1. Protocols and Protocol Layering
Part 2.2
Protocols and
Protocol Layering
Robert Probert, SITE, University of Ottawa
SEPT, 2003
CSI 4118

2. Protocol Agreement about communication Specifies Format of messages
Meaning of messages
Rules for exchange
Procedure for handling problems
SEPT, 2003
CSI 4118

3. Need for Protocols Hardware is low level Many problems can occur
Bits corrupted or destroyed
Entire packet lost
Packet duplicated
Packets delivered out of order
SEPT, 2003
CSI 4118

4. Need for Protocols (continued)
Need mechanisms to distinguish among
Multiple computers on a network
Multiple applications on a computer
Multiple copies of a single application on a computer
SEPT, 2003
CSI 4118

5. Set of Protocols Work together
Each protocol solves part of communication problem
Known as
Protocol suite
Protocol family
Designed in layers
SEPT, 2003
CSI 4118

6. Plan for Protocol Design
Intended for protocol designers
Divides protocols into layers
Each layer devoted to one subproblem
Example: ISO 7-layer reference model
SEPT, 2003
CSI 4118

7. Illustration of the 7-Layer Model
Defined early
Now somewhat dated
Does not include internet layer!
SEPT, 2003
CSI 4118

8. ISO Layers Layer 1: Physical Layer 2: Data Link (media access)
Underlying hardware
Layer 2: Data Link (media access)
Hardware frame definitions
Layer 3: Network
Packet forwarding
Layer 4: Transport
Reliability
SEPT, 2003
CSI 4118

9. ISO Layers (continued)
Layer 5: Session
Login and passwords
Layer 6: Presentation
Data representation
Layer 7: Application
Individual application program
SEPT, 2003
CSI 4118

10. Layers and Protocol Software
Protocol software follows layering model
One software module per layer
Modules cooperate
Incoming or outgoing data passes from one module to another
Entire set of modules known as stack
SEPT, 2003
CSI 4118

11. Illustration of Stacks
SEPT, 2003
CSI 4118

12. Layers and Packet Headers
Each layer
Prepends header to outgoing packet
Removes header from incoming packet
SEPT, 2003
CSI 4118

13. Scientific Layering Principle
Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source
SEPT, 2003
CSI 4118

14. Illustration of Layering Principle
SEPT, 2003
CSI 4118

15. Protocol Techniques For bit corruption For out-of-order delivery
Parity
Checksum
CRC
For out-of-order delivery
Sequence numbers
Duplication
SEPT, 2003
CSI 4118

16. Protocol Techniques (continued)
For lost packets
Positive acknowledgement and retransmission
For replay (excessive delay)
Unique message ID
For data overrun
Flow control
SEPT, 2003
CSI 4118

17. Flow Control Needed because Related to buffering Two forms
Sending computer system faster than receiving computer
Sending application faster than receiving application
Related to buffering
Two forms
Stop-and-go
Sliding window
SEPT, 2003
CSI 4118

18. Stop-And-Go Flow Control
Sending Side
Transmits one packet
Waits for signal from receiver
Receiving side
Receives and consumes packets
Transmits signal to sender
Inefficient
SEPT, 2003
CSI 4118

19. Sliding Window Flow Control
Receiving side
Establishes multiple buffers and informs sender
Sending side
Transmits packets for all available buffers
Only waits if no signal arrives before transmission
Sends signals as packets arrive
SEPT, 2003
CSI 4118

20. Illustration of Sliding Window on Sending Side
Window tells how many packets can be sent
Window moves as acknowledgements arrive
SEPT, 2003
CSI 4118

21. Performance Stop-and-go Sliding window Slow
Useful only in special cases
Sliding window
Fast
Needed in high-speed network
SEPT, 2003
CSI 4118

22. Comparison of Flow Control
SEPT, 2003
CSI 4118

23. Why Sliding Window? Simultaneously Speedup Tw = min(B, TG x W) where
Increase throughput
Control flow
Speedup
Tw = min(B, TG x W)
where
B is underlying hardware bandwidth
TW is sliding window throughput
TG is stop-and-go throughput
W is the window size
SEPT, 2003
CSI 4118

24. Congestion Fundamental problem in networks
Caused by traffic, not hardware failure
Analogous to congestion on a highway
Principle cause of delay
SEPT, 2003
CSI 4118

25. Illustration of Architecture That Can Experience Congestion
Multiple sources
Bottleneck
SEPT, 2003
CSI 4118

26. Congestion and Loss
Modern network hardware works well; most packet loss results from congestion, not from hardware failure
SEPT, 2003
CSI 4118