control
messages usually only have to travel a single hop and they do
not contain very much data. However, the algorithms developed
for TCP are sufficiently simple that their use is recommended
for ST as well; see [2]. An implementor might, for example,
choose to keep statistics separately for each
neighboring ST agent, or combined into a single statistic for
an attached network.
Estimating the packet round-trip time (RTT) is a key function
in reliable transport protocols such as TCP. Estimation must
be dynamic, since congestion and resource contention result in
varying delays. If RTT estimates are too low, packets will be
retransmitted too frequently, wasting network capacity. If RTT
estimates are too high, retransmissions will be delayed
reducing network throughput when transmission errors occur.
Article [11] identifies problems that arise when RTT estimates
are poor, outlines how RTT is used and how retransmission
timeouts (RTO) are estimated, and surveys several ways that RTT
and RTO estimates can be improved.
Note the HELLO/ACK mechanism described in Section 3.7.1.2 (page
49) can give an estimate of the RTT and its variance. These
estimates are also important for use with the delay and delay
variance entries in the FlowSpec.
3.7.7. Routing
ST requires access to routing information in order to select a
path from an origin to the destination(s). However, routing is
considered to be a separate issue and neither the routing
algorithm nor its implementation is specified here. ST should
operate equally well with any reasonable routing algorithm.
While ST may be capable of using several types of information
that are not currently available, the minimal information
required is that provided by IP, namely the ability to find an
interface and next hop router for a specified IP destination
address and Type of Service. Methods to make more information
available and to use it are left for further study. For
initial ST implementations, any routing information that is
required but not automatically provided will be assumed to be
manually configured into the ST agents.
3.7.8. Security
The ST Protocol by itself does not provide security services.
It is more vulnerable to misdelivery and denial of service than
IP since the ST Header only carries a 16-bit HID for
identification purposes. Any information, such as source and
destination addresses, which a higher-layer protocol might use
to detect misdelivery are the responsibility of either the
application or higher-layer protocol.
ST is less prone to traffic analysis than IP since the only
identifying information contained in the ST Header is a hop-
by-hop identifier (HID). However, the use of a HID is also
what makes ST more vulnerable to denial of service since an ST
agent has no reliable way to detect when bogus traffic is
injected into, and thus consumes bandwidth from, a user's
stream. Detection can be enhanced through use of per-interface
forwarding tables and verification of local network source and
destination addresses.
We envision that applications that require security services
will use facilities, such as the Secure Digital Networking
System (SDNS) layer 3 Security Protocol (SP3/D) [19] [20]. In
such an environment, ST PDUs would first be encapsulated in an
IP Header, using IP Protocol 5 (ST) as described in Section
3.7.5 (page 64). These IP datagrams would then be secured
using SP3/D, which results in another IP Protocol 5 PDU that
can be passed between ST agents.
This memo does not specify how an application invokes security
services.
3.8. ST Service Interfaces
ST has several interfaces to other modules in a communication
system. ST provides its services to applications or transport-
level protocols through its "upper" interface (or SAP). ST in
turn uses the services provided by network layers, management
functions (e.g., address translation and routing), and IP. The
interfaces to these modules are described in this section in the
form of subroutine calls. Note that this does not mean that an
implementation must actually be implemented as subroutines, but is
instead intended to identify the information to be passed between
the modules.
In this style of outlining the module interfaces, the information
passed into a module is shown as arguments to the subroutine call.
Return information and/or success/failure indications are listed
after the arrow ("->") that follows the subroutine call. In
several cases, a list of values must either be passed to or
returned from a module interface. Examples include a set of
target addresses, or the mappings from a target list to a set of
next hop addresses that span the route to the originally listed
targets. When such a list is appropriate, the values repeated for
each list element are bracketed and an asterisk is added to
indicate that zero, one, or many list elements can be passed
across the interface (e.g., "<target>*" means zero, one, or more
targets).
3.8.1. Access to Routing Information
The design of routing functions that can support a variety of
resource management algorithms is difficult. In this section
we suggest a set of preliminary interfaces suitable for use in
initial experiments. We expect that these interfaces will
change as we gain more insight into how routing, resource
allocation, and decision making elements are best divided.
Routing functions are required to identify the set of potential
routes to each destination site. The routing functions should
make some effort to identify routes that are currently
available and that meet the resource requirements. However,
these properties need not be confirmed until the actual
resource allocation and connection setup propagation are
performed.
The minimum capability required of the interface to routing is
to identify the network interface and next hop toward a given
target. We expect that the traditional routing table will need
to be extended to include information that ST requires such as
whether or not a next hop supports ST, and, if so, whether or
not IP encapsulation (see Section 3.7.5 (page 64)) is required
to communicate with it. In particular, host entries will be
required for hosts that can only support ST through
encapsulation because the IP software either is not capable of
demultiplexing datagrams based on the IP Version Number field,
or the application interface only supports access to raw IP
datagrams. This interface is illustrated by the function:
FindNextHop( destination, TOS )
-> result, < interface, next hop, ST-capable,
MustEncapsulate >*
However, the resource management functions can best tradeoff
among alternative routes when presented with a matrix of all
potential routes. The matrix entry corresponding to a
destination and a next hop would contain the estimated
characteristics of the corresponding pathway. Using this
representation, the resource management functions can quickly
determine the next hop sets that cover the entire destination
list, and compare the various parameters of the tradeoff
between the guarantees that can be promised by each set. An
interface that returns a compressed matrix, listing the
suitable routes by next hop and the destinations reachable
through each, is illustrated by the function:
FindNextHops( < destination >*, TOS )
-> result, < destination, < interface, next hop,
ST-capable, MustEncapsulate >* >*
We hope that routing protocols will be available that propagate
additional metrics of bandwidth, delay, bit/burst error rate,
and whether a router has ST capability. However, propagating
this information in a timely fashion is still a key research
issue.
3.8.2. Access to Network Layer Resource Reservation
The resources required to reach the next-hops associated with
the chosen routes must be allocated. These allocations will
generally be requested and released incrementally. As the
next-hop elements for the routes are chosen, the network
resources between the current node and the next-hops must be
allocated. Since the resources are not guaranteed to be
available -- a network or node further down the path might have
failed or needed resources might have been allocated since the
routing decisions where made -- some of these allocations may
have to be released, another route selected, and a new
allocation requested.
There are four basic interface functions needed for the network
resource allocator. The first checks to see if the required
resources are available, returning the likelihood that an
ensuing resource allocation will succeed. A probability of 0%
indicates the resources are not available or cannot promise to
meet the required guarantees. Low probabilities indicate that
most of the resource has been allocated or that there is a lot
of contention for using the resource. This call does not
actually reserve the resources:
ResourceProbe( requirements )
-> likelihood
Another call reserves the resources:
ResourceReserve( requirements )
-> result, reservation_id
The third call adjusts the resource guarantees:
ResourceAdjust( reservation_id, new requirements )
-> result
The final call allows the resources to be released:
ResourceRelease( reservation_id )
-> result
3.8.3. Network Layer Services Utilized
ST requires access to the usual network layer functions to send
and receive packets and to be informed of network status
information. In addition, it requires functions to enable and
disable reception of multicast packets. Such functions might
be defined as:
JoinLocalGroup( network level group-address )
-> result, multicast_id
LeaveLocalGroup( network level group-address )
-> result
RecvNet( SAP )
-> result, src, dst, len, BufPTR )
SendNet( src, dst, SAP, len, BufPTR )
-> result
GetNotification( SAP )
-> result, infop
3.8.4. IP Services Utilized
Since ST packets might be sent or received using IP
encapsulation, IP level routines to join and leave multicast
groups are required in addition to the usual services defined
in the IP specification (see the IP specification [2] [15] and
the IP multicast specification [6] for details).
JoinHostGroup( IP level group-address, interface )
-> result, multicast_id
LeaveHostGroup( IP level group-address, interface )
-> result
GET_SRCADDR( remote IP addr, TOS )
-> local IP address
SEND( src, dst, prot, TOS, TTL, BufPTR, len, Id, DF,
opt )
-> result
RECV( BufPTR, prot )
-> result, src, dst, SpecDest, TOS, len, opt
GET_MAXSIZES( local, remote, TOS )
-> MMS_R, MMS_S
ADVISE_DELIVPROB( problem, local, remote, TOS )
-> result
SEND_ICMP( src, dst, TOS, TTL, BufPTR, len, Id, DF, opt )
-> result
RECV_ICMP( BufPTR )
-> result, src, dst, len, opt
3.8.5. ST Layer Services Provided
Interface to the ST layer services may be modeled using a set
of subroutine calls (but need not be implemented as such).
When the protocol is implemented as part of an operating
system, these subroutines may be used directly by a higher
level protocol processing layer.
These subroutines might also be provided through system service
calls to provide a raw interface for use by an application.
Often, this will require further adaptation to conform with the
idiom of the particular operating system. For example, 4.3 BSD
UNIX (TM) provides sockets, ioctls and signals for network
programming.
open( connect/listen, SAPBytes, local SAP, local host,
account, authentication info, < foreign host,
SAPBytes, foreign SAP, options >*, flow spec,
precedence, group name, optional parameters )
-> result, id, stream name, < foreign host,
foreign SAPBytes, foreign SAP, result, flow spec,
rname, optional parameters >*
Note that an open by a target in "listen mode" may cause ST to
create a state block for the stream to facilitate rendezvous.
add( id, SAPBytes, local SAP, local host, < foreign host,
SAPBytes, foreign SAP, options >*, flow spec,
precedence, group name, optional parameters )
-> result, < foreign host, foreign SAPBytes,
foreign SAP, result,
flow spec, rname, optional parameters >*
send( id, buffer address, byte count, priority )
-> result, next send time, burst send time
recv( id, buffer address, max byte count )
-> result, byte count
recvsignal( id )
-> result, signal, info
receivecontrol( id )
-> result, id, stream name, < foreign host,
foreign SAPBytes, foreign SAP, result, flow spec,
rname, optional parameters >*
sendcontrol( id, flow spec, precedence, options,
< foreign host, SAPBytes, foreign SAP, options >*)
-> result, < foreign host, foreign SAPBytes,
foreign SAP, result, flow spec, rname,
optional parameters >*
change( id, flow spec, precedence, options,
< foreign host, SAPBytes, foreign SAP, options >*)
-> result, < foreign host, foreign SAPBytes,
foreign SAP, result, flow spec, rname,
optional parameters >*
close( id, < foreign host, SAPBytes, foreign SAP >*,
optional parameters )
-> result
status( id/stream name/group name )
-> result, account, group name, protocol,
< stream name, < foreign host, SAPbytes,
foreign SAP, state, options, flow spec,
routing info, rname >*, precedence, options >*
creategroup( members* )
-> result, group name
deletegroup( group name, members* )
-> result
[This page intentionally left blank.]
4. ST Protocol Data Unit Descriptions
The ST PDUs sent between ST agents consist of an ST Header
ncapsulating either a higher layer PDU or an ST Control Message.
Since ST operates as an extension of IP, the packet arrives at the
same network service access point that IP uses to receive IP
datagrams, e.g., ST would use the same ethertype (0x800) as does IP.
The two types of packets are distinguished by the IP Version Number
field (the first four bits of the packet); IP currently uses a value
of 4, while ST has been assigned the value 5 [18]. There is no
requirement for compatibility between IP and ST packet headers beyond
the first four bits.
The ST Header also includes an ST Version Number, a total length
field, a header checksum, and a HID, as shown in Figure 21. See
Appendix 1 (page 147) for an explanation of the notation.
ST is the IP Version Number assigned to identify ST packets. The
value for ST is 5.
Ver is the ST Version Number. This document defines ST Version 2.
Pri is the priority of the packet. It is used in data packets to
indicate those packets to drop if a stream is exceeding its
allocation. Zero is the lowest priority and 7 the highest.
T (bit 11) is used to indicate that a Timestamp is present
following the ST Header but before any next higher layer protocol
data. The Timestamp is not permitted on ST Control Messages
(which may use the OriginTimestamp option).
Bits 12 through 15 are spares and should be set to 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ST=5 Ver=2 Pri T Bits TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
HID HeaderChecksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+- Timestamp -+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 21. ST Header
TotalBytes is the length, in bytes, of the entire ST packet, it
includes the ST Header and optional Timestamp but does not include
any local network headers or trailers. In general, all length
fields in the ST Protocol are in units of bytes.
HID is the 16-bit hop-by-hop stream identifier. It is an
abbreviation for the Name of the stream and is used both to reduce
the packet header length and, by the receiver of the data packet,
to make the forwarding function more efficient. Control Messages
have a HID value of zero. HIDs are negotiated by the next-hop and
previous-hop agents to make the abbreviation unique. It is used
here in the ST Header and in various Control Messages. HID values
1-3 are reserved for future use.
HeaderChecksum covers only the ST Header and Timestamp, if
present. The ST Protocol uses 16-bit checksums here in the ST
Header and in each Control Message. The standard Internet
checksum algorithm is used: "The checksum field is the 16-bit
one's complement of the one's complement sum of all 16-bit Words
in the header. For purposes of computing the checksum, the value
of the checksum field is zero." See [1] [12] [15] for suggestions
for efficient checksum algorithms.
Timestamp is an optional timestamp inserted into data packets by
the origin. It is only present when the T bit, described above,
is set (1). Its use is negotiated at connection setup time; see
Sections 4.2.3.5 (page 108) and 4.2.3.1 (page 100). The Timestamp
has the NTP format; see [13].
4.1. Data Packets
ST packets whose HID is not zero to three are user data packets.
Their interpretation is a matter for the higher layer protocols
and consequently is not specified here. The data packets are not
protected by an ST checksum and will be delivered to the higher
layer protocol even with errors.
ST agents will not pass data packets over a new hop whose setup is
not complete, i.e., a HID must have been negotiated and either an
ACCEPT or REFUSE has been received for all targets specified in
the CONNECT.
4.2. ST Control Message Protocol Descriptions
ST Control Messages are between a previous-hop agent and its
next-hop agent(s) using a HID of zero. The control protocol
follows a request-response model with all requests expecting
responses. Retransmission after timeout (see Section 3.7.6 (page
66)) is used to allow for lost or ignored messages. Control
messages do not extend across packet boundaries; if a control
message is too large for the MTU of a hop, its information
(usually a TargetList) is partitioned and a control message per
partition is sent. All control messages have the following
format:
OpCode identifies the type of control message. Each is
described in detail in following sections.
Options is used to convey OpCode-specific variations for a
control message.
TotalBytes is the length of the control message, in bytes,
including all OpCode specific fields and optional parameters.
The value is always divisible by four.
RVLId is used to convey the Virtual Link Identifier of the
receiver of the control message, when known, or zero in the
case of an initial CONNECT or diagnostic message. The RVLId is
intended to permit efficient dispatch to the portion of a
stream's state machine containing information about a specific
operation in progress over the link. RVLId values 1-3 are
reserved; see Sections 3 (page 17) and 3.7.1.2 (page 49).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode Options TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+
: OpCode Specific Data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 22. ST Control Message Format
SVLId is used to convey the Virtual Link Identifier of the
sender of the control message. Except for ERROR-IN-REQUEST and
diagnostic messages, it must never be zero. SVLId values 1-3
are reserved; see Sections 3 (page 17) and 3.7.1.2 (page 49).
Reference is a transaction number. Each sender of a request
control message assigns a Reference number to the message that
is unique with respect to the stream. The Reference number is
used by the receiver to detect and discard duplicates. Each
acknowledgment carries the Reference number of the request
being acknowledged. Reference zero is never used, and
Reference numbers are assumed to be monotonically increasing
with wraparound so that the older-than and more-recent-than
relations are well defined.
LnkReference contains the Reference field of the request
control message that caused this request control message to be
created. It is used in situations where a single request leads
to multiple "responses". Examples are CONNECT and CHANGE
messages that must be acknowledged hop-by-hop and will also
lead to an ACCEPT or REFUSE from each target in the TargetList.
SenderIPAddress is the 32-bit IP address of the network
interface that the ST agent used to send the control message.
This value changes each time the packet is forwarded by an ST
agent (hop-by-hop).
Checksum is the checksum of the control message. Because the
control messages are sent in packets that may be delivered with
bits in error, each control message must be checked before it
is acted upon; see Section 4 (page 76).
OpCode Specific Data contains any additional information that
is associated with the control message. It depends on the
specific control message and is explained further below. In
some response control messages, fields of zero are included to
allow the format to match that of the corresponding request
message. The OpCode Specific Data may also contain any of the
optional Parameters defined in Section 4.2.2 (page 80).
4.2.1. ST Control Messages
The CONNECT and CHANGE messages are used to establish or modify
branches in the stream. They propagate in the direction from
the origin toward the targets. They are end-to-end messages
created by the origin. They propagate all the way to the
targets, and require ERROR-IN-REQUEST, ACK, HID-REJECT, HID-
APPROVE, ACCEPT, or REFUSE messages in response. The CONNECT
message is the stream setup message. The CHANGE message is
used to change the characteristics of an established stream.
The CONNECT message is also used to add one or more targets to
an existing stream and during recovery of a broken stream.
Both messages have a TargetList parameter and are processed
similarly.
The DISCONNECT message is used to tear down streams or parts of
streams. It propagates in the direction from the origin toward
the targets. It is either used as an end-to-end message
generated by the origin that is used to completely tear down a
stream, or is generated by an intermediate ST agent that
preempts a stream or detects the failure of its previous-hop
agent or network in the stream. In the latter case, it is used
to tear down the part of the stream from the failure to the
targets, thus the message propagates all the way to the
targets.
The REFUSE message is sent by a target to refuse to join or
remove itself from a stream; in these cases, it is an end-to-
end message. An intermediate ST agent issues a REFUSE if it
cannot find a route to a target, can only find a route to a
target through the previous-hop, preempts a stream, or detects
a failure in a next-hop ST agent or network. In all cases a
REFUSE propagates in the direction toward the origin.
The ACCEPT message is an end-to-end message generated by a
target and is used to signify the successful completion of the
setup of a stream or part of a stream, or the change of the
FlowSpec. There are no other messages that are similar to it.
The following sections contain descriptions of common fields
and parameters, followed by descriptions of the individual
control messages, both listed in alphabetical order. A brief
description of the use of the control message is given. The
packet format is shown graphically.
4.2.2. Common SCMP Elements
Several fields and parameters (referred to generically as
"elements") are common to two or more PDUs. They are described
in detail here instead of repeating their description several
times. In many cases, the presence of a parameter is optional.
To permit the parameters to be easily defined and parsed, each
is identified with a PCode byte that is followed by a PBytes
byte indicating the length of the parameter in bytes (including
the PCode, PByte, and any padding bytes). If the length of the
information is not a multiple of 4 bytes, the parameter is
padded with one to three zero (0) bytes. PBytes is thus always
a multiple of four. Parameters can be present in any order.
4.2.2.1. DetectorIPAddress
Several control messages contain the DetectorIPAddress
field. It is used to identify the agent that caused the
first instance of the message to be generated, i.e., before
it was propagated. It is copied from the received message
into the copy of the message that is to be propagated to a
previous-hop or next-hop. It use is primarily diagnostic.
4.2.2.2. ErroredPDU
The ErroredPDU parameter (PCode = 1) is used for diagnostic
purposes to encapsulate a received ST PDU that contained an
error. It may be included in the ERROR-IN-REQUEST, ERROR-
IN-RESPONSE, or REFUSE messages. It use is primarily
diagnostic.
PDUBytes indicates how many bytes of the PDUInError are
actually present.
ErrorOffset contains the number of bytes into the errored
PDU to the field containing the error. At least as much
of the PDU in error must be included to
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 1 PBytes PDUBytes ErrorOffset
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: PDUInError : Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 23. ErroredPDU
include the field or parameter identified by ErrorOffset;
an ErrorOffset of zero would imply a problem with the IP
Version Number or ST Version Number fields.
PDUInError is the PDU in error, beginning with the ST
Header.
4.2.2.3. FlowSpec & RFlowSpec
The FlowSpec is used to convey stream service requirements
end-to-end. We expect that other versions of FlowSpec will
be needed in the future, which may or may not be subsets or
supersets of the version described here. PBytes will allow
new constraints to be added to the end without having to
simultaneously update all implementations in the field.
Implementations are expected to be able to process in a
graceful manner a Version 4 (or higher) structure that has
more elements than shown here.
The FlowSpec parameter (PCode = 2) is used in several
messages to convey the FlowSpec.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode PBytes Version = 3 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DutyFactor ErrorRate Precedence Reliability
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Tradeoffs RecoveryTimeout
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LimitOnCost LimitOnDelay
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LimitOnPDUBytes LimitOnPDURate
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MinBytesXRate
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AccdMeanDelay
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
AccdDelayVariance
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DesPDUBytes DesPDURate
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 24. FlowSpec & RFlowSpec
The RFlowSpec parameter (PCode = 12) is used in conjunction
with the FDx option to convey the FlowSpec that is to be
used in the reverse direction.
Version identifies the version of the FlowSpec. Version
3 is defined here.
DutyFactor is the estimated proportion of the time that
the requested bandwidth will actually be in use. Zero is
taken to represent 256 and signify a duty factor of 1.
Other values are to be divided by 256 to yield the duty
factor.
ErrorRate expresses the error rate as the negative
exponent of 10 in the error rate. One (1) represents a
bit error rate of 0.1 and 10 represents 0.0000000001.
Precedence is the precedence of the connection being
established. Zero represents the lowest precedence.
Note that non-zero values of this parameter should be
subject to authentication and authorization checks, which
are not specified here. In general, the distinction
between precedence and priority is that precedence
specifies streams that are permitted to take previously
committed resources from another stream, while priority
identifies those PDUs that a stream is most willing to
have dropped when the stream exceeds its guaranteed
limits.
Reliability is modified by each intervening ST agent as a
measure of the probability that a given offered data
packet will be forwarded and not dropped. Zero is taken
to represent 256 and signify a probability of 1. Other
values are to be divided by 256 to yield the probability.
Tradeoffs is incompletely defined at this time. Bits
currently specified are as follows:
The most significant bit in the field, bit 0 in the
Figure 24, when one (1) means that each ST agent must
"implement" all constraints in the FlowSpec even if
they are not shown in the figure, e.g., when the
FlowSpec has been extended. When zero (0), unknown
constraints may be ignored.
The second most significant bit in the field, bit 1,
when one (1) means that one or more constraints are
unknown and have been ignored. When zero (0), all
constraints are known and have been processed.
The third most significant bit in the field, bit 2, is
used for RevChrg; see Section 3.6.5 (page 46).
Other bits are currently unspecified, and should be
set to zero (0) by the origin ST agent and not changed
by other agents unless those agents know their
meaning.
RecoveryTimeout specifies the nominal number of
milliseconds that the application is willing to wait for
a failed system component to be detected and any
corrective action to be taken.
LimitOnCost specifies the maximum cost that the origin is
willing to expend. A value of zero indicates that the
application is not willing to incur any direct charges
for the resources used by the stream. The meaning of
non-zero values is left for further study.
LimitOnDelay specifies the maximum end-to-end delay, in
milliseconds, that can be tolerated by the origin.
LimitOnPDUBytes is the smallest packet size, in terms of
ST-user data bytes, that can be tolerated by the origin.
LimitOnPDURate is the lowest packet rate that can be
tolerated by the origin, expressed as tenths of a packet
per second.
MinBytesXRate is the minimum bandwidth that can be
tolerated by the origin, expressed as a product of bytes
and tenths of a packet per second.
AccdMeanDelay is modified by each intervening ST agent.
This provides a means of reporting the total expected
delay, in milliseconds, for a data packet. Note that it
is implicitly assumed that the requested mean delay is
zero and there is no limit on the mean delay, so there
are no parameters to specify these explicitly.
AccdDelayVariance is also modified by each intervening ST
agent as a measure, in milliseconds squared, of the
packet dispersion. This quantity can be used by the
target or origin in determining whether the resulting
stream has an adequate quality of service to support the
application. Note that it is implicitly assumed that the
requested delay variance is zero and there is no limit on
the delay variance, so there are no parameters to specify
these explicitly.
DesPDUBytes is the desired PDU size in bytes. This is
not necessarily the same as the minimum necessary PDU
size. This value may be made smaller by intervening ST
agents so long as it is not made smaller than
LimitOnPDUBytes. The *PDUBytes limits measure the size
of the PDUs of next-higher protocol layer, i.e., the user
information contained in a data packet. An ST agent must
account for both the ST Header (including possible IP
encapsulation) and any local network headers and trailers
when comparing a network's MTU with *PDUBytes. In an
ACCEPT message, the value of this field will be no larger
than the MTU of the path to the specified target.
DesPDURate is the requested PDU rate, expressed as tenths
of a packet per second. This value may be made smaller
by intervening ST agents so long as it is not made
smaller than LimitOnPDURate.
It is expected that the next parameter to be added to the
FlowSpec will be a Burst Descriptor. This parameter will
describe the burstiness of the offered traffic. For
example, this may include the simple average rate, peak
rate and variance values, or more complete descriptions
that characterize the distribution of expected burst
rates and their expected duration. The nature of the
algorithms that deal with the traffic's burstiness and
the information that needs to be described by this
parameter will be subjects of further experimentation.
It is expected that a new FlowSpec with Version = 4 will
be defined that looks like Version 3 but has a Burst
Descriptor parameter appended to the end.
4.2.2.4. FreeHIDs
The FreeHIDs parameter (PCode = 3) is used to communicate to
the previous-hop suggestions for a HID. It consists of
BaseHID and FreeHIDBitMask fields. Experiments will
determine how long the mask should be for practical use of
this parameter. The parameter (if implemented) should be
included in all HID-REJECTs, and in HID-APPROVEs that are
linked to a multicast CONNECT, e.g., one containing the
MulticastAddress parameter.
BaseHID was the suggested value in a HID-CHANGE or
CONNECT. BaseHID is chosen to be the suggested HID value
to insure that the masks from multiple FreeHIDs
parameters will overlap.
FreeHIDBitMask identifies available HID values as
follows. Bit 0 in the FreeHIDBitMask corresponds to a
HID with a value equal to BaseHID with the 5 least
significant bits set to zero, bit 1 corresponds to that
value + 1, etc. This alignment of the mask on a 32-bit
boundary is used so that masks from several FreeHIDs
parameters might more easily be combined using a bit-wise
AND function to find a free HID.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 3 4+4*N BaseHID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FreeHIDBitMask :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 25. FreeHIDs
4.2.2.5. Group & RGroup
The Group parameter (PCode = 4) is an optional argument
used only for the creation of a stream. This parameter
contains a GroupName; the GroupName may be the same as the
Name of one of the group's streams. In addition, there
may be some number of <SubGroupId, Relation> tuples that
describe the meaning of the grouping and the relation
between the members of the group. The forms of grouping
are for further study.
The RGroup parameter (PCode = 13) is an optional argument
used only for the creation of a stream in the reverse
direction that is a member of a Group; see the FDx
option, Section 3.6.3 (page 45). This parameter has the
same format as the Group parameter.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode 12+4*N !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+
! GroupName !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SubGroupId Relation
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... : ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SubGroupId Relation
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 26. Group & RGroup
A GroupName has the same format as a Name; see Figure 29.
4.2.2.6. HID & RHID
The HID parameter (PCode = 5) is used in the NOTIFY message
when the notification is related to a HID, and possibly in
the STATUS-RESPONSE message to convey additional HIDs that
are valid for a stream when there are more than one. It
consists of the PCode and PBytes bytes prepended to a HID;
HIDs were described in Section 4 (page 76).
The RHID parameter (PCode = 14) is used in conjunction with
the FDx option to convey the HID that is to be used in the
reverse direction. It consists of the PCode and PBytes
bytes prepended to a HID.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode 4 HID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 27. HID & RHID
4.2.2.7. MulticastAddress
The MulticastAddress parameter (PCode = 6) is an optional
parameter that is used, when setting up a network level
multicast group, to communicate an IP and/or local network
multicast address to the next-hop agents that should become
members of the group.
LocalNetBytes is the length of the Local Net Multicast
Address.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 6 PBytes LocalNetBytes 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Local Net Multicast Address : Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 28. MulticastAddress
IP Multicast Address is described in [6]. This field is
zero (0) if no IP multicast address is known or is
applicable. The block of addresses 224.1.0.0 -
224.1.255.255 has been allocated for use by ST.
Local Net Multicast Address is the multicast address to
be used on the local network. It corresponds to the IP
Multicast Address when the latter is non-zero.
4.2.2.8. Name & RName
Each stream is uniquely (i.e., globally) identified by a
Name. A Name is created by the origin host ST agent and is
composed of 1) a 16-bit number chosen to make the Name
unique within the agent, 2) the IP address of the origin ST
agent, and 3) a 32-bit timestamp. If the origin has
multiple IP addresses, then any that can be used to reach
target may be used in the Name. The intent is that the
<Unique ID, IP Address> tuple be unique for the lifetime of
the stream. It is suggested that to increase robustness a
Unique ID value not be reused for a period of time on the
order of 5 minutes.
The Timestamp is included both to make the Name unique over
long intervals (e.g., forever) for purposes of network
management and accounting/billing, and to protect against
failure of an ST agent that causes knowledge of active
Unique IDs to be lost. The assumption is that all ST agents
have access to some "clock". If this is not the case, the
agent should have access to some form of non-volatile memory
in which it can store some number that at least gets
incremented per restart.
The Name parameter (PCode = 7) is used in most control
messages to identify a stream.
The RName parameter (PCode = 15) is used in conjunction with
the FDx option to convey the Name of the reverse stream in
an ACCEPT message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode 12 Unique ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Timestamp
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 29. Name & RName
4.2.2.9. NextHopIPAddress
The NextHopIPAddress parameter (PCode = 8) is an optional
parameter of NOTIFY (RouteBack) or REFUSE (RouteInconsist or
RouteLoop) and contains the IP address of a suggested next-
hop ST agent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 8 8 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next-hop IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 30. NextHopIPAddress
4.2.2.10. Origin
The Origin parameter (PCode = 9) is used to identify the
origin of the stream, the next higher protocol, and the SAP
being used in conjunction with that protocol.
NextPcol is an 8-bit field used in demultiplexing
operations to identify the protocol to be used above ST.
The values of NextPcol are in the same number space as
the IP Header's Protocol field and are consequently
defined in the Assigned Numbers RFC[18].
OriginSAPBytes specifies the length of the OriginSAP,
exclusive of any padding required to maintain 32-bit
alignment.
OriginIPAddress is (one of) the IP address of the origin.
OriginSAP identifies the origin's SAP associated with the
NextPcol protocol.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 9 PBytes NextPcol OriginSAPBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OriginIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: OriginSAP : Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 31. Origin
4.2.2.11. OriginTimestamp
The OriginTimestamp parameter (PCode = 10) is used to
indicate the time at which the control message was sent.
The units and format of the timestamp is that defined in the
NTP protocol specification [13]. Note that discontinuities
over leap seconds are expected.
Note that the time synchronization implied by the use of
such a parameter is the subject of systems management
functions not described in this memo, e.g., NTP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 10 12 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+- Timestamp -+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 32. OriginTimestamp
4.2.2.12. ReasonCode
Several errors may occur during protocol processing. All ST
error codes are taken from a single number space. The
currently defined values and their meaning is presented in
the list below. Note that new error codes may be defined
from time to time. All implementations are expected to
handle new codes in a graceful manner. If an unknown
ReasonCode is encountered, it should be assumed to be fatal.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 33. ReasonCode
Name Value Meaning
---------------- ----- ---------------------------------------
AcceptTimeout 2 An Accept has not been
acknowledged.
AccessDenied 3 Access denied.
AckUnexpected 4 An unexpected ACK was received.
ApplAbort 5 The application aborted the stream
abnormally.
ApplDisconnect 6 The application closed the stream
normally.
AuthentFailed 7 The authentication function
failed.
CantGetResrc 8 Unable to acquire (additional)
resources.
CantRelResrc 9 Unable to release excess
resources.
CksumBadCtl 10 A received control PDU has a bad
message checksum.
CksumBadST 11 A received PDU has a bad ST Header
checksum.
DropExcdDly 12 A received PDU was dropped because
it could not be processed within
the delay specification.
DropExcdMTU 13 A received PDU was dropped because
its size exceeds the MTU.
DropFailAgt 14 A received PDU was dropped because
of a failed ST agent.
DropFailHst 15 A received PDU was dropped because
of a host failure.
DropFailIfc 16 A received PDU was dropped because
of a broken interface.
DropFailNet 17 A received PDU was dropped because
of a network failure.
Name Value Meaning
---------------- ----- ---------------------------------------
DropLimits 18 A received PDU was dropped because
it exceeds the resource limits for
its stream.
DropNoResrc 19 A received PDU was dropped due to
no available resources (including
precedence).
DropNoRoute 20 A received PDU was dropped because
of no available route.
DropPriLow 21 A received PDU was dropped because
it has a priority too low to be
processed.
DuplicateIgn 22 A received control PDU is a
duplicate and is being
acknowledged.
DuplicateTarget 23 A received control PDU contains a
duplicate target, or an attempt to
add an existing target.
ErrorUnknown 1 An error not contained in this
list has been detected.
failure N/A An abbreviation used in the text
for any of the more specific
errors: DropFailAgt, DropFailHst,
DropFailIfc, DropFailNet,
IntfcFailure, NetworkFailure,
STAgentFailure, FailureRecovery.
FailureRecovery 24 A notification that recovery is
being attempted.
FlowVerBad 25 A received control PDU has a
FlowSpec Version Number that is
not supported.
GroupUnknown 26 A received control PDU contains an
unknown Group Name.
HIDNegFails 28 HID negotiation failed.
HIDUnknown 29 A received control PDU contains an
unknown HID.
Name Value Meaning
---------------- ----- ---------------------------------------
InconsistHID 30 An inconsistency has been detected
with a stream Name and
corresponding HID.
InconsistGroup 31 An inconsistency has been detected
with the streams forming a group.
IntfcFailure 32 A network interface failure has
been detected.
InvalidHID 33 A received ST PDU contains an
invalid HID.
InvalidSender 34 A received control PDU has an
invalid SenderIPAddress field.
InvalidTotByt 35 A received control PDU has an
invalid TotalBytes field.
LnkRefUnknown 36 A received control PDU contains an
unknown LnkReference.
NameUnknown 37 A received control PDU contains an
unknown stream Name.
NetworkFailure 38 A network failure has been
detected.
NoError 0 No error has occurred.
NoRouteToAgent 39 Cannot find a route to an ST
agent.
NoRouteToDest 40 Cannot find a route to the
destination.
NoRouteToHost 41 Cannot find a route to a host.
NoRouteToNet 42 Cannot find a route to a network.
OpCodeUnknown 43 A received control PDU has an
invalid OpCode field.
PCodeUnknown 44 A received control PDU has a
parameter with an invalid PCode.
ParmValueBad 45 A received control PDU contains an
invalid parameter value.
Name Value Meaning
---------------- ----- ---------------------------------------
PcolIdUnknown 46 A received control PDU contains an
unknown next-higher layer protocol
identifier.
ProtocolError 47 A protocol error was detected.
PTPError 48 Multiple targets were specified
for a stream created with the PTP
option.
RefUnknown 49 A received control PDU contains an
unknown Reference.
RestartLocal 50 The local ST agent has recently
restarted.
RemoteRestart 51 The remote ST agent has recently
restarted.
RetransTimeout 52 An acknowledgment to a control
message has not been received
after several retransmissions.
RouteBack 53 The routing function indicates
that the route to the next-hop is
through the same interface as the
previous-hop and is not the
previous-hop.
RouteInconsist 54 A routing inconsistency has been
detected, e.g., a route loop.
RouteLoop 55 A CONNECT was received that
specified an existing target.
SAPUnknown 56 A received control PDU contains an
unknown next-higher layer SAP
(port).
STAgentFailure 57 An ST agent failure has been
detected.
StreamExists 58 A stream with the given Name or
HID already exists.
StreamPreempted 59 The stream has been preempted by
one with a higher precedence.
Name Value Meaning
---------------- ----- ---------------------------------------
STVerBad 60 A received PDU is not ST Version
2.
TooManyHIDs 61 Attempt to add more HIDs to a
stream than the implementation
supports.
TruncatedCtl 62 A received control PDU is shorter
than expected.
TruncatedPDU 63 A received ST PDU is shorter than
the ST Header indicates.
UserDataSize 64 The UserData parameter is too
large to permit a control message
to fit into a network's MTU.
4.2.2.13. RecordRoute
The RecordRoute parameter (PCode = 11) may be used to
request that the route between the origin and a target be
recorded and returned to the agent specified in the
DetectorIPAddress field.
FreeOffset is the offset to the position where the next
next-hop IP address should be inserted. It is initialized
to four (4) and incremented by four each time an agent
inserts its IP address.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 11 PBytes 0 FreeOffset
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next-hop IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next-hop IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 34. RecordRoute
4.2.2.14. SrcRoute
The SrcRoute parameter is used, in the Target structure
shown in Figure 36, to specify the IP addresses of the ST
agents through which the stream to the target should pass.
There are two forms of the option, distinguished by the
PCode.
With loose source route (PCode = 18) each ST agent first
examines the first next-hop IP address in the option. If
the address is (one of) the address of the current ST agent,
that entry is removed, and the PBytes field reduced by four
(4). If the resulting PBytes field contains 4 (i.e., there
are no more next-hop IP addresses) the parameter is removed
from the Target. In either case, the Target's TargetBytes
field and the TargetList's PBytes field must be reduced
accordingly. The ST agent then routes toward the first
next-hop IP address in the option, if one exists, or toward
the target otherwise. Note that the target's IP address is
not included as the last entry in the list.
With a strict source route (PCode = 19) each ST agent first
examines the first next-hop IP address in the option. If
the address is not (one of) the address of the current ST
agent, a routing error has occurred and should be reported
with the appropriate reason code. Otherwise that entry is
removed, and the PBytes field reduced by four (4). If the
resulting PBytes field contains 4 (i.e., there are no more
next-hop IP addresses) the parameter is removed from the
Target. In either case, the Target's TargetBytes field and
the TargetList's PBytes field must be reduced accordingly.
The ST agent then routes toward the first next-hop IP
address in the option, if one exists, or toward the target
otherwise. Note that the target's IP address is not
included as the last entry in the list.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode 4+4*N 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next-hop IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
next-hop IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 35. SrcRoute
Since it is possible that a single hop between ST agents is
actually composed of multiple IP hops using IP
encapsulation, it might be necessary to also specify an IP
source routing option. Two additional PCodes are used in
this case. See [15] for a description of IP routing
options.
An IP Loose Source Route (PCode = 16) indicates that PDUs
for the next-hop ST agent should be encapsulated in IP and
that the IP datagram should contain an IP Loose Source Route
constructed from the list of IP router addresses contained
in this option.
An IP Strict Source Route (PCode = 17) is similarly used
when the corresponding IP Strict Source Route option should
be constructed.
Consequently, the "routing parameter" may consist of a
sequence of one or more separate parameters with PCodes 16,
17, 18, or 19.
4.2.2.15. Target and TargetList
Several control messages use a parameter called TargetList
(PCode = 20), which contains information about the targets
to which the message pertains. For each Target in the
TargetList, the information includes the IP addresses of the
target, the SAP applicable to the next higher layer
protocol, the length of the SAP (SAPBytes), and zero or more
optional SrcRoute parameters; see Section 4.2.2.14 (page
95). Consequently, a Target structure can be of variable
length. Each entry has the format shown in Figure 36.
The optional SrcRoute parameter is only meaningful in a
CONNECT messages; if present in other messages, they are
ignored. Note that the presence of SrcRoute parameter(s)
reduces the number of Targets that can be contained in a
TargetList since the maximum size of a TargetList is 256
bytes. Consequently an implementation should be prepared to
accept multiple TargetLists in a single message.
TargetIPAddress is the IP Address of the Target.
TargetBytes is the length of the Target structure,
beginning with the TargetIPAddress and including any
SrcRoute Parameter(s).
SAPBytes is the length of the SAP, excluding any padding
required to maintain 32-bit alignment. I.e.,
there would be no padding required for SAPs with lengths
of 2, 6, etc., bytes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TargetIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TargetBytes SAPBytes :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+-+-+-+-+-+-+-+-+
: SAP : Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: SrcRoute Parameter(s) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 36. Target
We assume that the ST agents must know the maximum packet
size of the networks to which they are connected (the MTU),
and those maximum sizes will restrict the number of targets
that can be specified in control messages. We feel that
this is not a serious drawback. High bandwidth networks
such as the Ethernet or the Terrestrial Wideband network
support packet sizes large enough to allow well over one
hundred targets to be specified, and we feel that
conferences with a larger number of participants will not
occur for quite some time. Furthermore, we expect that
future higher bandwidth networks will allow even larger
packet sizes. It may be desirable to send ST voice data
packets in individual B-ISDN ATM cells, which are small, but
network services on ATM will provide "adaptation layers" to
implement network-level fragmentation that may be used to
carry larger ST control messages.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 20 PBytes TargetCount = N
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Target 1 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Target N :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 37. TargetList
If a message must pass across a network whose maximum packet
size is too small, the message must be broken up into
multiple messages, each of which carries part of the
TargetList. The function of the message can still be
performed even if the message is so partitioned. The effect
in this partitioning is to compromise the performance, but
still allows proper operation. For example, if a CONNECT
message were partitioned, the first CONNECT would establish
the stream, and the rest of the CONNECTs would be processed
as additions to the first. The routing decisions might
suffer, however, since they would be made on partial
information. Nevertheless, the stream would be created.
4.2.2.16. UserData
The UserData parameter (PCode = 21) is an optional parameter
that may be used by the next higher protocol or an
application to convey arbitrary information to its peers.
Note that since the size of control messages is limited by
the smallest MTU in the path to the target(s), the maximum
size of this parameter cannot be specified a priori. If the
parameter is too large for some network's MTU, a
UserDataSize error will occur. The parameter must be padded
to a multiple of 32 bits.
UserBytes specifies the number of valid UserInformation
bytes.
UserInformation is arbitrary data meaningful to the next
higher protocol layer or application.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PCode = 21 PBytes UserBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserInformation : Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 38. UserData
4.2.3. ST Control Message PDUs
Each control message is described in a following section. See
Appendix 1 (page 147) for an explanation of the notation.
4.2.3.1. ACCEPT
ACCEPT (OpCode = 1) is issued by a target as a positive
response to a CONNECT message. It implies that the target
is prepared to accept data from the origin along the stream
that was established by the CONNECT. The ACCEPT includes
the FlowSpec that contains the cumulative information that
was calculated by the intervening ST agents as the CONNECT
made its way from the origin to the target, as well as any
modifications made by the application at the target. The
ACCEPT is relayed by the ST agents from the target to the
origin along the path established by the CONNECT but in the
reverse direction. The ACCEPT must be acknowledged with an
ACK at each hop.
The FlowSpec is not modified on this trip from the target
back to the origin. Since the cumulative FlowSpec
information can be different for different targets, no
attempt is made to combine the ACCEPTs from the various
targets. The TargetList included in each ACCEPT contains
the IP address of only the target that issued the ACCEPT.
Any SrcRoute parameters in the TargetList are ignored.
Since an ACCEPT might be the first response from a next-hop
on a control link (due to network reordering), the SVLId
field may be the first source of the Virtual Link Identifier
to be used in the RVLId field of subsequent control messages
sent to that next-hop.
When the FDx option has been selected to setup a second
stream in the reverse direction, the ACCEPT will contain
both RFlowSpec and RName parameters. Each agent should
update the state tables for the reverse stream with this
information.
TSR (bits 14 and 15) specifies the target's response for
the use of data packet timestamps; see Section 4 (page
76). Its values and semantics are:
00 Not implemented.
01 No timestamps are permitted.
10 Timestamps must always be present.
11 Timestamps may optionally be present.
Reference contains a number assigned by the agent sending
the ACCEPT for use in the acknowledging ACK.
LnkReference is the Reference number from the
corresponding CONNECT or CHANGE.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 1 0 TSR TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RecordRoute Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RFlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! RName Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserData Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 39. ACCEPT Control Message
4.2.3.2. ACK
ACK (OpCode = 2) is used to acknowledge a request. The
Reference in the header is the Reference number of the
control message being acknowledged.
Since a ACK might be the first response from a next-hop on a
control link, the SVLId field may be the first source of the
Virtual Link Identifier to be used in the RVLId field of
subsequent control messages sent to that next-hop.
ReasonCode is usually NoError, but other possibilities
exist, e.g., DuplicateIgn.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 2 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 40. ACK Control Message
4.2.3.3. CHANGE-REQUEST
CHANGE-REQUEST (OpCode = 4) is used by an intermediate or
target agent to request that the origin change the FlowSpec
of an established stream. The CHANGE-REQUEST message is
propagated hop-by-hop to the origin, with an ACK at each
hop.
Any SrcRoute parameters in the targets of the TargetList are
ignored.
G (bit 8) is used to request a global, stream-wide
change; the TargetList parameter may be omitted when the
G bit is specified.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 4 G 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserData Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 41. CHANGE-REQUEST Control Message
4.2.3.4. CHANGE
CHANGE (OpCode = 3) is used to change the FlowSpec of an
established stream. Parameters are the same as for CONNECT
but the TargetList is not required. The CHANGE message is
processed similarly to the CONNECT message, except that it
travels along the path of an established stream.
If the change to the FlowSpec is in a direction that makes
fewer demands of the involved networks, then the change has
a high probability of success along the path of the
established stream. Each ST agent receiving the CHANGE
message makes the necessary requested changes to the network
resource allocations, and if successful, propagates the
CHANGE message along the established paths. If the change
cannot be made then the ST agent must recover using
DISCONNECT and REFUSE messages as in the case of a network
failure. Note that a failure to change the resources
requested for a specific target(s) should not cause other
targets in the stream to be deleted. The CHANGE must be
ACKed.
If the CHANGE is a result of a CHANGE-REQUEST the
LnkReference field of the CHANGE will contain the value from
the Reference field of the CHANGE-REQUEST.
It is recommended that the origin only have one outstanding
CHANGE per target; if the application requests more that
one to be outstanding at a time, it is the application's
responsibility to deal with any sequencing problems that may
arise.
Any SrcRoute parameters in the targets of the
TargetListParameter are ignored.
G (bit 8) is used to request a global, stream-wide
change; the TargetList parameter may be omitted when the
G bit is specified.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 3 G 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserData Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 42. CHANGE Control Message
4.2.3.5. CONNECT
CONNECT (OpCode = 5) requests the setup of a new stream or
an addition to or recovery of an existing stream. Only the
origin can issue the initial set of CONNECTs to setup a
stream, and the first CONNECT to each next-hop is used to
convey the initial suggestion for a HID. If the stream's
data packets will be sent to some set of next-hop ST agents
by multicast then the CONNECTs to that set must suggest the
same HID. Otherwise, the HIDs in the various CONNECTs can
be different.
The CONNECT message must fit within the maximum allowable
packet size (MTU) for the intervening network. If a CONNECT
message is too large, it must be fragmented into multiple
CONNECT messages by partitioning the TargetList; see Section
4.2 (page 77). Any UserData parameter will be replicated in
each fragment for delivery to all targets.
The next-hop can initially respond with any of the following
five responses:
1 ERROR-IN-REQUEST, which implies that the CONNECT was
not valid and has been ignored,
2 ACK, which implies that the CONNECT with the H bit not
set was valid and is being processed,
3 HID-APPROVE, which implies that the CONNECT with the
H bit set was valid, and the suggested HID can be
used or was deferred,
4 HID-REJECT, which implies that the CONNECT with the H
bit set was valid but the suggested HID cannot be
used and another must be suggested in a subsequent
HID-CHANGE message, or
5 REFUSE, which implies that the CONNECT was valid but
the included list of targets in the REFUSE cannot be
processed for the stated reason.
The next-hop will later relay back either an ACCEPT or
REFUSE from each target not already specified in the REFUSE
of case 5 above (note multiple targets may be included in a
single REFUSE message).
An intermediate ST agent that receives a CONNECT selects the
next-hop ST agents, partitions the TargetList accordingly,
reserves network resources in the direction toward the
next-hop, updating the FlowSpec accordingly (see Section
4.2.2.3 (page 81)), selects a proposed HID for each next-
hop, and sends the resulting CONNECTs.
If the intermediate ST agent that is processing a CONNECT
fails to find a route to a target, then it responds with a
REFUSE with the appropriate reason code. If the next-hop to
a target is by way of the network from which it received the
CONNECT, then it sends a NOTIFY with the appropriate reason
code (RouteBack). In either case, the TargetList specifies
the affected targets. The intermediate ST agent will only
route to and propagate a CONNECT to the targets for which it
does not issue either an ERROR-IN-REQUEST or a REFUSE.
The processing of a received CONNECT message requires care
to avoid routing loops that could result from delays in
propagating routing information among ST agents. If a
received CONNECT contains a new Name, a new stream should be
created (unless the Virtual Link Identifier matches a known
link in which case an ERROR-IN-REQUEST should be sent). If
the Name is known, there are four cases:
1 the Virtual Link Identifier matches and the Target
matches a current Target -- the duplicate target
should be ignored.
2 the Virtual Link Identifier matches but the Target is
new -- the stream should be expanded to include the
new target.
3 the Virtual Link Identifier differs and the Target
matches a current Target -- an ERROR-IN-REQUEST
message should be sent specifying that the target is
involved in a routing loop. If a reroute, the old
path will eventually timeout and send a DISCONNECT;
a subsequent retransmission of the rerouted CONNECT
will then be processed under case 2 above.
4 the Virtual Link Identifier differs but the Target is
new -- a new (instance of the) stream should be
created for the target that is deliberately part of
a loop using a SrcRoute parameter.
Note that the test for a known or matching Target includes
comparing any SrcRoute parameter that might be present.
Option bits are specified by either the origin's service
user or by an intermediate agent, depending on the specific
option. Bits not specified below are currently unspecified,
and should be set to zero (0) by the origin agent and not
changed by other agents unless those agents know their
meaning.
H (bit 8) is used for the HID Field option; see Section
3.6.1 (page 44). It is set to one (1) only if the HID
field contains either zero (when the HID selection is
being deferred), or the proposed HID. This bit is zero
(0) if the HID field does not contain valid data and
should be ignored.
P (bit 9) is used for the PTP option; see Section 3.6.2
(page 44).
S (bit 10) is used for the NoRecovery option; see Section
3.6.4 (page 46).
TSP (bits 14 and 15) specifies the origin's proposal for
the use of data packet timestamps; see Section 4 (page
76). Its values and semantics are:
00 No proposal.
01 Cannot insert timestamps.
10 Must always insert timestamps.
11 Can insert timestamps if requested.
RVLId, the receiver's Virtual Link Identifier, is set to
zero in all CONNECT messages until its value arrives in
the SVLId field of an acknowledgment to the CONNECT.
SVLId, the sender's Virtual Link Identifier, is set to a
value chosen by each hop to facilitate efficient
dispatching of subsequent control messages.
HID is the identifier that will be used with data packets
moving through the stream in the direction from the
origin to the targets. It is a hop-by-hop shorthand
identifier for the stream's Name, and is chosen by each
agent for the branch to the next-hop agents. The
contents of the HID field are only valid, and a HID-
REJECT or HID-APPROVE reply may only be sent, when the
HID Field option (H bit) is set (1). If the HID Field
option is specified and the proposed HID is zero, the
selection of the HID is deferred to the receiving next-
hop agent. If the HID Field option is not set (H bit is
0), then the HID field does not contain valid data and
should be ignored; see Section 3.6.1 (page 44).
TargetList is the list of IP addresses of the target
processes. It is of arbitrary size up to the maximum
allowed for packets traveling across the specific
network.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 5 HPS 0 TSP TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId/0 SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum HID/0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Origin Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter(s) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Group Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: MulticastAddress Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RecordRoute Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RFlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RGroup Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! RHID Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserData Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 43. CONNECT Control Message
4.2.3.6. DISCONNECT
DISCONNECT (OpCode = 6) is used by an origin to tear down an
established stream or part of a stream, or by an
intermediate agent that detects a failure between itself and
its previous-hop, as distinguished by the ReasonCode. The
DISCONNECT message specifies the list of targets that are to
be disconnected. An ACK is required in response to a
DISCONNECT message. The DISCONNECT message is propagated
all the way to the specified targets. The targets are
expected to terminate their participation in the stream.
Note that in the case of a failure it may be advantageous to
retain state information as the stream should be repaired
shortly; see Section 3.7.2 (page 52).
G (bit 8) is used to request a DISCONNECT of all the
stream's targets; the TargetList parameter may be omitted
when the G bit is set (1).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 6 G 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserData Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 44. DISCONNECT Control Message
4.2.3.7. ERROR-IN-REQUEST
ERROR-IN-REQUEST (OpCode = 7) is sent in acknowledgment to a
request in which an error is detected. No action is taken
on the erroneous request and no state information for the
stream is retained. Consequently it is appropriate for the
SVLId to be zero (0). No ACK is expected.
An ERROR-IN-REQUEST is never sent in response to either an
ERROR-IN-REQUEST or an ERROR-IN-RESPONSE; however, the
event should be logged for diagnostic purposes. The
receiver of an ERROR-IN-REQUEST is encouraged to try again
without waiting for a retransmission timeout.
Reference is the Reference number of the erroneous
request.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 7 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId/0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ErroredPDU :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 45. ERROR-IN-REQUEST Control Message
4.2.3.8. ERROR-IN-RESPONSE
ERROR-IN-RESPONSE (OpCode = 8) is sent in acknowledgment to
a response in which an error is detected. No ACK is
expected. Action taken by the requester and responder will
vary with the nature of the request.
An ERROR-IN-REQUEST is never sent in response to either an
ERROR-IN-REQUEST or an ERROR-IN-RESPONSE; however, the
event should be logged for diagnostic purposes. The
receiver of an ERROR-IN-RESPONSE is encouraged to try again
without waiting for a retransmission timeout.
Reference identifies the erroneous response.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 8 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ErroredPDU :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 46. ERROR-IN-RESPONSE Control Message
4.2.3.9. HELLO
HELLO (OpCode = 9) is used as part of the ST failure
detection mechanism; see Section 3.7.1.2 (page 49).
R (bit 8) is used for the Restarted bit.
Reference is non-zero to inform the receiver that an ACK
should be promptly sent so that the sender can update its
round-trip time estimates. If the Reference is zero, no
ACK should be sent.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 9 R 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId/0 SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference/0 LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
HelloTimer
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! OriginTimestamp !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 47. HELLO Control Message
4.2.3.10. HID-APPROVE
HID-APPROVE (OpCode = 10) is used by the agent that is
responding to either a CONNECT or HID-CHANGE to agree to
either use the proposed HID or to the addition or deletion
of the specified HID. In all cases but deletion, the newly
approved HID is returned in the HID field; for deletion,
the HID field must be set to zero. The HID-APPROVE is the
acknowledgment of a CONNECT or HID-CHANGE.
The optional FreeHIDs parameter provides the previous-hop
agent with hints about what other HIDs are acceptable in
case a multicast HID is being negotiated; see Section
4.2.2.4 (page 84).
Since a HID-APPROVE might be the first response from a
next-hop on a control link, the SVLId field may be the first
source of the Virtual Link Identifier to be used in the
RVLId field of subsequent control messages sent to that
next-hop.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 10 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum HID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FreeHIDs Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 48. HID-APPROVE Control Message
4.2.3.11. HID-CHANGE-REQUEST
HID-CHANGE-REQUEST (OpCode = 12) is used by a next-hop agent
that would like, for administrative reasons, to change the
HID that is in use. The receiving previous-hop agent
acknowledges the request by either an ERROR-IN-REQUEST if it
is unwilling to make the requested change, or with a HID-
CHANGE if it can accommodate the request.
A (bit 8) is used to indicate that the specified HID
should be included in the set of HIDs for the specified
Name. When a HID is added, the acknowledging HID-APPROVE
should contain a HID field whose contents is the HID just
added.
D (bit 9) is used to indicate that the specified HID
should be removed in the set of HIDs for the specified
Name. When a HID is deleted, the acknowledging HID-
APPROVE should contain a HID field whose contents is
zero. Note that the Reference field may be used to
determine the HID that has been deleted.
If neither bit is set, the specified HID should replace
that currently in use with the specified Name.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 12 AD 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum HID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 49. HID-CHANGE-REQUEST Control Message
4.2.3.12. HID-CHANGE
HID-CHANGE (OpCode = 11) is used by the agent that issued a
CONNECT and received a HID-REJECT to attempt to negotiate a
suitable HID. The HID in the HID-CHANGE message must be
different from that in the CONNECT, or any previous HID-
CHANGE messages for the given Name. The agent receiving the
HID-CHANGE must respond with a HID-APPROVE if the new HID is
suitable, or a HID-REJECT if it is not. In case of an
error, either an ERROR-IN-REQUEST or a REFUSE may be
returned as an acknowledgment.
Since an agent may send CONNECT messages with the same HID
to several next-hops in order to use multicast data
transfer, any HID-CHANGE must also be sent to the same set
of next-hops. Therefore, a next-hop agent must be prepared
to receive a HID-CHANGE before or after it has sent a HID-
APPROVE response to the CONNECT or a previous HID-CHANGE.
Only the last HID-CHANGE is relevant. The previous-hop
agent will ignore HID-APPROVE or HID-REJECT messages to
previous CONNECT or HID-CHANGE messages.
A DISCONNECT can be sent instead of a HID-CHANGE, or a
REFUSE can be sent instead of a HID-APPROVE or HID-REJECT,
to terminate fatally the HID negotiation and the agent's
knowledge of the stream.
The A and D bits are used to change a HID, e.g., when adding
a new next-hop to a multicast group, in such a way that data
packets that are flowing through the network will not be
mishandled due to a race condition in processing the HID-
CHANGE messages between the previous-hop and its next-hops.
An implementation may choose to limit the number of
simultaneous HIDs associated with a stream, but must allow
at least two.
A (bit 8) is used to indicate that the specified HID
should be included in the set of HIDs for the specified
Name. When a HID is added, the acknowledging HID-APPROVE
should contain a HID field whose contents is the HID just
added.
D (bit 9) is used to indicate that the specified HID
should be removed from the set of HIDs for the specified
Name. When a HID is deleted, the acknowledging HID-
APPROVE should contain a HID field whose contents is
zero. Note that the Reference field may be used to
determine the HID that has been deleted.
If neither bit is set, the specified HID should replace
that currently in use for the specified Name.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 11 AD 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum HID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 50. HID-CHANGE Control Message
4.2.3.13. HID-REJECT
HID-REJECT (OpCode = 13) is used as an acknowledgment that a
CONNECT or HID-CHANGE was received and is being processed,
but means that the HID contained in the CONNECT or HID-
CHANGE is not acceptable. Upon receipt of this message the
agent that issued the CONNECT or HID-CHANGE must now issue a
HID-CHANGE to attempt to find a suitable HID. The HID-
CHANGE can cause another HID-REJECT but eventually the HID-
CHANGE must be acknowledged with a HID-APPROVE to end
successfully the HID negotiation. The agent that issued the
HID-REJECT may not issue an ACCEPT before it has found an
acceptable HID.
Since a HID-REJECT might be the first response from a next-
hop on a control link, the SVLId field may be the first
source of the Virtual Link Identifier to be used in the
RVLId field of subsequent control messages sent to that
next-hop.
Either agent may terminate the negotiation by issuing either
a DISCONNECT or a REROUTE. The agent that issued the HID-
REJECT may issue a REFUSE, or REROUTE at any time after the
HID-REJECT. In this case, the stream cannot be created, the
HID negotiation need not proceed, and the previous-hop need
not transmit any further messages; any further messages
that are received should be ignored.
The optional FreeHIDs parameter provides the previous-hop
agent with hints about what HIDs would have been acceptable;
see Section 4.2.2.4 (page 84).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 13 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum RejectedHID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FreeHIDs Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 51. HID-REJECT Control Message
4.2.3.14. NOTIFY
NOTIFY (OpCode = 14) is issued by a an agent to inform other
agents, the origin, or target(s) of events that may be
significant. The action taken by the receiver of a NOTIFY
depends on the ReasonCode. Possible events are suspected
routing problems or resource allocation changes that occur
after a stream has been established. These changes occur
when network components fail and when competing streams
preempt resources previously reserved by a lower precedence
stream. We also anticipate that NOTIFY can be used in the
future when additional resources become available, as is the
case when network components recover or when higher
precedence streams are deleted.
NOTIFY may contain a FlowSpec that reflects that revised
guarantee that can be promised to the stream. NOTIFY may
also identify those targets that are affected by the change.
In this way, NOTIFY is similar to ACCEPT.
NOTIFY may be relayed by the ST agents back to the origin,
along the path established by the CONNECT but in the reverse
direction. It is up to the origin to decide whether a
CHANGE should be submitted.
When NOTIFY is received at the origin, the application
should be notified of the target and the change in resources
allocated along the path to it, as specified in the FlowSpec
contained in the NOTIFY message. The application may then
use the information to either adjust or terminate the
portion of the stream to each affected target.
The NOTIFY may be propagated beyond the previous-hop or
next-hop agent; it must be acknowledged with an ACK.
Reference contains a number assigned by the agent sending
the NOTIFY for use in the acknowledging ACK.
ReasonCode identifies the reason for the notification.
LnkReference, when non-zero, is the Reference number from
a command that is the subject of the notification.
HID is present when the notification is related to a HID.
Name is present when the notification is related to a
stream.
NextHopIPAddress is an optional parameter and contains
the IP address of a suggested next-hop ST agent.
TargetList is present when the notification is related to
one or more targets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 14 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ErroredPDU :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! HID Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! NextHopIPAddress Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RecordRoute Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 52. NOTIFY Control Message
4.2.3.15. REFUSE
REFUSE (OpCode = 15) is issued by a target that either does
not wish to accept a CONNECT message or wishes to remove
itself from an established stream. It might also be issued
by an intermediate agent in response to a CONNECT or CHANGE
either to terminate fatally a failing HID negotiation, to
terminate a routing loop, or when a satisfactory next-hop to
a target cannot be found. It may also be a separate command
when an existing stream has been preempted by a higher
precedence stream or an agent detects the failure of a
previous-hop, next-hop, or the network between them. In all
cases, the TargetList specifies the targets that are
affected by the condition. Each REFUSE must be acknowledged
by an ACK.
The REFUSE is relayed by the agents from the originating
agent to the origin (or intermediate agent that created the
CONNECT or CHANGE) along the path traced by the CONNECT.
The agent receiving the REFUSE will process it differently
depending on the condition that caused it, as specified in
the ReasonCode field. In some cases, such as if a next-hop
cannot obtain resources, the agent can release any resources
reserved exclusively for transmissions in the stream in
question to the target specified in the TargetList, and the
previous-hop can attempt to find an alternate route. In
some cases, such as a routing failure, the previous-hop
cannot determine where the failure occurred, and must
propagate the REFUSE back to the origin, which can attempt
recovery of the stream by issuing a new CONNECT.
No special effort is made to combine multiple REFUSE
messages since it is considered most unlikely that separate
REFUSEs will happen to both pass through an agent at the
same time and be easily combined, e.g., have identical
ReasonCodes and parameters.
Since a REFUSE might be the first response from a next-hop
on a control link, the SVLId field may be the first source
of the Virtual Link Identifier to be used in the RVLId field
of subsequent control messages sent to that next-hop.
Reference contains a number assigned by the agent sending
the REFUSE for use in the acknowledging ACK.
LnkReference is either the Reference number from the
corresponding CONNECT or CHANGE, if it is the result of
such a message, or zero when the REFUSE was originated as
a separate command.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 15 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId SVLId
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum ReasonCode
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DetectorIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ErroredPDU :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: RecordRoute Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: UserData Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 53. REFUSE Control Message
4.2.3.16. STATUS
STATUS (OpCode = 16) is used to inquire about the existence
of a particular stream identified by either a HID (H bit
set) or Name (Name Parameter present).
When a stream has been identified, a STATUS-RESPONSE is
returned that will contain the specified HID and/or Name but
no other parameters if the specified stream is unknown, or
will otherwise contain the current HID(s), Name, FlowSpec,
TargetList, and possibly Group(s) of the stream. Note that
if a stream has no current HID, the HID field in the
STATUS-RESPONSE will contain zero; it will contain the
first, or only, HID if a valid HID exists; additional valid
HIDs will be returned in HID parameters.
Use of STATUS is intended for diagnostic purposes and to
assist in stream cleanup operations. Note that if both a
HID and Name are specified, but they do not correspond to
the same stream, an ERROR-IN-REQUEST with the appropriate
reason code (InconsistHID) would be returned.
It is possible in cases of multiple failures or network
partitioning for an ST agent to have information about a
stream after the stream has either ceased to exist or has
been rerouted around the agent. When an agent concludes
that a stream has not been used for a period of time and
might no longer be valid, it can probe the stream's
previous-hop or next-hop(s) to see if they believe that the
stream still exists through the interrogating agent. If
not, those hops would reply with a STATUS-RESPONSE that
contains the HID and/or Name but no other parameters;
otherwise, if the stream is still valid, the hops would
reply with the parameters of the stream.
H (bit 8) is used to indicate whether (when 1) or not
(when 0) a HID is present in the HID field.
Q (bit 9) is set to one (1) for remote diagnostic
purposes when the receiving agent should return a
stream's parameters, whether or not the source of the
message is believed to be a previous-hop or next-hop in
the specified stream. Note that this use has potential
for disclosure of sensitive information.
RVLId and SVLId may either or both be zero when STATUS is
used for diagnostic purposes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 16 HQ 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId/0 SVLId/0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum HID/0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 54. STATUS Control Message
4.2.3.17. STATUS-RESPONSE
STATUS-RESPONSE (OpCode = 17) is the reply to a STATUS
message. If the stream specified in the STATUS message is
not known, the STATUS-RESPONSE will contain the specified
HID and/or Name but no other parameters. It will otherwise
contain the current HID(s), Name, FlowSpec, TargetList, and
possibly Group of the stream. Note that if a stream has no
current HID, the H bit in the STATUS-RESPONSE will be zero.
The HID field will contain the first, or only, HID if a
valid HID exists; additional valid HIDs will be returned in
HID parameters.
H (bit 8) is used to indicate whether (when 1) or not
(when 0) a HID is present in the HID field.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OpCode = 17 HQ 0 TotalBytes
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RVLId/0 SVLId/0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reference LnkReference
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SenderIPAddress
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum HID/0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Name Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: FlowSpec Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Group Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! HID Parameter !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: TargetList Parameter :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 55. STATUS-RESPONSE Control Message
4.3. Suggested Protocol Constants
The ST Protocol uses several fields that must have specific values
for the protocol to work, and also several values that an
implementation must select. This section specifies the required
values and suggests initial values for others. It is recommended
that the latter be implemented as variables so that they may be
easily changed when experience indicates better values.
Eventually, they should be managed via the normal network
management facilities.
ST uses IP Version Number 5.
When encapsulated in IP, ST uses IP Protocol Number 5.
Value ST Command Message Name Value ST Element Name
------- ----------------------- ------- ---------------------
1 ACCEPT 1 ErroredPDU
2 ACK 2 FlowSpec
3 CHANGE 3 FreeHIDs
4 CHANGE-REQUEST 4 Group
5 CONNECT 5 HID
6 DISCONNECT 6 MulticastAddress
7 ERROR-IN-REQUEST 7 Name
8 ERROR-IN-RESPONSE 8 NextHopIPAddress
9 HELLO 9 Origin
10 HID-APPROVE 10 OriginTimestamp
11 HID-CHANGE 11 RecordRoute
12 HID-CHANGE-REQUEST 12 RFlowSpec
13 HID-REJECT 13 RGroup
14 NOTIFY 14 RHID
15 REFUSE 15 RName
16 STATUS 16 SrcRoute, IP Loose
17 STATUS-RESPONSE 17 SrcRoute, IP Strict
18 SrcRoute, ST Loose
19 SrcRoute, ST Strict
20 TargetList
21 UserData
A good choice for the minimum number of bits in the FreeHIDBitMask
element of the FreeHIDs parameter is not yet known. We suggest a
minimum of 64 bits, i.e., N in Figure 25 has a value of two (2).
HID value zero (0) is reserved for ST Control Messages. HID
values 1-3 are reserved for future use.
VLId value zero (0) may only be used in the RVLId field of an ST
Control Message when the appropriate value has not yet been
received from the other end of the virtual link;' except for an
ERROR-IN-REQUEST or diagnostic message, the SVLId field may never
contain a value of zero except in a diagnostic message. VLId
value 1 is reserved for use with HELLO messages by those agents
whose implementation wishes to have all HELLOs so identified.
VLId values 2-3 are reserved for future use.
The following permanent IP multicast addresses have been assigned
to ST:
224.0.0.7 All ST routers
224.0.0.8 All ST hosts
In addition, a block of transient IP multicast addresses,
224.1.0.0 - 224.1.255.255, has been allocated for ST multicast
groups. Note that in the case of Ethernet, an ST Multicast
address of 224.1.cc.dd maps to an Ethernet Multicast address of
01:00:5E:01:cc:dd (see [6]).
SCMP uses retransmission to effect reliability and thus has
several "retransmission timers". Each "timer" is modeled by an
initial time interval (ToXxx), which gets updated dynamically
through measurement of control traffic, and a number of times
(NXxx) to retransmit a message before declaring a failure. All
time intervals are in units of milliseconds.
Value Timeout Name Meaning
------- ---------------------- ----------------------------------
1000 ToAccept Initial hop-by-hop timeout for
acknowledgment of ACCEPT
3 NAccept ACCEPT retries before failure
1000 ToConnect Initial hop-by-hop timeout for
acknowledgment of CONNECT
5 NConnect CONNECT retries before failure
1000 ToDisconnect Initial hop-by-hop timeout for
acknowledgment of DISCONNECT
3 NDisconnect DISCONNECT retries before
failure
Value Timeout Name Meaning
------- ---------------------- ----------------------------------
1000 ToHIDAck Initial hop-by-hop timeout for
acknowledgment of
HID-CHANGE-REQUEST
3 NHIDAck HID-CHANGE-REQUEST retries
before failure
1000 ToHIDChange Initial hop-by-hop timeout for
acknowledgment of HID-CHANGE
3 NHIDChange HID-CHANGE retries before
failure
1000 ToNotify Initial hop-by-hop timeout for
acknowledgment of NOTIFY
3 NNotify NOTIFY retries before failure
1000 ToRefuse Initial hop-by-hop timeout for
acknowledgment of REFUSE
3 NRefuse REFUSE retries before failure
1000 ToReroute Timeout for receipt of ACCEPT or
REFUSE from targets during
failure recovery
5 NReroute CONNECT retries before failure
5000 ToEnd2End End-to-End timeout for receipt
of ACCEPT or REFUSE from targets
by origin
0 NEnd2End CONNECT retries before failure
Value Parameter Name Meaning
------- ---------------------- ----------------------------------
10 NHIDAbort Number of rejected HID proposals
before aborting the HID
negotiation process
10000 HelloTimerHoldDown Interval that Restarted bit must
be set after ST restart
5 HelloLossFactor Number of consecutively missed
HELLO messages before declaring
link failure
2000 DefaultRecoveryTimeout Interval between successive
HELLOs to/from active neighbors
2 DefaultHelloFactor HELLO filtering function factor
5. Areas Not Addressed
There are a number of issues that will need to be addressed in the
long run but are not addressed here. Some issues are network or
implementation specific. For example, the management of multicast
groups depends on the interface that a network provides to the ST
agent, and an UP/DOWN protocol based on ST HELLO messages depends on
the details of the ST agents. Both these examples may impact the ST
implementations, but we feel it is inappropriate to specify them
here.
In other cases we feel that appropriate solutions are not clear at
this time. The following are examples of such issues:
This document does not include a routing mechanism. We do not feel
that a routing strategy based on minimizing the number of hops from
the source to the destination is necessarily appropriate. An
alternative strategy is to minimize the consumption of internet
resources within some delay constraints. Furthermore, it would be
preferable if the routing function were to provide routes that
incorporated bandwidth, delay, reliability, and perhaps other
characteristics, not just connectivity. This would increase the
likelihood that a selected route would succeed. This requirement
would probably cause the ST agents to exchange more routing
information than currently implemented. We feel that further
research and experimentation will be required before an appropriate
routing strategy is well enough defined to be incorporated into the
ST specification.
Once the bandwidth for a stream has been agreed upon, it is not
sufficient to rely on the origin to transmit traffic at that rate.
The internet should not rely on the origin to operate properly.
Furthermore, even if the origin sources traffic at the agreed rate,
the packets may become aggregated unintentionally and cause local
congestion. There are several approaches to addressing this problem,
such as metering the traffic in each stream as it passes through each
agent. Experimentation is necessary before such a mechanism is
selected.
The interface between the agent and the network is very limited. A
mechanism is provided by which the ST layer can query the network to
determine the likelihood that a stream can be supported. However,
this facility will require practical experience before its
appropriate use is defined.
The simplex tree model of a stream does not easily allow for using
multiple paths to support a greater bandwidth. That is, at any given
point in a stream, the entire incoming bandwidth must be transmitted
to the same next-hop in order to get to some target. If the
bandwidth isn't available along any single path, the stream cannot be
built to that target. It may be the case that the bandwidth is not
available along a single path, but if the data
flow is split along multiple paths, and so multiple next-hops,
sufficient bandwidth would be available. As currently specified, the
ST agent at the point where the multiple flows converge will refuse
the second connection because it can only be interpreted as a routing
failure. A mechanism that allows multiple paths in a stream and can
protect against routing failures has not been defined.
If sufficient bandwidth is not available, both preemption and
rerouting are possible. However, it is not clear when to use one or
the other. As currently specified, an ST agent that cannot obtain
sufficient bandwidth will attempt to preempt lower precedence streams
before attempting to reroute around the bottleneck. This may lead to
an undesirably high number of preemptions. It may be that a higher
precedence stream can be rerouted around lower precedence streams and
still meet its performance requirements, whereas the preempted lower
precedence streams cannot be reconstructed and still meet their
performance requirements. A simple and effective algorithm to allow
a better decision has not been identified.
In case a stream cannot be completed, ST does not report to the
application the nature of the trouble in any great detail.
Specifically, the application cannot determine where the bottleneck
is, whether the problem is permanent or transitory, or the likely
time before the trouble may be resolved. The application can only
attempt to build the stream at some later time hoping that the
trouble has been resolved. Schemes can be envisioned by which
information is relayed back to the application. However, only
practical experience can evaluate the kind of trouble that is most
likely encountered and the nature of information that would be most
useful to the application.
A mechanism is also not defined for cases where a stream cannot be
completed not because of lack of resources but because of an
unexpected failure that results in an ERROR-IN-REQUEST message. An
ERROR-IN-REQUEST message is returned in cases when an ST agent issues
a malformed control message to a neighbor. Such an occurrence is
unexpected and may be caused by a bad or incomplete ST
implementation. In some cases a message, such as a NOTIFY should be
sent to the origin. Such a mechanism is not defined because it is
not clear what information can be extracted and what the origin
should do.
No special action is taken when a target is removed from a stream.
Removing a target may also remove a bottleneck either in bandwidth,
packet rate or packet size, but advantage of this opportunity is not
taken automatically. The application may initiate a change to the
stream's characteristics, but it is not in the best position to do
this because the application may not know the nature of the
bottleneck. The ST layer may have the best information, but a
mechanism to do this may be very complex. As a result, this concept
requires further thought.
An agent simply discards a stream's data packets if it cannot forward
them. The reason may be that the packets are too large or are
arriving at too high a rate. Alternative actions may include an
attempt to do something with the packets, such as fragmenting them,
or to notify the origin of the trouble. Corrective measures may be
too complex, so it may be preferable simply to notify the origin with
a NOTIFY message. However, if the incoming packet rate is causing
congestion, then the NOTIFY messages themselves may cause more
trouble. The nature of the communication has yet to be defined.
The FlowSpec includes a cost field, but its implementation has not
been identified. The units of cost can probably be defined
relatively easily. Cost of bandwidth can probably also be assigned.
It is not clear how cost is assigned to other functions, such as high
precedence or low delay, or how cost of the components of the stream
are combined together. It is clear that the cost to provide services
will become more important in the near future, but it is not clear at
this time how that cost is determined.
A number of parameters of the FlowSpec are intended to be used as
ranges, but some may be useful as discrete values. For example, the
FlowSpec may specify that bandwidth for a stream carrying voice
should be reserved in a range from 16Kbps to 64Kbps because the voice
codec has a variable coding rate. However, the voice codec may be
varied only among certain discrete values, such as 16Kbps, 32Kbps and
64Kbps. A stream that has 48Kbps of bandwidth is no better than one
with 32Kbps. The parameters of the FlowSpec where this may be
relevant should optionally specify discrete values. This is being
considered.
Groups are defined as a way to associate different streams, but the
nature of the association is left for further study. An example of
such an association is to allow streams whose traffic is inherently
not simultaneous to share the same allocated resources. This may
happen for example in a conference that has an explicit floor, such
that only one site can generate video or audio traffic at any given
time. The grouping facility can be implemented based on this
specification, but the implementation of the possible uses of groups
will require new functionality to be added to the ST agents. The
uses for groups and the implementation to support them will be
carried out as experience is gained and the need arises.
We hope that the ST we here propose will act as a vehicle to study
the use and performance of stream oriented services across packet
switched networks.
[This page intentionally left blank.]
6. Glossary
appropriate reason code
This phrase refers to one or perhaps a set of reason codes that
indicate why a particular action is being taken. Typically,
these result from detection of errors or anomalous conditions.
It can also indicate that an application component or agent has
presented invalid parameters.
DefaultRecoveryTimeout
The DefaultRecoveryTimeout is maintained by each ST agent. It
indicates the default time interval to use for sending HELLO
messages.
downstream
The direction in a stream from an origin toward its targets.
element
The fields and parameters of the ST control messages are
collectively called elements.
FlowSpec
The Flow Specification, abbreviated "FlowSpec" is used by an
application to specify required and desired characteristics of
the stream. The FlowSpec specifies bandwidth, delay, and
reliability parameters. Both minimal requirements and desired
characteristics are included. This information is then used to
guide route selection and resource allocation decisions. The
desired vs. required characteristics are used to guide tradeoff
decisions among competing stream requests.
group
A set of related streams can be associated as a group. This is
done by generating a Group Name and assigning it to each of the
related streams. The grouping information can then be used by
the ST agents in making resource management and other control
decisions. For example, when preemption is necessary to
establish a high precedence stream, we can exploit the group
information to minimize the number of stream groups that are
preempted.
Group Name
The Group Name is used to indicate that a collection of streams
are related. A Group Name is structured to ensure that it is
unique across all hosts: it includes the address of the host
where it was generated combined with a unique number generated
by that host. A timestamp is added to ensure that the overall
name is unique over all time. (A Group Name has the same format
as a stream Name.)
HelloLossFactor
The HelloLossFactor is a parameter maintained by each ST agent.
It identifies the expected number of consecutive HELLO messages
typically lost due to transient factors. Thus, an agent will be
assumed to be down after we miss more than HelloLossFactor
messages.
HelloTimer
The HelloTimer is a millisecond timer maintained by each ST
agent. It is included in each HELLO message. It represents the
time since the agent was restarted, modulo the precision of the
field. It is used to detect variations in the delay between the
two agents, by comparing the arrival interval of two HELLO
messages to the difference between their HelloTimer fields.
HelloTimerHoldDown
The HelloTimerHoldDown value is maintained by each ST agent.
When an ST agent is restarted, it will set the "Restarted" bit
in all HELLO messages it sends for HelloTimerHoldDown seconds.
HID
The Hop IDentifier, abbreviated as HID, is a numeric key stored
in the header of each ST packet. It is used by an ST agent to
associate the packet with one of the incoming hops managed by
the agent. It can be used by receiving agent to map to
the set of outgoing next-hops to which the message should be
forwarded. The HID field of an ST packet will generally need to
be changed as it passes through each ST agent since there may be
many HIDs associated with a single stream.
hop
A "hop" refers to the portion of a stream's path between two
neighbor ST agents. It is usually represented by a physical
network. However, a multicast hop can connect a single ST agent
to several next-hop ST agents.
host agents
Synonym for host ST agents.
host ST agents
Host ST agents are ST agents that provide services to higher
layer protocols and applications. The services include methods
for sourcing data from and sinking data to the higher layer or
application, and methods for requesting and modifying streams.
intermediate agents
Synonym for intermediate ST agents.
intermediate ST agents
Intermediate ST agents are ST agents that can forward ST
packets between the networks to which they are attached.
MTU
The abbreviation for Maximum Transmission Unit, which is the
maximum packet size in bytes that can be accepted by a given
network for transmission. ST agents determine the maximum
packet size for a stream so that data written to the stream can
be forwarded through the networks without fragmentation.
multi-destination simplex
The topology and data flow of ST streams are described as being
multi-destination simplex: all data flowing on the stream
originates from a single origin and is passed to one or more
destination targets. Only control information, invisible to the
application program, ever passes in the upstream direction.
NAccept
NAccept is an integer parameter maintained by each ST agent. It
is used to control retransmission of an ACCEPT message. Since
an ACCEPT request is relayed by agents back toward the origin,
it must be acknowledged by each previous-hop agent. If this ACK
is not received within the appropriate timeout interval, the
request will be resent up to NAccept times before giving up.
Name
Generally refers to the name of a stream. A stream Name is
structured to ensure that it is unique across all hosts: it
includes the address of the host where it was generated combined
with a unique number generated at that host. A timestamp is
added to ensure that the overall Name is unique over all time.
(A stream Name has the same format as a Group Name.)
NConnect
NConnect is an integer parameter maintained by each ST agent.
It is used to control retransmission of a CONNECT message. A
CONNECT request must be acknowledged by each next-hop agent as
it is propagated toward the targets. If a HID-ACCEPT,
HID-REJECT, or ACK is not received for the CONNECT between any
two agents within the appropriate timeout interval, the request
will be resent up to NConnect times before giving up.
NDisconnect
NDisconnect is an integer parameter maintained by each ST
agent. It is used to control retransmission of a DISCONNECT
message. A DISCONNECT request must be acknowledged by each
next-hop agent as it is propagated toward the targets. If this
ACK is not received for the DISCONNECT between any two agents
within the appropriate timeout interval, the request will be
resent up to NDisconnect times before giving up.
next protocol identifier
The next protocol identifier is used by a target ST agent to
identify to which of several higher layer protocols it should
pass data packets it receives the network. Examples of higher
layer protocols include the Network Voice Protocol and the
Packet Video Protocol. These higher layer protocols will
typically perform further demultiplexing among multiple
application processes as part of their protocol processing
activities.
next-hop
Synonym for next-hop ST agent.
next-hop ST agent
For each origin or intermediate ST agent managing a stream
there are a set of next-hop ST agents. The intermediate agent
forwards each data packet it receives to all the next-hop ST
agents, which in turn forward the data toward the target host
agent (if the particular next-hop agent is another intermediate
agent) or to the next higher protocol layer at the target (if
the particular next-hop agent is a host agent).
NextPcol
NextPcol is a field in each Target of the CONNECT message used
to convey the next protocol identifier. See definition of next
protocol identifier above for more details.
NHIDAbort
NHIDAbort is an integer parameter maintained by each ST agent.
It is the number of unacceptable HID proposals before an ST
agent aborts the HID negotiation process.
NHIDAck
NHIDAck is an integer parameter maintained by each ST agent.
It is used to control retransmission of HID-CHANGE-REQUEST
messages. HID-CHANGE-REQUEST is sent by an ST agent to the
previous-hop ST agent to request that the HID in use between
those agents be changed. The previous-hop acknowledges the
HID-CHANGE-REQUEST message by sending a HID-CHANGE message. If
the HID-CHANGE is not received within the appropriate timeout
interval, the request will be resent up to NHIDAck times before
giving up.
NHIDChange
NHIDChange is an integer parameter maintained by each ST agent.
It is used to control retransmission of the HID-CHANGE message.
A HID-CHANGE message must be acknowledged by the next-hop agent.
If this ACK is not received within the appropriate timeout
interval, the request will be resent up to NHIDChange times
before giving up.
NRefuse
NRefuse is an integer parameter maintained by each ST agent.
It is used to control retransmission of a REFUSE message. As a
REFUSE request is relayed by agents back toward the origin, it
must be acknowledged by each previous-hop agent. If this ACK is
not received within the appropriate timeout interval, the
request will be resent up to NRefuse times before giving up.
NRetryRoute
NRetryRoute is an integer parameter maintained by each ST
agent. It is used to control route exploration. When an agent
receives a REFUSE message whose ReasonCode indicates that the
originally selected route is not acceptable, the agent should
attempt to find an alternate route to the target. If the agent
has not found a viable route after a maximum of NRetryRoute
choices, it should give up and notify the previous-hop or
application that it cannot find an acceptable path to the
target.
origin
The origin of a stream is the host agent where an application
or higher level protocol originally requested that the stream be
created. The origin specifies the data to be sent through the
stream.
parameter
Parameters are additional values that may be included in
control messages. Parameters are often optional. They are
distinguished from fields, which are always present.
participants
Participants are the end-users of a stream.
PDU
Abbreviation for Protocol Data Unit, defined below.
peer
The term peer is used to refer to entities at the same protocol
layer. It is used here to identify instances of an application
or protocol layer above ST. For example, data is passed through
a stream from an originating peer process to its target peers.
previous-hop
Synonym for previous-hop ST agent.
previous-hop ST agent
The origin or intermediate agent from which an ST agent receives
its data.
protocol data unit
A protocol data unit (PDU) is the unit of data passed to a
protocol layer by the next higher layer protocol or user. It
consists of control information and possibly user data.
RecoveryTimeout
RecoveryTimeout is specified in the FlowSpec of each stream.
The minimum of these values over all streams between a pair of
adjacent agents determines how often those agents must send
HELLO messages to each other in order to ensure that failure of
one of the agents will be detected quickly enough to meet the
guarantee implied by the FlowSpec.
Restarted bit
The Restarted bit is part of the HELLO message. When set, it
indicates that the sending agent was restarted recently (within
the last HelloTimerHoldDown seconds).
round-trip time
The round-trip-time is the time it takes a message to be sent,
delivered, processed, and the acknowledgment received. It
includes both network and processing delays.
RTT
Abbreviation for round-trip-time.
RVLId
Abbreviation for Receiver's Virtual Link Identifier. It
uniquely identifies to the receiver the virtual link, and this
stream, used to send it a message. See definition for Virtual
Link Identifier below.
SAP
Abbreviation for Service Access Point.
SCMP
Abbreviation for ST Control Message Protocol, defined below.
Service Access Point
A point where a protocol service provider makes available the
services it offers to a next higher layer protocol or user.
setup phase
Before data can be transmitted through a stream, the ST agents
must distribute state information about the stream to all agents
along the path(s) to the target(s). This is the setup phase.
The setup phase ends when all the ACCEPT and REFUSE messages
sent by the targets have been delivered to the origin. At this
point, the data transfer phase begins and data can be sent.
Requests to modify the stream can be issued after the setup
phase has ended, i.e., during the data transfer phase without
disrupting the flow of data.
ST agent
An ST agent is an entity that implements the ST Protocol.
ST Control Message Protocol
The ST Control Message Protocol is the subset of the overall ST
Protocol responsible for creation, modification, maintenance,
and tear down of a stream. It also includes support for event
notification and status monitoring.
stream
A stream is the basic object managed by the ST Protocol for
transmission of data. A stream has one origin where data are
generated and one or more targets where the data are received
for processing. A flow specification, provided by the origin
and negotiated among the origin, intermediate, and target ST
agents, identifies the requirements of the application and the
guarantees that can be assured by the ST agents.
subsets
Subsets of the ST Protocol are permitted, as defined in various
sections of this specification. Subsets are defined to allow
simplified implementations that can still effectively
interoperate with more complete implementations without causing
disruption.
SVLId
Abbreviation for Sender's Virtual Link Identifier. It uniquely
identifies to the receiver the virtual link identifier that
should be placed into the RVLId field of all replies sent over
the virtual link for a given stream. See definition for Virtual
Link Identifier below.
target
An ST target is the destination where data supplied by the
origin will be delivered for higher layer protocol or
application processing.
tear down
The tear down phase of a stream begins when the origin indicates
that it has no further data to send and the ST agents through
which the stream passes should dismantle the stream and release
its resources.
ToAccept
ToAccept is a timeout in seconds maintained by each ST agent.
It sets the retransmission interval for ACCEPT messages.
ToConnect
ToConnect is a timeout in seconds maintained by each ST agent.
It sets the retransmission interval a CONNECT messages.
ToDisconnect
ToDisconnect is a timeout in seconds maintained by each ST
agent. It sets the retransmission interval for DISCONNECT
messages.
ToHIDAck
ToHIDAck is a timeout in seconds maintained by each ST agent.
It sets the retransmission interval for HID-CHANGE-REQUEST
messages.
ToHIDChange
ToHIDChange is a timeout in seconds maintained by each ST agent.
It sets the retransmission interval for HID-CHANGE messages.
ToRefuse
ToRefuse is a timeout in seconds maintained by each ST agent.
It sets the retransmission interval for REFUSE messages.
upstream
The direction in a stream from a target toward the origin.
Virtual Link
A virtual link is one edge of the tree describing the path of
data flow through a stream. A separate virtual link is assigned
to each pair of neighbor ST agents, even when multiple next-hops
are be reached through a single network level multicast group.
The virtual link allows efficient demultiplexing of ST Control
Message PDUs received from a single physical link or network.
Virtual Link Identifier
For each ST Control Message sent, the sender provides its own
virtual link identifier and that of the receiver (if known).
Either of these identifiers, combined with the address of the
corresponding host, can be used to identify uniquely the virtual
control link to the agent. However, virtual link identifiers
are chosen by the associated agent so that the agent may
precisely identify the stream, state machine, and other protocol
processing data elements managed by that agent, without regard
to the source of the control message. Virtual link identifiers
are not negotiated, and do not change during the lifetime of a
stream. They are discarded when the stream is torn down.
7. References
[1] Braden, B., Borman, D., and C. Partridge, "Computing the
Internet Checksum", RFC1071, USC/Information Sciences
Institute, Cray Research, BBN Laboratories, September
1988.
[2] Braden, R. (ed.), "Requirements for Internet Hosts --
Communication Layers", RFC1122, USC/Information Sciences
Institute, October 1989.
[3] Cheriton, D., "VMTP: Versatile Message Transaction Protocol
Specification", RFC1045, Stanford University, February 1988.
[4] Cohen, D., "A Network Voice Protocol NVP-II", USC/Information
Sciences Institute, April 1981.
[5] Cole, E., "PVP - A Packet Video Protocol", W-Note 28,
USC/Information Sciences Institute, August 1981.
[6] Deering, S., "Host Extensions for IP Multicasting", RFC1112,
Stanford University, August 1989.
[7] Edmond W., Seo K., Leib M., and C. Topolcic, "The DARPA
Wideband Network Dual Bus Protocol", accepted for presentation
at ACM SIGCOMM '90, September 24-27, 1990.
[8] Forgie, J., "ST - A Proposed Internet Stream Protocol",
IEN 119, M. I. T. Lincoln Laboratory, 7 September 1979.
[9] Jacobs I., Binder R., and E. Hoversten E., "General Purpose
Packet Satellite Network", Proc. IEEE, vol 66, pp 1448-1467,
November 1978.
[10] Jacobson, V., "Congestion Avoidance and Control", ACM
SIGCOMM-88, August 1988.
[11] Karn, P. and C. Partridge, "Round Trip Time Estimation",
ACM SIGCOMM-87, August 1987.
[12] Mallory, T., and A. Kullberg, "Incremental Updating of the
Internet Checksum", RFC1141, BBN Communications
Corporation, January 1990.
[13] Mills, D., "Network Time Protocol (Version 2) Specification
and Implementation", RFC1119, University of Delaware,
September 1989 (Revised February 1990).
[14] Pope, A., "The SIMNET Network and Protocols", BBN
Report No. 7102, BBN Systems and Technologies, July 1989.
[15] Postel, J., ed., "Internet Protocol - DARPA Internet Program
Protocol Specification", RFC791, DARPA, September 1981.
[16] Postel, J., ed., "Transmission Control Protocol - DARPA
Internet Program Protocol Specification", RFC793, DARPA,
September 1981.
[17] Postel, J., "User Datagram Protocol", RFC768,
USC/Information Sciences Institute, August 1980.
[18] Reynolds, J., Postel, J., "Assigned Numbers", RFC1060,
USC/Information Sciences Institute, March 1990.
[19] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
SDNS Secure Data Network System, Security Protocol 3 (SP3),
SDN.301, Rev. 1.5, 1989-05-15.
[20] SDNS Protocol and Signaling Working Group, SP3 Sub-Group,
SDNS Secure Data Network System, Security Protocol 3 (SP3)
Addendum 1, Cooperating Families, SDN.301.1, Rev. 1.2,
1988-07-12.
8. Security Considerations
See section 3.7.8.
9. Authors' Addresses
Stephen Casner
USC/Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292-6695
Phone: (213) 822-1511 x153
EMail: Casner@ISI.Edu
Charles Lynn, Jr.
BBN Systems and Technologies,
a division of Bolt Beranek and Newman Inc.
10 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-3367
EMail: CLynn@BBN.Com
Philippe Park
BBN Systems and Technologies,
a division of Bolt Beranek and Newman Inc.
10 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-2892
EMail: ppark@BBN.COM
Kenneth Schroder
BBN Systems and Technologies,
a division of Bolt Beranek and Newman Inc.
10 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-3167
EMail: Schroder@BBN.Com
Claudio Topolcic
BBN Systems and Technologies,
a division of Bolt Beranek and Newman Inc.
10 Moulton Street
Cambridge, MA 02138
Phone: (617) 873-3874
EMail: Topolcic@BBN.Com
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Appendix 1. Data Notations
The convention in the documentation of Internet Protocols is to
express numbers in decimal and to picture data with the most
significant octet on the left and the least significant octet on the
right.
The order of transmission of the header and data described in this
document is resolved to the octet level. Whenever a diagram shows a
group of octets, the order of transmission of those octets is the
normal order in which they are read in English. For example, in the
following diagram the octets are transmitted in the order they are
numbered.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 2 3 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5 6 7 8
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9 10 11 12
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 56. Transmission Order of Bytes
Whenever an octet represents a numeric quantity the left most bit in
the diagram is the high order or most significant bit. That is, the
bit labeled 0 is the most significant bit. For example, the
following diagram represents the value 170 (decimal).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
1 0 1 0 1 0 1 0
+-+-+-+-+-+-+-+-+
Figure 57. Significance of Bits
Similarly, whenever a multi-octet field represents a numeric quantity
the left most bit of the whole field is the most significant bit.
When a multi-octet quantity is transmitted the most significant octet
is transmitted first.
Fields whose length is fixed and fully illustrated are shown with a
vertical bar () at the end; fixed fields whose contents are
abbreviated are shown with an exclamation point (!); variable fields
are shown with colons (:).
Optional parameters are separated from control messages with a blank
line. The order of any optional parameters is not meaningful.
