HTTP Seeding

:Post-History:

Metadata Extension

"httpseeds"

In the main area of the metadata file and not part of the "info"

section, will be a new key, "httpseeds". This key will refer to a

list of URLs, and will contain a list of web addresses where torrent

data can be retrieved. This key may be safely ignored if the client

is not capable of using it.

Examples

  d['httpseeds'] = [ 'http://www.whatever.com/seed.php' ]

This specifies the client can retrieve data by accessing the given

URL with the parameters supplied in the protocol specification

below

  d['httpseeds'] = [ 'http://www.site1.com/source1.php',
                     'http://www.site2.com/source2.php'  ]

More than one URL may be specified; if so, the client will attempt

to access both URLs to download seed data.

Protocol

The client calls the URL given, in the following format

  <url>?info_hash=[hash]&piece=[piece]{&ranges=[start]-[end]{,[start]-[end]}...}

Examples

  http://www.whatever.com/seed.php?info_hash=%9C%D9i%8A%F5Uu%1A%91%86%AE%06lW%EA%21W%235%E0&piece=3
  http://www.whatever.com/seed.php?info_hash=%9C%D9i%8A%F5Uu%1A%91%86%AE%06lW%EA%21W%235%E0&piece=8&ranges=49152-131071,180224-262143

The URL would be for a script which has access to the files

contained in the torrent, and to the metadata (.torrent) file

itself, so that it may calculate what byte ranges to pull from

what files. One such script has been written by DeHackEd, and

is available at http://bt.degreez.net .

The script should return, if everything is okay, either a status

of 200 (OK) and a block of data (either the entire piece if no

ranges were given, or the ranges of data requested for that piece

appended together), in binary format, or 503 (Service Temporarily

Unavailable), with the body of the return being an ASCII integer

value specifying how long the client should wait before retrying.

The client should consider any other return code as an error.

In the case of an error, the client should retry, but should

retry less often if the failure to contact the seed continues.

Server-side Implementation Notes

The purpose of the http seed script is to limit access to the

data being downloaded so that the web server isn't overwhelmed

by clients asking for the data. If it weren't for this limiting,

there would be no way to prevent someone from coding a client

to try to download continuously or multiply, resulting in a

heavy load on the server. Limiting the download rate also

allows an http seed script to be run on a web account where

the total amount of data downloaded is restricted or may result

in extra service charges.

The script must provide three major functions:

1. Limit its average upload to a reasonable level.

2. Intelligently tell peers how long they should wait before

retrying.

3. translate from an info-hash and piece number to a byte range

within a file or set of files, and return those bytes.

Another highly desirable function is to check whether peers are

retrying too often, and to automatically ban those peers.

Other desirable features include a way of monitoring the tracker

the torrent is using and to stop uploading data if sufficient

P2P seeds exist, and a way to feed back to the tracker to show

a seed is present.

Client-side Implementation Notes

The prototype code base has a default retry time of 30 seconds;

after 3 retries with errors, the time is lengthened with each

cycle.

The prototype code will not display any errors with contacting

http seeds (unless the URL given in the .torrent is incorrect)

until it has received data from that seed. (The prototype code

also won't display any errors for any http reply that was

actually received.)

Current behavior is: Request the rarest piece you're missing

in entirety that you can locate. If you have no pieces that

aren't partially downloaded, skip one retry cycle, then start

requesting partials. If you receive a 503 response, set the

retry time equal to the integer value received in the response.

..

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