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rfc:rfc4111

Network Working Group L. Fang, Ed. Request for Comments: 4111 AT&T Labs. Category: Informational July 2005

                      Security Framework for
       Provider-Provisioned Virtual Private Networks (PPVPNs)

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document addresses security aspects pertaining to Provider-
 Provisioned Virtual Private Networks (PPVPNs).  First, it describes
 the security threats in the context of PPVPNs and defensive
 techniques to combat those threats.  It considers security issues
 deriving both from malicious behavior of anyone and from negligent or
 incorrect behavior of the providers.  It also describes how these
 security attacks should be detected and reported.  It then discusses
 possible user requirements for security of a PPVPN service.  These
 user requirements translate into corresponding provider requirements.
 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, this document defines a template
 that may be used to describe and analyze the security characteristics
 of a specific PPVPN technology.

Table of Contents

 1.  Introduction .................................................  2
 2.  Terminology ..................................................  4
 3.  Security Reference Model .....................................  4
 4.  Security Threats .............................................  6
     4.1.  Attacks on the Data Plane ..............................  7
     4.2.  Attacks on the Control Plane ...........................  9
 5.  Defensive Techniques for PPVPN Service Providers ............. 11
     5.1.  Cryptographic Techniques ............................... 12
     5.2.  Authentication ......................................... 20
     5.3.  Access Control Techniques .............................. 22
     5.4.  Use of Isolated Infrastructure ......................... 27

Fang Informational [Page 1] RFC 4111 PPVPN Security Framework July 2005

     5.5.  Use of Aggregated Infrastructure ....................... 27
     5.6.  Service Provider Quality Control Processes ............. 28
     5.7.  Deployment of Testable PPVPN Service ................... 28
 6.  Monitoring, Detection, and Reporting of Security Attacks ..... 28
 7.  User Security Requirements ................................... 29
     7.1.  Isolation .............................................. 30
     7.2.  Protection ............................................. 30
     7.3.  Confidentiality ........................................ 31
     7.4.  CE Authentication ...................................... 31
     7.5.  Integrity .............................................. 31
     7.6.  Anti-replay ............................................ 32
 8.  Provider Security Requirements ............................... 32
     8.1.  Protection within the Core Network ..................... 32
     8.2.  Protection on the User Access Link ..................... 34
     8.3.  General Requirements for PPVPN Providers ............... 36
 9.  Security Evaluation of PPVPN Technologies .................... 37
     9.1.  Evaluating the Template ................................ 37
     9.2.  Template ............................................... 37
 10. Security Considerations ...................................... 40
 11. Contributors ................................................. 41
 12. Acknowledgement .............................................. 42
 13. Normative References ......................................... 42
 14. Informative References ....................................... 43

1. Introduction

 Security is 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 [RFC4110] and [RFC4031].  These
 documents acknowledge that security is an important and integral
 aspect of PPVPN services, for both VPN customers and VPN service
 providers.  Both will benefit from a PPVPN Security Framework
 document that lists the customer and provider 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.
 First, we 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 providers.  An important part
 of security defense is the detection and report of a security attack,

Fang Informational [Page 2] RFC 4111 PPVPN Security Framework July 2005

 which is also addressed in this document.  Special considerations
 engendered by IP mobility within PPVPNs are not in the scope of this
 document.
 Then, we discuss the possible user and provider security requirements
 for a PPVPN service.  Users expectations must be met for the security
 characteristics of a VPN service.  These user requirements translate
 into corresponding requirements for the providers offering the
 service.  Furthermore, providers have security requirements to
 protect their network infrastructure, securing it 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 is to
 provide a common tool, in the form of a checklist, 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, thereby
 providing a common ground for comparison between different
 technologies.
 It is important to clarify that this document is limited to
 describing users' and providers' security requirements that pertain
 to PPVPN services.  It is not the intention to formulate precise
 "requirements" on each specific technology by defining the mechanisms
 and techniques that must be implemented to satisfy such users' and
 providers' requirements.
 This document is organized as follows.  Section 2 defines the
 terminology used in the document.  Section 3 defines the security
 reference model for security in PPVPN networks.  Section 4 describes
 the security threats that are specific of PPVPNs.  Section 5 reviews
 defense techniques that may be used against those threats.  Section 6
 describes how attacks may be detected and reported.  Section 7
 discusses the user security requirements that apply to PPVPN
 services.  Section 8 describes additional security requirements on
 the provider to guarantee the security of the network infrastructure
 providing PPVPN services.  In Section 9, we provide a template that
 may be used to describe the security characteristics of specific
 PPVPN technologies.  Finally, Section 10 discusses security
 considerations.

Fang Informational [Page 3] RFC 4111 PPVPN Security Framework July 2005

2. Terminology

 This document uses PPVPN-specific terminology.  Definitions and
 details specific to PPVPN terminology can be found in [RFC4026] and
 [RFC4110].  The most important definitions are repeated in this
 section; for other definitions, the reader is referred to
 [RFC4026] and [RFC4110].
    CE: Customer Edge device, a router or a switch in the customer
       network interfacing with the service provider's network.
    P: Provider Router.  The Provider Router is a router in the
       service provider's core network that does not have interfaces
       directly toward the customer.  A P router is used to
       interconnect the PE routers.  A P router does not have to
       maintain VPN state and is thus VPN unaware.
    PE: Provider Edge device, the equipment in the service provider's
       network that interfaces with the equipment in the customer's
       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: Virtual Private Network, which restricts communication
       between a set of sites using an IP backbone 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
 networks.
 A PPVPN core network is 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, it appears to the PPVPN users as a single zone of
 trust.  However, several service providers providing a common PPVPN
 core still have 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.

Fang Informational [Page 4] RFC 4111 PPVPN Security Framework July 2005

 A PPVPN user is a company, institution or residential client of the
 PPVPN service provider.
 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 wherein restricted and controlled communication
 is allowed between trusted zones, often via well-defined transit
 points.
 This document defines each PPVPN as a trusted zone and the PPVPN core
 as another trusted zone.  A primary concern is 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, PPVPN
 core networks often offer Internet access, in which case a transit
 point (marked "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 that are
 independent of whether a network is a PPVPN network or a private

Fang Informational [Page 5] RFC 4111 PPVPN Security Framework July 2005

 network.  For example, attacks from the Internet to a web server
 inside a given PPVPN will not be considered here, unless the
 provisioning of the PPVPN network could make a difference to the
 security of this server.

4. Security Threats

 This section discusses the various network security threats that may
 endanger PPVPNs.  The discussion is limited to 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:
  1. observation, modification, or deletion of PPVPN user data,
  1. replay of PPVPN user data,
  1. injection of non-authentic data into a PPVPN,
  1. traffic pattern analysis on PPVPN traffic,
  1. disruption of PPVPN connectivity, or
  1. degradation of PPVPN service quality.
 It is useful to consider that threats to a PPVPN, whether malicious
 or accidental, may come from different categories of sources.  For
 example they may come from:
  1. users of other PPVPNs provided by the same PPVPN service provider,
  1. the PPVPN service provider or persons working for it,
  1. other persons who obtain physical access to a service provider

site,

  1. other persons who use social engineering methods to influence

behavior of service provider personnel,

  1. users of the PPVPN itself, i.e., intra-VPN threats (such threats

are beyond the scope of this document), or

  1. others, i.e., attackers from the Internet at large.
 In the case of PPVPNs, some parties may be in more advantageous
 positions that enable them to launch types of attacks not available
 to others.  For example, users of different PPVPNs provided by the

Fang Informational [Page 6] RFC 4111 PPVPN Security Framework July 2005

 same service provider may be able to launch attacks that those who
 are 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.  There is at least a perceived difference in the likelihood
 of most types of attacks being successfully mounted in different
 environments, such as
  1. in a PPVPN contained within one service provider's network, or
  1. in a PPVPN transiting the public Internet.
 Most types of attacks become easier to mount, and hence more likely,
 as the shared infrastructure that provides VPN service expands from a
 single service provider to multiple cooperating providers, and then
 to the global Internet.  Attacks that may not be sufficiently likely
 to warrant concern in a closely controlled environment often merit
 defensive measures in broader, more open environments.
 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 unchanged.

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 into the VPN (or "spoofing") of packets
 that do not belong there, with the objective of having them accepted
 as legitimate by the recipient.  Also included in this category is

Fang Informational [Page 7] RFC 4111 PPVPN Security Framework July 2005

 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 significantly less of a concern than
 are 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, and CPU power.
 DoS attacks of the resource exhaustion type can be mounted against
 the data plane of a particular PPVPN by attempting to insert (spoof)
 an overwhelming quantity of non-authentic data into the VPN from
 outside of that VPN.  Potential results might be to exhaust the
 bandwidth available to that VPN or to overwhelm the cryptographic
 authentication mechanisms of 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.
 For example, a PPVPN user might be able to monopolize network data
 plane resources and thus to disrupt other PPVPNs.)

Fang Informational [Page 8] RFC 4111 PPVPN Security Framework July 2005

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 Network Infrastructure

 Control plane DoS attacks can be mounted specifically against the
 mechanisms that the service provider uses to provide PPVPNs (e.g.,
 IPsec, MPLS) 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 to the VPN service;
 otherwise, they are not a PPVPN-specific issue.
 Of special concern for PPVPNs is denial of service to one PPVPN user
 caused by the activities of another.  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.
 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 Service Provider Equipment via Management

      Interfaces
 This includes unauthorized access to service provider infrastructure
 equipment, in order, for example, to reconfigure the equipment or to
 extract information (statistics, topology, etc.) about one or more
 PPVPNs.
 This can be accomplished through malicious entrance of the systems,
 or as an inadvertent 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 Service Provider

      Infrastructure
 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

Fang Informational [Page 9] RFC 4111 PPVPN Security Framework July 2005

 VPNs than against VPNs that are self-provisioned by the customer at
 the IP layer.

4.2.4. Cross-Connection of Traffic between PPVPNs

 This refers to events where expected isolation between separate
 PPVPNs is breached.  This includes cases such as:
  1. a site being connected into the "wrong" VPN,
  1. two or more VPNs being improperly merged,
  1. a point-to-point VPN connecting the wrong two points, or
  1. any packet or frame being improperly delivered outside the VPN it

is sent in.

 Misconnection or cross-connection of VPNs may be caused by service
 provider or equipment vendor error, or by the malicious action of an
 attacker.  The breach may be physical (e.g., PE-CE links
 misconnected) or logical (improper device configuration).
 Anecdotal evidence suggests that the cross-connection threat is one
 of the largest security concerns of PPVPN users (or would-be users).

4.2.5. Attacks against PPVPN Routing Protocols

 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 are
 described in [RFC3889].

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 layer-3 VPN types for which 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

Fang Informational [Page 10] RFC 4111 PPVPN Security Framework July 2005

 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 have to be
 kept separate.  In layer-2 VPNs, the MAC address and VLAN spaces of
 different VPNs have 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:
  1. MPLS signaling (LDP, RSVP-TE),
  2. IPsec signaling (IKE) ,
  3. L2TP,
  4. BGP-based membership discovery, and
  5. 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

 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 in regard
 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 protects against
 those attacks that are most likely to occur or that could have the
 most dire consequences.  Absolute protection against these attacks is
 seldom achievable; more often it is sufficient to make the cost of a
 successful attack greater than what the adversary would be willing to
 expend.

Fang Informational [Page 11] RFC 4111 PPVPN Security Framework July 2005

 Successful defense against an attack does not necessarily mean that
 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 that are applicable to general network
 communications.  In general, these techniques can provide
 confidentiality (encryption) of communication between devices,
 authentication of the identities of the devices, and detection of a
 change of the protected data 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.  This section focuses on
 encryption; traffic separation is addressed separately.
 Several aspects of authentication are addressed in some detail in a
 separate "Authentication" section.
 Encryption adds complexity, and thus it may not be a standard
 offering within every PPVPN service.  There are a few reasons for
 this.  Encryption adds an additional computational burden to the
 devices performing encryption and decryption.  This may reduce the
 number of user VPN connections that 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.  Encrypting packets typically increases
 packet lengths, thereby increasing the network traffic load and the
 likelihood of packet fragmentation, with its increased overhead.
 (Packet length increase can often be mitigated to some extent by data
 compression techniques, but with 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.

Fang Informational [Page 12] RFC 4111 PPVPN Security Framework July 2005

 In particular, it determines where encryption should be applied, as
 follows.
  1. 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.

  1. 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 be encrypted.
  1. 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.
  1. If the PPVPN user requires remote access to a PPVPN from a

system that is not at 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 by using encryption, as well.
 Although CE-CE encryption provides confidentiality 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, to bypass encryption processing altogether, or to
 provide some means of sniffing or diverting unencrypted traffic.
 Thus, a PPVPN implementation using CE-CE encryption has to consider
 the trust relationship between the PPVPN user and provider.  PPVPN
 users and providers may wish to negotiate a service level agreement
 (SLA) for CE-CE encryption that 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 or a configuration lock-down ability, or it might allow
 both parties to verify the configuration.  In general, if the managed
 CE-CE model is used, the PPVPN user has to have a fairly high level
 of trust that the PPVPN provider will properly provision and manage
 the CE devices.

Fang Informational [Page 13] RFC 4111 PPVPN Security Framework July 2005

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 [RFC3631].  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, data encryption that is performed within the user's
 site is outside the scope of this document, as it is simply handled
 as user data by the PPVPN.  IPsec can also be used to protect IP
 traffic between a remote user and the PPVPN.
 IPsec does not itself specify an encryption algorithm.  It can use a
 variety of encryption algorithms with various key lengths, such as
 AES encryption.  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 confidentiality rather than to analyze the specific
 encryption techniques used in the PPVPN service.
 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.  With IPsec, authentication of messages is
 provided by the Authentication Header (AH) or by the Encapsulating
 Security Protocol (ESP) with authentication only.  Where control
 messages require authentication but do not use IPsec, 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].

Fang Informational [Page 14] RFC 4111 PPVPN Security Framework July 2005

 One recommended mechanism for providing a combination
 confidentiality, data origin authentication, and connectionless
 integrity is the use of AES in Cipher Block Chaining (CBC) Mode, with
 an explicit Initialization Vector (IV) [RFC3602], as the IPsec ESP.
 PPVPNs that provide differentiated services based on traffic type may
 encounter some conflicts with IPsec encryption of traffic.  As
 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 confidentiality.
  1. Secure Shell (SSH) offers protection for TELNET [STD8] or

terminal-like connections to allow device configuration.

  1. SNMP v3 [STD62] provides encrypted and authenticated protection

for SNMP-managed devices.

  1. Transport Layer Security (TLS) [RFC2246] and the closely-related

Secure Sockets Layer (SSL) are widely used for securing HTTP-based

    communication, and thus can provide support for most XML- and
    SOAP-based device management approaches.
  1. As of 2004, extensive work is proceeding in several organizations

(OASIS, W3C, WS-I, and others) on securing device management

    traffic within a "Web Services" framework.  This work uses a wide
    variety of security models and supports multiple security token
    formats, multiple trust domains, multiple signature formats, and
    multiple encryption technologies.
  1. IPsec provides the services with security and confidentiality at

the network layer. With regard to device management, its current

    use is primarily focused on in-band management of user-managed
    IPsec gateway devices.

Fang Informational [Page 15] RFC 4111 PPVPN Security Framework July 2005

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
 IPsec will generally not be applicable for CE-PE traffic in Layer-2
 PPVPNs.
 Encryption techniques for Layer-2 links are widely available but are
 not within the scope of this document or IETF documents in general.
 Layer-2 encryption could be applied to the links from CE to PE, or it
 could be applied from CE to CE, as long as the encrypted Layer-2
 packets can be handled properly 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, confidentiality 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 that encrypt the
 traffic passing between them.  The devices may be directly connected
 (over a single "hop"), or there may be intervening devices that
 transport the encrypted traffic between the pair of devices.  The
 extreme cases involve hop-by-hop encryption between every adjacent
 pair of devices along a given path or "end-to-end" encryption only
 between the end devices along a given path.  To keep this discussion
 within the scope of PPVPNs, we consider the "end to end" case to be
 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 that might be
 present along the path between two sites in a single VPN, operated by
 a single service provider (SP).
        Site_1---CE---PE---P---P---P---PE---CE---Site_2
                Figure 2: Simplified PPVPN topology

Fang Informational [Page 16] RFC 4111 PPVPN Security Framework July 2005

 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
 devices:
    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) can be combined, with encryption
       running from CE to PE, then from PE to PE, and then from PE to
       CE.
 Among the four feasible configurations, key tradeoffs in considering
 encryption include the following:
  1. Vulnerability to link eavesdropping: Assuming that an attacker can

observe the data in transit on the links, would it be protected by

    encryption?
  1. Vulnerability to device compromise: Assuming an attacker can get

access to a device (or freely alter its configuration), would the

    data be protected?
  1. Complexity of device configuration and management: Given Nce, the

number of sites per VPN customer, and Npe, the number of PEs

    participating in a given VPN, how many device configurations have
    to be created or maintained and how do those configurations scale?
  1. 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.
  1. 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 below for each configuration.

Fang Informational [Page 17] RFC 4111 PPVPN Security Framework July 2005

 1) Site-to-site (CE-to-CE) Configurations
    o  Link eavesdropping: Protected on all links.
    o  Device compromise: Vulnerable to CE compromise.
    o  Complexity: Single administration, responsible for one device
       per site (Nce devices), but overall configuration per VPN
       scales as Nce**2.
    o  Processing load: on each of two CEs, each packet is either
       encrypted or decrypted (2P).
    o  Enhanced services: Severely limited; typically only DiffServ
       markings are visible to SP, allowing some QoS services.
 2) Provider edge-to-edge (PE-to-PE) Configurations
    o  Link eavesdropping: Vulnerable on CE-PE links; protected on
       SP's network links.
    o  Device compromise: Vulnerable to CE or PE compromise.
    o  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
       combined.
    o  Processing load: On each of two PEs, each packet is either
       encrypted or decrypted (2P).
    o  Enhanced services: Full; SP can apply any enhancements based on
       detailed view of traffic.
 3) Access link (CE-to-PE) Configuration
    o  Link eavesdropping: Protected on CE-PE link; vulnerable on SP's
       network links.
    o  Device compromise: Vulnerable to CE or PE compromise.
    o  Complexity: Two administrations (customer and SP) with device
       configuration on each side (Nce + Npe devices to configure),
       but as there is no mesh, the overall configuration scales as
       Nce.

Fang Informational [Page 18] RFC 4111 PPVPN Security Framework July 2005

    o  Processing load: On each of two CEs, each packet is either
       encrypted or decrypted.  On each of two PEs, each packet is
       either encrypted or decrypted (4P).
    o  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).
    o  Link eavesdropping: Protected on all links.
    o  Device compromise: Vulnerable to CE or PE compromise.
    o  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.
    o  Processing load: On each of two CEs, each packet is either
       encrypted or decrypted.  On each of two PEs, each packet is
       both encrypted and decrypted (6P).
    o  Enhanced services: Full; SP can apply any enhancements based on
       detailed view of traffic.
 Given the tradeoffs discussed above, a few conclusions can be
 reached.
  1. Configurations 2 and 3, which are subsets of 4, may be appropriate

alternatives to 4 under certain threat models. The remainder of

    these conclusions compare 1 (CE-to-CE) with 4 (combined access
    links and PE-to-PE).
  1. If protection from link eavesdropping is most important, then

configurations 1 and 4 are equivalent.

  1. 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.
  1. If reducing complexity is most important and the size of the

network is very small, configuration 1 is the best. Otherwise,

    the comparison between options 1 and 4 is relatively complex ,
    based on a number of issues such as, how close the CE to CE
    communication is to a full mesh, and what tools are used for key
    management.  Option 1 requires configuring keys for each CE-CE

Fang Informational [Page 19] RFC 4111 PPVPN Security Framework July 2005

    pair that is communicating directly.  Option 4 requires
    configuring keys on both CE and PE devices but may offer benefit
    from the fact that the number of PEs is generally much smaller
    than the number of CEs.
    Also, under some PPVPN approaches, the scaling of 4 is further
    improved by sharing the same PE-PE mesh across all VPN contexts.
    The scaling characteristics of 4 may be increased or decreased in
    any given situation if the CE devices are simpler to configure
    than the PE devices, or vice versa.  Furthermore, with option 4,
    the impact of operational error may be significantly increased.
  1. If the overall processing load is a key factor, then 1 is best.
  1. 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 it comes at the cost of
 enhanced services and with additional operational complexity due to
 the Order(n**2) scaling of the mesh.
 This analysis of site-to-site vs. hop-by-hop encryption tradeoffs
 does not explicitly include cases where multiple providers cooperate
 to provide a PPVPN service, public Internet VPN connectivity, or
 remote access VPN service, but many of the tradeoffs will be similar.

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 for
 ensuring 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 it recommends authentication mechanisms for these scenarios.
 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
 exhaustion.  In fact, the processing required to decrypt 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).

Fang Informational [Page 20] RFC 4111 PPVPN Security Framework July 2005

5.2.1. VPN Member Authentication

 This category includes techniques for the CEs to verify that 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), 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 [RFC2865] and
 DIAMETER [RFC3588].  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
 [RFC2661] 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 any of several possible
 locations in the network.  The user-managed case is of limited
 interest within the PPVPN security framework, and it 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 have to be

Fang Informational [Page 21] RFC 4111 PPVPN Security Framework July 2005

 referenced in a secure manner by the PPVPN provider.  This can be
 accomplished through 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 it 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 certificates.
 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 elsewhere in this document.
 We distinguish between filtering and firewalls primarily by the
 direction of traffic flow.  We define filtering as being applicable
 to unidirectional traffic, whereas a firewall can analyze and control
 both sides of a conversation.
 There are two significant corollaries of this definition:
  1. 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 then
    operate upon traffic flowing in one direction without considering
    traffic in the reverse direction.

Fang Informational [Page 22] RFC 4111 PPVPN Security Framework July 2005

  1. Statefulness: Because it receives both sides of a conversation, a

firewall may be able to obtain a significant amount of information

    concerning that conversation and to use this information to
    control access.  A filter can maintain some limited state
    information on a unidirectional flow of packets, but it cannot
    determine the state of the bi-directional conversation as
    precisely as a firewall can.

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 that match the
 criteria associated with a particular filter may be either discarded
 or given special treatment.
 In discussing filters, it is useful to separate the filter
 characteristics that may be used to determine whether a packet
 matches a filter from the packet actions that are applied to packets
 that 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 determines whether a particular packet matches a
    filter based solely on the filter definition, on normal forwarding
    information (such as the next hop for a packet), and on 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 may also consider fields in the TCP or UDP header
    such as the Port fields and 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 commonly done in firewalls, although firewall technology
    may be added to routers.

Fang Informational [Page 23] RFC 4111 PPVPN Security Framework July 2005

 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 that may be taken based on that
    filter match.  Examples of such actions include:
  1. Discard
       In many cases, filters may be set to catch certain undesirable
       packets.  Examples may include packets with forged or invalid
       source addresses, packets that are part of a DoS or DDoS
       attack, or packets that are trying to access forbidden
       resources (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 also be counted
       and/or logged, of course.
  1. Set CoS
       A filter may be used to set the Class of Service associated
       with the packet.
  1. Count Packets and/or Bytes
  1. Rate Limit
       In some cases, the set of packets that match a particular
       filter may be limited to a specified bandwidth.  Packets and/or
       bytes would be counted and forwarded normally up to the
       specified limit.  Excess packets may be discarded or marked
       (for example, by setting a "discard eligible" bit in the IP ToS
       field or the MPLS EXP field).
  1. Forward and Copy
       It is useful in some cases not only to forward some set of
       packets normally, but also to 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 System.
 o  Other Issues Related to 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 due to differences between types or numbers

Fang Informational [Page 24] RFC 4111 PPVPN Security Framework July 2005

    of filters deployed.  For filtering to be useful, the 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 on 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.
    A key consideration with the use of packet filters is that they
    can provide few options for filtering packets carrying encrypted
    data.  Because the data itself is not accessible, only packet
    header information or other unencrypted fields can be used for
    filtering.

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, as 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, or acting as a TCP connection proxy.  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 it 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.

Fang Informational [Page 25] RFC 4111 PPVPN Security Framework July 2005

 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.
 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.
 Another consideration with the use of firewalls is that they can
 provide few options for handling packets carrying encrypted data.  As
 the data itself is not accessible, only packet header information,
 other unencrypted fields, or analysis of the flow of encrypted
 packets can be used for making decisions on accepting or rejecting
 encrypted traffic.

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 described in
 the section on authentication.  However, additional security may be
 provided by controlling access to management interfaces in other
 ways.
 Management interfaces, especially console ports on PPVPN devices, may
 be configured so that they are only accessible out of band, through a
 system that is physically 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.

Fang Informational [Page 26] RFC 4111 PPVPN Security Framework July 2005

5.4. Use of Isolated Infrastructure

 One way to protect the infrastructure used for support of VPNs is to
 separate the VPN support resources from the resources used for other
 purposes (such as support of Internet services).  In some cases, this
 may require the 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 they may use 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 that are
 dedicated to one specific VPN.  In many or most cases, CE-based VPNs
 may make use of normal Internet services to interconnect CE devices.

5.5. Use of Aggregated Infrastructure

 In general it is not feasible to use a completely separate set of
 resources for support of each VPN.  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, 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 use by different VPNs.  Thus, a well-behaved VPN
 user should be protected from possible misbehavior by other VPNs.
 This requires that limits be placed on the amount of resources that
 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 and CPU and memory resources may be reserved on a
 per-VPN basis.
 The techniques that 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.
 The use of aggregated infrastructure allows the service provider to
 benefit from stochastic multiplexing of multiple bursty flows and may

Fang Informational [Page 27] RFC 4111 PPVPN Security Framework July 2005

 also, 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 requires a relatively
 large amount of configuration by the service provider.  For example,
 the service provider has 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 misconfiguration.
 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 readily be tested for correct
 configuration.  For example, for a point-point VPN, checking that the
 intended connectivity is working largely ensures that there is not
 connectivity to some unintended site.

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
 add value for both providers and customers by implementing security-
 monitoring systems that detect and report on any security attacks
 that occur, regardless of whether the attacks are effective.
 Attackers often begin by probing and analyzing defenses, so systems
 that 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 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 to 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.

Fang Informational [Page 28] RFC 4111 PPVPN Security Framework July 2005

 Monitoring systems used to detect security attacks in PPVPNs will
 typically operate by collecting information from 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 notifications).  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 allow
 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.
 The mechanisms for reporting security attacks should be flexible
 enough to meet the needs of VPN service providers, VPN customers, and
 regulatory agencies.  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 translate into requirements 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 or all of these
 security requirements.  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.

Fang Informational [Page 29] RFC 4111 PPVPN Security Framework July 2005

7.1. Isolation

 A virtual private network usually defines "private" as isolation from
 other PPVPNs and the Internet.  More specifically, isolation has
 several components, which are discussed in the following sections.

7.1.1. Address Separation

 A given PPVPN can use the full Internet address range, including
 private address ranges [RFC1918], without interfering with other
 PPVPNs that use PPVPN services from the same service provider(s).
 When Internet access is provided (e.g., by the same service provider
 that is offering PPVPN service), NAT functionality may be needed.
 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, unless the zones are specifically
 engineered this way (e.g., for Internet access.)
 In layer-2 VPNs, the switching information must be kept separate
 between the trusted zones, so that switching information of one PPVPN
 does not influence other PPVPNs or the PPVPN core.

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
 made where zones are is specifically engineered that way (e.g., for
 extranet purposes or Internet access.)

7.2. Protection

 The common perception is that a completely separated "private"
 network has defined entry points and is only subject to attack or
 intrusion over those entry points.  By sharing a common core, a PPVPN
 appears to lose some of these clear interfaces to networks 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.

Fang Informational [Page 30] RFC 4111 PPVPN Security Framework July 2005

7.2.1. Protection against Intrusion

 An intrusion is defined here as the penetration of a trusted zone
 from outside.  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 (DoS) attack aims at making services or devices
 unavailable to legitimate users.  In the framework of this document,
 only those DoS attacks are considered that are a consequence of
 providing network service through a VPN.  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 it would be if the same network were private.

7.2.3. Protection against Spoofing

 It must not be possible to violate the integrity of a PPVPN by
 changing the sender identification (source address, source label,
 etc) of traffic in transit.  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.

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 such a manner that it cannot be
 altered or that any alteration may be detected at the receiver.

Fang Informational [Page 31] RFC 4111 PPVPN Security Framework July 2005

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 private
 and "virtually private" PPVPN services are compared, these
 requirements are only applicable if the comparable private service
 also included these services.  However, the fact that VPNs operate
 over a shared infrastructure may make some of these requirements more
 important in a VPN environment than in a private network environment.

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, and 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 core
 may lead to additional security requirements, if only because 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

  1. 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

Fang Informational [Page 32] RFC 4111 PPVPN Security Framework July 2005

    sessions may be authenticated by using TCP MD5 or IPsec.  If an
    MPLS core is used, LDP sessions may be authenticated by using 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 it will
    improve the security of the core.  If the core is dedicated to VPN
    services and there are no interconnects to third parties, then it
    may reduce the requirement for authentication of the core control
    plane.
  1. 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 by 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 should be taken for
    loop prevention.  This prevents the backbone addresses from being
    exposed through trace route; however, it 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, by using MPLS label
    switching for all traffic within the core and possibly by making
    the Internet a VPN within the PPVPN core itself.  This helps
    detach Internet access from PPVPN services.
    PE devices may implement separate control plane, data plane, and
    management plane functionality in terms of hardware and software,
    to improve security.  This may help limit the problems when one
    particular area is attacked, and it may allow each plane to
    implement additional security measurement separately.
    PEs are often more vulnerable to attack than P routers, since, by
    their very nature, PEs cannot be made unreachable to outside
    users.  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 8.2.3).

Fang Informational [Page 33] RFC 4111 PPVPN Security Framework July 2005

    In the PE, using separate routing processes for Internet and PPVPN
    service may help 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 on
    individual VPNs, it may help 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.  Providing PPVPN
    services introduces new security requirements for VPN services.
    Similar consideration apply to L2 VPN services.

8.1.2. Data Plane Protection

 PPVPN using IPsec technologies provides VPN users with encryption of
 secure user data.
 In today's MPLS, ATM, and Frame Relay networks, encryption is not
 provided as a basic feature.  Mechanisms can be used to secure the
 MPLS data plane and to secure the data carried over the MPLS core.
 Additionally, if the core is dedicated to VPN services and there are
 no external interconnects to third party networks, then there is no
 obvious need for encryption of the user data plane.
 Inter-working IPsec/L3 PPVPN technologies or IPsec/L2 PPVPN
 technologies may be used to provide PPVPN users with 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 an 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.
 Firewall/Filtering: Access control mechanisms can be used to filter
 out any packets destined for the service provider's infrastructure
 prefix or to eliminate routes identified as illegitimate.

Fang Informational [Page 34] RFC 4111 PPVPN Security Framework July 2005

 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 it supports VPN and Internet services on the same
 physical interfaces through different logical interfaces.

8.2.1. Link Authentication

 Authentication mechanisms can be employed to validate site access to
 the PPVPN network via fixed or logical (e.g., L2TP, IPsec)
 connections.  When 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, for example, by limiting it simply to an exchange in
 relation to the authentication phase and with other attributes (e.g.,
 optional filtering of RADIUS).

8.2.2. 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:
  1. Limiting the number of prefixes that may be advertised into the PE

on a per-access basis . Appropriate action may be taken should a

    limit be exceeded; for example, the PE might shut down the peer
    session to the CE.
  1. Applying route dampening at the PE on received routing updates.
  1. 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 may
 be applied between ASes.  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 both providers must agree on it.

Fang Informational [Page 35] RFC 4111 PPVPN Security Framework July 2005

8.2.3. Access QoS

 PPVPN providers offering QoS-enabled services require mechanisms to
 ensure that individual accesses are validated against their
 subscribed QOS profile and are granted 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 by marking,
 remarking, or discarding traffic that is outside of the profile.

8.2.4. 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 both to validate the incoming
 user and to limit the views available to that particular user's VPN.
 Mechanisms should also be considered to ensure that such access
 cannot be used to create a DoS attack (either malicious or
 accidental) on the PE itself.  This could be accomplished either
 through separation of these resources within the PE itself or via the
 capability to rate-limit such traffic on a per-VPN basis.

8.3. General Requirements for PPVPN Providers

 The PPVPN providers must support the users' security requirements as
 listed in Section 7.  Depending on the technologies used, these
 requirements may include the following.
  1. User control plane separation: Routing isolation.
  1. User address space separation: Supporting overlapping addresses

from different VPNs.

  1. User data plane separation: One VPN traffic cannot be intercepted

by other VPNs or any other users.

  1. Protection against intrusion, DoS attacks and spoofing.
  1. Access Authentication.
  1. Techniques highlighted through this document identify

methodologies for the protection of PPVPN resources and

    infrastructure.
 Hardware or software bugs in equipment that lead to security breaches
 are outside the scope of this document.

Fang Informational [Page 36] RFC 4111 PPVPN Security Framework July 2005

9. Security Evaluation of PPVPN Technologies

 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 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:
  1. The requirement is not applicable to the VPN approach because …

(fill in reason).

  1. The base VPN approach completely addresses the requirement by …

(fill in technique).

  1. The base VPN approach partially addresses the requirement by …

(fill in technique and extent to which it addresses the

    requirement).
  1. An optional extension to the VPN approach completely addresses the

requirement by … (fill in technique).

  1. An optional extension to the VPN approach partially addresses the

requirement by … (fill in technique and extent to which it

    addresses the requirement).
  1. 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, such as membership discovery, is
    done via configuration.  The protection afforded to the
    configuration would be beyond the scope of the VPN approach.).
  1. 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.

Fang Informational [Page 37] RFC 4111 PPVPN Security Framework July 2005

 2.  The approach provides complete L2 address space separation for
     each L2 VPN.
 3.  The approach provides complete VLAN ID space separation for each
     L2 VPN.
 4.  The approach provides complete IP route separation for each L3
     VPN.
 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, or
       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, or
       c. in the Internet used as PPVPN backbone.
 10. The approach provides sender authentication for PPVPN user data.
       a. in the CE-PE link,
       b. in a single- or multi-provider PPVPN backbone, or
       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, or
       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, or
       c. in the Internet used as PPVPN backbone.

Fang Informational [Page 38] RFC 4111 PPVPN Security Framework July 2005

 13. The approach provides protection against unauthorized traffic
     pattern analysis for PPVPN user data
       a. in the CE-PE link,
       b. in a single- or multi-provider PPVPN backbone, or
       c. in the Internet used as PPVPN backbone.
 14. The control protocol(s) used for each of the following functions
     provides 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, 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
     attacks):
       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.

Fang Informational [Page 39] RFC 4111 PPVPN Security Framework July 2005

       c. Protection against worms and probes originated in the PPVPN
          user networks, sent toward 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, that the approach provides for
     each.

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.  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 both successful and unsuccessful attacks.  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 and 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 offering.
 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 memo.

Fang Informational [Page 40] RFC 4111 PPVPN Security Framework July 2005

11. Contributors

 The following people made major contributions to writing this
 document:  Michael Behringer, Ross Callon, Fabio Chiussi, Jeremy De
 Clerque, Paul Hitchen, and Paul Knignt.
 Michael Behringer
 Cisco
 Village d'Entreprises Green Side,  Phone: +33.49723-2652
 400, Avenue Roumanille, Bat. T 3   EMail: mbehring@cisco.com
 06410 Biot, Sophia Antipolis
 France
 Ross Callon
 Juniper Networks
 10 Technology Park Drive           Phone: 978-692-6724
 Westford, MA  01886                EMail: rcallon@juniper.net
 Fabio Chiussi                      Phone: 1 978 367-8965
 Airvana                            EMail: fabio@airvananet.com
 19 Alpha Road
 Chelmsford, Massachusetts 01824
 Jeremy De Clercq
 Alcatel
 Fr. Wellesplein 1, 2018 Antwerpen  EMail: jeremy.de_clercq@alcatel.be
 Belgium
 Mark Duffy
 Sonus Networks
 250 Apollo Drive                   Phone: 1 978-614-8748
 Chelmsford, MA 01824               EMail: mduffy@sonusnet.com
 Paul Hitchen
 BT
 BT Adastral Park
 Martlesham Heath                   Phone: 44-1473-606-344
 Ipswich IP53RE                     EMail: paul.hitchen@bt.com
 UK
 Paul Knight
 Nortel
 600 Technology Park Drive          Phone: 978-288-6414
 Billerica, MA 01821                EMail: paul.knight@nortel.com

Fang Informational [Page 41] RFC 4111 PPVPN Security Framework July 2005

12. Acknowledgement

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

13. Normative References

 [RFC1918]    Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
              G., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, February 1996.
 [RFC2246]    Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.
 [RFC2401]    Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.
 [RFC2402]    Kent, S. and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.
 [RFC2406]    Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.
 [RFC2407]    Piper, D., "The Internet IP Security Domain of
              Interpretation for ISAKMP", RFC 2407, November 1998.
 [RFC2661]    Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol
              "L2TP"", RFC 2661, August 1999.
 [RFC2865]    Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.
 [RFC3588]    Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September
              2003.
 [RFC3602]    Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC
              Cipher Algorithm and Its Use with IPsec", RFC 3602,
              September 2003.

Fang Informational [Page 42] RFC 4111 PPVPN Security Framework July 2005

 [STD62]      Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC
              3411, December 2002.
              Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
              "Message Processing and Dispatching for the Simple
              Network Management Protocol (SNMP)", STD 62, RFC 3412,
              December 2002.
              Levi, D., Meyer, P., and B. Stewart, "Simple Network
              Management Protocol (SNMP) Applications", STD 62, RFC
              3413, December 2002.
              Blumenthal, U. and B. Wijnen, "User-based Security Model
              (USM) for version 3 of the Simple Network Management
              Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
              Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
              Access Control Model (VACM) for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3415, December
              2002.
              Presuhn, R., "Version 2 of the Protocol Operations for
              the Simple Network Management Protocol (SNMP)", STD 62,
              RFC 3416, December 2002.
              Presuhn, R., "Transport Mappings for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3417, December
              2002.
              Presuhn, R., "Management Information Base (MIB) for the
              Simple Network Management Protocol (SNMP)", STD 62, RFC
              3418, December 2002.
 [STD8]       Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, May 1983.

14. Informative References

 [RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
              Keyed-Hashing for Message Authentication", RFC 2104,
              February 1997.
 [RFC2411]    Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
              Document Roadmap", RFC 2411, November 1998.

Fang Informational [Page 43] RFC 4111 PPVPN Security Framework July 2005

 [RFC3174]    Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
              1 (SHA1)", RFC 3174, September 2001.
 [RFC3631]    Bellovin, S., Schiller, J., and C. Kaufman, "Security
              Mechanisms for the Internet", RFC 3631, December 2003.
 [RFC3889]    Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 3889, October 2004.
 [RFC4026]    Andersson, L. and T. Madsen, "Provider Provisioned
              Virtual Private Network (VPN) Terminology", RFC 4026,
              March 2005.
 [RFC4031]    Carugi, M. and D. McDysan, Eds., "Service Requirements
              for Layer 3 Provider Provisioned Virtual Private
              Networks (PPVPNs)", RFC 4031, April 2005.
 [RFC4110]    Callon, R. and M. Suzuki, "A Framework for Layer 3
              Provider Provisioned Virtual Private Networks", RFC
              4110, July 2005.

Author's Address

 Luyuan Fang
 AT&T Labs.
 200 Laurel Avenue, Room C2-3B35
 Middletown, NJ 07748
 Phone: 732-420-1921
 EMail: luyuanfang@att.com

Fang Informational [Page 44] RFC 4111 PPVPN Security Framework July 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 attempt made to obtain a general license or permission for the use of
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 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
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 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Fang Informational [Page 45]

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