21.12 Summary
A multicast application starts by joining the
multicast group assigned to the application. This tells the IP
layer to join the group, which in turns tells the datalink layer to
receive multicast frames that are sent to the corresponding
hardware layer multicast address. Multicasting takes advantage of
the hardware filtering present on most interface cards, and the
better the filtering, the fewer the number of undesired packets
received. Using this hardware filtering reduces the load on all the
other hosts that are not participating in the application.
Multicasting on a WAN requires multicast-capable
routers and a multicast routing protocol. Until all the routers on
the Internet are multicast-capable, multicast is only available to
a subset of Internet users. We use the term "IP multicast
infrastructure" to describe the set of all multicast-capable
systems on the Internet.
Nine socket options provide the API for
multicasting:
-
Join an any-source multicast group on an
interface
-
Leave a multicast group
-
Block a source from a joined group
-
Unblock a blocked source
-
Join a source-specific multicast group on an
interface
-
Leave a source-specific multicast group
-
Set the default interface for outgoing
multicasts
-
Set the TTL or hop limit for outgoing
multicasts
-
Enable or disable loopback of multicasts
The first six are for receiving, and the last
three are for sending. There is enough difference between the IPv4
socket options and the IPv6 socket options that
protocol-independent multicasting code becomes littered with
#ifdefs very quickly. We developed 12 functions of our
own, all beginning with mcast_, that can help in writing
multicast applications that work with either IPv4 or IPv6.
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