Merge branch 'tcp-default-RACK-loss-recovery'

Yuchung Cheng says:

====================
tcp: default RACK loss recovery

This patch set implements the features correspond to the
draft-ietf-tcpm-rack-03 version of the RACK draft.
https://datatracker.ietf.org/meeting/101/materials/slides-101-tcpm-update-on-tcp-rack-00

1. SACK: implement equivalent DUPACK threshold heuristic in RACK to
   replace existing RFC6675 recovery (tcp_mark_head_lost).

2. Non-SACK: simplify RFC6582 NewReno implementation

3. RTO: apply RACK's time-based approach to avoid spuriouly
   marking very recently sent packets lost.

4. with (1)(2)(3), make RACK the exclusive fast recovery mechanism to
   mark losses based on time on S/ACK. Tail loss probe and F-RTO remain
   enabled by default as complementary mechanisms to send probes in
   CA_Open and CA_Loss states. The probes would solicit S/ACKs to trigger
   RACK time-based loss detection.

All Google web and internal servers have been running RACK-only mode
(4) for a while now. a/b experiments indicate RACK/TLP on average
reduces recovery latency by 10% compared to RFC6675. RFC6675
is default-off now but can be enabled by disabling RACK (sysctl
net.ipv4.tcp_recovery=0) for unseen issues.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>