Re: Is the diffServ preemptive ?

I was answering a question about the standards, since this is
the standards WG list. Implementation discussions belong at
http://www.tip.csiro.au/dsimplementation/
  
   Brian

Zyad Dwekat wrote:
> 
> In the question below , I think he is trying to say what if we need to service two types of PHB in same router i.e we have different classes of AF but we have a traffic that we need to give it delay gurantee by EF PHB .so the question how to coordinate the requirements of both PHBs
> 
>  > Geunhyung Kim wrote:
>  >
>  > Hello all,
>  >
>  > I have basic question about diffServ architecture.
>  > As far as I know, AF flow may use the excess bandwidth when there is underload, end the minimum bandwidth
>  should be guaranteed even though
>  > the link state is overloaded.
> 
>  But that is not a question about the diffserv architecture. The
>  diffserv architecture (RFC 2475) applies equally to all PHBs, not
>  just AF, and says nothing about guarantees or bandwidth.
> 
>  >
>  > Is that correct ?
> 
>  Assuming your question applies to AF, i.e. RFC 2597, AF does not define
>  an AF "flow". It defines a set of AF behaviors allowing the creation
>  of a set of AF traffic aggregates. If I assume your question applies to
>  an AF traffic class (i.e. all the data packets carrying the code
>  points {AFn1,AFn2,AFn3} for a given n), then the relevant part of the RFC is:
> 
>  >    A DS node MUST allocate a configurable, minimum amount of forwarding
>  >    resources (buffer space and bandwidth) to each implemented AF class.
>  >    Each class SHOULD be serviced in a manner to achieve the configured
>  >    service rate (bandwidth) over both small and large time scales.
>  >
>  >    An AF class MAY also be configurable to receive more forwarding
>  >    resources than the minimum when excess resources are available either
>  >    from other AF classes or from other PHB groups.  This memo does not
>  >    specify how the excess resources should be allocated, but
>  >    implementations MUST specify what algorithms are actually supported
>  >    and how they can be parameterized.
>  >
>  So, the answer to your first question is more subtle than "yes" or "no".
> 
>  > If it is correct, when the higher priority flow enters into network, the excee bandwidth that the lower
>  priority flow used may be preempted by the higher priority.
>  > Even in this case, the higher priority flow can not use the bandwidth that the lower priority flow
>  required minimally.
>  >
>  > Is that correct ?
> 
>  Neither diffserv, nor AF, defines "priority" among traffic classes.
>  Let's be very clear about that: there is no priority ordering between
>  AF1x, AF2x, AF3x, AF4x. They each have their assigned resources as
>  defined in the above extract.
> 
>     Brian Carpenter
> 
>  >
>  > Thanks in advance,
>  >
>  > Geunhyung
>  >
>  > None of us is as smart as all of us
>  > ==========================================
>  > Geunhyung Kim
>  >
>  > E-mail: [email protected]
>  >
>  > Tel: +82-54-279-5655
>  > Fax: +82-54-279-5699
>  >
>  > Networking & Distributed Systems Lab.
>  > CSE
>  > POSTECH
>  > ===========================================
> 
>  --__--__--
> 
> Zyad Dwekat
> **********************************************************************
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> North Carolina State University                                      *
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