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Internet Engineering Task Force                             D. McPherson
Internet-Draft                                            Verisign, Inc.
Intended status: Informational                                   D. Oran
Expires: June 1, 2013                                      Cisco Systems
                                                               D. Thaler
                                                   Microsoft Corporation
                                                            E. Osterweil
                                                          Verisign, Inc.
                                                       November 28, 2012

               Architectural Considerations of IP Anycast


   This memo discusses architectural implications of IP anycast, and
   provides some historical analysis of anycast use by various IETF

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 1, 2013.

Copyright Notice

   Copyright (c) 2012 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

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   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.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Anycast History  . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Anycast in IPv6  . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  DNS Anycast  . . . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  BCP 126 on Operation of Anycast Services . . . . . . . . .  8
   3.  Principles . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Layering and Resiliency  . . . . . . . . . . . . . . . . .  8
     3.2.  Anycast Addresses as Destinations  . . . . . . . . . . . .  9
     3.3.  Anycast Addresses as Sources . . . . . . . . . . . . . . .  9
     3.4.  Service Discovery  . . . . . . . . . . . . . . . . . . . . 10
   4.  Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Regarding Widespread Anycast Use . . . . . . . . . . . . . 10
     4.2.  Transport Implications . . . . . . . . . . . . . . . . . . 11
     4.3.  Stateful Firewalls, Middleboxes and Anycast  . . . . . . . 11
     4.4.  Security Considerations  . . . . . . . . . . . . . . . . . 12
     4.5.  Deployment Considerations  . . . . . . . . . . . . . . . . 13
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   6.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 15
   Appendix A.  IAB Members . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Overview

   IP anycast is used for at least one critical Internet service, that
   of the Domain Name System [RFC1035] root servers.  As of early 2009,
   at least 10 of the 13 root name servers were using IP anycast
   [RSSAC29].  Use of IP anycast is growing for other applications as
   well.  It has been deployed for over a decade for DNS resolution
   services and is currently used by several DNS Top Level Domain (TLD)
   operators.  IP anycast is also used for other services in operational
   environments, including Network Time Protocol (NTP) [RFC1305]

   Anycast addresses are syntactically indistinguishable from unicast
   addresses.  Allocation of anycast addresses typically follows a model
   similar to that of unicast allocation policies.  Anycast addressing
   is largely equivalent to that of unicast in multiple locations, and
   leverages unicast's destination-based routing to deliver a packet to
   either zero or one interface among the set of interfaces asserting
   the reachability for the address.  The expectation of delivery is to
   the "closest" instance as determined by unicast routing topology
   metric(s), and there is also a possibility that various load-
   balancing techniques (e.g., per-packet, per-microflow) may be used
   among multiple equal cost routes to distribute load for an anycasted

   Unlike IP unicast, it is not considered an error to assert the same
   anycast address on multiple interfaces within the same or multiple

   Some consider anycast a "deceptively simple idea".  That is, when IP
   anycast is employed, many pitfalls and subtleties exist with
   applications and transports, as well as for routing configuration and
   operation.  In this document, we aim to capture many of the
   architectural implications of IP anycast.

   BCP 126 [RFC4786] discusses several different deployment models with
   IP anycast.  Two additional distinctions beyond that document involve
   "off-link anycast" and "on-link anycast".  "Off-link anycast" takes
   advantage of routing protocol preferences and IP's hop-by-hop
   destination-based forwarding paradigm in order to direct packets to
   the "closest" destination.  This is the traditional method of anycast
   largely considered in BCP 126 [RFC4786] and can be used for IPv4 and
   IPv6.  "On-link anycast" is the formal support of anycast in the
   address resolution (duplicate address detection) protocol and is only
   standardized for IPv6, with the introduction of designated anycast
   addresses on the anycasted hosts, and the Override flag in Neighbor
   Discovery (ND) Neighbor Advertisements (NAs) [RFC4861].  There is no
   standardized mechanism for this in IPv4.

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2.  Background

   As of this writing, the term "anycast" appears in 176 RFCs and 144
   active Internet-Drafts.  The following sections capture some of the
   key appearances and discussion of anycasting within the IETF over the

2.1.  Anycast History

   The first formal specification of anycast was provided in "Host
   Anycasting Service" [RFC1546].  The authors of this document did a
   good job of capturing most of the issues that exist with IP anycast

   One of the first documented uses of anycast was in 1994 for a "Video
   Registry" experiment [IMR9401].  In the experiment a UDP query was
   transmitted to an anycasted address to locate the topologically
   closest "supposedly equivalent network resource":

             "A video resource (for example, a catalog server that lists
             available video clips) sends an anycast UDP datagram to
             locate the nearest video registry.  At most one registry
             responds with a unicast UDP datagram containing the
             registry's IP address.  Said resource then opens a TCP
             connection to that [the received registry address] address
             and sends a request to register itself.  Every 5 minutes or
             so, each registry multicasts to all other registries all of
             the resources it knows from local registration requests.
             It also immediately announces newly registered resources.
             Remotely registered resources not heard about for 20
             minutes are dropped."

   There is also discussion that ISPs began using anycast for DNS
   resolution services around the same time, although no public
   references to support this are available.

   In 1997 the IAB clarified that IPv4 anycast addresses were pure
   "locators", and could never serve as an "identifier" (of a host, an
   interface, or anything else) [RFC2101].

   In 1998 the IAB conducted a routing workshop [RFC2902].  Of the
   conclusions and output action items from the report, an Anycast
   section is contained in Section 2.10.3.  Specifically called out is
   the need to describe the advantages and disadvantages of anycast, and
   the belief that local-scoped well-known anycast addresses will be
   useful to some applications.  In the subsequent section, an action
   item was outlined that suggested a BOF should be held to plan work on
   anycast, and if a working group forms, a paper on the advantages and

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   the disadvantages of anycast should be included as part of the

   As a result of the recommendation in [RFC2902], in November of 1999
   an Anycast BOF [ANYCAST BOF] was held at IETF 46.  A number of uses
   for anycast were discussed.  No firm conclusion was reached regarding
   use of TCP with anycasted services, but it was observed that
   anycasting was useful for DNS, although it did introduce some new
   complexities.  The use of global anycast was not expected to scale
   (see Section 4.1 below for more discussion), and hence was expected
   to be limited to a small number of key uses.

   In 2001, the Multicast and Anycast Group Membership [MAGMA] WG was
   chartered to address host-to-router signaling, including initial
   authentication and access control issues for multicast and anycast
   group membership, but other aspects of anycast, including
   architecture and routing, were outside the group's scope.

   SNTPv4 [RFC2030] defined how to use SNTP anycast for server
   discovery.  This was extended in [RFC4330] as an NTP-specific
   "manycast" service, in which anycast was used for the discovery part.

   IPv6 defined some reserved subnet anycast addresses [RFC2526] and
   assigned one to "Mobile IPv6 Home-Agents" [RFC3775].

   The original IPv6 transition mechanism [RFC2893] made use of IPv4
   anycast addresses as tunnel endpoints for IPv6 encapsulated in IPv4,
   but this was later removed [RFC4213].  The 6to4 tunneling protocol
   [RFC3056] was augmented by a 6to4 relay anycast prefix [RFC3068]
   aiming to simplify the configuration of 6to4 routers.  Incidentally,
   6to4 deployment has shown a fair number of operational and security
   issues [RFC3964] that result from using anycast as a discovery
   mechanism.  Specifically, one inference is that operational
   consideration is needed to ensure that anycast addresses get
   advertised and/or filtered in a way that produces the intended scope
   (e.g., only advertise a route for your 6to4 relay to ASes that
   conform to your own acceptable usage policy), an attribute that can
   easily become quite operationally expensive.

   In 2002, DNS' use of anycast was first specified in "Distributing
   Authoritative Name Servers via Shared Unicast Addresses" [RFC3258].
   It is notable that it used the term "shared unicast address" rather
   than "anycast address" for the service.  This distinction was made
   due to the IPv6 distinction in the on-link model.  "Shared unicast"
   addresses are unicast (not multicast) in the IPv6 model and,
   therefore, support the off-link anycast model (described earlier),
   but not the on-link anycast model.  At the same time, site-local-
   scoped well-known addresses began being used for recursive resolvers

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   [I-D.ietf-ipv6-dns-discovery], but this use was never standardized
   (see below in Section 3.4 for more discussion).

   Anycast was used for routing to rendezvous points (RPs) for PIM

   "Operation of Anycast Services" BCP 126 [RFC4786] deals with how the
   routing system interacts with anycast services, and the operation of
   anycast services.

   "Requirements for a Mechanism Identifying a Name Server Instance"
   [RFC4892] cites the use of anycast with DNS as a motivation to
   identify individual name server instances, and the Name Server ID
   (NSID) option was defined for this purpose [RFC5001].

   The IAB's "Reflections on Internet Transparency" [RFC4924] briefly
   mentions how violating transparency can also damage global services
   that use anycast.

2.2.  Anycast in IPv6

   Originally, the IPv6 addressing architecture [RFC1884] [RFC2373]
   [RFC3513] severely restricted the use of anycast addresses.  In
   particular, they provided that anycast addresses must not be used as
   a source address, and must not be assigned to an IPv6 host (i.e.,
   only routers).  These restrictions were later lifted in 2006

   In fact, the recent "IPv6 Transition/Co-existence Security
   Considerations" [RFC4942] overview now recommends:

             "To avoid exposing knowledge about the internal structure
             of the network, it is recommended that anycast servers now
             take advantage of the ability to return responses with the
             anycast address as the source address if possible."

   As discussed in the Overview, "on-link anycast" is employed expressly
   in IPv6 via ND NAs; see Section 7.2.7 of [RFC4861] for additional

2.3.  DNS Anycast

   "Distributed Authoritative Name Servers via Shared Unicast Addresses"
   [RFC3258] described how to reach authoritative name servers using
   anycast.  It made some interesting points, for example this text from
   Section 2.3:

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             "This document presumes that the usual DNS failover methods
             are the only ones used to ensure reachability of the data
             for clients.  It does not advise that the routes be
             withdrawn in the case of failure; it advises instead that
             the DNS process shutdown so that servers on other addresses
             are queried.  This recommendation reflects a choice between
             performance and operational complexity.  While it would be
             possible to have some process withdraw the route for a
             specific server instance when it is not available, there is
             considerable operational complexity involved in ensuring
             that this occurs reliably.  Given the existing DNS failover
             methods, the marginal improvement in performance will not
             be sufficient to justify the additional complexity for most

   Other assertions included:

   o  it asserted (as an advantage) that no routing changes were needed

   o  it recommended stopping DNS processes, rather than withdrawing
      routes, to deal with failures, data synchronization issues, and
      fail-over, as provided in the quoted text above.  The spirit of
      this advice was that DNS resolvers may (indeed) reach out and
      query unavailable DNS name servers, but as their queries timeout
      they will elect to pin themselves to other name server instances,
      which are available.

   o  it argued that failure modes involving state were not serious,

      *  the vast majority of DNS queries are UDP

      *  large routing metric disparity among authoritative server
         instances would localize queries to a single instance for most

      *  when the resolver tries TCP and it breaks, the resolver will
         try to move to a different server instance.  In order ensure
         that this is possible, it is important that an anycast zone be
         configured with multiple server addresses for different name
         server instances.  The advice given in
         [I-D.ietf-ipv6-dns-discovery] describes, in more detail, why
         using multiple addresses is important.  With these provisions,
         failed TCP connections to an anycast instance presumably won't
         break, just as with normal unicast failover.

   "Unique Per-Node Origin ASNs for Globally Anycasted Services"
   [RFC6382] makes recommendations regarding the use of per-node unique

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   origin ASNs for globally anycasted critical infrastructure services
   in order to provide routing system discriminators for a given
   anycasted prefix.  The object was to allow network management and
   monitoring techniques, or other operational mechanisms to employ this
   new origin AS as a discriminator in whatever manner fits their
   operating environment, either for detection or policy associated with
   a given anycasted node.

2.4.  BCP 126 on Operation of Anycast Services

   "Operation of Anycast Services" BCP 126 [RFC4786] was a product of
   the IETF's GROW working group.  The primary design constraint
   considered was that routing "be stable" for significantly longer than
   a "transaction time", where "transaction time" is loosely defined as
   "a single interaction between a single client and a single server".
   It takes no position on what applications are suitable candidates for
   anycast usage.

   Furthermore, it views anycast service disruptions as an operational
   problem: "Operators should be aware that, especially for long running
   flows, there are potential failure modes using anycast that are more
   complex than a simple 'destination unreachable' failure using

   The document primary deals with global Internet-wide services
   provided by anycast.  Where internal topology issues are discussed
   they're mostly regarding routing implications, rather than
   application design implications.  BCP 126 also views networks
   employing per-packet load balancing on equal cost paths as
   "pathological".  This was also discussed in [RFC2991].

3.  Principles

3.1.  Layering and Resiliency

   Preserving the integrity of a modular layered design for IP protocols
   on the Internet is critical to its continued success and flexibility.
   One such consideration is that of whether an application should have
   to adapt to changes in the routing system.

   Higher layer protocols should make minimal assumptions about lower
   layer protocols.  E.g., applications should make minimal assumptions
   about routing stability, just as they should make minimal assumptions
   about congestion and packet loss.  When designing applications, it
   would perhaps be safe to assume that the routing system may deliver
   each packet to a different service instance, in any pattern, with
   temporal re-ordering being a not-so-rare phenomenon.

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   Stateful transport protocols (e.g., TCP), without modification, do
   not understand the properties of anycast and hence will fail
   probabilistically, but possibly catastrophically, when using anycast
   addresses in the presence of "normal" routing dynamics.
   Specifically, if datagrams associated with a given active transaction
   are routed to a new anycasted end system and that end system lacks
   state data associated with the active transaction, the session will
   be reset and hence need to be reinitiated.

3.2.  Anycast Addresses as Destinations

   Anycast addresses are "safe" to use as destination addresses for an
   application if the following design points are all met:

   o  A request message or "one shot" message is self-contained in a
      single transport packet

   o  A stateless transport (e.g., UDP) is used for the above

   o  Replies are always sent to a unicast address; these can be multi-
      packet since the unicast destination is presumed to be associated
      with a single "stable" end system and not an anycasted source
      address.  Note that this constrains the use of anycast as source
      addresses in request messages, since reply messages sent back to
      that address may reach a device that was not the source that
      initially triggered it.

   o  The server side of the application keeps no hard state across

   o  Retries are idempotent; in addition to not assuming server state,
      they do not encode any assumptions about loss of requests versus
      loss of replies.

3.3.  Anycast Addresses as Sources

   Anycast addresses are "safe" to use as source addresses for an
   application if all of the following design points are met:

   o  No response message is generated by the receiver with the anycast
      source used as a destination unless the application has some
      private state synchronization that allows for the response message
      arriving at a different instance

   o  The source anycast address is reachable via the interface address
      if unicast reverse path forwarding (RPF) [RFC4778] checking is on,
      or the service address is explicitly provisioned to bypass RPF
      checks.  In addition to the application defined in [RFC4778],

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      Section 4.4.5 of BCP 126 [RFC4786] gives explicit consideration to
      RPF checks in anycasting operations.

3.4.  Service Discovery

   Applications able to tolerate an extra round trip time (RTT) to learn
   a unicast destination address for multi-packet exchanges might safely
   use anycast destination addresses for service instance discovery.
   For example, "instance discovery" messages are sent to an anycast
   destination address, and a reply is subsequently sent from the unique
   unicast source address of the interface that received the discovery
   message, or a reply is sent from the anycast source address of the
   interface that received the message, containing the unicast address
   to be used to invoke the service.  Only the latter of these will
   avoid potential NAT binding and stateful firewall issues.

   Section 3.3 of [RFC4339] proposes a "Well-known Anycast Address" for
   recursive DNS service configuration in clients to ease configuration
   and allow those systems to ship with these well-known addresses
   configured "from the beginning, as, say, factory default".  During
   publication the IESG requested that the following "IESG Note" be
   contained in the document:

             "This document describes three different approaches for the
             configuration of DNS name resolution server information in
             IPv6 hosts.

             There is not an IETF consensus on which approach is
             preferred.  The analysis in this document was developed by
             the proponents for each approach and does not represent an
             IETF consensus.

             The 'RA option' and 'Well-known anycast' approaches
             described in this document are not standardized.
             Consequently the analysis for these approaches might not be
             completely applicable to any specific proposal that might
             be proposed in the future."

4.  Analysis

4.1.  Regarding Widespread Anycast Use

   Widespread use of anycast for global Internet-wide services or inter-
   domain services has some scaling challenges.  Similar in ways to
   multicast, each service generates at least one unique route in the
   global BGP routing system.  As a result, additional anycast instances
   result in additional paths for a given prefix, which scales super-

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   linearly as a function of denseness of inter-domain interconnection
   within the routing system (i.e., more paths result in more resources,
   more network interconnections result in more paths).

   This is why the Anycast BOF concluded that "the use of global anycast
   addresses was not expected to scale and hence was expected to be
   limited to a small number of key uses."

4.2.  Transport Implications

   UDP is the "lingua franca" for anycast today.  Stateful transports
   could be enhanced to be more anycast friendly.  This was anticipated
   in Host Anycasting Services [RFC1546], specifically:

             "The solution to this problem is to only permit anycast
             addresses as the remote address of a TCP SYN segment
             (without the ACK bit set).  A TCP can then initiate a
             connection to an anycast address.  When the SYN-ACK is sent
             back by the host that received the anycast segment, the
             initiating TCP should replace the anycast address of its
             peer, with the address of the host returning the SYN-ACK.
             (The initiating TCP can recognize the connection for which
             the SYN-ACK is destined by treating the anycast address as
             a wildcard address, which matches any incoming SYN-ACK
             segment with the correct destination port and address and
             source port, provided the SYN-ACK's full address, including
             source address, does not match another connection and the
             sequence numbers in the SYN-ACK are correct.)  This
             approach ensures that a TCP, after receiving the SYN-ACK is
             always communicating with only one host."

   Multi-address transports (e.g., SCTP) might be more amenable to such
   extensions than TCP.

   Some similarities exist between what is needed for anycast and what
   is needed for address discovery when doing multi-homing in the
   transport layer.

4.3.  Stateful Firewalls, Middleboxes and Anycast

   Middleboxes (e.g., NATs) and stateful firewalls cause problems when
   used in conjunction with some ways to use anycast.  In particular, a
   server-side transition from an anycast source IP address to a unique
   unicast address may require new or additional session state, and this
   may not exist in the middlebox, as discussed previously in
   Section 3.4.

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4.4.  Security Considerations

   Anycast is often deployed to mitigate or at least localize the
   effects of distributed denial of service (DDOS) attacks.  For
   example, with the Netgear NTP fiasco [RFC4085] anycast was used in a
   distributed sinkhole model [RFC3882] to mitigate the effects of
   embedded globally-routed Internet addresses in network elements.

   "Internet Denial-of-Service Considerations" [RFC4732] notes: that "A
   number of the root nameservers have since been replicated using
   anycast to further improve their resistance to DoS".

   "Operation of Anycast Services" BCP 126 [RFC4786] cites DoS
   mitigation, constraining DoS to localized regions, and identifying
   attack sources using spoofed addresses as some motivations to deploy
   services using anycast.  Multiple anycast service instances such as
   those used by the root name servers also add resiliency when network
   partitioning occurs (e.g., as the result of transoceanic fiber cuts
   or natural disasters).

   It should be noted that there is a significant man in the middle
   (MITM) exposure in either variant of anycast discovery (see
   Section 3.4) that in many applications may necessitate the need for
   end to end security models [RFC2402] [RFC2406] that enable end
   systems to authenticate one another.

   Furthermore, as discussed earlier in this document, operational
   consideration needs to be given to ensure that anycast addresses get
   advertised and/or filtered in a way that produces intended scope (for
   example, only advertise a route to your 6to4 relay to ASes that
   conform to your own Acceptable Use Policy, AUP).  This seems to be
   operationally expensive, and is often vulnerable to errors outside of
   the local routing domain, in particular when anycasted services are
   deployed with the intent to scope associated announcements within
   some local or regional boundary.

   As previously discussed, [RFC6382] makes recommendations regarding
   the use of per-node unique origin ASNs 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 then
   employ this new discriminator in whatever manner fits their operating
   environment, either for detection or policy associated with a given
   anycasted node.

   Unlike multicast (but like unicast), anycast allows traffic stealing.
   That is, with multicast, joining a multicast group doesn't prevent
   anyone else who was receiving the traffic from continuing to receive

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   the traffic.  With anycast, adding an anycasted node to the routing
   system can prevent a previous recipient from continuing to receive
   traffic because it may now be delivered to the new node instead.  As
   such, if one allows unauthorized anycast nodes onto the network,
   traffic can be diverted thereby triggering DoS or other attacks.
   Section 6.3 of BCP 126 [RFC4786] provides expanded discussion on
   "Service Hijacking" and "traffic stealing".

   Unlike unicast (but like multicast), the desire is to allow
   applications to cause route injection (either directly or as a side
   effect of doing something else).  This combination is unique to
   anycast and presents new security concerns which are why MAGMA
   [MAGMA] only got so far.  The security concerns include:

   1.  Allowing route injection can cause DOS to a legitimate address

   2.  Allowing route injection consumes routing resources and can hence
       cause DOS to the routing system and impact legitimate
       communications as a result.

   These are two of the core issues that were part of the discussion
   during [RFC1884], the [ANYCAST BOF], and the MAGMA [MAGMA]

   Additional security considerations are scattered throughout the list
   of references provided herein.

4.5.  Deployment Considerations

   BCP 126 [RFC4786] provides some very solid guidance related to
   operations of anycasted services, and in particular DNS.

   This document covers issues associated with the architectural
   implications of anycast.  This document does not treat in any depth
   the fact that there are deployed services with TCP transport using
   anycast today.  While we believe that such practice is not "safe" in
   the traditional and architectural sense, these things are indeed
   relative, and we recognize it is not always the case that
   unpredictability in the routing system beyond the local
   administrative domain is unmanageable.  That is, despite the inherent
   architectural problems in the use of anycast with stateful transport
   and connection-oriented protocols, there is expanding deployment
   (e.g., for content distribution networks) and situations exist where
   it may make sense (e.g., such as with service discovery, short- lived
   transactions, or in cases where a client-base may need topological-
   specific service selection that can be implicitly handled by allowing
   a homogeneous anycast address configuration to dynamically direct

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   traffic to topologically optimal service instances).  In general,
   operators should consider the content and references provided herein,
   and evaluate the benefits and implications of anycast in their
   specific environments and applications.

   In addition, (as noted in Section 2.3) the issue of whether to
   withdraw anycast routes when there is a service failure is only
   briefly broached in [RFC3258].  The advice given is that routes
   should not be withdrawn, in order to reduce operational complexity.
   However, the issue of route advertisements and service outages
   deserves greater attention.  There is an inherent tradeoff that
   exists between the operational complexity of matching service outages
   with anycast route withdraws, and allowing anycast routes to persist
   for services that are no longer available.  [RFC3258] maintains that
   DNS' inherent failure recovery mechanism is sufficient to overcome
   failed nodes, but even this advice enshrines the notion that these
   decisions are both application specific and subject to the
   operational needs of each deployment.  Rather than prescribing advice
   that attempts to befit all situations, it is important to realize
   that anycast services are, inherently, cross-modal (i.e. they are
   visible in multiple protocol layers, or modes, of the network
   application).  The routing system plays a larger role in DNS when
   services are anycast.  Therefore, operational consideration must be
   given to the fact that relying on anycast for DNS deployment
   optimizations means that there are operational benefits to keeping
   route advertisements (and withdraws) symmetric with service
   availability.  In order to ensure, for example, the DNS resolvers in
   a failed anycast instance's catchment [RFC4786] are able to failover
   and reach a non-failed catchment, a route withdraw is almost
   certainly required.

5.  IANA Considerations

   No IANA actions are required.

6.  Conclusions

   In summary, operators and application vendors alike should consider
   the benefits and implications of anycast in their specific
   environments and applications, and also give forward consideration to
   how new network protocols and application functions may take
   advantage of anycast, or how they may be negatively impacted if
   anycasting is employed.

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7.  Acknowledgements

   Many thanks to Kurtis Lindqvist for his early review and feedback on
   this document.  Thanks to Brian Carpenter, Alfred Hoenes, and Joe
   Abley for their usual careful review and feedback as well as Mark
   Smith for his detailed review.

8.  Informative References

              Deering, S., "IAB Anycast BOF Announcement", October 1999,

              Durand, A., Hagino, J., and D. Thaler, "Well known site
              local unicast addresses for DNS resolver",
              draft-ietf-ipv6-dns-discovery-06 (work in progress),
              September 2002.

   [IMR9401]  "INTERNET MONTHLY REPORT", January 1994,

   [MAGMA]    "Multicast and Anycast Group Membership (MAGMA),
              concluded", April 2006,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, November 1993.

   [RFC1884]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 1884, December 1995.

   [RFC2030]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4
              for IPv4, IPv6 and OSI", RFC 2030, October 1996.

   [RFC2101]  Carpenter, B., Crowcroft, J., and Y. Rekhter, "IPv4
              Address Behaviour Today", RFC 2101, February 1997.

   [RFC2373]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

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   [RFC2402]  Kent, S. and R. Atkinson, "IP Authentication Header",
              RFC 2402, November 1998.

   [RFC2406]  Kent, S. and R. Atkinson, "IP Encapsulating Security
              Payload (ESP)", RFC 2406, November 1998.

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [RFC2893]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 2893, August 2000.

   [RFC2902]  Deering, S., Hares, S., Perkins, C., and R. Perlman,
              "Overview of the 1998 IAB Routing Workshop", RFC 2902,
              August 2000.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991, November 2000.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, June 2001.

   [RFC3258]  Hardie, T., "Distributing Authoritative Name Servers via
              Shared Unicast Addresses", RFC 3258, April 2002.

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.

   [RFC3882]  Turk, D., "Configuring BGP to Block Denial-of-Service
              Attacks", RFC 3882, September 2004.

   [RFC3964]  Savola, P. and C. Patel, "Security Considerations for
              6to4", RFC 3964, December 2004.

   [RFC4085]  Plonka, D., "Embedding Globally-Routable Internet
              Addresses Considered Harmful", BCP 105, RFC 4085,
              June 2005.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing

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              Architecture", RFC 4291, February 2006.

   [RFC4330]  Mills, D., "Simple Network Time Protocol (SNTP) Version 4
              for IPv4, IPv6 and OSI", RFC 4330, January 2006.

   [RFC4339]  Jeong, J., "IPv6 Host Configuration of DNS Server
              Information Approaches", RFC 4339, February 2006.

   [RFC4610]  Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol
              Independent Multicast (PIM)", RFC 4610, August 2006.

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.

   [RFC4778]  Kaeo, M., "Operational Security Current Practices in
              Internet Service Provider Environments", RFC 4778,
              January 2007.

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

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

   [RFC4924]  Aboba, B. and E. Davies, "Reflections on Internet
              Transparency", RFC 4924, July 2007.

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942,
              September 2007.

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

   [RFC6382]  McPherson, D., Donnelly, R., and F. Scalzo, "Unique Origin
              Autonomous System Numbers (ASNs) per Node for Globally
              Anycasted Services", BCP 169, RFC 6382, October 2011.

   [RSSAC29]  "RSSAC 29 Meeting Minutes", December 2007,

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Appendix A.  IAB Members

   Internet Architecture Board Members at the time this document was
   published were:


Authors' Addresses

   Danny McPherson
   Verisign, Inc.
   12061 Bluemont Way
   Reston, VA

   Email: dmcpherson@verisign.com

   Dave Oran
   Cisco Systems

   Email: oran@cisco.com

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA

   Email: dthaler@microsoft.com

   Eric Osterweil
   Verisign, Inc.
   12061 Bluemont Way
   Reston, VA

   Email: eosterweil@verisign.com

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