Host extensions for IP multicasting
RFC 988

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Network Working Group                                      S. E. Deering
Request for Comments: 988                            Stanford University
                                                               July 1986

                  Host Extensions for IP Multicasting

1.  STATUS OF THIS MEMO

   This memo specifies the extensions required of a host implementation
   of the Internet Protocol (IP) to support internetwork multicasting.
   This specification supersedes that given in RFC-966, and constitutes
   a proposed protocol standard for IP multicasting in the
   ARPA-Internet.  The reader is directed to RFC-966 for a discussion of
   the motivation and rationale behind the multicasting extension
   specified here.  Distribution of this memo is unlimited.

2.  INTRODUCTION

   IP multicasting is defined as the transmission of an IP datagram to a
   "host group", a set of zero or more hosts identified by a single IP
   destination address.  A multicast datagram is delivered to all
   members of its destination host group with the same "best-efforts"
   reliability as regular unicast IP datagrams, i.e. the datagram is not
   guaranteed to arrive at all members of the destination group or in
   the same order relative to other datagrams.

   The membership of a host group is dynamic; that is, hosts may join
   and leave groups at any time.  There is no restriction on the
   location or number of members in a host group, but membership in a
   group may be restricted to only those hosts possessing a private
   access key.  A host may be a member of more than one group at a time.
   A host need not be a member of a group to send datagrams to it.

   A host group may be permanent or transient.  A permanent group has a
   well-known, administratively assigned IP address.  It is the address,
   not the membership of the group, that is permanent; at any time a
   permanent group may have any number of members, even zero.  A
   transient group, on the other hand, is assigned an address
   dynamically when the group is created, at the request of a host.  A
   transient group ceases to exist, and its address becomes eligible for
   reassignment, when its membership drops to zero.

   The creation of transient groups and the maintenance of group
   membership information is the responsibility of "multicast agents",
   entities that reside in internet gateways or other special-purpose
   hosts.  There is at least one multicast agent directly attached to
   every IP network or subnetwork that supports IP multicasting.  A host
   requests the creation of new groups, and joins or leaves existing
   groups, by exchanging messages with a neighboring agent.

Deering                                                         [Page 1]



RFC 988                                                        July 1986
Host Extensions for IP Multicasting

   Multicast agents are also responsible for internetwork delivery of
   multicast IP datagrams.  When sending a multicast IP datagram, a host
   transmits it to a local network multicast address which identifies
   all neighboring members of the destination host group.  If the group
   has members on other networks, a multicast agent becomes an
   additional recipient of the local multicast and relays the datagram
   to agents on each of those other networks, via the internet gateway
   system.  Finally, the agents on the other networks each transmit the
   datagram as a local multicast to their own neighboring members of the
   destination group.

   This memo specifies the extensions required of a host IP
   implementation to support IP multicasting, where a "host" is any
   internet host or gateway other than those serving as multicast
   agents.  The algorithms and protocols used within and between
   multicast agents are transparent to non-agent hosts and will be
   specified in a separate document.  This memo also does not specify
   how local network multicasting is accomplished for all types of
   network, although it does specify the required service interface to
   an arbitrary local network and gives an Ethernet specification as an
   example.  Specifications for other types of network will be the
   subject of future memos.

3.  LEVELS OF CONFORMANCE

   There are three levels of conformance to this specification:

   Level 0: no support for IP multicasting.

      There is, at this time, no requirement that all IP implementations
      support IP multicasting.  Level 0 hosts will, in general, be
      unaffected by multicast activity.  The only exception arises on
      some types of local network, where the presence of level 1 or 2
      hosts may cause misdelivery of multicast IP datagrams to level 0
      hosts.  Such datagrams can easily be identified by the presence of
      a class D IP address in their destination address field; they
      should be quietly discarded by hosts that do not support IP
      multicasting.  Class D addresses are defined in section 4 of this