An application layer is an abstraction layer that specifies the shared

communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity. The protocol defines the rules, syntax, semantics (computer science), sem ...

s and interface 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 communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are ...

(TCP/IP) and the

OSI model The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems inter ...

. 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 ...

(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, forming a peer-to-peer network of Node ...

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 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 using the functionality of several 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 network, 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 ...

: * Hypertext documents: Hypertext Transfer Protocol (HTTP) * Remote login to hosts: Telnet, Secure Shell * 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 d ...

(FTP), Trivial File Transfer Protocol (TFTP) * Electronic mail transport: Simple Mail Transfer Protocol (SMTP) * Networking support: Domain Name System (DNS) * Host initialization: BOOTP * 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 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 ...

distributed file system protocol *AFP, Apple Filing Protocol *APPC, Advanced Program-to-Program Communication *AMQP, Advanced Message Queuing Protocol * Atom Publishing Protocol * BEEP, Block Extensible Exchange Protocol *

Bitcoin Bitcoin (abbreviation: BTC; Currency symbol, sign: ₿) is the first Decentralized application, decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 when an unknown entity published a white paper under ...

BitTorrent BitTorrent is a Protocol (computing), communication protocol for peer-to-peer file sharing (P2P), which enables users to distribute data and electronic files over the Internet in a Decentralised system, decentralized manner. The protocol is d ...

*CFDP, Coherent File Distribution Protocol *CoAP, Constrained Application Protocol *DDS, Data Distribution Service * DeviceNet * DNS, Domain Name Services * eDonkey *ENRP, Endpoint Handlespace Redundancy Protocol * FastTrack (KaZaa, Grokster, iMesh) *

Finger A finger is a prominent digit (anatomy), digit on the forelimbs of most tetrapod vertebrate animals, especially those with prehensile extremities (i.e. hands) such as humans and other primates. Most tetrapods have five digits (dactyly, pentadact ...

, User Information Protocol * Freenet * 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 t ...

*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 *IPFS, InterPlanetary File System *

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 (networking), no ...

*LDAP, Lightweight Directory Access Protocol *LPD, Line Printer Daemon 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 *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. Today, POP version 3 (POP3) is the most commonly used version. Together with IMAP, ...

*PNRP, Peer Name Resolution Protocol *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 computer network), which is written as if it were a ...

*RTMP, Real Time Messaging Protocol *RTP, Real-time Transport Protocol * RTPS, Real Time Publish Subscribe *RTSP, Real Time Streaming Protocol *SAP, Session Announcement Protocol *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, Secure Shell *SSMS, Secure SMS Messaging Protocol *TCAP, Transaction Capabilities Application Part *TDS, Tabular Data Stream * Tor (anonymity network) * Tox *TSP, Time Stamp Protocol *VTP, Virtual Terminal Protocol * Whois (and RWhois), Remote Directory Access Protocol * WebDAV * WebRTC * WebSocket * X.400, Message Handling Service Protocol * X.500, Directory Access Protocol (DAP) *XMPP, Extensible Messaging and Presence Protocol * Z39.50

References

External links

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