The top layer of the protocol stack in the Internet Protocol Suite model and the top three layers in the OSI Reference Model, Application, Presentation and Session, can be considered as a singular discrete layer, the application layer. These provide user level services situated on end hosts. Examples include HTTP \cite{rfc2616} and DNS \cite{rfc1035}. These provide for the transmission of material in the form of hypertext documents and other media and a distributed directory of names and addresses.
+\subsection{Simulation}
+
+Network simulation is a evolving area of research, with many different simulators and simulation techniques being used. It is rapidly becoming the most popular way of performing large scale network research for both local area networks, and the wider internet due to the low cost and speed of iteration of different tests that it allows. When testing networks of significant scale, the approach commonly taken is to verify it works on a small testbed, before scaling up under simulatuion.
+
+There are a number of different approaches to simulating a computer network architecture. The first is discrete event simulation. Here a simulator is primed with a topology and a number of event sources. These event sources fire events, which are executed in turn, which may then fire other events. The events are stored in a priority queue. Each event is executed according to it's time, with a global variable containing the current time of the simulation being advanced upon the executing of each event. This is continued until either there are no more events left in the queue, or some predefined time has passed. This simulation technique has the advantage that areas in which nothing of interest are passed over quickly, while events which are notable are where the majority of the computation is spent.
+
+Other types of network simulation include markov chain simulations which are useful for modelling queueing systems. These depend on systems with little state that can be modelled by a stochastic process. This isn't the case for packet simulation, where there is a large amount of state and no overall defined equation across the entire simulation.
+
+A further type of simulation is continuous simulation where the timestep is incremented, and events ocurring between the two states are modelled. This works well for physical phonomena, such as radio waves or propogation of signals in wires where the situation is continually evolving. However, they spend the same processing power on all periods of time during the simulation, whether the period is one of high activity or low activity. Also, in order to get a good representation of the underlying phonemena, you have to use very small time increments which gives a high simulation cost.
+
\section{Context}
The most important work in this area is set out by Scott et al \cite{moose1} which sets out the ideas behind MOOSE which this paper aims to test under simulation. A prototype NetFPGA implementation also exists by Wagner-Hall et al \cite{moose2} which outlines a practical implementation of MOOSE is described.
\subsection{Simulation}
-Network simulation is a evolving area of research, with many different simulators and simulation techniques being used. It is rapidly becoming the most popular way of performing large scale network research for both local area networks, and the wider internet due to the low cost and speed of iteration of different tests that it allows. When testing networks of significant scale, the approach commonly taken is to verify it works on a small testbed, before scaling up under simulatuion.
-
-There are a number of different approaches to simulating a computer network architecture. The first is discrete event simulation. Here a simulator is primed with a topology and a number of event sources. These event sources fire events, which are executed in turn, which may then fire other events. The events are stored in a priority queue. Each event is executed according to it's time, with a global variable containing the current time of the simulation being advanced upon the executing of each event. This is continued until either there are no more events left in the queue, or some predefined time has passed. This simulation technique has the advantage that areas in which nothing of interest are passed over quickly, while events which are notable are where the majority of the computation is spent.
-
-Other types of network simulation include markov chain simulations which are useful for modelling queueing systems. These depend on systems with little state that can be modelled by a stochastic process. This isn't the case for packet simulation, where there is a large amount of state and no overall defined equation across the entire simulation.
-
-A further type of simulation is continuous simulation where the timestep is incremented, and events ocurring between the two states are modelled. This works well for physical phonomena, such as radio waves or propogation of signals in wires where the situation is continually evolving. However, they spend the same processing power on all periods of time during the simulation, whether the period is one of high activity or low activity. Also, in order to get a good representation of the underlying phonemena, you have to use very small time increments which gives a high simulation cost.
-
-\subsection{Simulation}
-
Due to the large number of hosts, switches and links I will be aiming to simulate to demonstrate the weaknesses of Ethernet, and how it compares to MOOSE, I will be using simulation instead of implementing the topology physically. While simulators pose an enormous advantage in terms of the time it takes to change the topology, implement new protocols and explore larger topologies, there are a number of disadvantages, namely that as it is a simulation, it takes time to program a simulator, and this may approach the difficulty in creating a real network, execution can be computationally expensive, and statistical analysis of the simulation in order to verify the results can be difficult.
As discussed in the introduction, there are a number of different type of simulators - the main distinction being a discrete event simulation or a continuous event simulation. The most suitable for performing network simulation of a packet based architecture is a discrete event simulator, as each movement of a packet may be viewed of as an event in the system.
projects / ii-diss.git / commitdiff