A Quick Crash Detection Method for IKECheck Point Software Technologies Ltd.5 Hasolelim st.Tel Aviv67897Israelynir@checkpoint.comInternational Business Machines1701 North StreetEndicottNew York13760United Stateswierbows@us.ibm.com
Security Area
IPsecME Working GroupInternet-Draft This document describes an extension to the IKEv2 protocol that allows for faster
detection of SA desynchronization using a saved token. When an IPsec tunnel between two IKEv2 peers is disconnected due to a restart of one peer,
it can take as much as several minutes for the other peer to discover that the reboot has
occurred, thus delaying recovery. In this text we propose an extension to the protocol,
that allows for recovery immediately following the restart. IKEv2, as described in and its predecessor RFC 4306, has a
method for recovering from a reboot of one peer. As long as traffic flows in both
directions, the rebooted peer should re-establish the tunnels immediately. However, in many
cases the rebooted peer is a VPN gateway that protects only servers, or else the
non-rebooted peer has a dynamic IP address. In such cases, the rebooted peer will not be
able to re-establish the tunnels. describes how recovery works under
RFC 4306, and explains why it may take several minutes. The method proposed here, is to send an octet string, called a "QCD token" in the
IKE_AUTH exchange that establishes the tunnel. That token can be stored on the peer as part
of the IKE SA. After a reboot, the rebooted implementation can re-generate the token, and
send it to the peer, so as to delete the IKE SA. Deleting the IKE SA results is a quick
establishment of new IPsec tunnels. This is described in .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 . The term "token" refers to an octet string that an implementation can generate using
only the properties of a protected IKE message (such as IKE SPIs) as input. A conforming
implementation MUST be able to generate the same token from the same input even after
rebooting. The term "token maker" refers to an implementation that generates a token and sends it
to the peer as specified in this document. The term "token taker" refers to an implementation that stores such a token or a digest
thereof, in order to verify that a new token it receives is identical to the old token it
has stored. The term "non-volatile storage" in this document refers to a data storage module, that
persists across restarts of the token maker. Examples of such a storage module include an
internal disk, an internal flash memory module, an external disk and an external
database. A small non-volatile storage module is required for a token maker, but a larger
one can be used to enhance performance, as described in . When one peer loses state or reboots, the other peer does not get any notification, so
unidirectional IPsec traffic can still flow. The rebooted peer will not be able to
decrypt it, however, and the only remedy is to send an unprotected INVALID_SPI
notification as described in section 3.10.1 of . That section
also describes the processing of such a notification: Since the INVALID_SPI can only be used as a hint, the non-rebooted peer has to determine
whether the IPsec SA, and indeed the parent IKE SA are still valid. The method of doing
this is described in section 2.4 of . This method, called
"liveness check" involves sending a protected empty INFORMATIONAL message, and awaiting a
response. This procedure is sometimes referred to as "Dead Peer Detection" or DPD. Section 2.4 does not mandate how many times the liveness check message should be
retransmitted, or for how long, but does recommend the following: Those "at least several minutes" are a time during which both peers are active, but
IPsec cannot be used. Supporting implementations will send a notification, called a "QCD token", as described
in in the last IKE_AUTH exchange messages. These are the
final IKE_AUTH request and final IKE_AUTH response that contain the AUTH payloads. The
generation of these tokens is a local matter for implementations, but considerations are
described in . Implementations that send such a token will be
called "token makers". A supporting implementation receiving such a token MUST store it (or a digest thereof)
along with the IKE SA. Implementations that support this part of the protocol will be
called "token takers". has considerations for which
implementations need to be token takers, and which should be token makers. Implementation
that are not token takers will silently ignore QCD tokens. When a token maker receives a protected IKE request message with unknown IKE SPIs, it
SHOULD generate a new token that is identical to the previous token, and send it to the
requesting peer in an unprotected IKE message as described in . When a token taker receives the QCD token in an unprotected notification, it MUST verify
that the TOKEN_SECRET_DATA matches the token stored with the matching IKE SA. If the
verification fails, or if the IKE SPIs in the message do not match any existing IKE SA,
it SHOULD log the event. If it succeeds, it MUST silently delete the IKE SA associated
with the IKE_SPI fields, and all dependant child SAs. This event MAY also be logged. The
token taker MUST accept such tokens from any IP address and port combination, so as to
allow different kinds of high-availability configurations of the token maker. A supporting token taker MAY immediately create new SAs using an Initial exchange,
or it may wait for subsequent traffic to trigger the creation of new SAs. See for a short discussion about this extensions's
interaction with IKEv2 Session Resumption (). The notification payload called "QCD token" is formatted as follows:Protocol ID (1 octet) MUST be 1, as this message is related to an IKE SA.SPI Size (1 octet) MUST be zero, in conformance with section 3.10 of
.QCD Token Notify Message Type (2 octets) - MUST be xxxxx, the value assigned for QCD
token notifications. TBA by IANA.TOKEN_SECRET_DATA (16-128 octets) contains a generated token as described in
. For brevity, only the EAP version of an AUTH exchange will be presented here. The
non-EAP version is very similar. The figures below are based on appendix C.3 of
. Note that the QCD_TOKEN notification is marked as optional because it is not required
by this specification that every implementation be both token maker and token taker.
If only one peer sends the QCD token, then a reboot of the other peer will not be
recoverable by this method. This may be acceptable if traffic typically originates from
the other peer. In any case, the lack of a QCD_TOKEN notification MUST NOT be taken as an indication
that the peer does not support this standard. Conversely, if a peer does not understand
this notification, it will simply ignore it. Therefore a peer MAY send this notification
freely, even if it does not know whether the other side supports it. The QCD_TOKEN notification is related to the IKE SA and MUST follow the AUTH payload
and precede the Configuration payload and all payloads related to the child SA. After rekeying an IKE SA, the IKE SPIs are replaced, so the new SA also needs to have
a token. If only the responder in the rekey exchange is the token maker, this can be
done within the CREATE_CHILD_SA exchange. If the initiator is a token maker, then we
need an extra informational exchange. The following figure shows the CREATE_CHILD_SA exchange for rekeying the IKE SA. Only
the responder sends a QCD token. If the initiator is also a token maker, it SHOULD soon initiate an INFORMATIONAL
exchange as follows: For session resumption, as specified in , the situation is
similar. The responder, which is necessarily the peer that has crashed, SHOULD send a
new ticket within the protected payload of the IKE_SESSION_RESUME exchange. If the
Initiator is also a token maker, it needs to send a QCD_TOKEN in a separate
INFORMATIONAL exchange. The INFORMATIONAL exchange described in this section can also be used if QCD tokens
need to be replaced due to a key rollover. However, since token takers are required to
verify at least 4 QCD tokens, this is only necessary if secret QCD keys are rolled over
more than four times as often as IKE SAs are rekeyed. With some token generation methods, such as that described in ,
a QCD token may sometimes become invalid, although the IKE SA is still perfectly valid. In such a case, the token maker MUST send the new token in a protected message under
that IKE SA. That exchange could be a simple INFORMATIONAL, such as in the last figure
in the previous section, or else it can be part of a MOBIKE INFORMATIONAL exchange such
as in the following figure taken from section 2.2 of and
modified by adding a QCD_TOKEN notification: A token taker MUST accept such gratuitous QCD_TOKEN notifications as long as they are
carried in protected exchanges. A token maker SHOULD NOT generate them unless it is
no longer able to generate the old QCD_TOKEN. This QCD_TOKEN notification is unprotected, and is sent as a response to a protected
IKE request, which uses an IKE SA that is unknown. If child SPIs are persistently mapped to IKE SPIs as described in
, a token taker may get the following unprotected message
in response to an ESP or AH packet. The QCD_TOKEN and INVALID_IKE_SPI notifications are sent together to support both
implementations that conform to this specification and implementations that don't.
Similar to the description in section 2.21 of , The IKE SPI and
message ID fields in the packet headers are taken from the protected IKE request. To support a periodic rollover of the secret used for token generation, the token
taker MUST support at least four QCD_TOKEN notifications in a single packet. The token
is considered verified if any of the QCD_TOKEN notifications matches. The token maker
MAY generate up to four QCD_TOKEN notifications, based on several generations of keys. If the QCD_TOKEN verifies OK, an empty response MUST be sent. If the QCD_TOKEN
cannot be validated, a response MUST NOT be sent.
defines token verification. No token generation method is mandated by this document. Two method are documented in
the following sub-sections, but they only serve as examples. The following lists the requirements from a token generation mechanism: Tokens MUST be at least 16 octets long, and no more than 128 octets long, to
facilitate storage and transmission. Tokens SHOULD be indistinguishable from random data. It should not be possible for an external attacker to guess the QCD token generated
by an implementation. Cryptographic mechanisms such as PRNG and hash functions are
RECOMMENDED. The token maker, MUST be able to re-generate or retrieve the token based on the
IKE SPIs even after it reboots. The method of token generation MUST be such, that a collision of QCD tokens between
different pairs of IKE SPI will be highly unlikely. This describes a stateless method of generating a token: At installation or immediately after the first boot of the token maker, 32 random
octets are generated using a secure random number generator or a PRNG. Those 32 bytes, called the "QCD_SECRET", are stored in non-volatile storage on
the machine, and kept indefinitely. If key rollover is required by policy, the implementation MAY periodically generate
a new QCD_SECRET and keep up to 3 previous generations. When sending an unprotected
QCD_TOKEN, as many as 4 notification payloads may be sent, each from a different
QCD_SECRET. The TOKEN_SECRET_DATA is calculated as follows: This method is similar to the one in the previous section, except that the IP
address of the token taker is also added to the block being hashed. This has the
disadvantage that the token needs to be replaced (as described in
) whenever the token taker changes its address. The reason to use this method is described in .
When using this method, the TOKEN_SECRET_DATA field is calculated as follows: The IPaddr-T field specifies the IP address of the token taker. Secret rollover
considerations are similar to those in the previous section. The token is associated with a single IKE SA, and SHOULD be deleted by the token taker
when the SA is deleted or expires. More formally, the token is associated with the pair
(SPI-I, SPI-R). Making crash detection and recovery quick is a worthy goal, but since rebooting a
gateway takes a non-zero amount of time, many implementations choose to have a stand-by
gateway ready to take over as soon as the primary gateway fails for any reason.
describes consideration for such clusters of gateways with
synchronized state, but the rest of this section is relevant even when there is no
synchnorized state. If such a configuration is available, it is RECOMMENDED that the stand-by gateway be
able to generate the same token as the active gateway. if the method described in
is used, this means that the QCD_SECRET field is identical in both
gateways. This has the effect of having the crash recovery available immediately. Note that this refers to "high availability" configurations, where only one gateway is
active at any given moment. This is different from "load sharing" configurations where
more than one gateway is active at the same time. For load sharing configurations, please
see for security considerations. Instead of sending a QCD token, we could have the rebooted implementation start an
Initial exchange with the peer, including the INITIAL_CONTACT notification. This would
have the same effect, instructing the peer to erase the old IKE SA, as well as establishing
a new IKE SA with fewer rounds. The disadvantage here, is that in IKEv2 an authentication exchange MUST have
a piggy-backed Child SA set up. Since our use case is such that the rebooted implementation
does not have traffic flowing to the peer, there are no good selectors for such a Child
SA. Additionally, when authentication is asymmetric, such as when EAP is used, it is not
possible for the rebooted implementation to initiate IKE. Another proposal that was considered for this work item is the SIR extension,
which is described in . Under that proposal, the non-rebooted
peer sends a non-protected query to the possibly rebooted peer, asking whether the IKE
SA exists. The peer replies with either a positive or negative response, and the
absence of a positive response, along with the existence of a negative response is
taken as proof that the IKE SA has really been lost. The working group preferred the QCD proposal to this one. Birth Certificates is a method of crash detection that has never been formally
defined. Bill Sommerfeld suggested this idea in a mail to the IPsec mailing list on
August 7, 2000, in a thread discussing methods of crash detection: We believe that this method would have some problems. First, it requires Alice to
store the certificate, so as to be able to compare the public keys. That requires more
storage than does a QCD token. Additionally, the public-key operations needed to verify
the self-signed certificates are more expensive for Alice. We believe that a symmetric-key operation such as proposed here is more light-weight
and simple than that implied by the Birth Certificate idea. Some have suggested that the RFC 4306 procedure described in can
be tweaked by requiring fewer retransmissions over a shorter period of time for cases
of liveness check started because of an INVALID_SPI or INVALID_IKE_SPI notification. We believe that the default retransmission policy should represent a good balance
between the need for a timely discovery of a dead peer, and a low probability of false
detection. We expect the policy to be set to take the shortest time such that this
probability achieves a certain target. Therefore, reducing elapsed time and
retransmission count will create an unacceptably high probability of false detection,
and this can be triggered by a single INVALID_IKE_SPI notification. Additionally, even if the retransmission policy is reduced to, say, one minute, it
is still a very noticeable delay from a human perspective, from the time that the
gateway has come up until the tunnels are active, or from the time the backup
gateway has taken over until the tunnels are active. Session Resumption, specified in proposes to make setting
up a new IKE SA consume less computing resources. This is particularly useful in the case
of a remote access gateway that has many tunnels. A failure of such a gateway would
require all these many remote access clients to establish an IKE SA either with the
rebooted gateway or with a backup gateway. This tunnel re-establishment should occur
within a short period of time, creating a burden on the remote access gateway. Session
Resumption addresses this problem by having the clients store an encrypted derivative of
the IKE SA for quick re-establishment. What Session Resumption does not help, is the problem of detecting that the peer
gateway has failed. A failed gateway may go undetected for as long as the lifetime of a
child SA, because IPsec does not have packet acknowledgement, and applications cannot
signal the IPsec layer that the tunnel "does not work". Before establishing a new
IKE SA using Session Resumption, a client should ascertain that the gateway has indeed
failed. This could be done using either a liveness check (as in RFC 4306) or using the
QCD tokens described in this document. A remote access client conforming to both specifications will store QCD tokens, as well
as the Session Resumption ticket, if provided by the gateway. A remote access gateway
conforming to both specifications will generate a QCD token for the client. When the
gateway reboots, the client will discover this in either of two ways: The client does regular liveness checks, or else the time for some other IKE exchange
has come. Since the gateway is still down, the IKE exchange times out after several
minutes. In this case QCD does not help. Either the primary gateway or a backup gateway (see )
is ready and sends a QCD token to the client. In that case the client will quickly
re-establish the IPsec tunnel, either with the rebooted primary gateway or the backup
gateway as described in this document.
The full combined protocol looks like this: Throughout this document, we have referred to reboot time alternatingly as the time that
the implementation crashes and the time when it is ready to process IPsec packets and IKE
exchanges. Depending on the hardware and software platforms and the cause of the reboot,
rebooting may take anywhere from a few seconds to several minutes. If the implementation
is down for a long time, the benefit of this protocol extension is reduced. For this reason
critical systems should implement backup gateways as described in
. Implementing the "token maker" side of QCD makes sense for IKE implementation where protected
connections originate from the peer, such as inter-domain VPNs and remote access gateways.
Implementing the "token taker" side of QCD makes sense for IKE implementations where protected
connections originate, such as inter-domain VPNs and remote access clients. To clarify the requirements: A remote-access client MUST be a token taker and MAY be a token maker. A remote-access gateway MAY be a token taker and MUST be a token maker. An inter-domain VPN gateway MUST be both token maker and token taker. In order to limit the effects of DoS attacks, a token taker SHOULD limit the rate
of QCD_TOKENs verified from a particular source. If excessive amounts of IKE requests protected with unknown IKE SPIs arrive at a token
maker, the IKE module SHOULD revert to the behavior described in section 2.21 of
and either send an INVALID_IKE_SPI notification, or ignore it
entirely. After a reboot, it is more likely that an implementation receives IPsec packets than
IKE packets. In that case, the rebooted implementation will send an INVALID_SPI
notification, triggering a liveness check. The token will only be sent in a response to
the liveness check, thus requiring an extra round-trip. To avoid this, an implementation that has access to enough non-volatile storage MAY
store a mapping of child SPIs to owning IKE SPIs, or to generated tokens. If such a
mapping is available and persistent across reboots, the rebooted implementation SHOULD
respond to the IPsec packet with an INVALID_SPI notification, along with the appropriate
QCD_Token notifications. A token taker SHOULD verify the QCD token that arrives with an
INVALID_SPI notification the same as if it arrived with the IKE SPIs of the parent IKE
SA. However, a persistent storage module might not be updated in a timely manner, and
could be populated with tokens relating to IKE SPIs that have already been rekeyed. A
token taker MUST NOT take an invalid QCD Token sent along with an INVALID_SPI
notification as evidence that the peer is either malfunctioning or attacking, but it
SHOULD limit the rate at which such notifications are processed. This section describes the rationale for token generation methods such as the one
described in . Note that this section merely provides a possible
rationale, and does not specify or recommend any kind of configuration. Some configurations of security gateway use a load-sharing cluster of hosts, all
sharing the same IP addresses, where the SAs (IKE and child) are not synchronized between
the cluster members. In such a configuration, a single member does not know about all the
IKE SAs that are active for the configuration. A load balancer (usually a networking
switch) sends IKE and IPsec packets to the several members based on source IP address. In such a configuration, an attacker can send a forged protected IKE packet with the
IKE SPIs of an existing IKE SA, but from a different IP address. This packet will likely
be processed by a different cluster member from the one that owns the IKE SA. Since no
IKE SA state is stored on this member, it will send a QCD token to the attacker. If the
QCD token does not depend on IP address, this token can immediately be used to tell the
token taker to tear down the IKE SA using an unprotected QCD_TOKEN notification. To thwart this possible attack, such configurations should use a method that considers
the taker's IP address, such as the method described in . Tokens MUST be hard to guess. This is critical, because if an attacker can guess the
token associated with an IKE SA, she can tear down the IKE SA and associated tunnels at
will. When the token is delivered in the IKE_AUTH exchange, it is encrypted. When it is
sent again in an unprotected notification, it is not, but that is the last time this
token is ever used. An aggregation of some tokens generated by one maker together with the related IKE SPIs
MUST NOT give an attacker the ability to guess other tokens. Specifically, if one taker
does not properly secure the QCD tokens and an attacker gains access to them, this
attacker MUST NOT be able to guess other tokens generated by the same maker. This is the
reason that the QCD_SECRET in needs to be sufficiently long. The token taker MUST store the token in a secure manner. No attacker should be able to
gain access to a stored token. The QCD_SECRET MUST be protected from access by other parties. Anyone gaining
access to this value will be able to delete all the IKE SAs for this token maker. The QCD token is sent by the rebooted peer in an unprotected message. A message like
that is subject to modification, deletion and replay by an attacker. However, these
attacks will not compromise the security of either side. Modification is meaningless
because a modified token is simply an invalid token. Deletion will only cause the
protocol not to work, resulting in a delay in tunnel re-establishment as described in
. Replay is also meaningless, because the IKE SA has been deleted
after the first transmission. A token maker MUST NOT send a QCD token in an unprotected message for an existing IKE
SA. This implies that a conforming QCD token maker MUST be able to tell whether a
particular pair of IKE SPIs represent a valid IKE SA. This requirement is obvious and easy in the case of a single gateway. However, some
implementations use a load balancer to divide the load between several physical gateways.
It MUST NOT be possible even in such a configuration to trick one gateway into sending
a QCD token for an IKE SA which is valid on another gateway. This document does not specify how a load sharing sharing configuration of IPsec
gateways would work, but in order to support this specification, all members MUST be able
to tell whether a particular IKE SA is active anywhere in the cluster. One way to do it
is to synchronize a list of active IKE SPIs among all the cluster members. An attacker may try to attack QCD if the generation algorithm described in
is used. The attacker will send several fake IKE requests to the
gateway under attack, receiving and recording the QCD Tokens in the responses. This will
allow the attacker to create a dictionary of IKE SPIs to QCD Tokens, which can later be
used to tear down any IKE SA. Three factors mitigate this threat: The space of all possible IKE SPI pairs is huge: 2^128, so making such a dictionary
is impractical. Even if we assume that one implementation always generates predictable
IKE SPIs, the space is still at least 2^64 entries, so making the dictionary is
extremely hard. Throttling the amount of QCD_TOKEN notifications sent out, as discussed in
, especially when not soon after a crash will limit the
attacker's ability to construct a dictionary. The methods in and allow for a periodic
change of the QCD_SECRET. Any such change invalidates the entire dictionary. IANA is requested to assign a notify message type from the status types range
(16406-40959) of the "IKEv2 Notify Message Types" registry with name
"QUICK_CRASH_DETECTION". We would like to thank Hannes Tschofenig and Yaron Sheffer for their comments about
Session Resumption. Frederic D'etienne and Pratima Sethi contributed the ideas in
and . Others who have contrinuted valuable comments are, in alphabetical order, Lakshminath
Dondeti, Scott C Moonen and Dave Wierbowski. This section lists all changes in this document NOTE TO RFC EDITOR : Please remove this section in the final RFC First WG version. Addressed Scott C Moonen's concern about collisions of QCD tokens. Updated references to point to IKEv2bis instead of RFC 4306 and 4718. Also
converted draft reference for resumption to RFC 5723. Added Dave Wiebrowski as author, and removed Pratima and Frederic.Mostly editorial changes and cleaning up. Described QCD token enumeration, following a question by Lakshminath Dondeti. Added the ability to replace the QCD token for an existing IKE SA. Added tokens dependant on peer IP address and their interaction with MOBIKE. Removed stateless method. Added discussion of rekeying and resumption. Added discussion of non-synchronized load-balanced clusters of gateways in the
security considerations. Other wording fixes. Merged proposal with draft-detienne-ikev2-recovery Changed the protocol so that the rebooted peer generates the token. This has the
effect, that the need for persistent storage is eliminated. Added discussion of birth certificates. Changed name to reflect that this relates to IKE. Also changed from quick crash
recovery to quick crash detection to avoid confusion with IFARE. Added more operational considerations. Added interaction with IFARE. Added discussion of backup gateways.Key words for use in RFCs to Indicate Requirement LevelsHarvard University1350 Mass. Ave.CambridgeMA 02138- +1 617 495 3864sob@harvard.edu
General
keywordInternet Key Exchange Protocol: IKEv2MicrosoftVPN ConsortiumCheck PointNokiaIKEv2 Mobility and Multihoming Protocol (MOBIKE)NokiaIKEv2 Session ResumptionCheck PointNokia Siemens NetworksSafe IKE RecoveryCiscoCiscoCheck PointIPsec Cluster Problem StatementCheck Point