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INFORMATIONAL
Network Working Group                                        R. GravemanRequest for Comments: 4891                             RFG Security, LLCCategory: Informational                                 M. Parthasarathy                                                                   Nokia                                                               P. Savola                                                               CSC/FUNET                                                           H. Tschofenig                                                  Nokia Siemens Networks                                                                May 2007Using IPsec to Secure IPv6-in-IPv4 TunnelsStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2007).Abstract   This document gives guidance on securing manually configured IPv6-in-   IPv4 tunnels using IPsec in transport mode.  No additional protocol   extensions are described beyond those available with the IPsec   framework.Graveman, et al.             Informational                      [Page 1]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .32.  Threats and the Use of IPsec . . . . . . . . . . . . . . . . .32.1.  IPsec in Transport Mode  . . . . . . . . . . . . . . . . .42.2.  IPsec in Tunnel Mode . . . . . . . . . . . . . . . . . . .53.  Scenarios and Overview . . . . . . . . . . . . . . . . . . . .53.1.  Router-to-Router Tunnels . . . . . . . . . . . . . . . . .63.2.  Site-to-Router/Router-to-Site Tunnels  . . . . . . . . . .63.3.  Host-to-Host Tunnels . . . . . . . . . . . . . . . . . . .84.  IKE and IPsec Versions . . . . . . . . . . . . . . . . . . . .95.  IPsec Configuration Details  . . . . . . . . . . . . . . . . .105.1.  IPsec Transport Mode . . . . . . . . . . . . . . . . . . .115.2.  Peer Authorization Database and Identities . . . . . . . .126.  Recommendations  . . . . . . . . . . . . . . . . . . . . . . .137.  Security Considerations  . . . . . . . . . . . . . . . . . . .138.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . .149.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .1410. References . . . . . . . . . . . . . . . . . . . . . . . . . .1510.1. Normative References . . . . . . . . . . . . . . . . . . .1510.2. Informative References . . . . . . . . . . . . . . . . . .15Appendix A.  Using Tunnel Mode . . . . . . . . . . . . . . . . . .17A.1.  Tunnel Mode Implementation Methods . . . . . . . . . . . .17A.2.  Specific SPD for Host-to-Host Scenario . . . . . . . . . .18A.3.  Specific SPD for Host-to-Router Scenario . . . . . . . . .19Appendix B.  Optional Features . . . . . . . . . . . . . . . . . .20B.1.  Dynamic Address Configuration  . . . . . . . . . . . . . .20B.2.  NAT Traversal and Mobility . . . . . . . . . . . . . . . .20B.3.  Tunnel Endpoint Discovery  . . . . . . . . . . . . . . . .21Graveman, et al.             Informational                      [Page 2]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 20071.  Introduction   The IPv6 Operations (v6ops) working group has selected (manually   configured) IPv6-in-IPv4 tunneling [RFC4213] as one of the IPv6   transition mechanisms for IPv6 deployment.   [RFC4213] identified a number of threats that 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.   The first four sections of this document analyze the threats and   scenarios that can be addressed by IPsec and assumptions made by this   document for successful IPsec Security Association (SA)   establishment.Section 5 gives the details of Internet Key Exchange   (IKE) and IP security (IPsec) exchange with packet formats and   Security Policy Database (SPD) entries.Section 6 gives   recommendations.  Appendices further discuss tunnel mode usage and   optional extensions.   This document does not address the use of IPsec for tunnels that 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 bits [RFC4213]) from the encapsulated packets to the   tunnel path is out of scope.   The use of the word "interface" or the phrase "IP interface" refers   to the IPv6 interface that must be present on any IPv6 node to send   or receive IPv6 packets.  The use of the phrase "tunnel interface"   refers to the interface that receives the IPv6-in-IPv4 tunneled   packets over IPv4.2.  Threats and the Use of IPsec   [RFC4213] is mostly concerned about address spoofing threats:   1.  The IPv4 source address of the encapsulating ("outer") packet can       be spoofed.   2.  The IPv6 source address of the encapsulated ("inner") packet can       be spoofed.Graveman, et al.             Informational                      [Page 3]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   The reason threat (1) exists is the lack of universal deployment of   IPv4 ingress filtering [RFC3704].  The reason threat (2) exists is   that the IPv6 packet is encapsulated in IPv4 and hence may escape   IPv6 ingress filtering.  [RFC4213] 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., check 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,   confidentiality, replay protection, and origin protection between   tunnel endpoints.   IPsec can be used in two ways, in transport and tunnel mode; detailed   discussion about applicability in this context is provided inSection 5.2.1.  IPsec in Transport Mode   In transport mode, the IPsec Encapsulating Security Payload (ESP) or   Authentication Header (AH) 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 (e.g., 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, because the SA is   bound to the IPv4 source address.   This prevents threat (1) but not 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 using IPsec transport mode   to secure the tunnel can spoof the inner payload.  The packet will be   decapsulated successfully and accepted.   This shortcoming can be partially 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 endpoint, similar to   a Strict Reverse Path Forwarding (RPF) check [RFC3704].Graveman, et al.             Informational                      [Page 4]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   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   kinds of tunnels such as Generic Routing Encapsulation (GRE)   [RFC4023] and Layer 2 Tunneling Protocol (L2TP) [RFC3193].)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 (e.g.,   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 this   in tunnel 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 have come from the right   tunnel endpoint.   As described inSection 5, using tunnel mode is more difficult than   applying transport mode to a tunnel interface, and as a result this   document recommends transport mode.  Note that even though transport   rather than tunnel mode is recommended, an IPv6-in-IPv4 tunnel   specified by protocol 41 still exists [RFC4213].3.  Scenarios and Overview   There are roughly three scenarios:   1.  (Generic) router-to-router tunnels.   2.  Site-to-router or router-to-site tunnels.  These refer to tunnels       between a site's IPv6 (border) device and an IPv6 upstream       provider's router.  A degenerate case of a site is a single host.   3.  Host-to-host tunnels.Graveman, et al.             Informational                      [Page 5]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 20073.1.  Router-to-Router Tunnels   IPv6/IPv4 hosts and routers can tunnel IPv6 datagrams over regions of   IPv4 forwarding topology by encapsulating them within IPv4 packets.   Tunneling can be used in a variety of ways.   .--------.           _----_          .--------.   |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 generally 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.Graveman, et al.             Informational                      [Page 6]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007      _----_        .---------. 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-   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.   In the other direction, 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 IPv6 source   addresses coming from a well-known prefix, whereas the destination   addresses could be any nodes on the Internet.Graveman, et al.             Informational                      [Page 7]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   In this case, an IPsec tunnel mode SA could be bound to the prefix   that was allocated to the router at Site B, and Router A could 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, an 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 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 inSection 5.3.3.  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 interconnected by an IPv4 infrastructure can tunnel   IPv6 packets between themselves.  In this case, the tunnel spans the   entire end-to-end path.   In this case, the source and the destination IPv6 addresses are known   a priori.  A tunnel mode SA could be bound to these specific   addresses.  Address verification prevents IPv6 source address   spoofing completely.   As noted in the Introduction, automatic host-to-host tunneling   methods (e.g., 6to4) are out of scope for this memo.Graveman, et al.             Informational                      [Page 8]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 20074.  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 previously defined in [RFC2401]   and is now superseded by [RFC4301].  IKE was originally defined in   [RFC2409] (which is called IKEv1 in this document) and is now   superseded by [RFC4306] (called IKEv2; see also [RFC4718]).  There   are several differences between them.  The differences relevant to   this document are discussed below.   1.  [RFC2401] does not require allowing IP as the next layer protocol       in traffic selectors when an IPsec SA is negotiated.  In       contrast, [RFC4301] requires supporting IP as the next layer       protocol (like TCP or UDP) in traffic selectors.   2.  [RFC4301] assumes IKEv2, as some of the new features cannot be       negotiated using IKEv1.  It is valid to negotiate multiple       traffic selectors for a given IPsec SA in [RFC4301].  This is       possible only with IKEv2.  If IKEv1 is used, then multiple SAs       need to be set up, one for each traffic selector.   Note that the existing implementations based on IKEv1 may already be   able to support the [RFC4301] features described in (1) and (2).  If   appropriate, the deployment may choose to use either version of the   security architecture.   IKEv2 supports features useful for configuring and securing tunnels   not present with IKEv1.   1.  IKEv2 supports legacy authentication methods by carrying them in       Extensible Authentication Protocol (EAP) payloads.  This can be       used to authenticate hosts or sites to an 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.   Network Address Translation (NAT) traversal works with both the old   and revised IPsec architectures, but the negotiation is integrated   with IKEv2.   For the purposes of this document, where the confidentiality of ESP   [RFC4303] is not required, AH [RFC4302] can be used as an alternative   to ESP.  The main difference is that AH is able to provide integrity   protection for certain fields in the outer IPv4 header and IPv4   options.  However, as the outer IPv4 header will be discarded in anyGraveman, et al.             Informational                      [Page 9]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   case, and those particular fields are not believed to be relevant in   this particular application, there is no particular reason to use AH.5.  IPsec Configuration Details   This section describes the SPD entries for setting up the IPsec   transport mode SA to protect the IPv6 traffic.   Several requirements arise when IPsec is used to protect the IPv6   traffic (inner header) for the scenarios listed inSection 3.   1.  All of IPv6 traffic should be protected, including link-local       (e.g., Neighbor Discovery) and multicast traffic.  Without this,       an attacker can pollute the IPv6 neighbor cache causing       disruption in communication between the two routers.   2.  In router-to-router tunnels, the source and destination addresses       of the traffic could come from a wide range of prefixes that are       normally learned through routing.  As routing can always learn a       new prefix, one cannot assume that all the prefixes are known a       priori [RFC3884].  This mainly affects scenario (1).   3.  Source address selection depends on the notions of routes and       interfaces.  This implies that the reachability to the various       IPv6 destinations appear as routes in the routing table.  This       affects scenarios (2) and (3).   The IPv6 traffic can be protected using transport or tunnel mode.   There are many problems when using tunnel mode as implementations may   or may not model the IPsec tunnel mode SA as an interface as   described inAppendix A.1.   If IPsec tunnel mode SA is not modeled as an interface (e.g., as of   this writing, popular in many open source implementations), the SPD   entries for protecting all traffic between the two endpoints must be   described.  Evaluating against the requirements above, all link-local   traffic multicast traffic would need to be identified, possibly   resulting in a long list of SPD entries.  The second requirement is   difficult to satisfy, because the traffic needing protection is not   necessarily (e.g., router-to-router tunnel) known a priori [RFC3884].   The third requirement is also problematic, because almost all   implementations assume addresses are assigned on interfaces (rather   than configured in SPDs) for proper source address selection.   If the IPsec tunnel mode SA is modeled as interface, the traffic that   needs protection can be modeled as routes pointing to the interface.   But the second requirement is difficult to satisfy, because the   traffic needing protection is not necessarily known a priori.  TheGraveman, et al.             Informational                     [Page 10]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   third requirement is easily solved, because IPsec is modeled as an   interface.   In practice, (2) has been solved by protecting all the traffic   (::/0), but no interoperable implementations support this feature.   For a detailed list of issues pertaining to this, see [VLINK].   Because applying transport mode to protect a tunnel is a much simpler   solution and also easily protects link-local and multicast traffic,   we do not recommend using tunnel mode in this context.  Tunnel mode   is, however, discussed further inAppendix A.   This document assumes that tunnels are manually configured on both   sides and the ingress filtering is manually set up to discard spoofed   packets.5.1.  IPsec Transport Mode   Transport mode has typically been applied to L2TP, GRE, and other   tunneling methods, especially when the user wants to tunnel non-IP   traffic.  [RFC3884], [RFC3193], and [RFC4023] provide examples of   applying transport mode to protect tunnel traffic that spans only a   part of an end-to-end path.   IPv6 ingress filtering must be applied on the tunnel interface on all   the packets that pass the inbound IPsec processing.   The following SPD entries assume that there are two routers, Router1   and Router2, with tunnel endpoint IPv4 addresses denoted IPV4-TEP1   and IPV4-TEP2, respectively.  (In other scenarios, the SPDs are set   up similarly.)     Router1's SPD:                                  Next Layer     Rule     Local     Remote     Protocol   Action     ----     -----     ------    ---------- --------       1     IPV4-TEP1  IPV4-TEP2    ESP       BYPASS       2     IPV4-TEP1  IPV4-TEP2    IKE       BYPASS       3     IPv4-TEP1  IPV4-TEP2     41       PROTECT(ESP,transport)Graveman, et al.             Informational                     [Page 11]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007     Router2's SPD:                                  Next Layer     Rule     Local     Remote     Protocol   Action     ----     -----     ------    ---------- --------       1     IPV4-TEP2  IPV4-TEP1    ESP       BYPASS       2     IPV4-TEP2  IPV4-TEP1    IKE       BYPASS       3     IPv4-TEP2  IPV4-TEP1     41       PROTECT(ESP,transport)     In both SPD entries, "IKE" refers to UDP destination port 500     and possibly also port 4500 if NAT traversal is used.   The packet format is as shown in Table 1.    +----------------------------+------------------------------------+    | 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: Packet Format for IPv6/IPv4 Tunnels.   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.5.2.  Peer Authorization Database and Identities   The Peer Authorization Database (PAD) provides the link between SPD   and the key management daemon [RFC4306].  This is defined in   [RFC4301] and hence relevant only when used with IKEv2.   As there is currently no defined way to discover the PAD-related   parameters dynamically, it is assumed that these are manually   configured:   o  The Identity of the peer asserted in the IKEv2 exchange: Many      different types of identities can be used.  At least, the IPv4      address of the peer should be supported.   o  IKEv2 can authenticate the peer by several methods.  Pre-shared      key and X.509 certificate-based authentication is required by      [RFC4301].  At least, pre-shared key should be supported, because      it interoperates with a larger number of implementations.Graveman, et al.             Informational                     [Page 12]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   o  The child SA authorization data should contain the IPv4 address of      the peer.   IPv4 address should be supported as Identity during the key exchange.   As this does not provide Identity protection, main mode or aggressive   mode can be used with IKEv1.6.  Recommendations   InSection 5, we examined the differences between setting up an IPsec   IPv6-in-IPv4 tunnel using either transport or tunnel mode.  We   observe that applying transport mode to a tunnel interface is the   simplest and therefore recommended solution.   InAppendix A, we also explore what it would take to use so-called   Specific SPD (SSPD) tunnel mode.  Such usage is more complicated   because IPv6 prefixes need to be known a priori, and multicast and   link-local traffic do not work over such a tunnel.  Fragment handling   in tunnel mode is also more difficult.  However, because the Mobility   and Multihoming Protocol (MOBIKE) [RFC4555] supports only tunnel   mode, when the IPv4 endpoints of a tunnel are dynamic and the other   constraints are not applicable, using tunnel mode may be an   acceptable solution.   Therefore, our primary recommendation is to use transport mode   applied to a tunnel interface.  Source address spoofing can be   limited by enabling ingress filtering on the tunnel interface.   Manual keying must not be used as large amounts of IPv6 traffic may   be carried over the tunnels and doing so would make it easier for an   attacker to recover the keys.  IKEv1 or IKEv2 must be used for   establishing the IPsec SAs.  IKEv2 should be used where supported and   available; if not, IKEv1 may be used instead.7.  Security Considerations   When running 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 signaled   somehow (e.g., using authentication protocol and obtaining a static   v6 prefix), someone might be able to spoof the signaling (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.Graveman, et al.             Informational                     [Page 13]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   IPsec, with ESP, offers integrity and data origin authentication,   confidentiality, and optional (at the discretion of the receiver)   anti-replay features.  Using confidentiality without integrity is   discouraged.  ESP furthermore provides limited traffic flow   confidentiality.   IPsec provides access control mechanisms through the distribution of   keys and also through the application 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 [RFC4306].   Please note that using IPsec for the scenarios described in Figures   1, 2, and 3 does not aim to protect the end-to-end communication.  It   protects just the tunnel part.  It is still possible for an IPv6   endpoint not attached to the IPsec tunnel to spoof packets.8.  Contributors   The authors are listed in alphabetical order.   Suresh Satapati also participated in the initial discussions on this   topic.9.  Acknowledgments   The authors would like to thank Stephen Kent, Michael Richardson,   Florian Weimer, Elwyn Davies, Eric Vyncke, Merike Kaeo, Alfred   Hoenes, Francis Dupont, and David Black for their substantive   feedback.   We would like to thank Pasi Eronen for his text contributions and   suggestions for improvement.Graveman, et al.             Informational                     [Page 14]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 200710.  References10.1.  Normative References   [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.   [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.   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms              for IPv6 Hosts and Routers",RFC 4213, October 2005.   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",RFC 4303, December 2005.   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",RFC 4306, December 2005.10.2.  Informative References   [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.   [RFC3193]  Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth,              "Securing L2TP using IPsec",RFC 3193, November 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.Graveman, et al.             Informational                     [Page 15]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, "Encapsulating              MPLS in IP or Generic Routing Encapsulation (GRE)",RFC 4023, March 2005.   [RFC4302]  Kent, S., "IP Authentication Header",RFC 4302,              December 2005.   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol              (MOBIKE)",RFC 4555, June 2006.   [RFC4718]  Eronen, P. and P. Hoffman, "IKEv2 Clarifications and              Implementation Guidelines",RFC 4718, October 2006.   [TUNN-AD]  Palet, J. and M. Diaz, "Analysis of IPv6 Tunnel End-point              Discovery Mechanisms", Work in Progress, January 2005.   [VLINK]    Duffy, M., "Framework for IPsec Protected Virtual Links              for PPVPNs", Work in Progress, October 2002.Graveman, et al.             Informational                     [Page 16]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007Appendix A.  Using Tunnel Mode   First, we describe the different tunnel mode implementation methods.   We note that, in this context, only the so-called Specific SPD (SSPD)   model (without a tunnel interface) can be made to work, but it has   reduced applicability, and the use of a transport mode tunnel is   recommended instead.  However, we will describe how the SSPD tunnel   mode might look if one would like to use it in any case.A.1.  Tunnel Mode Implementation Methods   Tunnel mode could (in theory) 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" addresses ("::/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 similar to transport mode.       The SPDs must be interface-specific.  However, because IKE uses       IPv4 but the tunnel is IPv6, there is no standard solution to map       the IPv4 interface to IPv6 interface [VLINK] and this approach is       not feasible.   2.  "Specific SPDs": some implementations do not model the tunnel       mode SA as an IP interface.  Traffic selection is 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 Duplicate       Address Detection (DAD), Multicast Listener Discovery (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 it explicitly when   transport mode or option (1) is chosen.   In summary, there does not appear to be a standard solution in this   context for the first implementation approach.Graveman, et al.             Informational                     [Page 17]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   The second approach can be made to work, but is only applicable in   host-to-host or site-to-router/router-to-site scenarios (i.e., when   the IPv6 prefixes can be known a priori), and it offers only a   limited set of features (e.g., no multicast) compared with a   transport mode tunnel.   When tunnel mode is used, fragment handling [RFC4301] may also be   more difficult compared with transport mode and, depending on   implementation, may need to be reflected in SPDs.A.2.  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 IPV6-EP1 and IPV6-EP2 (global   addresses), and the IPV4 addresses of the tunnel endpoints are   denoted IPV4-TEP1 and IPV4-TEP2, respectively.   Host1's SPD:                                Next Layer   Rule     Local     Remote     Protocol   Action   ----     -----     ------    ---------- --------     1     IPV6-EP1  IPV6-EP2      ESP      BYPASS     2     IPV6-EP1  IPV6-EP2      IKE      BYPASS     3     IPv6-EP1  IPV6-EP2       41      PROTECT(ESP,                                            tunnel{IPV4-TEP1,IPV4-TEP2})   Host2's SPD:                                Next Layer   Rule     Local     Remote     Protocol   Action   ----     -----     ------    ---------- --------     1     IPV6-EP2  IPV6-EP1      ESP      BYPASS     2     IPV6-EP2  IPV6-EP1      IKE      BYPASS     3     IPv6-EP2  IPV6-EP1       41      PROTECT(ESP,                                            tunnel{IPV4-TEP2,IPV4-TEP1})   "IKE" refers to UDP destination port 500 and possibly also   port 4500 if NAT traversal is used.   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.Graveman, et al.             Informational                     [Page 18]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007A.3.  Specific SPD for Host-to-Router Scenario   The following SPD entries assume that the host has the IPv6 address   IPV6-EP1 and the tunnel endpoints 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 an IPV6-PREF/48 to the   host, the corresponding SPD entries can be derived by replacing IPV6-   EP1 with IPV6-PREF/48.   Please note the bypass entry for host's SPD, absent in router's SPD.   While this might be an implementation matter for host-to-router   tunneling, having a similar entry, "Local=IPV6-PREF/48 & Remote=IPV6-   PREF/48", is critical for site-to-router tunneling.   Host's SPD:                                Next Layer   Rule     Local     Remote     Protocol   Action   ----     -----     ------    ---------- --------     1     IPV6-EP1  IPV6-EP2      ESP      BYPASS     2     IPV6-EP1  IPV6-EP2      IKE      BYPASS     3     IPV6-EP1  IPV6-EP1      ANY      BYPASS     4     IPV6-EP1    ANY         ANY      PROTECT(ESP,                                            tunnel{IPV4-TEP1,IPV4-TEP2})   Router's SPD:                                Next Layer   Rule     Local     Remote     Protocol   Action   ----     -----     ------    ---------- --------     1     IPV6-EP2  IPV6-EP1      ESP      BYPASS     2     IPV6-EP2  IPV6-EP1      IKE      BYPASS     3       ANY     IPV6-EP1      ANY      PROTECT(ESP,                                            tunnel{IPV4-TEP1,IPV4-TEP2})   The IDci and IDcr payloads of IKEv1 carry the IPV6-EP1 and   ID_IPV6_ADDR_RANGE or ID_IPV6_ADDR_SUBNET as their phase 2   identities.  The starting address is zero 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.Graveman, et al.             Informational                     [Page 19]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007Appendix B.  Optional FeaturesB.1.  Dynamic Address Configuration   With the exchange of protected configuration payloads, IKEv2 is able   to provide the IKEv2 peer with Dynamic Host Configuration Protocol   (DHCP)-like information payloads.  These configuration payloads are   exchanged between the IKEv2 initiator and responder.   This could be used (for example) by the host in the host-to-router   scenario to obtain an IPv6 address from the ISP as part of setting up   the IPsec tunnel mode SA.  The details of these procedures are out of   scope for this memo.B.2.  NAT Traversal and Mobility   Network address (and port) translation devices are commonly found in   today's networks.  A detailed description of the problem and   requirements of IPsec-protected data traffic traversing a NAT is   provided in [RFC3715].   IKEv2 can detect the presence of a NAT automatically by sending   NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads in   the initial IKE_SA_INIT exchange.  Once a NAT is detected and both   endpoints 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 set up both       a secure tunnel and a tunnel that 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 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.Graveman, et al.             Informational                     [Page 20]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007   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, because 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.  MOBIKE [RFC4555] provides a solution when IKEv2 is   used, but it only supports tunnel mode.B.3.  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 endpoint for the configured IPv6-in-IPv4   tunnel.   This problem is also discussed at more length in [TUNN-AD].   However, simply discovering the tunnel endpoint is not sufficient for   establishing an IKE session with the peer.  The PAD information (seeSection 5.2) also needs to be learned dynamically.  Hence, currently,   automatic endpoint discovery provides benefit only if PAD information   is chosen in such a manner that it is not IP-address specific.Graveman, et al.             Informational                     [Page 21]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007Authors' Addresses   Richard Graveman   RFG Security, LLC   15 Park Avenue   Morristown, NJ  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   Finland   EMail: psavola@funet.fi   Hannes Tschofenig   Nokia Siemens Networks   Otto-Hahn-Ring 6   Munich, Bayern  81739   Germany   EMail: Hannes.Tschofenig@nsn.comGraveman, et al.             Informational                     [Page 22]

RFC 4891            IPsec with IPv6-in-IPv4 Tunnels             May 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Graveman, et al.             Informational                     [Page 23]

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