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Unique Origin Autonomous System Numbers (ASNs) per Node for Globally Anycasted Services
RFC 6382 also known as BCP 169

Document Type RFC - Best Current Practice (October 2011)
Authors Danny R. McPherson , Ryan Donnelly , Frank Scalzo
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Ron Bonica
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RFC 6382
Internet Engineering Task Force (IETF)                      D. McPherson
Request for Comments: 6382                                   R. Donnelly
BCP: 169                                                       F. Scalzo
Category: Best Current Practice                           Verisign, Inc.
ISSN: 2070-1721                                             October 2011

             Unique Origin Autonomous System Numbers (ASNs)
                per Node for Globally Anycasted Services


   This document makes recommendations regarding the use of unique
   origin autonomous system numbers (ASNs) per node for globally
   anycasted critical infrastructure services in order to provide
   routing system discriminators for a given anycasted prefix.  Network
   management and monitoring techniques, or other operational
   mechanisms, may employ this new discriminator in whatever manner best
   accommodates their operating environment.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2011 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
   ( 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.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................4
   3. Recommendation for Unique Origin ASNs ...........................5
   4. Additional Recommendations for Globally Anycasted Services ......6
   5. Security Considerations .........................................7
   6. Deployment Considerations .......................................7
   7. Acknowledgements ................................................9
   8. IANA Considerations .............................................9
   9. References ......................................................9
      9.1. Normative References .......................................9
      9.2. Informative References .....................................9

1.  Introduction

   IP anycasting [RFC4786] has been deployed for an array of network
   services since the early 1990s.  It provides a mechanism for a given
   network resource to be available in a more distributed manner,
   locally and/or globally, with a more robust and resilient footprint,
   commonly yielding better localization and absorption of systemic
   query loads, as well as better protections in the face of distributed
   denial-of-service (DDoS) attacks, network partitions, and other
   similar incidents.  A large part of the Internet root DNS
   infrastructure, as well as many other resources, has been anycasted
   for nearly a decade.

   While the benefits realized by anycasting network services is proven,
   some issues do emerge with asserting routing system reachability for
   a common network identifier from multiple locations.  Specifically,
   anycasting in BGP requires injection of reachability information in
   the routing system for a common IP address prefix from multiple
   locations.  These anycasted prefixes and network services have
   traditionally employed a common origin autonomous system number (ASN)
   in order to preserve historically scarce 16-bit AS number space
   utilized by BGP for routing domain identifiers in the global routing
   system.  Additionally, a common origin AS number was used in order to
   ease management overhead of resource operations associated with
   acquiring and maintaining multiple discrete AS numbers as well as to
   avoid triggering various operations-oriented reporting functions
   aimed at identifying "inconsistent origin AS announcements" observed
   in the routing system.  As a result, the representation of routing
   system path attributes associated with those service instances, and
   that anycasted prefix itself, typically bear no per-instance
   discriminators in the routing system (i.e., within the network
   control plane itself).

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   Service-level query capabilities may or may not provide a mechanism
   to identify which anycast node responded to a particular query,
   although this is likely both service (e.g., DNS or NTP) and
   implementation dependent.  For example, Name Server Daemon (NSD),
   Unbound, and BIND all provide 'hostname.bind or'
   [RFC4892] [RFC5001] query support that enables service-level
   identification of a given server.  Tools such as traceroute are also
   used to determine to which location a given query is being routed,
   although it may not reveal local-scope anycast instances, or if there
   are multiple servers within a given anycast node, which of the
   servers responded to a given query, in particular, when multiple
   servers within an anycast node are connected to a single IP router.
   When utilizing these service-level capabilities, query responses are
   typically both deterministic and inherently topology dependent;
   however, these service-level identifiers at the data plane provide no
   control plane (routing system) uniqueness.

   As more services are globally anycasted, and existing anycasted
   services realize wider deployment of anycast nodes for a given
   service address in order to accommodate growing system loads, the
   difficulty of providing safeguards and controls to better protect
   those resources expands.  Intuitively, the more widely distributed a
   given anycasted service address is, the more difficult it becomes for
   network operators to detect operational and security issues that
   affect that service.  Some examples of such security and operational
   issues include BGP route leaks affecting the anycasted service, rogue
   anycast nodes appearing for the service, or the emergence of other
   aberrant behavior in either the routing system, the forward query
   datapath, or query response datapath.  Diagnosis of the routing
   system issues is complicated by the fact that no unique
   discriminators exist in the routing system to identify a given local
   or global anycast node.  Furthermore, both datapath and routing
   system problem identification is compounded by the fact that these
   incident types can be topologically dependent, and only observable
   between a given client-server set.

   Additionally, while it goes without saying that many anycasted
   services strive for exact synchronization across all instances of an
   anycasted service address, if local policies or data plane response
   manipulation techniques were to "influence" responses within a given
   region in such a way that those responses are no longer authentic or
   that they diverge from what other nodes within an anycasted service
   were providing, then it should be an absolute necessity that those
   modified resources only be utilized by service consumers within that
   region or influencer's jurisdiction.

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   Mechanisms should exist at both the network- and service-layer to
   make it abundantly apparent to operators and users alike whether any
   of the query responses are not authentic.  For DNS, DNSSEC [RFC4033]
   provides this capability at the service layer with object-level
   integrity, assuming validation is being performed by recursive name
   servers, and DNSSEC deployment at the root and top-level domain (TLD)
   levels is well underway [DNSSEC-DEPLOY].  Furthermore, control plane
   discriminators should exist to enable operators to know toward which
   of a given set of instances a query is being directed, and to enable
   detection and alerting capabilities when this changes.  Such
   discriminators may also be employed to enable anycast node preference
   or filtering keys, should local operational policy require it.

2.  Terminology

   This document employs much of the following terminology, which was
   taken in full from Section 2 of [RFC4786].

      Service Address:  an IP address associated with a particular
         service (e.g., the destination address used by DNS resolvers to
         reach a particular authority server).

      Anycast:  the practice of making a particular Service Address
         available in multiple, discrete, autonomous locations, such
         that datagrams sent are routed to one of several available

      Anycast Node:  an internally-connected collection of hosts and
         routers that together provide service for an anycast Service
         Address.  An Anycast Node might be as simple as a single host
         participating in a routing system with adjacent routers, or it
         might include a number of hosts connected in some more
         elaborate fashion; in either case, to the routing system across
         which the service is being anycast, each Anycast Node presents
         a unique path to the Service Address.  The entire anycast
         system for the service consists of two or more separate Anycast

      Catchment:  in physical geography, an area drained by a river,
         also known as a drainage basin.  By analogy, as used in this
         document, the topological region of a network within which
         packets directed at an Anycast Address are routed to one
         particular node.

      Local-Scope Anycast:  reachability information for the anycast
         Service Address is propagated through a routing system in such
         a way that a particular anycast node is only visible to a
         subset of the whole routing system.

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      Local Node:  an Anycast Node providing service using a Local-Scope
         Anycast Address.

      Global Node:  an Anycast Node providing service using a Global-
         Scope Anycast Address.

      Global-Scope Anycast:  reachability information for the anycast
         Service Address is propagated through a routing system in such
         a way that a particular anycast node is potentially visible to
         the whole routing system.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Recommendation for Unique Origin ASNs

   In order to be able to better detect changes to routing information
   associated with critical anycasted resources, globally anycasted
   services with partitioned origin ASNs SHOULD utilize a unique origin
   ASN per node where possible, if appropriate in their operating
   environment and service model.

   Discrete origin ASNs per node provide a discriminator in the routing
   system that would enable detection of leaked or hijacked instances
   more quickly and would enable operators that so choose to proactively
   develop routing policies that express preferences or avoidance for a
   given node or set of nodes associated with an anycasted service.
   This is particularly useful when it is observed that local policy or
   known issues exist with the performance or authenticity of responses
   returned from a specific anycast node, or that enacted policies meant
   to affect service within a particular region are affecting users
   outside of that region as a result of a given anycast catchment
   expanding beyond its intended scope.

   Furthermore, inconsistent origin AS announcements associated with
   anycasted services for critical infrastructure SHOULD NOT be deemed
   undesirable by routing system reporting functions, but should instead
   be embraced in order to better identify the connectedness and
   footprint of a given anycasted service.

   While namespace conservation and reasonable use of AS number
   resources should always be a goal, the introduction of 32-bit ASNs
   significantly lessens concerns in this space.  Globally anycasted
   resources, in particular, those associated with critical
   infrastructure-enabling services such as root and TLD name servers,
   SHOULD warrant special consideration with regard to AS number
   allocation practices during policy development by the constituents of

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   those responsible organizations (e.g., the Regional Internet
   Registries).  Additionally, defining precisely what constitutes
   "critical infrastructure services" or "special consideration" (e.g.,
   some small range of 32-bit AS numbers might be provided) is left to
   the constituents of those organizations.  Additionally, critical
   infrastructure employment of 32-bit ASNs for new nodes might well
   help to foster more rapid adoption of native 32-bit ASN support by
   network operators.

   One additional benefit of unique origin AS numbers per anycast node
   is that Resource Public Key Infrastructure (RPKI) Secure Inter-domain
   Routing [SIDR] machinery, and, in particular, that of Route Origin
   Authorizations (ROAs), and routing policies that may be derived based
   on those ROAs, can be employed with per-anycast-node resolution,
   rather than relying on a single ROA and common origin AS to cover all
   instantiations of an anycasted prefix (possibly hundreds) within the
   global routing system.  For example, in the case of deployments that
   incorporate partitioned ASN anycast models that have a single ASN
   bound to all nodes but crossing organizational or political
   boundaries, a situation may arise where nobody would be deemed
   appropriate to hold the key for the ROA.  Additionally, a globally
   anycasted service within a given IP prefix that shares a common ASN
   might be taken totally offline because of the revocation of an ROA
   for that origin ASN.  Today's RPKI model already inherently
   accommodates issuance of multiple ROAs with unique origins for a
   given prefix.

4.  Additional Recommendations for Globally Anycasted Services

   Two additional recommendations for globally anycasted critical
   infrastructure services are related to publication of information
   associated with a given node's physical location, and with which
   adjacent upstream ASNs an origin AS interconnects.  The former would
   allow operators to better define and optimize preferences associated
   with a given node to align with local policy and service
   optimizations.  The latter would allow expression through policy such
   as Routing Policy Specification Language [RFC4012] specified in
   Internet Routing Registries (IRRs) in a manner that illustrates a
   discrete set of upstream ASNs for each anycast node, rather than the
   current model where all upstream ASNs associated with a common origin
   AS may or may not be expressed.  This information would provide an
   additional level of static routing policy or monitoring and detection
   models by network operators and perhaps explicit network-layer source
   address validation in the datapath.

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5.  Security Considerations

   The recommendations made in this memo aim to provide more flexibility
   for network operators hoping to better monitor and prevent issues
   related to globally anycasted critical infrastructure resources.
   Anycast itself provides considerable benefit in the face of certain
   attacks; yet, if a given instance of a service can appear at many
   points in the routing system and legitimate instances are difficult
   to distinguish from malicious ones, then anycast expands the
   service's attack surface rather than reducing it.

   The recommendations made in this document are expressed to assist
   with visibility and policy specification capabilities in order to
   improve the availability of critical Internet resources.  Use cases,
   where the recommendations outlined in this memo may have helped to
   more easily detect or scope the impact of a particular incident, are
   illustrated in [RENESYS-BLOG].

   Furthermore, while application-layer protection mechanisms such as
   DNS security extensions (DNSSEC) provide object-level integrity and
   authentication, they often do so at the cost of introducing more
   failure conditions.  For example, if a recursive name server is
   performing DNSSEC validator functions and receives a bogus response
   to a given query as a result of a man-in-the-middle (MITM) or
   injected spoofed response packet such as a cache-poisoning attempt,
   the possibility might exist that the response packet is processed by
   the server and results in some temporal or persistent DoS condition
   on the recursive name server and for its client set.  The unique
   origin AS mechanism outlined in this document provides the capability
   for network operators to expressly avoid anycast node catchments
   known to regularly elicit bogus responses, while allowing the
   anycasted service address to remain available otherwise.

6.  Deployment Considerations

   Maintenance of unique ASNs for each node within an anycasted service
   may be challenging for some critical infrastructure service operators
   initially, but for globally anycasted resources, there needs to be
   some type of per-node discriminator in the control plane to enable
   detection, remediation, and optimally, preventative controls for
   dealing with routing system anomalies that are intensified by the
   application of IP anycasting.  Additionally, this technique sets the
   stage to employ RPKI-enabled machinery and more secure and explicit
   routing policies, which all network operators should be considering.

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   The granularity of data publication related to anycast node location
   should be left to the devises of each services operator, and the
   value of this mechanism in each operator's unique environment, but
   some reasonable level of detail to enable operators and service
   consumers to make informed decisions that align with their security
   and operational objectives as outlined herein should be provided by
   each critical services operator.

   Adjacent AS information for a given origin AS can already be obtained
   through careful routing system analysis when prefixes are advertised
   via a given set of AS adjacencies, and therefore, should present no
   new threat.  However, network interconnection and peering policies
   may well present some challenges in this area.  For example, if a
   technique such as unique origin AS per node is employed, then a
   single organization may no longer have a single AS for
   interconnection at each location, and interconnection policies should
   expressly consider this.  That said, interconnection with networks
   that provide critical infrastructure services should certainly be
   given due consideration as such by network operators when evaluating
   interconnection strategies.

   Today, some root and TLD operators identify erroneous anycast prefix
   announcements by detecting prefix announcements with an origin AS
   other than the common origin AS shared via all nodes.  This detection
   model would need to be expanded to account for unique origin ASNs per
   node if a given service operator chooses to employ such a model.
   Given that AS paths are trivial to manipulate in the current system,
   the above technique would only assist in the event of unintentional
   configuration errors that reoriginate the route (e.g., it does not
   detect leaks that preserve the initial path elements).  In that case,
   work underway on routing security origin and path validation in the
   SIDR working group and beyond should be consulted.

   While local policy based on any BGP attributes, to include AS path
   information, can influence policy within a local administrative
   domain and possibly downstream, there exists a possibility that
   upstream nodes continue to use a route deemed undesirable by the
   local administrator once data packets reach that network.  Network
   operators must understand the implications of this property in their
   operating environment, as it is inherent in all Internet routing.

   Finally, anycast node presence at exchange points that employ route
   servers may make enumeration of adjacent ASNs for a given node
   challenging.  While this is understood, service operators should make
   every effort to enumerate the set of adjacent ASNs associated with a
   given anycast node's origin AS.  Without express understanding of
   legitimate AS interconnection and authorized origin AS information,
   more secure routing is difficult to achieve.

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7.  Acknowledgements

   Thanks to David Conrad, Steve Kent, Mark Kosters, Andrei Robachevsky,
   Paul Vixie, Brad Verd, Andrew Herrmann, Gaurab Raj Upadhaya, Joe
   Abley, Benson Schliesser, Shane Amante, Hugo Salgado, and Randy Bush
   for review and comments on this concept.

8.  IANA Considerations

   This document requires no direct IANA actions, although it does
   provide general guidance to number resource allocation and policy
   development organizations, and, in particular, Regional Internet
   Registries, regarding allocation of AS numbers for globally anycasted

9.  References

9.1.  Normative References

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

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

9.2.  Informative References

              "Root DNSSEC", <>

              Zmijewski, E., "Accidentally Importing Censorship",
              Renesys Blog, March 30, 2010.

   [RFC4012]  Blunk, L., Damas, J., Parent, F., and A. Robachevsky,
              "Routing Policy Specification Language next generation
              (RPSLng)", RFC 4012, March 2005.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4892]  Woolf, S. and D. Conrad, "Requirements for a Mechanism
              Identifying a Name Server Instance", RFC 4892, June 2007.

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   [RFC5001]  Austein, R., "DNS Name Server Identifier (NSID) Option",
              RFC 5001, August 2007.

   [SIDR]     Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", Work in Progress, May 2011.

Authors' Addresses

   Danny McPherson
   Verisign, Inc.
   21345 Ridgetop Circle
   Dulles, VA USA 20166
   Phone: +1 703.948.3200


   Ryan Donnelly
   Verisign, Inc.
   21345 Ridgetop Circle
   Dulles, VA USA 20166
   Phone: +1 703.948.3200


   Frank Scalzo
   Verisign, Inc.
   21345 Ridgetop Circle
   Dulles, VA USA 20166
   Phone: +1 703.948.3200


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