RFC5739

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Internet Engineering Task Force (IETF) P. Eronen Request for Comments: 5739 Nokia Category: Experimental J. Laganier ISSN: 2070-1721 QUALCOMM, Inc.

                                                           C. Madson
                                                       Cisco Systems
                                                       February 2010
IPv6 Configuration in Internet Key Exchange Protocol Version 2 (IKEv2)

Abstract

When Internet Key Exchange Protocol version 2 (IKEv2) is used for remote VPN access (client to VPN gateway), the gateway assigns the client an IP address from the internal network using IKEv2 configuration payloads. The configuration payloads specified in RFC 4306 work well for IPv4 but make it difficult to use certain features of IPv6. This document specifies new configuration attributes for IKEv2 that allows the VPN gateway to assign IPv6 prefixes to clients, enabling all features of IPv6 to be used with the client-gateway "virtual link".

Status of This Memo

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.

This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5739.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Introduction and Problem Statement

In typical remote access VPN use (client to VPN gateway), the client needs an IP address in the network protected by the security gateway. IKEv2 includes a feature called "configuration payloads" that allows the gateway to dynamically assign a temporary address to the client [IKEv2].

For IPv4, the message exchange would look as follows:

  Client      Gateway
 --------    ---------
  HDR(IKE_SA_INIT), SAi1, KEi, Ni  -->
           <--  HDR(IKE_SA_INIT), SAr1, KEr, Nr, [CERTREQ]
  HDR(IKE_AUTH),
  SK { IDi, CERT, [CERTREQ], AUTH, [IDr],
       CP(CFG_REQUEST) =
          { INTERNAL_IP4_ADDRESS(),
            INTERNAL_IP4_DNS() }, SAi2,
       TSi = (0, 0-65535, 0.0.0.0-255.255.255.255),
       TSr = (0, 0-65535, 0.0.0.0-255.255.255.255) }  -->
         <--  HDR(IKE_AUTH),
              SK { IDr, CERT, AUTH,
                   CP(CFG_REPLY) =
                      { INTERNAL_IP4_ADDRESS(192.0.2.234),
                        INTERNAL_IP4_DNS(198.51.100.33) },
                   SAr2,
                   TSi = (0, 0-65535, 192.0.2.234-192.0.2.234),
                   TSr = (0, 0-65535, 0.0.0.0-255.255.255.255) }
                   Figure 1: IPv4 Configuration

The IPv4 case has been implemented by various vendors and, in general, works well. IKEv2 also defines almost identical configuration payloads for IPv6:

  Client      Gateway
 --------    ---------
  HDR(IKE_AUTH),
  SK { IDi, CERT, [CERTREQ], AUTH, [IDr],
       CP(CFG_REQUEST) =
          { INTERNAL_IP6_ADDRESS(),
            INTERNAL_IP6_DNS() }, SAi2,
       TSi = (0, 0-65535,
              0:0:0:0:0:0:0:0 -
              FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF),
       TSr = (0,
              0-65535, 0:0:0:0:0:0:0:0 -
              FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF) }  -->
         <--  HDR(IKE_AUTH),
              SK { IDr, CERT, AUTH,
                   CP(CFG_REPLY) =
                      { INTERNAL_IP6_ADDRESS(2001:DB8:0:1:2:3:4:5,
                                             64),
                        INTERNAL_IP6_DNS(2001:DB8:9:8:7:6:5:4) },
                   SAr2,
                   TSi = (0, 0-65535,
                          2001:DB8:0:1:2:3:4:5 -
                          2001:DB8:0:1:2:3:4:5),
                   TSr = (0, 0-65535,
                          0:0:0:0:0:0:0:0 -
                          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF) }
                   Figure 2: IPv6 Configuration

In other words, IPv6 is basically treated as IPv4 with larger addresses. As noted in RFC4718, this does not fully follow the "normal IPv6 way of doing things", and it complicates or prevents using certain features of IPv6. Section 3 describes the limitations in detail.

This document specifies new configuration attributes for IKEv2 that allows the VPN gateway to assign IPv6 prefixes to clients, enabling all features of IPv6 to be used with the client-gateway "virtual link".

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 [KEYWORDS].

When messages containing IKEv2 payloads are described, optional payloads are shown in brackets (for instance, "[FOO]"); a plus sign indicates that a payload can be repeated one or more times (for instance, "FOO+").

This document uses the term "virtual interface" when describing how the client uses the IPv6 address(es) assigned by the gateway. While existing IPsec documents do not use this term, it is not a new concept. In order to use the address assigned by the VPN gateway, current VPN clients already create a local "virtual interface", as only addresses assigned to interfaces can be used, e.g., as source addresses for TCP connections. Note that this definition of "interface" is not necessarily identical with what some particular implementations call "interface".

Current Limitations and Goals

This section describes the limitations of the current IPv6 configuration mechanism and requirements for the new solution.

Multiple Prefixes

In Figure 2, only a single IPv6 address (from a single prefix) is assigned. The specification does allow the client to include multiple INTERNAL_IP6_ADDRESS attributes in its request, but the gateway cannot assign more addresses than the client requested.

Multiple prefixes are useful for site renumbering, host-based site multihoming [SHIM6], and unique local IPv6 addresses RFC4193. In all of these cases, the gateway has better information on how many different addresses (from different prefixes) the client should be assigned.

The solution should support assigning addresses from multiple prefixes, without requiring the client to know beforehand how many prefixes are needed.

Link-Local Addresses

The IPv6 addressing architecture [IPv6Addr] specifies that "IPv6 addresses of all types are assigned to interfaces, not nodes. [..] All interfaces are required to have at least one Link-Local unicast address".

Currently, the virtual interface created by IKEv2 configuration payloads does not have link-local addresses. This violates the requirements in [IPv6Addr] and prevents the use of protocols that require link-local addresses, such as [MLDv2] and [DHCPv6].

The solution should assign link-local addresses to the virtual interfaces and allow them to be used for protocols between the VPN client and gateway.

Interface Identifier Selection

In the message exchange shown in Figure 2, the gateway chooses the interface ID used by the client. It is also possible for the client to request a specific interface ID; the gateway then chooses the prefix part.

This approach complicates the use of Cryptographically Generated Addresses (CGAs) [CGA]. With CGAs, the interface ID cannot be calculated before the prefix is known. The client could first obtain a non-CGA address to determine the prefix and then send a separate CFG_REQUEST to obtain a CGA address with the same prefix. However, this approach requires that the IKEv2 software component provide an interface to the component managing CGAs; an ugly implementation dependency that would be best avoided.

Similar concerns apply to other cases where the client has some interest in what interface ID is being used, such as Hash-Based Addresses [HBA] and privacy addresses RFC4941.

Without CGAs and HBAs, VPN clients are not able to fully use IPv6 features such as [SHIM6] or enhanced Mobile IPv6 route optimization RFC4866.

The solution should allow the VPN client to easily obtain several addresses from a given prefix, where the interface IDs are selected by the client and may depend on the prefix.

Sharing VPN Access

Some VPN clients may want to share the VPN connection with other devices (e.g., from a cell phone to a laptop or vice versa) via some local area network connection (such as Wireless LAN or Bluetooth), if allowed by the security policy.

Quite obviously, sharing of VPN access requires more than one address (unless NAT is used). However, the current model where each address is requested separately is probably complex to integrate with a local area network that uses stateless address autoconfiguration [AUTOCONF]. Thus, obtaining a whole prefix for the VPN client and advertising that to the local link (something resembling [NDProxy]) would be preferable. With DHCPv6 prefix delegation RFC3633, even [NDProxy] and associated multi-link subnet issues would be avoided.

The solution should support sharing the VPN access over a local area network connection when the other hosts are using stateless address autoconfiguration.

General Goals

o The solution should avoid periodic messages over the VPN tunnel.

o Reauthentication should work, where the client can start a new IKE

  Security Association (SA) and continue using the same addresses as
  before.

o There should be compatibility with other IPsec uses. Configuring

  a virtual IPv6 link (with addresses assigned in IKEv2) should not
  prevent the same peers from using IPsec/IKEv2 for other uses (with
  other addresses).  In particular, the peers may have Security
  Policy Database (SPD) entries and Peer Authorization Database
  (PAD) Child SA Authorization Data entries that are not related to
  the virtual link; when a CHILD_SA is created, it should be
  unambiguous which entries are used.

o There should be compatibility with current IPv6 configuration.

  Although the current IPv6 mechanism is not widely implemented, new
  solutions should not preclude its use (e.g., by defining
  incompatible semantics for the existing payloads).

o The solution should have clean implementation dependencies. In

  particular, it should not require significant modifications to the
  core IPv6 stack (typically part of the operating system) or
  require the IKEv2 implementor to re-implement parts of the IPv6
  stack (e.g., to have access or control to functionality that is
  currently not exposed by interfaces of the IPv6 stack).

o Re-use existing mechanisms as much as possible, as described in

  [IPConfig].  Appendix A describes the rationale of why this
  document nevertheless uses IKEv2 configuration payloads for
  configuring the addresses.  However, Section 4.1 recommends using
  a DHCPv6 Information-Request message for obtaining other
  configuration information (such as DNS server addresses).

Non-Goals

Mobile IPv6 already defines how it interacts with IPsec/IKEv2 RFC4877, and the intent of this document is not to change that interaction in any way.

Additional Information

If the VPN client is assigned IPv6 address(es) from prefix(es) that are shared with other VPN clients, this results in some kind of multi-link subnet. [Multilink] describes issues associated with multi-link subnets and recommends that they be avoided.

The original 3GPP specifications for IPv6 assigned a single IPv6 address to each mobile phone, resembling current IKEv2 payloads. RFC3314 describes the problems with this approach and caused 3GPP to change the specifications to assign unique /64 prefix(es) for each phone.

Due to similar concerns, the IEEE 802.16 IPv6 Convergence Sublayer RFC5121 and Proxy Mobile IPv6 RFC5213 also assign unique prefixes.

Solution Details

Initial Exchanges

During IKE_AUTH, the client sends a new configuration attribute, INTERNAL_IP6_LINK, which requests a virtual link to be configured. The attribute contains the client's interface ID for the link-local address (other addresses may use other interface IDs). Typically, the client would also ask for the DHCPv6 server address; this is used only for configuration (such as DNS server addresses), not address assignment.

   CP(CFG_REQUEST) =
      { INTERNAL_IP6_LINK(Client's Link-Local Interface ID)
        INTERNAL_IP6_DHCP() }
   TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

If the client has sent the INTERNAL_IP6_LINK configuration attribute, the VPN gateway SHOULD ignore any INTERNAL_IP6_ADDRESS configuration attribute present in the request.

The VPN gateway MUST choose for itself a link-local interface identifier different than the client's, i.e., accept the link-local interface identifier proposed by the client. In case the VPN gateway cannot accept the link-local interface identifier the client proposed, the VPN gateway MUST fail the IPv6 address assignment by including a NOTIFY payload with the INTERNAL_ADDRESS_FAILURE message.

The VPN gateway then replies with an INTERNAL_IP6_LINK configuration attribute that contains the IKEv2 Link ID (an identifier selected by the VPN gateway, treated as an opaque octet string by the client -- this will be used for reauthentication and CREATE_CHILD_SA messages), the gateway's link-local interface identifier, and zero or more INTERNAL_IP6_PREFIX attributes. The traffic selectors proposed by the initiator are also narrowed to contain only the assigned prefixes and the client link-local address (FE80::<Client's Interface ID>) identifier.

   CP(CFG_REPLY) =
      { INTERNAL_IP6_LINK(Gateway's Link-Local Interface ID,
                          IKEv2 Link ID)
        INTERNAL_IP6_PREFIX(Prefix1/64),
        [INTERNAL_IP6_PREFIX(Prefix2/64),...],
        INTERNAL_IP6_DHCP(Address) }
   TSi = ((0, 0-65535,
           FE80::<Client's Interface ID> -
           FE80::<Client's Interface ID>)
          (0, 0-65535,
           Prefix1::0 -
           Prefix1::FFFF:FFFF:FFFF:FFFF),
          [(0, 0-65535,
            Prefix2::0 -
            Prefix2::FFFF:FFFF:FFFF:FFFF), ...])
   TSr = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

At this point, the client can configure its link-local address (FE80::<Client's Interface ID>) and other non-link-local unicast addresses from the assigned prefixes (with any proper interface identifier [IPv6Addr]). The VPN gateway MUST NOT simultaneously assign the same prefixes to any other client and MUST NOT itself configure addresses from these prefixes. Thus, the client does not have to perform Duplicate Address Detection (DAD). (This approach is based on [IPv6PPP].)

The prefixes remain valid through the lifetime of the IKE SA (and its continuations via rekeying). If the VPN gateway needs to remove a prefix it has previously assigned, or assign a new prefix, it can do so with reauthentication (either starting reauthentication itself or requesting the client to reauthenticate using RFC4478).

The client also contacts the DHCPv6 server. This is the RECOMMENDED way to obtain additional configuration parameters (such as DNS server addresses), as it allows easier extensibility and more options (such as the domain search list for DNS).

Reauthentication

When the client performs reauthentication (and wants to continue using the same "virtual link"), it includes the IKEv2 Link ID given by the gateway in the INTERNAL_IP6_LINK attribute.

  CP(CFG_REQUEST) =
     { INTERNAL_IP6_LINK(Client's Link Local Interface ID,
                         IKEv2 Link ID)
       INTERNAL_IP6_DHCP() }
  TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
         FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
  TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
         FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

At this point, the gateway MUST verify that the client is indeed allowed to use the link identified by the IKEv2 Link ID. The same situation occurs in [IKEv2] when the client wants to continue using the same IPv4 address with the INTERNAL_IP4_ADDRESS configuration attribute. Typically, the gateway would use the Link ID to look up relevant local state and compare the authenticated peer identity of the IKE_SA with the local state.

If the client is allowed to continue using this link, the gateway replies (see Section 4.1) with the same gateway's link-local interface ID and IKEv2 Link ID as used earlier and sends the IPv6 prefix(es) associated with this link. Usually, the IPv6 prefix(es) will also be the same as earlier, but this is not required.

If the client is not allowed to continue using this link, the gateway treats it as a request for a new virtual link, selects a different IKEv2 Link ID value, and replies as in Section 4.1.

Creating CHILD_SAs

When a CHILD_SA is created, the peers need to determine which SPD entries and PAD Child SA Authorization Data entries are used for this CHILD_SA. In the basic client-to-VPN-gateway uses, the situation is simple: all the matching SPD entries and Child SA Authorization Data entries are related to the "virtual link" between the VPN client and the VPN gateway. However, if the same peers are also using IPsec/ IKEv2 for other uses (with addresses not assigned inside IKEv2), they would also have SPD entries and PAD Child SA Authorization Data that is not related to the virtual link.

If one of the peers requests a CHILD_SA and proposes traffic selectors covering everything (like in Figure 2), should those be narrowed to the prefixes configured with INTERNAL_IP6_PREFIX or to

the other SPD/PAD entries? While some kind of heuristics are possible (see Appendix A for discussion), this document specifies an explicit solution:

The peers MUST include a LINK_ID notification, containing the IKEv2 Link ID, in all CREATE_CHILD_SA requests (including rekeys) that are related to the virtual link. The LINK_ID notification is not included in the CREATE_CHILD_SA response or when doing IKE_SA rekeying.

Relationship to Neighbor Discovery

Neighbor Discovery [IPv6ND] specifies the following mechanisms:

Router Discovery, Prefix Discovery, Parameter Discovery, and address autoconfiguration are not used, as the necessary functionality is implemented in IKEv2.

Address Resolution, Next-hop Determination, and Redirect are not used, as the virtual link does not have link-layer addresses and is a point-to-point link.

Neighbor Unreachability Detection could be used but is a bit redundant given IKEv2 Dead Peer Detection.

Duplicate Address Detection is not needed because this is a point-to- point link, where the VPN gateway does not assign any addresses from the global unicast prefixes, and the link-local interface identifier is negotiated separately.

Duplicate Address Detection is not needed for global unicast addresses formed from the global unicast prefix(es) configured as part of the IKEv2 exchange, because this is a point-to-point link, where the VPN gateway does not assign any addresses from the global unicast prefixes. Duplicate Address Detection may be needed for link-local addresses, e.g., when the client configures a link-local address as per RFC4941.

Thus, Duplicate Address Detection MAY be skipped for global unicast addresses formed from the global unicast prefix(es) configured as part of the IKEv2 exchange. However, Duplicate Address Detection for link-local unicast addresses MUST be performed as required per some other specifications, e.g., RFC4941.

Relationship to Existing IKEv2 Payloads

The mechanism described in this document is not intended to be used at the same time as the existing INTERNAL_IP6_ADDRESS attribute. For compatibility with gateways implementing only INTERNAL_IP6_ADDRESS, the VPN client MAY include attributes for both mechanisms in CFG_REQUEST. The capabilities and preferences of the VPN gateway will then determine which is used.

All other attributes except INTERNAL_IP6_ADDRESS (and INTENAL_ADDRESS_EXPIRY) from [IKEv2] remain valid, including the somewhat confusingly named INTERNAL_IP6_SUBNET (see Section 6.3 of RFC4718 for discussion).

Payload Formats

INTERNAL_IP6_LINK Configuration Attribute

The INTERNAL_IP6_LINK configuration attribute is formatted as follows:

                    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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ !R| Attribute Type ! Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link-Local | | Interface ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ IKEv2 Link ID ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

o Reserved (1 bit) - See [IKEv2].

o Attribute Type (15 bits) - INTERNAL_IP6_LINK (17).

o Length (2 octets) - Length in octets of the Value field (Link-

  Local Interface ID and IKEv2 Link ID); 8 or more.

o Link-Local Interface ID (8 octets) - The Interface ID used for

  link-local address (by the party that sent this attribute).

o IKEv2 Link ID (variable length) - The Link ID (may be empty when

  the client does not yet know the Link ID).  The Link ID is
  selected by the VPN gateway and is treated as an opaque octet
  string by the client.

INTERNAL_IP6_PREFIX Configuration Attribute

The INTERNAL_IP6_PREFIX configuration attribute is formatted as follows:

                    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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ !R| Attribute Type ! Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Prefix | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix Length | +-+-+-+-+-+-+-+-+

o Reserved (1 bit) - See [IKEv2].

o Attribute Type (15 bits) - INTERNAL_IP6_PREFIX (18).

o Length (2 octets) - Length in octets of the Value field; in this

  case, 17.

o Prefix (16 octets) - An IPv6 prefix assigned to the virtual link.

  The low-order bits of the prefix field that are not part of the
  prefix MUST be set to zero by the sender and MUST be ignored by
  the receiver.

o Prefix Length (1 octet) - The length of the prefix in bits;

  usually 64.

LINK_ID Notify Payload

The LINK_ID notification is included in CREATE_CHILD_SA requests to indicate that the SA being created is related to the virtual link. If this notification is not included, the CREATE_CHILD_SA requests are related to the real interface.

The Notify Message Type for LINK_ID is 16414. The Protocol ID and SPI Size fields are set to zero. The data associated with this notification is the IKEv2 Link ID returned in the INTERNAL_IP6_LINK configuration attribute.

IANA Considerations

This document defines two new IKEv2 configuration attributes, whose values have been allocated from the "IKEv2 Configuration Payload Attribute Types" namespace [IKEv2]:

                                   Multi-
  Value    Attribute Type          Valued  Length         Reference
  ------   ----------------------  ------  -------------  ---------
  17       INTERNAL_IP6_LINK       NO      8 or more      RFC5739
  18       INTERNAL_IP6_PREFIX     YES     17 octets      RFC5739

This document also defines one new IKEv2 notification, whose value has been allocated from the "IKEv2 Notify Message Types - Status Types" namespace [IKEv2]:

  Value   Notify Messages - Status Types   Reference
  ------  -------------------------------  ---------
  16414   LINK_ID                          RFC5739

This document does not create any new namespaces to be maintained by IANA.

Security Considerations

Since this document is an extension to IKEv2, the security considerations in [IKEv2] apply here as well.

The mechanism described in this document assigns each client a unique prefix, which makes using randomized interface identifiers RFC4941 ineffective from a privacy point of view: the client is still uniquely identified by the prefix. In some environments, it may be preferable to assign a VPN client the same prefix each time a VPN connection is established; other environments may prefer assigning a different prefix every time for privacy reasons. (This is basically a similar trade-off as in Mobile IPv6 -- using the same Home Address forever is simpler than changing it often, but has privacy implications.)

Acknowledgments

The authors would like to thank Patrick Irwin, Tero Kivinen, Chinh Nguyen, Mohan Parthasarathy, Yaron Sheffer, Hemant Singh, Dave Thaler, Yinghzhe Wu, and Fan Zhao for their valuable comments.

Many of the challenges associated with IPsec-protected "virtual interfaces" have been identified before, for example, in the context of protecting IPv6-in-IPv4 tunnels with IPsec RFC4891, Provider

Provisioned VPNs ([VLINK], RFC3884), and Mobile IPv6 RFC4877. Some of the limitations of assigning a single IPv6 address were identified in RFC3314.

References

Normative References

[IKEv2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",

            RFC 4306, December 2005.

[IPv6Addr] Hinden, R. and S. Deering, "IP Version 6 Addressing

            Architecture", RFC 4291, February 2006.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate

            Requirement Levels", BCP 14, RFC 2119, March 1997.

Informative References

[AUTOCONF] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless

            Address Autoconfiguration", RFC 4862, September 2007.

[CGA] Aura, T., "Cryptographically Generated Addresses (CGA)",

            RFC 3972, March 2006.

[DHCPv6] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,

            and M. Carney, "Dynamic Host Configuration Protocol for
            IPv6 (DHCPv6)", RFC 3315, July 2003.

[HBA] Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,

            June 2009.

[IPConfig] Aboba, B., Thaler, D., Andersson, L., and S. Cheshire,

            "Principles of Internet Host Configuration", RFC 5505,
            May 2009.

[IPv6ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,

            "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
            September 2007.

[IPv6PPP] Varada, S., Haskins, D., and E. Allen, "IP Version 6

            over PPP", RFC 5072, September 2007.

[MLDv2] Vida, R. and L. Costa, "Multicast Listener Discovery

            Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

[MOBIKE] Eronen, P., "IKEv2 Mobility and Multihoming Protocol

            (MOBIKE)", RFC 4555, June 2006.

[Multilink] Thaler, D., "Multi-Link Subnet Issues", RFC 4903,

            June 2007.

[NDProxy] Thaler, D., Talwar, M., and C. Patel, "Neighbor

            Discovery Proxies (ND Proxy)", RFC 4389, April 2006.

RFC3314 Wasserman, M., "Recommendations for IPv6 in Third

            Generation Partnership Project (3GPP) Standards",
            RFC 3314, September 2002.

RFC3456 Patel, B., Aboba, B., Kelly, S., and V. Gupta, "Dynamic

            Host Configuration Protocol (DHCPv4) Configuration of
            IPsec Tunnel Mode", RFC 3456, January 2003.

RFC3633 Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic

            Host Configuration Protocol (DHCP) version 6", RFC 3633,
            December 2003.

RFC3884 Touch, J., Eggert, L., and Y. Wang, "Use of IPsec

            Transport Mode for Dynamic Routing", RFC 3884,
            September 2004.

RFC4193 Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast

            Addresses", RFC 4193, October 2005.

RFC4478 Nir, Y., "Repeated Authentication in Internet Key

            Exchange (IKEv2) Protocol", RFC 4478, April 2006.

RFC4718 Eronen, P. and P. Hoffman, "IKEv2 Clarifications and

            Implementation Guidelines", RFC 4718, October 2006.

RFC4866 Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route

            Optimization for Mobile IPv6", RFC 4866, May 2007.

RFC4877 Devarapalli, V. and F. Dupont, "Mobile IPv6 Operation

            with IKEv2 and the Revised IPsec Architecture",
            RFC 4877, April 2007.

RFC4891 Graveman, R., Parthasarathy, M., Savola, P., and H.

            Tschofenig, "Using IPsec to Secure IPv6-in-IPv4
            Tunnels", RFC 4891, May 2007.

RFC4941 Narten, T., Draves, R., and S. Krishnan, "Privacy

            Extensions for Stateless Address Autoconfiguration in
            IPv6", RFC 4941, September 2007.

RFC5121 Patil, B., Xia, F., Sarikaya, B., Choi, JH., and S.

            Madanapalli, "Transmission of IPv6 via the IPv6
            Convergence Sublayer over IEEE 802.16 Networks",
            RFC 5121, February 2008.

RFC5213 Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury,

            K., and B. Patil, "Proxy Mobile IPv6", RFC 5213,
            August 2008.

[SHIM6] Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming

            Shim Protocol for IPv6", RFC 5533, June 2009.

[VLINK] Duffy, M., "Framework for IPsec Protected Virtual Links

            for PPVPNs", Work in Progress, October 2002.

Appendix A. Design Rationale (Non-Normative)

This appendix describes some of the reasons why the solution in Section 4 was selected and lists some alternative designs that were considered but were ultimately rejected.

Assigning a new IPv6 address to the client creates a new "virtual IPv6 interface" and "virtual link" between the client and the gateway. We will assume that the virtual link has the following properties:

o The link and its interfaces are created and destroyed by the IKEv2

  process.

o The link is not an IPsec SA; at any time, there can be zero or

  more IPsec SAs covering traffic on this link.

o The link is not a single IKE SA; to support reauthentication, it

  must be possible to identify the same link in another IKE SA.

o Not all IPsec-protected traffic between the peers is necessarily

  related to the virtual link (although in the simplest VPN client-
  to-gateway scenario, it will be).

Given these assumptions and the goals described in Section 3, it seems that the most important design choices to be made are the following:

o What link/subnet model is used; in other words, how relationships

  between VPN clients, IPv6 subnet prefixes, and link-local traffic
  (especially link-local multicast) are organized.

o How information about the IPv6 prefix(es) is distributed from the

  gateway to the clients.

o How to ensure unique IPv6 addresses for each client and keep

  forwarding state up-to-date accordingly.

o How layer 3 access control is done; in other words, where the

  mechanisms for preventing address spoofing by clients are placed
  architecturally.

Each of these is discussed next in turn.

A.1. Link Model

There are at least three main choices for how to organize the relationships between VPN clients, IPv6 subnet prefixes, and link- local traffic:

o Point-to-point link model: each VPN client is assigned one or more

  IPv6 prefixes.  These prefixes are not shared with other clients,
  and there is no link-local traffic between different VPN clients
  connected to the same gateway.

o Multi-access link model: multiple VPN clients share the same IPv6

  prefix.  Link-local multicast packets sent by one VPN client will
  be received by other VPN clients (VPN gateway will forward the
  packets, possibly with Multicast Listener Discovery (MLD) snooping
  to remove unnecessary packets).

o "Router aggregation" link model: one form of "multi-link" subnet

  [Multilink] where multiple VPN clients share the same IPv6 prefix.
  Link-local multicast will not be received by other VPN clients.

In the multi-access link model, VPN clients who are idle (i.e., not currently sending or receiving application traffic) could receive significant amounts of multicast packets from other clients (depending on how many other clients are connected). This is especially undesirable when the clients are battery-powered such as a PDA that keeps the VPN connection to corporate intranet active 24/7. For this reason, using the multi-access link model was rejected.

The configuration attributes specified in Section 4 use the point-to- point link model.

A.2. Distributing Prefix Information

Some types of addresses, such as CGAs, require knowledge about the prefix before an address can be generated. The prefix information could be distributed to clients in the following ways:

o IKEv2 messages (configuration payloads)

o Router Advertisement messages (sent over the IPsec tunnel)

o DHCPv6 messages (sent over the IPsec tunnel)

In Section 4, the prefix information is distributed in IKEv2 messages.

A.3. Unique Address Allocation

In the "multi-access" and "router aggregation" link models (where a single IPv6 prefix is shared between multiple VPN clients), mechanisms are needed to ensure that one VPN client does not use an address already used by some other client. Also, the VPN gateway has to know which client is using which addresses in order to correctly forward traffic.

The main choices seem to be the following:

o Clients receive the address(es) they are allowed to use in IKEv2

  messages (configuration payloads).  In this case, keeping track of
  which client is using which address is trivial.

o Clients receive the address(es) they are allowed to use in DHCPv6

  messages sent over the IPsec tunnel.  In case the DHCPv6 server is
  not integrated with the VPN gateway, the gateway may need to work
  as a relay agent to keep track of which client is using which
  address (and update its forwarding state accordingly).

o Clients can use stateless address autoconfiguration to configure

  addresses and perform Duplicate Address Detection (DAD).  This is
  easy to do in a multi-access link model and can be made to work
  with a router aggregation link model if the VPN gateway traps
  Neighbor Solicitation (NS) messages and spoofs Neighbor
  Advertisement (NA) replies.  The gateway keeps track of which
  client is using which address (and updates its forwarding state
  accordingly) by trapping these NS/NA messages.

In the point-to-point link model, the client can simply use any address from the prefix, and the VPN gateway only needs to know which client is using which prefix in order to forward packets correctly.

A.4. Layer 3 Access Control

It is almost always desirable to prevent one VPN client from sending packets with a source address that is used by another VPN client. In order to correctly forward packets destined to clients, the VPN gateway obviously has to know which client is using which address; the question is therefore where, architecturally, the mechanisms for ingress filtering are placed.

o Layer 3 access control could be enforced by IPsec Security

  Association Database (SAD) / SPD; the addresses/prefixes assigned
  to a VPN client would be reflected in the traffic selectors used
  in IPsec Security Association and Security Policy Database
  entries, as negotiated in IKEv2.

o The ingress filtering capability could be placed outside IPsec;

  the traffic selectors in SAD/SPD entries would cover traffic that
  would be dropped later by ingress filtering.

The former approach is used by the current IPv4 solution and the mechanism specified in Section 4.

A.5. Other Considerations

VPN gateway state

  In some combinations of design choices, the amount of state
  information required in the VPN gateway depends not only on the
  number of clients but also on the number of addresses used by one
  client.  With privacy addresses and potentially some uses of
  Cryptographically Generated Addresses (CGAs), a single client
  could have a large number of different addresses (especially if
  different privacy addresses are used with different destinations).

Virtual link identifier

  Reauthentication requires a way to uniquely identify the virtual
  link when a second IKE SA is created.  Some possible alternatives
  are the IKE Security Parameter Indexes (SPIs) of the IKE SA where
  the virtual link was "created" (assuming we can't have multiple
  virtual links within the same IKE SA), a new identifier assigned
  when the link is created, or any unique prefix or address that
  remains assigned to the link for its entire lifetime.  Section 4
  specifies that the gateway assigns a new IKEv2 Link ID when the
  link is created.  The client treats the Link ID as an opaque octet
  string; the gateway uses it to identify relevant local state when
  reauthentication is done.
  Note that the link is not uniquely identified by the IKE peer
  identities (because IDi is often a user identity that can be used
  on multiple hosts at the same time) or the outer IP addresses of
  the peers (due to NAT Traversal and [MOBIKE]).

Prefix lifetime

  Prefixes could remain valid either for the lifetime of the IKE SA,
  until explicitly cancelled, or for an explicitly specified time.
  In Section 4, the prefixes remain valid for the lifetime of the
  IKE SA (and its continuations via rekeying but not via
  reauthentication).  If necessary, the VPN gateway can thus add or
  remove prefixes by triggering reauthentication.  It is assumed
  that adding or removing prefixes is a relatively rare situation,
  and thus this document does not specify more complex solutions
  (such as explicit prefix lifetimes or use of CFG_SET/CFG_ACK).

Compatibility with other IPsec uses

  Compatibility with other IPsec uses probably requires that when a
  CHILD_SA is created, both peers can determine whether the CHILD_SA
  applies to the virtual interface (at the end of the virtual link)
  or the real interfaces over which IKEv2 messages are being sent.
  This is required to select the correct SPD to be used for traffic-
  selector narrowing and SA authorization in general.
  One straight-forward solution is to add an extra payload to
  CREATE_CHILD_SA requests, containing the virtual link identifier.
  Requests not containing this payload would refer to the real link
  (over which IKEv2 messages are being sent).
  Another solution is to require that the peer requesting a CHILD_SA
  proposes traffic selectors that identify the link.  For example,
  if TSi includes the peer's "outer" IP address, it's probably
  related to the real interface, not the virtual one.  Or if TSi
  includes any of the prefixes assigned by the gateway (or the link-
  local or multicast prefix), it is probably related to the virtual
  interface.
  These heuristics can work in many situations but have proved
  inadequate in the context of IPv6-in-IPv4 tunnels RFC4891,
  Provider Provisioned VPNs ([VLINK], RFC3884), and Mobile IPv6
  RFC4877.  Thus, Section 4 includes the virtual link identifier
  in all CREATE_CHILD_SA requests that apply to the virtual
  interface.

Example of other IPsec uses:

  If a VPN gateway receives a CREATE_CHILD_SA request associated
  with a physical Ethernet interface, requesting an SA for
  (TSi=FE80::something, dst=*), it would typically reject the
  request (or, in other words, narrow it to an empty set) because it
  doesn't have SPD/PAD entries that would allow [email protected]
  to request such CHILD_SAs.
  (However, it might have SPD/PAD entries that would allow
  "neighboring-router.example.com" to create such SAs to protect,
  for example, some routing protocol that uses link-local
  addresses.)
  However, the virtual interface created when [email protected]
  authenticated and sent INTERNAL_IP6_LINK would have a different
  SPD/PAD, which would allow [email protected] to create this SA.

A.6. Alternative Solution Sketches

A.6.1. Version -00 Sketch

The -00 version of this document contained the following solution sketch, which is basically a combination of (1) a point-to-point link model, (2) prefix information distributed in Neighbor Advertisements, and (3) access control enforced outside IPsec.

1. During IKE_AUTH, the client sends a new configuration attribute,

   INTERNAL_IP6_LINK, which requests a virtual link to be created.
   The attribute contains the client's interface ID for the link-
   local address (other addresses may use other interface IDs).
   CP(CFG_REQUEST) =
      { INTERNAL_IP6_LINK(Link-Local Interface ID) }
   TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

The VPN gateway replies with its own link-local interface ID (which has to be different from the client's) and an IKEv2 Link ID (which will be used for reauthentication).

   CP(CFG_REPLY) =
     { INTERNAL_IP6_LINK(Link-Local Interface ID, IKEv2 Link ID) }
   TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

At this point, both peers configure the virtual interface with the link-local addresses.

2. The next step is IPv6 stateless address autoconfiguration, that

   is, Router Solicitation and Router Advertisement messages sent
   over the IPsec SA.
   ESP(Router Solicitation:
       src=::,
       dst=FF02:0:0:0:0:0:0:2)  -->
   <-- ESP(Router Advertisement:
           src=FE80::<Gateway's Interface ID>
           dst=FF02:0:0:0:0:0:0:1,
           Prefix1, [Prefix2...])

After receiving the Router Advertisement, the client can configure unicast addresses from the advertised prefixes, using any proper interface ID. The VPN gateway does not simultaneously assign the same prefixes to any other client and does not itself configure addresses from these prefixes. Thus, the client does not have to perform Duplicate Address Detection (DAD).

3. Reauthentication works basically the same way as in Section 4;

   the client includes the IKEv2 Link ID in the INTERNAL_IP6_LINK
   attribute.

4. Creating and rekeying IPsec SAs works basically the same way as

   in Section 4.3; the client includes the IKEv2 Link ID in those
   CHILD_SA requests that are related to the virtual link.

Comments: This was changed in the -01 version of this document based on feedback from VPN vendors; while the solution looks nice on paper, it is claimed to be unnecessarily complex to implement when the IKE implementation and IPv6 stack are from different companies. Furthermore, enforcing access control outside IPsec is a significant architectural change compared to current IPv4 solutions.

A.6.2. Router Aggregation Sketch #1

Hemant Singh helped sketch the following solution during the IETF 70 meeting in Vancouver. It combines (1) the router aggregation link model, (2) prefix information distributed in IKEv2 messages, (3) unique address allocation with stateless address autoconfiguration (with VPN gateway trapping NS messages and spoofing NA replies), and (4) access control enforced (partly) outside IPsec.

1. During IKE_AUTH, the client sends a new configuration attribute,

   INTERNAL_IP6_LINK, which requests a virtual link to be created.
   The attribute contains the client's interface ID for the link-
   local address (other addresses may use other interface IDs).
   CP(CFG_REQUEST) =
      { INTERNAL_IP6_LINK(Link-Local Interface ID) }
   TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

The VPN gateway replies with its own Link-Local Interface ID (which has to be different from the client's), an IKEv2 Link ID (which will be used for reauthentication and CREATE_CHILD_SA messages), and zero or more INTERNAL_IP6_PREFIX attributes. The traffic selectors proposed by the initiator are also narrowed to contain only the assigned prefixes (and the link-local prefix).

   CP(CFG_REPLY) =
      { INTERNAL_IP6_LINK(Link-Local Interface ID, IKEv2 Link ID),
        INTERNAL_IP6_PREFIX(Prefix1/64),
        [INTERNAL_IP6_PREFIX(Prefix2/64),...] }
   TSi = ((0, 0-65535,
           FE80::<Client's Interface ID> -
           FE80::<Client's Interface ID>)
          (0, 0-65535,
           Prefix1::0 -
           Prefix1::FFFF:FFFF:FFFF:FFFF),
          [(0, 0-65535,
            Prefix2::0 -
            Prefix2::FFFF:FFFF:FFFF:FFFF), ...])
   TSr = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

2. The client now configures tentative unicast addresses from the

   prefixes given by the gateway, and performs Duplicate Address
   Detection (DAD) for them.
   The Neighbor Solicitation messages are processed by the VPN
   gateway; if the target address is already in use by some other
   VPN client, the gateway replies with a Neighbor Advertisement.
   If the target address is not already in use, the VPN gateway
   notes that it is now being used by this client and updates its
   forwarding state accordingly.

Comments: The main disadvantages of this solution are non-standard processing of NS messages (which are used to update the gateway's forwarding state), and performing access control partly outside IPsec.

A.6.3. Router Aggregation Sketch #2

This is basically similar to the version -00 sketch described above but uses the router aggregation link model. In other words, it combines (1) the router aggregation link model, (2) prefix information distributed in Neighbor Advertisements, (3) unique address allocation with stateless address autoconfiguration (with the VPN gateway trapping NS messages and spoofing NA replies), and (4) access control enforced outside IPsec.

1. During IKE_AUTH, the client sends a new configuration attribute,

   INTERNAL_IP6_LINK, which requests a virtual link to be created.
   The attribute contains the client's interface ID for the link-
   local address (other addresses may use other interface IDs).
   CP(CFG_REQUEST) =
      { INTERNAL_IP6_LINK(Link-Local Interface ID) }
   TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

The VPN gateway replies with its own Link-Local Interface ID (which has to be different from the client's) and an IKEv2 Link ID (which will be used for reauthentication).

   CP(CFG_REPLY) =
     { INTERNAL_IP6_LINK(Link-Local Interface ID, IKEv2 Link ID) }
   TSi = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, 0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

At this point, both peers configure the virtual interface with the link-local addresses.

2. The next step is IPv6 stateless address autoconfiguration, that

   is, Router Solicitation and Router Advertisement messages sent
   over the IPsec SA.
   ESP(Router Solicitation:
       src=::,
       dst=FF02:0:0:0:0:0:0:2)  -->
   <-- ESP(Router Advertisement:
           src=FE80::<Gateway's Interface ID>
           dst=FF02:0:0:0:0:0:0:1,
           Prefix1, [Prefix2...])

3. The client now configures tentative unicast addresses from the

   prefixes given by the gateway and performs Duplicate Address
   Detection (DAD) for them.
   The Neighbor Solicitation messages are processed by the VPN
   gateway; if the target address is already in use by some other
   VPN client, the gateway replies with a Neighbor Advertisement.
   If the target address is not already in use, the VPN gateway
   notes that it is now being used by this client and updates its
   forwarding state accordingly.

Comments: The main disadvantages of this solution are non-standard processing of NS messages (which are used to update the gateway's forwarding state) and performing access control outside IPsec.

A.6.4. IPv4-Like Sketch

This sketch resembles the current IPv4 configuration payloads and combines (1) the router aggregation link model, (2) prefix information distributed in IKEv2 messages, (3) unique address allocation with IKEv2 messages, and (4) access control enforced by IPsec SAD/SPD.

1. During IKE_AUTH, the client sends a new configuration attribute,

   INTERNAL_IP6_LINK, which requests a virtual link to be created.
   The attribute contains the client's interface ID for the link-
   local address (other addresses may use other interface IDs).
   CP(CFG_REQUEST) =
      { INTERNAL_IP6_LINK(Link-Local Interface ID) }
   TSi = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

The VPN gateway replies with its own Link-Local Interface ID (which has to be different from the client's), an IKEv2 Link ID (which will be used for reauthentication and CREATE_CHILD_SA messages), and zero or more INTERNAL_IP6_ADDRESS2 attributes. Each attribute contains one address from a particular prefix.

   CP(CFG_REPLY) =
      { INTERNAL_IP6_LINK(Link-Local Interface ID, IKEv2 Link ID),
        INTERNAL_IP6_ADDRESS2(Prefix1+Client's Interface ID1),
        [INTERNAL_IP6_ADDRESS2(Prefix2+Client's Interface ID2),...],
   TSi = ((0, 0-65535,
           FE80::<Client's Link-Local Interface ID> -
           FE80::<Client's Link-Local Interface ID>)
          (0, 0-65535,
           Prefix1::<Client's Interface ID1> -
           Prefix1::<Client's Interface ID1>),
          [(0, 0-65535,
            Prefix2::<Client's Interface ID2> -
            Prefix2::<Client's Interface ID2>), ...])
   TSr = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

Since the VPN gateway keeps track of address uniqueness, there is no need to perform Duplicate Address Detection.

2. If the client wants additional addresses later (for example, with

   a specific interface ID), it requests them in a separate
   CREATE_CHILD_SA exchange.  For example:
   CP(CFG_REQUEST) =
      { INTERNAL_IP6_ADDRESS2(Prefix1+Client's Interface ID3) }
   TSi = (0, 0-65535,
          Prefix1::0 -
          Prefix1::FFFF:FFFF:FFFF:FFFF>),
   TSr = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)  -->

If the requested address is not currently in use by some other client, the VPN gateway simply returns the same address and the appropriately narrowed traffic selectors.

   CP(CFG_REQUEST) =
      { INTERNAL_IP6_ADDRESS2(Prefix1+Client's Interface ID3) }
   TSi = ((0, 0-65535,
           Prefix1::<Client's Interface ID3> -
           Prefix1::<Client's Interface ID3>),
   TSr = (0, 0-65535,
          0:0:0:0:0:0:0:0 -
          FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

Comments: The main advantage of this solution is that it's quite close to the current IPv4 way of doing things. By adding explicit link creation (with Link ID for reauthentication/SPD selection and link-local addresses) and slightly changing the semantics (and also name) of the INTERNAL_IP6_ADDRESS attribute (which can return more attributes than was asked), we get much of the needed functionality.

The biggest disadvantages are probably potentially complex implementation dependency for interface ID selection (see Section 3.3) and the multi-link subnet model.

A.6.5. Sketch Based on RFC 3456

For completeness: a solution modeled after RFC3456 would combine (1) the router aggregation link model, (2) prefix information distribution and unique address allocation with DHCPv6, and (3) access control enforced by IPsec SAD/SPD.

Appendix B. Evaluation (Non-Normative)

Section 3 describes the goals and requirements for IPv6 configuration in IKEv2. This appendix briefly summarizes how the solution specified in Sections 4 and 5 meets these goals.

o (3.1) Assigning addresses from multiple prefixes is supported,

  without requiring the client to know beforehand how many prefixes
  are needed.

o (3.2) Link-local addresses are assigned and can be used for

  protocols between the VPN client and gateway.

o (3.3) The entire prefix is assigned to a single client, so the

  client can freely select any number of interface IDs (which may
  depend on the prefix).

o (3.4) This document does not specify how the VPN client would

  share the VPN connection with other devices.  However, since the
  entire prefix is assigned to a single client, the client could
  further assign addresses from it without requiring coordination
  with the VPN gateway.

o (3.5) The solution does not add any new periodic messages over the

  VPN tunnel.

o (3.5) Reauthentication works (see Section 4.2).

o (3.5) The solution is compatible with other IPsec uses since the

  LINK_ID notification makes it unambiguous which CHILD_SAs are
  related to the virtual link and which are not (see Sections 4.3
  and 5.3).

o (3.5) The new mechanisms do not prevent the VPN client and/or

  gateway from implementing the INTERNAL_IP6_ADDRESS configuration
  attribute as well; however, the two mechanisms are not intended to
  be used simultaneously (see Section 4.5).

o (3.5) Implementation dependencies are, obviously, implementation

  dependent (and their cleanliness somewhat subjective).  Possible
  drawbacks of some alternative solutions are discussed in
  Appendix A.6.

o (3.5) The mechanism for configuring the prefixes (configuration

  payloads) is specific to IKEv2, for reasons described in
  Appendix A.  However, Section 4.1 recommends using DHCPv6
  Information-Request message for obtaining other configuration
  information (such as DNS server addresses).

Authors' Addresses

Pasi Eronen Nokia Research Center P.O. Box 407 FIN-00045 Nokia Group Finland

EMail: [email protected]

Julien Laganier QUALCOMM Incorporated 5775 Morehouse Drive San Diego, CA 92121 USA

Phone: +1 858 658 3538 EMail: [email protected]

Cheryl Madson Cisco Systems, Inc. 510 MacCarthy Drive Milpitas, CA USA

EMail: [email protected]