21.1 Introduction
As shown in Figure 20.1, a unicast
address identifies a single IP
interface, a broadcast address identifies all IP interfaces on the subnet, and a
multicast address identifies a set
of IP interfaces. Unicasting and broadcasting are the extremes of
the addressing spectrum (one or all) and the intent of multicasting
is to allow addressing something in between. A multicast datagram
should be received only by those interfaces interested in the
datagram, that is, by the interfaces on the hosts running
applications wishing to participate in the multicast group. Also,
broadcasting is normally limited to a LAN, whereas multicasting can
be used on a LAN or across a WAN. Indeed, applications multicast
across a subset of the Internet on a daily basis.
The additions to the sockets API to support
multicasting are simple; they comprise nine socket options: three
that affect the sending of UDP datagrams to a multicast address and
six that affect the host's reception of multicast datagrams.
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