CS 447: NETWORKS AND DATA COMMUNICATIONS

1. CS 447: NETWORKS AND DATA COMMUNICATIONS Spring 2012 – TR 3:30-4:45 PM – EB 0012 Course Description Concepts of networks and data communications. Networking protocols and architecture; data encoding and transmission; network management; and distributed applications. Textbook Prerequisites CS 314 and CS 340, both with a minimum grade of C.

2. CHAPTER 1: COMPUTER NETWORKS AND THE INTERNET • Introduction • The Internet • Client-Server Systems • Access Networks • Circuit & Packet Switching • Network Delays • Protocol Layers • Security Issues

3. HISTORY OF THE INTERNET CS 447 Chapter 1 Page 3

4. INTERNET COMPONENTS CS 447 Chapter 1 Page 4

5. CLIENT-SERVER VS. PEER-TO-PEER SYSTEMS There are two basic models for conducting networked applications. Peer-to-Peer Systems Client-Server Systems Tasks are partitioned between the requesters of the service (the clients) and the providers of the service (the servers). Computers pool their resources (memory, processing power, bandwidth) without any centralized coordination. Centralization lends itself to easier maintenance, upgrading, and security, as well as better ease-of-use. P2P systems are impacted less by machine failures and actually benefit from expansion due to increased resources. CS 447 Chapter 1 Page 5

6. CLIENT-SERVER VS. PEER-TO-PEER APPLICATIONS Client-Server: E-Mail, Web Service, Database Access, File Processing, Facebook Peer-to-Peer: Content Delivery, File Sharing, Skype CS 447 Chapter 1 Page 6

7. ACCESS NETWORKS: DIAL-UP CONNECTION Dial-up Internet connections utilize modems (modulators/demodulators) to convert digital data into analog transmissions across telephone lines, and then back to digital signals at the receiver. • Requires no infrastructure except a telephone network • Uses handshaking protocol to establish connection prior to data transfer • Low bandwidth (56 kbit/sec) inhibits use for streaming, on-line gaming, etc. NetZero combats the bandwidth problem by prefetching data, compressing text files, and exploiting the user’s cache to prevent redownloading. CS 447 Chapter 1 Page 7

8. ACCESS NETWORKS: DIGITAL SUBSCRIBER LINE DSL uses standard telephone lines to provide large throughput downstream (to the customer) for Internet data and small throughput upstream (from the customer) for voice. • Their lack of symmetry causes most DSL connections to be called Asymmetric DSL (ADSL) • A higher frequency band (>25kHz) is used for the subscriber line, while a lower frequency band (<5kHz) is still used for POTS (Plain Old Telephone Service: voice, fax, analog modems) In May 2011, AT&T instituted a 150GB use cap for its DSL service, penalizing customers who consume “a disproportionate amount of bandwidth”. CS 447 Chapter 1 Page 8

9. ACCESS NETWORKS: CABLE MODEM Extra bandwidth on cable television systems may also be exploited to provide Internet access. • Television channels require 6MHz bands; downstream channels are allocated 6MHz as well (with upstream channels • allocated only 2 MHz) • Such cable systems are usually Hybrid Fiber-Coaxial (HFC), especially if they provide on-demand service. CS 447 Chapter 1 Page 9

10. ACCESS NETWORKS: FIBER-TO-THE-HOME Instead of using legacy coax systems to support broadband Internet access, FTTH systems provide fiber optic cable connections to individual residences. • Active Optical Networks route signals directly to the intended customer, and buffer signals coming from the customer. • Passive Optical Networks broadcast the same signal to multiple users, using encryption to prevent eavesdropping, while some multiple access protocol is used to deal with signals coming from customers. CS 447 Chapter 1 Page 10

11. ACCESS NETWORKS: ETHERNET Local area networks are typically used to connect end stations to an edge router, which, in turn, connects them to the Internet. • Ethernet, developed by Xerox in the late 1970s, is the most common LAN technology • Ethernet uses CSMA/CD: • Carrier Sense (CS) – Each end station “listens” for incoming messages, and for conflicting traffic whenever it wants to transmit • Multiple Access (MA) – A single bus is used to carry all messages • Collision Detection (CD) – When a station determines that its outgoing message has collided with that of another station, both stations wait a random amount of time and then retransmit CS 447 Chapter 1 Page 11

12. ACCESS NETWORKS: WIFI Wireless Internet access occurs via wireless local area networks or via wide area cellular networks. • In a WLAN, users transmit to and receive from an access point which is wired to the Internet. • A single WAP can usually handle about 30 end systems within a 100-meter radius. • Cellular systems process messages through base stations in cell towers, which forward the wired signals through switching centers to the Internet. • Greater distances, more sources of interference, and larger numbers of users complicate this approach. CS 447 Chapter 1 Page 12

13. ACCESS NETWORKS: WIMAX WiMAX (Worldwide Interoperability for Microwave Access) is being developed as a wireless alternative to DSL and cable modems for “last mile” broadband access. • While WiFi provides Internet access via an intervening WLAN, WiMAX provides direct Internet access. • WiMAX is sometimes called a 4G technology, but it fails to meet the 1Gbps requirement of 4G (although the planned WiMAX 2 does). • Similarly, WiMAX’s European counterpart, LTE (long Term Expansion) does not currently meet 4G specifications, but its next iteration, LTE Advanced, should. CS 447 Chapter 1 Page 13

14. CIRCUIT SWITCHING Traditional telephone networks use switching technologies based on dedicated circuits between communicating end stations, with potential wasted bandwidth, but with guaranteed capacity and consistent delays. Time-Division Multiplexing: Better line utilization via sequential channels based on time slices Frequency-Division Multiplexing: Better line utilization via simultaneous sub-band channels CS 447 Chapter 1 Page 14

15. PACKET SWITCHING To make better use of communication lines, most modern protocols have transmitting stations break messages down into manageable “packets” that are forwarded through the network and reassembled at the destination stations. Datagram Packet Switching (used in IP) allows each packet to find its own path to the destination Virtual Circuit Packet Switching (used in ATM) sets up a single route for all packets in the message to follow CS 447 Chapter 1 Page 15

16. NETWORK DELAYS Without the dedicated lines associated with circuit switching, packet-switched communication experiences inherent delays. • Processing Delay • Routers need time to process the packet headers • (encryption, error-checking, address translation) • Queuing Delay • Packets must await their turn to be transmitted • (one packet at a time; limited buffer capacity) • Transmission Delay • Pushing the packet’s bits onto the link takes time • (switches store packets before forwarding them) • Propagation Delay • Time it takes to traverse the links to the destination • (directly proportional to the end-to-end distance) CS 447 Chapter 1 Page 16

17. DELAY COMPARISON How do the four components of delay compare? Local Area Networks Lightly Loaded Processing Delay < Propagation Delay < Queuing Delay < Transmission Delay Heavily Loaded Processing Delay < Propagation Delay < Transmission Delay < Queuing Delay Wide Area Networks Lightly Loaded Processing Delay < Transmission Delay < Queuing Delay < Propagation Delay Heavily Loaded Processing Delay < Transmission Delay < Propagation Delay < Queuing Delay CS 447 Chapter 1 Page 17

18. PACKET LOSS: CAUSE & EFFECT One potentially serious source of delay is packet loss, where an expected packet fails to arrive at the destination. Causes Buffer Overflow Causes Packet Discard Effects Displacement of Streaming Signal Data Can Result in Jitter Signal Degradation Causes Packet Corruption Retransmission or Interpolated Recovery of Lost Packets Can Exacerbate Existing Performance Problems in VoIP, Online Gaming, Videoconferencing, Etc. CS 447 Chapter 1 Page 18

19. PROTOCOL LAYERS In an effort to simplify network analysis, networks are often modeled as layered hierarchies of protocols, with hardware-intensive protocols on bottom and user applications on top. Physical Medium The network end stations communicate directly via the highest protocol layers End stations communicate directly with network routers (and routers communicate directly with each other) via the mid-level protocol layers All network nodes communicate with those with which they are physically connected via the lowest protocol layers CS 447 Chapter 1 Page 19

20. TRANSMITTING VIA LAYERS Network nodes only communicate directly at the lowest (physical) layer, but the transmission source inserts certain relevant data as headers and trailers prior to sending. Physical Medium The end station prepares a message to be sent, using its currently active application. message Headers and trailers are added at each layer to facilitate that layer’s functionality. rh rt message Packet size restrictions may compel messages to be split at some layers. rha rta msga rhb rtb msgb rhc rtc msgc At some layers, headers and trailers may contain routing and priority data... bha rha rta bta msga bhb rhb rtb btb msgb bhc rhc rtc btc msgc ...while at others, they may contain error checking and sequencing data. cha bha rha rta bta cta msga chb bhb rhb rtb btb ctb msgb chc bhc rhc rtc btc ctc msgc At the lowest layer, the packets are physically transmitted. gha cha bha rha rta bta cta gta msga ghb chb bhb rhb rtb btb ctb gtb msgb ghc chc bhc rhc rtc btc ctc gtc msgc CS 447 Chapter 1 Page 20

21. RELAYING THE MESSAGE Intermediate network nodes receive incoming packets, perform the appropriate functions at their active protocol layers, and then prepare the packets for forwarding. Physical Medium Headers and trailers at higher layers are merely treated as part of the message, which is irrelevant to the low-level network node. cha’ cha bha rha rta bta cta cta’ msga At the lower layers, the headers and trailers are examined and replaced. chb’ chb bhb rhb rtb btb ctb’ ctb msgb chc chc’ bhc rhc rtc btc ctc ctc’ msgc gha gha’ cha cha’ bha bha rha rha rta rta bta bta cta’ cta gta’ gta msga msga The revised packets are then sent closer to their destination. ghb’ ghb chb’ chb bhb bhb rhb rhb rtb rtb btb btb ctb’ ctb gtb’ gtb msgb msgb ghc ghc’ chc chc’ bhc bhc rhc rhc rtc rtc btc btc ctc ctc’ gtc’ gtc msgc msgc CS 447 Chapter 1 Page 21

22. THE MESSAGE IS RECEIVED The receiving end station processes all of the headers and trailers at all of the protocol layers and, after reassembling the original message, applies the application to it. Physical Medium After dealing with headers & trailers, the receiver uses its application on the reassembled message. message When necessary, packets are reassembled into larger messages. rhw rtw message In some cases, problems may be corrected by the receiver without requiring retransmission. rhaw rtaw msga rhbw rtbw msgb rhcw rtcw msgc Problems with any headers or trailers are addressed, possibly via retransmission. bhax rhaw rtaw btax msga bhbx rhbw rtbw btbx msgb bhcx rhcw rtcw btcx msgc At each layer, the headers and trailers are removed and checked. chay bhax rhaw rtaw btax ctay msga chby bhbx rhbw rtbw btbx ctby msgb chcy bhcx rhcw rtcw btcx ctcy msgc Packets physically arrive, with modified headers and trailers. ghaz chay bhax rhaw rtaw btax ctay gtaz msga ghbz chby bhbx rhbw rtbw btbx ctby gtbz msgb ghcz chcy bhcx rhcw rtcw btcx ctcy gtcz msgc CS 447 Chapter 1 Page 22

23. OPEN SYSTEMS INTERCONNECTION The International Organization for Standardization developed a seven-layer model for communication systems. • End-user protocols, e.g., e-mail, file transfer, Web browsing, network management, videoconferencing. • Application Layer • Format data according to syntactical and semantic negotiations between source and destination; encrypt and decrypt messages. • Presentation Layer • Establish, maintain, and discontinue dialogues between a source and a destination; synchronize data transfer via checkpoints • Session Layer • Detect and handle end-to-end transmission errors; alter transmission rate when too much congestion is encountered. • Transport Layer • Route messages from their source to their destination; take measures to reroute traffic when congestion is encountered. • Network Layer • Detect and handle transmission errors between consecutive network nodes; control access to the shared transmission medium. • Data Link Layer • Transmit bits across physical medium; determine frequencies and amplitudes to use when transmitting; specify role of connector pins. • Physical Layer CS 447 Chapter 1 Page 23

24. TCP/IP The Defense Advanced Research Projects Agency developed the five-layer predecessor to OSI in the 1970s. • Higher layer protocols such as FTP, SMTP, and HTTP, as well as libraries to provide the equivalent of OSI’s Presentation and Session functionality. • Application Protocols • TCP (and its less reliable, connectionless counterpart, UDP) is responsible for providing end-to-end message transfer capabilities. • Transmission Control Protocol • IP deals with two major tasks, host addressing and identification, and routing packets to their destination • Internet Protocol • Also known as the Network Access Layer, these protocols specify the networking scope of the local network to which a host is connected. • Link Protocols • TCP/IP does not provide hardware specifications, instead assuming that it can be implemented in a hardware-independent manner. • Physical Protocols CS 447 Chapter 1 Page 24

25. SECURITY ISSUES: MALWARE Various types of malicious software has been designed to disrupt operations or gather private information. Trojan Horse Disguised as innocuous software, but concealing a harmful payload when downloaded Computer Virus A program that copies itself, infects a system, and then infects other systems via network access Computer Worm Self-replicating software that spreads on a network without human intervention or attaching to other software Adware Software that automatically downloads and displays advertisements Backdoor A method for bypassing normal authentication to remotely access a computer Spyware Undetected software that collects information about users without their knowledge CS 447 Chapter 1 Page 25

26. SECURITY ISSUES: DENIAL OF SERVICE ATTACKS A DoS attack is designed to make a computer resource unavailable to its intended users, typically by saturating the resources with external communication requests. • The attacker contacts compromised “handler” machines. • The handlers then contact pre-infected “zombie” agents.. • The zombies proceed to bombard the target server.. CS 447 Chapter 1 Page 26

27. SECURITY ISSUES: PACKET SNIFFING Packet analyzer software is used to intercept and log traffic passing through a network. Originally developed as a means for network administrators to monitor system usage, diagnose problems, and detect intrusion or misuse, this software can itself be misused to spy on other network users and to collect sensitive information such as passwords or e-mail messages. CS 447 Chapter 1 Page 27

28. SECURITY ISSUES: SPOOFING Because many TCP/IP protocols have no mechanism for authenticating source or destination information, it is possible for someone to falsify data and masquerade as someone else, potentially accessing data illegitimately. To determine how to send a message to destination B, source A sends a request containing B’s IP address to a server, which broadcasts the request. Hacker C responds to the server’s request, and the server naively associates B’s IP address with C’s location, so source A communicates with C, thinking that it is B. CS 447 Chapter 1 Page 28

29. SECURITY ISSUES: MAN-IN-THE-MIDDLE ATTACKS An extension of the spoofing attack has the hacker pretending to the client to be the server, as well as pretending to the server to be the client. This not only permits the hacker to intercept messages between the client and the server, but also to inject false messages of its own. CS 447 Chapter 1 Page 29