Network Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Intended status: Informational                              May 12, 2010
Expires: November 13, 2010

       IPsec High Availability and Load Sharing Problem Statement


   This document describes a requirement from IKE and IPsec to allow for
   more scalable and available deployments for VPNs.  It defines
   terminology for high availability and load sharing clusters
   implementing IKE and IPsec, and describes gaps in the existing

Status of this Memo

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   This Internet-Draft will expire on November 13, 2010.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions Used in This Document  . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  The Problem Statement  . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Lots of Long Lived State . . . . . . . . . . . . . . . . .  5
     3.2.  IKE Counters . . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Outbound SA Counters . . . . . . . . . . . . . . . . . . .  6
     3.4.  Inbound SA Counters  . . . . . . . . . . . . . . . . . . .  6
     3.5.  Missing Synch Messages . . . . . . . . . . . . . . . . . .  7
     3.6.  Simultaneous use of IKE and IPsec SAs by Different
           Members  . . . . . . . . . . . . . . . . . . . . . . . . .  7
       3.6.1.  Outbound SAs using counter modes . . . . . . . . . . .  8
     3.7.  Different IP addresses for IKE and IPsec . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   IKEv2, as described in [RFC4306] and [RFC4718], and IPsec, as
   described in [RFC4301] and others, allows deployment of VPNs between
   different sites as well as from VPN clients to protected networks.

   As VPNs become increasingly important to the organizations deploying
   them, there is a demand to make IPsec solutions more scalable and
   less prone to down time, by using more than one physical gateway to
   either share the load or back each other up.  Similar demands have
   been made in the past for other critical pieces of an organizations's
   infrastructure, such as DHCP and DNS servers, web servers, databases
   and others.

   IKE and IPsec are in particular less friendly to clustering than
   these other protocols, because they store more state, and that state
   is more volatile.  Section 2 defines terminology for use in this
   document, and in the envisioned solution documents.

   In general, deploying IKE and IPsec in a cluster requires such a
   large amount of information to be synchronized among the members of
   the cluster, that it becomes impractical.  Alternatively, if less
   information is synchronized, failover would mean a prolonged and
   intensive recovery phase, which negates the scalability and
   availability promises of using clusters.  In Section 3 we will
   describe this in more detail.

1.1.  Conventions Used in This Document

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

2.  Terminology

   "Single Gateway" is an implementation of IKE and IPsec enforcing a
   certain policy, as described in [RFC4301].

   "Cluster" is a set of two or more gateways, implementing the same
   security policy, and protecting the same domain.  Clusters exist to
   provide both high availability through redundancy, and scalability
   through load sharing.

   "Member" is one gateway in a cluster.

   "High Availability" is a condition of a system, not a configuration
   type.  A system is said to have high availability if its expected

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   down time is low.  High availability can be achieved in various ways,
   one of which is clustering.  All the clusters described in this
   document achieve high availability.

   "Fault Tolerance" is a condition related to high availability, where
   a system maintains service availability, even when a specified set of
   fault conditions occur.  In clusters, we expect the system to
   maintain service availability, when one or more of the cluster
   members fails.

   "Completely Transparent Cluster" is a cluster where the occurence of
   a fault is never visible to the peers.

   "Partially Transparent Cluster" is a cluster where the occurence of a
   fault may be visible to the peers.

   "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 the "active", whereas the others are referred to as "stand-
   bys".  [VRRP] is one method of building such a cluster.

   "Load Sharing Cluster", or "LS Cluster" is a cluster where more than
   one of the members may be active at the same time.  The term "load
   balancing" is also common, but it implies that the load is actually
   balanced between the members, and we don't want to even imply that
   this is a requirement.

   "Failover" is the event where a one member takes over some load from
   some other member.  In a hot standby cluster, this hapens when a
   standby memeber becomes active due to a failure of the former active
   member, or because of an administrator command.  In a load sharing
   cluster this usually happens because of a failure of one of the
   members, but certain load-balancing technologies may allow a
   particular load (such as all the flows associated with a particular
   child SA) to move from one member to another to even out the load,
   even without any failures.

   "Tight Cluster" is a cluster where all the members share an IP
   address.  This could be accomplished using configured interfaces with
   specialized protocols or hardware, such as VRRP, or through the use
   of multicast addresses, but in any case, peers need only be
   configured with one IP address in the PAD.

   "Loose Cluster" is a cluster where each member has a different IP
   address.  Peers find the correct member using some method such as DNS
   queries or [REDIRECT].  In some cases, members IP addresses may be
   allocated to other members at failover.

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   "Synch Channel" is a communications channel among the cluster
   members, used to transfer state information.  The synch channel may
   or may not be IP based, may or may not be encrypted, and may work
   over short or long distances.  The security and physical
   characteristics of this channel are out of scope for this document,
   but it is a requirement that its use be minimized for scalability.

3.  The Problem Statement

   This document will make no attempt to describe the problems in
   setting up a cluster.  The following subsections describe the
   problems related to the protocol itself.

   We also ignore the problem of synchronizing the policy between
   cluster members, as this is an administrative issue that is not
   particular to either clusters or to IPsec.

   Note that the interesting scenario here is VPN, whether tunneled
   site-to-site or remote access. host-to-host transport mode is not
   expected to benefit from this work.

3.1.  Lots of Long Lived State

   IKE and IPsec have a lot of long lived state:
   o  IKE SAs last for minutes, hours, or days, and carry keys and other
      information.  Some gateways may carry thousands to hundreds of
      thousands of IKE SAs.
   o  IPsec SAs last for minutes or hours, and carry keys, selectors and
      other information.  Some gateways may carry hundreds of thousands
      such IPsec SAs.
   o  SPD Cache entries.  While the SPD is unchanging, the SPD cache
      changes on the fly due to narrowing.  Entries last at least as
      long as the SAD entries, but tend to last even longer than that.

   A naive implementation of a high availability cluster would have no
   synchronized state, and a failover would produce an effect similar to
   that of a rebooted gateway. [resumption] describes how new IKE and
   IPsec SAs can be recreated in such a case.

3.2.  IKE Counters

   We can overcome the first problem described in Section 3.1, by
   synchronizing states - whenever an SA is created, we can synch this
   new state to all other members.  However, those states are not only
   long-lived, they are also ever changing.

   IKE has message counters.  A peer may not process message n until

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   after it has processed message n-1.  Skipping message IDs is not
   allowed.  So a newly-active member needs to know the last message IDs
   both received and transmitted.

   Often, it is feasible to synchronize the IKE message counters for
   every IKE exchange.  This way, the newly active member knows what
   messages it is allowed to process, and what message IDs to use on IKE
   requests, so that peers process them.

3.3.  Outbound SA Counters

   ESP and AH have an optional anti-replay feature, where every
   protected packet carries a counter number.  Repeating counter numbers
   is considered an attack, so the newly-active member must not use a
   replay counter number that has already been used.  The peer will drop
   those packets as duplicates and/or warn of an attack.

   Though it may be feasible to synchronize the IKE message counters, it
   is almost never feasible to synchronize the IPsec packet counters for
   every IPsec packet transmitted.  So we have to assume that at least
   for IPsec, the replay counter will not be up-to-date on the newly-
   active member, and the newly-active member may repeat a counter.

   A possible solution is to synch replay counter information, not for
   each packet emitted, but only at regular intervals, say, every 10,000
   packets or every 0.5 seconds.  After a failover, the newly-active
   member advances the counters for outbound SAs by 10,000.  To the peer
   this looks like up to 10,000 packets were lost, but this should be
   acceptable, as neither ESP nor AH guarantee reliable delivery.

3.4.  Inbound SA Counters

   An even tougher issue, is the synchronization of packet counters for
   inbound SAs.  If a packet arrives at a newly-active member, there is
   no way to determine whether this packet is a replay or not.  The
   periodic synch does not solve the problem at all, because suppose we
   synchronize every 10,000 packets, and the last synch before the
   failover had the counter at 170,000.  It is probable, though not
   certain, that packet number 180,000 has not yet been processed, but
   if packet 175,000 arrives at the newly- active member, it has no way
   of determining whether or not that packet has or has not already been
   processed.  The synchronization does prevent the processing of really
   old packets, such as those with counter number 165,000.  Ignoring all
   counters below 180,000 won't work either, because that's up to 10,000
   dropped packets, which may be very noticeable.

   The easiest solution is to learn the replay counter from the incoming
   traffic.  This is allowed by the standards, because replay counter

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   verification is an optional feature.  The case can even be made that
   it is relatively secure, because non-attack traffic will reset the
   counters to what they should be, so an attacker faces the dual
   challenge of a very narrow window for attack, and the need to time
   the attack to a failover event.  Unless the attacker can actually
   cause the failover, this would be very difficult.  It should be
   noted, though, that although this solution is acceptable as far as
   RFC 4301 goes, it is a matter of policy whether this is acceptable.

   Another possible solution to the inbound SA problem is to rekey all
   child SAs following a failover.  This may or may not be feasible
   depending on the implementation and the configuration.

3.5.  Missing Synch Messages

   The synch channel is very likely not to be infallible.  Before
   failover is detected, some synchronization messages may have been
   missed.  For example, the active member may have created a new Child
   SA using message n.  The new information (entry in the SAD and update
   to counters of the IKE SA) is sent on the synch channel.  Still, with
   every possible technology, the update may be missed before the

   This is a bad situation, because the IKE SA is doomed. the newly-
   active member has two problems:
   o  It does not have the new IPsec SA pair.  It will drop all incoming
      packets protected with such an SA.  This could be fixed by sending
      some DELETEs and INVALID_SPI notifications, if it wasn't for the
      other problem...
   o  The counters for the IKE SA show that only request n-1 has been
      sent.  The next request will get the message ID n, but that will
      be rejected by the peer.  After a sufficient number of
      retransmissions and rejections, the whole IKE SA with all
      associated IPsec SAs will get dropped.

   The above scenario may be rare enough that it is acceptable that on a
   configuration with thousands of IKE SAs, a few will need to be
   recreated from scratch or using session resumption techniques.
   However, detecting this may take a long time (several minutes) and
   this negates the goal of creating a high availability cluster in the
   first place.

3.6.  Simultaneous use of IKE and IPsec SAs by Different Members

   For load sharing clusters, all active members may need to use the
   same SAs, both IKE and IPsec.  This is an even greater problem than
   in the case of HA, because consecutive packets may need to be sent by
   different members to the same peer gateway.

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   The solution to the IKE SA issue is up to the application.  It's
   possible to create some locking mechanism over the synch channel, or
   else have one member "own" the IKE SA and manage the child SAs for
   all other members.  For IPsec, solutions fall into two broad

   The first is the "sticky" category, where all communications with a
   single peer, or all communications involving a certain SPD cache
   entry go through a single peer.  In this case, all packets that match
   any particular SA go through the same member, so no synchronization
   of the replay counter needs to be done.  Inbound processing is a
   "sticky" issue, because the packets have to be processed by the
   correct member based on peer and SPI.  Another issue is that
   commodity load balancers will not be able to match the SPIs of the
   encrypted side to the clear traffic, and so the wrong member may get
   the the other half of the flow.

   The other way, is to duplicate the child SAs, and have a pair of
   IPsec SAs for each active member.  Different packets for the same
   peer go through different members, and get protected using different
   SAs with the same selectors and matching the same entries in the SPD
   cache.  This has some shortcomings:
   o  It requires multiple parallel SAs, which the peer has no use for.
      Section 2.8 or [RFC4306] specifically allows this, but some
      implementation might have a policy against long term maintenance
      of redundant SAs.
   o  Different packets that belong to the same flow may be protected by
      different SAs, which may seem "weird" to the peer gateway,
      especially if it is integrated with some deep inspection
      middleware such as a firewall.  It is not known whether this will
      cause problems with current gateways.  It is also impossible to
      mandate against this, because the definition of "flow" varies from
      one implementation to another.
   o  Reply packets may arrive with an IPsec SA that is not "matched" to
      the one used for the outgoing packets.  Also, they might arrive at
      a different member.  This problem is beyond the scope of this
      document and should be solved by the application, perhaps by
      forwarding misdirected packets to the correct gateway for deep

3.6.1.  Outbound SAs using counter modes

   For SAs involving counter mode ciphers such as [CTR] or [GCM] there
   is yet another complication.  The initial vector for such modes must
   never be repeated, and senders use methods such as counters or LFSRs
   to ensure this.  An SA shared between more than one active member, or
   even failing over from one member to another need to make sure that
   they do not generate the same initial vector.  See [COUNTER_MODES]

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   for a discussion of this problem in another context.

3.7.  Different IP addresses for IKE and IPsec

   In many implementations there are separate IP addresses for the
   cluster, and for each member.  While the packets protected by tunnel
   mode child SAs are encapsulated in IP headers with the cluster IP
   address, the IKE packets originate from a specific member, and carry
   that member's IP address.  For the peer, this looks weird, as the
   usual thing is for the IPsec packets to come from the same IP address
   as the IKE packets.

   One obvious solution, is to use some fancy capability of the IKE host
   to change things so that IKE packets also come out of the cluster IP
   address.  This can be achieved through NAT or through assigning
   multiple addresses to interfaces.  This is not, however, possible for
   all implementations.

   [ARORA] discusses this problem in greater depth, and proposes another
   solution, that does involve protocol changes.

4.  Security Considerations

   Implementations running on clusters MUST be as secure as
   implementations running on single gateways.  In other words, no
   extension or interpretation used to allow operation in a cluster may
   facilitate attacks that are not possible for single gateways.

   Moreover, thought must be given to the synching requirements of any
   protocol extension, to make sure that it does not create an
   opportunity for denial of service attacks on the cluster.

   As mentioned in Section 3.4, allowing an inbound child SA to fail
   over to another member has the effect of disabling replay counter
   protection for a short time.  Though the threat is arguably low, it
   is a policy decision whether this is acceptable.

5.  IANA Considerations

   This document has no actions for IANA.

6.  Acknowledgements

   This document is the collective work, and includes contribution from
   many people who participate in the IPsecME working group.

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   The editor would particularly like to acknowledge the extensive
   contribution of the following people (in alphabetical order):
   Jitender Arora, Dan Harkins, Steve Kent, Tero Kivinen, Yaron Sheffer,
   Melinda Shore, and Rodney Van Meter.

7.  Change Log


   Version 00 was identical to draft-nir-ipsecme-ipsecha-ps-00, re-spun
   as an WG document.

   Version 01 included closing issues 177, 178 and 180, with updates to
   terminology, and added discussion of inbound SAs and the CTR issue.

   Version 02 includes comments by Yaron Sheffer and the acknowledgement

   Version 03 fixes some ID-nitsi, and adds the problem presented by
   Jitender Arora in [ARORA].

8.  Informative References

   [ARORA]    Arora, J. and P. Kumar, "Alternate Tunnel Addresses for
              IKEv2", draft-arora-ipsecme-ikev2-alt-tunnel-addresses
              (work in progress), April 2010.

              McGrew, D. and B. Weis, "Using Counter Modes with
              Encapsulating Security Payload (ESP) and Authentication
              Header (AH) to Protect Group Traffic",
              draft-ietf-msec-ipsec-group-counter-modes (work in
              progress), March 2010.

   [CTR]      Housley, R., "Using Advanced Encryption Standard (AES)
              Counter Mode", RFC 3686, January 2009.

   [GCM]      Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
              (GCM) in IPsec Encapsulating Security Payload (ESP)",
              RFC 4106, June 2005.

              Devarapalli, V. and K. Weniger, "Redirect Mechanism for
              IKEv2", RFC 5685, November 2009.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate

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              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

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

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

   [VRRP]     Nadas, S., "Virtual Router Redundancy Protocol (VRRP)",
              RFC 5798, March 2010.

              Sheffer, Y. and H. Tschofenig, "IKEv2 Session Resumption",
              RFC 5723, January 2010.

Author's Address

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


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