L2TP via RADIUS: Implementation of L2TP Compulsory Tunneling via RADIUS

Network Working Group                                          B. Aboba
Request for Comments: 2809                                    Microsoft
Category: Informational                                         G. Zorn
                                                                  Cisco
                                                             April 2000


         Implementation of L2TP Compulsory Tunneling via RADIUS

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document discusses implementation issues arising in the
   provisioning of compulsory tunneling in dial-up networks using the
   L2TP protocol.  This provisioning can be accomplished via the
   integration of RADIUS and tunneling protocols. Implementation issues
   encountered with other tunneling protocols are left to separate
   documents.

1. 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.

   Tunnel Network Server
              This is a server which terminates a tunnel. 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 L2TP
              terminology, a NAS performing compulsory tunneling is
              referred to as the L2TP Access Concentrator (LAC).

   RADIUS authentication server
              This is a server which provides for
              authentication/authorization via the protocol described in
              [1].

   RADIUS proxy
              In order to provide for the routing of RADIUS
              authentication 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.  Can be used to locate the tunnel
              endpoint when realm-based tunneling is used.

2.  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 [4].

3.  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
   purpose. 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
   networks 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
   tunneling, 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. Those
   attributes are defined in [3].

3.1.  Advantages of RADIUS-based compulsory tunneling

   Current proposals for routing of tunnel requests include static
   tunneling, where all users are automatically tunneled to a given
   endpoint, and realm-based tunneling, where the tunnel endpoint is
   determined from the realm portion of the userID. User-based tunneling
   as provided by integration of RADIUS and tunnel protocols offers
   significant 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 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
   management.  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 accommodated via user-based
   tunneling as enabled by the attributes defined in [3].

4.  Authentication alternatives

   RADIUS-based compulsory tunneling can support both single
   authentication, 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 authentication; CHAP/PAP(token) authentication; and EAP/EAP
   authentication, using the same EAP type for both authentications. EAP
   is described in [5].

   The alternatives are described in more detail below.

4.1.  Single authentication

   Single authentication alternatives include:

   NAS authentication
   NAS authentication with RADIUS reply forwarding
   Tunnel server authentication

4.1.1.  NAS authentication

   With this approach, authentication and authorization (including
   tunneling information) occurs once, at the NAS. The advantages of
   this approach are that it disallows network access for unauthorized
   NAS users, and permits accounting to done at the NAS.  Disadvantages
   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
   forwarding and tunnel authentication, both of which are supported in
   L2TP, described in [2].  Thus, the tunnel server can be set up to
   accept all calls occurring within authenticated tunnels, without
   requiring PPP authentication.  However, 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
   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
   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
   Configure-Request packet, described in [6].  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.

4.1.2.  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
   accounting to be done 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 Access-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
   result, this scheme is impractical.

4.1.2.1. 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
   following methods:

   Telephone-number based authentication
   UserID

4.1.2.2.  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 L2TP the tunnel
   server MAY choose to reject or accept the call based on the Dialed
   Number and Dialing Number included in the L2TP Incoming-Call-Request
   packet sent by the NAS.  Accounting can also take place based on the
   Calling-Station-Id and Called-Station-Id.

   RADIUS as defined in [1] 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
   initiation 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: L2TP Incoming-Call-Request
   Tunnel Server to NAS: L2TP Incoming-Call-Reply
   NAS to Tunnel Server: 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
   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
   tunnel.  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 an L2TP tunnel is indicated, the NAS will now
   bring up a control connection if none existed before. This is
   accomplished by sending an L2TP Start-Control-Connection-Request
   message to the tunnel server.  The tunnel server will then reply with
   an 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 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 an L2TP Incoming-Call-Reply message. If this
   message indicates an error, then the NAS MUST send an LCP-Terminate
   and disconnect the user. If no error is indicated, the NAS then
   replies with an L2TP Incoming-Call-Connected message.

   At this point, data can begin to flow through the tunnel. If LCP
   negotiation had been begun between the NAS and the client, then LCP
   forwarding 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
   negotiation.  Rather than sending an LCP CONFACK, the NAS will
   instead send an LCP Configure-Request Packet, described in [6].  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.

   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
   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 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.

   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
   negotiation, 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
   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

4.1.2.3.  User-Name

   Since authentication will occur only at the tunnel-server, tunnel
   initiation 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
   verify 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
   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
   supports compulsory tunneling, but does not provide for user-based
   tunneling.

   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.
   However, 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
   business relationship with the NAS owner. Thus this approach is
   incompatible 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: L2TP Incoming-Call-Request
   Tunnel Server to NAS: L2TP Incoming-Call-Reply
   NAS to Tunnel Server: 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
   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
   CONFACK, the NAS will instead send an LCP Configure-Request packet,
   described in [6].  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.  In single authentication
   compulsory tunneling, L2TP authentication 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 can 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

4.2.  Dual authentication

   In this scheme, authentication occurs both at the NAS and the tunnel
   server. This requires the dial-up client to handle dual
   authentication, 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
   network 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
   authentication combinations. Feasible combinations include
   PAP/PAP(token), PAP/CHAP, PAP/EAP, CHAP/PAP(token), CHAP/CHAP,
   CHAP/EAP, EAP/CHAP, and EAP/EAP.  EAP is described in [5].

   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: L2TP Incoming-Call-Request
   Tunnel Server to NAS: L2TP Incoming-Call-Reply
   NAS to Tunnel Server: 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 Tunnel Server: RADIUS Access-Accept/Access-Reject
   Client and Tunnel Server: NCP negotiation

   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 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 MAY begin
   to flow through the tunnel. The client and tunnel server MAY now
   renegotiate 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 Configure-Request packet,
   described in [6].  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.

   If L2TP is being used as the tunnel protocol, the NAS MAY in its
   initial 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
   authentication 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 tunneling 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 can start
   accounting.

   The interactions involved in initiation of a compulsory tunnel with
   dual authentication are summarized below.

                                  INITIATION SEQUENCE

   NAS                            Tunnel Server       RADIUS Server
   ---                            -------------       -------------
   Call accepted
   LCP starts
   PPP authentication
    phase starts
   Send RADIUS
    Access-Request
    with userID 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
   ENDIF

5.  Termination sequence

   The tear down of a compulsory tunnel involves an interaction between
   the client, NAS and Tunnel Server. This interaction is virtually
   identical regardless of whether telephone-number based
   authentication, single authentication, or dual authentication is
   being used.  In any of the cases, the following events occur:

        Tunnel Server to NAS: L2TP Call-Clear-Request (optional)
        NAS to Tunnel Server: L2TP Call-Disconnect-Notify

   Tunnel termination can occur due to a client request (PPP
   termination), 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-
   Disconnect-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.

   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
   ---                            -------------         -------------
   IF user disconnected
    send
    Call-Disconnect-Notify
    message to tunnel server
                                  Tear down the call
                                  stop accounting
   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
   ENDIF

6.  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.

6.1.  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
   tunnel authentications. One pair will be used in the initial PPP
   authentication with the NAS, and the second pair will be used for
   authentication at the tunnel server.

   This has implications if the NAS attempts to forward authentication
   information to the tunnel server in the initial setup notification.
   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.

   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. Rather than putting up an error message
   indicating an authentication failure, it is preferable to present a
   dialog requesting the tunnel userID/password combination.

   A similar issue arises when extended authentication methods are being
   used, as is enabled by EAP, described in [5]. In particular, when
   one-time passwords or cryptographic calculators are being used,
   different passwords will be used for the first and second
   authentications. Thus the user will need to be prompted to enter the
   second password.

6.2.  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
   correct 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.

7.  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.

   Similarly, [2] refers to use of the userID in determining the tunnel
   endpoint, although it does not provide guidelines for how RADIUS or
   tunnel routing is to be accomplished. Thus the possibility of
   conflicting interpretations exists.

   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
   flexibility.  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 routing the NAS RADIUS Access-Request to the same
   RADIUS server used by the tunnel server.

8.  References

   [1]  Rigney C., Rubens A., Simpson W. and S. Willens, "Remote
        Authentication Dial In User Service (RADIUS)", RFC 2138, April
        1997.

   [2]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
        Palter, B., "Layer Two Tunneling Protocol "L2TP"", RFC 2661,
        August 1999.

   [3]  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. and
        Goyret, I., "RADIUS Attributes for Tunnel Protocol Support",
        Work in Progress.

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

   [5]  Blunk, L. anf J. Vollbrecht, "PPP Extensible Authentication
        Protocol (EAP)", RFC 2284, March 1998.

   [6]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, July 1994.

9.  Security Considerations

   In PPP-based tunneling, PPP security is negotiated between the client
   and the tunnel server, and covers the entire length of the path. This
   is because the client does not have a way to know that they are being
   tunneled. Thus, any security the NAS may negotiate with the tunnel
   server will occur in addition to that negotiated between the client
   and NAS.

   In L2TP compulsory tunneling, this means that PPP encryption and
   compression will be negotiated between the client and the tunnel
   server.  In addition, the NAS may bring up an IPSEC security
   association between itself and the tunnel server. This adds
   protection against a number of possible attacks.

   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 support 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.

10.  Acknowledgements

   Thanks to Gurdeep Singh Pall of Microsoft for many useful discussions
   of this problem space, and to Allan Rubens of Tut Systems and
   Bertrand Buclin of AT&T Labs Europe for their comments on this
   document.

   Most of the work on this document was performed while Glen Zorn was
   employed by the Microsoft Corporation.

11.  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
   EMail: cdr@livingston.com

12.  Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425-936-6605
   EMail: bernarda@microsoft.com


   Glen Zorn
   Cisco Systems, Inc.
   500 108th Avenue N.E., Suite 500
   Bellevue, WA 98004
   USA

   Phone: +1 425 438 8218
   FAX:   +1 425 438 1848
   EMail: gwz@cisco.com

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