Introduction to Data Communications (ITS 323)

Assignment

Below is some information about the assignment which is in addition to the original instructions.

What To Look For In My Capture?

6 Sep 2012

Some hints on how to use the capture I provided:

There are multiple web page transfers captured (see the video). Some are simpler than others. You can filter by http.request and find the requests for different web pages that correspond to what I did on my laptop in the video. You only need to consider one web page transfer, not all of them.
Once you've found the http messages, try to understand what they mean, e.g. what is being sent between who?
Once you understand the web exchange, look for other related packets. Remember that HTTP is an application layer protocol. It uses protocols at lower layers to transfer the data. Find the transport protocol HTTP uses, and filter for that protocol. You should see some more packets related to the web transfer (in addition to the HTTP packets). Hint: the transport protocol that HTTP uses includes flow and error control mechanisms similar to what we covered in lectures.
Computers on the Internet use IP at the network layer. They identify each other using IP addresses. But humans commonly remember domain names, not IP addresses. So when I visit a web site, I type in a URL that includes a domain name and the file/directory that I want to visit. Before my computer can send IP datagrams to the webserver, it must discover the IP address of the web server. DNS is a system/protocol for discovering an IP address of a server, given the servers domain name.
There are many packets in my capture that are not relevant. Use the filters wisely.
Draw your own message (time) sequence diagram; don't rely on Wiresharks diagram. You can draw better than what Wireshark does. Use software for drawing the diagrams, e.g. technical drawing software like Dia (free) or MS Visio, or general drawing software like Powerpoint, OpenOffice Draw, Adobe software etc. (if you want to draw diagrams as inartistically as me, try XFig
For the packet diagrams you should identify the protocols used in each layer and label their headers. You don't need to give header fields, although sometimes its useful to give a selection of the most important header fields.

Throughput Experiments

For the wireless LAN throughput experiment, I suggest you transfer a large file between a nearby server (e.g on the same LAN as you) and your laptop. One approach is to use a direct wireless link (sometimes called adhoc) from one laptop to another, i.e. no access point, cables or other servers.

Example Wireless LAN Capture for Task 2

2 Sep 2012

Originally task B of the assignment required you to capture traffic on a wireless link while accessing a web page. Although you can do this on almost any computer, to see the details of what happens using the wireless LAN you normally need Linux, and in some cases specific wireless LAN cards/chips.

Rather than ask everyone to find a laptop that can correctly capture, instead I have performed a capture using my laptops and provide you that capture. You can use the capture below to complete Task B. That is, you can load the capture file in Wireshark and complete tasks B2, B4 and B5. Tasks B1 and B3 do not need to be completed (since I've done them for you). Of course, if you want to capture usingyour own computer you may (and should still complete tasks B1 and B3).

I captured all traffic seen by laptop 1 on its wireless LAN interface. The capture file to be loaded into Wireshark: ITS323Y12S1H16-Wireless-LAN-Capture.cap.

While capturing on laptop 1, on laptop 2 I connected to a wireless LAN access point (with SSID its323) and in my browser accessed several web pages. A video of this is available on YouTube. The time in the video is about 1 second behind the time in the capture, e.g. at time 47 seconds in the video when I press Enter to visit the website, the time in the capture is about 48 seconds.

The hardware address of laptop 2 is: 00:26:5e:8e:e4:95. It was assigned an IP address: 192.168.0.104. The hardware address of the access point is: 00:18:e7:8a:44:63.

Example Display Filters

2 Sep 2012

In Wireshark when there are more than 10's of packets captured, finding packets of interest requires search or filtering. Wireshark has a display filter the shows only those packets that match certain conditions. The conditions are written as an expression in the filter box at the top of Wireshark:

Clicking on the Expression... button takes you to a window where you can create the filter.

You select the header field or parameter for a specific protocol. If you want to display only packets which have this field present, then select the relation Is present and click ok. But you can be more specific, showing only packets where the field has a particilar (range of) values, using ==, !=, > etc. Once you start to learn the field names you don't need to use the expression editor: instead you can type the expression directly into the display filter box.

There are many protocols to chose from, and for each protocol usually many fields. Finding the field you want is not easy. To get you started, below are some examples of filters that may be useful.

Protocols: http, tcp, ip, arp, ...

Typing the protocol's acronym, in lowercase, will often filter packets using that protocol. Works for http, tcp, ip, arp, icmp, ssl, wlan and many others.

HTTP: http

http

This finds all packets that use HTTP. You can also find requests only (http.request) or responses only (http.response).

IP Address: ip.addr

ip.addr==192.168.0.104

This finds all packets where the source address or destination address match 192.168.0.104. Similarly, you can filter based on source (ip.src) and destination (ip.dst).

WLAN Address: wlan.addr

wlan.addr==00:26:5e:8e:e4:95

Finds all packets where the source or destination wireless LAN address matches 00:26:5e:8e:e4:95. Similarly, you can filter based on source address (wlan.sa), destination address (wlan.da), transmitter address (wlan.ta) and receiver address (wlan.ra). Note that for a laptop sending to a computer on a LAN, the destination address will be computers address, while the receiver address will be the WLAN access point (since the laptop sends to the computer via the AP).

WLAN Frame Types: wlan.fc.type and wlan.fc.subtype

Wireless LAN frames are categorised into different types and subtypes. Common frames you will see are listed below (here is a more complete list).

Frame	Type	Subtype
Management	0	-
Association Request	0	0
Association Response	0	1
Reassocation Request	0	2
Reassociation Response	0	3
Probe Request	0	4
Probe Response	0	5
Beacon	0	8
Authentication	0	11
Deauthentication	0	12
Control	1	-
RTS	1	11
CTS	1	12
ACK	1	13
Data	2	-
Data	2	0

wlan.fc.type==0

This displays all management frames, such as frames for discovering, joining and leaving the network.

wlan.fc.type==0 && wlan.fc.subtype==8

This displays all the Beacon frames (which are a subset of the management frames).

Group Numbers and Websites

General Comments

Login to IT Server

To login to the IT server you need a SSH client. On Unix/Linux operating systems (e.g. Ubuntu, MacOSX) a SSH client is normally included. Open a terminal and type:

ssh -l u5000000000 it.siit.tu.ac.th

but of course using your username and then supplying your password (same as used for Moodle). If successful you can run commands to view files, change into directories and edit files.

Windows doesn't come with a built-in SSH client. I recommend you download and use PuTTY (select the Windows Installer from the Download page).

Basics File Editing on IT Server

You have two options to create your website:

Write the web pages using a text editor directly on the IT server
Create the web pages (e.g. using Notepad++ or any text/HTML editor) on your own computer and then transfer the files to the IT server

To edit directly on the IT server, once you have logged in you can try the following commands. To list the files and directories in your present working directory:

ls

To change to another directory, for example the public_html directory:

cd public_html

To change back to your home directory (which is /home/students/u5000000000 in my example):

cd

To edit a file, such as index.html in a text editor (called nano):

nano index.html

Press Ctrl-O to save the file and Ctrl-X to exit the editor.

To make a new directory, for example, its323:

mkdir its323

Further basic commands can be found on pages 7, 8 and 9 of the ITS332 Lab Manual. A more detailed introduction is available in the free book by FSF Introduction to Command Line.

Transfer Files to IT Server

If you want to edit files on your computer and then upload them to IT server then you need a SFTP client. For Windows, WinSCP is ok. For cross-platform (Windows, MacOS, Linux) try FileZilla. With either of these installed you can easily connect to the IT server and drag-and-drop files to transfer them to/from the IT server.

You can also use the command line to copy files. Again, Linux/MacOS have scp available, while pscp.exe comes part of the PuTTY install on Windows. The following is an example with scp in a terminal on your computer to copy file mypage.html into the directory /home/students/u5000000000/public_html/its323/ on the IT server:

scp mypage.html u5000000000@it.siit.tu.ac.th:/home/students/u5000000000/public_html/its323/

Similar syntax on Windows, but use pscp.exe instead of scp (you may have to set your PATH variable for this to work - see me if problems).