MSRP chat: 'chatroom' attribute ABNF

I was doing some editorial fixes to the MSRP chat document, and then I 
went to validate the ABNF of the 'chatroom' attribute in SDP and the 
"Use-Nickname" header in MSRP.

When doing this, I noticed an open area, for which I would like to get 
input from the working group.

The current ABNF of the chatroom attribute is:

           attribute        /= chatroom-attr
           chatroom-attr     = chatroom-label ":" chat-token
                               *(SP chat-token)

The problem is that the syntax above always require a chat-token. So, it 
is not possible to write an SDP line without tokens:

         a=chatroom

However, the text has text that may lead to this case, for example:

    A conference focus that includes the 'nicknames' token in the session
    description is signaling that the MSRP switch supports and the chat
    room allows to use the procedures specified in Section 7.  A
    conference focus that includes the 'private-messages' in the SDP
    description is signaling that the MSRP switch supports and the chat
    room allows to use the procedures specified in Section 6.2.

So, one can derive that if a conference focus does not have support for 
nicknames nor for private messages, it should write an empty attribute: 
a=chatroom

The standalone a=chatroom signals that the MSRP switch understand this 
draft, in particular the convention to use Message/CPIM wrappers for 
sending regular messages.

So, I suggest to modify the ABNF to support a standalone attribute in the 
following way:

            attribute         =/ chatroom-attr
            chatroom-attr     = chatroom-label [":" chat-token
                                *(SP chat-token)]

This accommodates both standalone and non-standalone attributes, such as:

a=chatroom
a=chatroom:private-messages
a=chatroom:nickname private-messages

I also suggest to add a couple of paragraphs indicating that it is fine 
to include a standalone 'chatroom' attribute:

    An example of a 'chatroom' attribute for an MSRP media stream where
    the endpoint, e..g, an MSRP switch, does not allow neither nicknames
    nor private messages.

              a=chatroom

    The 'chatroom' attribute
    without further modifiers (e.g., tokens) indicates that the endpoint
    supports the procedures described in this document for transferring
    MSRP messages to/from a multi-party conference.


Does anyone have concerns with the above proposal?

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