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| author | Yuchung Cheng <ycheng@google.com> | 2012-05-02 13:30:03 +0000 |
|---|---|---|
| committer | David S. Miller <davem@davemloft.net> | 2012-05-02 20:56:10 -0400 |
| commit | eed530b6c67624db3f2cf477bac7c4d005d8f7ba (patch) | |
| tree | c07096807ead2adb9d85e85d1a9cd1ada85755ac /include/linux | |
| parent | 1fbc340514fc3003514bd681b372e1f47ae6183f (diff) | |
| download | op-kernel-dev-eed530b6c67624db3f2cf477bac7c4d005d8f7ba.zip op-kernel-dev-eed530b6c67624db3f2cf477bac7c4d005d8f7ba.tar.gz | |
tcp: early retransmit
This patch implements RFC 5827 early retransmit (ER) for TCP.
It reduces DUPACK threshold (dupthresh) if outstanding packets are
less than 4 to recover losses by fast recovery instead of timeout.
While the algorithm is simple, small but frequent network reordering
makes this feature dangerous: the connection repeatedly enter
false recovery and degrade performance. Therefore we implement
a mitigation suggested in the appendix of the RFC that delays
entering fast recovery by a small interval, i.e., RTT/4. Currently
ER is conservative and is disabled for the rest of the connection
after the first reordering event. A large scale web server
experiment on the performance impact of ER is summarized in
section 6 of the paper "Proportional Rate Reduction for TCP”,
IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf
Note that Linux has a similar feature called THIN_DUPACK. The
differences are THIN_DUPACK do not mitigate reorderings and is only
used after slow start. Currently ER is disabled if THIN_DUPACK is
enabled. I would be happy to merge THIN_DUPACK feature with ER if
people think it's a good idea.
ER is enabled by sysctl_tcp_early_retrans:
0: Disables ER
1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4.
2: (Default) reduce dupthresh like mode 1. In addition, delay
entering fast recovery by RTT/4.
Note: mode 2 is implemented in the third part of this patch series.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/linux')
| -rw-r--r-- | include/linux/tcp.h | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 278af9e..7859b41 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -372,6 +372,7 @@ struct tcp_sock { repair : 1, unused : 1; u8 repair_queue; + u8 do_early_retrans:1;/* Enable RFC5827 early-retransmit */ /* RTT measurement */ u32 srtt; /* smoothed round trip time << 3 */ |
