Application Layer Functionality and Protocols

Network Fundamentals Chapter 3 Click to Edit Master Subtitle Style

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Application LayerThe Open Systems Interconnection (OSI ) reference model is a layered, abstract representation created as a guideline for network protocol design. The OSI model divides the networking process into seven logical layers, each of which has unique functionality and to which are assigned specific services and protocols.

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Application LayerThe Presentation layer has three primary functions: Coding and conversion of data. Compression of data. Encryption and decryption of data.

Standards, used in the application layer such as MPEG and QuickTime, are not typically associated with a particular protocol stack. The Session layer creates and maintains dialogs between source and destination applications.

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Application LayerTwo forms of software programs or processes provide access to the network: applications and services. Applications are the software programs used by people to communicate over the network.

Services are the programs that interface with the network and Some applications are networkprepare the data for transfer. aware, they implement the Applications or network services Application layer protocols and uses protocols as the are able to communicate directly standards and data formats. with the lower layers of the protocol stack.

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The Client/Server modelIn the client/server model, the device requesting the information is called a client and the device responding to the request is called a server.

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Peer-to-PeerIn a peer-to-peer network, two or more computers are connected via a network and can share resources without a dedicated server. Every connected end device (known as a peer) can function as either a server or a client.

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Peer-to-Peer A peer-to-peer application (P2P), allows a device to act as both a client and a server within the same communication. Both can initiate a communication and are considered equal in the communication process. Peer-to-peer applications require that each end device provide a user interface and run a background service.

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Port Numbers

The Transport layer uses port numbers to identify applications and services that are the source and destination of data, common predefined port numbers are: Domain Name System (DNS) - TCP/UDP Port 53 Hypertext Transfer Protocol (HTTP) - TCP Port 80 Simple Mail Transfer Protocol (SMTP) - TCP Port 25 Post Office Protocol (POP) - TCP Port 110 Telnet - TCP Port 23 Dynamic Host Configuration Protocol (DHCP) - UDP Ports 67 and 68 File Transfer Protocol (FTP) - TCP Ports 20 and 21

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Domain Name SystemThe Domain Name System (DNS) uses a hierarchical system to create a name database to provide name resolution. The hierarchy looks like an inverted tree with the root at the top and branches below.

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Domain Name SystemA DNS server provides name resolution using the name daemon, named, (pronounced name-dee), it stores different types of resource records which contain the name, address, and type of record: A - an end device address NS - an authoritative name server CNAME the Fully Qualified Domain Name for an alias; used when multiple services have the single network address but each service has its own entry in DNS MX - mail exchange record; maps a domain name to a list of mail exchange servers for that domain

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Hypertext Transfer Protocol (HTTP)When a web address (or URL) is typed into a web browser, it connects to the web service running on the server using the HTTP protocol. URLs (Uniform Resource Locator) and URIs (Uniform Resource Identifier) are the names most people associate with web addresses that refers to a specific resource - a web page named index.html on a server.

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Hypertext Transfer Protocol (HTTP)HTTP specifies a request/response protocol. Three common message types in this protocol are GET, POST, and PUT. GET is a client request for data. POST includes the data (unencrypted) in the message sent to the server. PUT uploads resources or content to the web server. HTTP Secure (HTTPS) is used for secure accessing or posting of web server information, it uses authentication and encryption to secure data as it travels between the client and server.

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EmailE-mail requires several applications and services, two application layer protocols are Post Office Protocol (POP) and Simple Mail Transfer Protocol (SMTP). An e-mail client or Mail User Agent (MUA) is used to compose e-mail messages, it allows messages to be sent and places received messages into the client's mailbox. Sending e-mail from either a client or a server uses message formats and command strings defined by the SMTP protocol. The e-mail client uses POP to receive e-mail messages from an e-mail server. Usually an e-mail client provides the functionality of both protocols within one application.

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EmailPOP and POP3 (Post Office Protocol, version 3) are inbound mail delivery client/server protocols, which deliver e-mail from the e-mail server to the client (MUA). Once the client connects to the server the MDA delivers e-mail to the client. The Simple Mail Transfer Protocol (SMTP) handles outbound e-mail from the sending client to the e-mail server (MDA), and the transport between e-mail servers (MTA) making e-mail exchange over the Internet possible.

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FTP, DHCP, and TelnetFile Transfer Protocol (FTP) allows for file transfers between a client and a server, an FTP client application runs on a computer and is used to push and pull files from a server running the FTP daemon (FTPd). The Dynamic Host Configuration Protocol (DHCP) service enables devices on a network to obtain IP addresses, subnet masks, gateway and other IP networking parameters from a DHCP server.

A connection using Telnet is called a Virtual Terminal (VTY) sessionServer Network Connection Physical Connection Network Connection

Rather than using a physical device to connect to the server , Telnet uses software to create a virtual device to provide a terminal session with access to the server command line interface (CLI).

Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices.

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Server Message BlockThe Server Message Block (SMB) is a client/server file sharing protocol. Once aconnection is established, the user of the client can access the resources on the server as if the resource is local to the client host. SMB file-sharing and print services have become the mainstay of Microsoft networking, which uses DNS naming, allowing TCP/IP protocols to directly support SMB resource sharing

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