PPPEXT Working Group Baiju Patel INTERNET-DRAFT Intel Category: Standards Track Bernard Aboba <draft-ietf-pppext-l2tp-security-03.txt> Microsoft 2 February 1999 Securing L2TP using IPSEC 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working 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 material or to cite them other than as "work in progress." To view the list Internet-Draft Shadow Directories, see http://www.ietf.org/shadow.html. The distribution of this memo is unlimited. It is filed as <draft-ietf- pppext-l2tp-security-03.txt> and expires August 1, 1999. Please send comments to the authors. 2. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. 3. Abstract This document discusses how L2TP may utilize IPSEC to provide for tunnel authentication, privacy protection, integrity check and replay protection. Both the voluntary and compulsory tunneling cases are discussed. 4. Requirements language In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as Patel & Aboba Standards Track [Page 1]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 described in [2]. 5. Terminology Voluntary Tunneling In voluntary tunneling, a tunnel is created by the user, typically via use of a tunneling client. As a result, the user will send L2TP packets to the NAS which will forward them on to the LNS. In voluntary tunneling, the NAS does not need to support L2TP, and the LAC resides on the same machine as the user. Compulsory Tunneling In compulsory tunneling, a tunnel is created without any action from the user and without allowing the user any choice. As a result, the user will send PPP packets to the NAS/LAC, which will encapsulate them in L2TP and tunnel them to the LNS. In the compulsory tunneling case, the NAS/LAC must be L2TP capable. 6. Introduction L2TP, described in [1], is a protocol that tunnels PPP traffic over variety of networks (e.g., IP, SONET, ATM). Since the protocol encapsulates PPP, L2TP inherits PPP authentication, as well as the PPP Encryption Control Protocol (ECP) (described in [10]), and the Compression Control Protocol (CCP) (described in [9]). L2TP also includes support for tunnel authentication, which can be used to mutually authenticate the tunnel endpoints. However, L2TP does not define tunnel protection mechanisms. IPSEC is a protocol suite defined by IETF working group on IP security to secure communication at the network layer between communicating peers. This protocol is comprised of IP Security Architecture document [6], IKE, described in [7], IPSEC AH, described in [3] and IPSEC ESP, described in [4]. IKE is the key management protocol while AH and ESP are used to protect IP traffic. This draft proposes use of the IPSEC protocol suite for protecting L2TP traffic over IP and Non-IP networks, and discusses how IPSEC and L2TP should be used together. This document does not attempt to standardize end-to-end security. When end-to-end security is required, it is recommended that additional security mechanisms (such as IPSEC or TLS) be used inside the tunnel, in addition to L2TP tunnel security. Patel & Aboba Standards Track [Page 2]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 7. L2TP security requirements L2TP tunnels PPP traffic over the IP and non-IP public networks. Therefore, both the control and data packets of L2TP protocol are vulnerable to attack. Examples of attacks include: 1. The adversary may try to discover user identities by snooping data packets. 2. The adversary may try to modify packets (both control and data). 3. The adversary may try to hijack the L2TP tunnel or the PPP connection inside the tunnel. 4. An adversary can launch denial of service attacks by terminating PPP connections, or L2TP tunnels. 5. An adversary may attempt to disrupt the PPP ECP negotiation in order to weaken or remove confidentiality protection. Alternatively, an adversary may wish to disrupt the PPP LCP authentication negotiation so as to weaken the PPP authentication process or gain access to user passwords. To address these threats, the L2TP security protocol MUST be able to provide authentication, integrity and replay protection for control packets. In addition, it SHOULD be able to protect confidentiality of control packets. It MUST be able to provide integrity and replay protection of data packets, and MAY be able to protect confidentiality of data packets. An L2TP security protocol MUST also provide a scalable approach to key management. The L2TP protocol, and PPP authentication and encryption do not meet the security requirements for L2TP. L2TP authentication typically mutually authenticates LAC to LNS at tunnel origination and may periodically re- authenticate. Therefore, it does not protect control and data traffic on a per packet basis. Thus, L2TP tunnel authentication leaves L2TP tunnel vulnerable to attack. PPP authenticates the client to the LNS, but also does not provide per- packet authentication, integrity, or replay protection. PPP encryption meets confidentiality requirements for PPP traffic but does not address authentication, integrity and key management requirements. In addition, PPP ECP negotiation, outlined in [10] does not provide for a protected ciphersuite negotiation. Therefore, PPP encryption provides a weak security solution, and in addition does not assist in securing L2TP control channel. Key management facilities are not provided by the L2TP protocol. However, where L2TP tunnel authentication is desired, it is necessary to distribute tunnel passwords. Patel & Aboba Standards Track [Page 3]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 Note that several of the attacks outlined above can be carried out on PPP packets sent over the link between the client and the NAS/LAC, prior to encapsulation of the packets within an L2TP tunnel. While strictly speaking these attacks are outside the scope of L2TP security, in order to protect against them, the user SHOULD provide for confidentiality, authentication and integrity protection for PPP packets sent over the dial-up link. Authentication and integrity protection are not currently supported by PPP encryption methods, described in [11]-[13]. 7.1. L2TP Security Protocol The L2TP security protocol MUST provide authentication, integrity and replay protection for control packets. In addition, it SHOULD protect confidentiality of control packets. It MUST provide integrity and replay protection of data packets, and MAY protect confidentiality of data packets. An L2TP security protocol MUST also provide a scalable approach to key management. To meet above requirements, all L2TP security compliant implementations MUST implement IPSEC ESP for securing L2TP control packets and SHOULD implement IPSEC ESP for protection of L2Tp data packets. All mandated cipher suites, including NULL encryption, of IPSEC ESP MUST be supported. Note that if confidentiality is not required (e.g., L2TP data traffic), ESP with NULL encryption may be used. The implementations MUST implement replay protection mechanisms of IPSEC. L2TP security MUST meet the key management requirements of the IPSEC protocol suite. IKE SHOULD be supported for authentication, security association negotiation, and key management using the IPSEC DOI [5]. 7.2. Stateless compression and encryption Stateless encryption and/or compression is highly desirable when L2TP is run over IP. Since L2TP is a connection-oriented protocol, use of stateful compression/encryption is feasible, but when run over IP, this is not desirable. While providing better compression, and somewhat more secure encryption, when used without an underlying reliable delivery mechanism stateful methods magnify packet losses, and thus are problematic when used over the Internet where packet loss can be significant. In addition, although L2TP is connection oriented, the L2TP specification [1] does not mandate packet ordering, which can create difficulties in implementation of stateful compression/encryption schemes. However, these considerations are not as important when L2TP is run over non-IP media such as ATM, X.25, or Frame Relay, since these media guarantee ordering, and packet losses are typically low. Patel & Aboba Standards Track [Page 4]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 7.3. Implementation considerations for L2TP over Non-IP networks L2TP requires that a non-IP network supports packet transport, so that the non-IP network should be able to carry L2TP control and data packets. Since ESP functions are defined on the IP payload (excluding the IP header), the presence of an IP header is not a requirement for use of ESP. Therefore, L2TP implemented on non-IP networks MUST be able to transport IPSEC ESP packets. The next payload field of the ESP header MUST be set to the L2TP protocol number. IANA has assigned 115 as the protocol number for L2TP. IKE messages use UDP transport. Therefore, in order to be compliant with the IKE protocol on non-IP media, the non-IP media (which is capable of transporting packets) MUST support transport of UDP datagrams. Since the IP header is not present in the UDP datagram over non-IP media, the UDP checksum MUST be set to zero by the source and ignored by the destination. The exact mechanisms for enabling transport of ESP and UDP packets over non-IP media MUST be addressed in appropriate standards for L2TP over specific non-IP networks. 8. L2TP/IPSEC interoperability guidelines The following guidelines are established to meet L2TP security requirements using IPSEC in practical situations. Note that this section is not a requirement for an implementation to be L2TP security compliant. Its purpose to make the implementors aware of certain efficiency and security considerations. In the scenarios that follow, it is assumed that both L2TP clients and servers are able to set and get the properties of IPSEC security associations, as well as to influence the IPSEC security services negotiated. Furthermore, it is assumed that L2TP clients and servers are able to influence the negotiation process for PPP encryption and compression. 8.1. Compulsory tunnel In the case of a compulsory tunnel, the dial-in user will be sending PPP packets to the LAC, and will typically not be aware that its packets are being tunneled, nor that any security services are in place between the LAC and LNS. From the LNS's point of view, it will note arrival of a PPP data packet encapsulated in L2TP, which is itself encapsulated in an IP Patel & Aboba Standards Track [Page 5]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 packet. Assuming that the LNS is able to retrieve the properties of the Security Association set up between itself and the LAC, it will have knowledge of the security services in place between the LAC and itself. Thus in the compulsory tunneling case, the dial-in user and the LNS have unequal knowledge of the security services in place between them. Since the LNS is capable of knowing whether confidentiality, authentication, integrity and replay protection are in place between the LAC and itself, it can use this knowledge in order to modify its behavior during PPP ECP and CCP negotiation. For example, let us assume that LNS confidentiality policy can be described by one of the following terms: "Require Encryption," "Allow Encryption" or "Prohibit Encryption." If IPSEC confidentiality services are in place, then an LNS implementing a "Prohibit Encryption" policy will act as though the policy had been violated. Similarly, an LNS implementing a "Require Encryption" or "Allow Encryption" policy will act as though these policies were satisfied, and would not mandate use of PPP encryption or compression. Note however, that this is not the same as insisting that PPP encryption and compression be turned off, since this decision will depend on the policy of the dialin user. Since the dial-in user has no knowledge of the security services in place between the LAC and the LNS, and since it may not trust the LAC or the wire between itself and the LAC, the dial-in user will typically want to ensure sufficient security through use of end-to-end IPSEC or PPP encryption/compression between itself and the LNS. A dial-in user wishing to ensure security services over the entire travel path would not modify this behavior even if it had knowledge of the security services in place between the LAC and the LNS. This is because the dial-in user needs to negotiate confidentiality services between itself and the LNS in order to provide privacy on the wire between itself and the LAC. Similarly, the dial-in user needs to negotiate end-to-end security between itself and the endstation in order to ensure confidentiality on the portion of the path between the LNS and the endstation. Given that the dial-in user will typically not trust the LAC and will negotiate confidentiality and compression services on its own, the LAC may only wish to negotiate IPSEC ESP with null encryption (described in []) with the LNS, and the LNS will request replay protection. This will ensure that confidentiality and compression services will not be duplicated over the path between the LAC and the LNS. This will typically result in better scalability for the LAC, since encryption will be handled by the dial-in user and the LNS. The dial-in user can satisfy its desire for confidentiality services in one of two ways. If it knows that all endstations that it will Patel & Aboba Standards Track [Page 6]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 communicate with are IPSEC-capable (or if it refuses to talk to non- IPSEC capable endstations), then it can refuse to negotiate PPP encryption/compression and negotiate IPSEC ESP with the endstations instead. If the client does not know that all endstations it will contact are IPSEC capable (the most likely case), then it will negotiate PPP encryption/compression. This may result in duplicate compression/encryption which can only be eliminated if PPP compression/encryption can be turned off on a per-packet basis. Note that since the LNS knows that the dial-in user's packets are being tunneled but the dial-in user does not, the LNS can ensure that stateless compression/encryption is used by offering stateless compression/encryption methods if available in the ECP and CCP negotiations. 8.2. Voluntary tunnel In the case of a voluntary tunnel, the dial-in user will be sending L2TP packets to the NAS, which will route them to the LNS. Over a dialup link, these L2TP packets will be encapsulated in IP and PPP. Assuming that it is possible for the dial-in user to retrieve the properties of the Security Association between itself and the LNS, the dial-in user will have knowledge of any security services negotiated between itself and the LNS. It will also have knowledge of PPP encryption/compression services negotiated between itself and the NAS. >From the LNS's point of view, it will note a PPP packet encapsulated in L2TP, which is itself encapsulated in an IP frame. This situation is identical to the compulsory tunneling case. Assuming that the LNS is able to retrieve the properties of the Security Association set up between itself and the dial-in user, it will have knowledge of the security services in place between the dial-in user and itself. Thus in the voluntary tunneling case, the dial-in user and the LNS have symmetric knowledge of the security services in place between them. Since the LNS is capable of knowing whether confidentiality, authentication, integrity check or replay protection is in place between the dial-in user and itself, it is able to use this knowledge to modify its PPP ECP and CCP negotiation stance. If IPSEC confidentiality is in place, the LNS can behave as though a "Require Encryption" directive had been fulfilled, not mandating use of PPP encryption or compression. Typically the LNS will not insist that PPP encryption/compression be turned off, instead leaving this decision to the dial-in user. Since the dial-in user has knowledge of the security services in place between itself and the LNS, it can act as though a "Require Encryption" directive had been fulfilled if IPSEC ESP was already in place between itself and the LNS. Thus, it can request that PPP encryption and Patel & Aboba Standards Track [Page 7]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 compression not be negotiated. Note that if IP compression services cannot be negotiated, it will typically be desirable to turn off PPP compression if no stateless method is available, due to the undesirable effects of stateful PPP compression. Thus in the voluntary tunneling case the dial-in user and LNS will typically be able to avoid use of PPP encryption and compression, negotiating IPSEC Confidentiality, Authentication, and Integrity protection services instead, as well as IP Compression if available. This may result in duplicate encryption if the dial-in user is communicating with an IPSEC-capable endstation. In order to avoid duplicate encryption/compression, the dial-in user may negotiate two Security Associations with the LNS, one with ESP with null encryption, and one with confidentiality/compression. Packets going to an IPSEC- capable endstation would run over the ESP with null encryption security association, and packets to a non-IPSEC capable endstation would run over the other security association. Note that many IPSEC implementations cannot support this without allowing L2TP packets on the same tunnel to be originated from multiple UDP ports. This requires modifications to the L2TP specification. Also note that the dial-in user may wish to put confidentiality services in place for non-tunneled packets travelling between itself and the NAS. This will protect the dial-in user against eavesdropping on the wire between itself and the NAS. As a result, it may wish to negotiate PPP encryption and compression with the NAS. As in compulsory tunneling, this will result in duplicate encryption and possibly compression unless PPP compression/encryption can be turned off on a per-packet basis. 9. Security considerations This draft defines a security protocol. 10. Acknowledgements Thanks to Gurdeep Singh Pall, David Eitelbach, William Dixon, Peter Ford, and Sanjay Anand of Microsoft, John Richardson of Intel and Rob Adams of Cisco for many useful discussions of this problem space. 11. References [1] Townsley, W.M., Valencia, A., Rubens, A., Pall, G.S., Zorn, G., and Palter, B., "Layer Two Tunneling Protocol L2TP", Internet draft (work in Patel & Aboba Standards Track [Page 8]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 progress), draft-ietf-pppext-l2tp-13.txt, January 1999. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Kent,S., Atkinson, R., "IP Authentication Header", RFC 2402, November 1998. [4] Kent,S., Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [5] Piper, D., "The Internet IP Security Domain of Interpretation of ISAKMP." Internet draft (work in progress), draft-ietf-ipsec-ipsec- doi-09.txt, May 1998. [6] Atkinson, R., Kent, S., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [7] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [8] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [9] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962, June 1996. [10] Meyer, G., "The PPP Encryption Control Protocol (ECP)", RFC 1968, June 1996. [11] Sklower, K., Meyer, G., "The PPP DES Encryption Protocol (DESE)", RFC 1969, June 1996. [12] Sklower, K., Meyer, G., "The PPP DES Encryption Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998. [13] Hummert, K., "The PPP Triple-DES Encryption Protocol (3DESE)", RFC 2420, September 1998. 12. Authors' Addresses Baiju V. Patel Intel Corp 2511 NE 25th Ave Hillsboro, OR 97124 Phone: 503-264-2422 Patel & Aboba Standards Track [Page 9]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 Email: baiju.v.patel@intel.com Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: 425-936-6605 EMail: bernarda@microsoft.com 13. Intellectural Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards- related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. 14. Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Patel & Aboba Standards Track [Page 10]
INTERNET-DRAFT Securing L2TP Using IPSEC 2 February 1999 Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." 15. Expiration Date This memo is filed as <draft-ietf-pppext-l2tp-security-03.txt>, and expires August 1, 1999 Patel & Aboba Standards Track [Page 11]
