Problem found Re: [asterisk-users] Headaches with Video over SIP

[Steve Langstaff]

> -----Original Message-----
> From: asterisk-users-bounces at lists.digium.com 
> [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of 
> Peter Howard
> Sent: 14 November 2006 00:38
> To: Asterisk Users Mailing List - Non-Commercial Discussion
> Subject: Problem found Re: [asterisk-users] Headaches with 
> Video over SIP

> Found the problem.  Some other people on the list might be 
> interested in the problem.
> 
> Looking at the initial INVITE call from station 1 to asterisk 
> and from asterisk to station 2, you see the following discrepancy:
> 
> >From Polycom to asterisk:
> a=rtpmap:109 H264/90000
> a=rtpmap:96 H263-1998/90000
> 
> >From asterisk to other Polycom:
> a=rtpmap:103 h263-1998/90000
> a=rtpmap:99 H264/90000
> 
> The latter numbers match the numbers in rtp.c, which clearly 
> don't agree with the Polycom's opinion on life.  However the 
> other Polycom seems to accept asterisk's version, while the 
> first Polycom is still working on it's version.
> 
> I modified the initialisation of static_RTP_PT in rtp.c to 
> match the Polycom values and, hey presto!  I have video 
> working between the two stations.
> 
> However, this doesn't look like a "proper" solution, if there 
> are multiple opinions out there of what h263p and h264 (at 
> least) should be mapped to.  There's also the array 
> current_RTP_PT in rtp.c  What is that used for?  I would have 
> thought that either:
> 1) Asterisk should tell _both_ ends what mapping to use,
> 2) Asterisk should update it's own mapping based on what it's 
> told (though this would have to be on a call-by-call basis)

#2 is true (at least for audio, so I guess for video as well).

Codec identifiers >= 96 refer to dynamic payload types.

They have to be negotiated on each SDP offer/answer exchange.

So for the Polycom->Asterisk traffic, Asterisk should parse the SDP and
say to itself "Hey, the caller wants me to send it H264 marked with
payload type 109, and/or H263-1998 marked with payload type 96." and
adapt it's outgoing payload type marking accordingly.