Address Resolution Protocol

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Address Resolution Protocol

Unless IP is planning to initiate a full broadcast on the network, it has to have the physical address of the machine to which it is going to send datagrams. For this information, it relies on Address Resolution Protocol (ARP). ARP is responsible for mapping IP addresses on the network to physical addresses in memory. This way, whenever IP needs a physical address for a particular IP address, ARP can deliver. But ARP’s memory does not last indefinitely, and occasionally IP will ask for an IP address that is not in ARP’s memory. When this happens, ARP has to go out and find one. This is why ARP is represented by the detective icon in Figure 2.24.

ARP is responsible for finding a map to a local physical address for any local IP address that IP may request. If ARP does not have a map in memory it has to go find one on the network. ARP uses local broadcasts to find physical addresses of machines and maintains a cache in memory of recently mapped IP addresses to physical addresses. Although this cache does not last indefinitely, it enables ARP to not have to broadcast every time IP needs a physical address.

As long as the destination IP address is local, all ARP does is a local broadcast for that machine and returns the physical address to IP. IP, realizing that the destination IP address is local, simply formulates the datagram with the IP address above the physical address of the destination machine. Figure 2.28 shows how that process happens.

But IP does not always need to send datagrams to local IP addresses. In fact, often the destination address is on a remote network where the path may include several routers along the way. The hardest thing to realize conceptually is that ARP operates so close to the network interface layer that it is really only good for finding local physical addresses. This is true even in environments where routers exist. ARP never reports a physical address that exists on a remote network to IP. Figure 2.29 illustrates what would happen if ARP was capable of responding with a physical address from a remote network. IP datagrams specify exactly which physical address is supposed to listen to their message. In the example in Figure 2.29, then, the datagram is sent out onto the network, and the router, which also has a physical address, simply ignores the packet. Not exactly what was intended.

To get the packet to the other network, the router is supposed to listen to the packet and forward it on. The only way to get it to listen to the packet, though, is to either do a broadcast, or send the packet to the router’s physical address. IP is smart enough to realize that the destination IP address is on a remote network and that the datagram must be sent to the router. However, it has no idea what the physical address of the router is, and thus relies on ARP to discover that for it.

To route a packet, IP asks ARP whether it has the physical address of the router, not of the destination machine. This is one of the more subtle and elegant features of the TCP/IP suite, in that it cleverly redirects packets based upon what layer is being communicated with. After IP receives the physical address of the router from ARP, it formulates the datagram, placing the destination IP address directly above the router’s physical address. Figure 2.30 illustrates how this interaction actually works and how elegant this system of routing really is.

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