Subject: [Diffserv] QUESTION on draft-ietf-diffserv-model-04.txt

From: Michele Fontana <[email-masked]>
Date: Thu, 7 Sep 2000 18:46:46 +0200

Hi,

I have a doubt about the draft draft-ietf-diffserv-model-04.txt, where
it is said that a generalised TCB might consist of the following stages:

- classification stage
- metering stage
- action stage
- algorithmic dropping stage
- queueing stage
- scheduling stage

As each router (e.g. each hop) has to decrement the TTL and verify the
header checksum of every packet, I was wondering where these "stages"
take place in the case of a Diff-Serv edge router.

Is it correct to assume that the header checksum have to be verified
BEFORE the meter stage? in this case the packets with corrupted header
will not be metered.

Is it correct to assume that the TTL decrement (and consequent dropping
action due to expired TTL) has to take place AFTER the meter-action
stage.

Is there any other suggestion?

Thanks.

Michele

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 07 Sep 2000 13:27:56 -0500

Michele,

If the rate of bad checksums is significant, you have a much worse 
problem than diffserv conformance. TTL expiry is always a pathological 
case, so again it is a much worse problem than diffserv conformance.

So I think this is entirely an implementation detail, left to the
discretion of the router implementor. 

   Brian

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Brian E Carpenter 
Program Director, Internet Standards & Technology, IBM 
On assignment for IBM at http://www.iCAIR.org 
Board Chairman, Internet Society http://www.isoc.org
Non-IBM email: [email-masked]