Subject: [AVT] RFC 4445 - MDI --- clarifications on sum(sj) and DF

From: "Jorge Canas" <[email-masked]>
Date: Mon, 19 Mar 2007 15:58:25 -0400

Hello,

Section 3.1 of RFC 4445 has the following text:

   To further detail the calculation of DF, consider a virtual buffer VB
   used to buffer received packets of a stream.  When a packet P(i)
   arrives during a calculation interval, compute two VB values,
   VB(i,pre) and VB(i,post), defined as:

   VB(i,pre) = sum (Sj) - MR * Ti; where j=1..i-1
   VB(i,post) = VB(i,pre) + Si

   where Sj is the media payload size of the jth packet, Ti is the
   relative time at which packet i arrives in the interval, and MR is
   the nominal media rate.

   VB(i,pre) is the Virtual Buffer size just before the arrival of P(i).
   VB(i,post) is the Virtual Buffer size just after the arrival of P(i).

   The initial condition of VB(0) = 0 is used at the beginning of each
   measurement interval.  A measurement interval is defined from just
   after the time of arrival of the last packet during a nominal period
   (typically 1 second) as mentioned above to the time just after the
   arrival of the last packet of the next nominal period.

   During a measurement interval, if k packets are received, then there
   are 2*k+1 VB values used in deriving VB(max) and VB(min).  After
   determining VB(max) and VB(min) from the 2k+1 VB samples, DF for the
   measurement interval is computed and displayed as:

   DF = [VB(max) - VB(min)]/ MR

My questions are:

- what is sum(Sj)?  Is it the sum of the payload of packets 1 through j?
- Can VB(pre or post) ever be less than zero? Is VB(pre) always negative 
after the first packet is received? or is the first packet "packet 0" ?
- If VB cannot be negative, then when is VB(min) != 0?   It seems to me that 
since VB(0) = 0, and VB must be reset at the beginning of each interval, 
VB(min) is always 0.

Thanks.

-Jorge

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