[AVT] Re: Review requested: draft-ietf-avt-rtp-toffset-04.txt

Here is Colin's response to Dave's note. Again, the subject line was 
changed to correlate with the previous messages I sent on.

I do apologize for the inefficiency of all this. I've learned a few 
lessons for the next set of documents I shepherd through Last Call.

Tom

Colin Perkins wrote:
-----------------------

On 29 Jan 2007, at 10:28, Dave Singer wrote:
> At 11:41  +0000 26/01/07, Colin Perkins wrote:
>>
>> Qiaobing Xie sent the following comments on the toffset draft:
>
> Thanks, I missed this one.  Here are some proposed responses...
>
>>
>> 1. I agree with the use case analysis in the draft and agree in  
>> general this may be a useful extension.
>>
>> 2. I would like to see a more crisp definition of the  
>> "transmission timestamp" - what is it? Is it the time instance the  
>> first (or the last) octet of the packet is put on the wire by the  
>> physical interface at the RTP sender? Or is it the time instance  
>> the sending RTP stack passes the fully assembled packet into the  
>> out-going buffer? Or, is it the time instance the last media octet  
>> in the packet was assembled at the RTP stack? and so on.
>
> I really don't want to define it any more than the RTP  
> specification defines it.  This draft merely provides information  
> about it;  its definition is in 3550.

Agreed.

>> 3. The following statement in the draft:
>>
>>   "In the RTP specification [RFC3550] it is implied that packets are
>>    transmitted essentially in accordance with their RTP timestamps."
>>
>> is a little misleading since RFC3550 only says:
>>
>>   "The timestamp reflects the sampling instant of the first octet in
>>    the RTP data packet."
>>
>> I'd therefore suggest to say something like: "RFC 3550 does not  
>> specify a transmission timestamp."
>
> But this is missing the point.  It's true that 3550 does not have a  
> 'transmission timestamp' that is different from the media  
> timestamp; but jitter calculations etc. are all based on the  
> assumption that they are in 1-1 relationship.

Kind-of... The jitter calculation breaks if the transmission time
doesn't have a one-to-one mapping onto the sampling instant, but I
don't think 3550 mandates such a mapping.

...snip the rest, which seems reasonable...

-- 
Colin Perkins
http://csperkins.org/

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