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Versions: (draft-jokela-hip-esp) 00 01 02 03 04 05 06 RFC 5202

Network Working Group                                          P. Jokela
Internet-Draft                              Ericsson Research NomadicLab
Expires: December 25, 2005                                  R. Moskowitz
                                       ICSAlabs, a Division of TruSecure
                                                             Corporation
                                                             P. Nikander
                                            Ericsson Research NomadicLab
                                                           June 23, 2005


                  Using ESP transport format with HIP
                         draft-ietf-hip-esp-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on December 25, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo specifies an Encapsulated Security Payload (ESP) based
   mechanism for transmission of user data packets, to be used with the
   Host Identity Protocol (HIP).




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions used in this document  . . . . . . . . . . . . . .  5
   3.  Using ESP with HIP . . . . . . . . . . . . . . . . . . . . . .  6
     3.1   ESP Packet Format  . . . . . . . . . . . . . . . . . . . .  6
     3.2   ESP Packet Processing  . . . . . . . . . . . . . . . . . .  6
       3.2.1   Semantics of the Security Parameter Index (SPI)  . . .  7
     3.3   Security Association Establishment and Maintenance . . . .  7
       3.3.1   ESP Security Associations  . . . . . . . . . . . . . .  8
       3.3.2   Rekeying . . . . . . . . . . . . . . . . . . . . . . .  8
       3.3.3   Security Association Management  . . . . . . . . . . .  9
       3.3.4   Security Parameter Index (SPI) . . . . . . . . . . . .  9
       3.3.5   Supported Transforms . . . . . . . . . . . . . . . . .  9
       3.3.6   Sequence Number  . . . . . . . . . . . . . . . . . . . 10
       3.3.7   Lifetimes and Timers . . . . . . . . . . . . . . . . . 10
   4.  The Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1   ESP in HIP . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.1.1   Setting up an ESP Security Association . . . . . . . . 11
       4.1.2   Updating an Existing ESP SA  . . . . . . . . . . . . . 12
   5.  Parameter and Packet Formats . . . . . . . . . . . . . . . . . 13
     5.1   New Parameters . . . . . . . . . . . . . . . . . . . . . . 13
       5.1.1   ESP_INFO . . . . . . . . . . . . . . . . . . . . . . . 13
       5.1.2   ESP_TRANSFORM  . . . . . . . . . . . . . . . . . . . . 14
       5.1.3   NOTIFY Parameter . . . . . . . . . . . . . . . . . . . 15
     5.2   HIP ESP Security Association Setup . . . . . . . . . . . . 16
       5.2.1   Setup During Base Exchange . . . . . . . . . . . . . . 16
     5.3   HIP ESP Rekeying . . . . . . . . . . . . . . . . . . . . . 17
       5.3.1   Initializing Rekeying  . . . . . . . . . . . . . . . . 17
       5.3.2   Responding to the Rekeying Initialization  . . . . . . 18
     5.4   ICMP Messages  . . . . . . . . . . . . . . . . . . . . . . 18
       5.4.1   Unknown SPI  . . . . . . . . . . . . . . . . . . . . . 19
   6.  Packet Processing  . . . . . . . . . . . . . . . . . . . . . . 20
     6.1   Processing Outgoing Application Data . . . . . . . . . . . 20
     6.2   Processing Incoming Application Data . . . . . . . . . . . 20
     6.3   HMAC and SIGNATURE Calculation and Verification  . . . . . 21
     6.4   Processing Incoming ESP SA Initialization (R1) . . . . . . 21
     6.5   Processing Incoming Initialization Reply (I2)  . . . . . . 22
     6.6   Processing Incoming ESP SA Setup Finalization (R2) . . . . 22
     6.7   Dropping HIP Associations  . . . . . . . . . . . . . . . . 22
     6.8   Initiating ESP SA Rekeying . . . . . . . . . . . . . . . . 22
     6.9   Processing Incoming UPDATE Packets . . . . . . . . . . . . 24
       6.9.1   Processing UPDATE Packet: No  Outstanding Rekeying
               Request  . . . . . . . . . . . . . . . . . . . . . . . 24
     6.10  Finalizing Rekeying  . . . . . . . . . . . . . . . . . . . 25
     6.11  Processing NOTIFY Packets  . . . . . . . . . . . . . . . . 26
   7.  Keying Material  . . . . . . . . . . . . . . . . . . . . . . . 27
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28



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   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 30
     10.1  Normative references . . . . . . . . . . . . . . . . . . . 30
     10.2  Informative references . . . . . . . . . . . . . . . . . . 30
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 31
   A.  A Note on Implementation Options . . . . . . . . . . . . . . . 32
       Intellectual Property and Copyright Statements . . . . . . . . 33












































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1.  Introduction

   In the Host Identity Protocol Architecture [8], hosts are identified
   with public keys.  The Host Identity Protocol [5] base exchange
   allows any two HIP-supporting hosts to authenticate each other and to
   create a HIP association between themselves.  During the base
   exchange, the hosts generate a piece of shared keying material using
   an authenticated Diffie-Hellman exchange.

   The HIP base exchange specification [5] does not describe any
   transport formats, or methods for user data, to be used during the
   actual communication; it only defines that it is mandatory to
   implement the Encapsulated  Security Payload (ESP) [4] based
   transport format and method.  This document specifies how ESP is used
   with HIP to carry actual user data.

   To be more specific, this document specifies a set of HIP protocol
   extensions and their handling.  Using these extensions, a pair of ESP
   Security Associations (SAs) is created between the hosts during the
   base exchange.  The resulting ESP Security Associations use keys
   drawn from the keying material (KEYMAT) generated during the base
   exchange.  After the HIP association and required ESP SAs have been
   established between the hosts, the user data communication is
   protected using ESP.  In addition, this document specifies methods to
   update an existing ESP Security Association.

   It should be noted that representations of host identity are not
   carried explicitly in the headers of user data packets.  Instead, the
   ESP Security Parameter Index (SPI) is used to indicate the right host
   context.  The SPIs are selected during the HIP ESP setup exchange.
   For user data packets, ESP SPIs (in possible combination with IP
   addresses) are used indirectly to identify the host context, thereby
   avoiding any additional explicit protocol headers.


















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2.  Conventions used in this document

   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 [1].














































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3.  Using ESP with HIP

   The HIP base exchange is used to set up a HIP association between two
   hosts.  The base exchange provides two-way host authentication and
   key material generation, but it does not provide any means for
   protecting data communication between the hosts.  In this document we
   specify the use of ESP for protecting user data traffic after the HIP
   base exchange.  Note that this use of ESP is intended only for host-
   to-host traffic; security gateways are not supported.

   To support ESP use, the HIP base exchange messages require some minor
   additions to the parameters transported.  In the R1 packet, the
   responder adds the possible ESP transforms in a new ESP_TRANSFORM
   parameter before sending it to the Initiator.  The Initiator gets the
   proposed transforms, selects one of those proposed transforms, and
   adds it to the I2 packet in an ESP_TRANSFORM parameter.  In this I2
   packet, the Initiator also sends the SPI value that it wants to be
   used for ESP traffic flowing from the Responder to the Initiator.
   This information is carried using the new ESP_INFO parameter.  When
   finalizing the ESP SA setup, the Responder sends its SPI value to the
   Initiator in the R2 packet, again using ESP_INFO.

3.1  ESP Packet Format

   The ESP specification [4] defines the ESP packet format for IPsec.
   The HIP ESP packet looks exactly the same as the IPsec ESP transport
   format packet.  The semantics, however, are a bit different and are
   described in more detail in the next subsection.

3.2  ESP Packet Processing

   The ESP specification [4] defines packet processing for ESP, and
   defines two modes of operation:  tunnel mode and transport mode.
   This section reviews the main changes to ESP packet processing when
   ESP is combined with HIP.

   The main difference between standard ESP and HIP's use of ESP is the
   use by HIP of a new mode for operation, which has been called "Bound
   End-to-End Tunnel" (BEET) mode [12].  In BEET mode, the ESP packet is
   formatted as a transport mode packet, but the semantics of the
   connection are the same as for tunnel mode.  The "outer" addresses of
   the packet are the IP addresses and the "inner" addresses are the
   HITs.  The HITs do not need to be transmitted over the network,
   because the SPI value in the ESP packet is used as the hint for the
   correct HIT pair used for the connection.  This mode of operation
   avoids overhead typically associated with ESP tunnel mode.

   In ESP, the packet processing and SA lookup are based on IP



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   addresses.  In HIP, however, SAs are bound to end-host HITs instead
   of IP addresses.  When a HIP ESP SA packet arrives at the end-host,
   the host changes the IP addresses in the packet to the corresponding
   HITs before ESP processing.

   It should be noted that it is possible to support the HIP way of
   using ESP with a fully standards compliant IPsec implementation by
   adding the necessary header rewriting mechanisms below IPsec in the
   stack.  These mechanisms can be considered to be on the network side
   of IPsec, thus they cannot add any integrity or confidentiality
   problems that would not exist without them.  However, explicit care
   must be taken to avoid introducing any new denial-of-service attacks.

3.2.1  Semantics of the Security Parameter Index (SPI)

   SPIs are used in ESP to find the right Security Association for
   received packets.  The ESP SPIs have added significance when used
   with HIP; they are a compressed representation of a pair of HITs.
   Thus, SPIs MAY be used by intermediary systems in providing services
   like address mapping.  Note that since the SPI has significance at
   the receiver, only the < DST, SPI >, where DST is a destination IP
   address, uniquely identifies the receiver HIT at any given point of
   time.  The same SPI value may be used by several hosts.  A single <
   DST, SPI > value may denote different hosts and contexts at different
   points of time, depending on the host that is currently reachable at
   the DST.

   Each host selects for itself the SPI it wants to see in packets
   received from its peer.  This allows it to select different SPIs for
   different peers.  The SPI selection SHOULD be random; the rules of
   Section 2.1 of the ESP specification [4] must be followed.  A
   different SPI SHOULD be used for each HIP exchange with a particular
   host; this is to avoid a replay attack.  Additionally, when a host
   rekeys, the SPI MUST be changed.  Furthermore, if a host changes over
   to use a different IP address, it MAY change the SPI.

   One method for SPI creation that meets the above criteria would be to
   concatenate the HIT with a 32-bit random or sequential number, hash
   this (using SHA1), and then use the high order 32 bits as the SPI.

   The selected SPI is communicated to the peer in the third (I2) and
   fourth (R2) packets of the base HIP exchange.  Changes in SPI are
   signaled with ESP_INFO parameters.

3.3  Security Association Establishment and Maintenance






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3.3.1  ESP Security Associations

   In HIP, ESP Security Associations are setup between the HIP nodes
   during the base exchange [5].  Existing ESP SAs can be updated later
   using UPDATE messages.  The reason for updating the ESP SA later can
   be e.g. need for rekeying the SA because of sequence number rollover.

   Upon setting up a HIP association, each association is linked to two
   ESP SAs, one for incoming packets and one for outgoing packets.  The
   Initiator's incoming SA corresponds with the Responder's outgoing
   one, and vice versa.  The Initiator defines the SPI for the former
   association, as defined in Section 3.2.1.  This SA is called SA-RI,
   and the corresponding SPI is called SPI-RI.  Respectively, the
   Responder's incoming SA corresponds with the Initiator's outgoing SA
   and is called SA-IR, with the SPI being called SPI-IR.

   The Initiator creates SA-RI as a part of R1 processing, before
   sending out the I2, as explained in Section 6.4.  The keys are
   derived from KEYMAT, as defined in Section 7.  The Responder creates
   SA-RI as a part of I2 processing, see Section 6.5.

   The Responder creates SA-IR as a part of I2 processing, before
   sending out R2; see Section 6.5.  The Initiator creates SA-IR when
   processing R2; see Section 6.6.

   The initial session keys are drawn from the generated keying
   material, KEYMAT, after the HIP keys have been drawn as specified in
   [5].

   When the HIP association is removed, the related ESP SAs MUST also be
   removed.

3.3.2  Rekeying

   After the initial HIP base exchange and SA establishment, both hosts
   are in the ESTABLISHED state.  There are no longer Initiator and
   Responder roles and the association is symmetric.  In this
   subsection, the party that initiates the rekey procedure is denoted
   with I' and the peer with R'.

   An existing HIP-created ESP SA may need updating during the lifetime
   of the HIP association.  This document specifies the rekeying of an
   existing HIP-created ESP SA, using the UPDATE message.  The ESP_INFO
   parameter introduced above is used for this purpose.

   I' initiates the ESP SA updating process when needed (see
   Section 6.8).  It creates an UPDATE packet with required information
   and sends it to the peer node.  The old SAs are still in use, local



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   policy permitting.

   R', after receiving and processing the UPDATE (see Section 6.9),
   generates new SAs: SA-I'R' and SA-R'I'.  It does not take the new
   outgoing SA into use, but still uses the old one, so there
   temporarily exists two SA pairs towards the same peer host.  The SPI
   for the new outgoing SA, SPI-R'I', is specified in the received
   ESP_INFO parameter in the UPDATE packet.  For the new incoming SA, R'
   generates the new SPI value, SPI-I'R', and includes it in the
   response UPDATE packet.

   When I' receives a response UPDATE from R', it generates new SAs, as
   described in Section 6.9: SA-I'R' and SA-R'I'.  It starts using the
   new outgoing SA immediately.

   R' starts using the new outgoing SA when it receives traffic on the
   new incoming SA.  After this, R' can remove the old SAs.  Similarly,
   when the I' receives traffic from the new incoming SA, it can safely
   remove the old SAs.

3.3.3  Security Association Management

   An SA pair is indexed by the 2 SPIs and 2 HITs (both local and remote
   HITs since a system can have more than one HIT).  An inactivity timer
   is RECOMMENDED for all SAs.  If the state dictates the deletion of an
   SA, a timer is set to allow for any late arriving packets.

3.3.4  Security Parameter Index (SPI)

   The SPIs in ESP provide a simple compression of the HIP data from all
   packets after the HIP exchange.  This does require a per HIT-pair
   Security Association (and SPI), and a decrease of policy granularity
   over other Key Management Protocols like IKE.

   When a host updates the ESP SA, it provides a new inbound SPI to and
   gets a new outbound SPI from its partner.

3.3.5  Supported Transforms

   All HIP implementations MUST support AES [3] and HMAC-SHA-1-96 [2].
   If the Initiator does not support any of the transforms offered by
   the Responder, it should abandon the negotiation and inform the peer
   with a NOTIFY message about a non-supported transform.

   In addition to AES, all implementations MUST implement the ESP NULL
   encryption algorithm.  When the ESP NULL encryption is used, it MUST
   be used together with SHA1 or MD5 authentication as specified in
   Section 5.1.2



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3.3.6  Sequence Number

   The Sequence Number field is MANDATORY when ESP is used with HIP.
   Anti-replay protection MUST be used in an ESP SA established with
   HIP.  This means that each host MUST rekey before its sequence number
   reaches 2^32, or if extended sequence numbers are used, 2^64.

   In some instances, a 32-bit sequence number is inadequate.  In the
   ESP_TRANSFORM parameter, a peer MAY require that a 64-bit sequence
   numbers be used.  In this case the higher 32 bits are NOT included in
   the ESP header, but are simply kept local to both peers.  The 64-bit
   sequence number is required in fast networks when there is a risk
   that the sequence number will rollover too often.  See [10].

3.3.7  Lifetimes and Timers

   HIP does not negotiate any lifetimes.  All ESP lifetimes are local
   policy.  The only lifetimes a HIP implementation MUST support are
   sequence number rollover (for replay protection), and SHOULD support
   timing out inactive ESP SAs.  An SA times out if no packets are
   received using that SA.  The default timeout value is 15 minutes.
   Implementations MAY support lifetimes for the various ESP transforms.
   Each implementation SHOULD implement per-HIT configuration of the
   inactivity timeout, allowing statically configured HIP associations
   to stay alive for days, even when inactive.


























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4.  The Protocol

   In this section, the protocol for setting up an ESP association to be
   used with HIP association is described.

4.1  ESP in HIP

4.1.1  Setting up an ESP Security Association

   Setting up an ESP Security Association between hosts using HIP
   consists of three messages passed between the hosts.  The parameters
   are included in R1, I2, and R2 messages during base exchange.


                 Initiator                             Responder

                                   I1
                   ---------------------------------->

                             R1: ESP_TRANSFORM
                   <----------------------------------

                       I2: ESP_TRANSFORM, ESP_INFO
                   ---------------------------------->

                               R2: ESP_INFO
                   <----------------------------------


   Setting up an ESP Security Association between HIP hosts requires
   three messages to exchange the information that is required during an
   ESP communication.

   The R1 message contains the ESP_TRANSFORM parameter, in which the
   sending host defines the possible ESP transforms it is willing to use
   for the ESP SA.

   The I2 message contains the response to an ESP_TRANSFORM received in
   the R1 message.  The sender must select one of the proposed ESP
   transforms from the ESP_TRANSFORM parameter in the R1 message and
   include the selected one in the ESP_TRANSFORM parameter in the I2
   packet.  In addition to the transform, the host includes the ESP_INFO
   parameter, containing the SPI value to be used by the peer host.

   In the R2 message, the ESP SA setup is finalized.  The packet
   contains the SPI information required by the Initiator for the ESP
   SA.




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4.1.2  Updating an Existing ESP SA

   The update process is accomplished using two messages.  The HIP
   UPDATE message is used to update the parameters of an existing ESP
   SA.  The UPDATE mechanism and message is defined in [5] and the
   additional parameters for updating an existing ESP SA are described
   here.


       H1                                                          H2
            UPDATE: ESP_INFO [, DIFFIE_HELLMAN]
          ----------------------------------------------------->

            UPDATE: ESP_INFO [, DIFFIE_HELLMAN]
          <-----------------------------------------------------

   Not shown in the above figures are the corresponding SEQ and ACK
   parameters; at a minimum, the exchange shown above would include a
   SEQ parameter in the first packet shown, both a SEQ and an ACK in the
   second packet, and a third packet containing only an ACK.

   The host willing to update the ESP SA creates and sends an UPDATE
   message.  The message contains the ESP_INFO parameter, containing the
   old SPI value that was used, the new SPI value to be used, and the
   index value for the keying material, giving the point from where the
   next keys will be drawn.  If new keying material must be generated,
   the UPDATE message will also contain the DIFFIE_HELLMAN parameter,
   defined in [5].

   The host receiving the UPDATE message requesting update of an
   existing ESP SA, MUST reply with an UPDATE message.  In the reply
   message, the host sends the ESP_INFO parameter containing the
   corresponding values: old SPI, new SPI, and the keying material
   index.  If the incoming UPDATE contained a DIFFIE_HELLMAN parameter,
   the reply packet MUST also contain a DIFFIE_HELLMAN parameter.
















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5.  Parameter and Packet Formats

   In this section, new and modified HIP parameters are presented, as
   well as modified HIP packets.

5.1  New Parameters

   Two new HIP parameters are defined for setting up ESP transport
   format associations in HIP communication and for rekeying existing
   ones.  Also, the NOTIFY parameter, described in [5], has two new
   error parameters.

      Parameter         Type  Length     Data

      ESP_INFO          65    12         Remote's old SPI,
                                         new SPI and other info
      ESP_TRANSFORM     2048  variable   ESP Encryption and
                                         Authentication Transform(s)


5.1.1  ESP_INFO

   During the establishment and update of an ESP SA, the SPI value of
   both hosts must be transmitted between the hosts.  Additional
   information that is required when the hosts are drawing keys from the
   generated keying material is the index value into the KEYMAT from
   where the keys are drawn.  The ESP_INFO parameter is used to transmit
   this information between the hosts.

   During the initial ESP SA setup, the hosts send the SPI value that
   they want the peer to use when sending ESP data to them.  The value
   is set in the New SPI field of the ESP_INFO parameter.  In the
   initial setup, an old value for the SPI does not exist, thus the Old
   SPI value field is set to zero.  The Old SPI field value may also be
   zero when additional SAs are set up between HIP hosts, e.g. in case
   of multihomed HIP hosts [6].  However, such use is beyond the scope
   of this specification.

   The Keymat index value points to the place in the KEYMAT from where
   the keying material for the ESP SAs is drawn.  The Keymat index value
   is zero only when the ESP_INFO is sent during a rekeying process and
   new keying material is generated.

   During the life of an SA established by HIP, one of the hosts may
   need to reset the Sequence Number to one and rekey.  The reason for
   rekeying might be an approaching sequence number wrap in ESP, or a
   local policy on use of a key.  Rekeying ends the current SAs and
   starts new ones on both peers.



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   During the rekeying process, the ESP_INFO parameter is used to
   transmit the changed SPI values and the keying material index.



       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Reserved            |         Keymat Index          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Old SPI                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            New SPI                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           65
      Length         12
      Keymat Index   Index, in bytes, where to continue to draw ESP keys
                     from KEYMAT.  If the packet includes a new
                     Diffie-Hellman key and the ESP_INFO is sent in an
                     UPDATE packet, the field MUST be zero.  If the
                     ESP_INFO is included in base exchange messages, the
                     Keymat Index must have the index value of the point
                     from where the ESP SA keys are drawn. Note that the
                     length of this field limits the amount of
                     keying material that can be drawn from KEYMAT.  If
                     that amount is exceeded, the packet MUST contain
                     a new Diffie-Hellman key.
      Old SPI        Old SPI for data sent to address(es) associated
                     with this SA. If this is an initial SA setup, the
                     Old SPI value is zero.
      New SPI        New SPI for data sent to address(es) associated
                     with this SA."


5.1.2  ESP_TRANSFORM

   The ESP_TRANSFORM parameter is used during ESP SA establishment.  The
   first party sends a selection of transfrom families in the
   ESP_TRANSFORM parameter and the peer must select one of the proposed
   values and include it in the response ESP_TRANSFORM parameter.








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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Reserved           |E|           Suite-ID #1         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Suite-ID #2          |           Suite-ID #3         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Suite-ID #n          |             Padding           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type           2048
         Length         length in octets, excluding Type, Length, and
                        padding
         E              One if the ESP transform requires 64-bit
                        sequence numbers
                        (see
   Section 3.3.6

         Reserved       zero when sent, ignored when received
         Suite-ID       defines the ESP Suite to be used

   The following Suite-IDs are defined ([7],[9]):

            Suite-ID                          Value

            RESERVED                          0
            ESP-AES-CBC with HMAC-SHA1        1
            ESP-3DES-CBC with HMAC-SHA1       2
            ESP-3DES-CBC with HMAC-MD5        3
            ESP-BLOWFISH-CBC with HMAC-SHA1   4
            ESP-NULL with HMAC-SHA1           5
            ESP-NULL with HMAC-MD5            6

   There MUST NOT be more than six (6) ESP Suite-IDs in one
   ESP_TRANSFORM parameter.  The limited number of Suite-IDs sets the
   maximum size of ESP_TRANSFORM parameter.  The ESP_TRANSFORM MUST
   contain at least one of the mandatory Suite-IDs.

   Mandatory implementations: ESP-AES-CBC with HMAC-SHA1 and ESP-NULL
   with HMAC-SHA1.

5.1.3  NOTIFY Parameter

   The HIP base specification defines a set of NOTIFY error types.  The
   following error types are required for describing errors in ESP
   Transform crypto suites during negotiation.



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         NOTIFY PARAMETER - ERROR TYPES           Value
         ------------------------------           -----

         NO_ESP_PROPOSAL_CHOSEN                    18

            None of the proposed ESP Transform crypto suites was
            acceptable.

         INVALID_ESP_TRANSFORM_CHOSEN              19

            The ESP Transform crypto suite does not correspond to
            one offered by the responder.



5.2  HIP ESP Security Association Setup

   The ESP Security Association is set up during the base exchange.  The
   following subsections define the ESP SA setup procedure both using
   base exchange messages (R1, I2, R2) and using UPDATE messages.

5.2.1  Setup During Base Exchange

5.2.1.1  Modifications in R1

   The ESP_TRANSFORM contains the ESP modes supported by the sender, in
   the order of preference.  All implementations MUST support AES [3]
   with HMAC-SHA-1-96 [2].

   The following figure shows the resulting R1 packet layout.

      The HIP parameters for the R1 packet:

      IP ( HIP ( [ R1_COUNTER, ]
                 PUZZLE,
                 DIFFIE_HELLMAN,
                 HIP_TRANSFORM,
                 ESP_TRANSFORM,
                 HOST_ID,
                 [ ECHO_REQUEST, ]
                 HIP_SIGNATURE_2 )
                 [, ECHO_REQUEST ])


5.2.1.2  Modifications in I2

   The ESP_INFO contains the sender's SPI for this association as well
   as the keymat index from where the ESP SA keys will be drawn.  The



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   Old SPI value is set to zero.

   The ESP_TRANSFORM contains the ESP mode selected by the sender of R1.
   All implementations MUST support AES [3] with HMAC-SHA-1-96 [2].

   The following figure shows the resulting I2 packet layout.

      The HIP parameters for the I2 packet:

      IP ( HIP ( ESP_INFO,
                 [R1_COUNTER,]
                 SOLUTION,
                 DIFFIE_HELLMAN,
                 HIP_TRANSFORM,
                 ESP_TRANSFORM,
                 ENCRYPTED { HOST_ID },
                 [ ECHO_RESPONSE ,]
                 HMAC,
                 HIP_SIGNATURE
                 [, ECHO_RESPONSE] ) )


5.2.1.3  Modifications in R2

   The R2 contains an ESP_INFO parameter, which has the SPI value of the
   sender of the R2 for this association.  The ESP_INFO also has the
   keymat index value specifying where the ESP SA keys are drawn.

   The following figure shows the resulting R2 packet layout.

      The HIP parameters for the R2 packet:

      IP ( HIP ( ESP_INFO, HMAC_2, HIP_SIGNATURE ) )


5.3  HIP ESP Rekeying

   In this section, the procedure for rekeying an existing ESP SA is
   presented.

5.3.1  Initializing Rekeying

   When HIP is used with ESP, the UPDATE packet is used to initiate
   rekeying.  The UPDATE packet MUST carry an ESP_INFO and MAY carry a
   DIFFIE_HELLMAN parameter.

   Intermediate systems that use the SPI will have to inspect HIP
   packets for those that carry rekeying information.  The packet is



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   signed for the benefit of the intermediate systems.  Since
   intermediate systems may need the new SPI values, the contents cannot
   be encrypted.

   The following figure shows the contents of a rekeying initialization
   UPDATE packet.

      The HIP parameters for the UPDATE packet initiating rekeying:

      IP ( HIP ( ESP_INFO,
                 SEQ,
                 [DIFFIE_HELLMAN, ]
                 HMAC,
                 HIP_SIGNATURE ) )


5.3.2  Responding to the Rekeying Initialization

   The UPDATE ACK is used to acknowledge the received UPDATE rekeying
   initialization.  The acknowledgement UPDATE packet MUST carry an
   ESP_INFO and MAY carry a DIFFIE_HELLMAN parameter.

   Intermediate systems that use the SPI will have to inspect HIP
   packets for packets carrying rekeying information.  The packet is
   signed for the benefit of the intermediate systems.  Since
   intermediate systems may need the new SPI values, the contents cannot
   be encrypted.

   The following figure shows the contents of a rekeying acknowledgement
   UPDATE packet.

      The HIP parameters for the UPDATE packet:

      IP ( HIP ( ESP_INFO,
                 SEQ,
                 ACK,
                 [ DIFFIE_HELLMAN, ]
                 HMAC,
                 HIP_SIGNATURE ) )


5.4  ICMP Messages

   The ICMP message handling is mainly described in the HIP base
   specification [5].  In this section, we describe the actions related
   to ESP security associations.





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5.4.1  Unknown SPI

   If a HIP implementation receives an ESP packet that has an
   unrecognized SPI number, it MAY respond (subject to rate limiting the
   responses) with an ICMP packet with type "Parameter Problem", with
   the Pointer pointing to the the beginning of SPI field in the ESP
   header.












































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6.  Packet Processing

   Packet processing is mainly defined in the HIP base specification
   [5].  This section describes the changes and new requirements for
   packet handling when the ESP transport format is used.  Note that all
   HIP packets (currently protocol 99) MUST bypass ESP processing.

6.1  Processing Outgoing Application Data

   Outgoing application data handling is specified in the HIP base
   specification [5].  When ESP transport format is used, and there is
   an active HIP session for the given < source, destination > HIT pair,
   the outgoing datagram is protected using the ESP security
   association.  In a typical implementation, this will result in a
   BEET-mode ESP packet being sent.  BEET-mode [12] was introduced above
   in Section 3.2.

   1.  Detect the proper ESP SA using the HITs in the packet header or
       other information associated with the packet

   2.  Process the packet normally, as if the SA was a transport mode
       SA.

   3.  Ensure that the outgoing ESP protected packet has proper IP
       addresses in its IP header, e.g., by replacing HITs left by the
       ESP processing.  Note that this placement of proper IP addresses
       MAY also be performed at some other point in the stack, e.g.,
       before ESP processing.


6.2  Processing Incoming Application Data

   Incoming HIP user data packets arrive as ESP protected packets.  In
   the usual case the receiving host has a corresponding ESP security
   association, identified by the SPI and destination IP address in the
   packet.  However, if the host has crashed or otherwise lost its HIP
   state, it may not have such an SA.

   The basic incoming data handling is specified in the HIP base
   specification.  Additional steps are required when ESP is used for
   protecting the data traffic.  The following steps define the
   conceptual processing rules for incoming ESP protected datagrams
   targeted to an ESP security association created with HIP.

   1.  Detect the proper ESP SA using the SPI.  If the resulting SA is a
       non-HIP ESP SA, process the packet according to standard IPsec
       rules.  If there are no SAs identified with the SPI, the host MAY
       send an ICMP packet as defined in Section 5.4.  How to handle



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       lost state is an implementation issue.

   2.  If the SPI matches with an active HIP-based ESP SA, the IP
       addresses in the datagram are replaced with the HITs associated
       with the SPI.  Note that this IP-address-to-HIT conversion step
       MAY also be performed at some other point in the stack, e.g.,
       after ESP processing.  Note also that if the incoming packet has
       IPv4 addresses, the packet must be converted to IPv6 format
       before replacing the addresses with HITs (such that the transport
       checksum will pass if there are no errors).

   3.  The transformed packet is next processed normally by ESP, as if
       the packet were a transport mode packet.  The packet may be
       dropped by ESP, as usual.  In a typical implementation, the
       result of successful ESP decryption and verification is a
       datagram with the associated HITs as source and destination.

   4.  If LSIs are used, the address field is converted to contain the
       LSI value before the packet is sent to the application.

   5.  The datagram is delivered to the upper layer.  Demultiplexing the
       datagram to the right upper layer socket is performed as usual,
       except that the HITs are used in place of IP addresses during the
       demultiplexing.


6.3  HMAC and SIGNATURE Calculation and Verification

   The new HIP parameters described in this document, ESP_INFO and
   ESP_TRANSFORM, must be protected using HMAC and signature
   calculations.  In a typical implementation, they are included in R1,
   I2, R2, and UPDATE packet HMAC and SIGNATURE calculations as
   described in [5].

6.4  Processing Incoming ESP SA Initialization (R1)

   The ESP SA setup is initialized in the R1 message.  The receiving
   host (Initiator) select one of the ESP transforms from the presented
   values.  If no suitable value is found, the negotiation is
   terminated.  The selected values are subsequently used when
   generating and using encryption keys, and when sending the reply
   packet.  If the proposed alternatives are not acceptable to the
   system, it may abandon the ESP SA establishment negotiation, or it
   may resend the I1 message within the retry bounds.

   After selecting the ESP transform, and performing other R1
   processing, the system prepares and creates an incoming ESP security
   association.  It may also prepare a security association for outgoing



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   traffic, but since it does not have the correct SPI value yet, it
   cannot activate it.

6.5  Processing Incoming Initialization Reply (I2)

   The following steps are required to process the incoming ESP SA
   initialization replies in I2.  The steps below assume that the I2 has
   been accepted for processing (e.g., has not been dropped due to HIT
   comparisons as described in [5]).

   o  The ESP_TRANSFORM parameter is verified and it MUST contain a
      single value in the parameter and it MUST match one of the values
      offered in the initialization packet.

   o  The ESP_INFO New SPI field is parsed to obtain the SPI that will
      be used for the Security Association outbound from the Responder
      and inbound to the Initiator.  For this initial ESP SA
      establishment, the Old SPI value MUST be zero.  The Keymat Index
      field MUST contain the index value to the KEYMAT from where the
      ESP SA keys are drawn.

   o  The system prepares and creates both incoming and outgoing ESP
      security associations.

   o  Upon successful processing of the initialization reply message,
      the possible old Security Associations (as left over from an
      earlier incarnation of the HIP association) are dropped and the
      new ones are installed, and a finalizing packet, R2, is sent.
      Possible ongoing rekeying attempts are dropped.


6.6  Processing Incoming ESP SA Setup Finalization (R2)

   Before the ESP SA can be finalized, the ESP_INFO New SPI field is
   parsed to obtain the SPI that will be used for the ESP Security
   Association inbound to the sender of the finalization message R2.
   The system uses this SPI to create or activate the outgoing ESP
   security association used for sending packets to the peer.

6.7  Dropping HIP Associations

   When the system drops a HIP association, as described in the HIP base
   specification, the associated ESP SAs MUST also be dropped.

6.8  Initiating ESP SA Rekeying

   During ESP SA rekeying, the hosts draw new keys from the existing
   keying material, or a new keying material is generated from where the



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   new keys are drawn.

   A system may initiate the SA rekeying procedure at any time.  It MUST
   initiate a rekey if its incoming ESP sequence counter is about to
   overflow.  The system MUST NOT replace its keying material until the
   rekeying packet exchange successfully completes.

   Optionally, a system may include a new Diffie-Hellman key for use in
   new KEYMAT generation.  New KEYMAT generation occurs prior to drawing
   the new keys.

   The rekeying procedure uses the UPDATE mechanism defined in [5].
   Because each peer must update its half of the security association
   pair (including new SPI creation), the rekeying process requires that
   each side both send and receive an UPDATE.  A system will then rekey
   the ESP SA when it has sent parameters to the peer and has received
   both an ACK of the relevant UPDATE message and corresponding peer's
   parameters.  It may be that the ACK and the required HIP parameters
   arrive in different UPDATE messages.  This is always true if a system
   does not initiate ESP SA update but responds to an update request
   from the peer, but may also occur if two systems initiate update
   nearly simultaneously.  In such a case, if the system has an
   outstanding update request, it saves the one parameter and waits for
   the other before completing rekeying.

   The following steps define the processing rules for initiating an ESP
   SA update:

   1.  The system decides whether to continue to use the existing KEYMAT
       or to generate new KEYMAT.  In the latter case, the system MUST
       generate a new Diffie-Hellman public key.

   2.  The system creates an UPDATE packet, which contains the ESP_INFO
       parameter.  In addition, the host may include the optional
       DIFFIE_HELLMAN parameter.  If the UDPATE contains the
       DIFFIE_HELLMAN parameter, the Keymat Index in the ESP_INFO
       parameter MUST be zero, and the Diffie-Hellman group ID must be
       unchanged from that used in the initial handshake.  If the UPDATE
       does not contain DIFFIE_HELLMAN, the ESP_INFO Keymat Index MUST
       be greater or equal to the index of the next byte to be drawn
       from the current KEYMAT.

   3.  The system sends the UPDATE packet.  For reliability, the
       underlying UPDATE retransmission mechanism SHOULD be used.

   4.  The system MUST NOT delete its existing SAs, but continue using
       them if its policy still allows.  The rekeying procedure SHOULD
       be initiated early enough to make sure that the SA replay



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       counters do not overflow.

   5.  In case a protocol error occurs and the peer system acknowledges
       the UPDATE but does not itself send an ESP_INFO, the system may
       not finalize the outstanding ESP SA update request.  To guard
       against this, a system MAY re-initiate the ESP SA update
       procedure after some time waiting for the peer to respond, or it
       MAY decide to abort the ESP SA after waiting for an
       implementation-dependent time.  The system MUST NOT keep an
       oustanding ESP SA update request for an indefinite time.

   To simplify the state machine, a host MUST NOT generate new UPDATEs
   while it has an outstanding ESP SA update request, unless it is
   restarting the update process.

6.9  Processing Incoming UPDATE Packets

   When a system receives an UPDATE packet, it must be processed if the
   following conditions hold:

   1.  A corresponding HIP association must exist.  This is usually
       ensured by the underlying UPDATE mechanism.

   2.  The state of the HIP association is ESTABLISHED or R2-SENT.

   If the above conditions hold, the following steps define the
   conceptual processing rules for handling the received UPDATE packet:

   1.  If the received UPDATE contains a DIFFIE_HELLMAN parameter, the
       received Keymat Index MUST be zero and the Group ID must match
       the Group ID in use on the association.  If this test fails, the
       packet SHOULD be dropped and the system SHOULD log an error
       message.

   2.  If there is no outstanding rekeying request, the packet
       processing continues as specified in Section 6.9.1.

   3.  If there is an outstanding rekeying request, the UPDATE MUST be
       acknowledged, the received ESP_INFO (and possibly DIFFIE_HELLMAN)
       parameters must be saved, and the packet processing continues as
       specified in Section 6.10.


6.9.1  Processing UPDATE Packet: No  Outstanding Rekeying Request

   The following steps define the conceptual processing rules for
   handling a received UPDATE packet with ESP_INFO parameter:




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   1.  The system consults its policy to see if it needs to generate a
       new Diffie-Hellman key, and generates a new key (with same Group
       ID) if needed.  The system records any newly generated or
       received Diffie-Hellman keys, for use in KEYMAT generation upon
       finalizing the ESP SA update.

   2.  If the system generated new Diffie-Hellman key in the previous
       step, or it received a DIFFIE_HELLMAN parameter, it sets ESP_INFO
       Keymat Index to zero.  Otherwise, the ESP_INFO Keymat Index MUST
       be greater or equal to the index of the next byte to be drawn
       from the current KEYMAT.  In this case, it is RECOMMENDED that
       the host use the Keymat Index requested by the peer in the
       received ESP_INFO.

   3.  The system creates an UPDATE packet, which contains an ESP_INFO
       parameter, and the optional DIFFIE_HELLMAN parameter.

   4.  The system sends the UPDATE packet and stores any received
       ESP_INFO, and DIFFIE_HELLMAN parameters.  At this point, it only
       needs to receive an acknowledgement for the sent UPDATE to finish
       ESP SA update.  In the usual case, the acknowledgement is handled
       by the underlying UPDATE mechanism.


6.10  Finalizing Rekeying

   A system finalizes rekeying when it has both received the
   corresponding UPDATE acknowledgement packet from the peer and it has
   successfully received the peer's UPDATE.  The following steps are
   taken:

   1.  If the received UPDATE messages contains a new Diffie-Hellman
       key, the system has a new Diffie-Hellman key from initiating ESP
       SA update, or both, the system generates new KEYMAT.  If there is
       only one new Diffie-Hellman key, the old existing key is used as
       the other key.

   2.  If the system generated new KEYMAT in the previous step, it sets
       Keymat Index to zero, independent on whether the received UPDATE
       included a Diffie-Hellman key or not.  If the system did not
       generate new KEYMAT, it uses the greater Keymat Index of the two
       (sent and received) ESP_INFO parameters.

   3.  The system draws keys for new incoming and outgoing ESP SAs,
       starting from the Keymat Index, and prepares new incoming and
       outgoing ESP SAs.  The SPI for the outgoing SA is the new SPI
       value received in an ESP_INFO parameter.  The SPI for the
       incoming SA was generated when the ESP_INFO was sent to the peer.



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       The order of the keys retrieved from the KEYMAT during rekeying
       process is similar to that described in Section 7.  Note, that
       only IPsec ESP keys are retrieved during rekeying process, not
       the HIP keys.

   4.  The system cancels any timers protecting the UPDATE.

   5.  The system starts to send to the new outgoing SA and prepares to
       start receiving data on the new incoming SA.


6.11  Processing NOTIFY Packets

   The processing of NOTIFY packets is described in the HIP base
   specification.




































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7.  Keying Material

   The keying material is generated as described in the HIP base
   specification.  During the base exchange, the initial keys are drawn
   from the generated material.  After the HIP association keys have
   been drawn, the ESP keys are drawn in the following order:

      SA-gl ESP encryption key for HOST_g's outgoing traffic

      SA-gl ESP authentication key for HOST_g's outgoing traffic

      SA-lg ESP encryption key for HOST_l's outgoing traffic

      SA-lg ESP authentication key for HOST_l's outgoing traffic

   The four HIP keys are only drawn from KEYMAT during a HIP I1->R2
   exchange.  Subsequent rekeys using UPDATE will only draw the four ESP
   keys from KEYMAT.  Section 6.9 describes the rules for reusing or
   regenerating KEYMAT based on the rekeying.

   The number of bits drawn for a given algorithm is the "natural" size
   of the keys.  For the mandatory algorithms, the following sizes
   apply:

   AES 128 bits

   SHA-1 160 bits

   NULL 0 bits






















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8.  Security Considerations

   In this document the usage of ESP [4] between HIP hosts to protect
   data traffic is introduced.  The Security Considerations for ESP are
   discussed in the ESP specification.

   There are different ways to establish an ESP Security Association
   between two nodes.  This can be done, e.g. using IKE [[11]].  This
   document specifies how Host Identity Protocol is used to establish
   ESP Security Associations.

   The following issues are new, or changed from the standard ESP usage:

   o  Initial keying material generation

   o  Updating the keying material

   o  Using the BEET mode

   The initial keying material is generated using the Host Identity
   Protocol [5] using Diffie-Hellman procedure.  This document extends
   the usage of UDPATE packet, defined in the base specification, to
   modify existing ESP SAs.  The hosts may rekey, i.e. force the
   generation of new keying material using Diffie-Hellman procedure.
   The initial setup of ESP SA between the hosts is done during the base
   ecxhange and the message exchange is protected with using methods
   provided by base exchange.  Changing of connection parameters means
   basically that the old ESP SA is removed and a new one is generated
   once the UPDATE message exchange has been completed.  The message
   exchange is protected using the HIP association keys.  Both HMAC and
   signing of packets is used.

   IPsec ESP defines two modes of operation: tunnel mode and transport
   mode.  This document takes advantage of the so called Bound End-to-
   End Tunneling (BEET) [12], that is a combination of tunnel and
   transport modes.  The packet looks like a transport mode packet, but
   the semantics is like in tunnel mode packets.  The security issues
   are discussed in the Security Considerations section in the BEET
   specification.












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9.  IANA Considerations

   This document defines additional parameters for the Host Identity
   Protocol [5].  These parameters are defined in Section 5.1.1 and
   Section 5.1.2 with the following numbers:

   o  ESP_INFO is 65.

   o  ESP_TRANSFORM is 2048.










































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10.  References

10.1  Normative references

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

   [2]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP
        and AH", RFC 2404, November 1998.

   [3]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
        Algorithm and Its Use with IPsec", RFC 3602, September 2003.

   [4]  Kent, S., "IP Encapsulating Security Payload (ESP)",
        draft-ietf-ipsec-esp-v3-05 (work in progress), April 2003.

   [5]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-00
        (work in progress), June 2004.

   [6]  Nikander, P., "End-Host Mobility and Multi-Homing with Host
        Identity Protocol", draft-ietf-hip-mm-00 (work in progress),
        October 2004.

   [7]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
        draft-ietf-ipsec-ikev2-07 (work in progress), April 2003.

   [8]  Moskowitz, R., "Host Identity Protocol Architecture",
        draft-ietf-hip-arch-01 (work in progress), December 2004.

10.2  Informative references

   [9]   Bellovin, S. and W. Aiello, "Just Fast Keying (JFK)",
         draft-ietf-ipsec-jfk-04 (work in progress), July 2002.

   [10]  Kent, S., "Security Architecture for the Internet Protocol",
         draft-ietf-ipsec-rfc2401bis-00 (work in progress),
         October 2003.

   [11]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [12]  Nikander, P., "A Bound End-to-End Tunnel (BEET) mode for ESP",
         draft-nikander-esp-beet-mode-00 (expired) (work in progress),
         Oct 2003.







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Authors' Addresses

   Petri Jokela
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   Email: petri.jokela@nomadiclab.com


   Robert Moskowitz
   ICSAlabs, a Division of TruSecure Corporation
   1000 Bent Creek Blvd, Suite 200
   Mechanicsburg, PA
   USA

   Email: rgm@icsalabs.com


   Pekka Nikander
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   Email: pekka.nikander@nomadiclab.com
























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Appendix A.  A Note on Implementation Options

   It is possible to implement this specification in multiple different
   ways.  As noted above, one possible way of implementing is to rewrite
   IP headers below IPsec.  In such an implementation, IPsec is used as
   if it was processing IPv6 transport mode packets, with the IPv6
   header containing HITs instead of IP addresses in the source and
   destionation address fields.  In outgoing packets, after IPsec
   processing, the HITs are replaced with actual IP addresses, based on
   the HITs and the SPI.  In incoming packets, before IPsec processing,
   the IP addresses are replaced with HITs, based on the SPI in the
   incoming packet.  In such an implementation, all IPsec policies are
   based on HITs and the upper layers only see packets with HITs in the
   place of IP addresses.  Consequently, support of HIP does not
   conflict with other use of IPsec as long as the SPI spaces are kept
   separate.

   Another way for implementing is to use the proposed BEET mode (A
   Bound End-to-End mode for ESP) [12].  The BEET mode provides some
   features from both IPsec tunnel and transport modes.  The HIP uses
   HITs as the "inner" addresses and IP addresses as "outer" addresses
   like IP addresses are used in the tunnel mode.  Instead of tunneling
   packets between hosts, a conversion between inner and outer addresses
   is made at end-hosts and the inner address is never sent in the wire
   after the initial HIP negotiation.  BEET provides IPsec transport
   mode syntax (no inner headers) with limited tunnel mode semantics
   (fixed logical inner addresses - the HITs - and changeable outer IP
   addresses).

   Compared to the option of implementing the required address rewrites
   outside of IPsec, BEET has one implementation level benefit.  The
   BEET-way of implementing the address rewriting keeps all the
   configuration information in one place, at the SADB.  On the other
   hand, when address rewriting is implemented separately, the
   implementation must make sure that the information in the SADB and
   the separate address rewriting DB are kept in synchrony.  As a
   result, the BEET mode based way of implementing is RECOMMENDED over
   the separate implementation.













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Internet-Draft     Using ESP transport format with HIP         June 2005


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