An application layer is an

abstraction layer In computing, an abstraction layer or abstraction level is a way of hiding the working details of a subsystem. Examples of software models that use layers of abstraction include the OSI model for network protocols, OpenGL, and other graphics libr ...

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 synch ...

s and interface 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 (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-e ...

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 In computing, the robustness principle is a design guideline for software that states: "be conservative in what you do, be liberal in what you accept from others". It is often reworded as: "be conservative in what you send, be liberal in what you a ...

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 and presentation layer, 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 (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, p ...

: * 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 We ...

(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 ...

* 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 da ...

(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 netwo ...

(TFTP) * Electronic mail transport: Simple Mail Transfer Protocol (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 * Remote host management: Simple Network Management Protocol (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 Commo ...

over TCP (CMOT)

Examples

Additional notable application-layer protocols include the following: * 9P, Plan 9 from Bell Labs distributed file system protocol *AFP, Apple Filing Protocol *APPC, Advanced Program-to-Program Communication *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 *

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 pee ...

, 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 *

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 ran ...

* eDonkey *ENRP, Endpoint Handlespace Redundancy Protocol * FastTrack (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). Chamber ...

, User Information Protocol * Freenet *

, File Transfer Access and Management *FTP,

*Gemini, Gemini protocol *Gopher, Gopher protocol *HL7, Health Level Seven *HTTP,

*Hypercore, formerly dat:// *

H.323 H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols to provide audio-visual communication sessions on any packet network. The H.323 standard addresses call signaling and control, mu ...

, Packet-Based Multimedia Communications System * IMAP, 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 * Kademlia *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 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. Messag ...

and Secure MIME * Modbus *

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 *NIS, Network Information Service *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 Univer ...

*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 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 ...

* OSCAR,

AOL Instant Messenger Protocol AIM (AOL Instant Messenger) was an instant messaging and presence computer program created by AOL, which used the proprietary OSCAR instant messaging protocol and the TOC protocol to allow registered users to communicate in real time. AIM ...

*POP, Post Office Protocol *PNRP, Peer Name Resolution Protocol *RDP,

Remote Desktop Protocol Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft which provides a user with a graphical interface to connect to another computer over a network connection. The user employs RDP client software for this purpose, while ...

*RELP, Reliable Event Logging Protocol *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 Syste ...

, 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 *RTP, Real-time Transport Protocol * RTPS, Real Time Publish Subscribe *RTSP, Real Time Streaming Protocol *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. Ann ...

*SDP, Session Description Protocol *SIP, Session Initiation Protocol *SLP, Service Location Protocol *SMB, Server Message Block *SMTP, Simple Mail Transfer Protocol *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 (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 * WebSocket *

X.400 X.400 is a suite of ITU-T Recommendations that defines the ITU-T Message Handling System (MHS). At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP-based Internet e- ...

, Message Handling Service Protocol *

X.500 X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). ITU-T was formerly kno ...

, 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), i ...

Z39.50 Z39.50 is an international standard client–server, application layer communications protocol for searching and retrieving information from a database over a TCP/IP computer network, developed and maintained by the Library of Congress. It is ...

* DNS, Domain Name Services

References

External links

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