Network Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Intended status: Standards Track                          March 16, 2008
Expires: September 17, 2008

                 A Quick Crash Recovery Method for IKE

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This document describes an extension to the IKEv2 protocol that
   allows for faster crash recovery using a saved token method.

   When an IPsec tunnel between two IKEv2 implementations is
   disconnected due to a restart of one peer, it can take as much as
   several minutes to recover.  In this text we propose an extension to
   the protocol, that allows for recovery within a few seconds of the

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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  . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Notification Format  . . . . . . . . . . . . . . . . . . .  4
     4.2.  Authentication Exchange  . . . . . . . . . . . . . . . . .  5
     4.3.  Informational Exchange . . . . . . . . . . . . . . . . . .  7
   5.  Token Generation and Verification  . . . . . . . . . . . . . .  7
     5.1.  A Stateful Method of Token Generation  . . . . . . . . . .  7
     5.2.  A Stateless Method of Token Generation . . . . . . . . . .  8
     5.3.  Token Lifetime . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Alternative Solutions  . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Why not Save the Entire IKE SA . . . . . . . . . . . . . .  8
     6.2.  Initiating a new IKE SA  . . . . . . . . . . . . . . . . .  9
   7.  Operational Considerations . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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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 peers have a dynamic IP address.
   In such cases, the rebooted peer will not re-establish the tunnels.

   Section 2 describes the current procedure, and explains why crash
   recovery can take up to several minutes.  The method proposed here,
   is to send a token in the IKE_AUTH exchange that establishes the
   tunnel.  That token can be maintained on the peer in some kind of
   persistent storage such as a disk or a database, and can be used to
   delete the IKE SA after a crash.  Deleting the IKE SA results is a
   quick re-establishment of the IPsec tunnel.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  RFC 4306 Crash Recovery

   When one peer reboots, the other peer does not get any notification,
   so 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
   INFORMATIONAL exchange with an 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 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 refered to as "Dead Peer
   Detection" or DPD.

   Section 2.4 does not mandate how many times the INFORMATIONAL 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

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   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 "QCR
   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.

   A supporting implementation receiving such a token SHOULD store it in
   such a way, that it will survive a reboot.  When a supporting
   implementation receives a protected IKE request message with unknown
   IKE SPIs, it should scan its saved token store.  If a token matching
   the IKE SPIs is found, it SHOULD send it to the requesting peer in an
   unprotected IKE message as described in Section 4.3.

   When a supporting implementation receives the QCR notification token
   in an unprotected INFORMATIONAL exchange, it MUST verify that the
   TOKEN_SECRET_DATA field is associated with the IKE SPIs in the
   IKE_SPI fields of the IKE packet.  If the verification fails, 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.

   A supporting implementation 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].  The
   current proposal is orthogonal to Session Resumption, and in fact
   using Session Resumption instead of a regular IKE exchange, the new
   SA can be created with minimal overhead.

4.  Formats and Exchanges

4.1.  Notification Format

   The notification payload called "QCR token" is formatted as follows:

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                            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    ! QCR 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  QCR Token Notify Message Type (2 octets) - Must be xxxxx, the
      value assigned for QCR token notifications.  TBA by IANA.
   o  TOKEN_SECRET_DATA (16-256 octets) contains a generated token as
      described in Section 5.

4.2.  Authentication Exchange

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

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    first request       --> IDi,
                            [[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
                            SA, TSi, TSr,

    first response      <-- IDr, [CERT+], AUTH,

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

    last request        --> AUTH

    last response       <-- AUTH,
                            SA, TSi, TSr,

   Note that the QCR_TOKEN notification is marked as optional because it
   is not required by this specification that both sides send QCR
   tokens.  If only one peer sends the QCR 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 QCR_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 doesnOt know whether the other side supports it.

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4.3.  Informational Exchange

   This informational exchange is non-protected, and is sent as a
   response to a protected IKE request, which uses an IKE SA that is

               request             --> N(QCR_TOKEN)

               response            <--

   The QCR_TOKEN is the only notification in the request.  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

   If the QCR_TOKEN verifies OK, an empty response MUST be sent.  If the
   QCR_TOKEN cannot be validated, a response SHOULD NOT be sent.
   Section 5 defines token verification.

5.  Token Generation and Verification

   No token generation method is mandated by this document.  Two methods
   are documented in Section 5.1 and Section 5.2, but they only serve as

   The following lists the requirements from a token generation
   o  Tokens should be at least 16 octets log, and no more than 256
      octets long, to facilitate storage.
   o  It should not be possible for an external attacker to guess the
      QCR token generated by an implementation.  Cryptographic
      mechanisms such as PRNG and hash functions are RECOMMENDED.
   o  The peer that generated the QCR token, should be able to
      immediately verify it, provided that the IKE SPIs are given, and
      that the IKE SA has not expired or been otherwise deleted.

5.1.  A Stateful Method of Token Generation

   This describes a stateful method of generating a token:
   o  Before sending the QCR token, 32 random octets are generated using
      a secure random number generator or a PRNG.
   o  Those 32 bytes are used as the TOKEN_SECRET_DATA field, and stored
      as part of the IKE SA.
   o  For verification, the IKE implementation simply retrieves the IKE
      SA, and compares the TOKEN_SECRET_DATA field from the notification
      to the TOKEN_SECRET_DATA field stored with the SA.

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5.2.  A Stateless Method of Token Generation

   This describes a stateless method of generating a token.
   o  At startup, the IKE implementation generates a 32-octet random
      buffer using a cryptographically secure PRNG.  This buffer is
      called the QCR_SECRET.
   o  For each QCR token, the TOKEN_SECRET_DATA field is generated by
      calculating a SHA-256 hash over a concatenation of the QCR_SECRET
      and the IKE SPI as follows:


   o  Verification uses the same calculation, and works even if the IKE
      SA has been deleted.  Still, if the IKE SA is no longer valid, the
      notification MUST NOT be acknowledged, as this could be used in an
      attempt to guess the QCR_SECRET.

5.3.  Token Lifetime

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

6.  Alternative Solutions

6.1.  Why not Save the Entire IKE SA

   IKEv2 does not assume the existence of a persistent storage module.
   If we are adding such a module, why not use it to save the entire IKE
   SA across reboots, nullifying the need for a crash recovery

   There are several reasons why we believe that this is not a good
   1.  A token is only 16-256 octets, and is much more compact than all
       the data needed to store an IKE SA.
   2.  A token is valid for the life of an IKE SA.  An IKE SA state is
       updated whenever a message is sent, becuase of the requirement to
       keep the sequence of message IDs.  It may not be acceptable to
       update the persistent storage whenever an IKE message is sent.
   3.  A reboot is usually an unpredictable event, and as such, we
       cannot know how long it will last.  By the time the machine has
       rebooted, the peer may have attempted some type of protected
       exchange (liveness check, create-child-SA or delete), timed out,
       and deleted the SA.  It is far better to reboot without SAs and

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       with only a token for quick recovery.

6.2.  Initiating a new IKE SA

   Instead of sending a QCR 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 assymetric, such as when EAP is
   used, it is not possible for the rebooted implementation to initiate

7.  Operational Considerations

   To support this standard, an implementation needs to have access to a
   persistent storage module.  This could be an internal hard disk, a
   local or remote database application, or any other method that
   persists across reboots.  This storage module and the data links
   between the storage module and the IKE module must meet the
   performance requirements of the IKE module.  The storage module MUST
   support insertion and deletion rates equal to peek IKE SA setup rates
   and it SHOULD support query rates that are fast enough.

   See Section 8 for security considerations for this storage mechanism.

   In order to limit the effects of DoS attacks, an implementation
   SHOULD limit the rate of queries into the token storage so as not to
   overload it.  If excessive amounts of IKE requests protected with
   unknown IKE SPIs arrive, 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.

8.  Security Considerations

   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
   back in an informational exchange it is not encrypted, but that is

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   the last use of that token.

   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 QCR
   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 QCR_SECRET in Section 5.2 needs to be long.

   The persistent storage MUST be protected from access by other
   parties.  Anyone gaining access to the contents of the storage will
   be able to delete all the IKE SAs described in it.

   The tokens associated with expired and deleted IKE SAs MUST be
   deleted from the storage, so that a future compromise of the storage
   does not reveal enough tokens to facilitate an attack against the QCR

   The QCR 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.

9.  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_RECOVERY".

10.  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.

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

              Sheffer, Y., Tschofenig, H., Dondeti, L., and V.

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              Narayanan, "IPsec Gateway Failover Protocol",
              draft-sheffer-ipsec-failover-02 (work in progress),
              November 2007.

Author's Address

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

   Email: ynir@checkpoint.com

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