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 A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ( ...

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 suit ...

(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 h ...

(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 n ...

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 Association Control Service Element (ACSE) is the OSI method for establishing a call between one application programs. ACSE checks the identities and contexts of the application entities, and could apply an authentication security check. Reference ...

) * 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 FTAM, ISO standard 8571, is the OSI application layer protocol for file transfer, access and management. The goal of FTAM is to combine into a single protocol both file transfer, similar in concept to the Internet FTP, as well as remote acc ...

(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 MMS may refer to: Science and technology Network communication protocols * Multimedia Messaging Service for mobile phones * Microsoft Media Server, a content-streaming protocol (mms://) * Manufacturing Message Specification for real time proces ...

(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 control i ...

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 a ...

* 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 data ...

(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 to ...

(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 The Common Management Information Protocol (CMIP) is the OSI specified network management protocol. Defined iITU-T Recommendation X.711, ISO/IEC International Standard 9596-1 It provides an implementation for the services defined by the Common ...

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 In computing, Advanced Program to Program Communication or APPC is a protocol which computer programs can use to communicate over a network. APPC is at the application layer in the OSI model, it enables communications between programs on differe ...

*AMQP,

Advanced Message Queuing Protocol The Advanced Message Queuing Protocol (AMQP) is an open standard application layer protocol for message-oriented middleware. The defining features of AMQP are message orientation, queuing, routing (including point-to-point and publish-and-su ...

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 Bitcoin ( abbreviation: BTC; sign: ₿) is a decentralized digital currency that can be transferred on the peer-to-peer bitcoin network. Bitcoin transactions are verified by network nodes through cryptography and recorded in a public distr ...

* BitTorrent *CFDP, Coherent File Distribution Protocol *CoAP,

Constrained Application Protocol Constrained Application Protocol (CoAP) is a specialized Internet application protocol for constrained devices, as defined iRFC 7252 It enables those constrained devices called "nodes" to communicate with the wider Internet using similar protocols ...

*DDS,

Data Distribution Service The Data Distribution Service (DDS) for real-time systems is an Object Management Group (OMG) machine-to-machine (sometimes called middleware or connectivity framework) standard that aims to enable dependable, high-performance, interoperable, re ...

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 netwo ...

(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 ...

, 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 and ...

*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 servi ...

*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 commonl ...

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 The Network Configuration Protocol (NETCONF) is a network management protocol developed and standardized by the IETF. It was developed in the NETCONF working group and published in December 2006 as RFC 4741 and later revised in June 2011 and pub ...

*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 ...

*NNTP,

Network News Transfer Protocol The Network News Transfer Protocol (NNTP) is an application protocol used for transporting Usenet news articles (''netnews'') between news servers, and for reading/posting articles by the end user client applications. Brian Kantor of the Univers ...

*NTCIP,

National Transportation Communications for Intelligent Transportation System Protocol The National Transportation Communications for Intelligent Transportation System Protocol (NTCIP) is a family of standards designed to achieve interoperability and interchangeability between computers and electronic traffic control equipment from ...

*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 Reliable Event Logging Protocol (RELP), a networking protocol for computer data logging in computer networks, extends the functionality of the syslog protocol to provide reliable delivery of event messages. It is most often used in environments whic ...

*RFB, Remote Framebuffer Protocol *

Rlogin The Berkeley r-commands are a suite of computer programs designed to enable users of one Unix system to log in or issue commands to another Unix computer via TCP/IP computer network. The r-commands were developed in 1982 by the Computer Systems ...

, 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 (l ...

*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 The Session Announcement Protocol (SAP) is an experimental protocol for advertising multicast session information. SAP typically uses Session Description Protocol (SDP) as the format for Real-time Transport Protocol (RTP) session descriptions. Anno ...

*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 The Service Location Protocol (SLP, srvloc) is a service discovery protocol that allows computers and other devices to find services in a local area network without prior configuration. SLP has been designed to scale from small, unmanaged networks ...

*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 provides ...

*SMTP,

*SNTP,

Simple Network Time Protocol The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable- latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in c ...

*SSH,

*SSMS, Secure SMS Messaging Protocol *TCAP,

Transaction Capabilities Application Part Transaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the ...

*TDS,

Tabular Data Stream Tabular Data Stream (TDS) is an application layer Protocol (computing), protocol used to transfer data between a database server and a client. It was initially designed and developed by Sybase Inc. for their Sybase SQL Server relational database eng ...

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 Tox or TOX may refer to: Science and technology * TOX, a protein encoded by the TOX gene * Tox screen, medical diagnostic screening for toxic substances Computing * Tox (protocol), peer-to-peer instant messaging software * tox (Python testing ...

*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 standard, open communication protocol designed for instant messaging (IM), presence information, and contact list maintenance. Based on XML (Extensible Markup ...

* Z39.50 *

DNS 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 to ...

, Domain Name Services

References

External links

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