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rfc:rfc1221

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
 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.

Edmond [Page 6] RFC 1221 HAP2 April 1991

 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

Edmond [Page 7] RFC 1221 HAP2 April 1991

             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
             mechanism.
   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
             present.
   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.)

Edmond [Page 8] RFC 1221 HAP2 April 1991

   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

Edmond [Page 9] RFC 1221 HAP2 April 1991

   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
             octets.

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.

Edmond [Page 10] RFC 1221 HAP2 April 1991

 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.

Edmond [Page 11] RFC 1221 HAP2 April 1991

   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
             description.)
   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
             octets.

Edmond [Page 12] RFC 1221 HAP2 April 1991

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

Edmond [Page 13] RFC 1221 HAP2 April 1991

 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

Edmond [Page 14] RFC 1221 HAP2 April 1991

                 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).

Edmond [Page 15] RFC 1221 HAP2 April 1991

   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:

Edmond [Page 16] RFC 1221 HAP2 April 1991

                  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

6. The Service Agent

 Allocation of network resources, such as streams and groups, is
 accomplished via an exchange of datagram messages, called Setup
 messages, between the user host and the Service Agent (network
 address zero).  Setup operations include reserving, allocating,
 modifying, freeing, and deallocating resources.  The Service Agent
 causes the requested action to be carried out and serves as the
 intermediary between the user and the rest of the network.  In the
 process of implementing the requested action, various network data
 bases are updated to reflect the current state of the referenced
 resource.  The Service Agent also permits a host to inquire about
 resources it owns using Information Request and Information Reply
 messages.
 A setup interaction initiated by a host involves a 3-way exchange
 where: (1) the requesting host sends a Setup Request to the Service
 Agent, (2) the Service Agent returns a Setup Reply to the requesting
 host, and (3) the requesting host returns a Setup Acknowledgment to
 the Service Agent.  This procedure is used to ensure reliable
 transmission of Setup Requests and Replies.  In order to allow more
 than one Setup Request message from a host to be outstanding, each
 Request is assigned a unique Request ID.  The associated Reply and
 subsequent Acknowledgment are identified by the Request ID that they
 contain.  The requesting host should receive a reply to a setup
 request within 3 seconds.  The actual delay will depend on the nature
 of the request and the topology of the network.  For simple networks,
 the delay will often be less than one second.  The requesting host
 should respond to a Reply with a Setup Acknowledgment within one
 second.
 Setup exchanges initiated by the Service Agent involve a two-way
 exchange where: (1) the Service Agent sends a Notification to

Edmond [Page 17] RFC 1221 HAP2 April 1991

 affected hosts, and (2) the hosts return a Setup Acknowledgment to
 the Service Agent.  Notifications are used to inform a host of
 changes in the status of a network resource.  In order to allow more
 than one Notification to be outstanding, each is assigned a unique
 Notification ID.  The Setup Acknowledgment returned by the notified
 host to the Service Agent must contain the Notification ID.  The host
 should respond within one second.
 An information query is initiated by a host and involves a two-way
 exchange where: (1) the host sends an Information Request message to
 the Service Agent, and (2) the Service Agent sends back an
 Information Reply.  There is no acknowledgment mechanism, since this
 request does not change any resource allocation.  Furthermore, if
 there is an error in the request, only one response will be sent by
 the WPS, and the WPS will make no effort to check for or retransmit
 lost responses.  It is the responsibility of the host to wait a
 certain amount of time and then determine that an unanswered
 information request has been lost and to resend it.  (The time
 necessary to answer such a request is usually much less than one
 second.)  The WPS will return the message ID of the information
 request in the information reply message.
        The general format of all Service Agent messages is:
                       <DATAGRAM MESSAGE HEADER>
                        <SERVICE AGENT HEADER>
                            <MESSAGE BODY>
 The Protocol ID field in the datagram message header must be
 HAP_PROTO_SETUP (1) (see Appendix C) for messages sent to the Service
 Agent and will be HAP_PROTO_SETUP in messages received from the
 Service Agent.  The Service Agent does not recognize or support use
 of other higher level protocols (e.g., IP), in setup messages, and
 will discard messages containing such headers.
 Illustrations of message formats below show only the Service Agent
 Header header and message body and do not include the datagram
 message header.  As a reminder that the datagram header is not
 included, word offsets are prefixed with an "S".
 The format of the Service Agent Header is illustrated in Figure 6.
 The body of the message will depend on the particular message type.
 Stream Request and Reply messages are described in Section 6.1.
 Group Request and Reply messages are described in Section 6.2.  The
 format of Notifications is described in Section 6.3, and Setup
 Acknowledgments are described in Section 6.4.  Information Request
 and Reply messages are described in Section 6.5.

Edmond [Page 18] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |     MESSAGE TYPE      |          CODE         |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                    CHECKSUM                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   MESSAGE ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         SERVICE AGENT HEADER
                               Figure 6
   S0[0-7]   Message Type.  This field determines the type of
             message.
                  0 = Setup Acknowledgment
                  1 = Setup Request
                  2 = Setup Reply
                  3 = Notification
                  4 = Information Request
                  5 = Information Reply
   S0[8-15]  Code.  For Setup Requests, this field identifies the
             request type.
                  1 = Create group (multicast) address
                  2 = Delete group address
                  3 = Join group
                  4 = Leave group
                  5 = Create stream
                  6 = Delete stream
                  7 = Change stream
                  8 = Create shared stream
                  9 = Delete all streams owned by this host
                 10 = Add member to group
                 11 = Remove member from group
             For Setup Replies, this field provides the Reply Code.
             Some of the Reply Codes can be returned to any setup
             request and others are request specific.
                  0 = Group or stream created
                  1 = Group or stream deleted
                  2 = Host added to group
                  3 = Host deleted from group
                  4 = Stream changed

Edmond [Page 19] RFC 1221 HAP2 April 1991

                  5 = (Reserved)
                  6 = Request type invalid or unsupported
                  7 = (Reserved)
                  8 = Network trouble
                  9 = Bad group key
                 10 = Group address/stream ID nonexistent
                 11 = Not member of group/not creator of stream
                 12 = Stream precedence not being accepted
                 13 = (Reserved)
                 14 = (Reserved)
                 15 = (Reserved)
                 16 = Unable to add all the new hosts
                 17 = Insufficient network resources
                 18 = Requested bandwidth too large
                 19 = (Reserved)
                 20 = (Reserved)
                 21 = Maximum messages per interval too small
                 22 = Reply lost in network
                 23 = Illegal priority or precedence value
                 24 = Invalid address provided
             For Notifications, this field contains the Notification
             Type.  (See Section 6.3.)
             For Setup Acknowledgments, this field contains the
             Acknowledgment Type.  (See Section 6.4.)
             For Information Requests, this field contains the
             request type.  (See Section 6.5.)
             For Information Replies, this field contains the reply
             type.  (See Section 6.5.)
   S1[0-15]  Checksum.  The checksum is the 2's-complement of the
             2's-complement sum of the words in the Service Agent
             Header (excluding the checksum word itself) and the
             message body.  Messages received with bad checksums
             must be discarded.
   S2[0-15]  Message ID.  This field is assigned by the host to
             uniquely identify outstanding requests (Request ID) and
             by the Service Agent to uniquely identify outstanding
             notifications (Notification ID).

6.1. Stream Setup Messages

 Streams provide a means of reserving network resources for the
 delivery of traffic at a specified maximum throughput to a specified

Edmond [Page 20] RFC 1221 HAP2 April 1991

 list of recipients.  Traffic sent via a stream has priority over all
 non-stream traffic, and is delivered with the minimum end-to-end
 delay possible.  Hosts use streams to support applications that have
 predictable traffic loads (such as packet voice or video or other
 continuous media traffic) or that require minimum transmission delay
 and lowest delay variance.  Streams are typically used for traffic
 flows of moderate to long duration, where the cost of performing a
 stream Setup is acceptable.
 Streams must be set up before stream data messages can flow.  The
 stream setup messages, each of which has a Request and a Reply, are
 Create Stream, Delete Stream, Change Stream, and Delete All Streams.
 (Create Shared Stream Request is a planned future addition to the
 protocol.)  The use of these messages is illustrated in the scenario
 of exchanges between a host and the Service Agent shown in Figure 7
 where the host establishes a stream, sends some data, modifies the
 stream characteristics, sends some more data, and finally closes down
 the stream.  Not illustrated, but implicit in this scenario, are the
 optional A/R indications associated with each of the stream Setup
 messages.
                                            Service     Other
                                   Host      Agent      hosts
        Create Stream Request        ---------->
        Create Stream Reply          <----------
        Reply Acknowledgment         ---------->
        Stream Messages              --------------------->
           :   :
        Change Stream Request        ---------->
        Change Stream Reply          <----------
        Reply Acknowledgment         ---------->
        Stream Messages              --------------------->
           :   :
        Delete Stream Request        ---------->
        Delete Stream Reply          <----------
        Reply Acknowledgment         ---------->
                            STREAM EXAMPLE
                               Figure 7
 Streams have eight characteristic properties which are selected at
 stream setup time.  These properties are: (1) data words per time
 interval, (2) time interval, (3) reliability, (4) reliability length,
 (5) precedence, (6) maximum messages per interval, (7) the list of
 recipients, and (8) the set of other streams with which this stream
 shares resources.  To establish a stream, the host sends the Create

Edmond [Page 21] RFC 1221 HAP2 April 1991

 Stream Request message (Figure 8) to the Service Agent.  After the
 network has processed the Create Stream Request, the Service Agent
 will reply with a Create Stream Reply message (Figure 9).  If the
 reply code in the Create Stream Reply indicates that the stream has
 been created successfully, the host may proceed to transmit stream
 data messages after sending a Reply Acknowledgment.
 During the lifetime of a stream, the host which created it may decide
 that some of its characteristic properties should be modified.  All
 but one of the properties can be modified using the Change Stream
 Request message (Figure 10).  The one property that cannot be changed
 is whether or not the stream is willing to share its resources with
 other streams.  After the network has processed the Change Stream
 Request, the Service Agent will respond by sending a Change Stream
 Reply (Figure 11) to the host.  A host requesting a reduced channel
 allocation should decrease its sending rate immediately without
 waiting for receipt of the Change Stream Reply.  A host requesting an
 increased allocation should not proceed to transmit according to the
 new set of parameters without first having received a Reply Code
 indicating that the requested change has taken effect.
 When the host no longer needs the stream it created, it should first
 stop sending traffic via the stream and then send the Service Agent a
 Delete Stream Request message (Figure 12).  After the network has
 processed the Delete Stream Request, the Service Agent will respond
 by sending a Delete Stream Reply (Figure 13) to the host.
 If the host has crashed or restarted, it may no longer know what
 streams it owns.  The host may use an Information Request (see
 Section 6.5) to determine what streams it owns, or the host may use a
 Delete All Streams Request (Figure 14) to discard whatever stream
 resources it may own.  The format for the Delete All Streams Reply is
 shown in Figure 15.
 Note that streams, like all other resources allocated by the Service
 Agent, may be reclaimed by the network if unused.  Currently, if no
 traffic is sent to a stream in a 6 minute interval, and if the owner
 of the steam is down or unreachable, the stream may be deleted.

Edmond [Page 22] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           5           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |  MAX MES  | PRE | INT | RLY |      RLEN       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |            DATA WORDS PER INTERVAL            |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                 INTERVAL                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |           0           |  ADDRESS LIST LENGTH  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
   S7-SN     :            DESTINATION ADDRESS LIST           :
             |                                               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         CREATE STREAM REQUEST
                               Figure 8
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 5 (Create Stream).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-3]   Maximum Messages Per Interval (1-15).  This field
             specifies the maximum number of stream messages the
             host will deliver to the WPS in any single stream
             interval.
   S3[4-5]   Precedence.  This field specifies the precedence of the
             stream.  When there are insufficient network resources
             to support all the requested streams, requests for
             higher precedence streams will preempt existing lower
             precedence streams, and requests for streams with
             insufficient precedence will be rejected.  Medium
             precedence is recommended as the default choice.

Edmond [Page 23] RFC 1221 HAP2 April 1991

                  0 = Low Precedence
                  1 = Medium Precedence
                  2 = High Precedence
   S3[6-7]   Interval.  This field specifies the interval, in
             multiples of 21.22 milliseconds.  (For backward
             compatibility only.  New applications should use 3.
             Use of this field to specify an interval is being
             phased out.)
                  0 =  21.22 milliseconds
                  1 =  42.44 milliseconds
                  2 =  84.88 milliseconds
                  3 =  use interval in word S5
   S3[8-9]   Reliability.  This field specifies the basic bit-error
             rate requirement for the data portion of all messages
             in the stream.  The exact error rate obtained by each
             choice is not specified.
                  0 = Low Reliability
                  1 = Medium-Low Reliability
                  2 = Medium-High Reliability
                  3 = High Reliability
   S3[10-15] Reliability Length.  This field specifies how many
             words beyond the stream message header should be
             transmitted at maximum reliability for all messages in
             the host stream.
   S4[0-15]  Data words per interval.  This field specifies the
             maximum number of 16-bit words of this stream's data
             the network will need to carry during each interval,
             not counting HAP stream message header words.  The
             stream data may be carried in however many messages (up
             to MAX MES) in each interval the host chooses.
   S5[0-15]  Interval (125 microsecond units).  This field specifies
             the time interval over which the <data words per
             interval> data in <max mes> messages will be sent.  For
             backward compatibility, an interval of 0 selects an
             interval of 169.76 milliseconds.  This field is ignored
             unless the INT field is 3.
   S6[0-7]   Reserved.  Must be zero.
   S6[8-15]  Destination address list length.  This field specifies
             the number of entries in the Destination Address List

Edmond [Page 24] RFC 1221 HAP2 April 1991

             field.  Allowed values are 1-8.
   S7-SN     Destination address list.  This list must specify, at
             least indirectly, all the intended recipients of this
             stream's traffic.  At least one destination address
             must be supplied.  Any valid network address,
             specifically including group addresses, may be used
             (except the Service Agent's address, 0).  Messages sent
             in the stream are not limited to using the HAP
             addresses listed.  E.g., if the list consists of only
             group address G, and host A is a member of G, a stream
             message may be sent to A, which was not in the list.
 Caution: Group membership is only evaluated at setup time.  Changes
 in group membership do not cause the stream to be modified.
 Caution: Stream creation involves allocation of specific network
 resources along specific routes for delivery of that traffic.  A
 stream message sent to hosts other than those specified via Setup
 will probably be undeliverable.  A stream message to a group address
 that has gained new members since the stream's last Setup may be
 undeliverable to the new members.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |        0        |         STREAM ID           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |        0        |     ADDRESS LIST LENGTH     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
   S5-SN     :                 ADDRESS LIST                  :
             |                                               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          CREATE STREAM REPLY
                               Figure 9
   S0[0-7]   Setup Type = 2 (Reply).

Edmond [Page 25] RFC 1221 HAP2 April 1991

   S0[8-15]  Reply Code.  Any reply other than "Stream created"
             means the stream was not created.
                  0 = Stream created
                  8 = Network trouble
                 12 = Stream precedence not being accepted
                 17 = Insufficient network resources
                 18 = Requested bandwidth too large
                 21 = Max. messages per interval too small
                 22 = Reply lost in network
                 23 = Illegal precedence value
                 24 = Invalid destination address in list
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-5]   Reserved.  Must be zero.
   S3[6-15]  Stream ID.  This field contains a stream ID assigned by
             the network.  It must be included in all stream data
             messages sent by the host to allow the WPS to associate
             the message with stored stream characteristics and the
             resources reserved for that stream's traffic.
   S4[0-5]   Reserved.  Must be zero.
   S4[6-15]  Address list length.  The number of entries in the
             Address List field.
   S5-SN     Address list.  This contains the destination addresses
             from the Create Stream Request that were invalid or
             unreachable.  Unreachable destinations are listed as a
             group if every member of the group was unreachable, or
             individually otherwise; i.e., group addresses are
             expanded and the unreachable members are included in
             the list.  The list of unreachable destinations will be
             truncated, if needed, to limit this Reply to a single,
             maximum length HAP message.

Edmond [Page 26] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           7           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |        0        |         STREAM ID           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |  MAX MES  | PRE | INT | RLY |      RLEN       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |            DATA WORDS PER INTERVAL            |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                   INTERVAL                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S7        |           0           |  ADDRESS LIST LENGTH  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
   S8-SN     :            DESTINATION ADDRESS LIST           :
             |                                               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         CHANGE STREAM REQUEST
                               Figure 10
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 7 (Change Stream).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-5]   Reserved.  Must be zero.
   S3[6-15]  Stream ID.
   S4[0-3]   New Maximum Messages Per Interval.
   S4[4-5]   New Precedence.
   S4[6-7]   New Interval selection.
   S4[8-9]   New Reliability.

Edmond [Page 27] RFC 1221 HAP2 April 1991

   S4[10-15] New Reliability Length.
   S5[0-15]  New Data Words Per Interval.
   S6[0-15]  New Interval (ignored unless INT = 3).
   S7[0-7]   Reserved.  Must be zero.
   S7[8-15]  Destination Address List length.  This field specifies
             the number of entries in the new Destination Address
             List.  Allowed values are 0-8.  Use zero (indicating no
             addresses in the list) to avoid changing the list of
             recipient hosts.
   S8-SN     New Destination Address List.  The new, complete, list
             of recipient hosts.  Membership of group addresses is
             evaluated at setup execution time.  Subsequent changes
             in group membership do not cause the stream to be
             modified.  Note that using the same destination address
             list in the Change Stream Request as was used in the
             Create Stream Request can result in a change in the
             list of recipient hosts if membership in a group has
             changed.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |        0        |     ADDRESS LIST LENGTH     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
   S4-SN     :                 ADDRESS LIST                  :
             |                                               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          CHANGE STREAM REPLY
                               Figure 11
   S0[0-7]   Setup Type = 2 (Reply).

Edmond [Page 28] RFC 1221 HAP2 April 1991

   S0[8-15]  Reply Code.  The number in parentheses indicates the
             processing phase at the time of the error (see Caution
             below).  Phase zero and phase one errors leave the
             stream unchanged; errors from later phases may leave
             the stream partially modified.
                  4 = Stream changed
                  8 = (1) Network trouble
                 10 = (0) Stream ID nonexistent
                 11 = (0) Not creator of stream
                 12 = (0) Stream precedence not being accepted
                 16 = (3) Unable to add all the new recipients
                 17 = (2) Insufficient network resources
                 18 = (2) Requested bandwidth too large
                 21 = (0) Maximum messages per interval too small
                 22 = (2) Reply lost in network
                 23 = (0) Illegal precedence value
                 24 = (0) Invalid destination address in list
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-5]   Reserved.  Must be zero.
   S3[6-15]  Address list length.  This field specifies the number
             of addresses in the Address List.
   S4-SN     Address list.  This contains the destination addresses
             from the Change Stream Request that were invalid (phase
             0 errors) or unreachable (phase 3 errors).  Unreachable
             destinations are listed as a group if every member of
             the group was unreachable, or individually otherwise;
             i.e., group addresses are expanded and the unreachable
             members are included in the list.  The list of
             unreachable destinations will be truncated, if needed,
             to limit this Reply to a single, maximum length HAP
             message.
   Caution: The Change Stream Reply will indicate failure if any
   aspect of the requested changes did not occur.  However, the
   stream may have been partially modified.  Processing is performed
   in the following phases:
       0: check for invalid requests;
       1: drop former recipients that are not in the latest list;
       2: increase or decrease the stream's bandwidth allocation
           (decreases are normally successful); then
       3: extend the stream to any new recipients.

Edmond [Page 29] RFC 1221 HAP2 April 1991

   If phase 2 fails, phase 3 is not performed, the Reply Code will
   indicate an error and the stream parameters will be unchanged.
   If phase 3 fails, the Address List will contain the destinations,
   if any, from the latest list that the stream does not reach.
   Phase 1 only fails if the stream has been suspended (see
   Notifications) or the WPS is experiencing network connectivity
   problems.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           6           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |        0        |         STREAM ID           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         DELETE STREAM REQUEST
                               Figure 12
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 6 (Delete Stream).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-5]   Reserved.  Must be zero.
   S3[6-15]  Stream ID.

Edmond [Page 30] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          DELETE STREAM REPLY
                               Figure 13
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.  If the request was valid, the Service
             Agent will have marked the stream for deletion even if
             the stream resources have not actually been deleted
             yet.
                  1 = Stream deleted
                 10 = Stream ID nonexistent
                 11 = Not creator of stream
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           9           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                      DELETE ALL STREAMS REQUEST
                               Figure 14
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 9 (Delete All Streams).

Edmond [Page 31] RFC 1221 HAP2 April 1991

   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                       DELETE ALL STREAMS REPLY
                               Figure 15
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.  The Service Agent will have marked all of
             the host's streams for deletion, even if the stream
             resources have not actually been deleted yet.
                  1 = Streams deleted
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.

6.2. Group Setup Messages

 Group (multicast) addressing allows a host to send the same message
 to N different hosts without having to send N copies of the message.
 The network duplicates the message as required.  In addition to
 reducing the burden on the originating host, multicasting reduces the
 load on the network because the network no longer has to carry the
 duplicates along the common portions of the paths between the source
 and destinations.  Multicasting is particularly recommended for
 multi-site conferencing and distributed simulations.
 Group addresses are dynamically created and deleted via setup
 messages exchanged between the hosts and the Service Agent.
 Membership in a group may be any arbitrary subset of the network
 hosts.  A datagram message or stream message addressed to a group is
 delivered to all hosts that are members of that group (exception:
 stream messages sent to a group address that includes hosts the

Edmond [Page 32] RFC 1221 HAP2 April 1991

 stream was not set up to reach).  The group setup messages, each of
 which has a Request and a Reply, are Create Group, Delete Group, Join
 Group, Leave Group, Add Group Member, and Remove Group Member.
 Figure 16 shows a typical use of group setup messages.  The figure
 illustrates a scenario of exchanges between three hosts and the
 Service Agent.  In the scenario one host, Host A, creates a group
 which is joined by hosts B and C.  The hosts then exchange some data
 messages using the group address.  Note that multicast messages are
 not returned to their originator.  Hosts A and C then leave the
 group, and Host B decides to delete the group.  As in the scenario in
 Section 6.1, A/R indications have been omitted for clarity.
 Part of the group creation procedure involves the Service Agent
 returning to the creating host a 48-bit key along with the 16-bit
 group address.  The creating host must pass the key along with the
 group address to other hosts that want to join the group.  These
 other hosts must supply the key along with the group address in their
 Join Group Requests.  The key is used by the network to authenticate
 these operations and thereby minimize the probability that unwanted
 hosts will deliberately or inadvertently become members of the group.
 The procedure used by a host to distribute the group address and key
 is not within the scope of HAP.
 In the figure below, the network Service Agent is pictured as a
 single entity for simplicity.

Edmond [Page 33] RFC 1221 HAP2 April 1991

                                 Service   Host  Host  Host
                                  Agent     A     B     C
      Create Group Request         |<-------|
      Create Group Reply           |------->|
      Reply Acknowledgment         |<-------|
         :   :
      Distribute Group Adr & Key            |---->|
      Distribute Group Adr & Key            |---------->|
         :   :
      Join Group Request (C)       |<-------------------|
      Join Group Reply             |------------------->|
      Reply Acknowledgment         |<-------------------|
      Join Group Request (B)       |<-------------|
      Join Group Reply             |------------->|
      Reply Acknowledgment         |<-------------|
         :   :
      Data Message 1 (A to B and C)         |---->|---->|
      Data Message 2 (B to A and C)         |<----|---->|
      Data Message 3 (C to A and B)         |<----|<----|
         :   :
      Leave Group Request (C)      |<-------------------|
      Leave Group Reply            |------------------->|
      Reply Acknowledgment         |<-------------------|
      Leave Group Request (A)      |<-------|
      Leave Group Reply            |------->|
      Reply Acknowledgment         |<-------|
      Delete Group Request         |<-------------|
      Delete Group Reply           |------------->|
      Reply Acknowledgment         |<-------------|
                             GROUP EXAMPLE
                               Figure 16
 An alternative method of adding and removing group members is the use
 of Add Group Member and Remove Group Member.  These setup requests
 allow hosts that are already members of the group to add or delete
 other hosts.
 The Setup requests Join Group, Leave Group, Add Group Member, Remove
 Group Member, and Delete Group are authenticated using the 48-bit
 key.  Leave Group and Remove Group Member will remove a host from the
 group membership list but will not alter the existence of the group.
 Delete Group expunges all knowledge of the group from the network.
 HAP permits any host with the proper key to delete the group at any
 time.  Thus, group addresses can be deleted even if the host which
 originally created the group has left the group or has crashed.
 Moreover, groups may exist for which there are currently no members

Edmond [Page 34] RFC 1221 HAP2 April 1991

 because each member has executed a Leave while none has executed a
 Delete.  It is the responsibility of the hosts to coordinate and
 manage the use of group addresses.
 Note that group addresses, like all other resources allocated by the
 network, may be reclaimed by the network if unused for too long.
 Currently, if no traffic is sent to the group address in a 6 minute
 interval, the network may delete the group and notify all members
 that the group no longer exists.
 The Create Group Request (Figure 17) is used to establish a multicast
 address.  After the network has processed the Create Group Request,
 the Service Agent will respond by sending a Create Group Reply
 (Figure 18) to the host.
 A host may become a member of a group, once it knows the group
 address and the 48-bit key, by sending the Service Agent the Join
 Group Request message (Figure 19).  The Service Agent will respond to
 the Join Group Request with a Join Group Reply (Figure 20).  The host
 which creates a group automatically becomes a member of that group
 without any need for an explicit Join Group Request.
 A member host may add another host to the group by sending the
 Service Agent the Add Group Member Request message (Figure 21).  The
 Service Agent will respond with an Add Group Member Reply (Figure
 22).
 At any time after becoming a member of a group, a host may choose to
 drop out of the group.  To do this, the host sends the Service Agent
 a Leave Group Request (Figure 23).  The Service Agent will respond
 with a Leave Group Reply (Figure 24).
 One member host may expel another member of the group by sending the
 Service Agent the Remove Group Member Request message (Figure 25).
 The Service Agent will respond with a Remove Group Member Reply
 (Figure 26).
 A host can delete an existing group via a Delete Group Request
 (Figure 27).  The Service Agent will respond with a Delete Group
 Reply (Figure 28).  The Service Agent will also send the other
 members of the group, if any, a notification that the group has been
 deleted (see Section 6.3).

Edmond [Page 35] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           1           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         CREATE GROUP REQUEST
                               Figure 17
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 1 (Create Group).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                 GROUP ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          CREATE GROUP REPLY
                               Figure 18
   S0[0-7]   Setup Type = 2 (Reply).

Edmond [Page 36] RFC 1221 HAP2 April 1991

   S0[8-15]  Reply Code.
                  0 = Group created
                  8 = Network trouble
                 17 = Insufficient network resources
                 22 = Reply lost in network
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-15]  Group Address.  This field contains the 16-bit
             multicast address that any group member may use to
             reach the other group members.  Multicast addresses are
             dynamically assigned by the network.
   S4-S6     Key.  This field contains a 48-bit key assigned by the
             network which is associated with the group address.  It
             must be provided for subsequent Join Group, Leave
             Group, Add Group Member, Remove Group Member, and
             Delete Group requests which reference the group
             address.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           3           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                 GROUP ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S7        |                     0                   | MGP |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          JOIN GROUP REQUEST
                               Figure 19

Edmond [Page 37] RFC 1221 HAP2 April 1991

   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 3 (Join Group).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-15]  Group Address.  This is the group that the host wishes
             to join.  Upon successfully joining the group, the host
             may send messages to the group and will receive
             messages sent to the group when those messages have a
             priority of MGP or higher.
   S4-S6     Key.  This is the key associated with the group
             address.
   S7[0-13]  Reserved.  Must be zero.
   S7[14-15] Minimum group message priority.  The host will not
             receive messages sent to the group that have a message
             priority less than MGP.  Send another Join Group
             Request message to change the minimum priority.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   REQUEST ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                           JOIN GROUP REPLY
                               Figure 20
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.
                  2 = Host added to group
                  9 = Bad key
                 10 = Group address nonexistent
                 17 = Insufficient network resources

Edmond [Page 38] RFC 1221 HAP2 April 1991

   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           10          |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   REQUEST ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                 GROUP ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S7        |                  HOST ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                       ADD GROUP MEMBER REQUEST
                               Figure 21
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 3 (Join Group).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-15]  Group Address.  This is the group the host will join.
             Upon successfully joining the group, the host may send
             messages to the group and will receive messages sent to
             the group by other hosts (the initial minimum priority
             will be 0).
   S4-S6     Key.  This is the key associated with the group
             address.
   S7[0-15]  Host address.  The network address of the host to add

Edmond [Page 39] RFC 1221 HAP2 April 1991

             to the group.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                        ADD GROUP MEMBER REPLY
                               Figure 22
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.
                  2 = Host added to group (or was already a member)
                  9 = Bad key
                 10 = Group address nonexistent
                 11 = Requestor is not a member of the group
                 17 = Insufficient network resources
                 22 = Reply lost in network
                 24 = Host address was invalid
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.

Edmond [Page 40] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           4           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   REQUEST ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                 GROUP ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          LEAVE GROUP REQUEST
                               Figure 23
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 4 (Leave Group).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-15]  Group Address.  This is the group that the host wishes
             to cease being a member of.  After leaving the group,
             the host will cease receiving messages sent to the
             group and will be unable to send to the group.
   S4-S6     Key.  This is the key associated with the group
             address.

Edmond [Page 41] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2            |     REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                           LEAVE GROUP REPLY
                               Figure 24
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.
                  3 = Host deleted from group
                  9 = Bad key
                 10 = Invalid group address
                 11 = Not member of group
                 17 = Insufficient network resources
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.

Edmond [Page 42] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           11          |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   REQUEST ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                 GROUP ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S7        |                  HOST ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                      REMOVE GROUP MEMBER REQUEST
                               Figure 25
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 4 (Leave Group).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-15]  Group Address.  This is the group from which the host
             should be removed.  After leaving the group, that host
             will cease receiving messages sent to the group and
             will be unable to send to the group.
   S4-S6     Key.  This is the key associated with the group
             address.
   S7[0-15]  Host address.  The network address of the host to
             remove from the group.

Edmond [Page 43] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2            |     REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                       REMOVE GROUP MEMBER REPLY
                               Figure 26
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.
                  3 = Host deleted from group (or was not a member)
                  9 = Bad key
                 10 = Invalid group address
                 11 = Requestor is not a member of the group
                 17 = Insufficient network resources
                 22 = Reply lost in network
                 24 = Host address was invalid
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.

Edmond [Page 44] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           1           |           2           |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   REQUEST ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                 GROUP ADDRESS                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S4        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S5        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S6        |                      KEY                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         DELETE GROUP REQUEST
                               Figure 27
   S0[0-7]   Setup Type = 1 (Request).
   S0[8-15]  Request Type = 2 (Delete Group).
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.
   S3[0-15]  Group Address.  This is the multicast address to
             delete.  If the group is deleted, the other remaining
             members of the group, if any, will be notified of the
             group's deletion.
   S4-S6     Key.

Edmond [Page 45] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           2           |      REPLY CODE       |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                 SETUP CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                  REQUEST ID                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          DELETE GROUP REPLY
                               Figure 28
   S0[0-7]   Setup Type = 2 (Reply).
   S0[8-15]  Reply Code.
                  1 = Group deleted
                  8 = Network trouble
                  9 = Bad key
                 10 = Invalid group address
                 17 = Insufficient network resources
                 22 = Reply lost in network
   S1[0-15]  Setup Checksum.  (See setup header description.)
   S2[0-15]  Request ID.

6.3. Notifications

 Notifications are Setup exchanges initiated by the WPS to inform a
 host of changes in the status of a network resource.  The format of
 Notification messages is shown in Figure 29.

Edmond [Page 46] RFC 1221 HAP2 April 1991

               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           3           |          CODE         |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                    CHECKSUM                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                 NOTIFICATION ID               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |                NOTIFICATION INFO              |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         NOTIFICATION MESSAGE
                               Figure 29
   S0[0-7]   Message Type = 3 (Notification).
   S0[8-15]  Code.  This indicates what the Notification signifies.
                  0 = Stream suspended
                  1 = Stream resumed
                  2 = Stream deleted
                  3 = Group deleted by a host
                  4 = Group deleted by network
                  5 = All streams deleted
                  6 = All groups deleted
                  7 = Group changed by a host
                  8 = Group changed by network
   S1[0-15]  Checksum.  (See Service Agent Header description.)
   S2[0-15]  Notification ID.
   S3[0-15]  Notification Information.
             For notification types 0, 1, and 2, NOTIFICATION INFO
             contains the following:
                 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             S3  |        0        |         stream ID           |
                 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             For notification types 3, 4, 7, and 8, NOTIFICATION
             INFO contains the following:

Edmond [Page 47] RFC 1221 HAP2 April 1991

                 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             S3  |                  group address                |
                 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             For notification types 5 and 6, which refer to all
             streams or groups, NOTIFICATION INFO is zero.

6.4. Setup Acknowledgments

 The host must acknowledge receipt of Setup Replies and Notifications
 from the Service Agent, as described earlier.  The format for the
 Setup Acknowledgment message is shown in Figure 30.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           0           |           CODE        |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                    CHECKSUM                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   MESSAGE ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                         SETUP ACKNOWLEDGMENT
                               Figure 30
   S0[0-7]   Message Type = 0 (Acknowledgment).
   S0[8-15]  Code.  This field indicates the type of acknowledgment.
                 0 = Reply acknowledgment
                 1 = Notification acknowledgment
   S1[0-15]  Checksum.  (See Service Agent Header description.)
   S2[0-15]  Message ID.  This is either a Request ID or a
             Notification ID.

6.5. Information Request / Reply Messages

 The host may obtain information about WPS state and about what
 resources the WPS currently has allocated for the host by sending an
 Information Request message to the Service Agent.  The Information
 Reply that is returned will enable the host to determine 1) what

Edmond [Page 48] RFC 1221 HAP2 April 1991

 resources the WPS has allocated to the host, and 2) the current state
 of the network and, possibly, certain network parameters.  This
 allows the host to refrain from trying to use resources it no longer
 has, and to regain information it may have lost on its network
 resources.  This communication also informs the host of the network
 state so that it may make priority and routing decisions.
 Each Information Request (Figure 31) and Information Reply (Figure
 32) message deals with a single type of resource at a time.  The
 header of the Information Reply message contains the number of
 entries within the message, the number of 16-bit words in each entry,
 and an instance of the appropriate information structure for each
 resource the Information Reply message describes.  These information
 structures are described in Figures 33 and 34.
 Future versions of the HAP protocol may permit queries about network
 connectivity, estimated delay to a specified destination address
 under specified conditions, etc.  This is a section of the protocol
 that is likely to expand in the future.  Extensions are expected to
 be backward compatible provided implementors do not hard code the
 size of the returned information entries.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           4           |           CODE        |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                    CHECKSUM                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   MESSAGE ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                      INFORMATION REQUEST MESSAGE
                               Figure 31
   S0[0-7]   Message type = 4 (Information Request).
   S0[8-15]  Code.  This field identifies the Information Request
             Type.
                  1 = streams owned by host
                  2 = groups to which the host belongs
   S1[0-15]  Checksum.  (See Service Agent Header description.)

Edmond [Page 49] RFC 1221 HAP2 April 1991

   S2[0-15]  Message ID.  This field is assigned by the host to
             uniquely identify outstanding requests (Request ID).
             This ID is copied into Information Replies by the
             Service Agent.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S0        |           5           |          CODE         |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S1        |                    CHECKSUM                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S2        |                   MESSAGE ID                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   S3        |   NUMBER OF ENTRIES   |    WORDS PER ENTRY    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
   S4-SN     :              ENTRIES (0 or more)              :
             |                                               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                       INFORMATION REPLY MESSAGE
                               Figure 32
   S0[0-7]   Message type = 5 (Information Reply).
   S0[8-15]  Code.  This field identifies the Information Reply
             Type.
                  1 = streams owned by host
                  2 = groups to which the host belongs
                  3 = error in Information Request message
                  4 = network trouble
                  5 = access not allowed
   S1[0-15]  Checksum.  (See Service Agent Header description.)
   S2[0-15]  Message ID.  This field is assigned by the host in the
             Information Request message to uniquely identify
             outstanding requests.  This ID is copied into the
             Information Reply message by the Service Agent.
   S3[0-7]   Number of entries included in the Information Reply
             message.

Edmond [Page 50] RFC 1221 HAP2 April 1991

   S3[8-15]  Number of 16-bit words per entry.
   S4-SN     Zero or more instances of either the stream information
             or group information structure.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         |       0         |          STREAM ID          |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |          STREAM TYPE OF SERVICE WORD          |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   2         |        STREAM SIZE (bits per interval)        |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   3         |    STREAM INTERVAL (in units of 0.125 ms.)    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          STREAM INFORMATION
                               Figure 33
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         |                  GROUP ADDRESS                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                    0                    | MGP |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                           GROUP INFORMATION
                               Figure 34

7. Host Access Link Monitoring

 While the access link is operating, statistics on traffic load and
 error rate are maintained by the host and WPS.  Once a second, the
 host and WPS exchange this information via Status messages (Figure
 35).  This periodic exchange of Status messages permits both ends of
 the link to monitor flows in both directions.  The WPS also reports
 these monitoring statistics to the Network Operations Center (NOC).
 If either host or WPS fails to receive Status messages for ten
 seconds, the link will be restarted (see Section 8).

Edmond [Page 51] RFC 1221 HAP2 April 1991

 The link restart procedure initializes all internal WPS counts and
 statistics for that link to zero.  As data and control messages are
 processed, counts are updated to reflect the total number of messages
 sent, messages received correctly, and messages received with
 different classes of errors since the last link restart.  Whenever a
 Status message arrives, a snapshot is taken of the local WPS counts.
 The local receive counts, in conjunction with a sent count contained
 in the received Status message, permits the computation of traffic
 statistics in the one second update interval assuming that the set of
 counts at the time of the previous monitoring report have been saved.
 By including in the Status message sent (in the opposite direction)
 the receive counts and the received sent count that was used with
 them, the transmitting end of the access link as well as the
 receiving end can determine the link performance from sender to
 receiver.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         | 1|LB|GOPRI|           0           |     0     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                HEADER CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   2         |             MOST RECENT A/R SENT              |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   3         |                STREAM CAPACITY                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   4         |                   TIMESTAMP                   |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   5         |                      SBU                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   6         |                      STU                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   7         |                      RNE                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   8         |                      RWE                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   9         |                      BHC                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   10        |                      HEI                      |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                            STATUS MESSAGE
                               Figure 35

Edmond [Page 52] RFC 1221 HAP2 April 1991

   0[0]      Message Class = 1 (Control Message).
   0[1]      Loopback indicator.
   0[2-3]    Go-Priority.
   0[4-11]   Reserved.  Must be zero.
   0[12-15]  Control Message Type = 0 (Status).
   1[0-15]   Header Checksum.  The checksum is the 2's-complement of
             the 2's-complement sum of words 0-10 (excluding the
             checksum word itself).
   2[0-15]   Most Recent A/R Sent.  This field is a duplicate of the
             most recent acceptance/refusal word.  It is included in
             the periodic Status message in case previous
             transmissions containing A/R information were lost.
   3[0-15]   Stream Capacity.  When sent by the WPS, this field
             indicates how much stream capacity is unused, in units
             of data bits per millisecond.  There is no guarantee
             that a request for a stream of this size will succeed.
             Since available capacity depends directly on a variety
             of parameters that can be selected by the user, the
             value of this field is the maximum capacity that could
             be achieved if existing streams were expanded at low
             reliability.  This field is not meaningful in messages
             sent from the host to the WPS and must be set to zero.
   4[0-15]   Timestamp.  This field indicates the time that the
             Status message was generated.  When sent by a WPS, the
             time is in units of seconds since the last link
             restart.  The host should also timestamp its messages
             in units of seconds.
   5[0-15]   Sent By Us.  Count of messages sent by us since the
             last link restart (not including this one).
   6[0-15]   Sent To Us.  Count of messages sent to us since the
             last link restart.  This is the count from word 5 of
             the last Status message received.
   7[0-15]   Received, No Errors.  This is the count of messages
             received without errors (since the last link restart)
             at the time that the last Status message was received.
   8[0-15]   Received With Errors.  This is the count of messages

Edmond [Page 53] RFC 1221 HAP2 April 1991

             received with errors (since the last link restart) at
             the time the last Status message was received.
   9[0-15]   Bad Header Checksums.  This is the count of messages
             received with bad header checksums (since the last link
             restart) at the time the last Status message was
             received.
   10[0-15]  Hardware Error Indication.  This is the count of
             messages received with hardware CRC errors or hardware
             interface error indications (since the last link
             restart) at the time the last Status message was
             received.

8. Initialization

 The Host Access Protocol uses a number of state variables that must
 be initialized in order to function properly.  These variables are
 associated with the send and receive message numbers used by the
 acceptance/refusal mechanism and the statistics maintained to support
 link monitoring.  Link initialization should be carried out when a
 machine is initially powered up, when it does a system restart, when
 the ON state (see below) times out, when a loopback condition times
 out (see Section 9), or whenever the link transitions from non-
 operational to operational status.
 Initialization is accomplished by the exchange of Restart Request
 (RR) and Restart Complete (RC) messages between a host and a WPS.
 Either end (or both ends) may send an initial RR, and both ends must
 have sent and received an RC message in order to declare the link up.
 Because the RC message is a reply (to an RR or RC), receipt of an RC
 message by both ends guarantees that the physical link is operating
 in both directions.  The initialization state diagram that must be
 implemented by both WPS and host is shown in Figure 36.  Five states
 are identified in the state diagram:
   OFF       Entered upon recognition of a requirement to restart.
             The interface in the Host or WPS can recognize this
             requirement itself or be forced to restart by receipt
             of an RR message from the other end while in the ON
             state.
   INIT      Local state variables have been initialized but no RC
             messages have yet been sent or received.  If receipt of
             an RR initiated the restart, or if an RR has been
             received since this restart began, send an RC
             (optional, reduces startup time).  Otherwise, send an
             RR to alert the other end of the restart.

Edmond [Page 54] RFC 1221 HAP2 April 1991

   RR-SNT    A request to reinitialize (RR) has been sent to the
             other end, but no RR or RC messages have been received.
   RC-SNT    An RC has been sent to the other end in response to an
             RR.  The interface is waiting to receive an RC.
   ON        RC messages have been both sent and received.  Local
             counters have been zeroed.  Data and control messages
             can now be exchanged between the WPS and host.
 All states have 10-second timeouts (not illustrated) which return the
 protocol to the OFF state.  The occurrence of any events other than
 those indicated in the diagram are ignored.

Edmond [Page 55] RFC 1221 HAP2 April 1991

                            .-----.
       Any Timeout or ----->| OFF |<----------------------------+
       Device Down          `--+--'                             |
                               |                                |
                               | (When I/O Device Up)           |
                               V                                |
                           .-------.                            |
                           | INIT  |                            |
                           `---+---'                            |
                               |                                |
                 (Yes)         V            (No)                |
                +---------RR Received?----------+               |
                |                               |               |
                |                            Send RR            |
                |                               |               |
                |                               V               |
                |                           .--------.          |
             Send RC <-----+-------<--------+ RR-SNT |          |
                |          |       (Rcv RR) `---+----'          |
                |          |                    | (Rcv RC)      |
                V          |                    |               |
           .--------.      |                    |               |
           | RC-SNT +--->--+                 Send RC            |
           `----+---'  (Rcv RR)                 |               |
       (Rcv RC) |                               |               |
                |                               |               |
                +------->------+-------<--------+               |
                               |                                |
                    Initialize Status Counters                  |
                               |                                |
                               V                                |
                            .-----.   Rcv RR   or               |
            Rcv Any  +----->| ON  +---------------------->------+
            Other    |      `--+--'   Fail to Rcv Status message
                     +---------+      for 10 seconds
                    HAP LINK RESTART STATE DIAGRAM
                               Figure 36
 The Restart Request control message (Figure 37) is sent by either a
 host or a WPS when it wishes to restart a link.  The Restart Request
 causes all the monitoring statistics reported in the Status Message
 to be reset to zero and stops all traffic on the link in both
 directions.  The Restart Complete message (Figure 38) is sent in
 response to a received Restart Request or Restart Complete to
 complete link initialization.  The Restart Complete carries a field
 used by the host to enable or disable the acceptance/refusal

Edmond [Page 56] RFC 1221 HAP2 April 1991

 mechanism for the link being restarted (see Section 5).  After the
 Restart Complete is processed, traffic may flow on the link.
 The allocation and state of network resources (streams and groups)
 are separate from the state of the host's access link(s) to the WPS.
 The Information Request message (see Section 6.5) may be used by a
 host to determine what resources it has.  If the "SL" bit is set in
 the Restart Complete message from the WPS, and if the host believes
 it has resources allocated to it, the host is strongly encouraged to
 use an Information Request to verify that it still has its resources.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         | 1|LB|    0   |VERSION |     0     |     3     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                HEADER CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   2         |                 HOST ADDRESS                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   3         |                  LINK NUMBER                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                            RESTART REQUEST
                               Figure 37
   0[0]     Message Type = 1 (Control Message).
   0[1]     Loopback indicator.
   0[2-4]   Reserved.  Must be zero.
   0[5-7]   HAP version number.  Use 1.  Use of zero invokes
            backward compatibility code (see Appendix B).
   0[8-11]  Reserved.  Must be zero.
   0[12-15] Control Message Type = 3 (Restart Request).
   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]  Host Address.  The WPS inserts the primary network
            address of the host.  The host may insert any of its

Edmond [Page 57] RFC 1221 HAP2 April 1991

            network addresses in this field (hosts may have more
            than one logical address per physical port).  The WPS
            will only bring up the HAP link if the host address is
            valid for the port being used.
   3[0-15]  Link Number.  This field contains the sender's
            identification of the physical link being used.  This
            information is used to identify the link when reporting
            errors to the Network Operations Center (NOC).
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         | 1|LB|   0    |VERSION |  0  |SL|AR|     4     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                HEADER CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   2         |                 HOST ADDRESS                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   3         |                  LINK NUMBER                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                           RESTART COMPLETE
                               Figure 38
   0[0]     Message Type = 1 (Control Message).
   0[1]     Loopback indicator.
   0[2-4]   Reserved.  Must be zero.
   0[5-7]   HAP version number.  Use 1.  Use of zero invokes
            backward compatibility code (see Appendix B).
   0[8-9]   Reserved.  Must be zero.
   0[10]    Service loss alert (boolean) (WPS to host only; host
            must send zero).  If the WPS has any reason to believe
            that the resources allocated to the host may not match
            what the host believes is allocated, SL is set to one.
            If SL is one, a host that believes it owns any resources
            is strongly encouraged to use an Information Request to
            verify that the resources are still allocated.  SL will
            be one the first time a link is brought up after a WPS
            is restarted, and may be set in other cases.

Edmond [Page 58] RFC 1221 HAP2 April 1991

   0[11]    Acceptance/Refusal Control.  This bit is used by the
            host to enable or disable the acceptance/refusal
            mechanism for all traffic on the link.
                 0 = Disable acceptance/refusal
                 1 = Enable acceptance/refusal
   0[12-15] Control Message Type = 4 (Restart Complete).
   1[0-15]  Header Checksum.  Covers words 0-3.
   2[0-15]  Host Address.
   3[0-15]  Link Number.

9. Loopback Control

 The Host Access Protocol provides a Loopback Request control message
 which can be used by a WPS or a host to request the remote loopback
 of its HAP messages.  Such requests are usually the result of
 operator intervention for purposes of system fault diagnosis.  For
 clarity in the following discussion, the unit (WPS or host)
 requesting the remote loopback is referred to as the "transmitter"
 and the unit implementing (or rejecting) the loopback is referred to
 as the "receiver".
 When the host access link is remotely looped, all HAP messages will
 be returned, unmodified, over the access link by the receiver.
 (Messages that are too long to be valid HAP messages may be discarded
 instead of being returned.)  The receiver will not send any of its
 own messages to the transmitter while it is implementing the loop.
 WPS-generated messages are distinguished from host-generated messages
 by means of the Loopback indicator that is in every HAP message
 header.
 Two types of remote loopback may be requested: loopback at the
 receiver's interface hardware and loopback at the receiver's I/O
 driver software.  HAP does not specify the manner in which the
 receiver should implement these loops; additionally, some receivers
 may use interface hardware which is incapable of looping the
 transmitter's messages, only allowing the receiver to provide
 software loops.  A receiver may not be able to interpret the
 transmitter's messages as it is looping them back.  If such
 interpretation is possible, however, the receiver will not act on any
 of the transmitter's messages other than requests to reinitialize the
 WPS-host link (Restart Request (RR) control messages; see Section 8.)
 When a receiver initiates a loopback condition in response to a

Edmond [Page 59] RFC 1221 HAP2 April 1991

 loopback request, it makes an implicit promise to maintain the
 condition for the duration specified in the Loopback Request message.
 However, if an unanticipated condition such as a system restart
 occurs in either the transmitter or the receiver, the affected unit
 will try to reinitialize the WPS-host link by sending an RR message
 to the other unit.  If the RR message is recognized by the other
 unit, a link initialization sequence can be completed.  This will
 restore the link to an unlooped condition even if the specified loop
 duration has not yet expired.  If a receiver cannot interpret a
 transmitter's RR messages, and in the absence of operator
 intervention at the receiver, the loop will remain in place for its
 duration.
 HAP does not specify the characteristics of any loopback conditions
 that may be locally implemented by a given unit.  An example of such
 a condition is that obtained when a WPS commands its host interface
 to loop back its own messages.  If such local loop conditions also
 cause the reflection of messages received from the remote unit, the
 remote unit will detect the condition via the HAP header Loopback
 indicator.
 A specific sequence must be followed for setting up a remote
 loopback.  It begins after the HAP link has been initialized and a
 decision is made to request a remote loop.  The transmitter then
 sends a Loopback Request message (Figure 39) to the receiver and
 waits for either (1) a 10-second timer to expire, (2) a "Can't
 implement loop" Unnumbered Response message from the receiver, or (3)
 one of its own reflected messages.  If event (1) or (2) occurs the
 request has failed and the transmitter may, at its option, try again
 with a new Loopback Request message.  If event (3) occurs, the remote
 loopback condition has been established.  While waiting for one of
 these events, messages from the receiver are processed normally.
 Note that RR messages arriving from the receiver during this time
 will terminate the loopback request.
 When a receiver gets a Loopback Request message, it either implements
 the requested loop for the specified duration, or returns a "Can't
 implement loop" response without changing the state of the link.  The
 latter response would be returned, for example, if a receiver is
 incapable of implementing a requested hardware loop.  A receiver
 should initiate reinitialization of the link with an RR message(s)
 whenever a loopback condition times out.
 There is one asymmetry that is required in the above sequence to
 resolve the (unlikely) case where both WPS and host request a remote
 loopback at the same time. If a WPS receives a Loopback Request
 message from a host while it is itself waiting for an event of type
 (1)-(3), it will return a "Can't implement loop" response to the host

Edmond [Page 60] RFC 1221 HAP2 April 1991

 and will continue to wait.  A host in the converse situation,
 however, will abort its loopback request and will instead act on the
 WPS's loopback request.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         | 1|LB|GOPRI|     0     | LOOP TYPE |     8     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                HEADER CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   2         |                 LOOP DURATION                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                           LOOPBACK REQUEST
                               Figure 39
   0[0]      Message Type = 1 (Control Message).
   0[1]      Loopback indicator.
   0[2-3]    Go-Priority.
   0[4-7]    Reserved.  Must be zero.
   0[8-11]   Loop Type.  This field indicates the type of loop that
             is being requested as follows:
                  0 = Undefined
                  1 = Loop at interface (hardware loop)
                  2 = Loop at driver (software loop)
                  3-15 = Undefined
   0[12-15]  Control Message Type = 8 (Loopback Request).
   1[0-15]   Header Checksum.  The checksum is the 2's-complement of
             the 2's-complement sum of words 0-2 (excluding the
             checksum word itself).
   2[0-15]   Loop Duration.  The transmitter of a Loopback Request
             message uses this field to specify the number of
             seconds that the loop is to be maintained by the
             receiver.

Edmond [Page 61] RFC 1221 HAP2 April 1991

10. Other Control Messages

 Before a WPS or a host voluntarily disables a WPS-host link, it
 should send at least one Link Going Down control message (Figure 40)
 over that link.  HAP does not define the action(s) that should be
 taken by a WPS or a host when such a message is received; informing
 the Network Operations Center (NOC) and/or the network users of the
 impending event is a typical course of action.  Note that each Link
 Going Down message only pertains to the WPS-host link that it is sent
 over; if a host and a WPS are connected by multiple links, these
 links may be selectively disabled.
 A No Operation (NOP) control message (Figure 41) may be sent at any
 time by a WPS or a host.  A NOP message contains up to 32 words of
 arbitrary data which are undefined by HAP.  NOP messages may be
 required in some cases to clear the state of the WPS-host link
 hardware.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         | 1|LB|GOPRI|     0     |  REASON   |     7     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                HEADER CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   2         |               TIME UNTIL DOWN                 |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   3         |                DOWN DURATION                  |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                            LINK GOING DOWN
                               Figure 40
   0[0]      Message Type = 1 (Control Message).
   0[1]      Loopback indicator.
   0[2-3]    Go-Priority.
   0[4-7]    Reserved.  Must be zero.
   0[8-11]   Reason.  This field is used by the WPS or the host to
             indicate the reason for disabling this WPS-host link as
             follows:

Edmond [Page 62] RFC 1221 HAP2 April 1991

                  0 = Cancel previous notice, not going down
                  1 = Unspecified reason
                  2 = Scheduled PM
                  3 = Scheduled hardware work
                  4 = Scheduled software work
                  5 = Emergency restart
                  6 = Power outage
                  7 = Software breakpoint
                  8 = Hardware failure
                  9 = Not scheduled up
                 10 = Last warning:  The WPS or host will disable
                      the link in 10 seconds
                 11-15 = Undefined
   0[12-15]  Control Message Type = 7 (Link Going Down).
   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]   Time Until Down.  This field specifies the amount of
             time remaining until the WPS or host disables the link
             (in minutes).  An entry of zero indicates that there is
             less than a minute remaining.
   3[0-15]   Down Duration.  This field specifies the amount of time
             that the WPS-host link will be down (in minutes).  An
             entry of zero indicates that the down duration will be
             less than a minute.  An entry of -1 (all bits set)
             indicates an indefinite down duration.
               0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   0         | 1|LB|       0      |    LENGTH    |     6     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   1         |                HEADER CHECKSUM                |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |                                               |
   2-N       :                ARBITRARY DATA                 :
             |                                               |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                          NO OPERATION (NOP)
                               Figure 41

Edmond [Page 63] RFC 1221 HAP2 April 1991

   0[0]      Message Type = 1 (Control Message).
   0[1]      Loopback indicator.
   0[2-6]    Reserved.  Must be zero.
   0[7-11]   Length.  The number of words of arbitrary data.
   0[12-15]  Control Message Type = 6 (NOP).
   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).
   2-N       Arbitrary Data.  Up to 32 words of data may be sent.
             The data are undefined by HAP.

11. Appendix A – Future Extensions

 The extensions to HAP described below are included to provide
 additional context for the understanding of HAP's current
 capabilities, as well as suggest how HAP may be enhanced in the
 future to provide better support for multi-site conferencing.  These
 capabilities are not supported by TWBNET.
 One change under consideration is the addition of a "conference"
 resource, which would own some number of streams and groups and
 improve the network's ability to meet the needs of video conference
 users.  A single request to modify the "conference", such as to add a
 new member, would result in modifying all the streams in the
 conference to include the new member, modifying the conference's
 primary group address to add the new member, etc., in a single
 network operation.  Such a capability would not only simplify
 conference resource management for hosts, but also reduce the number
 of network setup operations, permit more nearly "atomic" decisions of
 whether a particular conference modification is possible, and reduce
 the problem of recovery if modification is not possible.
 Another change under consideration is the addition of "shared
 streams."  This capability would allow hosts to share a single
 allocation of network bandwidth (and other resources) wherever the
 streams shared a common communication path.  Hosts using a shared
 stream must be willing to restrict their total transmission rate to
 the rate of the shared bandwidth.  Multi-site conferences could use
 such a capability to avoid allocating full bandwidth for voice data
 for all conference members.  Instead, bandwidth for, say, four active
 voices at once could be allocated and shared, and voice messages
 would only be lost when more than four people tried to talk at once.

Edmond [Page 64] RFC 1221 HAP2 April 1991

 The Create Shared Stream Request would use a different request code
 than Create Stream Request, and the setup message would likely
 contain at least one additional field to identify the set of shared
 streams.  Change and Delete Stream requests could be used for both
 shared and non-shared streams.

12. Appendix B – Backward compatibility

 The WPS will support the use of HAP version 0 by hosts until all
 hosts have upgraded to version 1.  The WPS determines which HAP
 version the host is using by examining the Restart Request and/or
 Restart Complete control messages sent by the host to the WPS.  If
 the host initiates a restart and thus sends both a Restart Request
 and a Restart Complete, and if the HAP version numbers in the two
 messages differ, the version number in the Restart Complete will
 prevail.  The WPS will always set the version number to 1.  If the
 host sends 0 in the version number field, version 0 compatiblity mode
 will be invoked.
 Version 0 of HAP did not contain the PROTOCOL ID field in the
 datagram and stream message headers.  Instead, the IL bit in the Type
 of Service word was used to indicate the presence or absence of an
 Internet Protocol (IP) header (any version number) following the HAP
 header.  This is the original description of that bit:
   3[1]   Internet/Local Flag.  This flag is set by a source host to
          specify to a destination host whether the data portion of
          the message contains an Internet Protocol (IP) header [3].
          This field is passed transparently by the source and
          destination WPSen for traffic between network hosts.  This
          field is examined by WPS Agents in order to support
          Internet operation.
               0 = Internet
               1 = Local
 Conversion Algorithms
 Link control messages (e.g., Restart Request) do not require
 conversion.  Datagram and stream messages sent by or to a host
 running HAP version 0 will be converted by the WPS.  Message
 conversion will probably cause the maximum throughput of hosts using
 HAP version 0 to be somewhat lower than that of hosts using HAP
 version 1.
 HAP version 0 used the IL bit in the HAP Type of Service word to
 indicate the presence or absence of an IP header.  Version 1 uses the
 Protocol ID field.  To convert host-to-WPS messages, the IL bit will

Edmond [Page 65] RFC 1221 HAP2 April 1991

 be cleared, and the protocol ID field will be inserted, with the
 value indicated:
      IL was   Destination   Protocol ID set to:
      ------  -------------  ---------------------
        0          any       HAP_PROTO_IP  (0x800)
        1     Service Agent  HAP_PROTO_SETUP (1)
        1         other      HAP_PROTO_NONE  (0)
   To convert WPS-to-host messages, the protocol ID field will be
   deleted, and the IL bit will be set by:
             IL = (protocol_id was HAP_PROTO_IP) ? 0 : 1;
   HAP_PROTO_IP (see Appendix C) will be used for IP "versions" 3
   (GG protocol), 4 (IP), and 5 (ST).
 The datagram message header fields TTL and PRI have been swapped in
 HAP version 0 compared to version 1.  The conversion code swaps the
 contents of these two fields for hosts running version 0.
 The stream message header field TTL in HAP version 0 was replaced by
 the PRE field in version 1.  Since the only permitted value of TTL
 was 1, and it is a valid PRE value, no conversion is necessary.
 In HAP version 0, messages between a host and the Service Agent were
 allowed to contain Internet Protocol headers.  No hosts use that
 capability, so no provision will be made to accommodate IP headers in
 Setups between hosts and the Service Agent.
 In version 0, the Restart Request control message contained a "reason
 for restart" field.  That field was ignored in all current
 implementations and has been eliminated in version 1.
 Current implementations expect the WPS to insert an "incarnation
 count" in bits 5-10 of the first word of both Restart Request and
 Restart Complete messages.  This functionality has been replaced by
 the "SL" bit in the Restart Complete message in version 1.
 Compatibility code will be added if needed, but it is expected that
 none will be needed.

13. Appendix C – HAP Protocol ID Assigned Numbers

 This section lists the values of the PROTOCOL ID field.  This part of
 the specification will be obsolete when a version of the Assigned
 Numbers RFC containing HAP protocol ID numbers is issued.
 HAP adopts the Ether-type numbers in the 1500-65535 range.  Protocol
 IDs 256-511 identify ISO protocols.  Zero indicates the absence of a

Edmond [Page 66] RFC 1221 HAP2 April 1991

 higher level protocol header.  Other protocol IDs are reserved for
 future assignment.
           Protocol ID     Indicates
           -----------     ---------
                0          No higher level protocol
                1          For Network Service Agent messages
              2-255        Reserved
             256-511       ISO protocol identifier + 256
             512-1499      Reserved
            1500-65535     Identical to Ether-type [10].
                        HAP PROTOCOL ID NUMBERS
                               Figure 42

REFERENCES

  1. Falk, G., Groff, S., Koolish, R., and W. Milliken, "PSAT
     Technical Report", BBN Technical Report No. 4469, Chapter 4, May
     1981.
  2. Rees, T., Editor, "A Host Access Protocol Specification", BBN
     Laboratories, Inc., May 1987.  (A revision of RFC 907 that was
     distributed to DARPA and the WBNET user community but not
     resubmitted as an RFC.)
  3. Postel, J., Editor, "Internet Protocol - DARPA Internet Program
     Protocol Specification", RFC 791, USC/Information Sciences
     Institute, September 1981.
  4. Topolcic, C., Editor, "Experimental Internet Stream Protocol,
     Version 2 (ST-II)", RFC 1190, Bolt Beranek and Newman, Inc.,
     October 1990.
  5. Edmond, W., Seo, K., Leib, M., and C. Topolcic, "The DARPA
     Wideband Network Dual Bus Protocol", Proceedings of ACM SIGCOMM
     '90, pages 79-89, September 24-27, 1990.
  6. "Host/SATNET Protocol", Internet Engineering Note (IEN) 192, July
     1981.
  7. Evenchik, L., McNeill, D., Bressler, R., Owen, A., Rice, Jr., R.,
     Trout, G., Pavey, C., Damer, R., Deckelman, F., and T. Hughes,
     "MATNET, An Experimental Navy Shipboard Satellite Communications
     Network", Proceedings of INFOCOM '82, pages 3-11, March 30 -
     April 1, 1982.

Edmond [Page 67] RFC 1221 HAP2 April 1991

  8. Falk, G., Groff, J., Milliken, W., Nodine, M., Blumenthal, S.,
     and W. Edmond, "Integration of Voice and Data in the Wideband
     Packet Satellite Network", IEEE Journal on Selected Areas in
     Communications, Vol. SAC-1, No. 6, December 1983.
  9. "Interface Message Processor: Specifications for the
     Interconnection of a Host and an IMP", BBN Technical Report No.
     1822, October 1980.
 10. Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
     USC/Information Sciences Institute, March 1990.

Security Considerations

 Security issues are not discussed in this memo.

Author's Address

 Winston Edmond
 Bolt Beranek and Newman, Inc.
 Network Technologies Department
 10 Moulton Street
 Cambridge, Massachusetts 02138
 Phone: (617) 873-3000
 EMail: wbe@bbn.com

Edmond [Page 68]

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