Application Layer

1. Application layer
An application layer is an abstraction layer that speci-
fies the shared protocols and interface methods used by
hosts in a communications network. The application
layer abstraction is used in both of the standard mod-
els 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 respec-
tive highest level layer, the detailed definitions and pur-
poses 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 applica-
tion 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 specifi-
cation (in RFC 1123) does rely on and recommend the
robustness principle for application design.[1]
In the OSI model, the definition of the application layer
is narrower in scope. The OSI model defines the applica-
tion layer as the user interface responsible for displaying
received information to the user. In contrast, the Inter-
net Protocol model does not concern itself with such de-
tail. OSI also explicitly distinguishes additional function-
ality 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 separa-
tion of functionality at these layers and provides protocol
implementations for each layer.
1 TCP/IP
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
functionality of the early Internet.[1]
• Remote login to hosts: Telnet
• 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 Man-
agement Protocol (SNMP), Common Management
Information Protocol over TCP (CMOT)
2 Other protocol examples
• 9P, Plan 9 from Bell Labs distributed file system
protocol
• AFP, Apple Filing Protocol
• APPC, Advanced Program-to-Program Communi-
cation
• 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
• eDonkey
• ENRP, Endpoint Handlespace Redundancy Proto-
col
• FastTrack (KaZaa, Grokster, iMesh)
• Finger, User Information Protocol
• Freenet
• FTAM, File Transfer Access and Management
• Gopher, Gopher protocol
• HL7, Health Level Seven
• HTTP, HyperText Transfer Protocol
• H.323, Packet-Based Multimedia Communications
System
1

2. 2 4 EXTERNAL LINKS
• IRCP, Internet Relay Chat Protocol
• Kademlia
• KAP, Anonymous File Transfer over UDP/IP
(KickAss Protocol)
• LDAP, Lightweight Directory Access Protocol
• LPD, Line Printer Daemon Protocol
• MIME (S-MIME), Multipurpose Internet Mail Ex-
tensions 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
• 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)
• Tox
• TSP, Time Stamp Protocol
• VTP, Virtual Terminal Protocol
• Whois (and RWhois), Remote Directory Access
Protocol
• WebDAV
• X.400, Message Handling Service Protocol
• X.500, Directory Access Protocol (DAP)
• XMPP, Extensible Messaging and Presence Proto-
col
3 References
[1] Robert Braden, ed. (October 1989). “RFC 1123: Re-
quirements for Internet Hosts – Application and Support”.
Network Working Group of the IETF.
4 External links

3. 3
5 Text and image sources, contributors, and licenses
5.1 Text
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