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 intact 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.  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.  Those IP
   multicast addresses that are not reserved for permanent groups are
   available for dynamic assignment to transient groups which exist only
   as long as they have members.

   Internetwork forwarding of IP multicast datagrams is handled by
   "multicast routers" which may be co-resident with, or separate from,
   internet gateways.  A host transmits an IP multicast datagram as a
   local network multicast which reaches all immediately-neighboring
   members of the destination host group.  If the datagram has an IP
   time-to-live greater than 1, the multicast router(s) attached to the
   local network take responsibility for forwarding it towards all other
   networks that have members of the destination group.  On those other
   member networks that are reachable within the IP time-to-live, an

   attached multicast router completes delivery by transmitting the
   datagram as a local multicast.

   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 acting as multicast
   routers.  The algorithms and protocols used within and between
   multicast routers are transparent to hosts and will be specified in
   separate documents.  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.