What is the difference between the TCP/IP model and the OSI model?
The Open Systems Interconnection, or OSI, model is a standard reference model created by the International Organization for Standardization to describe how the different software and hardware components involved in a network communication should divide labor and interact with one another. It defines a seven-layer set of functional elements, ranging from the physical interconnections at Layer 1 -- also known as the physical layer, or PHY interface -- all the way up to Layer 7, the application layer.
Separately from the OSI model, TCP and IP are two of the network standards that define the internet. IP defines how computers can get data to each other over a routed, interconnected set of networks. TCP defines how applications can create reliable channels of communication across a network. IP basically defines addressing and routing, while TCP defines how to have a conversation across the link without garbling or losing data. TCP/IP grew out of research by the U.S. Department of Defense and is based on a loose, rather than strict, approach to network layering.
How the OSI model and TCP/IP model differ
The TCP/IP model doesn't map cleanly to the OSI model because it was developed in the 1970s, before the OSI model was created in the 1980s. TCP/IP was designed to solve a specific set of problems, not to function as a general description for all network communications.
- IP corresponds to a subset of OSI Layer 3, the network layer.
- TCP corresponds to OSI Layer 4 (transport) and some functions of Layer 5 (session).
- TCP/IP makes no assumptions about what happens above the level of a network session -- part of OSI Layer 5 -- while OSI defines several more layers of standardized functions.
- TCP/IP makes no prescriptions as to the link layers below IP, while OSI specifies two layers.
- If an application needs functions not found in TCP/IP, the application has to supply them. In the OSI model, it is assumed an application will never implement any functionality belonging in any defined layer, and because interfaces between layers abstract many details, it may not be able to anyway.
OSI vs. TCP/IP model
Additional key internet protocols
Many other key internet protocols -- including HTTP, which is the basic protocol of the web; and the Simple Mail Transfer Protocol, the core email transfer protocol -- are built on top of TCP. The User Datagram Protocol, a companion to TCP, sacrifices the guarantees of reliability that TCP makes in return for faster communications.
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