Members of the IETF SIPPING Working Group have published an updated draft defining SPIT authorization documents that use SAML. The problem of SPAM for Internet Telephony (SPIT) is an imminent challenge and only the combination of several techniques can provide a framework for dealing with unwanted communication. The responsibility for filtering or blocking calls can belong to different elements in the call flow and may depend on various factors. This document defines an authorization based policy language that allows end users to upload anti-SPIT policies to intermediaries, such as SIP proxies. These policies mitigate unwanted SIP communications. It extends the Common Policy authorization framework with additional conditions and actions. The new conditions match a particular Session Initiation Protocol (SIP) communication pattern based on a number of attributes. The range of attributes includes information provided, for example, by SIP itself, by the SIP identity mechanism, by information carried within SAML assertions... A SPIT authorization document is an XML document, formatted according to the schema defined in RFC 4745.
SPIT authorization documents inherit the MIME type of common policy documents, application/auth-policy+xml. As described in RFC 4745, this document is composed of rules which contain three parts -- conditions, actions, and transformations. Each action or transformation, which is also called a permission, has the property of being a positive grant to the authorization server to perform the resulting actions, be it allow, block etc . As a result, there is a well-defined mechanism for combining actions and transformations obtained from several sources. This mechanism therefore can be used to filter connection attempts thus leading to effective SPIT prevention... Policies are XML documents that are stored at a Proxy Server or a dedicated device. The Rule Maker therefore needs to use a protocol to create, modify and delete the authorization policies defined in this document. Such a protocol is available with the Extensible Markup Language (XML) Configuration Access Protocol (XCAP), per RFC 4825..."
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