Models OSI and TCP have a lot in common. Both models are based on the concept of a stack of independent protocols. Functionality levels similar. For example, in both models the levels, since transport and above, provide cross-cutting, independent of the network transport service to processes, those wishing to exchange information. These levels form a transport provider. Also in each of the model levels above transport are application users of transport services.

Despite this fundamental similarity, these models have some differences.

For model OSI are three Central concepts:

1. Service.

2. Interfaces.

3. Protocols.

 Probably, the biggest contribution of the OSI model became the clear separation of these three concepts.

Initially in the TCP/IP model was not a clear separation between services, interface and Protocol, although he made attempts to change it, to make it more similar to the OSI model. So, for example, the only real services, provide firewall level, are SEND IP PACKET (to send an IP packet) and RECEIVE IP PACKET (to receive the IP packet).

As a result, in the OSI model protocols are hidden better, than the TCP/IP model, and if the technology changes they can be relatively easily replaced. The ability to conduct such changes - one of the main goals of layered protocols.

OSI reference model was devised before, what were the protocols invented for her. This sequence of events means, this model was not configured to a specific set of protocols, what made it universal. The reverse side of this order of actions was, that the developers had little experience in this area and no clear idea of, what functions should be performed by each level.

For example, the level of data transfer initially worked only in networks with transfer from node to node. With the advent of broadcast networks in the model was required to enter a new sublevel. When based on the OSI model began to build a real network using existing protocols, found, they do not meet the required specifications of services. Therefore, in the model it was necessary to add sub-levels to solve the inconsistency. Finally, initially expected, each country will have one network, managed by the government and using the OSI protocols, so no one thought about the Association of various networks. In fact it was not so.

With the model TCP/IP was the exact opposite: first there were protocols, and then the model was created, describing the existing protocols. Thus no problems with line protocols model. They match perfectly. The only problem was, the model did not fit any other Protocol stacks. As a result, she was not used to describe any other networks, than TCP/IP.

If you look at these two models closer, first of all, pay attention to the difference in the number of levels: in the OSI model eight levels, in the model TCP/IP four. In both models there is a firewall, the transport and application levels, and the rest of the levels.

Another difference between the models lies in the sphere of the possibility of using communication connection-oriented communication and connectionless. The OSI model at the network layer supports both connection, and at the transport level - only a relationship based on the connection (since the transport service are visible to the user). In the model TCP/IP at the network layer there is only one mode of communication (connectionless), but at the transport level, it supports both modes, by giving users a choice. This is especially important for simple protocols "request — response".