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TOC 1. However, its highly centralized client-server approach also means, it is inadequate for mass publication of files, where a single point may expect to be requested by a critically large number of clients simultaneously. To remedy this situation many organizations either implement a cap on the number of simultaneous requests, or spread the load on multiple mirror servers.
Needless to say both approaches have their drawbacks, and a solution that addresses these problems is highly needed. The approach in BitTorrent Protocol BTP is to spread the load not on mirror servers, but to the clients themselves by having them upload bits of the file to each other while downloading it. Since the clients usually do not utilize their upload capacity while fetching a file, this approach does not put the clients in any disadvantage.
This has the added advantage that even small organizations with limited resources can publish large files on the Internet without having to invest in costly infrastructure. To the extent that these extensions have become part of what the BitTorrent community considers best practice they have been included in this document. However, many extensions have been omitted either because they have been deemed to lack interoperability with existing implementations, or because they are not regarded as being sufficiently mature.
Also, system administrators and architects may use this document to fully understand the implications of installing an implementation of BTP. In particular, it is advised to study the security implications in more detail, before installing an implementation on a machine that also contains sensitive data.
A peer is a node in a network participating in file sharing. It can simultaneously act both as a server and a client to other nodes on the network. Neighboring peers: Peers to which a client has an active point to point TCP connection.
A client is a user agent UA that acts as a peer on behalf of a user. A torrent is the term for the file single-file torrent or group of files multi-file torrent the client is downloading. A network of peers that actively operate on a given torrent.
A peer that has a complete copy of a torrent. A tracker is a centralized server that holds information about one or more torrents and associated swarms.
It functions as a gateway for peers into a swarm. Metainfo file: A text file that holds information about the torrent, e. It usually has the extension. Peer ID: A byte string that identifies the peer. How the peer ID is obtained is outside the scope of this document, but a peer must make sure that the peer ID it uses has a very high probability of being unique in the swarm. Info hash: A SHA1 hash that uniquely identifies the torrent. It is calculated from data in the metainfo file.
THP defines a method for contacting a tracker for the purposes of joining a swarm, reporting progress etc. PWP defines a mechanism for communication between peers, and is thus responsible for carrying out the actual download and upload of the torrent.
In order for a client to download a torrent the following steps must be carried through: A metainfo file must be retrieved. Instructions that will allow the client to contact other peers must be periodically requested from the tracker using THP. The torrent must be downloaded by connecting to peers in the swarm and trading pieces using PWP.
To publish a torrent the following steps must be taken: A tracker must be set up. A metainfo file pointing to the tracker and containing information on the structure of the torrent must be produced and published. At least one seeder with access to the complete torrent must be set up.
TOC 2. Bencoding Bencoding encodes data in a platform independent way. The format specifies two scalar types integers and strings and two compound types lists and dictionaries. Strings are encoded by prefixing the string content with the length of the string followed by a colon. Lists are an arbitrary number of bencoded elements prefixed with the letter “l” and postfixed with the letter “e”.
It follows that lists can contain nested lists and dictionaries. For instance “li2e3: All keys are bencoded strings while the associated value can be any bencoded element. All dictionary keys MUST be sorted. TOC 3. Pieces and Blocks This section describes how a torrent is organized in pieces and blocks.
The torrent is divided into one or more pieces. Each piece represents a range of data which it is possible to verify using a piece SHA1 hash. When distributing data over PWP pieces are divided into one or more blocks, as shown in the following diagram: Block M 3. The size of each piece in the torrent remains fixed and can be calculated using the following formula: Only the last piece of the torrent is allowed to have fewer bytes than the fixed piece size.
The size of a piece is determined by the publisher of the torrent. A good recommendation is to use a piece size so that the metainfo file does not exceed 70 kilobytes.
For the sake of calculating the correct position of a piece within a file, or files, the torrent is regarded as a single continuous byte stream. In case the torrent consists of multiple files, it is to be viewed as the concatenation of these files in the order of their appearance in the metainfo file. Conceptually, the torrent is only translated into files when all its pieces have been downloaded and verified using their respective SHA1 values; although in practice an implementation may choose a better approach in accordance with local operating system and filesystem specific demands.
Once a fixed size is defined, the number of blocks per piece can be calculated using the formula: The negation operator is used to ensure that the last factor only adds a value of 0 or 1 to the sum. Given the start offset of the block its index within a piece can be calculated using the formula: The Metainfo File The metainfo file provides the client with information on the tracker location as well as the torrent to be downloaded.
Besides listing which files will result from downloading the torrent, it also lists how the client should split up and verify individual pieces making up the complete torrent. How the client retrieves the metainfo file is beyond the scope of this document, however, the most user-friendly approach is for a client to find the file on a web page, click on it, and start the download immediately. This is a string value. It contains the announce URL of the tracker.
Each value is a URL pointing to a backup tracker. This is an optional string value and may contain the name and version of the program used to create the metainfo file. It contains the creation time of the torrent in standard Unix epoch format. This key points to a dictionary that contains information about the files to download. The entries are explained in the following sections. This is an integer value indicating the length of the file in bytes.
If included it must be a string of 32 characters corresponding to the MD5 sum of the file. A string containing the name of the file. An integer indicating the number of bytes in each piece. This is a string value containing the concatenation of the byte SHA1 hash value for all pieces in the torrent. For example, the first 20 bytes of the string represent the SHA1 value used to verify piece index 0. The complete file is derived by combining all the pieces into one string of bytes.
This is a list of dictionaries. Each file in the torrent has a dictionary associated to it having the following structure: This is an integer indicating the total length of the file in bytes. This is a list of string elements that specify the path of the file, relative to the topmost directory. The last element in the list is the name of the file, and the elements preceding it indicate the directory hierarchy in which this file is situated. It contains the name of the top-most directory in the file structure.
This is an integer value. It contains the number of bytes in each piece. The first 20 bytes of the string represent the SHA1 value used to verify piece index 0.
TOC 5. A tracker is a HTTP service that must be contacted by a peer in order to join a swarm. A tracker does not by itself provide access to any downloadable data. A tracker relies on peers sending regular requests. It may assume that a peer is dead if it misses a request.
The following parameters must be present in the request: In order to obtain this value the peer must calculate the SHA1 of the value of the “info” key in the metainfo file. The port number that the peer is listening to for incoming connections from other peers.
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