Chapter 11: Flow Control – can occur at layer
2 (data link) and at layer 4 (transport)
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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?
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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.
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Recall the layering of protocols
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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
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Byte oriented
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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
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What if flag was part of the data (non-text files)?
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Bit oriented
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More typical of streaming, binary files, graphics,
etc
Frame interpreted as a bit stream
Start and end of frame marked with a
flag=01111110
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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
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Acronyms
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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
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Has similarities to a streaming protocol
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Algorithm 11.1 Sender-site algorithm for the simplest protocol
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Algorithm 11.2 Receiver-site algorithm for the simplest protocol
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Figure 11.7 Flow diagram for Example 11.1
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No mechanism for error control or acknowledgments
Figure 11.8 Design of Stop-and-Wait Protocol
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Algorithm 11.3 Sender-site algorithm for Stop-and-Wait Protocol
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Algorithm 11.4 Receiver-site algorithm for Stop-and-Wait Protocol
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Figure 11.9 Flow diagram for Example 11.2
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Assumes no errors in frames
Assumes frames are not lost
Assumes Acks are not lost
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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
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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
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Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ
(continued)
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Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ
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(continued)
Option: If a NAK frame arrives, proceed as in the
timeout
Algorithm 11.6 Receiver-site algorithm for Stop-and-Wait ARQ Protocol
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11.27
Option: Do an error check and send either an Ack or
NAK frame
Figure 11.11 Flow diagram for Example 11.3
11.28
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What if an ack is not lost but just delayed past when
the timer expires?