Generating Multicast Traffic

If you're a protocol junkie like me, you probably enjoy investigating a variety of network traffic types. I don't encounter multicast traffic too often, so the following caught my eye.

I'm using Iperf for some simple testing, and I notice it has a multicast option. Here's how I used it.

In the following scenario, I have two hosts (cel433 and cel600) on the same segment. This is important because the router(s) in this test network are not configured to support multicast.

I set up cel433 as a Iperf server listening on multicast address 224.0.55.55.

cel433:/root# iperf -s -u -B 224.0.55.55 -i 1
------------------------------------------------------------
Server listening on UDP port 5001
Binding to local address 224.0.55.55
Joining multicast group  224.0.55.55
Receiving 1470 byte datagrams
UDP buffer size: 41.1 KByte (default)

Now I generate multicast traffic from cel600.

cel600:/root# iperf -c 224.0.55.55 -u -T 32 -t 3 -i 1
------------------------------------------------------------
Client connecting to 224.0.55.55, UDP port 5001
Sending 1470 byte datagrams
Setting multicast TTL to 32
UDP buffer size: 9.00 KByte (default)
------------------------------------------------------------
[  3] local 10.1.10.3 port 51296 connected with 224.0.55.55 port 5001
[  3]  0.0- 1.0 sec    129 KBytes  1.06 Mbits/sec
[  3]  1.0- 2.0 sec    128 KBytes  1.05 Mbits/sec
[  3]  2.0- 3.0 sec    128 KBytes  1.05 Mbits/sec
[  3]  0.0- 3.0 sec    386 KBytes  1.05 Mbits/sec
[  3] Sent 269 datagrams

Here is what cel433 sees:

------------------------------------------------------------
[  3] local 224.0.55.55 port 5001 connected with 10.1.10.3 port 51296
[  3]  0.0- 1.0 sec    128 KBytes  1.05 Mbits/sec  0.146 ms    0/   89 (0%)
[  3]  1.0- 2.0 sec    128 KBytes  1.05 Mbits/sec  0.100 ms    0/   89 (0%)
[  3]  2.0- 3.0 sec    128 KBytes  1.05 Mbits/sec  0.110 ms    0/   89 (0%)
[  3]  0.0- 3.0 sec    386 KBytes  1.05 Mbits/sec  0.098 ms    0/  268 (0%)
[  3]  0.0- 3.0 sec  1 datagrams received out-of-order

The traffic looks like this:

cel433:/root# tcpdump -n -i xl0 -s 1515 udp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on xl0, link-type EN10MB (Ethernet), capture size 1515 bytes
15:29:53.669508 IP 10.1.10.3.51296 > 224.0.55.55.5001: UDP, length 1470
15:29:53.680789 IP 10.1.10.3.51296 > 224.0.55.55.5001: UDP, length 1470
15:29:53.691934 IP 10.1.10.3.51296 > 224.0.55.55.5001: UDP, length 1470
...truncated...

This is a simple way to generate multicast traffic and ensure a member of the multicast group actually receives it.

Update: I forgot to show the IGMP messages one would see when starting a multicast listener.

This is the interface listening for multicast:

cel433:/root# ifconfig xl0
xl0: flags=8843 mtu 1500
        options=9
        inet6 fe80::2c0:4fff:fe1c:102b%xl0 prefixlen 64 scopeid 0x6 
        inet 10.1.10.2 netmask 0xffffff00 broadcast 10.1.10.255
        ether 00:c0:4f:1c:10:2b
        media: Ethernet autoselect (100baseTX )
        status: active

Here are IGMP report and leave messages.

cel433:/root# tcpdump -nevv -i xl0 -s 1515 igmp
tcpdump: listening on xl0, link-type EN10MB (Ethernet), capture size 1515 bytes
06:28:40.887868 00:c0:4f:1c:10:2b > 01:00:5e:00:37:37, ethertype IPv4 (0x0800),
 length 46: (tos 0x0, ttl   1, id 59915, offset 0, flags [none], proto: IGMP (2),
 length: 32, options
 ( RA (148) len 4 )) 10.1.10.2 > 224.0.55.55: igmp v2 report 224.0.55.55

06:28:42.196233 00:c0:4f:1c:10:2b > 01:00:5e:00:00:02, ethertype IPv4 (0x0800),
 length 46: (tos 0x0, ttl   1, id 59920, offset 0, flags [none], proto: IGMP (2),
 length: 32, options
 ( RA (148) len 4 )) 10.1.10.2 > 224.0.0.2: igmp leave 224.0.55.55

I used the -e option to show the MAC addresses. Notice the destination MAC for these multicast packets.

06:31:21.467919 00:b0:d0:14:b2:11 > 01:00:5e:00:37:37, ethertype IPv4 (0x0800),
 length 1512: (tos 0x0, ttl  32, id 1652, offset 0, flags [none], proto: UDP (17),
 length: 1498) 
 10.1.10.3.58479 > 224.0.55.55.5001: [udp sum ok] UDP, length 1470

The 01:00:5e:00:37:37 MAC address is a mapping derived from the 24-bit IANA multicast OUI 01:00:5e and the multicast IP address 224.0.55.55.