Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Informational September 15, 2009
Expires: March 19, 2010
IPsec High Availability Problem Statement
draft-nir-ipsecme-ipsecha-00
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Abstract
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
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implementing IKE and IPsec, and describes gaps in the existing
standards.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Problem Statement . . . . . . . . . . . . . . . . . . . . . 4
3.1. Lots of Long Lived State . . . . . . . . . . . . . . . . . 4
3.2. IKE and IPsec Counters . . . . . . . . . . . . . . . . . . 5
3.3. Missing Synch Messages . . . . . . . . . . . . . . . . . . 6
3.4. Simultaneous use of IKE and IPsec SAs by Different
Members . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
5. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Informative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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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",
"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.
"Member" is one gateway in a cluster.
"High Availability Cluster", or "HA 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".
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"Load Sharing Cluster", or "LS Cluster" is a cluster where more than
one of the members may be active at the same time.
"Failover" is the event where a stand-by member becomes active, and
the formerly active member becomes a stand-by.
"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].
"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.
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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 and IPsec Counters
We can overcome the first problem described in Section 3.1, by
synchronizing states - whenever an SA is created, we can share this
new state with all other members. There is, however, another
problem. Those states are not only long-lived, but they are ever
changing.
IKE has message counters. A peer may not process message n until 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.
ESP and AH have an anti-replay feature, where every encrypted packet
carries a counter number. Repeating counter numbers is considered an
attack, so the newly-active member SHOULD NOT use a replay counter
number that has already been used.
In some cases, it is feasible to synchronize the IKE message counters
for every IKE exchange, but it is almost never feasible to
synchronize the IPsec message counters for every IPsec packet
transmitted or received. So we have to assume that at least for
IPsec, the replay counter will not be up-to-date on the newly-active
member.
A possible solution to the IPsec problem is to send replay counter
information not for each packet processed, but only at regular
intervals, say, every 10,000 packets. 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 are reliable protocols.
This still has the problem of what to do with inbound IPsec packets,
for which the newly-active member is unable to determine if they are
replayed or not.
Another possible solution to the IPsec problem is to rekey all child
SAs following a failover. This may or may not be feasible depending
on the implementation and the configuration.
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3.3. 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, 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.
3.4. 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.
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
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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
inspection.
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.
5. Change Log
This is the first version
6. Informative References
[REDIRECT]
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Devarapalli, V. and K. Weniger, "Redirect Mechanism for
IKEv2", draft-ietf-ipsecme-ikev2-redirect (work in
progress), August 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.
[RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006.
[VRRP] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
RFC 3768, April 2004.
[resumption]
Sheffer, Y. and H. Tschofenig, "IKEv2 Session Resumption",
draft-ietf-ipsecme-ikev2-resumption (work in progress),
June 2009.
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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