RFC 988 (rfc988) - Page 2 of 20
Host extensions for IP multicasting
Alternative Format: Original Text Document
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.
Deering