Re: [AVT] draft-schierl-avt-rtp-multi-session-transmission-00.txt

["Ingemar Johansson S" <ingemar.s.johansson at ericsson.com>]

Hi

I have a comment on draft-schierl-avt-rtp-multi-session-transmission-00.txt


=============
7.2.6.  RTP header extension

   The RTP header extension may be used to add generic signaling about
   Data Alignment to RTP packets.

7.2.6.1.  Identified problems

   RTP header extensions are required to be ancillary information which
   can safely be discarded by receivers which do not understand them.
   Data alignment mechanisms do not satisfy this requirement.
=============

My question is ..how serious is the identified problem ?. 
My gut feeling is that a receiver that implements e.g decoding of G.718 content will likely update the RTP stack to recognize header extensions if header extensions are used to carry e.g the NTP timestamp related to the RTP timetstamp in the RTP header. 
I see many benefits with using the RTP header extension for this purpose: 
1) It is relatively compact 
2) Easy to make generic (I would believe that the NTP is all info that is needed)
3) The clock-skew issue is easily solved as you don't need to schedule the transmission of the header extension like for RTCP-SR. For instance for a layered audio codec you typically add the header extension for all layers corresponding to the same exact time instant.

Regards
Ingemar
******************************************* 
Ingemar Johansson 
Senior Research Engineer, IETF "nethead" 
EAB/TVP - Multimedia Technologies 
Ericsson Research Ericsson AB 
Box 920 S-971 28 Luleå, Sweden 
Tel: +46 (0)8 4043042 
ECN: 850-43042 
Mobile: +46 (0)730 783289 
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