IKEv2 Session Resumption
draft-ietf-ipsecme-ikev2-resumption-03.txt

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Abstract

The Internet Key Exchange version 2 (IKEv2) protocol has a certain computational and communication overhead with respect to the number of round-trips required and the cryptographic operations involved. In remote access situations, the Extensible Authentication Protocol (EAP) is used for authentication, which adds several more round trips and consequently latency.

To re-establish security associations (SAs) upon a failure recovery condition is time consuming especially when an IPsec peer (such as a VPN gateway) needs to re-establish a large number of SAs with various end points. A high number of concurrent sessions might cause additional problems for an IPsec peer during SA re-establishment.

In order to avoid the need to re-run the key exchange protocol from scratch it would be useful to provide an efficient way to resume an IKE/IPsec session. This document proposes an extension to IKEv2 that allows a client to re-establish an IKE SA with a gateway in a highly efficient manner, utilizing a previously established IKE SA.

A client can reconnect to a gateway from which it was disconnected. The proposed approach requires passing opaque data from the IKEv2 responder to the IKEv2 initiator, which is later made available to the IKEv2 responder for re-authentication. We use the term ticket to refer to the opaque data that is created by the IKEv2 responder. This document does not specify the format of the ticket but recommendations are provided.

Table of Contents

1.  Introduction
2.  Terminology
3.  Usage Scenario
4.  Protocol Details
    4.1.  Requesting a Ticket
    4.2.  Receiving a Ticket
    4.3.  Presenting a Ticket
    4.4.  IKE_SESSION_RESUME Details
    4.5.  Requesting a Ticket During Resumption
    4.6.  IP Address Change and NAT
    4.7.  IKE Notifications
        4.7.1.  TICKET_LT_OPAQUE Notify Payload
        4.7.2.  TICKET_OPAQUE Notify Payload
    4.8.  Computing the AUTH Payload
5.  Processing Guidelines for IKE SA Establishment
6.  The State After Resumption
7.  Ticket Handling
    7.1.  Ticket Content
    7.2.  Ticket Identity and Lifecycle
8.  IANA Considerations
9.  Security Considerations
    9.1.  Stolen Tickets
    9.2.  Forged Tickets
    9.3.  Denial of Service Attacks
    9.4.  Key Management for Tickets By Value
    9.5.  Ticket Lifetime
    9.6.  Ticket by Value Format
    9.7.  Identity Privacy, Anonymity, and Unlinkability
10.  Acknowledgements
11.  References
    11.1.  Normative References
    11.2.  Informative References
Appendix A.  Ticket Format
    A.1.  Example Ticket by Value Format
    A.2.  Example Ticket by Reference Format
Appendix B.  Change Log
    B.1.  -03
    B.2.  -02
    B.3.  -01
    B.4.  -00
    B.5.  -01
    B.6.  -00
    B.7.  -04
    B.8.  -03
    B.9.  -02
    B.10.  -01
    B.11.  -00
§  Authors' Addresses

1.  Introduction

The Internet Key Exchange version 2 (IKEv2) protocol has a certain computational and communication overhead with respect to the number of round-trips required and the cryptographic operations involved. In particular the Extensible Authentication Protocol (EAP) is used for authentication in remote access cases, which increases latency.

To re-establish security associations (SA) upon a failure recovery condition is time-consuming, especially when an IPsec peer, such as a VPN gateway, needs to re-establish a large number of SAs with various end points. A high number of concurrent sessions might cause additional problems for an IPsec responder.

In many failure cases it would be useful to provide an efficient way to resume an interrupted IKE/IPsec session. This document proposes an extension to IKEv2 that allows a client to re-establish an IKE SA with a gateway in a highly efficient manner, utilizing a previously established IKE SA.

A client can reconnect to a gateway from which it was disconnected. One way to ensure that the IKEv2 responder is able to recreate the state information is by maintaining IKEv2 state (or a reference into a state store) in a "ticket", an opaque data structure. This ticket is created by the server and forwarded to the client. The IKEv2 protocol is extended to allow a client to request and present a ticket. This document does not mandate the format of the ticket structure but a recommendation is provided. In Appendix A (Ticket Format) a ticket by value and a ticket by reference format is proposed.

This approach is similar to the one taken by TLS session resumption [RFC5077] (Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, “Transport Layer Security (TLS) Session Resumption without Server-Side State,” January 2008.) with the required adaptations for IKEv2, e.g., to accommodate the two-phase protocol structure. We have borrowed heavily from that specification.

The proposed solution should additionally meet the following goals:

• Using only symmetric cryptography to minimize CPU consumption.
• Providing cryptographic agility.
• Having no negative impact on IKEv2 security features.

The following are non-goals of this solution:

• Failover from one gateway to another. This use case may be added in a future specification.
• Providing load balancing among gateways.
• Specifying how a client detects the need for resumption.

2.  Terminology

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.).

This document uses terminology defined in [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) and [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). In addition, this document uses the following terms:

Ticket:

An IKEv2 ticket is a data structure that contains all the necessary information that allows an IKEv2 responder to re-establish an IKEv2 security association.

In this document we use the term "ticket" and thereby refer to an opaque data structure that may either contain IKEv2 state as described above or a reference pointing to such state.

3.  Usage Scenario

This specification envisions two usage scenarios for efficient IKEv2 and IPsec SA session re-establishment.

The first is similar to the use case specified in Section 1.1.3 of the IKEv2 specification [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), where the IPsec tunnel mode is used to establish a secure channel between a remote access client and a gateway; the traffic flow may be between the client and entities beyond the gateway. This scenario is further discussed below.

The second use case focuses on the usage of transport (or tunnel) mode to secure the communicate between two end points (e.g., two servers). The two endpoints have a client-server relationship with respect to a protocol that runs using the protections afforded by the IPsec SA.

 (a)

 +-+-+-+-+-+                          +-+-+-+-+-+
 !         !      IKEv2/IKEv2-EAP     !         !     Protected
 ! Remote  !<------------------------>!         !     Subnet
 ! Access  !                          ! Access  !<--- and/or
 ! Client  !<------------------------>! Gateway !     Internet
 !         !      IPsec tunnel        !         !
 +-+-+-+-+-+                          +-+-+-+-+-+


 (b)

 +-+-+-+-+-+                          +-+-+-+-+-+
 !         !    IKE_SESSION_RESUME    !         !
 ! Remote  !<------------------------>!         !
 ! Access  !                          ! Access  !
 ! Client  !<------------------------>! Gateway !
 !         !      IPsec tunnel        !         !
 +-+-+-+-+-+                          +-+-+-+-+-+

In the first use case above, an end host (an entity with a host implementation of IPsec [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.)) establishes a tunnel mode IPsec SA with a gateway in a remote network using IKEv2. The end host in this scenario is sometimes referred to as a remote access client. At a later stage when a client needs to re-establish the IKEv2 session it may choose to establish IPsec SAs using a full IKEv2 exchange or the IKE_SESSION_RESUME exchange (shown in Figure 1 (Resuming a Session with a Remote Access Gateway)).

4.  Protocol Details

This section provides protocol details and contains the normative parts. This document defines two protocol exchanges, namely requesting a ticket, see Section 4.1 (Requesting a Ticket), and presenting a ticket, see Section 4.3 (Presenting a Ticket).

4.1.  Requesting a Ticket

A client MAY request a ticket in the following exchanges:

• In an IKE_AUTH exchange, as shown in the example message exchange in Figure 2 (Example Message Exchange for Requesting a Ticket) below.
• In a CREATE_CHILD_SA exchange, when an IKE SA is rekeyed (and only when this exchange is initiated by the client).
• In an Informational exchange at any time, e.g. if the gateway previously replied with an N(TICKET_ACK) instead of providing a ticket, or when the ticket lifetime is about to expire. All such Informational exchanges MUST be initiated by the client.
• While resuming an IKE session, i.e. in the IKE_AUTH exchange that follows an IKE_SESSION_RESUME exchange, see Section 4.5 (Requesting a Ticket During Resumption).

Normally, a client requests a ticket in the third message of an IKEv2 exchange (the first of IKE_AUTH). Figure 2 (Example Message Exchange for Requesting a Ticket) shows the message exchange for this typical case.

  Initiator                Responder
  -----------              -----------
 HDR, SAi1, KEi, Ni  -->

                     <--    HDR, SAr1, KEr, Nr, [CERTREQ]

 HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,]
 AUTH, SAi2, TSi, TSr, N(TICKET_REQUEST)}     -->

The notification payloads are described in Section 4.7 (IKE Notifications). The above is an example, and IKEv2 allows a number of variants on these messages. Refer to [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) and [I‑D.ietf‑ipsecme‑ikev2bis] (Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen, “Internet Key Exchange Protocol: IKEv2,” April 2010.) for more details on IKEv2.

When an IKEv2 responder receives a request for a ticket using the N(TICKET_REQUEST) payload it MUST perform one of the following operations if it supports the extension defined in this document:

• it creates a ticket and returns it with the N(TICKET_LT_OPAQUE) payload in a subsequent message towards the IKEv2 initiator. This is shown in Figure 3 (Receiving a Ticket).
• it returns an N(TICKET_NACK) payload, if it refuses to grant a ticket for some reason.
• it returns an N(TICKET_ACK), if it cannot grant a ticket immediately, e.g., due to packet size limitations. In this case the client MAY request a ticket later using an Informational exchange, at any time during the lifetime of the IKE SA.

Regardless of this choice, there is no change to the behavior of the responder with respect to the IKE exchange, and the proper IKE response (e.g. an IKE_AUTH response or an error notification) MUST be sent.

4.2.  Receiving a Ticket

The IKEv2 initiator receives the ticket and may accept it, provided the IKEv2 exchange was successful. The ticket may be used later with an IKEv2 responder that supports this extension. Figure 3 (Receiving a Ticket) shows how the initiator receives the ticket.

  Initiator                Responder
  -----------              -----------
         <--    HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi,
                     TSr, N(TICKET_LT_OPAQUE) }

When a multi-round-trip IKE_AUTH exchange is used, the N(TICKET_REQUEST) payload MUST be included in the first IKE_AUTH request, and N(TICKET_LT_OPAQUE) (or TICKET_NACK/TICKET_ACK) MUST only be returned in the final IKE_AUTH response.

4.3.  Presenting a Ticket

A client MAY initiate a regular (non-ticket-based) IKEv2 exchange even if it is in possession of a valid ticket. Note that the client can only judge validity in the sense of the ticket lifetime. A client MUST NOT present a ticket when it knows that the ticket's lifetime has expired.

It is up to the client's local policy to decide when the communication with the IKEv2 responder is seen as interrupted and the session resumption procedure is to be initiated.

Tickets are intended for one-time use, i.e. a client MUST NOT reuse a ticket. A reused ticket SHOULD be rejected by a gateway. Note that a ticket is considered as used only when an IKE SA has been established successfully with it.

This document specifies a new IKEv2 exchange type called IKE_SESSION_RESUME whose value is TBA by IANA. This exchange is equivalent to the IKE_SA_INIT exchange, and MUST be followed by an IKE_AUTH exchange. The client SHOULD NOT use this exchange type unless it knows that the gateway supports it.

 Initiator                Responder
 -----------              -----------
 HDR, Ni, N(TICKET_OPAQUE) [,N+]   -->

The exchange type in HDR is set to 'IKE_SESSION_RESUME'. The initiator sets the SPIi value in the HDR to a new random value and the SPIr value is set to 0.

When the IKEv2 responder receives a ticket using the N(TICKET_OPAQUE) payload it MUST perform one of the following steps if it supports the extension defined in this document:

• If it is willing to accept the ticket, it responds as shown in Figure 4 (IKEv2 Responder accepts the ticket).
• It responds with an unprotected N(TICKET_NACK) notification, if it rejects the ticket for any reason. In that case, the initiator should re-initiate a regular IKE exchange. One such case is when the responder receives a ticket for an IKE SA that has previously been terminated on the responder itself, which may indicate inconsistent state between the IKEv2 initiator and the responder. However, a responder is not required to maintain the state for terminated sessions.

 Initiator                Responder
 -----------              -----------
                 <--  HDR, Nr [,N+]

Again, the exchange type in HDR is set to 'IKE_SESSION_RESUME'. The responder copies the SPIi value from the request, and the SPIr value is set to a new random value .

At this point the client MUST initiate an IKE_AUTH exchange, as per [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). See Section 4.8 (Computing the AUTH Payload) for guidelines on computing the AUTH payloads. The IDi value sent in this exchange MUST be identical to the value included in the ticket. Following this exchange, a new IKE SA is created by both parties, see Section 5 (Processing Guidelines for IKE SA Establishment), and a child SA is derived, per Section 2.17 of [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

When the responder receives a ticket for an IKE SA that is still active and if the responder accepts it, then the old SA SHOULD be silently deleted without sending a DELETE informational exchange. Consequently, all the dependent IPsec child SAs are also deleted. This happens after both peers have been authenticated.

4.4.  IKE_SESSION_RESUME Details

The IKE_SESSION_RESUME exchange behaves like the IKE_SA_INIT exchange in most respects. Specifically:

• The first message may be rejected in denial of service situations, with the initiator instructed to send a cookie.
• Notifications normally associated with IKE_SA_INIT can be sent. In particular, NAT detection payloads.
• The SPI values and Message ID fields behave similarly to IKE_SA_INIT.

4.5.  Requesting a Ticket During Resumption

When resuming a session, a client will typically request a new ticket immediately, so it is able to resume the session again in the case of a second failure. The N(TICKET_REQUEST) and N(TICKET_LT_OPAQUE) notifications will be included in the IKE_AUTH exchange that follows the IKE_SESSION_RESUME exchange, with similar behavior to a ticket request during a regular IKE exchange, Section 4.1 (Requesting a Ticket).

The returned ticket (if any) will correspond to the IKE SA created per the rules described in Section 5 (Processing Guidelines for IKE SA Establishment).

4.6.  IP Address Change and NAT

The client MAY resume the IKE exchange from an IP address different from its original address. The gateway MAY reject the resumed exchange if its policy depends on the client's address (although this rarely makes sense).

The client's NAT traversal status SHOULD be determined anew upon session resumption, by using the appropriate notifications. This status is explicitly not part of the session resumption state.

4.7.  IKE Notifications

This document defines a number of notifications. The notification numbers are TBA by IANA.

For all these notifications, the Protocol ID and the SPI Size fields MUST both be sent as 0.

4.7.1.  TICKET_LT_OPAQUE Notify Payload

The data for the TICKET_LT_OPAQUE Notify payload consists of the Notify message header, a Lifetime field and the ticket itself. The four octet Lifetime field contains a relative time value, the number of seconds until the ticket expires (encoded as an unsigned integer).

       0                     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 = 0  !    Notify Message Type        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                       Lifetime                                !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                        Ticket                                 ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.7.2.  TICKET_OPAQUE Notify Payload

The data for the TICKET_OPAQUE Notify payload consists of the Notify message header, and the ticket itself. Unlike the TICKET_LT_OPAQUE payload no lifetime value is included in the TICKET_OPAQUE Notify payload.

       0                     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 = 0  !    Notify Message Type        !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                                                               !
      ~                        Ticket                                 ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.8.  Computing the AUTH Payload

The value of the AUTH payload is derived in a manner similar to the usage of IKEv2 pre-shared secret authentication, as shown below:

         AUTH = prf(SK_px, <msg octets>)

Each of the initiator and responder uses its own SK_p value, taken from the newly generated IKE SA, Section 5 (Processing Guidelines for IKE SA Establishment).

The exact material to be signed is defined in Section 2.15 of [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.). The notation "IDr'" in RFC 4306 should be applied to the new IDr value included in the exchange, rather than the value in the ticket.

5.  Processing Guidelines for IKE SA Establishment

When a ticket is presented, the gateway needs to obtain the ticket state. In case a ticket by reference was provided by the client, the gateway needs to resolve the reference in order to obtain this state. In case the client has already provided a ticket per value, the gateway can parse the ticket to obtain the state directly. In either case, the gateway needs to process the ticket state in order to restore the state of the old IKE SA, and the client retrieves the same state from its local store. Both peers now create state for the new IKE SA as follows:

• The SA value (transforms etc.) is taken directly from the ticket.
• The Message ID values are reset to 0 in IKE_SESSION_RESUME, and subsequently incremented normally.
• The IDi value is obtained from the ticket.
• The IDr value is obtained from the new exchange. The gateway MAY make policy decisions based on the IDr value encoded in the ticket.
• The SPI values are created anew during IKE_SESSION_RESUME, similarly to a regular IKE_SA_INIT exchange. SPI values from the ticket MUST NOT be reused, and they are sent merely to help the gateway to locate the old state. The restriction on SPI reuse is to avoid problems caused by collisions with other SPI values used already by the initiator/responder.

The cryptographic material is refreshed based on the ticket and the nonce values, Ni, and Nr, from the current exchange. A new SKEYSEED value is derived as follows:

     SKEYSEED = prf(SK_d_old, "Resumption" | Ni | Nr)

where SK_d_old is taken from the ticket. The literal string is encoded as 10 ASCII characters, with no NULL terminator.

The keys are derived as follows, unchanged from IKEv2:

     {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr} =
                                 prf+(SKEYSEED, Ni | Nr | SPIi | SPIr)

where SPIi, SPIr are the SPI values created in the new IKE exchange.

See [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) for the notation. "prf" is determined from the SA value in the ticket.

6.  The State After Resumption

The following table, compiled by Pasi Eronen, describes the IKE and IPsec state of the peers after session resumption, and how it is related to their state before the IKE SA was interrupted. When the table mentions that a certain state item is taken "from the ticket", this should be construed as:

• The client retrieves this item from its local store.
• In the case of ticket by value, the gateway encodes this information in the ticket.
• In the case of ticket by reference, the gateway fetches this information from the ticket store.

Note 1:

Certificates don't need to be stored if the peer never uses them for anything after the IKE SA is up (but would be needed if exposed to applications via IPsec APIs).

Note 2:

If the certificate has an iPAddress SubjectAltName, and the implementation requires it to match the peer's source IP address, the same check needs to be performed on session resumption and the required information saved locally or in the ticket.

Note 3:

The client can request the address it was using earlier, and if possible, the gateway SHOULD honor the request.

Note 4:

IKEv2 features that affect only the IKE_AUTH exchange (including HTTP_CERT_LOOKUP_SUPPORTED, multiple authentication exchanges [RFC4739] (Eronen, P. and J. Korhonen, “Multiple Authentication Exchanges in the Internet Key Exchange (IKEv2) Protocol,” November 2006.), ECDSA authentication [RFC4754] (Fu, D. and J. Solinas, “IKE and IKEv2 Authentication Using the Elliptic Curve Digital Signature Algorithm (ECDSA),” January 2007.), and OCSP [RFC4806] (Myers, M. and H. Tschofenig, “Online Certificate Status Protocol (OCSP) Extensions to IKEv2,” February 2007.)) don't usually need any state in the IKE SA (after the IKE_AUTH exchanges are done), so resumption doesn't affect them.

Note 5:

Since information about CHILD SAs and configuration payloads is not resumed, IKEv2 features related to CHILD SA negotiation (such as IPCOMP_SUPPORTED, ESP_TFC_PADDING_NOT_SUPPORTED, ROHC-over-IPsec [I‑D.ietf‑rohc‑ikev2‑extensions‑hcoipsec] (Ertekin, E., Christou, C., Jasani, R., Kivinen, T., and C. Bormann, “IKEv2 Extensions to Support Robust Header Compression over IPsec,” February 2010.) and configuration aren't usually affected by session resumption.

Note 6:

New IKEv2 features that are not covered by notes 4 and 5 should specify how they interact with session resumption.

7.  Ticket Handling

7.1.  Ticket Content

When passing a ticket by value to the client, the ticket content MUST be integrity protected and encrypted.

A ticket by reference does not need to be encrypted, as it does not contain any sensitive material, such as keying material. However, access to the storage where that sensitive material is stored MUST be protected so that only unauthorized access is not allowed. We note that such a ticket is analogous to the concept of 'stub', as defined in [I‑D.xu‑ike‑sa‑sync] (Xu, Y., Yang, P., Ma, Y., Deng, H., and H. Deng, “IKEv2 SA Synchronization for session resumption,” October 2008.), or the concept of a Session ID from TLS.

Although not strictly required for cryptographic protection, it is RECOMMENDED to integrity-protect the ticket by reference. Failing to do so could result in various security vulnerabilities on the gateway side, depending on the format of the reference. Potential vulnerabilities include access by the gateway to unintended URLs (similar to cross-site scripting) or SQL injection.

When the state is passed by value, the ticket MUST encode at least the following state from an IKE SA. The same state MUST be stored in the ticket store, in the case of ticket by reference.

• IDi, IDr.
• SPIi, SPIr.
• SAr (the accepted proposal).
• SK_d.

The ticket by value MUST include a key identity field, so that keys for encryption and authentication can be changed, and when necessary, algorithms can be replaced.

7.2.  Ticket Identity and Lifecycle

Each ticket is associated with a single IKE SA. In particular, when an IKE SA is deleted, the client MUST delete its stored ticket. Similarly, when credentials associated with the IKE SA are invalidated (e.g. when a user logs out), the ticket MUST be deleted. When the IKE SA is rekeyed the ticket is invalidated, and the client SHOULD request a new ticket.

The lifetime of the ticket sent by the gateway SHOULD be the minimum of the IKE SA lifetime (per the gateway's local policy) and its re-authentication time, according to [RFC4478] (Nir, Y., “Repeated Authentication in Internet Key Exchange (IKEv2) Protocol,” April 2006.). Even if neither of these are enforced by the gateway, a finite lifetime MUST be specified for the ticket.

The key that is used to protect the ticket MUST have a lifetime that is significantly longer than the lifetime of an IKE SA.

In normal operation, the client will request a ticket when establishing the initial IKE SA, and then every time the SA is rekeyed or re-established because of re-authentication.

8.  IANA Considerations

This document requires a number of IKEv2 notification status types in Section 4.7 (IKE Notifications), to be registered by IANA. The "IKEv2 Notify Message Types - Status Types" registry was established by IANA.

The document defines a new IKEv2 exchange in Section 4.3 (Presenting a Ticket). The corresponding registry was established by IANA.

9.  Security Considerations

This section addresses security issues related to the usage of a ticket.

9.1.  Stolen Tickets

An man-in-the-middle may try to eavesdrop on an exchange to obtain a ticket by value and use it to establish a session with the IKEv2 responder. This can happen in different ways: by eavesdropping on the initial communication and copying the ticket when it is granted and before it is used, or by listening in on a client's use of the ticket to resume a session. However, since the ticket's contents is encrypted and the attacker does not know the corresponding secret key, a stolen ticket cannot be used by an attacker to successfully resume a session. An IKEv2 responder MUST use strong encryption and integrity protection of the ticket to prevent an attacker from obtaining the ticket's contents, e.g., by using a brute force attack.

A ticket by reference does not need to be encrypted. When an adversary is able to eavesdrop on an exchange, as described in the previous paragraph, then the ticket by reference may be obtained. A ticket by reference cannot be used by an attacker to successfully resume a session, for the same reasons as for a ticket by value. Moreover, the adversary MUST NOT be able to resolve the ticket via the reference, i.e., access control MUST be enforced to ensure disclosure only to authorized entities.

9.2.  Forged Tickets

A malicious user could forge or alter a ticket by value in order to resume a session, to extend its lifetime, to impersonate as another user, or to gain additional privileges. This attack is not possible if the content of the ticket by value is protected using a strong integrity protection algorithm.

In case of a ticket by reference an adversary may attempt to construct a fake ticket by reference to point to state information stored by the IKEv2 responder. This attack will fail because the adversary is not in possession of the keying material associated with the IKEv2 SA. As noted in Section 7.1 (Ticket Content), it is often useful to integrity-protect the ticket by reference, too.

9.3.  Denial of Service Attacks

An adversary could generate and send a large number of tickets by value to a gateway for verification. To minimize the possibility of such denial of service, ticket verification should be lightweight (e.g., using efficient symmetric key cryptographic algorithms).

When an adversary chooses to send a large number of tickets by reference then this may lead to an amplification attack as the IKEv2 responder is forced to resolve the reference to a ticket in order to determine that the adversary is not in possession of the keying material corresponding to the stored state or that the reference is void. To minimize this attack, the protocol to resolve the reference should be as lightweight as possible. and should not generate a large number of messages.

9.4.  Key Management for Tickets By Value

A full description of the management of the keys used to protect the ticket by value is beyond the scope of this document. A list of RECOMMENDED practices is given below.

• The keys should be generated securely following the randomness recommendations in [RFC4086] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.).
• The keys and cryptographic protection algorithms should be at least 128 bits in strength.
• The keys should not be used for any other purpose than generating and verifying tickets.
• The keys should be changed regularly.
• The keys should be changed if the ticket format or cryptographic protection algorithms change.

9.5.  Ticket Lifetime

An IKEv2 responder controls the validity period of the state information by attaching a lifetime to a ticket. The chosen lifetime is based on the operational and security requirements of the environment in which this IKEv2 extension is deployed. The responder provides information about the ticket lifetime to the IKEv2 initiator, allowing it to manage its tickets.

9.6.  Ticket by Value Format

Great care must be taken when defining a ticket format such that the requirements outlined in Section 7.1 (Ticket Content) are met. In particular, if confidential information, such as a secret key, is transferred to the client it MUST be done using channel security to prevent attackers from obtaining or modifying the ticket. Also, the ticket by value MUST have its integrity and confidentiality protected with strong cryptographic techniques to prevent a breach in the security of the system.

9.7.  Identity Privacy, Anonymity, and Unlinkability

Since opaque state information is passed around between the IKEv2 initiator and the IKEv2 responder it is important that leakage of information, such as the identities of an IKEv2 initiator and a responder, MUST be avoided.

When an IKEv2 initiator presents a ticket as part of the IKE_SESSION_RESUME exchange, confidentiality is not provided for the exchange. There is thereby the possibility for an on-path adversary to observe multiple exchange handshakes where the same state information is used and therefore to conclude that they belong to the same communication end points.

This document therefore requires that the ticket be presented to the IKEv2 responder only once; under normal circumstances (e.g. no active attacker), there should be no multiple use of the same ticket.

10.  Acknowledgements

We would like to thank Paul Hoffman, Pasi Eronen, Florian Tegeler, Stephen Kent, Sean Shen, Xiaoming Fu, Stjepan Gros, Dan Harkins, Russ Housely, Yoav Nir and Tero Kivinen for their comments. We would like to particularly thank Florian Tegeler and Stjepan Gros for their help with their implementation efforts and Florian Tegeler for his formal verification using the CASPER tool set.

We would furthermore like to thank the authors of [I‑D.xu‑ike‑sa‑sync] (Xu, Y., Yang, P., Ma, Y., Deng, H., and H. Deng, “IKEv2 SA Synchronization for session resumption,” October 2008.)(Yan Xu, Peny Yang, Yuanchen Ma, Hui Deng and Ke Xu) for their input on the stub concept.

We would like to thank Hui Deng, Tero Kivinen, Peny Yang, Ahmad Muhanna and Stephen Kent for their feedback regarding the ticket by reference concept.

11.  References

11.1. Normative References

11.2. Informative References

Appendix A.  Ticket Format

This document does not specify a mandatory-to-implement or a mandatory-to-use ticket format. The formats described in the following sub-sections are provided as useful examples.

A.1.  Example Ticket by Value Format

struct {
    [authenticated] struct {
        octet format_version;    // 1 for this version of the protocol
        octet reserved[3];       // sent as 0, ignored by receiver.
        octet key_id[8];         // arbitrary byte string
        opaque IV[0..255];       // actual length (possibly 0) depends
                                 // on the encryption algorithm

        [encrypted] struct {
            opaque IDi, IDr;     // the full payloads
            octet SPIi[8], SPIr[8];
            opaque SA;           // the full SAr payload
            octet SK_d[0..255];  // actual length depends on SA value
            int32 expiration;    // an absolute time value, seconds
                                 // since Jan. 1, 1970
        } ikev2_state;
    } protected_part;
    opaque MAC[0..255];          // the length (possibly 0) depends
                                 // on the integrity algorithm
} ticket;

Note that the key defined by "key_id" determines the encryption and authentication algorithms used for this ticket. Those algorithms are unrelated to the transforms defined by the SA payload.

The reader is referred to [I‑D.rescorla‑stateless‑tokens] (Rescorla, E., “How to Implement Secure (Mostly) Stateless Tokens,” March 2007.) that recommends a similar (but not identical) ticket format, and discusses related security considerations in depth.

A.2.  Example Ticket by Reference Format

For implementations that prefer to pass a reference to IKE state in the ticket, rather than the state itself, we suggest the following format:

struct {
      [authenticated] struct {
          octet format_version;  // 1 for this version of the protocol
          octet reserved[3];     // sent as 0, ignored by receiver.
          octet key_id[8];       // arbitrary byte string

          struct {
              opaque state_ref;  // reference to IKE state
              int32 expiration;  // an absolute time value, seconds
                                 // since Jan. 1, 1970
          } ikev2_state_ref;
      } protected_part;
      opaque MAC[0..255];        // the length depends
                                 // on the integrity algorithm
} ticket;

Appendix B.  Change Log

B.1.  -03

Changed the protocol from one to two round trips, to simplify the security assumptions. Eliminated security considerations associated with the previous version.

Closed issue #69, Clarify behavior of SPI and sequence numbers.

Closed issue #70, Ticket lifetime - explicit or not? (and ticket push from gateway).

Closed issue #99, Ticket example: downgrade.

Closed issue #76, IPsec child SAs during resumption.

Closed issue #77, Identities in draft-ietf-ipsecme-ikev2-resumption.

Closed issue #95, Minor nits for ikev2-resumption-02.

Closed issue #97, Clarify what state comes from where.

Closed issue #98, Replays in 1-RTT protocol.

Closed issue #100, NAT detection [and] IP address change.

Closed issue #101, Assorted issues by Tero.

B.2.  -02

Added a new TICKET_OPAQUE payload that does not have a lifetime field included.

Removed the lifetime usage from the IKE_SESSION_RESUME exchange (utilizing the TICKET_OPAQUE) when presenting the ticket to the gateway.

Removed IDi payloads from the IKE_SESSION_RESUME exchange.

Clarified that IPsec child SAs would be deleted once the old IKE SA gets deleted as well.

Clarified the behavior of SPI and sequence number usage.

B.3.  -01

Addressed issue#75, see http://tools.ietf.org/wg/ipsecme/trac/ticket/75. This included changes throughout the document to ensure that the ticket may contain a reference or a value.

B.4.  -00

Resubmitted document as a WG item.

B.5.  -01

Added reference to [I‑D.xu‑ike‑sa‑sync] (Xu, Y., Yang, P., Ma, Y., Deng, H., and H. Deng, “IKEv2 SA Synchronization for session resumption,” October 2008.)

Included recommended ticket format into the appendix

Various editorial improvements within the document

B.6.  -00

Issued a -00 version for the IPSECME working group. Reflected discussions at IETF#72 regarding the strict focus on session resumption. Consequently, text about failover was removed.

B.7.  -04

Editorial fixes; references cleaned up; updated author's contact address

B.8.  -03

Removed counter mechanism. Added an optional anti-DoS mechanism, based on IKEv2 cookies (removed previous discussion of cookies). Clarified that gateways may support reallocation of same IP address, if provided by network. Proposed a solution outline to the problem of key exchange for the keys that protect tickets. Added fields to the ticket to enable interoperability. Removed incorrect MOBIKE notification.

B.9.  -02

Clarifications on generation of SPI values, on the ticket's lifetime and on the integrity protection of the anti-replay counter. Eliminated redundant SPIs from the notification payloads.

B.10.  -01

Editorial review. Removed 24-hour limitation on ticket lifetime, lifetime is up to local policy.

B.11.  -00

Initial version. This draft is a selective merge of draft-sheffer-ike-session-resumption-00 and draft-dondeti-ipsec-failover-sol-00.

Authors' Addresses