|  | 8.6 | TCP/IP Fundamentals
| 8.6.3 | TCP, UDP, POP3, SMTP, SNMP, FTP, HTTP, and IP
TCP Transmission Control Protocol (TCP) is a connection-oriented protocol, which operates on the Transport Layer of the TCP/IP and OSI Models. TCP is the de facto standard of the Internet and provides full-duplex data transmission. When TCP receives data from the upper layers of the OSI Model, it guarantees reliable delivery to remote networks. TCP is useful for transmitting large amounts of data reliably, but with the penalty of large ACK overhead consuming bandwidth. UDP User Datagram Protocol (UDP) is a connectionless protocol, which operates on the Transport Layer of the TCP/IP and OSI Models. Because UDP is an unreliable delivery service, it does not require receiving protocols to acknowledge the receipt of a packet. An advantage UDP has over TCP is because it does not concentrate on establishing a connection, it can transmit more information in a less amount of time than TCP. Useful for transmitting small amounts of data where reliability is less crucial, UDP lacks the overhead caused by ACKs. POP3 Post Office Protocol Version 3 (POP3), which uses TCP port 110, is a mail protocol that is responsible for holding email until delivery. When an SMTP server sends an email message to a POP3 server, POP3 holds onto to the message until a request is made by the user to have the data delivered. Thus POP3 transfers mail files from a mail server to a mail client. SMTP The Simple Mail Transfer Protocol (SMTP), which uses TCP port 25, allows users to be able to send and receive email over the Internet. It is the SMTP's responsibility to make sure that the email is sent to the POP3 server. SNMP Simple Network Management Protocol (SNMP), which uses TCP port 161, allows simple maintenance and remote monitoring of any device on a network. Administrators are able address issues, such as problems with a network card in a server, a program or service on the server, or a device such as a hub or a router by using SNMP. Two approaches an administrator can take when managing a network device using SNMP are a central management system and the management information base (MIB). The management system allows the administrator to view performance and operation statistics of the network devices, enabling him or her to diagnose a network remotely. FTP File Transfer Protocol (FTP) is a fast, connection-oriented, error-free protocol and uses TCP ports 20 and 21. FTP allows data to be transferred between servers and clients. In order for FTP to connect to a remote server, the IP address or host name must be provided. FTP must be able to resolve IP addresses to host names in order to establish a connection. HTTP Hypertext Transfer Protocol (HTTP), which uses TCP port 80, allows clients to transfer documents written in Hypertext Markup Language (HTML) over the World Wide Web for display by a browser. It is the universal display language of the Internet. IP Internet Protocol (IP) is a routable (routed) connectionless protocol, which operates on the Network Layer of the OSI and TCP/IP Models. IP is the de facto standard for the Internet and provides packet delivery and addressing for source and destination. Because IP is a connectionless delivery service, it is unreliable and does not guarantee that the packets received will be in the order sent, if received at all.