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IETF RFC 4786
Operation of Anycast Services
Last modified on Thursday, December 21st, 2006
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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 © 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.
Abley & Lindqvist Best Current Practice PAGE 1 
RFC 4786 Anycast BCP December 2006
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
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RFC 4786 Anycast BCP December 2006
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 [RFC 1546], [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)
[RFC 4271]. Many of the issues for monitoring and data
synchronisation are common to both, but deployment issues differ
substantially.
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RFC 4786 Anycast BCP December 2006
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.
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RFC 4786 Anycast BCP December 2006
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)
[RFC 4271] 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;
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RFC 4786 Anycast BCP December 2006
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
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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.
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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.
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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.
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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 [RFC 793] 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
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RFC 4786 Anycast BCP December 2006
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.
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RFC 4786 Anycast BCP December 2006
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 [RFC 2439].
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 [RFC 4271]). 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.
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4.4.5. Reverse Path Forwarding Checks
Reverse Path Forwarding (RPF) checks, first described in [RFC 2267],
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 [RFC 2827]). 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
[RFC 3704].
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 [RFC 3704] 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 [RFC 1997] or
NOPEER [RFC 3765], or by arranging with peers to apply a conventional
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RFC 4786 Anycast BCP December 2006
"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
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RFC 4786 Anycast BCP December 2006
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 [RFC 1918] 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
[RFC 4291].
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,
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RFC 4786 Anycast BCP December 2006
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.
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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.
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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 [RFC 2821].
Provision of such in-band mechanisms for node identification is
strongly recommended for services to be distributed using anycast.
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RFC 4786 Anycast BCP December 2006
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;
Abley & Lindqvist Best Current Practice PAGE 19
RFC 4786 Anycast BCP December 2006
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., [RFC 2845], [RFC 3207]) or
message (e.g., [RFC 4033], [RFC 2311]) 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.
Abley & Lindqvist Best Current Practice PAGE 20
RFC 4786 Anycast BCP December 2006
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
[RFC 793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC 1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC 1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Communities Attribute", RFC 1997, August 1996.
[RFC 2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP
Route Flap Damping", RFC 2439, November 1998.
[RFC 2827] 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.
[RFC 3704] Baker, F. and P. Savola, "Ingress Filtering for
Multihomed Networks", BCP 84, RFC 3704, March 2004.
[RFC 4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC 4291] 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/>.
Abley & Lindqvist Best Current Practice PAGE 21
RFC 4786 Anycast BCP December 2006
[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.
[RFC 1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.
[RFC 2267] Ferguson, P. and D. Senie, "Network Ingress
Filtering: Defeating Denial of Service Attacks which
employ IP Source Address Spoofing", RFC 2267,
January 1998.
[RFC 2311] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
and L. Repka, "S/MIME Version 2 Message
Specification", RFC 2311, March 1998.
[RFC 2821] Klensin, J., "Simple Mail Transfer Protocol",
RFC 2821, April 2001.
[RFC 2845] Vixie, P., Gudmundsson, O., Eastlake, D., and B.
Wellington, "Secret Key Transaction Authentication
for DNS (TSIG)", RFC 2845, May 2000.
[RFC 3207] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207,
February 2002.
[RFC 3765] Huston, G., "NOPEER Community for Border Gateway
Protocol (BGP) Route Scope Control", RFC 3765,
April 2004.
Abley & Lindqvist Best Current Practice PAGE 22
RFC 4786 Anycast BCP December 2006
[RFC 4033] 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/
Abley & Lindqvist Best Current Practice PAGE 23
RFC 4786 Anycast BCP December 2006
Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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Abley & Lindqvist Best Current Practice PAGE 24
Operation of Anycast Services
RFC TOTAL SIZE: 56818 bytes
PUBLICATION DATE: Thursday, December 21st, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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