Re: [Diffserv-interest] QoS in diffserv network

Brian E Carpenter <> Sun, 09 November 2003 18:29 UTC

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Date: Sun, 09 Nov 2003 19:26:48 +0100
From: Brian E Carpenter <>
Organization: IBM
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To: Feng Y <>
Subject: Re: [Diffserv-interest] QoS in diffserv network
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Feng Y wrote:
> >>
> >> It is up to the carrier to monitor congestion
> periodically to prevent
> >> lowest-priority traffic from being too congested. EF
> should ALWAYS get
> >> through on-time, and AF CIR should always get through,
> if somewhat delayed
> >> and jittery. The cases when this is NOT true should be
> rare and bizarre
> >> (earthquakes, major network outages, etc.) unless the
> carrier is not doing
> >> their job... Which is simply to make sure they engineer
> total network
> >> bandwidth above the level of peak CIR.
> >
> >Exactly. Each traffic class (i.e. each queue) needs to be
> provisioned for
> >the expected load with appropriate over- or
> under-provisioning. The new
> >thing is that you can over-provision for some traffic and
> under-provision
> >for other traffic. (New to IP, that is. It's hardly a new
> concept.)
> >
> Can we always assume that a higher service class provides
> the same or a better service than any lower service class
> in AF? 

No. There is no "higher" or "lower" built into AF. There
is simply the option of having one or more mutually
independent AF service classes, with an arbitrary number
of four service classes being named AF1, AF2, AF3, AF4
if you choose to use the recommended code points.

If you happen to give each of these classes equal weights
in a WFQ scheme they are all offering the same level of service.

If you happen to give AF2 a higher weight than AF3, you are
choosing to offer more service to AF2 traffic.

> Can the following scenario happen in Diffserv
> network?
> The gold service was allocated x percent of a link’s
> bandwidth and silver service was allocated x/2 percent of
> the link’s bandwidth, but the traffic intensity of gold
> packets was 10 times higher than that of silver packets.

That can certainly happen, if you mean that the offered
load to the gold service is ten times greater than the
offered load to the silver service.

> Should the ISP assure that there are not so many gold
> packets in service?

It depends what the SLA for gold and silver say, doesn't
it? There is no rule. It's the ISP's business decision.

My assumption and what I would recommend to an ISP is:

If the offered load on silver equals the bandwidth assigned
to silver, then it should all be transmitted, unless the SLA says
otherwise. And the excess traffic offered to the gold service 
will be at high risk of being dropped, depending on the utilization
of the remaining (100-x-x/2)% of the line.


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