Failure Retry Extension

Abstract

This BEP provides a simple backward compatible extension for the

BitTorrent Tracker Protocol (BEP 0003) to provide a client with

more details on a failure, specifying if a failure is permanent or

temporary and when the request can be repeated.

Rationale

There have been resported cases where (un)intentionally the tracker

locations (hostnames and their resolved IP addresses, or IP addresses)

where actually not BitTorrent Trackers. When this happens the HTTP

server that is living at those addresses will receive a large amount

of /announce and /scrape requests which it cannot fulfull as there

is no BitTorrent Tracker present at that webserver. Most BitTorrent

clients do not check the HTTP errorcodes provided by the server and

this thus makes them ignore 404 (File Not Found), which is the general

case when the server is not supposed to be used as a tracker. Clients

then keep on retrying forever till the user finally gives up.

With a large enough number of clients this might overwhelm the

webserver from serving the content that it is really supposed to

perform.

Clients though might not want to parse the 404 or any other error

code as the developers of which claim that the 404 might be temporary

and that keeping retrying is more important.

This proposal addresses this problem by defining a BitTorrent Tracker

response "retry in" which allows site owners to return a static

response for /announce and /scrape telling the client that this server

is permanently not acting as a tracker, thus making the client never

return. The user/client can then also inform the source of the

.torrent specifying the faulty tracker that the tracker is not a

tracker.

This error message can also be used to specify to a client that it

should retry its request after a certain amount of time. This allows

a overwhelmed tracker to distribute load a little bit.

"retry in" extension to "failure reason"

The permanent failure error consists of a bencoded "failure reason",

which contains the reason and is backward compatible to clients

which don't support this extension.

The new field "retry in" specifies the number of minutes in which

a retry can be done for this tracker. This field is either a positive

integer or the value "never". The latter specifies that the client

should never send this query again.

Examples

(PHP style)

Return that the server is overloaded

	<?php
	bencode(array(
		"failure reason"	=> "Overloaded",
		"retry in"		=> "5"
		));
	?>

Return that the server is not a tracker

	<?php
	bencode(array(
		"failure reason"	=> "Not a tracker",
		"retry in"		=> "never"
		));
	?>

The result of both can be stored in static files and 'cheaply'

served by webservers, firewalls or loadbalancers.

References

Copyright

This document has been placed in the public domain.

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