[Docs] [txt|pdf] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03 04 05 06

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


   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.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 1, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

Moura, et al.             Expires June 1, 2019                  [Page 1]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.

Moura, et al.             Expires June 1, 2019                  [Page 2]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

               +-----+   +-----+   +-----+   +-----+
               | 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

Moura, et al.             Expires June 1, 2019                  [Page 3]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   engineering options, and points readers to the pertinent papers for

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

Moura, et al.             Expires June 1, 2019                  [Page 4]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   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

Moura, et al.             Expires June 1, 2019                  [Page 5]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   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

   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.

Moura, et al.             Expires June 1, 2019                  [Page 6]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   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

   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

Moura, et al.             Expires June 1, 2019                  [Page 7]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   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

Moura, et al.             Expires June 1, 2019                  [Page 8]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   (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

   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

Moura, et al.             Expires June 1, 2019                  [Page 9]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

      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

   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

   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

Moura, et al.             Expires June 1, 2019                 [Page 10]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

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

   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

   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

Moura, et al.             Expires June 1, 2019                 [Page 11]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   and datasets used in this research, as well as the funding agencies
   sponsoring the individual research works.

11.  References

11.1.  Normative References

              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,

   [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,

   [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,

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

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

Moura, et al.             Expires June 1, 2019                 [Page 12]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

11.2.  Informative References

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

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

              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,

              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,

              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,

              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,

Moura, et al.             Expires June 1, 2019                 [Page 13]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

              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,

              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
   Email: giovane.moura@sidn.nl

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

   Phone: +1 (530) 404-0099
   Email: ietf@hardakers.net

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

   Phone: +1 (310) 448-8708
   Email: johnh@isi.edu

Moura, et al.             Expires June 1, 2019                 [Page 14]

Internet-Draft          Recomm-Authoritative-Ops           November 2018

   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]

Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/