An Overview of Source-Specific Multicast (SSM)

Network Working Group                              S. Bhattacharyya, Ed.
Request for Comments: 3569                                        Sprint
Category: Informational                                        July 2003


             An Overview of Source-Specific Multicast (SSM)

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 (2003).  All Rights Reserved.

Abstract

   The purpose of this document is to provide an overview of
   Source-Specific Multicast (SSM) and issues related to its deployment.
   It discusses how the SSM service model addresses the challenges faced
   in inter-domain multicast deployment, changes needed to routing
   protocols and applications to deploy SSM and interoperability issues
   with current multicast service models.

1.  Introduction

   This document provides an overview of the Source-Specific Multicast
   (SSM) service and its deployment using the PIM-SM and IGMP/MLD
   protocols.  The network layer service provided by SSM is a "channel",
   identified by an SSM destination IP address (G) and a source IP
   address S.  An IPv4 address range has been reserved by IANA for use
   by the SSM service.  An SSM destination address range already exists
   for IPv6.  A source S transmits IP datagrams to an SSM destination
   address G.  A receiver can receive these datagrams by subscribing to
   the channel (Source, Group) or (S,G).  Channel subscription is
   supported by version 3 of the IGMP protocol for IPv4 and version2 of
   the MLD protocol for IPv6.  The interdomain tree for forwarding IP
   multicast datagrams is rooted at the source S, and is constructed
   using the PIM Sparse Mode [9] protocol.

   This document is not intended to be a standard for Source-Specific
   Multicast (SSM).  Instead, its goal is to serve as an introduction to
   SSM and its benefits for anyone interested in deploying SSM services.
   It provides an overview of SSM and how it solves a number of problems
   faced in the deployment of inter-domain multicast.  It outlines
   changes to protocols and applications both at end-hosts and routers
   for supporting SSM, with pointers to more detailed documents where
   appropriate.  Issues of interoperability with the multicast service
   model defined by RFC 1112 are also discussed.

   This memo is a product of the Source-Specific Multicast (SSM) Working
   Group of the Internet Engineering Task Force.

   The keywords "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as defined in BCP 14, RFC 2119 [28].

2.  Terminology

   This section defines some terms that are used in the rest of this
   document:

      Any-Source Multicast (ASM): This is the IP multicast service model
      defined in RFC 1112 [25].  An IP datagram is transmitted to a
      "host group", a set of zero or more end-hosts (or routers)
      identified by a single IP destination address (224.0.0.0 through
      239.255.255.255 for IPv4).  End-hosts may join and leave the group
      any time, and there is no restriction on their location or number.
      Moreover, this model supports multicast groups with arbitrarily
      many senders - any end-host (or router) may transmit to a host
      group, even if it is not a member of that group.

      Source-Specific Multicast (SSM): This is the multicast service
      model defined in [5].  An IP datagram is transmitted by a source S
      to an SSM destination address G, and receivers can receive this
      datagram by subscribing to channel (S,G).  SSM provides host
      applications with a "channel" abstraction, in which each channel
      has exactly one source and any number of receivers.  SSM is
      derived from earlier work in EXPRESS [1].  The address range 232/8
      has been assigned by IANA for SSM service in IPv4.  For IPv6, the
      range FF3x::/96 is defined for SSM services [21].

      Source-Filtered Multicast (SFM): This is a variant of the ASM
      service model, and uses the same address range as ASM
      (224.0.0.0-239.255.255.255).  It extends the ASM service model as
      follows.  Each "upper layer protocol module" can now request data
      sent to a host group G by only a specific set of sources, or can
      request data sent to host group G from all BUT a specific set of
      sources.  Support for source filtering is provided by version 3 of
      the Internet Group Management Protocol (or IGMPv3) [3] for IPv4,
      and version 2 of the Multicast Listener Discovery (or MLDv2) [22]
      protocol for IPv6.  We shall henceforth refer to these two
      protocols as "SFM-capable".  Earlier versions of these
      protocols - IGMPv1/IGMPv2 and MLDv1 - do not provide support for
      source-filtering, and are referred to as "non-SFM-capable".  Note
      that while SFM is a different model than ASM from a receiver
      standpoint, there is no distinction between the two for a sender.

   For the purpose of this document, we treat the scoped multicast model
   of [12] to be a variant of ASM since it does not explicitly restrict
   the number of sources, but only requires that they be located within
   the scope zone of the group.

3.  The IGMP/PIM-SM/MSDP/MBGP Protocol Suite for ASM

   As of this writing, all multicast-capable networks support the ASM
   service model.  One of the most common multicast protocol suites for
   supporting ASM consists of IGMP version 2 [2], PIM-SM [8,9], MSDP
   [13] and MBGP [26].  IGMPv2 is the most commonly used protocol for
   hosts to specify membership in a multicast group, and nearly all
   multicast routers support (at least) IGMPv2.  In case of IPv6, MLDv1
   [21] is the commonly used protocol.

   Although a number of protocols such as PIM-DM [10], CBT [24,11],
   DVMRP [6], etc. exist for building multicast tree among all receivers
   and sources in the same administrative domain, PIM-SM [8,9] is the
   most widely used protocol.  PIM-SM builds a spanning multicast tree
   rooted at a core rendezvous point or RP for all group members within
   a single administrative domain.  A 'first-hop' router adjacent to a
   multicast source sends the source's traffic to the RP for its domain.
   The RP forwards the data down the shared spanning tree to all
   interested receivers within the domain.  PIM-SM also allows receivers
   to switch to a source-based shortest path tree.

   As of this writing, multicast end-hosts with SFM capabilities are not
   widely available.  Hence a client can only specify interest in an
   entire host group and receives data sent from any source to this
   group.

   Inter-domain multicast service (i.e., where sources and receivers are
   located in different domains) requires additional protocols - MSDP
   [13] and MBGP [26] are the most commonly used ones.  An RP uses the
   MSDP protocol to announce multicast sources to RPs in other domains.
   When an RP discovers a source in a different domain transmitting data
   to a multicast group for which there are interested receivers in its
   own domain, it joins the shortest-path source based tree rooted at
   that source.  It then redistributes the data received to all
   interested receivers via the intra-domain shared tree rooted at
   itself.

   MBGP defines extensions to the BGP protocol to support the
   advertisement of reachability information for multicast routes.  This
   allows an autonomous system (AS) to support incongruent unicast and
   multicast routing topologies, and thus implement separate routing
   policies for each.

   However, the last-hop routers of interested receivers may eventually
   switch to a shortest-path tree rooted at the source that is
   transmitting the data.

4.  Problems with Current Architecture

   There are several deployment problems associated with current
   multicast architecture:

      A) Address Allocation:

         Address allocation is one of core deployment challenges posed
         by the ASM service model.  The current multicast architecture
         does not provide a deployable solution to prevent address
         collisions among multiple applications.  The problem is much
         less serious for IPv6 than for IPv4 since the size of the
         multicast address space is much larger.  A static address
         allocation scheme, GLOP [17] has been proposed as an interim
         solution for IPv4; however, GLOP addresses are allocated per
         registered AS, which is inadequate in cases where the number of
         sources exceeds the AS numbers available for mapping.  RFC 3138
         expands on RFC 2770 to allow routing registries to assign
         multicast addresses from the GLOP space corresponding to the
         RFC 1930 private AS space [27].  This space is referred to as
         the EGLOP (Extended GLOP) address space.  Proposed longer-term
         solutions such as the Multicast Address Allocation Architecture
         [14] are generally perceived as being too complex (with respect
         to the dynamic nature of multicast address allocation) for
         widespread deployment.

      B) Lack of Access control:

         In the ASM service model, a receiver cannot specify which
         specific sources it would like to receive when it joins a given
         group.  A receiver will be forwarded data sent to a host group
         by any source.  Moreover, even when a source is allocated a
         multicast group address to transmit on, it has no way of
         enforcing that no other source will use the same address.  This
         is true even in the case of IPv6, where address collisions are
         less likely due to the much larger size of the address space.

      C) Inefficient handling of well-known sources:

         In cases where the address of the source is well known in
         advance of the receiver joining the group, and when the
         shortest forwarding path is the preferred forwarding mode, then
         shared tree mechanisms are not necessary.

5.  Source Specific Multicast (SSM): Benefits and Requirements

   As mentioned before, the Source Specific Multicast (SSM) service
   model defines a "channel" identified by an (S,G) pair, where S is a
   source address and G is an SSM destination address.  Channel
   subscriptions are described using an SFM-capable group management
   protocol such as IGMPv3 or MLDv2.  Only source-based forwarding trees
   are needed to implement this model.

   The SSM service model alleviates all of the deployment problems
   described earlier:

      A) Address Allocation: SSM defines channels on a per-source basis,
         i.e., the channel (S1,G) is distinct from the channel (S2,G),
         where S1 and S2 are source addresses, and G is an SSM
         destination address.  This averts the problem of global
         allocation of SSM destination addresses, and makes each source
         independently responsible for resolving address collisions for
         the various channels that it creates.

      B) Access Control: SSM lends itself to an elegant solution to the
         access control problem.  When a receiver subscribes to an (S,G)
         channel, it receives data sent only by the source S.  In
         contrast, any host can transmit to an ASM host group.  At the
         same time, when a sender picks a channel (S,G) to transmit on,
         it is automatically ensured that no other sender will be
         transmitting on the same channel (except in the case of
         malicious acts such as address spoofing).  This makes it much
         harder to "spam" an SSM channel than an ASM multicast group.

      C) Handling of well-known sources: SSM requires only
         source-based forwarding trees; this eliminates the need for a
         shared tree infrastructure.  This implies that neither the
         RP-based shared tree infrastructure of PIM-SM nor the MSDP
         protocol is required.  Thus the complexity of the multicast
         routing infrastructure for SSM is low, making it viable for
         immediate deployment.  Note that there is no difference in how
         MBGP is used for ASM and SSM.

6.  SSM Framework

   Figure 1 illustrates the elements in an end-to-end implementation
   framework for SSM:

      --------------------------------------------------------------
       IANA assigned 232/8 for IPv4             ADDRESS ALLOCATION
            FF3x::/96 for IPv6
      --------------------------------------------------------------
                   |
                   v
          +--------------+ session directory/web page
          | source,group |                      SESSION DESCRIPTION
      --------------------------------------------------------------
                 ^ |
           Query | | (S,G)
                 | v
        +-----------------+ host
        |   SSM-aware app |                     CHANNEL DISCOVERY
      --------------------------------------------------------------
        |   SSM-aware app |                   SSM-AWARE APPLICATION
      --------------------------------------------------------------
        |   IGMPv3/MLDv2  |              IGMPv3/MLDv2 HOST REPORTING
        +-----------------+
                  |(source specific host report)
      --------------------------------------------------------------
                  v
        +-----------------+  Querier Router
        |   IGMPv3/MLDv2  |                         QUERIER
      --------------------------------------------------------------
          |   PIM-SSM  |                        PIM-SSM ROUTING
          +------------+     Designated Router
                  |
                  | (S,G) Join only
                  v
            +-----------+  Backbone Router
            |  PIM-SSM  |
            +-----------+
                  |
                  | (S,G) Join only
                  V

        Figure 1: SSM Framework: elements in end-to-end model

   We now discuss the framework elements in detail:

6.1.  Address Allocation

   For IPv4, the address range of 232/8 has been assigned by IANA for
   SSM.  To ensure global SSM functionality in 232/8, including in
   networks where routers run non-SFM-capable protocols, operational
   policies are being proposed [9] which recommend that routers should
   not send SSM traffic to parts of the network that do not have channel
   subscribers.

   Note that IGMPv3/MLDv2 does not limit (S,G) joins to only the 232/8
   range.  However, SSM service, as defined in [5], is available only in
   this address range for IPv4.

   In case of IPv6, [23] has defined an extension to the addressing
   architecture to allow for unicast prefix-based multicast addresses.
   See RFC 3306 for details.

6.2.  Session Description and Channel Discovery

   An SSM receiver application must know both the SSM destination
   address G and the source address S before subscribing to a channel.
   Channel discovery is the responsibility of applications.  This
   information can be made available in a number of ways, including via
   web pages, sessions announcement applications, etc.  This is similar
   to what is used for ASM applications where a multicast session needs
   to be announced so that potential subscribers can know of the
   multicast group address, encoding schemes used, etc.  In fact, the
   only additional piece of information that needs to be announced is
   the source address for the channel being advertised.  However, the
   exact mechanisms for doing this is outside the scope of this
   framework document.

6.3.  SSM-Aware Applications

   There are two main issues in making multicast applications
   "SSM-aware":

   -  An application that wants to receive an SSM session must first
      discover the channel address in use.

   -  A receiving application must be able to specify both a source
      address and a destination address to the network layer protocol
      module on the end-host.

      Specific API requirements are identified in [16].  [16] describes
      a recommended application programming interface for a host
      operating system to support the SFM service model.  Although it is
      intended for SFM, a subset of this interface is sufficient for
      supporting SSM.

6.4.  IGMPv3/MLDv2 Host Reporting and Querier

   In order to use SSM service, an end-host must be able to specify a
   channel address, consisting of a source's unicast address and an SSM
   destination address.  IGMP version 2 [3] and MLD version 1 [19]
   allows an end-host to specify only a destination multicast address.
   The ability to specify an SSM channel address c is provided by IGMP
   version 3 [3] and MLD version 2 [20].  These protocols support
   "source filtering", i.e., the ability of an end-system to express
   interest in receiving data packets sent only by SPECIFIC sources, or
   from ALL BUT some specific sources.  In fact, IGMPv3 provides a
   superset of the capabilities required to realize the SSM service
   model.

   A detailed discussion of the use of IGMPv3 in the SSM destination
   address range is provided in [4].

   The Multicast Listener Discovery (MLD) protocol used by an IPv6
   router to discover the presence of multicast listeners on its
   directly attached links, and to discover the multicast addresses that
   are of interest to those neighboring nodes.  MLD version 1 is derived
   from IGMPv2 and does not provide the source filtering capability
   required for the SSM service model.  MLD version 2 is derived from,
   and provides the same support for source-filtering as, IGMPv3.  Thus
   IGMPv3 (or MLDv2 for IPv6) provides a host with the ability to
   request the network for an SSM channel subscription.

6.5.  PIM-SSM Routing

   [9] provides guidelines for how a PIM-SM implementation should handle
   source-specific host reports as required by SSM.  Earlier versions of
   the PIM protocol specifications did not describe how to do this.

   The router requirements for operation in the SSM range are detailed
   in [5].  These rules are primarily concerned with preventing
   ASM-style behaviour in the SSM address range.  In order to comply
   with [5] several changes to the PIM-SM protocol are required, as
   described in [9].  The most important changes in PIM-SM required for
   compliance with [5] are:

   -  When a DR receives an (S,G) join request with the address G in the
      SSM address range, it MUST initiate a (S,G) join, and NEVER a
      (*,G) join.

   -  Backbone routers (i.e., routers that do not have directly attached
      hosts) MUST NOT propagate (*,G) joins for group addresses in the
      SSM address range.

   -  Rendezvous Points (RPs) MUST NOT accept PIM Register messages or
      (*,G) Join messages in the SSM address range.

   Note that only a small subset of the full PIM-SM protocol
   functionality is needed to support the SSM service model.  This
   subset is explicitly documented in [9].

7.  Interoperability with Existing Multicast Service Models

   Interoperability with ASM is one of the most important issues in
   moving to SSM deployment, since both models are expected to be used
   at least in the foreseeable future.  SSM is the ONLY service model
   for the SSM address range - the correct protocol behaviour for this
   range is specified in [5].  The ASM service model will be offered for
   the non-SSM address range, where receivers can issue (*,G) join
   requests to receive multicast data.  A receiver is also allowed to
   issue an (S,G) join request in the non-SSM address range; however, in
   that case there is no guarantee that it will receive service
   according to the SSM model.

   Another interoperability issue concerns the MSDP protocol, which is
   used between PIM-SM rendezvous points (RPs) to discover multicast
   sources across multiple domains.  MSDP is not needed for SSM, but is
   needed if ASM is supported.  [9] specifies operational
   recommendations to help ensure that MSDP does not interfere with the
   ability of a network to support the SSM service model.  Specifically,
   [9] states that RPs must not accept, originate or forward MSDP SA
   messages for the SSM address range.

8.  Security Considerations

   SSM does not introduce new security considerations for IP multicast.
   It can help in preventing denial-of-service attacks resulting from
   unwanted sources transmitting data to a multicast channel (S, G).
   However no guarantee is provided.

9.  Acknowledgments

   We would like to thank Gene Bowen, Ed Kress, Bryan Lyles, Timothy
   Roscoe, Hugh Holbrook, Isidor Kouvelas, Tony Speakman and Nidhi
   Bhaskar for participating in lengthy discussions and design work on
   SSM, and providing feedback on this document.  Thanks are also due to
   Mujahid Khan, Ted Seely, Tom Pusateri, Bill Fenner, Kevin Almeroth,
   Brian Levine, Brad Cain, Hugh LaMaster and Pekka Savola for their
   valuable insights and continuing support.

10.  References

10.1.  Informative References

   [1]  Holbrook, H. and D.R. Cheriton, "IP Multicast Channels: EXPRESS
        Support for Large-scale Single-Source Applications", In
        Proceedings of SIGCOMM 1999.

   [2]  Fenner, W., "Internet Group Management Protocol, Version 2", RFC
        2236, November 1997.

   [3]  Cain, B., Deering, S., Kouvelas, I. and A. Thyagarajan,
        "Internet Group Management Protocol, Version 3.", RFC 3376,
        October 2002.

   [4]  Holbrook, H. and B. Cain, "Using IGMPv3 and MLDv2 for
        Source-Specific Multicast", Work In Progress.

   [5]  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
        Work in Progress.

   [6]  Deering, S. and D. Cheriton,"Multicast Routing in Datagram
        Networks and Extended LANs", ACM Transactions on Computer
        Systems, 8(2):85-110, May 1990.

   [7]  Deering, S. et al., "PIM Architecture for Wide-Area Multicast
        Routing", IEEE/ACM Transaction on Networking, pages 153-162,
        April 1996.

   [8]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
        Handley, M., Jacobson, V., Liu, C., Sharma, P. and L. Wei,
        "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
        Specification", RFC 2362, June 1998.

   [9]  Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol
        Independent Multicast - Sparse Mode (PIM-SM): Protocol
        Specification (Revised)", Work In Progress.

   [10] Adams, A., Nicholas, J. and W. Siadek, "Protocol Independent
        Multicast - Dense Mode (PIM-DM): Protocol Specification
        (Revised)", Work In Progress.

   [11] Ballardie, A., "Core-Based Trees (CBT) Multicast Routing
        Architecture", RFC 2201, September 1997.

   [12] Meyer, D., "Adminstratively Scoped IP Multicast", BCP 23, RFC
        2365, July 1998.

   [13] Farinacci, D. et al., "Multicast Source Discovery Protocol",
        Work In Progress.

   [14] Thaler, D., Handley, M. and D. Estrin, "The Internet Multicast
        Address Allocation Architecture", RFC 2908, September 2000.

   [15] Diot, C., Levine, B., Lyles, B., Kassem, H. and D. Balensiefen,
        "Deployment Issues for the IP Multicast Service and
        Architecture", In IEEE Networks Magazine's Special Issue on
        Multicast, January, 2000.

   [16] Thaler, D., Fenner B. and B. Quinn, "Socket Interface Extensions
        for Multicast Source Filters", Work in Progress.

   [17] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", BCP 53,
        RFC 3180, September 2001.

   [18] Levine, B. et al., "Consideration of Receiver Interest for IP
        Multicast Delivery", In Proceedings of IEEE Infocom, March 2000.

   [19] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
        Discovery for IPv6", RFC 2710, October 1999.

   [20] Vida, R. et. al., "Multicast Listener Discovery Version 2(MLDv2)
        for IPv6", Work In Progress.

   [21] Haberman, B. and D. Thaler, "Unicast-Prefix-Based IPv6 Multicast
        Addresses", RFC 3306, August 1992.

   [22] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [23] Haberman, B., "Allocation Guidelines for IPv6 Multicast
        Addresses", RFC 3307, August 2002.

   [24] Ballardie, A., "Core-Based Trees (CBT Version 2) Multicast
        Routing -- Protocol Specification", RFC 2189, September 2001.

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

   [26] Bates, T., Rekhter, Y., Chandra, R. and D. Katz, "Multiprotocol
        Extensions for BGP-4", RFC 2858, June 2000.

   [27] Meyer, D., "Extended Assignments in 233/8", RFC 3138, June 2001.

10.2.  Normative References

   [28] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

11.  Contributors

   Christophe Diot
   Intel
   EMail: christophe.diot@intel.com

   Leonard Giuliano
   Juniper Networks
   EMail: lenny@juniper.net

   Greg Shepherd
   Procket Networks
   EMail: shep@procket.com

   Robert Rockell
   Sprint
   EMail: rrockell@sprint.net

   David Meyer
   Sprint
   EMail: dmm@1-4-5.net

   John Meylor
   Cisco Systems
   EMail: jmeylor@cisco.com

   Brian Haberman
   Caspian Networks
   EMail: bkhabs@nc.rr.com

12.  Editor's Address

   Supratik Bhattacharyya
   Sprint

   EMail: supratik@sprintlabs.com

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