    Network Working Group                                     V. Devarapalli
    Request for Comments: 5685                                      WiChorus
    Category: Standards Track                                     K. Weniger
                                                                Unaffiliated
                                                               November 2009

                            Redirect Mechanism for
              the Internet Key Exchange Protocol Version 2 (IKEv2)

    Abstract

       The Internet Key Exchange Protocol version 2 (IKEv2) is a protocol
       for setting up Virtual Private Network (VPN) tunnels from a remote
       location to a gateway so that the VPN client can access services in
       the network behind the gateway.  This document defines an IKEv2
       extension that allows an overloaded VPN gateway or a VPN gateway that
       is being shut down for maintenance to redirect the VPN client to
       attach to another gateway.  The proposed mechanism can also be used
       in Mobile IPv6 to enable the home agent to redirect the mobile node
       to another home agent.

    Status of This Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (c) 2009 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 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

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    RFC 5685                     IKEv2 Redirect                November 2009

       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.

    Table of Contents

       1. Introduction ....................................................2
       2. Terminology .....................................................3
       3. IKEv2 Initial Exchange with Redirect ............................3
       4. Use of Anycast Addresses with the Redirect Mechanism ............5
       5. Redirect during an Active Session ...............................6
       6. Redirect during IKE_AUTH Exchange ...............................7
       7. Handling Redirect Loops .........................................8
       8. Using the Redirect Mechanism with Mobile IPv6 ...................8
       9. Redirect Messages ...............................................9
          9.1. REDIRECT_SUPPORTED .........................................9
          9.2. REDIRECT ..................................................10
          9.3. REDIRECTED_FROM ...........................................11
       10. Use of the Redirect Mechanism between IKEv2 Peers .............12
       11. Security Considerations .......................................12
       12. IANA Considerations ...........................................13
       13. Acknowledgements ..............................................13
       14. References ....................................................14
          14.1. Normative References .....................................14
          14.2. Informative References ...................................14

    1.  Introduction

       IKEv2 [2] is used for setting up IPsec-based [7] VPNs.  The IP
       address of the VPN gateway can be configured on the VPN client.  But
       this does not scale well when the number of VPN gateways is large.
       Dynamic discovery of VPN gateways using DNS is quite widely used too.
       However, using DNS is not flexible when it comes to assigning a VPN
       gateway to the VPN client based on the load on the VPN gateways.  The
       VPN client typically tries to connect to the IP address of the VPN
       gateway that appears first in the DNS response.  If the VPN tunnel
       setup fails, then the VPN client tries to attach to the other VPN
       gateways returned in the DNS response.

       This document proposes a redirect mechanism for IKEv2 that enables a
       VPN gateway to redirect the VPN client to another VPN gateway, for
       example, based on the load condition.  The redirect can be done
       during the IKE_SA_INIT or the IKE_AUTH exchange.  Gateway-initiated

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       redirect in the middle of a session is also supported.  The redirect
       mechanism can also be used in conjunction with anycast addresses.  In
       this case, an anycast address for the cluster of VPN gateways is
       stored in the DNS instead of a list of unicast IP addresses of the
       VPN gateways.

       The redirect can also happen because of administrative or optimal-
       routing reasons.  This document does not attempt to provide an
       exhaustive list of reasons for redirecting a VPN client to another
       VPN gateway.

    2.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [1].

    3.  IKEv2 Initial Exchange with Redirect

       This section describes the use of the redirect mechanism during the
       IKE_SA_INIT exchange.  Gateway-initiated redirect during an active
       session and the use of redirect during IKE_AUTH exchange are
       explained in subsequent sections.

       The VPN client indicates support for the IKEv2 redirect mechanism and
       its willingness to be redirected by including a REDIRECT_SUPPORTED
       notification message in the initial IKE_SA_INIT request (see
       Section 9.1).  The gateway MUST keep track of those clients that
       indicated support for the redirect mechanism and those that didn't.

       To redirect an IKEv2 session to another VPN gateway, the VPN gateway
       that initially received the IKE_SA_INIT request selects another VPN
       gateway (how the selection is made is beyond the scope of this
       document) and replies with an IKE_SA_INIT response containing a
       REDIRECT notification message (see Section 9.2).  The notification
       includes information about the selected VPN gateway and the nonce
       data from the Ni payload in the IKE_SA_INIT request.  If the
       IKE_SA_INIT request did not indicate support for the redirect
       mechanism, the responder MUST NOT send the REDIRECT payload to the
       VPN client.  This is applicable to all REDIRECT scenarios described
       in this document.

       Note that when the IKE_SA_INIT response includes the REDIRECT
       notification, the exchange does not result in the creation of an
       IKE_SA and the responder Security Parameter Index (SPI) will be zero.

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           Initiator                    Responder (initial VPN GW)
           ---------                    -------------------------

        (IP_I:500 -> Initial_IP_R:500)
        HDR(A,0), SAi1, KEi, Ni,   -->
        N(REDIRECT_SUPPORTED)

                                  (Initial_IP_R:500 -> IP_I:500)
                              <-- HDR(A,0), N(REDIRECT, New_GW_ID, Ni_data)

       When the client receives the IKE_SA_INIT response, it MUST verify
       that the nonce data matches the value sent in the IKE_SA_INIT
       request.  If the values do not match, the client MUST silently
       discard the response (and keep waiting for another response).  This
       prevents certain denial-of-service (DoS) attacks on the initiator
       that could be caused by an attacker injecting IKE_SA_INIT responses
       with REDIRECT payloads.

       After verifying the nonce data, the client initiates a new
       IKE_SA_INIT exchange with the VPN gateway listed in the REDIRECT
       payload, provided this is allowed by its Peer Authorization Database
       (PAD) entries.  In the IKE_SA_INIT exchange with the new VPN gateway,
       the client MUST include the REDIRECTED_FROM payload (see
       Section 9.3).  The VPN client includes the IP address of the original
       VPN gateway that redirected the client in the REDIRECTED_FROM
       notification.  The IKEv2 exchange then proceeds as it would have
       proceeded with the original VPN gateway.

           Initiator                   Responder (Selected VPN GW)
           ---------                   ---------------------------

        (IP_I:500 -> IP_R:500)
        HDR(A,0), SAi1, KEi, Ni,   -->
        N(REDIRECTED_FROM, Initial_IP_R)

                                  (IP_R:500 -> IP_I:500)
                              <-- HDR(A,B), SAr1, KEr, Nr,[CERTREQ]

        (IP_I:500 -> IP_R:500)
        HDR(A,B), SK {IDi, [CERT,] [CERTREQ,]
        [IDr,]AUTH, SAi2, TSi, TSr} -->

                                  (IP_R:500 -> IP_I:500)
                              <-- HDR(A,B), SK {IDr, [CERT,] AUTH,
                                                     SAr2, TSi, TSr}

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       The client MAY get redirected again by the new VPN gateway if the new
       VPN gateway also cannot serve the client.  The client does not have
       to include the REDIRECT_SUPPORTED payload again in the IKE_SA_INIT
       exchange with the new gateway after a redirect.  The presence of the
       REDIRECT_FROM payload in the IKE_SA_INIT exchange with the new
       gateway indicates to the new gateway that the client supports the
       redirect mechanism.

       When the client gets redirected, it MUST use the same Peer
       Authorization Database (PAD) and Security Policy Database (SPD)
       entries as it would have used with the original gateway.  Receiving a
       redirect notification MUST NOT result in the modification of any PAD
       or SPD entries.  In practice, this means the new gateway either has
       to use the same responder identity (IDr) as the original gateway, or
       both should be part of a group of responders that are authorized by
       the same PAD entry.  See Section 4.4.3.1 of [7] on using DNS names to
       represent a group of peers in a PAD entry.

       This document allows the client to be redirected in several protocol
       states.  In some of them, the gateway is already authenticated at the
       point of redirect; in others, it is not.  We emphasize that the above
       rules regarding the identity of the new gateway and the PAD and SPD
       entries apply equally to all these scenarios.

    4.  Use of Anycast Addresses with the Redirect Mechanism

       Using anycast addresses will avoid the necessity of configuring a
       particular VPN gateway's IP address in the DNS.  Instead, the anycast
       address that represents the group of VPN gateways is stored in the
       DNS.  When the VPN client performs a DNS lookup for the VPN gateway,
       it receives the anycast address of the VPN gateway in the DNS
       response.

       If an anycast address is returned in response to the DNS resolution
       of a Fully Qualified Domain Name (FQDN), the VPN client sends the
       IKE_SA_INIT request to the anycast address.  The REDIRECT_SUPPORTED
       payload is included in the IKE_SA_INIT request sent to the anycast
       address.  The IKE_SA_INIT request is routed to one of the VPN
       gateways that is part of the anycast group.  The VPN gateway that
       receives the IKE_SA_INIT request responds with an IKE_SA_INIT reply
       from the anycast address.

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           Initiator                    Responder (any VPN GW)
           ---------                    -------------------------

        (IP_I:500 -> ANYCAST:500)
        HDR(A,0), SAi1, KEi, Ni)   -->
        N(REDIRECT_SUPPORTED)

                                  (ANYCAST:500 -> IP_I:500)
                              <-- HDR(A,0), N(REDIRECT, New_GW_ID, Ni_data)

       If the destination address on the IKE_SA_INIT request is an anycast
       address, the VPN gateway that received the IKE_SA_INIT request MUST
       include the REDIRECT payload to redirect the VPN client to a unicast
       address of one of the VPN gateways.  The VPN gateway that received
       the IKE_SA_INIT request MAY redirect the client to its own unicast
       address if it is not overloaded.

       The rest of the IKEv2 exchange is the same as described in Section 3.

    5.  Redirect during an Active Session

       The redirect mechanism may also be used by a VPN gateway to redirect
       the client to another VPN gateway in the middle of a session.  To
       redirect a client, the gateway should send an INFORMATIONAL message
       with the REDIRECT Notify payload.  The REDIRECT payload MUST carry
       information about the new VPN gateway.  The gateway MUST NOT include
       any nonce data in the REDIRECT payload, since it is a gateway-
       initiated redirect and is protected by the IKEv2 security
       association.  When the client receives this message, it sends a
       response (usually empty) to the gateway.  The gateway retransmits the
       redirect INFORMATIONAL message as described in [2], until it gets a
       response.  The following illustrates the INFORMATIONAL message
       exchange for gateway-initiated redirect.

           Initiator (VPN client)        Responder (VPN GW)
           ----------------------        ------------------

                                   <--  HDR, SK {N(REDIRECT, New_GW_ID)}

         HDR, SK {} -->

       The INFORMATIONAL message exchange described above is protected by
       the existing IKEv2 SA between the client and the gateway.

       Once the client sends an acknowledgement to the gateway, it SHOULD
       delete the existing security associations with the old gateway by
       sending an INFORMATIONAL message with a DELETE payload.  The gateway
       MAY also decide to delete the security associations without any

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       signaling from the client, again by sending an INFORMATIONAL message
       with a DELETE payload; however, it should allow sufficient time for
       the client to set up the required security associations with the new
       security gateway.  This time period should be configurable on the
       gateway.

    6.  Redirect during IKE_AUTH Exchange

       If the gateway decides to redirect the client during the IKE_AUTH
       exchange, based on the identity presented by the client in the
       IKE_AUTH request message, it prevents the creation of a CHILD SA and
       sends the REDIRECT payload in the IKE_AUTH response.  The gateway
       MUST verify the client's AUTH payload before sending the REDIRECT
       payload, and the client MUST verify the gateway's AUTH payload before
       acting on the REDIRECT payload.  Since the AUTH payloads were
       exchanged and successfully verified, the IKEv2 security association
       is valid.  When the client receives the IKE_AUTH response with the
       REDIRECT payload, it SHOULD delete the IKEv2 security association
       with the gateway by sending an INFORMATIONAL message with a DELETE
       payload.

            Initiator                    Responder (VPN GW)
            ---------                    ------------------

        (IP_I:500 -> IP_R:500)
        HDR(A,0), SAi1, KEi, Ni,   -->
        N(REDIRECTED_SUPPORTED)

                                      (IP_R:500 -> IP_I:500)
                                  <-- HDR(A,B), SAr1, KEr, Nr,[CERTREQ]

        (IP_I:500 -> IP_R:500)
        HDR(A,B), SK {IDi, [CERT,] [CERTREQ,]
        [IDr,]AUTH, SAi2, TSi, TSr} -->

                                      (IP_R:500 -> IP_I:500)
                                  <-- HDR(A,B), SK {IDr, [CERT,] AUTH,
                                               N(REDIRECT, New_GW_ID)}

       In case the IKE_AUTH exchange involves Extensible Authentication
       Protocol (EAP) authentication (as described in Section 2.16 of RFC
       4306 [2]) or multiple authentication methods (as described in RFC
       4739 [6]), the gateway may decide to redirect the client based on the
       interaction with the Authentication, Authorization, and Accounting
       (AAA) server or the external authentication server.  In this case,
       the gateway MUST send the REDIRECT Notify payload in either the first
       or the last IKE_AUTH response.  The client and the gateway MUST
       verify the AUTH payloads as described above.

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       When EAP is used, the gateway MAY also redirect the client based on
       the unauthenticated identity presented by the client in the first
       IKE_AUTH exchange, itself.  Since EAP is used as the authentication
       mechanism, the client does not include AUTH payload to authenticate
       its identity, but the server MUST still include its own AUTH payload,
       and the client MUST verify it.  Note that the IKEv2 SA is not created
       in this case and the client does not have to explicitly delete the
       IKEv2 SA.

       In all of the cases above, the client MUST accept the REDIRECT
       notification only in the first IKE_AUTH response or the last IKE_AUTH
       response.  It MUST NOT accept the REDIRECT notification in an
       intermediate IKE_AUTH response.

    7.  Handling Redirect Loops

       The client could end up getting redirected multiple times in a
       sequence, either because of a wrong configuration or a DoS attack.
       The client could even end up in a loop with two or more gateways
       redirecting the client to each other.  This could deny service to the
       client.  To prevent this, the client SHOULD be configured to not
       accept more than a certain number of redirects (MAX_REDIRECTS) within
       a short time period (REDIRECT_LOOP_DETECT_PERIOD) for a particular
       IKEv2 SA setup.  The default value for the MAX_REDIRECTS
       configuration variable is 5.  The default value for the
       REDIRECT_LOOP_DETECT_PERIOD configuration variable is 300 seconds.
       Client implementations may allow these variables to be configured,
       depending on a specific deployment or system configuration.

    8.  Using the Redirect Mechanism with Mobile IPv6

       Mobile IPv6 [3] may use IKEv2 for mutual authentication between the
       mobile node and the home agent, for home address configuration, and
       for setting up security associations for protecting Mobile IPv6
       signaling messages [4].  The IKEv2 exchange, if IKEv2 is used,
       precedes the exchange of Mobile IPv6 signaling messages.  Therefore,
       the mechanism described in this document can also be used by a Mobile
       IPv6 home agent to redirect a mobile node to another home agent.

       There is a Home Agent Switch mechanism available for redirecting a
       mobile node to another home agent, described in [5].  The Home Agent
       Switch mechanism can only be used after the binding cache has been
       created at the home agent for the mobile node.  The disadvantage with
       this is that quite a bit of state is created on the home agent before
       the mobile node can be redirected to another home agent.  The
       mechanism described in this document can be used for redirecting a
       mobile node before any state related to the Mobile IPv6 binding is
       created on the home agent.

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       When running IKEv2 between a Mobile IPv6 mobile node (MN) and home
       agent (HA), redirecting the IKEv2 exchange to another HA is not
       enough; the Mobile IPv6 signaling also needs to be sent to the new HA
       address.  The MN MAY treat the information received in the
       IKE_SA_INIT response in a similar way as it would treat HA discovery
       information received from other unauthenticated (and potentially
       untrustworthy) sources (such as DNS lookups not protected with DNS
       Security (DNSSEC)).  However, if the MN has authenticated information
       about its home agent, it MUST NOT be updated based on the IKE_SA_INIT
       response.

       If the REDIRECT notification is received during the IKE_AUTH exchange
       (after the HA has been authenticated; see Section 6), the MN MAY pass
       the new address to Mobile IPv6 and treat it in a similar fashion as
       information from the Home Agent Switch message [5].

       Gateway-initiated REDIRECT notifications exchanged in INFORMATIONAL
       exchanges (see Section 5) MUST NOT result in updating any Mobile IPv6
       state.  In such cases, the Home Agent Switch message specified in [5]
       is used instead.

    9.  Redirect Messages

    9.1.  REDIRECT_SUPPORTED

       The REDIRECT_SUPPORTED payload is included in the initial IKE_SA_INIT
       request by the initiator to indicate support for the IKEv2 redirect
       mechanism described in this document.

                             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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | Next Payload  |C|  RESERVED   |         Payload Length        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
       'Notify Message Type' fields are the same as described in Section
       3.10 of [2].  The 'SPI Size' field MUST be set to 0 to indicate that
       the SPI is not present in this message.  The 'Protocol ID' MUST be
       set to 0, since the notification is not specific to a particular
       security association.

       The 'Payload Length' field is set to the length in octets of the
       entire payload, including the generic payload header.  The 'Notify
       Message Type' field is set to indicate the REDIRECT_SUPPORTED payload
       (16406).

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    9.2.  REDIRECT

       When the responder wants to redirect the initiator to another VPN
       gateway, the REDIRECT payload is included in either an IKE_SA_INIT
       response from the responder or an INFORMATIONAL message from the
       responder.  The message includes the new responder's IP address or
       DNS name.

                             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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | Next Payload  |C|  RESERVED   |         Payload Length        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | GW Ident Type |  GW Ident Len |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
        ~                   New Responder GW Identity                   ~
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        ~                        Nonce Data                             ~
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
       'Notify Message Type' fields are the same as described in Section
       3.10 of [2].  The 'SPI Size' field MUST be set to 0 to indicate that
       the SPI is not present in this message.  The 'Protocol ID' MUST be
       set to 0, since the notification is not specific to a particular
       security association.

       The 'Payload Length' field is set to the length in octets of the
       entire payload, including the generic payload header.  The 'Notify
       Message Type' field is set to indicate the REDIRECT payload (16407).
       The 'GW Identity Type' field indicates the type of information that
       is sent to identify the new VPN gateway.  The following values are
       valid in the REDIRECT payload.

          1 - IPv4 address of the new VPN gateway

          2 - IPv6 address of the new VPN gateway

          3 - FQDN of the new VPN gateway

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       The 'GW Ident Len' field is set to the length of the gateway identity
       information.  The identity of the new VPN gateway is carried in the
       'New Responder GW Identity' field.  The IPv4 address, the IPv6
       address, or the FQDN of the new VPN gateway MUST be encoded as
       described in Section 3.5 of [2].

       The 'Nonce Data' field carries the nonce data from the Ni payload
       sent by the initiator.  The size of the nonce MUST be between 16 and
       256 bytes, as described in Section 3.9 of [2].  The 'Nonce Data'
       field is present in the REDIRECT payload only when the REDIRECT
       payload is sent in the IKE_SA_INIT response message.  It MUST NOT be
       included in the REDIRECT payload if sent in an IKE_AUTH response or
       in a gateway-initiated redirect message.

    9.3.  REDIRECTED_FROM

       The REDIRECTED_FROM Notify payload is included in the IKE_SA_INIT
       request from the initiator to the new VPN gateway to indicate the IP
       address of the original VPN gateway that redirected the initiator.
       The original VPN gateway's IP address is included in the message.  If
       the IKE_SA_INIT request was sent to any anycast address (see
       Section 4), then the anycast address is included in the message.
       This payload also serves the purpose of indicating support for the
       redirect mechanism to the new VPN gateway after a redirect.

                             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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | Next Payload  |C|  RESERVED   |         Payload Length        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | GW Ident Type |  GW Ident Len |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
        ~                   Original Responder GW Identity              ~
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
       'Notify Message Type' fields are the same as described in Section
       3.10 of [2].  The 'SPI Size' field MUST be set to 0 to indicate that
       the SPI is not present in this message.  The 'Protocol ID' MUST be
       set to 0, since the notification is not specific to a particular
       security association.

       The 'Payload Length' field is set to the length in octets of the
       entire payload, including the generic payload header.  The 'Notify
       Message Type' field is set to indicate the REDIRECTED_FROM payload

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       (16408).  The 'GW Identity Type' field indicates the type of
       information that is sent to identify the new VPN gateway.  The
       following values are valid in the REDIRECTED_FROM payload.

          1 - IPv4 address of the original VPN gateway

          2 - IPv6 address of the original VPN gateway

       The 'GW Ident Len' field is set to the length of the gateway identity
       information.  The identity of the original VPN gateway is carried in
       the 'Original Responder GW Identity' field.

    10.  Use of the Redirect Mechanism between IKEv2 Peers

       The redirect mechanism described in this document is mainly intended
       for use in client-gateway scenarios.  However, the mechanism can also
       be used between any two IKEv2 peers.  But this protocol is
       asymmetric, meaning that only the original responder can redirect the
       original initiator to another server.

    11.  Security Considerations

       An eavesdropper on the path between a VPN client and server may send
       a redirect to the client upon receiving an IKE_SA_INIT message from
       this client.  This is no problem regarding DoS attacks for the VPN
       connection, since an on-path-attacker can as well drop the
       IKE_SA_INIT requests to prevent VPN access for the client.  But an
       eavesdropper on the path between VPN client and server can redirect a
       large number of clients to a victim, which is then flooded with
       IKE_SA_INIT requests.  Flooding only happens if many clients initiate
       IKEv2 exchange at almost the same time, which is considered a rare
       event.  However, this may happen if a home agent / VPN server is
       shutdown for maintenance and all clients need to re-establish VPN
       connections with another home agent / VPN server, or if the on-path
       attacker forces all IPsec security associations to expire by dropping
       all received IKEv2 messages.

       The use of the REDIRECTED_FROM payload is intended to discourage a
       rogue VPN gateway from redirecting a large number of VPN clients to a
       particular VPN gateway.  It does not prevent such a DoS attack.

       The redirect mechanism MUST NOT update any state on the client apart
       from the VPN gateway information.  When used with Mobile IPv6, care
       must be taken to ensure that the home agent information that the
       mobile node has configured is not modified wrongly by the redirect
       message.

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       Redirecting based on the unauthenticated identities from the client
       might leak out information about the user when an active attacker,
       pretending to be a VPN client, can get information on the gateway to
       which the real user was redirected.  If redirection is based on some
       internal information of the user, it might leak information (that
       might not be available otherwise) about the user to the attacker.  To
       prevent these kinds of attacks, redirection based on unauthenticated
       IDs should be avoided and should be done only after the client has
       also authenticated itself.

    12.  IANA Considerations

       This document defines three new IKEv2 Notify Message Types, as
       described in Section 9.  The three Notify Message Types have been
       assigned the following values:

          16406 - REDIRECT_SUPPORTED

          16407 - REDIRECT

          16408 - REDIRECTED_FROM

       This document creates a new namespace called the "Gateway Identity
       Type".  This is used to indicate the type of information regarding
       the VPN gateway that is carried in the REDIRECT (Section 9.2) and
       REDIRECTED_FROM (Section 9.3) Notify payloads.  The following values
       have been assigned.

          1 - IPv4 address of the VPN gateway

          2 - IPv6 address of the VPN gateway

          3 - FQDN of the VPN gateway

       Value '0' is reserved.  Values 4-240 are unassigned.  New values can
       be allocated by Expert Review [8].  Values 241-255 are set aside for
       private use.  A specification that extends this registry MUST also
       mention which of the new values are valid in which Notify payload.

    13.  Acknowledgements

       The use of anycast addresses with IKEv2 was first proposed by K.
       Weniger and F. Dupont in the context of home agent assignment in
       Mobile IPv6 / Network Mobility (NEMO) bootstrapping.  It was then
       added to an early version of [4] and later removed before the RFC was
       published.  The authors of RFC 5026 are acknowledged.

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       Thanks to Pasi Eronen, with whom the solution described in this
       document was extensively discussed.  Thanks to Tero Kivinen for
       suggesting the use of the REDIRECTED_FROM payload and other comments
       that helped improve the document.  The authors would also like to
       thank Yaron Sheffer, Sunil Kumar, Fan Zhao, Yoav Nir, Richard
       Graveman, Kanagavel Rajan, Srini Addepalli, Raj Singh, and Arnaud
       Ebalard for their reviews and comments.

    14.  References

    14.1.  Normative References

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

       [2]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306,
            December 2005.

    14.2.  Informative References

       [3]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
            IPv6", RFC 3775, June 2004.

       [4]  Giaretta, G., Kempf, J., and V. Devarapalli, "Mobile IPv6
            Bootstrapping in Split Scenario", RFC 5026, October 2007.

       [5]  Haley, B., Devarapalli, V., Deng, H., and J. Kempf, "Mobility
            Header Home Agent Switch Message", RFC 5142, January 2008.

       [6]  Eronen, P. and J. Korhonen, "Multiple Authentication Exchanges
            in the Internet Key Exchange (IKEv2) Protocol", RFC 4739,
            November 2006.

       [7]  Kent, S. and K. Seo, "Security Architecture for the Internet
            Protocol", RFC 4301, December 2005.

       [8]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
            Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

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    Authors' Addresses

       Vijay Devarapalli
       WiChorus
       3590 North First St
       San Jose, CA  95134
       USA

       EMail: vijay@wichorus.com

       Kilian Weniger
       Unaffiliated

       EMail: kilian.weniger@googlemail.com

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