1. Introduction
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.
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.