[Udpcast] Image does not complete correctly (receiver hangs)

Ramon Bastiaans bastiaans at sara.nl
Tue Jan 3 17:29:45 CET 2006

Alain Knaff wrote:

> What exactly is "new"? New make of network card? New switch? More than 
> one switch between sender and receiver? Maybe even a router? Is it 
> possibly for you to test each change "one-by-one", to try to identify 
> which particular change brings the problem.

A extra networkcard in the server and 1 switch connected to it (for 
testing) with 4 machines connected to it (for testing).
There are no routers between them.

> It looks like the "return" traffic (from the receiver back to the 
> sender) is not working correctly. It's somewhat bizarre, because 
> apparently it did work at the beginning of the transfer (or else the 
> transfer could not have taken place at all, unless you used 
> asynchronous mode)
Indeed weird, only at the end the problem seems to arise.
I am not running in asyncrhonous mode.

> [...]
> From the IP addresses, I assume you are using unicast mode (one single 
> receiver).
> It would help if you would tell us more about your network. What 
> devices (switches, routers, etc.) are sitting between your sender and 
> your receiver? What netmasks are involved? Where was the tcpdump 
> observed (on sender? on receiver? on an unrelated box? If so, how was 
> that box connected?) Are you reasonably sure that the trace is 
> complete? (Many switches send unicast traffic only to the port where 
> that machine is connected, unless you use a specifically configured 
> monitoring port. That means that if you used an unrelated box to 
> observe, the trace might be incomplete, depending on how the 
> switch(es) has(ve) been set up)
> Alain

Yes, I tried this particular case in unicast mode, to eliminate any 
possible multicast issues that might arise.
I would figure a unicast setup should work, even if the switch has 
problems with multicasting.

The network on networkcard X is a /22 network (, the 
network on networkcard Y is a /21 network (

The tcpdump was done on the sending side/image server, so I should be 
able to see the return packets in the tcpdump (I see return packets 
during transferring earlier on).

The setup pretty much looks like this right now:

server networkcard Y ->| switch  |-> machine 1
                                                   |-> machine 2
                                                   |-> machine 3
                                                   |-> machine 4

Kind regards,
- Ramon.

There are really only three types of people:

  Those who make things happen,
   those who watch things happen,
   and those who say, "What happened?"

ing. R. Bastiaans
HPC  - Systems Programmer

SARA - Computing and Networking Services
Kruislaan 415      PO Box 194613
1098 SJ Amsterdam  1090 GP Amsterdam

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