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Network Working Group                              S. Bhattacharyya, Ed.Request for Comments: 3569                                        SprintCategory: Informational                                        July 2003An 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 routersBhattacharyya                Informational                      [Page 1]

RFC 3569                   An Overview of SSM                  July 2003   for supporting SSM, with pointers to more detailed documents where   appropriate.  Issues of interoperability with the multicast service   model defined byRFC 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 inBCP 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 inRFC 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 forBhattacharyya                Informational                      [Page 2]

RFC 3569                   An Overview of SSM                  July 2003      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.Bhattacharyya                Informational                      [Page 3]

RFC 3569                   An Overview of SSM                  July 2003   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 onRFC 2770 to allow routing registries to assign         multicast addresses from the GLOP space corresponding to theRFC 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.Bhattacharyya                Informational                      [Page 4]

RFC 3569                   An Overview of SSM                  July 2003      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.Bhattacharyya                Informational                      [Page 5]

RFC 3569                   An Overview of SSM                  July 20036.  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 modelBhattacharyya                Informational                      [Page 6]

RFC 3569                   An Overview of SSM                  July 2003   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.   SeeRFC 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.Bhattacharyya                Informational                      [Page 7]

RFC 3569                   An Overview of SSM                  July 2003      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:Bhattacharyya                Informational                      [Page 8]

RFC 3569                   An Overview of SSM                  July 2003   -  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.Bhattacharyya                Informational                      [Page 9]

RFC 3569                   An Overview of SSM                  July 20039.  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.  References10.1.  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",RFC2236, 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.Bhattacharyya                Informational                     [Page 10]

RFC 3569                   An Overview of SSM                  July 2003   [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,RFC2365, 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.Bhattacharyya                Informational                     [Page 11]

RFC 3569                   An Overview of SSM                  July 2003   [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,RFC1112, 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.comBhattacharyya                Informational                     [Page 12]

RFC 3569                   An Overview of SSM                  July 200312.  Editor's Address   Supratik Bhattacharyya   Sprint   EMail: supratik@sprintlabs.comBhattacharyya                Informational                     [Page 13]

RFC 3569                   An Overview of SSM                  July 200313.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  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 assignees.   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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Bhattacharyya                Informational                     [Page 14]