Question

In Address Resolution Protocol: ARP it was stated that, in newer implementations, “repeat ARP queries about a timed out entry are first sent unicast”, in order to reduce broadcast traffic. Suppose host A uses ARP to retrieve B’s LAN address. A short time later, B changes its LAN address, either through a hardware swap or through software reconfiguration.

(a). What will happen if A now sends a unicast repeat ARP query for B?

(b). What will happen if A now sends a broadcast repeat ARP query for B?

Answer 1

Explanation:

Definition:

The Address Resolution Protocol or ARP is a communication protocol for mappinga dynamic Internet Protocol or IP address to a permanent physical machine address called the Media Access Control or MAC address. typically ARP is used for mapping the link layer MAC address to the internet layer IPv4 address. ARP is one of the most important protocal of the Network layer of the OSI model.

Purpose:

The purpose of ARP is to essentially translate 32 bit addresses to 48 bit addresses and vice versa which is necessary because IPv4 addresses are 32 bits long whereas MAC addresses are 48 bit long.

Functioning:

The term address resolution refers to the process of finding the address of a computer in a network. The address is resolved using ARP in which a piece of information is sent by a client process executing on the local computer to the server process executing on a remote computer. The information received by the server allows it to uniquely identify the network system for which the address was required and therefore to provide the required address. The address resolution procedure is completed when the client receives a response from the server containing the required address.

The downside:

The distributed approach to address resolution and discovery of ARP results in considerable volume of network traffic where each host must discover every address on the network. If nodes do not cache information as a result of transmission of a neighbour, every node has the potential to generate n number of messages for a network with n nodes and adding to it will be n replies to a single requesting node. Thus, for a network with n nodes there will be 2(n-1) messages for a single ARP request.Complicating matters is the fact that ARP tables age out for nodes that are not routinely participating in message exchanges and refreshing those tables further add to network traffic.Consequently the routers are burdened heavily and this causes an unintended network delay. For this reason, repeat ARP queries about a timed out entry are first sent unicast, in order to reduce broadcast traffic.

The workaround:

When a new computer joins a LAN, it is assigned a unique IP address to use for identification and communication. When an incoming packet destined for a host machine in a particular LAN arrives at a gateway, the gateway program asks ARP to find the MAC address that matches the IP address. A table called the ARP cache maintains a record of each IP address and its corresponding MAC address. All operating systems inan IPv4 Etehrnet network keep an ARP cache. Every time a host requests a MAC address in order to send a data packet to another host in the LAN, it checks the ARP cache. If the cache already has the required data, ARP is not performed, else ARP is carried out.

The given situation:

In the given situation, host A uses ARP to retrieve B’s LAN address and a short time later, B changes its LAN address, either through a hardware swap or through software reconfiguration. So in this case, for a hardware swap both the MAC and IP addresses have changed whereas for a software reconfiguration only the IP address has changed.

Keeping in view the above discussions:

Ans.(a). What will happen if A now sends a unicast repeat ARP query for B?

If A now sends a unicast repeat ARP query for B, the unicast repeat will contain the previous IP address of B from the ARP cache of A and consequently fail to reach B, the destined host with a different IP address now and the intended network transmission will not be successful.

Ans.(b). What will happen if A now sends a broadcast repeat ARP query for B?

If A now sends a broadcast repeat ARP query for B, the message will reach all the nodes in the network, including B albeit with a different IP address. So in the higher networking layers ARP will discover the intended host B with a new IP address and deliver the packet. B in return will send an unicast reply back to A acknowledging the receipt and updating the ARP cache of A with the new IP Address for future successful transmissions.