Re: RE RE: [AVT] RTP - RTT

["Marshall Eubanks" <marshall.eubanks at gmail.com>]

On 9/14/07, Vikal Kumar Jain <vikalkj at slingmedia.com> wrote:
>
> I would like to use round trips but that is not possible in the scenario
> I mentioned below for the receiver (client).
>
> I may be able to do so by extending the RTCP headers in the scenario I
> mentioned below; I can't do that using the standard headers since there
> is no RTP data sent back from receiver to sender like in case of a video
> conferencing system.
>
> I am more interested in having a solution that exist and does not
> require me to extend any of the standard headers.

There is an old saying, you can't make bricks without straw.

All computers have a clock. Round trip times (RTT) do not require a
very accurate clock at all, as you are sending out a packet and
compare time stamps when you receive a response. That can be done even
if your clock is not set to the accurate time, has a rate error, etc.
All you need to do is to have some clock accuracy over a few hundred
milliseconds to a few seconds.

One way time requires that both ends have a clock, and that these
clocks be set to some time standard to better than the desired
accuracy. With NTP, you can set clocks to about a millisecond, so you
can do one way timing if everything is set up to about a millisecond.

If you have neither round trip nor a time standard, you will not be
able to do what you want. You are trying to make bricks without either
straw or clay.

Regards
Marshall


>
> Moreover, there may be no notion of a wall clock on many systems and I
> am not very sure of the complexities involved in using a NTP server to
> get NTP timestamps.
>
> Regards,
> Vikal.
>
>
>
> -----Original Message-----
> From: Marshall Eubanks [mailto:marshall.eubanks at gmail.com]
> Sent: Friday, September 14, 2007 5:05 PM
> To: Vikal Kumar Jain
> Cc: avt at ietf.org
> Subject: Re: [AVT] RTP - RTT
>
> Hello;
>
> On 9/14/07, Vikal Kumar Jain <vikalkj at slingmedia.com> wrote:
> >
> >
> >
> >
> > Hi all,
> >
> >
> >
> > I would like to know the ways to calculate RTT on a RTP receiver
> (client)
> > that does not send back packets to sender (Server) in RTP session.
> Since,
> > receiver does not send back any packets to the sender, the sender
> should not
> > include receiver report blocks in the SR RTCP packets and hence
> receiver
> > does not know the "delay on the sender since it received last RR" and
> can
> > not calculate RTT using the scheme mentioned in section "6.4.1 SR:
> Sender
> > Report RTCP Packet".
> >
> >
> >
> > How does a receiver that does not send back any packets back to the
> sender
> > calculate RTT? I will be obliged if some one could help me on the
> same.
> >
> >
> >
> >  I would like to add one more point.
> >
> >  RFC 3550 also mentions that NTP can be used to calculate RTT, but I
> think
> > it can be used to calculate only one side delay and multiplying it by
> 2 to
> > get RTT may not be       a great idea as the delay may be different.
> >
> >
>
> I may be missing something, but it seems that you want to calculate
> the round trip delay without actually having any round trips. Isn't
> doubling the one way delay (which of course may be inaccurate) the
> best you can do in that case ?
>
> Regards
> Marshall
>
> >
> > Regards,
> >
> > Vikal.
> >
> >
> > _______________________________________________
> > Audio/Video Transport Working Group
> > avt at ietf.org
> > https://www1.ietf.org/mailman/listinfo/avt
> >
> >
>

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