tcp/sack: Use RFC3517bis IsLost(snd_una) as fallback of early retransmit

Since we are less certain about whether is segment is lost or not when
using IsLost(snd_una), we do not send out other unSACKed segments except
the first unSACKed segment under this condition.  Sending out other
unSACKed segments could be too aggressive here; just wait for another
ACK to tick out more unSACKed segments.

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 38112c2..d687b68 100644 (file)
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -3379,6 +3379,8 @@ tcp_established(struct tcpcb *tp)
 static boolean_t
 tcp_fast_recovery(struct tcpcb *tp, tcp_seq th_ack, const struct tcpopt *to)
 {
+       boolean_t fast_sack_rexmt = TRUE;
+
        tcpstat.tcps_rcvdupack++;
 
        /*
@@ -3467,24 +3469,40 @@ fastretransmit:
                        tp->snd_nxt = old_snd_nxt;
                KASSERT(tp->snd_limited <= 2, ("tp->snd_limited too big"));
                if (TCP_DO_SACK(tp)) {
-                       tcp_sack_rexmt(tp);
+                       if (fast_sack_rexmt)
+                               tcp_sack_rexmt(tp);
                } else {
                        tp->snd_cwnd += tp->t_maxseg *
                            (tp->t_dupacks - tp->snd_limited);
                }
        } else if ((tcp_do_rfc3517bis && TCP_DO_SACK(tp)) || TCP_DO_NCR(tp)) {
+               /*
+                * The RFC3517bis recommends to reduce the byte threshold,
+                * and enter fast retransmit if IsLost(snd_una).  However,
+                * if we use IsLost(snd_una) based fast retransmit here,
+                * segments reordering will cause spurious retransmit.  So
+                * we defer the IsLost(snd_una) based fast retransmit until
+                * the extended limited transmit can't send any segments and
+                * early retransmit can't be done.
+                */
                if (tcp_rfc3517bis_rxt && tcp_do_rfc3517bis &&
                    tcp_sack_islost(&tp->scb, tp->snd_una))
                        goto fastretransmit;
+
                if (tcp_do_limitedtransmit || TCP_DO_NCR(tp)) {
-                       /* outstanding data */
-                       uint32_t ownd = tp->snd_max - tp->snd_una;
-
-                       if (!tcp_sack_limitedxmit(tp) &&
-                           need_early_retransmit(tp, ownd)) {
-                               ++tcpstat.tcps_sndearlyrexmit;
-                               tp->rxt_flags |= TRXT_F_EARLYREXMT;
-                               goto fastretransmit;
+                       if (!tcp_sack_limitedxmit(tp)) {
+                               /* outstanding data */
+                               uint32_t ownd = tp->snd_max - tp->snd_una;
+
+                               if (need_early_retransmit(tp, ownd)) {
+                                       ++tcpstat.tcps_sndearlyrexmit;
+                                       tp->rxt_flags |= TRXT_F_EARLYREXMT;
+                                       goto fastretransmit;
+                               } else if (tcp_do_rfc3517bis &&
+                                   tcp_sack_islost(&tp->scb, tp->snd_una)) {
+                                       fast_sack_rexmt = FALSE;
+                                       goto fastretransmit;
+                               }
                        }
                }
        } else if (tcp_do_limitedtransmit) {

diff --git a/sys/netinet/tcp_sack.c b/sys/netinet/tcp_sack.c
index 545604f..87167f1 100644 (file)
--- a/sys/netinet/tcp_sack.c
+++ b/sys/netinet/tcp_sack.c
@@ -475,15 +475,7 @@ tcp_sack_update_lostseq(struct scoreboard *scb, tcp_seq snd_una, u_int maxseg,
        int bytes_sacked = 0;
        int rxtthresh_bytes;
 
-       /*
-        * XXX
-        * The RFC3517bis recommends to reduce the byte threshold.
-        * However, it will cause extra spurious retransmit if
-        * segments are reordered.  Before certain DupThresh adaptive
-        * algorithm is implemented, we don't reduce the byte
-        * threshold (tcp_rfc3517bis_rxt is off by default).
-        */
-       if (tcp_do_rfc3517bis && tcp_rfc3517bis_rxt)
+       if (tcp_do_rfc3517bis)
                rxtthresh_bytes = (rxtthresh - 1) * maxseg;
        else
                rxtthresh_bytes = rxtthresh * maxseg;