TCP/IP Model

OSI Model Chart
OSI Model Description

TCP/IP Layers

The TCP/IP Model is easier to understand than the OSI model for the following reason:
  • Most protocols the reader is already familiar with a probably TCP/IP protocols, making examples easier.
  • Not all of the TCP/IP protocols fit the OSI model as well as the TCP/IP model
  • The TCP/IP model has less layers (4) than the OSI model (7)

Note that the TCP/IP Model predates the OSI model.

Application Layer
The top layer in the TCP/IP protocol suite is the Application Layer. Applications communicate with the Application Layer, which in turn communicates  with the Transport Layer.

Some application layer TCP/IP protocols are:
  • DNS
  • DHCP (Dynamic Host Configuration Protocol)
  • FTP (File Transfer Protocol)
  • HTTP
  • LDAP
  • NFS (Network File System)
  • NTP (Network Time Protocol)
  • SNMP (Simple Network Management Protocol)

Transport Layer
The second layer in the TCP/IP protocol suite is the Transport Layer. The transport layer protocols are designed to provide the following:
An interface to specific applications running on another computer
A mechanism for multiplexing/demultiplexing, allowing computers to simultaneously support multiple network applications. Also allows for a networked application to support to simultaneously maintain connections with more than one computer.
Error checking, flow control, and verification: to ensure delivery of data between computers. A Transport Layer protocol might provide extensive error control and flow control; TCP being the best example. Or, a Transport Layer protocol might maximize performance by minimizing error control and flow control; UDP being the best example.

Internet Layer
The third layer in the TCP/IP protocol suite is the Internet Layer. Two important protocols in the Internet Layer are IP and ARP. The IP protocol maintains a logical, hierarchical addressing scheme. These addresses are called IP addresses. ARP (Address Resolution Protocol) and RARP (Reverse ARP) create a table that maps IP addresses to physical addresses.

Network Access Layer
The fourth/bottom layer in the TCP/IP protocol suite is the Network Access Layer. The Network Access Layer converts the data to be consistent with the specifications of the physical network. For example, the Network Access layer may prepare the data for transmission through the hardware of the network adaptor card.

When Ethernet is used, the Network Access Layer:
  • Breaks the data into smaller chunks
  • Packages data into Ethernet frames

The hardware address (mac address) is part of the Network Access Layer.

Network traffic will follow this path on a TCP/IP network:
  1. From the application to a protocol in the Application Layer, such as HTTP
  2. The Application Layer protocol will use a Transport Layer protocol, such as TCP
  3. The Transport Layer protocol will use an Internet Layer protocol (almost certainly IP in conjunction with one or more routing protocols) to send the data to the right network segment
  4. Once the data is on the right network segment, Network Access protocols send the data to the correct network interface.

The company SolarFlare creates high-performance network interface cards. They may improve software used in or by the Network Access Layer, but they would not need to make changes to any Internet Layer protocols.

Virtual networks are based on improvements in protocols that function within the Internet Layer. Developers can improve this technology without being concerned about network interface cards or the Application Layer protocols.

OSI Model Chart
OSI Model Description