Re: accept-types [was Re: Fwd: Re: Adrian Farrel's Discuss on draft-ietf-simple-chat-16: (with DISCUSS and COMMENT)]

Ben, and the SIMPLE WG:

To clarify the usage of accept-types, I propose two paragraphs to Section 
5.2 (Joining a chat room). The first paragraph is new and discusses the 
participant's SDP. Here, I propose not to add normative text mandating 
the UA to add 'a=accept-types:Message/CPIM' in SDP. The reason is that 
such normative text would require a mechanism for the UA to know in 
advance that it is joining a chat room, perhaps by examining the chat 
room URI. But in general, such mechanism does not exist. The UA will 
create an INVITE request with SDP, not knowing if at the other end there 
is a chat room or a regular user. If this SDP contains 'm=message', the 
attributes included afterwards will be the same, no matter whether the 
participant is joining a chat room or just another endpoint. Please 
verify that the proposed words are acceptable: "...needs to include at 
least...

    This participant's INVITE request contains an SDP body.  This SDP
    body includes the description of MSRP 'message' media line as per RFC
    4975 procedures [RFC4975].  This specification requires all instant
    messages to be wrapped in a Message/CPIM wrapper [RFC3862],
    therefore, the 'accept-types' attribute for the MSRP message media in
    the participant's SDP offer needs to include at least the value
    'Message/CPIM', otherwise the conference focus will reject the
    request.  The actual instant message payload type is negotiated in
    the 'accept-wrapped-types' attribute in SDP (see RFC 4975 [RFC4975]
    for details).  There is no default wrapped type.  Typical wrapped
    type values can include: text/plain, text/html, image/jpeg, image/
    png, audio/mp3, etc.  It is RECOMMENDED that participant endpoints
    add an 'accept-wrapped-types' attribute to the MSRP 'message' media
    line in SDP, where the supported wrapped types are declared, as per
    RFC 4975 procedures [RFC4975].

Now, the second paragraph was already present in the previous version, 
and is now divided into two paragraphs. It tackles the conference focus. 
I have added text to indicate that the focus must only accept 
Message/CPIM, and also that the focus should add an 
'accept-wrapped-types'. The text reads:

    The conference focus of a chat room MUST only use a Message/CPIM
    [RFC3862] top-level wrapper as a payload of MSRP messages, and this
    needs to be declared in the SDP offer and answer as per regular RFC
    4975 procedures [RFC4975].  This implies that if the conference focus
    receives from a participant's endpoint an SDP offer that does not
    include the value 'Message/CPIM' in the 'accept-types' attribute for
    the MSRP message media line, the conference focus SHOULD either
    reject the MSRP message media stream or the complete SDP offer by
    using regular SIP or SDP procedures (e.g., creating an SDP answer
    that sets to zero the port of the MSRP message media line, responding
    the INVITE with a 488 response, etc.).

    If the conference focus accepts the participant's SDP offer, when the
    conference focus generates the SDP answer, it MUST set the 'accept-
    types' attribute for the MSRP message media line to a value of
    'Message/CPIM'.  This specification requires all instant messages to
    be wrapped in a Message/CPIM wrapper, therefore, the 'accept-types'
    attribute in this SDP body contains a single value of 'Message/CPIM'.
    The actual instant message payload type is negotiated in the 'accept-
    wrapped-types' attribute in SDP (see RFC 4975 [RFC4975] for details).
    The conference focus SHOULD also add an 'accept-wrapped-types'
    attribute to the MSRP message media line in SDP containing the
    supported wrapped types.


Please indicate if there are problems with the proposed text.

/Miguel



On 07/09/2012 10:41, Miguel A. Garcia wrote:
> On 06/09/2012 22:12, Ben Campbell wrote:
>> But the text talks about the what the conference focus sends, not UAs
>> in_general_. If the focus includes anything_other_  than Message/CPIM
>> in accept-types, it better be prepared to receive it. Is that the intent?
>
> Ben, I see your point. The conference focus MUST only send an
> accept-types of Message/CPIM. There is no reason for using any other value.
>
> I will revise this text to make clear the intention.
>
> BR,
>
>       Miguel

-- 
Miguel A. Garcia
+34-91-339-3608
Ericsson Spain