Network Working Group ST2 Working Group
Request for Comments: 1819 L. Delgrossi and L. Berger, Editors
Obsoletes: 1190, IEN 119 August 1995
Internet Stream Protocol Version 2 (ST2)
Protocol Specification - Version ST2+
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
This document is a revision of RFC1190. The charter of this effort
was clarifying, simplifying and removing errors from RFC1190 to
ensure interoperability of implementations.
NOTE WELL: Neither the version of the protocol described in this
document nor the previous version is an Internet Standard or under
consideration for that status.
Since the publication of the original version of the protocol, there
have been significant developments in the state of the art. Readers
should note that standards and technology addressing alternative
approaches to the resource reservation problem are currently under
development within the IETF.
This memo contains a revised specification of the Internet STream
Protocol Version 2 (ST2). ST2 is an experimental resource reservation
protocol intended to provide end-to-end real-time guarantees over an
internet. It allows applications to build multi-destination simplex
data streams with a desired quality of service. The revised version
of ST2 specified in this memo is called ST2+.
This specification is a product of the STream Protocol Working Group
of the Internet Engineering Task Force.
4.5.4 CONNECT Processing by an Intermediate ST agent 374.5.5 CONNECT Processing at the Targets 384.5.6 ACCEPT Processing by an Intermediate ST agent 384.5.7 ACCEPT Processing by the Origin 394.5.8 REFUSE Processing by the Intermediate ST agent 394.5.9 REFUSE Processing by the Origin 394.5.10 Other Functions during Stream Setup 404.6 Modifying an Existing Stream 404.6.1 The Origin Adding New Targets 414.6.2 The Origin Removing a Target 414.6.3 A Target Joining a Stream 4220.127.116.11 Intermediate Agent (Router) as Origin 434.6.4 A Target Deleting Itself 434.6.5 Changing a Stream's FlowSpec 444.7 Stream Tear Down 455 Exceptional Cases 455.1 Long ST Messages 455.1.1 Handling of Long Data Packets 455.1.2 Handling of Long Control Packets 465.2 Timeout Failures 475.2.1 Failure due to ACCEPT Acknowledgment Timeout 475.2.2 Failure due to CHANGE Acknowledgment Timeout 475.2.3 Failure due to CHANGE Response Timeout 485.2.4 Failure due to CONNECT Acknowledgment Timeout 485.2.5 Failure due to CONNECT Response Timeout 485.2.6 Failure due to DISCONNECT Acknowledgment Timeout 485.2.7 Failure due to JOIN Acknowledgment Timeout 485.2.8 Failure due to JOIN Response Timeout 495.2.9 Failure due to JOIN-REJECT Acknowledgment Timeout 495.2.10 Failure due to NOTIFY Acknowledgment Timeout 495.2.11 Failure due to REFUSE Acknowledgment Timeout 495.2.12 Failure due to STATUS Response Timeout 495.3 Setup Failures due to Routing Failures 505.3.1 Path Convergence 505.3.2 Other Cases 515.4 Problems due to Routing Inconsistency 525.5 Problems in Reserving Resources 535.5.1 Mismatched FlowSpecs 535.5.2 Unknown FlowSpec Version 535.5.3 LRM Unable to Process FlowSpec 535.5.4 Insufficient Resources 535.6 Problems Caused by CHANGE Messages 545.7 Unknown Targets in DISCONNECT and CHANGE 55
1.1 What is ST2?
The Internet Stream Protocol, Version 2 (ST2) is an experimental
connection-oriented internetworking protocol that operates at the
same layer as connectionless IP. It has been developed to support the
efficient delivery of data streams to single or multiple destinations
in applications that require guaranteed quality of service. ST2 is
part of the IP protocol family and serves as an adjunct to, not a
replacement for, IP. The main application areas of the protocol are
the real-time transport of multimedia data, e.g., digital audio and
video packet streams, and distributed simulation/gaming, across
ST2 can be used to reserve bandwidth for real-time streams across
network routes. This reservation, together with appropriate network
access and packet scheduling mechanisms in all nodes running the
protocol, guarantees a well-defined Quality of Service (QoS) to ST2
applications. It ensures that real-time packets are delivered within
their deadlines, that is, at the time where they need to be
presented. This facilitates a smooth delivery of data that is
essential for time- critical applications, but can typically not be
provided by best- effort IP communication.
DATA PATH CONTROL PATH
Upper +------------------+ +---------+
Layer | Application data | | Control |
SCMP | | SCMP | |
ST | ST | | | ST | | |
Figure 1: ST2 Data and Control Path
Just like IP, ST2 actually consists of two protocols: ST for the data
transport and SCMP, the Stream Control Message Protocol, for all
control functions. ST is simple and contains only a single PDU format
that is designed for fast and efficient data forwarding in order to
1.2 ST2 and IP
ST2 is designed to coexist with IP on each node. A typical
distributed multimedia application would use both protocols: IP for
the transfer of traditional data and control information, and ST2 for
the transfer of real-time data. Whereas IP typically will be accessed
from TCP or UDP, ST2 will be accessed via new end-to-end real-time
protocols. The position of ST2 with respect to the other protocols of
the Internet family is represented in Figure 2.
Both ST2 and IP apply the same addressing schemes to identify
different hosts. ST2 and IP packets differ in the first four bits,
which contain the internetwork protocol version number: number 5 is
reserved for ST2 (IP itself has version number 4). As a network layer
protocol, like IP, ST2 operates independently of its underlying
subnets. Existing implementations use ARP for address resolution, and
use the same Layer 2 SAPs as IP.
As a special function, ST2 messages can be encapsulated in IP
packets. This is represented in Figure 2 as a link between ST2 and
IP. This link allows ST2 messages to pass through routers which do
not run ST2. Resource management is typically not available for
these IP route segments. IP encapsulation is, therefore, suggested
only for portions of the network which do not constitute a system
In Figure 2, the RTP protocol is shown as an example of transport
layer on top of ST2. Others include the Packet Video Protocol (PVP)
[Cole81], the Network Voice Protocol (NVP) [Cohe81], and others such
as the Heidelberg Transport Protocol (HeiTP) [DHHS92].
1.3 Protocol History
The first version of ST was published in the late 1970's and was used
throughout the 1980's for experimental transmission of voice, video,
and distributed simulation. The experience gained in these
applications led to the development of the revised protocol version
ST2. The revision extends the original protocol to make it more
complete and more applicable to emerging multimedia environments. The
specification of this protocol version is contained in Internet RFC
1190 which was published in October 1990 [RFC1190].
With more and more developments of commercial distributed multimedia
applications underway and with a growing dissatisfaction at the
transmission quality for audio and video over IP in the MBONE,
interest in ST2 has grown over the last years. Companies have
products available incorporating the protocol. The BERKOM MMTS
project of the German PTT [DeAl92] uses ST2 as its core protocol for
the provision of multimedia teleservices such as conferencing and
mailing. In addition, implementations of ST2 for Digital Equipment,
IBM, NeXT, Macintosh, PC, Silicon Graphics, and Sun platforms are
In 1993, the IETF started a new working group on ST2 as part of
ongoing efforts to develop protocols that address resource
reservation issues. The group's mission was to clean up the existing
protocol specification to ensure better interoperability between the
existing and emerging implementations. It was also the goal to
produce an updated experimental protocol specification that reflected
the experiences gained with the existing ST2 implementations and
applications. Which led to the specification of the ST2+ protocol
contained in this document.
1.3.1 RFC1190 ST and ST2+ Major Differences
The protocol changes from RFC1190 were motivated by protocol
simplification and clarification, and codification of extensions in
existing implementations. This section provides a list of major
differences, and is probably of interest only to those who have
knowledge of RFC1190. The major differences between the versions are:
o Elimination of "Hop IDentifiers" or HIDs. HIDs added much complexity
to the protocol and was found to be a major impediment to
interoperability. HIDs have been replaced by globally unique
identifiers called "Stream IDentifiers" or SIDs.
o Elimination of a number of stream options. A number of options were
found to not be used by any implementation, or were thought to add
more complexity than value. These options were removed. Removed
options include: point-to-point, full-duplex, reverse charge, and
o Elimination of the concept of "subset" implementations. RFC1190
permitted subset implementations, to allow for easy implementation
and experimentation. This led to interoperability problems. Agents
implementing the protocol specified in this document, MUST implement
the full protocol. A number of the protocol functions are best-
effort. It is expected that some implementations will make more
effort than others in satisfying particular protocol requests.
o Clarification of the capability of targets to request to join a
steam. RFC1190 can be interpreted to support target requests, but
most implementors did not understand this and did not add support
for this capability. The lack of this capability was found to be a
significant limitation in the ability to scale the number of
participants in a single ST stream. This clarification is based on
work done by IBM Heidelberg.
o Separation of functions between ST and supporting modules. An effort
was made to improve the separation of functions provided by ST and
those provided by other modules. This is reflected in reorganization
of some text and some PDU formats. ST was also made FlowSpec
independent, although it does define a FlowSpec for testing and
o General reorganization and re-write of the specification. This
document has been organized with the goal of improved readability
and clarity. Some sections have been added, and an effort was made
to improve the introduction of concepts.
1.4 Supporting Modules for ST2
ST2 is one piece of a larger mosaic. This section presents the
overall communication architecture and clarifies the role of ST2 with
respect to its supporting modules.
ST2 proposes a two-step communication model. In the first step, the
real-time channels for the subsequent data transfer are built. This
is called stream setup. It includes selecting the routes to the
destinations and reserving the correspondent resources. In the second
step, the data is transmitted over the previously established
streams. This is called data transfer. While stream setup does not
have to be completed in real-time, data transfer has stringent real-
time requirements. The architecture used to describe the ST2
communication model includes:
o a data transfer protocol for the transmission of real-time data
over the established streams,
o a setup protocol to establish real-time streams based on the flow
o a flow specification to express user real-time requirements,
o a routing function to select routes in the Internet,
o a local resource manager to appropriately handle resources involved
in the communication.
This document defines a data protocol (ST), a setup protocol (SCMP),
and a flow specification (ST2+ FlowSpec). It does not define a
routing function and a local resource manager. However, ST2 assumes
Alternative architectures are possible, see [RFC1633] for an example
alternative architecture that could be used when implementing ST2.
1.4.1 Data Transfer Protocol
The data transfer protocol defines the format of the data packets
belonging to the stream. Data packets are delivered to the targets
along the stream paths previously established by the setup protocol.
Data packets are delivered with the quality of service associated
with the stream.
Data packets contain a globally unique stream identifier that
indicates which stream they belong to. The stream identifier is also
known by the setup protocol, which uses it during stream
establishment. The data transfer protocol for ST2, known simply as
ST, is completely defined by this document.
1.4.2 Setup Protocol
The setup protocol is responsible for establishing, maintaining, and
releasing real-time streams. It relies on the routing function to
select the paths from the source to the destinations. At each
host/router on these paths, it presents the flow specification
associated with the stream to the local resource manager. This causes
the resource managers to reserve appropriate resources for the
stream. The setup protocol for ST2 is called Stream Control Message
Protocol, or SCMP, and is completely defined by this document.
1.4.3 Flow Specification
The flow specification is a data structure including the ST2
applications' QoS requirements. At each host/router, it is used by
the local resource manager to appropriately handle resources so that
such requirements are met. Distributing the flow specification to all
resource managers along the communication paths is the task of the
setup protocol. However, the contents of the flow specification are
transparent to the setup protocol, which simply carries the flow
specification. Any operations on the flow specification, including
updating internal fields and comparing flow specifications are
performed by the resource managers.
This document defines a specific flow specification format that
allows for interoperability among ST2 implementations. This flow
specification is intended to support a flow with a single
transmission rate for all destinations in the stream. Implementations
may support more than one flow specification format and the means are
provided to add new formats as they are defined in the future.
However, the flow specification format has to be consistent
throughout the stream, i.e., it is not possible to use different flow
specification formats for different parts of the same stream.
1.4.4 Routing Function
The routing function is an external unicast route generation
capability. It provides the setup protocol with the path to reach
each of the desired destinations. The routing function is called on a
hop-by-hop basis and provides next-hop information. Once a route is
selected by the routing function, it persists for the whole stream
lifetime. The routing function may try to optimize based on the
number of targets, the requested resources, or use of local network
multicast or bandwidth capabilities. Alternatively, the routing
function may even be based on simple connectivity information.
The setup protocol is not necessarily aware of the criteria used by
the routing function to select routes. It works with any routing
function algorithm. The algorithm adopted is a local matter at each
host/router and different hosts/routers may use different algorithms.
The interface between setup protocol and routing function is also a
local matter and therefore it is not specified by this document.
This version of ST does not support source routing. It does support
route recording. It does include provisions that allow identification
of ST capable neighbors. Identification of remote ST hosts/routers is
not specifically addressed.
1.4.5 Local Resource Manager
At each host/router traversed by a stream, the Local Resource Manager
(LRM) is responsible for handling local resources. The LRM knows
which resources are on the system and what capacity they can provide.
o CPUs on end systems and routers to execute the application and
o main memory space for this software (as in all real-time systems,
code should be pinned in main memory, as swapping it out would have
detrimental effects on system performance),
o buffer space to store the data, e.g., communication packets, passing
through the nodes,
o network adapters, and
o transmission networks between the nodes. Networks may be as simple
as point-to-point links or as complex as switched networks such as
Frame Relay and ATM networks.
During stream setup and modification, the LRM is presented by the
setup protocol with the flow specification associated to the stream.
For each resource it handles, the LRM is expected to perform the
o Stream Admission Control: it checks whether, given the flow
specification, there are sufficient resources left to handle the new
data stream. If the available resources are insufficient, the new
data stream must be rejected.
o QoS Computation: it calculates the best possible performance the
resource can provide for the new data stream under the current
traffic conditions, e.g., throughput and delay values are computed.
o Resource Reservation: it reserves the resource capacities required
to meet the desired QoS.
During data transfer, the LRM is responsible for:
o QoS Enforcement: it enforces the QoS requirements by appropriate
scheduling of resource access. For example, data packets from an
application with a short guaranteed delay must be served prior to
data from an application with a less strict delay bound.
The LRM may also provide the following additional functions:
o Data Regulation: to smooth a stream's data traffic, e.g., as with the
leaky bucket algorithm.
o Policing: to prevent applications exceed their negotiated QoS, e.g.,
to send data at a higher rate than indicated in the flow
o Stream Preemption: to free up resources for other streams with
higher priority or importance.
The strategies adopted by the LRMs to handle resources are resource-
dependent and may vary at every host/router. However, it is necessary
that all LRMs have the same understanding of the flow specification.
The interface between setup protocol and LRM is a local matter at
every host and therefore it is not specified by this document. An
example of LRM is the Heidelberg Resource Administration Technique
It is also assumed that the LRM provides functions to compare flow
specifications, i.e., to decide whether a flow specification requires
a greater, equal, or smaller amount of resource capacities to be
Streams form the core concepts of ST2. They are established between a
sending origin and one or more receiving targets in the form of a
routing tree. Streams are uni-directional from the origin to the
targets. Nodes in the tree represent so-called ST agents, entities
executing the ST2 protocol; links in the tree are called hops. Any
node in the middle of the tree is called an intermediate agent, or
router. An agent may have any combination of origin, target, or
Figure 3 illustrates a stream from an origin to four targets, where
the ST agent on Target 2 also functions as an intermediate agent. Let
us use this Target 2/Router node to explain some basic ST2
terminology: the direction of the stream from this node to Target 3
and 4 is called downstream, the direction towards the Origin node
upstream. ST agents that are one hop away from a given node are
called previous-hops in the upstream, and next-hops in the downstream
Streams are maintained using SCMP messages. Typical SCMP messages are
CONNECT and ACCEPT to build a stream, DISCONNECT and REFUSE to close
a stream, CHANGE to modify the quality of service associated with a
stream, and JOIN to request to be added to a stream.
Each ST agent maintains state information describing the streams
flowing through it. It can actively gather and distribute such
information. It can recognize failed neighbor ST agents through the
use of periodic HELLO message exchanges. It can ask other ST agents
about a particular stream via a STATUS message. These ST agents then
send back a STATUS-RESPONSE message. NOTIFY messages can be used to
inform other ST agents of significant events.
ST2 offers a wealth of functionalities for stream management. Streams
can be grouped together to minimize allocated resources or to process
them in the same way in case of failures. During audio conferences,
for example, only a limited set of participants may talk at once.
Using the group mechanism, resources for only a portion of the audio
streams of the group need to be reserved. Using the same concept, an
entire group of related audio and video streams can be dropped if one
of them is preempted.
1.5.2 Data Transmission
Data transfer in ST2 is simplex in the downstream direction. Data
transport through streams is very simple. ST2 puts only a small
header in front of the user data. The header contains a protocol
identification that distinguishes ST2 from IP packets, an ST2 version
number, a priority field (specifying a relative importance of streams
in cases of conflict), a length counter, a stream identification, and
a checksum. These elements form a 12-byte header.
Efficiency is also achieved by avoiding fragmentation and reassembly
on all agents. Stream establishment yields a maximum message size for
data packets on a stream. This maximum message size is communicated
to the upper layers, so that they provide data packets of suitable
size to ST2.
Communication with multiple next-hops can be made even more efficient
using MAC Layer multicast when it is available. If a subnet supports
multicast, a single multicast packet is sufficient to reach all
next-hops connected to this subnet. This leads to a significant
reduction of the bandwidth requirements of a stream. If multicast is
not provided, separate packets need to be sent to each next-hop.
As ST2 relies on reservation, it does not contain error correction
mechanisms features for data exchange such as those found in TCP. It
is assumed that 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. Also, 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 when
1.5.3 Flow Specification
As part of establishing a connection, SCMP handles the negotiation of
quality-of-service parameters for a stream. In ST2 terminology, these
parameters form a flow specification (FlowSpec) which is associated
with the stream. Different versions of FlowSpecs exist, see
[RFC1190], [DHHS92] and [RFC1363], and can be distinguished by a
version number. Typically, they contain parameters such as average
and maximum throughput, end-to-end delay, and delay variance of a
stream. SCMP itself only provides the mechanism for relaying the
Three kinds of entities participate in the quality-of-service
negotiation: application entities on the origin and target sites as
the service users, ST agents, and local resource managers (LRM). The
origin application supplies the initial FlowSpec requesting a
particular service quality. Each ST agent which obtains the FlowSpec
as part of a connection establishment message, it presents the local
resource manager with it. ST2 does not determine how resource
managers make reservations and how resources are scheduled according
to these reservations; ST2, however, assumes these mechanisms as its
An example of the FlowSpec negotiation procedure is illustrated in
Figure 4. Depending on the success of its local reservations, the LRM
updates the FlowSpec fields and returns the FlowSpec to the ST agent,
which passes it downstream as part of the connection message.
Eventually, the FlowSpec is communicated to the application at the
target which may base its accept/reject decision for establishing the
connection on it and may finally also modify the FlowSpec. If a
target accepts the connection, the (possibly modified) FlowSpec is
propagated back to the origin which can then calculate an overall
service quality for all targets. The application entity at the origin
may later request a CHANGE to adjust reservations.
Origin Router Target 1
+------+ 1a +------+ 1b +------+
| |-------------->| |------------->| |
+------+ +------+ +------+
^ | ^ |
| | | 2 |
| | +------------------------------------------+
+-------------+ \ \ +-------------+ +-------------+
|Max Delay: 12| \ \ |Max Delay: 12| |Max Delay: 12|
|-------------| \ \ |-------------| |-------------|
|Min Delay: 2| \ \ |Min Delay: 5| |Min Delay: 9|
|-------------| \ \ |-------------| |-------------|
|Max Size:4096| + + |Max Size:2048| |Max Size:2048|
+-------------+ | | +-------------+ +-------------+
FlowSpec | | 1
2 | +------+
|Max Delay: 12|
|Min Delay: 4|
Figure 4: Quality-of-Service Negotiation with FlowSpecs
1.6 Outline of This Document
This document contains the specification of the ST2+ version of the
ST2 protocol. In the rest of the document, whenever the terms "ST" or
"ST2" are used, they refer to the ST2+ version of ST2.
The document is organized as follows:
o Section 2 describes the ST2 user service from an application point
o Section 3 illustrates the ST2 data transfer protocol, ST.
o Section 4 through Section 8 specify the ST2 setup protocol, SCMP.
o the ST2 flow specification is presented in Section 9.
o the formats of protocol elements and PDUs are defined in Section 10.
2. ST2 User Service Description
This section describes the ST user service from the high-level point
of view of an application. It defines the ST stream operations and
primitive functions. It specifies which operations on streams can be
invoked by the applications built on top of ST and when the ST
primitive functions can be legally executed. Note that the presented
ST primitives do not specify an API. They are used here with the only
purpose of illustrating the service model for ST.
2.1 Stream Operations and Primitive Functions
An ST application at the origin may create, expand, reduce, change,
send data to, and delete a stream. When a stream is expanded, new
targets are added to the stream; when a stream is reduced, some of
the current targets are dropped from it. When a stream is changed,
the associated quality of service is modified.
An ST application at the target may join, receive data from, and
leave a stream. This translates into the following stream operations:
o OPEN: create new stream [origin], CLOSE: delete stream [origin],
o ADD: expand stream, i.e., add new targets to it [origin],
o DROP: reduce stream, i.e., drop targets from it [origin],
o JOIN: join a stream [target], LEAVE: leave a stream [target],
o DATA: send data through stream [origin],
o CHG: change a stream's QoS [origin],
Each stream operation may require the execution of several primitive
functions to be completed. For instance, to open a new stream, a
request is first issued by the sender and an indication is generated
at one or more receivers; then, the receivers may each accept or
refuse the request and the correspondent indications are generated at
the sender. A single receiver case is shown in Figure 5 below.
Sender Network Receiver
| | |
OPEN.req | | |
|-----------------> | |
| |-----------------> |
| | | OPEN.ind
| | | OPEN.accept
| |<----------------- |
|<----------------- | |
OPEN.accept-ind | | |
| | |
Figure 5: Primitives for the OPEN Stream Operation
Table 1 defines the ST service primitive functions associated to each
stream operation. The column labelled "O/T" indicates whether the
primitive is executed at the origin or at the target.
|Primitive | Descriptive |O/T|
|OPEN.req | open a stream | O |
|OPEN.ind | connection request indication | T |
|OPEN.accept | accept stream | T |
|OPEN.refuse | refuse stream | T |
|OPEN.accept-ind| connection accept indication | O |
|OPEN.refuse-ind| connection refuse indication | O |
|ADD.req | add targets to stream | O |
|ADD.ind | add request indication | T |
|ADD.accept | accept stream | T |
|ADD.refuse | refuse stream | T |
|ADD.accept-ind | add accept indication | O |
|ADD.refuse-ind | add refuse indication | O |
|JOIN.req | join a stream | T |
|JOIN.ind | join request indication | O |
|JOIN.reject | reject a join | O |
|JOIN.reject-ind| join reject indication | T |
|DATA.req | send data | O |
|DATA.ind | receive data indication | T |
|CHG.req | change stream QoS | O |
|CHG.ind | change request indication | T |
|CHG.accept | accept change | T |
|CHG.refuse | refuse change | T |
|CHG.accept-ind | change accept indication | O |
|CHG.refuse-ind | change refuse indication | O |
|DROP.req | drop targets | O |
|DROP.ind | disconnect indication | T |
|LEAVE.req | leave stream | T |
|LEAVE.ind | leave stream indication | O |
|CLOSE.req | close stream | O |
|CLOSE.ind | close stream indication | T |
Table 1: ST Primitives
2.2 State Diagrams
It is not sufficient to define the set of ST stream operations. It is
also necessary to specify when the operations can be legally
executed. For this reason, a set of states is now introduced and the
transitions from one state to the others are specified. States are
defined with respect to a single stream. The previously defined
stream operations can be legally executed only from an appropriate
An ST agent may, with respect to an ST stream, be in one of the
o IDLE: the stream has not been created yet.
o PENDING: the stream is in the process of being established.
o ACTIVE: the stream is established and active.
o ADDING: the stream is established. A stream expansion is underway.
o CHGING: the stream is established. A stream change is underway.
Previous experience with ST has lead to limits on stream operations
that can be executed simultaneously. These restrictions are:
1. A single ADD or CHG operation can be processed at one time. If
an ADD or CHG is already underway, further requests are queued
by the ST agent and handled only after the previous operation
has been completed. This also applies to two subsequent
requests of the same kind, e.g., two ADD or two CHG operations.
The second operation is not executed until the first one has
2. Deleting a stream, leaving a stream, or dropping targets from a
stream is possible only after stream establishment has been
completed. A stream is considered to be established when all
the next-hops of the origin have either accepted or refused the
stream. Note that stream refuse is automatically forced after
timeout if no reply comes from a next-hop.
3. An ST agent forwards data only along already established paths
to the targets, see also Section 3.1. A path is considered to
be established when the next-hop on the path has explicitly
accepted the stream. This implies that the target and all other
intermediate ST agents are ready to handle the incoming data
packets. In no cases an ST agent will forward data to a
next-hop ST agent that has not explicitly accepted the stream.
To be sure that all targets receive the data, an application
should send the data only after all paths have been
established, i.e., the stream is established.
4. It is allowed to send data from the CHGING and ADDING states.
While sending data from the CHGING state, the quality of
service to the targets affected by the change should be assumed
to be the more restrictive quality of service. When sending
data from the ADDING state, the targets that receive the data
include at least all the targets that were already part of the
stream at the time the ADD operation was invoked.
The rules introduced above require ST agents to queue incoming
requests when the current state does not allow to process them
immediately. In order to preserve the semantics, ST agents have to
maintain the order of the requests, i.e., implement FIFO queuing.
Exceptionally, the CLOSE request at the origin and the LEAVE request
at the target may be immediately processed: in these cases, the queue
is deleted and it is possible that requests in the queue are not
The following state diagrams define the ST service. Separate diagrams
are presented for the origin and the targets.
The symbol (a/r)* indicates that all targets in the target list have
explicitly accepted or refused the stream, or refuse has been forced
after timeout. If the target list is empty, i.e., it contains no
targets, the (a/r)* condition is immediately satisfied, so the empty
stream is created and state ESTBL is entered.
The separate OPEN and ADD primitives at the target are for conceptual
purposes only. The target is actually unable to distinguish between
an OPEN and an ADD. This is reflected in Figure 7 and Table 3 through
the notation OPEN/ADD.