View source for Application layer

{{Use American English|date = March 2019}}
{{Short description|Highest layer of a networked communication model}}
{{Use mdy dates|date = March 2019}}

An '''application layer''' is an [[abstraction layer]] that specifies the shared [[communication protocol]]s and [[Interface (computing)|interface]] methods used by [[Host (network)|hosts]] in a communications network.<ref>{{Cite web|url=https://osi-model.com/application-layer/|title=Application Layer {{!}} Layer 7|website=The OSI-Model|language=en|access-date=2019-11-05}}</ref> An ''application layer'' abstraction is specified in both the [[Internet Protocol Suite]] (TCP/IP) and the [[OSI model]].<ref>{{Cite web|url=http://www.omnisecu.com/tcpip/tcpip-model.php|title=Four Layers of TCP/IP model, Comparison and Difference between TCP/IP and OSI models|website=www.omnisecu.com|access-date=2019-11-05}}</ref> Although both models use the same term for their respective highest-level layer, the detailed definitions and purposes are different.<ref>{{Cite web|url=https://searchnetworking.techtarget.com/answer/What-is-the-difference-between-TCP-IP-and-IP-protocol|title=What is the difference between TCP/IP and IP protocol?|website=SearchNetworking|language=en|access-date=2019-11-05}}</ref>

==Internet protocol suite==
{{IP stack}}
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]] (IP) computer network.<ref>{{Cite web|url=https://searchnetworking.techtarget.com/answer/What-is-the-difference-between-TCP-IP-and-IP-protocol|title=What is the difference between TCP/IP and IP protocol?|website=SearchNetworking|language=en|access-date=2019-11-05}}</ref> 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]] networking model.<ref>{{Cite web|url=https://www.lifewire.com/transmission-control-protocol-and-internet-protocol-816255|title=What Is Transmission Control Protocol/Internet Protocol (TCP/IP)?|last1=SEO|first1=Bradley Mitchell An MIT graduate who brings years of technical experience to articles on|last2=computers|website=Lifewire|language=en|access-date=2019-11-05|last3=Networking|first3=Wireless}}</ref> Though the TCP/IP application layer does not describe specific rules or data formats that applications must consider when communicating, the original specification (in {{IETF RFC|1123}}) does rely on and recommend the [[robustness principle]] for application design.<ref name=rfc1123>{{cite web
| title = RFC 1123: Requirements for Internet Hosts – Application and Support
| url = http://tools.ietf.org/html/rfc1123
| publisher = Network Working Group of the IETF
| editor = Robert Braden
| date = October 1989
}}</ref><ref>{{Cite web|url=https://www.techopedia.com/definition/6006/application-layer|title=What is the Application Layer?|website=www.tutorialspoint.com|access-date=2019-11-05}}</ref>

==OSI model==
{{OSI model}}
In the [[OSI model]], the definition of the application layer is narrower in scope.<ref>{{Cite web|url=https://www.networkworld.com/article/3239677/the-osi-model-explained-how-to-understand-and-remember-the-7-layer-network-model.html|title=The OSI model explained: How to understand (and remember) the 7 layer network model|last=Shaw|first=Keith|date=2018-10-22|website=Network World|language=en|access-date=2019-11-05}}</ref> The OSI model defines the application layer as only the interface responsible for communicating with host-based and user-facing applications.<ref name=":0">{{Cite web|url=https://www.cloudflare.com/learning/ddos/glossary/open-systems-interconnection-model-osi/|title=What Is The OSI Model?|date=2019|website=CloudFlare|access-date=November 4, 2019}}</ref> 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 [[OSI protocols|protocol implementations]] for each. In contrast, the Internet Protocol Suite compiles these functions into a single layer.<ref name=":0" /> 

===Sublayers===
Originally the OSI model consisted of two kinds of application layer services with their related protocols.<ref>{{Cite web|url=https://networkencyclopedia.com/application-layer/|title = Application Layer (Layer 7 of the OSI Model)|date = September 2, 2019}}</ref> These two sublayers are the common application service element (CASE) and specific application service element (SASE).<ref name="Hura">{{cite book | last = Hura | first = Gurdeep | title = Data and Computer Communications: Networking and Internetworking | publisher = CRC Press LLC | year = 2001 |chapter= Application Layer |pages = [https://archive.org/details/datacomputercomm0000hura/page/710 710]–712 | isbn = 9780849309281 | url = https://archive.org/details/datacomputercomm0000hura | url-access = registration }}</ref><!-- this belongs in application layer?? --> Generally, an application layer protocol is realized by the use of the functionality of a number of application service elements.<ref>{{Cite report|url=https://tools.ietf.org/rfc/rfc1095|title = Common Management Information Services and Protocol over TCP/IP (CMOT)|date = April 1989| doi=10.17487/RFC1095 | last1=Warrier | first1=U.S. | last2=Besaw | first2=L. }}</ref> Some application service elements invoke different procedures based on the version of the session service available.<ref>{{Cite web|url=https://tools.ietf.org/html/rfc1085|title = ISO presentation services on top of TCP/IP based internets|date = December 1988|last1 = Rose|first1 = Marshall T.}}</ref>

====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]])<ref name="Hura"/>
* 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)
* [[Virtual terminal|VT]] (Virtual Terminal)
* MOTIS (Message Oriented Text Interchange Standard)
* CMIP (Common Management Information Protocol)
* JTM (Job Transfer and Manipulation)<ref>[http://www.furniss.co.uk/jtm/index.html a former OSI standard]{{dl|fix-attempted=yes|date=July 2023}}</ref>
* [[Manufacturing Message Specification|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]]:<ref name=rfc1123 />
* Hypertext documents: [[Hypertext Transfer Protocol]] (HTTP)
* Remote login to hosts: [[Telnet]], [[Secure Shell]]
* File transfer: [[File Transfer Protocol]] (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:
{{div col|colwidth=20em}}
*[[9P (protocol)|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]]
*[[Atom Publishing Protocol]]
*[[BEEP]], Block Extensible Exchange Protocol
*[[Bitcoin]]
*[[BitTorrent]]
*CFDP, [[Coherent File Distribution Protocol]]
*CoAP, [[Constrained Application Protocol]]
*DDS, [[Data Distribution Service]]
*[[DeviceNet]]
*[[DNS]], Domain Name Services
*[[eDonkey network|eDonkey]]
*ENRP, [[Endpoint Handlespace Redundancy Protocol]]
*[[FastTrack]] (KaZaa, Grokster, iMesh)
*[[Finger protocol|Finger]], User Information Protocol
*[[Freenet]]
*[[FTAM]], 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 (software)|dat://]]
*[[H.323]], Packet-Based Multimedia Communications System
*[[IMAP]], Internet Message Access Protocol
*IRC, [[Internet Relay Chat]]
*IPFS, [[InterPlanetary File System]] 
*[[Kademlia]]
*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 protocol|OSCAR]], [[AOL Instant Messenger Protocol]]
*POP, [[Post Office Protocol]]
*PNRP, [[Peer Name Resolution Protocol]]
*RDP, [[Remote Desktop Protocol]]
*RELP, [[Reliable Event Logging Protocol]]
*RFB, [[RFB protocol|Remote Framebuffer Protocol]]
*[[Rlogin]], Remote Login in UNIX Systems
*RPC, [[Remote Procedure Call]]
*RTMP, [[Real Time Messaging Protocol]]
*RTP, [[Real-time Transport Protocol]]
*[[RTPS 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 (protocol)|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]] 
{{div col end}}

==References==
{{Reflist}}

==External links==
*{{Commons category-inline|Application layer protocols}}
*{{Wikiversity-inline|Application layer}}

[[Category:OSI model]]