When the part lands on a server, it initiates a periodic pulse -- a sequence of message sent from that server back to the base server from where the part had originated. Each message traverses the so far traveled path in the backward direction, and drops a little encrypted file that contains a reference to the previous server. This way, when a user gets the file from the store, the servers follow the tracks of each part.<p>When a part leaves a server to another server, it stops the current pulse -- the target server initiates a new pulse.<p>Each pulse erases the previous track file on each server it travels through.