1 Chapter 17-20 Internetworking Part 1 (Concept, IP Addressing, IP Routing, IP Datagrams, Address Resolution
Dec 26, 2015
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Chapter 17-20Chapter 17-20
Internetworking
Part 1(Concept, IP Addressing, IP Routing,
IP Datagrams, Address Resolution
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Motivation For InternetworkingMotivation For
InternetworkingLANs
Low costLimited distance
WANsHigh costUnlimited distance
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Heterogeneity is InevitableHeterogeneity is Inevitable
No single networking technology is best for all needs
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Universal ServiceUniversal Service
Fundamental concept in networkingPioneered by telephone systemArbitrary pairs of computers can communicateDesirableDifficult in a heterogeneous world
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Heterogeneity and Universal Service
Heterogeneity and Universal Service
Incompatibilities among networksElectrical propertiesSignaling and data encodingPacket formatsAddresses
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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
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An InternetworkAn Internetwork
Begin with heterogeneous network technologies
Connect the physical networksCreate software to make resulting system
appear homogeneousCalled an internetwork or internet
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Connecting Heterogeneous Networks
Connecting Heterogeneous Networks
Computer system usedSpecial-purposeDedicatedWorks with LAN and/or WAN technologiesKnown as
Internet routerInternet gateway
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Illustration of an Internet Router
Illustration of an Internet Router
Cloud denotes arbitrary network technologyOne interface per network
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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
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Internet ArchitectureInternet Architecture
MultipleNetworksRouters interconnecting networks
Host computer connects to a networkSingle router has insufficient
CPU power and memoryI/O capability
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InternetworkingInternetworking
Goal: communication systemSeamlessUniformGeneral-purposeUniversalHides heterogeneity from user
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The Internet ConceptThe Internet Concept
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To Hide HeterogeneityTo Hide Heterogeneity
Create “virtual” networkInvent
Addressing schemeNaming scheme
Implement with Protocol software
Note: protocol software needed on both hosts and routers
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Internet ProtocolsInternet Protocols
Known as TCP / IPMany protocols comprise suiteDesigned to work togetherDivided into five conceptual layers
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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.
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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
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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
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TCP/IP protocol suiteTCP/IP protocol suite
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Internet Protocol (IP)Internet Protocol (IP)
Main protocol at Layer 3Fundamental in suiteDefines
Internet addressingInternet packet formatInternet routing
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IP AddressingIP Addressing
AbstractionIndependent of hardware addressingUsed by
Higher-layer protocolsApplications
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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.
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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.
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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
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Original Classes of Addresses
Original Classes of Addresses
Initial bits determine classClass determines boundary between prefix
and suffix
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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
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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
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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
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Addressing ExamplesAddressing Examples
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Illustration of Router Addresses
Illustration of Router Addresses
Address prefix identifies networkNeed one router address per connection
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Special AddressesSpecial Addresses
Network address not used in packetsLoopback never leaves local computer
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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
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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
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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
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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
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Classless AddressingClassless Addressing
Goal: extend address spaceInvented in 1990sWorks throughout InternetAccommodates
Original classful addressesSubnet addressesOther forms
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Classless Addressing (continued)
Classless Addressing (continued)
TechniqueAllow arbitrary prefix sizeRepresent network address as pair
(address, mask_size)Known as Classless Inter-Domain Routing
(CIDR)
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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)
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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.
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Internet PacketsInternet Packets
AbstractionCreated and understood only by softwareContains sender and destination addressesSize depends on data being carriedCalled IP datagram
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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
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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)
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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
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Example of an IP Routing Table
Example of an IP Routing Table
Table (b) is for center router in part (a)
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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
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Datagram ForwardingDatagram Forwarding
Given a datagramExtract destination address field, DLook up D in routing tableFind next-hop address, NSend datagram to N
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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.
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IP SemanticsIP Semantics
IP is connectionlessDatagram contains identity of destinationEach datagram sent / handled independently
Routes can change at any time
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IP Sematics (continued)IP Sematics (continued)
IP allows datagrams to beDelayedDuplicatedDelivered out-of-orderLost
Called best-effort deliveryMotivation: accommodates all possible
networks
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Resolving AddressesResolving Addresses
Hardware only recognizes MAC addressesIP only uses IP addressesConsequence: software needed to perform
translationPart of network interfaceKnown as address resolution
NäringslivsträffNäringslivsträff
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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
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Address Resolution Techniques
Address Resolution Techniques
Table lookupHashed, orDirect indexing
Closed-form computationWorks well for configurable hardware addresses
Message exchangeServer based, orTruly distributed
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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
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ARP TableARP Table
Only contains entries for computers on local network
IP network prefix in all entries identical
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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
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Illustration of ARP Exchange
Illustration of ARP Exchange
W needs Y’s hardware addressRequest sent via broadcastReply sent via unicast
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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
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Transmission of ARP Message in a FrameTransmission of ARP Message in a Frame
ARP message sent in payload area of frameCalled encapsulation
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Frame TypeFrame Type
Frame type identifies message as ARPReceiver examines frame type
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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
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SummarySummary
InternetworkingSolves problem of heterogeneityIncludes LANs and WANs
Internet conceptVirtual networkSeamlessUniversal
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Summary (continued)Summary (continued)
Internet architectureMultiple networksInterconnected by routers
RouterSpecial-purpose computer systemInterconnects two or more networksUses table to forward datagrams
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Summary (continued)Summary (continued)
Internet Protocol (IP)Fundamental piece of TCP / IPDefines
Internet addressingDelivery semanticsInternet packet format (IP datagram)
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Summary (continued)Summary (continued)
Address resolutionNeeded to map IP address to equivalent
hardware addressPart of network interfaceUses tableAutomatically updates table entriesBroadcasts requests