draft-ietf-grow-anycast-00.txt   draft-ietf-grow-anycast-01.txt 
Network Working Group J. Abley Network Working Group J. Abley
Internet-Draft ISC Internet-Draft ISC
Expires: August 19, 2005 K. Lindqvist Expires: January 19, 2006 K. Lindqvist
Netnod Internet Exchange Netnod Internet Exchange
February 15, 2005 July 18, 2005
Operation of Anycast Services Operation of Anycast Services
draft-ietf-grow-anycast-00.txt draft-ietf-grow-anycast-01
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2005).
Abstract Abstract
As the Internet has grown, and as systems and networked services As the Internet has grown, and as systems and networked services
within enterprises have been more pervasive, many services with high within enterprises have become more pervasive, many services with
availability requirements have emerged. These requirements have high availability requirements have emerged. These requirements have
increased the demands on the reliability of the infrastructure on increased the demands on the reliability of the infrastructure on
which those services rely. which those services rely.
Various techniques have been employed to increase the availability of Various techniques have been employed to increase the availability of
services deployed on the Internet. This document presents a series services deployed on the Internet. This document presents commentary
of recommendations for distribution of services using anycast. and recommendations for distribution of services using anycast.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Anycast Service Distribution . . . . . . . . . . . . . . . . . 5 3. Anycast Service Distribution . . . . . . . . . . . . . . . . . 5
3.1 General Description . . . . . . . . . . . . . . . . . . . 5 3.1 General Description . . . . . . . . . . . . . . . . . . . 5
3.2 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1 Protocol Suitability . . . . . . . . . . . . . . . . . . . 7
4.1 Protocol Suitability . . . . . . . . . . . . . . . . . . . 6
4.2 Node Placement . . . . . . . . . . . . . . . . . . . . . . 7 4.2 Node Placement . . . . . . . . . . . . . . . . . . . . . . 7
4.3 Routing Systems . . . . . . . . . . . . . . . . . . . . . 8 4.3 Routing Systems . . . . . . . . . . . . . . . . . . . . . 8
4.3.1 Anycast within an IGP . . . . . . . . . . . . . . . . 8 4.3.1 Anycast within an IGP . . . . . . . . . . . . . . . . 8
4.3.2 Anycast within the Global Internet . . . . . . . . . . 9 4.3.2 Anycast within the Global Internet . . . . . . . . . . 9
4.4 Routing Considerations . . . . . . . . . . . . . . . . . . 9 4.4 Routing Considerations . . . . . . . . . . . . . . . . . . 9
4.4.1 Signalling Service Availability . . . . . . . . . . . 9 4.4.1 Signalling Service Availability . . . . . . . . . . . 9
4.4.2 Covering Prefix . . . . . . . . . . . . . . . . . . . 9 4.4.2 Covering Prefix . . . . . . . . . . . . . . . . . . . 10
4.4.3 Equal-Cost Paths . . . . . . . . . . . . . . . . . . . 10 4.4.3 Equal-Cost Paths . . . . . . . . . . . . . . . . . . . 10
4.4.4 Route Dampening . . . . . . . . . . . . . . . . . . . 11 4.4.4 Route Dampening . . . . . . . . . . . . . . . . . . . 11
4.4.5 Reverse Path Forwarding Checks . . . . . . . . . . . . 11 4.4.5 Reverse Path Forwarding Checks . . . . . . . . . . . . 12
4.4.6 Propagation Scope . . . . . . . . . . . . . . . . . . 12 4.4.6 Propagation Scope . . . . . . . . . . . . . . . . . . 12
4.4.7 Other Peoples' Networks . . . . . . . . . . . . . . . 13 4.4.7 Other Peoples' Networks . . . . . . . . . . . . . . . 13
4.4.8 Aggregation Risks . . . . . . . . . . . . . . . . . . 13 4.4.8 Aggregation Risks . . . . . . . . . . . . . . . . . . 14
4.5 Addressing Considerations . . . . . . . . . . . . . . . . 14 4.5 Addressing Considerations . . . . . . . . . . . . . . . . 14
4.6 Data Synchronisation . . . . . . . . . . . . . . . . . . . 14 4.6 Data Synchronisation . . . . . . . . . . . . . . . . . . . 15
4.7 Node Autonomy . . . . . . . . . . . . . . . . . . . . . . 15 4.7 Node Autonomy . . . . . . . . . . . . . . . . . . . . . . 15
4.8 Multi-Service Nodes . . . . . . . . . . . . . . . . . . . 15 4.8 Multi-Service Nodes . . . . . . . . . . . . . . . . . . . 16
4.8.1 Multiple Covering Prefixes . . . . . . . . . . . . . . 16 4.8.1 Multiple Covering Prefixes . . . . . . . . . . . . . . 16
4.8.2 Pessimistic Withdrawal . . . . . . . . . . . . . . . . 16 4.8.2 Pessimistic Withdrawal . . . . . . . . . . . . . . . . 16
4.8.3 Intra-Node Interior Connectivity . . . . . . . . . . . 16 4.8.3 Intra-Node Interior Connectivity . . . . . . . . . . . 17
5. Service Management . . . . . . . . . . . . . . . . . . . . . . 18
5. Service Management . . . . . . . . . . . . . . . . . . . . . . 17 5.1 Monitoring . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1 Monitoring . . . . . . . . . . . . . . . . . . . . . . . . 17 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6.1 Denial-of-Service Attack Mitigation . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 6.2 Service Compromise . . . . . . . . . . . . . . . . . . . . 19
6.1 Denial-of-Service Attack Mitigation . . . . . . . . . . . 17 6.3 Service Hijacking . . . . . . . . . . . . . . . . . . . . 19
6.2 Increased Risk of Service Compromise . . . . . . . . . . . 17 7. Protocol Considerations . . . . . . . . . . . . . . . . . . . 20
6.3 Service Hijacking . . . . . . . . . . . . . . . . . . . . 18 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9. Acknowlegements . . . . . . . . . . . . . . . . . . . . . . . 22
7. Protocol Considerations . . . . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1 Normative References . . . . . . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 10.2 Informative References . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
A. Change History . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . 21
1. Introduction 1. Introduction
To distribute a service using anycast, the service is first To distribute a service using anycast, the service is first
associated with a stable set of IP addresses, and reachability to associated with a stable set of IP addresses, and reachability to
those addresses is advertised in a routing system from multiple, those addresses is advertised in a routing system from multiple,
independent service nodes. Various techniques for anycast deployment independent service nodes. Various techniques for anycast deployment
of services are discussed in RFC 1546 [4], ISC-TN-2003-1 [14] and of services are discussed in [RFC1546], [ISC-TN-2003-1] and [ISC-TN-
ISC-TN-2004-1 [15]. 2004-1].
Anycast has in recent years become increasingly popular for adding Anycast has in recent years become increasingly popular for adding
redundancy to DNS servers to complement the redundancy which the DNS redundancy to DNS servers to complement the redundancy which the DNS
architecture itself already provides. Several root DNS server architecture itself already provides. Several root DNS server
operators have distributed their servers widely around the Internet, operators have distributed their servers widely around the Internet,
and both resolver and authority servers are commonly distributed and both resolver and authority servers are commonly distributed
within the networks of service providers. Anycast distribution has within the networks of service providers. Anycast distribution has
been used by commercial DNS authority server operators for several been used by commercial DNS authority server operators for several
years. The use of anycast is not limited to the DNS, although the years. The use of anycast is not limited to the DNS, although the
use of anycast imposes some additional limitations on the nature of use of anycast imposes some additional limitations on the nature of
the service being distributed, including transaction longevity, the service being distributed, including transaction longevity,
transaction state held on servers and data synchronisation transaction state held on servers and data synchronisation
capabilities. capabilities.
Although anycast is conceptually simple, its implementation Although anycast is conceptually simple, its implementation
introduces some pitfalls for operation of the service. For example, introduces some pitfalls for operation of services. For example,
monitoring the availability of the service becomes more difficult; monitoring the availability of the service becomes more difficult;
the observed availability changes according to the location of the the observed availability changes according to the location of the
client within the network, and the client catchment of individual client within the network, and the client catchment of individual
anycast nodes is not static, nor especially deterministic. anycast nodes is neither static, nor reliably deterministic.
This document will describe the use of anycast for both local scope This document will describe the use of anycast for both local scope
distribution of services using an Interior Gateway Protocol (IGP) and distribution of services using an Interior Gateway Protocol (IGP) and
global distribution using BGP [5]. Many of the issues for monitoring global distribution using BGP [RFC1771]. Many of the issues for
and data synchronisation are common to both, but deployment issues monitoring and data synchronisation are common to both, but
differ substantially. deployment issues differ substantially.
2. Terminology 2. Terminology
Service Address: an IP address associated with a particular service Service Address: an IP address associated with a particular service
(e.g. the address of a nameserver). (e.g. the destination address used by DNS resolvers to reach a
particular authority server).
Anycast: the practice of making a particular Service Address Anycast: the practice of making a particular Service Address
available in multiple, discrete, autonomous locations, such that available in multiple, discrete, autonomous locations, such that
datagrams sent are routed to one of several available locations. datagrams sent are routed to one of several available locations.
Anycast Node: an internally-connected collection of hosts and routers Anycast Node: an internally-connected collection of hosts and routers
which together provide service for an anycast Service Address. which together provide service for an anycast Service Address. An
The entire anycast system for the service consists of two or more Anycast Node might be as simple as a single host participating in
separate Anycast Nodes. a routing protocol 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.
Local-Scope Anycast: reachability information for the anycast Service Local-Scope Anycast: reachability information for the anycast Service
Address is propagated through a routing system in such a way that 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 a particular anycast node is only visible to a subset of the whole
routing system. routing system.
Local Node: an Anycast Node providing service using a Local-Scope Local Node: an Anycast Node providing service using a Local-Scope
Anycast address. Anycast address.
Global-Scope Anycast: reachability information for the anycast Global-Scope Anycast: reachability information for the anycast
Service Address is propagated through a routing system in such a Service Address is propagated through a routing system in such a
way that a particular anycast node is potentially visible to the way that a particular anycast node is potentially visible to the
whole routing system. whole routing system.
Global Node: an Anycast Node providing service using a Global-Scope Global Node: an Anycast Node providing service using a Global-Scope
Anycast address. Anycast address.
3. Anycast Service Distribution 3. Anycast Service Distribution
3.1 General Description 3.1 General Description
Anycast is the name given to the practice of making a Service Address 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 available to a routing system at Anycast Nodes in two or more
discrete locations. The service provided by each node is necessarily discrete locations. The service provided by each node is consistent
consistent regardless of the particular node chosen by the routing regardless of the particular node chosen by the routing system to
system to handle a particular request. handle a particular request.
For services distributed using anycast, there is no inherent For services distributed using anycast, there is no inherent
requirement for referrals to other servers or name-based service requirement for referrals to other servers or name-based service
distribution ("round-robin DNS"), although those techniques could be distribution ("round-robin DNS"), although those techniques could be
combined with anycast service distribution if an application required combined with anycast service distribution if an application required
it. The routing system decides which node is used for each request, it. The routing system decides which node is used for each request,
based on the topological design of the routing system and the point based on the topological design of the routing system and the point
in the network at which the request originates. in the network at which the request originates.
The Anycast Node chosen to service a particular query can be The Anycast Node chosen to service a particular query can be
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Load-balancing between Anycast Nodes is typically difficult to Load-balancing between Anycast Nodes is typically difficult to
achieve (load distribution between nodes is generally unbalanced in achieve (load distribution between nodes is generally unbalanced in
terms of request and traffic load). Distribution of load between terms of request and traffic load). Distribution of load between
nodes for the purposes of reliability, and coarse-grained nodes for the purposes of reliability, and coarse-grained
distribution of load for the purposes of making popular services distribution of load for the purposes of making popular services
scalable can often be achieved, however. scalable can often be achieved, however.
The scale of the routing system through which a service is anycast The scale of the routing system through which a service is anycast
can vary from a small Interior Gateway Protocol (IGP) connecting a can vary from a small Interior Gateway Protocol (IGP) connecting a
small handful of components, to the Border Gateway Protocol (BGP) [5] small handful of components, to the Border Gateway Protocol (BGP)
connecting the global Internet, depending on the nature of the [RFC1771] connecting the global Internet, depending on the nature of
service distribution that is required. the service distribution that is required.
3.2 Goals 3.2 Goals
A service may be anycast for a variety of reasons. A number of A service may be anycast for a variety of reasons. A number of
common objectives are: common objectives are:
1. Coarse ("unbalanced") distribution of load across nodes, to allow 1. Coarse ("unbalanced") distribution of load across nodes, to allow
infrastructure to scale to increased numbers of queries and to infrastructure to scale to increased numbers of queries and to
accommodate transient query peaks; accommodate transient query peaks;
2. Mitigation of non-distributed denial of service attacks by 2. Mitigation of non-distributed denial of service attacks by
localising damage to single anycast nodes; localising damage to single anycast nodes;
3. Constraint of distributed denial of service attacks or flash 3. Constraint of distributed denial of service attacks or flash
crowds to local regions around anycast nodes (perhaps restricting crowds to local regions around anycast nodes (perhaps restricting
query traffic to local peering links, rather than paid transit query traffic to local peering links, rather than paid transit
circuits); circuits);
4. To provide additional information to help locate location of 4. To provide additional information to help locate location of
traffic sources in the case of attack (or query) traffic which traffic sources in the case of attack (or query) traffic which
incorporates spoofed source addresses. This information is incorporates spoofed source addresses. This information is
derived from the property of anycast service distribution that derived from the property of anycast service distribution that
the the selection of the Anycast Node used to service a the the selection of the Anycast Node used to service a
particular query may be related to the topological source of the particular query may be related to the topological source of the
request. request.
5. Improvement of query response time, by reducing the network 5. Improvement of query response time, by reducing the network
distance between client and server with the provision of a local distance between client and server with the provision of a local
Anycast Node. The extent to which query response time is Anycast Node. The extent to which query response time is
improved depends on the way that nodes are selected for the improved depends on the way that nodes are selected for the
clients by the routing system. Topological nearness within the clients by the routing system. Topological nearness within the
routing system does not, in general, correlate to round-trip routing system does not, in general, correlate to round-trip
performance across a network; in some cases response times may performance across a network; in some cases response times may
see no reduction, and may increase. see no reduction, and may increase.
6. To reduce a list of servers to a single, distributed address. 6. To reduce a list of servers to a single, distributed address.
For example, a large number of authoritative nameservers for a For example, a large number of authoritative nameservers for a
zone may be deployed using a small set of anycast Service zone may be deployed using a small set of anycast Service
Addresses; this approach can increase the accessibility of zone Addresses; this approach can increase the accessibility of zone
data in the DNS without increasing the size of a referral data in the DNS without increasing the size of a referral
response from a nameserver authoritative for the parent zone. response from a nameserver authoritative for the parent zone.
4. Design 4. Design
4.1 Protocol Suitability 4.1 Protocol Suitability
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zone may be deployed using a small set of anycast Service zone may be deployed using a small set of anycast Service
Addresses; this approach can increase the accessibility of zone Addresses; this approach can increase the accessibility of zone
data in the DNS without increasing the size of a referral data in the DNS without increasing the size of a referral
response from a nameserver authoritative for the parent zone. response from a nameserver authoritative for the parent zone.
4. Design 4. Design
4.1 Protocol Suitability 4.1 Protocol Suitability
When a service is anycast between two or more nodes, the routing When a service is anycast between two or more nodes, the routing
system effectively makes the node selection decision on behalf of a system makes the node selection decision on behalf of a client.
client. Since it is usually a requirement that a single Since it is usually a requirement that a single client-server
client-server interaction is carried out between a client and the interaction is carried out between a client and the same server node
same server node for the duration of the transaction, it follows that for the duration of the transaction, it follows that the routing
the routing system's node selection decision ought to be stable for system's node selection decision ought to be stable for substantially
an order of magnitude longer than the expected transaction time, if longer than the expected transaction time, if the service is to be
the service is to be provided reliably. provided reliably.
Some services have very short transaction times, and may even be Some services have very short transaction times, and may even be
carried out using a single packet request and a single packet reply carried out using a single packet request and a single packet reply
in some cases (e.g. DNS transactions over UDP transport). Other in some cases (e.g. DNS transactions over UDP transport). Other
services involve far longer-lived transactions (e.g. bulk file services involve far longer-lived transactions (e.g. bulk file
downloads and audio-visual media streaming). downloads and audio-visual media streaming).
Some anycast deployments have very predictable routing systems, which Some anycast deployments have very predictable routing systems, which
can remain stable for long periods of time (e.g. anycast within an can remain stable for long periods of time (e.g. anycast within an
well-managed and topologically-simple IGP, where node selection well-managed and topologically-simple IGP, where node selection
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This document deliberately avoids prescribing rules as to which This document deliberately avoids prescribing rules as to which
protocols or services are suitable for distribution by anycast; to protocols or services are suitable for distribution by anycast; to
attempt to do so would be presumptuous. attempt to do so would be presumptuous.
4.2 Node Placement 4.2 Node Placement
Decisions as to where Anycast Nodes should be placed will depend to a Decisions as to where Anycast Nodes should be placed will depend to a
large extent on the goals of the service distribution. For example: large extent on the goals of the service distribution. For example:
o A DNS recursive resolver service might be distributed within an o A DNS recursive resolver service might be distributed within an
ISP's network, one Anycast Node per PoP. ISP's network, one Anycast Node per site.
o A root DNS server service might be distributed throughout the o A root DNS server service might be distributed throughout the
Internet with nodes located in regions with poor external Internet with nodes located in regions with poor external
connectivity, to ensure that the DNS functions adequately within connectivity, to ensure that the DNS functions adequately within
the region during times of external network failure. the region during times of external network failure.
o An FTP mirror service might include local nodes located at o An FTP mirror service might include local nodes located at
exchange points, so that ISPs connected to that exchange point exchange points, so that ISPs connected to that exchange point
could download bulk data more cheaply than if they had to use could download bulk data more cheaply than if they had to use
expensive transit circuits. expensive transit circuits.
In general node placement decisions should be made with consideration In general node placement decisions should be made with consideration
of likely traffic requirements, the potential for flash crowds or of likely traffic requirements, the potential for flash crowds or
denial-of-service traffic, the stability of the local routing system denial-of-service traffic, the stability of the local routing system
and the failure modes with respect to node failure, or local routing and the failure modes with respect to node failure, or local routing
system failure. system failure.
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provisioned can be made by network engineers without requiring such provisioned can be made by network engineers without requiring such
operational complexities as regional variances in the configuration operational complexities as regional variances in the configuration
of client computers, or deliberate DNS incoherence (causing DNS of client computers, or deliberate DNS incoherence (causing DNS
queries to yield different answers depending on where the queries queries to yield different answers depending on where the queries
originate). originate).
When a service is anycast within an IGP the routing system is When a service is anycast within an IGP the routing system is
typically under the control of the same organisation that is typically under the control of the same organisation that is
providing the service, and hence the relationship between service providing the service, and hence the relationship between service
transaction characteristics and network stability are likely to be transaction characteristics and network stability are likely to be
relatively well-understood. This technique is consequently well-understood. This technique is consequently applicable to a
applicable to a larger number of applications than Internet-wide larger number of applications than Internet-wide anycast service
anycast service distribution (see Section 4.1). distribution (see Section 4.1).
An IGP will generally have no inherent restriction on the length of An IGP will generally have no inherent restriction on the length of
prefix that can be introduced to it. There may well therefore be no prefix that can be introduced to it. There may well therefore be no
need to construct a covering prefix for particular Service Addresses; need to construct a covering prefix for particular Service Addresses;
host routes corresponding to the Service Address can instead be host routes corresponding to the Service Address can instead be
introduced to the routing system. See Section 4.4.2 for more introduced to the routing system. See Section 4.4.2 for more
discussion of the requirement for a covering prefix. discussion of the requirement for a covering prefix.
IGPs often feature little or no aggregation of routes, partly due to IGPs often feature little or no aggregation of routes, partly due to
algorithmic complexities in supporting aggregation. There is little algorithmic complexities in supporting aggregation. There is little
motiviation for aggregation in many networks' IGPs in any case, since motivation for aggregation in many networks' IGPs in any case, since
the amount of routing information carried in the IGP is small enough the amount of routing information carried in the IGP is small enough
that scaling concerns in routers do not arise. For discussion of that scaling concerns in routers do not arise. For discussion of
aggregation risks in other routing systems, see Section 4.4.8. aggregation risks in other routing systems, see Section 4.4.8.
By reducing the scope of the IGP to just the hosts providing service By reducing the scope of the IGP to just the hosts providing service
(together with one or more gateway routers) this technique can be (together with one or more gateway routers) this technique can be
applied to the construction of server clusters. This application is applied to the construction of server clusters. This application is
discussed in some detail in [15]. discussed in some detail in [ISC-TN-2004-1].
4.3.2 Anycast within the Global Internet 4.3.2 Anycast within the Global Internet
Service Addresses may be anycast within the global Internet routing Service Addresses may be anycast within the global Internet routing
system in order to distribute services across the entire network. system in order to distribute services across the entire network.
The principal differences between this application and the IGP-scope The principal differences between this application and the IGP-scope
distribution discussed in Section 4.3.1 are that: distribution discussed in Section 4.3.1 are that:
1. the routing system is, in general, controlled by other people; 1. the routing system is, in general, controlled by other people;
and
2. the routing protocol concerned (BGP), and commonly-accepted 2. the routing protocol concerned (BGP), and commonly-accepted
practices in its deployment, impose some additional constraints practices in its deployment, impose some additional constraints
(see Section 4.4). (see Section 4.4).
4.4 Routing Considerations 4.4 Routing Considerations
4.4.1 Signalling Service Availability 4.4.1 Signalling Service Availability
When a routing system is provided with reachability information for a When a routing system is provided with reachability information for a
Service Address from an individual node, packets addressed to that Service Address from an individual node, packets addressed to that
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Where a routing advertisement from a node corresponds to a single Where a routing advertisement from a node corresponds to a single
Service Address, this coupling might be such that availability of the Service Address, this coupling might be such that availability of the
service triggers the route advertisement, and non-availability of the service triggers the route advertisement, and non-availability of the
service triggers a route withdrawal. This can be achieved using service triggers a route withdrawal. This can be achieved using
routing protocol implementations on the same server which provide the routing protocol implementations on the same server which provide the
service being distributed, which are configured to advertise and service being distributed, which are configured to advertise and
withdraw the route advertisement in conjunction with the availability withdraw the route advertisement in conjunction with the availability
(and health) of the software on the host which processes service (and health) of the software on the host which processes service
requests. An example of such an arrangement for a DNS service is requests. An example of such an arrangement for a DNS service is
included in [15]. included in [ISC-TN-2004-1].
Where a routing advertisement from a node corresponds to two or more Where a routing advertisement from a node corresponds to two or more
Service Addresses, it may not be appropriate to trigger a route Service Addresses, it may not be appropriate to trigger a route
withdrawal due to the non-availability of a single service. Another withdrawal due to the non-availability of a single service. Another
approach is to route requests for the service which is down at one approach is to route requests for the service which is down at one
Anycast Node to a different Anycast Node at which the service is up. Anycast Node to a different Anycast Node at which the service is up.
This approach is discussed in Section 4.8. 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 4.4.2 Covering Prefix
In some routing systems (e.g. the BGP-based routing system of the 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 global Internet) it is not possible, in general, to propagate a host
route with confidence that availability of the route will be route with confidence that the route will propagate throughout the
signalled throughout the network. This is a consequence of network. This is a consequence of operational policy, and not a
operational policy, and not a protocol restriction. protocol restriction.
In such cases it is necessary to propagate a route which covers the In such cases it is necessary to propagate a route which covers the
Service Address, and which has a sufficiently short prefix that it Service Address, and which has a sufficiently short prefix that it
will not be discarded by commonly-deployed import policies. For IPv4 will not be discarded by commonly-deployed import policies. For IPv4
Service Addresses, this is often a 24-bit prefix, but there are other Service Addresses, this is often a 24-bit prefix, but there are other
well-documented examples of IPv4 import polices which filter on well-documented examples of IPv4 import polices which filter on
Regional Internet Registry (RIR) allocation boundaries, and hence Regional Internet Registry (RIR) allocation boundaries, and hence
some experimentation may be prudent. Corresponding import policies some experimentation may be prudent. Corresponding import policies
for IPv6 prefixes also exist. See Section 4.5 for more discussion of for IPv6 prefixes also exist. See Section 4.5 for more discussion of
IPv6 Service Addresses and corresponding anycast routes. IPv6 Service Addresses and corresponding anycast routes.
skipping to change at page 10, line 33 skipping to change at page 11, line 6
of that route and the individual services associated with the covered of that route and the individual services associated with the covered
host routes. The resulting impact on signaling availability of host routes. The resulting impact on signaling availability of
individual services is discussed in Section 4.4.1 and Section 4.8. individual services is discussed in Section 4.4.1 and Section 4.8.
4.4.3 Equal-Cost Paths 4.4.3 Equal-Cost Paths
Some routing systems support equal-cost paths to the same Some routing systems support equal-cost paths to the same
destination. Where multiple, equal-cost paths exist and lead to destination. Where multiple, equal-cost paths exist and lead to
different anycast nodes, there is a risk that different request different anycast nodes, there is a risk that different request
packets associated with a single transaction might be delivered to packets associated with a single transaction might be delivered to
more than one node. Services provided over TCP necessarily involve more than one node. Services provided over TCP [RFC0793] necessarily
transactions with multiple request packets, due to the TCP setup involve transactions with multiple request packets, due to the TCP
handshake. setup handshake.
Equal cost paths are commonly supported in IGPs. Multi-node Equal cost paths are commonly supported in IGPs. Multi-node
selection for a single transaction can be avoided in most cases by selection for a single transaction can be avoided in most cases by
careful consideration of IGP link metrics, or by applying equal-cost careful consideration of IGP link metrics, or by applying equal-cost
multi-path (ECMP) selection algorithms which cause a single node to multi-path (ECMP) selection algorithms which cause a single node to
be selected for a single multi-packet transaction. For a description be selected for a single multi-packet transaction. For an example of
of hash-based ECMP selection, see [15]. the use of hash-based ECMP selection in anycast service distribution,
see [ISC-TN-2004-1].
For services which are distributed across the global Internet using For services which are distributed across the global Internet using
BGP, equal-cost paths are normally not a consideration: BGP's exit BGP, equal-cost paths are normally not a consideration: BGP's exit
selection algorithm usually selects a single, consistent exit for a selection algorithm usually selects a single, consistent exit for a
single destination regardless of whether multiple candidate paths single destination regardless of whether multiple candidate paths
exist. Implementations of BGP exist that support multi-path exit exist. Implementations of BGP exist that support multi-path exit
selection, however, and corner cases where dual selected exits route selection, however, and corner cases where dual selected exits route
to different nodes are possible. Analysis of the likely incidence of to different nodes are possible. Analysis of the likely incidence of
such corner cases for particular distributions of Anycast Nodes are such corner cases for particular distributions of Anycast Nodes are
recommended for services which involve multi-packet transactions. recommended for services which involve multi-packet transactions.
4.4.4 Route Dampening 4.4.4 Route Dampening
Frequent advertisements and withdrawals of individual prefixes in BGP Frequent advertisements and withdrawals of individual prefixes in BGP
are known as flaps. Rapid flapping can lead to CPU exhaustion on are known as flaps. Rapid flapping can lead to CPU exhaustion on
routers quite remote from the source of the instability, and for this routers quite remote from the source of the instability, and for this
reason rapid route oscillations are frequently "damped", as described reason rapid route oscillations are frequently "dampened", as
in [10]. described in [RFC2439].
A dampened path will be suppressed by routers for an interval which A dampened path will be suppressed by routers for an interval which
increases according to the frequency of the observed oscillation; a increases according to the frequency of the observed oscillation; a
suppressed path will not propagate. Hence a single router can suppressed path will not propagate. Hence a single router can
prevent the propagation of a flapping prefix to the rest of an prevent the propagation of a flapping prefix to the rest of an
autonomous system, affording other routers in the network protection autonomous system, affording other routers in the network protection
from the instability. from the instability.
Some implementations of flap dampening penalises oscillating Some implementations of flap dampening penalise oscillating
advertisements based on the observed AS_PATH, and not on the NLRI. advertisements based on the observed AS_PATH, and not on the NLRI.
For this reason, network instability which leads to route flapping For this reason, network instability which leads to route flapping
from a single anycast node ought not to cause advertisements from from a single anycast node ought not to cause advertisements from
other nodes (which have different AS_PATH attributes) to be dampened. other nodes (which have different AS_PATH attributes) to be dampened.
As dampening works on advertisements with the same AS_PATH attribute, To limit the opportunity of such implementations to penalise
care should be taken so that the AS_PATH is as diverse as possible advertisements originating from different Anycast Nodes in response
for the anycasted nodes. The Anycasted nodes should have the same to oscillations from just a single node, care should be taken to
origin AS for their advertisements, but they should have different arrange that the AS_PATH attributes on routes from different nodes
upstream ASs for each node. If the upstream AS is the same at all are as diverse as possible. For example, Anycast Nodes should use
locations, there is a risk that the upstream AS will peer with the the same origin AS for their advertisements, but might have different
ASs at multiple locations and could therefore propagate the same upstream ASs.
AS_PATH, but for different Anycast nodes. This could render the
service for multiple Anycast nodes unavailable due to dampening
caused by only one of them.
Where different implementations of flap dampening are prevalent, Where different implementations of flap dampening are prevalent,
individual nodes' instability may result in stable nodes becoming individual nodes' instability may result in stable nodes becoming
unavailable. Judicious deployment of Local Nodes in combination with 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, especially stable Global Nodes (with high inter-AS path splay,
redundant hardware, power, etc) may help mitigate such problems. 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 adjcacent 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 4.4.5 Reverse Path Forwarding Checks
Reverse Path Forwarding (RPF) checks, first described in RFC 2267 Reverse Path Forwarding (RPF) checks, first described in [RFC2267],
[8], are commonly deployed as part of ingress interface packet are commonly deployed as part of ingress interface packet filters on
filters on routers in the global Internet in order to deny packets routers in the Internet in order to deny packets whose source
whose source addresses are spoofed (see also RFC 2827 [11]). addresses are spoofed (see also RFC 2827 [RFC2827]). Deployed
Deployed implementations of RPF make several modes of operation implementations of RPF make several modes of operation available
available (e.g. "loose" and "strict"). (e.g. "loose" and "strict").
Some modes of RPF can cause non-spoofed packets to be denied when Some modes of RPF can cause non-spoofed packets to be denied when
they originate from multi-homed site, since selected paths might they originate from multi-homed site, since selected paths might
legitimately not correspond with the ingress interface of non-spoofed legitimately not correspond with the ingress interface of non-spoofed
packets from the multi-homed site. This issue is discussed in RFC packets from the multi-homed site. This issue is discussed in
3704 [12]. [RFC3704].
A collection of anycast nodes deployed across the Internet is largely A collection of anycast nodes deployed across the Internet is largely
indistinguishable from a distributed, multi-homed site to the routing indistinguishable from a distributed, multi-homed site to the routing
system, and hence this risk also exists for anycast nodes, even if system, and hence this risk also exists for anycast nodes, even if
individual nodes are not multi-homed. Care should be taken to ensure 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 that each anycast node is treated as a multi-homed network, and that
the corresponding recommendations in RFC 3704 [12] with respect to the corresponding recommendations in [RFC3704] with respect to RPF
RPF checks are heeded. checks are heeded.
4.4.6 Propagation Scope 4.4.6 Propagation Scope
In the context of Anycast service distribution across the global In the context of Anycast service distribution across the global
Internet, Global Nodes are those which are capable of providing Internet, Global Nodes are those which are capable of providing
service to clients anywhere in the network; reachability information service to clients anywhere in the network; reachability information
for the service is propagated globally, without restriction, by for the service is propagated globally, without restriction, by
advertising the routes covering the Service Addresses for global advertising the routes covering the Service Addresses for global
transit to one or more providers. transit to one or more providers.
skipping to change at page 12, line 42 skipping to change at page 13, line 22
which only provides services to a local catchment of autonomous which only provides services to a local catchment of autonomous
systems, and which is deliberately not available to the entire systems, and which is deliberately not available to the entire
Internet; such nodes are referred to in this document as Local Nodes. Internet; such nodes are referred to in this document as Local Nodes.
An example of circumstances in which a Local Node may be appropriate An example of circumstances in which a Local Node may be appropriate
are nodes designed to serve a region with rich internal connectivity are nodes designed to serve a region with rich internal connectivity
but unreliable, congested or expensive access to the rest of the but unreliable, congested or expensive access to the rest of the
Internet. Internet.
Local Nodes advertise covering routes for Service Addresses in such a Local Nodes advertise covering routes for Service Addresses in such a
way that their propagation is restricted. This might be done using way that their propagation is restricted. This might be done using
well-known community string attributes such as NO_EXPORT [7] or well-known community string attributes such as NO_EXPORT [RFC1997] or
NOPEER [13], or by arranging with peers to apply a conventional NOPEER [RFC3765], or by arranging with peers to apply a conventional
"peering" import policy instead of a "transit" import policy, or some "peering" import policy instead of a "transit" import policy, or some
suitable combination of measures. suitable combination of measures.
Advertising reachability to Service Addresses from Local Nodes should Advertising reachability to Service Addresses from Local Nodes should
ideally be made using a routing policy that require presence of ideally be made using a routing policy that require presence of
explicit attributes for propagation, rather than reling on implicit explicit attributes for propagation, rather than reling on implicit
(default) policy. Inadvertant propagation of a route beyond its (default) policy. Inadvertant propagation of a route beyond its
intended horizon can result in capacity problems for Local Nodes intended horizon can result in capacity problems for Local Nodes
which might degrade service performance. which might degrade service performance network-wide.
4.4.7 Other Peoples' Networks 4.4.7 Other Peoples' Networks
When Anycast services are deployed across networks operated by When Anycast services are deployed across networks operated by
others, their reachability is dependent on routing policies and others, their reachability is dependent on routing policies and
topology changes (planned and unplanned) which are unpredictable and topology changes (planned and unplanned) which are unpredictable and
sometimes difficult to identify. Since the routing system may sometimes difficult to identify. Since the routing system may
include networks operated by multiple, unrelated organisations, the include networks operated by multiple, unrelated organisations, the
possibility of unforeseen interactions resulting from the possibility of unforeseen interactions resulting from the
combinations of unrelated changes also exists. combinations of unrelated changes also exists.
The stability and predictability of such a routing system should be The stability and predictability of such a routing system should be
taken into consideration when assessing the suitability of anycast as taken into consideration when assessing the suitability of anycast as
a distribution strategy for particular services and protocols (see a distribution strategy for particular services and protocols (see
also Section 4.1). also Section 4.1).
By way of mitigation, routing policies used by Anycast Nodes across By way of mitigation, routing policies used by Anycast Nodes across
such routing systems should be conservative, individual nodes' such routing systems should be conservative, individual nodes'
internal and external/connecting infrastructure should be scaled to internal and external/connecting infrastructure should be scaled to
support loads far in excess of the average, and the service should be support loads far in excess of the average, and the service should be
monitored proactively (Section 5.1) from many points in order to monitored proactively from many points in order to avoid unpleasant
avoid unpleasant surprises. surprises (see Section 5.1).
4.4.8 Aggregation Risks 4.4.8 Aggregation Risks
The propagation of a single route for each anycast service does not The propagation of a single route for each anycast service does not
scale well for routing systems in which the load of routing scale well for routing systems in which the load of routing
information which must be carried is a concern, and where there are information which must be carried is a concern, and where there are
potentially many services to distribute. For example, an autonomous potentially many services to distribute. For example, an autonomous
system which provides services to the Internet with N Service system which provides services to the Internet with N Service
Addresses covered by a single exported route, for example, would need Addresses covered by a single exported route, would need to advertise
to advertise (N+1) routes if each of those services were to be (N+1) routes if each of those services were to be distributed using
distributed using anycast. anycast.
The common practice of applying minimm prefix-length filters in The common practice of applying minimum prefix-length filters in
import policies on the Internet (see Section 4.4.2) means that for a 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 route covering a Service Address to be usefully propagated the prefix
length must be substantially less than that required to advertise length must be substantially less than that required to advertise
just the host route. Widespread advertisement of short prefixes for just the host route. Widespread advertisement of short prefixes for
individual services hence also has a negative impact on address individual services hence also has a negative impact on address
conservation. conservation.
Both of these issues can be mitigated to some extent by the use of a Both of these issues can be mitigated to some extent by the use of a
single covering prefix to accommodate multiple Service Addresses, as single covering prefix to accommodate multiple Service Addresses, as
described in Section 4.8). This implies a decoupling of the route described in Section 4.8. This implies a decoupling of the route
advertisement from individual service availability (see advertisement from individual service availability (see
Section 4.4.1), however, and can also impact the stability of the Section 4.4.1), however, with attendant risks to the stability of the
service as a whole (see Section 4.7). service as a whole (see Section 4.7).
In general, the scaling problems described here prevent anycast from In general, the scaling problems described here prevent anycast from
being a useful, general approach for service distribution on the being a useful, general approach for service distribution on the
global Internet. It remains, however, a useful technique for global Internet. It remains, however, a useful technique for
distributing a limited number of Internet-critical services. 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 4.5 Addressing Considerations
Service Addresses should be unique within the routing system that Service Addresses should be unique within the routing system that
connects all Anycast Nodes to all possible clients of the service. connects all Anycast Nodes to all possible clients of the service.
Service Addresses must also be chosen so that corresponding routes Service Addresses must also be chosen so that corresponding routes
will be allowed to propagate within that routing system. will be allowed to propagate within that routing system.
For an IPv4-numbered service deployed across the Internet, for For an IPv4-numbered service deployed across the Internet, for
example, an address might be chosen from a block where the minimum example, an address might be chosen from a block where the minimum
RIR allocation size is 24 bits, and reachability to that address RIR allocation size is 24 bits, and reachability to that address
might be provided by originating the covering 24-bit prefix. might be provided by originating the covering 24-bit prefix.
For an IPv4-numbered service deployed within a private network, a For an IPv4-numbered service deployed within a private network, a
locally-unused RFC1918 [6] address might be chosen, and rechability locally-unused [RFC1918] address might be chosen, and rechability to
to that address might be signalled using a (32-bit) host route. that address might be signalled using a (32-bit) host route.
For IPv6-numbered services, Anycast Addresses are not scoped For IPv6-numbered services, Anycast Addresses are not scoped
differently from unicast addresses (see RFC2713 [9]). As such the differently from unicast addresses [RFC3513]. As such the guidelines
guidelines presented for IPv4 with respect to address suitability presented for IPv4 with respect to address suitability follow for
follow for IPv6. IPv6.
RFC2713 [9] also imposes two restrictions on the use of anycast which
inhibit deployment of host-based services:
o An [IPv6] anycast address must not be used as the source address
of an IPv6 packet.
o An anycast address must not be assigned to an IPv6 host, that is,
it may be assigned to an IPv6 router only.
Despite these restrictions (and in violation of them), production
deployment of IPv6 anycast services across the Internet has taken
place.
4.6 Data Synchronisation 4.6 Data Synchronisation
Although some services have been deployed in localised form (such Although some services have been deployed in localised form (such
that clients from particular regions are presented with that clients from particular regions are presented with regionally-
regionally-relevant content) many services have the property that relevant content) many services have the property that responses to
responses to client requests should be consistent, regardless of client requests should be consistent, regardless of where the request
where the request originates. For a service distributed using originates. For a service distributed using anycast, that implies
anycast, that implies that different Anycast Nodes must operate in a that different Anycast Nodes must operate in a consistent manner and,
consistent manner and, where that consistent behaviour is based on a where that consistent behaviour is based on a data set, that the data
data set, that the data concerned be synchronised between nodes. concerned be synchronised between nodes.
The mechanism by which data is synchronised depends on the nature of The mechanism by which data is synchronised depends on the nature of
the service; examples are zone transfers for authoritative DNS the service; examples are zone transfers for authoritative DNS
servers and rsync for FTP archives. In general, the synchronisation servers and rsync for FTP archives. In general, the synchronisation
of data between Anycast Nodes will involve transactions between of data between Anycast Nodes will involve transactions between non-
non-anycast addresses. anycast addresses.
Data synchronisation across public networks should be carried out Data synchronisation across public networks should be carried out
with appropriate authentication and encryption. with appropriate authentication and encryption.
4.7 Node Autonomy 4.7 Node Autonomy
For an Anycast deployment whose goals include improved reliability For an Anycast deployment whose goals include improved reliability
through redundancy, it is important to minimise the opportunity for a through redundancy, it is important to minimise the opportunity for a
single defect to compromise many (or all) nodes, or for the failure single defect to compromise many (or all) nodes, or for the failure
of one node to provide a cascading failure bringing down additional of one node to provide a cascading failure bringing down additional
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The possibility of cascading failure due to load can also be reduced The possibility of cascading failure due to load can also be reduced
by the deployment of both Global and Local Nodes for a single by the deployment of both Global and Local Nodes for a single
service, since the effective fail-over path of traffic is, in service, since the effective fail-over path of traffic is, in
general, from Local Node to Global Node; traffic that might sink one 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 Local Node is unlikely to sink all Local Nodes, except in the most
degenerate cases. degenerate cases.
The chance of cascading failure due to a software defect in an The chance of cascading failure due to a software defect in an
operating system or server can be reduced in many cases by deploying operating system or server can be reduced in many cases by deploying
nodes running different software implementations. nodes running different implementations of operating system, server
software, routing protocol software, etc, such that a defect which
appears in a single component does not affect the whole system.
4.8 Multi-Service Nodes 4.8 Multi-Service Nodes
For a service distributed across a routing system where covering For a service distributed across a routing system where covering
prefixes are required to announce reachability to a single Service prefixes are required to announce reachability to a single Service
Address (see Section 4.4.2), special consideration is required in the Address (see Section 4.4.2), special consideration is required in the
case where multiple services need to be distributed across a single case where multiple services need to be distributed across a single
set of nodes. This results from the requirement to signal set of nodes. This results from the requirement to signal
availability of individual services to the routing system so that availability of individual services to the routing system so that
requests for service are not received by nodes which are not able to requests for service are not received by nodes which are not able to
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4.8.1 Multiple Covering Prefixes 4.8.1 Multiple Covering Prefixes
Each Service Address is chosen such that only one Service Address is Each Service Address is chosen such that only one Service Address is
covered by each advertised prefix. Advertisement and withdrawal of a covered by each advertised prefix. Advertisement and withdrawal of a
single covering prefix can be tightly coupled to the availability of single covering prefix can be tightly coupled to the availability of
the single associated service. the single associated service.
This is the most straightforward approach. However, since it makes This is the most straightforward approach. However, since it makes
very poor utilisation of globally-unique addresses, it is only very poor utilisation of globally-unique addresses, it is only
suitable for use for a small number of critical, infrastructural suitable for use for a small number of critical, infrastructural
services such as root DNS servers. General deployment of services services such as root DNS servers. General Internet-wide deployment
using this approach will not scale. of services using this approach will not scale.
4.8.2 Pessimistic Withdrawal 4.8.2 Pessimistic Withdrawal
Multiple Service Addresses are chosen such that they are covered by a Multiple Service Addresses are chosen such that they are covered by a
single prefix. Advertisement and withdrawl of the single covering single prefix. Advertisement and withdrawl of the single covering
prefix is coupled to the availability of all associated services; if prefix is coupled to the availability of all associated services; if
any individual service becomes unavailable, the covering prefix is any individual service becomes unavailable, the covering prefix is
withdrawn. withdrawn.
The coupling between service availability and advertisement of the The coupling between service availability and advertisement of the
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monitoring a non-distributed service, since the observed accuracy and monitoring a non-distributed service, since the observed accuracy and
availability of the service is, in general, different when viewed availability of the service is, in general, different when viewed
from clients attached to different parts of the network. When a from clients attached to different parts of the network. When a
problem is identified, it is also not always obvious which node problem is identified, it is also not always obvious which node
served the request, and hence which node is malfunctioning. served the request, and hence which node is malfunctioning.
It is recommended that distributed services are monitored from probes It is recommended that distributed services are monitored from probes
distributed representatively across the routing system, and, where distributed representatively across the routing system, and, where
possible, the identity of the node answering individual requests is possible, the identity of the node answering individual requests is
recorded along with performance and availability statistics. The recorded along with performance and availability statistics. The
RIPE NCC DNSMON service [16] is an example of such monitoring for the RIPE NCC DNSMON service [1] is an example of such monitoring for the
DNS. DNS.
Monitoring the routing system (from a variety of places, in the case Monitoring the routing system (from a variety of places, in the case
of routing systems where perspective counts) can also provide useful of routing systems where perspective is relevant) can also provide
diagnostics for troubleshooting service availability. This can be useful diagnostics for troubleshooting service availability. This
achieved using dedicated probes, or public route measurement can be achieved using dedicated probes, or public route measurement
facilities on the Internet such as the RIPE NCC Routing Information facilities on the Internet such as the RIPE NCC Routing Information
Service [17] and the University of Oregon Route Views Project [18]. Service [2] and the University of Oregon Route Views Project [3].
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. Security Considerations
6.1 Denial-of-Service Attack Mitigation 6.1 Denial-of-Service Attack Mitigation
This document describes mechanisms for deploying services on the This document describes mechanisms for deploying services on the
Internet which can be used to mitigate vulnerability to attack. Internet which can be used to mitigate vulnerability to attack:
6.2 Increased Risk of Service Compromise 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 which
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 which is not
distributed.
6.2 Service Compromise
The distribution of a service across several (or many) autonomous The distribution of a service across several (or many) autonomous
nodes imposes increased monitoring as well as an increased systems nodes imposes increased monitoring as well as an increased systems
administration burden on the operator of the service which might administration burden on the operator of the service which might
reduce the effectiveness of host and router security. reduce the effectiveness of host and router security.
The potential benefit of being able to take compromised servers The potential benefit of being able to take compromised servers off-
off-line without compromising the service can only be realised if line without compromising the service can only be realised if there
there are working procedures to do so quickly and reliably. are working procedures to do so quickly and reliably.
6.3 Service Hijacking 6.3 Service Hijacking
It is possible that an unauthorised party might advertise routes It is possible that an unauthorised party might advertise routes
corresponding to anycast Service Addresses across a network, and by corresponding to anycast Service Addresses across a network, and by
doing so capture legitimate request traffic or process requests in a doing so capture legitimate request traffic or process requests in a
manner which compromises the service (or both). A rogue Anycast Node manner which compromises the service (or both). A rogue Anycast Node
might be difficult to detect by clients or by the operator of the might be difficult to detect by clients or by the operator of the
service. service.
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routing system may make it more difficult to detect rogue nodes. routing system may make it more difficult to detect rogue nodes.
7. Protocol Considerations 7. Protocol Considerations
This document does not impose any protocol considerations. This document does not impose any protocol considerations.
8. IANA Considerations 8. IANA Considerations
This document requests no action from IANA. This document requests no action from IANA.
9. References 9. Acknowlegements
[1] Oran, D., "OSI IS-IS Intra-domain Routing Protocol", RFC 1142,
February 1990.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April The authors gratefully acknowledge the contributions from various
1992. participants of the grow working group, and in particular Geoff
Huston, Pekka Savola, Danny McPherson and Ben Black.
[3] Moy, J., "OSPF Version 2", RFC 1247, July 1991. This work was supported by the US National Science Foundation
(research grant SCI-0427144) and DNS-OARC.
[4] Partridge, C., Mendez, T. and W. Milliken, "Host Anycasting 10. References
Service", RFC 1546, November 1993.
[5] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", 10.1 Normative References
RFC 1771, March 1995.
[6] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
Lear, "Address Allocation for Private Internets", BCP 5, RFC 793, September 1981.
RFC 1918, February 1996.
[7] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities [RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
Attribute", RFC 1997, August 1996. (BGP-4)", RFC 1771, March 1995.
[8] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
Defeating Denial of Service Attacks which employ IP Source E. Lear, "Address Allocation for Private Internets",
Address Spoofing", RFC 2267, January 1998. BCP 5, RFC 1918, February 1996.
[9] Hinden, R. and S. Deering, "IP Version 6 Addressing [RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Architecture", RFC 2373, July 1998. Communities Attribute", RFC 1997, August 1996.
[10] Villamizar, C., Chandra, R. and R. Govindan, "BGP Route Flap [RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
Damping", RFC 2439, November 1998. Flap Damping", RFC 2439, November 1998.
[11] Ferguson, P. and D. Senie, "Network Ingress Filtering: [RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000. Address Spoofing", BCP 38, RFC 2827, May 2000.
[12] Baker, F. and P. Savola, "Ingress Filtering for Multihomed [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004. Networks", BCP 84, RFC 3704, March 2004.
[13] Huston, G., "NOPEER Community for Border Gateway Protocol (BGP) 10.2 Informative References
Route Scope Control", RFC 3765, April 2004.
[14] Abley, J., "Hierarchical Anycast for Global Service [ISC-TN-2003-1]
Abley, J., "Hierarchical Anycast for Global Service
Distribution", March 2003, Distribution", March 2003,
<http://www.isc.org/pubs/tn/isc-tn-2003-1.html>. <http://www.isc.org/pubs/tn/isc-tn-2003-1.html>.
[15] Abley, J., "A Software Approach to Distributing Requests for [ISC-TN-2004-1]
DNS Service using GNU Zebra, ISC BIND 9 and FreeBSD", March Abley, J., "A Software Approach to Distributing Requests
2004, <http://www.isc.org/pubs/tn/isc-tn-2004-1.html>. for DNS Service using GNU Zebra, ISC BIND 9 and FreeBSD",
March 2004,
<http://www.isc.org/pubs/tn/isc-tn-2004-1.html>.
[16] <http://dnsmon.ripe.net/> [RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.
[17] <http://ris.ripe.net> [RFC2267] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2267, January 1998.
[18] <http://www.route-views.org> [RFC3765] Huston, G., "NOPEER Community for Border Gateway Protocol
(BGP) Route Scope Control", RFC 3765, April 2004.
URIs
[1] <http://dnsmon.ripe.net/>
[2] <http://ris.ripe.net>
[3] <http://www.route-views.org>
Authors' Addresses Authors' Addresses
Joe Abley Joe Abley
Internet Systems Consortium, Inc. Internet Systems Consortium, Inc.
950 Charter Street 950 Charter Street
Redwood City, CA 94063 Redwood City, CA 94063
USA USA
Phone: +1 650 423 1317 Phone: +1 650 423 1317
skipping to change at page 20, line 4 skipping to change at page 25, line 24
Joe Abley Joe Abley
Internet Systems Consortium, Inc. Internet Systems Consortium, Inc.
950 Charter Street 950 Charter Street
Redwood City, CA 94063 Redwood City, CA 94063
USA USA
Phone: +1 650 423 1317 Phone: +1 650 423 1317
Email: jabley@isc.org Email: jabley@isc.org
URI: http://www.isc.org/ URI: http://www.isc.org/
Kurt Erik Lindqvist Kurt Erik Lindqvist
Netnod Internet Exchange Netnod Internet Exchange
Bellmansgatan 30 Bellmansgatan 30
118 47 Stockholm 118 47 Stockholm
Sweden Sweden
Email: kurtis@kurtis.pp.se Email: kurtis@kurtis.pp.se
URI: http://www.netnod.se/ URI: http://www.netnod.se/
Appendix A. Change History
This section should be removed before publication.
draft-kurtis-anycast-bcp-00: Initial draft. Discussed at IETF 61 in
the grow meeting and adopted as a working group document shortly
afterwards.
draft-ietf-grow-anycast-00: Missing and empty sections completed;
some structural reorganisation; general wordsmithing. Document
discussed at IETF 62.
draft-ietd-grow-anycast-01: This appendix added; acknowledgements
section added; commentary on [RFC3513] prohibition of anycast on
hosts removed; minor sentence re-casting and related jiggery-
pokery. This revision published for discussion at IETF 63.
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79. found in BCP 78 and BCP 79.
 End of changes. 

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