[lwip-devel] [bug #50837] TCP: zero window probe doesn't timeout

[Joel Cunningham]

Update of bug #50837 (project lwip):

             Assigned to:                    None => jcunningham            
                 Summary: LWIP TCP/IP race condition => TCP: zero window probe
doesn't timeout

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Follow-up Comment #25:

> My suggestion would be to use pcb->nrtx here, too. We could increase it for
every zero window probe and reset it for every matching rx segment if the
persist timer is running (as opposed to resetting it when new data is
acknowledged otherwise).

> With default settings, that would still take ~11 minutes to time out such a
connection (if my calculations with 'tcp_persist_backoff' and 'TCP_MAXRTX' are
correct).

I think I can put a patch together that does this.

I went back and looked at the original persist timer patch in an attempt to
understand the backoff and maximum timeout.

Originally it used tcp_output to send a 1 byte segment, then tcp_rexmit() to
send subsequent segments.  I believe this would have timed out since
tcp_rexmit() increments pcb->nrtx.  But when the patch was cleaned up and
submitted, this behavior was omitted.  See bug #20511.  TCP_MAXRTX seems to
have always been 12, so the upper bound doesn't seem to have changed from the
original patch

11 minutes seems very long to me, but I'm fine with adding the timeout
behavior first, then tuning the timeout (if we decide too).  RFC 1122 has
guidance on upper bound before closing a connection





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