[Bloat] tcp loss probes in linux 3.10

[Dave Taht]

While this appears to make a great deal of sense

http://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01

and just landed in

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6ba8a3b19e764b6a65e4030ab0999be50c291e6c

I was intrigued by several of the pieces of data that drive this stuff

Measurements on Google Web servers show that approximately 70% of
   retransmissions for Web transfers are sent after the RTO timer
   expires, while only 30% are handled by fast recovery.  Even on
   servers exclusively serving YouTube videos, RTO based retransmissions,

96% of the timeout episodes occur without any preceding duplicate ACKs or
other indication of losses at the sender




And especially this, in the context of a post-delay-aware-aqm world.



The key takeaway (with TLP) is: the
   average response time improved up to 7% and the 99th percentile
   improved by 10%.  Nearly all of the improvement for TLP is in the
   tail latency (post-90th percentile).  The varied improvements across
   services are due to different response-size distributions and traffic
   patterns.  For example, TLP helps the most for Images, as these are
   served by multiple concurrently active TCP connections which increase
   the chances of tail segment losses.


Application        Average   99%

   Google Web Search  -3%       -5%

   Google Maps        -5%       -10%

   Google Images      -7%       -10%


   TLP also improved performance in mobile networks -- by 7.2% for Web
   search and Instant and 7.6% for Images transferred over Verizon
   network.  To see why and where the latency improvements are coming
   from, we measured the retransmission statistics.  We broke down the
   retransmission stats based on nature of retransmission -- timeout
   retransmission or fast recovery.  TLP reduced the number of timeouts
   by 15% compared to the baseline, i.e. (timeouts_tlp -
   timeouts_baseline) / timeouts_baseline = 15%.  Instead, these losses
   were either recovered via fast recovery or by the loss probe
   retransmission itself.  The largest reduction in timeouts is when the
   sender is in the Open state in which it receives only insequence ACKs
   and no duplicate ACKs, likely because of tail losses.
   Correspondingly, the retransmissions occurring in the slow start
   phase after RTO reduced by 46% relative to baseline.  Note that it is
   not always possible for TLP to convert 100% of the timeouts into fast
   recovery episodes because a probe itself may be lost.  Also notable
   in our experiments is a significant decrease in the number of
   spurious timeouts -- the experiment had 61% fewer congestion window
   undo events.  The Linux TCP sender uses either DSACK or timestamps to
   determine if retransmissions are spurious and employs techniques for
   undoing congestion window reductions.  We also note that the total
   number of retransmissions decreased 7% with TLP because of the
   decrease in spurious retransmissions, and because the TLP probe
   itself plugs a hole.

-- 
Dave Täht

Fixing bufferbloat with cerowrt:
http://www.teklibre.com/cerowrt/subscribe.html
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