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Ch 9 Hardware Addressing and Frame Type Identification

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Specifying a RecipientSpecifying a RecipientAll stations on shared-media LAN receive all

transmissionsTo allow sender to specify destination

Each station assigned a unique numberKnown as station’s physical address, hardware

address, or media access address (MAC address)Each frame contains address of intended

recipient in the header (also the sender’s address)

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Conceptual Frame FormatConceptual Frame Format

Each LAN technology defines the exact formatHeader

Contains address and type informationFixed size

PayloadContains data being sentVariable in size

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Illustration Of Ethernet Frame

Illustration Of Ethernet Frame

Sender places64-bit alternating 1s and 0sSender’s address in sourceRecipient’s address in destinationType of data in frame typeCyclic redundancy check in CRC

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Ethernet AddressingEthernet Addressing

Standardized by IEEEEach station assigned by unique 48-bit

address (demo)Address assigned when network interface

card (NIC) manufacturedDemo: ipconfig /all

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Use Addresses to Filter Packets

Use Addresses to Filter Packets

LAN interface HW handles Sending and receiving framesChecks frame length and CRCCompare the destination address in the received

frame Accept or discard (filter packets)

NICCPU

MemoryLAN

Computer

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Ethernet Address Recognition

Ethernet Address Recognition

Each frame contains destination addressAll stations receive a transmissionStation discards any frame addresses to

another stationImportant: interface hardware, not software,

checks address

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Possible DestinationsPossible Destinations

Packet can be sent to:Single destination (unicast)All stations on network (broadcast)Subset of stations (multicast)

Address used to distinguish

A B C D E F

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Advantages of Address Alternatives

Advantages of Address Alternatives

UnicastEfficient for interaction between two computers

BroadcastEfficient for transmitting to all computers

MulticastEfficient for transmitting to a subset of

computers

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Broadcast on EthernetBroadcast on EthernetSender

Places broadcast address in frameAll 1s address specifies a broadcastTransmits one copy on shared networkAll stations receive the copy

Receiver always accepts BC frame

CRC

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Multicast on EthernetMulticast on EthernetA B C D E F

Group 1: AC 1000…01Group 2: CDF 1000…11

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Multicast on EthernetMulticast on EthernetHalf addresses reserved for multicast (start

with a 1 bit)Network interface card

Always accepts unicast and broadcastCan be programmed to accept zero or more

multicast addressesSoftware

Determines multicast address to acceptInforms NIC

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Promiscuous ModePromiscuous Mode

Allows interface to accept all framesDesigned for testing / debuggingAvailable on most interface hardware

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Network AnalyzerNetwork Analyzer

Listens in promiscuous mode for testing and maintenance

ProducesSummaries (e.g., % of broadcast frames)Specific items (e.g., frames from a given address)

ExampleEthereal http://www.ethereal.com/ (WinPcap_3_0.exe and ethereal-setup-0.10.2.exe)Sniffer

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ExerciseExercise

Use the Ethereal to find the physical address of computers on a LAN

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End

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Identifying Frame Contents

Identifying Frame Contents

Integer type field tells recipient the type of data being carried

For example, e-mail, web pages, or text filesTwo possibilities

Self-identifying or explicit type (network hardware includes a type field in each frame)

Implicit type (application sending data must handle type)

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Example Ethernet TypesExample Ethernet Types

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When Network HardwareDoes Not Include Types

When Network HardwareDoes Not Include Types

Sending and receiving computers must agreeTo only send single type of dataTo put type information in first few octets of

payload

QuestionsWhat size should the type information be?Meaning of type value? Multiple standards

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A Standard For Type Information

A Standard For Type Information

Defined by IEEE, called LLC / SNAP headerLLC portion specifies that a type field followsSNAP portion contains 2 fields

Organizationally Unique IdentifierTYPE