The MPLS WG Archive

[Jim Boyle <jboyle@Level3.net>]

That is the difference between diffserv and intserv, I believe.  Diffserv
doesn't aim to give hard guarentees to individual flows.  Diffserv, and
"E-LSPs", work.

Jim


On Wed, 24 Jan 2001, David Charlap wrote:

> Bora Akyol wrote:
> > 
> > >>>>> "David" == David Charlap <david.charlap@marconi.com> writes:
> > 
> >     David> If you need actual QoS guarantees, or if your
> >     David> switches only support L-LPSs, or if you want the two
> >     David> service levels to be routed independantly, then you
> >     David> need to create a separate LSP for each service level.
> >     David> CR-LDP or RSVP-TE can be used for signalling the QoS
> >     David> requirements for the two LSPs.
> > 
> > David
> > 
> > You are making assumptions about a particular switch
> > architecture when you assert that you can not do "actual" QoS
> > guarantees with E-LSPs. If you have a policer and a
> > shaper/scheduler that can process the EXP bits as well as the
> > label, then doing "actual" QoS is quite possible with
> > E-LSPs.
> > 
> > Not only that, but you can map traffic destined to the same
> > prefix to different EXP levels based either on Diffserv bits or
> > on packet content. This also applies to VLAN tags.
> 
> The meaning of the EXP bits is not signalled.  It must be preconfigured
> network-wide.  And in order to be useful, they must be identical across
> the entire network.  Which limits you to 8 different service levels,
> network-wide.
> 
> If you use the EXP bits to define a priority level and two flows are
> both assigned that level, they will be queued in best-effort fashion
> relative to each other.  There is no way to guarantee the individual
> flows any particular level of service.  You can only guarantee the
> aggregate of one class relative to another class.
> 
> Using L-LSPs, you can request a separate label, and a separate resource
> reservation for every flow, if needed.  This is the only way to
> guarantee this level of service.  (Unless you're willing to limit each
> DS code point to a single flow, which is a ridiculous concept.)
> 
> E-LSPs are useful, no doubt about that.  But they can not provide the
> level of QoS necessary to make sure that a single specific flow can be
> guaranteed the resources it requires.
> 
> -- David
>