Re: [lwip-users] Does it correct SYN packet with window size 0

[Kieran Mansley]

On Tue, 2007-07-17 at 16:52 +0200, Oleg Tychev wrote:
> Hello All,
> 
> Two devices tries to communicate.
> 1st device tries to initiate TCP connection. But instead SYN packet with 
> conventional window size = 65535 or MSS,
> it sends packet with window size = 0.
> 2nd device replies not.
> 
> I have reread RFCs and not found direct answer.
> Does ISN (initial sequence number) is data or not and should be 
> applicable window size for it?
> 
> Wireshark, for example, does not display information about window size 
> in first SYN packet.
> 
> The question is who is guilty 1st or 2nd? :)

And you don't even make it easy by telling us which end is lwIP!

The first is guilty, in my opinion.  The SYN-ACK the second would like
to send in reply needs available window to be sent as the SYN flag
consumes a byte of sequence space.  However, I don't have chapter and
verse on which is correct.

I can see the desirability of sending the SYN (and even more so if it
were a FIN which could have the same problem) even when there is no
advertised window space.  The first SYN in a connection for example is
sent when there is no advertised space for example (it has to be, as
it's the first packet to be sent), so the SYN-ACK following the same
logic would probably be a good idea.

My guess is that some other stack is being a bit unusual by not
advertising window space in its SYN packets, and lwIP is failing to
reply because it is simple and doesn't have special cases for stuff like
this.  Am I right?

Kieran