IKEv2 Resumption: Internet Key Exchange Protocol Version 2 (IKEv2) Session Resumption

Internet Engineering Task Force (IETF)                        Y. Sheffer
Request for Comments: 5723                                   Check Point
Category: Standards Track                                  H. Tschofenig
ISSN: 2070-1721                                   Nokia Siemens Networks
                                                            January 2010


           Internet Key Exchange Protocol Version 2 (IKEv2)
                           Session Resumption

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
   endpoints.  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 encodes partial IKE state into an opaque
   ticket, which can be stored on the client or in a centralized store,
   and 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 examples are provided.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5723.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................5
   3. Usage Scenario ..................................................5
   4. Protocol Sequences ..............................................7
      4.1. Requesting a Ticket ........................................7
      4.2. Receiving a Ticket .........................................8
      4.3. Presenting a Ticket ........................................9
           4.3.1. Prologue ............................................9
           4.3.2. IKE_SESSION_RESUME Exchange ........................10
           4.3.3. IKE_AUTH Exchange ..................................11
           4.3.4. Epilogue ...........................................12
   5. IKE and IPsec State after Resumption ...........................12
      5.1. Generating Cryptographic Material for the Resumed IKE SA ..15
   6. Ticket Handling ................................................16
      6.1. Ticket Content ............................................16
      6.2. Ticket Identity and Lifecycle .............................16
   7. IKE Notifications ..............................................17
      7.1. TICKET_LT_OPAQUE Notify Payload ...........................17
      7.2. TICKET_OPAQUE Notify Payload ..............................18
   8. IANA Considerations ............................................18
   9. Security Considerations ........................................19
      9.1. Stolen Tickets ............................................19
      9.2. Forged Tickets ............................................19
      9.3. Denial-of-Service Attacks .................................20
      9.4. Detecting the Need for Resumption .........................20
      9.5. Key Management for "Tickets by Value" .....................20
      9.6. Ticket Lifetime ...........................................21
      9.7. Tickets and Identity ......................................21
      9.8. Ticket Revocation .........................................21
      9.9. Ticket by Value Format ....................................21
      9.10. Identity Privacy, Anonymity, and Unlinkability ...........22
   10. Acknowledgements ..............................................22
   11. References ....................................................23
      11.1. Normative References .....................................23
      11.2. Informative References ...................................23
   Appendix A.  Ticket Format ........................................25
     A.1.  Example "Ticket by Value" Format ..........................25
     A.2.  Example "Ticket by Reference" Format ......................25

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 (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
   endpoints.  A high number of concurrent sessions might cause
   additional problems for an IPsec responder.  Usability is also
   affected when the re-establishment of an IKE SA involves user
   interaction for re-authentication.

   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.

   The client (IKEv2 initiator) stores the state about the previous IKE
   SA locally.  The gateway (IKEv2 responder) has two options for
   maintaining the IKEv2 state about the previous IKE SA:

   o  In the "ticket by reference" approach, the gateway stores the
      state locally, and gives the client a protected and opaque
      reference (e.g., an index to the gateway's table) that the gateway
      can later use to find the state.  The client includes this opaque
      reference when it resumes the session.

   o  In the "ticket by value" approach, the gateway stores its state in
      a ticket (data structure) that is encrypted and integrity-
      protected by a key known only to the gateway.  The ticket is
      passed to the client (who treats the ticket as an opaque string)
      and sent back to the gateway when the session is resumed.  The
      gateway can then decrypt the ticket and recover the state.

   Note that the client behaves identically in both cases, and in
   general does not know which approach the gateway is using.  Since the
   ticket (or reference) is only interpreted by the same party that
   created it, this document does not specify the exact format for it.
   However, Appendix A contains examples for both "ticket by reference"
   and "ticket by value" formats.

   This approach is similar to the one taken by Transport Layer Security
   (TLS) session resumption [RFC5077] 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  Providing cryptographic agility.

   o  Having no negative impact on IKEv2 security features.

   The following are non-goals of this solution:

   o  Failover from one gateway to another.  This use case may be added
      in a future specification.

   o  Providing load balancing among gateways.

   o  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].

   This document uses terminology defined in [RFC4301] and [RFC4306].
   In addition, this document uses the following term:

   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.  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 endpoints (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        |         |
    +-+-+-+-+-+                          +-+-+-+-+-+

         Figure 1: Resuming a Session with a Remote Access Gateway

   In the first use case above, 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).

   For either of the above use cases, there are multiple possible
   situations where the mechanism specified in this document could be
   useful.  These include the following (note that this list is not
   meant to be exhaustive, and any particular deployment may not care
   about all of these):

   o  If a client temporarily loses network connectivity (and the IKE SA
      times out through the liveness test facility, a.k.a. "dead peer
      detection"), this mechanism could be used to re-establish the SA
      with less overhead (network, CPU, authentication infrastructure)
      and without requiring user interaction for authentication.

   o  If the connectivity problems affect a large number of clients
      (e.g., a large remote access VPN gateway), when the connectivity
      is restored, all the clients might reconnect almost
      simultaneously.  This mechanism could be used to reduce the load
      spike for cryptographic operations and authentication
      infrastructure.

   o  Losing connectivity can also be predictable and planned; for
      example, putting a laptop to "stand-by" mode before traveling.
      This mechanism could be used to re-establish the SA when the
      laptop is switched back on (again, with less overhead and without
      requiring user interaction for authentication).  However, such
      user-level "resumption" may often be disallowed by policy.
      Moreover, this document requires the client to destroy the ticket
      when the user explicitly "logs out" (Section 6.2).

4.  Protocol Sequences

   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 4.3.

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 (and only
      when this exchange is initiated by the client).

   o  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, or
      following a gateway-initiated IKE rekey.  All such Informational
      exchanges MUST be initiated by the client.

   o  While resuming an IKE session, i.e., in the IKE_AUTH exchange that
      follows an IKE_SESSION_RESUME exchange, see Section 4.3.3.

   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 7.  The above is
   an example, and IKEv2 allows a number of variants on these messages.
   Refer to [RFC4306] and [IKEV2-BIS] 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:

   o  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.

   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.

   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 shows how
   the initiator receives the ticket.

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

                       Figure 3: Receiving a Ticket

   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.

   When the client accepts the ticket, it stores it in its local storage
   for later use, along with the IKE SA to which the ticket refers.
   Since the ticket itself is opaque to the client, the local storage
   MUST also include all items marked as "from the ticket" in the table
   of Section 5.

4.3.  Presenting a Ticket

   When the client wishes to recover from an interrupted session, it
   presents the ticket to resume the session.  This section describes
   the resumption process, consisting of some preparations, an
   IKE_SESSION_RESUME exchange, an IKE_AUTH exchange and finalization.

4.3.1.  Prologue

   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.

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

   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.

4.3.2.  IKE_SESSION_RESUME Exchange

   This document specifies a new IKEv2 exchange type called
   IKE_SESSION_RESUME whose value is 38.  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 (this condition is trivially true
   in the context of the current document, since the client always
   resumes into the same gateway that generated the ticket).

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

           Figure 4: IKEv2 Initiator Wishes to Resume an IKE SA

   The exchange type in HDR is set to 'IKE_SESSION_RESUME'.  The
   initiator sets the SPIi (Security Parameter Index, Initiator) value
   in the HDR to a new, unique 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:

   o  If it is willing to accept the ticket, it responds as shown in
      Figure 5.

   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.

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

               Figure 5: IKEv2 Responder Accepts the Ticket

   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, unique value.

   Where not specified otherwise, the IKE_SESSION_RESUME exchange
   behaves exactly like the IKE_SA_INIT exchange.  Specifically:

   o  The client MAY resume the IKE exchange from any IP address and
      port, regardless of its original address.  The gateway MAY reject
      the resumed exchange if its policy depends on the client's address
      (although this rarely makes sense).

   o  The first message MAY be rejected in denial-of-service (DoS)
      situations, with the initiator instructed to send a cookie.

   o  Notifications normally associated with IKE_SA_INIT can be sent.
      In particular, NAT detection payloads.

   o  The client's NAT traversal status SHOULD be determined anew in
      IKE_SESSION_RESUME.  If NAT is detected, the initiator switches to
      UDP encapsulation on port 4500, as per [RFC4306], Section 2.23.
      NAT status is explicitly not part of the session resumption state.

   o  The SPI values and Message ID fields behave similarly to
      IKE_SA_INIT.

   Although the IKE SA is not fully valid until the completion of the
   IKE_AUTH exchange, the peers must create much of the SA state
   (Section 5) now.  Specifically, the shared key values are required to
   protect the IKE_AUTH payloads.  Their generation is described in
   Section 5.1.

4.3.3.  IKE_AUTH Exchange

   Following the IKE_SESSION_RESUME exchange, the client MUST initiate
   an IKE_AUTH exchange, which is largely as specified in [RFC4306].
   This section lists the differences and constraints compared to the
   base document.

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

            AUTH = prf(SK_px, <message octets>)

   Each of the initiator and responder uses its own value for SK_px,
   namely SK_pi for the initiator and SK_pr for the responder.  Both are
   taken from the newly generated IKE SA (Section 5.1).

   The exact material to be signed is defined in Section 2.15 of
   [RFC4306].

   The IDi value sent in the IKE_AUTH exchange MUST be identical to the
   value included in the ticket.  A CERT payload MUST NOT be included in
   this exchange, and therefore a new IDr value cannot be negotiated
   (since it would not be authenticated).  As a result, the IDr value
   sent (by the gateway, and optionally by the client) in this exchange
   MUST also be identical to the value included in the ticket.

   When resuming a session, a client will typically request a new ticket
   immediately, so that 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.  The returned ticket (if any) will correspond to the IKE
   SA created per the rules described in Section 5.

4.3.4.  Epilogue

   Following the IKE_AUTH exchange, a new IKE SA is created by both
   parties, see Section 5, and a Child SA is derived, per Section 2.17
   of [RFC4306].

   When the responder receives a ticket for an IKE SA that is still
   active and if the responder accepts it (i.e., following successful
   completion of the IKE_AUTH exchange), the old SA SHOULD be silently
   deleted without sending a DELETE informational exchange.
   Consequently, all the dependent IPsec Child SAs are also deleted.

5.  IKE and IPsec State after Resumption

   During the resumption process, both peers create IKE and IPsec state
   for the resumed IKE SA.  Although the SA is only completed following
   a successful IKE_AUTH exchange, many of its components are created
   earlier, notably the SA's crypto material (Section 5.1).

   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 by 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.

   The following table 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:

   o  The client retrieves this item from its local store.

   o  In the case of "ticket by value", the gateway encodes this
      information in the ticket.

   o  In the case of "ticket by reference", the gateway fetches this
      information from the ticket store.

   +--------------------------------+----------------------------------+
   | State Item                     | After Resumption                 |
   +--------------------------------+----------------------------------+
   | IDi                            | From the ticket (but must also   |
   |                                | be exchanged in IKE_AUTH).  See  |
   |                                | also Note 1.                     |
   |                                |                                  |
   | IDr                            | From the ticket (but must also   |
   |                                | be exchanged in IKE_AUTH).       |
   |                                |                                  |
   | Authentication method (PKI,    | From the ticket.                 |
   | pre-shared secret, EAP,        |                                  |
   | PKI-less EAP [EAP-AUTH] etc.)  |                                  |
   |                                |                                  |
   | Certificates (when applicable) | From the ticket, see Note 2.     |
   |                                |                                  |
   | Local IP address/port, peer IP | Selected by the client, see Note |
   | address/port                   | 3.                               |
   |                                |                                  |
   | NAT detection status           | From new exchange.               |
   |                                |                                  |
   | SPIs                           | From new exchange, see Note 4.   |
   |                                |                                  |
   | Which peer is the "original    | Determined by the initiator of   |
   | initiator"?                    | IKE_SESSION_RESUME.              |
   |                                |                                  |
   | IKE SA sequence numbers        | Reset to 0 in                    |
   | (Message ID)                   | IKE_SESSION_RESUME, and          |
   |                                | subsequently incremented         |
   |                                | normally.                        |
   |                                |                                  |
   | IKE SA algorithms (SAr)        | From the ticket.                 |
   |                                |                                  |
   | IKE SA keys (SK_*)             | The old SK_d is obtained from    |
   |                                | the ticket and all keys are      |
   |                                | refreshed, see Section 5.1.      |
   |                                |                                  |
   | IKE SA window size             | Reset to 1.                      |
   |                                |                                  |
   | Child SAs (ESP/AH)             | Created in new exchange, see     |
   |                                | Note 6.                          |
   |                                |                                  |
   | Internal IP address            | Not resumed, but see Note 5.     |
   |                                |                                  |
   | Other Configuration Payload    | Not resumed.                     |
   | information                    |                                  |
   |                                |                                  |
   | Peer Vendor IDs                | Not resumed, resent in new       |
   |                                | exchange if required.            |
   |                                |                                  |
   | Peer supports MOBIKE [RFC4555] | From new exchange.               |
   |                                |                                  |
   | MOBIKE additional addresses    | Not resumed, should be resent by |
   |                                | client if necessary.             |
   |                                |                                  |
   | Time until re-authentication   | From new exchange (but ticket    |
   | [RFC4478]                      | lifetime is bounded by this      |
   |                                | duration).                       |
   |                                |                                  |
   | Peer supports redirects        | From new exchange.               |
   | [RFC5685]                      |                                  |
   +--------------------------------+----------------------------------+

   Note 1:  The authenticated peer identity used for policy lookups may
            not be the same as the IDi payload.  This is possible when
            using certain EAP methods, see Section 3.5 of [RFC4718].  If
            these identities are indeed different, then the
            authenticated client identity MUST be included in the
            ticket.  Note that the client may not have access to this
            value.

   Note 2:  Certificates don't need to be stored if the peer never uses
            them for anything after the IKE SA is up; however, if they
            are needed, e.g., if exposed to applications via IPsec APIs,
            they MUST be stored in the ticket.

   Note 3:  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 4:  SPI values of the old SA MAY be stored in the ticket, to
            help the gateway locate corresponding old IKE state.  These
            values MUST NOT be used for the resumed SA.

   Note 5:  The client can request the address it was using earlier, and
            if possible, the gateway SHOULD honor the request.

   Note 6:  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 [ROHCoIPsec]
            and configuration) aren't usually affected by session
            resumption.

   IKEv2 features that affect only the IKE_AUTH exchange (including
   HTTP_CERT_LOOKUP_SUPPORTED, multiple authentication exchanges
   [RFC4739], Elliptic Curve Digital Signature Algorithm (ECDSA)
   authentication [RFC4754], and the Online Certificate Status Protocol
   (OCSP) [RFC4806]) don't usually need any state in the IKE SA (after
   the IKE_AUTH exchanges are done), so resumption doesn't affect them.

   New IKEv2 features that are not covered by Note 6 or by the previous
   paragraph should specify how they interact with session resumption.

5.1.  Generating Cryptographic Material for the Resumed IKE SA

   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] for the notation. "prf" is determined from the SA value
   in the ticket.

6.  Ticket Handling

6.1.  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 authorized access is allowed.  We note that
   such a ticket is analogous to the concept of 'stub', as defined in
   [SA-SYNC], 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 all state
   information marked "from the ticket" in the table on Section 5.  The
   same state MUST be stored in the ticket store, in the case of "ticket
   by reference".

   A "ticket by value" MUST include a protected expiration time, which
   is an absolute time value and SHOULD correspond to the value included
   in the TICKET_LT_OPAQUE payload.

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

6.2.  Ticket Identity and Lifecycle

   Each ticket is associated with a single IKE SA.  In particular, when
   an IKE SA is deleted by the client or the gateway, 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.  When a
   client does not follow these rules, it might present an invalid
   ticket to the gateway.  See Section 9.8 for more about this issue.

   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].  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.

7.  IKE Notifications

   This document defines a number of notifications.  The following
   Notify Message types have been assigned by IANA.

              +-------------------+-------+-----------------+
              | Notification Name | Value | Data            |
              +-------------------+-------+-----------------+
              | TICKET_LT_OPAQUE  | 16409 | See Section 7.1 |
              |                   |       |                 |
              | TICKET_REQUEST    | 16410 | None            |
              |                   |       |                 |
              | TICKET_ACK        | 16411 | None            |
              |                   |       |                 |
              | TICKET_NACK       | 16412 | None            |
              |                   |       |                 |
              | TICKET_OPAQUE     | 16413 | See Section 7.2 |
              +-------------------+-------+-----------------+

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

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,
   in network byte order).

        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 6: TICKET_LT_OPAQUE Notify Payload

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                                 ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 7: TICKET_OPAQUE Notify Payload

8.  IANA Considerations

   Section 4.3.2 defines a new IKEv2 exchange type, IKE_SESSION_RESUME,
   whose value has been allocated from the "IKEv2 Exchange Types"
   registry.

   Section 7 defines several new IKEv2 notifications whose Message Type
   values have been allocated from the "IKEv2 Notify Message Types -
   Status Types" registry.

9.  Security Considerations

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

9.1.  Stolen Tickets

   A 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.  Since all exchanges where the client obtains a ticket are
   encrypted, this is only possible by listening in on a client's use of
   the ticket to resume a session.  However, since the ticket's contents
   are 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 a resumption attempt, 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", namely because the attacker would not be able to prove,
   during IKE_AUTH, its knowledge of the secret part of the IKE state
   embedded in the ticket.  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 the 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 6.1, 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.  Such an attack could burden
   the gateway's CPU, and/or exhaust its memory with half-open IKE
   state.  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.

   Note also that the regular IKEv2 cookie mechanism can be used to
   handle state-overflow DoS situations.

9.4.  Detecting the Need for Resumption

   Detecting when an old IKE SA is no longer usable and needs to be
   resumed is out of scope of the current document.  However, clients
   are warned against implementing a more liberal policy than that used
   to detect failed IKE SAs (Section 2.4 of RFC 4306).  In particular,
   untrusted messages MUST NOT be relied upon to make this decision.

9.5.  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.

   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.

9.6.  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.

9.7.  Tickets and Identity

   A ticket is associated with a certain identity, and MUST be managed
   securely on the client side.  Section 6.2 requires that a ticket be
   deleted when the credentials associated with the ticket's identity
   are no longer valid, e.g., when a user whose credentials were used to
   create the SA logs out.

9.8.  Ticket Revocation

   A misbehaving client could present a ticket in its possession to the
   gateway resulting in session resumption, even though the IKE SA
   associated with this ticket had previously been deleted.  This is
   disallowed by Section 6.2.  This issue is unique to "ticket by value"
   cases, since a "ticket by reference" will have been deleted from the
   ticket store.

   To avoid this issue for "ticket by value", an Invalid Ticket List
   (ITL) may be maintained by the gateway, see [TOKENS].  This can be a
   simple blacklist of revoked tickets.  Alternatively, [TOKENS]
   suggests to use Bloom Filters [Bloom70] to maintain the list in
   constant space.  Management of such lists is outside the scope of the
   current document.  Note that a policy that requires tickets to have
   shorter lifetimes (e.g., 1 hour) significantly mitigates this issue.

9.9.  Ticket by Value Format

   The ticket's format is not defined by this document, since this is
   not required for interoperability.  However, great care must be taken
   when defining a ticket format such that the requirements outlined in
   Section 6.1 are met.  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.

9.10.  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 endpoints.

   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.

   We are not aware of additional security issues associated with ticket
   reuse: the protocol guarantees freshness of the generated crypto
   material even in such cases.  As noted in Section 4.3.1, the gateway
   SHOULD prevent multiple uses of the same ticket.  But this is only an
   extra precaution, to ensure that clients do not implement reuse.  In
   other words, the gateway is not expected to cache old tickets for
   extended periods of time.

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, Peny Yang, Sean Turner, and Tero Kivinen for their
   comments.  We would like to particularly thank Florian Tegeler and
   Stjepan Gros for their implementation efforts and Florian Tegeler for
   a formal verification using the Casper tool set.

   We would furthermore like to thank the authors of [SA-SYNC] (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.

   Vidya Narayanan and Lakshminath Dondeti coauthored several past
   versions of this document, and we acknowledge their significant
   contribution.

11.  References

11.1.  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.

11.2.  Informative References

   [Bloom70]     Bloom, B., "Space/time trade-offs in hash coding with
                 allowable errors", Comm. ACM 13(7):422-6, July 1970.

   [EAP-AUTH]    Eronen, P., Tschofenig, H., and Y. Sheffer, "An
                 Extension for EAP-Only Authentication in IKEv2", Work
                 in Progress, October 2009.

   [IKEV2-BIS]   Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
                 "Internet Key Exchange Protocol: IKEv2", Work
                 in Progress, October 2009.

   [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.

   [RFC4478]     Nir, Y., "Repeated Authentication in Internet Key
                 Exchange (IKEv2) Protocol", RFC 4478, April 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.

   [RFC4739]     Eronen, P. and J. Korhonen, "Multiple Authentication
                 Exchanges in the Internet Key Exchange (IKEv2)
                 Protocol", RFC 4739, November 2006.

   [RFC4754]     Fu, D. and J. Solinas, "IKE and IKEv2 Authentication
                 Using the Elliptic Curve Digital Signature Algorithm
                 (ECDSA)", RFC 4754, January 2007.

   [RFC4806]     Myers, M. and H. Tschofenig, "Online Certificate Status
                 Protocol (OCSP) Extensions to IKEv2", RFC 4806,
                 February 2007.

   [RFC5077]     Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
                 "Transport Layer Security (TLS) Session Resumption
                 without Server-Side State", RFC 5077, January 2008.

   [RFC5685]     Devarapalli, V. and K. Weniger, "Redirect Mechanism for
                 the Internet Key Exchange Protocol Version 2 (IKEv2)",
                 RFC 5685, November 2009.

   [ROHCoIPsec]  Ertekin, E., Christou, C., Jasani, R., Kivinen, T., and
                 C. Bormann, "IKEv2 Extensions to Support Robust Header
                 Compression over IPsec (ROHCoIPsec)", Work in Progress,
                 December 2009.

   [SA-SYNC]     Xu, Y., Yang, P., Ma, Y., Deng, H., and H. Deng, "IKEv2
                 SA Synchronization for session resumption", Work
                 in Progress, October 2008.

   [TOKENS]      Rescorla, E., "How to Implement Secure (Mostly)
                 Stateless Tokens", Work in Progress, March 2007.

Appendix A.  Ticket Format

   This document does not specify a particular ticket format nor even
   the suggested contents of a ticket: both are entirely up to the
   implementer.  The formats described in the following sub-sections are
   provided as useful examples, and implementers are free to adopt them
   as-is or change them in any way necessary.

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
              enum ... authentication_method;
              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 [TOKENS] 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;


Authors' Addresses

   Yaron Sheffer
   Check Point Software Technologies Ltd.
   5 Hasolelim St.
   Tel Aviv  67897
   Israel

   EMail: yaronf@checkpoint.com


   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