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3.1 The TCP/IP Architecture

OSI and TCP/IP

As illustrated in the figure, the TCP/IP architecture consists of four layers, each of which can have several sublayers. These layers correlate roughly to layers in the OSI reference model and define similar functions. Some of the TCP/IP layers correspond directly to layers in the OSI reference model while others span several OSI layers. The four TCP/IP layers are:

• The TCP/IP Application Layer refers to communications services to applications and is the interface between the network and the application. It is also responsible for presentation and controlling communication sessions. It spans the Application Layer, Presentation Layer and Session Layer of the OSI reference model. Examples include: HTTP, POP3, and SNMP.
• The TCP/IP Transport Layer defines several functions, including the choice of protocols, error recovery and flow control. The transport layer may provide for retransmission,
• error recovery, and may use flow control to prevent unnecessary congestion by attempting to send data at a rate that the network can accommodate, or it might not, depending on the choice of protocols. Multiplexing of incoming data for different flows to applications on the same host is also performed. Reordering of the incoming data stream when packets arrive out of order is included. It correlates with the Transport Layer of the OSI reference model. Examples include: TCP and UDP, which are called Transport Layer, or Layer 4, protocols. TCP provides connection oriented service while UDP provides connectionless service in the Transport Layer.
• The TCP/IP Internetwork Layer, or Internet Layer, defines end-to-end delivery of packets and defines logical addressing to accomplish this. It also defines how routing works and how routes are learned; and how to fragment a packet into smaller packets to accommodate media with smaller maximum transmission unit sizes. It correlates with the Network Layer of the OSI reference model. However, while the OSI network-layer protocols provide connection-oriented or Connectionless Network Service (CLNS), IP provides only connectionless network service. The routing protocols are network layer protocols with an IP protocol number. One exception is the Border Gateway Protocol (BGP), which uses a TCP port number. Another is Intermediate System-to-Intermediate System (IS-IS), which resides over the data-link layer.
• The TCP/IP Network Interface Layer is concerned with the physical characteristics of the transmission medium as well as getting data across one particular link or medium. This layer defines delivery across an individual link as well as the physical layer specifications. It spans the Data Link Layer and Physical Layer of the OSI reference model. Examples include: Ethernet and Frame Relay.