Internet Engineering Task Force                                  L. Song
Internet-Draft                                Beijing Internet Institute
Intended status: Experimental                             August 2, 2018
Expires: February 3, 2019

       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.  An
   ATR Experiment was done to show how well it works and some
   operational issues are discussed in this memo as well.

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
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   This Internet-Draft will expire on February 3, 2019.

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

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   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The ATR mechanism . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Experiment on how well ATR works  . . . . . . . . . . . . . .   5
   4.  Operational considerations  . . . . . . . . . . . . . . . . .   6
     4.1.  ATR timer . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  ATR payload size  . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Less aggressiveness of ATR  . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  Considerations on Resolver awareness of ATR  . . . .  10
   Appendix B.  Revision history of this document  . . . . . . . . .  11
     B.1.  draft-song-atr-large-resp-01  . . . . . . . . . . . . . .  11
     B.2.  draft-song-atr-large-resp-02  . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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
   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.

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   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.

   [REMOVE BEFORE PUBLICATION] Note that ATR is not just a proposed
   idea.  Some advocates of ATR implemented it based on BIND9
   (  And
   Some verify it based on an large-scale experiment platform of APNIC
   lab Section 3 which is introduced in this memo.

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.

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   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

   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.

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   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.  Experiment on how well ATR works

   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
   on-line 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
   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

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4.  Operational considerations

   There are some operational consideration on ATR, 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.

   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

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   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.

   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.

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   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 aggressiveness of ATR

   There is a concern ATR sends TC=1 response too aggressively
   especially in the beginning of adoption.  ATR can be implemented as
   an optional and configurable feature at the disposal of authoritative
   server operator.  One of the idea to mitigate this aggressiveness,
   ATR may respond TC=1 responses at a low possibility, such as 10%.

   Another way 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
   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.  The
   extra packet is pretty small.  In the worst case, it's 50% more
   packets and they are small

   DNS cookies [RFC7873] and RRL on authoritative may be possible

6.  IANA considerations

   No IANA considerations for this memo

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.  Shane Kerr help author with the security

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   consideration.  Stephane Bortzmeyer gave thought of happyeyeballs on
   resolver side.

   Acknowledgments are also give to Mukund Sivaraman, Evan Hunt and Mark
   Andrews who implement it and maintained it in a brunch in BIND9 code

8.  References

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

   [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,

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   [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,

   [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.  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 MUST
   recognized ATR and react accordingly.  But it may helpful for some
   cases where a resolver is able to recognized ATR response, for
   example by checking the large edns0 payload size and TrunCation bit.

   One case is use ATR is used as troubleshooting tool by which resolver
   operators are able to flag problematic name servers.  The resolver
   operators is enable to log cases where ATR responses is received
   without a (reassembled) UDP response to a query.  In the case of
   receiving a ATR, RDNS can choose to restrict maximum EDNS to a lower
   value than the default 4096 that currently used.

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   Another 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".

   Note that a normal truncated response may be mistaken as ATR response
   when authoritative server truncated responses once the packets size
   surpasses a certain value.

   However resolver use case of ATR is currently outside of the scope of
   server-ATR proposal.  It needs further discussion.

Appendix B.  Revision history of this document

B.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

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      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: <

B.2.  draft-song-atr-large-resp-02

   Changes in 02 version of ATR draft:

   o  Remove the section of introduction of AT bit as well as
      requirement of IANA registration of that bit;

   o  Change the category of this document to experimental and move the
      introduction of APNIC's experiment from Appendix A to section 3;

   o  Add IANA consideration to register the AT bit;

   o  Add more names in Acknowledgments part after IETF102;

Author's Address

   Linjian Song
   Beijing Internet Institute


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