Subject: LINK Draft: TCP Performance Characteristics Formulae

From: Janne Peisa <[email-masked]>
Date: Wed, 10 May 2000 19:59:40 +0300

Hi all,

I have some problems trying to reproduce the example in "Analysis of
Link Layer Effects on TCP Performance" using the formulae given in TCP
performance Characteristics section. The text in current draft reads

>    For MSS, use 1000 bytes (remove the 40 bytes for TCP/IP headers,
>    which do not contribute to performance).
> 
>    For RTT, use 120 msec (80 msec for the Internet part, plus 20 msec
>    each way for the new wireless link).
> 
>    For p, use .01.  For C, assume 1.
> 
>    The simple formula gives:
> 
>    BW = (1000 * 8 bits) / (.120 sec * sqrt(.01)) = 666 kbit/sec
> 
>    The more complex formula gives:
> 
>    BW = 402.9 kbit/sec

First, using 

>                      MSS   1
>            BW = 0.93 --- -------
>                      RTT sqrt(p)

and plugging in MSS = 1000 bytes, RTT = 0.12 seconds and p=0.01 gives BW
= 620 kbps, which is not exactly 666 kbps. The text mentions that "For
C, assume 1.", though, and replacing the 0.93 with 1 indeed results in
666 kbps, so this is not a big deal. However, it would probably be
better to use the formula given in the draft.

Second, using

> 
>                                          MSS
>            BW = --------------------------------------------------------
>                 RTT*sqrt(1.33*p) + RTO*p*[1+32*p^2]*min[1,3*sqrt(.75*p)]
> 

with the values given above and assuming that the RTO = 5*RTT as
suggested in the text, gives BW = 519.4 kbps, which is quite far from
the value calculated in the example (402.9). I checked that the formula
is the same as in [PFTK98], so it's probably not simply a typo. Did I
make a mistake in assuming RT0 = 5*RTT, or is the result in example just
wrong? 

These are just minor issues, but being able to reproduce the results in
the example would give link layer designers (read: me) some confidence
in estimating the maximal TCP throughput on real life links.

Best Regards,

-- 
Janne Peisa                       mailto:[email-masked]
NomadicLab, Ericsson Research     phone: +358 40 549 2873
Oy LM Ericsson Ab                 fax: +358 9 299 3247
02420 JORVAS
Finland

---

From: Jamshid Mahdavi <[email-masked]>
Date: 10 May 2000 14:38:23 -0700

Good catch.  I'll send some suggested fixes to Phil next week.

--Jamshid