SSL allows applications to specify how I/O is to take place. This is done by specifying callback routines for receiving and sending data. The contents of this routine can be very unique per application. SSL has an internally defined default routine which is used if gsk_attribute_set_callback() is not used to override I/O routines. The default assumes that TCP/IP is being used. For reading it executes a recv() and for write a send(). If not using TCP/IP, applications should also consider the specification of the getpeername and setsocketoptions callback routine. It also depends on TCP/IP as being the transport layer protocol.