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authorNeal Cardwell <ncardwell@google.com>2020-06-24 12:42:02 -0400
committerDavid S. Miller <davem@davemloft.net>2020-06-25 16:08:47 -0700
commitb344579ca8478598937215f7005d6c7b84d28aee (patch)
treee1a60ff57731ccc21100c62ad70dc8a6d4a8fb8b /net
parent29a30bac564252d732f6c387617a3456e038bdf6 (diff)
downloadlinux-b344579ca8478598937215f7005d6c7b84d28aee.tar.bz2
tcp_cubic: fix spurious HYSTART_DELAY exit upon drop in min RTT
Mirja Kuehlewind reported a bug in Linux TCP CUBIC Hystart, where Hystart HYSTART_DELAY mechanism can exit Slow Start spuriously on an ACK when the minimum rtt of a connection goes down. From inspection it is clear from the existing code that this could happen in an example like the following: o The first 8 RTT samples in a round trip are 150ms, resulting in a curr_rtt of 150ms and a delay_min of 150ms. o The 9th RTT sample is 100ms. The curr_rtt does not change after the first 8 samples, so curr_rtt remains 150ms. But delay_min can be lowered at any time, so delay_min falls to 100ms. The code executes the HYSTART_DELAY comparison between curr_rtt of 150ms and delay_min of 100ms, and the curr_rtt is declared far enough above delay_min to force a (spurious) exit of Slow start. The fix here is simple: allow every RTT sample in a round trip to lower the curr_rtt. Fixes: ae27e98a5152 ("[TCP] CUBIC v2.3") Reported-by: Mirja Kuehlewind <mirja.kuehlewind@ericsson.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_cubic.c5
1 files changed, 2 insertions, 3 deletions
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 8f8eefd3a3ce..c7bf5b26bf0c 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -432,10 +432,9 @@ static void hystart_update(struct sock *sk, u32 delay)
if (hystart_detect & HYSTART_DELAY) {
/* obtain the minimum delay of more than sampling packets */
+ if (ca->curr_rtt > delay)
+ ca->curr_rtt = delay;
if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
- if (ca->curr_rtt > delay)
- ca->curr_rtt = delay;
-
ca->sample_cnt++;
} else {
if (ca->curr_rtt > ca->delay_min +