CS 43: Computer Networks. 16: Reliable Data Transfer October 8, 2018

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1 CS 43: Computer Networks 16: Reliable Data Transfer October 8, 2018

2 Reading Quiz Lecture 16 - Slide 2

3 Last class We are at the transport-layer protocol! provide services to the application layer interact with n/w layer below that provides routing UDP: User Datagram Protocol No frills transport protocol Simple 8-byte header with ports, length, checksum Checksum protects against most bit-flips one s complement of the total sum of 16-bit integers (data + payload) Lecture 16 - Slide 3

4 Layering and encapsulation Port no. Data Layer Message Application IP address TCP/UDP Data Segment Transport MAC address IP TCP/UDP Data Datagram Network Ethernet IP TCP/UDP Data Frame Link Bit stream Physical Lecture 16 - Slide 4

5 Today Principles of reliability The Two Generals Problem Automatic Repeat Requests Stop and Wait outs and Losses Pipelined Transmission Lecture 16 - Slide 5

6 The Two Generals Problem A B Two army divisions (blue) surround enemy (red) Each division led by a general Both must agree when to simultaneously attack If either side attacks alone, defeat Generals can only communicate via messengers Messengers may get captured (unreliable channel) Lecture 16 - Slide 6

7 The Two Generals Problem A B How to coordinate? Send messenger: Attack at dawn What if messenger doesn t make it? Lecture 16 - Slide 7

8 The Two Generals Problem A B How to be sure messenger made it? Send acknowledgment: I delivered message Lecture 16 - Slide 8

9 In the two generals problem, can the two armies reliably coordinate their attack? (using what we just discussed) A. Yes (explain how) B. No (explain why not) Lecture 16 - Slide 9

10 The Two Generals Problem A B Result Can t create perfect channel out of faulty one Can only increase probability of success Lecture 16 - Slide 10

11 Give up? No way! As humans, we like to face difficult problems. We can t control oceans, but we can build canals We can t fly, but we ve landed on the moon We just need engineering! (Unsinkable) Lecture 16 - Slide 11

12 Engineering Concerns Message corruption Message duplication Message loss Message reordering Performance Our toolbox Checksums outs Acks & Nacks Sequence numbering Pipelining Lecture 16 - Slide 12

13 Engineering Concerns Message corruption Message duplication Message loss Message reordering Performance Our toolbox Checksums outs Acks & Nacks Sequence numbering Pipelining We use these to build Automatic Repeat Request (ARQ) protocols. (We ll briefly talk about alternatives at the end.) Lecture 16 - Slide 13

14 Automatic Repeat Request (ARQ) Intuitively, ARQ protocols act like you would when using a cell phone with bad reception. Message garbled? Ask to repeat. Didn t hear a response? Speak again. Refer to book for building state machines. We ll look at TCP s states soon Lecture 16 - Slide 14

15 1. Stop-and-wait ARQ Broad Classifications Lecture 16 - Slide 15

16 Stop and Wait We have: a sender a receiver time: represented by downwards arrow Lecture 16 - Slide 16

17 Stop and Wait sends data and waits till they get the response message from the receiver. Data Response Lecture 16 - Slide 17

18 Stop and Wait Up next: concrete problems and mechanisms to solve them. These mechanisms will build upon each other Questions? Data Response Data Response Lecture 16 - Slide 18

19 Corruption? Error detection mechanism: checksum Data good receiver sends back ACK Data corrupt receiver sends back NACK Data ACK/NACK Lecture 16 - Slide 19

20 Could we do this with just ACKs or just NACKs? Error detection mechanism: checksum Data good receiver sends back ACK Data corrupt receiver sends back NACK A. No, we need them both. B. Yes, we could do without one of them, but we d need some other mechanism. C. Yes, we could get by without one of them. Data ACK/NACK Lecture 16 - Slide 20

21 Could we do this with just ACKs or just NACKs? With only ACK, we could get by with a timeout. With only NACK, we couldn t advance (no good). A. No, we need them both. B. Yes, we could do without one of them, but we d need some other mechanism. C. Yes, we could get by without one of them. Data ACK/NACK Lecture 16 - Slide 21

22 outs and Losses Data out ACK starts a clock. If no response, retry. Lecture 16 - Slide 22

23 outs and Losses out Data ACK out Data ACK Corruption? Send no response starts a clock. If no response, retry. Lecture 16 - Slide 23

24 outs and Losses out Data ACK out Data Corruption? Send no response Data starts a clock. If no response, retry. out ACK Probably not a great idea for handling corruption, but it works. Lecture 16 - Slide 24

25 outs and Losses Data out ACK outs help us handle message losses too! Lecture 16 - Slide 25

26 outs and Losses out Data ACK out Data Data outs help us handle message losses too! out ACK Lecture 16 - Slide 26

27 Adding timeouts might create new problems for us to worry about. How many? Examples? out Data ACK A. No new problems (why not?) B. One new problem (which is..) C. Two new problems (which are..) D. More than two new problems (which are..) Lecture 16 - Slide 27

28 Adding timeouts might create new problems for us to worry about. How many? Examples? out Data ACK out Data ACK A. No new problems (why not?) B. One new problem (which is..) C. Two new problems (which are..) out D. More than two new problems (which are..) Data ACK Lecture 16 - Slide 28

29 Sequence Numbering Add a monotonically increasing label to each msg Ignore messages with numbers we ve seen before When pipelining (a few slides from now) Detect gaps in the sequence (e.g., 1,2,4,5) Lecture 16 - Slide 29

30 What is our link utilization with a stop-and-wait protocol? A. < 0.1 % B. 0.1 % C. 1 % D % E. > 10 % System parameters: Link rate: 8 Mbps (one megabyte per second) RTT: 100 milli-seconds Segment size: 1024 bytes Lecture 16 - Slide 30

31 What is our link utilization with a stop-and-wait protocol? A. < 0.1 % B. 0.1 % C. 1 % System parameters: Link rate: 8 Mbps (one megabyte per second) RTT: 100 milli-seconds Segment size: 1024 bytes D % E. > 10 % Big Problem: Performance is determined by RTT, not channel capacity! Lecture 16 - Slide 31

32 Pipelined Transmission Data-0 Data-1 Data-2 Data-3 Keep multiple segments in flight Allows sender to make efficient use of the link Sequence numbers ensure receiver can distinguish segments We ll talk about how many next time (windowing). Lecture 16 - Slide 32

33 Pipelined Transmission Data-0 Data-1 Data-2 Data-3 Data-0 Data-1 Data-2 Data-3 Now what? Keep multiple segments in flight Allows sender to make efficient use of the link Sequence numbers ensure receiver can distinguish segments We ll talk about how many next time (windowing). Lecture 16 - Slide 33

34 What should the sender do here? Data-0 Data-1 Data-2 Data-3 What information does the sender need to make that decision? Now what? What is required by either party to keep track? A. Start sending all data again from 0. B. Start sending all data again from 2. C. Resend just 2, then continue with 4 afterwards. Lecture 16 - Slide 34

35 1. Stop-and-wait ARQ Broad Classifications 2. Go-back-N Lecture 16 - Slide 35

36 Go-Back-N Data-0 Data-1 Data-2 Retransmit from point of loss Segments between loss event and retransmission are ignored Go-back-N if a timeout event occurs Lecture 16 - Slide 36

37 Go-Back-N Data-0 Data-1 Data-2 Retransmit from point of loss Segments between loss event and retransmission are ignored Go-back-N if a timeout event occurs Lecture 16 - Slide 37

38 Go-Back-N Data-0 Data-1 Data-2 Data-3 Lecture 16 - Slide 38

39 Go-Back-N Data-0 Data-1 Data-2 Data-3 Data-4 Lecture 16 - Slide 39

40 Go-Back-N Data-0 Data-1 Data-2 Data-3 Data-4 Lecture 16 - Slide 40

41 Go-Back-N Data-0 Data-1 Data-2 Data-3 Data-4 Lecture 16 - Slide 41

42 Go-Back-N Data-0 Data-1 Data-2 out Data-3 Data-4 Lecture 16 - Slide 42

43 Go-Back-N Data-0 Data-1 Data-2 out Data-3 Data-4 Data-2 Data-3 Data-4 Lecture 16 - Slide 43

44 Go-Back-N out Data-0 Data-1 Data-2 Data-3 Data-4 Data-2 Data-3 Data-4 Retransmit from point of loss Segments between loss event and retransmission are ignored Go-back-N if a timeout event occurs Fast retransmit Don t wait for timeout if we get N duplicate ACKs Lecture 16 - Slide 44

45 Go-Back-N out Data-0 Data-1 Data-2 Data-3 Data-4 Data-2 Data-3 Data-4 Retransmit from point of loss Segments between loss event and retransmission are ignored Go-back-N if a timeout event occurs Fast retransmit Don t wait for timeout if we get N duplicate ACKs Lecture 16 - Slide 45

46 ARQ Broad Classifications 1. Stop-and-wait 2. Go-back-N 3. Selective repeat a.k.a selective reject, selective acknowledgement Lecture 16 - Slide 46

47 Data-0 Data-1 Data-2 Selective Repeat ACKs each segment individually (not cumulative) only resends those not ACKed Requires extra buffering and state on the receiver Lecture 16 - Slide 47

48 Selective Repeat Data-0 Data-1 Data-2 Lecture 16 - Slide 48

49 Selective Repeat Data-0 Data-1 Data-2 Data-3 Data-4 Lecture 16 - Slide 49

50 Selective Repeat Data-0 Data-1 Data-2 Data-3 Data-4 Lecture 16 - Slide 50

51 Selective Repeat Data-0 Data-1 Data-2 Data-3 Data-4 Data-5 Data-6 Lecture 16 - Slide 51

52 Selective Repeat out Data-0 Data-1 Data-2 Data-3 Data-4 Data-5 Data-6 Data-2 ACKs each segment individually (not cumulative) only resends those not ACKed Requires extra buffering and state on the receiver Lecture 16 - Slide 52

53 ARQ Alternatives Can t afford the RTT s or timeouts? When? Broadcasting, with lots of receivers Very lossy or long-delay channels (e.g., space) Use redundancy send more data Simple form: send the same message N times More efficient: use erasure coding For example, encode your data in 10 pieces such that the receiver can piece it together with any subset of size 8. Lecture 16 - Slide 53

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