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TCP/Internet Protocol and OSI

This explains the layers in TCP/IP and the OSI model, and what the differences are.

The ISO/OSI protocol is a 7 layer reference model that was designed after TCP/IP model, which only has 4 layers. That said, the differences between the two are only minor.

The 7 layers of the OSI Protocol Stack are as follows:

7. Application: end user services such as email
6. Presentation: data problems and data compression
5. Session: authentication and authorization
4. Transport: guarantee end-to-end delivery of packets
3. Network: packet routing
2. Data Link: transmit and receive packets
1. Physical: The physical connection or cable itself.

Of these 7 layers, the 4 layers the TCP/IP Protocol Stack use are as follows:

5. Application: authentication, compression, and end user services
4. Transport: handles the flow of data between systems and provides access to the network for applications via the BSD socket library
3. Network: packet routing
2. Link: Kernel OS/device driver interface to the network interface on the computer.

The main differences between OSI and TCP/IP are:

  • The application layer in TCP/IP handles the responsibilities of layers 5,6, and 7 in the OSI model.
  • The transport layer in TCP/IP does not always guarantee reliable delivery of packets as the transport layer in the OSI model does.

So as you can see there is not too much difference between the models. In terms of software implementation TCP/IP looks like this:

  • Application Layer: some of the applications covered are SMTP (mail), Telnet, FTP, Rlogin, NFS, NIS, and LPD.
  • Transport Layer: the transport uses two protocols, UDP (User Datagram Protocol) and TCP, which does not guarantee packet delivery and applications which use this must provide their own means of verifying delivery. TCP guarantees delivery of packets to the applications that use it.
  • Network Layer: the network layer is concerned with packet routing and used low level protocols such as ICMP, IP, and IGMP.
  • Link Layer: the link layer is concerned with the actual transmittal of packets as well as IP to Ethernet address translation. This layer is concerned with Arp, the device driver and Rarp.

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