Network Working Group G. Armitage
Request for Comments: 2269 LUCent Technologies
Category: Informational January 1998

Using the MARS Model in non-ATM NBMA Networks

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

Initially developed for IP over ATM, the RFC2022 (MARS) model is
also applicable to other NBMA networks that provide the equivalent of
switched, point to multipoint connections. This short document is
intended to state the obvious equivalences, and eXPlain the less
obvious implications. No changes to the MARS model per se are
suggested or required. RFC2022 is not required over NBMA networks
that offer Ethernet-like group addressing functionality.

1. Introduction

Most network layer models, like the one described in STD 5, RFC1112
[1] for IP multicasting, assume sources may send their packets to an
abstract 'multicast group addresses'. Link layer support for such an
abstraction is assumed to exist, and is provided by technologies such
as Ethernet.

Some NBMA networks (e.g. ATM using UNI3.0 or UNI3.1 signaling [4,5])
do not support a multicast (or group) address abstraction. In these
environments multicasting is typically supported through point to
multipoint calls (or emulated with multiple point to point calls).
The MARS model (RFC2022) [2] was originally developed by the IP over
ATM working group, and so uses ATM-centric descriptive language. For
completeness this memo explains how RFC2022 can be applied to other
NBMA technologies.

2. RFC2022's basic assumptions.

Section 3 of [2] describes the basic assumptions that the MARS model
makes about the services available from the link layer network (using
ATM as the specific case). In summary these are:

The ATM model broadly describes an 'AAL User' as any entity that
establishes and manages VCs and underlying AAL services to
exchange data. An IP over ATM interface is a form of 'AAL User'

The most fundamental limitations of UNI 3.0/3.1's multicast
support are:

Only point to multipoint, unidirectional VCs may be
established.

Only the root (source) node of a given VC may add or remove
leaf nodes.

Leaf nodes are identified by their unicast ATM addresses.

Given this point to multipoint call service, the MARS document goes
on to describe two architectures for emulating multipoint to
multipoint IP multicasting - the VC Mesh, and the Multicast Server.
In either case it was assumed that IP/ATM interfaces (whether in
routers or hosts) are allowed to originate and manage outgoing point
to multipoint calls without network operator intervention or manual
provisioning.

The MARS document also specifies that AAL5 be used for all SVCs,
implying a requirement that the underlying link service supports the
atomic exchange of PDUs.

3. Generalising the MARS model.

Any NBMA service that offers an equivalent to (or superset of) the
ATM point to multipoint call service can use the MARS model directly.
It must be possible to transmit atomic data units bi-directionally
with point to point calls, and unidirectionally (from root to leaves)
on point to multipoint calls.

A MARS is an entity known by its NBMA address.

A MARS Client is an entity known by its NBMA address.

An MCS (where needed) is an entity known by its NBMA address.

The MARS control messages defined in sections 4 onwards of the MARS
document are shown carrying ATM addresses. Using different mar$afn
(Address Family) values in the fixed header of MARS control messages
allows MARS entities to indicate they are carrying other types of
NBMA addresses (as done in NHRP [3]). As for NHRP, the
interpretation of the 'sub-address' fields shall be in the context of
the address family selected (which means it will often simply be
null).

In all cases where {IP, ATM.1, ATM.2, ...} mappings are referred to,
they may be interpreted as {IP, NBMA.1, NBMA.2, ...} in the context
of whatever NBMA network you are deploying MARS.

The MARS Cluster is defined in [2] as:

The set of ATM interfaces chosing to participate in direct ATM
connections to achieve multicasting of AAL_SDUs between
themselves.

It is trivial to observe that the cluster definition is independent
of the underlying link layer technology. A revised definition
becomes:

The set of NBMA interfaces chosing to participate in direct NBMA
connections to achieve multicasting of packets between themselves.

The term 'Cluster Member' continues to refer to an endpoint that is
currently using a specific MARS for multicast support. The potential
scope of a cluster may be the entire membership of a LIS, while the
actual scope of a cluster depends on which LIS members are actually
registered with the cluster's MARS at any given time.

Section 3.4 of [2] provided a set of mneumonics for the signalling
functions available to AAL Users. These mneumonics are then used in
the remainder of [2] to indicate link layer events to which MARS
entities might react. Recast from the perspective of an NBMA based
MARS entity, the descriptions would now read:

The following generic signalling functions are presumed to be
available to local MARS entities:

L_CALL_RQ Establish a pt-pt call to a specific endpoint.
L_MULTI_RQ Establish pt-mpt call to a specific endpoint.
L_MULTI_ADD Add new leaf node to previously established pt-mpt
call.
L_MULTI_DROP Remove specific leaf node from established pt-mpt
call.
L_RELEASE Release pt-pt call, or all Leaves of a pt-mpt call.

The signalling exchanges and local information passed between MARS
entity and NBMA signalling entity with these functions are outside
the scope of this document.

The following indications are assumed to be available to MARS
entities, generated by by the local NBMA signalling entity:

L_ACK Succesful completion of a local request.
L_REMOTE_CALL A new call has been established to the MARS
entity.
ERR_L_RQFAILED A remote NBMA endpoint rejected an L_CALL_RQ,
L_MULTI_RQ, or L_MULTI_ADD.
ERR_L_DROP A remote NBMA endpoint dropped off an existing
call.
ERR_L_RELEASE An existing call was terminated.

The signalling exchanges and local information passed between MARS
entity and NBMA signalling entity with these functions are outside
the scope of this document.

4. Open Issues.

The trade offs between VC Mesh and Multicast Server modes may look
quite different for each NBMA technology. This will be especially
true in the area of VC (or equivalent) resource consumption in the
NICs of hosts, routers, and endpoints supporting MARSs or MCss. The
use of VC mesh mode is most vulnerable to NBMA technologies that are
signalling intensive or resource challenged.

Sizing of Clusters (and hence LISes) will also be affected by a given
NBMA network's ability to support lots of pt-mpt calls.
Additionally, you cannot have more members in a cluster than you can
have leaf nodes on a pt-mpt call, without hacking the MARS model [6].

On going developments in server synchronisation protocols for
distributed RFC2022 implementations are expected to be applicable to
non-ATM NBMA networks.

Quality of service considerations are outside the scope of this
document. They will be very specific to each NBMA technology's
capabilities. Related work is being pursued outside the ION Working
Group.

If the NBMA network offers some sort of native multipoint to
multipoint service then use of the MARS model may not be optimal.
Such situations require further analysis.

Security Considerations

This memo is informational, and specifies no protocol for deployment.
No specific non-ATM NBMA network technologies or security
characteristics are discussed. Should enhancements to security be
required, they shall be added as an extension to the base
architecture in RFC2022, or described in documents explicitly
proposing use of RFC2022 over specific NBMA technologies.

Acknowledgments

This memo was substantially developed while the author was with Bell
Communications Research (Bellcore).

Author's Address

Grenville Armitage
Bell Laboratories, Lucent Technologies.
101 Crawfords Corner Rd,
Holmdel, NJ, 07733
USA

EMail: gja@lucent.com

References

[1] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, Stanford University, August 1989.

[2] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
Networks.", RFC2022, November 1996.

[3] Luciani, J., et. al., "NBMA Next Hop Resolution Protocol (NHRP)",
Work in Progress.

[4] ATM Forum, "ATM User-Network Interface Specification Version
3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993.

[5] ATM Forum, "ATM User Network Interface (UNI) Specification
Version 3.1", ISBN 0-13-393828-X, Prentice Hall, Englewood Cliffs,
NJ, June 1995.

[6] Armitage, G., "Issues affecting MARS Cluster Size", RFC2121,
March 1997.

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