home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Internet Info 1997 December
/
Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso
/
drafts
/
draft_ietf_q_t
/
draft-ietf-st2-state-02.txt
< prev
next >
Wrap
Text File
|
1996-10-28
|
106KB
|
2,463 lines
Internet Draft ST2+ Protocol State Machines October 26, 1996
M. Rajagopal
Expiration: April 27, 1997 S. Sergeant
File: draft-ietf-st2-state-02.txt
Internet Stream Protocol Version 2 (ST2)
Protocol State Machines - Version ST2+
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas
and Working Groups. Note that other groups may also distribute
working documents as Internet-Drafts. Internet-Drafts are draft
documents valid for a maximum of six months. Internet-Drafts may be
updated, replaced, or obsoleted by other documents at any time. It is
not appropriate to use Internet-Drafts as reference material or cite
them other than as "work in progress". To learn the current status
of any Internet-Draft, please check the "lid-abstracts.txt" listing
contained in the Internet-Drafts Shadow directories on
ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu
(US West Coast), or munnari.oz.au (Pacific Rim).
Abstract:
This memo contains a description of state machines for the revised
specification of the Internet STream Protocol Version 2 (ST2+)
described in RFC 1819. The state machines in this document are
descriptions of the ST2+ protocol states and message sequence
specifications for normal behavior. Exception processsing issues are
defined and discussed for protocol compliance and implementation
options.
Editor's Note:
This memo is available both in ASCII format (file: draft-ietf-ST-
state-02.txt) and in PostScript (file: draft-ietf-ST-state-02.ps).
The PostScript version contains the essential state diagrams and is
absolutely required.
Rajagopal, Sergeant Expires April 26, 1997[Page 1]
Internet Draft ST2+ Protocol State Machines October 26, 1996
TABLE OF CONTENTS
1 Introduction 4
2 ST Agent Architecture 6
2.1 ST Protocol Characteristics 6
2.2 The Stream FSM M odel 7
2.3 ST Agent Roles in an Internetwork 7
2.4 The ST Agent Model 8
2.5 Origin, Next Hop, Previous Hop and
Target Finite State 10
2.6 Stream Finite State Machines 11
2.6.1 Externally Communicating FSMs 11
2.6.2 Internally Communicating FSMs 11
2.7 Queues between External Communicating FSMs 11
2.8 Queues Inside an Agent 12
3 Stream Finite State Machines 12
3.1 Assumptions 14
3.2 State Machine Model Conventions 15
3.2.1 Naming Conventions and Notations 15
3.2.2 Transmissions and Receptions 15
3.2.3 Predicates 15
3.3 Normal Behavior versus Exception Processing 16
3.3.1 Context not Represented in Stream FSMs 17
3.3.2 Special Message Types 17
3.3.3 Classes of Response Types 18
3.4 Stream State Machines. 19
3.4.1 Origin State Machine (OSM) 19
3.4.2 Next Hop State Machine (NHSM) 22
3.4.3 Previous Hop State Machine (PHSM) 26
3.5 The Target State Machine (TSM) 27
4 ST Agent FSMs 29
4.1 Agent Database Context 29
4.2 ST Dispatcher role for incoming Packet-switching, 30
4.3 ST Dispatcher functions for outgoing
Packet switching, timer 33
4.4 Retry FSM- RFSM for datalink reliability of
PDU transmissions 33
4.5 Agent , Neighbor and Stream Supervision 35
4.5.1 The MonitorFSM (MFSM) for Agent and Stream
Supervision 35
4.5.2 The Nieghbor Detection Failure FSM for Neighbor
Management 36
4.5.3 Service Model Interactions 37
Rajagopal, Sergeant Expires April 26, 1997[Page 2]
Internet Draft ST2+ Protocol State Machines October 26, 1996
5 Exception Processing 37
5.1 Additional Exception Processing 38
5.1.1 ST Dispatcher detected inconsistencies
Reason Codes: 38
5.1.2 MonitorFSM issues with neighbor failure and
stream recovery 38
5.1.3 Retry and Timeout Failures Reason Codes: 39
5.1.4 Routing issues Reason Codes: 39
5.1.5 LRM issue Reason Codes: 40
6 APPENDIX 41
6.1 Glossary 41
6.2 ST Control Message Flow 43
6.2.1 Message Type 43
6.2.2 Response 44
6.2.3 Possible causes for messages 44
ACKNOWLEDGEMENTS and AUTHORS 45
LIST OF REFERENCES 45
Rajagopal, Sergeant Expires April 26, 1997[Page 3]
Internet Draft ST2+ Protocol State Machines October 26, 1996
1 Introduction
This section gives a brief overview of the ST protocol terms and the
protocol Finite State Machine (FSM) issues addressed in this document.
It is assumed that the reader is familiar with the ST2+ Protocol
Specification document listed in [1]. Unless otherwise stated, ST in
this document refers to the enhanced ST protocol (ST2+).
ST2+ is a connection-oriented internetworking protocol that operates
at the same layer as connectionless IP. An ST stream is defined as a
connection established between an Origin sending data to one or more
Targets. An ST Agent is a network node that participates in resource
reservation negotiations during stream setup along the path between
the Origin and Targets. The resource reservation request is based on
a Flow Specification sent by the Origin. The FlowSpec provides the
basis for the negotiated Quality of Service (QOS).
This QOS is only established, monitored and maintained by nodes with
ST Agent capabilities. Each hop in the ST stream routing tree is an
ST Agent. ST Agents that are one hop away from a given node are
called Previous-Hops in the upstream direction and Next-Hops in the
downstream direction. ST Agent Previous-Hop and Next-Hop Agents are
called ST neighbors.
Data transfer in the ST stream is simplex in the downstream
direction. As such, a single Origin sending data to many Targets is
similar to a media broadcast model. However, each ST Agent may
simultaneously need to perform Origin, Previous-Hop, Next-Hop and
Target functions for a number of different streams. These streams may
be part of a conference ( as in the telephone model) or Group of
related streams, such that resource reservation and routing issues
may be interrelated. The streams may also be unrelated to each other,
but ranked by Precedence within an internetwork in the event that
limited or changing resources need to be reallocated. Origin
applications may request an automatic Recovery option in the event of
network failure or a Change to the QOS after the original setup.
Target applications may send a request to a stream ST Agent to allow
that Target to Join the stream, with or without Notifying the Origin.
Thus, an ST Agent may be required to support a complex web of
intersecting streams with competing QOS requirements and changing
resource allocations or members. The ST Service Model supports the ST
protocol and ST QOS features for routing, resource management and
packet-switching. This Protocol State Machine document addresses the
ST protocol in any ST Agent, regardless of the implementation
specifics of the ST Service Model.
Stream Control Message Protocol (SCMP) messages form a request-
Rajagopal, Sergeant Expires April 26, 1997[Page 4]
Internet Draft ST2+ Protocol State Machines October 26, 1996
response protocol where the particulars of the Flow Specification, as
well as other Protocol Data Unit (PDU) parameters, are interpreted by
the chosen QOS algorithms for routing, Local Resource Management
(LRM) and packet-switching. The ST2+ Specification explicitly defines
all required and allowable, functions and sequences of SCMP message
operations. The SCMP message types are: ACCEPT, ACK, CHANGE, CONNECT,
DISCONNECT, ERROR, HELLO, JOIN, JOIN-REJECT, NOTIFY, REFUSE, STATUS,
STATUS-RESPONSE.
An ST Agent will direct incoming SCMP messages to the appropriate
FSMs for each stream. An Origin State Machine (OSM) is associated
with every Origin ST application. A Next-Hop State Machine (NHSM) is
associated with every downstream ST neighbor. A Previous-Hop State
Machine (PHSM) is associated with every upstream ST neighbor. A
Target State Machine (TSM) is associated with every Target ST
application.
The OSM, NHSM, PHSM and TSM have the same four fundamental states:
IDLE, ESTABLISHED, ADD and CHANGE. The basic transition from IDLE to
ESTABLISHED is through a CONNECT request with an ACCEPT response.
Additional CONNECT and JOIN requests may result in an ADD stream
state, while a CHANGE request would result in a CHANGE stream state.
DISCONNECT and REFUSE messages may remove one or more Targets while
the stream is in any state.
A Retry FSM is used to monitor the datalink reliability between ST
Agents. Each SCMP request-response sequence is defined with next hop
ACKnowledgement, ERROR, timeout and retry conditions. Such exception
processing is designed to resolve incomplete functions during times
of network or ST Agent failure.
A Monitor FSM is used to manage ST neighbor and stream Recovery
issues for all streams managed by the ST Agent. Each ST Agent
maintains state information describing the streams flowing through
it, and can actively gather and distribute such information.
If, for example, an Intermediate ST Agent fails, the neighboring
Agents can recognize this via HELLO messages that are periodically
exchanged between the ST Agents that share streams. STATUS packets
can be used to ask other ST Agents about a particular stream. These
agents then send back a STATUS-RESPONSE message. NOTIFY messages
serve to inform ST Agents of additional mangement information.
ST Reason Codes are used to inform other ST Agents of the source and
type of problem such that the correct response sequences will be
followed. These Reason Codes are inserted in the appropriate SCMP
PDUs and available to the ST Agent management functions. Thus an ST
Rajagopal, Sergeant Expires April 26, 1997[Page 5]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Agent not only manages the normal request- response protocol between
the Origin and Targets of each stream, but also is actively involved
in the detection and distribution of error and QOS implications.
The ST Agent architecture and FSM models contained in this document
have been chosen to illustrate a method for an ST2+ protocol
implementation. There are many alternative techniques. Every effort
has been made to note the relevant tradeoffs between protocol
requirements and implementation choices. The atomic components of this
model may be rearranged to accomodate platform and implementation
issues. Every effort has been made to ensure that the described state
tables accurately reflect state transitions of the ST2+ version of
SCMP, and that the described state diagrams accurately reflect the
state transition tables. If there are discrepancies, the tables take
precedence over the diagrams and the protocol specification takes
precedence over the tables.
Section 2 ST Agent Architecture describes the organization of the
ST Agent model. Section 3 Stream Finite State Machines describes the
OSM, NHSM, PHSM and TSM Section 4 Agent Finite State Machines
describes the Monitor and Retry FSMs. Section 5 Exception Processing
Issues details the Reason Codes by category.
2 ST Agent Architecture
This section describes the ST Agent Finite State Machines (FSMs). The
architectural descriptions are necessarily at a high level and are
meant to serve as a guide to the protocol implementer. The state
machine models are expected to provide the implementer with useful
information such as valid message sequences. The ST2+ Specification
provides the fully documented message detail.
2.1 ST Protocol Characteristics
The ST Agent FSM architecture is organized in a hierarchy of ST2
protocol characteristics. The characteristics are modeled as Agent
roles (i.e., Origin, Intermediate, Target, Previous Hop and Next
Hop), as well as protocol functions (e.g., individual SCMP
Request/Response patterns and reliability at both the PDU and Agent
level).
Figure 1. ST Agent Roles
Each ST Agent has an ST Dispatcher to filter incoming PDUs, intercept
semantic errors and direct valid PDUs to the appropriate queues and
FSMs. FIFO queues and a Message Separator/Combinator in each ST Agent
Rajagopal, Sergeant Expires April 26, 1997[Page 6]
Internet Draft ST2+ Protocol State Machines October 26, 1996
provide additional intra-Agent FSM communications.
Table 1 below shows the Request/Response patterns of each SCMP
message name by the Agent type generating the message. The message
type may be either a Request, a Response or a local control for the
Agent and PDU reliability functions. The message direction can be
downstream (towards a Target) or upstream (towards the Origin). The
Monitor FSM and the Retry FSM manage network, Agent and link
reliability status with the local control SCMP messages.
Table 1: Request/Response Patterns
2.2 The Stream FSM Model
Figure 2 below shows the relationship between the ST Agents and FSMs
in each stream.
Figure 2. Stream FSM Model
The Origin State Machine (OSM) provides communications between an
Origin application and one or more Next Hop State Machines (NHSM).
An Intermediate Agent's Previous Hop State Machine (PHSM) has
communications with an Origin Agent's Next Hop State Machine (NHSM)
through their common network link and the respective Agent's ST
Dispatchers. In the same fashion, an Intermediate Agent's Next Hop
State Machine (NHSM) has communications with Intermediate and/or
Target Agent's Previous Hop State Machines(PHSM).
The Target State Machine (TSM) provides communications between a
Target application and a Previous Hop State Machine (PHSM) in the
Target Agent.
Figure 3. Internetwork Diagram of ST Agent Roles
2.3 ST Agent Roles in an Internetwork
The internetwork diagram of ST Agents (Figure 3) indicates the Origin
(O), Intermediate (I) and Target (T) roles of each ST Agent in a
conference with 4 Origins. The Intermediate Agents I1, I2, I3 and I4
are each neighbors of an ST Agent acting as both an Origin and a
Rajagopal, Sergeant Expires April 26, 1997[Page 7]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Target for the other Origins. Each Origin is sending data in one
outgoing stream to three Targets, and simultaneously recieving data
on 3 incoming streams from the other Origins.
All ST Agents in this illustration have multiple ST neighbors,
streams and interfaces to manage. Each ST Agent may be required to
manage multiple FSMs for any one stream, as well as all competitive,
intersecting streams in the internetwork topology.
ST neighbor communications and SCMP exception processing can be used
to create a first line of defense for the ST stream FSMs. Normal
operations for stream FSMs may be protected and simplified. Network
errors and conflicting message sequences can be filtered out of the
fundamental stream state transitions.
2.4 The ST Agent Model
In Figure 4 , an ST Agent is depicted with an ST Dispatcher sending
and receiving ST PDUs from interface queues (and PDU surrogates from
application interfaces), representing a high order ST message
management scheme. This dispatcher unpacks or forwards incoming PDUs,
and creates and forwards outgoing PDUs.
Figure 4. ST Agent Model
The forwarding of data or the forwarding of certain command sequences
that are not following a negotiatied QOS path (i.e., JOIN and/or JOIN
flooding messages) requires a packet-forwarding mechanism, separate
from the stream operations that unpack, interpret or create PDUs. The
efficient packet switching of ST PDUs through Intermediate hops is
the main reason for this filtering priority.
A first level of filtering is designed to determine whether the
incoming PDU (or PDU surrogate) is data or one of the JOIN sequences
where the destination is, in fact, another ST Agent or an
application. Such PDUs are then forwarded to the destination,
whether a resident Target or for replication to multiple Next-hop
Targets.
The ST PDU validation and delivery functions manage information about
the the messaging success or failure, i.e. the Retry, timeout, ERROR
, or ACK status of messages. This information concerns datalink,
Agent and network reliability. Stream state transitions may occur as
a result.
Many Retry and Monitor FSM transitions may occur while any particular
Rajagopal, Sergeant Expires April 26, 1997[Page 8]
Internet Draft ST2+ Protocol State Machines October 26, 1996
stream state exists. The Monitor FSM interprets and sends messages
with information relevant to multiple streams, i.e., HELLO, STATUS,
STATUS-RESPONSE, NOTIFY.
Second-level filtering occurs when an ST Agent validates incoming
SCMP PDUs and sends the required ACKnowledge (or ERROR PDU, if there
are semantic errors) to the originator of the incoming PDU.
Conversely, incoming ACKnowledgement and ERROR PDUs trigger the Retry
FSM, where the timeout and retry values are updated or a signal is
generated to the appropriate stream FSM for specialized exception
processing.
All SCMP PDUs, except ACK, ERROR, HELLO, STATUS and STATUS-RESPONSE,
require an ACK from the next hop Agent upon receipt. An Agent or
datalink failure may be detected by either the Retry or the Monitor
FSM. A signal is then sent to the appropriate stream FSMs. The
Monitor FSM then manages a reCONNECT sequence for all streams that
have specified this Recovery option.
Once an ST Dispatcher has validated and filtered the PDUs, the stream
SCMP messages are separately queued into Requests (CONNECT, CHANGE,
JOIN) and Responses (ACCEPT, DISCONNECT, JOIN-REJECT, REFUSE).
Requests must wait for the completion of any preceding Requests for
the same stream, while Responses must be handled immediately without
regard to Request state transitions or queues.
The Request and Response queues are directed to the Origin (OSM),
Next Hop (NHSM), Previous Hop (PHSM) and Target (TSM) state machines.
These state machines are referred to as the stream state machines,
rather than the Agent state machines.
The stream state machines are designed to focus on normal (typical)
behavior rather than all pathological cases. Error control and
recovery in the architecture (i.e., ST Dispatcher filters, Monitor
and Retry FSMs) provide a firewall against many problems in the
stream FSMs.
However, when the architecture of a particular Agent platform has ST
intra-Agent communications that are actually between multiple
processors, the Next-hop and Previous-hop FSM communications may
require the concept of multiple ST Agents within what might otherwise
appear to be one ST Agent. Thus the rationale for the filtering and
queueing of the SCMP messages, not just the particular method of
illustration, is very important to the ST Agent Model.
The convention of discussing issues in terms of individual stream
state transistions will be used throughout Section 3 (Stream FSMs) as
a way of simplifying the discussion. Section 4 (ST Agent FSMs) and
Rajagopal, Sergeant Expires April 26, 1997[Page 9]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Section 5 (Exception Processing) will provide more detail about the
architecture, filtering and queues used with the stream FSMs, such
that network failures can be managed with competing and intersecting
streams.
2.5 Origin, Next Hop, Previous Hop and Target Finite State
Machines
Communicating Finite State Machine (CFSM) models have been
extensively used in the trade to formally describe protocol behavior
[2]. Many variations of the basic CFSM model exist and our model is
also a variation of the basic model. Our model uses the basic CFSM
model with FIFO queues combined with predicates. The model describes
the ST protocol behavior and consists of ST SCMP messages along with
a number of predicates. These predicates are not part of the formal
ST Protocol specifications but are useful mechanisms that simplify
the state machine specifications
Origin, Intermediate and Target ST Agents in Figure 2 are all modeled
separately. Because a stream diverges in a tree-like graph, every
Intermediate ST Agent has to communicate with one upstream ST Agent
and one or more downstream Agents. An Intermediate Agent will
therefore have exactly one PHSM and one or more NHSMs for each
stream. Note that, it is possible to have more than one NHSM per
physical interface, when that interface has more than one Agent on
the associated communications link.
The state machine model architecture at an Origin is similar to the
state machine architecture at an Intermediate (Figure 2). The Origin
may have one or more NHSMs. There is no PHSM in this case. However,
in the place of the PHSM there is an Origin State Machine (OSM) which
interfaces with the application. An OSM is a special case of the
PHSM.
The Target is modeled with one PHSM (Figure 2). There are no NHSMs in
this example. However, in the place of a NHSM there are one or more
Target State Machine (TSM) that interface with the application. The
TSM is a special case of the NHSM.
Because the role of each ST Agent (Origin, Intermediate, or Target)
is different, the finite state machine models are not identical.
However, the model for communication between FSMs inside or outside
an Agent is uniform.
Consider a stream topology shown in Figure 2. The figure shows an ST
Origin (O) connected to 2 Intermediate Agents (I1 and I3). I1 is also
connected to I2 and a target T2. I2 is connected to Target T1 and I3
is connected to Targets T3 and T4.
Rajagopal, Sergeant Expires April 21, 1996 [Page 10]
Internet Draft ST2+ Protocol State Machines October 26, 1996
The Origin is modeled with one OSM and 2 NHSMs (one per next hop).
Each Target is modeled with one PHSM and one or more TSMs. I1 and I3
are both modeled with one PHSM and 2 NHSMs; I2 is modeled with one
PHSM and 2 NHSMs.
2.6 Stream Finite State Machines
2.6.1 Externally Communicating FSMs
Communication between two ST agents is External Communication and
always happens between a NHSM and a PHSM pair (see Figure 2). Note
that, in the case of Origin and Target Agent as direct neighbors, it
is possible for a Target to be directly connected to an Origin. It
is also possible that one Target Agent is an Intermediate Agent for
another Target in which case an Agent will have a PHSM communicating
with a TSM and one or more NHSMs.
2.6.2 Internally Communicating FSMs
Communicating entities inside an ST Agent is different for each Agent
type, i.e., Origin, Intermediate or Target. However, all FSMs inside
an Agent communicate via a Message Separator/Combiner box (MS/C). The
function of the MS/C box is described later in this section.
Internal Communication within the Origin occurs:
o between the OSM and an Upper Layer module and
o between the OSM and one or more NHSMs via a MS/C box (Note
that the NHSMs themselves do not communicate with each other)
Internal Communication within a Target occurs:
o between one or more TSMs and an Upper Layer module and
o between the TSM and a PHSM via a MS/C box
Internal Communication within an Intermediate Agent occurs:
o between a PHSM and one or more NHSMs via a MS/C box (Note
that the NHSMs themselves do not communicate with each other)
2.7 Queues between External Communicating FSMs
For the purposes of modelling, assume that messages are filtered and
queued in FIFO queues for the case of external Communicating FSM
pairs, i.e. between any two ST Agents. However, as indicated in the
previous discussion and diagrams of the ST dispatcher and filtering
Rajagopal, Sergeant Expires April 21, 1996 [Page 11]
Internet Draft ST2+ Protocol State Machines October 26, 1996
hierarchy, it is somewhat more complex in reality. The concept shown
below in Figure 5 allows the discussion of the inter-Agent and
intra-Agent state machines to focus on the stream FSM issues without
regard to message and neighbor management issues.
Figure 5. Implicit Queues between an External Communicating FSM pair
2.8 Queues Inside an Agent
Each Agent is modeled with at least 2 state machines for each stream.
These state machines also need to communicate just like the external
communicating FSM pairs described above. The queue model in this case
also requires filtering mechanisms. This model requires a message
Separating and Combining function shown as Message Separator/Combiner
(MS/C) box in Figure 6.
Figure 6 pictorially describes the multi-stage FIFO queue model for
an Agent. Implicit FIFO queues are assumed between the PHSM and the
MS/C, and also between the MS/C and one or more NHSMs. Use of such
FIFO queues eliminates the need for a separate synchronizing state
machine that would normally be required to synchronize the flows.
Figure 6. Queues between Internal Communicating FSMs inside an Agent
The function of this Message Separator/Combiner box is many:
o Performing a multicasting function by replicating an OSM or
PHSM message and sending them to different NHSMs or TSMs
o Combining messages coming from different TSMs or NHSMs and
sending them to the appropriate OSM or PHSM
Designing the Agent to contain separate upstream and downstream state
machines (PHSM and NHSMs respectively) with FIFO queues as shown in
Figure 6, offers several benefits:
o It simplifies the Agent design considerably by separating the
neighbor upstream and downstream communications
o Use of FIFO queues simplifies the Agent management since no
other synchronization mechanisms need to be used to streamline
messages flowing through the Agent.
3 Stream Finite State Machines
Each ST Agent must maintain state for each stream supported by that
Rajagopal, Sergeant Expires April 21, 1996 [Page 12]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Agent. There are many ways to represent the state that must be
maintained by Agents. This section presents the OSM, NHSM, PHSM and
TSM as a reference set of state machines.
Implementations may support machines based on this section or may
even support a completely different set of state machines. These
stream FSMs represent normal operations for the stream request-
response scenarios without regard to the functions performed by the
Retry and Monitor FSMs. The model assumes that a data engine
separate from the control engine exists.
This section represents stream state through four state machines. The
defined machines are:
o The Origin State Machine, or OSM. It represents the state of
a stream at the Origin Agent.
o The Next-Hop State Machine, or NHSM. It represents the state
of the stream for Targets reached via a particular next-hop.
o The Previous-Hop State Machine, or PHSM. It represents the
state of a stream at an Intermediate Agent or a Target Agent. The OSM
is essentially a special case of the PHSM, where the delivery of SCMP
to the Origin is via an API.
o The Target State Machine, or TSM. It represents the state of
a stream for a particular target application at the Target Agent.
This state machine is essentially a special case of the NHSM, where
there is only a ever a single Target per TSM and delivery of SCMP to
the Target is via an API.
A number of NHSMs related to the same stream, could conceivably all
be running in parallel -one for each next hop. In some cases, where
there is a network-layer multipoint link (e.g., ethernet), it is even
possible to have more than one NHSM associated with the same physical
interface.
A Message Separator/Combiner (MS/C) box separates all downstream
messages modifying the Targetlist and placing them in the respective
NHSM FIFO queues. The MS/C box also functions as a combiner of
messages flowing up stream. In this role it multiplexes all local
messages and places them in the PHSM FIFO queues. Note that the MS/C
relies on separate routing and LRM functions to determine the
appropriate separation since route and resource computation is not
part of ST protocol. Full-duplex FIFO queues are assumed between the
MS/C box and PHSM, and also between the MS/C box and the NHSMs.
The multi-machine Agent model breaks the complexity that results with
Rajagopal, Sergeant Expires April 21, 1996 [Page 13]
Internet Draft ST2+ Protocol State Machines October 26, 1996
only one large model with the aid of the FIFO queue buffers and a
MS/C box. The FIFO queues eliminate the need for a separate
synchronizing state machine while reducing the complexity. The MS/C
reduces the explicit next-hop identification modelling that would
otherwise be required.
The Intermediate Agent PHSM always communicates with a NHSM on the
upstream side and the NHSM always communicates with a PHSM on the
downstream side.
3.1 Assumptions
Some basic assumptions were made as part of the development of the
enclosed state machines. These included:
o All state machines exist as part of an ST Agent and that the
Agent will instantiate state machines as needed to represent state on
a per stream basis.
o The ST Agent implements logic that unpacks incoming SCMP
packets, validates the contents, updates the Agent databases and
routes the message signal to the appropriate stream and it's
associated state machine.
o Detection and handling of messages that are broken,
duplicates, or not valid for a particular stream state does not
affect stream state and is not represented in the state machines. The
mechanisms to prevent such misleading signals to individual state
machines are described in the Architecture, Agent FSM and Exception
Processing Sections.
o All reliable delivery of intra- and inter-Agent SCMP messages
is handled by the ST Agent independent of the described state
machines except in the case where stream state is dependent on the
outcome of the message delivery.
o All communication within the same Agent should follow the
same Request Response paradigm as inter-Agent messages in order to be
as reliable as SCMP communications. This assumes that all API
communications and intra-agent communications recreate the
reliability available with the ACK, timeout and retry paradigms. It
is an implementation specific choice.
o The described state tables accurately reflect state
transitions of the ST2+ version of SCMP. The described state
diagrams accurately reflect the state transition tables for all
states, input trigger events and state transitions. Output events are
not shown in the diagrams, but are detailed in the tables. If there
Rajagopal, Sergeant Expires April 21, 1996 [Page 14]
Internet Draft ST2+ Protocol State Machines October 26, 1996
are discrepancies, the tables take precedence over the diagrams and
the protocol specification takes precedence over the tables.
o API notations for the Origin and Target ST applications are
shown to illustrate the OSM and TSM interactions. The actual
defintion of an ST application API is outside the scope of this
document.
3.2 State Machine Model Conventions
3.2.1 Naming Conventions and Notations
All state names are in bold and start out with a capital letter
followed by the lower case letters. All message names are in capitals
usually prefixed with a + or - sign. All messages with special
response conditions have suffixes indicating the condition, i.e.
_last, _all, _change. Predicates are in bold and lower case string.
Tables show states, events, output, and transitions. Diagrams show
states, events and transitions. Initial states are indicated by an
asterisk "*".
Messages that trigger events are proceeded by a plus sign "+".
Outputs are proceeded by a minus sign "-".
Transitions are represented by arrows in the diagrams and by ">>" in
the tables.
3.2.2 Transmissions and Receptions
In all the state machine models, the standard convention of prefixing
message transition labels, with a + or - symbol, is used to
explicitly indicate a transmission and reception respectively. The
prefixes are not part of the message syntax. In addition, the tables
will show both transmitted and received messages, but the diagrams
show only recieived messages. This simplifies the diagrams, but the
tables must be referenced for the message outputs.
3.2.3 Predicates
State transitions are sometimes dictated by conditions outside the
scope of the protocol specification. Predicates are mechanisms that
allow such transitions to occur. For example, terminating a protocol
session (a result of many conditions) should allow the Agent to
transition to either the initial state or some idle state. This
Rajagopal, Sergeant Expires April 21, 1996 [Page 15]
Internet Draft ST2+ Protocol State Machines October 26, 1996
decision is of course Application-initiated but a means should
nevertheless exist to allow transitioning to the correct state. In
the ST protocol there is no message which accomplishes this.
Predicates allow a state machine to express conditions and control
not explicitly possible with the protocol messages. Generally
speaking, they add clarity to the state diagram while reducing the
complexity in terms of states. The addition of control predicates
allows user defined change of states. Predicates are meant to give
hints to the protocol implementer and are not part of the ST
protocol. A Glossary in the Appendix can be used to check the
explicit meaning of each message or predicate.
API predicates are used to illustrate the OSM and TSM interactions
with ST applications. A predicate with an api_ prefix shows an API
message coming into the FSM. A predicate with an _api suffix shows a
message being sent to the API from an FSM.
For example, an api_open and an api_close predicate are defined for
the OSM as a means to transfer control to and from the Init state.
The Origin application may open or maintain a stream in the Establd
state without any Targets being active or in the TargetList.
NHSM, PHSM and TSM state machines have corresponding nh_open, ph_open
and tsm_open predicate definitions to allow the Agent to bring the
state machine into the Establd state when the Agent is ready to
process the initial CONNECT. Unlike the OSM, these state machines
return to the Init state when all Targets have been deleted, so no
predicate is required to close the NHSM, PHSM or TSM.
Some triggers and events are combinations of implicit and explicit
message conditions. This is particularly true for the RetryTimeout
mechanisms, as well as the requirement that responses from all
Targets in a Request be complete before the Request state can
complete. See Section 3.3 below.
No attempt has been made to illustrate the API interactions with
Routing and LRM functions. The results of these interactions affect
both how the TargetList is partitioned and what Reason Code has been
included in a DISCONNECT or REFUSE to indicate the source of a
Request failure. ST Agent management of such failures is discussed in
Section 4 ST Agent State Machines and Section 5 Exception Processing.
3.3 Normal Behavior versus Exception Processing
The stream FSMs describe the protocol under normal conditions. In
general, the architecture is designed to protect these stream FSMs
from error conditions handled in the Monitor and Retry FSMs. The SCMP
Rajagopal, Sergeant Expires April 21, 1996 [Page 16]
Internet Draft ST2+ Protocol State Machines October 26, 1996
messages STATUS, STATUS-RESPONSE, NOTIFY and ERROR, as well as
detailed error handling will be discussed in both Section 4 and
Section 5. Otherwise, if a core message transition is not specified
from a state, it implicitly means that this message is not allowed
from that state.
3.3.1 Context not Represented in Stream FSMs
The OSM, NHSM, PHSM and TSM diagrams and tables in this section
cannot represent state as the complete context of the stream. There
are context issues that are handled by the ST Agent Dispatcher, Retry
and Monitor FSMs and ST Agent database implementation. These stream
FSMs define the atomic elements of stream setup, maintenance and
teardown.
The G-bit (all Targets), the S-bit (stream Recovery), the I-bit and
E-bit ( CHANGE stream teardown risk) involve combinations of FSM and
stream database interactions. The implementor must consider the best
way to manage these conditions with the other elements of the ST
Service Model.
Stream Recovery by the Monitor FSM is modeled such that the
reconnection heuristics are outside of the basic CONNECT
functionality in the stream FSMs. The Monitor FSM initiates stream
teardown, and then initiates reCONNECT sequences. The individual
stream FSMs are not directly concerned with the Recovery option.
MTU size limitations may cause multiple SCMP PDUs for the same
transaction or an SCMP propagation failure. This type of problem is
managed by the Dispatcher and Retry FSM filtering.
Another issue not specifically addressed in this section is the
partitioning and management of the TargetList according to the NHSM
and ST Agent neighbor associated with each Target or set of Targets.
3.3.2 Special Message Types
In addition to the described predicates for API transactions and
state transitions, there are signals from the Retry FSM for ACK
failures, a signal for the timer expiration for the End-to-End
Response to a Request and special conditions for a REFUSE Response to
a CHANGE.
The Retry FSM issues a RetryTimeout signal when no ACK has been
received for a Request after the configured number of retries have
been attempted. This signal is an implicit REFUSE to appropriate PHSM
or NHSM.
Rajagopal, Sergeant Expires April 21, 1996 [Page 17]
Internet Draft ST2+ Protocol State Machines October 26, 1996
In the following FSM explanations, you will note that a RetryTimeout
is an indicated signal only to the NHSM and the PHSM. The NHSM and
PHSM provide the inter-Agent communications for the stream FSMs. A
RetryTimeout is generated by the Retry FSM and forwarded to the
appropriate PHSM or NHSM, and that FSM then generates the appropriate
DISCONNECT and REFUSE messages as intra-Agent communications. For
example, an OSM would receive a REFUSE with Reason Code 41
RetransTimeout as the result of an NHSM receiving a RetryTimeout.
Once an Origin (or Agent acting as an Origin) receives an ACK to a
Request in the Retry FSM, the End-to-End Response timer is set for
the maximum time to wait for Responses to this Request. If this End-
to-End timer expires before a Response has been recieved, the
E2ETimeout becomes an implicit REFUSE for all Targets that have not
yet Responded. The Retry FSM communicates this failure to OSM (or
PHSM, in the case of an Agent acting as Origin) as an E2ETimeout. The
OSM issues the appropriate messages to the API and NHSM.
If a CHANGE request is made with the I-bit set, the LRM may risk
losing the existing resources to allocate the requested resources. If
the I-bit is not set, application does not want to risk losing the
current resources for the sake of a CHANGE. Thus when a REFUSE to a
CHANGE is recieved, and the E-bit is zero, it means the REFUSE will
result in stream teardown. This is the normal result of a REFUSE.
However, if the the E-bit is set, it is a REFUSE_CHANGE, indicating
only that the CHANGE could not be completed, but the that the stream
still has the original QOS resources.
3.3.3 Classes of Response Types
The ST2+ protocol requires that all Responses be received from all
Targets in a TargetList before the Request state transition may be
completed and any other Request may be processed. The protocol,
however, allows immediate processing of all DISCONNECT and REFUSE
messages whether or not they are not initiated by the current
REQUEST. These requirements result in the need to differentiate three
classes of Responses.
The first class is a Response that does not have any signifigance for
state change, where such Responses are not specifically either the
last one required to complete the TargetList of the current Request,
nor a deletion of the last of all Targets associated with that
stream's FSM. Completion of the Responses for the current TargetList
is the second class. Removal of the last of all Targets for that
stream's FSM is the third class.
All Responses in the second and third class are defined by predicates
that identify the message type with a suffix for either last (class
Rajagopal, Sergeant Expires April 21, 1996 [Page 18]
Internet Draft ST2+ Protocol State Machines October 26, 1996
2) or all (class 3). All API references are illustrative and are not
intended to fully define the application interface.
Class 1 Responses:
api_accept, api_disconnect, api_refuse, ACCEPT, DISCONNECT, REFUSE,
REFUSE_CHANGE and RetryTimeout indicate that an individual TargetList
member has signaled a Response. The api_disconnect, api_refuse,
DISCONNECT and REFUSE messages may also be a Request to delete a
Target( whether or not it is in the current TargetList of an Add or
Change state transaction). Individual and/or Global Target deletion
may occur at any time, but any Global (G-bit set) Target Response or
deletion Request falls into one of the second two classes.
Class 2 Responses:
api_accept_last, api_disconnect_last, api_refuse_last, ACCEPT_LAST,
DISCONNECT_LAST, REFUSE_LAST, REFUSE_CHANGE_LAST, RetryTimeout_last,
E2E_Timeout_last are only relevant to the current stream Request and
refer to the completion of the Request state by occurring as the
Response that incidently completes the TargetList.
Class 3 responses:
api_disconnect_all, api_refuse_all, DISCONNECT_ALL and REFUSE_ALL
refer to the Requests or Responses that remove the last active Target
from that FSM for that stream.
These classes delineate the asynchronous Request/Response activity
that may occur. Network conditions may result in interruptions of any
stream FSM operation.
The OSM, NHSM, PHSM and TSM diagrams and tables in this section
define stream state as it relates to atomic setup and teardown
functions. Every attempt has been made to delineate the atomic SCMP
request-response specifications such that implementors may reorganize
the Agent architecture to address implementation-specific issues.
3.4 Stream State Machines
3.4.1 Origin State Machine (OSM)
The Origin State Machine (OSM) communicates with one or more NHSMs.
The OSM also talks to the Upper Layer module via primitives. This OSM
to Upper Layer Interface is outside the scope of this document, but
examples of API predicates are illustrated in the diagrams and
tables. All ST Dispatcher and MS/C Box diagrams have indicated that
API messages could be included. The actual mechanism used for API
Rajagopal, Sergeant Expires April 21, 1996 [Page 19]
Internet Draft ST2+ Protocol State Machines October 26, 1996
communications should be decided by implementation factors.
The OSM consists of a small number of states: Init, Establd, Add and
Change.
Init: The initial state is called Init. An api_open predicate moves
the control to the Establd state. An api_close is required to return
the stream to the Init state.
Establd: The Establd state is the stable state from which all
api_connect, JOIN and api_change requests may cause a transition to
the Add or Change states. All Requests that occur while a stream is
in either an Add or Change state will be queued up until the stream
returns to the Establd state. Data transfer may occur to established
Targets. The removal of Targets from previous operations or current
operations may occur in the Establd, Add or Change states with an
api_disconnect or a REFUSE.
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. A
JOIN message received by an OSM indicates that the Origin Agent
happens to be the first Agent for that stream in the path between the
JOIN originator and the Origin.
JOIN messages from potential Targets require the authorization
process to determine if the JOIN will be allowed. The OSM then issues
either a JOIN-REJECT message or a CONNECT message. If this validation
is complete and the stream JOIN option allows authorization to be
completed,the ST Agent at the Origin transitions to the Add state and
then issues a CONNECT message that contains the SID, the FlowSpec,
and the TargetList specifying the new Target, waiting an ACCEPT or
REFUSE response.
If this is not the case, a JOIN-REJECT message is sent to the Target
with the appropriate ReasonCode (e.g., JoinAuthFailure,
DuplicateTarget or RouteLoop). Issuing a JOIN-REJECT brings the OSM
back to the Establd state.
Add:Once in the Establd state the API may issue an api_connect. A
transition to Add will create a CONNECT message that is placed in the
FIFO queue between the OSM and the MS/C box. The CONNECT message
contains the SID, an updated FlowSpec, and a TargetList. The MS/C box
will then make a copy of the CONNECT message, partition the
Targetlist parameter and place it the NHSMs queues. The spliting (or
separating) information is derived from the implementation's routing
and LRM functions.
Once in the Add state the OSM waits to get ACCEPT or REFUSE
Rajagopal, Sergeant Expires April 21, 1996 [Page 20]
Internet Draft ST2+ Protocol State Machines October 26, 1996
responses. The stream will not transition back to the Establd state
until all Targets have responded. A REFUSE may be generated by the
local Routing or LRM functions, NHSM or Monitor FSMs as well as any
Agent in the path between the Origin and the Target. Normal
operations in the OSM treat all types of REFUSE responses to a
CONNECT in the same manner. The Monitor FSM will manage the Recovery
reCONNECT analysis and may also be expanded to include other ST
Service Model functions.
The OSM will record the status of each response from each Target. As
each ACCEPT is received, the OSM updates its database and records the
status of each 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 either an ACCEPT or REFUSE from all Targets has
been received at the Origin, the stream state returns to Establd and
any additional queued up requests may then be processed.
Figure 7. Origin State Machine (OSM)
Table 2: OSM
Once an ACCEPT is received by the OSM, the path to the Target is
considered to be established and the ST Agent is allowed to forward
the data along this path. When a REFUSE reaches the OSM, the OSM
notifies the Application that the Target is no longer part of the
stream. If there are no remaining Targets, the Application may wish
to terminate the stream or keep the stream active to allow stream
joining.
To ensure 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 all stream Targets.
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. 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-hops might have to be added to an
Rajagopal, Sergeant Expires April 21, 1996 [Page 21]
Internet Draft ST2+ Protocol State Machines October 26, 1996
existing multicast group. These issues are managed by the
implementation of the ST Service Model and stream state transitions
remain the same. Intermediate or Target ST Agents that are not
already nodes in the stream behave as in the case of stream setup.
The OSM may issue a DISCONNECT when an api_disconnect is received.
This message may be processed in any state. The OSM then records this
fact and appropriately updates its database.
A REFUSE message may arrive at the OSM asynchronously at any
time. This message is sent as a result of an Intermediate Agent
failure or a Target leaving a stream.
Change:The Application at the Origin may wish to change the FlowSpec
of an established stream. To do so, it informs the ST Agent at the
Origin of the new FlowSpec and of the list of Targets associated with
the change with an api_change. The Origin then issues one CHANGE
message with the new FlowSpec per next-hop and sends it to the
relevant next-hop Agents. The control flow to the Change state is
very similar to the control to the Add state from the Establd state.
Depending on the CHANGE options selected and the resources
avalailable in each of the stream paths, the CHANGE may result in
either a simple refusal of any change or the disconnect of the entire
stream. A REFUSE response to a CHANGE request with the E-bit set to
zero means that the stream has been torn down for that Target. A
REFUSE_CHANGE is a REFUSE with the E-bit set to 1 indicating that the
CHANGE has been refused but the prior stream resources are unchanged
3.4.2 Next Hop State Machine (NHSM)
The NHSM is pictorially shown in Figure 8. This model is common to
the Origin as well as an Intermediate Agent.The NHSM consists of the
same fundamental states as the OSM: Init, Establd, Add and Change.
Init:The state machine for each next hop enters its Init state at
Agent start-up time. An asterisk indicates that this is the initial
state. A nexthop_open predicate moves control to the Establd state
when the next hop associated with an NHSM is required by Targets in a
stream.
Establd:Once in the Establd state a number of things can happen.
Targets may be added by the Origin or Targets may request to join the
stream. However, the processing of a JOIN request is always handled
by either an OSM or a PHSM. Within each ST Agent, the ST Dispatcher
examines incoming JOIN requests and determines whether the stream
referenced is a stream that that Agent supports. If not, the JOIN is
forwarded on towards the Origin. Once a JOIN request reaches an Agent
that can process the JOIN, the ST Dispatcher ACKs the JOIN and queues
Rajagopal, Sergeant Expires April 21, 1996 [Page 22]
Internet Draft ST2+ Protocol State Machines October 26, 1996
it up to the resident OSM or PHSM. The NHSM only sees the resultant
CONNECT when stream authorization has completed successfully and the
OSM or PHSM has issued a CONNECT through the MS/C Box.
As previously described in the OSM, an ST Agent can handle only one
stream Add or Change at a time. If such a stream operation is already
underway, further requests are queued and handled when the previous
operation has been completed. Either a DISCONNECT or REFUSE for all
Targets transfers control from the Establd state to the Init state.
Add: A CONNECT that has been propagated from the NHSM Add state to
the next hop Agent PHSM and will require a Response in the form an
ACK. If an ACK is not received, the timeout and retry mechanisms of
the Retry FSM will invoke a RetryTimeout signal. Every PDU has a
unique reference number, so that all ACKs may be matched to the
appropriate Request or Response.
The CONNECT message contains the SID, an updated FlowSpec, and a
TargetList. 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. If the TargetList causes a PDU that is larger than the MTU
size, CONNECT message to be generated, the CONNECT message is
partitioned.
The ACK, if it is received, does not need to be reported to the NHSM.
However, if the ACK is not received and the retries are exhausted, a
RetryTimeout signal will be reported to the NHSM and interpreted as a
REFUSE. The NHSM will record all Target Responses until the last
Target in the TargetList has sent an ACCEPT or REFUSE (or an implicit
REFUSE due to Retry exhaustion). An Origin DISCONNECT may terminate
this process when the End-to-End Response timer is exceeded. A
DISCONNECT or REFUSE signal may be due to the failure of a next hop
or previous hop.
If an Application at a Target does not wish to participate in the
stream, it sends a REFUSE message back to the Origin with a
ReasonCode (ApplDisconnect). When an NHSM receives a REFUSE message
with ReasonCode (ApplDisconnect), the acknowledgement has already
been sent by the ST Dispatcher as an ACK to the next-hop. The Agent
considers which resources are to be released, deletes the Target
entry from the internal database, and propagates the REFUSE message
back to the OSM or PHSM.
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
Rajagopal, Sergeant Expires April 21, 1996 [Page 23]
Internet Draft ST2+ Protocol State Machines October 26, 1996
be released if network multicasting is being used since they may
still be required for traffic to other next-hops in the multicast
group.
Change: The Application at the Origin may wish to change the FlowSpec
of an established stream. To do so, it informs the OSM of the new
FlowSpec and of the list of Targets relative to the change. The OSM
then issues one CHANGE message with the new FlowSpec per next-hop and
sends it with the correct Targetlist. The MS/C box then places copies
(as required) of this in the NHSM queues. This takes the control to
the Change state from the Establd state. CHANGE messages are
structured and processed similar to CONNECT messages.
A next-hop agent that is an Intermediate Agent that receives a CHANGE
message similarly determines if it can implement the new FlowSpec
along the path 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 (or REFUSE) message
that is propagated back to the OSM.
Figure 8. Next Hop State Machine (NHSM)
At this point the Application decides whether all replies have been
received. If the change to the FlowSpec is in a direction that makes
fewer demands of the involved networks, then the change has a high
probability of success along the path of the established stream. Each
ST agent receiving the CHANGE message makes the necessary request
changes to the network resource allocations, and if successful,
propagates the CHANGE message along the established paths. If the
change cannot be made, but the E-bit indicates that stream should be
torn down, then the ST Agent must recover using DISCONNECT and REFUSE
messages as in the case of a network failure. Note that a failure to
change the resources requested for specific Targets should not cause
other targets in the stream to be deleted. A REFUSE response to a
CHANGE request with the E-bit set to zero means that the stream has
been torn down for that Target. A REFUSE_CHANGE is a REFUSE with the
E-bit set to 1 and the stream is unchanged
Table 3: NHSM
The Application at the Origin may specify a set of Targets that are
to be removed from the stream with an appropriate ReasonCode
(ApplDisconnect). The Targets are partitioned into multiple
DISCONNECT messages based on the next-hop route towards the
individual Targets. If the TargetList is too long to fit into one
DISCONNECT message, it is partitioned.
Rajagopal, Sergeant Expires April 21, 1996 [Page 24]
Internet Draft ST2+ Protocol State Machines October 26, 1996
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 the 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.
When the DISCONNECT reaches a Target, the Target Agent sends an ACK
to the upstream NHSM and notifies the Application (at target) that it
is no longer part of the stream and for which reason. The ST Agent at
the Target deletes the stream from its database after performing any
necessary management and accounting functions. Note that the stream
is not deleted if the ST Target Agent is also an Intermediate Agent
for the stream and there are remaining downstream Targets.
Data Forwarding: Once the Application or OSM determines that the
stream is established Data may be transferred to the targets. An
Application is not guaranteed that the data reaches its destinations:
ST is unreliable and it does not make any attempt to recover from
packet loss, e.g. due to the underlying network. In case the data
reaches its destination, it does it accordingly to the negotiated
quality of service. An ST Agent forwards the data only along already
established paths to Targets.
Since a path is considered to be established when the ST next-hop
agent on the path sends an ACCEPT message, it 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 case will an ST
Agent forward data to a next-hop Agent that has not explicitly
accepted the stream.
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 Agents. 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 SID. This SID was selected at
the Origin so that it is globally unique and thus can be used as an
index into the database, to obtain quickly the necessary replication
and forwarding information.
Rajagopal, Sergeant Expires April 21, 1996 [Page 25]
Internet Draft ST2+ Protocol State Machines October 26, 1996
The forwarding information will be network and implementation
specific, but must identify the next-hop Agents. 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 the next-hops by a (local network) multicast
address. If the network does not support multicast, or the next-hops
are on different networks, multiple copies of the data packet must be
sent.
No data fragmentation is supported during the data transfer phase.
The Application is expected to segment its 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 FlowSpec contained in the ACCEPT message. The minimum MTU over
all paths has to be calculated from the MTUs relative to the single
paths. If the Application at the Origin sends a too large data
packet, the ST Agent at the Origin generates an error and it does not
forward the data.
3.4.3 Previous Hop State Machine (PHSM)
The Previous Hop State Machine Model is common to a Target or
Intermediate Agent. A PHSM communicates with an upstream NHSM and
downstream with one or more NHSMs and/or a TSM via a MS/C box. When a
CONNECT message is received, the Intermediate ST Agent invokes the
routing function, reserves resources via the Local Resource Manager,
and then propagates the CONNECT messages to its next-hops. For the
most part the Intermediate Agent behaves like a relay. In the cases
when the Intermediate Agent is not able to successfully send out a
CONNECT message to a downstream PHSM, a REFUSE message from the PHSM
is sent to the upstream NHSM.
The PHSM consists of a small number of states: Init, Establd, Add and
Change.
Init: The ST Agent initially takes control from the Init state to the
Establd state via the phsm_open predicate. A DISCONNECT or REFUSE of
all Targets in a stream will take the stream from the Establd to a
terminating state which is also the Init state.
Establd:Once in the Establd state, Targets may be added or changed by
the Origin or Targets may request to join the stream. The processing
of a JOIN request is always handled by either an OSM or a PHSM.
Within each ST Agent, the ST Dispatcher examines incoming JOIN
requests and determines whether the stream referenced is a stream
that that Agent supports. If not, the JOIN is forwarded on towards
the Origin. Once a JOIN request reaches an Agent that can process the
Rajagopal, Sergeant Expires April 21, 1996 [Page 26]
Internet Draft ST2+ Protocol State Machines October 26, 1996
JOIN, the ST Dispatcher ACKs the JOIN and queues it up to the
resident OSM or PHSM. When stream authorization has completed
successfully, the PHSM issues a CONNECT through the MS/C Box to
either a NHSM or a TSM.
As previously described in the OSM, an ST Agent can handle only one
stream Add or Change at a time. If such a stream operation is already
underway, further requests are queued and handled when the previous
operation has been completed. Either a DISCONNECT or REFUSE for all
Targets transfers control from the Establd state to the Init state.
Add: Once in the Establd state the previous hop may relay a CONNECT
message. A transition to Add will create a CONNECT message that is
placed in the FIFO queue between the PHSM and the MS/C box. The
CONNECT message contains the SID, an updated FlowSpec, and a
TargetList. The MS/C box will then make a copy of the CONNECT
message, partition the Targetlist parameter and place it the NHSM
and/or TSM queues. The spliting (or separating) information is derived
from the implementation's routing and LRM functions.
Once in the Add state the OSM waits to get ACCEPT or REFUSE
responses. The stream will not transition back to the Establd state
until all Targets have responded. The expiration of the retry timer
and count (if the next hop is not ACKing the request) or the
expiration of the end-to-end timer will be interpreted as an implicit
refuse.
Change:The Application at the Origin may wish to change the FlowSpec
of an established stream. To do so, it informs the ST Agent at the
Origin of the new FlowSpec and of the list of Targets relative to the
change and this message will be propagated to through the NHSMs to
the PHSMs and TSMs. The control flow to the Change state is very
similar to the previous FSM discussions.
Figure 9. Previous Hop State Machine (PHSM)
Table 4: PHSM
3.5 The Target State Machine (TSM)
The Target State Machine (TSM) is a high level state machine which
communicates with a PHSM, or OSM if residing the same Agent as the
Origin. The TSM also talks to the Upper Layer module via primitives.
The TSM consists of a small number of states: Init, Establd, Add and
Change.
Init: The ST Agent initially takes control from the Init state to the
Rajagopal, Sergeant Expires April 21, 1996 [Page 27]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Establd state via the tsm_open predicate. A Target 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 together with
the FlowSpec. The Application directs the TSM to generate a JOIN
message containing the Application's request to join the stream and
sends it to the PHSM which in turn sends it upstream toward the
stream Origin.
An ST Agent receiving a JOIN message for which that Agent has a
matching stream, 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. Eventually,
an ST Agent traversed by the stream or the stream's Origin itself is
reached. In any case, the TSM will eventually receive a JOIN-REJECT
or CONNECT response. This is shown as transitions to the Establd
state and the Add state respectively.
Add: The TSM may receive a CONNECT message any time. The ST Agent
reserves local resources and inquires from the specified Application
process whether or not it is willing to accept the connection. In
particular, the Application must be presented with parameters from
the CONNECT, such as the SID, 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 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.
The TSM responds with an ACCEPT or REFUSE - a result of the Upper
Layer module decision.
Change: The TSM may receive a CHANGE message any time it is in a
Establd state. This happens always after a CONNECT. The TSM again
responds with an ACCEPT or REFUSE after informing the Upper Layer
Protocol.
The TSM may at any time want to terminate its membership in the
stream. This is handled by the TSM sending out a REFUSE message. On
the other hand it is possible for an Origin or IntermediateAgent to
disconnect the Target from the stream. This is accomplished by the
Agent or Origin sending a DISCONNECT message.
Rajagopal, Sergeant Expires April 21, 1996 [Page 28]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Figure 10. Target State Machine (TSM)
Table 5: TSM
4 ST Agent FSMs
This section describes the Retry FSM and the Monitor FSM for the
datalink and ST Agent neighbor reliability functions. The OSM, NHSM,
PHSM and TSM have been shown to model the stream specific Request-
Response pattern. The Retry FSM models the datalink reliability
provided by the ACK mechanisms with the associated timer and retry
count. The Monitor FSM models the ST Agent reliabilty provided by the
neighbor HELLO mechanisms, the STATUS and STATUS_RESPONSE messages,
as well as the stream Recovery timer and retry count.
The ST Agent FSMs are the unifying aspect of the total FSM model
architecture. These models are dependent on how the SCMP messages
traverse the ST Agents, and impact Agent databases and FSMs.
4.1 Agent Database Context
ST Agent stream database entries are intitated by the first CONNECT
for that Stream Id. The information initially correlated to each
StreamId entry includes:
ST Neighbor Previous Hop and Next Hops
FlowSpec, Group, MulticastAddress, Origin, TargetList,
ACK and Response timers
Stream Options for NoRecovery(S-bit) and Join
Authorization Level (J-bit, N-bit)
Routing results for each Target's Next Hop
LRM results for each Next Hop's resource allocation
Each Agent database is modified when the CONNECT Responses indicate
some variation specified by a downstream Agent or Target Response.
Subsequent Requests can also modify the database and include
additional CONNECT, JOIN and CHANGE requests. Origin, Network or
Agent Recovery and LRM initiated stream teardown can occur in the
form of explicit DISCONNECT, REFUSE and Recovery initiated CONNECT
messages or implicit conditions detected through the HELLO, STATUS,
STATUS-RESPONSE and NOTIFY messages.
The database context is then augmented with the history of Reason
Rajagopal, Sergeant Expires April 21, 1996 [Page 29]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Codes and prior stream characteristics. Transient state
characteristics can also include the G-bit (Global stream
TargetList), I-bit (CHANGE risking teardown of old resources)and E-
bit (CHANGE REFUSE without teardown), R-bit (Restarted Agent) or ACK
and Response retry values.
While all control messages may have an indirect effect on stream
state and databases, only ACCEPT, CHANGE, CONNECT, DISCONNECT,
JOIN-REJECT, JOIN and REFUSE directly affect each Agent's defintion
of each stream. ACK, ERROR, HELLO, NOTIFY, STATUS and STATUS-
RESPONSE are the control messages that are primarily used to maintain
ST Agent databases for datalink, neighbor and network management
functions.
As the ST PDUs traverse the network, each Agent presumably has a
platform specific interface-to-packet-switching function that must
intercept the ST packets for ST functions. The ST Dispatcher
represents ST PDU validation, filtering and packet-switching. The ST
Dispatcher in this model is organized as the Agent packet-switcher,
rather than as a per-interface or per-next-hop packet-switcher. This
function may be reorganized as a distributed function if the Agent
platform architecture requires such distribution.
4.2 ST Dispatcher role for incoming Packet-switching,
ACKnowledgement and PDU validation
An ST Dispatcher can validate an ST PDU for ST header and PDU syntax
and semantic validity, and then rapidly switch Data packets, .i.e to
a local Target application SAP or to the appropriate next hop
interface for remote Targets.
When the PDU syntax are in error, an ERROR PDU with the corresponding
Reason Code and the offending PDU contents are returned to the
SenderIpAddress (instead of an ACK for those SCMP messages that
require an ACK). The incoming PDU in ERROR is then discarded and does
not directly impact any FSM state. The following Reason Codes detail the
inconsistencies that could be reported in an ERROR Response:
2 ErrorUnknown An error not contained in this list
has been detected.
8 AuthentFailed The authentication function failed.
13 CksumBadCtl Control PDU has a bad message
checksum.
Rajagopal, Sergeant Expires April 21, 1996 [Page 30]
Internet Draft ST2+ Protocol State Machines October 26, 1996
14 CksumBadST PDU has a bad ST Header checksum.
23 InvalidSender Control PDU has an invalid
SenderIPAddress field.
24 InvalidTotByt Control PDU has an invalid
TotalBytes field.
* 26 LnkRefUnknown Control PDU contains an unknown
LnkReference.
31 OpCodeUnknown Control PDU has an invalid OpCode
field.
32 PCodeUnknown Control PDU has a parameter with an
invalid PCode.
33 ParmValueBad Control PDU contains an invalid
parameter value
35 ProtocolUnknown Control PDU contains an unknown
next-higher layer protocol identifier.
37 RefUnknown Control PDU contains an unknown
Reference.
45 SAPUnknown Control PDU contains an unknown next-
higher layer SAP (port)
* 46 SIDUnknown Control PDU contains an unknown SID.
48 STVer3Bad A received PDU is not ST Version 3.
54 TruncatedCtl Control PDU is shorter than expected.
55 TruncatedPDU A received ST PDU is shorter than the
ST Header indicates.
In some cases, RFC1819 specifically requires that an error in a PDU
Rajagopal, Sergeant Expires April 21, 1996 [Page 31]
Internet Draft ST2+ Protocol State Machines October 26, 1996
result in an ACK and then a response with the error code. An example
of this is when a CONNECT or CHANGE request with an unknown SID
results in an ACK followed by a REFUSE with Reason Code 46. In any
event, the ST Dispatcher function is to direct only valid PDUs to the
inidividual FSM logic.
Figure 11. ST Dispatcher Input
The next level of PDU analysis involves Agent and stream consistency.
The PDU is examined for content consistency with both Agent and
stream database information. The following detected inconsistencies
may result:
3 AccessDenied Access denied.
4 AckUnexpected An unexpected ACK was received.
15 DuplicateIgn Control PDU is a duplicate and is
being acknowledged.
16 DuplicateTarget Control PDU contains a duplicate
target, or an attempt to add an
existing target.
49 StreamExists A stream with the given SID already
exists.
51 TargetExists A CONNECT was received that specified
an existing target.
52 TargetUnknown A target is not a member of the
specified stream.
53 TargetMissing A target parameter was expected and
is not included, or is empty.
Most SCMP PDUs (except ACK, ERROR, HELLO, STATUS, STATUS-RESPONSE,)
will trigger an ACK to the ST neighbor that sent the PDU.
CONNECT, CHANGE and JOIN Requests will be directed to the appropriate
stream PHSM.
Rajagopal, Sergeant Expires April 21, 1996 [Page 32]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Incoming Responses are first correlated with any corresponding
Request Reference so that the appropriate next hop or Response timer
may be terminated. Then ACCEPT and REFUSE messages are queued up to
the appropriate stream NHSM, while DISCONNECT and JOIN-REJECT
messages are queued up to the appropriate stream PHSM.
ACK and ERROR messages are correlated with a PDU Reference,
terminating the appropriate timers, and then queued up to the stream
Retry FSM.
HELLO, STATUS and STATUS-RESPONSE messages are correlated with a PDU
Reference so that the appropriate timers may be terminated and then
queued up to the Monitor FSM.
4.3 ST Dispatcher functions for outgoing Packet switching, timer
and retry settings
Figure 12.
The ST Dispatcher also has the role of packaging and forwarding
outgoing PDUs to the apprropriate interfaces. The outgoing PDU must
be given it's own PDU Reference number and any correlated PDU
Referencer number, as well as the semantics and context of the PDU
database entries. This Agent architecture model assumes that the
Agent and stream databases are the intra-Agent repository of all
activities, such that the ST Dispatcher can efficiently create and
distribute the PDUs. However, it is entirely possible that the
accumulated contents of a PDU has exceeded an outgoing MTU
restriction and the PDU would be truncated with the following Reason
Codes:
6 UserDataSize UserData parameter too large to
permit a message to fit into a
network's MTU.
36 RecordRouteSize RecordRoute parameter is too long
to permit message to fit a network's
MTU.
4.4 Retry FSM- RFSM for datalink reliability of PDU transmissions
The following table provides a quick reference for ST Recovery and
Retry implications across ST Agent FSMs. Each SCMP message type that
requires an ACK has configured values for the ACK timer and Retry
Rajagopal, Sergeant Expires April 21, 1996 [Page 33]
Internet Draft ST2+ Protocol State Machines October 26, 1996
count. Each Request that requires an End-to-End Response has a
configured value for the End-to-End Response timer. End-to-End
Response timers are set by the Retry FSM when an ACK signals that the
Request has successfully gone out to the network. The Recovery
Option, STATUS and HELLO messages are managed by the Monitor FSM and
follow a different paradigm.
Table 6: Table of local control and End-to-End retry parameterss
The Retry FSM conditions are explicit when an ST Agent neighbor ACK
terminates the neighbor ACK timer and retry count for that
transaction. Any End-to-End Response will terminate the End-to-End
Response timer. Implicit conditions occur when any of the timer or
the retry count values have been exhausted. The general paradigm is
that an implicit REFUSE is generated for unsatisfied downstream
Requests and an implicit DISCONNECT is generated for unsatisfied
upstream Requests. The secondary consequence of a timeout is that
explicit REFUSE and DISCONNECT messages may also be issued.
Each table entry has its own variation of this basic paradigm. In
addition, the ST specification indicates many secondary and tertiary
implications for SCMP message failures. As a particular example, once
any ST Agent has completed a downstream Request-Response scenario, an
upstream propagation problem may or may not cause the stream to be
torn down. The I-bit (risk teardown)in CHANGE processing and the S-
bit (Recovery) are examples of causes for the secondary and tertiary
implications.
Figure 13. Retry State Machine (RFSM)
The Retry FSM has three states - Init, Ack-wait and Resp-wait. The
general paradigm for the Retry FSM is to move from the Init state to
the Ack-wait state whenever a PDU requiring an ACK is sent. The
STATUS message does not require an ACK, but the required STATUS-
RESPONSE performs the same function as an ACK.
The Retry FSM waits for the resultant ACKs, Responses and/or
timeouts. PDUs requiring ACKs cycle through resends for the
appropriate NAccept, NChange, NConnect, NDisconnect, NJoin,
NJoinReject, NNotify and NRefuse configured counts.
Since all ACKs are correlated by PDU Reference numbers, packets maybe
correlated to the outstanding Retry FSM by the same mechanism. Either
Rajagopal, Sergeant Expires April 21, 1996 [Page 34]
Internet Draft ST2+ Protocol State Machines October 26, 1996
an ACK or a RetryTimeout that is correlated to an ACCEPT, DISCONNECT,
JOIN-REJECT, NOTIFY or REFUSE results in the Retry FSM transitions to
Init. Such PDUs have no End-to-End response requirements and
generally have no secondary error processing when it can be assumed
that the neighbor Agent and/or link layer reliability is gone. An
ACCEPT is an exception. The failure of an ACCEPT is an implicit
REFUSE upstream and DISCONNECT downstream, since this ACCEPT was an
End-to-End Response that has now failed to completely traverse the
stream Agents.
An ACK on a CHANGE, CONNECT or JOIN causes the respective
ToChange,ToConnect or ToJoin End-to-End timers to be set, and a state
transition to Resp-wait.
A legitimate Response, or an E2ETimeout on CHANGE, CONNECT or JOIN
causes the transition to the Init state with the signal to be
replicated to the appropriate stream FSM.
4.5 Agent, Neighbor and Stream Supervision
4.5.1 The Monitor FSM (MFSM) for Agent and Stream Supervision
Each ST Agent must monitor its own status, network conditions,
neighbor Agent status and supervise the Recovery of streams whenever
required and possible during a network failure. This MFSM is intended
to be a general approach to these issues, rather than a fully
specified FSM since the particular network, platform and
implementation architecture will determine detail FSM considerations.
What this MFSM model does suggest is that the MFSM provides a
superstructure for the management of the Neighbor Detection Failure
FSM(NDFSM), as well as any Agent NOTIFY, STATUS or STATUS-RESPONSE
implications. The Service Model management (including application
issues, routing and LRM or other Agent implementation specific
issues), as well as datalink statistics analysis (e.g., broken or
dropped PDUs or accumulated routing errors) may also be incorporated
into this FSM.
At the very least, stream Recovery requires careful analysis of the
possible recursions in Agent, neighbor failure detection, routing and
LRM conditions. The ST2+ specification defines parameters for a
configured number of times that Recovery should be attempted
(NRetryRoute), the configured time to wait for each Response
(ToRetryRoute) and variations in the exception processing.
Rajagopal, Sergeant Expires April 21, 1996 [Page 35]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Figure 14. MFSM
During the course of a stream setup, the CONNECT contains a Recovery
Timeout, as specified by the Origin. The resultant ACCEPTs contain
the Agent's "supportable" Recovery Timeout such that the stream
Recovery Timeout becomes the smallest Recovery Timeout for all
Targets. The HELLO timer must be smaller than the smallest Recovery
Timeout for all streams between these Agents, but an Agent may have
various HELLO timers between different Agents, such that the
management function of such timers should fall into the MFSM also. A
Round Trip Time (RTT) estimation function is available with STATUS
and STATUS-RESPONSE messages to aid in this area.
The MFSM relies on the Nieghbor Detection Failure FSM (NDFSM) as the
primary notification vehicle for stream and neighbor management.
During the initial stream setup of any stream NHSM and PHSM, the MFSM
is signalled to begin monitoring of the FSM neighbor Agents involved
in the stream. The sending of HELLOs is begun once an ACCEPT is
forwarded upstream. The receiving of HELLOs is acceptable as soon as
an ACCEPT is received. HELLOs are terminated once an ACK is sent or
received for the DISCONNECT or REFUSE associated with the last of all
streams and Targets for that neighbor. This requires signalling and
coordination with the ST Dispatcher, Retry FSM and database context,
especially when the Restarted bit is active for either the local
Agent of a neighbor.
Agent network "inspection and repair" functions might also exist in
the MFSM to extend the mechanisms of the NDFSM before attempting
Recovery and/or stream teardown.
Group management for Bandwidth-sharing, Fate-sharing, Path-sharing
and Subnet resource- sharing can be intiated by any ST Agent and it
may be adviseable to incorporate optimization algorithms in the MFSM
to interact with Routing and LRM functions, thus allowing the MFSM to
monitor and gauge the impact on the stream Recovery analysis.
4.5.2 The Nieghbor Detection Failure FSM for Neighbor Management
This FSM has a more atomic focus in that ST neighbor HELLOs are
maintained and monitored only while there are one or more shared
streams active. When the neighbor HELLOs and subsequent STATUS
inquiry fails or the neighbor R-bit has been set, the neighbor is
considered down and the streams involved in that neighbor
relationship must be examined for Recovery conditions.
Figure 15. NFDSM
Rajagopal, Sergeant Expires April 21, 1996 [Page 36]
Internet Draft ST2+ Protocol State Machines October 26, 1996
Table 7: NFDSM
4.5.3 Service Model Interactions
Figure 16. MS/C Box Communications inside an Agent
The optimization of route and LRM functions can affect the selection
from multiple path routes to a Target on initial CONNECTs, as well as
CHANGE and Recovery procedures. This document's model follows a
sequential process of integrating the route and LRM services with the
MS/C Box for the atomic stream FSMs.
Additional algorithms may be used in the MFSM, such that algorithms
for Options and Group factors may be optimized in relation to the
stream Recovery decisions.
5 Exception Processing
Various types of exception processing conditions have been referenced
in the preceding sections. Not all have been spelled out in detail.
The general paradigms fall into several categories and all of this
document's models are based on a suggested approach. The secondary
and tertiary conditions of some apects of exception processsing are
especially subject to implementation preferences.
The first topic of discussion might be the category SCMP datalink
reliability as generally characterized in the Retry FSM. This
document favors maintaining a coordinating Retry FSM versus
incorporating the Retry states in each of the OSM, NHSM, PHSM and
TSM, which is naturally an alternative.
ERROR message generation for PDU semantics problems is discussed in
Section 5 as an ST Dispatcher function. A special case occurs in PDU
construction when the MTU size is exceeded, i.e.:
6 UserDataSize UserData parameter too large to
permit a message to fit into a
network's MTU.
36 RecordRouteSize RecordRoute parameter is too long
to permit message to fit a
network's MTU.
Rajagopal, Sergeant Expires April 21, 1996 [Page 37]
Internet Draft ST2+ Protocol State Machines October 26, 1996
However, all of the analysis and potential REFUSE message or signal
generation, still seems best suited to the ST Dispatcher.
5.1 Additional Exception Processing
5.1.1 ST Dispatcher detected inconsistencies Reason Codes:
The following errors can also be detected by the ST Dispatcher with
the careful analysis of all Agent and stream database values:
3 AccessDenied Access denied.
4 AckUnexpected An unexpected ACK was received.
15 DuplicateIgn Control PDU is a duplicate and is
being acknowledged.
16 DuplicateTarget Control PDU contains a duplicate
target, or an attempt to add
an existing target.
49 StreamExists A stream with the given SID already
exists.
51 TargetExists A CONNECT was received that specified
an existing target.
52 TargetUnknown A target is not a member of the
specified stream.
53 TargetMissing A target parameter was expected and
is not included, or is empty.
This means that the atomic FSMs do not have to incorporate this logic
and this approach simplifies the atomic FSM paradigms.
5.1.2 MonitorFSM issues with neighbor failure and stream recovery
Reason Codes:
The details of these specific instances can also be intertwined with
Retry, Routing and LRM failures.
Rajagopal, Sergeant Expires April 21, 1996 [Page 38]
Internet Draft ST2+ Protocol State Machines October 26, 1996
12 CantRecover Unable to recover failed stream.
22 IntfcFailure A network interface failure has been
detected.
27 NetworkFailure A network failure has been
detected.
39 RestartLocal The local ST agent has recently
restarted.
40 RestartRemote The remote ST agent has recently
restarted.
47 STAgentFailure An ST agent failure has been
detected.
5.1.3 Retry and Timeout Failures Reason Codes:
38 ResponseTimeout Control message has been
acknowledged but not answered
by an appropriate control message.
41 RetransTimeout An acknowledgment has not been
received after several retransmissions.
5.1.4 Routing issues Reason Codes:
Routing issues initiate special exception processing requirements.
Some of these have been addressed in the ST2+ specification, but each
implementation should consider the network and platform architecture,
also.
9 BadMcastAddress IP Multicast address is
unacceptable in CONNECT
28 NoRouteToAgent Cannot find a route to an ST agent.
29 NoRouteToHost Cannot find a route to a host.
30 NoRouteToNet Cannot find a route to a network.
34 PathConvergence Two branches of the stream join
during the CONNECT setup.
Rajagopal, Sergeant Expires April 21, 1996 [Page 39]
Internet Draft ST2+ Protocol State Machines October 26, 1996
42 RouteBack Route to next-hop through same interface
as previous-hop and is not previous-hop.
43 RouteInconsist A routing inconsistency has been
detected.
44 RouteLoop A routing loop has been detected.
5.1.5 LRM issue Reason Codes:
Optimization of routing and LRM issues can also initiate special
exception processing requirements. Some of these have been addressed
in the ST2+ specification, but each implementation should also
consider the network and platform architecture.
10 CantGetResrc Unable to acquire (additional)
resources.
11 CantRelResrc Unable to release excess resources.
17 FlowSpecMismatch FlowSpec in request does not
match existing FlowSpec.
18 FlowSpecError An error occurred while processing
the FlowSpec.
19 FlowVerUnknown Control PDU has a FlowSpec Version
Number that is not supported.
20 GroupUnknown Control PDU contains an unknown Group
Name.
21 InconsistGroup An inconsistency has been detected
with the streams forming a group.
Rajagopal, Sergeant Expires April 21, 1996 [Page 40]
Internet Draft ST2+ Protocol State Machines October 26, 1996
50 StreamPreempted The stream has been preempted by
one with ahigher precedence.
6 APPENDIX
6.1 Glossary
All stream FSMs have the following 4 states in common:
Init: The stream has no active Targets.
Establd: The stream is established and may or may not have Target
members.
Add: The stream is currently adding Targets as the result of a
Connect or Join initiated Connect.
Change: The stream is currently attempting to Change according to a
new FlowSpec.
A list of predicates, API interactions and combination conditions
include the following:
api_close - the Origin API explicitly terminates a stream, since a
stream with no Targets at the Origin may remain Established.
api_open - the Origin API explicitly establishes a stream to initiate
all database setup functions whether or not any Targets are initially
specified.
api_connect- the Origin API adds Targets.
api_change - the Origin API initiates a CHANGE to the FlowSpec.
api_disconnect - the Origin API initiates a DISCONNECT to Targets.
accept_api - the OSM propagates an ACCEPT received from either a TSM
or a NHSM to the Origin API.
notify_api - the OSM propagates a NOTIFY received from either a TSM
or a NHSM to the Origin API.
Rajagopal, Sergeant Expires April 21, 1996 [Page 41]
Internet Draft ST2+ Protocol State Machines October 26, 1996
refuse_api - the OSM propagates a REFUSE received from either a TSM
or a NHSM to the Origin API.
nexthop_open - the first time each unique NHSM is invoked for each
unique stream in an Agent, the Agent explicitly establishes a NHSM
database and Establd state.
prevhop_open - the first time the PHSM is invoked for each unique
stream in an Agent, the Agent explicitly establishes a PHSM database
and Establd state.
api_join - the Target API initiates a JOIN request.
api_refuse - the Target API initiates a REFUSE to the TSM.
api_refuse_change - the Target API initiates a REFUSE of a CHANGE
request to the TSM.
connect_api - the TSM propagates a CONNECT to the Target API.
change_api - the TSM propagates a CHANGE to the Target API.
join_reject_api - the TSM propagates a JOIN_REJECT to the Target API.
disconnect_api - the TSM propagates a DISCONNECT to the Target API.
JOIN_AUTH - a PHSM or OSM JOIN is authorized.
JOIN_NOT_AUTH - a PHSM or OSM JOIN is not authorized.
RetryTimeout - an FSM recieves an implicit REFUSE response to a
CONNECT or CHANGE request to one Target in the TargetList by
exceeding the ACK retry and timeout values (i.e., ToChange/NChange,
ToConnect/NConnect timers and retry counts) for that particular
transaction.
The Add and Change states cannot transition back to the Establd state
until all Targets have given implicit or explicit responses.
ACCEPT_LAST - the last Target in the TargetList for a CONNECT or
CHANGE has responded with an ACCEPT.
DISC_LAST - the last Target in the TargetList for a CONNECT or CHANGE
has responded with a DISCONNECT.
REFUSE_LAST - the last Target in the TargetList for a CONNECT or
Rajagopal, Sergeant Expires April 21, 1996 [Page 42]
Internet Draft ST2+ Protocol State Machines October 26, 1996
CHANGE has responded with a REFUSE.
RetryTimeout_last - the last Target in the TargetList for a CONNECT
or CHANGE has responded with an implicit REFUSE by exceeding the ACK
retry and timeout values (i.e., ToChange/NChange, ToConnect/NConnect
timers and retry counts).
E2E_Timeout_last - the last Target in the TargetList for a CONNECT or
CHANGE has responded with an implicit REFUSE by exceeding the End-
to-End timeout value (i.e., ToChangeResp, ToConnectResp timers).
Except in the case of the OSM (which must be explicitly closed by the
Origin API, the Establd, Add and Change states transition back to the
Init state when all Targets in the unique FSM TargetList have given
implicit or explicit stream teardown instructions.
DISC_ALL -a DISCONNECT has been received for the last Target in the
entire TargetList for a stream FSM (as opposed to the TargetList for
a particular CONNECT or CHANGE request).
REFUSE_ALL - a REFUSE has been received for the last Target in the
entire TargetList for a stream FSM (as opposed to the TargetList for
a particular CONNECT or CHANGE request
6.2 ST Control Message Flow
Control Message Types
ST control messages are generally of the Request -Response type.
Table 8 summarizes these control messages alphabetically. The table
has three major columns.
o Message type
o Response
o Possible causes for message
6.2.1 Message Type
Under the Message Type each control message is categorized either as
a:
- Request message
- Response message
Rajagopal, Sergeant Expires April 21, 1996 [Page 43]
Internet Draft ST2+ Protocol State Machines October 26, 1996
It is possible for a message to be more than one type depending on
the usage, although this is not apparent from this table.
6.2.2 Response
The Response to each control message is given in the next major
column under Response. Note that the Response to a message can be
interpreted to mean either:
1. a Response to another control message
2. a Response to indicate the condition of receipt of the
message, driven primarily by the error control function
The second interpretation of Response includes positive
acknowledgments and negative acknowledgments (error response). Thus,
this major column has the following categories:
- Error Response
- Mandatory Response
- Other response following mandatory response.
An X or an entry in the table indicates classification of a message
under a particular category shown under each major column.
6.2.3 Possible causes for messages
Finally, a control message might have been sent in response to
another control message. This is shown in the last column. Note that
it is possible that independently a number of control messages may be
the cause for this control message in question Note that an entry
does not necessarily mean that is the only cause. A blank entry in
this column for instance means that the message was not invoked by
another message.
For example, an ACCEPT message is a Response Type message to either a
CONNECT or a CHANGE message. It will be acknowledged (Mandatory
response) with an ACK. It may be responded with an ERROR in case of
error conditions. The state diagrams illustrate this sequencing more
completely. It may be noted that the sequencing of messages gives the
protocol semantics.
Table 8: Message Types: Requests, Responses and Others
Rajagopal, Sergeant Expires April 21, 1996 [Page 44]
Internet Draft ST2+ Protocol State Machines October 26, 1996
ACKNOWLEDGEMENTS and AUTHORS:
Many individuals have contributed to the work described in this memo.
We thank the participants in the ST Working Group for their input,
review, and constructive comments.
We would also like to thank Luca Delgrossi and Louis Berger for
allowing us to adopt the text from their [1] document.
We would like to acknowledge inputs from Mark Pullen and his graduate
students, Tim O'Malley, Eric Crowley, Muneyoshi Suzuki and many
others.
Murali Rajagopal EMail: murali@fbcs.com, Phone: 714-764-2952
Sharon Sergeant EMail:sergeant@xylogics.com, Phone: 617-893-6142
LIST OF REFERENCES:
[1] L. Delgrossi and L. Berger: Internet STream Protocol Version 2
(ST) - Protocol Specification- Version ST2+, RFC 1819, April 1995.
[2] D. Brand and P. Zafiropulo: On Communicating Finite-State
Machines, J.ACM, 30, No.2, April 1983
Rajagopal, Sergeant Expires April 21, 1996 [Page 45]