[lwip-devel] [bug #46948] Default TCP_SNDLOWAT invalid when TCP_SND_BUF > 2*65535

[Joel Cunningham]

URL:
  <http://savannah.nongnu.org/bugs/?46948>

                 Summary: Default TCP_SNDLOWAT invalid when TCP_SND_BUF >
2*65535
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: jcunningham
            Submitted on: Thu 21 Jan 2016 04:28:01 PM 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: git head

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Details:

The calculation in opt.h for TCP_SNDLOWAT results in a invalid configuration
when TCP_SND_BUF is greater than 2*66535.

Due to limitations of tcp_write, tcp_sndbuf() is limited to returning 65535
(even with large send buffers) and when TCP_SNDLOWAT is greater than 65535, we
will never pass any checks where tcp_sndbuf() is compared against
TCP_SNDLOWAT.  This happens once the send buffer is full and then drains, but
we never mark the netconn/socket as writable again.

I've changed the value of TCP_SNDLOWAT in my port, but it would be good to
update the opt.h calculation to prevent this from happening when TCP_SND_BUF
is tuned to be large





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