Editing Internet protocol suite (section)

==Application layer==
{{See also|Application layer#Internet protocol suite}}
The application layer includes the protocols used by most applications for providing user services or exchanging application data over the network connections established by the lower-level protocols. This may include some basic network support services such as [[routing protocol]]s and host configuration. Examples of application layer protocols include the [[Hypertext Transfer Protocol]] (HTTP), the [[File Transfer Protocol]] (FTP), the [[Simple Mail Transfer Protocol]] (SMTP), and the [[Dynamic Host Configuration Protocol]] (DHCP).<ref name="RxqD0">{{cite book|url=http://www.kohala.com/start/tcpipiv1.html|title=TCP/IP Illustrated: the protocols|isbn=0-201-63346-9|first=W. Richard|last=Stevens|author-link=W. Richard Stevens|date=February 1994|publisher=Addison-Wesley |access-date=April 25, 2012|archive-date=April 22, 2012|archive-url=https://web.archive.org/web/20120422024917/http://www.kohala.com/start/tcpipiv1.html|url-status=live}}</ref> Data coded according to application layer protocols are  [[encapsulation (networking)|encapsulated]] into transport layer protocol units (such as TCP streams or UDP datagrams), which in turn use [[lower layer protocol]]s to effect actual data transfer.

The TCP/IP model does not consider the specifics of formatting and presenting data and does not define additional layers between the application and transport layers as in the OSI model (presentation and session layers). According to the TCP/IP model, such functions are the realm of [[Library (computing)|libraries]] and [[application programming interface]]s. The application layer in the TCP/IP model is often compared to a combination of the fifth (session), sixth (presentation), and seventh (application) layers of the OSI model.

Application layer protocols are often associated with particular [[client–server]] applications, and common services have ''well-known'' port numbers reserved by the [[Internet Assigned Numbers Authority]] (IANA). For example, the [[HyperText Transfer Protocol]] uses server port 80 and [[Telnet]] uses server port 23. [[client (computing)|Clients]] connecting to a service usually use [[ephemeral port]]s, i.e., port numbers assigned only for the duration of the transaction at random or from a specific range configured in the application.

At the application layer, the TCP/IP model distinguishes between ''user protocols'' and ''support protocols''.{{Ref RFC|1122|rsection=1.1.3}} Support protocols provide services to a system of network infrastructure. User protocols are used for actual user applications. For example, FTP is a user protocol and DNS is a support protocol.

Although the applications are usually aware of key qualities of the transport layer connection such as the endpoint IP addresses and port numbers, application layer protocols generally treat the transport layer (and lower) protocols as [[black box]]es which provide a stable network connection across which to communicate. The transport layer and lower-level layers are unconcerned with the specifics of application layer protocols. Routers and [[network switch|switches]] do not typically examine the encapsulated traffic, rather they just provide a conduit for it. However, some [[Firewall (computing)|firewall]] and [[bandwidth throttling]] applications use [[deep packet inspection]] to interpret application data. An example is the [[Resource Reservation Protocol]] (RSVP).{{citation needed|date=May 2021}} It is also sometimes necessary for [[Network address translation#Applications affected by NAT|Applications affected by NAT]] to consider the application payload.