[Live-devel] SDP attribute extension

[Eric HEURTEL]

>> I have to deal with application-specific SDP attributes (a=my_attribute:value). I have my own subclass of MediaSubsession.
>> I was wondering if parsing of subsession description lines (c=, b=, a=rtpmap... ) could be done in a virtual new method (ie MediaSubsession::parseSDPline()) that could be overloaded, instead of being hardcoded in MediaSession::initializeWithSDP().
>> What do you think ?
> This is a possibility, but I consider changes like this - whose sole purpose is to support non-standard protocol extensions - to be low priority.
>
> What 'application-specific SDP attributes' do you have in mind?
We are targetting professional live audio for studio. For precise sync 
and low delay we have to carry information giving the relation between 
RTP timestamp and PTP timestamp (NTP timestamp carried in RTCP is not 
precise enough). I'll will not give details here, but we retrieve this 
constant delta through a new SDP "a=timesync:value" media-level 
attribute. It is a legal extension to the standard, not yet published.

You can also think to other extensions, like "a=orient:portrait" and the 
ones given in Section 6 of RFC456, or "a=cliprect:..." (Quicktime) or 
"a=crypto" (for RTP/SAVP RFC4568), or de-facto standard extensions, like 
"a=x-dimensions" that you've chosen to implement...

Ideally, it would be cumfortable to be able to extend "Media Level" and 
"Session Level" attributes through virtual methods.

Best wishes

E. HEURTEL
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