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Network Working Group                                           J. Melen
Internet-Draft                                                J. Ylitalo
Expires: January 28, 2009                                     P. Salmela
                                            Ericsson Research NomadicLab
                                                           July 27, 2008


    Security Parameter Index multiplexed Network Address Translation
                                (SPINAT)
                         draft-melen-spinat-01

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 January 28, 2009.

Copyright Notice

   Copyright (C) The IETF Trust (2008).












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Abstract

   This drafts defines a Security Parameter Index multiplexed Network
   Address Translator (SPINAT).  The SPINAT method uses the SPI value of
   ESP packets to de-multiplex multiple IP addresses on single IP
   address.  The solution presented in this draft requires a state in
   the SPINAT node and in the peer node.  The state establishment
   requires a control signaling messages carrying the SPI values.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  State Establishment  . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  State Establishment at SPINAT Node . . . . . . . . . . . .  6
     3.2.  State Establishment at End-Hosts . . . . . . . . . . . . .  7
   4.  Packet Processing  . . . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Control Signaling packet handling  . . . . . . . . . . . .  8
     4.2.  ESP packet processing  . . . . . . . . . . . . . . . . . .  9
       4.2.1.  IP Address and SPI Translation at SPINAT Nodes . . . .  9
       4.2.2.  SPI Translation at End-hosts . . . . . . . . . . . . . 10
   5.  Parameters and packet formats  . . . . . . . . . . . . . . . . 11
     5.1.  New Parameters . . . . . . . . . . . . . . . . . . . . . . 11
       5.1.1.  NAT_ESP_INFO . . . . . . . . . . . . . . . . . . . . . 11
     5.2.  HIP ESP Security Association Setup . . . . . . . . . . . . 11
       5.2.1.  Modifications in I2  . . . . . . . . . . . . . . . . . 11
     5.3.  HIP ESP Rekeying . . . . . . . . . . . . . . . . . . . . . 12
       5.3.1.  Initializing Rekeying  . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18














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1.  Introduction

   The IP Network Address Translator (Traditional NAT) functionality is
   explained in [RFC3022].  From the IP address translation point of
   view, it is possible to compare the traditional Network Address Port
   Translation (NAPT) method to SPINAT method, i.e. presented in this
   draft.  A SPINAT node connects a private link to a WAN link, like a
   NAPT node does.  The NAPT method uses port values in TCP/UDP headers
   for multiplexing IP addresses, while the SPINAT method uses Security
   Parameter Index (SPI) values in the ESP [RFC4303] headers for that
   purpose.

   There are few differences in the operation of SPINAT compared to
   NAPT.

      In NAPT the port values in the TCP/UDP headers are not integrity
      protected, unlike in the SPINAT case where the SPI values in ESP
      headers are integrity protected.  From the SPI translation point
      of view this is a problem, because the related ESP integrity
      protection keys are only shared between the end-points, not with
      the SPINAT nodes.  Therefore, the SPINAT nodes cannot
      transparently translate SPI values like traditional NAPT nodes
      translate port values.  To sustain the integrity of ESP headers
      and to support SPI translation, the SPINAT nodes MUST inform the
      sending end-hosts about the translation, unlike the NAPT method
      about the port translation.

      When the Security Associations are setup between the end-hosts,
      the end-hosts will use a separate control signaling to negotiate
      the SPI values to be used, unlike in NAPT case where the
      destination port values are well-known and source ports may be
      randomly selected and modified.  Additionally, the ESP header
      carries only the destination SPI value, thus a separate control
      signaling (key-exchange) is needed for state establishment at the
      SPINAT node, instead of using only ESP packets for state
      establishment.

   The SPINAT node that is in the forwarding path of the two peer nodes
   will create its state in two steps.  The first step is to create a
   soft-state for the SPI-to-IP address mapping for ESP payload traffic
   based on the control signaling (key-exchange).  However, the SPINAT
   method does not require explicit state establishment exchange between
   SPINAT node and the end-hosts.  Therefore, the SPINAT method does not
   increase the amount of signaling compared to a situation when there
   does not exist SPINAT nodes on the packet forwarding path.  The
   SPINAT node intercepts a control signaling message received from a
   private link that carries a SPI value.  The SPINAT adds a Type-
   Length-Value (TLV) field containing the SPI mapping information at



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   the end of the intercepted message before forwarding the message to
   the WAN link.  Next, the state is completed to a hard-state after
   receiving the first ESP payload packet that carries the SPI
   corresponding to the SPI-to-IP address mapping.

   The SPINAT operation does not require any modifications to the ESP
   processing at the host in the private network, but requires a
   modification at the peer host that is in the WAN side to allow the
   SPINAT node to re-write the SPI value on the received ESP packets.
   The original SPI value is selected by the receiving end-host in the
   private IP network, and the value is replaced by the SPINAT node with
   another SPI value.  As a result of the key-exchange, both the SPINAT
   node and the peer host establish a translation state.  The end-host
   implements the same SPI mapping as SPINAT node for integrity
   protected ESP packets, but in a reverse order.  The SPI translation
   MUST be made after the ESP integrity protection is computed using the
   original SPI values.


































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2.  Terminology

   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].

   client node
      The node residing in the private network.

   SPINAT node
      The SPI based Network Address Translation node.

   Peer node
      The node residing in the public network.





































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3.  State Establishment

3.1.  State Establishment at SPINAT Node

   This section describes the state establishment and SA mapping setup
   in the SPINAT node.  After the state has been setup the SPINAT node
   is ready to forward the packets between the peer nodes.


     +--------+                    +--------+                 +--------+
     |   Peer |                    | SPINAT |                 | client |
     +--------+                    +--------+                 +--------+
          |                             |                          |
          |            {SPI=3030-->1212}|                {SPI=3030}|
          |                            \|                         \|
          |<-Key-exchange message-------|--------------------------|
          |                             |                          |
          |                             |                          |
          |{SPI=4545}                   |                          |
          |/                            |                          |
          |-Key-exchange message------->|------------------------->|



      Figure 1: A round-trip of a key-exchange containing SPI values

   Figure 1 illustrates control signaling that contains the SPI values.
   As a result the SPINAT node creates translation state for the
   security associations (SAs).  The SPI needs to be translated only on
   the direction from peer to client because only in that directions
   there is a possibility that two or more clients behind the SPINAT
   node select the same SPI value for incoming ESP packets causing a SPI
   collision.


     +--------+                    +--------+                 +--------+
     |   Peer |                    | SPINAT |                 | client |
     +--------+                    +--------+                 +--------+
          |                             |                          |
          |(SA-out)       (SA-in-public)|(SA-out-private)   (SA-in)|
          |/          ESP              \|/                        \|
          |---------------------------->|------------------------->|
          |                             |                          |
          |                             |                          |
          |(SA-in)       (SA-out-public)|(SA-in-private)   (SA-out)|
          |/           ESP             \|/                        \|
          |<----------------------------|--------------------------|




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             Figure 2: Naming the Security Associations (SAs)

   The translation state contains a incoming SA and a outgoing SA.  This
   pair contains the information needed to forward the packet through
   the SPINAT node.  The information needed in the state is illustrated
   in the Figure 2

   The state on the direction from peer to client contains mapping for
   SPI value on the public side to a SPI value on the private side and
   the private address of the client.

   The SPI value selected for the SA-in-public MUST be unique for each
   ESP incoming association.  The SPINAT node selects the SPI value for
   each connection that goes through the SPINAT node.  The selected SPI
   value is carried in the control signaling message to the peer node.

   The state on the direction from client to peer contains mapping for
   client private address to the public address of SPINAT.  In this
   direction there is no need to translate the SPI.

   The SPI value used in the SA-out-public is selected by the peer node
   and thus it does not have to be translated.  The SPINAT node is able
   to use on both sides the same SPI value.

3.2.  State Establishment at End-Hosts

   This section describes the state needed in the end-hosts to support
   SPINAT translations.  Both End-Hosts create one incoming and outgoing
   SAs as can be seen from Figure 2.

   On the client node that resides behind the SPINAT node in the private
   network there is no additional state information that should be
   stored.  The client node operates as it would operate in other
   network and stores the same state data as it would store for any
   other SA.

   The peer node on the WAN side has to store the SPI mapping
   information that it has received from the SPINAT node in the control
   signaling.  As the SA is created to peer node it needs to store both
   the SPI value given by the client and the SPI value given by the
   SPINAT node to the SA.  The SPI value given by the client will be
   used when constructing the ESP packet and calculating message
   authentication code.  This SPI value will later be replaced by the
   SPI value given by the SPINAT node.







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4.  Packet Processing

4.1.  Control Signaling packet handling

   This section describes the packet processing of the control signaling
   packets.

   When a control packet is received from private network following
   processing steps will be done:

   1.  Upon receipt of a control signaling packet from private network,
       the SPINAT node parses the packet.

   2.  If there is no state for this pair of end-hosts it will create a
       state that is used to forward the control signaling packets
       through the SPINAT node.

   3.  If the packet contains a SPI value the SPINAT node MUST select a
       unique SPI from its free SPI space.  If the packet didn't contain
       SPI value it will be forwarded and no further processing is
       applied.

   4.  SPINAT node MUST add to the state that the mapping between the
       SPI selected by the client host and the SPI that it has chosen.

   5.  The SPINAT node adds the SPI TLV to the original control
       signaling packet.

   6.  The SPINAT node translates the source address of the packet.

   7.  SPINAT node forwards the packet to recipient.

   When a control packet is received from WAN network following
   processing steps will be done:

   1.  Upon receipt of a control signaling packet from WAN network, the
       SPINAT node looks up the state for the control signaling it has
       created from previously outgoing packet.  If no state is found
       the packet MUST be discarded.

   2.  The SPINAT node translates the destination address based on the
       information stored in to the state.

   3.  SPINAT forwards the packet to recipient.







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4.2.  ESP packet processing

4.2.1.  IP Address and SPI Translation at SPINAT Nodes

   This section describes the packet processing of the ESP packet.
   Figure 3 illustrates an example of SPINAT packet processing for ESP
   packets.


     +--------+                    +--------+                 +--------+
     |   Peer |                    | SPINAT |                 | client |
     +--------+                    +--------+                 +--------+
          |              {s=138.76.28.4,|           {s=10.0.0.10,  |
          |               d=138.76.29.7,|            d=138.76.29.7,|
          |       ESP     SPI=4545     \|            SPI=4545}    \|
          |<----------------------------|--------------------------|
          |                             |                          |
          | {s=138.76.29.7              | {s=138.76.29.7,          |
          | {d=138.76.29.4,             |  d=10.0.0.10,            |
          |/ SPI=1212}       ESP        |/ SPI=3030}               |
          |---------------------------->|------------------------->|



       Figure 3: SPINAT Operation for ESP protected payload packets

   When a ESP packet is received from private network following
   processing steps will be done:

   1.  Upon receipt of a ESP packet from the private network, the SPINAT
       determines the appropriate translation state, based on the SPI.
       If no valid state exists for this SPI the SPINAT node MUST
       discard the packet.

   2.  The SPINAT node translates the source address of the packet.

   3.  SPINAT node forwards the packet to recipient.

   When a ESP packet is received from WAN network following processing
   steps will be done:

   1.  Upon receipt of a ESP packet from the WAN network, the SPINAT
       determines the appropriate translation state, based on the SPI.
       If no valid state exists for this SPI the SPINAT node MUST
       discard the packet.

   2.  The SPINAT node translates the destination address and the SPI
       based on the information in the stored in to the state.



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   3.  SPINAT node forwards the packet on to the private network.

4.2.2.  SPI Translation at End-hosts

   This section describes the packet processing of ESP packets in the
   end-hosts.

   The client node processes the inbound and outbound packets as defined
   in sections 3.3 and 3.4 of [RFC4303] and in the client host there is
   no SPI translation takes place.

   The peer node processes the inbound packets as defined in section 3.4
   of [RFC4303] and no SPI translation takes place.  The outbound
   packets are first processed as defined in section 3.3 of [RFC4303].
   When the ESP payload has been constructed, the SPI will be translated
   to the one selected by SPINAT node before sending the packet to the
   client node.


































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5.  Parameters and packet formats

   The control signaling protocol packet carry the SPI value selected by
   SPINAT node.

5.1.  New Parameters

5.1.1.  NAT_ESP_INFO


    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                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Original SPI                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Translated SPI                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type           XXX
   Length         12
   Original SPI   Original SPI selected by the peer which is associated
                  with peer's address(es) to this SA.
   Translated SPI Translated SPI for data sent to address(es) associated
                  with this SA.

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 (I2) and using UPDATE messages.

5.2.1.  Modifications in I2

   When HIP is used with ESP, the I2 packet MUST carry an ESP_INFO
   parameter.  Intermediate SPINAT nodes MUST add NAT_ESP_INFO parameter
   to the I2 packet.  The packet is signed for the benefit of the
   intermediate SPINAT nodes to be able to verify the origin of the
   packet.

   The NAT_ESP_INFO contains the translated SPI for this association as
   well as the sender's original SPI.

   The following figure shows the contents of a I2 packet after it has
   passed the SPINAT node processing.



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      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],
                 NAT_ESP_INFO ) )

5.3.  HIP ESP Rekeying

   In this section, the procedure for rekeying an existing ESP SA is
   presented.  Only the first packet of the UPDATE packet exchange is
   modified by the SPINAT node.

5.3.1.  Initializing Rekeying

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

   Intermediate SPINAT nodes will have to inspect HIP UPDATE packets.
   Those that carry rekeying information the SPINAT node MUST add
   NAT_ESP_INFO parameter.  The packet is signed for the benefit of the
   intermediate SPINAT nodes to be able to verify the origin of the
   packet.

   The following figure shows the contents of a rekeying initialization
   UPDATE packet after it has passed the SPINAT node processing.

      The HIP parameters for the UPDATE packet initiating rekeying:

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







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6.  Security Considerations

   The translated SPI values included in the key-exchange messages and
   ESP headers are not integrity protected with signatures or HMAC
   computation.  Therefore, a Man-in-the-Middle (MitM) attacker MAY
   change the SPIs in the packets.  However, the SPI value is only an
   index to a specific IPsec Security Association (SA) at the receiving
   party.  The actual security is based on the shared session keys.
   Hence, an SPI changing attack does not affect the confidentiality or
   integrity properties of the protocol.

   It must be noted that changing SPI values is only possible for an on-
   path attacker that is able to modify packets on the fly.  Such an
   attacker is not only able to change the SPI values, but he can block
   all communications between the parties.  Therefore, having unsigned
   and changeable SPIs does not introduce new security vulnerabilities
   to ESP.  A host trusts a SPINAT device to change the SPI values in
   the same way it trusts the NAPT to change the port values.

































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7.  IANA Considerations


















































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8.  Acknowledgments

   A number of people have contributed to the text and ideas.  The list
   of these people include Pekka Nikander, Petri Jokela, Raimo
   Vuopionperae, Yuri Ismailov, Jan Hoeller and Hannes Tschofenig.  Our
   apologies to anyone whose name is missing.













































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9.  References

9.1.  Normative References

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

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

9.2.  Informative References

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.




































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Authors' Addresses

   Jan Melen
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   Email: jan.melen@nomadiclab.com


   Jukka Ylitalo
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   Email: jukka.ylitalo@nomadiclab.com


   Patrik Salmela
   Ericsson Research NomadicLab
   JORVAS  FIN-02420
   FINLAND

   Phone: +358 9 299 1
   Email: patrik.salmela@nomadiclab.com
























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Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
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