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End-to-end Publishing Using
Bittorrent
Bittorrent
•Bittorrent is a widely used peer-topeer network used to distribute files,
especially large ones
•It has a number of legal uses which
separate it from other P2P
Practical Applications
Distributing large files
Podcasting
Vlogging
Disk images
Legal distribution of movies (see
bittorrent.com)

Traditional vs. Bittorrent
•One server
provides
many clients
•Many clients
provide many
clients
Terminology
Swarm – clients downloading or uploading a
given file through Bittorrent
Tracker – centralized server that clients connect
to to ask for lists of other clients connected to
the swarm
Seed – A client that has a complete copy of the
file
Peer (Leecher) – A client that does not have a
complete copy of the file
Problem
Torrents that are less popular may
eventually “die” when there are no
longer any complete copies of the
file in the swarm
Everseed


Permanent seed
running on the
same server as the
tracker
Guarantees that
there will always
be a complete
copy of the file
Related Research
The creator of Bittorrent wrote a paper on the
process of downloading a file using Bittorrent at
http://www.bittorrent.org/protocol.html
Maintainers of various Bittorrent clients wrote
http://wiki.theory.org/BitTorrentSpecification, which is
like the official specification except far more in depth
Osprey (http://osprey.ibiblio.org/) seems to have
thought of something similar, but haven't made
much progress

Explanation
•The .torrent metadata file tells client
tracker URL & other data
•Client connects to tracker
•Tracker gives client a list of other clients
•Client downloads file from other clients
(not a centralized server)
•Periodic update with tracker
Goals
•Complete internet publishing solution
using Bittorrent
•Metadata file generator (.torrent)
•Tracker
•“Everseed”
•Web interface
.torrent File
•Official docs on bittorrent.org
•Metadata on the file to be
downloaded (tracker URL, filename,
size, checksum hashes)
•Stored as “bencoded” strings,
integers, lists, dictionaries
Bencoding
•Integer: 6 => “i6e”
•String: “hello” => “5:hello”
•List: [“hello”,”world”] =>
“l5:hello5:worlde”
•Dictionary: {“hello”:”world”} =>
“d5:hello5:worlde”
Bencoding implementation
•Python has good string manipulation
•Structure of a .torrent file is a
dictionary containing string keys and
integer, string, list, and dictionary
values
•Recursion to encode/decode
Tracker
•Makes use of the bencoding algorithm
•Handles two types of requests:
“announce” and “scrape”
•Stores data on peers and torrents in a
SQLite database
•No performance issues
Network performance
Peer List Size vs Time (seconds)
0.1
0.09
Peer List Size
0.08
Peer List
Size
0.07
0.06
0.05
Column B
0.04
0.03
0.02
0.01
0
0
2000
Time (seconds)
Database performance
Number of Peers Inserted vs. Time (seconds)
Number of Peers Inserted
0.5
0.4
0.3
Column B
0.2
0.1
0
0
2000
Time (seconds)
Announce request
•“Announces” client's presence to
tracker
•Used to get lists of IP addresses
and BT ports of other clients in
the swarm
Announce request
•“Compact” peer list response
•Recognition of seeding status
Scrape request
•“Scrapes” data from the tracker
•Used to get info on the different
torrents that the tracker is tracking
•# peers, # seeds, total
downloaded, total uploaded, #
completed etc...
Scrape request
•Client sends an HTTP GET request
to tracker's scrape URL
•Tracker urldecodes request, selects
the data the client is interested in
•Tracker responds with a bencoded
text/plain document
Summary
•Python
•Benefits of P2P technology
•“Everseed” concept
•.torrent files and bencoding
•Tracker