What is the difference between the OSI model and TCP?
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The Open Systems Interconnection (OSI) model is a standard "reference model" created by the International Organization for Standardization (ISO) 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.
The Transmission Control Protocol (TCP) and the Internet Protocol (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 such a network. Basically, IP 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. Dept. of Defense and is based on a loose rather than a strict approach to layering. Many other key Internet protocols, such as the Hypertext Transfer Protocol (HTTP), the basic protocol of the Web, and the Simple Mail Transfer Protocol (SMTP), the core email transfer protocol, are built on top of TCP. The User Datagram Protocol (UDP), a companion to TCP, sacrifices the guarantees of reliability that TCP makes in return for faster communications.
TCP/IP doesn't map cleanly to the OSI model, since it was developed before the OSI model and was designed to solve a specific set of problems, not to be a general description for all network communications.
OSI versus TCP/IP Layers
The relationships and differences between the OSI model and TCP/IP are:
- 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, where OSI specifies two.
- Where an application needs functions not found in TCP/IP, the application has to supply them. In the OSI model, it is assumed that an application will never implement any functionally belonging in any defined layer, and because interfaces between layers abstract many details, it may not be able to anyway.
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By making it possible to implement arbitrary, complex behaviors at any port in a network, does SDN make the OSI model more or less useful to your understanding of networks and networked applications?
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