[e2e] Are we doing sliding window in the Internet?

Lachlan Andrew lachlan.andrew at gmail.com
Tue Jan 2 21:15:27 PST 2007


This is probably not related to the original thread (on what happens
in real networks, as distinct from what *should* happen), but the word
"bug" bugged me...

On 02/01/07, Joe Touch <touch at isi.edu> wrote:

Fair enough.  I've just noticed that the default in 2.6.18 has been
changed to "off", possibly as a result of their experiments :)

> > delayed ACKs (as Linux receivers don't when the window is small),
> Delayed ACKs are strongly encouraged.
> Both good reasons to fix these bugs in Linux.

I don't follow the logic of that at all.  Linux deliberatly suppresses
delayed ACKs when it guesses that the sender is in slow start, which
sems generally correct, judging by the earlier posts in this thread.
In that phase, they harm performance, by making slow-start even slower
than it was intended to be.  Increasing the initial speed of slow
starts helps short flows at no long term cost to ongoing long flows.
When the window is large, Linux does use delayed ACKs, for the reasons
given in the RFCs.  Since this is fully standards compliant, I don't
see how it can be called a bug.

The fact that something is "encouraged" doesn't *of itself* seem a
good reason to do it, if there are clear reasons not to.  That isn't
to say that there may not indeed be good reasons to change Linux's
behaviour; I'd be interested to hear them.

(On a related note, this year's PFLDnet
<http://wil.cs.caltech.edu/pfldnet2007> has a panel session on the
implications of network stack implementors Linux and Microsoft setting
new de-facto flow control standards.  This seems analogous to what the
BSD Reno release did, implementing improvements well before Reno made
it into the RFCs.  The difference is that now a global infrastructure
rides on it...)


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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