[bmwg] RFC 2889, Appendix A

["Timmons C. Player" <timmons at monkeyplayground.net>]

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Howdy, all

So. I've been working through Appendix A of RFC 2889, and no offense to
anyone, but it's a mess.

* The formulas are incomplete.  The TXTIME formulas has no numerator.
(I'm guessing this is a known bug)

* The included example doesn't agree with the IBG formula

According to the RFC,

         [(100/LOAD - 1) * BURST * (IFG + 64 + 8*LENGTH)] + IFG
   IBG = -----------------------------------------------------------
                                  SPEED

Now, try the example:

   Example:

     LENGTH   = 64  bytes per frame
     LOAD     = 100 % offered load
     BURST    = 24  frames per burst
     SPEED    = 10  Mbits/sec (Ethernet)
     DURATION = 10  seconds test

you get IBG = IFG

I guess this makes sense for 100% load.  But given the answers in the
example, I think the load is supposed to be 50%.  However, even with 50%
load, the RFC formula gives IBG = 1622.4 microseconds which is off by 1
IFG from the example:

1622.4 micro - 9.6 micro = 1612.8 micro

This suggests that the example is using

        [(100/LOAD - 1) * BURST * (IFG + 64 + 8*LENGTH)]
   IBG = -----------------------------------------------------------
                                  SPEED

Anyway this leads me to the question:  What is the IBG supposed to be?

Does the IBG contain the IFG + extra?  Or is the IBG seperate from the IFG?

Put another way:  At 100% load is my IBG supposed to equal the IFG? (RFC
formula)
or does it equal 0? (RFC example)

Personally, I'm thinking that the IBG is really IFG + extra and that the
formulas  in the RFC should be:

         [(100/LOAD - 1) * BURST * (IFG + 64 + 8*LENGTH)] + IFG
   IBG = -----------------------------------------------------------
                                  SPEED

           BURST * (IFG + 64 + 8*LENGTH) - IFG
TXTIME = ------------------------------------------
                       SPEED



This means that for 50% load, the example values would have the
following answers:

IBG = 1622.4 micro (instead of 1612.8 micro)
TXTIME = 1603.2 micro
#OFBURSTS = 3101 (instead of 3110)

Or did I miss something?

* (NIT) The range for LOAD in the IBG equation should be written as (0,
100] since 0 makes the equation nonsensical, i.e. you can't divide by 0
(again, this is obvious, or at least should be)

* (NIT) Giving results in seconds seems silly when the Ethernet transmit
clock is +/- 100 ppm.  Bit times would at least be accurate.

Timmons
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