data com tutorials on sliding window protocol and error checking and flow control.

1. Module 15
Sliding Windows Protocol, and
Error Control
1

2. • Textbook sections
– LG Section 5.2 ARQ Protocols
• Topics
1. Sliding Window
• Overview
• Terminology
• The sliding of sliding window
• Considerations
2. Error Control
• Stop-and-wait ARQ
• Go-back-N ARQ
• Selective Repeat ARQ
2

3. 1.0 Sliding Windows - Overview
• Sliding window protocol (SWP):
– A protocol that allows several data units to be in transition before
receiving an acknowledgement. SWP is used in TCP for error
control and flow Control
– Send window
• Maintain by the sender
• A set of sequence number
• Represents a range of permissible sequence numbers for
transmitted but not-yet-acknowledged frames
• As the protocol operates, the window slides forward over the
sequence number space.
– Receive window:
• Maintain by the receiver
• A set of sequence number
• Represents a range of frames it is permitted to accept
• As the protocol operates, the window slides forward over the
sequence number space.
– Microsoft Windows NT use 8760 as the default window size
3

4. 1.0 Sliding Windows - Overview
– Typical scenario
• TCP sender accepts a data stream from the upper layers,
breaks large streams of data into smaller segments, sequence
each segment, and places this stream in a send window
• TCP sender adds this information in the transport layers
header, and then hands it down to IP to be formed into packets
• When TCP transmits each segment to IP, the TCP protocol
sets a retransmission timer, specifying how long it’ll wait for
an acknowledgement (an ACK) before the segment is
retransmitted. A copy of each segment remains inside the send
window until an acknowledgement is received
4

5. 1.0 Sliding Windows - Overview
– Typical scenario (continue)
• The sequence number that TCP adds to each segment are used
to put the segments back together at the receiver.
• An acknowledge for each segment is sent back to the sender.
• When the acknowledge is returned from the receiver and
received by the sender, the send window slides (hence the
name sliding windows) pass the acknowledged data to the
remaining data stream waiting to be sent.
• If the sender does not receive an acknowledgment with the
time originally set, the segment will be resent.
• Retransmitting segments takes up precious bandwidth on the
network.
5

6. 1.0 Sliding Windows - Overview
– Highlights
• Each byte of data is assigned a sequence number
• The sequence number of the first byte of data in a segment is
transmitted with that segment and is called the segment
sequence number
• Segment also carry an acknowledgement number which is the
sequence number of the next expected data byte of
transmission in the reverse direction.
• When the TCP transmits a segment containing data, it puts a
copy on a retransmission queue and starts a timer.
– When the acknowledgement for that data is received, the
segment is deleted from the queue.
– If the acknowledgement is not received before the timer runs out,
the segment is retransmitted.
• The TCP receiver reports a “window” to the TCP sender. This
window specifies the number of bytes, starting with the
acknowledge number, that the TCP receiver is currently
prepared to receive. (Note: This is the value in the window
size field from the receiver)
6

7. 1.0 Sliding Windows - Terminology
Send Sequence Space
1 2 3 4
------------|------------------|------------------|--------------
SND.UNA SND.NXT SND.UNA + SND.WND
SND.UNA: Send unacknowledged
SND.NXT: Send nest
SND.WND: Send window size
1 – old sequence numbers which have been acknowledged
2 – sequence numbers of unacknowledged data
3 – sequence numbers allowed for new data transmission
4 – future sequence numbers which are not yet allowed
The send window is the portion of the sequence space labeled 3.
7

8. 1.0 Sliding Windows - Terminology
Receive Sequence Space
1 2 3
------------|------------------|----------------
RCV.NXT RCV.NXT + RCV.WND
RCV.NXT: receive next
RCV.WND: receive window size
1 – old sequence numbers which have been acknowledged
2 – sequence numbers allowed for new reception
3 – future sequence numbers which are not yet allowed
The receive window is the portion of the sequence space labeled 2.
8

9. 1.0 Sliding Windows - The sliding of sliding window
Sliding Window
9

10. 1.0 Sliding Windows - The sliding of sliding window
Sender Sliding Window
Note:
• ACK controls the right boundary of the send window
• Sending frames controls the left boundary of the send window
10

11. 1.0 Sliding Windows - The sliding of sliding window
Receiver Sliding Window
Note:
• ACK controls the right boundary of the receive window
• Receiving frames controls the left boundary of the receive window
11

12. Sliding Window Example
12

13. 1.0 Sliding Windows - Considerations
• Considerations
– Sliding Window Protocol (SWP) mechanisms
• Acknowledgement (ACK): A response sent by the receiver to
indicate the successful receipt and acceptance of data.
• Negative acknowledgement (NAK): A message sent to
indicate the rejection of received data.
• Retransmission timer
• Sender application programs, receiver application programs,
sequence number, acknowledgement number, window size.
– The relationship between sequence number space and maximum
window size
13

14. 2. Error Control
14

15. 2. Error Control
• Purpose
– A technique used to ensure that a data stream is delivered accurately to
the user despite errors that occur during transmission
– Automatic Repeat Request (ARQ) forms the basis for peer-to-peer
protocols
– For both hop-by-hop and end-to-end approaches
– Information flow can be in one direction or both directions
• Assumptions
– Frames arrive at the receiver in the same order in which they are sent
• Types
– Stop-and-wait ARQ
– Go-back-N ARQ
– Selective Repeat ARQ
15

16. 2. Error Control
• For each type of ARQ, four scenarios are considered:
– Normal scenario
– Abnormal scenarios:
• Damaged frame
• Lost frame
• Lost ACK
16

17. 2. Error Control
LG Figure 5.8 Basic elements of ARQ
Information Error-free
Packet
frames packet
sequence
sequence
Transmitter Receiver
Station A Control Station B
frames
CRC CRC Header
Header Control frame
Information
packet
Information Frame 17

18. 2. Error Control – Stop-and-Wait ARQ
• Stop-and-Wait ARQ
– Sender uses: ACK, retransmission timer, and sequence number
– Scenario 1 (normal and Frame 1 lost)
Steps Events
1 Station A transmit frame 0 and then waits for an ACK frame from the receiver
2 Frame 0 is transmitted without error, so station B transmits an ACK frames.
3 The ACK from station B is also received without error, so station A knows the
frame 0 has been received correctly.
4 Station A now proceeds to transmit frame 1 and then resets the timer.
5 Frame 1 undergoes errors in transmission. It is possible that station B receives
frame 1 and detects the errors through the CRC check; it is also possible that frame
1 was so badly garbled that station is unaware of the transmission. In either case
station B does not take any action
6 The time-out period expires, and frame 1 is retransmitted
18

19. 2. Error Control – Stop-and-Wait ARQ
LG Figure 5.9 Possible ambiguities when frames are unnumbered
(a) Frame 1 lost Time-out
time
A frame frame frame frame
0 1 1 2
ACK ACK
B
(b) ACK lost Time-out
time
A frame frame frame frame
0 1 1 2
ACK ACK ACK
B
In parts (a) and (b) transmitting station A acts the same way, but part
(b) receiving station B accepts frame 1 twice.
19

20. 2. Error Control – Stop-and-Wait ARQ
– Scenario 2 (ambiguity due to ACK not received)
• Figure 5.9 (b)
• Problem: Loss of an ACK can result in the delivery of duplicated
packet to station B
• Solution: Include a sequence number in the header of each
frame
– Scenario 3 (ambiguity due to premature time-out and loss
of frame)
• Figure 5.10
• Problem: Premature time-outs combined with loss of I-frame can
result in gaps in the delivered packet sequence
• Solution: Provide a sequence number in the acknowledgement
frame that enables the transmitter to determine which frames have
been received.
20

21. 2. Error Control – Stop-and-Wait ARQ
Figure 5.10 Possible ambiguities when ACKs are unnumbered
time-out
time
A frame
0 frame frame frame
0 1 2
ACK
ACK
B
Transmitting station A misinterprets duplicate ACKs
21

22. Figure 5.11 System state information in Stop-and-wait ARQ
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
Slast Rnext
Timer
Slast
Transmitter Receiver
Station A Rnext Station B
(0,0) Error-free frame 0 (0,1)
arrives at receiver
Global State:
ACK for
(Slast, Rnext) ACK for frame 0
frame 1 arrives at
arrives at transmitter
transmitter Error-free frame 1
arrives at receiver
(1,0) (1,1)
22

23. 2. Error Control – Stop-and-Wait ARQ
• Delay-bandwidth product
– The product of the bit rate and the delay that elapses before an action
can take place
– A key performance parameter
– In stop-and-wait the delay-bandwidth product can be viewed as a
measure of lost opportunity in terms of transmitted bits.
23

24. 2. Error Control – Stop-and-Wait ARQ
Stop and Wait
EOT: End of Transmission
24

25. 2. Error Control – Stop-and-Wait ARQ
Damaged Frame
sequence space consists of 0 and 1
0
1
0
Error in Frame 0
0
0
25

26. 1.0 Sliding Windows
Lost Frame
0
0
0
0
26

27. 1.0 Sliding Windows
Lost ACK
0
0 second
27

28. 2. Error Control – Go-Back-N(GBN) ARQ
• Go-Back-N(GBN) ARQ
– Pipeline
• A procedure where the processing of a new task is begun before
the completion of the previous task
• In the context of ARQ, the sender will have to buffer packets that
have been transmitted, but not yet acknowledged. Buffering of
received packets may also be needed at the receiver.
– Description:
• In a GBN protocol, the sender is allowed to transmit multiple
packets (when available) without waiting for an
acknowledgement, but is constrained to have no more than some
maximum allowable number, N, of unacknowledged packets in
the pipeline.
28

29. 2. Error Control – Go-Back-N(GBN) ARQ
• Examples
– Example 1
• Figure 5.13 Basic Go-Back-N ARQ
• Typical case when N = WS = 4
– Example 2
• Figure 5.15 Go-Back-N ARQ
• Use timer
– Example 3
• Figure 5.16 The window size should be less than 2 m
• The receiver can determine the correct frame if the window
size is smaller than 2m where m is the number of bits in the
header which are allotted to the sequence number
representation
29

30. 2. Error Control – Go-Back-N(GBN) ARQ
– Example 4
• Figure 5.17 NAK error recovery
• The NAK procedure results in having the transmitter go back less
than WS frames
– Example 5
• Figure 5.18 System parameter in bidirectional Go-Back-N ARQ
• Piggybacking: The inclusion of acknowledgement on a data frame.
When a data frame arrives, instead of immediately sending a
separate control frame, the receiver restrains itself and wait until
the network layer passes it the next packet. The acknowledgement
is attached to the outgoing data frame (using the ACK field in the
frame header). In effect, the acknowledgement gets a free ride on
the next outgoing data gram.
30

31. 2. Error Control – Go-Back-N(GBN) ARQ
LG Figure 5.13 Basic Go-Back-N ARQ
Go-Back-4: 4 frames are outstanding; so go back 4
fr fr fr fr fr fr fr fr fr fr fr fr fr fr time
0 1 2 3 4 5 6 3 4 5 6 7 8 9
A
B
A A A Out-of-sequence frames A A A A A A
C C C C C C C C C
K K K K K K K K K
1 2 3 error 4 5 6 7 8 9
31

32. 2. Error Control – Go-Back-N(GBN) ARQ
LG Figure 5.15 Go-Back-N ARQ
Transmitter Receiver
Send Window
... Receive Window
Frames
transmitted S
last Srecent Slast+Ws-1
and ACKed
frames
Buffers Rnext
received
Timer Slast
Timer Slast+1 The receiver will only accept
... a frame that is error-free and
that has sequence number Rnext
Timer
Srecent
...
Slast+Ws-1
32

33. LG Figure 5.16 The window size should be less then 2 m
M =22 = 4, Go-Back - 4: Transmitter goes back 4
fr fr fr fr fr fr fr fr time
0 1 2 3 0 1 2 3
A
A A
B A
C C C
A
C Receiver has Rnext=0, but it does not
K K
K
2 3
K know whether its ACK for frame 0
1 4
was received, so it does not know
whether this is the old frame 0 or a
new frame 0
M=22=4, Go-Back-3: Transmitter goes back 3
fr fr fr fr fr fr time
0 1 2 0 1 2
A
A A
B A
C C C
K K K Receiver has Rnext=3 , so it rejects the old
1 2 3 frame 0
33

34. 2. Error Control – Go-Back-N(GBN) ARQ
LG Figure 5.17 NAK error recovery
Transmitter goes back to frame 1
Go-Back-7:
fr fr fr fr fr fr fr fr fr fr fr fr fr fr time
0 1 2 3 4 5 1 2 3 4 5 6 7 0
A
B
A N Out-of-sequence A A A A A A
C A frames C C C C C C
K K K K K K K K
1 1 2 3 4 5 6 7
error
34

35. LG Figure 5.18 System parameters in bidirectional Go-Back-N ARQ
Station A Station B
SArecent RA next
Transmitter Receiver
Receiver Transmitter
SBrecent RB next
ACKs are piggybacked in headers
“A” Receive Window “B” Receive Window
RA next RB next
“A” Send Window “B” Send Window
... ...
SA last SA last+WA s-1 SB last SB last+WB s-1
Buffers Buffers
Timer SA last Timer SB last
Timer SA last+1 Timer SBlast+1
... ...
Timer SArecent Timer
SBrecent
... ...
Timer SA last+WA s-1 Timer 35 +W
S B
last
B
-1
s

36. 2. Error Control – Go-Back-N(GBN) ARQ
Damaged Data Frame
36

37. 2. Error Control – Go-Back-N(GBN) ARQ
Lost Data Frame
37

38. 2. Error Control – Go-Back-N(GBN) ARQ
Lost ACK
38

39. 2. Error Control – Selective Repeat ARQ
• Selective Repeat ARQ
– Two new features
• The receiver windows is made larger than one frame so that the
receiver can accept frames that are out of order but error free
• The retransmission mechanism is modified so that only individual
frames are retransmitted
– Algorithm
39

40. 2. Error Control – Selective Repeat ARQ
Transmitter’s events and actions
Event Action
Data received from When data is received from above, the transmitter checks the next available
upper layer sequence number for the packet. If the sequence number is within the
transmitter’s send window, the data is packetized and sent. Otherwise it is either
buffered or returned to the upper layer for later transmission.
Ack received If an ACK is received, the transmitter marks that packet as having been received.
If the packet’s sequence number is equal to Slast,the send window is moved to the
unacknowledged packet with the smallest sequence number. If the window moves
and there are un-transmitted packets with sequence numbers that now fall within
the send window, these packets are transmitted.
Timeout or NAK • When the transmitter receives a NAK frame, it retransmits the specific frame.
received • When a packet’s logical timer timed-out, it will be retransmitted
40

41. 2. Error Control – Selective Repeat ARQ
Receiver’s Events and Actions
Event Action
Packet with • ACK packet is returned to the transmitter
sequence number in • If the packet was not previously received, it is buffered
receive window is
• If this packet has a sequence number equal to Rnext, then this packet and any
correctly received
previously buffered and consecutively numbered packets are delivered to the
upper layer.
• The receive window is then moved forward by the number of packets delivered
to the upper layer.
• If an out-of –sequence frame is observed at the receiver, A NAK can be sent
with sequence number Rnex
Packet with An ACK must be generated, even though this is a packet that the receiver has
sequence number previously acknowledged.
between Rnex –1 and
Rnex –WR is correctly
received
Otherwise Ignore the packet
41

42. 2. Error Control – Selective Repeat ARQ
• Selective Repeat ARQ
• Retransmission mechanism
– Timer: When the timer expires, only the corresponding frame
is retransmitted.
– NAK: Whenever an out-of-sequence frame is observed at the
receiver, a NAK frame is sent with sequence number Rnext.
When the transmission receives such a NAK frame, it
retransmits the specific frame, Rnext.
– Figure 5.21 Error recovery in Selective Repeat ARQ
– Maximum send window size
• WS = WR = 2m-1 = (2m)/2 = half the sequence number space
• Example: Figure 5.22 Maximum window size in Selective Repeat
ARQ
42

43. LG Figure 5.20 Selective Repeat ARQ
Transmitter Receiver
Receive Window
Send Window
...
Frames
transmitted S Frames
last Srecent Slast+Ws-1
and ACKed received Rnext Rnext +Wr-1
Buffers Buffers
Timer Slast Rnext+1
Timer Slast+1 Rnext+2
...
...
Timer
Srecent
Rnext+Wr-1
...
Slast+Ws-1
43

44. 2. Error Control – Selective Repeat ARQ
LG Figure 5.21 Error recovery in Selective Repeat ARQ
fr fr fr fr fr fr fr fr fr fr fr fr fr fr time
0 1 2 3 4 5 6 2 7 8 9 10 11 12
A
B
A A N A A A A A A A A A
C C A C C C C C C C C C
K K K K K K K K K K K K
1 2 2 2 2 2 7 8 9 1 1 1
error 0 1 2
44

45. 1.0 Sliding Windows
Selective Reject
45

46. LG Figure 5.22 Maximum window size in Selective Repeat ARQ
M=22=4, Selective Repeat: Send Window = Receive Window = 3
Frame 0 resent
fr fr fr fr time
0 1 2 0
A
A A A
B C C C
K K K
Receive Window {3,0,1}
1 2 3
Send Window = Receive Window = 2
Frame 0 resent
fr fr fr time
0 1 0
A
A
B A
C C frame 0 rejected
K
1
K
2 46
Receive Window {2,3}