Network Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track April 2, 2008
Expires: October 4, 2008
A Quick Crash Detection Method for IKE
draft-nir-ike-qcd-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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 within a few
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seconds of the reboot.
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. Formats and Exchanges . . . . . . . . . . . . . . . . . . . . 4
4.1. Notification Format . . . . . . . . . . . . . . . . . . . 5
4.2. Authentication Exchange . . . . . . . . . . . . . . . . . 5
4.3. Informational Exchange . . . . . . . . . . . . . . . . . . 7
5. Token Generation and Verification . . . . . . . . . . . . . . 7
5.1. A Stateful Method of Token Generation . . . . . . . . . . 7
5.2. A Stateless Method of Token Generation . . . . . . . . . . 8
5.3. Token Lifetime . . . . . . . . . . . . . . . . . . . . . . 8
6. Backup Gateways . . . . . . . . . . . . . . . . . . . . . . . 8
7. Alternative Solutions . . . . . . . . . . . . . . . . . . . . 8
7.1. Why not Save the Entire IKE SA . . . . . . . . . . . . . . 8
7.2. Initiating a new IKE SA . . . . . . . . . . . . . . . . . 9
8. Interaction with IFARE . . . . . . . . . . . . . . . . . . . . 9
9. Operational Considerations . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 13
13.1. Changes from draft-nir-qcr-00 . . . . . . . . . . . . . . 13
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
14.1. Normative References . . . . . . . . . . . . . . . . . . . 14
14.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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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 re-establish the tunnels.
Section 2 describes the current procedure, and explains why crash
recovery can take up to several minutes. The method proposed here,
is to send a token in the IKE_AUTH exchange that establishes the
tunnel. That token can be maintained on the peer in some kind of
persistent storage such as a disk or a database, and can be used to
delete the IKE SA on the non-rebooted peer after a crash. Deleting
the IKE SA results is a quick re-establishment of the IPsec tunnel.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. 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
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 INFORMATIONAL 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
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significant amount of time.
3. Protocol Outline
Supporting implementations will send a notification, called a "QCD
token", as described in Section 4.1 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 5. Implementations that send such a token will be called
"token makers".
A supporting implementation receiving such a token SHOULD store it in
such a way, that it will survive a reboot. If the implementation is
part of a configuration where there is a backup gateway as described
in Section 6 (such configurations are often referred to as high-
availability), then the persistent storage module SHOULD be
accessible to all implementations within the configuration. An
implementation supporting this part of the protocol will be called
"token taker".
When a token taker receives a protected IKE request message with
unknown IKE SPIs, it MUST scan its saved token store. If a token
matching the IKE SPIs is found, it SHOULD be sent to the requesting
peer in an unprotected IKE message as described in Section 4.3.
When a token maker receives the QCD token in an unprotected
notification, it MUST verify that the TOKEN_SECRET_DATA field is
associated with the IKE SPIs in the IKE_SPI fields of the IKE packet.
If the verification fails, 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 maker
MUST accept such tokens from any address, so as to allow different
kinds of high-availability configuration of the token taker.
A supporting implementation 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 protocol's interaction
with session resumption.
4. Formats and Exchanges
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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 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 5.
4.2. 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+]
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+]
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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4.3. Informational Exchange
This informational exchange is non-protected, and is sent as a
response to a protected IKE request, which uses an IKE SA that is
unknown.
request --> N(QCD_TOKEN)
response <--
The QCD_TOKEN is the only notification in the request. 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.
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 5 defines token verification.
5. Token Generation and Verification
No token generation method is mandated by this document. Two methods
are documented in Section 5.1 and Section 5.2, but they only serve as
examples.
The following lists the requirements from a token generation
mechanism:
o Tokens should be at least 16 octets log, and no more than 256
octets long, to facilitate storage.
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 peer that generated the QCD token, should be able to
immediately verify it, provided that the IKE SPIs are given, and
that the IKE SA has not expired or been otherwise deleted.
5.1. A Stateful Method of Token Generation
This describes a stateful method of generating a token:
o Before sending the QCD token, 32 random octets are generated using
a secure random number generator or a PRNG.
o Those 32 bytes are used as the TOKEN_SECRET_DATA field, and stored
as part of the IKE SA.
o For verification, the IKE implementation simply retrieves the IKE
SA, and compares the TOKEN_SECRET_DATA field from the notification
to the TOKEN_SECRET_DATA field stored with the SA.
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5.2. A Stateless Method of Token Generation
This describes a stateless method of generating a token.
o At startup, the IKE implementation generates a 32-octet random
buffer using a cryptographically secure PRNG. This buffer is
called the QCD_SECRET.
o For each QCD token, the TOKEN_SECRET_DATA field is generated by
calculating a SHA-256 hash over a concatenation of the QCD_SECRET
and the IKE SPI as follows:
TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R)
o Verification uses the same calculation, and works even if the IKE
SA has been deleted. Still, if the IKE SA is no longer valid, the
notification MUST NOT be acknowledged, as this could be used in an
attempt to guess the QCD_SECRET.
5.3. Token Lifetime
The token is associated with a single IKE SA, and SHOULD be deleted
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 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
persistent storage be shared between the primary and backup gateway.
This has the effect of having the crash recovery available
immediately. This recommendation is especially useful if the primary
and backup gateway either share an external IP address or reside on
the same LAN. If they are geographically remote, this may be less
practical.
7. Alternative Solutions
7.1. Why not Save the Entire IKE SA
IKEv2 does not assume the existence of a persistent storage module.
If we are adding such a module, why not use it to save the entire IKE
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SA across reboots, nullifying the need for a crash recovery
procedure?
There are several reasons why we believe that this is not a good
idea:
1. A token is only 16-256 octets, and is much more compact than all
the data needed to store an IKE SA.
2. A token is valid for the life of an IKE SA. An IKE SA state is
updated whenever a message is sent, because of the requirement to
maintain the sequence of message IDs. It may not be acceptable
to update the persistent storage whenever an IKE message is sent.
3. A reboot is usually an unpredictable event, and as such, we
cannot know how long it will last. By the time the machine has
rebooted, the peer may have attempted some type of protected
exchange (liveness check, create-child-SA or delete), timed out,
and deleted the SA. It is far better to reboot without SAs and
with only a token for quick recovery.
7.2. 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. 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.
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What IFARE does not help, is the problem of detecting that the peer
gateway has failed. A failed gateway may go undetected for an
unbounded amount of time, 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
generate QCD tokens, and store the IFARE state, if provided by the
gateway. A remote access gateway conforming to both specifications
will store the QCD token sent from 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. The IKE times out after
several minutes, if the gateway does not finish rebooting in
time. 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, the backup gateway as described in
this document or another gateway as described in [resumption]
The full combined protocol looks like this:
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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)]}
9. Operational Considerations
To support "token taker" part of this standard, an implementation
needs to have access to a persistent storage module. This could be
an internal hard disk, a local or remote database application, or any
other method that persists across reboots. This storage module and
the data links between the storage module and the IKE module must
meet the performance requirements of the IKE module. The storage
module MUST support insertion and deletion rates equal to peek IKE SA
setup rates and it SHOULD support query rates that are fast enough.
See Section 10 for security considerations for this storage
mechanism.
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
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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 6.
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 taker" 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 maker" 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 maker and MAY be a token
taker.
o A remote-access gateway MAY be a token maker and MUST be a token
taker.
o An inter-domain VPN gateway MUST be both token maker and token
taker.
In order to limit the effects of DoS attacks, an implementation
SHOULD limit the rate of queries into the token storage so as not to
overload it. If excessive amounts of IKE requests protected with
unknown IKE SPIs arrive, 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. Security Considerations
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
back in an informational exchange it is not encrypted, but that is
the last use of that token.
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 5.2 needs to be long.
The persistent storage MUST be protected from access by other
parties. Anyone gaining access to the contents of the storage will
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be able to delete all the IKE SAs described in it.
The tokens associated with expired and deleted IKE SAs MUST be
deleted from the storage, so that a future compromise of the storage
does not reveal enough tokens to facilitate an attack against the QCD
tokens.
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. 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".
12. Acknowledgements
We would like to thank Hannes Tschofenig and Yaron Sheffer for their
comments about IFARE.
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-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
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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.
[RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006.
14.2. Informative References
[resumption]
Sheffer, Y., Tschofenig, H., Dondeti, L., and V.
Narayanan, "IPsec Gateway Failover Protocol",
draft-sheffer-ipsec-failover-03 (work in progress),
March 2008.
Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
Email: ynir@checkpoint.com
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