Usually file sharing follows the peer-to-peer (P2P) model, where the data are stored on and served by personal computers of the peoples. Most users who engage in file sharing are also downloading files that other users share. Sometimes these two activities are linked together. P2P File sharing is distinct from file trading in that downloading files from a P2P network does not require uploading, although some networks either provide incentives for uploading such as credits or force the sharing of files being currently downloaded. In practice, many of the files shared on file sharing networks are copies of copyrighted music and movies. Sharing of these copies with strangers without authorization from the copyright holder is illegal in most jurisdictions. Most forms of protection against (and prevention of) trojans in the "file sharing" are based on a technique sometimes referred to as object reconciliation. Although the term might sound intimidating, it isn't. It is a fancy way of asking "Are things still just the way I left them?" Here is how it works: Objects are either files or directories. Reconciliation is the process of comparing those objects against themselves at some earlier (or later) date. For example, take a backup tape and compare the file PS as it existed in November 1995 to the PS that now resides on your drive. If the two differ, and no change has been made to the operating system, something is amiss. This technique is invariably applied to system files that are installed as part of the basic operating system.