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 Windows Modem Optimization Guide
==================================

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Authors: R a v e N and GoMoRRaH.
Date of Release: 10/12/1999.

Contact info
------------
R a v e N:  barakirs@netvision.net.il.

For comments about this tutorial and suggestions for future versions of it,
Email R a v e N.
For questions, go to blacksun.box.sk, find the message board and post your
questions there. DO NOT Email us directly with questions!


Introduction
============
Welcome to the exciting world of modem optimization. What? Why would you want
to "optimize" your modem? I'll tell you why!

Windows comes configured for optimal performance on LAN connections.
This means that although your modem will work perfectly fine, it will not run
with maximum efficiency, and will waste time on actions which can be optimized
or even skipped upon.

These settings aren't exactly so easy to change. You can't just click on
'control panel' and change them. You'll have to venture into the registry or
install external applications to change these. But believe me - it's worth it.
You'll get to optimize your modem and learn something new about networking.

So, are you ready to make your modem faster without reaching for your pocket?
Well, I certainly hope you are.

Note: some of the information in this tutorial was taken from the Internet
Boost '99 readme file.

Stuff You Need to Know
======================
Data on the internet travels in what is called 'packets'. Each TCP (Transfer
Control Protocol) packet has a 40 bytes header, whcih contains the source
address, destination address, routing information etc', and the data file,
which contains the actual information that is being transferred. Unless both
the sender and the receiver are on the same subnet (see
blacksun.box.sk/subnet.txt), the packet will have to be routed through
several hosts called 'routers'. Also, two packets with the same sender and the
same destination won't necessarily travel through the same route.

MTU
===
Yup, that's the first value we need to optimize. Each host has an MTU value.
MTU stands for Maximum Transmission Unit. When two computers begin a TCP
session so one of them could send a packet or several packets to the other,
they agree on a Maximum Transfer Unit value. How they do that? Well, each host
takes it's MTU value and compares it with the other host's value. If both
values are identical, then the two hosts will agree on this value. If the two
values are different, the lower value will be used for this session. The MTU
is the maximum number of bytes a packet can contain (including the header, of
course). The default MTU on most systems is 1500, which is great for LANs
(Local Area Networks) but is bad for Dial-Up access. I'll explain:

Suppose you agree on an MTU of 1500. You send this 1,500 bytes data packet on
the Internet, but on the way, it travels through some routers that have an MTU
of 576. These routers will have to divide your 1,500 packet into three 576
packets, each one with it's own header. Since 1,500 is not evenly divisible
with 576, the last packet will contain the last fragment of the original 1,500
packet. And since each packet must be acknowledged by the receiver, this would
take more time. If you don't understand, read this whole tutorial over again
until you do.

Now, we want to set our MTU to 576, right? But how do we do that?

Tuning your MTU
+++++++++++++++
There are two ways to change the MTU value:
a) Play around with your registry. See regedit.com for information on how to
optimize your networking configurations.
b) Get a program called Internet Boost. A search on altavista.com with the
keywords 'internet boost' found hundreds of download sites within seconds.

Path Maximum Transmission Unit (PMTU)
=====================================
Auto Detection
--------------
Auto Detection is Enabled by Default. This option enables the system to
automatically determine the Maximum Transmission Unit (MTU) value and detect
routers which do not return appropriate ICMP messages under some conditions.
Enabling this setting causes TCP to attempt to discover the Maximum
Transmission Unit (MTU or largest packet size) over the path to a remote
host. By discovering the Path MTU and limiting TCP segments to this size, TCP
is supposed to be able to eliminate fragmentation at routers along the path
that connect networks with different MTUs. This requires implementation of the
corresponding server side algorithm, however, and presupposes all of the
servers on the entire Internet only running MSFT server software with
server-side optimization features accessible from MSFT web-browser client
software. However, it is not recommended to disable this setting as it would
then cause an MTU of 576 bytes to be used for all connections that are not to
machines on the local subnet. Disabling this setting can cause severe
performance degradation because fragmentation may not be compensated for.

Black Hole Detection
--------------------
Black Hole Detection is Disabled by default. Enabling this option forces the
system to try to detect black hole routers. Black hole routers are routers
that do not send back ICMP (Internet Control Message Protocol)
fragmentation-needed messages while automatically determining the optimal MTU
value. This increases the maximum number of retransmissions performed for a
given segment. Setting this parameter when it is not needed can cause
performance degradation. So it is not recommended that you enable this option
unless you understand the implications of doing so.

Receive Window (RWIN)
=====================
When a TCP connection is being established, the sender and requester
negotiate the maximum data segment size that will be accepted by both hosts.
This value, called the Maximum Segment Size (MSS) is different from the MTU
because it only addresses the maximum size of the data portion of the packet,
and does not include the header. If each data segment, along with the header,
does not exceed the smallest MTU as it is being routed to it's destination,
then it will not be fragmented. Therefore, because the size of a TCP header is
40 bytes, the MSS is calculated as MTU minus 40. In turn, the MSS value is
useful in determining the size of the receive window (RWIN), which is the
maximum number of bytes of data that can be buffered, pending the receipt of
an acknowledgment packet back from the sender. Typically, the RWIN value
should be a multiple of 2-10 times the MSS value. If you don't know which one
to choose, just go for 4 times the MSS value. For example, a good RWIN value
for a system with an MTU of 576 (and an MSS of 536) would be 2144 (536 * 4).
The Receive Window (RWIN) value is the maximum number of bytes of data that
can be buffered, pending the receipt of an acknowledgment packet back from the
sender. Typically, the RWIN value should be a multiple of 2-10 times the
maximum segment size (MTU-40).

Usually, the default value would be 8192. This sets the RWIN value to the
Windows default. Under Windows, this value is 8192, which should not be used
(because it is not evenly divisible by the default maximum segment size, it
can force the packet to be resent repeatedly when the buffer is full). You can
improve performance by setting this to a lower value such as 4 times your MSS
(MSS = MTU - 40), by either playing with your registry (again, see
regedit.com) or using Internet Boost '99.

Time To Live (TTL)
==================
The Internet is a vast network. Data packets usually go through a lot of
routers before they reach their final destination. But what happens if one of
the routers screws up, and the packet goes wandering around the net forever?
We don't want THAT to happen, now do we?

This is where the TTL value comes into the picture. Each TCP packet has a TTL
value in it's header, which is decremented by one whenever it passes through a
router. When it reaches zero, the packet dies, and an error message is sent
back to the sender, which will send the packet again, only this time with a
higher TTL.

On Windows 95 and NT 4.0 (don't know about 98 or any newer versions), this
value is set to 32. This is bad, because the Internet has grown to be a huge
place since 1995, and today, it is likely that a data packet would go through
more than 32 hosts on the way. This means that you will have to resend your
packets many times. Of course, your computer can learn that if in the first
time it sent a packet on a TCP session it had to increase it's TTL value, it
should use the same TTL value for all the rest of the packets, but still -
this packet loss could get quite annoying.

Again, changing this value is the same as changing all the
previously-mentioned ones.




Well, looks like that's it. You optimized your dial-up connection under Windows
and you're finished just in time to watch the Teletubbies.   :-)

Interested in learning more about networking, Unix, security and hacking,
cracking, phreaking (phone hacking), hardware and encryption? Point your
browser to http://blacksun.box.sk and surf on!