    Internet Engineering Task Force (IETF)                           T. King
    Request for Comments: 7999                                    C. Dietzel
    Category: Informational                                           DE-CIX
    ISSN: 2070-1721                                              J. Snijders
                                                                         NTT
                                                                  G. Doering
                                                                 SpaceNet AG
                                                                  G. Hankins
                                                                       Nokia
                                                                October 2016

                              BLACKHOLE Community

    Abstract

       This document describes the use of a well-known Border Gateway
       Protocol (BGP) community for destination-based blackholing in IP
       networks.  This well-known advisory transitive BGP community named
       "BLACKHOLE" allows an origin Autonomous System (AS) to specify that a
       neighboring network should discard any traffic destined towards the
       tagged IP prefix.

    Status of This Memo

       This document is not an Internet Standards Track specification; it is
       published for informational purposes.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Not all documents
       approved by the IESG are a candidate for any level of Internet
       Standard; see Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7999.

    King, et al.                  Informational                     [Page 1]

    RFC 7999                   BLACKHOLE Community              October 2016

    Copyright Notice

       Copyright (c) 2016 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (http://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1. Introduction ....................................................3
          1.1. Requirements Language ......................................3
       2. BLACKHOLE Community .............................................4
       3. Operational Recommendations .....................................4
          3.1. IP Prefix Announcements with BLACKHOLE Community Attached ..4
          3.2. Local Scope of Blackholes ..................................4
          3.3. Accepting Blackholed IP Prefixes ...........................5
       4. Vendor Implementation Recommendations ...........................6
       5. IANA Considerations .............................................6
       6. Security Considerations .........................................6
       7. References ......................................................7
          7.1. Normative References .......................................7
          7.2. Informative References .....................................7
       Acknowledgements ...................................................8
       Authors' Addresses .................................................9

    King, et al.                  Informational                     [Page 2]

    RFC 7999                   BLACKHOLE Community              October 2016

    1.  Introduction

       Network infrastructures have been increasingly hampered by DDoS
       attacks.  In order to dampen the effects of these DDoS attacks, IP
       networks have offered blackholing with BGP [RFC4271] using various
       mechanisms such as those described in [RFC3882] and [RFC5635].

       DDoS attacks targeting a certain IP address may cause congestion of
       links used to connect to adjacent networks.  In order to limit the
       impact of such a scenario on legitimate traffic, networks adopted a
       mechanism called "BGP blackholing".  A network that wants to trigger
       blackholing needs to understand the triggering mechanism adopted by
       its neighboring networks.  Different networks provide different
       mechanisms to trigger blackholing, including but not limited to pre-
       defined blackhole next-hop IP addresses, specific BGP communities, or
       out-of-band BGP sessions with a special BGP speaker.

       Having several different mechanisms to trigger blackholing in
       different networks makes it an unnecessarily complex, error-prone,
       and cumbersome task for network operators.  Therefore, a well-known
       BGP community [RFC1997] is defined for operational ease.

       Having such a well-known BGP community for blackholing also further
       simplifies network operations because:

       o  Implementing and monitoring blackholing becomes easier when
          implementation and operational guides do not cover many variations
          to trigger blackholing.

       o  The number of support requests from customers about how to trigger
          blackholing in a particular neighboring network will be reduced as
          the codepoint for common blackholing mechanisms is unified and
          well-known.

    1.1.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
       be interpreted as described in [RFC2119] only when they appear in all
       upper case.  They may also appear in lower case or mixed case as
       English words, without normative meaning.

    King, et al.                  Informational                     [Page 3]

    RFC 7999                   BLACKHOLE Community              October 2016

    2.  BLACKHOLE Community

       This document defines the use of a new well-known BGP transitive
       community, BLACKHOLE.

       The semantics of this community allow a network to interpret the
       presence of this community as an advisory qualification to drop any
       traffic being sent towards this prefix.

    3.  Operational Recommendations

    3.1.  IP Prefix Announcements with BLACKHOLE Community Attached

       Accepting and honoring the BLACKHOLE community, or ignoring it, is a
       choice that is made by each operator.  This community MAY be used in
       all bilateral and multilateral BGP deployment scenarios.  In a
       bilateral peering relationship, use of the BLACKHOLE community MUST
       be agreed upon by the two networks before advertising it.  In a
       multilateral peering relationship, the decision to honor or ignore
       the BLACKHOLE community is to be made according to the operator's
       routing policy.  The community SHOULD be ignored, if it is received
       by a network that it not using it.

       When a network is under DDoS duress, it MAY announce an IP prefix
       covering the victim's IP address(es) for the purpose of signaling to
       neighboring networks that any traffic destined for these IP
       address(es) should be discarded.  In such a scenario, the network
       operator SHOULD attach the BLACKHOLE community.

       The BLACKHOLE community MAY also be used as one of the trigger
       communities in a destination-based Remote Triggered Blackhole (RTBH)
       [RFC5635] configuration.

    3.2.  Local Scope of Blackholes

       A BGP speaker receiving an announcement tagged with the BLACKHOLE
       community SHOULD add the NO_ADVERTISE or NO_EXPORT community as
       defined in [RFC1997], or a similar community, to prevent propagation
       of the prefix outside the local AS.  The community to prevent
       propagation SHOULD be chosen according to the operator's routing
       policy.

       Unintentional leaking of more specific IP prefixes to neighboring
       networks can have adverse effects.  Extreme caution should be used
       when purposefully propagating IP prefixes tagged with the BLACKHOLE
       community outside the local routing domain, unless policy explicitly
       aims at doing just that.

    King, et al.                  Informational                     [Page 4]

    RFC 7999                   BLACKHOLE Community              October 2016

    3.3.  Accepting Blackholed IP Prefixes

       It has been observed in provider networks running BGP that
       announcements of IP prefixes longer than /24 for IPv4 and /48 for
       IPv6 are usually not accepted on the Internet (see Section 6.1.3 of
       [RFC7454]).  However, blackhole prefix length should be as long as
       possible in order to limit the impact of discarding traffic for
       adjacent IP space that is not under DDoS duress.  The blackhole
       prefix length is typically as specific as possible, /32 for IPv4 or
       /128 for IPv6.

       BGP speakers in a bilateral peering relationship using the BLACKHOLE
       community MUST only accept and honor BGP announcements carrying the
       BLACKHOLE community under the two following conditions:

       o  The announced prefix is covered by an equal or shorter prefix that
          the neighboring network is authorized to advertise.

       o  The receiving party agreed to honor the BLACKHOLE community on the
          particular BGP session.

       In topologies with a route server or other multilateral peering
       relationships, BGP speakers SHOULD accept and honor BGP announcements
       under the same conditions.

       An operator MUST ensure that origin validation techniques (such as
       the one described in [RFC6811]) do not inadvertently block legitimate
       announcements carrying the BLACKHOLE community.

       The BLACKHOLE community is not intended to be used with Network Layer
       Reachability Information (NLRI) [RFC5575] to distribute traffic flow
       specifications.

       The error handling for this community follows the process in
       [RFC7606] that causes a malformed community to be treated as
       withdrawn.

       Operators are encouraged to store all BGP updates in their network
       carrying the BLACKHOLE community for long-term analysis or internal
       audit purposes.

    King, et al.                  Informational                     [Page 5]

    RFC 7999                   BLACKHOLE Community              October 2016

    4.  Vendor Implementation Recommendations

       Without an explicit configuration directive set by the operator,
       network elements SHOULD NOT discard traffic destined towards IP
       prefixes that are tagged with the BLACKHOLE community.  The operator
       is expected to explicitly configure the network element to honor the
       BLACKHOLE community in a way that is compliant with the operator's
       routing policy.

       Vendors MAY provide a shorthand keyword in their configuration
       language to reference the well-known BLACKHOLE community attribute
       value.  The suggested string to be used is "blackhole".

    5.  IANA Considerations

       The IANA has registered BLACKHOLE in the "BGP Well-known Communities"
       registry.

          BLACKHOLE (= 0xFFFF029A)

       The low-order two octets in decimal are 666, a value commonly
       associated with BGP blackholing among network operators.

    6.  Security Considerations

       BGP contains no specific mechanism to prevent the unauthorized
       modification of information by the forwarding agent.  This allows
       routing information to be modified or removed; it also allows false
       information to be added by forwarding agents.  Recipients of routing
       information are not able to detect this modification.  BGPsec
       [BGPSEC] does not resolve this situation.  Even when BGPsec is in
       place, a forwarding agent can alter, add, or remove BGP communities.

       The unauthorized addition of the BLACKHOLE community to an IP prefix
       by an adversary may cause a denial-of-service attack based on denial
       of reachability.

       In order to further limit the impact of unauthorized BGP
       announcements carrying the BLACKHOLE community, the receiving BGP
       speaker SHOULD verify by applying strict filtering (see
       Section 6.2.1.1.2 of [RFC7454]) that the peer announcing the prefix
       is authorized to do so.  If not, the BGP announcement should be
       filtered.

    King, et al.                  Informational                     [Page 6]

    RFC 7999                   BLACKHOLE Community              October 2016

       BGP announcements carrying the BLACKHOLE community should only be
       accepted and honored if the neighboring network is authorized to
       advertise the prefix.  The method of validating announcements is to
       be chosen according to the operator's routing policy.

       It is RECOMMENDED that operators use best common practices to protect
       their BGP sessions, such as the ones in [RFC7454].

    7.  References

    7.1.  Normative References

       [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
                  Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
                  <http://www.rfc-editor.org/info/rfc1997>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
                  Border Gateway Protocol 4 (BGP-4)", RFC 4271,
                  DOI 10.17487/RFC4271, January 2006,
                  <http://www.rfc-editor.org/info/rfc4271>.

       [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
                  Patel, "Revised Error Handling for BGP UPDATE Messages",
                  RFC 7606, DOI 10.17487/RFC7606, August 2015,
                  <http://www.rfc-editor.org/info/rfc7606>.

    7.2.  Informative References

       [BGPSEC]   Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
                  Specification", Work in Progress, draft-ietf-sidr-bgpsec-
                  protocol-18, August 2016.

       [RFC3882]  Turk, D., "Configuring BGP to Block Denial-of-Service
                  Attacks", RFC 3882, DOI 10.17487/RFC3882, September 2004,
                  <http://www.rfc-editor.org/info/rfc3882>.

       [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
                  and D. McPherson, "Dissemination of Flow Specification
                  Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
                  <http://www.rfc-editor.org/info/rfc5575>.

    King, et al.                  Informational                     [Page 7]

    RFC 7999                   BLACKHOLE Community              October 2016

       [RFC5635]  Kumari, W. and D. McPherson, "Remote Triggered Black Hole
                  Filtering with Unicast Reverse Path Forwarding (uRPF)",
                  RFC 5635, DOI 10.17487/RFC5635, August 2009,
                  <http://www.rfc-editor.org/info/rfc5635>.

       [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
                  Austein, "BGP Prefix Origin Validation", RFC 6811,
                  DOI 10.17487/RFC6811, January 2013,
                  <http://www.rfc-editor.org/info/rfc6811>.

       [RFC7454]  Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
                  and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
                  February 2015, <http://www.rfc-editor.org/info/rfc7454>.

    Acknowledgements

       The authors would like to gratefully acknowledge many people who have
       contributed discussions and ideas to the development of this
       document.  They include Petr Jiran, Yordan Kritski, Christian Seitz,
       Nick Hilliard, Joel Jaeggli, Christopher Morrow, Thomas Mangin, Will
       Hargrave, Niels Bakker, David Farmer, Jared Mauch, John Heasley, and
       Terry Manderson.

    King, et al.                  Informational                     [Page 8]

    RFC 7999                   BLACKHOLE Community              October 2016

    Authors' Addresses

       Thomas King
       DE-CIX Management GmbH
       Lichtstrasse 43i
       Cologne  50825
       Germany

       Email: thomas.king@de-cix.net

       Christoph Dietzel
       DE-CIX Management GmbH
       Lichtstrasse 43i
       Cologne  50825
       Germany

       Email: christoph.dietzel@de-cix.net

       Job Snijders
       NTT Communications
       Theodorus Majofskistraat 100
       Amsterdam  1065 SZ
       The Netherlands

       Email: job@ntt.net

       Gert Doering
       SpaceNet AG
       Joseph-Dollinger-Bogen 14
       Munich  80807
       Germany

       Email: gert@space.net

       Greg Hankins
       Nokia
       777 E. Middlefield Road
       Mountain View, CA  94043
       United States of America

       Email: greg.hankins@nokia.com

    King, et al.                  Informational                     [Page 9]