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Versions: 00 01 02 03 04 05 06 07 08 09 10                              
Routing Protocol Security                              B. Christian, Ed.
Requirements                                       KMC Telecom Solutions
Internet-Draft                                            T. Tauber, Ed.
Expires: April 28, 2006                                          Comcast
                                                        October 25, 2005

                       BGP Security Requirements

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on April 28, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   The security of BGP, the Border Gateway Protocol, 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
   routing 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

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   information carried within BGP.

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1.  Introduction

1.1.  System Description

   BGP is described in RFC1771 [3], and, more recently, in an updated
   specification,RFC4271 [7], as a path-vector routing protocol.  BGP
   speakers typically exchange information about reachable destinations
   (expressed as address prefixes) in an internetwork through pair-wise
   peering sessions.  Once this information has been exchanged, each BGP
   speaker locally determines a loop free path to each reachable
   destination, based on local policy or policy indicators which may be
   carried in the UPDATE, and the AS_PATH data carried in the BGP UPDATE

   Each BGP speaker represents an Autonomous System (AS).  All of the
   BGP speakers within an AS operate under a common administrative

1.2.  Threats

   Violations of security for network and information systems 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 security violations.  Attacks against
   communications are typically classified as passive or active
   wiretapping attacks.  Passive attacks are ones where an attacker
   simply observes information traversing the network, violating
   confidentiality or identifying a means of engaging in further
   attacks.  Active attacks are ones where the attacker modifies data in
   transit.  Such attacks include replay attacks, message insertion,
   message deletion, and message modification attacks.  Some attacks may
   be effected by sending data from any where in the Internet.  Other
   active attacks require a "man-in-the-middle" capability, i.e., the
   attacker must be in a position where traffic passes through an
   attacker-controlled device.  Attacks against BGP may be used by an
   attacker to facilitate a wide variety of active or passive
   wiretapping attacks against subscriber traffic.

   Attacks that do not involve direct manipulation of BGP, and the
   information contained within BGP, are outside the scope of this

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   Because ASes are autonomous in their operation, it is not possible to
   mandate secure operation by all ASes, nor would it be advisable to
   assume such operation.  Thus the primary goal of BGP security
   measures is to provide data to AS operators to enable BGP speakers to
   reject advertisements (UPDATE messages) that are not valid.  For
   example, UPDATE messages that represent erroneous binding of prefixes
   to an origin AS, or that advertise invalid paths (as defined later in
   this document) should be rejected.  Because BGP peering sessions take
   place in the context of TCP, the authentication and integrity
   guarantees usually association with TCP need to be provided in the
   face of possible active wiretapping attacks.  Using the terminology
   established in RFC 3552 [2], these peering sessions should be
   afforded data origin and peer entity authentication and connection-
   oriented integrity.

   Security for subscriber traffic is outside the scope of this
   document, and of BGP security in general.  IETF standards for
   subscriber data security, e.g., IPsec, TLS, and S/MIME should be
   employed for such purposes.  While adoption of BGP security measures
   may preclude certain classes of attacks on subscriber traffic, these
   measures are not a substitute for use of subscriber-based security
   mechanisms of the sort noted above.

1.3.  Areas to secure

   There are two primary points where BGP may be secured; the data
   payload of the protocol and the data semantics of the protocol.

   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 peer BGP speaker and not a replay of
   previously transmitted legitimate data.  There are several existing
   IETF standards to choose from to ensure that this system functions
   with greater effectiveness than the current system.  An example might
   be IPsec.  Some in the Operator community have expressed concerns
   that a requiring cryptographic validation could open another vector
   for a denial-of-service attack by flooding the processor with bogus
   packets which must be cryptographically invalidated before being

   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  Does the AS_PATH, received via an UPDATE, represent a valid path
      through the network?

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   The verification of AS_PATH validity falls into two distinct
   categories.  These categories are ordered from least to most

   o  Does the AS_PATH specified actually exist as a path in the network
      topology and, based on the AS_PATH, is it possible to traverse
      that path to reach a given prefix?  This AS_PATH Feasibility Check
      will be referred to later in this document.

   o  Has the UPDATE actually traveled the path?

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2.  Underlying Assumptions regarding BGP

   In order to properly identify security requirements it is important
   to articulate the fundamental aspects of BGP as related to security
   requirements.  The following list presents the basic parameters and
   application concepts of BGP that are assumed by this document.

   o  Peer Communication: BGP traffic travels over TCP between peers, so
      BGP speakers assume the 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, interior BGP (iBGP) is used to
      distribute prefix reachability information in conjunction with an
      Interior Gateway Protocol (IGP) and is used by a distinct network
      administrative entity to convey internal routing policy regarding
      external and internal information.  Exterior BGP (eBGP) is
      typically used to distribute route/prefix reachability information
      between two distinct routing entities and is used to signal eBGP
      preferences and policy decisions.

   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, with regard
      to a block of addresses, AS-A may send 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, i.e., to provide a
      backup path for the addresses in question.  However, it should be
      noted that while AS-A tries to influence the routing decisions of
      AS-B and downstream ASes, AS-A is only providing inputs to a local
      decision by AS-B, a decision that is very much influenced by
      AS-B's local policy over which AS-A has no control.  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 policy-based mechanisms
      between peers such as the NO-EXPORT community.  It is important to
      note that an UPDATE message itself generally is 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 (e.g.
      timestamps or sequence numbers).  This indicates that messages may
      appear valid at any point in the life of a BGP peering session.

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      While the AS_PATH information is typically transitive it is,
      currently, not clearly mandated and many times is removed for
      various utilitarian reasons.

   o  It is important to note that while preferences regarding routing
      can be explicitly managed with direct peers it is markedly more
      difficult to influence routing decisions by ASes that are not
      directly adjacent.

   o  Inter-AS withdrawal message assumptions: The processing model of
      BGP RFC4271 [7] indicates that only the peer advertising NLRI
      information may withdraw it.  There are several instances where a
      withdrawal may occur.  Typical reasons for withdrawal include the
      determination of a better path, peer session failure, or local
      policy change.  There is no specified mechanism for indicating to
      a peer the reason for a route withdrawal.  Each withdrawal
      received via a valid peering session must be taken at face value.
      There is no existing method to ensure that an AS will properly
      respond to a withdrawal message, e.g. withdraw the route and send
      such announcement to its neighbors.  Nor do mechanisms exist to
      ensure that old UPDATES are not re-propagated after a route was
      withdrawn before it is legitimately re-advertised.

   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 rightmost
      AS in the list is the origin AS, or at least the AS responsible
      for the announcement of the NLRI information contained in the
      UPDATE.  Specifications state that the AS routing graph MUST be
      loop free.  This indicates that UPDATES received from an external
      peer which contain the local AS will be rejected.  Prepending one
      or more instances of an AS number on inbound advertisements (where
      the external peer's AS number is prepended) and outbound
      advertisements (where the local AS number is prepended) is a
      commonly used method to bias routing.  Prepending a peer AS number
      on inbound UPDATEs is employed for biasing internal routing and
      forwarding management while prepending one's own AS number on
      outbound advertisement is typically used to bias forwarding and
      routing changes in external networks.  The latter practice is
      explicitly permitted by the BGP specification while the former is
      not.  Some operators, insert a remote AS number in an UPDATE, in
      order to cause the UPDATE to be dropped by that AS so that traffic
      will not traverse a given path.  Though this practice appears to
      be unintended in the design in BGP, anecdotal evidence is that its
      use is not totally insignificant.  While such a practice can be
      beneficial to legitimate operators, it presents a strong potential
      for misuse.  A proposed security system SHOULD address how to
      either address this concern or give specific information on this

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      topic for consideration by the Operational community.

   o  Route Origination: BGP speakers may originate routes based either
      configured internal data or via data received from peers via
      UPDATES.  An Autonomous System SHOULD only originate a prefix to
      its external peers if that prefix has been allocated to the
      administrators of that system, or if authorized by the prefix

   o  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, notable exceptions include:

      *  Deployments that make use of route servers which are separate
         from forwarding devices

      *  Deployments that use the propagation of prefixes in order to
         effectively block high bandwidth attacks (e.g.  DDoS) against
         specific IP addresses (and the associated oversubscription of

   o  Aggregation and de-aggregation: According to RFC4271 [7], 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 because some
      orignation information is removed (i.e. the more-specific
      information which triggered the generation of the aggregate).

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3.  Operational 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, etc.  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 route-table data and
      verifying the originating AS and the validity 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.

   It is recognized that achieving these goals might prove very
   difficult or even impossible.

3.2.  Incremental deployment

   It will not be feasible to deploy a newly secured BGP protocol
   throughout the public Internet instantaneously.  It also may not be
   possible to deploy a such a protocol to all routers in a large AS at
   one time.  Because of this, there are several requirements that any
   proposed mechanism to secure BGP must consider.

   o  A BGP security mechanism MUST enable each BGP speaker to configure
      use of the security mechanism on a per-peer basis.

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   o  A BGP security mechanism MUST provide backward compatibility in
      the message formatting, transmission, and processing of routing
      information carried through a mixed security environment.  Message
      formatting in a fully secured environment MAY be handled in a non-
      backward compatible fashion though care must be taken to ensure
      when traversing intermediate routers which don't support the new

   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 today's Internet 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 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.  Local controls for secure UPDATE acceptance

   Each secured environment (e.g. public Internet vs. private
   internetwork) 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 environments the rapidity of route installation may
   be of paramount importance, e.g. in order to facilitate the common
   occurrence of route withdrawal 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 so
      that routing preference and policy can be applied to its inclusion
      in the RIB.  Any given route should be trustable to a locally
      configured degree, based on the completeness of security
      information with a received UPDATE and other factors.

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   o  The security system SHOULD allow the operator to determine whether
      speed of convergence is more important than security or security
      is 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

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4.  Infrastructure Requirements

   BGP security mechanisms MAY make use of a security infrastructure to
   distribute authenticated data that is an input to routing decisions.
   Such data may be needed to verify whether a given AS is authorized to
   originate an advertisement for a specified prefix, whether an given
   organization is the recognized holder of a block of address space or
   of an AS number, etc.  Any infrastructure used to distribute data in
   support of BGP security is subject to the following criteria:

   o  It MUST be resilient to attacks on the integrity of the data it

   o  It MUST enable network operators to verify the origin of the data.

   o  It MUST be sufficiently available so as to not degrade the
      existing pace of network operations.

   o  It SHOULD not introduce new organizational entities that have to
      be trusted in order to establish the authenticity of the data.

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5.  The Trust Model

   In discussion with the operations community, concerns have emerged
   regarding the viability of a security system that requires agreement
   on a trust model dependent on a single root.  Current operational
   practice has many providers engaging in bilateral agreements that
   grant primacy to local policy choices.  The viability of a solution
   may well rest on the business imperatives of the provider community
   which may be unwilling to surrender their perceived autonomy or
   unable to come to communal agreement on this topic.

   In other environments, deployments may require an authority which has
   been decided by law or other institutional mandate.  Moreover, these
   two deployment types (single-rooted hierarchy or arbitrary
   association) may "touch" (i.e. be part of the same co-extensive BGP

   Solutions MUST account for these differing types of deployments.

   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.  Some
   acknowledgement is made that this requirement might render it
   difficult to discern an attack from a difference in trust model or
   implementation.  Any proposed solution MUST mitigate this risk.

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6.  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 NLRI (Network Layer
   Reachability Information, 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 not transitive and,
   as such, are protected by BGP session security.

   The AS_PATH for specific prefixes may be protected in any proposed
   security system in four ways:

   o  Authorization of Originating AS: For the purposes of authorization
      of the originating AS, verifiable means that it MUST be possible
      to verify that the origin AS has been authorized to originate the
      route by the prefix holder(s).

   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 the peer from which the UPDATE was

   o  AS_PATH Feasibility Check: The AS_PATH list MUST correspond to a
      valid list of autonomous systems according to the first
      verification category listed in the "Areas to Secure" Section

   o  Update Transit Check: 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.  This check is somewhat more rigorous
      than the "verifiable list of autonomous systems" above.

   All of these checks SHOULD be made available to network operators.
   Each network operator will decide, on a local basis, which of these
   checks to enable.

   There are many ways in which a differential between the speed of
   prefix/AS path attribute propagation and the information validating
   the prefix/AS_PATH attribute information can be exploited to attack
   the routing system on a transient basis.  These types of attacks
   primarily exploit the time it takes to follow the withdrawal of a
   route via an UPDATE.  As a result of this potential for temporary
   disruption, BGP security solutions MUST be capable of distributing
   security information at the same rate as the BGP announcements and
   withdrawals propagate.

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   All data needed by BGP routers to evaluate the validity of an
   advertisement MUST be made available to the routers in a timeframe
   consistent with the rate at which advertsisements characteristics
   change.  Two examples are:

   o  the distribution of information about the AS(es) authorized to
      advertise a given block of IP addresses (or an address space),

   o  the distribution of information about connectivity between
      autonomous systems and about autonomous system policies.

   Note that in today's operational Internet, the first two pieces of
   information, or their analogues, are not a part of the BGP routing
   system per se (e.g. information in Routing or Address registries.)
   They are consulted by operators on an irregular basis and are not
   consulted in real time by the routing system.  Policy information is
   explicitly carried in the routing system and inconsistently expressed
   and consulted by operators.  However, the ability to change the
   connectivity in real time is an important feature of the current

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7.  Address Allocation and Advertisement

   As part of the regular operation of the Internet, addresses allocated
   to one organization may be, and are quite commonly, advertised by
   different organizations.  Common reasons for this practice include
   multi-homing and route reduction for the purposes of resource
   conservation (e.g. aggregation).  There are two modes of delegation:

   o  A BGP speaker and listener have chosen to restrict the number 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 held by an autonomous system.  The holder 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 the ability of an address block holder to
   declare (in a secure fashion) the AS(es) that the holder authorizes
   to originate routes to its address block(s) or any portion thereof
   regardless of the relationship of the entities.

   An associated delegation criteria is the requirement to allow for
   non-BGP stub networks.  As a result, all secured BGP implementations
   MUST allow for the contemporaneous origination of a route for a
   prefix by more than one AS.

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8.  Logging

   In order to facilitate auditing and troubleshooting, a logging
   capability MUST be implemented that will indicate both negative and
   positive event behaviors.  This data SHALL be for consumption of the
   AS operating the device which is producing the logs.  Further, the
   information MAY be combined with data from other is ASes or devices
   with different implementations within the same AS for purposes of
   event correlation and tracking.  Here follow some considerations in
   this regard:

   o  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

   o  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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9.  NLRI and Path Attribute Tracking

   The ability for a receiver to know the identity of each AS that
   originates and/or forwards a routing UPDATE is a desirable trait.  In
   order to rapidly identify attack points and parties at fault for
   route table disruption, it is important to be able to track and log
   prefix origination information along with associated security

   This capability can be afforded by implementation of the
   aforementioned directive that any security system SHOULD provide a
   method to allow the receiver of an UPDATE to verify that the
   originator is actually authorized to originate the update, and that
   the AS's listed in the AS_PATH actually forwarded the update.

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10.  Transport Layer 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 (e.g.  TCP-MD5 [4] and GTSM
   [6]) are inadequate and require significant operator interaction to
   maintain a respectable level of security.

   Transport protection systems MAY 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

   Any proposed security mechanism MUST include provisions for securing
   both internal BGP and external BGP peering sessions.

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11.  Key Management

   Current implementations and deployments of TCP-MD5 [4] exhibit
   serious shortcomings with regard of key management as described in
   RFC 3562 [5].

   Key maintenance can be especially onerous for operators.  The number
   of keys required and the maintenance of keys (update/withdraw/renew)
   has had an additive effect as a barrier to deployment.  Thus
   automated means of managing keys, to reduce operational burdens, MUST
   be available throughout BGP security systems.  These security systems
   MUST be resistant to compromise of session-level or device-level
   keys, i.e., the security implications of such compromises MUST be

12.  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.

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

   [5]  Leech, "RFC 3562 - Key Management Considerations for the TCP MD5
        Signature Option", July 2003.

   [6]  Gill, Heasley, and Meyer, "RFC 3682 - The Generalized TTL
        Security Mechanism (GTSM)", February 2004.

   [7]  Rekhter, Li, and Hares, "RFC 4271 - A Border Gateway Protocol 4
        (BGP-4)", October 2005.

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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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Authors' Addresses

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

   Tony Tauber (editor)
   27 Industrial Avenue
   Chelmsford, MA  01824

   Email: ttauber@1-4-5.net

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Internet-Draft          BGP Security Requirements           October 2005

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