Metalink

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, like DownThemAll! (Firefox extension), GetRight, Free Download Manager, aria2, Phex, SmartFTP, Speed Download, & KGet, 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.

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.
• Add a file or multiple files to a download queue.
• Share a library of files between filesharing applications.
• Simplified download pages.

Here a Metalink selection dialog from DTA! is shown, with logo, description, publisher, license, OS/arch, and language information taken from the .metalink.

"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)." Geospatial Semantic Web

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:nettransport@202.107.248.221/NXSetup_multi.zip</url>
   </resources>
 </file>
</files>
</metalink>

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