From 625835c8b5e897e54a1a023788a3a9c3b16631c9 Mon Sep 17 00:00:00 2001 From: Michael Tuexen Date: Tue, 5 Nov 2024 09:52:42 +0100 Subject: [PATCH] tcp: fix the initial CWND when a SYN retransmission happened According to RFC 3390 the CWND should be set to one MSS if the SYN or SYN-ACK has been retransmitted. This is handled in the code by setting CWND to 1 and cc_conn_init() translates this to MSS. Unfortunately, cc_cong_signal() was overwriting the special value of 1 in case of a lost SYN, and therefore the initial CWND was not as it was supposed to be. Fix this by not overwriting the special value of 1. Reviewed by: cc, rscheff MFC after: 3 days Sponsored by: Netflix, Inc. Differential Revision: https://reviews.freebsd.org/D47439 --- sys/netinet/tcp_timer.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/sys/netinet/tcp_timer.c b/sys/netinet/tcp_timer.c index c5c65dda5b1a..4d8dafaec31d 100644 --- a/sys/netinet/tcp_timer.c +++ b/sys/netinet/tcp_timer.c @@ -810,7 +810,9 @@ tcp_timer_rexmt(struct tcpcb *tp) */ tp->t_rtttime = 0; - cc_cong_signal(tp, NULL, CC_RTO); + /* Do not overwrite the snd_cwnd on SYN retransmissions. */ + if (tp->t_state != TCPS_SYN_SENT) + cc_cong_signal(tp, NULL, CC_RTO); NET_EPOCH_ENTER(et); rv = tcp_output_locked(tp); NET_EPOCH_EXIT(et);