Module 15 Sliding Windows Protocol, and Error Control

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Module 15Sliding Windows Protocol, and Error
Control
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• Textbook sections
• LG Section 5.2 ARQ Protocols
• Topics
• Sliding Window
• Overview
• Terminology
• The sliding of sliding window
• Considerations
• Error Control
• Stop-and-wait ARQ
• Go-back-N ARQ
• Selective Repeat ARQ

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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

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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 itll 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

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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.

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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)

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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.

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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.

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1.0 Sliding Windows - The sliding of sliding
window
Sliding Window
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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

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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

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Sliding Window Example
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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

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2. Error Control
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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

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2. Error Control

• For each type of ARQ, four scenarios are
  considered
• Normal scenario
• Abnormal scenarios
• Damaged frame
• Lost frame
• Lost ACK

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

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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 0
frame 1
frame 1
frame 2
ACK
ACK
B
(b) ACK lost
Time-out
time
A
frame 0
frame 1
frame 1
frame 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.

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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.

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

• Transmitting station A misinterprets duplicate
  ACKs

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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
Rnext
Slast
Timer
Slast
Receiver
Transmitter
Rnext
Station B
Station A
Error-free frame 0 arrives at receiver
(0,0)
(0,1)
Global State (Slast, Rnext)
ACK for frame 0 arrives at transmitter
ACK for frame 1 arrives at transmitter
Error-free frame 1 arrives at receiver
(1,0)
(1,1)
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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.

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2. Error Control Stop-and-Wait ARQ
Stop and Wait
EOT End of Transmission
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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
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1.0 Sliding Windows
Lost Frame
0
0
0
0
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1.0 Sliding Windows
Lost ACK
0
second
0
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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.

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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
  2m
• 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

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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.

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2. Error Control Go-Back-N(GBN) ARQ
LG Figure 5.13 Basic Go-Back-N ARQ
4 frames are outstanding so go back 4
Go-Back-4
time
fr 0
fr 1
fr 2
fr 3
fr 4
fr 5
fr 3
fr 5
fr 4
fr 6
fr 6
fr 7
fr 8
fr 9
A
B
Out-of-sequence frames
ACK1
ACK2
ACK4
ACK5
ACK3
ACK7
ACK6
ACK9
ACK8
error
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2. Error Control Go-Back-N(GBN) ARQ
LG Figure 5.15 Go-Back-N ARQ
Transmitter
Receiver
Send Window
Receive Window
...
Frames transmitted and ACKed
Slast
Srecent
SlastWs-1
frames received
Buffers
Rnext
Slast
Timer
Slast1
The receiver will only accept a frame that is
error-free and that has sequence number Rnext
Timer
...
Srecent
Timer
...
SlastWs-1
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LG Figure 5.16 The window size should be less
then 2m
Transmitter goes back 4
M 22 4, Go-Back - 4
time
fr 0
fr 2
fr 1
fr 1
fr 3
fr 3
fr 2
fr 0
A
B
ACK2
ACK3
ACK4
ACK1
Receiver has Rnext0, but it does not know
whether its ACK for frame 0 was received, so it
does not know whether this is the old frame 0 or
a new frame 0
M224, Go-Back-3
Transmitter goes back 3
time
fr 0
fr 2
fr 1
fr 2
fr 0
fr 1
A
B
ACK2
ACK3
ACK1
Receiver has Rnext3 , so it rejects the old
frame 0
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2. Error Control Go-Back-N(GBN) ARQ
LG Figure 5.17 NAK error recovery
Transmitter goes back to frame 1
Go-Back-7
time
fr 0
fr 1
fr 2
fr 3
fr 4
fr 5
fr 1
fr 2
fr 4
fr 3
fr 5
fr 6
fr 7
fr 0
A
B
ACK2
ACK6
ACK1
ACK3
ACK4
ACK5
ACK7
Out-of-sequence frames
NAK1
error
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LG Figure 5.18 System parameters in bidirectional
Go-Back-N ARQ
Station B
Station A
SArecent RA next
Receiver
Transmitter
Transmitter
Receiver
SBrecent RB next
ACKs are piggybacked in headers
A Send Window
B Send Window
...
...
SA last
SB last
SA lastWA s-1
SB lastWB s-1
Buffers
Buffers
SA last
SB last
Timer
Timer
SA last1
SBlast1
Timer
Timer
...
...
SArecent
Timer
SBrecent
Timer
...
...
SA lastWA s-1
SB lastWB s-1
Timer
Timer
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2. Error Control Go-Back-N(GBN) ARQ
Damaged Data Frame
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2. Error Control Go-Back-N(GBN) ARQ
Lost Data Frame
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2. Error Control Go-Back-N(GBN) ARQ
Lost ACK
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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

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2. Error Control Selective Repeat
ARQTransmitters events and actions
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Receivers Events and Actions
2. Error Control Selective Repeat ARQ
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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

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LG Figure 5.20 Selective Repeat ARQ
Transmitter
Receiver
Send Window
...
Frames transmitted and ACKed
Slast
Srecent
SlastWs-1
Buffers
Buffers
Rnext1
Rnext2
...
RnextWr-1
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2. Error Control Selective Repeat ARQ
LG Figure 5.21 Error recovery in Selective Repeat
ARQ
time
fr 0
fr 1
fr 2
fr 3
fr 4
fr 5
fr 2
fr 8
fr 7
fr 6
fr 9
fr 10
fr 11
fr 12
A
B
ACK2
NAK2
ACK7
ACK8
ACK9
ACK10
ACK11
ACK12
ACK2
ACK2
ACK1
ACK2
error
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1.0 Sliding Windows
Selective Reject
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LG Figure 5.22 Maximum window size in Selective
Repeat ARQ
M224, Selective Repeat Send Window Receive
Window 3
Frame 0 resent
time
fr 0
fr 2
fr 1
fr 0
A
B
ACK2
ACK3
ACK1
3,0,1
Receive Window
Send Window Receive Window 2
Frame 0 resent
time
fr 0
fr 1
fr 0
A
B
ACK2
ACK1
frame 0 rejected
Receive Window
2,3