1. Network Architecture
Layered system with alternative abstractions available at a given layer

2. Abstraction and Layering
The University of Adelaide, School of Computer Science
9 June 2018
Abstraction and Layering
Abstraction
The hiding of details behind a well-defined interface
Define a model
Capture important aspects of the system
Abstractions naturally leads to layering
The general idea
Start from services offered by the underlying hardware
Add a sequence of layers, each providing a higher (i.e., more abstract) level of service.
Manageability and Mudularity
Chapter 2 — Instructions: Language of the Computer

3. Protocols
The abstract objects the make up the layers of a network system are called protocols
Building blocks of a network architecture
Each protocol object has two different interfaces
service interface: operations on this protocol
peer-to-peer interface: messages exchanged with peer (indirect communication, except for the hardware)

4. Interfaces
Service and Peer Interfaces

5. The University of Adelaide, School of Computer Science
9 June 2018
Protocols
Protocol Specification:
Written description (prose)
pseudo-code
state transition diagram
Packet format
RFCs: Request For Comments
IETF: Internet Engineering Task Force
Standardization body
Ex. RFC 2616 for HTTP protocol
Interoperable: when two or more protocols that implement the specification accurately
Chapter 2 — Instructions: Language of the Computer

6. Protocol Graph Example of a protocol graph
nodes are the protocols and links the “depends-on” relation

7. High-level messages are encapsulated inside of low-level messages
Encapsulation
High-level messages are encapsulated inside of low-level messages

8. Packets General Format
Header
Payload
Trailer
Peers talk to each other through Headers and Trailers

9. OSI – Open Systems Interconnection
OSI Architecture
The OSI 7-layer Model
OSI – Open Systems Interconnection

10. The University of Adelaide, School of Computer Science
9 June 2018
Description of Layers
Physical Layer
Handles the transmission of raw bits over a communication link
Data Link Layer
Collects a stream of bits into a larger aggregate called a frame
Network adaptor along with device driver in OS implement the protocol in this layer
Frames are actually delivered to hosts
Network Layer
Handles routing among nodes within a packet-switched network
Provides Host-to-Host connectivity
Unit of data exchanged between nodes in this layer is called a packet
The lower three layers are implemented on all network nodes
Chapter 2 — Instructions: Language of the Computer

11. Description of Layers Transport Layer
Implements a process-to-process channel
Unit of data exchanges in this layer is called a message
There is a disagreement on the top three
Session, Presentation, and Application
Mainly because the are not always present

12. The University of Adelaide, School of Computer Science
9 June 2018
Description of Layers
Session Layer
Provides a name space that is used to tie together the potentially different transport streams that are part of a single application
Ex., tie the audio and video together in videoconference
Presentation Layer
Concerned about the format of data exchanged between peers
Ex., integer formats, audio/video format, most/least significant
Application Layer
Include things like the Hypertext Transfer Protocol
Basis the world wide web
Used by web browsers
The transport layer and the higher layers typically run only on end-hosts and not on the intermediate switches and routers
Chapter 2 — Instructions: Language of the Computer