[e2e] Stupid Question: Why are missing ACKs not considered as indicator for congestion?

Lloyd Wood L.Wood at surrey.ac.uk
Wed Jan 31 17:40:56 PST 2007

At Wednesday 31/01/2007 16:34 -0800, Lachlan Andrew wrote:
>Greetings Lloyd,
>On 31/01/07, Lloyd Wood <L.Wood at surrey.ac.uk> wrote:
>>It's possible for the sender to infer that an ack has been lost, based on subsequent receiver behaviour in sending a cumulative ack including packets received that the sender didn't get individual acks for.
>No, that was my point.  We can't distinguish between ACKs which are
>lost and those which are never sent in the first place.

Yes, we can. If a SACK block is present, it tells you which datagrams were and weren't received.

If a datagram was received, an ack was sent (modulo the delack mechanism), and the datagram will not be called out in the SACK block.

If the datagram wasn't received, this will be reflected in the SACK block.

>Also, having a unique identifier (like a timestamp) isn't the same as
>having sequence numbers which can say "We're (not) consecutive".  The
>latter can detect loss but the former can't.

If you have timestamps on every ack and packet, what's the difference?

>Lachlan Andrew  Dept of Computer Science, Caltech
>1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
>Phone: +1 (626) 395-8820    Fax: +1 (626) 568-3603

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