Internet Engineering Task Force                                  L. Song
Internet-Draft                                Beijing Internet Institute
Intended status: Standards Track                             May 8, 2018
Expires: November 9, 2018

       ATR: Additional Truncation Response for Large DNS Response


   As the increasing use of DNSSEC and IPv6, there are more public
   evidence and concerns on IPv6 fragmentation issues due to larger DNS
   payloads over IPv6.  This memo introduces an simple improvement on
   DNS server by replying an additional truncated response just after
   the normal fragmented response.  It can be used to relieve users
   suffering on DNS latency and failures due to large DNS response.  It
   also can be utilized as a measuring and troubleshooting tool to
   locate the issue and conquer.

   REMOVE BEFORE PUBLICATION: The source of the document with test
   script is currently placed at GitHub [ATR-Github].  Comments and pull
   request are welcome.

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
   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 November 9, 2018.

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

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The ATR mechanism . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Indicating a ATR response . . . . . . . . . . . . . . . . . .   5
   4.  Operational considerations  . . . . . . . . . . . . . . . . .   6
     4.1.  ATR timer . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  ATR payload size  . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Less aggresiveness of ATR . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  How well does ATR actually work? . . . . . . . . . .  11
   Appendix B.  Considerations on Resolver awareness of ATR  . . . .  12
   Appendix C.  Revision history of this document  . . . . . . . . .  13
     C.1.  draft-song-atr-large-resp-01  . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Large DNS response is identified as a issue for a long time.  There
   is an inherent mechanism defined in [RFC1035] to handle large DNS
   response (larger than 512 octets) by indicating (set TrunCation bit)
   the resolver to fall back to query via TCP.  Due to the fear of cost
   of TCP, EDNS(0) [RFC6891] was proposed which encourages server to
   response larger response instead of falling back to TCP.  However, as
   the increasing use of DNSSEC and IPv6, there are more public evidence
   and concerns on user's suffering due to packets dropping caused by
   IPv6 fragmentation in DNS due to large DNS response.

   It is observed that some IPv6 network devices like firewalls
   intentionally choose to drop the IPv6 packets with fragmentation
   Headers[I-D.taylor-v6ops-fragdrop].  [RFC7872] reported more than 30%
   drop rates for sending fragmented packets.  Regarding IPv6
   fragmentation issue due to larger DNS payloads in response, one
   measurement [IPv6-frag-DNS] reported 35% of endpoints using
   IPv6-capable DNS resolver can not receive a fragmented IPv6 response
   over UDP.  Moreover, most of the underlying issues with fragments are
   unrevealed due to good redundancy and resilience of DNS.  It is hard

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   for DNS client and server operators to trace and locate the issue
   when fragments are blocked or dropped.  The noticeable DNS failures
   and latency experienced by end users are just the tip of the iceberg.

   Depending on retry model, the resolver's failing to receive
   fragmented response may experience long latency or failure due to
   timeout and reties.  One typical case is that the resolver finally
   got the answer after several retires and it falls back to TCP after
   deceasing the payload size in EDNS0.  To avoid that issue, some
   authoritative servers may adopt a policy ignoring the UDP payload
   size in EDNS0 extension and always truncating the response when the
   response size is large than a expected one.  However one study
   [Not-speak-TCP] shows that about 17% of resolvers in the samples can
   not ask a query in TCP when they receive truncated response.  It
   seems a dilemma to choose hurting either the users who can not
   receive fragments or the users without TCP fallback capacity.  There
   is also some voice of "moving all DNS over TCP".  But It is generally
   desired that DNS can keep the efficiency and high performance by
   using DNS UDP in most of time and fallback as soon as possible to TCP
   if necessary for some corner case.

   To relieve the problem, this memo introduces an small improvement on
   DNS responding process by replying an Additional Truncated Response
   (ATR) just after a normal large response which is to be fragmented.
   Generally speaking ATR provides a way to decouple the EDNS0 and TCP
   fallback in which they can work independently according to the server
   operator's requirement.  One goal of ATR is to relieve the hurt of
   users, both stub and recursive resolver, from the position of server,
   both authoritative and recursive server.  It does not require any
   changes on resolver and has a deploy-and-gain feature to encourage
   operators to implement it to benefit their resolvers.

   Another goal of ATR is to help troubleshooting for DNS operators
   where ATR can be deployed as a measurement tool to identify
   vulnerable servers and users.  A flag bit is required in EDNS0 OPT
   header to distinguish ATR response from a ordinary truncated
   response.  A resolver (or troubleshooter) can tell if there is any
   fragment not received in a certain transaction with a name server by
   receiving only ATR response without ordinary UDP response.  Another
   way of using ATR as measurement tool is to reply ATR response to
   specific group of resolvers and record the TCP connections it
   received during that period.  It can help identify vulnerable users
   and adopt ATR to them selectively.

   [REMOVE BEFORE PUBLICATION] Note that in Appendix A of this memo
   there is a brief introduction of the test done by APNIC on how well
   does ATR actually work.  And comments are also attached by the author

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   of this memo.  It may help people to understand what the benefit and
   tradeoff that ATR brings.

2.  The ATR mechanism

   The ATR mechanism is very simple that it involves a ATR module in the
   responding process of current DNS implementation . As show in the
   following diagram the ATR module is right after truncation loop if
   the packet is not going to be fragmented.

   A DNS +-------------+        +-------------+  Normal
   query |             | No     |             | response
   +------>  Truncation +-------->     ATR     +--------->
         |    loop     |        |    Module   |
         | truncation? |        | truncation? |
         +-------------+        +-------------+
             yes|                   yes|     +-----+
                |                      +-----+timer+-->
                |                            +-----+
                |                      Truncated Response
                 Truncated Response

                  Figure 1: High-Level Testbed Components

   The ATR responding process goes as follows:

   o  When an authoritative server receives a query and enters the
      responding process, it first go through the normal truncation loop
      to see whether the size of response surpasses the EDNS0 payload
      size.  If yes, it ends up with responding a truncated packets.  If
      no, it enters the ATR module.

   o  In ATR module, similar like truncation loop, the size of response
      is compared with a value called ATR payload size.  If the response
      of a query is larger than ATR payload size, the server firstly
      sends the normal response and then coin a truncated response with
      the same ID of the query.

   o  The server can reply the coined truncated response in no time.
      But considering the possible impact of network reordering, it is
      suggested a timer to delay the second truncated response, for
      example 10~50 millisecond which can be configured by local

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   Note that the choice of ATR payload size and timer SHOULD be
   configured locally.  And the operational consideration and guidance
   is discussed in Section 4.2 and Section 4.1 respectively.

   There are three typical cases of ATR-unaware resolver behavior when a
   resolver send query to an ATR server in which the server will
   generate a large response with fragments:

   o  Case 1: a resolver (or sub-resolver) will receive both the large
      response and a very small truncated response in sequence.  It will
      happily accepts the first response and drop the second one because
      the transaction is over.

   o  Case 2: In case a fragment is dropped in the middle, the resolver
      will end up with only receiving the small truncated response.  It
      will retry using TCP in no time.

   o  Case 3: For those (probably 30%*17% of them) who can not speak TCP
      and sitting behind a firewall stubbornly dropping fragments.  Just
      say good luck to them!

   In the case authoritative server truncated all response surpass
   certain value , for example setting IPv6-edns-size to 1220 octets,
   ATR will helpful for resolver with TCP capacity, because the resolver
   still has a fair chance to receive the large response.

3.  Indicating a ATR response

   As introduced in ATR it is necessary to distinguish ATR response in a
   special way from a ordinary truncated response.  It enables resolver
   operators to log cases where ATR responses is received without a
   (reassembled) UDP response to a query.  Without an indicator that
   distinguishes ATR response, there would be no way to avoid false
   alarms from authoritative servers that always and only return
   truncated responses when the message exceeds some size.  It is
   actually the use case where Google RDNS is considering.  Google RDNS
   would like to use such indications to flag problematic name servers
   where RDNS should restrict maximum EDNS to a lower value than the
   default 4096 that currently used.

   A simple way for that indicator of ATR response is to define a bit in
   the Z field on the EDNS0 OPT header in the response.  This has the
   virtue of simplicity, and only a minimal risk of breaking existing
   implementations.  This bit is referred to as the "ATR Response" (AT)
   bit.  In the context of the EDNS0 OPT meta-RR, just following the DO
   bit, the AT bit is the second bit of the third and fourth bytes of
   the "extended RCODE and flags" portion (section 6.1.3 of RFC6891) of
   the EDNS0 OPT meta-RR, structured as follows:

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                    +0 (MSB)                +1 (LSB)
         0: |   EXTENDED-RCODE      |       VERSION         |
         2: |DO|AT|                 Z                       |

       Figure 2: Wire format of extended RCODE and flags with AT bit

   The logic of AT bit is simple that setting the AT bit to one in a
   response indicates to the resolver that the response is an ATR
   response.  The AT bit cleared(set to zero) indicates the response is
   a ordinary response.  Note that AT bit and TC bit SHOULD be set and
   appear in the response as a pair.  The response will be ignored if
   only AT bit is set.

   The indication of ATR defined in this memo is for measurement and
   logging purpose.  But it is possible used by resolver operator in
   other aspects, because it signals the resolver the large response is
   fragmented and dropped on the path.  The resolver could act more
   actively if it is able to recognized that bit.  More is discussed in
   Appendix B.

4.  Operational considerations

   In previous sections, only behavior of ATR server and AT bit are
   specified.  There are lots of space for operational issues, such as
   the parameter of the ATR timer and ATR payload size, and policies on
   when ATR is triggered to avoid side-effect.

4.1.  ATR timer

   As introduced in Section 2 ATR timer is a way to avoid the impact of
   network reordering(RO).  The value of the timer is critical, because
   if the delay is too short, the ATR response may be received earlier
   than the fragmented response (the first piece), the resolver will
   fall back to TCP bearing the cost which should have been avoided.  If
   the delay is too long, the client may timeout and retry which negates
   the incremental benefit of ATR.  Generally speaking, the delay of the
   timer should be "long enough, but not too long".

   To the best knowledge of author, the nature of RO is characterized as
   follows hopefully helping ATR users understand RO and how to operate
   ATR appropriately in RO context.

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   o  RO is mainly caused by the parallelism in Internet components and
      links other than network anomaly [Bennett].  It was observed that
      RO is highly related to the traffic load of Internet components.
      So RO will long exists as long as the traffic load continue
      increase and the parallelism is used to enhance network

   o  The probability of RO varies largely depending on the different
      tests samples.  Some work shown RO probability below 2% [Paxson]
      [Tinta] and another work was above 90% [Bennett].  But it is
      agreed that RO is site-dependent and path-dependent.  It is
      observed in that when RO happens, it is mostly exhibited
      consistently in a small percentages of the paths.  It is also
      observed that higher rates smaller packets were more prone to RO
      because the sending inter-spacing time was small.

   o  It was reported that the inter-arrival time of RO varies from a
      few milliseconds to multiple tens of milliseconds [Tinta].  And
      the larger the packet the larger the inter-arrival time, since
      larger packets will take longer to be transmitted.

   Reasonably we can infer that firstly RO should be taken into account
   because it long exists due to middle Internet components which can
   not be avoided by end-to-end way.  Secondly the mixture of larger and
   small packets in ATR case will increase the inter-arrival time of RO
   as well as the its probability.  The good news is that the RO is
   highly site specific and path specific, and persistent which means
   the ATR operator is able to identify a few sites and paths, setup a
   tunable timer setting for them, or just put them into a blacklist
   without replying ATR response.

   Based on the above analysis it is hard to provide a perfect value of
   ATR timer for all ATR users due to the diversity of networks.  It
   seems OK to set the timer with a range from ten to hundreds ms, just
   below the timeout setting of typical resolver.  Is suggested that a
   decision should be made as operator-specific according to the
   statistic of the RTT of their users.  Some measurement shown
   [Brownlee][Liang] the mean of response time is below 50 ms for the
   sites with lots of anycast instance like L-root, .com and .net name
   servers.  For that sites, delay less than 50 ms is appropriate.

4.2.  ATR payload size

   Regarding the operational choice for ATR payload size, there are some
   good input from APNIC study [scoring-dns-root]on how to react to
   large DNS payload for authoritative server.  The difference in ATR is
   that ATR focuses on the second response after the ordinary response.

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   For IPv4 DNS server, it is suggested the study that do not truncate
   and fragment IPv4 UDP response with a payload up to 1472 octets which
   is Ethernet MTU(1500) minus the sum of IPv4 header(20) and UDP
   header(8).  The reason is to avoid gratuitously fragmenting outbound
   packets and TCP fallback at the source.

   In the case of ATR, the first ordinary response is emitted without
   knowing it be to fragmented or not on the path.  If a large value is
   set up to 1472 octets, payload size between 512 octets and the large
   value size will probably get fragmented by aggressive firewalls which
   leads losing the benefit of ATR.  If ATR payload size set exactly 512
   octets, in most of case ATR response and the single unfragmented
   packets are under a race at the risk of RO.

   Given IPv4 fragmentation issue is not so serious compared to IPv6, it
   is suggested in this memo to set ATR payload size 1472 octets which
   means ATR only fit large DNS response larger than 1500 octets in

   For IPv6 DNS server, similar to IPv4, the APNIC study is suggested
   that do not truncate IPv6 UDP packets with a payload up to 1,452
   octets which is Ethernet MTU(1500) minus the sum of IPv6 header(40)
   and UDP header(8). 1452 octets is chosen to avoid TCP fallback in the
   context that most TCP MSS in the root server is not set probably at
   that time.

   In the case of ATR considering the second truncated response, a
   smaller size: 1232 octets, which is IPv6 MTU for most network
   devices(1280) minus the sum of IPv6 header(40) and UDP
   header(8), should be chosen as ATR payload size to trigger necessary
   TCP fallback.  As a complementary requirement with ATR, the TCP MSS
   should be set 1220 octets to avoid Packet Too Big ICMP message as
   suggested in the APNIC study.

   In short, it is recommended that in IPv4 ATR payload size SHOULD be
   1472 octets, and in IPv6 the value SHOULD be 1232 octets.

4.3.  Less aggresiveness of ATR

   There is a concern ATR sends TC=1 response too aggressively
   especially in the beginning of adoption.  One of the idea to mitigate
   this aggressiveness, ATR may respond TC=1 responses at a low
   possibility, such as 10%.

   Another way to mitigating is to reply ATR response selectively.  It
   is observed that RO and IPv6 fragmentation issues are path specific
   and persistent due to the Internet components and middle box.  So it
   is reasonable to keep a ATR "whitelist" by counting the retries and

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   recording the IP destination address of that large response causing
   many retires.  ATR only acts to those queries from the IP address in
   the white list.

5.  Security Considerations

   There may be concerns on DDoS attack problem due to the fact that the
   ATR introduces multiple responses from authoritative server.  DNS
   cookies [RFC7873] and RRL on authoritative may be possible solutions

6.  IANA considerations

   EDNS(0) [RFC6891] defines 16 bits as extended flags in the OPT
   record, these bits are encoded into the TTL field of the OPT record.
   IETF Standards Action is required for assignments of new EDNS(0)

   This document reserves one of these bits as the AT bit.  It is
   requested that the second bit (left most) be allocated.  Thus the USE
   of the OPT record TTL field would look like:

                    +0 (MSB)                +1 (LSB)
         0: |   EXTENDED-RCODE      |       VERSION         |
         2: |DO|AT|                 Z                       |

         Figure 3: the USE of the OPT record TTL field for AT bit

7.  Acknowledgments

   Many thanks to reviewers and their comments.  Geoff Huston and Joao
   Damas did a testing on the question "How well does ATR actually
   work?".  Alexander Dupuy proposed the idea to distinguish ATR
   responses from normal ones.  Akira Kato contributed ideas on
   operational consideration.

8.  References

              "XML source file and test script of DNS ATR", September
              2017, <>.

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   [Bennett]  "Packet Reordering is Not Pathological Network Behavior",
              December 1999, <

              "Response time distributions for global name servers",

              APNIC, "How well does ATR actually work?", April 2018,

              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
              progress), December 2013.

              "Dealing with IPv6 fragmentation in the DNS", August 2017,

   [Liang]    Tsinghua University, "Measuring Query Latency of Top Level
              DNS Servers", February 2013,

              "A Question of DNS Protocols", August 2013,

   [Paxson]   "End-to-End Internet Packet Dynamics", August 1999,

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

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

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   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,

              APNIC, "Scoring the DNS Root Server System", November
              2016, <

   [Tinta]    "Characterizing End-to-End Packet Reordering with UDP
              Traffic", August 2009, <https://static.googleusercontent.c

Appendix A.  How well does ATR actually work?

   It is worth of mentioning APNIC report[How-ATR-Work] on "How well
   does ATR actually work?" done by Geoff Huston and Joao Damas after 00
   version of ATR draft.  It was reported firstly in IEPG meeting before
   IETF 101 and then posted in APNIC Blog later.

   It is said the test was performed over 55 million endpoints, using an
   online ad distribution network to deliver the test script across the
   Internet.  The result is positive that ATR works!  From the end
   users' perspective, in some 9% of IPv4 cases the use of ATR by the
   server will improve the speed of resolution of a fragmented UDP
   response by signaling to the client an immediate switch to TCP to
   perform a re-query.  The IPv6 behavior would improve the resolution
   times in 15% of cases.

   It also analyzed the pros and cons of ATR.  On one hand, It is said
   that ATR certainly looks attractive if the objective is to improve
   the speed of DNS resolution when passing large DNS responses.  And
   ATR is incrementally deployable in favor of decision made by each
   server operator.  On another hand, ATR also has some negative
   factors.  One issue is adding another DNS DDoS attack vector due to
   the additional packet sent by ATR, (author's note : very small adding
   actually.)  Another issue is risk of RO by the choice of the delay
   timer which is discussed fully in Section 4.1.

   As a conclusion, it is said that "ATR does not completely fix the
   large response issue.  If a resolver cannot receive fragmented UDP
   responses and cannot use TCP to perform DNS queries, then ATR is not

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   going to help.  But where there are issues with IP fragment
   filtering, ATR can make the inevitable shift of the query to TCP a
   lot faster than it is today.  But it does so at a cost of additional
   packets and additional DNS functionality".  "If a faster DNS service
   is your highest priority, then ATR is worth considering", said at the
   end of this report

   This test and report definitely made ATR more shiny attracting
   attention in the community.  But it is found that there are still
   something unknown on "How well does ATR actually work".  Firstly the
   test only counts the "success" case ("failure" otherwise) in which
   the "success" for large UDP case can be achieved by normal retries.
   The latency of that retries may reduced by ATR is not taken into

   Secondly more analysis could be done in future to compare ATR with
   TCP.  From the failure rate of users, we see ATR and TCP have similar
   performance (same for IPv4, and only 2% difference in IPv6).  But
   there are resolvers who are able to receive the fragments more sooner
   in ATR case , but they fall back to TCP in TCP case.  If not
   misunderstood, the two unknown parts underestimates the benefit of
   ATR in terms of speed of response.

   The third unknown part is about the ATR timer and RO impact.  In
   APNIC test, 10 ms was adopted as the delay of ATR timer according to
   00 version of this draft.  Different delay of ATR timer may not
   change the key result on gains of ATR (9% for IPv4 and 15% for IPv6).
   But the cost of RO is not measured.  In the majority cases ATR is not
   needed, say 87% in IPv4, and 79% in IPv6.  So it overestimate ATR in
   this regards if RO cost is not taken into account.

Appendix B.  Considerations on Resolver awareness of ATR

   ATR proposed in this memo is a server-side function which requires no
   change in resolver, so it is not required that resolver should
   recognized or use AT bit.  But it may helpful for some special cases
   where a resolver is able to recognized or use AT bit.

   One case is that when receiving a ATR response a ATR-aware resolver
   can adopt a "happyeyeballs" strategy by opening a separate
   transaction sending the query via TCP instead of falling back to TCP
   and closing the original UDP transaction.  Listen to port 53 on both
   TCP and UDP port 53 will enhance the availability and reduce the
   latency.  It will add more tolerance to network reordering issue as
   well.  However, it should be taken into account about the balance of
   resolver's resource.  Less priority should be given to that function
   when the resolver is "busy".

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   Another case is that a ATR-aware resolver is able to indicate its
   support for ATR to prudent servers who do not reply ATR response to
   every query and resolver.  If that requirement is valid, it is
   possible for resolver to re-use the AT bit defined in this memo as a
   indication asking ATR response from the server.

   However the two cases are currently outside of the scope of server-
   ATR specification.  It needs further discussion.

Appendix C.  Revision history of this document

C.1.  draft-song-atr-large-resp-01

   After receiving reviews and comments, changes of 01 version are shown
   as belows:

   o  Rewrite introduction and add another goal of ATR as a measuring

   o  Add section 3 indicating a ATR response.  An bit in the EDNS0 OPT
      header is defined as a indicator of ATR response.  The flag bit is
      called "ATR Response" (AT) bit;

   o  Add Section 4 Operation considerations, which discuss ATR timer ,
      ATR payload size, and less aggressiveness of ATR;

   o  Add IANA consideration to register the AT bit;

   o  Add section 7 Acknowledgments;

   o  Append a list of references regarding Network reordering, and
      APNIC's study on IPv6 and DNS;

   o  Add Appendix A, An introduce of APNIC testing work and author's

   o  Appendix B.  Considerations on Resolver awareness of ATR;

   o  Change the category="std" . It is said in RFC6891 IETF Standards
      Action is required for assignments of new EDNS(0) flags.  So the
      draft should be categorized as standard track if registering AT
      bit is desired in this document.

   Change history is also available in the public GitHub repository
   where this document is maintained: <

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Internet-DrATR: Additional Truncation Response for Large DNS    May 2018

Author's Address

   Linjian Song
   Beijing Internet Institute


Song                    Expires November 9, 2018               [Page 14]