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    dccp ccid-2: Replace broken RTT estimator with better algorithm · 1435562d
    Gerrit Renker 提交于
    The current CCID-2 RTT estimator code is in parts broken and lags behind the
    suggestions in RFC2988 of using scaled variants for SRTT/RTTVAR. 
    That code is replaced by the present patch, which reuses the Linux TCP RTT
    estimator code - reasons for this code duplication are given below.
    
    Further details:
    ----------------
     1. The minimum RTO of previously one second has been replaced with TCP's, since
        RFC4341, sec. 5 says that the minimum of 1 sec. (suggested in RFC2988, 2.4)
        is not necessary. Instead, the TCP_RTO_MIN is used, which agrees with DCCP's
        concept of a default RTT (RFC 4340, 3.4). 
     2. The maximum RTO has been set to DCCP_RTO_MAX (64 sec), which agrees with 
        RFC2988, (2.5). 
     3. De-inlined the function ccid2_new_ack().
     4. Added a FIXME: the RTT is sampled several times per Ack Vector, which will
        give the wrong estimate. It should be replaced with one sample per Ack.
        However, at the moment this can not be resolved easily, since     
        - it depends on TX history code (which also needs some work),
        - the cleanest solution is not to use the `sent' time at all (saves 4 bytes
          per entry) and use DCCP timestamps / elapsed time to estimated the RTT,
          which however is non-trivial to get right (but needs to be done).
    
    Reasons for reusing the Linux TCP estimator algorithm:   
    ------------------------------------------------------
    Some time was spent to find a better alternative, using basic RFC2988 as a first
    step. Further analysis and experimentation showed that the Linux TCP RTO
    estimator is superior to a basic RFC2988 implementation. A summary is on
    http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid2/rto_estimator/
    
    In addition, this estimator fared well in a recent empirical evaluation:
    
        Rewaskar, Sushant, Jasleen Kaur and F. Donelson Smith.
        A Performance Study of Loss Detection/Recovery in Real-world TCP
        Implementations. Proceedings of 15th IEEE International
        Conference on Network Protocols (ICNP-07). 2007.
    
    Thus there is significant benefit in reusing the existing TCP code.
    Signed-off-by: NGerrit Renker <gerrit@erg.abdn.ac.uk>
    1435562d
ccid2.c 19.7 KB