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A protocol is a set of standards for communication between network hosts. Protocols often provide services, such as e-mail or file transfer. Most protocols are not intended to be used alone, but instead rely on and interact with other dependent or complimentary protocols. A group of protocols that is intended to be used together is called a protocol suite.

The Internet protocol suite (normally referred to as TCP/IP) is the most widely used protocol suite today. The following table lists several protocols in the TCP/IP protocol suite.

Category Protocol Description
Web browsing HyperText Transfer Protocol (HTTP) HTTP is used by Web browsers and Web servers to exchange files (such as Web pages) through the World Wide Web and intranets. HTTP can be described as an information requesting and responding protocol. It is typically used to request and send Web documents, but is also used as the protocol for communication between agents using different TCP/IP protocols.
HTTP over SSL (HTTPS) HTTPS is a secure form of HTTP that uses SSL to encrypt data before it is transmitted.
Security protocols Secure Sockets Layer (SSL) SSL secures messages being transmitted on the Internet. It uses RSA for authentication and encryption. Web browsers use SSL (Secure Sockets Layer) to ensure safe Web transactions. URLs that begin withhttps:// trigger your Web browser to use SSL.
Transport Layer Security (TLS) TLS ensures that messages being transmitted on the Internet are private and tamper proof. TLS is implemented through two protocols:
  • TLS Record--Can provide connection security with encryption (with DES for example).
  • TLS Handshake--Provides mutual authentication and choice of encryption method.
TLS and SSL are similar but not interoperable.
File transfer File Transfer Protocol (FTP) FTP provides a generic method of transferring files. It can include file security through usernames and passwords, and it allows file transfer between dissimilar computer systems. FTP can transfer both binary and text files, including HTML, to another host. FTP URLs are preceded byftp:// followed by the DNS name of the FTP server. To log in to an FTP server, use:ftp://username@servername.
Trivial File Transfer Protocol (TFTP) TFTP is similar to FTP. It lets you transfer files between a host and an FTP server. However, it provides no user authentication and no error detection. TFTP is often used when transferring files such as video, audio, or images. Because it does not perform error detection, TFTP is faster than FTP, but might be subject to file errors.
Secure File Transfer Protocol (SFTP)

SFTP is a file transfer protocol that uses Secure Shell (SSH) to secure data transfers. SSH ensures that SFTP transmissions use encrypted commands and data which prevent data from being transmitted over the network in clear text.

Secure Copy (SCP) SCP is associated with Unix/Linux networks and used to transfer files between systems. Like SFTP, SCP relies on SSH to ensure that data and passwords are not transmitted over the network in clear text.
E-mail Simple Mail Transfer Protocol (SMTP) SMTP is used to route electronic mail through the internetwork. SMTP is used:
  • Between mail servers for sending and relaying mail.
  • By all e-mail clients to send mail.
  • By some e-mail client programs, such as Microsoft Outlook, for receiving mail from an Exchange server.
Internet Message Access Protocol (IMAP) IMAP is an e-mail retrieval protocol designed to enable users to access their e-mail from various locations without the need to transfer messages or files back and forth between computers. Messages remain on the remote mail server and are not automatically downloaded to a client system. Note: An e-mail client that uses IMAP for receiving mail uses SMTP for sending mail.
Post Office Protocol 3 (POP3) POP3 is part of the TCP/IP protocol suite and used to retrieve e-mail from a remote server to a local client over a TCP/IP connection. With POP3, e-mail messages are downloaded to the client. Note: An e-mail client that uses POP3 for receiving mail uses SMTP for sending mail.
Network services Dynamic Host Configuration Protocol (DHCP) DHCP is a method for automatically assigning addresses and other configuration parameters to network hosts. Using a DHCP server, hosts receive configuration information at startup, reducing the amount of manual configuration required on each host.
Domain Name System (DNS) DNS is a system that is distributed throughout the internetwork to provide address/name resolution. For example, the name would be identified with a specific IP address.
Network Time Protocol (NTP) NTP is used to communicate time synchronization information between systems on a network.
Network News Transport Protocol (NNTP) NNTP is the most widely-used protocol that manages notes posted on Usenet Newsgroups.
Lightweight Directory Access Protocol (LDAP) LDAP is used to allow searching and updating of a directory service. The LDAP directory service follows a client/server model. One or more LDAP servers contain the directory data, the LDAP client connects to an LDAP Server to make a directory service request.
Network management Simple Network Management Protocol (SNMP) SNMP is a protocol designed for managing complex networks. SNMP lets network hosts exchange configuration and status information. This information can be gathered by management software and used to monitor and manage the network.
Remote Terminal Emulation (Telnet) Telnet allows an attached computer to act as a dumb terminal, with data processing taking place on the TCP/IP host computer. It is still widely used to provide connectivity between dissimilar systems. Telnet can also be used to test a service by the use of HTTP commands.
Secure Shell (SSH) SSH allows for secure interactive control of remote systems. SSH uses RSA public key cryptography for both connection and authentication. SSH uses the IDEA algorithm for encryption by default, but is able to use Blowfish and DES. SSH is a secure and acceptable alternative to Telnet.
Transport protocols Transmission Control Protocol (TCP) TCP provides services that ensure accurate and timely delivery of network communications between two hosts. TCP provides the following services to ensure message delivery:
  • Sequencing of data packets
  • Flow control
  • Error checking
User Datagram Protocol (UDP) UDP is a host-to-host protocol like TCP. However, it does not include mechanisms for ensuring timely and accurate delivery. Because it has less overhead, it offers fast communications, but at the expense of possible errors or data loss.
Control protocols Internet Control Message Protocol (ICMP) ICMP works closely with IP in providing error and control information, by allowing hosts to exchange packet status information, which helps move the packets through the internetwork. Two common management utilities, ping and traceroute, use ICMP messages to check network connectivity. ICMP also works with IP to send notices when destinations are unreachable, when devices' buffers overflow, the route and hops packets take through the network, and whether devices can communicate across the network.
Internet Group Membership Protocol (IGMP) IGMP is a protocol for defining host groups. All group members can receive broadcast messages intended for the group (called multicasts). Multicast groups can be composed of devices within the same network or across networks (connected with a router).

The TCP/IP protocol suite was developed to work independently of the physical network architecture. You can use a wide variety of architectures with the TCP/IP protocol suite.