An application layer is an abstraction layer that specifies the shared

communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchr ...

s and

interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...

methods used by hosts in a communications network. An ''application layer'' abstraction is specified in both the

Internet Protocol Suite The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...

(TCP/IP) and the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

. Although both models use the same term for their respective highest-level layer, the detailed definitions and purposes are different.

Internet protocol suite

In the Internet protocol suite, the application layer contains the communications protocols and interface methods used in process-to-process communications across an

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

(IP) computer network. The application layer only standardizes communication and depends upon the underlying

transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end ...

protocols to establish host-to-host data transfer channels and manage the data exchange in a client–server or

peer-to-peer Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network. They are said to form a peer-to-peer ...

networking model. Though the TCP/IP application layer does not describe specific rules or data formats that applications must consider when communicating, the original specification (in ) does rely on and recommend the robustness principle for application design.

OSI model

In the

, the definition of the application layer is narrower in scope. The OSI model defines the application layer as only the interface responsible for communicating with host-based and user-facing applications. OSI then explicitly distinguishes the functionality of two additional layers, the

session layer In the seven-layer OSI model of computer networking, the session layer is layer 5. The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. ...

and

presentation layer In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves as the data translator for the network. It is sometimes called the syntax layer. Description Within the service layering semantics of the OSI netw ...

, as separate levels below the application layer and above the transport layer. OSI specifies a strict modular separation of functionality at these layers and provides protocol implementations for each. In contrast, the Internet Protocol Suite compiles these functions into a single layer.

Sublayers

Originally the OSI model consisted of two kinds of application layer services with their related protocols. These two sublayers are the common application service element (CASE) and specific application service element (SASE). Generally, an application layer protocol is realized by the use of the functionality of a number of application service elements. Some application service elements invoke different procedures based on the version of the session service available.

CASE

The common application service element sublayer provides services for the application layer and request services from the session layer. It provides support for common application services, such as: * ACSE ( Association Control Service Element) * ROSE (Remote Operation Service Element) * CCR (Commitment Concurrency and Recovery) * RTSE (Reliable Transfer Service Element)

SASE

The specific application service element sublayer provides application-specific services (protocols), such as: * FTAM (File Transfer, Access and Manager) * VT (Virtual Terminal) * MOTIS (Message Oriented Text Interchange Standard) * CMIP (Common Management Information Protocol) * JTM (Job Transfer and Manipulation)a former OSI standard
/ref> * MMS (Manufacturing Messaging Specification) * RDA (Remote Database Access) * DTP (Distributed Transaction Processing)

Protocols

The IETF definition document for the application layer in the Internet Protocol Suite is RFC 1123. It provided an initial set of protocols that covered the major aspects of the functionality of the early

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

: * Hypertext documents:

Hypertext Transfer Protocol The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, ...

(HTTP) * Remote login to hosts:

Telnet Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet contr ...

Secure Shell The Secure Shell Protocol (SSH) is a cryptographic network protocol for operating network services securely over an unsecured network. Its most notable applications are remote login and command-line execution. SSH applications are based on ...

* File transfer:

File Transfer Protocol The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and dat ...

(FTP),

Trivial File Transfer Protocol Trivial File Transfer Protocol (TFTP) is a simple lockstep File Transfer Protocol which allows a client to get a file from or put a file onto a remote host. One of its primary uses is in the early stages of nodes booting from a local area networ ...

(TFTP) * Electronic mail transport:

Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typica ...

(SMTP) * Networking support:

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...

(DNS) * Host initialization:

BOOTP The Bootstrap Protocol (BOOTP) is a computer networking protocol used in Internet Protocol networks to automatically assign an IP address to network devices from a configuration server. The BOOTP was originally defined in RFC 951. While some part ...

* Remote host management:

Simple Network Management Protocol Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behaviour. Devices that typically ...

(SNMP), Common Management Information Protocol over TCP (CMOT)

Examples

Additional notable application-layer protocols include the following: * 9P,

Plan 9 from Bell Labs Plan 9 from Bell Labs is a distributed operating system which originated from the Computing Science Research Center (CSRC) at Bell Labs in the mid-1980s and built on UNIX concepts first developed there in the late 1960s. Since 2000, Plan 9 has be ...

distributed file system protocol *AFP,

Apple Filing Protocol The Apple Filing Protocol (AFP), formerly AppleTalk Filing Protocol, is a proprietary network protocol, and part of the Apple File Service (AFS), that offers file services for macOS and the classic Mac OS. In Mac OS 9 and earlier, AFP was ...

*APPC, Advanced Program-to-Program Communication *AMQP, Advanced Message Queuing Protocol *

Atom Publishing Protocol The name Atom applies to a pair of related Web standards. The Atom Syndication Format is an XML language used for web feeds, while the Atom Publishing Protocol (AtomPub or APP) is a simple HTTP-based protocol for creating and updating web re ...

BEEP The Blocks Extensible Exchange Protocol (BEEP) is a framework for creating network application protocols. BEEP includes building blocks like framing, pipelining, multiplexing, reporting and authentication for connection and message-oriented pe ...

, Block Extensible Exchange Protocol * Bitcoin * BitTorrent *CFDP, Coherent File Distribution Protocol *CoAP, Constrained Application Protocol *DDS, Data Distribution Service *

DeviceNet DeviceNet is a network protocol used in the automation industry to interconnect control devices for data exchange. It utilizes the Common Industrial Protocol over a Controller Area Network media layer and defines an application layer to cover a rang ...

* eDonkey *ENRP,

Endpoint Handlespace Redundancy Protocol The Endpoint Handlespace Redundancy Protocol is used by the Reliable server pooling (RSerPool) framework for the communication between Pool Registrars to maintain and synchronize a handlespace. It is allocated on the application layer like the ...

FastTrack FastTrack is a peer-to-peer (P2P) protocol that was used by the Kazaa, Grokster, iMesh and Morpheus file sharing programs. FastTrack was the most popular file sharing network in 2003, and used mainly for the exchange of music mp3 files. The ne ...

(KaZaa, Grokster, iMesh) *

Finger A finger is a limb of the body and a type of digit, an organ of manipulation and sensation found in the hands of most of the Tetrapods, so also with humans and other primates. Most land vertebrates have five fingers ( Pentadactyly). Chambers ...

, User Information Protocol *

Freenet Freenet is a peer-to-peer platform for censorship-resistant, anonymous communication. It uses a decentralized distributed data store to keep and deliver information, and has a suite of free software for publishing and communicating on the Web ...

* FTAM, File Transfer Access and Management *FTP,

*Gemini, Gemini protocol *Gopher,

Gopher protocol The Gopher protocol () is a communication protocol designed for distributing, searching, and retrieving documents in Internet Protocol networks. The design of the Gopher protocol and user interface is menu-driven, and presented an alternative to ...

*HL7,

Health Level Seven Health Level Seven or HL7 refers to a set of international standards for transfer of clinical and administrative data between software applications used by various healthcare providers. These standards focus on the application layer, which is "la ...

*HTTP,

*Hypercore, formerly dat:// * H.323, Packet-Based Multimedia Communications System *

IMAP In computing, the Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection. IMAP is defined by . IMAP was designed with the goal of per ...

, Internet Message Access Protocol *IRC,

Internet Relay Chat Internet Relay Chat (IRC) is a text-based chat system for instant messaging. IRC is designed for group communication in discussion forums, called '' channels'', but also allows one-on-one communication via private messages as well as chat an ...

*IPFS,

InterPlanetary File System The InterPlanetary File System (IPFS) is a protocol, hypermedia and file sharing peer-to-peer network for storing and sharing data in a distributed file system. IPFS uses content-addressing to uniquely identify each file in a global namespace ...

Kademlia Kademlia is a distributed hash table for decentralized peer-to-peer computer networks designed by Petar Maymounkov and David Mazières in 2002. It specifies the structure of the network and the exchange of information through node lookups. Kademl ...

*LDAP,

Lightweight Directory Access Protocol The Lightweight Directory Access Protocol (LDAP ) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network. Directory ser ...

*LPD,

Line Printer Daemon The Line Printer Daemon protocol/Line Printer Remote protocol (or LPD, LPR) is a network printing protocol for submitting print jobs to a remote printer. The original implementation of LPD was in the Berkeley printing system in the BSD UNIX oper ...

Protocol *MIME (S-MIME),

Multipurpose Internet Mail Extensions Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Message ...

and Secure MIME *

Modbus Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a ''de facto'' standard communication protocol and is now a common ...

MQTT MQTT (originally an initialism of MQ Telemetry Transport) is a lightweight, publish-subscribe, machine to machine network protocol for Message queue/Message queuing service. It is designed for connections with remote locations that have devices ...

Protocol * Netconf *NFS,

Network File System Network File System (NFS) is a distributed file system protocol originally developed by Sun Microsystems (Sun) in 1984, allowing a user on a client computer to access files over a computer network much like local storage is accessed. NFS, lik ...

*NIS,

Network Information Service The Network Information Service, or NIS (originally called Yellow Pages or YP), is a client–server directory service protocol for distributing system configuration data such as user and host names between computers on a computer network. Sun M ...

*NNTP, Network News Transfer Protocol *NTCIP, National Transportation Communications for Intelligent Transportation System Protocol *NTP, Network Time Protocol *

OSCAR Oscar, OSCAR, or The Oscar may refer to: People * Oscar (given name), an Irish- and English-language name also used in other languages; the article includes the names Oskar, Oskari, Oszkár, Óscar, and other forms. * Oscar (Irish mythology) ...

, AOL Instant Messenger Protocol *POP,

Post Office Protocol In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. POP version 3 (POP3) is the version in common use, and along with IMAP the most common ...

*PNRP,

Peer Name Resolution Protocol Peer Name Resolution Protocol (PNRP) is a peer-to-peer protocol designed by Microsoft. PNRP enables dynamic name publication and resolution, and requires IPv6. PNRP was first mentioned during a presentation at a P2P conference in November 2001. It ...

*RDP, Remote Desktop Protocol *RELP, Reliable Event Logging Protocol *RFB, Remote Framebuffer Protocol * Rlogin, Remote Login in UNIX Systems *RPC,

Remote Procedure Call In distributed computing, a remote procedure call (RPC) is when a computer program causes a procedure ( subroutine) to execute in a different address space (commonly on another computer on a shared network), which is coded as if it were a normal ( ...

*RTMP,

Real Time Messaging Protocol Real-Time Messaging Protocol (RTMP) is a communication protocol for streaming audio, video, and data over the Internet. Originally developed as a proprietary protocol by Macromedia for streaming between Flash Player and the Flash Communication Ser ...

*RTP,

Real-time Transport Protocol The Real-time Transport Protocol (RTP) is a network protocol for delivering audio and video over IP networks. RTP is used in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applicati ...

* RTPS, Real Time Publish Subscribe *RTSP,

Real Time Streaming Protocol The Real Time Streaming Protocol (RTSP) is an application-level network protocol designed for multiplexing and packetizing multimedia transport streams (such as interactive media, video and audio) over a suitable transport protocol. RTSP is us ...

*SAP, Session Announcement Protocol *SDP,

Session Description Protocol The Session Description Protocol (SDP) is a format for describing multimedia communication sessions for the purposes of announcement and invitation. Its predominant use is in support of streaming media applications, such as voice over IP (VoIP) ...

*SIP, Session Initiation Protocol *SLP, Service Location Protocol *SMB,

Server Message Block Server Message Block (SMB) is a communication protocol originally developed in 1983 by Barry A. Feigenbaum at IBM and intended to provide shared access to files and printers across nodes on a network of systems running IBM's OS/2. It also provide ...

*SMTP,

*SNTP, Simple Network Time Protocol *SSH,

*SSMS, Secure SMS Messaging Protocol *TCAP, Transaction Capabilities Application Part *TDS, Tabular Data Stream *

Tor (anonymity network) Tor, short for The Onion Router, is free and open-source software for enabling anonymous communication. It directs Internet traffic through a free, worldwide, volunteer overlay network, consisting of more than seven thousand relays, to co ...

* Tox *TSP, Time Stamp Protocol *VTP, Virtual Terminal Protocol *

Whois WHOIS (pronounced as the phrase "who is") is a query and response protocol that is widely used for querying databases that store the registered users or assignees of an Internet resource, such as a domain name, an IP address block or an autonomou ...

(and RWhois), Remote Directory Access Protocol *

WebDAV WebDAV (Web Distributed Authoring and Versioning) is a set of extensions to the Hypertext Transfer Protocol (HTTP), which allows user agents to collaboratively author contents ''directly'' in an HTTP web server by providing facilities for con ...

WebRTC WebRTC (Web Real-Time Communication) is a free and open-source project providing web browsers and mobile applications with real-time communication (RTC) via application programming interfaces (APIs). It allows audio and video communication to wor ...

WebSocket WebSocket is a computer communications protocol, providing full-duplex communication channels over a single TCP connection. The WebSocket protocol was standardized by the IETF as in 2011. The current API specification allowing web applications ...

* X.400, Message Handling Service Protocol * X.500, Directory Access Protocol (DAP) *XMPP,

Extensible Messaging and Presence Protocol Extensible Messaging and Presence Protocol (XMPP, originally named Jabber) is an open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML (Extensible Markup Language), it ...

* Z39.50 * DNS, Domain Name Services

References

External links

* *{{Wikiversity-inline, Application layer OSI model