ARP

Address Resolution Protocol (ARP)

ARP table:

• Used to determine the interface’s MAC address given its IP address
• Entries are <IP address; MAC address; TTL>
• TTL - Time after which address mapping will be forgotten (typically 20 minutes)
  • Because devices can die without telling server

Steps

1. A wants to send datagram to B
   • B’s MAC address not in A’s ARP table
2. A broadcasts ARP query packet, containing B’s IP address
   • Destination MAC address = FF-FF-FF-FF-FF-FF
   • All nodes on LAN receive ARP query
   • Query includes A’s MAC address so B can respond
3. B receives ARP packet, replies to A with B’s MAC address
   • Frame sent to A’s MAC address (unicast)
4. A caches (saves) IP-to-MAC address pair in its ARP table until TTL
   • Soft state: Information that times out unless refreshed
5. ARP is “plug-and-play”:
   • Nodes create their own ARP tables without intervention from net administrator

ARP Packet Format

• HardwareType: type of physical network (e.g. Ethernet)
• HLen: length of physical address
• PLen: length of protocol address
• Operation: Request or Response