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

Abstract

   Initially developed for IP over ATM, the RFC 2022 (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. RFC 2022 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, RFC 1112
   [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 (RFC 2022) [2] was originally developed by the IP over
   ATM working group, and so uses ATM-centric descriptive language.  For
   completeness this memo explains how RFC 2022 can be applied to other
   NBMA technologies.

2. RFC 2022'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 RFC 2022 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 RFC 2022, or described in documents explicitly
   proposing use of RFC 2022 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.", RFC 2022, 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", RFC 2121,
   March 1997.

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