21.8 dg_cli Function Using
Multicasting
We modify our dg_cli function from
Figure 20.5 by just
removing the call to setsockopt. As we said earlier, none
of the multicast socket options needs to be set to send a multicast
datagram if the default settings for the outgoing interface, TTL,
and loopback option are acceptable. We run a modified UDP echo
server that joins the allhosts group, then run our program
specifying the all-hosts group as the destination address.
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macosx % udpcli01 224.0.0.1
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hi
there
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from 172.24.37.78: hi there
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MacOS X
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from 172.24.37.94: hi there
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FreeBSD
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We get a response from both systems on the
subnet. They are each running the multicast echo server. Each reply
is unicast because the source address of the request, which is used
by each server as the destination address of the reply, is a
unicast address.
IP Fragmentation and Multicasts
We mentioned at the end of Section 20.4
that most systems do not allow the fragmentation of a broadcast
datagram as a policy decision. Fragmentation is fine to use with
multicasting, as we can easily verify using the same file with a
2,000-byte line.
macosx % udpcli01 224.0.0.1 < 2000line
from 172.24.37.78: xxxxxxxxxx[...]
from 172.24.37.94: xxxxxxxxxx[...]
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