SPam over Internet Telephony (SPIT) Prevention Framework.

SPam over Internet Telephony (SPIT) is expected to become a threat inhibiting the delivery of voice services over the Internet in the near future both because of its technical and economical characteristics. Experiences with email SPAM and its analogies with SPIT suggest that SPIT will be difficult to detect with a single detection method. Moreover, personalized management of detection strategies will be needed to increase effectiveness and to adapt the methods to special environments. This paper presents a modular framework for SPIT prevention designed to be easily manageable and extensible. Additionally, the framework makes use of a two-stage architecture in order to exploit the knowledge coming both from the signaling and from the media flows while still allowing real-time delivery of the media content to the user. Following the experience gained with our prototype implementation we also present the most important design issues we encountered and suggest solutions to these issues. I. INTRODUCTION Everybody knows unsolicited bulk messages or SPAM emails which have become a serious problem not only for companies but also for private users. Many users already receive more SPAM than normal messages and some estimates put the percentage of SPAM at up to 80% of all email traffic. With the increasing deployment of Voice over IP there are concerns that it will also become a target of unsolicited advertisement which for the case of VoIP has been named SPIT (SPAM over Internet Telephony). SPAM already exists in the Public Switched Telephone Network (PSTN) but because of the relatively high cost for the caller the problem is limited. VoIP on the other hand is orders of magnitude cheaper (1) and therefore a much more attractive target. Among all the Voice over IP protocols available today we look at SIP (Session Initiation Protocol, (2)) in this paper because SIP solutions dominate today's enterprise market and will soon dominate the telecom business. The most obvious form of SPIT are marketing calls placed by a computer. Typically a list of SIP addresses is called one by one and every time a call is answered some marketing content is replayed. Creating a software application which can place such calls is a trivial exercise and any commodity hardware can be used to run it. The only additional requirement is an internet connection which is able to handle a certain number of simultaneous VoIP calls. Current standardization activities (1) suggest that strongly authenticated identities (3) together with white lists (to allow known users to call) and a consent framework (4) (to solve the introduction problem) are the solution to SPIT calls. But even if these methods provide a good starting point, their effectiveness cannot be fully exploited because strongly authenticated identities cannot be enforced universally. If a callee receives a call without an authenticated identity he does not know whether the caller did not support them or whether it was intentionally not authenticated. Furthermore, there is no standardized technique to establish networks of trust in order to support a consent framework. Some research effort in this direction has also been pre- sented in (5), where a reputation-based SIP social networks approach is discussed. However, this approach is still in an early phase and requires more detailed investigation.