RFC 1190 (rfc1190)
Experimental Internet Stream Protocol, Version 2 (ST-II)
Network Working Group CIP Working Group
Request for Comments: 1190 C. Topolcic, Editor
Obsoletes: IEN-119 October 1990
Experimental Internet Stream Protocol, Version 2 (ST-II)
Status Of This Memo 
top
This memo defines a revised version of the Internet Stream Protocol,
originally defined in IEN-119 [8], based on results from experiments
with the original version, and subsequent requests, discussion, and
suggestions for improvements. This is a Limited-Use Experimental
Protocol. Please refer to the current edition of the "IAB Official
Protocol Standards" for the standardization state and status of this
protocol. Distribution of this memo is unlimited.
1. Abstract top
This memo defines the Internet Stream Protocol, Version 2 (ST-II), an
IP-layer protocol that provides end-to-end guaranteed service across
an internet. This specification obsoletes IEN 119 "ST - A Proposed
Internet Stream Protocol" written by Jim Forgie in 1979, the previous
specification of ST. ST-II is not compatible with Version 1 of the
protocol, but maintains much of the architecture and philosophy of
that version. It is intended to fill in some of the areas left
unaddressed, to make it easier to implement, and to support a wider
range of applications.
1.1. Table Of Contents top
Status of this Memo . . . . . . . . . . . . 1
1. Abstract . . . . . . . . . . . . . . . 1
1.1. Table of Contents . . . . . . . . . . . 2
1.2. List of Figures . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . 7
2.1. Major Differences Between ST and ST-II . . . . 8
2.2. Concepts and Terminology . . . . . . . . . 9
2.3. Relationship Between Applications and ST . . . . 11
2.4. ST Control Message Protocol . . . . . . . . 12
2.5. Flow Specifications . . . . . . . . . . . 14
3. ST Control Message Protocol Functional Description . 17
3.1. Stream Setup . . . . . . . . . . . . . 18
3.1.1. Initial Setup at the Origin . . . . . . . 18
3.1.2. Invoking the Routing Function . . . . . . 19
3.1.3. Reserving Resources . . . . . . . . . . 19
3.1.4. Sending CONNECT Messages . . . . . . . . 20
3.1.5. CONNECT Processing by an Intermediate Agent . . 22
3.1.6. Setup at the Targets . . . . . . . . . 23
3.1.7. ACCEPT Processing by an Intermediate Agent . . 24
3.1.8. ACCEPT Processing by the Origin . . . . . . 26
3.1.9. Processing a REFUSE Message . . . . . . . 27
3.2. Data Transfer . . . . . . . . . . . . . 30
3.3. Modifying an Existing Stream . . . . . . . . 31
3.3.1. Adding a Target . . . . . . . . . . . 31
3.3.2. The Origin Removing a Target . . . . . . . 33
3.3.3. A Target Deleting Itself . . . . . . . . 35
3.3.4. Changing the FlowSpec . . . . . . . . . 36
3.4. Stream Tear Down . . . . . . . . . . . . 36
3.5. Exceptional Cases . . . . . . . . . . . 37
3.5.1. Setup Failure due to CONNECT Timeout . . . . 37
3.5.2. Problems due to Routing Inconsistency . . . . 38
3.5.3. Setup Failure due to a Routing Failure . . . 39
3.5.4. Problems in Reserving Resources . . . . . . 41
3.5.5. Setup Failure due to ACCEPT Timeout . . . . 41
3.5.6. Problems Caused by CHANGE Messages . . . . . 42
3.5.7. Notification of Changes Forced by Failures . . 42
3.6. Options . . . . . . . . . . . . . . . 44
3.6.1. HID Field Option . . . . . . . . . . . 44
3.6.2. PTP Option . . . . . . . . . . . . . 44
3.6.3. FDx Option . . . . . . . . . . . . . 45
3.6.4. NoRecovery Option . . . . . . . . . . 46
3.6.5. RevChrg Option . . . . . . . . . . . 46
3.6.6. Source Route Option . . . . . . . . . . 46
3.7. Ancillary Functions . . . . . . . . . . . 48
3.7.1. Failure Detection . . . . . . . . . . 48
3.7.1.1. Network Failures . . . . . . . . . . 48
3.7.1.2. Detecting ST Stream Failures . . . . . . 49
3.7.1.3. Subset . . . . . . . . . . . . . 51
3.7.2. Failure Recovery . . . . . . . . . . . 51
3.7.2.1. Subset . . . . . . . . . . . . . 55
3.7.3. A Group of Streams . . . . . . . . . . 56
3.7.3.1. Group Name Generator . . . . . . . . 57
3.7.3.2. Subset . . . . . . . . . . . . . 57
3.7.4. HID Negotiation . . . . . . . . . . . 58
3.7.4.1. Subset . . . . . . . . . . . . . 64
3.7.5. IP Encapsulation of ST . . . . . . . . . 64
3.7.5.1. IP Multicasting . . . . . . . . . . 65
3.7.6. Retransmission . . . . . . . . . . . 66
3.7.7. Routing . . . . . . . . . . . . . . 67
3.7.8. Security . . . . . . . . . . . . . 67
3.8. ST Service Interfaces . . . . . . . . . . 68
3.8.1. Access to Routing Information . . . . . . 69
3.8.2. Access to Network Layer Resource Reservation . 70
3.8.3. Network Layer Services Utilized . . . . . . 71
3.8.4. IP Services Utilized . . . . . . . . . 71
3.8.5. ST Layer Services Provided . . . . . . . 72
4. ST Protocol Data Unit Descriptions . . . . . . . 75
4.1. Data Packets . . . . . . . . . . . . . 76
4.2. ST Control Message Protocol Descriptions . . . . 77
4.2.1. ST Control Messages . . . . . . . . . . 79
4.2.2. Common SCMP Elements . . . . . . . . . 80
4.2.2.1. DetectorIPAddress . . . . . . . . . 80
4.2.2.2. ErroredPDU . . . . . . . . . . . . 80
4.2.2.3. FlowSpec & RFlowSpec . . . . . . . . 81
4.2.2.4. FreeHIDs . . . . . . . . . . . . 84
4.2.2.5. Group & RGroup . . . . . . . . . . 85
4.2.2.6. HID & RHID . . . . . . . . . . . . 86
4.2.2.7. MulticastAddress . . . . . . . . . . 86
4.2.2.8. Name & RName . . . . . . . . . . . 87
4.2.2.9. NextHopIPAddress . . . . . . . . . . 88
4.2.2.10. Origin . . . . . . . . . . . . . 88
4.2.2.11. OriginTimestamp . . . . . . . . . . 89
4.2.2.12. ReasonCode . . . . . . . . . . . . 89
4.2.2.13. RecordRoute . . . . . . . . . . . 94
4.2.2.14. SrcRoute . . . . . . . . . . . . 95
4.2.2.15. Target and TargetList . . . . . . . . 96
4.2.2.16. UserData . . . . . . . . . . . . 98
4.2.3. ST Control Message PDUs . . . . . . . . 99
4.2.3.1. ACCEPT . . . . . . . . . . . . . 100
4.2.3.2. ACK . . . . . . . . . . . . . . 102
4.2.3.3. CHANGE-REQUEST . . . . . . . . . . 103
4.2.3.4. CHANGE . . . . . . . . . . . . . 104
4.2.3.5. CONNECT . . . . . . . . . . . . . 105
4.2.3.6. DISCONNECT . . . . . . . . . . . . 110
4.2.3.7. ERROR-IN-REQUEST . . . . . . . . . . 111
4.2.3.8. ERROR-IN-RESPONSE . . . . . . . . . 112
4.2.3.9. HELLO . . . . . . . . . . . . . 113
4.2.3.10. HID-APPROVE . . . . . . . . . . . 114
4.2.3.11. HID-CHANGE-REQUEST . . . . . . . . . 115
4.2.3.12. HID-CHANGE . . . . . . . . . . . . 116
4.2.3.13. HID-REJECT . . . . . . . . . . . . 118
4.2.3.14. NOTIFY . . . . . . . . . . . . . 120
4.2.3.15. REFUSE . . . . . . . . . . . . . 122
4.2.3.16. STATUS . . . . . . . . . . . . . 124
4.2.3.17. STATUS-RESPONSE . . . . . . . . . . 126
4.3. Suggested Protocol Constants . . . . . . . . 127
5. Areas Not Addressed . . . . . . . . . . . . 131
6. Glossary . . . . . . . . . . . . . . . 135
7. References . . . . . . . . . . . . . . . 143
8. Security Considerations. . . . . . . . . . . 144
9. Authors' Addresses . . . . . . . . . . . . 145
Appendix 1. Data Notations . . . . . . . . . . 147
1.2. List Of Figures top
Figure 1. Protocol Relationships . . . . . . . . . 6
Figure 2. Topology Used in Protocol Exchange Diagrams . . 16
Figure 3. Virtual Link Identifiers for SCMP Messages . . 16
Figure 4. HIDs Assigned for ST User Packets . . . . . 18
Figure 5. Origin Sending CONNECT Message . . . . . . 21
Figure 6. CONNECT Processing by an Intermediate Agent . . 22
Figure 7. CONNECT Processing by the Target . . . . . . 24
Figure 8. ACCEPT Processing by an Intermediate Agent . . 25
Figure 9. ACCEPT Processing by the Origin . . . . . . 26
Figure 10. Sending REFUSE Message . . . . . . . . . 28
Figure 11. Routing Around a Failure . . . . . . . . 29
Figure 12. Addition of Another Target . . . . . . . . 32
Figure 13. Origin Removing a Target . . . . . . . . 34
Figure 14. Target Deleting Itself . . . . . . . . . 35
Figure 15. CONNECT Retransmission after a Timeout . . . . 38
Figure 16. Processing NOTIFY Messages . . . . . . . . 43
Figure 17. Source Routing Option . . . . . . . . . 47
Figure 18. Typical HID Negotiation (No Multicasting) . . . 60
Figure 19. Multicast HID Negotiation . . . . . . . . 61
Figure 20. Multicast HID Re-Negotiation . . . . 62
Figure 21. ST Header . . . . . . . . . . . . . 75
Figure 22. ST Control Message Format . . . . . . . . 77
Figure 23. ErroredPDU . . . . . . . . . . . . . 80
Figure 24. FlowSpec & RFlowSpec . . . . . . . . . . 81
Figure 25. FreeHIDs . . . . . . . . . . . . . . 85
Figure 26. Group & RGroup . . . . . . . . . . . . 85
Figure 27. HID & RHID . . . . . . . . . . . . . 86
Figure 28. MulticastAddress . . . . . . . . . . . 86
Figure 29. Name & RName . . . . . . . . . . . . 87
Figure 30. NextHopIPAddress . . . . . . . . . . . 88
Figure 31. Origin . . . . . . . . . . . . . . 88
Figure 32. OriginTimestamp . . . . . . . . . . . 89
Figure 33. ReasonCode . . . . . . . . . . . . . 89
Figure 34. RecordRoute . . . . . . . . . . . . . 94
Figure 35. SrcRoute . . . . . . . . . . . . . . 95
Figure 36. Target . . . . . . . . . . . . . . 97
Figure 37. TargetList . . . . . . . . . . . . . 97
Figure 38. UserData . . . . . . . . . . . . . . 98
Figure 39. ACCEPT Control Message . . . . . . . . . 101
Figure 40. ACK Control Message . . . . . . . . . . 102
Figure 41. CHANGE-REQUEST Control Message . . . . . . 103
Figure 42. CHANGE Control Message . . . . . . . . . 105
Figure 43. CONNECT Control Message . . . . . . . . . 109
Figure 44. DISCONNECT Control Message . . . . . . . . 110
Figure 45. ERROR-IN-REQUEST Control Message . . . . . . 111
Figure 46. ERROR-IN-RESPONSE Control Message . . . . . 112
Figure 47. HELLO Control Message . . . . . . . . . 113
Figure 48. HID-APPROVE Control Message . . . . . . . 114
Figure 49. HID-CHANGE-REQUEST Control Message . . . . . 115
Figure 50. HID-CHANGE Control Message . . . . . . . . 117
Figure 51. HID-REJECT Control Message . . . . . . . . 119
Figure 52. NOTIFY Control Message . . . . . . . . . 121
Figure 53. REFUSE Control Message . . . . . . . . . 123
Figure 54. STATUS Control Message . . . . . . . . . 125
Figure 55. STATUS-RESPONSE Control Message . . . . . . 126
Figure 56. Transmission Order of Bytes . . . . . . . 147
Figure 57. Significance of Bits . . . . . . . . . . 147
+--------------------+
| 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 | ... | | 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 1. Protocol Relationships
2. Introduction top
ST has been developed to support efficient delivery of streams of
packets to either single or multiple destinations in applications
requiring guaranteed data rates and controlled delay characteristics.
The motivation for the original protocol was that IP [2] [15] did not
provide the delay and data rate characteristics necessary to support
voice applications.
ST is an internet protocol at the same layer as IP, see Figure 1. ST
differs from IP in that IP, as originally envisioned, did not require
routers (or intermediate systems) to maintain state information
describing the streams of packets flowing through them. ST
incorporates the concept of streams across an internet. Every
intervening ST entity maintains state information for each stream
that passes through it. The stream state includes forwarding
information, including multicast support for efficiency, and resource
information, which allows network or link bandwidth and queues to be
assigned to a specific stream. This pre-allocation of resources
allows data packets to be forwarded with low delay, low overhead, and
a low probability of loss due to congestion. The characteristics of
a stream, such as the number and location of the endpoints, and the
bandwidth required, may be modified during the lifetime of the
stream. This allows ST to give a real time application the
guaranteed and predictable communication characteristics it requires,
and is a good vehicle to support an application whose communications
requirements are relatively predictable.
ST proved quite useful in several early experiments that involved
voice conferences in the Internet. Since that time, ST has also been
used to support point-to-point streams that include both video and
voice. Recently, multimedia conferencing applications have been
developed that need to exchange real-time voice, video, and pointer
data in a multi-site conferencing environment. Multimedia
conferencing across an internet is an application for which ST
provides ideal support. Simulation and wargaming applications [14]
also place similar requirements on the communication system. Other
applications may include scientific visualization between a number of
workstations and one or more remote supercomputers, and the
collection and distribution of real-time sensor data from remote
sensor platforms. ST may also be useful to support activities that
are currently supported by IP, such as bulk file transfer using TCP.
Transport protocols above ST include the Packet Video Protocol (PVP)
[5] and the Network Voice Protocol (NVP) [4], which are end-to-end
protocols used directly by applications. Other transport layer
protocols that may be used over ST include TCP [16], VMTP [3], etc.
They provide the user interface, flow control, and packet ordering.
This specification does not describe these higher layer protocols.
2.1. Major Differences Between ST And ST-II top
ST-II supports a wider variety of applications than did the
original ST. The differences between ST and ST-II are fairly
straight forward yet provide great improvements. Four of the more
notable differences are:
1 ST-II is decoupled from the Access Controller (AC). The
AC, as well as providing a rudimentary access control
function, also served as a centralized repository and
distributor of the conference information. If an AC is
necessary, it should be an entity in a higher layer
protocol. A large variety of applications such as
conferencing, distributed simulations, and wargaming can
be run without an explicit AC.
2 The basic stream construct of ST-II is a directed tree
carrying traffic away from a source to all the
destinations, rather than the original ST's omniplex
structure. For example, a conference is composed of a
number of such trees, one for traffic from each
participant. Although there are more (simplex) streams in
ST-II, each is much simpler to manage, so the aggregate is
much simpler. This change has a minimal impact on the
application.
3 ST-II defines a number of the robustness and recovery
mechanisms that were left undefined in the original ST
specification. In case of a network or ST Agent failure,
a stream may optionally be repaired automatically (i.e.,
without intervention from the user or the application)
using a pruned depth first search starting at the ST Agent
immediately preceding the failure.
4 ST-II does not make an inherent distinction between
streams connecting only two communicants and streams among
an arbitrary number of communicants.
This memo is the specification for the ST-II Protocol. Since
there should be no ambiguity between the original ST specification
and the specification herein, the protocol is simply called ST
hereafter.
ST is the protocol used by ST entities to exchange information.
The same protocol is used for communication among all ST entities,
whether they communicate with a higher layer protocol or forward
ST packets between attached networks.
The remainder of this section gives a brief overview of the ST
Protocol. Section 3 (page 17) provides a detailed description of
the operations required by the protocol. Section 4 (page 75)
provides descriptions of the ST Protocol Data Units exchanged
between ST entities. Issues that have not yet been fully
addressed are presented in Section 5 (page 131). A glossary and
list of references are in Sections 6 (page 135) and 7 (page 143),
respectively.
This memo also defines "subsets" of ST that can be implemented. A
subsetted implementation does not have full ST functionality, but
it can interoperate with other similarly subsetted
implementations, or with a full implementation, in a predictable
and consistent manner. This approach allows an implementation to
be built and provide service with minimum effort, and gives it an
immediate and well defined growth path.
2.2. Concepts And Terminology top
The ST packet header is not constrained to be compatible with the
IP packet header, except for the IP Version Number (the first four
bits) that is used to distinguish ST packets (IP Version 5) from
IP packets (IP Version 4). The ST packets, or protocol data units
(PDUs), can be encapsulated in IP either to provide connectivity
(possibly with degraded service) across portions of an internet
that do not provide support for ST, or to allow access to services
such as security that are not provided directly by ST.
An internet entity that implements the ST Protocol is called an
"ST Agent". We refer to two kinds of ST agents: "host ST
agents", also called "host agents" and "intermediate ST agents",
also called "intermediate agents". The ST agents functioning as
hosts are sourcing or sinking data to a higher layer protocol or
application, while ST agents functioning as intermediate agents
are forwarding data between directly attached networks. This
distinction is not part of the protocol, but is used for
conceptual purposes only. Indeed, a given ST agent may be
simultaneously performing both host and intermediate roles. Every
ST agent should be capable of delivering packets to a higher layer
protocol. Every ST agent can replicate ST data packets as
necessary for multi-destination delivery, and is able to send
packets whether received from a network interface or a higher
layer protocol. There are no other kinds of ST agents.
ST provides applications with an end-to-end flow oriented service
across an internet. This service is implemented using objects
called "streams". ST data packets are not considered to be
totally independent as are IP data packets. They are transmitted
only as part of a point-to-point or point-to-multi- point stream.
ST creates a stream during a setup phase before data is
transmitted. During the setup phase, routes are selected and
internetwork resources are reserved. Except for explicit changes
to the stream, the routes remain in effect until the stream is
explicitly torn down.
An ST stream is:
o the set of paths that data generated by an application
entity traverses on its way to its peer application
entity(s) that receive it,
o the resources allocated to support that transmission of
data, and
o the state information that is maintained describing that
transmission of data.
Each stream is identified by a globally unique "Name"; see
Section 4.2.2.8 (page 87). The Name is specified in ST control
operations, but is not used in ST data packets. A set of streams
may be related as members of a larger aggregate called a "group".
A group is identified by a "Group Name"; see Section 3.7.3 (page
56).
The end-users of a stream are called the "participants" in the
stream. Data travels in a single direction through any given
stream. The host agent that transmits the data into the stream is
called the "origin", and the host agents that receive the data are
called the "targets". Thus, for any stream one participant is the
origin and the others are the targets.
A stream is "multi-destination simplex" since data travels across
it in only one direction: from the origin to the targets. A
stream can be viewed as a directed tree in which the origin is the
root, all the branches are directed away from the root toward the
targets, which are the leaves. A "hop" is an edge of that tree.
The ST agent that is on the end of an edge in the direction toward
the origin is called the "previous-hop ST agent", or the
"previous-hop". The ST agents that are one hop away from a
previous-hop ST agent in the direction toward the targets are
called the "next-hop ST agents", or the "next-hops". It is
possible that multiple edges between a previous-hop and several
next-hops are actually implemented by a network level multicast
group.
Packets travel across a hop for one of two purposes: data or
control. For ST data packet handling, hops are marked by "Hop
IDentifiers" (HIDs) used for efficient forwarding instead of the
stream's Name. A HID is negotiated among several agents so that
data forwarding can be done efficiently on both a point-to-point
and multicast basis. All control message exchange is done on a
point-to-point basis between a pair of agents. For control
message handling, Virtual Link Identifiers are used to quickly
dispatch the control messages to the proper stream's state
machine.
ST requires routing decisions to be made at several points in the
stream setup and management process. ST assumes that an
appropriate routing algorithm exists to which ST has access; see
Section 3.8.1 (page 69). However, routing is considered to be a
separate issue. Thus neither the routing algorithm nor its
implementation is specified here. A routing algorithm may attempt
to minimize the number of hops to the target(s), or it may be more
intelligent and attempt to minimize the total internet resources
consumed. ST operates equally well with any reasonable routing
algorithm. The availability of a source routing option does not
eliminate the need for an appropriate routing algorithm in ST
agents.
2.3. Relationship Between Applications And ST top
It is the responsibility of an ST application entity to exchange
information among its peers, usually via IP, as necessary to
determine the structure of the communication before establishing
the ST stream. This includes:
o identifying the participants,
o determining which are targets for which origins,
o selecting the characteristics of the data flow between any
origin and its target(s),
o specifying the protocol that resides above ST,
o identifying the Service Access Point (SAP), port, or
socket relevant to that protocol at every participant, and
o ensuring security, if necessary.
The protocol layer above ST must pass such information down to the
ST protocol layer when creating a stream.
ST uses a flow specification, abbreviated herein as "FlowSpec", to
describe the required characteristics of a stream. Included are
bandwidth, delay, and reliability parameters. Additional
parameters may be included in the future in an extensible manner.
The FlowSpec describes both the desired values and their minimal
allowable values. The ST agents thus have some freedom in
allocating their resources. The ST agents accumulate information
that describes the characteristics of the chosen path and pass
that information to the origin and the targets of the stream.
ST stream setup control messages carry some information that is
not specifically relevant to ST, but is passed through the
interface to the protocol that resides above ST. The "next
protocol identifier" ("NextPcol") allows ST to demultiplex streams
to a number of possible higher layer protocols. The SAP
associated with each participant allows the higher layer protocol
to further demultiplex to a specific application entity. A
UserData parameter is provided; see Section 4.2.2.16 (page 98).
2.4. ST Control Message Protocol top
ST agents create and manage a stream using the ST Control Message
Protocol (SCMP). Conceptually, SCMP resides immediately above ST
(as does ICMP above IP) but is an integral part of ST. Control
messages are used to:
o create streams,
o refuse creation of a stream,
o delete a stream in whole or in part,
o negotiate or change a stream's parameters,
o tear down parts of streams as a result of router or
network failures, or transient routing inconsistencies,
and
o reroute around network or component failures.
SCMP follows a request-response model. SCMP reliability is
ensured through use of retransmission after timeout; see Section
3.7.6 (page 66).
An ST application that will transmit data requests its local ST
agent, the origin, to create a stream. While only the origin
requests creation of a stream, all the ST agents from the origin
to the targets participate in its creation and management. Since
a stream is simplex, each participant that wishes to transmit data
must request that a stream be created.
An ST agent that receives an indication that a stream is being
created must:
1 negotiate a HID with the previous-hop identifying the
stream,
2 map the list of targets onto a set of next-hop ST agents
through the routing function,
3 reserve the local and network resources required to
support the stream,
4 update the FlowSpec, and
5 propagate the setup information and partitioned target
list to the next-hop ST agents.
When a target receives the setup message, it must inquire from the
specified application process whether or not it is willing to
accept the stream, and inform the origin accordingly.
Once a stream is established, the origin can safely send data. ST
and its implementations are optimized to allow fast and efficient
forwarding of data packets by the ST agents using the HIDs, even
at the cost of adding overhead to stream creation and management.
Specifically, the forwarding decisions, that is, determining the
set of next-hop ST agents to which a data packet belonging to a
particular stream will be sent, are made during the stream setup
phase. The shorthand HIDs are negotiated at that time, not only
to reduce the data packet header size, but to access efficiently
the stream's forwarding information. When possible, network-layer
multicast is used to forward a data packet to multiple next-hop ST
agents across a network. Note that when network-layer multicast
is used, all members of the multicast group must participate in
the negotiation of a common HID.
An established stream can be modified by adding or deleting
targets, or by changing the network resources allocated to it. A
stream may be torn down by either the origin or the targets. A
target can remove itself from a stream leaving the others
unaffected. The origin can similarly remove any subset of the
targets from its stream leaving the remainder unaffected. An
origin can also remove all the targets from the stream and
eliminate the stream in its entirety.
A stream is monitored by the involved ST agents. If they detect a
failure, they can attempt recovery. In general, this involves
tearing down part of the stream and rebuilding it to bypass the
failed component(s). The rebuilding always occurs from the origin
side of the failure. The origin can optionally specify whether
recovery is to be attempted automatically by intermediate ST
agents or whether a failure should immediately be reported to the
origin. If automatic recovery is selected but an intermediate
agent determines it cannot effect the repair, it propagates the
failure information backward until it reaches an agent that can
effect repair. If the failure information propagates back to the
origin, then the application can decide if it should abort or
reattempt the recovery operation.
Although ST supports an arbitrary connection structure, we
recognize that certain stream topologies will be common and
justify special features, or options, which allow for optimized
support. These include:
o streams with only a single target (see Section 3.6.2 (page
44)), and
o pairs of streams to support full duplex communication
between two points (see Section 3.6.3 (page 45)).
These features allow the most frequently occurring topologies to
be supported with less setup delay, with fewer control messages,
and with less overhead than the more general situations.
2.5. Flow Specifications top
Real time data, such as voice and video, have predictable
characteristics and make specific demands of the networks that
must transfer it. Specifically, the data may be transmitted in
packets of a constant size that are produced at a constant rate.
Alternatively, the bandwidth may vary, due either to variable
packet size or rate, with a predefined maximum, and perhaps a
non-zero minimum. The variation may also be predictable based on
some model of how the data is generated. Depending on the
equipment used to generate the data, the packet size and rate may
be negotiable. Certain applications, such as voice, produce
packets at the given rate only some of the time. The networks
that support real time data must add minimal delay and delay
variance, but it is expected that they will be non-zero.
The FlowSpec is used for three purposes. First, it is used in the
setup message to specify the desired and minimal packet size and
rate required by the origin. This information is used by ST
agents when they attempt to reserve the resources in the
intervening networks. Second, when the setup message reaches the
target, the FlowSpec contains the packet size and rate that was
actually obtained along the path from the origin, and the accrued
mean delay and delay variance expected for data packets along that
path. This information is used by the target to determine if it
wishes to accept the connection. The target may reduce reserved
resources if it wishes to do so and if the possibility is still
available. Third, if the target accepts the connection, it
returns the updated FlowSpec to the origin, so that the origin can
decide if it still wishes to participate in the stream with the
characteristics that were actually obtained.
When the data transmitted by stream users is generated at varying
rates, including bursts of varying rate and duration, there is an
opportunity to provide service to more subscribers by providing
guaranteed service for the average data rate of each stream, and
reserving additional network capacity, shared among all streams,
to service the bursts. This concept has been recognized by analog
voice network providers leading to the principle of time assigned
speech interpolation (TASI) in which only the talkspurts of a
speech conversation are transmitted, and, during silence periods,
the circuit can be used to send the talkspurts of other
conversations. The FlowSpec is intended to assist algorithms that
perform similar kinds of functions. We do not propose such
algorithms here, but rather expect that this will be an area for
experimentation. To allow for experiments, and a range of ways
that application traffic might be characterized, a "DutyFactor" is
included in the FlowSpec and we expect that a "burst descriptor"
will also be needed.
The FlowSpec will need to be revised as experience is gained with
connections involving numerous participants using multiple media
across heterogeneous internetworks. We feel a change of the
FlowSpec does not necessarily require a new version of ST, it only
requires the FlowSpec version number be updated and software to
manage the new FlowSpec to be distributed. We further suggest
that if the change to the FlowSpec involves additional information
for improved operation, such as a burst descriptor, that it be
added to the end of the FlowSpec and that the current parameters
be maintained so that obsolete software can be used to process the
current parameters with minimum modifications.
**** ****
* * ST Agent 1 * * +---+
* *------- o ---------* *-------+ B |
* * * * +---+
* * ****
+---+ * * |
| | * * |
| A +---------* * o ST Agent 3
| | * * |
+---+ * * |
* * ***
* * * * +---+
* * ST Agent 2 * *-------+ C |
* *------- o --------* * +---+
* * * *
**** * *
* *
+---+ * * +---+
| E +--------* *-------+ D |
+---+ * * +---+
***
Figure 2. Topology Used in Protocol Exchange Diagrams
**** ST Agent 1 ****
* +--+---14--- o -----15--+----+--44---+---+
* | +-+--11--- -----16--+-+ * | B |
* | | * * |+-+--45---+---+
* | | * *++*
+---+ * | | * 34 ||32
| +----4----+--+ | * ||
| A +----6----+----+ * o ST Agent 3
| +----5----+---+ * |
+---+ * | * | 33
* | * ST *+*
* | * Agent * | *
* | * 2 -----24-+--+ * +---+
* +--+--23--- o -----25-+-----+--54---+ C |
* * -----26-+---+ * +---+
**** -----27-+-+ | *
* | | *
+---+ * | | * +---+
| E +---74---+-+ +-+--64---+ D |
+---+ * * +---+
***
Figure 3. Virtual Link Identifiers for SCMP Messages
3. ST Control Message Protocol Functional Description top
This section contains a functional description of the ST Control
Message Protocol (SCMP); Section 4 (page 75) specifies the formats of
the control message PDUs. We begin with a description of stream
setup. Mechanisms used to deal with the exceptional cases are then
presented. Complications due to options that an application or a ST
agent may select are then detailed. Once a stream has been
established, the data transfer phase is entered; it is described.
Once the data transfer phase has been completed, the stream must be
torn down and resources released; the control messages used to
perform this function are presented. The resources or participants
of a stream may be changed during the lifetime of the stream; the
procedures to make changes are described. Finally, the section
concludes with a description of some ancillary functions, such as
failure detection and recovery, HID negotiation, routing, security,
etc.
To help clarify the SCMP exchanges used to setup and maintain ST
streams, we have included a series of figures in this section. The
protocol interactions in the figures assume the topology shown in
Figure 2. The figures, taken together,
o Create a stream from an application at A to three peers at B,
C and D,
o Add a peer at E,
o Disconnect peers B and C, and
o D drops out of the stream.
Other figures illustrate exchanges related to failure recovery.
In order to make the dispatch function within SCMP more uniform and
efficient, each end of a hop is assigned, by the agent at that end, a
Virtual Link Identifier that uniquely (within that agent) identifies
the hop and associates it with a particular stream's state
machine(s). The identifier at the end of a link that is sending a
message is called the Sender Virtual Link Identifier (SVLId); that
at the receiving end is called the Receiver Virtual Link Identifier
(RVLId). Whenever one agent sends a control message for the other to
receive, the sender will place the receiver's identifier into the
RVLId field of the message and its own identifier in the SVLId field.
When a reply to the message is sent, the values in SVLId and RVLId
fields will be reversed, reflecting the fact the sender and receiver
roles are reversed. VLIds with values zero through three are
received and should not be assigned in response to CONNECT messages.
Figure 3 shows the hops that will be used in the examples and
summarizes the VLIds that will be assigned to them.
Similarly, Figure 4 summarizes the HIDs that will eventually be
negotiated as the stream is created.
**** ST Agent 1 ****
* +>+--1200-> o -------->+--->+-3600->+---+
* ^ * * * | B |
* | * * +->+-6000->+---+
* | * *+**
+---+ * | * ^
| +-------->+-->+ * |
| A | * * o St Agent 3
| +-------->+-->+ * ^
+---+ * | * | 4801
* | * *+*
* V * ST Agent 2 * ^ * +---+
* +>+--2400-> o ------->+->+->+-4800->+ C |
**** * | * 4801 +---+
* | *
+---+ * V * +---+
| E +<-4800--+<-+->+-4800->+ D |
+---+ * * 4801 +---+
***
Figure 4. HIDs Assigned for ST User Packets
Some of the diagrams that follow form a progression. For example,
the steps required initially to establish a connection are spread
across five figures. Within a progression, the actions on the first
diagram are numbered 1.1, 1.2, etc.; within the second diagram they
are numbered 2.1, 2.2, etc. Points where control leaves one diagram
to enter another are identified with a continuation arrow "-->>", and
are continued with "[a.b] >>-->" in the other diagram. The number in
brackets shows the label where control left the earlier diagram. The
reception of simple acknowledgments, e.g., ACKs, in one figure from
another is omitted for clarity.
3.1. Stream Setup top
This section presents a description of stream setup assuming that
everything succeeds -- HIDs are approved, any required resources
are available, and the routing is correct.
3.1.1. Initial Setup At The Origin top
As described in Section 2.3 (page 11), the application has
collected the information necessary to determine the
participants in the communication before passing it to the host
ST agent at the origin. The host ST agent will take this
information, allocate a Name for the stream (see Section
4.2.2.8 (page 87)), and create a stream.
3.1.2. Invoking The Routing Function top
An ST agent that is setting up a stream invokes a routing
function to find a path to reach each of the targets specified
in the TargetList. This is similar to the routing decision in
IP. However, in this case the route is to a multitude of
targets rather than to a single destination.
The set of next-hops that an ST agent would select 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.
The result of the routing function is a set of next-hop ST
agents and the parameters of the intervening network(s). The
latter permit the ST agent to determine whether the selected
network has the resources necessary to support the level of
service requested in the FlowSpec.
3.1.3. Reserving Resources top
The intent of ST is to provide a guaranteed level of service by
reserving internet resources for a stream during a setup phase
rather than on a per packet basis. The relevant resources are
not only the forwarding information maintained by the ST
agents, but also packet switch processor bandwidth and buffer
space, and network bandwidth and multicast group identifiers.
Reservation of these resources can help to increase the
reliability and decrease the delay and delay variance with
which data packets are delivered. The FlowSpec contains all
the information needed by the ST agent to allocate the
necessary resources. When and how these resources are
allocated depends on the details of the networks involved, and
is not specified here.
If an ST agent must send data across a network to a single
next-hop ST agent, then only the point-to-point bandwidth needs
to be reserved. If the agent must send data to multiple next-
hop agents across one network and network layer multicasting is
not available, then bandwidth must be reserved for all of them.
This will allow the ST agent to
use replication to send a copy of the data packets to each
next-hop agent.
If multicast is supported, its use will decrease the effort
that the ST agent must expend when forwarding packets and also
reduces the bandwidth required since one copy can be received
by all next-hop agents. However, the setup phase is more
complicated. A network multicast address must be allocated
that contains all those next-hop agents, the sender must have
access to that address, the next-hop agents must be informed of
the address so they can join the multicast group identified by
it (see Section 4.2.2.7 (page 86)), and a common HID must be
negotiated.
The network should consider the bandwidth and multicast
requirements to determine the amount of packet switch
processing bandwidth and buffer space to reserve for the
stream. In addition, the membership of a stream in a Group may
affect the resources that have to be allocated; see Section
3.7.3 (page 56).
Few networks in the Internet currently offer resource
reservation, and none that we know of offer reservation of all
the resources specified here. Only the Terrestrial Wideband
Network (TWBNet) [7] and the Atlantic Satellite Network
(SATNET) [9] offer(ed) bandwidth reservation. Multicasting is
more widely supported. No network provides for the reservation
of packet switch processing bandwidth or buffer space. We hope
that future networks will be designed to better support
protocols like ST.
Effects similar to reservation of the necessary resources may
be obtained even when the network cannot provide direct support
for the reservation. Certainly if total reservations are a
small fraction of the overall resources, such as packet switch
processing bandwidth, buffer space, or network bandwidth, then
the desired performance can be honored if the degree of
confidence is consistent with the requirements as stated in the
FlowSpec. Other solutions can be designed for specific
networks.
3.1.4. Sending CONNECT Messages top
A VLId and a proposed HID must be selected for each next-hop
agent. The control packets for the next-hop must carry the
VLId in the SVLId field. The data packets transmitted in the
stream to the next-hop must carry the HID in the ST Header.
The ST agent sends a CONNECT message to each of the ST agents
identified by the routing function. Each CONNECT message
contains the VLId, the proposed HID (the HID Field option bit
must be set, see Section 3.6.1 (page 44)), an updated FlowSpec,
and a TargetList. In general, the HID, FlowSpec, and
TargetList will depend on both the next-hop and the intervening
network. Each TargetList is a subset of the received (or
original) TargetList, identifying the targets that are to be
reached through the next-hop to which the CONNECT message is
being sent. Note that a CONNECT message to a single next-hop
might have to be fragmented into multiple CONNECTs if the
single CONNECT is too large for the intervening network's MTU;
fragmentation is performed by further dividing the TargetList.
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; see Section 4.2.3.5 (page 105).
However, since an identical copy of each ensuing data packet
will reach each member of the multicast group, all the CONNECT
messages must propose the same HID. See Section 3.7.4 (page
58) for a detailed discussion on HID selection.
In the example of Figure 2, the routing function might return
that B is reachable via Agent 1 and C and D are reachable via
Agent 2. Thus A would create two CONNECT messages, one each
for Agents 1 and 2, as illustrated in Figure 5. Assuming that
the proposed HIDs are available in the receiving agents, they
would each send a responding HID-APPROVE back to Agent A.
Application Agent A Agent 1 Agent 2
1.1. (open B,C,D)
V
1.2. +-> (routing to B,C,D)
V
1.3. +->(reserve resources from A to Agent 1)
| V
1.4. | +-> CONNECT B --------->>
| <RVLId=0><SVLId=4>
| <Ref=10><HID=1200>
V
1.5. +->(reserve resources from A to Agent 2)
V
1.6. +-> CONNECT C,D ------------------>>
<RVLId=0><SVLId=5>
<Ref=15><HID=2400>
Figure 5. Origin Sending CONNECT Message
3.1.5. CONNECT Processing By An Intermediate Agent top
An ST agent receiving a CONNECT message should, assuming no
errors, quickly select a VLId and respond to the previous-hop
with either an ACK, a HID-REJECT, or a HID-APPROVE message, as
is appropriate. This message must identify the CONNECT to
which it corresponds by including the CONNECT's Reference
number in its Reference field. Note that the VLId that this
agent selects is placed in the SVLId of the response, and the
previous-hop's VLId (which is contained in the SVLId of the
CONNECT) is copied into the RVLId of the response. If the
agent is not a target, it must then invoke the routing
function, reserve resources, and send a CONNECT message(s) to
its next-hop(s), as described in Sections 3.1.2-4 (pages 19-
20).
Agent A Agent 1 Agent B
[1.4] >>-> CONNECT B -------->+--+
<RVLId=0><SVLId=4> | V
2.1. <Ref=10><HID=1200> | (routing to B)
| V
2.2. V +->(reserve resources from 1 to B)
2.3. +<- HID-APPROVE <------+ V
2.4. <RVLId=4><SVLId=14> +-> CONNECT B ---------->>
<Ref=10><HID=1200> <RVLId=0><SVLId=15>
<Ref=110><HID=3600>
Agent A Agent 2 Agent C
[1.6] >>-> CONNECT C,D ------>+-+
<RVLId=0><SVLId=5> | V
2.5. <Ref=15><HID=2400> | (routing to C,D)
| V
2.6. V +-->(reserve resources from 2 to C)
2.7. +<- HID-APPROVE <------+ | V
2.8. <RVLId=5><SVLId=23> | +-> CONNECT C ---------->>
<Ref=15><HID=2400> | <RVLId=0><SVLId=25>
| <Ref=210><HID=4800>
|
| Agent D
V
2.9. +->(reserve resources from 2 to D)
V
2.10. +-> CONNECT D ---------->>
<RVLId=0><SVLId=26>
<Ref=215><HID=4800>
Figure 6. CONNECT Processing by an Intermediate Agent
The resources listed as Desired in a received FlowSpec may not
correspond to those actually reserved in either the ST agent
itself or in the network(s) used to reach the next-hop
agent(s). As long as the reserved resources are sufficient to
meet the specified Limits, the copy of the FlowSpec sent to a
next-hop must have the Desired resources updated to reflect the
resources that were actually obtained. For example, the
Desired bandwidth might be reduced because the network to the
next-hop could not provide all of the desired bandwidth. Also,
the delay and delay variance are appropriately increased, and
the link MTU may require that the DesPDUBytes field be reduced.
(The minimum requirements that the origin had entered into the
FlowSpec Limits fields cannot be altered by the intermediate or
target agents.)
3.1.6. Setup At The Targets top
An ST agent that is the target of a CONNECT, whether from an
intermediate ST agent, or directly from the origin host ST
agent, must respond first (assuming no errors) with either a
HID-REJECT or HID-APPROVE. After inquiring from the specified
application process whether or not it is willing to accept the
connection, the agent must also respond with either an ACCEPT
or a REFUSE.
In particular, the application must be presented with
parameters from the CONNECT, such as the Name, FlowSpec,
Options, and Group, to be used as a basis for its decision.
The application is identified by a combination of the NextPcol
field and the SAP field in the (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 a short hand identifier (the HID) that
can be mapped into this information and be used for their
delivery.
The responses to the CONNECT message are sent to the previous-
hop from which the CONNECT was received. An ACCEPT contains
the Name of the stream and the updated FlowSpec. Note that the
application might have reduced the desired level of service in
the received FlowSpec before accepting it. The target must not
send the ACCEPT until HID negotiation has been successfully
completed.
Since the ACCEPT or REFUSE message must be acknowledged by the
previous-hop, it is assigned a new Reference number that will
be returned in the ACK. The CONNECT to which the ACCEPT or
REFUSE is a reply is identified by placing the CONNECT's
Reference number in the LnkReference field of the ACCEPT or
REFUSE.
Agent 1 Agent B Application B
3.1. (proc B listening)
[2.4] >>-> CONNECT B ---------->+------------------+
<RVLId=0><SVLId=15> | |
3.2. <Ref=110><HID=3600> V (proc B accepts)
3.3. +<- HID-APPROVE <--------+ |
<RVLId=15><SVLId=44> |
<Ref=110><HID=3600> V
3.4. (wait until HID negotiated) <---+
V
3.5. <<--+<- ACCEPT B <-----------+
<RVLId=15><SVLId=44>
<Ref=410><LnkRef=110>
Agent 2 Agent C Application C
3.6. (proc C listening)
[2.8] >>-> CONNECT C ---------->+------------------+
<RVLId=0><SVLId=25> | |
3.7. <Ref=210><HID=4800> V (proc C accepts)
3.8. +<- HID-APPROVE <--------+ |
<RVLId=25><SVLId=54> |
<Ref=210><HID=4800> V
3.9. (wait until HID negotiated) <---+
V
3.10. <<--+<- ACCEPT C <-----------+
<RVLId=25><SVLId=54>
<Ref=510><LnkRef=210>
Agent 2 Agent D Application D
3.11. (proc D listening)
[2.10] >>-> CONNECT D ---------->+------------------+
<RVLId=0><SVLId=26> | |
3.12. <Ref=215><HID=4800> V (proc D accepts)
3.13. +<- HID-APPROVE <--------+ |
<RVLId=26><SVLId=64> |
<Ref=215><HID=4800> V
3.14. (wait until HID negotiated) <---+
V
3.15. <<--+<- ACCEPT D <-----------+
<RVLId=26><SVLId=64>
<Ref=610><LnkRef=215>
Figure 7. CONNECT Processing by the Target
3.1.7. ACCEPT Processing By An Intermediate Agent top
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-IN-
REPLY (LnkRefUnknown) message. Otherwise, it responds to the
next-hop ST agent with an ACK, and propagates
the ACCEPT message to the previous-hop along the same path
traced by the CONNECT but in the reverse direction toward the
origin. The ACCEPT should not be propagated until all HID
negotiations with the next-hop agent(s) have been successfully
completed.
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 by the agent when the CONNECT was
Agent A Agent 1 Agent B
+<-+<- ACCEPT B <-------<< [3.5]
V | <RVLId=15><SVLId=44>
4.1. (wait for ACCEPTS) V <Ref=410><LnkRef=110>
4.2. V +-> ACK --------------->+
4.3. (wait until HID negotiated)<-+ <RVLId=44><SVLId=15>
V <Ref=410>
4.4. <<--+<-- ACCEPT B <---------+
<RVLId=4><SVLId=14>
<Ref=115><LnkRef=10>
Agent A Agent 2 Agent C
+<-+<- ACCEPT C <------<< [3.10]
| | <RVLId=25><SVLId=54>
| V <Ref=510><LnkRef=210>
4.5. | +-> ACK --------------->+
| <Ref=510>
| <RVLId=54><SVLId=25>
|
| Agent D
V
+<-+<- ACCEPT D <------<< [3.15]
V | <RVLId=26><SVLId=64>
4.6. (wait for ACCEPTS) V <Ref=610><LnkRef=215>
4.7. V +-> ACK --------------->+
4.8. (wait until HID negotiated)<-+ <RVLId=64><SVLId=26>
V <Ref=610>
4.9. <<--+<- ACCEPT C <----------+
<RVLId=5><SVLId=23> |
<Ref=220><LnkRef=15>|
V
4.10. <<--+<- ACCEPT D <----------+
<RVLId=5><SVLId=23>
<Ref=225><LnkRef=15>
Figure 8. ACCEPT Processing by an Intermediate Agent
originally processed. However, the agent does not adjust the
reservation in response to the ACCEPT. It is expected that any
excess resource allocation will be released for use by other
stream or datagram traffic through an explicit CHANGE message
initiated by the application at the origin if it does not wish
to be charged for any excess resource allocations.
3.1.8. ACCEPT Processing By The Origin top
The origin will eventually receive an ACCEPT (or REFUSE or
ERROR-IN-REQUEST) message from each of the targets. As each
ACCEPT is received, the application should be 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 ACCEPTs (or failures) from all targets have been
received at the origin, the application is notified that stream
setup is complete, and that data may be sent.
Application A Agent A Agent 1 Agent 2
+<-- ACCEPT B <--------<< [4.4]
| <RVLId=4><SVLId=14>
V <Ref=115><LnkRef=10>
5.1. +--> ACK ----------------->+
| <RVLId=14><SVLId=4>
V <Ref=115>
5.2. +<-- (inform A of B's FlowSpec)
| +<-- ACCEPT C <----------------<< [4.9]
| | <RVLId=5><SVLId=23>
| V <Ref=220><LnkRef=15>
5.3. | +--> ACK ------------------------->+
| | <RVLId=23><SVLId=5>
| V <Ref=220>
5.4. +<-- (inform A of C's FlowSpec)
| +<-- ACCEPT D <----------------<< [4.10]
| | <RVLId=5><SVLId=23>
| V <Ref=225><LnkRef=15>
5.5. | +--> ACK ------------------------->+
| | <RVLId=23><SVLId=5>
| V <Ref=225>
5.6. +<-- (inform A of D's FlowSpec)
V
5.7. (wait until HIDs negotiated)
V
5.8. (inform A open to B,C,D)
Figure 9. ACCEPT Processing by the Origin
There are several pieces of information contained in the
FlowSpec that the application must combine before sending data
through the stream. The PDU size should be computed from the
minimum value of the DesPDUBytes field from all ACCEPTs and the
protocol layers above ST should be informed of the limit. It
is expected that the next higher protocol layer above ST will
segment its PDUs accordingly. Note, however, that the MTU may
decrease over the life of the stream if new targets are
subsequently added. Whether the MTU should be increased as
targets are dropped from a stream is left for further study.
The available bandwidth and packet rate limits must also be
combined. In this case, however, it may not be possible to
select a pair of values that may be used for all paths, e.g.,
one path may have selected a low rate of large packets while
another selected a high rate of small packets. The application
may remedy the situation by either tearing down the stream,
dropping some participants, or creating a second stream.
After any differences have been resolved (or some targets have
been deleted by the application to permit resolution), the
application at the origin should send a CHANGE message to
release any excess resources along paths to those targets that
exceed the resolved parameters for the stream, thereby reducing
the costs that will be incurred by the stream.
3.1.9. Processing A REFUSE Message top
REFUSE messages are used to indicate a failure to reach an
application at a target; they are propagated toward the origin
of a stream. They are used in three situations:
1 during stream setup or expansion to indicate that there
is no satisfactory path from an ST agent to a target,
2 when the application at the target either does not
exist does not wish to be a participant, or wants to
cease being a participant, and
3 when a failure has been detected and the agents are
trying to find a suitable path around the failure.
The cases are distinguished by the ReasonCode field and an
agent receiving a REFUSE message must examine that field in
order to determine the proper action to be taken. In
particular, if the ReasonCode indicates that the CONNECT
message reached the target then the REFUSE should be propagated
back to the origin, releasing resources as appropriate along
the way. If the ReasonCode indicates that
the CONNECT message did not reach the target then the
intermediate (origin) ST agent(s) should check for alternate
routes to the target before propagating the REFUSE back another
hop toward the origin. This implies that an agent must keep
track of the next-hops that it has tried, on a target by target
basis, in order not to get caught in a loop.
An ST agent that receives a REFUSE message must acknowledge it
by sending an ACK to the next-hop. The REFUSE must also be
propagated back to the previous-hop ST agent. Note that the ST
agent may not have any information about the target in
Appl. Agent A Agent 2 Agent E
(proc E NOT listening)
1. (add E)
2. +----->+-> CONNECT E ---------->+->+
<RVLId=23><SVLId=5> | |
<Ref=65> V |
3. +<-- ACK <---------------+ |
<RVLId=5><SVLId=23> V
4. <Ref=65> (routing to E)
V
5. (reserve resources 2 to E)
V
6. +--> CONNECT E --------->+
<RVLId=0><SVLId=27> |
<Ref=115><HID=4600> |
V
7. +<-+<- REFUSE B <-----------+
| | <RVLId=27><SVLId=74>
| | <Ref=705><LnkRef=115>
| V <RC=SAPUnknown>
8. | +-> ACK ---------------->+
| | <RVLId=74><SVLId=27> |
| V <Ref=705> |
9. | (free link 27) V
10. V (free link 74)
11. +<- REFUSE B <-----------+
| <RVLId=5><SVLId=23> |
| <Ref=550><LnkRef=65> V
12. | <RC=SAPUnknown> (free resources 2 to E)
V
13. +-> ACK --------------->+
| <RVLId=23><SVLId=5> |
| <Ref=550> V
14. V (keep link 23 for C,D)
15. (keep link 5 for C,D)
V
16. (inform application failed SAPUnknown)
Figure 10. Sending REFUSE Message
the TargetList. This may result from interacting DISCONNECT
and REFUSE messages and should be logged and silently ignored.
If, after deleting the specified target, the next-hop has no
remaining targets, then those resources associated with that
next-hop agent may be released. Note that network resources
may not actually be released if network multicasting is being
Appl. Agent A Agent 2 Agent 1 Agent 3 Agent B
1. (network from 1 to B fails)
2. (add B)
3. +-> CONNECT B ----------------->+
<RVLId=0><SVLId=6> |
<Ref=35><HID=100> |
3. +<- HID-APPROVE <---------------+
<RVLId=6><SVLId=11> |
<Ref=35><HID=100> V
4. (routing to B: no route)
V
5. +<-+-- REFUSE B ----------------+
| | <RVLId=6><SVLId=11>
| | <Ref=155><LnkRef=35>
| V <RC=NoRouteToDest>
6. | +-> ACK -------------------->+
| | <RVLId=11><SVLId=6> V
7. | V <Ref=155> (drop link 6)
8. V (drop link 11)
9. (find alternative route: via agent 2)
10. (resources from A to 2 already allocated:
V reuse control link & HID, no additional resources required)
11. +-> CONNECT B -------->+->+
<RVLId=23><SVLId=5>| |
<Ref=40> V |
12. +<- ACK <--------------+ |
<RVLId=5><SVLId=23> V
13. <Ref=40> (routing to B: via agent 3)
V
14. +-> CONNECT B -->+
15. <RVLId=0><SVLId=24> +-> CONNECT B --------->+
<Ref=245><HID=4801> V <RVLId=0><SVLId=32> |
16. +<- HID-APPROVE -+ <Ref=310><HID=6000> |
<RVLId=24><SVLId=33> |
<Ref=245><HID=4801> V
17. +<- HID-APPROVE --------+
<RVLId=32><SVLId=45>|
<Ref=310><HID=6000> V
18. (ACCEPT handling follows normally to complete stream setup)
Figure 11. Routing Around a Failure
used since they may still be required for traffic to other
next-hops in the multicast group.
When the REFUSE reaches a origin, the origin sends an ACK and
notifies the application via the next higher layer protocol
that the target listed in the TargetList 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.
Figure 10 illustrates the protocol exchanges for processing a
REFUSE generated at the target, either because the target
application is not running or that the target application
rejects membership in the stream. Figure 11 illustrates the
case of rerouting around a failure by an intermediate agent
that detects a failure or receives a refuse. The protocol
exchanges used by an application at the target to delete itself
from the stream is discussed in Section 3.3.3 (page 35).
3.2. Data Transfer top
At the end of the connection setup phase, the origin, each target,
and each intermediate ST agent has a database entry that allows it
to forward the data packets from the origin to the targets and to
recover from failures of the intermediate agents or networks. The
database should be optimized to make the packet forwarding task
most efficient. The time critical operation is an intermediate
agent receiving a packet from the previous-hop agent and
forwarding it to the next-hop agent(s). The database entry must
also contain the FlowSpec, utilization information, the address of
the origin and previous-hop, and the addresses of the targets and
next-hops, so it can perform enforcement and recover from
failures.
An ST agent receives data packets encapsulated by an ST header. A
data packet received by an ST agent contains the non-zero HID
assigned to the stream for the branch from the previous-hop to
itself. This HID was selected so that it is unique at the
receiving ST agent and thus can be used, e.g., as an index into
the database, to obtain quickly the necessary replication and
forwarding information.
The forwarding information will be network and implementation
specific, but must identify the next-hop agent or agents and their
respective HIDs. It is suggested that the cached information for
a next-hop agent include the local network address of the next-
hop. If the data packet must be forwarded to multiple next-hops
across a single network that supports multicast, the database may
specify a single HID and may identify the next-hops by a (local
network) multicast address.
If the network does not support multicast, or the next-hops are on
different networks, then the database must indicate multiple
(next-hop, HID) tuples. When multiple copies of the data packet
must be sent, it may be necessary to invoke a packet replicator.
Data packets should not require fragmentation as the next higher
protocol layer at the origin was informed of the minimum MTU over
all paths in the stream and is expected to segment its PDUs
accordingly. However, it may be the case that a data packet that
is being rerouted around a failed network component may be too
large for the MTU of an intervening network. This should be a
transient condition that will be corrected as soon as the new
minimum MTU has been propagated back to the origin. Disposition
by a mechanism other than dropping of the too large PDUs is left
for further study.
3.3. Modifying An Existing Stream top
Some applications may wish to change the parameters of a stream
after it has been created. Possible changes include adding or
deleting targets and changing the FlowSpec. These are described
below.
3.3.1. Adding A Target top
It is possible for an application to add a new target to an
existing stream any time after ST has incorporated information
about the stream into its database. At a high level, the
application entities exchanges whatever information is
necessary. Although the mechanism or protocol used to
accomplish this is not specified here, it is necessary for the
higher layer protocol to inform the host ST agent at the origin
of this event. The host ST agent at the target must also be
informed unless this had previously been done. Generally, the
transfer of a target list from an ST agent to another, or from
a higher layer protocol to a host ST agent, will occur
atomically when the CONNECT is received. Any information
concerning a new target received after this point can be viewed
as a stream expansion by the receiving ST agent. However, it
may be possible that an ST agent can utilize such information
if it is received before it makes the relevant routing
decisions. These implementation details are not specified
here, but implementations must be prepared to receive CONNECT
messages that represent expansions of streams that are still in
the process of being setup.
To expand an existing stream, the origin issues one or more
CONNECT messages that contain the Name, the VLId, the FlowSpec,
and the TargetList specifying the new target or targets. The
origin issues multiple CONNECT messages if
either the targets are to be reached through different next-hop
agents, or a single CONNECT message is too large for the
network MTU. The HID Field option is not set since the HID has
already been (or is being) negotiated for the hop;
consequently, the CONNECT is acknowledged with an ACK instead
of a HID-REJECT or HID-APPROVE.
Application Agent A Agent 2 Agent E
1. (open E)
2. V (proc E listening)
3. +->(routing to E)
V
4. +-> (check resources from A to Agent 2: already allocated,
V reuse control link & HID, no additional resources needed)
5. +-> CONNECT E --------->+->+
<RVLId=23><SVLId=5> | V
6. <Ref=20> V (routing to E)
7. +<- ACK <---------------+ V
<RVLId=5><SVLId=23> +->(reserve resources 2 to E)
<Ref=20> V
8. +-> CONNECT E --------->+
<RVLId=0><SVLId=27> |
<Ref=230><HID=4800> |
9. +<- HID-APPROVE <-------+
<RVLId=27><SVLId=74>|
<Ref=230><HID=4800> V
10. (proc E accepts)
11. (wait until HID negotiated)
V
12. +<-+<- ACCEPT E <----------+
V | <RVLId=27><SVLId=74>
13. (wait for ACCEPTS) V <Ref=710><LnkRef=230>
14. V +-> ACK --------------->+
15. (wait until HID negotiated)<-+ <RVLId=74><SVLId=27>
V <Ref=710>
16. +<- ACCEPT E <-------+
| <RVLId=5><SVLId=23>
V <Ref=235><LnkRef=20>
17. +-> ACK ------------>+
| <RVLId=23><SVLId=5>
V <Ref=235>
18. +<-(inform A of E's FlowSpec)
V
19. +<-(wait for ACCEPTS)
V
20. +<-(wait until HID negotiated)
V
21. (inform A open to E)
Figure 12. Addition of Another Target
An ST agent that is already a node in the stream recognizes the
RVLId and verifies that the Name of 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, then the receiver responds with an ERROR-IN-REQUEST with
the appropriate reason code (RouteLoop) that contains a
TargetList of those targets that were duplicates; see Section
4.2.3.5 (page 106).
For each new target in the TargetList, processing is much the
same as for the original CONNECT; see Sections 3.1.2-4 (pages
19-20). The CONNECT must be acknowledged, propagated, and
network resources must be reserved. However, it may be
possible to route to the new targets using previously allocated
paths or an existing multicast group. In that case, additional
resources do not need to be reserved but more next-hop(s) might
have to be added to an existing multicast group.
Nevertheless, the origin, or any intermediate ST agent that
receives a CONNECT for an existing stream, can make a routing
decision that is independent of any it may have made
previously. Depending on the routing algorithm that is used,
the ST agent may decide to reach the new target by way of an
established branch, or it may decide to create a new branch.
The fact that a new target is being added to an existing stream
may result in a suboptimal overall routing for certain routing
algorithms. We take this problem to be unavoidable since it is
unlikely that the stream routing can be made optimal in
general, and the only way to avoid this loss of optimality is
to redefine the routing of potentially the entire stream, which
would be too expensive and time consuming.
3.3.2. The Origin Removing A Target top
The application at the origin specifies a set of targets that
are to be removed from the stream and an appropriate reason
code (ApplDisconnect). The targets are partitioned into
multiple DISCONNECT messages based on the next-hop to the
individual targets. As with CONNECT messages, an ST agent that
is sending a DISCONNECT must make sure that the message fits
into the MTU for the intervening network. If the message is
too large, the TargetList must be further partitioned into
multiple DISCONNECT messages.
An ST agent that receives a DISCONNECT message must acknowledge
it by sending an ACK back to the previous-hop. The DISCONNECT
must also be propagated to the relevant next-hop ST agents.
Before propagating the message, however, the TargetList should
be partitioned based on next-hop ST
agent and MTU, as described above. Note that there may be
targets in the TargetList for which the ST agent has no
information. This may result from interacting DISCONNECT and
REFUSE messages and should be logged and silently ignored.
If, after deleting the specified targets, any next-hop has no
remaining targets, then those resources associated with that
next-hop agent may be released. Note that network resources
may not actually be released if network multicasting is being
used since they may still be required for traffic to other
next-hops in the multicast group.
Application Application
Agent A Agent 1 Agent 2 Agent B C
1. (close B,C ApplDisconnect)
V
2. +->+-+-> DISCONNECT B ----->+
3. | | <RVLId=14><SVLId=4>+-+-> DISCONNECT B ------>+
| | <Ref=25> | | <RVLId=44><SVLId=15>|
| V <RC=ApplDisconnect>| | <Ref=120> |
4. | (free A to 1 resrc.) | V <RC=ApplDisconnect> |
5. | V (free 1 to B resrc.) |
6. | +<- ACK <--------------+ V
7. | | <RVLId=4><SVLId=14>| +<- ACK <---------------+
| V <Ref=25> | | <RVLId=15><SVLId=44>|
8. | (free link 4) V | <Ref=120> |
9. | (free link 14) V |
10. | (free link 15) V
11. | (inform B that stream closed ApplDisconnect)
12. | (free link 44)
V
13. +<-+-+-> DISCONNECT C ---------->+
14. | | <RVLId=23><SVLId=5> +-+-> DISCONNECT C ------>+
| | <Ref=30> | | <RVLId=54><SVLId=25>|
| V <RC=ApplDisconnect> | | <Ref=240> |
15. | (keep A to 2 resrc for | V <RC=ApplDisconnect> |
16. | data going to D,E) | (free 2 to C resrc.) |
| V |
17. | +<- ACK <-------------------+ V
18. | | <RVLId=5><SVLId=23> | +<- ACK <---------------+
| V <Ref=30> | | <RVLId=25><SVLId=54>|
19. | (keep link 5 for D,E) V | <Ref=240> |
20. | (keep link 23 for D,E) V |
21. | (free link 25) V
22. | (inform C that stream closed ApplDisconnect>)
23. V (free link 54)
24. (inform A closed to B,C ApplDisconnect)
Figure 13. Origin Removing a Target
When the DISCONNECT reaches a target, the target sends an ACK
and notifies the application that it is no longer part of the
stream and the reason. The application should then inform ST
to terminate the stream, and ST should delete the stream from
its database after performing any necessary management and
accounting functions.
3.3.3. A Target Deleting Itself top
The application at the target may inform ST that it wants to be
removed from the stream and the appropriate reason code
(ApplDisconnect). The agent then forms a REFUSE message with
itself as the only entry in the TargetList. The REFUSE 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 such a REFUSE message must
acknowledge it by sending an ACK to the next-hop. The target
is deleted and, if the next-hop has no remaining targets, then
the those resources associated with that next-hop agent may be
released. Note that network resources may not actually be
released if network multicasting is being used since they may
still be required for traffic to other next-hops in the
multicast group. The REFUSE must also be propagated back to
the previous-hop ST agent.
Agent A Agent 2 Agent E
1. (close E ApplDisconnect)
V
2. +<- REFUSE E --+
| <RVLId=27><SVLId=74>
| <Ref=720>
V <RC=ApplDisconnect>
3. +<-+-> ACK ------>+
| | <RVLId=74><SVLId=27>
4. V V <Ref=720>
5. +<-+<- REFUSE E --+ (prune allocations)
| | <RVLId=5><SVLId=23>
| | <Ref=245>
| V <RC=ApplDisconnect>
6. | +-> ACK ------>+
| | <RVLId=23><SVLId=5>
| V <Ref=245>
7. V (prune allocations)
8. (inform application closed E ApplDisconnect)
Figure 14. Target Deleting Itself
When the REFUSE reaches the origin, the origin sends an ACK and
notifies the application that the target listed in the
TargetList is no longer part of the stream. If the stream has
no remaining targets, the application may choose to terminate
the stream.
3.3.4. Changing The FlowSpec top
An application may wish to change the FlowSpec of an
established stream. To do so, it informs ST of the new
FlowSpec and the list of targets that are to be changed. The
origin ST agent then issues one or more CHANGE messages with
the new FlowSpec and sends them to the relevant next-hop
agents. CHANGE messages are structured and processed similarly
to CONNECT messages. A next-hop agent that is an intermediate
agent and receives a CHANGE message similarly determines if it
can implement the new FlowSpec along the hop to each of its
next-hop agents, and if so, it propagates the CHANGE messages
along the established paths. If this process succeeds, the
CHANGE messages will eventually reach the targets, which will
each respond with an ACCEPT message that is propagated back to
the origin.
Note that since a CHANGE may be sent containing a FlowSpec with
a range of permissible values for bandwidth, delay, and/or
error rate, and the actual values returned in the ACCEPTs may
differ, then another CHANGE may be required to release excess
resources along some of the paths.
3.4. Stream Tear Down top
A stream is usually terminated by the origin when it has no
further data to send, but may also be partially torn down by the
individual targets. These cases will not be further discussed
since they have already been described in Sections 3.3.2-3 (pages
33-35).
A stream is also torn down if the application should terminate
abnormally. Processing in this case is identical to the previous
descriptions except that the appropriate reason code is different
(ApplAbort).
When all targets have left a stream, the origin notifies the
application of that fact, and the application then is responsible
for terminating the stream. Note, however, that the application
may decide to add a target(s) to the stream instead of terminating
it.
3.5. Exceptional Cases top
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 are lost, the
requested resources are not available, the routing fails or is
inconsistent.
In order for the ST Control Message Protocol to be reliable over
an unreliable internetwork, the problems of corruption,
duplication, loss, and ordering must be addressed. Corruption is
handled through use of checksumming, as described in Section 4
(page 76). Duplication of control messages is detected by
assigning a transaction number (Reference) to each control
message; duplicates are discarded. Loss is detected using a
timeout at the sender; messages that are not acknowledged before
the timeout expires are retransmitted; see Section 3.7.6 (page
66). If a message is not acknowledged after a few retransmissions
a fault is reported. The protocol does not have significant
ordering constraints. However, minor sequencing of control
messages for a stream is facilitated by the requirement that the
Reference numbers be monotonically increasing; see Section 4.2
(page 78).
3.5.1. Setup Failure Due To CONNECT Timeout top
If a response (an ERROR-IN-REQUEST, an ACK, a HID-REJECT, or a
HID-APPROVE) has not been received within time ToConnect, the
ST agent should retransmit the CONNECT message. If no response
has been received within NConnect retransmissions, then a fault
occurs and a REFUSE message with the appropriate reason code
(RetransTimeout) is sent back in the direction of the origin,
and, in place of the CONNECT, a DISCONNECT is sent to the
next-hop (in case the response to the CONNECT is the message
that was lost). The agent will expect an ACK for both the
REFUSE and the DISCONNECT messages. If it does not receive an
ACK after retransmission time ToRefuse and ToDisconnect
respectively, it will resend the REFUSE/DISCONNECT message. If
it does not receive ACKs after sending NRefuse/ NDisconnect
consecutive REFUSE/DISCONNECT messages, then it simply gives up
trying.
Sending Agent Receiving Agent
1. ->+----> CONNECT X ------>//// (message lost or garbled)
| <RVLId=0><SVLId=99>
V <Ref=1278><HID=1234>
2. (timeout)
V
3. +----> CONNECT X ------------>+
4. | <RVLId=0><SVLId=99> +----> CONNECT X ----------->+
| <Ref=1278><HID=1234> V <RVLId=0><SVLId=1010> |
5. | //<- HID-APPROVE <----------+ <Ref=6666><HID=6666> V
6. | <RVLId=99><SVLId=88> +<- HID-APPROVE <---------+
V <Ref=1278><HID=1234> <RVLId=1010><SVLId=1111>
7. (timeout) <Ref=6666><HID=6666>
V
8. +----> CONNECT X ------------>+
<RVLId=0><SVLId=99> |
<Ref=1278><HID=1234> V
9. +<-+<- HID-APPROVE <----------+
| <RVLId=99><SVLId=88>
V <Ref=1278><HID=1234>
(cancel timer)
Figure 15. CONNECT Retransmission after a Timeout
3.5.2. Problems Due To Routing Inconsistency top
When an intermediate agent receives a CONNECT, it selects the
next-hop agents based on the TargetList and the networks to
which it is connected. If the resulting next-hop to any of the
targets is across the same network from which it received the
CONNECT (but not the previous-hop itself), there may be a
routing problem. However, the routing algorithm at the
previous-hop may be optimizing differently than the local
algorithm would in the same situation. Since the local ST
agent cannot distinguish the two cases, it should permit the
setup but send back to the previous-hop agent an informative
NOTIFY message with the appropriate reason code (RouteBack),
pertinent TargetList, and in the NextHopIPAddress element the
address of the next-hop ST agent returned by its routing
algorithm.
The agent that receives such a NOTIFY should ACK it. If the
agent is using an algorithm that would produce such behavior,
no further action is taken; if not, the agent should send a
DISCONNECT to the next-hop agent to correct the problem.
Alternatively, if the next-hop returned by the routing function
is in fact the previous-hop, a routing inconsistency has been
detected. In this case, a REFUSE is sent back to
the previous-hop agent containing an appropriate reason code
(RouteInconsist), pertinent TargetList, and in the
NextHopIPAddress element the address of the previous-hop. When
the previous-hop receives the REFUSE, it will recompute the
next-hop for the affected targets. If there is a difference in
the routing databases in the two agents, they may exchange
CONNECT and REFUSE messages again. Since such routing errors
in the internet are assumed to be temporary, the situation
should eventually stabilize.
3.5.3. Setup Failure Due To A Routing Failure top
It is possible for an agent to receive a CONNECT message that
contains a known Name, but from an agent other than the
previous-hop agent of the stream with that Name. This may be:
1 that two branches of the tree forming the stream have
joined back together,
2 a deliberate source routing loop,
3 the result of an attempted recovery of a partially
failed stream, or
4 an erroneous routing loop.
The TargetList is used to distinguish the cases 1 and 2 (see
also Section 4.2.3.5 (page 107)) by comparing each newly
received target with those of the previously existing stream:
o if the IP address of the targets differ, it is case 1;
o if the IP address of the targets match but the source
route(s) are different, it is case 2;
o if the target (including any source route) matches a
target (including any source route) in the existing
stream, it may be case 3 or 4.
It is expected that the joining of branches will become more
common as routing decisions are based on policy issues and not
just simple connectivity. Unfortunately, there is no good way
to merge the two parts of the stream back into a single stream.
They must be treated independently with respect to processing
in the agent. In particular, a separate state machine is
required, the Virtual Link Identifiers and HIDs from the
previous-hops and to the next-hops must be different, and
duplicate resources must be reserved in both the agent and in
any next-hop networks. Processing is the same for a deliberate
source routing loop.
The remaining cases requiring recovery, a partially failed
stream and an erroneous routing loop, are not easily
distinguishable. In attempting recovery of a failed stream, an
agent may issue new CONNECT messages to the affected targets;
for a full explanation see also Section 3.7.2 (page 51),
Failure Recovery. Such a CONNECT may reach an agent downstream
of the failure before that agent has received a DISCONNECT from
the neighborhood of the failure. Until that agent receives the
DISCONNECT, it cannot distinguish between a failure recovery
and an erroneous routing loop. That agent must therefore
respond to the CONNECT with a REFUSE message with the affected
targets specified in the TargetList and an appropriate reason
code (StreamExists).
The agent immediately preceding that point, i.e., the latest
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 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. 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 error code (RouteLoop).
The REFUSE message with this error code (RouteLoop) is
propagated by each ST agent without retransmitting any CONNECT
messages. At each agent, it causes any resources reserved
exclusively for the listed targets to be released. The REFUSE
will be propagated to the origin in the case of an erroneous
routing loop. In the case of stream recovery, it will be
propagated to the ST agent that is attempting the recovery,
which may be an intermediate agent or the origin itself. In
the case of a stream recovery, the agent attempting the
recovery may issue new CONNECT messages to the same or to
different next-hops.
If an agent receives both a REFUSE message and a DISCONNECT
message with a target in common then it can release the
relevant resources and propagate neither the REFUSE nor the
DISCONNECT (however, we feel that it is unlikely that most
implementations will be able to detect this situation).
If the origin receives such a REFUSE message, it should attempt
to send a new CONNECT to all the affected targets. Since
routing errors in an internet are assumed to be temporary, the
new CONNECTs will eventually find acceptable routes to the
targets, if one exists. If no further routes exist after
NRetryRoute tries, the application should be
informed so that it may take whatever action it deems
necessary.
3.5.4. Problems In Reserving Resources top
If the network or ST agent resources are not available, an ST
agent may preempt one or more streams that have lower
precedence than the one being created. When it breaks a lower
precedence stream, it must issue REFUSE and DISCONNECT messages
as described in Sections 4.2.3.15 (page 122) and 4.2.3.6 (page
110). If there are no streams of lower precedence, or if
preempting them would not provide sufficient resources, then
the stream cannot be accepted by the ST agent.
If an intermediate agent detects that it cannot allocate the
necessary resources, then it sends a REFUSE that contains an
appropriate reason code (CantGetResrc) and the pertinent
TargetList to the previous-hop ST agent. For further study are
issues of reporting what resources are available, whether the
resource shortage is permanent or transitory, and in the latter
case, an estimate of how long before the requested resources
might be available.
3.5.5. Setup Failure Due To ACCEPT Timeout top
An ST agent that propagates an ACCEPT message backward toward
the origin expects an ACK from the previous-hop. If it does
not receive an ACK within a timeout, called ToAccept, it will
retransmit the ACCEPT. If it does not receive an ACK after
sending a number, called NAccept, of ACCEPT messages, then it
will replace the ACCEPT with a REFUSE, and will send a
DISCONNECT in the direction toward the target. Both the REFUSE
and DISCONNECT will identify the affected target(s) and specify
an appropriate reason code (AcceptTimeout). Both are also
retransmitted until ACKed with timeout ToRefuse/ ToDisconnect
and retransmit count NRefuse/NDisconnect. If they are not
ACKed, the agent simply gives up, letting the failure detection
mechanism described in Section 3.7.1 (page 48) take care of any
cleanup.
3.5.6. Problems Caused By CHANGE Messages top
An application must exercise care when changing a FlowSpec to
prevent a failure. A CHANGE might fail for two reasons. The
request may be for a larger amount of network resources when
those resources are not available; this failure may be
prevented by requiring that the current level of service be
contained within the ranges of the FlowSpec in the CHANGE.
Alternatively, the local network might require all the former
resources to be released before the new ones are requested and,
due to unlucky timing, an unrelated request for network
resources might be processed between the time the resources are
released and the time the new resources are requested, so that
the former resources are no longer available. There is not
much that an application or ST can do to prevent such failures.
If the attempt to change the FlowSpec fails then the ST agent
where the failure occurs must intentionally break the stream
and invoke the stream recovery mechanism using REFUSE and
DISCONNECT messages; see Section 3.7.2 (page 51). Note that
the reserved resources after the failure of a CHANGE may not be
the same as before, i.e., the CHANGE may have been partially
completed. The application is responsible for any cleanup
(another CHANGE).
3.5.7. Notification Of Changes Forced By Failures top
NOTIFY is issued by a an ST Agent to inform upsteam agents and
the origin that resource allocation changes have occurred after
a stream was established. These changes occur when network
components fail and when competing streams preempt resources
previously reserved by a lower precedence stream. We also
anticipate that NOTIFY can be used in the future when
additional resources become available, as is the case when
network components recover or when higher precedence streams
are deleted.
NOTIFY is also used to inform upstream agents that a routing
anomaly has occurred. Such an example was cited in Section
3.5.2 (page 38), where an agent notices that the next-hop agent
is on the same network as the previous-hop agent; the anomaly
is that the previous-hop should have connected directly to the
next-hop without using an intermediate agent. Delays in
propagating host status and routing information can cause such
anomalies to occur. NOTIFY allows ST to correct automatically
such mistakes.
NOTIFY reports a FlowSpec that reflects that revised guarantee
that can be promised to the stream. NOTIFY also
identifies those targets affected by the change. In this way,
NOTIFY is similar to ACCEPT. NOTIFY includes a ReasonCode to
identify the event that triggered the notification. It also
includes a TargetList, rather than a single Target, since a
single event can affect a branch leading to several targets.
NOTIFY is relayed by the ST agents back toward the origin,
along the path established by the CONNECT but in the reverse
direction. NOTIFY must be acknowledged with an ACK at each
hop. If intermediate agent corrects the situation without
causing any disruption to the data flow or guarantees, it can
choose to drop the notification message before it reaches the
origin. If the originating agent recei
