Subject: [AVT] RTP session bandwidth

From: Magnus Westerlund <[email-masked]>
Date: Thu, 07 Mar 2002 14:52:57 +0100

Hi,

I have problems with the session bandwidth definition in 6.2 and how 
RTCP bandwidth relates to this.
I have been looking at draft-ietf-avt-rtp-new-11.txt.

 From 6.2:

  "For each session, it is assumed that the data traffic is subject to
   an aggregate limit called the "session bandwidth" to be divided among
   the participants."

This I interpret as that session bandwidth is all RTP traffic but not 
RTCP traffic.

Three paragraphs below:
"It is RECOMMENDED that the fraction of the session bandwidth allocated 
to RTCP be fixed at 5%."

Yet another paragraph:
  "A profile MAY specify that the control traffic bandwidth may be a
   separate parameter of the session rather than a strict percentage of
   the session bandwidth."

Next last paragraph:
  "This algorithm may be used for sessions in which all participants are
   allowed to send. In that case, the session bandwidth parameter is the
   product of the individual sender's bandwidth times the number of
   participants, and the RTCP bandwidth is 5% of that."

I think the second and third quote contradicts what is expressed in the 
definition of the session bandwidth. Based on these two the bandwidth 
for the control traffic is part of the session bandwidth, i.e. 95 % is 
RTP traffic and 5% is control traffic. Looking at the fourth quote the 
RTCP bandwidth is 5% of the RTP traffic, resulting that the total 
bandwidth i 105% of the RTP bandwidth.

This will also effect the interpretation of the SDP parameter b=AS: when 
using RTP.

Is session bandwidth inclusive or exclusive the RTCP control traffic?

Regards 

Magnus Westerlund 

Multimedia Technologies, Ericsson Research ERA/T/VA
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From: "Marshall Eubanks" <[email-masked]>
Date: Thu, 07 Mar 2002 09:04:18 -0500

On Thu, 07 Mar 2002 14:52:57 +0100
 Magnus Westerlund <[email-masked]>
> I have been looking at draft-ietf-avt-rtp-new-11.txt.

Well, I always understood it as RTP+RTCP traffic.

Regards
Marshall Eubanks

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From: Magnus Westerlund <[email-masked]>
Date: Thu, 07 Mar 2002 16:14:25 +0100

>Well, I always understood it as RTP+RTCP traffic.
I think that the total bandwidth including the RTCP control traffic is 
the reasonable assumption. This makes it possible to use the value for 
example QoS reservations. But I have problems interpreting the text in 
6.2, as saying that. I hope it is not to late to get a clearer text into 
the RTP spec. Colin and Steve, what are your opinions?

Cheers

Magnus

-- 

Magnus Westerlund

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From: Stephen Casner <[email-masked]>
Date: Mon, 18 Mar 2002 20:56:34 -0800 (PST)

Magnus,

> I have been looking at draft-ietf-avt-rtp-new-11.txt.

Thanks for your comments.

> This I interpret as that session bandwidth is all RTP traffic but not

That is what I/we intended.

> "It is RECOMMENDED that the fraction of the session bandwidth allocated

I agree that this one might be interpreted to mean RTCP bandwidth is
subtracted from the session bandwidth leaving 95% for the data.  What
I meant was:

   The amount of bandwidth RTCP gets to use is 5% of what the data
   gets to use.

That statement by itself does not say definitely whether the 5% is
subtracted from or added to the session bandwidth.  My intention was
that it should be added, but different readers may make different
interpretations.  I need to make this more clear.
							-- Steve