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This document describes an extension to the IKEv2 protocol that allows for faster detection of SA desynchronization using a saved token.
When an IPsec tunnel between two IKEv2 peers is disconnected due to a restart of one peer, it can take as much as several minutes for the other peer to discover that the reboot has occurred, thus delaying recovery. In this text we propose an extension to the protocol, that allows for recovery immediately following the restart.
1.
Introduction
1.1.
Conventions Used in This Document
2.
RFC 4306 Crash Recovery
3.
Protocol Outline
4.
Formats and Exchanges
4.1.
Notification Format
4.2.
Passing a Token in the AUTH Exchange
4.3.
Replacing Tokens After Rekey or Resumption
4.4.
Presenting the Token in an INFORMATIONAL Exchange
5.
Token Generation and Verification
5.1.
A Stateless Method of Token Generation
5.2.
Token Lifetime
6.
Backup Gateways
7.
Alternative Solutions
7.1.
Initiating a new IKE SA
7.2.
Birth Certificates
8.
Interaction with Session Resumption
9.
Operational Considerations
9.1.
Who should implement this specification
9.2.
Response to unknown child SPI
10.
Security Considerations
10.1.
QCD Token Handling
10.2.
QCD Token Transmission
11.
IANA Considerations
12.
Acknowledgements
13.
Change Log
13.1.
Changes from draft-nir-ike-qcd-01
13.2.
Changes from draft-nir-ike-qcd-00
13.3.
Changes from draft-nir-qcr-00
14.
References
14.1.
Normative References
14.2.
Informative References
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
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IKEv2, as described in [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) 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 (RFC 4306 Crash Recovery) 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 (Protocol Outline).
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
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.
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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] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). 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] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). 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.
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Supporting implementations will send a notification, called a "QCD token", as described in Section 4.1 (Notification Format) 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 (Token Generation and Verification). 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 9.1 (Who should implement this specification) 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 4.4 (Presenting the Token in an INFORMATIONAL Exchange).
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 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] (Sheffer, Y., Tschofenig, H., Dondeti, L., and V. Narayanan, “IPsec Gateway Failover Protocol,” July 2008.). See Section 8 (Interaction with Session Resumption) for a short discussion about this protocol's interaction with session resumption.
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The notification payload called "QCD token" is formatted as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload !C! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol ID ! SPI Size ! QCD Token Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ TOKEN_SECRET_DATA ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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For clarity, only the EAP version of an AUTH exchange will be presented here. The non-EAP version is very similar. The figures below are based on appendix A.3 of [RFC4718] (Eronen, P. and P. Hoffman, “IKEv2 Clarifications and Implementation Guidelines,” October 2006.).
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.
The QCD_TOKEN notification is related to the IKE SA and MUST follow the AUTH payload and precede the Configuration payload and all payloads related to the child SA.
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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 before 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 stateless token.
request --> SA, Ni, [KEi] response <-- SA, Nr, [KEr], N(QCD_TOKEN)
If the initiator is also a token maker, it SHOULD soon initiate an INFORMATIONAL exchange as follows:
request --> N(QCD_TOKEN) response <--
For session resumption, as specified in [resumption] (Sheffer, Y., Tschofenig, H., Dondeti, L., and V. Narayanan, “IPsec Gateway Failover Protocol,” July 2008.), 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.
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This QCD_TOKEN notification is unprotected, and is sent as a response to a protected IKE request, which uses an IKE SA that is unknown.
request --> N(INVALID_IKE_SPI), N(QCD_TOKEN)+
If child SPIs are persistently mapped to IKE SPIs as described in Section 9.2 (Response to unknown child SPI), we may get the following exchange in response to an ESP or AH packet.
request --> 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 [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), The IKE SPI and message ID fields in the packet headers are taken from the protected IKE request.
To support a periodic rollover of the secret used for token generation, the token taker MUST support at least four QCD_TOKEN notifications in a single packet. The token is considered verified if any of the QCD_TOKEN notifications matches. The token maker MAY generate up to four QCD_TOKEN notifications, based on several generations of keys.
If the QCD_TOKEN verifies OK, an empty response MUST be sent. If the QCD_TOKEN cannot be validated, a response SHOULD NOT be sent. Section 5 (Token Generation and Verification) defines token verification.
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No token generation method is mandated by this document. A method is documented in Section 5.1 (A Stateless Method of Token Generation), but only serves as an example.
The following lists the requirements from a token generation mechanism:
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This describes a stateless method of generating a token:
TOKEN_SECRET_DATA = HASH(QCD_SECRET | SPI-I | SPI-R)
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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).
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Making crash detection and recovery quick is a worthy goal, but since rebooting a gateway takes a non-zero amount of time, many implementations choose to have a stand-by gateway ready to take over as soon as the primary gateway fails for any reason.
If such a configuration is available, it is RECOMMENDED that the stand-by gateway be able to generate the same token as the active gateway. if the method described in Section 5.1 (A Stateless Method of Token Generation) 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.
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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.
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Birth Certificates is a method of crash detection that has never been formally defined. Bill Sommerfeld suggested this idea in a mail to the IPsec mailing list on August 7, 2000, in a thread discussing methods of crash detection:
If we have the system sign a "birth certificate" when it
reboots (including a reboot time or boot sequence number),
we could include that with a "bad spi" ICMP error and in
the negotiation of the IKE SA.
We believe that this method would have some problems. First, it requires Alice to store the certificate, so as to be able to compare the public keys. That requires more storage than does a QCD token. Additionally, the public-key operations needed to verify the self-signed certificates are more expensive for Alice.
We believe that a symmetric-key operation such as proposed here is more light-weight and simple than that implied by the Birth Certificate idea.
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Session Resumption, specified in [resumption] (Sheffer, Y., Tschofenig, H., Dondeti, L., and V. Narayanan, “IPsec Gateway Failover Protocol,” July 2008.) proposes to make setting up a new IKE SA consume less computing resources. This is particularly useful in the case of a remote access gateway that has many tunnels. A failure of such a gateway would require all these many remote access clients to establish an IKE SA either with the rebooted gateway or with a backup gateway. This tunnel re-establishment should occur within a short period of time, creating a burden on the remote access gateway. Session Resumption addresses this problem by having the clients store an encrypted derivative of the IKE SA for quick re-establishment.
What Session Resumption does not help, is the problem of detecting that the peer gateway has failed. A failed gateway may go undetected for as long as the lifetime of a child SA, because IPsec does not have packet acknowledgement, and applications cannot signal the IPsec layer that the tunnel "does not work". Before establishing a new IKE SA using Session Resumption, a client 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 Session Resumption ticket, if provided by the gateway. A remote access gateway conforming to both specifications will generate a QCD token for the client. When the gateway reboots, the client will discover this in either of two ways:
The 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)]}
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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 (Backup Gateways). 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:
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] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) and either send an INVALID_IKE_SPI notification, or ignore it entirely.
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After a reboot, it is more likely that an implementation receives IPsec packets than IKE packets. In that case, the rebooted implementation will send an INVALID_SPI notification, triggering a liveness check. The token will only be sent in a response to the liveness check, thus requiring an extra round-trip.
To avoid this, an implementation that has access to non-volatile storage MAY store a mapping of child SPIs to owning IKE SPIs, or to generated toekns. 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 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.
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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 5.1 (A Stateless Method of Token Generation) 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 (RFC 4306 Crash Recovery). Replay is also meaningless, because the IKE SA has been deleted after the first transmission.
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A token maker MUST NOT send a QCD token in an unprotected message for an existing IKE SA. This implies that a conforming QCD token maker MUST be able to tell whether a particular pair of IKE SPIs represent a valid IKE SA.
This requirement is obvious and easy in the case of a single gateway. However, some implementations use a load balancer to divide the load between several physical gateways. It MUST NOT be possible even in such a configuration to trick one gateway into sending a QCD token for an IKE SA which is valid on another gateway.
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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".
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We would like to thank Hannes Tschofenig and Yaron Sheffer for their comments about Session Resumption.
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This section lists all changes in this document
NOTE TO RFC EDITOR : Please remove this section in the final RFC
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| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC4306] | Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” RFC 4306, December 2005 (TXT, HTML, XML). |
| [RFC4718] | Eronen, P. and P. Hoffman, “IKEv2 Clarifications and Implementation Guidelines,” RFC 4718, October 2006 (TXT, HTML, XML). |
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| [recovery] | Detienne, F., Sethi, P., and Y. Nir, “Safe IKE Recovery,” draft-detienne-ikev2-recovery (work in progress), July 2008 (TXT, HTML). |
| [resumption] | Sheffer, Y., Tschofenig, H., Dondeti, L., and V. Narayanan, “IPsec Gateway Failover Protocol,” draft-sheffer-ipsec-failover (work in progress), July 2008 (TXT, HTML). |
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| Yoav Nir | |
| Check Point Software Technologies Ltd. | |
| 5 Hasolelim st. | |
| Tel Aviv 67897 | |
| Israel | |
| Email: | ynir@checkpoint.com |
| Frederic Detienne | |
| Cisco Systems, Inc. | |
| De Kleetlaan, 7 | |
| Diegem B-1831 | |
| Belgium | |
| Phone: | +32 2 704 5681 |
| Email: | fd@cisco.com |
| Pratima Sethi | |
| Cisco Systems, Inc. | |
| O'Shaugnessy Road, 11 | |
| Bangalore, Karnataka 560027 | |
| India | |
| Phone: | +91 80 4154 1654 |
| Email: | psethi@cisco.com |
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