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Network Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Intended status: Standards Track                             F. Detienne
Expires: April 15, 2009                                         P. Sethi
                                                                   Cisco
                                                        October 12, 2008


                 A Quick Crash Detection Method for IKE
                          draft-nir-ike-qcd-03

Status of this Memo

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   This Internet-Draft will expire on April 15, 2009.

Abstract

   This document describes an extension to the IKEv2 protocol that
   allows for faster detection of SA desynchronization using a saved
   token.

   When an IPsec tunnel between two IKEv2 peers is disconnected due to a
   restart of one peer, it can take as much as several minutes for the
   other peer to discover that the reboot has occurred, thus delaying
   recovery.  In this text we propose an extension to the protocol, that
   allows for recovery immediately following the restart.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  RFC 4306 Crash Recovery  . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol Outline . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Formats and Exchanges  . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Notification Format  . . . . . . . . . . . . . . . . . . .  5
     4.2.  Passing a Token in the AUTH Exchange . . . . . . . . . . .  5
     4.3.  Replacing Tokens After Rekey or Resumption . . . . . . . .  7
     4.4.  Replacing the Token for an Existing SA . . . . . . . . . .  7
     4.5.  Presenting the Token in an INFORMATIONAL Exchange  . . . .  8
   5.  Token Generation and Verification  . . . . . . . . . . . . . .  9
     5.1.  A Stateless Method of Token Generation . . . . . . . . . .  9
     5.2.  A Stateless Method with IP addresses . . . . . . . . . . .  9
     5.3.  Token Lifetime . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Backup Gateways  . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Alternative Solutions  . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Initiating a new IKE SA  . . . . . . . . . . . . . . . . . 10
     7.2.  Birth Certificates . . . . . . . . . . . . . . . . . . . . 11
   8.  Interaction with Session Resumption  . . . . . . . . . . . . . 11
   9.  Operational Considerations . . . . . . . . . . . . . . . . . . 13
     9.1.  Who should implement this specification  . . . . . . . . . 13
     9.2.  Response to unknown child SPI  . . . . . . . . . . . . . . 13
     9.3.  Using Tokens that Depend on IP Addresses . . . . . . . . . 14
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     10.1. QCD Token Handling . . . . . . . . . . . . . . . . . . . . 15
     10.2. QCD Token Transmission . . . . . . . . . . . . . . . . . . 15
     10.3. QCD Token Enumeration  . . . . . . . . . . . . . . . . . . 15
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     13.1. Changes from draft-nir-ike-qcd-02  . . . . . . . . . . . . 16
     13.2. Changes from draft-nir-ike-qcd-01  . . . . . . . . . . . . 17
     13.3. Changes from draft-nir-ike-qcd-00  . . . . . . . . . . . . 17
     13.4. Changes from draft-nir-qcr-00  . . . . . . . . . . . . . . 17
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     14.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19










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1.  Introduction

   IKEv2, as described in [RFC4306] has a method for recovering from a
   reboot of one peer.  As long as traffic flows in both directions, the
   rebooted peer should re-establish the tunnels immediately.  However,
   in many cases the rebooted peer is a VPN gateway that protects only
   servers, or else the non-rebooted peer has a dynamic IP address.  In
   such cases, the rebooted peer will not be able to re-establish the
   tunnels.  Section 2 describes how recovery works under RFC 4306, and
   explains why it may take several minutes.

   The method proposed here, is to send a so-called "token" in the
   IKE_AUTH exchange that establishes the tunnel.  That token can be
   stored on the peer as part of the IKE SA.  After a reboot, the
   rebooted implementation can re-generate the token, and send it to the
   non-rebooted peer so as to delete the IKE SA.  Deleting the IKE SA
   results is a quick re-establishment of the IPsec tunnels.  This is
   described in Section 3.

1.1.  Conventions Used in This Document

   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 [RFC2119].

   The term "token" refers to an octet string that an implementation can
   generate using only the properties of a protected IKE message (such
   as IKE SPIs) as input.  A conforming implementation MUST be able to
   generate the same token from the same input even after rebooting.

   The term "token maker" refers to an implementation that generates a
   token and sends it to the peer as specified in this document.

   The term "token taker" refers to an implementation that stores such a
   token or a digest thereof, in order to verify that a new token it
   receives is identical to the old token it has stored.


2.  RFC 4306 Crash Recovery

   When one peer loses state or reboots, the other peer does not get any
   notification, so unidirectional IPsec traffic can still flow.  The
   rebooted peer will not be able to decrypt it, however, and the only
   remedy is to send an unprotected INVALID_SPI notification as
   described in section 3.10.1 of [RFC4306].  That section also
   describes the processing of such a notification: "If this
   Informational Message is sent outside the context of an IKE_SA, it
   should be used by the recipient only as a "hint" that something might



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   be wrong (because it could easily be forged)."

   Since the INVALID_SPI can only be used as a hint, the non-rebooted
   peer has to determine whether the IPsec SA, and indeed the parent IKE
   SA are still valid.  The method of doing this is described in section
   2.4 of [RFC4306].  This method, called "liveness check" involves
   sending a protected empty INFORMATIONAL message, and awaiting a
   response.  This procedure is sometimes referred to as "Dead Peer
   Detection" or DPD.

   Section 2.4 does not mandate how many times the liveness check
   message should be retransmitted, or for how long, but does recommend
   the following: "It is suggested that messages be retransmitted at
   least a dozen times over a period of at least several minutes before
   giving up on an SA".  Clearly, implementations differ, but all will
   take a significant amount of time.


3.  Protocol Outline

   Supporting implementations will send a notification, called a "QCD
   token", as described in Section 4.1 in the last packets of the
   IKE_AUTH exchange.  These are the final request and final response
   that contain the AUTH payloads.  The generation of these tokens is a
   local matter for implementations, but considerations are described in
   Section 5.  Implementations that send such a token will be called
   "token makers".

   A supporting implementation receiving such a token SHOULD store it
   (or a digest thereof) as part of the IKE SA.  Implementations that
   support this part of the protocol will be called "token takers".
   Section 9.1 has considerations for which implementations need to be
   token takers, and which should be token makers.  Implementation that
   are not token takers will silently ignore QCD tokens.

   When a token maker receives a protected IKE request message with
   unknown IKE SPIs, it MUST generate a new token that is identical to
   the previous token, and send it to the requesting peer in an
   unprotected IKE message as described in Section 4.5.

   When a token taker receives the QCD token in an unprotected
   notification, it MUST verify that the TOKEN_SECRET_DATA matches the
   token stored in the matching the IKE SA.  If the verification fails,
   or if the IKE SPIs in the message do not match any existing IKE SA,
   it SHOULD log the event.  If it succeeds, it MUST delete the IKE SA
   associated with the IKE_SPI fields, and all dependant child SAs.
   This event MAY also be logged.  The token taker MUST accept such
   tokens from any IP address and port combination, so as to allow



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   different kinds of high-availability configurations of the token
   maker.

   A supporting token taker MAY immediately create new SAs using an
   Initial exchange, or it may wait for subsequent traffic to trigger
   the creation of new SAs.

   There is ongoing work on IKEv2 Session Resumption ([resumption] or
   [stubs]).  See Section 8 for a short discussion about this protocol's
   interaction with session resumption.


4.  Formats and Exchanges

4.1.  Notification Format

   The notification payload called "QCD token" 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ! Next Payload  !C!  RESERVED   !         Payload Length        !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       !  Protocol ID  !   SPI Size    ! QCD Token Notify Message Type !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       !                                                               !
       ~                       TOKEN_SECRET_DATA                       ~
       !                                                               !
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Protocol ID (1 octet) MUST contain 1, as this message is related
      to an IKE SA.
   o  SPI Size (1 octet) MUST be zero, in conformance with [RFC4306].
   o  QCD Token Notify Message Type (2 octets) - MUST be xxxxx, the
      value assigned for QCD token notifications.  TBA by IANA.
   o  TOKEN_SECRET_DATA (16-128 octets) contains a generated token as
      described in Section 5.

4.2.  Passing a Token in the AUTH Exchange

   For brevity, only the EAP version of an AUTH exchange will be
   presented here.  The non-EAP version is very similar.  The figures
   below are based on appendix A.3 of [RFC4718].








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    first request       --> IDi,
                            [N(INITIAL_CONTACT)],
                            [[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
                            [IDr],
                            [CP(CFG_REQUEST)],
                            [N(IPCOMP_SUPPORTED)+],
                            [N(USE_TRANSPORT_MODE)],
                            [N(ESP_TFC_PADDING_NOT_SUPPORTED)],
                            [N(NON_FIRST_FRAGMENTS_ALSO)],
                            SA, TSi, TSr,
                            [V(SIR_VID)]
                            [V+]

    first response      <-- IDr, [CERT+], AUTH,
                            EAP,
                            [V(SIR_VID)]
                            [V+]

                      / --> EAP
    repeat 1..N times |
                      \ <-- EAP

    last request        --> AUTH
                            [N(QCD_TOKEN)]

    last response       <-- AUTH,
                            [N(QCD_TOKEN)]
                            [CP(CFG_REPLY)],
                            [N(IPCOMP_SUPPORTED)],
                            [N(USE_TRANSPORT_MODE)],
                            [N(ESP_TFC_PADDING_NOT_SUPPORTED)],
                            [N(NON_FIRST_FRAGMENTS_ALSO)],
                            SA, TSi, TSr,
                            [N(ADDITIONAL_TS_POSSIBLE)],
                            [V+]

   Note that the QCD_TOKEN notification is marked as optional because it
   is not required by this specification that every implementation be
   both token maker and token taker.  If only one peer sends the QCD
   token, then a reboot of the other peer will not be recoverable by
   this method.  This may be acceptable if traffic typically originates
   from the other peer.

   In any case, the lack of a QCD_TOKEN notification MUST NOT be taken
   as an indication that the peer does not support this standard.
   Conversely, if a peer does not understand this notification, it will
   simply ignore it.  Therefore a peer MAY send this notification
   freely, even if it does not know whether the other side supports it.



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   The QCD_TOKEN notification is related to the IKE SA and MUST follow
   the AUTH payload and precede the Configuration payload and all
   payloads related to the child SA.

4.3.  Replacing Tokens After Rekey or Resumption

   After rekeying an IKE SA, the IKE SPIs are replaced, so the new SA
   also needs to have a token.  If only the responder in the rekey
   exchange is the token maker, this can be done before within the
   CREATE_CHILD_SA exchange.  If the initiator is a token maker, then we
   need an extra informational exchange.

   The following figure shows the CREATE_CHILD_SA exchange for rekeying
   the IKE SA.  Only the responder sends a QCD token.

      request             --> SA, Ni, [KEi]

      response            <-- SA, Nr, [KEr], N(QCD_TOKEN)

   If the initiator is also a token maker, it SHOULD soon initiate an
   INFORMATIONAL exchange as follows:

      request             --> N(QCD_TOKEN)

      response            <--

   For session resumption, as specified in [resumption], the situation
   is similar.  The responder, which is necessarily the peer that has
   crashed, SHOULD send a new ticket within the protected payload of the
   IKE_SESSION_RESUME exchange.  If the Initiator is also a token maker,
   it needs to send a QCD_TOKEN in a separate INFORMATIONAL exchange.

4.4.  Replacing the Token for an Existing SA

   With some token generation methods, such as that described in
   Section 5.2, a QCD token may sometimes become invalid, although the
   IKE SA is still perfectly valid.

   In such a case, the token maker MUST send the new token in a
   protected message under that IKE SA.  That exchange could be a simple
   INFORMATIONAL, such as in the last figure in the previous section, or
   else it can be part of a MOBIKE INFORMATIONAL exchange such as in the
   following figure taken from section 2.2 of [RFC4555] and modified by
   adding a QCD_TOKEN notification:







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     (IP_I2:4500 -> IP_R1:4500)
     HDR, SK { N(UPDATE_SA_ADDRESSES),
               N(NAT_DETECTION_SOURCE_IP),
               N(NAT_DETECTION_DESTINATION_IP) }  -->

                           <-- (IP_R1:4500 -> IP_I2:4500)
                               HDR, SK { N(NAT_DETECTION_SOURCE_IP),
                                    N(NAT_DETECTION_DESTINATION_IP) }

                           <-- (IP_R1:4500 -> IP_I2:4500)
                               HDR, SK { N(COOKIE2), [N(QCD_TOKEN)] }

     (IP_I2:4500 -> IP_R1:4500)
     HDR, SK { N(COOKIE2), [N(QCD_TOKEN)] }  -->

   A token taker MUST accept such gratuitous QCD_TOKEN notifications as
   long as they are carried in protected exchanges.  A token maker
   SHOULD NOT generate them unless it will not be able to generate the
   old QCD_TOKEN after a crash.

4.5.  Presenting the Token in an INFORMATIONAL Exchange

   This QCD_TOKEN notification is unprotected, and is sent as a response
   to a protected IKE request, which uses an IKE SA that is unknown.

            request             --> N(INVALID_IKE_SPI), N(QCD_TOKEN)+

   If child SPIs are persistently mapped to IKE SPIs as described in
   Section 9.2, a token taker may get the following unprotected message
   in response to an ESP or AH packet.

            request             --> N(INVALID_SPI), N(QCD_TOKEN)+

   The QCD_TOKEN and INVALID_IKE_SPI notifications are sent together to
   support both implementations that conform to this specification and
   implementations that don't.  Similar to the description in section
   2.21 of [RFC4306], The IKE SPI and message ID fields in the packet
   headers are taken from the protected IKE request.

   To support a periodic rollover of the secret used for token
   generation, the token taker MUST support at least four QCD_TOKEN
   notifications in a single packet.  The token is considered verified
   if any of the QCD_TOKEN notifications matches.  The token maker MAY
   generate up to four QCD_TOKEN notifications, based on several
   generations of keys.

   If the QCD_TOKEN verifies OK, an empty response MUST be sent.  If the
   QCD_TOKEN cannot be validated, a response SHOULD NOT be sent.



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   Section 5 defines token verification.


5.  Token Generation and Verification

   No token generation method is mandated by this document.  A method is
   documented in Section 5.1, but only serves as an example.

   The following lists the requirements from a token generation
   mechanism:
   o  Tokens MUST be at least 16 octets long, and no more than 128
      octets long, to facilitate storage and transmission.  Tokens
      SHOULD be indistinguishable from random data.
   o  It should not be possible for an external attacker to guess the
      QCD token generated by an implementation.  Cryptographic
      mechanisms such as PRNG and hash functions are RECOMMENDED.
   o  The token maker, MUST be able to re-generate or retrieve the token
      based on the IKE SPIs even after it reboots.

5.1.  A Stateless Method of Token Generation

   This describes a stateless method of generating a token:
   o  At installation or immediately after the first boot of the IKE
      implementation, 32 random octets are generated using a secure
      random number generator or a PRNG.
   o  Those 32 bytes, called the "QCD_SECRET", are stored in non-
      volatile storage on the machine, and kept indefinitely.
   o  The TOKEN_SECRET_DATA is calculated as follows:


            TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R)


   o  If key rollover is required by policy, the implementation MAY
      periodically generate a new QCD_SECRET and keep up to 3 previous
      generations.  When sending an unprotected QCD_TOKEN, as many as 4
      notification payloads may be sent, each from a different
      QCD_SECRET.

5.2.  A Stateless Method with IP addresses

   This method is similar to the one in the previous section, except
   that the IP address of the token taker is also added to the block
   being hashed.  This has the disadvantage that the token needs to be
   replaced (as described in Section 4.4) whenever the token taker
   changes its address.

   The reason to use this method is described in Section 9.3.  When



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   using this method, the TOKEN_SECRET_DATA field is calculated as
   follows:


         TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R | IPaddr-T)


   The IPaddr-T field specifies the IP address of the token taker.
   Secret rollover considerations are similar to those in the previous
   section.

5.3.  Token Lifetime

   The token is associated with a single IKE SA, and SHOULD be deleted
   by the token taker when the SA is deleted or expires.  More formally,
   the token is associated with the pair (SPI-I, SPI-R).


6.  Backup Gateways

   Making crash detection and recovery quick is a worthy goal, but since
   rebooting a gateway takes a non-zero amount of time, many
   implementations choose to have a stand-by gateway ready to take over
   as soon as the primary gateway fails for any reason.

   If such a configuration is available, it is RECOMMENDED that the
   stand-by gateway be able to generate the same token as the active
   gateway. if the method described in Section 5.1 is used, this means
   that the QCD_SECRET field is identical in both gateways.  This has
   the effect of having the crash recovery available immediately.


7.  Alternative Solutions

7.1.  Initiating a new IKE SA

   Instead of sending a QCD token, we could have the rebooted
   implementation start an Initial exchange with the peer, including the
   INITIAL_CONTACT notification.  This would have the same effect,
   instructing the peer to erase the old IKE SA, as well as establishing
   a new IKE SA with fewer rounds.

   The disadvantage here, is that in IKEv2 an authentication exchange
   MUST have a piggy-backed Child SA set up.  Since our use case is such
   that the rebooted implementation does not have traffic flowing to the
   peer, there are no good selectors for such a Child SA.

   Additionally, when authentication is asymmetric, such as when EAP is



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   used, it is not possible for the rebooted implementation to initiate
   IKE.

7.2.  Birth Certificates

   Birth Certificates is a method of crash detection that has never been
   formally defined.  Bill Sommerfeld suggested this idea in a mail to
   the IPsec mailing list on August 7, 2000, in a thread discussing
   methods of crash detection:

       If we have the system sign a "birth certificate" when it
       reboots (including a reboot time or boot sequence number),
       we could include that with a "bad spi" ICMP error and in
       the negotiation of the IKE SA.

   We believe that this method would have some problems.  First, it
   requires Alice to store the certificate, so as to be able to compare
   the public keys.  That requires more storage than does a QCD token.
   Additionally, the public-key operations needed to verify the self-
   signed certificates are more expensive for Alice.

   We believe that a symmetric-key operation such as proposed here is
   more light-weight and simple than that implied by the Birth
   Certificate idea.


8.  Interaction with Session Resumption

   Session Resumption, specified in [resumption] proposes to make
   setting up a new IKE SA consume less computing resources.  This is
   particularly useful in the case of a remote access gateway that has
   many tunnels.  A failure of such a gateway would require all these
   many remote access clients to establish an IKE SA either with the
   rebooted gateway or with a backup gateway.  This tunnel re-
   establishment should occur within a short period of time, creating a
   burden on the remote access gateway.  Session Resumption addresses
   this problem by having the clients store an encrypted derivative of
   the IKE SA for quick re-establishment.

   What Session Resumption does not help, is the problem of detecting
   that the peer gateway has failed.  A failed gateway may go undetected
   for as long as the lifetime of a child SA, because IPsec does not
   have packet acknowledgement, and applications cannot signal the IPsec
   layer that the tunnel "does not work".  Before establishing a new IKE
   SA using Session Resumption, a client MUST ascertain that the gateway
   has indeed failed.  This could be done using either a liveness check
   (as in RFC 4306) or using the QCD tokens described in this document.




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   A remote access client conforming to both specifications will store
   QCD tokens, as well as the Session Resumption ticket, if provided by
   the gateway.  A remote access gateway conforming to both
   specifications will generate a QCD token for the client.  When the
   gateway reboots, the client will discover this in either of two ways:
   1.  The client does regular liveness checks, or else the time for
       some other IKE exchange has come.  Since the gateway is still
       down, the IKE times out after several minutes.  In this case QCD
       does not help.
   2.  Either the primary gateway or a backup gateway (see Section 6) is
       ready and sends a QCD token to the client.  In that case the
       client will quickly re-establish the IPsec tunnel, either with
       the rebooted primary gateway, the backup gateway as described in
       this document or another gateway as described in [resumption]

   The full combined protocol looks like this:

        Initiator                Responder
        -----------              -----------
       HDR, SAi1, KEi, Ni  -->

                           <--    HDR, SAr1, KEr, Nr, [CERTREQ]

       HDR, SK {IDi, [CERT,]
       [CERTREQ,] [IDr,]
       AUTH, N(QCD_TOKEN)
       SAi2, TSi, TSr,
       N(TICKET_REQUEST)}  -->
                           <--    HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi,
                                  TSr, N(TICKET_OPAQUE)
                                  [,N(TICKET_GATEWAY_LIST)]}

                ---- Reboot -----

       HDR, {}             -->
                           <--  HDR, N(QCD_Token)

       HDR, Ni, N(TICKET_OPAQUE),
       [N+,], SK {IDi, [IDr,]
       SAi2, TSi, TSr,
       [CP(CFG_REQUEST)]}  -->
                           <--  HDR, SK {IDr, Nr, SAr2, [TSi, TSr],
                                [CP(CFG_REPLY)]}








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9.  Operational Considerations

9.1.  Who should implement this specification

   Throughout this document, we have referred to reboot time
   alternatingly as the time that the implementation crashes and the
   time when it is ready to process IPsec packets and IKE exchanges.
   Depending on the hardware and software platforms and the cause of the
   reboot, rebooting may take anywhere from a few seconds to several
   minutes.  If the implementation is down for a long time, the benefit
   of this protocol extension is reduced.  For this reason critical
   systems should implement backup gateways as described in Section 6.
   Note that the lower-case "should" in the previous sentence is
   intentional, as we do not specify this in the sense of RFC 2119.

   Implementing the "token maker" side of QCD makes sense for IKE
   implementation where protected connections originate from the peer,
   such as inter-domain VPNs and remote access gateways.  Implementing
   the "token taker" side of QCD makes sense for IKE implementations
   where protected connections originate, such as inter-domain VPNs and
   remote access clients.

   To clarify the requirements:
   o  A remote-access client MUST be a token taker and MAY be a token
      maker.
   o  A remote-access gateway MAY be a token taker and MUST be a token
      maker.
   o  An inter-domain VPN gateway MUST be both token maker and token
      taker.

   In order to limit the effects of DoS attacks, a token taker SHOULD
   limit the rate of QCD_TOKENs verified from a particular source.

   If excessive amounts of IKE requests protected with unknown IKE SPIs
   arrive at a token maker, the IKE module SHOULD revert to the behavior
   described in section 2.21 of [RFC4306] and either send an
   INVALID_IKE_SPI notification, or ignore it entirely.

9.2.  Response to unknown child SPI

   After a reboot, it is more likely that an implementation receives
   IPsec packets than IKE packets.  In that case, the rebooted
   implementation will send an INVALID_SPI notification, triggering a
   liveness check.  The token will only be sent in a response to the
   liveness check, thus requiring an extra round-trip.

   To avoid this, an implementation that has access to non-volatile
   storage MAY store a mapping of child SPIs to owning IKE SPIs, or to



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   generated tokens.  If such a mapping is available and persistent
   across reboots, the rebooted implementation SHOULD respond to the
   IPsec packet with an INVALID_SPI notification, along with the
   appropriate QCD_Token notifications.  A token taker SHOULD verify the
   QCD token that arrives with an INVALID_SPI notification the same as
   if it arrived with the IKE SPIs of the parent IKE SA.

   However, a persistent storage module might not be updated in a timely
   manner, and could be populated with IKE SPIs that have already been
   rekeyed.  A token taker MUST NOT take an invalid QCD Token sent along
   with an INVALID_SPI notification as evidence that the peer is either
   malfunctioning or attacking, but it SHOULD limit the rate at which
   such notifications are processed.

9.3.  Using Tokens that Depend on IP Addresses

   This section will describe the rationale for token generation methods
   such as the one described in Section 5.2.  Note that this section
   merely provides a possible rationale, and does not specify or
   recommend any kind of configuration.

   Some configurations of security gateway use a load-sharing cluster of
   hosts, all sharing the same IP addresses, where the SAs (IKE and
   child) are not synchronized between the cluster members.  In such a
   configuration, a single member does not know about all the IKE SAs
   that are active for the configuration.  A load balancer (usually a
   networking switch) sends IKE and IPsec packets to the several members
   based on source IP address.

   In such a configuration, an attacker can send a forged protected IKE
   packet with the IKE SPIs of an existing IKE SA, but from a different
   IP address.  This packet will likely be processed by a different
   cluster member from the one that owns the IKE SA.  Since no IKE SA
   state is stored on this member, it will send a QCD token to the
   attacker.  If the QCD token does not depend on IP address, this token
   can immediately be used to tell the token taker to tear down the IKE
   SA using an unprotected QCD_TOKEN notification.

   To thwart this possible attack, such configurations should use a
   method that considers the taker's IP address, such as the method
   described in Section 5.2.


10.  Security Considerations







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10.1.  QCD Token Handling

   Tokens MUST be hard to guess.  This is critical, because if an
   attacker can guess the token associated with the IKE SA, she can tear
   down the IKE SA and associated tunnels at will.  When the token is
   delivered in the IKE_AUTH exchange, it is encrypted.  When it is sent
   again in an unprotected notification, it is not, but that is the last
   time this token is ever used.

   An aggregation of some tokens generated by one peer together with the
   related IKE SPIs MUST NOT give an attacker the ability to guess other
   tokens.  Specifically, if one peer does not properly secure the QCD
   tokens and an attacker gains access to them, this attacker MUST NOT
   be able to guess other tokens generated by the same peer.  This is
   the reason that the QCD_SECRET in Section 5.1 needs to be
   sufficiently long.

   The QCD_SECRET MUST be protected from access by other parties.
   Anyone gaining access to this value will be able to delete all the
   IKE SAs for this token maker.

   The QCD token is sent by the rebooted peer in an unprotected message.
   A message like that is subject to modification, deletion and replay
   by an attacker.  However, these attacks will not compromise the
   security of either side.  Modification is meaningless because a
   modified token is simply an invalid token.  Deletion will only cause
   the protocol not to work, resulting in a delay in tunnel re-
   establishment as described in Section 2.  Replay is also meaningless,
   because the IKE SA has been deleted after the first transmission.

10.2.  QCD Token Transmission

   A token maker MUST NOT send a QCD token in an unprotected message for
   an existing IKE SA.  This implies that a conforming QCD token maker
   MUST be able to tell whether a particular pair of IKE SPIs represent
   a valid IKE SA.

   This requirement is obvious and easy in the case of a single gateway.
   However, some implementations use a load balancer to divide the load
   between several physical gateways.  It MUST NOT be possible even in
   such a configuration to trick one gateway into sending a QCD token
   for an IKE SA which is valid on another gateway.

10.3.  QCD Token Enumeration

   An attacker may try to attack QCD if the generation algorithm
   described in Section 5.1 is used.  The attacker will send several
   fake IKE requests to the gateway under attack, receiving and



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   recording the QCD Tokens in the responses.  This will allow the
   attacker to create a dictionary of IKE SPIs to QCD Tokens, which can
   later be used to tear down any IKE SA.

   Three factors mitigate this threat:
   o  The space of all possible IKE SPI pairs is huge: 2^128, so making
      such a dictionary is impractical.  Even if we assume that one
      implementation is faulty and always generates predictable IKE
      SPIs, the space is still at least 2^64 entries, so making the
      dictionary is extremely hard.
   o  Throttling the amount of QCD_TOKEN notifications sent out, as
      discussed in Section 9.1, especially when not soon after a crash
      will limit the attacker's ability to construct a dictionary.
   o  The methods in Section 5.1 and Section 5.2 allow for a periodic
      change of the QCD_SECRET.  Any such change invalidates the entire
      dictionary.


11.  IANA Considerations

   IANA is requested to assign a notify message type from the error
   types range (43-8191) of the "IKEv2 Notify Message Types" registry
   with name "QUICK_CRASH_DETECTION".


12.  Acknowledgements

   We would like to thank Hannes Tschofenig and Yaron Sheffer for their
   comments about Session Resumption.


13.  Change Log

   This section lists all changes in this document

   NOTE TO RFC EDITOR : Please remove this section in the final RFC

13.1.  Changes from draft-nir-ike-qcd-02

   o  Described QCD token enumeration, following a question by
      Lakshminath Dondeti.
   o  Added the ability to replace the QCD token for an existing IKE SA.
   o  Added tokens dependant on peer IP address and their interaction
      with MOBIKE.







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13.2.  Changes from draft-nir-ike-qcd-01

   o  Removed stateless method.
   o  Added discussion of rekeying and resumption.
   o  Added discussion of non-synchronized load-balanced clusters of
      gateways in the security considerations.
   o  Other wording fixes.

13.3.  Changes from draft-nir-ike-qcd-00

   o  Merged proposal with draft-detienne-ikev2-recovery [recovery]
   o  Changed the protocol so that the rebooted peer generates the
      token.  This has the effect, that the need for persistent storage
      is eliminated.
   o  Added discussion of birth certificates.

13.4.  Changes from draft-nir-qcr-00

   o  Changed name to reflect that this relates to IKE.  Also changed
      from quick crash recovery to quick crash detection to avoid
      confusion with IFARE.
   o  Added more operational considerations.
   o  Added interaction with IFARE.
   o  Added discussion of backup gateways.


14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4306]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
              RFC 4306, December 2005.

   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, June 2006.

   [RFC4718]  Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
              Implementation Guidelines", RFC 4718, October 2006.

14.2.  Informative References

   [recovery]
              Detienne, F., Sethi, P., and Y. Nir, "Safe IKE Recovery",
              draft-detienne-ikev2-recovery (work in progress),
              July 2008.



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   [resumption]
              Sheffer, Y., Tschofenig, H., Dondeti, L., and V.
              Narayanan, "IKEv2 Session Resumption",
              draft-tschofenig-ipsecme-ikev2-resumption (work in
              progress), September 2008.

   [stubs]    Xu, Y., Yang, P., Ma, Y., Deng, H., and K. Xu, "IKEv2 SA
              Synchronization for session resumption",
              draft-xu-ike-sa-sync (work in progress), October 2008.


Authors' Addresses

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  67897
   Israel

   Email: ynir@checkpoint.com


   Frederic Detienne
   Cisco Systems, Inc.
   De Kleetlaan, 7
   Diegem  B-1831
   Belgium

   Phone: +32 2 704 5681
   Email: fd@cisco.com


   Pratima Sethi
   Cisco Systems, Inc.
   O'Shaugnessy Road, 11
   Bangalore, Karnataka  560027
   India

   Phone: +91 80 4154 1654
   Email: psethi@cisco.com











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