Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

20.1 TCP/IP PROTOCOL SUITE

The TCP/IP protocol suite was developed as part of the United States Department of Defense's project ARP Anet (Advanced Research Projects Agency Network), but the standards are publicly available. Due to fast spread of the Internet, the TCP/IP protocol suite has a very large installation base. The TCP/IP software is now an integral part of most operating systems, including Unix, Linux, Windows. The TCP/IP stack is also being embedded into systems running real-time operating systems such as VxWorks, RTLinux, and OS/9 and handheld operating systems such as Embedded XP, Palm OS, Symbian OS and so on.

The TCP/IP protocol suite is depicted in Figure 20.1. It consists of 5 layers:

• Physical layer

• Datalink layer (also referred to as the network layer)

• Internet Protocol (IP) layer

• Transport layer (TCP layer and UDP layer)

• Application layer

Figure 20.1: TCP/IP protocol suite.

The TCP/IP protocol suite consists of five layers: physical layer, datalink layer (also referred to as network layer), Internet Protocol (IP) layer, Transmission Control Protocol (TCP) layer, and application layer.

Physical layer: This layer defines the characteristics of the transmission such as data rate and signal-encoding scheme.

Datalink layer: This layer defines the logical interface between the end system and the subnetwork.

Internet Protocol (IP) layer: This layer routes the data from source to destination through routers. Addressing is an integral part of the routing mechanism. IP provides an unreliable service: the packets may be lost, arrive out of order, or have variable delay. The IP layer runs on every end system and every router.

Transport layer: This layer is also called the host-to-host layer because it provides end-to-end data transfer service between two hosts (or end systems) connected to the Internet. Because the IP layer does not provide a reliable service, it is the responsibility of the transport layer to incorporate reliability through acknowledgments, retransmissions, and so on. The transport layer software runs on every end system.

For applications that require reliable data transfer (such as most data applications), a connection-oriented transport protocol called Transmission Control Protocol (TCP) is defined. For connectionless service, user datagram protocol (UDP) is defined. Applications such as network management, that do not need very reliable packet transfer use the UDP layer.

Application layer: This layer differs from application to application. Two processes on two end systems communicate with the application layer as the interface.

As in OSI architecture, peer-to-peer communication applies to the TCP/IP architecture as well. The application process (such as for transferring a file) generates an application byte stream, which is divided into TCP segments and sent to the IP layer. The TCP segment is encapsulated in the IP datagram and sent to the datalink layer. The IP datagram is encapsulated in the datalink layer frame. Since datalink layer can be subdivided into LLC layer and MAC layer, IP datagram is encapsulated in the LLC layer and then passed on to the MAC layer. The MAC frame is sent over the physical medium. At the destination, each layer strips off the header, does the necessary processing based on the information in the header, and passes the remaining portion of the data to the higher layer. This mechanism for protocol encapsulation is depicted in Figure 20.2.

Figure 20.2: Protocol encapsulation in TCP/IP.

The complete TCP/IP protocol stack is shown in Figure 20.3 indicating various application layer protocols. The various application layer protocols are

• Simple Mail Transfer Protocol (SMTP), for electronic mail containing ASCII text.

• Multimedia Internet Mail Extension (MIME), for electronic mail with multimedia content.

• File Transfer Protocol (FTP) for file transfer.

• Telnet for remote login.

• Hypertext Transfer Protocol (HTTP) for World Wide Web service.

Figure 20.3: TCP/IP protocol stack.

In the TCP/IP protocol stack, the various application layer protocols are SMTP for e-mail, FTP for file transfer, Telnet for remote login, and HTTP for World Wide Web service.

In addition, the following protocols are also depicted:

• Border gateway protocol (BGP), a routing protocol to exchange routing information between routers. Exterior gateway protocol (EGP) is another routing protocol.

• Internet Control Message Protocol (ICMP), which is at the same level as IP but uses IP service.

• Simple Network Management Protocol (SNMP) for network management. Note that SNMP uses UDP and not TCP.