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Versions: (draft-aboba-radius-tunnel-imp) 00 01 02 03 04 05 07 RFC 2809

     RADIUS Working Group                                     Bernard Aboba
     INTERNET-DRAFT                                               Microsoft
     Category: Standards Track                                    Glen Zorn
     <draft-ietf-radius-tunnel-imp-07.txt>                        Microsoft
     18 May 1998
     
     
          Implementation of PPTP/L2TP Compulsory Tunneling via RADIUS
     
     
     1.  Status of this Memo
     
     This document is an Internet-Draft.  Internet-Drafts are working docu-
     ments of the Internet Engineering Task Force (IETF),  its  areas,  and
     its  working groups.  Note that other groups may also distribute work-
     ing documents as Internet-Drafts.
     
     Internet-Drafts are draft documents valid for a maximum of six  months
     and  may  be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet-Drafts as  reference  mate-
     rial or to cite them other than as ``work in progress.''
     
     To  learn  the  current status of any Internet-Draft, please check the
     ``1id-abstracts.txt'' listing contained in the Internet-Drafts  Shadow
     Directories   on   ds.internic.net   (US  East  Coast),  nic.nordu.net
     (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
     
     The  distribution  of  this memo is unlimited.  It is filed as <draft-
     ietf-radius-tunnel-imp-07.txt> and  expires December 1, 1998.   Please
     send comments to the authors.
     
     
     2.  Abstract
     
     This  document  discusses  implementation issues arising in the provi-
     sioning of compulsory tunneling in dial-up networks using the PPTP and
     L2TP  protocols.   This provisioning can be accomplished via the inte-
     gration of  RADIUS  and  tunneling  protocols.  Implementation  issues
     encountered  with other tunneling protocols are left to separate docu-
     ments.
     
     
     3.  Terminology
     
     
     Voluntary Tunneling
               In voluntary tunneling, a tunnel is  created  by  the  user,
               typically via use of a tunneling client.
     
     Compulsory Tunneling
               In  compulsory  tunneling,  a  tunnel is created without any
               action from the user  and  without  allowing  the  user  any
               choice.
     
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
     Roaming   "Roaming capability" can be loosely defined  as  the ability
               to use  any  one  of  multiple  Internet  service  providers
               (ISPs),  while maintaining a formal,  customer-vendor  rela-
               tionship  with  only one. Examples  of  cases  where roaming
               capaility might be required include ISP "confederations" and
               ISP-provided corporate network access support.
     
     Shared Use Network
               This is an IP dialup network whose use is shared by  two  or
               more organizations.  Shared use networks typically implement
               distributed authentication and accounting in order to facil-
               itate   the relationship  among  the  sharing parties. Since
               these facilities are also  required  for  implementation  of
               roaming,  implementation of shared use is frequently a first
               step toward development of roaming  capabilities. In   fact,
               one  of  the ways by which a provider can offer roaming ser-
               vice is to conclude shared use agreements with multiple net-
               works.  However,  to date the ability to accomplish this has
               been hampered by lack of interoperability among  shared  use
               implementations.
     
     Tunnel Network Server
               This  is  a server which terminates a tunnel. In PPTP termi-
               nology, this is known as the PPTP Network Server  (PNS).  In
               L2TP  terminology,  this is known as the L2TP Network Server
               (LNS).
     
     Network Access Server
               The Network Access Server (NAS) is the device  that  clients
               contact  in order to get access to the network. In PPTP ter-
               minology this is referred to as the PPTP Access Concentrator
               (PAC).  In  L2TP  terminology, the NAS is referred to as the
               L2TP Access Concentrator (LAC).
     
     RADIUS server
               This is a server which  provides  for  authentication/autho-
               rization via the protocol described in [3], and for account-
               ing as described in [4].
     
     RADIUS proxy
               In order to provide for the routing of RADIUS authentication
               and  accounting requests, a RADIUS proxy can be employed. To
               the NAS, the RADIUS proxy appears to act as a RADIUS server,
               and  to  the  RADIUS  server,  the proxy appears to act as a
               RADIUS client.
     
     Network Access Identifier
               In order to provide for the routing of RADIUS authentication
               and accounting requests, the userID field used in PPP and in
               the  subsequent   RADIUS   authentication   and   accounting
               requests,  known  as the Network Access Identifier (NAI) MAY
               contain structure. This structure provides a means by  which
               the  RADIUS  proxy  will locate the RADIUS server that is to
               receive the request. This same structure MAY also be used to
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
               locate  the  tunnel  endpoint when domain-based tunneling is
               used.
     
     
     4.  Requirements language
     
     In  this  document,  the  key  words  "MAY",  "MUST,    "MUST    NOT",
     "optional",  "recommended",  "SHOULD",  and  "SHOULD  NOT",  are to be
     interpreted as described in [9].
     
     
     5.  Introduction
     
     Many applications  of  tunneling  protocols  involve  dial-up  network
     access.  Some, such  as the provisioning of secure access to corporate
     intranets via the Internet, are characterized by voluntary  tunneling:
     the  tunnel  is created at the request of the user for a specific pur-
     pose. Other applications involve compulsory tunneling: the  tunnel  is
     created without any action from the user and without allowing the user
     any choice.
     
     Examples  of  applications  that might be implemented using compulsory
     tunnels  are  Internet software upgrade servers, software registration
     servers  and  banking services.  These are all services which, without
     compulsory  tunneling, would probably be provided using dedicated net-
     works  or  at least dedicated network access servers (NAS), since they
     are characterized by the need to limit user access to specific  hosts.
     
     Given  the  existence  of widespread support for compulsory tunneling,
     however,  these  types  of services could be accessed via any Internet
     service provider (ISP).  The most popular means of authorizing dial-up
     network  users  today  is  through  the  RADIUS  protocol. The use  of
     RADIUS allows the dial-up users' authorization and authentication data
     to be maintained in a central location,  rather  than on each NAS.  It
     makes sense to use RADIUS to centrally  administer   compulsory   tun-
     neling,   since   RADIUS  is   widely deployed  and  was  designed  to
     carry this type of information.  New RADIUS attributes are  needed  to
     carry  the  tunneling  information  from the  RADIUS server to the NAS
     and to transfer accounting data from the NAS to the RADIUS  accounting
     server; those attributes are defined in [7] and [13].
     
     
     5.1.  Advantanges of RADIUS-based compulsory tunneling
     
     The  use  of  RADIUS in provisioning of compulsory tunnels has several
     advantages. These include:
     
          User-based tunneling
          Auditing capabilities
     
     
     
     
     
     
     
     
     Aboba & Zorn                                                  [Page 3]


     INTERNET-DRAFT                                             18 May 1998
     
     
     5.1.1.  User-based Tunneling
     
     Current proposals for routing of tunnel requests include  static  tun-
     neling,  where  all  users  are automatically tunneled to a given end-
     point, and realm-based tunneling, where the tunnel endpoint is  deter-
     mined  from  the  realm portion of the userID. User-based tunneling as
     provided by integration of RADIUS and tunnel protocols offers signifi-
     cant advantages over both of these approaches.
     
     Static  tunneling  requires dedication of a NAS device to the purpose.
     In the case of an ISP, this is undesirable because it requires them to
     dedicate  a  NAS  to tunneling service for a given corporate customer,
     rather than allowing them to use existing NASes deployed in the field.
     As  a result static tunneling is likely to be costly for deployment of
     a global service.
     
     Realm-based tunneling assumes that all users within a given realm wish
     to be treated the same way. This limits flexibility in account manage-
     ment.  For example, BIGCO may desire to provide Janet with an  account
     that allows access to both the Internet and the intranet, with Janet's
     intranet access provided by a tunnel server located in the engineering
     department.  However  BIGCO may desire to provide Fred with an account
     that provides only access to the intranet, with Fred's intranet access
     provided  by  a tunnel network server located in the sales department.
     Such a situation cannot be accommodated  with  realm-based  tunneling,
     but  can  be  accomodated  via  user-based tunneling as enabled by the
     attributes defined in [7].  When deployed  in  concert  with  roaming,
     user-based  tunneling offers corporations the ability to provide their
     users with access to the corporate Intranet on a global basis.
     
     
     5.2.  Auditing Capabilities
     
     The integration of RADIUS and tunnel protocols allows the ISP and  the
     corporation  to  synchronize  their accounting activities so that each
     side receives a record of the user's resource consumption.  This  pro-
     vides the corporation with the means to audit ISP bills.
     
     In auditing, the User-Name, Acct-Tunnel-Connection, Tunnel-Client-End-
     point and Tunnel-Server-Endpoint  attributes  are  typically  used  to
     uniquely  identify  the  call, allowing the Accounting-Request sent by
     the NAS to be reconciled  with  the  corresponding  Accounting-Request
     sent by the tunnel server.
     
     When  implementing  L2TP/PPTP  tunneling  based  on  RADIUS, the Call-
     Serial-Number SHOULD be used in the Acct-Tunnel-Connection  attribute.
     In  L2TP, the Call-Serial-Number is a 32-bit field and in PPTP it is a
     16-bit field. As described in [6],  in  PPTP  the  combination  of  IP
     Address  and  Call-Serial-Number  SHOULD  be  unique,  but this is not
     required.  In addition, no method for determining the Call-Serial-Num-
     ber  is specified, which leaves open the possibility of wrapping after
     a reboot.
     
     
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
     Note that a 16-bit Call-Serial-Number is not sufficient to distinguish
     a  given  call from all other calls over an extended time period.  For
     example, if the Call-Serial-Number is assigned monotonically, the  NAS
     in  question  has  96 ports which are continually busy and the average
     call is of 20 minutes duration, then a 16-bit Call-Serial-Number  will
     wrap within 65536/(96 * 3 calls/hour * 24 hours/day) = 9.48 days.
     
     
     6.  Authentication alternatives
     
     RADIUS-based  compulsory tunneling can support both single authentica-
     tion, where the user is authenticated at the NAS or tunnel server,  or
     dual  authentication,  where the user is authenticated at both the NAS
     and the tunnel server. When  single  authentication  is  supported,  a
     variety  of  modes  are  possible,  including  telephone-number  based
     authentication.  When dual-authentication is used, a number  of  modes
     are available, including dual CHAP authentications; CHAP/EAP authenti-
     cation; CHAP/PAP(token) authentication;  and  EAP/EAP  authentication,
     using the same EAP type for both authentications.
     
     The alternatives are described in more detail below.
     
     
     6.1.  Single authentication
     
     Single authentication alternatives include:
     
          NAS authentication
          NAS authentication with RADIUS reply forwarding
          Tunnel server authentication
     
     
     6.1.1.  NAS authentication
     
     With  this  approach, authentication and authorization (including tun-
     neling information) occurs once, at the NAS. The  advantages  of  this
     approach  are  that  it  disallows network access for unauthorized NAS
     users, and allows RADIUS accounting to be used at the NAS.   Disadvan-
     tages are that it requires that the tunnel server trust the NAS, since
     no user authentication occurs at the tunnel server. Due to the lack of
     user authentication, accounting cannot take place at the tunnel server
     with strong assurance that the correct party is being billed.
     
     NAS-only authentication is most typically employed along with LCP for-
     warding  and  tunnel  authentication,  both  of which are supported in
     L2TP, described in [5].  Thus, the tunnel server  can  be  set  up  to
     accept  all  calls  occurring  within  authenticated  tunnels, without
     requiring PPP authentication.  This approach is  not  compatible  with
     roaming,  since  the  tunnel  server  will typically only be set up to
     accept tunnels from a restricted set of NASes.  A  typical  initiation
     sequence looks like this:
     
          Client and NAS: Call Connected
          Client and NAS: PPP LCP negotiation
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
          Client and NAS: PPP authentication
          NAS to RADIUS Server: RADIUS Access-request
          RADIUS server to NAS: RADIUS Access-Accept/Access-Reject
          NAS to Tunnel Server: L2TP Incoming-Call-Request w/LCP forwarding
          Tunnel Server to NAS: L2TP Incoming-Call-Reply
          NAS to Tunnel Server: L2TP  Incoming-Call-Connected
          Client and Tunnel Server: NCP negotiation
          NAS to RADIUS Server: RADIUS Accounting-Request with
            Acct-Status-Type=Tunnel-Link-Start
          RADIUS Server to NAS: RADIUS Accounting-Response
     
     The  process  begins with an incoming call to the NAS, and the PPP LCP
     negotiation between the client and the NAS. In order  to  authenticate
     the  client,  the  NAS will send a RADIUS Access-Request to the RADIUS
     server and  will  receive  a  RADIUS  Access-Accept  including  tunnel
     attributes, or an Access-Reject.
     
     In  the case where an L2TP tunnel is indicated, the NAS will now bring
     up a control connection if none existed before, and the NAS and tunnel
     server  will bring up the call. At this point, data will begin to flow
     through the tunnel.  The NAS will  typically  employ  LCP  forwarding,
     although it is also possible for the tunnel server to renegotiate LCP.
     If LCP renegotiation is to be permitted, the NAS SHOULD  NOT  send  an
     LCP  CONFACK  completing  LCP  negotiation. Rather than sending an LCP
     CONFACK, the NAS will instead send an LCP DOWN message. The Client MAY
     then  renegotiate  LCP,  and  from that point forward, all PPP packets
     originated from the client will be encapsulated and sent to the tunnel
     server.
     
     Since  address  assignment will occur at the tunnel server, the client
     and NAS MUST NOT begin NCP negotiation. Instead, NCP negotiation  will
     occur between the client and the tunnel server.
     
     
     6.1.2.  NAS authentication with RADIUS reply forwarding
     
     With  this  approach,  authentication and authorization occurs once at
     the NAS and the RADIUS reply is forwarded to the tunnel  server.  This
     approach disallows network access for unauthorized NAS users; does not
     require trust between the NAS and tunnel server; and allows for RADIUS
     accounting  to  be  used  at both ends of the tunnel. However, it also
     requires that both ends share the same secret with the RADIUS  server,
     since that is the only way that the tunnel server can check the RADIUS
     reply.
     
     In this approach,  the tunnel server will share secrets with  all  the
     NASes and associated RADIUS servers, and there is no provision for LCP
     renegotiation by the tunnel server. Also, the tunnel server will  need
     to know how to handle and verify RADIUS Access-Accept messages.
     
     While  this  scheme can be workable if the reply comes directly from a
     RADIUS server, it would become  unmanageable  if  a  RADIUS  proxy  is
     involved,  since  the  reply  would  be authenticated using the secret
     shared by the client and proxy, rather than the RADIUS  server.  As  a
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
     result, this scheme is impractical.
     
     
     6.1.3.  Tunnel server authentication
     
     In  this  scheme,  authentication and authorization occurs once at the
     tunnel server.  This requires that the NAS  determine  that  the  user
     needs  to  be  tunneled  (through  RADIUS or NAS configuration). Where
     RADIUS is used, the determination can be made using one of the follow-
     ing methods:
     
          Telephone-number based authentication
          User-Name
     
     
     6.1.3.1.  Telephone-number based authentication
     
     Using  the Calling-Station-Id and Called-Station-Id RADIUS attributes,
     authorization and subsequent tunnel attributes can  be  based  on  the
     phone  number  originating  the call, or the number being called. This
     allows the RADIUS server to authorize users based on the calling phone
     number or to provide tunnel attributes based on the Calling-Station-Id
     or Called-Station-Id.  Similarly, in PPTP/L2TP the tunnel  server  MAY
     choose  to  reject  or  accept the call based on the Dialed Number and
     Dialing Number included in the PPTP/L2TP Incoming-Call-Request  packet
     sent by the NAS.
     
     The  use  of  RADIUS  accounting  by  the NAS and/or the tunnel server
     allows for accounting to take place based  on  the  Calling-Station-Id
     and  Called-Station-Id.   RADIUS  as  defined  in [3] requires that an
     Access-Request packet contain a User-Name attribute as well as  either
     a  CHAP-Password  or User-Password attribute, which must be non-empty.
     To satisfy this requirement the Called-Station-Id or  Calling-Station-
     Id  may  be furnished in the User-Name attribute and a dummy value may
     be used in the User-Password or CHAP-Password attribute.
     
     In the case of telephone-number based authentication, a typical initi-
     ation sequence looks like this:
     
          Client and NS: Call Connected
          NAS to RADIUS Server: RADIUS Access-request
          RADIUS server to NAS: RADIUS Access-Accept/Access-Reject
          NAS to Tunnel Server: PPTP/L2TP Incoming-Call-Request
          Tunnel Server to NAS: PPTP/L2TP Incoming-Call-Reply
          NAS to Tunnel Server: PPTP/L2TP  Incoming-Call-Connected
          Client and Tunnel Server: PPP LCP negotiation
          Client and Tunnel Server: PPP authentication
          Tunnel Server to RADIUS Server: RADIUS Access-request (optional)
          RADIUS server to Tunnel Server: RADIUS Access-Accept/Access-Reject
          Client and Tunnel Server: NCP negotiation
          NAS to RADIUS Server: RADIUS Accounting-Request
            with Acct-Status-Type=Tunnel-Link-Start
          RADIUS Server to NAS: RADIUS Accounting-Response
          Tunnel Server to RADIUS Server: RADIUS Accounting-Request
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
             with Acct-Status-Type=Tunnel-Link-Start (Optional)
          RADIUS Server to Tunnel Server: RADIUS Accounting-Response
     
     The process begins with an incoming call to the NAS. If configured for
     telephone-number based authentication, the NAS sends a RADIUS  Access-
     Request  containing  the  Calling-Station-Id and the Called-Station-Id
     attributes. The RADIUS server will then respond with a RADIUS  Access-
     Accept or Access-Reject.
     
     The  NAS MUST NOT begin PPP authentication before bringing up the tun-
     nel. If timing permits, the NAS MAY  bring  up  the  tunnel  prior  to
     beginning  LCP  negotiation  with  the peer. If this is done, then LCP
     will not need to be renegotiated between the peer and  tunnel  server,
     nor will LCP forwarding need to be employed.
     
     If  the initial telephone-number based authentication is unsuccessful,
     the RADIUS server sends a RADIUS Access-Reject. In this case, the  NAS
     MUST send an LCP-Terminate and disconnect the user.
     
     In the case where tunnel attributes are included in the RADIUS Access-
     Accept, and a PPTP/L2TP tunnel is indicated, the NAS will now bring up
     a  control  connection if none existed before. This is accomplished by
     sending a PPTP/L2TP Start-Control-Connection-Request  message  to  the
     tunnel  server.   The  tunnel  server will then reply with a PPTP/L2TP
     Start-Control-Connection-Reply.  If this message indicates  an  error,
     or  if  the  control connection is terminated at any future time, then
     the NAS MUST send an LCP-Terminate and disconnect the user.
     
     The NAS will then send an PPTP/L2TP Incoming-Call-Request  message  to
     the  tunnel  server. Among other things, this message will contain the
     Call Serial Number, which along with the  NAS-IP-Address  and  Tunnel-
     Server-Endpoint  is  used  to  uniquely  identify the call. The tunnel
     server will reply with a  PPTP/L2TP  Incoming-Call-Reply  message.  If
     this  message indicates an error, then the NAS MUST send an LCP-Termi-
     nate and disconnect the user. If no error is indicated, the  NAS  then
     replies with a PPTP/L2TP Incoming-Call-Connected message.
     
     At this point, data MAY begin to flow through the tunnel. If LCP nego-
     tiation had been begun between the NAS and the client, then  LCP  for-
     warding  may  be  employed,  or  the client and tunnel server will now
     renegotiate LCP and begin PPP authentication.  Otherwise,  the  client
     and tunnel server will negotiate LCP for the first time, and then move
     on to PPP authentication.
     
     If a renegotiation is required, at the  time  that  the  renegotiation
     begins,  the  NAS  SHOULD  NOT have sent an LCP CONFACK completing LCP
     negotiation, and the client and NAS MUST NOT have begun  NCP  negotia-
     tion. Rather than sending an LCP CONFACK, the NAS will instead send an
     LCP DOWN message. The Client MAY then renegotiate LCP, and  from  that
     point  forward,  all  PPP  packets  originated from the client will be
     encapsulated and sent to the tunnel server.  When  LCP  re-negotiation
     has  been  concluded,  the NCP phase will begin, and the tunnel server
     will assign an address to the client.
     
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
     If L2TP is being used as the tunnel protocol, and LCP renegotiation is
     required, the NAS MAY in its initial setup notification include a copy
     of the LCP CONFACKs sent in each direction which completed LCP negoti-
     ation.  The  tunnel  server  MAY then use this information to avoid an
     additional LCP negotiation. With L2TP, the initial setup  notification
     can  also include the authentication information required to allow the
     tunnel server to authenticate the user and decide to accept or decline
     the connection. However, in telephone-number based authentication, PPP
     authentication MUST NOT occur prior to the NAS bringing up the tunnel.
     As a result, L2TP authentication forwarding MUST NOT be employed.
     
     In performing the PPP authentication, the tunnel server can access its
     own user database, or alternatively can send a RADIUS  Access-Request.
     The latter approach is useful in cases where authentication forwarding
     is enabled, such as with roaming or shared use networks. In this case,
     the  RADIUS  and  tunnel servers are under the same administration and
     are typically located  close  together,  possibly  on  the  same  LAN.
     Therefore having the tunnel server act as a RADIUS client provides for
     unified user administration. Note  that  the  tunnel  server's  RADIUS
     Access-Request  is  typically sent directly to the local RADIUS server
     rather than being forwarded via a proxy.
     
     After the tunnel has been brought up, the NAS and tunnel  server  will
     typically  start  accounting.  In the case of the NAS, this is done by
     sending a RADIUS Accounting-Request packet with  Acct-Status-Type=Tun-
     nel-Start to a RADIUS server. When an individual call is brought up, a
     RADIUS Accounting-Request packet is sent with Acct-Status-Type=Tunnel-
     Link-Start.  The tunnel server can produce its own accounting records,
     or it MAY send a RADIUS Accounting-Request packet to  a  local  RADIUS
     server.
     
     The  interactions  involved  in initiation of a compulsory tunnel with
     telephone-number based authentication are summarized below.  In  order
     to  simplify  the  diagram  that follows, we have left out the client.
     However, it is understood that the client participates via PPP negoti-
     ation,  authentication and subsequent data interchange with the Tunnel
     Server.
     
     
                                    INITIATION SEQUENCE
     
     NAS                            Tunnel Server       RADIUS Server
     ---                            -------------       -------------
     Call connected
     Send RADIUS
      Access-Request
      with Called-Station-Id,
      and/or Calling-Station-Id
     LCP starts
                                                        IF authentication
                                                        succeeds
                                                         Send ACK
                                                        ELSE Send NAK
     IF NAK DISCONNECT
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
     ELSE
      IF no control
       connection exists
       Send
       Start-Control-Connection-Request
       to Tunnel Server
                                  Send
                                  Start-Control-Connection-Reply
                                  to NAS
      ENDIF
     
     Send
     Incoming-Call-Request
     message to Tunnel Server
                                  Send Incoming-Call-Reply
                                  to NAS
     Send
     Incoming-Call-Connected
     message to Tunnel Server
     
     Send data through the tunnel
                                  Re-negotiate LCP,
                                  authenticate user,
                                  bring up IPCP,
                                  start accounting
                                  Send RADIUS
                                  Accounting-Request with
                                  Acct-Status-Type=Tunnel-Link-Start
                                  (optional)
                                                       Send
                                                       RADIUS
                                                       Accounting-Response
     Send a RADIUS
     Accounting-Request message
     with Acct-Status-Type=Tunnel-Link-Start
                                                       Send
                                                       RADIUS
                                                       Accounting-Response
     
     
     
     6.1.3.2.  User-Name
     
     Since authentication will occur only at the tunnel-server, tunnel ini-
     tiation  must  occur  prior  to  user  authentication at the NAS. As a
     result, this scheme typically uses either the domain  portion  of  the
     userID  or  attribute-specific processing on the RADIUS server.  Since
     the user identity is never verified by  the  NAS,  either  the  tunnel
     server  owner  must be willing to be billed for all incoming calls, or
     other information such as the Calling-Station-Id must be used to  ver-
     ify the user's identity for accounting purposes.
     
     In  attribute-specific  processing  RADIUS  may  be  employed  and  an
     attribute is used to signal tunnel initiation.   For  example,  tunnel
     
     
     
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     INTERNET-DRAFT                                             18 May 1998
     
     
     attributes  can be sent back if the User-Password attribute contains a
     dummy value (such as "tunnel"  or  "L2TP").  Alternatively,  a  userID
     beginning with a special character ('*') could be used to indicate the
     need to initiate a  tunnel.   When  attribute-specific  processing  is
     used, the tunnel server may need to renegotiate LCP.
     
     Another  solution involves using the domain portion of the userID; all
     users in domain X would be tunneled to address Y. This  proposal  sup-
     ports  compulsory  tunneling, but does not provide for user-based tun-
     neling.
     
     In order for the NAS to start accounting on the connection,  it  would
     need  to use the identity claimed by the user in authenticating to the
     tunnel server, since it did not verify the identity via  RADIUS.  How-
     ever,  in  order  for  that to be of any use in accounting, the tunnel
     endpoint needs to have an account relationship  with  the  NAS  owner.
     Thus even if a user has an account with the NAS owner, they cannot use
     this account for tunneling unless the tunnel endpoint also has a busi-
     ness relationship with the NAS owner. Thus this approach is incompati-
     ble with roaming.
     
     A typical initiation sequence involving use of the domain  portion  of
     the userID looks like this:
     
          Client and NAS: Call Connected
          Client and NAS: PPP LCP negotiation
          Client and NAS: Authentication
          NAS to Tunnel Server: PPTP/L2TP Incoming-Call-Request
          Tunnel Server to NAS: PPTP/L2TP Incoming-Call-Reply
          NAS to Tunnel Server: PPTP/L2TP  Incoming-Call-Connected
          Client and Tunnel Server: PPP LCP re-negotiation
          Client and Tunnel Server: PPP authentication
          Tunnel Server to RADIUS Server: RADIUS Access-request (optional)
          RADIUS server to Tunnel Server: RADIUS Access-Accept/Access-Reject
          Client and Tunnel Server: NCP negotiation
          NAS to RADIUS Server: RADIUS Accounting-Request
            with Acct-Status-Type=Tunnel-Link-Start
          RADIUS Server to NAS: RADIUS Accounting-Response
          Tunnel Server to RADIUS Server: RADIUS Accounting-Request
             with Acct-Status-Type=Tunnel-Link-Start (Optional)
          RADIUS Server to Tunnel Server: RADIUS Accounting-Response
     
     The  process  begins with an incoming call to the NAS, and the PPP LCP
     negotiation between the Client and  NAS.  The  authentication  process
     will then begin and based on the domain portion of the userID, the NAS
     will now bring up a control connection if none existed before, and the
     NAS  and tunnel server will bring up the call. At this point, data MAY
     begin to flow through the tunnel.  The client and  tunnel  server  MAY
     now renegotiate LCP and will complete PPP authentication.
     
     At  the  time  that  the renegotiation begins, the NAS SHOULD NOT have
     sent an LCP CONFACK completing LCP negotiation, and the client and NAS
     MUST  NOT  have begun NCP negotiation. Rather than sending an LCP CON-
     FACK, the NAS will instead send an LCP DOWN message.  The  Client  MAY
     
     
     
     Aboba & Zorn                                                 [Page 11]


     INTERNET-DRAFT                                             18 May 1998
     
     
     then  renegotiate  LCP,  and  from that point forward, all PPP packets
     originated from the client will be encapsulated and sent to the tunnel
     server.  In single authentication compulsory tunneling, L2TP authenti-
     cation forwarding MUST NOT be employed.  When LCP  re-negotiation  has
     been  concluded,  the NCP phase will begin, and the tunnel server will
     assign an address to the client.
     
     In performing the PPP authentication, the tunnel server can access its
     own  user  database, or it MAY send a RADIUS Access-Request. After the
     tunnel has been brought up,  the  NAS  and  tunnel  server  MAY  start
     accounting.
     
     The interactions are summarized below.
     
     
     
                                    INITIATION SEQUENCE
     
     NAS                            Tunnel Server       RADIUS Server
     ---                            -------------       -------------
     Call accepted
     LCP starts
     Authentication
      phase starts
     IF no control
      connection exists
      Send
      Start-Control-Connection-Request
      to Tunnel Server
     ENDIF
                                  IF no control
                                   connection exists
                                   Send
                                   Start-Control-Connection-Reply
                                   to NAS
                                  ENDIF
     
     Send
     Incoming-Call-Request
     message to Tunnel Server
                                  Send Incoming-Call-Reply
                                  to NAS
     Send
     Incoming-Call-Connected
     message to Tunnel Server
     
     Send data through the tunnel
                                  Re-negotiate LCP,
                                  authenticate user,
                                  bring up IPCP,
                                  start accounting
                                  Send RADIUS
                                  Accounting-Request with
                                  Acct-Status-Type=Tunnel-Link-Start
     
     
     
     Aboba & Zorn                                                 [Page 12]


     INTERNET-DRAFT                                             18 May 1998
     
     
                                  (optional)
                                                       Send
                                                       RADIUS
                                                       Accounting-Response
     Send a RADIUS
     Accounting-Request message
     with Acct-Status-Type=Tunnel-Link-Start
                                                       Send
                                                       RADIUS
                                                       Accounting-Response
     
     
     
     6.2.  Dual authentication
     
     In  this  scheme, authentication occurs both at the NAS and the tunnel
     server. This requires the dial-up client to  handle  dual  authentica-
     tion,  with  attendant  LCP re-negotiations. In order to allow the NAS
     and tunnel network server to authenticate against the  same  database,
     this  requires  RADIUS client capability on the tunnel network server,
     and possibly a RADIUS proxy on the NAS end.
     
     Advantages of dual authentication include support  for  authentication
     and   accounting  at  both  ends  of  the  tunnel;  use  of  a  single
     userID/password pair via implementation of RADIUS on the  tunnel  net-
     work server; no requirement for telephone-number based authentication,
     or attribute-specific processing on the RADIUS server.
     
     Dual authentication allows for accounting records to be  generated  on
     both  the  NAS  and tunnel server ends, making auditing possible. Also
     the tunnel endpoint does not need to have an account relationship with
     the NAS owner, making this approach compatible with roaming.
     
     A disadvantage of dual authentication is that unless LCP forwarding is
     used, LCP will need to be renegotiated; some clients do not support it
     at  all, and others only support only a subset of the dual authentica-
     tion  combinations.  Feasible  combinations  include   PAP/PAP(token),
     PAP/CHAP, PAP/EAP, CHAP/PAP(token), CHAP/CHAP, CHAP/EAP, EAP/CHAP, and
     EAP/EAP.
     
     In the case of a dual authentication, a  typical  initiation  sequence
     looks like this:
     
          Client and NAS: PPP LCP negotiation
          Client and NAS: PPP authentication
          NAS to RADIUS Server: RADIUS Access-request
          RADIUS server to NAS: RADIUS Access-Accept/Access-Reject
          NAS to Tunnel Server: PPTP/L2TP Incoming-Call-Request
          Tunnel Server to NAS: PPTP/L2TP Incoming-Call-Reply
          NAS to Tunnel Server: PPTP/L2TP  Incoming-Call-Connected
          Client and Tunnel Server: PPP LCP re-negotiation (optional)
          Client and Tunnel Server: PPP authentication
          Tunnel Server to RADIUS Server: RADIUS Access-request (optional)
          RADIUS server to Tunel Server: RADIUS Access-Accept/Access-Reject
     
     
     
     Aboba & Zorn                                                 [Page 13]


     INTERNET-DRAFT                                             18 May 1998
     
     
          Client and Tunnel Server: NCP negotiation
          NAS to RADIUS Server: RADIUS Accounting-Request
            with Acct-Status-Type=Tunnel-Link-Start
          RADIUS Server to NAS: RADIUS Accounting-Response
          Tunnel Server to RADIUS Server: RADIUS Accounting-Request
             with Acct-Status-Type=Tunnel-Link-Start (Optional)
          RADIUS Server to Tunnel Server: RADIUS Accounting-Response
     
     The  process  begins  with an incoming call to the NAS. The client and
     NAS then begin LCP negotiation. Subsequently  the  PPP  authentication
     phase starts, and the NAS sends a RADIUS Access-Request message to the
     RADIUS server. If the authentication is successful, the RADIUS  server
     responds with a RADIUS Access-Accept containing tunnel attributes.
     
     In  the  case  where a PPTP/L2TP tunnel is indicated, the NAS will now
     bring up a control connection if none existed before, and the NAS  and
     tunnel server will bring up the call. At this point, data MAY begin to
     flow through the tunnel. The client and tunnel server MAY now  renego-
     tiate  LCP  and go through another round of PPP authentication. At the
     time that this renegotiation begins, the NAS SHOULD NOT have  sent  an
     LCP  CONFACK  completing  LCP negotiation, and the client and NAS MUST
     NOT have begun NCP negotiation. Rather than sending  an  LCP  CONFACK,
     the  NAS  will  instead  send an LCP DOWN message. The Client MAY then
     renegotiate LCP, and from that point forward, all PPP  packets  origi-
     nated  from  the  client  will  be encapsulated and sent to the tunnel
     server.  When LCP re-negotiation has been  concluded,  the  NCP  phase
     will  begin,  and  the  tunnel  server  will  assign an address to the
     client.
     
     If L2TP is being used as the tunnel protocol, the NAS MAY in its  ini-
     tial  setup  notification  include  a copy of the LCP CONFACKs sent in
     each direction which completed LCP negotiation. The tunnel server  MAY
     then use this information to avoid an additional LCP negotiation. With
     L2TP, the initial setup notification can also include the  authentica-
     tion  information  required to allow the tunnel server to authenticate
     the user and decide to accept or decline the connection. However, this
     facility  creates  a  vulnerability  to replay attacks, and can create
     problems in the case where the  NAS  and  tunnel  server  authenticate
     against  different  RADIUS servers. As a result, where user-based tun-
     neling via  RADIUS  is  implemented,  L2TP  authentication  forwarding
     SHOULD NOT be employed.
     
     In performing the PPP authentication, the tunnel server can access its
     own user database, or it MAY send a RADIUS Access-Request.  After  the
     tunnel  has  been  brought  up,  the  NAS  and tunnel server MAY start
     accounting.
     
     The interactions involved in initiation of a  compulsory  tunnel  with
     dual authentication are summarized below.
     
     
                                    INITIATION SEQUENCE
     
     NAS                            Tunnel Server       RADIUS Server
     
     
     
     Aboba & Zorn                                                 [Page 14]


     INTERNET-DRAFT                                             18 May 1998
     
     
     ---                            -------------       -------------
     Call accepted
     LCP starts
     PPP authentication
      phase starts
     Send RADIUS
      Access-Request
      with username and
      authentication data
                                                        IF authentication
                                                        succeeds
                                                         Send ACK
                                                        ELSE Send NAK
     IF NAK DISCONNECT
     ELSE
      IF no control
       connection exists
       Send
       Start-Control-Connection-Request
       to Tunnel Server
                                  Send
                                  Start-Control-Connection-Reply
                                  to NAS
      ENDIF
     
     Send
     Incoming-Call-Request
     message to Tunnel Server
                                  Send Incoming-Call-Reply
                                  to NAS
     Send
     Incoming-Call-Connected
     message to Tunnel Server
     
     Send data through the tunnel
                                  Re-negotiate LCP,
                                  authenticate user,
                                  bring up IPCP,
                                  start accounting
                                  Send RADIUS
                                  Accounting-Request with
                                  Acct-Status-Type=Tunnel-Link-Start
                                  (optional)
                                                       Send
                                                       RADIUS
                                                       Accounting-Response
     Send a RADIUS
     Accounting-Request message
     with Acct-Status-Type=Tunnel-Link-Start
                                                       Send
                                                       RADIUS
                                                       Accounting-Response
     
     ENDIF
     
     
     
     Aboba & Zorn                                                 [Page 15]


     INTERNET-DRAFT                                             18 May 1998
     
     
     7.  Termination sequence
     
     The  tear  down of a compulsory tunnel involves an interaction between
     the client, NAS, Tunnel Server, and  RADIUS  Accounting  server.  This
     interaction  is  virtually  identical regardless of whether telephone-
     number based authentication, single authentication, or dual  authenti-
     cation  is  being  used.   In  any  of the cases, the following events
     occur:
     
          Tunnel Server to NAS: PPTP/L2TP Call-Clear-Request (optional)
          NAS to Tunnel Server: PPTP/L2TP Call-Disconnect-Notify
          NAS to RADIUS Server: RADIUS Accounting-Request
           with Acct-Status-Type=Tunnel-Link-Stop
          RADIUS Server to NAS: RADIUS Accounting-Response
          Tunnel Server to RADIUS Server: RADIUS Accounting-Request
             with Acct-Status-Type=Tunnel-Link-Stop (Optional)
          RADIUS Server to Tunnel Server: RADIUS Accounting-Response
     
     Tunnel termination can occur due to a  client  request  (PPP  termina-
     tion), a tunnel server request (Call-Clear-Request), or a line problem
     (call disconnect).
     
     In the case of a client-requested termination, the tunnel server  MUST
     terminate  the PPP session. The tunnel server MUST subsequently send a
     Call-Clear-Request to the NAS. The NAS MUST then send  a  Call-Discon-
     nect-Notify  message  to  the  tunnel  server, and will disconnect the
     call.
     
     The NAS MUST also respond with a  Call-Disconnect-Notify  message  and
     disconnection  if  it  receives  a  Call-Clear-Request from the tunnel
     server without a client-requested termination.
     
     In the case of a line problem or user hangup,  the  NAS  MUST  send  a
     Call-Disconnect-Notify to the tunnel server. Both sides will then tear
     down the  call.
     
     After call tear down is complete, if RADIUS accounting is  being  used
     then  the  NAS MUST send a RADIUS Accounting-Request with Acct-Status-
     Type=Tunnel-Link-Stop packet to a RADIUS accounting server.
     
     The tunnel server MAY produce its own accounting records,  or  it  MAY
     use  RADIUS  accounting.  If  RADIUS accounting is being used then the
     tunnel server MUST send a RADIUS Accounting-Request with  Acct-Status-
     Type=Tunnel-Link-Stop to a RADIUS accounting server.
     
     The  interactions  involved  in termination of a compulsory tunnel are
     summarized below. In order to simplify the diagram  that  follows,  we
     have  left  out  the client. However, it is understood that the client
     MAY participate via PPP termination and disconnection.
     
     
                                    TERMINATION SEQUENCE
     
     NAS                            Tunnel Server         RADIUS Server
     
     
     
     Aboba & Zorn                                                 [Page 16]


     INTERNET-DRAFT                                             18 May 1998
     
     
     ---                            -------------         -------------
     IF user disconnected
      send
      Call-Disconnect-Notify
      message to tunnel server
                                    Tear down the call
                                    stop accounting
                                    Send RADIUS
                                    Accounting-Request with
                                    Acct-Status-Type=Tunnel-Link-Stop
                                    (optional)
                                                         Send RADIUS
                                                         Accounting-Response
     ELSE IF client requests
      termination
                                    send
                                    Call-Clear-Request
                                    to the NAS
      Send
      Call-Disconnect-Notify
      message to tunnel server
      Disconnect the user
                                    Tear down the call
                                    stop accounting
                                    Send RADIUS
                                    Accounting-Request with
                                    Acct-Status-Type=Tunnel-Link-Stop
                                    (optional)
                                                         Send RADIUS
                                                         Accounting-Response
     
      Send a RADIUS
      Accounting-Request message
      with Acct-Status-Type=Tunnel-Link-Stop
                                                         Send RADIUS
                                                         Accounting-Response
     
     ENDIF
     
     
     8.  Use of distinct RADIUS servers
     
     In the case that the NAS and the  tunnel  server  are  using  distinct
     RADIUS  servers,  some interesting cases can arise in the provisioning
     of compulsory tunnels.
     
     
     8.1.  Distinct userIDs
     
     If distinct RADIUS servers are being used, it is likely that  distinct
     userID/password pairs will be required to complete the RADIUS and tun-
     nel authentications. One pair will be used in the initial PPP  authen-
     tication  with the NAS, and the second pair will be used for authenti-
     cation at the tunnel server.
     
     
     
     Aboba & Zorn                                                 [Page 17]


     INTERNET-DRAFT                                             18 May 1998
     
     
     However, in order to provide maximum ease of use in the case where the
     userID/password  pairs are identical, tunnel clients typically attempt
     authentication with the same userID/password pair as was used  in  the
     initial PPP negotiation. Only after this fails do they prompt the user
     for the second pair.
     
     In this case, tunnel client implementations SHOULD take  care  not  to
     put  up  error  messages  indicating  a bad password. Instead a dialog
     SHOULD be presented requesting the tunnel userID/password combination.
     
     In the case where token cards are being used for both authentications,
     the tunnel client MUST NOT attempt to present the token  used  in  the
     first  authentication  during  the  second authentication, since these
     will never be identical. Instead the user SHOULD be prompted to  enter
     the second token.
     
     The same issue arises in L2TP if the NAS attempts to forward authenti-
     cation information to the tunnel server in the initial setup notifica-
     tion. Since the userID/password pair used for tunnel authentication is
     different from that used to authenticate against the  NAS,  forwarding
     authentication  information  in  this  manner  will  cause  the tunnel
     authentication to fail. As a result, where  user-based  tunneling  via
     RADIUS  is  implemented,  L2TP authentication forwarding SHOULD NOT be
     employed.
     
     
     8.2.  Multilink PPP issues
     
     It is possible for the two RADIUS servers to  return  different  Port-
     Limit  attributes.  For example, it is conceivable that the NAS RADIUS
     server will only grant use of  a  single  channel,  while  the  tunnel
     RADIUS server will grant more than one channel. In this case, the cor-
     rect behavior is for the tunnel client to open a connection to another
     NAS  in order to bring up a multilink bundle on the tunnel server. The
     client MUST NOT indicate to the NAS that this additional link is being
     brought  up as part of a multilink bundle; this will only be indicated
     in the subsequent negotiation with the tunnel server.
     
     It is also conceivable that the  NAS  RADIUS  server  will  allow  the
     client  to  bring  up  multiple  channels,  but that the tunnel RADIUS
     server will allow fewer channels than the NAS RADIUS server.  In  this
     case, the client should terminate use of the excess channels.
     
     
     9.  UserID Issues
     
     In  the  provisioning  of  roaming and shared use networks, one of the
     requirements is to be able to route the authentication request to  the
     user's home RADIUS server. This authentication routing is accomplished
     based on the userID submitted by the user to the NAS  in  the  initial
     PPP  authentication.  The userID is subsequently relayed by the NAS to
     the RADIUS server in the User-Name attribute, as part  of  the  RADIUS
     Access-Request.
     
     
     
     
     Aboba & Zorn                                                 [Page 18]


     INTERNET-DRAFT                                             18 May 1998
     
     
     Similarly, references [5] and [6] refer to use of the userID in deter-
     mining the tunnel endpoint. However, since none  of  these  references
     provide  guidelines  for  how RADIUS or tunnel routing is to be accom-
     plished, the possibility of conflicting interpretations exists.
     
     
     9.1.  UserID convergence in user-based tunneling
     
     The use of RADIUS in provisioning of compulsory tunneling relieves the
     userID  from having to do double duty. Rather than being used both for
     routing of the RADIUS authentication/authorization request as well for
     determination  of  the  tunnel endpoint, the userID is now used solely
     for routing of RADIUS authentication/authorization  requests.   Tunnel
     attributes  returned  in  the  RADIUS Access-Response are then used to
     determine the tunnel endpoint.
     
     Since the framework described in this document allows  both  ISPs  and
     tunnel users to authenticate users as well as to account for resources
     consumed by  them,  and  provides  for  maintenance  of  two  distinct
     userID/password pairs, this scheme provides a high degree of flexibil-
     ity.  Where RADIUS proxies and tunneling are employed, it is  possible
     to  allow  the user to authenticate with a single userID/password pair
     at both the NAS and the tunnel endpoint. This is accomplished by rout-
     ing  the  NAS  RADIUS Access-Request to the same RADIUS server used by
     the tunnel server.
     
     As described  in  [8],  the  recommended  form  for  the  user  ID  is
     userID@realm, i.e. fred@bigco.com.
     
     
     10.  Security considerations
     
     In  compulsory tunneling, PAP authentication SHOULD NOT be used, since
     this will typically involve transmission of a cleartext password  over
     the  Internet.  A possible exception is where PAP is used to support a
     one time password or token.
     
     Where RADIUS proxies are deployed, the Access-Reply sent by the RADIUS
     server may be processed by one or more proxies prior to being received
     by the NAS.  In order to ensure that tunnel attributes arrive  without
     modification,  intermediate RADIUS proxies forwarding the Access-Reply
     MUST NOT modify tunnel attributes. If the RADIUS proxy does  not  sup-
     port tunnel attributes, then it MUST send an Access-Reject to the NAS.
     This is necessary to ensure that the user is only  granted  access  if
     the services requested by the RADIUS server can be provided.
     
     Since  RADIUS  tunnel  attributes  are  used for compulsory tunneling,
     address assignment is handled by the tunnel  server  rather  than  the
     NAS.  As  a  result,  if  tunnel  attributes are present, the NAS MUST
     ignore any address assignment attributes sent by the RADIUS server. In
     addition,  the  NAS  and  client MUST NOT begin NCP negotiation, since
     this could create a time window in which the client will be capable of
     sending  packets  to  the transport network, which is not permitted in
     compulsory tunneling.
     
     
     
     Aboba & Zorn                                                 [Page 19]


     INTERNET-DRAFT                                             18 May 1998
     
     
     11.  Acknowledgements
     
     Thanks to Gurdeep Singh Pall of Microsoft for many useful  discussions
     of  this  problem  space,  and  to Allan Rubens of Ascend and Bertrand
     Buclin of AT&T Labs Europe for his comments on this document.
     
     
     12.  References
     
     [1]  B. Aboba, J. Lu, J. Alsop, J. Ding, W. Wang.  "Review of  Roaming
     Implementations."   RFC 2194, Microsoft, Aimnet, i-Pass Alliance, Asi-
     ainfo, Merit, September 1997.
     
     [2]  B. Aboba, G. Zorn.  "Dialup Roaming Requirements." Internet draft
     (work   in  progress),  draft-ietf-roamops-roamreq-09.txt,  Microsoft,
     March 1998.
     
     [3]  C. Rigney, A. Rubens, W. Simpson, S. Willens.  "Remote  Authenti-
     cation  Dial  In  User Service (RADIUS)." RFC 2138, Livingston, Merit,
     Daydreamer, April 1997.
     
     [4]  C. Rigney.  "RADIUS  Accounting."  RFC  2139,  Livingston,  April
     1997.
     
     [5]  A.  Valencia,  K.  Hamzeh, A. Rubens, T. Kolar, M. Littlewood, W.
     Townsley, J. Taarud, G. Pall, B. Palter, W. Verthein.  "Layer Two Tun-
     neling  Protocol L2TP." Internet draft (work in progress), draft-ietf-
     pppext-l2tp-10.txt, Ascend Communications, Microsoft, Copper  Mountain
     Networks, U.S. Robotics, March 1998.
     
     [6]  K.  Hamzeh,  G.  S.  Pall,  J. Taarud, W. Verthein, W. A. Little.
     "Point-to-Point Tunneling Protocol -- PPTP." ." Internet  draft  (work
     in  progress),  draft-ietf-pppext-pptp-02.txt,  Ascend Communications,
     Microsoft, Copper Mountain Networks, U.S. Robotics, September 1997.
     
     [7] G. Zorn, D. Leifer, A. Rubens, J. Shriver.  "RADIUS Attributes for
     Tunnel  Protocol  Support."  Internet draft (work in progress), draft-
     ietf-radius-tunnel-auth-05.txt,  Microsoft,   Ascend   Communications,
     Shiva Corporation, April 1998.
     
     [8]  B. Aboba, M. A. Beadles.  "The Network Access Identifier." Inter-
     net   draft   (work   in   progress),   draft-ietf-roamops-nai-10.txt,
     Microsoft, CompuServe, March 1998.
     
     [9]  S.  Bradner.   "Key words for use in RFCs to Indicate Requirement
     Levels." RFC 2119, Harvard University, March, 1997.
     
     [10]   C.   Rigney,   W.   Willats.   "RADIUS  Extensions."  Work   in
     progress, draft-ietf-radius-ext-01.txt, Livingston, June, 1997.
     
     [11]  P.  Calhoun,  A.C. Rubens, B. Aboba.  "Extensible Authentication
     Protocol Support in RADIUS." Internet draft (work in progress), draft-
     ietf-radius-eap-05.txt,  Sun  Microsystems., Merit Network, Microsoft,
     May, 1998.
     
     
     
     Aboba & Zorn                                                 [Page 20]


     INTERNET-DRAFT                                             18 May 1998
     
     
     [12]  L. Blunk, J. Vollbrecht.  "PPP Extensible Authentication  Proto-
     col (EAP)." RFC 2284, Merit Network, Inc., March 1998.
     
     [13]   G. Zorn, D. Mitton. "RADIUS Accounting Modifications for Tunnel
     Protocol Support."  Internet draft  (work  in  progress),  draft-ietf-
     radius-tunnel-acct-00.txt, Microsoft, Bay Networks, November 1997.
     
     
     13.  Chair's Address
     
     The RADIUS Working Group can be contacted via the current chair:
     
     Carl Rigney
     Livingston Enterprises
     4464 Willow Road
     Pleasanton, California  94588
     
     Phone: +1 510 426 0770
     E-Mail: cdr@livingston.com
     
     
     14.  Authors' Addresses
     
     Bernard Aboba
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: +1 425 936 6605
     EMail: bernarda@microsoft.com
     
     Glen Zorn
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052
     
     Phone: +1 425 703 1559
     EMail: glennz@microsoft.com
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     
     Aboba & Zorn                                                 [Page 21]
     

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