Considerations for Large Authoritative DNS Servers Operators

Document Type Active Internet-Draft (individual)
Authors Giovane Moura  , Wes Hardaker  , John Heidemann  , Marco Davids 
Last updated 2021-02-19
Stream Independent Submission
Intended RFC status Informational
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Document shepherd Adrian Farrel
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Send notices to Adrian Farrel <>
DNSOP Working Group                                             G. Moura
Internet-Draft                                        SIDN Labs/TU Delft
Intended status: Informational                               W. Hardaker
Expires: August 23, 2021                                    J. Heidemann
                                      USC/Information Sciences Institute
                                                               M. Davids
                                                               SIDN Labs
                                                       February 19, 2021

      Considerations for Large Authoritative DNS Servers Operators


   Recent research work has explored the deployment characteristics and
   configuration of the Domain Name System (DNS).  This document
   summarizes the conclusions from these research efforts and offers
   specific, tangible advice to operators when configuring authoritative
   DNS servers.

   It is possible that the results presented in this document could be
   applicable in a wider context than just the DNS protocol, as some of
   the results may generically apply to any stateless/short-duration,
   anycasted service.

   This document is not an IETF consensus document: it is published for
   informational purposes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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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 August 23, 2021.

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Copyright Notice

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  C1: Deploy anycast in every authoritative server for better
       load distribution . . . . . . . . . . . . . . . . . . . . . .   5
   4.  C2: Routing can matter more than locations  . . . . . . . . .   6
   5.  C3: Collecting anycast catchment maps to improve design . . .   7
   6.  C4: When under stress, employ two strategies  . . . . . . . .   9
   7.  C5: Consider longer time-to-live values whenever possible . .  10
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
   10. IANA considerations . . . . . . . . . . . . . . . . . . . . .  13
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This document summarizes recent research work that explored the
   deployed DNS configurations and offers derived, specific tangible
   advice to DNS authoritative server operators (DNS operators
   hereafter).  The considerations (C1--C5) presented in this document
   are backed by published research work, which used wide-scale Internet
   measurements to draw their conclusions.  This document summarizes the
   research results and describes the resulting key engineering options.
   In each section, it points readers to the pertinent publications
   where additional details are presented.

   These considerations are designed for operators of "large"
   authoritative DNS servers.  In this context, "large" authoritative

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   servers refers to those with a significant global user population,
   like top-level domain (TLD) operators, run by either a single or
   multiple operators.  Typically these networks are deployed on wide
   anycast networks [RFC1546].  These considerations may not be
   appropriate for smaller domains, such as those used by an
   organization with users in one unicast network, or in one city or
   region, where operational goals such as uniform, global low latency
   are less required.

   It is possible that the results presented in this document could be
   applicable in a wider context than just the DNS protocol, as some of
   the results may generically apply to any stateless/short-duration,
   anycasted service.  Because the conclusions of the reviewed studies
   don't measure smaller networks, the wording in this document
   concentrates solely on disusing large-scale DNS authoritative
   services only.

   This document is not an IETF consensus document: it is published for
   informational purposes.

2.  Background

   The DNS has main two types of DNS servers: authoritative servers and
   recursive resolvers, shown by a representational deployment model in
   Figure 1.  An authoritative server (shown as AT1--AT4 in Figure 1)
   knows the content of a DNS zone, and is responsible for answering
   queries about that zone.  It runs using local (possibly automatically
   updated) copies of the zone and does not need to query other servers
   [RFC2181] in order to answer requests.  A recursive resolver (Re1--
   Re3) is a server that iteratively queries authoritative and other
   servers to answer queries received from client requests [RFC1034].  A
   client typically employs a software library called a stub resolver
   (stub in Figure 1) to issue its query to the upstream recursive
   resolvers [RFC1034].

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           +-----+  +-----+  +-----+  +-----+
           | AT1 |  | AT2 |  | AT3 |  | AT4 |
           +-----+  +-----+  +-----+  +-----+
             ^         ^        ^        ^
             |         |        |        |
             |      +-----+     |        |
             +------| Re1 |----+|        |
             |      +-----+              |
             |         ^                 |
             |         |                 |
             |      +----+   +----+      |
             +------|Re2 |   |Re3 |------+
                    +----+   +----+
                      ^          ^
                      |          |
                      | +------+ |
                      +-| stub |-+

        Figure 1: Relationship between recursive resolvers (Re) and
                     authoritative name servers (ATn)

   DNS queries issued by a client contribute to a user's perceived
   perceived latency and affect user experience [Sigla2014] depending on
   how long it takes for responses to be returned.  The DNS system has
   been subject to repeated Denial of Service (DoS) attacks (for
   example, in November 2015 [Moura16b]) in order to specifically
   degrade user experience.

   To reduce latency and improve resiliency against DoS attacks, the DNS
   uses several types of service replication.  Replication at the
   authoritative server level can be achieved with (i) the deployment of
   multiple servers for the same zone [RFC1035] (AT1---AT4 in Figure 1),
   (ii) the use of IP anycast [RFC1546][RFC4786][RFC7094] that allows
   the same IP address to be announced from multiple locations (each of
   referred to as an "anycast instance" [RFC8499]) and (iii) the use of
   load balancers to support multiple servers inside a single
   (potentially anycasted) instance.  As a consequence, there are many
   possible ways an authoritative DNS provider can engineer its
   production authoritative server network, with multiple viable choices
   and no necessarily single optimal design.

   In the next sections we cover the specific consideration (C1--C5) for
   conclusions drawn within the academic papers about large
   authoritative DNS server operators.

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3.  C1: Deploy anycast in every authoritative server for better load

   Authoritative DNS server operators announce their service using NS
   records[RFC1034].  Different authoritative servers for a given zone
   should return the same content; typically they stay synchronized
   using DNS zone transfers (AXFR[RFC5936] and IXFR[RFC1995]),
   coordinating the zone data they all return to their clients.

   DNS heavily relies upon replication to support high reliability,
   ensure capacity and to reduce latency [Moura16b].  DNS has two
   complementary mechanisms for service replication.  First, the DNs
   protocol itself supports nameserver replication through the use of
   multiple nameserver records (NS records), each operating on different
   IP addresses.  Second, each of these addresses can run at multiple
   physical locations through the use of IP
   anycast[RFC1546][RFC4786][RFC7094], by announcing the same IP address
   from each instance at multiple locations -- Internet routing
   (BGP[RFC4271]) associates the service's clients with their
   topologically nearest anycast instance.  Outside the DNS protocol,
   replication can also be achieved by deploying load balancers at each
   physical location.  Nameserver replication is strongly recommended
   for all zones (multiple NS records).  IP anycast is used by many
   large zones such as the DNS Root, most top-level domains[Moura16b]
   and many large commercial enterprises, governments and other

   Most DNS operators strive to reduce service latency for users.
   However, because they only have control over their authoritative
   servers, and not over the client recursive resolvers, 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 vendors and even different releases
   employ different criteria to chose the authoritative servers with
   which to communicate.

   Understanding how recursive resolvers choose authoritative servers is
   a key step in improving the effectiveness of authoritative server
   deployments.  To measure and evaluate server deployments,
   [Mueller17b] deployed seven unicast authoritative name servers in
   different global locations and then queried them from more than 9000
   RIPE authoritative server operators and their respective recursive

   [Mueller17b] found that recursive resolvers in the wild query all
   available authoritative servers, regardless of the observed latency.
   But the distribution of queries tends to be skewed towards
   authoritatives with lower latency: the lower the latency between a

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   recursive resolver and an authoritative server, the more often the
   recursive will send queries to that server.  These results were
   obtained by aggregating results from all of the vantage points and
   were not specific to any specific vendor or version.

   The authors believe this behavior is a consequence of combining the
   two main criteria employed by resolvers when selecting authoritative
   servers: resolvers regularly check all listed authoritative servers
   in an NS set to determine which is closer (the least latent) and when
   one isn't available selects one of the alternatives.

   For an authoritative DNS operator, this result means that the latency
   of all authoritative servers (NS records) matter, so they all must be
   similarly capable -- all available authoritatives will be queried by
   most recursive resolvers.  Unicasted services, unfortunately, cannot
   deliver good latency worldwide (a unicast authoritative server in
   Europe will always have high latency to resolvers in California and
   Australia, for example, given its geographical distance).
   [Mueller17b] recommends that DNS operators deploy equally strong IP
   anycast instances for every authoritative server (i.e., for each NS
   record).  Each large authoritative DNS server provider should phase
   out their usage of unicast and deploy a well engineered number of
   anycast instances with good peering strategies so they can provide
   good latency to their global clients.

   As a case study, the ".nl" TLD zone was originally served on seven
   authoritative servers with a mixed unicast/anycast setup.  In early
   2018, .nl moved to a setup with 4 anycast authoritative servers.

   [Mueller17b]'s contribution to DNS service engineering shows that
   because unicast cannot deliver good latency worldwide, anycast needs
   to be used to provide a low latency service worldwide.

4.  C2: Routing can matter more than locations

   When selecting an anycast DNS provider or setting up an anycast
   service, choosing the best number of anycast instances[RFC4786] to
   deploy is a challenging problem.  Selecting where and how many global
   locations to announce from using BGP is tricky.  Intuitively, one
   could naively think that the more instances the better and simply
   "more" will always lead to shorter response times.

   This is not necessarily true, however.  In fact, [Schmidt17a] found
   that proper route engineering can matter more than the total number
   of locations.  They analyzed the relationship between the number of
   anycast instances and service performance (measuring latency of the
   round-trip time (RTT)), measuring the overall performance of four DNS
   Root servers.  The Root DNS servers are implemented by 12 separate

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   organizations serving the DNS root zone at 13 different IPv4/IPv6
   address pairs.

   The results documented in [Schmidt17a] measured the performance of
   the {c,f,k,l} (hereafter, "C", "F", "K" and "L")
   servers from more than 7.9k RIPE Atlas probes.  RIPE Atlas is a
   Internet measurement platform with more than 12000 global vantage
   points called "Atlas Probes" -- it is used regularly by both
   researchers and operators [RipeAtlas15a] [RipeAtlas19a].

   [Schmidt17a] found that the C server, a smaller anycast deployment
   consisting of only 8 instances, provided very similar overall
   performance in comparison to the much larger deployments of K and L,
   with 33 and 144 instances respectively.  The median RTT for C, K and
   L root server were all between 30-32ms.

   Because RIPE Atlas is known to have better coverage in Europe than
   other regions, the authors specifically analyzed the results per
   region and per country (Figure 5 in [Schmidt17a]), and show that
   known Atlas bias toward Europe does not change the conclusion that
   properly selected anycast locations is more important to latency than
   the number of sites.

   The important conclusion of [Schmidt17a] is that when engineering
   anycast services for performance, factors other than just the number
   of instances (such as local routing connectivity) must be considered.
   They showed that 12 instances can provide reasonable latency,
   assuming they are globally distributed and have good local
   interconnectivity.  However, additional instances can still be useful
   for other reasons, such as when handling Denial-of-service (DoS)
   attacks [Moura16b].

5.  C3: Collecting anycast catchment maps to improve design

   An anycast DNS service may be deployed from anywhere from several
   locations to hundreds of locations (for example,
   has over 150 anycast instances at the time this was written).
   Anycast leverages Internet routing to distribute incoming queries to
   a service's hop-nearest distributed anycast locations.  However,
   usually queries are not evenly distributed across all anycast
   locations, as found in the case of L-Root [IcannHedge18].

   Adding locations to or removing locations from a deployed anycast
   network changes the load distribution across all of its locations.
   When a new location is announced by BGP, locations may receive more
   or less traffic than it was engineered for, leading to suboptimal
   service performance or even stressing some locations while leaving
   others underutilized.  Operators constantly face this scenario that

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   when expanding an anycast service.  Operators cannot easily directly
   estimate future query distributions based on proposed anycast network
   engineering decisions.

   To address this need and estimate the query loads based on changing,
   in particular expanding, anycast service changes [Vries17b] developed
   a new technique enabling operators to carry out active measurements,
   using an open-source tool called Verfploeter (available at
   [VerfSrc]).  The results allow the creation of detailed anycast maps
   and catchment estimates.  By running verfploeter combined with a
   published IPv4 "hit list", DNS can precisely calculate which remote
   prefixes will be matched to each anycast instance in a network.  At
   the moment of this writing, Verfploeter still does not support IPv6
   as the IPv4 hit lists used are generated via frequent large scale
   ICMP echo scans, which is not possible using IPv6.

   As proof of concept, [Vries17b] documents how it verfploeter was used
   to predict both the catchment and query load distribution for a new
   anycast instance deployed for  Using two anycast
   test instances in Miami (MIA) and Los Angeles (LAX), an ICMP echo
   query was sent from an IP anycast addresses to each IPv4 /24 network
   routing block on the Internet.

   The ICMP echo responses were recorded at both sites and analyzed and
   overlayed onto a graphical world map, resulting in an Internet scale
   catchment map.  To calculate expected load once the production
   network was enabled, the quantity of traffic received by b.root-'s single site at LAX was recorded based on a single day's
   traffic (2017-04-12, DITL datasets [Ditl17]).  [Vries17b] predicted
   that 81.6% of the traffic load would remain at the LAX site.  This
   estimate by verfploeter turned out to be very accurate; the actual
   measured traffic volume when production service at MIA was enabled
   was 81.4%.

   Verfploeter can also be used to estimate traffic shifts based on
   other BGP route engineering techniques (for example, AS path
   prepending or BGP community use) in advance of operational
   deployment.  [Vries17b] studied this using prepending with 1-3 hops
   at each instance and compared the results against real operational
   changes to validate the techniques accuracy.

   An important operational takeaway [Vries17b] provides is how DNS
   operators can make informed engineering choices when changing DNS
   anycast network deployments by using Verfploeter in advance.
   Operators can identify sub-optimal routing situations in advance with
   significantly better coverage than using other active measurement
   platforms such as RIPE Atlas.  To date, Verfploeter has been deployed

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   on a operational testbed (Anycast testbed) [AnyTest], on a large
   unnamed operator and is run daily at[Vries17b].

   Operators are encouraged to use active measurement techniques like
   Verfploeter in advance of potential anycast network changes to
   accurately measure the benefits and potential issues ahead of time.

6.  C4: When under stress, employ two strategies

   DDoS attacks are becoming bigger, cheaper, and more frequent
   [Moura16b].  The most powerful recorded DDoS attack against DNS
   servers to date reached 1.2 Tbps by using IoT devices [Perlroth16].
   How should a DNS operator engineer its anycast authoritative DNS
   server react to such a DDoS attack?  [Moura16b] investigates this
   question using empirical observations grounded with theoretical
   option evaluations.

   An authoritative DNS server deployed using anycast will have many
   server instances distributed over many networks.  Ultimately, the
   relationship between the DNS provider's network and a client's ISP
   will determine which anycast instance will answer queries for a given
   client, given that BGP is the protocol that maps clients to specific
   anycast instances by using routing information [RF:KDar02].  As a
   consequence, when an anycast authoritative server is under attack,
   the load that each anycast instance receives is likely to be unevenly
   distributed (a function of the source of the attacks), thus some
   instances 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 instances may have different capacity
   (bandwidth, CPU, etc.), making a decision about how to react to
   stress becomes even more difficult.

   In practice, an anycast instance is overloaded with incoming traffic,
   operators have two options:

   o  They can withdraw its routes, pre-prepend its AS route to some or
      all of its neighbors, perform other traffic shifting tricks (such
      as reducing route announcement propagation using BGP
      communities[RFC1997]), or by communicating with its upstream
      network providers to apply filtering (potentially using FlowSpec
      [RFC5575]).  These techniques shift both legitimate and attack
      traffic to other anycast instances (with hopefully greater
      capacity) or to block traffic entirely.

   o  Alternatively, operators can be become a degraded absorber by
      continuing to operate, knowing dropping incoming legitimate
      requests due to queue overflow.  However, this approach will also

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      absorb attack traffic directed toward its catchment, hopefully
      protecting the other anycast instances.

   [Moura16b] saw both of these behaviors deployed in practice by
   studying instance reachability and route-trip time (RTTs) in the DNS
   root events.  When withdraw strategies were deployed, the stress of
   increased query loads were displaced from one instance to multiple
   other sites.  In other observed events, one site was left to absorb
   the brunt of an attack leaving the other sites to remain relatively
   less affected.

   Operators should consider having both a anycast site withdraw
   strategy and a absorption strategy ready to be used before a network
   overload occurs.  Ideally, these should be encoded into operating
   playbooks with defined site measurement guidelines for which strategy
   to employ based on measured data from past events.

   [Moura16b] speculates that careful, explicit, and automated
   management policies may provide stronger defenses to overload events.
   DNS operators should be ready to employ both traditional filtering
   approaches and other routing load balancing techniques
   (withdraw/prepend/communities or isolate instances), where the best
   choice depends on the specifics of the attack.

   Note that this consideration refers to the operation of just one
   anycast service point, i.e., just one anycasted IP address block
   covering one NS record.  However, DNS zones with multiple
   authoritative anycast servers may also expect loads to shift from one
   anycasted server to another, as resolvers switch from on
   authoritative service point to another when attempting to resolve a
   name [Mueller17b].

7.  C5: Consider longer time-to-live values whenever possible

   Caching is the cornerstone of good DNS performance and reliability.
   A 50 ms response to a new DNS query may be considered fast, but a
   less than 1 ms response to a cached entry is far faster.  [Moura18b]
   showed that caching also protects users from short outages and even
   significant DDoS attacks.

   DNS record TTLs (time-to-live values) [RFC1034][RFC1035] directly
   control cache durations and affect latency, resilience, and the role
   of DNS in CDN server selection.  Some early work modeled caches as a
   function of their TTLs [Jung03a], and recent work has examined their
   interaction with DNS[Moura18b], but until [Moura19a] no research
   provided considerations about the benefits of various TTL value
   choices.  To study this, Moura et. al.  [Moura19a] carried out a
   measurement study investigating TTL choices and their impact on user

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   experiences in the wild.  They performed this study independent of
   specific resolvers (and their caching architectures), vendors, or

   First, they identified several reasons why operators and zone-owners
   may want to choose longer or shorter TTLs:

   o  As discussed, longer TTLs lead to a longer cache life, resulting
      in faster responses.  [Moura19a] measured this in the wild and
      showed that by increasing the TTL for .uy TLD from 5 minutes
      (300s) to 1 day (86400s) the latency measured from 15k Atlas
      vantage points changed significantly: the median RTT decreased
      from 28.7ms to 8ms, and the 75%ile decreased from 183ms to 21ms.

   o  Longer caching times also results in lower DNS traffic:
      authoritative servers will experience less traffic with extended
      TTLs, as repeated queries are answered by resolver caches.

   o  Consequently, longer caching results in a lower overall cost if
      DNS is metered: some DNS-As-A-Service providers charge a per query
      (metered) cost (often in addition to a fixed monthly cost).

   o  Longer caching is more robust to DDoS attacks on DNS
      infrastructure.  [Moura18b] also measured and show that DNS
      caching can greatly reduce the effects of a DDoS on DNS, provided
      that caches last longer than the attack.

   o  However, shorter caching supports deployments that may require
      rapid operational changes: An easy way to transition from an old
      server to a new one is to simply change the DNS records.  Since
      there is no method to remotely remove cached DNS records, the TTL
      duration represents a necessary transition delay to fully shift
      from one server to another.  Thus, low TTLs allow for more rapid
      transitions.  However, when deployments are planned in advance
      (that is, longer than the TTL), it is possible to lower the TTLs
      just-before a major operational change and raise them again

   o  Shorter caching can also help with a DNS-based response to DDoS
      attacks.  Specifically, some DDoS-scrubbing services use the DNS
      to redirect traffic during an attack.  Since DDoS attacks arrive
      unannounced, DNS-based traffic redirection requires the TTL be
      kept quite low at all times to allow operators to suddenly have
      their zone served by a DDoS-scrubbing service.

   o  Shorter caching helps DNS-based load balancing.  Many large
      services are known to rotate traffic among their servers using
      DNS-based load balancing.  Each arriving DNS request provides an

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      opportunity to adjust service load by rotating IP address records
      (A and AAAA) to the lowest unused server.  Shorter TTLs may be
      desired in these architectures to react more quickly to traffic
      dynamics.  Many recursive resolvers, however, have minimum caching
      times of tens of seconds, placing a limit on this form of agility.

   Given these considerations, the proper choice for a TTL depends in
   part on multiple external factors -- no single recommendation is
   appropriate for all scenarios.  Organizations must weigh these trade-
   offs and find a good balance for their situation.  Still, some
   guidelines can be reached when choosing TTLs:

   o  For general DNS zone owners, [Moura19a] recommends a longer TTL of
      at least one hour, and ideally 8, 12, or 24 hours.  Assuming
      planned maintenance can be scheduled at least a day in advance,
      long TTLs have little cost and may, even, literally provide a cost

   o  For registry operators: TLD and other public registration
      operators (for example most ccTLDs and .com, .net, .org) that host
      many delegations (NS records, DS records and "glue" records),
      [Moura19a] demonstrates that most resolvers will use the TTL
      values provided by the child delegations while the others some
      will choose the TTL provided by the parent's copy of the record.
      As such, [Moura19a] recommends longer TTLs (at least an hour or
      more) for registry operators as well for child NS and other

   o  Users of DNS-based load balancing or DDoS-prevention services may
      require shorter TTLs: TTLs may even need to be as short as 5
      minutes, although 15 minutes may provide sufficient agility for
      many operators.  There is always a tussle between shorter TTLs
      providing more agility against all the benefits listed above for
      using longer TTLs.

   o  Use of A/AAAA and NS records: The TTLs for A/AAAA records should
      be shorter to or equal to the TTL for the corresponding NS records
      for in-bailiwick authoritative DNS servers, since [Moura19a] finds
      that once an NS record expires, their associated A/AAAA will also
      be re-queried when glue is required to be sent by the parents.
      For out-of-bailiwick servers, A, AAAA and NS records are usually
      all cached independently, so different TTLs can be used
      effectively if desired.  In either case, short A and AAAA records
      may still be desired if DDoS-mitigation services are required.

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8.  Security considerations

   This document discusses applying measured research results to
   operational deployments.  Most of the considerations affect mostly
   operational practice, though a few do have security related impacts.

   Specifically, C4 discusses a couple of strategies to employ when a
   service is under stress from DDoS attacks and offers operators
   additional guidance when handling excess traffic.

   Similarly, C5 identifies the trade-offs with respect to the
   operational and security benefits of using longer time-to-live

9.  Privacy Considerations

   This document does not add any practical new privacy issues, aside
   from possible benefits in deploying longer TTLs as suggested in C5.
   Longer TTLs may help preserve a user's privacy by reducing the number
   of requests that get transmitted in both the client-to-resolver and
   resolver-to-authoritative cases.

10.  IANA considerations

   This document has no IANA actions.

11.  Acknowledgements

   This document is a summary of the main considerations of six research
   works performed by the authors and others.  This document would not
   have been possible without the hard work of these authors and co-

   o  Ricardo de O.  Schmidt

   o  Wouter B de Vries

   o  Moritz Mueller

   o  Lan Wei

   o  Cristian Hesselman

   o  Jan Harm Kuipers

   o  Pieter-Tjerk de Boer

   o  Aiko Pras

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   We would like also to thank the reviewers of this draft that offered
   valuable suggestions: Duane Wessels, Joe Abley, Toema Gavrichenkov,
   John Levine, Michael StJohns, Kristof Tuyteleers, Stefan Ubbink,
   Klaus Darilion and Samir Jafferali, and comments provided at the IETF
   DNSOP session (IETF104).

   Besides those, we would like thank those acknowledged in the papers
   this document summarizes for helping produce the results: RIPE NCC
   and DNS OARC for their tools and datasets used in this research, as
   well as the funding agencies sponsoring the individual research

12.  References

12.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, DOI 10.17487/RFC1546,
              November 1993, <>.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              DOI 10.17487/RFC1995, August 1996,

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

   [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,

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <>.

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   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

12.2.  Informative References

   [AnyTest]  Schmidt, R., "Anycast Testbed", December 2018,

   [Ditl17]   OARC, D., "2017 DITL data", October 2018,

              ICANN, ., "DNS-STATS - Hedgehog 2.4.1", October 2018,

   [Jung03a]  Jung, J., Berger, A., and H. Balakrishnan, "Modeling TTL-
              based Internet caches", ACM 2003 IEEE INFOCOM,
              DOI 10.1109/INFCOM.2003.1208693, July 2003,

              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,

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              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,

              Moura, G., Heidemann, J., Schmidt, R., and W. Hardaker,
              "Cache Me If You Can: Effects of DNS Time-to-Live",
              ACM 2019 Internet Measurement Conference,
              DOI 10.1145/3355369.3355568, October 2019,

              Moura, G., Heidemann, J., Hardaker, W., Bulten, J., Ceron,
              J., and C. Hesselman, "Old but Gold: Prospecting TCP to
              Engineer DNS Anycast (extended)", Technical Report ISI-
              TR-740 USC/Information Sciences Institute. , June 2020,

              Moura, G., Castro, S., Hardaker, W., Wullink, M., and C.
              Hesselman, "Clouding up the Internet: how centralized is
              DNS traffic becoming?", ACM 2020 Internet Measurement
              Conference, DOI 10.1145/3419394.3423625, October 2020,

              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,

              Perlroth, N., "Hackers Used New Weapons to Disrupt Major
              Websites Across U.S.", October 2016,

              Staff, R., "RIPE Atlas A Global Internet Measurement
              Network", September 2015, <

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              NCC, R., "Ripe Atlas - RIPE Network Coordination Centre",
              September 2019, <>.

              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,

              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,

   [VerfSrc]  Vries, W., "Verfploeter source code", November 2018,

              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,

Authors' Addresses

   Giovane C. M. Moura
   SIDN Labs/TU Delft
   Meander 501
   Arnhem  6825 MD
   The Netherlands

   Phone: +31 26 352 5500

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   Wes Hardaker
   USC/Information Sciences Institute
   PO Box 382
   Davis  95617-0382

   Phone: +1 (530) 404-0099

   John Heidemann
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina Del Rey  90292-6695

   Phone: +1 (310) 448-8708

   Marco Davids
   SIDN Labs
   Meander 501
   Arnhem  6825 MD
   The Netherlands

   Phone: +31 26 352 5500

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