Editing Internet protocol suite (section)

==Comparison of TCP/IP and OSI layering==
{{See also|OSI model#Comparison with TCP/IP model}}
The three top layers in the OSI model, i.e. the application layer, the presentation layer and the session layer, are not distinguished separately in the TCP/IP model which only has an application layer above the transport layer. While some pure OSI protocol applications, such as [[X.400]], also combined them, there is no requirement that a TCP/IP protocol stack must impose monolithic architecture above the transport layer. For example, the NFS application protocol runs over the [[External Data Representation]] (XDR) presentation protocol, which, in turn, runs over a protocol called [[Remote Procedure Call]] (RPC). RPC provides reliable record transmission, so it can safely use the best-effort UDP transport.

Different authors have interpreted the TCP/IP model differently, and disagree whether the link layer, or any aspect of the TCP/IP model, covers OSI layer 1 ([[physical layer]]) issues, or whether TCP/IP assumes a hardware layer exists below the link layer. Several authors have attempted to incorporate the OSI model's layers 1 and 2 into the TCP/IP model since these are commonly referred to in modern standards (for example, by [[IEEE]] and [[ITU]]). This often results in a model with five layers, where the link layer or network access layer is split into the OSI model's layers 1 and 2.{{cn|date=September 2024}}

The IETF protocol development effort is not concerned with strict layering. Some of its protocols may not fit cleanly into the OSI model, although RFCs sometimes refer to it and often use the old OSI layer numbers. The IETF has repeatedly stated<ref name="introduction-to-the-ietf" />{{failed verification|date=April 2024}} that Internet Protocol and architecture development is not intended to be OSI-compliant. RFC 3439, referring to the internet architecture, contains a section entitled: "Layering Considered Harmful".{{ref RFC|3439}}

For example, the session and presentation layers of the OSI suite are considered to be included in the application layer of the TCP/IP suite. The functionality of the session layer can be found in protocols like [[HTTP]] and [[SMTP]] and is more evident in protocols like [[Telnet]] and the [[Session Initiation Protocol]] (SIP). Session-layer functionality is also realized with the port numbering of the TCP and UDP protocols, which are included in the transport layer of the TCP/IP suite. Functions of the presentation layer are realized in the TCP/IP applications with the [[MIME]] standard in data exchange.

Another difference is in the treatment of [[routing protocol]]s. The OSI routing protocol [[IS-IS]] belongs to the network layer, and does not depend on [[CLNS]] for delivering packets from one router to another, but defines its own layer-3 encapsulation. In contrast, [[OSPF]], [[Routing Information Protocol|RIP]], [[BGP]] and other routing protocols defined by the IETF are transported over IP, and, for the purpose of sending and receiving routing protocol packets, routers act as hosts. As a consequence, routing protocols are included in the application layer.{{Ref RFC|1812}} Some authors, such as Tanenbaum in ''Computer Networks'', describe routing protocols in the same layer as IP, reasoning that routing protocols inform decisions made by the forwarding process of routers.

IETF protocols can be encapsulated recursively, as demonstrated by tunnelling protocols such as [[Generic Routing Encapsulation]] (GRE). GRE uses the same mechanism that OSI uses for tunnelling at the network layer.