1. CS4470 Computer Networking Protocols
1/1/2019
CS Computer Networking Protocols
15. TCP 2
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Principles of reliable transfer Stop and wait Go back N
1/1/2019
Outline
Principles of reliable transfer
Stop and wait
Go back N
15. TCP CS4470

3. Principles of reliable transfer
Errors
A segment is corrupted
A segment is lost
Automatic retransmission request (ARQ)
When errors occur, the receiver sends a message to the sender asking for retransmission
Types of ARQ
Stop and wait
Sliding window (Go back N)
15. TCP CS4470

4. A trivial reliable transfer protocol
Assumption
A segment may be corrupted.
NO segment gets lost
Sender
Sends a segment
Waits for the acknowledgement for the segment
If the acknowledgement is positive
Sends the next segment
If the acknowledgement is negative
Resends the segment
15. TCP CS4470

5. A trivial reliable transfer protocol
Receiver
Receives a segment
Is the segment corrupted?
Yes. Sends a negative acknowledgement (NAK) asking for retransmission
No. Sends a positive acknowledgement (ACK). Waits for the next segment
15. TCP CS4470

6. Problems with this protocol
What happens if ACK/NAK is corrupted?
sender doesn’t know what happened at receiver!
A possible approach
The sender just simply resends the segment .
This causes possible duplicate segments
The receiver cannot tell whether the received segment is the re-transmitted segment or a new segment
A solution to this problem
Add a field sequence number(1 bit) to the segment header
The receiver only need to check the sequence number to determine whether or not the received segment is a retransmission.
15. TCP CS4470

7. Stop-and-Wait Protocol v1
Characteristics
Use 1 bit sequence number
A NAK free protocol
The receiver always sends positive acknowledgements
ACK0: Seg1 is correctly received, waiting for seg 0
ACK1: Seg0 is correctly received, waiting for seg 1
15. TCP CS4470

8. Stop-and-Wait Protocol v1(cont.)
Sender:
Sender keeps track of the sequence number k (0,1) of the current segment being sent out
Sender sends out a segment k
If the sender receives ACK k, go to step 1
If the sender receives ACK k+1 , k=k+1 mod 2, go to step 1
15. TCP CS4470

9. Stop-and-Wait Protocol v1 (cont.)
Receiver:
keeps track of the sequence number of(0, 1) of the next expected segment
Receiver is expecting segment k
Receiver receives a segment .
If the sequence number of the segment is not k, discards the segment , sends ack k, go to step 1
If sequence number is k and the segment is OK, sends ACK k=k+1 mod 2 to the sender, go to step 1
If segment k is corrupted, sends ACK k
Go to step 1
15. TCP CS4470

10. Stop-and-Wait v1 Examples
Sender
Receiver
seg0
Sends seg0
Receives ack1
Sends seg1
seg1
ACK 0
ACK 1
Receives ack0
Receives seg0
Sends ack1
Receives seg1
Sends ack0
a. Without erros
Sender
Receiver
seg0
Sends seg0
Receives seg0
Sends ack1
ACK 1
seg1
Receives ack1
Sends seg1
Receives seg1
Seg1 has errors
Sends ack1
ACK 1
seg1
Receives ack1
resends seg1
b. With a seg corrupted
15. TCP CS4470

11. Stop-and-Wait v1 Examples (cont.)
Sender
Receiver
seg0
Sends seg0
Receives seg0
Sends ack1
ACK 1
seg1
Receives ack1
Sends seg1
Receives seg1
Sends ack0
ACK 0
Ack0 is corrupted
resends seg1
seg1
Receives seg1, discards it
Sends ack0
ACK 0
c. With an ack corrupted
15. TCP CS4470

12. Stop-and-Wait Protocol v2
Stop-and-wait protocol v1 only works if there is no data loss.
What if there is data loss?
A segment may be lost
An ACK may be lost
Stop-and-wait protocol v2
Works with data errors AND data loss
15. TCP CS4470

13. Stop-and-Wait Protocol v2 (cont.)
Approach:
sender waits “reasonable” amount of time for ACK
retransmits if no ACK received in this time
if segments(or ACKs) are just delayed (not lost):
retransmission will be duplicate, but use of seq. #’s already handles this
receiver must specify seq # of seg being ACKed
requires countdown timer
15. TCP CS4470

14. Stop-and-Wait examples
Sender
Receiver
Sender
Receiver
seg0
Sends seg0
Receives ack1
Sends seg1
seg1
ACK 0
ACK 1
Receives ack0
Receives seg0
Sends ack1
Receives seg1
Sends ack0
a. Without errors
seg0
Sends seg0
Receives seg0
Sends ack1
ACK 1
seg1
Receives ack1
Sends seg1
X loss
seg1
timeout
resends seg1
Receives seg1
Sends ack0
b. segment loss
15. TCP CS4470

15. Stop-and-Wait examples
Sender
Receiver
Sender
Receiver
seg0
Sends seg0
Receives seg0
Sends ack1
seg0
Sends seg0
ACK 1
Receives seg0
Sends ack1
ACK 1
seg1
Receives ack1
Sends seg1
Receives seg1
Sends ack0
seg1
Receives ack1
Sends seg1
Receives seg1
Sends ack0
ACK 0
ACK 0 lost
Timeout
Resends seg1
seg1
Receives seg1
Discards it
Sends ack0
Receives ack0
Sends seg0
seg0
seg1
timeout
Resends seg1
ACK 0
Receives ack0
Resends seg0
Receives seg0
Sends ack1
Receives seg1
Sends ack0
ACK 1
seg0
ACK 0
Receives ack1
Sends seg1
seg1
Receives seg0
Sends ack1
c. Ack loss
d. Premature timeout
15. TCP CS4470

16. Pros and Cons of Stop-and-wait
Stop-and-wait is a complete reliable transfer protocol
It’s very simple
Cons
Inefficient
While the sender is waiting for an ACK, it doesn’t do anything.
Why not the sender sends multiple segments at a time?
Need more than 1 bits for the sequence number.
15. TCP CS4470

17. RTT and transmission delay
1/1/2019
RTT and transmission delay
Definition of RTT:
time to send a small segment to travel from sender to receiver and back.
Transmission delay
R=link bandwidth (bps)
L=segment length (bits)
time to send bits into link = L/R
Total delay
RTT+L/R
15. TCP CS4470

18. RTT and transmission delay
first segment bit
transmitted, t = 0
first segment bit arrives
RTT
ACK arrives, send next
segment, t = RTT + L / R
15. TCP CS4470

19. Performance of stop-and-wait
sender
receiver
first segment bit transmitted, t = 0
last packet bit transmitted, t = L / R
first segment bit arrives
RTT
last packet bit arrives, send ACK
ACK arrives, send next
segment, t = RTT + L / R
15. TCP CS4470

20. Increased utilization
first segment bit transmitted, t = 0
sender
receiver
RTT
last bit transmitted, t = L / R
first segment bit arrives
last segment bit arrives, send ACK
ACK arrives, send next
segment, t = RTT + L / R
last bit of 2nd segment arrives, send ACK
last bit of 3rd frame arrives, send ACK
Increase utilization
by a factor of 3!
15. TCP CS4470

21. Go-Back-N Based on a sliding window
Use window to control number of segments to send
Number of segments that the sender can send
N-(nextseqnum – send_base)
15. TCP CS4470

22. Go-Back-N Cumulative ACKs
ACK (n): ACKs all segments up to, including seq # n - 1
Timer
Only one timer is used.
The timer is for the oldest transmitted, yet un-acked segment
15. TCP CS4470

23. Go-back-N: sender Needs to send segments
If the window is full, wait until it’s not full
If the window is not full, sends out as many segment as allowed, adjusts nextsegnum
Receive an ACK for un-acked segments
Slide the window forward by the number of acked segments
Start the timer
Adjust send_base
Timerout
Re-transmit all un-acked segments in the window
15. TCP CS4470

24. Go-back-N: receiver Waits for seg k Receives seg k, sends ack k+1
Receives, seg k+1, discards it and sends ack k
Receives seg k-1, discards it and sends ack k
15. TCP CS4470

25. Go-back-N example The window size is 4 ACK1 ACK2 ACK3 ACK4
15. TCP CS4470

26. 1/1/2019
Exercise
Consider a sender sends  7 segments to  a receiver. The RTT between the sender and the receiver is 2ms. 
The transmission time of each segment is 0.2ms.
The timeout value is 3 ms. 
In  each of the following cases, how long does it take for sender to transfer the 7 segments? (time between sending out the first segment   and receiving the ack of the last segment).
a) Stop-and-wait protocol is used.
b) Go-Back-N protocol is used.   The window size is 4. The 2nd  and the 5th segment are lost during the first transmission.
For each case, draw a diagram that shows the interaction between the sender and receiver. You also need to provide the information about which segment is sent and the ACK number.
15. TCP CS4470

27. For each segment, it takes 2+0. 2=2
For each segment, it takes 2+0.2=2.2 to send and receive the ack,  so the total time is 7x2.2 =15.4ms
15. TCP CS4470

28. 15. TCP CS4470