RFC 1221
Host Access Protocol (HAP) specification: Version 2
Network Working Group W. Edmond Request for Comments: 1221 BBN Updates: RFC 907 April 1991 Host Access Protocol (HAP) Specification - Version 2 Status of this Memo This memo describes the Host Access Protocol implemented in the Terrestrial Wideband Network (TWBNET). It obsoletes most but not all of RFC 907. This memo provides information for the Internet community. It does not specify an Internet standard. Distribution of this memo is unlimited. Preface This memo specifies the Host Access Protocol (HAP). HAP is a Network layer (OSI Layer 3 lower) access protocol that was first implemented about a decade ago for the DARPA/DCA sponsored Wideband Packet Satellite Network (WBNET), the precursor of the current Terrestrial Wideband Network (TWBNET). This version of the specification obsoletes references [1] and [2] in addition to most of RFC 907. HAP is a developmental protocol, and will be revised as new capabilities are added and unused features are eliminated or revised. One reason that HAP is being revised now is that, unlike the original WBNET's satellite channel, the TWBNET's T1 fiber links are not a broadcast medium. This has prompted some changes to the protocol that will permit greater efficiency in a mesh topology network. Another cause of revision is the need to make HAP able to support a variety of OSI layer 3 upper protocols, such as DECNET Phase V, ST, and CLNP, where before only Internet Protocol (IP) was used. Appendix B describes how backward compatibility with the older IP- only version of HAP is achieved. A third cause of protocol changes is the desire to simplify interaction between ST2 protocol (RFC 1190) agents and the TWBNET. This has mainly affected the way certain setup errors are handled. These changes are expected to be backward compatible. Appendix A describes two capabilities that may be added to HAP in the future. One of the protocol enhancements, "Group Streams", described in reference [2] has been eliminated. There are no known applications that use the feature. As described in Appendix A, a new mechanism, to be called "shared streams", capable of providing equivalent capabilities will be implemented if needed. Changes in [2] that have been retained include various query/reply control messages that permit a host to determine what resources it owns (mostly useful for
cleanup following a host reboot or crash). This document assumes the reader is familiar with DoD internetworking terminology. 1. Introduction The Host Access Protocol (HAP) is a network layer protocol (as is X.25). ("Network layer" here means ISO layer 3 lower, the protocol layer below the DoD Internet Protocol (IP) layer [3] and above any link layer protocol.) HAP defines the different types of host-to- network control messages and host-to-host data messages that may be exchanged over the access link connecting a host and the network packet switch node. The protocol establishes formats for these messages, and describes procedures for determining when each type of message should be transmitted and what it means when one is received. HAP has been implemented in the wide-area network called the Terrestrial Wideband Network (TWBNET) [5] and in the routers and other hosts that connect to TWBNET. The packet switch nodes that compose the TWBNET are called Wideband Packet Switches (WPS). Both the precursor to HAP, the Host/SATNET Protocol [6], used in the Atlantic Packet Satellite Network (SATNET) and the Mobile Access Terminal Network (MATNET [7]), and HAP, used in the original Wideband Satellite Network (WBNET) [8], were originally designed to provide efficient access to the single satellite channel each network used to connect all sites. The HAP protocol designers reflected some of the peculiarities of the single satellite channel environment in the HAP protocol itself. The current Terrestrial Wideband Network (TWBNET) utilizes T1-speed fiber connections between sites. Future networks and TWBNET may use a combination of terrestrial connections and satellite connections, and may have more than one of each. The HAP protocol has been changed to accommodate these extensions. Section 2 presents an overview of HAP. Details of HAP formats and message exchange procedures are contained in Sections 3 through 10. Further explanation of some of the topics addressed in this HAP specification can be found in reference [1]. Any protocol employed to provide sufficiently reliable message exchange over the Host-WPS link is assumed to be transparent to the protocol defined in this document. Examples of such link-level protocols are ARPANET 1822 local and distant host [9], ARPANET VDH protocol [9], and HDLC.
2. Overview HAP can be characterized as a full duplex, nonreliable protocol with an optional flow control mechanism. HAP messages flow simultaneously in both directions between the WPS and the host. Transmission is nonreliable in the sense that the protocol does not provide any guarantee of error-free sequenced delivery. If error-free delivery on the host's access link is required, it must be provided by the link layer protocol below HAP. (Use of link layer protocols for this purpose is not within the scope of this document.) HAP's flow control mechanism operates independently in each direction, but the choice to enable flow control or not applies to both directions together. HAP supports host-to-host communication in two modes corresponding to the two types of HAP data messages, datagram messages and stream messages. Each type of message can be up to 2048 octets in length. The basic transmission service in the network is datagram service. Datagrams are variable length, unsequenced, independent, and delivery is not guaranteed. The HAP header of each datagram determines the processing of the message. On this datagram service base a "stream" service is built. Stream service provides network bandwidth guarantees, but requires explicit setup and teardown operations to allocate and deallocate network resources. Stream traffic is best suited for continuous media traffic, but may also be used to obtain the lowest possible network delay. Host streams are established by a setup message exchange between the host and the network prior to the commencement of data flow. Although established host streams can have their characteristics modified by subsequent setup messages while they are in use, the fixed allocation properties of streams relative to datagrams impose rather strict requirements on the source of the traffic using the stream. Stream traffic arrivals must match the stream allocation both in interarrival time and message size if reasonable efficiency is to be achieved. The characteristics and use of datagrams and streams are described in detail in Sections 3 and 4 of this document. Both datagram and stream transmission in the network use logical addressing. Each host on the network is assigned a permanent 16-bit logical address which is independent of the physical port on the WPS to which it is attached. These 16-bit logical addresses are present in all Host-to-WPS and WPS-to-Host data messages. HAP supports multicast addressing via "groups". Multicast addressing is provided primarily to support the multi-destination delivery required for conferencing applications. Group addresses are
dynamically created and deleted by the use of setup messages exchanged between a host and the WPS. Membership in a group may be any arbitrary subset of the network hosts. A message addressed to a group address is delivered to all hosts that are members of that group, except the sender. Once a multicast address has been created, any member host may use that address, not just the creator. Although HAP does not guarantee error-free delivery, error control is an important aspect of the protocol design. HAP error control is concerned with both local transfers between a host and its local WPS and transfers through the network to the destination(s). The WPS offers users a choice of network error protection options based on the network's ability to selectively send messages over its transmission media at different forward error correction (FEC) rates. These FEC options are referred to as reliability levels. Four reliability levels (low, medium-low, medium-high, and high) are available. The precise error rate provided by each reliability level is not specified. Various checksum and CRC mechanisms are employed in the network to provide an error detection capability. A host has an opportunity when sending a message to indicate whether the message should be delivered to its destination or discarded if a data error is detected by the network. Each message received by a host from the network will have a flag indicating whether or not an error was detected in that particular message. A host can decide on a per-message basis whether or not it wants to accept or discard transmissions containing data errors. For connection of a host and WPS in close proximity, error rates due to external noise or hardware failures on the access circuit may reasonably be expected to be much smaller than the best network trunk circuit error rates. Thus for this case, little is gained by using error detection and retransmission on the access circuit. A 16-bit header checksum is provided, however, to ensure that WPSen do not act on incorrect control information. For relatively long distances or noisy connections, retransmissions over the access circuit may be required to optimize performance for both low and high reliability traffic. It is expected that link layer error control procedures (such as HDLC with retransmission) will be used for this purpose, but use of a reliable link layer protocol is not within the scope of this document. Each datagram message submitted to the WPS by a host is marked as being in one of three priority classes, from priority 2 (highest) through priority 0 (lowest). The priority class is used by the WPS for arbitrating contention for scarce network resources (e.g., link bandwidth). That is, if the network cannot deliver all of the
offered messages, high priority messages will be delivered in preference to low priority messages. Priority level affects the order of access to intersite link bandwidth and the order of message delivery at the destination WPS. Each stream message also has three priority classes, from priority 2 (highest) through priority 0 (lowest). In addition, streams themselves have three precedence classes, from precedence 2 (highest) through precedence 0. A stream of higher precedence can preempt a stream of lower precedence at setup time. Stream message priority provides a mechanism for a low-bandwidth host to receive a high- bandwidth stream and selectively discard messages marked as less important by the sender. Stream message priority does not affect the order of delivery of stream messages between the source and the destination. Datagram and stream messages being presented to the WPS by a host may not be accepted for a number of reasons: priority too low, destination dead, lack of buffers in the source WPS, etc. The host faces a similar situation with respect to handling messages from the WPS. To permit the receiver of a message to inform the sender of the local disposition of its message, an acceptance/refusal (A/R) mechanism is implemented. The mechanism is the external manifestation of the WPS's (or host's) internal flow and congestion control algorithm. If A/Rs are enabled, an explicit or implicit acceptance or refusal for each message is returned to the host by the WPS (and conversely). This allows the host (or WPS) to retry refused messages at its discretion and can provide information useful for optimizing the sending of subsequent messages when the reason for refusals is also provided. The A/R mechanism can be disabled to provide a "pure discard" interface. The host's choice to use the A/R mechanism or not does not limit its ability to send and receive messages to any other hosts. While the A/R mechanism allows control of individual message transfers, it does not facilitate regulation of priority flows. Such regulation is handled by passing advisory status information (GOPRI) across the Host-WPS interface indicating which priorities are currently being accepted. As long as this information, relative to the change in priority status, is passed frequently, the sender can avoid originating messages which are sure to be refused. HAP defines both data messages (datagram messages and stream messages) and link control messages. Data messages are used to send information between hosts on the network. Link control messages are exchanged between a host and the WPS to manage the local access link. Allocation of network resources, such as streams and groups, is
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.
Datagram priority determines which packets are delivered or discarded
when network resources do not permit handling all of the presented
traffic. It is expected that datagram messages will be used to
support the majority of computer-to-computer and terminal-to-computer
traffic which is bursty in nature.
The format of datagram messages and the purpose of each of the header
control fields is described in Figure 1.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0 | 0|LB|GOPRI| 0 | F| MESSAGE NUMBER |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
1 | HEADER CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2 | A/R |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
3 | 0|IL| D| E| PRI | TTL | RLY | RLEN |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4 | DESTINATION HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
5 | SOURCE HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
6 | PROTOCOL ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
7-N : DATA :
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
DATAGRAM MESSAGE
Figure 1
0[0] Message Class. This bit identifies the message as a
data message or a control message.
0 = Data Message
1 = Control Message
0[1] Loopback indicator. This bit allows the sender of a
message to determine if its own messages are being
looped back. The host and the WPS each use different
settings of this bit for their transmissions. If a
message arrives with the loopback bit set equal to its
outgoing value, then the message has been looped.
0 = Sent by Host
1 = Sent by WPS
0[2-3] Go-Priority. In WPS-to-Host messages, this field
provides advisory information concerning the lowest
priority currently being accepted by the WPS. The host
may optionally choose to provide similar priority
information to the WPS.
0 = Low Priority
1 = Medium Priority
2 = High Priority
3 = (Reserved.)
0[4-6] Reserved. Must be zero.
0[7] Reserved. Must be zero. Formerly used for WPS
diagnostic purposes.
0[8-15] Message Number. This field contains the identification
of the message used by the acceptance/refusal (A/R)
mechanism (when enabled). If the message number is
zero, A/R is disabled for this specific message. See
Section 5 for a detailed description of the A/R
mechanism.
1[0-15] Header Checksum. The checksum is the 2's-complement of
the 2's-complement sum of words 0-6 (excluding the
checksum word itself).
2[0-15] Piggybacked A/R. This field may contain an
acceptance/refusal word providing A/R status on traffic
flowing in the opposite direction. Its inclusion may
eliminate the need for a separate A/R control message
(see Section 5). A value of zero for this word is used
to indicate that no piggybacked A/R information is
present.
3[0] Data Message Type. This bit identifies whether the
message is a datagram message or a stream message.
0 = Datagram Message
1 = Stream Message
3[1] IL flag. Obsolete. Must be zero. (See Appendix B.)
3[2] Discard Flag. This flag allows a source host to
instruct the network (including the destination host)
what to do with the message when data errors are
detected (assuming the header checksum is correct).
0 = Discard message if data errors detected.
1 = Don't discard message if data errors detected.
The value of this flag, set by the source host, is
passed on to the destination host.
3[3] Data Error Flag. This flag is used in conjunction with
the Discard Flag to indicate to the destination host
whether any data errors have been detected in the
message prior to transmission over the destination's
WPS-to-Host access link. It is used only if Discard
Flag = 1. It should be set to zero by the source host.
0 = No Data Errors Detected
1 = Data Errors Detected
3[4-5] Priority. The source host uses this field to specify
the priority with which the message should be handled
within the network.
0 = Low Priority
1 = Medium Priority
2 = High Priority
3 = (Reserved.)
The priority of each message is passed to the
destination host by the destination WPS.
3[6-7] Time-to-Live Designator. The source host uses this
field to specify the maximum time that a message should
be allowed to exist within the network before being
deleted. Elapsed time begins when the message has been
received by the WPS from the source host (or is sent by
a WPS agent) and is last checked when the message is
queued for transmission out the I/O interface to the
destination host. If a message is multicast, each copy
is treated separately.
0 = 1 seconds
1 = 2 seconds
2 = 5 seconds
3 = 10 seconds
3[8-9] Reliability. The source host uses this field to
specify the basic bit error rate requirement for the
data portion of this message. The source WPS uses this
field to determine the trunk circuit transmission
parameters and forward error correction level required
to provide that bit error rate.
0 = Low Reliability
1 = Medium-Low Reliability
2 = Medium-High Reliability
3 = High Reliability
3[10-15] Reliability Length. The source host uses this field to
specify a portion of the user data which should be
transmitted at the highest reliability level (lowest
bit error rate). Both the HAP message header words and
the first 2*<Reliability Length> octets of user data
will be transmitted at high reliability while the
remainder of the user data will be transmitted at
whatever reliability level is specified in field 3[8-
9]. The reliability length mechanism gives the user
the ability to transmit private header information
(e.g., IP and TCP headers) at a higher reliability
level than the remainder of the data.
4[0-15] Destination Host Address. This field contains the
network logical address of the destination host.
5[0-15] Source Host Address. This field contains the network
logical address of the source host.
6[0-15] Protocol ID. This field specifies the next higher
level protocol. Protocol identifiers are assigned
administratively, except 0 which is reserved, and are
not part of this specification. See reference [10].
7-N Data. This field contains up to 16,384 bits (2048
octets) of user data, and must be an even number of
octets.
4. Stream Messages
Stream messages are the second message type provided by HAP, as
described in Section 2. Streams provide guaranteed bandwidth between
the source and destination(s), and provide the minimum delivery delay
and delay variance available in the network. Streams are suitable
for volatile traffic, such as speech, and for support of high duty
cycle applications that require throughput guarantees.
Streams must be created before stream messages can flow from host to host. The protocol to accomplish stream creation is described in Section 6.1. Once established, a stream is allocated specific network resources, such as bandwidth. Within the bounds of its stream allocation, a host is permitted considerable flexibility in how it may use the stream. Although the time to live, reliability, and reliability length of each stream message is fixed at stream setup time, the destination logical address can vary from stream message to stream message. A host can, therefore, multiplex a variety of logical flows onto a single stream, as long as the stream was set up to reach all the destination hosts. The format of stream messages is described in Figure 2. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 0 | 0|LB|GOPRI| 0 | MESSAGE NUMBER | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1 | HEADER CHECKSUM | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 2 | A/R | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 3 | 1|IL| D| E| PRI | HOST STREAM ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 4 | DESTINATION HOST ADDRESS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 5 | SOURCE HOST ADDRESS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 6 | PROTOCOL ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | 7-N : DATA : | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ STREAM MESSAGE Figure 2 0[0] Message Class = 0 (Data Message). 0[1] Loopback indicator. 0[2-3] Go-Priority.
0[4-7] Reserved.
0[8-15] Message Number. This field serves the same purpose as
the message number field in the datagram message.
Moreover, a single message number sequence is used for
both datagram and stream messages (see Section 5).
1[0-15] Header Checksum. (See datagram checksum for
description.)
2[0-15] Piggybacked A/R.
3[0] Data Message Type = 1 (Stream).
3[1] IL flag. Obsolete. Must be zero.
3[2] Discard Flag.
3[3] Data Error Flag.
3[4-5] Stream message priority. Note that all stream messages
have priority over any datagram message. Priority will
not affect the order of stream message delivery.
0 = Low priority
1 = Medium priority
2 = High priority
3 = Reserved
3[6-15] Stream ID. The WPS uses this field to identify the
preallocated network resources (bandwidth allocations,
queues, buffers, etc.) to use for delivery of the
message. Streams and their identifying numbers (stream
IDs) are established by an explicit Create Stream
request (see Section 6.1).
4[0-15] Destination Host Address.
5[0-15] Source Host Address.
6[0-15] Protocol ID.
7-N Data. This field contains up to 16,384 bits (2048
octets) of user data, and must be an even number of
octets.
5. Flow Control Messages The WPS supports an acceptance/refusal (A/R) mechanism in each direction on the host access link. The A/R mechanism is enabled for the link by the host by setting a bit in the Restart Complete control message (see Section 8). Each datagram and stream message contains an 8-bit message number used to identify the message for flow control purposes. When the A/R mechanism is enabled, the message number is incremented modulo 256 in successive messages, skipping over message number zero (zero indicates that A/R's are disabled for that message). Up to 127 messages may be outstanding (awaiting acceptance or refusal) in each direction. If the receiver of a message is unable to accept the message, a refusal indication containing the message number of the refused message and the reason for the refusal is returned. The refusal indication may be piggybacked on data messages in the opposite direction over the link or may be sent in a separate control message in the absence of reverse data traffic. Acceptance indications are returned in a similar manner, either piggybacked on data messages or in a separate control message. An acceptance is returned by the receiver to indicate that the identified message was received from the host access link and was not refused. Acceptance indications returned by the WPS are not an end- to-end acknowledgement and do not imply any guarantee of delivery to the destination host(s), or even any assurance that the message will not be intentionally discarded by the network. They are sent primarily to facilitate buffer management in the host. To reduce the number of A/R messages exchanged, a single A/R indication can be returned for multiple (lower numbered) previously unacknowledged messages. Explicit acceptance of message number N implies implicit acceptance of outstanding messages with numbers N-1, N-2, etc., according to the definition of acceptance outlined above. Analogous interpretation of the refusal message number allows the receiver of a group of messages to reject them as a group when they all are being refused for the same reason. As a further efficiency measure, HAP permits aggregation of any mix of A/R indications into a single A/R control message. Such a message might be used, for example, to reject a group of messages where the refusal code on each is different. In some circumstances the overhead associated with processing A/R messages may prove unattractive. For these cases, it is possible to disable the A/R mechanism and operate the HAP interface in a purely discard mode. The ability to effect this on a link basis has already been noted (see Sections 2 and 8). In addition, messages with sequence number zero are taken as messages for which the A/R mechanism is selectively disabled. To permit critical feedback, even
when operating in discard mode, HAP defines an "Unnumbered Response"
control message. Flow control information, and other information
which cannot be sent as an A/R indication, is sent in an Unnumbered
Response control message. The format of this type of message is
illustrated in Figure 5.
The format shown in Figure 3 is used both for A/R indications that
are piggybacked on data messages (word 2), and for aggregated A/R
information in A/R control messages. The format of A/R control
messages is shown in Figure 4.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|AR| REFUSAL CODE | A/R MESSAGE NUMBER |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
ACCEPTANCE/REFUSAL WORD
Figure 3
[0] Acceptance/Refusal Type. This field identifies whether
A/R information is an acceptance or a refusal.
0 = Acceptance
1 = Refusal
[1-7] Refusal Code. When the Acceptance/Refusal Type = 1,
this field gives the Refusal Code.
0 = Priority not being accepted
1 = Source WPS congestion
2 = Destination WPS congestion
3 = Destination host dead
4 = Destination WPS dead
5 = Illegal destination host address
6 = Destination host access not allowed
7 = Illegal source host address
8 = Message lost in access link
9 = Invalid stream ID
10 = Illegal source host for stream ID
11 = Message length too long
12 = Stream message too early
13 = Illegal control message type
14 = Illegal refusal code in A/R
15 = Can't implement loop
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).
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:
3: Destination Host Address
5: Destination Host Address
7: Source Host Address
9: Stream ID (right justified)
10: Stream ID (right justified)
13: Word 0 of offending message
15: Word 0 of Loopback Request message
3[0-15] Response Information. If Response Code is:
3,5,7, or 9: Undefined
10: Source Host Address
13: Word 3 of offending message, or 0 if no word 3
15: Word 2 of Loopback Request message
(page 17 continued on part 2)
