1

Chapter 17-20Chapter 17-20

Internetworking

Part 1(Concept, IP Addressing, IP Routing,

IP Datagrams, Address Resolution

2

Motivation For InternetworkingMotivation For

InternetworkingLANs

Low costLimited distance

WANsHigh costUnlimited distance

3

Heterogeneity is InevitableHeterogeneity is Inevitable

No single networking technology is best for all needs

4

Universal ServiceUniversal Service

Fundamental concept in networkingPioneered by telephone systemArbitrary pairs of computers can communicateDesirableDifficult in a heterogeneous world

5

Heterogeneity and Universal Service

Heterogeneity and Universal Service

Incompatibilities among networksElectrical propertiesSignaling and data encodingPacket formatsAddresses

6

The Bottom LineThe Bottom Line

Although universal service is highly desirable, incompatibilities among network hardware and physical addressing prevent an organization from building a bridged network that includes arbitrary technologies

7

An InternetworkAn Internetwork

Begin with heterogeneous network technologies

Connect the physical networksCreate software to make resulting system

appear homogeneousCalled an internetwork or internet

8

Connecting Heterogeneous Networks

Connecting Heterogeneous Networks

Computer system usedSpecial-purposeDedicatedWorks with LAN and/or WAN technologiesKnown as

Internet routerInternet gateway

9

Illustration of an Internet Router

Illustration of an Internet Router

Cloud denotes arbitrary network technologyOne interface per network

10

Important IdeaImportant Idea

A router can interconnect networks that use different technologies, including different media and media access techniques, physical addressing schemes, or frame formats

11

Internet ArchitectureInternet Architecture

MultipleNetworksRouters interconnecting networks

Host computer connects to a networkSingle router has insufficient

CPU power and memoryI/O capability

12

InternetworkingInternetworking

Goal: communication systemSeamlessUniformGeneral-purposeUniversalHides heterogeneity from user

13

The Internet ConceptThe Internet Concept

14

To Hide HeterogeneityTo Hide Heterogeneity

Create “virtual” networkInvent

Addressing schemeNaming scheme

Implement with Protocol software

Note: protocol software needed on both hosts and routers

15

Internet ProtocolsInternet Protocols

Known as TCP / IPMany protocols comprise suiteDesigned to work togetherDivided into five conceptual layers

16

Layering Used with TCP/IPLayering Used with TCP/IP

Note: TCP/IP layering often used today.The ISO model still used as reference and for some specific high level tasks.

17

TCP/IP LayersTCP/IP Layers

Layer 1: PhysicalBasic network hardware

Layer 2: Network interfaceMAC frame formatMAC addressingInterface between computer and network (NIC)

Layer 3: InternetFacilities to send packets across internet composed of

multiple routers

18

TCP/IP Layers (continued)TCP/IP Layers (continued)

Layer 4: TransportTransport from an application on one computer

to application on anotherLayer 5: Application

Everything else

19

TCP/IP protocol suiteTCP/IP protocol suite

20

Internet Protocol (IP)Internet Protocol (IP)

Main protocol at Layer 3Fundamental in suiteDefines

Internet addressingInternet packet formatInternet routing

21

IP AddressingIP Addressing

AbstractionIndependent of hardware addressingUsed by

Higher-layer protocolsApplications

22

IP AddressIP Address

VirtualOnly understood by software

Used for all communication32-bit integer (IPv4)Unique value for each hostThe version mostly used is v4, a newer

version, v6, is gaining more acceptance.

23

IP Address AssignmentIP Address Assignment

An IP address does not identify a specific computer. Instead, each IP address identifies a connection between a computer and a network. A computer with multiple network interconnections (e.g., a router) must be assigned one IP address for each connection.

24

IP Address DetailsIP Address Details

Divided into two partsPrefix identifies networkSuffix identifies host

Global authority assigns unique prefix to network

Local administrator assigns unique suffix to host

25

Original Classes of Addresses

Original Classes of Addresses

Initial bits determine classClass determines boundary between prefix

and suffix

26

Dotted Decimal NotationDotted Decimal Notation

Shorthand for IP addressAllows humans to avoid binaryRepresents each octet in decimal separated

by dotsNOT the same as names like

www.somewhere.com

27

Example of Dotted Decimal Notation

Example of Dotted Decimal Notation

Four decimal values per 32-bit addressEach decimal number

Represents eight bitsIs between 0 and 255

28

Classful Addresses and Network Sizes

Classful Addresses and Network Sizes

Maximum network size determined by class of address

Class A largeClass B mediumClass C small

29

Addressing ExamplesAddressing Examples

30

Illustration of Router Addresses

Illustration of Router Addresses

Address prefix identifies networkNeed one router address per connection

31

Special AddressesSpecial Addresses

Network address not used in packetsLoopback never leaves local computer

32

Subnet and Classless Addressing

Subnet and Classless Addressing

Not part of original schemeInvented to prevent address exhaustionAllow boundary between prefix and suffix to

occur on arbitrary bit boundaryRequire auxiliary information to identify

boundary

33

Address MaskAddress Mask

Accompanies IP address32 bit binary valueSpecifies prefix / suffix boundary

1 bits cover prefix0 bits cover suffix

Example: class B mask is255.255.0.0 = 11111111 11111111 0000000 00000000

34

Subnet AddressingSubnet Addressing

Goal: extend address spaceInvented in 1980sWorks within a siteTechnique

Assign single network prefix to siteDivide suffix into two parts: network at site and

hostTypical use: divide class B addresses

35

Example of Subnet Addressing

Example of Subnet Addressing

Single Class B number such as 128.10.0.0 assigned to site

Site chooses subnet boundary such as 24 bitsRouters and hosts configured with corresponding

subnet mask M=255.255.255.0Given destination address, D, extract prefix with

“logical and” operation D & M

36

Classless AddressingClassless Addressing

Goal: extend address spaceInvented in 1990sWorks throughout InternetAccommodates

Original classful addressesSubnet addressesOther forms

37

Classless Addressing (continued)

Classless Addressing (continued)

TechniqueAllow arbitrary prefix sizeRepresent network address as pair

(address, mask_size)Known as Classless Inter-Domain Routing

(CIDR)

38

CIDRCIDR

Uses slash notationExample

128.211.0.0/17

Means that the boundary between prefix and suffix occurs after the first 17 bits

Each network can be as large or small as needed (power of two)

39

Motivation for IP PacketsMotivation for IP Packets

Because it can connect heterogeneous networks, a router cannot transmit a copy of a hardware frame that arrives on one network across another. To accommodate heterogeneity, an internet must define a hardware-independent packet format.

40

Internet PacketsInternet Packets

AbstractionCreated and understood only by softwareContains sender and destination addressesSize depends on data being carriedCalled IP datagram

41

The Two Parts of an IP Datagram

The Two Parts of an IP Datagram

HeaderContains destination addressFixed-size fields

PayloadVariable size up to 64KNo minimum size

42

Datagram HeaderDatagram Header

Three key fieldsSource IP addressDestination IP addressType (Layer 4 protocol sending the datagram, UDP uses the number 17,

TCP uses 6, ICMP uses 1, IGRP uses 88 and OSPF uses 89)

43

IP Datagram ForwardingIP Datagram Forwarding

Performed by routersSimilar to WAN forwarding

Table-drivenEntry specifies next hop

Unlike WAN forwardingUses IP addressesNext-hop is router or destination

44

Example of an IP Routing Table

Example of an IP Routing Table

Table (b) is for center router in part (a)

45

Routing Table SizeRouting Table Size

Because each destination in a routing table corresponds to a network, the number of entries in a routing table is proportional to the number of networks in an internet

46

Datagram ForwardingDatagram Forwarding

Given a datagramExtract destination address field, DLook up D in routing tableFind next-hop address, NSend datagram to N

47

Key ConceptKey Concept

The destination address in a datagram header always refers to the ultimate destination. When a router forwards the datagram to another router, the address of the next hop does not appear in the datagram header.

48

IP SemanticsIP Semantics

IP is connectionlessDatagram contains identity of destinationEach datagram sent / handled independently

Routes can change at any time

49

IP Sematics (continued)IP Sematics (continued)

IP allows datagrams to beDelayedDuplicatedDelivered out-of-orderLost

Called best-effort deliveryMotivation: accommodates all possible

networks

50

Resolving AddressesResolving Addresses

Hardware only recognizes MAC addressesIP only uses IP addressesConsequence: software needed to perform

translationPart of network interfaceKnown as address resolution

NäringslivsträffNäringslivsträff

51

52

Address ResolutionAddress Resolution

Resides somewhere in the border between layer 2 and 3

GivenA locally-connected network, NIP address C of computer on N

FindHardware address for C

TechniqueAddress Resolution Protocol

53

Address Resolution Techniques

Address Resolution Techniques

Table lookupHashed, orDirect indexing

Closed-form computationWorks well for configurable hardware addresses

Message exchangeServer based, orTruly distributed

54

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP)

Key bindings in tableTable entry contains pair of addresses for

one computerIP addressHardware address

Build table automatically as needed

55

ARP TableARP Table

Only contains entries for computers on local network

IP network prefix in all entries identical

56

ARP Lookup AlgorithmARP Lookup Algorithm

Look for target IP address, T, in ARP tableIf not found

Send ARP request message to TReceive reply with T’s hardware addressAdd entry to table

Return hardware address from table

57

Illustration of ARP Exchange

Illustration of ARP Exchange

W needs Y’s hardware addressRequest sent via broadcastReply sent via unicast

58

ARP Message Format (For Ethernet)

ARP Message Format (For Ethernet)

Length of Hardware Address field depend on network type, Ethernet uses 48-bit address

Length of Protocol Address field depend on protocol, IPv4 uses 32-bit address

59

Transmission of ARP Message in a FrameTransmission of ARP Message in a Frame

ARP message sent in payload area of frameCalled encapsulation

60

Frame TypeFrame Type

Frame type identifies message as ARPReceiver examines frame type

61

Important NoteImportant Note

Because ARP software is part of the network interface software, all higher-layer protocols and applications can use IP addresses exclusively, and remain completely unaware of hardware addresses

62

SummarySummary

InternetworkingSolves problem of heterogeneityIncludes LANs and WANs

Internet conceptVirtual networkSeamlessUniversal

63

Summary (continued)Summary (continued)

Internet architectureMultiple networksInterconnected by routers

RouterSpecial-purpose computer systemInterconnects two or more networksUses table to forward datagrams

64

Summary (continued)Summary (continued)

Internet Protocol (IP)Fundamental piece of TCP / IPDefines

Internet addressingDelivery semanticsInternet packet format (IP datagram)

65

Summary (continued)Summary (continued)

Address resolutionNeeded to map IP address to equivalent

hardware addressPart of network interfaceUses tableAutomatically updates table entriesBroadcasts requests