The Application Protocols Networking

Application layer protocols are built on top of and into the TCP/IP protocol suite and are available on most implementations. The following list includes such protocols

  • SNMP
  • FTP
  • TFTP
  • SFTP
  • SMTP
  • POP3
  • IMAP
  • LPD
  • NFS
  • Telnet
  • SSH
  • HTTP
  • NTP
  • NNTP
  • SCP
  • LDAP
  • IGMP
  • LPR

Simple Network Management Protocol(SNMP)

Simple Network Management Protocol (SNMP) allows network administrators to collect information about the network. It is a communications protocol for collecting information about devices on the network, including hubs, routers, and bridges. Each piece of information to be collected about a device is defined in a Management Information Base (MIB). SNMP uses UDP to send and receive messages on the network.

File Transfer Protocol(FTP)

File Transfer Protocol (FTP) provides a mechanism for single or multiple file transfers between computer systems; when written in lowercase as “ftp,” it is also the name of the client software used to access the FTP server running on the remote host. The FTP package provides all the tools needed to look at files and directories, change to other directories, and transfer text and binary files from one system to another. FTP uses TCP to actually move the files.

Trivial File Transfer Protocol(TFTP)

Trivial File Transfer Protocol (TFTP) is a “stripped down” version of FTP, primarily used to boot diskless workstations and to transfer boot images to and from routers. It uses a reduced feature set (fewer commands and a smaller overall program size). In addition to its reduced size, it also uses UDP instead of TCP, which makes for faster transfers but with no reliability.

Secure File Transfer Protocol(SFTP)

Secure File Transfer Protocol (SFTP) is used when you need to transfer files over an encrypted connection. It uses an SSH session (more on this later) which encrypts the connection. The SFTP protocol then is used to transfer files over this encrypted connection. Apart from that, it functions exactly as the FTP protocol does: It is used to transfer files between computers.

Simple Mail Transfer Protocol(SMTP)

Simple Mail Transfer Protocol(SMTP) allows for a simple e-mail service and is responsible for moving messages from one e-mail server to another. The e-mail servers run either Post Office Protocol (POP) or Internet Mail Access Protocol (IMAP) to distribute e-mail messages to users.

Post Office Protocol (POP)

Post Office Protocol(POP) provides a storage mechanism for incoming mail; the latest version of the standard is known as POP3. When a client connects to a POP3 server, all the messages addressed to that client are downloaded; there is no way to download messages selectively. Once the messages are downloaded, the user can delete or modify messages without further interaction with the server. In some locations, POP3 is being replaced by another standard, IMAP.

Internet Message Access Protocol, Version 4 (IMAP4)

Internet Message Access Protocol (IMAP) allows users to download mail selectively, look at the message header, download just a part of a message, store messages on the e-mail server in a hierarchical structure, and link to documents and Usenet newsgroups. Search commands are also available so that users can locate messages based on their subject, header or content. IMAP has strong authentication features and supports the Kerberos authentication scheme originally developed at MIT. The current version of IMAP is version 4.

Line Printer Daemon (LPD)

Another TCP/IP upper-layer service that is in widespread use is the Line Printer Daemon (LPD). It resides on a network printer or print server and responds to TCP/IP printing requests from the printing clients(known as LPR clients). It was developed as the printing services for UNIX. But, because of the tight marriage between UNIX and TCP/IP, the LPD service became the default print service used with TCP/IP.

Network File System (NFS)

UNIX systems are unique in the way they access files and are actually fairly elegant. The Network File System (NFS) Application layer protocol was originally designed to allow shared file systems on UNIX servers to appear as local file systems on UNIX clients.


Telnet is a terminal emulation protocol that provides a remote logon to another host over the network. It allows a user to connect to a remote host over a TCP/IP connection as if they were sitting right at that host. Keystrokes typed into a Telnet program will be transmitted over a TCP/IP network to the host. The visual responses are sent back by the host to the Telnet client to be displayed.

Secure Shell (SSH)

The Secure Shell (SSH) protocol is used to establish a secure Telnet session over a standard TCP/IP connection. It is used to run programs on remote systems, log in to other systems, and move files from one system to another, all while maintaining a strong, encrypted connection. It replaces such utilities as rsh and rlogin as well as Telnet.

Hypertext Transfer Protocol (HTTP)

Hypertext Transfer Protocol (HTTP) is the command and control protocol used to manage communications between a web browser and a web server. When you access a web page on the Internet or on a corporate intranet, you see a mixture of text, graphics, and links to other documents or other Internet resources. HTTP is the mechanism that opens the related document when you select a link, no matter where that document is actually located.

Hypertext Transfer Protocol Secure (HTTPS)

Hypertext Transfer Protocol Secure (HTTPS), also referred to as Secure Hypertext Transfer Protocol (which you will see abbreviated as SHTTP or S-HTTP), is a secure version of HTTP that provides a variety of security mechanisms to the transactions between a web browser and the server. HTTPS allows browsers and servers to sign, authenticate, and encrypt an HTTP message.

Network Time Protocol(NTP)

Network Time Protocol (NTP), originally developed by Professor David Mills at the University of Delaware, is used to synchronize (or set) computer clocks to some standard time source, which is usually a nuclear clock. This protocol (along with synchronization utilities) keeps all computers on a network set to the same time. Time synchronization is important because many transactions are time and date stamped (in a database, for example). If the time on a server is out of synchronization with the time on two different computers, even by just a few seconds, the server will get confused. For example, one computer can seemingly enter a transaction, but the server will indicate that it occurred before it actually did. Because this time problem will crash the database server, it is important that these servers (and workstations) use NTP.

Network News Transfer Protocol(NNTP)

The Network News Transfer Protocol(NNTP) is the TCP/IP protocol used to access Usenet news servers. Usenet news servers contain thousands of individual message boards known as newsgroups. Each newsgroup is about a particular subject (cars, dating, computers, etc.). Chances are, if you have an interest, there is a newsgroup about it. The details of the NNTP protocol are specified in RFC 977.

Secure Copy Protocol (SCP)

While FTP is easy to use to transfer files, it has a major security problem in that the username and password are sent along with the file request in clear text (i.e., not encrypted). It would be a relatively simple matter for someone to intercept that information and use it for other purposes.Secure Copy Protocol (SCP) was designed to overcome this limitation. It uses SSH to establish and maintain an encrypted connection between hosts. The file transfer can then take place without fear of password or data interception.

Lightweight Directory Access Protocol (LDAP)

In large networks, most administrators have set up some kind of directory that keeps track of users and resources (e.g., NDS, Active Directory). In order to have a standard method of accessing directories, the Lightweight Directory Access Protocol (LDAP) was developed. It allows clients to perform object lookups with a directory using a standard method. LDAP was originally specified as RFCs 1487 (version 1) and 1777 (version 2), with RFC 3377 proposing the more commonly used third version, which fixes a number of shortcomings in the protocol.

Internet Group Management Protocol(IGMP)

The Internet Group Management Protocol (IGMP) is a TCP/IP protocol that is used to manage IP multicast sessions. It uses special IGMP messages to learn the layout of the multicast groups and which hosts belong to which groups. Additionally, the individual hosts in an IP network use IGMP messages to join and leave a multicast group. IGMP messages help keep track of group membership and active multicast streams. IGMP is in its second version, as specified in RFC 2236, with a third version (RFC 3376), currently proposed.

Line Printer Remote(LPR)

When using pure TCP/IP printing (as with UNIX workstations or when used for cross-platform printing), the LPD/LPR pairing is used most often. The Line Printer Daemon (LPD) is installed on the print device and manages the printer as well as the print jobs. The Line Printer Remote (LPR) software is the printing client that sends the print jobs to the LPD via TCP/IP

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