Hi,

Metalink is an XML format that lists multiple ways (FTP/HTTP/P2P/rsync) 
to retrieve files, along with checksums and other metadata, to increase 
availability and guarantee integrity in downloads. It is used by 
download programs such as download managers, P2P & FTP programs, but was 
designed for web browsers.

Around 21 programs have Metalink support. Metalink is frequently used 
for large downloads, such as Linux/BSD ISOs (600 megs to multiple gigs). 
Projects/sites such as OpenOffice.org, cURL, openSUSE, DOFUS, Eiffel 
Software, and others use it.

DownThemAll! 1.0 Beta [1] is a Firefox extension, and the easiest way to 
try Metalink out. It supports download selection and whole file 
verification.

Some basic features the information in a metalink can be used for:
- Listing multiple mirrors for higher availability.
- Automatic error recovery and repair of corrupted downloads.
- More information about a download for users. [2]
- Add a file or multiple files to a download queue.
- Share a library of files between filesharing applications.
- Simplified download pages.

"Metalink solves the first problem — how to find the most speedy way to 
download a file — by grouping different download protocols into one 
protocol. This enables Metalink clients to automatically switch between 
different mirror servers without explicit user instructions. Not only 
multiple server information is described but also meta-data (e.g., the 
location of the server, document tags and license)." [3]

An example .metalink:

<?xml version="1.0" encoding="utf-8"?>
<metalink version="3.0" generator="Metalink Editor version 1.0.0" 
xmlns="http://www.metalinker.org/";>
   <publisher>
     <name>Xi Software</name>
     <url>http://www.xi-soft.com/</url>
   </publisher>
   <identity>Net Transport</identity>
   <version>2.43</version>
   <copyright>2005-2007</copyright>
   <description>Net Transport (NetXfer) is a downloading tool.</description>
   <files>
     <file name="NXSetup_multi.zip">
       <size>2014232</size>
       <os>Windows-x86</os>
       <verification>
         <hash type="md5">18148ed6dc0fc6a48f345579f099d6bd</hash>
         <hash type="sha1">01586b2c0ec5d8e985138204404878f5ecbeef58</hash>
         <pieces type="sha1" length="262144">
           <hash piece="0">ce50319f58e846e3e0c66a9ada9015e601126864</hash>
           <hash piece="1">c9e73e336831a364cedcd889259b7330d48f90fd</hash>
           <hash piece="2">a7f5281c629ea63c791f320db3b0df85fdb01861</hash>
           <hash piece="3">e6cf5571185db0ec79b551222596462db445bdd6</hash>
           <hash piece="4">8982e850d2e3ea005119beea3cc05daca28de474</hash>
           <hash piece="5">a0e7a3a3a46f1ae5fb4eb8529b6fb8ee82fa43a8</hash>
           <hash piece="6">96540cfade1ce9d3846f41510fbed46965b29568</hash>
           <hash piece="7">87108128d4dde5ba62ffec1f1cc3fc292fc298ca</hash>
         </pieces>
       </verification>
       <resources>
         <url type="http" 
preference="100">http://www.xi-soft.com/downloads/NXSetup_multi.zip</url>
         <url type="http" location="gb" 
preference="100">http://nettransport.abangadek.com/NXSetup_multi.zip</url>
         <url type="http" location="hu" 
preference="100">http://www.axitech.hu/nettransport/downloads/NXSetup_multi.zip</url>
         <url type="ftp" location="cn" 
preference="100">ftp://nettransport:[EMAIL PROTECTED]/NXSetup_multi.zip</url>
       </resources>
     </file>
   </files>
</metalink>

For more information: http://www.metalinker.org and 
http://en.wikipedia.org/wiki/Metalink

Thanks,
-- 
(( Anthony Bryan ... Metalink [ http://www.metalinker.org ]
   )) Easier, More Reliable, Self Healing Downloads

[1] http://www.downthemall.net/main/install-it/downthemall-10-beta/
[2] http://code.downthemall.net/maierman/metaselect4.png
[3] 
http://www.geospatialsemanticweb.com/2007/02/25/metalink-unifies-internet-downloads
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