SIPPING WG                                                     J. Elwell
Internet-Draft                         Siemens Enterprise Communications
Updates:  RFC 3325                                          GmbH & Co KG
(if approved)                                              April 4,                                               May 16, 2008
Intended status:  Informational
Expires:  October 6,  November 17, 2008

 Updates to Asserted Identity in the Session Initiation Protocol (SIP)
                  draft-ietf-sipping-update-pai-01.txt
                  draft-ietf-sipping-update-pai-02.txt

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Abstract

   SIP has a mechanism for conveying the asserted identity of the
   originator of a request by means of the P-Asserted-Identity header
   field.  This header field is specified for use in requests using a
   number of SIP methods, in particular the INVITE method.  However, RFC
   3325 does not specify the insertion of this header field by a trusted
   UAC, does not specify the use of this header field with the SIP
   UPDATE, REGISTER, MESSAGE or PUBLISH methods, and is unclear on the
   use of this header field in responses.  This document extends RFC
   3325 to cover these situations.

   This work is being discussed on the sipping@ietf.org mailing list.

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  Inclusion of P-Asserted-Identity by a UAC  . . . . . . . .  3  4
     3.2.  Inclusion of P-Asserted-Identity in an UPDATE request  . .  4
     3.3.  Inclusion of P-Asserted-Identity in a REGISTER request . .  5
     3.4.  Inclusion of P-Asserted-Identity or
           P-Preferred-Identity in a MESSAGE request  . . . . . . . .  5
     3.5.  Inclusion of P-Asserted-Identity or
           P-Preferred-Identity in a PUBLISH request  . . . . . . . .  5  6
     3.6.  Inclusion of P-Asserted-Identity in an ACK request . . . .  6
     3.7.  Inclusion of P-Asserted-Identity or
           P-Preferred-Identity in a response . . . . . . . . . . . .  5  6
   4.  Behaviour  . . . . . . . . . . . . . . . . . . . . . . . . . .  7  8
     4.1.  UAC Behaviour  . . . . . . . . . . . . . . . . . . . . . .  7  8
       4.1.1.  Request handling . . . . . . . . . . . . . . . . . . .  7  8
       4.1.2.  Response handling  . . . . . . . . . . . . . . . . . .  7  9
     4.2.  Proxy Behaviour  . . . . . . . . . . . . . . . . . . . . .  8  9
       4.2.1.  Request handling . . . . . . . . . . . . . . . . . . .  8  9
       4.2.2.  Response handling  . . . . . . . . . . . . . . . . . .  8 10
     4.3.  Registrar Behaviour  . . . . . . . . . . . . . . . . . . .  9 10
     4.4.  UAS Behaviour  . . . . . . . . . . . . . . . . . . . . . .  9 10
       4.4.1.  Request handling . . . . . . . . . . . . . . . . . . .  9 10
       4.4.2.  Response handling  . . . . . . . . . . . . . . . . . .  9 10
   5.  IANA considerations  . . . . . . . . . . . . . . . . . . . . .  9 11
   6.  Security considerations  . . . . . . . . . . . . . . . . . . .  9 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 11
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . 10 . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 12 14

1.  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].

   This document uses the concepts of Trust Domain and Spec(T), as
   specified in section 2.3 of RFC 3324 [RFC3324].

2.  Introduction

   SIP [RFC3261] has

   The Session Initiation Protocol (SIP) is specified in RFC 3261
   [RFC3261].  RFC 3325 [RFC3325] specifies a mechanism for conveying
   within a Trust Domain the asserted identity of the originator of a request
   SIP request.  This is achieved by means of the P-Asserted-Identity
   header field [RFC3325].  This header field field, which is specified for use in requests using a number
   of SIP methods, in particular the INVITE method.  However,

   RFC 3325 does not specify the insertion of this the P-Asserted-Identity
   header field by a UAC in the same Trust Domain as the first proxy.
   Also RFC 3325 does not specify the use of the P-Asserted-Identity
   header field with the SIP UPDATE method [RFC3311], the SIP REGISTER
   method, the SIP MESSAGE method [RFC3428] or [RFC3428], the SIP PUBLISH method
   [RFC3903],
   [RFC3903] or the SIP ACK method, and is unclear on the use of this
   header field in responses.  There are similar omissions concerning
   the P-Preferred-
   Identity P-Preferred-Identity header field.

   This document extends RFC 3325 by allowing inclusion of the
   P-Asserted-Identity header field by a UAC in the same Trust Domain as
   the first proxy, allowing use of this header field in UPDATE,
   REGISTER, MESSAGE and MESSAGE, PUBLISH and ACK requests and, under certain
   conditions, allowing use of this header field in SIP responses.  It  This
   document also allows the use of the P-Preferred-Identity header field
   in some of these situations.

   This document does not alter the fact that the asserted identity
   mechanism has limited applicability, i.e., within a Trust Domain.
   For general applicability, including operation outside a Trust Domain
   (e.g., over the public Internet) or between different Trust Domains,
   a different mechanism is needed.  RFC 4474 [RFC4474] specifies the
   Identity header field, in conjunction with the From header field, for
   providing authenticated identity in such circumstances.

3.  Discussion

3.1.  Inclusion of P-Asserted-Identity by a UAC

   RFC 3325 does not include procedures for a UAC to include the
   P-Asserted-Identity header field in a request.  This can be
   meaningful if the UAC is in the same Trust Domain as the first proxy.
   downstream SIP entity.  Examples of types of UAC that are often
   suitable for inclusion in a Trust Domain are:

   o  PSTN gateways;

   o  media servers;

   o  application servers (or B2BUAs) that act as URI list servers
      [I-D.ietf-sipping-uri-services];

   o  application servers (or B2BUAs) that perform third party call
      control.

   In the particular case of a PSTN gateway, the PSTN gateway might be
   able to assert an identity received from the PSTN, the proxy itself
   having no means to authenticate such an identity.  Likewise, in the
   case of certain application server or B2BUA arrangements, the
   application server or B2BUA may be in a position to assert an
   identity of a user on the other side of that application server or
   B2BUA.

   In accordance with RFC 3325, nodes within a Trust Domain must be
   connected using TLS behave
   in accordance with a certain cipher suite, Spec(T), and this principle needs to apply to the connection
   between a UAC and its proxy as part of the condition for considering
   the UAC to be within the same Trust Domain.  Normal proxy procedures
   of RFC 3325 ensure that the header field is removed or replaced if
   the first proxy considers the UAC to be outside the Trust Domain.

   This update to RFC 3325 clarifies that a UAC may include a
   P-Asserted-Identity header field in a request in certain
   circumstances.

3.2.  Inclusion of P-Asserted-Identity in an UPDATE request

   There are several use cases that would benefit from the use of the
   P-Asserted-Identity header field in an UPDATE request.  These use
   cases apply within a Trust Domain where the use of asserted identity
   is appropriate (see RFC 3325).

   In one example, an established call passes through a gateway to the
   PSTN.  The gateway becomes aware that the remote party in the PSTN
   has changed, e.g., due to call transfer.  By including the
   P-Asserted-Identity header field in an UPDATE request, the gateway
   can convey the identity of the new remote party to the peer SIP UA.

      Note that the (re-)INVITE method could be used in this situation.
      However, this forces an offer-answer exchange, which typically is
      not required in this situation.  Also it involves 3 messages
      rather than 2.

   In another example, a B2BUA that provides third party call control
   (3PCC) [RFC3725] wishes to join two calls together, one of which is
   still waiting to be answered and potentially is forked to different
   UAs.  At this point in time it is not possible to trigger the normal offer-
   answer
   offer-answer exchange between the two joined parties, because of the
   mismatch between a single dialog on the one side and potentially
   multiple early dialogs on the other side, so this action must wait
   until one of the called UAs answers.  However, it would be useful to
   give an early indication to each user concerned of the identity of
   the user to which they will become connected when the call is
   answered.  This can be achieved by  In other words, it would provide the B2BUA new calling UA with
   the identity of the new called user and provide the new called UA(s)
   with the identity of the new calling user.  This can be achieved by
   the B2BUA sending an UPDATE request with a P-Asserted-Identity header
   field on the dialogs concerned.

   This update to RFC 3325 allows a P-Asserted-Identity header field to
   be included in an UPDATE request.

3.3.  Inclusion of P-Asserted-Identity in a REGISTER request

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in a REGISTER request between an edge proxy
   that has authenticated the source of the request and the registrar.

   This update to RFC 3325 allows a P-Asserted-Identity header field to
   be included in a REGISTER request.

3.4.  Inclusion of P-Asserted-Identity or P-Preferred-Identity in a
      MESSAGE request

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in a MESSAGE request to assert the source of a
   page mode instant message.  This would complement its use in an
   INVITE request to assert the source of an instant message session or
   any other form of session.  Similarly, between a UAC and first proxy
   that are not within the same Trust Domain, a P-Preferred-Identity
   header field could be used in a MESSAGE request to express a
   preference when the user has several identities.

   This update to RFC 3325 allows a P-Asserted-Identity or P-Preferred-
   Identity header field to be included in a MESSAGE request.

3.5.  Inclusion of P-Asserted-Identity or P-Preferred-Identity in a
      PUBLISH request

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in a PUBLISH request to assert the source of
   published state information.  This would complement its use in
   SUBSCRIBE and NOTIFY requests.  Similarly, between a UAC and first
   proxy that are not within the same Trust Domain, a P-Preferred-
   Identity header field could be used in a PUBLISH request to express a
   preference when the user has several identities.

   This update to RFC 3325 allows a P-Asserted-Identity or P-Preferred-
   Identity header field to be included in a PUBLISH request.

3.6.  Inclusion of P-Asserted-Identity in an ACK request

   Within a Trust Domain, a P-Asserted-Identity header field could
   advantageously be used in an ACK request.  Considering the 3PCC
   scenario in Flow I of [RFC3725], the asserted identity of user B may
   not be known when the B2BUA (controller) sends the initial INVITE
   request to UA A, but might be known when the B2BUA sends the ACK
   request to UA A (having received it in the 200 response from UA B).

   This update to RFC 3325 allows a P-Asserted-Identity header field to
   be included in an ACK request.

3.7.  Inclusion of P-Asserted-Identity or P-Preferred-Identity in a
      response

   There are cases where the inclusion of the P-Asserted-Identity header
   field in responses would be useful.  Retargeting of a request can
   result in the responding entity having a different identity from that
   placed in the To URI of the request.  Inclusion of asserted identity
   in a response would provide the UAC with the identity of the sender.
   responder.  Some examples of the benefits to be gained include:

   o  Asserted identity in a 2xx response to an INVITE request would
      indicate the identity of the connected user.

   o  Asserted identity in a provisional response to an INVITE request
      would indicate the contacted (e.g., alerted) user.

   o  Asserted identity in a 2xx response to a MESSAGE request would
      give
      provide confirmation of where the message was delivered to.

   o  Asserted identity in certain 4xx/5xx/6xx responses would provide
      an indication of where the response originated.

   In the case of a request that results in the formation of a dialog, a
   mid-dialog request (e.g., UPDATE) in the reverse direction can
   provide the identity of the user at the destination end of that
   dialog, and therefore the need to include asserted identity in a
   response to the dialog-forming request to identify the connected user
   is debatable.  There can be some benefits in terms of ease of
   interworking with PSTN, where such information is placed in the
   response to a call establishment request.  For other responses,
   including successful responses to requests such as MESSAGE and
   PUBLISH and unsuccessful responses, the use of a request in the
   reverse direction is unsuitable.

      Note that when the authenticated identity of the connected user is
      to be provided using the From and Identity header fields (as
      opposed to providing asserted identity using the P-Asserted-
      Identity header field), RFC 4916 [RFC4916] requires this to be
      done in a mid-dialog request (e.g., UPDATE) in the reverse
      direction.  This is because the Identity header field is defined
      only for use in requests.

   RFC 3325 is ambiguous on inclusion of P-Asserted-Identity in a
   response.  For example, section 4 of RFC 3325 talks about inclusion
   of the header field in messages, as opposed to requests.  Moreover
   section 5 explicitly mentions "message (request or response)".
   However, there are other places (e.g., sections 6, 7 and 8) that talk only about
   mention requests.

   Section 5 of RFC 3325 requires a proxy to authenticate the originator
   of a message before adding a P-Asserted-Identity header field to the
   forwarded message.  In practice there is no SIP means to authenticate
   the sender of a SIP response message.  However, authentication may be
   possible by other means.  For example, if the proxy has TLS
   connectivity with the originator of the response and has previously
   authenticated the connected entity (e.g., using SIP digest
   authentication at registration time), then the originator of the
   response can be considered to be authenticated.  In such
   circumstances it is permissible for a proxy to insert a P-Asserted-
   Identity header field in a SIP response.

   It should also be permissible for a UAS to insert a P-Asserted-
   Identity header field into a response if it is within the same Trust
   Domain as the proxy SIP entity from which the request was received (the last
   proxy). received.

   Between a UAS and last proxy a SIP entity that are not within the same Trust
   Domain, a P-Preferred-Identity header field could be used in a
   response, in order to express a preference when the authenticated
   user has several identities.

   This update to RFC 3325 allows a P-Asserted-Identity or P-Preferred-
   Identity header field to be included in a response in certain
   circumstances.

4.  Behaviour

   This document updates RFC 3325 by allowing a P-Asserted-Identity
   header field to be included by a UAC within the same Trust Domain, by
   allowing a P-Asserted-Identity header field to appear in an UPDATE,
   MESSAGE or PUBLISH request, and by allowing a P-Asserted-Identity
   header field to appear in a response in certain circumstances.  It
   also allows a P-Preferred-Identity header field to appear in a
   MESSAGE or PUBLISH request or in a response.

4.1.  UAC Behaviour

4.1.1.  Request handling

   A UAC MAY include a P-Asserted-Identity header field in a request to
   report the identity of the user on behalf of which the UAC is acting
   and whose identity the UAC is in a position to assert.  A UAC SHOULD
   do so only in cases where it believes it is in the same Trust Domain
   as the first proxy SIP entity to which it sends the request and is connected to the first proxy
   that SIP entity in accordance with the security requirements of RFC
   3325.  A UAC SHOULD NOT do so in other circumstances and might
   instead use the P-Preferred-Identity header field.  A UAC MUST NOT
   include both header fields.

   A

   In addition to the methods specified in RFC 3325, a UAC MAY include a
   P-Asserted-Identity header field in an UPDATE request to report a
   changed identity mid-dialog.  This can be an UPDATE request sent
   specially for this purpose or an UPDATE request sent for some other
   purpose.  A UAC SHOULD do so only in cases where it believes it is in
   the same Trust Domain as the first proxy SIP entity to which it sends the request
   and is connected to the first proxy that SIP entity in accordance with the security
   requirements of RFC 3325.

   A

   In addition to the methods specified in RFC 3325, a UAC MAY include a
   P-Asserted-Identity or P-Preferred-Identity header field in a MESSAGE or
   MESSAGE, PUBLISH or ACK request.  A UAC SHOULD include a
   P-Asserted-Identity P-Asserted-
   Identity header field only in cases where it believes it is in the
   same Trust Domain as the first proxy SIP entity to which it sends the request and
   is connected to
   the first proxy that SIP entity in accordance with the security
   requirements of RFC 3325.

4.1.2.  Response handling

   Typically a UA renders the value of a P-Asserted-Identity header
   field that it receives in a response to its user.  It may consider
   the identity provided by a Trust Domain to be privileged, or
   intrinsically more trustworthy than other information in the
   response.  However, any particular behaviour is specific to
   implementations or services.  This document also does not mandate any
   UA handling for multiple P-Asserted-Identity header field values that
   happen to appear in a response (such as a SIP URI alongside a tel
   URL).

   However, if a UAC receives a response from a previous element outside
   the Trust Domain, it does
   not trust, it MUST NOT use the P-Asserted-Identity header field in
   any way.

   If a UA is part of the Trust Domain from which it received a response
   containing a P-Asserted-Identity header field, then it can use the
   value freely internally but it MUST ensure that it does not forward the
   information to any element that is not part of the Trust Domain if
   the responding user has requested that asserted identity information
   be kept private.

4.2.  Proxy Behaviour

4.2.1.  Request handling

   If a proxy receives a request from a UAC within the Trust Domain it
   MUST behave as for a request from any other node within the Trust
   Domain, in accordance with the rules of RFC 3325 for a proxy.

      Note that this implies that the proxy must have authenticated the
      sender of the request in accordance with the Spec(T) in force for
      the Trust Domain and determined that the sender is indeed part of
      the Trust Domain.

   If a proxy receives an UPDATE, REGISTER, MESSAGE or MESSAGE, PUBLISH or ACK
   request containing a P-Asserted-Identity header field, it MUST behave
   as for any other request in accordance with the rules of RFC 3325 for
   a proxy.

   If a proxy receives a MESSAGE or PUBLISH request containing a
   P-Preferred-Identity header field, it MUST behave as for any other
   request in accordance with the rules of RFC 3325 for a proxy.

4.2.2.  Response handling

   The proxy behaviour specified in RFC 3325 is applicable to responses
   with the following qualifications.  A proxy that receives a response
   from a node outside the Trust Domain cannot directly authenticate the
   UAS by SIP means.  Therefore it MUST NOT include a P-Asserted-
   Identity header field when forwarding the response unless it has
   authenticated the UAS by other means.  If a proxy receives a response
   from a UAS within the Trust Domain it MUST behave as for a response
   from any other node within the Trust Domain, in accordance with the
   rules of RFC 3325 for a proxy.

      One possible circumstance in which a proxy can include a
      P-Asserted-Identity header field when forwarding a response from a
      node outside the Trust Domain is when the proxy has direct TLS
      connectivity with the UAS and has authenticated the UA by some
      other means (e.g., SIP digest authentication) during that same TLS
      session.

   The proxy behaviour specified in RFC 3325 for handling a received
   P-Preferred-Identity header field is applicable also to responses,
   subject to the qualification above concerning authentication of the
   UAS as a pre-requisite for inserting a P-Asserted-Identity header
   field.

4.3.  Registrar Behaviour

   If a registrar receives a REGISTER request containing a P-Asserted-
   Identity header field, it MUST disregard the asserted identity unless
   received over a secure transport from a proxy within the Trust
   Domain.  Otherwise it MAY use this as evidence that the registering
   UA has been authenticated as representing the identity asserted in
   the header field.

4.4.  UAS Behaviour

4.4.1.  Request handling

   If a UAS receives an UPDATE, MESSAGE or MESSAGE, PUBLISH or ACK request
   containing a P-Asserted-Identity header field, it MUST behave as for
   any other request in accordance with the rules of RFC 3325 for a UAS.

4.4.2.  Response handling

   A UAS MAY include a P-Asserted-Identity or P-Preferred-Identity
   header field in a response to report the identity of the user on
   behalf of which the UAS is acting and whose identity the UAS is in a
   position to assert.  A UAS SHOULD include a P-Asserted-Identity
   header field only in cases where it believes it is in the same Trust
   Domain as the last proxy SIP entity from which it received the request and is
   connected to the last proxy that SIP entity in accordance with the security
   requirements of RFC 3325.

5.  IANA considerations

   None

   This document requires no IANA actions.

6.  Security considerations

   The use of asserted identity raises a number of security
   considerations, which are discussed fully in [RFC3325].  This
   document raises the following additional security considerations.

   When receiving a request or response containing a P-Asserted-Identity
   header field directly from a UA (rather than from another proxy), field, a proxy will trust the UA assertion only if it the source is
   known to be within the Trust Domain and is connected by means of TLS as specified behaves in RFC 3325. accordance with a
   Spec(T), which defines the security requirements.  This applies
   regardless of the nature of the resource (UA or proxy).  One example
   where this a trusted source might be true is a UA that is a PSTN gateway.  In this case
   the UA can assert an identity received from the PSTN, the proxy
   itself having no means to authenticate such an identity.  A
   proxy SIP
   entity must not trust an identity asserted by a UA source outside the
   Trust Domain.  Typically a UA under the control of an individual user
   (such as a desk phone or mobile phone) should not be considered part
   of a Trust Domain.

   When receiving a response from a node outside the Trust Domain, a
   proxy has no direct SIP means to authenticate the node.  However, if
   authentication has taken place by other means (e.g., an earlier use
   of SIP digest authentication) and the entity sending the response is
   known to be the same entity (e.g., connected via the same TLS
   session) this can be sufficient grounds for asserting an identity.
   In other circumstances a proxy must not assert identity for a
   responding user.

   When receiving a REGISTER request containing a P-Asserted-Identity
   header field, a proxy will trust the asserted identity only if
   received over a secure connection from a proxy within the Trust
   Domain.

7.  Acknowledgements

   Useful comments were received from Francois Audet, Jeroen van Bemmel,
   Hans Erik van Elburg, Vijay Gurbani, Cullen Jennings, Hadriel Kaplan,
   Paul Kyzivat and Kyzivat, Jonathan Rosenberg and Thomas Stach during drafting. drafting and
   review.

8.  References

8.1.  Normative References

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

   [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.

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, October 2002.

   [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
              Identity", RFC 3324, November 2002.

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              November 2002.

   [RFC3428]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", RFC 3428, December 2002.

   [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.

   [I-D.ietf-sipping-uri-services]
              Camarillo, G. and A. Roach, "Framework and Security
              Considerations for Session Initiation Protocol (SIP)
              Uniform Resource Identifier (URI)-List Services",
              draft-ietf-sipping-uri-services-07 (work in progress),
              November 2007.

8.2.  Informative References

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, April 2004.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

Author's Address

   John Elwell
   Siemens Enterprise Communications GmbH & Co KG
   Hofmannstrasse 51
   D-81379 Munich
   Germany

   Phone:  +44 115 943 4989
   Email:  john.elwell@siemens.com

Full Copyright Statement

   Copyright (C) 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
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