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Re: few comments on draft-ietf-tewg-interas-mpls-te-req-01.txt

Hi Yakov,
Thanks for the response...

[snip...]

Perhaps I missed it, but the -02 version still does not include
this "good point".

Sorry... -02 was posted right before your email to list showed up. I'll make sure this point is well reflected in the text (in section 6.1)

> >With this in mind the following in 6.1 should be removed:
> >
> >    It MUST meet all the requirements described in section 5 each
> >    time a MUST is specified.
> >
> >and be replaced with the following:
> >
> >    It MUST document whether it meets each requirement described
> >    in section 5.
>
> Well in a requirements doc, is it not the case that when there is a MUST,
> then the solution MUST meet the requirement ?

Unless there is an existence proof that it is possible to have a
practical solution that meets all the requirements in section 5
that are "MUST", saying that a solution "MUST meet all the requirements
described in section 5 each time a MUST is specified" is nothing
but a wishful thinking.

There are only a few "MUST" in section 5 of -02 now:
1. Section 5.1.1. " ..the derived solution MUST be such that it will interoperate seamlessly with current intra-AS MPLS TE mechanism..."
2. Section 5.1.1. "The proposed solution MUST allow to provision at the Head/Tail end with end-to-end RSVP signaling (eventually with loose paths)
traversing across the interconnected ASBRs, without further provisioning required along the transit path."
3. Section 5.1.5. "...the solution MUST be able to re-optimize the LSP accordingly and non-disruptively, either upon expiration of a configurable timer or
triggered by a network event or a manual request at the TE tunnel Head-end."
4. Section 5.1.10.1 "This new MIB MUST extend (and not reinvent) the existing MIBs to accommodate this new functionality."

5. Section 5.2.2.1 "The signaling of a non policy compliant request MUST trigger the generation of a RSVP Path Error message by the policy enforcing
node towards the Head-end LSR, indicating the cause."
6. Section 5.2.2.2. "the re-writing node MUST generate a RSVP Path Error Message towards the Head-end LSR indicating the cause in terms
of types of changes made so as to maintain the end-to-end integrity of inter-AS TE LSP."

In -03, we will change "MUST" to "SHOULD" for item 5 and 6 above.

As for 1, 2, 3 and 4, they constitute, in our thinking, a basic set of requirements for the deployments of inter-AS TE. It would be great if you could clarify further of each of item 1, 2, 3 and 4 on any technical limitations or impracticality such that these "MUST" requirement can not all be met in a solution ?

> But to the merit of your first point, maybe we can propose to add this in
> section 6.1 : "if a solution can fulfill just a subset of those
> requirements in section 5, then it MUST be explicitly documented"
>
> >2. Since this is a *requirements* document, other than relying on the
> >existing MPLS/DiffServ TE mechanisms, the document should not presume
> >a particular solution.
>
> Indeed. This point was mentioned before. As a result, references as such in
> the latest revision of the document have been reworded as examples, merely
> to illustrate or help in understanding the corresponding requirements.
>
> >Therefore references to such specific solutions like PCS (e.g., in
> >section 5.1.2) should be removed from the document.
>
> Similarly, the mention of this is merely an example to illustrate the
> requirements in path computation...

Sorry, but the -02 version of the document still talks "... require
a discovery mechanism of some PCS using IGP extensions".

Also, may I suggest that the requirements document should focus on the
requirements and not on the specific examples of how to meet the
requirements.

Right. To this point, I will simplify the wording of the examples in this section
(tho I don't want to remove the example entirely since I feel it does help in
further illustrating the stated requirement). How about the following wording ?

Section 5.1.2
"...
For example, one may provide a manual description of all or some of the hops (loose routing) the TE LSP must traverse, allowing to keep the information related to the intra-AS resources confidential while still leaving intra-AS routing decisions to local operators. Another example is an automated way of setting up the TE LSP without any static configuration on the Head-End LSR by ways of requesting the computation of an inter-AS TE LSP to some path computation elements..."

> >Likewise, the following in 5.1.5 has to be removed:
> >
> > The solution SHOULD support the ability for intermediate nodes to
> > signal the respective Head-End LSRs of the existence of a more
> > optimal path.
> >
> >(as it presuppose a particular way of establishing an optimal path).
>
> This is actually one of our requirements that a SP with inter-AS TE HE
> would like to be notified if there is a more optimal path opening up over a
> transiting AS. This way the HE SP can 1). LSR can have a configurable
> choice to optimize or not depending upon the available capacity in its own
> AS should the ASBR exist point change for the re-opt path. 2). in case the
> more optimal path yields for example, a worse delay (over a longer path due
> to some mis-configured TE metrics, e.g.), the notification would provide
> additional clue for HE SP to determine the cause of problem
>
> Also in some cases, the Head-end LSR may want to control the reoptimization
> process, especially when carrying sensitive traffic (because
> reoptimization, although not traffic disruptive (thanks to make before
> break), does generate jitter and potentially packet reordering, something
> not desirable for some TE LSPs. In that case, the solution SHOULD provide a
> mode whereby intermediate nodes can signal the existence of a better path
> and then the head-end LSR will decide whether to perform a reoptimization
> or not. Note that this is a requirement from various SPs on this draft and
> is listed as a SHOULD.

Let's not confuse requirements with the mechanisms to support the
requirements.

Right... I would also like to further clarify that the requirement calls for the HE LSR of the inter-AS TE LSP to have control of re-optimization not only for the AS segment in its own AS, but also for transit AS segments as well.


The requirement here is for the Head-end LSR under control of local
configuration to be able to reoptimize the path, if and when a "better"
path becomes available.

Requiring intermediate nodes to provide the information about
a more optimal path over a transit AS to the Head-end LSR is *one
possible mechanim* to support the requirement.

Yakov, I do see your point regarding "mechanism". But we'd still like to keep the requirement in our response above. Can I then propose the following wording to replace this paragraph ?

"The solution SHOULD provide an option for the Head-End LSRs to control if re-optimizing or not should there exist a more optimal path in one of the transit ASes along the inter-AS TE LSP path."


> >3. In Section 5.1.3 the document needs to explicitly state whether
> >it is a requirement to maintain optimal path all the time, and if
> >not then what percentage of overall path life time.
>
> Well, for some LSPs (not all, but some LSPs) it is desirable to have a
> solution being able to compute an optimal end to end path (a requirement
> for SPs on the draft). The question of whether the TE LSPs should always be
> on the optimal is a question of implementation agreement and parameter
> setting. The requirement here is just that the solution must offer the
> possibility on compute an optimal path end to end (again by optimal we man
> "shortest path"). Note this is a requirement listed as a "SHOULD".

If the requirement here is to compute an optimal path end to end
just for the sake of computation, then what are the practical
benefits of such a requirement ?

And if the requirement here is to compute an optimal path end to
end for the sake of using it (means establishing an LSP along such
a path), then the requirement has to be very explicit with respect
to whether it expects path optimality to be maintained all the time
or not, and if not then what percentage of the time.

This requirement is indeed for the ability to establish an optimal path e2e for the inter-AS TE LSP
("using it"). For example, this ability may be required for a SP with multiple ASes to be able to deploy
a set of meshed TE LSPs optimally across their own AS boundaries and once established, it should be
maintained at all times. However, in some cases it is also a matter of tuning, trade-off between optimality
and network stability, ... on whether path optimality should be maintained "all the time" or triggered
after some timer has elapsed. At the end, what we are actually after in the req's draft is that a solution
should offer this capability of computing an optimal inter-AS TE LSP path ("shortest") end to end.

Will have some additional wordings then to reflect what we corresponded above. Your thoughts ?

>
> >4. Sections 6.2 ("Scalability and Extensibility"), 6.3 ("Complexity
> >and Risks"), and 6.5 ("Backward Compatibility") talk about requirements
> >on scalability and extensibility, and therefore should be moved
> >into Section 5.
>
> Will do.. Thanks.

-02 version still doesn't reflect this.

It will be in -03, thanks.

Regards,
Raymond