Network Working Group G. Meyer
Request for Comments: 1582 Spider Systems
Category: Standards Track February 1994
Extensions to RIP to Support Demand Circuits
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
Running routing protocols on connection oriented Public Data
Networks, for example X.25 packet switched networks or ISDN, can be
eXPensive if the standard form of periodic broadcasting of routing
information is adhered to. The high cost arises because a connection
has to all practical intents and purposes be kept open to every
destination to which routing information is to be sent, whether or
not it is being used to carry user data.
Routing information may also fail to be propagated if the number of
destinations to which the routing information is to be sent exceeds
the number of channels available to the router on the Wide Area
Network (WAN).
This memo defines a generalized modification which can be applied to
Bellman-Ford (or distance vector) algorithm information broadcasting
protocols, for example IP RIP, Netware RIP or Netware SAP, which
overcomes the limitations of the traditional methods described above.
The routing protocols support a purely triggered update mechanism on
demand circuits on WANs. The protocols run UNMODIFIED on LANs or
fixed point-to-point links.
Routing information is sent on the WAN when the routing database is
modified by new routing information received from another interface.
When this happens a (triggered) update is sent to a list of
destinations on other WAN interfaces. Because routing protocols are
datagram based they are not guaranteed to be received by the peer
router on the WAN. An acknowledgement and retransmission mechanism
is provided to ensure that routing updates are received.
The WAN circuit manager advises the routing applications on the
reachability and non-reachability of destinations on the WAN.
Acknowledgements
I would like to thank colleagues at Spider, in particular Richard
Edmonstone, Tom Daniel and Alam Turland, Yakov Rekhter (IBM), Martha
Steenstrup (BBN), and members of the RIP-2 working group of the IETF
for stimulating discussions and comments which helped to clarify this
memo.
Conventions
The following language conventions are used in the items of
specification in this document:
o MUST -- the item is an absolute requirement of the specification.
MUST is only used where it is actually required for interoperation,
not to try to impose a particular method on implementors
where not required for interoperability.
o SHOULD -- the item should be followed for all but exceptional cir-
cumstances.
o MAY or optional -- the item is truly optional and may be followed
or ignored according to the needs of the implementor.
The Words "should" and "may" are also used, in lower case, in their
more ordinary senses.
Table of Contents
1. IntrodUCtion ........................................... 3
2. Running a routing Protocol on the WAN .................. 4
2.1. Overview ......................................... 4
2.2. Presumption of Reachability ...................... 6
2.3. WAN Router list .................................. 7
2.4. Triggered Updates and Unreliable Delivery ........ 8
2.5. Guaranteeing delivery of Routing Updates ......... 8
2.6. The Routing Database ............................. 9
2.7. New Packet Types ................................. 10
2.8. Fragmentation .................................... 12
2.9. Preventing Queue Overload ........................ 13
3. IP Routing Information Protocol Version 1 .............. 13
4. IP Routing Information Protocol Version 2 .............. 16
5. Netware Routing Information Protocol ................... 17
6. Netware Service Advertising Protocol ................... 20
7. Timers ................................................. 24
7.1. Database Timer ................................... 24
7.2. Retransmission Timer ............................. 25
7.3. Reassembly Timer ................................. 26
8. Implementation Considerations ...........................27
9. Security Considerations ................................ 27
10. References ............................................. 28
11. Author's Address ....................................... 29
1. Introduction
Routers are used on connection oriented networks, such as X.25 packet
switched networks and ISDN networks, to allow potential connectivity
to a large number of remote destinations. Circuits on the Wide Area
Network (WAN) are established on demand and are relinquished when the
traffic subsides. Depending on the application, the connection
between any two sites for user data might actually be short and
relatively infrequent.
Practical experience of routing shows that periodic 'broadcasting' of
routing updates on the WAN is unsatisfactory on several counts:
o Running a routing protocol like RIP is expensive if the standard
form of transmitting routing information to every next hop router
every 30 seconds is adhered to. The more routers there are
wishing to exchange information the worse the problem. If there
are N routers on the WAN, N * (N - 1) routing updates are sent over
N * (N - 1)/2 connections every broadcast period.
The expense arises because a circuit has to be kept open to each
destination to which routing information is to be sent. Routing
updates are sufficiently frequent that little benefit is oBTainable
on most networks by attempting to set up a call purely for
the duration of the routing update. (There are often minimum call
charges, or there is a charge to set up a call irrespective of
what data is sent.)
The option of reducing the 'broadcast' frequency, while reducing
the cost, would make the system less responsive.
o The number of networks to be connected (N) on the WAN can easily
exceed the number of simultaneous calls (M) which the interface
can support. If this happens the routing 'broadcast' will only
reach M next hop routers, and (N - M) next hop routers will not
receive the routing update.
A basic rate ISDN interface can support 2 simultaneous calls, and
even the number of logical channels most users subscribe to on an
X.25 network is not large. There is no fundamental reason why
routing protocols should restrict the user to routing between so
few sites.
o Since there is no broadcast facility on the WAN, 'broadcasting' of
routing information actually consists of sending the updates
separately to all known locations. This means that N routing
updates are queued for transmission on the WAN link (in addition
to any data which might be queued).
Routers take a pragmatic view on queuing datagrams for the WAN.
If the queue length gets too long, so that datagrams at the end of
the queue would take too long be transmitted in a reasonable time
(say 1 to 2 seconds) the router may start discarding them. On an
X.25 network, with slow line speeds (perhaps 9600 baud), it may
not take too many routing updates to fulfill this condition if the
link is also actively being used to carry user data.
This memo addresses all the above problems.
The approach taken is to modify the routing protocols so as to send
information on the WAN only when there has been an update to the
routing database OR a change in the reachability of a next hop router
is indicated by the task which manages connections on the WAN.
Because datagrams are not guaranteed to get through on all WAN media,
an acknowledgement and retransmission system is required to provide
reliability.
This memo describes the modifications required for Bellman-Ford (or
distance vector) algorithm information broadcasting protocols, such
as IP RIP [1,2] or Netware RIP and SAP [3] on the WAN. The protocols
run unmodified on Local Area Networks (LANs) or fixed point-to-point
links, and so interoperate transparently with implementations
adhering to the original specifications.
2. Running Routing Protocols on the WAN
2.1 Overview
Multiprotocol routers are used on connection oriented Wide Area
Networks (WANs), such as X.25 packet switched networks and ISDN
networks, to interconnect LANs. By using the multiplexing properties
of the underlying WAN technology, several LANs can be interconnected
simultaneously through a single physical interface on the router.
A circuit manager provides an interface between the connectionless
network layers (IP, IPX, CLNP etc) and the connection oriented WAN
(X.25 or ISDN). Figure 1 shows a schematic representative stack
showing the relationship between routing protocols, the network
layers, the circuit manager and the connection oriented WAN.
-------------- --------- ---------
RIP RIP SAP
-------------- --------- ---------
--------------
UDP
--------------
-------------- ----------------
IP IPX
-------------- ----------------
-------------------------------------------
Circuit Manager
-------------------------------------------
---------------------------
Connection Oriented
WAN stack
---------------------------
A WAN circuit manager will support a variety of network layer
protocols, on its upper interface. On its lower interface, it
may support one or more subnetworks. A subnetwork may support a
number of Virtual Circuits.
Figure 1. Representative Multiprotocol Router stack
The router has a translation table which relates the network layer
address of the next hop router to the physical address used to
establish a Virtual Circuit (VC) to it. Datagrams may be
encapsulated in a header to distinguish the network layer protocol
[5].
The circuit manager takes datagrams from the connectionless network
layer protocols and (if one is not currently available) opens a VC to
the next hop router. A VC can carry all traffic between two end-
point routers for a given network layer protocol (or with appropriate
encapsulation all network layer protocols). An idle timer is used to
close the VC when the datagrams stop arriving at the circuit manager.
Running routing protocols on the WAN has traditionally consisted of
making small modifications to the methods used on LANs. Where
routing information would be broadcast periodically on a LAN
interface, it is converted to a series of periodic updates sent to a
list of addresses on the WAN.
This memo targets two areas:
o Eliminating the overkill inherent in periodic transmission of
routing updates.
o Overcoming the bandwidth limitations on the WAN: the number of
simultaneous VCs to next hop routers and restricted data
throughput which the WAN link can support.
The first of these is overcome by transmitting routing updates
(called routing responses) only when required:
o Firstly, when a specific request for a routing update has been
received.
o Secondly, when the routing database is modified by new
information from another interface.
Update information received in this way is not normally
propagated on other interfaces immediately, but is delayed for a
few seconds to allow information from several updates to be
grouped.
o Thirdly, when the circuit manager indicates that a destination
has changed from an unreachable (circuit down) to a reachable
(circuit up) state.
Because of the inherent unreliability of a datagram based system,
both routing requests and routing responses require acknowledgement,
and retransmission in the event of NOT receiving an acknowledgement.
To overcome the bandwidth limitations the routing application can
perform a form of self-imposed flow control, to spread routing
updates out over a period of time.
2.2 Presumption of Reachability
If a routing update is received from a next hop router on the WAN,
entries in the update are thereafter always considered to be
reachable, unless proven otherwise:
o If in the normal course of routing datagrams, the circuit manager
fails to establish a connection to the next hop router, it
notifies the routing application that the next hop router is not
reachable through an internal circuit down message.
The routing application then goes through a process of timing out
database entries to make them unreachable in the routing sense.
o If the circuit manager is subsequently able to establish a
connec tion to the next hop router, it will notify the routing
applica tion that the next hop router is reachable through an
internal circuit up message.
The routing application will then exchange messages with the next
hop router so as to re-prime their respective routing databases
with up-to-date information.
Handling of circuit up and circuit down messages requires that the
circuit manager takes responsibility for establishing (or
reestablishing) the connection in the event of a next hop router
becoming unreachable. A description of the processes the circuit
manager adopts to perform this task is outside the scope of this
memo.
2.3 WAN Router list
The routing task MAY be provided with a list of routers to send
routing updates to on the WAN. It will comprise of the logical
addresses of next hop routers for which the router has a logical to
physical address mapping. Entries in the list SHOULD be categorized
(on a per-peer basis) as follows:
o Running the standard routing protocol, namely transmitting
updates periodically with the packet formats used in LAN
broadcasts.
This option is supported to allow interoperability with existing
routing implementations, and might also be appropriate if some
of the destinations are using Permanent Virtual Circuits (PVCs)
rather than SVCs.
o Running the triggered update routing protocol proposed in this
memo.
Omitting an address from both of these categories is equivalent to
not running the routing protocols.
If routing packets arrive from a destination not supporting the
appropriate variant they MUST be discarded.
2.4 Triggered Updates and Unreliable Delivery
If triggered update information is sent to next hop routers on the
WAN only once it can fail to arrive for one of the following reasons:
o A free VC resource might not be available, because of a
restricted number of X.25 logical channels or ISDN B-channels.
o The transmit queue might be full - requiring the datagram to be
discarded.
o The VC might be pre-empted (in favour of establishing a VC to
another next hop router) while the datagram is in a queue,
resulting in the queue being flushed and the datagram
discarded.
o In cases where the method of transport is not guaranteed, for
example with PPP where there is no acknowledgement and
retransmission of HDLC frames, a corrupted frame will result in
the loss of the datagram.
2.5 Guaranteeing delivery of Routing Updates
To guarantee delivery of routing updates on the WAN an
acknowledgement and retransmission scheme MUST be used:
o Send a routing update to a next hop router on the WAN.
o The other router responds with an acknowledgement packet.
The original router receives the acknowledgement.
o Otherwise the original router retransmits the update until an
acknowledgement is received.
Retransmission timer values are covered in section 7.
In cases where the routing database is modified before an
acknowledgement is received a new routing update with an
updated sequence number is sent out. If an acknowledgement for
the old routing update is received it is ignored.
o A router only updates its routing database when it receives a
complete update, which may consist of several fragments. Each
fragment is individually acknowledged.
The above mechanism caters for cases where the datagram is lost
because of a frame error or is discarded because of an over-full
queue. The routing update and acknowledgement will eventually both
get through.
In cases where the circuit manager cannot establish a connection, a
mechanism is provided to allow the circuit manager to inform the
routing task of the failure to make a connection so that it can
suppress retransmissions until a circuit becomes available.
2.6 The Routing Database
A requirement of using triggered updates for propagating routing
information is that NO routing information ever gets LOST or
DISCARDED.
The routing database MUST adopt one of the following strategies:
o It must keep ALL alternative routing information it learns from
any routing updates from the LAN and the WAN, so that if the
best route disappears an alternative route (if available) can
replace it as the new best route.
o If the amount of memory this consumes is problematic the routing
application must keep SOME alternative routing information - say
a best route and two alternatives.
If the router ever has to discard routing information about a
route it should note the fact. If the routes that have been
kept disappear because they have become unreachable, the router
MUST issue a request on all interfaces to try and obtain
discarded alternatives.
It is recommended that the request is issued BEFORE all routes
to a destination have been lost.
Entries in the routing database can either be permanent or temporary.
Entries learned from broadcasts on LANs are temporary. They will
expire if not periodically refreshed by further broadcasts.
Entries learned from a triggered response on the WAN are 'permanent'.
They MUST not time out in the normal course of events. The entries
state MUST be changed to 'temporary' by the following events:
o The arrival of a routing update containing the entry set to
unreachable.
The normal hold down timer MUST be started, after which the
entry disappears from the routing database.
o The arrival of a routing update with the entry absent.
If the hold down timer is not already running, the entry MUST be
set to unreachable and the hold down timer started.
o A message sent from the circuit manager, to indicate that it
failed to make a connection in normal running.
The routing table MUST be scanned for all routes via that next
hop router. Aging of these routing entries MUST commence. If
the aging timer expires the entry MUST be set to unreachable and
the hold down timer started. If the hold down timer expires the
entry disappears from the routing database.
o If the interface goes down, the circuit manager should indicate
that all circuits on that interface have gone down.
Database timer values are covered in section 7.
2.7 New Packet Types
To support triggered updates, three new packet types MUST be
supported:
TRIGGERED REQUEST
A request to the responding system to send all
appropriate elements in its routing database.
A triggered request is retransmitted at periodic
intervals until a triggered response is received.
Routing requests are transmitted in the following
circumstances:
o Firstly when the router is powered on.
o Secondly when the circuit manager indicates a
destination has been in an unreachable (circuit down)
state for an extended period and changes to a
reachable (circuit up) state.
o Thirdly in the event of all routing update fragments
failing to arrive within a set period.
o It may also send triggered requests at other times to
compensate for discarding non-optimal routing
information.
TRIGGERED RESPONSE
A message containing all appropriate elements of the
routing database. An appropriate element is an entry
NOT learned from the interface to which the routing
information is being sent out. This is known as "split
horizon".
Stability is improved by adding "poisoned reverse" on
routes learned from a destination. This consists of also
including some routes learned from a destination in
routing updates sent back to that destination, but
setting the routes as unreachable. A route is only
poisoned if it is the best route (rather than an inferior
alternative route) in the database.
A triggered response message may be sent in response to a
triggered request, or it may be an update message issued
because of a change in the routing database.
A triggered response message MUST be sent in response to
a triggered request message even if there are no routes
to propagate. This would be the case for a host which
had a WAN interface only, but which wished to run the
triggered update protocol.
A triggered response is retransmitted at periodic
intervals until a triggered acknowledgement is received.
TRIGGERED ACKNOWLEDGEMENT
A message sent in response to every triggered response
packet received.
Triggered response and triggered acknowledgement packets MUST contain
additional fields for a sequence number, fragment number and number
of fragments.
If a triggered request or response is not acknowledged after 10
retransmissions, routes to the destination should be marked as
unreachable for the duration of a hold down timer before being
deleted.
The destination should then be polled at a lower frequency using
triggered request packets. When a triggered response is received,
the router should prime the next hop router my sending its routing
database through triggered response packets.
Strictly speaking polling should occur indefinitely to guarantee
database integrity. However the administrator MAY wish the router to
cease polling after a few attempts believing that the lack of
response is due to a mis-configuration of the next hop router. The
destination should be marked as NOT supporting the mechanism and no
further routing messages should be sent to that destination.
Before marking the destination as not supporting the mechanism, at
least 5 triggered request polls (without acknowledgement) should be
sent.
If a destination marked as not supporting the mechanism, subsequently
sends a valid 'triggered' message, the destination should be marked
as supporting the mechanism once more (to allow for the next hop
router's configuration being changed). It should be sent a triggered
request and a triggered response to obtain and propagate up-to-date
routing information.
2.8 Fragmentation
If a routing update is sufficiently large, the information MUST be
fragmented over several triggered response packets:
o Each fragment MUST be individually acknowledged with a triggered
acknowledgement packet.
The sender of the routing update MUST periodically retransmit
fragments which have not been acknowledged (or until the
destination is marked as not supporting the mechanism).
o A router receiving fragments MUST re-assemble them before
updating its routing database.
o If all fragments are not received within four times the
retransmit period, they MUST be discarded.
A triggered request packet MUST then be sent to the originator
of the routing update.
On receiving the triggered request packet, the originator of the
routing update MUST retransmit ALL fragments.
o If a fragment with an updated sequence number is received, ALL
fragments with the earlier sequence number MUST be discarded.
An updated sequence number is defined as any sequence number
that is different. There is no concept of the value of the
sequence number conveying its age.
Fragmentation timer values are covered in section 7.
2.9 Preventing Queue Overload
In order to prevent too many routing messages being queued at a WAN
interface, the routing task MAY operate a scheme whereby
'broadcasting' of a triggered request or triggered response to a WAN
interface is staggered. All routing requests or routing responses
are not sent to ALL next hop routers on the interface in a single
batch:
o The routing task should limit the number of outstanding triggered
request messages for which a triggered response has not been
received.
o The routing task should limit the number of outstanding triggered
response messages for which a triggered acknowledgement has not
been received.
As outstanding messages are appropriately acknowledged, further
messages can be sent out to other next hop routers, until all next
hop routers have been sent the message and have acknowledged it.
The maximum number of outstanding messages transmitted without
acknowledgement is a function of the link speed and the number of
other routing protocols operating the triggered update mechanism.
Messages should always be acknowledged immediately (even if it causes
the limit to be exceeded), since a connection is almost certainly
available. This has the potential benefit of allowing the VC to
close sooner (on its idle timer).
Sending all triggered request fragments to a destination at once is
also beneficial.
3. IP Routing Information Protocol Version 1
This section should be read in conjunction with reference [1].
IP RIP is a UDP-based protocol which generally sends and receives
datagrams on UDP port number 520.
To support the mechanism outlined in this proposal the packet format
for RIP version 1 [1] is modified as shown in Figure 2.
Every Routing Information Protocol datagram contains the following:
COMMAND Commands supported in RIP Version 1 are: request (1),
response (2), traceon (3), traceoff (4), SUN reserved (5).
The fields sequence number, fragment number and number of
fragments MUST NOT be included in packets with these
command values.
The following new commands (with values in brackets) are
required:
TRIGGERED REQUEST (6)
A request for the responding system to send all of its
routing database.
Only the first 4 octets of the packet format shown in
figure 2 are sent, since all routing information is
implied by this request type.
TRIGGERED RESPONSE (7)
A message containing all of the sender's routing
database, excluding those entries learned from the
interface to which the routing information is being
sent.
This message may be sent in response to a triggered
request, or it may be an update message resulting
from a change in the routing database.
A triggered response message MUST be sent in response
to a triggered request message even if there are no
routes to propagate. This would be the case for a
host which had a WAN interface only, but which wished
to run the triggered update protocol.
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
command (1) version (1) must be zero (2)
+---------------+---------------+-------------------------------+
The following new fields are inserted for some commands
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
sequence number (2) fragment (1) no of frags (1)
+-------------------------------+-------------------------------+
Followed by up to 25 routing entries (each 20 octets)
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
address family identifier (2) must be zero (2)
+-------------------------------+-------------------------------+
IP address (4)
+---------------------------------------------------------------+
must be zero (4)
+---------------------------------------------------------------+
must be zero (4)
+---------------------------------------------------------------+
metric (4)
+---------------------------------------------------------------+
.
.
The format of an IP RIP datagram in octets, with each tick mark
representing one bit. All fields are in network order.
The four octets: sequence number (2), fragment number (1) and
number of fragments (1) are not present in the original RIP
specification. They are only present if command takes the
values 7 or 8.
Figure 2. IP Routing Information Protocol packet format
TRIGGERED ACKNOWLEDGEMENT (8)
A message sent in response to every triggered response
packet received.
Only the first 8 octets of the packet format shown in
figure 2 are sent.
VERSION In this instance Version 1.
SEQUENCE NUMBER
This is a new field inserted if command takes the values 7
or 8.
The sequence number MUST be incremented every time updated
information is sent out on a WAN. The sequence number
wraps round at 65535.
When a triggered acknowledgement is sent the sequence
number is set to the same value as the triggered response
packet being acknowledged.
The sequence number MUST be identical over fragments. If a
fragment is retransmitted the sequence number MUST not
change.
FRAGMENT NUMBER
The fragment number is one for the first fragment of a
routing update, and is incremented for each subsequent
fragment. A fragment can contain up to 25 routing entries.
When a triggered acknowledgement is sent the fragment
number is set to the same value as the triggered response
packet being acknowledged.
NUMBER OF FRAGMENTS
In a triggered response packet this indicates the number of
packets required to complete the routing update.
This field has no relevance for triggered acknowledgement
packets so should be set to zero.
For triggered response packets the rest of the datagram contains a
list of destinations, with information about each. Each entry in
this list contains the address family identifier (2 for IP), a
destination network or host, and the metric for it. The packet
format is intended to allow RIP to carry routing information for
several different protocols, identifiable by the family identifier.
The IP address is the usual Internet address, stored as 4 octets in
network order. The metric field contains a value between 1 and 15
inclusive, specifying the current metric for the destination, or the
value 16 (representing 'infinity'), which indicates that the
destination is not reachable. Each route sent by a router supersedes
any previous route to the same destination from the same router.
The maximum datagram size is 508 octets, excluding UDP and IP
headers.
4. IP Routing Information Protocol Version 2
An enhancement to IP RIP to include subnetting has recently become
available [2]. This section only describes differences from that
RFC.
The triggered update mechanism can be supported by including the
triggered request (6), triggered response (7) and triggered
acknowledgement (8) commands described in the previous section.
The sequence number, fragment number and number of fragments fields
are included in triggered response and triggered acknowledgement
commands.
The triggered request packet should also contain the 4 extra octets
corresponding to the sequence number, fragment number and number of
fragments fields - but set to zero.
Because additional security information is included in RIP Version 2
packets, this MUST be appended to the triggered request and triggered
acknowledgement packets, as well as being present in the triggered
response packet.
The version number becomes 2. Other ASPects of packet layout follow
reference [2].
5. Netware Routing Information Protocol
This section should be read in conjunction with references [3], since
it only describes differences from the specification.
Netware [3] is the trade name of Novell Research's protocols for
computer communication which are derived and extended from Xerox
Network System's (XNS) protocols [4].
Netware supports a mechanism that allows routers on an internetwork
to exchange routing information using the Routing Information
Protocol (RIP) which runs over the Internetwork Packet Exchange (IPX)
protocol using socket number 453h.
Netware RIP and IP RIP share a common heritage, in that they are both
based on XNS RIP, but there is some divergence, mostly at the packet
format level to reflect the differing addressing schemes.
The triggered update mechanism can be applied to Netware RIP. To
support the mechanism outlined in this proposal the packet format for
Netware RIP is modified as shown in Figure 3.
Every datagram contains the following:
RIP OPERATION
Operations supported in standard Netware RIP are: request
(1) and response (2).
The fields sequence number, fragment number and number of
fragments MUST NOT be included in packets with these
operation values.
The following new operations are required (with values
chosen to be the same as for IP RIP commands):
0 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
operation (2)
+---------------+---------------+
The following new fields are inserted for some operations
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
sequence number (2) fragment (1) no of frags (1)
+-------------------------------+-------------------------------+
Followed by up to 50 routing entries (each 8 octets)
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
network number (4)
+---------------------------------------------------------------+
number of hops (2) number of ticks (2)
+---------------------------------------------------------------+
.
.
The format of a Netware RIP datagram in octets, with each tick
mark representing one bit. All fields are in network order.
The four octets: sequence number (2), fragment number (1) and
number of fragments (1) are not present in the original RIP
specification. They are only present if operation takes the
values 7 or 8.
Figure 3. Netware Routing Information Protocol packet format
TRIGGERED REQUEST (6)
A request for the responding system to send all of its
routing database.
Only the first 2 octets of the packet format shown in
figure 3 are sent, since all routing information is
implied by this request type.
TRIGGERED RESPONSE (7)
A message containing all of the sender's routing
database, excluding those entries learned from the
interface to which the routing information is being
sent.
This message may be sent in response to a triggered
request, or it may be an update message resulting
from a change in the routing database.
A triggered response message MUST be sent in response
to a triggered request message even if there are no
routes to propagate. This would be the case for a
host which had a WAN interface only, but which wished
to run the triggered update protocol.
TRIGGERED ACKNOWLEDGEMENT (8)
A message sent in response to every triggered
response packet received.
Only the first 6 octets of the packet format shown in
figure 3 are sent.
SEQUENCE NUMBER
This is a new field inserted if operation takes the
values 7 or 8.
The sequence number MUST be incremented every time
updated information is sent out on a WAN. The sequence
number wraps round at 65535.
When a triggered acknowledgement is sent the sequence
number is set to the same value as the triggered response
packet being acknowledged.
The sequence number MUST be identical over fragments. If
a fragment is retransmitted the sequence number MUST not
change.
FRAGMENT NUMBER
The fragment number is one for the first fragment of a
routing update, and is incremented for each subsequent
fragment. A fragment can contain up to 50 routing entries.
When a triggered acknowledgement is sent the fragment
number is set to the same value as the triggered response
packet being acknowledged.
NUMBER OF FRAGMENTS
In a triggered response packet this indicates the number
of packets required to complete the routing update.
This field has no relevance for triggered acknowledgement
packets so should be set to zero.
For triggered response packets the rest of the datagram contains a
list of networks, with information about each. Each entry in this
list contains a destination network, and the number of hops and
number of ticks for each.
The maximum datagram size is 406 octets, excluding the IPX header (a
further 30 octets).
6. Netware Service Advertising Protocol
This section should be read in conjunction with references [3], since
it only describes differences from the specification.
Netware [3] also supports a mechanism that allows servers on an
internetwork to advertise their services by name and type using the
Service Advertising Protocol (SAP) which runs over the Internetwork
Packet Exchange (IPX) protocol using socket number 452h.
SAP operates on similar principals to running RIP. Routers act as
SAP agents, collecting service information from different networks
and relay it to interested parties.
To support the triggered update mechanism outlined in this proposal
the packet format for Netware SAP is modified as shown in Figure 4.
Every Service Advertising Protocol datagram contains the following:
SAP OPERATION
Operations supported in standard Netware SAP are: general
service query (1), general service response (2), nearest
service query (3) and nearest service response (4).
The fields sequence number, fragment number and number of
fragments MUST NOT be included in packets with these
operation values.
The following new operations are required:
TRIGGERED GENERAL SERVICE QUERY (6)
A request for the responding system to send the
identities of all servers of all types.
Only the first 2 octets of the packet format shown in
figure 4 are sent, since all service types are
implied by this request type.
0 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
operation (2)
+---------------+---------------+
The following new fields are inserted for some operations
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
sequence number (2) fragment (1) no of frags (1)
+-------------------------------+-------------------------------+
Followed by up to 8 service entries (each 66 octets)
0 1 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Service Type (4)
+---------------------------------------------------------------+
Service Name (48)
+ +
.
.
+---------------------------------------------------------------+
Network Address (4)
+---------------------------------------------------------------+
Node Address (6)
+ +-------------------------------+
Socket Address (2)
+---------------------------------------------------------------+
Hops to Server (2)
+-------------------------------+
.
.
The format of a Netware SAP datagram in octets, with each tick
mark representing one bit. All fields are in network order.
The four octets: sequence number (2), fragment number (1) and
number of fragments (1) are not present in the original SAP
specification. They are only present if operation takes the
values 7 or 8.
Figure 4. Netware Service Advertising Protocol packet format
TRIGGERED GENERAL SERVICE RESPONSE (7)
A message containing all of the sender's services
table, excluding those entries learned from the
interface to which the service advertising
information is being sent out.
This message may be sent in response to a triggered
general service query, or it may be an update message
resulting from a change in the service advertising
database.
A triggered general service response message MUST be
sent in response to a triggered general request
message even if there are no services to advertise.
This would be the case for a router with a LAN
network which had work stations but no servers on it.
TRIGGERED GENERAL SERVICE ACKNOWLEDGEMENT (8)
A message sent in response to every triggered general
service response packet received.
Only the first 6 octets of the packet format shown in
figure 4 are sent.
SEQUENCE NUMBER
This is a new field inserted if operation takes the values
7 or 8.
The sequence number MUST be incremented every time updated
information is sent out on a WAN. The sequence number
wraps round at 65535.
When a triggered general service acknowledgement is sent
the sequence number is set to the same value as the
triggered general service response packet being
acknowledged.
The sequence number MUST be identical over fragments. If
a fragment is retransmitted the sequence number MUST not
change.
FRAGMENT NUMBER
The fragment number is one for the first fragment of a
triggered general service response update, and is
incremented for each subsequent fragment. A fragment can
contain up to 8 service entries.
When a triggered general service acknowledgement is sent,
the fragment number is set to the same value as the
triggered general service response packet being
acknowledged.
NUMBER OF FRAGMENTS
In a triggered response packet this indicates the number of
packets required to complete the service update.
This field has no relevance for triggered acknowledgement
packets so should be set to zero.
For triggered general service response packets the rest of the
datagram contains a list of services, with information about each.
Each entry in this list contains the service type, service name, full
address (network, node and socket), and the number of hops to the
server.
The maximum datagram size is 534 octets, excluding the IPX header (a
further 30 octets).
7. Timers
A number of timers are supported to handle the triggered update
mechanism:
o Database timers.
o Retransmission timer.
o Reassembly timer.
In this section appropriate timer values for IP RIP are suggested.
For other routing protocols, only the database timer should need to
take different values. The database timer values are chosen to match
equivalent timer operation for using the protocol on a LAN. The
behaviour of a routing entry when a timer is running becomes
indistinguishable from a routing entry learned from a broadcast
update.
Implementations MAY make timer values configurable - and hence
different from the values suggested here - but interoperability
requires that all timers on a sub-network should be the same in all
routers.
7.1 Database Timers
Routes learned by a triggered response command (7) are normally
considered to be permanent - that is they do NOT time out unless
activated by one of the following events:
o If the circuit manager indicates that a next hop router cannot be
contacted, all routes learned from that next hop router should
start timing out as if they had (just) been learned from a
conventional response command (2).
Namely each route exists while the database entry timer is
running and is advertised on other interfaces as if still
present. The route is then advertised as unreachable while a
further hold down timer is allowed to expire, at which point the
entry is deleted.
If the circuit manager indicates that the next hop router can be
contacted while the database entry timer is running, the routes
are reinstated as permanent entries.
If the database entry timer has expired and the circuit manager
indicates that the next hop router is reachable, the routing
application MUST issue a triggered request. The routes will be
reinstated on the basis of any triggered response packet(s)
received.
o If a triggered response packet is received in which a route is
marked unreachable, the hold down timer MUST be started and the
entry is advertised as unreachable on other interfaces. On
expiry of the hold down timer the entry is deleted.
If a triggered response packet is received in which an existing
route is ABSENT, the hold down timer MUST also be started and
the entry is advertised as unreachable on other interfaces. On
expiry of the hold down timer the entry is deleted.
For IP RIP the hold down timer should always run for 120 seconds, to
be consistent with RIP usage on broadcast networks. The database
entry timer should by default run for 180 seconds. The network can
be made more responsive by reducing the database entry timer value.
However, making this timer too short can lead to network
instabilities. The duration of the database entry timer allows a
period of grace in which contention for network resources can be
resolved by the circuit manager.
7.2 Retransmission Timer
The routing task runs a retransmission timer:
o When a triggered request is sent it will be retransmitted
periodically while a triggered response packet is not received.
o When a triggered response is sent a note of the sequence number
and fragment number(s) of the routing update is kept.
Fragments will be retransmitted at periodic intervals while a
triggered acknowledgement packet is not received for the
appropriate fragment.
With call set up time on the WAN being of the order of a second, a
value of 5 seconds for the retransmission timer is appropriate.
If no response is received after 10 retransmissions, routes via the
next hop router are marked as unreachable, the hold down timer MUST
be started and the entry is advertised as unreachable on other
interfaces. On expiry of the hold down timer the entry is deleted.
The next hop router is then polled using a triggered request packet
at 60 second intervals. If a response is received the routers should
exchange routing information using triggered response packets.
It may not be desirable to poll indefinitely, since a lack of
response (when a circuit is up) is most likely caused by incorrect
configuration of the next hop router. An administrator definable
number of polls (5 or greater) should be provided.
If the circuit manager indicates that the next hop router is
unreachable, the retransmission is suppressed until the circuit
manager indicates that the next hop router is reachable once more.
Counting of the number of retransmissions continues from where it
left off prior to the circuit down indication.
7.3 Reassembly Timer
When a router receives a triggered response update it MUST
acknowledge each fragment. If the routing update is fragmented over
more than one packet, the receiving router MUST store the fragments
until ALL fragments are received.
On receiving the first fragment a timer should be started. If all
fragments of the routing update are not received within that period
they are discarded - and a triggered request is sent back to the
originator (with retransmissions if necessary). The originator MUST
then resend ALL triggered response fragments.
The reassembly timer should be set to four times the value of the
retransmission timer. With a suggested retransmission timer value of
5 seconds, the suggested reassembly timer value SHOULD be 20 seconds.
Implementations MAY allow the reassembly timer and retransmission
timer to be configurable (in the 1:4 ratio), but interoperability
will be compromised on WANs where all participating routers DO NOT
support the same values for these timers.
Fragments MUST also be discarded if a new fragment with a different
sequence number is received. A triggered request MUST not be sent in
this instance.
8. Implementation Considerations
In the implementation described in this memo, it is assumed that
there is a close binding between the circuit manager and the routing
applications - that they are in some way the same 'program'. This is
not necessarily true of all products which are routers.
In particular there are UNIX host implementations in which the
routing application is distinct from the kernel, where the circuit
manager is likely to be installed. In such systems it is possible to
stop (or crash) the routing applications independently of what is
happening in the kernel.
Other implementations might have the circuit manager on a separate
card which again may give the circuit manager a life of its own.
In implementations where the applications and circuit manager have
independent lives, a keep-alive mechanism MUST be provided between
the applications and the circuit manager, so that if the application
or network layer dies and is subsequently re-started they can
resynchronize their state tables.
Ideally, when an application dies, the circuit manager should close
all existing VCs appropriate to the application and make no further
outgoing calls and reject incoming calls until the application is
running again.
If the circuit manager is using some form of encapsulation, several
applications may be sharing the same VC. If this is the case the
circuit manager may wish to filter out datagrams for the appropriate
network layer if only one of the applications is affected. But this
is not an ideal solution.
Conversely if the application believes the circuit manager has died,
it should mark all routes via the circuit manager as unreachable and
advertise them on other interfaces for the duration of the hold down
timer before deleting them.
9. Security Considerations
Security is provided my a number of aspects:
o The circuit manager is required to be provided with a list of
physical addresses to enable it to establish a call to the next
hop router on an X.25 SVC or ISDN B-channel.
The circuit manager SHOULD only allow incoming calls to be
accepted from the same well defined list of routers.
Elsewhere in the system there will be a set of logical address
and physical address tuples to enable the network protocols to
run over the correct circuit. This may be a lookup table, or in
some instances there may be an algorithmic conversion between
the two addresses.
o The routing (or service advertising) task MUST be provided with a
list of logical addresses to which triggered updates are to be
sent on the WAN. The list MAY be a subset of the list of next
hop routers maintained by the circuit manager.
There MAY also be a separate list of next hop routers to which
traditional broadcasts of routing (or service advertising)
updates should be sent. Next hop routers omitted from either
list are assumed to be not participating in routing (or service
advertising) updates.
The list (or lists) doubles as a list of routers from which
routing updates are allowed to be received from the WAN. Any
routing information received from a router not in the
appropriate list MUST be discarded.
10. References
[1] Hedrick. C., "Routing Information Protocol", STD 34, RFC1058,
Rutgers University, June 1988.
[2] Malkin. G., "RIP Version 2 - Carrying Additional Information",
RFC1388, Xylogics, January 1993.
[3] Novell Incorporated., "IPX Router Specification", Version 1.10,
October 1992.
[4] Xerox Corporation., "Internet Transport Protocols", Xerox System
Integration Standard XSIS 028112, December 1981.
[5] Malis. A., Robinson. D., and R. Ullmann, "Multiprotocol
Interconnect on X.25 and ISDN in the Packet Mode", RFC1356, BBN
Communications, Computervision Systems Integration, Process
Software Corporation, August 1992.
11. Author's Address
Gerry Meyer
Spider Systems
Stanwell Street
Edinburgh EH6 5NG
Scotland, UK
Phone: (UK) 31 554 9424
Fax: (UK) 31 554 0649
EMail: gerry@spider.co.uk
