APPLICATION LAYER

APPLICATION LAYER

1. 15.1 APPLICATION, PRESENTATION, AND SESSION

   • Application Layer
     • The application layer provides the interface between the applications used to communicate, and the underlying network over which messages are transmitted.
   • The presentation layer
     • • Formatting, or presenting, data at the source device into a compatible format for receipt by the destination device.
       • • Compressing data in a way that can be decompressed by the destination device.
         • • Encrypting data for transmission and decrypting data upon receipt.
   • The session layer
     • • It creates and maintains 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.
   • TCP/IP Application Layer Protocols
     • The TCP/IP application protocols specify the format and control information necessary for many common internet communication functions.

2. 15.2 PEER-TO-PEER

   • 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.
   • Peer-to-Peer Networks
     • In a peer-to-peer (P2P) network, two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server.
       • Every connected end device (known as a peer) can function as both a server and a client.
   • Common P2P Applications
     • With P2P applications, each computer in the network that is running the application can act as a client or a server for the other computers in the network that are also running the application.
       • Common P2P networks include the following: • BitTorrent • Direct Connect • eDonkey • Freenet

3. 15.5 FILE SHARING SERVICES

   • File Transfer Protocol
     • FTP was developed to allow for data transfers between a client and a server. An FTP client is an application which runs on a computer that is being used to push and pull data from an FTP server.
   • Server Message Block
     • The Server Message Block (SMB) is a client/server, request- response file sharing protocol. Servers can make their own resources available to clients on the network.

4. 15.4 IP ADDRESSING SERVICES

   • Domain Name Service
     • Domain names were created to convert the numeric IP addresses into a simple, recognizable name.
       • DNS Message Format
         • The DNS server stores different types of resource records that are used to resolve names. These records contain the name, address, and type of record.
           • DNS Hierarchy
             • • DNS uses a hierarchical system to create a database to provide name resolution.
             • The nslookup Command
               • Nslookup is a computer operating system utility that allows a user to manually query the DNS servers configured on the device to resolve a given host name.
         • Dynamic Host Configuration Protocol
           • The Dynamic Host Configuration Protocol (DHCP) for IPv4 service automates the assignment of IPv4 addresses, subnet masks, gateways, and other IPv4 networking parameters.
             • DHCP Operation

5. 15.3 WEB AND EMAIL PROTOCOLS

   • Hypertext Transfer Protocol and Hypertext Markup Language
     • Web and Email Protocols 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.
       • HTTP and HTTPS
         • HTTP is a request/response protocol that specifies the message types used for that communication. The three common message types are GET, POST, and PUT
         • Email Protocols
           • Email is a store-and-forward method of sending, storing, and retrieving electronic messages across a network. The email protocols used for operation are: • Simple Mail Transfer Protocol (SMTP) – used to send mail. • Post Office Protocol (POP) & IMAP – used for clients to receive mail.
             • SMTP
               • SMTP message formats require a message header and a message body. Although the message body can contain any amount of text, the message header must have a properly formatted recipient email address and a sender address.
             • POP
               • 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 the default operation of POP.
             • IMAP
               • 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

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