Topics (2) 3. Retransmissions. 4. MulFple Access. 5. Switching. Handling loss. Classic Ethernet, Modern Ethernet

Transcription

1 3. Retransmissions Handling loss 4. MulFple Access Classic Ethernet, Switching Modern Ethernet Topics (2) CSE 461 University of Washington 1

2 Topic Two strategies to handle errors: 1. Detect errors and retransmit frame (AutomaFc Repeat request, ARQ) 2. Correct errors with an error correcfng code Done this CSE 461 University of Washington 2

3 Context on Reliability Where in the stack should we place reliability funcfons? ApplicaFon Transport Network Link Physical CSE 461 University of Washington 3

4 Context on Reliability (2) Everywhere! It is a key issue Different layers contribute differently ApplicaFon Transport Network Link Physical Recover acfons (correctness) Mask errors (performance opfmizafon) CSE 461 University of Washington 4

5 ARQ ARQ o[en used when errors are common or must be corrected E.g., WiFi, and TCP (later) Rules at sender and receiver: Receiver automafcally acknowledges correct frames with an ACK Sender automafcally resends a[er a Fmeout, unfl an ACK is received CSE 461 University of Washington 5

6 Normal operafon (no loss) Sender Receiver Frame Timeout Time ACK ARQ (2) CSE 461 University of Washington 6

7 Loss and retransmission Sender Receiver Frame X Timeout Time Frame ARQ (3) ACK CSE 461 University of Washington 7

8 So What s Tricky About ARQ? Two non-trivial issues: How long to set the Fmeout?» How to avoid accepfng duplicate frames as new frames» Want performance in the common case and correctness always CSE 461 University of Washington 8

9 Timeouts Timeout should be: Not too big (link goes idle) Not too small (spurious resend) Fairly easy on a LAN Clear worst case, lible variafon Fairly difficult over the Internet Much variafon, no obvious bound We ll revisit this with TCP (later) CSE 461 University of Washington 9

10 Duplicates What happens if an ACK is lost? Sender Frame Receiver Timeout X ACK CSE 461 University of Washington 10

11 Duplicates (2) What happens if an ACK is lost? Sender Frame Receiver Timeout X ACK Frame ACK New Frame?? CSE 461 University of Washington 11

12 Or the Fmeout is early? Duplicates (3) Timeout Sender Frame ACK Receiver CSE 461 University of Washington 12

13 Or the Fmeout is early? Duplicates (4) Timeout Sender Frame ACK Frame ACK Receiver New Frame?? CSE 461 University of Washington 13

14 Sequence Numbers Frames and ACKs must both carry sequence numbers for correctness To disfnguish the current frame from the next one, a single bit (two numbers) is sufficient Called Stop-and-Wait CSE 461 University of Washington 14

15 In the normal case: Sender Receiver Stop-and-Wait Time CSE 461 University of Washington 15

16 In the normal case: Stop-and-Wait (2) Sender Receiver Timeout Frame 0 ACK 0 Time Frame 1 ACK 1 CSE 461 University of Washington 16

17 With ACK loss: Stop-and-Wait (3) Sender Receiver Frame 0 Timeout X ACK 0 CSE 461 University of Washington 17

18 With ACK loss: Stop-and-Wait (4) Sender Receiver Frame 0 Timeout X ACK 0 Frame 0 ACK 0 It s a Resend! CSE 461 University of Washington 18

19 With early Fmeout: Stop-and-Wait (5) Sender Frame 0 Timeout ACK 0 Receiver CSE 461 University of Washington 19

20 With early Fmeout: Stop-and-Wait (6) Sender Frame 0 Timeout ACK 0 Receiver OK Frame 0 ACK 0 It s a Resend CSE 461 University of Washington 20

21 LimitaFon of Stop-and-Wait It allows only a single frame to be outstanding from the sender: Good for LAN, not efficient for high BD Ex: R=1 Mbps, D = 50 ms How many frames/sec? If R=10 Mbps? CSE 461 University of Washington 21

22 Sliding Window GeneralizaFon of stop-and-wait Allows W frames to be outstanding Can send W frames per RTT (=2D) Various opfons for numbering frames/acks and handling loss Will look at along with TCP (later) CSE 461 University of Washington 22

23 Topic MulFplexing is the network word for the sharing of a resource Classic scenario is sharing a link among different users Time Division MulFplexing (TDM)» Frequency Division MulFplexing (FDM)» CSE 461 University of Washington 23

24 Time Division MulFplexing (TDM) Users take turns on a fixed schedule CSE 461 University of Washington 24

25 Frequency Division MulFplexing (FDM) Put different users on different frequency bands Overall FDM channel CSE 461 University of Washington 25

26 TDM versus FDM In TDM a user sends at a high rate a fracfon of the Fme; in FDM, a user sends at a low rate all the Fme Rate TDM FDM Time CSE 461 University of Washington 26

27 TDM versus FDM (2) In TDM a user sends at a high rate a fracfon of the Fme; in FDM, a user sends at a low rate all the Fme Rate TDM FDM Time CSE 461 University of Washington 27

28 TDM/FDM Usage StaFcally divide a resource Suited for confnuous traffic, fixed number of users Widely used in telecommunicafons TV and radio stafons (FDM) GSM (2G cellular) allocates calls using TDM within FDM CSE 461 University of Washington 28

29 MulFplexing Network Traffic Network traffic is bursty ON/OFF sources Load varies greatly over Fme Rate Rate Time Time CSE 461 University of Washington 29

30 MulFplexing Network Traffic (2) Network traffic is bursty Inefficient to always allocate user their ON needs with TDM/FDM Rate Rate R Time R Time CSE 461 University of Washington 30

31 MulFplexing Network Traffic (3) MulFple access schemes mulfplex users according to their demands for gains of stafsfcal mulfplexing Two users, each need R Rate Rate R Time R Time Together they need R < 2R Rate R <2R Time CSE 461 University of Washington 31

32 MulFple Access We will look at two kinds of mulfple access protocols 1. Randomized. Nodes randomize their resource access abempts Good for low load situafons 2. ContenFon-free. Nodes order their resource access abempts Good for high load or guaranteed quality of service situafons CSE 461 University of Washington 32

33 Topic How do nodes share a single link? Who sends when, e.g., in WiFI? Explore with a simple model Assume no-one is in charge; this is a distributed system CSE 461 University of Washington 33

34 Topic (2) We will explore random mulfple access control (MAC) protocols This is the basis for classic Ethernet Remember: data traffic is bursty Busy! Zzzz.. Ho hum CSE 461 University of Washington 34

35 ALOHA Network Seminal computer network connecfng the Hawaiian islands in the late 1960s When should nodes send? A new protocol was devised by Norm Abramson Hawaii CSE 461 University of Washington 35

36 ALOHA Protocol Simple idea: Node just sends when it has traffic. If there was a collision (no ACK received) then wait a random Fme and resend That s it! CSE 461 University of Washington 36

37 Some frames will be lost, but many may get through ALOHA Protocol (2) Good idea? CSE 461 University of Washington 37

38 ALOHA Protocol (3) Simple, decentralized protocol that works well under low load! Not efficient under high load Analysis shows at most 18% efficiency Improvement: divide Fme into slots and efficiency goes up to 36% We ll look at other improvements CSE 461 University of Washington 38

39 Classic Ethernet ALOHA inspired Bob Metcalfe to invent Ethernet for LANs in 1973 Nodes share 10 Mbps coaxial cable Hugely popular in 1980s, 1990s : 2009 IEEE CSE 461 University of Washington 39

40 CSMA (Carrier Sense MulFple Access) Improve ALOHA by listening for acfvity before we send (Doh!) Can do easily with wires, not wireless So does this eliminate collisions? Why or why not? CSE 461 University of Washington 40

41 CSMA (2) SFll possible to listen and hear nothing when another node is sending because of delay CSE 461 University of Washington 41

42 CSMA (3) CSMA is a good defense against collisions only when BD is small X CSE 461 University of Washington 42

43 CSMA/CD (with Collision DetecFon) Can reduce the cost of collisions by detecfng them and aborfng (Jam) the rest of the frame Fme Again, we can do this with wires Jam! X X X X X X X X Jam! CSE 461 University of Washington 43

44 CSMA/CD ComplicaFons Want everyone who collides to know that it happened Time window in which a node may hear of a collision is 2D seconds X CSE 461 University of Washington 44

45 CSMA/CD ComplicaFons (2) Impose a minimum frame size that lasts for 2D seconds So node can t finish before collision Ethernet minimum frame is 64 bytes X CSE 461 University of Washington 45

46 CSMA Persistence What should a node do if another node is sending? What now? Idea: Wait unfl it is done, and send CSE 461 University of Washington 46

47 CSMA Persistence (2) Problem is that mulfple waifng nodes will queue up then collide More load, more of a problem Now! Uh oh Now! CSE 461 University of Washington 47

48 CSMA Persistence (3) IntuiFon for a beber solufon If there are N queued senders, we want each to send next with probability 1/N Send p=½ Whew Send p=½ CSE 461 University of Washington 48

49 Binary ExponenFal Backoff (BEB) Cleverly esfmates the probability 1st collision, wait 0 or 1 frame Fmes 2nd collision, wait from 0 to 3 Fmes 3rd collision, wait from 0 to 7 Fmes BEB doubles interval for each successive collision Quickly gets large enough to work Very efficient in pracfce CSE 461 University of Washington 49

50 Classic Ethernet, or IEEE Most popular LAN of the 1980s, 1990s 10 Mbps over shared coaxial cable, with baseband signals MulFple access with 1-persistent CSMA/CD with BEB CSE 461 University of Washington 50

51 Ethernet Frame Format Has addresses to idenffy the sender and receiver CRC-32 for error detecfon; no ACKs or retransmission Start of frame idenffied with physical layer preamble Packet from Network layer (IP) CSE 461 University of Washington 51

52 Modern Ethernet Based on switches, not mulfple access, but sfll called Ethernet We ll get to it in a later segment Switch Switch ports Twisted pair CSE 461 University of Washington 52

53 Topic How do wireless nodes share a single link? (Yes, this is WiFi!) Build on our simple, wired model Send? Send? CSE 461 University of Washington 53

54 Wireless ComplicaFons Wireless is more complicated than the wired case (Surprise!) 1. Nodes may have different areas of coverage doesn t fit Carrier Sense» 2. Nodes can t hear while sending can t Collision Detect» CSMA/CD CSE 461 University of Washington 54

55 Different Coverage Areas Wireless signal is broadcast and received nearby, where there is sufficient SNR CSE 461 University of Washington 55

56 Hidden Terminals Nodes A and C are hidden terminals when sending to B Can t hear each other (to coordinate) yet collide at B We want to avoid the inefficiency of collisions CSE 461 University of Washington 56

57 Exposed Terminals B and C are exposed terminals when sending to A and D Can hear each other yet don t collide at receivers A and D We want to send concurrently to increase performance CSE 461 University of Washington 57

58 Nodes Can t Hear While Sending With wires, detecfng collisions (and aborfng) lowers their cost More wasted Fme with wireless Wired Collision X X Resend Wireless Collision XXXXXXXXX Time XXXXXXXXX Resend CSE 461 University of Washington 58

59 Possible SoluFon: MACA MACA uses a short handshake instead of CSMA (Karn, 1990) uses a refinement of MACA (later) Protocol rules: 1. A sender node transmits a RTS (Request-To-Send, with frame length) 2. The receiver replies with a CTS (Clear-To-Send, with frame length) 3. Sender transmits the frame while nodes hearing the CTS stay silent Collisions on the RTS/CTS are sfll possible, but less likely CSE 461 University of Washington 59

60 MACA Hidden Terminals AàB with hidden terminal C 1. A sends RTS, to B A B C D CSE 461 University of Washington 60

61 MACA Hidden Terminals (2) AàB with hidden terminal C 2. B sends CTS, to A, and C too A RTS B C D CSE 461 University of Washington 61

62 MACA Hidden Terminals (3) AàB with hidden terminal C 2. B sends CTS, to A, and C too A RTS CTS B CTS Alert! C D CSE 461 University of Washington 62

63 MACA Hidden Terminals (4) AàB with hidden terminal C 3. A sends frame while C defers Frame Quiet... CSE 461 University of Washington 63

64 MACA Exposed Terminals BàA, CàD as exposed terminals B and C send RTS to A and D A B C D CSE 461 University of Washington 64

65 MACA Exposed Terminals (2) BàA, CàD as exposed terminals A and D send CTS to B and C A RTS B C RTS D CSE 461 University of Washington 65

66 MACA Exposed Terminals (3) BàA, CàD as exposed terminals A and D send CTS to B and C All OK All OK A RTS CTS B C RTS CTS D CSE 461 University of Washington 66

67 MACA Exposed Terminals (4) BàA, CàD as exposed terminals A and D send CTS to B and C A Frame B C Frame D CSE 461 University of Washington 67

68 802.11, or WiFi Very popular wireless LAN started in the 1990s Clients get connecfvity from a (wired) AP (Access Point) It s a mulf-access problem J Various flavors have been developed over Fme Faster, more features Access Point Client To Network CSE 461 University of Washington 68

69 Physical Layer Uses 20/40 MHz channels on ISM bands b/g/n on 2.4 GHz a/n on 5 GHz OFDM modulafon (except legacy b) Different amplitudes/phases for varying SNRs Rates from 6 to 54 Mbps plus error correcfon n uses mulfple antennas; see with MulFple Antennas for Dummies CSE 461 University of Washington 69

70 Link Layer MulFple access uses CSMA/CA (next); RTS/CTS opfonal Frames are ACKed and retransmibed with ARQ Funky addressing (three addresses!) due to AP Errors are detected with a 32-bit CRC Many, many features (e.g., encrypfon, power save) Packet from Network layer (IP) CSE 461 University of Washington 70

71 CSMA/CA for MulFple Access Sender avoids collisions by inserfng small random gaps E.g., when both B and C send, C picks a smaller gap, goes first Send? Send? Time CSE 461 University of Washington 71

72 The Future of (Guess) Likely ubiquitous for Internet connecfvity Greater diversity, from low- to high-end devices InnovaFon in physical layer drives speed And power-efficient operafon too More seamless integrafon of connecfvity Too manual now, and limited (e.g., device-to-device) CSE 461 University of Washington 72

73 Topic How do we connect nodes with a switch instead of mulfple access Uses mulfple links/wires Basis of modern (switched) Ethernet Switch CSE 461 University of Washington 73

74 Switched Ethernet Hosts are wired to Ethernet switches with twisted pair Switch serves to connect the hosts Wires usually run to a closet Switch Switch ports Twisted pair CSE 461 University of Washington 74

75 What s in the box? Remember from protocol layers: Hub, or repeater Physical Physical All look like this: Switch Link Link Router Network Link Network Link CSE 461 University of Washington 75

76 Inside a Hub All ports are wired together; more convenient and reliable than a single shared wire CSE 461 University of Washington 76

77 Inside a Switch Uses frame addresses to connect input port to the right output port; mulfple frames may be switched in parallel... Fabric CSE 461 University of Washington 77

78 Inside a Switch (2) Port may be used for both input and output (full-duplex) Just send, no mulfple access protocol à 4 and 2 à 3 CSE 461 University of Washington 78

79 Inside a Switch (3) Need buffers for mulfple inputs to send to one output Input Output Input Buffer Fabric Output Buffer CSE 461 University of Washington 79

80 Inside a Switch (4) Sustained overload will fill buffer and lead to frame loss XXX Loss! Input Output Input Buffer Fabric Output Buffer CSE 461 University of Washington 80

81 Advantages of Switches Switches and hubs have replaced the shared cable of classic Ethernet Convenient to run wires to one locafon More reliable; wire cut is not a single point of failure that is hard to find Switches offer scalable performance E.g., 100 Mbps per port instead of 100 Mbps for all nodes of shared cable / hub CSE 461 University of Washington 81

82 Switch Forwarding Switch needs to find the right output port for the desfnafon address in the Ethernet frame. How? Want to let hosts be moved around readily; don t look at IP Ethernet Frame Source DesFnaFon CSE 461 University of Washington 82

83 Backward Learning Switch forwards frames with a port/address table as follows: 1. To fill the table, it looks at the source address of input frames 2. To forward, it sends to the port, or else broadcasts to all ports CSE 461 University of Washington 83

84 1: A sends to D Backward Learning (2) Switch D Address Port A B C D CSE 461 University of Washington 84

85 2: D sends to A Backward Learning (3) Switch D Address Port A 1 B C D CSE 461 University of Washington 85

86 3: A sends to D Backward Learning (4) Switch D Address Port A 1 B C D 4 CSE 461 University of Washington 86

87 3: A sends to D Backward Learning (5) Switch D Address Port A 1 B C D 4 CSE 461 University of Washington 87

88 Learning with MulFple Switches Just works with mulfple switches and a mix of hubs assuming no loops, e.g., A sends to D then D sends to A Switch CSE 461 University of Washington 88

89 Learning with MulFple Switches (2) Just works with mulfple switches and a mix of hubs assuming no loops, e.g., A sends to D then D sends to A Switch CSE 461 University of Washington 89

90 Topic How can we connect switches in any topology so they just work This is part 2 of switched Ethernet Loops yikes! CSE 461 University of Washington 90

91 Problem Forwarding Loops May have a loop in the topology Redundancy in case of failures Or a simple mistake Want LAN switches to just work Plug-and-play, no changes to hosts But loops cause a problem Redundant Links CSE 461 University of Washington 91

92 Forwarding Loops (2) Suppose the network is started and A sends to F. What happens? A B C D Le[ / Right E F CSE 461 University of Washington 92

93 Forwarding Loops (3) Suppose the network is started and A sends to F. What happens? A à C à B, D-le[, D-right D-le[ à C-right, E, F D-right à C-le[, E, F C-right à D-le[, A, B C-le[ à D-right, A, B D-le[ à D-right à A E C D B Le[ / Right F CSE 461 University of Washington 93

94 Spanning Tree SoluFon Switches collecfvely find a spanning tree for the topology A subset of links that is a tree (no loops) and reaches all switches They switches forward as normal on the spanning tree Broadcasts will go up to the root of the tree and down all the branches CSE 461 University of Washington 94

95 Spanning Tree (2) Topology One ST Another ST CSE 461 University of Washington 95

96 Spanning Tree (3) Topology One ST Another ST Root CSE 461 University of Washington 96

97 Spanning Tree Algorithm Rules of the distributed game: All switches run the same algorithm They start with no informafon Operate in parallel and send messages Always search for the best solufon Ensures a highly robust solufon Any topology, with no configurafon Adapts to link/switch failures, CSE 461 University of Washington 97

98 Radia Perlman (1952 ) Key early work on roufng protocols RouFng in the ARPANET Spanning Tree for switches (next) Link-state roufng (later) Now focused on network security CSE 461 University of Washington 98

99 Outline: Spanning Tree Algorithm (2) 1. Elect a root node of the tree (switch with the lowest address) 2. Grow tree as shortest distances from the root (using lowest address to break distance Fes) 3. Turn off ports for forwarding if they aren t on the spanning tree CSE 461 University of Washington 99

100 Spanning Tree Algorithm (3) Details: Each switch inifally believes it is the root of the tree Each switch sends periodic updates to neighbors with: Its address, address of the root, and distance (in hops) to root Switches favors ports with shorter distances to lowest root Uses lowest address as a Fe for distances Hi, I m C, the root is A, it s 2 hops away or (C, A, 2) C CSE 461 University of Washington 100

101 Spanning Tree Example 1 st round, sending: A sends (A, A, 0) to say it is root B, C, D, E, and F do likewise 1 st round, receiving: A sfll thinks is it (A, A, 0) B sfll thinks (B, B, 0) C updates to (C, A, 1) D updates to (D, C, 1) E updates to (E, A, 1) F updates to (F, B, 1) A,A,0 E,E,0 C,C,0 D,D,0 B,B,0 F,F,0 CSE 461 University of Washington 101

102 Spanning Tree Example (2) 2 nd round, sending Nodes send their updated state 2 nd round receiving: A remains (A, A, 0) B updates to (B, A, 2) via C C remains (C, A, 1) D updates to (D, A, 2) via C E remains (E, A, 1) F remains (F, B, 1) A,A,0 E,A,1 C,A,1 D,C,1 B,B,0 F,B,1 CSE 461 University of Washington 102

103 Spanning Tree Example (3) 3 rd round, sending Nodes send their updated state 3 rd round receiving: A remains (A, A, 0) B remains (B, A, 2) via C C remains (C, A, 1) D remains (D, A, 2) via C-le[ E remains (E, A, 1) F updates to (F, A, 3) via B A,A,0 E,A,1 C,A,1 D,A,2 B,A,2 F,B,1 CSE 461 University of Washington 103

104 Spanning Tree Example (4) 4 th round Steady-state has been reached Nodes turn off forwarding that is not on the spanning tree A,A,0 C,A,1 B,A,2 Algorithm confnues to run Adapts by Fming out informafon E.g., if A fails, other nodes forget it, and B will become the new root E,A,1 D,A,2 F,A,3 CSE 461 University of Washington 104

105 Spanning Tree Example (5) Forwarding proceeds as usual on the ST IniFally D sends to F: A,A,0 B,A,2 C,A,1 And F sends back to D: D,A,2 E,A,1 F,A,3 CSE 461 University of Washington 105

106 Spanning Tree Example (6) Forwarding proceeds as usual on the ST IniFally D sends to F: D à C-le[ C à A, B A à E B à F A,A,0 C,A,1 B,A,2 And F sends back to D: F à B B à C C à D (hm, not such a great route) E,A,1 D,A,2 F,A,3 CSE 461 University of Washington 106