Brian E Carpenter writes:
> Michael Thomas wrote:
> >
> > Brian E Carpenter writes:
> > > Excellent summary. We originally chose a 6 bit diffserv field partly because
> > > it was available in both IPv4 and IPv6, and partly because it allows for
> > > very efficient classification in *core* routers, with the more demanding
> > > multi-field classification being left to border routers.
> > >
> > > The question before the house (in the end that means both ipng and diffserv)
> > > is whether the added complexity of adding the flow label to the diffserv
> > > model is justified by the gain in expressiveness. It doesn't do anything
> > > for the trust model.
> >
> > I haven't heard about any imminent shortage of DSCP's.
> > Indeed, it seems that there's only a small handful
> > (2-6) that I've ever heard people contemplating. Maybe
> > I just don't travel in the right circles...
>
> We have deliberately been *very* conservative about defining standard PHBs, since
> we want to be very sure about what we standardise. But there are a potentially
> infinite number of local-use PHBs. That is why the DSCP value is mappable, and why
> the PHB ID was defined, so that local-use PHBs can be registered with IANA and
> signalled. The idea here is to stretch the semantics of the PHB ID just a little.
Right. Even if the number of PHB's grows beyond
6-8 bits (which seems pretty unlikely to me), in
order to be a real problem you would have to posit
a network/domain which actually wanted to use more
than could be currently mapped into the DSCP. I've
been working on a lot of voip stuff in the last
few years and the number of DSCP's I've heard has
been in the range of about 2 to 5 or 6 that I
recall, with higher end of the range seeming
pretty extravagant.
> Normal semantics of a PHB ID:
>
> "This is local-use PHB number 379"
>
> Stretched semantics:
>
> "This is local-use PHB number 379 and according to our SLA, that gets classified
> as real-time traffic needing at least a 10 Mbit/s rate"
Ah. This strikes me as trying to overload the
semantics of DiffServ. Another way to go about
this is to use signaled diffserv (either the
COPS-PR or RSVP varieties) with policers at the
proper edges of the network. That way, the traffic
classes (ie the actual PHB) stays small.
Mike
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