Subject: [MMUSIC] Grouping framework, BUNDLE, and disabled m= lines

From: Roman Shpount <[email-masked]>
Date: Tue, 27 Nov 2018 19:34:12 -0500

Hi All,

I had a question related to grouping framework RFC 5888: Is it safe to
remove "mid" attribute from the disabled media stream?

Part of the problem is that RFC 3264 says that stream description MAY omit
all attributes in the refused media stream (
https://tools.ietf.org/html/rfc3264#section-8.2):

Existing media streams are removed by creating a new SDP with the port
number for that stream set to zero.  The stream description MAY omit all
attributes present previously, and MAY list just a single media format.

In https://tools.ietf.org/html/rfc5888#section-9.1 it says:

The "mid" attribute is an identifier for a particular media stream.
 Therefore, the "mid" value in the offer MUST be the same as the "mid"
value in the answer.
...
If a media stream that contained a particular "mid" identifier in the offer
contains a different identifier in the answer, the application ignores all
of the "mid" and "group" lines that might appear in the session description.

Furthermore,  it looks like no group will contain this specific "mid"
value, since based on https://tools.ietf.org/html/rfc5888#section-9.2:

SIP entities refuse media streams by setting the port to zero in the
corresponding "m" line. "a=group" lines MUST NOT contain
identification-tags that correspond to "m" lines with the port set to zero.

So, since the "mid" value in the refused media stream is not going to
be used for anything, is it allowed for the end point to omit the
"mid" in the answer or "mid" MUST be present even if media stream is
refused? Is absence of "mid" attribute in the answer the same thing as
different "mid" value?

Thank You,

_____________

Roman Shpount