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Network Working Group                                   Jeremy De Clercq
INTERNET DRAFT                                         Olivier Paridaens
<draft-ietf-l3vpn-ce-based-02.txt>                               Alcatel
                                                        Andrew Krywaniuk
                                                              Cliff Wang

                                                           February 2004
                                                    Expires August, 2004


                          An Architecture for
         Provider Provisioned CE-based Virtual Private Networks
                              using IPsec

                  <draft-ietf-l3vpn-ce-based-02.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months. Internet-Drafts may be updated, replaced, or obsoleted by
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   ``working draft'' or ``work in progress.''

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au(Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

   Distribution of this memo is unlimited.

Abstract

   This document describes the procedures for a Service Provider to
   offer Virtual Private Network Services to its customers by
   provisioning the CE devices on behalf of the customer. The IPsec
   technology is used to protect the customer traffic.

Table of Contents




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1.     Introduction ................................................  2
2.     Reference Model .............................................  3
2.1    Entities in the Reference Model .............................  3
2.2    IP Connectivity between CE and PE devices ...................  5
2.3    Assumed Service Provider's Infrastructure ...................  7
3.     Configuring the CE-based VPN ................................  8
3.1    Initializing the SP's VPN database ..........................  8
3.2    Pre-configuration of the CE device ..........................  9
3.3    Fetching the VPN configuration information .................. 10
3.4    Establishing the (secure) VPN tunnels ....................... 10
3.5    Updating the VPN configuration information .................. 13
3.6    Removing an existing VPN site ............................... 13
4.     Exchanging and maintaining VPN routes ....................... 14
4.1    The CE device and VPN routing ............................... 15
4.2    IPsec and routing ........................................... 16
4.3    Exchanging VPN routes between VPN sites ..................... 16
5.     Tunneling IP traffic (user data) among VPN sites ............ 17
6.     CE-based VPN and Internet ................................... 19
6.1    Allowing both VPN connectivity and Internet connectivity .... 19
6.2    Prohibiting or restricting Internet connectivity from within
       a CE-based VPN .............................................. 22
7.     Security Considerations ..................................... 24
8.     Acknowledgements ............................................ 25
9.     References .................................................. 25
10.    Authors' Addresses .......................................... 26

1. Introduction

   The L3VPN framework document [FRAMEWORK] identifies three basic
   provider provisioned VPN types : Provider Provisioned Network Based
   (also termed PE-based) Layer 3 VPNs, Provider Provisioned Layer 2
   VPNs and Provider Provisioned CE-based VPNs.

   This document describes a method enabling a Service Provider to offer
   IP VPN services to its customers by provisioning the CE devices on
   behalf of the customer (Provider Provisioned CE-based VPNs). This
   document describes which parameters need to be provisioned, but not
   which protocol to use for the provisioning.

   For a CE-based VPN to be set up under the SP's control, the VPN
   customer informs the Service Provider of which sites (identified by a
   set of CE devices) should become part of the considered VPN and what
   the requested topology of the VPN should look like. The SP then
   configures and updates its VPN database, and then provisions and
   manages the Customer's VPN.

   The model proposed in this document uses the IPsec protocol suite for
   the purpose of securely tunneling the customer VPN traffic and the



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   inter-site reachability information distribution.

2. Reference Model

   The reference model upon which the mechanisms and procedures
   described in this document are based, is taken from the CE-based VPN
   reference model described in [FRAMEWORK]. The most important aspects
   of that framework model and the restrictions that are relevant to
   this document are described in this section.

    +---------+  +------------------------------------+  +---------+
    |         |  |                                    |  |         |
    |         |  |                     +------+     +------+  : +------+
+------+ :    |  |                     |      |     |      |  : |  CE  |
|  CE  | :    |  |                     |   P  |     |  PE  |  : |device|
|device| :  +------+    VPN tunnel     |router|     |router|  : |  of  |
|  of  |=:====================================================:=|VPN  A|
|VPN  A| :  |      |                   +------+     +------+  : +------+
+------+ :  |  PE  |                                  |  |    :    |
+------+ :  |router|                                  |  |    :    |
|  CE  | :  |      |              VPN tunnel        +------+  : +------+
|device|=:====================================================:=|  CE  |
|  of  | :  +------+                                |  PE  |  : |device|
|VPN  B| :    |  |                                  |router|  : |  of  |
+------+ :    |  |  +------------+   +------------+ |      |  : |VPN  B|
    |    :    |  |  |  Customer  |   |   Network  | +------+  : +------+
    |Customer |  |  | management |   | management |   |  |    :    |
    |interface|  |  |  function  |   |  function  |   |  |Customer |
    |         |  |  +------------+   +------------+   |  |interface|
    |         |  |                                    |  |         |
    +---------+  +------------------------------------+  +---------+
    | Access  |  |<---------- SP network(s) --------->|  | Access  |
    | network |  |                                    |  | network |

   Figure 1: Reference model for provider provisioned CE-based VPNs

2.1 Entities in the reference model and Terminology

   o Customer Edge (CE) device

      In the context of this solution, a CE device is a router located
      at the edge of a customer site, that has IP connectivity  with a
      SP's PE device (not necessarily Internet connectivity). A CE
      device maintains one or more VPN tunnel endpoints. The VPN-
      specific functions in the CE device are provisioned by the SP.

      Note that other functions that are normally applied by the PE
      router may need to be performed by the CE device in this context



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      (e.g. NAT functionality, QoS classification, etc.). These
      functions may be managed by the SP or alternatively be managed by
      the VPN customer, depending on the applicable service contract.

      The CE device may also provide general (non VPN-oriented) Internet
      connectivity for the customer network. Such connectivity may be
      achieved via the SP's PE router that provides the VPN connectivity
      or some other router (of the same or another SP). In such a
      situation, the CE device must be able to distinguish between
      traffic to be sent through a VPN and traffic to be sent outside
      any VPN. Section 6 of this document discusses this in more
      details.

      CE devices in a CE-based VPN model differ from CE devices in a
      PE-based VPN model in that they need to support VPN-specific
      functions. With CE-based PPVPNs, the VPN awareness is pushed even
      further towards the edges of the provider networks.

   o Provider Edge (PE) router

      In the context of Provider Provisioned CE-based VPNs, a PE router
      is a router, located at the edge of the Service Provider's
      network, that does not have any VPN-specific functionality. A PE
      router is attached via an access connection to one or more CE
      devices, and offers possibly limited or restricted  IP
      connectivity over the access connections to these CE devices.

   o SP network

      A SP network is a network administrated by a single service
      provider. In the context of PP CE-based VPNs, the SP who owns the
      SP network can also be the VPN provider (managing the CE devices).
      This can lead to operational advantages (e.g. for offering QoS).
      Alternatively, the SP owning the SP network may be an ISP offering
      Internet connectivity, while an other entity may provision the VPN
      service. This configuration allows for inter-SP and Internet-wide
      VPN scenarios.

   o Access connection

      An access connection represents a layer 2 connectivity between a
      CE device and a PE router. This includes dedicated physical
      circuits, logical circuits (such as Frame Relay and ATM), IP
      tunnels (e.g., using IPsec, L2TP) and shared medium access (such
      as Ethernet-based access). In the context of provider provisioned
      CE-based VPNs, the CE device and the PE router have layer 3
      connectivity over the Access Connection.




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   o VPN tunnel

      A VPN tunnel is a logical link between two entities which is
      created by encapsulating packets within an encapsulating header
      for purpose of transmission between those two entities for support
      of VPNs. In the context of provider provisioned CE-based VPNs, a
      VPN tunnel is an IP tunnel (e.g., using IPsec [IPSEC], L2TP
      [L2TP], GRE [GRE], IP-in-IP [IPinIP]) between two CE devices over
      the SP's network. In the context of this document, a VPN tunnel is
      achieved using IPsec in tunnel mode or via an IP-in-IP tunnel
      protected by IPsec in transport mode between two CE devices.
      [GRE-CE] describes how to use GRE encapsulation for CE to CE
      tunneling in a CE-based IPsec VPN scenario.

   o Security Association (SA)

      Throughout this document, the acronym SA will be used to denote an
      IPsec Security Association.

2.2 IP connectivity between CE and PE devices

   CE devices operating in a PP CE-based VPN will operate in two
   independent IP routing spaces.

   The first routing space is the VPN routing space. Hosts and routers
   within the VPN will use IP addresses that belong to this VPN routing
   space. The CE router will participate in this VPN routing space, and
   will create VPN tunnels (virtual links) to be used as virtual
   interfaces by this VPN routing space.

   The second routing space is the SP's routing space. Every CE device
   that belongs to a PP CE-based VPN is identified by an IP address that
   is routable in the SP's network. This IP address may be a global IP
   address or a private IP address. The CE device MUST be reachable from
   the SP's core network via this IP address.

   In order to easily differentiate between these two routing spaces,
   this document uses the following convention: IP addresses belonging
   to the VPN's routing realm will be followed by a 'v' between
   brackets: address (v); IP addresses belonging to the service
   provider's routable space will be followed by a 's' between brackets:
   address (s).

   These two routing spaces may use overlapping address spaces and thus
   need to be kept separate in the CE devices. The way this is done is
   largely implementation dependent. This may be by using two separate
   sets of (virtual) routing and forwarding tables (figure 2). These
   routing tables may then run independent routing protocols.



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   Only the CE's IP address (s) needs to be reachable in the provider's
   core network. This means that this approach requires only one IP
   address (s) per CE device to be injected in the core network. A CE
   device should not inject other routes into the SP's network (one
   exception is for Internet Access scenarios, that are discussed in
   section 6). In many cases, this CE's IP address (s) may be an IP
   address assigned by the SP who owns the core network. As such,
   aggregate routes can be distributed by the PE devices into the core
   network.

   The CE device and the PE device may be routing protocol peers in the
   SP's routing space. Alternatively, a default route (s) (towards the
   PE) may be statically configured in the SP's routing space on the CE
   device, and the CE device's IP address (s) statically configured on
   the PE. The CE device should not inject SP's routes (s) towards the
   other routers within its VPN site (except in the context of the
   Internet Access scenarios described in section 6).

   Note that, when the CE device is attached to only one PE device, via
   only one (sub-)interface, the CE's implementation can be fairly
   straightforward (see figure 3). With regards to the SP routing space,
   the CE device then acts as a host, having only one outgoing
   interface. The source IP address (s) (of the _outer_ IP header) of
   all packets leaving the CE device MUST always be the CE's identifier,
   and the IP next hop will always be the PE device to which it is
   attached. On the CE, no routing decisions need to be performed in the
   provider's routing space and only one forwarding action is possible.

   The following figures give an overview of the routing spaces in the
   CE device. Note that this description is merely an example and is not
   meant to specify a particular implementation: every implementation
   that results in the same behaviour described throughout this
   specification is acceptable.

   Section 5 describes the end-to-end processing of customer data-
   packets in more details.















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                     CE device
      ----------------------------------------------------------
     |   ____________    ========   |I|      ______________   __|__ PE_1
     |  |routing and |   ========   |P|     |routing and   | /  |
     |  |forwarding  |   ========   |s|     |forwaring in  |<   |
     |  |in VPN space|   ========   |e| --- |provider space| |__|__ PE_2
     |  |   IP(v)    |   ========   |c|     |    IP(s)     |    |
     |   ------------    = = = =    | |      --------------     |
     |                IP(v)-in-IP(s)                            |
     |__________________________________________________________|

   <- - - - - - - - - - - - -><- - - - - - - - - - - - - - - - - - - - >
           VPN space                           SP space
              figure (2): routing spaces in the CE device
              (''VPN space'' = customer ''private'' space)


      ----------------------------------------------
     |   ____________    ========== to CE_2  |I|    |
     |  |routing and |   ========== to CE_3  |P|    |
     |  |forwarding  |   ========== to CE_4  |s|----|--- PE
     |  |in VPN space|   ========== to CE_5  |e|    |
     |  |   IP(v)    |   ========== to CE_6  |c|    |
     |   ------------    = = = = =           | |    |
     |                  IP(v)-in-IP(s)              |
     |______________________________________________|

   <- - - - - -  - - - - - - ><- - - - - - - - - - - - - - - ->
            VPN space                    SP space
         figure (3): the CE is connected to only one PE device

   Note that there are no routing protocols operating in both routing
   spaces simultaneously. Packets can only go from one routing space to
   the other routing space via either (IP-in-IP) tunneling or after
   firewall and possibly NAT processing (as described in section 6).

   This approach enables the CE devices to reach each other via tunnels
   over the SP's network, but does not prevent the interconnection of CE
   devices via so-called "backdoor routes". CE devices belonging to the
   same VPN MAY be interconnected via "backdoor routes". If "backdoor
   routes" are present in a certain VPN, the VPN's routing protocol
   metrics will dictate which routes will be used as the prefered routes
   for certain destinations.

2.3 Assumed Service Provider's infrastructure

   The service provider maintains a secured VPN database (e.g. on a
   centralized server). One such VPN database may be used for the



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   provisioning of many VPNs. As the number of VPNs to be provisioned
   grows, other servers may be deployed. As such, the scalability of no
   single device is dependent on the total number of VPNs.

   In order to provide a reliable service, the SP may choose to deploy
   backup VPN database servers that it keeps synchronized with the
   primary server.

   The Service Provider's VPN management infrastructure needs to have a
   secure provisioning channel to every attached CE device. This secure
   provisioning channel will be used to exchange VPN-specific
   configuration information between the SP's VPN database and the CE
   devices.

   Note that this document does not prescribe one particular protocol
   for this provisioning channel. Some examples are: SOAP/XML/HTTP/TLS,
   CLI/Telnet/SSH, an IPsec-protected remote configuration protocol,
   etc.

   As the SP will be responsible for provisioning the secure tunnels
   between the CE devices, it needs to deploy a key management system.

3. Configuring the CE-based VPN

   As was noted before, this specification does not describe the
   protocol to use as a remote management protocol to provision CE
   devices. It does however describe with which information CE devices
   need to be pre-provisioned, and which parameters need to be
   configurable via this management protocol by the Service Provider.

3.1 Initializing the VPN database

   As a first step in the VPN configuration process, the Service
   Provider configures its VPN database with a new VPN entry and with
   the IP addresses (s) of the CE devices belonging to the VPN, and with
   a description of the VPN's topology.

   For every CE device, the following information must be configured and
   maintained in the VPN database:

      - the security information that is necessary for the secure remote
      management protocol. This information should allow for mutual
      authentication between CE and SP's VPN server, and for encryption
      of the management data. The detials of this information will
      depend on the particular protocol (stack) used for remote
      management

      - the security information that is necessary for the CE device to



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      establish and maintain Security Associations with its peer CE
      devices belonging to the same VPN; section 3.3 defines which is
      the minimal set of information a CE device should be able to
      retrieve/receive fromt the SP's VPN management server.

3.2 Pre-configuration of the CE device

   This document uses the term "pre-configuration" for the initial
   provisioning of a CE device. This pre-configuration happens before a
   CE is attached to a VPN (before the considered site actively belongs
   to the VPN). This pre-configuration can be performed by the SP before
   shipping the CE device to the customer's premises. Alternatively, the
   SP can pre-provision the CE device manually at the customer's
   premises. Another possibility is for the SP to tell the customer how
   to pre-provision its CE device. Finally other scenarios such as
   remote management with for example secured SNMP are also possible.

   Every CE device participating in a VPN needs to be pre-provisioned
   with the necessary configuration information that enables it to
   establish a secure communication path with the SP's VPN server.

      The CE device must be configured with the IP address (s) of the
      Service Provider's VPN server or with a URL to the required CE's
      VPN information on the Service Provider's VPN database.

      The CE device must be configured with the security information
      required by the SP's secure remote management protocol (stack).

   And finally, the CE device must be ''pre-configured'' with the CE's
   IP address (s) in the SP's space.

   As mentioned before, the CE device is identified by an IP address (s)
   that belongs to the Service Provider's routing space. This IP address
   (s) may be an IP address assigned by the SP and manually configured
   on the CE device, together with the other (pre-) configuration
   information (this would require this IP address (s) to be configured
   as a static route on the attached PE too). Alternatively, the CE may
   dynamically obtain this IP address (s), using for example DHCP or
   IPCP over the CE-PE link. Yet another possibility is that the CE
   device has obtained a (global) IP address (s) from an ISP, and that
   the VPN customer communicates this IP address (s) to the VPN Service
   Provider. Note that the CE device needs to maintain this same IP
   address (s) at least for the duration of its VPN membership.

   Note that other information, such as timer-parameters etc. may be
   configurable by the SP. These parameters can be provisioned by the SP
   at pre-configuration time.




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3.3 Fetching the VPN configuration information

   The VPN service is initialized by the CE device by retrieving the VPN
   configuration information from the SP's VPN database using the
   appropriate secure remote configuration channel.

   The CE device will retrieve from the SP's VPN server the information
   that is necessary to establish IPsec-secured tunnels with the other
   CE devices that belong to the same VPN (and to which it should
   establish a virtual VPN link - dependend on the VPN topology). The SP
   may choose to let the CE devices authenticate the IKE negotiations
   between CE devices using (i) pre-shared keys or (ii) digital
   signatures and certificates. The IPsec implementation on the CE
   devices SHOULD support both modes of authentication.

   (i) in case of pre-shared keys, the following information is to be
   retrieved from the SP's VPN server:

      - a list of <peer CE IP address (s), pre-shared key, SA
      information, tunnel information> tuplets

         (SA information = the necessary information to negotiate a SA
         with the peer CE: security protocol, Diffie-Hellman group,
         IPsec transforms, etc. The (optional) presence of this
         information will overwrite possible default values in the CE)

         (tunnel information : traffic-driven tunnel or 'permanent'
         tunnel; tunnel mode IPsec or transport mode IPsec over an IP-
         in-IP encapsulation; dynamic routing trough the tunnel or not)

   (ii) in case of digital signature authentication, the following
   information is to be retrieved from the SP's VPN server:

      - a <private key, public key> pair

      - a certificate for the public key

      - a public key from the Certificate Authority

      - a list of <peer CE IP address (s), SA information, tunnel
      information> tuplets

   The above information is maintained on the SP's VPN server, and sent
   to the CE device when necessary.

3.4 Establishing the (secure) VPN tunnels/SAs

   When one Site sends traffic to another Site belonging to the same



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   VPN, these IP packets will be secured via IPsec. This means that an
   IPsec Security Association is needed between each pair of sites that
   directly exchange private VPN data with each other.

   The Internet Key Exchange protocol (IKE, [IKE]) or its successor
   IKEv2 [IKEv2] will be used for the purpose of automatic setup of
   security associations between VPN sites within the same VPN. The CE
   devices will use the information that they have retrieved (or
   received) from the SP's VPN server to negotiate SAs with their peers,
   using IKE(v2).

   The succesfull establishment of such a 'VPN' IPsec SA between two CEs
   will result in the auto-configuration of a new VPN tunnel (or virtual
   link) between the two considered CE devices.

   As explained in section 5 of this memo, a 'VPN tunnel' is either an
   IP-in-IP tunnel protected by an IPsec transport mode SA or
   alternatively a tunnel mode IPsec SA. In both cases, the VPN tunnel
   is established once the protecting SA is established.

   These dynamically established SAs can be set-up and maintained
   independently of the presence of actual inter-site user traffic,
   resulting in 'permanent' IPsec tunnels. These tunnels are then always
   available and not traffic-triggered. It is then required to
   frequently re-negotiate the SA (via IKE(v2)) before the IPsec timers
   of the connection time out. The set-up of a 'permanent' IPsec tunnel
   will be triggered by the configuration of a new peer CE device within
   the same VPN. An advantage of this method is that the IPsec tunnel is
   always available, and that eventual traffic does not encounter an
   extra delay due to the setup time of a new SA. The use of 'permanent'
   IPsec tunnels is recommended for CE-based site-to-site VPNs.

   A CE device that first joins a VPN must retrieve the initial VPN
   configuration information from the SP's VPN server. Next, for
   'permanent' IPsec tunnels, the considered CE SHOULD subsequently
   establish "VPN tunnel SAs" (using IKE) with every peer CE device
   listed in the VPN configuration information.

      o if the IKE negotiation is accepted and authentication succeeds,
      the SA is successfuly established.

      o if the IKE negotiation is refused or the authentication fails,
      the IKE negotiation must be stopped and the SA not be established;
      the CE device SHOULD then wait for a time interval larger than a
      certain minimum value (to be configured, depending on e.g. the
      responsiveness of the auto-discovery mechanism) and then try
      negotiating the SA with the considered peer again. After a new
      failure, the CE device SHOULD retry after a certain period of time



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      (t1, to be configured). This process SHOULD continue with
      exponential backoff of t1 until a certain limit (to be configured)
      upon which an alarm MUST trigger human interaction.

   Provider provisioned CE-based IPsec VPNs as described by this
   document SHOULD use 'permanent' IPsec Security Associations when
   dynamic routing through IPsec-secured tunnels is used.

   Alternatively, the IPsec SA setup can be triggered by the effective
   (data) traffic flow going from one site to another. In this case, the
   arrival of data packets at the CE device, coming from within the VPN
   site and going to another VPN site, will be noticed by the CE's IPsec
   or routing database, and an IKE exchange will be initiated to set up
   an IPsec secured connection between both parties. Once the secure
   tunnel is set up, the data packets can flow from one site to the
   other in a secure way. When no traffic flows for a certain duration
   of time, the secure tunnel will be torn down again. An advantage of
   this method is that an IPsec tunnel is only to be maintained when
   there is effectively traffic flowing. A disadvantage is the extra
   delay introduced for the traffic during IKE signaling and the
   difficult interaction with the tunneled inter-site VPN routing
   information distribution.

   Provider provisioned CE-based IPsec VPNs as described by this
   document MAY use traffic-driven IPsec SA establishment when static
   intra VPN inter-site routing is used (no dynamic routing through the
   IPsec tunnels), see section 4.3. Provider provisioned CE-based IPsec
   VPNs as described by this document SHOULD NOT use traffic-driven
   IPsec SA establishment when dynamic site-to-site routing through the
   IPsec-secured tunnels is used.

   The CE configuration determines whether traffic-driven SA
   establishment is used or not, and whether dynamic routing through
   IPsec tunnels is used or not.

   The procedures described in this memo can be used together with
   [IPSEC-DPD] that offers a mechanism to efficiently keep IPsec SAs
   alive.

   Note that IPsec tunnels are unidirectional in nature, but that within
   the application of this specificiation, the set-up of one direction
   MUST be accompanied by the set-up of the reverse direction IPsec
   tunnel.

   This document describes two possible ways to use IPsec in CE-based
   VPN scenarios (see section 5): in 'transport mode' or in 'tunnel
   mode'. The CE configuration, IKE exchange and resulting SA's must
   specify which mode will be used.



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   Note that the number of peer CE devices with which a specific CE
   device must have an IPsec connection to secure the data traffic, is
   dependent on the specific 'role' of the site in the considered VPN. A
   hub CE will for example have a larger number of tunnels to support
   than a spoke device.

3.5 Updating VPN configuration information

   An important requirement for the scalability of L3VPNs is the
   availability of an 'auto-discovery' mechanism. Such an 'auto-
   discovery' mechanism should for example make sure that the
   addition/deletion of a VPN site to/from an existing VPN is possible
   by only configuring the 'new' CE device (and the SP's VPN database):
   the existing VPN sites should automatically 'discover' the new site
   in a reliable and secure manner.

   The precise autodiscovery mechanism and related protocol actions will
   highly depend on the remote management protocol in use. As such this
   document does not describe a specific autodiscovery mechanism, and
   the principles of this document remain interoperable with any
   autodiscovery mechanism.

   The remote management protocol can operate in a 'push' model (when a
   new CE device is added to the VPN, the VPN server pushes the new VPN
   configuration information to all existing CE devices from that VPN),
   in a 'pull' model (CE devices periodically download their VPN
   configuration information from the SP's VPN server, or after
   receiving tunnel establishment requests from unknown CE devices), or
   in a combined mode (the SP's VPN server sends a 'notification' to the
   CE devices that tells them to update their VPN configuration
   information by downloading it from the VPN server). The different
   modes and the applied protocol dynamics will have different
   reliability characteristics.

3.6 Removing an existing VPN site

   When the VPN customer wants to remove an existing site from a certain
   VPN, this customer first informs the VPN SP. The SP will then update
   the VPN database on the centralized server.

   Different approaches can then be used. The SP can provision the
   considered CE device to delete its VPN information and to tear-down
   the IPsec SA's using IKE(v2). After completion of the IKE tear-down
   proces, the peering CE devices must not attempt to re-establish the
   deleted SA. At this stage, the VPN tunnels are actually removed, and
   the routing protocols operating through the tunnels in the VPN's
   routing space will notice the topology change and react
   appropriately. The periodical retrieval of the VPN configuration



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   information from the VPN database by the other CE devices will then
   make sure that the removed CE's information is no longer available.
   The discussed provisioning action can happen in the same way as the
   pre-provisioning action described in section 3.1, i.e. via manual
   configuration, via remote management or via interaction with the
   customer.

   Alternatively, the SP will not provision the to-be-removed CE
   individually but the removal of the information relevant to the
   considered CE from the VPN database will ultimately automatically
   result in the removal of the CE from the VPN: peer CEs will notice
   the removal of the particular CE from their updated configuration
   file and will tear-down the appropriate SA using IKE(v2); the
   deletion of active SAs will effectively remove the VPN tunnels and
   the routing protocols running through the VPN tunnels will discover
   the topology changes and react accordingly. The to-be-removed CE will
   not be able to retrieve VPN information from the VPN database and
   will delete all its VPN information and try to tear-down the
   remaining SAs.

4. Exchanging and maintaining VPN routes

   One of the requirements for PP CE-based VPNs is that dynamic routing
   is not only supported within individual VPN sites, but also between
   the different VPN sites of a specific VPN. This means that when a
   change in the routing information in a specific site occurs, the
   other sites that belong to the same VPN must be notified of that
   change.

   This section deals with the exchange of routing information in the
   customer VPN's routing space (v). As depicted in figure 4, this
   exchange of routing information happens over the VPN tunnels and is
   as such transparant for the SP's network. CE devices MUST NOT leak
   VPN routes into the SP's network and MUST NOT leak routes from the
   SP's routing space into the VPN sites, unless explicitly configured
   to do so (as e.g. explained in section 6 of this document).















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                        routing adjacency (VPN space)
         ______________________________________________________
CE_1    |                                                      |   CE_2
 -------|-----------------                      ---------------|-------
|  _____V____   ===  |I|  |                    | |I| ===   ____V_____  |
| |routing/  |=======|P|==|===== VPN tunnel ===|=|P|======|routing/  | |
| |forwarding|  ===  |s|  |                    | |s| ===  |forwarding| |
| |VPN space |  ===  |e|  |-- PE - core - PE --| |e| ===  |VPN space | |
|  ----------   =====|c|==|=                   | |c| = =   ----------  |
|            IP-in-IP     | ||                 |    IPinIP             |
|_________________________|   == to CE_3        -----------------------

--- VPN space ---><--------- SP's routing space ------><-- VPN space --

     figure 4: tunnelled routing adjacency in the VPN routing space

   This document assumes that the routing within a VPN site is
   controlled by the VPN customer, and thus is outside of the scope of
   this specification.

4.1 The CE device and VPN routing

   On the customer network side, a CE router connects to internal
   networks of an enterprise, where one or more subnets can reside. Many
   times, the CE router may interact with another internal router. And
   sometimes, "backdoor links" between routers of different sites of the
   same VPN exist.

   In the VPN routing space (v), the CE is involved in (i) the intra-
   site routing, (ii) the VPN tunnel termination, and (iii) the inter-
   site VPN routing.

   The CE device could be an integrated device providing both routing
   and IPsec tunnel termination. Sometimes, a dedicated VPN terminator
   may be used. Implementations in which the VPN tunnel terminator
   resides on a firewall are also very common. For the sake of
   simplicity, we assume that the CE router is an integrated device that
   participates in the intra-site routing (e.g. via an IGP) and at the
   same time terminates VPN tunnels.

   In the context of this document, the routing aspects within a VPN
   site (intra-site routing information distribution) are controlled by
   the VPN customer.

   As was explained earlier, the SP's dynamic VPN discovery scheme and
   tunnel establishment mechanism provides the CE device with secure
   (virtual) links towards other CE devices in the same VPN. Whether the
   intra-VPN inter-site routing aspects that make use of these virtual



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   links are managed by the customer or by the SP is dependent on the
   service contract. In many situations, the SP will configure the
   necessary routing protocol information at pre-configuration time (see
   section 3.1), in close colaboration with the customer.

   An important requirement for the routing protocol implementation that
   is configured to exchange reachability information through the
   inter-site tunnels, is that it must be able to autonomously deal with
   dynamically created new inter-site links.

4.2 IPsec and routing

   IPsec is a layer 3 security protocol, which operates purely at the IP
   layer. The IETF IPsec Working Group currently specifies the revisions
   of the IPsec standards ([IPSEC], [RFC2402], [RFC2406], [RFC2407],
   [RFC2408], and [IKE]). The interaction between IPsec and layer 3
   routing was often troublesome and has been described in [TOUCH],
   [KNIGHT] and [DUFFY]. Depending on individual implementations,
   difficulty may arise when an IPsec user wants to support robust
   routing across IPsec-interconnected VPNs sites.

   Details regarding the problems of the interaction between VPN routing
   and VPN tunneling, and some proposed solutions to counter these
   issues, can be found in [TOUCH], [DUFFY] and [KNIGHT].

4.3 Exchanging VPN routes between VPN sites

   In the proposed mechanism to exchange VPN reachability information
   between VPN sites, routing protocol messages are tunneled through the
   IPsec-secured tunnels between peering sites. The CE-to-CE IPsec-
   secured tunnels between VPN sites are then being seen as point-to-
   point links by the customer networks and are interpreted as such by
   the routing protocol functions of the CE devices. This means that
   when a change in the reachability occurs in one particular site, a
   routing protocol (such as RIP, OSPF, etc.) will take care of the
   distribution of the new reachability information within the site, but
   also to all other sites, through the VPN tunnels that the considered
   CE is possibly maintaining.

   As the described architecture allows for the dynamic creation of
   inter-site (IPsec-protected) VPN links, the routing protocol
   implementation(s) operating on the CE device MUST be able to support
   this.

   Although very often it will be the SP's responsibility to configure
   the CE's routing information at pre-configuration time, the service
   aggreement MAY specify that routing on the CE device falls under the
   customer's management.



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   When routing protocol messages are tunneled through the IPsec-secured
   tunnels ('dynamic routing through IPsec-secured tunnels') as per this
   section, IPsec transport mode secured IP-in-IP tunnels as per [TOUCH]
   SHOULD be used (in contrast to IPsec tunnel mode tunnels).

   Note that other approaches may use a combination of dynamic routing
   and IPsec tunnel mode tunnels, though it is not clear how
   interoperability will then be assured.

   Another issue to consider is the fact that using a traffic-driven
   tunnel establishment mechanism and at the same time using an approach
   whereby a routing protocol (with a keep-alive mechanism) runs on top
   of the VPN tunnel, does not seem currently applicable: the delay
   introduced by the tunnel establishment phase could lead to a loss of
   routing updates and the routing protocol's keep-alive mechanism could
   interact with the tunnel establishment in an undesired way.
   Therefore, when dynamic routing is used through IPsec-secured CE-to-
   CE tunnels, traffic-driven SA establishment SHOULD NOT be used.

5. Tunneling IP traffic (user data) among VPN sites

   This section describes the processes that an IP packet that is sent
   from one VPN site to another will go through. This is dependent on
   the way that IPsec is used. This document describes two possible ways
   to use IPsec in CE-based VPN implementations: IPsec in tunnel mode,
   and IPsec in transport mode.

   An IP packet that is sent by an IP device in a certain site and
   destined for an IP device in another site belonging to the same VPN,
   will be forwarded as follows.

   The device in the sending site sends an IP packet (possibly using a
   private address space) on its LAN network. The next hop for this
   destination IP address will (at some point in time) be the site's CE
   device (according to the routing/forwarding in the VPN site). The
   processing by the CE device now is dependent on the implemented mode
   for IPsec.

   The use of IPsec in tunnel mode has the advantage that the complete
   range of SPD-checks remain usuable, but has the disadvantage that
   dynamic routing through the tunnels is not supported. The use of
   IPsec in transport mode secured IP-in-IP tunnels has the advantage
   that dynamic routing through the tunnels is fully supported, but has
   the disadvantage that the complete range of SPD-checks is not
   supported.

   Note that the following description is not meant to specify an
   implementation strategy; any implementation procedure wich produces



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   the same results is acceptable.

   o IPsec in transport mode (see also [TOUCH] for a detailed
      specification)

      When IPsec is used in transport mode in this context, the CE
      device must first analyze the private IP packets arriving from
      within the site and select the appropriate outgoing interface and
      required encapsulation, based on the VPN routing/forwarding
      information. For a destination located in an other site, the
      outgoing interface will be a virtual interface (a VPN tunnel) and
      the required encapsulation will be IP-in-IP, using the considered
      CE's IP address (s) as the source address in the outer IP
      encapsulation header and a peer CE's IP address (s) in the outer
      IP encapsulation header's destination address field. The CE device
      then processes this new IP packet to its IPsec driver.

      The IPsec driver in the CE device must then do the following:

      - analyze the IP packets that have been IP-in-IP encapsulated and
      select the appropriate SA (based on the packet's outer header
      destination address (s)).

      - authenticate and/or encrypt the private IP packet according to
      the (transport mode-specific) rules described in the SA and insert
      an appropriate IPsec header (according to IPsec in transport
      mode).

   o IPsec in tunnel mode

      When IPsec is used in tunnel mode in this context, the IPsec
      driver in the CE device must do the following:

      - analyze the private IP packets arriving from within the site and
      select/setup an appropriate SA with the appropriate destination CE
      device.

      - authenticate and/or encrypt the private IP packet according to
      the (tunnel mode-specific) rules described in the SA, AND
      encapsulate the packet in an IPsec header AND encapsulate the
      packet in a new 'outer' IP header. This 'outer' IP header has the
      CE's non-private (i.e. routable in the SP's realm) IP address in
      the source IP address field and the destination CE's non-private
      (i.e. routable in the SP's realm) IP address in the destination IP
      address field.


   The CE device then sends the IPsec packet to the PE device, and the



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   IPsec packet will then be forwarded using 'normal' IP forwarding
   across the SP's network, based on the outer header's IP destination
   address (s), that is the destination CE's 'global' (i.e. routable in
   the SP's realm) IP address. The packet will be forwarded to the
   egress PE who will also only examine the outer IP header and send the
   IP(sec) packet to the destined CE device. The egress CE device will
   recognize itself as the destination node (the IP packet has the CE's
   IP address (s) in the outer IP destination address field) and process
   the IPsec packet to the IPsec driver that will then, based on the
   appropriate Security Association (identified by the packet's SPI
   field in the IPsec header), perform IPsec authentication and/or
   decryption. Dependent on whether tunnel mode or transport mode IPsec
   is used, the packet will be decapsulated by the IPsec driver itself
   or sent to the IP-in-IP decapsulation function. The resulting
   (private) IP packet (v) will then be further processed in the CE's
   VPN IP forwarding table and send on the LAN network to the
   appropriate next hop router or destination IP device.

   Note that IPsec tunnels might unintentionally terminate or break.
   When dynamic routing is not supported through the inter-site VPN
   tunnels, this may have serious consequences if VPN membership and VPN
   routing information are not changed accordingly within the VPN.
   Indeed, the unnoticed termination of a VPN tunnel can result in the
   creation of black holes.

   This means that a mechanism must exist to monitor the state of the
   VPN tunnels. When dynamic inter-site VPN routing is used, the routing
   protocol that runs on top of the IPsec VPN tunnels will serve that
   purpose. When dynamic inter-site routing is not used, the following
   alternatives are possible: (i) the use of an IPsec-specific keep-
   alive mechanism [IPSEC-DPD] and (ii) a SP-proprietary mechanism.

6. CE-based VPN and Internet

6.1 Allowing both VPN connectivity and Internet connectivity

   In many VPNs, sites will need to both access the public Internet as
   well as to access other sites within the same VPN.

   In order to achieve this, some sites within the VPN will obtain
   Internet Access by means of an "Internet Gateway" that is attached
   via one of its interfaces to an ISP's PE device. Such an Internet
   Gateway may for example be a firewall and may need to implement
   network address translation functions. The ISP may be the same SP
   that offers the VPN service, or it may be a different SP. The PE to
   which the Internet Gateway is connected may be the same PE to which
   the CE is connected or it may be an other PE.




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   The Internet Gateway may be a separate device, or alternatively the
   Internet Gateway functions may be integrated into the CE device. When
   the Internet Gateway functions are integrated into the CE device, the
   CE-PE interface used by the Internet Gateway functions may be the
   same or a different interface than the interface used by the VPN
   tunnels. In further discussions, we'll assume that the Internet
   Gateway is a separate device.

   The service contract will define whether the Internet Gateway will be
   managed by the SP or by the VPN customer.

   Note that when Internet Access is offered within a VPN, the address
   spaces used within the VPN must be non-overlapping. This means that
   the VPN SHOULD either use global addresses that have been assigned to
   the VPN customer, or private addressing in combination with NAT
   [NAT].

   The sites that have Internet Access via an Internet Gateway will have
   a default route (v) pointing to their Internet Gateway and may be
   distributing a default route via their CE towards the other CEs of
   the same VPN through the VPN tunnels. This provides Internet Access
   for all the VPN sites. Note that other sites (that don't have their
   own Internet Gateway) must not distribute default routes in this
   scenario. A site that has distributed a default route to other sites
   for Internet Access should have either a default route to its
   Internet Gateway or Internet routes (leading to its Internet Gateway)
   in its forwarding table (of the VPN routing space).
























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                                      VPN site <---- :
                                                     :
                          ----------  to Internet    :
                    _____| Internet |----------------:-- PE_2
                   |     | Gateway  |                :
                   |      ----------                 :
           --------|---------------------------      :
          |     default                        |     :
          |      route              to         |     :
          |   _____|______                     |     :
          |  |            |   ===== CE2 |I|    |     :
      ----|--|routing and |   ===== CE3 |P|    |     :
          |  |forwarding  |   ===== CE4 |s| -->|-----:-- PE_1
      ----|--|in VPN space|   ===== CE5 |e|    |     :
          |   ------------    = = =     |c|    |     :
          |                  IP-in-IP          |     :
          |______CE device_____________________|     :
     <---                                            :
      intra                                          :
      site                                           :
                                                     :
                figure 5: Internet Access from within a VPN

   The Internet Gateway will process (e.g. firewall + NAT) all traffic
   coming from within the VPN and, if accepted, send it to the PE with
   which it interfaces. As such the Internet Gateway effectively is the
   device that interfaces between the VPN routing space and the
   SP's/Internet routing space. Note that traffic that leaves a VPN via
   an Internet Gateway will not be IP-in-IP encapsulated and will not be
   IPsec processed. The traffic coming from the gateway will then be
   forwarded according to the PE's (default/Internet) forwarding table.

   In order to allow for traffic in the reverse direction (from the
   Internet to the VPN sites), the ISP connected to the Internet Gateway
   must distribute, to the Internet, routes that lead to addresses that
   are within the VPN. NAT-like techniques may also be used. As such
   there will be routes that will lead from the Internet to the site's
   Internet Gateway. The Internet Gateway will process traffic coming
   from the Internet and, if accepted, send it into the VPN site where
   intra-VPN routing and forwarding will lead the packets to their
   destination. This distribution of routes that lead to addresses
   within the VPN towards the Internet is independent of any intra-VPN
   route distribution as described elsewhere within this specification.
   Note also that normally the internal structure of the VPN will remain
   invisible to the outside world.

   When the Internet Gateway functions are implemented in the CE device
   and the CE device is attached via only one (sub-)interface towards



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   only one PE device, inspection of the packets coming from the PE will
   indicate whether the concerned traffic is intra-VPN traffic (when the
   packet is an IPsec packet with the CE device's own IP address (s) in
   the outer header's destination address field and the encapsulated
   payload is an IP-in-IP encapsulated private IP packet (v), and a
   matching SA is found), or control-plane traffic (IKE(v2) or VPN
   remote management traffic: when the inspected packets conform to the
   control plane's policies), or VPN <--> Internet traffic (then the
   Internet Gateway function will decide whether the considered packets
   will be accepted, (translated), and forwarded or not).

   In the above discussed procedures, some sites will access the
   Internet via a VPN tunnel that leads to another site of the same VPN,
   because they don't have an own Internet Gateway, and will forward the
   traffic according to the default route. Ultimately though, Internet
   traffic will always go via an Internet Gateway before
   entering/leaving a VPN.

   Further note that the PE to which the Internet Gateway is attached
   doesn't necessarily need to carry all the Internet routes; a default
   route to an other Internet router suffices.

6.2 Prohibiting or restricting Internet connectivity from within a CE-
   based VPN

   In the approach described in this document, the CE device sends IP
   packets (s) to the VPN-unaware PE device and receives IP packets from
   that PE device. The PE device forwards these packets based on the IP
   addresses (s) in the (outer) IP header. The packets received by the
   PE are as such either packets that are routable within the SP's
   private scope, or either in the public Internet's scope. This section
   discusses the implications hereof with regards to security and access
   control.

   o traffic that the CE sends to the PE

      Following the procedures described in this document, three types
      of 'VPN' traffic can be sent by the CE device towards the PE
      device:

      (i) customer VPN traffic: intra-VPN traffic sent from one VPN site
      to an other VPN site; these packets will always have the sending
      CE's IP address (s) in the IP header's source IP address field,
      the IP address (s) of a peer CE device of the same VPN in the IP
      header's destination IP address field, and will always contain an
      IPsec header;

      (ii) secure remote management traffic: this comprises both the



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      traffic to establish the secure management channel (e.g. IPsec or
      secure TLS) and the traffic to download the VPN configuration
      file; these packets will always have the CE's IP address (s) in
      the IP header's source IP address field;

      (iii) IKE(v2) traffic: the IP packets sent between CE devices in
      order to establish SAs; these packets will always have the CE's IP
      address (s) in the IP header's source IP address field.

   o traffic that the CE receives from the PE

      Following the procedures described in this document, the same
      three types of traffic can be received by the CE device from the
      PE device. As such, the CE device should perform the following
      actions:

      + for IP packets that have the CE's own IP address (s) in the
      outer IP header's destination address field and that have an IPsec
      header: process the packets through the CE router's IPsec deamon
      where conformance with an existing SA will be checked, and the
      packets further processed;

      + for IKE(v2) packets that have the CE's own IP address (s) in the
      outer IP header's destination address field: process according to
      the tunnel establishment procedures described in this
      specification;

      + for IP packets that have the CE's own IP address (s) in the
      outer IP header's destination address field and that correspond to
      secured management traffic: process according to the VPN secure
      remote management procedures, which will depend on the used
      management protocols;

      + for CE devices that have an integrated Internet Gateway role:
      process all other packets to the Internet Gateway module;

      + for CE devices that don't have an integrated Internet Gateway
      role: drop all other IP packets, unless explicitly allowed by
      complementary procedures that are out of scope of this memo.

   o SP's control over CE initiated traffic

      Note that with this specification's concepts, the PE device that
      receives traffic from a CE device has no means to verify whether
      the received traffic is intra-VPN traffic, or traffic that is sent
      to for example an other VPN or e.g. to the Internet.

      From a VPN data privacy point of view, this has no implications,



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      as the security is enforced at the CE devices themselves: traffic
      that doesn't conform to the security associations or other policy
      rules will be dropped at the CE.

      One remaining issue is that customers might use CE devices (that
      have been granted VPN access) to access services they have not
      been granted access for, via the PE device. Although this would
      possibly compromise the security of the customer's own VPN, the SP
      may want to deploy measures to prevent this without bringing full
      VPN knowledge to the PE. One way of doing this would be by using
      specific IP address ranges for VPN purposes and to have specific
      access lists configured on the PE devices (this has inter-SP and
      Internet transparency issues though). Note that maintaining, at
      every PE, a list of <CE device IP address, VPN-ID> would add a
      considerable management burden and is as such not advised. Another
      strategy for the SP would be not to care about the particularities
      of the traffic and treat it as it treats public Internet traffic
      (and as such to only control the total of the resources consumed
      by particular access connections).

      Taking into consideration that in many cases, VPNs will also need
      to be able to access the public Internet, and that the above
      problem does not seem to be an important thread for the SP nor the
      VPN customer, this issue is not considered as a major drawback for
      the deployment of the discussed VPN approach.

7. Security Considerations

   The security aspects of what is presented in this document are
   implicitly discussed in most of the sections. This draft is for a
   large part focussing on security aspects.

   Note that the security of the mechanisms presented here is highly
   dependent on the following factors:

      - the security of the 'management channel', used by the management
      protocol to configure the VPN CE devices.

      - the security of the site and of the CE-device itself

      - the security aspects of the credentials: the IPsec credential
      must be generated, provisioned, updated, and stored securely

      - for a VPN with a complex topology, every tunnel must use the
      same grade of security strength, otherwise, a single weak link
      degrades the whole VPN





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8. Acknowledgements

   The authors would like to thank the following persons for their
   valuable contributions to this document: Lars Eggert, Brian Gleeson,
   Archana Khetan, Sankar Ramamoorthi, Eric Rosen, Michael Choung Shieh,
   Joe Touch, Eric Vyncke, S. Felix Wu, Yu-Shun Wang, Cliff Wang, Alex
   Zinin.

9. References

   [DUFFY] Duffy, M., "Framework for IPsec Protected Virtual Links for
   PPVPNs", draft-duffy-ppvpn-ipsec-vlink-00.txt, October 2002, Work in
   Progress

   [FRAMEWORK] Callon, R. et al., "A Framework for Provider Provisioned
   Virtual Private Networks", draft-ietf-ppvpn-framework-0x.txt, Work in
   progress

   [GRE] Farinacci, D. et al., "Generic Route Encapsulation", March
   2000, RFC 2784

   [GRE-CE] Khetan, A., et al., "Use of GRE for routing support in IPsec
   VPNs", draft-khetan-sp-greipsec, Work in progress

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

   [IPinIP] Perkins, C., "IP encapsulation within IP", October 1996, RFC
   2003

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

   [IPSEC-DPD] Huang, G., Beaulieu, S., Rochefort, D., "A Traffic-Based
   Method of Detecting Dead IKE Peers", draft-ietf-ipsec-dpd-0x.txt,
   Work in progress

   [KNIGHT] Knight, P., Gleeson, B., "A Method to Signal and Provide
   Dynamic Routing in IPsec VPNs", draft-knight-ppvpn-ipsec-dynroute,
   Work in progress

   [LEE-DHCP] Lee, C.Y., "Customer Equipment Auto-configuration", March
   2002, work in progress

   [L2TP] Lau, J., et al., "Layer Two Tunneling Protocol (Version 3)",
   draft-ietf-l2tpext-l2tp-base-0x.txt, Work in progress

   [NAT] Srisuresh, P., Egevang, K., "Traditional IP Network Address



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   Translator (Traditional NAT)", January 2001, RFC 3022

   [RFC2402] Kent, S., Atkinson, R., "IP Authentication Header",
   November 1998, RFC 2402

   [RFC2406] Kent, S., Atkinson, R., "IP Encapsulating Security Payload
   (ESP)", November 1998, RFC 2406

   [RFC2407] Piper, D., "The Internet IP Security Domain of
   Interpretation for ISAKMP" November 1998, RFC 2407

   [RFC2408] Maughan, D., et al., "Internet Security Association and Key
   Management Protocol (ISAKMP)", November 1998, RFC 2408

   [TOUCH] Touch, J. and Eggert, L., "Use of IPSEC transport mode for
   Virtual Networks", draft-touch-ipsec-vpn-0x.txt, Work in progress

10. Authors' Addresses

   Jeremy De Clercq
   Alcatel
   Fr. Wellesplein 1, 2018 Antwerpen, Belgium
   E-mail: jeremy.de_clercq@alcatel.be

   Olivier Paridaens
   Alcatel
   Fr. Wellesplein 1, 2018 Antwerpen, Belgium
   E-mail: olivier.paridaens@alcatel.be

   Cliff Wang
   E-mail: cliff.wang@us.army.mil




















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