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rfc:rfc3376

Network Working Group B. Cain Request for Comments: 3376 Cereva Networks Obsoletes: 2236 S. Deering Category: Standards Track I. Kouvelas

                                                         Cisco Systems
                                                             B. Fenner
                                                  AT&T Labs - Research
                                                        A. Thyagarajan
                                                              Ericsson
                                                          October 2002
           Internet Group Management Protocol, Version 3

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 (2002).  All Rights Reserved.

Abstract

 This document specifies Version 3 of the Internet Group Management
 Protocol, IGMPv3.  IGMP is the protocol used by IPv4 systems to
 report their IP multicast group memberships to neighboring multicast
 routers.  Version 3 of IGMP adds support for "source filtering", that
 is, the ability for a system to report interest in receiving packets
 *only* from specific source addresses, or from *all but* specific
 source addresses, sent to a particular multicast address.  That
 information may be used by multicast routing protocols to avoid
 delivering multicast packets from specific sources to networks where
 there are no interested receivers.
 This document obsoletes RFC 2236.

Cain, et. al. Standards Track [Page 1] RFC 3376 IGMPv3 October 2002

Table of Contents

 1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  The Service Interface for Requesting IP Multicast Reception .   3
 3.  Multicast Reception State Maintained by Systems . . . . . . .   5
 4.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .   7
 5.  Description of the Protocol for Group Members . . . . . . . .  19
 6.  Description of the Protocol for Multicast Routers . . . . . .  24
 7.  Interoperation with Older Versions of IGMP. . . . . . . . . .  35
 8.  List of Timers, Counters, and Their Default Values. . . . . .  40
 9.  Security Considerations . . . . . . . . . . . . . . . . . . .  43
 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  47
 12. Normative References. . . . . . . . . . . . . . . . . . . . .  47
 13. Informative References. . . . . . . . . . . . . . . . . . . .  47
     Appendix A. Design Rationale. . . . . . . . . . . . . . . . .  49
     Appendix B. Summary of changes from IGMPv2. . . . . . . . . .  50
     Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  52
     Full Copyright Statement. . . . . . . . . . . . . . . . . . .  53

1. Introduction

 The Internet Group Management Protocol (IGMP) is used by IPv4 systems
 (hosts and routers) to report their IP multicast group memberships to
 any neighboring multicast routers.  Note that an IP multicast router
 may itself be a member of one or more multicast groups, in which case
 it performs both the "multicast router part" of the protocol (to
 collect the membership information needed by its multicast routing
 protocol) and the "group member part" of the protocol (to inform
 itself and other, neighboring multicast routers of its memberships).
 IGMP is also used for other IP multicast management functions, using
 message types other than those used for group membership reporting.
 This document specifies only the group membership reporting functions
 and messages.
 This document specifies Version 3 of IGMP.  Version 1, specified in
 [RFC-1112], was the first widely-deployed version and the first
 version to become an Internet Standard.  Version 2, specified in
 [RFC-2236], added support for "low leave latency", that is, a
 reduction in the time it takes for a multicast router to learn that
 there are no longer any members of a particular group present on an
 attached network.  Version 3 adds support for "source filtering",
 that is, the ability for a system to report interest in receiving
 packets *only* from specific source addresses, as required to support
 Source-Specific Multicast [SSM], or from *all but* specific source
 addresses, sent to a particular multicast address.  Version 3 is
 designed to be interoperable with Versions 1 and 2.

Cain, et. al. Standards Track [Page 2] RFC 3376 IGMPv3 October 2002

 Multicast Listener Discovery (MLD) is used in a similar way by IPv6
 systems.  MLD version 1 [MLD] implements the functionality of IGMP
 version 2; MLD version 2 [MLDv2] implements the functionality of IGMP
 version 3.
 The capitalized 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].  Due to the lack of italics, emphasis is indicated herein
 by bracketing a word or phrase in "*" characters.

2. The Service Interface for Requesting IP Multicast Reception

 Within an IP system, there is (at least conceptually) a service
 interface used by upper-layer protocols or application programs to
 ask the IP layer to enable and disable reception of packets sent to
 specific IP multicast addresses.  In order to take full advantage of
 the capabilities of IGMPv3, a system's IP service interface must
 support the following operation:
    IPMulticastListen ( socket, interface, multicast-address,
                        filter-mode, source-list )
 where:
 o "socket" is an implementation-specific parameter used to
   distinguish among different requesting entities (e.g., programs or
   processes) within the system; the socket parameter of BSD Unix
   system calls is a specific example.
 o "interface" is a local identifier of the network interface on which
   reception of the specified multicast address is to be enabled or
   disabled.  Interfaces may be physical (e.g., an Ethernet interface)
   or virtual (e.g., the endpoint of a Frame Relay virtual circuit or
   the endpoint of an IP-in-IP "tunnel").  An implementation may allow
   a special "unspecified" value to be passed as the interface
   parameter, in which case the request would apply to the "primary"
   or "default" interface of the system (perhaps established by system
   configuration).  If reception of the same multicast address is
   desired on more than one interface, IPMulticastListen is invoked
   separately for each desired interface.
 o "multicast-address" is the IP multicast address, or group, to which
   the request pertains.  If reception of more than one multicast
   address on a given interface is desired, IPMulticastListen is
   invoked separately for each desired multicast address.

Cain, et. al. Standards Track [Page 3] RFC 3376 IGMPv3 October 2002

 o "filter-mode" may be either INCLUDE or EXCLUDE.  In INCLUDE mode,
   reception of packets sent to the specified multicast address is
   requested *only* from those IP source addresses listed in the
   source-list parameter.  In EXCLUDE mode, reception of packets sent
   to the given multicast address is requested from all IP source
   addresses *except* those listed in the source-list parameter.
 o "source-list" is an unordered list of zero or more IP unicast
   addresses from which multicast reception is desired or not desired,
   depending on the filter mode.  An implementation MAY impose a limit
   on the size of source lists, but that limit MUST NOT be less than
   64 addresses per list.  When an operation causes the source list
   size limit to be exceeded, the service interface MUST return an
   error.
 For a given combination of socket, interface, and multicast address,
 only a single filter mode and source list can be in effect at any one
 time.  However, either the filter mode or the source list, or both,
 may be changed by subsequent IPMulticastListen requests that specify
 the same socket, interface, and multicast address.  Each subsequent
 request completely replaces any earlier request for the given socket,
 interface and multicast address.
 Previous versions of IGMP did not support source filters and had a
 simpler service interface consisting of Join and Leave operations to
 enable and disable reception of a given multicast address (from *all*
 sources) on a given interface.  The equivalent operations in the new
 service interface follow:
 The Join operation is equivalent to
    IPMulticastListen ( socket, interface, multicast-address,
                        EXCLUDE, {} )
 and the Leave operation is equivalent to:
    IPMulticastListen ( socket, interface, multicast-address,
                        INCLUDE, {} )
 where {} is an empty source list.
 An example of an API providing the capabilities outlined in this
 service interface is in [FILTER-API].

Cain, et. al. Standards Track [Page 4] RFC 3376 IGMPv3 October 2002

3. Multicast Reception State Maintained by Systems

3.1. Socket State

 For each socket on which IPMulticastListen has been invoked, the
 system records the desired multicast reception state for that socket.
 That state conceptually consists of a set of records of the form:
    (interface, multicast-address, filter-mode, source-list)
 The socket state evolves in response to each invocation of
 IPMulticastListen on the socket, as follows:
 o If the requested filter mode is INCLUDE *and* the requested source
   list is empty, then the entry corresponding to the requested
   interface and multicast address is deleted if present.  If no such
   entry is present, the request is ignored.
 o If the requested filter mode is EXCLUDE *or* the requested source
   list is non-empty, then the entry corresponding to the requested
   interface and multicast address, if present, is changed to contain
   the requested filter mode and source list.  If no such entry is
   present, a new entry is created, using the parameters specified in
   the request.

3.2. Interface State

   In addition to the per-socket multicast reception state, a system
   must also maintain or compute multicast reception state for each of
   its interfaces.  That state conceptually consists of a set of
   records of the form:
      (multicast-address, filter-mode, source-list)
   At most one record per multicast-address exists for a given
   interface.  This per-interface state is derived from the per-socket
   state, but may differ from the per-socket state when different
   sockets have differing filter modes and/or source lists for the
   same multicast address and interface.  For example, suppose one
   application or process invokes the following operation on socket
   s1:
      IPMulticastListen ( s1, i, m, INCLUDE, {a, b, c} )
   requesting reception on interface i of packets sent to multicast
   address m, *only* if they come from source a, b, or c.  Suppose
   another application or process invokes the following operation on
   socket s2:

Cain, et. al. Standards Track [Page 5] RFC 3376 IGMPv3 October 2002

      IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} )
   requesting reception on the same interface i of packets sent to the
   same multicast address m, *only* if they come from sources b, c, or
   d.  In order to satisfy the reception requirements of both sockets,
   it is necessary for interface i to receive packets sent to m from
   any one of the sources a, b, c, or d.  Thus, in this example, the
   reception state of interface i for multicast address m has filter
   mode INCLUDE and source list {a, b, c, d}.
   After a multicast packet has been accepted from an interface by the
   IP layer, its subsequent delivery to the application or process
   listening on a particular socket depends on the multicast reception
   state of that socket [and possibly also on other conditions, such
   as what transport-layer port the socket is bound to].  So, in the
   above example, if a packet arrives on interface i, destined to
   multicast address m, with source address a, it will be delivered on
   socket s1 but not on socket s2.  Note that IGMP Queries and Reports
   are not subject to source filtering and must always be processed by
   hosts and routers.
   Filtering of packets based upon a socket's multicast reception
   state is a new feature of this service interface.  The previous
   service interface [RFC1112] described no filtering based upon
   multicast join state; rather, a join on a socket simply caused the
   host to join a group on the given interface, and packets destined
   for that group could be delivered to all sockets whether they had
   joined or not.
   The general rules for deriving the per-interface state from the
   per-socket state are as follows:  For each distinct (interface,
   multicast-address) pair that appears in any socket state, a per-
   interface record is created for that multicast address on that
   interface.  Considering all socket records containing the same
   (interface, multicast-address) pair,
 o if *any* such record has a filter mode of EXCLUDE, then the filter
   mode of the interface record is EXCLUDE, and the source list of the
   interface record is the intersection of the source lists of all
   socket records in EXCLUDE mode, minus those source addresses that
   appear in any socket record in INCLUDE mode.  For example, if the
   socket records for multicast address m on interface i are:
      from socket s1:  ( i, m, EXCLUDE, {a, b, c, d} )
      from socket s2:  ( i, m, EXCLUDE, {b, c, d, e} )
      from socket s3:  ( i, m, INCLUDE, {d, e, f} )

Cain, et. al. Standards Track [Page 6] RFC 3376 IGMPv3 October 2002

   then the corresponding interface record on interface i is:
                       ( m, EXCLUDE, {b, c} )
   If a fourth socket is added, such as:
      from socket s4:  ( i, m, EXCLUDE, {} )
   then the interface record becomes:
                       ( m, EXCLUDE, {} )
 o if *all* such records have a filter mode of INCLUDE, then the
   filter mode of the interface record is INCLUDE, and the source list
   of the interface record is the union of the source lists of all the
   socket records.  For example, if the socket records for multicast
   address m on interface i are:
      from socket s1:  ( i, m, INCLUDE, {a, b, c} )
      from socket s2:  ( i, m, INCLUDE, {b, c, d} )
      from socket s3:  ( i, m, INCLUDE, {e, f} )
   then the corresponding interface record on interface i is:
                       ( m, INCLUDE, {a, b, c, d, e, f} )
   An implementation MUST NOT use an EXCLUDE interface record to
   represent a group when all sockets for this group are in INCLUDE
   state.  If system resource limits are reached when an interface
   state source list is calculated, an error MUST be returned to the
   application which requested the operation.
 The above rules for deriving the interface state are (re-)evaluated
 whenever an IPMulticastListen invocation modifies the socket state by
 adding, deleting, or modifying a per-socket state record.  Note that
 a change  of socket state does not necessarily result in a change of
 interface state.

4. Message Formats

 IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol
 number of 2.  Every IGMP message described in this document is sent
 with an IP Time-to-Live of 1, IP Precedence of Internetwork Control
 (e.g., Type of Service 0xc0), and carries an IP Router Alert option
 [RFC-2113] in its IP header.  IGMP message types are registered by
 the IANA [IANA-REG] as described by [RFC-3228].

Cain, et. al. Standards Track [Page 7] RFC 3376 IGMPv3 October 2002

 There are two IGMP message types of concern to the IGMPv3 protocol
 described in this document:
    Type Number (hex)   Message Name
    -----------------   ------------
          0x11          Membership Query
          0x22          Version 3 Membership Report
 An implementation of IGMPv3 MUST also support the following three
 message types, for interoperation with previous versions of IGMP (see
 section 7):
         0x12          Version 1 Membership Report    [RFC-1112]
         0x16          Version 2 Membership Report    [RFC-2236]
         0x17          Version 2 Leave Group          [RFC-2236]
 Unrecognized message types MUST be silently ignored.  Other message
 types may be used by newer versions or extensions of IGMP, by
 multicast routing protocols, or for other uses.
 In this document, unless otherwise qualified, the capitalized words
 "Query" and "Report" refer to IGMP Membership Queries and IGMP
 Version 3 Membership Reports, respectively.

4.1. Membership Query Message

 Membership Queries are sent by IP multicast routers to query the
 multicast reception state of neighboring interfaces.  Queries have
 the following format:

Cain, et. al. Standards Track [Page 8] RFC 3376 IGMPv3 October 2002

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Type = 0x11  | Max Resp Code |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Group Address                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Resv  |S| QRV |     QQIC      |     Number of Sources (N)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Source Address [1]                      |
    +-                                                             -+
    |                       Source Address [2]                      |
    +-                              .                              -+
    .                               .                               .
    .                               .                               .
    +-                                                             -+
    |                       Source Address [N]                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1.1. Max Resp Code

 The Max Resp Code field specifies the maximum time allowed before
 sending a responding report.  The actual time allowed, called the Max
 Resp Time, is represented in units of 1/10 second and is derived from
 the Max Resp Code as follows:
 If Max Resp Code < 128, Max Resp Time = Max Resp Code
 If Max Resp Code >= 128, Max Resp Code represents a floating-point
 value as follows:
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |1| exp | mant  |
    +-+-+-+-+-+-+-+-+
 Max Resp Time = (mant | 0x10) << (exp + 3)
 Small values of Max Resp Time allow IGMPv3 routers to tune the "leave
 latency" (the time between the moment the last host leaves a group
 and the moment the routing protocol is notified that there are no
 more members).  Larger values, especially in the exponential range,
 allow tuning of the burstiness of IGMP traffic on a network.

Cain, et. al. Standards Track [Page 9] RFC 3376 IGMPv3 October 2002

4.1.2. Checksum

 The Checksum is the 16-bit one's complement of the one's complement
 sum of the whole IGMP message (the entire IP payload).  For computing
 the checksum, the Checksum field is set to zero.  When receiving
 packets, the checksum MUST be verified before processing a packet.
 [RFC-1071]

4.1.3. Group Address

 The Group Address field is set to zero when sending a General Query,
 and set to the IP multicast address being queried when sending a
 Group-Specific Query or Group-and-Source-Specific Query (see section
 4.1.9, below).

4.1.4. Resv (Reserved)

 The Resv field is set to zero on transmission, and ignored on
 reception.

4.1.5. S Flag (Suppress Router-Side Processing)

 When set to one, the S Flag indicates to any receiving multicast
 routers that they are to suppress the normal timer updates they
 perform upon hearing a Query.  It does not, however, suppress the
 querier election or the normal "host-side" processing of a Query that
 a router may be required to perform as a consequence of itself being
 a group member.

4.1.6. QRV (Querier's Robustness Variable)

 If non-zero, the QRV field contains the [Robustness Variable] value
 used by the querier, i.e., the sender of the Query.  If the querier's
 [Robustness Variable] exceeds 7, the maximum value of the QRV field,
 the QRV is set to zero.  Routers adopt the QRV value from the most
 recently received Query as their own [Robustness Variable] value,
 unless that most recently received QRV was zero, in which case the
 receivers use the default [Robustness Variable] value specified in
 section 8.1 or a statically configured value.

4.1.7. QQIC (Querier's Query Interval Code)

 The Querier's Query Interval Code field specifies the [Query
 Interval] used by the querier.  The actual interval, called the
 Querier's Query Interval (QQI), is represented in units of seconds
 and is derived from the Querier's Query Interval Code as follows:

Cain, et. al. Standards Track [Page 10] RFC 3376 IGMPv3 October 2002

 If QQIC < 128, QQI = QQIC
 If QQIC >= 128, QQIC represents a floating-point value as follows:
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |1| exp | mant  |
    +-+-+-+-+-+-+-+-+
 QQI = (mant | 0x10) << (exp + 3)
 Multicast routers that are not the current querier adopt the QQI
 value from the most recently received Query as their own [Query
 Interval] value, unless that most recently received QQI was zero, in
 which case the receiving routers use the default [Query Interval]
 value specified in section 8.2.

4.1.8. Number of Sources (N)

 The Number of Sources (N) field specifies how many source addresses
 are present in the Query.  This number is zero in a General Query or
 a Group-Specific Query, and non-zero in a Group-and-Source-Specific
 Query.  This number is limited by the MTU of the network over which
 the Query is transmitted.  For example, on an Ethernet with an MTU of
 1500 octets, the IP header including the Router Alert option consumes
 24 octets, and the IGMP fields up to including the Number of Sources
 (N) field consume 12 octets, leaving 1464 octets for source
 addresses, which limits the number of source addresses to 366
 (1464/4).

4.1.9. Source Address [i]

 The Source Address [i] fields are a vector of n IP unicast addresses,
 where n is the value in the Number of Sources (N) field.

4.1.10. Additional Data

 If the Packet Length field in the IP header of a received Query
 indicates that there are additional octets of data present, beyond
 the fields described here, IGMPv3 implementations MUST include those
 octets in the computation to verify the received IGMP Checksum, but
 MUST otherwise ignore those additional octets.  When sending a Query,
 an IGMPv3 implementation MUST NOT include additional octets beyond
 the fields described here.

Cain, et. al. Standards Track [Page 11] RFC 3376 IGMPv3 October 2002

4.1.11. Query Variants

 There are three variants of the Query message:
 1. A "General Query" is sent by a multicast router to learn the
    complete multicast reception state of the neighboring interfaces
    (that is, the interfaces attached to the network on which the
    Query is transmitted).  In a General Query, both the Group Address
    field and the Number of Sources (N) field are zero.
 2. A "Group-Specific Query" is sent by a multicast router to learn
    the reception state, with respect to a *single* multicast address,
    of the neighboring interfaces.  In a Group-Specific Query, the
    Group Address field contains the multicast address of interest,
    and the Number of Sources (N) field contains zero.
 3. A "Group-and-Source-Specific Query" is sent by a multicast router
    to learn if any neighboring interface desires reception of packets
    sent to a specified multicast address, from any of a specified
    list of sources.  In a Group-and-Source-Specific Query, the Group
    Address field contains the multicast address of interest, and the
    Source Address [i] fields contain the source address(es) of
    interest.

4.1.12. IP Destination Addresses for Queries

 In IGMPv3, General Queries are sent with an IP destination address of
 224.0.0.1, the all-systems multicast address.  Group-Specific and
 Group-and-Source-Specific Queries are sent with an IP destination
 address equal to the  multicast address of interest.  *However*, a
 system MUST accept and  process any Query whose IP Destination
 Address field contains *any* of the addresses (unicast or multicast)
 assigned to the interface on which the Query arrives.

4.2. Version 3 Membership Report Message

 Version 3 Membership Reports are sent by IP systems to report (to
 neighboring routers) the current multicast reception state, or
 changes in the multicast reception state, of their interfaces.
 Reports have the following format:

Cain, et. al. Standards Track [Page 12] RFC 3376 IGMPv3 October 2002

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Type = 0x22  |    Reserved   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Reserved            |  Number of Group Records (M)  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Group Record [1]                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Group Record [2]                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               .                               |
    .                               .                               .
    |                               .                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                        Group Record [M]                       .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Cain, et. al. Standards Track [Page 13] RFC 3376 IGMPv3 October 2002

 where each Group Record has the following internal format:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Record Type  |  Aux Data Len |     Number of Sources (N)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Multicast Address                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Source Address [1]                      |
    +-                                                             -+
    |                       Source Address [2]                      |
    +-                                                             -+
    .                               .                               .
    .                               .                               .
    .                               .                               .
    +-                                                             -+
    |                       Source Address [N]                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                         Auxiliary Data                        .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2.1. Reserved

 The Reserved fields are set to zero on transmission, and ignored on
 reception.

4.2.2. Checksum

 The Checksum is the 16-bit one's complement of the one's complement
 sum of the whole IGMP message (the entire IP payload).  For computing
 the checksum, the Checksum field is set to zero.  When receiving
 packets, the checksum MUST be verified before processing a message.

4.2.3. Number of Group Records (M)

 The Number of Group Records (M) field specifies how many Group
 Records are present in this Report.

4.2.4. Group Record

 Each Group Record is a block of fields containing information
 pertaining to the sender's membership in a single multicast group on
 the interface from which the Report is sent.

Cain, et. al. Standards Track [Page 14] RFC 3376 IGMPv3 October 2002

4.2.5. Record Type

 See section 4.2.12, below.

4.2.6. Aux Data Len

 The Aux Data Len field contains the length of the Auxiliary Data
 field in this Group Record, in units of 32-bit words.  It may contain
 zero, to indicate the absence of any auxiliary data.

4.2.7. Number of Sources (N)

 The Number of Sources (N) field specifies how many source addresses
 are present in this Group Record.

4.2.8. Multicast Address

 The Multicast Address field contains the IP multicast address to
 which this Group Record pertains.

4.2.9. Source Address [i]

 The Source Address [i] fields are a vector of n IP unicast addresses,
 where n is the value in this record's Number of Sources (N) field.

4.2.10. Auxiliary Data

 The Auxiliary Data field, if present, contains additional information
 pertaining to this Group Record.  The protocol specified in this
 document, IGMPv3, does not define any auxiliary data.  Therefore,
 implementations of IGMPv3 MUST NOT include any auxiliary data (i.e.,
 MUST set the Aux Data Len field to zero) in any transmitted Group
 Record, and MUST ignore any auxiliary data present in any received
 Group Record.  The semantics and internal encoding of the Auxiliary
 Data field are to be defined by any future version or extension of
 IGMP that uses this field.

4.2.11. Additional Data

 If the Packet Length field in the IP header of a received Report
 indicates that there are additional octets of data present, beyond
 the last Group Record, IGMPv3 implementations MUST include those
 octets in the computation to verify the received IGMP Checksum, but
 MUST otherwise ignore those additional octets.  When sending a
 Report, an IGMPv3 implementation MUST NOT include additional octets
 beyond the last Group Record.

Cain, et. al. Standards Track [Page 15] RFC 3376 IGMPv3 October 2002

4.2.12. Group Record Types

 There are a number of different types of Group Records that may be
 included in a Report message:
 o A "Current-State Record" is sent by a system in response to a Query
   received on an interface.  It reports the current reception state
   of that interface, with respect to a single multicast address.  The
   Record Type of a Current-State Record may be one of the following
   two values:
      Value  Name and Meaning
      -----  ----------------
        1    MODE_IS_INCLUDE - indicates that the interface has a
             filter mode of INCLUDE for the specified multicast
             address.  The Source Address [i] fields in this Group
             Record contain the interface's source list for the
             specified multicast address, if it is non-empty.
        2    MODE_IS_EXCLUDE - indicates that the interface has a
             filter mode of EXCLUDE for the specified multicast
             address.  The Source Address [i] fields in this Group
             Record contain the interface's source list for the
             specified multicast address, if it is non-empty.
 o A "Filter-Mode-Change Record" is sent by a system whenever a local
   invocation of IPMulticastListen causes a change of the filter mode
   (i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to
   INCLUDE), of the interface-level state entry for a particular
   multicast address.  The Record is included in a Report sent from
   the interface on which the change occurred.  The Record Type of a
   Filter-Mode-Change Record may be one of the following two values:
        3    CHANGE_TO_INCLUDE_MODE - indicates that the interface
             has changed to INCLUDE filter mode for the specified
             multicast address.  The Source Address [i] fields
             in this Group Record contain the interface's new
             source list for the specified multicast address,
             if it is non-empty.
        4    CHANGE_TO_EXCLUDE_MODE - indicates that the interface
             has changed to EXCLUDE filter mode for the specified
             multicast address.  The Source Address [i] fields
             in this Group Record contain the interface's new
             source list for the specified multicast address,
             if it is non-empty.

Cain, et. al. Standards Track [Page 16] RFC 3376 IGMPv3 October 2002

 o A "Source-List-Change Record" is sent by a system whenever a local
   invocation of IPMulticastListen causes a change of source list that
   is *not* coincident with a change of filter mode, of the
   interface-level state entry for a particular multicast address.
   The Record is included in a Report sent from the interface on which
   the change occurred.  The Record Type of a Source-List-Change
   Record may be one of the following two values:
        5    ALLOW_NEW_SOURCES - indicates that the Source Address
             [i] fields in this Group Record contain a list of the
             additional sources that the system wishes to
             hear from, for packets sent to the specified
             multicast address.  If the change was to an INCLUDE
             source list, these are the addresses that were added
             to the list; if the change was to an EXCLUDE source
             list, these are the addresses that were deleted from
             the list.
        6    BLOCK_OLD_SOURCES - indicates that the Source Address
             [i] fields in this Group Record contain a list of the
             sources that the system no longer wishes to
             hear from, for packets sent to the specified
             multicast address.  If the change was to an INCLUDE
             source list, these are the addresses that were
             deleted from  the list; if the change was to an
             EXCLUDE source list, these are the addresses that
             were added to the list.
 If a change of source list results in both allowing new sources and
 blocking old sources, then two Group Records are sent for the same
 multicast address, one of type ALLOW_NEW_SOURCES and one of type
 BLOCK_OLD_SOURCES.
 We use the term "State-Change Record" to refer to either a Filter-
 Mode-Change Record or a Source-List-Change Record.
 Unrecognized Record Type values MUST be silently ignored.

4.2.13. IP Source Addresses for Reports

 An IGMP report is sent with a valid IP source address for the
 destination subnet.  The 0.0.0.0 source address may be used by a
 system that has not yet acquired an IP address.  Note that the
 0.0.0.0 source address may simultaneously be used by multiple systems
 on a LAN.  Routers MUST accept a report with a source address of
 0.0.0.0.

Cain, et. al. Standards Track [Page 17] RFC 3376 IGMPv3 October 2002

4.2.14. IP Destination Addresses for Reports

 Version 3 Reports are sent with an IP destination address of
 224.0.0.22, to which all IGMPv3-capable multicast routers listen.  A
 system that is operating in version 1 or version 2 compatibility
 modes sends version 1 or version 2 Reports to the multicast group
 specified in the Group Address field of the Report.  In addition, a
 system MUST accept and process any version 1 or version 2 Report
 whose IP Destination Address field contains *any* of the addresses
 (unicast or multicast) assigned to the interface on which the Report
 arrives.

4.2.15. Notation for Group Records

 In the rest of this document, we use the following notation to
 describe the contents of a Group Record pertaining to a particular
 multicast address:
    IS_IN ( x )  -  Type MODE_IS_INCLUDE, source addresses x
    IS_EX ( x )  -  Type MODE_IS_EXCLUDE, source addresses x
    TO_IN ( x )  -  Type CHANGE_TO_INCLUDE_MODE, source addresses x
    TO_EX ( x )  -  Type CHANGE_TO_EXCLUDE_MODE, source addresses x
    ALLOW ( x )  -  Type ALLOW_NEW_SOURCES, source addresses x
    BLOCK ( x )  -  Type BLOCK_OLD_SOURCES, source addresses x
 where x is either:
 o a capital letter (e.g., "A") to represent the set of source
   addresses, or
 o a set expression (e.g., "A+B"), where "A+B" means the union of sets
   A and B, "A*B" means the intersection of sets A and B, and "A-B"
   means the removal of all elements of set B from set A.

4.2.16. Membership Report Size

 If the set of Group Records required in a Report does not fit within
 the size limit of a single Report message (as determined by the MTU
 of the network on which it will be sent), the Group Records are sent
 in as many Report messages as needed to report the entire set.
 If a single Group Record contains so many source addresses that it
 does not fit within the size limit of a single Report message, if its
 Type is not MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, it is split
 into multiple Group Records, each containing a different subset of
 the source addresses and each sent in a separate Report message.  If
 its Type is MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single Group
 Record is sent, containing as many source addresses as can fit, and

Cain, et. al. Standards Track [Page 18] RFC 3376 IGMPv3 October 2002

 the remaining source addresses are not reported; though the choice of
 which sources to report is arbitrary, it is preferable to report the
 same set of sources in each subsequent report, rather than reporting
 different sources each time.

5. Description of the Protocol for Group Members

 IGMP is an asymmetric protocol, specifying separate behaviors for
 group members -- that is, hosts or routers that wish to receive
 multicast packets -- and multicast routers.  This section describes
 the part of IGMPv3 that applies to all group members.  (Note that a
 multicast router that is also a group member performs both parts of
 IGMPv3, receiving and responding to its own IGMP message
 transmissions as well as those of its neighbors.  The multicast
 router part of IGMPv3 is described in section 6.)
 A system performs the protocol described in this section over all
 interfaces on which multicast reception is supported, even if more
 than one of those interfaces is connected to the same network.
 For interoperability with multicast routers running older versions of
 IGMP, systems maintain a MulticastRouterVersion variable for each
 interface on which multicast reception is supported.  This section
 describes the behavior of group member systems on interfaces for
 which MulticastRouterVersion = 3.  The algorithm for determining
 MulticastRouterVersion, and the behavior for versions other than 3,
 are described in section 7.
 The all-systems multicast address, 224.0.0.1, is handled as a special
 case.  On all systems -- that is all hosts and routers, including
 multicast routers -- reception of packets destined to the all-systems
 multicast address, from all sources, is permanently enabled on all
 interfaces on which multicast reception is supported.  No IGMP
 messages are ever sent regarding the all-systems multicast address.
 There are two types of events that trigger IGMPv3 protocol actions on
 an interface:
 o a change of the interface reception state, caused by a local
   invocation of IPMulticastListen.
 o reception of a Query.
 (Received IGMP messages of types other than Query are silently
 ignored, except as required for interoperation with earlier versions
 of IGMP.)

Cain, et. al. Standards Track [Page 19] RFC 3376 IGMPv3 October 2002

 The following subsections describe the actions to be taken for each
 of these two cases.  In those descriptions, timer and counter names
 appear in square brackets.  The default values for those timers and
 counters are specified in section 8.

5.1. Action on Change of Interface State

 An invocation of IPMulticastListen may cause the multicast reception
 state of an interface to change, according to the rules in section
 3.2.  Each such change affects the per-interface entry for a single
 multicast address.
 A change of interface state causes the system to immediately transmit
 a State-Change Report from that interface.  The type and contents of
 the Group Record(s) in that Report are determined by comparing the
 filter mode and source list for the affected multicast address before
 and after the change, according to the table below.  If no interface
 state existed for that multicast address before the change (i.e., the
 change consisted of creating a new per-interface record), or if no
 state exists after the change (i.e., the change consisted of deleting
 a per-interface record), then the "non-existent" state is considered
 to have a filter mode of INCLUDE and an empty source list.
   Old State         New State         State-Change Record Sent
   ---------         ---------         ------------------------
   INCLUDE (A)       INCLUDE (B)       ALLOW (B-A), BLOCK (A-B)
   EXCLUDE (A)       EXCLUDE (B)       ALLOW (A-B), BLOCK (B-A)
   INCLUDE (A)       EXCLUDE (B)       TO_EX (B)
   EXCLUDE (A)       INCLUDE (B)       TO_IN (B)
 If the computed source list for either an ALLOW or a BLOCK State-
 Change Record is empty, that record is omitted from the Report
 message.
 To cover the possibility of the State-Change Report being missed by
 one or more multicast routers, it is retransmitted [Robustness
 Variable] - 1 more times, at intervals chosen at random from the
 range (0, [Unsolicited Report Interval]).
 If more changes to the same interface state entry occur before all
 the retransmissions of the State-Change Report for the first change
 have been completed, each such additional change triggers the
 immediate transmission of a new State-Change Report.

Cain, et. al. Standards Track [Page 20] RFC 3376 IGMPv3 October 2002

 The contents of the new transmitted report are calculated as follows.
 As was done with the first report, the interface state for the
 affected group before and after the latest change is compared.  The
 report records expressing the difference are built according to the
 table above.  However these records are not transmitted in a message
 but instead merged with the contents of the pending report, to create
 the new State-Change report.  The rules for merging the difference
 report resulting from the state change and the pending report are
 described below.
 The transmission of the merged State-Change Report terminates
 retransmissions of the earlier State-Change Reports for the same
 multicast address, and becomes the first of [Robustness Variable]
 transmissions of State-Change Reports.
 Each time a source is included in the difference report calculated
 above, retransmission state for that source needs to be maintained
 until [Robustness Variable] State-Change reports have been sent by
 the host.  This is done in order to ensure that a series of
 successive state changes do not break the protocol robustness.
 If the interface reception-state change that triggers the new report
 is a filter-mode change, then the next [Robustness Variable] State-
 Change Reports will include a Filter-Mode-Change record.  This
 applies even if any number of source-list changes occur in that
 period.  The host has to maintain retransmission state for the group
 until the [Robustness Variable] State-Change reports have been sent.
 When [Robustness Variable] State-Change reports with Filter-Mode-
 Change records have been transmitted after the last filter-mode
 change, and if source-list changes to the interface reception have
 scheduled additional reports, then the next State-Change report will
 include Source-List-Change records.
 Each time a State-Change Report is transmitted, the contents are
 determined as follows.  If the report should contain a Filter-Mode-
 Change record, then if the current filter-mode of the interface is
 INCLUDE, a TO_IN record is included in the report, otherwise a TO_EX
 record is included.  If instead the report should contain Source-
 List-Change records, an ALLOW and a BLOCK record are included.  The
 contents of these records are built according to the table below.
    Record   Sources included
    ------   ----------------
    TO_IN    All in the current interface state that must be forwarded
    TO_EX    All in the current interface state that must be blocked
    ALLOW    All with retransmission state that must be forwarded
    BLOCK    All with retransmission state that must be blocked

Cain, et. al. Standards Track [Page 21] RFC 3376 IGMPv3 October 2002

 If the computed source list for either an ALLOW or a BLOCK record is
 empty, that record is omitted from the State-Change report.
 Note: When the first State-Change report is sent, the non-existent
 pending report to merge with, can be treated as a source-change
 report with empty ALLOW and BLOCK records (no sources have
 retransmission state).

5.2. Action on Reception of a Query

 When a system receives a Query, it does not respond immediately.
 Instead, it delays its response by a random amount of time, bounded
 by the Max Resp Time value derived from the Max Resp Code in the
 received Query message.  A system may receive a variety of Queries on
 different interfaces and of different kinds (e.g., General Queries,
 Group-Specific Queries, and Group-and-Source-Specific Queries), each
 of which may require its own delayed response.
 Before scheduling a response to a Query, the system must first
 consider previously scheduled pending responses and in many cases
 schedule a combined response.  Therefore, the system must be able to
 maintain the following state:
 o A timer per interface for scheduling responses to General Queries.
 o A per-group and interface timer for scheduling responses to Group-
   Specific and Group-and-Source-Specific Queries.
 o A per-group and interface list of sources to be reported in the
   response to a Group-and-Source-Specific Query.
 When a new Query with the Router-Alert option arrives on an
 interface, provided the system has state to report, a delay for a
 response is randomly selected in the range (0, [Max Resp Time]) where
 Max Resp Time is derived from Max Resp Code in the received Query
 message.  The following rules are then used to determine if a Report
 needs to be scheduled and the type of Report to schedule.  The rules
 are considered in order and only the first matching rule is applied.
 1. If there is a pending response to a previous General Query
    scheduled sooner than the selected delay, no additional response
    needs to be scheduled.
 2. If the received Query is a General Query, the interface timer is
    used to schedule a response to the General Query after the
    selected delay.  Any previously pending response to a General
    Query is canceled.

Cain, et. al. Standards Track [Page 22] RFC 3376 IGMPv3 October 2002

 3. If the received Query is a Group-Specific Query or a Group-and-
    Source-Specific Query and there is no pending response to a
    previous Query for this group, then the group timer is used to
    schedule a report.  If the received Query is a Group-and-Source-
    Specific Query, the list of queried sources is recorded to be used
    when generating a response.
 4. If there already is a pending response to a previous Query
    scheduled for this group, and either the new Query is a Group-
    Specific Query or the recorded source-list associated with the
    group is empty, then the group source-list is cleared and a single
    response is scheduled using the group timer.  The new response is
    scheduled to be sent at the earliest of the remaining time for the
    pending report and the selected delay.
 5. If the received Query is a Group-and-Source-Specific Query and
    there is a pending response for this group with a non-empty
    source-list, then the group source list is augmented to contain
    the list of sources in the new Query and a single response is
    scheduled using the group timer.  The new response is scheduled to
    be sent at the earliest of the remaining time for the pending
    report and the selected delay.
 When the timer in a pending response record expires, the system
 transmits, on the associated interface, one or more Report messages
 carrying one or more Current-State Records (see section 4.2.12), as
 follows:
 1. If the expired timer is the interface timer (i.e., it is a pending
    response to a General Query), then one Current-State Record is
    sent for each multicast address for which the specified interface
    has reception state, as described in section 3.2.  The Current-
    State Record carries the multicast address and its associated
    filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
    Multiple Current-State Records are packed into individual Report
    messages, to the extent possible.
    This naive algorithm may result in bursts of packets when a system
    is a member of a large number of groups.  Instead of using a
    single interface timer, implementations are recommended to spread
    transmission of such Report messages over the interval (0, [Max
    Resp Time]).  Note that any such implementation MUST avoid the
    "ack-implosion" problem, i.e., MUST NOT send a Report immediately
    on reception of a General Query.

Cain, et. al. Standards Track [Page 23] RFC 3376 IGMPv3 October 2002

 2. If the expired timer is a group timer and the list of recorded
    sources for the that group is empty (i.e., it is a pending
    response to a Group-Specific Query), then if and only if the
    interface has reception state for that group address, a single
    Current-State Record is sent for that address.  The Current-State
    Record carries the multicast address and its associated filter
    mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
 3. If the expired timer is a group timer and the list of recorded
    sources for that group is non-empty (i.e., it is a pending
    response to a Group-and-Source-Specific Query), then if and only
    if the interface has reception state for that group address, the
    contents of the responding Current-State Record is determined from
    the interface state and the pending response record, as specified
    in the following table:
                       set of sources in the
    interface state   pending response record   Current-State Record
    ---------------   -----------------------   --------------------
     INCLUDE (A)                B                   IS_IN (A*B)
     EXCLUDE (A)                B                   IS_IN (B-A)
 If the resulting Current-State Record has an empty set of source
 addresses, then no response is sent.
 Finally, after any required Report messages have been generated, the
 source lists associated with any reported groups are cleared.

6. Description of the Protocol for Multicast Routers

 The purpose of IGMP is to enable each multicast router to learn, for
 each of its directly attached networks, which multicast addresses are
 of interest to the systems attached to those networks.  IGMP version
 3 adds the capability for a multicast router to also learn  which
 *sources* are of interest to neighboring systems, for packets sent to
 any particular multicast address.  The information gathered by IGMP
 is provided to whichever multicast routing protocol is being used by
 the router, in order to ensure that multicast packets are delivered
 to all networks where there are interested receivers.
 This section describes the part of IGMPv3 that is performed by
 multicast routers.  Multicast routers may also themselves become
 members of multicast groups, and therefore also perform the group
 member part of IGMPv3, described in section 5.

Cain, et. al. Standards Track [Page 24] RFC 3376 IGMPv3 October 2002

 A multicast router performs the protocol described in this section
 over each of its directly-attached networks.  If a multicast router
 has more than one interface to the same network, it only needs to
 operate this protocol over one of those interfaces.  On each
 interface over which this protocol is being run, the router MUST
 enable reception of multicast address 224.0.0.22, from all sources
 (and MUST perform the group member part of IGMPv3 for that address on
 that interface).
 Multicast routers need to know only that *at least one* system on an
 attached network is interested in packets to a particular multicast
 address from a particular source; a multicast router is not required
 to keep track of the interests of each individual neighboring system.
 (However, see Appendix A.2 point 1 for discussion.)
 IGMPv3 is backward compatible with previous versions of the IGMP
 protocol.  In order to remain backward compatible with older IGMP
 systems, IGMPv3 multicast routers MUST also implement versions 1 and
 2 of the protocol (see section 7).

6.1. Conditions for IGMP Queries

 Multicast routers send General Queries periodically to request group
 membership information from an attached network.  These queries are
 used to build and refresh the group membership state of systems on
 attached networks.  Systems respond to these queries by reporting
 their group membership state (and their desired set of sources) with
 Current-State Group Records in IGMPv3 Membership Reports.
 As a member of a multicast group, a system may express interest in
 receiving or not receiving traffic from particular sources.  As the
 desired reception state of a system changes, it reports these changes
 using Filter-Mode-Change Records or Source-List-Change Records.
 These records indicate an explicit state change in a group at a
 system in either the group record's source list or its filter-mode.
 When a group membership is terminated at a system or traffic from a
 particular source is no longer desired, a multicast router must query
 for other members of the group or listeners of the source before
 deleting the group (or source) and pruning its traffic.
 To enable all systems on a network to respond to changes in group
 membership, multicast routers send specific queries.  A Group-
 Specific Query is sent to verify there are no systems that desire
 reception of the specified group or to "rebuild" the desired
 reception state for a particular group.  Group-Specific Queries are
 sent when a router receives a State-Change record indicating a system
 is leaving a group.

Cain, et. al. Standards Track [Page 25] RFC 3376 IGMPv3 October 2002

 A Group-and-Source Specific Query is used to verify there are no
 systems on a network which desire to receive traffic from a set of
 sources.  Group-and-Source Specific Queries list sources for a
 particular group which have been requested to no longer be forwarded.
 This query is sent by a multicast router to learn if any systems
 desire reception of packets to the specified group address from the
 specified source addresses.  Group-and-Source Specific Queries are
 only sent in response to State-Change Records and never in response
 to Current-State Records.  Section 4.1.11 describes each query in
 more detail.

6.2. IGMP State Maintained by Multicast Routers

 Multicast routers implementing IGMPv3 keep state per group per
 attached network.  This group state consists of a filter-mode, a list
 of sources, and various timers.  For each attached network running
 IGMP, a multicast router records the desired reception state for that
 network.  That state conceptually consists of a set of records of the
 form:
    (multicast address, group timer, filter-mode, (source records))
 Each source record is of the form:
    (source address, source timer)
 If all sources within a given group are desired, an empty source
 record list is kept with filter-mode set to EXCLUDE.  This means
 hosts on this network want all sources for this group to be
 forwarded.  This is the IGMPv3 equivalent to a IGMPv1 or IGMPv2 group
 join.

6.2.1. Definition of Router Filter-Mode

 To reduce internal state, IGMPv3 routers keep a filter-mode per group
 per attached network.  This filter-mode is used to condense the total
 desired reception state of a group to a minimum set such that all
 systems' memberships are satisfied.  This filter-mode may change in
 response to the reception of particular types of group records or
 when certain timer conditions occur.  In the following sections, we
 use the term "router filter-mode" to refer to the filter-mode of a
 particular group within a router.  Section 6.4 describes the changes
 of a router filter-mode per group record received.

Cain, et. al. Standards Track [Page 26] RFC 3376 IGMPv3 October 2002

 Conceptually, when a group record is received, the router filter-mode
 for that group is updated to cover all the requested sources using
 the least amount of state.  As a rule, once a group record with a
 filter-mode of EXCLUDE is received, the router filter-mode for that
 group will be EXCLUDE.
 When a router filter-mode for a group is EXCLUDE, the source record
 list contains two types of sources.  The first type is the set which
 represents conflicts in the desired reception state; this set must be
 forwarded by some router on the network.  The second type is the set
 of sources which hosts have requested to not be forwarded.  Appendix
 A describes the reasons for keeping this second set when in EXCLUDE
 mode.
 When a router filter-mode for a group is INCLUDE, the source record
 list is the list of sources desired for the group.  This is the total
 desired set of sources for that group.  Each source in the source
 record list must be forwarded by some router on the network.
 Because a reported group record with a filter-mode of EXCLUDE will
 cause a router to transition its filter-mode for that group to
 EXCLUDE, a mechanism for transitioning a router's filter-mode back to
 INCLUDE must exist.  If all systems with a group record in EXCLUDE
 filter-mode cease reporting, it is desirable for the router filter-
 mode for that group to transition back to INCLUDE mode.  This
 transition occurs when the group timer expires and is explained in
 detail in section 6.5.

6.2.2. Definition of Group Timers

 The group timer is only used when a group is in EXCLUDE mode and it
 represents the time for the *filter-mode* of the group to expire and
 switch to INCLUDE mode.  We define a group timer as a decrementing
 timer with a lower bound of zero kept per group per attached network.
 Group timers are updated according to the types of group records
 received.
 A group timer expiring when a router filter-mode for the group is
 EXCLUDE means there are no listeners on the attached network in
 EXCLUDE mode.  At this point, a router will transition to INCLUDE
 filter-mode.  Section 6.5 describes the actions taken when a group
 timer expires while in EXCLUDE mode.
 The following table summarizes the role of the group timer.  Section
 6.4 describes the details of setting the group timer per type of
 group record received.

Cain, et. al. Standards Track [Page 27] RFC 3376 IGMPv3 October 2002

    Group
    Filter-Mode      Group Timer Value      Actions/Comments
    -----------      -----------------      ----------------
    INCLUDE          Timer >= 0             All members in INCLUDE
                                            mode.
    EXCLUDE          Timer > 0              At least one member in
                                            EXCLUDE mode.
    EXCLUDE          Timer == 0             No more listeners to
                                            group.  If all source
                                            timers have expired then
                                            delete Group Record.
                                            If there are still
                                            source record timers
                                            running, switch to
                                            INCLUDE filter-mode
                                            using those source records
                                            with running timers as the
                                            INCLUDE source record
                                            state.

6.2.3. Definition of Source Timers

 A source timer is kept per source record and is a decrementing timer
 with a lower bound of zero.  Source timers are updated according to
 the type and filter-mode of the group record received.  Source timers
 are always updated (for a particular group) whenever the source is
 present in a received record for that group.  Section 6.4 describes
 the setting of source timers per type of group records received.
 A source record with a running timer with a router filter-mode for
 the group of INCLUDE means that there is currently one or more
 systems (in INCLUDE filter-mode) which desire to receive that source.
 If a source timer expires with a router filter-mode for the group of
 INCLUDE, the router concludes that traffic from this particular
 source is no longer desired on the attached network, and deletes the
 associated source record.
 Source timers are treated differently when a router filter-mode for a
 group is EXCLUDE.  If a source record has a running timer with a
 router filter-mode for the group of EXCLUDE, it means that at least
 one system desires the source.  It should therefore be forwarded by a
 router on the network.  Appendix A describes the reasons for keeping
 state for sources that have been requested to be forwarded while in
 EXCLUDE state.

Cain, et. al. Standards Track [Page 28] RFC 3376 IGMPv3 October 2002

 If a source timer expires with a router filter-mode for the group of
 EXCLUDE, the router informs the routing protocol that there is no
 longer a receiver on the network interested in traffic from this
 source.
 When a router filter-mode for a group is EXCLUDE, source records are
 only deleted when the group timer expires.  Section 6.3 describes the
 actions that should be taken dependent upon the value of a source
 timer.

6.3. IGMPv3 Source-Specific Forwarding Rules

 When a multicast router receives a datagram from a source destined to
 a particular group, a decision has to be made whether to forward the
 datagram onto an attached network or not.  The multicast routing
 protocol in use is in charge of this decision, and should use the
 IGMPv3 information to ensure that all sources/groups desired on a
 subnetwork are forwarded to that subnetwork.  IGMPv3 information does
 not override multicast routing information; for example, if the
 IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward
 packets for excluded sources to a transit subnet.
 To summarize, the following table describes the forwarding
 suggestions made by IGMP to the routing protocol for traffic
 originating from a source destined to a group.  It also summarizes
 the actions taken upon the expiration of a source timer based on the
 router filter-mode of the group.
    Group
    Filter-Mode    Source Timer Value    Action
    -----------    ------------------    ------
    INCLUDE        TIMER > 0             Suggest to forward traffic
                                         from source
    INCLUDE        TIMER == 0            Suggest to stop forwarding
                                         traffic from source and
                                         remove source record.  If
                                         there are no more source
                                         records for the group, delete
                                         group record.
    INCLUDE        No Source Elements    Suggest to not forward source
    EXCLUDE        TIMER > 0             Suggest to forward traffic
                                         from source

Cain, et. al. Standards Track [Page 29] RFC 3376 IGMPv3 October 2002

    EXCLUDE        TIMER == 0            Suggest to not forward
                                         traffic from source
                                         (DO NOT remove record)
    EXCLUDE        No Source Elements    Suggest to forward traffic
                                         from source

6.4. Action on Reception of Reports

6.4.1. Reception of Current-State Records

 When receiving Current-State Records, a router updates both its group
 and source timers.  In some circumstances, the reception of a type of
 group record will cause the router filter-mode for that group to
 change.  The table below describes the actions, with respect to state
 and timers that occur to a router's state upon reception of Current-
 State Records.
 The following notation is used to describe the updating of source
 timers.  The notation ( A, B ) will be used to represent the total
 number of sources for a particular group, where
 A = set of source records whose source timers > 0 (Sources that at
     least one host has requested to be forwarded)
 B = set of source records whose source timers = 0 (Sources that IGMP
     will suggest to the routing protocol not to forward)
 Note that there will only be two sets when a router's filter-mode for
 a group is EXCLUDE.  When a router's filter-mode for a group is
 INCLUDE, a single set is used to describe the set of sources
 requested to be forwarded (e.g., simply (A)).
 In the following tables, abbreviations are used for several variables
 (all of which are described in detail in section 8).  The variable
 GMI is an abbreviation for the Group Membership Interval, which is
 the time in which group memberships will time out.  The variable LMQT
 is an abbreviation for the Last Member Query Time, which is the total
 time spent after Last Member Query Count retransmissions.  LMQT
 represents the  "leave latency", or the difference between the
 transmission of a membership change and the change in the information
 given to the routing protocol.
 Within the "Actions" section of the router state tables, we use the
 notation 'A=J', which means that the set A of source records should
 have their source timers set to value J.  'Delete A' means that the
 set A of source records should be deleted.  'Group Timer=J' means
 that the Group Timer for the group should be set to value J.

Cain, et. al. Standards Track [Page 30] RFC 3376 IGMPv3 October 2002

 Router State   Report Rec'd  New Router State         Actions
 ------------   ------------  ----------------         -------
 INCLUDE (A)    IS_IN (B)     INCLUDE (A+B)            (B)=GMI
 INCLUDE (A)    IS_EX (B)     EXCLUDE (A*B,B-A)        (B-A)=0
                                                       Delete (A-B)
                                                       Group Timer=GMI
 EXCLUDE (X,Y)  IS_IN (A)     EXCLUDE (X+A,Y-A)        (A)=GMI
 EXCLUDE (X,Y)  IS_EX (A)     EXCLUDE (A-Y,Y*A)        (A-X-Y)=GMI
                                                       Delete (X-A)
                                                       Delete (Y-A)
                                                       Group Timer=GMI

6.4.2. Reception of Filter-Mode-Change and Source-List-Change Records

 When a change in the global state of a group occurs in a system, the
 system sends either a Source-List-Change Record or a Filter-Mode-
 Change Record for that group.  As with Current-State Records, routers
 must act upon these records and possibly change their own state to
 reflect the new desired membership state of the network.
 Routers must query sources that are requested to be no longer
 forwarded to a group.  When a router queries or receives a query for
 a specific set of sources, it lowers its source timers for those
 sources to a small interval of Last Member Query Time seconds.  If
 group records are received in response to the queries which express
 interest in receiving traffic from the queried sources, the
 corresponding timers are updated.
 Similarly, when a router queries a specific group, it lowers its
 group timer for that group to a small interval of Last Member Query
 Time seconds.  If any group records expressing EXCLUDE mode interest
 in the group are received within the interval, the group timer for
 the group is updated and the suggestion to the routing protocol to
 forward the group stands without any interruption.
 During a query period (i.e., Last Member Query Time seconds), the
 IGMP component in the router continues to suggest to the routing
 protocol that it forwards traffic from the groups or sources that it
 is querying.  It is not until after Last Member Query Time seconds
 without receiving a record expressing interest in the queried group
 or sources that the router may prune the group or sources from the
 network.

Cain, et. al. Standards Track [Page 31] RFC 3376 IGMPv3 October 2002

 The following table describes the changes in group state and the
 action(s) taken when receiving either Filter-Mode-Change or Source-
 List-Change Records.  This table also describes the queries which are
 sent by the querier when a particular report is received.
 We use the following notation for describing the queries which are
 sent.  We use the notation 'Q(G)' to describe a Group-Specific Query
 to G.  We use the notation 'Q(G,A)' to describe a Group-and-Source
 Specific Query to G with source-list A.  If source-list A is null as
 a result of the action (e.g., A*B) then no query is sent as a result
 of the operation.
 In order to maintain protocol robustness, queries sent by actions in
 the table below need to be transmitted [Last Member Query Count]
 times, once every [Last Member Query Interval].
 If while scheduling new queries, there are already pending queries to
 be retransmitted for the same group, the new and pending queries have
 to be merged.  In addition, received host reports for a group with
 pending queries may affect the contents of those queries.  Section
 6.6.3 describes the process of building and maintaining the state of
 pending queries.

Router State Report Rec'd New Router State Actions ———— ———— —————- ——-

INCLUDE (A) ALLOW (B) INCLUDE (A+B) (B)=GMI

INCLUDE (A) BLOCK (B) INCLUDE (A) Send Q(G,A*B)

INCLUDE (A) TO_EX (B) EXCLUDE (A*B,B-A) (B-A)=0

                                                  Delete (A-B)
                                                  Send Q(G,A*B)
                                                  Group Timer=GMI

INCLUDE (A) TO_IN (B) INCLUDE (A+B) (B)=GMI

                                                  Send Q(G,A-B)

EXCLUDE (X,Y) ALLOW (A) EXCLUDE (X+A,Y-A) (A)=GMI

EXCLUDE (X,Y) BLOCK (A) EXCLUDE (X+(A-Y),Y) (A-X-Y)=Group Timer

                                                  Send Q(G,A-Y)

EXCLUDE (X,Y) TO_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y)=Group Timer

                                                  Delete (X-A)
                                                  Delete (Y-A)
                                                  Send Q(G,A-Y)
                                                  Group Timer=GMI

Cain, et. al. Standards Track [Page 32] RFC 3376 IGMPv3 October 2002

EXCLUDE (X,Y) TO_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI

                                                  Send Q(G,X-A)
                                                  Send Q(G)

6.5. Switching Router Filter-Modes

 The group timer is used as a mechanism for transitioning the router
 filter-mode from EXCLUDE to INCLUDE.
 When a group timer expires with a router filter-mode of EXCLUDE, a
 router assumes that there are no systems with a *filter-mode* of
 EXCLUDE present on the attached network.  When a router's filter-mode
 for a group is EXCLUDE and the group timer expires, the router
 filter-mode for the group transitions to INCLUDE.
 A router uses source records with running source timers as its state
 for the switch to a filter-mode of INCLUDE.  If there are any source
 records with source timers greater than zero (i.e., requested to be
 forwarded), a router switches to filter-mode of INCLUDE using those
 source records.  Source records whose timers are zero (from the
 previous EXCLUDE mode) are deleted.
 For example, if a router's state for a group is EXCLUDE(X,Y) and the
 group timer expires for that group, the router switches to filter-
 mode of INCLUDE with state INCLUDE(X).

6.6. Action on Reception of Queries

6.6.1. Timer Updates

 When a router sends or receives a query with a clear Suppress
 Router-Side Processing flag, it must update its timers to reflect the
 correct timeout values for the group or sources being queried.  The
 following table describes the timer actions when sending or receiving
 a Group-Specific or Group-and-Source Specific Query with the Suppress
 Router-Side Processing flag not set.
    Query      Action
    -----      ------
    Q(G,A)     Source Timer for sources in A are lowered to LMQT
    Q(G)       Group Timer is lowered to LMQT
 When a router sends or receives a query with the Suppress Router-Side
 Processing flag set, it will not update its timers.

Cain, et. al. Standards Track [Page 33] RFC 3376 IGMPv3 October 2002

6.6.2. Querier Election

 IGMPv3 elects a single querier per subnet using the same querier
 election mechanism as IGMPv2, namely by IP address.  When a router
 receives a query with a lower IP address, it sets the Other-Querier-
 Present timer to Other Querier Present Interval and ceases to send
 queries on the network if it was the previously elected querier.
 After its Other-Querier Present timer expires, it should begin
 sending General Queries.
 If a router receives an older version query, it MUST use the oldest
 version of IGMP on the network.  For a detailed description of
 compatibility issues between IGMP versions see section 7.

6.6.3. Building and Sending Specific Queries

6.6.3.1. Building and Sending Group Specific Queries

 When a table action "Send Q(G)" is encountered, then the group timer
 must be lowered to LMQT.  The router must then immediately send a
 group specific query as well as schedule [Last Member Query Count -
 1] query retransmissions to be sent every [Last Member Query
 Interval] over [Last Member Query Time].
 When transmitting a group specific query, if the group timer is
 larger than LMQT, the "Suppress Router-Side Processing" bit is set in
 the query message.

6.6.3.2. Building and Sending Group and Source Specific Queries

 When a table action "Send Q(G,X)" is encountered by a querier in the
 table in section 6.4.2, the following actions must be performed for
 each of the sources in X of group G, with source timer larger than
 LMQT:
 o Set number of retransmissions for each source to [Last Member Query
   Count].
 o Lower source timer to LMQT.
 The router must then immediately send a group and source specific
 query as well as schedule [Last Member Query Count - 1] query
 retransmissions to be sent every [Last Member Query Interval] over
 [Last Member Query Time].  The contents of these queries are
 calculated as follows.

Cain, et. al. Standards Track [Page 34] RFC 3376 IGMPv3 October 2002

 When building a group and source specific query for a group G, two
 separate query messages are sent for the group.  The first one has
 the "Suppress Router-Side Processing" bit set and contains all the
 sources with retransmission state and timers greater than LMQT.  The
 second has the "Suppress Router-Side Processing" bit clear and
 contains all the sources with retransmission state and timers lower
 or equal to LMQT.  If either of the two calculated messages does not
 contain any sources, then its transmission is suppressed.
 Note: If a group specific query is scheduled to be transmitted at the
 same time as a group and source specific query for the same group,
 then transmission of the group and source specific message with the
 "Suppress Router-Side Processing" bit set may be suppressed.

7. Interoperation With Older Versions of IGMP

 IGMP version 3 hosts and routers interoperate with hosts and routers
 that have not yet been upgraded to IGMPv3.  This compatibility is
 maintained by hosts and routers taking appropriate actions depending
 on the versions of IGMP operating on hosts and routers within a
 network.

7.1. Query Version Distinctions

 The IGMP version of a Membership Query message is determined as
 follows:
    IGMPv1 Query: length = 8 octets AND Max Resp Code field is zero
    IGMPv2 Query: length = 8 octets AND Max Resp Code field is
                  non-zero
    IGMPv3 Query: length >= 12 octets
 Query messages that do not match any of the above conditions (e.g., a
 Query of length 10 octets) MUST be silently ignored.

7.2. Group Member Behavior

7.2.1. In the Presence of Older Version Queriers

 In order to be compatible with older version routers, IGMPv3 hosts
 MUST operate in version 1 and version 2 compatibility modes.  IGMPv3
 hosts MUST keep state per local interface regarding the compatibility
 mode of each attached network.  A host's compatibility mode is

Cain, et. al. Standards Track [Page 35] RFC 3376 IGMPv3 October 2002

 determined from the Host Compatibility Mode variable which can be in
 one of three states:  IGMPv1, IGMPv2 or IGMPv3.  This variable is
 kept per interface and is dependent on the version of General Queries
 heard on that interface as well as the Older Version Querier Present
 timers for the interface.
 In order to switch gracefully between versions of IGMP, hosts keep
 both an IGMPv1 Querier Present timer and an IGMPv2 Querier Present
 timer per interface.  IGMPv1 Querier Present is set to Older Version
 Querier Present Timeout seconds whenever an IGMPv1 Membership Query
 is received.  IGMPv2 Querier Present is set to Older Version Querier
 Present Timeout seconds whenever an IGMPv2 General Query is received.
 The Host Compatibility Mode of an interface changes whenever an older
 version query (than the current compatibility mode) is heard or when
 certain timer conditions occur.  When the IGMPv1 Querier Present
 timer expires, a host switches to Host Compatibility mode of IGMPv2
 if it has a running IGMPv2 Querier Present timer.  If it does not
 have a running IGMPv2 Querier Present timer then it switches to Host
 Compatibility of IGMPv3.  When the IGMPv2 Querier Present timer
 expires, a host switches to Host Compatibility mode of IGMPv3.
 The Host Compatibility Mode variable is based on whether an older
 version General query was heard in the last Older Version Querier
 Present Timeout seconds.  The Host Compatibility Mode is set
 depending on the following:
 Host Compatibility Mode       Timer State
 -----------------------       -----------
       IGMPv3 (default)        IGMPv2 Querier Present not running
                               and IGMPv1 Querier Present not running
       IGMPv2                  IGMPv2 Querier Present running
                               and IGMPv1 Querier Present not running
       IGMPv1                  IGMPv1 Querier Present running
 If a host receives a query which causes its Querier Present timers to
 be updated and correspondingly its compatibility mode, it should
 switch compatibility modes immediately.
 When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3
 protocol on that interface.  When Host Compatibility Mode is IGMPv2,
 a host acts in IGMPv2 compatibility mode, using only the IGMPv2
 protocol, on that interface.  When Host Compatibility Mode is IGMPv1,
 a host acts in IGMPv1 compatibility mode, using only the IGMPv1
 protocol on that interface.

Cain, et. al. Standards Track [Page 36] RFC 3376 IGMPv3 October 2002

 An IGMPv1 router will send General Queries with the Max Resp Code set
 to 0.  This MUST be interpreted as a value of 100 (10 seconds).
 An IGMPv2 router will send General Queries with the Max Resp Code set
 to the desired Max Resp Time, i.e., the full range of this field is
 linear and the exponential algorithm described in section 4.1.1 is
 not used.
 Whenever a host changes its compatibility mode, it cancels all its
 pending response and retransmission timers.

7.2.2. In the Presence of Older Version Group Members

 An IGMPv3 host may be placed on a network where there are hosts that
 have not yet been upgraded to IGMPv3.  A host MAY allow its IGMPv3
 Membership Record to be suppressed by either a Version 1 Membership
 Report, or a Version 2 Membership Report.

7.3. Multicast Router Behavior

7.3.1. In the Presence of Older Version Queriers

 IGMPv3 routers may be placed on a network where at least one router
 on the network has not yet been upgraded to IGMPv3.  The following
 requirements apply:
 o If any older versions of IGMP are present on routers, the querier
   MUST use the lowest version of IGMP present on the network.  This
   must be administratively assured; routers that desire to be
   compatible with IGMPv1 and IGMPv2 MUST have a configuration option
   to act in IGMPv1 or IGMPv2 compatibility modes.  When in IGMPv1
   mode, routers MUST send Periodic Queries with a Max Resp Code of 0
   and truncated at the Group Address field (i.e., 8 bytes long), and
   MUST ignore Leave Group messages.  They SHOULD also warn about
   receiving an IGMPv2 or IGMPv3 query, although such warnings MUST be
   rate-limited.  When in IGMPv2 mode, routers MUST send Periodic
   Queries truncated at the Group Address field (i.e., 8 bytes long),
   and SHOULD also warn about receiving an IGMPv3 query (such warnings
   MUST be rate-limited).  They also MUST fill in the Max Resp Time in
   the Max Resp Code field, i.e., the exponential algorithm described
   in section 4.1.1 is not used.
 o If a router is not explicitly configured to use IGMPv1 or IGMPv2
   and hears an IGMPv1 Query or IGMPv2 General Query, it SHOULD log a
   warning.  These warnings MUST be rate-limited.

Cain, et. al. Standards Track [Page 37] RFC 3376 IGMPv3 October 2002

7.3.2. In the Presence of Older Version Group Members

 IGMPv3 routers may be placed on a network where there are hosts that
 have not yet been upgraded to IGMPv3.  In order to be compatible with
 older version hosts, IGMPv3 routers MUST operate in version 1 and
 version 2 compatibility modes.  IGMPv3 routers keep a compatibility
 mode per group record.  A group's compatibility mode is determined
 from the Group Compatibility Mode variable which can be in one of
 three states:  IGMPv1, IGMPv2 or IGMPv3.  This variable is kept per
 group record and is dependent on the version of Membership Reports
 heard for that group as well as the Older Version Host Present timer
 for the group.
 In order to switch gracefully between versions of IGMP, routers keep
 an IGMPv1 Host Present timer and an IGMPv2 Host Present timer per
 group record.  The IGMPv1 Host Present timer is set to Older Version
 Host Present Timeout seconds whenever an IGMPv1 Membership Report is
 received.  The IGMPv2 Host Present timer is set to Older Version Host
 Present Timeout seconds whenever an IGMPv2 Membership Report is
 received.
 The Group Compatibility Mode of a group record changes whenever an
 older version report (than the current compatibility mode) is heard
 or when certain timer conditions occur.  When the IGMPv1 Host Present
 timer expires, a router switches to Group Compatibility mode of
 IGMPv2 if it has a running IGMPv2 Host Present timer.  If it does not
 have a running IGMPv2 Host Present timer then it switches to Group
 Compatibility of IGMPv3.  When the IGMPv2 Host Present timer expires
 and the IGMPv1 Host Present timer is not running, a router switches
 to Group Compatibility mode of IGMPv3.  Note that when a group
 switches back to IGMPv3 mode, it takes some time to regain source-
 specific state information.  Source-specific information will be
 learned during the next General Query, but sources that should be
 blocked will not be blocked until [Group Membership Interval] after
 that.
 The Group Compatibility Mode variable is based on whether an older
 version report was heard in the last Older Version Host Present
 Timeout seconds.  The Group Compatibility Mode is set depending on
 the following:

Cain, et. al. Standards Track [Page 38] RFC 3376 IGMPv3 October 2002

 Group Compatibility Mode      Timer State
 ------------------------      -----------
       IGMPv3 (default)        IGMPv2 Host Present not running
                               and IGMPv1 Host Present not running
       IGMPv2                  IGMPv2 Host Present running
                               and IGMPv1 Host Present not running
       IGMPv1                  IGMPv1 Host Present running
 If a router receives a report which causes its older Host Present
 timers to be updated and correspondingly its compatibility mode, it
 SHOULD switch compatibility modes immediately.
 When Group Compatibility Mode is IGMPv3, a router acts using the
 IGMPv3 protocol for that group.
 When Group Compatibility Mode is IGMPv2, a router internally
 translates the following IGMPv2 messages for that group to their
 IGMPv3 equivalents:
     IGMPv2 Message                IGMPv3 Equivalent
     --------------                -----------------
       Report                        IS_EX( {} )
       Leave                         TO_IN( {} )
 IGMPv3 BLOCK messages are ignored, as are source-lists in TO_EX()
 messages (i.e., any TO_EX() message is treated as TO_EX( {} )).
 When Group Compatibility Mode is IGMPv1, a router internally
 translates the following IGMPv1 and IGMPv2 messages for that group to
 their IGMPv3 equivalents:
     IGMP Message                  IGMPv3 Equivalent
     ------------                  -----------------
       v1 Report                      IS_EX( {} )
       v2 Report                      IS_EX( {} )
 In addition to ignoring IGMPv3 BLOCK messages and source-lists in
 TO_EX() messages as in IGMPv2 Group Compatibility Mode, IGMPv2 Leave
 messages and IGMPv3 TO_IN() messages are also ignored.

Cain, et. al. Standards Track [Page 39] RFC 3376 IGMPv3 October 2002

8. List of Timers, Counters and Their Default Values

 Most of these timers are configurable.  If non-default settings are
 used, they MUST be consistent among all systems on a single link.
 Note that parentheses are used to group expressions to make the
 algebra clear.

8.1. Robustness Variable

 The Robustness Variable allows tuning for the expected packet loss on
 a network.  If a network is expected to be lossy, the Robustness
 Variable may be increased.  IGMP is robust to (Robustness Variable -
 1) packet losses.  The Robustness Variable MUST NOT be zero, and
 SHOULD NOT be one.  Default: 2

8.2. Query Interval

 The Query Interval is the interval between General Queries sent by
 the Querier.  Default: 125 seconds.
 By varying the [Query Interval], an administrator may tune the number
 of IGMP messages on the network; larger values cause IGMP Queries to
 be sent less often.

8.3. Query Response Interval

 The Max Response Time used to calculate the Max Resp Code inserted
 into the periodic General Queries.  Default: 100 (10 seconds)
 By varying the [Query Response Interval], an administrator may tune
 the burstiness of IGMP messages on the network; larger values make
 the traffic less bursty, as host responses are spread out over a
 larger interval.  The number of seconds represented by the [Query
 Response Interval] must be less than the [Query Interval].

8.4. Group Membership Interval

 The Group Membership Interval is the amount of time that must pass
 before a multicast router decides there are no more members of a
 group or a particular source on a network.
 This value MUST be ((the Robustness Variable) times (the Query
 Interval)) plus (one Query Response Interval).

Cain, et. al. Standards Track [Page 40] RFC 3376 IGMPv3 October 2002

8.5. Other Querier Present Interval

 The Other Querier Present Interval is the length of time that must
 pass before a multicast router decides that there is no longer
 another multicast router which should be the querier.  This value
 MUST be ((the Robustness Variable) times (the Query Interval)) plus
 (one half of one Query Response Interval).

8.6. Startup Query Interval

 The Startup Query Interval is the interval between General Queries
 sent by a Querier on startup.  Default: 1/4 the Query Interval.

8.7. Startup Query Count

 The Startup Query Count is the number of Queries sent out on startup,
 separated by the Startup Query Interval.  Default: the Robustness
 Variable.

8.8. Last Member Query Interval

 The Last Member Query Interval is the Max Response Time used to
 calculate the Max Resp Code inserted into Group-Specific Queries sent
 in response to Leave Group messages.  It is also the Max Response
 Time used in calculating the Max Resp Code for Group-and-Source-
 Specific Query messages.  Default: 10 (1 second)
 Note that for values of LMQI greater than 12.8 seconds, a limited set
 of values can be represented, corresponding to sequential values of
 Max Resp Code.  When converting a configured time to a Max Resp Code
 value, it is recommended to use the exact value if possible, or the
 next lower value if the requested value is not exactly representable.
 This value may be tuned to modify the "leave latency" of the network.
 A reduced value results in reduced time to detect the loss of the
 last member of a group or source.

8.9. Last Member Query Count

 The Last Member Query Count is the number of Group-Specific Queries
 sent before the router assumes there are no local members.  The Last
 Member Query Count is also the number of Group-and-Source-Specific
 Queries sent before the router assumes there are no listeners for a
 particular source.  Default: the Robustness Variable.

Cain, et. al. Standards Track [Page 41] RFC 3376 IGMPv3 October 2002

8.10. Last Member Query Time

 The Last Member Query Time is the time value represented by the Last
 Member Query Interval, multiplied by the Last Member Query Count.  It
 is not a tunable value, but may be tuned by changing its components.

8.11. Unsolicited Report Interval

 The Unsolicited Report Interval is the time between repetitions of a
 host's initial report of membership in a group.  Default: 1 second.

8.12. Older Version Querier Present Timeout

 The Older Version Querier Interval is the time-out for transitioning
 a host back to IGMPv3 mode once an older version query is heard.
 When an older version query is received, hosts set their Older
 Version Querier Present Timer to Older Version Querier Interval.
 This value MUST be ((the Robustness Variable) times (the Query
 Interval in the last Query received)) plus (one Query Response
 Interval).

8.13. Older Host Present Interval

 The Older Host Present Interval is the time-out for transitioning a
 group back to IGMPv3 mode once an older version report is sent for
 that group.  When an older version report is received, routers set
 their Older Host Present Timer to Older Host Present Interval.
 This value MUST be ((the Robustness Variable) times (the Query
 Interval)) plus (one Query Response Interval).

8.14. Configuring Timers

 This section is meant to provide advice to network administrators on
 how to tune these settings to their network.  Ambitious router
 implementations might tune these settings dynamically based upon
 changing characteristics of the network.

8.14.1. Robustness Variable

 The Robustness Variable tunes IGMP to expected losses on a link.
 IGMPv3 is robust to (Robustness Variable - 1) packet losses, e.g., if
 the Robustness Variable is set to the default value of 2, IGMPv3 is
 robust to a single packet loss but may operate imperfectly if more

Cain, et. al. Standards Track [Page 42] RFC 3376 IGMPv3 October 2002

 losses occur.  On lossy subnetworks, the Robustness Variable should
 be increased to allow for the expected level of packet loss. However,
 increasing the Robustness Variable increases the leave latency of the
 subnetwork.  (The leave latency is the time between when the last
 member stops listening to a source or group and when the traffic
 stops flowing.)

8.14.2. Query Interval

 The overall level of periodic IGMP traffic is inversely proportional
 to the Query Interval.  A longer Query Interval results in a lower
 overall level of IGMP traffic.  The Query Interval MUST be equal to
 or longer than the Max Response Time inserted in General Query
 messages.

8.14.3. Max Response Time

 The burstiness of IGMP traffic is inversely proportional to the Max
 Response Time.  A longer Max Response Time will spread Report
 messages over a longer interval.  However, a longer Max Response Time
 in Group-Specific and Source-and-Group-Specific Queries extends the
 leave latency.  (The leave latency is the time between when the last
 member stops listening to a source or group and when the traffic
 stops flowing.)  The expected rate of Report messages can be
 calculated by dividing the expected number of Reporters by the Max
 Response Time.  The Max Response Time may be dynamically calculated
 per Query by using the expected number of Reporters for that Query as
 follows:
    Query Type            Expected number of Reporters
    ----------            ----------------------------
    General Query         All systems on subnetwork
    Group-Specific Query  All systems that had expressed interest
                          in the group on the subnetwork
    Source-and-Group-     All systems on the subnetwork that had
    Specific Query        expressed interest in the source and group
 A router is not required to calculate these populations or tune the
 Max Response Time dynamically; these are simply guidelines.

9. Security Considerations

 We consider the ramifications of a forged message of each type, and
 describe the usage of IPSEC AH to authenticate messages if desired.

Cain, et. al. Standards Track [Page 43] RFC 3376 IGMPv3 October 2002

9.1. Query Message

 A forged Query message from a machine with a lower IP address than
 the current Querier will cause Querier duties to be assigned to the
 forger.  If the forger then sends no more Query messages, other
 routers' Other Querier Present timer will time out and one will
 resume the role of Querier.  During this time, if the forger ignores
 Leave Messages, traffic might flow to groups with no members for up
 to [Group Membership Interval].
 A DoS attack on a host could be staged through forged Group-and-
 Source-Specific Queries.  The attacker can find out about membership
 of a specific host with a general query.  After that it could send a
 large number of Group-and-Source-Specific queries, each with a large
 source list and the Maximum Response Time set to a large value.  The
 host will have to store and maintain the sources specified in all of
 those queries for as long as it takes to send the delayed response.
 This would consume both memory and CPU cycles in order to augment the
 recorded sources with the source lists included in the successive
 queries.
 To protect against such a DoS attack, a host stack implementation
 could restrict the number of Group-and-Source-Specific Queries per
 group membership within this interval, and/or record only a limited
 number of sources.
 Forged Query messages from the local network can be easily traced.
 There are three measures necessary to defend against externally
 forged Queries:
 o Routers SHOULD NOT forward Queries.  This is easier for a router to
   accomplish if the Query carries the Router-Alert option.
 o Hosts SHOULD ignore v2 or v3 Queries without the Router-Alert
   option.
 o Hosts SHOULD ignore v1, v2 or v3 General Queries sent to a
   multicast address other than 224.0.0.1, the all-systems address.

9.2. Current-State Report messages

 A forged Report message may cause multicast routers to think there
 are members of a group on a network when there are not.  Forged
 Report messages from the local network are meaningless, since joining
 a group on a host is generally an unprivileged operation, so a local
 user may trivially gain the same result without forging any messages.
 Forged Report messages from external sources are more troublesome;
 there are two defenses against externally forged Reports:

Cain, et. al. Standards Track [Page 44] RFC 3376 IGMPv3 October 2002

 o Ignore the Report if you cannot identify the source address of the
   packet as belonging to a network assigned to the interface on which
   the packet was received.  This solution means that Reports sent by
   mobile hosts without addresses on the local network will be
   ignored.  Report messages with a source address of 0.0.0.0 SHOULD
   be accepted on any interface.
 o Ignore Report messages without Router Alert options [RFC-2113], and
   require that routers not forward Report messages.  (The requirement
   is not a requirement of generalized filtering in the forwarding
   path, since the packets already have Router Alert options in them.)
   This solution breaks backwards compatibility with implementations
   of IGMPv1 or earlier versions of IGMPv2 which did not require
   Router Alert.
 A forged Version 1 Report Message may put a router into "version 1
 members present" state for a particular group, meaning that the
 router will ignore Leave messages.  This can cause traffic to flow to
 groups with no members for up to [Group Membership Interval].  This
 can be solved by providing routers with a configuration switch to
 ignore Version 1 messages completely.  This breaks automatic
 compatibility with Version 1 hosts, so should only be used in
 situations where "fast leave" is critical.
 A forged Version 2 Report Message may put a router into "version 2
 members present" state for a particular group, meaning that the
 router will ignore IGMPv3 source-specific state messages.  This can
 cause traffic to flow from unwanted sources for up to [Group
 Membership Interval].  This can be solved by providing routers with a
 configuration switch to ignore Version 2 messages completely.  This
 breaks automatic compatibility with Version 2 hosts, so should only
 be used in situations where source include and exclude is critical.

9.3. State-Change Report Messages

 A forged State-Change Report message will cause the Querier to send
 out Group-Specific or Source-and-Group-Specific Queries for the group
 in question.  This causes extra processing on each router and on each
 member of the group, but can not cause loss of desired traffic.
 There are two defenses against externally forged State-Change Report
 messages:

Cain, et. al. Standards Track [Page 45] RFC 3376 IGMPv3 October 2002

 o Ignore the State-Change Report message if you cannot identify the
   source address of the packet as belonging to a subnet assigned to
   the interface on which the packet was received.  This solution
   means that State-Change Report messages sent by mobile hosts
   without addresses on the local subnet will be ignored.  State-
   Change Report messages with a source address of 0.0.0.0 SHOULD be
   accepted on any interface.
 o Ignore State-Change Report messages without Router Alert options
   [RFC-2113], and require that routers not forward State-Change
   Report messages.  (The requirement is not a requirement of
   generalized filtering in the forwarding path, since the packets
   already have Router Alert options in them.)

9.4. IPSEC Usage

 In addition to these measures, IPSEC in Authentication Header mode
 [AH] may be used to protect against remote attacks by ensuring that
 IGMPv3 messages came from a system on the LAN (or, more specifically,
 a system with the proper key).  When using IPSEC, the messages sent
 to 224.0.0.1 and 224.0.0.22 should be authenticated using AH.  When
 keying, there are two possibilities:
 1. Use a symmetric signature algorithm with a single key for the LAN
    (or a key for each group).  This allows validation that a packet
    was sent by a system with the key.  This has the limitation that
    any system with the key can forge a message; it is not possible to
    authenticate the individual sender precisely.  It also requires
    disabling IPSec's Replay Protection.
 2. When appropriate key management standards have been developed, use
    an asymmetric signature algorithm.  All systems need to know the
    public key of all routers, and all routers need to know the public
    key of all systems.  This requires a large amount of key
    management but has the advantage that senders can be authenticated
    individually so e.g., a host cannot forge a message that only
    routers should be allowed to send.
 This solution only directly applies to Query and Leave messages in
 IGMPv1 and IGMPv2, since Reports are sent to the group being reported
 and it is not feasible to agree on a key for host-to-router
 communication for arbitrary multicast groups.

Cain, et. al. Standards Track [Page 46] RFC 3376 IGMPv3 October 2002

10. IANA Considerations

 All IGMP types described in this document are already assigned in
 [IANA-REG].

11. Acknowledgments

 We would like to thank Ran Atkinson, Luis Costa, Toerless Eckert,
 Dino Farinacci, Serge Fdida, Wilbert de Graaf, Sumit Gupta, Mark
 Handley, Bob Quinn, Michael Speer, Dave Thaler and Rolland Vida for
 comments and suggestions on this document.
 Portions of the text of this document were copied from [RFC-1112] and
 [RFC-2236].

12. Normative References

 [AH]         Kent, S. and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.
 [IANA-REG]   http://www.iana.org/assignments/igmp-type-numbers
 [RFC-1112]   Deering, S., "Host Extensions for IP Multicasting", STD
              5, RFC 1112, August 1989.
 [RFC-2113]   Katz, D., "IP Router Alert Option," RFC 2113, February,
              1997.
 [RFC-2119]   Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC-2236]   Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, November 1997.
 [RFC-3228]   Fenner, B., "IANA Considerations for IPv4 Internet Group
              Management Protocol (IGMP)", BCP 57, RFC 3228, February
              2002.

13. Informative References

 [RFC-1071]   Braden, R., Borman, D. and C.  Partridge, "Computing the
              Internet checksum", RFC 1071, September 1988.
 [FILTER-API] Thaler, D., B. Fenner, and B. Quinn, "Socket Interface
              Extensions for Multicast Source Filters", Work in
              Progress.

Cain, et. al. Standards Track [Page 47] RFC 3376 IGMPv3 October 2002

 [SSM]        Bhattacharyya, S., et. al., "An Overview of Source-
              Specific Multicast (SSM)", Work in Progress.
 [MLD]        Deering, S., Fenner, W. and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710, October
              1999.
 [MLDV2]      Vida, R., L. Costa, S. Fdida, S. Deering, B. Fenner, I.
              Kouvelas, and B. Haberman, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", Work in Progress.

Cain, et. al. Standards Track [Page 48] RFC 3376 IGMPv3 October 2002

Appendix A. Design Rationale

A.1 The Need for State-Change Messages

 IGMPv3 specifies two types of Membership Reports: Current-State and
 State Change.  This section describes the rationale for the need for
 both these types of Reports.
 Routers need to distinguish Membership Reports that were sent in
 response to Queries from those that were sent as a result of a change
 in interface state.  Membership reports that are sent in response to
 Membership Queries are used mainly to refresh the existing state at
 the router; they typically do not cause transitions in state at the
 router.  Membership Reports that are sent in response to changes in
 interface state require the router to take some action in response to
 the received report (see Section 6.4).
 The inability to distinguish between the two types of reports would
 force a router to treat all Membership Reports as potential changes
 in state and could result in increased processing at the router as
 well as an increase in IGMP traffic on the network.

A.2 Host Suppression

 In IGMPv1 and IGMPv2, a host would cancel sending a pending
 membership reports if a similar report was observed from another
 member on the network.  In IGMPv3, this suppression of host
 membership reports has been removed.  The following points explain
 the reasons behind this decision.
 1. Routers may want to track per-host membership status on an
    interface.  This allows routers to implement fast leaves (e.g.,
    for layered multicast congestion control schemes) as well as track
    membership status for possible accounting purposes.
 2. Membership Report suppression does not work well on bridged LANs.
    Many bridges and Layer2/Layer3 switches that implement IGMP
    snooping do not forward IGMP messages across LAN segments in order
    to prevent membership report suppression.  Removing membership
    report suppression eases the job of these IGMP snooping devices.
 3. By eliminating membership report suppression, hosts have fewer
    messages to process; this leads to a simpler state machine
    implementation.

Cain, et. al. Standards Track [Page 49] RFC 3376 IGMPv3 October 2002

 4. In IGMPv3, a single membership report now bundles multiple
    multicast group records to decrease the number of packets sent.
    In comparison, the previous versions of IGMP required that each
    multicast group be reported in a separate message.

A.3 Switching Router Filter Modes from EXCLUDE to INCLUDE

 If there exist hosts in both EXCLUDE and INCLUDE modes for a single
 multicast group in a network, the router must be in EXCLUDE mode as
 well (see section 6.2.1).  In EXCLUDE mode, a router forwards traffic
 from all sources unless that source exists in the exclusion source
 list.  If all hosts in EXCLUDE mode cease to exist, it would be
 desirable for the router to switch back to INCLUDE mode seamlessly
 without interrupting the flow of traffic to existing receivers.
 One of the ways to accomplish this is for routers to keep track of
 all sources desired by hosts that are in INCLUDE mode even though the
 router itself is in EXCLUDE mode.  If the group timer now expires in
 EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on
 the network (otherwise a membership report from that host would have
 refreshed the group timer).  The router can then switch to INCLUDE
 mode seamlessly with the list of sources currently being forwarded in
 its source list.

Appendix B. Summary of Changes from IGMPv2

 While the main additional feature of IGMPv3 is the addition of source
 filtering, the following is a summary of other changes from RFC 2236.
 o State is maintained as Group + List-of-Sources, not simply Group as
   in IGMPv2.
 o Interoperability with IGMPv1 and IGMPv2 systems is defined as
   operations on the IGMPv3 state.
 o The IP Service Interface has changed to allow specification of
   source-lists.
 o The Querier includes its Robustness Variable and Query Interval in
   Query packets to allow synchronization of these variables on non-
   Queriers.
 o The Max Response Time in Query messages has an exponential range,
   changing the maximum from 25.5 seconds to about 53 minutes, for use
   on links with huge numbers of systems.
 o Hosts retransmit state-change messages for increased robustness.

Cain, et. al. Standards Track [Page 50] RFC 3376 IGMPv3 October 2002

 o Additional data sections are defined to allow later extensions.
 o Report packets are sent to 224.0.0.22, to assist layer-2 switches
   in "snooping".
 o Report packets can contain multiple group records, to allow
   reporting of full current state using fewer packets.
 o Hosts no longer perform suppression, to simplify implementations
   and permit explicit membership tracking.
 o New Suppress Router-Side Processing (S) flag in Query messages
   fixes robustness issues which were also present in IGMPv2.

Cain, et. al. Standards Track [Page 51] RFC 3376 IGMPv3 October 2002

Authors' Addresses

 Brad Cain
 Cereva Networks
 Steve Deering
 Cisco Systems, Inc.
 170 Tasman Drive
 San Jose, CA 95134-1706
 Phone: +1-408-527-8213
 EMail: deering@cisco.com
 Bill Fenner
 AT&T Labs - Research
 75 Willow Rd.
 Menlo Park, CA 94025
 Phone: +1-650-330-7893
 EMail: fenner@research.att.com
 Isidor Kouvelas
 Cisco Systems, Inc.
 170 Tasman Drive
 San Jose, CA 95134-1706
 Phone: +1-408-525-0727
 EMail: kouvelas@cisco.com
 Ajit Thyagarajan
 Ericsson IP Infrastructure

Cain, et. al. Standards Track [Page 52] RFC 3376 IGMPv3 October 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  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
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 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 assigns.
 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
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Cain, et. al. Standards Track [Page 53]

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