Re: [xrblock] 答复: Comments on draft-ietf-xrblock-rtcp-xr-synchronization-00

Kevin Gross <kevin.gross@avanw.com> Tue, 06 November 2012 18:35 UTC

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Date: Tue, 06 Nov 2012 13:34:30 -0500
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From: Kevin Gross <kevin.gross@avanw.com>
To: Qin Wu <bill.wu@huawei.com>
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Cc: Colin Perkins <csp@csperkins.org>, "xrblock@ietf.org" <xrblock@ietf.org>
Subject: Re: [xrblock] 答复: Comments on draft-ietf-xrblock-rtcp-xr-synchronization-00
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Sounds promising. Can you suggest some revisions for the draft?

Kevin Gross
+1-303-447-0517
Media Network Consultant
AVA Networks - www.AVAnw.com <http://www.avanw.com/>, www.X192.org



On Tue, Nov 6, 2012 at 12:01 AM, Qin Wu <bill.wu@huawei.com> wrote:

>  Hi,****
>
> For a group of session, maybe we can pick the session with longest report
> interval as the reference session and choose the joining time by the
> participant for that reference session as starting point for measurement.*
> ***
>
> ** **
>
> Regards!****
>
> -Qin****
>
> *发件人:* xrblock-bounces@ietf.org [mailto:xrblock-bounces@ietf.org] *代表 *Colin
> Perkins
> *发送时间:* 2012年10月31日 19:07
> *收件人:* Kevin Gross
> *抄送:* xrblock@ietf.org
> *主题:* Re: [xrblock] Comments on
> draft-ietf-xrblock-rtcp-xr-synchronization-00****
>
> ** **
>
> On 30 Oct 2012, at 18:49, Kevin Gross wrote:****
>
> The accuracy requirement we've discussed for presentation times is 10
> microseconds or better. I apologize that I have not yet drafted an I-D I
> promised summarizing these requirements. Required range is on the order of
> several seconds, maybe up to a minute. We need most of the bits to the
> right of the decimal in the NTP timestamp but we don't need many to the
> left. The reason we included all 64 is because the 64-bit NTP timestamp is
> the smallest standard timestamp format that gives us the resolution
> required. A non-standard format as small as 40 bits would suffice but RTCP
> is not a particularly bandwidth constrained protocol and the standard
> format and 32-bit word alignment as proposed in the draft should simplify
> implementation.****
>
> ** **
>
> It looks like I've overlooked issues with Synchronization Delay and
> Synchronization Offset in my previous review.****
>
> ** **
>
> The Synchronization Delay definition needs to provide a more specific
> description of what's being referred to in RFC 6051 for defining the
> reference point for this measurement. Specifically the draft needs to
> define "beginning of session". I don't find such a concept in RFC 6051. RFC
> 6051 section 2.1.1 defines when sessions are joined but probably not in
> enough detail to know how to do a measurement referenced to this point.***
> *
>
>  ** **
>
> The definition in RFC 6051 Section 2.1.1 gives a join time for a single
> session for a sender, but this draft could define a similar metric for a
> receiver. In any case, that gives the join time for a single RTP session,
> but the initial synchronisation delay metric is likely for a group of RTP
> sessions, and so needs to be defined with reference to some specific RTP
> session.****
>
>
>
> ****
>
> The text in 4.2 of the delay draft, "from the RTCP packets received on all
> of the components RTP sessions to the beginning of session" might lead some
> implementers to compute a negative Synchronization Delay. Revers the from
> and to clauses in the text or add a formula to clarify.****
>
> ** **
>
> There seems to be a dimensional problem in the Synchronization Offset
> definition. Everything in the definition is in units of RTP clock but since
> we're using an NTP timestamp for the report, I assume this is supposed to
> define a quantity in units of NTP time.****
>
> ** **
>
> Kevin Gross****
>
> +1-303-447-0517****
>
> Media Network Consultant****
>
> AVA Networks - www.AVAnw.com <http://www.avanw.com/>, www.X192.org****
>
>
>
> ****
>
> On Mon, Oct 29, 2012 at 10:38 PM, Hitoshi Asaeda <asaeda@sfc.wide.ad.jp>
> wrote:****
>
> Hi Colin and Rachel,****
>
>
> >>>> Hi folks,
> >>>>
> >>>> I have 2 comments for draft-ietf-xrblock-rtcp-xr-synchronization:
> >>>>
> >>>> 1.       In RTP flows synchronization offset metric block, only one
> SSRC set to the SSRC of the reference RTP stream has been specified. IMO,
>  the SSRC of the reporting stream should be also required.
> >>>> 2.       Synchronization offset is a 64-bit unsigned fixed-point
> number. No indication shows which stream, the reporting stream or the
> reference stream,  is lag behind. So I propose to split one bit from
> "Reserved" field to indicate the offset direction.
> >>>
> >>> It would probably be easier to implement if you made the
> Synchronisation Offset a signed 64 bit value, rather than an unsigned 64
> bit value and a separate sign bit.
> >>>
> >> [Rachel]: The reason I proposed that is because the synchronization
> offset is represented using the timestamp format of NTP (a 64-bit unsigned
> fixed-point number), which I think it's better to keep it unchanged.  Are
> you suggesting to use a nonstandard format instead of the standard NTP
> format? But you're right that it may be easier for implementations to use a
> signed value to tell fast or slow from 2 streams.
> >
> >
> > Thinking about this more, I'm confused why you need a 64 bit NTP format
> timestamp here. With the addition of a sign bit, this choice allows you to
> represent synchronisation offsets up to 136 years in either direction with
> picosecond accuracy. That range seems unnecessary. What is the actual
> requirement?
> >
> > [Rachel]: The reason of using 64 bit NTP format timestamp is for
> accuracy, which has been discussed in the list before (
> http://www.ietf.org/mail-archive/web/xrblock/current/msg00504.html ). But
> I agree with you that an additional sign bit is unnecessary. Your proposal
> makes sense here.****
>
> Well, the reason we use a 64 bit NTP format timestamp is just its
> simplicity. While we don't need such long format, it'd be easier to
> reuse it for the implementation, I thought.
> But I also don't have no objection to make it signed.
>
> Regards,
> --
> Hitoshi Asaeda****
>
> _______________________________________________
> xrblock mailing list
> xrblock@ietf.org
> https://www.ietf.org/mailman/listinfo/xrblock****
>
> ** **
>
> ** **
>
> ** **
>
> -- ****
>
> Colin Perkins****
>
> http://csperkins.org/****
>
> ** **
>
> ** **
>
> ** **
>