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Host Access Protocol (HAP) specification: Version 2

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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 [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

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   cleanup following a host reboot or crash).

   This document assumes the reader is familiar with DoD internetworking

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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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

   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

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   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

   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

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   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

   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

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   accomplished via an exchange of datagram messages, called Setups,
   between the user host and an agent inside the WPS called the "Service
   Agent."  Setups are used to reserve, allocate, modify, free, and
   deallocate network resources.  Each allocated resource has a unique
   identifier which, when placed in an appropriate field in a message
   header, allows that message to use the resource.  E.g., after an
   exchange of Setups to create a group address, a message may be sent
   to the group by placing the group address in the destination field of
   that message.  The Service Agent also permits a host to inquire about
   resources it owns.

   Every HAP message consists of an integral number of 16-bit words
   (i.e., an even number of octets).  The first several words of the
   message always contain control information and are referred to as the
   message header.  The first word of the message header identifies the
   type of message which follows.  The second word of the message header
   is a checksum which covers all header information.  Any message whose
   received header checksum does not match the checksum computed on the
   received header information must be discarded.  The format of the
   rest of the header depends on the specific message type.

   The formats and use of the individual message types are detailed in
   the following sections.  A common format description is used for this
   purpose.  Words in a message are numbered starting at zero (i.e.,
   zero is the first word of a message header).  Bits within a word are
   numbered from zero (most significant) to fifteen (least significant).
   The notation used to identify a particular field location is:

     <WORD#>{-<WORD#>}  [ <BIT#>{-<BIT#>} ]  <description>

   where optional elements in {} are used to specify the (inclusive)
   upper limit of a range.  The reader should refer to these field
   identifiers for precise field size specifications.  Fields which are
   common to several message types are defined in the first section
   which uses them.  Only the name of the field will usually appear in
   the descriptions in subsequent sections.

   Link-level protocols used to support HAP can differ in the order in
   which they transmit the bits constituting HAP messages.  The words of
   the message are transmitted from word 0 to word N.

3. Datagram Messages

   Datagrams are one of the two message types provided by HAP, as
   described in the previous section.  Because network resources are not
   reserved in advance for datagram traffic, delivery of datagram
   traffic is subject to greater delivery delays and delay variance than
   stream traffic, and is subject to flow and congestion controls.

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   Datagram priority determines which packets are delivered or discarded
   when network resources do not permit handling all of the presented
   traffic.  It is expected that datagram messages will be used to
   support the majority of computer-to-computer and terminal-to-computer
   traffic which is bursty in nature.

   The format of datagram messages and the purpose of each of the header
   control fields is described in Figure 1.

                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
     0         | 0|LB|GOPRI|    0   | F|     MESSAGE NUMBER    |
     1         |                HEADER CHECKSUM                |
     2         |                      A/R                      |
     3         | 0|IL| D| E| PRI | TTL | RLY |      RLEN       |
     4         |            DESTINATION HOST ADDRESS           |
     5         |              SOURCE HOST ADDRESS              |
     6         |                  PROTOCOL ID                  |
               |                                               |
     7-N       :                      DATA                     :
               |                                               |

                             DATAGRAM MESSAGE
                                 Figure 1

     0[0]      Message Class.  This bit identifies the message as a
               data message or a control message.

                    0 = Data Message
                    1 = Control Message

     0[1]      Loopback indicator.  This bit allows the sender of a
               message to determine if its own messages are being
               looped back.  The host and the WPS each use different
               settings of this bit for their transmissions.  If a
               message arrives with the loopback bit set equal to its

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               outgoing value, then the message has been looped.

                    0 = Sent by Host
                    1 = Sent by WPS

     0[2-3]    Go-Priority.  In WPS-to-Host messages, this field
               provides advisory information concerning the lowest
               priority currently being accepted by the WPS.  The host
               may optionally choose to provide similar priority
               information to the WPS.

                    0 = Low Priority
                    1 = Medium Priority
                    2 = High Priority
                    3 = (Reserved.)

     0[4-6]    Reserved.  Must be zero.

     0[7]      Reserved.  Must be zero.  Formerly used for WPS
               diagnostic purposes.

     0[8-15]   Message Number.  This field contains the identification
               of the message used by the acceptance/refusal (A/R)
               mechanism (when enabled).  If the message number is
               zero, A/R is disabled for this specific message.  See
               Section 5 for a detailed description of the A/R

     1[0-15]   Header Checksum.  The checksum is the 2's-complement of
               the 2's-complement sum of words 0-6 (excluding the
               checksum word itself).

     2[0-15]   Piggybacked A/R.  This field may contain an
               acceptance/refusal word providing A/R status on traffic
               flowing in the opposite direction.  Its inclusion may
               eliminate the need for a separate A/R control message
               (see Section 5).  A value of zero for this word is used
               to indicate that no piggybacked A/R information is

     3[0]      Data Message Type.  This bit identifies whether the
               message is a datagram message or a stream message.

                    0 = Datagram Message
                    1 = Stream Message

     3[1]      IL flag.  Obsolete.  Must be zero.  (See Appendix B.)

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     3[2]      Discard Flag.  This flag allows a source host to
               instruct the network (including the destination host)
               what to do with the message when data errors are
               detected (assuming the header checksum is correct).

                    0 = Discard message if data errors detected.
                    1 = Don't discard message if data errors detected.

               The value of this flag, set by the source host, is
               passed on to the destination host.

     3[3]      Data Error Flag.  This flag is used in conjunction with
               the Discard Flag to indicate to the destination host
               whether any data errors have been detected in the
               message prior to transmission over the destination's
               WPS-to-Host access link.  It is used only if Discard
               Flag = 1.  It should be set to zero by the source host.

                    0 = No Data Errors Detected
                    1 = Data Errors Detected

     3[4-5]    Priority.  The source host uses this field to specify
               the priority with which the message should be handled
               within the network.

                    0 = Low Priority
                    1 = Medium Priority
                    2 = High Priority
                    3 = (Reserved.)

               The priority of each message is passed to the
               destination host by the destination WPS.

     3[6-7]    Time-to-Live Designator.  The source host uses this
               field to specify the maximum time that a message should
               be allowed to exist within the network before being
               deleted.  Elapsed time begins when the message has been
               received by the WPS from the source host (or is sent by
               a WPS agent) and is last checked when the message is
               queued for transmission out the I/O interface to the
               destination host.  If a message is multicast, each copy
               is treated separately.

                    0 = 1 seconds
                    1 = 2 seconds
                    2 = 5 seconds
                    3 = 10 seconds

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     3[8-9]    Reliability.  The source host uses this field to
               specify the basic bit error rate requirement for the
               data portion of this message.  The source WPS uses this
               field to determine the trunk circuit transmission
               parameters and forward error correction level required
               to provide that bit error rate.

                    0 = Low Reliability
                    1 = Medium-Low Reliability
                    2 = Medium-High Reliability
                    3 = High Reliability

     3[10-15]  Reliability Length.  The source host uses this field to
               specify a portion of the user data which should be
               transmitted at the highest reliability level (lowest
               bit error rate).  Both the HAP message header words and
               the first 2*<Reliability Length> octets of user data
               will be transmitted at high reliability while the
               remainder of the user data will be transmitted at
               whatever reliability level is specified in field 3[8-
               9].  The reliability length mechanism gives the user
               the ability to transmit private header information
               (e.g., IP and TCP headers) at a higher reliability
               level than the remainder of the data.

     4[0-15]   Destination Host Address.  This field contains the
               network logical address of the destination host.

     5[0-15]   Source Host Address.  This field contains the network
               logical address of the source host.

     6[0-15]   Protocol ID.  This field specifies the next higher
               level protocol.  Protocol identifiers are assigned
               administratively, except 0 which is reserved, and are
               not part of this specification.  See reference [10].

     7-N       Data.  This field contains up to 16,384 bits (2048
               octets) of user data, and must be an even number of

4. Stream Messages

   Stream messages are the second message type provided by HAP, as
   described in Section 2.  Streams provide guaranteed bandwidth between
   the source and destination(s), and provide the minimum delivery delay
   and delay variance available in the network.  Streams are suitable
   for volatile traffic, such as speech, and for support of high duty
   cycle applications that require throughput guarantees.

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   Streams must be created before stream messages can flow from host to
   host.  The protocol to accomplish stream creation is described in
   Section 6.1.  Once established, a stream is allocated specific
   network resources, such as bandwidth.  Within the bounds of its
   stream allocation, a host is permitted considerable flexibility in
   how it may use the stream.  Although the time to live, reliability,
   and reliability length of each stream message is fixed at stream
   setup time, the destination logical address can vary from stream
   message to stream message.

   A host can, therefore, multiplex a variety of logical flows onto a
   single stream, as long as the stream was set up to reach all the
   destination hosts.  The format of stream messages is described in
   Figure 2.

                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
     0         | 0|LB|GOPRI|     0     |     MESSAGE NUMBER    |
     1         |               HEADER CHECKSUM                 |
     2         |                      A/R                      |
     3         | 1|IL| D| E| PRI |       HOST STREAM ID        |
     4         |            DESTINATION HOST ADDRESS           |
     5         |              SOURCE HOST ADDRESS              |
     6         |                  PROTOCOL ID                  |
               |                                               |
     7-N       :                      DATA                     :
               |                                               |

                              STREAM MESSAGE
                                 Figure 2

     0[0]      Message Class = 0 (Data Message).

     0[1]      Loopback indicator.

     0[2-3]    Go-Priority.

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     0[4-7]    Reserved.

     0[8-15]   Message Number.  This field serves the same purpose as
               the message number field in the datagram message.
               Moreover, a single message number sequence is used for
               both datagram and stream messages (see Section 5).

     1[0-15]   Header Checksum.  (See datagram checksum for

     2[0-15]   Piggybacked A/R.

     3[0]      Data Message Type = 1 (Stream).

     3[1]      IL flag.  Obsolete.  Must be zero.

     3[2]      Discard Flag.

     3[3]      Data Error Flag.

     3[4-5]    Stream message priority.  Note that all stream messages
               have priority over any datagram message.  Priority will
               not affect the order of stream message delivery.

                    0 = Low priority
                    1 = Medium priority
                    2 = High priority
                    3 = Reserved

     3[6-15]   Stream ID.  The WPS uses this field to identify the
               preallocated network resources (bandwidth allocations,
               queues, buffers, etc.) to use for delivery of the
               message.  Streams and their identifying numbers (stream
               IDs) are established by an explicit Create Stream
               request (see Section 6.1).

     4[0-15]   Destination Host Address.

     5[0-15]   Source Host Address.

     6[0-15]   Protocol ID.

     7-N       Data.  This field contains up to 16,384 bits (2048
               octets) of user data, and must be an even number of

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5. Flow Control Messages

   The WPS supports an acceptance/refusal (A/R) mechanism in each
   direction on the host access link.  The A/R mechanism is enabled for
   the link by the host by setting a bit in the Restart Complete control
   message (see Section 8).  Each datagram and stream message contains
   an 8-bit message number used to identify the message for flow control
   purposes.  When the A/R mechanism is enabled, the message number is
   incremented modulo 256 in successive messages, skipping over message
   number zero (zero indicates that A/R's are disabled for that
   message).  Up to 127 messages may be outstanding (awaiting acceptance
   or refusal) in each direction.  If the receiver of a message is
   unable to accept the message, a refusal indication containing the
   message number of the refused message and the reason for the refusal
   is returned.  The refusal indication may be piggybacked on data
   messages in the opposite direction over the link or may be sent in a
   separate control message in the absence of reverse data traffic.

   Acceptance indications are returned in a similar manner, either
   piggybacked on data messages or in a separate control message.  An
   acceptance is returned by the receiver to indicate that the
   identified message was received from the host access link and was not
   refused.  Acceptance indications returned by the WPS are not an end-
   to-end acknowledgement and do not imply any guarantee of delivery to
   the destination host(s), or even any assurance that the message will
   not be intentionally discarded by the network.  They are sent
   primarily to facilitate buffer management in the host.

   To reduce the number of A/R messages exchanged, a single A/R
   indication can be returned for multiple (lower numbered) previously
   unacknowledged messages.  Explicit acceptance of message number N
   implies implicit acceptance of outstanding messages with numbers N-1,
   N-2, etc., according to the definition of acceptance outlined above.
   Analogous interpretation of the refusal message number allows the
   receiver of a group of messages to reject them as a group when they
   all are being refused for the same reason.  As a further efficiency
   measure, HAP permits aggregation of any mix of A/R indications into a
   single A/R control message.  Such a message might be used, for
   example, to reject a group of messages where the refusal code on each
   is different.

   In some circumstances the overhead associated with processing A/R
   messages may prove unattractive.  For these cases, it is possible to
   disable the A/R mechanism and operate the HAP interface in a purely
   discard mode.  The ability to effect this on a link basis has already
   been noted (see Sections 2 and 8).  In addition, messages with
   sequence number zero are taken as messages for which the A/R
   mechanism is selectively disabled.  To permit critical feedback, even

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   when operating in discard mode, HAP defines an "Unnumbered Response"
   control message.  Flow control information, and other information
   which cannot be sent as an A/R indication, is sent in an Unnumbered
   Response control message.  The format of this type of message is
   illustrated in Figure 5.

   The format shown in Figure 3 is used both for A/R indications that
   are piggybacked on data messages (word 2), and for aggregated A/R
   information in A/R control messages.  The format of A/R control
   messages is shown in Figure 4.

                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
               |AR|    REFUSAL CODE    |  A/R MESSAGE NUMBER   |

                          ACCEPTANCE/REFUSAL WORD
                                 Figure 3

     [0]       Acceptance/Refusal Type.  This field identifies whether
               A/R information is an acceptance or a refusal.

                    0 = Acceptance
                    1 = Refusal

     [1-7]     Refusal Code.  When the Acceptance/Refusal Type = 1,
               this field gives the Refusal Code.

                    0 = Priority not being accepted
                    1 = Source WPS congestion
                    2 = Destination WPS congestion
                    3 = Destination host dead
                    4 = Destination WPS dead
                    5 = Illegal destination host address
                    6 = Destination host access not allowed
                    7 = Illegal source host address
                    8 = Message lost in access link
                    9 = Invalid stream ID
                   10 = Illegal source host for stream ID
                   11 = Message length too long
                   12 = Stream message too early
                   13 = Illegal control message type
                   14 = Illegal refusal code in A/R
                   15 = Can't implement loop

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                   16 = Destination host congestion
                   17 = Delivery refused
                   18 = Odd byte length packet (not allowed)
                   19 = Invalid stream time-to-live value
                   20 = "Reliability length" exceeds message length

     [8-15]    A/R Message Number.  This field contains the number of
               the message to which this acceptance/refusal refers.
               It also applies to all outstanding messages with
               earlier numbers.  Note that this field can never be
               zero since a message number of zero implies that the
               A/R mechanism is disabled.

                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
     0         | 1|LB|GOPRI|     0     |  LENGTH   |     1     |
     1         |                HEADER CHECKSUM                |
               |                                               |
     2-N       :                     A/R's                     :
               |                                               |

                        ACCEPTANCE/REFUSAL MESSAGE
                                 Figure 4

     0[0]      Message Class = 1 (Control Message).

     0[1]      Loopback indicator.

     0[2-3]    Go-Priority.

     0[4-7]    Reserved.

     0[8-11]   Message Length.  This field contains the total length
               of this message in words (N+1).

     0[12-15]  Control Message Type = 1 (Acceptance/Refusal).

     1[0-15]   Header Checksum.  The checksum is the 2's-complement of
               the 2's-complement sum of words 0-N (excluding the
               checksum word itself).

Top       Page 16 
     2[0-15]   Acceptance/Refusal Word.

     3-N       Additional Acceptance/Refusal Words (optional).

                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
     0         | 1|LB|GOPRI|     0     | RES-CODE  |     5     |
     1         |                HEADER CHECKSUM                |
     2         |                 RESPONSE INFO                 |
     3         |                 RESPONSE INFO                 |

                            UNNUMBERED RESPONSE
                                 Figure 5

     0[0]      Message Class = 1 (Control Message).

     0[1]      Loopback indicator.

     0[2-3]    Go-Priority.

     0[4-7]    Reserved.

     0[8-11]   Response Code.

                    3 = Destination unreachable
                    5 = Illegal destination host address
                    7 = Illegal source host address
                    9 = Nonexistent stream ID
                   10 = Illegal stream ID
                   13 = Protocol violation
                   15 = Can't implement loop

     0[12-15]  Control Message Type = 5 (Unnumbered Response).

     1[0-15]   Header Checksum.  The checksum is the 2's-complement of
               the 2's-complement sum of words 0-3 (excluding the
               checksum word itself).

     2[0-15]   Response Information. If Response Code is:

Top       Page 17 
                    3: Destination Host Address
                    5: Destination Host Address
                    7: Source Host Address
                    9: Stream ID (right justified)
                   10: Stream ID (right justified)
                   13: Word 0 of offending message
                   15: Word 0 of Loopback Request message

     3[0-15]   Response Information. If Response Code is:

                    3,5,7, or 9: Undefined
                    10: Source Host Address
                    13: Word 3 of offending message, or 0 if no word 3
                    15: Word 2 of Loopback Request message

(page 17 continued on part 2)

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