Project 1: BitTorrent Client Implementation

Name: Mrunal M Pagnis uname: mmpagnis

Name: Sri Laxmi Chintala uname: chintals

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Contents:

1. Introduction
2. Files used
3. Description of code
4. Implementation

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1. Introduction

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BitTorrent is a peer-to-peer file sharing protocol develop by Bram Cohen. This project implements a core subset of the BitTorrent protocol by implementing a BitTorrentclient. In this, BitTorrent client will take a pre-selected list of peers as a command line argument. A BitTorrent client is the application run by down-loaders/up-loaders that exchanges pieces of a file.

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2. Files used

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bt_client.c : This is the main portion of the client.

bt_setup.[c|h] : This is a C library that provides functionality for parsing command line arguments and peer strings.

bt_lib.[c|h] : This is the core BitTorrent client library and header file. It contains relevant structures for messages and arguments to BitTorrent client.

Files used for testing:

file.txt.torrent songs.torrent moby_dick.txt.torrent download.mp3.torrent

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3. Description of code

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In this project, the seeder and leecher functionality is implemented in different functions.

Firstly, a metainfo file(bencoded dictionary with keys and values) is parsed and the results of parsing the file by listing the length, name, piece length of the file and number of pieces are displayed in the verbose output.

Leecher functionality is implemented in leecher() in which a request for connection is made and handshake message is sent to the seeder. For the handshake to be sent info_hash(SHA1 hash of the bencoded info value in the metafile) and peer_ID(for identifying the client) should be computed. These values are computed in cal_info_hash() and calc_id(). The handshake to be generated is symmetric and is same for both sides. After the handshake is successful, messages are sent and received for communicating. The different types of messages used are defined in lib.c file.

Seeder will first generate the bitfield message with the available pieces, sends the message, and then it receives the interested message from leecher. Leecher will now request the piece it want with respect to the bitfield. Also, it can request pieces in any order. Sending request and storing the received piece is implemented in sendRequest(). After sending the request, seeder receives the request, checks for the type of message and sends the piece to the leecher which is implemented in sendPiece(). Filling the piece is implemented is fillPiece(). When the leecher receives the requested piece it is stored and is implemented in storePiece(). For every piece that leecher receives, it notifies that it has that piece.

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4. Implementation

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This project is implemented in C language.

The commands for compiling and running the program are as follows:

Compiling the server program(-lssl is required to compile when openssl/hmac.h library is used for creating the MAC)

Compile

gcc -lssl bt_client.c bt_setup.c bt_lib.c -o bt_client

Run the program with the IP address, port and file name(using -b(for binding) and -v(verbose) options)

./bt_client -v -lssl -b 127.0.0.1:6500 download.mp3.torrent

After running the seeder in one instance, run leecher in another instance with -p option(for peer) and -s option(save file) as follows:

./bt_client -v -lssl -p 127.0.0.1:6500 download.mp3.torrent -s somesong.mp3

Default save file name is output.