[MMUSIC] Clarification for multiple lang attributes in rfc4566bis
Gunnar Hellström <gunnar.hellstrom@omnitor.se> Sun, 04 October 2015 22:00 UTC
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From: Gunnar Hellström <gunnar.hellstrom@omnitor.se>
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Date: Sun, 04 Oct 2015 23:59:49 +0200
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Subject: [MMUSIC] Clarification for multiple lang attributes in rfc4566bis
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There is some uncertainty about interpretation of multiple lang attributes in RFC 4566 that would need to be sorted out in rfc4566bis. The uncertainty has caused stox to select another mechanism and now also slim is hesitating. (see slim and its real-time draft for a deeper discussion) The current text in section 6.12 of rfc4566bis is: -----------------------------current text from 6.12-------------------------------- Multiple lang attributes can be provided either at session or media level if the session or media use multiple languages, in which case the order of the attributes indicates the order of importance of the various languages in the session or media, from most important to least important. As a session-level attribute, it specifies the default language for the session being described. As a media-level attribute, it specifies the language for that media, overriding any session-level languages specified. The "lang" attribute value must be a single [RFC5646] language tag in US-ASCII. A "lang" attribute SHOULD be specified when a session is of sufficient scope to cross geographic boundaries where the language of recipients cannot be assumed, or where the session is in a different language from the locally assumed norm. ----------------------------------------------------------------------------------------------- The uncertainty has been around what the wording "use multiple languages" means. It can mean that all are provided simultaneously, but it can also mean that it is a list of capabilities and that events during the session can decide which languages will really be used. There is also uncertainty about if a negotiation can take place to agree on one or a set of common languages to select from as default language(s). In order to bring some clarity, I suggest to change to the following: -----------------------------proposed replacement text for 6.12-------------------------------- Multiple lang attributes can be provided either at session or media level if the session or media has capabilities to use multiple languages, in which case the order of the attributes indicates the order of preference of the various languages in the session or media, from most preferred to least preferred. As a session-level attribute, lang specifies a language capability for the session being described. As a media-level attribute, it specifies a language capability for that media, overriding any session-level language(s) specified. The "lang" attribute value must be a single [RFC5646] language tag in US-ASCII. A "lang" attribute SHOULD be specified when a session is of sufficient scope to cross geographic boundaries where the language of recipients cannot be assumed, or where the session has capabilities in languages different from the locally assumed norm. Events during the session may influence which language(s) are used, and the participants are not strictly bound to only use the declared languages. ----------------------------------------------------------------------------------------------- My conclusion from looking at the background of the wording for lang in RFC 4566 is that we can safely assume that for real-time sessions, the meaning of multiple lang specifications is a range of capabilities, and not a requirement to use all languages. By that interpretation, we can also assume that an offer/answer exchange can negotiate the language(s) to be suitable for use in a session to be a subset of what the offer contained. I traced the introduction of the wording from an announcement in mmusic on 1997-11-23, and then the wording appeared in draft-ietf-mmusic-sdp-05 on 1997-12-23. There was no further discussion and the wording has stayed like that with merely a swap of the paragraphs in rfc4566bis. In the mail announcing the introduction it was said that the intention was to fulfill requirements from a draft that then became RFC 2277. A relevant section in RFC 2277 is: " 4.4. Considerations for language negotiation Protocols where users have text presented to them in response to user actions MUST provide for support of multiple languages. How this is done will vary between protocols; for instance, in some cases, a negotiation where the client proposes a set of languages and the server replies with one is appropriate; in other cases, a server may choose to send multiple variants of a text and let the client pick which one to display. Negotiation is useful in the case where one side of the protocol exchange is able to present text in multiple languages to the other side, and the other side has a preference for one of these; the most common example is the text part of error responses, or Web pages that are available in multiple languages. Negotiating a language should be regarded as a permanent requirement of the protocol that will not go away at any time in the future. In many cases, it should be possible to include it as part of the connection establishment, together with authentication and other preferences negotiation. " So it is apparent that the intention for real-time conversational cases is a negotiation with an offer and a selection, either in an answer or by human agreement in the session. The central word creating the hesitation in interpretation is that all languages must be used is "use" in this phrase: "if the session description or media*use* multiple languages". It is hard to believe that it was the intention that all must be used. The proposed wording is made to make the interpretation clear that it is a set of capabilities.. There are multiple reasons for that interpretation: 1. It aligns with the requirement for language negotiation in RFC 2277 that was said to be the requirement behind the wording. 2. It is usually unrealistic to use multiple languages at the same time in a medium. It is more in line at least with real-time conversation that events during the conversation causes decision to use just one language. 3. The discussion in the original wording of importance of languages lose its meaning if all must be used. "the order of the attributes indicates the order of importance of the various languages in the session or media". There is no use of knowing how important a language is if they all must be presented. It is more likely that this relates to a possibility to select languages and that the "importance" is a grading of preference. This would mean that we can safely assume that the use of multiple lang attributes in SDP were and are intended as a selection and that negotiation through offer/answer is possible. I realize that the language about capabilities in my proposal does not match an sdp that belongs to a static session with recorded material equally well as it does for a real-time session between human participants, but I want anyway propose the wording. /Gunnar Hellström
- [MMUSIC] Clarification for multiple lang attribut… Gunnar Hellström
- Re: [MMUSIC] Clarification for multiple lang attr… Ali C. Begen
- Re: [MMUSIC] Clarification for multiple lang attr… Peter Saint-Andre - &yet
- Re: [MMUSIC] Clarification for multiple lang attr… Ali C. Begen
- Re: [MMUSIC] Clarification for multiple lang attr… Gunnar Hellström