OSI and TCP/IP Layering Differences

From Wikipedia
Jakub Dziwior

OSI and TCP/IP Layering Differences

The three top layers in the OSI model - the application layer, the presentation layer and the session layer - usually are lumped into one layer in the TCP/IP model. While some pure OSI protocol applications, such as X.400, also lumped them together, there is no requirement that a TCP/IP protocol stack needs to be monolithic above the transport layer. For example, the Network File System (NFS) application protocol runs over the eXternal Data Representation (XDR) presentation protocol, which, in turn, runs over a protocol with session layer functionality, Remote Procedure Call (RPC). RPC provides reliable record transmission, so it can run safely over the best-effort User Datagram Protocol (UDP) transport.

The session layer roughly corresponds to the Telnet virtual terminal functionality, which is part of text based protocols such as 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. The presentation layer has similarities to the MIME standard, which also is used in HTTP and SMTP.





Since the IETF protocol development effort is not concerned with strict layering, some of its protocols may not appear to fit cleanly into the OSI model. These conflicts, however, are more frequent when one only looks at the original OSI model, ISO 7498, without looking at 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 ARP and RARP.

IETF protocols can be applied recursively, as demonstrated by tunneling protocols such as Generic Routing Encapsulation (GRE). While basic OSI documents do not consider tunneling, there is some concept of tunneling in yet another extension to the OSI architecture, specifically the transport layer gateways within the International Standardized Profile framework [8]. The associated OSI development effort, however, has been abandoned given the real-world adoption of TCP/IP protocols.