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Versions: 00 01

Network Working Group                                       M. Behringer
Internet-Draft                                         Cisco Systems Inc
Intended status: Informational                             July 10, 2008
Expires: January 11, 2009


                   BGP Session Security Requirements
              draft-ietf-rpsec-bgp-session-sec-req-01.txt

Status of this Memo

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   This Internet-Draft will expire on January 11, 2009.

Abstract

   The document "BGP security requirements" (draft-ietf-rpsec-bgpsecrec)
   specifies general security requirements for BGP.  However, specific
   security requirements for single BGP sessions, i.e., the connection
   between two BGP peers, are only touched on briefly in the section
   "transport layer protection".  This document expands on this
   particular aspect of BGP security, defining the security requirements
   between two BGP peers.








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Table of Contents

   1.  Introduction and Problem Statement . . . . . . . . . . . . . .  3
   2.  The Threat Model . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  BGP Session Security Requirements  . . . . . . . . . . . . . .  5
     3.1.  BGP Speaker Identity . . . . . . . . . . . . . . . . . . .  5
     3.2.  Peer Authentication  . . . . . . . . . . . . . . . . . . .  6
     3.3.  Integrity  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Confidentiality  . . . . . . . . . . . . . . . . . . . . .  6
     3.5.  Anti-Replay  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.6.  Availability and Restricting IP Reachability . . . . . . .  7
     3.7.  Key Management and Operational Considerations  . . . . . .  7
     3.8.  Logging and Alerting . . . . . . . . . . . . . . . . . . .  8
     3.9.  General Considerations . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12






























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1.  Introduction and Problem Statement

   The document "BGP security requirements" [I-D.ietf-rpsec-bgpsecrec]
   specifies general security requirements for BGP.  However, specific
   security requirements for single BGP sessions, i.e., the connection
   between two BGP peers, are only touched on briefly in the section
   "transport layer protection".  This document expands on this
   particular aspect of BGP security, defining the security requirements
   between two BGP peers.

   It is important to note that security requirements between BGP peers
   are not limited to the BGP protocol itself or a particular layer of
   the OSI stack.  Crafted ICMP messages for example may have an impact
   on an established BGP session: An ICMP port unreachable, referring to
   the BGP port on the peer router, would tear down the BGP session, if
   no additional security measures are taken to prevent this.  A similar
   effect can be achieved with ICMP source quench messages.  Some of the
   mechanims currently employed to secure a BGP session are on the TCP
   layer (e.g., MD5), some on the IP layer (e.g., GTSM).  This document
   provides an overall, practical view on the security requirements for
   BGP sessions, not limited to the BGP protocol.

   Previous work in this area includes most notably the following
   documents:
   o  "Protection of BGP Sessions via the TCP MD5 Signature Option"
      [RFC2385]
   o  "Key Management Considerations for the TCP MD5 Signature Option"
      [RFC3562]
   o  "Key Change Strategies for TCP-MD5" [RFC4808]
   o  "The Generalized TTL Security Mechanism (GTSM)" [RFC5082]
   o  "Problem Statement and Requirements for a TCP Authentication
      Option" [I-D.bellovin-tcpsec]
   o  "The TCP Authentication Option" [I-D.ietf-tcpm-tcp-auth-opt].
   o  "BGP Security Requirements" [I-D.ietf-rpsec-bgpsecrec]
   o  "Generic Security Requirements for Routing Protocols"
      [I-D.ietf-rpsec-generic-requirements]
   o  "An Attack Tree for the Border Gateway Protocol"
      [I-D.ietf-rpsec-bgpattack]
   o  "Automated key selection extension for the TCP Enhanced
      Authentication Option" [I-D.weis-tcp-auth-auto-ks]
   o  "Backbone Infrastructure Attacks and Protections"
      [I-D.savola-rtgwg-backbone-attacks]

   Current implementations of BGP include a combination of some of these
   mechanisms.  However, while the security level achieved with these is
   probably currently acceptable for most providers, they pose some
   operational challenges which limit the effectiveness of BGP point to
   point security.  Current problems with BGP session security (between



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   BGP peers) include:
   o  The use of static keys, which tend to be changed infrequently, and
      often not at all.  [RFC3562] states that keys SHOULD be changed at
      least every 90 days.  However, the relative complexity of changing
      MD5 keys on all BGP peering sessions, specifically when securing
      sessions to routers maintained by two different organisations,
      means that keys are often not changed at all.  This makes long
      term brute force attacks feasible.
   o  The key change process needs to be coordinated between both sides
      of the BGP session, making this an operationally expensive
      exercise.
   o  The keys are typically chosen by humans, and expressed in ASCII;
      therefore, the entropy is typically small, making the keys easier
      to guess.  [RFC3562] outlines this problem.
   o  Dependence on the MD5 algorithm, which brings two problems: MD5 is
      not considered strong enough for the future.  ([RFC4278] states
      that "the IESG believes that [RFC2385], though adequate for BGP
      deployments at this moment, is not strong enough for general
      use".)  And, security architectures should also allow a choice of
      algorithms, to have an alternative in case serious vulnerabilities
      are discobered in an algorithm.
   o  Where confidentiality of BGP routing information is required, this
      can only be achieved today by securing the BGP session with IPsec.
      Other ways to provide confidentiality may be required in the
      future.

   This document lists the requirements for BGP session security, with
   the goal to provide a guideline for flexible, operationally
   manageable, and secure algorithms for BGP session protection.

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].


2.  The Threat Model

   The threat model presented here is based on the document "Generic
   Threats to Routing Protocols" [RFC4593], which explains generic
   threats to routing protocols.  That document provides a
   categorization and classification of threat sources, threat actions,
   threat consequences, threat consequence zones, and threat consequence
   periods.

   Security threats to point to point BGP sessions can be classified
   with the following parameters:





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   o  Threat source: Any host in the Internet that can reach one of the
      BGP peers.  (By using The Generalized TTL Security Mechanism
      (GTSM) [RFC5082] the threat source must be within the configured
      IP hop count, in the ideal case directly connected.)  A threat
      source can also be a wire tap agent, either passively listening to
      the BGP session, or actively modifying BGP data in transit.
   o  Threat action: Sending of forged BGP packets, or sniffing BGP
      traffic.
   o  Threat consequence: Break of confidentiality by wire tapping,
      break of integrity by faking BGP messages or hijacking a session,
      or denial of service, for example by sending fake RST packets,
      terminating a BGP session abnormally.
   o  Threat consequence zones: The BGP peering session itself, the BGP
      tables on the affected peers, or potentially larger areas of the
      BGP routing system.
   o  Threat consequence period: Depending on the attack and the
      implemented counter measures, a threat might be preliminarily
      mitigated by changing the MD5 key, unless it is a threat against
      MD5 itself, in which case the threat period is indefinite.

   Threats not considered in this document include:
   o  Attacks from legitimate BGP speakers, in other words, attacks from
      other BGP speakers, which are trusted (implicitly or explicitly).
      The source of the attack in this case could be either a
      misconfiguration, or an attacker gaining illegitimate access to a
      router, for example through SSH brute force attacks.

   The document "Backbone Infrastructure Attacks and Protections"
   [I-D.savola-rtgwg-backbone-attacks] describes general attack forms
   against backbones, not limited to the BGP protocol.  It provides
   useful background information to this threat model.


3.  BGP Session Security Requirements

3.1.  BGP Speaker Identity

   A BGP speaker MUST have a unique identity to present to its peer.
   This serves as a base for subsequent security mechanisms, such as
   peer authentication.  At this moment this identity is tied to the IP
   address used for the BGP peering session.  This address can be either
   the IP address of a loopback interface, or a physical interface.

   Any point to point security mechanism for BGP MUST refer to and use a
   specific BGP ID.  This ID MUST be unique for the BGP peers, it SHOULD
   be unique within an autonomous system, and it MAY be globally unique.

   A BGP speaker SHOULD be capable of using different IDs to different



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   peers, because a single router identity (the same ID for all peers)
   may not be sufficient from an operational point of view.  For example
   internally a provider may want to use address space which should not
   be seen from or accessible to the outside of his network.
   Alternatively, a provider who uses private address space [RFC1918]
   inside his network for iBGP sessions, may want to use public address
   space for eBGP sessions on the same router.

   Although currently the IP address used for the BGP peering is used as
   an identifier, it is entirely possible to use an alternative BGP ID,
   for example based on public/private key pairs, or the HIP
   architecture [RFC4423].  The document "AS-wide Unique BGP Identifier
   for BGP-4" [I-D.ietf-idr-bgp-identifier] for example proposes a
   4-byte unsigned, non-zero integer as an identity, which should be
   unique in the autonomous system.

   Note that this document does not mandate or recommend the use of a
   particular type of BGP ID, nor does it discuss the differences
   between the various approaches.  It only specifies the generic
   requirements for BGP IDs.

3.2.  Peer Authentication

   A BGP speaker MUST have a way to authenticate messages from its peer.
   Currently this is achieved by [RFC2385] derived mechanisms, however,
   several alternatives are conceivable and partly under discussion, for
   example [I-D.ietf-tcpm-tcp-auth-opt].  Also IPsec [RFC4301] provides
   peer authentication, as does TLS [RFC4346] or SSH [RFC4251].  (Note:
   Key management is discussed below.)

3.3.  Integrity

   A BGP speaker MUST have methods to ensure integrity of messages in
   transit, to avoid insertion of fake messages in the transport layer.
   This requirement is currently addressed by RFC 2385-derived
   mechanisms.  However, new methods should avoid the operational issues
   mentioned in the introduction of that RFC.  It MUST be possible to
   use various algorithms, either statically by specifying the
   algorithms used for integrity services, or by dynamic negotiation.
   (Note: Key management is discussed below.)

3.4.  Confidentiality

   A BGP speaker MAY have mechanisms to encrypt BGP messages in transit,
   thus providing confidentiality.  This service is rarely used today,
   but since BGP is used increasingly for more applications, amongst
   which for example signaling security measures, it is conceivable that
   the need for confidentiality for BGP sessions will increase.  If



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   confidentiality services are provided, they MUST allow for different
   crypto algorithms, and negotiation of which algorithm to use.  (Note:
   Key management is discussed below.)

3.5.  Anti-Replay

   A BGP speaker MUST have methods to detect and prevent replay of
   messages, to avoid for example an attacker saving a session reset
   message, and replaying it later, to produce a denial of service
   attack.

3.6.  Availability and Restricting IP Reachability

   A BGP speaker SHOULD have mechanisms to protect the BGP session
   against denial of service attacks, targeting the availability of the
   BGP session.  More specifically, a BGP speaker SHOULD have mechanisms
   to drop non-session packets efficiently (with minimum CPU impact,
   specifically before any crypto operations).  This includes access
   control lists (ACL) on layer 3/4 and possibly layer 2, providing easy
   protection against some forms of attack.  It also includes TTL based
   mechanisms such as proposed in [RFC5082].  Any reachability
   restriction of these types MUST be carried out before more CPU
   intensive tasks such as crypto operations, to be effective against
   denial of service attacks.

   Fragmentation attacks can bypass layer 4 ACLs, by fragmenting packets
   in a way that no fragment is recognized by the ACL.  (Note that layer
   4 ACLs still provide operationally useful filtering, and should
   therefore be supported.)  Fragmentation cannot occur on a single-hop
   BGP session, therefore a BGP speaker MUST have the capability to drop
   fragments on a single-hop BGP session.  On multi-hop BGP sessions
   fragmentation should not occur, if the network has been correctly
   designed, therefore a BGP speaker SHOULD also be capable of dropping
   fragements on a multi-hop BGP session.  Also on other, related,
   protocols fragmentation needs to be specially considered, since it
   can bypass some forms of ACLs.

   The document "Service Provider Infrastructure Security"
   [I-D.ietf-opsec-infrastructure-security] provides an overview of best
   practices regarding infrastructure protection, and is useful
   background material.

3.7.  Key Management and Operational Considerations

   Some of the requirements above may, in turn, require mechanisms that
   employ shared keys between the BGP peers.  Currently, statically-
   defined and manually configured keys are used for this purpose.
   [RFC3562] explains possible shortcomings of such keys, and gives



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   recommendations to improve security.  Key selection is also discussed
   in [I-D.weis-tcp-auth-auto-ks], as an extension to
   [I-D.ietf-tcpm-tcp-auth-opt].

   For any new mechanism aimed at securing BGP sessions it is highly
   desirable to use automated key generation and negotiation mechanisms,
   based on the BGP speaker identity.

   Mechanisms based on key lists with defined life times for keys, for
   example as defined by the document "Authentication-Key Rollover
   mechanism for Routing and Management Protocols"
   [I-D.viswanathan-keyrollover] may be acceptable if there are good
   reasons to avoid automated key negotiation.

3.8.  Logging and Alerting

   To be able to detect attempts of security breaks, BGP speakers MUST
   have be able to produce related alerts or logging messages.  General
   considerations to logging apply here: There should be summarization
   of events, to avoid for example a message to be sent for each packet
   that is not authenticated.  When available, secure syslog should be
   used to guarantee delivery of those messages to the management
   center.

3.9.  General Considerations

   For many of the above mentioned security requirements there are a
   vast range of protocol and implementation options, with varying
   degrees of effective security.  While strong security is desirable,
   there are several considerations to be taken into account for a BGP
   implementation:
   o  Efficient use of system resources: Stronger security mechanisms
      may require more system resources (CPU, memory, bandwidth) than
      more light-weight versions, or the unprotected BGP protocol.  A
      security mechanism may lead to an excessively higher exposure to
      denial of service attacks than the unprotected protocol, or
      another security mechanims.When considering security mechanisms,
      the "cost" in terms of system resources should be taken into
      account.
   o  Ease of configuration: Complicated configurations increase the
      likelihood for misconfigurations, with potential security
      vulnerabilities.  BGP security mechanisms should therefore be easy
      to configure, where "easy" referes to both the length of the
      configuration, as well as the complexity of it.

   Both efficient use of system resources and ease of configuration
   cannot be judged on their own, but rather represent additional
   variables to judge an overall BGP security implementation.  In other



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   words, a highly secure, but also highly complex and resource-
   consuming solution may be less preferrable over a somewhat less
   secure but simple and light-weight solution.  This has to be decided
   case-by-case.


4.  Security Considerations

   This document is entirely about security requirements for BGP point-
   to-point connections.  No new security exposures are created by this
   document.


5.  Acknowledgements

   The author would like to thank Russ White, Alvaro Retana, Brian Weis,
   Carlos Pignataro, Stephen Kent, and Joe Touch for their feedback and
   support.


6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

6.2.  Informative References

   [I-D.bellovin-tcpsec]
              Bellovin, S., "Problem Statement and Requirements for a
              TCP Authentication Option", draft-bellovin-tcpsec-01 (work
              in progress), July 2007.

   [I-D.ietf-idr-bgp-identifier]
              Chen, E. and J. Yuan, "AS-wide Unique BGP Identifier for
              BGP-4", draft-ietf-idr-bgp-identifier-09 (work in
              progress), May 2008.

   [I-D.ietf-opsec-infrastructure-security]
              Lewis, D., "Service Provider Infrastructure Security",
              draft-ietf-opsec-infrastructure-security-01 (work in
              progress), April 2007.

   [I-D.ietf-rpsec-bgpattack]
              Convery, S., "An Attack Tree for the Border Gateway
              Protocol", draft-ietf-rpsec-bgpattack-00 (work in
              progress), April 2004.



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   [I-D.ietf-rpsec-bgpsecrec]
              Christian, B. and T. Tauber, "BGP Security Requirements",
              draft-ietf-rpsec-bgpsecrec-09 (work in progress),
              November 2007.

   [I-D.ietf-rpsec-generic-requirements]
              McPherson, D., "Generic Security Requirements for Routing
              Protocols", draft-ietf-rpsec-generic-requirements-01 (work
              in progress), January 2005.

   [I-D.ietf-tcpm-tcp-auth-opt]
              Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", draft-ietf-tcpm-tcp-auth-opt-00
              (work in progress), November 2007.

   [I-D.savola-rtgwg-backbone-attacks]
              Savola, P., "Backbone Infrastructure Attacks and
              Protections", draft-savola-rtgwg-backbone-attacks-03 (work
              in progress), January 2007.

   [I-D.viswanathan-keyrollover]
              Viswanathan, S., "Authentication-Key Rollover mechanism
              for Routing and Management Protocols",
              draft-viswanathan-keyrollover-00 (work in progress),
              October 2006.

   [I-D.weis-tcp-auth-auto-ks]
              Weis, B., Appanna, C., McGrew, D., and A. Ramaiah,
              "Automated key selection extension for the TCP Enhanced
              Authentication  Option", draft-weis-tcp-auth-auto-ks-03
              (work in progress), October 2007.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

   [RFC3562]  Leech, M., "Key Management Considerations for the TCP MD5
              Signature Option", RFC 3562, July 2003.

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4278]  Bellovin, S. and A. Zinin, "Standards Maturity Variance
              Regarding the TCP MD5 Signature Option (RFC 2385) and the
              BGP-4 Specification", RFC 4278, January 2006.



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   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4423]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
              (HIP) Architecture", RFC 4423, May 2006.

   [RFC4593]  Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
              Routing Protocols", RFC 4593, October 2006.

   [RFC4808]  Bellovin, S., "Key Change Strategies for TCP-MD5",
              RFC 4808, March 2007.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.


Author's Address

   Michael H. Behringer
   Cisco Systems Inc
   Village d'Entreprises Green Side
   400, Avenue Roumanille, Batiment T 3
   Biot - Sophia Antipolis  06410
   France

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




















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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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   This document and the information contained herein are provided on an
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