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Versions: (draft-devarapalli-ipsec-ikev2-redirect) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5685

Network Working Group                                     V. Devarapalli
Internet-Draft                                                  WiChorus
Intended status: Standards Track                              K. Weniger
Expires: February 5, 2010                                 August 4, 2009


                      Redirect Mechanism for IKEv2
                draft-ietf-ipsecme-ikev2-redirect-13.txt

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   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   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
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14












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1.  Introduction

   IKEv2 [2] is used for setting up IPsec [8] based 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
   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, 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 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
   the 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.



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   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 SPI will be zero.

       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 attacks on the initiator that
   could be caused by an attacker injecting IKE_SA_INIT responses with
   the 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 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.







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       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}

   The client MAY get redirected again by the new VPN gateway if the new
   VPN gateway cannot also 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 [8] on using DNS names to
   represent a group of peers in a PAD entry.


4.  Use of Anycast Addresses with the Redirect Mechanism

   The use of anycast addresses will avoid having to configure 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 DNS resolution of an
   FQDN, the VPN client sends the IKE_SA_INIT request to the anycast



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   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.

       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 gateway.  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 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 {} -->




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   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
   signaling from the client, again by sending an INFORMATIONAL message
   with a DELETE payload.  However, it should allow sufficient time for
   the client to setup 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 EAP authentication as



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   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 AAA server or
   the external authentication server.  In this case, the gateway MUST
   send the REDIRECT Notification payload in either the first or the
   last IKE_AUTH response.  The client and the gateway MUST verify the
   AUTH payloads as described above.

   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
   his identity, but the server still MUST include his own AUTH payload,
   and 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 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 not to
   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 MAX_REDIRECTS configuration
   variable is 5.  The default value for 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.



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   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 had 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.

   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 signalling also needs to be sent to the new
   HA address.  The MN MAY treat the information received in the
   IKE_SA_INIT response in similar way as it would treat HA discovery
   information received from other unauthenticated (and potentially
   untrustworthy) sources (such as DNS lookups not protected with
   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 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



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   the '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
   <value to be assigned by IANA>.

9.2.  REDIRECT

   The REDIRECT payload is included in an IKE_SA_INIT response from the
   responder or an INFORMATIONAL message from the responder, when the
   responder wants to redirect the initiator to another VPN gateway.
   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
   the '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.  'Notify
   Message Type' field is set to indicate the REDIRECT payload <value to
   be assigned by IANA>.  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.



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      1 - IPv4 address of the new VPN gateway
      2 - IPv6 address of the new VPN gateway
      3 - FQDN of the new VPN gateway

   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 notification 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
   the '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.



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   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
   <value to be assigned by IANA>.  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 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 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



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   must be taken to ensure that the home agent information that the
   mobile node has configured is not modified wrongly by the redirect
   message.

   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 to which gateway
   the real user was redirected to.  If redirection is based on some
   internal information of the user, it might leak information to
   attacker about the user which might not be available otherwise.  To
   prevent these kind of attacks, redirection based on unauthenticated
   ID should be avoided and should be done only after the client has
   also authenticated itself.


12.  IANA Considerations

   This document defines three new IKEv2 Notification Message types as
   described in Section 9.  The three Notify Message Types must be
   assigned values between 16396 and 40959.

   o  REDIRECT_SUPPORTED
   o  REDIRECT
   o  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) Notification payloads.  The following
   values are assigned.

      1 - IPv4 address of the new VPN gateway
      2 - IPv6 address of the new VPN gateway
      3 - FQDN of the new VPN gateway

   Values '0', and 4-240 are reserved.  New values can be allocated by
   Expert Review [9].  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 Notification payload.


13.  Acknowledgements

   The use of anycast address with IKEv2 was first described in [7].  It
   was then added to an early draft version of RFC 5026 and later
   removed before the RFC was published.  Therefore the authors of [7]
   and 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 REDIRECTED_FROM payload and other comments
   which 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]  Weniger, K. and F. Dupont, "IKEv2-based Home Agent Assignment in
        Mobile IPv6/NEMO Bootstrapping", draft-dupont-ikev2-haassign-02
        (work in progress), January 2007.

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

   [9]  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

   Email: kilian.weniger@googlemail.com





































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