[46773] in North American Network Operators' Group
Re: packet reordering at exchange points
daemon@ATHENA.MIT.EDU (Stephen Sprunk)
Wed Apr 10 12:49:03 2002
Message-ID: <01b701c1e0af$6d2bcc10$e1876540@amer.cisco.com>
From: "Stephen Sprunk" <ssprunk@cisco.com>
To: "Peter Galbavy" <peter.galbavy@knowtion.net>,
"Richard A Steenbergen" <ras@e-gerbil.net>,
"E.B. Dreger" <eddy+public+spam@noc.everquick.net>,
"Mathew Lodge" <mathew@cplane.com>
Cc: "Paul Vixie" <paul@vix.com>, <nanog@merit.edu>
Date: Wed, 10 Apr 2002 11:44:11 -0500
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Thus spake "Mathew Lodge" <mathew@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
