packet reordering at exchange points

Wed Apr 10 16:44:11 UTC 2002

Thus spake "Mathew Lodge" <mathew at cplane.com>
> At 03:48 PM 4/10/2002 +0100, Peter Galbavy wrote:
> >Why ?
> >
> >I am still waiting (after many years) for anyone to explain to me
> >the issue of buffering. It appears to be completely unneccesary
> >in a router.
>
> Well, that's some challenge but I'll have a go :-/
>
> As far as I can tell, the use of buffering has to do with traffic
> shaping vs. rate limiting. If you have a buffer on the interface,
> you are doing traffic shaping -- whether or not your vendor calls
> it that. ... If you have no queue or a very small queue ... This is
> rate limiting.

Well, that's implicit shaping/policing if you wish to call it that.  It's
only common to use those terms with explicit shaping/policing, i.e. when you
need to shape/police at something other than line rate.

> except for the owner of the routers who wanted to know why
> they had to buy the more expensive ATM card  (i.e. why
> couldn't the ATM core people couldn't put more buffering on
> their ATM access ports).

The answer here lies in ATM switches being designed primarily for carriers
(and by people with a carrier mindset).  Carriers, by and large, do not want
to carry unfunded traffic across their networks and then be forced to buffer
it; it's much easier (and cheaper) to police at ingress and buffer nothing.

It would have been nice to see a parallel line of switches (or cards) with
more buffers.  However, anyone wise enough to buy those was wise enough to
ditch ATM altogether :)

S