Subject: [Tsvwg] ACK Heuristic

From: "Tze-Ven Poh" <[email-masked]>
Date: Sun, 17 Jul 2005 21:30:49 +0100

I would like some confirmation as to whether the ACK Heuristic as described in
RFC 3782 is correct:

1*)  If the Cumulative Acknowledgement field didn't cover more than
        "recover", check to see if the congestion window is greater than
        SMSS bytes and the difference between highest_ack and
        prev_highest_ack is at most 4*SMSS bytes.  If true, duplicate
        ACKs indicate a lost segment (proceed to Step 1A in Section 3).
        Otherwise, duplicate ACKs likely result from unnecessary
        retransmissions (proceed to Step 1B in Section 3).

According to the paper -- "Resolving Acknowledgment Ambiguity in non-SACK TCP",
the difference between highest_ack and prev_highest_ack must be at most 3*SMSS
bytes to invoke fast retransmit/fast recovery. In my calculation, value 3 should
be the correct one.

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