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RFC 5296 - EAP Extensions for EAP Re-authentication Protocol (ER


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Network Working Group                                       V. Narayanan
Request for Comments: 5296                                    L. Dondeti
Category: Standards Track                                 Qualcomm, Inc.
                                                             August 2008

        EAP Extensions for EAP Re-authentication Protocol (ERP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

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 not repeat
   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  ERP Description  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  ERP With the Home ER Server  . . . . . . . . . . . . . . .  6
     3.2.  ERP with a Local ER Server . . . . . . . . . . . . . . . .  8
   4.  ER Key Hierarchy . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  rRK Derivation . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  rRK Properties . . . . . . . . . . . . . . . . . . . . . . 12
     4.3.  rIK Derivation . . . . . . . . . . . . . . . . . . . . . . 12
     4.4.  rIK Properties . . . . . . . . . . . . . . . . . . . . . . 13
     4.5.  rIK Usage  . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.6.  rMSK Derivation  . . . . . . . . . . . . . . . . . . . . . 14
     4.7.  rMSK Properties  . . . . . . . . . . . . . . . . . . . . . 15
   5.  Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  ERP Bootstrapping  . . . . . . . . . . . . . . . . . . . . 15
     5.2.  Steps in ERP . . . . . . . . . . . . . . . . . . . . . . . 18
       5.2.1.  Multiple Simultaneous Runs of ERP  . . . . . . . . . . 20
       5.2.2.  ERP Failure Handling . . . . . . . . . . . . . . . . . 21
     5.3.  New EAP Packets  . . . . . . . . . . . . . . . . . . . . . 22
       5.3.1.  EAP-Initiate/Re-auth-Start Packet  . . . . . . . . . . 23
       5.3.2.  EAP-Initiate/Re-auth Packet  . . . . . . . . . . . . . 25
       5.3.3.  EAP-Finish/Re-auth Packet  . . . . . . . . . . . . . . 26
       5.3.4.  TV and TLV Attributes  . . . . . . . . . . . . . . . . 29
     5.4.  Replay Protection  . . . . . . . . . . . . . . . . . . . . 30
     5.5.  Channel Binding  . . . . . . . . . . . . . . . . . . . . . 30
   6.  Lower-Layer Considerations . . . . . . . . . . . . . . . . . . 31
   7.  Transport of ERP Messages  . . . . . . . . . . . . . . . . . . 32
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 33
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 39
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 39
     11.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Appendix A.  Example ERP Exchange  . . . . . . . . . . . . . . . . 42

1.  Introduction

   The Extensible Authentication Protocol (EAP) is a 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 delays in
   handover times.  Some EAP methods specify the use of state from the
   initial authentication to optimize re-authentications by reducing the
   computational overhead, but method-specific re-authentication takes
   at least 2 round trips with the original EAP server in most cases
   (e.g., [6]).  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, compromise of an
   authenticator results in compromise of keying material established
   via other authenticators.  Other solutions for fast re-authentication
   exist in the literature [7] [8].

   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.  The EAP
   re-authentication problem statement is described in detail in [9].

   This document specifies EAP Re-authentication Extensions (ERXs) for
   efficient re-authentication using EAP.  The protocol that uses these
   extensions itself is referred to as the EAP Re-authentication
   Protocol (ERP).  It supports EAP method-independent re-authentication
   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 IEEE802.1x specification must be revised and RFC
   4306 must be updated to carry ERP messages.

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 [1].

   This document uses the basic EAP terminology [2] and EMSK keying
   hierarchy terminology [3].  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;
      the home ER server is located on the same backend authentication
      server as the EAP server in the home domain.  The local ER server
      may not necessarily be a full EAP server.

      ERX - EAP re-authentication extensions.

      ERP - EAP Re-authentication Protocol that uses the
      re-authentication extensions.

      rRK - re-authentication Root Key, derived from the EMSK or 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 [3]; 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 NAI.

      Domain - Refers to a "key management domain" as defined in [3].
      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.

3.  ERP Description

   ERP allows a peer and server to mutually verify proof of possession
   of keying 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
   described in RFC 3748 [2].  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.

   Figure 2 shows the protocol exchange.  The first time the peer
   attaches to any network, it performs a full EAP exchange (shown in
   Figure 1) 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 a re-authentication Root Key (rRK).  More precisely, a
   re-authentication Root Key is derived from the EMSK or from a
   Domain-Specific Root Key (DSRK), which itself is 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, a re-authentication
   Integrity Key (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.

   When the ER server is in the home domain, the peer and the server use
   the rIK and rRK derived from the EMSK; and when the ER server is not
   in the home 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

   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 1: EAP Authentication

   Peer               Authenticator                      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 2: ERP Exchange

   Two new 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 2;
   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.

   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, it will send the EAP-Request/
   Identity message.  Note that this avoids having two EAP messages in
   flight at the same time [2].  The authenticator may send the EAP-
   Initiate/Re-auth-Start message and wait for a short, locally
   configured amount of time.  If the peer does not already know, 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
   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 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 drops EAP-Initiate/
   Re-auth messages [2] as the EAP code of those packets is greater than
   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 stage onwards, 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 in the IEEE 802.1X specification
   [10].

   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 [4] 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 a
   re-authentication MSK (rMSK) from the rRK using the sequence number
   as an input to the key derivation.  The server 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.  Ongoing work in [11] describes 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 server for the rMSK lifetime.  If so, the ER
   server sends the rMSK lifetime in the EAP-Finish/Re-auth message.

   In an ERP bootstrap exchange, the peer MAY request 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 replay protection 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.

3.2.  ERP with a Local ER Server

   The defined ER extensions allow executing the ERP with an ER server
   in the local domain (access network).  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 the lower layer or by means of
   ERP bootstrapping.  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 3 shows the full EAP and subsequent
   local ERP exchange; Figure 4 shows it with a local ER server.

   Peer        EAP Authenticator     Local ER Server     Home EAP Server
   ====        =================     ===============     ===============

   <-- EAP-Request/ --
        Identity

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

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

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

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

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

            Figure 3: Local ERP Exchange, Initial EAP Exchange

   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 4: 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 authenticator and the
   home EAP server of the peer).  In that case, the ER server requests
   the DSRK by sending the domain name to the EAP server.  In response,
   the EAP server computes the DSRK by following the procedure specified
   in [3] and sends the DSRK and the key name, EMSKname, to the ER
   server in the claimed domain.  The local domain is responsible for
   announcing that same domain name via the lower layer to the peer.

   If the peer does not know the domain name (did not receive the domain
   name via the lower-layer announcement, due to a missed announcement
   or lack of support for domain name announcements in a specific lower
   layer), it SHOULD initiate ERP bootstrap exchange with the home ER
   server to obtain the domain name.  The local ER server SHALL request
   the home AAA server for the DSRK by sending the domain name in the
   AAA message that carries the EAP-Initiate/Re-auth bootstrap message.
   The local ER server MUST be in the path from the peer to the home ER
   server.  If it is not, it cannot request the DSRK.

   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, 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.

   Subsequently, when the peer 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.

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) in accordance with [3] for re-authentication.
   The USRK designated for re-authentication is the re-authentication
   root key (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 they are derived for, 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 5: Re-authentication Key Hierarchy

   The derivations in this document are according to [3].  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 [3].

   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
   [3].

   The KDF and algorithm agility for the KDF are as defined in [3].

   An rRK derived from the DSRK is referred to as a DS-rRK in the rest
   of the document.  All 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 derivation of 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 re-authentication Integrity Key (rIK) is used for integrity
   protecting the ERP exchange.  This serves as the proof of possession
   of valid keying material from a previous full EAP exchange by the
   peer to the server.

   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 encoded as
   specified in [3].

   The cryptosuite and length of the rIK are part of the input to the
   key derivation function to ensure cryptographic separation of keys if
   different rIKs of different lengths 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 EMSKname-NAI within the context of ERP
   messages.  The username part of EMSKname-NAI is the EMSKname; the
   realm is the domain name of the ER server.  In case of ERP with the
   home ER server, the peer uses the realm from its original NAI; in
   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 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.

   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 whose possession 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 case of hash-based MAC algorithms, the key
   is first hashed to the required key length as specified in [5].  In
   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 EAP Re-auth Protocol
   exchange.

   The rMSK is derived as follows.

   rMSK = KDF (K, S), where

      K = rRK and

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

   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.  The
   length field is encoded as specified in [3].

   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.  All 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 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
   bootstrapping.  In implicit bootstrapping, the local ER server SHOULD
   include its domain name and SHOULD request the DSRK from the home AAA
   server during the initial EAP exchange, in the AAA message
   encapsulating the first EAP Response message sent by the peer.  If
   the EAP exchange is successful, the server sends the DSRK for the
   local ER server (derived using the EMSK and the domain name as
   specified in [3]), EMSKname, and DSRK lifetime along with the EAP-
   Success message.  The local ER server MUST extract the DSRK,
   EMSKname, and DSRK lifetime (if present) before forwarding the EAP-
   Success message to the peer, as specified in [12].  Note that the MSK
   (also present along 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 via lower-layer
   announcements.  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 does not know the domain name (did not receive the domain
   name via the lower-layer announcement, due to a missed announcement
   or lack of support for domain name announcements in a specific lower
   layer), it SHOULD initiate ERP bootstrap exchange (ERP exchange with
   the bootstrap flag turned on) with the home ER server to obtain the

   domain name.  The local ER server behavior is the same as described
   above.  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 turned on.  The bootstrap message is always
      sent to the home AAA 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  When an ERP-capable authenticator receives the EAP-Initiate/
      Re-auth message from a peer, it copies the contents of the
      keyName-NAI into the User-Name attribute of RADIUS [13].  The rest
      of the process is similar to that described in [14] and [12].

   o  If a local ER server is present, the local ER server MUST include
      the DSRK request and 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 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 server then
      verifies to ensure that the cryptosuite used by the peer is
      acceptable.  Next, it verifies the origin authentication of the
      message by looking up the rIK.  If any of the checks fail, the
      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 and a DSRK request is received,
         the server MUST include the domain name to which the DSRK is
         being sent.

      *  If the home ER server verifies the authorization of a local
         domain 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.

   o  If the ERP exchange is successful, and the local ER server sent a
      DSRK request, the home ER server MUST include the DSRK for the
      local ER server (derived using the EMSK and the domain name as
      specified in [3]), EMSKname, and DSRK lifetime along with 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 [12].

   o  The local ER server MUST extract the DSRK, EMSKname, and DSRK
      lifetime (if present), before forwarding the EAP-Finish/Re-auth
      message to the peer, as specified in [12].

   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 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 the ERP and obtaining relevant
      keys using RADIUS and Diameter are specified in [12] and [15],
      respectively.

   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,
   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 the 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.

   The EAP Re-auth Protocol has the following steps:

      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 over the message.

      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
      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.

      The authenticator copies the contents of the value field of the
      keyName-NAI TLV into the User-Name RADIUS attribute in the AAA
      message to the ER server.

      The 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 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 server MUST then verify to ensure that the
      cryptosuite used by the peer is acceptable.  The 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 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.

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

         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 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 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.

      The server transports the rMSK along with this message to the
      authenticator.  The rMSK is transported in a manner similar to the
      MSK transport along with the EAP-Success message in a regular EAP
      exchange.

      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.

      The peer uses the sequence number to compute the rMSK.

      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 SEQ number values within a window of allowed values.
   Recall that the SEQ 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.  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.

   The peer, upon receiving an EAP-Finish/Re-auth message with the
   Result flag set to '1', MUST verify the sequence number and 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 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 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 imply 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.

5.3.  New EAP Packets

   Two new 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 RFC 3748 [2].

   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 6: EAP Packet

      Code

         5 Initiate

         6 Finish

         Two new code values are defined for the purpose of ERP.

      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 a Finish message.  Any new
         (non-retransmission) Initiate message MUST use a new Identifier
         field.

         The Identifier field of the Finish message MUST match that of
         the currently outstanding Initiate message.  A peer or
         authenticator receiving a Finish message whose Identifier value
         does not match that of the currently outstanding 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.

      Type

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

      Type-Data

         The Type-Data field varies with the Type of re-authentication
         packet.

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

   The EAP-Initiate/Re-auth-Start packet contains the parameters shown
   in Figure 7.

   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 7: EAP-Initiate/Re-auth-Start Packet

      Type = 1.

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

      One or more 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 [4].  The Domain-Name
         attribute SHOULD be present in an EAP-Initiate/Re-auth-Start
         message.

         In addition, channel binding information MAY be included; see
         Section 5.5 for discussion.  See Figure 11 for parameter
         specification.

5.3.1.1.  Authenticator Operation

   The authenticator MAY 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 lower-layer support or the ERP
   bootstrapping exchange.

   The authenticator MAY include channel binding information so that the
   peer can send the information to the server in the EAP-Initiate/
   Re-auth message so that the server can verify whether the
   authenticator is claiming the same identity to both parties.

   The authenticator MAY re-transmit 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 Initiate code value, it silently
   discards the message.  If the peer has already sent the EAP-Initiate/
   Re-auth message to begin the ERP exchange, it silently discards the
   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 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 the peer has already
   initiated an ERP exchange with the home ER server, it MAY choose to
   not start an ERP exchange with the local ER server.

5.3.2.  EAP-Initiate/Re-auth Packet

   The EAP-Initiate/Re-auth packet contains the parameters 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      |R|B|L| Reserved|             SEQ               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 1 or more TVs or TLVs                         ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | cryptosuite  |        Authentication Tag                     ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 8: 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 rest of the 5 bits are set to 0 and ignored on reception.

      SEQ: A 16-bit sequence number is used for replay protection.  The
      SEQ number field is initialized to 0 every time a new rRK is
      derived.

      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 length and so
         the username portion takes up 128 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 follows [4].  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 11 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.

5.3.3.  EAP-Finish/Re-auth Packet

   The EAP-Finish/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-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 rest of the 5 bits are set to 0 and ignored on reception.

      SEQ: A 16-bit sequence number is used for replay protection.  The
      SEQ number field is initialized to 0 every time a new rRK is
      derived.

      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 follows [4].  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 is a 32-bit field and contains 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 is a 32-bit field and contains 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 [4].  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 8.
         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 to the peer about its cryptographic algorithm
         capabilities.

         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 ER 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 that the DSRK is being derived for.

         In addition, channel binding information MAY be included: see
         Section 5.5 for discussion.  See Figure 11 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 10: TV Attribute Format

   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 11: 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 and Finish/Re-auth messages.
      Below are the current assignments (all of them are TLVs):

         '128' - Called-Station-Id [13]

         '129' - Calling-Station-Id [13]

         '130' - NAS-Identifier [13]

         '131' - NAS-IP-Address [13]

         '132' - NAS-IPv6-Address [16]

   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 per rIK and is initialized to zero in both
   directions.  In the first EAP-Initiate/Re-auth message, the peer uses
   the sequence number zero or higher.  Note that the when the sequence
   number rotates, the rIK MUST be changed by running 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 accepts 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 pre-configured number of times before giving
   up.  However, it is plausible that the server itself may have
   responded to the message and it 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.  When the sequence number rotates, 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 in Section 7.15 of [2].  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 IANA managed based on IETF Consensus [17].

   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.

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 may not know whether the peer supports
   ERP; in those cases, the peer may be silently dropping 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 (this is based on the recommendation
   of [2]).  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 [2] 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

   Note that to support ERP, lower-layer specifications may need to be
   revised.  Specifically, the IEEE802.1x specification must be revised
   to allow carrying EAP messages of the new codes defined in this
   document in order to support ERP.  Similarly, RFC 4306 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; peers may learn whether an
   authenticator supports ERP via configuration, 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 an 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.,
   piggy-backed on a beacon) or through negotiation.  Alternatively,
   clients may recognize environments where ERP is available based on
   pre-configuration.  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.  Transport of ERP Messages

   AAA Transport of ERP messages is specified in [11] and [12].

8.  Security Considerations

   This section provides an analysis of the protocol in accordance with
   the AAA key management requirements specified in [18].

      Cryptographic algorithm independence

         The EAP Re-auth Protocol 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 with Failure indication.  Algorithm
         agility for the KDF is specified in [3].  Only when the
         algorithms used are acceptable, the server proceeds with
         derivation of keys and verification of the proof of possession
         of relevant keying material 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 a different rMSK
         at any time.

      Limit 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 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.

      Authenticate all parties

         The EAP Re-auth Protocol provides mutual authentication of the
         peer and the server.  Both parties need to possess the keying
         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.

      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 that
         exchanged within the AAA protocol.

      Keying 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 an DSRK retroactively
         compromises all ERP keys.

         It is RECOMMENDED that the AAA protocol be protected using
         IPsec or 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 ER 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 as
         that received the DSRK originally.  If the home ER 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 ER server includes the Authorization Indication TLV to
         assure the peer that DSRK delivery is secure.

      Confirm cryptosuite selection

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

      Prevent the domino effect

         The compromise of one peer does not result in the compromise of
         keying 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.  Hence, the
         EAP Re-auth Protocol 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,
         i.e., the DSRK being sent to a local ER server.

      Bind key to its context

         All the keys derived for ERP are bound to the appropriate
         context using appropriate key labels.  Lifetime of a child key
         is less than or equal to that of its parent key as specified in
         RFC 4962 [18].  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 the use of
         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 the ERP protocol.  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.

      Authorization restriction

         All the keys derived 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.  Any other restrictions of session
         keys may be imposed by the specific lower layer and are out of
         scope for this specification.

   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 RADIUS-Access-Request to the server; in response,
   the server may send an EAP-Finish/Re-auth with Failure indication in
   a RADIUS Access-Reject message.  Note that such attacks may be
   plausible 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 RADIUS Access-Reject 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 keying materials 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 the bogus EAP-Initiate/Re-auth message.

   When a DSRK is sent from a home ER server to a local domain server or
   when a 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 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

   This document specifies IANA registration of two new 'Packet Codes'
   from the EAP registry:

   o  5 (Initiate)

   o  6 (Finish)

   These values are in accordance with [2].

   This document also specifies creation of a new table, Message Types,
   in the EAP registry with the following assigned numbers:

   o  0 Reserved

   o  1 (Re-auth-Start, applies to Initiate Code only)

   o  2 (Re-auth, applies to Initiate and Finish Codes)

   o  3-191 IANA managed and assigned based on IETF Consensus [17]

   o  192-255 Private use

   Next, we specify creation of a new table, EAP Initiate and Finish
   Attributes, associated with EAP Initiate and Finish messages in the
   EAP registry with the following assigned numbers.

   o  0: Reserved

   o  keyName-NAI: This is a TLV payload.  The Type is 1.

   o  rRK Lifetime: This is a TV payload.  The Type is 2.

   o  rMSK Lifetime: This is a TV payload.  The Type is 3.

   o  Domain name: This is a TLV payload.  The Type is 4.

   o  Cryptosuite list: This is a TLV payload.  The Type is 5.

   o  Authorization Indication: This is a TLV payload.  The Type is 6.

   o  7-127: Used to carry other non-channel-binding-related attributes.
      IANA managed and assigned based on IETF Consensus [17].

   o  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.
      Below are the current assignments (all of them are TLVs):

      *  Called-Station-Id: 128

      *  Calling-Station-Id: 129

      *  NAS-Identifier: 130

      *  NAS-IP-Address: 131

      *  NAS-IPv6-Address: 132

      133-191: Used to carry other channel-binding-related attributes.
      IANA managed and assigned based on IETF Consensus [17].

   o  192-255: Reserved for Private use.

   We specify creation of another registry, 'Re-authentication
   Cryptosuites', with the following assigned numbers:

   o  0: Reserved

   o  1: HMAC-SHA256-64

   o  2: HMAC-SHA256-128

   o  3: HMAC-SHA256-256

   o  4-191: IANA managed and assigned based on IETF consensus [17]

   o  192-255: Reserved for Private use.

   Further, this document registers a Re-auth usage label from the "USRK
   Key Labels" name space with a value

      EAP Re-authentication Root Key@ietf.org

   and DSRK-authorized delivery key from the "USRK Key Labels" name
   space

      DSRK Delivery Authorized Key@ietf.org

   in accordance with [3].

10.  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.  The credit for the idea to use EAP-
   Initiate/Re-auth-Start goes to Charles Clancy, and the multiple link-
   layer SAs idea to mitigate the DoS attack 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 rIKname when sent as
   part of keyName-NAI field.  Thanks to Bernard Aboba for suggestions
   in clarifying the EAP lock-step operation, and Joe Salowey and Glen
   Zorn for help in specifying AAA transport of ERP messages.  Thanks to
   Sam Hartman for the DSRK Authorization Indication mechanism.

11.  References

11.1.  Normative References

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

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

   [3]   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.

   [4]   Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network
         Access Identifier", RFC 4282, December 2005.

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

11.2.  Informative References

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

   [7]   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.

   [8]   Arbaugh, W. and B. Aboba, "Handoff Extension to RADIUS", Work
         in Progress, October 2003.

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

   [10]  Institute of Electrical and Electronics Engineers, "IEEE
         Standards for Local and Metropolitan Area Networks: Port based
         Network Access Control, IEEE Std 802.1X-2004", December 2004.

   [11]  Nakhjiri, M. and Y. Ohba, "Derivation, delivery and management
         of EAP based keys for handover and re-authentication", Work
         in Progress, February 2008.

   [12]  Gaonkar, K., Dondeti, L., Narayanan, V., and G. Zorn, "RADIUS
         Support for EAP Re-authentication Protocol", Work in Progress,
         February 2008.

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

   [14]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
         In User Service) Support For Extensible Authentication Protocol
         (EAP)", RFC 3579, September 2003.

   [15]  Dondeti, L. and H. Tschofenig, "Diameter Support for EAP Re-
         authentication Protocol", Work in Progress, November 2007.

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

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

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

Appendix A.  Example ERP Exchange

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

   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.

Authors' Addresses

   Vidya Narayanan
   Qualcomm, Inc.
   5775 Morehouse Dr.
   San Diego, CA  92121
   USA

   Phone: +1 858-845-2483
   EMail: vidyan@qualcomm.com

   Lakshminath Dondeti
   Qualcomm, Inc.
   5775 Morehouse Dr.
   San Diego, CA  92121
   USA

   Phone: +1 858-845-1267
   EMail: ldondeti@qualcomm.com

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