ECE 428 Computer Networks and Security

Transcription

1 ECE 428 Computer Networks and Security 1

2 Instructor: Sagar Naik About the Instructor Office: EIT 4174, ECE Dept. Other courses that I teach ECE 355: Software Engineering ECE 453/CS 447/ SE 465: Software Testing and QA ECE 454: Distributed and Network-Centric Computing ECE 750-4: Protocols, Software, and Issues in Mobile Comp. Research interest: Computer networks, mobile computing, wireless communication, network-based applications Book: Software Testing and QA (Wiley, August 2008) 2

3 Objective #1 Course Outline Fundamentals of transporting messages from one process to another process on another computer. Important communication protocols to access the Internet Objective #2 Fundamentals of network security Objective #3 Advance topics in computer networks 3

4 Objective #1 Course Outline To transport messages from a process on one machine to another process on another machine IP TCP Server Process Client Process DLC IP IP PHY/MAC DNS DNS 4

5 Course Outline: Realize Objective #1 Physical +Medium Access Control (MAC) layers Data Link Control (DLC) layer Internet Protocol (IP) layer Transmission Control Protocol (TCP) layer Application Layer Protocols 5

6 Course Outline Objective #2: Achieve secure communication Security User Authentication Privacy (Confidentiality) Data Authentication (Data Integrity) Techniques DES, Public-key cryptography, key generation, security protocols 6

7 Objective #3 Course Outline Advance topics in communication networks Cellular communication network GSM/ GPRS Wireless backbone networks WiMAX Vehicular networks 7

8 Evaluation Two assignments 15% Mid-term exam 25% Final 60% 8

9 Reading Resources Text Computer Networks, A. S. Tanenbaum, Prentice Hall Computer Networking, Kurose and Ross, Addison Wesley TCP/IP Protocol Suite, B. A. Forouzan, McGraw Hill, 3 rd ed. Online notes to be posted Course URL 9

10 Teaching Assistants Towhidul Islam Rajesh Palit 10

11 Teaching Style Balanced coverage of functional, non-functional and design aspects of network protocols Functional aspect: what the protocols do Non-functional aspect: Quality of Service (QoS) Design aspects: choices and parameters View this course as a window to the larger world of computer networks. Stay close to real protocols with a combination of abstraction and details. 11

12 Near exam times Tutorial Style Exam related questions and answers Otherwise Protocols and their details related to in-class materials 12

13 My assumption about class level Beginner Moderate Topically high 13

14 Physical + MAC layers C C C Wired media C C C Wireless medium Ethernet cable C C C Hub Wireless Access Point 14

15 Physical layer Physical + MAC layers Actual movement of bits over comm. medium Different communication media Wire (fiber optics, coaxial cable, telephone line) Free space wireless communication Main function Bit stream electrical signal/ electromagnetic waves Strive for media efficiency Concepts: Frequency, Time, and Code division multiplexing 15

16 Media Physical + MAC layers Shared: Ethernet, free space Dedicated: a dial-up link MAC layer Uses the services of the underlying PHY layer. Tells the PHY layer WHEN to transmit. Transmission may not be successful: packet loss. Mechanisms for reducing packet loss 16

17 DLC layer Data Link Control layer Runs on top of a MAC layer (unreliable) Provides a sense of reliability: ACK mechanism Synchronize source transmission rate with the sink s acceptance rate. Flow control Data multiplexing/ demultiplexing Data is broken up into frames 17

18 IP (Network) layer IP layer Runs on top of a DLC layer. Route packets from one computer to another. Builds routing tables for hop-by-hop routing. 18

19 TCP layer Transport layer Runs on top of IP layer. Provides end-to-end communication symantic. Packet delivery Lossless In-order Single copy (No duplicate) Confirmation Two control mechanisms: flow and congestion 19

20 Applications Internet Directory Service DNS Web Access HTTP protocol Session Initiated Protocol (SIP) 20

21 Communication Protocols Medium Access Control (MAC) Lets two nodes communicate directly. Node: a computer or a router Data (Logical) Link Control (DLC/LLC) Lets us have a reliable link between two adjacent nodes. Internet Protocol (IP) Routes packets from one computer to another. Uses the services of BGP, OSPF, and RIP Transmission Control Protocol (TCP) Lets us have end-to-end semantics between applications. 21

22 Media/Physical Layer Computers are physically connected by a medium Wired medium (Ex.: Ethernet Local Area Network) Wireless medium (Ex.: Free space WLAN, Cellular network) Physical layer A layer of protocol controlling the hardware system that actually moves bits from one computer to another Abstraction: Transmitter and Receiver Tx Rx Node Node 2 Rx Medium Tx 22

23 Medium Access Control (MAC) Protocols for LAN and WLAN 23

24 Some Basic Ideas Concept of layering Segment/ Packet/ Frame Packet Header Five Basic Assumptions Aloha Protocols Throughput 24

25 Concept of Layering App. TCP IP DLC1 MAC1 DLC1 MAC1 IP DLC2 MAC2 App. TCP IP DLC2 MAC2 PHY PHY PHY PHY LAN1 Note: Protocols are executed in - hardware, kernel space, and user space LAN2 25

26 Concepts of segment/packet/frame, and header Packet: A formatted stream of bits with the following info. H - Type: The receiver knows what to do with this. E A - Addresses: Destination and source D E R - Control information: Layer related - Data: optional App. TCP IP DLC1 MAC1 PHY User Data User Data User Data User Data User Data User Data User Data User Data User Data User Data User Data User Data App. TCP IP DLC1 MAC1 PHY 26

27 MAC Protocols Function: Send packets from one node to another sharing the same medium Sharing => Nodes compete for transmission Key problem to resolve Each node finds a good time for transmission with the hope that it will not collide with another. The decision must be made largely independently Ignore what others are doing Look for possible Tx from others (sense carrier) Take permission from the receiver 27

28 MAC Protocols (5 key assumptions) Station Model Nodes are independent. When a node generates a packet for Tx, subsequent packets are queued up for Tx. Single channel model only one can Tx at a time Collision assumption Two packets are transmitted at the same time => signal is potentially garbled (collision occurs) at the receiver. Collision occurs at receivers. NOTE Transmitter needs to know the occurrence of collision A Tx can t actually detect a collision A Tx can sometimes infer a collision Collision detection is not possible in WLAN 28

29 MAC Protocols (5 key assumptions) Time model Continuous: A packet Tx can begin at any instant Slotted: Packets are transmitted at well-known instants defined by slots. Carrier sense No carrier sense: Don t try to detect an on-going Tx Carrier sense Mechanism to sense carrier Utilize its absence 29

30 We will study Aloha protocols MAC Protocols CSMA/CD (Carrier Sense Multiple Access/ Collision Detection) CSMA/CA (CSMA/ Collision Avoidance) 30

31 Aloha Protocol Developed in the 1970s at U of Hawaii To interconnect terminals with mainframes LAN/ WLAN: Possible, but not used GSM: Phones use this protocol to request a channel from the base stations Two types Pure Aloha (Continuous time) Slotted Aloha 31

32 Pure Aloha Exponential backoff Wait T B = R*Tp R = Random(0,2 i -1) No Start: i = 0 Transmit Frame Start a timer T = 2*Tp + Δ i > Kmax i++ Timeout ACK received Cancel timer Error Yes Success 32

33 (Pure Aloha: Performance Measure) Throughput: A commonly used performance metric. Input/ Output System Input I/O System Output Throughput: The output rate. Output rate Input rate 33

34 Pure Aloha: Performance Measure Throughput Throughput = Total input rate (G) * Prob. of successful packet transmission G*e -2G Throughput peaks at G = 0.5 units of packet/x sec, where X is the packet transmission time. Max throughput = packets/x sec. Oblivious to collision while transmitting => Loss of available bandwidth 34

35 Slotted Aloha Protocol Similar to pure Aloha Difference Time is slotted A terminal transmits a packet at the beginning of a slot. Throughput G*e -G Peaks at G = 1, and throughput = packets/x sec 35

36 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 36

37 CSMA/CD Concepts of Carrier Sense and Collision Detection Collision? MAC/ PHY Data Compare Carrier? Tx Rx Medium 37

38 10Base5 IEEE 802.3: PHY Thick co-ax (10 mm diameter) 500 m segment Bus 10BaseT Twisted pair 100 m Star 38

39 CSMA/CD Assumption A Tx from one node can reach all on the LAN. 39

40 CSMA/CD Start: i = 0 Wait T B = R*Tp Sense medium Y Busy N Wait R = Random(0,2 i -1) No i > Kmax Yes Error i++ Collision Send jamming Signal + Abort Transmit frame WHILE detecting collsion No collision Success 40

41 CSMA/CD Time for collision detection 2 * T p (where T p is max propagation time between two nodes) Propagation time Includes transceiver delay + physical propagation time Of the order of 25 microseconds Minimum packet length Frame transmission time is T f T f = 2* T p Whatever can be transmitted in T f 41

42 Aloha vs. CSMA/CD No carrier sense ACK Oblivious to collision No jamming signal Low throughput Carrier sense No ACK Collision => Abort Jamming signal Let all hear the collision Higher throughput 42

43 Wireless LAN IEEE /a/b/g 43

44 Reminder Note CSMA/CD Assumption Signal from one node can reach all nodes. The assumption may not hold in WLAN Collision detection is out of question (Use CA) Two nodes may not be able to comm. directly Use an intermediate node <= AP (Access Point) 44

45 WLAN View C: Computer, AP: Access Point C Radio range of the AP Access Point IEEE protocol C Router To the rest of the network Other AP Basic Service Set (BSS): BSSID = MAC address of AP Independent BSS (IBSS)= BSS - AP Extended Service Set (ESS): A collection of BSS connected by a Distribution System 45

46 IEEE /a/b/g Family IEEE Technique Band Rate Mbps DSSS FHSS 2.4 GHz 2.4 GHz 1 and 2 1 and a OFDM GHz b DSSS 2.4 GHz 5.5 and g OFDM 2.4 GHz 22 and 54 46

47 Different Modes of Operation of MAC in IEEE Modes of IEEE MAC Distributed Coordination Function (DCF) mode Point Coordination Function (PCF) mode With Hand-shake Without Hand-shake 47

48 The AP PCF Mode: Optional Acts as the central controller for all nodes within its range. Decides who transmits and when. Can follow a round-robin policy to allocate slots. Note: There is no contention for medium access. This mode Can support real-time traffic due to periodic scheduling. Leads to waste of bandwidth if a scheduled node has no traffic. Is optional <= Spec. 48

49 An AP DCF Mode: Mandatory Not necessarily to be used. Computers can communicate among themselves <= Ad hoc. Is used to provide connectivity to the Internet. In DCF All nodes, including the AP, compete for medium access. The AP does not act as a central controller. Contention => No guarantee of bandwidth» Delivery is best effort 49

50 Alternative use of PCF and DCF The WLAN operates In the PCF mode for T1 seconds Bandwidth guarantee for some nodes In the DCF mode for T2 seconds Nodes with additional traffic can contend for a share of the bandwidth PCF DCF PCF DCF PCF DCF T1 T2 T3 T4 Time 50

51 DCF with hand-shake A sender obtains permission from the receiver before transmitting a data frame. Hand-shake mechanism Sender transmits a Request To Send (RTS) frame Receiver gives permission by sending back a Clear To Send (CTS) frame Used to increase the probability of successful Tx when Traffic is high Packet length is long. ( dotrtsthreshold holds the value.) Incurs additional cost loss of some bandwidth 51

52 DCF with and without hand-shake The two modes are not mutually exclusive. A node decides what mode to use on a frame/frame basis. The MAC management database contains a variable dotrtsthreshold: integer in bytes Length of a data frame >= dotrtsthreshold» Use hand-shake Length of a data frame < dotrtsthreshold» Do not use hand-shake PCF DCF PCF DCF PCF No hand-shake Hand-shake } Mode of operation of the same node 52

53 DCF without hand-shake A sender does not obtain permission from the receiver before transmitting a data frame. RTS/CTS mechanism is NOT used. There is no prior coordination between sender and receiver A sender transmits a frame when some medium sensing conditions are satisfied. To follow When traffic is low OR data frames are short Use this to save bandwidth 53

54 Problems in WLAN Hidden Terminal Problem Exposed Terminal Problem Inability to detect collision (at the receiver) Assumption All nodes have identical radio range Note how far away their signal can be received The assumption does not cause the problems to surface. Without this assumption, the problems become worse. 54

55 Hidden Terminal Problem D A B C Tx Problem - C is transmitting a frame to B. - A is unaware of C s Tx. - Now, if A transmits, A s Tx will collide with C s at B The above problem is due to C being hidden from A. 55

56 Exposed Terminal Problem A B C D Tx Problem - A is transmitting a frame to D. - B is aware of the ongoing Tx. - If B transmits a frame to C, no harm is caused. - However, B does not transmit because it is unaware of D s location. The above problem is due to B being exposed to A s Tx. 56

57 No collision detection Fact: Collision occurs at receivers. In a wired LAN Collision is indirectly detected by the sender by enforcing the following assumption: Signal from one node can reach all nodes. In a WLAN The assumption does not hold. Evidence: The hidden terminal problem Collision is avoided (CA), rather than detected 57

58 WLAN MAC: CSMA/CA In CSMA/CA, collision is avoided using PHY-level carrier sensing: Done by receiver hardware Virtual carrier sensing: Done by Processing all frame headers (RTS, CTS, DATA) A duration field in frame headers indicates for how long the sender of the frame may use the medium. A Network Allocation Vector is managed using duration fields Each node has a NAV essentially an integer NAV > 0: A node had announced its intention to use the medium now. NAV = 0: Nobody had announced its intention to use the medium now. Transmit condition: When medium is idle (Absence of carrier) AND (NAV = 0) 58

59 NAV Update Mechanism Each node has its own NAV. NAV represents the length of time for which the medium is likely to remain busy Initially: NAV = 0. With each passing μs NAV = NAV 1 Decrementing stops if NAV = 0. NAV is updated using the duration field in a received frame NAV = Max(NAV, duration) 59

60 Frame format RTS and CTS Frames RTS Frame Control Duration RA TA FCS bytes CTS/ ACK Frame Control Duration RA FCS bytes FCS: Frame Check Sequence 60

61 DATA Frame Frame format Frame Control Duration/ ID Seq. A1 A2 A3 A4 Control Frame Body FCS RA TA 61

62 Timing Intervals The IEEE MAC defines 4 timing intervals 2 at the PHY level SIFS: Short InterFrame Space aslot 2 at the MAC level PIFS: Priority (in PCF) IFS DIFS: Distributed IFS 62

63 Hand-shake using RTS/CTS Value of duration in RTS A DIFS RTS SIFS DATA Value of duration in CTS B SIFS CTS SIFS ACK C Value of NAV of C D Value of NAV of D Time 63

64 DCF with Hand-shake: Tx F: a new data frame to be transmitted i = 0, CW = CW min NAV =0? End of backoff Idle medium for DIFS interval? Yes Send an RTS Start a timer No Random Backoff CTS is received ACK is received Cancel timer Send DATA (F) Start a timer Timeout Timeout i: Retry count, CW: Contention Window CWmin: Minimum value of CW (typical value is 32) CWmax: Maximum value of CW (typical value is 256) DIFS: Distributed Interframe Space SIFS < DIFS Important note Wait for fairness to others Cancel timer Wait for a random interval i = i+1 CW = CW min *2 i (At some point, CW saturates at CW max.) 64

65 Backoff Mechanism Backoff Time Counter (BTC) = Random(0,CW) The time unit of BTC is aslottime aslottime: propagation + transceiver switching time BTC is decremented as follows: Medium is idle for aslottime: BTC = BTC 1 Medium is busy: Stop decrementing Resume decrementing BTC after finding the medium to be idle for DIFS interval. Subsequent decrementing is done for every aslottime of idleness of the medium. 65

66 Backoff Mechanism Ch. Busy (A) Ch. Busy (C) Time B DIFS DIFS X X X X BTC = If the channel is busy, it has to remain idle for DIFS interval for BTC to be decremented by 1. X = aslottime If the channel is idle for at least DIFS interval, it has to remain idle for aslottime interval for BTC to be decremented by 1. 66

67 DCF with Hand-shake: Rx Receive an RTS Remain silent. NAV = 0? No Receive a DATA frame Yes Channel is idle for SIFS and the computer is ready to receive? No Ch. idle for SIFS? Yes No Yes Send an ACK Send a CTS Note: The above two fragments of flow-charts can be easily merged. 67

68 DCF Mode without Hand-shake A special case of DCF with hand-shake RTS/CTS frames are not exchanged The idea of NAV is still used in this mode All stations process the received RTS/CTS of others NOTE: A node may broadcast a DATA frame to all Done in DCF without hand-shake Receivers do not send back an ACK. 68

69 PCF Mode of Operation An AP acts as the controller of a BSS as follows AP alternates between PCF and DCF modes AP assumes the role of a controller as follows AP senses the medium at the start of a CF (Contention-Free) period for a PIFS (Priority IFS) interval. SIFS < PIFS < DIFS PIFS = SIFS + aslottime DIFS = SIFS + 2*aSlotTime If the medium is idle for PIFS, transmit a beacon frame Beacon contains a CFPMaxDuration field (Nodes receiving a beacon update their NAV to CFPMaxDuration)» These nodes perceive the medium to be busy for CFPMaxDuration 69

70 PCF Mode of Operation (Contd.) After transmitting a beacon, AP waits for SIFS before transmitting one of the following DATA frame CF Poll frame DATA+CF Poll frame ACK frame CF End frame 70

71 PCF Mode of Operation (Contd.) CF Poll frame AP User 1 User 2 AP User 1 CF Poll CF Poll SIFS DATA DATA SIFS ACK SIFS SIFS ACK The polled user sends data to another user. The polled user sends data to the AP. 71

72 PCF Mode of Operation (Contd.) DATA + CF Poll frame AP User 1 User 2 DATA+ CF Poll ACK SIFS DATA ACK SIFS The polled user receives data from the AP and sends data to another user. 72

73 PCF Mode of Operation (Contd.) DATA frame Contains user data from AP to a specific station. The receiver sends back an ACK after SIFS interval. AP does not receive an ACK Retransmit the DATA after a PIFS interval AP can broadcast a DATA frame These are not ACKed. 73

74 PCF Mode of Operation (Contd.) CF Poll frame AP grants permission to another node to transmit DATA to the AP or to a third node. Receiver of DATA frame sends an ACK to the sender. If the polled node has no data to send, it sends a null DATA frame. If the polled station does not receive an ACK, it can not retransmit its data frame until it is polled again. 74

75 PCF Mode of Operation (Contd.) CF End frame Identifies the end of CF period Sent by an AP under two conditions An AP has no data to send and no node to poll Can happen before the pre-announced CFPMaxDuration Receivers of CF End reset their NAV to 0. Normal end of CF period (Initially: CFPDurRemaining = CFPMaxDuration) CFPDurRemaining time expires 75

76 A node joining a WLAN with an AP (No need for such a procedure in a wired LAN) Two ways for a node to join a WLAN Passive scanning Scan a channel for a Beacon frame If a Beacon frame is received Negotiate Authentication and Association processes Active Scanning Transmit a Probe frame If a Probe Response is received Negotiate Authentication and Authorization processes 76