BEP:31
Title:Failure Retry Extension
Version: 0226195b420b961716ed3198593d752161e10d5b
Last-Modified:Thu Oct 15 19:34:11 2009 +0000
Author: Jeroen Massar <jeroen@unfix.org>
Status: Draft
Type:Standards Track
Content-Type:text/x-rst
Created:15-Oct-2009
Post-History:

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