[SIPForum-techwg] Reliable provisional responses in SIPconnect 1.1

[Elwell, John]

At present there is some text in 15.2 concerning reliable provisional
responses. It indicates that this capability is optional on both sides.
It was suggested at the meeting that there was no need to mention
capabilities that are optional on both sides - there are many other SIP
capabilities that are optional but not mentioned in the document.

There was also an opposing view that suggested that RFC 3262 support
should be mandatory for IPPBXs. The reason given was that a service
provider wanting to provide early media would want to be sure that SDP
answer in a provisional response reaches the IP-PBX. However, this would
raise the bar for adoption by IP-PBXs. The mechanism is know to be
rather complex, and the need for it will be diminished or even removed
in the future as ICE and DTLS-SRTP get deployed (since in-band
mechanisms will achieve the desired result).

Furthermore, assuming TCP transport, the only benefit of reliable
provisional responses is when a SIP intermediary somehow "loses" the
SDP. Also the mechanism is not helpful if SIP intermediaries (B2BUA)
generate PRACK on a hop-by-hop basis, rather than ensuring it operates
end-to-end.

In my opinion we should not mandate reliable provisional responses and
we should not even mention the capability in this specification.

John