Network Working Group R. Cole
Request for Comments: 1932 D. Shur
Category: Informational AT&T Bell Laboratories
C. Villamizar
ANS
April 1996

IP over ATM: A Framework Document

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

The discussions of the IP over ATM working group over the last
several years have prodUCed a diverse set of proposals, some of which
are no longer under active consideration. A categorization is
provided for the purpose of focusing discussion on the various
proposals for IP over ATM deemed of primary interest by the IP over
ATM working group. The intent of this framework is to help clarify
the differences between proposals and identify common features in
order to promote convergence to a smaller and more mutually
compatible set of standards. In summary, it is hoped that this
document, in classifying ATM approaches and issues will help to focus
the IP over ATM working group's direction.

1. Introduction

The IP over ATM Working Group of the Internet Engineering Task Force
(IETF) is chartered to develop standards for routing and forwarding
IP packets over ATM sub-networks. This document provides a
classification/taxonomy of IP over ATM options and issues and then
describes various proposals in these terms.

The remainder of this memorandum is organized as follows:

o Section 2 defines several terms relating to networking and
internetworking.

o Section 3 discusses the parameters for a taxonomy of the
different ATM models under discussion.

o Section 4 discusses the options for low level encapsulation.

o Section 5 discusses tradeoffs between connection oriented and
connectionless approaches.

o Section 6 discusses the various means of providing direct
connections across IP subnet boundaries.

o Section 7 discusses the proposal to extend IP routing to better
accommodate direct connections across IP subnet boundaries.

o Section 8 identifies several prominent IP over ATM proposals that
have been discussed within the IP over ATM Working Group and
their relationship to the framework described in this document.

o Section 9 addresses the relationship between the documents
developed in the IP over ATM and related working groups and the
various models discussed.

2. Definitions and Terminology

We define several terms:

A Host or End System: A host delivers/receives IP packets to/from
other systems, but does not relay IP packets.

A Router or Intermediate System: A router delivers/receives IP
packets to/from other systems, and relays IP packets among
systems.

IP Subnet: In an IP subnet, all members of the subnet are able to
transmit packets to all other members of the subnet directly,
without forwarding by intermediate entities. No two subnet
members are considered closer in the IP topology than any other.
From an IP routing and IP forwarding standpoint a subnet is
atomic, though there may be repeaters, hubs, bridges, or switches
between the physical interfaces of subnet members.

Bridged IP Subnet: A bridged IP subnet is one in which two or
more physically disjoint media are made to appear as a single IP
subnet. There are two basic types of bridging, media Access
control (MAC) level, and proxy ARP (see section 6).

A Broadcast Subnet: A broadcast network supports an arbitrary
number of hosts and routers and additionally is capable of
transmitting a single IP packet to all of these systems.

A Multicast Capable Subnet: A multicast capable subnet supports
a facility to send a packet which reaches a subset of the
destinations on the subnet. Multicast setup may be sender

initiated, or leaf initiated. ATM UNI 3.0 [4] and UNI 3.1
support only sender initiated while IP supports leaf initiated
join. UNI 4.0 will support leaf initiated join.

A Non-Broadcast Multiple Access (NBMA) Subnet: An NBMA supports
an arbitrary number of hosts and routers but does not
natively support a convenient multi-destination connectionless
transmission facility, as does a broadcast or multicast capable
subnetwork.

An End-to-End path: An end-to-end path consists of two hosts which
can communicate with one another over an arbitrary number of
routers and subnets.

An internetwork: An internetwork (small "i") is the concatenation
of networks, often of various different media and lower level
encapsulations, to form an integrated larger network supporting
communication between any of the hosts on any of the component
networks. The Internet (big "I") is a specific well known
global concatenation of (over 40,000 at the time of writing)
component networks.

IP forwarding: IP forwarding is the process of receiving a packet
and using a very low overhead decision process determining how
to handle the packet. The packet may be delivered locally
(for example, management traffic) or forwarded externally. For
traffic that is forwarded externally, the IP forwarding process
also determines which interface the packet should be sent out on,
and if necessary, either removes one media layer encapsulation
and replaces it with another, or modifies certain fields in the
media layer encapsulation.

IP routing: IP routing is the exchange of information that takes
place in order to have available the information necessary to
make a correct IP forwarding decision.

IP address resolution: A quasi-static mapping exists between IP
address on the local IP subnet and media address on the local
subnet. This mapping is known as IP address resolution.
An address resolution protocol (ARP) is a protocol supporting
address resolution.

In order to support end-to-end connectivity, two techniques are used.
One involves allowing direct connectivity across classic IP subnet
boundaries supported by certain NBMA media, which includes ATM. The
other involves IP routing and IP forwarding. In essence, the former
technique is extending IP address resolution beyond the boundaries of
the IP subnet, while the latter is interconnecting IP subnets.

Large internetworks, and in particular the Internet, are unlikely to
be composed of a single media, or a star topology, with a single
media at the center. Within a large network supporting a common
media, typically any large NBMA such as ATM, IP routing and IP
forwarding must always be accommodated if the internetwork is larger
than the NBMA, particularly if there are multiple points of
interconnection with the NBMA and/or redundant, diverse
interconnections.

Routing information exchange in a very large internetwork can be
quite dynamic due to the high probability that some network elements
are changing state. The address resolution space consumption and
resource consumption due to state change, or maintenance of state
information is rarely a problem in classic IP subnets. It can become
a problem in large bridged networks or in proposals that attempt to
extend address resolution beyond the IP subnet. Scaling properties
of address resolution and routing proposals, with respect to state
information and state change, must be considered.

3. Parameters Common to IP Over ATM Proposals

In some discussion of IP over ATM distinctions have made between
local area networks (LANs), and wide area networks (WANs) that do not
necessarily hold. The distinction between a LAN, MAN and WAN is a
matter of geographic dispersion. Geographic dispersion affects
performance due to increased propagation delay.

LANs are used for network interconnections at the the major Internet
traffic interconnect sites. Such LANs have multiple administrative
authorities, currently exclusively support routers providing transit
to multihomed internets, currently rely on PVCs and static address
resolution, and rely heavily on IP routing. Such a configuration
differs from the typical LANs used to interconnect computers in
corporate or campus environments, and emphasizes the point that prior
characterization of LANs do not necessarily hold. Similarly, WANs
such as those under consideration by numerous large IP providers, do
not conform to prior characterizations of ATM WANs in that they have
a single administrative authority and a small number of nodes
aggregating large flows of traffic onto single PVCs and rely on IP
routers to avoid forming congestion bottlenecks within ATM.

The following characteristics of the IP over ATM internetwork may be
independent of geographic dispersion (LAN, MAN, or WAN).

o The size of the IP over ATM internetwork (number of nodes).

o The size of ATM IP subnets (LIS) in the ATM Internetwork.

o Single IP subnet vs multiple IP subnet ATM internetworks.

o Single or multiple administrative authority.

o Presence of routers providing transit to multihomed internets.

o The presence or absence of dynamic address resolution.

o The presence or absence of an IP routing protocol.

IP over ATM should therefore be characterized by:

o Encapsulations below the IP level.

o Degree to which a connection oriented lower level is available
and utilized.

o Type of address resolution at the IP subnet level (static or
dynamic).

o Degree to which address resolution is extended beyond the IP
subnet boundary.

o The type of routing (if any) supported above the IP level.

ATM-specific attributes of particular importance include:

o The different types of services provided by the ATM Adaptation
Layers (AAL). These specify the Quality-of-Service, the
connection-mode, etc. The models discussed within this document
assume an underlying connection-oriented service.

o The type of virtual circuits used, i.e., PVCs versus SVCs. The
PVC environment requires the use of either static tables for
ATM-to-IP address mapping or the use of inverse ARP, while the
SVC environment requires ARP functionality to be provided.

o The type of support for multicast services. If point-to-point
services only are available, then a server for IP multicast is
required. If point-to-multipoint services are available, then
IP multicast can be supported via meshes of point-to-multipoint
connections (although use of a server may be necessary due to
limits on the number of multipoint VCs able to be supported or to
maintain the leaf initiated join semantics).

o The presence of logical link identifiers (VPI/VCIs) and the
various information element (IE) encodings within the ATM SVC
signaling specification, i.e., the ATM Forum UNI version 3.1.

This allows a VC originator to specify a range of "layer"
entities as the destination "AAL User". The AAL specifications
do not prohibit any particular "layer X" from attaching
directly to a local AAL service. Taken together these points
imply a range of methods for encapsulation of upper layer
protocols over ATM. For example, while LLC/SNAP encapsulation is
one approach (the default), it is also possible to bind virtual
circuits to higher level entities in the TCP/IP protocol stack.
Some examples of the latter are single VC per protocol binding,
TULIP, and TUNIC, discussed further in Section 4.

o The number and type of ATM administrative domains/networks, and
type of addressing used within an administrative domain/network.
In particular, in the single domain/network case, all attached
systems may be safely assumed to be using a single common
addressing format, while in the multiple domain case, attached
stations may not all be using the same common format,
with corresponding implications on address resolution. (See
Appendix A for a discussion of some of the issues that arise
when multiple ATM address formats are used in the same logical
IP subnet (LIS).) Also security/authentication is much more of a
concern in the multiple domain case.

IP over ATM proposals do not universally accept that IP routing over
an ATM network is required. Certain proposals rely on the following
assumptions:

o The widespread deployment of ATM within premises-based networks,
private wide-area networks and public networks, and

o The definition of interfaces, signaling and routing protocols
among private ATM networks.

The above assumptions amount to ubiquitous deployment of a seamless
ATM fabric which serves as the hub of a star topology around which
all other media is attached. There has been a great deal of
discussion over when, if ever, this will be a realistic assumption
for very large internetworks, such as the Internet. Advocates of
such approaches point out that even if these are not relevant to very
large internetworks such as the Internet, there may be a place for
such models in smaller internetworks, such as corporate networks.

The NHRP protocol (Section 8.2), not necessarily specific to ATM,
would be particularly appropriate for the case of ubiquitous ATM
deployment. NHRP supports the establishment of direct connections
across IP subnets in the ATM domain. The use of NHRP does not
require ubiquitous ATM deployment, but currently imposes topology
constraints to avoid routing loops (see Section 7). Section 8.2

describes NHRP in greater detail.

The Peer Model assumes that internetwork layer addresses can be
mapped onto ATM addresses and vice versa, and that reachability
information between ATM routing and internetwork layer routing can be
exchanged. This approach has limited applicability unless ubiquitous
deployment of ATM holds. The peer model is described in Section 8.4.

The Integrated Model proposes a routing solution supporting an
exchange of routing information between ATM routing and higher level
routing. This provides timely external routing information within
the ATM routing and provides transit of external routing information
through the ATM routing between external routing domains. Such
proposals may better support a possibly lengthy transition during
which assumptions of ubiquitous ATM access do not hold. The
Integrated Model is described in Section 8.5.

The Multiprotocol over ATM (MPOA) Sub-Working Group was formed by the
ATM Forum to provide multiprotocol support over ATM. The MPOA effort
is at an early stage at the time of this writing. An MPOA baseline
document has been drafted, which provides terminology for further
discussion of the architecture. This document is available from the
FTP server ftp.atmforum.com in pub/contributions as the file atm95-
0824.ps or atm95-0824.txt.

4. Encapsulations and Lower Layer Identification

Data encapsulation, and the identification of VC endpoints,
constitute two important issues that are somewhat orthogonal to the
issues of network topology and routing. The relationship between
these two issues is also a potential sources of confusion. In
conventional LAN technologies the 'encapsulation' wrapped around a
packet of data typically defines the (de)multiplexing path within
source and destination nodes (e.g. the Ethertype field of an
Ethernet packet). Choice of the protocol endpoint within the
packet's destination node is essentially carried 'in-band'.

As the multiplexing is pushed towards ATM and away from LLC/SNAP
mechanism, a greater burden will be placed upon the call setup and
teardown capacity of the ATM network. This may result in some
questions being raised regarding the scalability of these lower level
multiplexing options.

With the ATM Forum UNI version 3.1 service the choice of endpoint
within a destination node is made 'out of band' - during the Call
Setup phase. This is quite independent of any in-band encapsulation
mechanisms that may be in use. The B-LLI Information Element allows
Layer 2 or Layer 3 entities to be specified as a VC's endpoint. When

faced with an incoming SETUP message the Called Party will search
locally for an AAL User that claims to provide the service of the
layer specified in the B-LLI. If one is found then the VC will be
accepted (assuming other conditions such as QoS requirements are also
met).

An obvious approach for IP environments is to simply specify the
Internet Protocol layer as the VCs endpoint, and place IP packets
into AAL--SDUs for transmission. This is termed 'VC multiplexing' or
'Null Encapsulation', because it involves terminating a VC (through
an AAL instance) directly on a layer 3 endpoint. However, this
approach has limitations in environments that need to support
multiple layer 3 protocols between the same two ATM level endpoints.
Each pair of layer 3 protocol entities that wish to exchange packets
require their own VC.

RFC-1483 [6] notes that VC multiplexing is possible, but focuses on
describing an alternative termed 'LLC/SNAP Encapsulation'. This
allows any set of protocols that may be uniquely identified by an
LLC/SNAP header to be multiplexed onto a single VC. Figure 1 shows
how this works for IP packets - the first 3 bytes indicate that the
payload is a Routed Non-ISO PDU, and the Organizationally Unique
Identifier (OUI) of 0x00-00-00 indicates that the Protocol Identifier
(PID) is derived from the EtherType associated with IP packets
(0x800). ARP packets are multiplexed onto a VC by using a PID of
0x806 instead of 0x800.
.---------------.
: :
: IP Packet :
: :
---------------
: :
: :
8 byte header V V
.-------------.-------------.------------.---------------.
: : : : :
: : : : Encapsulated :
: 0xAA-AA-03 : 0x00-00-00 : 0x08-00 : Payload :
: : : : :
-------------^-------------^------------^---------------
: : : :
: (LLC) (OUI) (PID) : : :
V V V V
.----------------------------------------------------------.
: :
: AAL SDU :
: :
----------------------------------------------------------
Figure 1: IP packet encapsulated in an AAL5 SDU

.----------. .----------. .---------. .----------.
: : : : : : : :
: IP : : ARP : :AppleTalk: : etc... :
: : : : : : : :
---------- ---------- --------- ----------
^ : ^ : ^ : ^ :
: : : : : : : :
: V : V : V : V
.-----------------------------------------------------------.
: :
: 0x800 0x806 0x809 other :
: :
: Instance of layer using LLC/SNAP header to :
: perform multiplexing/demultiplexing :
: :
-----------------------------------------------------------
^ :
: :
: V
.------------------.
: :
: Instance of AAL5 :
: terminating :
: one VCC :
: :
------------------

Figure 2: LLC/SNAP encapsulation allows more than just
IP or ARP per VC.

Whatever layer terminates a VC carrying LLC/SNAP encapsulated traffic
must know how to parse the AAL--SDUs in order to retrieve the
packets. The recently approved signalling standards for IP over ATM
are more eXPlicit, noting that the default SETUP message used to
establish IP over ATM VCs must carry a B-LLI specifying an ISO 8802/2
Layer 2 (LLC) entity as each VCs endpoint. More significantly, there
is no information carried within the SETUP message about the identity
of the layer 3 protocol that originated the request - until the
packets begin arriving the terminating LLC entity cannot know which
one or more higher layers are packet destinations.

Taken together, this means that hosts require a protocol entity to
register with the host's local UNI 3.1 management layer as being an
LLC entity, and this same entity must know how to handle and generate
LLC/SNAP encapsulated packets. The LLC entity will also require
mechanisms for attaching to higher layer protocols such as IP and
ARP. Figure 2 attempts to show this, and also highlights the fact
that such an LLC entity might support many more than just IP and ARP.

In fact the combination of RFC1483 LLC/SNAP encapsulation, LLC
entities terminating VCs, and suitable choice of LLC/SNAP values, can
go a long way towards providing an integrated approach to building
multiprotocol networks over ATM.

The processes of actually establishing AAL Users, and identifying
them to the local UNI 3.1 management layers, are still undefined and
are likely to be very dependent on operating system environments.

Two encapsulations have been discussed within the IP over ATM working
group which differ from those given in RFC-1483 [6]. These have the
characteristic of largely or totally eliminating IP header overhead.
These models were discussed in the July 1993 IETF meeting in
Amsterdam, but have not been fully defined by the working group.

TULIP and TUNIC assume single hop reachability between IP entities.
Following name resolution, address resolution, and SVC signaling, an
implicit binding is established between entities in the two hosts.
In this case full IP headers (and in particular source and
destination addresses) are not required in each data packet.

o The first model is "TCP and UDP over Lightweight IP" (TULIP)
in which only the IP protocol field is carried in each packet,
everything else being bound at call set-up time. In this
case the implicit binding is between the IP entities in each
host. Since there is no further routing problem once the binding
is established, since AAL5 can indicate packet size, since
fragmentation cannot occur, and since ATM signaling will handle
exception conditions, the absence of all other IP header fields
and of ICMP should not be an issue. Entry to TULIP mode would
occur as the last stage in SVC signaling, by a simple extension
to the encapsulation negotiation described in RFC-1755 [10].

TULIP changes nothing in the abstract architecture of the IP
model, since each host or router still has an IP address which is
resolved to an ATM address. It simply uses the point-to-point
property of VCs to allow the elimination of some per-packet
overhead. The use of TULIP could in principle be negotiated on a
per-SVC basis or configured on a per-PVC basis.

o The second model is "TCP and UDP over a Nonexistent IP
Connection" (TUNIC). In this case no network-layer information
is carried in each packet, everything being bound at virtual
circuit set-up time. The implicit binding is between two
applications using either TCP or UDP directly over AAL5 on a
dedicated VC. If this can be achieved, the IP protocol field has
no useful dynamic function. However, in order to achieve binding
between two applications, the use of a well-known port number

in classical IP or in TULIP mode may be necessary during call
set-up. This is a subject for further study and would require
significant extensions to the use of SVC signaling described in
RFC-1755 [10].

Encapsulation In setup message Demultiplexing
-------------+--------------------------+------------------------
SNAP/LLC _ nothing _ source and destination
_ _ address, protocol
_ _ family, protocol, ports
_ _
NULL encaps _ protocol family _ source and destination
_ _ address, protocol, ports
_ _
TULIP _ source and destination _ protocol, ports
_ address, protocol family _
_ _
TUNIC - A _ source and destination _ ports
_ address, protocol family _
_ protocol _
_ _
TUNIC - B _ source and destination _ nothing
_ address, protocol family _
_ protocol, ports _

Table 1: Summary of Encapsulation Types

TULIP/TUNIC can be presented as being on one end of a continuum opposite
the SNAP/LLC encapsulation, with various forms of null encapsulation
somewhere in the middle. The continuum is simply a matter of how much
is moved from in-stream demultiplexing to call setup demultiplexing.
The various encapsulation types are presented in Table 1.

Encapsulations such as TULIP and TUNIC make assumptions with regard to
the desirability to support connection oriented flow. The tradeoffs
between connection oriented and connectionless are discussed in Section
5.

5. Connection Oriented and Connectionless Tradeoffs

The connection oriented and connectionless approaches each offer
advantages and disadvantages. In the past, strong advocates of pure
connection oriented and pure connectionless architectures have argued
intensely. IP over ATM does not need to be purely connectionless or
purely connection oriented.

APPLICATION Pure Connection Oriented Approach
----------------+-------------------------------------------------
General _ Always set up a VC
_
Short Duration _ Set up a VC. Either hold the packet during VC
UDP (DNS) _ setup or drop it and await a retransmission.
_ Teardown on a timer basis.
_
Short Duration _ Set up a VC. Either hold packet(s) during VC
TCP (SMTP) _ setup or drop them and await retransmission.
_ Teardown on detection of FIN-ACK or on a timer
_ basis.
_
Elastic (TCP) _ Set up a VC same as above. No clear method to
Bulk Transfer _ set QoS parameters has emerged.
_
Real Time _ Set up a VC. QoS parameters are assumed to
(audio, video) _ precede traffic in RSVP or be carried in some
_ form within the traffic itself.

Table 2: Connection Oriented vs. Connectionless - a) a pure
connection oriented approach

ATM with basic AAL 5 service is connection oriented. The IP layer
above ATM is connectionless. On top of IP much of the traffic is
supported by TCP, a reliable end-to-end connection oriented protocol.
A fundamental question is to what degree is it beneficial to map
different flows above IP into separate connections below IP. There is
a broad spectrum of opinion on this.

As stated in section 4, at one end of the spectrum, IP would remain
highly connectionless and set up single VCs between routers which are
adjacent on an IP subnet and for which there was active traffic flow.
All traffic between the such routers would be multiplexed on a single
ATM VC. At the other end of the spectrum, a separate ATM VC would be
created for each identifiable flow. For every unique TCP or UDP
address and port pair encountered a new VC would be required. Part of
the intensity of early arguments has been over failure to recognize
that there is a middle ground.

ATM offers QoS and traffic management capabilities that are well
suited for certain types of services. It may be advantageous to use
separate ATM VC for such services. Other IP services such as DNS, are
ill suited for connection oriented delivery, due to their normal very
short duration (typically one packet in each direction). Short
duration transactions, even many using TCP, may also be poorly suited
for a connection oriented model due to setup and state overhead. ATM
QoS and traffic management capabilities may be poorly suited for
elastic traffic.

APPLICATION Middle Ground
----------------+-------------------------------------------------
General _ Use RSVP or other indication which clearly
_ indicate a VC is needed and what QoS parameters
_ are appropriate.
_
Short Duration _ Forward hop by hop. RSVP is unlikely to precede
UDP (DNS) _ this type of traffic.
_
Short Duration _ Forward hop by hop unless RSVP indicates
TCP (SMTP) _ otherwise. RSVP is unlikely to precede this
_ type of traffic.
_
Elastic (TCP) _ By default hop by hop forwarding is used.
Bulk Transfer _ However, RSVP information, local configuration
_ about TCP port number usage, or a locally
_ implemented method for passing QoS information
_ from the application to the IP/ATM driver may
_ allow/suggest the establishment of direct VCs.
_
Real Time _ Forward hop by hop unless RSVP indicates
(audio, video) _ otherwise. RSVP will indicate QoS requirements.
_ It is assumed RSVP will generally be used for
_ this case. A local decision can be made as to
_ whether the QoS is better served by a separate
_ VC.

Table 3: Connection Oriented vs. Connectionless - b) a middle ground
approach

APPLICATION Pure Connectionless Approach
----------------+-------------------------------------------------
General _ Always forward hop by hop. Use queueing
_ algorithms implemented at the IP layer to
_ support reservations such as those specified by
_ RSVP.
_
Short Duration _ Forward hop by hop.
UDP (DNS) _
_
Short Duration _ Forward hop by hop.
TCP (SMTP) _
_
Elastic (TCP) _ Forward hop by hop. Assume ability of TCP to
Bulk Transfer _ share bandwidth (within a VBR VC) works as well
_ or better than ATM traffic management.
_
Real Time _ Forward hop by hop. Assume that queueing
(audio, video) _ algorithms at the IP level can be designed to
_ work with sufficiently good performance
_ (e.g., due to support for predictive
_ reservation).

Table 4: Connection Oriented vs. Connectionless - c) a pure
connectionless approach

Work in progress is addressing how QoS requirements might be
expressed and how the local decisions might be made as to whether
those requirements are best and/or most cost effectively accomplished
using ATM or IP capabilities. Table 2, Table 3, and Table 4 describe
typical treatment of various types of traffic using a pure connection
oriented approach, middle ground approach, and pure connectionless
approach.

The above qualitative description of connection oriented vs
connectionless service serve only as examples to illustrate differing
approaches. Work in the area of an integrated service model, QoS and
resource reservation are related to but outside the scope of the IP
over ATM Work Group. This work falls under the Integrated Services
Work Group (int-serv) and Reservation Protocol Work Group (rsvp), and
will ultimately determine when direct connections will be
established. The IP over ATM Work Group can make more rapid progress
if concentrating solely on how direct connections are established.

6. Crossing IP Subnet Boundaries

A single IP subnet will not scale well to a large size. Techniques
which extend the size of an IP subnet in other media include MAC
layer bridging, and proxy ARP bridging.

MAC layer bridging alone does not scale well. Protocols such as ARP
rely on the media broadcast to exchange address resolution
information. Most bridges improve scaling characteristics by
capturing ARP packets and retaining the content, and distributing the
information among bridging peers. The ARP information gathered from
ARP replies is broadcast only where explicit ARP requests are made.
This technique is known as proxy ARP.

Proxy ARP bridging improves scaling by reducing broadcast traffic,
but still suffers scaling problems. If the bridged IP subnet is part
of a larger internetwork, a routing protocol is required to indicate
what destinations are beyond the IP subnet unless a statically
configured default route is used. A default route is only applicable
to a very simple topology with respect to the larger internet and
creates a single point of failure. Because internets of enormous
size create scaling problems for routing protocols, the component
networks of such large internets are often partitioned into areas,
autonomous systems or routing domains, and routing confederacies.

The scaling limits of the simple IP subnet require a large network to
be partitioned into smaller IP subnets. For NBMA media like ATM,
there are advantages to creating direct connections across the entire
underlying NBMA network. This leads to the need to create direct
connections across IP subnet boundaries.

.----------.
---------< Non-ATM :
.-------. / /-< Subnet >-
:Sub-ES >--/ : ---------- :
------- : :
: :
.--^---. .--^---.
:Router: :Router:
-v-v-- -v-v--
: : : :
.--------. : : .--------. : : .--------.
.-------. : >-/ -< >-/ -< : .-------.
:Sub-ES :---: Subnet :-----: Subnet :-----: Subnet :---:Sub-ES :
------- : : : : : : -------
-------- ---v---- --------
:
.--^----.
:Sub-ES :
-------

Figure 3: A configuration with both ATM-based and non-ATM based
subnets.

For example, figure 3 shows an end-to-end configuration consisting of
four components, three of which are ATM technology based, while the
fourth is a standard IP subnet based on non-ATM technology. End-
systems (either hosts or routers) attached to the ATM-based networks
may communicate either using the Classical IP model or directly via
ATM (subject to policy constraints). Such nodes may communicate
directly at the IP level without necessarily needing an intermediate
router, even if end-systems do not share a common IP-level network
prefix. Communication with end-systems on the non-ATM-based
Classical IP subnet takes place via a router, following the Classical
IP model (see Section 8.1 below).

Many of the problems and issues associated with creating such direct
connections across subnet boundaries were originally being addressed
in the IETF's IPLPDN working group and the IP over ATM working group.
This area is now being addressed in the Routing over Large Clouds
working group. Examples of work performed in the IPLPDN working
group include short-cut routing (proposed by P. Tsuchiya) and
directed ARP RFC-1433 [5] over SMDS networks. The ROLC working group
has produced the distributed ARP server architectures and the NBMA
Address Resolution Protocol (NARP) [7]. The Next Hop Resolution
Protocol (NHRP) is still work in progress, though the ROLC WG is
considering advancing the current document. Questions/issues
specifically related to defining a capability to cross IP subnet
boundaries include:

o How can routing be optimized across multiple logical IP subnets
over both a common ATM based and a non-ATM based infrastructure.
For example, in Figure 3, there are two gateways/routers between
the non-ATM subnet and the ATM subnets. The optimal path
from end-systems on any ATM-based subnet to the non ATM-based
subnet is a function of the routing state information of the two
routers.

o How to incorporate policy routing constraints.

o What is the proper coupling between routing and address
resolution particularly with respect to off-subnet communication.

o What are the local procedures to be followed by hosts and
routers.

o Routing between hosts not sharing a common IP-level (or L3)
network prefix, but able to be directly connected at the NBMA
media level.

o Defining the details for an efficient address resolution
architecture including defining the procedures to be followed by
clients and servers (see RFC-1433 [5], RFC-1735 [7] and NHRP).

o How to identify the need for and accommodate special purpose SVCs
for control or routing and high bandwidth data transfers.

For ATM (unlike other NBMA media), an additional complexity in
supporting IP routing over these ATM internets lies in the
multiplicity of address formats in UNI 3.0 [4]. NSAP modeled address
formats only are supported on "private ATM" networks, while either 1)
E.164 only, 2) NSAP modeled formats only, or 3) both are supported on
"public ATM" networks. Further, while both the E.164 and NSAP
modeled address formats are to be considered as network points of
attachment, it seems that E.164 only networks are to be considered as
subordinate to "private networks", in some sense. This leads to some
confusion in defining an ARP mechanism in supporting all combinations
of end-to-end scenarios (refer to the discussion in Appendix A on the
possible scenarios to be supported by ARP).

7. Extensions to IP Routing

RFC-1620 [3] describes the problems and issues associated with direct
connections across IP subnet boundaries in greater detail, as well as
possible solution approaches. The ROLC WG has identified persistent
routing loop problems that can occur if protocols which lose
information critical to path vector routing protocol loop suppression
are used to accomplish direct connections across IP subnet

boundaries.

The problems may arise when a destination network which is not on the
NBMA network is reachable via different routers attached to the NBMA
network. This problem occurs with proposals that attempt to carry
reachability information, but do not carry full path attributes (for
path vector routing) needed for inter-AS path suppression, or full
metrics (for distance vector or link state routing even if path
vector routing is not used) for intra-AS routing.

For example, the NHRP protocol may be used to support the
establishment of direct connections across subnetwork boundaries.
NHRP assumes that routers do run routing protocols (intra and/or
inter domain) and/or static routing. NHRP further assumes that
forwarding tables constructed by these protocols result in a steady
state loop-free forwarding. Note that these two assumptions do not
impose any additional requirements on routers, beyond what is
required in the absence of NHRP.

NHRP runs in addition to routing protocols, and provides the
information that allows the elimination of multiple IP hops (the
multiple IP hops result from the forwarding tables constructed by the
routing protocols) when traversing an NBMA network. The IPATM and
ROLC WGs have both expended considerable effort in discussing and
coming to understand these limitations.

It is well-known that truncating path information in Path Vector
protocols (e.g., BGP) or losing metric information in Distance Vector
protocols (e.g., RIP) could result in persistent forwarding loops.
These loops could occur without ATM and without NHRP.

The combination of NHRP and static routing alone cannot be used in
some topologies where some of the destinations are served by multiple
routers on the NBMA. The combination of NHRP and an intra-AS routing
protocol that does not carry inter-AS routing path attributes alone
cannot be used in some topologies in which the NBMA will provide
inter-AS transit connectivity to destinations from other AS served by
multiple routers on the NBMA.

Figure 4 provides an example of the routing loops that may be formed
in these circumstances. The example illustrates how the use of NHRP
in the environment where forwarding loops could exist even without
NHRP (due to either truncated path information or loss of metric
information) would still produce forwarding loops.

There are many potential scenarios for routing loops. An example is
given in Figure 4. It is possible to produce a simpler example where
a loop can form. The example in Figure 4 illustrates a loop which

will persist even if the protocol on the NBMA supports redirects or
can invalidate any route which changes in any way, but does not
support the communication of full metrics or path attributes.

.----. .----.
: H1 >----< S1 : Notes:
---- vvvv H#n == host #n
/ : R#n == router #n
/ : S#n == subnet #n
/------/ :
: : S2 to R3 breaks
.--^---. .----. .-^--.
: : : R4 : : R6 :
: NBMA : --v- --v- See the text for
: : : : details of the
-v--v- = = looping conditions
: = SLOW = and mechanisms
: .-^--. = LINK =
: : R2 : = =
: --v- : :
: : .--^-. .--^-.
.-^--. : : R5 : : R7 :
: R8 : : --v- --v-
--v- : :
: / :
.-^^-. .--^-.
: S2 : : S4 :
--v- --v-
/
/
.^--^.
: R3 : path before the break is
-v-- H1->S1->R1->NBMA->R2->S2->R3->H2
/
.----. .-^^-. path after the break is
: H2 >---< S3 : H1->S1->R1->NBMA->R2->S2->R5->R4->S1
---- ---- ------<--the-loop--<-------/

Figure 4: A Routing Loop Due to Lost PV Routing Attributes.

In the example in Figure 4, Host 1 is sending traffic toward Host 2.
In practice, host routes would not be used, so the destination for
the purpose of routing would be Subnet 3. The traffic travels by way
of Router 1 which establishes a "cut-through" SVC to the NBMA next-
hop, shown here as Router 2. Router 2 forwards traffic destined for
Subnet 3 through Subnet 2 to Router 3. Traffic from Host 1 would
then reach Host 2.

Router 1's cut-through routing implementation caches an association
between Host 2's IP address (or more likely all of Subnet 3) and
Router 2's NBMA address. While the cut-through SVC is still up, Link
1 fails. Router 5 loses it's preferred route through Router 3 and
must direct traffic in the other direction. Router 2 loses a route
through Router 3, but picks up an alternate route through Router 5.
Router 1 is still directing traffic toward Router 2 and advertising a
means of reaching Subnet 3 to Subnet 1. Router 5 and Router 2 will
see a route, creating a loop.

This loop would not form if path information normally carried by
interdomain routing protocols such as BGP and IDRP were retained
across the NBMA. Router 2 would reject the initial route from Router
5 due to the path information. When Router 2 declares the route to
Subnet 3 unreachable, Router 1 withdraws the route from routing at
Subnet 1, leaving the route through Router 4, which would then reach
Router 5, and would reach Router 2 through both Router 1 and Router
5. Similarly, a link state protocol would not form such a loop.

Two proposals for breaking this form of routing loop have been
discussed. Redirect in this example would have no effect, since
Router 2 still has a route, just has different path attributes. A
second proposal is that is that when a route changes in any way, the
advertising NBMA cut-through router invalidates the advertisement for
some time period. This is similar to the notion of Poison Reverse in
distance vector routing protocols. In this example, Router 2 would
eventually readvertise a route since a route through Router 6 exists.
When Router 1 discovers this route, it will advertise it to Subnet 1
and form the loop. Without path information, Router 1 cannot
distinguish between a loop and restoration of normal service through
the link L1.

The loop in Figure 4 can be prevented by configuring Router 4 or
Router 5 to refuse to use the reverse path. This would break backup
connectivity through Router 8 if L1 and L3 failed. The loop can also
be broken by configuring Router 2 to refuse to use the path through
Router 5 unless it could not reach the NBMA. Special configuration of
Router 2 would work as long as Router 2 was not distanced from Router
3 and Router 5 by additional subnets such that it could not determine
which path was in use. If Subnet 1 is in a different AS or RD than
Subnet 2 or Subnet 4, then the decision at Router 2 could be based on
path information.

.--------. .--------.
: Router : : Router :
--v-v--- ---v-v--
: : : :
.--------. .--------. : : .--------. : : .--------. .--------.
: Sub-ES :---: Subnet :-/ -: Subnet :-/ -: Subnet :---: Sub-ES :
-------- -------- -------- -------- --------

Figure 5: The Classical IP model as a concatenation of three separate
ATM IP subnets.

In order for loops to be prevented by special configuration at the
NBMA border router, that router would need to know all paths that
could lead back to the NBMA. The same argument that special
configuration could overcome loss of path information was posed in
favor of retaining the use of the EGP protocol defined in the now
historic RFC-904 [11]. This turned out to be unmanageable, with
routing problems occurring when topology was changed elsewhere.

8. IP Over ATM Proposals

8.1 The Classical IP Model

The Classical IP Model was suggested at the Spring 1993 IETF meeting
[8] and retains the classical IP subnet architecture. This model
simply consists of cascading instances of IP subnets with IP-level
(or L3) routers at IP subnet borders. An example realization of this
model consists of a concatenation of three IP subnets. This is shown
in Figure 5. Forwarding IP packets over this Classical IP model is
straight forward using already well established routing techniques
and protocols.

SVC-based ATM IP subnets are simplified in that they:

o limit the number of hosts which must be directly connected at any
given time to those that may actually exchange traffic.

o The ATM network is capable of setting up connections between
any pair of hosts. Consistent with the standard IP routing
algorithm [2] connectivity to the "outside" world is achieved
only through a router, which may provide firewall functionality
if so desired.

o The IP subnet supports an efficient mechanism for address
resolution.

Issues addressed by the IP Over ATM Working Group, and some of the
resolutions, for this model are:

o Methods of encapsulation and multiplexing. This issue is
addressed in RFC-1483 [6], in which two methods of encapsulation
are defined, an LLC/SNAP and a per-VC multiplexing option.

o The definition of an address resolution server (defined in
RFC-1577).

o Defining the default MTU size. This issue is addressed in
RFC-1626 [1] which proposes the use of the MTU discovery
protocol (RFC-1191 [9]).

o Support for IP multicasting. In the summer of 1994, work began
on the issue of supporting IP multicasting over the SVC LATM
model. The proposal for IP multicasting is currently defined by
a set of IP over ATM WG Works in Progress, referred to collectively
as the IPMC documents. In order to support IP multicasting the
ATM subnet must either support point-to- multipoint SVCs, or
multicast servers, or both.

o Defining interim SVC parameters, such as QoS parameters and
time-out values.

o Signaling and negotiations of parameters such as MTU size
and method of encapsulation. RFC-1755 [10] describes an
implementation agreement for routers signaling the ATM network
to establish SVCs initially based upon the ATM Forum's UNI
version 3.0 specification [4], and eventually to be based
upon the ATM Forum's UNI version 3.1 and later specifications.
Topics addressed in RFC-1755 include (but are not limited to)
VC management procedures, e.g., when to time-out SVCs, QOS
parameters, service classes, explicit setup message formats for
various encapsulation methods, node (host or router) to node
negotiations, etc.

RFC-1577 is also applicable to PVC-based subnets. Full mesh PVC
connectivity is required.

For more information see RFC-1577 [8].

8.2 The ROLC NHRP Model

The Next Hop Resolution Protocol (NHRP), currently a work in progress
defined by the Routing Over Large Clouds Working Group (ROLC),
performs address resolution to accomplish direct connections across
IP subnet boundaries. NHRP can supplement RFC-1577 ARP. There has
been recent discussion of replacing RFC-1577 ARP with NHRP. NHRP can
also perform a proxy address resolution to provide the address of the
border router serving a destination off of the NBMA which is only

served by a single router on the NBMA. NHRP as currently defined
cannot be used in this way to support addresses learned from routers
for which the same destinations may be heard at other routers,
without the risk of creating persistent routing loops.

8.3 "Conventional" Model

The "Conventional Model" assumes that a router can relay IP packets
cell by cell, with the VPI/VCI identifying a flow between adjacent
routers rather than a flow between a pair of nodes. A latency
advantage can be provided if cell interleaving from multiple IP
packets is allowed. Interleaving frames within the same VCI requires
an ATM AAL such as AAL3/4 rather than AAL5. Cell forwarding is
accomplished through a higher level mapping, above the ATM VCI layer.

The conventional model is not under consideration by the IP/ATM WG.
The COLIP WG has been formed to develop protocols based on the
conventional model.

8.4 The Peer Model

The Peer Model places IP routers/gateways on an addressing peer basis
with corresponding entities in an ATM cloud (where the ATM cloud may
consist of a set of ATM networks, inter-connected via UNI or P-NNI
interfaces). ATM network entities and the attached IP hosts or
routers exchange call routing information on a peer basis by
algorithmically mapping IP addressing into the NSAP space. Within
the ATM cloud, ATM network level addressing (NSAP-style), call
routing and packet formats are used.

In the Peer Model no provision is made for selection of primary path
and use of alternate paths in the event of primary path failure in
reaching multihomed non-ATM destinations. This will limit the
topologies for which the peer model alone is applicable to only those
topologies in which non-ATM networks are singly homed, or where loss
of backup connectivity is not an issue. The Peer Model may be used
to avoid the need for an address resolution protocol and in a proxy-
ARP mode for stub networks, in conjunction with other mechanisms
suitable to handle multihomed destinations.

During the discussions of the IP over ATM working group, it was felt
that the problems with the end-to-end peer model were much harder
than any other model, and had more unresolved technical issues.
While encouraging interested individuals/companies to research this
area, it was not an initial priority of the working group to address
these issues. The ATM Forum Network Layer Multiprotocol Working
Group has reached a similar conclusion.

8.5 The PNNI and the Integrated Models

The Integrated model (proposed and under study within the
Multiprotocol group of ATM Forum) considers a single routing protocol
to be used for both IP and for ATM. A single routing information
exchange is used to distribute topological information. The routing
computation used to calculate routes for IP will take into account
the topology, including link and node characteristics, of both the IP
and ATM networks and calculates an optimal route for IP packets over
the combined topology.

The PNNI is a hierarchical link state routing protocol with multiple
link metrics providing various available QoS parameters given current
loading. Call route selection takes into account QoS requirements.
Hysteresis is built into link metric readvertisements in order to
avoid computational overload and topological hierarchy serves to
subdivide and summarize complex topologies, helping to bound
computational requirements.

Integrated Routing is a proposal to use PNNI routing as an IP routing
protocol. There are several sets of technical issues that need to be
addressed, including the interaction of multiple routing protocols,
adaptation of PNNI to broadcast media, support for NHRP, and others.
These are being investigated. However, the ATM Forum MPOA group is
not currently performing this investigation. Concerned individuals
are, with an expectation of bringing the work to the ATM Forum and
the IETF.

PNNI has provisions for carrying uninterpreted information. While
not yet defined, a compatible extension of the base PNNI could be
used to carry external routing attributes and avoid the routing loop
problems described in Section 7.

++++++++++++++++++++++++++++++++++++++++++
+ .------------. .------------. +
.---------. + .-: :-. .-: :-. +
: Host or >-+-< : Single ATM : >--< : Single ATM : >-+-----
: Router : + : : Domain : : : : Domain : : + :
--------- + -: :- -: :- + .---^----.
+ ------------ ------------ + : Router :
+ .------------. + ---v----
.---------. + .-: :-. + :
: Host or >-+- ... ... --< : Single ATM : >-+-----/
: Router : + : : Domain : : +
--------- + ATM Cloud -: :- +
+ ------------ +
++++++++++++++++++++++++++++++++++++++++++

Note: IS within ATM cloud are ATM IS

Figure 6: The ATM transition model assuming the presence of gateways
or routers between the ATM networks and the ATM peer networks.

8.6 Transition Models

Finally, it is useful to consider transition models, lying somewhere
between the Classical IP Models and the Peer and Integrated Models.
Some possible architectures for transition models have been suggested
by Fong Liaw. Others are possible, for example Figure 6 showing a
Classical IP transition model which assumes the presence of gateways
between ATM networks and ATM Peer networks.

Some of the models described in the prior sections, most notably the
Integrated Model, anticipate the need for mixed environment with
complex routing topologies. These inherently support transition
(possibly with an indefinite transition period). Models which
provide no transition support are primarily of interest to new
deployments which make exclusive, or near exclusive use of ATM or
deployments capable of wholesale replacement of existing networks or
willing to retain only non-ATM stub networks.

For some models, most notably the Peer Model, the ability to attach
to a large non-ATM or mixed internetwork is infeasible without
routing support at a higher level, or at best may pose
interconnection topology constraints (for example: single point of
attachment and a static default route). If a particular model
requires routing support at a higher level a large deployment will
need to be subdivided to provide scalability at the higher level,
which for some models degenerates back to the Classical model.

9. Application of the Working Group's and Related Documents

The IP Over ATM Working Group has generated several Works in Progress
and RFCs. This section identifies the relationship of these and
other related documents to the various IP Over ATM Models identified
in this document. The documents and RFCs produced to date are the
following references, RFC-1483 [6], RFC-1577 [8], RFC-1626 [1], RFC-
1755 [10] and the IPMC documents. The ROLC WG has produced the NHRP
document. Table 5 gives a summary of these documents and their
relationship to the various IP Over ATM Models.

Acknowledgments

This memo is the direct result of the numerous discussions of the IP
over ATM Working Group of the Internet Engineering Task Force. The
authors also had the benefit of several private discussions with H.
Nguyen of AT&T Bell Laboratories. Brian Carpenter of CERN was kind
enough to contribute the TULIP and TUNIC sections to this memo.
Grenville Armitage of Bellcore was kind enough to contribute the
sections on VC binding, encapsulations and the use of B-LLI
information elements to signal such bindings. The text of Appendix A
was pirated liberally from Anthony Alles' of Cisco posting on the IP
over ATM discussion list (and modified at the authors' discretion).
M. Ohta provided a description of the Conventional Model (again which
the authors modified at their discretion). This memo also has
benefitted from numerous suggestions from John T. Amenyo of ANS, Joel
Halpern of Newbridge, and Andy Malis of Ascom-Timplex. Yakov Rekhter
of Cisco provided valuable comments leading to the clarification of
normal loop free NHRP operation and the potential for routing loop
problems only with the improper use of NHRP.

Documents Summary
----------------+-------------------------------------------------
RFC-1483 _ How to identify/label multiple
_ packet/frame-based protocols multiplexed over
_ ATM AAL5. Applies to any model dealing with IP
_ over ATM AAL5.
_
RFC-1577 _ Model for transporting IP and ARP over ATM AAL5
_ in an IP subnet where all nodes share a common
_ IP network prefix. Includes ARP server/Inv-ARP
_ packet formats and procedures for SVC/PVC
_ subnets.
_
RFC-1626 _ Specifies default IP MTU size to be used with
_ ATM AAL5. Requires use of PATH MTU discovery.
_ Applies to any model dealing with IP over ATM
_ AAL5

_
RFC-1755 _ Defines how implementations of IP over ATM
_ should use ATM call control signaling
_ procedures, and recommends values of mandatory
_ and optional IEs focusing particularly on the
_ Classical IP model.
_
IPMC _ Defines how to support IP multicast in Classical
_ IP model using either (or both) meshes of
_ point-to-multipoint ATM VCs, or multicast
_ server(s). IPMC is work in progress.
_
NHRP _ Describes a protocol that can be used by hosts
_ and routers to determine the NBMA next hop
_ address of a destination in "NBMA
_ connectivity"
_ of the sending node. If the destination is not
_ connected to the NBMA fabric, the IP and NBMA
_ addresses of preferred egress points are
_ returned. NHRP is work in progress (ROLC WG).

Table 5: Summary of WG Documents

References

[1] Atkinson, R., "Default IP MTU for use over ATM AAL5", RFC1626,
Naval Research Laboratory, May 1994.

[2] Braden, R., and J. Postel, "Requirements for Internet Gateways",
STD 4, RFC1009, USC/Information Sciences Institute, June 1987.

[3] Braden, R., Postel, J., and Y. Rekhter, "Internet Architecture
Extensions for Shared Media", RFC1620, USC/Information Sciences
Institute, IBM Research, May 1994.

[4] ATM Forum, "ATM User-Network Interface Specification", Prentice
Hall, September 1993.

[5] Garrett, J., Hagan, J., and J. Wong, "Directed ARP", RFC1433,
AT&T Bell Labs, University of Pennsylvania, March 1993.

[6] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
Layer 5", RFC1483, Telecom Finland, July 1993.

[7] Heinanen, J., and R. Govindan, "NBMA Address Resolution Protocol
(NARP)", RFC1735, Telecom Finland, USC/Information Sciences
Institute, December 1994.

[8] Laubach, M., "Classical IP and ARP over ATM", RFC1577,
Hewlett-Packard Laboratories, January 1994.

[9] Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,
DECWRL, Stanford University, November 1990.

[10] Perez, M., Liaw, F., Grossman, D., Mankin, A., and A. Hoffman,
"ATM signalling support for IP over ATM", RFC1755,
USC/Information Sciences Institute, FORE Systems, Inc., Motorola
Codex, Ascom Timeplex, Inc., January 1995.

[11] Mills, D., "Exterior Gateway Protocol Formal Specification",
STD 18, RFC904, BBN, April 1984.

A Potential Interworking Scenarios to be Supported by ARP

The architectural model of the VC routing protocol, being defined by
the Private Network-to-Network Interface (P-NNI) working group of the
ATM Forum, categorizes ATM networks into two types:

o Those that participate in the VC routing protocols and use NSAP
modeled addresses UNI 3.0 [4] (referred to as private networks,
for short), and

o Those that do not participate in the VC routing protocol.
Typically, but possibly not in all cases, public ATM networks
that use native mode E.164 addresses UNI 3.0 [4] will fall into
this later category.

The issue for ARP, then is to know what information must be returned
to allow such connectivity. Consider the following scenarios:

o Private host to Private Host, no intervening public transit
network(s): Clearly requires that ARP return only the NSAP
modeled address format of the end host.

o Private host to Private host, through intervening public
networks: In this case, the connection setup from host A to host
B must transit the public network(s). This requires that at
each ingress point to the public network that a routing decision
be made as to which is the correct egress point from that public
network to the next hop private ATM switch, and that the native
E.164 address of that egress point be found (finding this is a VC
routing problem, probably requiring configuration of the public
network links and connectivity information). ARP should return,
at least, the NSAP address of the endpoint in which case the
mapping of the NSAP addresses to the E.164 address, as specified
in [4], is the responsibility of ingress switch to the public

network.

o Private Network Host to Public Network Host: To get connectivity
between the public node and the private nodes requires the
same kind of routing information discussed above - namely, the
directly attached public network needs to know the (NSAP format)
ATM address of the private station, and the native E.164 address
of the egress point from the public network to that private
network (or to that of an intervening transit private network
etc.). There is some argument, that the ARP mechanism could
return this egress point native E.164 address, but this may
be considered inconsistent for ARP to return what to some is
clearly routing information, and to others is required signaling
information.

In the opposite direction, the private network node can use, and
should only get, the E.164 address of the directly attached public
node. What format should this information be carried in? This
question is clearly answered, by Note 9 of Annex A of UNI 3.0 [4],
vis:

"A call originated on a Private UNI destined for an host which
only has a native (non-NSAP) E.164 address (i.e. a system
directly attached to a public network supporting the native E.164
format) will code the Called Party number information element in
the (NSAP) E.164 private ATM Address Format, with the RD, AREA,
and ESI fields set to zero. The Called Party Subaddress
information element is not used."

Hence, in this case, ARP should return the E.164 address of the
public ATM station in NSAP format. This is essentially implying an
algorithmic resolution between the native E.164 and NSAP addresses of
directly attached public stations.

o Public network host to Public network host, no intervening
private network: In this case, clearly the Q.2931 requests would
use native E.164 address formats.

o Public network host to Public network host, intervening private
network: same as the case immediately above, since getting
to and through the private network is a VC routing, not an
addressing issue.

So several issues arise for ARP in supporting arbitrary connections
between hosts on private and public network. One is how to
distinguish between E.164 address and E.164 encoded NSAP modeled
address. Another is what is the information to be supplied by ARP,
e.g., in the public to private scenario should ARP return only the

private NSAP modeled address or both an E.164 address, for a point of
attachment between the public and private networks, along with the
private NSAP modeled address.

Authors' Addresses

Robert G. Cole
AT&T Bell Laboratories
101 Crawfords Corner Road, Rm. 3L-533
Holmdel, NJ 07733

Phone: (908) 949-1950
Fax: (908) 949-8887
EMail: rgc@qsun.att.com

David H. Shur
AT&T Bell Laboratories
101 Crawfords Corner Road, Rm. 1F-338
Holmdel, NJ 07733

Phone: (908) 949-6719
Fax: (908) 949-5775
EMail: d.shur@att.com

Curtis Villamizar
ANS
100 Clearbrook Road
Elmsford, NY 10523

EMail: curtis@ans.net