HLA interfaces for ns-3: Introduction
- Student: Mudit Raj Gupta
- Mentors: Tommaso Pecorella
- Code: http://code.nsnam.org/muditraajgupta/ns-3-dev-hla/codeRev/
- Mid-Term Report:https://www.nsnam.org/wiki/index.php/GSOC2012HLAMidTerm
- Abstract: The project deals with developing interfaces for High Level Architecture (HLA) for ns-3 project and thus enabling it to participate in a distributed simulation (Federation) involving various federates over Run Time infrastructure (RTI). The project involves three phases. Firstly, creating a Federate and Federate Ambassador for ns-3 to communicate with the RTI. Secondly, creating a synchroniser and simulator implementation that uses time from the RTI clock. Finally, creating a helper classes and modules to simplify the process of sharing and using objects in a distributed simulation. Detail proposal and initial plan can be found here here.
- About me: I'm a fourth year student presently working for my M.Sc (H) in Chemistry and B.E. (H) in Electronics and Instrumentation Engineering at Birla Institute of Technology and Science - Pilani. I am interested in modelling and simulation of complex systems, learning in multi-agent systems and evolutionary algorithms.
HLA, which is an IEEE standard, facilitates interoperability among simulations and promotes reuse of simulation models. Using HLA, a large-scale distributed simulation can be constructed by linking together a number of distributed simulation components (or federates) into an aggregate simulation (federation). .
Run-time infrastructure (RTI) is a middleware that is required when implementing the High Level Architecture. RTI is the fundamental component of HLA. It provides a set of software services that are necessary to support federates to coordinate their operations and data exchange during a runtime execution. In other sense, it is the implementation of the HLA interface specification but is not itself part of specification
Federates can communicate to RTI through RTI ambassador. Request of a service from/to federate to/from any other federate is done through interactions. There are two basic ways to communicate:
- Interchange messages between federates
- Publication of objects that are shared between federates
poRTIco is an open-source Java implementation of the above mentioned standard and it offers libraries to implement a federate ambassador. It is an interface between the RTI and the simulator. Federates can communicate to RTI through RTI ambassador.
Ns-3 HLA Model
The User Creates a script to share an object with other simulator using RTI and HLA interfaces. The first phase is to use the Rti Simulator implementation as it is responsible for synchronisation of RTI Clock with ns-3 clock. Next, use the HLA API provided in Helper to communicate with Rti and create a shared object.
This is how it looks on the surface. The HlaHelper talks with LinkToRti, to send the details of the object and other information through a socket and proper serialisation framework (MsgPack) to Java Block. The Synchroniser part also communicates with LinkToRti in order to receive messages from RTI.
The messages from C++ blocks travels to JAVA through sockets and are serialised using Message Pack, a standard tool for serialisation, de-serialisation. These are now handled by class j. This is in turn parsed and used by ns3 Federate to initialise the federate and j also is used by ns3FedAmb when it receives a call back from RTI. The two classes handles communication with RTI
As it is quite clear from the diagram. The Java block communicates with the RTI. The Ns3Federate is the class which uses the portico Java API to communicate with the portico RTI. At present the sequence of events followed by Ns-3 Federate are are follows:
- Create Federation
- Join Federation
- Synchronise with RTI
- Enable Time Policies
- Start Signal for C++ Block
- Publish and Subscribe to Objects
- Wait for ns-3 C++ Block Time Advance Request
- Send Time Advance Requests To RTI
- Resign From Federation
- Destroy Federation
The other class Ns3FedAmb handles Callbacks from RTI whenever an event occurs that is initiated by the other federate or RTI has to convey a request grant. The Call backs supported at present are:
- Synchronisation (Synchronised, Registered, Failed, Ready)
- Time (Time Advance Request Granted, Time Policies Enabled)
- Object (Object Discovered, Object Attributes Changed, Object Deleted)
An other helper class is j. This class is a Java Side link between the Java Block and C++ Block. It has various utility functions to set up a socket connection and receive and send messages. It also has various Getter methods to extract the value of the incoming messages into variables. It basically has functions for:
- Socket Communication (Establish Connection, Notify Changes to ns-3, Start ns-3, Send Time Advance to C++ block, read Time Advance, Initial Values)
- Getters (Extract Values like Federation Name, Federate Name, Fom File, Object Name, Attribute Names etc.)
As the diagram shows, it consists of basically two parts: The Synchroniser and The Helper. It also has a class which act as a C++ link to the connection between Java and C++ block. The code is well documented in doxygen format.
The Helper .cc .h contain functions to set various variables that are used to initialise the federation, shared objects and attributes, It also has functions to access the variables and their values. So basically,
- Setters (Set values like federation name, Federate Name, Fom File, Object Name, Attribute Names etc.)
- Getters (Retrieve current values of attributes, their parents, modified time etc.)
The values of Hla Helper are updated as and when they are changed by the other federates.
The Synchroniser is responsible for synchronising the event times with the RTI clock. The RTI Synchroniser is inspired by the wall-clock-synchronizer-impl, in it's case it fetches the RTI time from the RTI clock variable maintained in LinkToRti class. It is used by the functions in RTI Simulator Implementation. Whenever an event has to happen, this code block checks if the event time is more than the time in RTI. If yes, the event is processed and if no the a Time Advance Request is sent to the RTI. Other standard functions for simulator implementation are also present, quite similar to realtime-simulator-impl.
- Multi threading: It is implemented for simultaneous listening and sending of messages over the socket.
- Packing and Sending: It uses Message Pack for serialisation of the attributes of the object, implements various functions for sending data.
- Callbacks: The class handle callbacks from RTI (Time Advance, Update, Start Signal)
Apart from regular ns-3 helper scripts, HLA module needs a helper script hla.sh. This script is written to remind the end-user of the path variables to set. The script also provides a waf like build system to build java code. User does not require build tools like ant and maven for building java code and producing .class and .jar files. Once ns3 is build, this script can be used to automatically create .class and .jar files in $ns3/build/src/hla/model and running the java code. It has commands like:
- ./hla.sh --clean (Cleans .class, .jar files and RTI log files)
- ./hla.sh --build (Builds HLA interfaces, Java Code)
- ./hla.sh --run scratch/<filename> (.fed file has to be in scratch directory)
To know more about usage read below or this tutorial.
- System Integration Test
The hla model at present has a dummy federate. It has two classes Federate and FederateAbassador. The ns-3 hla model can be checked without creating a federate in other simulator. This federates just creates a federation and modify a shared object PingPong 's attributes - Msg and MsgType. These attributes are toggles every second. The values are:
- MsgType : Ping, Pong
- Msg : "Client: Hello!!!", "Server: Hello!!!"
It can be executed using this helper script
All scripts that use HLA should also have a corresponding .fed file, which is nothing but a simple object description. This .fed file used for the examples and can be modified for more scripts. It contains the shared object "PingPong" and two attributes "Msg" and "MsgType".
- Simple Shared Object Example
This example demonstrates how to create a shared object using the HLA API and how to publish the object. The example creates a simple object PingPong with attributes Msg and MsgType and shares with other federates and sets simulator to RTI simulator Implementation. It demonstrates the concepts of Class Handles and Attribute Handles. A tutorial on the model can be found here.
- Shared Object With CallBack
In a real scenario you need to use HLA interfaces with some ns-3 model and use the attributes shared with other federates in your model. The example uses a simple mobility model and prints the object's attributes in the Callback function. One can see the attribute changes when the external federate modify it.
Ns-3 scripts can use HLA API and RTI Simulator Implementation to communicate with other federates. HLA module uses the following external packages/software:
Please refer to package installation documents from respective websites (You just need portico.jar for portico).
Once the packages are set up. You need to set two environment variables:
- JAR_PATH=/path/to/portico.jar file
Try and set both of these as your system wide variables, a small post talks about this for Ubuntu.
Once you are done create a new ns-3 script that uses HLA helper classes along with other models, say ns3-hla-script.cc. Write an object description file similar to one in the examples, Ns3HlaScript.fed. Copy both in $ns3/scratch/ folder. Open two terminal windows, lets call them JavaShell and CppShell.
For running your ns-3 script use the following command, in any shell (say CppShell):
CppShell$ ./waf --run scratch/ns3-hla-script
Now, once you receive the following message:
CppShell$ Connecting to RTI ...
Start Java code in the other shell (JavaShell). There is hla.sh script in your $ns3 folder, it provides a waf like build to the java code.
JavaShell$ ./hla.sh --clean JavaShell$ Removing HLA Bindings and Old Log files JavaShell$ ./hla.sh --build JavaShell$ Building HLA interfaces JavaShell$ ./hla.sh --run scratch/Ns3HlaScript
If you are using HLA for the first time, you will have to build it first using the "--build" command. "--clean" will remove old files. If HLA (java) is build once you can just run your scripts using "--run". The scripts mimics waf build system for the convenience of the user.
This will get your ns-3 federate started. Once you are done with this you can start your other federates or if other federates are running on the same federation, ns-3 federate will just join them automatically.
Again a small tutorial on using HLA in ns3 from writing code to running can be found here.
The deliverables mentioned above in the "components" section
- HLA API
- Ns3Federate and Ns3Federate Ambassador
- C++ and Java Links to the C++ Java connecting pipe
- Scripts, Tests and Examples
- Week 1 : May.21 -- May.27
* Read Code related to Scheduler and various implementations like Map Scheduler and List Scheduler * Started Coding on RTI Scheduler, implemented basic functions (similar to Map Scheduler) * Read about MessagePack Message Format.
- Week 2 : May.28 -- Jun.03
* Changed the default scheduler from map to list and finally rti by small changes in simulator.cc * Modified map-scheduler and wrote a basic rti-scheduler.cc and rti-scheduler.h implementing scheduler.cc it is basic skeleton for implementing thread for handling RTI request. Tried making thread in the class, some problems. * Tested. Discussed progress with the mentor and figured out a flaw in design. * no scheduler required, last 10 days work not so useful. * Finalised a new design started working fresh.
- Week 3 : Jun.4 -- Jun.10
* Installed portico and ran demo programs in JAVA * Tested with multiple federates, wrote a simple dummy federate.
- Week 4 : Jun.11 -- Jun.17
* Completed work on writing ns3Federate and ns3FederateAmbassador in JAVA using portico * Added functionality to dummy by sharing object between the two federates and changing attributes * Wrote a test for checking socket implementation
- Week 5 : Jun.18 -- Jun.24
* Integrating the server code to one of the NS3 examples. * Tested and debugged of the entire code.
- Week 6 : Jun.25 -- Jul.01
* Completed work on writing a C++ code for reading socket values * Completed work on integrating the above code with link-to-rti and decoding messages * Completed deliverable 1.2 and 1.3
- Week 7 : Jul.02 -- Jul.08
* Migrated code repo from code.google.com to code.nsnam.org * Time advance request handled and wrote various functions to decode the RTI messages * Created various functions to start simulation, stop simulation and RTI_CLOCK
- Week 8 : Jul.09 -- Jul.15
* Finding and designing the best way to synchronise ns-3 with RTI * Read about JNI basics
- Week 9 : Jul.16 -- Jul.22
* Modified realtime-simulator-impl to schedule events according to RTI time and send time advance requests. * Modified wall-clock-syncronizer to fetch RTI time. * Tested the new implementation and synchronisation. * Designed the basic API
- Week 10 : Jul.23 -- Jul.29
* Started working on the HLA API, some issues to be discussed with the mentor. * Started working on hla-interface, it creates an object and add attributes to the attributes vector which is published by and modified by other federates * Created an example (not completely working), just to have an idea ho create various components of the API * Read about callbacks in ns-3 and Mobility.
- Week 11 : Jul.30 -- Aug.6
* Read documentation about Mobility model * Tried an alternate RTI - CERTI, downloaded, build and tried a few example * Tried using the C++ API of portico * Read through and tried a few xmlparsers and MessagePack * Further modified the design of the API
- Week 12 : Aug.7 -- Aug.12
* Finally went with portico and JAVA API * Read and implemented msgpack for serialisation * Designed the basic API (Helper)
- Week 13 : Aug.13 -- Aug.19
* Cleaning Code * Completing documentation * Preparing code for review
- Ns-3 as a source - At present ns-3 acts as a sink while in a federation. This means that ns-3 can only receive updates on the objects but cannot modify the attributes of the object. The source property was not added because of two reasons, firstly, because we could not identify a scenario in which such a feature would be useful and secondly, due to the limited time span of GSoC. Although, adding a source property will be similar to sink property and can be implemented by adding a "RTI register object instance" and "Update Values" functions.
- In a federations federates communicate using objects and interactions at present the code supports publishing objects and interactions can be added, if required by writing functions for interactions that mimic object functions.
-  http://www.cs.bham.ac.uk/research/projects/hlarepast/
-  Distributed Repast Agent Based Simulation with HLA - R. Minson and G. K. Theodoropoulos, CONCURRENCY AND COMPUTATION: PRACTICE AND EXPERIENCE
-  Large Scale Agent Based Simulation on the Grid - Dan Chena , Georgios K. Theodoropoulos, Stephen J. Turner, Wentong Cai, Robert Minson, Future Generation Computer Systems, 24 (2008), 658-51
-  MobileOnRealEnvironment-GIS: a federated mobile network simulator of mobile nodes on real geographic data- Emiliano Casalicchio, Emanuele Galli and Vittorio Ottaviani Dipartimento di Informatica, Sistemi e Produzione (DISP), 2009 13th IEEE/ACM International Symposium on Distributed Simulation and Real Time Applications
-  http://porticoproject.org/index.php?title=Main_Page
-  http://msgpack.org/