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    Proportional Rate Reduction for TCP. · a262f0cd
    Nandita Dukkipati 提交于
    This patch implements Proportional Rate Reduction (PRR) for TCP.
    PRR is an algorithm that determines TCP's sending rate in fast
    recovery. PRR avoids excessive window reductions and aims for
    the actual congestion window size at the end of recovery to be as
    close as possible to the window determined by the congestion control
    algorithm. PRR also improves accuracy of the amount of data sent
    during loss recovery.
    
    The patch implements the recommended flavor of PRR called PRR-SSRB
    (Proportional rate reduction with slow start reduction bound) and
    replaces the existing rate halving algorithm. PRR improves upon the
    existing Linux fast recovery under a number of conditions including:
      1) burst losses where the losses implicitly reduce the amount of
    outstanding data (pipe) below the ssthresh value selected by the
    congestion control algorithm and,
      2) losses near the end of short flows where application runs out of
    data to send.
    
    As an example, with the existing rate halving implementation a single
    loss event can cause a connection carrying short Web transactions to
    go into the slow start mode after the recovery. This is because during
    recovery Linux pulls the congestion window down to packets_in_flight+1
    on every ACK. A short Web response often runs out of new data to send
    and its pipe reduces to zero by the end of recovery when all its packets
    are drained from the network. Subsequent HTTP responses using the same
    connection will have to slow start to raise cwnd to ssthresh. PRR on
    the other hand aims for the cwnd to be as close as possible to ssthresh
    by the end of recovery.
    
    A description of PRR and a discussion of its performance can be found at
    the following links:
    - IETF Draft:
        http://tools.ietf.org/html/draft-mathis-tcpm-proportional-rate-reduction-01
    - IETF Slides:
        http://www.ietf.org/proceedings/80/slides/tcpm-6.pdf
        http://tools.ietf.org/agenda/81/slides/tcpm-2.pdf
    - Paper to appear in Internet Measurements Conference (IMC) 2011:
        Improving TCP Loss Recovery
        Nandita Dukkipati, Matt Mathis, Yuchung Cheng
    Signed-off-by: NNandita Dukkipati <nanditad@google.com>
    Signed-off-by: NDavid S. Miller <davem@davemloft.net>
    a262f0cd
tcp.h 14.5 KB