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RFC 4786 - Operation of Anycast Services


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Network Working Group                                           J. Abley
Request for Comments: 4786                                Afilias Canada
BCP: 126                                                    K. Lindqvist
Category: Best Current Practice                 Netnod Internet Exchange
                                                           December 2006

                     Operation of Anycast Services

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

   As the Internet has grown, and as systems and networked services
   within enterprises have become more pervasive, many services with
   high availability requirements have emerged.  These requirements have
   increased the demands on the reliability of the infrastructure on
   which those services rely.

   Various techniques have been employed to increase the availability of
   services deployed on the Internet.  This document presents commentary
   and recommendations for distribution of services using anycast.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................4
   3. Anycast Service Distribution ....................................5
      3.1. General Description ........................................5
      3.2. Goals ......................................................5
   4. Design ..........................................................6
      4.1. Protocol Suitability .......................................6
      4.2. Node Placement .............................................7
      4.3. Routing Systems ............................................8
           4.3.1. Anycast within an IGP ...............................8
           4.3.2. Anycast within the Global Internet ..................9
      4.4. Routing Considerations .....................................9
           4.4.1. Signalling Service Availability .....................9
           4.4.2. Covering Prefix ....................................10
           4.4.3. Equal-Cost Paths ...................................10
           4.4.4. Route Dampening ....................................12
           4.4.5. Reverse Path Forwarding Checks .....................13
           4.4.6. Propagation Scope ..................................13
           4.4.7. Other Peoples' Networks ............................14
           4.4.8. Aggregation Risks ..................................14
      4.5. Addressing Considerations .................................15
      4.6. Data Synchronisation ......................................15
      4.7. Node Autonomy .............................................16
      4.8. Multi-Service Nodes .......................................17
           4.8.1. Multiple Covering Prefixes .........................17
           4.8.2. Pessimistic Withdrawal .............................17
           4.8.3. Intra-Node Interior Connectivity ...................18
      4.9. Node Identification by Clients ............................18
   5. Service Management .............................................19
      5.1. Monitoring ................................................19
   6. Security Considerations ........................................19
      6.1. Denial-of-Service Attack Mitigation .......................19
      6.2. Service Compromise ........................................20
      6.3. Service Hijacking .........................................20
   7. Acknowledgements ...............................................21
   8. References .....................................................21
      8.1. Normative References ......................................21
      8.2. Informative References ....................................21

1.  Introduction

   This document is addressed to network operators who are considering
   whether to deploy or operate a distributed service using anycast.  It
   describes the best current practice for doing so, but does not
   recommend whether any particular service should or should not be
   deployed using anycast.

   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 [RFC1546], [ISC-TN-2003-1], and
   [ISC-TN-2004-1].

   The techniques and considerations described in this document apply to
   services reachable over both IPv4 and IPv6.

   Anycast has in recent years become increasingly popular for adding
   redundancy to DNS servers to complement the redundancy that the DNS
   architecture itself already provides.  Several root DNS 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 limitations on the nature of
   the service being distributed, including transaction longevity,
   transaction state held on servers, and data synchronisation
   capabilities.

   Although anycast is conceptually simple, its implementation
   introduces some pitfalls for operation of services.  For example,
   monitoring the availability of the service becomes more difficult;
   the observed availability changes according to the location of the
   client within the network, and the population of clients using
   individual anycast nodes is neither static, nor reliably
   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 the Border Gateway Protocol (BGP)
   [RFC4271].  Many of the issues for monitoring and data
   synchronisation are common to both, but deployment issues differ
   substantially.

2.  Terminology

   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.

   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 a Service Address
   available to a routing system at Anycast Nodes in two or more
   discrete locations.  The service provided by each node is generally
   consistent regardless of the particular node chosen by the routing
   system to handle a particular request (although some services may
   benefit from deliberate differences in the behaviours of individual
   nodes, in order to facilitate locality-specific behaviour; see
   Section 4.6).

   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 decides which node is 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 that 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 achieved, 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)
   [RFC4271] connecting the global Internet, depending on the nature of
   the 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
       localising damage to single Anycast Nodes;

   3.  Constraint of distributed denial-of-service attacks or flash
       crowds to local regions around Anycast Nodes.  Anycast
       distribution of a service provides the opportunity for traffic to
       be handled closer to its source, perhaps using high-performance
       peering links rather than oversubscribed, paid transit circuits;

   4.  To provide additional information to help identify the location
       of traffic sources in the case of attack (or query) traffic which
       incorporates spoofed source addresses.  This information is
       derived from the property of anycast service distribution that
       the selection of the Anycast Node used to service a particular
       query may be related to the topological source of the request.

   5.  Improvement of query response time, by reducing the network
       distance between client and server with the provision of a local
       Anycast Node.  The extent to which query response time is
       improved depends on the way that nodes are selected for the
       clients by the routing system.  Topological nearness within the
       routing system does not, in general, correlate to round-trip
       performance across a network; in some cases, response times may
       see no reduction, and may increase.

   6.  To reduce 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 nameserver authoritative for the parent zone.

4.  Design

4.1.  Protocol Suitability

   When a service is anycast between two or more nodes, the routing
   system 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 and 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 substantially
   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
   (e.g., DNS transactions over UDP transport).  Other services involve

   far longer-lived transactions (e.g., bulk file downloads and audio-
   visual media streaming).

   Services may be anycast within very predictable routing systems,
   which can remain stable for long periods of time (e.g., anycast
   within a well-managed and topologically-simple IGP, where node
   selection changes only occur as a response to node failures).  Other
   deployments have far less predictable characteristics (see
   Section 4.4.7).

   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.  In
   some cases, for new protocols, it may be practical to split large
   transactions into an initialisation phase that is handled by anycast
   servers, and a sustained phase that is provided by non-anycast
   servers, perhaps chosen during the initialisation phase.

   This document deliberately avoids prescribing rules as to which
   protocols or services are suitable for distribution by anycast; to
   attempt to do so would be presumptuous.

   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 unicast.

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 DNS recursive resolver service might be distributed within an
      ISP's network, one Anycast Node per site.
   o  A root DNS server service might be distributed throughout the
      Internet; Anycast Nodes could be 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 in order to avoid congesting wide-
       area links.

   In each case, the decisions as to where and how services are
   provisioned can be made by network engineers without requiring such
   operational complexities as regional variances in the configuration
   of client computers, or deliberate DNS incoherence (causing DNS
   queries to yield different answers depending on where the queries
   originate).

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

   An IGP will generally have no inherent restriction on the length of
   prefix that can be introduced to it.  In this case, there is no need
   to construct a covering prefix for particular Service Addresses; host
   routes corresponding to the Service Address can instead be introduced
   to the routing system.  See Section 4.4.2 for more discussion of the
   requirement for a covering prefix.

   IGPs often feature little or no aggregation of routes, partly due to
   algorithmic complexities in supporting aggregation.  There is little
   motivation for aggregation in many networks' IGPs in many cases,
   since the amount of routing information carried in the IGP is small
   enough that scaling concerns in routers do not arise.  For discussion
   of aggregation risks in other routing systems, see Section 4.4.8.

   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 [ISC-TN-2004-1].

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
   essential 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 server.  These
   implementations provide the service being distributed and are
   configured to advertise and withdraw the route advertisement in
   conjunction with the availability (and health) of the software on the
   host that processes service requests.  An example of such an
   arrangement for a DNS service is included in [ISC-TN-2004-1].

   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 in the case where the service is down at one Anycast Node is
   to route requests to a different Anycast Node where the service is
   working normally.  This approach is discussed in Section 4.8.

   Rapid advertisement/withdrawal oscillations can cause operational
   problems, and nodes should be configured such that rapid oscillations
   are avoided (e.g., by implementing a minimum delay following a
   withdrawal before the service can be re-advertised).  See
   Section 4.4.4 for a discussion of route oscillations in BGP.

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 the route will propagate 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 that covers the
   Service Address, and that has a sufficiently short prefix that it
   will not be discarded by commonly-deployed import policies.  For IPv4
   Service Addresses, this is often a 24-bit prefix, but there are other
   well-documented examples of IPv4 import polices that filter on
   Regional Internet Registry (RIR) allocation boundaries, and hence
   some experimentation may be prudent.  Corresponding import policies
   for IPv6 prefixes also exist.  See Section 4.5 for more discussion of
   IPv6 Service Addresses and corresponding anycast routes.

   The propagation of a single route per service has some associated
   scaling issues, which are discussed in Section 4.4.8.

   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 signalling availability of
   individual services is discussed in Section 4.4.1 and Section 4.8.

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 different request
   packets associated with a single transaction might be delivered to
   more than one node.  Services provided over TCP [RFC0793] necessarily
   involve transactions with multiple request packets, due to the TCP
   setup handshake.

   For services that 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.

   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 an example of
   the use of hash-based ECMP selection in anycast service distribution,
   see [ISC-TN-2004-1].

   Other ECMP selection algorithms are commonly available, including
   those in which packets from the same flow are not guaranteed to be
   routed towards the same destination.  ECMP algorithms that select a
   route on a per-packet basis rather than per-flow are commonly
   referred to as performing "Per Packet Load Balancing" (PPLB).

   With respect to anycast service distribution, some uses of PPLB may
   cause different packets from a single multi-packet transaction sent
   by a client to be delivered to different Anycast Nodes, effectively
   making the anycast service unavailable.  Whether this affects
   specific anycast services will depend on how and where Anycast Nodes
   are deployed within the routing system, and on where the PPLB is
   being performed:

   1.  PPLB across multiple, parallel links between the same pair of
       routers should cause no node selection problems;

   2.  PPLB across diverse paths within a single autonomous system (AS),
       where the paths converge to a single exit as they leave the AS,
       should cause no node selection problems;

   3.  PPLB across links to different neighbour ASes, where the
       neighbour ASes have selected different nodes for a particular
       anycast destination will, in general, cause request packets to be
       distributed across multiple Anycast Nodes.  This will have the
       effect that the anycast service is unavailable to clients
       downstream of the router performing PPLB.

   The uses of PPLB that have the potential to interact badly with
   anycast service distribution can also cause persistent packet
   reordering.  A network path that persistently reorders segments will
   degrade the performance of traffic carried by TCP [Allman2000].  TCP,
   according to several documented measurements, accounts for the bulk
   of traffic carried on the Internet ([McCreary2000], [Fomenkov2004]).
   Consequently, in many cases, it is reasonable to consider networks
   making such use of PPLB to be pathological.

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 "dampened", as
   described in [RFC2439].

   A dampened path will be suppressed by routers for an interval that
   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.

   Some implementations of flap dampening penalise oscillating
   advertisements based on the observed AS_PATH, and not on Network
   Layer Reachability Information (NLRI; see [RFC4271]).  For this
   reason, network instability that leads to route flapping from a
   single Anycast Node, will not generally cause advertisements from
   other nodes (which have different AS_PATH attributes) to be dampened
   by these implementations.

   To limit the opportunity of such implementations to penalise
   advertisements originating from different Anycast Nodes in response
   to oscillations from just a single node, care should be taken to
   arrange that the AS_PATH attributes on routes from different nodes
   are as diverse as possible.  For example, Anycast Nodes should use
   the same origin AS for their advertisements, but might have different
   upstream ASes.

   Where different implementations of flap dampening are prevalent,
   individual nodes' instability may result in stable nodes becoming
   unavailable.  In mitigation, the following measures may be useful:

   1.  Judicious deployment of Local Nodes in combination with
       especially stable Global Nodes (with high inter-AS path splay,
       redundant hardware, power, etc.) may help limit oscillation
       problems to the Local Nodes' limited regions of influence;

   2.  Aggressive flap-dampening of the service prefix close to the
       origin (e.g., within an Anycast Node, or in adjacent ASes of each
       Anycast Node) may also help reduce the opportunity of remote ASes
       to see oscillations at all.

4.4.5.  Reverse Path Forwarding Checks

   Reverse Path Forwarding (RPF) checks, first described in [RFC2267],
   are commonly deployed as part of ingress interface packet filters on
   routers in the Internet in order to deny packets whose source
   addresses are spoofed (see also RFC 2827 [RFC2827]).  Deployed
   implementations of RPF make several modes of operation available
   (e.g., "loose" and "strict").

   Some modes of RPF can cause non-spoofed packets to be denied when
   they originate from multi-homed sites, since selected paths might
   legitimately not correspond with the ingress interface of non-spoofed
   packets from the multi-homed site.  This issue is discussed in
   [RFC3704].

   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 multi-homed.  Care should be taken to ensure
   that each Anycast Node is treated as a multi-homed network, and that
   the corresponding recommendations in [RFC3704] with respect to RPF
   checks are heeded.

4.4.6.  Propagation Scope

   In the context of anycast service distribution across the global
   Internet, Global Nodes are those that 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 that
   only provides services to a local catchment of autonomous systems,
   and that 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
   Internet.

   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 [RFC1997] or
   NOPEER [RFC3765], or by arranging with peers to apply a conventional

   "peering" import policy instead of a "transit" import policy, or some
   suitable combination of measures.

   Advertising reachability to Service Addresses from Local Nodes should
   ideally be done using a routing policy that requires presence of
   explicit attributes for propagation, rather than relying on implicit
   (default) policy.  Inadvertent propagation of a route beyond its
   intended horizon can result in capacity problems for Local Nodes,
   which might degrade service performance network-wide.

4.4.7.  Other Peoples' Networks

   When anycast services are deployed across networks operated by
   others, their reachability is dependent on routing policies and
   topology changes (planned and unplanned), which are unpredictable and
   sometimes difficult to identify.  Since the routing system may
   include networks operated by multiple, unrelated organisations, the
   possibility of unforeseen interactions resulting from the
   combinations of unrelated changes also exists.

   The stability and predictability of such a routing system should be
   taken into consideration when assessing the suitability of anycast as
   a distribution strategy for particular services and protocols (see
   also Section 4.1).

   By way of mitigation, routing policies used by Anycast Nodes across
   such routing systems 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 from many points in order to avoid unpleasant
   surprises (see Section 5.1).

4.4.8.  Aggregation Risks

   The propagation of a single route for each anycast service does not
   scale well for routing systems in which the load of routing
   information that must be carried is a concern, and where there are
   potentially many services to distribute.  For example, an autonomous
   system that provides services to the Internet with N Service
   Addresses covered by a single exported route would need to advertise
   (N+1) routes, if each of those services were to be distributed using
   anycast.

   The common practice of applying minimum prefix-length filters in
   import policies on the Internet (see Section 4.4.2) means that for a
   route covering a Service Address to be usefully propagated the prefix
   length must be substantially less than that required to advertise
   just the host route.  Widespread advertisement of short prefixes for

   individual services, hence, also has a negative impact on address
   conservation.

   Both of these issues can be mitigated to some extent by the use of a
   single covering prefix to accommodate multiple Service Addresses, as
   described in Section 4.8.  This implies a de-coupling of the route
   advertisement from individual service availability (see
   Section 4.4.1), however, with attendant risks to the stability of the
   service as a whole (see Section 4.7).

   In general, the scaling problems described here prevent anycast from
   being a useful, general approach for service distribution on the
   global Internet.  It remains, however, a useful technique for
   distributing a limited number of Internet-critical services, as well
   as in smaller networks where the aggregation concerns discussed here
   do not apply.

4.5.  Addressing Considerations

   Service Addresses should be unique within the routing system that
   connects all Anycast Nodes to all possible clients of the service.
   Service Addresses must also be chosen so that corresponding routes
   will be allowed to propagate within that routing system.

   For an IPv4-numbered service deployed across the Internet, for
   example, an address might be chosen from a block where the minimum
   RIR allocation size is 24 bits, and reachability to that address
   might be provided by originating the covering 24-bit prefix.

   For an IPv4-numbered service deployed within a private network, a
   locally-unused [RFC1918] address might be chosen, and reachability to
   that address might be signalled using a (32-bit) host route.

   For IPv6-numbered services, Anycast Addresses are not scoped
   differently from unicast addresses.  As such, the guidelines for
   address suitability presented for IPv4 follow for IPv6.  Note that
   historical prohibitions on anycast distribution of services over IPv6
   have been removed from the IPv6 addressing specification in
   [RFC4291].

4.6.  Data Synchronisation

   Although some services have been deployed in localised form (such
   that clients from particular regions are presented with regionally-
   relevant content), many services have the property that responses to
   client requests should be consistent, regardless of where the request
   originates.  For a service distributed using anycast, that implies
   that different Anycast Nodes must operate in a consistent manner and,

   where that consistent behaviour is based on a data set, the data
   concerned be synchronised between nodes.

   The mechanism by which data is synchronised depends on the nature of
   the service; examples are zone transfers for authoritative DNS
   servers and rsync for FTP archives.  In general, the synchronisation
   of data between Anycast Nodes will involve transactions between non-
   anycast addresses.

   Data synchronisation across public networks should be carried out
   with appropriate authentication and encryption.

4.7.  Node Autonomy

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

   Co-dependencies 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 implementations of operating system, server
   software, routing protocol software, etc., such that a defect that
   appears in a single component does not affect the whole system.

   It should be noted that these approaches to increase node autonomy
   are, to varying degrees, contrary to the practical goals of making a
   deployed service straightforward to operate.  A service that is
   overly complex is more likely to suffer from operator error than a
   service that is more straightforward to run.  Careful consideration
   should be given to all of these aspects so that an appropriate
   balance may be found.

4.8.  Multi-Service Nodes

   For a service distributed across a routing system where covering
   prefixes are required to announce reachability to a single Service
   Address (see Section 4.4.2), special consideration is required in the
   case where multiple services need to be distributed across a single
   set of nodes.  This results from the requirement to signal
   availability of individual services to the routing system so that
   requests for service are not received by nodes that are not able to
   process them (see Section 4.4.1).

   Several approaches are described in the following sections.

4.8.1.  Multiple Covering Prefixes

   Each Service Address is chosen such that only one Service Address is
   covered by each advertised prefix.  Advertisement and withdrawal of a
   single covering prefix can be tightly coupled to the availability of
   the single associated service.

   This is the most straightforward approach.  However, since it makes
   very poor utilisation of globally-unique addresses, it is only
   suitable for use for a small number of critical, infrastructural
   services such as root DNS servers.  General Internet-wide deployment
   of services using this approach will not scale.

4.8.2.  Pessimistic Withdrawal

   Multiple Service Addresses are chosen such that they are covered by a
   single prefix.  Advertisement and withdrawal of the single covering
   prefix is coupled to the availability of all associated services; if
   any individual service becomes unavailable, the covering prefix is
   withdrawn.

   The coupling between service availability and advertisement of the
   covering prefix is complicated by the requirement that all Service
   Addresses must be available -- the announcement needs to be triggered
   by the presence of all component routes, and not just a single
   covered route.

   The fact that a single malfunctioning service causes all deployed
   services in a node to be taken off-line may make this approach
   unsuitable for many applications.

4.8.3.  Intra-Node Interior Connectivity

   Multiple Service Addresses are chosen such that they are covered by a
   single prefix.  Advertisement and withdrawal of the single covering
   prefix is coupled to the availability of any one service.  Nodes have
   interior connectivity, e.g., using tunnels.  Host routes for Service
   Addresses are distributed using an IGP that extends to include
   routers at all nodes.

   In the event that a service is unavailable at one node, but available
   at other nodes, a request may be routed over the interior network
   from the receiving node towards some other node for processing.

   In the event that some local services in a node are down and the node
   is disconnected from other nodes, continued advertisement of the
   covering prefix might cause requests to become black-holed.

   This approach allows reasonable address utilisation of the netblock
   covered by the announced prefix, at the expense of reduced autonomy
   of individual nodes; the IGP in which all nodes participate can be
   viewed as a single point of failure.

4.9.  Node Identification by Clients

   From time to time, all clients of deployed services experience
   problems, and those problems require diagnosis.  A service
   distributed using anycast imposes an additional variable on the
   diagnostic process over a simple, unicast service -- the particular
   Anycast Node that is handling a client's request.

   In some cases, common network-level diagnostic tools such as
   traceroute may be sufficient to identify the node being used by a
   client.  However, the use of such tools may be beyond the abilities
   of users at the client side of a transaction, and, in any case,
   network conditions at the time of the problem may change by the time
   such tools are exercised.

   Troubleshooting problems with anycast services is greatly facilitated
   if mechanisms to determine the identity of a node are designed into
   the protocol.  Examples of such mechanisms include the NSID option in
   DNS [NSID] and the common inclusion of hostname information in SMTP
   servers' initial greeting at session initiation [RFC2821].

   Provision of such in-band mechanisms for node identification is
   strongly recommended for services to be distributed using anycast.

5.  Service Management

5.1.  Monitoring

   Monitoring a service that 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.  The
   RIPE NCC DNSMON service [DNSMON] is an example of such monitoring for
   the DNS.

   Monitoring the routing system (from a variety of places, in the case
   of routing systems where perspective is relevant) 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 the RIPE NCC Routing Information
   Service [RIS] and the University of Oregon Route Views Project
   [ROUTEVIEWS].

   Monitoring the health of the component devices in an anycast
   deployment of a service (hosts, routers, etc.) is straightforward,
   and can be achieved using the same tools and techniques commonly used
   to manage other network-connected infrastructure, without the
   additional complexity involved in monitoring anycast Service
   Addresses.

6.  Security Considerations

6.1.  Denial-of-Service Attack Mitigation

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

   1.  An Anycast Node can act as a sink for attack traffic originated
       within its sphere of influence, preventing nodes elsewhere from
       having to deal with that traffic;

   2.  The task of dealing with attack traffic whose sources are widely
       distributed is itself distributed across all the nodes that
       contribute to the service.  Since the problem of sorting between
       legitimate and attack traffic is distributed, this may lead to
       better scaling properties than a service that is not distributed.

6.2.  Service Compromise

   The distribution of a service across several (or many) autonomous
   nodes imposes increased monitoring as well as an increased systems
   administration burden on the operator of the service, which might
   reduce the effectiveness of host and router security.

   The potential benefit of being able to take compromised servers off-
   line without compromising the service can only be realised if there
   are working procedures to do so quickly and reliably.

6.3.  Service Hijacking

   It is possible that an unauthorised party might advertise routes
   corresponding to anycast Service Addresses across a network, and by
   doing so, capture legitimate request traffic or process requests in a
   manner that compromises the service (or both).  A rogue Anycast Node
   might be difficult to detect by clients or by the operator of the
   service.

   The risk of service hijacking by manipulation of the routing system
   exists regardless of whether a service is distributed using anycast.
   However, the fact that legitimate Anycast Nodes are observable in the
   routing system may make it more difficult to detect rogue nodes.

   Many protocols that incorporate authentication or integrity
   protection provide those features in a robust fashion, e.g., using
   periodic re-authentication within a single session, or integrity
   protection at either the channel (e.g., [RFC2845], [RFC3207]) or
   message (e.g., [RFC4033], [RFC2311]) levels.  Protocols that are less
   robust may be more vulnerable to session hijacking.  Given the
   greater opportunity for undetected session hijack with anycast
   services, the use of robust protocols is recommended for anycast
   services that require authentication or integrity protection.

7.  Acknowledgements

   The authors gratefully acknowledge the contributions from various
   participants of the grow working group, and in particular Geoff
   Huston, Pekka Savola, Danny McPherson, Ben Black, and Alan Barrett.

   This work was supported by the US National Science Foundation
   (research grant SCI-0427144) and DNS-OARC.

8.  References

8.1.  Normative References

   [RFC0793]        Postel, J., "Transmission Control Protocol", STD 7,
                    RFC 793, September 1981.

   [RFC1918]        Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
                    G., and E. Lear, "Address Allocation for Private
                    Internets", BCP 5, RFC 1918, February 1996.

   [RFC1997]        Chandrasekeran, R., Traina, P., and T. Li, "BGP
                    Communities Attribute", RFC 1997, August 1996.

   [RFC2439]        Villamizar, C., Chandra, R., and R. Govindan, "BGP
                    Route Flap Damping", RFC 2439, November 1998.

   [RFC2827]        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.

   [RFC3704]        Baker, F. and P. Savola, "Ingress Filtering for
                    Multihomed Networks", BCP 84, RFC 3704, March 2004.

   [RFC4271]        Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
                    Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4291]        Hinden, R. and S. Deering, "IP Version 6 Addressing
                    Architecture", RFC 4291, February 2006.

8.2.  Informative References

   [Allman2000]     Allman, M. and E. Blanton, "On Making TCP More
                    Robust to Packet Reordering", January 2000, <http://
                    www.icir.org/mallman/papers/tcp-reorder-ccr.ps>.

   [DNSMON]         "RIPE NCC DNS Monitoring Services",
                    <http://dnsmon.ripe.net/>.

   [Fomenkov2004]   Fomenkov, M., Keys, K., Moore, D., and k. claffy,
                    "Longitudinal Study of Internet Traffic from 1999-
                    2003", January 2004, <http://www.caida.org/
                    outreach/papers/2003/nlanr/nlanr_overview.pdf>.

   [ISC-TN-2003-1]  Abley, J., "Hierarchical Anycast for Global Service
                    Distribution", March 2003,
                    <http://www.isc.org/pubs/tn/isc-tn-2003-1.html>.

   [ISC-TN-2004-1]  Abley, J., "A Software Approach to Distributing
                    Requests for DNS Service using GNU Zebra, ISC BIND 9
                    and FreeBSD", March 2004,
                    <http://www.isc.org/pubs/tn/isc-tn-2004-1.html>.

   [McCreary2000]   McCreary, S. and k. claffy, "Trends in Wide Area IP
                    Traffic Patterns: A View from Ames Internet
                    Exchange", September 2000, <http://www.caida.org/
                    outreach/papers/2000/AIX0005/AIX0005.pdf>.

   [NSID]           Austein, R., "DNS Name Server Identifier Option
                    (NSID)", Work in Progress, June 2006.

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

   [RFC2267]        Ferguson, P. and D. Senie, "Network Ingress
                    Filtering: Defeating Denial of Service Attacks which
                    employ IP Source Address Spoofing", RFC 2267,
                    January 1998.

   [RFC2311]        Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
                    and L. Repka, "S/MIME Version 2 Message
                    Specification", RFC 2311, March 1998.

   [RFC2821]        Klensin, J., "Simple Mail Transfer Protocol",
                    RFC 2821, April 2001.

   [RFC2845]        Vixie, P., Gudmundsson, O., Eastlake, D., and B.
                    Wellington, "Secret Key Transaction Authentication
                    for DNS (TSIG)", RFC 2845, May 2000.

   [RFC3207]        Hoffman, P., "SMTP Service Extension for Secure SMTP
                    over Transport Layer Security", RFC 3207,
                    February 2002.

   [RFC3765]        Huston, G., "NOPEER Community for Border Gateway
                    Protocol (BGP) Route Scope Control", RFC 3765,
                    April 2004.

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

   [RIS]            "RIPE NCC Routing Information Service (RIS)",
                    <http://ris.ripe.net>.

   [ROUTEVIEWS]     "University of Oregon Route Views Project",
                    <http://www.routeviews.org/>.

Authors' Addresses

   Joe Abley
   Afilias Canada, Corp.
   204 - 4141 Yonge Street
   Toronto, ON  M2P 2A8
   Canada

   Phone: +1 416 673 4176
   EMail: jabley@ca.afilias.info
   URI:   http://afilias.info/

   Kurt Erik Lindqvist
   Netnod Internet Exchange
   Bellmansgatan 30
   118 47 Stockholm
   Sweden

   EMail: kurtis@kurtis.pp.se
   URI:   http://www.netnod.se/

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