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9.2. Network Emulators and Simulators
Basically, an emulator is a device that sits on a network and mimics the behavior of network devices or the behavior of part of a system, e.g., subnets. Actual traffic measurements are made on a network whose behavior is controlled, in part, by the emulator. Simulators are software systems that model with software the behavior of the system or networks. A simulator is a totally artificial or synthetic environment. At best, network emulators and simulators are very unlikely troubleshooting tools. But for the extremely ambitious (or desperate), it is possible to investigate the behavior of a network using these tools. Neither of these approaches is for the fainthearted or novice. Generally an expensive and complex proposition, there are two projects that are making these approaches more accessible. If you are really interested in making the investment in time and effort needed to use emulators or simulators, read on. There is a continuum of approaches to investigating the behavior of a network, ranging from direct measurement at one extreme through emulation to simulation at the opposite extreme. It's not unusual for emulators to provide limited simulation features or for simulators to have emulation features. This is certainly true for the two tools briefly described here. We have already discussed measurement techniques. But while real measurements have an unquestionable authenticity, a number of problems are associated with real measurements. Lack of reproducibility is one problem. Scale problems, such as the cost of increasing the size of the test network, are another concern. If you are considering implementation issues, then direct measurement can only be done late in the development cycle, compounding the cost of mistakes. Emulation and simulation offer lower-cost alternatives. Simulators have the advantages of being relatively cheap, providing highly reproducible results, scaling very well and inexpensively, and giving results quickly. It is generally very straightforward to customize the degree of detail in reports so you can focus on just what is of interest. Simulations vary in degree of abstraction. The greater the degree of abstraction, the easier it is to focus on what is of interest at the cost of lost realism. However, if a simulation is poorly designed, the results can have little basis in reality. Also, some simulators may be implemented primarily for one type of use and may not be appropriate for other uses. From a troubleshooting perspective, you might use a simulator to further investigate a hypothesis. Simulators would provide a way to closely examine behavior to confirm or refute the hypothesis without creating problems on a production network. Emulators lie between simulators and live systems. They allow controlled experiments with a high degree of reproducibility. They make it much easier to create the type of traffic or events of interest. They also provide a mechanism to test real systems effectively. For example, an emulator might duplicate or approximate the behavior of an attached device or network. A router emulator might drop traffic or inject traffic into the actual test network. On the downside, some emulators tend to be very specialized and are usually platform specific. For troubleshooting, an emulator could be used to stress a network.9.2.1. NISTNet
NIST Network Emulation Tool (NISTNet) is a general purpose tool that can be used to emulate the dynamics in an IP network. It was developed by the National Institute of Standards and Technology (NIST) and is implemented as an extension to the Linux operating system through a kernel module. Unlike many emulators, NISTNet supports a fairly heterogeneous approach to emulation. And since it will run on a fairly standard platform, it is remarkably inexpensive to set up and use. NISTNet allows you to use a Linux system configured as a router, through an X Window interface, to model or emulate a number of different scenarios. For example, you can program both fixed and variable packet delays and random reordering of packets. Packets can be dropped either randomly (uniform distribution) or based on congestion.[37] Random duplication of packets, bandwidth limitations, or asymmetric bandwidth can all be programmed into NISTNet. You can also program in jitter and do basic quality-of-service measurements. NISTNet can be driven by traces from measurements from existing networks. User-defined packet handlers can be added to the system to add timestamps, do data collection, generate responses for emulated clients, and so forth.[37]Gateway emulators that support this kind of behavior are sometimes less charitably called flakeways.
9.2.2. ns and nam
If you want to consider simulations, you should first look into a pair of programs, ns and nam. ns is a network simulator, while nam is a network visualization tool. Both are under development by the Virtual InterNetwork Testbed (VINT) project, a DARPA-funded research project whose goal is to produce a simulator for studying scale and protocol interactions. VINT is a collaborative project that involves USC, Xerox PARC, LBNL, and UCB. ns is derived from earlier simulation projects such as REAL and has gone through a couple of incarnations. The kernel is written in C++, while user scripts are written in MIT's Object Tool Command Language (OTCL), an object-oriented version of Tcl. With any simulation software, you should expect a steep learning curve, and ns is no exception. You'll need to learn how to use the product, and you will also need a broad knowledge of simulations in general. To use ns, you'll need to learn how to write scripts in OTCL. Fortunately, the ns project provides a wealth of documentation. The Unix manpage is more than 30 pages and displays the typical unreadable terseness associated with Unix manual pages -- great for looking up something you already know (arguably the intended use) but abysmal for learning something new. There is also a downloadable manual that runs more than 300 pages. However, the best place to start is with Marc Greis's tutorial. It is a more manageable 50 pages and introduces the scripting language in a series of readable examples. One problem with simulations is that they can produce an overwhelming amount of information. Even worse, simulation results describe dynamic events that are difficult to interpret when viewed statically. nam is a visualization tool that animates network simulations. It is hard to convey the real flavor of nam from a single black-and-white snapshot, but Figure 9-1 should give you some idea of its value.
