1. TCP/IP Protocol Suite 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 4 Introduction to Network Layer (Prepared by Yu-Chee Tseng, CS/NCTU)

2. OBJECTIVES: TCP/IP Protocol Suite 2  To introduce packet switching as the mechanism of data delivery in the network layer.  Two types of services of a packet-switch network: 1.connectionless service: routers forward packets in a connectionless packet-switch network using the destination address of the packet and a routing table. 2.connection-oriented service: routers forward packets in a connection-oriented packet-switch network using the label on the packet and a routing table.  To discuss issues or services that are not directly provided in the network layer protocol, but are sometimes provided by some auxiliary protocols or some protocols added later to the Internet.

3. TCP/IP Protocol Suite 3 4-1 INTRODUCTION At the conceptual level, we can think of the global Internet as a black box network that connects millions (if not billions) of computers in the world together. Figure 4.1 Internet as a block box

4. Let’s Decompose the Black Box TCP/IP Protocol Suite 4 Figure 4.2 Internet as a combination of LANs and WANs connected together

5. TCP/IP Protocol Suite 5 4-2 SWITCHING (1)When a message reaches a connecting device, a decision needs to be made to select one of the output ports through which the packet needs to be send out. In other words, the connecting device acts as a switch that connects one port to another port. (2)Two techniques: Circuit switching Packet switching

6. TCP/IP Protocol Suite 6 In circuit switching, the whole message is sent from the source to the destination without being divided into packets. Note In packet switching, the message is first divided into manageable packets at the source before being transmitted. The packets are assembled at the destination.

7. TCP/IP Protocol Suite 7 (1) A good example of a circuit-switched network is the early telephone systems in which the path was established between a caller and a callee when the telephone number of the callee was dialed by the caller. (2) When the callee responded to the call, the circuit was established. (3)The voice message could now flow between the two parties, in both directions, while all of the connecting devices maintained the circuit. (4)When the caller or callee hung up, the circuit was disconnected. (5)(The telephone network is not totally a circuit-switched network today.) Example Example 4.1: Circuit-Switched Network

8. TCP/IP Protocol Suite 8 4-3 PACKET SWITHING (1)The network layer is designed as a packet-switched network. This means that the packet at the source is divided into manageable packets, normally called datagrams. (2)Individual datagrams are then transferred from the source to the destination. (3)The received datagrams are assembled at the destination before recreating the original message. (4)The packet-switched network layer of the Internet was originally designed as a connectionless service, but recently there is a tendency to change this to a connection-oriented service.

9. Connectionless Network Packets may not travel along the same path. 9 Figure 4.3 A connectionless packet-switched network

10. 10 Forwarding decision is based on DA Forwarding Process in a Connectionless Network

11. TCP/IP Protocol Suite 11 Delay in a Connectionless Network

12. TCP/IP Protocol Suite 12 Figure 4.6 A connection-oriented packet switched network Connection-Oriented Network In a connection-oriented packet switched network, the forwarding decision is based on the label of the packet. A virtual circuit has to be set up first.

13. Packet Forwarding Rule Using incoming port and label, the output port is decided and the outgoing label in the packet is updated SA and DA are kept for compatibility reason. 13 Figure 4.7 Forwarding process in a connection-oriented network

14. 4 Phases in Connection-Oriented Networks Setup Phase (by Request packet) Acknowledgement Phase (by ACK packet) Data Transfer Phase Teardown Phase ** In the following examples, we assume that how to reach from one port of a switch to the next port is already known. (Refer to future chapters.)

15. TCP/IP Protocol Suite 15 Note : In each switch, an available incoming label is selected. (R1 sets label = 14) (R3 sets label = 66)(R4 sets label = 22) Setup Phase: Sending Request Packet

16. 16 ACK Phase: Returning Acknowledgement If B is ready to receive, it will pick a label, say 77.

17. TCP/IP Protocol Suite 17 Data Transfer Phase: Flow of a Packet in an Established Circuit

18. TCP/IP Protocol Suite 18 Delay in a Connection-Oriented Network

19. TCP/IP Protocol Suite 19 4-4 NETWORK LAYER SERVICES In this section, we briefly discuss services provided by the network layer. Our discussion is mostly based on the connectionless service, the dominant service in today ’ s Internet. At source side At router side At destination side

20. TCP/IP Protocol Suite 20 Figure 4.12 An Imaginary Internet Alice  LAN  Cable WAN  regional ISP  SONET  national ISP  router  ATM switch  router  SONET  regional ISP  DSL  LAN  Bob

21. TCP/IP Protocol Suite 21 Figure 4.13 Services provided at Source Computer

22. TCP/IP Protocol Suite 22 Figure 4.14 Services Provided at Router Note: 每個 router 的 fragmentation 可能不 同

23. TCP/IP Protocol Suite 23 Figure 4.15 Services Provided at Destination Computer

24. TCP/IP Protocol Suite 24 4-5 OTHER SERVICES (1)These issues actually represent services that are normally discussed for the network layer, but they are either partially implemented at the network layer or not implemented at all. (2)Some services are provided by some auxiliary protocols or by protocols added to the Internet later. (See future chapters.) Error Control Flow Control Congestion Control Routing Security

25. Summary Circuit Switching Packet Switching – Connectionless – Connection-oriented ( 羚羊峽谷, 2005.07)