IKEv2 crash detection: A Quick Crash Detection Method for the Internet Key Exchange Protocol (IKE)

Internet Engineering Task Force (IETF)                       Y. Nir, Ed.
Request for Comments: 6290                                   Check Point
Category: Standards Track                                  D. Wierbowski
ISSN: 2070-1721                                                      IBM
                                                             F. Detienne
                                                                P. Sethi
                                                                   Cisco
                                                               June 2011


                 A Quick Crash Detection Method for the
                  Internet Key Exchange Protocol (IKE)

Abstract

   This document describes an extension to the Internet Key Exchange
   Protocol version 2 (IKEv2) that allows for faster detection of
   Security Association (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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6290.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  RFC 5996 Crash Recovery  . . . . . . . . . . . . . . . . . . .  4
   3.  Protocol Outline . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Formats and Exchanges  . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Notification Format  . . . . . . . . . . . . . . . . . . .  6
     4.2.  Passing a Token in the AUTH Exchange . . . . . . . . . . .  7
     4.3.  Replacing Tokens after Rekey or Resumption . . . . . . . .  8
     4.4.  Replacing the Token for an Existing SA . . . . . . . . . .  9
     4.5.  Presenting the Token in an Unprotected Message . . . . . .  9
   5.  Token Generation and Verification  . . . . . . . . . . . . . . 10
     5.1.  A Stateless Method of Token Generation . . . . . . . . . . 11
     5.2.  A Stateless Method with IP Addresses . . . . . . . . . . . 11
     5.3.  Token Lifetime . . . . . . . . . . . . . . . . . . . . . . 12
   6.  Backup Gateways  . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Interaction with Session Resumption  . . . . . . . . . . . . . 13
   8.  Operational Considerations . . . . . . . . . . . . . . . . . . 14
     8.1.  Who Should Implement This Specification  . . . . . . . . . 14
     8.2.  Response to Unknown Child SPI  . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     9.1.  QCD Token Generation and Handling  . . . . . . . . . . . . 16
     9.2.  QCD Token Transmission . . . . . . . . . . . . . . . . . . 17
     9.3.  QCD Token Enumeration  . . . . . . . . . . . . . . . . . . 18
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     12.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  The Path Not Taken  . . . . . . . . . . . . . . . . . 20
     A.1.  Initiating a New IKE SA  . . . . . . . . . . . . . . . . . 20
     A.2.  SIR  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     A.3.  Birth Certificates . . . . . . . . . . . . . . . . . . . . 20
     A.4.  Reducing Liveness Check Length . . . . . . . . . . . . . . 21

1.  Introduction

   IKEv2, as described in [RFC5996] and its predecessor RFC 4306, 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, so all traffic is inbound.
   In other cases, the non-rebooted peer has a dynamic IP address, so
   the rebooted peer cannot initiate IKE because its current IP address
   is unknown.  In such cases, the rebooted peer will not be able to
   re-establish the tunnels.  Section 2 describes how recovery works
   under RFC 5996, and explains why it may take several minutes.

   The method proposed here is to send an octet string, called a "QCD
   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 peer, so as to delete the IKE SA.  Deleting the IKE SA
   results in a quick establishment of new 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 Security Parameter Indexes (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.

   The term "non-volatile storage" in this document refers to a data
   storage module that persists across restarts of the token maker.
   Examples of such a storage module include an internal disk, an
   internal flash memory module, an external disk, and an external
   database.  A small non-volatile storage module is required for a
   token maker, but a larger one can be used to enhance performance, as
   described in Section 8.2.

2.  RFC 5996 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 [RFC5996].  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 [RFC5996].  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...

   Those "at least several minutes" are a time during part of which both
   peers are active, but IPsec cannot be used.

   Especially in the case of a reboot (rather than fail-over or
   administrative clearing of state), the peer does not recover
   immediately.  Reboot, depending on the system, may take from a few
   seconds to a few minutes.  This means that at first the peer just
   goes silent, i.e., does not send or respond to any messages.  IKEv2
   implementations can detect this situation and follow the rules given
   in Section 2.4:

      If there has only been outgoing traffic on all of the SAs
      associated with an IKE SA, it is essential to confirm liveness of
      the other endpoint to avoid black holes.  If no cryptographically
      protected messages have been received on an IKE SA or any of its
      Child SAs recently, the system needs to perform a liveness check
      in order to prevent sending messages to a dead peer.

   [RFC5996] does not mandate any time limits, but it is possible that
   the peer will start liveness checks even before the other end is
   sending INVALID_SPI notification, as it detected that the other end
   is not sending any packets anymore while it is still rebooting or
   recovering from the situation.

   This means that the several minutes recovery period is overlapping
   the actual recover time of the other peer; i.e., if the security
   gateway requires several minutes to boot up from the crash, then the
   other peers have already finished their liveness checks before the
   crashing peer even has a chance to send INVALID_SPI notifications.

   There are cases where the peer loses state and is able to recover
   immediately; in those cases it might take several minutes to recreate
   the IPsec SAs.

   Note that the IKEv2 specification specifically gives no guidance for
   the number of retries or the length of timeouts, as these do not
   affect interoperability.  This means that implementations are allowed
   to use the hints provided by the INVALID_SPI messages to shorten
   those timeouts (i.e., a different environment and situation requiring
   different rules).

   Some existing IKEv2 implementations already do that (i.e., shorten
   timeouts or limit number of retries) based on these kinds of hints
   and also start liveness checks quickly after the other end goes
   silent.  However, see Appendix A.4 for a discussion of why this may
   not be enough.

3.  Protocol Outline

   Supporting implementations will send a notification, called a "QCD
   token", as described in Section 4.1 in the first IKE_AUTH exchange
   messages.  These are the first IKE_AUTH request and final IKE_AUTH
   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 MUST store it (or
   a digest thereof) along with the IKE SA.  Implementations that
   support this part of the protocol will be called "token takers".
   Section 8.1 has considerations for which implementations need to be
   token takers, and which should be token makers.  Implementations 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 SHOULD 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 with the matching 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 silently delete the
   IKE SA associated with the IKE_SPI fields and all dependent 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
   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.

   See Section 7 for a short discussion about this extension's
   interaction with IKEv2 Session Resumption ([RFC5723]).

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 be 1, as this message is related to an
      IKE SA.

   o  SPI Size (1 octet) MUST be zero, in conformance with Section 3.10
      of [RFC5996].

   o  QCD Token Notify Message Type (2 octets) - MUST be 16419, the
      value assigned for QCD token notifications.

   o  TOKEN_SECRET_DATA (variable) contains a generated token as
      described in Section 5.

4.2.  Passing a Token in the AUTH Exchange

   For brevity, only the Extensible Authentication Protocol (EAP)
   version of an AUTH exchange will be presented here.  The non-EAP
   version is very similar.  The figures below are based on Appendix C.3
   of [RFC5996].

    first request       --> IDi,
                            [N(INITIAL_CONTACT)],
                            [[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
                            [IDr],
                            [N(QCD_TOKEN)]
                            [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+]

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

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

    last request        --> AUTH

    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.

   The QCD_TOKEN notification is related to the IKE SA and should 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 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 initiate an
   INFORMATIONAL exchange immediately after the CREATE_CHILD_SA exchange
   as follows:

      request             --> N(QCD_TOKEN)

      response            <--

   For session resumption, as specified in [RFC5723], 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.

   The INFORMATIONAL exchange described in this section can also be used
   if QCD tokens need to be replaced due to a key rollover.  However,
   since token takers are required to verify at least 4 QCD tokens, this
   is only necessary if secret QCD keys are rolled over more than four
   times as often as IKE SAs are rekeyed.  See Section 5.1 for an
   example method that uses secret keys that may require rollover.

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:

     (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 is no longer able to generate the
   old QCD_TOKEN.

4.5.  Presenting the Token in an Unprotected Message

   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.

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

   If child SPIs are persistently mapped to IKE SPIs as described in
   Section 8.2, a token taker may get the following unprotected message
   in response to an Encapsulating Security Payload (ESP) or
   Authentication Header (AH) packet.

            message             --> 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 [RFC5996], 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, the receiver MUST silently discard the
   IKE SA and all associated child SAs.  If the QCD_TOKEN cannot be
   validated, a response MUST NOT be sent, and the event may be logged.
   Section 5 defines token verification.

5.  Token Generation and Verification

   No token generation method is mandated by this document.  Two methods
   are documented in the following sub-sections, but they only serve as
   examples.

   The following lists the requirements for 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 a pseudo-random number generator (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.

   o  The method of token generation MUST be such that a collision of
      QCD tokens between different pairs of IKE SPI will be highly
      unlikely.

   For verification, the token taker makes a bitwise comparison of the
   token stored along with the IKE SA with the token sent in the
   unprotected message.  Multihomed takers might flip back-and-forth
   between several addresses, and have their tokens replaced as
   described in Section 4.4.  To help avoid the case where the latest
   stored token does not match the address used after the maker lost
   state, the token taker MAY store several earlier tokens associated
   with the IKE SA, and silently discard the SA if any of them matches.

5.1.  A Stateless Method of Token Generation

   The following describes a stateless method of generating a token.  In
   this case, 'stateless' means not maintaining any per-tunnel state,
   although there is a small amount of non-volatile storage required.

   o  At installation or immediately after the first boot of the token
      maker, 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  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.

   o  The TOKEN_SECRET_DATA is calculated as follows:

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

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.

   See Section 9.2 for a discussion of a use-case for this method.  When
   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.

   Note that with a multihomed token taker, the QCD token matches just
   one of the token taker IP addresses.  Usually this is not a problem,
   as packets sent to the token maker come out the same IP address.  If
   for some reason this changes, then the token maker can replace the
   token as described in Section 4.4.  If IKEv2 Mobility and Multihoming
   (MOBIKE) is used, replacing the tokens SHOULD be piggybacked on the
   INFORMATIONAL exchange with the UPDATE_SA_ADDRESSES notifications.

   There is a corner case where the token taker begins using a new IP
   address (because of multihoming, roaming, or normal network
   operations) and the token maker loses state before replacing the
   token.  In that case, it will send a correct QCD token, but the token
   taker will still have the old token.  In that case, the extension
   will not work, and the peers will revert to RFC 5996 recovery.

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 standby gateway ready to take over
   as soon as the primary gateway fails for any reason.  [RFC6027]
   describes considerations for such clusters of gateways with
   synchronized state, but the rest of this section is relevant even
   when there is no synchronized state.

   If such a configuration is available, it is RECOMMENDED that the
   standby 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.

   Note that this refers to "high-availability" configurations, where
   only one gateway is active at any given moment.  This is different
   from "load sharing" configurations where more than one gateway is
   active at the same time.  For load sharing configurations, please see
   Section 9.2 for security considerations.

7.  Interaction with Session Resumption

   Session resumption, specified in [RFC5723], allows the setting up of
   a new IKE SA to 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 requires all these many
   remote access clients to establish an IKE SA either with the rebooted
   gateway or with a backup.  This tunnel re-establishment occurs 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 an arbitrarily long time, because IPsec does not have packet
   acknowledgement, and applications cannot signal the IPsec layer that
   the tunnel "does not work".  Section 2.4 of RFC 5996 does not specify
   how long an implementation needs to wait before beginning a liveness
   check, and only says "not recently" (see full quote in Section 2).
   In practice, some mobile devices wait a very long time before
   beginning a liveness check, in order to extend battery life by
   allowing parts of the device to remain in low-power modes.

   QCD tokens provide a way to detect the failure of the peer in the
   case where a liveness check has not yet ended (or begun).

   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 exchange 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 or the backup gateway as described
       in this document.

   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,
                                  N(QCD_TOKEN), SAr2, TSi, TSr,
                                  N(TICKET_LT_OPAQUE) }

                ---- Reboot -----

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

       HDR, [N(COOKIE),]
       Ni, N(TICKET_OPAQUE)
       [,N+]               -->
                           <--  HDR, Nr [,N+]

8.  Operational Considerations

8.1.  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.

   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 this discussion:

   o  For remote-access clients it makes sense to implement the token
      taker role.

   o  For remote-access gateways it makes sense to implement the token
      maker role.

   o  For inter-domain VPN gateways it makes sense to implement both
      roles, because it can't be known in advance where the traffic
      originates.

   o  It is perfectly valid to implement both roles in any case, for
      example, when using a single library or a single gateway to
      perform several roles.

   In order to limit the effects of Denial-of-Service (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 [RFC5996] and either send an
   INVALID_IKE_SPI notification or ignore it entirely.

   Section 9.2 requires that token makers never send a QCD token in the
   clear for a valid IKE SA and describes some configurations where this
   could occur.  Implementations that may be installed in such
   configurations SHOULD automatically detect this and disable this
   extension in unsafe configurations and MUST allow the user to control
   whether the extension is enabled or disabled.

8.2.  Response to Unknown Child SPI

   After a reboot, it is more likely that an implementation will receive
   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 enough non-
   volatile storage MAY store a mapping of child SPIs to owning IKE
   SPIs, or to 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 tokens relating to 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.  Security Considerations

   The extension described in this document must not reduce the security
   of IKEv2 or IPsec.  Specifically, an eavesdropper must not learn any
   non-public information about the peers.

   The proposed mechanism should be secure against attacks by a passive
   man in the middle (MITM) (eavesdropper).  Such an attacker must not
   be able to disrupt an existing IKE session, either by resetting the
   session or by introducing significant delays.  This requirement is
   especially significant, because this document introduces a new way to
   reset an IKE SA.

   The mechanism need not be similarly secure against an active MITM,
   since this type of attacker is already able to disrupt IKE sessions.

9.1.  QCD Token Generation and Handling

   Tokens MUST be hard to guess.  This is critical, because if an
   attacker can guess the token associated with an IKE SA, they 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 maker together with
   the related IKE SPIs MUST NOT give an attacker the ability to guess
   other tokens.  Specifically, if one taker 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 maker.
   This is the reason that the QCD_SECRET in Section 5.1 needs to be
   sufficiently long.

   The token taker MUST store the token in a secure manner.  No attacker
   should be able to gain access to a stored token.

   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.

9.2.  QCD Token Transmission

   A token maker MUST NOT send a valid QCD token in an unprotected
   message for an existing 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 valid QCD
   token for an IKE SA that is valid on another gateway.  This is true
   whether the attempt to trick the gateway uses the token taker's IP
   address or a different IP address.

   IPsec failure detection is not applicable to deployments where the
   QCD secret is shared by multiple gateways and the gateways cannot
   assess whether the token can be legitimately sent in the clear while
   another gateway may actually still own the SA's.  Load balancing
   configurations typically fall in this category.  In order for a load
   balancing configuration of IPsec gateways to support this
   specification, all members MUST be able to tell whether a particular
   IKE SA is active anywhere in the cluster.  One way to do this is to
   synchronize a list of active IKE SPIs among all the cluster members.

   Because it includes the token taker's IP address in the token
   generation, the method in Section 5.2 can (under certain conditions)
   prevent revealing the QCD token for an existing pair of IKE SPIs to
   an attacker who is using a different IP address, even in a load-
   sharing cluster without state synchronization.  That method does not
   prevent revealing the QCD token to an active attacker who is spoofing
   the token taker's IP address.  Such an attacker may attempt to direct
   messages to a cluster member other than the member responsible for
   the IKE SA in an attempt to trick that gateway into sending a QCD
   token for a valid IKE SA.  That method should not be used unless the
   load balancer guarantees that IKE packets from the same source IP
   address always go to the same cluster member.

9.3.  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
   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 always generates predictable IKE SPIs, the space is
      still at least 2^64 entries, so making the dictionary is extremely
      hard.  To ensure this, token makers MUST generate unpredictable
      IKE SPIs by using a cryptographically strong pseudo-random number
      generator.

   o  Throttling the amount of QCD_TOKEN notifications sent out, as
      discussed in Section 8.1, 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.

10.  IANA Considerations

   IANA has assigned a notify message type (16419) from the status types
   range (16406-40959) of the "IKEv2 Notify Message Types" registry with
   the name "QUICK_CRASH_DETECTION".

11.  Acknowledgements

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

   Others who have contributed valuable comments are, in alphabetical
   order, Lakshminath Dondeti, Paul Hoffman, Tero Kivinen, Scott C
   Moonen, Magnus Nystrom, and Keith Welter.

12.  References

12.1.  Normative References

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

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

   [RFC5996]   Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
               "Internet Key Exchange Protocol Version 2 (IKEv2)",
               RFC 5996, September 2010.

12.2.  Informative References

   [RFC5723]   Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
               Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
               January 2010.

   [RFC6027]   Nir, Y., "IPsec Cluster Problem Statement", RFC 6027,
               October 2010.

   [recovery]  Detienne, F., Sethi, P., and Y. Nir, "Safe IKE Recovery",
               Work in Progress, July 2009.

Appendix A.  The Path Not Taken

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

A.2.  SIR

   Another proposal that was considered for this work item is the SIR
   extension, which is described in [recovery].  Under that proposal,
   the non-rebooted peer sends a non-protected query to the possibly
   rebooted peer, asking whether the IKE SA exists.  The peer replies
   with either a positive or negative response, and the absence of a
   positive response, along with the existence of a negative response,
   is taken as proof that the IKE SA has really been lost.

   The working group preferred the QCD proposal to this one.

A.3.  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.

A.4.  Reducing Liveness Check Length

   Some implementations require fewer retransmissions over a shorter
   period of time for cases of liveness check started because of an
   INVALID_SPI or INVALID_IKE_SPI notification.

   We believe that the default retransmission policy should represent a
   good balance between the need for a timely discovery of a dead peer,
   and a low probability of false detection.  We expect the policy to be
   set to take the shortest time such that this probability achieves a
   certain target.  Therefore, we believe that reducing the elapsed time
   and retransmission count may create an unacceptably high probability
   of false detection, and this can be triggered by a single
   INVALID_IKE_SPI notification.

   Additionally, even if the retransmission policy is reduced to, say,
   one minute, it is still a very noticeable delay from a human
   perspective, from the time that the gateway has come up (i.e., is
   able to respond with an INVALID_SPI or INVALID_IKE_SPI notification)
   and until the tunnels are active, or from the time the backup gateway
   has taken over until the tunnels are active.  The use of QCD tokens
   can reduce this delay.

Authors' Addresses

   Yoav Nir (editor)
   Check Point Software Technologies, Ltd.
   5 Hasolelim st.
   Tel Aviv  67897
   Israel

   EMail: ynir@checkpoint.com


   David Wierbowski
   International Business Machines
   1701 North Street
   Endicott, New York  13760
   United States

   EMail: wierbows@us.ibm.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