• Yuchung Cheng's avatar
    tcp: early retransmit · eed530b6
    Yuchung Cheng authored
    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: default avatarYuchung Cheng <ycheng@google.com>
    Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    eed530b6
tcp.h 48.2 KB