The ns-2 Network Simulator

1. The ns-2 Network Simulator • Plan: • Discuss discrete-event network simulation • Discuss ns-2 simulator in particular • Demonstration and examples: • Download, installation, and configuration • Network topology and traffic modeling • TCP protocol models, and wireless models • Simulation results • Automating simulation scripts

2. Performance Evaluation Analytical Methods Simulation Methods Experimental Methods ... Time-Driven Event-Driven Monte Carlo Sequential Parallel Distributed ... ... ns-2

3. Discrete-Event Simulation (recap) • A system is modeled as a set of entities that affect each other via events • Each entity can have a set of states • Events happen at specific points in time and trigger state changes in the system • Very general technique, well-suited to modeling discrete systems and networks • Simulator has an event list, and processes events in timestamp order

4. Network Simulation • In networking research, the simulation is often of the Internet (or part thereof!) • Entities: hosts, routers, links, buffers,… • Messages: packets sent by protocols • Events: state changes of the network as packets move through the system • A network simulation models the movement of data and/or control packets through the network, according to the protocols being modeled

5. ns-2 • ns-2: one example of network simulator • Originally developed at USC/ISI • Public-domain (free!), with open-source code contributed by many people • Versions of ns: • ns: original version of ns • ns-2: current version, widely used • ns-3: currently under development/testing • pdns: parallel/distributed ns execution

6. Basic Information about ns-2 • Widely used in networking research • Written mostly in C++ • Over 50,000 lines of code • Can download, compile, and install on most any Linux system (or Windows?) • Also has tcl/tk interface for defining and manipulating models • Optional network animation tool (nam) for tracing, playback, and visualization

7. Using ns-2 • Download from ns-2 site • Extract and compile • Look at examples provided • Look at documentation available • Look at ns-2 book (U of Manitoba) • Configure some simple experiments • Try some simulation scripts

8. Strengths of ns-2 • Everything you need to get started! • Models for hosts, links, routers, buffers • Detailed protocol models for TCP • Usable for configuring topology • Usable for configuring network traffic • Some modeling for wireless, mobility • Tracing and visualization support • Large community of users and resources available (doc, examples, etc)

9. Weaknesses of ns-2 • Too much stuff to get started! (code,doc) • Minimal protocol models for IP • Weak models for wireless, mobility • Few models for application-layer traffic • Minimal validation of any ns-2 models • Extracting results and statistics is hard • Debugging large and complex models • Cryptic error messages and crashes • Compatibility across ns-2 versions

10. Some ns-2 Examples (1 of 2) • Example 1: TCP Throughput Testing • Simple dumbbell network topology • One-way TCP traffic • Bulk data transfer • Fixed RTT • Bottleneck link has finite DropTail buffer • Throughput is primary metric

11. Some ns-2 Examples (2 of 2) • Example 2: Web Proxy Caching • Hierarchical network topology • Two-way TCP traffic • Web-like data transfers (server to clients) • Heterogeneous RTTs • DropTail router buffers • Response time is primary metric

12. Summary • The ns-2 network simulator is a widely used platform for networking researchers • Large and cumbersome, yet very useful • Good starting point for many projects • Detailed TCP models available • Limited validation of most ns-2 models • Try it if you want!