RFC tcp: timer restart for tail loss

This is a request for comments.

RTO and TLP restart is a modification to the restart process of the RTO and TLP timers in TCP. Currently, both timers are restarted with its corresponding timeout value when an acknowledgment (ACK) for correctly received data is received. In many situations, resetting the timers on incoming ACKs will add an implicit offset of at least RTT seconds to the loss recovery process.

The goal of the modified restart is to provide quicker loss recovery for segments lost in the end of a burst/connection, where the limited feedback from a receiver inhibits the use of fast/early retransmit. To accomplish this the algorithm adjusts the RTO and PTO (TLP's timer) values on each rearm of the timers to allow them to expire after exactly RTO and PTO ms, respectively.

The restart behavior is controlled by the new tcp_timer_restart sysctl. tcp_timer_restart==0; disables RTOR and TLPR.

		 ==1; enables RTOR.
		 ==2; enables TLPR.
		 ==3; enables both RTOR and TLPR. [DEFAULT]

The new restart behavior has been approved by the IETF for publication as an experimental RFC (for the RTO part of the mechanism) [1], and experiments have been conducted to show both the benefit of this strategy (in terms of reduced loss-recovery delays) and the limited negative impact (in terms of spurious retransmissions) [2]. More information regarding the mechanism can be found at [3].

Basic functionality tests, using packetdrill, are available at:
https://github.com/perhurt/packetdrill/tree/master/gtests/net/packetdrill/tests/linux/timer_restart

[1] https://datatracker.ietf.org/doc/draft-ietf-tcpm-rtorestart/
[2] http://www.sigcomm.org/sites/default/files/ccr/papers/2015/January/0000000-0000000.pdf
[3] http://riteproject.eu/resources/rto-restart/

Per Hurtig (2):
tcp: RTO Restart (RTOR)
tcp: TLP restart (TLPR)

 Documentation/networking/ip-sysctl.txt | 12 ++++++++++++
 include/net/tcp.h                      |  8 +++++++-
 net/ipv4/sysctl_net_ipv4.c             | 10 ++++++++++
 net/ipv4/tcp_input.c                   | 31 ++++++++++++++++++++++++++++++-
 net/ipv4/tcp_output.c                  | 12 ++++++++++--
 5 files changed, 69 insertions(+), 4 deletions(-)

1.9.1