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INFORMATIONAL
Network Working Group                                          W. EdmondRequest for Comments: 1221                                           BBNUpdates: RFC907                                              April 1991Host Access Protocol (HAP) Specification - Version 2Status of this Memo   This memo describes the Host Access Protocol implemented in the   Terrestrial Wideband Network (TWBNET).  It obsoletes most but not all   ofRFC 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 ofRFC 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 inAppendix 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 forEdmond                                                          [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 Sections3 through10.   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 19912. 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 Sections3 and4   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 areEdmond                                                          [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 theEdmond                                                          [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, isEdmond                                                          [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 itsEdmond                                                          [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.  SeeSection 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               (seeSection 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.  (SeeAppendix 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 secondsEdmond                                                          [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 inSection 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 inSection 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 (seeSection 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 (seeSection 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 19915. 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 (seeSection 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 Sections2 and8).  In addition, messages with   sequence number zero are taken as messages for which the A/R   mechanism is selectively disabled.  To permit critical feedback, evenEdmond                                                         [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 loopEdmond                                                         [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 message6. 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 toEdmond                                                         [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) (seeAppendix 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 inSection 6.1.   Group Request and Reply messages are described inSection 6.2.  The   format of Notifications is described inSection 6.3, and Setup   Acknowledgments are described inSection 6.4.  Information Request   and Reply messages are described inSection 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 changedEdmond                                                         [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.  (SeeSection 6.3.)               For Setup Acknowledgments, this field contains the               Acknowledgment Type.  (SeeSection 6.4.)               For Information Requests, this field contains the               request type.  (SeeSection 6.5.)               For Information Replies, this field contains the reply               type.  (SeeSection 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 specifiedEdmond                                                         [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 CreateEdmond                                                         [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 (seeSection 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 ListEdmond                                                         [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 theEdmond                                                         [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 inSection 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 membersEdmond                                                         [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 (seeSection 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 19Edmond                                                         [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 resourcesEdmond                                                         [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 addEdmond                                                         [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) whatEdmond                                                         [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 347. 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 (seeSection 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 35Edmond                                                         [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 messagesEdmond                                                         [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 (seeSection 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/refusalEdmond                                                         [Page 56]

RFC 1221                          HAP2                        April 1991   mechanism for the link being restarted (seeSection 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 (seeSection 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 (seeAppendix 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 itsEdmond                                                         [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 (seeAppendix 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; seeSection 8.)   When a receiver initiates a loopback condition in response to aEdmond                                                         [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 hostEdmond                                                         [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 199110. 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 41Edmond                                                         [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 willEdmond                                                         [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 (seeAppendix 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 aEdmond                                                         [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 42REFERENCES    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 ofRFC 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.comEdmond                                                         [Page 68]

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