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RADIUS Working Group Bernard Aboba INTERNET-DRAFT Microsoft Category: Standards Track Glen Zorn <draft-ietf-radius-tunnel-imp-03.txt> Microsoft 26 July 1997 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-03.txt> and expires February 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. Aboba & Zorn [Page 1] INTERNET-DRAFT 26 July 1997 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 Aboba & Zorn [Page 2] INTERNET-DRAFT 26 July 1997 locate the tunnel endpoint when domain-based tunneling is used. 4. Requirements language This specification uses the same words as [9] for defining the signif- icance of each particular requirement. These words are: MUST This word, or the adjectives "REQUIRED" or "SHALL", means that the definition is an absolute requirement of the speci- fication. MUST NOT This phrase, or the phrase "SHALL NOT", means that the defi- nition is an absolute prohibition of the specification. SHOULD This word, or the adjective "RECOMMENDED", means that there MAY exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. SHOULD NOT This phrase means that there MAY exist valid reasons in par- ticular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before imple- menting any behavior described with this label. MAY This word, or the adjective "OPTIONAL", means that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interop- erate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another imple- mentation which does not include the option.(except, of course, for the feature the option provides) Silently Discard The implementation discards the datagram without further processing, and without indicating an error to the sender. The implementation SHOULD provide the capability of logging the error, including the contents of the discarded datagram, and SHOULD record the event in a statistics counter. An implementation is not compliant if it fails to satisfy one or more of the MUST or MUST NOT requirements for the protocols it implements. An implementation that satisfies all the MUST, MUST NOT, SHOULD and SHOULD NOT requirements for its protocols is said to be Aboba & Zorn [Page 3] INTERNET-DRAFT 26 July 1997 "unconditionally compliant"; one that satisfies all the MUST and MUST NOT requirements but not all the SHOULD or SHOULD NOT requirements for its protocols is said to be "conditionally compliant." 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]. 5.1. Tunneling attributes As described in [7], provisioning of compulsory tunneling with RADIUS requires no new RADIUS messages, and implementation of five new RADIUS attributes: Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-End- point, Tunnel-Server-Endpoint, and Acct-Tunnel-Connection-Id. The Tunnel-Type attribute indicates the tunneling protocol(s) to be used (PPTP, L2F, L2TP, ATMP, VTP, IPSEC AH, IP-IP). It MAY be included in Access-Request, Access-Accept, and Access-Challenge packets. The Tunnel-Medium-Type Attribute indicates which transport medium to use (IP, X.25, ATM, Frame Relay) when creating a tunnel for those pro- tocols (such as L2TP) that can operate over multiple transports. It MAY be included in in Access-Request, Access-Accept, and Access- Challenge packets. Aboba & Zorn [Page 4] INTERNET-DRAFT 26 July 1997 Acct-Tunnel-Client-Endpoint contains the address of the NAS end of the tunnel. It SHOULD be included in Accounting-Request packets which contain Acct-Status-Type attributes with values of either Start or Stop. This Attribute, along with the Tunnel-Server-Endpoint and Acct-Tunnel-Connection-ID attributes, may be used to provide a glob- ally unique means to identify a tunnel for accounting purposes. Tunnel-Server-Endpoint indicates the address of the server end of the tunnel. It SHOULD be included in Accounting-Request packets which contain Acct-Status-Type attributes with values of either Start or Stop. Acct-Tunnel-Connection-ID indicates the identifier assigned to the call. It SHOULD be included in Accounting-Request packets which contain Acct-Status-Type attributes with values of either Start or Stop. 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 under compulsory tunneling. 5.2. 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 Telephone-number based authentication and accounting Single or dual authentication 5.2.1. 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 Aboba & Zorn [Page 5] INTERNET-DRAFT 26 July 1997 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 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 a corporation's flexibility in managing the account rights of their users. 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. On the other hand, user-based tunneling as enabled by the attributes defined in [7] provides BIGCO with the flexibility to administer tun- neling behavior on a per-user basis. When deployed in concert with roaming, user-based tunneling offers corporations the ability to pro- vide their users with access to the corporate Intranet on a global basis. 5.3. 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. The Acct-Tunnel-Client-Endpoint and Acct-Tunnel-Connection-ID attributes were introduced in [7] in order to support auditing. These attributes are included within the RADIUS Accounting-Request packet along with other attributes such as the User-Name and Tunnel-Server- Endpoint. During accounting, the User-Name, Acct-Tunnel-Connection- ID, Acct-Tunnel-Client-Endpoint and Tunnel-Server-Endpoint attributes uniquely identify the call. This allows the Accounting-Request sent by the NAS to be reconciled with the corresponding Accounting-Request sent by the tunnel server. When implementing L2TP compulsory tunneling based on RADIUS, the NAS MUST transmit the Call-Serial-Number in the Acct-Tunnel-Connection-ID attribute in the Accounting-Request packet. In order to protect against wrapping due to reboots or call volume, a 64-bit NTP timestamp SHOULD be used as the Call-Serial Number. This feasible in L2TP since the Call-Serial-Number string is of variable length. When implementing PPTP compulsory tunneling based on RADIUS, the NAS MUST transmit the Call-Serial-Number in the Acct-Tunnel-Connection-ID Aboba & Zorn [Page 6] INTERNET-DRAFT 26 July 1997 attribute in the Accounting-Request packet. While [6] states that the Call-Serial-Number SHOULD be unique, this is not required. In addi- tion, no method for determining the Call-Serial-Number is specified, which leaves open the possibility of wrapping after a reboot. Since the Call-Serial-Number is a 16-bit value, the Call-Serial-Number is not sufficient to distinguish a given call from all other calls over an extended time period. For example, let us assume that the Call Serial Number is assigned monotonically, and that the NAS in question has 96 ports. If the NAS ports are kept continually busy and the aver- age call is of 20 minutes duration, then the Call Serial Number will wrap within 65536/(96 * 3 calls/hour * 24 hours/day) = 9.5 days. In order to rectify this, it is recommended that another message be added to PPTP so that the NAS could provide the tunnel server with an Acct-Tunnel-Connection-ID unique over an extended time period. It is recommended that a 64-bit NTP timestamp be used for this purpose. 5.4. Telephone-number based authentication and accounting 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 (with or without a userID/password combination), or to select the Tunnel-Type, Tunnel-Medium-Type, and Tunnel-Server-Endpoint attributes based on the Calling-Station-Id or Called-Station-Id. Sim- ilarly, 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. 5.5. Single or dual authentication As described below, RADIUS-based compulsory tunneling can support a variety of authentication configurations. These include single authen- tication, where the user is authenticated at the 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 described above, or EAP-based authentication. When dual-authentication is used, a number of modes are available, includ- ing dual CHAP authentications; CHAP/EAP authentication; CHAP/PAP(token) authentication; and EAP/EAP authentication, using the same EAP type for both authentications. PAP/PAP, CHAP/PAP and EAP/PAP dual authentications SHOULD NOT be used, since these combinations involve transmission of cleartext passwords over the Internet. Aboba & Zorn [Page 7] INTERNET-DRAFT 26 July 1997 6. Authentication alternatives in compulsory tunneling There are several authentication alternatives for integration of RADIUS and tunneling: Authenticate at the NAS Authenticate at the NAS, and forward the RADIUS reply Authenticate at the tunnel network server Authenticate at both the NAS and the tunnel network server We discuss each of these alternatives in turn. 6.1. Authenticate at the NAS 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 network server trust the NAS, since no user authentication occurs on the tunnel network server end of the tunnel. Another disadvantage is that this does not allow LCP parameters to be re-negotiated by the tunnel network server. Also, due to the lack of user authentication, accounting cannot take place at the tunnel network server with strong assurance that the correct party is being billed. As a result, it does not appear that this scheme can be practically employed. 6.2. Authenticate at the NAS, and forward the RADIUS reply With this approach, authentication and authorization occurs once, at the NAS end of the tunnel and the RADIUS reply is forwarded to the tunnel network server. This approach disallows network access for unauthorized NAS users; does not require trust between the NAS and tunnel network server ends of the tunnel; 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 network server could check the RADIUS reply. In this approach, the tunnel network server MUST share secrets with all the NASes and associated RADIUS servers, and there is no provision for LCP renegotiation by the tunnel network server. Also, the tunnel network server MUST 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, it does not appear that this scheme can be practically employed. Aboba & Zorn [Page 8] INTERNET-DRAFT 26 July 1997 6.3. Authenticate at the tunnel network server In this scheme, authentication and authorization occurs once at the tunnel network 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 EAP Identity 6.3.1. Telephone-number based authentication Where telephone-number based authentication is used, the Calling-Sta- tion-Id or Called-Station-Id attributes included in the RADIUS Access- Request are used to determine whether the call will be accepted or rejected, and if accepted, where the user is to be tunneled. Where telephone-number based authentication is used, the User-Name and User- Password or CHAP-Password attributes need not be present. In cases where telephone-number authentication may be employed, accounting may be accomplished on the NAS side via the Calling-Sta- tion-Id or Called-Station-Id, and on the tunnel server side, via the userID. Thus this scheme is capable of providing both authentication and accounting, and appears practical to implement. 6.3.2. User-Name Where the User-Name attribute is present, RADIUS as defined in [3] requires that either a CHAP-Password or User-Password attribute be present in an Access-Request message, as well as requiring that the password be non-empty. Thus, this scheme either requires attribute- specific processing on the RADIUS server, or addition of an "Autho- rize-Only" message. In attribute-specific processing an attribute is used to signal tunnel initiation. For example, tunnel attributes can be sent back if the PAP password is empty (or "tunnel" or "L2TP"). Alternatively, a user could prefix the userID with some special character ('*') if he wanted to be tunneled. This particular solution does not support compulsory tunneling. Another solution involves keying on 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. An Authorize-Only message would not include a CHAP or PAP password; nevertheless, in response the RADIUS server would return the attribute list. In order to prevent password guessing attacks, an Authorize-Only message would need to be authenticated via the RADIUS shared secret. This could be accomplished via the Signature attribute described in [10]. Aboba & Zorn [Page 9] INTERNET-DRAFT 26 July 1997 Note that when either attribute-specific processing or an authorize- only message is used, the tunnel network server would need to renego- tiate LCP. Note also that 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 network server, since it did not ver- ify the identity via RADIUS. However, in order for that to be of any use in accounting, the tunnel endpoint needs to have an account rela- tionship 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. Without putting in place a public key infrastructure, it is not clear how the NAS would be able to verify the existence of such a business relationship in a scalable manner. Thus this approach nullifies many of the benefits of roaming. Due to these difficulties, it does not appear that this scheme can be practically employed. 6.3.3. EAP Identity Where EAP is used as described in [11], the NAS may include an EAP- Message/Identity attribute in the RADIUS Access-Request. Based on the Identity included in the Access-Request, the RADIUS server may send tunneling attributes within the RADIUS Access-Challenge packet. The NAS can then set up the tunnel and EAP authentication may continue between the client and the tunnel server. This method avoids having to use attribute-specific processing or an authorize-only message. However, in this case, the EAP identity is never verified by the NAS, and so 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. Where either of these conditions holds true, this scheme may be prac- tically employed. 6.4. Authenticate at both the NAS and the tunnel network server In this scheme, authentication occurs both at the NAS and the tunnel network server. This requires the dial-up PPP peer to handle dual authentications, 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 this scheme are that it allows secure authentication and accounting at both ends of the tunnel; allows the use of a single userID/password pair via implementation of RADIUS on the tunnel net- work server; and does not require telephone-number based authentica- tion, use of EAP, attribute-specific processing or addition of an Authorize-Only message on the RADIUS server. This scheme allows for accounting records to be generated on both the NAS and tunnel server ends allowing for auditing. Also, in contrast to the previous scheme, the tunnel endpoint does not need to have an account relationship with Aboba & Zorn [Page 10] INTERNET-DRAFT 26 July 1997 the NAS owner. Thus this scheme is compatible with roaming. Disadvan- tages are that unless LCP forwarding is used, LCP will need to be renegotiated, and that dual authentications are required. The use of dual authentication can be complex, since 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. Assuming that the required dual authentication capabilities are supported, this scheme may be practi- cally employed. 7. Telephone-number based authentication 7.1. Initiation sequence In the case of telephone-number based authentication, a typical initi- ation sequence looks like this: Client and NAS: 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=Start RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=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 MUST send a RADIUS Access-Request containing the Calling-Station-Id and the Called-Sta- tion-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 client. If this is done, then LCP will not need to be renegotiated between the client and tunnel server. 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. Aboba & Zorn [Page 11] INTERNET-DRAFT 26 July 1997 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. The control connection will be of the type indicated by Tunnel-Type, over the medium indi- cated by Tunnel-Medium-Type, to the tunnel server indicated by Tunnel- Server-Endpoint. This is accomplished by sending a PPTP/L2TP Start- Control-Connection-Request message to the tunnel server. The tunnel server will then with a PPTP/L2TP Start-Control-Connection-Reply. If this message indicates an error, or if the control connection is ter- minated at any future time, then the NAS MUST send an LCP-Terminate and disconnect the user. The NAS will then proceed to send a 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-Ter- minate 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, the client and tunnel server MAY 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. 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 it MAY 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. Aboba & Zorn [Page 12] INTERNET-DRAFT 26 July 1997 Therefore having the tunnel server act as a RADIUS client provides for unified user administration. Other methods are also available, such as use of LDAP, described in [12], by both the tunnel and RADIUS servers. Note that the tunnel server's RADIUS Access-Request is typically sent directly to the local RADIUS server rather than being forwarded via proxy. After the tunnel has been brought up, the NAS and tunnel server MAY start accounting. In the case of the NAS, this is done by sending a RADIUS Accounting-Request packet with Acct-Status-Type=Start to a RADIUS server. The Accounting-Request packet MUST include the follow- ing attributes: Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client- Endpoint, Tunnel-Server-Endpoint, and Acct-Tunnel-Connection-Id. The Accounting-Request packet MAY also include the Calling-Station-Id and Called-Station-Id attributes. The tunnel server can produce its own accounting records, or it MAY send a RADIUS Accounting-Request packet with Acct-Status-Type=Start to a local RADIUS server. In the latter case, the RADIUS Accounting- Request packet MUST contain the following attributes: Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-End- point, and Acct-Tunnel-Connection-Id. 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 with Tunnel-Type, Tunnel-Medium-Type, Tunnel-Server-Endpoint, ELSE Send NAK IF NAK DISCONNECT ELSE IF no control connection exists Send Start-Control-Connection-Request Aboba & Zorn [Page 13] INTERNET-DRAFT 26 July 1997 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=Start (optional) Send RADIUS Accounting-Response Send a RADIUS Accounting-Request message with Acct-Status-Type=Start containing Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, Acct-Tunnel-Connection-Id, Calling-Station-Id, Called-Station-Id. Send RADIUS Accounting-Response ENDIF 7.2. EAP support It is expected that the Extensible Authentication Protocol (EAP) described in [13] will frequently be used along with telephone number- based authentication and tunneling in order to provide additional security. In this case, EAP authentication may be used in the tunnel authentication only, using EAP code present on the NAS, or via support of EAP within RADIUS described in [11]. In this case, the initiation sequence will look like this: Aboba & Zorn [Page 14] INTERNET-DRAFT 26 July 1997 Client and NAS: Call Connected Client and NAS: PPP LCP negotiation NAS to RADIUS Server: RADIUS Access-Request RADIUS server to NAS: RADIUS Access-Reply/Tunnel attributes 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: EAP authentication Tunnel Server to RADIUS Server: RADIUS Access-Request/EAP-start RADIUS server to Tunnel Server: RADIUS Access-Challenge/EAP-Message Tunnel Server to Client: EAP-Request Client to Tunnel Server: EAP-Response Tunnel Server to RADIUS Server: RADIUS Access-Request/EAP-Message RADIUS server to Tunnel Server: RADIUS Access-Accept/EAP-Message/EAP-Success Client and Tunnel Server: NCP negotiation NAS to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=Start RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=Start (Optional) RADIUS Server to Tunnel Server: RADIUS Accounting-Response 8. Single authentication 8.1. Initiation sequence In the case of a single authentication compulsory tunnel, a typical initiation sequence looks like this: Client and NAS: Call Connected Client and NAS: PPP LCP negotiation Client and NAS: EAP authentication NAS to RADIUS Server: RADIUS Access-request RADIUS server to NAS: RADIUS Access-Challenge/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: EAP authentication Tunnel Server to RADIUS Server: RADIUS Access-request (optional) RADIUS server to Tunnel Server: RADIUS Access-Challenge/Access-Reject Client and Tunnel Server: NCP negotiation NAS to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=Start RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=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. At this point, the NAS must Aboba & Zorn [Page 15] INTERNET-DRAFT 26 July 1997 offer EAP, and the Client must accept if the negotiation is to pro- ceed. If the Client is incapable of authenticating via EAP, then the NAS MUST send an LCP-Terminate and disconnect the user. The NAS will now typically send an EAP-Request/Identity packet to the Client, and the client will respond with an EAP-Response/MyId packet. The NAS now sends an Access-Request/EAP-Message/EAP-Response/MyId packet to the RADIUS server, which responds with an Access-Challenge or an Access-Reject. If single-authentication tunneling is to be carried out, the Access- Challenge packet MUST contain the Tunnel-Type, Tunnel-Medium-Type, and Tunnel-Server-Endpoint Attributes. The presence of tunneling attributes in the Access-Challenge indicates to the NAS that a tunnel MUST be brought up prior to continuation of the EAP conversation. As a result, if the Access-Challenge contains an EAP-Message attribute, then the NAS MUST NOT send the EAP-Request encapsulated in the EAP- Message prior to bringing up the tunnel. Since after the tunnel is brought up LCP will be renegotiated, EAP-Message attributes are effec- tively ignored whenever the Access-Challenge also contains 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 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 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. If the tunnel server and NAS both are using the same RADIUS server, the RADIUS server will respond to the tunnel server's Access-Request with an Access-Challenge packet containing tunnel attributes and an EAP-Message attribute, as before. On receiving the Access-Challenge including tunnel attributes, the tunnel server will check whether the tunnel matches the attributes returned by the RADIUS server; if so, the tunnel attributes will be ignored, and the EAP-Request specified in the EAP-Message attribute will be sent to the Client. If the tunnel attributes do not match, then the tunnel server MUST disconnect the call. 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 Aboba & Zorn [Page 16] INTERNET-DRAFT 26 July 1997 accounting. The interactions involved in initiation of a compulsory tunnel with single authentication are summarized below. INITIATION SEQUENCE NAS Tunnel Server RADIUS Server --- ------------- ------------- Call accepted LCP starts EAP authentication phase starts Send RADIUS Access-Request with EAP-Message/MyId attribute IF MyId is known, send RADIUS Access-Challenge with Tunnel-Type, Tunnel-Medium-Type, Tunnel-Server-Endpoint, EAP-Message (optional) 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 via EAP, bring up IPCP, start accounting Send RADIUS Aboba & Zorn [Page 17] INTERNET-DRAFT 26 July 1997 Accounting-Request with Acct-Status-Type=Start (optional) Send RADIUS Accounting-Response Send a RADIUS Accounting-Request message with Acct-Status-Type=Start containing Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, and Acct-Tunnel-Connection-Id. Send RADIUS Accounting-Response ENDIF 9. Dual authentication 9.1. Initiation sequence 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 Tunnel Server: RADIUS Access-Accept/Access-Reject Client and Tunnel Server: NCP negotiation NAS to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=Start RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=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. For users requiring mandatory Aboba & Zorn [Page 18] INTERNET-DRAFT 26 July 1997 tunneling, the Access-Accept MUST contain the the Tunnel-Type, Tunnel- Medium-Type, and Tunnel-Server-Endpoint 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 --- ------------- ------------- Call accepted LCP starts PPP authentication phase starts Send RADIUS Access-Request with username and authentication data IF authentication succeeds Aboba & Zorn [Page 19] INTERNET-DRAFT 26 July 1997 Send ACK with Tunnel-Type, Tunnel-Medium-Type, Tunnel-Server-Endpoint, 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=Start (optional) Send RADIUS Accounting-Response Send a RADIUS Accounting-Request message with Acct-Status-Type=Start containing Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, and Acct-Tunnel-Connection-Id. Send RADIUS Accounting-Response ENDIF Aboba & Zorn [Page 20] INTERNET-DRAFT 26 July 1997 9.2. EAP support It is expected that the Extensible Authentication Protocol (EAP) described in [13] will frequently be used along with tunneling in order to provide additional security. EAP authentication may be used in the initial NAS authentication, in the tunnel authentication, or both, with support of EAP within RADIUS described in [11]. In the case where EAP authentication is carried out between the NAS and client, the initiation sequence will look like this: Client and NAS: PPP LCP negotiation Client and NAS: EAP Authentication NAS to RADIUS Server: RADIUS Access-Request/EAP-start RADIUS server to NAS: RADIUS Access-Challenge/EAP-Message NAS to Client: EAP-Request Client to NAS: EAP-Response NAS to RADIUS Server: RADIUS Access-Request/EAP-Message RADIUS server to NAS: RADIUS Access-Accept/EAP-Message/EAP-Success 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 Tunnel Server: RADIUS Access-Accept/Access-Reject Client and Tunnel Server: NCP negotiation NAS to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=Start RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request With Acct-Status-Type=Start(Optional) RADIUS Server to Tunnel Server: RADIUS Accounting-Response In the case where EAP authentication is carried out between the client and tunnel server, the initiation sequence will look 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 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: EAP authentication Tunnel Server to RADIUS Server: RADIUS Access-Request/EAP-start RADIUS server to Tunnel Server: RADIUS Access-Challenge/EAP-Message Tunnel Server to Client: EAP-Request Client to Tunnel Server: EAP-Response Tunnel Server to RADIUS Server: RADIUS Access-Request/EAP-Message RADIUS server to Tunnel Server: RADIUS Access-Accept/EAP-Message/EAP-Success Client and Tunnel Server: NCP negotiation NAS to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=Start RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request with Aboba & Zorn [Page 21] INTERNET-DRAFT 26 July 1997 Acct-Status-Type=Start (Optional) RADIUS Server to Tunnel Server: RADIUS Accounting-Response For the negotiations described above to succeed, the client must be capable of renegotiating EAP with the tunnel server after an initial authentication with the NAS. 10. 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=Stop RADIUS Server to NAS: RADIUS Accounting-Response Tunnel Server to RADIUS Server: RADIUS Accounting-Request with Acct-Status-Type=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=Stop packet to a RADIUS accounting server. The Accounting- Request packet MUST include the following attributes: Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-End- point, and Acct-Tunnel-Connection-Id. 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=Stop to a RADIUS accounting server. The Accounting-Request packet MUST contain the following attributes: Tunnel-Type, Tunnel-Medium- Aboba & Zorn [Page 22] INTERNET-DRAFT 26 July 1997 Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, and Acct- Tunnel-Connection-Id. 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 Send RADIUS Accounting-Request with Acct-Status-Type=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=Stop (optional) Send RADIUS Accounting-Response Send a RADIUS Accounting-Request message with Acct-Status-Type=Stop containing Tunnel-Type, Tunnel-Medium-Type, Acct-Tunnel-Client-Endpoint, Tunnel-Server-Endpoint, and Acct-Tunnel-Connection-Id. Send RADIUS Accounting-Response ENDIF Aboba & Zorn [Page 23] INTERNET-DRAFT 26 July 1997 11. 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. 11.1. 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 the tunnel authentication. 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 smart 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. 11.2. 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. Aboba & Zorn [Page 24] INTERNET-DRAFT 26 July 1997 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. 12. 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, 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. 12.1. 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. The Tun- nel-Server-Endpoint attribute returned in the RADIUS Access-Response Is 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. 13. 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. Aboba & Zorn [Page 25] INTERNET-DRAFT 26 July 1997 14. References [1] B. Aboba, J. Lu, J. Alsop, J. Ding, W. Wang. "Review of Roaming Implementations." Work in progress, draft-ietf-roamops-imprev-04.txt, Microsoft, Aimnet, i-Pass Alliance, Asiainfo, Merit, June, 1997. [2] B. Aboba, G. Zorn. "Dialup Roaming Requirements." Internet draft (work in progress), draft-ietf-roamops-roamreq-05.txt, Microsoft, July, 1997. [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] K. Hamzeh, T. Kolar, M. Littlewood, G. S. Pall, J. Taarud, A. J. Valencia, W. Verthein. "Layer Two Tunneling Protocol -- L2TP." ." Internet draft (work in progress), draft-ietf-pppext-l2tp-04.txt, Ascend Communications, Microsoft, Copper Mountain Networks, U.S. Robotics, June, 1997. [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, July, 1997. [7] G. Zorn. "RADIUS Attributes for Tunnel Protocol Support." Inter- net draft (work in progress), draft-ietf-radius-tunnel-auth-02.txt, Microsoft, July, 1997. [8] B. Aboba, M. A. Beadles. "The Network Access Identifier." Inter- net draft (work in progress), draft-ietf-roamops-nai-06.txt, Microsoft, CompuServe, July, 1997. [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-00.txt, Livingston, January, 1997. [11] P. Calhoun, A.C. Rubens, B. Aboba. "Extensible Authentication Protocol Support in RADIUS." Internet draft (work in progress), draft- ietf-radius-eap-02.txt, US Robotics Access Corp., Merit Network, Microsoft, May, 1997. [12] M. Wahl, T. Howes, S. Kille. "Lightweight Directory Access Pro- tocol (v3)." ." Internet draft (work in progress), draft-ietf-asid- ldapv3-protocol-04.txt, Critical Angle Inc., Netscape, Isode Limited, March, 1997. [13] L. J. Blunk, J. R. Vollbrecht. "PPP Extensible Authentication Protocol (EAP)." ." Internet draft (work in progress), draft-ietf- Aboba & Zorn [Page 26] INTERNET-DRAFT 26 July 1997 pppext-eap-auth-02.txt, Merit Network, Inc., June, 1996. 15. Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: 425-936-6605 EMail: bernarda@microsoft.com Glen Zorn Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: 425-703-1559 EMail: glennz@microsoft.com Aboba & Zorn [Page 27]