“Computational Puzzles for SPAM Reduction in SIP”, C. Jennings, October 2005, draft-jennings-sip-hashcash-03
Abstract: One of the techniques used in SPAM prevention and various solutions for denial of service attacks is to force the SIP client requesting a service to perform a calculation that limits the rate and increases the cost of the request. This draft defines a way to allow a UAS to ask the UAC to compute a computationally expensive hash based function and present the result to the UAS. Although the computation is expensive for the UAC to compute, it is cheap for the UAS to verify. The solution also allows for proxies to compute and check the puzzle on behalf of the UAC or UAS.