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

[Lloyd Wood]

At Wednesday 31/01/2007 20:23 +0000, Jon Crowcroft wrote:
>its clear we should devise a schmee for disguising data packets as acks

which is what piggybacking acks on data packets already does.

(ns one-way tcp doesn't simulate this. Try Fulltcp.)


>a they'd 
>1/ advance the  congestion window and so on
>2/ get highrer priority than data packets
>
>otoh, how do we do this - compression, perhaps? how well would VJ's compressed
>tcp./ip headers scale over multiple hops? intersting to thin kabout sratge
>recovery ( a la  nat state recovery) too...
>
>also, what would happen if this was typical behaviour? virtual circuit IP?
>MPLS on IP? who knows?


who cares?

>In missive <aa7d2c6d0701291418xf8f715eu447b669ae977160b at mail.gmail.com>, "Lachlan Andrew" 
>typed:
>
> >>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=B4t do so with missing ACKs.
> >>>
> >>> If a TCP packet is dropped, this is reckognized as congestion
> >>> indication. Shouldn=B4t 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
> >>congested.
> >>
> >>
> >>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.
> >>
> >>Cheers,
> >>Lachlan
> >>
> >>--=20
> >>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
> >>
>
> cheers
>
>   jon