Assignment

Task 1

Create and simulate a network similar to the one shown here, using the Netml system.

Links

The links from A (node_0) to X (node_1), C to X, Y (node_3) to B (node_2), and Y to D (node_5) should have a capacity of 10 (in Mbit/sec), and the link from X to Y should have a capacity of 2.

Traffic

The traffic from C to D represents UDP traffic which sends a series of ping packets at regular intervals. This trafficstream should have the following parameters set:

Type udp

Start 1

Finish 9

PacketSize 1000

MaxPackets 10000

Interval 0.1

• The traffic from A to B and also from C to D represent bulk ftp transfers and they should have the following parameters set:
type bulkftp

Start 1

Finish 9

MaxBytes 10000000

• The Output

• When we run the simulation, we want to collection information about what is happening. There are many ways to do this, so if we are not careful we will collect so much information that we will not be able to make sense of it. For example, we can turn on logging of events relating to different modules in the simulation by making appropriate selections in the ns3 dialog box. It is a good idea to do this, to make sure that the simulation is working as expected. It is a requirement of this assignment that you do this and report on the information in the simulation log which tells you whether the simulation is working as you expect

• In this case, the main interest is on the middle link, which can easily be overloaded, because the links which feed into it have higher capacity than it. We therefore would like to know the behaviour of the buffer, at the head of the link from X to Y. To observe this turn on tracing of this buffer, which you can do using the menu of the link from X to Y. The graphs which are produced in this way should be included in your report and explanations provided of what they tell us about the simulation.

Task 2

This task builds on the previous one. The network is a little more complex, as depicted at the right. There is an extra node which is a lot further away. In the diagram at the right, the labels indicate the propagation delay of each link.

• The link capacities are all 10 Mbps except for the middle link, which has capacity 2 Mbps. The top traffic stream and bottom traffic stream are both bulk ftp transfers of 10 Mbytes and the other traffic stream is a UDP flow of 100000 packets with an interval between packets of 0.1 second.

• The issue we wish to explore in this assignment is whether a traffic flow can be disadvantaged by passing along a path with larger propagation delay than the other flows which share the network.

• In addition, we would like to explore ways in which this performance problem (if it exists) might be alleviated. The methods we wish to explore as follows:

o replace the default DropTail queue at the congested node, X, by a RED queue;

o try different values of the propagation delay from Z to X starting at 50ms and going up to 500ms (including at least, 100ms), to see if the problem becomes more serious with delay (and less serious as the delay differential decreases);

o check if the problem is different depending on which of the two flows (the one with a long delay and the one with a short delay) starts first.

Network Setup

All the links in the network should have a data rate of 10 Mbps and a propagation delay of 10 ms except the link from X to Y, which should have a data rate of 2 Mbps, and the link from Z to X, which should have a propagation delay of 50ms initially, and then varying up to 500 ms.
The top and bottom traffic flows should be ftp transfers of 10,000,000 bytes each. The one at the top should start at 1s, and the one below at 4s (although this will be varied during the experiments). Both flows should have a finish time of 10s and the PacketSize parameter of both should be 1000. The other traffic stream should be a UDP Ping flow, with an Interval of 0.1, starting at time 0.9s, and finishing at time 10 and with many packets: 100,000.

The criteria for this assignment are as follows:

Aspect 1

A satisfactory assignment document has been prepared, a network has been created and is depicted in a diagram in the assignment document.

Aspect 2

Graphs which explain the behaviour of the network in its original setup are presented including the throughput of the three flows.

Aspect 3

Graphs of dropped packets, and the congestion window between A and B and between Z and F, are included.

Aspect 4

A description of the normal behaviour of the network is included in the assignment document. This should include a comparison of the throughput of the flow with a long delay to the flow with a short delay.

Aspect 5

A description of the behaviour of the network when the DropTail queue is replaced by a RED queue is provided. This should include plots of key performance indicators such as throughput for both FTP flows.

Aspect 6

Experiments have been conducted to determine the effect of the delay difference (increasing the delay from 50ms to 500ms), between the two ftp flows, on the different performance each flow experiences.

Aspect 7

Experiments have been conducted to show if it makes a difference if the flow from A to B starts before or after the flow from Z to F.

Aspect 8

The assignment includes a discussion section in which the issue of fair access to network resources by flows with different path lengths and different start times is discussed.