





Network Working Group                                           B. Patel
Request for Comments: 3193                                         Intel
Category: Standards Track                                       B. Aboba
                                                                W. Dixon
                                                               Microsoft
                                                                 G. Zorn
                                                                S. Booth
                                                           Cisco Systems
                                                           November 2001


                       Securing L2TP using IPsec

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document discusses how L2TP (Layer Two Tunneling Protocol) may
   utilize IPsec to provide for tunnel authentication, privacy
   protection, integrity checking and replay protection. Both the
   voluntary and compulsory tunneling cases are discussed.




















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Table of Contents

   1. Introduction ..................................................  2
   1.1 Terminology ..................................................  3
   1.2 Requirements language ........................................  3
   2. L2TP security requirements  ...................................  4
   2.1 L2TP security protocol .......................................  5
   2.2 Stateless compression and encryption .........................  5
   3. L2TP/IPsec inter-operability guidelines .......................  6
   3.1. L2TP tunnel and Phase 1 and 2 SA teardown ...................  6
   3.2. Fragmentation Issues ........................................  6
   3.3. Per-packet security checks ..................................  7
   4. IPsec Filtering details when protecting L2TP ..................  7
   4.1. IKE Phase 1 Negotiations ....................................  8
   4.2. IKE Phase 2 Negotiations ....................................  8
   5. Security Considerations ....................................... 15
   5.1 Authentication issues ........................................ 15
   5.2 IPsec and PPP interactions ................................... 18
   6. References .................................................... 21
   Acknowledgments .................................................. 22
   Authors' Addresses ............................................... 23
   Appendix A: Example IPsec Filter sets ............................ 24
   Intellectual Property Statement .................................. 27
   Full Copyright Statement ......................................... 28

1.  Introduction

   L2TP [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 which is used to secure communication at
   the network layer between two 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 document proposes use of the IPsec protocol suite for protecting
   L2TP traffic over IP networks, and discusses how IPsec and L2TP
   should be used together.  This document does not attempt to






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   standardize end-to-end security.  When end-to-end security is
   required, it is recommended that additional security mechanisms (such
   as IPsec or TLS [14]) be used inside the tunnel, in addition to L2TP
   tunnel security.

   Although L2TP does not mandate the use of IP/UDP for its transport
   mechanism, the scope of this document is limited to L2TP over IP
   networks.  The exact mechanisms for enabling security for non-IP
   networks must be addressed in appropriate standards for L2TP over
   specific non-IP networks.

1.1.  Terminology

   Voluntary Tunneling
             In voluntary tunneling, a tunnel is created by the user,
             typically via use of a tunneling client.  As a result, the
             client 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 client.  Another example of voluntary
             tunneling is the gateway to gateway scenario.  In this case
             the tunnel is created by a network device, typically a
             router or network appliance.  In this scenario either side
             may start the tunnel on demand.

   Compulsory Tunneling
             In compulsory tunneling, a tunnel is created without any
             action from the client and without allowing the client any
             choice.  As a result, the client 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.

   Initiator The initiator can be the LAC or the LNS and is the device
             which sends the SCCRQ and receives the SCCRP.

   Responder The responder can be the LAC or the LNS and is the device
             which receives the SCCRQ and replies with a SCCRP.

1.2.  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
   "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [2].







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2.  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] An adversary may try to discover user identities by snooping data
       packets.

   [2] An adversary may try to modify packets (both control and data).

   [3] An 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
   for 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 tunnel authentication
   provides mutual authentication between the LAC and the LNS at tunnel
   origination.  Therefore, it does not protect control and data traffic
   on a per packet basis.  Thus, L2TP tunnel authentication leaves the
   L2TP tunnel vulnerable to attacks.  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, replay protection 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.






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   Key management facilities are not provided by the L2TP protocol.
   However, where L2TP tunnel authentication is desired, it is necessary
   to distribute tunnel passwords.

   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 client SHOULD provide
   for confidentiality, authentication, replay and integrity protection
   for PPP packets sent over the dial-up link.  Authentication, replay
   and integrity protection are not currently supported by PPP
   encryption methods, described in [11]-[13].

2.1.  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 the above requirements, all L2TP security compliant
   implementations MUST implement IPsec ESP for securing both L2TP
   control and data packets.  Transport mode MUST be supported; tunnel
   mode MAY be supported.  All the IPsec-mandated ciphersuites
   (described in RFC 2406 [4] and RFC 2402 [3]), including NULL
   encryption MUST be supported.  Note that although an implementation
   MUST support all IPsec ciphersuites, it is an operator choice which
   ones will be used.  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].

2.2.  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, when used
   without an underlying reliable delivery mechanism, stateful methods
   magnify packet losses.  As a result, they are problematic when used
   over the Internet where packet loss can be significant.  Although
   L2TP [1] is connection oriented, packet ordering is not mandatory,



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   which can create difficulties in implementation of stateful
   compression/encryption schemes.  These considerations are not as
   important when L2TP is run over non-IP media such as IEEE 802, ATM,
   X.25, or Frame Relay, since these media guarantee ordering, and
   packet losses are typically low.

3.  L2TP/IPsec inter-operability guidelines

   The following guidelines are established to meet L2TP security
   requirements using IPsec in practical situations.

3.1.  L2TP tunnel and Phase 1 and 2 SA teardown

   Mechanisms within PPP and L2TP provide for both graceful and non-
   graceful teardown.  In the case of PPP, an LCP TermReq and TermAck
   sequence corresponds to a graceful teardown.  LCP keep-alive messages
   and L2TP tunnel hellos provide the capability to detect when a non-
   graceful teardown has occurred.  Whenever teardown events occur,
   causing the tunnel to close, the control connection teardown
   mechanism defined in [1] must be used.  Once the L2TP tunnel is
   deleted by either peer, any phase 1 and phase 2 SA's which still
   exist as a result of the L2TP tunnel between the peers SHOULD be
   deleted.  Phase 1 and phase 2 delete messages SHOULD be sent when
   this occurs.

   When IKE receives a phase 1 or phase 2 delete message, IKE should
   notify L2TP this event has occurred.  If the L2TP state is such that
   a ZLB ack has been sent in response to a STOPCCN, this can be assumed
   to be positive acknowledgment that the peer received the ZLB ack and
   has performed a teardown of any L2TP tunnel state associated with the
   peer.  The L2TP tunnel state and any associated filters can now be
   safely removed.

3.2.  Fragmentation Issues

   Since the default MRU for PPP connections is 1500 bytes,
   fragmentation can become a concern when prepending L2TP and IPsec
   headers to a PPP frame.  One mechanism which can be used to reduce
   this problem is to provide PPP with the MTU value of the
   ingress/egress interface of the L2TP/IPsec tunnel minus the overhead
   of the extra headers.  This should occur after the L2TP tunnel has
   been setup and but before LCP negotiations begin.  If the MTU value
   of the ingress/egress interface for the tunnel is less than PPP's
   default MTU, it may replace the value being used.  This value may
   also be used as the initial value proposed for the MRU in the LCP
   config req.





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   If an ICMP PMTU is received by IPsec, this value should be stored in
   the SA as proposed in [6].  IPsec should also provide notification of
   this event to L2TP so that the new MTU value can be reflected into
   the PPP interface.  Any new PTMU discoveries seen at the PPP
   interface should be checked against this new value and processed
   accordingly.

3.3.  Per-packet security checks

   When a packet arrives from a tunnel which requires security, L2TP
   MUST:

   [1] Check to ensure that the packet was decrypted and/or
       authenticated by IPsec.  Since IPsec already verifies that the
       packet arrived in the correct SA, L2TP can be assured that the
       packet was indeed sent by a trusted peer and that it did not
       arrive in the clear.

   [2] Verify that the IP addresses and UDP port values in the packet
       match the socket information which was used to setup the L2TP
       tunnel.  This step prevents malicious peers from spoofing packets
       into other tunnels.

4.  IPsec Filtering details when protecting L2TP

   Since IKE/IPsec is agnostic about the nuances of the application it
   is protecting, typically no integration is necessary between the
   application and the IPsec protocol.  However, protocols which allow
   the port number to float during the protocol negotiations (such as
   L2TP), can cause problems within the current IKE framework.  The L2TP
   specification [1] states that implementations MAY use a dynamically
   assigned UDP source port.  This port change is reflected in the SCCRP
   sent from the responder to the initiator.

   Although the current L2TP specification allows the responder to use a
   new IP address when sending the SCCRP, implementations requiring
   protection of L2TP via IPsec SHOULD NOT do this.  To allow for this
   behavior when using L2TP and IPsec, when the responder chooses a new
   IP address it MUST send a StopCCN to the initiator, with the Result
   and Error Code AVP present.  The Result Code MUST be set to 2
   (General Error) and the Error Code SHOULD be set to 7 (Try Another).
   If the Error Code is set to 7, then the optional error message MUST
   be present and the contents MUST contain the IP address (ASCII
   encoded) that the Responder desires to use for subsequent
   communications.  Only the ASCII encoded IP address should be present
   in the error message.  The IP address is encoded in dotted decimal
   format for IPv4 or in RFC 2373 [17] format for IPv6.  The initiator
   MUST parse the result and error code information and send a new SCCRQ



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   to the new IP address contained in the error message.  This approach
   reduces complexity since now the initiator always knows precisely the
   IP address of its peer.  This also allows a controlled mechanism for
   L2TP to tie IPsec filters and policy to the same peer.

   The filtering details required to accommodate this behavior as well
   as other mechanisms needed to protect L2TP with IPsec are discussed
   in the following sections.

4.1.  IKE Phase 1 Negotiations

   Per IKE [7], when using pre-shared key authentication, a key must be
   present for each peer to which secure communication is required.
   When using Main Mode (which provides identity protection), this key
   must correspond to the IP address for the peer.  When using
   Aggressive Mode (which does not provide identity protection), the
   pre-shared key must map to one of the valid id types defined in the
   IPsec DOI [5].

   If the initiator receives a StopCCN with the result and error code
   AVP set to "try another" and a valid IP address is present in the
   message, it MAY bind the original pre-shared key used by IKE to the
   new IP address contained in the error-message.

   One may may wish to consider the implications for scalability of
   using pre-shared keys as the authentication method for phase 1.  As
   the number of LAC and LNS endpoints grow, pre-shared keys become
   increasingly difficult to manage.  Whenever possible, authentication
   with certificates is preferred.

4.2.  IKE Phase 2 Negotiations

   During the IKE phase 2 negotiations, the peers agree on what traffic
   is to be protected by the IPsec protocols.  The quick mode IDs
   represent the traffic which the peers agree to protect and are
   comprised of address space, protocol, and port information.

   When securing L2TP with IPsec, the following cases must be
   considered:












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   Cases:

   +--------------------------------------------------+
   | Initiator Port | Responder Addr | Responder Port |
   +--------------------------------------------------+
   |      1701      |     Fixed      |     1701       |
   +--------------------------------------------------+
   |      1701      |     Fixed      |    Dynamic     |
   +--------------------------------------------------+
   |      1701      |    Dynamic     |     1701       |
   +--------------------------------------------------+
   |      1701      |    Dynamic     |    Dynamic     |
   +--------------------------------------------------+
   |     Dynamic    |     Fixed      |     1701       |
   +--------------------------------------------------+
   |     Dynamic    |     Fixed      |    Dynamic     |
   +--------------------------------------------------+
   |     Dynamic    |    Dynamic     |     1701       |
   +--------------------------------------------------+
   |     Dynamic    |    Dynamic     |    Dynamic     |
   +--------------------------------------------------+

   By solving the most general case of the above permutations, all cases
   are covered.  The most general case is the last one in the list.
   This scenario is when the initiator chooses a new port number and the
   responder chooses a new address and port number.  The L2TP message
   flow which occurs to setup this sequence is as follows:

   -> IKE Phase 1 and Phase 2 to protect Initial SCCRQ

           SCCRQ ->         (Fixed IP address, Dynamic Initiator Port)
                 <- STOPCCN (Responder chooses new IP address)

   -> New IKE Phase 1 and Phase 2 to protect new SCCRQ

           SCCRQ ->         (SCCRQ to Responder's new IP address)

   <- New IKE Phase 2 to for port number change by the responder

                 <- SCCRP   (Responder chooses new port number)
           SCCCN ->         (L2TP Tunnel Establishment completes)

   Although the Initiator and Responder typically do not dynamically
   change ports, L2TP security must accommodate emerging applications
   such as load balancing and QoS.  This may require that the port and
   IP address float during L2TP tunnel establishment.





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   To support the general case, mechanisms must be designed into L2TP
   and IPsec which allow L2TP to inject filters into the IPsec filter
   database.  This technique may be used by any application which floats
   ports and requires security via IPsec, and is described in the
   following sections.

   The responder is not required to support the ability to float its IP
   address and port.  However, the initiator MUST allow the responder to
   float its port and SHOULD allow the responder to choose a new IP
   address (see section 4.2.3, below).

   Appendix A provides examples of these cases using the process
   described below.

4.2.1.  Terminology definitions used for filtering statements

   I-Port      The UDP port number the Initiator chooses to
               originate/receive L2TP traffic on.  This can be a static
               port such as 1701 or an ephemeral one assigned by the
               socket.

   R-Port      The UDP port number the Responder chooses to
               originate/receive L2TP traffic on.  This can be the port
               number 1701 or an ephemeral one assigned by the socket.
               This is the port number the Responder uses after
               receiving the initial SCCRQ.

   R-IPAddr1   The IP address the Responder listens on for initial
               SCCRQ.  If the responder does not choose a new IP
               address, this address will be used for all subsequent
               L2TP traffic.

   R-IPAddr2   The IP address the Responder chooses upon receiving the
               SCCRQ.  This address is used to send the SCCRP and all
               subsequent L2TP tunnel traffic is sent and received on
               this address.

   R-IPAddr    The IP address which the responder uses for sending and
               receiving L2TP packets.  This is either the initial value
               of R-IPAddr1 or a new value of R-IPAddr2.

   I-IPAddr    The IP address the Initiator uses to communicate with for
               the L2TP tunnel.

   Any-Addr    The presence of Any-Address defines that IKE should
               accept any single address proposed in the local address
               of the quick mode IDs sent by the peer during IKE phase 2
               negotiations.  This single address may be formatted as an



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               IP Single address, an IP Netmask address with the Netmask
               set to 255.255.255.255, and IP address Range with the
               range being 1, or a hostname which can be resolved to one
               address.  Refer to [5] for more information on the format
               for quick mode IDs.

   Any-Port    The presence of Any-Port defines that IKE should accept a
               value of 0 or a specific port value for the port value in
               the port value in the quick mode IDs negotiated during
               IKE phase 2.

   The filters defined in the following sections are listed from highest
   priority to lowest priority.

4.2.2.  Initial filters needed to protect the SCCRQ

   The initial filter set on the initiator and responder is necessary to
   protect the SCCRQ sent by the initiator to open the L2TP tunnel.
   Both the initiator and the responder must either be pre-configured
   for these filters or L2TP must have a method to inject this
   information into the IPsec filtering database.  In either case, this
   filter MUST be present before the L2TP tunnel setup messages start to
   flow.

      Responder Filters:
         Outbound-1: None.  They should be be dynamically created by IKE
         upon successful completion of phase 2.

      Inbound-1:  From Any-Addr,  to R-IPAddr1, UDP, src Any-Port, dst
         1701

      Initiator Filters:
         Outbound-1: From I-IPAddr,  to R-IPAddr1, UDP, src I-Port,
         dst 1701

         Inbound-1:  From R-IPAddr1, to I-IPAddr,  UDP, src 1701,
         dst I-Port
         Inbound-2:  From R-IPAddr1, to I-IPAddr,  UDP, src Any-Port,
         dst I-Port

   When the initiator uses dynamic ports, L2TP must inject the filters
   into the IPsec filter database, once its source port number is known.
   If the initiator uses a fixed port of 1701, these filters MAY be
   statically defined.

   The Any-Port definition in the initiator's inbound-2 filter statement
   is needed to handle the potential port change which may occur as the
   result of the responder changing its port number.



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   If a phase 2 SA bundle is not already present to protect the SCCRQ,
   the sending of a SCCRQ by the initiator SHOULD cause IKE to setup the
   necessary SAs to protect this packet.  Alternatively, L2TP may also
   request IKE to setup the SA bundle.  If the SA cannot be setup for
   some reason, the packet MUST be dropped.

   The port numbers in the Quick Mode IDs sent by the initiator MUST
   contain the specific port numbers used to identify the UDP socket.
   The port numbers would be either I-Port/1701 or 1701/1701 for the
   initial SCCRQ.  The quick mode IDs sent by the initiator will be a
   subset of the Inbound-1 filter at the responder.  As a result, the
   quick mode exchange will finish and IKE should inject a specific
   filter set into the IPsec filter database and associate this filter
   set with the phase 2 SA established between the peers.  These filters
   should persist as long as the L2TP tunnel exists.  The new filter set
   at the responder will be:

      Responder Filters:
         Outbound-1: From R-IPAddr1, to I-IPAddr,  UDP, src 1701,
         dst I-Port

         Inbound-1:  From I-IPAddr,  to R-IPAddr1, UDP, src I-Port,
         dst 1701
         Inbound-2:  From Any-Addr,  to R-IPAddr1, UDP, src Any-Port,
         dst 1701

   Mechanisms SHOULD exist between L2TP and IPsec such that L2TP is not
   retransmitting the SCCRQ while the SA is being established.  L2TP's
   control channel retransmit mechanisms should start once the SA has
   been established.  This will help avoid timeouts which may occur as
   the result of slow SA establishment.

   Once the phase 2 SA has been established between the peers, the SCCRQ
   should be sent from the initiator to the responder.

   If the responder does not choose a new IP address or a new port
   number, the L2TP tunnel can now proceed to establish.

4.2.3.  Responder chooses new IP Address

   This step describes the process which should be followed when the
   responder chooses a new IP address.  The only opportunity for the
   responder to change its IP address is after receiving the SCCRQ but
   before sending a SCCRP.

   The new address the responder chooses to use MUST be reflected in the
   result and error code AVP of a STOPCCN message.  The Result Code MUST
   be set to 2 (General Error) and the Error Code MUST be set to 7 (Try



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   Another).  The optional error message MUST be present and the
   contents MUST contain the IP address (ASCII encoded) the Responder
   desires to use for subsequent communications.  Only the ASCII encoded
   IP address should be present in the error message.  The IP address is
   encoded in dotted decimal format for IPv4 or in RFC 2373 [17] format
   for IPv6.

   The STOPCCN Message MUST be sent using the same address and UDP port
   information which the initiator used to send the SCCRQ.  This message
   will be protecting using the initial SA bundle setup to protect the
   SCCRQ.

   Upon receiving the STOPCCN, the initiator MUST parse the IP address
   from the Result and Error Code AVP and perform the necessary sanity
   checks to verify this is a correctly formatted address.  If no errors
   are found L2TP should inject a new set of filters into the IPsec
   filter database.  If using pre-shared key authentication, L2TP MAY
   request IKE to bind the new IP address to the pre-shared key which
   was used for the original IP address.

   Since the IP address of the responder changed, a new phase 1 and
   phase 2 SA must be established between the peers before the new SCCRQ
   is sent.

   Assuming the initial tunnel has been torn down and the filters needed
   to create the tunnel removed, the new filters for the initiator and
   responder will be:

      Initiator Filters:
         Outbound-1: From I-IPAddr,  to R-IPAddr2, UDP, src I-Port,
         dst 1701

         Inbound-1:  From R-IPAddr2, to I-IPAddr,  UDP, src 1701,
         dst I-Port
         Inbound-2:  From R-IPAddr2, to I-IPAddr,  UDP, src Any-Port,
         dst I-Port

   Once IKE phase 2 completes, the new filter set at the responder will
   be:

      Responder Filters:
         Outbound-1: From R-IPAddr2, to I-IPAddr,  UDP, src 1701,
         dst I-Port

         Inbound-1:  From I-IPAddr,  to R-IPAddr2, UDP, src I-Port,
         dst 1701
         Inbound-2:  From Any-Addr,  to R-IPAddr1, UDP, src Any-Port,
         dst 1701



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   If the responder chooses not to move to a new port number, the L2TP
   tunnel setup can now complete.

4.2.4.  Responder chooses new Port Number

   The responder MAY choose a new UDP source port to use for L2TP tunnel
   traffic.  This decision MUST be made before sending the SCCRP.  If a
   new port number is chosen, then L2TP must inject new filters into the
   IPsec filter database.  The responder must start new IKE phase 2
   negotiations with the initiator.

   The final filter set at the initiator and responder is as follows.

      Initiator Filters:
         Outbound-1: From I-IPAddr, to R-IPAddr, UDP, src I-Port,   dst
         R-Port
         Outbound-2: From I-IPAddr, to R-IPAddr, UDP, src I-Port,   dst
         1701


         Inbound-1:  From R-IPAddr, to I-IPAddr, UDP, src R-Port,   dst
         I-Port
         Inbound-2:  From R-IPAddr, to I-IPAddr, UDP, src 1701,     dst
         I-Port
         Inbound-3:  From R-IPAddr, to I-IPAddr, UDP, src Any-Port, dst
         I-Port

         The Inbound-1 filter for the initiator will be injected by IKE
         upon successful completion of the phase 2 negotiations
         initiated by the peer.

      Responder Filters:
         Outbound-1: From R-IPAddr, to I-IPAddr,  UDP, src R-Port,   dst
         I-Port
         Outbound-2: From R-IPAddr, to I-IPAddr,  UDP, src 1701,     dst
         I-Port

         Inbound-1:  From I-IPAddr, to R-IPAddr,  UDP, src I-Port,   dst
         R-Port
         Inbound-2:  From I-IPAddr, to R-IPAddr,  UDP, src I-Port,   dst
         1701
         Inbound-3:  From Any-Addr, to R-IPAddr1, UDP, src Any-Port, dst
         1701

   Once the negotiations have completed, the SCCRP is sent and the L2TP
   tunnel can complete establishment.  After the L2TP tunnel has been
   established, any residual SAs and their associated filters may be
   deleted.



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4.2.5.  Gateway-gateway and L2TP Dial-out considerations

   In the gateway-gateway or the L2TP dial-out scenario, either side may
   initiate L2TP.  The process outlined in the previous steps should be
   followed with one addition.  The initial filter set at both sides
   MUST include the following filter:

      Inbound Filter:
         1: From Any-Addr, to R-IPAddr1, UDP, src Any-Port, dst 1701

   When either peer decides to start a tunnel, L2TP should inject the
   necessary inbound and outbound filters to protect the SCCRQ.  Tunnel
   establishment then proceeds exactly as stated in the previous
   sections.

5.  Security Considerations

5.1.  Authentication issues

   IPsec IKE negotiation MUST negotiate an authentication method
   specified in the IKE RFC 2409 [7].  In addition to IKE
   authentication, L2TP implementations utilize PPP authentication
   methods, such as those described in [15]-[16].  In this section, we
   discuss authentication issues.

5.1.1.  Differences between IKE and PPP authentication

   While PPP provides initial authentication, it does not provide per-
   packet authentication, integrity or replay protection.  This implies
   that the identity verified in the initial PPP authentication is not
   subsequently verified on reception of each packet.

   With IPsec, when the identity asserted in IKE is authenticated, the
   resulting derived keys are used to provide per-packet authentication,
   integrity and replay protection.  As a result, the identity verified
   in the IKE conversation is subsequently verified on reception of each
   packet.

   Let us assume that the identity claimed in PPP is a user identity,
   while the identity claimed within IKE is a machine identity.  Since
   only the machine identity is verified on a per-packet basis, there is
   no way to verify that only the user authenticated within PPP is using
   the tunnel.  In fact, IPsec implementations that only support machine
   authentication typically have no way to enforce traffic segregation.
   As a result, where machine authentication is used, once an L2TP/IPsec
   tunnel is opened, any user on a multi-user machine will typically be
   able to send traffic down the tunnel.




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   If the IPsec implementation supports user authentication, this
   problem can be averted.  In this case, the user identity asserted
   within IKE will be verified on a per-packet basis.  In order to
   provide segregation of traffic between users when user authentication
   is used, the client MUST ensure that only traffic from that
   particular user is sent down the L2TP tunnel.

5.1.2.  Certificate authentication in IKE

   When X.509 certificate authentication is chosen within IKE, the LNS
   is expected to use an IKE Certificate Request Payload (CRP) to
   request from the client a certificate issued by a particular
   certificate authority or may use several CRPs if several certificate
   authorities are trusted and configured in its IPsec IKE
   authentication policy.

   The LNS SHOULD be able to trust several certificate authorities in
   order to allow tunnel client end-points to connect to it using their
   own certificate credential from their chosen PKI.  Client and server
   side certificate revocation list checking MAY be enabled on a per-CA
   basis, since differences in revocation list checking exist between
   different PKI providers.

   L2TP implementations MAY use dynamically assigned ports for both
   source and destination ports only if security for each source and
   destination port combination can be successfully negotiated by IKE.

5.1.3.  Machine versus user certificate authentication in IKE

   The certificate credentials provided by the L2TP client during the
   IKE negotiation MAY be those of the machine or of the L2TP user.
   When machine authentication is used, the machine certificate is
   typically stored on the LAC and LNS during an enrollment process.
   When user certificates are used, the user certificate can be stored
   either on the machine or on a smartcard.

   Since the value of a machine certificate is inversely proportional to
   the ease with which an attacker can obtain one under false pretenses,
   it is advisable that the machine certificate enrollment process be
   strictly controlled.  For example, only administrators may have the
   ability to enroll a machine with a machine certificate.

   While smartcard certificate storage lessens the probability of
   compromise of the private key, smartcards are not necessarily
   desirable in all situations.  For example, some organizations
   deploying machine certificates use them so as to restrict use of
   non-approved hardware.  Since user authentication can be provided




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   within PPP (keeping in mind the weaknesses described earlier),
   support for machine authentication in IPsec makes it is possible to
   authenticate both the machine as well as the user.

   In circumstances in which this dual assurance is considered valuable,
   enabling movement of the machine certificate from one machine to
   another, as would be possible if the machine certificate were stored
   on a smart card, may be undesirable.

   Similarly, when user certificate are deployed, it is advisable for
   the user enrollment process to be strictly controlled.  If for
   example, a user password can be readily used to obtain a certificate
   (either a temporary or a longer term one), then that certificate has
   no more security value than the password.  To limit the ability of an
   attacker to obtain a user certificate from a stolen password, the
   enrollment period can be limited, after which password access will be
   turned off.  Such a policy will prevent an attacker obtaining the
   password of an unused account from obtaining a user certificate once
   the enrollment period has expired.

5.1.4.  Pre-shared keys in IKE

   Use of pre-shared keys in IKE main mode is vulnerable to man-in-the-
   middle attacks when used in remote access situations.  In main mode
   it is necessary for SKEYID_e to be used prior to the receipt of the
   identification payload.  Therefore the selection of the pre-shared
   key may only be based on information contained in the IP header.
   However, in remote access situations, dynamic IP address assignment
   is typical, so that it is often not possible to identify the required
   pre-shared key based on the IP address.

   Thus when pre-shared keys are used in remote access scenarios, the
   same pre-shared key is shared by a group of users and is no longer
   able to function as an effective shared secret.  In this situation,
   neither the client nor the server identifies itself during IKE phase
   1; it is only known that both parties are a member of the group with
   knowledge of the pre-shared key.  This permits anyone with access to
   the group pre-shared key to act as a man-in-the-middle.

   This vulnerability does not occur in aggressive mode since the
   identity payload is sent earlier in the exchange, and therefore the
   pre-shared key can be selected based on the identity.  However, when
   aggressive mode is used the user identity is exposed and this is
   often considered undesirable.

   As a result, where main mode is used with pre-shared keys, unless PPP
   performs mutual authentication, the server is not authenticated.
   This enables a rogue server in possession of the group pre-shared key



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   to successfully masquerade as the LNS and mount a dictionary attack
   on legacy authentication methods such as CHAP [15].  Such an attack
   could potentially compromise many passwords at a time.  This
   vulnerability is present in some existing IPsec tunnel mode
   implementations.

   To avoid this problem, L2TP/IPsec implementations SHOULD NOT use a
   group pre-shared key for IKE authentication to the LNS.  IKE pre-
   shared authentication key values SHOULD be protected in a manner
   similar to the user's account password used by L2TP.

5.2.  IPsec and PPP security interactions

   When L2TP is protected with IPsec, both PPP and IPsec security
   services are available.  Which services are negotiated depends on
   whether the tunnel is compulsory or voluntary.  A detailed analysis
   of voluntary and compulsory tunneling scenarios is included below.
   These scenarios are non-normative and do not create requirements for
   an implementation to be L2TP security compliant.

   In the scenarios below, 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.

5.2.1.  Compulsory tunnel

   In the case of a compulsory tunnel, the client sends PPP frames to
   the LAC, and will typically not be aware that the frames are being
   tunneled, nor that any security services are in place between the LAC
   and LNS.  At the LNS, a data packet will arrive, which includes a PPP
   frame encapsulated in L2TP, which is itself encapsulated in an IP
   packet.  By obtaining the properties of the Security Association set
   up between the LNS and the LAC, the LNS can obtain information about
   security services in place between itself and the LAC.  Thus in the
   compulsory tunneling case, the client 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
   itself and the LAC, it can use this knowledge in order to modify its
   behavior during PPP ECP [10] and CCP [9] negotiation.  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



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   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.  This is not the same as
   insisting that PPP encryption and compression be turned off, since
   this decision will depend on client policy.

   Since the client 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 client will typically want
   to ensure sufficient security through use of end-to-end IPsec or PPP
   encryption/compression between itself and the LNS.

   A client 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.  The client
   negotiates confidentiality services between itself and the LNS in
   order to provide privacy on the wire between itself and the LAC.  The
   client negotiates end-to-end security between itself and the end-
   station in order to ensure confidentiality on the portion of the path
   between the LNS and the end-station.

   The client will typically not trust the LAC and will negotiate
   confidentiality and compression services on its own.  As a result,
   the LAC may only wish to negotiate IPsec ESP with null encryption
   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 results
   in better scalability for the LAC, since encryption will be handled
   by the client and the LNS.

   The client can satisfy its desire for confidentiality services in one
   of two ways.  If it knows that all end-stations that it will
   communicate with are IPsec-capable (or if it refuses to talk to non-
   IPsec capable end-stations), then it can refuse to negotiate PPP
   encryption/compression and negotiate IPsec ESP with the end-stations
   instead.  If the client does not know that all end-stations 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 client's packets are being tunneled
   but the client 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.





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5.2.2.  Voluntary tunnel

   In the case of a voluntary tunnel, the client will be send 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 client to retrieve the
   properties of the Security Association between itself and the LNS,
   the client 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 point of view, it will note a PPP frame encapsulated in
   L2TP, which is itself encapsulated in an IP packet.  This situation
   is identical to the compulsory tunneling case.  If LNS retrieves the
   properties of the Security Association set up between itself and the
   client, it can be informed of the security services in place between
   them.  Thus in the voluntary tunneling case, the client 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 client 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 client.

   Since the client 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 compression not be negotiated.  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 client 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 client is
   communicating with an IPsec-capable end-station.  In order to avoid
   duplicate encryption/compression, the client may negotiate two



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   Security Associations with the LNS, one with ESP with null
   encryption, and one with confidentiality/compression.  Packets going
   to an IPsec- capable end-station would run over the ESP with null
   encryption security association, and packets to a non-IPsec capable
   end-station 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 client may wish to put confidentiality services in
   place for non-tunneled packets traveling between itself and the NAS.
   This will protect the client 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.

6.  References

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

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

   [3]   Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

   [4]   Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406, November 1998.

   [5]   Piper, D., "The Internet IP Security Domain of Interpretation
         of ISAKMP", RFC 2407, November 1998.

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

   [7]   Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
         RFC 2409, November 1998.

   [8]   Simpson, W., "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.




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   [10]  Meyer, G., "The PPP Encryption Control Protocol (ECP)", RFC
         1968, June 1996.

   [11]  Sklower, K. and G. Meyer, "The PPP DES Encryption Protocol
         (DESE)", RFC 1969, June 1996.

   [12]  Sklower, K. and G. Meyer, "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.

   [14]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, November 1998.

   [15]  Simpson, W., "PPP Challenge Handshake Authentication Protocol
         (CHAP)," RFC 1994, August 1996.

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

   [17]  Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.

Acknowledgments

   Thanks to Gurdeep Singh Pall, David Eitelbach, Peter Ford, and Sanjay
   Anand of Microsoft, John Richardson of Intel and Rob Adams of Cisco
   for useful discussions of this problem space.






















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Authors' Addresses

   Baiju V. Patel
   Intel Corp
   2511 NE 25th Ave
   Hillsboro, OR 97124

   Phone: +1 503 702 2303
   EMail: baiju.v.patel@intel.com


   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 706-6605
   EMail: bernarda@microsoft.com


   William Dixon
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 703 8729
   EMail: wdixon@microsoft.com


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

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


   Skip Booth
   Cisco Systems
   7025 Kit Creek Road
   RTP, NC 27709

   Phone: +1 919 392 6951
   EMail: ebooth@cisco.com





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Appendix A: Example IPsec Filter sets for L2TP Tunnel Establishment

   This section provides examples of IPsec filter sets for L2TP tunnel
   establishment.  While example filter sets are for IPv4, similar
   examples could just as easily be constructed for IPv6.

A.1 Initiator and Responder use fixed addresses and ports

   This is the most simple of the cases since nothing changes during
   L2TP tunnel establishment.  Since the initiator does not know whether
   the responder will change its port number, it still must be prepared
   for this case.  In this example, the initiator will use an IPv4
   address of 1.1.1.1 and the responder will use an IPv4 address of
   2.2.2.1.

   The filters for this scenario are:

A.1.1 Protect the SCCRQ

   Initiator Filters:
      Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 1701,     dst 1701

      Inbound-1:  From 2.2.2.1, to 1.1.1.1, UDP, src 1701,     dst 1701
      Inbound-2:  From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 1701

   Responder Filters:
      Outbound-1: None, dynamically injected when IKE Phase 2 completes

      Inbound-1:  From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701

   After IKE Phase 2 completes the filters at the initiator and
   responder will be:

   Initiator Filters:
      Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 1701,     dst 1701

      Inbound-1:  From 2.2.2.1, to 1.1.1.1, UDP, src 1701,     dst 1701
      Inbound-2:  From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 1701

   Responder Filters:
      Outbound-1: From 2.2.2.1,  to 1.1.1.1, UDP, src 1701,     dst 1701

      Inbound-1:  From 1.1.1.1,  to 2.2.2.1, UDP, src 1701,     dst 1701
      Inbound-2:  From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701







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A.2 Gateway to Gateway Scenario where Initiator and Responder use
    dynamic ports

   In this scenario either side is allowed to initiate the tunnel.
   Since dynamic ports will be used, an extra phase 2 negotiation must
   occur to protect the SCCRP sent from the responder to the initiator.
   Other than the additional phase 2 setup, the only other difference is
   that L2TP on the responder must inject an additional filter into the
   IPsec database once the new port number is chosen.

   This example also shows the additional filter needed by the initiator
   which allows either side to start the tunnel.  In either the dial-out
   or the gateway to gateway scenario this additional filter is
   required.

   For this example, assume the dynamic port given to the initiator is
   5000 and his IP address is 1.1.1.1.  The responder will use an IP
   address of 2.2.2.1 and a port number of 6000.

   The filters for this scenario are:

A.2.1 Initial Filters to allow either side to respond to negotiations

   In this case both peers must be able to accept phase 2 negotiations
   to from L2TP peers.  My-IPAddr is defined as whatever IP address the
   device is willing to accept L2TP negotiations on.

   Responder Filters present at both peers:
     Inbound-1: From Any-Addr, to My-IPAddr, UDP, src Any-Port, dst 1701

   Note: The source IP in the inbound-1 filter above for gateway to
   gateway tunnels can be IP specific, such as 1.1.1.1, not necessarily
   Any-Addr.

A.2.2 Protect the SCCRQ, one peer is now the initiator

   Initiator Filters:
      Outbound-1: From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 1701

      Inbound-1:  From 2.2.2.1,  to 1.1.1.1, UDP, src 1701,     dst 5000
      Inbound-2:  From 2.2.2.1,  to 1.1.1.1, UDP, src Any-Port, dst 5000
      Inbound-3:  From Any-Addr, to 1.1.1.1, UDP, src Any-Port, dst 1701

   Responder Filters:
      Outbound-1: None, dynamically injected when IKE Phase 2 completes

      Inbound-1:  From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701




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   After IKE Phase 2 completes the filters at the initiator and
   responder will be:

   Initiator Filters:
      Outbound-1: From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 1701

      Inbound-1:  From 2.2.2.1,  to 1.1.1.1, UDP, src 1701,     dst 5000
      Inbound-2:  From 2.2.2.1,  to 1.1.1.1, UDP, src Any-Port, dst 5000

      Inbound-3:  From Any-Addr, to 1.1.1.1, UDP, src Any-Port, dst 1701

   Responder Filters:
      Outbound-1: From 2.2.2.1,  to 1.1.1.1, UDP, src 1701,     dst 5000

      Inbound-1:  From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 1701
      Inbound-2:  From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701

A.2.3 Protect the SCCRP after port change

   At this point the responder knows which port number it is going to
   use.  New filters should be injected by L2TP to reflect this new port
   assignment.

   The new filter set at the responder is:

   Responder Filters:
      Outbound-1: From 2.2.2.1,  to 1.1.1.1, UDP, src 6000,     dst 5000
      Outbound-2: From 2.2.2.1,  to 1.1.1.1, UDP, src 1701,     dst 5000

      Inbound-1:  From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 6000
      Inbound-2:  From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 1701
      Inbound-3:  From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701

   The second phase 2 will start once L2TP sends the SCCRP.  Once the
   phase 2 negotiations complete, the new filter set at the initiator
   and the responder will be:

   Initiator Filters:
      Outbound-1: From 1.1.1.1, to 2.2.2.1, UDP, src 5000,     dst 6000
      Outbound-2: From 1.1.1.1, to 2.2.2.1, UDP, src 5000,     dst 1701

      Inbound-1:  From 2.2.2.1, to 1.1.1.1, UDP, src 6000,     dst 5000
      Inbound-2:  From 2.2.2.1, to 1.1.1.1, UDP, src 1701,     dst 5000
      Inbound-3:  From 2.2.2.1, to 1.1.1.1, UDP, src Any-Port, dst 1701







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   Responder Filters:
      Outbound-1: From 2.2.2.1,  to 1.1.1.1, UDP, src 6000,     dst 5000
      Outbound-2: From 2.2.2.1,  to 1.1.1.1, UDP, src 1701,     dst 5000

      Inbound-1:  From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 6000
      Inbound-2:  From 1.1.1.1,  to 2.2.2.1, UDP, src 5000,     dst 1701
      Inbound-3:  From Any-Addr, to 2.2.2.1, UDP, src Any-Port, dst 1701

   Once the L2TP tunnel has been successfully established, the original
   phase 2 may be deleted.  This allows the Inbound-2 and Outbound-2
   filter statements to be removed as well.

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RFC 3193               Securing L2TP using IPsec           November 2001


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