Network Working Group                                     K. Grewal
Internet Draft                                    Intel Corporation
Intended status: Standards Track                      G. Montenegro
Expires: May 30, 2010                         Microsoft Corporation
                                                          M. Bhatia
                                                  November 30, 2009

                   Wrapped ESP for Traffic Visibility

Status of this Memo

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   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-
   info). Please review these documents carefully, as they describe
   your rights and restrictions with respect to this document.


   This document describes the Wrapped Encapsulating Security
   Payload (WESP) protocol, which builds on the Encapsulating
   Security Payload (ESP) [RFC4303], and is designed to allow
   intermediate devices to (1) ascertain if data confidentiality is
   being employed within ESP and if not, (2) inspect the IPsec
   packets for network monitoring and access control functions.
   Currently in the IPsec ESP standard, there is no way to
   differentiate between encrypted and unencrypted payloads by
   simply examining a packet. This poses certain challenges to the
   intermediate devices that need to deep inspect the packet before
   making a decision on what should be done with that packet
   (Inspect and/or Allow/Drop). The mechanism described in this
   document can be used to easily disambiguate integrity-only ESP
   from ESP-encrypted packets, without compromising on the security
   provided by ESP.

Table of Contents

   1. Introduction...................................................3
      1.1. Requirements Language.....................................4
      1.2. Applicability Statement...................................4
   2. Wrapped ESP (WESP) Header format...............................5
      2.1. UDP Encapsulation.........................................8
      2.2. Transport and Tunnel Mode Considerations..................9
         2.2.1. Transport Mode Processing...........................10
         2.2.2. Tunnel Mode Processing..............................11
      2.3. IKE Considerations.......................................12
   3. Security Considerations.......................................12
   4. IANA Considerations...........................................13
   5. Acknowledgments...............................................13
   6. References....................................................14
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      6.1. Normative References.....................................14
      6.2. Informative References...................................14

1. Introduction

   Use of ESP within IPsec [RFC4303] specifies how ESP packet
   encapsulation is performed.  It also specifies that ESP can
   provide data confidentiality and data integrity services. Data
   integrity without data confidentiality ("integrity-only ESP") is
   possible via the ESP-NULL encryption algorithm [RFC2410] or via
   combined-mode algorithms such as AES-GMAC [RFC4543]. The exact
   encapsulation and algorithms employed are negotiated out-of-band
   using, for example, IKEv2 [RFC4306] and based on policy.

   Enterprise environments typically employ numerous security
   policies (and tools for enforcing them), as related to access
   control, content screening, firewalls, network monitoring
   functions, deep packet inspection, Intrusion Detection and
   Prevention Systems (IDS and IPS), scanning and detection of
   viruses and worms, etc.  In order to enforce these policies,
   network tools and intermediate devices require visibility into
   packets, ranging from simple packet header inspection to deeper
   payload examination.  Network security protocols which encrypt
   the data in transit prevent these network tools from performing
   the aforementioned functions.

   When employing IPsec within an enterprise environment, it is
   desirable to employ ESP instead of AH [RFC4302], as AH does not
   work in NAT environments. Furthermore, in order to preserve the
   above network monitoring functions, it is desirable to use
   integrity-only ESP. In a mixed-mode environment, some packets
   containing sensitive data employ a given encryption cipher
   suite, while other packets employ integrity-only ESP. For an
   intermediate device to unambiguously distinguish which packets
   are using integrity-only ESP requires knowledge of all the
   policies being employed for each protected session. This is
   clearly not practical. Heuristics-based methods can be employed
   to parse the packets, but these can be very expensive, requiring
   numerous rules based on each different protocol and payload.
   Even then, the parsing may not be robust in cases where fields
   within a given encrypted packet happen to resemble the fields
   for a given protocol or heuristic rule.  In cases where the
   packets may be encrypted, it is also wasteful to check against
   heuristics-based rules, when a simple exception policy (e.g.,
   allow, drop or redirect) can be employed to handle the encrypted
   packets. Because of the non-deterministic nature of heuristics-
   based rules for disambiguating between encrypted and non-
   encrypted data, an alternative method for enabling intermediate
   devices to function in encrypted data environments needs to be
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   defined. Additionally there are many types and classes of
   network devices employed within a given network and a
   deterministic approach provides a simple solution for all of
   them. Enterprise environments typically use both stateful and
   stateless packet inspection mechanisms. The previous
   considerations weigh particularly heavy on stateless mechanisms
   such as router ACLs and NetFlow exporters. Nevertheless, a
   deterministic approach provides a simple solution for the myriad
   types of devices employed within a network, regardless of their
   stateful or stateless nature.

   This document defines a mechanism to provide additional
   information in relevant IPsec packets so intermediate devices
   can efficiently differentiate between encrypted and integrity-
   only packets. Additionally and in the interest of consistency,
   this extended format can also be used to carry encrypted packets
   without loss in disambiguation.

   The document is consistent with the operation of ESP in NAT
   environments [RFC3947].

   The design principles for this protocol are the following:

   o  Allow easy identification and parsing of integrity-only IPsec

   o  Leverage the existing hardware IPsec parsing engines as much
   as possible to minimize additional hardware design costs

   o  Minimize the packet overhead in the common case

1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   "OPTIONAL" in this document are to be interpreted as described
   in RFC 2119 [RFC2119].

1.2. Applicability Statement

   The document is applicable only to the wrapped ESP header
   defined below, and does not describe any changes to either ESP
   [RFC4303] nor IP Authentication Header (AH) [RFC4302].

   There are two ways to enable intermediate security devices to
   distinguish between encrypted and unencrypted ESP traffic:

   - The heuristics approach [Heuristics I-D] has the intermediate
   node inspect the unchanged ESP traffic, to determine with
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   extremely high probability whether or not the traffic stream is

   - The Wrapped ESP (WESP) approach described in this document, in
   contrast, requires the ESP endpoints to be modified to support
   the new protocol. WESP allows the intermediate node to
   distinguish encrypted and unencrypted traffic deterministically,
   using a simpler implementation for the intermediate node.

   Both approaches are being documented simultaneously by the IP
   Security Maintenance and Extensions (IPsecME) Working Group,
   with WESP being put on Standards Track while the heuristics
   approach is being published as an Informational RFC. While
   endpoints are being modified to adopt WESP, we expect both
   approaches to coexist for years, because the heuristic approach
   is needed to inspect traffic where at least one of the endpoints
   has not been modified. In other words, intermediate nodes are
   expected to support both approaches in order to achieve good
   security and performance during the transition period.

2. Wrapped ESP (WESP) Header format

   Wrapped ESP encapsulation (WESP) uses protocol number (TBD via
   IANA). Accordingly, the (outer) protocol header (IPv4, IPv6, or
   Extension) that immediately precedes the WESP header SHALL
   contain the value (TBD via IANA) in its Protocol (IPv4) or Next
   Header (IPv6, Extension) field. WESP provides additional
   attributes in each packet to assist in differentiating between
   encrypted and non-encrypted data, and to aid parsing of the
   packet. WESP follows RFC 4303 for all IPv6 and IPv4
   considerations (e.g., alignment considerations).

   This extension essentially acts as a wrapper to the existing ESP
   protocol and provides an additional 4 octets at the front of the
   existing ESP packet.

   This may be depicted as follows:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |                       Wrapped ESP Header                      |
  |                      Existing ESP Encapsulation               |
  ~                                                               ~
  |                                                               |

                     Figure 1 WESP Packet Format
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   By preserving the body of the existing ESP packet format, a
   compliant implementation can simply add in the new header,
   without needing to change the body of the packet. The value of
   the new protocol used to identify this new header is TBD via
   IANA. Further details are shown below:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |  Next Header  |   HdrLen      |  TrailerLen   |     Flags     |
  |                       Padding (optional)                      |
  |                      Existing ESP Encapsulation               |
  ~                                                               ~
  |                                                               |

                   Figure 2 Detailed WESP Packet Format


   Next Header, 8 bits: This field MUST be the same as the Next
   Header field in the ESP trailer when using ESP in the Integrity
   only mode. When using ESP with encryption, the "Next Header"
   field looses this name and semantics and becomes an empty field
   which MUST be initialized to all zeros. The receiver MUST do
   some sanity checks before the WESP packet is accepted. The
   receiver MUST ensure that the Next Header field in the WESP
   header and the Next Header field in the ESP trailer match when
   using ESP in the Integrity only mode. The packet MUST be dropped
   if the two do not match. Similarly, the receiver MUST ensure
   that the Next Header field in the WESP header is an empty field
   initialized to zero if using WESP with encryption. The WESP
   flags dictate if the packet is encrypted.

   HdrLen, 8 bits: Offset from the beginning of the WESP header to
   the beginning of the Rest of Payload Data (i.e., past the IV, if
   present) within the encapsulated ESP header, in octets. HdrLen
   MUST be set to zero when using ESP with encryption. When using
   integrity-only ESP, the following HdrLen values are invalid: any
   value less than 12; any value that is not a multiple of 4; any
   value that is not a multiple of 8 when using IPv6. The receiver
   MUST ensure that this field matches with the header offset
   computed from using the negotiated SA and MUST drop the packet
   in case it does not match.

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   TrailerLen, 8 bits: TrailerLen contains the size of the ICV
   being used by the negotiated algorithms within the IPsec SA, in
   octets. TrailerLen MUST be set to zero when using ESP with
   encryption. The receiver MUST only accept the packet if this
   field matches with the value computed from using the negotiated
   SA. This insures that sender is not deliberately setting this
   value to obfuscate a part of the payload from examination by a
   trusted intermediary device.

   Flags, 8 bits: The bits are defined most-significant-bit (MSB)
   first, so bit 0 is the most significant bit of the flags octet.

   0 1 2 3 4 5 6 7
  |V V|E|P| Rsvd  |

   Figure 3 Flags format

       Version (V), 2 bits: MUST be sent as 0 and checked by the
   receiver. If the version is different than an expected version
   number (e.g. negotiated via the control channel), then the
   packet MUST be dropped by the receiver. Future modifications to
   the WESP header may require a new version number. Intermediate
   nodes dealing with unknown versions are not necessarily able to
   parse the packet correctly. Intermediate treatment of such
   packets is policy-dependent (e.g., it may dictate dropping such

       Encrypted Payload (E), 1 bit: Setting the Encrypted Payload
   bit to 1 indicates that the WESP (and therefore ESP) payload is
   protected with encryption. If this bit is set to 0, then the
   payload is using integrity-only ESP. Setting or clearing this
   bit also impacts the value in the WESP Next Header field, as
   described above. The recipient MUST ensure consistency of this
   flag with the negotiated policy and MUST drop the incoming
   packet otherwise.

      Padding Present (P), 1 bit: If P=1 (Padding Present flag),
   the first octet of the Padding field holds the padding length,
   in octets (including the length octet). All other Padding
   octets MUST be sent as zero, and MUST be ignored by the
   receiver and intermediate devices (however, this field is
   intended to allow future extensibility, so these restrictions
   may be relaxed in a future document updating this RFC).

   The padding length may be any length between 4 and 252 that
   preserves the alignment of the ESP header (4 octet alignment
   for IPv4, 8 octet alignment for IPv6). This padding MUST be
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   used with IPv6 in order to preserve IPv6 8-octet alignment. If
   WESP is being used with UDP encapsulation (see 2.1 below) and
   IPv6, the Protocol Identifier (0x00000002) occupies four octets
   so the padding is not needed, as the header is already on an 8-
   octet boundary.

       Rsvd, 4 bits: Reserved for future use.  The reserved bits
   MUST be sent as 0, and ignored by the receiver. Future documents
   defining any of these bits MUST NOT affect the distinction
   between encrypted and unencrypted packets. Intermediate nodes
   dealing with unknown reserved bits are not necessarily able to
   parse the packet correctly. Intermediate treatment of such
   packets is policy-dependent (e.g., it may dictate dropping such

   Future versions of this protocol may change the Version number
   and/or the reserved bits sent, possibly by negotiating them over
   the control channel.

   As can be seen, the WESP format extends the standard ESP header
   by the first 4 octets for IPv4 and optionally (see above) by 8
   octets for IPv6. The WESP header is integrity protected, along
   with all the fields specified for ESP in RFC 4303.

   Modifying the integrity protection in ESP to include the
   additional WESP header octets means that ESP implementations
   cannot be simply reused. The chosen tradeoff errs on the side of
   caution instead of treating ESP as a completely modular

2.1. UDP Encapsulation

   This section describes a mechanism for running the new packet
   format over the existing UDP encapsulation of ESP as defined in
   RFC 3948. This allows leveraging the existing IKE negotiation of
   the UDP port for NAT-T discovery and usage [RFC3947, RFC4306],
   as well as preserving the existing UDP ports for ESP (port
   4500).  With UDP encapsulation, the packet format can be
   depicted as follows.

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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |        Src Port (4500)        | Dest Port (4500)              |
  |             Length            |          Checksum             |
  |          Protocol Identifier (value = 0x00000002)             |
  |  Next Header  |   HdrLen      |  TrailerLen   |    Flags      |
  |                      Existing ESP Encapsulation               |
  ~                                                               ~
  |                                                               |

                Figure 4 UDP-Encapsulated WESP Header


   Source/Destination port (4500) and checksum: describes the UDP
   encapsulation header, per RFC3948.

   Protocol Identifier: new field to demultiplex between UDP
   encapsulation of IKE, UDP encapsulation of ESP per RFC 3948, and
   the UDP encapsulation in this specification.

   According to RFC 3948, clause 2.2, a 4 octet value of zero (0)
   immediately following the UDP header indicates a Non-ESP marker,
   which can be used to assume that the data following that value
   is an IKE packet.  Similarly, a value greater then 255 indicates
   that the packet is an ESP packet and the 4-octet value can be
   treated as the ESP SPI. However, RFC 4303, clause 2.1 indicates
   that the values 1-255 are reserved and cannot be used as the
   SPI.  We leverage that knowledge and use one of these reserved
   values to indicate that the UDP encapsulated ESP header contains
   this new packet format for ESP encapsulation.

   The remaining fields in the packet have the same meaning as per
   section 2 above.

2.2. Transport and Tunnel Mode Considerations

   This extension is equally applicable to transport and tunnel mode
   where the ESP Next Header field is used to differentiate between
   these modes, as per the existing IPsec specifications.

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   In the diagrams below, "WESP ICV" refers to the ICV computation as
   modified by this specification. Namely, the ESP ICV computation is
   augmented to include the four octets that constitute the WESP header.
   Otherwise, the ICV computation is as specified by ESP [RFC4303].

2.2.1. Transport Mode Processing

   In transport mode, ESP is inserted after the IP header and before a
   next layer protocol, e.g., TCP, UDP, ICMP, etc. The following
   diagrams illustrate how WESP is applied to the ESP transport mode for
   a typical packet, on a "before and after" basis.

        |orig IP hdr  |     |      |
        |(any options)| TCP | Data |

        |orig IP hdr  | WESP | ESP |     |      |   ESP   |WESP|
        |(any options)| Hdr  | Hdr | TCP | Data | Trailer | ICV|
                                   |<---- encryption ---->|
                      |<----------- integrity ----------->|

        | orig |hop-by-hop,dest*,|dest|   |    |
        |IP hdr|routing,fragment.|opt*|TCP|Data|

      | orig |hop-by-hop,dest*,|    |   |dest|   |    | ESP   |WESP|
      |IP hdr|routing,fragment.|WESP|ESP|opt*|TCP|Data|Trailer| ICV|
                                        |<---- encryption --->|
                               |<-------- integrity --------->|

               * = if present, could be before WESP, after ESP, or both

   All other considerations are as per RFC 4303.

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2.2.2. Tunnel Mode Processing

   In tunnel mode, ESP is inserted after the new IP header and before
   the original IP header, as per RFC 4303. The following diagram
   illustrates how WESP is applied to the ESP tunnel mode for a typical
   packet, on a "before and after" basis.

       | orig IP hdr*  |   |    |
       | (any options) |TCP|Data|

      |new IP hdr*  |    |   | orig IP hdr*  |   |    | ESP   |WESP|
      |(any options)|WESP|ESP| (any options) |TCP|Data|Trailer| ICV|
                             |<--------- encryption --------->|
                    |<--------------- integrity ------------->|

        | orig*|orig ext |   |    |
        |IP hdr| hdrs *  |TCP|Data|

  | new* |new ext |    |   | orig*|orig ext |   |    | ESP   |WESP|
  |IP hdr| hdrs*  |WESP|ESP|IP hdr| hdrs *  |TCP|Data|Trailer| ICV|
                           |<--------- encryption ---------->|
                  |<--------------- integrity -------------->|

   * = if present, construction of outer IP hdr/extensions and

   modification of inner IP hdr/extensions is discussed in

   the Security Architecture document.

   All other considerations are as per RFC 4303.

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2.3. IKE Considerations

   This document assumes that WESP negotiation is performed using
   IKEv2. In order to negotiate the new format of ESP encapsulation
   via IKEv2 [RFC4306], both parties need to agree to use the new
   packet format. This can be achieved using a notification method
   similar to USE_TRANSPORT_MODE defined in RFC 4306.

   The notification, USE_WESP_MODE (value TBD) MAY be included in a
   request message that also includes an SA payload requesting a
   CHILD_SA using ESP.  It requests that the CHILD_SA use WESP mode
   rather than ESP for the SA created.  If the request is accepted,
   the response MUST also include a notification of type
   USE_WESP_MODE. If the responder declines the request, the
   CHILD_SA will be established using ESP, as per RFC 4303.  If
   this is unacceptable to the initiator, the initiator MUST delete
   the SA.  Note: Except when using this option to negotiate  WESP
   mode, all CHILD_SAs will use standard ESP.

   Negotiation of WESP in this manner preserves all other
   negotiation parameters, including NAT-T [RFC3948]. NAT-T is
   wholly compatible with this wrapped frame format and can be used
   as-is, without any modifications, in environments where NAT is
   present and needs to be taken into account.

3. Security Considerations

   As this document augments the existing ESP encapsulation format,
   UDP encapsulation definitions specified in RFC 3948 and IKE
   negotiation of the new encapsulation, the security observations
   made in those documents also apply here. In addition, as this
   document allows intermediate device visibility into IPsec ESP
   encapsulated frames for the purposes of network monitoring
   functions, care should be taken not to send sensitive data over
   connections using definitions from this document, based on
   network domain/administrative policy. A strong key agreement
   protocol, such as IKEv2, together with a strong policy engine
   should be used to in determining appropriate security policy for
   the given traffic streams and data over which it is being

   ESP is end-to-end and it will be impossible for the intermediate
   devices to verify that all the fields in the WESP header are
   correct. It is thus possible to modify the WESP header so that
   the packet sneaks past a firewall if the fields in the WESP
   header are set to something that the firewall will allow. The
   endpoint thus must verify the sanity of the WESP header before
   accepting the packet. In an extreme case, someone colluding with
   the attacker, could change the WESP fields back to the original
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   values so that the attack goes unnoticed. However, this is not a
   new problem and it already exists IPsec.

4. IANA Considerations

   The WESP protocol number is assigned by IANA out of the IP
   Protocol Number space (and as recorded at the IANA web page at
   http://www.iana.org/assignments/protocol-numbers) is: TBD.

   The USE_WESP_MODE notification number is assigned out of the
   "IKEv2 Notify Message Types - Status Types" registry's 16384-
   40959 (Expert Review) range: TBD.

   The SPI value of 2 is assigned by IANA out of the reserved SPI
   range from the SPI values registry to indicate use of the WESP
   protocol within a UDP encapsulated, NAT-T environment.

   This specification requests that IANA create a new registry for
   "WESP Flags" to be managed as follows:

   The first 2 bits are the WESP Version Number. The value 0 is
   assigned to the version defined in this specification. Further
   assignments of the WESP Version Number are to be managed via the
   IANA Policy of "Standards Action" [RFC5226]. For WESP version
   numbers, the unassigned values are 1, 2 and 3. The Encrypted
   Payload bit is used to indicate if the payload is encrypted or
   using integrity-only ESP. The Padding Present bit is used to
   signal the presence of padding. The remaining 4 bits of the WESP
   Flags are undefined and future assignment is to be managed via
   the IANA Policy of "Specification Required".

5. Acknowledgments

   The authors would like to acknowledge the following people for
   their feedback on updating the definitions in this document.

   David McGrew, Brian Weis, Philippe Joubert, Brian Swander, Yaron
   Sheffer, Pasi Eronen, Men Long, David Durham, Prashant Dewan,
   Marc Millier among others.

   This document was prepared using 2-Word-v2.0.template.doc.

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6. References

6.1. Normative References

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

   [RFC2410] Glenn, R. and Kent, S., "The NULL Encryption Algorithm
             and Its Use With IPsec", RFC 2410, November 1998.

   [RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and
             M. Stenberg, "UDP Encapsulation of IPsec ESP Packets",
             RFC 3948, January 2005.

   [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
             RFC 4303, December 2005.

   [RFC4543] McGrew, D. and Viega J., "The Use of Galois Message
             Authentication Code (GMAC) in IPsec ESP and AH", RFC
             4543, May 2006.

   [RFC5226] Narten, T., Alverstrand, H., "Guidelines for Writing
             an IANA Considerations Section in RFCs",  RFC 5226,
             May 2008.

6.2. Informative References

   [RFC3947] Kivinen, T., Swander, B., Huttunen, A., and V. Volpe,
             "Negotiation of NAT-Traversal in the IKE", RFC 3947,
             January 2005.

   [RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
             December 2005.

   [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
             RFC 4306, December 2005.

   [Heuristics I-D] Kivinen, T., McDonald, D., "Heuristics for Detecting
             ESP-NULL packets", Internet Draft, April 2009.

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Author's Addresses

   Ken Grewal
   Intel Corporation
   2111 NE 25th Avenue, JF3-232
   Hillsboro, OR  97124

   Email: ken.grewal@intel.com

   Gabriel Montenegro
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052

   Email: gabriel.montenegro@microsoft.com

   Manav Bhatia
   Manyata Embassy
   Nagawara Bangalore


   Email: manav.bhatia@alcatel-lucent.com

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