STANAG 5066 (Profile for High Frequency (HF) Radio Data Communication) is a NATO specification to enable applications to communicate efficiently over HF radio. STANAG 5066 provides peer protocols that operate above an HF modem and below the application level. STANAG 5066 includes the mandatory SIS (Subnet Interface Sublayer, sometimes called Subnet Interface Service) protocol that enables an application to connect to an HF modem through a STANAG 5066 server over TCP/IP. This enables a clean separation between application and modem.