University of Quebec in Outaouais
Voice over IP telephony reduces communications costs, but it is also subject to spam, i.e. unwanted, unsolicited calls. This paper discusses the problem of spam over IP telephony and reviews published techniques to reject likely nuisance callers. The techniques investigated include content filtering, black lists, white lists, gray lists, call rate monitoring, IP/domain correlation, reputation systems, consent-based communications, address obfuscation, limited-use addresses, computational puzzles, Turing tests, payments at risk, legislation, centralized SIP providers, circles of trust and authenticated identity.