1. Internetworking Concept and Architectural Model
Chapter 3

2. Raise level of abstraction
Looked at low-level details of transmission across individual data networks
Raise level of abstraction
System that hides underlying details while providing universal communication services
Result: high level abstraction; framework for all design decisions
Later chapters:
Use the abstraction to build layers of internet communication software
Show how applications use the comm system

3. Application-Level Interconnection
Two approaches to hiding details
Have applications do the work
Hide details in the operating system
Application-level connections require application gateways
Application running on each computer in the network
Understands details of network connections

4. Seems natural at first, but:
New functionality means new application program for every computer
New network hardware means modifying all existing programs
With hundreds or thousands of networks, no way to write all possible application programs
Intermediate application programs between the source and destination must operate correctly

5. Network-Level Interconnection
Alternative to application-level
Provides a mechanism to deliver small data packets from source to destination
No intermediate application programs

6. Key is abstraction of internetworking
Several advantages
Efficient (scheme maps onto underlying HW)
Separates data communication from applications
Flexible (allows general-purpose comm facilities)
Modifying network does not affect applications
Key is abstraction of internetworking
Internetwork (internet) concept is powerful
Detaches comm from network technologies
Hides low-level details from user
Drives software design decisions
Helps handle physical addresses and routes

7. Motivation for internetworking comes from two observations:
No single network hardware technology can satisfy all needs
Example: LAN vs WAN
Users desire universal interconnection
Goal:
Build unified interconnection of networks to support universal communication
Each network has own technologies
Software between networks and application programs gives impression of single large network

8. Internet Properties Universal service Hidden underlying architecture
No forced network interconnection topology
Adding network should be easy
Intermediate networks not directly connected to source or destination computers
Set of shared, universal machine identifiers
Operations and applications independent of underlying network technologies

9. Routers Use a special computer to connect networks
Called internet gateways or internet routers
IP router & IP gateway used interchangeably
Routers must know the topology beyond the networks to which it connects
Key:
Routers use the destination network, not the destination computer, for forwarding packets

11. Will study in detail in later chapters For now:
Assume it is possible and practical for routers to do their job
Also assume that only routers connect physical networks

12. User’s View
Want user to see only one network to which all machines are connected
Network level interconnections means:
Applications that communicate run without change on any computer
Internet SW worries about physical connections
TCP/IP internet protocols treat all networks equally

15. Internetworking Summary
Internet is more than collection of networks interconnected by computers
Need conventions that allow communication
Need universal identifiers and set of procedures for moving data to destination
IP routers (or gateways) connect networks and forward packets

16. Details As Yet Unanswered
Form of internet addresses?
How addresses relate to Ethernet, ATM, etc.?
What does a packet look like?
What if packets come too fast for a computer or router to handle?
How do routers learn routes?
How do multiple applications executing con-currently on one computer send/receive packets to multiple destinations without problems?