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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 4381

Network Working Group                                       M. Behringer
Internet-Draft                                         Cisco Systems Inc
Expires: May 28, 2004                                  November 28, 2003


              Analysis of the Security of BGP/MPLS IP VPNs
                  draft-behringer-mpls-security-05.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on May 28, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document analyses the security of the BGP/MPLS IP VPN
   architecture as described in RFC 2547bis [13], especially in
   comparison with other VPN technologies such as ATM and Frame Relay.
   The target audience is service providers and VPN users. The document
   consists of two main parts: First the requirements for security in
   VPN services are defined, second BGP/MPLS IP VPNs are examined with
   respect to these requirements.

   The analysis shows that BGP/MPLS IP VPN networks can be equally
   secured as traditional layer-2 VPN networks such as ATM and Frame
   Relay.





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Table of Contents

   1.  Scope and Introduction . . . . . . . . . . . . . . . . . . . .  3
   2.  Security Requirements of VPN Networks  . . . . . . . . . . . .  4
   2.1 Address Space, Routing and Traffic Separation  . . . . . . . .  4
   2.2 Hiding of the Core Infrastructure  . . . . . . . . . . . . . .  4
   2.3 Resistance to Attacks  . . . . . . . . . . . . . . . . . . . .  5
   2.4 Impossibility of Label Spoofing  . . . . . . . . . . . . . . .  5
   3.  Analysis of BGP/MPLS IP VPN Security . . . . . . . . . . . . .  7
   3.1 Address Space, Routing and Traffic Separation  . . . . . . . .  7
   3.2 Hiding of the BGP/MPLS IP VPN Core Infrastructure  . . . . . .  8
   3.3 Resistance to Attacks  . . . . . . . . . . . . . . . . . . . .  9
   3.4 Label Spoofing . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.5 Comparison with ATM/FR VPNs  . . . . . . . . . . . . . . . . . 12
   4.  Security of advanced BGP/MPLS IP VPN architectures . . . . . . 14
   4.1 Carriers' Carrier (CsC)  . . . . . . . . . . . . . . . . . . . 14
   4.2 Inter-provider backbones . . . . . . . . . . . . . . . . . . . 15
   5.  What BGP/MPLS IP VPNs Do Not Provide . . . . . . . . . . . . . 18
   5.1 Protection against Misconfigurations of the Core and
       Attacks 'within' the Core  . . . . . . . . . . . . . . . . . . 18
   5.2 Data Encryption, Integrity and Origin Authentication . . . . . 18
   5.3 Customer Network Security  . . . . . . . . . . . . . . . . . . 19
   6.  Layer 2 security considerations  . . . . . . . . . . . . . . . 20
   7.  Summary and Conclusions  . . . . . . . . . . . . . . . . . . . 22
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 25
       Intellectual Property and Copyright Statements . . . . . . . . 26























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1. Scope and Introduction

   As MPLS (multi protocol label switching) is becoming a more
   wide-spread technology for providing IP VPN (virtual private network)
   services, the security of the BGP/MPLS IP VPN architecture is of
   increasing concern to service providers and VPN customers. This
   document gives an overview of the security of the BGP/MPLS IP VPN
   architecture as described in RFC 2547bis [13] for both service
   providers and MPLS users, and compares it with traditional layer-2
   services such as ATM or Frame Relay from a security perspective.

   The term "MPLS core" is defined for this document as the set of PE
   and P routers which are used to provide an BGP/MPLS IP VPN service,
   typically under the control of a single service provider. This
   document assumes that the MPLS core network is trusted and provided
   in a secure manner. Thus it does not address basic security concerns
   such as securing the network elements against unauthorised access,
   misconfigurations of the core, internal (within the core) attacks and
   the likes. Should a customer not wish to assume the service provider
   network as trusted it becomes necessary to use additional security
   mechanisms such as IPsec over the MPLS infrastructure. One way to
   implement IPsec over BGP/MPLS is described in
   draft-guichard-ce-ce-ipsec [16].

   Analysis of the security features of routing protocols is only
   covered to the extend where it influences BGP/MPLS IP VPNs. IPsec
   technology is also not covered, except to highlight the combination
   of MPLS VPNs with IPsec.

   The overall security of a system depends on three parts: the
   architecture, the implementation, and the operation of the system.
   Security issues can exist in either part. This document analyses the
   architectural security of BGP/MPLS IP VPNs. It does not cover
   implementation issues nor operational issues.

   This document is targeted at technical staff of service providers and
   enterprises. Knowledge of the basic BGP/MPLS IP VPN architecture as
   described in RFC 2547bis [13] is required to understand this
   document.












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2. Security Requirements of VPN Networks

   Both service providers offering any type of VPN services and
   customers using them have specific demands for security. Mostly they
   compare MPLS based solutions with traditional layer 2 based VPN
   solutions such as Frame Relay and ATM, since these are widely
   deployed and accepted. This section outlines the security
   requirements that are typically made in VPN networks. The following
   section discusses if and how BGP/MPLS IP VPNs address these
   requirements, for both the MPLS core and the connected VPNs.

2.1 Address Space, Routing and Traffic Separation

   Between two non-intersecting layer 3 VPNs of an VPN service it is
   assumed that the address space between different VPNs is entirely
   independent. This means that for example two non-intersecting VPNs
   must be able to both use the 10/8 network without any interference.
   In addition traffic from one VPN must never enter another VPN. This
   includes separation of routing protocol information, so that also
   routing tables are separate per VPN. Specifically:

   o  Any VPN must be able to use the same address space as any other
      VPN.

   o  Any VPN must be able to use the same address space as the MPLS
      core.

   o  Traffic from one VPN must never flow to another VPN.

   o  Routing information, as well as distribution and processing of
      that information, for one VPN instance must be independent from
      any other VPN instance.

   o  Routing information, as well as distribution and processing of
      that information, for one VPN instance must be independent from
      the core.

   From a security point of view the basic requirement is to avoid that
   packets destined to a host a.b.c.d within a given VPN reach a host
   with the same address in another VPN or the core, or get routed to
   another VPN even if this address does not exist there.

2.2 Hiding of the Core Infrastructure

   The internal structure of the core network (in the case of MPLS PE
   and P elements) should not be visible to outside networks (Internet
   or any connected VPN). Whilst a breach of this requirement does not
   lead to a security problem itself, many service providers feel that



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   it is advantageous if the internal addressing and network structure
   remains hidden to the outside world. A strong argument is that DoS
   attacks against a core router for example are much easier to carry
   out if an attacker knows the address. Where addresses are not known,
   they can be guessed, but with this attacks become more difficult.
   Ideally the core should be as invisible to the outside world as a
   comparable layer 2 (e.g., frame relay, ATM) infrastructure. Core
   network elements should also not be accessible from a VPN.

   Note that security should never rely on obscurity, i.e., the hiding
   of information. On the contrary services should be equally secure if
   the implementation is known. However, there is a strong market
   perception that hiding of details is advantageous. This point
   addresses that market perception.

2.3 Resistance to Attacks

   There are two basic types of attacks: Denial-of-Service (DoS)
   attacks, where resources become unavailable to authorised users, and
   intrusions, where resources become available to un-authorised users.

   For attacks that give unauthorised access to resources (intrusions)
   there are two basic ways to protect the network: Firstly, to harden
   protocols that could be abused (e.g., telnet to a router), secondly
   to make the network as inaccessible as possible. The latter is
   achieved by a combination of packet filtering or firewalling and
   address hiding, as discussed above.

   DoS attacks are easier to execute, since in the simplest case a known
   IP address might be enough to attack a machine. This can be done
   using normal "allowed" traffic, but higher than normal packet rates,
   so that other users cannot access the targeted machine. The only way
   to be certain not be vulnerable to this kind of attack is to make
   sure that machines are not reachable, again by packet filtering and
   optionally address hiding.

   BGP/MPLS IP VPN networks must provide at least the same level of
   protection against both forms of attack as current layer 2 networks.
   Note that this document concentrates on protecting the core network
   against attacks from the "outside", i.e., the Internet and connected
   VPNs. Protection against attacks from the "inside", i.e., if an
   attacker has logical or physical access to the core network is not
   considered here, since any network can be attacked with access from
   the inside.

2.4 Impossibility of Label Spoofing

   Assuming the address and traffic separation as discussed above, a



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   potential attacker might try to gain access to other VPNs by
   inserting packets with a label that he does not "own". This could be
   done from the outside, i.e., another CE router or from the Internet,
   or from within the MPLS core. The latter case (from within the core)
   will not be discussed, since the assumption is that the core network
   is provided in a secure manner. Should protection against an insecure
   core be required it is necessary to run IPsec across the MPLS
   infrastructure, at least from CE to CE, since the PEs belong to the
   core.

   Depending on the way several CEs are connected to a PE router, it
   might be technically possible to intrude into another VPN that is
   also connected on that PE, based on layer 2 attack mechanisms.
   Examples are 802.1Q - label spoofing, or ATM VPI/VCI spoofing. Layer
   2 security issues will be discussed in section 6.

   It is required that VPNs cannot abuse the MPLS label mechanisms or
   protocols to gain un-authorised access to other VPNs or the core.

































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3. Analysis of BGP/MPLS IP VPN Security

   In this section the BGP/MPLS IP VPN architecture is analysed with
   respect to the security requirements listed above.

3.1 Address Space, Routing and Traffic Separation

   BGP/MPLS allows distinct IP VPNs to use the same address space, which
   can also be private address space (RFC 1918 [2]). This is achieved by
   adding a 64 bit route distinguisher (RD) to each IPv4 route, making
   VPN-unique addresses also unique in the MPLS core. This "extended"
   address is also called a "VPN-IPv4 address". Thus customers of an
   BGP/MPLS IP VPN service do not need to change current addressing in
   their networks.

   There is only one exception, which is the IP addresses of the PE
   routers the CE routers are peering with, in the case of using routing
   protocols between CE and PE routers (for static routing between PE
   and CE this is not an issue). Routing protocols on the CE routers
   need to have configured the address of the peer PE router in the
   core, to be able to "talk" to the PE router. This address must be
   unique from the CE router's perspective. In an environment where the
   service provider manages also the CE routers as CPE, this can be made
   invisible to the customer. The address space on the CE-PE link
   (including the peering PE address) must be considered as part of the
   VPN address space. However, since address space can overlap between
   VPNs, also the CE-PE link addressing can overlap between VPNs. (Note
   that for practical management considerations SPs typically choose to
   address all CE-PE links from a global pool, keeping them unique
   across the entire core. The considerations of CE-PE addressing are
   discussed in detail in draft-guichard-pe-ce-addr [17].

   Routing separation between the VPNs can also be achieved. Every PE
   router maintains a separate Virtual Routing and Forwarding instance
   (VRF) for each connected VPN. Each VRF on the PE router is populated
   with routes from one VPN, through statically configured routes or
   through routing protocols that run between the PE and the CE router.
   Since every VPN results in a separate VRF there will be no
   interferences between the VPNs on the PE router.

   Across the core to the other PE routers this separation is maintained
   by adding unique VPN identifiers in multi-protocol BGP, such as the
   route distinguisher. VPN routes are exclusively exchanged by MP-BGP
   across the core, and this BGP information is not re-distributed to
   the core network but only to the other PE routers, where the
   information is kept again in VPN specific VRFs. Thus routing across
   an BGP/MPLS network is separate per VPN.




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   On the data plane traffic separation is achieved by the ingress PE
   prepending a VPN-specific label to the packets. The packets with the
   VPN labels are sent through the core to the egress PE, where the VPN
   label is used to determine the correct VPN.

   Given the addressing, routing and traffic separation across an BGP/
   MPLS IP VPN core network, it can be assumed that this architecture
   offers in this respect the same security as comparable layer-2 VPNs
   such as ATM or Frame Relay. It is not possible to intrude from a VPN
   or the core into other VPNs through the BGP/MPLS IP VPN network,
   unless this has been configured specifically.

3.2 Hiding of the BGP/MPLS IP VPN Core Infrastructure

   For reasons of security service providers and end-customers do not
   normally want their network topology revealed to the outside. This is
   done to make attacks more difficult: If an attacker doesn't know the
   target he can only guess the IP addresses to attack. Since most DoS
   attacks don't provide direct feedback to the attacker it would be
   difficult to attack the network. It has to be mentioned specifically
   that information hiding as such does not provide security. However,
   in the market this is a perceived requirement.

   With a known IP address a potential attacker can launch a DoS attack
   more easily against that device. So the ideal is to not reveal any
   information of the internal network to the outside. This applies
   equally to the customer networks as to the core. In practice a number
   of additional security measures have to be taken, most of all
   extensive packet filtering.

   For security reasons it is recommended for any core network - MPLS
   based or not - to filter packets from the "outside" (Internet or
   connected VPNs) destined to the core infrastructure, where possible.
   This makes it very hard to attack the core, although some potentially
   desired functionality such as pinging core routers will be lost.
   Traceroute across the core still works, since it addresses a
   destination outside the core.

   MPLS does not reveal unnecessary information to the outside, not even
   to customer VPNs. The addressing of the core can be done with private
   addresses (RFC 1918 [2]) or public addresses. Since the interface to
   the VPNs as well as potentially to the Internet is BGP, there is no
   need to reveal any internal information. The only information
   required in the case of a routing protocol between PE and CE is the
   address of the PE router. If this is not desired, and if no dynamic
   routing protocol is required, static routing on unnumbered interfaces
   can be configured between the PE and CE. With this measure the BGP/
   MPLS IP VPN core can be kept completely hidden.



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   Customer VPNs will have to advertise their routes as a minimum to the
   BGP/MPLS IP VPN core (dynamically or statically), to ensure
   reachability across their VPN. Whilst this could be seen as "too
   open", the following has to be noted: Firstly, the information known
   to the core is not about specific hosts, but networks (routes); this
   offers some degree of abstraction. Secondly, in a VPN-only BGP/MPLS
   IP VPN network (i.e., no shared Internet access) this is equal to
   existing layer-2 models, where the customer has to trust the service
   provider to some degree. Also in a FR or ATM network routing
   information about the VPNs can be seen on the core network.

   In a VPN service with shared Internet access the service provider
   will typically announce the routes of customers that wish to use the
   Internet to his upstream or peer providers. This can be done via a
   NAT function to further obscure the addressing information of the
   customers' networks. In this case the customer does not reveal more
   information to the general Internet than with a general Internet
   service. Core information will still not be revealed at all, except
   for the peering address(es) of the PE router(s) that hold(s) the
   peering with the Internet.

   In summary, in a pure MPLS-VPN service, where no Internet access is
   provided, the information hiding is as good as on a comparable FR or
   ATM network: No addressing information is revealed to third parties
   or the Internet. If a customer chooses to access the Internet via the
   BGP/MPLS IP VPN core he will have to reveal the same addressing
   structure as for a normal Internet service. NAT can be used for
   further address hiding. Being reachable from the Internet
   automatically exposes a customer network to additional security
   threats. Appropriate security mechanisms have to be deployed such as
   firewalls and intrusion detection systems. But this is true for any
   Internet access, over MPLS or direct.

   If a BGP/MPLS IP VPN network has no interconnections to the Internet,
   the security is equal to FR or ATM VPN networks. With an Internet
   access from the MPLS cloud the service provider has to reveal at
   least one IP address (of the peering PE router) to the next provider,
   and thus the outside world.

3.3 Resistance to Attacks

   Section 3.1 shows that it is not possible to directly intrude into
   other VPNs. Another possibility is to attack the MPLS core, and try
   to attack other VPNs from there. As shown above it is not possible to
   address a P router directly. The only reachable address from a VPN or
   the Internet are the peering addresses of the PE routers. Thus there
   are two basic ways the BGP/MPLS IP VPN core can be attacked:




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   1.  By attacking the PE routers directly.

   2.  By attacking the signaling mechanisms of MPLS (mostly routing)

   To attack an element of an BGP/MPLS IP VPN network it is first
   necessary to know this element, that is, its address. As discussed in
   section 3.2 the addressing structure of the BGP/MPLS IP VPN core is
   hidden to the outside world. Thus an attacker does not know the IP
   address of any router in the core that he wants to attack. The
   attacker could now guess addresses and send packets to these
   addresses. However, due to the address separation of MPLS each
   incoming packet will be treated as belonging to the address space of
   the customer. Thus it is impossible to reach an internal router, even
   through IP address guessing. There is only one exception to this
   rule, which is the peer interface of the PE router. This address of
   the PE is the only attack point from the outside (a VPN or Internet).

   The routing between a VPN and the BGP/MPLS IP VPN core can be
   configured two ways:

   1.  Static; in this case the PE routers are configured with static
       routes to the networks behind each CE, and the CEs are configured
       to statically point to the PE router for any network in other
       parts of the VPN (mostly a default route).  There are now two
       sub-cases: The static route can point to the IP address of the PE
       router, or to an interface of the CE router (e.g., serial0).

   2.  Dynamic; here a routing protocol (e.g., RIP, OSPF, BGP) is used
       exchange the routing information between the CE and the PE at
       each peering point.

   In the case of a static route from the CE router to the PE router,
   which points to an interface, the CE router doesn't need to know any
   IP address of the core network, not even of the PE router. This has
   the disadvantage of a more extensive (static) configuration, but from
   a security point of view is preferable to the other cases. It is now
   possible to configure packet filters on the PE interface to deny any
   packet to the PE interface. This way the router and the whole core
   cannot be attacked.

   In all other cases, each CE router needs to know at least the router
   ID (RID; peer IP address) of the PE router in the core, and thus has
   a potential destination for an attack. One could imagine various
   attacks on various services running on a router. In practice access
   to the PE router over the CE-PE interface can be limited to the
   required routing protocol by using ACLs (access control lists). This
   limits the point of attack to one routing protocol, for example BGP.
   A potential attack could be to send an extensive number of routes, or



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   to flood the PE router with routing updates. Both could lead to a
   denial-of-service, however, not to unauthorised access.

   To restrict this risk it is necessary to configure the routing
   protocol on the PE router as securely as possible. This can be done
   in various ways:

   o  By ACL, allow the routing protocol only from the CE router, not
      from anywhere else. Furthermore, no access other than that should
      be allowed to the PE router in the inbound ACL on each CE
      interface.

   o  Where available, configure MD-5 authentication for routing
      protocols. This is available for BGP (RFC 2385 [6]), OSPF (RFC
      2154 [4]) and RIP2 (RFC 2082 [3]) for example. It avoids that
      packets could be spoofed from other parts of the customer network
      than the CE router. Note that this requires service provider and
      customer to agree on a shared secret between all CE and PE
      routers. Note that it is necessary to do this for all VPN
      customers, it is not sufficient to do this for the customer with
      the highest security requirements.

   o  To configure where available parameters of the routing protocol,
      to further secure this communication. For example the rate of
      routing updates should be restricted where possible (in BGP this
      can be done through dampening). Also, a maximum number of routes
      accepted per VRF should be configured where possible.

   In summary, it is not possible to intrude from one VPN into other
   VPNs, or the core. However, it is theoretically possible to exploit
   the routing protocol to execute a DoS attack against the PE router.
   This in turn might have negative impact on other VPNs on this PE
   router. For this reason PE routers must be extremely well secured,
   especially on their interfaces to the CE routers. ACLs must be
   configured to limit access only to the port(s) of the routing
   protocol, and only from the CE router. MD5 authentication in routing
   protocols should be used on all PE-CE peerings. With all these
   security measures the only possible attack is a DoS attack against
   the routing protocol itself. However, BGP for example has a number of
   counter-meassures such as prefix filtering and dampening built into
   the protocol, to assure stability. It is also easily possible to
   track the source of such a potential DoS attack. Without dynamic
   routing between CEs and PEs the security is equivalent to the
   security of ATM or Frame Relay networks.

3.4 Label Spoofing

   Within the MPLS network packets are not forwarded based on the IP



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   destination address, but based on labels that are pre-pended to the
   IP packets by the inbound PE routers. Similar to IP spoofing attacks,
   where an attacker replaces the source or destination IP address of a
   packet, it is also theoretically possible to spoof the label of an
   MPLS packet. In the first section the assumption was made that the
   core network is trusted. If this assumption cannot be made IPsec must
   be run over the MPLS cloud. Thus in this section the emphasis is on
   whether it is possible to insert packets with (spoofed) labels into
   the MPLS network from the outside, i.e., from a VPN (CE router) or
   from the Internet.

   Principally the interface between any CE router and its peering PE
   router is an IP interface, i.e., without labels. The CE router is
   unaware of the MPLS core, and thinks it is sending IP packets to a
   simple router. The "intelligence" is done in the PE device, where
   based on the configuration, the label is chosen and pre-pended to the
   packet. This is the case for all PE routers, towards CE routers as
   well as the upstream service provider. All interfaces into the MPLS
   cloud only require IP packets, without labels.

   For security reasons a PE router should never accept a packet with a
   label from a CE router. RFC 3031 [11] specifies: "Therefore, when a
   labeled packet is received with an invalid incoming label, it MUST be
   discarded, UNLESS it is determined by some means (not within the
   scope of the current document) that forwarding it unlabeled cannot
   cause any harm." Since accepting labels on the CE interface would
   allow passing packets to other VPNs it is not permitted by the RFC.

   Thus it is impossible for an outside attacker to send labelled
   packets into the BGP/MPLS IP VPN core.

   There remains the possibility to spoof the IP address of a packet
   that is being sent to the MPLS core. However, since there is strict
   addressing separation within the PE router, and each VPN has its own
   VRF, this can only do harm to the VPN the spoofed packet originated
   from, in other words, a VPN customer can attack himself. MPLS doesn't
   add any security risk here.

   The Inter-AS and CsC cases are special cases, since on the interfaces
   between providers typically packets with labels are exchanged. See
   section 4 for an analysis of these architectures.

3.5 Comparison with ATM/FR VPNs

   ATM and FR VPN services often enjoy a very high reputation in terms
   of security. Although ATM and FR VPNs can also be provided in a
   secure manner, it has been reported that also these technologies can
   have severe security vulnerabilities [20]. Also in ATM/FR the



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   security depends on the configuration of the network being secure,
   and errors can also lead to security problems.

















































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4. Security of advanced BGP/MPLS IP VPN architectures

   The BGP/MPLS IP VPN architecture as described in RFC 2547 [7] defines
   the PE-CE interface as the only external interface, as seen from the
   service provider network. In this case, the PE treats the CE as
   untrusted, and only accepts pure IP packets from the CE. The IP range
   however is treated as belonging to the VPN of the CE, thus the PE
   maintains full control over VPN separation.

   Subsequently, RFC 2547bis [13] has defined more complex
   architectures, with more open interfaces. These interfaces allow the
   exchange of label information and labelled packets to and from
   devices outside the control of the service provider. This section
   discusses the security implications of these architectures.

4.1 Carriers' Carrier (CsC)

   In the CsC architecture the CE is linked to a VRF on the PE. The CE
   may send labeled packets to the PE. The label has been previously
   assigned by the PE to the CE, and represents the LSP from this CE to
   the remote CE via the carrier's network.

   RFC 2547bis [13] specifies for this case: "When the PE receives a
   labeled packet from a CE, it must verify that the top label is one
   that was distributed to that CE." This ensures that the CE can only
   use labels that the PE correctly associates with the corresponding
   VPN. Packets with incorrect labels will be discarded, and thus label
   spoofing is not possible.

   The use of label-maps on the PE equally leaves the control of the
   label information entirely with the PE, so that this has no impact on
   the security of the solution.

   The packet underneath the top label will - as in standard  networks -
   remain local to the carrier's VPN and not be looked at in the
   carriers' carrier core. Consequently potential spoofing of subsequent
   labels or IP addresses remains also local to the carrier's VPN, and
   has no implication on the carriers' carrier core, nor on other VPNs
   on that core. This is specifically stated in RFC 2547bis [13] in
   section 6.

   Note that if the PE and CE are interconnected using a shared layer 2
   infrastructure such as a switch, attacks are possible on layer 2,
   which might enable a third party on the shared layer 2 network to
   intrude into a VPN on that PE router. RFC 2547bis [13] specifies
   therefore that either all devices on a shared layer 2 network have to
   be part of the same VPN, or the layer 2 network must be split
   logically to avoid this issue. This will be discussed in more detail



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   in section 6.

   In the CsC architecture the carrier needs to trust the carriers'
   carrier for correct configuration and operation. The customer of the
   carrier thus implicitely needs to trust both his carrier and the
   carriers' carrier.

   In summary, a correctly configured carriers' carrier network provides
   the same level of security as comparable layer 2 networks, or
   traditional  networks.

4.2 Inter-provider backbones

   RFC 2547bis [13] specifies three sub-cases for the inter-provider
   backbone (Inter-AS) case.

   a) VRF-to-VRF connections at the AS border routers

   In this case each PE sees and treats the other PE as a CE; each will
   not accept labelled packets, and there is no signalling between the
   PEs other than inside the VRFs on both sides. Thus the separation of
   the VPNs on both sides and the security of those are the same as on a
   single AS  network. This has already been shown above to have the
   same security properties as traditional layer 2 VPNs.

   This solution has potential scalability issues in that the ASBRs need
   to maintain a VRF per VPN, and all of the VRFs need to hold all
   routes of the specific VPNs. Thus an ASBR can run into memory
   problems affecting all VPNs if one single VRF contains too many
   routes. Thus the service providers needs to assure that the ASBRs are
   properly dimensioned, and apply appropriate security meassures such
   as limiting the number of routes per VRF.

   The two service providers connecting their VPNs in this way must
   trust each other. Since the VPNs are physically separated on
   different (sub-)interfaces all signalling between ASBRs remains
   within a given VPN. This means that no dynamic cross-VPN security
   breaches are possible. However, it is conceivable that a service
   provider connects a specific connection from a given VPN to a wrong
   interface, thus interconnecting two VPNs that should not be
   connected. This has to be controlled operationally.

   b) EBGP redistribution of labeled VPN-IPv4 routes from AS to
   neighboring AS

   In this case the ASBRs on both sides hold the full routing
   information for all VPNs on both sides, but not in separate VRFs, but
   in the BGP database. (Note this is typically limited to the Inter-AS



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   VPNs through filtering.) The separation inside the PE is maintained
   through the use of VPN-IPv4 addresses. The control plane between the
   ASBRs is using MP-eBGP, exchanging the VPN routes as VPN-IPv4
   addresses, and their own addresses as BGP next hop IPv4 addresses,
   plus labels to be used on the data plane.

   The data plane is separated through the use of a single label,
   representing a VRF or a subset thereof. RFC 2547bis [13] states that
   an ASBR should only accept packets with a label that it has assigned
   to this router. This prevents the insertion of packets with unknown
   labels, but it is possible for a service provider to use any label
   that the ASBR of the other provider has passed on to the other ASBR.
   This allows one provider to insert packets into any VPN of the other
   provider to which it has a label.

   Also this solution needs to consider the security on layer 2 at the
   interconnection. The RFC states that this type of interconnection
   should only be implemented on private interconnection points. See
   section 6 for more details.

   RFC 2547bis [13] states for this case that a trust relationship
   between the two connecting ASes must exist for this model to work
   securely. Effectively all ASes interconnected in this way form
   together one single zone of trust. The VPN customer needs to trust
   all the service providers involved in this architecture.

   c) PEs exchange labeled VPN-IPv4 routes, ASBRs only exchange
   loopbacks of PEs with labels.

   In this solution there are effectively two control connections
   between ASes. The route reflectors (RRs) exchange via multihop eBGP
   the VPN-IPv4 routes. The ASBRs only exchange the labeled addresses of
   those PE routers that hold VPN routes which are shared between those
   ASes. This maintains scalability for the ASBR routers, since they do
   not need to know the VPN-IPv4 routes.

   In this solution the top label specifies an LSP to an egress PE
   router, the second label specifies a VPN connected to this egress PE.
   The security of the ASBR connection has the same constraints as in
   solution b): An ASBR should only accept packets with top labels that
   it has assigned to the other router, thus verifying that the packet
   is addressed to a valid PE router. But any label which was assigned
   to the other ASBR router will be accepted, thus it is not possible
   for an ASBR to distinguish between different egress PEs, nor between
   different VPNs on those PEs. A malicious service provider of one AS
   could therefore introduce packets into any VPN of the other AS to
   which it holds valid information on its ASBR and PEs.




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   This means that such an ASBR-ASBR connection can only be made with a
   trusted party, over a private interface, as described in b).

   In addition this solution exchanges labeled VPN-IPv4 addresses
   between route reflectors (RR) via MP-eBGP. The control plane itself
   can be protected via routing authentication (RFC 2385 [6]), which
   ensures that the routing information has been originated by the
   expected RR and has not been modified in transit. But the received
   VPN information cannot be verified, as in the previous case. The ASes
   need to trust each other to configure their respective networks
   correctly. Again all ASes involved in this design form together one
   trusted zone. The customer therefore needs to trust all the service
   providers involved.

   The difference between case b) and case c) is that in b) the ASBRs
   act as iBGP next-hops for their AS, thus each SP needs to know of the
   other SP's core only the addresses of the ASBRs. In case c) the SPs
   exchange the loopback addresses of their PE routers, thus each SP
   reveals information to the other of his PE routers, and these routers
   must be accessible from the other AS. As stated above, accessibility
   does not necessarily mean insecurity, and networks should never rely
   on "security through obscurity". So if the PE routers are
   appropriately secured this should not be an issue. However, there is
   an increasing perception that network devices should generally not be
   accessible.

   In addition for case c) scalability considerations, for example for
   the number of BGP peerings, have now to be made for the overall
   network including all ASes linked this way. So SPs on both sides need
   to work together in defining a scalable architecture, probably with
   route reflectors.

   In summary all of these Inter-AS solutions logically merge several
   provider networks together. For all cases of Inter-AS configuration
   all ASes together form a single zone of trust, and service providers
   need to trust each other. For the VPN customer the security of the
   overall solution is equal to the security of traditional  networks,
   but he needs to trust all service providers involved.













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5. What BGP/MPLS IP VPNs Do Not Provide

5.1 Protection against Misconfigurations of the Core and Attacks
    'within' the Core

   The security mechanisms discussed here assume correct configuration
   of the involved network elements on the core network (PE and P
   routers). Deliberate or inadvertent misconfigurations from SP staff
   may result in undesired behaviour including severe security leaks.

   Note that this paragraph specifically refers to the core network,
   i.e., the PE and P elements. Misconfiguration of any of the customer
   side elements such as the CE router are covered by the security
   mechanisms above. This means that a potential attacker must have
   access to either PE or P routers to gain advantage from
   misconfigurations. If an attacker has access to core elements, or is
   able to insert into the core additional equipment, he will be able to
   attack both the core network as well as the connected VPNs. Thus the
   following is important:

   o  To avoid the risk of misconfigurations it is important that the
      equipment is easy to configure, and that SP staff have the
      appropriate training and experience when configuring the network.
      Also, proper tools are required for configuring the core network.

   o  To avoid the risk of "internal" attacks the core network must be
      properly secured. This includes network element security,
      management security, physical security of the service provider
      infrastructure, access control to service provider installations
      and other standard SP security mechanisms.

   BGP/MPLS IP VPNs can only provide a secure service if the core
   network is provided in a secure fashion. This document assumes this
   to be the case.

   There are various approaches to control the security of a core if the
   VPN customer cannot or does not want to trust the service provider.
   IPsec from customer controlled devices is one of them. draft-
   ietf-l3vpn-auth [19] proposes a CE based authentication scheme based
   on tokens, aimed at detecting misconfigurations in the MPLS core.
   draft-behringer-mpls-vpn-auth [18] proposes a similar scheme based on
   using the MD5 routing authentication. Both schemes aim to detect and
   prevent misconfigurations in the core.

5.2 Data Encryption, Integrity and Origin Authentication

   BGP/MPLS IP VPNs themselves does not provide encryption, integrity or
   authentication services. If these are required IPsec should be used



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   over the MPLS infrastructure. The same applies to ATM and Frame
   Relay: Also here IPsec can provide these missing services.

5.3 Customer Network Security

   BGP/MPLS IP VPNs can be secured so that they are comparable with
   other VPN services. However, the security of the core network is only
   one factor for the overall security of a customer's network. Threats
   in today's networks do not only come from the "outside" connection,
   but also from the "inside" and from other entry points (modems for
   example). To reach a good security level for a customer network in an
   BGP/MPLS infrastructure, MPLS security is necessary but not
   sufficient. The same applies to other VPN technologies like ATM or
   frame relay. See also RFC 2196 [5] for more information on how to
   secure a network.




































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6. Layer 2 security considerations

   In most cases of Inter-AS or Carrier's Carrier solutions a network
   will be interconnected to other networks via a point-to-point private
   connection, that is, a connection which cannot be interfered by third
   parties. It is important to understand that the use of any shared
   medium layer 2 technology for such interconnections, such as ethernet
   switches, may carry addtional security risks.

   There are two types of risks involved in a layer 2 infrastructure:

   a) Attacks against layer 2 protocols or mechanisms

   Risks in a layer 2 environment include many different forms of ARP
   attacks, VLAN trunking attacks, or CAM overflow attacks. For example
   ARP spoofing allows an attacker to re-direct traffic between two
   routers through his device, thus being able to see all packets
   between those two routers.

   All of those can be prevented by appropriate security meassures, but
   often these security concerns are overlooked. It is of utmost
   importance that if a shared medium such as a switch is used in the
   above scenarios, that all available layer 2 security mechanisms are
   used to prevent layer 2 based attacks.

   b) Traffic insertion attacks

   Where many routers share a common layer 2 network, for example on an
   Internet exchange point, it is possible for a third party to
   introduce packets into a network. This has been abused in the past on
   traditional exchange points by some service providers to default to
   another provider on this exchange point. In effect they are sending
   all their traffic into the other SPs network, even though the control
   plane (routing) might not allow that.

   For this reason routers on exchange points or other shared layer 2
   connections should only accept non-labelled IP packets into the
   global routing table. Any labelled packet must be discarded. This
   maintains VPN security of connected networks.

   However, some of the designs above require the exchange of labelled
   packets. This would make it possible for a third party to introduce
   labelled packets, which when correctly crafted might be associated
   with certain VPNs on an BGP/MPLS IP VPN network, effectively
   introducing false packets into a VPN.

   The current recommendation is therefore to not accept labelled
   packets on generic shared medium layer 2 networks such as Internet



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   exchange points (IXPs). Where labelled packets are required it is
   strongly recommended to use private connections.

















































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7. Summary and Conclusions

   BGP/MPLS IP VPNs provide full address and traffic separation as in
   traditional layer-2 VPN services. It hides addressing structures of
   the core and other VPNs, and it is in today's understanding not
   possible from the outside to intrude into the core or other VPNs
   abusing the BGP/MPLS mechanisms. It is also not possible to intrude
   into the MPLS core if this is properly secured. However, there is a
   significant difference between BGP/MPLS based IP VPNs and for example
   FR or ATM based VPNs: The control structure of the core is on layer 3
   in the case of MPLS. This caused significant skepticism in the
   industry towards MPLS, since this might open the architecture to DoS
   attacks from other VPNs or the Internet (if connected).

   As shown in this document, it is possible to secure an BGP/MPLS IP
   VPN infrastructure to the same level of security than a comparable
   ATM or FR service. It is also possible to offer Internet connectivity
   to MPLS VPNs in a secure manner, and to interconnect different VPNs
   via firewalls. Although ATM and FR services have a strong reputation
   with regard to security, it has been shown that also in these
   networks security problems can exist [20].

   As far as attacks from within the MPLS core are concerned, all VPN
   classes (BGP/MPLS, FR, ATM) have the same problem: If an attacker can
   install a sniffer, he can read information in all VPNs, and if he has
   access to the core devices, he can execute a large number of attacks,
   from packet spoofing to introducing a new peer routers. There are a
   number of precautions measures outlined above that a service provider
   can use to tighten security of the core, but the security of the BGP/
   MPLS IP VPN architecture depends on the security of the service
   provider. If the service provider is not trusted, the only way to
   fully secure a VPN against attacks from the "inside" of the VPN
   service is to run IPsec on top, from the CE devices or beyond.

   This document discussed many aspects of BGP/MPLS IP VPN security. It
   has to be noted explicitly that the overall security of this
   architecture depends on all components, and is determined by the
   security of the weakest part of the solution. For example a perfectly
   secured static BGP/MPLS IP VPN network with secured Internet access
   and secure management is still open to many attacks if there is a
   weak remote access solution in place.










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8. Acknowledgements

   The author would like to thank everybody who has provided input to
   this document. Specific thanks go to Yakov Rekhter for his continued
   strong support, and Eric Rosen, Loa Andersson, Alexander Manhenke,
   Jim Guichard, Monique Morrow and Eric Vyncke for their extended
   feedback and support.












































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References

   [1]   Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
         RFC 1771, March 1995.

   [2]   Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E.
         Lear, "Address Allocation for Private Internets", BCP 5, RFC
         1918, February 1996.

   [3]   Baker, F., Atkinson, R. and G. Malkin, "RIP-2 MD5
         Authentication", RFC 2082, January 1997.

   [4]   Murphy, S., Badger, M. and B. Wellington, "OSPF with Digital
         Signatures", RFC 2154, June 1997.

   [5]   Fraser, B., "Site Security Handbook", RFC 2196, September 1997.

   [6]   Heffernan, A., "Protection of BGP Sessions via the TCP MD5
         Signature Option", RFC 2385, August 1998.

   [7]   Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March
         1999.

   [8]   Ferguson, P. and D. Senie, "Network Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Source
         Address Spoofing", BCP 38, RFC 2827, May 2000.

   [9]   Shirey, R., "Internet Security Glossary", RFC 2828, May 2000.

   [10]  Killalea, T., "Recommended Internet Service Provider Security
         Services and Procedures", BCP 46, RFC 3013, November 2000.

   [11]  Rosen, E., Viswanathan, A. and R. Callon, "Multiprotocol Label
         Switching Architecture", RFC 3031, January 2001.

   [12]  Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
         Thomas, "LDP Specification", RFC 3036, January 2001.

   [13]  Rosen, E., "BGP/MPLS IP VPNs", draft-ietf-l3vpn-rfc2547bis-01
         (work in progress), September 2003.

   [14]  Fang, L., "Security Framework for Provider Provisioned Virtual
         Private Networks", draft-ietf-l3vpn-security-framework-00 (work
         in progress), September 2003.

   [16]  Guichard, J., "CE-CE IPSec within an RFC-2547 Network",
         draft-guichard-ce-ce-ipsec-00 (work in progress), May 2003.




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   [17]  Guichard, J., "Address Allocation for PE-CE links within a
         Provider Provisioned VPN  Network",
         draft-guichard-pe-ce-addr-03 (work in progress), July 2003.

   [18]  Behringer, M., Guichard, J. and P. Marques, "MPLS VPN Import/
         Export Verification", draft-behringer-mpls-vpn-auth-02 (work in
         progress), April 2003.

   [19]  Bonica, R. and Y. Rekhter, "CE-to-CE Member Verification for
         Layer 3 VPNs", draft-ietf-l3vpn-auth-00 (work in progress),
         February 2003.

   [20]  DataComm, "Data Communications Report, Vol 15, No 4: Frame
         Relay and ATM: Are they really secure?", February 2000.


Author's Address

   Michael H. Behringer
   Cisco Systems Inc
   Avenida de la Vega 15
   Alcobendas, Madrid  28100
   Spain

   EMail: mbehring@cisco.com
   URI:   http://www.cisco.com

























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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

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