📄 rfc907.txt
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RFC 907
HOST ACCESS PROTOCOL SPECIFICATION
July 1984
prepared for
Defense Advanced Research Projects Agency
1400 Wilson Boulevard
Arlington, Virginia 22209
by
Bolt Beranek and Newman Laboratories
10 Moulton Street
Cambridge, Massachusetts 02238
RFC 907 Host Access Protocol
July 1984 Specification
Preface (Status of this Memo)
This document specifies the Host Access Protocol (HAP).
Although HAP was originally designed as the network-access level
protocol for the DARPA/DCA sponsored Wideband Packet Satellite
Network, it is intended that it evolve into a standard interface
between hosts and packet-switched satellite networks such as
SATNET and TACNET (aka MATNET) as well as the Wideband Network.
The HAP specification presented here is a minor revision of, and
supercedes, the specification presented in Chapter 4 of BBN
Report No. 4469, the "PSAT Technical Report". As such, the
details of the current specification are still most closely
matched to the characteristics if the Wideband Satellite Network.
Revisions to the specification in the "PSAT Technical Report"
include the definition of three new control message types
(Loopback Request, Link Going Down, and NOP), a "Reason" field in
Restart Request control messages, new Unnumbered Response codes,
and new values for the setup codes used to manage streams and
groups.
HAP is an experimental protocol, and will undergo further
revision as new capabilities are added and/or different satellite
networks are supported. Implementations of HAP should be
performed in coordination with satellite network development and
operations personnel.
RFC 907 Host Access Protocol
July 1984 Specification
Table of Contents
1 Introduction.......................................... 1
2 Overview.............................................. 3
3 Datagram Messages..................................... 8
4 Stream Messages...................................... 14
5 Flow Control Messages................................ 17
6 Setup Level Messages................................. 24
6.1 Stream Setup Messages.............................. 32
6.2 Group Setup Messages............................... 44
7 Link Monitoring...................................... 58
8 Initialization....................................... 62
9 Loopback Control..................................... 68
10 Other Control Messages.............................. 72
i
RFC 907 Host Access Protocol
July 1984 Specification
FIGURES
DATAGRAM MESSAGE.......................................... 9
STREAM MESSAGE........................................... 15
ACCEPTANCE/REFUSAL WORD.................................. 19
ACCEPTANCE/REFUSAL MESSAGE............................... 21
UNNUMBERED RESPONSE...................................... 22
SETUP MESSAGE HEADER..................................... 26
NOTIFICATION MESSAGE..................................... 29
SETUP ACKNOWLEDGMENT..................................... 31
STREAM EXAMPLE........................................... 33
CREATE STREAM REQUEST.................................... 35
CREATE STREAM REPLY...................................... 37
CHANGE STREAM PARAMETERS REQUEST......................... 39
CHANGE STREAM PARAMETERS REPLY........................... 41
DELETE STREAM REQUEST.................................... 42
DELETE STREAM REPLY...................................... 43
GROUP EXAMPLE............................................ 45
CREATE GROUP REQUEST..................................... 47
CREATE GROUP REPLY....................................... 48
JOIN GROUP REQUEST....................................... 50
JOIN GROUP REPLY......................................... 52
LEAVE GROUP REQUEST...................................... 53
LEAVE GROUP REPLY........................................ 55
DELETE GROUP REQUEST..................................... 56
DELETE GROUP REPLY....................................... 57
STATUS MESSAGE........................................... 59
HAP LINK RESTART STATE DIAGRAM........................... 64
RESTART REQUEST.......................................... 65
RESTART COMPLETE......................................... 67
LOOPBACK REQUEST......................................... 71
LINK GOING DOWN.......................................... 73
NO OPERATION (NOP)....................................... 75
ii
RFC 907 Host Access Protocol
July 1984 Specification
1 Introduction
The Host Access Protocol (HAP) specifies the network-access
level communication between an arbitrary computer, called a host,
and a packet-switched satellite network. The satellite network
provides message delivery services for geographically separated
hosts: Messages containing data which are meaningful to the hosts
are submitted to the network by an originating (source) host, and
are passed transparently through the network to an indicated
destination host. To utilize such services, a host interfaces to
the satellite network via an access link to a dedicated packet-
switching computer, known as a Satellite Interface Message
Processor (Satellite IMP or SIMP). HAP defines the different
types of control messages and (host-to-host) data messages that
may be exchanged over the access link connecting a host and a
SIMP. 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.
The term "Interface Message Processor" originates in the
ARPANET, where it refers to the ARPANET's packet-switching nodes.
SIMPs differ from ARPANET IMPs in that SIMPs form a network via
connections to a common multiaccess/broadcast satellite channel,
whereas ARPANET IMPs are interconnected by dedicated point-to-
point terrestrial communications lines. This fundamental
difference between satellite-based and ARPANET-style networks
results in different mechanisms for the delivery of messages from
source to destination hosts and for internal network
coordination. Additionally, satellite networks tend to offer
different type of service options to their connected hosts than
do ARPANET-style networks. These options are included in the
Host Access Protocol presented here.
Several types of Satellite IMPs have been developed on a
variety of processors for the support of three different packet-
switched satellite networks. The original SIMP was employed in
the Atlantic Packet Satellite Network (SATNET). It was developed
from one of the models of ARPANET IMP, and was implemented on a
Honeywell 316 minicomputer. The 316 SIMPs were succeeded in
SATNET by SIMPs based on BBN C/30 Communications Processor
hardware. The C/30 SIMPs have also been employed in the Mobile
1
RFC 907 Host Access Protocol
July 1984 Specification
Access Terminal Network (MATNET). The SATNET and MATNET SIMPs
implement a network-access level protocol known as Host/SATNET
Protocol. Host/SATNET Protocol is the precursor to HAP and is
documented in Internet Experiment Note (IEN) No. 192. The
Wideband Satellite Network, like SATNET, has undergone an
evolution in the development of its SIMP hardware and software.
The original Wideband Network SIMP is known as the Pluribus
Satellite IMP, or PSAT, having been implemented on the BBN
Pluribus Multiprocessor. Its successor, the BSAT, is based on
the BBN Butterfly Multiprocessor. Both the PSAT and the BSAT
communicate with their connected network hosts via HAP.
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 many of the topics
addressed in this HAP specification can be found in BBN Report
No. 4469, the "PSAT Technical Report".
The protocol used to provide sufficiently reliable message
exchange over the host-SIMP link is assumed to be transparent to
the network-access protocol defined in this document. Examples
of such link-level protocols are ARPANET 1822 local and distant
host, ARPANET VDH protocol, and HDLC.
2
RFC 907 Host Access Protocol
July 1984 Specification
2 Overview
HAP can be characterized as a full duplex nonreliable
protocol with an optional flow control mechanism. HAP messages
flow simultaneously in both directions between the SIMP and the
host. Transmission is nonreliable in the sense that the protocol
does not provide any guarantee of error-free sequenced delivery.
To the extent that this functionality is required on the access
link (e.g., non-collocated SIMP and host operating over a
communication circuit), it must be supported by the link-level
protocol below HAP. The flow control mechanism operates
independently in each direction except that enabling or disabling
the mechanism applies to both sides of the interface.
HAP supports host-to-host communication in two modes
corresponding to the two types of HAP data messages, datagram
messages and stream messages. Each type of message can be up to
approximately 16K bits in length. Datagram messages provide the
basic transmission service in the satellite network. Datagram
messages transmitted by a host experience a nominal two satellite
hop end-to-end network delay. (Note that this delay, of about 0.6
sec excluding access link delay, is associated with datagram
transmission between hosts on different SIMPs. The transmission
delay between hosts on the same SIMP will be much smaller
assuming the destination is not a group address. See Section 3
and 6.2.) A datagram control header, passed to the SIMP by the
host along with message text, determines the processing of the
message within the satellite network independent of any previous
exchanges.
Stream messages provide a one satellite hop delay
(approximately 0.3 sec) for volatile traffic, such as speech,
which cannot tolerate the delay associated with datagram
transmission. Hosts may also use streams to support high duty
cycle applications which require guaranteed channel bandwidth.
Host streams are established by a setup message exchange between
the host and the network prior to the commencement of data flow.
Although established host streams can have their characteristics
modified by subsequent setup messages while they are in use, the
fixed allocation properties of streams relative to datagrams
impose rather strict requirements on the source of the traffic
3
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