Return unique identifier when packet is received from outside system

[qual25]

Hello,

I'd like to know if there is a way to use the firewall to return a unique identifier when a specific outside system sends a packet.

As my internet connection is based on PPPoE and upon reconnect there is an IP address change and delay until dyndns updates, I'm trying to figure out the best/most secure way to always let the outside system know if it is actually trying to connect to the correct system.

Thanks in advance for any help!