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Code Issues Releases Activity

tcp: fix duplicate retransmissions when RTO happens during SACK loss recovery

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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
This commit is contained in:
Richard Scheffenegger 2024-10-10 12:50:22 +02:00
parent d9daa28c36
commit 440f4ba18e
4 changed files with 41 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
sys/netinet/tcp_input.c
View File

@ -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) &&

25
sys/netinet/tcp_output.c
View File

@ -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) {
/*

22
sys/netinet/tcp_sack.c
View File

@ -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);
}
/*

2
sys/netinet/tcp_var.h
View File

@ -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 *);