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GESTION DU TEMPS ET DES EVENEMENTS EN HLA
(SYNCHRONISATION) A. Nketsa LAAS, Groupe OCSD 7,
Avenue du Colonel Roche 31077 Toulouse Cédex
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PLAN ? RAPPEL
HLA ? NOTIONS DE TEMPS ET MODES D'EXECUTION ?
DIFFERENTS MECHANISMES DE GESTION DU TEMPS
EN SIMULATION ? GESTION DU TEMPS EN HLA (RTI) ?
TYPES DE MESSAGES ? AVANCEMENT DU TEMPS
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High Level Architecture (HLA)

• based on a composable system of systems
  approach
• no single simulation can satisfy all user needs
• support interoperability and reuse among DoD
  simulations
• federations of simulations (federates)
• pure software simulations
• human-in-the-loop simulations (virtual
  simulators)
• live components (e.g., instrumented weapon
  systems)
• mandated as the standard reference architecture
  for all MS in the U.S. Department of Defense
  (September 1996)

• The HLA consists of
• rules that simulations (federates) must follow to
  achieve proper interaction during a federation
  execution
• Object Model Template (OMT) defines the format
  for specifying the set of common objects used by
  a federation (federation object model), their
  attributes, and relationships among them
• Interface Specification (IFSpec) provides
  interface to the Run-Time Infrastructure (RTI),
  that ties together federates during model
  execution

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HLA Federation
Interconnecting autonomous simulators
Federation
Simulation (federate)
Simulation (federate)
Simulation (federate)
Interface Specification
Interface Specification
Runtime Infrastructure(RTI)

• Services to create and manage the execution of
  the federation
• Federation setup / tear down
• Transmitting data among federates
• Synchronization (time management)

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Interface Specification
Category
Functionality
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A Typical Federation Execution

• initialize federation
• Create Federation Execution (Federation Mgt)
• Join Federation Execution (Federation Mgt)

• declare objects of common interest among
  federates
• Publish Object Class (Declaration Mgt)
• Subscribe Object Class Attribute (Declaration Mgt)

• exchange information
• Update/Reflect Attribute Values (Object Mgt)
• Send/Receive Interaction (Object Mgt)
• Time Advance Request, Time Advance Grant (Time
  Mgt)
• Request Attribute Ownership Assumption (Ownership
  Mgt)
• Modify Region (Data Distribution Mgt)

• terminate execution
• Resign Federation Execution (Federation Mgt)
• Destroy Federation Execution (Federation Mgt)

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NOTIONS DE TEMPS ET MODES D'EXECUTION
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Notions de temps

• temps physique temps dans le système physique
• temps simulé représentation du temps
  physique pendant la simulation
• temps absolu (horloge) temps d'exécution de la
  simulation (fourni par une horloge matérielle
  (e.g., GPS))

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Modes d'exécution d'une simulation

• Aussi rapide que possible (As-fast-as-possible
  execution) pas de relation directe dans
  l'avancement du temps entre le temps simulé et
  une horloge.
• synchronisation temporelle (respect du
  principe de causalité)
• Temps réel (Real-time execution)
• synchronisation événementielle (pas de temps
  explicite)

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Différents mécanismes de gestion de temps en
simulation
Simulation orientée événement
Evénements traités dans l'ordre des estampilles
(Causalité) Avancement du temps
estampille Simulation orientée temps
Evénements traités par paquet
Avancement par pas de durée fixée
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Différents mécanismes de gestion de temps en
simulation
Simulation distribuée Evénements
traités en parallèle Risque de
violation du principe de causalité
Nécessité de synchronisation
synchronisation conservative (base HLA)
synchronisation optimiste (Extension
HLA) Simulation temps réel Temps de
réponse compatible avec l'exécution
Synchronisation des horloges
Ordonnancement temps réel
compensation du temps (des délais)
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CONCLUSIONS POUR HLA - RTI
RTI Implémentation des fonctions
de base de gestion du temps indépendante
des modèles utilisés Proposition
service minimal, délai de communication
prédictif calcul minimal pour la
synchronisation et la communication
pas de traitement de synchronisation inutile
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CONCLUSIONS POUR HLA - RTI
RTI (suite)
Types de messages Receiver order (RO)
Time stamp order (TSO) Services de gestion de
temps système d'ordre des messages
estampillés protocole - le fédéré
demande explicite d'avancement du temps
- le RTI accord d'avancement (butée
simulation)
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RTI (Suite)
Services de gestion du temps traitement du
temps simulé coordination
avancement du temps simulé mise à
disposition des données estampillées support
de plusieurs types de gestion du temps
pas de gestion explicite du temps (temps réel)
synchronisation conservative
synchronisation optimiste
scrutation des activités (simulation orientée
horloge) mélanges des types
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COMPLEMENT GESTION TEMPS DE SIMULATION
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Lower Bound on Time Stamp (LBTS)

• Pas de null messages, chaque LP peut envoyer
  des messages à n'importe quel autre LP
• LP bloqué, calculer lower bound on time stamp
  (LBTS) des messages à recevoir plus tard (avec
  estampille lt LBTS )

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Lower Bound on Time Stamp (LBTS)
LBTS min (6, 10, 7) ( zero lookahead)

• LBTS minimum parmi
• Estampille de tous les messages transitoires
  (envoyés, mais non encore reçus)
• LP non bloqués temps courant de simulation
  lookahead
• LP bloqués Time of next event
  lookahead

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Lookahead
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TYPES DE MESSAGES
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HLA Message Ordering Services

• The HLA provides two types of message ordering
• receive order (unordered) messages passed to
  federate in an arbitrary order
• time stamp order (TSO) sender assigns a time
  stamp to message successive messages passed to
  each federate have non-decreasing time stamps

• receive order minimizes latency, does not prevent
  temporal anomalies
• TSO prevents temporal anomalies, but has somewhat
  higher latency

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Related Object Management Services

• Sending and Receiving Messages
• Update Attribute Values Reflect Attribute
  Values
• Send Interaction Receive Interaction
• Message Order (Receive Order or Time Stamp Order)
• Preferred Order Type default order type
  specified in fed file for each attribute and
  interaction
• Sent Message Order Type
• TSO if preferred order type is TSO and the
  federate is time regulating and a time stamp was
  used in the Update Attribute Values or Send
  Interaction call
• RO otherwise
• Received Message Order Type
• TSO if sent message order type is TSO and
  receiver is time constrained
• RO otherwise

indicates callback to federate
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Federated vs. RTI Initiated Services

• Some services are initiated by the federate,
  others by the RTI
• Federate invoked services
• Publish, subscribe, register, update
• Not unlike calls to a library
• Procedures defined in the RTI ambassador
• RTI invoked services
• Discover, reflect
• Federate defined procedures, in Federate
  Ambassador

Federate
Federate ambassador
Update
Reflect
RTI ambassador
RTI
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Example Receiving a Message
Tick() transfer execution to RTI to perform RTI
functions, perform callbacks
/ code sketch, receiving messages / Boolean
Waiting4Message Waiting4Message
TRUE while (Waiting4Message) Tick() /
Federate ambassador / Proc ReflectAttributeValues
() save incoming message in
buffer Waiting4Message FALSE
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AVANCEMENT DU TEMPS SIMULE
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Advancing Logical Time

• HLA TM services define a protocol for federates
  to advance logical time logical time only
  advances when that federate explicitly requests
  an advance
• Time Advance Request time stepped federates
• Next Event Request event stepped federates
• Time Advance Grant RTI invokes to acknowledge
  logical time advances

If the logical time of a federate is T, the RTI
guarantees no more TSO messages will be passed to
the federate with time stamp lt T Federates
responsible for pacing logical time advances with
wallclock time in real-time executions
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HLA Time Management Services
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Time Regulating and Time Constrained Federates

• Federates must declare their intent to utilize
  time management services by setting their time
  regulating and/or time constrained flags
• Time regulating federates can send TSO messages
• Can prevent other federates from advancing their
  logical time
• Enable Time Regulation Time Regulation
  Enabled 
• Disable Time Regulation
• Time constrained federates can receive TSO
  messages
• Time advances are constrained by other federates
• Enable Time Constrained Time Constrained
  Enabled
• Disable Time Constrained
• Each federate in a federation execution can be
• Time regulating only (e.g., message source)
• Time constrained only (e.g., Stealth)
• Both time constrained and regulating (common case
  for analytic simulations)
• Neither time constrained nor regulating (e.g.,
  DIS-style training simulations)

indicates callback to federate
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Régulateur du temps
Vrai
faux
Reception des événements Pas de génération
Synchonisation temporelle stricte Conservative
Optimiste
Temps contraint
Vrai
Synchronisation externe pas de gestion de temps
Générateur événements Régulateur
Faux
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Synchronizing Message Delivery

• Goal process all events (local and incoming
  messages) in time stamp order To support this,
  RTI will
• Deliver messages in time stamp order (TSO)
• Synchronize delivery with simulation time advances

next TSO message
RTI
TSO messages
next local event
T
federate
local events
logical time
T
current time

• Federate next local event has time stamp T
• If no TSO messages w/ time stamp lt T, advance to
  T, process local event
• If there is a TSO message w/ time stamp T ? T,
  advance to T and process TSO message

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Next Event Request (NER)

• Federate invokes Next Event Request (T) to
  request its logical time be advanced to time
  stamp of next TSO message, or T, which ever is
  smaller
• If next TSO message has time stamp T ? T
• RTI delivers next TSO message, and all others
  with time stamp T
• RTI issues Time Advance Grant (T)
• Else
• RTI advances federates time to T, invokes Time
  Advance Grant (T)

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Code Example Event Stepped Federate
sequential simulator T current simulation
time PES pending event set While (simulation
not complete) T time of next event in
PES process next event in PES End-While
federated simulator While (simulation not
complete) T time of next event in
PES PendingNER TRUE NextEventRequest(T) whil
e (PendingNER) Tick() process next event in
PES End-While / the following
federate-ambassador procedures are called by the
RTI / Procedure ReflectAttributeValues
() place event in PES Procedure
TimeAdvanceGrant () PendingNER False
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ACCELERATION SIMULATION
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Lookahead in the HLA

• Each federate must declare a non-negative
  lookahead value
• Any TSO sent by a federate must have time stamp
  at least the federates current time plus its
  lookahead
• Lookahead can change during the execution (Modify
  Lookahead)
• increases take effect immediately
• decreased do not take effect until the federate
  advances its logical time

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Federate/RTI Guarantees

• Federate at logical time T (with lookahead L)
• All outgoing TSO messages must have time stamp
  TL (Lgt0)
• Time Advance Request (T)
• Once invoked, federate cannot send messages with
  time stamp less than T plus lookahead
• Next Event Request (T)
• Once invoked, federate cannot send messages with
  time stamp less than T plus the federates
  lookahead unless a grant is issued to a time less
  than T
• Time Advance Grant (T) (after TAR or NER service)
• All TSO messages with time stamp less than or
  equal to T have been delivered

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Minimum Next Event Time and LBTS
LBTS010
MNET08

• LBTSi Lower Bound on Time Stamp of TSO messages
  that could later be placed into the TSO queue for
  federate i
• TSO messages w/ TS LBTSi eligible for delivery
• RTI ensures logical time of federate i never
  exceeds LBTSi

• MNETi Minimum Next Event Time is a lower bound
  on the time stamp of any message that could later
  be delivered to federate i.
• Minimum of LBTSi and minimum time stamp of
  messages in TSO queue

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Simultaneous Events

• Simultaneous events are events containing the
  same time stamp
• Ordering of simultaneous events often important
• RTI does not have sufficient information to
  intelligently order simultaneous events
• HLA ordering simultaneous events left to the
  federate
• Grant to time T (after TAR/NER) all events with
  time stamp T delivered to federate
• Simultaneous events delivered to federate in an
  arbitrary order (may vary from one execution to
  the next)
• Federate must have a deterministic way to order
  simultaneous to ensure repeatable executions
  (e.g., sort events by type or other event
  properties)

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Zero Lookahead

• Zero lookahead a federate at time T can send TSO
  messages with time stamp T
• If zero lookahead is allowed, a Time Advance
  Grant to time T cannot guarantee delivery of all
  events with time stamp equal to T

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Zero Lookahead in the HLATAR Available and NER
Available

• Zero lookahead allowed in HLA federations
• Next Event Request (NER), Time Advance Request
  (TAR)
• grant to time T guarantees delivery of all TSO
  messages w/ time stamp T (or less)
• constraint once a Time Advance Grant to time T
  is issued for these requests, subsequent
  messages sent by the federate must have time
  stamp strictly greater than T.
• Two new services Time Advance Request Available
  (TARA) and Next Event Request Available (NERA)
• TARA (NERA) advance logical time, similar to TAR
  (NER)
• federate can send zero lookahead messages after
  receiving grant
• grant to time T does not guarantee all messages
  with time stamp T have been delivered, only those
  available at the time of the call
• order that TSO messages are delivered to the
  federate is arbitrary

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Representation of Logical Time

• Time represented as a federation-defined abstract
  data type
• Federation must agree upon a common
  representation of logical time during federation
  development
• Time value (e.g., 0900, 12 May 1973)
• Time duration (e.g., 30 minutes)
• Federation specifies
• Data type of time values and duration (structure
  types allowed)
• Comparison (time values) and addition (value and
  duration) operators

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Event Retraction

• Previously sent events can be unsent via the
  Retract service
• Update Attribute Values and Send Interaction
  return a handle to the scheduled event
• Handle can be used to Retract (unschedule) the
  event
• Can only retract event if its time stamp gt
  current time lookahead
• Retracted event never delivered to destination
  (unless Flush Queue used)

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Optimistic Time Management

• Mechanisms to ensure events are processed in time
  stamp order
• conservative block to avoid out of order event
  processing
• optimistic detect out-of-order event processing,
  recover (e.g., Time Warp)

• Primitives for optimistic time management
• Optimistic event processing
• Deliver (and process) events without time stamp
  order delivery guarantee
• HLA Flush Queue Request
• Rollback
• Deliver message w/ time stamp T, other
  computations already performed at times gt T
• Must roll back (undo) computations at logical
  times gt T
• HLA (local) rollback mechanism must be
  implemented within the federate
• Anti-messages secondary rollbacks
• Anti-message message sent to cancel (undo) a
  previously sent message
• Causes rollback at destination if cancelled
  message already processed
• HLA Retract service deliver retract request if
  cancelled message already delivered
• Global Virtual Time
• Lower bound on future rollback to commit I/O
  operations, reclaim memory
• HLA Query Next Event Time service gives current
  value of GVT

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Optimistic Time Management in the HLA

• HLA Support for Optimistic Federates
• federations may include conservative and/or
  optimistic federates
• federates not aware of local time management
  mechanism of other federates (optimistic or
  conservative)
• optimistic events (events that may be later
  canceled) will not be delivered to conservative
  federates that cannot roll back
• optimistic events can be delivered to other
  optimistic federates
• individual federates may be sequential or
  parallel simulations

Flush Queue Request similar to NER except (1)
deliver all messages in RTIs local message
queues, (2) need not wait for other federates
before issuing a Time Advance Grant
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Summary HLA Time Management

• Functionality
• allows federates with different time management
  requirements (and local TM mechanisms) to be
  combined within a single federation execution
• DIS-style training simulations
• simulations with hard real-time constraints
• event-driven simulations
• time-stepped simulations
• optimistic simulations

• HLA Time Management services
• Event order
• receive order delivery
• time stamp order delivery
• Logical time advance mechanisms
• TAR/TARA unconditional time advance
• NER/NERA advance depending on message time stamps