]> Cisco Systems, Inc.

170 West Tasman Drive San Jose CA 95134 USA dwing@cisco.com

Network Laboratories, NEC Europe Ltd.

Kurfuersten-Anlage 36 Heidelberg 69115 Germany +49 (0) 6221 4342 118 saverio.niccolini@netlab.nec.de http://www.netlab.nec.de

Network Laboratories, NEC Europe Ltd.

Kurfuersten-Anlage 36 Heidelberg 69115 Germany +49 (0) 6221 4342 113 stiemerling@netlab.nec.de http://www.netlab.nec.de

Nokia Siemens Networks

Linnoitustie 6 Espoo 02600 Finland +358 (50) 4871445 Hannes.Tschofenig@nsn.com http://www.tschofenig.com

RUCUS Exploratory Working Group This document defines a mechanism for SIP proxies to communicate a spam score to downstream SIP proxies and to SIP user agents. This information can then be used as input to other decision making engines, for example, to provide alternate call routing or call handling.

It is desirable for SIP proxies to insert a spam score so that downstream SIP proxies and downstream SIP user agents can use a high score to decide that special handling is required. For example, a score above 20 might cause one of the spam avoidance techniques described in to be triggered for this call. This specification allows each SIP proxy to contribute spam scoring information that can be useful to downstream SIP proxies and the SIP user agent (UA). The downstream SIP proxies or SIP UA might ignore that information (e.g., it doesn't trust the SIP proxy that generated the spam score) or might use it. Note that this document does not make the attempt to define how the spam score was derived nor to distribute information that could be used to verify the spam score generation. Furthermore, this document does not attempt to cryptographically bind the identity of the entity generating the score to the value itself. Hence, its usage is likely to be useful only between neighboring administrative domains communicating such a score.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

A SIP proxy evaluates an incoming SIP request and generates a spam score using a local mechanism. This score is between 0 (indicating the message is not spam) and 100 (indicating the message is spam). Values between 0 and 100 indicate the 'likelihood' that the SIP request is spam, with higher values indicating a higher likelihood the message is spam. This spam score is inserted into the new "Spam-Score" header. This header field contains a summary spam score and optionally contains detail information. The detail information is implementation dependent. The detail information is valuable for debugging and to provide the SIP user agent or SIP proxy with additional information regarding how the spam-scoring SIP proxy's local mechanism arrived at the summary spam score.

A downstream proxy or the SIP user agent MAY use the spam score or spam-detail information to change call routing or call handling. It is envisioned that some form of policies indicate the trusted proxies in order to decide which spam scores to consider for special call treatment. In some jurisdictions, the end user needs to authorize call handling, including rejection of a call based on a spam score. Mechanisms to allow users to influence such policies are, however, out of scope of this document. The behavior of the SIP proxy or user agent when the spam score is above a certain value is a local policy matter. Examples of behavior include: a SIP request with a high spam score might cause a proxy or user agent to redirect the SIP request to company's main telephone extension or to the user's voicemail a user agent might alert the user by flashing the phone (without audible ringing) a user agent might allow calls with a spam score below a certain value during daylight hours, but deny such calls at night. a proxy might challenge the caller to complete a Turing test.

ABNF using the ABNF syntax of :

The following example shows a SIP score generated and inserted by two SIP proxies, sip.example.com and sip.example.net. In this example, sip.example.com is owned by a spammer who is trying to fool downstream systems with their low spam score (0). However, the example.net proxies and user agents only pay attention to spam scores from Spam-Score headers generated by example.net proxies, so example.com's attempts to fool the downstream proxies (with its low spam score) are in vain. From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710@pc33.example.com CSeq: 314159 INVITE Contact: Content-Type: application/sdp Content-Length: 142 [... SDP elided from this example...] ]]>

SIP proxies and SIP user agents need to ignore spam scores generated by proxies that aren't trusted. [[This section will be completed in a later version of this document.]]

Thanks to Joachim Charzinski, Daniel Quinlan, and S. Moonesamy for their suggestions to improve this document.

[[This section will be completed in a later version of this document.]]

&rfc2119; &rfc3261; &rfc5039;

Note to RFC Editor: please remove this section prior to publication.

Changed scoring from positive/negative to 0-100 range. Moved score from a "Via:" extension to a new header "Spam-Score:". Changed from Standards Track to Experimental.