[AVT] (no subject)

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

Hi

As a follow up from todays discussion just in order to "cast the iron while it's hot"

I can see that there are essentially two options regarding what constitutes a non-compound RTCP
 1) One RTCP / UDP
 2) RTCP that deviate from the SR/RR + SDES rules. 

1) is quite strict in that it only specifies one RTCP / UDP, quite a simple approach but may be limited in certain situations.
2) Essentially says that anything that is not SR+SDES(CNAME) or RR+SDES(CNAME)  is a non-compound RTCP. This makes it possible to stack several feedback messages on top of one another.

If one would follow 2) one could actually envision an "imaginary" rewrite of RFC4585 section 3.1 in line with the below: 

=====================
a) Minimal compound RTCP feedback packet 
...
......
.....
b) (Full) compound RTCP feedback packet
...
......
.....
(c) Non-compound RTCP feedback packet
A non-compound RTCP does not (generally) contain the items specified in a) for a Minimal compound RTCP. A Non-compound RTCP can be used to convey one or more feedback (or control) messages with the sole purpose to minimize the RTCP overhead associated with the RR, SR and SDES RTCP packet types. Although more than one feedback message can be conveyed in a non-compound RTCP it is still RECOMMENDED to keep the size of non-compound RTCP as small as possible with a preference for single feedback messages in non-compound RTCPs. 
=====================
More text is needed, esp. regarding enforcing RTCP of type a) above and also regarding the possiblility to do some minimal verification of middle box behavior using non-compound SR and R. 
The text above is just sort of an initial "bait". Version 1) above (1 RTCP/UDP) would lead to a more strict wording in the imaginary text above. 
Personally I am more in favor or version 2) above as it is more in line with how the feedback messages can be stacked in RFC4585 and in the codec control messages draft... with the disclaimer that single messages are preferred. 

Comments are very welcome.
/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
ECC: 850-43074
Mobile: +46 (0)730 783289
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