1. Protocol Strategies

2. Idle RQ (Stop-and-Wait)
Transmission
Initiated
Ack Received
Send Next
Send N
Send N+1
Fig 1.23

3. Message is lost
Send N
Repeat N
Send N+1
Fig 1.23

4. Ack is lost
Send N
Repeat N
Send N+1
Fig 1.23

5. Message Propagates First bit of message arrives at receiver
-> Propagation delay
Fig 1.24

6. Message Transmission
Transmission time
SAME Transmission time
Fig 1.24

7. Message Transmission Plus Propagation
Transmission + Propagation time
Fig 1.24

8. Receiver Processes
Turnaround time
Fig 1.24

9. Receiver Send Ack
Short time to transmit ACK
Fig 1.24

10. ACK Propagates First bit of ACK arrives at receiver
-> Propagation delay… same as before
Fig 1.24

11. Continuous RQ Sender continues sending, eventually acks arrive
from the receiver At the
same rate as they depart
from the transmitter
No Errors
Idealistic
Fig 1.26

12. Aggressive Sending Strategy but messages are lost
What is the strategy
being used by the
receiver when
receiving messages
out of sequence?
Receiver recognizes
Sender knows N+1 problem
Sends in next slot
Fig 1.27

13. Now ACK is lost!
Fig 1.27

14. A different retransmission strategy
Information frame lost
Fig 1.28

15. Again ACK is Lost
Fig 1.28

16. Strategies Idle RQ -> Stop and Wait Continuous RQ->
Selective Repeat
Go-Back-N

17. Idle RQ Send a message Do not send the next until ACK is received
Messages and ACKS numbered 0/1/0/1/….
If “a” is large,
capacity of link may be severely underutilized
Overall user delay may be excessive (why wait)

18. Selective repeat Aggressive sending
Multiple outstanding messages (w/o ACK)
If error at receiver
Receiver keeps later messages buffered at a lower level and only passes them up when the missing frames arrive
Sender only resends error messages
Extra management effort required at receiver

19. Go-Back-N Aggressive sending Multiple outstanding messages (w/o ACK)
If error at receiver
Receiver REJECTs out of order messages
Sender only resends ALL messages from point of error
Simple management effort required at receiver

20. How do you number frames?
Fig 1.29

21. SIZE not NAMES
Fig 1.29

22. This Shows How To Name
Fig 1.30

23. N=4, K=3 (will lead to a problem)
Sender
Receiver
Send M1, M2 and M3
Recv M1, M2 and M3
ACK M1, M2 and M3
Recv ACK M1, M2 and M3
Send M4, M1* and M2*
time

24. N=4, K=3 ACKs are lost Sender Receiver Send M1, M2 and M3
Recv M1, M2 and M3
ACK M1, M2 and M3
Recv ACK M1, M2 and M3
ReSend M1, M2 and M3
time

25. N=7, K=3 (NO problems) Sender Receiver Send M1, M2 and M3
Recv M1, M2 and M3
ACK M1, M2 and M3
Recv ACK M1, M2 and M3
Send M4, M5 and M6
time

26. N=7, K=3 ACKs lost.. No problems
Sender
Receiver
Send M1, M2 and M3
Recv M1, M2 and M3
ACK M1, M2 and M3
Recv ACK M1, M2 and M3
ReSend M1, M2 and M3
time

27. Naming… A summary
Fig 1.30

28. What about 2-way traffic?

29. PiggyBacked ACKs Traffic frequently going in the reverse direction
No send to send an entire new frame with additional overhead.
Typically, a message contains the name of the message in transit AND the name of the ack message, message expected in the other direction

30. What if nothing in the other driection?
Set a timer waiting for return travel
If no return travel before the timer goes off,
Send a separate ACK when the timer expires.
ELSE
Piggyback on return traffic.

31. What about reverse traffic
Discussion only focuses on traffic in one direction
Problem is symmetric.
Software is tricky but the send and receive code are integrated into the same process.

32. Protocol Design and Software Development

33. Link Layer Complexity
SubLayers
Link
Fig 1.31

34. Link Layer Complexity
Timer to alert sender to resend
Fig 1.31

35. Events
From Side
From Above
From Below
Fig 1.32

36. State Diagram
event
action
state change
Fig 1.33

37. State Machine and Events
Figs 1.32 and 1.33

38. Actions Create Outgoing Events (Events at the receiver)
Figs 1.32 and 1.33

39. These Outgoing Events Are INCOMING to the receiver via MAC layer
sender
receiver
Figs 1.33 and 1.34

40. One more time Ls_User Creates LdataReq TxFrame to destination
iRCVD from MAC_provider to destination

41. Focus on State Machines

42. Three Ways = Same
State Transition Diagram
Program
Event-State Table

43. Event-State Table
Fig 1.33

44. Program Choose -action -next State based on -current state -event Main
Loop
Fig 1.33

45. An Example : HDLC

46. 1-Connect 2- Data 3-Disconnect

47. network
link
physical
physical
link
network
network
link
link
network

48. 1-Connect 2- Data 3-Disconnect

49. HDLC Frame Format

50. Frame Types

51. 16 bit version

52. Network<-> Link Link <-> Link

53. Entire State Machine (Sender AND Receiver)

54. Information Exchange Station A Station B SEND I Frames N(R)=0 N(S)=0
Frame Damaged
N(R)=1 N(S)=2
Frame Rejected
Send REJ with N(R)=1
SEND I Frames
N(R)=2 N(S)=1
(no activity)
N(R)=2 N(S)=2
Send RRJ with N(R)=3
N(R)=2 N(S)=3