FIT1047 Tutorial 8 – Sample Solution
Topics and goals
• Network Layers and Protocols, Encapsulation and Decapsulation, Addresses at various layers. • Applicationlayer–HTTP
• Familiarize yourself with some of the networking tools
Instructions
The tasks are done in groups of 2 to 3 students. But for Online Classes it will be done individually.
Task 1: Basic knowledge
(i) Bit Rate & Bandwidth
Bit rate: the numbers of bits that can be transmitted over a data communications channel per time unit, usually measured in bits per second (or kilobits, megabits, gigabits per second).
Bandwidth: strictly speaking, the width of a range of fre-quencies (i.e. highest minus lowest frequency). E.g. Each channel in WiFi has a bandwidth of 20 MHz. But it’s often used as a synonym for bit rate.
(ii) Latency
The time it takes for a message from being sent to being received. Note that a network can have a very high bit rate but still feel slow: e.g. consider using a satellite connection for a video conference: you can stream very high quality video over the satellite connection, but due to the high latency, you could experience a delay of several seconds, which makes the connection almost unusable for real-time communication. This is also often a problem when playing real-time online video games.
(iii) Protocol and Protocol Layering.
A protocol: A protocol defines the rules that both the sender and receiver and all the intermediate devices need to follow to be able to communicate effectively.
Protocol layering: Protocol layering is dividing a single complex protocol into multiple simpler protocols stacked together to perform the same task.
(iv) What are the benefits of a layered architecture?
● Division of tasks: Divide a complex task into several smaller and simpler tasks.
● Modularity (flexibility): allowing a layer to be replaced by another without the need to
change the entire protocol layer stack
● Hiding complexity: protocol layering allows the services to be separated from
implementation which hides the complexity of how a service is implemented in the lower layer from the higher layer
(v) Is there any disadvantage to a layered architecture?
Yes, it adds some overhead. Each layer needs to keep track of its communication with the corresponding layer on other devices (host-to-host, or hop-to-hop). Some of the tasks may be repeated in some of the layers (e.g. error detection at data link layer and transport layer).
(vi) What is a logical connection?
The layer-to-layer communication is referred to as a logical connection. Logical since the actual communication occurs at each layer by passing the Protocol Data Units (PDUs) down to the layer below (or up on receiver). So the actual data exchange is between the layers of the protocol stack. A connection since the corresponding layers follow the same protocol when communicating with one another (host-to-host, or hop-to-hop). The term connection here is used loosely.
(vii) What is Addressing in a layered architecture?
Any communication that requires two parties (any two from a larger pool of entities), needs a mechanism to specify the source address and destination address. As we have logical connections, we have various forms of addressing in each layer.
(viii) PDU & Encapsulation
PDU: Short for Protocol Data Unit. The term used to describe data as it moves from one layer of the TCP/IP model to another. In this reference, PDU is often used synonymously with packets.
Encapsulation is the process of taking data from one layer protocol and translating it into another layer protocol, so the data can continue across a network. For example, a TCP/IP packet contained within an ATM frame is a form of encapsulation.
Figure 1: The network where the traffic was captured
Task 2: Packet Sniffing
A packet analyser (sometimes also called “packet sniffer”) is a program that can log all packets that are received and transmitted over a network interface. We will be using Wireshark, a very popular open- source tool for packet analysis. You can download it on your own computer from www.wireshark.org, or use the version installed on the Monash lab computers.
This week, we will analyse a sequence of packets captured on Guido’s network at home. You can download the log file that Wireshark produced from the FIT1047 Moodle site (week 7). The file is called Wireshark http example.pcap. The diagram in Figure 1 explains how Guido’s computer is connected to the Monash web server.
1. Start Wireshark and load the capture file (see Fig. 2). After opening the file, the main window should look like the one in Fig. 3.
2. Select “frame” number 6 (as in Fig. 3). This frame shows a request sent from Guido’s home computer to the web server at www.csse.monash.edu, requesting his homepage.
3. Familiarise yourself with the three main sections (panes) of the Wireshark window:
Figure 2: Wireshark File menu
• Thepacketlistpanedisplaysasummaryofeachpacketcaptured.Whenyouclickonapacket here, the other two panes are updated with the details for that packet.
• The packet details pane below shows information about the selected packet.
• The packet bytes pane displays the raw data for the selected packet. It highlights the data for
the field that is selected in the packet details pane.
Navigate between the panes and explore the relationships between the displayed pieces of information.
4. Click on frame 6.
a) How long is it in bytes? 432 bytes
b) Which application layer protocol does it use? HTTP
5. There are five lines in the packet details pane for frame 6, each of which can be expanded by
clicking on the “+” or “>” symbol.
Figure 3: Wireshark main window
a) Can you identify what they stand for?
From top to bottom: the entire frame captured by Wireshark; the data link layer; the network layer; the transport layer; the application layer.
b) Which protocols are being used?
From top to bottom: Ethernet; IP version 4; TCP; HTTP.
c) What are the names of the PDUs for each protocol?
Ethernet: frame; IP: packet; TCP: segment; HTTP: message.
d) Recall that each protocol layer encapsulates the message from the layer above and adds a header. What are the sizes of the headers for each PDU used in frame 6?
You can find the size by clicking on the relevant protocol line and counting the number of bytes highlighted in the bottom pane. Some protocols also tell you the size when you open the details view. Ethernet: 14 bytes; IP: 20 bytes; 32 bytes; HTTP: we can consider the entire contents to be the actual message, there are no additional headers (such as the ones for TCP or IP).