Spam Score for SIP
draft-wing-sipping-spam-score-01

Status of this Memo

By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.

This Internet-Draft will expire on August 16, 2008.

Abstract

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.

Table of Contents

1.  Introduction
2.  Terminology
3.  Operation of Spam-Scoring Proxy
4.  Operation of Proxy or User Agent
5.  Grammar
6.  Examples
7.  Security Considerations
8.  Acknowledgements
9.  IANA Considerations
10.  References
    10.1.  Normative References
    10.2.  Informational References
Appendix A.  Changes
    A.1.  Changes from -00 to -01
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

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 [RFC5039] (Rosenberg, J. and C. Jennings, “The Session Initiation Protocol (SIP) and Spam,” January 2008.) 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.

2.  Terminology

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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Operation of Spam-Scoring Proxy

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.

4.  Operation of Proxy or User Agent

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.

5.  Grammar

ABNF using the ABNF syntax of [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.):

  extension-header   = spam-score [ SP ";" spam-detail ]

  spam-score         = score SP "by" SP hostname
  score              = 1*3DIGIT [ "." 0*3DIGIT ]

  spam-detail        = "detail" EQUAL detail
  detail             = QUOTE mech SEMI rule-score
                       *(COMMA rule-score) QUOTE
                       ; mathematical average of the rule-scores
                       ; MUST be same as spam-score

  rule-score         = rule [ "=" score ]
  mech               = token
  rule               = token

6.  Examples

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.

  INVITE sip:bob@example.net SIP/2.0
  Via: SIP/2.0/UDP sip.example.net;branch=z9hG4bKnashds8
    ;received=192.0.2.1
  Spam-Score: 75 by sip.example.net
    ;detail="SIPfilter-1.0;call_volume=75"
  Via: SIP/2.0/UDP sip.example.com;branch=z9hG4bKfjzc
    ;received=192.0.2.127
  Max-Forwards: 70
  To: Bob <sip:bob@example.net>
  From: Alice <sip:alice@example.com>;tag=1928301774
  Call-ID: a84b4c76e66710@pc33.example.com
  CSeq: 314159 INVITE
  Contact: <sip:alice@pc33.example.com>
  Content-Type: application/sdp
  Content-Length: 142

  [... SDP elided from this example...]

7.  Security Considerations

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.]]

8.  Acknowledgements

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

9.  IANA Considerations

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

10.  References

10.1. Normative References

10.2. Informational References

Appendix A.  Changes

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

A.1.  Changes from -00 to -01

• 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.

Authors' Addresses

Full Copyright Statement

Copyright © The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.

Acknowledgment

This document was produced using xml2rfc v1.35 (of http://xml.resource.org/) from a source in RFC-2629 XML format.