Application Layer | Layer 7 | The OSI-Model

OSI Layer 7 – Application Layer

An application layer is an abstraction layer that specifies the shared protocols and interface methods used by hosts in a communications network. The application layer abstraction is used in both of the standard models of computer networking; the Internet Protocol Suite (TCP/IP) and the Open Systems Interconnection model (OSI model).

Although both models use the same term for their respective highest level layer, the detailed definitions and purposes are different.

In TCP/IP, the application layer contains the communications protocols and interface methods used in process-to-process communications across an Internet Protocol (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 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 RFC 1123) does rely on and recommend the robustness principle for application design.

In the OSI model, the definition of the application layer is narrower in scope. The OSI model defines the application layer as the user interface responsible for displaying received information to the user. In contrast, the Internet Protocol model does not concern itself with such detail. OSI also explicitly distinguishes additional functionality below the application layer, but above the transport layer at two additional levels; the session layer and the presentation layer. OSI specifies a strict modular separation of functionality at these layers and provides protocol implementations for each layer.

Wikipedia

Popular Application Layer Protocols

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

Digital currency

BitTorrent

peer-to-peer file sharing

CFDP

Coherent File Distribution Protocol

CoAP

Constrained Application Protocol

DDS

Data Distribution Service

DeviceNet

automation industry protocol

eDonkey

classic file sharing protocol

ENRP

Endpoint Handlespace Redundancy Protocol

FastTrack

filesharing, known from KaZaa and more

Finger

User Information Protocol

Freenet

censorship resistant p2p network

FTAM

File Transfer Access and Management

Gopher

Gopher protocol

HL7

Health Level Seven

HTTP

HyperText Transfer Protocol

H.323

Packet-Based Multimedia Communications System

IRCP

Internet Relay Chat Protocol

Kademlia

p2p hashtables

LDAP

Lightweight Directory Access Protocol

LPD

Line Printer Daemon Protocol

MIME (S-MIME)

Multipurpose Internet Mail Extensions and Secure MIME

Modbus

Serial communications protocol

MQTT Protocol

MQ Telemetry Transport

Netconf

Network Configuration Protocol

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

PNRP

Peer Name Resolution Protocol

RDP

Remote Desktop Protocol

RELP

Reliable Event Logging Protocol

RIP

Routing Information Protocol

Rlogin

Remote Login in UNIX Systems

RPC

Remote Procedure Call

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

TSP

Time Stamp Protocol

VTP

Virtual Terminal Protocol

Whois (and RWhois)

Remote Directory Access Protocol

WebDAV

Web Distributed Authoring and Versioning

X.400

Message Handling Service Protocol

X.500

Directory Access Protocol (DAP)

XMPP

Extensible Messaging and Presence Protocol

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