Binding Protocol Addresses ARP

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Binding Protocol Addresses(ARP)
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Protocol Versus Physical

• Protocol addresses are software abstractions
• Physical addresses are not
• Protocol addresses must be translated into
  physical addresses for delivery on a given
  hardware network
• Either final or next hop
• Termed address resolution - binding

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Address Resolution

• Typically local to a network

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Techniques

• Each network might have different approach
• Done on per-NIC basis
• Table lookup, stored in memory
• Closed-form computation, addresses correspond to
  each other mathematical function maps
• Message exchange, request/response - dynamic

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Table Lookup

• Entry for each machine on LAN
• Different table for each physical network you are
  on
• Easy to program and use
• Can index or hash
• Downside static
• When new host added must change all tables

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Table Example
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Indexing Example
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Closed Form

• Not all network types have static physical and
  protocol addresses
• For Networks with configurable hardware
  addresses, can make the two related
• Example have physical address have IP address as
  subset

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Message Exchange

• Other two approaches are static
• Another approach dynamic distributed information
• Two approaches
• Server-based
• Completely distributed

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Server-Based

• Whenever resolution need, consult server
• Must know where servers are
• Or broadcast
• Servers must be up
• Centralization may help with administration

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Decentralized

• Each computer knows its two addresses
• Each responds to requests
• Requests are broadcast
• New machines can be added without modifying
  servers

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ARP

• TCP/IP can use each of the three methods
• Table static ARP entries
• Closed-form specific networks
• Message exchange Address Resolution Protocol
• Focus on dynamic fully distributed paradigm
• Request/response

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ARP Continued

• Request contains IP address, asks for physical
  address
• Reply contains requested IP address and physical
  address
• Request broadcast (layer 2) to all on LAN
• Each computer examines and compares to own IP
  address

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ARP Continued

• If IP address matches, send response to only
  requesting machine
• ARP not specific to Ethernets very general
• Has variable sized addresses whose length
  specified in header
• Although the 4 and 6 limits seem designed for
  Ethernets

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ARP Message
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ARP Transport

• ARP message carried inside layer 2 frame
• Encapsulated in underlying frame
• Frame must specify it contains ARP message type
  806

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Encapsulation
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Identifier
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Caching

• For each IP packet, three packets sent
• ARP request
• ARP reply
• IP packet
• To reduce traffic, each host has cache of recent
  resolutions
• What cache means
• New data constantly replaces old
• Limited space oldest data removed
• Greatly speeds process

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Caching Continued

• Linux 1 minute
• NT 2 minutes
• Cisco 4 hours
• Machines also cache reply hosts
• Will typically need it very soon for two-way
  communication
• Use of arp utility (arp a)

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Handling A RP Messages

• If sender binding in cache refresh with new
  data
• If request build response and send (fill in
  sender HW address)
• Then cache sender binding

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Address Resolution

• Hides details of physical mapping from higher
  layers
• Boundary between hardware in- and dependence
  the line between layer 2 (network interface
  layer) and layer 3 (IP layer) everyone above
  uses IP addresses everyone below uses HW
  addresses

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Problems

• 19.2, 19.3, 19.4, 19.5
• Use arp utility