[xiph-rtp] Theora RTP payload format

Aaron Colwell acolwell at real.com
Mon Apr 18 08:51:00 PDT 2005

On Mon, Apr 18, 2005 at 11:30:26AM -0400, Steve Kann wrote:
> Hi, List,
>    I've been working on building an implementation of a 
> video-conferencing endpoint using Theora, and have been working with the 
> draft-kerr-avt-theora-rtp-00 spec.
>    I've also read the archives of this list, about some of the proposed 
> changes.   I'd like to describe here what I'm planning on doing, and see 
> how this might fit into your design.
>    Basically, what I'm working with is a project called "iaxclient".  
> iaxclient is a library for a VoIP softphone, which presently supports 
> only audio, but I am extending to support video as well.  It uses the 
> IAX2 protocol, which is a lightweight VoIP protocol that does _not_ use 
> RTP.  However,  the payload format for IAX2 is generally compatible with 
> the payload format for RTP.  Asterisk (the open-source PBX) includes 
> support for RTP-based VoIP protocols (SIP, H.323, etc), as well as 
> non-RTP-based VoIP protocols (IAX2, others).
>    There are basically two use cases for users making videoconferencing 
> calls using the application:
>     1) Point-to-Point calls:  This case seems to be pretty easy to 
> handle, and fits into most of the designs I've seen so far:
>    2) Multi-party conferences:  This is where some of the designs I've 
> seen so far seem to work well, and some of them do not.
>    The basic idea for multi-party conferences is that each user 
> maintains a virtual connection to a "conference engine"  (this is 
> already in place for audio conferences).  The conference engine 
> intelligently  receives audio from the clients and sends audio to the 
> clients, so each client can hear the audio of any other speaking 
> participants.
>    The idea for video is that the clients each send their video to the 
> conference engine, and the conference engine will send zero or one video 
> stream to each participant, in one of two "modes"
>       a) Automatic mode:   The conference engine will use some 
> heurestics to decide whose video should be shown to the participant -- 
> Generally, this will be the only participant who is presently speaking  
> (in the case of multiple active speakers, or zero active speakers, there 
> will be some secondary criteria).
>      b) Request mode:  The client itself will notify the conference 
> engine (perhaps out-of-band) and request to see a particular speaker's 
> video.
>    What this means for the video stream (and this works just fine for 
> any other video format, (i.e. h.26x, etc), is that we would like to be 
> able to change the video source at any time (or, at any keyframe at 
> least). 
> The whole setup headers business, of course, makes this design 
> particularly difficult.   With the present draft-kerr-avt-theora-rtp-00 
> format, though, I think I could probably (with a great deal of 
> unnecessary overhead), send the setup headers occassionally, and then 
> switch at any time.  The clients could then use "header caching", and, 
> if they've seen these headers before (matching CRC32), they could use 
> their cached copy, and if not, they'd just have to wait a few seconds to 
> get them before they could start decoding.
>    *Note:  I also suspect, but I haven't researched, that if all the 
> clients are using the same version of the theora encoder, and the same 
> settings, that their setup headers would likely be the same;  If this is 
> the case, then their CRC32's would be the same, and they could start 
> decoding at any keyframe..
> With the latest idea I've read, though, it makes this process much more 
> inconvenient, because _each_ client would have their own 16bit "chain 
> ID", and these chain ID's would be duplicated in the streams sent by 
> each client, and therefore the server would need to deeply understand 
> and parse each of the streams in order to put them together, etc.

What did you do in the case where the CRC32 was different from each of the
clients? This is basically the same scenario isn't it?

I know nothing about IAX2, but I would assume that it has some sort of 
offer/answer model to negotiate codec parameters and such. You could easily
put the chain ID in this negotiation so that all users in the conference use
the same codebook.

> I think that my use case isn't all that unusual though; it's somewhat 
> like the properties you might have in multicasting, I think.
> 1) It would be ideal if the RTP payload format could be made independent 
> of SDP.

It is currently independent of SDP if you use inline codebook transmission. The
info in the SDP just allows you to know ahead of time what the info and setup 
headers are going to be for each chain. It also provides a mechanism to grab
the codebooks ahead of time. You also can save bits if you don't want
to periodically transmit codebooks.

>     [the present theora rtp format exhibits this property; if you use 
> periodic inline
>        setup header transmission]
> 2) It would be ideal if the RTP payload format continued to allow inline 
> setup header
>    transmission.

To my knowledge we weren't going to get rid of inline transmission. I had 
always intended to keep it.

> It would be most convenient, if there were a "fixed setup" mode for 
> theora, where you could ask the theora-encoder to use fixed setup header 
> set, and have it act like other codecs in this respect.  I understand 
> the flexibility that the setup headers give you in encoder design, but 
> it would be nice if there were a way to configure it otherwise..

If the encoder allowed you to specify a codebook on initialization, you could
effectively do this. Basically your app could just always specify the same
codebook to the encoder and then sent the hash to the other participants. 
They would then verify that your hash matches the hash of their codebook and
then your done. This is basically the codebook cache hit scenario. If you get
a miss then you just make connection to the conference fail.


> -SteveK
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