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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 /Documentation/networking | |
| parent | 1fbc340514fc3003514bd681b372e1f47ae6183f (diff) | |
| download | linux-eed530b6c67624db3f2cf477bac7c4d005d8f7ba.tar.bz2 | |
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 'Documentation/networking')
| -rw-r--r-- | Documentation/networking/ip-sysctl.txt | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt index 9b569a2d9c60..34916e792d9d 100644 --- a/Documentation/networking/ip-sysctl.txt +++ b/Documentation/networking/ip-sysctl.txt @@ -190,6 +190,20 @@ tcp_cookie_size - INTEGER tcp_dsack - BOOLEAN Allows TCP to send "duplicate" SACKs. +tcp_early_retrans - INTEGER + Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold + for triggering fast retransmit when the amount of outstanding data is + small and when no previously unsent data can be transmitted (such + that limited transmit could be used). + Possible values: + 0 disables ER + 1 enables ER + 2 enables ER but delays fast recovery and fast retransmit + by a fourth of RTT. This mitigates connection falsely + recovers when network has a small degree of reordering + (less than 3 packets). + Default: 2 + tcp_ecn - INTEGER Enable Explicit Congestion Notification (ECN) in TCP. ECN is only used when both ends of the TCP flow support it. It is useful to |