Network Working Group ST2 Working Group
Request for Comments: 1819 L. Delgrossi and L. Berger, Editors
Obsoletes: 1190, IEN 119 August 1995
Category: Experimental
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.
IESG NOTE
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.
Abstract
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.
Delgrossi & Berger, Editors Experimental [Page 1]
RFC 1819 ST2+ Protocol Specification August 1995
Table of Contents
1 Introduction 6
1.1 What is ST2? 6
1.2 ST2 and IP 8
1.3 Protocol History 8
1.3.1 RFC1190 ST and ST2+ Major Differences 9
1.4 Supporting Modules for ST2 10
1.4.1 Data Transfer Protocol 11
1.4.2 Setup Protocol 11
1.4.3 Flow Specification 11
1.4.4 Routing Function 12
1.4.5 Local Resource Manager 12
1.5 ST2 Basic Concepts 15
1.5.1 Streams 16
1.5.2 Data Transmission 16
1.5.3 Flow Specification 17
1.6 Outline of This Document 19
2 ST2 User Service Description 19
2.1 Stream Operations and Primitive Functions 19
2.2 State Diagrams 21
2.3 State Transition Tables 25
3 The ST2 Data Transfer Protocol 26
3.1 Data Transfer with ST 26
3.2 ST Protocol Functions 27
3.2.1 Stream Identification 27
3.2.2 Packet Discarding based on Data Priority 27
4 SCMP Functional Description 28
4.1 Types of Streams 29
4.1.1 Stream Building 30
4.1.2 Knowledge of Receivers 30
4.2 Control PDUs 31
4.3 SCMP Reliability 32
4.4 Stream Options 33
4.4.1 No Recovery 33
4.4.2 Join Authorization Level 34
4.4.3 Record Route 34
4.4.4 User Data 35
4.5 Stream Setup 35
4.5.1 Information from the Application 35
4.5.2 Initial Setup at the Origin 35
4.5.2.1 Invoking the Routing Function 36
4.5.2.2 Reserving Resources 36
4.5.3 Sending CONNECT Messages 37
4.5.3.1 Empty Target List 37
Delgrossi & Berger, Editors Experimental [Page 2]
RFC 1819 ST2+ Protocol Specification August 1995
4.5.4 CONNECT Processing by an Intermediate ST agent 37
4.5.5 CONNECT Processing at the Targets 38
4.5.6 ACCEPT Processing by an Intermediate ST agent 38
4.5.7 ACCEPT Processing by the Origin 39
4.5.8 REFUSE Processing by the Intermediate ST agent 39
4.5.9 REFUSE Processing by the Origin 39
4.5.10 Other Functions during Stream Setup 40
4.6 Modifying an Existing Stream 40
4.6.1 The Origin Adding New Targets 41
4.6.2 The Origin Removing a Target 41
4.6.3 A Target Joining a Stream 42
4.6.3.1 Intermediate Agent (Router) as Origin 43
4.6.4 A Target Deleting Itself 43
4.6.5 Changing a Stream's FlowSpec 44
4.7 Stream Tear Down 45
5 Exceptional Cases 45
5.1 Long ST Messages 45
5.1.1 Handling of Long Data Packets 45
5.1.2 Handling of Long Control Packets 46
5.2 Timeout Failures 47
5.2.1 Failure due to ACCEPT Acknowledgment Timeout 47
5.2.2 Failure due to CHANGE Acknowledgment Timeout 47
5.2.3 Failure due to CHANGE Response Timeout 48
5.2.4 Failure due to CONNECT Acknowledgment Timeout 48
5.2.5 Failure due to CONNECT Response Timeout 48
5.2.6 Failure due to DISCONNECT Acknowledgment Timeout 48
5.2.7 Failure due to JOIN Acknowledgment Timeout 48
5.2.8 Failure due to JOIN Response Timeout 49
5.2.9 Failure due to JOIN-REJECT Acknowledgment Timeout 49
5.2.10 Failure due to NOTIFY Acknowledgment Timeout 49
5.2.11 Failure due to REFUSE Acknowledgment Timeout 49
5.2.12 Failure due to STATUS Response Timeout 49
5.3 Setup Failures due to Routing Failures 50
5.3.1 Path Convergence 50
5.3.2 Other Cases 51
5.4 Problems due to Routing Inconsistency 52
5.5 Problems in Reserving Resources 53
5.5.1 Mismatched FlowSpecs 53
5.5.2 Unknown FlowSpec Version 53
5.5.3 LRM Unable to Process FlowSpec 53
5.5.4 Insufficient Resources 53
5.6 Problems Caused by CHANGE Messages 54
5.7 Unknown Targets in DISCONNECT and CHANGE 55
Delgrossi & Berger, Editors Experimental [Page 3]
RFC 1819 ST2+ Protocol Specification August 1995
6 Failure Detection and Recovery 55
6.1 Failure Detection 55
6.1.1 Network Failures 56
6.1.2 Detecting ST Agents Failures 56
6.2 Failure Recovery 58
6.2.1 Problems in Stream Recovery 60
6.3 Stream Preemption 62
7 A Group of Streams 63
7.1 Basic Group Relationships 63
7.1.1 Bandwidth Sharing 63
7.1.2 Fate Sharing 64
7.1.3 Route Sharing 65
7.1.4 Subnet Resources Sharing 65
7.2 Relationships Orthogonality 65
8 Ancillary Functions 66
8.1 Stream ID Generation 66
8.2 Group Name Generator 66
8.3 Checksum Computation 67
8.4 Neighbor ST Agent Identification and
Information Collection 67
8.5 Round Trip Time Estimation 68
8.6 Network MTU Discovery 68
8.7 IP Encapsulation of ST 69
8.8 IP Multicasting 70
9 The ST2+ Flow Specification 71
9.1 FlowSpec Version #0 - (Null FlowSpec) 72
9.2 FlowSpec Version #7 - ST2+ FlowSpec 72
9.2.1 QoS Classes 73
9.2.2 Precedence 74
9.2.3 Maximum Data Size 74
9.2.4 Message Rate 74
9.2.5 Delay and Delay Jitter 74
9.2.6 ST2+ FlowSpec Format 75
10 ST2 Protocol Data Units Specification 77
10.1 Data PDU 77
10.1.1 ST Data Packets 78
10.2 Control PDUs 78
10.3 Common SCMP Elements 80
10.3.1 FlowSpec 80
10.3.2 Group 81
10.3.3 MulticastAddress 82
10.3.4 Origin 82
10.3.5 RecordRoute 83
10.3.6 Target and TargetList 84
Delgrossi & Berger, Editors Experimental [Page 4]
RFC 1819 ST2+ Protocol Specification August 1995
10.3.7 UserData 85
10.3.8 Handling of Undefined Parameters 86
10.4 ST Control Message PDUs 86
10.4.1 ACCEPT 86
10.4.2 ACK 88
10.4.3 CHANGE 89
10.4.4 CONNECT 89
10.4.5 DISCONNECT 92
10.4.6 ERROR 93
10.4.7 HELLO 94
10.4.8 JOIN 95
10.4.9 JOIN-REJECT 96
10.4.10 NOTIFY 97
10.4.11 REFUSE 98
10.4.12 STATUS 100
10.4.13 STATUS-RESPONSE 100
10.5 Suggested Protocol Constants 101
10.5.1 SCMP Messages 102
10.5.2 SCMP Parameters 102
10.5.3 ReasonCode 102
10.5.4 Timeouts and Other Constants 104
10.6 Data Notations 105
11 References 106
12 Security Considerations 108
13 Acknowledgments and Authors' Addresses 108
Delgrossi & Berger, Editors Experimental [Page 5]
RFC 1819 ST2+ Protocol Specification August 1995
1. Introduction
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
internets.
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 |
+------------------+ +---------+
| |
| V
| +-------------------+
SCMP | | SCMP | |
| +-------------------+
| |
V V
+-----------------------+ +------------------------+
ST | ST | | | ST | | |
+-----------------------+ +------------------------+
D-bit=1 D-bit=0
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
Delgrossi & Berger, Editors Experimental [Page 6]
RFC 1819 ST2+ Protocol Specification August 1995
achieve low communication delays. SCMP, however, is more complex than
IP's ICMP. As with ICMP and IP, SCMP packets are transferred within
ST packets as shown in Figure 1.
+--------------------+
| Conference Control |
+--------------------+
+-------+ +-------+ |
| Video | | Voice | | +-----+ +------+ +-----+ +-----+ Application
| Appl | | Appl | | | SNMP| |Telnet| | FTP | ... | | Layer
+-------+ +-------+ | +-----+ +------+ +-----+ +-----+
| | | | | | |
V V | | | | | ------------
+-----+ +-----+ | | | | |
| PVP | | NVP | | | | | |
+-----+ +-----+ + | | | |
| \ | \ \ | | | |
| +-----|--+-----+ | | | |
| Appl.|control V V V V V
| ST data | +-----+ +-------+ +-----+
| & control| | UDP | | TCP | ... | RTP | Transport
| | +-----+ +-------+ +-----+ Layer
| /| / | \ / / | / /|
|\ / | +------+--|--\-----+-/--|--- ... -+ / |
| \ / | | | \ / | / |
| \ / | | | \ +----|--- ... -+ | -----------
| \ / | | | \ / | |
| V | | | V | |
| +------+ | | | +------+ | +------+ |
| | SCMP | | | | | ICMP | | | IGMP | | Internet
| +------+ | | | +------+ | +------+ | Layer
| | | | | | | | |
V V V V V V V V V
+-----------------+ +-----------------------------------+
| STream protocol |->| Internet Protocol |
+-----------------+ +-----------------------------------+
| \ / |
| \ / |
| X | ------------
| / \ |
| / \ |
VV VV
+----------------+ +----------------+
| (Sub-) Network |...| (Sub-) Network | (Sub-)Network
| Protocol | | Protocol | Layer
+----------------+ +----------------+
Figure 2. Protocol Relationships
Delgrossi & Berger, Editors Experimental [Page 7]
RFC 1819 ST2+ Protocol Specification August 1995
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
bottleneck.
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
Delgrossi & Berger, Editors Experimental [Page 8]
RFC 1819 ST2+ Protocol Specification August 1995
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
available.
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
source route.
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
Delgrossi & Berger, Editors Experimental [Page 9]
RFC 1819 ST2+ Protocol Specification August 1995
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
interoperability purposes.
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
specification,
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
their existence.
Delgrossi & Berger, Editors Experimental [Page 10]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 11]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Resources include:
o CPUs on end systems and routers to execute the application and
protocol software,
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
Delgrossi & Berger, Editors Experimental [Page 12]
RFC 1819 ST2+ Protocol Specification August 1995
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
following functions:
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
specification.
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
(HeiRAT) [VoHN93].
Delgrossi & Berger, Editors Experimental [Page 13]
RFC 1819 ST2+ Protocol Specification August 1995
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
reserved.
Delgrossi & Berger, Editors Experimental [Page 14]
RFC 1819 ST2+ Protocol Specification August 1995
1.5 ST2 Basic Concepts
The following sections present at an introductory level some of the
fundamental ST2 concepts including streams, data transfer, and flow
specification.
Hosts Connections... : ...and Streams
==================== : ==============
data Origin : Origin
packets +-----------+ : +----+
+----|Application| : | |
| |-----------| : +----+
+--->| ST Agent | : | |
+-----------+ : | |
| : | |
V : | |
+-------------+ : | |
| | : | |
+-------------| Network A | : +-------+ +--+
| | | : | |
| +-------------+ : | Target 2|
| | Target 2 : | & Router|
| Target 1 | and Router : | |
| +-----------+ | +-----------+ : V V
| |Application|<-+ | |Application|<-+ : +----+ +----+
| |-----------| | | |-----------| | : | | | |
+->| ST Agent |--+ +->| ST Agent |--+ : +----+ +----+
+-----------+ +-----------+ :Target 1 | |
| : | |
V : | |
+-------------+ : | |
| | : | |
+-------------| Network B | : +-----+ |
| | | : | |
| +-------------+ : | |
| Target 3 | Target 4 : | |
| +-----------+ | +-----------+ : V V
| |Application|<-+ | |Application|<-+ : +----+ +----+
| |-----------| | | |-----------| | : | | | |
+->| ST Agent |--+ +->| ST Agent |--+ : +----+ +----+
+-----------+ +-----------+ : Target 3 Target 4
:
Figure 3: The Stream Concept
Delgrossi & Berger, Editors Experimental [Page 15]
RFC 1819 ST2+ Protocol Specification August 1995
1.5.1 Streams
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
intermediate capabilities.
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
direction.
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
Delgrossi & Berger, Editors Experimental [Page 16]
RFC 1819 ST2+ Protocol Specification August 1995
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
needed.
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
quality-of-service parameters.
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
Delgrossi & Berger, Editors Experimental [Page 17]
RFC 1819 ST2+ Protocol Specification August 1995
basis.
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
| +---------------+
| |
| V
2 | +------+
+---------------| |
+------+
Target 2
+-------------+
|Max Delay: 12|
|-------------|
|Min Delay: 4|
|-------------|
|Max Size:4096|
+-------------+
Figure 4: Quality-of-Service Negotiation with FlowSpecs
Delgrossi & Berger, Editors Experimental [Page 18]
RFC 1819 ST2+ Protocol Specification August 1995
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
of view.
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],
Delgrossi & Berger, Editors Experimental [Page 19]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 20]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 21]
RFC 1819 ST2+ Protocol Specification August 1995
stream operations can be legally executed only from an appropriate
state.
An ST agent may, with respect to an ST stream, be in one of the
following states:
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
been completed.
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.
Delgrossi & Berger, Editors Experimental [Page 22]
RFC 1819 ST2+ Protocol Specification August 1995
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
processed.
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.
Delgrossi & Berger, Editors Experimental [Page 23]
RFC 1819 ST2+ Protocol Specification August 1995
+------------+
| |<-------------------+
+---------->| IDLE |-------------+ |
| | | OPEN.req | |
| +------------+ | |
CLOSE.req | CLOSE.req ^ ^ CLOSE.req V | CLOSE.req
| | | +---------+ |
| | | | PENDING |-|-+ JOIN.reject
| | -------------| |<|-+
| JOIN.reject | +---------+ |
| DROP.req +----------+ | |
| +-----| | | |
| | | ESTDL | OPEN.(a/r)* | |
| +---->| |<------------+ |
| +----------+ |
| | ^ | ^ |
| | | | | |
+----------+ CHG.req| | | | Add.(a/r)* +----------+
| |<-------+ | | +-------------- | |
| CHGING | | | | ADDING |
| |-----------+ +----------------->| |
+----------+ CHG.(a/r)* JOIN.ind +----------+
| ^ ADD.req | ^
| | | |
+---+ +---+
DROP.req DROP.req
JOIN.reject JOIN.reject
Figure 6: ST Service at the Origin
+--------+
| |-----------------------+
| IDLE | |
| |<---+ | OPEN/ADD.ind
+--------+ | CLOSE.ind | JOIN.req
^ | OPEN/ADD.refuse |
| | JOIN.refect-ind |
CLOSE.ind | | V
DROP.ind | | +---------+
LEAVE.req | +-------------| |
| | PENDING |
+-------+ | |
| | +---------+
| ESTBL | OPEN/ADD.accept |
| |<-----------------------+
+-------+
Figure 7: ST Service at the Target
Delgrossi & Berger, Editors Experimental [Page 24]
RFC 1819 ST2+ Protocol Specification August 1995
2.3 State Transition Tables
Table 2 and Table 3 define which primitives can be processed from
which states and the possible state transitions.
+======================================================================+
|Primitive |IDLE| PENDING | ESTBL | CHGING | ADDING |
|======================================================================|
|OPEN.req | ok | - | - | - | - |
|OPEN.accept-ind| - |if(a,r)*->ESTBL| - | - | - |
|OPEN.refuse-ind| - |if(a,r)*->ESTBL| - | - | - |
|ADD.req | - | queued |->ADDING| queued | queued |
|ADD.accept-ind | - | - | - | - |if(a,r)* |
| | - | - | - | - |->ESTBL |
|ADD.refuse-ind | - | - | - | - |if(a,r)* |
| | - | - | - | - |->ESTBL |
|JOIN.ind | - | queued |->ADDING| queued |queued |
|JOIN.reject | - | OK | ok | ok | ok |
|DATA.req | - | - | ok | ok | ok |
|CHG.req | - | queued |->CHGING| queued |queued |
|CHG.accept-ind | - | - | - |if(a,r)* | - |
| | - | - | - |->ESTBL | - |
|CHG.refuse.ind | - | - | - |if(a,r)* | - |
| | - | - | - |->ESTBL | - |
|DROP.req | - | - | ok | ok | ok |
|LEAVE.ind | - | OK | ok | ok | ok |
|CLOSE.req | - | OK | ok | ok | ok |
+----------------------------------------------------------------------+
Table 2: Primitives and States at the Origin
+======================================================+
| Primitive | IDLE | PENDING | ESTBL |
|======================================================|
| OPEN/ADD.ind | ->PENDING | - | - |
| OPEN/ADD.accept | - | ->ESTBL | - |
| OPEN/ADD.refuse | - | ->IDLE | - |
| JOIN.req | ->PENDING | - | - |
| JOIN.reject-ind |- | ->IDLE | - |
| DATA.ind | - | - | ok |
| CHG.ind | - | - | ok |
| CHG.accept | - | - | ok |
| DROP.ind | - | ok | ok |
| LEAVE.req | - | ok | ok |
| CLOSE.ind | - | ok | ok |
| CHG.ind | - | - | ok |
+------------------------------------------------------+
Table 3: Primitives and States at the Target
Delgrossi & Berger, Editors Experimental [Page 25]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 26]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 27]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 28]
RFC 1819 ST2+ Protocol Specification August 1995
+---------+ +---+
| |----| 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.
Delgrossi & Berger, Editors Experimental [Page 29]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 30]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 31]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 32]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 33]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 34]
RFC 1819 ST2+ Protocol Specification August 1995
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),
Delgrossi & Berger, Editors Experimental [Page 35]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 36]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 37]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 38]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 39]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 40]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 41]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 42]
RFC 1819 ST2+ Protocol Specification August 1995
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
Delgrossi & Berger, Editors Experimental [Page 43]
RFC 1819 ST2+ Protocol Specification August 1995
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.
Delgrossi & Berger, Editors Experimental [Page 44]
RFC 1819 ST2+ Protocol Specification August 1995
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.
5. Exceptional Cases
The previous descriptions covered the simple cases where everything
worked. We now discuss what happens when things do not succeed.
Included are situations where messages exceed a network MTU, are
lost, the requested resources are not available, the routing fails or
is inconsistent.
5.1 Long ST Messages
It is possible that an ST agent, or an application, will need to send
a message that exceeds a network's Maximum Transmission Unit (MTU).
This case must be handled but not via generic fragmentation, since
ST2 does not support generic fragmentation of either data or control
messages.
5.1.1 Handling of Long Data Packets
ST agents discard data packets that exceed the MTU of the next-hop
network. No error message is generated. Applications should avoid
sending data packets larger than the minimum MTU supported by a given
Delgrossi & Berger, Editors Experimental [Page 45]
RFC 1819 ST2+ Protocol Specification August 1995
stream. The application, both at the origin and targets, can learn
the stream minimum MTU through the MTU discovery mechanism described
in Section 8.6.
5.1.2 Handling of Long Control Packets
Each ST agent knows the MTU of the networks to which it is connected,
and those MTUs restrict the size of the SCMP message it can send. An
SCMP message size can exceed the MTU of a given network for a number
of reasons:
o the TargetList parameter (Section 10.3.6) may be too long;
o the RecordRoute parameter (Section 10.3.5) may be too long.
o the UserData parameter (Section 10.3.7) may be too long;
o the PDUInError field of the ERROR message (Section 10.4.6) may be
too long;
An ST agent receiving or generating a too long SCMP message should:
o break the message into multiple messages, each carrying part of the
TargetList. Any RecordRoute and UserData parameters are replicated
in each message for delivery to all targets. Applications that
support a large number of targets may avoid using long TargetList
parameters, and are expected to do so, by exploiting the stream
joining functions, see Section 4.6.3. One exception to this rule
exists. In the case of a long TargetList parameter to be included in
a STATUS-RESPONSE message, the TargetList parameter is just
truncated to the point where the list can fit in a single message,
see Section 8.4.
o for down stream agents: if the TargetList parameter contains a
single Target element and the message size is still too long, the ST
agent should issue a REFUSE message with ReasonCode
(RecordRouteSize) if the size of the RecordRoute parameter causes
the SCMP message size to exceed the network MTU, or with ReasonCode
(UserDataSize) if the size of the UserData parameter causes the SCMP
message size to exceed the network MTU. If both RecordRoute and
UserData parameters are present the ReasonCode (UserDataSize) should
be sent. For messages generated at the target: the target ST agent
must check for SCMP messages that may exceed the MTU on the complete
target-to-origin path, and inform the application that a too long
SCMP messages has been generated. The format for the error reporting
is a local implementation issue. The error codes are the same as
previously stated.
Delgrossi & Berger, Editors Experimental [Page 46]
RFC 1819 ST2+ Protocol Specification August 1995
ST agents generating too long ERROR messages, simply truncate the
PDUInError field to the point where the message is smaller than the
network MTU.
5.2 Timeout Failures
As described in Section 4.3, SCMP message delivery is made reliable
through the use of acknowledgments, timeouts, and retransmission. The
ACCEPT, CHANGE, CONNECT, DISCONNECT, JOIN, JOIN-REJECT, NOTIFY, and
REFUSE messages must always be acknowledged, see Section 4.2. In
addition, for some SCMP messages (CHANGE, CONNECT, JOIN) the sending
ST agent also expects a response back (ACCEPT/REFUSE, CONNECT/JOIN-
REJECT) after an ACK has been received. Also, the STATUS message must
be answered with a STATUS-RESPONSE message.
The following sections describe the handling of each of the possible
failure cases due to timeout situations while waiting for an
acknowledgment or a response. The timeout related variables, and
their names, used in the next sections are for reference purposes
only. They may be implementation specific. Different implementations
are not required to share variable names, or even the mechanism by
which the timeout and retransmission behavior is implemented.
5.2.1 Failure due to ACCEPT Acknowledgment Timeout
An ST agent that sends an ACCEPT message upstream expects an ACK from
the previous-hop ST agent. If no ACK is received before the ToAccept
timeout expires, the ST agent should retry and send the ACCEPT
message again. After NAccept unsuccessful retries, the ST agent sends
a REFUSE message toward the origin, and a DISCONNECT message toward
the targets. Both REFUSE and DISCONNECT must identify the affected
targets and specify the ReasonCode (RetransTimeout).
5.2.2 Failure due to CHANGE Acknowledgment Timeout
An ST agent that sends a CHANGE message downstream expects an ACK
from the next-hop ST agent. If no ACK is received before the ToChange
timeout expires, the ST agent should retry and send the CHANGE
message again. After NChange unsuccessful retries, the ST agent
aborts the change attempt by sending a REFUSE message toward the
origin, and a DISCONNECT message toward the targets. Both REFUSE and
DISCONNECT must identify the affected targets and specify the
ReasonCode (RetransTimeout).
Delgrossi & Berger, Editors Experimental [Page 47]
RFC 1819 ST2+ Protocol Specification August 1995
5.2.3 Failure due to CHANGE Response Timeout
Only the origin ST agent implements this timeout. After correctly
receiving the ACK to a CHANGE message, an ST agent expects to receive
an ACCEPT, or REFUSE message in response. If one of these messages is
not received before the ToChangeResp timer expires, the ST agent at
the origin aborts the change attempt, and behaves as if a REFUSE
message with the E-bit set and with ReasonCode (ResponseTimeout) is
received.
5.2.4 Failure due to CONNECT Acknowledgment Timeout
An ST agent that sends a CONNECT message downstream expects an ACK
from the next-hop ST agent. If no ACK is received before the
ToConnect timeout expires, the ST agent should retry and send the
CONNECT message again. After NConnect unsuccessful retries, the ST
agent sends a REFUSE message toward the origin, and a DISCONNECT
message toward the targets. Both REFUSE and DISCONNECT must identify
the affected targets and specify the ReasonCode (RetransTimeout).
5.2.5 Failure due to CONNECT Response Timeout
Only the origin ST agent implements this timeout. After correctly
receiving the ACK to a CONNECT message, an ST agent expects to
receive an ACCEPT or REFUSE message in response. If one of these
messages is not received before the ToConnectResp timer expires, the
origin ST agent aborts the connection setup attempt, acts as if a
REFUSE message is received, and it sends a DISCONNECT message toward
the targets. Both REFUSE and DISCONNECT must identify the affected
targets and specify the ReasonCode (ResponseTimeout).
5.2.6 Failure due to DISCONNECT Acknowledgment Timeout
An ST agent that sends a DISCONNECT message downstream expects an ACK
from the next-hop ST agent. If no ACK is received before the
ToDisconnect timeout expires, the ST agent should retry and send the
DISCONNECT message again. After NDisconnect unsuccessful retries, the
ST agent simply gives up and it assumes the next-hop ST agent is not
part in the stream any more.
5.2.7 Failure due to JOIN Acknowledgment Timeout
An ST agent that sends a JOIN message toward the origin expects an
ACK from a neighbor ST agent. If no ACK is received before the ToJoin
timeout expires, the ST agent should retry and send the JOIN message
again. After NJoin unsuccessful retries, the ST agent sends a JOIN-
REJECT message back in the direction of the target with ReasonCode
(RetransTimeout).
Delgrossi & Berger, Editors Experimental [Page 48]
RFC 1819 ST2+ Protocol Specification August 1995
5.2.8 Failure due to JOIN Response Timeout
Only the target agent implements this timeout. After correctly
receiving the ACK to a JOIN message, the ST agent at the target
expects to receive a CONNECT or JOIN-REJECT message in response. If
one of these message is not received before the ToJoinResp timer
expires, the ST agent aborts the stream join attempt and returns an
error corresponding with ReasonCode (RetransTimeout) to the
application.
Note that, after correctly receiving the ACK to a JOIN message,
intermediate ST agents do not maintain any state on the stream
joining attempt. As a consequence, they do not set the ToJoinResp
timer and do not wait for a CONNECT or JOIN-REJECT message. This is
described in Section 4.6.3.
5.2.9 Failure due to JOIN-REJECT Acknowledgment Timeout
An ST agent that sends a JOIN-REJECT message toward the target
expects an ACK from a neighbor ST agent. If no ACK is received before
the ToJoinReject timeout expires, the ST agent should retry and send
the JOIN-REJECT message again. After NJoinReject unsuccessful
retries, the ST agent simply gives up.
5.2.10 Failure due to NOTIFY Acknowledgment Timeout
An ST agent that sends a NOTIFY message to a neighbor ST agent
expects an ACK from that neighbor ST agent. If no ACK is received
before the ToNotify timeout expires, the ST agent should retry and
send the NOTIFY message again. After NNotify unsuccessful retries,
the ST agent simply gives up and behaves as if the ACK message was
received.
5.2.11 Failure due to REFUSE Acknowledgment Timeout
An ST agent that sends a REFUSE message upstream expects an ACK from
the previous-hop ST agent. If no ACK is received before the ToRefuse
timeout expires, the ST agent should retry and send the REFUSE
message again. After NRefuse unsuccessful retries, the ST agent gives
up and it assumes it is not part in the stream any more.
5.2.12 Failure due to STATUS Response Timeout
After sending a STATUS message to a neighbor ST agent, an ST agent
expects to receive a STATUS-RESPONSE message in response. If this
message is not received before the ToStatusResp timer expires, the ST
agent sends the STATUS message again. After NStatus unsuccessful
retries, the ST agent gives up and assumes that the neighbor ST agent
Delgrossi & Berger, Editors Experimental [Page 49]
RFC 1819 ST2+ Protocol Specification August 1995
is not active.
5.3 Setup Failures due to Routing Failures
It is possible for an ST agent to receive a CONNECT message that
contains a known SID, but from an ST agent other than the previous-
hop ST agent of the stream with that SID. This may be:
1. that two branches of the tree forming the stream have joined
back together,
2. the result of an attempted recovery of a partially failed
stream, or
3. a routing loop.
The TargetList contained in the CONNECT is used to distinguish the
different cases by comparing each newly received target with those of
the previously existing stream:
o if the IP address of the target(s) differ, it is case #1;
o if the target matches a target in the existing stream, it may be
case #2 or #3.
Case #1 is handled in Section 5.3.1, while the other cases are
handled in Section 5.3.2.
5.3.1 Path Convergence
It is possible for an ST agent to receive a CONNECT message that
contains a known SID, but from an ST agent other than the previous-
hop ST agent of the stream with that SID. This might be the result of
two branches of the tree forming the stream have joined back
together. Detection of this case and other possible sources was
discussed in Section 5.2.
SCMP does not allow for streams which have converged paths, i.e.,
streams are always tree-shaped and not graph-like. At the point of
convergence, the ST agent which detects the condition generates a
REFUSE message with ReasonCode (PathConvergence). Also, as a help to
the upstream ST agent, the detecting agent places the IP address of
one of the stream's connected targets in the ValidTargetIPAddress
field of the REFUSE message. This IP address will be used by upstream
ST agents to avoid splitting the stream.
An upstream ST agent that receives the REFUSE with ReasonCode
(PathConvergence) will check to see if the listed IP address is one
Delgrossi & Berger, Editors Experimental [Page 50]
RFC 1819 ST2+ Protocol Specification August 1995
of the known stream targets. If it is not, the REFUSE is propagated
to the previous-hop agent. If the listed IP address is known by the
upstream ST agent, this ST agent is the ST agent that caused the
split in the stream. (This agent may even be the origin.) This agent
then avoids splitting the stream by using the next-hop of that known
target as the next-hop for the refused targets. It sends a CONNECT
with the affected targets to the existing valid next-hop.
The above process will proceed, hop by hop, until the
ValidTargetIPAddress matches the IP address of a known target. The
only case where this process will fail is when the known target is
deleted prior to the REFUSE propagating to the origin. In this case
the origin can just reissue the CONNECT and start the whole process
over again.
5.3.2 Other Cases
The remaining cases including a partially failed stream and a routing
loop, are not easily distinguishable. In attempting recovery of a
failed stream, an ST agent may issue new CONNECT messages to the
affected targets. Such a CONNECT may reach an ST agent downstream of
the failure before that ST agent has received a DISCONNECT from the
neighborhood of the failure. Until that ST agent receives the
DISCONNECT, it cannot distinguish between a failure recovery and an
erroneous routing loop. That ST agent must therefore respond to the
CONNECT with a REFUSE message with the affected targets specified in
the TargetList and an appropriate ReasonCode (StreamExists).
The ST agent immediately preceding that point, i.e., the latest ST
agent to send the CONNECT message, will receive the REFUSE message.
It must release any resources reserved exclusively for traffic to the
listed targets. If this ST agent was not the one attempting the
stream recovery, then it cannot distinguish between a failure
recovery and an erroneous routing loop. It should repeat the CONNECT
after a ToConnect timeout, see Section 5.2.4. If after NConnect
retransmissions it continues to receive REFUSE messages, it should
propagate the REFUSE message toward the origin, with the TargetList
that specifies the affected targets, but with a different ReasonCode
(RouteLoop).
The REFUSE message with this ReasonCode (RouteLoop) is propagated by
each ST agent without retransmitting any CONNECT messages. At each ST
agent, it causes any resources reserved exclusively for the listed
targets to be released. The REFUSE will be pr |
