Re: [lwip-devel] [bug #50837] TCP: zero window probe doesn't timeout

[address@hidden]

Joel Cunningham wrote:
> There was some discussion of that in the original bug adding the 
> Persist Timer:

Thanks for the reminder. I had the thought of this being talked about 
somewhere...

> It looks like there was hesitation about whether the remote TCP could 
> handle this even though Steven's says the receiver should trim the 
> segment to what will fit.

By now I'd say we should just try it. There are three possibilites: the 
segment will not be ACKed, the segment will be partially ACKed or the 
segment wil be fully ACKed. Of course, we would have to be prepared for 
all those.

> If LwIP was modified to behave this way, it wouldn't be able to remove 
> the segment from the unacked queue when receiving the zero window 
> update because we only support receiving an ACK covering the full 
> segment.  Maybe this would be ok since in tcp_slowtmr(), the persist 
> timer effectively overrides the RTO timer when active and we wouldn't 
> retransmit during the zero window.

Hmm, that would only work if the remote host ACKed the data inside the 
window - not more and not less.

> Also, calculations for determining the in-flight data would show an 
> amount exceeding the receiver's advertised window, not sure if that 
> will cause any problems, needs more investigation.

That's also true. Given all that, this solution probably is the same 
mess as our current behaviour of "early-zero-window". Any preference?

Simon