Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Protocol Version 2 (MLDv2) for Source-Specific Multicast
RFC 4604

Network Working Group                                        H. Holbrook
Request for Comments: 4604                                 Arastra, Inc.
Updates: 3376, 3810                                              B. Cain
Category: Standards Track                                Acopia Networks
                                                             B. Haberman
                                                                 JHU APL
                                                             August 2006

      Using Internet Group Management Protocol Version 3 (IGMPv3)
      and Multicast Listener Discovery Protocol Version 2 (MLDv2)
                     for Source-Specific Multicast

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Internet Group Management Protocol Version 3 (IGMPv3) and the
   Multicast Listener Discovery Protocol Version 2 (MLDv2) are protocols
   that allow a host to inform its neighboring routers of its desire to
   receive IPv4 and IPv6 multicast transmissions, respectively.
   Source-specific multicast (SSM) is a form of multicast in which a
   receiver is required to specify both the network-layer address of the
   source and the multicast destination address in order to receive the
   multicast transmission.  This document defines the notion of an
   "SSM-aware" router and host, and clarifies and (in some cases)
   modifies the behavior of IGMPv3 and MLDv2 on SSM-aware routers and
   hosts to accommodate source-specific multicast.  This document
   updates the IGMPv3 and MLDv2 specifications.

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1.  Introduction

   The Internet Group Management Protocol (IGMP) [RFC1112, IGMPv2,
   IGMPv3] allows an IPv4 host to communicate IP multicast group
   membership information to its neighboring routers.  IGMP version 3
   (IGMPv3) [IGMPv3] provides the ability for a host to selectively
   request or filter traffic from individual sources within a multicast
   group.

   The Multicast Listener Discovery Protocol (MLD) [RFC2710, MLDv2]
   offers similar functionality for IPv6 hosts.  MLD version 2 (MLDv2)
   provides the analogous "source filtering" functionality of IGMPv3 for
   IPv6.

   Due to the commonality of function, the term "Group Management
   Protocol", or "GMP", will be used to refer to both IGMP and MLD.  The
   term "Source Filtering GMP", or "SFGMP", will be used to refer
   jointly to the IGMPv3 and MLDv2 group management protocols.

   The use of source-specific multicast is facilitated by small changes
   to the SFGMP protocols on both hosts and routers.  [SSM] defines
   general requirements that must be followed by systems that implement
   the SSM service model; this document defines the concrete application
   of those requirements to systems that implement IGMPv3 and MLDv2.  In
   doing so, this document defines modifications to the host and router
   portions of IGMPv3 and MLDv2 for use with SSM, and presents a number
   of clarifications to their behavior when used with SSM addresses.
   This document updates the IGMPv3 and MLDv2 specifications.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   In order to emphasize the parts of this document that modify the
   existing protocol specifications ([RFC2710, MLDv2, IGMPv3]), as
   opposed to merely clarify them, any protocol modifications are marked
   with the tag "MODIFICATION".

2.  Host Requirements for Source-Specific Multicast

   This section defines the notion of an "SSM-aware" host and then goes
   on to describe the API requirements and the SFGMP protocol
   requirements of an SSM-aware host.  It is important to note that SSM
   can be used by any host that supports source filtering APIs and whose
   operating system supports the appropriate SFGMP.  The SFGMP

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RFC 4604                IGMPv3/MLDv2 for SSM                 August 2006

   modifications described in this section make SSM work better on an
   SSM-aware host, but they are not strict prerequisites for the use of
   SSM.

   The 232/8 IPv4 address range is currently allocated for SSM by IANA
   [IANA-ALLOCATION].  In IPv6, the FF3x::/32 range (where 'x' is a
   valid IPv6 multicast scope value) is reserved for SSM semantics
   [RFC3306], although today SSM allocations are restricted to
   FF3x::/96.  ([SSM] has a more thorough discussion of this topic.)  A
   host that knows the SSM address range and is capable of applying SSM
   semantics to it is described as an "SSM-aware" host.

   A host or router may be configured to apply SSM semantics to
   addresses other than those in the IANA-allocated range.  The GMP
   module on a host or router SHOULD have a configuration option to set
   the SSM address range(s).  If this configuration option exists, it
   MUST default to the IANA-allocated SSM range.  The mechanism for
   setting this configuration option MUST at least allow for manual
   configuration.  Protocol mechanisms to set this option may be defined
   in the future.

2.1.  API Requirements

   If the host IP module of an SSM-aware host receives a non-source-
   specific request to receive multicast traffic sent to an SSM
   destination address, it SHOULD return an error to the application, as
   specified in [MSFAPI] (MODIFICATION).  On a non-SSM-aware host, an
   application that uses the wrong API (e.g., "join(G)",
   "IPMulticastListen(G,EXCLUDE(S1))" for IGMPv3, or
   "IPv6MulticastListen(G,EXCLUDE(S2))" for MLDv2) to request delivery
   of packets sent to an SSM address will not receive the requested
   service, because an SSM-aware router (following the rules of this
   document) will refuse to process the request, and the application
   will receive no indication other than a failure to receive the
   requested traffic.

2.2.  GMP Requirements

   This section defines the behavior of the SFGMP protocol module on an
   SSM-aware host, including two modifications to the protocols as
   described in [IGMPv3, MLDv2].  It also includes a number of
   clarifications of protocol operations.  In doing so, it documents the
   behavior of an SSM-aware host with respect to sending and receiving
   the following GMP message types:

      - IGMPv1/v2 and MLDv1 Reports (2.2.1)
      - IGMPv3 and MLDv2 Reports (2.2.2)
      - IGMPv1 Queries, IGMPv2 and MLDv1 General Queries (2.2.3)

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      - IGMPv2 Leave and MLDv1 Done (2.2.4)
      - IGMPv2 and MLDv1 Group Specific Query (2.2.5)
      - IGMPv3 and MLDv2 Group Specific Query (2.2.6)
      - IGMPv3 and MLDv2 Group-and-Source Specific Query (2.2.7)

2.2.1.  IGMPv1/v2 and MLDv1 Reports

   An SSM-aware host operating according to [IGMPv3, MLDv2] could send
   an IGMPv1, IGMPv2, or MLDv1 report for an SSM address when it is
   operating in "older-version compatibility mode."  This is an
   exceptional (error) condition, indicating that the router(s) cannot
   provide the SFGMP support needed for SSM, and an error is logged when
   the host enters compatibility mode for an SSM address, as described
   below.  In this situation, it is likely that traffic sent to a
   channel (S,G) will not be delivered to a receiving host that has
   requested to receive channel (S,G).

   [IGMPv3] and [MLDv2] specify that a host MAY allow an older-version
   report to suppress its own IGMPv3 or MLDv2 Membership Record.  An
   SSM-aware host, however, MUST NOT allow its report to be suppressed
   in this situation (MODIFICATION).  Suppressing reports in this
   scenario would provide an avenue for an attacker to deny SSM service
   to other hosts on the link.

2.2.2.  IGMPv3 and MLDv2 Reports

   A host implementation may report more than one SSM channel in a
   single report either by including multiple sources within a group
   record or by including multiple group records.

   A Group Record for a source-specific destination address may (under
   normal operation) be any of the following types:

      - MODE_IS_INCLUDE as part of a Current-State Record

      - ALLOW_NEW_SOURCES as part of a State-Change Record

      - BLOCK_OLD_SOURCES as part of a State-Change Record

   A report may include both SSM destination addresses and non-source-
   specific, i.e., Any-Source Multicast (ASM) destination addresses, in
   the same message.

   Additionally, a CHANGE_TO_INCLUDE_MODE record may be sent by a host
   in some cases, for instance, when the SSM address range is changed
   through configuration.  A router should process such a record
   according to the normal SFGMP rules.

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   An SSM-aware host SHOULD NOT send any of the following record types
   for an SSM address.

      - MODE_IS_EXCLUDE as part of a Current-State Record

      - CHANGE_TO_EXCLUDE_MODE as part of a Filter-Mode-Change Record

   This is a MODIFICATION to [IGMPv3, MLDv2], imposing a restriction on
   its use for SSM destination addresses.  The rationale is that EXCLUDE
   mode does not apply to SSM addresses, and an SSM-aware router will
   ignore MODE_IS_EXCLUDE and CHANGE_TO_EXCLUDE_MODE requests in the SSM
   range, as described below.

2.2.3.  IGMPv1 Queries, IGMPv2 and MLDv1 General Queries

   If an IGMPv1 Query, or an IGMPv2 or MLDv1 General Query is received,
   the SFGMP protocol specifications require the host to revert to the
   older (IGMPv1, IGMPv2, or MLDv1) mode of operation on that interface.
   If this occurs, the host will stop reporting source-specific
   subscriptions on that interface and will start using IGMPv1, IGMPv2,
   or MLDv1 to report interest in all SSM destination addresses,
   unqualified by a source address.  As a result, SSM semantics will no
   longer be applied to the multicast group address by the router.

   A router compliant with this document would never generate an IGMPv1,
   IGMPv2, or MLDv1 query for an address in the SSM range; thus, this
   situation only occurs either if the router is not SSM-aware, or if
   the host and the router disagree about the SSM address range (for
   instance, if they have inconsistent manual configurations).

   A host SHOULD log an error if it receives an IGMPv1, IGMPv2, or MLDv1
   query for an SSM address (MODIFICATION).

   In order to mitigate this problem, it must be administratively
   assured that all routers on a given shared-medium network are
   compliant with this document and are in agreement about the SSM
   address range.

2.2.4.  IGMPv2 Leave and MLDv1 Done

   IGMP Leave and MLD Done messages are not processed by hosts.  IGMPv2
   Leave and MLDv1 Done messages should not be sent for an SSM address,
   unless the sending host has reverted to older-version compatibility
   mode, with all the caveats described above.

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2.2.5.  IGMPv2 and MLDv1 Group Specific Query

   If a host receives an IGMPv2 or MLDv1 Group Specific Query for an
   address in any configured source-specific range, it should process
   the query normally, as per [IGMPv3, MLDv2], even if the group queried
   is a source-specific destination address.  The transmission of such a
   query likely indicates either that the sending router is not
   compliant with this document or that it is not configured with the
   same SSM address range(s) as the receiving host.  A host SHOULD log
   an error in this case (MODIFICATION).

2.2.6.  IGMPv3 and MLDv2 Group-Specific Query

   If an SSM-aware host receives an SFGMP Group-Specific Query for an
   SSM address, it must respond with a report if the group matches the
   source-specific destination address of any of its subscribed source-
   specific channels, as specified in [IGMPv3, MLDv2].

   The rationale for this is that, although in the current SFGMP
   protocol specifications a router would have no reason to send one,
   the semantics of such a query are well-defined in this range and
   future implementations may have reason to send such a query.  Be
   liberal in what you accept.

2.2.7.  IGMPv3 and MLDv2 Group-and-Source-Specific Query

   An SFGMP router typically uses a Group-and-Source-Specific Query to
   query an SSM channel that a host has requested to leave via a
   BLOCK_OLD_SOURCES record.  A host must respond to a Group-and-
   Source-Specific Query for which the group and source in the query
   match any channel for which the host has a subscription, as required
   by [IGMPv3, MLDv2].  The use of an SSM address does not change this
   behavior.

   A host must be able to process a query with multiple sources listed
   per group, again as required by [IGMPv3, MLDv2].  The use of an SSM
   address does not modify the behavior of the SFGMPs in this regard.

3.  Router Requirements for Source-Specific Multicast

   Routers must be aware of the SSM address range in order to provide
   the SSM service model.  A router that knows the SSM address range and
   is capable of applying SSM semantics to it as described in this
   section is described as an "SSM-aware" router.  An SSM-aware router
   MAY have a configuration option to apply SSM semantics to addresses
   other than the IANA-allocated range, but if such an option exists, it
   MUST default to the IANA-allocated range.

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   This section documents the behavior of routers with respect to the
   following types of SFGMP messages for source-specific destination
   addresses:

      - IGMPv3 and MLDv2 Reports (3.1)
      - IGMPv3 and MLDv2 General Query (3.2)
      - IGMPv3 and MLDv2 Group-Specific Query (3.3)
      - IGMPv3 and MLDv2 Group-and-Source Specific Query (3.4)
      - IGMPv1/v2 and MLDv1 Reports (3.5)
      - IGMPv1/v2 and MLDv1 Queries (3.6)
      - IGMPv2 Leave and MLDv1 Done (3.7)

3.1.  IGMPv3 and MLDv2 Reports

   SFGMP Reports are used to report source-specific subscriptions in the
   SSM address range.  A router SHOULD ignore a group record of either
   of the following types if it refers to an SSM destination address:

         - MODE_IS_EXCLUDE Current-State Record

         - CHANGE_TO_EXCLUDE_MODE Filter-Mode-Change Record

   A router MAY choose to log an error in either case.  It MUST process
   any other group records within the same report.  These behaviors are
   MODIFICATIONS to [IGMPv3, MLDv2] to prevent non-source-specific
   semantics from being applied to SSM addresses, and to avoid reverting
   to older-version compatibility mode.

   A CHANGE_TO_INCLUDE_MODE Filter-Mode-Change Record is processed per
   the normal SFGMP rules; Section 2.2.2 describes a legitimate scenario
   when this could occur.

3.2.  IGMPv3 and MLDv2 General Queries

   An SSM router sends periodic SFGMP General Queries as per the IGMPv3
   and MLDv2 specifications.  No change in behavior is required for SSM.

3.3.  IGMPv3 and MLDv2 Group-Specific Queries

   SFGMP routers that support source-specific multicast may send group-
   specific queries for addresses in the source-specific range.  This
   specification does not explicitly prohibit such a message, although,
   at the time of this writing, a router conformant to [IGMPv3, MLDv2]
   would not send one.

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3.4.  IGMPv3 and MLDv2 Group-and-Source-Specific Queries

   SFGMP Group-and-Source-Specific Queries are used when a receiver has
   indicated that it is no longer interested in receiving traffic from a
   particular (S,G) pair to determine if there are any remaining
   directly-attached hosts with interest in that (S,G) pair.  Group-
   and-Source-Specific Queries are used within the source-specific
   address range when a router receives a BLOCK_OLD_SOURCES Record for
   one or more source-specific groups.  These queries are sent normally,
   as per [IGMPv3, MLDv2].

3.5.  IGMPv1/v2 and MLDv1 Reports

   An IGMPv1/v2 or MLDv1 report for an address in the source-specific
   range could be sent by a non-SSM-aware host.  A router SHOULD ignore
   all such reports and specifically SHOULD NOT use them to establish IP
   forwarding state.  This is a MODIFICATION to [IGMPv3, MLDv2].  A
   router MAY log an error if it receives such a report (also a
   MODIFICATION).

3.6.  IGMPv1/v2 and MLDv1 Queries

   An SFGMP router that loses the querier election to a lower version
   router must log an error, as specified by [IGMPv3, MLDv2].

3.7.  IGMPv2 Leave and MLDv1 Done

   An IGMPv2 Leave or MLDv1 Done message may be sent by a non-SSM-aware
   host.  A router SHOULD ignore all such messages in the source-
   specific address range and MAY log an error (MODIFICATION).

4.  Security Considerations

   The specific protocol modifications described in this document are
   not known to create any security concerns that are not already
   present when IGMPv3 or MLDv2 is used with ASM-style multicast.  The
   reader is referred to [SSM] for an analysis of SSM-specific security
   issues.

   It is important that a router not accept non-source-specific
   reception requests for an SSM destination address.  The rules of
   [IGMPv3] and [MLDv2] require a router, upon receiving such a
   membership report, to revert to earlier version compatibility mode
   for the group in question.  If the router were to revert in this
   situation, it would prevent an IGMPv3-capable host from receiving SSM
   service for that destination address, thus creating a potential for
   an attacker to deny SSM service to other hosts on the same link.

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5.  Acknowledgements

   The authors would like to thank Vince Laviano, Nidhi Bhaskar, Steve
   Deering, Toerless Eckert, and Pekka Savola for their input and
   careful review.

6.  Normative References

   [IGMPv2]     Fenner, W., "Internet Group Management Protocol, Version
                2", RFC 2236, November 1997.

   [IGMPv3]     Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
                Thyagarajan, "Internet Group Management Protocol,
                Version 3", RFC 3376, October 2002.

   [MSFAPI]     Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
                Extensions for Multicast Source Filters", RFC 3678,
                January 2004.

   [RFC1112]    Deering, S., "Host extensions for IP multicasting", STD
                5, RFC 1112, August 1989.

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SSM]        Holbrook, H. and B. Cain, "Source-Specific Multicast for
                IP", RFC 4607, August 2006.

   [MLDv2]      Vida, R. and L. Costa, "Multicast Listener Discovery
                Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC2710]    Deering, S., Fenner, W., and B. Haberman, "Multicast
                Listener Discovery (MLD) for IPv6", RFC 2710, October
                1999.

8.  Informative References

   [IANA-ALLOC] Internet Assigned Numbers Authority,
                http://www.iana.org/assignments/multicast-addresses.

   [RFC3306]    Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
                Multicast Addresses", RFC 3306, August 2002.

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Authors' Addresses

   Hugh Holbrook
   Arastra, Inc.
   P.O. Box 10905
   Palo Alto, CA 94303

   Phone: +1 650 331-1620
   EMail: holbrook@arastra.com

   Brad Cain
   Acopia Networks

   EMail: bcain99@gmail.com

   Brian Haberman
   Johns Hopkins University Applied Physics Lab
   11100 Johns Hopkins Road
   Laurel, MD  20723-6099

   EMail: brian@innovationslab.net

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