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

[Yakov Rekhter <yakov@juniper.net>]

Raymond,

> Sorry for the delayed response and many thanks for the comments.  Please 
> see the responses in lines below...
> 
> At 01:17 PM 11/17/2003 -0800, Yakov Rekhter wrote:
> >Folks,
> >
> >Here are few comments on draft-ietf-tewg-interas-mpls-te-req-01.txt.
> >
> >1. The document should state up front the following:
> >
> >      This document doesn't make any claims with respect to whether
> >      it is possible to have a practical solution that meets all the
> >      requirements listed in this document.
> 
> Good point, although this point may have been largely reflected in 
> requirements specified with "SHOULDs" in section 5...

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

> >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.

> 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.

> >Ditto for the reference to Forwarding Adjacencies in 5.1.8.
> 
> Already got that one. Thanks.
> 
> >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.

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.

> >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.

> 
> >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.

> >5. I would suggest to include 5.1.8 into 6.2, as 6.2 talks about
> >scalability, and 5.1.8 talks about one particular aspect of scalability -
> >scalability in the forwarding plane.
> 
> Good point, thanks.

Ditto.

> >6. In 5.1.5 replace:
> >
> >    Furthermore the solution SHOULD provide the ability of manually
> >    rejecting re-optimization at AS boundaries.
> >
> >with the following:
> >
> >    Furthermore, the solution MUST provide the ability to reject
> >    re-optimizatization at AS boundaries.
> 
> Right.  Will incorporate, thanks...

Ditto.

> >7. In 6.2 replace "SPF" with "Constrained SPF" or "CSPF".
> 
> Thanks for pointing this out.  Will update...

Ditto.

Yakov.