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However, hardware or software implementation is not stated in the protocols or the layered model. High-performance routers are to a large extent based on fast non-programmable digital electronics, carrying out link level switching.
==OSI and TCP/IP layering differences==
The three top layers in the OSI model—the application layer, the [[presentation layer]] and the [[session layer]]—are not distinguished separately in the TCP/IP model where it is just the application 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|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 run safely over the best-effort UDP transport.
The session layer roughly corresponds to the Telnet [[virtual terminal]] functionality{{Citation needed|date=June 2009}}, which is part of text based protocols such as the HTTP and [[SMTP]] TCP/IP model application layer protocols. It also corresponds to TCP and UDP port numbering, which is considered as part of the transport layer in the TCP/IP model. Some functions that would have been performed by an OSI presentation layer are realized at the Internet application layer using the [[MIME]] standard, which is used in application layer protocols such as HTTP and SMTP.
The IETF protocol development effort is not concerned with strict layering, and some of its protocols may not fit cleanly into the OSI model. These conflicts, however, are apparent also in the original OSI model, ISO 7498, when not considering the annexes to this model (e.g., ISO 7498/4 Management Framework), or the ISO 8648 Internal Organization of the Network layer (IONL). When the IONL and Management Framework documents are considered, the ICMP and IGMP are neatly defined as layer management protocols for the network layer. In like manner, the IONL provides a structure for "subnetwork dependent convergence facilities" such as [[Address Resolution Protocol|ARP]] and [[Reverse Address Resolution Protocol|RARP]].
IETF protocols can be encapsulated recursively, as demonstrated by tunneling protocols such as [[Generic Routing Encapsulation]] (GRE). GRE uses the same mechanism that OSI uses for tunneling at the network layer.
==Layer names and number of layers in the literature==
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