Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Informational June 10, 2010
Expires: December 12, 2010
IPsec Cluster Problem Statement
draft-ietf-ipsecme-ipsec-ha-05
Abstract
This document defines terminology, problem statement and requirements
for implementing 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. An agreed terminology, problem statement and
requirements will allow the IPSECME WG to consider development of
IPsec/IKEv2 mechanisms to simplify cluster implementations.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 12, 2010.
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the Trust Legal Provisions and are provided without warranty as
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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. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Lots 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 . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . 9
3.9. Allocation of SPIs . . . . . . . . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Informative References . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
IKEv2, as described in [RFC4306] and [IKEv2bis], 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, 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 [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.
"Availability" is a measure of a system's ability to perform the
service for which it was designed. It is measured as the percentage
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of time a service is available, from the time it is supposed to be
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 condition 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
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 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 other(s) 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 this is not 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 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 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
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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, a member's IP address(es) may
be allocated to another member at failover.
"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 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 tunneled site-to-site
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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3.2. 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 (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. [resumption] 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 may 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.
In some cases, it is feasible 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. We
leave it for future discussion to determine if it is always feasible
to do so.
3.4. 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
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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. 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 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
verification is an optional feature (see section 3.2 in [RFC4301]).
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 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
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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 LS 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 HS 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 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
categories.
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 most
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.
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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, 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
inspection.
3.7.1. 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
NOT 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]
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. 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
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all implementations.
[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.
As mentioned in Section 3.5, 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.
The editor would particularly like to acknowledge the extensive
contribution of the following people (in alphabetical order):
Jitender Arora, Jean-Michel Combes, Dan Harkins, Steve Kent, Tero
Kivinen, Yaron Sheffer, Melinda Shore, and Rodney Van Meter.
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7. Change Log
NOTE TO RFC EDITOR: REMOVE THIS SECTION BEFORE PUBLICATION
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
section.
Version 03 fixes some ID-nits, and adds the problem presented by
Jitender Arora in [ARORA].
Version 04 fixes a spelling mistake, moves the scope discussion to a
subsection of its own (Section 3.1), and adds a short discussion of
the duplicate SPI problem, presented by Jean-Michel Combes.
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.
[COUNTER_MODES]
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.
[IKEv2bis]
Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol: IKEv2",
draft-ietf-ipsecme-ikev2bis (work in progress), May 2010.
[REDIRECT]
Devarapalli, V. and K. Weniger, "Redirect Mechanism for
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IKEv2", RFC 5685, November 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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.
[VRRP] Nadas, S., "Virtual Router Redundancy Protocol (VRRP)",
RFC 5798, March 2010.
[resumption]
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
Israel
Email: ynir@checkpoint.com
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