RFC 988 (rfc988) - Page 1 of 20

Host extensions for IP multicasting

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

                  Host Extensions for IP Multicasting


   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.


   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.


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