An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network. An ''application layer'' abstraction is specified in both the Internet Protocol Suite (TCP/IP) and the OSI model. 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 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 OSI model, 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) * 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 An application layer is an abstraction layer that specifies the shared communications protocols and Interface (computing), interface methods used by Host (network), hosts in a communi ...

(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 ''internetworking, network of networks'' that consists ...

: * Hypertext documents: Hypertext Transfer Protocol (HTTP) * Remote login to hosts: Telnet,

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 (FTP), Trivial File Transfer Protocol (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 ...

(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 p ...

* Remote host management: Simple Network Management Protocol (SNMP), Common Management Information Protocol 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 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 t ...

*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-subsc ...

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 r ...

* BEEP, 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 di ...

* BitTorrent *CFDP, Coherent File Distribution Protocol *CoAP, Constrained Application Protocol *DDS, Data Distribution Service * DeviceNet * 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). Chambers 1 ...

, User Information Protocol * Freenet *

, File Transfer Access and Management *FTP, File Transfer Protocol *Gemini, Gemini protocol *Gopher, Gopher protocol *HL7, Health Level Seven *HTTP, Hypertext Transfer Protocol *Hypercore, formerly dat:// * H.323, 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 a ...

*IPFS, InterPlanetary File System * Kademlia *LDAP, Lightweight Directory Access Protocol *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 opera ...

Protocol *MIME (S-MIME), Multipurpose Internet Mail Extensions and Secure MIME * Modbus * MQTT Protocol * Netconf *NFS, Network File System *NIS, Network Information Service *NNTP, Network News Transfer Protocol *NTCIP, National Transportation Communications for Intelligent Transportation System Protocol *NTP, Network Time Protocol * 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 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 *RTMP, Real Time Messaging Protocol *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 *SAP, Session Announcement Protocol *SDP, Session Description Protocol *SIP,

Session Initiation Protocol The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, and terminating communication sessions that include voice, video and messaging applications. SIP is used in Internet telephony, in private IP telep ...

*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 * 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-m ...

, 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 ITU-T, Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T). ITU-T was former ...

, Directory Access Protocol (DAP) *XMPP, Extensible Messaging and Presence Protocol *

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 c ...

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