Re: [rtcweb] Why SDP answer needs and ack … was Re: SDP Offer/Answer draft-jennings-rtcweb-signaling

[Randell Jesup <[e-mail]>]

On 10/19/2011 8:43 PM, Cullen Jennings wrote:
>
> I guess the question I am thinking about is how important is this use case. I'm a bit of
 > the fence on the topic.  The primary use of INVITE with no offer is 
when doing a gateway
 > to some forms of  H.323. It seem the odds of a gateway from RTCWeb to 
SIP to H323 is
 > pretty low so I have not been thinking about how to deal with
 > INVITE with no
 > offer much.  I should think about it more but too few hours in the 
day :-)

The primary use of bare INVITE in SIP I've seen is for an SBC to force a 
renegotiation by the two sides:

A                        SBC                        B
| <-------------------- media --------------------> |
| <--- INVITE w/o SDP --- |                         |
| ----- 200 OK w/OFFER -> |                         |
|                         | ------ INVITE w/OFFER-> |
|                         | <--- 200 OK w/ANSWER -- |
| <---- ACK w/ANSWER ---- |                         |
|                         | -------- ACK ---------> |
| <--------------------- media -------------------> |

This can let the SBC modify things in the process (like move media
relaying to a new IP/port, change hold state), and other 3PCC things.


-- 
Randell Jesup
[e-mail]