GROW K. Lindqvist
Internet-Draft Netnod Internet Exchange
Expires: April 22, 2005 J. Abley
ISC
October 22, 2004
Operation of Anycast Services
draft-kurtis-anycast-bcp-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
As the Internet has grown, many services with high availability
requirements have emerged. The requirements of these services have
increased the demands on the reliability of the infrastructure on
which those services rely.
Many techniques have been employed to increase the availability of
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services deployed on the Internet. This document presents
operational experience of wide-scale service distribution using
anycast, and proposes a series of recommendations for others using
this approach.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Anycast Service Distribution . . . . . . . . . . . . . . . . . 4
3.1 General Description . . . . . . . . . . . . . . . . . . . 4
3.2 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Protocol Suitability . . . . . . . . . . . . . . . . . . . 5
4.2 Node Placement . . . . . . . . . . . . . . . . . . . . . . 6
4.3 Routing Systems . . . . . . . . . . . . . . . . . . . . . 6
4.3.1 Anycast within an IGP . . . . . . . . . . . . . . . . 6
4.3.2 Anycast within the Global Internet . . . . . . . . . . 7
4.4 Routing Considerations . . . . . . . . . . . . . . . . . . 7
4.4.1 Signalling Service Availability . . . . . . . . . . . 7
4.4.2 Covering Prefix . . . . . . . . . . . . . . . . . . . 8
4.4.3 Equal-Cost Paths . . . . . . . . . . . . . . . . . . . 8
4.4.4 Route Dampening . . . . . . . . . . . . . . . . . . . 9
4.4.5 Reverse Path Forwarding Checks . . . . . . . . . . . . 10
4.4.6 Propagation Scope . . . . . . . . . . . . . . . . . . 10
4.4.7 Other Peoples' Networks . . . . . . . . . . . . . . . 11
4.5 Data Synchronisation . . . . . . . . . . . . . . . . . . . 11
4.6 Node Autonomy . . . . . . . . . . . . . . . . . . . . . . 11
5. Service Management . . . . . . . . . . . . . . . . . . . . . . 12
5.1 Monitoring . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2 Self-Healing Nodes . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Protocol Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . 16
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1. Introduction
To distribute a service using anycast, the service is first
associated with a stable set of IP addresses, and reachability to
those addresses is advertised in a routing system from multiple,
independent service nodes. Various techniques for anycast deployment
of services are discussed in RFC 1546 [4], ISC-TN-2003-1 [12] and
ISC-TN-2004-1 [13].
Anycast has in recent years become increasingly popular for adding
redundancy to DNS servers. Several root server operators have
distributed their servers widely around the Internet, and both
resolver and authority servers are commonly distributed within the
networks of service providers. Anycast distribution has been used by
commercial DNS authority server operators for several years. The use
of anycast is not limited to the DNS, although the use of anycast
imposes some additional requirements on the nature of the service
being distributed, including transaction longevity, transaction state
held on servers and data synchronization capabilities.
Although anycast is conceptually simple, its implementation
introduces some pitfalls for operation of the service. For example,
monitoring the availability of the service becomes more difficult;
the observed availability changes according to the source of the
query, and the client catchment of individual anycast nodes is not
static, nor especially deterministic.
This document will describe the use of anycast for both local scope
distribution of services using an Interior Gateway Protocol (IGP) and
global distribution using BGP [5]. Many of the issues for monitoring
and data synchronization are common to both, but deployment issues
differ substantially.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Service Address: an IP address associated with a particular service
(e.g. the address of a nameserver).
Anycast: the practice of making a particular Service Address
available in multiple, discrete, autonomous locations, such that
datagrams sent are routed to one of several available locations.
Anycast Node: an internally-connected collection of hosts and routers
which together provide service for an anycast service address.
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Local-Scope Anycast: reachability information for the anycast service
address is propagated through a routing system in such a way that
a particular anycast node is only visible to a subset of the whole
routing system.
Local Node: an Anycast Node providing service using a Local-Scope
Anycast address.
Global-Scope Anycast: reachability information for the anycast
service address is propagated through a routing system in such a
way that a particular anycast node is potentially visible to the
whole routing system.
Global Node: an Anycast Node providing service using a Global-Scope
Anycast address.
3. Anycast Service Distribution
3.1 General Description
Anycast is the name given to the practice of making one or more
Service Addresses available to a routing system at Anycast Nodes in
two or more discrete locations. The service provided by each node is
necessarily consistent regardless of the particular node chosen by
the routing system to handle a particular request.
For services distributed using anycast, there is no inherent
requirement for referrals to other servers or name-based service
distribution ("round-robin DNS"), although those techniques could be
combined with anycast service distribution if an application required
it. The routing system makes the decision of the node to be used for
each request, based on the topological design of the routing system
and the point in the network at which the request originates.
The Anycast Node chosen to service a particular query can be
influenced by the traffic engineering capabilities of the routing
protocols which make up the routing system. The degree of influence
available to the operator of the node depends on the scale of the
routing system within which the Service Address is anycast.
Load-balancing between Anycast Nodes is typically difficult to
achieve (load distribution between nodes is generally unbalanced in
terms of request and traffic load). Distribution of load between
nodes for the purposes of reliability, and coarse-grained
distribution of load for the purposes of making popular services
scalable can often be accommodated, however.
The scale of the routing system through which a service is anycast
can vary from a small Interior Gateway Protocol (IGP) connecting a
small handful of components, to the Border Gateway Protocol (BGP) [5]
connecting the global Internet, depending on the nature of the
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service distribution that is required.
3.2 Goals
A service may be anycast for a variety of reasons. A number of
common objectives are:
1. Coarse ("unbalanced") distribution of load across nodes, to allow
infrastructure to scale to increased numbers of queries and to
accommodate transient query peaks;
2. Mitigation of non-distributed denial of service attacks by
localizing damage to single anycast nodes;
3. Constraint of distributed denial of service attacks or flash
crowds to local regions around anycast nodes (perhaps restricting
query traffic to local peering links, rather than paid transit
circuits);
4. Triangulation of traffic sources, in the case of attack (or
query) traffic which incorporates spoofed source addresses;
5. Improvement of query response time, by reducing the network RTT
between client and server with the provision of a local Anycast
Node.
6. Reduction of a list of servers to a single, distributed address.
For example, a large number of authoritative nameservers for a
zone may be deployed using a small set of anycast service
addresses; this approach can increase the accessibility of zone
data in the DNS without increasing the size of a referral
response from a parent nameserver.
4. Design
4.1 Protocol Suitability
When a service is anycast between two or more nodes, the routing
system effectively makes the node selection decision on behalf of a
client. Since it is usually a requirement that a single
client-server interaction is carried out between a client the same
server node for the duration of the transaction, it follows that the
routing system's node selection decision ought to be stable for an
order of magnitude longer than the expected transaction time, if the
service is to be provided reliably.
Some services have very short transaction times, and may even be
carried out using a single packet request and a single packet reply
in some cases (the DNS is an example of this). Other services
involve far longer-lived transactions (e.g. bulk file downloads and
audio-visual media streaming).
Some anycast deployments have very predictable routing systems, which
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can remain stable for long periods of time (e.g. anycast within an
IGP, where node selection changes only occur as a response to node
failures). Other deployments have far less predictable
characteristics (e.g. a densely-deployed array of nodes across the
global Internet).
The stability of the routing system together with the transaction
time of the service should be carefully compared when deciding
whether a service is suitable for distribution using anycast.
4.2 Node Placement
Decisions as to where Anycast Nodes should be placed will depend to a
large extent on the goals of the service distribution. For example:
o A recursive resolver service might be distributed within an ISP's
network, one Anycast Node per PoP.
o A root server service might be distributed throughout the Internet
with nodes located in regions with poor external connectivity, to
ensure that the DNS functions adequately within the region during
times of external network failure.
o An FTP mirror service might include local nodes located at
exchange points, so that ISPs connected to that exchange point
could download bulk data more cheaply than if they had to use
expensive transit circuits.
In general node placement decisions should be made with consideration
of likely traffic requirements, the potential for flash crowds or
denial-of-service traffic, the stability of the local routing system
and the failure modes with respect to node failure, or local routing
system failure.
4.3 Routing Systems
4.3.1 Anycast within an IGP
There are several common motivations for the distribution of a
Service Address within the scope of an IGP:
1. to improve service response times, by hosting a service close to
other users of the network;
2. to improve service reliability by providing automatic fail-over
to backup nodes; and
3. to keep service traffic local, to avoid congesting wide-area
links.
In each case the decisions as to where and how services are
provisioned can be made by network engineers without requiring such
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operational complexities as regional variances in the configuration
of client computers, or DNS tricks which respond differently to
requests from clients in different locations.
When a service is anycast within an IGP the routing system is
typically under the control of the same organization who is providing
the service, and hence the relationship between service transaction
characteristics and network stability are likely to be relatively
well-understood. This technique is consequently applicable to a
larger number of applications than Internet-wide anycast service
distribution (see Section 4.1).
By reducing the scope of the IGP to just the hosts providing service
(together with one or more gateway routers) this technique can be
applied to the construction of server clusters. This application is
discussed in some detail in [13].
4.3.2 Anycast within the Global Internet
Service Addresses may be anycast within the global Internet routing
system in order to distribute services across the entire network.
The principal differences between this application and the IGP-scope
distribution discussed in Section 4.3.1 are that:
1. the routing system is, in general, controlled by other people;
and
2. the routing protocol concerned (BGP), and commonly-accepted
practices in its deployment, impose some additional constraints
(see Section 4.4).
4.4 Routing Considerations
4.4.1 Signalling Service Availability
When a routing system is provided with reachability information for a
Service Address from an individual node, packets addressed to that
Service Address will start to arrive at the node. Since it is
desirable for the node to be ready to accept requests before they
start to arrive, a coupling between the routing information and the
availability of the service at a particular node is desirable.
Where a routing advertisement from a node corresponds to a single
Service Address, this coupling might be such that availability of the
service triggers the route advertisement, and non-availability of the
service triggers a route withdrawal. This can be achieved using
routing protocol implementations on the same servers which provide
the service being distributed.
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Where a routing advertisement from a node corresponds to two or more
Service Addresses, it may not be appropriate to trigger a route
withdrawal due to the non-availability of a single service. Another
approach is to tunnel requests from nodes that cannot handle
individual services to other nodes that can, perhaps using an IGP
which extends over tunnels between nodes, in which servers
participate. Circumstances which might lead to multiple Service
Addresses being covered by a single route are discussed in Section
4.4.2.
4.4.2 Covering Prefix
In some routing systems (e.g. the BGP-based routing system of the
global Internet) it is not possible, in general, to propagate a host
route with confidence that availability of the route will be signaled
throughout the network. This is a consequence of operational policy,
and not a protocol restriction.
In such cases it is necessary to propagate a route which covers the
Service Address, and which has a sufficiently short prefix that it
will not be caught by commonly-deployed import policies. In many
cases this will be a 24-bit prefix, but there are other
well-documented examples of import polices which filter on RIR
allocation boundaries, and hence some experimentation may be prudent.
Where multiple Service Addresses are covered by the same covering
route, there is no longer a tight coupling between the advertisement
of that route and the individual services associated with the covered
host routes. The resulting impact on signaling availability of
individual services is discussed in Section 4.4.1.
4.4.3 Equal-Cost Paths
Some routing systems support equal-cost paths to the same
destination. Where multiple, equal-cost paths exist and lead to
different anycast nodes, there is a risk that request packets
associated with a single transaction might be delivered to more than
one node. Services provided over TCP necessarily involve
transactions with multiple request packets, due to the TCP setup
handshake.
Equal cost paths are commonly supported in IGPs. Multi-node
selection for a single transaction can be avoided in most cases by
careful consideration of IGP link metrics, or by applying equal-cost
multi-path (ECMP) selection algorithms which cause a single node to
be selected for a single multi-packet transaction. For a description
of hash-based ECMP selection, see [13].
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For services which are distributed across the global Internet using
BGP, equal-cost paths are normally not a consideration: BGP's exit
selection algorithm usually selects a single, consistent exit for a
single destination regardless of whether multiple candidate paths
exist. Implementations of BGP exist that support multi-path exit
selection, however, and corner cases where dual selected exits route
to different nodes are possible. Analysis of the likely incidence of
such corner cases for particular distributions of Anycast Nodes are
recommended for services which involve multi-packet transactions.
4.4.4 Route Dampening
Frequent advertisements and withdrawals of individual prefixes in BGP
are known as flaps. Rapid flapping can lead to CPU exhaustion on
routers quite remote from the source of the instability, and for this
reason rapid route oscillations are frequently "damped", as described
in [9].
A dampened path will be suppressed by routers for an interval which
increases according to the frequency of the observed oscillation; a
suppressed path will not propagate. Hence a single router can
prevent the propagation of a flapping prefix to the rest of an
autonomous system, affording other routers in the network protection
from the instability.
Common implementations of flap dampening penalizes oscillating
advertisements based on the observed AS_PATH, and not on the NLRI.
For this reason, network instability which leads to route flapping
from a single anycast node ought not to cause advertisements from
other nodes (which have different AS_PATH attributes) to be dampened.
As dampening works on advertisements with the same AS_PATH attribute,
care should be taken so that the AS_PATH is as diverse as possible
for the anycasted nodes. The Anycasted nodes should have the same
origin AS for their advertisements, but they should have different
upstream AS:es for each node. If the upstream AS is the same at all
locations, there is a risk that the upstream AS will peer with the
AS:es at multiple locations and could therefor propagate the same
AS_PATH, but for different Anycast nodes. This could render the
service for multiple Anycast nodes unavailable due to dampening
caused by only one of them.
It is possible that other implementations of flap dampening may
become prevalent in the future, causing individual nodes' instability
to result in stable nodes becoming unavailable. Judicious deployment
of Local Nodes in combination with especially stable Global Nodes
may help mitigate such problems, should they ever arise.
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4.4.5 Reverse Path Forwarding Checks
Reverse Path Forwarding (RPF) checks, first described in [8], are
commonly deployed as part of ingress interface packet filters on
routers in the global Internet in order to deny packets whose source
addresses are spoofed (see also [10]). Deployed implementations of
RPF make available two modes of operation: a loose mode, and a strict
mode.
Strict-mode RPF checks can cause non-spoofed packets to be denied
when they originate from multi-homed site, since selected paths might
legitimately not correspond with the ingress interface of non-spoofed
packets from the multi-homed site. A collection of anycast nodes
deployed across the Internet is largely indistinguishable from a
distributed, multi-homed site to the routing system, and hence this
risk also exists for anycast nodes, even if individual nodes are not
multi-homed.
Care should be taken to ensure that strict-mode RPF is not enabled in
peer networks connecting to anycast nodes.
4.4.6 Propagation Scope
In the context of Anycast service distribution across the global
Internet, Global Nodes are those which are capable of providing
service to clients anywhere in the network; reachability information
for the service is propagated globally, without restriction, by
advertising the routes covering the Service Addresses for global
transit to one or more providers.
More than one Global Node can exist for a single service (and indeed
this is often the case, for reasons of redundancy and load-sharing).
In contrast, it is sometimes desirable to deploy an Anycast Node
which only provides services to a local catchment of autonomous
systems, and which is deliberately not available to the entire
Internet; such nodes are referred to in this document as Local Nodes.
An example of circumstances in which a Local Node may be appropriate
are nodes designed to serve a region with rich internal connectivity
but unreliable, congested or expensive access to the rest of the
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 [6] or
NOPEER [11], or by arranging with peers to apply a conventional
"peering" import policy instead of a "transit" import policy, or some
suitable combination of measures.
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4.4.7 Other Peoples' Networks
When Anycast services are deployed across networks operated by
others, their reachability is dependent on routing polices and
topology changes (planned and unplanned) which are unpredictable and
sometimes difficult to identify. Consequently, routing policies used
by Anycast Nodes should be conservative, individual nodes' internal
and external/connecting infrastructure should be scaled to support
loads far in excess of the average, and the service should be
monitored proactively (Section 5.1) from many points in order to
avoid unpleasant surprises.
4.5 Data Synchronisation
As a client contacting a anycasted service will expect all possible
servers to serve the same data, the Anycast service needs to assure
data consistency across all Anycast Nodes. This includes periodic
updating of all data, and verification of a successful transfer of
data.
How data is synchronized depends on the service being Anycasted. The
methods used could for example be a zone transfer for an
authoritative set of DNS-servers, rsync for a FTP archive or no
synchronization needed for a DNS resolver service. In the DNS
examples, synchronization comes with the service and the associated
protocol. For other services, this will be an external mechanism to
the protocol. In both cases, the synchronization needs to be run
from a local IP address that is not the service address. The data
transfer should be authenticated in order to prevent spoofing of the
data on the Anycasted nodes and the data should be verified.
Verification can be done with for example TSIG for DNS, or for
example a MD5 hash[2] for verification of other data. The method
might vary but should verify that all data was transfered, and that
the data is correct and not manipulated.
Authentication of the data source can be based either on the protocol
in use, as is the case with TSIG for DNS, or some other external
mechanism. For example a IP tunnel protected by authentication and
encryption as described in [7].
4.6 Node Autonomy
For an Anycast deployment whose goals include improved reliability
through redundancy, it is important to minimize the opportunity for a
single defect to compromise many (or all) nodes, or for the failure
of one node to provide a cascading failure bringing down additional
successive nodes until the service as a whole is defeated.
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Codependencies are avoided by making each node as autonomous and
self-sufficient as possible. The degree to which nodes can survive
failure elsewhere depends on the nature of the service being
delivered, but for services which accommodate disconnected operation
(e.g. the timed propagation of changes between master and slave
servers in the DNS) a high degree of autonomy can be achieved.
The possibility of cascading failure due to load can also be reduced
by the deployment of both Global and Local Nodes for a single
service, since the effective fail-over path of traffic is, in
general, from Local Node to Global Node; traffic that might sink one
Local Node is unlikely to sink all Local Nodes, except in the most
degenerate cases.
The chance of cascading failure due to a software defect in an
operating system or server can be reduced in many cases by deploying
nodes running different software implementations.
5. Service Management
5.1 Monitoring
Monitoring a service which is distributed is more complex than
monitoring a non-distributed service, since the observed accuracy and
availability of the service is, in general, different when viewed
from clients attached to different parts of the network. When a
problem is identified, it is also not always obvious which node
served the request, and hence which node is malfunctioning.
It is recommended that distributed services are monitored from probes
distributed representatively across the routing system, and, where
possible, the identity of the node answering individual requests is
recorded along with performance and availability statistics.
Monitoring the routing system (from a variety of places, in the case
of routing systems where perspective counts) can also provide useful
diagnostics for troubleshooting service availability. This can be
achieved using dedicated probes, or public route measurement
facilities on the Internet such as RIPE's Routing
Information Service [14] and the University of
Oregon Route
Views Project [15].
5.2 Self-Healing Nodes
As is described in having the Anycast Node avoid black-holing
traffic in the event of a failure on the software or subsystem
providing the service should be avoided. As described, this can be
done with withdrawing the announcement of the prefix corresponding to
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the service address, or the covering route. However, the nodes could
also try and handle the failure in a number of ways. This can be
with as also previously described tunneling to other instances of the
Anycasted service, and using a IGP over the tunnels, route incoming
client queries to the other destination. The Anycasted node could
also contain separate systems for trying to restart the service in
question, and if successful again re-announce the service prefix.
6. Security Considerations
This document describes mechanisms for deploying services on the
Internet which can be used to mitigate vulnerability to attack.
The distribution of a service across several (or many) autonomous
nodes imposes an increased monitoring load on the operator of the
service, and which also imposes an additional systems administration
load on the service operator which might reduce the effectiveness of
host and router security. It is recommended that these factors be
taken into account when assessing the risks and benefits of
distributing services using anycast.
7. Protocol Considerations
This document does not impose any protocol considerations.
8. IANA Considerations
This document requests no action from IANA.
9 References
[1] Oran, D., "OSI IS-IS Intra-domain Routing Protocol", RFC 1142,
February 1990.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[3] Moy, J., "OSPF Version 2", RFC 1247, July 1991.
[4] Partridge, C., Mendez, T. and W. Milliken, "Host Anycasting
Service", RFC 1546, November 1993.
[5] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[6] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
Attribute", RFC 1997, August 1996.
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[7] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[8] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2267, January 1998.
[9] Villamizar, C., Chandra, R. and R. Govindan, "BGP Route Flap
Damping", RFC 2439, November 1998.
[10] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[11] Huston, G., "NOPEER Community for Border Gateway Protocol (BGP)
Route Scope Control", RFC 3765, April 2004.
[12] Abley, J., "Hierarchical Anycast for Global Service
Distribution", March 2003,
<http://www.isc.org/pubs/tn/isc-tn-2003-1.html>.
[13] 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>.
[14] <http://ris.ripe.net>
[15] <http://www.route-views.org>
Authors' Addresses
Kurt Erik Lindqvist
Netnod Internet Exchange
Bellmansgatan 30
118 47 Stockholm
Sweden
EMail: kurtis@kurtis.pp.se
URI: http://www.netnod.se/
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Joe Abley
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 423 1317
EMail: jabley@isc.org
URI: http://www.isc.org/
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Lindqvist & Abley Expires April 22, 2005 [Page 16]