Kieran Mansley - kjm25@cam.ac.uk - 12th September 2004

Applied patch from Sam Jansen as detailed in
http://lists.gnu.org/archive/html/lwip-users/2004-07/msg00106.html
to correctly handle retransmission after a retransmission timeout

src/core/tcp.c

@@ -590,7 +590,6 @@ tcp_slowtmr(void)
         if (pcb->state != SYN_SENT) {
           pcb->rto = ((pcb->sa >> 3) + pcb->sv) << tcp_backoff[pcb->nrtx];
         }
-        tcp_rexmit(pcb);
         /* Reduce congestion window and ssthresh. */
         eff_wnd = LWIP_MIN(pcb->cwnd, pcb->snd_wnd);
         pcb->ssthresh = eff_wnd >> 1;
@@ -600,7 +599,10 @@ tcp_slowtmr(void)
         pcb->cwnd = pcb->mss;
         LWIP_DEBUGF(TCP_CWND_DEBUG, ("tcp_slowtmr: cwnd %u ssthresh %u\n",
                                 pcb->cwnd, pcb->ssthresh));
-      }
+ 
+        /* The following needs to be called AFTER cwnd is set to one mss - STJ */
+        tcp_rexmit_rto(pcb);
+     }
     }
     /* Check if this PCB has stayed too long in FIN-WAIT-2 */
     if (pcb->state == FIN_WAIT_2) {

src/core/tcp_in.c

@@ -818,10 +818,6 @@ tcp_receive(struct tcp_pcb *pcb)
        rationale is that lwIP puts all outstanding segments on the
        ->unsent list after a retransmission, so these segments may
        in fact have been sent once. */
-    /* KJM 13th July 2004
-       I don't think is is necessary as we no longer move all unacked
-       segments on the unsent queue when performing retransmit */
-#if 0
     while (pcb->unsent != NULL &&
            /*TCP_SEQ_LEQ(ntohl(pcb->unsent->tcphdr->seqno) + TCP_TCPLEN(pcb->unsent), ackno) &&
              TCP_SEQ_LEQ(ackno, pcb->snd_max)*/
@@ -846,7 +842,6 @@ tcp_receive(struct tcp_pcb *pcb)
         pcb->snd_nxt = htonl(pcb->unsent->tcphdr->seqno);
       }
     }
-#endif
     /* End of ACK for new data processing. */
 
     LWIP_DEBUGF(TCP_RTO_DEBUG, ("tcp_receive: pcb->rttest %u rtseq %lu ackno %lu\n",

src/core/tcp_out.c

@@ -462,8 +462,16 @@ tcp_output(struct tcp_pcb *pcb)
         pcb->unacked = seg;
         useg = seg;
       } else {
-        useg->next = seg;
-        useg = useg->next;
+        /* In the case of fast retransmit, the packet should not go to the end
+         * of the unacked queue, but rather at the start. We need to check for
+         * this case. -STJ Jul 27, 2004 */
+        if (TCP_SEQ_LT(ntohl(seg->tcphdr->seqno), ntohl(useg->tcphdr->seqno))){
+          seg->next = pcb->unacked;
+          pcb->unacked = seg;
+        } else {
+          useg->next = seg;
+          useg = useg->next;
+        }
       }
     } else {
       tcp_seg_free(seg);
@@ -568,6 +576,33 @@ tcp_rst(u32_t seqno, u32_t ackno,
   LWIP_DEBUGF(TCP_RST_DEBUG, ("tcp_rst: seqno %lu ackno %lu.\n", seqno, ackno));
 }
 
+void
+tcp_rexmit_rto(struct tcp_pcb *pcb)
+{
+  struct tcp_seg *seg;
+
+  if (pcb->unacked == NULL) {
+    return;
+  }
+
+  /* Move all unacked segments to the unsent queue. */
+  for (seg = pcb->unacked; seg->next != NULL; seg = seg->next);
+  seg->next = pcb->unsent;
+  pcb->unsent = pcb->unacked;
+  pcb->unacked = NULL;
+
+  pcb->snd_nxt = ntohl(pcb->unsent->tcphdr->seqno);
+
+  ++pcb->nrtx;
+
+  /* Don't take any rtt measurements after retransmitting. */
+  pcb->rttest = 0;
+
+  /* Do the actual retransmission. */
+  tcp_output(pcb);
+
+}
+
 void
 tcp_rexmit(struct tcp_pcb *pcb)
 {
@@ -595,6 +630,7 @@ tcp_rexmit(struct tcp_pcb *pcb)
 
 }
 
+
 void
 tcp_keepalive(struct tcp_pcb *pcb)
 {

src/include/lwip/tcp.h

@@ -105,6 +105,7 @@ void             tcp_input   (struct pbuf *p, struct netif *inp);
 /* Used within the TCP code only: */
 err_t            tcp_output  (struct tcp_pcb *pcb);
 void             tcp_rexmit  (struct tcp_pcb *pcb);
+void             tcp_rexmit_rto  (struct tcp_pcb *pcb);