Author: andre
Date: Sun Oct 28 17:25:08 2012
New Revision: 242250
URL: http://svn.freebsd.org/changeset/base/242250

Log:
  When SYN or SYN/ACK had to be retransmitted RFC5681 requires us to
  reduce the initial CWND to one segment.  This reduction got lost
  some time ago due to a change in initialization ordering.
  
  Additionally in tcp_timer_rexmt() avoid entering fast recovery when
  we're still in TCPS_SYN_SENT state.
  
  MFC after:    2 weeks

Modified:
  head/sys/netinet/tcp_input.c
  head/sys/netinet/tcp_syncache.c
  head/sys/netinet/tcp_timer.c

Modified: head/sys/netinet/tcp_input.c
==============================================================================
--- head/sys/netinet/tcp_input.c        Sun Oct 28 17:16:09 2012        
(r242249)
+++ head/sys/netinet/tcp_input.c        Sun Oct 28 17:25:08 2012        
(r242250)
@@ -345,10 +345,16 @@ cc_conn_init(struct tcpcb *tp)
        /*
         * Set the initial slow-start flight size.
         *
-        * RFC3390 says only do this if SYN or SYN/ACK didn't got lost.
-        * XXX: We currently check only in syncache_socket for that.
-        */
-       if (V_tcp_do_rfc3390)
+        * RFC5681 Section 3.1 specifies the default conservative values.
+        * RFC3390 specifies slightly more aggressive values.
+        *
+        * If a SYN or SYN/ACK was lost and retransmitted, we have to
+        * reduce the initial CWND to one segment as congestion is likely
+        * requiring us to be cautious.
+        */
+       if (tp->snd_cwnd == 1)
+               tp->snd_cwnd = tp->t_maxseg;            /* SYN(-ACK) lost */
+       else if (V_tcp_do_rfc3390)
                tp->snd_cwnd = min(4 * tp->t_maxseg,
                    max(2 * tp->t_maxseg, 4380));
        else {

Modified: head/sys/netinet/tcp_syncache.c
==============================================================================
--- head/sys/netinet/tcp_syncache.c     Sun Oct 28 17:16:09 2012        
(r242249)
+++ head/sys/netinet/tcp_syncache.c     Sun Oct 28 17:25:08 2012        
(r242250)
@@ -852,11 +852,12 @@ syncache_socket(struct syncache *sc, str
        tcp_mss(tp, sc->sc_peer_mss);
 
        /*
-        * If the SYN,ACK was retransmitted, reset cwnd to 1 segment.
+        * If the SYN,ACK was retransmitted, indicate that CWND to be
+        * limited to one segment in cc_conn_init().
         * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
         */
        if (sc->sc_rxmits > 1)
-               tp->snd_cwnd = tp->t_maxseg;
+               tp->snd_cwnd = 1;
 
 #ifdef TCP_OFFLOAD
        /*

Modified: head/sys/netinet/tcp_timer.c
==============================================================================
--- head/sys/netinet/tcp_timer.c        Sun Oct 28 17:16:09 2012        
(r242249)
+++ head/sys/netinet/tcp_timer.c        Sun Oct 28 17:25:08 2012        
(r242250)
@@ -539,7 +539,13 @@ tcp_timer_rexmt(void * xtp)
        }
        INP_INFO_RUNLOCK(&V_tcbinfo);
        headlocked = 0;
-       if (tp->t_rxtshift == 1) {
+       if (tp->t_state == TCPS_SYN_SENT) {
+               /*
+                * If the SYN was retransmitted, indicate CWND to be
+                * limited to 1 segment in cc_conn_init().
+                */
+               tp->snd_cwnd = 1;
+       } else if (tp->t_rxtshift == 1) {
                /*
                 * first retransmit; record ssthresh and cwnd so they can
                 * be recovered if this turns out to be a "bad" retransmit.
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