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An early pair of architectural documents, {{IETF RFC|1122}} and {{IETF RFC|1123|plainlink=yes}}, titled ''Requirements for Internet Hosts'', emphasizes architectural principles over layering.{{Ref RFC|1958}} RFC 1122/23 are structured in sections referring to layers, but the documents refer to many other architectural principles, and do not emphasize layering. They loosely defines a four-layer model, with the layers having names, not numbers, as follows:{{ref RFC|1122}}{{ref RFC|1123}}
* The [[application layer]] is the scope within which applications, or [[Process (computing)|processes]], create user data and communicate this data to other applications on another or the same host. The applications make use of the services provided by the underlying lower layers, especially the transport layer which provides [[Reliability (computer networking)|reliable or unreliable]] ''pipes'' to other processes. The communications partners are characterized by the application architecture, such as the [[client–server model]] and [[peer-to-peer]] networking. This is the layer in which all application protocols, such as SMTP, FTP, SSH, and HTTP, operate. Processes are addressed via ports which essentially represent [[Service (systems architecture)|services]].
* The [[transport layer]] performs host-to-host communications on either the local network or remote networks separated by routers.<ref name="AoJD3">{{cite book |last=Hunt |first=Craig |date=2002 |title=TCP/IP Network Administration |edition=3rd |publisher=O'Reilly |pages=9–10 |isbn=9781449390785}}</ref> It provides a channel for the communication needs of applications. The [[User Datagram Protocol]] (UDP) is the most basic{{Citation needed|date=June 2025|reason=Which source states UDP is the most basic transport layer protocol ? I refer to Jim Kurose (2012), Computer Networking: A Top-Down Approach, 7th Edition, Chapter 3: Transport layer, Section 3.1 Introduction and Transport-layer services), which states that both TCP and UDP are two best known transport layer protocols, which made up many application layer protocols like HTTP, IMAP, POP3, and SMTP. So what makes UDP is the "most basic", compared to TCP ?}} transport layer protocol, providing an unreliable [[connectionless]] datagram service. The [[Transmission Control Protocol]] (TCP) provides flow-control, connection establishment, and reliable transmission of data.
* The [[internet layer]] exchanges datagrams across network boundaries. It provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. It is therefore also the layer that establishes internetworking. Indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines [[IP address]]es.<ref>{{Cite journal |last=Guttman |first=E. |date=1999 |title=Service ___location protocol: automatic discovery of IP network services |url=http://dx.doi.org/10.1109/4236.780963 |journal=IEEE Internet Computing |volume=3 |issue=4 |pages=71–80 |doi=10.1109/4236.780963 |issn=1089-7801|url-access=subscription }}</ref>{{failed verification|date=April 2024}}<ref name=kz>{{Cite journal |last=Zheng |first=Kai |date=July 2017 |title=Enabling "Protocol Routing": Revisiting Transport Layer Protocol Design in Internet Communications |url=http://dx.doi.org/10.1109/mic.2017.4180845 |journal=IEEE Internet Computing |volume=21 |issue=6 |pages=52–57 |doi=10.1109/mic.2017.4180845 |issn=1089-7801|url-access=subscription }}</ref> Its function in routing is to transport datagrams to the next host, functioning as an IP router, that has the connectivity to a network closer to the final data destination.<ref name=kz/>{{failed verification|date=April 2024}}
* The [[link layer]] defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer includes the protocols used to describe the local network topology and the interfaces needed to effect the transmission of internet layer datagrams to next-neighbor hosts.<ref>{{Cite journal |last=Huang |first=Jing-lian |date=2009-04-07 |title=Cross layer link adaptation scheme in wireless local area network |url=http://dx.doi.org/10.3724/sp.j.1087.2009.00518 |journal=Journal of Computer Applications |volume=29 |issue=2 |pages=518–520 |doi=10.3724/sp.j.1087.2009.00518 |doi-broken-date=November 1, 2024 |issn=1001-9081|url-access=subscription }}</ref>
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