RFC 1221 (rfc1221) - Page 2 of 68

Host Access Protocol (HAP) specification: Version 2

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.






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