Layer 7- Application Layer Application Layer Protocols

Application Layer Protocols & Functionality Learning Objectives Describe how the functions of the three upper OSI model layers provide network services to end user applications. Describe how the TCP/IP Application Layer protocols provide the services specified by the upper layers of the OSI model. Define how people use the Application Layer to communicate across the information network. Describe the function of well-known TCP/IP applications, such as the World Wide Web and email, and their related services (HTTP, DNS, SMB, DHCP, SMTP/POP, and Telnet).

Applications – The Interface Between Human and Data Networks The Application layer is the top layer of both the OSI and TCP/IP models Application layer protocols are used to exchange data between programs running on the source and destination hosts. There are many Application layer protocols and new protocols are always being developed.

Application OSI TCP/IP Application: Layer 7 Interface to operating System Application Presentation: Layer 6 Coding, encryption, compression Although the TCP/IP protocol suite was developed prior to the definition of the OSI model, the functionality of the TCP/IP application layer protocols fit roughly into the framework of the top three layers of the OSI model: Application, Presentation and Session layers. Most TCP/IP application layer protocols were developed before the emergence of personal computers, graphical user interfaces and multimedia objects. As a result, these protocols implement very little of the functionality that is specified in the OSI model Presentation and Session layers. Session: Layer 5 Create and maintain dialogues between applications

Well Known Application Protocols File Transfer: File Transfer Protocol (FTP) Trivial File Transfer Protocol (TFTP) Transfer to Application Virtual Terminal Session, Compression & Encryption Establish, send, close session, Authenticators, Master / Slave Email: Simple Mail Transfer Protocol (SMTP) Post Office Protocol 3 (POP3) TRANSPORT Web Browsing: Hyper Text Transfer Protocol (HTTP) INTERNET The most widely-known TCP/IP Application layer protocols are those that provide for the exchange of user information. These protocols specify the format and control information necessary for many of the common Internet communication functions. Among these TCP/IP protocols are: File Transfer protocols – allow the sharing of files between PCs using different operating systems. Email – SMTP transmits mail, POP3 receives it. Web browsing – HTTP transfers web pages written in HTML. Network Management - remote monitoring of network devices. DNS – resolution of URLs to IP addresses. The protocols in the TCP/IP suite are generally defined by Requests for Comments (RFCs). The Internet Engineering Task Force maintains the RFCs as the standards for the TCP/IP suite. Network Management: Simple Network Management Protocol (SNMP) LOCAL NETWORK N I C Name Resolution: Domain Name Service (DNS)

Client / Server Processes Application layer protocols describe the format of the requests and responses between clients and servers Host A Host B Host C 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 control information, such as user authentication and the identification of a data file to be transferred. FTP Server Download Data flowing from a server to a client is known as download.

Client / Server Processes In a client/server network, the server runs a service, or process, sometimes called a server daemon , typically running in the background. Daemons are described as "listening" for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon. When a daemon "hears" a request from a client, it exchanges appropriate messages with the client, as required by its protocol, and proceeds to send the requested data to the client in the proper format. In a general networking context, any device that responds to requests from client applications is functioning as a server. A server is usually a computer that contains information to be shared with many client systems. For example, web pages, documents, databases, pictures, video, and audio files can all be stored on a server and delivered to requesting clients. In other cases, such as a network printer, the print server delivers the client print requests to the specified printer. Additionally, servers typically have multiple clients requesting information at the same time. For example, a WWW server may have many clients requesting connections to it. These individual client requests must be handled simultaneously and separately for the network to succeed. The Application layer processes and services rely on support from lower layer functions to successfully manage the multiple conversations.

Peer-to-Peer Networks In a peer-to-peer network, two or more computers are connected via a network and can share resources (i.e. printers and files) without having a dedicated server. Every connected end device (known as a peer) can function as either a server or a client. One computer might assume the role of server for one transaction while simultaneously serving as a client for another. The roles of client and server are set on a per request basis. Unlike the client/server model, which uses dedicated servers, peer-to-peer networks decentralize the resources on a network. Instead of locating information to be shared on dedicated servers, information can be located anywhere on any connected device. Most of the current operating systems support file and print sharing without requiring additional server software. Because peer-to-peer networks usually do not use centralized user accounts, permissions, or monitors, it is difficult to enforce security and access policies in networks containing more than just a few computers. User accounts and access rights must be set individually on each peer device.

Application Layer Services

WWW and HTTP The World Wide Web (WWW) is a repository of information stored on web pages, linked together from points all over the world. Web pages are written in a language called Hypertext Mark-Up Language (HTML), and stored on web servers. To retrieve an HTML web page, a client/server protocol called Hyper Text Transfer Protocol (HTTP) is used. HTML web pages are displayed on a users PC by web browser software clients.

Web Browsing Processes Web browser uses HTTP to request a particular web-page from a web server. Web server responds, using HTTP to send the HTML web page page to the web browser. The web browser formats the web page for display on the client PC. Client Web Server HTML WWW Page Web Browser HTTP Request HTML WWW Page

Hyper Text Mark-up Language (HTML) Language used for creating web pages. Mark-up language formats a web page independently from the process that created it – provides a standard way for web browsers to interpret web pages. Uses only ASCII characters for both the main web page text and the formatting instructions. ‘Mark-up’ is a term borrowed from the publishing industry, where an editor marks a new manuscript indicating the formatting required by the printers. HTML uses ASII to represent all the data and mark-ups presented on a web page- hence why HTTP is designed to transfer ASCII characters.

Hyper Text Transfer Protocol (HTTP) HTTP is used mainly to access data on the WWW. Functions like a combination of FTP and SMTP. Used to transfer files using TCP and well-known port 80 – there is no control connection required. Web browser utilises an HTTP client, while a web server runs an HTTP server. The HTML data transferred by HTTP is not readable by the user – it has to be interpreted by a web-browser. The Hypertext Transfer Protocol (HTTP), one of the protocols in the TCP/IP suite, was originally developed to publish and retrieve HTML pages and is now used for distributed, collaborative information systems. HTTP is used across the WWW for data transfer and is one of the most used application protocols. HTTP specifies a request/response protocol. When a client, typically a web browser, sends a request message to a server, the HTTP protocol defines the message types the client uses to request the web page and also the message types the server uses to respond. The three common message types are GET, POST, and PUT.

Client – Web Browser WWW Server Data (80) Web Page Transfer Phases: HTML Web Page Web Page Transfer Phases: Connection Establishment – Client makes connection to TCP port 80 on the web server. Server commences the connection phase. HTTP Transfer – Server transfers HTML web page using HTTP Connection Termination – After web page is transferred successfully, the client terminates the connection. HTTP Client HTTP Server Request Response Request Response Data (80) TRANSPORT HTTP specifies a request/response protocol. When a client, typically a web browser, sends a request message to a server, the HTTP protocol defines the message types the client uses to request the web page and also the message types the server uses to respond. The three common message types are GET, POST, and PUT. HTTP is not a secure protocol. The POST messages upload information to the server in plain text that can be intercepted and read. Similarly, the server responses, typically HTML pages, are also unencrypted. For secure communication across the Internet, the Secure HTTP (HTTPS) protocol is used for accessing or posting web server information. HTTPS can use authentication and encryption to secure data as it travels between the client and server. HTTPS specifies additional rules for passing data between the Application layer and the Transport Layer. INTERNET LOCAL NETWORK

Uniform Resource Locater (URL) The navigation of web pages is achieved using locators called Uniform Resource Locators (URLs). These allow a user to access sites without using IP addresses. The URL is a standard for specifying any kind of information on the Internet, and defines four things: protocol, host computer, port and path.

:// : / Uniform Resource Locater (URL) URLs are presented in a standard format: Method :// Host : Port / Path Method – the protocol used to retrieve data (usually HTTP) Host – alias of the web server where the data is located. Normally prefixed with ‘WWW’ signifying a web server. Port – Transport layer port that the web server is using – not a mandatory field, as port 80 is used by default. Path – the location of the data on the web server – the ‘/’ indicates directories and subdirectories. On the Internet domain names, such as www.cisco.com , are much easier for people to remember than 198.132.219.25, which is the actual numeric address for this server. Also, if Cisco decides to change the numeric address, it is transparent to the user, since the domain name will remain www.cisco.com . The new address will simply be linked to the existing domain name and connectivity is maintained. Accessing a web site, so HTTP is used. RFC 1738 lists all the methods that can be used within an URL (FTP, telnet, etc). The novatech server using the default web name ‘WWW’. The company has also registered the name novatech within the UK, and they are a commercial organisation. Overall control of domain names is maintained by the Internet Assigned Numbering Authority (IANA) The web page is located within the /novatech file on the web server. 16

Simple Name Resolution Host Table: Phill - 192.168.1.100 Lisa - 192.168.1.101 Bazil - 192.168.1.102 Host Table: Phill - 192.168.1.100 Lisa - 192.168.1.101 Bazil - 192.168.1.102 Host Table: Phill - 192.168.1.100 Lisa - 192.168.1.101 Bazil - 192.168.1.102 Phill 192.168.1.100 Lisa 192.168.1.101 Bazil 192.168.1.102 Name Resolution can be achieved using a host table, mapping all the host names in a network to their respective IP addresses All host tables need to be changed every time a new PC is added to the network- this can be time consuming on a large network IP addresses are not easy to remember, so having to remember where services are located on a network, based on IP addresses would be frustrating. Hostnames were devised early in the development of TCP/IP – a host is assigned an easily-remembered alphanumeric name. When a user requests a access to a network location based on a hostname, the host refers to a host-table, which contains mappings for all the hostname – IP address pairs. This has to be configured individually on each host in the network – time consuming in a large network. Switch 17

Name Resolution Using DNS DNS Server: 192.168.1.103 What is IP Address of Bazil? DNS Server: 192.168.1.103 DNS Server: 192.168.1.103 Phill 192.168.1.100 Lisa 192.168.1.101 Bazil 192.168.1.102 DNS Client DNS Client DNS Client A single DNS server holds the host table. Client DNS services request host/IP address mappings from the server. In data networks, devices are labeled with numeric IP addresses, so that they can participate in sending and receiving messages over the network. However, most people have a hard time remembering this numeric address. Hence, domain names were created to convert the numeric address into a simple, recognizable name. The DNS protocol defines an automated service that matches resource names with the required numeric network address. It includes the format for queries, responses, and data formats. DNS protocol communications use a single format called a message. This message format is used for all types of client queries and server responses, error messages, and the transfer of resource record information between servers. DNS servers provide name resolution services for the network. If a host encounters an unknown hostname, it sends a request to the DNS server, asking for the IP address required. If the DNS server has the address, it sends it back to the requesting computer. Much easier to maintain a single DNS server host file. DNS Server 192.168.1.103 Host Table: Phill - 192.168.1.100 Lisa - 192.168.1.101 Bazil - 192.168.1.102 Bazil = 192.168.1.103 Switch 18

Conclusions Today we looked at how the functions of the three upper OSI model layers provide network services to end user applications Protocol used by WWW