[lwip-devel] [bug #27329] dupacks by unidirectional data transmit

[Kieran Mansley]

Follow-up Comment #6, bug #27329 (project lwip):

OK, diff makes a bad job of displaying my changes, so here's what the code
block that deals with dupacks now looks like:


    /* (From Stevens TCP/IP Illustrated Vol II, p970.) Its only a
     * duplicate ack if:
     * 1) It doesn't ACK new data 
     * 2) length of received packet is zero (i.e. no payload) 
     * 3) the advertised window hasn't changed 
     * 4) There is outstanding unacknowledged data (retransmission timer
running)
     * 5) The ACK is == biggest ACK sequence number so far seen (snd_una)
     * 
     * If it passes all five, should process as a dupack: 
     * a) dupacks < 3: do nothing 
     * b) dupacks == 3: fast retransmit 
     * c) dupacks > 3: increase cwnd 
     * 
     * If it only passes 1-3, should reset dupack counter (and add to
     * stats, which we don't do in lwIP)
     *
     * If it only passes 1, should reset dupack counter
     *
     */

    /* Clause 1 */
    if (TCP_SEQ_LEQ(ackno, pcb->lastack)) {
      pcb->acked = 0;
      /* Clause 2 */
      if (tcplen == 0) {
        /* Clause 3 */
        if (pcb->snd_wl2 + pcb->snd_wnd == right_wnd_edge){
          /* Clause 4 */
          if (pcb->rtime >= 0) {
            /* Clause 5 */
            if (pcb->lastack == ackno) {
              found_dupack = 1;
              if (pcb->dupacks + 1 > pcb->dupacks)
                ++pcb->dupacks;
              if (pcb->dupacks > 3) {
                /* Inflate the congestion window, but not if it means that
                   the value overflows. */
                if ((u16_t)(pcb->cwnd + pcb->mss) > pcb->cwnd) {
                  pcb->cwnd += pcb->mss;
                }
              } else if (pcb->dupacks == 3) {
                /* Do fast retransmit */
                tcp_rexmit_fast(pcb);
              }
            }
          }
        }
      }
      /* If Clause (1) or more is true, but not a duplicate ack, reset
       * count of consecutive duplicate acks */
      if (!found_dupack) {
        pcb->dupacks = 0;
      }
    } else if (TCP_SEQ_BETWEEN(ackno, pcb->lastack+1, pcb->snd_nxt)){
      /* We come here when the ACK acknowledges new data. */



    _______________________________________________________

Reply to this item at:

  <http://savannah.nongnu.org/bugs/?27329>

_______________________________________________
  Message sent via/by Savannah
  http://savannah.nongnu.org/