ARP

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ARP.or

• Whats your MAC address?

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Packet Transmission

• NICs transmit packets from

Destination
Source
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• Each host on the network checks packet to see if
• the MAC address matches its own
• its a broadcast to all hosts

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• Where does MAC address checking occur?
• TCP/IP - network interface layer
• OSI - Data Link Layer
• MAC address uniquely identifies a specific device
  on the network.
• IP address identifies a specific host on a
  specific network
• Both MAC and IP addresses are required

5
Routed Protocols

• On TCP/IP networks
• Packets of routed protocols
• Contain Internetwork layer addressing info
• Allows user traffic to be directed from one
  network to another
• Define structure and use of fields within packet
• Types of routed protocols
• ARP
• RARP

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ARP

• Address Resolution Protocol
• Maps IP addresses to MAC addresses
• ARP tables
• are maintained in networking devices (I.e.
  routers)
• Also called ARP cache
• Maintained in RAM
• May be modified by network administrators but
  not typical usually maintained by device.

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ARP request

• If computer cannot locate IP to MAC address
  mapping in its ARP table
• Must obtain correct mapping
• Initiates an ARP request
• Source computer broadcasts ARP request to all
  hosts on local segment
• Each device checks if packet is destined for its
  own IP address
• if not ignores
• If yes - responds

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ARP frame

• Logical configuration
• Frame header

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• Real packets
• 28 octets
• Hardware types
• Protocol types
• Message lengths

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• Source host does not know the MAC address of the
  destination.
• ARP request frame is a broadcast to all MAC
  addresses

MAC header Destination Ff-ff-ff-ff-ff-f
f Source 00-00-BC-12-34-56
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• All devices on the local segment will investigate
  the frame to check if their own IP address
  matches the destination IP address.

IP Header Destination 192.168.1.100 Source 192.168
.1.205
ME?
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• If no discard the packet
• If yes
• Reads rest of packet
• Returns ARP reply
• ARP reply
• Supplies MAC address of the host.

no
no
Hey my IP matches
Source Arp Request
My MAC address
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ARP Cache Life

• Source checks its local ARP cache before sending
  request
• Must contain current information else data
  packets could be routed to the wrong host.
• ARP table directs data packets to a specific
  network card based on mapping
• Mapping must be correct

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• Host addresses can change so old ARP table
  mappings can cause misrouted packets.
• To avoid problems
• Networking devices place a TTL (time-to-live) on
  ARP entries.
• Ex. Microsoft operating systems use a two-to
  ten-minute TTL.
• Using TTLs the operating system ensures that ARP
  cache is never outdated for more than 10 min
• Process of removing ARP entries from an ARP table
  is called aging out.
• In addition, devices on network replace ARP
  entries whenever they receive new information.
• Ex. If computer receives packet from host with
  new info, old info is replaced.

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RARP

• Reverse Address Resolution Protocol
• Similar to ARP (used to bind MAC addresses to IP
  addresses)
• Used primarily by diskless workstations.
• NIC with MAC address
• No IP addresses

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• Diskless Workstations
• Dont have hard drives that hold IP
  configurations
• Must obtain IP address each time they boot up.
• Clients IP config must be stored on a RARP
  server.
• RARP server maintains table of IP address to MAC
  address mappings for RARP clients.
• During boot-up RARP clients call RARP server to
  obtain their IP config information.

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RARP Frame vs. ARP Frame
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More info.

• Note IP header does not have specific
  destination or source IP
• Source does not know which device is RARP server
  
• must broadcast to all devices on the segment

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RARP Client

• RARP client receives RARP reply
• Configures its IP networking
• Copies its IP address config info into local RAM
• After settings have been received, client can use
  MAC and IP address to send packets on the network
  

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• When diskless workstation reboots, or is shut
  down
• IP config is lost
• Each time workstation reboots it must obtain IP
  config from RARP server.

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Compare ARP RARP

• Similarities
• Both are concerned with mapping IP to MAC
  addresses
• Both use same packet format
• Both use broadcast addresses for their task.

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• Differences
• ARP - obtains MAC address of other clients by
  using an IP address
• RARP obtains its own (local host) IP address by
  using its own (local hosts) MAC address
• ARP packets are broadcast on local network by
  using broadcast MAC address (ff-ff-ff-ff-ff-ff)
• RARP uses broadcast IP address (255.255.255.255)
  as well as broadcast MAC address
  (ff-ff-ff-ff-ff-ff)

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• ARP table maintained by local host
• RARP table maintained by RARP server
• ARP reply is used by local host to update its ARP
  table and to send packets to destination.
• RARP reply is used to configure the IP protocol
  of a local host.