1. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Chapter 3 Transport Layer

2. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Understand 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: O BJECTIVES 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.

3. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 APPLICATION-TO-APPLICATIONDELIVERYAPPLICATION-TO-APPLICATIONDELIVERY 3.1

4. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-1 Transport layer in the Internet model The transport layer provides application-to-applocation Delivery in the Internet.

5. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-2 Application-to-application delivery The application program is aware of only one layer (the transport layer)involve in this delivery.

6. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 DUTIESDUTIES 3.2

7. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-3 Duties of the transport layer

8. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Packetizing : Packetizing devides a long message into smaller ones ; Packetizing devides a long message into smaller ones ; these smaller units are encapsulated into the data field of the these smaller units are encapsulated into the data field of the transport-layer packet and headers are added. transport-layer packet and headers are added. Creating connection : connection-oriented : connection-oriented : - A connection-oriented protocol first establishes a connection - A connection-oriented protocol first establishes a connection (a virtual path) between the sender and receiver. (a virtual path) between the sender and receiver. - The process is much like a telephone call. - The process is much like a telephone call. connectionless : connectionless : Connectionless delivery means that there is no virtual Connectionless delivery means that there is no virtual connection (no session) between two application programs. connection (no session) between two application programs.

9. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-4 Connection establishment (Three-way handshake)

10. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-5 Connection termination

11. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Connection is closely related to reliability: A connectionless protocol cannot be reliable because the relationship between packets provides reliability. Note:

12. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Addressing : Local Addresses : Local Addresses : The client needs the address of the remote computer, The client needs the address of the remote computer, which must be unique to distinguish it from all the computer which must be unique to distinguish it from all the computer in the world. in the world. Port Number : Port Number : - In the Internet model, the addresses at the transport layer - In the Internet model, the addresses at the transport layer are called port unmber. are called port unmber. - A port number is a number between 0 and 65,535. - A port number is a number between 0 and 65,535.

13. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-6 Application programs

14. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. Note:

15. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-7 Port numbers

16. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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 Temporary ports: 49,152 to 66,535 Registered ports: 1,024 to 49,151

17. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Business Focus: Well-Known Ports Some well-known port numbers are shown below: SMTP: 25 TFTP: 69 HTTP: 80 FTP: 20 and 21 TELNET: 23

18. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A client uses a temporary port number; a server uses a well- known port number. Note:

19. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Reliability : The service can be reliable or unreliable : Unreliable Service : Unreliable Service : - Some transport-layer protocols (such as UDP) are designed - Some transport-layer protocols (such as UDP) are designed for speed, not reliability. for speed, not reliability. - They do not guarantee reliability. - They do not guarantee reliability. - This unreliable service provide by the transport layer does - This unreliable service provide by the transport layer does not accept responsibility for the packets. not accept responsibility for the packets. Reliable Service : Reliable Service : - Some transport-layer protocols (such as TCP) are designed - Some transport-layer protocols (such as TCP) are designed for reliability. for reliability. - Reliability is sacrificed for quick service. - Reliability is sacrificed for quick service.

20. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 For reliable service, the transport layer needs to number packets belonging to a connection using sequence numbers. Note:

21. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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:

22. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-8 Damage control

23. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 INTERNETPROTOCOLSINTERNETPROTOCOLS 3.3

24. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 The Internet specifies two protocols for the transport layer : User Datagram protocol (UDP) User Datagram protocol (UDP) - UDP is a connectionless, unreliable transport protocol. - UDP is a connectionless, unreliable transport protocol. - If an error is detected, the packet is simply discarded with - If an error is detected, the packet is simply discarded with no notification to the sender. no notification to the sender. - UDP is used when an application needs quick action. - UDP is used when an application needs quick action. - For example, TFTP and SNMP. - For example, TFTP and SNMP. Transmission Control Protocol (TCP) Transmission Control Protocol (TCP) - UDP is a connection-oriented, reliable transport protocol. - UDP is a connection-oriented, reliable transport protocol. - It establishes a connection before data transfer and terminates - It establishes a connection before data transfer and terminates the connection after data transfer. the connection after data transfer. - It provides damage control, loss control, order control, and - It provides damage control, loss control, order control, and duplicate control. duplicate control. - For example, SMTP, HTTP, FTP, and TELNET. - For example, SMTP, HTTP, FTP, and TELNET.

25. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-9 UDP and TCP in the Internet model

26. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-10 User datagram

27. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: User Datagram The fields in a user datagram are as follows: Source port number Length Destination port number Checksum

28. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 3-11 Segment

29. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Segment The fields in a segment are as follows: Source port number Destination port number Sequence number Header length Control flags Checksum Option Urgent pointer