Internetworking Concept Internetworking Concept and Architectural Modeland Architectural Model

Chapter 3Chapter 3

Looked at low-level details of transmission Looked at low-level details of transmission across individual data networksacross individual data networks

Raise level of abstractionRaise level of abstraction System that hides underlying details while System that hides underlying details while

providing universal communication servicesproviding universal communication services Result: high level abstraction; framework for Result: high level abstraction; framework for

all design decisionsall design decisions

Later chapters:Later chapters: Use the abstraction to build layers of internet Use the abstraction to build layers of internet

communication softwarecommunication software Show how applications use the comm systemShow how applications use the comm system

Application-Level Application-Level InterconnectionInterconnection

Two approaches to hiding detailsTwo approaches to hiding details Have applications do the workHave applications do the work Hide details in the operating systemHide details in the operating system

Application-level connections require Application-level connections require application gatewaysapplication gateways Application running on each computer in the Application running on each computer in the

networknetwork Understands details of network connectionsUnderstands details of network connections

Seems natural at first, but:Seems natural at first, but: New functionality means new application New functionality means new application

program for every computerprogram for every computer New network hardware means modifying all New network hardware means modifying all

existing programsexisting programs With hundreds or thousands of networks, no way With hundreds or thousands of networks, no way

to write all possible application programsto write all possible application programs Intermediate application programs between the Intermediate application programs between the

source and destination must operate correctlysource and destination must operate correctly

Network-Level InterconnectionNetwork-Level Interconnection

Alternative to application-levelAlternative to application-level

Provides a mechanism to deliver small Provides a mechanism to deliver small data packets from source to destinationdata packets from source to destination

No intermediate application programsNo intermediate application programs

Several advantagesSeveral advantages Efficient (scheme maps onto underlying HW)Efficient (scheme maps onto underlying HW) Separates data communication from applicationsSeparates data communication from applications Flexible (allows general-purpose comm facilities)Flexible (allows general-purpose comm facilities) Modifying network does not affect applicationsModifying network does not affect applications

Key is abstraction of Key is abstraction of internetworkinginternetworking InternetworkInternetwork (internet) concept is powerful (internet) concept is powerful Detaches comm from network technologiesDetaches comm from network technologies Hides low-level details from userHides low-level details from user Drives software design decisionsDrives software design decisions Helps handle physical addresses and routesHelps handle physical addresses and routes

Motivation for internetworking comes from Motivation for internetworking comes from two observations:two observations: No single network hardware technology can No single network hardware technology can

satisfy all needssatisfy all needsExample: LAN vs WANExample: LAN vs WAN

Users desire universal interconnectionUsers desire universal interconnection

Goal: Goal: Build unified interconnection of networks to Build unified interconnection of networks to

support universal communicationsupport universal communicationEach network has own technologiesEach network has own technologies

Software between networks and application Software between networks and application programs gives impression of single large networkprograms gives impression of single large network

Internet PropertiesInternet PropertiesUniversal serviceUniversal service

Hidden underlying architectureHidden underlying architecture

No forced network interconnection topologyNo forced network interconnection topology Adding network should be easyAdding network should be easy Intermediate networks not directly connected to Intermediate networks not directly connected to

source or destination computerssource or destination computers

Set of shared, universal machine identifiersSet of shared, universal machine identifiers

Operations and applications independent of Operations and applications independent of underlying network technologiesunderlying network technologies

RoutersRoutersUse a special computer to connect Use a special computer to connect networksnetworks Called Called internet gatewaysinternet gateways or or internet routersinternet routers IP router & IP gateway used interchangeablyIP router & IP gateway used interchangeably

Routers must know the topology beyond Routers must know the topology beyond the networks to which it connectsthe networks to which it connectsKey:Key: Routers use the destination Routers use the destination networknetwork, not the , not the

destination computer, for forwarding packetsdestination computer, for forwarding packets

Will study in detail in later chaptersWill study in detail in later chapters

For now:For now: Assume it is possible and practical for Assume it is possible and practical for

routers to do their jobrouters to do their job Also assume that only routers connect Also assume that only routers connect

physical networksphysical networks

User’s ViewUser’s ViewWant user to see only one network to Want user to see only one network to which all machines are connectedwhich all machines are connected

Network level interconnections means:Network level interconnections means: Applications that communicate run without Applications that communicate run without

change on any computerchange on any computer Internet SW worries about physical Internet SW worries about physical

connectionsconnections

TCP/IP internet protocols treat all TCP/IP internet protocols treat all networks equallynetworks equally

Internetworking SummaryInternetworking SummaryInternet is more than collection of networks Internet is more than collection of networks interconnected by computersinterconnected by computers

Need conventions that allow communicationNeed conventions that allow communication

Need universal identifiers and set of Need universal identifiers and set of procedures for moving data to destinationprocedures for moving data to destination

IP routers (or gateways) connect networks IP routers (or gateways) connect networks and forward packetsand forward packets

Details As Yet UnansweredDetails As Yet Unanswered

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