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Routing Protocol Security                              B. Christian, Ed.
Requirements                                       KMC Telecom Solutions
Internet-Draft                                                  Feb 2004
Expires: August 1, 2005

                       BGP Security Requirements

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   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   The security of BGP is critical to the proper operation of
   large-scale internetworks, both public and private.  While securing
   the information transmitted between two BGP speakers is a relatively
   easy technical matter, securing BGP, as a routed system, is more
   complex.  This document describes a set of requirements for securing
   BGP, including securing peering relationships between BGP speakers,
   and authenticating the routing information carried within BGP.

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1.  Introduction

1.1  System Description

   BGP is described in RFC1771 [3] as a routing protocol.  Essentially,
   BGP speakers exchange information about reachable destinations in an
   internetwork through a peering session.  Once this information has
   been exchanged, each BGP speaker locally determines a loop free path
   to each reachable destination, based on local policy, policy
   indicators (or policies) carried in the update, and the AS Path
   carried in the BGP update.

1.2  Threats

   Threats to networking protocols generally fall under one of the three
   categories as defined in RFC 2196 [1]:
   o  Unauthorized access to resources and/or information
   o  Unintended and/or unauthorized disclosure of information
   o  Denial of service

   A number of attacks can be realized which, if exploited, can lead to
   one of the above mentioned threats.  These are typically classified
   as passive attacks and active attacks.  Passive attacks are ones
   where an attacker simply reads information off the network and
   obtains confidential and/or private information.  Active attacks are
   ones where the attacker writes data to the network and can include
   replay attacks, message insertion, message deletion, message
   modification and man-in-the-middle attacks.  These attacks are often

   Attacks that do not involve direct manipulation of BGP, and the
   information contained within it, are outside the scope of this
   document.  When possible, the requirements will attempt to minimize
   the extent of the damage that occurs when end systems come under

   The intent of this requirements document is to prevent attacks that
   originate false data or create invalid routing paths and therefore
   addresses issues relating to data integrity and peer entity
   authentication.  As described in RFC 3552 [2], data integrity
   protection ensures that data is not modified in transit and peer
   entity authentication ensures that there is a reasonable guarantee
   that the sender and recipient of the data are the intended parties.

   Guaranteed packet delivery is not part of the BGP protocol security
   model.  Just because a packet is addressed to a specific destination
   does not mean it will be received, even with a "secure" route.  For
   example: an attacker could have compromised an intermediate router

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   and installed a static route for target address A.B.C.D pointing to
   an inappropriate direction or an attacker might splice into a circuit
   between two secure routers and install a device that diverts A.B.C.D
   traffic without requiring the compromise of control plane devices.

1.3  Areas to secure

   There are two primary points where BGP may be secured.  If we examine
   the system description presented above those points are as follows.
   o  The session between two BGP speakers can be secured such that the
      BGP data received by the BGP speakers can by cryptographically
      verified to have been transmitted by the other speaker.
   o  The originator and the propagators of prefix information may have
      their policy information verified such that the intent of the
      policy with respect to the specific prefix is preserved

   There are also several questions we can ask about the information
   contained within a received update.
   o  Is the originating Autonomous system authorized to propagate the
      prefix we have received?
   o  Is the AS path, received via an UPDATE, valid?

   The verification of AS-Path validity falls into three distinct
   o  Does the AS-Path specified actually exist and, based on the
      AS-PATH, is it possible to traverse that path to reach a given
   o  Has the update actually travelled the path?
   o  Was the update authorized to traverse the given path by the
      originator of the prefix?

   In this draft we do not attempt to set requirements dealing with the
   third question presented above.  However, the needs of the operators
   of the systems that use BGP are such that the first two questions are
   of key importance to any secured interdomain routing system.

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2.  Application Concepts

   In order to properly identify security requirements it is important
   to cover the fundamental aspects of BGP as related to security
   requirements.  The following list presents the basic parameters and
   application concepts of BGP that will be covered by this document.
   o  Peer Communication: BGP traffic travels over TCP between peers, so
      BGP speakers assume the TCP data delivery guarantees of TCP in a
      benign environment.  This includes ordered, error-free delivery of
      application traffic from a peer identified by an IP address, plus
      integrity of the control aspects of TCP.  From a security
      perspective, these guarantees need to be enforced in the context
      of possible active wiretapping.
   o  Routing and Reachability: BGP is a protocol used to convey routing
      and reachability information both internal and external to an
      Autonomous System.  Typically, internal BGP is used to distribute
      prefix reachability information in conjunction with an IGP and is
      used by a distinct network administrative entity to convey
      internal routing policy regarding external and internal
      information.  External BGP is typically used to distribute route/
      prefix reachability information between two distinct routing
      entities and is used to signal forwarding preferences and policy
   o  Inter-AS UPDATE Message assumptions: When an AS distributes
      reachability information to a peer it is done with the intent of
      affecting routing decisions by the peer.  For example, an AS-A
      sends peer AS-B a less specific advertisement and peer AS-C a
      "more" specific advertisement.  This prefix distribution decision
      may have been made to provide a means for failure resolution
      between AS-A and AS-C.  Update messages are sent between AS peers
      with the implicit assumption that those messages will be forwarded
      to others.  A notable exception to this assumption is the use of
      various policy based mechanisms between peers such as the
      NO-EXPORT community.  In this document an important aspect of the
      UPDATE message to note is that the specific UPDATE message itself
      is typically not re-transmitted.  Instead, the specific UPDATE
      message is regenerated continually as it passes from BGP speaker
      to BGP speaker.  Furthermore, UPDATE messages have no mechanism
      for freshness (i.e timestamps or sequence numbers).  This
      indicates that messages may appear valid at any point in the life
      of a BGP peering session.
   o  Inter-AS withdraw message assumptions: The processing model of BGP
      RFC1771 [3] indicates that only the peer advertising NLRI
      information may withdraw it.  There are several instances where a
      withdraw may occur.  Typical reasons for withdraw include the
      determination of a better path, peer session failure, or local
      policy change.  There is no specified mechanism for indicating, to
      an external peer, the reason for a route withdraw.  Each withdraw

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      received from a valid peering session must be taken at face value.
      There is no, existing, method to ensure that an AS will properly
      propagate withdraw messages received from it's external peers.
   o  AS-Path assumptions: Aside from the use of AS-Set, the AS-Path is
      typically considered to be an ordered list of the Autonomous
      Systems that an update has traversed.  In most cases the first AS
      in the list is the origin AS, or at least the AS responsible for
      the management of the NLRI information associated with the first
      AS.  Specifications state that the AS topology must be loop free.
      This indicates that the appearance of the local AS in an update
      received from an external peer is generally not permitted.  The
      prepending of AS information for received updates and transmitted
      updates is generally permitted and is common practice.  Prepended
      AS information on inbound advertisements (where the external peers
      AS is prepended) and outbound advertisements (where the local AS
      number is prepended) is a commonly used method to effect
      forwarding changes.
   o  Route Origination: Originating a route without the ability to
      forward the traffic associated with that route is, in most cases,
      in conflict with the intent of the BGP specification.  BGP
      speakers may originate routes based on various internal and
      external data.  An Autonomous System should only originate a
      prefix to it's external peers if that prefix has been somehow
      allocated to the administrators of that system, or authorized by
      the prefix holders.
   o  Aggregation: Aggregation, and deaggregation, of prefixes can cause
      significant issues with proposed security mechanisms.  According
      to RFC 1771, if a BGP speaker chooses to aggregate a set of more
      specific prefixes into a less specific prefix then the
      ATOMIC_AGGREGATE attribute SHOULD be set.  This creates a
      significant potential loophole in an attempt to secure BGP based
      on the RFC specifications.

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3.  Deployment Requirements

   We have determined, through discussion with several large
   internetwork operators and equipment vendors, that the following
   attributes are important to the ongoing performance of interdomain
   routing systems such as BGP

3.1  Convergence speed

   Convergence speed is a major concern to many operators of large scale
   internetworking systems.  Networks, and internetworks, are carrying
   ever increasing amounts of information that is time and delay
   sensitive; increasing convergence times can adversely affect the
   usability of the network, and the ability of an internetwork to grow.
   BGP's convergence speed, with a security system in operation, SHOULD
   be equivalent to BGP running without the security system in
   operation.  This includes the preservation of optimizations currently
   used to produce acceptable convergence speeds on current hardware,
   including update packing, peer groups, and others.  Current timers,
   including hold timers, keepalive timers, and the peering process,
   SHOULD NOT be impacted by the security system.  Two types of
   verification MAY be offered for the NLRI and the AS_PATH in order to
   allow for a selection of optimizations:
   o  Contents of the UPDATE message SHOULD be authenticated in
      real-time as the UPDATE message is processed.
   o  The route information base MAY be authenticated periodically or in
      an event driven manner by scanning the data and verifying the
      originating AS and the verifiability of the AS-PATH list.

   All BGP implementations that implement security MUST utilize at least
   one of the above methods for validating routing information.  Real
   time verification is preferred in order to prevent transitive
   failures based on periodic or event driven scan intervals.

3.2  Incremental deployment

   We will not be able to deploy a newly secured BGP protocol
   instantaneously and will be unable to dictate a partitioning of large
   internetworks by the operators.  Because of this, there are several
   requirements that any proposed mechanism to secure BGP must consider.
   o  BGP MUST support transmitting, receiving, and acting on both
      secured and unsecured routing information with the security system
      in place.
   o  The security system MUST allow the forming of peering
      relationships between peers regardless of whether the security
      system is in place
   o  MUST be backward compatible in the message formatting,
      transmission, and processing of routing information carried

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      through a mixed security environment.  Message formatting in a
      fully secured environment MAY be handled in a non-backward
      compatible fashion.
   o  In an environment where both secured and non-secured systems are
      interoperating a mechanism MUST exist for secured systems to
      identify whether an originator intended the information to be

3.3  Conditions for initialization

   A key factor in the robust nature of the existing internal and
   external relationships maintained in todays Internet provider space
   is the ability to maintain and return to a significantly converged
   state without the need to rely on systems external to the routing
   system (the physical equipment that is performing the forwarding).
   In order to ensure the rapid initialization and/or return to service
   of failed nodes it is important to reduce reliance on external
   systems to the greatest extent possible.  Therefore, proposed systems
   SHOULD NOT require connections to external systems, beyond those
   directly involved in peering relationships, in order to return to
   full service.  Proposed systems MAY require post initialization
   synchronization with external systems in order to synchronize
   security information.

3.4  Trust level variability

   Each secured environment may have different levels of requirements in
   terms of what is acceptable or unacceptable.  In environments that
   require strict security it may not be acceptable to temporarily route
   to a destination while waiting for security verification to be
   performed.  However, in many commercial environments the rapidity of
   route installation may be of paramount importance; in order to
   facilitate the more common occurence of route withdrawl due to
   network failure.  Based on the two divergent requirements, the
   following criteria apply.
   o  The security system MUST support a range of possible outputs for
      local determination of the trust level for a specific route.  Any
      given route should be trustable to a locally configured degree,
      based on the completeness of security information for the update
      and other factors.
   o  The security system SHOULD allow the operator to determine whether
      the speed of convergence is more important than security
      operations, or security operations are more important than the
      speed of convergence.  This facilitates the incremental deployment
      of security on systems not designed to support increased
      processing requirements imposed by the security system.

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4.  The Trust Model

   In examining the various environments in which BGP is deployed, and
   through discussions with various operators working with the context
   of the public Internet, and other internetworks, it is apparent that
   trust models are largely environment specific.  For instance, in the
   public Internet, a distributed trust model, following the current
   transitive trust pattern of contractual and peering arrangements,
   would fit the the business models of the participants.  In other
   environments a hierarchical trust model would work better.  Thus, any
   trust system specified in a security mechanism designed for BGP must
   be flexible, and support both a true distributed trust model and a
   fully hierarchical trust model.

   Since hierarchical trust models are a subset (or a special case of) a
   distributed trust model, any security system designed for BGP MUST
   support a distributed trust model, and MUST also support a
   hierarchical trust model, if desired.

   If two internetworks using differing trust models are interconnected
   they MUST be able to interoperate using locally determined levels of
   trust to compensate for differences in their trust models.

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5.  The AS-Path Attribute and NLRI Authentication

   BGP distributes routing information across the Internet (between BGP
   speakers) using BGP UPDATE messages.  The UPDATE message contains
   withdrawn routes, path attributes and one or more NLRIs (Network
   Layer Reachability Information is synonymous with advertised prefix).
   For the remainder of this section, we will focus on the AS-Path
   Attribute and the NLRI.  Attributes such as local pref are locally
   specific and, as such, are protected by BGP session security.

   The AS_PATH for specific prefixes must be protected in any proposed
   security system in three ways:
   o  Authorization of Originating AS: For all prefixes announced in
      BGP, the originating AS MUST be verifiable through the trust model
      as the authorized announcer of the prefix.  The verification
      mechanism must account for existing BGP mechanisms such as
      summarization.  For the purpose of this document the term
      verifiable is defined as the resultant of a secured routing
      systems as described in this document.  The term specifically
      indicates the ability to validate the originator of a specific
      prefix (or block of IP addresses) and the ability to validate the
      session through which the prefix was received
   o  The AS_PATH list MUST correspond to a verifiable list of
      autonomous systems based on the peering topology of the network.
   o  Announcing AS Check: For all BGP peers, a BGP Implementation MUST
      ensure that the first element of the AS_PATH list corresponds to
      the locally configured AS of that peer.
   o  Routing information carried through BGP SHOULD include information
      that can be used to verify the readvertisement or modification by
      each autonomous system through which the UPDATE has passed.

   There are many ways in which a differential between the speed of
   prefix/AS path attribute propagation and the information validating
   the the prefix/AS path attribute information can be exploited to
   attack the routing system on a temporary basis.  These types of
   attacks are dominantly exploitative of the moment in time it takes to
   follow the withdraw of a NLRI with an update.  As a result of this
   potential for temporary disruption, BGP security solutions MUST
   propagate security information at the same rate as the BGP updates
   and withdrawls.  The following items are required to propagate at the
   same rate:
   o  The distribution of key information used by individual actors
      within the system, including the keys used by individual
      autonomous systems to sign certificates and other objects
   o  The distribution of information about the AS authorized to
      advertise a given block of IP addresses (or an address space)
   o  The distribution of information about connectivity between
      autonomous systems and autonomous system polices, if such

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      information is to be distributed within the security system.

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6.  Address Allocation and advertisement

   As part of the regular operation of the Internet, addresses that are
   allocated to an organization may be, and are quite commonly,
   advertised by a different organizations.  Common reasons for this
   practice include multi-homing and route reduction for the purposes of
   resource conservation.  There are two modes of delegation:
   o  A BGP speaker and listener have chosen to restrict the amount of
      received prefixes for the listener.  The listener has chosen to
      honor route announcements sent in a summary fashion by the
   o  Address space that is being delegated is part of a larger
      allocation that is owned by an autonomous system.  The owner then
      delegates the smaller block to another AS for purposes of
      advertisement.  This mode is commonly observed in multi-homing.

   These two modes lead to a single common requirement: Any BGP Security
   solution MUST support delegation of an address block of any size
   regardless of its relationship to other address blocks to another
   entity via verifiable means.

   An associated delegation criteria is the requirement to allow for
   non-BGP IP end user implementations.  As a result, all secured BGP
   implementations MUST allow for the propagation of a prefix by more
   than one originator AS within normal network convergence times.

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7.  NLRI and Path Attribute Tracking

   The ability for a receiver to know exactly who originated and
   forwarded a routing update, is a desirable trait.  In order to
   rapidly identify agressors and parties at fault for route table
   disruption it is important to track and log prefix origination
   information along with associated security information.

   Any security system SHOULD provide a method to allow the receiver of
   an update to verify that the originator actually originated the
   update, and that the AS's listed in the AS_PATH actually forwarded
   the update.

   The data generated by logging may be very large depending on the
   number of peers, the number of prefixes received, the authentication
   model used, and routing policies.  As such, efficient data structures
   and storage mechanisms MUST be developed to allow for an effective
   means of reproducing incidents and outages

   Path and NLRI attributes MUST be logged using a standard format.  The
   format must be scalable with the amount of data logged and the
   frequency of log generation.  The frequency of log generation should
   be controllable by the operator.  The logging mechanisms for the
   tracked information MUST be standardized across all platforms.
   Logging ability both on and off line is considered highly desirable.

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8.  Transport Protection

   Transport protection is an important aspect of BGP routing protocol
   security.  The potential to create a linked transport/NLRI/AS-PATH
   authentication mechanism should not be overlooked and may provide for
   the accelerated deployment of a BGP security system.  Current
   security mechanisms for BGP transport are inadequate and require
   significant operator interaction to maintain a respectable level of

   Any proposed security mechanism MUST include provisions for securing
   both internal BGP and external BGP peering sessions.  Key maintenance
   can be especially onerous to the operators.  The number of keys
   required and the maintenance of keys (update/withdraw/renew) may have
   an additive affect to a barrier to deployment.  A highly securable
   BGP routing system SHOULD require no more than three keys and each
   key should be updateable within similar timeframes as prefix
   propagation.  The preferred number of keys is ONE per AS.

   Transport protection systems SHOULD function as a component of the
   BGP routing protocol security mechanism.  This includes the use of
   the same key generation/management systems as the rest of the
   security system.

9  References

   [1]  Fraser, "RFC 2196 - Site Security Handbook", September 1997.

   [2]  Rescorla, Korver and Internet Architecture Board, "RFC 3552 -
        Guidelines for Writing RFC Text on Security Considerations",
        July 2003.

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

Author's Address

   Blaine Christian (editor)
   KMC Telecom Solutions
   1545 U.S. Highway 206
   Bedminster, NJ  07921

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Appendix A.  Acknowledgements

   The following individuals contributed to the development and review
   of this draft.  Steve Kent, Russ White, Sandy Murphy, Jeff Haas, Bora
   Akyol, Susan Hares, Mike Tibodeau, Thomas Renzy, Kaarthik Sivakumar,
   Tao Wan, Radia Perlman, and Merike Kaeo.

   This draft was developed based on conversations with various network
   operators including Chris Morrow, Jared Mauch, Tim Battles, and Ryan

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