1. A method of compiling a data structure that is performed by a socket library in a computer system, the computer system being arranged to perform protocol processing and transmit data via an interface on the basis of instructions in accordance with a route, the method that is performed by the socket library comprising: receiving a first instruction comprising a system call and an indication of a descriptor; determining a route indicated by the first instruction; identifying, in response to the determination, an interface within the determined route; attempting to determine the ownership of the interface wherein determining ownership of the interface comprises determining an entity to which the interface is allocated; and causing an entry to be made in the data structure stored in the socket library, the entry including an identifier of the descriptor and a state of the descriptor, wherein the state represents the result of the attempting step and indicates an entity to which the socket library should send instructions including an indication of the descriptor.