STANAG 5066
STANAG 5066 (Profile for High Frequency (HF) Radio Data Communication) is a NATO Standardization Agreement 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. The standard also defines two more layers, CAS which is intended to establish connections to  other HF nodes and control the status of these connections, and DTS, which controls all the data manipulation for transmission (slicing, directioning, timing...) and the reconstruction in reception. There are two basic modes of transmission defined by this standard. ARQ and NON-ARQ. STANAG 5066 defines a SIS-to-SIS package size of 2048 bytes maximum, when using point-to-point transmitting mode (ARQ or NON-ARQ), and 4096 bytes when using broadcast (NON-ARQ only).