• Priyaranjan Jha's avatar
    tcp: higher throughput under reordering with adaptive RACK reordering wnd · 1f255691
    Priyaranjan Jha authored
    Currently TCP RACK loss detection does not work well if packets are
    being reordered beyond its static reordering window (min_rtt/4).Under
    such reordering it may falsely trigger loss recoveries and reduce TCP
    throughput significantly.
    
    This patch improves that by increasing and reducing the reordering
    window based on DSACK, which is now supported in major TCP implementations.
    It makes RACK's reo_wnd adaptive based on DSACK and no. of recoveries.
    
    - If DSACK is received, increment reo_wnd by min_rtt/4 (upper bounded
      by srtt), since there is possibility that spurious retransmission was
      due to reordering delay longer than reo_wnd.
    
    - Persist the current reo_wnd value for TCP_RACK_RECOVERY_THRESH (16)
      no. of successful recoveries (accounts for full DSACK-based loss
      recovery undo). After that, reset it to default (min_rtt/4).
    
    - At max, reo_wnd is incremented only once per rtt. So that the new
      DSACK on which we are reacting, is due to the spurious retx (approx)
      after the reo_wnd has been updated last time.
    
    - reo_wnd is tracked in terms of steps (of min_rtt/4), rather than
      absolute value to account for change in rtt.
    
    In our internal testing, we observed significant increase in throughput,
    in scenarios where reordering exceeds min_rtt/4 (previous static value).
    Signed-off-by: default avatarPriyaranjan Jha <priyarjha@google.com>
    Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
    Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    1f255691
tcp_minisocks.c 26.6 KB