Unit 3: The Application Layer
In this unit, we will examine the application layer of the TCP/IP stack. The application layer is where all network processes and applications run. We will explore five of this layer's prominent applications: the Domain Name System (DNS), e-mail protocols, the World Wide Web's Hypertext Transfer Protocol (HTTP), Simple Network Management Protocol (SNMP), and Secure Shell (SSH). Finally, we will discuss socket programming and how it can be used to develop network applications.
Completing this unit should take you approximately 5 hours.
Upon successful completion of this unit, you will be able to:
- use the Domain Name System (DNS) protocol to map hostnames to IP addresses;
- compare the Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and Internet Message Access (IMAP) protocols to send and retrieve email;
- describe the use of Hypertext Transfer Protocol (HTTP) for the generation and management of web-based applications;
- use the telnet and File Transfer Protocol (FTP) applications to open remote connections and transfer files between hosts in a network;
- improve system reliability by using client-server and peer-to-peer models;
- illustrate the use of Session Initiation Protocol (SIP) to initiate and control multimedia sessions;
- describe Secure Shell (SSH)-based applications;
- describe and use the Simple Network Management Protocol (SNMP); and
- explain the role of socket programming in application processing.
3.1: Principles of the Application Layer and the Peer-to-Peer Model
As you read this section, consider these questions: What is the client-server model? What is the function of the client? Where does the server reside?
This section introduces the peer-to-peer model, which will be discussed further.
Peer-to-peer (P2P) communication allows direct communication between two parties, which bypasses the need for a middleman, such as a server. Using P2P, a message can be sent online to a friend, going straight to their device as if they were having a face-to-face conversation or using an audio telecommunications device. Pay attention to the history of P2P, beginning with Napster. Then, review the role P2P plays in Bitcoin, a digital currency. The big deal here is control. How is P2P data stored that allows for control, and how does this make it harder for others to access it without consent? Since P2P promotes independence and security, consider the industries that could benefit from P2P technology while watching the video.
As you read, try to answer the following question: how does the peer-to-peer model change or improve the client-server model discussed in the previous section?
3.2: Application-Level Protocols
There are several application protocols available, but so far, we have only talked about three: Domain Name System (DNS), Electronic Mail (SMTP and POP3), and HyperText Transfer Protocol (HTTP). The DNS protocol allows you to reference resources outside of your network quickly. E-mail protocols allow you to reach out and communicate with friends and family, and HTTP is the popular protocol for accessing webpages on other systems.
We will also look at the SNMP and SSH protocols. These two protocols are being used more and more in Internet applications. In particular, the SNMP protocol allows administrators to manage an extensive network more efficiently, and the SSH protocol allows us to design virtual private networks over public networks.
As you read, think about your social networking activities. How do these protocols allow you to communicate using various media, equipment, and file types/software?
To access a website, you need to know the website's IP address. IP addresses can be somewhat cumbersome and difficult to remember since they are simply four numbers separated by periods (for example, 120.755.3.9). The Domain Name System (DNS) is the application that has solved this problem by allowing us to use "human readable" names for websites. What is the domain name for your home page on your work, school, or home LAN? What is the IP address for each one? Which would you rather use, the IP address or the domain name?
DNS is a global network that translates domain names into IP addresses. The video explains how the DNS functions like a GPS for the Internet, where domain names are your destination and IP addresses are the coordinates guiding you to your destination. It also describes how DNS is crucial for Content Delivery Networks (CDNs) to deliver content efficiently by providing geographically relevant IP addresses. After viewing, consider the question: What is the effect on web navigation right after a DNS update occurs?
This section examines the e-mail application and explores two more popular protocols: SMTP and POP3. How does e-mail work? How long does it take for a message to reach its destination? Then what happens?
HTTP is a text-based protocol, in which the client sends a request and the server returns a response. Try browsing the links on this page, such as by navigating through this course, to see how this is accomplished.
This video introduces Simple Network Management Protocol (SNMP), which is critical for maintaining a network's health. It allows the collection, organization, and modification of data about network devices. SNMP agents, embedded in devices like routers and switches, send alerts and how these are structured within a Management Information Base (MIB). Pay particular attention to the transition from SNMPv1 and v2 to the more secure SNMPv3, noting the security mechanisms it introduces. How do the security features in SNMPv3 enhance the protocol's effectiveness in protecting network management activities?
Secure shell (SSH) is a protocol enabling a protected connection to another computer across an unsecured network. A good analogy of sending data over the internet without SSH is like placing information in an unlocked box that anyone could open. SSH secures the process by allowing the box to be locked so only the intended recipient can open it with a special key. Through SSH, data is encrypted to ensure that sensitive information remains confidential. The primary goal of SSH is to ensure that the data sent can only be read by the person intended to receive it. Why might it be important to use SSH when connecting to a computer remotely over a public network?
Ports act as access points for services, similar to phone extension numbers, while protocols are the languages used for communication between applications. This video covers some well-known ports and dynamic port assignments that enable multiple connections, such as FTP, TFTP, SNMP, Telnet, SSH, DNS, DHCP, RDP, and SMB, alongside their default port numbers, that will enhance your understanding of network communication and management.
3.3: Writing Simple Networked Applications
Read this tutorial on how to make a simple network application program with a Python socket.
3.4: Practice Exercises
These exercises further expand the key principles in this chapter. As a computer professional, you will enjoy the challenges and higher-level discussions in this section. For the novice, wander through the presentations and spend more time on the topics that are meaningful to you.
It will take a significant amount of time for you to finish these exercises. We recommend that you schedule 4 hours and attempt a few challenging exercises.
Unit 3 Assessment
- Receive a grade
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.