1. Introduction

The Host Access Protocol (HAP) is a network layer protocol (as is X.25). ("Network layer" here means ISO layer 3 lower, the protocol layer below the DoD Internet Protocol (IP) layer [3] and above any link layer protocol.) HAP defines the different types of host-to- network control messages and host-to-host data messages that may be exchanged over the access link connecting a host and the network packet switch node. The protocol establishes formats for these messages, and describes procedures for determining when each type of message should be transmitted and what it means when one is received.

HAP has been implemented in the wide-area network called the Terrestrial Wideband Network (TWBNET) [5] and in the routers and other hosts that connect to TWBNET. The packet switch nodes that compose the TWBNET are called Wideband Packet Switches (WPS).

Both the precursor to HAP, the Host/SATNET Protocol [6], used in the Atlantic Packet Satellite Network (SATNET) and the Mobile Access Terminal Network (MATNET [7]), and HAP, used in the original Wideband Satellite Network (WBNET) [8], were originally designed to provide efficient access to the single satellite channel each network used to connect all sites. The HAP protocol designers reflected some of the peculiarities of the single satellite channel environment in the HAP protocol itself. The current Terrestrial Wideband Network (TWBNET) utilizes T1-speed fiber connections between sites. Future networks and TWBNET may use a combination of terrestrial connections and satellite connections, and may have more than one of each. The HAP protocol has been changed to accommodate these extensions.

Section 2 presents an overview of HAP. Details of HAP formats and message exchange procedures are contained in Sections 3 through 10. Further explanation of some of the topics addressed in this HAP specification can be found in reference [1].

Any protocol employed to provide sufficiently reliable message exchange over the Host-WPS link is assumed to be transparent to the protocol defined in this document. Examples of such link-level protocols are ARPANET 1822 local and distant host [9], ARPANET VDH protocol [9], and HDLC.