rfc1221.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 1,439 行 · 第 1/5 页

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.

Preface

   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 [1] and [2] 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 [2] 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 [2] that have
   been retained include various query/reply control messages that
   permit a host to determine what resources it owns (mostly useful for



Edmond                                                          [Page 1]

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                                                          [Page 2]

RFC 1221                          HAP2                        April 1991


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 WPS and the host.  Transmission is
   nonreliable in the sense that the protocol does not provide any
   guarantee of error-free sequenced delivery.  If error-free delivery
   on the host's access link is required, it must be provided by the
   link layer protocol below HAP.  (Use of link layer protocols for this
   purpose is not within the scope of this document.)  HAP's flow
   control mechanism operates independently in each direction, but the
   choice to enable flow control or not applies to both directions
   together.

   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 2048 octets in length.
   The basic transmission service in the network is datagram service.
   Datagrams are variable length, unsequenced, independent, and delivery
   is not guaranteed.  The HAP header of each datagram determines the
   processing of the message.

   On this datagram service base a "stream" service is built.  Stream
   service provides network bandwidth guarantees, but requires explicit
   setup and teardown operations to allocate and deallocate network
   resources.  Stream traffic is best suited for continuous media
   traffic, but may also be used to obtain the lowest possible network
   delay.  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 using the stream.  Stream traffic arrivals must match the
   stream allocation both in interarrival time and message size if
   reasonable efficiency is to be achieved.  The characteristics and use
   of datagrams and streams are described in detail in Sections 3 and 4
   of this document.

   Both datagram and stream transmission in the network use logical
   addressing.  Each host on the network is assigned a permanent 16-bit
   logical address which is independent of the physical port on the WPS
   to which it is attached.  These 16-bit logical addresses are present
   in all Host-to-WPS and WPS-to-Host data messages.

   HAP supports multicast addressing via "groups".  Multicast addressing
   is provided primarily to support the multi-destination delivery
   required for conferencing applications.  Group addresses are



Edmond                                                          [Page 3]

RFC 1221                          HAP2                        April 1991


   dynamically created and deleted by the use of setup messages
   exchanged between a host and the WPS.  Membership in a group may be
   any arbitrary subset of the network hosts.  A message addressed to a
   group address is delivered to all hosts that are members of that
   group, except the sender.  Once a multicast address has been created,
   any member host may use that address, not just the creator.

   Although HAP does not guarantee error-free delivery, error control is
   an important aspect of the protocol design.  HAP error control is
   concerned with both local transfers between a host and its local WPS
   and transfers through the network to the destination(s).  The WPS
   offers users a choice of network error protection options based on
   the network's ability to selectively send messages over its
   transmission media at different forward error correction (FEC) rates.
   These FEC options are referred to as reliability levels.  Four
   reliability levels (low, medium-low, medium-high, and high) are
   available.  The precise error rate provided by each reliability level
   is not specified.

   Various checksum and CRC mechanisms are employed in the network to
   provide an error detection capability.  A host has an opportunity
   when sending a message to indicate whether the message should be
   delivered to its destination or discarded if a data error is detected
   by the network.  Each message received by a host from the network
   will have a flag indicating whether or not an error was detected in
   that particular message.  A host can decide on a per-message basis
   whether or not it wants to accept or discard transmissions containing
   data errors.

   For connection of a host and WPS in close proximity, error rates due
   to external noise or hardware failures on the access circuit may
   reasonably be expected to be much smaller than the best network trunk
   circuit error rates.  Thus for this case, little is gained by using
   error detection and retransmission on the access circuit.  A 16-bit
   header checksum is provided, however, to ensure that WPSen do not act
   on incorrect control information.  For relatively long distances or
   noisy connections, retransmissions over the access circuit may be
   required to optimize performance for both low and high reliability
   traffic.  It is expected that link layer error control procedures
   (such as HDLC with retransmission) will be used for this purpose, but
   use of a reliable link layer protocol is not within the scope of this
   document.

   Each datagram message submitted to the WPS by a host is marked as
   being in one of three priority classes, from priority 2 (highest)
   through priority 0 (lowest).  The priority class is used by the WPS
   for arbitrating contention for scarce network resources (e.g., link
   bandwidth).  That is, if the network cannot deliver all of the



Edmond                                                          [Page 4]

RFC 1221                          HAP2                        April 1991


   offered messages, high priority messages will be delivered in
   preference to low priority messages.  Priority level affects the
   order of access to intersite link bandwidth and the order of message
   delivery at the destination WPS.

   Each stream message also has three priority classes, from priority 2
   (highest) through priority 0 (lowest).  In addition, streams
   themselves have three precedence classes, from precedence 2 (highest)
   through precedence 0.  A stream of higher precedence can preempt a
   stream of lower precedence at setup time.  Stream message priority
   provides a mechanism for a low-bandwidth host to receive a high-
   bandwidth stream and selectively discard messages marked as less
   important by the sender.  Stream message priority does not affect the
   order of delivery of stream messages between the source and the
   destination.

   Datagram and stream messages being presented to the WPS by a host may
   not be accepted for a number of reasons: priority too low,
   destination dead, lack of buffers in the source WPS, etc.  The host
   faces a similar situation with respect to handling messages from the
   WPS.  To permit the receiver of a message to inform the sender of the
   local disposition of its message, an acceptance/refusal (A/R)
   mechanism is implemented.  The mechanism is the external
   manifestation of the WPS's (or host's) internal flow and congestion
   control algorithm.  If A/Rs are enabled, an explicit or implicit
   acceptance or refusal for each message is returned to the host by the
   WPS (and conversely).  This allows the host (or WPS) to retry refused
   messages at its discretion and can provide information useful for
   optimizing the sending of subsequent messages when the reason for
   refusals is also provided.  The A/R mechanism can be disabled to
   provide a "pure discard" interface.  The host's choice to use the A/R
   mechanism or not does not limit its ability to send and receive
   messages to any other hosts.

   While the A/R mechanism allows control of individual message
   transfers, it does not facilitate regulation of priority flows.  Such
   regulation is handled by passing advisory status information (GOPRI)
   across the Host-WPS interface indicating which priorities are
   currently being accepted.  As long as this information, relative to
   the change in priority status, is passed frequently, the sender can
   avoid originating messages which are sure to be refused.

   HAP defines both data messages (datagram messages and stream
   messages) and link control messages.  Data messages are used to send
   information between hosts on the network.  Link control messages are
   exchanged between a host and the WPS to manage the local access link.

   Allocation of network resources, such as streams and groups, is



Edmond                                                          [Page 5]

RFC 1221                          HAP2                        April 1991


   accomplished via an exchange of datagram messages, called Setups,
   between the user host and an agent inside the WPS called the "Service