What is the difference between the TCP/IP model and the OSI model?
TCP and IP are two of the network standards that make the internet possible. The Internet Protocol (IP) defines how computers can get data to each other over an interconnected set of networks. The Transmission Control Protocol (TCP) defines how applications can create reliable channels of communication across an IP network. IP basically defines addressing and routing, while TCP defines how to have a conversation across an IP-mediated link without garbling or losing data. TCP/IP grew out of U.S. Department of Defense networking research.
The Open Systems Interconnection (OSI) model describes how the different software and hardware components involved in a network communication should divide their labor and otherwise interact. It was created by the International Organization for Standardization (ISO) to encourage network vendors and developers to create interoperable and interchangeable systems. (It is defined in the ISO/IEC 7498-1 standard.)
The OSI model defines a network as a set of seven functional elements, or layers of service,. These layers range from physical interconnection of nodes (e.g., via network interface cards or Bluetooth radio interface) at Layer 1 -- also known as the physical layer -- all the way up to Layer 7, called the application layer. Ideally, a component at any given layer provides services to the layer above it, consumes services from the layer below it, and never reaches directly for any other layers or provides functions belonging in them.
Mapping TCP/IP to the OSI model: Similarities and differences
The TCP/IP model doesn't map cleanly to the OSI model. It was developed in the 1970s to solve a specific set of problems, while the OSI model was created in the 1980s. TCP/IP is not intended to function as a general description for all network communications, so it does not cover all the functions the OSI model does, nor does it divide functionality as finely or as broadly.
- IP corresponds to a subset of OSI Layer 3, the network layer.
IP describes only the protocol used for the internet, while OSI Layer 3 also encompasses non-internet protocols such as the Datagram Delivery Protocol (DDP) or the Internetwork Packet Exchange (IPX).
OSI Layer 4, the transport layer, ensures delivery of data from one node to another by doing things like assigning sequential numbers to packets, checking to make sure all sent packets arrive and retransmitting lost or damaged packets. TCP is responsible for these functions as well.
OSI Layer 5, the session layer, sets up and terminates connections between nodes, as does TCP, but also handles authentication and authorization, which TCP does not.
- TCP/IP makes no assumptions about what happens above the level of a network session -- part of OSI Layer 5.
TCP/IP assigns all higher aspects of network use to applications while the OSI model defines two more layers of standardized functions: Layer 6, the presentation layer, and Layer 7, the application layer. TCP/IP applications essentially talk to Layer 4, while OSI applications talk to Layer 7.
If an application needs functions not found in TCP/IP, the application has to supply them.
The OSI model assumes an application will never implement any functionality belonging in any defined layer.
- Under TCP/IP, there is one layer below IP, the Network Access layer; the OSI model breaks its functions into two layers: the data link layer (2) and the physical layer (1).
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, and underlies many streaming media applications.
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