Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clksrc-04.txt
Magnus Westerlund <magnus.westerlund@ericsson.com> Mon, 13 May 2013 08:45 UTC
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Date: Mon, 13 May 2013 10:45:19 +0200
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clksrc-04.txt
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Hi, I have reivewed the changes and I don't think Section 6.1 nor 6.2 is clear. Intro and the clarification of the relationship between media clock and reference clock is good. "Usage of reference clock and media clock signalling in offer/answer allows the offerer to declare the reference clock and media clock in use and allows the answerer to acknowledge that a connection according to these declarations will be successful or, in limited cases described below, specify a simplified synchronization mode." This implies that the rules also for reference clock handling is to be specified. That is not the case. There is no explanation what an Reference clock offer means and what the limiations on that offer is, nor what the answerer should send back. To my understanding something like this is needed. An offer MAY include multiple a=ts-refclk attributes, however if there is more than one attribute, they MUST be for multiple instances of the same clock type and reference type and be possible to use interchangeable. If an answerer supports and can access the proposed reference clock it SHALL include all reference clock attributes offered that it supports. If none are supported and accessible then the a=ts-refclk attribute SHALL NOT be included in the answer. Question: Should as alternative to no a=ts-refclk in the answer a=ts-refclk:private be allowed as indication that the answer will run on its clock? And in this case when it is not supported, are there no benefit in the answerer identifying the clock it will actually use, despite that a common clock sync was not achieved? Then I think it makes sense to follow this by: "While full negotiation of reference clock and media clock attributes is not directly supported in the clock source signalling defined in this document, negotiation MAY be accomplished using capabilities negotiation procedures defined in [RFC5939]." Then we have the use of SHOULD: An answer to an offer with direct-referenced media clock SHOULD include the same media clock and reference clock signalling in which case a connection is established using the specified synchronisation. Alternatively the answer MAY omit both signals or include only the reference clock specified in the offer. In this case, a connection with asynchronous media clock is established. What are the exceptions for this SHOULD? Using SHOULD allows for other cases of excluding it then specified. If there are a one or few exceptions, it is much better to write SHALL, except for ... At least make clear what the exceptions are and how to interpret when it not present. So what does it mean to the answerer to receive an OFFER with different Media clock and reference clock. How should it answer it. An answer to an offer with stream-referenced media clock signalling SHOULD include the same media clock specification. If the offer contains reference clock signalling, the answer SHOULD include the same reference clock specification. The answer MAY omit reference clock signalling. If reference clock signalling is not present in the answer, either due to not being present in the offer or due to being dropped in the answer, the stream may be established as rate synchronised but not time synchronised. I also think this above section mixes up reference clock signalling (a=ts-refclk) with a=mediaclk. I think it would better with a clearer full spec for just the reference clock, like my proposal above, then followed by the a=mediaclk rules, especially in relation to the ts-refclk. It might be also good to write more rules on what applies for the Offerer constructing the offer. 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] I-D Action: draft-ietf-avtcore-clksrc-0… internet-drafts
- Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clks… Magnus Westerlund
- Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clks… Magnus Westerlund
- Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clks… Kevin Gross
- Re: [AVTCORE] I-D Action: draft-ietf-avtcore-clks… Magnus Westerlund