RFC 1221
Host Access Protocol (HAP) specification: Version 2
6. The Service Agent Allocation of network resources, such as streams and groups, is accomplished via an exchange of datagram messages, called Setup messages, between the user host and the Service Agent (network address zero). Setup operations include reserving, allocating, modifying, freeing, and deallocating resources. The Service Agent causes the requested action to be carried out and serves as the intermediary between the user and the rest of the network. In the process of implementing the requested action, various network data bases are updated to reflect the current state of the referenced resource. The Service Agent also permits a host to inquire about resources it owns using Information Request and Information Reply messages. A setup interaction initiated by a host involves a 3-way exchange where: (1) the requesting host sends a Setup Request to the Service Agent, (2) the Service Agent returns a Setup Reply to the requesting host, and (3) the requesting host returns a Setup Acknowledgment to the Service Agent. This procedure is used to ensure reliable transmission of Setup Requests and Replies. In order to allow more than one Setup Request message from a host to be outstanding, each Request is assigned a unique Request ID. The associated Reply and subsequent Acknowledgment are identified by the Request ID that they contain. The requesting host should receive a reply to a setup request within 3 seconds. The actual delay will depend on the nature of the request and the topology of the network. For simple networks, the delay will often be less than one second. The requesting host should respond to a Reply with a Setup Acknowledgment within one second. Setup exchanges initiated by the Service Agent involve a two-way exchange where: (1) the Service Agent sends a Notification to
affected hosts, and (2) the hosts return a Setup Acknowledgment to
the Service Agent. Notifications are used to inform a host of
changes in the status of a network resource. In order to allow more
than one Notification to be outstanding, each is assigned a unique
Notification ID. The Setup Acknowledgment returned by the notified
host to the Service Agent must contain the Notification ID. The host
should respond within one second.
An information query is initiated by a host and involves a two-way
exchange where: (1) the host sends an Information Request message to
the Service Agent, and (2) the Service Agent sends back an
Information Reply. There is no acknowledgment mechanism, since this
request does not change any resource allocation. Furthermore, if
there is an error in the request, only one response will be sent by
the WPS, and the WPS will make no effort to check for or retransmit
lost responses. It is the responsibility of the host to wait a
certain amount of time and then determine that an unanswered
information request has been lost and to resend it. (The time
necessary to answer such a request is usually much less than one
second.) The WPS will return the message ID of the information
request in the information reply message.
The general format of all Service Agent messages is:
<DATAGRAM MESSAGE HEADER>
<SERVICE AGENT HEADER>
<MESSAGE BODY>
The Protocol ID field in the datagram message header must be
HAP_PROTO_SETUP (1) (see Appendix C) for messages sent to the Service
Agent and will be HAP_PROTO_SETUP in messages received from the
Service Agent. The Service Agent does not recognize or support use
of other higher level protocols (e.g., IP), in setup messages, and
will discard messages containing such headers.
Illustrations of message formats below show only the Service Agent
Header header and message body and do not include the datagram
message header. As a reminder that the datagram header is not
included, word offsets are prefixed with an "S".
The format of the Service Agent Header is illustrated in Figure 6.
The body of the message will depend on the particular message type.
Stream Request and Reply messages are described in Section 6.1.
Group Request and Reply messages are described in Section 6.2. The
format of Notifications is described in Section 6.3, and Setup
Acknowledgments are described in Section 6.4. Information Request
and Reply messages are described in Section 6.5.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S0 | MESSAGE TYPE | CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S1 | CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S2 | MESSAGE ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
SERVICE AGENT HEADER
Figure 6
S0[0-7] Message Type. This field determines the type of
message.
0 = Setup Acknowledgment
1 = Setup Request
2 = Setup Reply
3 = Notification
4 = Information Request
5 = Information Reply
S0[8-15] Code. For Setup Requests, this field identifies the
request type.
1 = Create group (multicast) address
2 = Delete group address
3 = Join group
4 = Leave group
5 = Create stream
6 = Delete stream
7 = Change stream
8 = Create shared stream
9 = Delete all streams owned by this host
10 = Add member to group
11 = Remove member from group
For Setup Replies, this field provides the Reply Code.
Some of the Reply Codes can be returned to any setup
request and others are request specific.
0 = Group or stream created
1 = Group or stream deleted
2 = Host added to group
3 = Host deleted from group
4 = Stream changed
5 = (Reserved)
6 = Request type invalid or unsupported
7 = (Reserved)
8 = Network trouble
9 = Bad group key
10 = Group address/stream ID nonexistent
11 = Not member of group/not creator of stream
12 = Stream precedence not being accepted
13 = (Reserved)
14 = (Reserved)
15 = (Reserved)
16 = Unable to add all the new hosts
17 = Insufficient network resources
18 = Requested bandwidth too large
19 = (Reserved)
20 = (Reserved)
21 = Maximum messages per interval too small
22 = Reply lost in network
23 = Illegal priority or precedence value
24 = Invalid address provided
For Notifications, this field contains the Notification
Type. (See Section 6.3.)
For Setup Acknowledgments, this field contains the
Acknowledgment Type. (See Section 6.4.)
For Information Requests, this field contains the
request type. (See Section 6.5.)
For Information Replies, this field contains the reply
type. (See Section 6.5.)
S1[0-15] Checksum. The checksum is the 2's-complement of the
2's-complement sum of the words in the Service Agent
Header (excluding the checksum word itself) and the
message body. Messages received with bad checksums
must be discarded.
S2[0-15] Message ID. This field is assigned by the host to
uniquely identify outstanding requests (Request ID) and
by the Service Agent to uniquely identify outstanding
notifications (Notification ID).
6.1. Stream Setup Messages
Streams provide a means of reserving network resources for the
delivery of traffic at a specified maximum throughput to a specified
list of recipients. Traffic sent via a stream has priority over all
non-stream traffic, and is delivered with the minimum end-to-end
delay possible. Hosts use streams to support applications that have
predictable traffic loads (such as packet voice or video or other
continuous media traffic) or that require minimum transmission delay
and lowest delay variance. Streams are typically used for traffic
flows of moderate to long duration, where the cost of performing a
stream Setup is acceptable.
Streams must be set up before stream data messages can flow. The
stream setup messages, each of which has a Request and a Reply, are
Create Stream, Delete Stream, Change Stream, and Delete All Streams.
(Create Shared Stream Request is a planned future addition to the
protocol.) The use of these messages is illustrated in the scenario
of exchanges between a host and the Service Agent shown in Figure 7
where the host establishes a stream, sends some data, modifies the
stream characteristics, sends some more data, and finally closes down
the stream. Not illustrated, but implicit in this scenario, are the
optional A/R indications associated with each of the stream Setup
messages.
Service Other
Host Agent hosts
Create Stream Request ---------->
Create Stream Reply <----------
Reply Acknowledgment ---------->
Stream Messages --------------------->
: :
Change Stream Request ---------->
Change Stream Reply <----------
Reply Acknowledgment ---------->
Stream Messages --------------------->
: :
Delete Stream Request ---------->
Delete Stream Reply <----------
Reply Acknowledgment ---------->
STREAM EXAMPLE
Figure 7
Streams have eight characteristic properties which are selected at
stream setup time. These properties are: (1) data words per time
interval, (2) time interval, (3) reliability, (4) reliability length,
(5) precedence, (6) maximum messages per interval, (7) the list of
recipients, and (8) the set of other streams with which this stream
shares resources. To establish a stream, the host sends the Create
Stream Request message (Figure 8) to the Service Agent. After the network has processed the Create Stream Request, the Service Agent will reply with a Create Stream Reply message (Figure 9). If the reply code in the Create Stream Reply indicates that the stream has been created successfully, the host may proceed to transmit stream data messages after sending a Reply Acknowledgment. During the lifetime of a stream, the host which created it may decide that some of its characteristic properties should be modified. All but one of the properties can be modified using the Change Stream Request message (Figure 10). The one property that cannot be changed is whether or not the stream is willing to share its resources with other streams. After the network has processed the Change Stream Request, the Service Agent will respond by sending a Change Stream Reply (Figure 11) to the host. A host requesting a reduced channel allocation should decrease its sending rate immediately without waiting for receipt of the Change Stream Reply. A host requesting an increased allocation should not proceed to transmit according to the new set of parameters without first having received a Reply Code indicating that the requested change has taken effect. When the host no longer needs the stream it created, it should first stop sending traffic via the stream and then send the Service Agent a Delete Stream Request message (Figure 12). After the network has processed the Delete Stream Request, the Service Agent will respond by sending a Delete Stream Reply (Figure 13) to the host. If the host has crashed or restarted, it may no longer know what streams it owns. The host may use an Information Request (see Section 6.5) to determine what streams it owns, or the host may use a Delete All Streams Request (Figure 14) to discard whatever stream resources it may own. The format for the Delete All Streams Reply is shown in Figure 15. Note that streams, like all other resources allocated by the Service Agent, may be reclaimed by the network if unused. Currently, if no traffic is sent to a stream in a 6 minute interval, and if the owner of the steam is down or unreachable, the stream may be deleted.
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.
0 = Low Precedence
1 = Medium Precedence
2 = High Precedence
S3[6-7] Interval. This field specifies the interval, in
multiples of 21.22 milliseconds. (For backward
compatibility only. New applications should use 3.
Use of this field to specify an interval is being
phased out.)
0 = 21.22 milliseconds
1 = 42.44 milliseconds
2 = 84.88 milliseconds
3 = use interval in word S5
S3[8-9] Reliability. This field specifies the basic bit-error
rate requirement for the data portion of all messages
in the stream. The exact error rate obtained by each
choice is not specified.
0 = Low Reliability
1 = Medium-Low Reliability
2 = Medium-High Reliability
3 = High Reliability
S3[10-15] Reliability Length. This field specifies how many
words beyond the stream message header should be
transmitted at maximum reliability for all messages in
the host stream.
S4[0-15] Data words per interval. This field specifies the
maximum number of 16-bit words of this stream's data
the network will need to carry during each interval,
not counting HAP stream message header words. The
stream data may be carried in however many messages (up
to MAX MES) in each interval the host chooses.
S5[0-15] Interval (125 microsecond units). This field specifies
the time interval over which the <data words per
interval> data in <max mes> messages will be sent. For
backward compatibility, an interval of 0 selects an
interval of 169.76 milliseconds. This field is ignored
unless the INT field is 3.
S6[0-7] Reserved. Must be zero.
S6[8-15] Destination address list length. This field specifies
the number of entries in the Destination Address List
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).
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.
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.
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).
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.
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.
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).
S1[0-15] Setup Checksum. (See setup header description.)
S2[0-15] Request ID.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S0 | 2 | REPLY CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S1 | SETUP CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S2 | REQUEST ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
DELETE ALL STREAMS REPLY
Figure 15
S0[0-7] Setup Type = 2 (Reply).
S0[8-15] Reply Code. The Service Agent will have marked all of
the host's streams for deletion, even if the stream
resources have not actually been deleted yet.
1 = Streams deleted
S1[0-15] Setup Checksum. (See setup header description.)
S2[0-15] Request ID.
6.2. Group Setup Messages
Group (multicast) addressing allows a host to send the same message
to N different hosts without having to send N copies of the message.
The network duplicates the message as required. In addition to
reducing the burden on the originating host, multicasting reduces the
load on the network because the network no longer has to carry the
duplicates along the common portions of the paths between the source
and destinations. Multicasting is particularly recommended for
multi-site conferencing and distributed simulations.
Group addresses are dynamically created and deleted via setup
messages exchanged between the hosts and the Service Agent.
Membership in a group may be any arbitrary subset of the network
hosts. A datagram message or stream message addressed to a group is
delivered to all hosts that are members of that group (exception:
stream messages sent to a group address that includes hosts the
stream was not set up to reach). The group setup messages, each of which has a Request and a Reply, are Create Group, Delete Group, Join Group, Leave Group, Add Group Member, and Remove Group Member. Figure 16 shows a typical use of group setup messages. The figure illustrates a scenario of exchanges between three hosts and the Service Agent. In the scenario one host, Host A, creates a group which is joined by hosts B and C. The hosts then exchange some data messages using the group address. Note that multicast messages are not returned to their originator. Hosts A and C then leave the group, and Host B decides to delete the group. As in the scenario in Section 6.1, A/R indications have been omitted for clarity. Part of the group creation procedure involves the Service Agent returning to the creating host a 48-bit key along with the 16-bit group address. The creating host must pass the key along with the group address to other hosts that want to join the group. These other hosts must supply the key along with the group address in their Join Group Requests. The key is used by the network to authenticate these operations and thereby minimize the probability that unwanted hosts will deliberately or inadvertently become members of the group. The procedure used by a host to distribute the group address and key is not within the scope of HAP. In the figure below, the network Service Agent is pictured as a single entity for simplicity.
Service Host Host Host
Agent A B C
Create Group Request |<-------|
Create Group Reply |------->|
Reply Acknowledgment |<-------|
: :
Distribute Group Adr & Key |---->|
Distribute Group Adr & Key |---------->|
: :
Join Group Request (C) |<-------------------|
Join Group Reply |------------------->|
Reply Acknowledgment |<-------------------|
Join Group Request (B) |<-------------|
Join Group Reply |------------->|
Reply Acknowledgment |<-------------|
: :
Data Message 1 (A to B and C) |---->|---->|
Data Message 2 (B to A and C) |<----|---->|
Data Message 3 (C to A and B) |<----|<----|
: :
Leave Group Request (C) |<-------------------|
Leave Group Reply |------------------->|
Reply Acknowledgment |<-------------------|
Leave Group Request (A) |<-------|
Leave Group Reply |------->|
Reply Acknowledgment |<-------|
Delete Group Request |<-------------|
Delete Group Reply |------------->|
Reply Acknowledgment |<-------------|
GROUP EXAMPLE
Figure 16
An alternative method of adding and removing group members is the use
of Add Group Member and Remove Group Member. These setup requests
allow hosts that are already members of the group to add or delete
other hosts.
The Setup requests Join Group, Leave Group, Add Group Member, Remove
Group Member, and Delete Group are authenticated using the 48-bit
key. Leave Group and Remove Group Member will remove a host from the
group membership list but will not alter the existence of the group.
Delete Group expunges all knowledge of the group from the network.
HAP permits any host with the proper key to delete the group at any
time. Thus, group addresses can be deleted even if the host which
originally created the group has left the group or has crashed.
Moreover, groups may exist for which there are currently no members
because each member has executed a Leave while none has executed a Delete. It is the responsibility of the hosts to coordinate and manage the use of group addresses. Note that group addresses, like all other resources allocated by the network, may be reclaimed by the network if unused for too long. Currently, if no traffic is sent to the group address in a 6 minute interval, the network may delete the group and notify all members that the group no longer exists. The Create Group Request (Figure 17) is used to establish a multicast address. After the network has processed the Create Group Request, the Service Agent will respond by sending a Create Group Reply (Figure 18) to the host. A host may become a member of a group, once it knows the group address and the 48-bit key, by sending the Service Agent the Join Group Request message (Figure 19). The Service Agent will respond to the Join Group Request with a Join Group Reply (Figure 20). The host which creates a group automatically becomes a member of that group without any need for an explicit Join Group Request. A member host may add another host to the group by sending the Service Agent the Add Group Member Request message (Figure 21). The Service Agent will respond with an Add Group Member Reply (Figure 22). At any time after becoming a member of a group, a host may choose to drop out of the group. To do this, the host sends the Service Agent a Leave Group Request (Figure 23). The Service Agent will respond with a Leave Group Reply (Figure 24). One member host may expel another member of the group by sending the Service Agent the Remove Group Member Request message (Figure 25). The Service Agent will respond with a Remove Group Member Reply (Figure 26). A host can delete an existing group via a Delete Group Request (Figure 27). The Service Agent will respond with a Delete Group Reply (Figure 28). The Service Agent will also send the other members of the group, if any, a notification that the group has been deleted (see Section 6.3).
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).
S0[8-15] Reply Code.
0 = Group created
8 = Network trouble
17 = Insufficient network resources
22 = Reply lost in network
S1[0-15] Setup Checksum. (See setup header description.)
S2[0-15] Request ID.
S3[0-15] Group Address. This field contains the 16-bit
multicast address that any group member may use to
reach the other group members. Multicast addresses are
dynamically assigned by the network.
S4-S6 Key. This field contains a 48-bit key assigned by the
network which is associated with the group address. It
must be provided for subsequent Join Group, Leave
Group, Add Group Member, Remove Group Member, and
Delete Group requests which reference the group
address.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S0 | 1 | 3 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S1 | SETUP CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S2 | REQUEST ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S3 | GROUP ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S4 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S5 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S6 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S7 | 0 | MGP |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
JOIN GROUP REQUEST
Figure 19
S0[0-7] Setup Type = 1 (Request).
S0[8-15] Request Type = 3 (Join Group).
S1[0-15] Setup Checksum. (See setup header description.)
S2[0-15] Request ID.
S3[0-15] Group Address. This is the group that the host wishes
to join. Upon successfully joining the group, the host
may send messages to the group and will receive
messages sent to the group when those messages have a
priority of MGP or higher.
S4-S6 Key. This is the key associated with the group
address.
S7[0-13] Reserved. Must be zero.
S7[14-15] Minimum group message priority. The host will not
receive messages sent to the group that have a message
priority less than MGP. Send another Join Group
Request message to change the minimum priority.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S0 | 2 | REPLY CODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S1 | SETUP CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S2 | REQUEST ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
JOIN GROUP REPLY
Figure 20
S0[0-7] Setup Type = 2 (Reply).
S0[8-15] Reply Code.
2 = Host added to group
9 = Bad key
10 = Group address nonexistent
17 = Insufficient network resources
S1[0-15] Setup Checksum. (See setup header description.)
S2[0-15] Request ID.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S0 | 1 | 10 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S1 | SETUP CHECKSUM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S2 | REQUEST ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S3 | GROUP ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S4 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S5 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S6 | KEY |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
S7 | HOST ADDRESS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
ADD GROUP MEMBER REQUEST
Figure 21
S0[0-7] Setup Type = 1 (Request).
S0[8-15] Request Type = 3 (Join Group).
S1[0-15] Setup Checksum. (See setup header description.)
S2[0-15] Request ID.
S3[0-15] Group Address. This is the group the host will join.
Upon successfully joining the group, the host may send
messages to the group and will receive messages sent to
the group by other hosts (the initial minimum priority
will be 0).
S4-S6 Key. This is the key associated with the group
address.
S7[0-15] Host address. The network address of the host to add
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.
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.
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.
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.
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.
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.
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.
(page 46 continued on part 3)
