COMPUTER NETWORK 7BITE2A UNIT V APPLICATION LAYER NETWORK SECURITY CRYPTOGRAPHY SECRET AND PUBLIC KEY ALGORITHMS DNS SNMP ELECTRONIC MAIL ELECTRONIC MAIL PRIVACY WORLD WIDE WEB CLIENT SIDE SERVER SIDE MULTIMEDIA AUDIO VIDEO DATA COMPRESSION JPEG MPEG STANDARDS APPLICATION LAYER The application layer in the OSI model is the closest layer to the end user which means that the application layer and end user can interact directly with the software application The application layer programs are based on client and servers The Application layer includes the following functions Identifying communication partners The application layer identifies the availability of communication partners for an application with data to transmit Determining resource availability The application layer determines whether sufficient network resources are available for the requested communication Synchronizing communication All the communications occur between the applications requires cooperation which is managed by an application layer Services of Application Layers Network Virtual terminal An application layer allows a user to log on to a remote host To do so the application creates a software emulation of a terminal at the remote host The user s computer talks to the software terminal which in turn talks to the host The remote host thinks that it is communicating with one of its own terminals so it allows the user to File Transfer Access and Management FTAM An application allows a user to access files in a remote computer to retrieve files from a computer and to manage files in a remote computer FTAM defines a hierarchical virtual file in terms of file structure file attributes and the kind of operations performed on the files and their attributes Addressing To obtain communication between client and server there is a need for addressing When a client made a request to the server the request contains the server address and its own address The server response to the client request the request contains the destination address i e client log on address To achieve this kind of addressing DNS is used Mail Services An application layer provides Email forwarding and storage Directory Services An application contains a distributed database that provides access for global information about various objects and services Authentication It authenticates the sender or receiver s message or both Manipulation of data information in various ways is done in this layer which enables user or software It is the top most layer of OSI Model to get access to the network Some services provided by this layer includes E Mail transferring files distributing the results to user directory services network resources etc S Priyadharshini MCA M Phil 1 COMPUTER NETWORK 7BITE2A The Application Layer contains a variety of protocols that are commonly needed by users One widely used application protocol is HTTP HyperText Transfer Protocol which is the basis for the World Wide Web When a browser wants a web page it sends the name of the page it wants to the server using HTTP The server then sends the page back Other Application protocols that are used are File Transfer Protocol FTP Trivial File Transfer Protocol TFTP Simple Mail Transfer Protocol SMTP TELNET Domain Name System DNS etc Functions of Application Layer Mail Services This layer provides the basis for E mail forwarding and storage Network Virtual Terminal It allows a user to log on to a remote host The application creates software emulation of a terminal at the remote host User s computer talks to the software terminal which in turn talks to the host and vice versa Then the remote host believes it is communicating with one of its own terminals and allows user to log Directory Services This layer provides access for global information about various services File Transfer Access and Management FTAM It is a standard mechanism to access files and on manages it Users can access files in a remote computer and manage it They can also retrieve files from a remote computer Design Issues with Application Layer There are commonly reoccurring problems that occur in the design and implementation of Application Layer protocols and can be addressed by patterns from several different pattern languages o Pattern Language for Application level Communication Protocols o Service Design Patterns o Patterns of Enterprise Application Architecture o Pattern Oriented Software Architecture NETWORK SECURITY Various business services are now offered online though client server applications The most popular forms are web application and e mail While using a service from any server application the client and server exchange a lot of information In both applications the client communicates to the designated server and obtains services on the underlying intranet or Internet We are aware of fact that these information transactions are vulnerable to various attacks Network security entails securing data against attacks while it is in transit on a network S Priyadharshini MCA M Phil 2 COMPUTER NETWORK 7BITE2A To achieve this goal many real time security protocols have been designed Such protocol needs to provide at least the following primary objectives The parties can negotiate interactively to authenticate each other Establish a secret session key before exchanging information on network Exchange the information in encrypted form For example S MIME protocol works at Application layer SSL protocol is developed to work at transport layer and IPsec protocol works at Network layer E mail Security Interestingly these protocols work at different layers of networking model Nowadays e mail has become very widely used network application E mail Infrastructure The simplest way of sending an e mail would be sending a message directly from the sender s machine to the recipient s machine In this case it is essential for both the machines to be running on the network simultaneously However this setup is impractical as users may occasionally connect their machines to the network Hence the concept of setting up e mail servers arrived In this setup the mail is sent to a mail server which is permanently available on the network When the recipient s machine connects to the network it reads the mail from the mail server In general the e mail infrastructure consists of a mesh of mail servers also termed as Message Transfer Agents MTAs and client machines running an e mail program comprising of User Agent UA and local MTA Typically an