Recommendations for Authoritative Servers Operators
draft-moura-dnsop-authoritative-recommendations-01

DNSOP Working Group                                             G. Moura
Internet-Draft                                        SIDN Labs/TU Delft
Intended status: Informational                               W. Hardaker
Expires: June 23, 2019                                      J. Heidemann
                                      USC/Information Sciences Institute
                                                               M. Davids
                                                               SIDN Labs
                                                       December 20, 2018

          Recommendations for Authoritative Servers Operators
           draft-moura-dnsop-authoritative-recommendations-01

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.

Ed note

   Text inside square brackets ([RF:ABC]) refers to individual comments
   we have received about the draft, and enumerated under
   <https://github.com/gmmoura/draft-moura-dnsop-authoritative-
   recommendations/blob/master/reviews/reviews-dnsop.md>.  They will be
   removed before publication.

   This draft is being hosted on GitHub - <https://github.com/gmmoura/
   draft-moura-dnsop-authoritative-recommendations>, where the most
   recent version of the document and open issues can be found.  The
   authors gratefully accept pull requests.

Status of This Memo

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 23, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  R1:  Use equaly strong IP anycast in every authoritative
       server to achieve even load distribution  . . . . . . . . . .   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  . . . . . . . .   8
   6.  R5: Consider longer time-to-live values whenever possible . .   9
   7.  R6: Shared Infrastructure Risks Collateral Damage During
       Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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

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   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.

               +-----+   +-----+   +-----+   +-----+
               | 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 user's perceived 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][RFC4786][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).
   [RF:JAb2]], [RF:MSJ1], [RF:DW2].  The recommendations (R1-R6)
   presented in this document are backed by previous research work,
   which used wide-scale Internet measurements upon which to draw their
   conclusions.  This document describes the key engineering options,
   and points readers to the pertinent papers for details.

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   [RF:JAb1, Issue#2].  These recommendations are designed for operators
   of "large" authoritative servers for domains like TLDs.  "Large"
   authoritative servers mean those with a significant global user
   population.  These recommendations may not be appropriate for smaller
   domains, such as those used by an organization with users in one city
   or region, where goals such as uniform low latency are less strict.

   It is likely that these recommendations might be useful in a wider
   context, such as for any stateless/short-duration, anycasted service.
   Because the conclusions of the studies don't verify this fact, the
   wording in this document discusses DNS authoritative services only.

2.  R1: Use equaly strong IP anycast in every authoritative server to
    achieve even load distribution

   Authoritative DNS servers operators announce their authoritative
   servers in the form of Name Server (NS)records{ {RFC1034}}. Different
   authoritatives for a given zone 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][RFC4786][RFC7094], 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 (multiple NS records), 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.

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   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 the observed latency.  But the
   distribution of queries tend to be skewed towards authoritatives with
   lower latency: the lower the latency between a recursive resolver and
   an authoritative server, the more often the recursive will send
   queries to that authoritative.  These results were obtained by
   aggregating results from all vantage points and not specific to any
   vendor/version.

   The 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 recursive resolvers.
   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,
   consequently, to phase out unicast), so they can deliver latency
   values to global clients.  However, [Mueller17b] also notes 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.

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   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
   33 and 144 sites respectively.  A median RTT was measured between
   30ms and 32ms for C, K and L roots, and 25ms for F.

   [Schmidt17a] 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 DDoS attacks [Moura16b].

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 aan open-source tool called Verfploeter
   (available at [VerfSrc]).  Verfploeter maps a large portion of the

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   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.
   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.

   To date, Verfploeter has been deployed on B-Root[Vries17b], on a
   operational testbed (Anycast testbed) [AnyTest], and on a large
   unnamed operator.

   The recommendation is therefore to deploy 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.

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5.  R4: When under stress, employ two strategies

   DDoS attacks are becoming bigger, cheaper, and more frequent
   [Moura16b].  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
   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, ([RF:Issue3]) perform other traffic shifting tricks
      (such as reducing the propagation of its announcements using BGP
      communities[RFC1997]) which shrinks portions of its catchment),
      use FlowSpec or other upstream communication mechanisms to deploy
      upstream filtering.  The goals of these techniques is to perform
      some combination of shifting of both legitimate and attack traffic
      to other anycast sites (with hopefully greater capacity) or to
      block the traffic entirely.

   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 route-trip time
   (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.

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   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
   (withdraw/prepend/communities or isolate sites), and the best choice
   depends on the specifics of the attack.

6.  R5: Consider longer time-to-live values whenever possible

   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, it has been argued that caching plays
   a significant role in protecting users during DDoS 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

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   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 , under normal operations, caching works as
   expected 70% of the times in the wild.  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.

   [Moura18b] also emulated DDoS attacks on authoritative servers were
   emulated by dropping all incoming packets for various TTLs values.
   For experiments when all authoritative servers were completely
   unreachable, they found that TTL value on the DNS records determined
   how long clients received responses, together with the status of the
   cache at the attack 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 expires, 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.

   They also emulated partial-failure DDoS failures were also emulated
   (similar to Dyn 2016 [Perlroth16], by dropping packet at rates of
   50-90%, for various TTL values.  They found that:

   o  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  Recursives retries was also a key part of resilience: 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.  With 90% packet drop on both
      authoritatives (with TTL of 60s), 27% of clients still got an
      answer due to retries, at the price of increased response times.
      However, this came with a price for authroritative servers: a 8.1
      times increase in normal traffic during a 90% packet drop with TTL
      of 60s, as recursives attempt to resolve queries - thus
      effectively creating "friendly fire".

   Altogether, these results help to explain why previous attacks
   against the Roots were not noticed by most users [Moura18b] and why
   other attacks (such as Dyn 2016 [Perlroth16]) had significant impact
   on users experience: records on the Root zone have TTL values ranging

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   from 1 to 6 days, while some of unreachable Dyn clients had TTL
   values ranging from 120 to 300s, which limit how long records ought
   to be cached.

   Therefore, given the important role of the TTL on user's experience
   during a DDoS attack (and in reducing ''friendly fire''), it is
   recommended that DNS zone owners set their TTL values carefully,
   using reasonable TTL values (at least 1 hour) whenever possible,
   given its role in DNS resilience against DDoS attacks.  However, the
   choice of the value depends on the specifics of each operator (CDNs
   are known for using TTL values in the range of few minutes).  The
   drawback of setting larger TTL values is that changes on the
   authoritative system infrastructure (e.g.: adding a new authoritative
   server or changing IP address) will take at least as long as the TTL
   to propagate among clients.

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.

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8.  Security considerations

   o  to be added

9.  IANA considerations

   This document has no IANA actions.

10.  Acknowledgements

   4 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
   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>.

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   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <https://www.rfc-editor.org/info/rfc1997>.

   [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>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/info/rfc4786>.

   [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>.

11.2.  Informative References

   [AnyTest]  Schmidt, R., "Anycast Testbed", December 2018,
              <http://www.anycast-testbed.com/>.

   [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., 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>.

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   [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]
              Schmidt, R., Heidemann, J., and J. Kuipers, "Anycast
              Latency - How Many Sites Are Enough. In Proceedings of the
              Passive and Active Measurement Workshop", PAM Passive and
              Active Measurement Conference, March 2017,
              <https://www.isi.edu/%7ejohnh/PAPERS/Schmidt17a.pdf>.

   [Sigla2014]
              Singla, A., Chandrasekaran, B., Godfrey, P., and B. Maggs,
              "The Internet at the speed of light. In Proceedings of the
              13th ACM Workshop on Hot Topics in Networks (Oct 2014)",
              ACM Workshop on Hot Topics in Networks, October 2014,
              <http://speedierweb.web.engr.illinois.edu/cspeed/papers/
              hotnets14.pdf>.

   [VerfSrc]  Vries, W., "Verfploeter source code", November 2018,
              <https://github.com/Woutifier/verfploeter>.

   [Vries17b]
              Vries, W., Schmidt, R., Hardaker, W., Heidemann, J., Boer,
              P., and A. Pras, "Verfploeter - Broad and Load-Aware
              Anycast Mapping", ACM 2017 Internet Measurement
              Conference, DOI 10.1145/3131365.3131371, October 2017,
              <https://www.isi.edu/%7ejohnh/PAPERS/Vries17b.pdf>.

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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

   Marco Davids
   SIDN Labs
   Meander 501
   Arnhem  6825 MD
   The Netherlands

   Phone: +31 26 352 5500
   Email: marco.davids@sidn.nl

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