[sr-dev] rtpengine alt stream question

[Richard Fuchs]

On 10/02/14 02:32, Alex Balashov wrote:
> Hello,
> 
> (Sorry for re-posting this here from sr-users; in retrospect, it seemed
> more appropriate to do so.)
> 
> I recently encountered SDP attributes that look like this:
> 
>    Media Attribute (a): alt:1 3 : njfxofkg bavpjfxg 192.168.2.59 41978
>    Media Attribute (a): alt:2 2 : fpukfyaj dqmiarjx 192.168.111.1 41978
>    Media Attribute (a): alt:3 1 : euvrwenk jctmhavh 192.168.238.1 41978
> 
> I was not familiar with these. However, RFC 4796 says:
> 
>    alt:  the media stream is taken from the alternative source.  A
>    typical use case for this is an event where the ambient sound is
>    separated from the main sound.  The alternative audio stream could
>    be, for example, the sound of a jungle.  Another example is the
>    video of a conference room, while the main stream carries the
>    video of the speaker.  This is similar to the 'live' role in
>    H.239.
> 
> RFC 6064 http://tools.ietf.org/html/rfc6064#section-4.4 speaks to this a
> little as well.
> 
> This does not really make any clearer to me what these mean in this
> scenario, and unfortunately I cannot find out from the user.
> 
> My question is: I assume that rtpengine (both Kamailio and daemon-side)
> does not support this, correct? Are there any plans to in the future?
> Can anyone with more experience with video, perhaps, give some insight
> into the meaning and application of this attribute?

I didn't reply originally because I don't have much insight to offer,
other than that rtpengine doesn't support this at all and that RFC 4796
talks about an "a=content" attribute, not "a=alt". Older versions of the
ANAT RFC [1] talk about this attribute, but the final RFC [2] doesn't.

cheers

[1] https://tools.ietf.org/html/draft-ietf-mmusic-anat-02
[2] http://tools.ietf.org/html/rfc4091