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Versions: 00 01                                                         
HIP                                                          P. Nikander
Internet-Draft                                                  Ericsson
Expires: August 18, 2005                                   H. Tschofenig
                                                                 Siemens
                                                            T. Henderson
                                                      The Boeing Company
                                                               L. Eggert
                                                                     NEC
                                                             J. Laganier
                                                                     SUN
                                                       February 14, 2005


            Preferred Alternatives for Tunnelling HIP (PATH)
                     draft-nikander-hip-path-00.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  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 become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on August 18, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract



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   With the extensions defined in this document Host Identity Protocol
   (HIP) can traverse legacy Network Address Translators (NATs) and
   certain Firewalls.  The extension will be useful as part of the base
   exchange and with the HIP Registration Extension.  By using a
   rendezvous server an additional entity inside the network is
   utilized, which not only allows but also supports more restrictive
   NATs to be traversed.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.   Protocol Extensions  . . . . . . . . . . . . . . . . . . . .   6
     3.1  UDP Encapsulation of HIP . . . . . . . . . . . . . . . . .   6
     3.2  UDP-REA parameter  . . . . . . . . . . . . . . . . . . . .   6
     3.3  S-UDP-REA parameter  . . . . . . . . . . . . . . . . . . .   8
   4.   Message Handling Rules . . . . . . . . . . . . . . . . . . .  10
   5.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1  HIP Initiator behind a NAT . . . . . . . . . . . . . . . .  11
     5.2  PATH Server Registration and Keep Alive  . . . . . . . . .  11
     5.3  Message flow for data receiver behind a NAT  . . . . . . .  13
     5.4  Mobility and multihoming message flow  . . . . . . . . . .  16
   6.   Security Considerations  . . . . . . . . . . . . . . . . . .  18
     6.1  Third Party Bombing  . . . . . . . . . . . . . . . . . . .  18
     6.2  Black hole . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.3  Man-in-the-middle attack . . . . . . . . . . . . . . . . .  19
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  21
   8.   IAB Considerations . . . . . . . . . . . . . . . . . . . . .  22
     8.1  Problem Definition . . . . . . . . . . . . . . . . . . . .  22
     8.2  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . .  22
     8.3  Brittleness Introduced by PATH . . . . . . . . . . . . . .  23
     8.4  Requirements for a Long Term Solution  . . . . . . . . . .  24
     8.5  Issues with Existing NAPT Boxes  . . . . . . . . . . . . .  25
     8.6  In Closing . . . . . . . . . . . . . . . . . . . . . . . .  25
   9.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  27
   10.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .  28
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1   Normative References . . . . . . . . . . . . . . . . . .  29
     11.2   Informative References . . . . . . . . . . . . . . . . .  29
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  30
        Intellectual Property and Copyright Statements . . . . . . .  32










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1.  Introduction

   This document defines extensions and allows the Host Identity
   Protocol (HIP) to be used in an environment where legacy NATs or
   Firewalls are presents.  To support this functionality it is
   necessary to provide
   o  UDP encapsulation for HIP signaling messages
   o  UDP encapsulation for IPsec traffic
   The problems of IPsec protected traffic and also the problems for a
   signaling protocol (namely IKEv1) traversing a NA[P]T are well
   described in [5].  A proposal for UDP encapsulation of IPsec
   protected traffic is described in [6].  It is possible to design an
   optimized version of it for usage with HIP.  This aspect is, however,
   outside the scope of this document.

   This document tries to accomplish the following goals:
   o  Make HIP work through legacy NATs (and possibly through some
      firewalls)
   o  Make HIP hosts reachable behind NATs

   By using a rendezvous server an additional entity inside the network
   is introduced which interacts with the HIP message exchange.  The
   interaction of HIP with the rendezvous server is described in [1].
   This allows a complete NAT/Firewall traversal to be accomplished.
   Two approaches are possible with respect to this rendezvous server
   concept.

   o  First, it is possible to combine a HIP rendezvous server (i.e.,
      PATH server) and a STUN server [7], TURN server [8] or NSIS NATFW
      [9] node.  The client obviously needs to support the clide part of
      the protocol as well.  The STUN, TURN or NSIS NATFW protocol is
      used to allow the PATH client to learn the public IP address (and
      port number) created at the NAT.
   o  Second, the HIP registration protocol can integrate a NAT
      detection check.  NAT-T support is provided by IKEv2 [10] and the
      corresponding extensions have been designed for IKEv1 (see [5] and
      [11]).

   This document uses the later approach to avoid the integration of
   another protocol and additional message exchanges but does not
   rule-out the former approach.  An integration with STUN and TURN
   would not add more security to the protocol exchange.  To support NAT
   detection a new parameter UDP-REA is introduced.

   To also allow the client to inform the server about its public IP
   address and port in a secure fashion (where this is possible and
   appropriate) another parameter has been defined S-UDP-REA, a secure
   version of the UDP-REA parameter.  Using this parameter a secure



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   traversal of legacy NATs is supported, for example by interworking
   with NSIS or MIDCOM.  Both, MIDCOM and the NSIS NATFW NSLP provide
   better security properties.  The interaction with these protocols is
   outside the scope of this document.

   Please note that this document tries to accomplish a different goal
   than [12] where middleboxes (such as NATs and firewalls) are assumed
   to be HIP-aware and participate in the HIP message exchange.  As a
   consequence, the security properties of these protocols are different
   as well.









































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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 [2].

   When interaction with a rendezvous server might be possible then we
   denote these entities as the PATH client and the PATH server
   traversing some type of legacy NAT.  A PATH client is a HIP-aware
   device which supports the extensions defined in this document in
   addition to the HIP Registration Extension [13].  The PATH client
   might be located behind a legacy NAT and initiates the protocol
   exchange with the PATH server.  The PATH server interacts with the
   client in the way specified in this document.

   Different types of NATs (e.g., full cone, restricted NAT) are
   deployed today and [7] assigns these NAT boxes to certain categories
   based on their data traffic forwarding or blocking behavior.  The
   existence of different NAT types has an impact on the protocol.
































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3.  Protocol Extensions

   This section explains the necessary protocol extensions to support
   the above-mentioned functionality.

3.1  UDP Encapsulation of HIP

   In order to deal with NA[P]Ts, it is necessary that the HIP signaling
   messages are UDP encapsulated and moreover the source port and the
   destination port MUST NOT be expected at a fixed port number.  This
   aspect of NAT traversal is known from IPsec/IKE and also reflected in
   the design of IKEv2.

   It is a policy issue whether to enable UDP encapsulation immediately
   when the first HIP base message is sent (i.e., the I1 message).

   For IPv4, the packet format is shown in Appendix E of [3].  The same
   specification states that UDP encapsulation is forbidden for IPv6 but
   might still be necessary particuarly particularly for IPv4-IPv6
   transition.

3.2  UDP-REA parameter

   This section defines the UDP-REA parameter which will be used in the
   traversal of legacy NATs.


























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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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Address Lifetime                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           HASH                                ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                          Padding                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type (2 bytes):
         This parameter has the value of TBD.

      Length (2 bytes)
         Represents the length in octets,
         excluding Type and Length fields.

      Address Lifetime (4 bytes):
         This field represents the address lifetime, in seconds.

      HASH (variable):
         This field of variable length contains the hash of
         IP address and port information.

      Padding (variable):
         Padding information following the HASH value

   The HASH is calculated as follows:

   HASH = PRF(RANDOM | Source IP | Destination IP | Source Port |
   Destination Port)

   using the negotiated hash algorithm (denoted as PRF) as part of the
   HIP_TRANSFORM payload (see Section 6.2.8 of [3]).  The hashed data is
   in the network byte-order.  The IP address is 4 octets for an IPv4
   address and 16 octets for an IPv6 address.  The port number is
   encoded as a 2 octet number in network byte-order.

   The necessary padding length depends on the selected hash algorithm.
   It MUST be ensured that the total length (including padding) of the
   UDP-REA parameter is 11 + Length - (Length + 3) % 8.

   The RANDOM value is used to prevent precomputation attacks.  The
   puzzle mechanism could be used for this purpose.





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3.3  S-UDP-REA parameter

   This section defines the S-UDP-REA parameter, a secure UDP-REA
   parameter version.  An end host might be able to retrieve address
   information securely using some protocols, such as MIDCOM or the NSIS
   NATFW NSLP.  These protocols enable the PATH client to create and
   retrieve a NAT binding in a secure fashion.  This information is then
   communicated from the PATH client to the PATH server experiencing
   integrity protection.  Furthermore, this extension might also be used
   when a stateful packet filtering firewall is known to be along the
   path which requires UDP encapsulation in order to perform properly.
   UDP-REA Section 3.2 would not be able to detect or act accordingly in
   such a situation.






































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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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Address Lifetime                      |T|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Source Port                |    Destination Port           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~    Source Address                                             ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~    Destination Address                                        ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type (2 bytes):
         This parameter has the value of TBD.

      Length (2 bytes)
         Represents the length in octets,
         excluding Type and Length fields.

      Address Lifetime (4 bytes):
         This field represents the address lifetime, in seconds.

      Type (T) Flag (1 bit):
         If this bit is set to 1 then the value in the Address
         field is an IPv6 address otherwise an IPv4 address.

      Source Port (2 bytes):
         This field contains the source port.

      Destination Port (2 bytes):
         This field contains the destination port.

       Source Address (4 or 16 bytes):
         This field contains either an IPv4 or an IPv6 address.

       Destination Address (4 or 16 bytes):
         This field contains either an IPv4 or an IPv6 address.












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4.  Message Handling Rules

   The PATH client attaches the UDP-REA payload to indicate support for
   legacy NAT traversal.  Thereby it creates a hash value over the
   source IP address, source port, destination IP and destination port
   from the IP header of the HIP packet.  When the HIP message traverse
   a NAT along the path between the client and the server, the IP header
   will be modified.  When the server receives the HIP message, it will
   compare the hash value carried in the HASH field of the UDP-REA
   parameter and the value computed on the IP address header
   information.  If the two values do not match then the server is
   assured that someone along the path modified the IP header (and
   hopefully a NAT and not an adversary).  The server will then use the
   information in the IP header to return a response to the client.  If
   the two values are equal then it is assumed that no NAT is located
   along the path and UDP encapsulation might not be necessary.

   This section provides further information on the message handling.
   o  Checksum and length field are provided in the UDP header and might
      not need to be repeated in the HIP header.
   o  HIP version determined by the destination port used when sending
      the I1 packet
   o  The digital signature and the keyed message digest is computed
      over the original payload.  First, a "normal" HIP packet is
      constructed, then the the HMAC and the digital signatures are
      computed.  Afterwards the the HIP packet is encapsulated into the
      UDP format.
   o  Short timeout (e.g., 200ms) after first packet and therefore
      encourage NAT-less operation.
   o  If preferred source address is in RFC 1918 address space then send
      I1 over IP and I1 over UDP a few milliseconds apart.




















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5.  Examples

5.1  HIP Initiator behind a NAT

   This figure shows the usage of the UDP-REA parameter by the Initiator
   and the Responder to detect the presence of a NAT along the path.  In
   this case the HIP Initiator is behind a NAT.  In this example, the
   HIP Initiator is behind a NAT and we assume the HIP Initiator
   immediately starts with UDP encapsulation.


                Private                          Public
                Addressing                       Addressing
     HIP        Realm          Network Address   Realm           HIP
     Initiator                    Translator                   Responder
       |                             |                             |
       | I1:  Trigger exchange       | I1:  Trigger exchange       |
       |      over UDP               |      over UDP               |
       | --------------------------> | --------------------------> |
       |                             |                             |
       | R1:  {Puzzle,DH(R),HI(R)    | R1:  {Puzzle,DH(R),HI(R)    |
       |       HIP Transform}SIG     |       HIP Transform}SIG     |
       |       UDP-REA(R)            |       UDP-REA(R)            |
       | <-------------------------- | <-------------------------- |
       |                             |                             |
       | I2:  {Solution,DH(I),       | I2:  {Solution,DH(I),       |
       |       HIP Transform         |       HIP Transform         |
       |      {H(I)},CERT(I)}SIG     |      {H(I)},CERT(I)}SIG     |
       |       UDP-REA(I)            |       UDP-REA(I)            |
       | --------------------------> | --------------------------> |
       |                             |                             |
       |  R2:  {HMAC}SIG             |  R2:  {HMAC}SIG             |
       | <-------------------------- | <-------------------------- |
       |                             |                             |

           Figure 3: HIP Initiation behind a NAT Message Flow


5.2  PATH Server Registration and Keep Alive

   This section illustrates the message exchange for a PATH client
   registering with a PATH server, as introduced with [13].  After the
   protocol exchange is finalized both peers will be mutually
   authenticated and authorized each other and a security association
   for HIP has been established.

   When the PATH client starts to interact with the PATH server the
   client might not be aware of the presence of the legacy NAT along the



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   path.  The I1 registration messages will most likely be dropped by
   the NA[P]T.  After some retransmissions the PATH client will switch
   to an UDP encapsulated registration protocol exchange.

   Figure 4 shows such a message exchange.

    PATH                      Network Address                   PATH
    Client                       Translator                     Server
      |                             |                             |
      | I1: Trigger exchange        |                             |
      |      over IP                |                             |
      | --------------------------> | ...I1 dropped               |
      |                             |                             |
      | ..retransmissions..         |                             |
      | --------------------------> | ...I1 dropped               |
      |                             |                             |
      | I1: Trigger exchange        | I1: Trigger exchange        |
      |      over UDP               |      over UDP               |
      | --------------------------> | --------------------------> |
      |                             |                             |
      | R1: {Puzzle,DH(R),HI(R)     | R1: {Puzzle,DH(R),HI(R)     |
      |       HIP Transform}SIG     |       HIP Transform}SIG     |
      | <-------------------------- | <-------------------------- |
      |                             |                             |
      | I2: {Solution,DH(I),        | I2: {Solution,DH(I),        |
      |       HIP Transform         |       HIP Transform         |
      |      {H(I)},CERT(I)}SIG     |      {H(I)},CERT(I)}SIG     |
      | --------------------------> | --------------------------> |
      |                             |                             |
      |  R2: {HMAC}SIG              |  R2: {HMAC}SIG              |
      | <-------------------------- | <-------------------------- |
      |                             |                             |
      |                             |                             |

              Figure 4: Registration Protocol Message Flow

   Additional payloads defined with the HIP Registration Extension are:
   REG_INFO (carried in the R1 message), REQ_REQ (carried in the I2
   message) and REQ_RESP (carried in the R2 message).  These payloads
   are not shown in the above message exchange.

   Note that this protocol exchange implicitly indicates that the PATH
   client will use the source IP address of the I1 and I2 messages as
   the preferred address.  The PATH server will use the source IP
   address of the incoming packet as the preferred address even though
   it was not authenticated (i.e., integrity protected).  The HIP
   middlebox registration protocol exchange already ensures that this
   address is authorized via a return routability test.



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5.3  Message flow for data receiver behind a NAT

   This section shows a message flow where one HIP node acting as the
   data receiver is behind a NAT.  The registration with the PATH server
   is not shown in the figure.  Figure 5 only shows the HIP base
   exchange between the HIP Initiator and the HIP Responder interacting
   with the PATH server.  Figure 5 shows such a protocol exchange taken
   from [4].

   Figure 5 shows that the HIP base exchange between the HIP Initiator
   and the PATH server does not use UDP encapsulation.  UDP
   encapsulation for HIP signaling messages and for the IPsec data
   traffic is only enabled between the PATH server and the HIP Responder
   which is enabled with this extension to the HIP registration
   protocol.  Note that IPsec data traffic will traverse the PATH server
   to experience UDP encapsulation.  The main advantage of this approach
   is two-fold: (1) the HIP Initiator does not need to support the
   extension defined in this document and (2) traversal of more
   restrictive NATs can be supported when the PATH server also changes
   IP address information































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   HIP                  PATH            Network Address       HIP
   Initiator            Server             Translator         Responder
      |                   |                   |                   |
      |  I1 over IP       |                   |                   |
      | ----------------> |   I1 over UDP     |    I1 over UDP    |
      |                   | ----------------> | ----------------> |
      |                   |                   |                   |
      |                   |    R1 over UDP    |    R1 over UDP    |
      |  R1 over IP       |    with UDP-REA   |    with UDP-REA   |
      |  without UDP-REA  | <---------------- | <---------------- |
      | <---------------- |                   |                   |
      |                   |                   |                   |
      |  I2 over IP       |                   |                   |
      |  without UDP-REA  |  I2 over UDP      |  I2 over UDP      |
      | ----------------> |  without UDP-REA  |  without UDP-REA  |
      |                   | ----------------> | ----------------> |
      |                   |                   |                   |
      |                   |  R2 over UDP      |  R2 over UDP      |
      |  R2 over IP       | <---------------- | <---------------- |
      | <---------------- |                   |                   |
      |                   |                   |                   |
      |    IPsec ESP      |  IPsec ESP        |  IPsec ESP        |
      | <===============> |  over UDP         |  over UDP         |
      |                   | <================ | ================> |
      |                   |                   |                   |
      |                   |                   |                   |

   Legend:
     -->: HIP signaling messages
     ==>: Data traffic

                  Figure 5: Establishing contact (1/3)

   Figure 6 modifies the message flow described in Figure 5 whereby R2
   is already sent from the HIP Responder to the HIP Initiator directly.
   The responder thereby creates the necessary NAT binding at the NAT to
   potentially allow IPsec protected traffic from the initiator towards
   the responder to traverse the NAT.  IPsec protected data traffic is
   sent only directly between the HIP Initiator and the HIP Responder.












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   HIP                  PATH            Network Address       HIP
   Initiator            Server             Translator         Responder
      |                   |                   |                   |
      |  I1 over IP       |                   |                   |
      | ----------------> |   I1 over UDP     |    I1 over UDP    |
      |                   | ----------------> | ----------------> |
      |                   |                   |                   |
      |                   |    R1 over UDP    |    R1 over UDP    |
      |    R1 over IP     |    with UDP-REA   |    with UDP-REA   |
      |    with UDP-REA   | <---------------- | <---------------- |
      | <---------------- |                   |                   |
      |                   |                   |                   |
      |  I2 over IP       |                   |                   |
      |  without UDP-REA  |  I2 over UDP      |  I2 over UDP      |
      | ----------------> |  without UDP-REA  |  without UDP-REA  |
      |                   | ----------------> | ----------------> |
      |                   |                   |                   |
      |  R2 over UDP      |  R2 over UDP      |  R2 over UDP      |
      | <------------------------------------ | <---------------- |
      |                   |                   |                   |
      |    IPsec ESP      |  IPsec ESP        |  IPsec ESP        |
      |    over UDP       |  over UDP         |  over UDP         |
      | <==================================== | ================> |
      |                   |                   |                   |
      |                   |                   |                   |

                  Figure 6: Establishing contact (2/3)

   Figure 7 extends Figure 6 further by allowing I2 to be sent directly
   to the HIP Responder.  This is only possible if the NAT forwards
   packets with a different source IP address and source port than the
   packets seen from the PATH server towards the HIP Responder.



















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   HIP                  PATH            Network Address       HIP
   Initiator            Server             Translator         Responder
      |                   |                   |                   |
      |  I1 over IP       |                   |                   |
      | ----------------> |   I1 over UDP     |    I1 over UDP    |
      |                   | ----------------> | ----------------> |
      |                   |                   |                   |
      |                   |    R1 over UDP    |    R1 over UDP    |
      |    R1 over IP     |    with UDP-REA   |    with UDP-REA   |
      |    with UDP-REA   | <---------------- | <---------------- |
      | <---------------- |                   |                   |
      |                   |                   |                   |
      |  I2 over UDP      |  I2 over UDP      |  I2 over UDP      |
      |  with UDP-REA     |  with UDP-REA     |  with UDP-REA     |
      | ------------------------------------> | ----------------> |
      |                   |                   |                   |
      |  R2 over UDP      |  R2 over UDP      |  R2 over UDP      |
      |  with UDP-REA     |  with UDP-REA     |  with UDP-REA     |
      | <------------------------------------ | <---------------- |
      |                   |                   |                   |
      |    IPsec ESP      |  IPsec ESP        |  IPsec ESP        |
      |    over UDP       |  over UDP         |  over UDP         |
      | <==================================== | ================> |
      |                   |                   |                   |
      |                   |                   |                   |

                  Figure 7: Establishing contact (3/3)

   FOR DISCUSSION: It needs to be decided which approach is the best one
   and if there are multiple preferred ways how we
      (a) can detect which approach is applicable and
      (b) what protocol extensions are required.
   The answer most likely depends on the types of NATs we are willing to
   support.  For example, sending the IPsec protected data traffic via
   the PATH server is useful if a NAT is very restrictive but, as a
   default approach, probably not the best choice -- except if the PATH
   server is along the path anyway (see discussion about discovery
   exchange in the HIP registration protocol).  Finally, the message
   flows need to add info about the information conveyed to the end
   hosts about the public IP address and port of it respective peer.

5.4  Mobility and multihoming message flow

   After the PATH client has registered itself to the PATH server, as
   described in Figure 4, the PATH client might roam within a network or
   roam outside a network.  Whenever the PATH client obtains a new IP
   address (either due to mobility, IP address reconfiguration or
   switching of interfaces) a REA message will be sent towards the PATH



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   server to update the stored IP address information.  Note that the
   initial registration procedure might be executed without a NAT along
   the path.  Hence, the messages are carried over IP and do not require
   UDP encapsulation.  When the PATH client roams to a new network UDP
   encapsulation might be required due to the presence of a NAT.  Hence,
   it is required to have the capability to enable UDP encapsulation for
   the HIP exchange (and for the IPsec protected data traffic) not only
   during the initial protocol exchange.

   Figure 8 shows such a protocol exchange which is taken from [4].

    PATH                      Network Address                   PATH
    Client                       Translator                     Server
      |                             |                             |
      | UPDATE(REA, SEQ)            |                             |
      | over IP                     |                             |
      | --------------------------> | ...UPDATE/REA dropped       |
      |                             |                             |
      | ..retransmissions..         |                             |
      | --------------------------> | ...UPDATE/REA dropped       |
      |                             |                             |
      | UPDATE(REA, SEQ)            | UPDATE(REA, SEQ)            |
      | over UDP                    | over UDP                    |
      | --------------------------> | --------------------------> |
      |                             |                             |
      | UPDATE(SPI, SEQ,            | UPDATE(SPI, SEQ,            |
      | ACK, ECHO_REQUEST)          | ACK, ECHO_REQUEST)          |
      | <-------------------------- | <-------------------------- |
      |                             |                             |
      | UPDATE(ACK, ECHO_RESPONSE)  | UPDATE(ACK, ECHO_RESPONSE)  |
      | --------------------------> | --------------------------> |
      |                             |                             |
      |                             |                             |

                    Figure 8: Mobility Message Flow
















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6.  Security Considerations

   Currently this text in this section focuses on the attacks between
   the PATH client and the PATH server since they differ from the
   description of threats provided in the past about NAT traversal for
   mobility protocols.  The latter one have been investigated in context
   of IKE, IKEv2 and various other protocols and will be summarized in a
   future version of the document.

   Attacks on the interaction between the PATH client and the PATH
   server can be classified as denial of service and might be launched
   against the PATH server itself, against third parties or against the
   PATH client.

   PATH servers create state through the HIP registration protocol.  A
   number of counter-measures are built-in into HIP registration
   protocol.  A PATH server might use the client-puzzle mechanism to
   prevent a certain degree of DoS attacks.  Additionally, it might be
   reasonable to limit the number of registrations at a PATH server
   itself.  Since the PATH server needs to be discovered somehow it
   needs to be ensured that some security mechanisms are provided for
   this procedure.  For example, if the PATH server is discovered using
   DNS SRV records then an attacker can compromise the DNS, it can
   inject fake records which map a domain name to the IP address of a
   PATH server run by the attacker.  This will allow it to inject fake
   responses to launch a number of the attacks.  This discovery
   procedure might, however, be part of the HIP Registration protocol.
   A detailed discussion about the security properties of the HIP
   registration protocol is outside the scope of this document.  Even
   though the base HIP registration protocol is outside the scope of
   this document some of its security properties are highly relevant and
   applicable for this discussion.  This document extends the
   capabilities of the registration protocol that might raise security
   concerns.  This section mostly focuses on the security properties of
   the UDP-REA parameter and it's semantic.

6.1  Third Party Bombing

   Threat:

      Third party bombing is also of concern when legacy NAT traversal
      mechanisms are in place.  These attacks have been discovered in
      the context of Mobile IP and a threat description can be found in
      [14].  The main problem described in [14] is caused by the missing
      integrity protection of the IP address communicated from the PATH
      client to the PATH server.  The PATH client cannot protect the IP
      address (without relying on additional protocol) since a NA[P]T is
      supposed to change the header's IP address (source, possibly



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      destination IP address and transport protocol identifiers).
      Instead of using the protected IP address inside the signaling
      message the PATH server is supposed to use IP header information.
      An adversary might provide change the IP header address to point
      to the intended target.  Data sent to the PATH server will be send
      to the target rather than to the true IP address of the client.
   Countermeasures:

      To prevent third party bombing, the address provided by the PATH
      client via the IP header needs to be verified using a
      return-routability check.  This check might either be provided as
      part of the base exchange (which involves two roundtrips) or as
      part of the REA message exchange which also provides mechanisms to
      execute such a test.  This return-routability test MUST be
      performed in order to ensure that this and other attacks can be
      thwarted.  A third party entity cannot respond to any of these HIP
      messages due to the cryptographic properties of the HIP base
      protocol and the multi-homing and mobility extensions.

6.2  Black hole

   Threat:

      This attack again exploits the ability for an adversary to act as
      a NAT and to modify the IP address information in the header.
      This information will then be used by the PATH server to sent
      traffic towards the indicated address.  If this address is not
      used by any entity (and particularly by the legitimate PATH
      client) then the traffic will be dropped.  This attack is a denial
      of service attack.
   Countermeasures:

      This threat can be avoided using the same counter measures as
      third party bombing.

6.3  Man-in-the-middle attack

   Threat:

      This attack again requires the adversary to modify the IP header
      of the HIP registration protocol messages exchanged between the
      PATH server and the PATH client.  Instead of pointing to a black
      hole or to a third party the adversary provides his address.  This
      allows the adversary to eavesdrop the data traffic.  However, in
      order to launch the attack, the adversary must have already been
      able to observe packets from the PATH client to the PATH server.
      In most cases (such as when the attack is launched from an access
      network), this means that the attacker could already observe



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      packets sent to the client.
   Countermeasures:

      It is possible that an adversary modifies the IP address
      information in such a way that it will receive the all traffic for
      a particular PATH client.  Therefore, it is necessary for the
      adversary to be along the path to mount the initial attack.  This
      will allow the adversary to eavesdrop both the HIP message
      exchange and the subsequent data traffic.  However, the HIP
      exchange is a cryptographic protocol which is resistant against
      these types of attack.  The data traffic is IPsec protected and
      therefore the adversary will gain very little profit from this
      attack.  To make things worse for the adversary, if the PATH
      client roams and uses the HIP registration protocol or the REA
      message to update state at the PATH server the adversary needs to
      be located somewhere along the path where it can observe this
      exchange and to modify it.  As a consequence, this attack is not
      particular useful for the adversary.

   The S-UDP-REA parameter does not suffer from the same threats as the
   UDP-REA parameter since it aims to provide a secure mechanism for the
   PATH server and the PATH client to communicate addressing
   information.  Still, the PATH server might want to authorize the
   parameters provided by the PATH client by either executing a
   return-routability check or by using other techniques (e.g.,
   authorization certificates) to ensure that the PATH client is indeed
   reachable at the indicated addresses.  A malicious PATH client might
   add wrong addressing information to redirect traffic to a black hole
   or a third party.  This threat has a different degree than the
   previously discussed threats in the sense that the PATH server will
   most likely know the identity of the PATH client, if we assume that
   only authenticated and authorized clients are allowed to use the PATH
   server.  If the PATH server is able to detect the malicious behavior
   it can act accordingly.

   Finally, it is necessary to add a remark on the usage of NAT/Firewall
   signaling protocols in relationship with the S-UDP-REA parameter
   usage.  If the PATH client uses these protocols in an insecure or
   inadequate way then the envisioned security of the S-UDP-REA
   parameter is seriously affected.  A discussion of the security
   properties of various NAT/Firewall signaling protocols is outside the
   scope of the document (in the same way as these protocols are outside
   the scope of this document).








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7.  IANA Considerations

   This document extends the HIP registration protocol by defining a new
   parameter (the UDP-REA and the S-UDP-REA parameter).  These
   parameters need IANA registration:

   TBD:

   Changes to the PATH protocol are made through a standards track
   revision of this specification.  This document does not create new
   IANA registries.








































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8.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a client attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism (RFC 3424 [15]).  PATH is
   an example of a protocol that performs this type of function.  The
   IAB has mandated that any protocols developed for this purpose
   document a specific set of considerations.  This section meets those
   requirements.

   The text in this section heavily borrows from [7].

8.1  Problem Definition

   From RFC 3424 [15], any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why "short term fixes
      usually aren't".

   The specific problem being solved by PATH is to provide a means for a
   PATH client to detect the presence of one or more NATs between it and
   a PATH server.  The purpose of such detection is to determine the
   need for UDP encapsulation by the PATH server (i.e., rendezvous
   server).

   PATH affect both UDP encapsulation of data traffic (which is IPsec
   protected) and HIP signaling messages.

8.2  Exit Strategy

   From [15], any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   PATH comes with its own built in exit strategy.  This strategy is the
   detection operation that is performed as a precursor to the actual
   UNSAF address-fixing operation.  The discovery operation, described
   in Section 3.2, attempts to discover the existence of, and type of,
   any NATS between the client and the PATH server.  PATH does not aim
   to detect the type of NAT (due to known deficiencies) and the
   discovery of the existence of NAT is itself quite robust.  As NATs
   are phased out through the deployment of IPv6, the discovery
   operation will return immediately with the result that there is no



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   NAT, and no further operations are required.  Indeed, the discovery
   operation itself can be used to help motivate deployment of IPv6; if
   a user detects a NAT between themselves and the public Internet, they
   can call up their access provider and complain about it.

   PATH can also help to facilitate the introduction of MICOM or NSIS.
   As MIDCOM or NSIS-capable NATs are deployed, HIP end hosts will,
   instead of using UDP-REA, first allocate an address binding using
   MIDCOM or NSIS and use S-UDP-REA.  However, it is a well-known
   limitation of MIDCOM that it only works when the agent knows the
   middleboxes through which its traffic will flow.  This issue is fixed
   with the path-coupled approach followed in NSIS.  Once bindings have
   been allocated from those middleboxes, a PATH detection procedure can
   validate that there are no additional middleboxes on the path from
   the PATH server to the PATH client.  If this is the case, the HIP end
   host can continue operation using the address bindings allocated from
   MIDCOM or NSIS.  If it is not the case, PATH provides a mechanism for
   self-address fixing through the remaining MIDCOM or NSIS-unaware
   middleboxes.  Thus, PATH provides a way to help transition to full
   MIDCOM or NSIS-aware networks.

8.3  Brittleness Introduced by PATH

   From [15], any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   PATH introduces brittleness into the system in several ways:

      [EDITOR's NOTE: Depending on the signaling flow and the
      involvement of the PATH server some behavior is assumed by NATs.
      There could be other types of NATs that are deployed that would
      not work well with some of the proposed signaling message flows.
      For some of the message flows the binding acquisition usage of
      PATH does not work for all NAT types.  It will work for any
      application for full cone NATs only.  For restricted cone and port
      restricted cone NAT, it may work for some cases.  For symmetric
      NATs, the binding acquisition will not yield a usable address (in
      case that not all the signaling messages and the entire data
      traffic is routed through the PATH server).  The tight dependency
      on the specific type of NAT makes the protocol brittle.]
      PATH assumes that the server exists on the public Internet.  If
      the server is located in another private address realm, the HIP
      end host may or may not be able to use the established state at
      the PATH server.  This heavily depends on the protocol interaction



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      between the other HIP end host and possibly other PATH servers
      than are cascaded.
      The bindings allocated from the NAT need to be continuously
      refreshed.  Since the timeouts for these bindings is
      implementation specific, the refresh interval cannot easily be
      determined.  When the binding is not being actively used to
      receive traffic, but to wait for an incoming message, the binding
      refresh will needlessly consume network bandwidth.
      The use of the PATH server as an additional network element
      introduces another point of potential security attack.  These
      attacks are largely prevented by the security measures provided
      the HIP registration protocol, but not entirely.
      The use of the PATH server as an additional network element
      introduces another point of failure.  If the client cannot locate
      a PATH server, or if the server should be unavailable due to
      failure, no interaction can be performed.
      The use of PATH to enable UDP encapsulation for IPsec protected
      data traffic and for HIP messages introduces an additional
      bandwidth consumption which might be problematic in certain
      wireless networks.  The modified packet forwarding through the
      PATH server, which might be necessary to ensure traversal of
      certain NAT types, might represent a non-optimal route and may
      increase latency for some applications (depending on the location
      of the PATH server).

8.4  Requirements for a Long Term Solution

   From [15], any UNSAF proposal must provide:

      Identify requirements for longer term, sound technical solutions -
      contribute to the process of finding the right longer term
      solution.

   Our experience with PATH has led to the following requirements for a
   long term solution to the NAT problem:

      Requests for bindings and control of other resources in a NAT need
      to be explicit.  Much of the brittleness in PATH derives from its
      guessing at the parameters of the NAT, rather than telling the NAT
      what parameters to use.
      Control needs to be "in-band".  There are far too many scenarios
      in which the client will not know about the location of
      middleboxes ahead of time.  Instead, control of such boxes needs
      to occur in-band, traveling along the same path as the data will
      itself travel.  This guarantees that the right set of middleboxes
      are controlled.  NSIS exactly provides a solution for this
      purpose.  Third-party controls are best handled using the MIDCOM
      framework.



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      Control needs to be limited.  Users will need to communicate
      through NATs which are outside of their administrative control.
      In order for providers to be willing to deploy NATs which can be
      controlled by users in different domains, the scope of such
      controls needs to be extremely limited - typically, allocating a
      binding to reach the address where the control packets are coming
      from.
      Simplicity is paramount.  The control protocol will need to be
      implement in very simple clients.  The servers will need to
      support extremely high loads.  The protocol will need to be
      extremely robust, being the precursor to a host of application
      protocols.  As such, simplicity is key.

8.5  Issues with Existing NAPT Boxes

   From [15], any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   Several of the practical issues with PATH involve future proofing -
   breaking the protocol when new NAT types get deployed.  Fortunately,
   this is not an issue at the current time, since most of the deployed
   NATs are of the types assumed by PATH.  The primary usage PATH has
   been found in the area of VoIP, to facilitate allocation of addresses
   for receiving RTP [12] traffic.  In that application, the periodic
   keepalives are provided by the RTP traffic itself.  However, several
   practical problems arise for RTP.  First, RTP assumes that RTCP
   traffic is on a port one higher than the RTP traffic.  This pairing
   property cannot be guaranteed through NATs that are not directly
   controllable.  As a result, RTCP traffic may not be properly
   received.  Protocol extensions to SDP have been proposed which
   mitigate this by allowing the client to signal a different port for
   RTCP [18].  However, there will be interoperability problems for some
   time.

   For VoIP, silence suppression can cause a gap in the transmission of
   RTP packets.  This could result in the loss of a binding in the
   middle of a call, if that silence period exceeds the binding timeout.
   This can be mitigated by sending occasional silence packets to keep
   the binding alive.  However, the result is additional brittleness;
   proper operation depends on the silence suppression algorithm in use,
   the usage of a comfort noise codec, the duration of the silence
   period, and the binding lifetime in the NAT.

8.6  In Closing

   Some of the limitations of PATH are not design flaws.  Due to the



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   properties of HIP, PATH is fairly secure and robust form of legacy
   NAT traversal compared to other approach such as STUN.  Some
   limitations are, however, related to the lack of standardized
   behaviors and controls in NATs.  The result of this lack of
   standardization has been a proliferation of devices whose behavior is
   highly unpredictable, extremely variable, and uncontrollable.  PATH
   does the best it can in such a hostile environment.  Ultimately, the
   solution is to make the environment less hostile, and to introduce
   controls and standardized behaviors into NAT.  However, until such
   time as that happens, PATH provides a good short term solution given
   the terrible conditions under which it is forced to operate.  PATH
   also offers a long-term solution if NATs are NSIS or MIDCOM aware.
   The main benefit is increased secure and a less brittle protocol
   operation since the NAT (or even firewalls) can be controlled and
   should then behave according to respective middlebox signaling
   protocol.  Ultimately, NAT boxes might be HIP aware.



































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9.  Acknowledgements

   The authors would like to thank Julien Laganier, Aarthi Nagarajan and
   Murugaraj Shanmugam for their feedback on this document.















































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10.  Open Issues

   This document is a first attempt in defining HIP NAT/Firewall
   traversal extensions.  The document raises some questions with regard
   to the usage of the PATH server and the later flow of data packets.
   The document still lacks a number of details which would benefit from
   hands-on experience.












































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11.  References

11.1  Normative References

   [1]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
        Rendezvous Extensions", Internet-Draft draft-ietf-hip-rvs-00,
        October 2004.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", March 1997.

   [3]  Moskowitz, R., "Host Identity Protocol",
        Internet-Draft draft-ietf-hip-base-01, October 2004.

   [4]  Nikander, P., "End-Host Mobility and Multi-Homing with Host
        Identity Protocol", Internet-Draft draft-ietf-hip-mm-00, October
        2004.

11.2  Informative References

   [5]   Aboba, B. and W. Dixon, "IPsec-Network Address Translation
         (NAT) Compatibility Requirements", RFC 3715, March 2004.

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

   [7]   Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
         Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

   [8]   Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         Internet-Draft draft-rosenberg-midcom-turn-06, October 2004.

   [9]   Stiemerling, M., "A NAT/Firewall NSIS Signaling Layer Protocol
         (NSLP)", Internet-Draft draft-ietf-nsis-nslp-natfw-04, October
         2004.

   [10]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
         Internet-Draft draft-ietf-ipsec-ikev2-17, October 2004.

   [11]  Kivinen, T., "Negotiation of NAT-Traversal in the IKE",
         Internet-Draft draft-ietf-ipsec-nat-t-ike-08, February 2004.

   [12]  Tschofenig, H., Nagarajan, A., Torvinen, V. and J. Ylitalo,
         "NAT and Firewall Traversal for HIP",
         Internet-Draft draft-tschofenig-hiprg-natfw-traversal-01.txt,
         February 2005.



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   [13]  Laganier, J., Koponen, T. and L. Eggert, "A Middlebox
         Registration Protocol for HIP",
         Internet-Draft draft-koponen-hip-registration-00.txt, February
         2005.

   [14]  Dupont, F., "A note about 3rd party bombing in Mobile IPv6",
         Internet-Draft draft-dupont-mipv6-3bombing-01, January 2005.

   [15]  Daigle, L. and IAB, "IAB Considerations for UNilateral
         Self-Address Fixing (UNSAF) Across Network Address
         Translation", RFC 3424, November 2002.

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


Authors' Addresses

   Pekka Nikander
   Ericsson Research Nomadic Lab
   Hirsalantie 11
   Turku FIN  FIN-02420 JORVAS
   Finland

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


   Hannes Tschofenig
   Siemens
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com


   Thomas R. Henderson
   The Boeing Company
   P.O. Box 3707
   Seattle, WA
   USA

   Email: thomas.r.henderson@boeing.com





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   Lars Eggert
   NEC Network Laboratories
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Phone: +49 6221 90511 43
   Email: lars.eggert@netlab.nec.de


   Julien Laganier
   Sun Labs (Sun Microsystems) and LIP (CNRS/INRIA/ENSL/UCBL)
   180, Avenue de l'Europe
   Saint Ismier CEDEX  38334
   France

   Phone: +33 476 188 815
   Email: ju@sun.com
   URI:   http://research.sun.com
































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