





Internet Engineering Task Force (IETF)                            Z. Cao
Request for Comments: 6696                                  China Mobile
Obsoletes: 5296                                                    B. He
Category: Standards Track                                           CATR
ISSN: 2070-1721                                                   Y. Shi
                                                              Q. Wu, Ed.
                                                                  Huawei
                                                            G. Zorn, Ed.
                                                             Network Zen
                                                               July 2012


      EAP Extensions for the EAP Re-authentication Protocol (ERP)

Abstract

   The Extensible Authentication Protocol (EAP) is a generic framework
   supporting multiple types of authentication methods.  In systems
   where EAP is used for authentication, it is desirable to avoid
   repeating the entire EAP exchange with another authenticator.  This
   document specifies extensions to EAP and the EAP keying hierarchy to
   support an EAP method-independent protocol for efficient re-
   authentication between the peer and an EAP re-authentication server
   through any authenticator.  The re-authentication server may be in
   the home network or in the local network to which the peer is
   connecting.

   This memo obsoletes RFC 5296.

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/rfc6696.









Cao, et al.                  Standards Track                    [Page 1]

RFC 6696                 EAP Extensions for ERP                July 2012


Copyright Notice

   Copyright (c) 2012 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.





































Cao, et al.                  Standards Track                    [Page 2]

RFC 6696                 EAP Extensions for ERP                July 2012


Table of Contents

   1. Introduction ....................................................4
      1.1. Changes from RFC 5296 ......................................5
   2. Terminology .....................................................5
   3. ERP Description .................................................7
      3.1. ERP with the Home ER Server ...............................10
      3.2. ERP with a Local ER Server ................................11
   4. ER Key Hierarchy ...............................................13
      4.1. rRK Derivation ............................................13
      4.2. rRK Properties ............................................14
      4.3. rIK Derivation ............................................14
      4.4. rIK Properties ............................................15
      4.5. rIK Usage .................................................16
      4.6. rMSK Derivation ...........................................16
      4.7. rMSK Properties ...........................................17
   5. Protocol Details ...............................................17
      5.1. ERP Bootstrapping .........................................17
      5.2. Steps in ERP ..............................................20
           5.2.1. Multiple Simultaneous Runs of ERP ..................23
           5.2.2. ERP Failure Handling ...............................23
      5.3. EAP Codes .................................................25
           5.3.1. EAP-Initiate/Re-auth-Start Packet ..................26
                  5.3.1.1. Authenticator Operation ...................27
                  5.3.1.2. Peer Operation ............................27
           5.3.2. EAP-Initiate/Re-auth Packet ........................28
           5.3.3. EAP-Finish/Re-auth Packet ..........................30
           5.3.4. TV and TLV Attributes ..............................32
      5.4. Replay Protection .........................................33
      5.5. Channel Binding ...........................................34
   6. Lower-Layer Considerations .....................................35
   7. AAA Transport of ERP Messages ..................................36
   8. Security Considerations ........................................36
   9. IANA Considerations ............................................41
   10. Contributors ..................................................41
   11. Acknowledgments ...............................................42
   12. References ....................................................42
      12.1. Normative References .....................................42
      12.2. Informative References ...................................42
   Appendix A. RFC 5296 Acknowledgments ..............................45
   Appendix B. Sample ERP Exchange ...................................46










Cao, et al.                  Standards Track                    [Page 3]

RFC 6696                 EAP Extensions for ERP                July 2012


1.  Introduction

   The Extensible Authentication Protocol (EAP) is an authentication
   framework that supports multiple authentication methods.  The primary
   purpose is network access authentication, and a key-generating method
   is used when the lower layer wants to enforce access control.  The
   EAP keying hierarchy defines two keys to be derived by all
   key-generating EAP methods: the Master Session Key (MSK) and the
   Extended MSK (EMSK).  In the most common deployment scenario, an EAP
   peer and an EAP server authenticate each other through a third party
   known as the EAP authenticator.  The EAP authenticator or an entity
   controlled by the EAP authenticator enforces access control.  After
   successful authentication, the EAP server transports the MSK to the
   EAP authenticator; the EAP authenticator and the EAP peer establish
   Transient Session Keys (TSKs) using the MSK as the authentication
   key, key derivation key, or a key transport key, and use the TSK for
   per-packet access enforcement.

   When a peer moves from one authenticator to another, it is desirable
   to avoid a full EAP authentication to support fast handovers.  The
   full EAP exchange with another run of the EAP method can take several
   round trips and significant time to complete, causing increased
   handover times.  Some EAP methods specify the use of state from the
   initial authentication to optimize re-authentications by reducing the
   computational overhead (e.g., EAP Authentication and Key Agreement
   (EAP-AKA) [RFC4187]), but method-specific re-authentication takes at
   least 2 round trips with the original EAP server in most cases.  It
   is also important to note that several methods do not offer support
   for re-authentication.

   Key sharing across authenticators is sometimes used as a practical
   solution to lower handover times.  In that case, however, the
   compromise of one authenticator results in the compromise of key
   material established via other authenticators.  Other solutions for
   fast re-authentication exist in the literature: for example, see
   Lopez, et al. [MSKHierarchy]; Clancy, et al. have described the EAP
   re-authentication problem statement in detail [RFC5169].

   In conclusion, to achieve low latency handovers, there is a need for
   a method-independent re-authentication protocol that completes in
   less than 2 round trips, preferably with a local server.

   This document specifies EAP Re-authentication Extensions (ERXs) for
   efficient re-authentication using EAP.  The protocol that uses these
   extensions is itself referred to as the EAP Re-authentication
   Protocol (ERP).  It supports EAP method-independent re-authentication





Cao, et al.                  Standards Track                    [Page 4]

RFC 6696                 EAP Extensions for ERP                July 2012


   for a peer that has valid, unexpired key material from a previously
   performed EAP authentication.  The protocol and the key hierarchy
   required for EAP re-authentication are described in this document.

   Note that to support ERP, lower-layer specifications may need to be
   revised to allow carrying EAP messages that have a code value higher
   than 4 and to accommodate the peer-initiated nature of ERP.
   Specifically, the Internet Key Exchange (IKE) protocol [RFC5996] must
   be updated to carry ERP messages; work is in progress on this project
   [IKE-EXT-for-ERP].

1.1.  Changes from RFC 5296

   This document obsoletes RFC 5296 but is fully backward compatible
   with that document.  The changes introduced in this document focus on
   fixing issues that have surfaced since the publication of the
   original ERP specification [RFC5296].  An overview of some of the
   major changes is given below.

   o  Co-location of the home EAP Re-authentication (ER) and EAP servers
      is no longer required (see the "ER Server" entry in Section 2).

   o  The behavior of the authenticator and local ER server during the
      bootstrapping process has been clarified (Section 5.1); in
      particular, the authenticator and/or local ER server is now
      required to check for current possession of the root keys.

   o  The authenticator is now recommended, rather than just allowed, to
      initiate the ERP conversation by means of the EAP-Initiate/
      Re-auth-Start message (Section 5.3.1.1).

   In addition, many editorial changes have been made to improve the
   clarity of the document and to eliminate perceived ambiguities.  A
   comprehensive list of changes is not given here for practical
   reasons.

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 RFC 2119 [RFC2119].










Cao, et al.                  Standards Track                    [Page 5]

RFC 6696                 EAP Extensions for ERP                July 2012


   This document uses the basic EAP terminology [RFC3748] and EMSK
   keying hierarchy terminology [RFC5295].  In addition, this document
   uses the following terms:

   ER Peer -  An EAP peer that supports the EAP Re-authentication
      Protocol.  All references to "peer" in this document imply an ER
      peer, unless specifically noted otherwise.

   ER Authenticator -  An entity that supports the authenticator
      functionality for EAP re-authentication described in this
      document.  All references to "authenticator" in this document
      imply an ER authenticator, unless specifically noted otherwise.

   ER Server -  An entity that performs the server portion of ERP
      described here.  This entity may or may not be an EAP server.  All
      references to "server" in this document imply an ER server, unless
      specifically noted otherwise.  An ER server is a logical entity;
      it may not necessarily be co-located with, or physically part of,
      a full EAP server.

   ERX -  EAP re-authentication extensions.

   ERP -  EAP Re-authentication Protocol.  Uses the re-authentication
      extensions.

   rRK -  re-authentication Root Key, derived from the EMSK or the
      Domain-Specific Root Key (DSRK).

   rIK -  re-authentication Integrity Key, derived from the rRK.

   rMSK -  re-authentication MSK.  This is a per-authenticator key,
      derived from the rRK.

   keyName-NAI -  ERP messages are integrity protected with the rIK or
      the DS-rIK.  The use of rIK or DS-rIK for integrity protection of
      ERP messages is indicated by the EMSKname [RFC5295]; the protocol,
      which is ERP; and the realm, which indicates the domain name of
      the ER server.  The EMSKname is copied into the username part of
      the Network Access Identifier (NAI).

   Domain -  Refers to a "key management domain" as defined in Salowey,
      et al. [RFC5295].  For simplicity, it is referred to as "domain"
      in this document.  The terms "home domain" and "local domain" are
      used to differentiate between the originating key management
      domain that performs the full EAP exchange with the peer and the
      local domain to which a peer may be attached at a given time.





Cao, et al.                  Standards Track                    [Page 6]

RFC 6696                 EAP Extensions for ERP                July 2012


3.  ERP Description

   ERP allows a peer and server to mutually verify proof of possession
   of key material from an earlier EAP method run and to establish a
   security association between the peer and the authenticator.  The
   authenticator acts as a pass-through entity for the re-authentication
   protocol in a manner similar to that of an EAP authenticator as
   described in Aboba, et al. [RFC3748].  ERP is a single round-trip
   exchange between the peer and the server; it is independent of the
   lower layer and the EAP method used during the full EAP exchange.
   The ER server may be in the home domain or in the same (visited)
   domain as the peer and the authenticator (i.e., the local domain).

   Figure 1 shows the protocol exchange.  The first time the peer
   attaches to any network, it performs a full EAP exchange (shown in
   Figure 2) with the EAP server; as a result, an MSK is distributed to
   the EAP authenticator.  The MSK is then used by the authenticator and
   the peer to establish TSKs as needed.  At the time of the initial EAP
   exchange, the peer and the server also derive an EMSK, which is used
   to derive an rRK.  More precisely, an rRK is derived from the EMSK or
   from a DSRK, which is itself derived from the EMSK.  The rRK is only
   available to the peer and the ER server and is never handed out to
   any other entity.  Further, an rIK is derived from the rRK; the peer
   and the ER server use the rIK to provide proof of possession while
   performing an ERP exchange.  The rIK is also never handed out to any
   entity and is only available to the peer and server.

   Peer             ER Authenticator                   ER Server
   ====             ================                   =========

     <-- EAP-Initiate/ -----
        Re-auth-Start
    [<-- EAP-Request/ ------
        Identity]


    ---- EAP-Initiate/ ----> ----AAA(EAP-Initiate/ ---------->
          Re-auth/                  Re-auth/
         [Bootstrap]              [Bootstrap])

    <--- EAP-Finish/ ------> <---AAA(rMSK,EAP-Finish/---------
          Re-auth/                   Re-auth/
        [Bootstrap]                [Bootstrap])

   Note: [] brackets indicate optionality.

                          Figure 1: ERP Exchange




Cao, et al.                  Standards Track                    [Page 7]

RFC 6696                 EAP Extensions for ERP                July 2012


   EAP Peer           EAP Authenticator                 EAP Server
   ========           =================                 ==========

    <--- EAP-Request/ ------
            Identity

    ----- EAP Response/ --->
            Identity          ---AAA(EAP Response/Identity)-->

    <--- EAP Method ------->  <------ AAA(EAP Method -------->
           exchange                    exchange)

                              <----AAA(MSK, EAP-Success)------

    <---EAP-Success---------

                       Figure 2: EAP Authentication

   Two EAP codes -- EAP-Initiate and EAP-Finish -- are specified in this
   document for the purpose of EAP re-authentication.  When the peer
   identifies a target authenticator that supports EAP
   re-authentication, it performs an ERP exchange, as shown in Figure 1;
   the exchange itself may happen when the peer attaches to a new
   authenticator supporting EAP re-authentication, or prior to
   attachment.  The peer initiates ERP by itself; it may also do so in
   response to an EAP-Initiate/Re-auth-Start message from the new
   authenticator.  The EAP-Initiate/Re-auth-Start message allows the
   authenticator to trigger the ERP exchange.  The EAP-Finish message
   also can be used by the authenticator to announce the local domain
   name.

   It is plausible that the authenticator does not know whether the peer
   supports ERP and whether the peer has performed a full EAP
   authentication through another authenticator.  The authenticator MAY
   initiate the ERP exchange by sending the EAP-Initiate/Re-auth-Start
   message and if there is no response MAY send the EAP-Request/Identity
   message.  Note that this avoids having two EAP messages in flight at
   the same time [RFC3748].  The authenticator may send the
   EAP-Initiate/Re-auth-Start message and wait for a short, locally
   configured amount of time.  This message indicates to the peer that
   the authenticator supports ERP.  In response to this trigger from the
   authenticator, the peer can initiate the ERP exchange by sending an
   EAP-Initiate/Re-auth message.  If there is no response from the peer
   after the necessary number of retransmissions (see Section 6), the
   authenticator MUST initiate EAP by sending an EAP-Request message,
   typically the EAP-Request/Identity message.  Note that the
   authenticator may receive an EAP-Initiate/Re-auth message after it
   has sent an EAP-Request/Identity message.  If the authenticator



Cao, et al.                  Standards Track                    [Page 8]

RFC 6696                 EAP Extensions for ERP                July 2012


   supports ERP, it MUST proceed with the ERP exchange.  When the
   EAP-Request/Identity times out, the authenticator MUST NOT close the
   connection if an ERP exchange is in progress or has already succeeded
   in establishing a re-authentication MSK.

   If the authenticator does not support ERP, it will silently discard
   EAP-Initiate/Re-auth messages (Section 5.3.2), since the EAP code of
   those packets is greater than 4 ([RFC3748], Section 4).  An ERP-
   capable peer will exhaust the EAP-Initiate/Re-auth message
   retransmissions and fall back to EAP authentication by responding to
   EAP-Request/Identity messages from the authenticator.  If the peer
   does not support ERP or if it does not have unexpired key material
   from a previous EAP authentication, it drops EAP-Initiate/
   Re-auth-Start messages.  If there is no response to the EAP-Initiate/
   Re-auth-Start message, the authenticator SHALL send an EAP-Request
   message (typically EAP-Request/Identity) to start EAP authentication.
   From this point onward, RFC 3748 rules apply.  Note that this may
   introduce some delay in starting EAP.  In some lower layers, the
   delay can be minimized or even avoided by the peer initiating EAP by
   sending messages such as EAPoL-Start [IEEE_802.1X].

   The peer sends an EAP-Initiate/Re-auth message that contains the
   keyName-NAI to identify the ER server's domain and the rIK used to
   protect the message, and a sequence number for replay protection.
   The EAP-Initiate/Re-auth message is integrity protected with the rIK.
   The authenticator uses the realm in the keyName-NAI field to send the
   message to the appropriate ER server.  The server uses the keyName to
   look up the rIK.  The server, after verifying proof of possession of
   the rIK and freshness of the message, derives an rMSK from the rRK
   using the sequence number as an input to the key derivation.  The
   server then updates the expected sequence number to the received
   sequence number plus one.

   In response to the EAP-Initiate/Re-auth message, the server sends an
   EAP-Finish/Re-auth message; this message is integrity protected with
   the rIK.  The server transports the rMSK along with this message to
   the authenticator.  The rMSK is transported in a manner similar to
   that of the MSK along with the EAP-Success message in a full EAP
   exchange.  Hoeper, et al. [RFC5749] discuss an additional key
   distribution protocol that can be used to transport the rRK from an
   EAP server to one of many different ER servers that share a trust
   relationship with the EAP server.

   The peer MAY request the rMSK lifetime from the server.  If so, the
   ER server sends the rMSK lifetime in the EAP-Finish/Re-auth message.






Cao, et al.                  Standards Track                    [Page 9]

RFC 6696                 EAP Extensions for ERP                July 2012


   In an ERP bootstrap exchange, the peer MAY ask the server for the rRK
   lifetime.  If so, the ER server sends the rRK lifetime in the
   EAP-Finish/Re-auth message.

   The peer verifies the sequence number and the integrity of the
   message.  It then uses the sequence number in the EAP-Finish/Re-auth
   message to compute the rMSK.  The lower-layer security association
   protocol is ready to be triggered after this point.

   The ER server is located either in the home domain or in the visited
   domain.  When the ER server is in the home domain and there is no
   local ER server in the visited domain, the peer and the server use
   the rIK and rRK derived from the EMSK; and when the ER server is in
   the local domain, they use the DS-rIK and DS-rRK corresponding to the
   local domain.  The domain of the ER server is identified by the realm
   portion of the keyName-NAI in ERP messages.

3.1.  ERP with the Home ER Server

   If the peer is in the home domain or there is no local server in the
   same domain as the peer, it SHOULD initiate an ERP bootstrap exchange
   with the home ER server to obtain the domain name.

   The defined ER extensions allow executing ERP with an ER server in
   the home domain.  The home ER server may be co-located with a home
   Authentication, Authorization, and Accounting (AAA) server.  ERP with
   the home ER server is similar to the ERP exchange described in
   Figure 1.

   Peer             ER Authenticator                   Home ER Server
   ====             ================                   ==============

     <-- EAP-Initiate/ -----
        Re-auth-Start
    [<-- EAP-Request/ ------
        Identity]


    ---- EAP-Initiate/ ----> ----AAA(EAP-Initiate/ ---------->
          Re-auth/                  Re-auth/
          Bootstrap                Bootstrap)

    <--- EAP-Finish/ ------> <---AAA(rMSK,EAP-Finish/---------
          Re-auth/                   Re-auth/
         Bootstrap                  Bootstrap)

             Figure 3: ER Explicit Bootstrapping Exchange/ERP
                          with the Home ER Server



Cao, et al.                  Standards Track                   [Page 10]

RFC 6696                 EAP Extensions for ERP                July 2012


3.2.  ERP with a Local ER Server

   The defined ER extensions allow the execution of ERP with an ER
   server in the local domain (access network) if the peer moves out of
   the home domain and a local ER server is present in the visited
   domain.  The local ER server may be co-located with a local AAA
   server.  The peer may learn about the presence of a local ER server
   in the network and the local domain name (or ER server name) either
   via a lower-layer advertisement or by means of an ERP exchange.  The
   peer uses the domain name and the EMSK to compute the DSRK and, from
   that key, the DS-rRK; the peer also uses the domain name in the realm
   portion of the keyName-NAI for using ERP in the local domain.
   Figure 4 shows the ER implicit bootstrapping exchange through a local
   ER server; Figure 5 shows ERP with a local ER server.

               EAP Authenticator     Local AAA Agent
   Peer         /ER Authenticator    /Local ER Server    Home EAP Server
   ====        ==================    ================    ===============

   <-- EAP-Request/ --
        Identity

   -- EAP Response/-->
        Identity      --AAA(EAP Response/-->
                            Identity,       --AAA(EAP Response/ -->
                        [domain name])             Identity,
                                                [DSRK Request,
                                              domain name])

   <------------------------ EAP Method exchange------------------>

                                            <---AAA(MSK, DSRK, ----
                                                   EMSKname,
                                                 EAP-Success)

                       <---  AAA(MSK,  -----
                            EAP-Success)

   <---EAP-Success-----

    Figure 4: Implicit Bootstrapping ERP Exchange, Initial EAP Exchange










Cao, et al.                  Standards Track                   [Page 11]

RFC 6696                 EAP Extensions for ERP                July 2012


   Peer                ER Authenticator            Local ER Server
   ====                ================            ===============

    <-- EAP-Initiate/ --------
        Re-auth-Start
   [<-- EAP-Request/ ---------
        Identity]


    ---- EAP-Initiate/ -------> ----AAA(EAP-Initiate/ -------->
          Re-auth                        Re-auth)


    <--- EAP-Finish/ ---------- <---AAA(rMSK,EAP-Finish/-------
          Re-auth                        Re-auth)

                       Figure 5: Local ERP Exchange

   As shown in Figure 4, the local ER server may be present in the path
   of the full EAP exchange (e.g., this may be one of the AAA entities,
   such as AAA proxies, in the path between the EAP authenticator and
   the home EAP server of the peer).  In that case, the local ER server
   requests the DSRK by sending the domain name to the home EAP server
   by means of a AAA message.  In response, the home EAP server computes
   the DSRK by following the procedure specified in RFC 5295 and sends
   the DSRK and the key name, EMSKname, to the ER server in the claimed
   domain (i.e., the local ER server).  The local domain is responsible
   for announcing that same domain name to the peer via a lower layer
   (for example, through DHCP-based local domain name discovery
   [RFC6440] or through the EAP-Initiate/Re-auth-Start message with the
   local ER server).

   After receiving the DSRK and the EMSKname, the local ER server
   computes the DS-rRK and the DS-rIK from the DSRK as defined in
   Sections 4.1 and 4.3 below.  After receiving the domain name, the
   peer also derives the DSRK, the DS-rRK, and the DS-rIK.  These keys
   are referred to by a keyName-NAI formed as follows: the username part
   of the NAI is the EMSKname, and the realm portion of the NAI is the
   domain name.  Both parties also maintain a sequence number
   (initialized to zero) corresponding to the specific keyName-NAI.

   If the peer subsequently attaches to an authenticator within the
   local domain, it may perform an ERP exchange with the local ER server
   to obtain an rMSK for the new authenticator.  ERP with the local ER
   server is similar to the ERP exchange illustrated in Figure 1.






Cao, et al.                  Standards Track                   [Page 12]

RFC 6696                 EAP Extensions for ERP                July 2012


4.  ER Key Hierarchy

   Each time the peer re-authenticates to the network, the peer and the
   authenticator establish an rMSK.  The rMSK serves the same purposes
   that an MSK, which is the result of full EAP authentication, serves.
   To prove possession of the rRK, we specify the derivation of another
   key, the rIK.  These keys are derived from the rRK.  Together they
   constitute the ER key hierarchy.

   The rRK is derived from either the EMSK or a DSRK as specified in
   Section 4.1.  For the purpose of rRK derivation, this document
   specifies derivation of a Usage-Specific Root Key (USRK) or a Domain-
   Specific USRK (DSUSRK) [RFC5295] for re-authentication.  The USRK
   designated for re-authentication is the rRK.  A DSUSRK designated for
   re-authentication is the DS-rRK available to a local ER server in a
   particular domain.  For simplicity, the keys are referred to without
   the DS label in the rest of the document.  However, the scope of the
   various keys is limited to just the respective domains for which they
   are derived, in the case of the domain-specific keys.  Based on the
   ER server with which the peer performs the ERP exchange, it knows the
   corresponding keys that must be used.

   The rRK is used to derive an rIK and rMSKs for one or more
   authenticators.  The figure below shows the key hierarchy with the
   rRK, rIK, and rMSKs.

                            rRK
                             |
                    +--------+--------+
                    |        |        |
                   rIK     rMSK1 ...rMSKn

                 Figure 6: Re-authentication Key Hierarchy

   The derivations in this document are from RFC 5295.  Key derivations
   and field encodings, where unspecified, default to that document.

4.1.  rRK Derivation

   The rRK may be derived from the EMSK or DSRK.  This section provides
   the relevant key derivations for that purpose.

   The rRK is derived as specified in RFC 5295.








Cao, et al.                  Standards Track                   [Page 13]

RFC 6696                 EAP Extensions for ERP                July 2012


   rRK = KDF (K, S), where

      K = EMSK or K = DSRK and

      S = rRK Label | "\0" | length

   The rRK Label is an IANA-assigned 8-bit ASCII string:

      EAP Re-authentication Root Key@ietf.org

   assigned from the "USRK Key Labels" name space in accordance with the
   policy stated in RFC 5295.

   The Key Derivation Function (KDF) and algorithm agility for the KDF
   are as defined in RFC 5295.

   An rRK derived from the DSRK is referred to as a DS-rRK in the rest
   of the document.  All of the key derivation and properties specified
   in this section remain the same.

4.2.  rRK Properties

   The rRK has the following properties.  These properties apply to the
   rRK regardless of the parent key used to derive it.

   o  The length of the rRK MUST be equal to the length of the parent
      key used to derive it.

   o  The rRK is to be used only as a root key for re-authentication and
      never used to directly protect any data.

   o  The rRK is only used for the derivation of the rIK and rMSK as
      specified in this document.

   o  The rRK MUST remain on the peer and the server that derived it and
      MUST NOT be transported to any other entity.

   o  The lifetime of the rRK is never greater than that of its parent
      key.  The rRK is expired when the parent key expires and MUST be
      removed from use at that time.

4.3.  rIK Derivation

   The rIK is used for integrity protecting the ERP exchange.  This
   serves as the proof of possession of valid key material from a
   previous full EAP exchange by the peer to the server.





Cao, et al.                  Standards Track                   [Page 14]

RFC 6696                 EAP Extensions for ERP                July 2012


   The rIK is derived as follows:

   rIK = KDF (K, S), where

      K = rRK and

      S = rIK Label | "\0" | cryptosuite | length

   The rIK Label is the 8-bit ASCII string:

      Re-authentication Integrity Key@ietf.org

   The length field refers to the length of the rIK in octets and is
   encoded as specified in RFC 5295.

   The cryptosuite and length of the rIK are part of the input to the
   KDF to ensure cryptographic separation of keys if different rIKs of
   different lengths (for example, for use with different Message
   Authentication Code (MAC) algorithms) are derived from the same rRK.
   The cryptosuite is encoded as an 8-bit number; see Section 5.3.2 for
   the cryptosuite specification.

   The rIK is referred to by the EMSKname-NAI within the context of ERP
   messages.  The username part of the EMSKname-NAI is the EMSKname; the
   realm is the domain name of the ER server.  In the case of ERP with
   the home ER server, the peer uses the realm from its original NAI; in
   the case of a local ER server, the peer uses the domain name received
   at the lower layer or through an ERP bootstrapping exchange.

   An rIK derived from a DS-rRK is referred to as a DS-rIK in the rest
   of the document.  All of the key derivation and properties specified
   in this section remain the same.

4.4.  rIK Properties

   The rIK has the following properties:

   o  The length of the rIK MUST be equal to the length of the rRK.

   o  The rIK is only used for authentication of the ERP exchange as
      specified in this document.

   o  The rIK MUST NOT be used to derive any other keys.

   o  The rIK must remain on the peer and the server and MUST NOT be
      transported to any other entity.





Cao, et al.                  Standards Track                   [Page 15]

RFC 6696                 EAP Extensions for ERP                July 2012


   o  The rIK is cryptographically separate from any other keys derived
      from the rRK.

   o  The lifetime of the rIK is never greater than that of its parent
      key.  The rIK MUST be expired when the EMSK expires and MUST be
      removed from use at that time.

4.5.  rIK Usage

   The rIK is the key the possession of which is demonstrated by the
   peer and the ERP server to the other party.  The peer demonstrates
   possession of the rIK by computing the integrity checksum over the
   EAP-Initiate/Re-auth message.  When the peer uses the rIK for the
   first time, it can choose the integrity algorithm to use with the
   rIK.  The peer and the server MUST use the same integrity algorithm
   with a given rIK for all ERP messages protected with that key.  The
   peer and the server store the algorithm information after the first
   use, and they employ the same algorithm for all subsequent uses of
   that rIK.

   If the server's policy does not allow the use of the cryptosuite
   selected by the peer, the server SHALL reject the EAP-Initiate/
   Re-auth message and SHOULD send a list of acceptable cryptosuites in
   the EAP-Finish/Re-auth message.

   The rIK length may be different from the key length required by an
   integrity algorithm.  In the case of hash-based MAC algorithms, the
   key is first hashed to the required key length using the HMAC
   algorithm [RFC2104].  In the case of cipher-based MAC algorithms, if
   the required key length is less than 32 octets, the rIK is hashed
   using HMAC-SHA256 and the first k octets of the output are used,
   where k is the key length required by the algorithm.  If the required
   key length is more than 32 octets, the first k octets of the rIK are
   used by the cipher-based MAC algorithm.

4.6.  rMSK Derivation

   The rMSK is derived at the peer and server and delivered to the
   authenticator.  The rMSK is derived following an ERP exchange.

   The rMSK is derived as follows:

   rMSK = KDF (K, S), where

      K = rRK and

      S = rMSK Label | "\0" | SEQ | length




Cao, et al.                  Standards Track                   [Page 16]

RFC 6696                 EAP Extensions for ERP                July 2012


   The rMSK Label is the 8-bit ASCII string:

      Re-authentication Master Session Key@ietf.org

   The length field refers to the length of the rMSK in octets and is
   encoded as specified in RFC 5295.

   SEQ is the sequence number sent by the peer in the EAP-Initiate/
   Re-auth message.  This field is encoded as a 16-bit number in network
   byte order (see Section 5.3.2).

   An rMSK derived from a DS-rRK is referred to as a DS-rIK in the rest
   of the document.  The key derivation and properties specified in this
   section remain the same.

4.7.  rMSK Properties

   The rMSK has the following properties:

   o  The length of the rMSK MUST be equal to the length of the rRK.

   o  The rMSK is delivered to the authenticator and is used for the
      same purposes that an MSK serves when the MSK is used at an
      authenticator.

   o  The rMSK is cryptographically separate from any other keys derived
      from the rRK.

   o  The lifetime of the rMSK is less than or equal to that of the rRK.
      It MUST NOT be greater than the lifetime of the rRK.

   o  If a new rRK is derived, subsequent rMSKs MUST be derived from the
      new rRK.  Previously delivered rMSKs MAY still be used until the
      expiry of the lifetime.

   o  A given rMSK MUST NOT be shared by multiple authenticators.

5.  Protocol Details

5.1.  ERP Bootstrapping

   We identify two types of bootstrapping for ERP: explicit and
   implicit.  In implicit bootstrapping, the ER-capable authenticator or
   local ER server MUST verify whether it has a valid rMSK or rRK
   corresponding to the peer.  If the ER-capable authenticator or the
   local ER server has the key material corresponding to the peer, it
   MUST be able to respond directly in the same way as the home AAA
   server does without forwarding the DSRK Request to the home domain;



Cao, et al.                  Standards Track                   [Page 17]

RFC 6696                 EAP Extensions for ERP                July 2012


   if not, the ER-capable authenticator or local ER server SHOULD
   include its domain name in the AAA message encapsulating the first
   EAP Response message sent by the peer and request the DSRK from the
   home EAP server during the initial EAP exchange.  If such an EAP
   exchange is successful, the home EAP server sends the DSRK for the
   specified local AAA client or agent (derived using the EMSK and the
   domain name as specified in RFC 5295), EMSKname, and DSRK lifetime
   along with the EAP-Success message.  The local AAA client or agent
   MUST extract the DSRK, EMSKname, and DSRK lifetime (if present)
   before forwarding the EAP-Success message to the peer.  Note that the
   MSK (also present with the EAP-Success message) is extracted by the
   EAP authenticator as usual.  The peer learns the domain name through
   the EAP-Initiate/Re-auth-Start message or by means of a lower-layer
   announcement (for example, DHCP [RFC6440]).  When the domain name is
   available to the peer during or after the full EAP authentication, it
   attempts to use ERP when it associates with a new authenticator.

   If the peer knows there is no local ER server present in the visited
   domain, it SHOULD initiate ERP explicit bootstrapping (ERP exchange
   with the bootstrap flag turned on) with the home ER server to obtain
   the rRK.  The peer MAY also initiate bootstrapping to fetch
   information such as the rRK lifetime from the AAA server.

   The following steps describe the ERP explicit bootstrapping process:

   o  The peer sends the EAP-Initiate/Re-auth message with the
      bootstrapping flag set (1).  The bootstrap message is always sent
      to the home ER server, and the keyName-NAI attribute in the
      bootstrap message is constructed as follows: the username portion
      of the NAI contains the EMSKname, and the realm portion contains
      the home domain name.

   o  In addition, the message MUST contain a sequence number for replay
      protection, a cryptosuite, and an integrity checksum.  The
      cryptosuite indicates the authentication algorithm.  The integrity
      checksum indicates that the message originated at the claimed
      entity, the peer indicated by the Peer-ID, or the rIKname.

   o  The peer MAY additionally set the lifetime flag to request the key
      lifetimes.

   o  Upon receipt of the EAP-Initiate/Re-auth message from a peer, the
      ERP-capable authenticator verifies whether it has the local domain
      name and valid key material corresponding to the peer.  If it
      knows the local domain name and has valid key material
      corresponding to the peer, it MUST be able to respond directly in
      the same way as the home ER does, with the local domain name
      included.  If not, it copies the contents of the keyName-NAI into



Cao, et al.                  Standards Track                   [Page 18]

RFC 6696                 EAP Extensions for ERP                July 2012


      the appropriate AAA attribute and may include its domain name in
      the AAA message encapsulating the EAP-Initiate/Re-auth message
      sent by the peer.

   o  Upon receipt of an EAP-Initiate/Re-auth message, the home ER
      server verifies whether the message is fresh or is a replay by
      evaluating whether the received sequence number is equal to or
      greater than the expected sequence number for that rIK.  The home
      ER server then verifies that the cryptosuite used by the peer is
      acceptable.  Next, it verifies the integrity of the message by
      looking up the rIK and checking the integrity checksum contained
      in the Authentication Tag field.  If any of the checks fail, the
      home ER server sends an EAP-Finish/Re-auth message with the Result
      flag set to '1'.  Please refer to Section 5.2.2 for details on
      failure handling.  This error MUST NOT have any correlation to any
      EAP-Success message that may have been received by the EAP
      authenticator and the peer earlier.  If the EAP-Initiate/Re-auth
      message is well formed and valid, the server prepares the
      EAP-Finish/Re-auth message.  The bootstrap flag MUST be set to
      indicate that this is a bootstrapping exchange.  The message
      contains the following fields:

      *  A sequence number for replay protection.

      *  The same keyName-NAI as in the EAP-Initiate/Re-auth message.

      *  If the lifetime flag was set in the EAP-Initiate/Re-auth
         message, the ER server SHOULD include the rRK lifetime and the
         rMSK lifetime in the EAP-Finish/Re-auth message.  The server
         may have a local policy for the network to maintain and enforce
         lifetime unilaterally.  In such cases, the server need not
         respond to the peer's request for the lifetime.

      *  If the bootstrap flag is set, the ER server MUST include the
         domain name to which the DSRK is being sent along with the
         EAP-Finish/Re-auth message.

      *  If the ER server verifies the authorization of a local ER
         server, it MAY include the Authorization Indication TLV to
         indicate to the peer that the server that received the DSRK and
         that is advertising the domain included in the Domain name TLV
         is authorized.

      *  An authentication tag MUST be included to prove that the
         EAP-Finish/Re-auth message originates at a server that
         possesses the rIK corresponding to the EMSKname-NAI.





Cao, et al.                  Standards Track                   [Page 19]

RFC 6696                 EAP Extensions for ERP                July 2012


   o  If the home ER server is involved in the ERP exchange and the ERP
      exchange is successful, the home ER server SHOULD request the DSRK
      from the home EAP server; the home EAP server MUST provide the
      DSRK for the home ER server (derived using the EMSK and the domain
      name as specified in RFC 5295), EMSKname, and DSRK lifetime for
      inclusion in the AAA message.  The home ER server SHOULD obtain
      them before sending the EAP-Finish/Re-auth message.

   o  In addition, the rMSK is sent along with the EAP-Finish/Re-auth
      message in a AAA attribute (for an example, see Bournelle,
      et al. [DIAMETER-ERP]).

   o  The authenticator receives the rMSK.

   o  When the peer receives an EAP-Finish/Re-auth message with the
      bootstrap flag set, if a local domain name is present, it MUST use
      that name to derive the appropriate DSRK, DS-rRK, DS-rIK, and
      keyName-NAI, and initialize the replay counter for the DS-rIK.  If
      not, the peer SHOULD derive the domain-specific keys using the
      domain name it learned via the lower layer or from the
      EAP-Initiate/Re-auth-Start message.  If the peer does not know the
      domain name, it must assume that there is no local ER server
      available.

   o  The peer MAY also verify the Authorization Indication TLV.

   o  The procedures for encapsulating ERP and obtaining relevant keys
      using Diameter are specified in Bournelle, et al. [DIAMETER-ERP].

   Since the ER bootstrapping exchange is typically done immediately
   following the full EAP exchange, it is feasible that the process is
   completed through the same entity that served as the EAP
   authenticator for the full EAP exchange.  In this case, the lower
   layer may already have established TSKs based on the MSK received
   earlier.  The lower layer may then choose to ignore the rMSK that was
   received with the ER bootstrapping exchange.  Alternatively, the
   lower layer may choose to establish a new TSK using the rMSK.  In
   either case, the authenticator and the peer know which key is used
   based on whether or not a TSK establishment exchange is initiated.
   The bootstrapping exchange may also be carried out via a new
   authenticator, in which case, the rMSK received SHOULD trigger a
   lower-layer TSK establishment exchange.

5.2.  Steps in ERP

   When a peer that has an active rRK and rIK associates with a new
   authenticator that supports ERP, it may perform an ERP exchange with
   that authenticator.  ERP is typically a peer-initiated exchange,



Cao, et al.                  Standards Track                   [Page 20]

RFC 6696                 EAP Extensions for ERP                July 2012


   consisting of an EAP-Initiate/Re-auth and an EAP-Finish/Re-auth
   message.  The ERP exchange may be performed with a local ER server
   (when one is present) or with the original EAP server.

   It is plausible for the network to trigger the EAP re-authentication
   process, however.  An ERP-capable authenticator SHOULD send an
   EAP-Initiate/Re-auth-Start message to indicate support for ERP.  The
   peer may or may not wait for these messages to arrive to initiate the
   EAP-Initiate/Re-auth message.

   The EAP-Initiate/Re-auth-Start message SHOULD be sent by an ERP-
   capable authenticator.  The authenticator may retransmit it a few
   times until it receives an EAP-Initiate/Re-auth message in response
   from the peer.  The EAP-Initiate/Re-auth message from the peer may
   have originated before the peer receives either an EAP-Request/
   Identity or an EAP-Initiate/Re-auth-Start message from the
   authenticator.  Hence, the Identifier value in the EAP-Initiate/
   Re-auth message is independent of the Identifier value in the
   EAP-Initiate/Re-auth-Start or EAP-Request/Identity messages.

   Operational Considerations at the Peer:

   ERP requires that the peer maintain retransmission timers for
   reliable transport of EAP re-authentication messages.  The
   reliability considerations of Section 4.3 of RFC 3748 apply with the
   peer as the retransmitting entity.

   ERP has the following steps:

   o  The ERP-capable authenticator sends the EAP-Initiate/Re-auth-Start
      message to trigger the ERP exchange.

   o  The peer sends an EAP-Initiate/Re-auth message.  At a minimum, the
      message SHALL include the following fields:

      *  a 16-bit sequence number for replay protection.

      *  keyName-NAI as a TLV attribute to identify the rIK used to
         integrity protect the message.

      *  cryptosuite to indicate the authentication algorithm used to
         compute the integrity checksum.

      *  authentication tag computed over the message.

   o  When the peer is performing ERP with a local ER server, it MUST
      use the corresponding DS-rIK it shares with the local ER server.
      The peer SHOULD set the lifetime flag to request the key lifetimes



Cao, et al.                  Standards Track                   [Page 21]

RFC 6696                 EAP Extensions for ERP                July 2012


      from the server.  The peer can use the rRK lifetime to know when
      to trigger an EAP method exchange and the rMSK lifetime to know
      when to trigger another ERP exchange.

   o  The authenticator copies the contents of the value field of the
      keyName-NAI TLV into an appropriate attribute (e.g., User-Name
      [RFC2865]) in the AAA message to the ER server.

   o  The ER server uses the keyName-NAI to look up the rIK.  It MUST
      first verify whether the sequence number is equal to or greater
      than the expected sequence number.  If the ER server supports a
      sequence number window size greater than 1, it MUST verify whether
      the sequence number falls within the window and has not been
      received before.  The ER server MUST then verify that the
      cryptosuite used by the peer is acceptable.  The ER server then
      proceeds to verify the integrity of the message using the rIK,
      thereby verifying proof of possession of that key by the peer.  If
      any of these verifications fail, the ER server MUST send an
      EAP-Finish/Re-auth message with the Result flag set to '1'
      (Failure).  Please refer to Section 5.2.2 for details on failure
      handling.  Otherwise, it MUST compute an rMSK from the rRK using
      the sequence number as the additional input to the key derivation.

   o  In response to a well-formed EAP-Initiate/Re-auth message, the ER
      server MUST send an EAP-Finish/Re-auth message with the following
      contents:

      *  a 16-bit sequence number for replay protection, which MUST be
         the same as the received sequence number.  The local copy of
         the sequence number MUST be incremented by 1.  If the ER server
         supports multiple simultaneous ERP exchanges, it MUST instead
         update the sequence number window.

      *  keyName-NAI as a TLV attribute to identify the rIK used to
         integrity protect the message.

      *  cryptosuite to indicate the authentication algorithm used to
         compute the integrity checksum.

      *  authentication tag computed over the message.

      *  If the lifetime flag was set in the EAP-Initiate/Re-auth
         message, the ER server SHOULD include the rRK lifetime and the
         rMSK lifetime.







Cao, et al.                  Standards Track                   [Page 22]

RFC 6696                 EAP Extensions for ERP                July 2012


   o  The ER server causes the rMSK along with this message to be
      transported to the authenticator.  The rMSK is transported in a
      manner similar to the MSK and the EAP-Success message in a regular
      EAP exchange.

   o  The peer looks up the sequence number to verify whether it is
      expecting an EAP-Finish/Re-auth message with that sequence number
      protected by the keyName-NAI.  It then verifies the integrity of
      the message.  If the verifications fail, the peer logs an error
      and stops the process; otherwise, it proceeds to the next step.

   o  The peer uses the sequence number to compute the rMSK.

   o  The lower-layer security association protocol can be triggered at
      this point.

5.2.1.  Multiple Simultaneous Runs of ERP

   When a peer is within the range of multiple authenticators, it may
   choose to run ERP via all of them simultaneously to the same ER
   server.  In that case, it is plausible that the ERP messages may
   arrive out of order, resulting in the ER server rejecting legitimate
   EAP-Initiate/Re-auth messages.

   To facilitate such operation, an ER server MAY allow multiple
   simultaneous ERP exchanges by accepting all EAP-Initiate/Re-auth
   messages with sequence number values within a window of allowed
   values.  Recall that the sequence number allows replay protection.
   Replay window maintenance mechanisms are a local matter.

5.2.2.  ERP Failure Handling

   If the processing of the EAP-Initiate/Re-auth message results in a
   failure, the ER server MUST send an EAP-Finish/Re-auth message with
   the Result flag set to '1'.  If the server has a valid rIK for the
   peer, it MUST integrity protect the EAP-Finish/Re-auth failure
   message.  If the failure is due to an unacceptable cryptosuite, the
   server SHOULD send a list of acceptable cryptosuites (in a TLV of
   Type 5) along with the EAP-Finish/Re-auth message.  In this case, the
   server MUST indicate the cryptosuite used to protect the EAP-Finish/
   Re-auth message in the Cryptosuite field of that message.  The rIK
   used with the EAP-Finish/Re-auth message in this case MUST be
   computed as specified in Section 4.3 using the new cryptosuite.  If
   the server does not have a valid rIK for the peer, the EAP-Finish/
   Re-auth message indicating a failure will be unauthenticated; the
   server MAY include a list of acceptable cryptosuites in the message.





Cao, et al.                  Standards Track                   [Page 23]

RFC 6696                 EAP Extensions for ERP                July 2012


   The peer, upon receiving an EAP-Finish/Re-auth message with the
   Result flag set to '1', MUST verify the sequence number and, if
   possible, the authentication tag to determine the validity of the
   message.  If the peer supports the cryptosuite, it MUST verify the
   integrity of the received EAP-Finish/Re-auth message.  If the
   EAP-Finish message contains a TLV of Type 5, the peer SHOULD retry
   the ERP exchange with a cryptosuite picked from the list included by
   the server.  The peer MUST use the appropriate rIK for the subsequent
   ERP exchange by computing it with the corresponding cryptosuite, as
   specified in Section 4.3.  If the Pseudo-Random Function (PRF) in the
   chosen cryptosuite is different from the PRF originally used by the
   peer, it MUST derive a new DSRK (if required), rRK, and rIK before
   proceeding with the subsequent ERP exchange.

   If the peer cannot verify the integrity of the received message, it
   MAY choose to retry the ERP exchange with one of the cryptosuites in
   the list of acceptable cryptosuites (in a TLV of Type 5), after a
   failure has been clearly determined following the procedure in the
   next paragraph.

   If the replay or integrity checks fail, the failure message may have
   been sent by an attacker.  It may also mean that the server and peer
   do not support the same cryptosuites; however, the peer cannot
   determine if that is the case.  Hence, the peer SHOULD continue the
   ERP exchange per the retransmission timers before declaring a
   failure.

   When the peer runs explicit bootstrapping (ERP with the bootstrapping
   flag on), there may not be a local ER server available to send a DSRK
   Request and the domain name.  In that case, the server cannot send
   the DSRK and MUST NOT include the Domain name TLV.  When the peer
   receives a response in the bootstrapping exchange without a Domain
   name TLV, it assumes that there is no local ER server.  The home ER
   server sends an rMSK to the ER authenticator, however, and the peer
   SHALL run the TSK establishment protocol as usual.
















Cao, et al.                  Standards Track                   [Page 24]

RFC 6696                 EAP Extensions for ERP                July 2012


5.3.  EAP Codes

   Two EAP codes are defined for the purpose of ERP: EAP-Initiate and
   EAP-Finish.  The packet format for these messages follows the EAP
   packet format defined in Aboba, et al. [RFC3748].

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Type-Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                           Figure 7: EAP Packet

      Code

         Two code values are defined for the purpose of ERP:

         5 Initiate

         6 Finish

      Identifier

         The Identifier field is one octet.  The Identifier field MUST
         be the same if an EAP-Initiate packet is retransmitted due to a
         timeout while waiting for an EAP-Finish message.  Any new
         (non-retransmission) EAP-Initiate message MUST use a new
         Identifier field.

         The Identifier field of the EAP-Finish message MUST match that
         of the currently outstanding EAP-Initiate message.  A peer or
         authenticator receiving an EAP-Finish message whose Identifier
         value does not match that of the currently outstanding
         EAP-Initiate message MUST silently discard the packet.

         In order to avoid confusion between new EAP-Initiate messages
         and retransmissions, the peer must choose an Identifier value
         that is different from the previous EAP-Initiate message,
         especially if that exchange has not finished.  It is
         RECOMMENDED that the authenticator clear EAP Re-auth state
         after 300 seconds.







Cao, et al.                  Standards Track                   [Page 25]

RFC 6696                 EAP Extensions for ERP                July 2012


      Type

         This field indicates that this is an ERP exchange.  Two type
         values are defined in this document for this purpose --
         Re-auth-Start (Type 1) and Re-auth (Type 2).

      Type-Data

         The Type-Data field varies according to the value of the Type
         field in the re-authentication packet.

5.3.1.  EAP-Initiate/Re-auth-Start Packet

   The EAP-Initiate/Re-auth-Start packet contains the fields shown in
   Figure 8.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |   Reserved    |     1 or more TVs or TLVs     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 8: EAP-Initiate/Re-auth-Start Packet

      Type = 1.

      Reserved:  MUST be zero.  Set to zero on transmission and ignored
         on reception.

      One or more Type/Values (TVs) or TLVs are used to convey
      information to the peer; for instance, the authenticator may send
      the domain name to the peer.

      TVs or TLVs:  In the TV payloads, there is a 1-octet type payload
         and a value with type-specific length.  In the TLV payloads,
         there is a 1-octet type payload and a 1-octet length payload.
         The length field indicates the length of the value expressed in
         number of octets.

         Domain name:  This is a TLV payload.  The Type is 4.  The
            domain name is to be used as the realm in an NAI [RFC4282].
            The Domain name TLV SHOULD be present in an EAP-Initiate/
            Re-auth-Start message.






Cao, et al.                  Standards Track                   [Page 26]

RFC 6696                 EAP Extensions for ERP                July 2012


         In addition, channel binding information MAY be included; see
         Section 5.5 for discussion.  See Figure 12 for parameter
         specification.

5.3.1.1.  Authenticator Operation

   In order to minimize ERP failure times, the authenticator SHOULD send
   the EAP-Initiate/Re-auth-Start message to indicate support for ERP to
   the peer and to initiate ERP if the peer has already performed full
   EAP authentication and has unexpired key material.  The authenticator
   SHOULD include the Domain name TLV to allow the peer to learn it
   without requiring either lower-layer support or the ERP bootstrapping
   exchange.

   The authenticator MAY include channel binding information so that the
   server can verify whether the authenticator is claiming the same
   identity to both parties.

   The authenticator MAY retransmit the EAP-Initiate/Re-auth-Start
   message a few times for reliable transport.

5.3.1.2.  Peer Operation

   The peer SHOULD send the EAP-Initiate/Re-auth message in response to
   the EAP-Initiate/Re-auth-Start message from the authenticator.  If
   the peer does not recognize the EAP-Initiate code value or if the
   peer has already sent the EAP-Initiate/Re-auth message to begin the
   ERP exchange, it MUST silently discard the EAP-Initiate/Re-auth-Start
   message.

   If the EAP-Initiate/Re-auth-Start message contains the domain name,
   and if the peer does not already have the domain information, the
   peer SHOULD use the domain name contained in the message to compute
   the DSRK and use the corresponding DS-rIK to send an EAP-Initiate/
   Re-auth message to start an ERP exchange with the local ER server.
   If there is a local ER server between the peer and the home ER server
   and the peer has already initiated an ERP exchange with the local ER
   server, it SHOULD NOT start an ERP exchange with the home ER server.













Cao, et al.                  Standards Track                   [Page 27]

RFC 6696                 EAP Extensions for ERP                July 2012


5.3.2.  EAP-Initiate/Re-auth Packet

   The EAP-Initiate/Re-auth packet contains the parameters shown in
   Figure 9.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |R|B|L| Reserved|             SEQ               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 1 or more TVs or TLVs                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Cryptosuite  |        Authentication Tag                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 9: EAP-Initiate/Re-auth Packet

      Type = 2.

      Flags

         'R' -  The R flag is set to 0 and ignored upon reception.

         'B' -  The B flag is used as the bootstrapping flag.  If the
                flag is turned on, the message is a bootstrap message.

         'L' -  The L flag is used to request the key lifetimes from the
                server.

         The remaining 5 bits are set to 0 on transmission and ignored
         on reception.

      SEQ:  An unsigned 16-bit sequence number is used for replay
         protection.  The SEQ field is initialized to 0 every time a new
         rRK is derived.  The field is encoded in network byte order.

      TVs or TLVs:  In the TV payloads, there is a 1-octet type payload
         and a value with type-specific length.  In the TLV payloads,
         there is a 1-octet type payload and a 1-octet length payload.
         The length field indicates the length of the value expressed in
         number of octets.

         keyName-NAI:  This is carried in a TLV payload.  The Type is 1.
            The NAI is variable in length, not exceeding 253 octets.
            The EMSKname is in the username part of the NAI and is
            encoded in hexadecimal values.  The EMSKname is 64 bits in



Cao, et al.                  Standards Track                   [Page 28]

RFC 6696                 EAP Extensions for ERP                July 2012


            length, and so the username portion takes up 16 octets.  If
            the rIK is derived from the EMSK, the realm part of the NAI
            is the home domain name, and if the rIK is derived from a
            DSRK, the realm part of the NAI is the domain name used in
            the derivation of the DSRK.  The NAI syntax is specified in
            Aboba, et al. [RFC4282].  Exactly one keyName-NAI attribute
            SHALL be present in an EAP-Initiate/Re-auth packet.

         In addition, channel binding information MAY be included; see
         Section 5.5 for discussion.  See Figure 12 for parameter
         specification.

      Cryptosuite:  This field indicates the integrity algorithm used
         for ERP.  Key lengths and output lengths are either indicated
         or are obvious from the cryptosuite name.  We specify some
         cryptosuites below:

         *  0 RESERVED

         *  1 HMAC-SHA256-64

         *  2 HMAC-SHA256-128

         *  3 HMAC-SHA256-256

      HMAC-SHA256-128 is mandatory to implement and SHOULD be enabled in
      the default configuration.

      Authentication Tag:  This field contains the integrity checksum
         over the ERP packet, excluding the Authentication Tag field
         itself.  The length of the field is indicated by the
         cryptosuite.



















Cao, et al.                  Standards Track                   [Page 29]

RFC 6696                 EAP Extensions for ERP                July 2012


5.3.3.  EAP-Finish/Re-auth Packet

   The EAP-Finish/Re-auth packet contains the parameters shown in
   Figure 10.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Code      |  Identifier   |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |R|B|L| Reserved |             SEQ              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 1 or more TVs or TLVs                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Cryptosuite  |        Authentication Tag                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 10: EAP-Finish/Re-auth Packet

      Type = 2.

      Flags

         'R' -  The R flag is used as the Result flag.  When set to 0,
                it indicates success, and when set to '1', it indicates
                a failure.

         'B' -  The B flag is used as the bootstrapping flag.  If the
                flag is turned on, the message is a bootstrap message.

         'L' -  The L flag is used to indicate the presence of the rRK
                lifetime TLV.

         The remaining 5 bits are set to 0 on transmission and ignored
         on reception.

      SEQ:  An unsigned 16-bit sequence number is used for replay
         protection.  The SEQ field is initialized to 0 every time a new
         rRK is derived.  The field is encoded in network byte order.












Cao, et al.                  Standards Track                   [Page 30]

RFC 6696                 EAP Extensions for ERP                July 2012


      TVs or TLVs:  In the TV payloads, there is a 1-octet type payload
         and a value with type-specific length.  In the TLV payloads,
         there is a 1-octet type payload and a 1-octet length payload.
         The length field indicates the length of the value expressed in
         number of octets.

         keyName-NAI:  This is carried in a TLV payload.  The Type is 1.
            The NAI is variable in length, not exceeding 253 octets.
            EMSKname is in the username part of the NAI and is encoded
            in hexadecimal values.  The EMSKname is 64 bits in length,
            and so the username portion takes up 16 octets.  If the rIK
            is derived from the EMSK, the realm part of the NAI is the
            home domain name, and if the rIK is derived from a DSRK, the
            realm part of the NAI is the domain name used in the
            derivation of the DSRK.  The NAI syntax is specified in
            [RFC4282].  Exactly one instance of the keyName-NAI
            attribute SHALL be present in an EAP-Finish/Re-auth message.

         rRK Lifetime:  This is a TV payload.  The Type is 2.  The value
            field contains an unsigned 32-bit integer in network byte
            order representing the lifetime of the rRK in seconds.  If
            the 'L' flag is set, the rRK Lifetime attribute SHOULD be
            present.

         rMSK Lifetime:  This is a TV payload.  The Type is 3.  The
            value field contains an unsigned 32-bit integer in network
            byte order representing the lifetime of the rMSK in seconds.
            If the 'L' flag is set, the rMSK Lifetime attribute SHOULD
            be present.

         Domain name:  This is a TLV payload.  The Type is 4.  The
            domain name is to be used as the realm in an NAI [RFC4282].
            The Domain name attribute MUST be present in an EAP-Finish/
            Re-auth message if the bootstrapping flag is set and if the
            local ER server sent a DSRK Request.

         List of cryptosuites:  This is a TLV payload.  The Type is 5.
            The value field contains a list of cryptosuites, each of
            size 1 octet.  The cryptosuite values are as specified in
            Figure 9.  The server SHOULD include this attribute if the
            cryptosuite used in the EAP-Initiate/Re-auth message was not
            acceptable and the message is being rejected.  The server
            MAY include this attribute in other cases.  The server MAY
            use this attribute to signal its cryptographic algorithm
            capabilities to the peer.






Cao, et al.                  Standards Track                   [Page 31]

RFC 6696                 EAP Extensions for ERP                July 2012


         Authorization Indication:  This is a TLV payload.  The Type
            is 6.  This attribute MAY be included in the EAP-Finish/
            Re-auth message when a DSRK is delivered to a local ER
            server and if the home EAP server can verify the
            authorization of the local ER server to advertise the domain
            name included in the domain TLV in the same message.  The
            value field in the TLV contains an authentication tag
            computed over the entire packet, starting from the first bit
            of the code field to the last bit of the Cryptosuite field,
            with the value field of the Authorization Indication TLV
            filled with all 0s for the computation.  The key used for
            the computation MUST be derived from the EMSK with key label
            "DSRK Delivery Authorized Key@ietf.org" and optional data
            containing an ASCII string representing the key management
            domain for which the DSRK is being derived.

         In addition, channel binding information MAY be included: see
         Section 5.5 for discussion.  See Figure 12 for parameter
         specification.  The server sends this information so that the
         peer can verify the information seen at the lower layer, if
         channel binding is to be supported.

      Cryptosuite:  This field indicates the integrity algorithm and the
         PRF used for ERP.  Key lengths and output lengths are either
         indicated or are obvious from the cryptosuite name.

      Authentication Tag:  This field contains the integrity checksum
         over the ERP packet, excluding the Authentication Tag field
         itself.  The length of the field is indicated by the
         cryptosuite.

5.3.4.  TV and TLV Attributes

   The TV attributes that may be present in the EAP-Initiate or
   EAP-Finish messages are of the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |              Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 11: TV Attribute Format








Cao, et al.                  Standards Track                   [Page 32]

RFC 6696                 EAP Extensions for ERP                July 2012


   The TLV attributes that may be present in the EAP-Initiate or
   EAP-Finish messages are of the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |            Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 12: TLV Attribute Format

   The following Types are defined in this document:

      '1' - keyName-NAI: This is a TLV payload.

      '2' - rRK Lifetime: This is a TV payload.

      '3' - rMSK Lifetime: This is a TV payload.

      '4' - Domain name: This is a TLV payload.

      '5' - Cryptosuite list: This is a TLV payload.

      '6' - Authorization Indication: This is a TLV payload.

      The TLV type range of 128-191 is reserved to carry channel binding
      information in the EAP-Initiate/Re-auth and EAP-Finish/Re-auth
      messages.  Below are the current assignments (all of them are
      TLVs):

         '128' - Called-Station-Id [RFC2865]

         '129' - Calling-Station-Id [RFC2865]

         '130' - NAS-Identifier [RFC2865]

         '131' - NAS-IP-Address [RFC2865]

         '132' - NAS-IPv6-Address [RFC3162]

   The length field indicates the length of the value part of the
   attribute in octets.

5.4.  Replay Protection

   For replay protection, ERP uses sequence numbers.  The sequence
   number is maintained on a per rIK basis and is initialized to zero in
   both directions.  In the first EAP-Initiate/Re-auth message, the peer



Cao, et al.                  Standards Track                   [Page 33]

RFC 6696                 EAP Extensions for ERP                July 2012


   uses a sequence number value of zero or higher.  Note that when the
   sequence number wraps back to zero, the rIK MUST be changed by
   running a full EAP authentication.  The server expects a sequence
   number of zero or higher.  When the server receives an EAP-Initiate/
   Re-auth message, it uses the same sequence number in the EAP-Finish/
   Re-auth message.  The server then sets the expected sequence number
   to the received sequence number plus 1.  The server MUST accept
   sequence numbers greater than or equal to the expected sequence
   number.

   If the peer sends an EAP-Initiate/Re-auth message but does not
   receive a response, it retransmits the request (with no changes to
   the message itself) a preconfigured number of times before giving up.
   However, it is plausible that the server itself may have responded to
   the message and the response was lost in transit.  Thus, the peer
   MUST increment the sequence number and use the new sequence number to
   send subsequent EAP re-authentication messages.  The peer SHOULD
   increment the sequence number by 1; however, it may choose to
   increment by a larger number.  If the sequence number wraps back to
   zero, the peer MUST run full EAP authentication.

5.5.  Channel Binding

   ERP provides a protected facility to carry channel binding (CB)
   information, according to the guidelines provided by Aboba,
   et al. (see Section 7.15 of [RFC3748]).  The TLV type range of
   128-191 is reserved to carry CB information in the EAP-Initiate/
   Re-auth and EAP-Finish/Re-auth messages.  Called-Station-Id,
   Calling-Station-Id, NAS-Identifier, NAS-IP-Address, and
   NAS-IPv6-Address are some examples of channel binding information
   listed in RFC 3748, and they are assigned values 128-132.  Additional
   values are managed by IANA, based on IETF Review (formerly called
   "IETF Consensus") [RFC5226].

   The authenticator MAY provide CB information to the peer via the
   EAP-Initiate/Re-auth-Start message.  The peer sends the information
   to the server in the EAP-Initiate/Re-auth message; the server
   verifies whether the authenticator identity available via AAA
   attributes is the same as the identity provided to the peer.

   If the peer does not include the CB information in the EAP-Initiate/
   Re-auth message, and if the local ER server's policy requires channel
   binding support, it SHALL send the CB attributes for the peer's
   verification.  The peer attempts to verify the CB information if the
   authenticator has sent the CB parameters, and it proceeds with the
   lower-layer security association establishment if the attributes
   match.  Otherwise, the peer SHALL NOT proceed with the lower-layer
   security association establishment.



Cao, et al.                  Standards Track                   [Page 34]

RFC 6696                 EAP Extensions for ERP                July 2012


6.  Lower-Layer Considerations

   The authenticator is responsible for retransmission of EAP-Initiate/
   Re-auth-Start messages.  The authenticator MAY retransmit the message
   a few times or until it receives an EAP-Initiate/Re-auth message from
   the peer.  The authenticator might not know if the peer supports ERP;
   in those cases, the peer could be silently discarding the
   EAP-Initiate/Re-auth-Start packets.  Thus, retransmission of these
   packets should be kept to a minimum.  The exact number is up to each
   lower layer.

   The Identifier value in the EAP-Initiate/Re-auth packet is
   independent of the Identifier value in the EAP-Initiate/Re-auth-Start
   packet.

   The peer is responsible for retransmission of EAP-Initiate/Re-auth
   messages.

   Retransmitted packets MUST be sent with the same Identifier value in
   order to distinguish them from new packets.  By default, where the
   EAP-Initiate message is sent over an unreliable lower layer, the
   retransmission timer SHOULD be dynamically estimated.  A maximum of
   3-5 retransmissions is suggested [RFC3748].  Where the EAP-Initiate
   message is sent over a reliable lower layer, the retransmission timer
   SHOULD be set to an infinite value so that retransmissions do not
   occur at the EAP layer.  Please refer to RFC 3748 for additional
   guidance on setting timers.

   The Identifier value in the EAP-Finish/Re-auth packet is the same as
   the Identifier value in the EAP-Initiate/Re-auth packet.

   If an authenticator receives a valid duplicate EAP-Initiate/Re-auth
   message for which it has already sent an EAP-Finish/Re-auth message,
   it MUST resend the EAP-Finish/Re-auth message without reprocessing
   the EAP-Initiate/Re-auth message.  To facilitate this, the
   authenticator SHALL store a copy of the EAP-Finish/Re-auth message
   for a finite amount of time.  The actual value of time is a local
   matter; this specification recommends a value of 100 milliseconds.

   The lower layer may provide facilities for exchanging information
   between the peer and the authenticator about support for ERP, for the
   authenticator to send the domain name information and channel binding
   information to the peer.








Cao, et al.                  Standards Track                   [Page 35]

RFC 6696                 EAP Extensions for ERP                July 2012


   Note that to support ERP, lower-layer specifications may need to be
   revised.  Specifically, RFC 5996 must be updated to include EAP code
   values higher than 4 in order to use ERP with Internet Key Exchange
   Protocol version 2 (IKEv2).  IKEv2 may also be updated to support
   peer-initiated ERP for optimized operation.  Other lower layers may
   need similar revisions.

   Our analysis indicates that some EAP implementations are not RFC 3748
   compliant in that instead of silently dropping EAP packets with code
   values higher than 4, they may consider it an error.  To accommodate
   such non-compliant EAP implementations, additional guidance has been
   provided below.  Furthermore, it may not be easy to upgrade all the
   peers in some cases.  In such cases, authenticators may be configured
   to not send EAP-Initiate/Re-auth-Start messages; peers may learn
   whether an authenticator supports ERP via configuration or from
   advertisements at the lower layer.

   In order to accommodate implementations that are not compliant to
   RFC 3748, such lower layers SHOULD ensure that both parties support
   ERP; this is trivial, for instance, when using a lower layer that is
   known to always support ERP.  For lower layers where ERP support is
   not guaranteed, ERP support may be indicated through signaling (e.g.,
   piggybacked on a beacon) or through negotiation.  Alternatively,
   clients may recognize environments where ERP is available based on
   preconfiguration.  Other similar mechanisms may also be used.  When
   ERP support cannot be verified, lower layers may mandate falling back
   to full EAP authentication to accommodate EAP implementations that
   are not compliant to RFC 3748.

7.  AAA Transport of ERP Messages

   AAA transport of ERP messages is specified by Hoeper,
   et al. [RFC5749] and Bournelle, et al. [DIAMETER-ERP].

8.  Security Considerations

   This section provides an analysis of the protocol in accordance with
   the AAA key management guidelines described by Housley & Aboba
   [RFC4962].

      Cryptographic algorithm independence

         ERP satisfies this requirement.  The algorithm chosen by the
         peer for the MAC generation is indicated in the EAP-Initiate/
         Re-auth message.  If the chosen algorithm is unacceptable, the
         EAP server returns an EAP-Finish/Re-auth message indicating a
         failure.  Algorithm agility for the KDF is specified in
         Salowey, et al. [RFC5295].  Only when the algorithms used are



Cao, et al.                  Standards Track                   [Page 36]

RFC 6696                 EAP Extensions for ERP                July 2012


         deemed acceptable does the server proceed with the derivation
         of keys and verification of the proof of possession of relevant
         key material presented by the peer.  A full-blown negotiation
         of algorithms cannot be provided in a single round-trip
         protocol.  Hence, while the protocol provides algorithm
         agility, it does not provide true negotiation.

      Strong, fresh session keys

         ERP results in the derivation of strong, fresh keys that are
         unique for the given session.  An rMSK is always derived on
         demand when the peer requires a key with a new authenticator.
         The derivation ensures that the compromise of one rMSK does not
         result in the compromise of another rMSK at any time.

      Limited key scope

         The scope of all the keys derived by ERP is well defined.  The
         rRK and rIK are never shared with any entity and always remain
         on the peer and the server.  The rMSK is provided only to the
         authenticator through which the peer performs the ERP exchange.
         No other authenticator is authorized to use that rMSK.

      Replay detection mechanism

         For replay protection of ERP messages, a sequence number
         associated with the rIK is used.  The sequence number is
         maintained by the peer and the server and is initialized to
         zero when the rIK is generated.  The peer increments the
         sequence number by one after it sends an ERP message.  The
         server sets the expected sequence number to the received
         sequence number plus one after verifying the validity of the
         received message and responds to the message.

      Authenticating all parties

         ERP provides mutual authentication of the peer and the server.
         Both parties need to possess the key material that resulted
         from a previous EAP exchange in order to successfully derive
         the required keys.  Also, both the EAP re-authentication
         Response and the EAP re-authentication Information messages are
         integrity protected so that the peer and the server can verify
         each other.  When the ERP exchange is executed with a local ER
         server, the peer and the local server mutually authenticate
         each other via that exchange in the same manner.  The peer and
         the authenticator authenticate each other in the secure
         association protocol executed by the lower layer, just as in
         the case of a regular EAP exchange.



Cao, et al.                  Standards Track                   [Page 37]

RFC 6696                 EAP Extensions for ERP                July 2012


      Peer and authenticator authorization

         The peer and authenticator demonstrate possession of the same
         key material without disclosing it, as part of the lower-layer
         secure association protocol.  Channel binding with ERP may be
         used to verify consistency of the identities exchanged, when
         the identities used in the lower layer differ from those
         exchanged within the AAA protocol.

      Key material confidentiality

         The peer and the server derive the keys independently using
         parameters known to each entity.  The AAA server sends the DSRK
         of a domain to the corresponding local ER server via the AAA
         protocol.  Likewise, the ER server sends the rMSK to the
         authenticator via the AAA protocol.

         Note that compromise of the DSRK results in compromise of all
         keys derived from it.  Moreover, there is no forward secrecy
         within ERP.  Thus, compromise of a DSRK retroactively
         compromises all ERP keys.

         It is RECOMMENDED that the AAA protocol be protected using
         IPsec or Transport Layer Security (TLS) so that the keys are
         protected in transit.  Note, however, that keys may be exposed
         to AAA proxies along the way, and compromise of any of those
         proxies may result in compromise of keys being transported
         through them.

         The home EAP server MUST NOT hand out a given DSRK to a local
         domain server more than once, unless it can verify that the
         entity receiving the DSRK after the first time is the same
         entity that received the DSRK originally.  If the home EAP
         server verifies authorization of a local domain server, it MAY
         hand out the DSRK to that domain more than once.  In this case,
         the home EAP server includes the Authorization Indication TLV
         to assure the peer that DSRK delivery is secure.

      Confirming cryptosuite selection

         Cryptographic algorithms for integrity and key derivation in
         the context of ERP MAY be the same as that used by the EAP
         method.  In that case, the EAP method is responsible for
         confirming the cryptosuite selection.  Furthermore, the
         cryptosuite is included in the ERP exchange by the peer and
         confirmed by the server.  The protocol allows the server to
         reject the cryptosuite selected by the peer and provide
         alternatives.  When a suitable rIK is not available for the



Cao, et al.                  Standards Track                   [Page 38]

RFC 6696                 EAP Extensions for ERP                July 2012


         peer, the alternatives may be sent in an unprotected fashion.
         The peer is allowed to retry the exchange using one of the
         allowed cryptosuites.  However, in this case, any en route
         modifications to the list sent by the server will go
         undetected.  If the server does have an rIK available for the
         peer, the list will be provided in a protected manner and this
         issue does not apply.

      Uniquely named keys

         All keys produced within the context of ERP can be referred to
         uniquely as specified in this document.  Also, the key names do
         not reveal any part of the key material.

      Preventing the domino effect

         The compromise of one peer does not result in the compromise of
         key material held by any other peer in the system.  Also, the
         rMSK is meant for a single authenticator and is not shared with
         any other authenticator.  Hence, the compromise of one
         authenticator does not lead to the compromise of sessions or
         keys held by any other authenticator in the system, and ERP
         thereby allows prevention of the domino effect by appropriately
         defining key scope.

         However, if keys are transported using hop-by-hop protection,
         compromise of a proxy may result in compromise of key material,
         e.g., the DSRK being sent to a local ER server.

      Binding a key to its context

         All the keys derived for ERP are bound to the appropriate
         context using appropriate key labels.  The lifetime of a child
         key is less than or equal to that of its parent key as
         specified in RFC 4962 [RFC4962].  The key usage, lifetime, and
         the parties that have access to the keys are specified.

      Confidentiality of identity

         Deployments where privacy is a concern may find that the use of
         the rIKname-NAI to route ERP messages serves their privacy
         requirements.  Note that it is plausible to associate multiple
         runs of ERP messages, since the rIKname is not changed as part
         of ERP.  There was no consensus for that requirement at the
         time of development of this specification.  If the rIKname is
         not used and the Peer-ID is used instead, the ERP exchange will
         reveal the Peer-ID over the wire.




Cao, et al.                  Standards Track                   [Page 39]

RFC 6696                 EAP Extensions for ERP                July 2012


      Authorization restriction

         All the derived keys are limited in lifetime by that of the
         parent key or by server policy.  Any domain-specific keys are
         further restricted for use only in the domain for which the
         keys are derived.  All the keys specified in this document are
         meant for use in ERP only.  Other restrictions on the use of
         session keys may be imposed by the specific lower layer but are
         out of scope for this specification.

      Preventing a DoS attack

         A denial-of-service (DoS) attack on the peer may be possible
         when using the EAP-Initiate/Re-auth message.  An attacker may
         send a bogus EAP-Initiate/Re-auth message, which may be carried
         by the authenticator in a AAA-Request to the server; in
         response, the server may send in a AAA reply an EAP-Finish/
         Re-auth message indicating failure.  Note that such attacks may
         be possible with the EAPoL-Start capability of IEEE 802.11 and
         other similar facilities in other link layers and where the
         peer can initiate EAP authentication.  An attacker may use such
         messages to start an EAP method run, which fails and may result
         in the server sending a rejection message, thus resulting in
         the link-layer connections being terminated.

         To prevent such DoS attacks, an ERP failure should not result
         in deletion of any authorization state established by a full
         EAP exchange.  Alternatively, the lower layers and AAA
         protocols may define mechanisms to allow two link-layer
         Security Associations (SAs) derived from different EAP key
         material for the same peer to exist so that smooth migration
         from the current link-layer SA to the new one is possible
         during rekey.  These mechanisms prevent the link-layer
         connections from being terminated when a re-authentication
         procedure fails due to a bogus EAP-Initiate/Re-auth message.

      Key material transport

         When a DSRK is sent from the home EAP server to a local domain
         server or when an rMSK is sent from an ER server to an
         authenticator, in the absence of end-to-end security between
         the entity that is sending the key and the entity receiving the
         key, it is plausible for other entities to get access to keys
         being sent to an ER server in another domain.  This mode of key
         transport is similar to that of MSK transport in the context of
         EAP authentication.  We further observe that ERP is for access
         authentication and does not support end-to-end data security.
         In typical implementations, the traffic is in the clear beyond



Cao, et al.                  Standards Track                   [Page 40]

RFC 6696                 EAP Extensions for ERP                July 2012


         the access control enforcement point (the authenticator or an
         entity delegated by the authenticator for access control
         enforcement).  The model works as long as entities in the
         middle of the network do not use keys intended for other
         parties to steal service from an access network.  If that is
         not achievable, key delivery must be protected in an end-to-end
         manner.

9.  IANA Considerations

   The previous version of this document -- [RFC5296] -- performed the
   following IANA [IANA] actions:

   1.  It registered Packet Codes "Initiate" and "Finish" in the EAP
       Registry.  Those codes are referred to as "EAP-Initiate" and
       "EAP-Finish" throughout this document.

   2.  It created a Message Types table in the EAP Registry and
       registered several items in that table.  Those items are referred
       to as "Re-auth-start" and "Re-auth" throughout this document.

   3.  It created an EAP-Initiate and Finish Attributes table in the EAP
       registry and registered several items in that table.  Those items
       are recorded in this document in Section 5.3.4.

   4.  It created a Re-authentication Cryptosuites table in the EAP
       registry and registered several items in that table.  Those items
       are recorded in this document at the end of Section 5.3.2.

   5.  It registered two items in the USRK Key Labels registry:

       *  Re-auth usage label "EAP Re-authentication Root Key@ietf.org",
          recorded in this document in Section 4.1.

       *  DSRK-authorized delivery key "DSRK Delivery Authorized
          Key@ietf.org", recorded in this document in the description of
          "Authorization Indication" in Section 5.3.3.

10.  Contributors

   Barry Leiba contributed all of the text in Section 9 and, as
   Applications Area Director, insisted upon its inclusion as a
   condition of publication.








Cao, et al.                  Standards Track                   [Page 41]

RFC 6696                 EAP Extensions for ERP                July 2012


11.  Acknowledgments

   This document is largely based upon RFC 5296; thanks to all who
   participated in that effort (see Appendix A).  In addition, thanks to
   Yaron Sheffer, Sebastien Decugis, Ralph Droms, Stephen Farrell,
   Charlie Kaufman, and Yoav Nir for (mostly) useful comments and
   review.

12.  References

12.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, June 2004.

   [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.

12.2.  Informative References

   [DIAMETER-ERP]
              Bournelle, J., Morand, L., Decugis, S., Wu, Q., and G.
              Zorn, "Diameter Support for the EAP Re-authentication
              Protocol (ERP)", Work in Progress, June 2012.

   [IANA]     "Internet Assigned Numbers Authority",
              <http://www.iana.org/>.

   [IEEE_802.1X]
              Institute of Electrical and Electronics Engineers, "IEEE
              Standard for Local and Metropolitan Area Networks:
              Port-Based Network Access Control", IEEE Std 802.1X-2010,
              February 2010.





Cao, et al.                  Standards Track                   [Page 42]

RFC 6696                 EAP Extensions for ERP                July 2012


   [IKE-EXT-for-ERP]
              Nir, Y. and Q. Wu, "An IKEv2 Extension for Supporting
              ERP", Work in Progress, May 2012.

   [MSKHierarchy]
              Lopez, R., Skarmeta, A., Bournelle, J., Laurent-
              Maknavicus, M., and J. Combes, "Improved EAP keying
              framework for a secure mobility access service",
              IWCMC '06, Proceedings of the 2006 International
              Conference on Wireless Communications and Mobile
              Computing, New York, NY, USA, 2006.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC3162]  Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
              RFC 3162, August 2001.

   [RFC4187]  Arkko, J. and H. Haverinen, "Extensible Authentication
              Protocol Method for 3rd Generation Authentication and Key
              Agreement (EAP-AKA)", RFC 4187, January 2006.

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

   [RFC5169]  Clancy, T., Nakhjiri, M., Narayanan, V., and L. Dondeti,
              "Handover Key Management and Re-Authentication Problem
              Statement", RFC 5169, March 2008.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5296]  Narayanan, V. and L. Dondeti, "EAP Extensions for EAP
              Re-authentication Protocol (ERP)", RFC 5296, August 2008.

   [RFC5749]  Hoeper, K., Ed., Nakhjiri, M., and Y. Ohba, Ed.,
              "Distribution of EAP-Based Keys for Handover and
              Re-Authentication", RFC 5749, March 2010.










Cao, et al.                  Standards Track                   [Page 43]

RFC 6696                 EAP Extensions for ERP                July 2012


   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [RFC6440]  Zorn, G., Wu, Q., and Y. Wang, "The EAP Re-authentication
              Protocol (ERP) Local Domain Name DHCPv6 Option", RFC 6440,
              December 2011.












































Cao, et al.                  Standards Track                   [Page 44]

RFC 6696                 EAP Extensions for ERP                July 2012


Appendix A.  RFC 5296 Acknowledgments

   In writing this document, we benefited from discussing the problem
   space and the protocol itself with a number of folks including
   Bernard Aboba, Jari Arkko, Sam Hartman, Russ Housley, Joe Salowey,
   Jesse Walker, Charles Clancy, Michaela Vanderveen, Kedar Gaonkar,
   Parag Agashe, Dinesh Dharmaraju, Pasi Eronen, Dan Harkins, Yoshi
   Ohba, Glen Zorn, Alan DeKok, Katrin Hoeper, and other participants of
   the HOKEY Working Group.  Credit for the idea to use EAP-Initiate/
   Re-auth-Start goes to Charles Clancy, and credit for the idea to use
   multiple link-layer SAs to mitigate DoS attacks goes to Yoshi Ohba.
   Katrin Hoeper suggested the use of the windowing technique to handle
   multiple simultaneous ER exchanges.  Many thanks to Pasi Eronen for
   the suggestion to use hexadecimal encoding for the rIKname when sent
   as part of the keyName-NAI field.  Thanks to Bernard Aboba for
   suggestions in clarifying the EAP lock-step operation, and to Joe
   Salowey and Glen Zorn for help in specifying AAA transport of ERP
   messages.  Thanks to Sam Hartman for the DSRK Authorization
   Indication mechanism.
































Cao, et al.                  Standards Track                   [Page 45]

RFC 6696                 EAP Extensions for ERP                July 2012


Appendix B.  Sample ERP Exchange

   0.  Authenticator --> Peer:
         EAP-Initiate/Re-auth-Start [Optional]

   1.  Peer --> Authenticator:
         EAP-Initiate/Re-auth(SEQ, keyName-NAI, cryptosuite,
                              Auth-tag*)

   1a. Authenticator --> Re-auth-Server:
         AAA-Request
         {
            Authenticator-Id,
            EAP-Initiate/Re-auth(SEQ, keyName-NAI, cryptosuite,
                                  Auth-tag*)
          }

   2.  ER-Server --> Authenticator:
         AAA-Response
         {
            rMSK,
            EAP-Finish/Re-auth(SEQ, keyName-NAI, cryptosuite, [CB-Info],
                                Auth-tag*)
         }

   2b. Authenticator --> Peer:
         EAP-Finish/Re-auth(SEQ, keyName-NAI, cryptosuite, [CB-Info],
                            Auth-tag*)

   * Auth-tag computation is over the entire EAP-Initiate/Finish
     message; the code values for Initiate and Finish are different,
     and thus reflection attacks are mitigated.



















Cao, et al.                  Standards Track                   [Page 46]

RFC 6696                 EAP Extensions for ERP                July 2012


Authors' Addresses

   Zhen Cao
   China Mobile
   No. 32, Xuanwumenxi Ave., Xicheng District
   Beijing  100053
   P.R. China
   EMail: caozhen@chinamobile.com


   Baohong He
   China Academy of Telecommunication Research
   Beijing
   P.R. China
   Phone: +86 10 62300050
   EMail: hebaohong@catr.cn


   Yang Shi
   Huawei Technologies Co., Ltd.
   156 Beiqing Road, Zhongguancun, Haidian District
   Beijing
   P.R. China
   Phone: +86 10 60614043
   EMail: shiyang1@huawei.com


   Qin Wu (editor)
   Huawei Technologies Co., Ltd.
   101 Software Avenue, Yuhua District
   Nanjing, JiangSu  210012
   China
   Phone: +86-25-84565892
   EMail: bill.wu@huawei.com


   Glen Zorn (editor)
   Network Zen
   227/358 Thanon Sanphawut
   Bang Na, Bangkok  10260
   Thailand
   Phone: +66 (0) 909 201060
   EMail: glenzorn@gmail.com








Cao, et al.                  Standards Track                   [Page 47]

