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Versions: (draft-tschofenig-v6ops-secure-tunnels) 00 01 02 03 04 05 RFC 4891

IPv6 Operations WG                                           R. Graveman
Internet-Draft                                         RFG Security, LLC
Expires: February 26, 2006                              M. Parthasarathy
                                                                   Nokia
                                                               P. Savola
                                                               CSC/FUNET
                                                           H. Tschofenig
                                                                 Siemens
                                                         August 25, 2005


               Using IPsec to Secure IPv6-in-IPv4 Tunnels
                 draft-ietf-v6ops-ipsec-tunnels-01.txt

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
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   This Internet-Draft will expire on February 26, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document gives guidance on securing manually configured IPv6-in-
   IPv4 tunnels using IPsec.  No additional protocol extensions are
   described beyond those available with the IPsec framework.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Threats and the Use of IPsec . . . . . . . . . . . . . . . . .  3
     2.1.  IPsec in Transport Mode  . . . . . . . . . . . . . . . . .  4
     2.2.  IPsec in Tunnel Mode . . . . . . . . . . . . . . . . . . .  5
   3.  Scenarios and Overview . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Router-to-Router Tunnels . . . . . . . . . . . . . . . . .  5
     3.2.  Site-to-Router/Router-to-Site Tunnels  . . . . . . . . . .  6
     3.3.  Host-to-Host Tunnels . . . . . . . . . . . . . . . . . . .  8
   4.  IKE and IPsec Versions . . . . . . . . . . . . . . . . . . . .  8
   5.  IPsec Configuration Details  . . . . . . . . . . . . . . . . .  9
     5.1.  Transport vs Tunnel Mode . . . . . . . . . . . . . . . . . 10
     5.2.  IPsec Transport Mode . . . . . . . . . . . . . . . . . . . 11
     5.3.  IPsec Tunnel Mode  . . . . . . . . . . . . . . . . . . . . 11
       5.3.1.  Generic SPDs for Tunnel Mode . . . . . . . . . . . . . 12
   6.  Dynamic Address Configuration  . . . . . . . . . . . . . . . . 12
   7.  NAT Traversal and Mobility . . . . . . . . . . . . . . . . . . 13
   8.  Tunnel Endpoint Discovery  . . . . . . . . . . . . . . . . . . 14
   9.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 14
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     14.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Specific SPDs for Tunnel Mode . . . . . . . . . . . . 17
     A.1.  Specific SPD for Host-to-Host Scenario . . . . . . . . . . 18
     A.2.  Specific SPD for Host-to-Router scenario . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22



















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1.  Introduction

   The IPv6 operations (v6ops) working group has selected (manually
   configured) IPv6-in-IPv4 tunneling [I-D.ietf-v6ops-mech-v2] as one of
   the IPv6 transition mechanisms for IPv6 deployment.

   [I-D.ietf-v6ops-mech-v2] identified a number of threats which had not
   been adequately analyzed or addressed in its predecessor, [RFC2893].
   The most complete solution is to use IPsec to protect IPv6-in-IPv4
   tunneling.  The document was intentionally not expanded to include
   the details on how to set up an IPsec-protected tunnel in an
   interoperable manner, but instead the details were deferred to this
   memo.

   First this document analyses the threats and scenarios that can be
   addressed by IPsec.  Next, this document discusses some of the
   assumptions made by this document for successful IPsec Security
   Association (SA) establishment.  Then, it gives the details of
   Internet Key Exchange (IKE) and IP security (IPsec) exchange with
   packet formats and Security Policy Database (SPD) entries.  Finally,
   it discusses the usage of IPsec NAT-traversal mechanism that can be
   used with configured tunnels in some scenarios.

   This document does not address the use of IPsec for tunnels which are
   not manually configured (e.g., 6to4 tunnels [RFC3056]).  Presumably,
   some form of opportunistic encryption or "better-than-nothing
   security" might or might not be applicable.  Similarly, propagating
   quality of service attributes (apart from Explicit Congestion
   Notification (ECN) bits [I-D.ietf-v6ops-mech-v2]) from the
   encapsulated packets to the tunnel path is out of scope.


2.  Threats and the Use of IPsec

   [I-D.ietf-v6ops-mech-v2] is mostly concerned about address spoofing
   threats:

   1.  IPv4 address of the encapsulating ("outer") packet can be
       spoofed.

   2.  IPv6 address of the encapsulated ("inner") packet can be spoofed.

   IPsec can obviously also provide payload integrity and
   confidentiality as well for the part of the end-to-end path that is
   tunneled.

   The reason for threat (1) is the lack of widespread deployment of
   IPv4 ingress filtering [RFC3704].  The reason for threat (2) is that



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   the IPv6 packet is encapsulated in IPv4 and hence may escape IPv6
   ingress filtering.  [I-D.ietf-v6ops-mech-v2] specifies the following
   strict address checks as mitigating measures:

   o  To mitigate threat (1), the decapsulator verifies that the IPv4
      source address of the packet is the same as the address of the
      configured tunnel endpoint.  The decapsulator may also implement
      IPv4 ingress filtering, i.e., checks whether the packet is
      received on a legitimate interface.

   o  To mitigate threat (2), the decapsulator verifies whether the
      inner IPv6 address is a valid IPv6 address and also applies IPv6
      ingress filtering before accepting the IPv6 packet.

   This memo proposes using IPsec for providing stronger security in
   preventing these threats and additionally providing integrity and
   confidentiality.  IPsec can be used in two ways, in transport and
   tunnel mode; further comparison is done in Section 5.1.

2.1.  IPsec in Transport Mode

   In transport mode, the IPsec security association (SA) is established
   to protect the traffic defined by (IPv4-source, IPv4-dest, protocol =
   41).  On receiving such an IPsec packet, the receiver first applies
   the IPsec transform (ESP) and then matches the packet against the
   Security Parameter Index (SPI) and the inbound selectors associated
   with the SA to verify that the packet is appropriate for the SA via
   which it was received.  A successful verification implies that the
   packet came from the right IPv4 endpoint as the SA is bound to the
   IPv4 source address.

   This prevents threat (1) but not the threat (2).  IPsec in transport
   mode does not verify the contents of the payload itself where the
   IPv6 addresses are carried, that is, two nodes that are using IPsec
   transport mode to secure the tunnel can spoof the inner payload.  The
   packet will be decapsulated successfully and accepted.

   The shortcoming can be mitigated by IPv6 ingress filtering i.e.,
   check that the packet is arriving from the interface in the direction
   of the route towards the tunnel end-point, similar to a Strict
   Reverse Path Forwarding (RPF) check [RFC3704].

   In most implementations, a transport mode SA is applied to a normal
   IPv6-in-IPv4 tunnel.  Therefore, ingress filtering can be applied in
   the tunnel interface.  (Transport mode is often also used in other
   kind of tunnels such as GRE and L2TP.)





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2.2.  IPsec in Tunnel Mode

   In tunnel mode, the IPsec SA is established to protect the traffic
   defined by (IPv6-source, IPv6-destination).  On receiving such an
   IPsec packet, the receiver first applies the IPsec transform (ESP)
   and then matches the packet against the SPI and the inbound selectors
   associated with the SA to verify that the packet is appropriate for
   the SA via which it was received.  The successful verification
   implies that the packet came from the right endpoint.

   The outer IPv4 addresses may be spoofed and IPsec cannot detect it in
   this mode; the packets will be demultiplexed based on the SPI and
   possibly the IPv6 address bound to the SA.  Thus, the outer address
   spoofing is irrelevant as long as the decryption succeeds and the
   inner IPv6 packet can be verified to come from the right tunnel
   endpoint.

   Tunnel mode SA can be used in two ways depending on whether it is
   modelled as an interface or not.  These are described in section
   Section 5.3.


3.  Scenarios and Overview

   There are roughly three kinds of scenarios:

   1.  (generic) router-to-router tunnels.

   2.  site-to-route/router-to-site tunnels.  This refers to a tunnel
       between a site's IPv6 (border) device to an IPv6 upstream
       provider's router.  A degenerate case of a site is a single host.

   3.  Host-to-host tunnels.

3.1.  Router-to-Router Tunnels

   IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of
   IPv4 routing topology by encapsulating them within IPv4 packets.
   Tunneling can be used in a variety of ways.












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   .--------.           _----_          .--------.
   |v6-in-v4|         _( IPv4 )_        |v6-in-v4|
   | Router | <======( Internet )=====> | Router |
   |   A    |         (_      _)        |   B    |
   '--------'           '----'          '--------'
       ^        IPsec tunnel between        ^
       |        Router A and Router B       |
       V                                    V

   Figure 1: Router-to-Router Scenario

   IPv6/IPv4 routers interconnected by an IPv4 infrastructure can tunnel
   IPv6 packets between themselves.  In this case, the tunnel spans one
   segment of the end-to-end path that the IPv6 packet takes.

   The source and destination addresses of the IPv6 packets traversing
   the tunnel could come from a wide range of IPv6 prefixes, so binding
   IPv6 addresses to be used to the SA is not feasible.  IPv6 ingress
   filtering must be performed to mitigate the IPv6 address spoofing
   threat.

   A specific case of router-to-router tunnels, when one router resides
   at an end site, is described in the next section.

3.2.  Site-to-Router/Router-to-Site Tunnels

   This is a generalization of host-to-router and router-to-host
   tunneling, because the issues when connecting a whole site (using a
   router), and connecting a single host are roughly equal.

      _----_        .---------. IPsec     _----_    IPsec  .-------.
    _( IPv6 )_      |v6-in-v4 | Tunnel  _( IPv4 )_  Tunnel | V4/V6  |
   ( Internet )<--->| Router  |<=======( Internet )=======>| Site B |
    (_      _)      |   A     |         (_      _)         '--------'
      '----'        '---------'           '----'
        ^
        |
        V
    .--------.
    | Native |
    | IPv6   |
    | node   |
    '--------'

   Figure 2: Router-to-Site Scenario

   IPv6/IPv4 routers can tunnel IPv6 packets to their final destination
   IPv6/IPv4 site.  This tunnel spans only the last segment of the end-



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   to-end path.

                                   +---------------------+
                                   |      IPv6 Network   |
                                   |                     |
   .--------.        _----_        |     .--------.      |
   | V6/V4  |      _( IPv4 )_      |     |v6-in-v4|      |
   | Site B |<====( Internet )==========>| Router |      |
   '--------'      (_      _)      |     |   A    |      |
                     '----'        |     '--------'      |
           IPsec tunnel between    |         ^           |
           IPv6 Site and Router A  |         |           |
                                   |         V           |
                                   |     .-------.       |
                                   |     |  V6    |      |
                                   |     |  Hosts |      |
                                   |     '--------'      |
                                   +---------------------+

   Figure 3: Site-to-Router Scenario

   Respectively, IPv6/IPv4 hosts can tunnel IPv6 packets to an
   intermediary IPv6/IPv4 router that is reachable via an IPv4
   infrastructure.  This type of tunnel spans the first segment of the
   packet's end-to-end path.

   The hosts in the site originate the packets with source addresses
   coming from a well known prefix whereas the destination address could
   be any node on the Internet.

   In this case, the IPsec tunnel mode SA can be bound to the prefix
   that was allocated to the router at Site B and router A can verify
   that the source address of the packet matches the prefix.  Site B
   will not be able to do a similar verification for the packets it
   receives.  This may be quite reasonable for most of the deployment
   cases, for example, the Internet Service Provider (ISP) allocating a
   /48 to a customer.  The Customer Premises Equipment (CPE) where the
   tunnel is terminated "trusts" (in a weak sense) the ISP's router and
   the ISP's router can verify that the Site B is the only one that can
   originate packets within the /48.

   IPv6 spoofing must be prevented, and setting up ingress filtering may
   require some amount of manual configuration; see more of these
   options in Section 5.







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3.3.  Host-to-Host Tunnels

     .--------.           _----_          .--------.
     | V6/V4  |         _( IPv4 )_        | V6/V4  |
     | Host   | <======( Internet )=====> | Host   |
     |   A    |         (_      _)        |   B    |
     '--------'           '----'          '--------'
                  IPsec tunnel between
                  Host A and Host B

   Figure 4: Host-to-Host Scenario

   IPv6/IPv4 hosts that are interconnected by an IPv4 infrastructure can
   tunnel IPv6 packets between themselves.  In this case, the tunnel
   spans the entire end-to-end path that the packet takes.

   In this case, the source and the destination IPv6 address are known a
   priori.  A tunnel mode SA can be bound to the specific address.  The
   address verification prevents IPv6 address spoofing completely.

   As noted in Introduction, automatic host-to-host tunneling methods
   (e.g., 6to4) are out of scope of this memo.


4.  IKE and IPsec Versions

   This section discusses the different versions of the IKE and IPsec
   security architecture and their applicability to this document.

   The IPsec security architecture was originally defined in [RFC2401]
   and now superseded by [I-D.ietf-ipsec-rfc2401bis].  IKE was
   originally defined in [RFC2409] (which is referred to as IKEv1 in
   this document) and is now superseded by [I-D.ietf-ipsec-ikev2]
   (referred to as IKEv2).  There are several differences between them.
   The differences relevant to this document are discussed below.

   1.  [RFC2401] does not allow IP as the next layer protocol in traffic
       selectors when IPsec SA is negotiated.  [I-D.ietf-ipsec-
       rfc2401bis] also allows IP as the next layer protocol like TCP or
       UDP in traffic selectors.

   2.  [RFC2401] does not support transport mode SAs between hosts and
       security gateways.  [I-D.ietf-ipsec-rfc2401bis] supports
       transport mode SA between hosts and security gateway to provide
       link security e.g., IP-IP tunnel protected with IPsec.

   3.  [I-D.ietf-ipsec-rfc2401bis] assumes IKEv2, as some of the new
       features cannot be negotiated using IKEv1.  It is valid to



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       negotiate multiple traffic selectors for a given IPsec SA in
       [I-D.ietf-ipsec-rfc2401bis].  This is possible only with
       [I-D.ietf-ipsec-ikev2].  If [RFC2409] is used, then multiple SAs
       need to be set up for each traffic selector.

   Note that the existing implementations based on [RFC2409] may already
   be able to support the [I-D.ietf-ipsec-rfc2401bis] features described
   in (1) and (2).  If appropriate, the deployment may choose to use the
   two versions of the security architecture.

   IKEv2 supports features that are useful for configuring and securing
   tunnels which are not present with IKEv1.

   1.  IKEv2 supports legacy authentication methods by carrying them in
       EAP payloads.  This can be used to authenticate the hosts/sites
       to the ISP using EAP methods that support username and password.

   2.  IKEv2 supports dynamic address configuration which may be used to
       configure the IPv6 address of the host.

   NAT traversal works with both the old and revised IPsec
   architectures, but the negotiation is integrated with IKEv2.

   We do not consider the usage of the IP Authentication Header (AH)
   [I-D.ietf-ipsec-rfc2402bis] as ESP [I-D.ietf-ipsec-esp-v3] provides
   security services (such as integrity protection without
   confidentiality protection using 'NULL' encryption) which are
   comparable with AH.  This is explicitly stated in [I-D.ietf-ipsec-
   rfc2401bis].


5.  IPsec Configuration Details

   This section describes details about establishment of an IPsec tunnel
   for the protection of IPv4/IPv6 data traffic.  However, first we will
   take a look at the packet format on the wire, and the salient
   differences between transport and tunnel modes.

   The packet format is the same for both transport mode and tunnel mode
   as shown in Table 1.











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    +----------------------------+------------------------------------+
    | Components (first to last) |              Contains              |
    +----------------------------+------------------------------------+
    |         IPv4 header        | (src = IPV4-TEP1, dst = IPV4-TEP2) |
    |         ESP header         |                                    |
    |         IPv6 header        |  (src = IPV6-EP1, dst = IPV6-EP2)  |
    |          (payload)         |                                    |
    +----------------------------+------------------------------------+

                                  Table 1

5.1.  Transport vs Tunnel Mode

   Transport mode is typically used by setting up a regular IPv6-in-IPv4
   (or GRE, L2TP, ...) tunnel, and then applying a transport mode SA to
   protect the packets before they are sent out over an interface.

   Tunnel mode can be deployed in two very different ways depending on
   the implementation:

   1.  "Generic SPDs": some implementations model the tunnel mode SA as
       an IP interface.  In this case, an IPsec tunnel interface is
       created and used with "any" address ("::/0 <-> ::/0" ) as IPsec
       traffic selectors while setting up the SA.  Though this allows
       all traffic between the two nodes to be protected by IPsec, the
       routing table would decide what traffic gets sent over the
       tunnel.  Ingress filtering must be separately applied on the
       tunnel interface as the IPsec policy checks do not check the IPv6
       addresses at all.  Routing protocols, multicast, etc. will work
       through this tunnel.  This mode is very similar to the transport
       mode.

   2.  "Specific SPDs": some implementations don't model the tunnel mode
       SA as an IP interface.  Traffic selection is done based on
       specific SPD entries, e.g., "2001:db8:1::/48 <-> 2001:db8:
       2::/48".  As the IPsec session between two endpoints does not
       have an interface (though an implementation may have a common
       pseudo-interface for all IPsec traffic), there is no DAD, MLD, or
       link-local traffic to protect; multicast is not possible over
       such a tunnel.  Ingress filtering is performed automatically by
       the IPsec traffic selectors.

   Ingress filtering is guaranteed by IPsec processing when option (2)
   is chosen whereas the operator has to enable them explicitly when
   transport mode or option (1) of tunnel mode SA is chosen.

   We describe the specific SPD case in Appendix A due to its length and
   relative complexity compared to transport mode or generic SPD tunnel



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   mode.

5.2.  IPsec Transport Mode

   The transport mode has typically been applied to L2TP, GRE, and other
   kind of tunneling methods, especially when the user wants to tunnel
   non-IP traffic.  [RFC3884] provides an example of applicability.

   IPv6 ingress filtering must be applied on the tunnel interface on all
   the packets which pass the inbound IPsec processing.

   The following SPD entries assume that there are two routers Router1
   and Router2, with tunnel endpoint IPv4 addresses are denoted by IPV4-
   TEP1 and IPV4-TEP2 respectively.  (In other scenarios, the SPDs are
   set up in a similar fashion.)  Implementations that are strictly
   conformant to [RFC2401] may not be able to setup the IPsec transport
   mode SA.


   Router1's SPD OUT :

   IF SRC = IPV4-TEP1 && DST = IPV4-TEP2 && protocol = 41
       THEN USE ESP TRANSPORT MODE SA

   Router1's SPD IN:

   IF SRC = IPV4-TEP2 && DST = IPV4-TEP1 && protocol = 41
       THEN USE ESP TRANSPORT MODE SA


   Router2's SPD OUT:

   IF SRC = IPV4-TEP2 && DST = IPV4-TEP1 && protocol = 41
       THEN USE ESP TRANSPORT MODE SA

   Router2's SPD IN:

   IF SRC = IPV4-TEP1 && DST = IPV4-TEP2 && protocol = 41
       THEN USE ESP TRANSPORT MODE SA

   The IDci and IDcr payloads of IKEv1 carry the IPv4-TEP1, IPV4-TEP2
   and protocol value 41 as phase 2 identities.  With IKEv2, the traffic
   selectors are used to carry the same information.

5.3.  IPsec Tunnel Mode

   As we described above, tunnel mode can be used either with "generic"
   or "specific" SPDs.  We describe the generic approach below, and



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   specific SPDs in Appendix A.

   Implementations may or may not model a tunnel mode SA as a separate
   interface between each IPsec peer.  A separate interface for each is
   simple as long as generic SPDs are used.  However, with specific
   SPDs, having an interface becomes highly problematic.  That is,
   interfaces must always have link-local addresses, run Duplicate
   Address Detection, etc. -- which results in packets which must be
   secured.  These would require a set-up of a number of complex SPDs
   because link-local addresses are not unique.  Therefore, this memo
   restricts to describing only the scenario where SPD tunnel mode is
   not modelled as separate interfaces.

   Routing protocols, multicast, etc. work fine over generic SPD tunnel
   mode, but are not feasible with specific SPDs.

5.3.1.  Generic SPDs for Tunnel Mode

   In the generic SPD case, for any scenario, SPDs are not really used
   for traffic selectors.  All the SPD entries match all the traffic,
   i.e., "src = ::/0 & destination = ::/0" (we do not write these out as
   the SPD entries are trivial).  We assume that the tunnel is modelled
   as an interface, one for each IPsec session.  Instead of SPDs, the
   routing table is used to perform outbound traffic selection, and all
   the traffic that is passed to the interface, gets IPsec-protected.

   Similarly, the inbound SPD matches everything, so demultiplexing is
   done based on the SPI.  This is secure; while an attacker could spoof
   packets with the correct SPI (and even tunnel source/destination
   addresses), the attacker would not know the keying material and such
   packets would fail IPsec processing.

   This mode obviously does not prevent an attacker from spoofing IPv6
   addresses, as any traffic sent by the IPsec peer is accepted.
   Therefore, ingress filtering must be applied on the tunnel interface.

   As all (IP) traffic will pass on this kind of tunnel, routing
   protocols, multicast, etc. will work without problems.


6.  Dynamic Address Configuration

   With the exchange of protected configuration payloads, IKEv2 is able
   to provide the IKEv2 peer with DHCP-like information payloads.  These
   configuration payloads are exchanged between the IKEv2 initiator and
   the responder.

   This can be used (for example) by the host in the host-to-router



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   scenario to obtain the IPv6 address from the ISP as part of setting
   up the IPsec tunnel mode SA.  The details of these procedures are out
   of scope of this memo.


7.  NAT Traversal and Mobility

   Network address (and port) translation devices are commonly found in
   today's networks.  A detailed description of the problem of IPsec
   protected data traffic traversing a NAT including requirements are
   discussed in [RFC3715].

   IKEv2 can detect the presence of a NAT automatically by sending an
   Informational exchange with NAT_DETECTION_SOURCE_IP and
   NAT_DETECTION_DESTINATION_IP payloads before establishing an IPsec
   SA.  These payloads are processed in the same way as in the initial
   IKE_SA_INIT exchange.  Once a NAT is detected and both end points
   support IPsec NAT traversal extensions UDP encapsulation can be
   enabled.

   More details about UDP encapsulation of IPsec protected IP packets
   can be found in [RFC3948].

   For IPv6-in-IPv4 tunneling, NAT traversal is interesting for two
   reasons:

   1.  One of the tunnel endpoints is often behind a NAT, and configured
       tunneling, using protocol 41, is not guaranteed to traverse the
       NAT.  Hence, using IPsec tunnels would enable one to both set-up
       a secure tunnel, and set-up a tunnel where it might not always be
       possible without other tunneling mechanisms.

   2.  Using NAT traversal allows the outer address to change without
       having to renegotiate the SAs.  This could be very beneficial for
       a crude form of mobility, and in scenarios where the NAT changes
       the IP addresses frequently.  However, as the outer address may
       change, this might introduce new security issues, and using
       tunnel mode would be most appropriate.

   When NAT is not applied, the second benefit would still be desirable.
   In particular, using manually configured tunneling is an operational
   challenge with dynamic IP addresses as both ends need to be
   reconfigured if an address changes.  Therefore an easy and efficient
   way to re-establish the IPsec tunnel if the IP address changes would
   be desirable.  The IETF MOBIKE working group is looking into
   providing a solution for IKEv2 but the work is still in progress
   [I-D.ietf-mobike-protocol].




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8.  Tunnel Endpoint Discovery

   The IKEv2 initiator needs to know the address of the IKEv2 responder
   to start IKEv2 signaling.  A number of ways can be used to provide
   the initiator with this information, for example:

   o  Using out-of-band mechanisms, e.g., from the ISP's web page.

   o  Using DNS to look up a service name by appending it to the DNS
      search path provided by DHCPv4 (e.g. "tunnel-
      service.example.com").

   o  Using a DHCP option.

   o  Using a pre-configured or pre-determined IPv4 anycast address.

   o  Using other, unspecified or proprietary methods.

   For the purpose of this document it is assumed that this address can
   be obtained somehow.  Once the address has been learned, it is
   configured as the tunnel end-point for the configured IPv6-in-IPv4
   tunnel.

   This problem is also discussed at more length in [I-D.palet-v6ops-
   tun-auto-disc].


9.  Recommendations

   In Section 5 we examined the differences of setting up an IPsec IPv6-
   in-IPv4 using either tunnel or transport mode.  We observe that the
   transport mode and tunnel mode with generic SPDs are very similar;
   multicast and routing protocols work over both, and ingress filtering
   must be applied on the tunnel interface manually.

   Tunnel mode with specific SPDs is slightly more complicated.  The
   approach does not seem feasible if modelled as an interface, so we do
   not recommend it.  Without an interface, the main benefit is that it
   automatically applies ingress filtering within the IPsec processing.
   However, multicast, routing protocols, etc. are not feasible with
   this approach, so its applicability is limited to host-to-host or
   edge tunnel cases.

   Tunnel mode may be more attractive when the IPv4 tunnel endpoint
   addresses change, as MOBIKE only supports tunnel mode.

   Therefore our primary recommendation is to use either tunnel mode
   with generic SPDs or transport mode, and apply ingress filtering on



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   the tunnel.


10.  IANA Considerations

   This memo makes no request to IANA. [[ RFC-editor: please remove this
   section prior to publication. ]]


11.  Security Considerations

   When you run IPv6-in-IPv4 tunnels (unsecured) over the Internet, it
   is possible to "inject" packets into the tunnel by spoofing the
   source address (data plane security), or if the tunnel is signalled
   somehow (e.g., some messages where you authenticate to the server, so
   that you would get a static v6 prefix), someone might be able to
   spoof the signalling (control plane security).

   The IPsec framework plays an important role in adding security to
   both the protocol for tunnel setup and data traffic.

   Either IKEv1 or IKEv2 provides a secure signaling protocol for
   establishing, maintaining and deleting an IPsec tunnel.

   IPsec, with the Encapsulating Security Payload (ESP), offers
   integrity and data origin authentication, confidentiality, with
   optional (at the discretion of the receiver) anti-replay features.
   The usage of confidentity-only is discouraged.  ESP furthermore
   provides limited traffic flow confidentality.

   IPsec provides access control mechanisms through the distribution of
   keys and also through the usage of policies dictated by the Security
   Policy Database (SPD).

   The NAT traversal mechanism provided by IKEv2 introduces some
   weaknesses into IKE and IPsec.  These issues are discussed in more
   detail in [I-D.ietf-ipsec-ikev2].

   Please note that the usage of IPsec for the scenarios described in
   Figure 3, Figure 2 and Figure 1 does not aim to protect the end-to-
   end communication.  It protects just the tunnel part.  It is still
   possible for an IPv6 endpoint that is not attached to the IPsec
   tunnel to spoof packets.


12.  Contributors

   The authors are listed in alphabetical order.



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   Suresh Satapati also participated in the initial discussions on the
   topic.


13.  Acknowledgments

   The authors would like to thank Stephen Kent, Michael Richardson,
   Florian Weimer, Elwyn Davies, and Eric Vyncke for their substantive
   feedback.

   We would like to thank Pasi Eronen for his text contributions.


14.  References

14.1.  Normative References

   [I-D.ietf-ipsec-esp-v3]
              Kent, S., "IP Encapsulating Security Payload (ESP)",
              draft-ietf-ipsec-esp-v3-10 (work in progress), March 2005.

   [I-D.ietf-ipsec-ikev2]
              Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              draft-ietf-ipsec-ikev2-17 (work in progress),
              October 2004.

   [I-D.ietf-ipsec-rfc2401bis]
              Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", draft-ietf-ipsec-rfc2401bis-06 (work
              in progress), April 2005.

   [I-D.ietf-v6ops-mech-v2]
              Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-07
              (work in progress), March 2005.

   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,



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              October 1999.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

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

14.2.  Informative References

   [I-D.ietf-ipsec-rfc2402bis]
              Kent, S., "IP Authentication Header",
              draft-ietf-ipsec-rfc2402bis-11 (work in progress),
              March 2005.

   [I-D.ietf-mobike-protocol]
              Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", draft-ietf-mobike-protocol-01 (work in
              progress), July 2005.

   [I-D.palet-v6ops-tun-auto-disc]
              Palet, J. and M. Diaz, "Analysis of IPv6 Tunnel End-point
              Discovery Mechanisms", draft-palet-v6ops-tun-auto-disc-03
              (work in progress), January 2005.

   [RFC2893]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 2893, August 2000.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3715]  Aboba, B. and W. Dixon, "IPsec-Network Address Translation
              (NAT) Compatibility Requirements", RFC 3715, March 2004.

   [RFC3884]  Touch, J., Eggert, L., and Y. Wang, "Use of IPsec
              Transport Mode for Dynamic Routing", RFC 3884,
              September 2004.


Appendix A.  Specific SPDs for Tunnel Mode

   We describe the specific SPD case in an appendix due to its length
   and relative complexity compared to transport mode or generic SPD
   tunnel mode.

   We assume that this kind of IPsec association is not modelled an
   interface, because then the link-local traffic would require very



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   complex SPDs as well.

A.1.  Specific SPD for Host-to-Host Scenario

   The following SPD entries assume that there are two hosts Host1 and
   Host2, whose IPv6 addresses are denoted by IPV6-EP1 and IPV6-EP2
   (global addresses) and IPV4 addresses of the tunnel endpoints are
   denoted by IPV4-TEP1 and IPV4-TEP2 respectively.

   The outbound SPD will encrypt the traffic to the specified global
   IPv6 address.


   Host1's SPD OUT :

   IF SRC = IPV6-EP1 & DST = IPV6-EP2
       THEN USE ESP TUNNEL MODE SA:
           outer source = IPv4-TEP1
           outer dest   = IPV4-TEP2

   Host1's SPD IN:

   IF SRC = IPV6-EP2 && DST = IPV6-EP1
       THEN USE ESP TUNNEL MODE SA
           outer source = IPV4-TEP2
           outer dest   = IPV4-TEP1

   Host2's SPD OUT:

   IF SRC = IPV6-EP2 & DST = IPV6-EP1
       THEN USE ESP TUNNEL MODE SA:
           outer source = IPv4-TEP2
           outer dest   = IPV4-TEP1

   Host2's SPD IN:

   IF SRC = IPV6-EP1 && DST = IPV6-EP2
       THEN USE ESP TUNNEL MODE SA:
           outer source = IPv4-TEP1
           outer dest   = IPV4-TEP2

   The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and IPV6-TEP2
   as phase 2 identities.  With IKEv2, the traffic selectors are used to
   carry the same information.

A.2.  Specific SPD for Host-to-Router scenario

   The following SPD entries assume that the host has the IPv6 address



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   IPV6-EP1 and the tunnel end points of the host and router are IPV4-
   TEP1 and IPV4-TEP2 respectively.  If the tunnel is between a router
   and a host where the router has allocated a IPV6-PREF/48 to the host,
   the corresponding SPD entries can be derived by substituting IPV6-EP1
   by IPV6-PREF/48.

   Please note the bypass entry for host's outbound SPD, and the
   corresponding router's inbound SPD.  While this might be an
   implementation matter for host-to-router tunneling, having a similar
   entry, "SRC=IPV6-PREF/48 & destination=IPV6-PREF/48" would be
   critical for site-to-router tunneling.


   Host's SPD OUT:

   IF SRC=IPV6-EP1 & DST = IPV6-EP1
       THEN BYPASS

   IF SRC = IPV6-EP1 & DST = any
       THEN USE ESP TUNNEL MODE SA:
           outer source = IPv4-TEP1
           outer dest   = IPV4-TEP2

   Host's SPD IN:

   IF SRC = any && DST = IPV6-EP1
       THEN use ESP TUNNEL MODE SA
           outer source = IPV4-TEP2
           outer dest   = IPV4-TEP1

   Router's SPD OUT:

   IF SRC = any & DST = IPV6-EP1
       THEN USE ESP TUNNEL MODE SA:
           outer source = IPv4-TEP2
           outer dest   = IPV4-TEP1

   Router's SPD IN:

   IF SRC=IPV6-EP1 & DST = IPV6-EP1
       THEN BYPASS

   IF SRC = IPV6-EP1 && DST = any
       THEN use ESP TUNNEL MODE SA
           outer source = IPV4-TEP1
           outer dest   = IPV4-TEP2

   The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and



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   ID_IPV6_ADDR_RANGE or ID_IPV6_ADDR_SUBNET as its phase 2 identity.
   The starting address is zero IP address and the end address is all
   ones for ID_IPV6_ADDR_RANGE.  The starting address is zero IP address
   and the end address is all zeroes for ID_IPV6_ADDR_SUBNET.  With
   IKEv2, the traffic selectors are used to carry the same information.














































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Authors' Addresses

   Richard Graveman
   RFG Security, LLC
   15 Park Avenue
   Morristown, New Jersey  07960
   USA

   Email: rfg@acm.org


   Mohan Parthasarathy
   Nokia
   313 Fairchild Drive
   Mountain View CA-94043
   USA

   Email: mohanp@sbcglobal.net


   Pekka Savola
   CSC/FUNET
   Espoo
   Finnland

   Email: psavola@funet.fi


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

   Email: Hannes.Tschofenig@siemens.com
















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