1. Network Protocol Hierarchies
Network software structure
Networks are organized as a series of layers.
Layer n on a host communicates with layer n on another host.
Layer n protocol: Rules and conventions for this communication.
The entities on each host implementing the layer n protocol are peers.

2. Layers, protocols, and interfaces
Host 1
Host 2
Physical
Communication
Layer 4 protocol
Layer 4
Layer 4
Layer 3/4 interface
Layer 3 protocol
Layer 3
Layer 3
Virtual
Communication
Layer 2/3 interface
Layer 2 protocol
Layer 2
Layer 2
Layer 1/2 interface
Layer 1 protocol
Layer 1
Layer 1
Physical medium

3. Philosopher - Translator - Secretary Architecture

4. Flow of info supporting virtual communication in layer 4
Layer 4 Protocol M M
Layer 3 Protocol H3 M H3 M
Layer 2 Protocol H2 H3 M1 H2 M2 H2 H3 M1 H2 M2
Source Machine
Destination Machine

5. Layer Design Issues
Mechanism for identifying senders and receivers (addressing).
Simplex, half-duplex, or full-duplex communication.
Error detection/correction.
Fragmentation
Preserving message order.
Fast sender vs. slow receiver.
Routing

6. Connection-Oriented vs. Connectionless Services
Connection-oriented service
Connection established and terminated.
Modeled after the telephone system.
Message arrive in the the order they are sent.
e.g. TCP
Connectionless service (datagram service)
Modeled after the postal system.
No guarantees on message ordering.
Unreliable: Messages may be lost.
e.g. UDP

7. OSI Reference Model ISO OSI 7 layer model Importance of Open Standards
Each layer performs a well-defined function in the context of the overall communication subsystem.
Peers exchange messages according to protocol.
Message = user data + control information.
Lower layers are network dependent.
Higher layers are application oriented.
Each layer provides an interface for the layer above.

8. OSI Reference Model 7 6 5 4 3 2 1 HOST A HOST B Application
Application protocol
Application
Application 6
Presentation protocol
Presentation
Presentation 5
Session protocol
Session
Session
Transport protocol 4
Transport
Transport
Communication subnet boundary 3
Network
Network
Network
Network 2
Data link
Data link
Data link
Data link 1
Physical
Physical
Physical
Physical
Physical
HOST A
Router
Router
HOST B

9. Network Dependent Layers
Physical Layer (exchanged unit: bit)
Transmits/Receives “raw” bits over comm. channel.
Mechanical and electrical network interface definitions.
Data Link Layer (exchanged unit: frame)
Transforms (i.e. “cleans up”) raw transmission.
Frames data using special bit patterns at the beginning and end of the frame.
Serial/Parallel conversion.
Medium Access Sublayer (broadcast networks)
controlling shared channel

10. Network Dependent Layers, cont.
Network Layer (exchanged unit: packet)
Operation of the subnet.
Addressing
Routing (point-to-point networks).
Transport Layer
End-to-end message transfer.
Connection management
Error control
Fragmentation

11. Application Oriented Layers
Session Layer
Dialogue control
Synchronization (i.e. “who does what when”)
“Thin” layer
Presentation Layer
Syntax and semantics of transferred data.
Data representation transformations.
e.g. Floating point number format.
e.g. Encryption

12. Application Oriented Layers, cont.
Application Layer
Variety of protocols:
File Transfer
Remote access and management
Document/message interchange

13. OSI layer index Application Presentation Session Transport
Network (packets)
Data Link (frames)
Physical (raw bits)

14. Finish reading Chap 1