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mfc42d 2004-04-21
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Metainfo file and tracker responses are both sent in a simple, efficient, and extensible format called bencoding (pronounced 'bee encoding'). Bencoded messages are nested dictionaries and lists (as in Python), which can contain strings and integers. Extensibility is supported by ignoring unexpected dictionary keys, so additional optional ones can be added later.
Bencoding is done as follows:
Strings are length-prefixed base ten followed by a colon and the string. For example 4:spam corresponds to 'spam'.
Integers are represented by an 'i' followed by the number in base 10 followed by an 'e'. For example i3e corresponds to 3 and i-3e corresponds to -3. Integers have no size limitation. i-0e is invalid. All encodings with a leading zero, such as i03e, are invalid, other than i0e, which of course corresponds to 0.
Lists are encoded as an 'l' followed by their elements (also bencoded) followed by an 'e'. For example l4:spam4:eggse corresponds to ['spam', 'eggs'].
Dictionaries are encoded as a 'd' followed by a list of alternating keys and their corresponding values followed by an 'e'. For example, d3:cow3:moo4:spam4:eggse corresponds to {'cow': 'moo', 'spam': 'eggs'} and d4:spaml1:a1:bee corresponds to {'spam': ['a', 'b']} . Keys must be strings and appear in sorted order (sorted as raw strings, not alphanumerics).
Metainfo files are bencoded dictionaries with the following keys:
announce
The URL of the tracker.
info
This maps to a dictionary, with keys described below.
The name key maps to a string which is the suggested name to save the file (or directory) as. It is purely advisory.
piece length maps to the number of bytes in each piece the file is split into. For the purposes of transfer, files are split into fixed-size pieces which are all the same length except for possibly the last one which may be truncated. Piece length is almost always a power of two, most commonly 218 = 256 K (BitTorrent prior to version 3.2 uses 220 = 1 M as default).
pieces maps to a string whose length is a multiple of 20. It is to be subdivided into strings of length 20, each of which is the SHA1 hash of the piece at the corresponding index.
There is also a key length or a key files, but not both or neither. If length is present then the download represents a single file, otherwise it represents a set of files which go in a directory structure.
In the single file case, length maps to the length of the file in bytes.
For the purposes of the other keys, the multi-file case is treated as only having a single file by concatenating the files in the order they appear in the files list. The files list is the value files maps to, and is a list of dictionaries containing the following keys:
length
The length of the file, in bytes.
path
A list of strings corresponding to subdirectory names, the last of which is the actual file name (a zero length list is an error case).
In the single file case, the name key is the name of a file, in the muliple file case, it's the name of a directory.
Tracker queries are two way. The tracker receives information via HTTP GET parameters and returns a bencoded message. Note that although the current tracker implementation has its own web server, the tracker could run very nicely as, for example, an apache module.
Tracker GET requests have the following keys:
info_hash
The 20 byte sha1 hash of the bencoded form of the info value from the metainfo file. Note that this is a substring of the metainfo file. This value will almost certainly have to be escaped.
peer_id
A string of length 20 which this downloader uses as its id. Each downloader generates its own id at random at the start of a new download. This value will also almost certainly have to be escaped.
ip
An optional parameter giving the IP (or dns name) which this peer is at. Generally used for the origin if it's on the same machine as the tracker.
port
The port number this peer is listening on. Common behavior is for a downloader to try to listen on port 6881 and if that port is taken try 6882, then 6883, etc. and give up after 6889.
uploaded
The total amount uploaded so far, encoded in base ten ascii.
downloaded
The total amount downloaded so far, encoded in base ten ascii.
left
The number of bytes this peer still has to download, encoded in base ten ascii. Note that this can't be computed from downloaded and the file length since it might be a resume, and there's a chance that some of the downloaded data failed an integrity check and had to be re-downloaded.
event
This is an optional key which maps to started, completed, or stopped (or empty, which is the same as not being present). If not present, this is one of the announcements done at regular intervals. An announcement using started is sent when a download first begins, and one using completed is sent when the download is complete. No completed is sent if the file was complete when started. Downloaders send an announcement using 'stopped' when they cease downloading.
Tracker responses are bencoded dictionaries. If a tracker response has a key failure reason, then that maps to a human readable string which explains why the query failed, and no other keys are required. Otherwise, it must have two keys: interval, which maps to the number of seconds the downloader should wait between regular rerequests, and peers. peers maps to a list of dictionaries corresponding to peers, each of which contains the keys peer id, ip, and port, which map to the peer's self-selected ID, IP address or dns name as a string, and port number, respectively. Note that downloaders may rerequest on nonscheduled times if an event happens or they need more peers.
Bencoding is done as follows:
Strings are length-prefixed base ten followed by a colon and the string. For example 4:spam corresponds to 'spam'.
Integers are represented by an 'i' followed by the number in base 10 followed by an 'e'. For example i3e corresponds to 3 and i-3e corresponds to -3. Integers have no size limitation. i-0e is invalid. All encodings with a leading zero, such as i03e, are invalid, other than i0e, which of course corresponds to 0.
Lists are encoded as an 'l' followed by their elements (also bencoded) followed by an 'e'. For example l4:spam4:eggse corresponds to ['spam', 'eggs'].
Dictionaries are encoded as a 'd' followed by a list of alternating keys and their corresponding values followed by an 'e'. For example, d3:cow3:moo4:spam4:eggse corresponds to {'cow': 'moo', 'spam': 'eggs'} and d4:spaml1:a1:bee corresponds to {'spam': ['a', 'b']} . Keys must be strings and appear in sorted order (sorted as raw strings, not alphanumerics).
Metainfo files are bencoded dictionaries with the following keys:
announce
The URL of the tracker.
info
This maps to a dictionary, with keys described below.
The name key maps to a string which is the suggested name to save the file (or directory) as. It is purely advisory.
piece length maps to the number of bytes in each piece the file is split into. For the purposes of transfer, files are split into fixed-size pieces which are all the same length except for possibly the last one which may be truncated. Piece length is almost always a power of two, most commonly 218 = 256 K (BitTorrent prior to version 3.2 uses 220 = 1 M as default).
pieces maps to a string whose length is a multiple of 20. It is to be subdivided into strings of length 20, each of which is the SHA1 hash of the piece at the corresponding index.
There is also a key length or a key files, but not both or neither. If length is present then the download represents a single file, otherwise it represents a set of files which go in a directory structure.
In the single file case, length maps to the length of the file in bytes.
For the purposes of the other keys, the multi-file case is treated as only having a single file by concatenating the files in the order they appear in the files list. The files list is the value files maps to, and is a list of dictionaries containing the following keys:
length
The length of the file, in bytes.
path
A list of strings corresponding to subdirectory names, the last of which is the actual file name (a zero length list is an error case).
In the single file case, the name key is the name of a file, in the muliple file case, it's the name of a directory.
Tracker queries are two way. The tracker receives information via HTTP GET parameters and returns a bencoded message. Note that although the current tracker implementation has its own web server, the tracker could run very nicely as, for example, an apache module.
Tracker GET requests have the following keys:
info_hash
The 20 byte sha1 hash of the bencoded form of the info value from the metainfo file. Note that this is a substring of the metainfo file. This value will almost certainly have to be escaped.
peer_id
A string of length 20 which this downloader uses as its id. Each downloader generates its own id at random at the start of a new download. This value will also almost certainly have to be escaped.
ip
An optional parameter giving the IP (or dns name) which this peer is at. Generally used for the origin if it's on the same machine as the tracker.
port
The port number this peer is listening on. Common behavior is for a downloader to try to listen on port 6881 and if that port is taken try 6882, then 6883, etc. and give up after 6889.
uploaded
The total amount uploaded so far, encoded in base ten ascii.
downloaded
The total amount downloaded so far, encoded in base ten ascii.
left
The number of bytes this peer still has to download, encoded in base ten ascii. Note that this can't be computed from downloaded and the file length since it might be a resume, and there's a chance that some of the downloaded data failed an integrity check and had to be re-downloaded.
event
This is an optional key which maps to started, completed, or stopped (or empty, which is the same as not being present). If not present, this is one of the announcements done at regular intervals. An announcement using started is sent when a download first begins, and one using completed is sent when the download is complete. No completed is sent if the file was complete when started. Downloaders send an announcement using 'stopped' when they cease downloading.
Tracker responses are bencoded dictionaries. If a tracker response has a key failure reason, then that maps to a human readable string which explains why the query failed, and no other keys are required. Otherwise, it must have two keys: interval, which maps to the number of seconds the downloader should wait between regular rerequests, and peers. peers maps to a list of dictionaries corresponding to peers, each of which contains the keys peer id, ip, and port, which map to the peer's self-selected ID, IP address or dns name as a string, and port number, respectively. Note that downloaders may rerequest on nonscheduled times if an event happens or they need more peers.
mfc42d 2004-04-21
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A BitTorrent file distribution consists of these entities:
An ordinary web server
A static 'metainfo' file
A BitTorrent tracker
An 'original' downloader
The end user web browsers
The end user downloaders
There are ideally many end users for a single file.
To start serving, a host goes through the following steps:
Start running a tracker (or, more likely, have one running already).
Start running an ordinary web server, such as apache, or have one already.
Associate the extension .torrent with mimetype application/x-bittorrent on their web server (or have done so already).
Generate a metainfo (.torrent) file using the complete file to be served and the URL of the tracker.
Put the metainfo file on the web server.
Link to the metainfo (.torrent) file from some other web page.
Start a downloader which already has the complete file (the 'origin').
To start downloading, a user does the following:
Install BitTorrent (or have done so already).
Surf the web.
Click on a link to a .torrent file.
Select where to save the file locally, or select a partial download to resume.
Wait for download to complete.
Tell downloader to exit (it keeps uploading until this happens).
The connectivity is as follows:
The web site is serving up static files as normal, but kicking off the BitTorrent helper app on the clients.
The tracker is receiving information from all downloaders and giving them random lists of peers. This is done over HTTP or HTTPS.
Downloaders are periodically checking in with the tracker to keep it informed of their progress, and are uploading to and downloading from each other via direct connections. These connections use the BitTorrent peer protocol, which operates over TCP.
The origin is uploading but not downloading at all, since it has the entire file. The origin is necessary to get the entire file into the network. Often for popular downloads the origin can be taken down after a while since several downloads may have completed and been left running indefinitely.
An ordinary web server
A static 'metainfo' file
A BitTorrent tracker
An 'original' downloader
The end user web browsers
The end user downloaders
There are ideally many end users for a single file.
To start serving, a host goes through the following steps:
Start running a tracker (or, more likely, have one running already).
Start running an ordinary web server, such as apache, or have one already.
Associate the extension .torrent with mimetype application/x-bittorrent on their web server (or have done so already).
Generate a metainfo (.torrent) file using the complete file to be served and the URL of the tracker.
Put the metainfo file on the web server.
Link to the metainfo (.torrent) file from some other web page.
Start a downloader which already has the complete file (the 'origin').
To start downloading, a user does the following:
Install BitTorrent (or have done so already).
Surf the web.
Click on a link to a .torrent file.
Select where to save the file locally, or select a partial download to resume.
Wait for download to complete.
Tell downloader to exit (it keeps uploading until this happens).
The connectivity is as follows:
The web site is serving up static files as normal, but kicking off the BitTorrent helper app on the clients.
The tracker is receiving information from all downloaders and giving them random lists of peers. This is done over HTTP or HTTPS.
Downloaders are periodically checking in with the tracker to keep it informed of their progress, and are uploading to and downloading from each other via direct connections. These connections use the BitTorrent peer protocol, which operates over TCP.
The origin is uploading but not downloading at all, since it has the entire file. The origin is necessary to get the entire file into the network. Often for popular downloads the origin can be taken down after a while since several downloads may have completed and been left running indefinitely.
mfc42d 2004-04-21
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BitTorrent is a protocol for distributing files. It identifies content by URL and is designed to integrate seamlessly with the web. Its advantage over plain HTTP is that when multiple downloads of the same file happen concurrently, the downloaders upload to each other, making it possible for the file source to support very large numbers of downloaders with only a modest increase in its load.
hollysky 2004-04-21
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thanks
已经有一个大概的了解了
我希望了解一下客户端与种子发布服务器是用什么协议,http?
在下载时是用TCP连接吗?
已经有一个大概的了解了
我希望了解一下客户端与种子发布服务器是用什么协议,http?
在下载时是用TCP连接吗?
_xiao 2004-04-20
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其实种子的制作(文件信息，分块信息)是在客户端完成的，服务器端只负责发布该种子的信息，在服务器端发布该种子信息以后，原客户端就与其它客户端完全平等，只不过由于该客户段已“下载”完所有的数据块所以不会再要求到其它客户端处下载，只提供下载服务，同理如果某个客户端已下载完了所有数据块仍不关闭BT继续进行“下载”，那么这个客户端就与种子客户端毫无差别了
所以人人都可以制作BT种子，但该种子信息需要找个服务器进行发布，这样别人才能知道你提供了一个种子，称之为"Announce URL"，很多BT网站都提供种子信息发布的。。。
所以人人都可以制作BT种子，但该种子信息需要找个服务器进行发布，这样别人才能知道你提供了一个种子，称之为"Announce URL"，很多BT网站都提供种子信息发布的。。。
hollysky 2004-04-20
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需要独立的服务器否?
还是每个客户端都可以做种子?
还是每个客户端都可以做种子?
_xiao 2004-04-20
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需要有个人先做种子，然后其它客户端登录它，做种子的服务器需要将整个文件分解为大小适当的数据块，并记录每个客户端已下载的文件块以及每一块的位置，以及该客户正在给多少其它客户提供下载，要下载的客户端先登录该服务器得到这个记录列表，然后根据列表从中选择几个有自己所需的数据块的客户，连接这些客户并开始下载，同时报告给种子服务器自己当前已下载的数据块和正在下载的数据块，并随时准备迎接其它客户的连接，提供下载服务。。。
