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Internet Protocol (IP): IP is responsible for several tasks, most importantly determining a route to the description. In addition, IP is responsible for the packing of messages into small network-transportable packets, called datagrams. IP is used with almost all TCP protocols, sitting at the bottom of the TCP protocol stack just above the network-layers. IP has no control over whether messages sent and received are intact. All IP does is handle the sending and receiving, leaving it up to the next higher layer, usually TCP or UDP, to take care of any problems that occur with lost or damaged data. Internet Control Message Protocol(ICMP): ICMP is a special form of IP used to handle error and status messages between IP layers on different machines. Whenever one IP layer has to send information to another, it uses ICMP. Also, whenever IP software detects an error of some sort, it uses ICMP to send reports to the other machine. Probably the most common use of ICMP is for the ping command, which checks whether a machine is responsive by sending a small ICMP message to the machine and waiting for a reply. Transmission Control Protocol (TCP): TCP is used primarily to verify that whatever was sent by the sending machine is received intact by the destination. TCP is called a reliable delivery protocol, meaning that it makes sure everything sent was received properly. TCP adds a header to the front of each message that contains checksums, numbering, and other reliability information to ensure that every packet sent is received without modification. If there is a transmission problem, TCP takes care of resending the information. TCP sits between the application and the IP layer on each machine, acting as a packaging layer for application data and a delivery mechanism of sending packets to an application. TCP usually runs with IP, but it can work with other protocols. TCP is a connection-based protocol, meaning that the sending and the destination machines communicate with each other by sending status messages back and forth. If the connection is lost because of routing problems or machine failures, errors are sent to the applications that use TCP. Some service use TCP to maintain a connection between two machines, notably FTP or Telnet, both of which enable you to move files and commands back and forth between two machines as if you were logged into both at the same time. User Datagram Protocol (UDP): UDP is an alternative to TCP. It is a connection-less protocol, meaning that the sending and receiving machine are not constantly connected to each other. They can send status messages back and forth to indicate reception of packets, but there is no constant connection maintained. UDP is used by services that do not require a connection, such as the TFTP, DNS, NFS, and RPC. Because of the lack of a connection, UDP is often thought of as a less reliable delivery protocol than TCP, although other protocols can pick up the tasks that TCP offers. UDP sits in the layer between the applications and IP. UDP usually uses IP to handle its packets. Telnet: The Telnet service provides a remote login capability. This lets a user on one machine log into another machine and act as if they are directly in front of the second machine. The connection can be anywhere on the local network, or on another network anywhere in the world, as long as the user has permission to log into the remote system. Telnet uses TCP to maintain a connection between two machines. File Transfer Protocol (FTP): FTP enables a file on one system to be copied to another system. Users don't actually log in as full users to the machine they want to access but instead use the FTP service to provide access. The remote machine must be set up with the permissions necessary to provide the user access to the files. FTP uses TCP to create and maintain a connection between source and destination machines. Once the connection to a remote machine has been established, FTP enables you to copy one or more files to your machine. The term transfer implies that the file is moved from one system to another, but the original is not affected, files are copied from one system to another. Simple Mail Transfer Protocol (SMTP): SMTP is one protocol used for transferring electronic mail. Transparent to the user. SMTP connects to different machines and transfers mail messages, much like FTP transfers files. Domain Name System(DNS): DNS enables a device with a common name to be converted to a special network address. DNS provides the conversion from a common local name to the unique physical address of the device's network connection. Simple Network Management Protocol (SNMP): SNMP is a network management protocol. SNMP uses UDP as a transport mechanism. SNMP relies on several terms from TCP/IP standard specifications, working with managers and agents instead of clients and servers. An agent provides information about a device, whereas a manager communicates across the network. Network File Server (NFS):NFS is used to transparently enable multiple machines to access each other's directories. NFS accomplishes this by using a distributed filesystem scheme. NFS systems are common in large corporate environments. Remote Procedure Calls(RPC): RPC are programming functions that enable an application to communicate with another machine, the server. They provide the programming functions, return codes, and predefined variables to support distributed computing. Trivial File Transfer Protocol (TFTP): TFTP is a very simple, unsophisticated file transfer protocol that lacks ant security. It uses UDP as a transport. Although not as sophisticated or as fast as FTP, TFTP can be used on many systems that do not enable FTP access. In some ways, TFTP can be analogous to an e-mail message requesting and receiving a file instead of a text body. BOOT Protocol (BOOTP): The BOOT Protocol, called BOOTP, is used to start up machines on a network that do not have their own hard drives or storage devices containing operating systems and network information. BOOTP is used for X-terminals and other diskless workstations. Address Resolution Protocol(ARP): ARP is one of several protocols that helps determine addresses on a network. ARP works with IP to set routes to a destination. ARP converts an IP address to a network interface hardware address. Reverse address Resolution Protocol(RARP): RARP as its name suggest, is the reverse process of ARP. RARP uses a network interface hardware address and from that produces the IP address, whereas ARP produces the IP address from the hardware address. Network Time Protocol(NTP): NTP is used to synchronise clocks across a network. This is important because many packets have a prespectified amount of time to reach their routes. If a clock on one machine is inaccurate, the timers in the packet might expire prematurely. Time is also used to build efficient routing tables that let IP determine the fastest route to a destination.

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