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rfc:rfc6867

Internet Engineering Task Force (IETF) Y. Nir Request for Comments: 6867 Check Point Category: Experimental Q. Wu ISSN: 2070-1721 Huawei

                                                          January 2013
        An Internet Key Exchange Protocol Version 2 (IKEv2)
     Extension to Support EAP Re-authentication Protocol (ERP)

Abstract

 This document updates the Internet Key Exchange Protocol version 2
 (IKEv2) described in RFC 5996.  This extension allows an IKE Security
 Association (SA) to be created and authenticated using the Extensible
 Authentication Protocol (EAP) Re-authentication Protocol extension,
 as described in RFC 6696.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  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).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see 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/rfc6867.

Nir & Wu Experimental [Page 1] RFC 6867 ERP for IKE January 2013

Copyright Notice

 Copyright (c) 2013 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

1. Introduction

 IKEv2, as specified in [RFC5996], allows (Section 2.16)
 authentication of the initiator using an EAP method.  Using EAP
 significantly increases the count of round trips required to
 establish the IPsec SA and also may require user interaction.  This
 makes it inconvenient to allow a single remote access client to
 create multiple IPsec tunnels with multiple IPsec gateways that
 belong to the same domain.
 The EAP Re-authentication Protocol (ERP), as described in [RFC6696],
 allows an EAP peer to authenticate to multiple authenticators while
 performing the full EAP method only once.  Subsequent authentications
 require fewer round trips and no user interaction.
 Bringing these two technologies together allows a remote access IPsec
 client to create multiple tunnels with different gateways that belong
 to a single domain as well as using the keys from other contexts of
 using EAP, such as network access within the same domain, to
 transparently connect to VPN gateways within this domain.
 Additionally, it allows for faster set up of new tunnels when
 previous tunnels have been torn down due to things like network
 outage, device suspension, or a temporary move out of range.  This is
 similar to the session resumption mechanism described in [RFC5723].
 One exception being that instead of a ticket stored by the client,
 the re-authentication Master Session Key (rMSK) (see Section 4.6 of
 [RFC6696]) is used as the session key stored on both the client and
 the Authentication, Authorization, and Accounting (AAA) server.

Nir & Wu Experimental [Page 2] RFC 6867 ERP for IKE January 2013

1.1. Conventions Used in This Document

 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].

2. Usage Scenarios

 This work is motivated by the following scenarios:
 o  Multiple tunnels for a single remote access VPN client.  Suppose a
    company has offices in New York City, Paris, and Shanghai.  For
    historical reasons, the email server is located in the Paris
    office, most of the servers hosting the company's intranet are
    located in Shanghai, and the finance department servers are in New
    York City.  An employee using a remote access VPN may need to
    connect to servers from all three locations.  While it is possible
    to connect to a single gateway, and have that gateway route the
    requests to the other gateways (perhaps through site to site VPN),
    this is not efficient; it is more desirable to have the client
    initiate three different tunnels.  It is, however, not desirable
    to have the user type in a password three times.
 o  Roaming.  In these days of mobile phones and tablets, users often
    move from the wireless LAN in their office, where access may be
    granted through 802.1x, to a cellular network, where a VPN is
    necessary, and back again.  Both the VPN server and the 802.1x
    access point are authenticators that connect to the same AAA
    servers.  So it makes sense to make the transition smooth, without
    requiring user interaction.  The device still needs to detect
    whether it is within the protected network, in which case it
    should not use VPN.  However, this process is beyond the scope of
    this document.  [SECBEAC] is a now-abandoned attempt at this.
 o  Resumption.  If a device gets disconnected from an IKE peer, ERP
    can be used to reconnect to the same gateway without user
    intervention.

3. Protocol Outline

 Supporting EAP Re-authentication Extension (ERX) requires an EAP
 payload in the first IKE_AUTH request.  This is a deviation from the
 rules in [RFC5996], so support needs to be indicated through a Notify
 payload in the IKE_SA_INIT response.  This Notify serves the same
 purpose as the EAP-Initiate/Re-auth-Start message of ERX, as
 specified in Section 5.3.1 of [RFC6696].  The "Domain Name" field
 contains the content of the Domain-Name TLV as specified in Section
 5.3.1.1 of the same document.

Nir & Wu Experimental [Page 3] RFC 6867 ERP for IKE January 2013

 A supporting initiator that has unexpired keys for this domain will
 send the EAP-Initiate/Re-auth message in an EAP payload in the first
 IKE_AUTH request.
 The responder sends the EAP payload content to a backend AAA server.
 If that server has a valid rMSK for that session, it sends those
 along with an EAP-Finish/Re-auth message.  The responder then
 forwards the EAP-Finish/Re-auth message to the initiator in an EAP
 payload within the first IKE_AUTH response.
 The initiator then sends an additional IKE_AUTH request that includes
 the AUTH payload, which has been calculated using the rMSK in the
 role of the MSK as described in Sections 2.15 and 2.16 of [RFC5996].
 The responder replies similarly, and the IKE_AUTH exchange is
 finished.
 If the backend AAA server does not have valid keys for the Re-auth-
 Start message, it sends back a normal EAP request, and no rMSK key.
 EAP flow continues as in [RFC5996].
 The following figure is adapted from Appendixes C.1 and C.3 of
 [RFC5996], with most of the optional payloads removed.  Note that the
 EAP-Initiate/Re-auth message is added.
 IKE_SA_INIT Exchange:
 | init request         --> SA, KE, Ni,
 |
 | init response       <-- SA, KE, Nr,
 |                         N[ERX_SUPPORTED]
 IKE_AUTH Exchanges:
 | first request       --> EAP(EAP-Initiate/Re-auth),
 |                         IDi,
 |                         SA, TSi, TSr
 |
 | first response      <-- IDr, [CERT+], AUTH,
 |                         EAP(EAP-Finish/Re-auth)
 |
 | last request        --> AUTH
 |
 | last response       <-- AUTH,
 |                         SA, TSi, TSr
 The IDi payload MUST have ID Type ID_RFC822_ADDR, and the data field
 MUST contain the same value as the KeyName-NAI TLV in the EAP-
 Initiate/Re-auth message.  See Section 3.2 for details.

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3.1. Clarification about EAP Codes

 Section 3.16 of [RFC5996] enumerates the EAP codes in EAP messages
 that are carried in EAP payloads.  The enumeration goes only to 4.
 It is not clear whether or not that list is supposed to be
 exhaustive.
 To clarify, an implementation conforming to this specification MUST
 accept and transmit EAP messages with at least the codes for Initiate
 and Finish (5 and 6) from Section 5.3 of [RFC6696], in addition to
 the four codes enumerated in [RFC5996].  This document is
 intentionally silent about other EAP codes that are not enumerated in
 those documents.

3.2. Username in the Protocol

 The authors, as well as participants of the HOKEY and IPsecME working
 groups, believe that all use cases for this extension to IKE have a
 single backend AAA server doing both the authentication and the re-
 authentication.  The reasoning behind this is that IKE runs over the
 Internet and would naturally connect to the user's home network.
 This section addresses instances where this is not the case.
 Section 5.3.2 of [RFC6696] describes the EAP-Initiate/Re-auth packet,
 which, in the case of IKEv2, is carried in the first IKE_AUTH
 request.  This packet contains the KeyName-NAI TLV.  This TLV
 contains the username used in authentication.  It is relayed to the
 AAA server in the AccessRequest message and is returned from the AAA
 server in the AccessAccept message.
 The username part of the Network Access Identifier (NAI) within the
 TLV is the EMSKName [RFC5295] encoded in hexadecimal digits.  The
 domain part is the domain name of the home domain of the user.  The
 username part is ephemeral in the sense that a new one is generated
 for each full authentication.  This ephemeral value is not a good
 basis for making policy decisions, and it is also a poor source of
 user identification for the purposes of logging.
 Instead, it is up to the implementation in the IPsec gateway to make
 policy decisions based on other factors.  The following list is by no
 means exhaustive:
 o  In some cases, the home domain name may be enough to make policy
    decisions.  If all users with a particular home domain get the
    same authorization, then policy does not depend on the real
    username.  Meaningful logs can still be issued by correlating VPN
    gateway IKE events with AAA servers access records.

Nir & Wu Experimental [Page 5] RFC 6867 ERP for IKE January 2013

 o  Sometimes users receive different authorizations based on groups
    to which they belong.  The AAA server can communicate such
    information to the VPN gateway, for example, using the CLASS
    attribute [RFC2865] in RADIUS and Diameter [RFC6733].  Logging
    again depends on correlation with AAA servers.
 o  AAA servers may support extensions that allow them to communicate
    with their clients (in our case -- the VPN gateway) to push user
    information.  For example, a certain product integrates a RADIUS
    server with the Lightweight Directory Access Protocol (LDAP)
    [RFC4511], so a client could query the server using LDAP and
    receive the real record for this user.  Others may provide this
    data through vendor-specific extensions to RADIUS or Diameter.
 In any case, authorization is a major issue in deployments, if the
 backend AAA server supporting the re-authentication is different from
 the AAA server that had supported the original authentication.  It is
 up to the re-authenticating AAA server to provide the necessary
 information for authorization.  A conforming implementation of this
 protocol MAY reject initiators for which it is unable to make policy
 decisions because of these reasons.

4. ERX_SUPPORTED Notification

 The Notify payload is as described in [RFC5996]:
                          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    !    ERX Notify Message Type    !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !                            Domain Name                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 o  Protocol ID (1 octet) - MUST be zero, as this message is related
    to an IKE SA.
 o  Security Parameter Index (SPI) Size (1 octet) - MUST be zero, in
    conformance with Section 3.10 of [RFC5996].
 o  ERX Notify Message Type (2 octets) - MUST be 16427, the value
    assigned for ERX.
 o  Domain Name (variable) - contains the domain name or realm, as
    these terms are used in [RFC6696] and encoded as ASCII, as
    specified in [RFC4282].

Nir & Wu Experimental [Page 6] RFC 6867 ERP for IKE January 2013

5. Operational Considerations

 This specification changes the behavior of IKE peers, both initiators
 and responders.  The behavior of backend AAA servers is not changed
 by this specification, but they are required to support [RFC6696].
 The same goes for the EAP client, if it's not integrated into the IKE
 initiator (for example, if the EAP client is an operating system
 component).
 This specification is silent about key storage and key lifetimes on
 either the EAP client or the EAP server.  These issues are covered in
 Sections 3, 4, and 5 of [RFC6696].  The key lifetime may be
 communicated from the AAA server to the EAP client via the Lifetime
 attribute in the EAP-Finish/Re-auth message.  If the server does not
 have a valid key, while the client does have one, regular EAP is used
 (see Section 3).  This should not happen if lifetimes are
 communicated.  In such a case, the IKEv2 initiator / EAP client MAY
 alert the user and MAY log the event.  Note that this does not
 necessarily indicate an attack.  It could simply be a loss of state
 on the AAA server.

6. Security Considerations

 The protocol extension described in this document extends the
 authentication from one EAP context, which may or may not be part of
 IKEv2, to an IKEv2 context.  Successful completion of the protocol
 proves to the authenticator, which in our case is a VPN gateway, that
 the supplicant or VPN client has authenticated in some other EAP
 context.
 The protocol supplies the authenticator with the domain name with
 which the supplicant has authenticated, but does not supply it with a
 specific identity.  Instead, the gateway receives an EMSKName, which
 is an ephemeral ID.  With this variant of the IKEv2 protocol, the
 initiator never sends its real identity on the wire while the server
 does.  This is different from the usual IKEv2 practice of the
 initiator revealing its identity first.
 If the domain name is sufficient to make access control decisions,
 this is enough.  If not, then the gateway needs to find out either
 the real name or authorization information for that particular user.
 This may be done using the AAA protocol or by some other federation
 protocol, which is out of scope for this specification.

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7. IANA Considerations

 IANA has assigned a notify message type of 16427 from the "IKEv2
 Notify Message Types - Status Types" registry with the name
 "ERX_SUPPORTED".

8. Acknowledgements

 The authors would like to thank Yaron Sheffer for comments and
 suggested text that have contributed to this document.
 Thanks also to Juergen Schoenwaelder for his OPS-DIR review comments.

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.
 [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
            Network Access Identifier", RFC 4282, December 2005.
 [RFC5295]  Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
            "Specification for the Derivation of Root Keys from an
            Extended Master Session Key (EMSK)", RFC 5295,
            August 2008.
 [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
            "Internet Key Exchange Protocol Version 2 (IKEv2)",
            RFC 5996, September 2010.
 [RFC6696]  Cao, Z., He, B., Shi, Y., Wu, Q., and G. Zorn, "EAP
            Extensions for the EAP Re-authentication Protocol (ERP)",
            RFC 6696, July 2012.

9.2. Informative References

 [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
            "Remote Authentication Dial In User Service (RADIUS)",
            RFC 2865, June 2000.
 [RFC4511]  Sermersheim, J., "Lightweight Directory Access Protocol
            (LDAP): The Protocol", RFC 4511, June 2006.
 [RFC5723]  Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
            Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
            January 2010.

Nir & Wu Experimental [Page 8] RFC 6867 ERP for IKE January 2013

 [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
            "Diameter Base Protocol", RFC 6733, October 2012.
 [SECBEAC]  Sheffer, Y. and Y. Nir, "Secure Beacon: Securely Detecting
            a Trusted Network", Work in Progress, June 2009.

Authors' Addresses

 Yoav Nir
 Check Point Software Technologies Ltd.
 5 Hasolelim st.
 Tel Aviv  67897
 Israel
 EMail: ynir@checkpoint.com
 Qin Wu
 Huawei Technologies Co., Ltd.
 101 Software Avenue, Yuhua District
 Nanjing, JiangSu  210012
 China
 EMail: sunseawq@huawei.com

Nir & Wu Experimental [Page 9]

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