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.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.

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.

   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.

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.

   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].

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.

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.

   [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