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Network Working Group                                      G. ArmitageRequest for Comments: 2269                         Lucent TechnologiesCategory: Informational                                   January 1998Using the MARS Model in non-ATM NBMA NetworksStatus 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, theRFC 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 howRFC 2022 can be applied to other   NBMA technologies.Armitage                     Informational                      [Page 1]

RFC 2269          MARS Model in non-ATM NBMA Networks       January 19982.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.Armitage                     Informational                      [Page 2]

RFC 2269          MARS Model in non-ATM NBMA Networks       January 1998   The MARS control messages defined in sections4 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.Armitage                     Informational                      [Page 3]

RFC 2269          MARS Model in non-ATM NBMA Networks       January 1998      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   distributedRFC 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.Armitage                     Informational                      [Page 4]

RFC 2269          MARS Model in non-ATM NBMA Networks       January 1998Security 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 inRFC 2022, or described in documents explicitly   proposing use ofRFC 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.comReferences   [1] Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC1112, 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.Armitage                     Informational                      [Page 5]

RFC 2269          MARS Model in non-ATM NBMA Networks       January 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Armitage                     Informational                      [Page 6]