host
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... 3] and above any
link layer protocol.) HAP defines the different types of host-to-
network control messages ...
... network control messages and host-to-host data messages that may be
exchanged over the access link ...
... network control messages and host-to-host data messages that may be
exchanged over the access link ...
... data messages that may be
exchanged over the access link connecting a host and the network
packet switch ...
...
Both the precursor to HAP, the Host/SATNET Protocol [6], used in the
Atlantic Packet Satellite Network ...
...
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 ...
... 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 ...
... delivery. If error-free delivery
on the host's access link is required, it must be provided by the
link layer ...
...
HAP supports host-to-host communication in two modes corresponding to
the two types of HAP data messages, datagram ...
...
HAP supports host-to-host communication in two modes corresponding to
the two types of HAP data messages, datagram ...
... 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 ...
... 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 ...
... 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
...
... 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 ...
... group may be
any arbitrary subset of the network hosts. A message addressed to a
group address is delivered to all hosts ...
... hosts. A message addressed to a
group address is delivered to all hosts that are members of that
group, except the sender ...
... multicast address has been created,
any member host may use that address, not just the creator.
...
... 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 ...
... 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 ...
... 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
...
... 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
...
...
For connection of a host and WPS in close proximity, error rates due
to external noise or hardware failures on the access circuit ...
...
Each datagram message submitted to the WPS by a host is marked as
being in one of three priority classes ...
... message priority
provides a mechanism for a low-bandwidth host to receive a high-
bandwidth stream ...
... 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 ...
... 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
...
... 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
...
... 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 ...
... 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.
...
... flows. Such
regulation is handled by passing advisory status information (GOPRI)
across the Host-WPS interface indicating which priorities are
...
... link control messages. Data messages are used to send
information between hosts on the network. Link control messages are
...
... network. Link control messages are
exchanged between a host and the WPS to manage the local access link.
...
... 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
...
... destination field of
that message. The Service Agent also permits a host to inquire about
resources it owns.
...
... TTL | RLY | RLEN |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4 | DESTINATION HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
5 | SOURCE HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... 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
...
... 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 ...
... provides advisory information concerning the lowest
priority currently being accepted by the WPS. The host
may optionally choose to provide similar priority
...
... 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 ...
... host to
instruct the network (including the destination host)
what to do with the message when data errors are
detected (assuming the header checksum ...
... 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.
...
... The value of this flag, set by the source host, is
passed on to the destination host.
3[3] Data Error ...
... 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 ...
... 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 ...
... access link. It is used only if Discard
Flag = 1. It should be set to zero by the source host.
0 = No Data ...
...
3[4-5] Priority. The source host uses this field to specify
the priority with which the message should be handled
...
...
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 ...
... 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.
...
...
3[8-9] Reliability. The source host uses this field to
specify the basic bit error rate requirement ...
...
3[10-15] Reliability Length. The source host uses this field to
specify a portion of the user data which should be
...
... level than the remainder of the data.
4[0-15] Destination Host Address. This field contains the
network ...
... network
logical address of the source host.
6[0-15] Protocol ID. This field specifies the next higher
...
... created before stream messages can flow from host to
host. The protocol to accomplish stream ...
... flow from host to
host. The protocol to accomplish stream creation is described in
Section 6.1. Once established, a stream ...
... 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 ...
... 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.
...
... +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
3 | 1|IL| D| E| PRI | HOST STREAM ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... STREAM ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
4 | DESTINATION HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
5 | SOURCE HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... request (see Section 6.1).
4[0-15] Destination Host Address.
...
...
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 ...
... access link. The A/R mechanism is enabled for
the link by the host by setting a bit in the Restart Complete control
message ...
... 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
...
... destination host address
6 = Destination host access not allowed
7 = Illegal source host address ...
... 6 = Destination host access not allowed
7 = Illegal source host address
8 = Message lost in access link ...
... 14 = Illegal refusal code in A/R
15 = Can't implement loop
16 = Destination host congestion
17 = Delivery ...
...
3,5,7, or 9: Undefined
10: Source Host Address
13: Word 3 of offending message, or 0 if no word 3
...
... accomplished via an exchange of datagram messages, called Setup
messages, between the user host and the Service Agent (network
...
... 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 ...
... 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
...
... 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 ...
... 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
...
... 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 ...
... 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.
...
... Service Agent sends a Notification to
affected hosts, and (2) the hosts return a Setup Acknowledgment to
the Service Agent ...
... Notification to
affected hosts, and (2) the hosts return a Setup Acknowledgment to
the Service Agent. Notifications ...
... the Service Agent. Notifications are used to inform a host of
changes in the status of a network resource. In order to allow more
...
... Notification ID. The Setup Acknowledgment returned by the notified
host to the Service Agent must contain the Notification ID. The host ...
... 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 ...
... 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 ...
... 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
...
... 14 = (Reserved)
15 = (Reserved)
16 = Unable to add all the new hosts
17 = Insufficient network resources
...
...
S2[0-15] Message ID. This field is assigned by the host to
uniquely identify outstanding requests (Request ID) and
by the Service Agent ...
... 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 ...
... 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 ...
... host and the Service Agent shown in Figure 7
where the host establishes a stream, sends some data, modifies the
stream ...
... During the lifetime of a stream, the host which created it may decide
that some of its characteristic properties should be modified. All
...
... Service Agent will respond by sending a Change Stream
Reply (Figure 11) to the host. A host requesting a reduced channel
...
... Stream
Reply (Figure 11) to the host. A host requesting a reduced channel
allocation should decrease its sending rate ...
... 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 ...
...
If the host has crashed or restarted, it may no longer know what
streams it owns. The host may use an Information Request (see
...
... If the host has crashed or restarted, it may no longer know what
streams it owns. The host may use an Information Request (see
Section 6.5) to determine what streams it owns, or the host may use a
...
... streams it owns. The host may use an Information Request (see
Section 6.5) to determine what streams it owns, or the host may use a
Delete All Streams Request (Figure 14) to discard whatever stream ...
... specifies the maximum number of stream messages the
host will deliver to the WPS in any single stream
interval.
...
... 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
...
... 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.
...
... traffic. A
stream message sent to hosts other than those specified via Setup
will probably be undeliverable. A stream message to a group address ...
... 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
...
... SN New Destination Address List. The new, complete, list
of recipient hosts. Membership of group addresses is
evaluated at setup execution time. Subsequent changes
...
... Create Stream Request can result in a change in the
list of recipient hosts if membership in a group has
changed.
...
... 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 ...
... Group (multicast) addressing allows a host to send the same message
to N different hosts without having to send N copies of the message.
...
... 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
...
... The network duplicates the message as required. In addition to
reducing the burden on the originating host, multicasting reduces the
load on the network ...
... created and deleted via setup
messages exchanged between the hosts and the Service Agent.
Membership in a group ...
... group may be any arbitrary subset of the network
hosts. A datagram message or stream message addressed to a group ...
... stream message addressed to a group is
delivered to all hosts that are members of that group (exception:
stream ...
... stream messages sent to a group address that includes hosts the
stream was not set up to reach). The group ...
... 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 ...
... creates a group
which is joined by hosts B and C. The hosts then exchange some data
messages using the group address ...
... group
which is joined by hosts B and C. The hosts then exchange some data
messages using the group address. Note that multicast ...
... group address. Note that multicast messages are
not returned to their originator. Hosts A and C then leave the
group, and Host ...
... Hosts A and C then leave the
group, and Host B decides to delete the group. As in the scenario in
...
... group creation procedure involves the Service Agent
returning to the creating host a 48-bit key along with the 16-bit
...
... bit
group address. The creating host must pass the key along with the
group address to other hosts ...
... host must pass the key along with the
group address to other hosts that want to join the group. These
...
... join the group. These
other hosts must supply the key along with the group address in their
Join ...
... 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 ...
... 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.
...
... Remove Group Member. These setup requests
allow hosts that are already members of the group to add or delete
...
... Remove Group Member will remove a host from the
group membership list but will not alter the existence of the group ...
... time. Thus, group addresses can be deleted even if the host which
originally created the group ...
... 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.
...
...
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 ...
... host may choose to
drop out of the group. To do this, the host sends the Service Agent
a Leave Group ...
... S3[0-15] Group Address. This is the group that the host wishes
to join. Upon successfully joining the group ...
... to join. Upon successfully joining the group, the host
may send messages to the group and will receive
...
... S7[14-15] Minimum group message priority. The host will not
receive messages sent to the group that have a message
priority ...
... S6 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S7 | HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... S3[0-15] Group Address. This is the group the host will join.
Upon successfully joining the group ...
... join.
Upon successfully joining the group, the host may send
messages to the group and will receive messages sent to
...
... group and will receive messages sent to
the group by other hosts (the initial minimum priority
will be 0).
...
... S3[0-15] Group Address. This is the group that the host wishes
to cease being a member of. After leaving the group,
...
... to cease being a member of. After leaving the group,
the host will cease receiving messages sent to the
group ...
... S6 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S7 | HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... S3[0-15] Group Address. This is the group from which the host
should be removed. After leaving the group ...
... should be removed. After leaving the group, that host
will cease receiving messages sent to the group ...
... 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 ...
...
The host may obtain information about WPS state and about what
resources the WPS currently has allocated for the host ...
... 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 ...
... Request message to the Service Agent. The Information
Reply that is returned will enable the host to determine 1) what
resources the WPS has allocated to the host, and 2) the current state ...
... Reply that is returned will enable the host to determine 1) what
resources the WPS has allocated to the host, and 2) the current state
of the network ...
... 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
...
... has, and to regain information it may have lost on its network
resources. This communication also informs the host of the network
state ...
...
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
...
...
S2[0-15] Message ID. This field is assigned by the host in the
Information Request message to uniquely identify
...
... Host Access Link Monitoring ...
... 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 ...
... 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 ...
... these monitoring statistics to the Network Operations Center (NOC).
If either host or WPS fails to receive Status messages for ten
seconds, the link ...
... reliability. This field is not meaningful in messages
sent from the host to the WPS and must be set to zero.
...
...
The Host Access Protocol uses a number of state variables that must
be initialized in order to function properly. These variables are
...
... 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
...
... 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:
...
... restart.
The interface in the Host or WPS can recognize this
requirement itself or be forced to restart ...
... Local
counters have been zeroed. Data and control messages
can now be exchanged between the WPS and host.
...
... Restart Request control message (Figure 37) is sent by either a
host or a WPS when it wishes to restart a link. The Restart ...
... initialization. The Restart Complete carries a field
used by the host to enable or disable the acceptance/refusal
mechanism for the link being restarted (see Section 5). After the
...
... groups)
are separate from the state of the host's access link(s) to the WPS.
The Information Request message ...
... The Information Request message (see Section 6.5) may be used by a
host to determine what resources it has. If the "SL" bit is set in
...
... 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
...
... 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.
...
... HEADER CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2 | HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... HEADER CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2 | HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
...
... 0[10] Service loss alert (boolean) (WPS to host only; host
must send zero). If the WPS has any reason to believe
...
... 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 ...
... 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 ...
... that the resources allocated to the host may not match
what the host believes is allocated, SL is set to one.
If SL ...
... 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 ...
... 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 ...
... 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 ...
...
When the host access link is remotely looped, all HAP messages will
be returned, unmodified, over the access link ...
... 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 ...
... of the transmitter's messages other than requests to reinitialize the
WPS-host link (Restart Request (RR ...
... transmitter or the receiver, the affected unit
will try to reinitialize the WPS-host link by sending an RR message
...
... 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
...
...
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 ...
... loopback at the same time. If a WPS receives a Loopback Request
message from a host while it is itself waiting for an event of type
(1)-(3), it will return a "Can't implement loop" response to the host
...
... Request
message from a host while it is itself waiting for an event of type
(1)-(3), it will return a "Can't implement loop" response to the host
and will continue to wait. A host in the converse situation,
...
... (1)-(3), it will return a "Can't implement loop" response to the host
and will continue to wait. A host in the converse situation,
however, will abort its loopback request and will instead act on the
...
...
Before a WPS or a host voluntarily disables a WPS-host link, it
should send at least one Link ...
... 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 ...
... 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 ...
... host link that it is sent
over; if a host and a WPS are connected by multiple links, these
links ...
... 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 ...
... 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[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 ...