SPAM over Internet Telephony (SPIT) is a strange beast: Everybody knows it is a threat, but in real life SIP installations, it has hardly been observed. In other networks it is not uncommon, Skype needs to police its users to get a grip on abuse, and all Instant Messaging networks have to deal with IM spam, and once they offer voice capability, SPIT as well.
So, SPIT over SIP hasn’t been a problem so far. After all, there are millions of SIP devices out there replacing PSTN phone service. Why?
- In many end-user installations, SIP is just a replacement for the old analog last-mile technology. There may be SIP accounts provisioned into these devices (ATAs, hardphones), but the user does not see them. Nor do they know their own SIP AoR. None of the people calling them knows their SIP AoR. It wouldn’t help them anyway, as these networks are usually not reachable via SIP from the Internet
- The same is true for IP enabled PBXs: They talk SIP on the inside, and maybe to other
PBXs in other offices of the same company (usually via some QoS-enabled VPN). In rare cases, people build SIP trunks to upstream carriers or even establish manual cross-links to the PBXs of other companies.
- Fully-featured SIP installations which implement inter-domain SIP routing as standardized by the IETF in RFC 3263 are the exceptions. They are the enthusiast’s Asterisk installations, some enterprising VoIP operators (perhaps even using User-ENUM) and that’s it.
Summary: the vast majority of SIP devices out there is not reachable over the public Internet.
Corrolary: There is no SPIT problem, as the target audience is too small to make SPIT profitable.
This is some sort of vicious circle: Operators don’t want to open up their SIP ingress elements, because they fear SPIT. On the other hand, it is hard to build defenses against SPIT, whose characteristics and weak points you don’t really know. And it is completely impossible to prove that certain clever anti-SPIT measures work if you can’t test them in practice.
There has been no shortage on clever ideas concerning SPIT defenses. Academic papers galore, internet drafts, and even products claiming to solve the problem.
We are still in the same vicious circle.
Will RUCUS help?