From 440f4ba18e3ab7be912858bbcb96a419fcf14809 Mon Sep 17 00:00:00 2001 From: Richard Scheffenegger Date: Thu, 10 Oct 2024 12:50:22 +0200 Subject: [PATCH] tcp: fix duplicate retransmissions when RTO happens during SACK loss recovery When snd_nxt doesn't track snd_max, partial SACK ACKs may elicit unexpected duplicate retransmissions. This is usually masked by LRO not necessarily ACKing every individual segment, and prior to RFC6675 SACK loss recovery, harder to trigger even when an RTO happens while SACK loss recovery is ongoing. Address this by improving the logic when to start a SACK loss recovery and how to deal with a RTO, as well as improvements to the adjusted congestion window during transmission selection. Reviewed By: tuexen, cc, #transport Sponsored by: NetApp, Inc. MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D43355 --- sys/netinet/tcp_input.c | 18 ++++++++++-------- sys/netinet/tcp_output.c | 25 ++++++++++++++++--------- sys/netinet/tcp_sack.c | 22 ++++++++++++++-------- sys/netinet/tcp_var.h | 2 +- 4 files changed, 41 insertions(+), 26 deletions(-) diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c index b64a71837ab..5d14664f3fc 100644 --- a/sys/netinet/tcp_input.c +++ b/sys/netinet/tcp_input.c @@ -444,10 +444,7 @@ cc_cong_signal(struct tcpcb *tp, struct tcphdr *th, uint32_t type) case CC_RTO: tp->t_dupacks = 0; tp->t_bytes_acked = 0; - if ((tp->t_rxtshift > 1) || - !((tp->t_flags & TF_SACK_PERMIT) && - (!TAILQ_EMPTY(&tp->snd_holes)))) - EXIT_RECOVERY(tp->t_flags); + EXIT_RECOVERY(tp->t_flags); if (tp->t_flags2 & TF2_ECN_PERMIT) tp->t_flags2 |= TF2_ECN_SND_CWR; break; @@ -2747,7 +2744,6 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th, } if (tcp_is_sack_recovery(tp, &to)) { TCPSTAT_INC(tcps_sack_recovery_episode); - tp->snd_recover = tp->snd_nxt; tp->snd_cwnd = maxseg; (void) tcp_output(tp); if (SEQ_GT(th->th_ack, tp->snd_una)) { @@ -2790,8 +2786,12 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th, __func__)); if (tp->t_dupacks == 1) tp->snd_limited = 0; - tp->snd_cwnd = - (tp->snd_nxt - tp->snd_una) + + if ((tp->snd_nxt == tp->snd_max) && + (tp->t_rxtshift == 0)) + tp->snd_cwnd = + SEQ_SUB(tp->snd_nxt, + tp->snd_una); + tp->snd_cwnd += (tp->t_dupacks - tp->snd_limited) * maxseg; /* @@ -2840,7 +2840,9 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th, * counted as dupacks here. */ if (tcp_is_sack_recovery(tp, &to) && - (sack_changed != SACK_NOCHANGE)) { + (((tp->t_rxtshift == 0) && (sack_changed != SACK_NOCHANGE)) || + ((tp->t_rxtshift > 0) && (sack_changed == SACK_NEWLOSS))) && + (tp->snd_nxt == tp->snd_max)) { tp->t_dupacks++; /* limit overhead by setting maxseg last */ if (!IN_FASTRECOVERY(tp->t_flags) && diff --git a/sys/netinet/tcp_output.c b/sys/netinet/tcp_output.c index 78ecfe5289b..a048abf5f97 100644 --- a/sys/netinet/tcp_output.c +++ b/sys/netinet/tcp_output.c @@ -259,6 +259,7 @@ tcp_default_output(struct tcpcb *tp) } } again: + sendwin = 0; /* * If we've recently taken a timeout, snd_max will be greater than * snd_nxt. There may be SACK information that allows us to avoid @@ -266,12 +267,12 @@ tcp_default_output(struct tcpcb *tp) */ if ((tp->t_flags & TF_SACK_PERMIT) && SEQ_LT(tp->snd_nxt, tp->snd_max)) - tcp_sack_adjust(tp); + sendwin = tcp_sack_adjust(tp); sendalot = 0; tso = 0; mtu = 0; off = tp->snd_nxt - tp->snd_una; - sendwin = min(tp->snd_wnd, tp->snd_cwnd); + sendwin = min(tp->snd_wnd, tp->snd_cwnd + sendwin); flags = tcp_outflags[tp->t_state]; /* @@ -288,7 +289,8 @@ tcp_default_output(struct tcpcb *tp) sack_bytes_rxmt = 0; len = 0; p = NULL; - if ((tp->t_flags & TF_SACK_PERMIT) && IN_FASTRECOVERY(tp->t_flags) && + if ((tp->t_flags & TF_SACK_PERMIT) && + (IN_FASTRECOVERY(tp->t_flags) || SEQ_LT(tp->snd_nxt, tp->snd_max)) && (p = tcp_sack_output(tp, &sack_bytes_rxmt))) { uint32_t cwin; @@ -387,7 +389,7 @@ tcp_default_output(struct tcpcb *tp) * in which case len is already set. */ if (sack_rxmit == 0) { - if (sack_bytes_rxmt == 0) { + if ((sack_bytes_rxmt == 0) || SEQ_LT(tp->snd_nxt, tp->snd_max)) { len = ((int32_t)min(sbavail(&so->so_snd), sendwin) - off); } else { @@ -1636,11 +1638,16 @@ tcp_default_output(struct tcpcb *tp) tp->snd_max = tp->snd_nxt + xlen; } if ((error == 0) && - (TCPS_HAVEESTABLISHED(tp->t_state) && - (tp->t_flags & TF_SACK_PERMIT) && - tp->rcv_numsacks > 0)) { - /* Clean up any DSACK's sent */ - tcp_clean_dsack_blocks(tp); + (tp->rcv_numsacks > 0) && + TCPS_HAVEESTABLISHED(tp->t_state) && + (tp->t_flags & TF_SACK_PERMIT)) { + /* Clean up any DSACK's sent */ + tcp_clean_dsack_blocks(tp); + } + if ((error == 0) && + sack_rxmit && + SEQ_LT(tp->snd_nxt, SEQ_MIN(p->rxmit, p->end))) { + tp->snd_nxt = SEQ_MIN(p->rxmit, p->end); } if (error) { /* diff --git a/sys/netinet/tcp_sack.c b/sys/netinet/tcp_sack.c index 53a181703fe..09e172ad460 100644 --- a/sys/netinet/tcp_sack.c +++ b/sys/netinet/tcp_sack.c @@ -1006,7 +1006,7 @@ tcp_sack_partialack(struct tcpcb *tp, struct tcphdr *th, u_int *maxsegp) highdata--; highdata = SEQ_MIN(highdata, tp->snd_recover); if (SEQ_LT(th->th_ack, highdata)) { - tp->snd_fack = th->th_ack; + tp->snd_fack = SEQ_MAX(th->th_ack, tp->snd_fack); if ((temp = tcp_sackhole_insert(tp, SEQ_MAX(th->th_ack, highdata - maxseg), highdata, NULL)) != NULL) { tp->sackhint.hole_bytes += @@ -1076,41 +1076,47 @@ tcp_sack_output(struct tcpcb *tp, int *sack_bytes_rexmt) * After a timeout, the SACK list may be rebuilt. This SACK information * should be used to avoid retransmitting SACKed data. This function * traverses the SACK list to see if snd_nxt should be moved forward. + * In addition, cwnd will be inflated by the sacked bytes traversed when + * moving snd_nxt forward. This prevents a traffic burst after the final + * full ACK, and also keeps ACKs coming back. */ -void +int tcp_sack_adjust(struct tcpcb *tp) { + int sacked = 0; struct sackhole *p, *cur = TAILQ_FIRST(&tp->snd_holes); INP_WLOCK_ASSERT(tptoinpcb(tp)); if (cur == NULL) { /* No holes */ - return; + return (0); } if (SEQ_GEQ(tp->snd_nxt, tp->snd_fack)) { /* We're already beyond any SACKed blocks */ - return; + return (tp->sackhint.sacked_bytes); } - /*- + /* * Two cases for which we want to advance snd_nxt: * i) snd_nxt lies between end of one hole and beginning of another * ii) snd_nxt lies between end of last hole and snd_fack */ while ((p = TAILQ_NEXT(cur, scblink)) != NULL) { if (SEQ_LT(tp->snd_nxt, cur->end)) { - return; + return (sacked); } + sacked += p->start - cur->end; if (SEQ_GEQ(tp->snd_nxt, p->start)) { cur = p; } else { tp->snd_nxt = p->start; - return; + return (sacked); } } if (SEQ_LT(tp->snd_nxt, cur->end)) { - return; + return (sacked); } tp->snd_nxt = tp->snd_fack; + return (tp->sackhint.sacked_bytes); } /* diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h index 8ff015b90f8..3fd6edccf36 100644 --- a/sys/netinet/tcp_var.h +++ b/sys/netinet/tcp_var.h @@ -1508,7 +1508,7 @@ void tcp_update_sack_list(struct tcpcb *tp, tcp_seq rcv_laststart, tcp_seq rcv_lastend); void tcp_clean_dsack_blocks(struct tcpcb *tp); void tcp_clean_sackreport(struct tcpcb *tp); -void tcp_sack_adjust(struct tcpcb *tp); +int tcp_sack_adjust(struct tcpcb *tp); struct sackhole *tcp_sack_output(struct tcpcb *tp, int *sack_bytes_rexmt); void tcp_do_prr_ack(struct tcpcb *, struct tcphdr *, struct tcpopt *, sackstatus_t, u_int *);