The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

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