GeezerVison

TL:DR;

Top Level APIs; Application Networking

Services, applications and network management. The application layer provides functions for applications. This layer provides the connection to the lower layers. At this level, the data input and output takes place. The applications themselves are not in the layer.

Typical Applications: web browser, email client, ...

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

Layer 7   Application Layer

Layer 6   Presentation Layer

Layer 5   Session Layer

Layer 4   Transport Layer

Layer 3   Network Layer

Layer 2   Data Link Layer

Layer 1   Physical Layer