chapter 11: flow control – can occur at layer 2 (data link) and at layer 4 (transport)
Post on 23-Mar-2016
33 Views
Preview:
DESCRIPTION
TRANSCRIPT
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
2.4
Figure 2.4 An exchange using the OSI model
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 datathat the sender can send before
waiting for acknowledgment.
11.10
Figure 11.5 Taxonomy of protocols discussed in this chapter
Acronyms ARQ – Automatic Repeat reQuest ACK – acknowledgment NAK – negative acknowledgment (indicates a
problem with a frame – damaged or never arrived)
11.12
Figure 11.6 The design of the simplest protocol with no flow or error control
Has similarities to a streaming protocol
11.13
Algorithm 11.1 Sender-site algorithm for the simplest protocol
11.14
Algorithm 11.2 Receiver-site algorithm for the simplest protocol
11.15
Figure 11.7 Flow diagram for Example 11.1
No mechanism for error control or acknowledgments
11.17
Figure 11.8 Design of Stop-and-Wait Protocol
11.18
Algorithm 11.3 Sender-site algorithm for Stop-and-Wait Protocol
11.19
Algorithm 11.4 Receiver-site algorithm for Stop-and-Wait Protocol
11.20
Figure 11.9 Flow diagram for Example 11.2
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 follows0,….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
11.24
Figure 11.10 Design of the Stop-and-Wait ARQ Protocol
11.25
Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ
(continued)
11.26
Algorithm 11.5 Sender-site algorithm for Stop-and-Wait ARQ(continued)
Option: If a NAK frame arrives, proceed as in the timeout
11.27
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
11.28
Figure 11.11 Flow diagram for Example 11.3
What if an ack is not lost but just delayed past when the timer expires?
top related