packet reordering at exchange points

Tue Apr 9 18:00:31 UTC 2002

> Date: Tue, 9 Apr 2002 07:13:38 +0200
> From: Jesper Skriver <jesper at skriver.dk>

> We're talking parallel GigE links between switches which are located
> close to each other.
> 
> And we're talking real life applications, which perhaps sends 100 pps
> in one stream, which means that you need to have ~ 10 ms different
> transmission delay on the individual links, before the risk of out of
> order packets for a given stream arise.

Hmmmm.  You're right.  I lost sight of the original thread...
GigE inter-switch trunking at PAIX.  In that case, congestion
_should_ be low, and there shouldn't be much queue depth.

But this _does_ bank on current "real world" behavior.  If
endpoints ever approach GigE speeds (of course requiring "low
enough" latency and "big enough" windows)...

Then again, last mile is so slow that we're probably a ways away
from that happening.

> > IIRC, 802.3ad DOES NOT allow round robin distribution;
> 
> That is not what we're talking about, we're talking about the impact of
> doing it.

Yes, I was incomplete in that part.  Intended point was that IEEE
at least [seemingly] found round robin inappropriate for general
case.

> > it uses hashes.  Sure, hashed distribution isn't perfect.
> 
> It's broken in a IX environment where you have few src/dst pairs, and
> where a single src/dst pair can easily use several hundreds of Mbps,
> if you have a few of those going of the same link due to the hashing
> algorithm, you will have problems.

In the [extreme] degenerate case, yes, one goes from N links to 1
effective link.

> A large IX in Europe have this exact problem on their Foundry swiches,
> which doesn't support round robin, and is currently forced to moving for

Can you state how many participants?  With N x GigE, what sort of
[im]balance is there over the N lines?

Of course, I'd hope that individual heavy pairs would establish
private interconnects instead of using public switch fabric, but
I know that's not always { an option | done | ... }.

> 10 GigE due to this very fact.

I'm going to have to play with ISL RR...

> /Jesper

--
Eddy

Brotsman & Dreger, Inc. - EverQuick Internet Division
Phone: +1 (316) 794-8922 Wichita/(Inter)national
Phone: +1 (785) 865-5885 Lawrence

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