Each enclosure has a URL of data represented by this enclosure. Usually this information is sufficient to download it. In most cases URL schema (e.g ``http:'' or ``ftp:'') detrmines download protocol.
However some file sharing technologies (most notably ``BitTorrent'') use special ``seed'' file which should be downloaded first. This "seed" file contains additional infromation on how real content should be downloaded. This file will be specified by a regular ``http:'' or ``ftp:'' URL. However, it is important for a client application processing enclosures to determine if both enclsoure content type and transport protocol are supported. In particular, in the case of multiple alternative enclosures it could be used in the decision process to select which one of them will be downloaded.
The goal of the transport attribute is to provide information about the transport type used to download this enclosure. The value of this attribute would be the content type of a ``seed'' file (e.g. ``application/x-bittorrent'' for BitTorrent). It should not be confused with the ``type'' attribute, which would specify content type of actual content ultimately downloaded via this protocol (e.g. ``audio/x-realaudio'').
The ``transport'' attribute is optional, and may be omitted in cases where a download URL points directly to the data, not to ``seed'' file (e.g. direct ``http:'', ``ftp'' downloads or ``ed2k:'' URLs). If omitted, it defaults to the value of ``type'' attribute.