ECT 373 – Introduction to Data Communications and Networking
Lab 1
Due: 2/8/2016 Points: 30
Activity 1
Purpose: the purpose of this part of the lab is to learn about RFC and how to locate them on the
internet.
To locate a particular RFC:
1. Use a network browser such as Firefox or Microsoft’s Internet Explorer to access the Internet.
2. Access the www.rfc-editor.org/cgi-bin/rfcsearch.pl Web site.
3. Type RFC1 in the search field. Make sure that all or RFC is selected for the category to
search, and then click the Search button.
4. Click RFC0001 in the list.
5. What is the title of the RFC? Who wrote it, and when did they write it?
6. In the index search box, type rfc1.txt, and then press Enter.
7. Click rfc1.txt, which is the only RFC listed.
8. Read the RFC (read the summary, you can read the entire RFC at your own convenience).
Activity 2
Purpose: To learn how protocols and layering are represented in packets.
Requirements
Wireshark: This lab uses the Wireshark software tool to capture and examine a packet trace. A
packet trace is a record of traffic at a location on the network, as if a snapshot was taken of all
the bits that passed across a particular wire. The packet trace records a timestamp for each
packet, along with the bits that make up the packet, from the lower-layer headers to the higherlayer
contents. Wireshark runs on most operating systems, including Windows, Mac and Linux.
It provides a graphical UI that shows the sequence of packets and the meaning of the bits when
interpreted as protocol headers and data. It color-codes packets by their type, and has various
ways to filter and analyze packets to let you investigate the behavior of network protocols.
Wireshark is widely used to troubleshoot networks. You can download it from
www.wireshark.org if it is not already installed on your computer.
wget / curl: This lab uses wget (Linux and Windows) and curl (Mac) to fetch web resources.
wget and curl are command-line programs that let you fetch a URL. Unlike a web browser,
which fetches and executes entire pages, wget and curl give you control over exactly which
URLs you fetch and when you fetch them. Under Linux, wget can be installed via your package
manager. Under Windows, wget is available as a binary; look for download information on
http://www.gnu.org/software/wget/. Under Mac, curl comes installed with the OS. Both have
many options (try “wget –help” or “curl –help” to see) but a URL can be fetched simply with
“wget URL” or “curl URL ”.
Step 1: Capture a Trace
Proceed as follows to capture a trace of network traffic; alternatively, you may use a supplied
trace. This trace will look at the protocol structure of packets. A simple Web fetch of a URL
from a server of your choice to your computer, which is the client, will serve as traffic.
1. Pick a URL and fetch it with wget or curl. For example, “wget http://www.google.com” or
“curl http://www.google.com”. This will fetch the resource and either write it to a file
(wget) or to the screen (curl). You are checking to see that the fetch works and retrieves
some content. A successful example is shown below (with added highlighting) for wget.
You want a single response with status code “200 OK”. If the fetch does not work then
try a different URL; if no URLs seem to work then debug your use of wget/curl or your
Internet connectivity.
Figure 1: Using wget to fetch a URL
2. Close unnecessary browser tabs and windows. By minimizing browser activity you will
stop your computer from fetching unnecessary web content, and avoid incidental traffic
in the trace.
3. Launch Wireshark and start a capture with a filter of “tcp port 80” and check “enable
network name resolution”. This filter will record only standard web traffic and not other
kinds of packets that your computer may send. The checking will translate the addresses
of the computers sending and receiving packets into names, which should help you to
recognize whether the packets are going to or from your computer. Your capture window
should be similar to the one pictured below, other than our highlighting. Select the
interface from which to capture as the main wired or wireless interface used by your
computer to connect to the Internet. If unsure, guess and revisit this step later if your
capture is not successful. Uncheck “capture packets in promiscuous mode”. This mode is
useful to overhear packets sent to/from other computers on broadcast networks. We only
want to record packets sent to/from your computer. Leave other options at their default
values. The capture filter, if present, is used to prevent the capture of other traffic your
computer may send or receive. On Wireshark 1.8, the capture filter box is present directly
on the options screen, but on Wireshark 1.9, you set a capture filter by double-clicking on
the interface.
Figure 2: Setting up the capture options
4. When the capture is started, repeat the web fetch using wget/curl above. This time, the
packets will be recorded by Wireshark as the content is transferred.
5. After the fetch is successful, return to Wireshark and use the menus or buttons to stop the
trace. If you have succeeded, the upper Wireshark window will show multiple packets,
and most likely it will be full. How many packets are captured will depend on the size of
the web page, but there should be at least 8 packets in the trace, and typically 20-100, and
many of these packets will be colored green. An example is shown below.
Congratulations, you have captured a trace!
Figure 3: Packet trace of wget traffic
Step 2: Inspect the Trace
Wireshark will let us select a packet (from the top panel) and view its protocol layers, in terms of
both header fields (in the middle panel) and the bytes that make up the packet (in the bottom
panel). In the figure above, the first packet is selected (shown in blue). Note that we are using
“packet” as a general term here. Strictly speaking, a unit of information at the link layer is called
a frame. At the network layer it is called a packet, at the transport layer a segment, and at the
application layer a message. Wireshark is gathering frames and presenting us with the higherlayer
packet, segment, and message structures it can recognize that are carried within the frames.
We will often use “packet” for convenience, as each frame contains one packet and it is often the
packet or higher-layer details that are of interest.
Select a packet for which the Protocol column is “HTTP” and the Info column says it is a GET.
It is the packet that carries the web (HTTP) request sent from your computer to the server. (You
can click the column headings to sort by that value, though it should not be difficult to find an
HTTP packet by inspection.) Let’s have a closer look to see how the packet structure reflects the
protocols that are in use.
Since we are fetching a web page, we know that the protocol layers being used are as shown
below. That is, HTTP is the application layer web protocol used to fetch URLs. Like many
Internet applications, it runs on top of the TCP/IP transport and network layer protocols. The link
and physical layer protocols depend on your network, but are typically combined in the form of
Ethernet (shown) if your computer is wired, or 802.11 (not shown) if your computer is wireless.
Figure 4: Protocol stack for a web fetch
With the HTTP GET packet selected, look closely to see the similarities and differences between
it and our protocol stack as described next. The protocol blocks are listed in the middle panel.
You can expand each block (by clicking on the “+” expander or icon) to see its details.
? The first Wireshark block is “Frame”. This is not a protocol, it is a record that describes
overall information about the packet, including when it was captured and how many bits
long it is.
? The second block is “Ethernet”. This matches our diagram! Note that you may have
taken a trace on a computer using 802.11 yet still see an Ethernet block instead of an
802.11 block. Why? It happens because we asked Wireshark to capture traffic in Ethernet
format on the capture options, so it converted the real 802.11 header into a pseudoEthernet
header.
? Then come IP, TCP, and HTTP, which are just as we wanted. Note that the order is from
the bottom of the protocol stack upwards. This is because as packets are passed down the
stack, the header information of the lower layer protocol is added to the front of the
HTTP
TCP
IP
Ethernet
Client Server
HTTP
TCP
IP
Ethernet
Packet
information from the higher layer protocol. That is, the lower layer protocols come first
in the packet “on the wire”.
Now find another HTTP packet, the response from the server to your computer, and look at the
structure of this packet for the differences compared to the HTTP GET packet. This packet
should have “200 OK” in the Info field, denoting a successful fetch. In our trace, there are two
extra blocks in the detail panel as seen in the next figure.
? The first extra block says “[11 reassembled TCP segments …]”. Details in your capture
will vary, but this block is describing more than the packet itself. Most likely, the web
response was sent across the network as a series of packets that were put together after
they arrived at the computer. The packet labeled HTTP is the last packet in the web
response, and the block lists packets that are joined together to obtain the complete web
response. Each of these packets is shown as having protocol TCP even though the
packets carry part of an HTTP response. Only the final packet is shown as having
protocol HTTP when the complete HTTP message may be understood, and it lists the
packets that are joined together to make the HTTP response.
? The second extra block says “Line-based text data …”. Details in your capture will vary,
but this block is describing the contents of the web page that was fetched. In our case it i

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