Luyuan Fang (editor)

                                                   Michael Behringer

                                                         Ross Callon

                                                       Fabio Chiussi
                                                 Lucent Technologies

                                                    Jeremy De Clercq

                                                          Mark Duffy
                                                 Quarry Technologies

   L3VPN WG                                             Paul Hitchen

   Internet Draft                                        Paul Knight
                                                     Nortel Networks
   Expires: August 2004                               February 2004

        Security Framework for Provider Provisioned Virtual Private

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-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in

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   The list of current Internet-Drafts can be accessed at
   The list of Internet-Draft Shadow Directories can be accessed at


   This draft addresses security aspects pertaining to Provider
   Provisioned Virtual Private Networks (PPVPNs). We first describe
   the security threats that are relevant in the context of PPVPNs,
   and the defensive techniques that can be used to combat those
   threats. We consider security issues deriving both from malicious
   behavior of anyone and from negligent or incorrect behavior of the
   providers. We also describe how these security attacks should be
   detected and reported. We then discuss the possible user
   requirements in terms of security in a PPVPN service. These user
   requirements translate into corresponding requirements for the
   providers. In addition, the provider may have additional
   requirements to make its network infrastructure secure to a level
   that can meet the PPVPN customer's expectations. Finally, we define
   a template that may be used to analyze the security characteristics
   of a specific PPVPN technology and describe them in a manner
   consistent with this framework.

Table of Contents

   Status of this Memo...............................................1
   Conventions used in this document.................................3
   1. Introduction..................................................3
   2. Terminology...................................................4
   3. Security Reference Model......................................5
   4. Security Threats..............................................7
   4.1.  Attacks on the Data Plane..................................8
   4.2.  Attacks on the Control Plane...............................9
   5. Defensive Techniques for PPVPN Service Providers.............11
   5.1.  Cryptographic techniques..................................12
   5.2.  Authentication............................................19
   5.3.  Access Control techniques.................................21
   5.4.  Use of Isolated Infrastructure............................25
   5.5.  Use of Aggregated Infrastructure..........................25
   5.6.  Service Provider Quality Control Processes................26
   5.7.  Deployment of Testable PPVPN Service......................26
   6. Monitoring, Detection, and Reporting of Security Attacks.....27
   7. User Security Requirements...................................28
   7.1.  Isolation.................................................28

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   7.2.  Protection................................................29
   7.3.  Confidentiality...........................................30
   7.4.  CE Authentication.........................................30
   7.5.  Integrity.................................................30
   7.6.  Anti-Replay...............................................30
   8. Provider Security Requirements...............................30
   8.1.  Protection within the Core Network........................31
   8.2.  Protection on the User Access Link........................32
   8.3.  General Requirements for PPVPN Providers..................34
   9. Security Evaluation of PPVPN Technologies....................34
   9.1.  Evaluating the Template...................................35
   9.2.  Template..................................................35
   10.  Security Considerations.....................................38
   11.  Acknowledgement.............................................38
   Author's Addresses...............................................40
   Full Copyright Statement.........................................41

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [1].

1. Introduction

   Security is clearly an integral aspect of Provider Provisioned
   Virtual Private Network (PPVPN) services.

   The motivation and rationale for both Provider Provisioned Layer-2
   VPN and Provider Provisioned Layer-3 VPN services are provided by
   [L3VPN-FW] and [L3VPN-REQ].

   [L3VPN-FW] and [L3VPN-REQ] acknowledge that security is an
   important and integral aspect of PPVPN services. Security is a
   concern for both VPN customers and VPN Service Providers. Both will
   benefit from a PPVPN Security Framework document that lists the
   customer's and provider's security requirements related to PPVPN
   services, and that can be used to assess how much a particular
   technology protects against security threats and fulfills the
   security requirements.

   In this document, we first describe the security threats that are
   relevant in the context of PPVPNs, and the defensive techniques
   that can be used to combat those threats. We consider security
   issues deriving both from malicious or incorrect behavior of users
   and other parties and from negligent or incorrect behavior of the

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   providers. An important part of security defense is the detection
   and report of a security attack, which is also addressed in this

   We then discuss the possible user and provider security
   requirements in a PPVPN service. The users have expectations that
   need to be met on the security characteristics of a VPN service.
   These user requirements translate into corresponding requirements
   for the providers in order to offer the service. Furthermore,
   providers have security requirements to protect their network
   infrastructure, and make it secure to the level required to provide
   the PPVPN services, in addition to other services.

   Finally, we define a template that may be used to describe the
   security characteristics of a specific PPVPN technology in a manner
   consistent with the security framework described in this document.
   It is not within the scope of this document to analyze the security
   properties of specific technologies; instead, our intention with
   this template is to provide a common tool, in the form of a check
   list, that may be used in other documents dedicated to an in-depth
   security analysis of individual PPVPN technologies to describe
   their security characteristics in a comprehensive and coherent way,
   and to provide a common ground for comparison between different

   It is important to clarify that, in this document, we limit
   ourselves to describing the users and providers' security
   requirements that pertain to PPVPN services. It is not our
   intention, however, to formulate precise "requirements" on each
   specific technology in terms of defining the mechanisms and
   techniques that must be implemented to satisfy such users and
   providers' requirements.

   This document is organized as follows. In Section 2, we
   define the terminology used in the document. In section 3, we
   define the security reference model for security in PPVPN networks,
   which we use in the rest of the document. In Section 4, we describe
   the security threats that are specific of PPVPNs. In Section 5, we
   review defense techniques that may be used against those threats.
   In Section 6, we describe how attacks may be detected and reported.
   In Section 7, we discuss the user security requirements that apply
   to PPVPN services. In Section 8, we describe additional security
   requirements that the provider may have in order to guarantee the
   security of the network infrastructure to provide PPVPN services.
   In Section 9, we provide a template that may be used to describe
   the security characteristics of specific PPVPN technologies.
   Finally, in Section 10, we discuss security considerations.

2. Terminology

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   This document uses PPVPN-specific terminology. Definitions and
   details about PPVPN-specific terminology can be found in [PPVPN-
   term] and [L3VPN-FW]. The most important definitions are repeated
   in this section, for other definitions the reader is referred to
   [PPVPN-term] and [L3VPN-FW].

   CE: Customer Edge device. A Customer Edge device is a router or a
   switch in the customer network interfacing with the Service
   ProviderÆs network.

   P: Provider Router. The Provider Router is a router in the Service
   ProviderÆs core network that does not have interfaces directly
   towards the customer. A P router is used to interconnect the PE
   routers. A P router does not need to maintain VPN state, and is
   thus VPN unaware.

   PE: Provider Edge device. The Provider Edge device is the equipment
   in the Service ProviderÆs network that interfaces with the
   equipment in the customerÆs network.

   PPVPN: Provider Provisioned Virtual Private Network. A VPN that is
   configured and managed by the Service Provider (and thus not by the
   customer itself).

   SP: Service Provider.

   VPN: Restricted communication between a set of sites, making use of
   an IP backbone which is shared by traffic that is not going to or
   coming from those sites.

3. Security Reference Model

   This section defines a reference model for security in PPVPN

   A PPVPN core network is defined here as the central network
   infrastructure (P and PE routers) over which PPVPN services are
   delivered. A PPVPN core network consists of one or more SP
   networks. All network elements in the core are under the
   operational control of one or more PPVPN service providers. Even if
   the PPVPN core is provided by several service providers, towards
   the PPVPN users it appears as a single zone of trust. However,
   several service providers providing together a PPVPN core still
   need to secure themselves against the other providers. PPVPN
   services can also be delivered over the Internet, in which case the
   Internet forms a logical part of the PPVPN core.

   A PPVPN user is a company, institution or residential client of the
   PPVPN service provider.

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   A PPVPN service is a private network service made available by a
   service provider to a PPVPN user.  The service is implemented using
   virtual constructs built on a shared PPVPN core network.  A PPVPN
   service interconnects sites of a PPVPN user.

   Extranets are VPNs in which multiple sites are controlled by
   different (legal) entities. Extranets are another example of PPVPN
   deployment scenarios where restricted and controlled communication
   is allowed between trusted zones, often via well-defined transit

   This document defines each PPVPN as a trusted zone, and the PPVPN
   core as another trusted zone. A primary concern is about security
   aspects that relate to breaches of security from the "outside" of a
   trusted zone to the "inside" of this zone. Figure 1 depicts the
   concept of trusted zones within the PPVPN framework.

   +------------+                             +------------+
   | PPVPN      +-----------------------------+      PPVPN |
   | user           PPVPN                             user |
   | site       +---------------------XXX-----+       site |
   +------------+  +------------------XXX--+  +------------+
                   |   PPVPN core     | |  |
                   +------------------| |--+
                                      | |
                                      | +------\
                                      +--------/  Internet

   Figure 1: The PPVPN trusted zone model

   In principle the trusted zones should be separate, however, often
   PPVPN core networks also offer Internet access, in which case a
   transit point (marked with "XXX" in the figure) is defined.

   The key requirement of a "virtual private" network (VPN) is that
   the security of the trusted zone of the VPN is not compromised by
   sharing the core infrastructure with other VPNs.

   Security against threats that originate within the same trusted
   zone as their targets (for example, attacks from a user in a PPVPN
   to other users within the same PPVPN, or attacks entirely within
   the core network) is outside the scope of this document.

   Also outside the scope are all aspects of network security which
   are independent of whether a network is a PPVPN network or a
   private network (for example, attacks from the Internet to a web-
   server inside a given PPVPN will not be considered here, unless the
   way the PPVPN network is provisioned could make a difference to the
   security of this server).

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4. Security Threats

   This section discusses the various network security threats that
   may endanger PPVPNs.  The discussion is limited to those threats
   that are unique to PPVPNs, or that affect PPVPNs in unique ways.

   A successful attack on a particular PPVPN or on a service
   provider's PPVPN infrastructure may cause one or more of the
   following ill effects:

    - Observation, modification, or deletion of PPVPN user data.
    - Replay of PPVPN user data.
    - Injection of non-authentic data into a PPVPN.
    - Traffic pattern analysis on PPVPN traffic.
    - Disruption of PPVPN connectivity.
    - Degradation of PPVPN service quality

   It is useful to consider that threats, whether malicious or
   accidental, to a PPVPN may come from different categories of
   sources.  For example they may come from:

   - Users of other PPVPNs provided by the same PPVPN service
   - The PPVPN service provider or persons working for it.
   - Other persons who obtain physical access to a service provider
   - Other persons who use social engineering methods to influence
   behavior of service provider personnel.
   - Users of the PPVPN itself, i.e. intra-VPN threats.  (Such threats
   are beyond the scope of this document.)
   - Others i.e. attackers from the Internet at large.

   In the case of PPVPNs, some parties may be in more advantaged
   positions that enable them to launch types of attacks not available
   to others.  For example users of different PPVPNs provided by the
   same service provider may be able to launch attacks that those
   completely outside the network cannot.

   Given that security is generally a compromise between expense and
   risk, it is also useful to consider the likelihood of different
   attacks occurring.  There is at least a perceived difference in the
   likelihood of most types of attacks being successfully mounted in
   different environments, such as:

    - In a PPVPN contained within one service provider's network
    - In a PPVPN transiting the public Internet

   Most types of attacks become easier to mount and hence more likely
   as the shared infrastructure via which VPN service is provided
   expands from a single service provider to multiple cooperating

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   providers to the global Internet.  Attacks that may not be of
   sufficient likeliness to warrant concern in a closely controlled
   environment often merit defensive measures in broader, more open

   The following sections discuss specific types of exploits that
   threaten PPVPNs.

4.1.    Attacks on the Data Plane

   This category encompasses attacks on the PPVPN user's data, as
   viewed by the service provider.  Note that from the PPVPN user's
   point of view, some of this might be control plane traffic, e.g.
   routing protocols running from PPVPN user site to PPVPN user site
   via an L2 PPVPN.

4.1.1.  Unauthorized Observation of Data Traffic

   This refers to "sniffing" VPN packets and examining their contents.
   This can result in exposure of confidential information.  It can
   also be a first step in other attacks (described below) in which
   the recorded data is modified and re-inserted, or re-inserted as-

4.1.2.  Modification of Data Traffic

   This refers to modifying the contents of packets as they traverse
   the VPN.

4.1.3.  Insertion of Non-Authentic Data Traffic: Spoofing and Replay

   This refers to the insertion (or "spoofing") into the VPN of
   packets that do not belong there, with the objective of having them
   accepted by the recipient as legitimate.  Also included in this
   category is the insertion of copies of once-legitimate packets that
   have been recorded and replayed.

4.1.4.  Unauthorized Deletion of Data Traffic

   This refers to causing packets to be discarded as they traverse the
   VPN.  This is a specific type of Denial of Service attack.

4.1.5.  Unauthorized Traffic Pattern Analysis

   This refers to "sniffing" VPN packets and examining aspects or
   meta-aspects of them that may be visible even when the packets
   themselves are encrypted.  An attacker might gain useful
   information based on the amount and timing of traffic, packet
   sizes, source and destination addresses, etc.  For most PPVPN
   users, this type of attack is generally considered to be

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   significantly less of a concern than the other types discussed in
   this section.

4.1.6.  Denial of Service Attacks on the VPN

   Denial of Service (DOS) attacks are those in which an attacker
   attempts to disrupt or prevent the use of a service by its
   legitimate users.  Taking network devices out of service, modifying
   their configuration, or overwhelming them with requests for service
   are several of the possible avenues for DOS attack.

   Overwhelming the network with requests for service, otherwise known
   as a "resource exhaustion" DOS attack, may target any resource in
   the network e.g. link bandwidth, packet forwarding capacity,
   session capacity for various protocols, CPU power, and so on.

   DOS attacks of the resource exhaustion type can be mounted against
   the data plane of a particular PPVPN by inserting an overwhelming
   quantity of non-authentic data into the VPN.

   Data plane resource exhaustion attacks can also be mounted by
   overwhelming the service provider's general (VPN-independent)
   infrastructure with traffic.  These attacks on the general
   infrastructure are not usually a PPVPN-specific issue, unless the
   attack is mounted by another PPVPN user from a privileged position.
   (E.g. a PPVPN user might be able to monopolize network data plane
   resources and thus disrupt other PPVPNs.)

4.2.    Attacks on the Control Plane

   This category encompasses attacks on the control structures
   operated by the PPVPN service provider.

4.2.1.  Denial of Service Attacks on the Network Infrastructure

   Control plane DOS attacks can be mounted specifically against the
   mechanisms the service provider uses to provide PPVPNs e.g. IPsec,
   MPLS, etc., or against the general infrastructure of the service
   provider e.g. P routers or shared aspects of PE routers.  (Attacks
   against the general infrastructure are within the scope of this
   document only if the attack happens in relation with the VPN
   service, otherwise is not a PPVPN-specific issue.)

   Of special concern for PPVPNs is denial of service to one PPVPN
   user caused by the activities of another PPVPN user.  This can
   occur for example if one PPVPN user's activities are allowed to
   consume excessive network resources of any sort that are also
   needed to serve other PPVPN users.

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   The attacks described in the following sections may each have
   denial of service as one of their effects.  Other DOS attacks are
   also possible.

4.2.2.  Attacks on the Service Provider Equipment Via Management

   This includes unauthorized access to service provider
   infrastructure equipment, which access can be used to reconfigure
   the equipment, or to extract information (statistics, topology,
   etc.) about one or more PPVPNs.

   This can be accomplished through malicious entering of the systems,
   or inadvertently as a consequence of inadequate inter-VPN isolation
   in a PPVPN user self-management interface.  (The former is not
   necessarily a PPVPN-specific issue.)

4.2.3.  Social Engineering Attacks on the Service Provider

   Attacks in which the service provider network is reconfigured or
   damaged, or in which confidential information is improperly
   disclosed, may be mounted through manipulation of service provider
   personnel. These types of attacks are PPVPN-specific if they affect
   PPVPN-serving mechanisms.  It may be observed that the
   organizational split (customer, service provider) that is inherent
   in PPVPNs may make it easier to mount such attacks against
   provider-provisioned VPNs than against VPNs that are customer self-
   provisioned at the IP layer.

4.2.4.  Cross-connection of Traffic Between PPVPNs

   This refers to the event where expected isolation between separate
   PPVPNs is breached.  This includes cases such as:

    - A site being connected into the "wrong" VPN
    - Two or more VPNs being improperly merged together
    - A point-to-point VPN connecting the wrong two points
    - Any packet or frame being improperly delivered outside the VPN
      it is sent in.

   Mis-connection or cross-connection of VPNs may be caused by service
   provider or equipment vendor error, or by the malicious action of
   an attacker.

   Anecdotal evidence suggests that the cross-connection threat is one
   of the largest security concerns of PPVPN users (or would-be

4.2.5.  Attacks Against PPVPN Routing Protocols

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   This encompasses attacks against routing protocols that are run by
   the service provider and that directly support the PPVPN service.
   In layer 3 VPNs this typically relates to membership discovery or
   to the distribution of per-VPN routes.  In layer 2 VPNs this
   typically relates to membership and endpoint discovery.  (Attacks
   against the use of routing protocols for the distribution of
   backbone (non-VPN) routes are beyond the scope of this document.)
   Specific attacks against popular routing protocols have been widely
   studied and described in [Beard].

4.2.6.  Attacks on Route Separation

   "Route separation" refers here to keeping the per-VPN topology and
   reachability information for each PPVPN separate from, and
   unavailable to, any other PPVPN (except as specifically intended by
   the service provider).  This concept is only a distinct security
   concern for those layer 3 VPN types where the service provider is
   involved with the routing within the VPN (i.e. VR, BGP-MPLS, routed
   version of IPsec).  A breach in the route separation can reveal
   topology and addressing information about a PPVPN.  It can also
   cause black hole routing or unauthorized data plane cross-
   connection between PPVPNs.

4.2.7.  Attacks on Address Space Separation

   In Layer 3 VPNs, the IP address spaces of different VPNs need to be
   kept separate.  In Layer 2 VPNs, the MAC address and VLAN spaces of
   different VPNs need to be kept separate. A control plane breach in
   this addressing separation may result in unauthorized data plane
   cross-connection between VPNs.

4.2.8.  Other Attacks on PPVPN Control Traffic

   Besides routing and management protocols (covered separately in the
   previous sections) a number of other control protocols may be
   directly involved in delivering the PPVPN service (e.g. for
   membership discovery and tunnel establishment in various PPVPN
   approaches).  These include but may not be limited to:

    - MPLS signaling (LDP, RSVP-TE)
    - IPsec signaling (IKE)
    - L2TP
    - BGP-based membership discovery
    - Database-based membership discovery (e.g. RADIUS-based)

   Attacks might subvert or disrupt the activities of these protocols,
   for example via impersonation or DOS attacks.

5. Defensive Techniques for PPVPN Service Providers

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   The defensive techniques discussed in this document are intended to
   describe methods by which some security threats can be addressed.
   They are not intended as requirements for all PPVPN
   implementations.  The PPVPN provider should determine the
   applicability of these techniques to the provider's specific
   service offerings, and the PPVPN user may wish to assess the value
   of these techniques to the user's VPN requirements.

   The techniques discussed here include encryption, authentication,
   filtering, firewalls, access control, isolation, aggregation, and
   other techniques.

   Nothing is ever 100% secure.  Defense therefore involves protecting
   against those attacks that are most likely to occur and/or that
   have the most dire consequences if successful.  For those attacks
   that are protected against, absolute protection is seldom
   achievable; more often it is sufficient just to make the cost of a
   successful attack greater than what the adversary will be willing
   to expend.

   Successfully defending against an attack does not necessarily mean
   the attack must be prevented from happening or from reaching its
   target.  In many cases the network can instead be designed to
   withstand the attack.  For example, the introduction of non-
   authentic packets could be defended against by preventing their
   introduction in the first place, or by making it possible to
   identify and eliminate them before delivery to the PPVPN user's
   system.  The latter is frequently a much easier task.

5.1.    Cryptographic techniques

   PPVPN defenses against a wide variety of attacks can be enhanced by
   the proper application of cryptographic techniques.  These are the
   same cryptographic techniques which are applicable to general
   network communications.  In general, these techniques can provide
   privacy (encryption) of communication between devices,
   authentication of the identities of the devices, and can ensure
   that it will be detected if the data being communicated is changed
   during transit.

   Privacy is a key part (the middle name!) of any Virtual Private
   Network.  In a PPVPN, privacy can be provided by two mechanisms:
   traffic separation and encryption.  In this section we focus on
   encryption, while traffic separation is addressed separately.

   Several aspects of authentication are addressed in some detail in a
   separate "Authentication" section.

   Encryption adds complexity to a service, and thus it may not be a
   standard offering within every PPVPN service. There are a few
   reasons why encryption may not be a standard offering within every

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   PPVPN service. Encryption adds an additional computational burden
   to the devices performing encryption and decryption.  This may
   reduce the number of user VPN connections which can be handled on a
   device or otherwise reduce the capacity of the device, potentially
   driving up the provider's costs.  Typically, configuring encryption
   services on devices adds to the complexity of the device
   configuration and adds incremental labor cost.  Packet lengths are
   typically increased when the packets are encrypted, increasing the
   network traffic load and adding to the likelihood of packet
   fragmentation with its increased overhead.  (This packet length
   increase can often be mitigated to some extent by data compression
   techniques, but at the expense of additional computational burden.
   Finally, some PPVPN providers may employ enough other defensive
   techniques, such as physical isolation or filtering/firewall
   techniques, that they may not perceive additional benefit from
   encryption techniques.

   The trust model among the PPVPN user, the PPVPN provider, and other
   parts of the network is a key element in determining the
   applicability of encryption for any specific PPVPN implementation.
   In particular, it determines where encryption should be applied:
   -  If the data path between the user's site and the provider's PE
      is not trusted, then encryption may be used on the PE-CE link.
   -  If some part of the backbone network is not trusted,
      particularly in implementations where traffic may travel across
      the Internet or multiple provider networks, then the PE-PE
      traffic may encrypted.
   -  If the PPVPN user does not trust any zone outside of its
      premises, it may require end-to-end or CE-CE encryption service.
      This service fits within the scope of this PPVPN security
      framework when the CE is provisioned by the PPVPN provider.
   -  If the PPVPN user requires remote access to a PPVPN from a
      system at a location which is not a PPVPN customer location (for
      example, access by a traveler) there may be a requirement for
      encrypting the traffic between that system and an access point
      on the PPVPN or at a customer site. If the PPVPN provider
      provides the access point, then the customer must cooperate with
      the provider to handle the access control services for the
      remote users. These access control services are usually
      implemented using encryption, as well.

   Although CE-CE encryption provides privacy against third-party
   interception, if the PPVPN provider has complete management control
   over the CE (encryption) devices, then it may be possible for the
   provider to gain access to the user's VPN traffic or internal
   network. Encryption devices can potentially be configured to use
   null encryption, bypass encryption processing altogether, or
   provide some means of sniffing or diverting unencrypted traffic.
   Thus a PPVPN implementation using CE-CE encryption needs to
   consider the trust relationship between the PPVPN user and
   provider. PPVPN users and providers may wish to negotiate a service

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   level agreement (SLA) for CE-CE encryption which will provide an
   acceptable demarcation of responsibilities for management of
   encryption on the CE devices. The demarcation may also be affected
   by the capabilities of the CE devices. For example, the CE might
   support some partitioning of management, a configuration lock-down
   ability, or allow both parties to verify the configuration. In
   general, the PPVPN user needs to have a fairly high level of trust
   that the PPVPN provider will properly provision and manage the CE
   devices, if the managed CE-CE model is used.

5.1.1.  IPsec in PPVPNs

   IPsec [RFC2401] [RFC2402] [RFC2406] [RFC2407] [RFC2411] is the
   security protocol of choice for encryption at the IP layer (Layer
   3), as discussed in [SECMECH].  IPsec provides robust security for
   IP traffic between pairs of devices.  Non-IP traffic must be
   converted to IP packets or it cannot be transported over IPsec.
   Encapsulation is a common conversion method.

   In the PPVPN model, IPsec can be employed to protect IP traffic
   between PEs, between a PE and a CE, or from CE to CE.  CE-to-CE
   IPsec may be employed in either a provider-provisioned or a user-
   provisioned model.  The user-provisioned CE-CE IPsec model is
   outside the scope of this document, and outside the scope of the
   PPVPN Working Group.  Likewise, encryption of data which is
   performed within the user's site is outside the scope of this
   document, since it is simply handled as user data by the PPVPN.
   IPsec can also be used to protect IP traffic between a remote user
   who is not located at a PPVPN site and the PPVPN.

   IPsec does not itself specify an encryption algorithm.  It can use
   a variety of encryption algorithms, with various key lengths.
   There are trade-offs between key length, computational burden, and
   the level of security of the encryption.  A full discussion of
   these trade-offs is beyond the scope of this document.  In order to
   assess the level of security offered by a particular IPsec-based
   PPVPN service, some PPVPN users may wish to know the specific
   encryption algorithm and effective key length used by the PPVPN
   provider.  However, in practice, any currently recommended IPsec
   encryption offers enough security to substantially reduce the
   likelihood of being directly targeted by an attacker; other weaker
   links in the chain of security are likely to be attacked first.
   PPVPN users may wish to use a Service Level Agreement (SLA)
   specifying the Service Provider's responsibility for ensuring data
   privacy, rather than analyzing the specific encryption techniques
   used in the PPVPN service.

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   For many of the PPVPN provider's network control messages and some
   PPVPN user requirements, cryptographic authentication of messages
   without encryption of the contents of the message may provide
   acceptable security.  Using IPsec, authentication of messages is
   provided by the Authentication Header (AH) or through the use of
   the Encapsulating Security Protocol (ESP) with authentication only.
   Where control messages require authentication but do not use IPsec,
   then other cryptographic authentication methods are available.
   Message authentication methods currently considered to be secure
   are based on hashed message authentication codes (HMAC) [RFC2104]
   implemented with a secure hash algorithm such as Secure Hash
   Algorithm 1 (SHA-1) [RFC3174].

   PPVPNs which provide differentiated services based on traffic type
   may encounter some conflicts with IPsec encryption of traffic.
   Since encryption hides the content of the packets, it may not be
   possible to differentiate the encrypted traffic in the same manner
   as unencrypted traffic.  Although DiffServ markings are copied to
   the IPsec header and can provide some differentiation, not all
   traffic types can be accommodated by this mechanism.

5.1.2.  Encryption for device configuration and management

   For configuration and management of PPVPN devices, encryption and
   authentication of the management connection at a level comparable
   to that provided by IPsec is desirable.

   Several methods of transporting PPVPN device management traffic
   offer security and privacy.
   -  Secure Shell (SSH) offers protection for TELNET [STD-8] or
      terminal-like connections to allow device configuration.
   -  SNMP v3 [STD62] provides encrypted and authenticated protection
      for SNMP-managed devices.
   -  Transport Layer Security (TLS) (also known as Secure Sockets
      Layer or SSL) [RFC-2246] is probably the emerging standard for
      securing HTTP-based communication, and thus can provide support
      for most XML- and SOAP-based device management approaches.
   -  IPsec provides security and privacy services at the network
      layer. With regards to device management, its current use is
      primarily focused on in-band management of user-managed IPsec
      gateway devices.

5.1.3.  Cryptographic techniques in Layer 2 PPVPNs

   Layer 2 PPVPNs will generally not be able to use IPsec to provide
   encryption throughout the entire network.  They may be able to use
   IPsec for PE-PE traffic where it is encapsulated in IP packets, but

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   IPsec will generally not be applicable for CE-PE traffic in Layer 2

   Encryption techniques for Layer 2 links are widely available, but
   are not within the scope of this document, nor of IETF documents in
   general.  Layer 2 encryption could be applied to the links from CE
   to PE, or could be applied from CE to CE, as long as the encrypted
   Layer 2 packets can be properly handled by the intervening PE
   devices.  In addition, the upper layer traffic transported by the
   Layer 2 VPN can be encrypted by the user.  In this case privacy
   will be maintained; however, this is transparent to the PPVPN
   provider and is outside the scope of this document.

5.1.4.  End-to-end vs. hop-by-hop encryption tradeoffs in PPVPNs

   In PPVPNs, encryption could potentially be applied to the VPN
   traffic at several different places.  This section discusses some
   of the tradeoffs in implementing encryption in several different
   connection topologies among different devices within a PPVPN.

   Encryption typically involves a pair of devices which encrypt the
   traffic passing between them.  The devices may be directly
   connected (over a single "hop"), or there may be intervening
   devices which transport the encrypted traffic between the pair of
   devices.  The extreme cases involve using encryption between every
   adjacent pair of devices along a given path (hop-by-hop), or using
   encryption only between the end devices along a given path (end-to-
   end).  To keep this discussion within the scope of PPVPNs, the
   latter ("end-to-end") case considered here is CE-to-CE rather than
   fully end-to-end.

   Figure 2 depicts a simplified PPVPN topology showing the Customer
   Edge (CE) devices, the Provider Edge (PE) devices, and a variable
   number (three are shown) of Provider core (P) devices which might
   be present along the path between two sites in a single VPN,
   operated by a single service provider (SP).


   Figure 2: Simplified PPVPN topology

   Within this simplified topology, and assuming that P devices are
   not to be involved with encryption, there are four basic feasible
   configurations for implementing encryption on connections among the

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   1) Site-to-site (CE-to-CE) - Encryption can be configured between
   the two CE devices, so that traffic will be encrypted throughout
   the SP's network.

   2) Provider edge-to-edge (PE-to-PE) - Encryption can be configured
   between the two PE devices.  Unencrypted traffic is received at one
   PE from the customer's CE, then it is encrypted for transmission
   through the SP's network to the other PE, where it is decrypted and
   sent to the other CE.

   3) Access link (CE-to-PE) - Encryption can be configured between
   the CE and PE, on each side (or on only one side).

   4) Configurations 2 and 3 above can also be combined, with
   encryption running from CE to PE, then PE to PE, then PE to CE.

   Among the four feasible configurations, key tradeoffs in
   considering encryption include:

   - Vulnerability to wiretap - assuming an attacker can tap the data
   in transit between devices, would it be protected by encryption?

   - Vulnerability to device compromise - assuming an attacker can get
   access to a device (or freely alter its configuration), would the
   data be protected?

   - Complexity of device configuration and management - given the
   number of sites per VPN customer as Nce and the number of PEs
   participating in a given VPN as Npe, how many device configurations
   need to be created or maintained, and how do those configurations

   - Processing load on devices - how many encryption or decryption
   operations must be done given P packets? - This influences
   considerations of device capacity and perhaps end-to-end delay.

   - Ability of SP to provide enhanced services (QoS, firewall,
   intrusion detection, etc.) - Can the SP inspect the data in order
   to provide these services?

   These tradeoffs are discussed for each configuration, below:

   1) Site-to-site (CE-to-CE)

   Wiretap - protected on all links
   Device compromise - vulnerable to CE compromise
   Complexity - single administration, responsible for one device per
        site (Nce devices), but overall configuration per VPN scales as
   Processing load - on each of two CEs, each packet is either
        encrypted or decrypted (2P)

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   Enhanced services û severely limited; typically only Diffserv
        markings are visible to SP, allowing some QoS services

   2) Provider edge-to-edge (PE-to-PE)

   Wiretap - vulnerable on CE-PE links; protected on SP's network
   Device compromise - vulnerable to CE or PE compromise
   Complexity - single administration, Npe devices to configure.
        (Multiple sites may share a PE device so Npe is typically much
        less than Nce.)  Scalability of the overall configuration
        depends on the PPVPN type: If the encryption is separate per
        VPN context, it scales as Npe^^2 per customer VPN.  If the
        encryption is per-PE, it scales as Npe^^2 for all customer VPNs
   Processing load - on each of two PEs, each packet is either
        encrypted or decrypted (2P)
   Enhanced services - full; SP can apply any enhancements based on
        detailed view of traffic

   3) Access link (CE-to-PE)

   Wiretap - protected on CE-PE link; vulnerable on SP's network links
   Device compromise - vulnerable to CE or PE compromise
   Complexity - two administrations (customer and SP) with device
        configuration on each side (Nce + Npe devices to configure) but
        since there is no mesh the overall configuration scales as Nce.
   Processing load - on each of two CEs, each packet is either
        encrypted or decrypted, plus on each of two PEs, each packet is
        either encrypted or decrypted (4P)
   Enhanced services - full; SP can apply any enhancements based on
        detailed view of traffic

   4) Combined Access link and PE-to-PE (essentially hop-by-hop)

   Wiretap - protected on all links
   Device compromise - vulnerable to CE or PE compromise
   Complexity - two administrations (customer and SP) with device
        configuration on each side (Nce + Npe devices to configure).
        Scalability of the overall configuration depends on the PPVPN
        type: If the encryption is separate per VPN context, it scales
        as Npe^^2 per customer VPN.  If the encryption is per-PE, it
        scales as Npe^^2 for all customer VPNs combined.
   Processing load - on each of two CEs, each packet is either
        encrypted or decrypted, plus on each of two PEs, each packet is
        both encrypted and decrypted (6P)
   Enhanced services - full; SP can apply any enhancements based on
        detailed view of traffic

   Given the tradeoffs discussed above, a few conclusions can be made:

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   - Configurations 2 and 3 are subsets of 4 that may be appropriate
   alternatives to 4 under certain threat models; the remainder of
   these conclusions compare 1 (CE-to-CE) vs. 4 (combined access links
   and PE-to-PE).

   - If protection from wiretaps is most important, then
   configurations 1 and 4 are equivalent.

   - If protection from device compromise is most important and the
   threat is to the CE devices, both cases are equivalent; if the
   threat is to the PE devices, configuration 1 is best.

   - If reducing complexity is most important, and the size of the
   network is very small, configuration 1 is the best.  Otherwise
   configuration 4 is the best because rather than a mesh of CE
   devices it requires a smaller mesh of PE devices.  Also under some
   PPVPN approaches the scaling of 4 is further improved by sharing
   the same PE-PE mesh across all VPN contexts.

   - If the overall processing load is a key factor, then 1 is best.

   - If the availability of enhanced services support from the SP is
   most important, then 4 is best.

   As a quick overall conclusion, CE-to-CE encryption provides greater
   protection against device compromise but this comes at the cost of
   enhanced services and at the cost of operational complexity due to
   the Order(n^^2) scaling of a larger mesh.

   This analysis of site-to-site vs. hop-by-hop encryption tradeoffs
   does not explicitly include cases of multiple providers cooperating
   to provide a PPVPN service, public Internet VPN connectivity, or
   remote access VPN service, but many of the tradeoffs will be

5.2.    Authentication

   In order to prevent security issues from some Denial-of-Service
   attacks or from malicious misconfiguration, it is critical that
   devices in the PPVPN should only accept connections or control
   messages from valid sources.  Authentication refers to methods to
   ensure that message sources are properly identified by the PPVPN
   devices with which they communicate.  This section focuses on
   identifying the scenarios in which sender authentication is
   required, and recommends authentication mechanisms for these

   Cryptographic techniques (authentication and encryption) do not
   protect against some types of denial of service attacks,
   specifically resource exhaustion attacks based on CPU or bandwidth

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   exhaustion. In fact, the processing required to decrypt and/or
   check authentication may in some cases increase the effect of these
   resource exhaustion attacks. Cryptographic techniques may however,
   be useful against resource exhaustion attacks based on exhaustion
   of state information (e.g., TCP SYN attacks).

5.2.1.  VPN Member Authentication

   This category includes techniques for the CEs to verify they are
   connected to the expected VPN.  It includes techniques for CE-PE
   authentication, to verify that each specific CE and PE is actually
   communicating with its expected peer.

5.2.2.  Management System Authentication

   Management system authentication includes the authentication of a
   PE to a centrally-managed directory server, when directory-based
   "auto-discovery" is used.  It also includes authentication of a CE
   to its PPVPN configuration server, when a configuration server
   system is used.

5.2.3.  Peer-to-peer Authentication

   Peer-to-peer authentication includes peer authentication for
   network control protocols (e.g. LDP, BGP, etc.), and other peer
   authentication (i.e. authentication of one IPsec security gateway
   by another).

5.2.4.  Authenticating Remote Access VPN members

   This section describes methods for authentication of remote access
   users connecting to a VPN.

   Effective authentication of individual connections is a key
   requirement for enabling remote access to a PPVPN from an arbitrary
   Internet address (for instance, by a traveler).

   There are several widely used standards-based protocols to support
   remote access authentication.  These include RADIUS [ref] and
   DIAMETER [ref].  Digital certificate systems also provide
   authentication.  In addition there has been extensive development
   and deployment of mechanisms for securely transporting individual
   remote access connections within tunneling protocols, including
   L2TP [ref] and IPsec.

   Remote access involves connection to a gateway device, which
   provides access to the PPVPN. The gateway device may be managed by
   the user at a user site, or by the PPVPN provider at several
   possible locations in the network. The user-managed case is of

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   limited interest within the PPVPN security framework, and is not
   considered at this time.

   When a PPVPN provider manages authentication at the remote access
   gateway, this implies that authentication databases, which are
   usually extremely confidential user-managed systems, will need to
   be referenced in a secure manner by the PPVPN provider. This can be
   accomplished by the use of proxy authentication services, which
   accept an encrypted authentication credential from the remote
   access user, pass it to the PPVPN user's authentication system, and
   receive a yes/no response as to whether the user has been
   authenticated.  Thus the PPVPN provider does not have access to the
   actual authentication database, but can use it on behalf of the
   PPVPN user to provide remote access authentication.

   Specific cryptographic techniques for handling authentication are
   described in the following sections.

5.2.5.  Cryptographic techniques for authenticating identity

   Cryptographic techniques offer several mechanisms for
   authenticating the identity of devices or individuals. These
   include the use of shared secret keys, one-time keys generated by
   accessory devices or software, user-ID and password pairs, and a
   range of public-private key systems. Another approach is to use a
   hierarchical Certificate Authority system to provide digital

   This section describes or provides references to the specific
   cryptographic approaches for authenticating identity.  These
   approaches provide secure mechanisms for most of the authentication
   scenarios required in operating a PPVPN.

5.3.    Access Control techniques

   Access control techniques include packet-by-packet or packet-flow-
   by-packet-flow access control by means of filters and firewalls, as
   well as by means of admitting a "session" for a
   control/signaling/management protocol that is being used to
   implement PPVPNs. Enforcement of access control by isolated
   infrastructure addresses is discussed in another section of this

   In this document, we distinguish between filtering and firewalls
   based primarily on the direction of traffic flow.  We define
   filtering as being applicable to unidirectional traffic, while a
   firewall can analyze and control both sides of a conversation.

   There are two significant corollaries of this definition:

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   - Routing or traffic flow symmetry: A firewall typically requires
   routing symmetry, which is usually enforced by locating a firewall
   where the network topology assures that both sides of a
   conversation will pass through the firewall.  A filter can operate
   upon traffic flowing in one direction, without considering traffic
   in the reverse direction.
   - Statefulness: Since it receives both sides of a conversation, a
   firewall may be able to interpret a significant amount of
   information concerning the state of that conversation, and use this
   information to control access.  A filter can maintain some limited
   state information on a unidirectional flow of packets, but cannot
   determine the state of the bi-directional conversation as precisely
   as a firewall.

5.3.1.  Filtering

   It is relatively common for routers to filter data packets. That
   is, routers can look for particular values in certain fields of the
   IP or higher level (e.g., TCP or UDP) headers. Packets which match
   the criteria associated with a particular filter may either be
   discarded or given special treatment.

   In discussing filters, it is useful to separate the Filter
   Characteristics which may be used to determine whether a packet
   matches a filter from the Packet Actions which are applied to those
   packets which match a particular filter.

   o Filter Characteristics

   Filter characteristics are used to determine whether a particular
   packet or set of packets matches a particular filter.

   In many cases filter characteristics may be stateless. A stateless
   filter is one which determines whether a particular packet matches
   a filter based solely on the filter definition, normal forwarding
   information (such as the next hop for a packet), and the
   characteristics of that individual packet. Typically stateless
   filters may consider the incoming and outgoing logical or physical
   interface, information in the IP header, and information in higher
   layer headers such as the TCP or UDP header. Information in the IP
   header to be considered may for example include source and
   destination IP address, Protocol field, Fragment Offset, and TOS
   field. Filters also may consider fields in the TCP or UDP header
   such as the Port fields as well as the SYN field in the TCP header.

   Stateful filtering maintains packet-specific state information, to
   aid in determining whether a filter has been met. For example, a
   device might apply stateless filters to the first fragment of a
   fragmented IP packet. If the filter matches, then the data unit ID
   may be remembered and other fragments of the same packet may then
   be considered to match the same filter. Stateful filtering is more

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   commonly done in firewalls, although firewall technology may be
   added to routers.

   o Actions based on Filter Results

   If a packet, or a series of packets, match a specific filter, then
   there are a variety of actions which may be taken based on that
   filter match. Examples of such actions include:

     - Discard

   In many cases filters may be set to catch certain undesirable
   packets. Examples may include packets with forged or invalid source
   addresses, packets which are part of a DOS or DDOS attack, or
   packets which are trying to access resources which are not
   permitted (such as network management packets from an unauthorized
   source). Where such filters are activated, it is common to silently
   discard the packet or set of packets matching the filter. The
   discarded packets may of course also be counted and/or logged.

     - Set CoS

   A filter may be used to set the Class of Service associated with
   the packet.

     - Count packets and/or bytes

     - Rate Limit

   In some cases the set of packets which match a particular filter
   may be limited to a specified bandwidth. In this case packets
   and/or bytes would be counted, and would be forwarded normally up
   to the specified limit. Excess packets may be discarded, or may be
   marked (for example by setting a "discard eligible" bit in the IP
   ToS field or the MPLS EXP field).

     - Forward and Copy

   It is useful in some cases to forward some set of packets normally,
   but to also send a copy to a specified other address or interface.
   For example, this may be used to implement a lawful intercept
   capability, or to feed selected packets to an Intrusion Detection

   o Other Issues related to Use of Packet Filters

   There may be a very wide variation in the performance impact of
   filtering. This may occur both due to differences between
   implementations, and also due to differences between types or
   numbers of filters deployed. For filtering to be useful, the

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   performance of the equipment has to be acceptable in the presence
   of filters.

   The precise definition of "acceptable" may vary from service
   provider to service provider, and may depend upon the intended use
   of the filters. For example, for some uses a filter may be turned
   on all the time in order to set CoS, to prevent an attack, or to
   mitigate the effect of a possible future attack. In this case it is
   likely that the service provider will want the filter to have
   minimal or no impact on performance. In other cases, a filter may
   be turned on only in response to a major attack (such as a major
   DDOS attack). In this case a greater performance impact may be
   acceptable to some service providers.

5.3.2.  Firewalls

   Firewalls provide a mechanism for control over traffic passing
   between different trusted zones in the PPVPN model, or between a
   trusted zone and an untrusted zone.  Firewalls typically provide
   much more functionality than filters, since they may be able to
   apply detailed analysis and logical functions to flows, and not
   just to individual packets.  They may offer a variety of complex
   services, such as threshold-driven denial-of-service attack
   protection, virus scanning, acting as a TCP connection proxy, etc.

   As with other access control techniques, the value of firewalls
   depends on a clear understanding of the topologies of the PPVPN
   core network, the user networks, and the threat model.  Their
   effectiveness depends on a topology with a clearly defined inside
   (secure) and outside (not secure).

   Within the PPVPN framework, traffic typically is not allowed to
   pass between the various user VPNs.  This inter-VPN isolation is
   usually not performed by a firewall, but is a part of the basic VPN
   mechanism.  An exception to the total isolation of VPNs is the case
   of "extranets", which allow specific external access to a user's
   VPN, potentially from another VPN.  Firewalls can be used to
   provide the services required for secure extranet implementation.

   In a PPVPN, firewalls can be applied between the public Internet
   and user VPNs, in cases where Internet access services are offered
   by the provider to the VPN user sites.  In addition, firewalls may
   be applied between VPN user sites and any shared network-based
   services offered by the PPVPN provider.

   Firewalls may be applied to help protect PPVPN core network
   functions from attacks originating from the Internet or from PPVPN
   user sites, but typically other defensive techniques will be used
   for this purpose.

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   Where firewalls are employed as a service to protect user VPN sites
   from the Internet, different VPN users, and even different sites of
   a single VPN user, may have varying firewall requirements.  The
   overall PPVPN logical and physical topology, along with the
   capabilities of the devices implementing the firewall services,
   will have a significant effect on the feasibility and manageability
   of such varied firewall service offerings.

5.3.3.  Access Control to management interfaces

   Most of the security issues related to management interfaces can be
   addressed through the use of authentication techniques as described
   in the section on authentication.  However, additional security may
   be provided by controlling access to management interfaces in other

   Management interfaces, especially console ports on PPVPN devices,
   may be configured so they are only accessible out-of-band, through
   a system which is physically and/or logically separated from the
   rest of the PPVPN infrastructure.

   Where management interfaces are accessible in-band within the PPVPN
   domain, filtering or firewalling techniques can be used to restrict
   unauthorized in-band traffic from having access to management
   interfaces.  Depending on device capabilities, these filtering or
   firewalling techniques can be configured either on other devices
   through which the traffic might pass, or on the individual PPVPN
   devices themselves.

5.4.    Use of Isolated Infrastructure

   One way to protect the infrastructure used for support of VPNs is
   to separate the resources for support of VPNs from the resources
   used for other purposes (such as support of Internet services). In
   some cases this may make use of physically separate equipment for
   VPN services, or even a physically separate network.

   For example, PE-based L3 VPNs may be run on a separate backbone not
   connected to the Internet, or may make use of separate edge routers
   from those used to support Internet service. Private IP addresses
   (local to the provider and non-routable over the Internet) are
   sometimes used to provide additional separation.

   It is common for CE-based L3VPNs to make use of CE devices which
   are dedicated to one specific VPN. In many or most cases CE-based
   VPNs may make use of normal Internet services to interconnect CE

5.5.    Use of Aggregated Infrastructure

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   In general it is not feasible to use a completely separate set of
   resources for support of each VPN. In fact, one of the main reasons
   for VPN services is to allow sharing of resources between multiple
   users, including multiple VPNs. Thus even if VPN services make use
   of a separate network from Internet services, nonetheless there
   will still be multiple VPN users sharing the same network
   resources. In some cases VPN services will share the use of network
   resources with Internet services or other services.

   It is therefore important for VPN services to provide protection
   between resource utilization by different VPNs. Thus a well-behaved
   VPN user should be protected from possible misbehavior by other
   VPNs. This requires that limits are placed on the amount of
   resources which can be used by any one VPN. For example, both
   control traffic and user data traffic may be rate limited. In some
   cases or in some parts of the network where a sufficiently large
   number of queues are available each VPN (and optionally each VPN
   and CoS within the VPN) may make use of a separate queue. Control-
   plane resources such as link bandwidth as well as CPU and memory
   resources may be reserved on a per-VPN basis.

   The techniques which are used to provision resource protection
   between multiple VPNs served by the same infrastructure can also be
   used to protect VPN services from Internet services.

   In general the use of aggregated infrastructure allows the service
   provider to benefit from stochastic multiplexing of multiple bursty
   flows, and also may in some cases thwart traffic pattern analysis
   by combining the data from multiple VPNs.

5.6.    Service Provider Quality Control Processes

   Deployment of provider-provisioned VPN services in general requires
   a relatively large amount of configuration by the service provider.
   For example, the service provider needs to configure which VPN each
   site belongs to, as well as QoS and SLA guarantees. This large
   amount of required configuration leads to the possibility of

   It is important for the service provider to have operational
   processes in place to reduce the potential impact of
   misconfiguration. CE to CE authentication may also be used to
   detect misconfiguration when it occurs.

5.7.    Deployment of Testable PPVPN Service.

   This refers to solutions that can be readily tested to make sure
   they are configured correctly.  E.g. for a point-point VPN,
   checking that the intended connectivity is working pretty much
   ensures that there is not connectivity to some unintended site.

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6. Monitoring, Detection, and Reporting of Security Attacks

   A PPVPN service may be subject to attacks from a variety of
   security threats.  Many threats are described in another part of
   this document.  Many of the defensive techniques described in this
   document and elsewhere provide significant levels of protection
   from a variety of threats.  However, in addition to silently
   employing defensive techniques to protect against attacks, PPVPN
   services can also add value for both providers and customers by
   implementing security monitoring systems which detect and report on
   any security attacks which occur, regardless of whether the attacks
   are effective.

   Attackers often begin by probing and analyzing defenses, so systems
   which can detect and properly report these early stages of attacks
   can provide significant benefits.

   Information concerning attack incidents, especially if available
   quickly, can be useful in defending against further attacks.  It
   can be used to help identify attackers and/or their specific
   targets at an early stage.  This knowledge about attackers and
   targets can be used to further strengthen defenses against specific
   attacks or attackers, or improve the defensive services for
   specific targets on an as-needed basis.  Information collected on
   attacks may also be useful in identifying and developing defenses
   against novel attack types.

   Monitoring systems used to detect security attacks in PPVPNs will
   typically operate by collecting information from the Provider Edge
   (PE), Customer Edge (CE), and/or Provider backbone (P) devices.
   Security monitoring systems should have the ability to actively
   retrieve information from devices (e.g., SNMP get) or to passively
   receive reports from devices (e.g., SNMP traps).  The specific
   information exchanged will depend on the capabilities of the
   devices and on the type of VPN technology.  Particular care should
   be given to securing the communications channel between the
   monitoring systems and the PPVPN devices.

   The CE, PE, and P devices should employ efficient methods to
   acquire and communicate the information needed by the security
   monitoring systems.  It is important that the communication method
   between PPVPN devices and security monitoring systems be designed
   so that it will not disrupt network operations.  As an example,
   multiple attack events may be reported through a single message,
   rather than allowing each attack event to trigger a separate
   message, which might result in a flood of messages, essentially
   becoming a denial-of-service attack against the monitoring system
   or the network.

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   The mechanisms for reporting security attacks should be flexible
   enough to meet the needs of VPN service providers, VPN customers,
   and regulatory agencies, if applicable.  The specific reports will
   depend on the capabilities of the devices, the security monitoring
   system, the type of VPN, and the service level agreements between
   the provider and customer.

7. User Security Requirements

   This section defines a list of security related requirements that
   the users of PPVPN services may have for their PPVPN service.
   Typically, these user requirements translate into requirement for
   the provider in offering the service.

   The following sections detail various requirements that ensure the
   security of a given trusted zone. Since in real life there are
   various levels of security, a PPVPN may fulfill any number or all
   of these security requirements. Specifically this document does not
   state that a PPVPN must fulfill all of these requirements to be
   secure. As mentioned in the Introduction, it is not within the
   scope of this document to define the specific requirements that
   each VPN technology must fulfill in order to be secure.

7.1.    Isolation

   A virtual private network usually defines the "private" as being
   isolated from other PPVPNs and the Internet. More specifically,
   isolation has several components:

7.1.1.  Address Separation

   Within a PPVPN service, a given PPVPN can use the full Internet
   address range, including private address ranges [RFC1918], without
   interfering with other PPVPNs that use the same PPVPN service. When
   using Internet access through the PPVPN core a NAT functionality
   can be implemented.

   In layer 2 VPNs the same requirement exists for the layer 2
   addressing schemes, such as MAC addresses.

7.1.2.  Routing Separation

   A PPVPN core must maintain routing separation between the trusted
   zones. This means that routing information must not leak from any
   trusted zone to any other trusted zone, unless this is specifically
   engineered this way, for example for Internet access.

   In layer 2 VPNs the switching information must be kept separate
   between the trusted zones, such that switching information of one
   PPVPN does not influence other PPVPNs or the PPVPN core.

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7.1.3.  Traffic Separation

   Traffic from a given trusted zone must never leave this zone, and
   traffic from another zone must never enter this zone. Exceptions
   are where this is specifically engineered that way, for example for
   extranet purposes or Internet access.

7.2.    Protection

   The perception of a completely separated, "private" network is that
   it has defined entry points, and only over those is can be attacked
   or intruded. By sharing a common core a PPVPN appears to lose some
   of this clear interfaces to parts outside the trusted zone. Thus
   one of the key security requirements of PPVPN services is that they
   offer the same level of protection as private networks.

7.2.1.  Protection against intrusion

   An intrusion is defined here as the penetration of a trusted zone
   from outside this zone. This could be from the Internet, another
   PPVPN, or the core network itself.

   The fact that a network is "virtual" must not expose it to
   additional threats over private networks. Specifically, it must not
   add new interfaces to other parts outside the trusted zone.
   Intrusions from known interfaces such as Internet gateways are
   outside the scope of this document.

7.2.2.  Protection against Denial of Service attacks

   A denial of service attack aims at making services or devices un-
   available to legitimate users. In the framework of this document
   only those DoS attacks are considered which are a consequence of
   providing the network in a virtual way. DoS attacks over the
   standard interfaces into a trusted zone are not considered here.

   The requirement is that a PPVPN is not more vulnerable against DoS
   attacks than if the same network would be private.

7.2.3.  Protection against spoofing

   It is not possible to change the sender identification (source
   address, source label, etc) of traffic in transit, such that by
   this spoofing the integrity of a PPVPN gets violated. For example,
   if two CEs are connected to the same PE, it must not be possible
   for one CE to send crafted packets that make the PE believe those
   packets are coming from the other CE, thus inserting them into the
   wrong PPVPN.

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7.3.    Confidentiality

   This requirement means that data must be cryptographically secured
   in transit over the PPVPN core network to avoid eavesdropping.

7.4.    CE Authentication

   Where CE authentication is provided it is not possible for an
   outsider to install a CE and pretend to belong to a specific PPVPN,
   to which this CE does not belong in reality.

7.5.    Integrity

   Data in transit must be secured in a manner such that it cannot be
   altered, or that any alteration may be detected at the receiver.

7.6.    Anti-Replay

   Anti-replay means that data in transit cannot be recorded and
   replayed later. To protect against anti-replay attacks the data
   must be cryptographically secured.

   Note: Even private networks do not necessarily meet the
   requirements of confidentiality, integrity and anti-reply. Thus
   when comparing private to "virtually private" PPVPN services these
   requirements are only applicable if the comparable private service
   also included these services.

8. Provider Security Requirements

   In this section, we discuss additional security requirements that
   the provider may have in order to secure its network infrastructure
   as it provides PPVPN services.

   The PPVPN service provider requirements defined here are the
   requirements for the PPVPN core in the reference model.  The core
   network can be implemented with different types of network
   technologies, and each core network may use different technologies
   to provide the PPVPN services to users with different levels of
   offered security. Therefore, a PPVPN service provider may fulfill
   any number of the security requirements listed in this section.
   This document does not state that a PPVPN must fulfill all of these
   requirements to be secure.

   These requirements are focused on: 1) how to protect the PPVPN core
   from various attacks outside the core including PPVPN users and
   non-PPVPN alike, both accidentally and maliciously, 2) how to
   protect the PPVPN user VPNs and sites themselves. Note that a PPVPN
   core is not more vulnerable against attacks than a core that does
   not provide PPVPNs. However providing PPVPN services over such a

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   core may need lead to additional security requirements, for the
   mere fact that most users are expecting higher security standards
   in a core delivering PPVPN services.

8.1.    Protection within the Core Network

8.1.1.  Control Plane Protection

   - Protocol authentication within the core:

   PPVPN technologies and infrastructure must support mechanisms for
   authentication of the control plane. For an IP core, IGP and BGP
   sessions may be authenticated by using TCP MD5 or IPsec. If an MPLS
   core is used, LDP sessions may be authenticated by use TCP MD5, in
   addition, IGP and BGP authentication should also be considered. For
   a core providing Layer 2 services, PE to PE authentication may also
   be used via IPsec.

   With the cost of authentication coming down rapidly, the
   application of control plane authentication may not increase the
   cost of implementation for providers significantly, and will help
   to improve the security of the core. If the core is dedicated to
   VPN services and without any interconnects to third parties then
   this may reduce the requirement for authentication of the core
   control plane.

   - Elements protection

   Here we discuss means to hide the provider's infrastructure nodes.

   A PPVPN provider may make the infrastructure routers (P and PE
   routers) unreachable from outside users and unauthorized internal
   users. For example, separate address space may be used for the
   infrastructure loopbacks.

   Normal TTL propagation may be altered to make the backbone look
   like one hop from the outside, but caution needs to be taken for
   loop prevention. This prevents the backbone addresses to be exposed
   through trace route, however this must also be assessed against
   operational requirements for end to end fault tracing.

   An Internet backbone core may be re-engineered to make Internet
   routing an edge function, for example, using MPLS label switching
   for all traffic within the core and possibly make the Internet a
   VPN within the PPVPN core itself. This helps to detach Internet
   access from PPVPN services.

   Separating control plane, data plane, and management plane
   functionality in terms of hardware and software may be implemented
   on the PE devices to improve security. This may help to limit the

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   problems when attacked in one particular area, and may allow each
   plane to implement additional security measurement separately.

   PEs are often more vulnerable to attack than P routers, since PEs
   cannot be made unreachable to outside users by their very nature.
   Access to core trunk resources can be controlled on a per user
   basis by the application of inbound rate-limiting/shaping, this can
   be further enhanced on a per Class of Service basis (see section

   In the PE, using separate routing processes for Internet and PPVPN
   service may help to improve the PPVPN security and better protect
   VPN customers. Furthermore, if the resources, such as CPU and
   Memory, may be further separated based on applications, or even
   individual VPNs, it may help to provide improved security and
   reliability to individual VPN customers.

   Many of these were not particular issues when an IP core was
   designed to support Internet services only. When providing PPVPN
   services, new requirements are introduced to satisfy the security
   needs for VPN services. Similar consideration apply to L2 VPN

8.1.2.  Data Plane Protection

   PPVPN using IPsec technologies provide VPN users with encryption of
   secure user data.

   In today's MPLS, ATM, or Frame Relay networks, encryption is not
   provided as a basic feature. Mechanisms can be used to secure the
   MPLS data plane to secure the data carried over MPLS core.
   Additionally, if the core is dedicated to VPN services and without
   any external interconnects to third party networks then there is no
   obvious need for encryption of the user data plane.

   IPsec / L3 PPVPN technologies inter-working, or IPsec /L2 PPVPN
   technologies inter-working may be used to provide PPVPN users end-
   to-end PPVPN services.

8.2.    Protection on the User Access Link

   Peer / Neighbor protocol authentication may be used to enhance
   security. For example, BGP MD5 authentication may be used to
   enhance security on PE-CE links using eBGP. In the case of Inter-
   provider connection, authentication / encryption mechanisms between
   ASes, such as IPsec, may be used.

   WAN link address space separation for VPN and non-VPN users may be
   implemented to improve security in order to protect VPN customers
   if multiple services are provided on the same PE platform.

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   Firewall / Filtering: access control mechanisms can be used to
   filter out any packets destined for the service provider's
   infrastructure prefix or eliminate routes identified as
   illegitimate routes.

   Rate limiting may be applied to the user interface/logical
   interfaces against DDOS bandwidth attack. This is very helpful when
   the PE device is supporting both VPN services and Internet
   Services, especially when supporting VPN and Internet Services on
   the same physical interfaces through different logical interfaces.

   7.2.1 Link Authentication

   Authentication mechanisms can be employed to validate site access
   to the PPVPN network via fixed or logical (e.g. L2TP, IPsec)
   connections. Where the user wishes to hold the 'secret' associated
   to acceptance of the access and site into the VPN, then PPVPN based
   solutions require the flexibility for either direct authentication
   by the PE itself or interaction with a customer PPVPN
   authentication server. Mechanisms are required in the latter case
   to ensure that the interaction between the PE and the customer
   authentication server is controlled e.g. limiting it simply to an
   exchange in relation to the authentication phase and with other
   attributes e.g. RADIUS optionally being filtered.

   7.2.2 Access Routing

   Mechanisms may be used to provide control at a routing protocol
   level e.g. RIP, OSPF, BGP between the CE and PE. Per neighbor and
   per VPN routing policies may be established to enhance security and
   reduce the impact of a malicious or non-malicious attack on the PE,
   in particular the following mechanisms should be considered:
    - Limiting the number of prefixes that may be advertised on a per
       access basis into the PE. Appropriate action may be taken should
       a limit be exceeded e.g. the PE shutting down the peer session
       to the CE
    - Applying route dampening at the PE on received routing updates
    - Definition of a per VPN prefix limit after which additional
       prefixes will not be added to the VPN routing table.

   In the case of Inter-provider connection, access protection, link
   authentication, and routing policies as described above may be
   applied. Both inbound and outbound firewall/filtering mechanism
   between ASes may be applied. Proper security procedures must be
   implemented in Inter-provider VPN interconnection to protect the
   providers' network infrastructure and their customer VPNs. This may
   be custom designed for each Inter-Provider VPN peering connection,
   and must be agreed by both providers.

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   7.2.3 Access QoS

   PPVPN providers offering QoS enabled services require mechanisms to
   ensure that individual accesses are validated against their
   subscribed QOS profile and as such gain access to core resources
   that match their service profile.  Mechanisms such as per Class of
   service rate limiting/traffic shaping on ingress to the PPVPN core
   are one option in providing this level of control.  Such mechanisms
   may require the per Class of Service profile to be enforced either
   by marking, remarking or discard of traffic outside of profile.

   7.2.4 Customer VPN monitoring tools

   End users requiring visibility of VPN specific statistics on the
   core e.g. routing table, interface status, QoS statistics, impose
   requirements for mechanisms at the PE to both validate the incoming
   user and limit the views available to that particular users VPN.
   Mechanisms should also be considered to ensure that such access
   cannot be used a means of a DOS attack (either malicious or
   accidental) on the PE itself. This could be accomplished through
   either separation of these resources within the PE itself or via
   the capability to rate-limit on a per VPN basis such traffic.

8.3.    General Requirements for PPVPN Providers

   The PPVPN providers must support the users security requirements as
   listed in Section 6. Depending on the technologies used, these
   requirements may include:

   - User control plane separation û routing isolation
   - User address space separation û supporting overlapping addresses
      from different VPNs
   - User data plane separation û one VPN traffic cannot be
      intercepted by other VPNs or any other users.
   - Protection against intrusion, DOS attacks and spoofing
   - Access Authentication
   - Techniques highlighted through this document identify
      methodologies for the protection of PPVPN resources and
      infrastructure. By following these approaches a secure VPN
      service can be delivered without the absolute need for
      cryptographic techniques

   Equipment hardware/software bugs leading to breaches in security
   are not within the scope of this document.

9. Security Evaluation of PPVPN Technologies

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   This section presents a brief template that may be used to evaluate
   and summarize how a given PPVPN approach (solution) measures up
   against the PPVPN Security Framework.  An evaluation of a given
   PPVPN approach using this template should appear in the
   applicability statement for each PPVPN approach.

9.1.    Evaluating the Template

   The first part of the template is in the form of a list of security
   assertions.  For each assertion the approach is assessed and one or
   more of the following ratings is assigned:

   - The requirement is not applicable to the VPN approach because ...
      (fill in reason)

   - The base VPN approach completely addresses the requirement by ...
      (fill in technique)

   - The base VPN approach partially addresses the requirement by ...
      (fill in technique and extent to which it addresses the

   - An optional extension to the VPN approach completely addresses
      the requirement by ...  (fill in technique)

   - An optional extension to the VPN approach partially addresses the
      requirement by ...  (fill in technique and extent to which it
      addresses the requirement)

   - In the VPN approach, the requirement is addressed in a way that
      is beyond the scope of the VPN approach.  (Explain)  (One
      example of this would be a VPN approach in which some aspect,
      say membership discovery, is done via configuration.  The
      protection afforded to the configuration would be beyond the
      scope of the VPN approach.)

   - The VPN approach does not meet the requirement.

9.2.    Template

   The following assertions solicit responses of the types listed in
   the previous section.

   1. The approach provides complete IP address space separation for
      each L3 VPN.

   2. The approach provides complete L2 address space separation for
      each L2 VPN.

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   3. The approach provides complete VLAN ID space separation for each
      L2 VPN.

   4. The approach provides complete IP route separation for each L3

   5. The approach provides complete L2 forwarding separation for each
      L2 VPN.

   6. The approach provides a means to prevent improper cross-
      connection of sites in separate VPNs.

   7. The approach provides a means to detect improper cross-
      connection of sites in separate VPNs.

   8. The approach protects against the introduction of unauthorized
      packets into each VPN.

       a. In the CE-PE link
       b. In a single- or multi- provider PPVPN backbone
       c. In the Internet used as PPVPN backbone

   9. The approach provides confidentiality (secrecy) protection for
      PPVPN user data.

       a. In the CE-PE link
       b. In a single- or multi- provider PPVPN backbone
       c. In the Internet used as PPVPN backbone

   10. The approach provides sender authentication for PPVPN user

       a. In the CE-PE link
       b. In a single- or multi- provider PPVPN backbone
       c. In the Internet used as PPVPN backbone

   11. The approach provides integrity protection for PPVPN user data.

       a. In the CE-PE link
       b. In a single- or multi- provider PPVPN backbone
       c. In the Internet used as PPVPN backbone

   12. The approach provides protection against replay attacks for
      PPVPN user data.

       a. In the CE-PE link
       b. In a single- or multi- provider PPVPN backbone
       c. In the Internet used as PPVPN backbone

   13. The approach provides protection against unauthorized traffic
      pattern analysis for PPVPN user data.

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       a. In the CE-PE link
       b. In a single- or multi- provider PPVPN backbone
       c. In the Internet used as PPVPN backbone

   14. The control protocol(s) used for each of the following
      functions provide for message integrity and peer authentication:

       a. VPN membership discovery
       b. Tunnel establishment
       c. VPN topology and reachability advertisement
          i.  PE-PE
          ii. PE-CE
       d. VPN provisioning and management
       e. VPN monitoring and attack detection and reporting
       f. Other VPN-specific control protocols, if any.  (list)

   The following questions solicit free-form answers.

   15. Describe the protection, if any, the approach provides against
      PPVPN-specific DOS attacks (i.e. Inter-trusted-zone DOS

      a. Protection of the service provider infrastructure against
         Data Plane or Control Plane DOS attacks originated in a
         private (PPVPN user) network and aimed at PPVPN mechanisms.
      b. Protection of the service provider infrastructure against
         Data Plane or Control Plane DOS attacks originated in the
         Internet and aimed at PPVPN mechanisms.
      c. Protection of PPVPN users against Data Plane or Control Plane
         DOS attacks originated from the Internet or from other PPVPN
         users and aimed at PPVPN mechanisms.

   16. Describe the protection, if any, the approach provides against
      unstable or malicious operation of a PPVPN user network:

      a. Protection against high levels of, or malicious design of,
         routing traffic from PPVPN user networks to the service
         provider network.
      b. Protection against high levels of, or malicious design of,
         network management traffic from PPVPN user networks to the
         service provider network.
      c. Protection against worms and probes originated in the PPVPN
         user networks, sent towards the service provider network.

   17. Is the approach subject to any approach-specific
      vulnerabilities not specifically addressed by this template?  If
      so describe the defense or mitigation, if any, the approach
      provides for each.

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10.     Security Considerations

   Security considerations constitute the sole subject of this memo
   and hence are discussed throughout.  Here we recap what has been
   presented and explain at a very high level the role of each type of
   consideration in an overall secure PPVPN system.

   The document describes a number of potential security threats.
   Some of these threats have already been observed occurring in
   running networks; others are largely theoretical at this time.  DOS
   attacks and intrusion
   attacks from the Internet against service provider infrastructure
   have been seen to occur.  DOS "attacks" (typically not malicious)
   have also been seen in which CE equipment overwhelms PE equipment
   with high quantities or rates of packet traffic or routing
   information.  Operational/provisioning errors are cited by service
   providers as one of their prime concerns.

   The document describes a variety of defensive techniques that may
   be used to counter the suspected threats.  All of the techniques
   presented involve mature and widely implemented technologies that
   are practical to implement.

   The document describes the importance of detecting, monitoring, and
   reporting attacks, both successful and unsuccessful.  These
   activities are essential for "understanding one's enemy",
   mobilizing new defenses, and obtaining metrics about how secure the
   PPVPN service is.  As such they are vital components of any
   complete PPVPN security system.

   The document evaluates PPVPN security requirements from a customer
   perspective as well as from a service provider perspective.  These
   sections re-evaluate the identified threats from the perspectives
   of the various stakeholders and are meant to assist equipment
   vendors and service providers, who must ultimately decide what
   threats to protect against in any given equipment or service

   Finally, the document includes a template for use by authors of
   PPVPN technical solutions for evaluating how those solutions
   measure up against the security considerations presented in this

11.     Acknowledgement

   The authors would also like to acknowledge the helpful comments and
   suggestions from Paul Hoffman, Eric Gray, Ron Bonica, Chris Chase,
   Jerry Ash, Stewart Bryant, and the IETF Security Directorate.

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   [Beard] D. Beard and Y. Yang, "Known Threats to Routing Protocols,"
   draft-beard-rpsec-routing-threats-00.txt, Oct. 2002.

   [GDOI] M. Baugher, T. Hardjono, H. Harney, B. Weis, "The Group
   Domain of Interpretation," draft-ietf-msec-gdoi-07.txt, December

   [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
   for Message Authentication," February 1997.

   [RFC-2246] T. Dierks and C. Allen, "The TLS Protocol Version 1.0",
   RFC 2246, January 1999.

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

   [RFC2402] S. Kent, R. Atkinson, "IP Authentication Header,"

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

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

   [RFC2411] R. Thayer, N. Doraswamy, R. Glenn,  "IP Security Document
   Roadmap," November 1998.

   [RFC3174] D. Eastlake, 3rd, and P. Jones, "US Secure Hash Algorithm
   1 (SHA1)," September 2001.

   [SECMECH] S. Bellovin, C. Kaufman, J. Schiller, "Security
   Mechanisms for the Internet," draft-iab-secmech-02.txt, January

   [STD62] "Simple Network Management Protocol, Version 3," RFCs 3411-
   3418, December 2002.

   [STD-8] J. Postel and J. Reynolds, "TELNET Protocol Specification",
   STD 8, May 1983.

   [L3VPN-FW] R. Callon et al, "A Framework for Layer 3 Provider
   Provisioned Virtual Private Networks," Internet-draft <draft-ietf-
   l3vpn-framework-00.txt>, March 2003.

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   [L3VPN-REQ] M. Carugi et al, "Service Requirements for Layer 3
   Provider Provisioned Virtual Private Networks," Internet-draft
   <draft-ietf-l3vpn-requirements-00.txt>, April 2003.

Author's Addresses

   Luyuan Fang
   200 Laurel Avenue, Room C2-3B35      Phone: 732-420-1921
   Middletown, NJ 07748                 Email:

   Michael Behringer
   Avda de la Vega 15                   Phone: 34-639-659-822
   28100 Alcobendas, Madrid             Email:

   Ross Callon
   Juniper Networks
   10 Technology Park Drive             Phone: 978-692-6724
   Westford, MA  01886                  Email:

   Fabio Chiussi
   Lucent Technologies
   101 Crawfords Corner Rd, Room 4G502  Phone: 732-949-2407
   Holmdel, NJ 07733                    Email:

   Jeremy De Clercq
   Fr. Wellesplein 1, 2018 Antwerpen    E-mail:

   Mark Duffy
   Quarry Technologies
   8 New England Executive Park         Phone: 781-359-5052
   Burlington, MA 01803                 Email:

   Paul Hitchen
   BT Adastral Park
   Martlesham Heath                     Phone: 44-1473-606-344
   Ipswich IP53RE                       Email:

   Paul Knight
   Nortel Networks
   600 Technology Park Drive    Phone: 978-288-6414

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   Billerica, MA 01821          Email:

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