[tcp] Avoid rewinding sequence numbers on receiving old duplicate ACKs

[people/peper/gpxe.git] / src / net / tcp.c

diff --git a/src/net/tcp.c b/src/net/tcp.c
index 5a64c83..e86cdf8 100644 (file)
--- a/src/net/tcp.c
+++ b/src/net/tcp.c
@@ -726,40 +726,44 @@ static int tcp_rx_ack ( struct tcp_connection *tcp, uint32_t ack,
        size_t len;
        unsigned int acked_flags;
 
-       /* Determine acknowledged flags and data length */
-       len = ack_len;
-       acked_flags = ( TCP_FLAGS_SENDING ( tcp->tcp_state ) &
-                       ( TCP_SYN | TCP_FIN ) );
-       if ( acked_flags )
-               len--;
-
-       /* Stop retransmission timer if necessary */
-       if ( ack_len == 0 ) {
-               /* Duplicate ACK (or just a packet that isn't
-                * intending to ACK any new data).  If the
-                * retransmission timer is running, leave it running
-                * so that we don't immediately retransmit and cause a
-                * sorceror's apprentice syndrome.
-                */
-       } else if ( ack_len <= tcp->snd_sent ) {
-               /* ACK of new data.  Stop the retransmission timer. */
-               stop_timer ( &tcp->timer );
-       } else {
-               /* Out-of-range (or old duplicate) ACK.  Leave the
-                * timer running, as for the ack_len==0 case, to
-                * handle old duplicate ACKs.
-                */
+       /* Check for out-of-range or old duplicate ACKs */
+       if ( ack_len > tcp->snd_sent ) {
                DBGC ( tcp, "TCP %p received ACK for %08x..%08zx, "
                       "sent only %08x..%08x\n", tcp, tcp->snd_seq,
                       ( tcp->snd_seq + ack_len ), tcp->snd_seq,
                       ( tcp->snd_seq + tcp->snd_sent ) );
-               /* Send RST if an out-of-range ACK is received on a
-                * not-yet-established connection.
-                */
-               if ( ! TCP_HAS_BEEN_ESTABLISHED ( tcp->tcp_state ) )
+
+               if ( TCP_HAS_BEEN_ESTABLISHED ( tcp->tcp_state ) ) {
+                       /* Just ignore what might be old duplicate ACKs */
+                       return 0;
+               } else {
+                       /* Send RST if an out-of-range ACK is received
+                        * on a not-yet-established connection, as per
+                        * RFC 793.
+                        */
                        return -EINVAL;
+               }
        }
 
+       /* Ignore ACKs that don't actually acknowledge any new data.
+        * (In particular, do not stop the retransmission timer; this
+        * avoids creating a sorceror's apprentice syndrome when a
+        * duplicate ACK is received and we still have data in our
+        * transmit queue.)
+        */
+       if ( ack_len == 0 )
+               return 0;
+
+       /* Stop the retransmission timer */
+       stop_timer ( &tcp->timer );
+
+       /* Determine acknowledged flags and data length */
+       len = ack_len;
+       acked_flags = ( TCP_FLAGS_SENDING ( tcp->tcp_state ) &
+                       ( TCP_SYN | TCP_FIN ) );
+       if ( acked_flags )
+               len--;
+
        /* Update SEQ and sent counters, and window size */
        tcp->snd_seq = ack;
        tcp->snd_sent = 0;