[lwip-devel] [bug #20429] Potential problem with IP Fragmentation and TCPRetransmit

[Simon Goldschmidt]

URL:
  <http://savannah.nongnu.org/bugs/?20429>

                 Summary: Potential problem with IP Fragmentation and
TCPRetransmit
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: goldsimon
            Submitted on: Dienstag 10.07.2007 um 08:18
                Category: IPv4
                Severity: 4 - Important
              Item Group: Crash Error
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any

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Details:

Tom Hennen wrote on lwip-users
(http://lists.nongnu.org/archive/html/lwip-users/2007-07/msg00011.html):

I've run into what may be a problem with IP Fragmentation when combined with
TCP Retransmit in lwip 1.1.1 (I'm not sure if this problem exists in 1.2).

It seems that ip_frag (via copy_from_pbuf) modifies the pbuf given to it. 
During normal operation this is fine; it causes problems when TCP tries to
retransmit a segment that has already been fragmented.  In that case the
pbuf's len and payload pointers will be incorrect (thanks to the first call
to ip_frag) and this can cause errors during the *second* call to ip_frag.

I'm not quite sure what the correct solution is.  Either a copy of the pbuf
chain could be made (minus the payloads) or copy_from_pbuf could be modified
so as not to change the state of the pbuf (or at least restore it once it's
done).

Thoughts?

Thanks,

Tom Hennen





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