Internetworking Concept and Architectural Model

1. Internetworking Concept and Architectural Model Chapter 3

2. 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. 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?