Subject: Re: [Fwd: L2TP Diffserv Draft]

From: Danny McPherson <[email-masked]>
Date: Fri, 15 Sep 2000 16:48:44 -0600

> I sent this out several weeks ago but got no response. This time can
> anyone respond (not just the authors :-).

I just picked it up from Ken & Pat so I'll give it a shot.

> > 1. Is the DSCP AVP a six or eight bit value?

The format as currently specified is as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|1|0|0|        Length         |              43               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                1              |        DSCP  Value            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This means that the total length is 8 octets, with 2 octets representing the 
DSCP value.  The value will remain at least one octet, as two octets seem to 
enable little more than tidy packets encoding pictures.

> > 2. What DSCP value is used for tunnel control messages? If the
> >    session DSCP has a 'higher priority' than the tunnels then
> >    payload packets may arrive before session setup completion.

The next version of the draft, which is hopefully no more than a couple weeks 
out, will specify the behavior in this scenario.

> > 3. Why is this AVP restricted to just sessions? One could conceive
> >    bringing up a tunnel for each PHB and forwarding all tunnel
> >    traffic (control and payload) with one of the PHB class DSCPs.

Agreed.  And this is something that we were just discussing.  I'll ensure that 
the upcoming draft provides provisions for this as well (comments welcome!).

> > 4. This draft makes a big assumption that the tunnel traverses a
> >    single DS domain. If the tunnel traverses multiple DS domains
> >    the DSCP to PHB mappings could be different (this may mean
> >    that each tunnel endpoint uses different DSCP marks that get
> >    translated at some DS boundary). How is this resolved?

Multiple DS domains is of course is a very well-know and interesting problem, 
and again, the next version of the draft will take a stab at addressing this 
as well.  We plan to do our best to be in alignment with similar work being 
done in the diffserv WG.  (Again, comments welcome!).

> > 5. Related to 4 it would seem that it may be more appropriate that
> >    the tunnel initiator ask for a PHB rather than a DSCP and then
> >    let each endpoint decide how that PHB maps to a DSCP.

Though you still have lots of problems with this model as well when traverses 
multiple DS domains.  Again, we'll take a stab at it and work with the DS 
folks to ensure that we're aligned.  Comments here are welcome!

Thanks for the input, hope to have something useful within a couple of weeks 
at most.

-danny