Acking out-of-order packets?

Phil Karn (
Wed, 2 Mar 88 18:58:20 PST

During conversations with Van Jacobsen here at the IETF meeting, I discovered
an abiguity in the TCP spec (RFC-793 at least), to wit:

What response (if any) should you make when you receive an out-of-order
segment? RFC-793 says pretty clearly that you should return an empty ACK
(indicating the sequence number you expect to receive next) if the packet
is "unacceptable" (i.e., not in the window) but it doesn't say whether
you should respond if the packet is in the window but not the next one
expected. It says only that they should be kept for future processing.

My TCP in fact makes no response to an out-of-order data segment. Others
apparently respond with "do-nothing" ACKs. One of Van's performance tricks
depends on that behavior. Any guidance on what is the Right Thing here?


Phil Karn

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