Chapter 21
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| 21.1
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When we run the program, there is no output. To
prevent accidental reception of multicast datagrams that a server
is not expecting, the kernel does not deliver multicast groups to a
socket that has never performed any multicast operations (e.g.,
joining a group). What is happening here is that the destination
address of the UDP datagram is 224.0.0.1, the all-hosts group that
all multicast-capable nodes must join. The UDP datagram is sent as
a multicast Ethernet frame and all the multicast-capable nodes
receive the datagram since they all belong to the group. However,
the kernel drops the received datagram since the process bound to
the daytime port has not set any multicast options.
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| 21.2
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Figure
E.14 shows a simple modification to the main function
to bind the multicast address and port 0.
Figure E.14
UDP client main function that binds a multicast
address.
mcast/udpcli06.c
1 #include "unp.h"
2 int
3 main(int argc, char **argv)
4 {
5 int sockfd;
6 socklen_t salen;
7 struct sockaddr *cli, *serv;
8 if (argc != 2)
9 err_quit("usage: udpcli06 <IPaddress>");
10 sockfd = Udp_client(argv[1], "daytime", (void **) &serv, &salen);
11 cli = Malloc(salen);
12 memcpy(cli, serv, salen); /* copy socket address struct */
13 sock_set_port(cli, salen, 0); /* and set port to 0 */
14 Bind(sockfd, cli, salen);
15 dg_cli(stdin, sockfd, serv, salen);
16 exit(0);
17 }
Unfortunately, on the three systems on which
this was tried鈥擣reeBSD 4.8, MacOS X, and Linux 2.4.7鈥攁ll allowed
the bind and then sent the UDP datagrams with a multicast
source IP address.
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| 21.3
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21.3 If we do
this from our host aix, which is multicast-capable, we get
the following:
aix % ping 224.0.0.1
PING 224.0.0.1: 56 data bytes
64 bytes from 192.168.42.2: icmp_seq=0 ttl=255 time=0 ms
64 bytes from 192.168.42.1: icmp_seq=0 ttl=64 time=1 ms (DUP!)
^C
----224.0.0.1 PING Statistics----
1 packets transmitted, 1 packets received, +1 duplicates, 0% packet loss
round-trip min/avg/max = 0/0/0 ms
Both systems on the right-hand Ethernet in
Figure 1.16
respond.
To prevent certain denial-of-service attacks,
some systems do not respond to broadcast or multicast pings by
default. To get freebsd to respond, we had to configure it
with
freebsd % sysctl net.inet.icmp.bmcastecho=1
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| 21.5
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The value 1,073,741,824 is converted to a
floating-point number and divided by 4,294,967,296, yielding 0.250.
This is multiplied by 1,000,000, yielding 250,000, which in
microseconds is one-quarter of a second.
The largest fraction is 4,294,967,295, which
divided by 4,294,967,296 yields 0.99999999976716935634. Multiplying
this by 1,000,000 and truncating to an integer yields 999,999, the
largest value for the number of microseconds.
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