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rfc:rfc3193

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.

Patel, et al. Standards Track [Page 1] RFC 3193 Securing L2TP using IPsec November 2001

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

Patel, et al. Standards Track [Page 2] RFC 3193 Securing L2TP using IPsec November 2001

 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].

Patel, et al. Standards Track [Page 3] RFC 3193 Securing L2TP using IPsec November 2001

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.

Patel, et al. Standards Track [Page 4] RFC 3193 Securing L2TP using IPsec November 2001

 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,

Patel, et al. Standards Track [Page 5] RFC 3193 Securing L2TP using IPsec November 2001

 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.

Patel, et al. Standards Track [Page 6] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 7] RFC 3193 Securing L2TP using IPsec November 2001

 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:

Patel, et al. Standards Track [Page 8] RFC 3193 Securing L2TP using IPsec November 2001

 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:
  1. > IKE Phase 1 and Phase 2 to protect Initial SCCRQ
         SCCRQ ->         (Fixed IP address, Dynamic Initiator Port)
               <- STOPCCN (Responder chooses new IP address)
  1. > 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.

Patel, et al. Standards Track [Page 9] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 10] RFC 3193 Securing L2TP using IPsec November 2001

             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.

Patel, et al. Standards Track [Page 11] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 12] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 13] RFC 3193 Securing L2TP using IPsec November 2001

 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.

Patel, et al. Standards Track [Page 14] RFC 3193 Securing L2TP using IPsec November 2001

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.

Patel, et al. Standards Track [Page 15] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 16] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 17] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 18] RFC 3193 Securing L2TP using IPsec November 2001

 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.

Patel, et al. Standards Track [Page 19] RFC 3193 Securing L2TP using IPsec November 2001

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

Patel, et al. Standards Track [Page 20] RFC 3193 Securing L2TP using IPsec November 2001

 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.

Patel, et al. Standards Track [Page 21] RFC 3193 Securing L2TP using IPsec November 2001

 [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.

Patel, et al. Standards Track [Page 22] RFC 3193 Securing L2TP using IPsec November 2001

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

Patel, et al. Standards Track [Page 23] RFC 3193 Securing L2TP using IPsec November 2001

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

Patel, et al. Standards Track [Page 24] RFC 3193 Securing L2TP using IPsec November 2001

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

Patel, et al. Standards Track [Page 25] RFC 3193 Securing L2TP using IPsec November 2001

 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

Patel, et al. Standards Track [Page 26] RFC 3193 Securing L2TP using IPsec November 2001

 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.

Intellectual 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
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 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.

Patel, et al. Standards Track [Page 27] RFC 3193 Securing L2TP using IPsec November 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
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Patel, et al. Standards Track [Page 28]

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