Network Working Group C. Brazdziunas
Request for Comments: 1680 Bellcore
Category: Informational August 1994
IPng Support for ATM Services
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This document was submitted to the IETF IPng area in response to RFC
1550. Publication of this document does not imply acceptance by the
IPng area of any ideas eXPressed within. Comments should be
submitted to the big-internet@munnari.oz.au mailing list.
Executive Summary
This white paper describes engineering considerations for IPng as
solicited by RFC1550 [1]. IPng should provide support for existing
and emerging link technologies that it will be transported over. Link
technologies like Ethernet simply multiplex traffic from upper layer
protocols onto a single channel. "Sophisticated" link technologies
like ATM are emerging in the marketplace allowing several virtual
channels to be established over a single wire (or fiber) potentially
based on an applications' network performance objectives.
Support for both "sophisticated" (ATM) and existing link technologies
needs to be considered in an IPng candidate. End-to-end applications
will communicate through a network where IPng packets travel across
subnetworks sUCh as Ethernet and Hippi and also more "sophisticated"
link levels such as ATM. Though initial support for IPng over ATM
subnetworks will not facilitate a virtual circuit per application,
the hooks to provide such a mapping should be in place while also
maintaining support for the transport of IPng packets across
conventional subnetworks. Application support for QOS-based link
level service requires that the following types of ATM information
be mappable (or derivable) from the higher level protocol(s) such as
IPng: source and destination(s) addresses, connection quality of
service parameters, connection state, and ATM virtual circuit
identifier. Some of these mappings may be derivable from information
provided by proposed resource reservation protocols supporting an
integrated services Internet [4]. However, the ATM virtual circuit
identifier should be efficiently derivable from IPng packet
information.
An IPng candidate should provide evidence that the mapping from an
applications' IPng packets to ATM virtual circuit(s) can be
accomplished in a heterogeneous Internet architecture keeping in
consideration the gigabit/sec rates that IPng/ATM subnetworks will
eventually be operating at.
1. Introduction
This paper describes parameters that are needed to map IPng (or any
protocol operating above the link level) to ATM services. ATM is a
"sophisticated" link level technology which provides the potential
capability for applications at the TCP/UDP level to map to a single
ATM virtual circuit for transport across an ATM network(s) customized
to the network performance and traffic requirements for that
application. This is a step above many of today's existing link
technologies which can only support a single level of network
performance that must be shared by all applications operating on a
single endpoint.
The future Internet will be comprised of both conventional and
"sophisticated" link technologies. The "sophisticated" features of
link layers like ATM need to be incorporated into an internet where
data travels not only across an ATM network but also several other
existing LAN and WAN technologies. Future networks are likely to be a
combination of subnetworks providing best-effort link level service
such as Ethernet and also sophisticated subnetworks that can support
quality of service-based connections like ATM. One can envision data
originating from an Ethernet, passing through an ATM network, FDDI
network, another ATM network, and finally arriving at its destination
residing on a HIPPI network. IPng packets will travel through such a
list of interconnected network technologies as ATM is incorporated as
one of the components of the future Internet.
To support per application customizable link level connections, four
types of ATM information should be derivable from the higher level
protocol(s) like IPng. This ATM information includes: source and
destination ATM addresses, connection quality of service parameters,
connection state, and an ATM virtual circuit identifier which maps to
a single IPng application (i.e., single TCP/UDP application). Some of
these mapping could potentially be derivable through information
provided by proposed resource reservation protocols supporting an
integrated services Internet [4]. However, the ATM virtual circuit
identifier needs to be efficiently mappable from IPng packet
information.
Organization of this white paper is as follows. First the
characteristics of ATM are described focusing on functions that are
not provided in today's LAN technologies. This section provides
background information necessary for the following section describing
the parameters needed to map IPng services to ATM services.
2. Terminology
In this white paper, the term "application" refers to a process or
set of collective processes operating at the TCP/UDP level or above
in the protocol stack. For example, each instance of "telnet" or
"FTP" session running on an end station is a distinct application.
3. Characteristics of ATM Service
ATM has several characteristics which differentiates it from current
link level technologies. First of all, ATM has the capability of
providing many virtual channels to transmit information over a single
wire (or fiber). This is very similar to X.25, where many logical
channels can be established over a single physical media. But unlike
X.25, ATM allows for each of these channels or circuits to have a
customizable set of performance and quality of service
characteristics. Link level technologies like Ethernet provide a
single channel with a single performance and quality of service
characteristic. In a sense, a single ATM link level media appears
like an array of of link level technologies each with customizable
characteristics.
ATM virtual circuits can be established dynamically utilizing its
signaling protocol. ATM signaling is a source initiated negotiation
process for connection establishment. This protocol informs elements
in the network of the characteristics for the desired connection. ATM
signaling does not provide any guidelines for how network elements
decide whether it can accept a call or where a signaling request
should be forwarded if the end destination (from the link level
perspective) has not been reached. In short, ATM signaling does not
support any routing functionality of network admission control.
ATM signaling establishes a "hard state" in the network for a call.
"Hard state" implies that the state of a connection in intermediate
switching equipment can be set and once established it will be
maintained until a message is received by one of the ends of the call
requesting a change in state for the connection [2]. As a result, an
ATM end system (this could be a workstation with an ATM adapter or a
router with an ATM interface) receives guaranteed service from the
ATM network. The ATM network is responsible for maintaining the
connection state. The price the ATM termination points pay for this
guarantee is the responsibility of changing the state of the
connection, specifically informing the ATM network to establish,
alter, or tear-down the connection.
Each ATM end point in a network has an ATM address associated with it
to support dynamic connection establishment via signaling. These
addresses are hierarchical in structure and globally unique [3]. As a
result, these addresses are routable. This allows ATM networks to
eventually support a large number of ATM endpoints once a routing
architecture and protocols to support it become available.
The ATM User-Network Interface (UNI) signaling protocol based on
ITU-TS Q.93B allows many different service parameters to be
specified for describing connection characteristics. [3] These
parameters can be grouped into several categories: ATM adaptation
layer (AAL) information, network QOS objectives, connection traffic
descriptor, and transit network selector. The AAL information
specifies negotiable parameters such as AAL type and maximum packet
sizes. The network QOS objectives describe the service that the ATM
user expects from the network. Q.93B allows for one of five service
classes to be selected by the ATM user. The service classes are
defined as general traffic types such as circuit emulation (class A),
variable bit rate audio and video (class B), connection-oriented data
transfer (class C), connectionless data transfer (class D), best
effort service (class X), and unspecified [3]. Each of these
categories are further specified through network provider objectives
for various ATM performance parameters. These parameters may include
cell transfer delay, cell delay variation, and cell loss ratio. The
connection traffic descriptor specifies characteristics of the data
generated by the user of the connection. This information allows the
ATM network to commit the resources necessary to support the traffic
flow with the quality of service the user expects. Characteristics
defined in the ATM Forum UNI specification include peak cell rate,
sustainable cell rate, and maximum and minimum burst sizes [3].
Lastly, the transit network selection parameter allows an ATM user to
select a preferred network provider to service the connection [3].
4. Parameters Required to Map IPng to ATM
There are several parameters required to map ATM services from a
higher level service like IPng. These ATM parameters can be
categorized in the following manner: addressing parameters,
connection QOS-related parameters, connection management information,
and ATM virtual circuit identifier. The first three categories
provide support for ATM signaling. The last parameter, a connection
identifier that maps IPng packets to ATM virtual circuits, provides
support for an ATM virtual circuit per application when the end-to-
end connection travels across an ATM subnetwork(s) (this does not
assume that ATM is the only type of subnetwork that this connection
travels across). Below, mapping issues for each of these parameters
will be described.
4.1. Addressing
ATM supports routable addresses to each ATM endpoint to facilitate
the dynamic establishment of connections. These addresses need to be
derived from a higher level address such as an IPng address and IPng
routing information. This type of mapping is not novel. It is a
mapping that is currently done for support of current IP over link
technologies such as Ethernet. An IP over ATM address resolution
protocol (ARP) has been described in the Internet Standard,
"Classical IP over ATM" [5]. In addition, support for IP routing over
large ATM networks is being worked in the IETF's "Routing over Large
Clouds" working group.
4.2. Quality of Service
As described in section 3, an ATM virtual circuit is established
based upon a user's traffic characteristics and network performance
objectives. These characteristics which include delay and throughput
requirements can only be defined by the application level (at the
transport level or above) as opposed to the internetworking (IPng)
level. For instance, a file transfer application transferring a 100
MB file has very different link level performance requirements than a
network time application. The former requires a high throughput and
low error rate connection whereas the latter could perhaps be
adequately serviced utilizing a best-effort service. Current IP does
not provide much support for a quality of service specification and
provides no support for the specification of link level performance
needs by an application directly. This is due to the fact that only a
single type of link level performance is available with link
technologies like Ethernet. As a result, all applications over IP
today receive the same level of link service.
IPng packets need not explicitly contain information parameters
describing an application's traffic characteristics and network
performance objectives (e.g., delay = low, throughput = 10 Mb/s).
This information could potentially be mapped from resource
reservation protocols that operate at the IP (and potentially IPng)
level [4].
4.3. Connection Management
The establishment and release of ATM connections should ultimately be
controlled by the applications utilizing the circuits. As described
in section 3, ATM signaling establishes a "hard state" in the network
which is controlled by the ATM termination points [2]. Currently, IP
provides no explicit mechanism for link level connection management.
Future support for link level connection management could be
accomplished through resource reservation protocols and need not
necessarily be supported directly via information contained in the
IPng protocol.
4.4. Connection Identifier
A mapping function needs to exist between IPng packets and ATM so
that application flows map one-to-one to ATM virtual circuits.
Currently, application traffic flows are identified at the transport
level by UDP/TCP source and destination ports and IP protocol
identifiers. This level of identification should also be available
at the IPng level so that information in the IPng packets identify an
application's flow and map to an ATM virtual circuit supporting that
flow when the IPng packets travels across an ATM subnetwork(s).
Using the current IP protocol, identifying an application's traffic
flow requires the combination of the following five parameters:
source and destination IP addresses, source and destination UDP/TCP
ports, and IP protocol identifier. This application connection
identifier for IP is complex and could potentially be costly to
implement in IP end stations and routers. The IPng connection
identifier should be large enough so that all application level
traffic from an IPng end point can be mapped into the IPng packet.
Currently, ATM provides 24 bits for virtual circuit identification
(VPI and VCI). This provides sufficient capacity for 2^24
(16,777,216) connections [6]. The actual number of bits that are used
for the ATM virtual circuit however is established through
negotiation between the ATM endpoint and ATM network. This number is
useful as an upper bound for the number of mappings that are needed
to be supported by IPng.
An IPng candidate should be able to identify how IPng packets from an
application can map to an ATM virtual circuit. In addition, this
mapping should be large enough to support a mapping for every IPng
application on an end system to an ATM virtual circuit. Careful
consideration should be given to complexity of this mapping for IPng
to ATM since it needs to eventually support gigabit/sec rates.
References
[1] Bradner, S., and A. Mankin, "IP: Next Generation (IPng) White
Paper Solicitation", RFC1550, Harvard University, NRL, December
1993.
[2] Clark, D., "The Design Philosophy of the DARPA Internet
Protocols", Proc. ATM SIGCOMM '88, August 1988.
[3] "ATM User-Network Interface Specification, Version 3.0", ATM
Forum, September 10, 1993.
[4] Zhang, L., Estrin, D., Herzog, S., and S. Jamin, "Resource
ReSerVation Protocol (RSVP) - Version 1 Functional
Specification", Work in Progress, October 1993.
[5] Laubach, M., "Classical IP and ARP over ATM", RFC1577, Hewlett-
Packard Laboratories, January 1994.
[6] "Asynchronous Transfer Mode (ATM) and ATM Adaptation Layer (AAL)
Protocols Generic Requirements", Bellcore Technical Advisory TA-
NWT-001113, Issue 1, June 1993.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Christina Brazdziunas
Bellcore
445 South Street
Morristown, NJ 07960
Phone: (201) 829-4173
