In this chapter, we will explore the role of the application layer and how the applications, services, and protocols within the application layer make robust communication across data networks possible.
The Application Layer
The application layer is closest to the end user. As shown in the figure, it is the layer that provides the interface between the applications used to communicate and the underlying network over which messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts.
The upper three layers of the OSI model (application, presentation, and session) define functions of the single TCP/IP application layer.
Client-Server Model
In the client-server model, the device requesting the information is called a client and the device responding to the request is called a server. Client and server processes are considered to be in the application layer. The client begins the exchange by requesting data from the server, which responds by sending one or more streams of data to the client. Application layer protocols describe the format of the requests and responses between clients and servers. In addition to the actual data transfer, this exchange may also require user authentication and the identification of a data file to be transferred.
Peer-to-Peer Applications
A P2P application allows a device to act as both a client and a server within the same communication, as shown in the figure. In this model, every client is a server and every server a client. P2P applications require that each end device provide a user interface and run a background service.
Hypertext Transfer Protocol and Hypertext Markup Language
When a web address or uniform resource locator (URL) is typed into a web browser, the web browser establishes a connection to the web service running on the server using the HTTP protocol. URLs and Uniform Resource Identifier (URIs) are the names most people associate with web addresses.
To better understand how the web browser and web server interact, we can examine how a web page is opened in a browser. For this example, use the http://www.cisco.com/index.html URL.
First, as shown in Figure 1, the browser interprets the three parts of the URL:
- http (the protocol or scheme)
- http://www.cisco.com (the server name)
- index.html (the specific filename requested)
As shown in Figure 2, the browser then checks with a name server to convert http://www.cisco.com into a numeric IP address, which it uses to connect to the server. Using HTTP requirements, the browser sends a GET request to the server and asks for the index.html file. The server, as shown in Figure 3, sends the HTML code for this web page to the browser. Finally, as shown in Figure 4, the browser deciphers the HTML code and formats the page for the browser window.
HTTP and HTTPS
HTTP is a request/response protocol. When a client, typically a web browser, sends a request to a web server, HTTP specifies the message types used for that communication. The three common message types are GET, POST, and PUT (see the figure):
- GET – A client request for data. A client (web browser) sends the GET message to the web server to request HTML pages.
- POST – Uploads data files to the web server such as form data.
- PUT – Uploads resources or content to the web server such as an image.
Email Protocols
Email supports three separate protocols for operation: Simple Mail Transfer Protocol (SMTP), Post Office Protocol (POP), and IMAP. The application layer process that sends mail uses SMTP. A client retrieves email, however, using one of the two application layer protocols: POP or IMAP.
SMTP Operation
SMTP message formats require a message header and a message body. While the message body can contain any amount of text, the message header must have a properly formatted recipient email address and a sender address.
When a client sends email, the client SMTP process connects with a server SMTP process on well-known port 25.
POP Operation
POP is used by an application to retrieve mail from a mail server. With POP, mail is downloaded from the server to the client and then deleted on the server. This is how POP operates, by default.
The server starts the POP service by passively listening on TCP port 110 for client connection requests.
IMAP Operation
IMAP is another protocol that describes a method to retrieve email messages. Unlike POP, when the user connects to an IMAP-capable server, copies of the messages are downloaded to the client application. The original messages are kept on the server until manually deleted. Users view copies of the messages in their email client software.
File Transfer Protocol
FTP is another commonly used application layer protocol. FTP was developed to allow for data transfers between a client and a server. An FTP client is an application that runs on a computer that is used to push and pull data from an FTP server.
As the figure illustrates, to successfully transfer data, FTP requires two connections between the client and the server, one for commands and replies, the other for the actual file transfer:
- The client establishes the first connection to the server for control traffic using TCP port 21, consisting of client commands and server replies.
- The client establishes the second connection to the server for the actual data transfer using TCP port 20. This connection is created every time there is data to be transferred.
The data transfer can happen in either direction. The client can download (pull) data from the server, or the client can upload (push) data to the server.
Server Message Block
The Server Message Block (SMB) is a client/server file sharing protocol that describes the structure of shared network resources, such as directories, files, printers, and serial ports. It is a request-response protocol. All SMB messages share a common format. This format uses a fixed-sized header, followed by a variable-sized parameter and data component.
SMB messages can:
- Start, authenticate, and terminate sessions
- Control file and printer access
- Allow an application to send or receive messages to or from another device
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