Internetworking Concept and Architectural Model Chapter 3

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

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

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

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

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

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

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

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

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

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

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

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?