RFC 1221 (rfc1221) - Page 2 of 68
Host Access Protocol (HAP) specification: Version 2
Alternative Format: Original Text Document
RFC 1221 HAP2 April 1991
cleanup following a host reboot or crash).
This document assumes the reader is familiar with DoD internetworking
terminology.
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.
Edmond