Network Working Group Y. Sheffer Internet-Draft Check Point Intended status: Standards Track H. Tschofenig Expires: May 21, 2009 Nokia Siemens Networks L. Dondeti V. Narayanan QUALCOMM, Inc. November 17, 2008 IKEv2 Session Resumption draft-ietf-ipsecme-ikev2-resumption-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 21, 2009. 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 (SA) upon a failure recovery Sheffer, et al. Expires May 21, 2009 [Page 1] Internet-Draft IKEv2 Session Resumption November 2008 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 uses a IKEv2 state (or a reference into a state store). to store state information that is later made available to the IKEv2 responder for re-authentication. Restoring state information by utilizing a ticket is one possible way. This document does not specify the format of the ticket but recommendations are provided. Sheffer, et al. Expires May 21, 2009 [Page 2] Internet-Draft IKEv2 Session Resumption November 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Requesting a Ticket . . . . . . . . . . . . . . . . . . . 7 4.2. Presenting a Ticket . . . . . . . . . . . . . . . . . . . 8 4.2.1. Protection of the IKE_SESSION_RESUME Exchange . . . . 9 4.2.2. Presenting a Ticket: The DoS Case . . . . . . . . . . 10 4.2.3. Requesting a ticket during resumption . . . . . . . . 10 4.3. IKE Notifications . . . . . . . . . . . . . . . . . . . . 11 4.4. TICKET_OPAQUE Notify Payload . . . . . . . . . . . . . . . 11 4.5. Processing Guidelines for IKE SA Establishment . . . . . . 11 5. Ticket Recommendations . . . . . . . . . . . . . . . . . . . . 12 5.1. Ticket Content . . . . . . . . . . . . . . . . . . . . . . 12 5.2. Ticket Identity and Lifecycle . . . . . . . . . . . . . . 13 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 7. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7.1. Stolen Tickets . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Forged Tickets . . . . . . . . . . . . . . . . . . . . . . 14 7.3. Denial of Service Attacks . . . . . . . . . . . . . . . . 14 7.4. Ticket Protection Key Management . . . . . . . . . . . . . 14 7.5. Ticket Lifetime . . . . . . . . . . . . . . . . . . . . . 14 7.6. Ticket Format . . . . . . . . . . . . . . . . . . . . . . 15 7.7. Identity Privacy, Anonymity, and Unlinkability . . . . . . 15 7.8. Replay Protection in the IKE_SESSION_RESUME Exchange . . . 15 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Normative References . . . . . . . . . . . . . . . . . . . 16 9.2. Informative References . . . . . . . . . . . . . . . . . . 16 Appendix A. Ticket Format . . . . . . . . . . . . . . . . . . . . 17 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 18 B.1. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 B.2. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 B.3. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 B.4. -04 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 B.5. -03 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 B.6. -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 B.7. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 B.8. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . . . 21 Sheffer, et al. Expires May 21, 2009 [Page 3] Internet-Draft IKEv2 Session Resumption November 2008 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 approach is similar to the one taken by TLS session resumption [RFC4507] 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: o Using only symmetric cryptography to minimize CPU consumption. o Allowing a gateway to push state to clients. o Providing cryptographic agility. o Having no negative impact on IKEv2 security features. The following are non-goals of this solution: o Providing load balancing among gateways. o Specifying how a client detects the need for a failover. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", Sheffer, et al. Expires May 21, 2009 [Page 4] Internet-Draft IKEv2 Session Resumption November 2008 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. This document uses terminology defined in [RFC4301], [RFC4306], and [RFC4555]. 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], 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. Sheffer, et al. Expires May 21, 2009 [Page 5] Internet-Draft IKEv2 Session Resumption November 2008 (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 ! ! +-+-+-+-+-+ +-+-+-+-+-+ Figure 1: Resuming a Session with a Remote Access Gateway In this scenario, an end-host (an entity with a host implementation of IPsec [RFC4301] ) 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). 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. Sheffer, et al. Expires May 21, 2009 [Page 6] Internet-Draft IKEv2 Session Resumption November 2008 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, and presenting a ticket, see Section 4.2. 4.1. Requesting a Ticket A client MAY request a ticket in the following exchanges: o In an IKE_AUTH exchange, as shown in the example message exchange in Figure 2 below. o In a CREATE_CHILD_SA exchange, when an IKE SA is rekeyed. o In an Informational exchange, if the gateway previously replied with an N(TICKET_ACK) instead of providing a ticket. o In an Informational exchange, when the ticket lifetime is about to expire. o In an IKE_SESSION_RESUME exchange, see Section 4.2.3. Normally, a client requests a ticket in the third message of an IKEv2 exchange (the first of IKE_AUTH). Figure 2 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)} --> Figure 2: Example Message Exchange for Requesting a Ticket The notification payloads are described in Section 4.3. 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]. 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: o 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. Sheffer, et al. Expires May 21, 2009 [Page 7] Internet-Draft IKEv2 Session Resumption November 2008 o it returns an N(TICKET_NACK) payload, if it refuses to grant a ticket for some reason. o 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. Provided the IKEv2 exchange was successful, the IKEv2 initiator can accept the requested ticket. The ticket may be used later with an IKEv2 responder that supports this extension. Figure 3 shows how the initiator receives the ticket. Initiator Responder ----------- ----------- <-- HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi, TSr, N(TICKET_OPAQUE) [,N(TICKET_GATEWAY_LIST)]} Figure 3: Receiving a Ticket 4.2. Presenting a Ticket A client MAY initiate a regular (non-ticket-based) IKEv2 exchange even if it is in possession of a valid ticket. 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: a client MUST NOT reuse a ticket, either with the same or with a different gateway. A gateway SHOULD reject a reused ticket. 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 ----------- ----------- Sheffer, et al. Expires May 21, 2009 [Page 8] Internet-Draft IKEv2 Session Resumption November 2008 HDR, Ni, N(TICKET_OPAQUE), [N+,] SK {IDi, [IDr,] SAi2, TSi, TSr [, CP(CFG_REQUEST)]} --> The exchange type in HDR is set to 'IKE_SESSION_RESUME'. See Section 4.2.1 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: o If it is willing to accept the ticket, it responds as shown in Figure 4. o 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. o 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. Initiator Responder ----------- ----------- <-- HDR, SK {IDr, Nr, SAr2, [TSi, TSr], [CP(CFG_REPLY)]} Figure 4: IKEv2 Responder accepts the ticket Again, the exchange type in HDR is set to 'IKE_SESSION_RESUME'. The SK payload is protected using the cryptographic parameters derived from the ticket, see Section 4.2.1 below. At this point a new IKE SA is created by both parties, see Section 4.5. This is followed by normal derivation of a child SA, per Section 2.17 of [RFC4306]. 4.2.1. 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: Sheffer, et al. Expires May 21, 2009 [Page 9] Internet-Draft IKEv2 Session Resumption November 2008 {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 4.5. See [RFC4306] for the notation. "prf" is determined from the SA value in the ticket. 4.2.2. 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), IDi, [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 4.2.1. See Section 2.6 and Section 3.10.1 of [RFC4306] for more guidance regarding the usage and syntax of the cookie. Note that the cookie is completely independent of the IKEv2 ticket. 4.2.3. 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. Sheffer, et al. Expires May 21, 2009 [Page 10] Internet-Draft IKEv2 Session Resumption November 2008 The returned ticket (if any) will correspond to the IKE SA created per the rules described in Section 4.5. 4.3. IKE Notifications This document defines a number of notifications. The notification numbers are TBA by IANA. +-------------------+--------+-----------------+ | Notification Name | Number | Data | +-------------------+--------+-----------------+ | TICKET_OPAQUE | TBA1 | See Section 4.4 | | TICKET_REQUEST | TBA2 | None | | TICKET_ACK | TBA3 | None | | TICKET_NACK | TBA4 | None | +-------------------+--------+-----------------+ 4.4. 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 ~ ! ! +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: TICKET_OPAQUE Notify Payload 4.5. Processing Guidelines for IKE SA Establishment When a ticket is presented, the gateway parses the ticket to retrieve 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: Sheffer, et al. Expires May 21, 2009 [Page 11] Internet-Draft IKEv2 Session Resumption November 2008 o The SA value (transforms etc.) is taken directly from the ticket. o The sequence numbers are reset to 0. o The IDi value is obtained from the ticket. o The IDr value is obtained from the new exchange. The gateway MAY make policy decisions based on the IDr value encoded in the ticket. o The SPI values are created anew, similarly to a regular IKE exchange. SPI values from the ticket SHOULD NOT be reused. This restriction is to avoid problems caused by collisions with other SPI values used already by the initiator/responder. The SPI value should only be reused if collision avoidance can be ensured through other means. 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 4.2.1). 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] for the notation. "prf" is determined from the SA value in the ticket. 5. Ticket Recommendations 5.1. Ticket Content As noted above, the ticket represents a partial IKEv2 state, either by reference or by value. When passing the state by value, the ticket MUST contain an integrity-protected reference to partial IKEv2 state, containing the state items described further in this section. We note that such a ticket is analogous to the concept of 'stub', as defined in [I-D.xu-ike-sa-sync], 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. These values MUST be encrypted Sheffer, et al. Expires May 21, 2009 [Page 12] Internet-Draft IKEv2 Session Resumption November 2008 and authenticated. o IDi, IDr. o SPIi, SPIr. o SAr (the accepted proposal). o SK_d. In addition, the ticket MUST encode a protected ticket expiration value. The ticket MUST include a key identity field, so that encryption and authentication can be changed, and when necessary, algorithms can be replaced. 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]. Even if neither of these are enforced by the gateway, a finite lifetime MUST be specified for the ticket. 6. IANA Considerations This document requires a number of IKEv2 notification status types in Section 4.3, to be registered by IANA. The corresponding registry was established by IANA. The document defines a new IKEv2 exchange in Section 4.2. The corresponding registry was established by IANA. 7. Security Considerations This section addresses security issues related to the usage of a ticket. The text below assumes that the IKE state is contained explicitly in the ticket ("passed by value"). In most cases, there are similar risks when the state is passed by reference. 7.1. Stolen Tickets An eavesdropper or man-in-the-middle may try to obtain a ticket and use it to establish a session with the IKEv2 responder. This can Sheffer, et al. Expires May 21, 2009 [Page 13] Internet-Draft IKEv2 Session Resumption November 2008 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. 7.2. Forged Tickets A malicious user could forge or alter a ticket 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 ticket is protected using a strong integrity protection algorithm. 7.3. Denial of Service Attacks An adversary could generate and send a large number of tickets 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). 7.4. Ticket Protection Key Management A full description of the management of the keys used to protect the ticket is beyond the scope of this document. A list of RECOMMENDED practices is given below. o The keys should be generated securely following the randomness recommendations in [RFC4086]. o The keys and cryptographic protection algorithms should be at least 128 bits in strength. o The keys should not be used for any other purpose than generating and verifying tickets. o The keys should be changed regularly. o The keys should be changed if the ticket format or cryptographic protection algorithms change. 7.5. Ticket Lifetime An IKEv2 responder controls the lifetime of a ticket, based on the operational and security requirements of the environment in which it is deployed. The responder provides information about the ticket lifetime to the IKEv2 initiator, allowing it to manage its tickets. Sheffer, et al. Expires May 21, 2009 [Page 14] Internet-Draft IKEv2 Session Resumption November 2008 7.6. Ticket Format Great care must be taken when defining a ticket format such that the requirements outlined in Section 5.1 are met. In particular, if confidential information, such as a secret key, is transferred to the client, it MUST be done using secure communication to prevent attackers from obtaining or modifying the key. Also, the ticket 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. Sheffer, et al. Expires May 21, 2009 [Page 15] Internet-Draft IKEv2 Session Resumption November 2008 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] (Yan Xu, Peny Yang, Yuanchen Ma, Hui Deng and Ke Xu) for their input on the stub concept. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005. 9.2. Informative References [I-D.rescorla-stateless-tokens] Rescorla, E., "How to Implement Secure (Mostly) Stateless Tokens", draft-rescorla-stateless-tokens-01 (work in progress), March 2007. [I-D.xu-ike-sa-sync] Xu, Y., Yang, P., Ma, Y., Deng, H., and H. Deng, "IKEv2 SA Synchronization for session resumption", draft-xu-ike-sa-sync-01 (work in progress), October 2008. [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. Sheffer, et al. Expires May 21, 2009 [Page 16] Internet-Draft IKEv2 Session Resumption November 2008 [RFC4478] Nir, Y., "Repeated Authentication in Internet Key Exchange (IKEv2) Protocol", RFC 4478, April 2006. [RFC4507] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 4507, May 2006. [RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)", RFC 4555, June 2006. [RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and Implementation Guidelines", RFC 4718, October 2006. Appendix A. Ticket Format This document does not specify a mandatory-to-implement or a mandatory-to-use ticket format. The following format is RECOMMENDED. 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 a recent draft [I-D.rescorla-stateless-tokens] that recommends a similar (but not identical) ticket format, and discusses related security Sheffer, et al. Expires May 21, 2009 [Page 17] Internet-Draft IKEv2 Session Resumption November 2008 considerations in depth. 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. -00 Resubmitted document as a WG item. B.2. -01 Added reference to [I-D.xu-ike-sa-sync] Included recommended ticket format into the appendix Various editorial improvements within the document B.3. -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. Sheffer, et al. Expires May 21, 2009 [Page 18] Internet-Draft IKEv2 Session Resumption November 2008 B.4. -04 Editorial fixes; references cleaned up; updated author's contact address B.5. -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.6. -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.7. -01 Editorial review. Removed 24-hour limitation on ticket lifetime, lifetime is up to local policy. B.8. -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 Yaron Sheffer Check Point Software Technologies Ltd. 5 Hasolelim St. Tel Aviv 67897 Israel Email: yaronf@checkpoint.com Sheffer, et al. Expires May 21, 2009 [Page 19] Internet-Draft IKEv2 Session Resumption November 2008 Hannes Tschofenig Nokia Siemens Networks Linnoitustie 6 Espoo 02600 Finland Phone: +358 (50) 4871445 Email: Hannes.Tschofenig@gmx.net URI: http://www.tschofenig.priv.at Lakshminath Dondeti QUALCOMM, Inc. 5775 Morehouse Dr San Diego, CA USA Phone: +1 858-845-1267 Email: ldondeti@qualcomm.com Vidya Narayanan QUALCOMM, Inc. 5775 Morehouse Dr San Diego, CA USA Phone: +1 858-845-2483 Email: vidyan@qualcomm.com Sheffer, et al. Expires May 21, 2009 [Page 20] Internet-Draft IKEv2 Session Resumption November 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Sheffer, et al. Expires May 21, 2009 [Page 21]