Following extension is supported by BitComet.
Last Update: JAN-05-2004
Author: ~RnySmile~

1. Torrent Format Extension
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1.1 Unicode Support (by ulion)
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add ".utf-8" for those utf-8 encoding strings. UTF-8 values have higher periority 
than MBCS strings. e.g:

* "name"
* "name.utf-8"

* "path"
* "path.uft-8"

* "comment"
* "comment.utf-8"

* "publisher"
* "publisher.utf-8"

* "publisher-url"
* "publisher-url.utf-8"

1.2 Multitracker (by John Hoffman) 
(all format supported, but all trackers treated equally)
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* "announce-list"

In addition to the standard "announce" key, in the main area of the
metadata file and not part of the "info" section, will be a new key,
"announce-list".  This key will refer to a list of lists of URLs, and
will contain a list of tiers of announces.  If the client is compatible
with the multitracker specification, and if the "announce-list" key is
present, the client will ignore the "announce" key and only use the
URLs in "announce-list".

2. Tracker Communication Extension
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2.1 [Request] Local IP Support
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* "&localip="

Add internal IP announce so that peers in one subnet could connect to each other.

2.2 [Request] Hide myself
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* "&hide=1"

Tell tracker do not tell my IP to others. e.g. Listening port is closed, no one can connected to me. 

2.3 [Response] Torrent Statistics Report
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* "num peers"

Totoal Connected Peer Number.

* "done peers"

Totoal seeds.


References
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http://home.elp.rr.com/tur/multitracker-spec.txt
http://62.216.18.38/btp_info_extension.txt




planned extension: 



1.3 piece hash less (by Olaf van der Spek)
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* "info_hash"

The info-pieces key of a .torrent is optional. When it's not present, 
a 20 bytes info_hash key should be present instead containing the SHA1 
hash of the info-pieces key.