RFC 988 (rfc988) - Page 2 of 20

Host extensions for IP multicasting

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
      memo.










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