===== RFC1112 - Host Extensions for IP Multicasting =====
[[dido:public:ra:xapend:xapend.b_stds:tech:ietf:start| return to the IETF Standards ]]

<table>
<caption>Data sheet for Host Extensions for IP Multicasting</caption>
| Title                      | Host Extensions for IP Multicasting |
| Acronym                    | |
| Version                    | 1989 |
| Document Number            | RFC1112 |
| Release Date               | August 1989  |
| Reference                  | [[https://tools.ietf.org/html/rfc1112]] |
</table>

  : **Note**: The following is an excerpt from the official IETF RFC. It is provided here as a convenience and is not authoritative. Refer to the original document as the authoritative reference.


==== Introduction ====
  : //IP multicasting is 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 [[dido:public:ra:xapend:xapend.a_glossary:g:gateway|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 [[dido:public:ra:xapend:xapend.a_glossary:i:interface|interface]] to an
    arbitrary local network and gives an [[dido:public:ra:xapend:xapend.a_glossary:e:ethernet|Ethernet]] specification as an
    example.  Specifications for other types of network will be the
    subject of future memos.//


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