[AVT] Comments on draft-ietf-avt-tfrc-profile-01.txt

[Magnus Westerlund <magnus.westerlund at ericsson.com>]

Hi Ladan,

Here is some comments I have on the draft for the AVPCC profile.

1. I think we should consider to scrap all restrictions on RTCP in 
regards to needed bandwidth for feedback, and instead ensure that it is 
congestion controlled. If one congestion control RTCP then it is the 
situation in both data and feedback channel that determine the available 
bandwidth. If I remember TFRC correctly, it stipulates that the feedback 
is sent either every RTT or for each received packet, what ever is the 
longer time interval. With a congestion controlled RTCP there would be 
no problem with the feedback traffic even if much larger than 5% of the 
session bandwidth as long as it actually is available in the feedback 
channel. Also 5% of the session bandwidth is rather difficult to define 
when it is changing due to the congestion control.

2. On what is each TFRC congestion control state calculated over? With 
the current text, I get the impression that the TFRC calculation is done 
for each SSRC, rather than all traffic from A->B. I think the later is 
more appropriate. However it creates some problem, as one has to take 
multiple sequence number spaces into account. Here is also a excellent 
hook to include RTCP in these calculations. Independently what is the 
most appropriate, the feedback needs to be clarified.

3. the RTP header addition. I think the data header addition should only 
use 16 bits, not also another 16 padding as shown in the figure. For the 
RTP header 32 bit alignment is not a strict requirement. Unless you have 
some other information, like being explicit about the RTT. Although the 
Quad counter allows a receiver to decide how to calculate the loss rate, 
do it suffice to allow the receiver to determine when to send feedback 
packets?

4. You write that if TFRC-PS becomes reality it should be supported. How 
do you intend the different versions to be indicated and handled? I know 
this is part of the draft that is TBD, but do you have any ideas?

5. I would have expected that you include a normative reference to AVP, 
as you are extenending it.

6. I would also think that an open issue section is appropriate for this 
draft, so that readers get a picture of what remains to be done.

Cheers

Magnus

--

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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Ericsson AB                | Phone +46 8 4048287
Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: magnus.westerlund at ericsson.com


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