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

Lachlan Andrew lachlan.andrew at gmail.com
Mon Jan 29 14:18:09 PST 2007

Greetings Detlef,

On 29/01/07, Detlef Bosau <detlef.bosau at web.de> wrote:
> In TCP, lost / dropped packets are recognised as congestion indicator.
> We don´t do so with missing ACKs.
> If a TCP packet is dropped, this is reckognized as congestion
> indication. Shouldn´t be a dropped ACK packet seen as congestion
> indication as well?

Because ACKs are cumulative, we don't know that separate ACKs were
sent for each packet.

For example, high-end NICs typically have "interrupt coalescence",
which delivers a large bunch of packets simultaneously to reduce CPU
overhead.  A single "fat ACK" is sent which cumulatively acknowledges
all of these packets.  This happens even when the receiver is not

Another factor is that ACKs are typically small compared with data
packets.  The total network throughput is much greater if we throttle
only the sources contributing most to a given link's congestion,
namely those sending full data packets over the link.


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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