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? 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. 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 Figure 2.4 An exchange using the OSI model 2.4 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 What if flag was part of the data (non-text files)? 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 Again, what if flag is part of the data? Stuff a bogus 0 after 5 consecutive 1s. 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. Figure 11.5 Taxonomy of protocols discussed in this chapter 11.10 Acronyms ARQ – Automatic Repeat reQuest ACK – acknowledgment NAK – negative acknowledgment (indicates a problem with a frame – damaged or never arrived) Figure 11.6 The design of the simplest protocol with no flow or error control Has similarities to a streaming protocol 11.12 Algorithm 11.1 Sender-site algorithm for the simplest protocol 11.13 Algorithm 11.2 Receiver-site algorithm for the simplest protocol 11.14 Figure 11.7 Flow diagram for Example 11.1 11.15 No mechanism for error control or acknowledgments Figure 11.8 Design of Stop-and-Wait Protocol 11.17 Algorithm 11.3 Sender-site algorithm for Stop-and-Wait Protocol 11.18 Algorithm 11.4 Receiver-site algorithm for Stop-and-Wait Protocol 11.19 Figure 11.9 Flow diagram for Example 11.2 11.20 Assumes no errors in frames Assumes frames are not lost Assumes Acks are not lost 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 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 Figure 11.10 Design of the Stop-and-Wait ARQ Protocol 11.24 Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ (continued) 11.25 (continued) Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ Option: If a NAK frame arrives, proceed as in the timeout 11.26 Algorithm 11.6 Receiver-site algorithm for Stop-and-Wait ARQ Protocol Option: Do an error check and send either an Ack or NAK frame 11.27 Figure 11.11 Flow diagram for Example 11.3 11.28 What if an ack is not lost but just delayed past when the timer expires?
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