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tcp: remove obsolete and unused RFC3517/RFC6675 loss recovery code

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RACK-TLP loss detection has been enabled as the default loss detection
algorithm for Linux TCP since 2018, in:

 commit b38a51fec1 ("tcp: disable RFC6675 loss detection")

In case users ran into unexpected bugs or performance regressions,
that commit allowed Linux system administrators to revert to using
RFC3517/RFC6675 loss recovery by setting net.ipv4.tcp_recovery to 0.

In the seven years since 2018, our team has not heard reports of
anyone reverting Linux TCP to use RFC3517/RFC6675 loss recovery, and
we can't find any record in web searches of such a revert.

RACK-TLP was published as a standards-track RFC, RFC8985, in February
2021.

Several other major TCP implementations have default-enabled RACK-TLP
at this point as well.

RACK-TLP offers several significant performance advantages over
RFC3517/RFC6675 loss recovery, including much better performance in
the common cases of tail drops, lost retransmissions, and reordering.

It is now time to remove the obsolete and unused RFC3517/RFC6675 loss
recovery code. This will allow a substantial simplification of the
Linux TCP code base, and removes 12 bytes of state in every tcp_sock
for 64-bit machines (8 bytes on 32-bit machines).

To arrange the commits in reasonable sizes, this patch series is split
into 3 commits. The following 2 commits remove bookkeeping state and
code that is no longer needed after this removal of RFC3517/RFC6675
loss recovery.

Suggested-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250615001435.2390793-2-ncardwell.sw@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Neal Cardwell
2025-06-14 20:14:33 -04:00
committed by Jakub Kicinski
parent 90b4e1cf6d
commit 1c120191dc
2 changed files with 15 additions and 130 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
Documentation/networking/ip-sysctl.rst
View File

@@ -645,9 +645,11 @@ tcp_recovery - INTEGER
features.
========= =============================================================
RACK: 0x1 enables the RACK loss detection for fast detection of lost
retransmissions and tail drops. It also subsumes and disables
RFC6675 recovery for SACK connections.
RACK: 0x1 enables RACK loss detection, for fast detection of lost
retransmissions and tail drops, and resilience to
reordering. currently, setting this bit to 0 has no
effect, since RACK is the only supported loss detection
algorithm.
RACK: 0x2 makes RACK's reordering window static (min_rtt/4).