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Versions: 00 01 02 03 04 05 rfc3569                                     
INTERNET-DRAFT                                    Supratik Bhattacharyya
Expires 04 May 2003                                      Christophe Diot
                                                              Sprint ATL
                                                        Leonard Giuliano
                                                        Juniper Networks
                                                             Rob Rockell
                                                             John Meylor
                                                           Cisco Systems
                                                             David Meyer
                                                           Greg Shepherd
                                                        Juniper Networks
                                                          Brian Haberman
                                                        Caspian Networks
                                                        04 November 2002

             An Overview of Source-Specific Multicast (SSM)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

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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.

Copyright Notice

Copyright (C) The Internet Society (2002).  All Rights Reserved.


   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 (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]

   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 and its benefits for anyone interested in deploying SSM
   services.  It provides an overview of SSM and 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.

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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 [27]. 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 ( through 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 [23].

  Source-Filtered Multicast (SFM) : This is a variant of the ASM service
  model, and uses the same address range as ASM
  (  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

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.

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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 [28], PIM-SM [8,9], MSDP
   [13] and MBGP [29] protocols.  IGMPv2 [2] 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 [26,11],
   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

   Inter-domain multicast service (i.e., where sources and receivers are
   located in multiple domains) requires additional protocols - MSDP
   [13] and MBGP [29] are the most commonly used ones. An RP uses the
   MSDP [13] 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.

   The MBGP protocol [29] 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.

4. Problems with Current Architecture

   There are several deployment problems associated with current

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   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 [18] 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. 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 and MSDP 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 :

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      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 by a only 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. In terms of the IGMP/PIM-SM/MSDP/MBGP protocol
      suite, 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 MBGP is
      still required for distribution of multicast reachability

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
       +--------------+ session directory/web page
       | source,group |                      SESSION DESCRIPTION
              ^ |
        Query | | (S,G)
              | v
     +-----------------+ host
     |   SSM-aware app |                     CHANNEL DISCOVERY

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     |   SSM-aware app |                   SSM-AWARE APPLICATION
     |   IGMPv3/MLDv2  |              IGMPv3/MLDv2 HOST REPORTING
               |(source specific host report)
     +-----------------+  Querier Router
     |   IGMPv3/MLDv2  |                         QUERIER
       |   PIM-SSM  |                        PIM-SSM ROUTING
       +------------+     Designated Router
               | (S,G) Join only
         +-----------+  Backbone Router
         |  PIM-SSM  |
               | (S,G) Join only

     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 [20] which recommend that routers should
   not send SSM traffic to parts of the network that do not have channel

   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, [25] has defined an extension to the addressing
   architecture to allow for unicast prefix-based multicast addresses.
   Bytes 0-3 (starting from the least significant byte) of the IP
   address are used to specify a multicast group id, bytes 4-11 are used
   to specify a unicast address prefix (of up to 64 bits) that owns this
   multicast group id, and byte 12 is used to specify the length of the
   prefix. A source-specific multicast address is specified by setting

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   both the unicast address prefix field and the prefix length field to

   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 adddres, 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-

      -- An application that wants to received an SSM session must first
      discover the channel address in use. Any of the mechanisms
      described in Section 5.2 can be used for this purpose.

      -- 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. In other words, the application must be

      Specific API requirements are identified in [17]. [17] 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

   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 [28] and MLD version 1
      [21] 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 [22]. These

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      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.  Version 1 of MLD
      [21] is  derived from IGMPv2 and does not provide the source
      filtering capability required for the SSM service model. Version 2
      of MLD [22] 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 guideliness 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].

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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 adddress 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. [20] specifies operational
   recommendations to help ensure that MSDP does not interfere with the
   ability of a network to support the SSM service model. Specifically,
   [20] states that RPs must not accept, originate or forward MSDP SA
   messages for the SSM address range [20].

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 and Timothy
   Roscoe at Sprintlabs, Hugh Holbrook, Isidor Kouvelas, Tony Speakman
   and Nidhi Bhaskar at Cisco Systems for participating in lengthy
   discussions and design work on SSM, and providing feedback on this
   document. Thanks are also due to Mujahid Khan and Ted Seely at
   SprintLink, Tom Pusateri at Juniper Networks, Bill Fenner at AT&T
   Research, Kevin Almeroth at the University of California Santa
   Barbara, Brian Levine at the University of  Massachusetts Amherst,
   Brad Cain at Cereva Networks and Hugh LaMaster at NASA for their
   valuable insights and continuing support.

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10. References:

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

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

   [2] B. Cain and S. Deering, I. Kouvelas and A. Thyagarajan, "Internet
   Group Management Protocol, Version 3.", Work in Progress.

   [4] H. Holbrook and B. Cain, "IGMPv3 for SSM", Work in Progress.

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

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

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

   [8] D. Estrin et al., "Protocol Independent Multicast - Sparse Mode
   (PIM-SM) : Protocol Specification", RFC 2362.

   [9] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas, "Protocol
   Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification
   (Revised)", Work In Progress, 2000.

   [10] S. Deering et al., "Protocol Independent Multicast Version 2
   Dense Mode Specification", Work in Progress.

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

   [12] D. Meyer, "Adminstratively Scoped IP Multicast", RFC 2365.

   [13] Farinacci et al., "Multicast Source Discovery Protocol", Work in

   [14] M. Handley, D. Thaler and D. Estrin, "The Internet Multicast
   Address Allocation Architecture", Work in Progress.

   [15] C. Diot, B. Levine, B. Lyles, H. Kassem and D. Balensiefen,
   "Deployment Issues for the IP Multicast Service and Architecture", In

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   IEEE Networks Magazine's Special Issue on Multicast, January, 2000.

   [16] H. Sandick and B. Cain, "PIM-SM Rules for Support of Single-
   Source Multicast", Work in Progress.

   [17] Dave Thaler, Bill Fenner and Bob Quinn, "Socket Interface
   Extensions for Multicast Source Filters", Work in Progress.

   [18] D. Meyer and P. Lothberg, "GLOP Addressing in 233/8", Request
   For Comments 2770.

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

   [20]   G. Shepherd et al., "Source-Specific Protocol Independent
   Multicast in 232/8", Work in Progress.

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

   [22] R. Vida, et. al., "Multicast Listener Discovery Version 2(MLDv2)
    for IPv6", Work in progress.

   [23] B. Haberman and D. Thaler, "Unicast-Prefix-Based IPv6 Multicast
   Addresses", Work in Progress.

   [24] S. Kent, R. Atkinson, "Security Architecture for the Internet
   Protocol", Request for Comments 2401.

   [25] B. Haberman, "Dynamic Allocation Guidelines for IPv6 Multicast
   Addresses", Work in Progress.

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

   [27] S. Deering, "Host Extensions for IP Multicasting", RFC 1112.

   [28] W. Fenner, "Internet Group Management Protocol, Version 2", RFC

   [29] T. Bates, R. Chandra, D. Katz, and Y. Rekhter, "Multiprotocol
   Extensions for BGP-4", RFC 2283.

Bhattacharyya et. al.                                          [Page 12]

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12. Authors' Address:

   Supratik Bhattacharyya
   Christophe Diot
   Sprint Advanced Technology Labs
   One Adrian Court
   Burlingame CA 94010 USA

   Leonard Giuliano
   Greg Shepherd
   Juniper Networks, Inc.
   1194 North Mathilda Avenue
   Sunnyvale, CA 94089 USA

   Robert Rockell
   David Meyer
   Sprint E|Solutions
   Reston Virginia USA

   John Meylor
   Cisco Systems
   San Jose CA USA

   Brian Haberman
   Caspian Networks

Bhattacharyya et. al.                                          [Page 13]