On Fri, 2008-09-05 at 17:56 +0200, address@hidden wrote:
Hi there
Kieran asked me to do further investigations considering the topic
"Deadlocked tcp_retransmit due to exceeded pcb->cwnd" (see
http://lists.gnu.org/archive/html/lwip-users/2008-07/msg00098.html).
Thanks for taking the time to produce such a detailed and helpful
analysis.
9. We get the third dupack for 10085. According to RFC2581 we shall
start a fast retransmission now
10. For fast retransmission tcp_process() calls tcp_receive() calls
tcp_rexmit() calls tcp_output()
11. Cause tcp_output() was invoked by an initial tcp_input() it bailes
out on
if (tcp_input_pcb == pcb)
==> !!! This violates RFC2581 IMHO !!!
Yes, that's a problem. We'll need to fix that somehow.
12. But tcp_rexmit() already tinkered our queues by placing the first
unacked segment to the
unsent queue.
unsent->10085
unacked->11453->12818->14183
13. The next few ouput attempts bail out in tcp_output() due to the
nagle algorith
(tcp_do_output_nagle()). Thus nothing more hapens till a
retransmission timeout occurs
14. tcp_slowtmr() requires a retransmission (pcb->rtime >= pcb->rto).
This shrinks down the
congestion window to the maximum segment size (1390 in my case).
BTW: A retransmission is triggered by segment 14183 and not by
10085 in this case
which is an aftereffect of the underlying bug IMHO.
15. tcp_slowtmr() calls tcp_rexmit_rto(). The rto function moves all
unacked segments to the head
of the unsent queue. This is final step causing the deadlock in
tcp_output() cause the
smallest sequence number is now at the end of the queue.
Unsent->11453->12818->14183->10085.
And that looks to be the fundamental cause of this bug.
I needed a quick fix for our project and therefore I reordered the
queue in tcp_output before the
While loop in tcp_output. However this is just a quick fix to fight
the symptoms. Therefore I ask
for other suggestions or perhaps a patch.
Your solution isn't that bad, other than the amount of CPU required to
sort the queue. I'll try and find some time to look at this though and
come up with something better. Other people's suggestions are always
welcome though as time is often in short supply!
If there's a bug open for this, could you add these details to the bug.
If there's no open bug, could you open one!
Thanks
Kieran
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