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Chapter 11: Flow Control – can occur at layer 2 (data link) and at layer 4 (transport)

Chapter 11: Flow Control – can occur at layer 2 (data link) and at layer 4 (transport). You saw the need in the most recent assignment. Data packets can be damaged, but it’s not only data that can be changed. If the sequence number is changed how do you know what packet was damaged?

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Chapter 11: Flow Control – can occur at layer 2 (data link) and at layer 4 (transport)

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  1. Chapter 11: Flow Control – can occur at layer 2 (data link) and at layer 4 (transport) • You saw the need in the most recent assignment. • Data packets can be damaged, but it’s not only data that can be changed. • If the sequence number is changed how do you know what packet was damaged? • What if the acknowledgment is damaged?

  2. What if a data packet is lost? • What if an acknowledgment is lost? • How many data packets can we acknowledge with one acknowledgment packet? • Book does some calculation of bit rates; you can skip that stuff. We’ll focus on the protocols. • Main thing is that the download speed is a function of not only the raw bit rate, but the flow control protocol used.

  3. Recall the layering of protocols • Frame – layer 2 unit of transmission • Packet – layer 3 unit of transmission • Text uses layer 2 context for flow control, but it does occur at layer 4 (TCP) as well

  4. Figure 2.4 An exchange using the OSI model

  5. Byte oriented • Frame interpreted as a sequence of bytes • Each byte means something • Old protocol typical of transferring text files • Flags (e.g. 01111110) delimit start and end of frame

  6. What if flag was part of the data (non-text files)?

  7. Bit oriented • More typical of streaming, binary files, graphics, etc • Frame interpreted as a bit stream • Start and end of frame marked with a flag=01111110

  8. Again, what if flag is part of the data? • Stuff a bogus 0 after 5 consecutive 1s.

  9. Flow control Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.

  10. Figure 11.5 Taxonomy of protocols discussed in this chapter

  11. Acronyms • ARQ – Automatic Repeat reQuest • ACK – acknowledgment • NAK – negative acknowledgment (indicates a problem with a frame – damaged or never arrived)

  12. Figure 11.6 The design of the simplest protocol with no flow or error control • Has similarities to a streaming protocol

  13. Algorithm 11.1Sender-site algorithm for the simplest protocol

  14. Algorithm 11.2Receiver-site algorithm for the simplest protocol

  15. Figure 11.7 Flow diagram for Example 11.1

  16. No mechanism for error control or acknowledgments

  17. Figure 11.8 Design of Stop-and-Wait Protocol

  18. Algorithm 11.3Sender-site algorithm for Stop-and-Wait Protocol

  19. Algorithm 11.4Receiver-site algorithm for Stop-and-Wait Protocol

  20. Figure 11.9 Flow diagram for Example 11.2

  21. Assumes no errors in frames • Assumes frames are not lost • Assumes Acks are not lost

  22. Each frame has a sequence number • Sequence nos range from 0 to 2m-1, where m is the number of bits used to represent the sequence number • If m=3, sequence nos are as follows 0,….7, 0,….7, 0,….7, etc

  23. ARQ: adds simple error control • Distinguish Ack frames from NAK frames • Implement a timer if neither of the above does not arrive in timely fashion

  24. Figure 11.10 Design of the Stop-and-Wait ARQ Protocol

  25. Algorithm 11.5Sender-site algorithm for Stop-and-Wait ARQ (continued)

  26. (continued) Algorithm 11.5Sender-site algorithm for Stop-and-Wait ARQ • Option: If a NAK frame arrives, proceed as in the timeout

  27. Algorithm 11.6Receiver-site algorithm for Stop-and-Wait ARQ Protocol • Option: Doan error check and send either an Ack or NAK frame

  28. Figure 11.11 Flow diagram for Example 11.3

  29. What if an ack is not lost but just delayed past when the timer expires?

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