GROW                                                        K. Lindqvist
Internet-Draft                                  Netnod Internet Exchange
Expires: April 22, 2005                                         J. Abley
                                                        October 22, 2004

                     Operation of Anycast Services

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).


   As the Internet has grown, many services with high availability
   requirements have emerged.  The requirements of these services have
   increased the demands on the reliability of the infrastructure on
   which those services rely.

   Many techniques have been employed to increase the availability of

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   services deployed on the Internet.  This document presents
   operational experience of wide-scale service distribution using
   anycast, and proposes a series of recommendations for others using
   this approach.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3

   3.  Anycast Service Distribution . . . . . . . . . . . . . . . . .  4
     3.1   General Description  . . . . . . . . . . . . . . . . . . .  4
     3.2   Goals  . . . . . . . . . . . . . . . . . . . . . . . . . .  5

   4.  Design . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1   Protocol Suitability . . . . . . . . . . . . . . . . . . .  5
     4.2   Node Placement . . . . . . . . . . . . . . . . . . . . . .  6
     4.3   Routing Systems  . . . . . . . . . . . . . . . . . . . . .  6
       4.3.1   Anycast within an IGP  . . . . . . . . . . . . . . . .  6
       4.3.2   Anycast within the Global Internet . . . . . . . . . .  7
     4.4   Routing Considerations . . . . . . . . . . . . . . . . . .  7
       4.4.1   Signalling Service Availability  . . . . . . . . . . .  7
       4.4.2   Covering Prefix  . . . . . . . . . . . . . . . . . . .  8
       4.4.3   Equal-Cost Paths . . . . . . . . . . . . . . . . . . .  8
       4.4.4   Route Dampening  . . . . . . . . . . . . . . . . . . .  9
       4.4.5   Reverse Path Forwarding Checks . . . . . . . . . . . . 10
       4.4.6   Propagation Scope  . . . . . . . . . . . . . . . . . . 10
       4.4.7   Other Peoples' Networks  . . . . . . . . . . . . . . . 11
     4.5   Data Synchronisation . . . . . . . . . . . . . . . . . . . 11
     4.6   Node Autonomy  . . . . . . . . . . . . . . . . . . . . . . 11

   5.  Service Management . . . . . . . . . . . . . . . . . . . . . . 12
     5.1   Monitoring . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.2   Self-Healing Nodes . . . . . . . . . . . . . . . . . . . . 12

   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13

   7.  Protocol Considerations  . . . . . . . . . . . . . . . . . . . 13

   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13

   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14

       Intellectual Property and Copyright Statements . . . . . . . . 16

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1.  Introduction

   To distribute a service using anycast, the service is first
   associated with a stable set of IP addresses, and reachability to
   those addresses is advertised in a routing system from multiple,
   independent service nodes.  Various techniques for anycast deployment
   of services are discussed in RFC 1546 [4], ISC-TN-2003-1 [12] and
   ISC-TN-2004-1 [13].

   Anycast has in recent years become increasingly popular for adding
   redundancy to DNS servers.  Several root server operators have
   distributed their servers widely around the Internet, and both
   resolver and authority servers are commonly distributed within the
   networks of service providers.  Anycast distribution has been used by
   commercial DNS authority server operators for several years.  The use
   of anycast is not limited to the DNS, although the use of anycast
   imposes some additional requirements on the nature of the service
   being distributed, including transaction longevity, transaction state
   held on servers and data synchronization capabilities.

   Although anycast is conceptually simple, its implementation
   introduces some pitfalls for operation of the service.  For example,
   monitoring the availability of the service becomes more difficult;
   the observed availability changes according to the source of the
   query, and the client catchment of individual anycast nodes is not
   static, nor especially deterministic.

   This document will describe the use of anycast for both local scope
   distribution of services using an Interior Gateway Protocol (IGP) and
   global distribution using BGP [5].  Many of the issues for monitoring
   and data synchronization are common to both, but deployment issues
   differ substantially.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

   Service Address: an IP address associated with a particular service
      (e.g.  the address of a nameserver).
   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
      which together provide service for an anycast service address.

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

3.  Anycast Service Distribution

3.1  General Description

   Anycast is the name given to the practice of making one or more
   Service Addresses available to a routing system at Anycast Nodes in
   two or more discrete locations.  The service provided by each node is
   necessarily consistent regardless of the particular node chosen by
   the routing system to handle a particular request.

   For services distributed using anycast, there is no inherent
   requirement for referrals to other servers or name-based service
   distribution ("round-robin DNS"), although those techniques could be
   combined with anycast service distribution if an application required
   it.  The routing system makes the decision of the node to be used for
   each request, based on the topological design of the routing system
   and the point in the network at which the request originates.

   The Anycast Node chosen to service a particular query can be
   influenced by the traffic engineering capabilities of the routing
   protocols which make up the routing system.  The degree of influence
   available to the operator of the node depends on the scale of the
   routing system within which the Service Address is anycast.

   Load-balancing between Anycast Nodes is typically difficult to
   achieve (load distribution between nodes is generally unbalanced in
   terms of request and traffic load).  Distribution of load between
   nodes for the purposes of reliability, and coarse-grained
   distribution of load for the purposes of making popular services
   scalable can often be accommodated, however.

   The scale of the routing system through which a service is anycast
   can vary from a small Interior Gateway Protocol (IGP) connecting a
   small handful of components, to the Border Gateway Protocol (BGP) [5]
   connecting the global Internet, depending on the nature of the

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   service distribution that is required.

3.2  Goals

   A service may be anycast for a variety of reasons.  A number of
   common objectives are:

   1.  Coarse ("unbalanced") distribution of load across nodes, to allow
       infrastructure to scale to increased numbers of queries and to
       accommodate transient query peaks;
   2.  Mitigation of non-distributed denial of service attacks by
       localizing damage to single anycast nodes;
   3.  Constraint of distributed denial of service attacks or flash
       crowds to local regions around anycast nodes (perhaps restricting
       query traffic to local peering links, rather than paid transit
   4.  Triangulation of traffic sources, in the case of attack (or
       query) traffic which incorporates spoofed source addresses;
   5.  Improvement of query response time, by reducing the network RTT
       between client and server with the provision of a local Anycast
   6.  Reduction of a list of servers to a single, distributed address.
       For example, a large number of authoritative nameservers for a
       zone may be deployed using a small set of anycast service
       addresses; this approach can increase the accessibility of zone
       data in the DNS without increasing the size of a referral
       response from a parent nameserver.

4.  Design

4.1  Protocol Suitability

   When a service is anycast between two or more nodes, the routing
   system effectively makes the node selection decision on behalf of a
   client.  Since it is usually a requirement that a single
   client-server interaction is carried out between a client the same
   server node for the duration of the transaction, it follows that the
   routing system's node selection decision ought to be stable for an
   order of magnitude longer than the expected transaction time, if the
   service is to be provided reliably.

   Some services have very short transaction times, and may even be
   carried out using a single packet request and a single packet reply
   in some cases (the DNS is an example of this).  Other services
   involve far longer-lived transactions (e.g.  bulk file downloads and
   audio-visual media streaming).

   Some anycast deployments have very predictable routing systems, which

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   can remain stable for long periods of time (e.g.  anycast within an
   IGP, where node selection changes only occur as a response to node
   failures).  Other deployments have far less predictable
   characteristics (e.g.  a densely-deployed array of nodes across the
   global Internet).

   The stability of the routing system together with the transaction
   time of the service should be carefully compared when deciding
   whether a service is suitable for distribution using anycast.

4.2  Node Placement

   Decisions as to where Anycast Nodes should be placed will depend to a
   large extent on the goals of the service distribution.  For example:

   o  A recursive resolver service might be distributed within an ISP's
      network, one Anycast Node per PoP.
   o  A root server service might be distributed throughout the Internet
      with nodes located in regions with poor external connectivity, to
      ensure that the DNS functions adequately within the region during
      times of external network failure.
   o  An FTP mirror service might include local nodes located at
      exchange points, so that ISPs connected to that exchange point
      could download bulk data more cheaply than if they had to use
      expensive transit circuits.

   In general node placement decisions should be made with consideration
   of likely traffic requirements, the potential for flash crowds or
   denial-of-service traffic, the stability of the local routing system
   and the failure modes with respect to node failure, or local routing
   system failure.

4.3  Routing Systems

4.3.1  Anycast within an IGP

   There are several common motivations for the distribution of a
   Service Address within the scope of an IGP:

   1.  to improve service response times, by hosting a service close to
       other users of the network;
   2.  to improve service reliability by providing automatic fail-over
       to backup nodes; and
   3.  to keep service traffic local, to avoid congesting wide-area

   In each case the decisions as to where and how services are
   provisioned can be made by network engineers without requiring such

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   operational complexities as regional variances in the configuration
   of client computers, or DNS tricks which respond differently to
   requests from clients in different locations.

   When a service is anycast within an IGP the routing system is
   typically under the control of the same organization who is providing
   the service, and hence the relationship between service transaction
   characteristics and network stability are likely to be relatively
   well-understood.  This technique is consequently applicable to a
   larger number of applications than Internet-wide anycast service
   distribution (see Section 4.1).

   By reducing the scope of the IGP to just the hosts providing service
   (together with one or more gateway routers) this technique can be
   applied to the construction of server clusters.  This application is
   discussed in some detail in [13].

4.3.2  Anycast within the Global Internet

   Service Addresses may be anycast within the global Internet routing
   system in order to distribute services across the entire network.
   The principal differences between this application and the IGP-scope
   distribution discussed in Section 4.3.1 are that:

   1.  the routing system is, in general, controlled by other people;
   2.  the routing protocol concerned (BGP), and commonly-accepted
       practices in its deployment, impose some additional constraints
       (see Section 4.4).

4.4  Routing Considerations

4.4.1  Signalling Service Availability

   When a routing system is provided with reachability information for a
   Service Address from an individual node, packets addressed to that
   Service Address will start to arrive at the node.  Since it is
   desirable for the node to be ready to accept requests before they
   start to arrive, a coupling between the routing information and the
   availability of the service at a particular node is desirable.

   Where a routing advertisement from a node corresponds to a single
   Service Address, this coupling might be such that availability of the
   service triggers the route advertisement, and non-availability of the
   service triggers a route withdrawal.  This can be achieved using
   routing protocol implementations on the same servers which provide
   the service being distributed.

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   Where a routing advertisement from a node corresponds to two or more
   Service Addresses, it may not be appropriate to trigger a route
   withdrawal due to the non-availability of a single service.  Another
   approach is to tunnel requests from nodes that cannot handle
   individual services to other nodes that can, perhaps using an IGP
   which extends over tunnels between nodes, in which servers
   participate.  Circumstances which might lead to multiple Service
   Addresses being covered by a single route are discussed in Section

4.4.2  Covering Prefix

   In some routing systems (e.g.  the BGP-based routing system of the
   global Internet) it is not possible, in general, to propagate a host
   route with confidence that availability of the route will be signaled
   throughout the network.  This is a consequence of operational policy,
   and not a protocol restriction.

   In such cases it is necessary to propagate a route which covers the
   Service Address, and which has a sufficiently short prefix that it
   will not be caught by commonly-deployed import policies.  In many
   cases this will be a 24-bit prefix, but there are other
   well-documented examples of import polices which filter on RIR
   allocation boundaries, and hence some experimentation may be prudent.

   Where multiple Service Addresses are covered by the same covering
   route, there is no longer a tight coupling between the advertisement
   of that route and the individual services associated with the covered
   host routes.  The resulting impact on signaling availability of
   individual services is discussed in Section 4.4.1.

4.4.3  Equal-Cost Paths

   Some routing systems support equal-cost paths to the same
   destination.  Where multiple, equal-cost paths exist and lead to
   different anycast nodes, there is a risk that request packets
   associated with a single transaction might be delivered to more than
   one node.  Services provided over TCP necessarily involve
   transactions with multiple request packets, due to the TCP setup

   Equal cost paths are commonly supported in IGPs.  Multi-node
   selection for a single transaction can be avoided in most cases by
   careful consideration of IGP link metrics, or by applying equal-cost
   multi-path (ECMP) selection algorithms which cause a single node to
   be selected for a single multi-packet transaction.  For a description
   of hash-based ECMP selection, see [13].

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   For services which are distributed across the global Internet using
   BGP, equal-cost paths are normally not a consideration: BGP's exit
   selection algorithm usually selects a single, consistent exit for a
   single destination regardless of whether multiple candidate paths
   exist.  Implementations of BGP exist that support multi-path exit
   selection, however, and corner cases where dual selected exits route
   to different nodes are possible.  Analysis of the likely incidence of
   such corner cases for particular distributions of Anycast Nodes are
   recommended for services which involve multi-packet transactions.

4.4.4  Route Dampening

   Frequent advertisements and withdrawals of individual prefixes in BGP
   are known as flaps.  Rapid flapping can lead to CPU exhaustion on
   routers quite remote from the source of the instability, and for this
   reason rapid route oscillations are frequently "damped", as described
   in [9].

   A dampened path will be suppressed by routers for an interval which
   increases according to the frequency of the observed oscillation; a
   suppressed path will not propagate.  Hence a single router can
   prevent the propagation of a flapping prefix to the rest of an
   autonomous system, affording other routers in the network protection
   from the instability.

   Common implementations of flap dampening penalizes oscillating
   advertisements based on the observed AS_PATH, and not on the NLRI.
   For this reason, network instability which leads to route flapping
   from a single anycast node ought not to cause advertisements from
   other nodes (which have different AS_PATH attributes) to be dampened.

   As dampening works on advertisements with the same AS_PATH attribute,
   care should be taken so that the AS_PATH is as diverse as possible
   for the anycasted nodes.  The Anycasted nodes should have the same
   origin AS for their advertisements, but they should have different
   upstream AS:es for each node.  If the upstream AS is the same at all
   locations, there is a risk that the upstream AS will peer with the
   AS:es at multiple locations and could therefor propagate the same
   AS_PATH, but for different Anycast nodes.  This could render the
   service for multiple Anycast nodes unavailable due to dampening
   caused by only one of them.

   It is possible that other implementations of flap dampening may
   become prevalent in the future, causing individual nodes' instability
   to result in stable nodes becoming unavailable.  Judicious deployment
   of Local Nodes in combination with especially stable Global Nodes
   may help mitigate such problems, should they ever arise.

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4.4.5  Reverse Path Forwarding Checks

   Reverse Path Forwarding (RPF) checks, first described in [8], are
   commonly deployed as part of ingress interface packet filters on
   routers in the global Internet in order to deny packets whose source
   addresses are spoofed (see also [10]).  Deployed implementations of
   RPF make available two modes of operation: a loose mode, and a strict

   Strict-mode RPF checks can cause non-spoofed packets to be denied
   when they originate from multi-homed site, since selected paths might
   legitimately not correspond with the ingress interface of non-spoofed
   packets from the multi-homed site.  A collection of anycast nodes
   deployed across the Internet is largely indistinguishable from a
   distributed, multi-homed site to the routing system, and hence this
   risk also exists for anycast nodes, even if individual nodes are not

   Care should be taken to ensure that strict-mode RPF is not enabled in
   peer networks connecting to anycast nodes.

4.4.6  Propagation Scope

   In the context of Anycast service distribution across the global
   Internet, Global Nodes are those which are capable of providing
   service to clients anywhere in the network; reachability information
   for the service is propagated globally, without restriction, by
   advertising the routes covering the Service Addresses for global
   transit to one or more providers.

   More than one Global Node can exist for a single service (and indeed
   this is often the case, for reasons of redundancy and load-sharing).

   In contrast, it is sometimes desirable to deploy an Anycast Node
   which only provides services to a local catchment of autonomous
   systems, and which is deliberately not available to the entire
   Internet; such nodes are referred to in this document as Local Nodes.
   An example of circumstances in which a Local Node may be appropriate
   are nodes designed to serve a region with rich internal connectivity
   but unreliable, congested or expensive access to the rest of the

   Local Nodes advertise covering routes for Service Addresses in such a
   way that their propagation is restricted.  This might be done using
   well-known community string attributes such as NO_EXPORT [6] or
   NOPEER [11], or by arranging with peers to apply a conventional
   "peering" import policy instead of a "transit" import policy, or some
   suitable combination of measures.

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4.4.7  Other Peoples' Networks

   When Anycast services are deployed across networks operated by
   others, their reachability is dependent on routing polices and
   topology changes (planned and unplanned) which are unpredictable and
   sometimes difficult to identify.  Consequently, routing policies used
   by Anycast Nodes should be conservative, individual nodes' internal
   and external/connecting infrastructure should be scaled to support
   loads far in excess of the average, and the service should be
   monitored proactively (Section 5.1) from many points in order to
   avoid unpleasant surprises.

4.5  Data Synchronisation

   As a client contacting a anycasted service will expect all possible
   servers to serve the same data, the Anycast service needs to assure
   data consistency across all Anycast Nodes.  This includes periodic
   updating of all data, and verification of a successful transfer of

   How data is synchronized depends on the service being Anycasted.  The
   methods used could for example be a zone transfer for an
   authoritative set of DNS-servers, rsync for a FTP archive or no
   synchronization needed for a DNS resolver service.  In the DNS
   examples, synchronization comes with the service and the associated
   protocol.  For other services, this will be an external mechanism to
   the protocol.  In both cases, the synchronization needs to be run
   from a local IP address that is not the service address.  The data
   transfer should be authenticated in order to prevent spoofing of the
   data on the Anycasted nodes and the data should be verified.

   Verification can be done with for example TSIG for DNS, or for
   example a MD5 hash[2] for verification of other data.  The method
   might vary but should verify that all data was transfered, and that
   the data is correct and not manipulated.

   Authentication of the data source can be based either on the protocol
   in use, as is the case with TSIG for DNS, or some other external
   mechanism.  For example a IP tunnel protected by authentication and
   encryption as described in [7].

4.6  Node Autonomy

   For an Anycast deployment whose goals include improved reliability
   through redundancy, it is important to minimize the opportunity for a
   single defect to compromise many (or all) nodes, or for the failure
   of one node to provide a cascading failure bringing down additional
   successive nodes until the service as a whole is defeated.

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   Codependencies are avoided by making each node as autonomous and
   self-sufficient as possible.  The degree to which nodes can survive
   failure elsewhere depends on the nature of the service being
   delivered, but for services which accommodate disconnected operation
   (e.g.  the timed propagation of changes between master and slave
   servers in the DNS) a high degree of autonomy can be achieved.

   The possibility of cascading failure due to load can also be reduced
   by the deployment of both Global and Local Nodes for a single
   service, since the effective fail-over path of traffic is, in
   general, from Local Node to Global Node; traffic that might sink one
   Local Node is unlikely to sink all Local Nodes, except in the most
   degenerate cases.

   The chance of cascading failure due to a software defect in an
   operating system or server can be reduced in many cases by deploying
   nodes running different software implementations.

5.  Service Management

5.1  Monitoring

   Monitoring a service which is distributed is more complex than
   monitoring a non-distributed service, since the observed accuracy and
   availability of the service is, in general, different when viewed
   from clients attached to different parts of the network.  When a
   problem is identified, it is also not always obvious which node
   served the request, and hence which node is malfunctioning.

   It is recommended that distributed services are monitored from probes
   distributed representatively across the routing system, and, where
   possible, the identity of the node answering individual requests is
   recorded along with performance and availability statistics.

   Monitoring the routing system (from a variety of places, in the case
   of routing systems where perspective counts) can also provide useful
   diagnostics for troubleshooting service availability.  This can be
   achieved using dedicated probes, or public route measurement
   facilities on the Internet such as RIPE's Routing
   Information Service [14] and the University of
          Oregon Route
   Views Project [15].

5.2  Self-Healing Nodes

   As is described in  having the Anycast Node avoid black-holing
   traffic in the event of a failure on the software or subsystem
   providing the service should be avoided.  As described, this can be
   done with withdrawing the announcement of the prefix corresponding to

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   the service address, or the covering route.  However, the nodes could
   also try and handle the failure in a number of ways.  This can be
   with as also previously described tunneling to other instances of the
   Anycasted service, and using a IGP over the tunnels, route incoming
   client queries to the other destination.  The Anycasted node could
   also contain separate systems for trying to restart the service in
   question, and if successful again re-announce the service prefix.

6.  Security Considerations

   This document describes mechanisms for deploying services on the
   Internet which can be used to mitigate vulnerability to attack.

   The distribution of a service across several (or many) autonomous
   nodes imposes an increased monitoring load on the operator of the
   service, and which also imposes an additional systems administration
   load on the service operator which might reduce the effectiveness of
   host and router security.  It is recommended that these factors be
   taken into account when assessing the risks and benefits of
   distributing services using anycast.

7.  Protocol Considerations

   This document does not impose any protocol considerations.

8.  IANA Considerations

   This document requests no action from IANA.

9  References

   [1]   Oran, D., "OSI IS-IS Intra-domain Routing Protocol", RFC 1142,
         February 1990.

   [2]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April

   [3]   Moy, J., "OSPF Version 2", RFC 1247, July 1991.

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

   [5]   Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
         RFC 1771, March 1995.

   [6]   Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
         Attribute", RFC 1997, August 1996.

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   [7]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

   [8]   Ferguson, P. and D. Senie, "Network Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Source
         Address Spoofing", RFC 2267, January 1998.

   [9]   Villamizar, C., Chandra, R. and R. Govindan, "BGP Route Flap
         Damping", RFC 2439, November 1998.

   [10]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Source
         Address Spoofing", BCP 38, RFC 2827, May 2000.

   [11]  Huston, G., "NOPEER Community for Border Gateway Protocol (BGP)
         Route Scope Control", RFC 3765, April 2004.

   [12]  Abley, J., "Hierarchical Anycast for Global Service
         Distribution", March 2003,

   [13]  Abley, J., "A Software Approach to Distributing Requests for
         DNS Service using GNU Zebra, ISC BIND 9 and FreeBSD", March
         2004, <>.

   [14]  <>

   [15]  <>

Authors' Addresses

   Kurt Erik Lindqvist
   Netnod Internet Exchange
   Bellmansgatan 30
   118 47 Stockholm


Lindqvist & Abley        Expires April 22, 2005                [Page 14]

Internet-Draft                  Anycast                     October 2004

   Joe Abley
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063

   Phone: +1 650 423 1317

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Internet-Draft                  Anycast                     October 2004

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