From eed530b6c67624db3f2cf477bac7c4d005d8f7ba Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Wed, 2 May 2012 13:30:03 +0000 Subject: tcp: early retransmit MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- Documentation/networking/ip-sysctl.txt | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'Documentation/networking') 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 -- cgit v1.2.3