From ccce324dabfe2143519daf50ed8b1ef1d0c542f7 Mon Sep 17 00:00:00 2001 From: David Morley Date: Tue, 9 May 2023 18:05:58 +0000 Subject: tcp: make the first N SYN RTO backoffs linear Currently the SYN RTO schedule follows an exponential backoff scheme, which can be unnecessarily conservative in cases where there are link failures. In such cases, it's better to aggressively try to retransmit packets, so it takes routers less time to find a repath with a working link. We chose a default value for this sysctl of 4, to follow the macOS and IOS backoff scheme of 1,1,1,1,1,2,4,8, ... MacOS and IOS have used this backoff schedule for over a decade, since before this 2009 IETF presentation discussed the behavior: https://www.ietf.org/proceedings/75/slides/tcpm-1.pdf This commit makes the SYN RTO schedule start with a number of linear backoffs given by the following sysctl: * tcp_syn_linear_timeouts This changes the SYN RTO scheme to be: init_rto_val for tcp_syn_linear_timeouts, exp backoff starting at init_rto_val For example if init_rto_val = 1 and tcp_syn_linear_timeouts = 2, our backoff scheme would be: 1, 1, 1, 2, 4, 8, 16, ... Signed-off-by: David Morley Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Tested-by: David Morley Reviewed-by: Eric Dumazet Link: https://lore.kernel.org/r/20230509180558.2541885-1-morleyd.kernel@gmail.com Signed-off-by: Paolo Abeni --- Documentation/networking/ip-sysctl.rst | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) (limited to 'Documentation/networking/ip-sysctl.rst') diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index 6ec06a33688a..3f6d3d5f5626 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -881,9 +881,10 @@ tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs tcp_syn_retries - INTEGER Number of times initial SYNs for an active TCP connection attempt will be retransmitted. Should not be higher than 127. Default value - is 6, which corresponds to 63seconds till the last retransmission - with the current initial RTO of 1second. With this the final timeout - for an active TCP connection attempt will happen after 127seconds. + is 6, which corresponds to 67seconds (with tcp_syn_linear_timeouts = 4) + till the last retransmission with the current initial RTO of 1second. + With this the final timeout for an active TCP connection attempt + will happen after 131seconds. tcp_timestamps - INTEGER Enable timestamps as defined in RFC1323. @@ -946,6 +947,16 @@ tcp_pacing_ca_ratio - INTEGER Default: 120 +tcp_syn_linear_timeouts - INTEGER + The number of times for an active TCP connection to retransmit SYNs with + a linear backoff timeout before defaulting to an exponential backoff + timeout. This has no effect on SYNACK at the passive TCP side. + + With an initial RTO of 1 and tcp_syn_linear_timeouts = 4 we would + expect SYN RTOs to be: 1, 1, 1, 1, 1, 2, 4, ... (4 linear timeouts, + and the first exponential backoff using 2^0 * initial_RTO). + Default: 4 + tcp_tso_win_divisor - INTEGER This allows control over what percentage of the congestion window can be consumed by a single TSO frame. -- cgit v1.2.3