[MMUSIC] Some issues with draft-ietf-mmusic-trickle-ice-sip-02

[Paul Kyzivat <[e-mail]>]

This is not a complete review of the document - I'm focusing only on the 
sdpfrag related stuff.

Section 8.1 says:

    A valid application/sdpfrag part could be obtained by starting with
    some valid SDP session description [I-D.ietf-mmusic-rfc4566bis] and
    deleting any number of lines.

Then section 8.2 says:

    The ABNF of an 'application/sdpfrag' body is based on
    [I-D.ietf-mmusic-rfc4566bis] with the following modification

       ; SDPfrag Syntax
              sdp-frag = *1proto-version
                         *1origin-field
                         *1session-name-field
                         *1information-field
                         *1uri-field
                         *1email-fields
                         *1phone-fields
                         *1connection-field
                         *1bandwidth-fields
                         *1time-fields
                         *1key-field
                         *1attribute-fields
                         *1media-descriptions

Those two sections are inconsistent. Some of the individual elements 
above (media-descriptions and time-fields) expand into more than one 
line. This leads to the possibility that you could take valid SDP body 
and delete some lines, leading to something that doesn't match the above 
syntax.

For example, media-descriptions allows a variety of other lines 
following an m= line. If you take a valid SDP and delete the m= lines 
you can get something that has a c= following an a=.

Similarly, time-fields only allows an r= line following a t= line. 
According to 8.1 you could take an SDP containing t=,r= and delete the 
t=, leaving a "naked" r= line. But the sdp-frag syntax in 8.2 can't 
generate that.

Beyond that, both of those introduce context-sensitivity. E.g., an a= 
following an m= has a different significance than one preceding the 
first m=. If you removed all the m= lines from an SDP the result would 
be incomprehensible. So the notion of stripping arbitrary lines from and 
SDP and making sense of the result is nonsense.

It does turn out that the definition of trickle-ice-sdpfrag is 
consistent with the syntax of sdp-frag. So there isn't a big problem. If 
you remove/replace the textual definition of sipfrag in 8.1 then things 
are consistent and make sense.

But is there any need for application/sdpfrag? Wouldn't it be more 
straightforward to define application/trickle-ice-sdpfrag?

	Thanks,
	Paul