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Network Working Group P. Srisuresh Request for Comments: 2888 Campio Communications Category: Informational August 2000

                   Secure Remote Access with L2TP

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.


 L2TP protocol is a virtual extension of PPP across IP network
 infrastructure. L2TP makes possible for an access concentrator (LAC)
 to be near remote clients, while allowing PPP termination server
 (LNS) to be located in enterprise premises. L2TP allows an enterprise
 to retain control of RADIUS data base, which is used to control
 Authentication, Authorization and Accountability (AAA) of dial-in
 users. The objective of this document is to extend security
 characteristics of IPsec to remote access users, as they dial-in
 through the Internet. This is accomplished without creating new
 protocols and using the existing practices of Remote Access and
 IPsec. Specifically, the document proposes three new RADIUS
 parameters for use by the LNS node, acting as Secure Remote Access
 Server (SRAS) to mandate network level security between remote
 clients and the enterprise. The document also discusses limitations
 of the approach.

1. Introduction and Overview

 Now-a-days, it is common practice for employees to dial-in to their
 enterprise over the PSTN (Public Switched Telephone Network) and
 perform day-to-day operations just as they would if they were in
 corporate premises. This includes people who dial-in from their home
 and road warriors, who cannot be at the corporate premises. As the
 Internet has become ubiquitous, it is appealing to dial-in through
 the Internet to save on phone charges and save the dedicated voice
 lines from being clogged with data traffic.

Srisuresh Informational [Page 1] RFC 2888 Secure Remote Access with L2TP August 2000

 The document suggests an approach by which remote access over the
 Internet could become a reality. The approach is founded on the
 well-known techniques and protocols already in place. Remote Access
 extensions based on L2TP, when combined with the security offered by
 IPSec can make remote access over the Internet a reality. The
 approach does not require inventing new protocol(s).
 The trust model of remote access discussed in this document is viewed
 principally from the perspective of an enterprise into which remote
 access clients dial-in. A remote access client may or may not want to
 enforce end-to-end IPsec from his/her end to the enterprise.
 However, it is in the interest of the enterprise to mandate security
 of every packet that it accepts from the Internet into the
 enterprise.  Independently, remote users may also pursue end-to-end
 IPsec, if they choose to do so. That would be in addition to the
 security requirement imposed by the enterprise edge device.
 Section 2 has reference to the terminology used throughout the
 document. Also mentioned are the limited scope in which some of these
 terms may be used in this document. Section 3 has a brief description
 of what constitutes remote access. Section 4 describes what
 constitutes network security from an enterprise perspective.  Section
 5 describes the model of secure remote access as a viable solution to
 enterprises. The solution presented in section 5 has some
 limitations. These limitations are listed in section 6.  Section 7 is
 devoted to describing new RADIUS attributes that may be configured to
 turn a NAS device into Secure Remote Access Server.

2. Terminology and scope

 Definition of terms used in this document may be found in one of (a)
 L2TP Protocol document [Ref 1], (b) IP security Architecture document
 [Ref 5], or (c) Internet Key Exchange (IKE) document [Ref 8].
 Note, the terms Network Access Server (NAS) and  Remote Access
 Server(RAS) are used interchangeably throughout the document.  While
 PPP may be used to carry a variety of network layer packets, the
 focus of this document is limited to carrying IP datagrams only.
 "Secure Remote Access Server" (SRAS) defined in this document refers
 to a NAS that supports tunnel-mode IPsec with its remote clients.
 Specifically, LNS is the NAS that is referred. Further, involuntary
 tunneling is assumed for L2TP tunnel setup, in that remote clients
 initiating PPP session and the LAC that tunnels the PPP sessions are
 presumed to be distinct physical entities.

Srisuresh Informational [Page 2] RFC 2888 Secure Remote Access with L2TP August 2000

 Lastly, there are a variety of transport mediums by which to tunnel
 PPP packets between a LAC and LNS. Examples include Frame Relay or
 ATM cloud and IP network infrastructure. For simplicity, the document
 assumes a public IP infrastructure as the medium to transport PPP
 packets between LAC and LNS. Security of IP packets (embedded within
 PPP) in a trusted private transport medium is less of a concern for
 the purposes of this document.

3. Remote Access operation

 Remote access is more than mere authentication of remote clients by a
 Network Access Server(NAS). Authentication, Authorization, Accounting
 and routing are integral to remote access. A client must first pass
 the authentication test before being granted link access to the
 network. Network level services (such as IP) are granted based on the
 authorization characteristics specified for the user in RADIUS.
 Network Access Servers use RADIUS to scale for large numbers of users
 supported. NAS also monitors the link status of the remote access
 There are a variety of techniques by which remote access users are
 connected to their enterprise and the Internet. At a link level, the
 access techniques include ISDN digital lines, analog plain-old-
 telephone-service lines, xDSL lines, cable and wireless to name a
 few. PPP is the most common Layer-2 (L2)protocol used for carrying
 network layer packets over these remote access links. PPP may be used
 to carry a variety of network layer datagrams including IP, IPX and
 AppleTalk. The focus of this document is however limited to IP
 datagrams only.
 L2TP is a logical extension of PPP over an IP infrastructure. While a
 LAC provides termination of Layer 2 links,  LNS provides the logical
 termination of PPP. As a result, LNS becomes the focal point for (a)
 performing the AAA operations for the remote users, (b) assigning IP
 address and monitoring the logical link status (i.e., the status of
 LAC-to-LNS tunnel and the link between remote user and LAC), and (c)
 maintaining host-route to remote user network and providing routing
 infrastructure into the enterprise.
 L2TP uses control messages to establish, terminate and monitor the
 status of the logical PPP sessions (from remote user to LNS). These
 are independent of the data messages. L2TP data messages contain an
 L2TP header, followed by PPP packets. The L2TP header identifies the
 PPP session (amongst other things) to which the PPP packet belongs.
 The IP packets exchanged from/to the remote user are carried within
 the PPP packets.  The L2TP data messages, carrying end-to-end IP
 packets in an IP transport medium may be described as follows. The
 exact details of L2TP protocol may be found in [Ref 1].

Srisuresh Informational [Page 3] RFC 2888 Secure Remote Access with L2TP August 2000

    | IP Header            |
    | (LAC <->LNS)         |
    | UDP Header           |
    | L2TP Header          |
    | (incl. PPP Sess-ID)  |
    | PPP Header           |
    | (Remote User<->LNS)  |
    | End-to-end IP packet |
    | (to/from Remote User)|

4. Requirements of an enterprise Security Gateway

 Today's enterprises are aware of the various benefits of connecting
 to the Internet. Internet is a vast source of Information and a means
 to disseminate information and make available certain resources to
 the external world. However, enterprises are also aware that security
 breaches (by being connected to the Internet) can severely jeopardize
 internal network.
 As a result, most enterprises restrict access to a pre-defined set of
 resources for external users. Typically, enterprises employ a
 firewall to restrict access to internal resources and place
 externally accessible servers in the DeMilitarized Zone (DMZ), in
 front of the firewall, as described below in Figure 1.

Srisuresh Informational [Page 4] RFC 2888 Secure Remote Access with L2TP August 2000

  1. —————

( )

                    (                  )
                   (      Internet      )
                    (                  )
                     (_______________ )
                     WAN  |
              |Enterprise Router|
                  |   DMZ - Network
              |            |                |
             +--+         +--+         +----------+
             |__|         |__|         | Firewall |
            /____\       /____\        +----------+
            DMZ-Name     DMZ-Web  ...    |
            Server       Server          |
                             (                  )
                            (  Internal Network  )
                           (   (private to the    )
                            (   enterprise)      )
                             (_________________ )
       Figure 1: Security model of an Enterprise using Firewall
 Network Access Servers used to allow direct dial-in access (through
 the PSTN) to employees are placed within the private enterprise
 network so as to avoid access restrictions imposed by a firewall.
 With the above model, private resources of an enterprise are
 restricted for access from the Internet. Firewall may be configured
 to occasionally permit access to a certain resource or service but is
 not recommended on an operational basis as that could constitute a
 security threat to the enterprise. It is of interest to note that
 even when the firewall is configured to permit access to internal
 resources from pre-defined external node(s), many internal servers,
 such as NFS, enforce address based authentication and do not co-
 operate when the IP address of the external node is not in corporate
 IP address domain. In other words, with the above security model, it

Srisuresh Informational [Page 5] RFC 2888 Secure Remote Access with L2TP August 2000

 becomes very difficult to allow employees to access corporate
 resources, via the Internet, even if you are willing to forego
 security over the Internet.
 With the advent of IPsec, it is possible to secure corporate data
 across the Internet by employing a Security Gateway within the
 enterprise. Firewall may be configured to allow IKE and IPsec packets
 directed to a specific  Security Gateway behind the firewall. It then
 becomes the responsibility of the Security Gateway to employ the
 right access list for external connections seeking entry into the
 enterprise. Essentially, the access control functionality for IPsec
 secure packets would be shifted to the Security Gateway (while the
 access control for clear packets is retained with the firewall). The
 following figure illustrates the model where a combination of
 Firewall and Security Gateway control access to internal resources.

Srisuresh Informational [Page 6] RFC 2888 Secure Remote Access with L2TP August 2000

  1. ———–

( )

                    (              )
                   (    Internet    )
                    (              )
                     (___________ )
                     WAN  |
              |Enterprise Router|
                  |   DMZ - Network
          |            |                     |
         +--+         +--+              +----------+
         |__|         |__|              | Firewall |
             /____\       /____\             +----------+
             DMZ-Name     DMZ-Web   ...         |
             Server       Server etc.           | LAN
                    |                          |
               +----------+         +------------------+
               |   LNS    |         | Security Gateway |
               |  Server  |         |      (SGW)       |
               +----------+         +------------------+
                                  (                  )
                                 (  Internal Network  )
                                (   (Private to the    )
                                 (   enterprise)      )
                                  (_________________ )
   Figure 2: Security Model based on Firewall and Security Gateway
 In order to allow employee dial-in over the Internet, an LNS may be
 placed behind a firewall, and the firewall may be configured to allow
 UDP access to the LNS from the Internet. Note, it may not be possible
 to know all the IP addresses of the LACs located on the Internet at
 configuration time. Hence, the need to allow UDP access from any node
 on the Internet. The LNS may be configured to process only the L2TP
 packets and drop any UDP packets that are not L2TP.

Srisuresh Informational [Page 7] RFC 2888 Secure Remote Access with L2TP August 2000

 Such a configuration allows remote access over the Internet. However,
 the above setup is prone to a variety of security attacks over the
 Internet. It is easy for someone on the Internet to steal a remote
 access session and gain  access to precious resources of the
 enterprise. Hence it is important that all packets are preserved with
 IPsec to a security Gateway (SGW) behind the LNS, so the Security
 Gateway will not allow IP packets into corporate network unless it
 can authenticate the same.
 The trust model of secure remote access assumes that the enterprise
 and the end user are trusted domains. Everything in between is not
 trusted. Any examination of the end-to-end packets by the nodes
 enroute would violate this trust model. From this perspective, even
 the LAC node enroute must not be trusted with the end-to-end IP
 packets. Hence, location and operation of LAC is not relevant for the
 discussion on security. On the other hand, location and operation of
 LNS and the Security Gateway (SGW) are precisely the basis for
 Having security processing done on an independent Security gateway
 has the following shortcomings.
 1. Given the trust model for remote access, the SGW must be
    configured with a set of security profiles, access control lists
    and IKE authentication parameters for each user. This mandates an
    independent provisioning of security parameters on a per-user
    basis. This may not be able to take advantage of the user-centric
    provisioning on RADIUS, used by the LNS node.
 2. Unlike the LNS, SGW may not be in the routing path of remote
    access packets. I.e., there is no guarantee that the egress IP
    packets will go through the chain of SGW and LNS before they are
    delivered to remote user. As a result, packets may be subject to
    IPSec in one direction, but not in the other. This can be a
    significant threat to the remote access trust model.
 3. Lastly, the SGW node does not have a way to know when a remote
    user node(s) simply died or the LAC-LNS tunnel failed. Being
    unable to delete the SAs for users that no longer exist could
    drain the resources of the SGW. Further, the LNS cannot even
    communicate the user going away to the SGW because, the SGW
    maintains its peer nodes based on IKE user ID, which could be
    different the user IDs employed by the LNS node.

Srisuresh Informational [Page 8] RFC 2888 Secure Remote Access with L2TP August 2000

5. Secure Remote Access

 Combining the functions of IPsec Security Gateway and LNS into a
 single system promises to offer a viable solution for secure remote
 access. By doing this, remote access clients will use a single node
 as both (a) PPP termination point providing NAS service, and (b) the
 Security gateway node into the enterprise. We will refer this node as
 "Secure Remote Access Server" (SRAS).
 The SRAS can benefit greatly from the confluence of PPP session and
 IPsec tunnel end points. PPP session monitoring capability of L2TP
 directly translates to being able to monitor IPsec tunnels. Radius
 based user authorization ability could be used to configure the
 security characteristics for IPsec tunnel. This includes setting
 access control filters and security preferences specific to each
 user. This may also be extended to configuring IKE authentication and
 other negotiation parameters, when automated key exchange is
 solicited. Security attributes that may be defined in Radius are
 discussed in detail in section 7. Needless to say, the centralized
 provisioning capability and scalability of Radius helps in the
 configuration of IPsec.
 As for remote access, the benefit is one of IPsec security as
 befitting the trust model solicited by enterprises for the end-to-end
 IP packets traversing the Internet. You may use simply AH where there
 is no fear of external eaves-dropping, but you simply need to
 authenticate packet data, including the source of packet. You may use
 ESP (including ESP-authentication), where there is no trust of the
 network and you do not want to permit eaves-dropping on corporate
 Operation of SRAS requires that the firewall be configured to permit
 UDP traffic into the SRAS node. The SRAS node in turn will process
 just the L2TP packets and drop the rest. Further, the SRAS will
 require all IP packets embedded within PPP to be one of AH and ESP
 packets, directed to itself. In addition, the SRAS will also permit
 IKE UDP packets (with source and destination ports sets to 500)
 directed to itself in order to perform IKE negotiation and generate
 IPsec keys dynamically. All other IP packets embedded within PPP will
 be dropped. This enforces the security policy for the enterprise by
 permitting only the secure remote access packets into the enterprise.
 When a PPP session is dropped, the IPsec and ISAKMP SAs associated
 with the remote access user are dropped from the SRAS. All the
 shortcomings listed in the previous section with LNS and SGW on two
 systems disappear withe Secure Remote Access Server. Figure 3 below
 is a typical description of an enterprise supporting remote access
 users using SRAS system.

Srisuresh Informational [Page 9] RFC 2888 Secure Remote Access with L2TP August 2000

  1. ———–

Remote Access +————-+ ( )

      +--+______   Link    | Local Access|     (              )
      |__|     /___________| Concentrator|----(    Internet    )
     /____\                |    (LAC)    |     (              )
     RA-Host               +-------------+      (____________)
                                WAN  |
                         |Enterprise Router|
                             |   DMZ - Network
          |            |                     |
         +--+         +--+              +----------+
         |__|         |__|              | Firewall |
             /____\       /____\             +----------+
             DMZ-Name     DMZ-Web   ...         |
             Server       Server etc.           | LAN
                              | Secure Remote |
                              | Access Server |
                              |    (SRAS)     |
                            (                     )
               +--+       (    Internal Network    )
               |__|------(     (Private to the      )
              /____\      (     enterprise)        )
              Ent-Host     (______________________)
   Figure 3: Secure Remote Access Server operation in an Enterprise
 The following is an illustration of secure remote access data flow as
 end-to-end IP packets traverse the Internet and the SRAS. The example
 shows IP packet tunneling and IPsec transformation as packets are
 exchanged between a remote Access host (RA-Host) and a host within
 the enterprise (say, Ent-Host).

Srisuresh Informational [Page 10] RFC 2888 Secure Remote Access with L2TP August 2000

 Note, the IP packets originating from or directed to RA-Host are
 shown within PPP encapsulation, whereas, all other packets are shown
 simply as IP packets.  It is done this way to highlight the PPP
 packets encapsulated within L2TP tunnel. The PPP headers below are
 identified by their logical source and destination in parenthesis.
 Note, however, the source and recipient information of the PPP data
 is not a part of PPP header. This is described thus, just for
 clarity. In the case of an L2TP tunnel, the L2TP header carries the
 PPP session ID, which indirectly identifies the PPP end points to the
 LAC and the LNS. Lastly, the IPsec Headers section below include the
 tunneling overhead and the AH/ESP headers that are attached to the

Srisuresh Informational [Page 11] RFC 2888 Secure Remote Access with L2TP August 2000

 RA-Host to Ent-Host Packet traversal:
 RA-Host              LAC                   SRAS              Ent-Host
 | PPP Header           |
 | (RA-Host ->SRAS)     |
 | Tunnel-Mode IPsec    |
 | Hdr(s)(RA-Host->SRAS)|
 | End-to-end IP packet |
 | transformed as needed|
 | (RA-Host->Ent-Host)  |
                 | IP Header            |
                 | (LAC->SRAS)          |
                 | UDP Header           |
                 | L2TP Header          |
                 | (incl. PPP Sess-ID)  |
                 | PPP Header           |
                 | (RA-Host ->SRAS)     |
                 | Tunnel-Mode IPsec    |
                 | Hdr(s)(RA-Host->SRAS)|
                 | End-to-end IP packet |
                 | transformed as needed|
                 | (RA-Host->Ent-Host)  |
                                    | End-to-end IP packet |
                                    | (RA-Host->Ent-Host)  |

Srisuresh Informational [Page 12] RFC 2888 Secure Remote Access with L2TP August 2000

 Ent-Host to RA-Host Packet traversal:
 Ent-Host             SRAS                  LAC               RA-Host
 | End-to-end IP packet |
 | (Ent-Host->Ra-Host)  |
                 | IP Header            |
                 | (SRAS->LAC)          |
                 | UDP Header           |
                 | L2TP Header          |
                 | (incl. PPP Sess-ID)  |
                 | PPP Header           |
                 | (SRAS->RA-Host)      |
                 | Tunnel-Mode IPsec    |
                 | Hdr(s)(SRAS->RA-Host)|
                 | End-to-end IP packet |
                 | transformed as needed|
                 | (Ent-Host->RA-Host)  |
                                   | PPP Header           |
                                   | (SRAS->RA-Host)      |
                                   | Tunnel-Mode IPsec    |
                                   | Hdr(s)(SRAS->RA-Host)|
                                   | End-to-end IP packet |
                                   | transformed as needed|
                                   | (Ent-Host->RA-Host)  |

Srisuresh Informational [Page 13] RFC 2888 Secure Remote Access with L2TP August 2000

6. Limitations to Secure Remote Access using L2TP

 The SRAS model described is not without its limitations. Below is a
 list of the limitations.
 1. Tunneling overhead: There is considerable tunneling overhead on
    the end-to-end IP packet. Arguably, there is overlap of
    information between tunneling headers. This overhead will undercut
    packet throughput.
    The overhead is particularly apparent at the LAC and SRAS nodes.
    Specifically, the SRAS has the additional computational overhead
    of IPsec processing on all IP packets exchanged with remote users.
    This can be a significant bottleneck in the ability of SRAS to
    scale for large numbers of remote users.
 2. Fragmentation and reassembly: Large IP packets may be required to
    undergo Fragmentation and reassembly at the LAC or the LNS as a
    result of multiple tunnel overhead tagged to the packet.
    Fragmentation and reassembly can havoc on packet throughput and
    latency. However, it is possible to avoid the overhead by reducing
    the MTU permitted within PPP frames.
 3. Multiple identity and authentication requirement: Remote Access
    users are required to authenticate themselves to the SRAS in order
    to be obtain access to the link. Further, when they require the
    use of IKE to automate IPsec key exchange, they will need to
    authenticate once again with the same or different ID and a
    distinct authentication approach. The authentication requirements
    of IKE phase 1 [Ref 8] and LCP [Ref 3] are different.
    However, it is possible to have a single authentication approach
    (i.e., a single ID and authentication mechanism) that can be
    shared between LCP and IKE phase 1.  The Extended Authentication
    Protocol(EAP) [Ref 4] may be used as the base to transport IKE
    authentication mechanism into PPP. Note, the configuration
    overhead is not a drag on the functionality perse.
 4. Weak security of Link level authentication: As LCP packets
    traverse the Internet, the Identity of the remote user and the
    password (if a password is used) is sent in the clear. This makes
    it a target for someone on the net to steal the information and
    masquerade as remote user. Note, however, this type of password
    stealing will not jeopardize the security of the enterprise per
    se, but could result in denial of service to remote users. An
    intruder can collect the password data and simply steal the link,
    but will not be able to run any IP applications subsequently, as
    the SRAS will fail non-IPsec packet data.

Srisuresh Informational [Page 14] RFC 2888 Secure Remote Access with L2TP August 2000

    A better approach would be to employ Extended Authentication
    Protocol (EAP) [Ref 4] and select an authentication technique that
    is not prone to stealing over the Internet. Alternately, the LAC
    and the SRAS may be independently configured to use IPsec to
    secure all LCP traffic exchanged between themselves.

7. Configuring RADIUS to support Secure Remote Access.

 A centralized RADIUS database is used by enterprises to maintain the
 authentication and authorization requirements of the dial-in Users.
 It is also believed that direct dial-in access (e.g., through the
 PSTN network is) safe and trusted and does not need any scrutiny
 outside of the link level authentication enforced in LCP. This belief
 is certainly not shared with the dial-in access through the Internet.
 So, while the same RADIUS database may be used for a user directly
 dialing-in or dialing in through the Internet, the security
 requirements may vary. The following RADIUS attributes may be used to
 mandate IPsec for the users dialing-in through the Internet.  The
 exact values for the attributes and its values may be obtained from
 IANA (refer Section 10).

7.1. Security mandate based on access method

 A new RADIUS attribute IPSEC_MANDATE (91) may be defined for each
 user. This attribute may be given one of the following values.
    NONE            (=0)     No IPsec mandated on the IP packets
                             embedded within PPP.
    LNS_AS_SRAS     (=1)     Mandates Tunnel mode IPsec on the IP
                             packets embedded within PPP, only so
                             long as the PPP session terminates
                             at an LNS. LNS would be the tunnel
                             mode IPsec end point.
    SRAS            (=2)     Mandates Tunnel mode IPsec on the IP
                             packets embedded within PPP,
                             irrespective of the NAS type the PPP
                             terminates in. I.e., the IPsec mandate
                             is not specific to LNS alone, and is
                             applicable to any NAS, terminating
                             PPP. NAS would be the tunnel mode
                             IPsec end point.

Srisuresh Informational [Page 15] RFC 2888 Secure Remote Access with L2TP August 2000

 When IPSEC_MANDATE attribute is set to one of LNS_AS_SRAS or SRAS,
 that would direct the NAS to drop any IP packets in PPP that are not
 associated with an AH or ESP protocol. As an exception, the NAS will
 continue to process IKE packets (UDP packets, with source and
 destination port set to 500) directed from remote users. Further, the
 security profile parameter, defined in the following section may add
 additional criteria for which security is not mandatory.

7.2. Security profile for the user

 A new SECURITY_PROFILE (92) parameter may be defined in RADIUS to
 describe security access requirements for the users. The profile
 could contain information such as the access control security
 filters, security preferences and the nature of Keys (manual or
 automatic generated via the IKE protocol) used for security purposes.
 The SECURITY-PROFILE attribute can be assigned a filename, as a
 string of characters. The contents of the file could be vendor
 specific. But, the contents should include (a) a prioritized list
 access control security policies, (b) Security Association security
 preferences associated with each security policy.

7.3. IKE negotiation profile for the user

 If the security profile of a user requires dynamic generation of
 security keys, the parameters necessary for IKE negotiation may be
 configured separately using a new IKE_NEGOTIATION_PROFILE (93)
 parameter in RADIUS. IKE-NEGOTIATION_PROFILE attribute may be
 assigned a filename, as a string of characters. The contents of the
 file could however be vendor specific. The contents would typically
 include (a) the IKE ID of the user and  SRAS, (b) preferred
 authentication approach and the associated parameters, such as a
 pre-shared-key or a pointer to X.509 digital Certificate, and, (c)
 ISAKMP security negotiation preferences for phase I.

8. Acknowledgements

 The author would like to express sincere thanks to Steve Willens for
 initially suggesting this idea. The author is also thankful to Steve
 for the many informal conversations which were instrumental in the
 author being able to appreciate the diverse needs of the Remote
 Access area.

Srisuresh Informational [Page 16] RFC 2888 Secure Remote Access with L2TP August 2000

9. Security Considerations

 This document is about providing secure remote access to enterprises
 via the Internet. However, the document does not address security
 issues for network layers other than IP. While the document focus is
 on security over the Internet, the security model provided is not
 limited to the Internet or the IP infrastructure alone. It may also
 be applied over other transport media such as Frame Relay and ATM
 clouds. If the transport media is a trusted private network
 infrastructure, the security measures described may not be as much of
 an issue. The solution suggested in the document is keeping in view
 the trust model between a remote user and enterprise.

10. IANA Considerations

 This document proposes a total of three new RADIUS attributes to be
 maintained by the IANA. These attributes IPSEC_MANDATE,
 values 91, 92 and 93 respectively so as not to conflict with the
 definitions for recognized radius types, as defined in
 The following sub-section explains the criteria to be used by the
 IANA to assign additional numbers as values to the IPSEC-MANDATE
 attribute described in section 7.1.

10.1. IPSEC-MANDATE attribute Value

 Values 0-2 of the IPSEC-MANDATE-Type Attribute are defined in Section
 7.1; the remaining values [3-255] are available for assignment by the
 IANA with IETF Consensus [Ref 11].


 [1]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
      B. Palter, "Layer Two Tunneling Protocol L2TP", RFC 2661, August
 [2]  Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
      Authentication Dial In User Service (RADIUS)", RFC 2138, April
 [3]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
      1661, July 1994.
 [4]  Blunk, L. and Vollbrecht, J. "PPP Extensible Authentication
      Protocol (EAP)", RFC 2284, March 1998.

Srisuresh Informational [Page 17] RFC 2888 Secure Remote Access with L2TP August 2000

 [5]  Kent, S. and R. Atkinson, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
 [6]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
      (ESP)", RFC 2406, November 1998.
 [7]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
      November 1998.
 [8]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
      RFC 2409, November 1998.
 [9]  Piper, D., "The Internet IP Security Domain of Interpretation
      for ISAKMP", RFC 2407, November 1998.
 [10] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
      October 1994.
      See also
 [11] Narten, T. and H. Alvestrand, "Guidelines for writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 [12] Meyer, G., "The PPP Encryption Control Protocol (ECP)", RFC
      1968, June 1996.
 [13] Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol,
      Version 2 (DESE-bis)", RFC 2419, September 1998.

Author's Address

 Pyda Srisuresh
 Campio Communications
 630 Alder Drive
 Milpitas, CA 95035
 Phone: +1 (408) 519-3849

Srisuresh Informational [Page 18] RFC 2888 Secure Remote Access with L2TP August 2000

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Srisuresh Informational [Page 19]

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