https://www.rfc-editor.org/rfc/rfc1112.html
S. Deering
Request for Comments: 1112 Stanford University
Obsoletes: RFCs 988, 1054 August 1989
Host Extensions for IP Multicasting
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STATUS OF THIS MEMO
This memo specifies the extensions required of a host implementation
of the Internet Protocol (IP) to support multicasting. It is the
recommended standard for IP multicasting in the Internet.
Distribution of this memo is unlimited.
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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.