re-work the fast retransmission code to follow algorithm from TCP/IP

Illustrated

CHANGELOG

@@ -45,6 +45,9 @@ HISTORY
     to a human-readable string.
 
   ++ Bugfixes:
+  2009-10-28: Kieran Mansley
+  * tcp_in.c, tcp_out.c, tcp.h: re-work the fast retransmission code
+    to follow algorithm from TCP/IP Illustrated
 
   2009-10-27: Kieran Mansley
   * tcp_in.c: fix BUG#27445: grow cwnd with every duplicate ACK

src/core/tcp_in.c

@@ -763,6 +763,7 @@ tcp_receive(struct tcp_pcb *pcb)
   u32_t right_wnd_edge;
   u16_t new_tot_len;
   u8_t accepted_inseq = 0;
+  int found_dupack = 0;
 
   if (flags & TCP_ACK) {
     right_wnd_edge = pcb->snd_wnd + pcb->snd_wl2;
@@ -789,55 +790,62 @@ tcp_receive(struct tcp_pcb *pcb)
 #endif /* TCP_WND_DEBUG */
     }
 
-    if (pcb->lastack == ackno) {
+    /* (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 (pcb->snd_wl2 + pcb->snd_wnd == right_wnd_edge){
+      if (tcplen == 0) {
-        ++pcb->dupacks;
+        /* Clause 3 */
-
+        if (pcb->snd_wl2 + pcb->snd_wnd == right_wnd_edge){
-        if (pcb->dupacks >= 3) {
+          /* Clause 4 */
-          if (!(pcb->flags & TF_INFR) && pcb->unacked != NULL) {
+          if (pcb->rtime >= 0) {
-            /* This is fast retransmit. Retransmit the first unacked segment. */
+            /* Clause 5 */
-            LWIP_DEBUGF(TCP_FR_DEBUG, ("tcp_receive: dupacks %"U16_F" (%"U32_F"), fast retransmit %"U32_F"\n",
+            if (pcb->lastack == ackno) {
-                                       (u16_t)pcb->dupacks, pcb->lastack,
+              found_dupack = 1;
-                                       ntohl(pcb->unacked->tcphdr->seqno)));
+              if (pcb->dupacks + 1 > pcb->dupacks)
-            tcp_rexmit(pcb);
+                ++pcb->dupacks;
-            /* Set ssthresh to max (FlightSize / 2, 2*SMSS) */
+              if (pcb->dupacks > 3) {
-            /*pcb->ssthresh = LWIP_MAX((pcb->snd_max -
+                /* Inflate the congestion window, but not if it means that
-              pcb->lastack) / 2,
+                   the value overflows. */
-              2 * pcb->mss);*/
+                if ((u16_t)(pcb->cwnd + pcb->mss) > pcb->cwnd) {
-            /* Set ssthresh to half of the minimum of the current cwnd and the advertised window */
+                  pcb->cwnd += pcb->mss;
-            if (pcb->cwnd > pcb->snd_wnd)
+                }
-              pcb->ssthresh = pcb->snd_wnd / 2;
+              } else if (pcb->dupacks == 3) {
-            else
+                /* Do fast retransmit */
-              pcb->ssthresh = pcb->cwnd / 2;
+                tcp_rexmit_fast(pcb);
-            
+              }
-            /* The minimum value for ssthresh should be 2 MSS */
-            if (pcb->ssthresh < 2*pcb->mss) {
-              LWIP_DEBUGF(TCP_FR_DEBUG, ("tcp_receive: The minimum value for ssthresh %"U16_F" should be min 2 mss %"U16_F"...\n", pcb->ssthresh, 2*pcb->mss));
-              pcb->ssthresh = 2*pcb->mss;
-            }
-            
-            pcb->cwnd = pcb->ssthresh + 3 * pcb->mss;
-            pcb->flags |= TF_INFR;
-          } else {
-            /* 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;
             }
           }
         }
-
+      }
-        if (pcb->unacked == NULL && pcb->unsent == NULL)
+      /* If Clause (1) or more is true, but not a duplicate ack, reset
-          pcb->dupacks = 0;
+       * count of consecutive duplicate acks */
-      } else {
+      if (!found_dupack) {
-        LWIP_DEBUGF(TCP_FR_DEBUG, ("tcp_receive: dupack averted %"U32_F" %"U32_F"\n",
+        pcb->dupacks = 0;
-                                   pcb->snd_wl2 + pcb->snd_wnd, right_wnd_edge));
       }
     } else if (TCP_SEQ_BETWEEN(ackno, pcb->lastack+1, pcb->snd_nxt)){
       /* We come here when the ACK acknowledges new data. */
-      
+
       /* Reset the "IN Fast Retransmit" flag, since we are no longer
          in fast retransmit. Also reset the congestion window to the
          slow start threshold. */

src/core/tcp_out.c

@@ -858,6 +858,46 @@ tcp_rexmit(struct tcp_pcb *pcb)
   tcp_output(pcb);
 }
 
+
+/**
+ * Handle retransmission after three dupacks received
+ *
+ * @param pcb the tcp_pcb for which to retransmit the first unacked segment
+ */
+void 
+tcp_rexmit_fast(struct tcp_pcb *pcb)
+{
+  if (pcb->unacked != NULL && !(pcb->flags & TF_INFR)) {
+    /* This is fast retransmit. Retransmit the first unacked segment. */
+    LWIP_DEBUGF(TCP_FR_DEBUG, 
+                ("tcp_receive: dupacks %"U16_F" (%"U32_F
+                 "), fast retransmit %"U32_F"\n",
+                 (u16_t)pcb->dupacks, pcb->lastack,
+                 ntohl(pcb->unacked->tcphdr->seqno)));
+    tcp_rexmit(pcb);
+
+    /* Set ssthresh to half of the minimum of the current
+     * cwnd and the advertised window */
+    if (pcb->cwnd > pcb->snd_wnd)
+      pcb->ssthresh = pcb->snd_wnd / 2;
+    else
+      pcb->ssthresh = pcb->cwnd / 2;
+    
+    /* The minimum value for ssthresh should be 2 MSS */
+    if (pcb->ssthresh < 2*pcb->mss) {
+      LWIP_DEBUGF(TCP_FR_DEBUG, 
+                  ("tcp_receive: The minimum value for ssthresh %"U16_F
+                   " should be min 2 mss %"U16_F"...\n",
+                   pcb->ssthresh, 2*pcb->mss));
+      pcb->ssthresh = 2*pcb->mss;
+    }
+    
+    pcb->cwnd = pcb->ssthresh + 3 * pcb->mss;
+    pcb->flags |= TF_INFR;
+  } 
+}
+
+
 /**
  * Send keepalive packets to keep a connection active although
  * no data is sent over it.

src/include/lwip/tcp.h

@@ -125,6 +125,7 @@ void             tcp_input   (struct pbuf *p, struct netif *inp);
 err_t            tcp_output  (struct tcp_pcb *pcb);
 void             tcp_rexmit  (struct tcp_pcb *pcb);
 void             tcp_rexmit_rto  (struct tcp_pcb *pcb);
+void             tcp_rexmit_fast (struct tcp_pcb *pcb);
 u32_t            tcp_update_rcv_ann_wnd(struct tcp_pcb *pcb);
 
 /**