INTERNET PROTOCOLS & APPLICATIONS

* TCP/IP: The TCP/IP suite of protocols has become the dominant standard for internetworking. TCP/IP represents a set of public standards that specify how packets of information are exchanged between computers over one or more networks. * IPX/SPX: Internetwork Packet Exchange/Sequenced Packet Exchange is the protocol suite originally employed by Novell Corporation’s network operating system, NetWare. It delivers functions similar to those included in TCP/IP. Novell in its current releases supports the TCP/IP suite. A large installed base of NetWare networks continue to use IPX/SPX. * NetBEUI: NetBIOS Extended User Interface is a protocol used primarily on small Windows NT networks. NetBEUI cannot be routed or used by routers to talk to each other on a large network. NetBEUI is suitable for small peer-to-peer networks, involving a few computers directly connected to each other. It can be used in conjunction with another routable protocol such as TCP/IP. This gives the network administrator the advantages of the high performance of NetBEUI within the local network and the ability to communicate beyond the LAN over TCP/IP. * AppleTalk: AppleTalk is a protocol suite to network Macintosh computers. It is composed of a comprehensive set of protocols that span the seven layers of the Open Systems Interconnection (OSI) reference model. The AppleTalk protocol was designed to run over LocalTalk, which is the Apple LAN physical topology. This protocol is also designed to run over major LAN types, notably Ethernet and Token Ring. * HTTP: Hypertext Transfer Protocol governs how files such as text, graphics, sound, and video are exchanged on the World Wide Web (WWW). The Internet Engineering Task Force (IETF) developed the standards for HTTP. * FTP: File Transfer Protocol provides services for file transfer and manipulation. FTP allows multiple simultaneous connections to remote file systems. * SSH: Secure Shell is used to securely connect to a remote computer. * Telnet: An application used to connect to a remote computer that lacks security features. * POP3: Post Office Protocol is used to download e-mail from a remote mail server. * IMAP: Internet Message Access Protocol is also used to download e-mail from a remote mail server. * SMTP: Simple Mail Transfer Protocol is used to send e-mail to a remote e-mail server. * Internet Control Message Protocol (ICMP): Send control and error messages to computers and servers. ICMP has several different uses, such as announcing network errors, announcing network congestion, and troubleshooting.

TCP/IP Model
The TCP/IP reference model provides a common frame of reference for developing the protocols used on the Internet. It consists of layers that perform functions necessary to prepare data for transmission over a network. Table 8-6 describes the four layers of the TCP/IP model.

A message begins at the top layer, the application layer, and moves down the TCP/IP layers to the bottom layer, the network access layer. Header information is added to the message as it moves down through each layer and then is transmitted. After reaching the destination, the message travels back up through each layer of the TCP/IP model. The header information that was added to the message is stripped away as the message moves up through the layers toward its destination.

TCP/IP Model Layer Description
Application 4 Where high-level protocols such as SMTP and FTP operate
Transport 3 Where flow-control and connection protocols exist
Internet 2 Where IP addressing and routing take place
Network access 1 Where MAC addressing and physical network components exist

Application Protocols
Application layer protocols provide network services to user applications such as web browsers and e-mail programs. Here are some of the application protocols that operate at the TCP/IP application layer:

* Hypertext Transfer Protocol (HTTP) governs how files such as text, graphics, sound, and video are exchanged on the Internet or World Wide Web (WWW). HTTP is an application layer protocol. A web server runs an HTTP service or daemon. A daemon is a program that services HTTP requests. These requests are transmitted by HTTP client software, which is another name for a web browser. * Telnet is an application that you can use to access, control, and troubleshoot remote computers and network devices. * File Transfer Protocol (FTP) is a set of rules governing how files are transferred. FTP allows multiple simultaneous connections to remote file systems. * Simple Mail Transport Protocol (SMTP) provides messaging services over TCP/IP and supports most Internet e-mail programs. * Domain Name System (DNS) translates domain names, such as www.cisco.com, to IP addresses. * Hypertext Markup Language (HTML) is a page description language. Web designers use HTML to indicate to web browser software how the page should look. HTML includes tags to indicate boldface and italic type, line breaks, paragraph breaks, hyperlinks, and insertion of tables, among other instructions.

Transport Protocols

Transport layer protocols provide end-to-end management of the data. One of the functions of these protocols is to divide the data into manageable segments for easier transport across the network. Here are the two transport protocols that operate at the TCP/IP transport layer:

■ Transmission Control Protocol (TCP) is the primary Internet protocol for the reliable delivery of data. TCP includes facilities for end-to-end connection establishment, error detection and recovery, and metering the rate of data flow into the network. Many standard applications, such as e-mail, web browser, file transfer, and Telnet, depend on the services of TCP.
■ User Datagram Protocol (UDP) offers a connectionless service for delivery of data. UDP uses lower overhead than TCP and doesn’t handle issues of reliability. Network management applications, network file system, and simple file transport use UDP.

Internet Protocols

Internet layer protocols operate at the third layer from the top in the TCP/IP model. Internet protocols are used to provide connectivity between hosts in the network. Here are some of the protocols that operate at the TCP/IP Internet layer:

■ Internet Protocol (IP) provides source and destination addressing, much like the address and return address on a postal envelope. In conjunction with routing protocols, IP provides packet-forwarding information from one network to another.
■ Internet Control Message Protocol (ICMP) is used for network testing and troubleshooting. It enables diagnostic and error messages. The ping application uses ICMP echo messages to test if a remote device can be reached.
■ Routing information Protocol (RIP) operates between router devices to discover paths between networks. In an intranet, routers depend on a routing protocol to build and maintain information about how to forward packets toward the destination. RIP chooses routes based on the distance or hop count to the destination.
■ Address Resolution Protocol (ARP) is used to map the MAC address of a node on the network when its IP address is known. End stations as well as routers use ARP to discover MAC addresses.

Network Access Protocols

Network access protocols describe the standards that hosts use to access the physical media. The IEEE 802.3 Ethernet standards and technologies, such as CSMA/CD and 10BASE-T, are defined at this layer.

The OSI Model
The OSI model is an industry-standard framework that is used to divide network communications into seven distinct layers. Although other models exist, most network vendors today build their products using this framework.

A system that implements protocol behavior consisting of a series of these layers is known as a protocol stack. Protocol stacks can be implemented in either hardware or software, or a combination of both. Typically, only the lower layers are implemented in hardware, and the higher layers are implemented in software.

Each layer is responsible for part of the processing to prepare data for transmission on the network.

OSI Model | Layer | Description | Application | 7 | Responsible for network services to applications | Presentation | 6 | Transforms data formats to provide a standard interface for the application layer | Session | 5 | Establishes, manages, and terminates the connections between the local and remote applications | Transport | 4 | Provides reliable transport and flow control across a network | Network | 3 | Responsible for logical addressing and the domain of routing | Data link | 2 | Provides physical addressing and media access procedures | Physical | 1 | Defines all the electrical and physical specifications for devices |

In the OSI model, when data is transferred, it is said to virtually travel down the OSI model layers of the sending computer and up the OSI model layers of the receiving computer.

When a user wants to send data, such as an e-mail, the encapsulation process starts at the application layer. The application layer provides network access to applications. Information flows through the top three layers and is considered to be data when it gets down to the transport layer.

At the transport layer, the data is broken into more manageable segments, or transport layer protocol data units (PDU), for orderly transport across the network. A PDU describes data as it moves from one layer of the OSI model to another. The transport layer PDU also contains information such as port numbers, sequence numbers, and acknowledgment numbers, which is used for reliable data transport.

At the network layer, each segment from the transport layer becomes a packet. The packet contains logical addressing and other Layer 3 control information.

At the data link layer, each packet from the network layer becomes a frame. The frame contains physical address and error-correction information.

At the physical layer, the frame becomes bits. These bits are transmitted one at a time across the network medium.

At the receiving computer, the de-encapsulation process reverses the process of encapsulation. The bits arrive at the physical layer of the OSI model of the receiving computer. The process of virtually traveling up the OSI model of the receiving computer brings the data to the application layer, where an e-mail program displays the e-mail.

Compare the OSI and TCP/IP Models
The OSI model and the TCP/IP model are both reference models used to describe the data communication process. The TCP/IP model is used specifically for the TCP/IP suite of protocols. The OSI model is used to develop standard communication for equipment and applications from different vendors.

The TCP/IP model performs the same process as the OSI model, but it uses four layers instead of seven.