when generating an ACK-only TCP packet, TCP should always fill in the

sequence number with the highest sequence number that it has ever sent
(except if it is doing a zero window probe). (from dab@cray.com)

SCCS-vsn: sys/netinet/tcp_output.c 8.2

diff --git a/usr/src/sys/netinet/tcp_output.c b/usr/src/sys/netinet/tcp_output.c
index 7c40796..6bc0fe1 100644 (file)
--- a/usr/src/sys/netinet/tcp_output.c
+++ b/usr/src/sys/netinet/tcp_output.c
@@ -4,7 +4,7 @@
  *
  * %sccs.include.redist.c%
  *
  *
  * %sccs.include.redist.c%
  *

- *     @(#)tcp_output.c        8.1 (Berkeley) %G%
+ *     @(#)tcp_output.c        8.2 (Berkeley) %G%

  */
 
 #include <sys/param.h>
  */
 
 #include <sys/param.h>


@@ -364,7 +364,23 @@ send:
        if (flags & TH_FIN && tp->t_flags & TF_SENTFIN && 
            tp->snd_nxt == tp->snd_max)
                tp->snd_nxt--;
        if (flags & TH_FIN && tp->t_flags & TF_SENTFIN && 
            tp->snd_nxt == tp->snd_max)
                tp->snd_nxt--;

-       ti->ti_seq = htonl(tp->snd_nxt);
+       /*
+        * If we are doing retransmissions, then snd_nxt will
+        * not reflect the first unsent octet.  For ACK only
+        * packets, we do not want the sequence number of the
+        * retransmitted packet, we want the sequence number
+        * of the next unsent octet.  So, if there is no data
+        * (and no SYN or FIN), use snd_max instead of snd_nxt
+        * when filling in ti_seq.  But if we are in persist
+        * state, snd_max might reflect one byte beyond the
+        * right edge of the window, so use snd_nxt in that
+        * case, since we know we aren't doing a retransmission.
+        * (retransmit and persist are mutually exclusive...)
+        */
+       if (len || (flags & (TH_SYN|TH_FIN)) || tp->t_timer[TCPT_PERSIST])
+               ti->ti_seq = htonl(tp->snd_nxt);
+       else
+               ti->ti_seq = htonl(tp->snd_max);

        ti->ti_ack = htonl(tp->rcv_nxt);
        if (optlen) {
                bcopy((caddr_t)opt, (caddr_t)(ti + 1), optlen);
        ti->ti_ack = htonl(tp->rcv_nxt);
        if (optlen) {
                bcopy((caddr_t)opt, (caddr_t)(ti + 1), optlen);