Transport Layer

Chapter 3TransportLayerUnderstand the position of the transport layer in the Internet model.Understand the rationale for the existence of the transport layer.Understand the concept of application-to-application delivery.Understand the duties of the transport layer: packetizing,addressing, connection creation, and reliable delivery.After reading this chapter, the reader should be able to:OBJECTIVESKnow which application layer program can use UDP and whichcan use TCP. Distinguish between the two transport-layer protocols usedin the Internet: UDP and TCP.APPLICATION-TO-APPLICATIONDELIVERY3.1Figure 3-1Transport layer in the Internet modelFigure 3-2Application-to-application deliveryDUTIES3.2Figure 3-3Duties of the transport layerFigure 3-4Connection establishmentFigure 3-5Connection terminationConnection is closely related to reliability: A connectionless protocol cannot be reliable because the relationship between packets provides reliability. Note:Figure 3-6Application programsThe addresses of client and server programs are defined at the transport layer. These addresses are local to the computer running the programs. The addresses must be unique locally but not universally. Note:Figure 3-7Port numbersTechnical Focus: Range of Port NumbersThe port numbers range from 0 to 65535 and are divided into three ranges:Well-known ports: 0 to 1023Temporary ports: 49,152 to 66,535Registered ports: 1,024 to 49,151Business Focus: Well-Known PortsSome well-known port numbers are shown below:SMTP: 25TFTP: 69HTTP: 80FTP: 20 and 21TELNET: 23A client uses a temporary port number; a server uses a well-known port number. Note:Figure 3-8Damage controlFor reliable service, the transport layer needs to number packets belonging to a connection using sequence numbers.Note:A reliable transport protocol provides damage control, loss control, order control, and duplicate control even if the underlying networking technology and lower-level protocols are not reliable.This is done through sequence numbers, timers, error detection, and retransmission.Note:INTERNETPROTOCOLS3.3Figure 3-9UDP and TCP in the Internet modelFigure 3-10User datagramTechnical Focus: User DatagramThe fields in a user datagram are as follows:Source port numberLengthDestination port numberChecksumFigure 3-11SegmentTechnical Focus: SegmentThe fields in a segment are as follows:Source port numberDestination port numberSequence numberHeader lengthControl flagsChecksumOptionUrgent pointer