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rfc:rfc5840

Internet Engineering Task Force (IETF) K. Grewal Request for Comments: 5840 Intel Corporation Category: Standards Track G. Montenegro ISSN: 2070-1721 Microsoft Corporation

                                                             M. Bhatia
                                                        Alcatel-Lucent
                                                            April 2010
Wrapped Encapsulating Security Payload (ESP) for Traffic Visibility

Abstract

 This document describes the Wrapped Encapsulating Security Payload
 (WESP) protocol, which builds on the Encapsulating Security Payload
 (ESP) RFC 4303 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 deterministic 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.

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 5741.
 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/rfc5840.

Grewal, et al. Standards Track [Page 1] RFC 5840 WESP for Traffic Visibility April 2010

Copyright Notice

 Copyright (c) 2010 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
 to this document.  Code Components extracted from this document must
 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

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 ...........................9
         2.2.2. Tunnel Mode Processing .............................10
    2.3. IKE Considerations ........................................11
 3. Security Considerations ........................................12
 4. IANA Considerations ............................................13
 5. Acknowledgments ................................................13
 6. References .....................................................14
    6.1. Normative References ......................................14
    6.2. Informative References ....................................14

Grewal, et al. Standards Track [Page 2] RFC 5840 WESP for Traffic Visibility April 2010

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,
 Internet Key Exchange Protocol version 2 (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
 that 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 Authentication Header (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
 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

Grewal, et al. Standards Track [Page 3] RFC 5840 WESP for Traffic Visibility April 2010

 typically use both stateful and stateless packet inspection
 mechanisms.  The previous considerations weigh particularly heavy on
 stateless mechanisms such as router Access Control Lists (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.
 This 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
    traffic
 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 NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "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] or
 the IP Authentication Header (AH) [RFC4302].
 There are two well-accepted ways to enable intermediate security
 devices to distinguish between encrypted and unencrypted ESP traffic:
  1. The heuristics approach [Heuristics] has the intermediate node

inspect the unchanged ESP traffic, to determine with extremely high

   probability whether or not the traffic stream is encrypted.

Grewal, et al. Standards Track [Page 4] RFC 5840 WESP for Traffic Visibility April 2010

  1. 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 (this document) as a Standards Track RFC while the heuristics
 approach is expected to be 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 (WESP) encapsulation uses protocol number 141.
 Accordingly, the (outer) protocol header (IPv4, IPv6, or Extension)
 that immediately precedes the WESP header SHALL contain the value
 (141) 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
 in 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 for IPv4.  For IPv6, additional padding may be
 required and this is described below.
 The packet format 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

Grewal, et al. Standards Track [Page 5] RFC 5840 WESP for Traffic Visibility April 2010

 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 141.  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
 Where:
 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 that 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
 and any other WESP options defined in the future) 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
 Security Association (SA) and MUST drop the packet in case it does
 not match.

Grewal, et al. Standards Track [Page 6] RFC 5840 WESP for Traffic Visibility April 2010

 TrailerLen, 8 bits: TrailerLen contains the size of the Integrity
 Check Value (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 ensures 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 require a
 new version number.  In particular, the version of WESP defined in
 this document does not allow for any extensions.  However, old
 implementations will still be able to find the encapsulated cleartext
 packet using the HdrLen field from the WESP header, when the 'E' bit
 is not set.  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 packets).
 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 header (P), 1 bit: If set (value 1), the 4-octet padding is
 present.  If not set (value 0), the 4-octet padding is absent.  This
 padding MUST be used with IPv6 in order to preserve IPv6 8-octet
 alignment.  If WESP is being used with UDP encapsulation (see Section
 2.1 below) and IPv6, the Protocol Identifier (0x00000002) occupies 4
 octets so the IPv6 padding is not needed, as the header is already on
 an 8-octet boundary.  This padding MUST NOT be used with IPv4, as it
 is not needed to guarantee 4-octet IPv4 alignment.

Grewal, et al. Standards Track [Page 7] RFC 5840 WESP for Traffic Visibility April 2010

 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 or the semantics of HdrLen.  In other words,
 even if new bits are defined, old implementations will be able to
 find the encapsulated packet correctly.  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 packets).
 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.

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 Network Address Translation Traversal (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.
     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

Grewal, et al. Standards Track [Page 8] RFC 5840 WESP for Traffic Visibility April 2010

 Where:
 Source/Destination port (4500) and checksum: describes the UDP
 encapsulation header, per RFC 3948.
 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, Section 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 Security Parameter Index (SPI).  However, RFC 4303, Section 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.

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.

Grewal, et al. Standards Track [Page 9] RFC 5840 WESP for Traffic Visibility April 2010

    BEFORE APPLYING WESP -IPv4
          -------------------------------------------------
          |orig IP hdr  | ESP |     |      |   ESP   | ESP|
          |(any options)| Hdr | TCP | Data | Trailer | ICV|
          -------------------------------------------------
                              |<---- encryption ---->|
                        |<------- integrity -------->|
    AFTER APPLYING WESP - IPv4
          --------------------------------------------------------
          |orig IP hdr  | WESP | ESP |     |      |   ESP   | ESP|
          |(any options)| Hdr  | Hdr | TCP | Data | Trailer | ICV|
          --------------------------------------------------------
                                     |<---- encryption ---->|
                               |<------- integrity -------->|
    BEFORE APPLYING WESP - IPv6
        --------------------------------------------------------------
        | orig |hop-by-hop,dest*,|   |dest|   |    | ESP   | ESP|
        |IP hdr|routing,fragment |ESP|opt*|TCP|Data|Trailer| ICV|
        --------------------------------------------------------------
                                     |<---- encryption --->|
                                 |<----- integrity ------->|
    AFTER APPLYING WESP - IPv6
        --------------------------------------------------------------
        | orig |hop-by-hop,dest*,|    |   |dest|   |    | ESP   | ESP|
        |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.

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.

Grewal, et al. Standards Track [Page 10] RFC 5840 WESP for Traffic Visibility April 2010

    BEFORE APPLYING WESP - IPv4
        ---------------------------------------------------------
        |new IP hdr*  |   | orig IP hdr*  |   |    | ESP   | ESP|
        |(any options)|ESP| (any options) |TCP|Data|Trailer| ICV|
        ---------------------------------------------------------
                          |<--------- encryption --------->|
                      |<----------- integrity ------------>|
    AFTER APPLYING WESP - IPv4
        --------------------------------------------------------------
        |new IP hdr*  |    |   | orig IP hdr*  |   |    | ESP   | ESP|
        |(any options)|WESP|ESP| (any options) |TCP|Data|Trailer| ICV|
        --------------------------------------------------------------
                               |<--------- encryption --------->|
                           |<----------- integrity ------------>|
    BEFORE APPLYING WESP - IPv6
    -----------------------------------------------------------------
    |new IP|new ext |   |orig IP|orig ext|   |    | ESP   | ESP|
    | hdr* | hdrs*  |ESP|  hdr* | hdrs * |TCP|Data|Trailer| ICV|
    -----------------------------------------------------------------
                        |<--------- encryption ---------->|
                    |<------------- integrity ----------->|
    AFTER APPLYING WESP - IPv6
    -----------------------------------------------------------------
    |new IP|new ext |    |   |orig IP|orig ext|   |    | ESP   | ESP|
    | hdr* | hdrs*  |WESP|ESP|  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.

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.

Grewal, et al. Standards Track [Page 11] RFC 5840 WESP for Traffic Visibility April 2010

 The notification, USE_WESP_MODE (value 16415) MUST be included in a
 request message that also includes an SA payload requesting a
 CHILD_SA using ESP.  It signals that the sender supports the WESP
 version defined in the current document and 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 format and can be used as-is, without any
 modifications, in environments where NAT is present and needs to be
 taken into account.
 WESP version negotiation is not introduced as part of this
 specification.  If the WESP version is updated in a future
 specification, then that document MUST specify how the WESP version
 is negotiated.

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 in determining appropriate security
 policy for the given traffic streams and data over which it is being
 employed.
 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

Grewal, et al. Standards Track [Page 12] RFC 5840 WESP for Traffic Visibility April 2010

 the WESP fields back to the original 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 assigned by IANA out of the IP Protocol
 Number space is 141.
 The USE_WESP_MODE notification number assigned out of the "IKEv2
 Notify Message Types - Status Types" registry's 16384-40959 (Expert
 Review) range is 16415.
 The SPI value of 2 has been 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.
 IANA has created 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 "IETF
 Review" [RFC5226].

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, Russ Housley, and Jari Arkko, among others.
 Manav Bhatia would also like to acknowledge Swati and Maitri for
 their continued support.

Grewal, et al. Standards Track [Page 13] RFC 5840 WESP for Traffic Visibility April 2010

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 S. Kent, "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 J. Viega, "The Use of Galois Message
              Authentication Code (GMAC) in IPsec ESP and AH", RFC
              4543, May 2006.
 [RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, 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., Ed., "Internet Key Exchange (IKEv2)
              Protocol", RFC 4306, December 2005.
 [Heuristics] Kivinen, T. and D. McDonald, "Heuristics for Detecting
              ESP-NULL packets", Work in Progress, March 2010.

Grewal, et al. Standards Track [Page 14] RFC 5840 WESP for Traffic Visibility April 2010

Authors' Addresses

 Ken Grewal
 Intel Corporation
 2111 NE 25th Avenue, JF3-232
 Hillsboro, OR  97124
 USA
 EMail: ken.grewal@intel.com
 Gabriel Montenegro
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 USA
 EMail: gabriel.montenegro@microsoft.com
 Manav Bhatia
 Alcatel-Lucent
 Manyata Embassy
 Nagawara Bangalore
 India
 EMail: manav.bhatia@alcatel-lucent.com

Grewal, et al. Standards Track [Page 15]

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