Bit-Torrent (BT) is a widely used protocol for file shar- ing over the network in an efficient way where a single file is effectively accessed by large number of users. Bit-Torrent is based on peer to peer networking (P2P) and it improves the transfer speed drastically by splitting the file into segments and collecting the segment needed from the other peers that already have them. The Bit-Torrent technique shows effec- tive resource utilization among hundreds of connected peers accessing the same file by not compensating on their down- load speed. This paper discusses the working principle of Bit-Torrent , framework behind its operation, proposed modifications in its working and what the future might hold for this innovative approach in order to serve files over the peer to peer social network.

1. Sharing large files through Bit Torrent in Peer-to-Peer social networks
Kailaash Balachandran
Project Group - A Peer-to-Peer Framework for Social Networks
Universit¨at Paderborn, Supervisor: Dr.-Ing. Kalman Graffi
Email: kailaash@mail.uni-paderborn.de
Abstract
Bit-Torrent (BT) is a widely used protocol for file shar-
ing over the network in an efficient way where a single file is
effectively accessed by large number of users. Bit-Torrent is
based on peer to peer networking (P2P) and it improves the
transfer speed drastically by splitting the file into segments
and collecting the segment needed from the other peers that
already have them. The Bit-Torrent technique shows effec-
tive resource utilization among hundreds of connected peers
accessing the same file by not compensating on their down-
load speed. This paper discusses the working principle
of Bit-Torrent , framework behind its operation, proposed
modifications in its working and what the future might hold
for this innovative approach in order to serve files over the
peer to peer social network.
1 Introduction
In recent times, Bit-Torrent has become one of the most
popular peer to peer content distribution technique over the
web and it has higher share of Internet traffic than all other
peer to peer networks. Bit-Torrent was installed on 28.20
percent of computers worldwide in September 2007 and
this number is certainly higher given the current trend of
Torrent usage [1]. A recent Internet traffic trend report re-
leased by the Canadian broadband management company
Sandvine reveals that bandwidth usage patterns during peak
hours of Internet usage show that of 29.97 percent of the
upstream stream can be attributed to Bit-torrent [2] and the
average download speed is about 30 KB/s, which is quite a
acceptable rate to get large files in a day [3]. Bit-torrent’s
popularity has paved way for a number of third party torrent
clients available for free download which provides ease to
use interfaces and one click download feature to their users,
which, on the other hand, contributed to the success of Bit-
Torrent.
The Bit-Torrent’s operation is based on hybrid decentral-
ization of the content which makes it different from the tra-
ditional server client download technique. In traditional
file sharing technique, to download a file, the client ,usually
,the browser requests the server that holds the file to trans-
fer a copy of the file to the clients system. This transfer is
handled by protocols such as FTP ( File Transfer Protocol)
or HTTP ( Hyper Text Transfer Protocol). Here, the down-
load speed is greatly affected by a number of factors such
as amount of traffic on the server , the protocol used for the
transfer and also the number of computers trying to down-
load the file at the same time. The transfer speed varies de-
pending on the demands of the server. However , in peer to
peer file sharing method, a P2P client is used on every par-
ticipating nodes. The client sends out a request for the file
to download and to locate the file, the client queries other
nodes that are connected. When the client finds a node that
have the requested file,the download begins. The shared file
is chopped into pieces and every computer that downloads
these pieces, also uploads them to each other. These pieces
are replicated on different computers as soon as possible.
Once a computer has a complete piece, it is then traded with
other computers that needs them. Thus every computer that
participates in the transfer acts as both, server and client.
A major factor behind Bit-Torrent’s success is the built-in
incentives mechanism, implemented by its choking algo-
rithm, also called the Tit-for-tat algorithm which encour-
ages every active node to upload pieces to each other. The
file transfer load is shared among the computers exchanging
files, but can cause bottlenecks when a node downloads the
file and immediately disconnect without allowing others to
obtain files. This limits the number of computers the soft-
ware can search for the requested file. Here, we’ll discuss
the working principle of Bit-torrent and how it solves these
problems.
The rest of this paper is organized as follows. Section
2 provides a description of the terminologies, operation of
the Bit-Torrent protocol and an explanation of its piece se-
lection and peer selection policies as implemented in the of-
ficial Bit-Torrent client. Section 3 describes the drawbacks
with conventional method along with the proposed and de-

2. ployed modifications to bit-torrent’s framework. Section 4
provides an overview on file sharing in a collaborative P2P
social networks, introduces the new paradigm of friend to
friend network and lists out challenges faced in implement-
ing an effectve file sharing feature. Section 5 presents bit-
torrent in P2P social network by providing a basic paradigm
on how a bit-torrent system reacts in a social environment
and lists some of the social community biased optimizations
to its framework that can be adapted in our project. Section
6 concludes.
2 Bit-Torrent Operation
2.1 Terminologies
Here, we define the terminologies used throughout this
paper though they are not of standardized definitions.
• Peer: A peer is a active computer node that participates
in the torrent file transfer. A peer can be in one of the
two states. The leecher state and the seed state.
• Seeder: A seeder is a peer that has a complete copy of
the torrent and still offers it for upload. The more the
number of seeders, the better are the chances of getting
a higher download rate.
• Swarm: The group of peers active on a torrent. Active
means those peers that transfer content associated with
the torrent either as a seed or a peer.
• Leecher: Leecher is a peer that still downloads pieces
of the content and becomes a seed once it gets the com-
plete file.
• Tracker: A tracker is a server that keeps track of which
seeds and peers that are in the swarm. The tracker is
centralized and a new peer wishing to join the torrent,
contacts the tracker to get the list of peers that have the
pieces of the file.
• .Torrent file: The .torrent file, also referred to as a
Metainfo file, has all informations necessary to down-
load the content and the number of pieces. A torrent
file does not contain the content but only the informa-
tion about those files, their names, sizes, structure, and
the hash values for verifying file integrity.
• Index: An index is a list of .torrent files published and
managed by a website. It acts as a search engine for
users to search and download .torrent files.
2.2 Torrent Client
A Torrent client is a software program that facilitates
peer to peer based file sharing over Bit-Torrent protocol.
Most of these Bit-torrent clients are free and open source
softwares. They provide a clean, user friendly interfaces
Figure 1. Overview of Bit-Torrent’s Operation,
Source:[4]
where the users launch by clicking on a hyper-link and are
given a standard ’Save as’ dialog [5]. These torrent clients
vary in their functions providing unique features and also in
their design framework which shows difference in its oper-
ating behaviors. The paper [6] , observes the performances
of two major famous clients, namely uTorrent and Vuze,
where uTorrent users achieve more download speed than
the users of Vuze for the same set of torrents [6]. Perfor-
mance of each client differs mainly on (a) How they man-
age their neighborhood size (b) How to find new connected
peers (c) When do they close a connection and (d) How do
they distribute upload capacity. Thus, the design choices
and framework of bit-torrent clients have significant effect
on the download speed.
2.3 Content Publishing
When a user decides to share a file using bit-torrent,
the peer distributing the file uses torrent client to chop the
file into identical sized pieces, creates hash for every single
piece and is recorded in the .torrent file. Peers that has the
complete file are called seeders, and the peer providing the
first copy, generally the torrent owner, is called the initial
seeder. These Torrents have an ”announce” section where
URL of the tracker is mentioned followed by an info sec-
tion containing name of the files, number of pieces, their
size and hashes for each piece. These are verified by clients
to check integrity of the data that they receive. Torrent files
are typically uploaded on websites such as index sites, with
at least one tracker. The tracker maintains lists of the peers
currently participating in the torrent. The initial seeder who
has the complete file should start seeding to make the file
available for other connected peers to download.
2.4 Role of a Tracker
A tracker is a server that helps in the communication
among peers participating in the transfer [7][8]. To make
it simple, a tracker just keeps track of what is happening

3. on the network. Peers regularly update the tracker to nego-
tiate with the new peers and they contain complete statis-
tics of the transfer. When users create a torrent file, they
would need to specify a tracker URL to the announce sec-
tion in the client. The more trackers you specify, the faster
the download speed will be. Some index sites provide their
own trackers where it also hosts the torrent file providing all
needed features to start a torrent download. Torrent clients
also allow its users to create their own tracker by fetching
the IP address of the system and the listening port number
followed by announce section.
2.4.1 Public Tracker
The public tracker, as the name specifies , is open to pub-
lic, Any one can use the tracker where the DHT and local
peer discovery are enabled in order to spread far and wide.
Public trackers are more prone to Free riding, where peers
benefits from resources (download) without paying for the
cost of the benefit (upload). In short, free riders are those
selfish peers that only download pieces without contributing
to its reciprocal upload.
2.4.2 Private Tracker
The defining factor of private bit-torrent trackers is that
they’re selective about who gets to use the site. Unlike pub-
lic trackers, on which anyone can search and download tor-
rents, private trackers require you to be invited by another
user or create an account in order to use the service. Each
site using the private tracker will have their own set of rules
about who and how the site can be accessed. Because of
these rules, you get a lot of benefits, including fast down-
load speed, quality content, privacy and a great community.
2.4.3 Multi-trackers
Multi-tracker torrents contain multiple trackers in a single
torrent file. This provides redundancy in the case that one
tracker fails, the other trackers can continue to maintain the
swarm for the torrent. One disadvantage to this is that it be-
comes possible to have multiple unconnected swarms for a
single torrent where some users can connect to one specific
tracker while being unable to connect to another. This can
create a disjoint set which can impede the efficiency of a
torrent to transfer the files it describes [8].
2.4.4 Tracker-less torrent
Even though bit-torrent works on decentralized P2P net-
work, it highly relies on the centralized tracker servers. The
simple solution is to go tracker less [9], which means there
is no central tracker server. DeCentralized or Trackerless
torrent is discussed elaborately in section 5.2.
2.5 Piece Selection Stratergy
In torrents, the file to be shared is broken into pieces and
further into sub pieces and these pieces are negotiated and
transferred between the peers. When a peer starts to down-
load, it defines free spaces for the whole file, then it search-
ing for different pieces of the file from the connected peers
to download. Peers download these pieces in a random or-
der , not necessarily from start to end piece. Adopting a
appropriate piece selection strategy plays a major role in
the overall performance.
2.5.1 Strict Priority
A single piece may have a number of sub pieces associ-
ated with it and different peer may contain these sub pieces.
When a particular peer requests for a single sub piece, all
the remaining sub pieces of the parent piece are requested
before requesting any other sub pieces of other pieces. This
policy helps downloading a complete piece as quickly as
possible.
2.5.2 Rarest First
After allocating required space for the whole file, the peer
now searches for pieces to download from connected peers.
While selecting the pieces, the peer usually selects the piece
which is found in rare in its connected network. This rarest
first technique makes sure that all pieces are available on
the network and gets replicated such that the overall perfor-
mance is not affected even when a seeder who has the com-
plete file goes off line. In some practical deployments, the
initial seeder goes offline due to cost measures leaving the
current leechers to exchange and share pieces among each
other. In such cases, rarest first approach plays a significant
role by not loosing a piece to go out of the network.
2.5.3 Random piece first
This is an exception to rarest first approach. When a peer
starts the download, it has got nothing to upload for the
other peers. The rarest pieces are present generally on fewer
peers and it takes longer download time as it rate will be
slow. So the peer selects a random piece and downloads it
as quickly as possible so that it gets its first complete piece
, thus joining the swarm of leechers and starts to upload to
other peers. Once it gets the first complete piece, it adopts
to rarest first technique.
2.5.4 End Game Mode
When a peer is about to complete the transfer, it will left
with fewer number of pieces to download. Sometimes those
pieces will be requested from a peer with slow transfer rate
rather than a peer with higher rate. It will drastically delay

4. Figure 2. Combined overview of piece selec-
tion stratergies used by a peer from down-
load start to end, Source: [4]
the download finish. In end game piece selection algorithm ,
the peer sends out request to all peers for the sub pieces that
is needed to complete the download. Once the piece gets
downloaded from peer that has higher transfer rates, cancel
messages are sent to all appending peers to avoid redundant
sends. The end game mode is very short and doesn’t have
much impact on the overall download process [10].
2.6 Peer Selection Strategy
Bit-torrent is a decentralized network and it does no cen-
tral resource allocation. Each participating peer should be
responsible to maximize their download rate [5]. Peers
download pieces from different peers and upload simulta-
neously to a limited number of peers. In general economics,
Pareto efficiency refers to the context where no two counter
parties can make an exchange and both be happier at the
same time. Seeking pareto efficiency is a local optimization
algorithm in which pairs of counterparties see if they can
improve their lot together,and such algorithms tend to lead
to global optima. Specifically, if two peers are both getting
poor reciprocation for some of the upload they are provid-
ing, they can often start uploading to each other instead and
both get a better download rate than they had before. Chok-
ing algorithms in bit-torrent allows a peer to select other
peer to which they can exchange data thus achieving Pareto
efficiency [5].
2.6.1 The Choking Algorithm
In a Bit-torrent environment, every peer need to download
and upload at the same time. To cooperate peers upload,
to not to cooperate they choke [5]. For example, peer ’A’
chokes peer ’B’ if peer A doesn’t want to upload pieces to
peer B but download continues to happen. Choking is anal-
ogous to temporary refusal to upload to other peers. Chok-
ing mechanism follows Tit for Tat policy where a peer up-
loads to another peer only if the second peer uploads pieces
in exchange. As a consequence, free riders, i.e., peers that
never upload, are penalized as they get choked.
A peer unchokes a number of connected peers (4 peers
by default) to exchange pieces with and the number is lim-
ited to avoid TCP congestions over the network. Out of 4
unchokes, 3 peers are unchoked purely based on their trans-
fer rate and 1 is randomly unchoked regardless the transfer
rate which is called the optimistic unchoke ( discussed in
section 2.6.2). A peer decides about which peer to unchoke
purely based on their transfer rate which is calculated for
every 10 seconds. Those peer that show good transfer rates
gets unchoked and re-unchoking happens when the next ten
second period is up. Choking Algorithms differs for seeders
and leechers. A seeder usually uploads pieces to all peers
in a round robin fashion as they have the complete file and
does not require any uploads to them.
2.6.2 Optimistic Choking
Regular Unchoking method may miss out some unused con-
nections that have better transfer rates than the ones that are
being unchoked. To fix this issue, bit-torrent uses optimistic
unchoking mechanism where a peer is unchoked in random
and the optimistic unchoke is recycled every third rechoke
period , which is thirty seconds ,claiming that thirty sec-
onds is a long enough period of time for TCP to ramp up
new transfers to their full capacity and new connections are
three times as likely to start at the current optimistic un-
choke as any other connection in the rotation. The opti-
mistic unchoke peer selection has two purposes. It allows
to evaluate the download capacity of new peers in the peer
set, and it allows to bootstrap new peers that do not have
any piece to share by giving them their first piece [11] [12].
2.6.3 Anti Snubbing
Sometimes a peer with higher transfer rates will get choked
for longer duration affecting its download speed. Such peer,
that receives nothing from a particular peer for more than
one minute, is assumed to be snubbed and only a optimistic
unchoke can release the peer from being snubbed. This re-
sults in more than one optimistic unchoke to recover the
download rates of the snubbed peer.
3 Proposed Modifications in Bit-Torrent’s
working
Bit-torrent systems are highly popular and are consid-
ered to be a viable alternative to traditional client-server
model as they are decentralized. As the content is not stored
in a central server, every participating peer is dependent

5. on each other as they download and upload pieces of file
. Even though Bit-torrent provides incentives to ensure fair-
ness among participating peers, there exists peers called
free riders who downloads resources from other peers but
do not share ( upload ) to any. This causes degradation in
the overall performance of the system. Also despite the im-
plementation of the Tit-for-tat principle, the fairness among
peers doesn’t seem to be impressive. Much research has
been done to improve fairness among peers but it has been
observed that peers with high upload bandwidth frequently
upload much more data than they download, with the op-
posite being the case for peers with low upload bandwidths
[11]. The following algorithms have been proposed to im-
prove the working of bit torrent.
3.1 Conditional Optimistic Unchoke
In Conditional Optimistic Unchoke, the Instantaneous
Fairness Ratio (IFR) is calculated for for an individual peer
as the ratio of data uploaded to data downloaded during the
last 10 seconds. Therefore, an IFR of less than 1 indicates
a peer is downloading an excessive amount and an IFR of
greater than 1 indicates a peer is downloading an insufficient
amount. Optimistic Unchoke happens only if the peer’s IFR
is greater than 1. The Peer need to be cautious for not down-
loading excessively and choking other peers may change the
set of peers from where its downloading thus reducing its
download amount [11].
3.2 Variable Number of Outgoing Con-
nections
Instead of having fixed number of connections for each
peer, this method proposes having variable number of con-
nections depending on the current upload capacity of the
peer. A peer with high upload capacity might have connec-
tion with more number of low capacity peers so that they
can serve them better whereas a low capacity peer connect
to limited number of peers. A pair of peers have multiple
connections with each other thus enabling peers with more
capacity can transfer data to one another at high rates[11]
3.3 Treat Before Trick
Treat Before Trick solves the problem of free riding to
some extent. Each file is encrypted by the owner or the
seeder. The key used in the encryption mechanism is again
split into ’n’ sub keys and pieces are sent along with sub
keys, i.e each piece has a subkey. These sub keys have
no dependence on the piece they have been sent. The le
owner uploads both the le pieces and subkeys to a set of re-
questing peers, called leechers. Now the leechers can barter
pieces for subkeys with each other such that every leecher
will have to upload the pieces in order to get the subkey to
decrypt the piece.The rule Data First Key Later is followed
by peers thus making every peer to upload a piece before re-
questing for the key. Here, peers swap le pieces for subkeys
and vice versa, which are needed in order to decrypt the le
pieces [13].
3.4 Super Seeding
This technique helps when we need to seed a large tor-
rent with limited upload capacity. Seed acts as normal
leechers by not claiming to have the complete file. When
the seed gets connected to leechers, it uploads a piece to a
leecher that has been never uploaded before and does not
upload another piece until it gets acknowledgement from
other leechers that the sent piece has been uploaded again.
This helps great for the seeds that have less upload capac-
ity or have to pay for the upload bandwidth as this method
limits the number of duplicated pieces[14]. Super seeding
can help save an upload ratio of around 20 percent of the
inital seeder but it stalls when there are very less leechers.
It works best when the upload speed of the seed is greater
than that of individual peers.
3.5 Tracker Protocol Extension
When a peer enters a torrent, it is called a leecher since
it does not contain any piece. It contacts the tracker for the
random list of peers and to achieve more optimal bandwidth
utilization, it connects to the peers that have similar trans-
fer rates. If there are very less peers with similar rates are
present, it takes time to discover them and the only way to
do so is by optimistic unchokes. It would be more prefer-
able if the tracker that the peer initially connects to can per-
form this operations as it might save more time rather than
waiting for an optimistic unchoke. The new peers that con-
tacts the tracker to join the torrent, reports its upload capac-
ity to the tracker and the tracker will reply with list of peers
with similar transfer capacities. Optimistic Unchoke is then
performed every 30 seconds so as to discover new partners
sooner with same upload capabilities[14].
3.6 Team Enhanced Protocol
This is a slight modification over the tracker protocol
extension. Trackers coordinates peers to form teams with
same upload limit while joining the torrent. In addition to
that, tracker enforces rules for a peer joining the team in or-
der to avoid free riding. Firstly, it limits a peer to change
teams frequently by tracking its history of team joins within
a short interval of time. Secondly, the tracker do not al-
low a peer to join a team if it doesnt satisfy or found non-
compliant with the team protocol in the past. For example
if the peer had lied about its upload bandwidth before , the
peer is blacklisted by the tracker and is not allowed to join
in the team. Thus it prevents the entry of a peer that tries to
free ride [4].
4 File Sharing in a social P2P environment
Online centralized file sharing social networks certainly
have limitations over privacy concerns. Particularly privacy

6. and protection from massive data-mining and big-brotherly
treatment of the users by the social networking service
providers. By mapping a peer-to-peer application to a peer-
to-peer infrastructure, direct connections can be exploited
such that locality can be taken into account. Even though
peer-to-peer file sharing systems do exist, they focus more
on the technical aspects and therefore unable to exploit the
power of social phenomena. Even though the bit-torrent
methodology has been proven successful, it is still vulner-
able to free riding and tracker offline scenarios. Integrat-
ing Bit-torrent technology in a peer-to-peer social network
faces a lot of challenges where security plays the major role.
4.1 F2F Network
Friends-friends network is a new paradigm after peer-
to-peer(P2P). In an F2F network [15] two computers can
communicate with one another only if their owners are re-
lated in the social network. Since the two owners know one
another they can communicate either in person, through an
instant messaging service or any other means. This side
channel can be used to set up the connections between users
computers, i.e. by exchanging their network addresses and
possibly some cryptographic data for securing the link.
4.2 Social trust becomes P2P trust
One of the biggest and largely unsolved problems of P2P
systems is security. Because any peer can connect to any
other peer this makes the systems extremely vulnerable.
Peers can arbitrarily deviate from the P2P protocols, appear
in the network under many identities or selfishly use the re-
sources of others while not contributing their own. Even
when peer misbehavior is detected the rogue peers need to
be isolated from the network, which is extremely hard to do
without having a stable peer identity. Open P2P systems are
essentially groups of strangers talking with each other, trust
is volatile and difficult to build. In F2F networks the peers
have a well defined identity and the users bring the social
trust into the system. A user in the system knows that the
computers that her computer is communicating with belong
to her friends and can be trusted to work as expected. If
one of the computers does misbehave and this is detected,
the users can be notified about it and undertake the neces-
sary action, e.g. possibly contact the owner of the rogue
computer. Strong identity allows for self-policing and user-
mediated repair in F2F systems. F2F greatly reduces most
of the P2P systems vulnerabilities by bringing the social
trust into them. This happens at a cost though. The clas-
sical P2P systems rely on being able to connect to any peer
in the network while in F2F, connections are only possible
among friends.
4.3 Challenges faced in implementing file
sharing feature in P2P social networks
• The Availability Problem:Guaranteeing the availabil-
ity of P2P system is a major challenge in implementing
file sharing feature. The system should not rely on the
availability of seeds or any central resource manager.
In [16], it is estimated that less than 4 percent of the
peers have an uptime of over 10 hours. More number
of peers tend to leave the system quite often leaving the
system with less seeders or no seeders in worst cases.
Hence, the availability problem is critical. Providing
more incentives such as social recognition and awards
could make users to run the P2P software for longer
duration, thus improving the availability of the system
as a whole.
• System Integrity: Maintaining the integrity of the sys-
tem is the another major concern and how well the sys-
tem can be trusted. The only solution for this is to
achieve trust among peers. Free riders are the so called
peers that do not cooperate with the system by self-
ishly not donating resources to other peers. So all peers
cannot be trusted and maintaining system integrity has
proven to be a great challenge in operational systems.
This problem could be solved in a social based P2P
network as users know each other or the group and can
also select trusted users as representatives [17].
5 Bit-Torrent for P2P Social Networks
5.1 Private Trackers
Usage of private trackers, though centralized , provides
greater benefits include larger selections of files, faster
downloads, higher quality files, and strong communities be-
hind them. They come with a lot of rules and regulations.
These rules may include Rules regarding ratio which corre-
sponds to upload/download ratio, rules regarding who can
access the site, rules regarding the quality of the file and
even more, which, in other hand improves wide selection of
nonpoisoned torrents and provides a lot of benefits such as
• High Quality Files: Because of the rules, only high
quality torrents gets uploaded. If a user uploaded tor-
rent doesn’t satisfy the quality rule, it is deleted auto-
matically.
• Faster Downloads: The rule of Private torrent is all
about seeding and have mechanisms to prevent free
riding resulting in faster downloads..
• Increased Privacy: Here, torrents are kept private from
the public eye. Compared to other public torrents, pri-
vate torrent sites are less traveled and users are forced
to register or get invite to access the site, which again
improves privacy.
5.2 DeCentralized Trackers
Bit-Torrent is a peer-to-peer approach and even though
it is claimed to be decentralized, the download process still

7. relies in part on central servers called trackers that can crash
or go offline for a variety of reasons. To address this vul-
nerability, bit-torrent downloads should no longer require
a central server, meaning that no central trackers. In to-
days deployment, Tribler client allow users to search for
torrents and download files without the need for any cen-
tral server, but the current client still lacks a solution for
decentralized tracking of swarms . In the paper [9] , they
have proposed completely removing the tracker and replac-
ing it with a set of distributed protocols based on random
walks, accomplishing the work of the tracker without ex-
plicitly tracking every peer in the system.
The distributed tracking algorithm , 2-Hop TorrentSmell,
is used in the implementation of Tribler Client [18] . Mod-
ern bit-torrent clients can exchange their neighborhood sets
through Peer Exchange (PEX). PEX greatly reduces the re-
liance of peers on a tracker by allowing each peer to directly
update others in the swarm as to which peers are currently
in the swarm. In 2-Hop TorrentSmell algorithm [19] , we
track peers in the swarm by RePex ( Reconnect to peers en-
countered before ) and find the distributed trackers tracking
the swarm. Much research has been done in the field of de-
centralizing trackers but they are not deployed anywhere of-
ficially outside the research domain. If such system comes
in the future, there is definitely no way for anyone to mon-
itor, ban or bring down a P2P network as it is completely
decentralized.
5.3 De-Indexing
There are bit-torrent search engines and indexes such as
The Pirate Bay and isoHunt. They search for the requested
content and download the .torrent file. Decentralization of
the index search can be done using the gossip protocol [20]
that can act over the bit-torrent engine. The searched key
word will be passed on to the neighboring connected nodes
like a gossip. It spreads on throughout the swarm return-
ing the result of the search. Gossip protocols are designed
to operate in very large, decentralised networks. A peer to
peer network keeps changing rapidly Ias new nodes arrive
and existing nodes depart after download or even abruptly.
In such cases, , gossip protocols perform well as informa-
tion exchange happens in a random periodic way. Loss of
data wouldn’t affect the system as no nodes are assigned
with specific tasks and are not given any roles . In spite
of loosing a data, copies of the same can be received from
multiple nodes. If a node fails or goes off line, it doesn’t
affect other nodes from sending messaged thus there won’t
be any single point of failure in the system.
5.4 BuddyCast algorithm
The list of content a person downloads via bit-torrent
can be considered the taste of the user.Through its down-
loads the user builds up a preference list of content. The
preference list contains by default all downloaded les from
which the user can add or remove entries. These preference
lists are exchanged freely amongst peers using the Buddy-
cast algorithm. Using this algorithm the user builds Cache.
The recommender component uses the Preference Cache to
calculate both similarity between peers and to recommend
certain content the user is predicted to like, using a special
collaborative ltering algorithm. When a certain peer has a
preference list with high similarity to the users they have
the same download taste. We call such similar peers taste
buddies. [17]
To enable effective content based discovery, every peer
provides a list of preference of files , by default, with re-
cently downloaded files. In Buddycast algorithm, Each peer
maintains a list of its top-N most similar peers along with
their current preference lists. Periodically, a peer connects
to either (a) one of its buddies to exchange preference lists
(exploitation), or (b) to a new peer, randomly chosen, to
exchange this information (exploration). To maximize the
exploration of the social network, every peer also maintains
a list with the K most recently visited random peers, and
avoids reconnecting to a peer already present in the list. In
contrast to other epidemic protocols such as Newscast, we
use both exploitation and exploration branches, we limit the
randomness of peer selection during the exploration, and we
implicitly cluster peers into (trusted) social groups [17].
5.5 Collaborative Downloads
In Collaborative downloads, each peer has to take one of
two roles; either a Collector or a helper. The collector is the
peer that needs the complete file and in order to improve the
download performance , the collector peer collaborates with
group of other peers called the helpers. These helpers down-
load pieces from various sources and sends those pieces to
the collector. A helper peer doesn’t seek anything in re-
turn from the collector, give more priority to collector re-
quests and show no interest in the content the collector is
downloading. The collector selects the best source from the
helpers thus optimizing its download performance.
Figure 3. Overview of cooperative download-
ing, Source:[17]

8. The fairness of bandwidth sharing is enforced by a hy-
brid approach, as the bandwidth is accounted differently
between non-collaborating peers, and between a collector
and its helpers. Peers that are not members of the same
collaboration can exchange chunks with the standard bit-
torrent’s tit-for-tat algorithm. The BitTorrent tit-for-tat bar-
tering strategy ensures that for a peer the amount of incom-
ing data is roughly equal to the amount of outgoing data.
The tit-for-tat mechanism guarantees fairness only within a
single download session (download of one file). The asym-
metric nature of the collector-helper relation implies that the
bandwidth contributed by the helper to obtain the chunks
for the collector in the current download can be reclaimed
only during later downloads. We base the fairness between
collaborating peers on a notion of a promise. A collector re-
cruits helpers by giving them a promise that the bandwidth
invested in boosting the collectors download performance
will be returned in the future. Helpers contribute their cur-
rently idle bandwidth knowing that later the roles may be
reversed a helper will become a collector and the current
collector will pay back for the consumed bandwidth by par-
ticipating in the download as a helper. The role of a promise
is to provide an incentive for collaboration. This role can
be fulfilled only if the system provides guarantees that the
promise will be delivered. [20] [17]
6 Conclusion
In this paper, we discussed the operation of bit-torrent,
the framework behind its working nature and have listed
the proposed modifications to deal with performance issues
, research challenges and factors to be considered while im-
plementing bit-torrent in a P2P social network. Bit torrent
has been slowly migrating from its conventional public file
sharing role to a social network. In todays world, recogni-
tion of having a more distributed and decentralized social
network has grown largely among people. We have pre-
sented a paradigm for the design of bit-torrent style sharing
feature in P2P social network. Exploiting the feasibility of
having a complete de-centralized robust torrent feature has
not been deployed officially and is open to further research.
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