[Sis-SCPS-INTEREST] SCPS data rate anomalies: please explain??

[Dan Miller]

Okay, now that I've gotten rid of the collisions, I can finally start 
evaluating this system.  I'm seeing several odd things that I'm hoping 
someone can clarify for me.  I started out with the rfile that I'll 
include at the end of this message.

My satellite connection is 3072000 bits/second in each direction. 
I have 550 msec RT delay (via an external hardware satellite simulator).
Theoretical max thruput thus is 384KB/sec (assuming no data overhead).
My target thruput is roughly 85% (~330KB/sec) based on past experience 
with other systems.

So originally I configured both gateways as shown below.  However, I 
wasn't seeing what I expected to see:

    * My data throughput with these settings was very low
    * Consistency is terrible!!  Data rates not only are low, they vary
      all over the place

I base my expectations on two measures: 
1. What do I see with the same hardware, no tcp acceleration, and no RT 
delay.  This gives me a measure of the capabilities of the linux 
drivers, our drivers, and our hardware.
2. We previously used a proprietary tcp acceleration package which we 
are trying to get away from. 

In both of these cases we see very high consistency; basically, 
variations in data rate are in the third significant digit across many 
runs.  Case 1 gives over 88% Max, case 2 gives 82% to 88% depending on 
configuration, but any given configuration is very consistent.

Anyway, to try to improve my throughput, I've been playing with BIF_RATE 
and BIF_BUF;
(all thruputs in KBytes/second)

BIF_RATE      BIF_BUF      min thruput      max thruput
========      ========      ========      =========
4096000         422400            170                  230
8000000         600000            270                  320
8000000         800000            290                  340

I had tried some intermediate rates and buffer sizes that I don't 
remember numbers for, but they were in between the results I show here.  
You can see that with my current settings I'm getting up to the rates 
that I'm expecting, but the variation suggests to me that I still have 
something wrong.  Also, the rate/buffers that I'm specifying have *no* 
correlation to real data, which is a problem if I need to configure for 
different settings...

Can anyone shed light on any of this??

Dan Miller


# GW Resource File - comments are allowed, starting with '#'.
#    Comments can be anywhere EXCEPT between parameter values.

# interface A info
AIF_NAME    eth0
AIF_RATE    100000000
AIF_BUF        32768
AIF_CC        1
AIF_TCPONLY    1
AIF_MTU     1500
#AIF_TAP_NAME    tap0

# interface B info
BIF_NAME     eth1
BIF_RATE     3072000
BIF_BUF     422400
BIF_CC         0
BIF_MINRTO     600000
#BIF_TAP_NAME    tap1


# IPFW info
C_DIVPORT    52000     
C_TAP_REMOTE_ACCESS 1