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Seminar Report: Bit Torrent | 7

Bit Torrent
peer-to-peer file sharing protocol

Introduction

BitTorrent is a peer-to-peer file sharing (P2P) communications protocol.
BitTorrent is a method of distributing large amounts of data widely without the
original distributor incurring the entire costs of hardware, hosting and bandwidth
resources. Instead, when data is distributed using the BitTorrent protocol,
recipients each supply data to newer recipients, reducing the cost and burden on
any given individual source, providing redundancy against system problems, and
reducing dependence upon the original distributor. The protocol was designed in
April 2001, implemented and first released 2 July 2001 by programmer Bram
Cohen, and is now maintained by BitTorrent, Inc. Usage of the protocol accounts
for significant traffic on the Internet, but the precise amount has proven difficult to
measure. There are numerous compatible BitTorrent clients, written in a variety of
programming languages, and running on a variety of computing platforms.

Gaurav Srivastava 0600710033


Operation

A BitTorrent client is any program which implements the BitTorrent protocol.
Each client is capable of preparing, requesting, and transmitting any type of
Computer file over a network, using the protocol. A peer is any computer running
an instance of a client. To share a file or group of files, a peer first creates a
"torrent." This is a small file which contains metadata about the files to be shared,
and about the tracker, the computer that coordinates the file distribution. Peers that
want to download the file first obtain a torrent file for it, and connect to the
specified tracker which tells them from which other peers to download the pieces
of the file.

Though both ultimately transfer files over a network, a BitTorrent download
differs from a classic full-file HTTP request in several fundamental ways:
BitTorrent makes many small P2P requests over different TCP sockets, while web-
browsers typically make a single HTTP GET request over a single TCP socket.
BitTorrent downloads in a random or "rarest-first" approach that ensures high
availability, while HTTP downloads in a contiguous manner. Taken together,
BitTorrent achieves much lower cost, much higher redundancy, and much greater
resistance to abuse or "flash crowds" than a regular HTTP server. However, this



protection comes at a cost: downloads take time to ramp up to full speed because
these many peer connections take time to establish, and it takes time for a node to
get sufficient data to become an effective uploader. As such, a typical BitTorrent
download will gradually ramp up to very high speeds, and then slowly ramp back
down toward the end of the download.

This contrasts with an HTTP server that, while more vulnerable to overload and
abuse, ramps up to full speed very quickly and maintains this speed throughout. In
general, BitTorrent's non-contiguous download methods prevented it from
supporting "progressive downloads" or "streaming playback". But recent
comments by Bram Cohen and new developments by Red Swoosh suggest that
streaming torrent downloads will soon be commonplace.



Creating and publishing torrents

The peer distributing a data file treats it as a number of identically-sized pieces,
typically between 64 kB and 1 MB each. A piece size of greater than 512 kB will
reduce the size of a torrent file for a very large payload, but is claimed to reduce
the efficiency of the protocol . The peer creates a checksum for each piece, using a
hashing algorithm, and records it in the torrent file. When another peer later
receives that piece, its checksum is compared to the recorded checksum to test that
it is error-free Peers that provide a complete file are called seeders, and the peer
providing the initial copy is called the initial seeder. The exact information
contained in the torrent file depends on the version of the BitTorrent protocol. By
convention, the name of a torrent file has the suffix .torrent. Torrent files contain
an "announce" section, which specifies the URL of the tracker, and an "info"
section which contains (suggested) names for the files, their lengths, the piece
length used, and a SHA-1 hash code for each piece, which clients should use to
verify the integrity of the data they receive.

Completed torrent files are typically published on websites or elsewhere, and
registered with a tracker. The tracker maintains lists of the clients currently



participating in the torrent. Alternatively, in a trackerless system (decentralized
tracking) every peer acts as a tracker. This is implemented by the BitTorrent,
μTorrent, BitComet and KTorrent clients through the distributed hash table (DHT)
method. Azureus also supports a trackerless method which is incompatible (as of

April 2007) with the DHT offered by all other supporting clients. In November
2006, BitTorrent Inc. introduced its "Publish Torrent" service which creates and
hosts a torrent file (seeded from an existing web-hosted media file), and tracks
downloads. The service (http://www.bittorrent.com/publish) requires a client
which supports web-seeding (currently only the official client, Azureus, and
μTorrent).



Downloading torrents and sharing files

Users browse the web to find a torrent of interest, download it, and open it with a
BitTorrent client. The client connects to the tracker(s) specified in the torrent file,
from which it receives a list of peers currently transferring pieces of the file(s)
specified in the torrent. The client connects to those peers to obtain the various
pieces. Such a group of peers connected to each other to share a torrent is called a
swarm. If the swarm contains only the initial seeder, the client connects directly to
it and begins to request pieces. As peers enter the swarm, they begin to trade pieces
with one another, instead of downloading directly from the seeder.

Clients incorporate mechanisms to optimize their download and upload rates; for
example they download pieces in a random order, to increase the opportunity to
exchange data, which is only possible if two peers have different pieces of the file.

The effectiveness of this data exchange depends largely on the policies that clients
use to determine to whom to send data. Clients may prefer to send data to peers



that send data back to them (a tit for tat scheme), which encourages fair trading.
But strict policies often result in suboptimal situations, where newly joined peers
are unable to receive any data (because they don't have any pieces yet to trade
themselves) and two peers with a good connection between them do not exchange
data simply because neither of them wants to take the initiative. To counter these
effects, the official BitTorrent client program uses a mechanism called “optimistic
unchoking,” where the client reserves a portion of its available bandwidth for
sending pieces to random peers (not necessarily known-good partners, so called
preferred peers), in hopes of discovering even better partners and to ensure that
newcomers get a chance to join the swarm.



Adoption of BitTorrent

A growing number of individuals and organizations are using BitTorrent to
distribute their own or licensed material. The most notable of these is BitTorrent
Inc. which has amassed a number of licenses from Hollywood studios for
distributing popular content at the company's website. Independent adopters report
that without using BitTorrent technology, with its dramatically reduced demands
on networking hardware and bandwidth, they could not afford to distribute their
files.

Many sites imitating big BitTorrent trackers have turned to distribution of only
legal material in general. With several different categories they often provide a
working distribution method for parties who want to have big material packets
available for large audience without investing large amounts of money. The
Amazon S3 "Simple Storage Service" is a scalable Internet-based storage service
with a simple web service interface, equipped with built-in BitTorrent support.
BitTorrent is often used to distribute free and open source software, especially
ISO images of Linux distributions. Most Linux distributions offer BitTorrent
as one of the download methods for installation CDs. Sub Pop Records, the record
label credited with popularizing grunge rock and current publisher of The Shins,
The Postal Service and other popular bands, releases tracks and videos via
BitTorrent Inc. to distribute its 1000+ albums. The band Ween uses the website
Browntracker.net to distribute free audio and video recordings of live shows.
Blizzard's World of Warcraft video game utilizes the BitTorrent protocol to
send game updates to clients.

Anime fansub communities often use BitTorrent for their releases, and the most
popular announce sites like Animesuki have RSS feeds. Blog Torrent offers a
simplified BitTorrent tracker to enable bloggers and non-technical users to host a
tracker on their site. Blog Torrent also allows visitors to download a "stub" loader
which acts as a BitTorrent client to download the desired file, allowing users
without BitTorrent software to use the protocol. This is similar to the concept of a
self-extracting archive.

The game GunZ The Duel has a built-in BitTorrent client. Podcasting software is
starting to integrate BitTorrent to help podcasters deal with the download demands
of their MP3 "radio" programs. Specifically, Juice and Democracy Player support
automatic processing of .torrent files from RSS feeds. Similarly, some BitTorrent
clients, such as μTorrent, are able to process web feeds and automatically
download content found within them. The creator of the BitTorrent protocol, Bram



Cohen, at one time worked for Valve Software. Valve uses the BitTorrent protocol
in their Steam media streaming frontend.



Network impact

CableLabs, the research organization of the North American cable industry,
estimates that BitTorrent represents 18% of all broadband traffic. In 2004,
CacheLogic put that number at roughly 35% of all traffic on the Internet. The
discrepancies in these numbers are caused by differences in the methodology used
to measure P2P traffic on the Internet. Routers on the Internet, including those used
by home users, are characterized by how many packets per second they can switch
between networks. When the MS Blaster virus crippled the Internet, it was not as a
result of exceeding the bandwidth of the Internet routers; it was a result of
exceeding their maximum packet switching capacity. With this in mind, any
protocol, like BitTorrent, that sends small packets reduces the carrying capacity of
theInternet.

Routers that use NAT, Network Address Translation, must maintain tables of
source and destination IP addresses and ports. Typical home routers are limited to
about 2000 table entries while some more expensive routers have larger table
capacities. BitTorrent frequently contacts 300-500 servers per second rapidly
filling the NAT tables. This is a common cause of home routers locking up.



Legal issues: Copyright enforcement

BitTorrent trackers have been subjected to raids and shutdowns due to claims
of copyright infringement. BitTorrent metafiles do not store copyrighted data,
so it is claimed that BitTorrent trackers, which only store and track the metafiles
and usually do not share any potentially copyrighted data, must therefore be legal.
Despite this claim, there has been tremendous legal pressure, usually on behalf of
the MPAA and RIAA, to shut down numerous BitTorrent trackers. In December
2004, the Finnish police raided a major BitTorrent site, Finreactor.

The case is before the courts, and 32 people, in September 2006, mostly
administrators and moderators, are facing charges. Software and media companies
are seeking damages worth 3.5 million euros (about 4.6 million USD) in total. Two
defendants were acquitted by reason of being underage at the time, but they are
being held liable for legal fees and compensation for illegal distribution ranging up
to 60,000 euros. The court set their fine at 10% of the retail price of products
distributed. Suprnova.org, one of the most popular early BitTorrent sites, closed in
December 2004, purportedly due to the pressure felt by Sloncek, the founder and
administrator of the site.

In December 2004, Sloncek revealed that the Suprnova computer servers had in
fact been confiscated by Slovenian authorities. LokiTorrent, arguably the biggest
torrent source after the demise of Suprnova, closed down soon after Suprnova.
Allegedly, after threats from the MPAA, Edward Webber (known as 'lowkee'),
webmaster of the site, was ordered by the court to pay a fine and supply the MPAA
with logs (the IP addresses of began a fundraising campaign to pay legal fees in a
legal battle against the MPAA. Webber raised approximately US$45,000 through a
PayPal-based donation system. It is unclear how much of that money went to the
MPAA. Following the agreement, the MPAA changed the LokiTorrent website to
display a message intended to discourage filesharers from downloading illegal
content. Webber did not comment on this change.



Legal defenses

There are two major differences between BitTorrent and many other peer-topeer
file-trading systems, which advocates suggest make it less useful to those sharing
copyrighted material without authorization. First, BitTorrent itself does not offer a
search facility to find files by name. A user must find the initial torrent file by
other means, such as a web search. Second, BitTorrent makes no attempt to
conceal the host ultimately responsible for facilitating the sharing: a person who
wishes to make a file available must run a tracker on a specific host or hosts and
distribute the tracker address(es) in the .torrent file. Because it is possible to
operate a tracker on a server that is located in a jurisdiction where the copyright
holder cannot take legal action, the protocol does offer some vulnerability that
other protocols lack. It is far easier to request that the server's ISP shut down the
site than it is to find and identify every user sharing a file on a peer-to-peer
network. However, with the use of a distributed hash table (DHT), trackers are no
longer required, though often used for client software that does not support DHT to
connect to the stream.



Limitations and security vulnerabilities

BitTorrent does not offer its users anonymity. It is possible to obtain the IP
addresses of all current, and possibly previous, participants in a swarm from the
tracker. This may expose users with insecure systems to attacks. Another drawback
is that BitTorrent file sharers, compared to users of client/server technology, often
have little incentive to become seeders after they finish downloading. The result of
this is that torrent swarms gradually die out, meaning a lower possibility of
obtaining older torrents. Some BitTorrent websites have attempted to address this
by recording each user's download and upload ratio for all or just the user to see, as
well as the provision of access to older torrent files to people with better ratios.
Also, users who have low upload ratios may see slower download speeds until they
upload more.

This prevents (statistical) leeching, since after a while they become unable to
download much faster than 1-10 kB/s on a high-speed connection. Some trackers
exempt dial-up users from this policy, because they cannot upload faster than 1-3
kB/s. BitTorrent is best suited to continuously connected broadband environments,
since dial-up users find it less efficient due to frequent disconnects and slow
download rates.



Technologies built on BitTorrent

The BitTorrent protocol is still under development and therefore may still acquire
new features and other enhancements such as improved efficiency.

Trackerless torrents

In May 2005, Bram Cohen released a new beta version of BitTorrent that liminated
the need for web site hosting of centralized servers known as "trackers." It is now
possible to have a torrent up in minutes, with a file, a website, and no
understanding of how it works. Cohen explained that the "trackerless" feature is
part of his ongoing effort to make publishing files online "painless and disruptively
cheap". The move is only one of several designed to remove BitTorrent's
dependence on centralized trackers. In June 2005, software version 4.2.0 was
released, supporting "trackerless" torrents, featured a DHT implementation that
allows the client to download torrents that have been created without using a
BitTorrent tracker. BitTorrent Mainline DHT: BitTorrent client (5.0.7), μTorrent
(1.6.1.0), BitComet (0.90), and BitSpirit (3.0+) all share DHT which is based on an
implementation of the Kademlia DHT, for trackerless torrents. This change is said
to cause some trouble in the legal efforts to shut down illegal file sharing.
However, Tarun Sawney, BSA Asia anti-copyright infringement director, said
BitTorrent files could still be identified, since with or without the tracker sites,
actual users still host the infringing files.

Search

In addition, Cohen launched a BitTorrent search engine on

http://search.bittorrent.com

which searches the web for .torrent files, including those on popular BitTorrent
trackers. BitTorrent has also licensed content which it commingles with search
results on its website at bittorrent.com. Opera Software ASA now incorporates
BitTorrent downloads through its popular browser software.

Indexing

The BitTorrent protocol provides no way to index torrent files. As a result, a



comparatively small number of websites have hosted the large majority of torrents
linking to (possibly) copyrighted material, rendering those sites especially
vulnerable to lawsuits. In response, some developers have sought ways to make
publishing of files more anonymous while still retaining BitTorrent's speed
advantage. The Shareaza client supports three alternative protocols to BitTorrent:
eDonkey2000, Gnutella, and Shareaza's native network, Gnutella2(G2). If the
tracker is down, it can finish the file using the other protocols, and/or find new
(Shareaza) peers over G2. The use of distributed trackers is also one of the goals
for Azureus 2.3.0.2 and BitTorrent 4.1.2. Another interesting idea that has surfaced
recently in Azureus is virtual torrent. This idea is based on the distributed tracker
approach and is used to describe some web resource. Right now, it is used for
instant messaging. It is implemented using a special messaging protocol and
requires an appropriate plugin. Anatomic P2P is another approach, which uses a
decentralized network of nodes that route traffic to dynamic trackers.

Web seeding

One recently implemented feature of BitTorrent is web seeding. The advantage of
this feature is that a site may distribute a torrent for a particular file or batch of files
and make those files available for download from that same web server
application; this can simplify seeding and load balancing greatly once support for
this feature is implemented in the various BitTorrent clients. In theory, this would
make using BitTorrent almost as easy for a web publisher as simply creating a
direct download while allowing some of the upload bandwidth demands to be
placed upon the downloaders (who normally use only a very small portion of their
upload bandwidth capacity). This feature was created by TheSHAD0W, who
created BitTornado. From version 5.0 onward the Mainline BitTorrent client also
supports web seeds and the BitTorrent web site has a simple publishing tool that
creates web seeded torrents. The latest version of the popular download manager
GetRight supports downloading a file from both HTTP/FTP protocols and using
BitTorrent.

Broadcatching

Another proposed feature combines RSS with the BitTorrent protocol to create a
content delivery system, further simplifying and automating content distribution.
Steve Gillmor explained the concept in a column for Ziff-Davis in December,
2003. The discussion spread quickly among bloggers (Techdirt, Ernest Miller,



Chris Pirillo, etc.). In an article entitled Broadcatching with BitTorrent, Scott
Raymond explained:

"I want RSS feeds of BitTorrent files. A script would periodically check the feed for
new items, and use them to start the download. Then, I could find a trusted
publisher of an Alias RSS feed, and 'subscribe' to all new episodes of the show,
which would then start downloading automatically

— like the 'season pass' feature of the TiVo."

The RSS feed will track the content, while BitTorrent ensures content integrity
with cryptographic hashing of all data, so subscribers to a feed receive uncorrupted
content. An early implementor of this approach is the IPTV show mariposaHD,
which uses BitTorrent to distribute large (2-4 GB) WMVHD files of high-
definition video. One of the first software clients ( free and open source) for
broadcatching is Democracy Player. Other free software clients such as
PenguinTV and KatchTV are also now supporting broadcatching.

APIs

The BitTorrent web-service MoveDigital has the ability to make torrents available
to any web application capable of parsing XML through its standard
Representational State Transfer (REST) based interface. Additionally, Torrenthut
is developing a similar torrent API which will provide the same features, as well as
further intuition to help bring the torrent community to Web 2.0 standards.
Alongside this release is a first PHP application built using the API called PEP
which will parse any Really Simple Syndication (RSS 2.0) feed and automatically
create and seed a torrent for each enclosure found in that feed.

Encryption

Protocol header encrypt (PHE) and Message stream encryption/Protocol
encryption (MSE/PE) are features of some BitTorrent clients that attempt to
make BitTorrent hard to throttle. At the moment Azureus, Bitcomet, kTorrent,
μTorrent and rtorrent support MSE/PE encryption.

Some ISPs throttle BitTorrent traffic because it makes up a large proportion of
total traffic and the ISPs don't want to spend money purchasing extra capacity.



Encryption makes BitTorrent traffic harder to detect and therefore harder to
throttle. Recently, ISPs have announced possible future hardware upgrades in order
to minimize BitTorrent traffic. Several universities have already taken these steps,
including the University of Maryland at College Park, Emory University, Brigham
Young University, ASU, UTC, University of Washington, Washington University
in St. Louis, and WPI.

Tor workaround

It is possible to route BitTorrent traffic into the .onion domain via Tor, although
some consider the use of BitTorrent over TOR to be counter to the goals of the
TOR project. However, client and server support for this is nearly non-existent,
and there is generally a loss in network efficiency when doing so. Future versions
of BitTorrent may support Tor, but this may take two or three years to happen.

Peer exchange

Peer exchange (PEX) is another method to gather peers for BitTorrent in addition
to trackers and DHT. Peer exchange checks with known peers to see if they know
of any other peers.

Multitracker

Another unofficial feature is an extension to the BitTorrent metadata format
proposed by John Hoffman and implemented by several indexing websites. It
allows the use of multiple trackers per file, so if one tracker fails, others can
ontinue supporting file transfer. It is implemented in several clients, such as
BitTornado, KTorrent and μTorrent. Trackers are placed in groups, or tiers, with a
tracker randomly chosen from the top tier and tried, moving to the next tier if all
the trackers in the top tier fail.

Torrents with multiple trackers, called MultiTorrents by indexing website
myBittorrent.com, can decrease the time it takes to download a file, but also
has a few consequences: Users have to contact more trackers, leading to more
overhead-traffic. Torrents from closed trackers suddenly become downloadable by
nonmembers, as they can connect to a seed via an open tracker.



Future technologies

Similarity Enhanced Transfer

An as yet (April, 2009) unimplemented unofficial feature is Similarity Enhanced
Transfer (SET), a technique for improving the speed at which peer-to-peer file
sharing and content distribution systems can share data. SET, proposed by
researchers Pucha, Andersen, and Kaminsky, works by spotting chunks of identical
data in files that are an exact or near match to the one needed and transferring this
data to the client if the 'exact' data is not present. Their experiments suggested that
SET will help greatly with less popular files, but not as much for popular data,
where many peers are already downloading it. Andersen believes that this
technique could be immediately used by developers with the bit torrent file sharing
system



ADVANTAGES & DISADVANTAGES

Advantages

• VERY good for zero day releases because they are typically so popular. The
Queen can begin propogating the file exponentionally quicker than with
FastTrack. Certain episodes of television programs have been available to the
West Coast on BT before the West Coast is even able to watch it on television
(due to time zones).

• Most BT clients and servers are not only freeware and contain no spyware but
are also Open Source and avaible for editing.

• The Queen need only transfer 100% of the file before it can be further
populated.

• Highly efficient.

Disadvantages

• Sites hosting .torrent files are often flaky and bogged down due to excessive
popularity.

• A Queen can only seed one or two files at a time unless they have massive
upload bandwidth available.

• If the Queen leaves the swarm too early, no one will be able to use the file.

CONCLUSION



BitTorrent was conceived as a way of distributing large files more quickly,
efficiently, and reliably. Creator Bram Cohen has made it clear that it was not
intended for online piracy, though that is likely what it has been used for the most.
Thanks to BitTorrent though, the download of large files has become more
feasible, and people have become accustomed to acquiring video through the net.
At this point it is virtually impossible for anyone to bring an end to illegal
downloads. whether or not BitTorrent will continue to play a role in this is yet to
be seen.Effective distribution means more commercial applications. Already many
places use BitTorrent to make available their own material because it is affordable.
As a result, size and quality of the files freely distributed online have been going
up.

A few of the things going on now include: free software projects, game files, films/
TV from major studios like Warner Bros., fansubs, and music exposure by indie
artists.

References



• Schiesel, Seth . "File Sharing's New Face." New York Times, link
• Thompson, Clive (January, 2005). "The BitTorrent Effect." Wired
Magazine, link.
• BBC News (May 13, 2005). "TV download sites hit by lawsuits" BBC News,
link.
• www.wikipedia.com
• BBC News (April 13, 2006). "BitTorrent battles over bandwidth." BBC
News, link.
• Rietjens, Bob (2005) "Give and Ye Shall Receive! The Copyright
Implications of BitTorrent", 2:3 SCRIPT-ed 364. link
• Roth, Daniel (November 14, 2005). "Torrential Rain." Fortune, p. 91–96.
• Pouwelse, Johan (December 18, 2004). "A detailed study of the BitTorrent


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