FreeBSD TCP ignores zero window size

[Darren Reed]

Andre Oppermann wrote:
> On 30.03.2012 16:22, Darren Reed wrote:
>> I've been tracking down some problems with FreeBSD's sending
>> of TCP packets and seem to have come to the conclusion that
>> in FreeBSD 8.2-RELEASE, when the system is working with a
>> TCP connection that has a moderate delay in it, FreeBSD's
>> TCP ignores the other end telling it that the window size
>> is now 0 and continues to send data. I suspect that this is
>> meant to make sense because it is expecting that the ACK
>> that will open up the window is already in transit. But that
>> only accounts for the condition where the TCP on FreeBSD can
>> compute and decide that the remote TCP will have its buffer
>> full. What I find harder to accept is that when FreeBSD's
>> TCP receives a TCP packet from the remote end advertising
>> a window of 0, FreeBSD's response is to send more data and
>> not a window probe or is that now the expected behaviour?
>> And whilst you might say "ok" for a packet of data, I'm
>> somewhat hard pressed to explain why FreeBSD's TCP sends
>> multiple packets with data in them after receiving a TCP
>> packet from the other end advertising a zero window size.
>>
>> However this causes a problem with firewalls (;_) that are
>> close to the FreeBSD end because for them, it appears that
>> FreeBSD is sending data outside of its window.
>>
>> Is this a known problem?
>> If so, has it been fixed in a later version of FreeBSD?
>> (No, I haven't tested anything other than 8.2)
>
> The window update acceptance test is too restrictive.  In your case
> the last updated seq# tracking gets it wrong and prevents the update.
>
> The code hasn't changed for a long time and newer versions behave the
> same.
>
> The concept patch below simplifies the logic, better tracks the seq#
> and is a bit more permissive.
>

This patch does not apply cleanly against 8.2 (BYTES_THIS_ACK
is not present in 8.2.)

I'll add in the obvious missing #defines and see how I go.

Darren