Re: draft-ott-mmusic-cap-00.txt
Dirk Kutscher <dku@informatik.uni-bremen.de> Wed, 18 August 1999 16:16 UTC
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To: Graham Klyne <GK@Dial.pipex.com>
Cc: confctrl@ISI.EDU, megaco@standards.nortelnetworks.com
Subject: Re: draft-ott-mmusic-cap-00.txt
References: <3.0.32.19990726142846.01606ad0@pop.dial.pipex.com>
From: Dirk Kutscher <dku@informatik.uni-bremen.de>
Date: Wed, 18 Aug 1999 18:15:26 +0200
In-Reply-To: Graham Klyne's message of Mon, 26 Jul 1999 14:32:28 +0100
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>>>>> "Graham" == Graham Klyne <GK@dial.pipex.com> writes: Graham> I have just read through your above-mentioned draft with Graham> some interest, as it closely parallels work with which I Graham> have been involved. For the most part, I shall not make Graham> detailed comments, but shall try to address some broader Graham> issues about what I see as the relationship between your Graham> draft and the IETF CONNEG work. Graham, Thank you for your comments on the draft and please excuse the rather long delay in answering. Most of your notes make a lot of sense to me. Here are some comments from my side: You correctly identified some deficits of the specification that need to be worked on: The relation between the system model, the mentioned requirements and the specified description language will be made explicit. You are right in emphasizing the similarities between the conneg work and our draft. Graham> - In section 1.3 where you discuss relationship to other Graham> developments, while what you say about the CONNEG work Graham> seems generally reasonable, you do not explain what is Graham> accomplished by your proposal that is not accomplished by Graham> the CONNEG work. The last paragraph in that section states that we aim for a simpler solution, so simplicity and probability of being implemented are the main characteristics that we thought that our approach can accomplish in a better way. We have therefore deliberately chosen the primitive "basic description language" as a representation. Graham> - 2.4. Collapsing Algorithm Graham> The objective of the collapsing algorithm is to take Graham> capability description sets from each end system in Graham> order to find a set of media-types, encodings and Graham> features that are supported by all end system, or, if Graham> this is not possible, to find a subset that would Graham> exclude as few systems as possible. Graham> How does your algorithm act to find a subset to "exclude Graham> as few systems as possible"? Yes, this is still to be defined. Graham> I also note that your collapsing algorithm does not handle Graham> cases where different constraints are applied to some Graham> given feature (e.g. one party might specify "fps<=30" and Graham> another might specify "fps=15" for a video frame rate). Right, it would be required to use "fps" consistently either with a less-or-equal-than-comparable or a selection-of-fixed-values type. For simplification we decided not to allow symbolic values to be less-or-equal-than-comparable. Graham> - 3. Specification of the Decription Language Graham> As far as I can tell, your description language is Graham> semantically a subset of that described in RFC 2533; your Graham> "Basic description language" being equivalent to Graham> Disjunctive Normal Form of the CONNEG syntax, and your Graham> "Concise Description Language" being closer to the general Graham> CONNEG syntax form. True, the idea was exactly to provide a subset of RFC 2533's functionality for expressing feature sets, while allowing for "non-collapsing parameters" and the definition of "simultaneous capabilities". The proposal we made in the draft actually tries to accomplish a set of different objectives: 1) provide an abstract model of capability negotiation and session descriptions for multiparty conferences that can be used to gather some requirements for a capability description framework. 2) define a syntax and a framwork for capability definitions; 3) define a syntax for session descriptions and provide a migration path from currently deployed standards. While the draft proposes to consider a successor of the Session Description Protocol (SDP) it is unclear at this time if the mmusic working group is going to adopt this (or the topic of multiparty capability negotiation in general) as a work item. Providing session description functionality does not only involve feature set definitions and feature match algorithms as currently defined in RFC 2533 but also additional facilities to allow for unnegotiable parameters of session descriptions. As you noted, the draft specification does currently no meet all of the mentioned demands itself, because some things are in a rather premature state, requiring further discussion in the mmusic group. Graham> - 6. Composed Configurations Graham> If I understand your intent correctly, I note that the Graham> CONNEG work includes a concept of auxiliary predicates Graham> that can be used to label configurations. Graham> We are also working on another proposal Graham> <draft-ietf-conneg-feature-hash-xx.txt> for labelling of Graham> arbitrary feature sets using an MD5-hash-based identifier. Right, auxiliary predicates can be used to factor out filter expression. As I understand RFC2533 and draft-ietf-conneg-feature-hash-02, the main objective is to allow for more convenient notations and to enable the use of references to external feature set expressions. Since auxiliary predicates are expanded before the conversion to DNF for the feature matching process, any reference to the named predicate is lost. When expressing capabilities for redundant encodings (which we deemed to be one application of composed configurations) this is problematic. (One should note that, at this time, our proposal does not contain a solution either). Also, I have a problem with what we call "non-collapsing parameters". I note that RFC 2533 has the concept of "parameters" that can be attached to any filter value in a predicate. However, I don't see how these parameters, including q-values for user preferences, are handled in a feature match process. So, summarizing, we acknowledge the semantic simliarities between RFC 2533 and our proposal. It is certainly worthwhile to explore the possibility to build on RFC 2533 for specifying a description and negotiation framework for multiparty cooperation. Resting upon some of conneg's work we decided not to do so for this first draft because it was not completely understood how to implement all aspects of the system model. The objective was to raise interest in mmusic group and to initiate the discussion. We would be happy to discuss the open issues within the conneg and/or mmusic group as soon as a requirement and working group agenda has taken place in mmusic concerning this topic. -- Dirk
- draft-ott-mmusic-cap-00.txt Graham Klyne
- Re: draft-ott-mmusic-cap-00.txt Dirk Kutscher
- Re: draft-ott-mmusic-cap-00.txt Graham Klyne