What is the difference between TCP/IP model vs. OSI model?

What is the OSI model?

The Open Systems Interconnection (OSI) model is just that -- a model. It describes how the different tasks involved in network communications can be divided across distinct layers of software and hardware components. The International Organization for Standardization (ISO) created the OSI model 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 include the physical interconnection of nodes 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.

OSI model layers

In the idealized world of the OSI model, a networked application, such as a web browser, would talk to the top layer of the network stack to get data from a web server somewhere. Then, the following processes occur:

• Layer 7 uses Layer 6, the presentation layer, to send data for its request.
• Layer 6 compresses, encrypts and manipulates the data and passes it on to Layer 5, the session layer.
• Layer 5 manages the communication session with the other system, making sure the system stays up while it's needed and is dropped when it is not. Layer 5 reaches the other system using Layer 4.
• Layer 4, the transport layer, knows how to send data across the network(s) connecting the two systems via Layer 3. Layer 4 breaks the data up into packets for transmission and watches for errors and lost packets. In a reliable network protocol, Layer 4 also ensures packets get delivered, retransmitting as needed.
• Layer 3, the network layer, adds addressing and routing information as needed to communicate with systems on other networks. It uses Layer 2 to send data to another system.
• Layer 2, the data link layer, puts data onto the local network -- via Layer 1 -- to travel to other devices on the local network, including devices like routers that will send the data to other networks. Ethernet can be considered a Layer 2 network protocol, and MAC addresses are referred to as Layer 2 addresses.
• Layer 1, the physical layer, comprises the network interfaces that send bits across a medium -- like copper, fiber, light, microwaves or radio -- and those media.

No actual version of the OSI stack is used in the world.

On the other end of the connection, data makes it to the web server via the same layers in reverse order, entering at Layer 1, rising layer by layer through the stack and reaching the actual web server application via Layer 7.

This is all theoretical, of course, because no actual version of the OSI stack is used in the world. The OSI model is useful mainly as a way of understanding the different types of work that must happen in successful network communications, and as a common frame of reference for discussions about network issues and functions.

TCP/IP model layers

Broadly speaking, TCP and IP are the only layers in the TCP/IP model. TCP is transport control, and it's responsible for setting up network communications sessions using IP, the internetworking layer, to talk to those other systems.

Mapping TCP/IP to the OSI model

The model embodied in the TCP/IP stack doesn't map easily to the OSI model. TCP/IP was created before the OSI model to solve a specific set of problems in actual working networked devices, and has since evolved to address new problems in working systems. The OSI model was created in the 1980s as a design exercise and explanatory reference model.

Further, because TCP/IP is an actual, implemented set of protocols that originated before the OSI model, and because the OSI model is only a model -- neither a set of protocols nor a suite of functioning software tools -- mapping from one model to the other isn't clean.

TCP/IP is not intended to function as a general description for all network communications, so it doesn't cover all the functions the OSI model does -- nor does it divide functionality as finely or broadly.

Key similarities between TCP/IP vs. OSI model

IP controls networking and corresponds to a subset of functions associated with OSI Layer 3, the network layer.

TCP corresponds to OSI Layer 4, the transport layer, but it also covers some functions of Layer 5, the OSI session layer. OSI Layer 4 ensures data delivery from one node to another by 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.

TCP also handles Layer 5 tasks such as setting up and terminating connections.

Key differences between TCP/IP vs. OSI model

IP describes only the protocol used for the internet. Other non-internet protocols, such as the Datagram Delivery Protocol (DDP) or the Internetwork Packet Exchange (IPX), also correspond to OSI Layer 3.

Although TCP does some OSI Layer 5 work, it doesn't do all Layer 5 tasks, like authentication and authorization, for example.

More fundamentally, TCP/IP makes no assumptions about what happens above the level of a network session -- the piece of OSI Layer 5 that TCP controls. 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. 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 network layer.

Similarly, TCP/IP does not dictate what happens below the network (IP) layer. IP assumes the existence of a network access layer, but it doesn't define it. In contrast, the OSI model breaks its functions into two further defined layers: the data link layer and the physical layer.

Additional key internet protocols

Many other key internet protocols are built on top of TCP. These include the Hypertext Transfer Protocol, which is the basic protocol of the web, and the Simple Mail Transfer Protocol, the core email transfer protocol.

The User Datagram Protocol (UDP) is a companion to TCP. Built atop IP, UDP sacrifices the guarantees of reliable delivery that TCP makes in return for faster communications. UDP underlies many streaming media applications.