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IETF RFC 6027
IPsec Cluster Problem Statement
Last modified on Thursday, October 14th, 2010
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Internet Engineering Task Force (IETF) Y. Nir
Request for Comments: 6027 Check Point
Category: Informational October 2010
ISSN: 2070-1721
IPsec Cluster Problem Statement
 Abstract
This document defines the terminology, problem statement, and
requirements for implementing Internet Key Exchange (IKE) and IPsec
on clusters. It also describes gaps in existing standards and their
implementation that need to be filled in order to allow peers to
interoperate with clusters from different vendors. Agreed upon
terminology, problem statement, and requirements will allow IETF
working groups to consider development of IPsec/IKEv2 mechanisms to
simplify cluster implementations.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/RFC 6027.
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RFC 6027 IPsec Cluster Problem Statement October 2010
Copyright Notice
Copyright (c) 2010 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. Terminology .....................................................3
3. The Problem Statement ...........................................5
3.1. Scope ......................................................5
3.2. A Lot of Long-Lived State ..................................6
3.3. IKE Counters ...............................................6
3.4. Outbound SA Counters .......................................6
3.5. Inbound SA Counters ........................................7
3.6. Missing Synch Messages .....................................8
3.7. Simultaneous Use of IKE and IPsec SAs by Different
Members ....................................................8
3.7.1. Outbound SAs Using Counter Modes ....................9
3.8. Different IP Addresses for IKE and IPsec ..................10
3.9. Allocation of SPIs ........................................10
4. Security Considerations ........................................10
5. Acknowledgements ...............................................11
6. References .....................................................11
6.1. Normative References ......................................11
6.2. Informative References ....................................11
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RFC 6027 IPsec Cluster Problem Statement October 2010
1. Introduction
IKEv2, as described in [RFC 5996], and IPsec, as described in
[RFC 4301] 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, forming a "cluster" (see
Section 2). Similar demands have been made in the past for other
critical pieces of an organization'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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2. Terminology
"Single Gateway" is an implementation of IKE and IPsec enforcing a
certain policy, as described in [RFC 4301].
"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.
"Availability" is a measure of a system's ability to perform the
service for which it was designed. It is measured as the percentage
of time a service is available from the time it is supposed to be
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RFC 6027 IPsec Cluster Problem Statement October 2010
available. Colloquially, availability is sometimes expressed in
"nines" rather than percentage, with 3 "nines" meaning 99.9%
availability, 4 "nines" meaning 99.99% availability, etc.
"High Availability" is a property of a system, not a configuration
type. A system is said to have high availability if its expected
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. What "high" means depends on the
application, but usually is 4 to 6 "nines" (at most 0.5-50 minutes of
down time per year in a system that is supposed to be available all
the time.
"Fault Tolerance" is a property 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 occurrence of
a fault is never visible to the peers.
"Partially Transparent Cluster" is a cluster where the occurrence 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 "active" member, whereas the other(s) are referred to as
"standbys". The Virtual Router Redundancy Protocol (VRRP)
([RFC 5798]) 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 this is not a requirement.
"Failover" is the event where one member takes over some load from
some other member. In a hot standby cluster, this happens when a
standby member 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 Security Association (SA)) to move from one member to another
to even out the load, even without any failures.
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RFC 6027 IPsec Cluster Problem Statement October 2010
"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 Peer Authentication Database.
"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 the IKEv2 redirect mechanism ([RFC 5685]). In some cases,
a member's IP address(es) may be allocated to another member at
failover.
"Synch Channel" is a communications channel among the cluster
members, which is 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 section starts by scoping the problem, and goes on to list each
of the issues encountered while setting up a cluster of IPsec VPN
gateways.
3.1. Scope
This document will make no attempt to describe the problems in
setting up a generic cluster. It describes only problems related to
the IKE/IPsec protocols.
The problem of synchronizing the policy between cluster members is
out of scope, as this is an administrative issue that is not
particular to either clusters or to IPsec.
The interesting scenario here is VPN, whether inter-domain or remote
access. Host-to-host transport mode is not expected to benefit from
this work.
We do not describe in full the problems of the communication channel
between cluster members (the Synch Channel), nor do we intend to
specify anything in this space later. Specifically, mixed-vendor
clusters are out of scope.
The problem statement anticipates possible protocol-level solutions
between IKE/IPsec peers in order to improve the availability and/or
performance of VPN clusters. One vendor's IPsec endpoint should be
able to work, optimally, with another vendor's cluster.
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RFC 6027 IPsec Cluster Problem Statement October 2010
3.2. A Lot 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 of such IPsec SAs.
o SPD (Security Policy Database) 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 (Security Association
Database) entries, but tend to last even longer than that.
A naive implementation of a cluster would have no synchronized state,
and a failover would produce an effect similar to that of a rebooted
gateway. [RFC 5723] describes how new IKE and IPsec SAs can be
recreated in such a case.
3.3. IKE Counters
We can overcome the first problem described in Section 3.2, 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 MUST NOT process message n until
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.
One possible solution is to synchronize information about the IKE
message counters after 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. This
solution may be appropriate in some cases, but may be too onerous in
systems with a lot of SAs. It also has the drawback that it never
recovers from the missing synch message problem, which is described
in Section 3.6.
3.4. Outbound SA Counters
The Encapsulating Security Payload (ESP) and Authentication Header
(AH) have an optional anti-replay feature, where every protected
packet carries a counter number. Repeating counter numbers is
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RFC 6027 IPsec Cluster Problem Statement October 2010
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 IPsec SAs by 10,000
packets. 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.
3.5. Inbound SA Counters
An even tougher issue is the synchronization of packet counters for
inbound IPsec SAs. If a packet arrives at a newly active member,
there is no way to determine whether or not this packet is a replay.
The periodic synch does not solve this 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 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
verification is an optional feature (see Section 3.2 in [RFC 4301]).
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.
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RFC 6027 IPsec Cluster Problem Statement October 2010
Another possible solution to the inbound IPsec 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.
3.6. 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
failover.
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 cluster in the first place.
3.7. 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 hot standby clusters, because consecutive packets may
need to be sent by different members to the same peer gateway.
The solution to the IKE SA issue is up to the implementation. 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
categories.
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RFC 6027 IPsec Cluster Problem Statement October 2010
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 (no pun intended), because the packets have to be
processed by the correct member based on peer and the Security
Parameter Index (SPI), and most load balancers will not be able to
match the SPIs to the correct member, unless stickiness extends to
all traffic with a particular peer. Another disadvantage of sticky
solutions is that the load tends to not distribute evenly, especially
if one SA covers a significant portion of IPsec traffic.
The second is the "duplicate" category, where the child SA is
duplicated for each 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, for which the peer has no use.
Section 2.8 of [RFC 5996] 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
inspection.
3.7.1. Outbound SAs Using Counter Modes
For SAs involving counter mode ciphers such as Counter Mode (CTR)
([RFC 3686]) or Galois/Counter Mode (GCM) ([RFC 4106]) there is yet
another complication. The initial vector for such modes MUST NOT be
repeated, and senders use methods such as counters or linear feedback
shift registers (LFSRs) to ensure this. For an SA shared between
more than one active member, or even failing over from one member to
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RFC 6027 IPsec Cluster Problem Statement October 2010
another, the cluster members need to make sure that they do not
generate the same initial vector. See [COUNTER_MODES] for a
discussion of this problem in another context.
3.8. 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. This may be done so that IPsec traffic
bypasses the load balancer for greater scalability. 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. Unmodified peers may
drop such 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, and will not reduce load on the balancer.
[ARORA] discusses this problem in greater depth, and proposes another
solution, that does involve protocol changes.
3.9. Allocation of SPIs
The SPI associated with each child SA, and with each IKE SA, MUST be
unique relative to the peer of the SA. Thus, in the context of a
cluster, each cluster member MUST generate SPIs in a fashion that
avoids collisions (with other cluster members) for these SPI values.
The means by which cluster members achieve this requirement is a
local matter, outside the scope of this document.
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.
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RFC 6027 IPsec Cluster Problem Statement October 2010
As mentioned in Section 3.5, allowing an inbound child SA to failover
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.
Section 3.7 describes the problem of the two directions of a flow
being protected by two SAs that are not part of a matched pair or
that are not even being processed by the same cluster member. This
is not a security problem as far as IPsec is concerned because IPsec
has policy at the IP, protocol and port level only. However, many
IPsec implementations are integrated with stateful firewalls, which
need to see both sides of a flow. Such implementations may have to
forward packets to other members for the firewall to properly inspect
the traffic.
5. Acknowledgements
This document is the collective work, and includes contribution from
many people who participate in the IPsecME working group.
The editor would particularly like to acknowledge the extensive
contribution of the following people (in alphabetical order):
Jitender Arora, Jean-Michel Combes, Dan Harkins, David Harrington,
Steve Kent, Tero Kivinen, Alexey Melnikov, Yaron Sheffer, Melinda
Shore, and Rodney Van Meter.
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC 5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
September 2010.
6.2. Informative References
[ARORA] Arora, J. and P. Kumar, "Alternate Tunnel Addresses for
IKEv2", Work in Progress, April 2010.
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RFC 6027 IPsec Cluster Problem Statement October 2010
[COUNTER_MODES]
McGrew, D. and B. Weis, "Using Counter Modes with
Encapsulating Security Payload (ESP) and Authentication
Header (AH) to Protect Group Traffic", Work in Progress,
March 2010.
[RFC 3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode", RFC 3686, January 2009.
[RFC 4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC 5685] Devarapalli, V. and K. Weniger, "Redirect Mechanism for
IKEv2", RFC 5685, November 2009.
[RFC 5723] Sheffer, Y. and H. Tschofenig, "IKEv2 Session Resumption",
RFC 5723, January 2010.
[RFC 5798] Nadas, S., "Virtual Router Redundancy Protocol (VRRP)",
RFC 5798, March 2010.
Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
EMail: ynir@checkpoint.com
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IPsec Cluster Problem Statement
RFC TOTAL SIZE: 27497 bytes
PUBLICATION DATE: Thursday, October 14th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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