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IETF RFC 6382
Last modified on Tuesday, October 25th, 2011
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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
 Abstract
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
http://www.rfc-editor.org/info/RFC 6382.
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
(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.
McPherson, et al. Best Current Practice PAGE 1 
RFC 6382 Unique ASNs for Anycasted Services October 2011
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 [RFC 4786] 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).
McPherson, et al. Best Current Practice PAGE 2
RFC 6382 Unique ASNs for Anycasted Services October 2011
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 hostname.id'
[RFC 4892] [RFC 5001] 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.
McPherson, et al. Best Current Practice PAGE 3
RFC 6382 Unique ASNs for Anycasted Services October 2011
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 [RFC 4033]
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 [RFC 4786].
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
locations.
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
Nodes.
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.
McPherson, et al. Best Current Practice PAGE 4
RFC 6382 Unique ASNs for Anycasted Services October 2011
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
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
McPherson, et al. Best Current Practice PAGE 5
RFC 6382 Unique ASNs for Anycasted Services October 2011
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 [RFC 4012] 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.
McPherson, et al. Best Current Practice PAGE 6
RFC 6382 Unique ASNs for Anycasted Services October 2011
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.
McPherson, et al. Best Current Practice PAGE 7
RFC 6382 Unique ASNs for Anycasted Services October 2011
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.
McPherson, et al. Best Current Practice PAGE 8
RFC 6382 Unique ASNs for Anycasted Services October 2011
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
services.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, December 2006.
9.2. Informative References
[DNSSEC-DEPLOY]
"Root DNSSEC", <http://www.root-dnssec.org/>
[RENESYS-BLOG]
Zmijewski, E., "Accidentally Importing Censorship",
Renesys Blog, March 30, 2010.
<http://www.renesys.com/blog/2010/03/
fouling-the-global-nest.shtml>
[RFC 4012] Blunk, L., Damas, J., Parent, F., and A. Robachevsky,
"Routing Policy Specification Language next generation
(RPSLng)", RFC 4012, March 2005.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC 4892] Woolf, S. and D. Conrad, "Requirements for a Mechanism
Identifying a Name Server Instance", RFC 4892, June 2007.
McPherson, et al. Best Current Practice PAGE 9
RFC 6382 Unique ASNs for Anycasted Services October 2011
[RFC 5001] 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
EMail: dmcpherson@verisign.com
Ryan Donnelly
Verisign, Inc.
21345 Ridgetop Circle
Dulles, VA USA 20166
Phone: +1 703.948.3200
EMail: rdonnelly@verisign.com
Frank Scalzo
Verisign, Inc.
21345 Ridgetop Circle
Dulles, VA USA 20166
Phone: +1 703.948.3200
EMail: fscalzo@verisign.com
McPherson, et al. Best Current Practice PAGE 10
RFC TOTAL SIZE: 25224 bytes
PUBLICATION DATE: Tuesday, October 25th, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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