North American Network Operators Group

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Re: packet reordering at exchange points

  • From: Jesper Skriver
  • Date: Tue Apr 09 01:15:43 2002

On Mon, Apr 08, 2002 at 11:19:56PM +0000, E.B. Dreger wrote:
> 
> > Date: Tue, 9 Apr 2002 00:32:50 +0200 (CEST)
> > From: Iljitsch van Beijnum <[email protected]>
> 
> > But how is packet reordering on two parallell gigabit interfaces
> > ever going to translate into reordered packets for individual
> > streams? Packets
>
> Queue depths.  Varying paths.

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.

> 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.

> 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.

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
10 GigE due to this very fact.

/Jesper

-- 
Jesper Skriver, jesper(at)skriver(dot)dk  -  CCIE #5456
Work:    Network manager   @ AS3292 (Tele Danmark DataNetworks)
Private: FreeBSD committer @ AS2109 (A much smaller network ;-)

One Unix to rule them all, One Resolver to find them,
One IP to bring them all and in the zone to bind them.