Chapter 3 TransportLayer
OBJECTIVES 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. Know which application layer program can use UDP and which can use TCP. Distinguish between the two transport-layer protocols used in the Internet: UDP and TCP. Understand the position of the transport layer in the Internet model. After reading this chapter, the reader should be able to:
3.1 APPLICATION-TO- APPLICATION DELIVERY
Figure 3-1 Transport layer in the Internet model
Figure 3-2 Application-to-application delivery
Figure 3-3 Duties of the transport layer
Figure 3-4 Connection establishment
Figure 3-5 Connection termination
Note: Connection is closely related to reliability: A connectionless protocol cannot be reliable because the relationship between packets provides reliability.
Figure 3-6 Application programs
Note: The 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.
Figure 3-7 Port numbers
Technical Focus:Range of Port Numbers The port numbers range from 0 to 65535 and are divided into three ranges: Well-known ports: 0 to 1023 Registered ports: 1,024 to 49,151 Temporary ports: 49,152 to 66,535
SMTP: 25 TFTP: 69 HTTP: 80 FTP: 20 and 21 TELNET: 23 Business Focus:Well-Known Ports Some well-known port numbers are shown below:
Note: A client uses a temporary port number; a server uses a well-known port number.
Figure 3-8 Damage control
Note: For 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.
3.3 INTERNET PROTOCOLS
Figure 3-9 UDP and TCP in the Internet model
Figure 3-10 User datagram
Technical Focus:User Datagram The fields in a user datagram are as follows: Source port number Destination port number Length Checksum
Figure 3-11 Segment
Source port number Destination port number Sequence number Header length Control flags Checksum Option Urgent pointer Technical Focus:Segment The fields in a segment are as follows: