[lwip-users] snd_wnd updation problem

[Mumtaz Ahmad]

Dear all

 

I have observed while testing with windows client that some times on retransmission of packets from window the window advertises smaller window .

In that case lwip does not update its snd_wnd which ultimately leads to more data sent by lwip than the advertised window by windows .The window discrads the extra data ,which does not fit in its window ,and ask lwip to send this lost data again as soon its window opens. But some times this demanded sequence number   does not lie on segment boundary . Lwip resend a segement which contains that sequence somewhere in middle ,as lwip cannot fragment that segment to send the exact sequence , but windows keep on insisting for exact sequence .In that case the deadlock is created and ultimately the connection gets stuck .

 

The check that seems responsible for this is in tcp_in.c in function tcp_receive

 

if (TCP_SEQ_LT(pcb->snd_wl1, seqno) ||
        (pcb->snd_wl1 == seqno && TCP_SEQ_LT(pcb->snd_wl2, ackno)) ||
        (pcb->snd_wl2 == ackno && seg->tcphdr->wnd > pcb->snd_wnd))
    {
      pcb->snd_wnd = seg->tcphdr->wnd;
      pcb->snd_wl1 = seqno;
      pcb->snd_wl2 = ackno;

 

 

The if check is failed for  retransmitted received packets with a smaller advertised window causing a no window update and causing a dead lock.

 

This problem may be solved if we add a new check before this check like

 

if(seg->tcphdr->wnd < pcb->snd_wnd)

    pcb->snd_wnd = seg->tcphdr->wnd;

 

This new if sets the snd_wnd whenever any small window is advertised

 

Any one comment please

 

 

Best Regards

Mumtaz Ahmad

Manager Network Systems Group

SN Pakistan

92-52-3335228327