ns_by_example

NS by Example Jae Chung and Mark Claypool Purpose NS (version 2) is an object-oriented, discrete event driven network simulator developed at UC Berkely written in C++ and OTcl, which is very useful for simulatry local and wide area networks. Although NS is fairly easy to use once you get to know the simulator, it is quite difficult for a first time user, because there are few user-friendly manuals. Even though there is a lot of documentation written by the developers which has in depth explanation of the simulator, it is written with the depth of a skilled NS user. The purpose of this project is to give a new user some basic idea of how the simultor works, how to setup simulation networks, where to look for further information about network components in simulator codes, how to create new network components, etc., mainly by giving simple examples and brief explanations based on our experiences. Although all the usage of the simulator or possible network simulation setups may not be covered in this project, the project should help a new user to get started quickly. Contents 1. Overview 2 2. Basics 2.1 OTcl: The User Language 5 2.2 Simple Simulation Example 8 2.3 Event Scheduler 14 2.4 Network Components 16 2.5 Packet 21 3. Post Simulation 3.1 Trace Analysis Example 22 3.2 RED Queue Monitor Example 25 4. Extending NS 4.1 Where to Find What? 28 4.2 OTcl Linkage 30 4.3 Add New Application and Agent 35 4.4 Add New Queue 42 5. More Examples 5.1 LAN 45 5.2 Multicasting 45 5.3 Web Server 46 6. Hot Links 47 1. Overview NS is an event driven network simulator developed at UC Berkeley that simulates variety of IP networks. It implements network protocols such as TCP and UPD, traffic source behavior such as FTP, Telnet, Web, CBR and VBR, router queue management mechanism such as Drop Tail, RED and CBQ, routing algorithms such as Dijkstra, and more. NS also implements multicasting and some of the MAC layer protocols for LAN simulations. The NS project is now a part of the VINT project that develops tools for simulation results display, analysis and converters that convert network topologies generated by well-known generators to NS formats. Currently, NS (version 2) written in C++ and OTcl (Tcl script language with Object-oriented extensions developed at MIT) is available. This document talks briefly about the basic structure of NS, and explains in detail how to use NS mostly by giving examples. Most of the figures that are used in describing the NS basic structure and network components are from the 5th VINT/NS Simulator Tutorial/Workshop slides and the NS Notes and Documentation, modified little bit as needed. For more information about NS and the related tools, visit the VINT project home page. Figure 1. Simplified User's View of NS As shown in Figure 1, in a simplified user's view, NS is Object-oriented Tcl (OTcl) script interpreter that has a simulation event scheduler and network component object libraries, and network setup (plumbing) module libraries (actually, plumbing modules are implemented as member functions of the base simulator object). In other s, to use NS, you program in OTcl script language. To setup and run a simulation network, a user should write an OTcl script that initiates an event scheduler, sets up the network topology using the network objects and the plumbing functions in the library, and tells traffic sources when to start and stop transmitting packets through the event scheduler. The term "plumbing" is used for a network setup, because setting up a network is plumbing possible data paths among network objects by setting the "neighbor" pointer of an object to the address of an appropriate object. When a user wants to make a new network object, he or she can easily make an object either by writing a new object or by making a compound object from the object library, and plumb the data path through the object. This may sound like complicated job, but the plumbing OTcl modules actually make the job very easy. The power of NS comes from this plumbing. Another major component of NS beside network objects is the event scheduler. An event in NS is a packet ID that is unique for a packet with scheduled time and the pointer to an object that handles the event. In NS, an event scheduler keeps track of simulation time and fires all the events in the event queue scheduled for the current time by invoking appropriate network components, which usually are the ones who issued the events, and let them do the appropriate action associated with packet pointed by the event. Network components communicate with one another passing packets, however this does not consume actual simulation time. All the network components that need to spend some simulation time handling a packet (i.e. need a delay) use the event scheduler by issuing an event for the packet and waiting for the event to be fired to itself before doing further action handling the packet. For example, a network switch component that simulates a switch with 20 microseconds of switching delay issues an event for a packet to be switched to the scheduler as an event 20 microsecond later. The scheduler after 20 microsecond dequeues the event and fires it to the switch component, which then passes the packet to an appropriate output link component. Another use of an event scheduler is timer. For example, TCP needs a timer to keep track of a packet transmission time out for retransmission (transmission of a packet with the same TCP packet number but different NS packet ID). Timers use event schedulers in a similar manner that delay does. The only difference is that timer measures a time value associated with a packet and does an appropriate action related to that packet after a certain time goes by, and does not simulate a delay. NS is written not only in OTcl but in C++ also. For efficiency reason, NS separates the data path implementation from control path implementations. In order to reduce packet and event processing time (not simulation time), the event scheduler and the basic network component objects in the data path are written and compiled using C++. These compiled objects are made available to the OTcl interpreter through an OTcl linkage that creates a matching OTcl object for each of the C++ objects and makes the control functions and the configurable variables specified by the C++ object act as member functions and member variables of the corresponding OTcl object. In this way, the controls of the C++ objects are given to OTcl. It is also possible to add member functions and variables to a C++ linked OTcl object. The objects in C++ that do not need to be controlled in a simulation or internally used by another object do not need to be linked to OTcl. Likewise, an object (not in the data path) can be entirely implemented in OTcl. Figure 2 shows an object hierarchy example in C++ and OTcl. One thing to note in the figure is that for C++ objects that have an OTcl linkage forming a hierarchy, there is a matching OTcl object hierarchy very similar to that of C++. Figure 2. C++ and OTcl: The Duality Figure 3. Architectural View of NS Figure 3 shows the general architecture of NS. In this figure a general user (not an NS developer) can be thought of standing at the left bottom corner, designing and running simulations in Tcl using the simulator objects in the OTcl library. The event schedulers and most of the network components are implemented in C++ and available to OTcl through an OTcl linkage that is implemented using tclcl. The whole thing together makes NS, which is an OO extended Tcl interpreter with network simulator libraries. This section briefly examined the general structure and architecture of NS. At this point, one might be wondering about how to obtain NS simulation results. As shown in Figure 1, when a simulation is finished, NS produces one or more text-based output files that contain detailed simulation data, if specified to do so in the input Tcl (or more specifically, OTcl) script. The data can be used for simulation analysis (two simulation result analysis examples are presented in later sections) or as an input to a graphical simulation display tool called Network Animator (NAM) that is developed as a part of VINT project. NAM has a nice graphical user interface similar to that of a CD player (play, fast forward, rewind, pause and so on), and also has a display speed controller. Furthermore, it can graphically present information such as throughput and number of packet drops at each link, although the graphical information cannot be used for accurate simulation analysis. 2. Basics 2.1 OTcl: The User Language As mentioned in the overview section, NS is basically an OTcl interpreter with network simulation object libraries. It is very useful to know how to program in OTcl to use NS. This section shows an example Tcl and OTcl script, from which one can get the basic idea of programming in OTcl. These examples are from the 5th VINT/NS Simulation Tutorial/Workshop. This section and the sections after assumes that the reader installed NS, and is familiar with C and C++. Example 1 is a general Tcl script that shows how to create a procedure and call it, how to assign values to variables, and how to make a loop. Knowing that OTcl is Object-oriented extension of Tcl, it is obvious that all Tcl commands work on OTcl - the relationship between Tcl and Otcl is just same as C and C++. To run this script you should download ex-tcl.tcl, and type "ns ex-tcl.tcl" at your shell prompt - the command "ns" starts the NS (an OTcl interpreter). You will also get the same results if you type "tcl ex-tcl.tcl", if tcl8.0 is installed in your machine. Example 1. A Sample Tcl Script In Tcl, the keyword proc is used to define a procedure, followed by an procedure name and arguments in curly brackets. The keyword set is used to assign a value to a variable. [expr ...] is to make the interpreter calculate the value of expression within the bracket after the keyword. One thing to note is that to get the value assinged to a variable, $ is used with the variable name. The keyword puts prints out the following string within double quotation marks. The following shows the result of Example 1. The next example is an object-oriented programming example in OTcl. This example is very simple, but shows the way which an object is created and used in OTcl. As an ordinary NS user, the chances that you will write your own object might be rare. However, since all of the NS objects that you will use in a NS simulation programming, whether or not they are written in C++ and made available to OTcl via the linkage or written only in OTcl, are essentially OTcl objects, understanding OTcl object is helpful. Example 2. A Sample OTcl Script Example 2 is an OTcl script that defines two object classes, "mom" and "kid", where "kid" is the child class of "mom", and a member function called "greet" for each class. After the class definitions, each object instance is declared, the "age" variable of each instance is set to 45 (for mom) and 15 (for kid), and the "greet" member function of each object instance is called. The keyword Class is to create an object class and instproc is to define a member function to an object class. Class inheritance is specified using the keyword -superclass. In defining member functions, $self acts same as the "this" pointer in C++, and instvar checks if the following variable name is already declared in its class or in its superclass. If the variable name given is already declared, the variable is referenced, if not a new one is declared. Finally, to create an object instance, the keyword new is used as shown in the example. Downloading ex-otcl.tcl and executing "ns ex-otcl.tcl" will give you the following result: 2.2 Simple Simulation Example This section shows a simple NS simulation script and explains what each line does. Example 3 is an OTcl script that creates the simple network configuration and runs the simulation scenario in Figure 4. To run this simulation, download "ns-simple.tcl" and type "ns ns-simple.tcl" at your shell prompt. Figure 4. A Simple Network Topology and Simulation Scenario This network consists of 4 nodes (n0, n1, n2, n3) as shown in above figure. The duplex links between n0 and n2, and n1 and n2 have 2 Mbps of bandwidth and 10 ms of delay. The duplex link between n2 and n3 has 1.7 Mbps of bandwidth and 20 ms of delay. Each node uses a DropTail queue, of which the maximum size is 10. A "tcp" agent is attached to n0, and a connection is established to a tcp "sink" agent attached to n3. As default, the maximum size of a packet that a "tcp" agent can generate is 1KByte. A tcp "sink" agent generates and sends ACK packets to the sender (tcp agent) and frees the received packets. A "udp" agent that is attached to n1 is connected to a "null" agent attached to n3. A "null" agent just frees the packets received. A "ftp" and a "cbr" traffic generator are attached to "tcp" and "udp" agents respectively, and the "cbr" is configured to generate 1 KByte packets at the rate of 1 Mbps. The "cbr" is set to start at 0.1 sec and stop at 4.5 sec, and "ftp" is set to start at 1.0 sec and stop at 4.0 sec. Example 3. A Simple NS Simulation Script The following is the explanation of the script above. In general, an NS script starts with making a Simulator object instance. • set ns [new Simulator]: generates an NS simulator object instance, and assigns it to variable ns (italics is used for variables and values in this section). What this line does is the following: • Initialize the packet format (ignore this for now) • Create a scheduler (default is calendar scheduler) • Select the default address format (ignore this for now) The "Simulator" object has member functions that do the following: • Create compound objects such as nodes and links (described later) • Connect network component objects created (ex. attach-agent) • Set network component parameters (mostly for compound objects) • Create connections between agents (ex. make connection between a "tcp" and "sink") • Specify NAM display options • Etc. Most of member functions are for simulation setup (referred to as plumbing functions in the Overview section) and scheduling, however some of them are for the NAM display. The "Simulator" object member function implementations are located in the "ns-2/tcl/lib/ns- lib.tcl" file. • $ns color fid color: is to set color of the packets for a flow specified by the flow id (fid). This member function of "Simulator" object is for the NAM display, and has no effect on the actual simulation. • $ns namtrace-all file-descriptor: This member function tells the simulator to record simulation traces in NAM input format. It also gives the file name that the trace will be written to later by the command $ns flush-trace. Similarly, the member function trace-all is for recording the simulation trace in a general format. • proc finish {}: is called after this simulation is over by the command $ns at 5.0 "finish". In this function, post-simulation processes are specified. • set n0 [$ns node]: The member function node creates a node. A node in NS is compound object made of address and port classifiers (described in a later section). Users can create a node by separately creating an address and a port classifier objects and connecting them together. However, this member function of Simulator object makes the job easier. To see how a node is created, look at the files: "ns-2/tcl/libs/ns-lib.tcl" and "ns-2/tcl/libs/ns- node.tcl". • $ns duplex-link node1 node2 bandwidth delay queue-type: creates two simplex links of specified bandwidth and delay, and connects the two specified nodes. In NS, the output queue of a node is implemented as a part of a link, therefore users should specify the queue- type when creating links. In the above simulation script, DropTail queue is used. If the reader wants to use a RED queue, simply replace the word DropTail with RED. The NS implementation of a link is shown in a later section. Like a node, a link is a compound object, and users can create its sub-objects and connect them and the nodes. Link source codes can be found in "ns-2/tcl/libs/ns-lib.tcl" and "ns-2/tcl/libs/ns-link.tcl" files. One thing to note is that you can insert error modules in a link component to simulate a lossy link (actually users can make and insert any network objects). Refer to the NS documentation to find out how to do this. • $ns queue-limit node1 node2 number: This line sets the queue limit of the two simplex links that connect node1 and node2 to the number specified. At this point, the authors do not know how many of these kinds of member functions of Simulator objects are available and what they are. Please take a look at "ns-2/tcl/libs/ns-lib.tcl" and "ns-2/tcl/libs/ns-link.tcl", or NS documentation for more information. • $ns duplex-link-op node1 node2 ...: The next couple of lines are used for the NAM display. To see the effects of these lines, users can comment these lines out and try the simulation. Now that the basic network setup is done, the next thing to do is to setup traffic agents such as TCP and UDP, traffic sources such as FTP and CBR, and attach them to nodes and agents respectively. • set tcp [new Agent/TCP]: This line shows how to create a TCP agent. But in general, users can create any agent or traffic sources in this way. Agents and traffic sources are in fact basic objects (not compound objects), mostly implemented in C++ and linked to OTcl. Therefore, there are no specific Simulator object member functions that create these object instances. To create agents or traffic sources, a user should know the class names these objects (Agent/TCP, Agnet/TCPSink, Application/FTP and so on). This information can be found in the NS documentation or partly in this documentation. But one shortcut is to look at the "ns- 2/tcl/libs/ns-default.tcl" file. This file contains the default configurable parameter value settings for available network objects. Therefore, it works as a good indicator of what kind of network objects are available i