2. Copyright by Jorg Liebeherr 98, 99
Need for Protocols
Protocols are a set of rules and
conventions. By enforcing that
communicating parties adhere to a
common protocol, communication
is made possible.
3. Copyright by Jorg Liebeherr 98, 99
Network Architecture
A Network Architecture is a
structured set of protocols that
implement the exchange of
information between computers
4. Copyright by Jorg Liebeherr 98, 99
Programming the
Communication
Any complex programming task is made
simpler by using several levels of abstraction
Abstraction hides the programming details of
lower levels
Communication tasks give rise to standard
abstractions
Committees such as the IETF (Internet
Engineering Task Force) are in charge for
standardizing the abstractions related to
internet communication
5. Copyright by Jorg Liebeherr 98, 99
The Levels of Abstraction
Multiple levels of communication abstractions
have been defined by standardization bodies
Each level is enforced by a corresponding
protocol
Each level uses the abstract services of the
level below
Abstraction achieves separation of concerns
A protocol needn’t know how the lower-level
abstractions are implemented and what the higher
level abstractions are.
6. Copyright by Jorg Liebeherr 98, 99
Layered Network Architecture
In a Layered Network Architecture, the
services are grouped in a hierarchy of layers.
A protocol of layer N uses only services of layer N-1.
A protocol of layer N provides services only to layer
N+1.
A
C
E
B
D E
B layer 3
layer 2
layer 1
not layered layered
A
C
D
7. Copyright by Jorg Liebeherr 98, 99
Service Primitives
N+1 Layer
Entity
N+1 Layer
Entity
N Layer
Entity
N Layer
Entity
N+1 Layer Protocol
X. Request X. Indication X. Response
X. Confirm
Assume that some protocol implements
service “X”.
8. Copyright by Jorg Liebeherr 98, 99
Service Primitives
Recall: A layer N+1 protocol entity sees the
lower layers only as an abstract service
provider
N+1 Layer
Entity
N+1 Layer
Entity
N+1 Layer Protocol
X. Request X. Indication
X. Confirm
Service Provider
X. Response
9. Copyright by Jorg Liebeherr 98, 99
Example: Sending a Letter
Bob’s
mailbox
Bob Alice
Alice’s
mailbox
Postman
Logical flow of information
10. Copyright by Jorg Liebeherr 98, 99
(Un-)Acknowledged Service
Unconfirmed service:
Acknowledged service:
US Mail
L.Request L.Indicate
L.Indicate
L.Confirm
US Mail
L.Request L.Indicate
11. Copyright by Jorg Liebeherr 98, 99
Protocol Architectures
There are only few protocol
architectures that are relevant
today:
OSI Reference Model
TCP/IP Protocols Suite
ATM Protocol Stack
12. Copyright by Jorg Liebeherr 98, 99
OSI Reference Model
The OSI model defines seven layers:
Layer 7: Application Layer
Layer 6: Presentation Layer
Layer 5: Session Layer
Layer 4: Transport Layer
Layer 3: Network Layer
Layer 2: Data Link Layer
Layer 1: Physical Layer
Interconnection Media
13. Copyright by Jorg Liebeherr 98, 99
OSI Layers
Phy sical
Lay er
Phy sical
Lay er
Phy sical
Lay er
Phy sical
Lay er
Data Link
Lay er
Data Link
Lay er
Data Link
Lay er
Data Link
Lay er
Network
Lay er
Session
Lay er
Session
Lay er
Transport
Lay er
Transport
Lay er
Application
Lay er
Application
Lay er
Presentation
Lay er
Presentation
Lay er
Network
Lay er
Network
Lay er
Network
Lay er
Application
HOST
NODE
HOST
NODE
Application
14. Copyright by Jorg Liebeherr 98, 99
OSI Layers and Encapsulation
Phy sical
Lay er
Phy sical
Lay er
Data Link
Lay er
Data Link
Lay er
Network
Lay er
Session
Lay er
Session
Lay er
Transport
Lay er
Transport
Lay er
Application
Lay er
Application
Lay er
Presentation
Lay er
Presentation
Lay er
Network
Lay er
Application Application
AH
PH
SH
TH
NH
DH
Bits
Data
AH Data
PH AH Data
SH PH AH Data
TH SH PH AH Data
NH TH SH PH AH Data
Data
Application
Data
Header of
Application
Layer
15. Copyright by Jorg Liebeherr 98, 99
OSI Model in a Switched
Communication Network
Transport
Session
Presentation
Application
Transport
Session
Presentation
Application
Physical
Data Link
Network
Physical
Data Link
Network
Physical
Data Link
Network
Physical
Data Link
Network
Physical
Data Link
Network
Physical
Data Link
Network
Station (Host)
Node (Router)
16. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Physical Layer (Layer 1):
Converts bits into electrical or optical
signals
Transmits these signals over the hardware
communication medium
Example: RS-232
17. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Data Link Layer (Layer 2):
Reliably transfers frames over a link
Performs synchronization, error control,
flow control
Example: PPP
18. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Network Layer (Layer 3):
Moves packets inside the network
Performs routing, addressing, switching,
congestion control
Example: IP
19. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Transport Layer (Layer 4):
Controls delivery of data between hosts
Connection management, error control,
flow control, multiplexing, TCP, UDP.
20. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Session Layer (Layer 5):
Support the dialog between cooperating
application programs
Session management, synchronization
Example: RPC
21. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Presentation Layer (Layer 6):
Data conversion into application format
Encryption/decryption
Secure sockets
22. Copyright by Jorg Liebeherr 98, 99
A Tour of the OSI Layers
Application Layer (Layer 7):
Provides network access to application
programs
Everything is application specific
Example: File Transfer, Electronic Mail
23. Copyright by Jorg Liebeherr 98, 99
TCP/IP Protocol Suite
The TCP/IP protocol suite
was first defined in 1974
The TCP/IP protocol suite
is the protocol
architecture of the
Internet
The TCP/IP suite has four
layers:
Application, Transport,
Internet, and Network
Interface Layer
Application
Layer
Transport
Layer
Internet
Network
Interface
telnet, ftp, email
TCP, UDP
IP, ICMP, IGMP
Device Drivers
24. Copyright by Jorg Liebeherr 98, 99
Example: File Transfer
FTP
program
TCP
IP
Ethernet
Driver
Ethernet
Driver
Ethernet
Driver
IP
FTP
program
TCP
IP
Ethernet
Driver
FTP protocol
TCP protocol
IP protocol IP protocol
Ethernet
protocol
Ethernet
protocol
Host A Router Host B
25. Copyright by Jorg Liebeherr 98, 99
Encapsulation in the TCP/IP
Suite
Application
TCP
IP
Ethernet
Driver
User data
User data
Application
Header
Application data
TCP Header
Application data
TCP Header
IP Header
Application data
TCP Header
IP Header
Ethernet
Header
Ethernet
Trailer
IP datagram
TCP segment
Ethernet frame
26. Copyright by Jorg Liebeherr 98, 99
TCP/IP Protocol Suite
Application
Layer
Network
Layer
Link Layer
IP
ARP
Hardware
Interface
RARP
Media
ICMP IGMP
Transport
Layer
TCP UDP
User
Process
User
Process
User
Process
User
Process
27. Copyright by Jorg Liebeherr 98, 99
Comparison of OSI Model and
TCP/IP Suite
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Internetwork
Network
Access
Physical
OSI TCP/IP
28. Copyright by Jorg Liebeherr 98, 99
Quiz
Name one function of the network layer
