[SIPForum-techwg] Proposed new text for SIPconnect 1.1 section15.2 (Ringback tone and early media)

[Elwell, John]

I was actioned to propose new text here. Those at the meeting will
recall that there were problems with the present text on multiple
provisional responses and with the table.

The following is a replacement for the whole of 15.2, up to and
including the table. Note that I think the table is redundant -
behaviour is adequately prescribed by the text below. 

Concerning the text below the table, much of this is to do with reliable
provisional responses, which will be the subject of a separate thread.

Proposed text:

"The delivery of in-band announcements and call progress tones to a
caller before a call is answered is achieved through early media.

When acting as a call originator, the IP-PBX, upon receipt of a 180
provisional response message (reliable or unreliable) MUST instruct the
Media Endpoint to play local ringback tone to the user.
Upon receipt of SDP in any 18x provisional response message (reliable or
unreliable), the IP-PBX MUST forward this information to the Media
Endpoint.

A Media Endpoint, on receipt of an instruction to play local ringback
tone, MUST do so until it receives valid RTP packets or is instructed by
the IP-PBX that the call has been answered. On receipt of valid RTP
packets, a Media Endpoint MUST disable any local ringback tone and play
the received media. A Media Endpoint, on receipt of information
concerning received SDP, MAY use the information to determine whether
RTP packets received are valid and MAY discard RTP packets arriving
before that time."

I think this is a considerable simplification. Further points to
consider.
1. This assumes that a 181, 182, 183 arriving after a 180 does not cause
local ringback to be disabled (unless RTP is received).
2. This assumes that if RTP is already being received, a 180 will not
result in local ringback tone.
3. The text should still work if, in the future, RTP is preceded by DTLS
or ICE.
4. At present we have a single sentence below the table covering early
media in the other direction (for incoming calls to the IP-PBX).
However, it does not specify how the Service Provider should behave. The
text proposal above could, with minor adaptation, be used to specify
service provider behaviour.

Comments?

John