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YARP supports UDP and MCAST communication. These are unreliable network protocols, but can be quite efficient. We discuss here steps that are useful for making your network friendly to UDP and MCAST. But if you find yourself spending a lot of time on this, you may need to reconsider whether you have made the right choice of protocol.


Check List

  • Do all your computers permit programs to allocate large receiver buffers? 640x480 color images at 30Hz just won't work in UDP/MCAST without a big receiver buffer, if your program ever does any actual processing. See "YARP's suggestion" below.
  • Do you know your Maximum Transmission Unit (MTU) Size? See "Useful Links" below.
  • Have you become familiar with the output of the network throughput test program iperf on your system? Again, see "Useful Links" below.

Using iperf

Checking TCP

Example of use: pick two machines, choose one (arbitrarily) to be the server. Find its IP address or domain name -- let's say it is Run the following commands:

 [On server, let's suppose it is] iperf -s
 [On client, let's suppose it is] iperf -c

This will run for a while and then describe your TCP throughput, for example:

 Client connecting to, TCP port 5001
 TCP window size: 16.0 KByte (default)
 [  3] local port 34160 connected with port 5001
 [  3]  0.0-10.0 sec  1.08 GBytes    924 Mbits/sec

Checking UDP

Now let's try UDP. Add the "-u" flag to the client and server. To the server, we can add "-i 1" to get reports at regular intervals. To the client, you can specify target bandwidth, e.g. "-b 50m" for a target of 50 MBits/sec.

 [On server,] iperf -s -u -i 1
 [On client,] iperf -c -b 50m

Example result:

 Sending 1470 byte datagrams
 UDP buffer size:   108 KByte (default)
 [  3] local port 32820 connected with port 5001
 [  3]  0.0-10.0 sec  59.7 MBytes  50.0 Mbits/sec
 [  3] Sent 42555 datagrams
 [  3] Server Report:
 [  3]  0.0-10.0 sec    119 MBytes    100 Mbits/sec  0.030 ms    0/42554 (0%)
 [  3]  0.0-10.0 sec  1 datagrams received out-of-order

A key test for YARP is that you can send big datagrams okay. This is necessary for efficient transmission of large images using UDP or MCAST. To check 63KB datagrams, do:

 [On server,] iperf -s -u -l 63K -w 200K
 [On client,] iperf -c -u -l 63K -w 200K

Make sure you try this in both directions! We've seen problems going from a Linux machine to a Windows machine (upgrading kernel fixed it; maybe NIC driver issue?). If you see problems, reduce the "-l" size until things work just to understand your system, but if you want to send large images you'll need to tweak your configuration.

Checking Multicast

For testing multicast, make sure you have a version of iperf >= 2.0.2. Try this:

 iperf -c -u -T 32 -t 100 -i 1 -l 60k 
 iperf -s -u -B -i 1 -T 32 -l 60k

On the client ("-c") side, you should see something like:

 Server listening on UDP port 5001
 Binding to local address
 Joining multicast group
 Receiving 61440 byte datagrams
 UDP buffer size:   108 KByte (default)
 [  3] local port 5001 connected with port 32821
 [  3]  0.0- 1.0 sec    120 KBytes    983 Kbits/sec  0.003 ms    0/    2 (0%)
 [  3]  1.0- 2.0 sec    120 KBytes    983 Kbits/sec  0.030 ms    0/    2 (0%)
 [  3]  2.0- 3.0 sec    120 KBytes    983 Kbits/sec  0.052 ms    0/    2 (0%)

YARP's suggestion

If MCAST/UDP packets drop, YARP may suggest this:

 The UDP/MCAST system buffer limit on your system is low.
 You may get packet loss under heavy conditions.
 To change the buffer limit on linux: sysctl -w net.core.rmem_max=8388608
 (Might be something like: sudo /sbin/sysctl -w net.core.rmem_max=8388608)
 To change the limit use: systcl for Linux/FreeBSD, ndd for Solaris, no for AIX

This controls the maximum allowed size for a RECEIVER buffer, so that a number of packets adding up to a large amount of data can be received and stored while your program is doing something else. YARP will try to get a large buffer, and your operating system may not comply unless you do the above. This configuration step is not necessary on MS-Windows.

Testing bandwith for UDP and Multicast

The tests above have concentrated just on connectivity. To test whether a specific data rate is possible, add the "-b" option. This applies to UDP and multicast. For example, on a gigabit network, to see how things work with 500MB/s of data, try adding:

 -b 500m

Relevant source code

In case you need to fix something, the relevant source code in YARP is:

  • src/libYARP_OS/src/DgramTwoWayStream.cpp
  • src/libYARP_OS/include/yarp/os/DgramTwoWayStream.h

We appreciate patches to these files. We only have one network to test on, and this stuff seems quite network-dependent.

Useful Links

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