IKEv2 Session Resumption
draft-ietf-ipsecme-ikev2-resumption-02.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
        4.3.1.  Protection of the IKE_SESSION_RESUME Exchange
        4.3.2.  Presenting a Ticket: The DoS Case
        4.3.3.  Requesting a Ticket during Resumption
    4.4.  IKE Notifications
    4.5.  TICKET_OPAQUE Notify Payload
    4.6.  TICKET_OPAQUE' Notify Payload
    4.7.  Processing Guidelines for IKE SA Establishment
5.  Ticket Recommendations
    5.1.  Ticket Content
    5.2.  Ticket Identity and Lifecycle
6.  IANA Considerations
7.  Security Considerations
    7.1.  Stolen Tickets
    7.2.  Forged Tickets
    7.3.  Denial of Service Attacks
    7.4.  Key Management for Tickets By Value
    7.5.  Ticket Lifetime
    7.6.  Ticket by Value Format
    7.7.  Identity Privacy, Anonymity, and Unlinkability
    7.8.  Replay Protection in the IKE_SESSION_RESUME Exchange
8.  Acknowledgements
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  Ticket Format
    A.1.  Recommended Ticket by Value Format
    A.2.  Recommended Ticket by Reference Format
Appendix B.  Change Log
    B.1.  -02
    B.2.  -01
    B.3.  -00
    B.4.  -01
    B.5.  -00
    B.6.  -04
    B.7.  -03
    B.8.  -02
    B.9.  -01
    B.10.  -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.
• Allowing a gateway to push state to clients.
• Providing cryptographic agility.
• Having no negative impact on IKEv2 security features.

The following are non-goals of this solution:

• Providing load balancing among gateways.
• Specifying how a client detects the need for a failover.

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.), [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.), and [RFC4555] (Eronen, P., “IKEv2 Mobility and Multihoming Protocol (MOBIKE),” June 2006.). 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.

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 this scenario, 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)).

In this scenario, the client needs to get an IP address from the remote network so that traffic can be encapsulated by the remote access gateway before reaching the client. In the initial exchange, the gateway may acquire IP addresses from the address pool of a local DHCP server. The session resumption exchange may need to support the assignment of a new IP address.

The protocol defined in this document supports the re-allocation of an IP address to the client, if this capability is provided by the network. This capability is implicit in the use of the IKE configuration mechanism, which allows the client to present its existing IP address and receive the same address back, if allowed by the 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.
• In an Informational exchange, if the gateway previously replied with an N(TICKET_ACK) instead of providing a ticket.
• In an Informational exchange, when the ticket lifetime is about to expire.
• In an IKE_SESSION_RESUME exchange, see Section 4.3.3 (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 4.4 (IKE Notifications). The above is an example, and IKEv2 allows a number of variants on these messages. A complete description of IKEv2 can be found in [RFC4718] (Eronen, P. and P. Hoffman, “IKEv2 Clarifications and Implementation Guidelines,” October 2006.).

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_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.

4.2.  Receiving a Ticket

The IKEv2 initiator receives the ticket and may accept it provided the IKEv2 exchange was successful, as described in Section 4.1 (Requesting a Ticket). 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_OPAQUE) }

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 a new exchange needs to be initiated and the session resumption procedure 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.

This document specifies a new IKEv2 exchange type called IKE_SESSION_RESUME whose value is TBA by IANA. This exchange is somewhat similar to the IKE_AUTH exchange, and results in the creation of a Child SA. The client SHOULD NOT use this exchange type unless it knows that the gateway supports it.

 Initiator                Responder
 -----------              -----------
 HDR, Ni, N(TICKET_OPAQUE'), [N+,]
      SK { [IDr,] SAi2, TSi, TSr [, CP(CFG_REQUEST)]} -->

Note: The initiator presents the TICKET_OPAQUE' payload to the responder as the lifetime field attached to the ticket is not relevant to the responder as it is included in the protected form inside the ticket.

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.

See Section 4.3.1 (Protection of the IKE_SESSION_RESUME Exchange) for details on computing the protected (SK) payload.

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.
• When the responder receives a ticket for an IKE SA that is still active and if the responder accepts it, then the old SAs SHOULD be silently deleted without sending a DELETE informational exchange. Consequently, all the IPsec child SAs are deleted as well once the old IKE SA is deleted.

 Initiator                Responder
 -----------              -----------
                 <--  HDR, SK {Nr, SAr2, [TSi, TSr],
                      [CP(CFG_REPLY)]}

Again, 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.

The SK payload is protected using the cryptographic parameters derived from the ticket, see Section 4.3.1 (Protection of the IKE_SESSION_RESUME Exchange) below.

At this point a new IKE SA is created by both parties, see Section 4.7 (Processing Guidelines for IKE SA Establishment). This is followed by normal derivation of a child SA, per Section 2.17 of [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.).

4.3.1.  Protection of the IKE_SESSION_RESUME Exchange

The two messages of this exchange are protected by a "subset" IKE SA. The key material is derived from the ticket, as follows:

     {SK_d2 | SK_ai | SK_ar | SK_ei | SK_er} = prf+(SK_d_old, Ni)

where SK_d_old is the SK_d value of the original IKE SA, as retrieved from the ticket. Ni guarantees freshness of the key material. SK_d2 is used later to derive the new IKE SA, see Section 4.7 (Processing Guidelines for IKE SA Establishment).

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

4.3.2.  Presenting a Ticket: The DoS Case

When receiving the first message of the IKE_SESSION_RESUME exchange, the gateway may decide that it is under a denial-of-service attack. In such a case, the gateway SHOULD defer the establishment of session state until it has verified the identity of the client. We use a variation of the IKEv2 Cookie mechanism, whereby the cookie is protected.

In the two messages that follow, the gateway responds that it is unwilling to resume the session until the client is verified, and the client resubmits its first message, this time with the cookie:

 Initiator                Responder
 -----------              -----------
                 <-- HDR, SK{N(COOKIE)}

HDR, Ni, N(TICKET_OPAQUE), [N+,]
     SK {N(COOKIE), [IDr,] SAi2, TSi, TSr [, CP(CFG_REQUEST)]} -->

Assuming the cookie is correct, the gateway now replies normally.

This now becomes a 4-message exchange. The entire exchange is protected as defined in Section 4.3.1 (Protection of the IKE_SESSION_RESUME Exchange).

See Section 2.6 and Section 3.10.1 of [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) for more guidance regarding the usage and syntax of the cookie. Note that the cookie is completely independent of the IKEv2 ticket.

4.3.3.  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. Therefore, the N(TICKET_REQUEST) and N(TICKET_OPAQUE) notifications may be piggybacked as protected payloads to the IKE_SESSION_RESUME exchange.

The returned ticket (if any) will correspond to the IKE SA created per the rules described in Section 4.7 (Processing Guidelines for IKE SA Establishment).

4.4.  IKE Notifications

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

4.5.  TICKET_OPAQUE Notify Payload

The data for the TICKET_OPAQUE Notify payload consists of the Notify message header, a lifetime field and the ticket itself. The four octet lifetime field contains 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                                 ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.6.  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_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                                 ~
      !                                                               !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.7.  Processing Guidelines for IKE SA Establishment

When a ticket is presented, the gateway needs to obtain the ticket per value. In case a ticket by reference was provided by the client the gateway needs to resolve the reference in order to obtain the ticket by value. In case the client has already provided the ticket per value it can parses the ticket. In either case, the gateway needs to process the ticket by value in order to restore the state of the old IKE SA, and the client retrieves this 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 sequence numbers are reset to 0.
• 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, similarly to a regular IKE exchange. SPI values from the ticket MUST NOT be reused. This restriction 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_d2, Ni | Nr)

where SK_d2 was computed earlier (Section 4.3.1 (Protection of the IKE_SESSION_RESUME Exchange)).

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.

5.  Ticket Recommendations

5.1.  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.

When the state is passed by value, the ticket MUST encode at least the following state from an IKE SA.

• 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.

In addition, the ticket by value and the ticket by reference MUST contain a protected ticket expiration value that is readable for the client.

5.2.  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. A ticket is therefore associated with the tuple (IDi, IDr).

The lifetime of the ticket carried in the N(TICKET_OPAQUE) notification 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 gateway SHOULD set the expiration date for the ticket to a larger value than the lifetime of the IKE SA. The key that is used to protect the ticket MUST have a lifetime that is significantly longer than the lifetime of an IKE SA.

6.  IANA Considerations

This document requires a number of IKEv2 notification status types in Section 4.4 (IKE Notifications), to be registered by IANA. The corresponding 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.

7.  Security Considerations

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

7.1.  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 succesfully 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.

Since 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. 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.

7.2.  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 faked 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.

7.3.  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 value then this may lead to an amplification attack as the IKEv2 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.

7.4.  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.

7.5.  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.

7.6.  Ticket by Value Format

Great care must be taken when defining a ticket format such that the requirements outlined in Section 5.1 (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.

7.7.  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 (e.g., with the help of encryption. Thus, it prevents the disclosure of potentially sensitive information.

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 envisions that the ticket is presented to the IKEv2 responder only once; multiple usage of the ticket is not provided.

7.8.  Replay Protection in the IKE_SESSION_RESUME Exchange

A major design goal of this protocol extension has been the two-message exchange for session resumption. There is a tradeoff between this abbreviated exchange and replay protection. It is RECOMMENDED that an IKEv2 responder should cache tickets, and reject replayed ones. However, some gateways may not do that in order to reduce state size. An adversary may attempt to replay a ticket. To mitigate these risks a client may be required by the gateway to show that it knows the ticket's secret, before any state is committed on the gateway side. Note that this is a stronger guarantee than the regular IKE cookie mechanism, which only shows IP return routability of the client. This is enabled by including the cookie in the protected portion of the message.

For performance reasons, the cookie mechanism is optional, and invoked by the gateway only when it suspects that it is the subject of a denial-of-service attack.

In any case, a ticket replayed by an adversary only causes partial IKE state to be created on the gateway. The IKE exchange cannot be completed and an IKE SA cannot be created unless the client knows the ticket's secret values.

8.  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 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.

9.  References

9.1. Normative References

9.2. Informative References

Appendix A.  Ticket Format

This document does not specify a mandatory-to-implement or a mandatory-to-use ticket format. The format described in the sub-sections are RECOMMENDED.

A.1.  Recommended 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.  Recommended Ticket by Reference Format

For implementations that prefer to pass a reference to IKE state in the ticket, rather than the state itself, we RECOMMEND 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.  -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.2.  -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.3.  -00

Resubmitted document as a WG item.

B.4.  -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.5.  -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.6.  -04

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

B.7.  -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.8.  -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.9.  -01

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

B.10.  -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