[AVTCORE] Comments on draft-ietf-avtcore-clksrc-01
Magnus Westerlund <magnus.westerlund@ericsson.com> Fri, 09 November 2012 11:40 UTC
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Date: Fri, 09 Nov 2012 06:40:31 -0500
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: [AVTCORE] Comments on draft-ietf-avtcore-clksrc-01
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Hi, I have review the Clock Source document draft-ietf-avtcore-clksrc-01 and have the following comments: 1. General comment on all the use of hanging lists. Please put in some separator characters like ":" between the label and the bullet text. 2. Section 3. media stream Yes, it is made clear above that this is SDP terminology. But as "media stream" is also used for the sequnce of packets comming from an SSRC in RTP I think it is prudent to call this type of media stream "SDP media stream" to avoid confusion in the reader. 3. Section 4.6 This is really a question of how traceable clocks works. I know in Sweden we have "Swedish Time" which is produced by a governmental body using a number of time production centers and time sources. Several of the time sources are also used to help produce UTC. Commonly the difference between Swedish time and UTC will be minimal, i.e on the nano second level. However, in the case Sweden would be isolated from UTC users of Swedish time will be able to use a common time-base. This is clearly tracable, but how do you deal with the fact that there are different sources of traceable time. For example, my home computer's clocks runs of the Swedish strata one NTP servers, but I think it is traceable also. 4. Section 4.8: clksrc = ntp / ptp / gps / gal / local / private I think this should ad a syntax element that make clear the syntax of a future extension shall have. An example: clksrc = ntp / ptp / gps / gal / local / private / clksrc-ext clksrc-ext = token ; token as in SDP The similar extensibility I think should exist in the following rules: ptp-version ptp-server 5. Section 4.8: hostname = *( domainlabel "." ) toplabel [ "." ] Where is this rule taken from. I try to remember why you need a hostname to end on a ".". 6. Section 4.8 port = 1*DIGIT I would recommend that we restrict the port number to 5 digits maximum length, i.e. port = 1*5DIGIT Or do you see any need for totally unlimited number of digits? 7. I wonder if it should be more explicit that the a=clksrc attribute actually are defined to usage at the source (a=SSRC) level in the definition text by including a mention of source-specific and a ref to RFC 5576? 8. Section 5.3: In a given system, master clock identifiers must be unique. Such identifiers MAY be manually configured, however 17 octet string identifiers SHOULD be generated according to the "short-term persistent RTCP CNAME" algorithm as described in RFC6222 [7]. I can see that a short term persistent is sufficient, but aren't it desired to actually use a long term persistent identifier for clock sources so that one can track their usage over different RTP sessions? 9. Section 5.4: mediaclock = refclk / streamid / sender I think the above ABNF syntax rules shall include a syntax definition for future extensions of the attribute. 10. Section 4 and 5: The SDP attributes are lacking clear rules for interpretation in a SDP Offer/Answer context and thus also the declarative usage should be contrasted and clear. But, I think this document is shaping up nicely and the authors are doing an excellent work with it. So lets get these last things settled so that we can last call it. Cheers Magnus Westerlund ---------------------------------------------------------------------- Multimedia Technologies, Ericsson Research EAB/TVM ---------------------------------------------------------------------- Ericsson AB | Phone +46 10 7148287 Färögatan 6 | Mobile +46 73 0949079 SE-164 80 Stockholm, Sweden| mailto: magnus.westerlund@ericsson.com ----------------------------------------------------------------------
- [AVTCORE] Comments on draft-ietf-avtcore-clksrc-01 Magnus Westerlund
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Kevin Gross