Re: [lwip-users] snd_queuelen does not count the number of segments...

[Kieran Mansley]

On Tue, 2006-11-28 at 08:53 -0500, Tom Hennen wrote:
> I think I understand you're reasoning, after a fashion.  
>  
> It does seem to be a problem for me because I'm queuing the pbufs in
> my link layer driver (using pbuf_queue).  As a result the length of
> the pbuf chain reported by pbuf_clen is not what tcp_in expects it to
> be and so pcb->snd_queuelen ends up wrapping around from 0 to ~255
> whenever pbuf_clen(next->p) > pcb->snd_queuelen.

That packets that you pass in to lwIP from the link layer should have no
direct effect on snd_queuelen, so how you construct them should be of no
consequence.

When an ACK is passed in from your link layer snd_queuelen is
decremented by the number of pbufs in ***the packet that is
acknowledged***, not the number of pbufs in the packet that you've just
passed in that does the acknowledging.

snd_queuelen was earlier (in tcp_out.c:tcp_enqueue()) incremented by the
number of pbufs in the same packet (when it was enqueued on the
unacknowledged or unsent list) so there is something really wrong if
you've ended up with snd_queuelen < the number of pbufs in one of the
packets in those queues.  We should probably be asserting this in
tcp_in.c:tcp_receive().

For this to happen, my guess is your packet queues are getting
corrupted, most likely due to insufficient locking and protection of the
stack resulting in two threads accessing it at the same time.  

Kieran