From: Neal Cardwell ncardwell@google.com
[ Upstream commit b344579ca8478598937215f7005d6c7b84d28aee ]
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/tcp_cubic.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index 78bfadfcf342..8b5ba0a5cd38 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -403,6 +403,8 @@ 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 == 0 || ca->curr_rtt > delay) ca->curr_rtt = delay;