Internet Group Management Protocol, Version 2
RFC 2236

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Network Working Group                                            W. Fenner
Request for Comments: 2236                                      Xerox PARC
Updates: 1112                                                November 1997
Category: Standards Track

             Internet Group Management Protocol, Version 2

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 (1997).  All Rights Reserved.

Abstract

   This memo documents IGMPv2, used by IP hosts to report their
   multicast group memberships to routers.  It updates STD 5, RFC 1112.

   IGMPv2 allows group membership termination to be quickly reported to
   the routing protocol, which is important for high-bandwidth multicast
   groups and/or subnets with highly volatile group membership.

   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).

1.  Definitions

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

2.  Introduction

   The Internet Group Management Protocol (IGMP) is used by IP hosts to
   report their multicast group memberships to any immediately-
   neighboring multicast routers.  This memo describes only the use of
   IGMP between hosts and routers to determine group membership.
   Routers that are members of multicast groups are expected to behave

Fenner                      Standards Track                     [Page 1]
RFC 2236           Internet Group Management Protocol      November 1997

   as hosts as well as routers, and may even respond to their own
   queries.  IGMP may also be used between routers, but such use is not
   specified here.

   Like ICMP, IGMP is a integral part of IP.  It is required to be
   implemented by all hosts wishing to receive IP multicasts.  IGMP
   messages are encapsulated in IP datagrams, with an IP protocol number
   of 2.  All IGMP messages described in this document are sent with IP
   TTL 1, and contain the IP Router Alert option [RFC 2113] in their IP
   header.  All IGMP messages of concern to hosts 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     | Max Resp Time |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Group Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1.  Type

   There are three types of IGMP messages of concern to the host-
   router interaction:

   0x11 = Membership Query
        There are two sub-types of Membership Query messages:
        - General Query, used to learn which groups have members on an
          attached network.
        - Group-Specific Query, used to learn if a particular group
          has any members on an attached network.

        These two messages are differentiated by the Group Address, as
        described in section 1.4 .  Membership Query messages are
        referred to simply as "Query" messages.

   0x16 = Version 2 Membership Report

   0x17 = Leave Group

   There is an additional type of message, for backwards-compatibility
   with IGMPv1:

   0x12 = Version 1 Membership Report

Fenner                      Standards Track                     [Page 2]
RFC 2236           Internet Group Management Protocol      November 1997

   This document refers to Membership Reports simply as "Reports".  When
   no version is specified, the statement applies equally to both
   versions.

   Unrecognized message types should be silently ignored.  New message
   types may be used by newer versions of IGMP, by multicast routing
   protocols, or other uses.

2.2.  Max Response Time

   The Max Response Time field is meaningful only in Membership Query
   messages, and specifies the maximum allowed time before sending a
   responding report in units of 1/10 second.  In all other messages, it
   is set to zero by the sender and ignored by receivers.

   Varying this setting allows IGMPv2 routers to tune the "leave
   latency" (the time between the moment the last host leaves a group
   and when the routing protocol is notified that there are no more
   members), as discussed in section 7.8.  It also allows tuning of the