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Versions: 00 01 02 03 04 05 06 RFC 6311

Network Working Group                                      R. Singh, Ed.
Internet-Draft                                                G. Kalyani
Intended status: Standards Track                                   Cisco
Expires: August 12, 2011                                          Y. Nir
                                                             Check Point
                                                              Y. Sheffer
                                                             Independent
                                                                D. Zhang
                                                                  Huawei
                                                        February 8, 2011


         Protocol Support for High Availability of IKEv2/IPsec
                 draft-ietf-ipsecme-ipsecha-protocol-03

Abstract

   The IPsec protocol suite is widely used for business-critical network
   traffic.  In order to make IPsec deployments highly available, more
   scalable and failure-resistant, they are often implemented as IPsec
   High Availability (HA) clusters.  However there are many issues in
   IPsec HA clustering, and in particular in IKEv2 clustering.  An
   earlier document, "IPsec Cluster Problem Statement", enumerates the
   issues encountered in the IKEv2/IPsec HA cluster environment.  This
   document attempts to resolve these issues with the least possible
   change to the protocol.

   This document proposes an extension to the IKEv2 protocol to solve
   the main issues of "IPsec Cluster Problem Statement" in the commonly
   deployed hot-standby cluster, and provides implementation advice for
   other issues.  The main issues to be solved are the synchronization
   of IKEv2 Message ID counters, and of IPsec Replay Counters.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on August 12, 2011.

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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Issues Resolved from IPsec Cluster Problem Statement . . . . .  6
   4.  The IKEv2/IPsec SA Counter Synchronization Problem . . . . . .  7
   5.  SA Counter Synchronization Solution  . . . . . . . . . . . . .  8
     5.1.  Processing Rules for IKE Message ID Synchronization  . . . 10
     5.2.  Processing Rules for IPsec Replay Counter
           Synchronization  . . . . . . . . . . . . . . . . . . . . . 11
   6.  IKEv2/IPsec Synchronization Notification Payloads  . . . . . . 11
     6.1.  The IKEV2_MESSAGE_ID_SYNC_SUPPORTED Notification . . . . . 11
     6.2.  The IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED Notification . . . 12
     6.3.  The IKEV2_MESSAGE_ID_SYNC Notification . . . . . . . . . . 12
     6.4.  The IPSEC_REPLAY_COUNTER_SYNC Notification . . . . . . . . 13
   7.  Implementation Details . . . . . . . . . . . . . . . . . . . . 14
   8.  IKE SA and IPsec SA Message Sequencing . . . . . . . . . . . . 14
     8.1.  Handling of Pending IKE Messages . . . . . . . . . . . . . 15
     8.2.  Handling of Pending IPsec Messages . . . . . . . . . . . . 15
     8.3.  IKE SA Inconsistencies . . . . . . . . . . . . . . . . . . 15
   9.  Step by Step Details . . . . . . . . . . . . . . . . . . . . . 15
   10. Interaction with other drafts  . . . . . . . . . . . . . . . . 16
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     14.1. Draft -03  . . . . . . . . . . . . . . . . . . . . . . . . 18
     14.2. Draft -02  . . . . . . . . . . . . . . . . . . . . . . . . 18
     14.3. Draft -01  . . . . . . . . . . . . . . . . . . . . . . . . 18
     14.4. Draft  -00 . . . . . . . . . . . . . . . . . . . . . . . . 19
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     15.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  IKEv2 Message ID Sync Examples  . . . . . . . . . . . 20
     A.1.  Normal Failover - Example 1  . . . . . . . . . . . . . . . 20
     A.2.  Normal Failover - Example 2  . . . . . . . . . . . . . . . 20
     A.3.  Simultaneous Failover  . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21













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1.  Introduction

   The IPsec protocol suite, including IKEv2, is a major building block
   of virtual private networks (VPNs).  In order to make such VPNs
   highly available, more scalable and failure-resistant, these VPNs are
   implemented as IKEv2/IPsec Highly Available (HA) cluster.  However
   there are many issues with the IKEv2/IPsec HA cluster.  The problem
   statement draft Section 4 enumerates the issues around the IKEv2/
   IPsec HA cluster solution.

   In the case of a hot-standby cluster implementation of IKEv2/IPsec
   based VPNs, the IKEv2/IPsec session is first established between the
   peer and the active member of the cluster.  Later, the active member
   continuously syncs/updates the IKE/IPsec SA state to the standby
   member of the cluster.  This primary SA state sync-up takes place
   upon each SA bring-up and/or rekey.  Performing the SA state
   synchronization/update for every single IKE and IPsec message is very
   costly, so normally it is done periodically.  As a result, when the
   failover event happens, this is first detected by the standby member
   and, possibly after a considerable amount of time, it becomes the
   active member.  During this failover process the peer is unaware of
   the failover event, and keeps sending IKE requests and IPsec packets
   to the cluster, as in fact it is allowed to do because of the IKEv2
   windowing feature.  After the newly-active member starts, it detects
   the mismatch in IKE Message ID values and IPsec replay counters and
   needs to resolve this situation.  Please see Section 4 for more
   details of the problem.

   This document proposes an extension to the IKEv2 protocol to solve
   the main issues of IKE Message ID synchronization and IPsec SA replay
   counter synchronization and gives implementation advice for others.
   Following is a summary of the solutions provided in this document:

   o  IKEv2 Message ID synchronization: this is done by syncing up the
      expected send and receive Message ID values with the peer, and
      updating the values at the newly active cluster member.
   o  IPsec Replay Counter synchronization: this is done by incrementing
      the cluster's outgoing SA replay counter values by a "large"
      number; in addition, the newly-active member requests the peer to
      increment the replay counter values it is using for the peer's
      outgoing traffic.

   Although this document describes the IKEv2 Message ID and IPsec
   replay counter synchronization in the context of an IPsec HA cluster,
   the solution provided is generic and can be used in other scenarios
   where IKEv2 Message ID or IPsec SA replay counter synchronization may
   be required.




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   Implementations differ on the need to synchronize the IKEv2 Message
   ID and/or IPsec replay counters.  Both of these problems are handled
   separately, using a separate notification for each capability.  This
   provides the flexibility of implementing either or both of these
   solutions.


2.  Terminology

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

   "SA Counter Synchronization Request/Response" are the request viz.
   response of the informational exchange defined in this document to
   synchronize the IKEv2/IPsec SA counter information between one member
   of the cluster and the peer.

   Some of the terms listed below are reused from [2] with further
   clarification in the context of the current document.

   o  "Hot Standby Cluster", or "HS Cluster" is a cluster where only one
      of the members is active at any one time.  This member is also
      referred to as the "active" member, whereas the other(s) are
      referred to as "standby" members.  VRRP [5] is one method of
      building such a cluster.  The goal of the Hot Standby Cluster is
      to create the illusion of a single virtual gateway to the peer(s).
   o  "Active Member" is the primary member in the Hot-Standby cluster.
      It is responsible for forwarding packets on behalf of the virtual
      gateway.
   o  "Standby Member" is the primary backup member.  This member takes
      control, i.e. becomes the active member, after the failover event.
   o  "Peer" is an IKEv2/IPsec endpoint that maintains an IPsec
      connection with the Hot-Standby cluster.  The Peer identifies the
      cluster by the cluster's (single) IP address.  If a failover event
      occurs, the standby member of the cluster becomes active, and the
      peer normally doesn't notice that failover has taken place.
      Although we treat the peer as a single entity, it may also be a
      cluster.
   o  "Multiple failover" is the situation where, in a cluster with
      three or more members, multiple failover events happen in rapid
      succession, e.g. from M1 to M2, and then to M3.  It is our goal
      that the implementation should be able to handle this situation,
      i.e. to handle the new failover event even if it is still
      processing the old failover.
   o  "Simultaneous failover" is the situation where two clusters have
      an IPsec connection between them, and failover happens at both
      ends at the same time.  It is our goal that implementations should



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      be able to handle simultaneous failover.

   The generic term "IKEv2/IPsec SA Counters" is used throughout this
   document.  This term refers to both IKEv2 Message ID counters and
   IPsec replay counters.  According to the IPsec standards, the IKEv2
   Message ID counter is mandatory, and used to ensure reliable delivery
   as well as to protect against message replay in IKEv2; the IPsec SA
   replay counters are optional, and are used to provide the IPsec anti-
   replay feature.


3.  Issues Resolved from IPsec Cluster Problem Statement

   The IPsec Cluster Problem Statement [2] enumerates the problems
   raised by IPsec clusters.  The following table lists the problem
   statement's sections that are resolved by this document.
   o  3.2.  Lots of Long Lived State
   o  3.3.  IKE Counters
   o  3.4.  Outbound SA Counters
   o  3.5.  Inbound SA Counters
   o  3.6.  Missing Synchronization Messages
   o  3.7.  Simultaneous use of IKE and IPsec SAs by Different Members
      *  3.7.1.  Outbound SAs using counter modes
   o  3.8.  Different IP addresses for IKE and IPsec
   o  3.9.  Allocation of SPIs

   The main problem areas are solved using the protocol extension
   defined below; additionally, this document provides implementation
   advice for other issues, as follows.
   o  Section 3.2 of the Problem Statement mentions that there is a
      large amount of state that needs to be synchronized.  However if
      state is not synchronized, this is not really an interesting
      cluster: failover is equivalent to a reboot of the cluster member,
      and so the issue need not be solved with a protocol extension.
   o  3.3, 3.4,3.5, and 3.6 are solved by this document.  Please see
      Section 4, for more details.
   o  3.7 is an implementation problem that needs to be solved while
      building IPsec clusters.  However, the peers should be required to
      accept multiple parallel SAs for 3.7.1.
   o  3.8 can be solved by using the IKEv2 Redirect mechanism [6].
   o  3.9 discusses the avoidance of collisions where the same SPI value
      is used by multiple cluster members.  This is outside the
      document's scope since the problem needs to be solved internally
      to the cluster and does not involve the peer.







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4.  The IKEv2/IPsec SA Counter Synchronization Problem

   The IKEv2 protocol [3] states that "An IKE endpoint MUST NOT exceed
   the peer's stated window size for transmitted IKE requests".

   All IKEv2 messages are required to follow a request-response
   paradigm.  The initiator of an IKEv2 request MUST retransmit the
   request, until it has received a response from the peer.  IKEv2
   introduces a windowing mechanism that allows multiple requests to be
   outstanding at a given point of time, but mandates that the sender's
   window should not move until the oldest message it has sent is
   acknowledged.  Loss of even a single message leads to repeated
   retransmissions followed by an IKEv2 SA teardown if the
   retransmissions remain unacknowledged.

   An IPsec Hot Standby Cluster is required to ensure that in the case
   of failover, the standby member becomes active immediately.  The
   standby member is expected to have the exact value of the Message ID
   counter as the active member had before failover.  Even assuming the
   best effort to update the Message ID values from active to standby
   member, the values at the standby member can still be stale due to
   the following reasons:
   o  The standby member is unaware of the last message that was
      received and acknowledged by the previously active member, as the
      failover event could have happened before the standby member could
      be updated.
   o  The standby member does not have information about on-going
      unacknowledged requests sent by the previously active member.  As
      a result after the failover event, the newly active member cannot
      retransmit those requests.

   When a standby member takes over as the active member, it can only
   initialize the Message ID values from the previously updated values.
   This would make it reject requests from the peer when these values
   are stale.  Conversely, the standby member may end up reusing a stale
   Message ID value which would cause the peer to drop the request.
   Eventually there is a high probability of the IKEv2 and corresponding
   IPsec SAs getting torn down simply because of a transitory Message ID
   mismatch and retransmission of requests, negating the benefits of the
   high availability cluster despite the periodic update between the
   cluster members.

   A similar issue is also observed with IPsec anti-replay counters if
   anti-replay protection is enabled, which is commonly the case.
   Regardless of how well the ESP and AH SA counters are synchronized
   from the active to the standby member, there is a chance that the
   standby member would end up with stale counter values.  The standby
   member would then use those stale counter values when sending IPsec



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   packets.  The peer would reject/drop such packets since when the
   anti-replay protection feature is enabled, duplicate use of counters
   is not allowed.  Note that IPsec allows the sender to skip some
   counter values and continue sending with higher counter values.

   We conclude that a mechanism is required to ensure that the standby
   member has correct Message ID and IPsec counter values when it
   becomes active, so that sessions are not torn down as a result of
   mismatched counters.


5.  SA Counter Synchronization Solution

   This document proposes two separate approaches to resolving the
   issues of mismatched IKE Message ID values and IPsec counter values.

   o  In the case of IKE Message ID values, the newly active cluster
      member and the peer negotiate a pair of new values so that future
      IKE messages will not be dropped.
   o  For IPsec counter values, the newly-active member and the peer
      both increment their respective counter values, "skipping forward"
      by a large number, to ensure that no IPsec counters are ever
      reused.

   Although conceptually separate, the two synchronization processes
   would typically take place simultaneously.

   First, the peer and the active member of the cluster negotiate their
   ability to support IKEv2 Message ID synchronization and/or IPsec
   Replay Counter synchronization.  This is done by exchanging one or
   both of the IKEV2_MESSAGE_ID_SYNC_SUPPORTED and
   IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED notifications during the IKE_AUTH
   exchange.  When negotiating these capabilities, the responder MUST
   NOT assert support of a capability unless such support was asserted
   by the initiator.  Only a capability whose support was asserted by
   both parties can be used during the lifetime of the SA.

   This per-IKE SA information is shared with the other cluster members.


Peer                                                  Active Member
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
HDR, SK {IDi, [CERT], [CERTREQ], [IDr], AUTH,
     [N(IKEV2_MESSAGE_ID_SYNC_SUPPORTED),]
     [N(IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED),]
     SAi2, TSi, TSr} ---------->

<-------- HDR, SK {IDr, [CERT+], [CERTREQ+], AUTH,



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               [N(IKEV2_MESSAGE_ID_SYNC_SUPPORTED),]
               [N(IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED),] SAr2, TSi, TSr}



   After a failover event, the standby member MAY use the IKE Message ID
   and/or IPsec Replay Counter synchronization capability when it
   becomes the active member, and provided support for the capabilities
   used has been negotiated.  Following that, the peer MUST respond to
   any synchronization message it receives from the newly-active cluster
   member, subject to the rules noted below.

   After the failover event, when the standby member becomes active, it
   has to synchronize its SA counters with the peer.  There are now
   three possible cases:

   1.  The cluster member wishes to only perform IKE Message ID value
       synchronization.  In this case it initiates an Informational
       exchange, with Message ID zero and the sole notification
       IKEV2_MESSAGE_ID_SYNC.
   2.  If the newly-active member wishes to perform only IPsec replay
       counter synchronization, it generates a regular IKEv2
       Informational exchange using the current Message ID values, and
       containing the IPSEC_REPLAY_COUNTER_SYNC notification.
   3.  If synchronization of both counters is needed, the cluster member
       generates a zero-Message ID message as in case #1, and includes
       both notifications in this message.

   This figure contains the IKE message exchange used for SA counter
   synchronization.  The following subsections describe the details of
   the sender and receiver processing of each message.


   Standby [Newly Active] Member                            Peer
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   HDR, SK {N(IKEV2_MESSAGE_ID_SYNC),
        [N(IPSEC_REPLAY_COUNTER_SYNC)]} -------->

                <--------- HDR, SK {N(IKEV2_MESSAGE_ID_SYNC)}


   Alternatively, if only IPsec Replay Counter synchronization is
   desired, a normal Informational exchange is used, where the Message
   ID is non-zero:







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   Standby [Newly Active] Member                            Peer
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   HDR, SK{N(IPSEC_REPLAY_COUNTER_SYNC)} -------->

                <--------- HDR


5.1.  Processing Rules for IKE Message ID Synchronization

   The newly-active member sends a request containing two counter value,
   one for the member (itself) and another for the peer, as well as a
   random nonce.  We denote the values M1 and P1.  The peer responds
   with a message containing two counter values, M2 and P2.  The goal of
   the rules below is to prevent an attacker from replaying a
   synchronization message, thereby invalidating IKE messages that are
   currently in process.

   o  M1 is the next sender's Message ID to be used by the member.  M1
      MUST be chosen so that it is larger than any value known to have
      been used.  It is RECOMMENDED to increment the known value at
      least by the size of the IKE sender window.
   o  P1 SHOULD be 1 more than the last Message ID value received from
      the peer, but may be any higher value.
   o  The member SHOULD communicate the sent values to the other cluster
      members, so that if a second failover event takes place, the
      synchronization message is not replayed.  Such a replay would
      result in the eventual deletion of the IKE SA (see below).
   o  The peer MUST reject any received synchronization message if M1 is
      lower than or equal to the highest value it has seen from the
      cluster.  This includes any previous received synchronization
      messages.
   o  M2 MUST be at least the higher of the received M1, and one more
      than the highest sender value received from the cluster.  This
      includes any previous received synchronization messages.
   o  P2 MUST be the higher of the received P1 value, and one more than
      the highest sender value used by the peer.
   o  The request contains a Nonce field.  This field MUST be returned
      in the response, unchanged.  A response MUST be silently dropped
      if the received Nonce does not match the one that was sent.
   o  Both the request and the response MUST NOT contain any additional
      payloads, other than an optional IPSEC_REPLAY_COUNTER_SYNC
      notification in the request.
   o  The request and the response MUST both be sent with a Message ID
      value of zero.







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5.2.  Processing Rules for IPsec Replay Counter Synchronization

   Upon failover, the newly-active member MUST increment its own Replay
   Counter (the counter used for outgoing traffic), so as to prevent the
   case of its traffic being dropped by the peer as replay.  We note
   that IPsec allows the replay counter to skip forward by any amount.
   The estimate is based on the outgoing IPsec bandwidth and the
   frequency of synchronization between cluster members.  In those
   implementations where it is difficult to estimate this value, the
   counter can be incremented by a very large number, e.g. 2**30.  In
   the latter case, a rekey SHOULD follow shortly afterwards, to ensure
   that the counter never wraps around.

   Next, the cluster member estimates the number of incoming messages it
   might have missed, using similar logic.  The member sends out a
   IPSEC_REPLAY_COUNTER_SYNC notification, either stand-alone or
   together with a IKEV2_MESSAGE_ID_SYNC notification.

   If the IPSEC_REPLAY_COUNTER_SYNC is included in the same message as
   IKEV2_MESSAGE_ID_SYNC, the peer MUST process the Message ID
   notification first (which might cause the entire message to be
   dropped as a replay).  Then, it MUST increment the replay counters
   for all Child SAs associated with the current IKE SA by the amount
   requested by the cluster member.


6.  IKEv2/IPsec Synchronization Notification Payloads

   This section lists the new notification payload types defined by this
   extension.

6.1.  The IKEV2_MESSAGE_ID_SYNC_SUPPORTED Notification

   This notification payload is included in the IKE_AUTH request/
   response to indicate support of the IKEv2 Message ID synchronization
   mechanism described in this document.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
   'Notify Message Type' fields are the same as described in Section 3
   of [3].  The 'SPI Size' field MUST be set to 0 to indicate that the



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   SPI is not present in this message.  The 'Protocol ID' MUST be set to
   0, since the notification is not specific to a particular security
   association.  The 'Payload Length' field is set to the length in
   octets of the entire payload, including the generic payload header.
   The 'Notify Message Type' field is set to indicate
   IKEV2_MESSAGE_ID_SYNC_SUPPORTED, value TBD by IANA.  There is no data
   associated with this notification.

6.2.  The IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED Notification

   This notification payload is included in the IKE_AUTH request/
   response to indicate support for the IPsec SA Replay Counter
   synchronization mechanism described in this document.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 'Next Payload', 'Payload Length', 'Protocol ID', 'SPI Size', and
   'Notify Message Type' fields are the same as described in Section 3
   of [3] .  The 'SPI Size' field MUST be set to 0 to indicate that the
   SPI is not present in this message.  The 'Protocol ID' MUST be set to
   0, since the notification is not specific to a particular security
   association.  The 'Payload Length' field is set to the length in
   octets of the entire payload, including the generic payload header.
   The 'Notify Message Type' field is set to indicate
   IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED, value TBD by IANA.  There is no
   data associated with this notification.

6.3.  The IKEV2_MESSAGE_ID_SYNC Notification

   This notification payload type (value TBD by IANA) is defined to
   synchronize the IKEv2 Message ID values between the newly-active
   (formerly standby) cluster member and the peer.













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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(=0)| SPI Size (=0) |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Nonce Data                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             EXPECTED_SEND_REQ_MESSAGE_ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             EXPECTED_RECV_REQ_MESSAGE_ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   It contains the following data.
   o  Nonce Data (4 octets): the random nonce data.  The data should be
      identical in the synchronization request and response.
   o  EXPECTED_SEND_REQ_MESSAGE_ID (4 octets): this field is used by the
      sender of this notification payload to indicate the Message ID it
      will use in the next request that it will send to the other
      protocol peer.
   o  EXPECTED_RECV_REQ_MESSAGE_ID (4 octets): this field is used by the
      sender of this notification payload to indicate the Message ID it
      is expecting in the next request to be received from the other
      protocol peer.

6.4.  The IPSEC_REPLAY_COUNTER_SYNC Notification

   This notification payload type (value TBD by IANA) is defined to
   synchronize the IPsec SA Replay Counters between the newly-active
   (formerly standby) cluster member and the peer.  Since there may be
   numerous IPsec SAs established under a single IKE SA, we do not
   directly synchronize the value of each one.  Instead, a delta value
   is sent and all Replay Counters for Child SAs of this IKE SA are
   incremented by the same value.  Note that this solution requires that
   all these Child SAs either use or do not use Extended Sequence
   Numbers [4].  This notification is only sent by the cluster.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Incoming IPsec SA delta value                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The notification payload contains the following data.
   o  Incoming IPsec SA delta value (4 or 8 octets): The sender requests
      that the peer should increment all the Child SA Replay Counters
      for the sender's incoming (the peer's outgoing) traffic by this
      value.  The size of this field depends on the ESN bit associated
      with the Child SAs: if the ESN bit is 1, the field's size is 8
      octets, otherwise it is 4 octets.  We note that this constrains
      the Child SAs of each IKE SA to either all have the ESN bit on or
      off.


7.  Implementation Details

   This protocol does not change any of the existing IKEv2 rules
   regarding Message ID values.

   The standby member can initiate the synchronization of IKEv2 Message
   ID's under different circumstances.
   o  When it receives a problematic IKEv2/IPsec packet, i.e. a packet
      outside its expected receive window.
   o  When it has to send the first IKEv2/IPsec packet after a failover
      event.
   o  When it has just received control from the active member and
      wishes to update the values proactively, so that it need not start
      this exchange later, when sending or receiving the request.

   The standby member can initiate the synchronization of IPsec SA
   Replay Counters:
   o  If there has been traffic using the IPsec SA in the recent past
      and the standby member suspects that its Replay Counter may be
      stale.

   Since there can be a large number of sessions at the standby member,
   and sending synchronization exchanges for all of them may result in
   overload, the standby member can choose to initiate the exchange in a
   "lazy" fashion: only when it has to send or receive the request.  In
   general, the standby member is free to initiate this exchange at its
   discretion.


8.  IKE SA and IPsec SA Message Sequencing

   The straightforward definitions of message sequence numbers,
   retransmissions and replay protection in IPsec and IKEv2 are strained
   by the failover scenarios described in this document.  This section
   describes some policy choices that need to be made by implementations
   in this setting.




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8.1.  Handling of Pending IKE Messages

   After sending its "receive" counter, the cluster member MUST reject
   any incoming IKE messages that are outside its declared window.  A
   similar rule applies to the peer.  Local policies vary, and strict
   implementations will reject any incoming IKE message arriving before
   Message ID synchronization is complete.

8.2.  Handling of Pending IPsec Messages

   For IPsec, there is often a trade-off between security and
   reliability of the protected protocols.  Here again there is some
   leeway for local policy.  Some implementations might accept incoming
   traffic that is outside the replay window for some time after the
   failover event.  Strict implementations will only accept traffic
   that's inside the "safe" window.

8.3.  IKE SA Inconsistencies

   IKEv2 is normally a reliable protocol.  As long as an IKE SA is
   valid, both peers share a single, consistent view of the IKE SA and
   all associated Child SAs.  Failover situations as described in this
   document may involve forced deletion of IKE messages, resulting in
   inconsistencies, such as Child SAs that exist on only one of the
   peers.  Such SAs would cause an INVALID_SPI to be returned when used
   by that peer.

   The Working Group discussed at some point a proposed set of rules for
   dealing with such situations.  However we believe that these
   situations should be rare in practice; as a result the "default"
   behavior of tearing down the entire IKE SA is to be preferred over
   the complexity of dealing with a multitude of edge cases.


9.  Step by Step Details

   This section goes through the sequence of steps of a typical failover
   event, looking at a case where the IKEv2 Message ID values are
   synchronized.
   o  The active cluster member and the peer device establish the
      session.  They both announce the capability to synchronize counter
      information by sending the IKEV2_MESSAGE_ID_SYNC_SUPPORTED
      notification in the IKE_AUTH Exchange.
   o  Some time later, the active member dies, and a standby member
      takes over.  The standby member sends its own idea of the IKE
      Message IDs (both incoming and outgoing) to the peer in an
      Informational message exchange with Message ID zero.




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   o  The peer first authenticates the message.  The peer compares the
      received values with the values available locally and picks the
      higher value.  It then updates its Message IDs with the higher
      values and also propose the same values in its response.
   o  The peer should not wait for any pending responses while
      responding with the new Message ID values.  For example, if the
      window size is 5 and the peer's window is 3-7, and if the peer has
      sent requests 3, 4, 5, 6, 7 and received responses only for 4, 5,
      6, 7 but not for 3, then it should include the value 8 in its
      EXPECTED_SEND_REQ_MESSAGE_ID payload and should not wait for a
      response to message 3 anymore.
   o  Similarly, the peer should also not wait for pending (incoming)
      requests.  For example if the window size is 5 and the peer's
      window is 3-7 and if the peer has received requests 4, 5, 6, 7 but
      not 3, then it should send the value 8 in the
      EXPECTED_RECV_REQ_MESSAGE_ID payload, and should not expect to
      receive message 3 anymore.


10.  Interaction with other drafts

   The usage scenario of the IKEv2/IPsec SA counter synchronization
   proposal is that an IKEv2 SA has been established between the active
   member of a hot-standby cluster and a peer, then a failover event
   occurred with the standby member becoming active.  The proposal
   further assumes that the IKEv2 SA state was continuously synchronized
   between the active and standby members of the cluster before the
   failover event.
   o  Session resumption [7] assumes that a peer (client or initiator)
      detects the need to re-establish the session.  In IKEv2/IPsec SA
      counter synchronization, it is the newly-active member (a gateway
      or responder) that detects the need to synchronize the SA counter
      after the failover event.  Also in a hot-standby cluster, the peer
      establishes the IKEv2/IPsec session with a single IP address that
      represents the whole cluster, so the peer normally does not detect
      the event of failover in the cluster unless the standby member
      takes too long to become active and the IKEv2 SA times out by use
      of the IKEv2 liveness check mechanism.  To conclude, session
      resumption and SA counter synchronization after failover are
      mutually exclusive.
   o  The IKEv2 Redirect mechanism for load-balancing [6] can be used
      either during the initial stages of SA setup (the IKE_SA_INIT and
      IKE_AUTH exchanges) or after session establishment.  SA counter
      synchronization is only useful after the IKE SA has been
      established and a failover event has occurred.  So, unlike
      Redirect, it is irrelevant during the first two exchanges.
      Redirect after the session has been established is mostly useful
      for timed or planned shutdown/maintenance.  A real failover event



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      cannot be detected by the active member ahead of time, and so
      using Redirect after session establishment is not possible in the
      case of failover.  So, Redirect and SA counter synchronization
      after failover are mutually exclusive.
   o  IKEv2 Failure Detection [8] solves a similar problem where the
      peer can rapidly detect that a cluster member has crashed based on
      a token.  It is unrelated to the current scenario because the goal
      in failover is for the peer not to notice that a failure has
      occurred.


11.  Security Considerations

   Since Message ID synchronization messages need to be sent with
   Message ID zero, they are potentially vulnerable to replay attacks.
   Because of the semantics of this protocol, these can only be denial-
   of-service (DoS) attacks, and we are aware of two variants.
   o  Replay of Message ID synchronization request: This is countered by
      the requirement that the Send counter sent by the cluster member
      should always be monotonically increasing, a rule that the peer
      enforces by silently dropping messages that contradict it.
   o  Replay of the Message ID synchronization response: This is
      countered by sending the nonce data along with the synchronization
      payload.  The same nonce data has to be returned in the response.
      Thus the standby member will accept a reply only for the current
      request.  After it receives a valid response, it MUST NOT process
      the same response again and MUST discard any additional responses.


12.  IANA Considerations

   This document introduces four new IKEv2 Notification Message types as
   described in Section 6.  The new Notify Message Types must be
   assigned values between 16396 and 40959.

           +-------------------------------------+-------------+
           | Name                                | Value       |
           +-------------------------------------+-------------+
           | IKEV2_MESSAGE_ID_SYNC_SUPPORTED     | TBD by IANA |
           | IPSEC_REPLAY_COUNTER_SYNC_SUPPORTED | TBD by IANA |
           | IKEV2_MESSAGE_ID_SYNC               | TBD by IANA |
           | IPSEC_REPLAY_COUNTER_SYNC           | TBD by IANA |
           +-------------------------------------+-------------+








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13.  Acknowledgements

   We would like to thank Pratima Sethi and Frederic Detienne for their
   review comments and valuable suggestions for the initial version of
   the document.

   We would also like to thank the following people (in alphabetical
   order) for their review comments and valuable suggestions: Dan
   Harkins, Paul Hoffman, Steve Kent, Tero Kivinen, David McGrew, and
   Pekka Riikonen.


14.  Change Log

   This section lists all the changes in this document.

   NOTE TO RFC EDITOR: Please remove this section before publication.

14.1.  Draft -03

   Clarified the rules for Message ID sync, so that replay attacks can
   be avoided without a failover counter.

   Added wording regarding inconsistent IKE state (basically choosing to
   ignore the problem) and further rules dealing with pending traffic.

   The IPsec replay counter delta value now refers to incoming traffic.
   The associated notification is only sent from the cluster to the
   peer, and not back.

14.2.  Draft -02

   Addressed comments by Yaron Sheffer posted on the WG mailing list.

   Numerous editorial changes.

14.3.  Draft -01

   Added "Multiple and Simultaneous failover' scenarios as pointed out
   by Pekka Riikonen.

   Now document provides a mechanism to sync either IKEv2 message or
   IPsec replay counter or both to cater different types of
   implementations.

   HA cluster's "failover count' is used to encounter replay of sync
   requests by attacker.




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   The sync of IPsec SA replay counter optimized to to have just one
   global bumped-up outgoing IPsec SA counter of ALL Child SAs under an
   IKEv2 SA.

   The examples added for IKEv2 Message ID sync to provide more clarity.

   Some edits as per comments on mailing list to enhance clarity.

14.4.  Draft  -00

   Version 00 is identical to
   draft-kagarigi-ipsecme-ikev2-windowsync-04, started as WG document.

   Added IPSECME WG HA design team members as authors.

   Added comment in Introduction to discuss the window sync process on
   WG mailing list to solve some concerns.


15.  References

15.1.  Normative References

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

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

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

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

15.2.  Informative References

   [5]  Nadas, S., "Virtual Router Redundancy Protocol (VRRP) Version 3
        for IPv4 and IPv6", RFC 5798, March 2010.

   [6]  Devarapalli, V. and K. Weniger, "Redirect Mechanism for the
        Internet Key Exchange Protocol Version 2 (IKEv2)", RFC 5685,
        November 2009.

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

   [8]  Nir, Y., Wierbowski, D., Detienne, F., and P. Sethi, "A Quick



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        Crash Detection Method for IKE",
        draft-ietf-ipsecme-failure-detection-03 (work in progress),
        January 2011.


Appendix A.  IKEv2 Message ID Sync Examples

   This (non-normative) section presents some examples that illustrate
   how the IKEv2 Message ID values are synchronized.  We use a tuple
   notation, denoting the two counters EXPECTED_SEND_REQ_MESSAGE_ID and
   EXPECTED_RECV_REQ_MESSAGE_ID on a member as
   (EXPECTED_SEND_REQ_MESSAGE_ID, EXPECTED_RECV_REQ_MESSAGE_ID).

A.1.  Normal Failover - Example 1


   Standby (Newly Active) Member                            Peer
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   Sync Request (2, 3) -------->

                             Peer has the values (4, 5) so it sends
                <------------- (4, 5) as the Sync Response


A.2.  Normal Failover - Example 2


   Standby (Newly Active) Member                            Peer
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   Sync Request (2, 5) -------->

                             Peer has the values (2, 4) so it sends
                <-------------(5, 4) as the Sync Response


A.3.  Simultaneous Failover

   In the case of simultaneous failover, both sides send the
   synchronization request, but whichever side has the higher value will
   be eventually synchronized.











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   Standby (Newly Active) Member                            Peer
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

   Sync Request (4,4)     ----->

                    <-------------- Sync Request (5,5)

   Sync Response (5,5)    ---->

                        <--------  Sync Response (5,5)



Authors' Addresses

   Raj Singh (Editor)
   Cisco Systems, Inc.
   Divyashree Chambers, B Wing, O'Shaugnessy Road
   Bangalore, Karnataka  560025
   India

   Phone: +91 80 4301 3320
   Email: rsj@cisco.com


   Kalyani Garigipati
   Cisco Systems, Inc.
   Divyashree Chambers, B Wing, O'Shaugnessy Road
   Bangalore, Karnataka  560025
   India

   Phone: +91 80 4426 4831
   Email: kagarigi@cisco.com


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

   Email: ynir@checkpoint.com









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   Yaron Sheffer
   Independent

   Email: yaronf.ietf@gmail.com


   Dacheng Zhang
   Huawei Technologies Ltd.

   Email: zhangdacheng@huawei.com









































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