When ARP Cache Empty

The Address Resolution Protocol (ARP) module caches all received IP addresses to an internal cache buffer, which stores the IP addresses and Ethernet addresses (MAC).

When the application starts, the ARP Cache buffer is normally empty. The ARP module does not yet know the target MAC address for the first UDP data packet being sent from the application. It sends the ARP request to the network. The first and any subsequent UDP data packets sent from the user application are lost until the target MAC address is resolved. This is because the UDP does not buffer outgoing packets.

An ARP request must be sent to the network and the MAC address for the target IP address must be resolved before the first UDP data packet is sent to the network. This is only required if no packets were received from a destination IP. Every received IP or ARP packet is also processed by the ARP module, and the IP and MAC addresses are cached internally.

All cached IP addresses are by default temporary IP cache entries. After a timeout, which is set in the Ethernet Network Interface - ARP Definitions, such entries are automatically deleted from the cache. You can use the function arp_cache_ip() to force an ARP request. You can also use the function to change the cache entry attribute to a fixed IP address rather than a temporary IP address.

Fixed IP entries are automatically refreshed by the ARP module on timeout. When timeout expires, the ARP module sends an ARP request to the target again to verify whether the target is still active and able to accept packets.

Once the function arp_cache_ip() returns the value of __TRUE, the remote IP address is resolved and cached in the ARP cache buffer. If the Cache Entry attribute is set to ARP_FIXED_IP, there is no need to take care of resolving the IP address when a timeout expires. The ARP module does this automatically.