[AVTCORE] Comments on draft-ietf-avtcore-clksrc-03
Magnus Westerlund <magnus.westerlund@ericsson.com> Tue, 09 April 2013 09:27 UTC
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Date: Tue, 09 Apr 2013 11:27:37 +0200
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: [AVTCORE] Comments on draft-ietf-avtcore-clksrc-03
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Hi, I promised doing an review of the O/A section. I also found some other issues to consider. 1. Section 5.4: Figure 5 shows the ABNF [5] grammar for the SDP media clock source information. mediaclk-master = "a=ssrc:" integer SP clk-master-id clk-master-id = "mediaclk:master-id=" master-id timestamp-mediaclk = "a=mediaclk:" mediaclock mediaclock = sender / refclk / streamid / mediaclock-ext sender = "sender" sender-ext sender-ext = token refclk = "direct" [ "=" 1*DIGIT ] [rate] [direct-ext] rate = [ SP "rate=" integer "/" integer ] direct-ext = token streamid = "master-id=" master-id streamid =/ "IEEE1722=" avb-stream-id streamid =/ streamid-ext master-id = EUI48 avb-stream-id = EUI64 EUI48 = 5(2HEXDIG ":") 2HEXDIG EUI64 = 7(2HEXDIG ":") 2HEXDIG streamid-ext = token mediaclock-ext = token I wonder if not the mediaclock-ext construction is to restrictive. As the refclk produces a sting with white-spaces in it, why isn't other mediaclock-ext allowed the same freedom? I think the mediaclock-ext can be basically byte-string, i.e. any character allowed on an SDP line should be possible to use. Likely the only resteriction should be that it starts with a token. Thus using: mediaclock-ext = token [SP byte-string] This would require a token for identifying the method followed by a space and then full freedom for anything that is allowed on an SDP attribute value. 2. Section 6: Purpose of signalling? I think this type of signalling can select two levels of goals with signalling: 1) Declaring what each SDP O/A uses on its side 2) Negotiating to arrive at the best but available mechanism on both sides. The currently proposed solution does neither of these. It enables the offerer to express to declare I intended to use X and for the answer to say I know that, or simply say I can't tell you because we are not having the same clocks. I think we need to be clear what our goals are here. And I will propose some goals with the signalling based on my understanding of how these defined timestamp reference clocks and media stream clocks can be used. And we do need to be aware that different application may have different needs. Goal with signalling: 1. Enable each media sender to declare its actually used ts-refclk, i.e. what clock is base for the RTCP SR NTP timestamps 2. Enable each media sender to identify what clock reference is used to check/drive the media sampling, i.e. RTP timestamp advancement. 3. Enable two media sender using SDP O/A to determine what common clocks they actually have so they can be used in either of ts-refclk or mediaclk. The purpose of doing three (3) is really so that one can do the following. A. has three ts-refclk it can use. B has two ts-refclk available. They are going to watch social TV. To make it at all possible for the social TV service to play out content at A and B in sync they need to have either of these two things be available. a) Each has a common clock with the TV service b) A and B has a common clock In either of the scenario the TV service will be able to ensure that the media is played out is sync. As a technical solution to these issues there are two main solution tracks: 1) Change the respective SDP attributes to provide info on all possible clocks for that it could be using and let the answer select the most preferred that both are supporting 2) Leave the current SDP attributes as they are and only use them for declaration that: I use this! Then add new SDP attributes to negotiate the list of commonly supported clocks. At this stage I think 2) is what is simplest and require the least changes to what already exist. I will note that this is likely to require a second round of O/A messages to update used clocks if the arrived common clock(s) are not the default used ones. But, I don't see a way around these. In SIP one could use OPTIONS to provide a SDP which includes the negotiation attribute to enable the other side to select using a clock that is supported by both. But when that isn't done, it will take two rounds or other a'priori knowledge. So my proposal is simply that one create two new SDP attributes one for each type of clock usage (ts-refclk and mediaclk) and include a list of clock type and any ID of the clock in a priority order. The O/A is basic negotiated and the answer removes all clocks it doesn't support and adds all it supports. Hopefully there is someone in the list that is common. The highest priority between the two parties should be used when common clock is required. IF that is not, then these attributes should not be included. This for example allows one to use a common ts-refclk, but not require common mediaclk. We should also discuss if the answerer may change the priority of commonly supported, and if it should or should not add additional clocks it supports that are not common. I really appreciate feedback on these comments and ideas. 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-03 Magnus Westerlund
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Kevin Gross
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Magnus Westerlund
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Roni Even
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Kevin Gross
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Kevin Gross
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Roni Even
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Roni Even
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Magnus Westerlund
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Kevin Gross
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Roni Even
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Magnus Westerlund
- Re: [AVTCORE] Comments on draft-ietf-avtcore-clks… Kevin Gross