[xiph-rtp] Theora RTP payload format

[Steve Kann]

Aaron Colwell wrote:

>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?
>  
>
In that case, the "setup header ident" of the payloads would be 
different, and the receiver would know that it needs to wait until it 
receives setup headers matching the CRC of these frames before decoding. 
So, the only time we could accidentally try to decode frames using an 
incorrect set of setup headers would be in the case of a CRC collision 
(P ~= 0).

If the payload format only includes a "chain ID", then the chances of 
two streams having the same "chain ID" when coming from different 
sources is pretty much P==1. So, the server that's doing the switching 
would need to actually muck with the payload in order to give each 
sender a different Chain ID, and then keep track of which was which, 
etc. It makes it impossible to just switch senders in the server without 
the server understanding the internals of the codec payload.

>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.
>  
>
Presently, it's pretty simple, where it allows negotiation of the codec, 
but not codec parameters. In practice, it hasn't been necessary to do 
that. In the future, it might need to be extended to do so.

But, consider that users will join and leave the conference at arbitrary 
times, so the conference engine can't know in advance all the codebooks 
that might be used.

Also, as you elude to below, there's no way to seed an encoder with a 
particular codebook (AFAIK).

>>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.
>  
>
I don't think so, I thought the latest proposal called for replacing the 
"setup ident" field (32 bits) with a "chain ID" field (16 bits or so), 
where the "chain ID" field would refer to a "chain-info" item in the SDP.

This would mean, that even for a RTP and SDP based conference 
application like mine, if a client joined the conference with a 
different codebook, then all the clients would need to re-fetch the SDP 
in order to identify the codebook that's needed.

But, I guess, I haven't seen (maybe it hasn't been written yet), how the 
inline codebook transfer would work.

In general, I don't think that my issues are unique to IAX2; Nor do I 
think that they are things that can't be made to work with whatever 
format you have. But the questions are 1) how complex will these 
implementations need to be, and 2) how will they perform.

Most video codecs have the property where a "switch" (which is basically 
what my conferencing application is), can "switch" between streams from 
different sources, at any keyframe, as long as the width, height, and 
framerates are the same (and in some cases even if they're not), without 
needing to negotiate with the receiver at all. This can make a switch 
fairly simple; It only needs to know, for each frame, whether it's a 
keyframe or not, and then treat the whole thing as opaque data.

Theora has already moved away from this goal a bunch with the whole 
codebook thing, but it would be nice to at least minimize the 
inconvenience of dealing with the codebooks as much as possible.

>>    [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.
>  
>

Would the format be the same as it is now (+- the setup header ident 
field)? Would there be some way outside of SDP to indicate which 
codebooks belonged to which "chain id?"

>  
>
>>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.
>  
>
Right. Something like this would allow for the most bit-efficient 
method, because we could (rarely, if ever) retransmit codebooks if we 
can control all the clients, and force them to use the same codebook.

One of the other things I'll need to do eventually is to "record" these 
conferences, into some container, and make that format 
forward-compatible; If all the clients use the same codebooks, that also 
makes things much simpler, because we could write this all out as one 
"chain".


-SteveK


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