Module15: Sliding Windows Protocol and Error Control

1

Module 15Sliding Windows Protocol, and

Error Control

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

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

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

5


– 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.

6


– 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)

7

1.0 Sliding Windows - Terminology

Send Sequence Space

1 2 3 4------------|------------------|------------------|--------------

SND.UNA SND.NXT SND.UNA + SND.WND

SND.UNA: Send unacknowledgedSND.NXT: Send nestSND.WND: Send window size

1 – old sequence numbers which have been acknowledged2 – sequence numbers of unacknowledged data3 – sequence numbers allowed for new data transmission4 – future sequence numbers which are not yet allowed

The send window is the portion of the sequence space labeled 3.

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.

9

Sliding Window

1.0 Sliding Windows - The sliding of sliding window

10

Sender Sliding Window


Note: • ACK controls the right boundary of the send window• Sending frames controls the left boundary of the send window

11

Receiver Sliding Window


Note:• ACK controls the right boundary of the receive window• Receiving frames controls the left boundary of the receive window

12

Sliding Window Example

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

14

2. Error Control

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

16

2. Error Control

• For each type of ARQ, four scenarios are considered:– Normal scenario

– Abnormal scenarios:

• Damaged frame

• Lost frame

• Lost ACK

17

Packet sequence

Error-free packet

sequence

Informationframes

Control frames

Transmitter Receiver

CRC

Informationpacket

Header

Station A Station B

Information Frame

Control frame

CRC Header

LG Figure 5.8 Basic elements of ARQ

2. Error Control

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

19

(a) Frame 1 lost

A

B

frame 0

frame1

ACK

frame1

ACK

timeTime-out

frame2

(b) ACK lost

A

B

frame 0

frame1

ACK

frame1

ACK

timeTime-out

frame2

ACK

In parts (a) and (b) transmitting station A acts the same way, but part (b) receiving station B accepts frame 1 twice.

LG Figure 5.9 Possible ambiguities when frames are unnumbered


20


– 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.

21

A

B

frame 0 frame

0ACK

frame1

ACK

timetime-out

frame2

Transmitting station A misinterprets duplicate ACKs

Figure 5.10 Possible ambiguities when ACKs are unnumbered


22


SlastRnext

0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

(0,0) (0,1)

(1,0) (1,1)

Timer

Global State:(Slast, Rnext)

Error-free frame 0arrives at receiver

ACK forframe 0arrives attransmitter

ACK forframe 1arrives attransmitter Error-free frame 1

arrives at receiver

Station A Station BRnext

Slast

Figure 5.11 System state information in Stop-and-wait ARQ

23


• 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.

24

Stop and Wait


EOT: End of Transmission

25

Damaged Framesequence space consists of 0 and 1


0

1

0

Error in Frame 0

0

0

26

Lost Frame

1.0 Sliding Windows

0

0

00

27

Lost ACK

1.0 Sliding Windows

second0

0

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.

29


• 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

30


– 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.

31

A

B

fr0

timefr1

fr2

fr3

fr4

fr5

fr6

fr3

ACK1 error

Out-of-sequence frames

Go-Back-4: 4 frames are outstanding; so go back 4

fr5

fr6

fr4

fr7

fr8

fr9

ACK2

ACK3

ACK4

ACK5

ACK6

ACK7

ACK8

ACK9

LG Figure 5.13 Basic Go-Back-N ARQ


32

Timer Slast

Slast+1

Srecent

Slast+Ws-1

Timer

Timer


...

Receive Window

Buffers

Slast Slast+Ws-1

...Send Window

Srecent

Framestransmittedand ACKed

Rnext

framesreceived

The receiver will only accepta frame that is error-free and that has sequence number Rnext

...

LG Figure 5.15 Go-Back-N ARQ


33

A

B

fr0

timefr1

fr2

fr3

fr0

fr1

fr2

fr3

ACK1

M =22 = 4, Go-Back - 4:

ACK4

ACK2

ACK3

Transmitter goes back 4

Receiver has Rnext=0, 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

A

B

fr0

timefr1

fr2

fr0

fr1

fr2

ACK1

M=22=4, Go-Back-3:

ACK2

ACK3

Transmitter goes back 3

Receiver has Rnext=3 , so it rejects the old frame 0

LG Figure 5.16 The window size should be less then 2m

34

A

B

fr0

timefr1

fr2

fr3

fr4

fr5

fr1

fr2

ACK1

error

Out-of-sequenceframes

Go-Back-7:

fr4

fr5

fr3

fr6

fr7

fr0

NAK1

ACK3

ACK4

ACK5

ACK6

ACK7

ACK2

Transmitter goes back to frame 1

LG Figure 5.17 NAK error recovery


35


TransmitterReceiver

ACKs are piggybacked in headers

Station A Station B

RA next

“A” Receive Window

RB next

“B” Receive Window

SA last

SA last

SA last+1

SArecent

SA last+WA s-1Timer

SA last+WA s-1

...

...

Buffers

“A” Send Window

...

SB last

SB last

SBlast+1

SBrecent

SB last+WB s-1

SB last+WB s-1

...

...

Buffers

“B” Send Window

...

SArecent RA next

SBrecent RB next

Timer

Timer

Timer

Timer

Timer

Timer

Timer

LG Figure 5.18 System parameters in bidirectional Go-Back-N ARQ

36

Damaged Data Frame


37

Lost Data Frame


38

Lost ACK


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

40


Transmitter’s events and actions

Event Action

Data received from upper layer

When data is received from above, the transmitter checks the next available 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 received

• When the transmitter receives a NAK frame, it retransmits the specific frame.

• When a packet’s logical timer timed-out, it will be retransmitted

41

Receiver’s Events and Actions

Event Action

Packet with sequence number in receive window is correctly received

• ACK packet is returned to the transmitter

• If the packet was not previously received, it is buffered

• If this packet has a sequence number equal to Rnext, then this packet and any 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 sequence number between Rnex –1 and

Rnex –WR is correctly received

An ACK must be generated, even though this is a packet that the receiver has previously acknowledged.

Otherwise Ignore the packet


42


• 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

43


Buffers

Slast Slast+Ws-1

...Send Window

Srecent

Framestransmittedand ACKed

Timer Slast

Slast+1

Srecent

Slast+Ws-1

Timer

Timer

...

...

Receive Window

Rnext

Framesreceived Rnext +Wr-1

Rnext+1

Rnext+2

Rnext+Wr-1

...

Buffers

LG Figure 5.20 Selective Repeat ARQ

44

A

B

fr0

timefr1

fr2

fr3

fr4

fr5

fr6

fr2

ACK1 error

fr8

fr9

fr7

fr10

fr11

fr12

ACK2

NAK2

ACK7

ACK8

ACK9

ACK10

ACK11

ACK12

ACK2

ACK2

ACK2

LG Figure 5.21 Error recovery in Selective Repeat ARQ


45

Selective Reject

1.0 Sliding Windows

46

A

B

fr0

timefr1

fr2

fr0

ACK1

M=22=4, Selective Repeat: Send Window = Receive Window = 3

ACK2

ACK3

Receive Window {3,0,1}

Frame 0 resent

A

B

fr0

timefr1

fr0

ACK1

Send Window = Receive Window = 2

ACK2

Receive Window {2,3}

Frame 0 resent

frame 0 rejected

LG Figure 5.22 Maximum window size in Selective Repeat ARQ

Documents

Module15: Sliding Windows Protocol and Error Control