Subject: [AVT] RTP - RTT

From: "Vikal Kumar Jain" <[email-masked]>
Date: Thu, 13 Sep 2007 22:57:36 -0700

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. 

Regards,

Vikal.

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: "Marshall Eubanks" <[email-masked]>
Date: Fri, 14 Sep 2007 07:34:49 -0400

Hello;

On 9/14/07, Vikal Kumar Jain <[email-masked]> 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.
>
>
>
>  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

---

From: "Vikal Kumar Jain" <[email-masked]>
Date: Fri, 14 Sep 2007 06:28:12 -0700

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. 

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:[email-masked]] 
Sent: Friday, September 14, 2007 5:05 PM
To: Vikal Kumar Jain
Cc: [email-masked]
Subject: Re: [AVT] RTP - RTT

Hello;

> I would like to know the ways to calculate RTT on a RTP receiver
(client)
> How does a receiver that does not send back any packets back to the
sender
>  RFC 3550 also mentions that NTP can be used to calculate RTT, but I
think

Regards
Marshall

---

From: "Marshall Eubanks" <[email-masked]>
Date: Fri, 14 Sep 2007 09:49:53 -0400

> 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

---

From: "Vikal Kumar Jain" <[email-masked]>
Date: Fri, 14 Sep 2007 07:05:46 -0700

> milliseconds to a few seconds.

As, I mentioned there are many embedded system that don't have RTC
(real-time clock) and If NTP is used to adjust the system clock (not a
RTC) that 
has it's own complexities I am not aware of. 
 
>able to do what you want. You are trying to make bricks without either

As I mentioned earlier it can still be done and would be more accurate
by extending RTCP headers by adding "delay since last RR" in the SR
report even if server does not get any packets back from the client. 

It is possible to calculate the RTT on the Sender (Server) using the
scheme described in RFC 3550 section "6.4.1 SR: Sender Report RTCP
Packet" even for the scenario I mentioned below in which receiver is not
sending any RTP packet back to server without any extensions.

Regards,
Vikal.
-----Original Message-----
From: Marshall Eubanks [mailto:[email-masked]] 
Sent: Friday, September 14, 2007 7:20 PM
To: Vikal Kumar Jain
Cc: [email-masked]
Subject: Re: RE RE: [AVT] RTP - RTT

Regards
Marshall

> > does not know the "delay on the sender since it received last RR"
and
> can
> > it can be used to calculate only one side delay and multiplying it
by
> 2 to

---

From: Colin Perkins <[email-masked]>
Date: Fri, 14 Sep 2007 15:08:47 +0100

> require me to extend any of the standard headers.

Do the reports in RFC 3611 section 4.4 help?

> am not very sure of the complexities involved in using a NTP server to

There's no requirement to use an NTP server - see the discussion on  
NTP timestamps in RFC 3550.

Colin

---

From: Dave Singer <[email-masked]>
Date: Fri, 14 Sep 2007 17:29:55 +0200

If there is no information flowing back, then not only is the 
round-trip time without meaning, even the one-way time is unknowable 
in the absence of synchronized clocks.  And even with the 
synchronized clocks, only the recipient can know it, since (by 
definition of the problem) there is no back traffic to carry the 
knowledge back to the sender.

I think the problem is either mis-stated or strange...

-- 
David Singer
Apple/QuickTime

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From: "Vikal Kumar Jain" <[email-masked]>
Date: Fri, 14 Sep 2007 08:39:29 -0700

There is no data/information flowing back on RTP session but RR are reported back on RTCP session. 

________________________________

From: Dave Singer [mailto:[email-masked]]
Sent: Fri 9/14/2007 8:29 AM
To: Marshall Eubanks; Vikal Kumar Jain
Cc: [email-masked]
Subject: Re: [AVT] RTP - RTT

--
David Singer
Apple/QuickTime