BitTorrent Protocol Overview

BitTorrent Protocol
1. Intorduction
1.1 Overview
BitTorrent is a peer-to-peer file sharing protocol used to distribute large amounts
of data. BitTorrent is one of the most common protocols for transferring large files. Its main
usage is for the transfer of large sized files. It makes transfer of such files easier by
implementing a different approach. A user can obtain multiple files simultaneously without
any considerable loss of the transfer rate. It is said to be a lot better than the conventional file
transfer methods because of a different principle that is followed by this protocol. It also
evens out the way a file is shared by allowing a user not just to obtain it but also to share it
with others. This is what has made a big difference between this and the conventional file
transfer methods. It makes a user to share the file he is obtaining so that the other users who
are trying to obtain the same file would find it easier and also in turn making these users to
involve themselves in the file sharing process. Thus the larger the number of users the more
is the demand and more easily a file can be transferred between them.
BitTorrent protocol has been built on a technology which makes it possible to
distribute large amounts of data without the need of a high capacity server, and expensive
bandwidth. This is the most striking feature of this file transfer protocol. The transferring of
files will never depend on a single source which is supposed the original copy of the file but
instead the load will be distributed across a number of such sources. Here not just the sources
are responsible for file transfer but also the clients or users who want to obtain the file are
involved in this process. This makes the load get distributed evenly across the users and thus
making the main source partially free from this process which will reduce the network traffic
imposed on it. Because of this, BitTorrent has become one of the most popular file transfer
mechanisms in today’s world. Though the mechanism itself is not as simple as an ordinary
file transfer protocol, it has gained its popularity because of the sharing policy that it imposes
on its users. This fact is quite obvious, since the recent surveys made by various
organizations show that 35% of the overall internet traffic is because of BitTorrent. This
shows that the amount of files that are being transferred and shared by users through
BitTorrent is very huge.
1 Dept of CS&E, VVCE

BitTorrent Protocol
1.2 History
BitTorrent was created by a programmer named Bram Cohen. After inventing this
new technology he said, "I decided I finally wanted to work on a project that people would
actually use, would actually work and would actually be fun". Before this was invented, there
were other techniques for file sharing but they were not utilizing the bandwidth effectively.
The bandwidth had become a bottleneck in such methods. Even other peer to peer file sharing
systems like Napster and Kazaa had the capability of sharing files by making the users
involve in the sharing process, but they required only a subset of users to share the files not
all. This meant that most of the users can simply download the files without being needed to
upload. So this again put a lot of network load on the original sources and on small number of
users. This led to inefficient usage of bandwidth of the remaining users. This was the main
intention behind Cohen’s invention, i.e., to make the maximum utilization of all the users’
bandwidth who are involved in the sharing of files. By doing so, every person who wants to
download a file had to contribute towards the uploading process also. This new and novel
concept of Cohen gave birth to a new peer to peer file sharing protocol called BitTorrent.
Cohen invented this protocol in April 2001. The first usable version of BitTorrent appeared in
October 2002, but the system needed a lot of fine-tuning. BitTorrent really started to take off
in early 2003 when it was used to distribute a new version of Linux and fans of Japanese
anime started relying on it to share cartoons. The most important part of this protocol that
matters a lot about this is that it makes it possible for people with limited bandwidth to supply
very popular files. This means that if you are a small software developer you can put up a
package, and if it turns out that millions of people want it, they can get it from each other in
an automated way. Thus the bandwidth which used to be a bottleneck in previous systems no
longer poses a problem.
2. BitTorrent and Other approaches

BitTorrent Protocol
2.1 Other P2P Methods
The most common method by which files are transferred on the Internet is the client-
server model. A central server sends the entire file to each client that requests it, this is how
both http and ftp work. The clients only speak to the server, and never to each other. The
main advantages of this method are that it's simple to set up, and the files are usually always
available since the servers tend to be dedicated to the task of serving, and are always on and
connected to the Internet. However, this model has a significant problem with files that are
large or very popular, or both. Namely, it takes a great deal of bandwidth and server
resources to distribute such a file, since the server must transmit the entire file to each client.
Perhaps you may have tried to download a demo of a new game just released, or CD images
of a new Linux distribution, and found that all the servers report "too many users," or there is
a long queue that you have to wait through. The concept of mirrors partially addresses this
shortcoming by distributing the load across multiple servers. But it requires a lot of
coordination and effort to set up an efficient network of mirrors, and it's usually only feasible
for the busiest of sites.
Another method of transferring files has become popular recently: the peer-to-peer
network, systems such as Kazaa, eDonkey, Gnutella, Direct Connect, etc. In most of these
networks, ordinary Internet users trade files by directly connecting one-to-one. The advantage
here is that files can be shared without having access to a proper server, and because of this
there is little accountability for the contents of the files. Hence, these networks tend to be
very popular for illicit files such as music, movies, pirated software, etc. Typically, a
downloader receives a file from a single source, however the newest version of some clients
allow downloading a single file from multiple sources for higher speeds. The problem
discussed above of popular downloads is somewhat mitigated, because there's a greater
chance that a popular file will be offered by a number of peers. The breadth of files available
tends to be fairly good, though download speeds for obscure files tend to be low. Another
common problem sometimes associated with these systems is the significant protocol
overhead for passing search queries amongst the peers, and the number of peers that one can
reach is often limited as a result. Partially downloaded files are usually not available to other

BitTorrent Protocol
peers, although some newer clients may offer this functionality. Availability is generally
dependent on the goodwill of the users, to the extent that some of these networks have tried to
enforce rules or restrictions regarding send/receive ratios.
Use of the Usenet binary newsgroups is yet another method of file distribution, one
that is substantially different from the other methods. Files transferred over Usenet are often
subject to miniscule windows of opportunity. Typical retention time of binary news servers
are often as low as 24 hours, and having a posted file available for a week is considered a
long time. However, the Usenet model is relatively efficient, in that the messages are passed
around a large web of peers from one news server to another, and finally fanned out to the
end user from there. Often the end user connects to a server provided by his or her ISP,
resulting in further bandwidth savings. Usenet is also one of the more anonymous forms of
file sharing, and it too is often used for illicit files of almost any nature. Due to the nature
of NNTP, a file's popularity has little to do with its availability and hence downloads from
Usenet tend to be quite fast regardless of content. The downsides of this method include a set
of rules and procedures, and requires a certain amount of effort and understanding from the
user. Patience is often required to get a complete file due to the nature of splitting big files
into a huge number of smaller posts. Finally, access to Usenet often must be purchased due to
the extremely high volume of messages in the binary groups.
BitTorrent is closest to Usenet. It is best suited to newer files, of which a number of
people have interest in. Obscure or older files tend to not be available. Perhaps as the
software matures a more suitable means of keeping torrents seeded will emerge, but currently
the client is quite resource-intensive, making it cumbersome to share a number of files.
BitTorrent also deals well with files that are in high demand, especially compared to the other
methods.
2.2 A Typical HTTP File Transfer
The most common type of file transfer is through a HTTP server. In this method, a
HTTP server listens to the client’s requests and serves them. Here the client can only depend
on the lone server that is providing the file. The overall download scheme will be limited to
the limitations of that server. Also this kind of transfer of file is subjected to single point of
failure, where if the server crashes then the whole download process will seize. A single
server can handle many such clients and serve the requested file simultaneously to all the

BitTorrent Protocol
clients. The file being served will be available as one single piece, which means that if the
download process stops abruptly in the middle the whole file has to be downloaded again.
BitTorrent protocol has overcome all these shortcomings seen in this type and thus it is more
robust due to which it is chosen by many people over this traditional method of file transfer.
Fig 2.1 : HTTP/FTP File Transfer
2.3 The DAP method
Download Accelerator Plus (DAP) is the world's most popular download accelerator.
DAP's key features include the ability to accelerate downloading of files in FTP and HTTP
protocols, to pause and resume downloads, and to recover from dropped internet connections.
On the Internet the same file is often hosted on numerous mirror sites, such as at
universities and on ISP servers. DAPimmediately senses when a user begins downloading a
file and identifies available mirror sites that host the requested file. As soon as it is
triggered, DAP's client side optimization begins to determine - in real time - which mirror
sites offer the fastest response for the specific user's location. The file is downloaded in
several segments simultaneously through multiple connections from the most responsive
server(s) and reassembled at the user's PC. This results in better utilization of the user's
available bandwidth. This ensures that each available mirror server is utilized to serve the
users that most benefit. This in turn effects an efficient balancing of the load among available

BitTorrent Protocol
servers across the entire World Wide Web, and reduces download times for users while
allowing them to receive maximum benefit from their available bandwidth. DAP'sResume
functionality and the ability to continue downloading even when one of the participating
connections has dropped also provides users with a more reliable download experience.
2.4 The BitTorrent Approach
In BitTorrent, the data to be shared is divided into many equal-sized portions called
pieces. Each piece is further sub-divided into equal-sized sub-pieces called blocks. All clients
interested in sharing this data are grouped into a swarm, each of which is managed by a
central entity called the tracker. BitTorrent has revolutionized the way files are shared
between people. It does not require a user to download a file completely from a single server.
Instead a file can be downloaded from many such users who are indeed downloading the
same file. A user who has the complete file, called the seed will initiate the download by
transferring pieces of file to the users. Once a user has some considerable number of such
pieces of a file then even he can start sharing them with other users who are yet to receive
those pieces. This concept enables a client not to depend on a server completely and also it
reduces overall load on the server.
Fig 2.2 : BitTorrent File Transfer
Each client independently sends a file, called a torrent, that contains the location of
the tracker along with a hash of each piece. Clients keep each other updated on the status of
their download. Clients download blocks from other (randomly chosen) clients who claim
they have the corresponding data. Accordingly, clients also send data that they have

BitTorrent Protocol
previously downloaded to other clients. Once a client receives all the blocks for a given
piece, he can verify the hash of that piece against the provided hash in the torrent. Thus once
a client has downloaded and verified all pieces, he can be confident that he has the complete
data.
Both BitTorrent and DAP download files from multiple sources. Also the files are
divided into pieces in both approaches. But BitTorrent has many such features that DAP
doesn’t, which has made it the most popular one. In BitTorrent the users participate actively
in sharing files along with servers. This is the uniqueness of this protocol. Also this needs an
implementation of a dedicated server called tracker to handle the peers connected in the
network. The file transfer in DAP takes place through the traditional HTTP or FTP protocol
which means that the transfer rate will always be limited by the server’s bandwidth. If these
servers are flooded with requests then the breakdown and the transaction will terminate. This
is not the case in BitTorrent since the whole process is not depending on servers alone. The
load is distributed across the network between peers and servers. This makes BitTorrent far
better than its competing peers like DAP and others.
3. Working of BitTorrent :
As previously explained, BitTorrent’s design makes it extremely efficient in the
sharing of large data files among interested peers. Looking under the hood, BitTorrent is a
protocol with some complexity where modeling is useful to gain a better understanding of its

BitTorrent Protocol
performance. BitTorrent scales well and is a superior method for transferring and
disseminating files between interested peers while limiting free riding (peers who download
but do not upload) between those same peers. BitTorrent’s is based on a “tit for tat”
reciprocity agreement between users that ultimately results in pareto efficiency. Pareto
efficiency is an important economic concept that maximizes resource allocation among peers
to their mutual advantage. Pareto efficiency is the crown jewel of BitTorrent and is the
driving force behind the protocol’s popularity and success. Cohen’s vision of peers
simultaneously helping each other by uploading and downloading has been realized by the
BitTorrent system.
Fig 3.1 : A Typical BitTorrent System
The protocol shares data through what are known as torrents. For a torrent to be alive
or active it must have several key components to function. These components include a
tracker server, a .torrent file, a web server where the .torrent file is stored and a complete
copy of the file being exchanged. Each of these components is described in the following
paragraphs.
The file being exchanged is the essence of the torrent and a complete copy is referred
to as a seed. A seed is a peer in the BitTorrent network willing to share a file with other peers
in the network. Why seed owners choose to share their files is debatable, as the BitTorrent
protocol does not reward seed behavior. In fact, some researchers believe the protocol lacks
any incentive mechanism for encouraging seeds to remain in torrents. Some argue that the

BitTorrent Protocol
lack of incentive in the protocol is a fundamental design flaw that leads to the punishment of
seeds.
Peers lacking the file and seeking it from seeds are called leechers. While seeds only
upload to leechers, leechers may both download from seeds and upload to other leechers.
BitTorrent’s protocol is designed so leeching peers seek each other out for data transfer in a
process known as “optimistic unchoking”. Together seeds and leechers engaged in file
transfer are referred to as a swarm. A swarm is coordinated by a tracker server serving the
particular torrent and interested peers find the tracker via metadata known as a .torrent file.
Since BitTorrent has no built in search functionality, .torrent files are usually located via
HTTP through search engines or trackers.
The first step in the BitTorrent exchange occurs when a peer downloads a .torrent file
from a server. The role of .torrent files is to provide the metadata that allows the protocol to
function; .torrent files can be viewed as surrogates for the files being shared. These .torrent
files contain key pieces of data to function correctly including file length, assigned name,
hashing information about the file and the URL of the tracker coordinating the torrent
activity. Torrent files can be created using a program such as MakeTorrent, another open
source tool available under the free software model.
When a .torrent file is opened by the peer’s client software, the peer then connects to
the tracker server responsible for coordinating activity for that specific torrent. The tracker
and client communicate by a protocol layered on top of HTTP and the tracker’s key role is to
coordinate peers seeking the same file for Cohen envisioned “The tracker’s responsibilities
are strictly limited to helping peers find each other”. In reality the tracker’s role is a bit more
complex as many trackers collect data about peers engaged in a swarm. Additionally, some of
the newer tracker software being released has integrated the functions of the tracker and
.torrent server.
Leechers and seeds are coordinated by the tracker server and the peers periodically
update the tracker on their status allowing the tracker to have a global view of the system.
The data monitored by the tracker can include peer IP addresses, amount of data
uploaded/downloaded for specific peers, data transfer rates among peers, the percentage of
the total file downloaded, length of time connected to the tracker, and the ratio of sharing
among peers. Usually a tracker coordinates multiple torrents and the most popular trackers
are busy coordinating thousands of swarms simultaneously.

BitTorrent Protocol
It should be noted that .torrent files are not the actual file being shared; rather .torrent
files are the metadata information which allow which trackers and peers to coordinate their
activities. As previously mentioned, the complete file is actually stored on peer seed nodes
and not the tracker server. Since .torrent files are small and require little space to store, one
server can easily host thousands of .torrent files without prohibitive server or bandwidth
requirements. There is some issue with bandwidth usage to host a tracker, however,
especially if the tracker becomes popular and begins to see heavy usage. Regardless, the
tracker’s bandwidth requirements are much less than hosting the complete files in a
traditional client-server model such as one would encounter with an FTP site.
While trackers and .torrent files serve as mechanisms to assist the BitTorrent protocol,
the process of actually transferring data is handled by the peers engaged in the swarm.
Cohen’s vision of “tit for tat” is the sole incentive measure he saw necessary for the
protocol’s success. Peers seek tit for tat behavior from others and discourage free riding by a
“choke/unchoke” policy. This choke policy uses a process known as “optimistic unchoking”
to constantly seek other swarm peers who may have more beneficial connections to offer an
interested peer.
There has been some research of the tit for tat algorithm by modeling rational users
whose behavior is then studied. This work defined rational users as those peer nodes
manipulating their client software beyond default settings. The fact that many newer
BitTorrent clients allow for custom tweaking of specific upload or download speed indicates
that perhaps the original tit for tat coding was too good, and thus detrimental to other peer
node functions such as normal HTTP traffic. Some BitTorrent FAQs recommend limiting
uploads to approximately 80% of known capacity and personal tests indicate this strategy
does benefit download speeds.
The final important aspect of the BitTorrent protocol’s architecture is its use of a
“rarest piece first” algorithm when a peer begins a file download. The rarest first algorithm
has as its goal the uniform distribution of data across peers, also known as the “endgame
mode”. A rarest first policy requires a seed to upload new file chunks (those not yet uploaded
to a swarm) to the newest peer connecting to a torrent. This policy encourages distribution of
the file further across peer nodes.. The rarest first algorithm is an interesting aspect of
BitTorrent that when combined with optimistic unchoking may explain why the protocol has
achieved such success.

BitTorrent Protocol
4. Terminology
These are the common terms that one would come across while making a typical
BitTorrent file transfer.

BitTorrent Protocol
 Torrent : this refers to the small metadata file you receive from the web server
(the one that ends in .torrent.) Metadata here means that the file contains
information about the data you want to download, not the data itself.
 Peer : A peer is another computer on the internet that you connect to and
transfer data. Generally a peer does not have the complete file.
 Leeches : They are similar to peers in that they won’t have the complete file.
But the main difference between the two is that a leech will not upload once
the file is downloaded.
 Seed : A computer that has a complete copy of a certain torrent. Once a client
downloads a file completely, he can continue to upload the file which is called
as seeding. This is a good practice in the BitTorrent world since it allows other
users to have the file easily.
 Reseed : When there are zero seeds for a given torrent, then eventually all the
peers will get stuck with an incomplete file, since no one in the swarm has the
missing pieces. When this happens, a seed must connect to the swarm so that
those missing pieces can be transferred. This is called reseeding.
 Swarm : The group of machines that are collectively connected for a particular
file.
 Tracker : A server on the Internet that acts to coordinate the action of
BitTorrent clients. The clients are in constant touch with this server to know
about the peers in the swarm.
 Share ratio : This is ratio of amount of a file downloaded to that of uploaded.
A ratio of 1 means that one has uploaded the same amount of a file that has
been downloaded.
 Distributed copies : Sometimes the peers in a swarm will collectively have a
complete file. Such copies are called distributed copies.
 Choked : It is a state of an uploader where he does not want to send anything
on his link. In such cases, the connection is said to be choked.
 Interested : This is the state of a downloader which suggests that the other end
has some pieces that the downloader wants. Then the downloader is said to be
interested in the other end.

BitTorrent Protocol
 Snubbed : If the client has not received anything after a certain period, it
marks a connection as snubbed, in that the peer on the other end has chosen
not to send in a while.
 Optimistic unchoking : Periodically, the client shakes up the list of uploaders
and tries sending on different connections that were previously choked, and
choking the connections it was just using. This is called optimistic unchoking.
5. Architecture of BitTorrent
The BitTorrent protocol can be split into the following five main components:
 Metainfo File - a file which contains all details necessary for the protocol to operate.
 Tracker - A server which helps manage the BitTorrent protocol.

BitTorrent Protocol
 Peers - Users exchanging data via the BitTorrent protocol.
 Data - The files being transferred across the protocol.
 Client - The program which sits on a peers computer and implements the protocol.
Peers use TCP (Transport Control Protocol) to communicate and send data. This protocol
is preferable over other protocols such as UDP (User Datagram Protocol) because TCP
guarantees reliable and in-order delivery of data from sender to receiver. UDP cannot give
such guarantees, and data can become scrambled, or lost all together. The tracker allows
peers to query which peers have what data, and allows them to begin communication. Peers
communicate with the tracker via the plain text via HTTP (Hypertext Transfer Protocol) The
following diagram illustrates how peers interact with each other, and also communicate with
a central tracker.
Fig 5.1 : Architecture of a BitTorrent System
5.1 Metainfo File
When someone wants to publish data using the BitTorrent protocol, they must create a
metainfo file. This file is specific to the data they are publishing, and contains all the

BitTorrent Protocol
information about a torrent, such as the data to be included, and IP address of the tracker to
connect to. A tracker is a server which 'manages' a torrent, and is discussed in the next
section. The file is given a '.torrent' extension, and the data is extracted from the file by a
BitTorrent client. This is a program which runs on the user computer, and implements the
bittorrent protocol. Every metainfo file must contain the following information, (or 'keys'):
• info: A dictionary which describes the file(s) of the torrent. Either for the single file,
or the directory structure for more files. Hashes for every data piece, in SHA 1 format
are stored here.
• announce: The announce URL of the tracker as a string
The following are optional keys which can also be used:
• announce-list: Used to list backup trackers
• creation date: The creation time of the torrent by way of UNIX time stamp (integer
seconds since 1-Jan-1970 00:00:00 UTC)
• comment: Any comments by the author
• created by: Name and Version of programme used to create the metainfo file
These keys are structured in the metainfo file as follows:
{'info': {'piece length': 131072, 'length': 38190848L, 'name':
'Cory_Doctorow_Microsoft_Research_DRM_talk.mp3', 'pieces':
'xcbxfazrx9bxe1x9axe1x83x91~xed@.....', } 'announce':
'http://tracker.var.cc:6969/announce', 'creation date': 1089749086L }
Instead of transmitting the keys in plan text format, the keys contained in the metainfo
file are encoded before they are sent. Encoding is done using bittorrent specific method
known as 'bencoding'.

BitTorrent Protocol
5.1.1 Bencoding :
Bencoding is used by bittorrent to send loosely structured data between the BitTorrent
client and a tracker. Bencoding supports byte strings, integers, lists and dictionaries.
Bencoding uses the beginning delimiters 'i' / 'l' / 'd' for integers, lists and dictionaries
respectively. Ending delimiters are always 'e'. Delimiters are not used for byte strings.
Bencoding Structure:
• Byte Strings : <string length in base ten ASCII> : <string data>
• Integers: i<base ten ASCII>e
• Lists: l<bencoded values>e
• Dictionaries: d<bencoded string><bencoded element>e
Minus integers are allowed, but prefixing the number with a zero is not permitted.
However '0' is allowed.
Examples of bencoding:
4:spam // represents the string "spam"
i3e // represents the integer "3"
l4:spam4:eggse // represents the list of two strings: ["spam","eggs"]
d4:spaml1:a1:bee // represents the dictionary {"spam" => ["a" , "b"] }
5.1.2 Metainfo File Distribution :
Because all information which is needed for the torrent is included in a single file, this
file can easily be distributed via other protocols, and as the file is replicated, the number of
peers can increase very quickly. The most popular method of distribution is using a public
indexing site which hosts the metainfo files. A seed will upload the file, and then others can
download a copy of the file over the HTTP protocol and participate in the torrent.

BitTorrent Protocol
5.2 Tracker
A tracker is used to manage users participating in a torrent (know as peers). It stored
statistics about the torrent, but its main role is allow peers to 'find each other' and start
communication, i.e. to find peers with the data they require. Peers know nothing of each other
until a response is received from the tracker. Whenever a peer contacts the tracker, it reports
which pieces of a file they have. That way, when another peer queries the tracker, it can
provide a random list of peers who are participating in the torrent, and have the required
piece.
Fig 5.2 : Tracker
A tracker is a HTTP/HTTPS service and typically works on port 6969. The address of
the tracker managing a torrent is specified in the metainfo file, a single tracker can manage
multiple torrents. Multiple trackers can also be specified, as backups, which are handled by
the BitTorrent client running on the users computer. BitTorrent clients communicate with the
tracker using HTTP GET requests, which is a standard CGI method. This consists of
appending a "?" to the URL, and separating parameters with a "&".

BitTorrent Protocol
The parameters accepted by the tracker are:
• info_hash: 20-byte SHA1 hash of the info key from the metainfo file.
• peer_id: 20-byte string used as a unique ID for the client.
• port: The port number the client is listed on.
• uploaded: The total amount uploaded since the client sent the 'started' event to the
tracker in base ten ASCII.
• downloaded: The total amount downloaded since the client sent the 'started' event to
the tracker in base ten ASCII.
• left: The number of bytes the client till has to download, in base ten ASCII.
• compact: Indicates that the client accepts compacted responses. The peer list can then
be replaced by a 6 bytes per peer. The first 4 bytes are the host, and the last 2 bytes
are port.
• event: If specified, must be one of the following: started, stopped, completed.
• ip: (optional) The IP address of the client machine, in dotted format.
• numwant: (optional) The number of peers the client wishes to receive from the
tracker.
• key: (optional) Allows a client to identify itself if their IP address changes.
• trackerid: (optional) If previous announce contained a tracker id, it should be set
here.
The tracker then responds with a "text/plain" document with the following keys:
• failure message: If present, then no other keys are included. The value is a human
readable error message as to why the request failed.
• warning message: Similar to failure message, but response still gets processed.
• interval: The number of seconds a client should wait between sending regular
requests to the tracker.

BitTorrent Protocol
• min interval: Minimum announce interval.
• tracker id: A string that the client should send back with its next announce.
• complete: Number of peers with the complete file.
• incomplete: number of non-seeding peers (leechers)
• peers: A list of dictionaries including: peer id, IP and ports of all the peers.
5.2.1 Scraping
Scraping is the process of querying the state of a given torrent (or all torrents) that the
tracker is managing. The result is known as a "scrape page". To get the scrape, you must start
with the announce URL, find the last '/' and if the text immediately following the '/' is
'announce', then this can be substituted for 'scrape' to find the scrape page.
Examples:
Announce URL Scrape URL
http://example.com/annnounce  http://example.com/scrape
http://example.com/a/annnounce  http://example.com/a/scrape
http://example.com/announce.php  http://example.com/scrape.php
The tracker then responds with a "text/plain" document with the following bencoded keys:
• files: A dictionary containing one key pair for each torrent. Each key is made up of a
20-byte binary hash value. The value of that key is then a nested dictionary with the
following keys:
• complete: number of peers with the entire file (seeds)
• downloaded: total number of times the entire file has been downloaded.
• incomplete: the number of active downloaders (lechers)

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• name: (optional) the torrent name
5.3 Peers
Peers are other users participating in a torrent, and have the partial file, or the
complete file (known as a seed). Pieces are requested from peers, but are not guaranteed to be
sent, depending on the status of the peer. BitTorrent uses TCP (Transmission Control
Protocol) ports 6881-6889 to send messages and data between peers, and unlike other
protocols, does not use UDP (User Datagram Protocol)
5.3.1 Piece Selection
Peers continuously queue up the pieces for download which they require. Therefore
the tracker is constantly replying to the peer with a list of peers who have the requested
pieces. Which piece is requested depends upon the BitTorrent client. There are three stages of
piece selection, which change depending on which stage of completion a peer is at.
5.3.2 Random First Piece
When downloading first begins, as the peer has nothing to upload, a piece is selected
at random to get the download started. Random pieces are then chosen until the first piece is
completed and checked. Once this happens, the 'rarest first' strategy begins.
5.3.3 Rarest First
When a peer selects which piece to download next, the rarest piece will be chosen
from the current swarm, i.e. the piece held by the lowest number of peers. This means that the
most common pieces are left until later, and focus goes to replication of rarer pieces.
At the beginning of a torrent, there will be only one seed with the complete file. There
would be a possible bottle neck if multiple downloaders were trying to access the same piece.
rarest first avoids this because different peers have different pieces. As more peers connect,
rarest first will the some load off of the tracker, as peers begin to download from one another.

BitTorrent Protocol
Eventually the original seed will disappear from a torrent. This could be because of
cost reasons, or most commonly because of bandwidth issues. Losing a seed runs the risk of
pieces being lost if no current downloaders have them. Rarest first works to prevent the loss
of pieces by replicating the pieces most at risk as quickly as possible. If the original seed goes
before at least one other peer has the complete file, then no one will reach completion, unless
a seed re-connects.
5.3.4 Endgame Mode
When a download nears completion, and waiting for a piece from a peer with slow
transfer rates, completion may be delayed. To prevent this, the remaining sub-pieces are
request from all peers in the current swarm.
5.3.5 Peer Distribution
The role of the tracker ends once peers have 'found each other'. From then on,
communication is done directly between peers, and the tracker is not involved. The set of
peers a BitTorrent client is in communication with is known as a swarm.
To maintain the integrity of the data which has been downloaded, a peer does not
report that they have a piece until they have performed a hash check with the one contained
in the metainfo file.
Peers will continue to download data from all available peers that they can, i.e. peers
that posses the required pieces. Peers can block others from downloading data if necessary.
This is known as choking.
5.3.6 Choking
When a peer receives a request for a piece from another peer, it can opt to refuse to
transmit that piece. If this happens, the peer is said to be choked. This can be done for
different reasons, but the most common is that by default, a client will only maintain a default
number of simultaneous uploads (max_uploads) All further requests to the client will be
marked as choked. Usually the default for max_uploads is 4.

BitTorrent Protocol
Fig 5.3 : Choking by a peer
The peer will then remain choked until an unchoke message is sent. Another example
of when a peer is choked would be when downloading from a seed, and the seed requires no
pieces. To ensure fairness between peers, there is a system in place which rotates which peers
are downloading. This is know as optimistic unchoking.
5.3.7 Optimistic Unchoking
To ensure that connections with the best data transfer rates are not favoured, each peer
has a reserved 'optimistic unchoke' which is left unchoked regardless of the current transfer
rate. The peer which is assigned to this is rotated every 30 seconds. This is enough time for
the upload / download rates to reach maximum capacity.
The peers then cooperate using the tit for tat strategy, where the downloader responds
in one period with the same action the uploader used in the last period.

BitTorrent Protocol
5.3.8 Communication Between Peers
Peers which are exchanging data are in constant communication. Connections are
symmetrical, and therefore messages can be exchanged in both directions. These messages
are made up of a handshake, followed by a never-ending stream of length-prefixed messages.
5.3.9 Handshaking
Handshaking is performed as follows:
1. The handshake starts with character 19 (base 10) followed by the string 'BitTorrent
Protocol'.
2. A 20 byte SHA1 hash of the bencoded info value from the metainfo is then sent. If
this does not match between peers the connection is closed.
3. A 20 byte peer id is sent which is then used in tracker requests and included in peer
requests. If the peer id does not match the one expected, the connection is closed.
5.3.10 Message Stream
This constant stream of messages allows all peers in the swarm to send data, and
control interactions with other peers.
Prefix Message Structure
Additional
Information
0 choke <len=0001><id=0>
Fixed length, no
payload. This
enables a peer
to block another
peers request
for data.

BitTorrent Protocol
1 unchoke <len=0001><id=1>
Fixed length, no
payload.
Unblock peer,
and if they are
still interested
in the data,
upload will
begin.
2 interested <len=0001><id=2>
Fixed length, no
payload. A user
is interested if a
peer has the
data they
require.
3
not
interested
<len=0001><id=3>
Fixed length, no
payload. The
peer does not
have any data
required.
4 have <len=0005><id=4><piece index>
Fixed length.
Payload is the
zero-based
index of the
piece. Details
the pieces that
peer currently
has.
5 bitfield <len=0001+X><id=5><bitfield>
Sent
immediately
after
handshaking.
Optional, and
only sent if
client has
pieces. Variable
length, X is the
length of
bitfield.
Payload
represents
pieces that have
been
successfully
downloaded.

BitTorrent Protocol
6 request <len=0013><id=6><index><begin><length>
Fixed length,
used to request
a block of
pieces. The
payload
contains integer
values
specifying the
index, begin
location and
length.
7 piece <len=0009+X><id=7><index><begin><block>
Sent together
with request
messages. Fixed
length, X is the
length of the
block. The
payload
contains integer
values
specifying the
index, begin
location and
length.
8 cancel <len=13><id=8><index><begin><length>
Fixed length,
used to cancel
block requests.
payload is the
same as
‘request’.
Typically used
during ‘end
game’ mode.
A peer will be 'interested' in data if there is a peer which has the required pieces. If the
peer which has this data is not choked, then data will be transferred. After handshaking, by
default, connections start out as choked, and not interested.

BitTorrent Protocol
5.4 Data
BitTorrent is very versatile, and can be used to transfer a single file, of multiple files
of any type, contained within any number of directories. File sizes can vary hugely, from
kilobytes to hundreds of gigabytes.
5.4.1 Piece Size
Data is split into smaller pieces which sent between peers using the bittorrent
protocol. These pieces are of a fixed size, which enables the tracker to keep tabs on who has
which pieces of data. This also breaks the file into verifiable pieces, each piece can then be
assigned a hash code, which can be checked by the downloader for data integrity. These
hashes are stored as part of the 'metinfo file' which is discussed in the next section.
The size of the pieces remains constant throughout all files in the torrent except for
the final piece which is irregular. The piece size a torrent is allocated depends on the amount
of data. Piece sizes which are too large will cause inefficiency when downloading (larger risk
of data corruption in larger pieces due to fewer integrity checks), whereas if the piece sizes
are too small, more hash checks will need to be run.
As the number of pieces increase, more hash codes need to be stored in the metainfo
file. Therefore, as a rule of thumb, pieces should be selected so that the metainfo file is no
larger than 50 - 75kb. The main reason for this is to limit the amount of hosting storage and
bandwidth needed by indexing servers. The most common piece sizes are 256kb, 512kb and
1mb. The number of pieces is therefore: total length / piece size. Pieces may overlap file
boundaries.
For example, a 1.4Mb file could be split into the following pieces. This shows
5 * 256kb pieces, and a final piece of 120kb.

BitTorrent Protocol
Fig 5.4 : Pieces of a file
5.5 BitTorrent Clients
A BitTorrent client is an executable program which implements the BitTorrent
protocol. It runs together with the operating system on a users machine, and handles
interactions with the tracker and peers. The client is sits on the operating system and is
responsible for controlling the reading / writing of files, opening sockets etc.
A metainfo file must be opened by the client to start partaking in a torrent. Once the
file is read, the necessary data is extracted, and a socket must be opened to contact the
tracker. BitTorrent clients use TCP ports 6881-6999. To find an available port, the client will
start at the lowest port, and work upwards until it finds one it can use. This means the client
will only use one port, and opening another BitTorrent client will use another port. A client
can handle multiple torrents running concurrently.
Clients come in many flavours, and can range from basic applications with few
features to very advanced, customisable ones. For example, some advanced features are
metainfo file wizards and inbuilt trackers. These additional features means different clients
behave very differently, and may use multiple ports, depending on the number of processes it
is running. As all applications implement the same protocol, there is no incompatibility
issues, however because of various tweaks and improvements between clients, a peer may
experience better performance from peers running the same client.
5.6 Sub Protocols :
BitTorrent can be described in terms of two sub-protocols: one which describes
interactions between the tracker and all clients, and one which describes all client-to-client
interactions.
5.6.1 THP: Tracker HTTP Protocol
The tracker protocol is implemented on top of HTTP/HTTPS. This means that the
machine running the tracker runs a HTPP or HTTPS server, and has the behaviour described
below:

BitTorrent Protocol
1. The client sends a GET request to the tracker URL, with certain CGI variables and
values added to the URL. This is done in the standard way, i.e., if the base URL is
“http://some.url.com/announce”, the full URL would be of this form:
“http://some.url.com/announce?var1=value1&var2=value2&var3=value3”.
2. The tracker responds with a “text/plain” document, containing a bencoded dictionary.
This dictionary has all the information required for the client.
3. The client then sends re-requests, either on regular intervals, or when an event occurs,
and the tracker responds.
The CGI variables and values added to the base URL by the client sending a GET
request are:
 info_hash: The 20 byte SHA1 hash calculated from whatever value the info key maps
to in the metainfo file.
 peer_id: A 20 character long id of the downloading client, random generated at start
of every download. There is no formal definition on how to generate this id, but some
client applications have adapted some semiformal standards on how to generate this
id.
 ip: This is an optional variable, giving the IP address of the client. This can usually be
extracted from the TCP connection, but this field is useful if the client and tracker are
on the same machine, or behind the same NAT gateway. In both cases, the tracker
then might publish an unroutable IP address to the client.
 port: The port number that the client is listening on. This is usually in the range 6881-
6889.
 uploaded: The amount of data uploaded so far by the client. There is no official
definition on the unit, but generally bytes are used
 left: How much the user has left for the download to be complete, in bytes.
 event: An optional variable, corresponding to one of four possibilities:
• started: Sent when the client starts the download
• stopped: Sent when the client stops downloading
• completed: Sent when the download is complete. If the download is complete
at start up, this message should not be sent.

BitTorrent Protocol
• empty: Has the same effect as if the event key is nonexistent. In either case,
the message in question is one of the messages sent with regular intervals.
There are some optional variables that can be sent along with the GET request that are
not specified in the official description of the protocol, but are implemented by some tracker
servers:
 numwant: The number of peers the client wants in the response.
 key: An identification key that is not published to other peers. peer_id is public, and
is thus useless as authorization. key is used if the peer changes IP number to prove it’s
identity to the tracker.
 trackerid: If a tracker previously gave its trackerid, this should be given here.
As mentioned earlier, the response is a “text/plain” response with a bencoded dictionary.
This dictionary contains the following keys:
 failure reason: If this key is present, no other keys are included. The value mapped to
this key is a human readable string with the reason to why the connection failed.
 interval: The number of seconds that the client should wait between regular
rerequests.
 peers: Maps to a list of dictionaries, that each represent a peer, where each dictionary
has the keys:
• peer_id: The id of the peer in question. The tracker obtained this by the
peer_id variable in the GET request sent to the tracker.
• ip: The address of the peer, either the IP address or the DNS domain name.
• port: The port number that the peer listens on.
These are the keys required by the official protocol specification, but here as well
there are optional extensions:
 min interval: If present, the client must do rereqests more often than this.
 warning message: Has the same information as failure reason, but the other keys in
the dictionary are present.
 tracker id: A string identificating the tracker. A client should resend it in the
trackerid variable to the tracker.
 complete: This is the number of peers that have the complete file available for upload.

BitTorrent Protocol
 incomplete: The number of peers that not have the complete file yet.
5.6.2 PWP: Peer Wire Protocol
The peer wire (peer to peer) protocol runs over TCP. Message passing is symmetric,
i.e. messages are the same sent in both directions. When a client wants to initiate a
connection, it sets up the TCP connection and sends a handshake message to the other peer. If
the message is acceptable, the receiving side sends a handshake message back. If the initiator
accepts this handshake, message passing can initiate, and continues indefinitely. All integers
are encoded as four byte big-endian, except the first length prefix in the handshake.
Handshake message
The handshake message consists of five parts:
 A single byte, containing the decimal value 19. This is the length of the character
string following this byte.
 A character string “BitTorrent protocol”, which describes the protocol. Newer
protocols should follow this convention to facilitate easy identification of protocols.
 Eight reserved bytes for further extension of the protocol. All bytes are zero in current
implementations.
 A 20 byte SHA1 hash of the value mapping to the info key in the torrent file. This is
the same hash sent to the tracker in the info_hash variable.
 The 20 byte character string representing the peer id. This is the same value sent to
the tracker.
If a peer is the first recipient to a handshake, and the info_hash doesn’t match any
torrent it is serving, it should break the connection. If the initiator of the connection receives a
handshake where the peer id doesn’t match with the id received from the tracker, the
connection should be dropped. Each peer needs to keep the state of each connection. The
state consists of two values, interested and choking. A peer can be either interested or not in
another peer, and either choke or not choke the other peer. Choking means that no requests
will be answered, and interested means that the peer is interested in downloading pieces of
the file from the other peer.
This means that each peer needs four Boolean values for each connection to keep
track of the state.
• am_interested

BitTorrent Protocol
• am_choking
• peer_interested
• peer_choking
All connections start out as not interested and choking for both peers. Clients should keep
the am_interested value updated continuously, and report changes to the other peer. The
messages sent after the handshaking are structured as: [message length as an integer] [single
byte describing message type] [payload] Keep alive messages are sent with regular intervals,
and they are simply a message with length 0, and no type or payload.
Type 0, 1, 2, 3 are choke, unchoke, interested and not interested respectively. All of
them have length 1 and no payload. These messages simply describe changes in state.
Type 4 is a have. This message has length = 5, and a payload that is a single integer,
giving the integer index of which piece of the file the peer has successfully downloaded and
verified.
Type 5 is bitfield. This message is only sent directly after handshake. It contains a
bitfield representation of which pieces the peer has. The payload is of variable length, and
consists of a bitmap, where byte 0 corresponds to piece 0-7, byte 1 to piece 8-15 etc. A bit set
to 1 represents having the piece. Peers that have no pieces can neglect to send this message.
Type 6 is a request. The payload consists of three integers, piece index, begin and
length. The piece index decides within which piece the client wants to download, begin gives
the byte offset within the piece, and length gives the number of bytes the client wants to
download. Length is usually a power of two.
Type 7 is a block. This message follows a request. The payload contains piece index,
length and the data itself that was requested. Type 8 is cancel. This message has the same
payload as request messages, and it is used to cancel requests made. Peers should
continuously update their interested status to neighbours, so that clients know which peers
will begin downloading when unchoked.

BitTorrent Protocol
6. Vulnerabilities of BitTorrent
6.1Attacks on BitTorrent
As we have seen so far, BitTorrent is one of most favoured file transfer protocol in
today’s world. But it has been exposed to various attacks in the recent past due to the
vulnerabilities that are being exploited by the hacker community. Here are some of the
attacks that are commonly seen.
6.1.1 Pollution attack
1. The peers receive the peer list from the tracker.
2. One peer contacts the attacker for a chunk of the file.
3. The attacker sends back a false chunk.
4. This false chunk will fail its hash and will be discarded.
5. Attacker requests all chunks from swarm and wastes their upload bandwidth.
Pollution attacks have become increasingly popular and have been used by
anti-piracy groups. In 2005 HBO used pollution attacks to prevent people from downloading
their show Rome.
6.1.2 DDOS attack
DDOS stands for Distributed denial of service. This attack is possible because
of the fact that BitTorrent Tracker has no mechanism for validating peers. This means there is
no way to trace the culprit in these kind of attacks. Also attacks of this stature are possible
because of the modifications that can be done to the client software.
1. The attacker downloads a large number of torrent files from a web server.
2. The attacker parses the torrent files with a modified BitTorrent client and
spoofs his IP address and port number with the victims as he announces he is
joining the swarm.

BitTorrent Protocol
3. As the tracker receives requests for a list of participating peers from other
clients it sends the victims IP and port number.
4. The peers then attempt to connect to the victim to try and download a chunk of
the file.
6.1.3 Bandwidth Shaping
Many ISPs don’t encourage the use of BitTorrent from their users. This is because
BitTorrent is usually used to transfer large sized files due to which the traffic over the ISPs
increase to a large extent. To avoid such exploding traffic on their servers many ISPs have
started to avoid the traffic caused by BitTorrent. This can be done by sniffing the packets that
pass through and detecting whether they oblige BitTorrent protocol. ISPs make use of filters
to find out such packets and block them from passing their servers. This has resulted in many
file transfer breakdowns across the world.
6.2 Solutions
Many of the attacks that BitTorrent suffers have been dealt with and some measures
have been taken to avoid such attacks. Here are a few solutions to the attacks that were
discussed above.
6.2.1 Pollution attack
The peers which perform such attacks are identified by tracing their IPs. Then, such
IPs are blacklisted to avoid further communication with them. These blacklisted IPs are
blocked by denying them connections with other peers. This is done by using software like
Peer Guardian or moBlock, which download the list of blacklisted IPs from internet.
6.2.2 DDOS attack
The main solution to this kind of attack is to have clients parse the response from the
tracker. In the case where a host (tracker) does not respond to a peer’s request with a valid
BitTorrent protocol message it should be inferred that this host is not running BitTorrent. The
peer should then exclude hat address from its tracker list, or set a high retry interval for that
specific tracker. Another fix would be for web sites hosting torrents to check and report
whether all trackers are active, or even remove the on-responding trackers from the tracker

BitTorrent Protocol
list in the torrent. Another measure could be to restrict the size of the tracker list to reduce the
effectiveness of such an attack.
6.2.3 Bandwidth Shaping
There are broadly two approaches followed to counter this type of attacks. The first
method is to encrypt the packets sent by the means of BitTorrent protocol. By doing this, the
filters that sniff packets will not be able to detect such packets belonging to BitTorrent
protocol. This means that the filters are fooled by the encrypted packets and thus packets can
sneak through such filters. Another approach is to make use of tunnels. Tunnels are dedicated
paths where the filters are avoided by using VPN software which connects to the unfiltered
networks. This results in successfully bypassing the filters and thus the packets are
guaranteed to be transmitted across networks.

BitTorrent Protocol
7. Conclusion
BitTorrent pioneered mesh-based file distribution that effectively utilizes all the
uplinks of participating nodes. Most followon research used similar distributed and
randomized algorithms for peer and piece selection, but with different emphasis or twists.
This work takes a different approach to the mesh-based file distribution problem by
considering it as a scheduling problem, and strives to derive an optimal schedule that could
minimize the total elapsed time. By comparing the total elapsed time of BitTorrent and CSFD
in a wide variety of scenarios, we are able to determine how close BitTorrent is to the
theoretical optimum. In addition, the study of applicability of BitTorrent to real-time media
streaming applications, shows that with minor modifications, BitTorrent can serve as an
effective media streaming tool as well.
BitTorrent’s application in this information sharing age is almost priceless. However,
it is still not perfected as it is still prone to malicious attacks and acts of misuse. Moreover,
the lifespan of each torrent is still not satisfactory, which means that the length of file
distribution can only survive for a limited period of time. Thus, further analysis and a more
thorough study in the protocol will enable one to discover more ways to improve it.

BitTorrent Protocol
8. References
1. BitTorrent Inc. (2006) http://www.bittorrent.com
2. BitTorrent.Org (2006) http://www.bittorrent.org/protocol.htm
3. Cohen, Bram (2003) Incentives Build Robustness in BitTorrent, May 22 2003
http://www.bitconjurer.org/BitTorrent/bittorrentecon.pdf
4. Cachelogic, BitTorrent bandwidth usage
http://www.cachelogic.com/research/2005_slide06.php
5. Information on BitTorrent Protocol
en.wikipedia.org/wiki/BitTorrent_(protocol)
6. BitTorrent FAQ: http://btfaq.com
7. BitTorrent Specifications http://wiki.theory.org/BitTorrentSpecification
8. Other Information http://www.dessent.net/btfaq/#compare

BitTorrent Protocol Overview

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