Mobility with Traversal Using Relays around NAT (TURN)
RFC 8016

Internet Engineering Task Force (IETF)                          T. Reddy
Request for Comments: 8016                                         Cisco
Category: Standards Track                                        D. Wing
ISSN: 2070-1721
                                                                P. Patil
                                                            P. Martinsen
                                                                   Cisco
                                                           November 2016

         Mobility with Traversal Using Relays around NAT (TURN)

Abstract

   It is desirable to minimize traffic disruption caused by changing IP
   address during a mobility event.  One mechanism to minimize
   disruption is to expose a shorter network path to the mobility event
   so that only the local network elements are aware of the changed IP
   address and the remote peer is unaware of the changed IP address.

   This document provides such an IP address mobility solution using
   Traversal Using Relays around NAT (TURN).  This is achieved by
   allowing a client to retain an allocation on the TURN server when the
   IP address of the client changes.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8016.

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
   3.  Mobility Using TURN . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Creating an Allocation  . . . . . . . . . . . . . . . . .   5
       3.1.1.  Sending an Allocate Request . . . . . . . . . . . . .   5
       3.1.2.  Receiving an Allocate Request . . . . . . . . . . . .   6
       3.1.3.  Receiving an Allocate Success Response  . . . . . . .   6
       3.1.4.  Receiving an Allocate Error Response  . . . . . . . .   7
     3.2.  Refreshing an Allocation  . . . . . . . . . . . . . . . .   7
       3.2.1.  Sending a Refresh Request . . . . . . . . . . . . . .   7
       3.2.2.  Receiving a Refresh Request . . . . . . . . . . . . .   7
       3.2.3.  Receiving a Refresh Response  . . . . . . . . . . . .   9
     3.3.  New STUN Attribute MOBILITY-TICKET  . . . . . . . . . . .   9
     3.4.  New STUN Error Response Code  . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Example of Ticket Construction . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   When moving between networks, the endpoint's IP address can change
   or, due to NAT, the endpoint's public IP address can change.  Such a
   change of IP address breaks upper-layer protocols such as TCP and
   RTP.  Various techniques exist to prevent this breakage, all tied to
   making the endpoint's IP address static (e.g., Mobile IP, Proxy
   Mobile IP, Locator/ID Separation Protocol (LISP)).  Other techniques
   exist, which make the change in IP address agnostic to the upper-
   layer protocol (e.g., Stream Control Transmission Protocol (SCTP)).
   The mechanism described in this document is in that last category.

   A server using Traversal Using Relays around NAT (TURN) [RFC5766]
   relays media packets and is used for a variety of purposes, including
   overcoming NAT and firewall traversal issues.  The existing TURN
   specification does not permit a TURN client to reuse an allocation
   across client IP address changes.  Due to this, when the IP address
   of the client changes, the TURN client has to request a new
   allocation, create permissions for the remote peer, create channels,
   etc.  In addition, the client has to re-establish communication with
   its signaling server and send an updated offer to the remote peer
   conveying the newly relayed candidate address.  Then, the remote side
   has to re-gather all candidates and signal them to the client, and
   the endpoints have to perform Interactive Connectivity Establishment
   (ICE) [RFC5245] checks.  If the ICE continuous nomination procedure
   [NOMBIS] is used, then the newly relayed candidate address would have
   to be "trickled" (i.e., incrementally provisioned as described in
   [TRICKLE-SIP]), and ICE checks would have to be performed according
   to [TRICKLE-ICE] by the endpoints to nominate pairs for selection by
   ICE.

   This specification describes a mechanism to seamlessly reuse
   allocations across client IP address changes without any of the
   hassles described above.  A critical benefit of this technique is
   that the remote peer does not have to support mobility or deal with
   any of the address changes.  The client, which is subject to IP
   address changes, does all the work.  The mobility technique works
   across and between network types (e.g., between 3G and wired Internet
   access), so long as the client can still access the TURN server.  The
   technique should also work seamlessly when (D)TLS is used as a
   transport protocol for Session Traversal Utilities for NAT (STUN)
   [RFC5389].  When there is a change in IP address, the client uses
   (D)TLS Session Resumption without Server-Side State as described in
   [RFC5077] to resume secure communication with the TURN server, using
   the changed client IP address.

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2.  Notational Conventions

   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 terminology defined in [RFC5245] and the following
   additional terminology:

   Break Before Make: The old communication path is broken ("break")
   before new communication can be created ("make").  Such changes
   typically occur because a network's physical cable is disconnected,
   radio transmission is turned off, or a client moves out of radio
   range.

   Make Before Break: A new communication path is created ("make")
   before the old communication path is broken ("break").  Such changes
   typically occur because a network is reconnected with a physical
   cable, radio transmission is turned on, or a client moves into radio
   range.

3.  Mobility Using TURN

   To achieve mobility, a TURN client should be able to retain an
   allocation on the TURN server across changes in the client IP address
   as a consequence of movement to other networks.

   When the client sends the initial Allocate request to the TURN
   server, it will include a new STUN attribute MOBILITY-TICKET (with
   zero length value), which indicates that the client is capable of
   mobility and desires a ticket.  The TURN server provisions a ticket
   that is sent inside the new STUN attribute MOBILITY-TICKET in the
   Allocate success response to the client.  The ticket will be used by
   the client when it wants to refresh the allocation but with a new
   client IP address and port.  This ensures that an allocation can only
   be refreshed by the same client that allocated the relayed transport
   address.  When a client's IP address changes due to mobility, it
   presents the previously obtained ticket in a Refresh request to the
   TURN server.  If the ticket is found to be valid, the TURN server
   will retain the same relayed address/port for the new IP address/port
   allowing the client to continue using previous channel bindings --
   thus, the TURN client does not need to obtain new channel bindings.
   Any data from the external peer will be delivered by the TURN server
   to this new IP address/port of the client.  The TURN client will
   continue to send application data to its peers using the previously
   allocated channelBind Requests.

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          TURN                                 TURN           Peer
          client                               server          A
            |-- Allocate request --------------->|             |
            |   + MOBILITY-TICKET (length=0)     |             |
            |                                    |             |
            |<--------------- Allocate failure --|             |
            |                 (401 Unauthorized) |             |
            |                                    |             |
            |-- Allocate request --------------->|             |
            |   + MOBILITY-TICKET (length=0)     |             |
            |                                    |             |
            |<---------- Allocate success resp --|             |
            |            + MOBILITY-TICKET       |             |
           ...                                  ...           ...
        (changes IP address)
            |                                    |             |
            |-- Refresh request ---------------->|             |
            |   + MOBILITY-TICKET                |             |
            |                                    |             |
            |<----------- Refresh success resp --|             |
            |   + MOBILITY-TICKET                |             |
            |                                    |             |

                       Figure 1: Mobility Using TURN

   In Figure 1, the client sends an Allocate request with a MOBILITY-
   TICKET attribute to the server without credentials.  Since the server
   requires that all requests be authenticated using STUN's long-term
   credential mechanism, the server rejects the request with a 401
   (Unauthorized) error code.  The client then tries again, this time
   including credentials (not shown).  This time, the server accepts the
   Allocate request and returns an Allocate success response and a
   ticket inside the MOBILITY-TICKET attribute.  Sometime later, the
   client IP address changes, and the client decides to refresh the
   allocation, and thus sends a Refresh request to the server with a
   MOBILITY-TICKET attribute containing the ticket it received from the
   server.  The refresh is accepted, and the server replies with a
   Refresh success response and a new ticket inside the MOBILITY-TICKET
   attribute.

3.1.  Creating an Allocation

3.1.1.  Sending an Allocate Request

   In addition to the process described in Section 6.1 of [RFC5766], the
   client includes the MOBILITY-TICKET attribute with a length of zero.
   This indicates that the client is a mobile node and wants a ticket.

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3.1.2.  Receiving an Allocate Request

   In addition to the process described in Section 6.2 of [RFC5766], the
   server does the following:

   If the MOBILITY-TICKET attribute is included, and has a length of
   zero, but TURN session mobility is forbidden by local policy, the
   server will reject the request with the new error code 405 (Mobility
   Forbidden).  If the MOBILITY-TICKET attribute is included and has a
   non-zero length, then the server will generate an error response with
   an error code of 400 (Bad Request).  Following the rules specified in
   [RFC5389], if the server does not understand the MOBILITY-TICKET
   attribute, it ignores the attribute.

   If the server can successfully process the request and create an
   allocation, the server replies with a success response that includes
   a STUN MOBILITY-TICKET attribute.  The TURN server can store system-
   internal data in the ticket that is encrypted by a key known only to
   the TURN server and sends the ticket in the STUN MOBILITY-TICKET
   attribute as part of the Allocate success response.  An example of
   ticket construction is discussed in Appendix A.  The ticket is opaque
   to the client, so the structure is not subject to interoperability
   concerns, and implementations may diverge from this format.  The
   client could be roaming across networks with a different path MTU and
   from one address family to another (e.g., IPv6 to IPv4).  The TURN
   server to support mobility must assume that the path MTU is unknown
   and use a ticket length in accordance with the published guidance on
   STUN UDP fragmentation (Section 7.1 of [RFC5389]).

   Note: There is no guarantee that the fields in the ticket are going
   to be decodable to a client, and therefore attempts by a client to
   examine the ticket are unlikely to be useful.

3.1.3.  Receiving an Allocate Success Response

   In addition to the process described in Section 6.3 of [RFC5766], the
   client will store the MOBILITY-TICKET attribute, if present, from the
   response.  This attribute will be presented by the client to the
   server during a subsequent Refresh request to aid mobility.

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3.1.4.  Receiving an Allocate Error Response

   If the client receives an Allocate error response with error code 405
   (Mobility Forbidden), the error is processed as follows:

      405 (Mobility Forbidden): The request is valid, but the server is
      refusing to perform it, likely due to administrative restrictions.
      The client considers the current transaction as having failed.

      The client can notify the user or operator.  The client SHOULD NOT
      retry sending the Allocate request containing the MOBILITY-TICKET
      with this server until it believes the problem has been fixed.

   All other error responses must be handled as described in [RFC5766].

3.2.  Refreshing an Allocation

3.2.1.  Sending a Refresh Request

   If a client wants to refresh an existing allocation and update its
   time-to-expiry or delete an existing allocation, it sends a Refresh
   request as described in Section 7.1 of [RFC5766].  If the client's IP
   address or source port has changed and the client wants to retain the
   existing allocation, the client includes the MOBILITY-TICKET
   attribute received in the Allocate success response in the Refresh
   request.  If there has been no IP address or source port number
   change, the client MUST NOT include a MOBILITY-TICKET attribute, as
   this would be rejected by the server and the client would need to
   retransmit the Refresh request without the MOBILITY-TICKET attribute.

3.2.2.  Receiving a Refresh Request

   In addition to the process described in Section 7.2 of [RFC5766], the
   server does the following:

   If the STUN MOBILITY-TICKET attribute is included in the Refresh
   request, and the server configuration changed to forbid mobility or
   the server transparently fails over to another server instance that
   forbids mobility, then the server rejects the Refresh request with a
   405 (Mobility Forbidden) error and the client starts afresh with a
   new allocation.

   If the STUN MOBILITY-TICKET attribute is included in the Refresh
   request, then the server will not retrieve the 5-tuple from the
   packet to identify an associated allocation.  Instead, the TURN
   server will decrypt the received ticket, verify the ticket's
   validity, and retrieve the 5-tuple allocation using the ticket.  If
   this 5-tuple obtained does not identify an existing allocation, then

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   the server MUST reject the request with a 437 (Allocation Mismatch)
   error.  If the ticket is invalid, then the server MUST reject the
   request with a 400 (Bad Request) error.

   If the source IP address and port of the Refresh request with the
   STUN MOBILITY-TICKET attribute is the same as the stored 5-tuple
   allocation, then the TURN server rejects the request with a 400 (Bad
   Request) error.  If the source IP address and port of the Refresh
   request is different from the stored 5-tuple allocation, the TURN
   server proceeds with a MESSAGE-INTEGRITY validation to identify that
   it is the same user that had previously created the TURN allocation.
   If the above check is not successful, then the server MUST reject the
   request with a 441 (Wrong Credentials) error.

   If all of the above checks pass, the TURN server understands that the
   client either has moved to a new network and acquired a new IP
   address (Break Before Make) or is in the process of switching to a
   new interface (Make Before Break).  The source IP address of the
   request could be either the host transport address or the server-
   reflexive transport address.  The server then updates its state data
   with the new client IP address and port but does not discard the old
   5-tuple from its state data.  The TURN server calculates the ticket
   with the new 5-tuple and sends the new ticket in the STUN MOBILITY-
   TICKET attribute as part of Refresh success response.  The new ticket
   sent in the refresh response MUST be different from the old ticket.

   The TURN server MUST continue receiving and processing data on the
   old 5-tuple and MUST continue transmitting data on the old-5 tuple
   until it receives a Send Indication or ChannelData message from the
   client on the new 5-tuple or a message from the client to close the
   old connection (e.g., a TLS fatal alert or TCP RST).  After receiving
   any of those messages, a TURN server discards the old ticket and the
   old 5-tuple associated with the old ticket from its state data.  Data
   sent by the client to the peer is accepted on the new 5-tuple and
   data received from the peer is forwarded to the new 5-tuple.  If the
   refresh request containing the MOBILITY-TICKET attribute does not
   succeed (e.g., the packet is lost if the request is sent over UDP, or
   the server is unable to fulfill the request), then the client can
   continue to exchange data on the old 5-tuple until it receives the
   Refresh success response.

   The old ticket can only be used for the purposes of retransmission.
   If the client wants to refresh its allocation with a new server-
   reflexive transport address, it MUST use the new ticket.  If the TURN
   server has not received a Refresh request with the STUN MOBILITY-
   TICKET attribute but receives Send indications or ChannelData
   messages from a client, the TURN server MAY discard or queue those
   Send indications or ChannelData messages (at its discretion).  Thus,

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   it is RECOMMENDED that the client avoid transmitting a Send
   indication or ChannelData message until it has received an
   acknowledgement for the Refresh request with the STUN MOBILITY-TICKET
   attribute.

   To accommodate the potential loss of Refresh responses, a server must
   retain the old STUN MOBILITY-TICKET attribute for a period of at
   least 30 seconds to be able to recognize a retransmission of the
   Refresh request with the old STUN MOBILITY-TICKET attribute from the
   client.

3.2.3.  Receiving a Refresh Response

   In addition to the process described in Section 7.3 of [RFC5766], the
   client will store the MOBILITY-TICKET attribute, if present, from the
   response.  This attribute will be presented by the client to the
   server during a subsequent Refresh request to aid mobility.

3.3.  New STUN Attribute MOBILITY-TICKET

   This attribute is used to retain an allocation on the TURN server.
   It is exchanged between the client and server to aid mobility.  The
   value of the MOBILITY-TICKET is encrypted and is of variable length.

3.4.  New STUN Error Response Code

   This document defines the following new error response code:

      405 (Mobility Forbidden): Mobility request was valid but cannot be
      performed due to administrative or similar restrictions.

4.  IANA Considerations

   IANA has added the following attribute to the "STUN Attributes"
   registry [IANA-STUN]:

   o  MOBILITY-TICKET (0x8030, in the comprehension-optional range)

   Also, IANA has added a new STUN error code "Mobility Forbidden" with
   the value 405 to the "STUN Error Codes" registry [IANA-STUN].

5.  Security Considerations

   The TURN server MUST always ensure that the ticket is authenticated
   and encrypted using strong cryptographic algorithms to prevent
   modification or eavesdropping by an attacker.  The ticket MUST be
   constructed such that it has strong entropy to ensure that nothing
   can be gleaned by looking at the ticket alone.

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   An attacker monitoring the traffic between the TURN client and server
   can impersonate the client and refresh the allocation using the
   ticket issued to the client with the attacker's IP address and port.
   The TURN client and server MUST use the STUN long-term credential
   mechanism [RFC5389], the STUN Extension for Third-Party Authorization
   [RFC7635], or a (D)TLS connection to prevent malicious users from
   impersonating the client.  With any of those three mechanisms, when
   the server receives the Refresh request with the STUN MOBILITY-TICKET
   attribute from the client, it identifies that it is indeed the same
   client but with a new IP address and port using the ticket it had
   previously issued to refresh the allocation.  If (D)TLS is not used
   or the (D)TLS handshake fails, and authentication also fails, then
   the TURN client and server MUST fail and not proceed with TURN
   mobility.

   Security considerations described in [RFC5766] are also applicable to
   this mechanism.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <http://www.rfc-editor.org/info/rfc5245>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <http://www.rfc-editor.org/info/rfc5389>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <http://www.rfc-editor.org/info/rfc5766>.

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6.2.  Informative References

   [IANA-STUN]
              IANA, "Session Traversal Utilities for NAT (STUN)
              Parameters",
              <http://www.iana.org/assignments/stun-parameters>.

   [NOMBIS]   Uberti, J. and J. Lennox, "Improvements to ICE Candidate
              Nomination", Work in Progress,
              draft-uberti-mmusic-nombis-00, March 2015.

   [RFC7635]  Reddy, T., Patil, P., Ravindranath, R., and J. Uberti,
              "Session Traversal Utilities for NAT (STUN) Extension for
              Third-Party Authorization", RFC 7635,
              DOI 10.17487/RFC7635, August 2015,
              <http://www.rfc-editor.org/info/rfc7635>.

   [TRICKLE-ICE]
              Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
              "Trickle ICE: Incremental Provisioning of Candidates for
              the Interactive Connectivity Establishment (ICE)
              Protocol", Work in Progress, draft-ietf-ice-trickle-04,
              September 2016.

   [TRICKLE-SIP]
              Ivov, E., Stach, T., Marocco, E., and C. Holmberg, "A
              Session Initiation Protocol (SIP) usage for Trickle ICE",
              Work in Progress, draft-ietf-mmusic-trickle-ice-sip-06,
              October 2016.

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Appendix A.  Example of Ticket Construction

   The TURN server uses two different keys: one 128-bit key for Advance
   Encryption Standard (AES) in Cipher Block Chaining (CBC) mode
   (AES_128_CBC) and a 256-bit key for HMAC-SHA-256-128 for integrity
   protection.  The ticket can be structured as follows:

         struct {
             opaque key_name[16];
             opaque iv[16];
             opaque encrypted_state<0..2^16-1>;
             opaque mac[16];
         } ticket;

                          Figure 2: Ticket Format

   Here, key_name serves to identify a particular set of keys used to
   protect the ticket.  It enables the TURN server to easily recognize
   tickets it has issued.  The key_name should be randomly generated to
   avoid collisions between servers.  One possibility is to generate new
   random keys and key_name every time the server is started.

   The TURN state information (which is either self-contained or a
   handle) in encrypted_state is encrypted using 128-bit AES in CBC mode
   with the given Initialization Vector (IV).  The Message
   Authentication Code (MAC) is calculated using HMAC-SHA-256-128 over
   key_name (16 octets) and IV (16 octets), followed by the length of
   the encrypted_state field (2 octets) and its contents (variable
   length).

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Acknowledgements

   Thanks to Alfred Heggestad, Lishitao, Sujing Zhou, Martin Thomson,
   Emil Ivov, Oleg Moskalenko, Dave Waltermire, Pete Resnick, Antoni
   Przygienda, Alissa Cooper, Ben Campbell, Suresh Krishnan, Mirja
   Kuehlewind, Jonathan Lennox, and Brandon Williams for review and
   comments.

Authors' Addresses

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: tireddy@cisco.com

   Dan Wing

   Email: dwing-ietf@fuggles.com

   Prashanth Patil
   Cisco Systems, Inc.
   Bangalore
   India

   Email: praspati@cisco.com

   Paal-Erik Martinsen
   Cisco Systems, Inc.
   Philip Pedersens vei 22
   Lysaker, Akershus  1325
   Norway

   Email: palmarti@cisco.com

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