Algorithm used for Routing across different Autonomous Systems (ASs).
Such a protocol is different than an Intra-Autonomous System Routing Protocol, which is usually strictly performance oriented, as it allows the ISPs owning the AS to control its routing, e.g. BGP routes are mostly dependant on the local preference value set by policy. ISPs might for example hide routes that would mean carrying the cost of transferring traffic between non-costumers.