2004.09_Bittorrent-Understanding Distributed Downloading Techinques.pdf

KNOW HOW

BitTorrent

Distributed Load

When a Linux distribution starts to

become popular, and the number of

users starts to increase, the demand

for downloading can be expensive.

To meet the needs of the users, the

distributors need faster download

capabilities. This can cause a financial

problem that could end the project

early as the bandwidth fees start to

rise, at least for smaller distributors.

BY CARSTEN SCHNOBER

does require enormous amounts of

bandwidth to support the hordes

of data hungry users downloading the

latest version from the distributor’s

servers. Data transfer can be an issue – if

distributors fail to distribute the load.

Users wanting to download a large file,

such as a CD image, without a flat-rate

account will need a lot of patience, and a

substantial amount of money, if the

download freezes despite a nominally

quick Internet connection. Developers

who are really interested in distributing

their software to the Community will

want to avoid this scenario.

amounts of data – such as one or multi-

ple CD images – are the first “seeds” in

BitTorrent speak. The files are split up

into smaller packages. After download-

ing the first package, the BitTorrent

downloader sets itself up as a “peer” and

offers that package to other machines.

This is something like the role played by

a mirror server , although the machine

only has part of the complete download.

The next user wanting to download

the file can access either of these sources

– the seed or the peer – to download the

packages, thus becoming a peer itself.

After collecting all the required pack-

ages, and provided you do not terminate

the BitTorrent client at this point, your

machine automatically becomes a server,

offering the complete download without

needing to download anything itself. In

other words, your machine becomes a

seed.

This design allows the total bandwidth

to be distributed across the whole group

of users. The more people who are inter-

ested in a file, the more peers there will

be. When a sufficiently large pool of

peers and seeds has been established,

the originator can forget bandwidth

issues on the original seed, while “cus-

tomers” are happy about the quick

download speed. The number of peers

tends to be driven by the amount of

interest in an offering; thus the primary

source should not experience difficulty,

even though there might be a restricted

number of peers.

A so-called tracker coordinates the

whole process. A tracker is a server that

talks to peers, asking them what pack-

The Theory…

The BitTorrent concept can help both

sides. People who need to host large

GLOSSARY

Mirror server: To offload some of the work

from one server, mirror servers provide the

same content at a different location.

Python: A programming language for Linux

and other Unix-based systems, Windows,

OS/2, and Mac OS. As Python is a scripting lan-

guage, it does not require a compiler to trans-

late the program code into a

machine-readable format, but it does need

the Python interpreter to run the program.

Hash: A sequence of numbers generated from

a file by an irreversible mathematical algo-

rithm. The same input will always produce

the same hash code and different input will

always produce different hashes. Thus, a file

can be uniquely identified by inspecting its

hash code.

September 2004

www.linux-magazine.com

File Downloads with BitTorrent

P opularity is a good thing, but it

BitTorrent

KNOW HOW

to handle window-dressing. Suse calls

this package python-wxGTK , and Man-

drake refers to it as wxPythonGTK .

Next, save the Torrent file on your

hard disk and run the GUI-based BitTor-

rent-Client in the new BitTorrent-3.4.2

directory to start the download:

ing the script, simply specify the name of

a Torrent file to view the names and

sizes of the files available in that file:

python btshowmetainfo.py U

tettnang-binary-i386-iso.torrent

Bartering

Torrent offerings that become so wide-

spread as to have a free peer or seed for

each download request, allow requesting

clients to achieve very efficient down-

load speeds. However, if a peer receives

several download requests at the same

time, it needs a way of deciding which

request to handle first. The peer will

always select the candidate that offers

the most packages in exchange. Thus,

the first package for a new peer might

take a while to load, but the download

rate will increase with every additional

package it has on offer itself.

This technique prevents

clients from downloading

without offering anything in

exchange, which is a good

thing as it would effectively

prevent load balancing.

Things start to become more

complex if you try to down-

load from behind a firewall or

router that prevents you from

contacting the outside world.

You will be unable to offer

package downloads yourself,

and that means having to live

with low download speeds, if

there are more requests than offers.

If you are in a position to configure

your firewall, you can resolve this issue

by allowing incoming connections to

TCP ports 6881-6889 for BitTorrent. Oth-

erwise, peers that support external

access will not get to know their hidden

colleagues until they receive a request

from them, as peers automatically com-

municate their offerings when they talk.

So clients hidden behind firewalls may

still receive a positive response to their

download requests, although they will

remain less well-known and thus not

profit as quickly from the download

speeds more open peers can expect.

python btdownloadgui.py U

tettnang-binary-i386-iso.torrent

Figure 1: BitTorrent allows fast download speeds.

ages they have downloaded and are now

in a position to offer. Whenever a new

peer contacts the tracker, it receives a list

of active peers and seeds where it can

start to download.

In order to find the appropriate tracker,

a Torrent file containing critical informa-

tion such as the tracker’s Internet

address, the available files and their

sizes, and a Hash code for

the purpose of unique identi-

fication, exists for each

offering. As the tracker does

not support downloading

itself, it needs a permanent

Internet connection, but very

little bandwidth.

This example downloads the “Tettnang”

( Core 2 ) Fedora edition (see Figure 1).

Debian users can type instead:

btdownloadgui tettnang-binary- U

i386-iso.torrent

Point to the desired target directory to

start the download.

Besides the GUI-based client, BitTor-

…and Practice

A user needs the Torrent file

for a specific offering in order

to start downloading. These

files are available on Web

servers distributed through-

out the Internet, and easily

locatable via search engines or links.

Let’s suppose you are looking for the

Fedora Linux distribution, and you query

your favorite search engine for Fedora

and Torrent . This should quickly take

you to the right address, but if not, you

should check the BitTorrent homepage

[1] for more links.

Debian Sarge, or later, includes a

ready-to-run BitTorrent program pack-

age, which can be installed by entering

the apt-get install bittorrent command.

Users of other distributions can type tar

-xzf BitTorrent-3.4.2.tar.gz to unpack a

downloaded archive into their home

directories.

The BitTorrent client was written in

the Python programming language and

thus requires other resources. Besides

the basic Python package, the GUI client

needs the wxWidgets add-in for Python

rent also supports text-based downloads.

Use btdownloadcurses.py instead of

btdownloadgui.py to download in a text

window, or run the command line pro-

gram, btdownloadheadless.py .

The following syntax launches multi-

ple parallel downloads:

python btlaunchmanycurses.py U

~/torrents

This example uses all the Torrent files in

the ~/torrents/ directory, and launches

the downloads in a single text window

(see Figure 2). There is no GUI-support

for multiple downloads, although you

can run multiple clients at the same time

without any trouble.

The btshowmetainfo.py tool provides

you with additional information on the

content of a Torrent offering. When call-

■

INFO

[1] BitTorrent:

http://bitconjurer.org/BitTorrent/

www.linux-magazine.com September 2004

Figure 2: Multiple downloads in parallel in a text window.

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