The MPLS WG Archive

[Eric Gray <eric.gray@sandburst.com>]

David,

    I'm sure some smart person or another has written a paper
on this, but I don't have the time to do a full-blown literature
search right now.  While what you are saying is technically
true, I believe it has very little applicability in practice.
You can see that this works by imagining that an implementation
might treat MTU propagation time exactly the same way that ATM
treats cell propagation time.

    But this is very ugly.  It is so coarse that many would be
hard pressed to consider it a useful guarantee.  It also seems
to require deliberately inserting latency (N times an MTU
propagation time) for all affected traffic (to allow for packet
shuffling to accomodate the guarantee).  And it would be
very difficult to allocate forwarding resources efficiently and
still deliver guaranteed delay variation deterministically.

    Without making any moral judgement about why it is so,
many people are more concerned about having a bounded
delay than they are about having a bounded delay variation.
And with a bounded delay, you get bounded delay variation
for free.

    My opinion: there are two models here and trying to make
technologies support both models may not be a good idea.

--
Eric Gray

You wrote:

> Eric Gray wrote:
> >
> >     You can't exactly do ATM style QoS over non-ATM media.
> >
> >     The thing that allows an ATM switch to contemplate delay
> > variation separately from delay is the fact that ATM switches
> > switch data in fixed size chunks.  When the size of data chunks
> > can vary, the only practical way to control delay variation is to
> > try to control delay.  Therefore, with IP packets on non-ATM
> > media, you can't even begin support two knobs for delay and
> > delay variation.  So why would you undertake to add complexity
> > by trying?
>
> Variable-size frames don't make it impossible.  It just means you are
> forced to work with a coarser granularity (defined by your configured
> MTU size.)
>
> -- david