Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clksrc-06.txt

Magnus Westerlund <magnus.westerlund@ericsson.com> Wed, 11 September 2013 07:14 UTC

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Date: Wed, 11 Sep 2013 09:14:27 +0200
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clksrc-06.txt
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Hi,

Great to see a new version. I have checked this version against my
previous comments and believe that it cares of all except the below two
issues.

Section 5.2:

   The signalling optionally indicates a media clock offset value.  The
   offset indicates the RTP timestamp value at the epoch (time of
   origin) of the reference clock.  If no offset is signalled, the
   offset can be inferred at the receiver by examining RTCP sender
   reports which contain NTP and RTP timestamps which combined define a
   mapping.


> First, I am not certain that I understand the offset. So the indicated
> value is an RTP timestamp, i.e. the integer representation of the 32-bit
> value of the RTP timestamp. So the epoch time is dependent on the clock
> system, but the current epoch for an NTP reference clock would be 1 jan
> at 0:00 year 1900. Thus you need to take all the leap seconds etc into
> account to determine how many time has actually passed since then and
> then calculate how many wrapping of the timestamp this means. This seems
> error prone, at least without being more explicit of how to do it for
> the various reference clocks.

I still think I need an author response on this to determine if there is
an issue here.


> 5. Section 5.2 and Figure 7:
> 
>    A rate modifier may be specified.  The modifier is expressed as the
>    ratio of two integers and modifies the rate specified or implied by
>    the media description by this ratio.  If omitted, the rate is assumed
>    to be the exact rate specified or implied by the media format.  For
>    example, without a rate specification, the media clock for an 8 kHz
>    G.711 audio stream will advance exactly 8000 units for each second
>    advance in the reference clock from which it is derived.
> 
> and looking at this example in Figure 7:
> 
>           m=audio 5004 RTP/AVP 96
>           a=rtpmap:96 L24/44100/2
>           a=sendonly
>           a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
>           a=mediaclk:direct=963214424 rate=1000/1001
> 
> I realize that I don't understand what the rate modifier means. Without
> the rate modifier there would be 44100 RTP timestamp ticks per second of
> reference clock. Does the rate modifier multiply with the nominal rate
> so that this example means 441000*1000/1001 RTP TS ticks per second of
> reference clock?
> 
> Some clarification may be required here.

Also this I would like to receive some answer to.

Cheers

Magnus


On 2013-09-11 02:13, internet-drafts@ietf.org wrote:
> 
> A New Internet-Draft is available from the on-line Internet-Drafts directories.
>  This draft is a work item of the Audio/Video Transport Core Maintenance Working Group of the IETF.
> 
> 	Title           : RTP Clock Source Signalling
> 	Author(s)       : Aidan Williams
>                           Kevin Gross
>                           Ray van Brandenburg
>                           Hans Stokking
> 	Filename        : draft-ietf-avtcore-clksrc-06.txt
> 	Pages           : 28
> 	Date            : 2013-09-10
> 
> Abstract:
>    NTP format timestamps are used by several RTP protocols for
>    synchronisation and statistical measurements.  This memo specifies
>    SDP signalling identifying timestamp reference clock sources and SDP
>    signalling identifying the media clock sources in a multimedia
>    session.
> 
> 
> 
> The IETF datatracker status page for this draft is:
> https://datatracker.ietf.org/doc/draft-ietf-avtcore-clksrc
> 
> There's also a htmlized version available at:
> http://tools.ietf.org/html/draft-ietf-avtcore-clksrc-06
> 
> A diff from the previous version is available at:
> http://www.ietf.org/rfcdiff?url2=draft-ietf-avtcore-clksrc-06
> 
> 
> Please note that it may take a couple of minutes from the time of submission
> until the htmlized version and diff are available at tools.ietf.org.
> 
> Internet-Drafts are also available by anonymous FTP at:
> ftp://ftp.ietf.org/internet-drafts/
> 
> _______________________________________________
> Audio/Video Transport Core Maintenance
> avt@ietf.org
> https://www.ietf.org/mailman/listinfo/avt
> 
> 


-- 

Magnus Westerlund

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