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RFC 2661

Layer Two Tunneling Protocol "L2TP"

Pages: 80
Proposed Standard
Part 4 of 4 – Pages 69 to 80
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9.0 Security Considerations

L2TP encounters several security issues in its operation. The general approach of L2TP to these issues is documented here.
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9.1 Tunnel Endpoint Security

The tunnel endpoints may optionally perform an authentication procedure of one another during tunnel establishment. This authentication has the same security attributes as CHAP, and has reasonable protection against replay and snooping during the tunnel establishment process. This mechanism is not designed to provide any authentication beyond tunnel establishment; it is fairly simple for a malicious user who can snoop the tunnel stream to inject packets once an authenticated tunnel establishment has been completed successfully. For authentication to occur, the LAC and LNS MUST share a single secret. Each side uses this same secret when acting as authenticatee as well as authenticator. Since a single secret is used, the tunnel authentication AVPs include differentiating values in the CHAP ID fields for each message digest calculation to guard against replay attacks. The Assigned Tunnel ID and Assigned Session ID (See Section 4.4.3) SHOULD be selected in an unpredictable manner rather than sequentially or otherwise. Doing so will help deter hijacking of a session by a malicious user who does not have access to packet traces between the LAC and LNS.

9.2 Packet Level Security

Securing L2TP requires that the underlying transport make available encryption, integrity and authentication services for all L2TP traffic. This secure transport operates on the entire L2TP packet and is functionally independent of PPP and the protocol being carried by PPP. As such, L2TP is only concerned with confidentiality, authenticity, and integrity of the L2TP packets between its tunnel endpoints (the LAC and LNS), not unlike link-layer encryption being concerned only about protecting the confidentiality of traffic between its physical endpoints.

9.3 End to End Security

Protecting the L2TP packet stream via a secure transport does, in turn, also protect the data within the tunneled PPP packets while transported from the LAC to the LNS. Such protection should not be considered a substitution for end-to-end security between communicating hosts or applications.
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9.4 L2TP and IPsec

When running over IP, IPsec provides packet-level security via ESP and/or AH. All L2TP control and data packets for a particular tunnel appear as homogeneous UDP/IP data packets to the IPsec system. In addition to IP transport security, IPsec defines a mode of operation that allows tunneling of IP packets. The packet level encryption and authentication provided by IPsec tunnel mode and that provided by L2TP secured with IPsec provide an equivalent level of security for these requirements. IPsec also defines access control features that are required of a compliant IPsec implementation. These features allow filtering of packets based upon network and transport layer characteristics such as IP address, ports, etc. In the L2TP tunneling model, analogous filtering is logically performed at the PPP layer or network layer above L2TP. These network layer access control features may be handled at the LNS via vendor-specific authorization features based upon the authenticated PPP user, or at the network layer itself by using IPsec transport mode end-to-end between the communicating hosts. The requirements for access control mechanisms are not a part of the L2TP specification and as such are outside the scope of this document.

9.5 Proxy PPP Authentication

L2TP defines AVPs that MAY be exchanged during session establishment to provide forwarding of PPP authentication information obtained at the LAC to the LNS for validation (see Section 4.4.5). This implies a direct trust relationship of the LAC on behalf of the LNS. If the LNS chooses to implement proxy authentication, it MUST be able to be configured off, requiring a new round a PPP authentication initiated by the LNS (which may or may not include a new round of LCP negotiation).

10.0 IANA Considerations

This document defines a number of "magic" numbers to be maintained by the IANA. This section explains the criteria to be used by the IANA to assign additional numbers in each of these lists. The following subsections describe the assignment policy for the namespaces defined elsewhere in this document.

10.1 AVP Attributes

As defined in Section 4.1, AVPs contain vendor ID, Attribute and Value fields. For vendor ID value of 0, IANA will maintain a registry
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   of assigned Attributes and in some case also values. Attributes 0-39
   are assigned as defined in Section 4.4. The remaining values are
   available for assignment through IETF Consensus [RFC 2434].

10.2 Message Type AVP Values

As defined in Section 4.4.1, Message Type AVPs (Attribute Type 0) have an associated value maintained by IANA. Values 0-16 are defined in Section 3.2, the remaining values are available for assignment via IETF Consensus [RFC 2434]

10.3 Result Code AVP Values

As defined in Section 4.4.2, Result Code AVPs (Attribute Type 1) contain three fields. Two of these fields (the Result Code and Error Code fields) have associated values maintained by IANA.

10.3.1 Result Code Field Values

The Result Code AVP may be included in CDN and StopCCN messages. The allowable values for the Result Code field of the AVP differ depending upon the value of the Message Type AVP. For the StopCCN message, values 0-7 are defined in Section 4.4.2; for the StopCCN message, values 0-11 are defined in the same section. The remaining values of the Result Code field for both messages are available for assignment via IETF Consensus [RFC 2434].

10.3.2 Error Code Field Values

Values 0-7 are defined in Section 4.4.2. Values 8-32767 are available for assignment via IETF Consensus [RFC 2434]. The remaining values of the Error Code field are available for assignment via First Come First Served [RFC 2434].

10.4 Framing Capabilities & Bearer Capabilities

The Framing Capabilities AVP and Bearer Capabilities AVPs (defined in Section 4.4.3) both contain 32-bit bitmasks. Additional bits should only be defined via a Standards Action [RFC 2434].

10.5 Proxy Authen Type AVP Values

The Proxy Authen Type AVP (Attribute Type 29) has an associated value maintained by IANA. Values 0-5 are defined in Section 4.4.5, the remaining values are available for assignment via First Come First Served [RFC 2434].
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10.6 AVP Header Bits

There are four remaining reserved bits in the AVP header. Additional bits should only be assigned via a Standards Action [RFC 2434].

11.0 References

[DSS1] ITU-T Recommendation, "Digital subscriber Signaling System No. 1 (DSS 1) - ISDN user-network interface layer 3 specification for basic call control", Rec. Q.931(I.451), May 1998 [KPS] Kaufman, C., Perlman, R., and Speciner, M., "Network Security: Private Communications in a Public World", Prentice Hall, March 1995, ISBN 0-13-061466-1 [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13, RFC 1034, November 1987. [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662, July 1994. [RFC1663] Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994. [RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. See also: [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996. [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996.
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   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2138] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
             Authentication Dial In User Service (RADIUS)", RFC 2138,
             April 1997.

   [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
             Languages", BCP 18, RFC 2277, January 1998.

   [RFC2341] Valencia, A., Littlewood, M. and T. Kolar, "Cisco Layer Two
             Forwarding (Protocol) L2F", RFC 2341, May 1998.

   [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

   [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W.
             and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)",
             RFC 2637, July 1999.

   [STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I The
             Protocols", Addison-Wesley Publishing Company, Inc., March
             1996, ISBN 0-201-63346-9

12.0 Acknowledgments

The basic concept for L2TP and many of its protocol constructs were adopted from L2F [RFC2341] and PPTP [PPTP]. Authors of these are A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, and G. Zorn. Dory Leifer made valuable refinements to the protocol definition of L2TP and contributed to the editing of this document. Steve Cobb and Evan Caves redesigned the state machine tables. Barney Wolff provided a great deal of design input on the endpoint authentication mechanism. John Bray, Greg Burns, Rich Garrett, Don Grosser, Matt Holdrege, Terry Johnson, Dory Leifer, and Rich Shea provided valuable input and review at the 43rd IETF in Orlando, FL., which led to improvement of the overall readability and clarity of this document.
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13.0 Authors' Addresses

Gurdeep Singh Pall Microsoft Corporation Redmond, WA EMail: Bill Palter RedBack Networks, Inc 1389 Moffett Park Drive Sunnyvale, CA 94089 EMail: Allan Rubens Ascend Communications 1701 Harbor Bay Parkway Alameda, CA 94502 EMail: W. Mark Townsley cisco Systems 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709 EMail: Andrew J. Valencia cisco Systems 170 West Tasman Drive San Jose CA 95134-1706 EMail: Glen Zorn Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail:
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Appendix A: Control Channel Slow Start and Congestion Avoidance

Although each side has indicated the maximum size of its receive window, it is recommended that a slow start and congestion avoidance method be used to transmit control packets. The methods described here are based upon the TCP congestion avoidance algorithm as described in section 21.6 of TCP/IP Illustrated, Volume I, by W. Richard Stevens [STEVENS]. Slow start and congestion avoidance make use of several variables. The congestion window (CWND) defines the number of packets a sender may send before waiting for an acknowledgment. The size of CWND expands and contracts as described below. Note however, that CWND is never allowed to exceed the size of the advertised window obtained from the Receive Window AVP (in the text below, it is assumed any increase will be limited by the Receive Window Size). The variable SSTHRESH determines when the sender switches from slow start to congestion avoidance. Slow start is used while CWND is less than SSHTRESH. A sender starts out in the slow start phase. CWND is initialized to one packet, and SSHTRESH is initialized to the advertised window (obtained from the Receive Window AVP). The sender then transmits one packet and waits for its acknowledgement (either explicit or piggybacked). When the acknowledgement is received, the congestion window is incremented from one to two. During slow start, CWND is increased by one packet each time an ACK (explicit ZLB or piggybacked) is received. Increasing CWND by one on each ACK has the effect of doubling CWND with each round trip, resulting in an exponential increase. When the value of CWND reaches SSHTRESH, the slow start phase ends and the congestion avoidance phase begins. During congestion avoidance, CWND expands more slowly. Specifically, it increases by 1/CWND for every new ACK received. That is, CWND is increased by one packet after CWND new ACKs have been received. Window expansion during the congestion avoidance phase is effectively linear, with CWND increasing by one packet each round trip. When congestion occurs (indicated by the triggering of a retransmission) one half of the CWND is saved in SSTHRESH, and CWND is set to one. The sender then reenters the slow start phase.
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Appendix B: Control Message Examples

B.1: Lock-step tunnel establishment

In this example, an LAC establishes a tunnel, with the exchange involving each side alternating in sending messages. This example shows the final acknowledgment explicitly sent within a ZLB ACK message. An alternative would be to piggyback the acknowledgement within a message sent as a reply to the ICRQ or OCRQ that will likely follow from the side that initiated the tunnel. LAC or LNS LNS or LAC ---------- ---------- SCCRQ -> Nr: 0, Ns: 0 <- SCCRP Nr: 1, Ns: 0 SCCCN -> Nr: 1, Ns: 1 <- ZLB Nr: 2, Ns: 1

B.2: Lost packet with retransmission

An existing tunnel has a new session requested by the LAC. The ICRP is lost and must be retransmitted by the LNS. Note that loss of the ICRP has two impacts: not only does it keep the upper level state machine from progressing, but it also keeps the LAC from seeing a timely lower level acknowledgment of its ICRQ. LAC LNS --- --- ICRQ -> Nr: 1, Ns: 2 (packet lost) <- ICRP Nr: 3, Ns: 1 (pause; LAC's timer started first, so fires first) ICRQ -> Nr: 1, Ns: 2 (Realizing that it has already seen this packet, the LNS discards the packet and sends a ZLB)
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                                         <-       ZLB
                                         Nr: 3, Ns: 2

                       (LNS's retransmit timer fires)

                                         <-      ICRP
                                         Nr: 3, Ns: 1
       ICCN      ->
       Nr: 2, Ns: 3

                                         <-       ZLB
                                         Nr: 4, Ns: 2
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Appendix C: Intellectual Property Notice

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 implementers 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. The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights.
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