[Users] RTP fixes for mediaproxy

[Jeff Williams]

Hi,

I was having a few issues with mediaproxy when it came to re-invites and
call setup where the SDP does not get sent with the first INVITE. I
noticed this was due a couple couple of assumptions in mediaproxy which
don't always hold:

1) That SDP is always set up first by the caller (i.e. in the INVITE)
then updated by the called party (i.e. in the 200). This is not the
case. The first SDP can be sent in the 200 response by the called party
and the caller send their SDP in the ACK.
2) That RTP streams don't change once set up. This is wrong for reINVITEs.

This would cause problems when a user agent sent a reINVITE to redirect
RTP to on hold music for example - the RTP change would be ignored. I
think this had an impact on some fax setups as well, but I haven't
tested it. I found a patch to fix this by storing the streams in a hash
by media type, but that is a limiting assumption also (maybe I want 2
audi streams?).

The attached patch fixes this by changing the following:

1) This pathc allows a mediaproxy RTPSession to be set up by either the
caller or called party. A 'caller' argument is passed to the RTPSession
functions telling it if the caller or called party is doing the setup or
update.

2) The official mediaproxy does not even look at SDP details for streams
which are already set up. This patch lets every SDP packet update all of
the RTP streams for that party (either caller or called).

If you have issues with one-way-audio with media proxy after reINVITEs,
have calls the first SDP from the called party or are just plain
adventurous, please give it a go. It has been used in a production
environment for a bit over a month now. The patch was made against
mediaproxy 1.7.2, but will work against 1.8.0 as well since none of the
rtphander.py code changed.

Regards,
Jeff
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