Lab 4.5.3 – Application and Transport Layer Protocols Examination (Answers)

Lab 4.5.3 – Application and Transport Layer Protocols Examination (Answers)

Topology Diagram

Lab 4.5.3 - Application and Transport Layer Protocols Examination (Answers) 12

Addressing Table

Device Interface IP Address Subnet Mask Default Gateway
R1-ISP S0/0/0 N/A
Fa0/0 N/A
R2-Central S0/0/0 N/A
Fa0/0 N/A
Eagle Server N/A
hostPod#A N/A 172.16.Pod#.1
hostPod#B N/A 172.16.Pod#.2
S1-Central N/A

Learning Objectives

Upon completion of this lab, you will be able to:

  • Configure the host computer to capture Application layer protocols.
  • Capture and analyze HTTP communication between the pod host computer and a web server.
  • Capture and analyze FTP communication between the pod host computer and an FTP server.
  • Observe TCP establish and manage communication channels with HTTP and FTP connections


The primary function of the Transport Layer is to keep track of multiple application conversations on the same host. However, different applications have different requirements for their data, and therefore different Transport protocols have been developed to meet these requirements.
Application layer protocols define the communication between network services, such as a web server and client, and an FTP server and client. Clients initiate communication to the appropriate server, and the server responds to the client. For each network service there is a different server listening on a different port for client connections. There may be several servers on the same end device. A user may open several client applications to the same server, yet each client communicates exclusively with a session established between the client and server.

Application layer protocols rely on lower level TCP/IP protocols, such as TCP or UDP. This lab will examine two popular Application Layer protocols, HTTP and FTP, and how Transport Layer protocols TCP and UDP manage the communication channel. Also examined are popular client requests and corresponding server responses.


In this lab, you will use client applications to connect to eagle-server network services. You will monitor the communication with Wireshark and analyze the captured packets.

A web browser such as Internet Explorer or Firefox will be used to connect to the eagle-server network service. Eagle-server has several network services preconfigured, such as HTTP, waiting to respond to client requests.

The web browser will also be used to examine the FTP protocol, as well as the FTP command line client. This exercise will demonstrate that although clients may differ the underlying communication to the server remains the same.

Task 1: Configure the Pod Host Computer to Capture Application Layer Protocols.

The lab should be configured as shown in the Topology Diagram and logical address table. If it is not, ask the instructor for assistance before proceeding.

Depending on the classroom situation, the lab topology may have been modified before this class. It is best to use one host to verify infrastructure connectivity. If the default web page cannot be accessed from, troubleshoot end-to-end network connectivity:

1. Verify that all network equipment is powered on, and eagle-server is on.

2. From a known good host computer, ping eagle-server. If the ping test fails, ping S1-Central, R2-Central, R1-ISP, and finally eagle-server. Take corrective action on devices that fail ping tests.

3. If an individual host computer cannot connect to eagle-server, check the cable connection
between the host and S1-Central. Verify that the host computer has the correct IP address, shown in the logical addressing table above, and can ping R2-Central, Verify that the host computer has the correct Gateway IP address,, and can ping R1-ISP, Finally, verify that the host has the correct DNS address, and can ping

Step 1: Download and install wireshark.

Lab 4.5.3 - Application and Transport Layer Protocols Examination (Answers) 13

Figure 1. FTP Download for Wireshark

If Wireshark is not installed on the pod host computer, it can be downloaded from See Figure 1. The download URL is

1. Right-click the wireshark filename, then save the file to the host pod computer.

2. When the file has downloaded, double-click the filename and install Wireshark with the default settings.

Step 2: Start Wireshark and configure the Capture Interface.

1. Start Wireshark from Start > All Programs > Wireshark > Wireshark.

2. When the opening screen appears, set the correct Capture Interface. The interface with the IP address of the pod host computer is the correct interface. See Figure 2.

If students ask why the Wireshark icon is a directory folder, it is because the file is a link, or short-cut, to another file. This is also why the file has no physical size.

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Figure 2. Wireshark Interface Capture Screen

Wireshark can be started by clicking the interface Start button. Thereafter, the interface is used as the default and does not need to be changed.

Wireshark should begin to log data.

3. Stop Wireshark for the moment. Wireshark will be used in upcoming tasks.

Task 2: Capture and Analyze HTTP Communication Between the Pod Host Computer and a Web Server.

HTTP is an Application layer protocol, relying on lower level protocols such as TCP to establish and manage the communication channel. HTTP version 1.1 is defined in RFC 2616, dated 1999. This part of the lab will demonstrate how sessions between multiple web clients and the web server are kept separate.

Step 1: Start Wireshark captures.

Start a Wireshark capture. Wireshark will display captures based on packet type.

Step 2: Start the pod host web browser.

1. Using a web browser such as Internet Explorer or Firefox, connect to URL A web page similar to Figure 3 will be displayed. Do not close this web browser until instructed to do so.

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Figure 3. Web Browser Connected to Web Server

2. Click the web browser Refresh button. There should be no change to the display in the web client.

3. Open a second web browser, and connect to URL This will display a different web page.

Do not close either browser until Wireshark capture is stopped.

Step 3: Stop Wireshark captures and analyze the captured data.

1. Stop Wireshark captures.

2. Close the web browsers.

The resulting Wireshark data will be displayed. There were actually at least three HTTP sessions created in Step 2. The first HTTP session started with a connection to
The second session occurred with a refresh action. The third session occurred when the second web browser accessed

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Figure 4. Captured HTTP Session

A sample captured HTTP session is shown in Figure 4. Before HTTP can begin, the TCP session must be created. This is seen in the first three session lines, numbers 10, 11, and 12. Use your capture or similar Wireshark output to answer the following questions:

3. Fill in the following table from the information presented in the HTTP session:

Web browser IP address
Web server IP address
Transport layer protocol (UDP/TCP) TCP
Web browser port number 1056
Web server port number 80

4. Which computer initiated the HTTP session, and how? started the HTTP session to with the SYN flag set. This is the beginning of the TCP three-way handshake.

5. Which computer initially signaled an end to the HTTP session, and how? signaled an end to the session with the FIN ACK flags set, acknowledged and repeated by

6. Highlight the first line of the HTTP protocol, a GET request from the web browser. In Figure 4 above, the GET request is on line 13. Move into the second (middle) Wireshark window to examine the layered protocols. If necessary, expand the fields.

7. Which protocol is carried (encapsulated) inside the TCP segment?
Hypertext Transfer Protocol

8. Expand the last protocol record, and any subfields. This is the actual information sent to the web server. Complete the following table using information from the protocol.

Protocol Version HTTP/1.1
Request Method GET
* Request UR \
Language en-us

* Request URI is the path to the requested document. In the first browser, the path is the root directory of the web server. Although no page was requested, some web servers are configured to display a default file if one is available.

The web server responds with the next HTTP packet. In Figure 4, this is on line 15. A response to the web browser is possible because the web server (1) understands the type of request and (2) has a file to return. Crackers sometimes send unknown or garbled requests to web servers in an attempt to stop the server or gain access to the server command line. Also, a request for an unknown web page will result in an error message.

9. Highlight the web server response, and then move into the second (middle) window. Open all collapsed sub-fields of HTTP. Notice the information returned from the server. In this reply, there are only a few lines of text (web server responses can contain thousands or millions of bytes).

The web browser understands and correctly formats the data in the browser window. .

10. What is the web server response to the web client GET request?
200 OK. From RFC 2616.

11. What does this response mean?
The request has succeeded.
GET an entity corresponding to the requested resource is sent in the response.

12. Scroll down the top window of Wireshark until the second HTTP session, refresh, is visible. A sample capture is shown in Figure 5.

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Figure 5. Captured HTTP Session for Refresh

The significance of the refresh action is in the server response, 304 Not Modified. With a single packet returned for both the initial GET request and refresh, the bandwidth used is minimal. However, for an initial response that contains millions of bytes, a single reply packet can save significant bandwidth.

Because this web page was saved in the web client’s cache, the GET request contained the following additional instructions to the web server:

If-modified-since: Fri, 26 Jan 2007 06:19:33 GMT\r\n
If-None-Match: “98072-b8-82da8740”\r\n <- page tag number (ETAG)

13. What is the ETAG response from the web server?

Task 3: Capture and Analyze FTP Communication Between the Pod Host Computer and a Web Server.

The Application layer protocol FTP has undergone significant revision since it first appeared in RFC 114, in 1971. FTP version 5.1 is defined in RFC 959, dated October, 1985.

The familiar web browser can be used to communicate with more than just the HTTP server. In this task, the web browser and a command line FTP utility will be used to download data from an FTP server.

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Figure 6. Windows Command Line Screen

In preparation for this task, open a command line on the host pod computer. This can be accomplished by clicking Start > Run, then typing CMD and clicking OK. A screen similar to Figure 6 will be displayed.

Step 1: Start Wireshark captures.

If necessary, refer to Task 1, Step 2, to open Wireshark.

Step 2: Start the pod host command line FTP client.

1. Start a pod host computer FTP session with the FTP server, using the Windows FTP client utility. To authenticate, use userid anonymous. In response to the password prompt, press <ENTER>.

Connected to
220 Welcome to the eagle-server FTP service.
User ( anonymous
331 Please specify the password.
Password: <ENTER>
230 Login successful.

2. The FTP client prompt is ftp>. This means that the FTP client is waiting for a command to send to the FTP server. To view a list of FTP client commands, type help <ENTER>:

ftp> help
Commands may be abbreviated. Commands are:
!          delete       literal   prompt       send
?          debug        ls        put          status
append     dir          mdelete   pwd          trace
ascii      disconnect   mdir      quit         type
bell       get          mget      quote        user
binary     glob         mkdir     recv         verbose
bye        hash         mls       remotehelp
cd         help         mput      rename
close      lcd          open      rmdir

Unfortunately, the large number of FTP client commands makes using the command line utility difficult for a novice. We will only use a few commands for Wireshark evaluation.

3. Type the command dir to display the current directory contents:

ftp> dir
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxr-xr-x   3  0         0          4096 Jan 12 04:32 pub

If the instructor is asked what the values at the beginning of the line mean, the values are Unix file permissions. The first character, ‘d’, indicates that the file is a directory. The permissions are arranged as r (read), w (write), x (access), for the owner, group, and world. World (anyone), can read and access the directory.

The FTP client is at the root directory of the FTP server. This is not the real root directory of the server—only the highest point that user anonymous can access. User anonymous has been placed into a root jail, prohibiting access outside of the current directory.

4. Subdirectories can be traversed, however, and files transferred to the pod host computer. Move into directory pub/eagle_labs/eagle1/chapter2, download a file, and exit.

ftp> cd pub/eagle_labs/eagle1/chapter2
250 Directory successfully changed.
ftp> dir
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
-rw-r--r-- 1 0 100 5853 Jan 12 04:26 ftptoeagle-server.pcap
-rw-r--r-- 1 0 100 4493 Jan 12 04:27 http to eagle-server.pcap
-rw-r--r-- 1 0 100 1486 Jan 12 04:27 ping to
-rw-r--r-- 1 0 100 15163750 Jan 12 04:30 wireshark-setup-0.99.4.exe
226 Directory send OK.
ftp: 333 bytes received in 0.04Seconds 8.12Kbytes/sec.
ftp> get "ftptoeagle-server.pcap"
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for ftptoeagle-server.pcap (5853 bytes).
226 File send OK.
ftp: 5853 bytes received in 0.34Seconds 17.21Kbytes/sec.
ftp> quit
221 Goodbye.

5. Close the command line window with the exit command.

6. Stop Wireshark captures, and save the captures as FTP_Command_Line_Client.

Step 3: Start the pod host web browser.

1. Start Wireshark captures again.

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Figure 7. Web Browser Used as an FTP Client

2. Open a web browser as shown in Figure 7, and type in URL A browser window opens with the pub directory displayed. Also, the web browser logged into the FTP server as user Anonymous as shown on the bottom of the screen capture.

3. Using the browser, go down the directories until the URL path is pub/eagle-labs/eagle1/chapter2. Double-click the file ftptoeagle-server.pcap and save the file.

4. When finished, close the web browser.

5. Stop Wireshark captures, and save the captures as FTP_Web_Browser_Client.

Step 4: Stop Wireshark captures and analyze the captured data.

1. If not already opened, open the Wireshark capture FTP_Web_Browser_Client.

2. On the top Wireshark window, select the FTP capture that is the first FTP protocol transmission, Response: 220. In Figure 8, this is line 23.

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Figure 8. Wireshark Capture of an FTP Session with a Web Browser

3. Move into the middle Wireshark window and expand the FTP protocol. FTP communicates using codes, similar to HTTP.

What is the FTP server response 220?
Service is ready for a new user.

When the FTP server issued a Response: 331 Please specify the password, what was the web browser reply?

Which port number does the FTP client use to connect to the FTP server port 21?
Port number will vary. In Figure 8 the port number is 1073.

When data is transferred or with simple directory listings, a new port is opened. This is called the transfer mode. The transfer mode can be either active or passive. In active mode, the server opens a TCP session to the FTP client and transfers data across that port. The FTP server source port number is 20, and the FTP client port number is some number above 1023. In passive mode, however, the client opens a new port to the server for data transfer. Both port numbers are above 1023.

What is the FTP-DATA port number used by the FTP server?
The port number is 20.
The Windows FTP client used for this lab did not support the PASSIVE transfer mode. If possible, students should be shown a PASSIVE transfer mode.

4. Open the Wireshark capture FTP_Web_Browser_Client, and observe the FTP communication. Although the clients are different, the commands are similar.

Step 5: FTP active and passive transfer modes

The implications between the two modes are very important from an information security perspective. The transfer mode sets how the data port is configured.

In active transfer mode, a client initiates an FTP session with the server on well-known TCP port 21. For data transfer, the server initiates a connection from well-known TCP port 20 to a client’s high port, a port number above 1023. See Figure 9.

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Figure 9.

Unless the FTP client firewall is configured to permit connections from the outside, data transfer may fail. To establish connectivity for data transfer, the FTP client must permit either FTP-related connections (implying stateful packet filtering), or disable blocking.

In passive transfer mode, a client initiates an FTP session with the server on well-known TCP port 21, the same connection used in the active transfer mode. For data transfer, however, there are two significant changes. First, the client initiates the data connection to the server. Second, high ports are used on both ends of the connection. See Figure 10.

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Figure 10.

Unless the FTP server is configured to permit a connection to a random high port, data transfer will fail. Not all FTP client applications support changes to the transfer mode.

Task 4: Reflection

Both HTTP and FTP protocols rely on TCP to communicate. TCP manages the connection between client and server to ensure datagram delivery.

A client application may be either a web browser or command line utility, but each must send and receive messages that can be correctly interpreted. The communication protocol is normally defined in an RFC.

The FTP client must authenticate to the FTP server, even if the authentication is open to the world. User Anonymous normally has restricted access to the FTP server and cannot upload files.

An HTTP session begins when a request is made to the HTTP server and ends when the response has been acknowledged by the HTTP client. An FTP session, however, lasts until the client signals that it is leaving with the quit command.

HTTP uses a single protocol to communicate with the HTTP server. The server listens on port 80 for client connections. FTP, however, uses two protocols. The FTP server listens on TCP port 21, as the command line. Depending on the transfer mode, the server or client may initiate the data connection.

Multiple Application layer protocols can be accessed through a simple web browser. While only HTTP and FTP were examined, Telnet and Gopher may also be supported on the browser. The browser acts as a client to the server, sending requests and processing replies.

Task 5: Challenge

Enabling Wireshark capture, use a web browser to browse to R2 at or use a Telnet client to connect to a Cisco device such as S1-Central or R2-Central. Observe the HTTP or Telnet protocol behavior. Issue some commands to observe the results.

How is the Application layer protocol Telnet similar to HTTP and FTP? How is TELNET different?
Answers will vary, but Telnet is similar to HTTP and FTP in that all three use the TCP protocol for connection-oriented, guaranteed delivery of datagrams.

Task 6: Clean Up

If Wireshark was installed on the pod host computer for this lab, the instructor may want the application removed. To remove Wireshark, click Start > Control Panel > Add or Remove Programs. Scroll to the bottom of the list, right-click on Wireshark, and click Remove.

If downloaded files need to be removed from the host pod computer, delete all files retrieved from the FTP server.

Unless directed otherwise by the instructor, turn off power to the host computers. Remove anything that was brought into the lab, and leave the room ready for the next class.


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