Overview

For this exercise, you'll be describing the OSI layers which a number of network tasks go through on their journey from client to server and back again.

You will need to refer to materials explaining the OSI model (you can start with module NG01d) and investigate various protocols used on the Internet.

There is a worksheet you'll use to compile the answers and submit to your instructor. This is an HTML page which can either be printed and filled in by hand (neatly, please!) or saved and modified using an HTML/text editor. The worksheet has tables which look like this:

Layer	Name	Protocol	Up/Dn	What the layer does in this context
7
6
5
4
3
2
1

You'll notice that the layers are listed from top to bottom rather than side to side. This is because the OSI model is commonly shown either way, and it's good practice to get accustomed to both.

What to put in each column:

• Name: The name of the layer
• Protocol: The name of the protocol, application program, port number, or physical device that plays the major role in this layer
• Up/Dn: The direction in which the message is travelling.
  • Up = 1-2-3-4-5-6-7
  • Down = 7-6-5-4-3-2-1
• What the layer does in this context: a brief explanation of happens to the message at this point in its journey. This does not need to be detailed - just explain, to the best of your understanding, what's going on here.

Example Task:

Your have clicked on a link in your browser, and the IP address of the server is known. Show the progress of the file request through the OSI layers on your computer until it reaches the physical network. Assume your computer is connected to a local area network (LAN) via ethernet.

Layer	Name	Protocol	Up/Dn	What the layer does in this context
7	Application	browser	Dn	Browser initiates request for a file
6	Presentation	HTTP	Dn	Creates HTTP-format request for file
5	Session	Port 80	Dn	Creates session and session request for server
4	Transport	TCP	Dn	Creates TCP packet with header and CRC
3	Network	IP	Dn	Finds IP address of next stop; puts it in header
2	Data Link	ethernet	Dn	Encodes packet for ethernet; sends to hardware
1	Physical	Cat-5	Dn	Transmits electrical impulses over Cat-5 wire

Find the worksheet at NG01hw (http://courses.wccnet.org/computer/mod/ng01hw.htm).

 

When a domain name (such as www.wccnet.edu) is used to refer to a network device, the IP (physical) address must be determined before any packets can be routed to that device. This requires asking a Domain Name System server to look up the domain name and return the IP address. This process is referred to as a "DNS lookup". (The IP address of one or more DNS servers is always entered into a computer when it is first set up for Internet connectivity.)

Trace a DNS lookup, using one worksheet table for each of the steps shown below. Label each table as Task 1, Step A, B, or C.

Steps:

1. You have clicked in your browser on a link to thomas.loc.gov. Your browser initiates a DNS lookup. Show the progress of the lookup through the OSI layers on your computer until it reaches the physical network. Assume your computer is connected to a local area network (LAN) via ethernet.
2. There is one network router between your computer and the nearest DNS server. Trace the request's upward progress from the physical layer until the packet is sent back down. (Note that the request does not go all the way from layers 1 to 7 in a router.)
3. The DNS server receives the lookup request. Trace the progress of the packet from the bottom to the top. (You do not need to trace the server's response back to your computer.)

Once your computer receives the IP address of thomas.loc.gov, it is able to send a request for a file to that server. Assume your request has arrived at the server, the server has found the file you want, and is ready to send it back to you.

Trace the file on its way back to your computer. Make another copy of the worksheet, and label each table as Task 2, Steps A, B, or C.

Steps:

1. The Web server at thomas.loc.gov has the file you asked for. Trace its progress on the server from the Application Layer to the Physical Layer.
2. There are probably several routers and network devices between thomas.loc.gov and your computer. Show the last device in the chain: this time, assume it is a dial-up modem controller at your ISP, and trace the progress of your file downward from the highest layer needed on a device of this kind.
3. Trace the file from your computer's modem connection to your browser. (Be sure to note the different low-level protocols in this configuration, which is different from the ethernet configuration in Task 1.)

Now you need to send a file via FTP from your computer to a Web server.

Trace the file on its way from your computer to the server. Make another copy of the worksheet, and label each table as Task 3, Steps A, B, or C.

Steps:

1. You are running WS-FTP and have connected to your Web server, stu.wccnet.edu. You have selected the file you want to send to the server, and click the arrow to send it. Trace the progress of the file from Application Layer to Physical Layer, assuming that you're back on a computer with an ethernet connection.
2. Trace the file's progress (upward only) through one of the routers it passes through on the way to the server.
3. And finally, trace the file all the way from Physical to Application Layer.

When you're done, double-check that all three tasks meet the specifications above, and that all links are functional.

You may submit this assignment either on paper or as an attachment to email, depending on whether you filled in the worksheet information by hand or using an editor.