SIPPING                                                    H. Tschofenig
Internet-Draft                                               E. Leppanen
Intended status: Standards Track                  Nokia Siemens Networks
Expires: January 3, 2008                                    July 2, 2007


  Completely Automated Public Turing Test to Tell Computers and Humans
   Apart (CAPTCHA) based Robot Challenges for the Session Initiation
                             Protocol (SIP)
                draft-tschofenig-sipping-captcha-00.txt

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 January 3, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Spam over Internet Telephony (SPIT) is one of the foreseen future
   forms of spamming that SIP networks may have to handle.  SPIT also
   has more impact on users than email spam since it is more intrusive.
   Email as a store-and-forward communication mechanism allows for
   several filtering mechanisms to be applied to the full content before
   being presented to the user.  Session Initiation Protocol (SIP)



Tschofenig & Leppanen    Expires January 3, 2008                [Page 1]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   interaction is, in contrast, real-time communication and therefore
   does not provide much information prior to the transmission of the
   content, making it both harder to filter and more annoying to users.
   The responsibility for filtering, blocking calls, or taking any other
   preventive action can belong to different elements in the call flow
   and may depend on various factors.

   This document concentrates on "Completely Automated Public Turing
   Test to Tell Computers and Humans Apart" (CAPTCHA) tests, which
   require human interaction.  The document proposes an approach how to
   apply those challenges to SIP communication for handling the SPIT
   problem.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Technical requirements and available solutions . . . . . .  5
     3.2.  Overview of the Proposed Solution  . . . . . . . . . . . .  6
   4.  Client and Proxy Operations  . . . . . . . . . . . . . . . . .  8
     4.1.  Operation of SPIT Proxy  . . . . . . . . . . . . . . . . .  8
     4.2.  Operation of UAC . . . . . . . . . . . . . . . . . . . . .  8
   5.  XML Structure  . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Structure of XML-Encoded CAPTCHA challenge . . . . . . . .  9
     5.2.  MIME Type for CAPTCHA Challenge Document . . . . . . . . .  9
     5.3.  The <challenge> Root Element . . . . . . . . . . . . . . .  9
     5.4.  The <media> element  . . . . . . . . . . . . . . . . . . . 10
     5.5.  The <uri> element  . . . . . . . . . . . . . . . . . . . . 10
     5.6.  The <data> element . . . . . . . . . . . . . . . . . . . . 10
     5.7.  Values . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Internationalization Support . . . . . . . . . . . . . . . . . 15
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Captcha Header . . . . . . . . . . . . . . . . . . . . . . 15
     11.2. 4xx Response . . . . . . . . . . . . . . . . . . . . . . . 16
     11.3. Namespace  . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.4. Content-Type registration for
           'application/captcha-challenge+xml'  . . . . . . . . . . . 16
     11.5. CAPTCHA Schema Registration  . . . . . . . . . . . . . . . 18
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     13.1. Normative references . . . . . . . . . . . . . . . . . . . 18
     13.2. Informative references . . . . . . . . . . . . . . . . . . 19



Tschofenig & Leppanen    Expires January 3, 2008                [Page 2]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21

















































Tschofenig & Leppanen    Expires January 3, 2008                [Page 3]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


1.  Introduction

   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 attempts.
   [I-D.ietf-sipping-spam] provides four core recommendations that need
   to be considered for a SPIT solution, namely,
   o  Strong Identity
   o  White Lists
   o  Solve the Introduction Problem
   o  Don't Wait Until its Too Late.

   The human interaction required challenges are mainly used for solving
   the introduction problem targeting to handle requests from user
   agents with whom the recipient do not have former relations.  E.g.,
   the challenge is initiated towards user agents that are not yet white
   or black-listed, or based on some other criteria.

   The [I-D.tschofenig-sipping-framework-spit-reduction] provides a
   framework for defining SPIT prevention policy.  The policy contains
   rules which are applied to requests if the conditions of a given rule
   matches.  The actions of the matching rules are executed.  One of the
   actions could be to provide a challenge which must be soved by a
   human before the request is forwarded to the destination.

   There are different techniques already developed for challenging user
   agents.  "Completely Automated Public Turing Test to Tell Computes
   and Humans Apart" (CAPTCHA) [captcha] typically provides a human a
   task either to recognize something or a question to be answered using
   different media types.  [Inaccessibility-of-CAPTCHA] provides
   alternatives to visual test for allowing systems to test for human
   users while preserving access by users with disabilities.  Hashcash
   challenge [hashcash] requires user agents to perform CPU-intensive
   computational puzzles making it difficult to send large amounts of
   requests.  The hashcash concept has been proposed for usage with SIP
   in [I-D.jennings-sip-hashcash].  XMPP has already adopted some of the
   techiques as extensions, see [XEP-0158].

   The Session Initiation Protocol (SIP) [RFC3261] provides the ability
   for the users to initiate, manage and terminate communication
   sessions.  The scope of this document is to provide a mechanism for
   SIP based communication to apply SPIT reduction related challenges
   requiring human interaction.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",



Tschofenig & Leppanen    Expires January 3, 2008                [Page 4]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119
   [RFC2119] and indicate requirement levels for compliant
   implementations.

   This document makes also use of the vocabulary defined in RFC3261
   [RFC3261].


3.  Overview

   This section describes the technical requirements, available
   solutions and gives an overview of the proposed solution.  The
   section is informational in nature.  It does not contain any
   normative statements.

3.1.  Technical requirements and available solutions

   Adopting CAPTCHA or Hashcash type of techniques for SIP communication
   requires a mechanism for enabling user interaction type of function
   to be associated with SIP requests.  When a proxy or user agent
   server (UAS) server receives a SIP request which needs to be
   challenged, the proxy or UAS sends a challenge to the originator of
   the SIP request before continue handling of the request.  After
   getting the answer to the challenge from the user, the user agent
   client (UAC) needs to provide the answer to the proxy or UAS in order
   to get the request passed to the recipient.

   The challenge should offer multiple choices for the UACs to select
   depending on the capabilities of the device where the UAC is running.
   Also, the UAC should be able to authenticate and authorize the source
   of challenge.  The UAC may receive the challenge via a URL or as
   direct media compoment(s).

   The main target is to support SIP dialog creating request such as SIP
   INVITE, but ideally the solution should also cover non-dialog
   creating requests, e.g., SIP MESSAGE.  It is also important that the
   mechanism functions with the existing UAC implementations.

   There are existing mechanisms specified to be considered as a
   solution for the technical requirements discussed above.

   [I-D.ietf-sipping-app-interaction-framework] defines a framework for
   interaction between users and SIP based applications.  The framework
   covers both the "presentation capable" and "presentation free" user
   interfaces (UI) having different solutions to both.  The user
   interaction with the presentation capable UI is handled by using SIP
   REFER and HTTP while the presentation free UI case utilize SIP events



Tschofenig & Leppanen    Expires January 3, 2008                [Page 5]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   [RFC3265] (SIP SUBSCRIBE and NOTIFY).  Since there are different
   solutions for different cases, the UAC needs to indicate the
   supported application user interaction mechamisms when issuing a SIP
   request.  This might be too a heavy requirement for solving the user
   interaction needs related to SPIT challenges.  Also, the application
   interaction framework requires that a dialog exists before initiating
   or accepting any user interaction requests.  In case of SPIT
   challenges the user interaction must happen during the dialog
   establishment so it seems that the application interaction framework
   cannot be directly used as a solution.

   [XEP-0158] provides a XMPP based solution for SPIT challenges.  The
   XML format defined for CAPTCHAs and Hashcash can be considered as
   basis also for the SIP based approach.

3.2.  Overview of the Proposed Solution

   The Figure 1 and Figure 2 present high level messages flows for
   conveying a challenge (e.g., CAPTCHA) to the SIP UAC which initiated
   a dialog forming SIP request.  In Figure 1 the challenge is included
   in the body of the SIP 4xx response while in Figure 2 describes a
   case when the challenge is fetched via URL which was provided with
   the response.  After the user has managed to solve the challenge the
   UAC re-issues the request with the solution.  The SPIT proxy removes
   the solution before proxying the request to the SIP UAS.


























Tschofenig & Leppanen    Expires January 3, 2008                [Page 6]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


    SIP                       SPIT                      SIP
    UAC                       Proxy                     UAS

     |  1) SIP INVITE          |                         |
     |------------------------>|                         |
     |                         |                         |
     |  2) 4xx + CAPTCHA       |                         |
     |<------------------------|                         |
     |                         |                         |
     |                         |                         |
     |                         |                         |
     |                         |                         |
     |                         |                         |
     |                         |                         |
     |  3) Re-INVITE + Solution|                         |
     |------------------------>|  4) SIP INVITE          |
     |                         |------------------------>|
     |                         |  5) 200 OK              |
     |  6) 200 OK              |<------------------------|
     |<------------------------|                         |

    Figure 1: A case where the SPIT Proxy returns the CAPTCHA directly
                             with the response


   SIP                       SPIT                      SIP
       UAC                       Proxy                     UAS

        |  1) SIP INVITE          |                         |
        |------------------------>|                         |
        |                         |                         |
        |  2) 4xx + CAPTCHA-Ref.  |                         |
        |<------------------------|                         |
        |                         |                         |
    +---+----+                    |                         |
    |3) Fetch|                    |                         |
    |CAPTCHA |                    |                         |
    +---+----+                    |                         |
        |                         |                         |
        | 4) Re-INVITE + Solution |                         |
        |------------------------>|  5) SIP INVITE          |
        |                         |------------------------>|
        |                         |  6) 200 OK              |
        |  7) 200 OK              |<------------------------|
        |<------------------------|                         |

   Figure 2: A case where the SPIT Proxy returns a link to the CAPTCHA.
               The UAC fetches the CAPTCHA e.g. using HTTP.



Tschofenig & Leppanen    Expires January 3, 2008                [Page 7]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


4.  Client and Proxy Operations

4.1.  Operation of SPIT Proxy

   When the SPIT Proxy receives a SIP request from a UAC, its
   authorization engine applies the authorization policy to the SIP
   request as defined in
   [I-D.tschofenig-sipping-framework-spit-reduction].  When the actions
   of the authorization policy results to request the proxy to generate
   a challenge to the UAC, the proxy sends a 4xx response with an XML
   document containing the challenge in the body.  The Content-Type used
   for the XML document is 'application/captcha-challenge+xml'.

   OPEN ISSUE: define the value of the new response code.

   When the SPIT Proxy receives a re-issued SIP request from the UAC, it
   validates the answer provided by the UAC in the Captcha header field.
   In case the answer and other possible policies allow the request to
   get proxied further to the UAS, the SPIT Proxy removes the Captcha
   header.  Depending on the policies and functionality of the SPIT
   Proxy, the proxy may update the authorization policy according to the
   decision, e.g., insert the AoR of the user of the UAC to a white or
   block list.  In case the answer was not satisfactory, the UAS acts
   according to a defined policy, e.g., rejects the request.

   If the SPIT proxy does not get a re-issued SIP request and/or because
   of a timeout the SPIT proxy may consider the request ignored.  The
   proxy might in some cases also consider adding the AoR of the user of
   the UAC to a block or observation list as a potential SPIT address.

4.2.  Operation of UAC

   When the UAC receives a 4xx response with a MIME type 'application/
   captcha-challenge+xml' in the body to be solved, the UAC first
   authenticates and authorizes the sender of the challenge.

      TODO: describe the authentication and authorization more.

   The UAC selects the challenges marked as mandatory and possibly some
   additional ones for UAC's execution or to be rendered to the user
   based on e.g. the device capabilities.  The UAC may also need to
   fetch the challenges from which URL links were provided.  When the
   challenge gets solved, the UAC provides an answer in the Captcha
   header field by re-issuing the SIP request, e.g. by sending a SIP re-
   INVITE.






Tschofenig & Leppanen    Expires January 3, 2008                [Page 8]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


      TODO: describe the selection more when solution details known.


5.  XML Structure

   The XML Schema for the CAPTCHA challenge XML document is defined in
   Section 8.

5.1.  Structure of XML-Encoded CAPTCHA challenge

   A CAPTCHA challenge is an XML document [XML] that MUST be well-formed
   and MUST be valid according to schemas, including extension schemas
   available to the validater and applicable to the XML document.  The
   XML documents MUST be based on XML 1.0 and MUST be encoded using
   UTF-8.

   The namespace identifier for elements defined by this specification
   is a URN [RFC2141], using the namespace identifier 'ietf' defined by
   [RFC2648] and extended by [RFC3688].  This urn is:
   urn:ietf:params:xml:ns:captcha.

5.2.  MIME Type for CAPTCHA Challenge Document

   The MIME type for the XML document is 'application/
   capcha-challenge+xml'.

      OPEN ISSUE: is there a need to indicate support for the MIME type
      in the initial request?

5.3.  The <challenge> Root Element

   The root element of the XML document is <challenge>.

   The <challenge> element contains the namespace definition mentioned
   in Section 5.1.  It also contains a mandatory 'id' attribute for
   correlating the challenge and the answer, and the 'min-tests'
   attribute which default value is 1.  With the 'min-tests' attribute,
   it is possible to define the minimum amount of tests which needs to
   be solved.

   The <challenge> element MUST have at least one child element.  This
   document defines the <media> element as a child element.  The
   <challenge> element may contain one or more <media> elements.

   The <challenge> element may also be extended by XML elements or
   attributes defined with other namespaces.





Tschofenig & Leppanen    Expires January 3, 2008                [Page 9]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


5.4.  The <media> element

   The <media> element contains one child element.  This document
   defines the <uri> and <data> elements as child elements for allowing
   the CAPTCHA challenge be provided directly as content or as a
   reference to an external content.

   The <media> element contains a mandatory 'var' attribute indicating
   the type of the challenge (see values from the 'var' column of
   Figure 3).  It may also contain optional 'width' and 'height'
   attributes for providing the size of the content.  In addition, the
   element may contain an 'instr' attribute which purpose is to provide
   instructions related to the challenge (see the 'example generic
   instruction' column from Figure 3).  The required tests can be
   indicated by setting the value of the 'required' attribute to 'true'.

   The <media> element may also be extended by XML elements or
   attributes defined with other namespaces.

5.5.  The <uri> element

   The <uri> element contains a mandatory 'type' attribute indicating
   the MIME type of the challenge.  See values from the 'MIME type'
   column of Figure 3.  The value of the <uri> element is a URL where
   the challenge can be fetched.

   The <uri> element may also be extended by XML attributes defined with
   other namespaces.

5.6.  The <data> element

   The <data> element contains a mandatory 'type' attribute indicating
   the MIME type of the challenge.  See typical values from the 'MIME
   type' column of Figure 3.

   The value of the <data> element is the content of the challenge.

   The <data> element may also be extended by XML attributes defined
   with other namespaces.

5.7.  Values

   The following table copied from [XEP-0158] presents typical values
   for the CAPTCHA challenge.  The 'var' column lists values for the
   'var' attribute of the <media> element.  The 'MIME type' column
   contains values of the corresponding 'type' attribute of the <uri> or
   <data> elements.




Tschofenig & Leppanen    Expires January 3, 2008               [Page 10]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


+---------------+-------------+-------+--------+------------------------+
| 'var'         | Name        | Media | MIME   | Example generic        |
|               |             | type  | type   | instructions           |
+---------------+-------------+-------+--------+------------------------+
| ocr*          | Optical Char| image | image/ | Enter the code         |
|               | Recognition |       | jpeg   | you see                |
+---------------+-------------+-------+--------+------------------------+
| picture_recog | Picture     | image | image/ | Describe               |
|               | Recognition |       | jpeg   | the picture            |
+---------------+-------------+-------+--------+------------------------+
| video_recog   | Video       | video | video/ | Describe               |
|               | Recognition |       | mpeg   | the video              |
+---------------+-------------+-------+--------+------------------------+
| speech_recog  | Speech      | audio | audio/ | Enter the              |
|               | Recognition |       | x-wav  | words you hear         |
+---------------+-------------+-------+--------+------------------------+
| audio_recog   | Audio       | audio | audio/ | Describe the           |
|               | Recognition |       | x-wav  | sound you hear         |
+---------------+-------------+-------+--------+------------------------+
| picture_q     | Picture     | image | image/ | Answer the             |
|               | Question    |       | jpeg   | question you see       |
+---------------+-------------+-------+--------+------------------------+
| video_q       | Video       | video | video/ | Answer the             |
|               | Question    |       | mpeg   | question in video      |
+---------------+-------------+-------+--------+------------------------+
| speech_q      | Speech      | audio | audio/ | Answer the             |
|               | Question    |       | x-wav  | question you hear      |
+---------------+-------------+-------+--------+------------------------+
| qa            | Text Q & A  | text  | text/  | Answer the question    |
|               |             |       | plain  |                        |
+---------------+-------------+-------+--------+------------------------+

* The image portrays random characters that humans can read but OCR
software cannot. To pass the challenge, the user must simply type the
characters. The correct answer SHOULD NOT depend on the language
specified by the 'xml:lang' attribute of the challenge.


                Figure 3: Information of CAPTCHA challenges


6.  Syntax

   The Captcha header field carries the solution information.  It has
   parameters called 'id' and 'answer'.  The 'id' parameter value is set
   to the same as the 'id' attribute of the CAPTCHA challenge sent to
   the UAC.  The 'answer' parameter value is set to the answer of the
   CAPTCHA challenge.



Tschofenig & Leppanen    Expires January 3, 2008               [Page 11]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


      OPEN ISSUE: the answer parameter to be thought more, e.g. whether
      there can be several answers?  And if possible, then also 'var'
      information should be provided.

   Example:

   Captcha: id="rjffe32"; answer="2";

   The ABNF for the header is:

   Captcha       = "Captcha" HCOLON captcha-parm *(COMMA captcha-param)
   captcha-param =  captcha-id SEMI captcha-answer *(SEMI generic-param)
   captcha-id   = "id" EQUAL quoted-string
   captcha-answer = "answer" EQUAL quoted-string

   This document updates the Table 2 of [RFC3261] by adding the
   following:


       Header field         where   proxy   ACK  BYE  CAN  INV  OPT  REG
       ------------         -----   -----   ---  ---  ---  ---  ---  ---
       Captcha               R       dr      o    o    -    o    o    o

                                            SUB  NOT  REF  INF  UPD  PRA
                                            ---  ---  ---  ---  ---  ---
                                             o    o    o    o    o    o


7.  Example

   The following XML document shows the content that is provided of a
   CAPTCHA the challenge message sent towards the sending party as shown
   in message (2) of Figure 2.


















Tschofenig & Leppanen    Expires January 3, 2008               [Page 12]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   <?xml version="1.0" encoding="UTF-8"?>
   <challenge xmlns="urn:ietf:params:xml:ns:captcha"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       id="73DE28A2">

       <media var="urn:ietf:params:xml:ns:captcha:ocr"
                   width="290" height="80">
           <uri type="image/jpeg">
                   http://www.example.com/challenges/ocr.jpeg?F3A6292C
           </uri>
       </media>

       <media var="urn:ietf:params:xml:ns:captcha:audio_recog">
           <uri type="audio/x-wav">
                   http://www.example.com/challenges/audio.wav?F3A6292C
           </uri>
       </media>

       <media var="urn:ietf:params:xml:ns:captcha:qa">
           <data type="text/plain">Type the color of a stop light</data>
       </media>

   </challenge>


8.  XML Schema

   This document defines the XML Schema based on the schema defined in
   Section 12 of [XEP-0158].

   OPEN ISSUE: should it be possible to define which tests are
   mandatory, and the minimun amount of tests to be executed?


   <?xml version='1.0' encoding='UTF-8'?>

   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
       targetNamespace="urn:ietf:params:xml:ns:captcha"
       xmlns="urn:ietf:params:xml:ns:captcha"
       elementFormDefault="qualified">

       <xs:element name="challenge" type="challengeType"/>

       <xs:complexType name="challengeType">
           <xs:complexContent>
               <xs:restriction base="xs:anyType">
                   <xs:sequence>
                       <xs:element ref="media"



Tschofenig & Leppanen    Expires January 3, 2008               [Page 13]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


                           minOccurs="1" maxOccurs="unbounded"/>
                       <xs:any namespace="##other"
                           minOccurs="0" processContents="lax"/>
                   </xs:sequence>
                   <xs:attribute name="id"
                       use="required" type="xs:string"/>
                   <xs:attribute name="min_tests" type="xs:unsignedInt"
                       default="1" use="optional" />

               </xs:restriction>
           </xs:complexContent>
       </xs:complexType>

       <xs:element name="media" type="mediaType"/>

       <xs:complexType name="mediaType">
           <xs:complexContent>
               <xs:restriction base="xs:anyType">
                   <xs:choice minOccurs="1" maxOccurs="1">
                       <xs:element ref="uri"
                           minOccurs="0" maxOccurs="unbounded"/>
                       <xs:element ref="data"
                           minOccurs="0" maxOccurs="unbounded"/>
                       <xs:any namespace="##other" minOccurs="0"
                           processContents="lax"/>
                   </xs:choice>
                   <xs:attribute name="var"
                       use="required" type="xs:anyURI"/>
                   <xs:attribute name="required" type="xs:boolean"
                       default="false" use="optional"/>
                   <xs:attribute name="height"
                       type="xs:string" use="optional"/>
                   <xs:attribute name="width"
                       type="xs:string" use="optional"/>
                   <xs:attribute name="instr"
                       type="xs:string" use="optional"/>
                   <xs:anyAttribute namespace="##any"
                       processContents="lax"/>
               </xs:restriction>
           </xs:complexContent>
       </xs:complexType>

       <xs:element name="uri">
           <xs:complexType>
               <xs:simpleContent>
                   <xs:extension base="xs:string">
                       <xs:attribute name="type" use="required"/>
                       <xs:anyAttribute namespace="##any"



Tschofenig & Leppanen    Expires January 3, 2008               [Page 14]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


                           processContents="lax"/>
                   </xs:extension>
               </xs:simpleContent>
           </xs:complexType>
       </xs:element>

       <xs:element name="data">
           <xs:complexType>
               <xs:simpleContent>
                   <xs:extension base="xs:string">
                       <xs:attribute name="type" use="required"/>
                       <xs:anyAttribute namespace="##any"
                           processContents="lax"/>
                   </xs:extension>
               </xs:simpleContent>
           </xs:complexType>
       </xs:element>

   </xs:schema>


9.  Internationalization Support

   [Editor's Note: A future version of this document will describe
   internationalization considerations.]


10.  Security Considerations

   [Editor's Note: A future version of this document will describe
   security considerations.]


11.  IANA Considerations

   This specification registers a new header and a new response code.
   IANA is requested to make the following updates in the registry at:
   http://www.iana.org/assignments/sip-parameters.  It also registers a
   new namespace and a content type.

   OPEN ISSUE: A registry for the 'var' tokens is needed.

11.1.  Captcha Header

   Add the following entry to the header sub-registry.

        Header Name        compact    Reference
        -----------------  -------    ---------



Tschofenig & Leppanen    Expires January 3, 2008               [Page 15]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


        Captcha                       [RFC-XXXX]

11.2.  4xx Response

   Add the following entry to the response code sub-registry under the
   "Request Failure 4xx" heading.

   4xx CAPTCHA required [RFC-XXXX]

11.3.  Namespace

   This section registers a new XML namespace per the procedures in
   [RFC3688].

   URI: urn:ietf:params:xml:ns:captcha

      Registrant Contact: IETF SIPPING Working Group, Hannes Tschofenig
         (hannes.tschofenig@nsn.com).

      XML:

      BEGIN
      <?xml version="1.0"?>
      <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
        "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
      <html xmlns="http://www.w3.org/1999/xhtml">
      <head>
        <meta http-equiv="content-type"
              content="text/html;charset=iso-8859-1"/>
        <title>Namespace for CAPTCHA Challenge</title>
      </head>
      <body>
        <h1>Namespace for providing CAPTCHA challenge</h1>
        <h2>urn:ietf:params:xml:ns:captcha</h2>
      <p>See <a href="[URL of published RFC]">RFCXXXX
          [NOTE TO IANA/RFC-EDITOR:
           Please replace XXXX with the RFC number of this
          specification.]</a>.</p>
      </body>
      </html>
      END

11.4.  Content-Type registration for 'application/captcha-challenge+xml'

   This specification requests the registration of a new MIME type
   according to the procedures of RFC 2048 [RFC2048] and guidelines in
   RFC 3023 [RFC3023].




Tschofenig & Leppanen    Expires January 3, 2008               [Page 16]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   MIME media type name: application

   MIME subtype name: captcha-challenge+xml
   Mandatory parameters: none
   Optional parameters: charset
     Indicates the character encoding of enclosed XML. Default is UTF-8.
   Encoding considerations:

      Uses XML, which can employ 8-bit characters, depending on the
      character encoding used. See RFC 3023 <xref target="RFC3023"/>,
      Section 3.2.

   Security considerations:

      This content type is designed to carry challenges for
      the user agent clients to solve in order to give a proof
      of being a human behind the generated request. This
      action is a part of a spam preventing mechanism.
      Appropriate precautions should be adopted to limit
      disclosure of this information. Please refer to RFCXXXX
      [NOTE TO IANA/RFC-EDITOR: Please replace XXXX
      with the RFC number of this specification.]
      Security Considerations section for more information.

   Interoperability considerations: none

   Published specification: RFCXXXX [NOTE TO IANA/RFC-EDITOR:
   Please replace XXXX with the RFC number of this specification.]
   this document

   Applications which use this media type: SIP applications

   Additional information:

      Magic Number: None
      File Extension: .xml
      Macintosh file type code: 'TEXT'

   Personal and email address for further information: Hannes
      Tschofenig, Hannes.Tschofenig@nsn.com
   Intended usage: LIMITED USE

   Author/Change controller:

      This specification is a work item of the IETF SIPPING working
      group, with mailing list address <xxxxx @ietf.org>.





Tschofenig & Leppanen    Expires January 3, 2008               [Page 17]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


11.5.  CAPTCHA Schema Registration

   URI:  urn:ietf:params:xml:schema:captcha
   Registrant Contact:  IETF SIPPING Working Group, Hannes Tschofenig
      (Hannes.Tschofenig@nsn.com).
   XML:  The XML schema to be registered is contained in Section 8.  Its
      first line is

   <?xml version="1.0" encoding="UTF-8"?>

      and its last line is

   </xs:schema>


12.  Acknowledgments

   Years ago CAPTCHAs have been introduced for XMPP, see 'XEP-0158:
   Robot Challenges' [XEP-0158].  The authors of this document believe
   that there is value in re-using it for SIP for Spam prevention.
   Hence, the authors would like to thank the XMPP community for their
   work on this subject.  In particular, all credits go to Ian Paterson
   (ian.paterson@clientside.co.uk), the author of [XEP-0158].


13.  References

13.1.  Normative references

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2141]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [RFC2648]  Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
              August 1999.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.



Tschofenig & Leppanen    Expires January 3, 2008               [Page 18]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [XML]      Bray, T., "Exensible Markup Language (XML) 1.0 (Second
              Edition)", W3C CR CR-xml11-20011006, October 2000.

13.2.  Informative references

   [I-D.ietf-sipping-app-interaction-framework]
              Rosenberg, J., "A Framework for Application Interaction in
              the Session Initiation Protocol  (SIP)",
              draft-ietf-sipping-app-interaction-framework-05 (work in
              progress), July 2005.

   [I-D.ietf-sipping-spam]
              Jennings, C. and J. Rosenberg, "The Session Initiation
              Protocol (SIP) and Spam", draft-ietf-sipping-spam-04 (work
              in progress), February 2007.

   [I-D.jennings-sip-hashcash]
              Jennings, C., "Computational Puzzles for SPAM Reduction in
              SIP", draft-jennings-sip-hashcash-05 (work in progress),
              June 2007.

   [I-D.tschofenig-sipping-framework-spit-reduction]
              Tschofenig, H., "A Framework for Reducing Spam for
              Internet Telephony",
              draft-tschofenig-sipping-framework-spit-reduction-00 (work
              in progress), June 2007.

   [Inaccessibility-of-CAPTCHA]
              May, M., "Inaccessibility of CAPTCHA; Alternatives to
              Visual Turing Tests on the Web",
              html http://www.w3.org/TR/turingtest/, November 2005.

   [RFC3265]  Roach, A., "SIP-Specific Event Notification", RFC 3265,
              June 2002.

   [XEP-0158]
              Paterson, I., "XEP-0158: Robot Challenges",
              html http://wiki.jabber.org/index.php/Robot Challenges
              (XEP-0158), October 2006.

   [captcha]  von Ahn, L., Blum, M., and J. Langford, "Telling Humans
              and Computers Apart Automatically",
              html http://www.captcha.net, February 2004.

   [hashcash]



Tschofenig & Leppanen    Expires January 3, 2008               [Page 19]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


              Back, A., "Hashcash - A Denial of Service Counter-
              Measure", html http://hashcash.org, August 2002.


Authors' Addresses

   Hannes Tschofenig
   Nokia Siemens Networks
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@nsn.com
   URI:   http://www.tschofenig.com


   Eva Leppanen
   Nokia Siemens Networks
   P.O BOX 785
   Tampere
   Finland

   Email: eva.leppanen@nsn.com




























Tschofenig & Leppanen    Expires January 3, 2008               [Page 20]

Internet-Draft   CAPTCHA based Robot Challenges for SIP        July 2007


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   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

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





Tschofenig & Leppanen    Expires January 3, 2008               [Page 21]