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Joining a Multicast group


Purpose of the Lab:

The purpose of the lab is to observe multicast datagrams being transmitted from a single sender to multiple receivers.

Lab Setup:

  1. Start tcpdump on hostC1: sudo tcpdump –n –e –v port 1234

  2. Start sock UDP servers on hostC2 and hostC3. The servers listen on the multicast address 225.1.2.3 with port 1234 sock -u -s -j 225.1.2.3 1234 note that –j 225.1.2.3 means that the server “joins” the multicast group 225.1.2.3. Use non-overlapping small terminal windows so that a screen snapshot will show all windows.

  3. Start a sock UDP client on hostC1 and send a message to 225.1.2.3 on port 1234 sock -u 225.1.2.3 1234 hello from hostC1 Make sure that this terminal window does not overlap with the ones from the previous step so that a screen snapshot will show all windows.

  4. Take snapshot of the screen showing the UDP sender and the two receivers.

  5. Observe the tcpdump output.

Results and Discussion:

tcpdump output:

15:18:03.649312 00:00:0a:01:00:21 > 01:00:5e:01:02:03, ethertype IPv4 (0x0800), length 60: (tos 0x0, ttl 1, id 4342, offset 0, flags [none], proto UDP (17), length 46)

10.1.3.1.31094 > 225.1.2.3.1234: UDP, length 18

Discussion:

Frame 1 is the UDP datagram with IP destination 225.1.2.3 and Ethernet destination 01:00:5e:01:02:03.

The TTL is 1 which is a default for multicast.

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