DNSOP Working Group G. Moura
Internet-Draft SIDN Labs/TU Delft
Intended status: Informational W. Hardaker
Expires: June 1, 2019 J. Heidemann
USC/Information Sciences Institute
M. Davids
SIDN Labs
November 28, 2018
Recommendations for Authoritative Servers Operators
draft-moura-dnsop-authoritative-recommendations-00
Abstract
This document summarizes recent research work exploring DNS
configurations and offers specific, tangible recommendations to
operators for configuring authoritative servers.
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. R1: All authoritative server should have similar latency . . 4
3. R2: Routing Can Matter More Than Locations . . . . . . . . . 5
4. R3: Collecting Detailed Anycast Catchment Maps Ahead of
Actual Deployment Can Improve Engineering Designs . . . . . . 6
5. R4: When under stress, employ two strategies . . . . . . . . 7
6. R5: Be careful on how to choose your records time-to-live
values . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. R6: Shared Infrastructure Risks Collateral Damage During
Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Security considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The domain name system (DNS) has main two types of DNS servers:
authoritative servers and recursive resolvers. Figure 1 shows their
relationship. An authoritative server knows the content of a DNS
zone from local knowledge, and thus can answer queries about that
zone needing to query other servers [RFC2181]. A recursive resolver
is a program that extracts information from name servers in response
to client requests [RFC1034]. A client, in Figure 1, is shown as
stub, which is shorthand for stub resolver [RFC1034] that is
typically located within the client software.
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+-----+ +-----+ +-----+ +-----+
| AT1 | | AT2 | | AT3 | | AT4 |
+--+--+ +--+--+ +---+-+ +--+--+
^ ^ ^ ^
| | | |
| +--+--+ | |
+------+ Rn +-------+ |
| +--^--+ |
| | |
| +--+--+ +-----+ |
+------+R1_1 | |R1_2 +------+
+-+---+ +----+
^ ^
| |
| +------+ |
+-+ stub +--+
+------+
Figure 1: Relationship between recursive resolvers (R) and
authoritative name servers (AT)
DNS queries contribute to web latency and affect user experience
[Sigla2014], and the DNS system has been subject to repeated Denial
of Service (DoS) attacks (for example, in November 2015 [Moura16b])
in order to degrade user experience. To reduce latency and improve
resiliency against DoS attacks, DNS uses several types of server
replication. Replication at the authoritative server level can be
achieved with the deployment of multiple servers for the same zone
[RFC1035] (AT1--AT4 in Figure 1), the use of IP anycast
[RFC1546][RFC7094] and by using load balancers to support multiple
servers inside a single (potentially anycasted) site. As a
consequence, there are many possible ways a DNS provider can engineer
its production authoritative server network, with multiple viable
choices and no single optimal design.
This document summarizes recent research work exploring DNS
configurations and offers specific tangible recommendations to DNS
authoritative servers operators (DNS operators hereafter). It
presents recommendations derived from multiple studies with the goal
of improving DNS engineering, to promote understanding about how
anycast reacts to DoS attacks[Moura16b], how anycast affects query
latency[Schmidt17a], how to accurately map anycast network reanch
[Vries17b], how recursive and authoritative resolvers
interact[Mueller17b], and how recursive resolver caching and retries
help clients during DDoS attacks on authoritatives[Moura18b]. The
recommendations (R1-R6) presented in this document are backed by
these studies, which used wide-scale Internet measurements upon which
to draw their conclusions. This document describes the key
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engineering options, and points readers to the pertinent papers for
details.
2. R1: All authoritative server should have similar latency
Authoritative DNS servers operators, such as Top-level domain (TLD)
operators (e.g.,: .org and .nl), announce their authoritative servers
in the form of Name Server (NS) records. Different authoritatives
should return the same content, typically by staying synchronized
using DNS zone transfers (AXFR[RFC5936] and IXFR[RFC1995]) to
coordinate the authoritative zone data to return to their clients.
DNS heavily relies upon replication to support high reliability,
capacity and to reduce latency [Moura16b]. DNS has two complementary
mechanisms to replicate the service. First, the protocol itself
supports nameserver replication of DNS service for a DNS zone through
the use of multiple nameservers that each operate on different IP
addresses, listed by a zone's NS records. Second, each of these
network addresses can run from multiple physical locations through
the use of IP anycast[RFC1546], by announcing the same IP address
from each site and allowing Internet routing (BGP[RFC4271]) to
associate clients with their topologically nearest anycast site.
Outside the DNS protocol, replication can be achieved by deploying
load balancers at each physical location. Nameserver replication is
recommended for all zones, and IP anycast is used by most large zones
such as the DNS Root, most top-level domains[Moura16b] and large
commercial enterprises, governments and other organizations.
Most DNS operators strive to reduce latency for users of their
service. However, because they control only their authoritative
servers, and not the recursive resolvers communicating with those
servers, it is difficult to ensure that recursives will be served by
the closest authoritative server. Server selection is up to the
recursive resolver's software implementation, and different software
vendors and releases employ different criteria to chose which
authoritative servers with which to communicate.
Knowing how recursives choose authoritative servers is a key step to
better engineer the deployment of authoritative servers.
[Mueller17b] evaluates this with a measurement study in which they
deployed seven unicast authoritative name servers in different global
locations and queried these authoritative servers from more than
9,000 RIPE Atlas probes (Vantage Points--VPs) and their respective
recursive resolvers.
In the wild, [Mueller17b] found that recursives query all available
authoritative servers, regardless of latency. But the distribution
of queries tend to be skewed towards authoritatives with lower
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latency: the lower the latency between a recursive resolver and an
authoritative server, the more often the recursive will send queries
to that authoritative. Our hypothesis is that this behavior is a
consequence of two main criteria employed by resolvers when choosing
authoritatives: performance (lower latency) and diversity of
authoritatives, where a resolver checks all recursives to determine
which is closer and to provide alternatives if one is unavailable.
For a DNS operator, this policy means that latency of all
authoritatives matter, so all must be similarly capable, since all
available authoritatives will be queried by most recursives. Since
unicast cannot deliver good latency worldwide (a site in Europe will
always have a high latency to resolvers in California, for example),
[Mueller17b] recommends to DNS operators that they deploy equally
strong IP anycast in every authoritative server (and thus to phase
out unicast), so they can deliver similar {xxx: I don't think similar
is the right word here. "good" or "best possible"? - Wes} latency
values to recursives. Having one or few unicast authoritative will
limit the worst-case latency for most users {xxx: I don't understand
what this sentence is trying to bring up as a point but can't fix it
without potentially changing the meaning in a way that wasn't
intended - Wes}. Note that DNS operators should also take
architectural considerations into account when planning for deploying
anycast [RFC1546].
This recommendation was deployed at the ".nl" TLD zone, which
originally had a mixed unicast/anycast setup; since early 2018 it now
has 4 anycast authoritative name servers.
3. R2: Routing Can Matter More Than Locations
A common metric when choosing an anycast DNS provider or setting up
an anycast service is the number of anycast sites, i.e., the number
of global locations from which the same address is announced with
BGP. Intuitively, one could think that more sites will lead to
shorter response times.
However, this is not necessarily true. In fact, [Schmidt17a] found
that routing can matter more than the total number of locations.
They analyzed the relationship between the number of anycast sites
and the performance of a service (latency-wise, RTT) and measured the
overall performance of four DNS Root servers, namely C, F, K and L,
from more than 7.9K RIPE Atlas probes.
[Schmidt17a] found that C-Root, a smaller anycast deployment
consisting of only 8 sites, provided a very similar overall
performance than that of the much larger deployments of K and L, with
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33 and 144 sites respectively. A median RTT was measured between
30ms and 32ms for C, K and L roots, and 25ms for F.
Their recommendation for DNS operators when engineering anycast
services is consider factors other than just the number of sites
(such as local routing connectivity) when designing for performance.
They showed that 12 sites can provide reasonable latency, given they
are globally distributed and have good local interconnectivity.
However, more sites can be useful for other reasons, such as when
handling DoS attacks [Mueller17b].
4. R3: Collecting Detailed Anycast Catchment Maps Ahead of Actual
Deployment Can Improve Engineering Designs
An anycast DNS service may have several dozens or even hundreds sites
(such as L-Root does). Anycast leverages Internet routing to
distribute the incoming queries to a service's distributed anycast
sites; in theory, BGP (the Internet's defacto routing protocol)
forwards incoming queries to a nearby anycast site (in terms of BGP
distance). However, usually queries are not evenly distributed
across all anycast sites, as found in the case of L-Root
[IcannHedge18].
Adding new sites to an anycast service may change the load
distribution across all sites, leading to suboptimal usage of the
service or even stressing some sites while others remain
underutilized. This is a scenario that operators constantly face
when expanding an anycast service. Besides, when setting up a new
anycast service instance, operators cannot directly estimate the
query distribution among the sites in advance of enabling the site.
To estimate the query loads across sites of an expanding service or a
when setting up an entirely new service, operators need detailed
anycast maps and catchment estimates (i.e., operators need to know
which prefixes will be matched to which anycast site). To do that,
[Vries17b] developed a new technique enabling operators to carry out
active measurements, using a technique and tool called Verfploeter.
Verfploeter maps a large portion of the IPv4 address space, allowing
DNS operators to predict both query distribution and clients
catchment before deploying new anycast sites.
[Vries17b] shows how this technique was used to predict both the
catchment and query load distribution for the new anycast service of
B-Root. Using two anycast sites in Miami (MIA) and Los Angeles (LAX)
from the operational B-Root server, they sent ICMP echo packets to IP
addresses from each IPv4 /24 in on the Internet using a source
address within the anycast prefix. Then, they recorded which site
the ICMP echo replies arrived at based on the Internet's BGP routing.
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This analysis resulted in an Internet wide catchment map. Weighting
was then applied to the incoming traffic prefixes based on of 1 day
of B-Root traffic (2017-04-12, DITL datasets [Ditl17]). The
combination of the created catchment mapping and the load per prefix
created an estimate predicting that 81.6% of the traffic would go to
the LAX site. The actual value was 81.4% of traffic going to LAX,
showing that the estimation was pretty close and the Verfploeter
technique was a excellent method of predicting traffic loads in
advance of a new anycast instance deployment.
Besides that, Verfploeter can also be used to estimate how traffic
shifts among sites when BGP manipulations are executed, such as AS
Path prepending that is frequently used by production networks during
DDoS attacks. A new catchment mapping for each prepending
configuration configuration: no prepending, and prepending with 1, 2
or 3 hops at each site. Then, [Vries17b] shows that this mapping can
accurately estimate the load distribution for each configuration.
An important operational takeaway from [Vries17b] is that DNS
operators can make informed choices when engineering new anycast
sites or when expending new ones by carrying out active measurements
using Verfploeter in advance of operationally enabling the fully
anycast service. Operators can spot sub-optimal routing situations
early, with a fine granularity, and with significantly better
coverage than using traditional measurement platforms such as RIPE
Atlas.
Deploying a small test Verfploeter-enabled platform in advance at a
potential anycast site may reveal the realizable benefits of using
that site as an anycast interest, potentially saving significant
financial and labor costs of deploying hardware to a new site that
was less effective than as had been hoped.
5. R4: When under stress, employ two strategies
DDoS attacks are becoming bigger, cheaper, and more frequent
[Mueller17b]. The most powerful recorded DDoS attack to DNS servers
to date reached 1.2 Tbps, by using IoT devices [Perlroth16]. Such
attacks call for an answer for the following question: how should a
DNS operator engineer its anycast authoritative DNS server react to
the stress of a DDoS attack? This question is investigated in study
[Moura16b] in which empirical observations are grounded with the
following theoretical evaluation of options.
An authoritative DNS server deployed using anycast will have many
server instances distributed over many networks and sites.
Ultimately, the relationship between the DNS provider's network and a
client's ISP will determine which anycast site will answer for
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queries for a given client. As a consequence, when an anycast
authoritative server is under attack, the load that each anycast site
receives is likely to be unevenly distributed (a function of the
source of the attacks), thus some sites may be more overloaded than
others which is what was observed analyzing the Root DNS events of
Nov. 2015 [Moura16b]. Given the fact that different sites may have
different capacity (bandwidth, CPU, etc.), making a decision about
how to react to stress becomes even more difficult.
In practice, an anycast site under stress, overloaded with incoming
traffic, has two options:
o It can withdraw or pre-prepend its route to some or to all of its
neighbors, shrinking its catchment (the number of clients that BGP
maps to it), shifting both legitimate and attack traffic to other
anycast sites. The other sites will hopefully have greater
capacity and be able to service the queries.
o Alternatively, it can be become a degraded absorber, continuing to
operate, but with overloaded ingress routers, dropping some
incoming legitimate requests due to queue overflow. However,
continued operation will also absorb traffic from attackers in its
catchment, protecting the other anycast sites.
[Moura16b] saw both of these behaviors in practice in the Root DNS
events, observed through site reachability and RTTs. These options
represent different uses of an anycast deployment. The withdrawal
strategy causes anycast to respond as a waterbed, with stress
displacing queries from one site to others. The absorption strategy
behaves as a conventional mattress, compressing under load, with some
queries getting delayed or dropped.
Although described as strategies and policies, these outcomes are the
result of several factors: the combination of operator and host ISP
routing policies, routing implementations withdrawing under load, the
nature of the attack, and the locations of the sites and the
attackers. Some policies are explicit, such as the choice of local-
only anycast sites, or operators removing a site for maintenance or
modifying routing to manage load. However, under stress, the choices
of withdrawal and absorption can also be results that emerge from a
mix of explicit choices and implementation details, such as BGP
timeout values.
[Moura16b] speculates that more careful, explicit, and automated
management of policies may provide stronger defenses to overload, an
area currently under study. For DNS operators, that means that
besides traditional filtering, two other options are available
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(withdraw/prepend or isolate sites), and the best choice depends on
the specifics of the attack.
6. R5: Be careful on how to choose your records time-to-live values
In a DNS response, each resource record is accompanied by a time-to-
live value (TTL), which "describes how long a RR can be cached before
it should be discarded" [RFC1034]. The TTL values are set by zone
owners in their zone files - either specifically per record or by
using default values for the entire zone. Sometimes the same
resource record may have different TTL values - one from the parent
and one from the child DNS server. In this cases, resolvers are
expected to prioritize the answer according to Section 5.4.1 in
[RFC2181].
While set by authoritative server operators (labeled "AT"s in
Figure 1), the TTL value in fact influences the behavior of recursive
resolvers (and their operators - "Rn" in the same figure), by setting
an upper limit on how long a record should be cached before
discarded. In this sense, caching can be seen as a sort of
"ephemeral replication", i.e., the contents of an authoritative
server are placed at a recursive resolver cache for a period of time
up to the TTL value. Caching improves response times (by avoiding
repeated queries between recursive resolvers and authoritative).
Besides improving performance, caching may play a significant role
during DoS attacks against authoritative servers. To investigate
that, [Moura18b] evaluates the role of caching (and retries) in DNS
resiliency to DDoS attacks. Two authoritative servers were
configured for a newly registered domain and a series of experiments
were carried out using various TTL values (60,1800, 3600, 86400s) for
records. Unique DNS queries were sent from roughly 15,000 vantage
points, using RIPE Atlas.
[Moura18b] found that caching, in the wild, works as expected 70% of
the times - for various TTL values. It is believe that Complex
recursive infrastructure (such as anycast recursives with fragmented
cache), besides cache flushing and hierarchy explains these other 30%
of the non-cached records. The results from the experiments were
confirmed by analyzing authoritative traffic for the .nl TLD, which
showed similar figures.
DDoS attacks on authoritative servers were emulated by dropping all
incoming pakcets for various TTLs values. The results showed:
o When 100% of requests were dropped, the TTL value of the record
set by the zone owner determined how long clients received
responses, together with the status of the cache at the attack
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time. Given the TTL value decreases as time passes at the cache,
it protected clients for up to its value in cache.
* Once the TTL values expired, there was some evidence of some
recursives serving stale content
[I-D.ietf-dnsop-terminology-bis]. Serving stale is the only
viable option when TTL values expire in recursive caches and
authoritative servers became completely unavailable.
Partial-failure DDoS failures were also emulated (similar to Dyn 2016
[Perlroth16]), simulating when authoritative are partially available,
by dropping packet at rates of 50-90%, for various TTL values. The
results showed:
o For various TTL values, caching was a key component in the success
of queries. For example, with a 50% packet drop rate at the
authoritatives, most clients eventually got an answer.
o When caching could not help (for a scenario with TTL of 60s, and
time in between probing of 10 minutes), recursive servers kept
retrying queries to authoritatives: at 90% packet drop with TTL of
60s, 27% of clients still got an answer, at the price of increased
response times.
o The study also showed that these retries have a significant effect
on the authoriative side: A 8.1x times increase was seen in normal
traffic during a 90% packet drop with TTL of 60s, as recursives
attempt to resolve queries - thus effectively creating "friendly
fire".
Therefore, given the important role of the TTL, it is recommended
that DNS zone owners set their TTL values carefully, knowing that
they will influence (i) the success of client's queries and (ii) the
amount of "friendly fire" traffic they will receive. Many operators
may as well reconsider their 10x overprovision metric for DNS
servers, given this significant increase legitimate traffic during
DDoS.
XXX: WJH: I don't understand what that last sentence is trying to
suggest. Specifically "reconsider their 10x overprovision metric".
Is it suggested that it doesn't need to be that high if the TTL
values are chosen more carefully? We should state that specifically,
if that's the case as it's hard to derive from that sentence. Or is
it trying to say it needs to be higher than 10x with short TTLs? Or
both!
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7. R6: Shared Infrastructure Risks Collateral Damage During Attacks
Co-locating services, such as authoritative servers, creates some
degree of shared risk, in that stress on one service may spill over
into another, resulting in collateral damage. Collateral damage is a
common side-effect of DDoS, and data centers and operators strive to
minimize collateral damage through redundancy, overcapacity, and
isolation.
This has been seen in practice during the DDoS attack against the
Root DNS system in November 2015 [Moura16b]. In this study, it was
shown that two services not directly targeted by the attack, namely
D-Root and the .nl TLD, suffered collateral damage. These services
showed reduced end-to-end performance (i.e., higher latency and
reduced reachability) with timing consistent with the DDoS event,
strongly suggesting a shared resource with original targets of the
attack.
Another example of collateral damage was the 1.2 Tbps attack against
Dyn, a major DNS provider on October 2017 [Perlroth16]. As a result,
many of their customers, including Airbnb, HBO, Netflix, and Twitter
experienced issues with clients failing to resolve their domains,
since the servers partially shared the same infrastructure.
It is recommended, therefore, when choosing third-party DNS
providers, operators should be aware of shared infrastructure risks.
By sharing infrastructure, there is an increased attack surface.
8. Security considerations
o to be added
9. IANA considerations
This document has no IANA actions.
10. Acknowledgements
This document is a summary of the main lessons of the research works
mentioned on each recommendation here provided. As such, each author
of each paper has a clear contribution. Here we mention the papers
co-authors and thank them for their work: Ricardo de O Schmidt,
Wouter B de Vries, Moritz Mueller, Lan Wei, Cristian Hesselman, Jan
Harm Kuipers, Pieter-Tjerk de Boer and Aiko Pras.
Besides those, we would like thank those who have been individually
thanked in each research work, RIPE NCC and DNS OARC for their tools
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and datasets used in this research, as well as the funding agencies
sponsoring the individual research works.
11. References
11.1. Normative References
[I-D.ietf-dnsop-terminology-bis]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-ietf-dnsop-terminology-bis-14 (work in
progress), September 2018.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1546] Partridge, C., Mendez, T., and W. Milliken, "Host
Anycasting Service", RFC 1546, DOI 10.17487/RFC1546,
November 1993, <https://www.rfc-editor.org/info/rfc1546>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<https://www.rfc-editor.org/info/rfc1995>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>.
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11.2. Informative References
[Ditl17] OARC, D., "2017 DITL data", October 2018,
<https://www.dns-oarc.net/oarc/data/ditl/2017>.
[IcannHedge18]
ICANN, ., "DNS-STATS - Hedgehog 2.4.1", October 2018,
<http://stats.dns.icann.org/hedgehog/>.
[Moura16b]
Moura, G., Schmidt, R., Heidemann, J., Vries, W., Mueller,
M., Wei, L., and C. Hesselman, "Anycast vs DDoS Evaluating
the November 2015 Root DNS Events.", ACM 2016 Internet
Measurement Conference, DOI /10.1145/2987443.2987446,
October 2016,
<https://www.isi.edu/~johnh/PAPERS/Moura16b.pdf>.
[Moura18b]
Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M.
Davids, "When the Dike Breaks: Dissecting DNS Defenses
During DDos", ACM 2018 Internet Measurement Conference,
DOI 10.1145/3278532.3278534, October 2018,
<https://www.isi.edu/~johnh/PAPERS/Moura18b.pdf>.
[Mueller17b]
Mueller, M., Moura, G., Schmidt, R., and J. Heidemann,
"Recursives in the Wild- Engineering Authoritative DNS
Servers.", ACM 2017 Internet Measurement Conference,
DOI 10.1145/3131365.3131366, October 2017,
<https://www.isi.edu/%7ejohnh/PAPERS/Mueller17b.pdf>.
[Perlroth16]
Perlroth, N., "Hackers Used New Weapons to Disrupt Major
Websites Across U.S.", October 2016,
<https://www.nytimes.com/2016/10/22/business/
internet-problems-attack.html>.
[Schmidt17a]
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[Vries17b]
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Authors' Addresses
Giovane C. M. Moura
SIDN Labs/TU Delft
Meander 501
Arnhem 6825 MD
The Netherlands
Phone: +31 26 352 5500
Email: giovane.moura@sidn.nl
Wes Hardaker
USC/Information Sciences Institute
PO Box 382
Davis 95617-0382
U.S.A.
Phone: +1 (530) 404-0099
Email: ietf@hardakers.net
John Heidemann
USC/Information Sciences Institute
4676 Admiralty Way
Marina Del Rey 90292-6695
U.S.A.
Phone: +1 (310) 448-8708
Email: johnh@isi.edu
Moura, et al. Expires June 1, 2019 [Page 14]
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Marco Davids
SIDN Labs
Meander 501
Arnhem 6825 MD
The Netherlands
Phone: +31 26 352 5500
Email: marco.davids@sidn.nl
Moura, et al. Expires June 1, 2019 [Page 15]