Author: rstone Date: Sat Feb 11 17:05:08 2017 New Revision: 313646 URL: https://svnweb.freebsd.org/changeset/base/313646
Log: Don't zero out srtt after excess retransmits If the TCP stack has retransmitted more than 1/4 of the total number of retransmits before a connection drop, it decides that its current RTT estimate is hopelessly out of date and decides to recalculate it from scratch starting with the next ACK. Unfortunately, it implements this by zeroing out the current RTT estimate. Drop this hack entirely, as it makes it significantly more difficult to debug connection issues. Instead check for excessive retransmits at the point where srtt is updated from an ACK being received. If we've exceeded 1/4 of the maximum retransmits, discard the previous srtt estimate and replace it with the latest rtt measurement. Differential Revision: https://reviews.freebsd.org/D9519 Reviewed by: gnn Sponsored by: Dell EMC Isilon Modified: head/sys/netinet/tcp_input.c head/sys/netinet/tcp_timer.c head/sys/netinet/tcp_timer.h Modified: head/sys/netinet/tcp_input.c ============================================================================== --- head/sys/netinet/tcp_input.c Sat Feb 11 15:25:49 2017 (r313645) +++ head/sys/netinet/tcp_input.c Sat Feb 11 17:05:08 2017 (r313646) @@ -3494,7 +3494,7 @@ tcp_xmit_timer(struct tcpcb *tp, int rtt TCPSTAT_INC(tcps_rttupdated); tp->t_rttupdated++; - if (tp->t_srtt != 0) { + if ((tp->t_srtt != 0) && (tp->t_rxtshift <= TCP_RTT_INVALIDATE)) { /* * srtt is stored as fixed point with 5 bits after the * binary point (i.e., scaled by 8). The following magic Modified: head/sys/netinet/tcp_timer.c ============================================================================== --- head/sys/netinet/tcp_timer.c Sat Feb 11 15:25:49 2017 (r313645) +++ head/sys/netinet/tcp_timer.c Sat Feb 11 17:05:08 2017 (r313646) @@ -845,20 +845,16 @@ tcp_timer_rexmt(void * xtp) (tp->t_rxtshift == 3)) tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_SACK_PERMIT); /* - * If we backed off this far, our srtt estimate is probably bogus. - * Clobber it so we'll take the next rtt measurement as our srtt; - * move the current srtt into rttvar to keep the current - * retransmit times until then. + * If we backed off this far, notify the L3 protocol that we're having + * connection problems. */ - if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) { + if (tp->t_rxtshift > TCP_RTT_INVALIDATE) { #ifdef INET6 if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0) in6_losing(tp->t_inpcb); else #endif in_losing(tp->t_inpcb); - tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT); - tp->t_srtt = 0; } tp->snd_nxt = tp->snd_una; tp->snd_recover = tp->snd_max; Modified: head/sys/netinet/tcp_timer.h ============================================================================== --- head/sys/netinet/tcp_timer.h Sat Feb 11 15:25:49 2017 (r313645) +++ head/sys/netinet/tcp_timer.h Sat Feb 11 17:05:08 2017 (r313646) @@ -119,6 +119,13 @@ #define TCPTV_DELACK ( hz/10 ) /* 100ms timeout */ +/* + * If we exceed this number of retransmits for a single segment, we'll consider + * the current srtt measurement no longer valid and will recalculate from + * scratch starting with the next ACK. + */ +#define TCP_RTT_INVALIDATE (TCP_MAXRXTSHIFT / 4) + #ifdef TCPTIMERS static const char *tcptimers[] = { "REXMT", "PERSIST", "KEEP", "2MSL", "DELACK" }; _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
