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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 3376

Internet Engineering Task Force           Inter-Domain Multicast Routing
                                                           Working Group
INTERNET-DRAFT                                Brad Cain, Nortel Networks
                                            Steve Deering, Cisco Systems
draft-ietf-idmr-igmp-v3-01.txt        Ajit Thyagarajan, Torrent Networks
                                                           February 1999
                                                     Expires August 1999


              Internet Group Management Protocol, Version 3
                   <draft-ietf-idmr-igmp-v3-01.txt>


STATUS OF THIS MEMO

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

  Internet-Drafts are working documents  of  the  Internet  Engineering
  Task  Force  (IETF),  its  areas,  and its working groups.  Note that
  other groups may  also  distribute  working  documents  as  Internet-
  Drafts.

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

  The  list   of   current   Internet-Drafts   can   be   accessed   at
  http://www.ietf.org/ietf/1id-abstracts.txt

  The list of Internet-Draft Shadow  Directories  can  be  accessed  at
  http://www.ietf.org/shadow.html.


                                  Abstract

This document specifies Version 3 of the Internet Group Management
Protocol, IGMPv3.  IGMP is the protocol used by IP 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 is a product of the Inter-Domain Multicast Routing working
group within the Internet Engineering Task Force.  Comments are
solicited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the author(s).

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1.  INTRODUCTION

The Internet Group Management Protocol (IGMP) is used by IP 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 [IGMPv2],
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, 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.



2.  THE API FOR REQUESTING IP MULTICAST RECEPTION

Within an IP system, there is (at least conceptually) an Application
Programmming Interface or API 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 API must
support the following operation (or any logical equivalent):

       IPMulticastListen ( socket, interface, multicast-address,
                           filter-mode, source-list )

where

   "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.

   "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

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   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.

  "multicast-address" is the IP multicast address 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.

   "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.

   "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.

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.

Previous versions of IGMP did not support source filters and had a
simpler API consisting of Join and Leave operations to enable and
disable reception of a given multicast address (from *all* sources) on
a given interface.  Those Join and Leave operations are supported by
the new API as follows:

   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.



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It is recommended that implementations continue to support the old API,
(perhaps as calls on the new API) for compatibility with pre-existing IP
multicast applications.

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 updated 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)

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:

          IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} )

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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 may be delivered on socket s1 but not on socket
s2.)

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} )

    then the corresponding interface record on interface i is:

                           ( m, EXCLUDE, {b, c} )

  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} )

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    *However*, there is one exception to this rule: if the source list
    in an interface record in INCLUDE mode would contain too many
    addresses to fit in an IGMP Version 3 Membership Report message (see
    paragraph 4.2.5), an interface record of the following form is
    created instead:

                           ( m, EXCLUDE, {} )

    That is, reception of packets to multicast address m from *all*
    sources is enabled on the interface, whenever the specific list of      desired sources is too long to include in an IGMP Report.   The
    exact limit depends on the MTU of the attached network; on Ethernet,    for example, the limit is 366 addresses.

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-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 IP 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, and carries an IP Router Alert option
[RFC2113] in its IP header.

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 6):

          0x12          Version 1 Membership Report    [RFC-1112]

          0x16          Version 2 Membership Report    [IGMPv2]

          0x17          Version 2 Leave Group          [IGMPv2]

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.

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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:

    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 Time |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Group Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Reserved           |     Number of Sources (N)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address [1]                      |
   +-                                                             -+
   |                       Source Address [2]                      |
   +-                              .                              -+
   .                               .                               .
   .                               .                               .
   +-                                                             -+
   |                       Source Address [N]                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.1.1   Max Response Time

     The Max Response Time field specifies the maximum time allowed
     before sending a responding report, in units of 1/10 second.

     Varying this setting allows 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).  It also allows tuning of the burstiness of IGMP
     traffic on a network.

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.

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

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     sending a Group-Specific Query or Group-and-Source-Specific Query
     (see section 4.1.8, below).

4.1.4  Reserved

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

4.1.5  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).

4.1.6  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.

4.1.7  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.

4.1.8  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

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        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.

4.1.9  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 and Group-and-
Source 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:

    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]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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where each Group Record has the following internal format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Record Type  |    Reserved   |     Number of Sources (N)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Multicast Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Source Address [1]                      |
   +-                                                             -+
   |                       Source Address [2]                      |
   +-                              .                              -+
   .                               .                               .
   .                               .                               .
   +-                                                             -+
   |                       Source Address [N]                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

4.2.4  Record Type

    See section 4.2.9, below.

4.2.5  Number of Sources (N)

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

4.2.6  Multicast Address

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

4.2.7  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.

4.2.8  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.

4.2.9  Group Record Types

There are a number of different types of Group Records that may be
included in a Report message:

    (1) 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.

    (2) 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:

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            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.

    (3) 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.

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4.2.10  IP Destination Addresses for Reports

Version 3 Reports are sent with an IP destination address of 224.0.0.?,
the all-membership-reports multicast group address.  A system that is
operating in version 1 or version 2 compatibility modes sends version 1
and 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.

4.2.11  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:

        - a capital letter (e.g., "A") to represent the set of source
          addresses,

        - a list of zero or more lower-case letters enclosed in braces
          (e.g., "{a, b, c}" or "{}") to represent the individual source
          addresses in the list, or

        - a set expression (e.g., "A + B"), where "x + y" means the
          union of x and y,  "x * y" means the intersection of x and y,
          and "x - y" means the removal of all elements of set y from
          set x.

4.2.12 Membership Report Size

If the entire list of Group Records does not fit within the MTU limits
of a single Report message, the Group Records are sent in as many Report
messages required to report the entire list of Group Records.  Each
Report is filled with the maximum number of Group Records within the
MTU limit.

Similarly, if the list of source addresses comprising a single Group
Record does not fit within the MTU limits of a single Report message,
the Group Record is sent in as many Report messages required to report
the entire list of sources.


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5.  PROTOCOL DESCRIPTION

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.  This
information is then 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.

IGMP version 3 adds the capability for a multicast router to learn
which *sources* are of interest to neighboring systems, for packets
sent to any particular multicast address.

It is important to understand that a multicast router needs 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;
the multicast router does not need to keep track of the interests of
each individual neighboring system.

IGMP is an asymmetric protocol, defining separate behaviors for group
members -- that is, hosts or routers that wish to receive multicast
packets -- and multicast routers.  A multicast router that is also a
group member performs both parts of the protocol, receiving its own
IGMP message transmissions as well as those of its neighbors.

If a multicast router has more than one interface on the same network,
it needs to operate the multicast router part of IGMP over only one of
those interfaces.  A group member, on the other hand, must operate the
group member part of IGMP over all interfaces from which reception of
multicast packets has been requested by IPMulticastListen invocations.

Membership in the all-systems multicast group, address 224.0.0.1, is
handled as a special case: 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 reporting the all-systems multicast address.

Note that, in the following descriptions, timer and counter names
appear in square brackets.  The default values for those timers and
counters are specified in section 7.


5.1  Group Member Behavior

For interoperability with multicast routers running older versions of
IGMP, systems maintain a MulticastRouterVersion variable for each
interface on which multicast reception is desired.  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 6.1.

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There are three types of events that trigger IGMP protocol actions on
an interface on which multicast reception is enabled:

  o a change of the interface reception state, caused by a local
    invocation of IPmulticastListen.

  o reception of a Query.

  o reception of a Report.

The following subsections describe the actions to be taken for each of
these cases.

5.1.1  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 immediately to transmit a
State-Change Report from that interface, with the Group field of the
Report carrying the affected multicast address.  The contents of the
rest of the report are determined by comparing the filter mode and
source list for that 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-existant" state is considered to have a filter mode of
INCLUDE and an empty source list.

  Old State         New State         State-Change Report 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)

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 State-Change Report reflecting the difference between
the newest state and the state *before* the first change for which

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retransmissions were not completed.  The transmission of the newer
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 the newer Report.


5.1.2  Action on Reception of a Query

A system may receive any of the three variants of IGMPv3 Query messages:
a General Query, a Group-Specific Query, or a Group-and-Source-Specific
Query (see section 4.1.8).

IGMPv3 hosts report multiple groups within the same "report packet".
Because of this "group bundling", a global report delay timer is used.
This timer is reset upon reception of a general query.  When a global
group delay timer expires, a report is sent for all groups for which
the host is joined on that interface.  For handling of Group-Specific
and Group-and-Source Specific Queries a per group timer is kept.

When a system receives a General Query, it adjusts the global "report
delay timer" on the interface from which the Query was received.  If
the timer is not running, it is set to a random value, using the finest
clock granularity available on the system, from the range (0,
MaxResponseTime], where MaxResponseTime is obtained from the Query
message.  If the global report timer is already running, it is reset to
a new random value only if the requested MaxResponseTime is less than
the remaining value of the running timer.  Otherwise, it is left
running with its current value.

When the global report timer expires, the system transmits one or more
Reports reporting for each multicast address present on that interface,
its filter mode (either MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and the
list of sources associated with the multicast address; any group-
specific report delay timers, if running, on that interface are deleted
as well.  If there is a change in filter mode or source address list of
a multicast group with the global report delay timer still running, a
State-Change Report is sent for that multicast group address as
specified in Section 5.1.1; the global report delay timer is not
deleted.

When a system receives a Group-Specific Query, it performs a similar
set of actions as it does when receiving a General Query, *except* that
the actions are performed only for the multicast address that was
specified in the Group-Specific Query and using the "group-specific
report delay timer" instead of the global timer.

When a multicast group report delay timer expires, the system
transmits a Report, reporting that multicast address and the filter mode
(either MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and the entire list of
sources addresses associated with the multicast address.


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When a system receives a Group-and-Source-Specific Query, it performs
the same actions as it does when receiving a Group-Specific Query,
*except*:

  o the pending report record and report delay timer it creates are
    *in addition to* any record and timer it may have created for the
    same multicast address in response to a General or Group-Specific
    Query.

  o the initial contents of the newly-created pending report record are
    determined by comparing the source list in the received Query with
    the local state, as specified in the following table:

    Local State       Query Source List       Pending Report Record
    -----------       -----------------       ---------------------

    INCLUDE (A)             B                 IS_IN (A*B)

    EXCLUDE (A)             B                 IS_IN (B-A)


5.1.3  Action on Reception of a Report

Earlier drafts of IGMPv3 specified that hosts perform suppression on
reports.  This was removed due to the complexity of the merging
operations involved.  IGMPv3 hosts send reports to the all-membership-
reports multicast address and do not perform suppression.


5.2  Multicast Router Behavior

5.2.1 Overview

Multicast routers use the IGMP protocol to periodically query the
multicast reception state of hosts attached to local subnetworks.
General Queries are sent periodically; Group-Specific and Group-and-
Source-Specific Queries are sent in response to Filter-Mode-Change and
Source-List-Change records.

It is important to understand that a multicast router needs 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;
the multicast router does not need to keep track of the interests of
each individual neighboring system.  IGMPv3 router MAY track the
memberships of individual hosts (at the expense of additional state).
If a router chooses to keep individual membership information, it may
omit Group-Specific and Group-and-Source-Specific Queries from its
implementation.  A router MUST implement IGMPv2 Group-Specific
queries for backwards compatibility.

IGMPv3 is backward compatible with previous versions of the IGMP
protocol.  In order to remain backward compatible with older IGMP

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hosts and routers, IGMPv3 routers must also implement versions 1 and 2
of the protocol (see section 6).


5.2.2  Conditions for IGMP Queries

IGMPv3 routers send General Queries periodically to request group
membership information.  Hosts respond to these queries with Current-
State Reports.  As hosts update the state of individual Group Records,
they may send send Filter-Mode-Change Records or Source-List-Change
Records.  In order for all hosts on a subnet to respond to changes in
group membership, a router sends specific queries.  A Group-Specific
Query is sent to verify there are no neighboring interfaces that desire
reception of the specified group.  A Group-and-Source Specific Query
lists sources within a group that have been requested to be deleted
(i.e. not forwarded).  This query is sent by a router to learn if any
neighboring interfaces desire reception of packets to the specified
group address from the specified source addresses.  Section 4.1.8
describes each query in more detail.


5.2.3  IGMP State Maintained by Multicast Routers

Routers implementing the IGMPv3 protocol keep keep state per group
per interface.  This state consists of a filter-mode, list of sources,
and various timers.  For each interface on which IGMP exists, a router
records the desired reception state for that locally attached network.
That state conceptually consists of a set of records of the form:

      (multicast address, group timer, filter-mode, source-element list)

Each member of the source-element list is a record of the form:

      (source address, source timer)

If all sources within a given group are desired, an empty source
element list is kept with the filter-mode set to EXCLUSION.  This means
forward all sources for this group.  This is equivalent to the older
version join of single group (i.e.  as in previous versions of IGMP).


5.2.3.1 Definition of Router Filter-Mode

To reduce internal state, IGMPv3 routers keep a filter-mode per group
per interface.  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.  The filter-mode of a group record may
change when receiving new reports, or when certain timer conditions
occur.  Upon receiving a host report, the filter-mode is updated to
represent the most number of sources desired with the least amount of
state.  In general, once a filter-mode of EXCLUSION is received, the
router filter-mode for that group record will be EXCLUSION.  A
mechanism to transition back to an INCLUSION filter-mode is specified

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as all hosts with EXCLUSION filter-mode may cease reporting, leaving
the router filter-mode in an EXCLUSION state.  Section 5.2.5 describes
the changes of a router-filter mode per host report received.  Section
5.2.6 describes the change in filter-mode when certain timer conditions
are met.


5.2.3.2 Definition Group Timers

A group timer is kept per group per interface state record.  A group
timer is a decrementing timer with a lower bound of zero.  Group timers
are updated according to the filter-mode of the group record and type of
host report received.  Group timers are not always updated per a
received group report.  A group timer expiring when the filter-mode is
INCLUSION means there are no listeners on the attached subnet for that
group.  A group timer expiring when the filter-mode is EXCLUSION means
there are no listeners on the attached subnet in EXCLUSION mode.
Section 5.2.5 details the exact setting of the group timer per filter
mode and report received.  Section 5.2.6 details the events when a group
timer expires while in EXCLUSION mode.


5.2.3.3 Definition of Source Timers

A source timer is kept per source element record.  A source timer is a
decrementing timer with a lower bound of zero.  Source timers are
updated according to the filter-mode of the group record and type of
host report received.  Unlike group timers, source timers are always
updated for a particular group whenever the source is present in a
received record for that group.  Section 5.2.5 details the exact
setting of the source timer per filter mode and report received.

The actions performed upon the expiry of source element timers depend
upon the group filter-mode.  If a source timer expires with the group
in INCLUSION filter-mode, the router concludes that traffic from thisi
particular source is no longer desired on the locally attached
interface, and deletes the associated record.  If a source timer expires
with the group in EXCLUSION filter-mode, the router stops forwarding
traffic from this source onto the interface, but does not delete its
record.  Section 5.2.4 details the actions that should be taken,
dependant upon the value of a source timer.

5.2.4  IGMPv3 Source Specific Forwarding Rules

When a router receives a datagram from a source destined to a particular
group, a decision has to be made whether to forward the datagram onto
the subnetwork or not.  This decision is first dependent on any
multicast routing protocols present.  Assuming that there are no other
routers downstream (or that downstream routers support source-specific
pruning/grafting), the forwarding decision depends on the presence of a
group record and/or a source-specific record.  If no group record
exists, the datagram is not forwarded on the subnetwork.  Even if a
group record exists, a source-specific record may or may not exist for

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the particular source.

To summarize, the following table details the actions to be performed
for traffic originating from a source destined to a group, and upon
the expiry of a source-specific timer, based on the group mode and the
value of the source-specific timer, if it exists:

  Group
  Filter-Mode      Source Timer Value        Action
  -----------      ------------------        ------

  INCLUDE          TIMER > 0                 Forward source

  INCLUDE          TIMER == 0                Stop forwarding source and
                                               remove source record

  INCLUDE          None                      Don't forward source

  EXCLUDE          TIMER > 0                 Don't forward source

  EXCLUDE          TIMER == 0                Start forwarding source

  EXCLUDE          None                      Forward source


5.2.5  Actions upon Reception of Reports

5.2.5.1 Reception of Current-State reports

When receiving Current-State Reports, routers update both group timers
and source timers.  Depending on the routers current filter-mode for a
group and the filter-mode of a Current-State Report received for that
group, a router may change to a different filter-mode per group.  The
table below details the actions, with respect to state and timers, that
occur at a router when receiving Current-State Reports.

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 sources whose timers > 0 (being forwarded)
          B = set of sources whose timers = 0 (not being forwarded)

The variable TQ is one query period plus the time to report field of
the last General Query.  The variable RI is the time to report field of
the last send Query.






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

  INCLUDE (A)    IS_IN (B)     INCLUDE (A+B)            (B) = TQ
                                                        Group Timer = TQ

  INCLUDE (A)    IS_EX (B)     EXCLUDE (A*B,B-A)        (B-A) = 0
                                                        Group Timer = TQ

  EXCLUDE (X,Y)  IS_IN (A)     EXCLUDE (X+A,Y-A)        (A) = TQ

  EXCLUDE (X,Y)  IS_EX (B)     EXCLUDE (X*B,Y-B)        Group Timer = TQ


5.2.5.2 Reception of Filter-Mode-Change and Source-List-Change Reports

When a change in the global state of a group occurs in a host, the host
sends either a Source-List-Change Report or a Filter-Mode-Change Report
for that group.  As with Current-State Reports, routers must act upon
these reports and possibly change their state to reflect the new
desired membership state.

Routers must query any sources that are requested to be deleted from the
network.  When a router queries a specific set of sources, it sets the
source timers to a small interval (5 secs).  If host membership reports
are received which express interest in receiving traffic from the
specific sources, the corresponding timers are updated.  Similarly, when
a router queries a specific group, it sets the group timer to a small
interval (5 secs).  If any membership reports expressing interest in
the group are received within the interval, the group timer is updated
and traffic is forwarded without any interruption.  During the interval,
the router continues to forward traffic from the sources that were
requested to be deleted to prevent traffic from being interrupted to
other interested hosts.

The following table details the change in state and actions that are
taken when receiving either Filter-Mode-Change or Source-List-Change
Reports dependent upon the current state of the group.














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

  INCLUDE (A)     ALLOW (B)    INCLUDE (A+B)              (B)=TQ
                                                          Group Timer=TQ

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

  INCLUDE (A)     ALLOW (B),   INCLUDE (A+B)              (B)=TQ
                  BLOCK (C)                               (A*C)=RI
                                                          Send Q(G,A*C)
                                                          Group Timer=TQ

  INCLUDE (A)     TO_EX (B)    EXCLUDE (A*B,B-A)          (A*B)=RI
                                                          Send Q(G,A*B )
                                                          Group Timer=TQ

  INCLUDE (A)     TO_IN (B)    INCLUDE (A+B)              (B)=TQ
                                                          (A-B)=RI
                                                          Send Q(G,A-B)
                                                          Group Timer=TQ

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

  EXCLUDE (X,Y)   BLOCK (A)    EXCLUDE (X+(A-Y),Y)        (A-Y)=RI
                                                          Send Q(G,A-Y)

  EXCLUDE (X,Y)   ALLOW (A),   EXCLUDE (X+A+(B-Y),Y-A)    (A)=TQ
                  BLOCK (B)                               (B-Y)=RI
                                                          Send Q(G,B-Y)

  EXCLUDE (X,Y)   TO_EX (A)    EXCLUDE (X*A,Y*A)          (A-Y)=RI
                                                          Send Q(G,A-Y)
                                                          Group Timer=TQ

  EXCLUDE (X,Y)   TO_IN (A)    EXCLUDE (X+A,Y-A)          (A)=TQ
                                                          (X-A)=RI
                                                          Send Q(G,*)


5.2.6  Switching Filter-Modes

The IGMPv3 group timer is used as a mechanism for transitioning from
EXCLUSION filter-mode to INCLUSION filter-mode.  When a group timer
expires with the group in INCLUSION filter-mode, a router concludes that
there are no group members present on the locally attached interface and
deletes the group record and the associated source records.

When a group timer expires with the group in EXCLUSION filter-mode, a
router assumes that there are no hosts in EXCLUSION filter-mode are

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present on the attached subnetwork, and switches to INCLUSION filter-
mode.  In addition, all source-element records with zero timer values
are deleted and the group timer assumes the maximum value among all the
remaining source-element records corresponding to that group.  A Group-
Specific Query is then sent for the group to determine the desired group
and source state for that group.


5.2.7  Action of Reception of Queries

IGMPv3 uses the same querier election method as previous versions.  Upon
receiving an IGMPv3 General Query from another router, the querier
ceases to send General Queries and sets the OTHER_QUERIER_PRESENT timer.
Upon expiry of OTHER_QUERIER_PRESENT timer, a router becomes the
querier.  Routers who are not the acting querier reset
OTHER_QUERIER_PRESENT timer upon reception of a IGMPv3 General Query.
For details of compatibility between versions see section 6.



6.  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.


6.1  Query Version Distinctions

The IGMP version of a Host Membership Query message is determined as
follows:

   IGMPv1 Query: length = 8 octets AND Max Response Time field is zero

   IGMPv2 Query: length = 8 octets AND Max Response Time field is
                 non-zero

   IGMPv3 Query: length >= 12 octets AND Reserved field is zero

Query messages that do not match any of the above conditions (e.g., a
Query of length 10 octets) must be silently ignored.


6.2  Group Member Behavior

IGMPv3 hosts can operate in version 1 and version 2 compatibility modes.
The mode in which a host operates is governed by the version of the
querier router on an interface.  The version of the querier can be
determined from a Host Membership Query.


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IGMPv3 hosts keep state per local interface regarding the version of
querier on the attached network.  Hosts can be in one of three states
depending on the version of querier on their attached networks.  This
state is reflected by Querier Version, a state variable kept per
interface describing the version of querier on the attached network.
This state variable can have only one of three values: IGMPv3, IGMPv2,
IGMPv1.  When Querier Version is IGMPv3, a host acts using the IGMPv3
protocol.  When Querier Version is IGMPv2, a host acts in IGMPv2
compatibility mode, using only the IGMPv2 protocol.  When Querier
Version is IGMPv1, a host acts in IGMPv1 compatibility mode, using the
IGMPv1 protocol.

If a lower version query (as compared to Querier Version) is received on
an interface, this state will change immediately to reflect the older
version querier and the host will operate in that lower version
compatibility mode.  However, if a higher version query (as compared to
Querier Version) is received, it will not immediately change it's state.
This is to prevent the problem when newer version queries are sent by a
router restarting and having not yet yielded to an older version
querier.

Each time a non-version 3 query is received, a host sets a timer: Older
Version Querier Present Timeout.  The state variable, Querier Version,
reflecting the version of querier on an interface will be based on this
timer.  If a host hears a newer version query (as compared to Querier
Version), it will not change its operating state until after the timer
expires.

6.2.1  In the Presence of Older Version Queriers

An IGMPv3 host may be placed on a network where the Querier router has
not yet been upgraded to IGMPv3.  The following requirements apply:

     An older version router expects older version Membership Reports in
     response to its Queries, and will not understand Version 3
     Membership Reports.  Therefore, a state variable MUST be kept for
     each interface, describing whether the multicast Querier on that
     interface is running IGMPv1, IGMPv2, or IGMPv3.  This variable
     MUST be based upon whether or not the older version query was
     heard in the last [Older Version Querier Present Timeout] seconds,
     and MUST NOT be based upon the type of the last Query heard.  This
     state variable MUST be used to decide what type of Membership
     Reports to send for unsolicited Membership Reports as well as
     Membership Reports in response to Queries.

     Version 1 Querier

     An IGMPv1 router will send General Queries with the Max Response
     Time set to 0.  This MUST be interpreted as a value of 100 (10
     seconds).

     IGMPv3 hosts must send IGMPv1 Membership reports when an IGMPv1
     router is present.  This is IGMPv1 compatibility mode.

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     Version 2 Querier

     IGMPv3 hosts must send IGMPv2 Membership reports when an IGMPv2
     router is present.  IGMPv3 hosts must use IGMPv2 Leave Group
     messages when an IGMPv2 router is present.  This is IGMPv2
     compability mode.

The following table summarizes an IGMPv3 host response to different
types of queries:

  Querier
  Version  Query Type                         Host Response
  -------  ----------                         -------------
  IGMPv1   Host Membership Query              IGMPv1 Membership Report
  IGMPv2   Host Membership Query              IGMPv2 Membership Report
  IGMPv2   Group-Specific Membership Query    IGMPv2 Membership Report
  IGMPv3   Host Membership Query              IGMPv3 Membership Report
  IGMPv3   Group-Specific Membership Query    IGMPv3 Membership Report
  IGMPv3   Source-Specific Membership Query   IGMPv3 Membership Report

6.2.2  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 Report to be suppressed by either a Version 1 Membership
Report, or a Version 2 Membership Report.

If a host is a member of any sources within a group reported in a V1 or
V2 membership report, then it may suppress its report by marking the
group so that it is not reported when the next IGMPv3 report is sent.






















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6.2.3  Group Member Compatibility State Transition Diagram


                                   RQ3
                               -----------
                               |         |
                               |         V
               RQ1/ST      -------------------   RQ2/ST
            ---------------|                 |----------------
            |              |     V3 Mode     |               |
            |      ------->|                 |<-------       |
            |      |       -------------------       |       |
            |      |                                 |       |
            |      |TE/DT                       TE/DT|       |
            |      |                                 |       |
            V      |                                 |       V
        -------------------                    ------------------
    ----|                 |                    |                |----
 RQ3|   |    V1 Mode      |<-------------------|     V2 Mode    |   |RQ3
    --->|                 |       RQ1/RT       |                |<---
        -------------------                    ------------------
          |    ^   |    ^                          |         ^
          |    |   |    |                          |         |
          ------   ------                          -----------
          RQ1/RT    RQ2                              RQ2/RT

  Actions
  -------
  ST : start Older Version Querier Present Timer
  DT : delete Older Version Querier Present Timer
  TE : Older Version Querier Present Timer expires
  RT : reset Older Version Querier Present Timer

  Events
  ------
  RQ1 : Receive IGMPv1 Host Membership Query
  RQ2 : Receive IGMPv2 Host Membership Query
  RQ3 : Receive IGMPv3 Host Membership Query

  States
  ------
  V3 Mode : An IGMPv3 router is the present querier on an interface.
            The host uses IGMPv3 Membership Reports

  V2 Mode : An IGMPv2 router is the present querier on an interface.
            The host uses IGMPv2 Membership Reports and IGMPv2 Leave
            Group messages.

  V1 Mode : An IGMPv1 router is the present querier on an interface.
            The host uses IGMPv1 Membership Reports.


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6.3  Multicast Router Behavior

6.3.1  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 not yet been upgraded to IGMPv3.  The following
requirements apply:

      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 send IGMPv1 and IGMPv2 queries.  When in IGMPv1 mode, routers
      MUST send Periodic Queries with a Max Response Time of 0, and MUST
      ignore Leave Group messages.  They SHOULD also warn about
      receiving an IGMPv2 or IGMPv3 query, although such warnings MUST        be rate-limited.

      If a router is not explicitly configured to use IGMPv1 and hears
      an IGMPv1 Query or IGMPv2 Query, it SHOULD log a warning.  These
      warnings MUST be rate-limited.


6.3.2  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.  The following requirements apply:

      IGMPv3 routers MUST keep state per group being forwarded per
      interface regarding the lowest version of IGMP heard.  For each
      group being forwarded per interface, the state variable Oldest
      Host Present is kept.  Groups can be in one of three states
      reflected by the state variable: Oldest Host Present.

      Routers MUST act in a compatibility mode on a per group per
      interface.  The following table summarizes the types of messages
      to be used dependent on the value of Oldest Host Present.

      Oldest Host Present         Messages Utilized
      -------------------         -----------------
          IGMPv1                  Version 1 Host Membership Queries
          IGMPv2                  Version 2 Host Membership Queries,
                                  Version 2 Group-Specific Host
                                   Membership Queries
          IGMPv3                  Version 3 General Membership Queries,
                                  Version 3 Group-Specific Membership
                                   Queries,
                                  Version 3 Group-and-Source Specific
                                   Membership Queries

      A router MUST keep a timer per group, Older Host Present Timeout,
      if it hears an non-version 3 report for a group.  This SHOULD be

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      set to the value of two query periods.  If a router does not hear
      a lower version report for the length of two query periods, it
      assumes that the older version members have left and reverts to
      version 3 operation for that group.

6.3.3  Router Compatibility State Transition Diagram*


                                RV3MR
                             -----------
                             |         |
                             |         V
             RV1MR/ST     -------------------   RV2MR/ST
             -------------|                 |------------
             |            |     V3 Mode     |           |
             |    ------->|                 |<-------   |
             |    |       -------------------       |   |
             |    |                                 |   |
             |    |TE/DT                       TE/DT|   |
             |    |                                 |   |
             V    |                                 |   V
            -----------------                    -------------
        ----|               |                    |           |---
   RV3MR|   |    V1 Mode    |<-------------------| V2 Mode   |  |RV3MR
        --->|               |     RV1MR/RT       |           |<--
            -----------------                    -------------
            |    ^   |    ^                        |     ^
            |    |   |    |                        |     |
            ------   ------                        -------
            RV1MR/RT   RV2MR                       RV2MR/RT


                * with respect to a single multicast group

  Actions
  -------
  ST : start Older Host Present Timer
  DT : delete Older Host Present Timer
  TE : Older Host Present Timer expires
  RT : reset Older Host Present Timer

  Events
  ------
  RV1MR : Receive Version 1 Membership Report
  RV2MR : Receive Version 2 Membership Report
  RV3MR : Receive Version 3 Membership Report

  States
  ------
  V3 Mode : A router operates using only IGMPv3 messages for this group.

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  V2 Mode : An IGMPv2 Membership Report has been heard for this group
            within the last Older Host Present Timeout seconds.  A
            router operates using only IGMPv2 messages for this group.

  V1 Mode : An IGMPv1 Membership Report has been heard for this group
            within the last Older Host Present Timeout seconds.  A
            router operates using only IGMPv1 messages for this group.

  Note: In V1 Mode and V2 Mode, the Host Membership Query is still a
        version 3 query.

7.  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.

7.1.  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

7.2.  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.

7.3.  Query Response Interval

The Max Response Time 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 lest bursty, as host responses are spread out over a larger
interval.  The number of seconds represented by the [Query Response
Interval] must me less than the [Query Interval].

7.4.  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
on a network.  This value MUST be ((the Robustness Variable) times (the
Query Interval)) plus (one Query Response Interval).

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7.5.  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).

7.6.  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.

7.7.  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.

7.8.  Last Member Query Interval

The Last Member Query Interval is the Max Response Time inserted into
Group-Specific Queries sent in response to Leave Group messages, and is
also the amount of time between Group-Specific Query messages.  Default:
10 (1 second)

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.

7.9.  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.  Default: the
Robustness Variable.

7.10.  Unsolicited Report Interval

The Unsolicited Report Interval is the time between repetitions of a
host's initial report of membership in a group.  Default: 10 seconds.

7.11.  Version 1 Router Present Timeout

The Version 1 Router Present Timeout is how long a host must wait after
hearing a Version 1 Query before it may send any IGMPv2 messages.
Value: 400 seconds.

8.  SECURITY CONSIDERATIONS

We consider the ramifications of a forged message of each type.


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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].

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:

     - 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.
     - Ignore Report messages without Router Alert options [RFC2113],
       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 earlier versions of this specification
       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.

Leave messages:

     A forged Leave message will cause the Querier to send out 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.



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9.  ACKNOWLEDGMENTS

Some of the text of this document was copied from [RFC1112] and [IGMPv2]
.


10.  REFERENCES

[RFC1112]  Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
           August 1989.

[IGMPv2]   Fenner, W., "Internet Group Management Protocol, Version 2",
           Internet-Draft, October 1996.

[RFC2113]  Katz, D., "IP Router Alert Option," RFC 2113, April 1996.


11.  APPENDIX A.

This section elaborates on the need for the various types of Group
Records described in Section 4.2.9 by considering an alternate approach
in sending Group Membership Report messages.

In this approach, the Group Records included within each Group
Membership Report consist of the filter mode (either MODE_IS_INCLUDE or
MODE_IS_EXCLUDE) and the complete list of sources specified for that
address.  In other words, each Report conveys the "Full State" of the
Group Record on an interface.  There are only two types of Group
Records in this approach, and they follow the filter mode of the
interface multicast address state.

While this approach reduces the number of types of Group Records along
with the processing required to "compute" the membership report by a
system, it unfortunately requires additional processing by the router.
Since the Group Records are only of two types, it is not possible to
distinguish Reports that were sent in response to Queries from those
that were sent by a change of interface state.  Membership reports
which are sent in response to Membership Queries are only used to
refresh the existing state and typically do not modify the multicast
address state on the router.  Reports that are sent in response to
changes in interface state of a multicast address require that the
router take some action in response to the received report (see Section
5.2.5).  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 unnecessary queries from the
router.





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12.  AUTHORS' ADDRESSES

   Brad Cain
   Nortel Networks, Inc.
   3 Federal Street
   Billerica, MA 01821
   phone: +1-978-916-1316
   email: bcain@baynetworks.com

   Steve Deering
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA 95134-1706
   phone: +1-408-527-8213
   email: deering@cisco.com

   Ajit Thyagarajan
   Torrent Networking Technologies
   8181 Professional Place
   Landover, MD 20785
   phone: +1-301-429-0246
   email: ajit@torrentnet.com






























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