1 ch 9 hardware addressing and frame type identification
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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