[lwip-users] Re: [lwip] TCP congestion window calculation

[Adam Dunkels]

Hi Biren!

On Monday 24 December 2001 20.22, you wrote:
> The way it works is, when I send data bytes less than the size of the MSS
> from lwIP, it works fine. But if I send data equals to size of MSS, then
> the congestion window comes out as one byte shorter than the data size. So
> after three-way hand shake, lwIP waits for the window update from peer.
> Below is the code from tcp_output() function which checks that.
>
> while(seg != NULL &&
>       ntohl(seg->tcphdr->seqno) - pcb->snd_ack + seg->len <= wnd)
>
> Here for example, say seg->len is 1460 and MSS is also 1460. Now after
> three-way handshake (ntohl(seg->tcphdr->seqno) - pcb->snd_ack) comes as 1
> which makes the final result 1461 which is greater than congestion window
> 1460. So it doesn't go inside the while loop and then data just sits in
> queue.

This is a bit weird - the first data segment on the pcb->unsent queue 
*should* have the sequence number ISS+1, which would be the same sequence 
number that the remote end acknowledged after the three way handshake. So 
ntohl(seg->tcphdr->seqno) - pcb->snd_ack should be 0, really. Are you able to 
run the code in a debugger? If so, would it be possible to inspect the first 
segment on the unsent queue? It should be a pure data segment (i.e., no SYN 
segment) and the sequence number should be the seqno of the SYN + 1.

I would assume that you initiate your connection from within lwIP - there 
have been a few bugs in that code. In fact, now that I looked into your 
problem I found a few new bugs and fixed them (connection attempts that went 
unanswered did not propagate the error back to the application if the 
application was using the sequential API).

Would it be possible to test your application against the 0.5.0 version? 
Perhaps your problem is a sideeffect of some other bug that has been fixed 
there.

/adam
-- 
Adam Dunkels <address@hidden>
http://www.sics.se/~adam
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