[lwip-devel] [bug #20515] TCP delayed ack does not work as expected

[Simon Goldschmidt]

Follow-up Comment #13, bug #20515 (project lwip):

> I'm happy to incorporate a "only increase the advertised window
> by a minimum of a segment" rule in there though, which should do 
> silly window avoidance.

I also think so. Although that isn't enough (because it would mean we would
send an ACK with every segment).

I think I already worked on this a bit, but it's been a while ago...

RFC 1122 chapter 4.2.3.2 says "the delay MUST be less than 0.5 seconds, and
in a stream of full-sized segments there SHOULD be an ACK for at least every
second segment."

This would mean we need
- the last advertised window and
- a timer
and send an ACK if the last advertised window is <= current_wnd - (2 * mss)
OR the timer has expired.

These are 2*16 bit plus a little code to check the timer in tcp_slow_tmr, I
don't think it's too much: it saves a lot of ACKs, after all!

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