This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
EID 3448, EID 3449
Internet Engineering Task Force (IETF) Y. Nir, Ed.
Request for Comments: 6290 Check Point
Category: Standards Track D. Wierbowski
ISSN: 2070-1721 IBM
F. Detienne
P. Sethi
Cisco
June 2011
A Quick Crash Detection Method for the
Internet Key Exchange Protocol (IKE)
Abstract
This document describes an extension to the Internet Key Exchange
Protocol version 2 (IKEv2) that allows for faster detection of
Security Association (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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6290.
Copyright Notice
Copyright (c) 2011 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
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. RFC 5996 Crash Recovery . . . . . . . . . . . . . . . . . . . 4
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 Unprotected Message . . . . . . 9
5. Token Generation and Verification . . . . . . . . . . . . . . 10
5.1. A Stateless Method of Token Generation . . . . . . . . . . 11
5.2. A Stateless Method with IP Addresses . . . . . . . . . . . 11
5.3. Token Lifetime . . . . . . . . . . . . . . . . . . . . . . 12
6. Backup Gateways . . . . . . . . . . . . . . . . . . . . . . . 12
7. Interaction with Session Resumption . . . . . . . . . . . . . 13
8. Operational Considerations . . . . . . . . . . . . . . . . . . 14
8.1. Who Should Implement This Specification . . . . . . . . . 14
8.2. Response to Unknown Child SPI . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9.1. QCD Token Generation and Handling . . . . . . . . . . . . 16
9.2. QCD Token Transmission . . . . . . . . . . . . . . . . . . 17
9.3. QCD Token Enumeration . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. The Path Not Taken . . . . . . . . . . . . . . . . . 20
A.1. Initiating a New IKE SA . . . . . . . . . . . . . . . . . 20
A.2. SIR . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
A.3. Birth Certificates . . . . . . . . . . . . . . . . . . . . 20
A.4. Reducing Liveness Check Length . . . . . . . . . . . . . . 21
1. Introduction
IKEv2, as described in [RFC5996] 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, so all traffic is inbound.
In other cases, the non-rebooted peer has a dynamic IP address, so
the rebooted peer cannot initiate IKE because its current IP address
is unknown. In such cases, the rebooted peer will not be able to
re-establish the tunnels. Section 2 describes how recovery works
under RFC 5996, 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 in 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 Security Parameter Indexes (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 8.2.
2. RFC 5996 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 [RFC5996]. 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 [RFC5996]. 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 part of which both
peers are active, but IPsec cannot be used.
Especially in the case of a reboot (rather than fail-over or
administrative clearing of state), the peer does not recover
immediately. Reboot, depending on the system, may take from a few
seconds to a few minutes. This means that at first the peer just
goes silent, i.e., does not send or respond to any messages. IKEv2
implementations can detect this situation and follow the rules given
in Section 2.4:
If there has only been outgoing traffic on all of the SAs
associated with an IKE SA, it is essential to confirm liveness of
the other endpoint to avoid black holes. If no cryptographically
protected messages have been received on an IKE SA or any of its
Child SAs recently, the system needs to perform a liveness check
in order to prevent sending messages to a dead peer.
[RFC5996] does not mandate any time limits, but it is possible that
the peer will start liveness checks even before the other end is
sending INVALID_SPI notification, as it detected that the other end
is not sending any packets anymore while it is still rebooting or
recovering from the situation.
This means that the several minutes recovery period is overlapping
the actual recover time of the other peer; i.e., if the security
gateway requires several minutes to boot up from the crash, then the
other peers have already finished their liveness checks before the
crashing peer even has a chance to send INVALID_SPI notifications.
There are cases where the peer loses state and is able to recover
immediately; in those cases it might take several minutes to recreate
the IPsec SAs.
Note that the IKEv2 specification specifically gives no guidance for
the number of retries or the length of timeouts, as these do not
affect interoperability. This means that implementations are allowed
to use the hints provided by the INVALID_SPI messages to shorten
those timeouts (i.e., a different environment and situation requiring
different rules).
Some existing IKEv2 implementations already do that (i.e., shorten
timeouts or limit number of retries) based on these kinds of hints
and also start liveness checks quickly after the other end goes
silent. However, see Appendix A.4 for a discussion of why this may
not be enough.
3. Protocol Outline
Supporting implementations will send a notification, called a "QCD
token", as described in Section 4.1 in the first IKE_AUTH exchange
messages. These are the first 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) along with the IKE SA. Implementations that
support this part of the protocol will be called "token takers".
Section 8.1 has considerations for which implementations need to be
token takers, and which should be token makers. Implementations 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 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 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 dependent 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 Section 7 for a short discussion about this extension's
interaction with IKEv2 Session Resumption ([RFC5723]).
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 ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) MUST be 0.
EID 3449 (Verified) is as follows:Section: 4.1
Original Text:
o Protocol ID (1 octet) MUST be 1, as this message is related to an
IKE SA.
Corrected Text:
o Protocol ID (1 octet) MUST be 0.
Notes:
RFC5996 (IKEv2) in section 3.10 while describing Protocol ID field in Notify Payload specifies that "If the SPI field is empty, this field MUST be sent as zero and MUST be ignored on receipt". As this RFC requires SPI field to be empty (later in section 4.1), Protocol ID should be zero to be consistent with RFC5996.
o SPI Size (1 octet) MUST be zero, in conformance with Section 3.10
of [RFC5996].
o QCD Token Notify Message Type (2 octets) - MUST be 16419, the
value assigned for QCD token notifications.
o TOKEN_SECRET_DATA (variable) contains a generated token as
described in Section 5.
4.2. Passing a Token in the AUTH Exchange
For brevity, only the Extensible Authentication Protocol (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 [RFC5996].
first request --> IDi,
[N(INITIAL_CONTACT)],
[[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
[IDr],
[N(QCD_TOKEN)]
[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
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.
The QCD_TOKEN notification is related to the IKE SA and should 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 initiate an
INFORMATIONAL exchange immediately after the CREATE_CHILD_SA exchange
as follows:
request --> N(QCD_TOKEN)
response <--
For session resumption, as specified in [RFC5723], the situation is
similar. The responder, which is necessarily the peer that has
crashed, SHOULD send a new QCD_TOKEN in the IKE_AUTH exchange
that immediately followes the IKE_SESSION_RESUME exchange.
If the Initiator is also a token maker, it needs to send a QCD_TOKEN in
the same IKE_AUTH exchange.
EID 3448 (Verified) is as follows:Section: 4.3
Original Text:
For session resumption, as specified in [RFC5723], 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.
Corrected Text:
For session resumption, as specified in [RFC5723], the situation is
similar. The responder, which is necessarily the peer that has
crashed, SHOULD send a new QCD_TOKEN in the IKE_AUTH exchange
that immediately followes the IKE_SESSION_RESUME exchange.
If the Initiator is also a token maker, it needs to send a QCD_TOKEN in
the same IKE_AUTH exchange.
Notes:
Original text mixes up terms "ticket" (as Session Resumption ticket from RFC5723) and "token" (as QCD token from this RFC). As QCD token must never be sent in an unprotected message (see section 9.2 from this RFC) it cannot be sent in the IKE_SESSION_RESUME exchange because this exchange is done in clear. So, QCD token must be sent in the IKE_AUTH exchange that immediately followes the IKE_SESSION_RESUME exchange. In this case there is no need for the separate INFORMATIONAL exchange the Initiator's QCD token (if any) to be sent in, because it could be sent in the same IKE_AUTH 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. See Section 5.1 for an
example method that uses secret keys that may require rollover.
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 Unprotected Message
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.
message --> N(INVALID_IKE_SPI), N(QCD_TOKEN)+
If child SPIs are persistently mapped to IKE SPIs as described in
Section 8.2, a token taker may get the following unprotected message
in response to an Encapsulating Security Payload (ESP) or
Authentication Header (AH) packet.
message --> N(INVALID_SPI), N(QCD_TOKEN)+
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 [RFC5996], 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, the receiver MUST silently discard the
IKE SA and all associated child SAs. If the QCD_TOKEN cannot be
validated, a response MUST NOT be sent, and the event may be logged.
Section 5 defines token verification.
5. Token Generation and Verification
No token generation method is mandated by this document. Two methods
are documented in the following sub-sections, but they only serve as
examples.
The following lists the requirements for 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 a pseudo-random number generator (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.
o The method of token generation MUST be such that a collision of
QCD tokens between different pairs of IKE SPI will be highly
unlikely.
For verification, the token taker makes a bitwise comparison of the
token stored along with the IKE SA with the token sent in the
unprotected message. Multihomed takers might flip back-and-forth
between several addresses, and have their tokens replaced as
described in Section 4.4. To help avoid the case where the latest
stored token does not match the address used after the maker lost
state, the token taker MAY store several earlier tokens associated
with the IKE SA, and silently discard the SA if any of them matches.
5.1. A Stateless Method of Token Generation
The following describes a stateless method of generating a token. In
this case, 'stateless' means not maintaining any per-tunnel state,
although there is a small amount of non-volatile storage required.
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.
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.
See Section 9.2 for a discussion of a use-case for this method. 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.
Note that with a multihomed token taker, the QCD token matches just
one of the token taker IP addresses. Usually this is not a problem,
as packets sent to the token maker come out the same IP address. If
for some reason this changes, then the token maker can replace the
token as described in Section 4.4. If IKEv2 Mobility and Multihoming
(MOBIKE) is used, replacing the tokens SHOULD be piggybacked on the
INFORMATIONAL exchange with the UPDATE_SA_ADDRESSES notifications.
There is a corner case where the token taker begins using a new IP
address (because of multihoming, roaming, or normal network
operations) and the token maker loses state before replacing the
token. In that case, it will send a correct QCD token, but the token
taker will still have the old token. In that case, the extension
will not work, and the peers will revert to RFC 5996 recovery.
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 standby gateway ready to take over
as soon as the primary gateway fails for any reason. [RFC6027]
describes considerations for such clusters of gateways with
synchronized state, but the rest of this section is relevant even
when there is no synchronized state.
If such a configuration is available, it is RECOMMENDED that the
standby 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
from "load sharing" configurations where more than one gateway is
active at the same time. For load sharing configurations, please see
Section 9.2 for security considerations.
7. Interaction with Session Resumption
Session resumption, specified in [RFC5723], allows the setting up of
a new IKE SA to 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 requires all these many
remote access clients to establish an IKE SA either with the rebooted
gateway or with a backup. This tunnel re-establishment occurs 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 an arbitrarily long time, because IPsec does not have packet
acknowledgement, and applications cannot signal the IPsec layer that
the tunnel "does not work". Section 2.4 of RFC 5996 does not specify
how long an implementation needs to wait before beginning a liveness
check, and only says "not recently" (see full quote in Section 2).
In practice, some mobile devices wait a very long time before
beginning a liveness check, in order to extend battery life by
allowing parts of the device to remain in low-power modes.
QCD tokens provide a way to detect the failure of the peer in the
case where a liveness check has not yet ended (or begun).
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:
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+]
8. Operational Considerations
8.1. Who Should Implement This Specification
Throughout this document, we have referred to reboot time
alternatingly as the time that the implementation crashes and the
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 this discussion:
o For remote-access clients it makes sense to implement the token
taker role.
o For remote-access gateways it makes sense to implement the token
maker role.
o For inter-domain VPN gateways it makes sense to implement both
roles, because it can't be known in advance where the traffic
originates.
o It is perfectly valid to implement both roles in any case, for
example, when using a single library or a single gateway to
perform several roles.
In order to limit the effects of Denial-of-Service (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 [RFC5996] and either send an
INVALID_IKE_SPI notification or ignore it entirely.
Section 9.2 requires that token makers never send a QCD token in the
clear for a valid IKE SA and describes some configurations where this
could occur. Implementations that may be installed in such
configurations SHOULD automatically detect this and disable this
extension in unsafe configurations and MUST allow the user to control
whether the extension is enabled or disabled.
8.2. Response to Unknown Child SPI
After a reboot, it is more likely that an implementation will receive
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.
9. Security Considerations
The extension described in this document must not reduce the security
of IKEv2 or IPsec. Specifically, an eavesdropper must not learn any
non-public information about the peers.
The proposed mechanism should be secure against attacks by a passive
man in the middle (MITM) (eavesdropper). Such an attacker must not
be able to disrupt an existing IKE session, either by resetting the
session or by introducing significant delays. This requirement is
especially significant, because this document introduces a new way to
reset an IKE SA.
The mechanism need not be similarly secure against an active MITM,
since this type of attacker is already able to disrupt IKE sessions.
9.1. 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, they 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 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.
9.2. QCD Token Transmission
A token maker MUST NOT send a valid QCD token in an unprotected
message for an existing 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 valid QCD
token for an IKE SA that is valid on another gateway. This is true
whether the attempt to trick the gateway uses the token taker's IP
address or a different IP address.
IPsec failure detection is not applicable to deployments where the
QCD secret is shared by multiple gateways and the gateways cannot
assess whether the token can be legitimately sent in the clear while
another gateway may actually still own the SA's. Load balancing
configurations typically fall in this category. In order for a load
balancing configuration of IPsec gateways 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 this is to
synchronize a list of active IKE SPIs among all the cluster members.
Because it includes the token taker's IP address in the token
generation, the method in Section 5.2 can (under certain conditions)
prevent revealing the QCD token for an existing pair of IKE SPIs to
an attacker who is using a different IP address, even in a load-
sharing cluster without state synchronization. That method does not
prevent revealing the QCD token to an active attacker who is spoofing
the token taker's IP address. Such an attacker may attempt to direct
messages to a cluster member other than the member responsible for
the IKE SA in an attempt to trick that gateway into sending a QCD
token for a valid IKE SA. That method should not be used unless the
load balancer guarantees that IKE packets from the same source IP
address always go to the same cluster member.
9.3. 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
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 always generates predictable IKE SPIs, the space is
still at least 2^64 entries, so making the dictionary is extremely
hard. To ensure this, token makers MUST generate unpredictable
IKE SPIs by using a cryptographically strong pseudo-random number
generator.
o Throttling the amount of QCD_TOKEN notifications sent out, as
discussed in Section 8.1, 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.
10. IANA Considerations
IANA has assigned a notify message type (16419) from the status types
range (16406-40959) of the "IKEv2 Notify Message Types" registry with
the name "QUICK_CRASH_DETECTION".
11. Acknowledgements
We would like to thank Hannes Tschofenig and Yaron Sheffer for their
comments about Session Resumption.
Others who have contributed valuable comments are, in alphabetical
order, Lakshminath Dondeti, Paul Hoffman, Tero Kivinen, Scott C
Moonen, Magnus Nystrom, and Keith Welter.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
12.2. Informative References
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
January 2010.
[RFC6027] Nir, Y., "IPsec Cluster Problem Statement", RFC 6027,
October 2010.
[recovery] Detienne, F., Sethi, P., and Y. Nir, "Safe IKE Recovery",
Work in Progress, July 2009.
Appendix A. The Path Not Taken
A.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 piggybacked 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.
A.2. SIR
Another proposal that was considered for this work item is the SIR
extension, which is described in [recovery]. 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.
A.3. 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.
A.4. Reducing Liveness Check Length
Some implementations require 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, we believe that reducing the elapsed time
and retransmission count may 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 (i.e., is
able to respond with an INVALID_SPI or INVALID_IKE_SPI notification)
and until the tunnels are active, or from the time the backup gateway
has taken over until the tunnels are active. The use of QCD tokens
can reduce this delay.
Authors' Addresses
Yoav Nir (editor)
Check Point Software Technologies, Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
EMail: ynir@checkpoint.com
David Wierbowski
International Business Machines
1701 North Street
Endicott, New York 13760
United States
EMail: wierbows@us.ibm.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