[lwip-devel] [bug #51154] ABC (Appropriate Byte Counting) fails to work with TCP_WND set to maximum.

[Tim Cussins]

URL:
  <http://savannah.nongnu.org/bugs/?51154>

                 Summary: ABC (Appropriate Byte Counting) fails to work with
TCP_WND set to maximum.
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: tcussins
            Submitted on: Thu 01 Jun 2017 10:09:00 AM UTC
                Category: TCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

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Details:

Hi,

Our deployed configuration of lwIP sets TCP_WND = 64k-1, which is the maximum
allowable value (value is stored as 16bit unsigned int). This delivers a large
performance increase over the default value (4 * MSS).

The Appropriate Byte Counting patch (de90d03e) breaks our configuration: A
thread dedicated to tcp rx quickly stalls.

Reducing our TCP_WND to 60k appears to resolve the issue, which would suggest
an arithmetic overflow in the ABC code due to our large TCP_WND.

If we can establish what the maximum allowable TCP_WND is, lwIP could enforce
it at compile time.

Thanks,
Tim




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