I am using scapy and a Wi-Fi card in monitor mode to extract data from probe requests and beacon frames travelling across a Wi-Fi network. Is it possible to use the RSSI to estimate distance of the device sending packets from the device I am using to pick them up? How does the value given in RSSI work - does it decrease over the life of the packet?
The RSSI is a measurement of the power present in a received radio signal.
It is not a protocol mechanism (like the TTL).
Moreover, the value you will get is related to a physic variable (the radio signal power) which is not related to the distance. By example, a far radio station with a high radio power could have a stronger RSSI than a close radio station with a low radio power.