Internet-Draft Grenville Armitage Bellcore May 23rd, 1996 Using the MARS model in non-ATM NBMA networks. Status of this Memo This document was submitted to the IETF IP over NBMA WG. Publication of this document does not imply acceptance by the IP over NBMA WG of any ideas expressed within. Comments should be submitted to the ion@nexen.com mailing list. Distribution of this memo is unlimited. This memo is an internet draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress". Please check the lid-abstracts.txt listing contained in the internet-drafts shadow directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim) to learn the current status of any Internet Draft. Abstract The MARS model developed by the IP over ATM working group 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. The MARS model is not required for NBMA networks that offer a link level group addressing service that maps directly onto the IP multicast model. This document is informational, and may influence the development of MARSv2/NHRPv2 in line with the new ION charter and 'goals and Armitage Expires November 23rd, 1996 [Page 1] Internet Draft May 23rd, 1996 milestones' timeline. 1. Introduction. Most network layer models, like the one described in 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) do not support a multicast (or group) address abstraction. In these environments multicasting is supported through point to multipoint calls. The MARS model [2] was originally developed by the IP over ATM working group. For completeness this memo explains how the MARS model and protocol can be applied to other NBMA technologies that offer similar, limited multicast support. 2. The MARS model's basic assumptions. Section 3 of [2] describes the basic assumptions that the MARS model makes about the services available from the (ATM) link layer network. In summary (from the intro to section 3), these were: 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' (although the default LLC/SNAP encapsulation mode specified in RFC1755 really requires that an 'LLC entity' is the AAL User, which in turn supports the IP/ATM interface). 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. Armitage Expires November 23rd, 1996 [Page 2] Internet Draft May 23rd, 1996 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 simply an entity with an NBMA address. A MARS Client is simply an entity with an NBMA address. An MCS (where needed) is simply an entity with an 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 'hardware' addresses. In a manner analagous to that described in NHRP [3], 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. This document does not provide any additional information on how to safely build a cluster that spans IP unicast subnet boundaries, the existing caveat that a Cluster == a LIS remains. Armitage Expires November 23rd, 1996 [Page 3] Internet Draft May 23rd, 1996 The term 'Cluster Member' continues to refer to an endpoint that is currently using a 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 endpoints are actually cluster members at any given time. Section 3.4 of [2] provided a somewhat stylised 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 Armitage Expires November 23rd, 1996 [Page 4] Internet Draft May 23rd, 1996 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 (e.g. because of ClusterControlVC). On going developments in server synchronisation protocols for redundant MARS and MCS entities 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. Look to the ISSLL working group for answers here. 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. Use of NBMA networks other than ATM does not imply that the problems associated with 'cut through' for multicast have been solved. This is still an open issue. Security Consideration Security consideration are not addressed in this document. Acknowledgments Author's Address Grenville Armitage Bellcore, 445 South Street Morristown, NJ, 07960 USA Email: gja@thumper.bellcore.com Ph. +1 201 829 2635 References Armitage Expires November 23rd, 1996 [Page 5] Internet Draft May 23rd, 1996 [1] S. Deering, "Host Extensions for IP Multicasting", RFC 1112, Stanford University, August 1989. [2] G.J. Armitage, "Support for Multicast over UNI 3.0/3.1 based ATM Networks.", Bellcore, INTERNET DRAFT, draft-ietf-ipatm-ipmc-12.txt, February 1996 [3] J. Luciani, et al, "NBMA Next Hop Resolution Protocol (NHRP)", INTERNET DRAFT, draft-ietf-rolc-nhrp-07.txt, December 1995. Armitage Expires November 23rd, 1996 [Page 6]