Network Working Group                                     K. Grewal
Internet Draft                                    Intel Corporation
Intended status: Standards Track                      G. Montenegro
Expires: April 22, 2009                       Microsoft Corporation
                                                   October 22, 2008

                   Wrapped ESP for Traffic Visibility

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   This document describes an ESP encapsulation for IPsec, allowing
   intermediate devices to ascertain if ESP-NULL is being employed
   and hence inspect the IPsec packets for network monitoring and
   access control functions.  Currently in the IPsec standard,
   there is no way to differentiate between ESP encryption and ESP
   NULL encryption by simply examining a packet.

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Table of Contents

   1. Introduction................................................2
      1.1. Requirements Language..................................3
      1.2. Applicability Statement................................4
   2. Wrapped ESP (WESP) Header format............................4
      2.1. UDP Encapsulation......................................5
      2.2. Tunnel and Transport mode of considerations............7
      2.3. IKE Considerations.....................................7
   3. Security Considerations.....................................7
   4. IANA Considerations.........................................7
   5. Acknowledgments.............................................8
   6. References..................................................8
      6.1. Normative References...................................8
      6.2. Informative References.................................8

1. Introduction

   Use of ESP within IPsec [RFC4303] specifies how ESP packet
   encapsulation is performed.  It also specifies that ESP can use
   NULL encryption [RFC2410] while preserving data integrity and
   authenticity.  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, 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

   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 ESP-
   NULL. In a mixed mode environment some packets containing
   sensitive data employ a given encryption cipher suite, while
   other packets employ ESP-NULL. For an intermediate device to
   unambiguously distinguish which packets are leveraging ESP-NULL,
   they would require knowledge of all the policies being employed
   for each protected session. This is clearly not practical.
   Heuristic-based methods can be employed to parse the packets,
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   but these can be very expensive, containing 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.  This is even more problematic when different
   length Initialization Vectors (IVs), Integrity Check Values
   (ICVs) and padding are used for different security associations,
   making it difficult to determine the start and end of the
   payload data, let alone attempting any further parsing.
   Furthermore, storage, lookup and cross-checking a set of
   comprehensive rules against every packet adds cost to hardware
   implementations and degrades performance. 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. 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.

   This document defines a mechanism to prove additional
   information in relevant IPsec packets so intermediate devices
   can efficiently differentiate between encrypted ESP packets and
   ESP packets with NULL encryption.

   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].

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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 AH [RFC4302].

2. Wrapped ESP (WESP) Header format

   The proposal is to define a protocol number for Wrapped ESP
   encapsulation (WESP), which provides additional attributes in
   each packet to assist in differentiating between encrypted and
   non-encrypted data, as well as aid parsing of the packet.  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 simply 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

   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         |
  |                      Existing ESP Encapsulation               |
  ~                                                               ~
  |                                                               |

                   Figure 2 Detailed WESP Packet Format

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   Next Header:  next protocol header (encrypted in ESP trailer,
   but in the clear in header), providing easy access to a HW
   parser to extract the upper layer protocol. Note: For security
   concerns, this value may optionally be set to zero, in which
   case the next header an be extracted from the ESP trailer.

   HdrLen: includes the new header + full ESP header + the IV (if
   present).  It is an offset to the beginning of the Payload Data.

   TrailerLen: Offset from the end of the packet including the ICV,
   pad length, and any padding.  It is an offset from the end of
   the packet to the last byte of the payload data.


       2 bits: Version

       6 bits: reserved for future use.  These MUST be set to zero
   per this specification, but usage may be defined by other

   As can be seen, this wrapped ESP format simply extends the
   standard ESP header by the first 4 octets.

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], 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)              |
  |                           Checksum                            |
  |          Protocol Identifier (value = 0x00000001)             |
  | Next Header   |  HdrLen       | TrailerLen    | Flags         |
  |                      Existing ESP Encapsulation               |
  ~                                                               ~
  |                                                               |

                Figure 3 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 of non-zero 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 a value of 1 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 2.0 above.

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2.2. Tunnel and Transport mode of considerations

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

2.3. IKE Considerations

   In order to negotiate the new format of ESP encapsulation via
   IKE [RFC4306], both parties need to agree to use the new packet
   format. This can be achieved by proposing a new protocol ID
   within the existing IKE proposal structure as defined by RFC
   4306, clause 3.3.1. The existing proposal substructure in this
   clause allows negotiation of ESP/AH (among others) by using
   different protocol Ids for these protocols. By using the same
   protocol substructure in the proposal payload and using a new
   value (TBD) for this encapsulation, the existing IKE negotiation
   can be leverage with minimal changes to support negotiation of
   this encapsulation.

   Furthermore, because the negotiation is at the protocol level,
   other transforms remain valid for this new encapsulation and
   consistent with IKEv2 [RFC4306]. Additionally, NAT-T [RFC3948]
   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 IKE, 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

4. IANA Considerations

   Reserving an appropriate value for this encapsulation as well as
   a new value for the protocol in the IKE negotiation is TBD by

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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, Men Long, David Durham, Prashant Dewan, Marc Millier
   among others.

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.

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

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.

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

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

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

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Author's Addresses

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


   Gabriel Montenegro
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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