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Physical Address Resolution

Based on the destination IP address and the route determination process, IP determines the forwarding IP address and interface to be used to forward the packet. IP then hands the IP packet, the forwarding IP address, and the interface to ARP.

If the forwarding IP address is the same as the destination IP address, then ARP performs a direct delivery. In a direct delivery, the MAC address corresponding to the destination IP address must be resolved.

If the forwarding IP address is not the same as the destination IP address, then ARP performs an indirect delivery. The forwarding IP address is the IP address of a router between the current IP node and the final destination. In an indirect delivery, the MAC address corresponding to the IP address of the router must be resolved.

To resolve a forwarding IP address to its MAC address, ARP uses the broadcasting facility on shared access networking technologies (such as Ethernet or Token Ring) to send out a broadcasted ARP Request frame. An ARP Reply, containing the MAC address corresponding to the desired forwarding IP address, is sent back to the sender of the ARP Request.

The ARP Cache

To keep the number of broadcasted ARP Request frames to a minimum, many TCP/IP protocol stacks incorporate an ARP cache, a table of recently resolved IP addresses and their corresponding MAC addresses. The ARP cache is checked first before sending an ARP Request frame. Each interface has its own ARP cache.

Depending on the vendor implementation, the ARP cache can have the following qualities:

• ARP cache entries can be dynamic (based on ARP Replies) or static. Static ARP entries are permanent and are manually added using a TCP/IP utility such as the ARP utility provided with Windows NT. Static ARP cache entries are used to prevent ARP Requests for commonly used local IP addresses, such as routers and servers. The problem with static ARP entries is that they have to be manually updated when network interface equipment changes.
• Dynamic ARP cache entries have a time-out value associated with them to remove entries in the cache after a specified period of time. Dynamic ARP cache entries for Windows NT TCP/IP are given a maximum time of ten minutes before being removed.

To view the ARP cache on a Windows NT computer, type arp -a at a Windows NT command prompt.

The ARP Process

IP sends information to ARP. ARP receives the IP packet, the forwarding IP address, and the interface to be used to forward the packet. Whether performing a direct or indirect delivery, ARP performs the following process as displayed in Figure 15:

1. Based on the interface and the forwarding IP address, ARP consults the appropriate ARP cache for an entry for the forwarding IP address. If an entry is found, ARP skips to step 6.

2. If the mapping is not found, ARP builds an ARP Request frame containing the MAC Address of the interface sending the ARP Request, the IP address of the interface sending the ARP Request, and the forwarding IP address. ARP then broadcasts the ARP Request using the appropriate interface.

3. All hosts receive the broadcasted frame and the ARP Request is processed. If the receiving host's IP address matches the requested IP address (the forwarding IP address), its ARP cache is updated with the address mapping of the sender of the ARP Request.

If the receiving host's IP address does not match the requested IP address, the ARP request is silently discarded.

4. An ARP Reply is formulated containing the requested MAC address and sent directly to the sender of the ARP Request.

5. When the ARP Reply is received by the sender of the ARP Request, it updates its ARP cache with the address mapping.

Between the ARP Request and the ARP Reply, both hosts have each other's address mappings in their ARP caches.

6. The IP packet is sent to the MAC address of the forwarding host by addressing it to the resolved MAC address.

Figure 15 The ARP process

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