Application Layer Functionality and Protocols

Most of us experience the Internet through the World Wide Web, e-mail services, and file-sharing programs. These applications, and many others, provide the human interface to the underlying network, enabling us to send and receive information with relative ease. Typically the applications that we use are intuitive, meaning we can access and use them without knowing how they work. However, for network professionals, it is important to know how an application is able to format, transmit and interpret messages that are sent and received across the network.


OSI – (Open System Interconnection) international standardization program created by ISO and ITU-T to develop standards for data networking that facilitate multivendor equipment interoperability. It divides the networking process into seven logical layers, each of which has unique functionality.

Application Layer – uses protocols that are implemented within applications and services. It provides the interface between the applications on either end of the network.

Presentation Layer – ensure that data from the source device can be interpreted by the appropriate application on the destination device.

Session Layer – create and maintain dialogs between source and destination applications. It handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

DNS – (Domain Name Service) is used to resolve internet names to IP addresses. It matches resource names with the required IP address.

HTTP – (Hypertext Transfer Protocol) is used to transfer files that make up the web pages of the World Wide Web. It transfer data from a web server to a client.

SMTP – (Simple Mail Transfer Protocol) is used for the transfer of mail messages and attachments.

Telnet – is used to provide remote access to servers and networking devices. But it does not support encryption.

FTP – (File Transfer Protocol) is commonly used to support for file transfer between a client and a server.

Client-server model – the device requesting the information is called a client and the device responding to the request is called a server.
• centralized administration
• security is easier to enforce

Peer-to-peer – two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server.
• Act as both a client and server within the same communication.
• Hybrid mode includes a centralized directory of files.
• Can be used in client-server networks



Nslookup – a utility of a computer operating systems that allows the user to manually query the name servers to resolve a given host name. This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers.



MTU – (Mail User Agent) allow messages to be sent and places received messages into the clients mailbox, both of which are distinct processes. In order to receive e-mail messages from an email server, the email client can use POP. Sending e-mail from either a client or a server uses message formats and command strings defined by the SMTP protocol. Usually an e-mail client provides the functionality of both protocols within one application.




Protocols operate at application layer of the OSI model:
• DNS
• SMTP
• POP

The e-mail server operates two separate processes:
• Mail Transfer Agent (MTA)
• Mail Delivery Agent (MDA)
The Mail Transfer Agent (MTA) process is used to forward e-mail. As shown in the figure, the MTA receives messages from the MUA or from another MTA on another e-mail server. Based on the message header, it determines how a message has to be forwarded to reach its destination. If the mail is addressed to a user whose mailbox is on the local server, the mail is passed to the MDA. If the mail is for a user not on the local server, the MTA routes the e-mail to the MTA on the appropriate server.

DHCP – (Dynamic Host Configuration Protocol) enables devices on a network to obtain IP addresses and other information from a DHCP server. This service automates the assignment of IP addresses, subnet masks, gateway and other IP networking parameters.

SMB – (Server Message Block) is a client/server file sharing protocol. It describe the structure of shared network resources, such as directories, files, printers and serial ports. SMB file-sharing and print services have become the mainstay of Microsoft networking.