1. ECE 4450:427/527 - Computer Networks Spring 2017
Dr. Nghi Tran
Department of Electrical & Computer Engineering
Lecture 5.3: Reliable Transmission
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

2. Link Layer: Five Common Problems
Basic problem: you can’t just send IP datagrams over the link!
We first consider how to encode bits into the signal at the source and recover bits at the receiving node
Once it is possible to transmit bits, we need to figure out how to package these bits into FRAME
Assume each node is able to recognize the collections of bits making up a frame, the third problem is to determine if those bits are in error: Error Detection and Correction
If frames arriving at destination contain errors, how to recover from such losses: ARQ
Final problem related to multiple-access link: how mediate access to a shared link so that all nodes have a chance to transmit: We focus on Ethernet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

3. Discussions We have learned so far:
Error Detection to detect errors in Frames
Error Correction to correct errors in Frames
But sometimes, errors are too severe to be corrected and corrupted frames must be discarded.
Now we consider a link-level protocol to deal with this issue so that frames can be delivered reliably:
A combination of two fundamental mechanisms: Acknowledgement (ACK) and Timeout – Automatic Repeat reQuest (ARQ).
Error detection?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

4. Acknowledgement
An acknowledgement (ACK for short) is a small control frame that a protocol sends back to its peer saying that it has received the earlier frame.
A control frame is a frame with header only (no data).
The receipt of an acknowledgement indicates to the sender of the original frame that its frame was successfully delivered.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

5. Timeout
If the sender does not receive an acknowledgment after a reasonable amount of time, then it retransmits the original frame.
The action of waiting a reasonable amount of time is called a timeout.
The general strategy of using acknowledgements and timeouts to implement reliable delivery is sometimes called Automatic Repeat reQuest (ARQ).
We now study different ARQ algorithms. We shall use generic language, i.e., do not give detail about header fields.
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

6. Stop-and-Wait Idea of stop-and-wait protocol is straightforward
After transmitting one frame, the sender waits for an acknowledgement before transmitting the next frame.
If the acknowledgement does not arrive after a certain period of time, the sender times out and retransmits the original frame
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

7. Stop-and-Wait Protocol
Let assume we transmit a frame. How many scenarios we can have with Stop-and-Wait?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

8. Stop-and-Wait Protocol: Issue
ACK is lost or arrives after timeout: what happen?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

9. Stop-and-Wait Protocol: Issue
How to deal with this issue: duplicate copies of frames will be delivered
Use 1 bit sequence number (0 or 1) in the header
When the sender retransmits frame 0, the receiver can determine that it is seeing a second copy of frame 0 rather than the first copy of frame 1 and therefore can ignore it (the receiver still acknowledges it, in case the first acknowledgement was lost)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

10. Stop-and-Wait Protocol: Inefficiency
How many outstanding frames on the link at a time?
This may be far below the link’s capacity
Consider a 1.5 Mbps link with a 45 ms RTT
The link has a delay  bandwidth product of 67.5 Kb or approximately 8 KB
Since the sender can send only one frame per RTT and assuming a frame size of 1 KB
Maximum Sending rate
Bits per frame  Time per frame = 1024  8  = 182 Kbps
Or about one-eighth of the link’s capacity
How can we use the link fully?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

11. Sliding-Window Protocol
Main idea: Leave up to N frames unacknowledged at any given time: Sender ready to send (N+1)th frame at the same moment ACK for the first frame arrives
first packet bit transmitted
sender
receiver
RTT
last packet bit transmitted
first packet bit arrives
last packet bit arrives, send ACK
ACK arrives, send next
Packet
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

12. Sliding-Window Protocol
Main idea: Leave up to N frames/packets unacknowledged at any given time: Sender ready to send (N+1)th frame at the same moment ACK for the first frame arrives
sender
receiver
RTT
last bit transmitted
first packet bit arrives
last packet bit arrives, send ACK
ACK arrives, send next
packet
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
first packet bit transmitted
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

13. Sliding-Window Protocol
Perhaps the best known algorithm in computer networking: Need to understanding it thoroughly!!!
It can be used to serve different roles in different layers:
Reliable delivery at Link Layer
Flow control: managing the rate of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver
TCP: Also reliable deliver
We shall consider those later on
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

14. Sliding-Window Protocol: Algorithm
Sender assigns a sequence number denoted as SeqNum to each frame
Sender maintains three variables
Sending Window Size (SWS)
Upper bound on the number of outstanding (unacknowledged) frames that the sender can transmit
Last Acknowledgement Received (LAR)
Sequence number of the last acknowledgement received
Last Frame Sent (LFS)
Sequence number of the last frame sent
Sender also maintains the following invariant
LFS – LAR ≤ SWS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

15. Sliding-Window Protocol
Sliding Window on Sender
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

16. Sliding-Window Protocol
When an acknowledgement arrives
the sender moves LAR to right, thereby allowing the sender to transmit another frame
Also the sender associates a timer with each frame it transmits
It retransmits the frame if the timer expires before the ACK is received
Note that the sender has to be willing to buffer up to SWS frames
WHY?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

17. Sliding-Window Protocol: Example
first packet bit transmitted
sender
receiver
RTT
last bit transmitted
first packet bit arrives
last packet bit arrives, send ACK
ACK arrives, send next
packet
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

18. Sliding-Window Protocol
Receiver maintains three variables
Receiving Window Size (RWS)
Upper bound on the number of out-of-order frames that the receiver is willing to accept
Largest Acceptable Frame (LAF)
Sequence number of the largest acceptable frame
Last Frame Received (LFR)
Sequence number of the last frame received (and ACK to the sender)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

19. Sliding-Window Protocol
Receiver also maintains the following invariant
LAF – LFR ≤ RWS
Sliding Window on Receiver
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

20. Sliding-Window Protocol
When a frame with sequence number SeqNum arrives, what does the receiver do?
If SeqNum ≤ LFR or SeqNum > LAF
Discard it (the frame is outside the receiver window)
If LFR < SeqNum ≤ LAF
Accept it
Now the receiver needs to decide whether or not to send an ACK
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

21. Sliding-Window Protocol
Let SeqNumToAck
Denote the largest sequence number not yet acknowledged, such that all frames with sequence number less than or equal to SeqNumToAck have been received
The receiver acknowledges the receipt of SeqNumToAck . This acknowledgement is said to be cumulative.
The receiver then sets
LFR = SeqNumToAck and adjusts
LAF = LFR + RWS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

22. Sliding-Window Protocol
For example, suppose LFR = 5 and RWS = 4 ->LAF = 9
(i.e. the last ACK that the receiver sent was for seq. no. 5)
If frames 7 and 8 arrive, they will be buffered because they are within the receiver window
But no ACK will be sent since frame 6 is yet to arrive
Frames 7 and 8 are out of order
Frame 6 arrives (it is late for some reason)
Now Receiver Acknowledges Frame 8
and bumps LFR to 8
and LAF to 12
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

23. Sliding-Window Protocol: Issues
When timeout occurs, the amount of data in transit decreases
Since the sender is unable to advance its window
When the packet loss occurs, this scheme is no longer keeping the pipe full
The longer it takes to notice that a packet loss has occurred, the more severe the problem becomes
How to improve this
Negative Acknowledgement (NAK)
Additional Acknowledgement
Selective Acknowledgement
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

24. Sliding-Window Protocol: Issues
Negative Acknowledgement (NAK)
Receiver sends NAK for frame 6 as soon as frame 7 arrive (in the previous example)
Additional Acknowledgement
Receiver sends additional ACK for frame 5 when frame 7 arrives
Sender uses duplicate ACK as a clue for frame loss
Selective Acknowledgement
Receiver will acknowledge exactly those frames it has received, rather than the highest number frames
Receiver will acknowledge frames 7 and 8
Sender knows frame 6 is lost
Sender can keep the pipe full, but…..??
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

25. Sliding-Window: Comment Settings
In practice, how to select SWS?
Depending on a quite number of factors (which?)
Now for RWS:
RWS=1: Go-back-N protocol. But what does this mean if RWS=1?
Should we set RWS>SWS?
RWS=SWS with Selective ACK: Selective Repeat
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

26. Maximum Sequence Number MaxSeqNum
We already discussed Stop-and-Wait: We need only 1 bit-header: Two sequence numbers 0 and 1, and reuse them - MaxSeqNum=2;
Now, for Sliding-Window Protocol: Should sequence numbers go to infinity?
We have a finite sequence number:
Example, 3-bit header: 8 sequences
Need to reuse or wrap around
So how we select MaxSeqNum?
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

27. Maximum Sequence Number MaxSeqNum
SWS + 1 ≤ MaxSeqNum
Is this sufficient?
Depends on RWS
If RWS = 1, then sufficient
If RWS = SWS, then not good enough
For example, we have eight sequence numbers
1, 2, 3, 4, 5, 6, 7,8
RWS = SWS = 7
Sender sends 1, 2, …, 7
Receiver receives 1, 2, … ,7
Receiver acknowledges 1, 2, …, 7
ACK (1, 2, …, 7) are lost
Sender retransmits 1, 2, …, 7
Receiver is expecting 8, 1, …., 6
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

28. Maximum Sequence Number MaxSeqNum
To avoid this,
If RWS = SWS
SWS < (MaxSeqNum + 1)/2
Note that this rule is specific to the situation where RWS=SWS. We might see a more general rule for arbitrary values of RWS and SWS in assignment
Also, at this moment, we assume frames are not re-ordered in transit, which is ony correct for direct point to point. For different environment, we need another rule (e.g., in TCP).
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

29. Sliding Window Protocol: Summary
Sender maintains three variables
Sending Window Size (SWS)
Last Acknowledgement Received (LAR)
Last Frame Sent (LFS)
Sender also maintains the following invariant
LFS – LAR ≤ SWS
When ACK arrives
Move LAR to right, transmit another frame
Retransmit frame if time expires (time out)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

30. Sliding-Window Protocol: Summary
Receiver maintains three variables
Receiving Window Size (RWS)
Largest Acceptable Frame (LAF)
Last Frame Received (LFR)
Receiver also maintains the following invariant
LAF – LFR ≤ RWS
When a frame with sequence number SeqNum arrives, only accept it if
If LFR < SeqNum ≤ LAF
ACK?
SeqNumToAck : The largest sequence number not yet acknowledged, such that all frames with sequence number less than or equal to SeqNumToAck have been received
The receiver then sets
LFR = SeqNumToAck and adjusts LAF = LFR + RWS
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

31. Sliding-Window Protocol: Example 1
Considering the case that SWS=RWS=3. Timeout interval is about 2RTT. Draw a timeline diagram of the following
Frame 4 is lost
Frames 4-6 are lost
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

32. Sliding-Window Protocol: Issues
When timeout occurs, the amount of data in transit decreases
Since the sender is unable to advance its window
When the packet loss occurs, this scheme is no longer keeping the pipe full
The longer it takes to notice that a packet loss has occurred, the more severe the problem becomes
How to improve this
Negative Acknowledgement (NAK)
Additional Acknowledgement
Selective Acknowledgement
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

33. Sliding-Window Protocol: Issues
Negative Acknowledgement (NAK)
Receiver sends NAK for frame 6 as soon as frame 7 arrive (in the previous example)
Additional Acknowledgement
Receiver sends additional ACK for frame 5 when frame 7 arrives
Sender uses duplicate ACK as a clue for frame loss
Selective Acknowledgement
Receiver will acknowledge exactly those frames it has received, rather than the highest number frames
Receiver will acknowledge frames 7 and 8
Sender knows frame 6 is lost
Sender can keep the pipe full, but…..??
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

34. Sliding-Window Protocol: Example 2
In the following example, we will see the efficiency of using NAK or additional ACK
Considering the case that SWS=RWS=4 and frame 2 is lost. Timeout interval is about 2RTT.
First, draw a timeline diagram with re-transmission taking place upon time out as usual
Now, assume NAK[2]/DUPACK[2] is sent after frame 3 is received. Re-transmission now take paces either upon time out or upon receipt of NAK[2]/DUPACK[2]
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

35. Sliding-Window: Comment Settings
In practice, how to select SWS?
Depending on a quite number of factors!!
Now for RWS:
RWS=1: Go-back-N protocol
Should we set RWS>SWS?
RWS=SWS with Selective ACK: Selective Repeat
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

36. Go-Back-N Protocol
It is Sliding Window Protocol with RWS=1: Receiver will not buffer any frames that arrive out of order
Demo:
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

37. Selective Repeat
Window size can be very large for nets with large delay x bandwidth
Inefficient to retransmit all N frames if one is lost
Selective repeat allows the re-transmission of only the lost packets
Accepts out-of-order packets
Simply increase the RWS up to SWS (does not make sense to allow for RWS > SWS)
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

38. Example with Selective Repeat
Demo:
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

39. Sliding-Window Protocol
Perhaps the best known algorithm in computer networking
Serves three different roles
Reliable
Preserve the order: The receiver makes sure that it does not pass a frame up to the next higher-level protocol until it has already passed up all frames with a smaller sequence number
Frame control: Receiver is able to throttle sender- Keeps sender from overrunning receiver from transmitting more data than the receiver is able to process
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527

40. Sliding-Window Protocol
Perhaps the best known algorithm in computer networking
It can be used to serve different roles in different layers:
Reliable delivery at Link Layer
TCP: Also reliable deliver
Up to now, we already learned quite a few techniques/protocols considered in Transport Layer
Dr. Nghi Tran (ECE-University of Akron)
ECE 4450:427/527