Subject: [Pce] Response Liaison Statement on the PCE Working group

From: "Adrian Farrel" <[email-masked]>
Date: Sun, 22 May 2005 12:32:13 +0100

To:       Hing-Kam Lam, Rapporteur Q14/15
         Malcolm Betts, Rapporteur Q12/15
         Jerry Shrimpton, Rapporteur Q6/15
From:     Adrian Farrel and JP Vasseur, co-chairs IETF PCE working group
Cc:       Alex Zinin and JP Vasseur, IETF Routing Area Directors
Subject:  Response Liaison Statement on the PCE Working group
For:      Action
Deadline: July 30th 2005 

The IETF's PCE Working Group would like to thank Questions 6,
12 and 14 of ITU-T Study Group 15 for their response liaison
on the PCE working group. We welcome the prospect of cooperation
between our organisations on this subject. 

Although this liaison may be too late for consideration in your
current meeting, we hope that it may be circulated for information,
perhaps as a WD, and considered more fully at later meetings. 

We note your reference to Recommendation G.8080 and observe that
there is, indeed, mention of the 'Route Table Query interface'
that is used to allow a Signaling Controller to resolve an
'unresolved portion of the route.' 

Nevertheless, our reading of G.8080 is that the 'Route Table Query
interface' exists between the Signaling Controller and the Routing
Controller. If this is the case, it does not match the
architectural model that we are proposing within the PCE working
group. That is, we are making a distinction between the distribution
and maintenance of TE routing information by the Routing Controller,
and the computation of paths as performed by the Path Computation
Element. Thus, in the PCE architecture, there may be a physical
separation between the Routing Controller responsible for the
routing-plane management of a transport entity (that is the RC
entity responsible for announcing local topology information
generated by the NE), and the PCE responsible for satisfying Path
Computation Requests issued by the Signaling Controller responsible
for the signaling-plane management of the transport entity. This
means that we are implying a functional separation between the items
listed at the head of section 7.3.2 of G.8080 where such a separation
is not recognized within the ASON architecture as currently stated.
If we have misunderstood the architecture as stated in G.8080 we
would appreciate clarification from you. 

In any case, we would welcome your review and comments on our
architecture draft which can be found at
http://www.ietf.org/internet-drafts/draft-ietf-pce-architecture-00.txt 

Question 6 asked a specific question about the objective metrics
referred to in the PCE charter... 

> Q.6/15 (Characteristics of optical systems for terrestrial transport
> networks) noted that your charter contains the following item:
> "- Definition of objective metrics to evaluate various criteria such
> as the measurement of path quality, response time, robustness and 
> scalability of path computation models."   
> 
> Could you please clarify the meaning of "path quality" in the above
> definition? If this includes optical monitoring aspects, please be 
> aware that ITU-T Q.6/15 has developed a Recommendation, G.697, 
> Optical monitoring for DWDM systems which may be of interest to the
> PCE working group. This Recommendation is currently in force.

Path quality in this context is intended to describe what is sometimes
termed the 'optimality' of the path ('route' in G.8080 terminology).
Measures of path quality reflect the ability of a PCE model to deliver
a path that satisfies the constraints specified by the requesting
client. One such quality might be 'shortest path', and it should be
clear that once Routing Areas or Layered Networks are in use, different
computation models will have different abilities to compute shortest
paths. 

Thus, there is no intention to measure the active quality of the 'path'
within the data plane: This is out side the scope of the PCE working
group. Note that there is also no intention to measure the
effectiveness of the algorithms in use (although there is a clear
requirement to control the effectiveness according to the requirements
of the requester), but rather to measure the likely ability of a
particular interaction between PCEs to produce an adequate path. 

Your liaison goes on to ask a further question about the intentions of
the PCE working group with respect to optical path quality. 

> In addition, we would appreciate a better understanding of whether
> the PCE working group will study mechanisms by which optical path 
> quality can be calculated or only mechanisms for communicating the
> results of such calculations.

We assume that here, too, you are refering the quality parameters of
an established optical trail, and not to the quality of the computed
path as expressed above. The PCE working group is limited to the
architectural models for computation of paths ('routes') through
networks, and will not examine the measurement of optical
characteristics. 

Note, however, that path computation as applied to GMPLS systems
falls within the scope of the PCE working group. It is likely that
future GMPLS control planes will require to compute paths bearing in
mind optical constraints such as (but not limited to) wavelength
continuity, dispersion characteristics, etc. In these cases, it will
possibly be necessary for some optical characteristic information to
be gathered from the data plane, and distributed within the routing
plane. These two functions will remain outside the scope of the PCE
working group, but may be the responsiblity of other IETF working
groups. In this regard, Quesiton 6 may be interested in a new RFC
that has been published: RFC 4054 is titled "Impairments and Other
Constraints on Optical Layer Routing" and can be found at
http://www.ietf.org/rfc/rfc4054.txt 

Anyone interested in discussing the work of the PCE working group
further is invited to join the PCE mailing list. Details can be
found at http://www.ietf.org/html.charters/pce-charter.html 

Best regards,
Adrian Farrel and JP Vasseur

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