ARP - Address resolution protocol

ARP - Address resolution protocol

1. Definição

   • Mapping Internet Addresses To Physical Addresses
   • In practice, ARP is only used to map 32-bit IPv4 addresses to 48-bit Ethernet addresses.

2. Resolution for IPV6

   • uses a technique known as direct mapping - porção do MAC dentro do IP
     • As the name direct mapping implies, the mapping can be performed without reference to external data. Direct mapping has the advantage that new computers can be added to a network without changing existing assignments and without propagating new information to existing computers.
   • IPv6 does not need ARP

3. Resolution for IPV4 (ARP)

   • The Address Resolution Protocol, ARP, allows a host to find the physical address of a target host on the same physical network, given only the target’s IP address.
   • direct mapping cannot be used with IPv4
     • Ethernet MAC address cannot be directly mapped into an IPv4 address because a MAC address is 48 bits long and an IPv4 address is only 32 bits long.
   • The Address Resolution Protocol, ARP, allows a host to find the physical address of a target host on the same physical network, given only the target’s IP address.

4. The ARP Cache

   • ARP Cache Timeout
     • when the timer expires, the information is deleted
     • se 1 destino cai, não tem-se como detectar e por isso usa-se esta tecnica
   • ARP Caches In Layer 3 Switches
   • it records the answer and reuses the information for successive transmissions
     • Senão está no cache usa broadcast

5. ARP Implementation

   • ARP software
     • address resolution for outgoing packets
       • given the IP address of a computer on the network, it finds the hardware address of the computer.
       • If an address is not in the cache, it sends a request.
     • incoming ARP packets
       • It updates the cache, answers requests from other computers on the network, and checks whether a reply matches an outstanding request

6. Reverse Address Resolution (RARP)

   • RARP uses the same packet format as ARP†. The only difference is that RARP uses Ethernet type 0x8035
   • RARP was once an essential protocol used to bootstrap systems that did not have stable storage (i.e., diskless devices) - No longer used.
   • used in cloud data centers
     • In a data center, when a Virtual Machine migrates from one PC to another, the VM retains the same Ethernet address it was using before.

7. Proxy ARP

   • How can proxy ARP be used for security? Proxy ARP can be used for a firewall or on a VPN connection. The idea is that because a proxy ARP machine impersonates machines on the second network, all packets must travel though the proxy ARP machine where they can be checked. In Figure 6.4, for example, a site could place all hosts on network 2 and put firewall software in machine P. Whenever a packet arrives from the Internet, the packet will go through P (where the packet can be examined and firewall can be rules applied) on its way to the destination host.
   • Proxy ARP can be used for
     • Firewall
     • VPN

8. IPv6 Neighbor Discovery

   • IPv6’s Neighbor Discovery Protocol (NDP) replaces ARP and allows a host to map between an IPv6 address and a hardware address†.

9. OSI

   • ARP is a low-level protocol that hides the underlying addressing used by network hardware, permitting us to assign an arbitrary IP address to every machine. We think of ARP as associated with the physical network system rather than as part of the Internet protocols.
   • We think of ARP as associated with the physical network system rather than as part of the Internet protocols.

10. ARP Caches In Layer 3 Switches

    • redução de ARP traffic por Sw L3
      • Escuta ARP e armazena
      • SW responde ARP request sem broadcast
      • MAC só nuda se PC power down
      • In general, a switch can create its own cache of ARP information and can answer requests.