MAC Addresses and ARP

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MAC Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination IP subnet
• MAC (or LAN or physical or Ethernet) address
• used to get datagram from one interface to
  another physically-connected interface (same
  network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

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LAN Addresses and ARP
Each adapter on LAN has unique LAN address
Broadcast address FF-FF-FF-FF-FF-FF
adapter
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LAN Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address
• MAC flat address ? portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on IP subnet to which node is attached

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ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

237.196.7.78
1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
237.196.7.88
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ARP protocol Same LAN (network)

• A wants to send datagram to B, and Bs MAC
  address not in As ARP table.
• A broadcasts ARP query packet, containing B's IP
  address
• Dest MAC address FF-FF-FF-FF-FF-FF
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) MAC address
• frame sent to As MAC address (unicast)

• A caches (saves) IP-to-MAC address pair in its
  ARP table until information becomes old (times
  out)
• ARP is plug-and-play
• nodes create their ARP tables without
  intervention from net administrator

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Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows B IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
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• A creates datagram with source A, destination B
• A uses ARP to get Rs MAC address for
  111.111.111.110
• A creates link-layer frame with R's MAC address
  as dest, frame contains A-to-B IP datagram
• As adapter sends frame
• Rs adapter receives frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs MAC address
• R creates frame containing A-to-B IP datagram
  sends to B

A
R
B
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Extending Networks(Repeaters, Bridges, Switches)

• LAN technologies are designed with constraints of
  speed, distance and costs
• Typical LAN technology can span, at most, a few
  hundred meters
• How can a network be extended to cover longer
  distances e.g., the UF campus?

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LAN design for distance

• Many LANs use shared medium - Ethernet, token
  ring
• Length of medium affects fair, shared access to
  medium
• CSMA/CD - delay between frames, minimum frame
  length
• Token passing - circulation time for token
• Length of medium affects strength of electrical
  signals and noise immunity

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Fiber Optic Extensions

• Optical fiber
• Device fiber modem (electronic circuitry convert
  signal to digit, optical driver convert digit to
  light pulse)
• Has low delay
• Has high bandwidth
• Provide two-way communication
• Extend connection of a LAN to a computer in
  another building

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Repeater

• Repeater - bidirectional, analog amplifier that
  retransmits analog signals
• Connects two LAN segments
• Amplifies and copies signals from one segment to
  the other
• Connection can be extended with fiber modem

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Repeater

• Ethernet standard includes limit of 4 repeaters
  between any two Ethernet stations
• Can't extend Ethernet with repeaters indefinitely
  
• CSMA/CD requires low delay
• Drawbacks
• Do not understand frame formats nor physical
  address
• Transmits false or interference signals
  (collision, noise)
• Do not allow simultaneous transmission for two
  segments

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Bridge

• Hardware device like computer
• CPU, memory, and two network interfaces
• Connects two LAN segments
• Forwards complete frames
• Does not forward interference or collisions
• Learns addresses and filters
• Allows parallel transmission in separate segments

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Frame filtering

• Listen in promiscuous mode
• Watch source address in incoming frames
• Make list of computers on each segment
• Only forward if necessary
• Always forward broadcast / multicast
• Adaptive or learning bridges
• Learn the locations of computers automatically

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Illustration of a Bridge
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Illustration of a Bridge

• Bridge set up address table
• Learns location of stations by watching all
  frames.
• Bridge examines source address in each frame
• Adds entry to list for LAN segment
• Looks up destination of incoming frame in the
  table
• forward any frame whose destination is not in the
  list on every interface

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Principle of Propagation

• Propagation principle for bridged network
• A bridge forwards each frame only as far as
  necessary
• Planning a bridged network
• Permit simultaneous communication on separate
  segments
• A set of computers that interact frequently are
  attached to one segment

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Principle of Propagation

• Filtering bridge allows concurrent use of
  different segment
• U and V can exchange frames at the same time as X
  and Y exchange frames

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Extending a Bridge

• Typically optical fiber
• Can span buildings
• Advantages low cost, easy change, parallel
  transmission

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Satellite Bridging

• Can span arbitrary distance

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Bridges and cycles

• Can use multiple bridges to interconnect many LAN
  segments
• Station of segment c sends frames to station on
  segment g through B2, B1, B3 and B6
• Broadcasts are forwarded through all bridges
• Suppose another bridge connects g and f?

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A cycle of Bridges
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Eliminating broadcast cycles

• Bridges must cooperate to broadcast frames
  exactly once on each segment
• Solution from graph theory - spanning tree - used
  to determine which bridges will forward
  broadcasts
• As each bridge joins the network, it communicates
  with other bridges on special hardware (typically
  multicast) address
• Learns network topology
• Performs spanning tree computation
• Determines if bridge will form a cycle

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bridge traffic isolation

• Bridge installation breaks subnet into LAN
  segments
• bridge filters packets
• same-LAN-segment frames not usually forwarded
  onto other LAN segments
• segments become separate collision domains

collision domain
collision domain
collision domain
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Bridges vs. Routers

• both store-and-forward devices
• routers network layer devices (examine network
  layer headers)
• bridges are link layer devices
• routers maintain routing tables, implement
  routing algorithms
• bridges maintain switch tables, implement
  filtering, learning algorithms

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Switch

• Effectively a separate LAN segment for each port
• With switching, multiple stations can transmit
  simultaneously
• Provides much higher aggregate bandwidth

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Conceptual Switch Function

• Conceptual operation
• One LAN segment per host
• Bridge interconnects each pair of segments