One document matched: draft-nir-ike-qcd-01.txt
Differences from draft-nir-ike-qcd-00.txt
Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track F. Detienne
Expires: January 14, 2009 P. Sethi
Cisco
July 13, 2008
A Quick Crash Detection Method for IKE
draft-nir-ike-qcd-01.txt
Status of this Memo
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Abstract
This document describes an extension to the IKEv2 protocol that
allows for faster crash recovery using a saved token.
When an IPsec tunnel between two IKEv2 implementations 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 reboot.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. RFC 4306 Crash Recovery . . . . . . . . . . . . . . . . . . . 3
3. Protocol Outline . . . . . . . . . . . . . . . . . . . . . . . 4
4. Stateless Variant Outline . . . . . . . . . . . . . . . . . . 5
4.1. Introducing CHECK_SPI . . . . . . . . . . . . . . . . . . 5
4.2. Stateless Recovery . . . . . . . . . . . . . . . . . . . . 6
4.3. Wait before rekey . . . . . . . . . . . . . . . . . . . . 6
4.4. Throttling and Dampening . . . . . . . . . . . . . . . . . 7
4.4.1. Invalid SPI throttling . . . . . . . . . . . . . . . . 8
4.4.2. Dampening . . . . . . . . . . . . . . . . . . . . . . 8
4.4.3. User controls . . . . . . . . . . . . . . . . . . . . 9
5. Formats and Exchanges . . . . . . . . . . . . . . . . . . . . 9
5.1. Notification Format . . . . . . . . . . . . . . . . . . . 9
5.2. check_fmt . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Stateless IKE Recovery VendorID . . . . . . . . . . . . . 10
5.4. Authentication Exchange . . . . . . . . . . . . . . . . . 10
5.5. Informational Exchange . . . . . . . . . . . . . . . . . . 12
6. Token Generation and Verification . . . . . . . . . . . . . . 12
6.1. A Stateless Method of Token Generation . . . . . . . . . . 13
6.2. Token Lifetime . . . . . . . . . . . . . . . . . . . . . . 13
7. Backup Gateways . . . . . . . . . . . . . . . . . . . . . . . 13
8. Alternative Solutions . . . . . . . . . . . . . . . . . . . . 13
8.1. Initiating a new IKE SA . . . . . . . . . . . . . . . . . 14
8.2. Birth Certificates . . . . . . . . . . . . . . . . . . . . 14
9. Interaction with IFARE . . . . . . . . . . . . . . . . . . . . 14
10. Operational Considerations . . . . . . . . . . . . . . . . . . 15
10.1. Who should implement this specification . . . . . . . . . 15
10.2. Response to unknown child SPI . . . . . . . . . . . . . . 16
10.3. Stateless IKE Recovery cookie . . . . . . . . . . . . . . 17
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11.1. Security Considerations for the Stateful Method . . . . . 18
11.2. Security Considerations for the Stateless Method . . . . . 18
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 19
14.1. Changes from draft-nir-ike-qcd-00 . . . . . . . . . . . . 19
14.2. Changes from draft-nir-qcr-00 . . . . . . . . . . . . . . 19
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
15.1. Normative References . . . . . . . . . . . . . . . . . . . 19
15.2. Informative References . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 22
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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 takes several minutes.
The method proposed here, is to send a 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 non-rebooted peer so as to
delete the IKE SA. Deleting the IKE SA results is a quick re-
establishment of the IPsec tunnels. This is described in Section 3.
Finally, Section 4 describes a variant that does not require storing
state on the non-rebooted peer, but does require an extra round-trip.
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 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 in the IKE_AUTH exchange.
The term "token taker" refers to an implementation that stores such a
token or a digest thereof, after receiving it in an IKE_AUTH
exchange.
2. RFC 4306 Crash Recovery
When one peer reboots, the other peer does not get any notification,
so 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
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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". Clearly, implementations differ, but all will
take a significant amount of time.
3. Protocol Outline
Supporting implementations will send a notification, called a "QCD
token", as described in Section 5.1 in the last packets of the
IKE_AUTH exchange. These are the final request and final response
that contain the AUTH payloads. The generation of these tokens is a
local matter for implementations, but considerations are described in
Section 6. Implementations that send such a token will be called
"token makers".
A supporting implementation receiving such a token SHOULD store it as
part of the IKE SA. Implementations that support this part of the
protocol will be called "token takers". Section 10.1 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 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 5.5.
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 the 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 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
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tokens from any address, so as to allow different kinds of high-
availability configuration 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 9 for a short discussion about this protocol's interaction
with session resumption.
4. Stateless Variant Outline
Sometimes, a QCD token is not available to the non-rebooted
implementation. This can happen for several reasons:
o Perhaps the rebooted peer has not implemented the "token maker"
part of the protocol.
o Perhaps the non-rebooted peer is resource-constrained, and cannot
spare the memory needed to save the token, so it did not implement
the "token taker" part of the protocol.
In such cases, we also define a stateless variant of the protocol,
that does not require any state on the non-rebooted peer, but does
require an extra round-trip.
A supporting implementation will advertise this capability with a
special VID payload as defined in Section 5.3. When such an
implementation reboots and sends an INVALID_SPI or INVALID_IKE_SPI
notification to the non-rebooted peer, which has no QCD token, the
non-rebooted peer uses a CHECK_SPI notification (see Section 4.1) to
poll its peer about whether or not the SPI is actually invalid.
4.1. Introducing CHECK_SPI
In order to achieve stateless IKE recovery, this memo introduces a
new notify type called CHECK_SPI. The CHECK_SPI payload carries an
SPI (IKE_SA or Child SA) and one of three sub-types (QUERY, ACK,
NACK). The semantic of the CHECK_SPI subtypes is the following:
o QUERY: a peer queries the remote peer SA DB for the presence of
the SA whose value is in the payload.
o ACK: a peer confirms it has the SA specified in the payload.
o NACK: a peer confirms it does not have the SA specified in the
payload.
The payload format of the CHECK_SPI notify is covered in Section 5.2.
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4.2. Stateless Recovery
After receiving the INVALID_SPI or INVALID_IKE_SPI notifications, the
non-rebooted peer (called Peer Y in the figure) will send an
unprotected IKE message as follows. Note that Peer Y MUST NOT send
this unless Peer X has advertised this capability in the IKE_AUTH
exchange.
Peer X Peer Y
HDR(A,B) INVALID_IKE_SPI(A,B)
-------------------------------------------->
HDR(A,B) CHECK_SPI(QUERY,(A,B)), N(Cookie)
<--------------------------------------------
HDR(A,B) CHECK_SPI(ACK|NACK,(A,B)), N(Cookie)
-------------------------------------------->
In this figure, A & B represent the IKE SPIs, and the Cookie is a
stateless cookie with similar considerations as the stateless cookie
described in section 2.6 of RFC 4306. The cookie SHOULD depend on
the IKE SPIs and a saved secret.
A similar exchange happens when the peer sends an INVALID_SPI
notification:
Peer X Peer Y
HDR(0,0) INVALID_SPI(a)
-------------------------------------------->
HDR(A,B) CHECK_SPI(QUERY,(A,B)), N(Cookie)
<--------------------------------------------
HDR(A,B) CHECK_SPI(ACK|NACK,(A,B)), N(Cookie)
-------------------------------------------->
The difference here is that Peer Y had to locate the IKE SPIs
associated with the SPI mentioned in the INVALID_SPI notification.
4.3. Wait before rekey
There exists a particular attack where a man-in-the-middle can snoop
and inject traffic but can not block or drop packets. This attack
can spoof INVALID_SPI (allegedly from X), forcing a CHECK_SPI(QUERY)
from Y. The attacker would spoof back CHECK_SPI(NACK) to force an
undue rekey. Since the attacker can not block packets, the
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INVALID_SPI will also reach Alice, who will reply with
CHECK_SPI(ACK).
Y receives CHECK_SPI(NACK) first and MAY wait for a few msec before
creating a new SA. Y will eventually receive BOTH a CHECK_SPI(ACK)
and a CHECK_SPI(NACK), Which is dubious. The SIR process should then
stop and log an error, saving the SA.
The process is illustrated below:
X Attacker Y
Inv SPI
------------------>
CHECK_SPI(QUERY)
<-------------------------------------
CHECK_SPI(NACK)
------------------> Should rekey
but wait a few msec
CHECK_SPI(ACK)
-------------------------------------> Hint of attack
=> no rekey
Ideally, the round-trip-time should be measured during the IKE
exchange and Y wait for a full RTT before initiating a rekey.
Given that IKE itself is subject to DH computation by a man-in-the-
middle, also considering that SA's are dampened after creation (see
Section 4.4.2), the staging complexity and limited interest of this
attack makes it rather impractical. An implementation MAY decided to
implement this final safety wait but this is strictly optional.
4.4. Throttling and Dampening
An important aspect of the security in stateless IKE recovery has to
do with limiting the CPU utilization. In order to thwart flood types
denial of service attacks, strict rate limiting and throttling
mechanisms have to be enforced.
All the notifications that are exchanged during IKE recovery SHOULD
be rate limited. This paragraph provides information on the way rate
limiting should take place.
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4.4.1. Invalid SPI throttling
The sending of all Invalid SPI notifies MUST be rate limited one way
or an other. The rate limiting SHOULD be performed on a per peer
basis but dynamic state creation SHOULD be avoided as much as
possible. A recommended tradeoff is to limit the number of flows
that can undergo recovery at one point in time and avoid sending
Invalid SPI notifies for flows that are potentially already under
recovery.
Invalid SPI rate limiting protects against natural dangling SA
occurences. I.e. normal traffic conditions may cause unrecognized
SPI's to be received and this message is the most important to
protect. Indeed, it is not realistic to send one notification per
bad ESP packet received. On high speed links, this could mean
thousands of IKE notifies sent for the same offending SPI.
The receiving of unauthenticated Invalid SPI notifies MUST as well be
rate limited. Again, the rate limiting SHOULD be performed on a per
peer basis without dynamic state creation. In normal circumstances,
the peer receiving Invalid SPI notifies has an SA with the peer
sendig those notifies and already maintains peer-related data
structures that can help in maintaining adequate counters.
Authenticated Invalid SPI notifies can be accepted without
throttling.
4.4.2. Dampening
After one of the following conditions:
o the natural creation or rekey of one or more SA's
o the recovery of one or more SA's
o the failure in recovering an SA owned by the local security
gateway
o the logging of an error or warning message involving an SA owned
by the local security gateway
The peer with which SA's were created, attempted or against which a
log was emitted SHOULD be dampened, which means that all the
unauthenticated Invalid SPI and Check SPI messages emitted by that
peer MUST be ignored for a chosen duration.
This protection prevents a man-in-the-middle from forcing the fast
recreation of SA's and potentially depleting the entropy of systems
under attack. It also deals efficently with race conditions that may
occur after a rekey.
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4.4.3. User controls
Because throttling at large is related to speed, the network
implementation around the security gateways has a major influence on
the pertinence of the paremeters controlling rate limiting. It is
difficult to provide good absolute values for the rate limiters,
considering that these are implementation dependent.
As such, for the sake of fitness in practical deployments, a system
implementing this memo MUST provide administrative controls over the
rate limiter parameters.
5. Formats and Exchanges
5.1. 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 ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) MUST contain 1, as this message is related
to an IKE SA.
o SPI Size (1 octet) MUST be zero, in conformance with [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 (16-256 octets) contains a generated token as
described in Section 6.
5.2. check_fmt
The notification payload called "CHECK_SPI" is formatted as follows:
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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 ! CHECK_SPI Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Operation !
+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) MUST contain 1, as this message is related
to an IKE SA.
o SPI Size (1 octet) MUST be zero, in conformance with [RFC4306].
o CHECK_SPI Notify Message Type (2 octets) - MUST be xxxxx, the
value assigned for CHECK_SPI notifications. TBA by IANA.
o Operation (1 Octet) - This field determines the operation being
performed (Query, Reply_ACK, Reply_NACK)
The list of operations and their corresponding value:
o Query: 0
o Reply_ACK: 1
o NACK: 2
5.3. Stateless IKE Recovery VendorID
The stateless IKE recovery VendorID or SIR_VID is as follows:
"SIR STATELESS" hex: 53 49 52 20 53 54 41 54 45 4c 45 53 53
This VendorID payload MUST be sent in the first IKE_AUTH message of
any implementation that supports the stateless variant of this
protocol.
5.4. Authentication Exchange
For clarity, only the EAP version of an AUTH exchange will be
presented here. The non-EAP version is very similar. The figure
below is based on appendix A.3 of [RFC4718].
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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(SIR_VID)]
[V+]
first response <-- IDr, [CERT+], AUTH,
EAP,
[V(SIR_VID)]
[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+]
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.
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5.5. 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)+
response <--
If child SPIs are persistently mapped to IKE SPIs as described in
Section 10.2, we may get the following exchange in response to an ESP
or AH packet.
request --> N(INVALID_SPI), N(QCD_TOKEN)+
response <--
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 token generation constants, 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 SHOULD NOT be sent.
Section 6 defines token verification.
6. Token Generation and Verification
No token generation method is mandated by this document. A method is
documented in Section 6.1, but only serves as an example.
The following lists the requirements from a token generation
mechanism:
o Tokens MUST be at least 16 octets log, 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.
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o The token maker, MUST be able to re-generate or retrieve the token
based on the IKE SPIs even after it reboots.
6.1. 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 IKE
implementation, 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 The TOKEN_SECRET_DATA is calculated as follows:
TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R)
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.
6.2. 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).
7. Backup Gateways
Making crash recovery quick is important, 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 6.1 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.
8. Alternative Solutions
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8.1. 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.
8.2. Birth Certificates
Here we should explain why not Birth Certificates.
9. Interaction with IFARE
IFARE, 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. IFARE addresses this problem by having the clients store an
encrypted derivative of the IKE SA for quick re-establishment.
What IFARE 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. Before establishing a new IKE SA using IFARE, a
client MUST 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 IFARE state, 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:
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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 times out after several minutes. In this case QCD
does not help.
2. Either the primary gateway or a backup gateway (see Section 7) 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, the backup gateway as described in
this document or another gateway as described in [resumption]
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, SAr2, TSi,
TSr, N(TICKET_OPAQUE)
[,N(TICKET_GATEWAY_LIST)]}
---- Reboot -----
HDR, {} -->
<-- HDR, N(QCD_Token)
HDR, Ni, N(TICKET_OPAQUE),
[N+,], SK {IDi, [IDr,]
SAi2, TSi, TSr,
[CP(CFG_REQUEST)]} -->
<-- HDR, SK {IDr, Nr, SAr2, [TSi, TSr],
[CP(CFG_REPLY)]}
10. Operational Considerations
10.1. 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 are reduced. For this reason critical
systems should implement backup gateways as described in Section 7.
Note that the lower-case "should" in the previous sentence is
intentional, as we do not specify this in the sense of RFC 2119.
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.
10.2. 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. If
such a mapping is available and persistent across reboots, the
rebooted implementation MAY 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.
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However, a persistent storage module might not be updated in a timely
manner, and could be populated with 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.
10.3. Stateless IKE Recovery cookie
The cookie information is chosen by the peer that emits it. As such,
the cookie has strictly no meaning for the remote peer and can thus
be chosen as seen fit. This section provides recommendations on how
to generate and validate those cookies.
When an IKE endpoint X sends an unauthenticated CHECK_SPI, the cookie
payload following the notify is computed as follow:
Cookie = VersionIDofSecret
| H( SECRET | CHECK_SPI(..., Query)
| ip.src | ip.dst
| udp.src | udp.dst)
where
o SECRET is a randomly generated secret known only to the
implementation and periodically changed.
o VersionIDofSecret should be changed whenever SECRET is
regenerated.
o CHECK_SPI(..., Query) is the content of the CHECK_SPI notify
payload where the operation subtype has been set to Query (cf.
Section 4.1)
o ip.src is the source ip address of the IKE packet.
o ip.dst is the destination ip address of the IKE packet.
o udp.src is the source udp post of the IKE packet.
o udp.dst is the destination udp port of the IKE packet.
Upon reception of a CHECK_SPI notify (ACK or NACK) followed by a
N(Cookie), a peer can verify whether this is the reply to a Query it
placed by recomputing the cookie and comparing it to the COOKIE in
the IKE message.
In order to minimize the range of cryptographic attacks on SECRET,
its value SHOULD have a limited life time.
11. Security Considerations
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11.1. Security Considerations for the Stateful Method
Tokens MUST be hard to guess. This is critical, because if an
attacker can guess the token associated with the 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 peer together with the
related IKE SPIs MUST NOT give an attacker the ability to guess other
tokens. Specifically, if one peer 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 peer. This is
the reason that the QCD_SECRET in Section 6.1 needs to be
sufficiently long.
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.
11.2. Security Considerations for the Stateless Method
IKE recovery self-protection is discussed all along the document and
contains many mechanism to thwart denial of service attacks.
IKE recovery is subject to a man-in-the-middle attack that can let
the attacker trigger a renegotiation. It has to be noticed that an
attacker able to block ESP and/or IKE packets can cause IKE itself to
also tear down and trigger a rekey of IKE SA's. With throttling and
dampening enabled, IKE recovery is able to reduce the amount of
rekeys/negotiations to as low a rate as IKEv2.
Overall, IKE Recovery is not more vulnerable than IKEv2 and even
improves on the security of IKEv2 by resynchronizing SA's more
rapidly which is important with dynamic polices.
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12. IANA Considerations
IANA is requested to assign a notify message type from the error
types range (43-8191) of the "IKEv2 Notify Message Types" registry
with name "QUICK_CRASH_DETECTION".
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 "CHECK_SPI".
13. Acknowledgements
We would like to thank Hannes Tschofenig and Yaron Sheffer for their
comments about IFARE.
14. Change Log
This section lists all changes in this document
NOTE TO RFC EDITOR : Please remove this section in the final RFC
14.1. 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.
14.2. 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.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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.
15.2. Informative References
[recovery]
Detienne, F. and P. Sethi, "Safe IKE Recovery",
draft-detienne-ikev2-recovery-00 (work in progress),
June 2008.
[resumption]
Sheffer, Y., Tschofenig, H., Dondeti, L., and V.
Narayanan, "IPsec Gateway Failover Protocol",
draft-sheffer-ipsec-failover-03 (work in progress),
March 2008.
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
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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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