Subject: [Tsvwg] sack I-D revisited

From: Mark Allman <[email-masked]>
Date: Sat, 28 Jul 2001 07:35:03 -0400

Folks-

Ethan and I have revised the SACK I-D we have been working on based
on discussions with Kevin Fall and Jamshid Mahdavi.  The current rev
is -07.  The bottom line is that we ripped out the "threshold
retransmission" mechanism that we inserted when it was suggested
(and discussed) by Jamshid.  We remove it for two basic reasons:

  * It is more expensive.  Not only does the sender's TCP
    implementation have to track the blocks of the sequence space
    that have been SACKed, it has to track the un-ACKed/SACKed
    "holes", as well (and update their state fairly often).

  * The mechanism doesn't seem to buy you much protection against
    reordering when using fast recovery style transmission.  In
    other words, we watch half a window of ACKs (/SACKs) go by
    before retransmitting.  So, the likelihood of retransmitting
    before getting the required 3 "over SACKs" is fairly low to
    start with.  So, it is not clear that threshold retransmission
    is adding a whole lot here.  (Although, note that it *is* adding
    an explicit test at run-time to make sure our assumption holds.)

We'd love to hear comments either way on this change!

Thanks,
allman

---
Mark Allman -- NASA GRC/BBN -- http://roland.grc.nasa.gov/~mallman/

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