SV: [Asterisk-video] sip/iax2 h.264 video?

[Gunnar Hellström]

You say: " the IAX2 protocol was designed to negotiate only one  
codec at a time.  With video, you need to negotiate two codecs in  
parallel"
When extending it, please make it handle at least three. There is work on
adding real-time text support to Asterisk, and that comes as a separate RTP
stream, as described in RFC 4103. 
It is very handy to have a text area available in any call for typing things
that may be hard get over by voice or video.

--

What are the thoughts about support for sdp variations for video parameters?
For H.264 I think it is well defined in the payload RFC for H.264.
But for H.263, three formats for sdp details occur. 
The one that is about to become an RFC is RFC2429-bis
(draft-ietf-avt-rfc2429-bis-09.txt )
The earlier ones are draft-koskelainen-sdp263-02.txt and 
draft-even-avt-h263-h261-options-00.txt

If support for only one of them is implemented, rfc2429-bis should be
chosen, because it is the one to become a standard.
But then we will have a risk to end up in coarse QCIF video after
negotiating with videophones that only support one of the older
specifications. 

I do not know any good resolution of this conflict. You cannot beforehand
know what format a device handles, so you make the Invite with a preferred
format that may not be understood by the Invited part. When answering, a
simple analysis of the syntax in the sdp parameters can reveal what
specification the sender follows. The answer can then be formatted to follow
the same specification. 

Gunnar


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[mailto:asterisk-video-bounces at lists.digium.com] För Mihai Balea
Ämne: Re: [Asterisk-video] sip/iax2 h.264 video?

>
> It's broken for all codecs?  That's not good.  My employers have had a
> change of heart and decided that maybe another codec like h.263  
> would be
> a safer choice, so I was just about to give that a try..
It's not a matter of which codec as much as a matter of codec  
negotiation.  the IAX2 protocol was designed to negotiate only one  
codec at a time.  With video, you need to negotiate two codecs in  
parallel (one for video and one for audio).  I posted more details  
about it on this list, you can look for it in the archives.

Bottom line is IAX2 needs changes to support video codec negotiation  
in parallel to audio.  There are several ways of doing it - I  
modified Asterisk in the simplest, "let's change as little as we can"  
way as possible.  I don't think it's the best way, but it works for  
us and I'm not going to change it until the community decides which  
way it wants to go.

> Thanks for all the work on iaxclient, it's at least making my life
> easier! :)
Glad you find it useful. Please let me know how it goes, we could use  
all the feedback we can get.

Mihai
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