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Chapter 16
Protocols and
Protocol Layering
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Protocol
Agreement about communication
Specifies
Format
of messages (syntax)
Meaning of messages (semantics)
Rules for exchange
Procedure for handling problems
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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
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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
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Set of Protocols
Work together
Each protocol solves part
of communication
problem
Known as
Protocol
suite
Protocol family
Designed in layers
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Plan for Protocol Design
Intended for
protocol designers
Divides protocols into layers
Each layer devoted to one subproblem
Example: ISO 7-layer reference model
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Illustration of the 7-Layer Model
Defined early
Now somewhat dated
Does
not include internet layer!
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ISO Layers
Layer 1:
Physical
Underlying
Layer 2:
Data Link (media access)
Hardware
Layer 3:
hardware
frame definitions
Network
Packet
forwarding
Layer 4:
Transport
Reliability
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ISO Layers (continued)
Layer 5:
Login
Layer
Session
and passwords
6: Presentation
Data
representation
Layer 7:
Application
Services
for common applications
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TCP/IP protocol suite
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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
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Illustration of Stacks
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Layers and Packet Headers
Each layer
Prepends
header to outgoing packet
Removes header from incoming packet
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Example of encapsulation
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Scientific Layering Principle
Software implementing layer N at the
destination receives exactly the message sent
by software implementing layer N at the source
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Illustration of Layering Principle
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Protocol Techniques
For bit
corruption
Parity
Checksum
CRC
For out-of-order delivery
Sequence
numbers
Duplication
Sequence
numbers
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Protocol Techniques (continued)
For lost packets
Positive
acknowledgement and retransmission
For replay
Unique
For data
Flow
(excessive delay)
message ID
overrun
control
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Flow Control
Needed because
Sending
computer system faster than receiving
computer
Sending application faster than receiving
application
Related to
buffering
Two forms
Stop-and-go
Sliding
window
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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
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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
Receiving side
Sends
signals as packets arrive
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Illustration of Sliding
Window on Sending Side
Window tells how many packets can
be sent
Window moves as acknowledgements arrive
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Performance
Stop-and-go
Slow
Useful
only in special cases
Sliding window
Fast
Needed
in high-speed network
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Comparison of Flow Control
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Why Sliding Window?
 Simultaneously
 Increase
throughput
 Control flow
 Speedup
Tw = min(B, TG * W)
where
B
is underlying hardware bandwidth
 TW is sliding window throughput
 TG is stop-and-go throughput
 W is the window size
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Congestion
Fundamental problem in networks
Caused by
traffic, not hardware failure
Analogous to congestion on a highway
Principle cause of delay
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Illustration of Architecture
That Can Experience Congestion
Multiple sources
Bottleneck
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Dealing with Congestion
Congestion results in filled buffers in packet
switches => packets will be discarded =>
retransmission => more packets…..
Solutions
Increasing
buffer space
Reducing the amount of packets in the network
Packet switches inform senders, or
 Senders use packet loss as an indication

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Congestion and Loss
Modern network hardware works well; most
packet loss results from congestion, not from
hardware failure
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