[PANRG] Two thoughts about draft-irtf-panrg-questions
Adrian Farrel <adrian@olddog.co.uk> Thu, 04 February 2021 10:25 UTC
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From: Adrian Farrel <adrian@olddog.co.uk>
To: panrg@irtf.org
Date: Thu, 04 Feb 2021 10:25:20 -0000
Organization: Old Dog Consulting
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Subject: [PANRG] Two thoughts about draft-irtf-panrg-questions
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Hi PANRG, I recently had cause to read draft-irtf-panrg-questions (apologies for coming late to this as a tourist) and it raised two issues for me. You may already have discussed these, in which case fine and good. On the other hand, this may give you pause for thought. 1. What is a Path? I know this is a somewhat fundamental question in the context of this work, but that probably means it should be front and centre of the document. As I'm sure you know, the term "path" means so many different things to different people. A lot depends upon the layer of the stack you are operating in. A fair amount depends on the virtualisation/abstraction model you are using and how your overlays are built. draft-irtf-panrg-questions is pretty clear about what "path-aware networking" is, but makes no mention of what a "path" is. Conversely, draft-irtf-panrg-path-properties relies on draft-irtf-panrg-questions for an explanation of "path-aware networking", but provides a reasonable explanation of "path" setting the concept in general and abstract terms. It wasn't clear to me, reading draft-irtf-panrg-questions, whether the work was intended to apply at the routing layer, in transport terms (such as MPTCP), in a tunnel overlay, etc., etc., and that significantly limited the value I could get from the document. It is also possible (probable?) that many readers and reviewers bring their own definition of "path" to the document and so are subjecting the work to their own world-view and not picking up on the intended meaning. This is probably resolved quite simply by adding a normative reference to draft-irtf-panrg-path-properties for the definition of "path". 2. Excluding the Path Computation Element draft-irtf-panrg-questions makes a single mention of PCE right at the top of the Introduction. I fear that the context ("the interfaces to these are generally administrative as opposed to endpoint-exposed") expresses a fundamental misunderstanding of what a PCE is and how it can be used. A PCE is an abstract functional element that enables its user to make choices between available paths based on a set of constraints. The underlying data sets are nodes, links, and metrics, and they can be subjected to any algorithm and any set of constraints in order to determine the chosen path. (To give an example, I am aware of a use of PCE that determines the best paths through a sewerage system - sadly not publicly documented.) The architecture being considered by PANRG seems to assume (Section 2.2) that the available paths will be presented to the application as complete things. That is, that some entity will have constructed these paths presumably by processing the information about nodes, links, and metrics within the network. But you haven't asked the question: How will those paths be computed? Of course, my answer would be, "By a PCE". This may be a reasonable architectural compromise, but it begs the question: How does the entity supplying the application with paths know what sort of paths the application wants? That question even extends to knowledge of the destination the application is trying to reach! An option here is clearly that the application makes a request to the PCE for a set of potential paths. This would fit within the issues raised in Section 2.3, since if the PCE is operated by the network, the paths offered can be limited to only those that the network trusts the application to use. There is, however, an architectural alternative where the application has access to more detailed information: not just the paths it can use, but a network topology over which it is allowed to operate. That topology could be the whole network (that is, the actual nodes, links, and metrics of the network) but it could also be an abstraction of the network (as a filter of the full network, or as a virtual overlay). The application is able to process this topology to derive its own preferred paths (by using its own internal PCE, or by invoking an external PCE). The paths that it generates and selects will be trusted by the network because they can only use the resources of the abstracted network that were supplied to the application by the network. I'd like to observe that many years' work has been spent considering these issues in the PCE and TEAS working groups. While some people might prefer to think of PANRG not overlapping with Traffic Engineering, I have to point out that steering traffic onto one path or another is a fundamental aspect of TE in the Internet (see draft-ietf-teas-rfc3272bis). Hope this helps. Best, Adrian
- [PANRG] Two thoughts about draft-irtf-panrg-quest… Adrian Farrel