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

["Lai, Wai S (Waisum), ALSVC" <wlai@att.com>]

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.

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.)

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.  

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.

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.