Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track F. Detienne
Expires: January 12, 2011 P. Sethi
Cisco
July 11, 2010
A Quick Crash Detection Method for IKE
draft-nir-ike-qcd-07
Abstract
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2011.
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
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions Used in This Document . . . . . . . . . . . . 4
2. RFC 4306 Crash Recovery . . . . . . . . . . . . . . . . . . . 5
3. Protocol Outline . . . . . . . . . . . . . . . . . . . . . . . 5
4. Formats and Exchanges . . . . . . . . . . . . . . . . . . . . 6
4.1. Notification Format . . . . . . . . . . . . . . . . . . . 6
4.2. Passing a Token in the AUTH Exchange . . . . . . . . . . . 7
4.3. Replacing Tokens After Rekey or Resumption . . . . . . . . 8
4.4. Replacing the Token for an Existing SA . . . . . . . . . . 9
4.5. Presenting the Token in an INFORMATIONAL Exchange . . . . 9
5. Token Generation and Verification . . . . . . . . . . . . . . 10
5.1. A Stateless Method of Token Generation . . . . . . . . . . 10
5.2. A Stateless Method with IP addresses . . . . . . . . . . . 11
5.3. Token Lifetime . . . . . . . . . . . . . . . . . . . . . . 11
6. Backup Gateways . . . . . . . . . . . . . . . . . . . . . . . 11
7. Alternative Solutions . . . . . . . . . . . . . . . . . . . . 12
7.1. Initiating a new IKE SA . . . . . . . . . . . . . . . . . 12
7.2. Birth Certificates . . . . . . . . . . . . . . . . . . . . 12
7.3. Reducing Liveness Check Length . . . . . . . . . . . . . . 13
8. Interaction with Session Resumption . . . . . . . . . . . . . 13
9. Operational Considerations . . . . . . . . . . . . . . . . . . 14
9.1. Who should implement this specification . . . . . . . . . 14
9.2. Response to unknown child SPI . . . . . . . . . . . . . . 15
9.3. Using Tokens that Depend on IP Addresses . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
10.1. QCD Token Generation and Handling . . . . . . . . . . . . 16
10.2. QCD Token Transmission . . . . . . . . . . . . . . . . . . 17
10.3. QCD Token Enumeration . . . . . . . . . . . . . . . . . . 17
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18
13.1. Changes from draft-nir-ike-qcd-03 and -04 . . . . . . . . 18
13.2. Changes from draft-nir-ike-qcd-02 . . . . . . . . . . . . 18
13.3. Changes from draft-nir-ike-qcd-01 . . . . . . . . . . . . 19
13.4. Changes from draft-nir-ike-qcd-00 . . . . . . . . . . . . 19
13.5. Changes from draft-nir-qcr-00 . . . . . . . . . . . . . . 19
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
14.1. Normative References . . . . . . . . . . . . . . . . . . . 19
14.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
IKEv2, as described in [RFC4306] 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. Section 2 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 Section 3.
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].
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 Section 9.2.
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2. RFC 4306 Crash Recovery
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 [RFC4306]. That section also
describes the processing of such a notification:
"If this Informational Message is sent outside the
context of an IKE_SA, it should be used by the recipient
only as a "hint" that something might be wrong (because it
could easily be forged)."
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 [RFC4306]. 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:
"It is
suggested that messages be retransmitted at least a dozen times over
a period of at least several minutes before giving up on an SA..."
Those "at least several minutes" are a time during which both peers
are active, but IPsec cannot be used.
3. Protocol Outline
Supporting implementations will send a notification, called a "QCD
token", as described in Section 4.1 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 Section 5. 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) as part of the IKE SA. Implementations that
support this part of the protocol will be called "token takers".
Section 9.1 has considerations for which implementations need to be
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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 MUST 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 Section 4.5.
When a token taker receives the QCD token in an unprotected
notification, it MUST verify that the TOKEN_SECRET_DATA matches the
token stored in 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.
There is ongoing work on IKEv2 Session Resumption ([resumption]).
See Section 8 for a short discussion about this extensions's
interaction with session resumption.
4. Formats and Exchanges
4.1. Notification Format
The notification payload called "QCD token" is formatted as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol ID ! SPI Size ! QCD Token Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ TOKEN_SECRET_DATA ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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o Protocol ID (1 octet) MUST be 1, as this message is related to an
IKE SA.
o SPI Size (1 octet) MUST be zero, in conformance with section 3.10
of [RFC4306].
o QCD Token Notify Message Type (2 octets) - MUST be xxxxx, the
value assigned for QCD token notifications. TBA by IANA.
o TOKEN_SECRET_DATA (16-128 octets) contains a generated token as
described in Section 5.
4.2. Passing a Token in the AUTH Exchange
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 A.3 of [RFC4718].
first request --> IDi,
[N(INITIAL_CONTACT)],
[[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
[IDr],
[CP(CFG_REQUEST)],
[N(IPCOMP_SUPPORTED)+],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)],
SA, TSi, TSr,
[V+]
first response <-- IDr, [CERT+], AUTH,
EAP,
[V+]
/ --> EAP
repeat 1..N times |
\ <-- EAP
last request --> AUTH
[N(QCD_TOKEN)]
last response <-- AUTH,
[N(QCD_TOKEN)]
[CP(CFG_REPLY)],
[N(IPCOMP_SUPPORTED)],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)],
SA, TSi, TSr,
[N(ADDITIONAL_TS_POSSIBLE)],
[V+]
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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.
4.3. Replacing Tokens After Rekey or Resumption
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.
request --> SA, Ni, [KEi]
response <-- SA, Nr, [KEr], N(QCD_TOKEN)
If the initiator is also a token maker, it SHOULD soon initiate an
INFORMATIONAL exchange as follows:
request --> N(QCD_TOKEN)
response <--
For session resumption, as specified in [resumption], 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
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is only necessary if secret QCD keys are rolled over more than four
times as often as IKE SAs are rekeyed.
4.4. Replacing the Token for an Existing SA
With some token generation methods, such as that described in
Section 5.2, 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 [RFC4555] and modified by
adding a QCD_TOKEN notification:
(IP_I2:4500 -> IP_R1:4500)
HDR, SK { N(UPDATE_SA_ADDRESSES),
N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) } -->
<-- (IP_R1:4500 -> IP_I2:4500)
HDR, SK { N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP) }
<-- (IP_R1:4500 -> IP_I2:4500)
HDR, SK { N(COOKIE2), [N(QCD_TOKEN)] }
(IP_I2:4500 -> IP_R1:4500)
HDR, SK { N(COOKIE2), [N(QCD_TOKEN)] } -->
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.
4.5. Presenting the Token in an INFORMATIONAL Exchange
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.
request --> N(INVALID_IKE_SPI), N(QCD_TOKEN)+
If child SPIs are persistently mapped to IKE SPIs as described in
Section 9.2, a token taker may get the following unprotected message
in response to an ESP or AH packet.
request --> N(INVALID_SPI), N(QCD_TOKEN)+
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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 [RFC4306], 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.
Section 5 defines token verification.
5. Token Generation and 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:
o 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.
o 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.
o The token maker, MUST be able to re-generate or retrieve the token
based on the IKE SPIs even after it reboots.
5.1. A Stateless Method of Token Generation
This describes a stateless method of generating a token:
o 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.
o Those 32 bytes, called the "QCD_SECRET", are stored in non-
volatile storage on the machine, and kept indefinitely.
o 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.
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o The TOKEN_SECRET_DATA is calculated as follows:
TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R)
5.2. A Stateless Method with IP addresses
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 Section 4.4) whenever the token taker
changes its address.
The reason to use this method is described in Section 9.3. When
using this method, the TOKEN_SECRET_DATA field is calculated as
follows:
TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R | IPaddr-T)
The IPaddr-T field specifies the IP address of the token taker.
Secret rollover considerations are similar to those in the previous
section.
5.3. Token Lifetime
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).
6. Backup Gateways
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.
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 Section 5.1 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
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from "load sharing" configurations where more than one gateway is
active at the same time. This is also different from high
availability configurations where the SAs are synchronized. For load
sharing configurations, please see Section 10.2 for security
considerations.
7. Alternative Solutions
7.1. Initiating a new IKE SA
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.
7.2. Birth Certificates
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:
If we have the system sign a "birth certificate" when it
reboots (including a reboot time or boot sequence number),
we could include that with a "bad spi" ICMP error and in
the negotiation of the IKE SA.
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.
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7.3. Reducing Liveness Check Length
Some have suggested that the RFC 4306 procedure described in
Section 2 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.
8. Interaction with Session Resumption
Session Resumption, specified in [resumption] 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
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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:
1. 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.
2. Either the primary gateway or a backup gateway (see Section 6) 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:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK {IDi, [CERT,]
[CERTREQ,] [IDr,]
AUTH, N(QCD_TOKEN)
SAi2, TSi, TSr,
N(TICKET_REQUEST)} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
N(QCD_TOKEN), SAr2, TSi, TSr,
N(TICKET_LT_OPAQUE) }
---- Reboot -----
HDR, {} -->
<-- HDR, N(QCD_TOKEN)
HDR, [N(COOKIE),]
Ni, N(TICKET_OPAQUE)
[,N+] -->
<-- HDR, Nr [,N+]
9. Operational Considerations
9.1. Who should implement this specification
Throughout this document, we have referred to reboot time
alternatingly as the time that the implementation crashes and the
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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 Section 6.
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:
o A remote-access client MUST be a token taker and MAY be a token
maker.
o A remote-access gateway MAY be a token taker and MUST be a token
maker.
o 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 [RFC4306] and either send an
INVALID_IKE_SPI notification, or ignore it entirely.
9.2. Response to unknown child SPI
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 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
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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.
9.3. Using Tokens that Depend on IP Addresses
This section describes the rationale for token generation methods
such as the one described in Section 5.2. 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 Section 5.2.
10. Security Considerations
10.1. QCD Token Generation and Handling
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
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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 Section 5.1 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 Section 2. Replay is also meaningless,
because the IKE SA has been deleted after the first transmission.
10.2. QCD Token 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.
10.3. QCD Token Enumeration
An attacker may try to attack QCD if the generation algorithm
described in Section 5.1 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
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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:
o 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 is faulty and always generates predictable IKE
SPIs, the space is still at least 2^64 entries, so making the
dictionary is extremely hard.
o Throttling the amount of QCD_TOKEN notifications sent out, as
discussed in Section 9.1, especially when not soon after a crash
will limit the attacker's ability to construct a dictionary.
o The methods in Section 5.1 and Section 5.2 allow for a periodic
change of the QCD_SECRET. Any such change invalidates the entire
dictionary.
11. IANA Considerations
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".
12. Acknowledgements
We would like to thank Hannes Tschofenig and Yaron Sheffer for their
comments about Session Resumption.
13. Change Log
This section lists all changes in this document
NOTE TO RFC EDITOR : Please remove this section in the final RFC
13.1. Changes from draft-nir-ike-qcd-03 and -04
Mostly editorial changes and cleaning up.
13.2. Changes from draft-nir-ike-qcd-02
o Described QCD token enumeration, following a question by
Lakshminath Dondeti.
o Added the ability to replace the QCD token for an existing IKE SA.
o Added tokens dependant on peer IP address and their interaction
with MOBIKE.
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13.3. Changes from draft-nir-ike-qcd-01
o Removed stateless method.
o Added discussion of rekeying and resumption.
o Added discussion of non-synchronized load-balanced clusters of
gateways in the security considerations.
o Other wording fixes.
13.4. Changes from draft-nir-ike-qcd-00
o Merged proposal with draft-detienne-ikev2-recovery [recovery]
o Changed the protocol so that the rebooted peer generates the
token. This has the effect, that the need for persistent storage
is eliminated.
o Added discussion of birth certificates.
13.5. Changes from draft-nir-qcr-00
o Changed name to reflect that this relates to IKE. Also changed
from quick crash recovery to quick crash detection to avoid
confusion with IFARE.
o Added more operational considerations.
o Added interaction with IFARE.
o Added discussion of backup gateways.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.
[RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006.
14.2. Informative References
[recovery]
Detienne, F., Sethi, P., and Y. Nir, "Safe IKE Recovery",
draft-detienne-ikev2-recovery (work in progress),
August 2008.
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[resumption]
Sheffer, Y. and H. Tschofenig, "IKEv2 Session Resumption",
draft-ietf-ipsecme-ikev2-resumption (work in progress),
June 2009.
Authors' Addresses
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
Email: ynir@checkpoint.com
Frederic Detienne
Cisco Systems, Inc.
De Kleetlaan, 7
Diegem B-1831
Belgium
Phone: +32 2 704 5681
Email: fd@cisco.com
Pratima Sethi
Cisco Systems, Inc.
O'Shaugnessy Road, 11
Bangalore, Karnataka 560027
India
Phone: +91 80 4154 1654
Email: psethi@cisco.com
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