Subject: [MMUSIC] RTSP: 300 (Multiple Choices) error response and body

From: Magnus Westerlund <[email-masked]>
Date: Mon, 01 Dec 2008 14:52:31 +0100

Hi,

RTSP 2.0 currently specifies the response code 300 (Multiple choices) by
a reference to the HTTP specification. However, if one actually follows
this one find the following:

   The requested resource corresponds to any one of a set of
   representations, each with its own specific location, and agent-
   driven negotiation information (section 12) is being provided so that
   the user (or user agent) can select a preferred representation and
   redirect its request to that location.

   Unless it was a HEAD request, the response SHOULD include an entity
   containing a list of resource characteristics and location(s) from
   which the user or user agent can choose the one most appropriate. The
   entity format is specified by the media type given in the Content-
   Type header field. Depending upon the format and the capabilities of
   the user agent, selection of the most appropriate choice MAY be
   performed automatically. However, this specification does not define
   any standard for such automatic selection.

   If the server has a preferred choice of representation, it SHOULD
   include the specific URI for that representation in the Location
   field; user agents MAY use the Location field value for automatic
   redirection. This response is cacheable unless indicated otherwise.

This is quite problematic for RTSP 2.0. First of all it doesn't have any
obvious supported format for representing choices to users. Secondly,
what does one present to the client or user that allow it to make a
choice of which is the best choice. I also fear that if we dive into
this topic we can get lost about capabilities of clients and preferences.

I can see two reasonable choices:

1. Remove 300 as response code completely.

2. Keep it and mandate a specific version html for response to be
displayed to users to let them perform the selection.

I so far have a preference for 1. Comments or suggestions?

Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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From: "Rémi Denis-Courmont" <[email-masked]>
Date: Mon, 1 Dec 2008 16:14:20 +0200

> I so far have a preference for 1. Comments or suggestions?

Not an HTTP expert, but I believe this is not used in real-life. Apache does 
implement it in its own way. But no website uses it, due to the highly 
confusing experience on the browser side and "unusual" file names for the 
webmaster. In practice, especially language autodetection always return a 2xx 
error code with a default choice made on the server side.

This is even more problematic for RTSP as the user agent might well not have 
HTML parsing/rendering capability. I would also remove it from RTSP.

-- 
Rémi Denis-Courmont
Maemo Software, Nokia Devices R&D
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From: Sean Sheedy <[email-masked]>
Date: Mon, 1 Dec 2008 14:11:09 -0800

On Mon, 2008-12-01 at 06:14 -0800, Rémi Denis-Courmont wrote:
> On Monday 01 December 2008 15:52:31 Magnus Westerlund wrote:

Remove it.  Rémi's point is definitely the case in our environment;
there's no way a typical set-top box client in a cable TV deployment
could do anything useful with such HTML information.  Any resource
ambiguity should to be resolved by another mechanism (such as SDP); it
doesn't belong here.

Sean

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Sean Sheedy
Staff System Software Engineer, Media and Communications Systems
[email-masked] | +1-503-495-9454 (Phone)
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