1. 1 Chapter 1: Foundation Dr. Rocky K. C. Chang 30 January 2004

2. 2 1. What is a computer network? Many networks: –Telephone networks –Satellite networks –Mobile telephone networks –Cable TV networks –Internet, intranets, and extranets –Virtual private network Data networks vs. telecommunications networks Convergence of services

3. 3 2. Classifications of computer networks According to the network size: –Interconnection networks: connecting multiple processors –System/Storage area networks (SAN): connecting processors to storage servers –Local area networks (LAN): connecting a limited number of hosts in a close proximity –Metropolitan area networks (MAN): connecting a limited number of LANs in a close proximity –Wide area networks (WAN): connecting hosts situated anywhere

4. 4 2. Classifications of computer networks According to mobility: –Fixed networks –Wireless networks –Mobile networks According to the network speed: –Dial-up networks (kbps) –High-speed networks (Mbps) –Gigabit networks (Gbps)

5. 5 2. Classifications of computer networks According to the “networking software”: –Novell NetWare (IPX) –IBM’s SNA –Xerox’s XNS –DEC’s DECnet –Apple’s AppleTalk –Microsoft’s NetBIOS and NetBEUI –TCP/IP

6. 6 3. Four basic requirements Connectivity Cost-effective resource sharing Support for common services Performance Other considerations, e.g., –Security –Accounting for resources –Economical factors

7. 7 3.1 Connectivity Connectivity includes directly and indirectly connected nodes. –Directly connected (a) (b) …

8. 8 3.1 Connectivity –Indirectly connected switched networks: packet switching vs. circuit switching internetworks

9. 9 3.1 Connectivity Network connection components: –Cables: UTP, 10baseT/2/5, coaxial cables, fibers, free space, etc. –Modems –Network interface cards (NIC) –Hubs (repeaters, layer one) –Switches/bridges (layer two) –Routers (layer three) –Layer-4 switches

10. 10 3.2 Cost-effective resource sharing Multiplexing: a process of sharing a system resource among multiple users. L1 L2 L3 R1 R2 R3 Switch 1Switch 2

11. 11 3.2 Cost-effective resource sharing Multiplexing approaches: –Synchronous time-division multiplexing (STDM) –Frequency-division multiplexing (FDM) –Statistical multiplexing On demand, rather than basing on a fixed time schedule or a fixed frequency assignment. Packet switching, rather than message switching, and the packet size is limited to avoid monopoly. Need other mechanisms for packet transmission. –Packet scheduling –Medium access mechanisms

12. 12 3.3 Support for common services From an application’s point of view, the underlying network provides a logical channel between two application processes. Host Application Host Application Host Channel

13. 13 3.3 Support for common services Application requirements vary: –A data file transfer through the FTP –A webpage transfer through HTTP –A video streaming through RTP Requirements: –Total reliability –Timing constraints: delay and delay jitter Results for the Internet protocol: –Provide a best-effort service at IP (connectivity). –Provide a reliable service on top of IP.

14. 14 3.4 Performance Network performance measured in two ways: –Throughput (in bits per second) –Latency (or delay, in time units) Bandwidth and throughput –Bandwidth of a channel in terms of Hz –Bandwidth of a channel in terms of bits/second –Throughput of a channel, in terms of bits/second, is the maximum data rate realized.

15. 15 3.4 Performance 1 second (a) 1 second (b)

16. 16 3.4 Performance Latency measures the time it takes a message/packet to travel from a source to a destination. –Round-trip time (RTT) Latency is a sum of –queueing delays: waiting time for its turn of transmission –transmission delays, and –propagation delay: time for propagating a packet from a source to a destination

17. 17 3.4 Performance For example, transfer a 1-MB file in a network of 10Mbps with 5000m apart. –Ignoring queueing and node processing delays –Transmission delay: (1x2 20 x8)/10x10 6 = 0.839s –Propagation delay: 5000/2x10 8 = 0.025ms –Latency = 0.839s. Now change 10Mbps to 10 Gps: –Transmission delay: (1x2 20 x8)/10x10 9 = 0.839ms –Latency = 0.864ms

18. 18 3.4 Performance Combine data size, bandwidth, and propagation delay: 10,000 5000 2000 1000 500 200 100 50 20 10 5 2 1 10010 RTT (ms) 1-MB object, 1.5-Mbps link 1-MB object, 10-Mbps link 2-KB object, 1.5-Mbps link 2-KB object, 10-Mbps link 1-byte object, 1.5-Mbps link 1-byte object, 10-Mbps link Perceived latency (ms)

19. 19 3.4 Performance Delay-bandwidth product: gives the volume of the pipe--the maximum number of bits it holds. For example, a transcontinental channel with one way latency of 50 ms and a bandwidth of 45 Mbps, the product is 2.25x10 6 bits (280 KB). Short fat pipe, long fat pipe, long thin pipe

20. 20 4. Network architecture Layered architecture –Layering decomposes the problem of building a network into more management components. –Layering provides a more modular design. Application programs Process-to-process channels Host-to-host connectivity Hardware

21. 21 4. Network architecture –Each layer provides a service to the upper layer. –Often, there are multiple services provided by a layer. Application programs Host-to-host connectivity Hardware Reliable channelUnreliable channel

22. 22 4.1 Internet architecture A four-layer architecture (a protocol graph) … FTPHTTPNV TFTP TCP UDP IP NET 1 2 n Application Transport Network Data-link

23. 23 4.1 Internet architecture Layers: –Data-link layer provides framing and address resolution services. –Network layer provides connectivity service. –Transport layer provides reliability or unreliable service, and flow control service. –Application layer provides application-specific services.

24. 24 4.2 Protocols The service provided by each layer is implemented by protocols at that layer. –A protocol is a series of steps, involving two or more parties, designed to accomplish a task. –A protocol for batch textbook purchase Each protocol defines two interfaces: –Service interface defines the operations that local objects can perform on the protocol. –Peer interface defines the form and meaning of messages exchanged between protocol peers to implement the communication service.

25. 25 4.2 Protocols

26. 26 4.2 Protocols Involve three parties (students, Rocky, and the Bookshop). Involve a series of steps, which must be executed in a pre-determined order (steps 1-6). Accomplish a task (purchasing textbooks) Every party involved in a protocol must know the protocol and must follow all steps. The protocol must be unambiguous and each step is well defined. The protocol must be complete.

27. 27 4.2 Protocols

28. 28 4.2 Protocols FTP/TCP/IP/Ethernet and FDDI protocols R1 ETH FDDI IP ETH TCP R2 FDDI ETH IP H1 IP ETH TCP H2 FTP FTP protocol TCP IP FDDI protocol

29. 29 4.3 Protocols at different layers Encapsulation –A header is attached to a message passed down from the upper layer. –The message to be encapsulated is called payload or protocol data unit (PDU). –A low-level protocol does not interpret the message it is given by some high-level protocol.

30. 30 4.3 Protocols at different layers FTP/TCP/IP/Ethernet IP ETH TCP FTP Appl. data TCP hdr Appl. dataTCP hdrIP hdr Appl. dataTCP hdrIP hdrEth. hdr user input Send out to the network interface

31. 31 4.3 Protocols at different layers Demultiplexing: A reverse process of encapsulation Appl. data TCP hdr Appl. dataTCP hdrIP hdr Appl. dataTCP hdrIP hdrEth. hdr Received from the network interface Other nonIP network protocols UDP-based applications Other TCP-based application processes

32. 32 4.4 The 7-layer OSI architecture The Open Systems Interconnection architecture serves as a reference model. Except for DECnet, the OSI was not implemented as a working product, because –The upper three layers were not generally agreed upon. –A very complex protocol architecture; a large number of layers implies inefficiency. –Significant implementation effort –TCP/IP was readily available then.

33. 33 4.4 The 7-layer OSI architecture Application Presentation Session Transport End host One or more nodes within the network Network Data link Physical Network Data link Physical Network Data link Physical Application Presentation Session Transport End host Network Data link Physical

34. 34 5. Network programming Two most common network APIs: Sockets and X/Open Transport Interface (XTI), a slight modification of AT&T’s Transport Layer Interface (TLI). The APIs allows programmers to easily make “connections” with another application process, without knowing how the underlying network operates.

35. 35 5.1 Basic socket calls for a client socket connect recv send peer sockaddr_in{} obtain a socket establish a connection to the peer receive and send data

36. 36 5.2 Basic socket calls for a server socket listen recv send peer sockaddr_in{} obtain a socket mark the socket as a listening socket receive and send data bind accept local sockaddr_in{} bind the server’s IP address and port to the socket accept new connections

37. 37 6. Organization of the textbook Host-to-host communications –Chapter 2: On a directly connected network (a single LAN segment, data-link layer) –Chapter 3: On multiple non-directly connected networks of the same type (multiple LAN segments, data-link layer) –Chapter 4: On multiple non-directly connected networks of different types (global Internet, network layer) Process-to-process communications –Chapter 5: Transport layer

38. 38 6. Organization of the textbook Other issues: –Chapter 6 (network congestion) –Chapter 7 (presentation formatting) –Chapter 8 (security) –Chapter 9 (application protocols)