Re: Issues in draft-ietf-tewg-diff-te-reqts-02.txt

[Jim Boyle <jboyle@pdnets.com>]

My $.02 inline.

regards,

Jim


On Tue, 27 Nov 2001, Lai, Wai S (Waisum), ALSVC wrote:

> During the preparation of DS-TE Requirements
> <draft-ietf-tewg-diff-te-reqts-02.txt>, some issues needed further
> discussion and hence were not incorporated into the text.  To facilitate
> more discussion, they are listed below.
>
> Please comment on the issues.  If there are no objections raised, we suggest
> that the changes be incorporated into the next update of the above I-D.
>
> Thanks, Wai Sum and Jerry.
>
> Issue 1: CT definition
> ----------------------
> In Section 3.2, Class-Type (CT) is defined as
> "the set of Traffic Trunks which share the exact same Bandwidth Constraint,
> or set of Bandwidth Constraints, on all links, for the purpose of Constraint
> Based Routing and Admission Control."
>
> The above definition is technically incorrect.  A given LSP instance has the
> same bandwidth "on all links," but a class type cannot have the same
> constraints "on all links," as exemplified by Scenario 2.  A workable
> definition is provided below.

Diffserv is by definition hop by hop, but I see no need to limit CTs as
such.  I think the existing text is fine.  A certain CT, in my view,
consist of all the resources in a network available to the CT, and all
of the LSPs that call upon those resources.

 >
> Class Type (CT): the set of Traffic Trunks crossing a link that is governed
> by a specific set of bandwidth constraints and whose packets receive either
> the same or closely related DiffServ forwarding treatment.  CT is used for
> purposes of link bandwidth allocation, constraint based routing, and
> admission control.
>
> Issue 2: Split EF
> -----------------
> Also in Section 3.2, another example can be added as follows:
>
> A network operator may split the EF traffic into two different sets of
> traffic trunks, so that one set of traffic trunks is given higher priority
> access to bandwidth than the other set, to satisfy some QoS objectives.  In
> this case, two distinct CTs are defined: one for the EF traffic with high
> priority, the other for EF traffic with normal priority.
>
> (This example of split-EF traffic was already discussed within the author
> team and no objections were raised.)

This falls into the category of "Does one CT correspond to one and only
one OA", or stated another way "Are we interested in tying DSTE lock-step
to Diffserv, or is it something more flexible"

Several have expressed a desire to make it more general, along the lines
of what CT is refered to in the principles draft.  I agree, and think the
above scenario is a valid way that a carrier might want to have two TE
"classes" which are tangible and distinct, but both of which are one OA.
You are right that we need to generalize CTs.

 >
> Issue 3: Max number of BW constraints
> -------------------------------------
> It is proposed to add the following requirement in Section 3.3.
>
> By definition of CT, each CT is assigned either a Bandwidth Constraint, or a
> set of Bandwidth Constraints.  Since there is a maximum of 8 CTs, there is
> correspondingly a maximum of 8 sets of Bandwidth Constraints.  That is,
> maxCT = maxBC = 8, for IGP advertisement purposes.
>
> There is the possibility that some related CTs may be grouped together and
> share a common set of bandwidth constraints.  However, such kind of grouping
> can be handled and enforced purely inside a router without being advertised
> by the IGP.  This is because the use of such information in the computation
> of unreserved bandwidths is a local matter.

I agree that we should try to limit the amount of bandwidth constraints
that are advertised, though additional "internal" constraints might be in
use within an LSR (for instance, grouping bandwidth usage of two CTs).

 >
> Issue 4: Suppression of preemption
> ----------------------------------
> It is proposed to add the following requirement in Section 3.4.
>
> A service provider preferring not to use preemption for user traffic should
> not be burdened by the overheads associated with preemption.  Thus, a
> service provider should not need to set all class types to the same
> preemption level to avoid using preemption.

As in the rsvp draft and with some implementations, use of preemption
should be optional.

>
> Issue 5: Relationship between preemption and CT
> -----------------------------------------------
> In Section 3.4 it says:
>
> "In other words, DS-TE must offer eight(8) preemption priority levels to be
> used by an LSP, that are completely orthogonal to that any other LSP's
> attributes (eg LSP's OA, LSP's Class Type, etc.)."
>
> The utility of the concept of preemption being orthogonal to CT is not
> clear.  If each CT is assigned one preemption level only, it appears that
> such concept is no longer necessary.  A one-to-one mapping between CT and
> preemption level does not preclude the possibility that some related CTs may
> be grouped together, as discussed in Issue 3.  We propose that the statement
> of orthogonality be removed.
>

Maybe they are not so much orthoginal, as not necessarily unique?

I do like limiting the amount of external information, and of course I
like reusing the existing advertisement structure and codepoints.
Decoupling the advertised values from the actual priority could be useful.