RFC 1819
Internet Stream Protocol Version 2 (ST2) Protocol Specification - Version ST2+
3. The ST2 Data Transfer Protocol This section presents the ST2 data transfer protocol, ST. First, data transfer is described in Section 3.1, then, the data transfer protocol functions are illustrated in Section 3.2. 3.1 Data Transfer with ST Data transmission with ST is unreliable. An application is not guaranteed that the data reaches its destinations and ST makes no attempts to recover from packet loss, e.g., due to the underlying network. However, if the data reaches its destination, it should do so according to the quality of service associated with the stream. Additionally, ST may deliver data corrupted in transmission. Many types of real-time data, such as digital audio and video, require partially correct delivery only. In many cases, retransmitted packets would arrive too late to meet their real-time delivery requirements. On the other hand, depending on the data encoding and the particular application, a small number of errors in stream data are acceptable. In any case, reliability can be provided by layers on top of ST2 if needed. Also, no data fragmentation is supported during the data transfer phase. The application is expected to segment its data PDUs according to the minimum MTU over all paths in the stream. The application receives information on the MTUs relative to the paths to the targets as part of the ACCEPT message, see Section 8.6. The minimum MTU over all paths can be calculated from the MTUs relative to the single paths. ST agents silently discard too long data packets, see also Section 5.1.1. An ST agent forwards the data only along already established paths to targets. A path is considered to be established once the next-hop ST agent on the path sends an ACCEPT message, see Section 2.2. This implies that the target and all other intermediate ST agents on the path to the target are ready to handle the incoming data packets. In no cases will an ST agent forward data to a next-hop ST agent that has not explicitly accepted the stream. To be reasonably sure that all targets receive the data with the desired quality of service, an application should send the data only after the whole stream has been established. Depending on the local API, an application may not be prevented from sending data before the completion of stream setup, but it should be aware that the data could be lost or not reach all intended targets. This behavior may actually be desirable to applications, such as those application that have multiple targets which can each process data as soon as it is
available (e.g., a lecture or distributed gaming). It is desirable for implementations to take advantage of networks that support multicast. If a network does not support multicast, or for the case where the next-hops are on different networks, multiple copies of the data packet must be sent. 3.2 ST Protocol Functions The ST protocol provides two functions: o stream identification o data priority 3.2.1 Stream Identification ST data packets are encapsulated by an ST header containing the Stream IDentifier (SID). This SID is selected at the origin so that it is globally unique over the Internet. The SID must be known by the setup protocol as well. At stream establishment time, the setup protocol builds, at each agent traversed by the stream, an entry into its local database containing stream information. The SID can be used as a reference into this database, to obtain quickly the necessary replication and forwarding information. Stream IDentifiers are intended to be used to make the packet forwarding task most efficient. The time-critical operation is an intermediate ST agent receiving a packet from the previous-hop ST agent and forwarding it to the next-hop ST agents. The format of data PDUs including the SID is defined in Section 10.1. Stream IDentifier generation is discussed in Section 8.1. 3.2.2 Packet Discarding based on Data Priority ST provides a well defined quality of service to its applications. However, there may be cases where the network is temporarily congested and the ST agents have to discard certain packets to minimize the overall impact to other streams. The ST protocol provides a mechanism to discard data packets based on the Priority field in the data PDU, see Section 10.1. The application assigns each data packet with a discard-priority level, carried into the Priority field. ST agents will attempt to discard lower priority packets first during periods of network congestion. Applications may choose to send data at multiple priority levels so that less important data may be discarded first.
4. SCMP Functional Description ST agents create and manage streams using the ST Control Message Protocol (SCMP). Conceptually, SCMP resides immediately above ST (as does ICMP above IP). SCMP follows a request-response model. SCMP messages are made reliable through the use of retransmission after timeout. This section contains a functional description of stream management with SCMP. To help clarify the SCMP exchanges used to setup and maintain ST streams, we include an example of a simple network topology, represented in Figure 8. Using the SCMP messages described in this section it will be possible for an ST application to: o Create a stream from A to the peers at B, C and D, o Add a peer at E, o Drop peers B and C, and o Let F join the stream o Delete the stream.
+---------+ +---+
| |----| B |
+---------+ +----------+ | | +---+
| |------| Router 1 |---| Subnet2 |
| | +----------+ | |
| | | |
| | +---------+
| | |
| Subnet1 | |
| | +----------+
| | | Router 3 |
+---+ | | +----------+
| A |---| | +----------+ |
+---+ | |----| Router 2 | |
| | +----------+ |
+---------+ | |
| |
| +----------+ +---+
+----------| |----| C |
| | +---+
+---------+ | Subnet3 |
+---+ | | +---+ | | +---+
| F |---| Subnet4 |---| E |--| |----| D |
+---+ | | +---+ +----------+ +---+
+---------+
Figure 8: Sample Topology for an ST Stream
We first describe the possible types of stream in Section 4.1;
Section 4.2 introduces SCMP control message types; SCMP reliability
is discussed in Section 4.3; stream options are covered in Section
4.4; stream setup is presented in Section 4.5; Section 4.6
illustrates stream modification including stream expansion,
reduction, changes of the quality of service associated to a stream.
Finally, stream deletion is handled in Section 4.7.
4.1 Types of Streams
SCMP allows for the setup and management of different types of
streams. Streams differ in the way they are built and the information
maintained on connected targets.
4.1.1 Stream Building Streams may be built in a sender-oriented fashion, receiver-oriented fashion, or with a mixed approach: o in the sender-oriented fashion, the application at the origin provides the ST agent with the list of receivers for the stream. New targets, if any, are also added from the origin. o in the receiver-oriented approach, the application at the origin creates an empty stream that contains no targets. Each target then joins the stream autonomously. o in the mixed approach, the application at the origin creates a stream that contains some targets and other targets join the stream autonomously. ST2 provides stream options to support sender-oriented and mixed approach steams. Receiver-oriented streams can be emulated through the use of mixed streams. The fashion by which targets may be added to a particular stream is controlled via join authorization levels. Join authorization levels are described in Section 4.4.2. 4.1.2 Knowledge of Receivers When streams are built in the sender-oriented fashion, all ST agents will have full information on all targets down stream of a particular agent. In this case, target information is relayed down stream from agent-to-agent during stream set-up. When targets add themselves to mixed approach streams, upstream ST agents may or may not be informed. Propagation of information on targets that "join" a stream is also controlled via join authorization levels. As previously mentioned, join authorization levels are described in Section 4.4.2. This leads to two types of streams: o full target information is propagated in a full-state stream. For such streams, all agents are aware of all downstream targets connected to the stream. This results in target information being maintained at the origin and at intermediate agents. Operations on single targets are always possible, i.e., change a certain target, or, drop that target from the stream. It is also always possible for any ST agent to attempt recovery of all downstream targets.
o in light-weight streams, it is possible that the origin and other
upstream agents have no knowledge about some targets. This results
in less maintained state and easier stream management, but it limits
operations on specific targets. Special actions may be required to
support change and drop operations on unknown targets, see Section
5.7. Also, stream recovery may not be possible. Of course, generic
functions such as deleting the whole stream, are still possible. It
is expected that applications that will have a large number of
targets will use light-weight streams in order to limit state in
agents and the number of targets per control message.
Full-state streams serve well applications as video conferencing or
distributed gaming, where it is important to have knowledge on the
connected receivers, e.g., to limit who participates. Light-weight
streams may be exploited by applications such as remote lecturing or
playback applications of radio and TV broadcast where the receivers
do not need to be known by the sender. Section 4.4.2 defines join
authorization levels, which support two types of full-state streams
and one type of light-weight stream.
4.2 Control PDUs
SCMP defines the following PDUs (the main purpose of each PDU is also
indicated):
1. ACCEPT to accept a new stream
2. ACK to acknowledge an incoming message
3. CHANGE to change the quality of service associated with
a stream
4. CONNECT to establish a new stream or add new targets to
an existing stream
5. DISCONNECT to remove some or all of the stream's targets
6. ERROR to indicate an error contained in an incoming
message
7. HELLO to detect failures of neighbor ST agents
8. JOIN to request stream joining from a target
9. JOIN-REJECT to reject a stream joining request from a target
10. NOTIFY to inform an ST agent of a significant event
11. REFUSE to refuse the establishment of a new stream
12. STATUS to query an ST agent on a specific stream
13. STATUS-RESPONSE to reply queries on a specific stream
SCMP follows a request-response model with all requests expecting
responses. Retransmission after timeout is used to allow for lost or
ignored messages. Control messages do not extend across packet
boundaries; if a control message is too large for the MTU of a hop,
its information is partitioned and a control message per partition is
sent, as described in Section 5.1.2.
CONNECT and CHANGE request messages are answered with ACCEPT messages
which indicate success, and with REFUSE messages which indicate
failure. JOIN messages are answered with either a CONNECT message
indicating success, or with a JOIN-REJECT message indicating failure.
Targets may be removed from a stream by either the origin or the
target via the DISCONNECT and REFUSE messages.
The ACCEPT, CHANGE, CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY
and REFUSE messages must always be explicitly acknowledged:
o with an ACK message, if the message was received correctly and it
was possible to parse and correctly extract and interpret its
header, fields and parameters,
o with an ERROR message, if a syntax error was detected in the header,
fields, or parameters included in the message. The errored PDU may
be optionally returned as part of the ERROR message. An ERROR
message indicates a syntax error only. If any other errors are
detected, it is necessary to first acknowledge with ACK and then
take appropriate actions. For instance, suppose a CHANGE message
contains an unknown SID: first, an ACK message has to be sent, then
a REFUSE message with ReasonCode (SIDUnknown) follows.
If no ACK or ERROR message are received before the correspondent
timer expires, a timeout failure occurs. The way an ST agent should
handle timeout failures is described in Section 5.2.
ACK, ERROR, and STATUS-RESPONSE messages are never acknowledged.
HELLO messages are a special case. If they contain a syntax error, an
ERROR message should be generated in response. Otherwise, no
acknowledgment or response should be generated. Use of HELLO messages
is discussed in Section 6.1.2.
STATUS messages containing a syntax error should be answered with an
ERROR message. Otherwise, a STATUS-RESPONSE message should be sent
back in response. Use of STATUS and STATUS-RESPONSE are discussed in
Section 8.4.
4.3 SCMP Reliability
SCMP is made reliable through the use of retransmission when a
response is not received in a timely manner. The ACCEPT, CHANGE,
CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY, and REFUSE messages
all must be answered with an ACK message, see Section 4.2. In
general, when sending a SCMP message which requires an ACK response,
the sending ST agent needs to set the Toxxxx timer (where xxxx is the
SCMP message type, e.g., ToConnect). If it does not receive an ACK
before the Toxxxx timer expires, the ST agent should retransmit the SCMP message. If no ACK has been received within Nxxxx retransmissions, then a SCMP timeout condition occurs and the ST agent enters its SCMP timeout recovery state. The actions performed by the ST agent as the result of the SCMP timeout condition differ for different SCMP messages and are described in Section 5.2. For some SCMP messages (CONNECT, CHANGE, JOIN, and STATUS) the sending ST agent also expects a response back (ACCEPT/REFUSE, CONNECT/JOIN- REJECT) after ACK has been received. For these cases, the ST agent needs to set the ToxxxxResp timer after it receives the ACK. (As before, xxxx is the initiating SCMP message type, e.g., ToConnectResp). If it does not receive the appropriate response back when ToxxxxResp expires, the ST agent updates its state and performs appropriate recovery action as described in Section 5.2. Suggested constants are given in Section 10.5.4. The timeout and retransmission algorithm is implementation dependent and it is outside the scope of this document. Most existing algorithms are based on an estimation of the Round Trip Time (RTT) between two agents. Therefore, SCMP contains a mechanism, see Section 8.5, to estimate this RTT. Note that the timeout related variable names described above are for reference purposes only, implementors may choose to combine certain variables. 4.4 Stream Options An application may select among some stream options. The desired options are indicated to the ST agent at the origin when a new stream is created. Options apply to single streams and are valid during the whole stream's lifetime. The options chosen by the application at the origin are included into the initial CONNECT message, see Section 4.5.3. When a CONNECT message reaches a target, the application at the target is notified of the stream options that have been selected, see Section 4.5.5. 4.4.1 No Recovery When a stream failure is detected, an ST agent would normally attempt stream recovery, as described in Section 6.2. The NoRecovery option is used to indicate that ST agents should not attempt recovery for the stream. The protocol behavior in the case that the NoRecovery option has been selected is illustrated in Section 6.2. The NoRecovery option is specified by setting the S-bit in the CONNECT message, see Section 10.4.4. The S-bit can be set only by the origin and it is never modified by intermediate and target ST agents.
4.4.2 Join Authorization Level When a new stream is created, it is necessary to define the join authorization level associated with the stream. This level determines the protocol behavior in case of stream joining, see Section 4.1 and Section 4.6.3. The join authorization level for a stream is defined by the J-bit and N-bit in the CONNECT message header, see Section 10.4.4. One of the following authorization levels has to be selected: o Level 0 - Refuse Join (JN = 00): No targets are allowed to join this stream. o Level 1 - OK, Notify Origin (JN = 01): Targets are allowed to join the stream. The origin is notified that the target has joined. o Level 2 - OK (JN = 10): Targets are allowed to join the stream. No notification is sent to the stream origin. Some applications may choose to maintain tight control on their streams and will not permit any connections without the origin's permission. For such streams, target applications may request to be added by sending an out-of-band, i.e., via regular IP, request to the origin. The origin, if it so chooses, can then add the target following the process described in Section 4.6.1. The selected authorization level impacts stream handling and the state that is maintained for the stream, as described in Section 4.1. 4.4.3 Record Route The RecordRoute option can be used to request the route between the origin and a target be recorded and delivered to the application. This option may be used while connecting, accepting, changing, or refusing a stream. The results of a RecordRoute option requested by the origin, i.e., as part of the CONNECT or CHANGE messages, are delivered to the target. The results of a RecordRoute option requested by the target, i.e., as part of the ACCEPT or REFUSE messages, are delivered to the origin. The RecordRoute option is specified by adding the RecordRoute parameter to the mentioned SCMP messages. The format of the RecordRoute parameter is shown in Section 10.3.5. When adding this parameter, the ST agent at the origin must determine the number of entries that may be recorded as explained in Section 10.3.5.
4.4.4 User Data The UserData option can be used by applications to transport application specific data along with some SCMP control messages. This option can be included with ACCEPT, CHANGE, CONNECT, DISCONNECT, and REFUSE messages. The format of the UserData parameter is shown in Section 10.3.7. This option may be included by the origin, or the target, by adding the UserData parameter to the mentioned SCMP messages. This option may only be included once per SCMP message. 4.5 Stream Setup This section presents a description of stream setup. For simplicity, we assume that everything succeeds, e.g., any required resources are available, messages are properly delivered, and the routing is correct. Possible failures in the setup phase are handled in Section 5.2. 4.5.1 Information from the Application Before stream setup can be started, the application has to collect the necessary information to determine the characteristics for the connection. This includes identifying the participants and selecting the QoS parameters of the data flow. Information passed to the ST agent by the application includes: o the list of the stream's targets (Section 10.3.6). The list may be empty (Section 4.5.3.1), o the flow specification containing the desired quality of service for the stream (Section 9), o information on the groups in which the stream is a member, if any (Section 7), o information on the options selected for the stream (Section 4.4). 4.5.2 Initial Setup at the Origin The ST agent at the origin then performs the following operations: o allocates a stream ID (SID) for the stream (Section 8.1), o invokes the routing function to determine the set of next-hops for the stream (Section 4.5.2.1), o invokes the Local Resource Manager (LRM) to reserve resources (Section 4.5.2.2),
o creates local database entries to store information on the new
stream,
o propagates the stream creation request to the next-hops determined
by the routing function (Section 4.5.3).
4.5.2.1 Invoking the Routing Function
An ST agent that is setting up a stream invokes the routing function
to find the next-hop to reach each of the targets specified by the
target list provided by the application. This is similar to the
routing decision in IP. However, in this case the route is to a
multitude of targets with QoS requirements rather than to a single
destination.
The result of the routing function is a set of next-hop ST agents.
The set of next-hops selected by the routing function is not
necessarily the same as the set of next-hops that IP would select
given a number of independent IP datagrams to the same destinations.
The routing algorithm may attempt to optimize parameters other than
the number of hops that the packets will take, such as delay, local
network bandwidth consumption, or total internet bandwidth
consumption. Alternatively, the routing algorithm may use a simple
route lookup for each target.
Once a next-hop is selected by the routing function, it persists for
the whole stream lifetime, unless a network failure occurs.
4.5.2.2 Reserving Resources
The ST agent invokes the Local Resource Manager (LRM) to perform the
appropriate reservations. The ST agent presents the LRM with
information including:
o the flow specification with the desired quality of service for the
stream (Section 9),
o the version number associated with the flow specification
(Section 9).
o information on the groups the stream is member in, if any
(Section 7),
The flow specification contains information needed by the LRM to
allocate resources. The LRM updates the flow specification contents
information before returning it to the ST agent. Section 9.2.3
defines the fields of the flow specification to be updated by the
LRM.
The membership of a stream in a group may affect the amount of resources that have to be allocated by the LRM, see Section 7. 4.5.3 Sending CONNECT Messages The ST agent sends a CONNECT message to each of the next-hop ST agents identified by the routing function. Each CONNECT message contains the SID, the selected stream options, the FlowSpec, and a TargetList. The format of the CONNECT message is defined by Section 10.4.4. In general, the FlowSpec and TargetList depend on both the next-hop and the intervening network. Each TargetList is a subset of the original TargetList, identifying the targets that are to be reached through the next-hop to which the CONNECT message is being sent. The TargetList may be empty, see Section 4.5.3.1; if the TargetList causes a too long CONNECT message to be generated, the CONNECT message is partitioned as explained in Section 5.1.2. If multiple next-hops are to be reached through a network that supports network level multicast, a different CONNECT message must nevertheless be sent to each next-hop since each will have a different TargetList. 4.5.3.1 Empty Target List An application at the origin may request the local ST agent to create an empty stream. It does so by passing an empty TargetList to the local ST agent during the initial stream setup. When the local ST agent receives a request to create an empty stream, it allocates the stream ID (SID), updates its local database entries to store information on the new stream and notifies the application that stream setup is complete. The local ST agent does not generate any CONNECT message for streams with an empty TargetList. Targets may be later added by the origin, see Section 4.6.1, or they may autonomously join the stream, see Section 4.6.3. 4.5.4 CONNECT Processing by an Intermediate ST agent An ST agent receiving a CONNECT message, assuming no errors, responds to the previous-hop with an ACK. The ACK message must identify the CONNECT message to which it corresponds by including the reference number indicated by the Reference field of the CONNECT message. The intermediate ST agent calls the routing function, invokes the LRM to reserve resources, and then propagates the CONNECT messages to its next-hops, as described in the previous sections.
4.5.5 CONNECT Processing at the Targets An ST agent that is the target of a CONNECT message, assuming no errors, responds to the previous-hop with an ACK. The ST agent invokes the LRM to reserve local resources and then queries the specified application process whether or not it is willing to accept the connection. The application is presented with parameters from the CONNECT message including the SID, the selected stream options, Origin, FlowSpec, TargetList, and Group, if any, to be used as a basis for its decision. The application is identified by a combination of the NextPcol field, from the Origin parameter, and the service access point, or SAP, field included in the correspondent (usually single remaining) Target of the TargetList. The contents of the SAP field may specify the port or other local identifier for use by the protocol layer above the host ST layer. Subsequently received data packets will carry the SID, that can be mapped into this information and be used for their delivery. Finally, based on the application's decision, the ST agent sends to the previous-hop from which the CONNECT message was received either an ACCEPT or REFUSE message. Since the ACCEPT (or REFUSE) message has to be acknowledged by the previous-hop, it is assigned a new Reference number that will be returned in the ACK. The CONNECT message to which ACCEPT (or REFUSE) is a reply is identified by placing the CONNECT's Reference number in the LnkReference field of ACCEPT (or REFUSE). The ACCEPT message contains the FlowSpec as accepted by the application at the target. 4.5.6 ACCEPT Processing by an Intermediate ST agent When an intermediate ST agent receives an ACCEPT, it first verifies that the message is a response to an earlier CONNECT. If not, it responds to the next-hop ST agent with an ERROR message, with ReasonCode (LnkRefUnknown). Otherwise, it responds to the next-hop ST agent with an ACK, and propagates the individual ACCEPT message to the previous-hop along the same path traced by the CONNECT but in the reverse direction toward the origin. The FlowSpec is included in the ACCEPT message so that the origin and intermediate ST agents can gain access to the information that was accumulated as the CONNECT traversed the internet. Note that the resources, as specified in the FlowSpec in the ACCEPT message, may differ from the resources that were reserved when the CONNECT was originally processed. Therefore, the ST agent presents the LRM with the FlowSpec included in the ACCEPT message. It is expected that each LRM adjusts local reservations releasing any excess resources. The
LRM may choose not to adjust local reservations when that adjustment may result in the loss of needed resources. It may also choose to wait to adjust allocated resources until all targets in transition have been accepted or refused. In the case where the intermediate ST agent is acting as the origin with respect to this target, see Section 4.6.3.1, the ACCEPT message is not propagated upstream. 4.5.7 ACCEPT Processing by the Origin The origin will eventually receive an ACCEPT (or REFUSE) message from each of the targets. As each ACCEPT is received, the application is notified of the target and the resources that were successfully allocated along the path to it, as specified in the FlowSpec contained in the ACCEPT message. The application may then use the information to either adopt or terminate the portion of the stream to each target. When an ACCEPT is received by the origin, the path to the target is considered to be established and the ST agent is allowed to forward the data along this path as explained in Section 2 and in Section 3.1. 4.5.8 REFUSE Processing by the Intermediate ST agent If an application at a target does not wish to participate in the stream, it sends a REFUSE message back to the origin with ReasonCode (ApplDisconnect). An intermediate ST agent that receives a REFUSE message with ReasonCode (ApplDisconnect) acknowledges it by sending an ACK to the next-hop, invokes the LRM to adjusts reservations as appropriate, deletes the target entry from the internal database, and propagates the REFUSE message back to the previous-hop ST agent. In the case where the intermediate ST agent is acting as the origin with respect to this target, see Section 4.6.3.1, the REFUSE message is only propagated upstream when there are no more downstream agents participating in the stream. In this case, the agent indicates that the agent is to be removed from the stream propagating the REFUSE message with the G-bit set (1). 4.5.9 REFUSE Processing by the Origin When the REFUSE message reaches the origin, the ST agent at the origin sends an ACK and notifies the application that the target is no longer part of the stream and also if the stream has no remaining targets. If there are no remaining targets, the application may wish to terminate the stream, or keep the stream active to allow addition
of targets or stream joining as described in Section 4.6.3. 4.5.10 Other Functions during Stream Setup Some other functions have to be accomplished by an ST agent as CONNECT messages travel downstream and ACCEPT (or REFUSE) messages travel upstream during the stream setup phase. They were not mentioned in the previous sections to keep the discussion as simple as possible. These functions include: o computing the smallest Maximum Transmission Unit size over the path to the targets, as part of the MTU discovery mechanism presented in Section 8.6. This is done by updating the MaxMsgSize field of the CONNECT message, see Section 10.4.4. This value is carried back to origin in the MaxMsgSize field of the ACCEPT message, see Section 10.4.1. o counting the number of IP clouds to be traversed to reach the targets, if any. IP clouds are traversed when the IP encapsulation mechanism is used. This mechanism described in Section 8.7. Encapsulating agents update the IPHops field of the CONNECT message, see Section 10.4.4. The resulting value is carried back to origin in the IPHops field of the ACCEPT message, see Section 10.4.1. o updating the RecoveryTimeout value for the stream based on what can the agent can support. This is part of the stream recovery mechanism, in Section 6.2. This is done by updating the RecoveryTimeout field of the CONNECT message, see Section 10.4.4. This value is carried back to origin in the RecoveryTimeout field of the ACCEPT message, see Section 10.4.1. 4.6 Modifying an Existing Stream Some applications may wish to modify a stream after it has been created. Possible changes include expanding a stream, reducing it, and changing its FlowSpec. The origin may add or remove targets as described in Section 4.6.1 and Section 4.6.2. Targets may request to join the stream as described in Section 4.6.3 or, they may decide to leave a stream as described in Section 4.6.4. Section 4.6.5 explains how to change a stream's FlowSpec. As defined by Section 2, an ST agent can handle only one stream modification at a time. If a stream modification operation is already underway, further requests are queued and handled when the previous operation has been completed. This also applies to two subsequent requests of the same kind, e.g., two subsequent changes to the FlowSpec.
4.6.1 The Origin Adding New Targets It is possible for an application at the origin to add new targets to an existing stream any time after the stream has been established. Before new targets are added, the application has to collect the necessary information on the new targets. Such information is passed to the ST agent at the origin. The ST agent at the origin issues a CONNECT message that contains the SID, the FlowSpec, and the TargetList specifying the new targets. This is similar to sending a CONNECT message during stream establishment, with the following exceptions: the origin checks that a) the SID is valid, b) the targets are not already members of the stream, c) that the LRM evaluates the FlowSpec of the new target to be the same as the FlowSpec of the existing stream, i.e., it requires an equal or smaller amount of resources to be allocated. If the FlowSpec of the new target does not match the FlowSpec of the existing stream, an error is generated with ReasonCode (FlowSpecMismatch). Functions to compare flow specifications are provided by the LRM, see Section 1.4.5. An intermediate ST agent that is already a participant in the stream looks at the SID and StreamCreationTime, and verifies that the stream is the same. It then checks if the intersection of the TargetList and the targets of the established stream is empty. If this is not the case, it responds with a REFUSE message with ReasonCode (TargetExists) that contains a TargetList of those targets that were duplicates. To indicate that the stream exists, and includes the listed targets, the ST agent sets to one (1) the E-bit of the REFUSE message, see Section 10.4.11. The agent then proceeds processing each new target in the TargetList. For each new target in the TargetList, processing is much the same as for the original CONNECT. The CONNECT is acknowledged, propagated, and network resources are reserved. Intermediate or target ST agents that are not already participants in the stream behave as in the case of stream setup (see Section 4.5.4 and Section 4.5.5). 4.6.2 The Origin Removing a Target It is possible for an application at the origin to remove existing targets of a stream any time after the targets have accepted the stream. The application at the origin specifies the set of targets that are to be removed and informs the local ST agent. Based on this information, the ST agent sends DISCONNECT messages with the ReasonCode (ApplDisconnect) to the next-hops relative to the targets.
An ST agent that receives a DISCONNECT message must acknowledge it by sending an ACK to the previous-hop. The ST agent updates its state and notifies the LRM of the target deletion so that the LRM can modify reservations as appropriate. When the DISCONNECT message reaches the target, the ST agent also notifies the application that the target is no longer part of the stream. When there are no remaining targets that can be reached through a particular next-hop, the ST agent informs the LRM and it deletes the next-hop from its next-hops set. SCMP also provides a flooding mechanism to delete targets that joined the stream without notifying the origin. The special case of target deletion via flooding is described in Section 5.7. 4.6.3 A Target Joining a Stream An application may request to join an existing stream. It has to collect information on the stream including the stream ID (SID) and the IP address of the stream's origin. This can be done out-of-band, e.g., via regular IP. The information is then passed to the local ST agent. The ST agent generates a JOIN message containing the application's request to join the stream and sends it toward the stream origin. An ST agent receiving a JOIN message, assuming no errors, responds with an ACK. The ACK message must identify the JOIN message to which it corresponds by including the Reference number indicated by the Reference field of the JOIN message. If the ST agent is not traversed by the stream that has to be joined, it propagates the JOIN message toward the stream's origin. Once a JOIN message has been acknowledged, ST agents do not retain any state information related to the JOIN message. Eventually, an ST agent traversed by the stream or the stream's origin itself is reached. This agent must respond to a received JOIN first with an ACK to the ST agent from which the message was received, then, it issues either a CONNECT or a JOIN-REJECT message and sends it toward the target. The response to the join request is based on the join authorization level associated with the stream, see Section 4.4.2: o If the stream has authorization level #0 (refuse join): The ST agent sends a JOIN-REJECT message toward the target with ReasonCode (JoinAuthFailure). o If the stream has authorization level #1 (ok, notify origin): The ST agent sends a CONNECT message toward the target with a TargetList including the target that requested to join the stream.
This eventually results in adding the target to the stream. When
the ST agent receives the ACCEPT message indicating that the new
target has been added, it does not propagate the ACCEPT message
backwards (Section 4.5.6). Instead, it issues a NOTIFY message
with ReasonCode (TargetJoined) so that upstream agents, including
the origin, may add the new target to maintained state
information. The NOTIFY message includes all target specific
information.
o If the stream has authorization level #2 (ok):
The ST agent sends a CONNECT message toward the target with a
TargetList including the target that requested to join the stream.
This eventually results in adding the target to the stream. When
the ST agent receives the ACCEPT message indicating that the new
target has been added, it does not propagate the ACCEPT message
backwards (Section 4.5.6), nor does it notify the origin. A NOTIFY
message is generated with ReasonCode (TargetJoined) if the target
specific information needs to be propagated back to the origin. An
example of such information is change in MTU, see Section 8.6.
4.6.3.1 Intermediate Agent (Router) as Origin
When a stream has join authorization level #2, see Section 4.4.2, it
is possible that the stream origin is unaware of some targets
participating in the stream. In this case, the ST intermediate agent
that first sent a CONNECT message to this target has to act as the
stream origin for the given target. This includes:
o if the whole stream is deleted, the intermediate agent must
disconnect the target.
o if the stream FlowSpec is changed, the intermediate agent must
change the FlowSpec for the target as appropriate.
o proper handling of ACCEPT and REFUSE messages, without propagation
to upstream ST agents.
o generation of NOTIFY messages when needed. (As described above.)
The intermediate agent behaves normally for all other targets added
to the stream as a consequence of a CONNECT message issued by the
origin.
4.6.4 A Target Deleting Itself
The application at the target may inform the local ST agent that it
wants to be removed from the stream. The ST agent then forms a REFUSE
message with the target itself as the only entry in the TargetList
and with ReasonCode (ApplDisconnect). The REFUSE message is sent back to the origin via the previous-hop. If a stream has multiple targets and one target leaves the stream using this REFUSE mechanism, the stream to the other targets is not affected; the stream continues to exist. An ST agent that receives a REFUSE message acknowledges it by sending an ACK to the next-hop. The target is deleted and the LRM is notified so that it adjusts reservations as appropriate. The REFUSE message is also propagated back to the previous-hop ST agent except in the case where the agent is acting as the origin. In this case a NOTIFY may be propagated instead, see Section 4.6.3. When the REFUSE reaches the origin, the origin sends an ACK and notifies the application that the target is no longer part of the stream. 4.6.5 Changing a Stream's FlowSpec The application at the origin may wish to change the FlowSpec of an established stream. Changing the FlowSpec is a critical operation and it may even lead in some cases to the deletion of the affected targets. Possible problems with FlowSpec changes are discussed in Section 5.6. To change the stream's FlowSpec, the application informs the ST agent at the origin of the new FlowSpec and of the list of targets relative to the change. The ST agent at the origin then issues one CHANGE message per next-hop including the new FlowSpec and sends it to the relevant next-hop ST agents. If the G-bit field of the CHANGE message is set (1), the change affects all targets in the stream. The CHANGE message contains a bit called I-bit, see Section 10.4.3. By default, the I-bit is set to zero (0) to indicate that the LRM is expected to try and perform the requested FlowSpec change without risking to tear down the stream. Applications that desire a higher probability of success and are willing to take the risk of breaking the stream can indicate this by setting the I-bit to one (1). Applications that require the requested modification in order to continue operating are expected to set this bit. An intermediate ST agent that receives a CHANGE message first sends an ACK to the previous-hop and then provides the FlowSpec to the LRM. If the LRM can perform the change, the ST agent propagates the CHANGE messages along the established paths.
If the whole process succeeds, the CHANGE messages will eventually reach the targets. Targets respond with an ACCEPT (or REFUSE) message that is propagated back to the origin. In processing the ACCEPT message on the way back to the origin, excess resources may be released by the LRM as described in Section 4.5.6. The REFUSE message must have the ReasonCode (ApplRefused). SCMP also provides a flooding mechanism to change targets that joined the stream without notifying the origin. The special case of target change via flooding is described in Section 5.7. 4.7 Stream Tear Down A stream is usually terminated by the origin when it has no further data to send. A stream is also torn down if the application should terminate abnormally or if certain network failures are encountered. Processing in this case is identical to the previous descriptions except that the ReasonCode (ApplAbort, NetworkFailure, etc.) is different. When all targets have left a stream, the origin notifies the application of that fact, and the application is then responsible for terminating the stream. Note, however, that the application may decide to add targets to the stream instead of terminating it, or may just leave the stream open with no targets in order to permit stream joins.
(page 45 continued on part 3)
