• Damian Lukowski's avatar
    Revert Backoff [v3]: Calculate TCP's connection close threshold as a time value. · 6fa12c85
    Damian Lukowski authored
    RFC 1122 specifies two threshold values R1 and R2 for connection timeouts,
    which may represent a number of allowed retransmissions or a timeout value.
    Currently linux uses sysctl_tcp_retries{1,2} to specify the thresholds
    in number of allowed retransmissions.
    
    For any desired threshold R2 (by means of time) one can specify tcp_retries2
    (by means of number of retransmissions) such that TCP will not time out
    earlier than R2. This is the case, because the RTO schedule follows a fixed
    pattern, namely exponential backoff.
    
    However, the RTO behaviour is not predictable any more if RTO backoffs can be
    reverted, as it is the case in the draft
    "Make TCP more Robust to Long Connectivity Disruptions"
    (http://tools.ietf.org/html/draft-zimmermann-tcp-lcd).
    
    In the worst case TCP would time out a connection after 3.2 seconds, if the
    initial RTO equaled MIN_RTO and each backoff has been reverted.
    
    This patch introduces a function retransmits_timed_out(N),
    which calculates the timeout of a TCP connection, assuming an initial
    RTO of MIN_RTO and N unsuccessful, exponentially backed-off retransmissions.
    
    Whenever timeout decisions are made by comparing the retransmission counter
    to some value N, this function can be used, instead.
    
    The meaning of tcp_retries2 will be changed, as many more RTO retransmissions
    can occur than the value indicates. However, it yields a timeout which is
    similar to the one of an unpatched, exponentially backing off TCP in the same
    scenario. As no application could rely on an RTO greater than MIN_RTO, there
    should be no risk of a regression.
    Signed-off-by: default avatarDamian Lukowski <damian@tvk.rwth-aachen.de>
    Acked-by: default avatarIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    6fa12c85
tcp_timer.c 14.5 KB