Host Access Protocol (HAP) specification: Version 2 (RFC1221)
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
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  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.
Network Working Group W. Edmond
Request for Comments: 1221 BBN
Updates: RFC 907 April 1991
Host Access Protocol (HAP) Specification - Version 2
Status of this Memo
This memo describes the Host Access Protocol implemented in the
Terrestrial Wideband Network (TWBNET). It obsoletes most but not all
of RFC 907. This memo provides information for the Internet
community. It does not specify an Internet standard. Distribution
of this memo is unlimited.
This memo specifies the Host Access Protocol (HAP). HAP is a Network
layer (OSI Layer 3 lower) access protocol that was first implemented
about a decade ago for the DARPA/DCA sponsored Wideband Packet
Satellite Network (WBNET), the precursor of the current Terrestrial
Wideband Network (TWBNET). This version of the specification
obsoletes references  and  in addition to most of RFC 907.
HAP is a developmental protocol, and will be revised as new
capabilities are added and unused features are eliminated or revised.
One reason that HAP is being revised now is that, unlike the original
WBNET's satellite channel, the TWBNET's T1 fiber links are not a
broadcast medium. This has prompted some changes to the protocol
that will permit greater efficiency in a mesh topology network.
Another cause of revision is the need to make HAP able to support a
variety of OSI layer 3 upper protocols, such as DECNET Phase V, ST,
and CLNP, where before only Internet Protocol (IP) was used.
Appendix B describes how backward compatibility with the older IP-
only version of HAP is achieved. A third cause of protocol changes
is the desire to simplify interaction between ST2 protocol (RFC 1190)
agents and the TWBNET. This has mainly affected the way certain
setup errors are handled. These changes are expected to be backward
compatible. Appendix A describes two capabilities that may be added
to HAP in the future.
One of the protocol enhancements, "Group Streams", described in
reference  has been eliminated. There are no known applications
that use the feature. As described in Appendix A, a new mechanism,
to be called "shared streams", capable of providing equivalent
capabilities will be implemented if needed. Changes in  that have
been retained include various query/reply control messages that
permit a host to determine what resources it owns (mostly useful for
cleanup following a host reboot or crash).
This document assumes the reader is familiar with DoD internetworking