[lwip-devel] [bug #26649] TCP fails when TCP_MSS > TCP_SND_BUF

[Robert Sprowson]

URL:
  <http://savannah.nongnu.org/bugs/?26649>

                 Summary: TCP fails when TCP_MSS > TCP_SND_BUF
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: sprow
            Submitted on: Sat 23 May 2009 18:24:35 GMT
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.3.0

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Details:

I'm assuming it is valid to have TCP_MSS > TCP_SND_BUF, if not a sanity check
in "core/init.c" might be worthwhile.

Anyway, the placement of parenthesis means that

  #define TCP_SND_QUEUELEN  (4 * (TCP_SND_BUF/TCP_MSS))

in "opt.h" evaluates to zero when the send buffer is larger than the segment
size. Likewise in "core/init.c" the sanity check 

  if (TCP_SND_QUEUELEN < (2 * (TCP_SND_BUF/TCP_MSS)))

would boil down to 

  if (0 < 0)

Proposed fixes:

  #define TCP_SND_QUEUELEN  ((4 * TCP_SND_BUF)/TCP_MSS)

and

  if (TCP_SND_QUEUELEN < ((2 * TCP_SND_BUF)/TCP_MSS))

with these in place everything seems to work fine when TCP_SND_BUF=256 and
TCP_MSS=512.




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