1. Introduction

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

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

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

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

1.1. Terminology

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

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