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Network Working Group H. Holbrook Request for Comments: 4607 Arastra, Inc. Category: Standards Track B. Cain

                                                       Acopia Networks
                                                           August 2006
                  Source-Specific Multicast for IP

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).


 IP version 4 (IPv4) addresses in the 232/8 ( to range are designated as source-specific multicast
 (SSM) destination addresses and are reserved for use by source-
 specific applications and protocols.  For IP version 6 (IPv6), the
 address prefix FF3x::/32 is reserved for source-specific multicast
 use.  This document defines an extension to the Internet network
 service that applies to datagrams sent to SSM addresses and defines
 the host and router requirements to support this extension.

Holbrook & Cain Standards Track [Page 1] RFC 4607 Source-Specific Multicast August 2006

Table of Contents

 1. Introduction ....................................................3
 2. Semantics of Source-Specific Multicast Addresses ................5
 3. Terminology .....................................................6
 4. Host Requirements ...............................................7
    4.1. Extensions to the IP Module Interface ......................7
    4.2. Requirements on the Host IP Module .........................8
    4.3. Allocation of Source-Specific Multicast Addresses ..........9
 5. Router Requirements ............................................10
    5.1. Packet Forwarding .........................................10
    5.2. Protocols .................................................10
 6. Link-Layer Transmission of Datagrams ...........................11
 7. Security Considerations ........................................12
    7.1. IPsec and SSM .............................................12
    7.2. SSM and RFC 2401 IPsec Caveats ............................12
    7.3. Denial of Service .........................................13
    7.4. Spoofed Source Addresses ..................................13
    7.5. Administrative Scoping ....................................14
 8. Transition Considerations ......................................14
 9. IANA Considerations ............................................15
 10. Acknowledgements ..............................................15
 11. Normative References ..........................................16
 12. Informative References ........................................17

Holbrook & Cain Standards Track [Page 2] RFC 4607 Source-Specific Multicast August 2006

1. Introduction

 The Internet Protocol (IP) multicast service model is defined in RFC
 1112 [RFC1112].  RFC 1112 specifies that a datagram sent to an IP
 multicast address ( through G is delivered
 to each "upper-layer protocol module" that has requested reception of
 datagrams sent to address G.  RFC 1112 calls the network service
 identified by a multicast destination address G a "host group".  This
 model supports both one-to-many and many-to-many group communication.
 This document uses the term "Any-Source Multicast" (ASM) to refer to
 model of multicast defined in RFC 1112.  RFC 3513 [RFC3513] specifies
 the form of IPv6 multicast addresses with ASM semantics.
 IPv4 addresses in the 232/8 ( to range are
 currently designated as source-specific multicast (SSM) destination
 addresses and are reserved for use by source-specific applications
 and protocols [IANA-ALLOC].
 For IPv6, the address prefix FF3x::/32 is reserved for source-
 specific multicast use, where 'x' is any valid scope identifier, by
 [IPv6-UBM].  Using the terminology of [IPv6-UBM], all SSM addresses
 must have P=1, T=1, and plen=0.  [IPv6-MALLOC] mandates that the
 network prefix field of an SSM address also be set to zero, hence all
 SSM addresses fall in the FF3x::/96 range.  Future documents may
 allow a non-zero network prefix field if, for instance, a new IP-
 address-to-MAC-address mapping is defined.  Thus, address allocation
 should occur within the FF3x::/96 range, but a system should treat
 all of FF3x::/32 as SSM addresses, to allow for compatibility with
 possible future uses of the network prefix field.
 Addresses in the range FF3x::4000:0001 through FF3x::7FFF:FFFF are
 reserved in [IPv6-MALLOC] for allocation by IANA.  Addresses in the
 range FF3x::8000:0000 through FF3x::FFFF:FFFF are allowed for dynamic
 allocation by a host, as described in [IPv6-MALLOC].  Addresses in
 the range FF3x::0000:0000 through FF3x::3FFF:FFFF are invalid IPv6
 SSM addresses.  ([IPv6-MALLOC] indicates that FF3x::0000:0001 to
 FF3x::3FFF:FFFF must set P=0 and T=0, but for SSM, [IPv6-UBM]
 mandates that  P=1 and T=1, hence their designation as invalid.)  The
 treatment of a packet sent to such an invalid address is undefined --
 a router or host MAY choose to drop such a packet.
 Source-specific multicast delivery semantics are provided for a
 datagram sent to an SSM address.  That is, a datagram with source IP
 address S and SSM destination address G is delivered to each upper-
 layer "socket" that has specifically requested the reception of
 datagrams sent to address G by source S, and only to those sockets.
 The network service identified by (S,G), for SSM address G and source

Holbrook & Cain Standards Track [Page 3] RFC 4607 Source-Specific Multicast August 2006

 host address S, is referred to as a "channel".  In contrast to the
 ASM model of RFC 1112, SSM provides network-layer support for one-
 to-many delivery only.
 The benefits of source-specific multicast include:
    Elimination of cross-delivery of traffic when two sources
    simultaneously use the same source-specific destination address.
    The simultaneous use of an SSM destination address by multiple
    sources and different applications is explicitly supported.
    Avoidance of the need for inter-host coordination when choosing
    source-specific addresses, as a consequence of the above.
    Avoidance of many of the router protocols and algorithms that are
    needed to provide the ASM service model.  For instance, the
    "shared trees" and Rendezvous Points of the PIM - Sparse Mode
    (PIM-SM) protocol [PIM-SM] are not necessary to support the
    source-specific model.  The router mechanisms required to support
    SSM are in fact largely a subset of those that are used to support
    ASM.  For example, the shortest-path tree mechanism of the PIM-SM
    protocol can be adapted to provide SSM semantics.
 Like ASM, the set of receivers is unknown to an SSM sender.  An SSM
 source is provided with neither the identity of receivers nor their
 SSM is particularly well-suited to dissemination-style applications
 with one or more senders whose identities are known before the
 application begins.  For instance, a data dissemination application
 that desires to provide a secondary data source in case the primary
 source fails over might implement this by using one channel for each
 source and advertising both of them to receivers.  SSM can be used to
 build multi-source applications where all participants' identities
 are not known in advance, but the multi-source "rendezvous"
 functionality does not occur in the network layer in this case.  Just
 like in an application that uses unicast as the underlying transport,
 this functionality can be implemented by the application or by an
 application-layer library.
 Multicast resource discovery of the form in which a client sends a
 multicast query directly to a "service location group" to which
 servers listen is not directly supported by SSM.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [RFC2119].

Holbrook & Cain Standards Track [Page 4] RFC 4607 Source-Specific Multicast August 2006

 This document defines the semantics of source-specific multicast
 addresses and specifies the policies governing their use.  In
 particular, it defines an extension to the Internet network service
 that applies to datagrams sent to SSM addresses and defines host
 extensions to support the network service.  Hosts, routers,
 applications, and protocols that use these addresses MUST comply with
 the policies outlined in this document.  Failure of a host to comply
 may prevent that host or other hosts on the same LAN from receiving
 traffic sent to an SSM channel.  Failure of a router to comply may
 cause SSM traffic to be delivered to parts of the network where it is
 unwanted, unnecessarily burdening the network.

2. Semantics of Source-Specific Multicast Addresses

 The source-specific multicast service is defined as follows:
    A datagram sent with source IP address S and destination IP
    address G in the SSM range is delivered to each host socket that
    has specifically requested delivery of datagrams sent by S to G,
    and only to those sockets.
 Where, using the terminology of [IGMPv3],
    "socket" is an implementation-specific parameter used to
    distinguish among different requesting entities (e.g., programs or
    processes or communication end-points within a program or process)
    within the requesting host; the socket parameter of BSD Unix
    system calls is a specific example.
 Any host may send a datagram to any SSM address, and delivery is
 provided according to the above semantics.
 The IP module interface to upper-layer protocols is extended to allow
 a socket to "Subscribe" to or "Unsubscribe" from a particular channel
 identified by an SSM destination address and a source IP address.
 The extended interface is defined in Section 4.1.  It is meaningless
 for an application or host to request reception of datagrams sent to
 an SSM destination address G, as is supported in the any-source
 multicast model, without also specifying a corresponding source
 address, and routers MUST ignore any such request.
 Multiple source applications on different hosts can use the same SSM
 destination address G without conflict because datagrams sent by each
 source host Si are delivered only to those sockets that requested
 delivery of datagrams sent to G specifically by Si.

Holbrook & Cain Standards Track [Page 5] RFC 4607 Source-Specific Multicast August 2006

 The key distinguishing property of the model is that a channel is
 identified (addressed) by the combination of a unicast source address
 and a multicast destination address in the SSM range.  So, for
 example, the channel
    S,G = (,
 differs from
    S,G = (,,
 even though they have the same destination address portion.
 Similarly, for IPv6,
    S,G = (2001:3618::1, FF33::1234)
    S,G = (2001:3618::2, FF33::1234)
 are different channels.

3. Terminology

 To reduce confusion when talking about the any-source and source-
 specific multicast models, we use different terminology when
 discussing them.
 We use the term "channel" to refer to the service associated with an
 SSM address.  A channel is identified by the combination of an SSM
 destination address and a specific source, e.g., an (S,G) pair.
 We use the term "host group" (used in RFC 1112) to refer to the
 service associated with "regular" ASM multicast addresses (excluding
 those in the SSM range).  A host group is identified by a single
 multicast address.
 Any host can send to a host group, and similarly, any host can send
 to an SSM destination address.  A packet sent by a host S to an ASM
 destination address G is delivered to the host group identified by G.
 A packet sent by host S to an SSM destination address G is delivered
 to the channel identified by (S,G).  The receiver operations allowed
 on a host group are called "join(G)" and "leave(G)" (as per RFC
 1112).  The receiver operations allowed on a channel are called
 "Subscribe(S,G)" and "Unsubscribe(S,G)".

Holbrook & Cain Standards Track [Page 6] RFC 4607 Source-Specific Multicast August 2006

 The following table summarizes the terminology:
    Service Model:        any-source          source-specific
    Network Abstraction:  group               channel
    Identifier:           G                   S,G
    Receiver Operations:  Join, Leave         Subscribe, Unsubscribe
 We note that, although this document specifies a new service model
 available to applications, the protocols and techniques necessary to
 support the service model are largely a subset of those used to
 support ASM.

4. Host Requirements

 This section describes requirements on hosts that support source-
 specific multicast, including:
  1. Extensions to the IP Module Interface
  1. Extensions to the IP Module
  1. Allocation of SSM Addresses

4.1. Extensions to the IP Module Interface

 The IP module interface to upper-layer protocols is extended to allow
 protocols to request reception of all datagrams sent to a particular
    Subscribe ( socket, source-address, group-address, interface )
    Unsubscribe ( socket, source-address, group-address, interface )
    "socket" is as previously defined in Section 2,
 and, paraphrasing [IGMPv3],
    "interface" is a local identifier of the network interface on
    which reception of the channel identified by the (source-
    address,group-address) pair is to be enabled or disabled.  A
    special value may be used to indicate a "default" interface.  If
    reception of the same channel is desired on multiple interfaces,
    Subscribe is invoked once for each.

Holbrook & Cain Standards Track [Page 7] RFC 4607 Source-Specific Multicast August 2006

 The above are strictly abstract functional interfaces -- the
 functionality can be provided in an implementation-specific way.  On
 a host that supports the multicast source filtering application
 programming interface of [MSFAPI], for instance, the Subscribe and
 Unsubscribe interfaces may be supported via that API.  When a host
 has been configured to know the SSM address range (whether the
 configuration mechanism is manual or through a protocol), the host's
 operating system SHOULD return an error to an application that makes
 a non-source-specific request to receive multicast sent to an SSM
 destination address.
 A host that does not support these IP module interfaces (e.g., ASM-
 only hosts) and their underlying protocols cannot expect to reliably
 receive traffic sent on an SSM channel.  As specified below in
 Section 5.2, routers will not set up SSM forwarding state or forward
 datagrams in response to an ASM join request.
 Widespread implementations of the IP packet reception interface
 (e.g., the recvfrom() system call in BSD Unix) do not allow a
 receiver to determine the destination address to which a datagram was
 sent.  On a host with such an implementation, the destination address
 of a datagram cannot be inferred when the socket on which the
 datagram is received is Subscribed to multiple channels.  Host
 operating systems SHOULD provide a way for a host to determine both
 the source and the destination address to which a datagram was sent.
 (As one example, the Linux operating system provides the destination
 of a packet as part of the response to the recvmsg() system call.)
 Until this capability is present, applications may be forced to use
 higher-layer mechanisms to identify the channel to which a datagram
 was sent.

4.2. Requirements on the Host IP Module

 An incoming datagram destined to an SSM address MUST be delivered by
 the IP module to all sockets that have indicated (via Subscribe) a
 desire to receive data that matches the datagram's source address,
 destination address, and arriving interface.  It MUST NOT be
 delivered to other sockets.
 When the first socket on host H subscribes to a channel (S,G) on
 interface I, the host IP module on H sends a request on interface I
 to indicate to neighboring routers that the host wishes to receive
 traffic sent by source S to source-specific multicast destination G.
 Similarly, when the last socket on a host unsubscribes from a channel
 on interface I, the host IP module sends an unsubscription request
 for that channel to interface I.

Holbrook & Cain Standards Track [Page 8] RFC 4607 Source-Specific Multicast August 2006

 These requests will typically be Internet Group Management Protocol
 version 3 (IGMPv3) messages for IPv4, or Multicast Listener Discovery
 Version 2 (MLDv2) messages for IPv6 [IGMPv3,MLDv2].  A host that
 supports the SSM service model MUST implement the host portion of
 [IGMPv3] for IPv4 and [MLDv2] for IPv6.  It MUST also conform to the
 IGMPv3/MLDv2 behavior described in [GMP-SSM].

4.3. Allocation of Source-Specific Multicast Addresses

 The SSM destination address is reserved, and it must not be
 used as a destination address.  Similarly, FF3x::4000:0000 is also
 reserved.  The goal of reserving these two addresses is to preserve
 one invalid SSM destination for IPv4 and IPv6, which can be useful in
 an implementation as a null value.  The address range - is currently reserved for allocation by IANA.  SSM
 destination addresses in the range FF3x::4000:0001 through
 FF3x::7FFF:FFFF are similarly reserved for IANA allocation
 [IPv6-MALLOC].  The motivation to reserve these addresses is outlined
 below in Section 9, "IANA Considerations".
 The policy for allocating the rest of the SSM addresses to sending
 applications is strictly locally determined by the sending host.
 When allocating SSM addresses dynamically, a host or host operating
 system MUST NOT allocate sequentially starting at the first allowed
 address.  It is RECOMMENDED to allocate SSM addresses to applications
 randomly, while ensuring that allocated addresses are not given
 simultaneously to multiple applications (and avoiding the reserved
 addresses).  For IPv6, the randomization should apply to the lowest
 31 bits of the address.
 As described in Section 6, the mapping of an IP packet with SSM
 destination address onto a link-layer multicast address does not take
 into account the datagram's source IP address (on commonly-used link
 layers like Ethernet).  If all hosts started at the first allowed
 address, then with high probability, many source-specific channels on
 shared-medium local area networks would use the same link-layer
 multicast address.  As a result, traffic destined for one channel
 subscriber would be delivered to another's IP module, which would
 then have to discard the datagram.
 A host operating system SHOULD provide an interface to allow an
 application to request a unique allocation of a channel destination
 address in advance of a session's commencement, and this allocation
 database SHOULD persist across host reboots.  By providing persistent
 allocations, a host application can advertise the session in advance

Holbrook & Cain Standards Track [Page 9] RFC 4607 Source-Specific Multicast August 2006

 of its start time on a web page or in another directory.  (We note
 that this issue is not specific to SSM applications -- the same
 problem arises for ASM.)
 This document neither defines the interfaces for requesting or
 returning addresses nor specifies the host algorithms for storing
 those allocations.  One plausible abstract API is defined in RFC 2771
 [RFC2771].  Note that RFC 2771 allows an application to request an
 address within a specific range of addresses.  If this interface is
 used, the starting address of the range SHOULD be selected at random
 by the application.
 For IPv6, administratively scoped SSM channel addresses are created
 by choosing an appropriate scope identifier for the SSM destination
 address.  Normal IPv6 multicast scope boundaries [SCOPINGv6] are
 applied to traffic sent to an SSM destination address, including any
 relevant boundaries applied to both the source and destination
 No globally agreed-upon administratively-scoped address range
 [ADMIN-SCOPE] is currently defined for IPv4 source-specific
 multicast.  For IPv4, administrative scoping of SSM addresses can be
 implemented within an administrative domain by filtering outgoing SSM
 traffic sent to a scoped address at the domain's boundary routers.

5. Router Requirements

5.1. Packet Forwarding

 A router that receives an IP datagram with a source-specific
 destination address MUST silently drop it unless a neighboring host
 or router has communicated a desire to receive packets sent from the
 source and to the destination address of the received packet.

5.2. Protocols

 Certain IP multicast routing protocols already have the ability to
 communicate source-specific joins to neighboring routers (in
 particular, PIM-SM [PIM-SM]), and these protocols can, with slight
 modifications, be used to provide source-specific semantics.  A
 router that supports the SSM service model MUST implement the PIM-SSM
 subset of the PIM-SM protocol from [PIM-SM] and MUST implement the
 router portion of [IGMPv3] for IPv4 and [MLDv2] for IPv6.  An SSM
 router MUST also conform to the IGMPv3/MLDv2 behavior described in

Holbrook & Cain Standards Track [Page 10] RFC 4607 Source-Specific Multicast August 2006

 With PIM-SSM, successful establishment of an (S,G) forwarding path
 from the source S to any receiver depends on hop-by-hop forwarding of
 the explicit join request from the receiver toward the source.  The
 protocol(s) and algorithms that are used to select the forwarding
 path for this explicit join must provide a loop-free path.  When
 using PIM-SSM, the PIM-SSM implementation MUST (at least) support the
 ability to use the unicast topology database for this purpose.
 A network can concurrently support SSM in the SSM address range and
 any-source multicast in the rest of the multicast address space, and
 it is expected that this will be commonplace.  In such a network, a
 router may receive a non-source-specific, or "(*,G)" in conventional
 terminology, request for delivery of traffic in the SSM range from a
 neighbor that does not implement source-specific multicast in a
 manner compliant with this document.  A router that receives such a
 non-source-specific request for data in the SSM range MUST NOT use
 the request to establish forwarding state and MUST NOT propagate the
 request to other neighboring routers.  A router MAY log an error in
 such a case.  This applies both to any request received from a host
 (e.g., an IGMPv1 or IGMPv2 [IGMPv2] host report) and to any request
 received from a routing protocol (e.g., a PIM-SM (*,G) join).  The
 inter-router case is further discussed in Section 8, "Transition
 It is essential that all routers in the network give source-specific
 semantics to the same range of addresses in order to achieve the full
 benefit of SSM.  To comply with this specification, a router MUST
 treat ALL IANA-allocated SSM addresses with source-specific

6. Link-Layer Transmission of Datagrams

 Source-specific multicast packets are transmitted on link-layer
 networks as specified in RFC 1112 for IPv4 and as in [ETHERv6] for
 IPv6.  On most shared-medium link-layer networks that support
 multicast (e.g., Ethernet), the IP source address is not used in the
 selection of the link-layer destination address.  Consequently, on
 such a network, all packets sent to destination address G will be
 delivered to any host that has subscribed to any channel (S,G),
 regardless of S.  Therefore, the IP module MUST filter packets it
 receives from the link layer before delivering them to the socket

Holbrook & Cain Standards Track [Page 11] RFC 4607 Source-Specific Multicast August 2006

7. Security Considerations

 This section outlines security issues pertaining to SSM.  The
 following topics are addressed: IPsec, denial-of-service attacks,
 source spoofing, and security issues related to administrative

7.1. IPsec and SSM

 The IPsec Authentication Header (AH) and Encapsulating Security
 Payload (ESP) can be used to secure SSM traffic, if a multicast-
 capable implementation of IPsec (as required in [RFC4301]) is used by
 the receivers.

7.2. SSM and RFC 2401 IPsec Caveats

 For existing implementations of RFC 2401 IPsec (now superseded by
 [RFC4301]), there are a few caveats related to SSM.  They are listed
 here.  In RFC 2401 IPsec, the source address is not used as part of
 the key in the SAD lookup.  As a result, two senders that happen to
 use the same SSM destination address and the same Security Parameter
 Index (SPI) will "collide" in the SAD at any host that is receiving
 both channels.  Because the channel addresses and SPIs are both
 allocated autonomously by the senders, there is no reasonable means
 to ensure that each sender uses a unique destination address or SPI.
 A problem arises if a receiver subscribes simultaneously to two
 unrelated channels using IPsec whose sources happen to be using the
 same IP destination address (IPDA) and the same IPsec SPI.  Because
 the channel destination addresses are allocated autonomously by the
 senders, any two hosts can simultaneously use the same destination
 address, and there is no reasonable means to ensure that this does
 not happen.  The <IPDA,SPI> tuple, however, consists of 56 bits that
 are generally randomly chosen (24 bits of the IP destination and 32
 bits of the SPI), and a conflict is unlikely to occur through random
 If such a collision occurs, a receiver will not be able to
 simultaneously receive IPsec-protected traffic from the two colliding
 sources.  A receiver can detect this condition by noticing that it is
 receiving traffic from two different sources with the same SPI and
 the same SSM destination address.

Holbrook & Cain Standards Track [Page 12] RFC 4607 Source-Specific Multicast August 2006

7.3. Denial of Service

 A subscription request creates (S,G) state in a router to record the
 subscription, invokes processing on that router, and possibly causes
 processing at neighboring routers.  A host can mount a denial-of-
 service attack by requesting a large number of subscriptions.  Denial
 of service can result if:
  1. a large amount of traffic arrives when it was otherwise

undesired, consuming network resources to deliver it and host

      resources to drop it;
  1. a large amount of source-specific multicast state is created in

network routers, using router memory and CPU resources to store

      and process the state; or
  1. a large amount of control traffic is generated to manage the

source-specific state, using router CPU and network bandwidth.

 To reduce the damage from such an attack, a router MAY have
 configuration options to limit, for example, the following items:
  1. The total rate at which all hosts on any one interface are

allowed to initiate subscriptions (to limit the damage caused by

      forged source-address attacks).
  1. The total number of subscriptions that can be initiated from any

single interface or host.

 Any decision by an implementor to artificially limit the rate or
 number of subscriptions should be taken carefully, however, as future
 applications may use large numbers of channels.  Tight limits on the
 rate or number of channel subscriptions would inhibit the deployment
 of such applications.
 A router SHOULD verify that the source of a subscription request is a
 valid address for the interface on which it was received.  Failure to
 do so would exacerbate a spoofed-source address attack.
 We note that these attacks are not unique to SSM -- they are also
 present for any-source multicast.

7.4. Spoofed Source Addresses

 By forging the source address in a datagram, an attacker can
 potentially violate the SSM service model by transmitting datagrams
 on a channel belonging to another host.  Thus, an application
 requiring strong authentication should not assume that all packets

Holbrook & Cain Standards Track [Page 13] RFC 4607 Source-Specific Multicast August 2006

 that arrive on a channel were sent by the requested source without
 higher-layer authentication mechanisms.  The IPSEC Authentication
 Header [RFC2401, RFC4301] may be used to authenticate the source of
 an SSM transmission, for instance.
 Some degree of protection against spoofed source addresses in
 multicast is already fairly widespread, because the commonly deployed
 IP multicast routing protocols [PIM-DM, PIM-SM, DVMRP] incorporate a
 "reverse-path forwarding check" that validates that a multicast
 packet arrived on the expected interface for its source address.
 Routing protocols used for SSM SHOULD incorporate such a check.
 Source Routing [RFC791] (both Loose and Strict) in combination with
 source address spoofing may be used to allow an impostor of the true
 channel source to inject packets onto an SSM channel.  An SSM router
 SHOULD by default disallow source routing to an SSM destination
 address.  A router MAY have a configuration option to allow source
 routing.  Anti-source spoofing mechanisms, such as source address
 filtering at the edges of the network, are also strongly encouraged.

7.5. Administrative Scoping

 Administrative scoping should not be relied upon as a security
 measure [ADMIN-SCOPE]; however, in some cases it is part of a
 security solution.  It should be noted that no administrative scoping
 exists for IPv4 source-specific multicast.  An alternative approach
 is to manually configure traffic filters to create such scoping if
 Furthermore, for IPv6, neither source nor destination address scoping
 should be used as a security measure.  In some currently-deployed
 IPv6 routers (those that do not conform to [SCOPINGv6]), scope
 boundaries are not always applied to all source address (for
 instance, an implementation may filter link-local addresses but
 nothing else).  Such a router may incorrectly forward an SSM channel
 (S,G) through a scope boundary for S.

8. Transition Considerations

 A host that complies with this document will send ONLY source-
 specific host reports for addresses in the SSM range.  As stated
 above, a router that receives a non-source-specific (e.g., IGMPv1 or
 IGMPv2 or MLDv1 [RFC2710]) host report for a source-specific
 multicast destination address MUST ignore these reports.  Failure to
 do so would violate the SSM service model promised to the sender:
 that a packet sent to (S,G) would only be delivered to hosts that
 specifically requested delivery of packets sent to G by S.

Holbrook & Cain Standards Track [Page 14] RFC 4607 Source-Specific Multicast August 2006

 During a transition period, it would be possible to deliver SSM
 datagrams in a domain where the routers do not support SSM semantics
 by simply forwarding any packet destined to G to all hosts that have
 requested subscription of (S,G) for any S.  However, this
 implementation risks unduly burdening the network infrastructure by
 delivering (S,G) datagrams to hosts that did not request them.  Such
 an implementation for addresses in the SSM range is specifically not
 compliant with Section 5.2 of this document.

9. IANA Considerations

 IANA allocates IPv4 addresses in the range through and IPv6 addresses in the range FF3x:4000:0001 to
 FF3x::7FFF:FFFF.  These addresses are allocated according to IETF
 Consensus [IANA-CONSID].  These address ranges are reserved for
 services with wide applicability that either require that or would
 strongly benefit if all hosts use a well-known SSM destination
 address for that service.  Any proposal for allocation must consider
 the fact that, on an Ethernet network, all datagrams sent to any SSM
 destination address will be transmitted with the same link-layer
 destination address, regardless of the source.  Furthermore, the fact
 that SSM destinations in and use the same
 link-layer addresses as the reserved IP multicast group range must also be considered.  Similar consideration should
 be given to the IPv6 reserved multicast addresses. and
 FF3x::4000:0000 should not be allocated, as suggested above.
 Except for the aforementioned addresses, IANA SHALL NOT allocate any
 SSM destination address to a particular entity or application.  To do
 so would compromise one of the important benefits of the source-
 specific model: the ability for a host to simply and autonomously
 allocate a source-specific multicast address from a large flat
 address space.

10. Acknowledgements

 The SSM service model draws on a variety of prior work on alternative
 approaches to IP multicast, including the EXPRESS multicast model of
 Holbrook and Cheriton [EXPRESS], Green's [SMRP], and the Simple
 Multicast proposal of Perlman, et al. [SIMPLE].  We would also like
 to thank Jon Postel and David Cheriton for their support in
 reassigning the 232/8 address range to SSM.  Brian Haberman
 contributed to the IPv6 portion of this document.  Thanks to Pekka
 Savola for a careful review.

Holbrook & Cain Standards Track [Page 15] RFC 4607 Source-Specific Multicast August 2006

11. Normative References

 [ETHERv6]     Crawford, M., "Transmission of IPv6 Packets over
               Ethernet Networks", RFC 2464, December 1998.
 [GMP-SSM]     Holbrook, H. and B. Cain, "Using Internet Group
               Management Protocol Version 3 (IGMPv3) and Multicast
               Listener Discovery Protocol Version 2 (MLDv2) for
               Source-Specific Multicast", RFC 4604, August 2006.
 [IGMPv3]      Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
               Thyagarajan, "Internet Group Management Protocol,
               Version 3", RFC 3376, October 2002.
 [IPv6-UBM]    Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
               Multicast Addresses", RFC 3306, August 2002.
 [IPv6-MALLOC] Haberman, B., "Allocation Guidelines for IPv6 Multicast
               Addresses", RFC 3307, August 2002.
 [MLDv2]       Vida, R. and L. Costa, "Multicast Listener Discovery
               Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
 [PIM-SM]      Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas.
               "Protocol Independent Multicast - Sparse Mode (PIM-SM):
               Protocol Specification (Revised)", RFC 4601, August
 [RFC791]      Postel, J., "Internet Protocol", STD 5, RFC 791,
               September 1981.
 [RFC1112]     Deering, S., "Host extensions for IP multicasting", STD
               5, RFC 1112, August 1989.
 [RFC2401]     Kent, S. and R. Atkinson, "Security Architecture for
               the Internet Protocol", RFC 2401, November 1998.
 [RFC3513]     Hinden, R. and S. Deering, "Internet Protocol Version 6
               (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.

Holbrook & Cain Standards Track [Page 16] RFC 4607 Source-Specific Multicast August 2006

12. Informative References

 [ADMIN-SCOPE] Meyer, D., "Administratively Scoped IP Multicast", BCP
               23, RFC 2365, July 1998.
 [DVMRP]       Waitzman, D., Partridge, C., and S. Deering, "Distance
               Vector Multicast Routing Protocol", RFC 1075, November
 [EXPRESS]     Holbrook, H., and Cheriton, D.  "Explicitly Requested
               Source-Specific Multicast: EXPRESS support for Large-
               scale Single-source Applications."  Proceedings of ACM
               SIGCOMM '99, Cambridge, MA, September 1999.
 [IANA-ALLOC]  Internet Assigned Numbers Authority,
 [IANA-CONSID] Narten, T. and H. Alvestrand, "Guidelines for Writing
               an IANA Considerations Section in RFCs", BCP 26, RFC
               2434, October 1998.
 [IGMPv2]      Fenner, W., "Internet Group Management Protocol,
               Version 2", RFC 2236, November 1997.
 [MSFAPI]      Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
               Extensions for Multicast Source Filters", RFC 3678,
               January 2004.
 [PIM-DM]      Adams, A., Nicholas, J., and W. Siadak, "Protocol
               Independent Multicast - Dense Mode (PIM-DM): Protocol
               Specification (Revised)", RFC 3973, January 2005.
 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2710]     Deering, S., Fenner, W., and B. Haberman, "Multicast
               Listener Discovery (MLD) for IPv6", RFC 2710, October
 [RFC2771]     Finlayson, R., "An Abstract API for Multicast Address
               Allocation", RFC 2771, February 2000.
 [SCOPINGv6]   Deering, S., Haberman, B., Jinmei, T., Nordmark, E.,
               and B. Zill, "IPv6 Scoped Address Architecture", RFC
               4007, March 2005.

Holbrook & Cain Standards Track [Page 17] RFC 4607 Source-Specific Multicast August 2006

 [SIMPLE]      R. Perlman, C-Y. Lee, A. Ballardie, J. Crowcroft, Z.
               Wang, T. Maufer, C. Diot, and M. Green, "Simple
               Multicast: A Design for Simple, Low-Overhead
               Multicast", Work in Progress, October 1999.
 [SMRP]        Green, M.  "Method and System of Multicast Routing for
               Groups with a Single Transmitter."  United States
               Patent Number 5,517,494.

Authors' Addresses

 Brad Cain
 Acopia Networks
 Hugh Holbrook
 Arastra, Inc.
 P.O. Box 10905
 Palo Alto, CA 94303
 Phone: +1 650 331-1620

Holbrook & Cain Standards Track [Page 18] RFC 4607 Source-Specific Multicast August 2006

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Holbrook & Cain Standards Track [Page 19]

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