TCP issue

Martin Kocian kocian at
Tue Sep 13 22:43:34 UTC 2011


I discovered an issue with TCP where RTEMS hangs when after an idle 
period it tries to send a small message followed by a large message.
Looking at tcp_output.c I think I understand what's going on:
When TCP is idle for awhile the congestion window gets reset to t_maxseg.
Then the first message I send is smaller than t_maxseg so it is sent out 
in a single packet. The receiver (Linux) only sends an ACK on every other 
packet so no ACK comes back which means the RTEMS congestion window stays 
at t_maxseg.
Now we send a message that is larger than t_maxseg. Len (the number of 
bytes to be sent) gets set to congestion window size (t_maxseg) minus 
the length of the first (unacknowledged) message.
The conditions in the code to actually send the next packet are:
- len = t_maxseg 
- This is the last part in a send and idle or NODELAY set 
- Time-out
- We have more then 1/2 the maximum send window's worth of data
- Retransmitting

All of the conditions are false so the send hangs until the Linux side 
times out and sends the ACK.
This looks like a bug to me but I'm no TCP expert so any confirmation or
suggestion on how to fix this would be greatly appreciated.

Thank you,


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