path: root/Documentation
diff options
authorYuchung Cheng <ycheng@google.com>2012-05-02 13:30:03 +0000
committerDavid S. Miller <davem@davemloft.net>2012-05-02 20:56:10 -0400
commiteed530b6c67624db3f2cf477bac7c4d005d8f7ba (patch)
treec07096807ead2adb9d85e85d1a9cd1ada85755ac /Documentation
parent1fbc340514fc3003514bd681b372e1f47ae6183f (diff)
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')
1 files changed, 14 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 9b569a2d9c6..34916e792d9 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