Selective Acks. (SACK and RWIN)

Discussion in 'Cisco Small Business Routers and VPN Solutions' started by Toxic, Apr 28, 2006.

  1. Toxic

    Toxic Administrator Staff Member

    Ok we have found that selective Acks are disabled in some firmwares of the rv0xx models.

    I have seen somewhere in a TCP IT book a mention of SACKS and RWIN are linked. so searching the Net I came up with a compromise that SHOULD work. Please test this process and report back

    Q: Can you set RWIN too high?
    A: YES!

    Here is the issue:

    * Large RWIN values (e.g. 150K) can results in 100+ outstanding TCP segments
    * *Culmultive* ACKs used by default in many TCP stacks
    * Drop just one segment (e.g. #50 of 100) and 50-100 have to be retransmitted
    * Even modest error rates (<1%) result in using 33% of bandwidth for retransmissions
    * *Selective* ACKs (SACKs) minimize this issue, but they may not be enabled

    Conclusion:

    * Determine if you have SACK enabled
    * If yes, then you can be aggressive (RWIN=64KB+)
    * If not, evaluate your error rate
    * If low (<.01%) then you can still be aggressive (RWIN=64KB+)
    * If high (>.01%) then you should be conservative (RWIN=8-16KB)

    Note: RWIN values >64KB are seldom necessary, so don't set it higher unless your bandwidth*delay warrants.

    more info on RWIN is here: http://www.dslreports.com/tweaks/RWIN
     
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