DNSOP Working Group                                     Paul Vixie, ISC
   INTERNET-DRAFT                                         Akira Kato, WIDE
                           DNS Response Size Issues

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      Copyright (C) The Internet Society (2006).  All Rights Reserved.

                                    Abstract

      With a mandated default minimum maximum message size of 512 octets,
      the DNS protocol presents some special problems for zones wishing to
      expose a moderate or high number of authority servers (NS RRs).  This
      document explains the operational issues caused by, or related to
      this response size limit.

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   1 - Introduction and Overview

   1.1. The DNS standard (see [RFC1035 4.2.1]) limits message size to 512
   octets.  Even though this limitation was due to the required minimum UDP IP
   reassembly limit for IPv4, it is became a hard DNS protocol limit and is
   not implicitly relaxed by changes in transport, for example to IPv6.

   1.2. The EDNS0 protocol extension (see [RFC2671 2.3, 4.5]) permits
   larger responses by mutual agreement of the requestor and responder.
   However, deployment of EDNS0 cannot be expected to reach every Internet
   resolver in the short or medium term.  The 512 octet message size limit
   remains in practical effect at this time.

   1.3. Since DNS responses include a copy of the request, the space
   available for response data is somewhat less than the full 512 octets.
   Negative responses are quite small, but for positive and delegation
   responses, every octet must be carefully and sparingly allocated.  This
   document specifically addresses delegation response sizes.

   2 - Delegation Details

   2.1. A delegation response will include the following elements:

      Header Section: fixed length (12 octets)
      Question Section: original query (name, class, type)
      Answer Section: (empty)
      Authority Section: NS RRset (nameserver names)
      Additional Section: A and AAAA RRsets (nameserver addresses)

   2.2. If the total response size would exceed 512 octets, and if the data
   that would not fit belonged in the answer or authority section, was "required", then the TC bit will be set
   (indicating truncation) which may truncation).  This will usually cause the requestor to retry
   using TCP, depending on what information was desired and what
   information was omitted.  (For example, truncation in the authority
   section is of no interest to a stub resolver who only plans to consume
   the answer section.)  If a retry using TCP is needed, the total cost of
   the transaction is much higher.  (See  See [RFC1123 6.1.3.2] for details on
   the requirement that UDP be attempted before falling back to TCP.) TCP.

   2.3. RRsets are never sent partially unless TC bit set to indicate
   truncation.  When TC bit is set, the final apparent RRset in the final
   nonempty section must be considered "possibly damaged" (see [RFC1035
   6.2], [RFC2181 9]).

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   2.4. With or without truncation, the glue present in the additional data
   section should be considered "possibly incomplete", and requestors
   should be prepared to re-query for any damaged or missing RRsets.  For
   multi-transport name or mail services, this can mean querying for an
   IPv6 (AAAA) RRset even when an IPv4 (A) RRset  Note
   that truncation of the additional data section might not be signalled
   via the TC bit since additional data is present.

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   2.4. often optional.

   2.5. DNS label compression allows a domain name to be instantiated only
   once per DNS message, and then referenced with a two-octet "pointer"
   from other locations in that same DNS message.  If all nameserver names
   in a message are similar (for example, all ending in ".ROOT-
   SERVERS.NET"), then more space will be available for uncompressable data
   (such as nameserver addresses).

   2.5.

   2.6. The query name can be as long as 255 characters of presentation
   data, which can be up to 256 octets of network data.  In this worst case
   scenario, the question section will be 260 octets in size, which would
   leave only 240 octets for the authority and additional sections (after
   deducting 12 octets for the fixed length header.)

   2.6.

   2.7. Average and maximum question section sizes can be predicted by the
   zone owner, since they will know what names actually exist, and can
   measure which ones are queried for most often.  For cost and performance
   reasons, the majority of requests should be satisfied without truncation
   or TCP retry.

   2.8. Some queries to non-existing names can be large, however, but this is not a
   problem because the negative responses include a SOA record in the need not contain any answer,
   authority section.

   2.7. Requestors who deliberately send large queries to force truncation
   are only increasing their own costs, and cannot effectively attack or additional records.  (See [RFC2308 2.1] for more
   information about the
   resources format of an authority server since the requestor would have to retry
   using TCP to complete the attack.  An attack that always used TCP would
   have a lower cost.

   2.8. negative responses.)

   2.9. The minimum useful number of address records name servers is two, since giving
   only one address undermines the for redundancy requirement.  Implicit
   truncation (truncation without setting TC bit) which occurs after two
   (see [RFC1034 4.1]).  In case of multihomed name servers, it is
   advantageous to include an address record from each of several name
   servers before including several address records have been added for any one name
   server.  If address records for more than one transport (for example, A
   and AAAA) are available, then it is advantageous to include records of
   both types early on, before the additional data section message is
   therefore less operationally significant than truncation which occurs
   earlier.

   2.9. full.

   2.10. The best case is no truncation at all.  This is because many
   requestors will retry using TCP by reflex, or will automatically re-
   query for RRsets that are "possibly truncated", without considering
   whether the omitted data was actually necessary.

   2.10.

   2.11. Each added NS RR for a zone will add a minimum of between 16 and
   44 octets to every untruncated referral or negative response from the
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   zone's authority servers (16 octets for an NS RR, 16 octets for an A RR,
   and 28 octets for an AAAA RR), in addition to whatever space is taken by
   the nameserver name (NS NSDNAME as well as A or AAAA owner name).

   2.12. While DNS distinguishes between necessary and optional resource
   records, this distinction is according to protocol elements necessary to
   signify facts, and takes no official notice of protocol content
   necessary to ensure correct operation.  For example, a nameserver name
   that is in or below the zone cut being described by a delegation is
   "necessary content," since there is no way to reach that zone unless the
   parent zone's delegation includes "glue records" describing that name
   server's addresses.

   2.13. It is also necessary to distinguish between "explicit truncation"
   where a message could not contain enough records to convey its intended
   meaning, and so the TC bit has been set, and "silent truncation", where
   the message was not large enough to contain some records which were "not
   required", and so the TC bit was not set.

   2.14. An delegation response should prioritize glue records as follows.

   first
      All glue RRsets for one name server whose name is in or below the
      zone being delegated, or which has multiple address RRsets (currently
      A and AAAA), or preferrably both;

   second
      Alternate between adding all glue RRsets for any name servers whose
      names are in or below the zone being delegated, and all glue RRsets
      for any name servers who have multiple address RRsets (currently A
      and AAAA);

   thence
      All other glue RRsets, in any order.

   The goal of this priority scheme is to offer "necessary" glue first,
   avoiding silent truncation for this glue if possible.

   2.15. If any "necessary content" is silently truncated, then it is
   advisable that the TC bit be set in order to force a TCP retry, rather
   than have the zone be unreachable.  Note that a parent server's proper
   response to a query for in-child glue or below-child glue is a referral
   rather than an answer, and that this referral MUST be able to contain
   the in-child or below-child glue, and that in outlying cases, only EDNS
   or TCP will be large enough to contain that data.

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   3 - Analysis

   3.1. An instrumented protocol trace of a best case delegation response
   follows.  Note that 13 servers are named, and 13 addresses are given.
   This query was artificially designed to exactly reach the 512 octet
   limit.

      ;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13
      ;; QUERY SECTION:
      ;;  [23456789.123456789.123456789.\
           123456789.123456789.123456789.com A IN]        ;; @80

      ;; AUTHORITY SECTION:
      com.                 86400 NS  E.GTLD-SERVERS.NET.  ;; @112
      com.                 86400 NS  F.GTLD-SERVERS.NET.  ;; @128
      com.                 86400 NS  G.GTLD-SERVERS.NET.  ;; @144
      com.                 86400 NS  H.GTLD-SERVERS.NET.  ;; @160
      com.                 86400 NS  I.GTLD-SERVERS.NET.  ;; @176
      com.                 86400 NS  J.GTLD-SERVERS.NET.  ;; @192
      com.                 86400 NS  K.GTLD-SERVERS.NET.  ;; @208
      com.                 86400 NS  L.GTLD-SERVERS.NET.  ;; @224
      com.                 86400 NS  M.GTLD-SERVERS.NET.  ;; @240
      com.                 86400 NS  A.GTLD-SERVERS.NET.  ;; @256
      com.                 86400 NS  B.GTLD-SERVERS.NET.  ;; @272
      com.                 86400 NS  C.GTLD-SERVERS.NET.  ;; @288
      com.                 86400 NS  D.GTLD-SERVERS.NET.  ;; @304

      ;; ADDITIONAL SECTION:
      A.GTLD-SERVERS.NET.  86400 A   192.5.6.30           ;; @320
      B.GTLD-SERVERS.NET.  86400 A   192.33.14.30         ;; @336
      C.GTLD-SERVERS.NET.  86400 A   192.26.92.30         ;; @352
      D.GTLD-SERVERS.NET.  86400 A   192.31.80.30         ;; @368
      E.GTLD-SERVERS.NET.  86400 A   192.12.94.30         ;; @384
      F.GTLD-SERVERS.NET.  86400 A   192.35.51.30         ;; @400
      G.GTLD-SERVERS.NET.  86400 A   192.42.93.30         ;; @416
      H.GTLD-SERVERS.NET.  86400 A   192.54.112.30        ;; @432
      I.GTLD-SERVERS.NET.  86400 A   192.43.172.30        ;; @448
      J.GTLD-SERVERS.NET.  86400 A   192.48.79.30         ;; @464
      K.GTLD-SERVERS.NET.  86400 A   192.52.178.30        ;; @480
      L.GTLD-SERVERS.NET.  86400 A   192.41.162.30        ;; @496
      M.GTLD-SERVERS.NET.  86400 A   192.55.83.30         ;; @512

      ;; MSG SIZE  sent: 80  rcvd: 512
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   3.2. For longer query names, the number of address records supplied will
   be lower.  Furthermore, it is only by using a common parent name (which
   is GTLD-SERVERS.NET in this example) that all 13 addresses are able to
   fit.  The following output from a response simulator demonstrates these
   properties:

      % perl respsize.pl a.dns.br b.dns.br c.dns.br d.dns.br
      a.dns.br requires 10 bytes
      b.dns.br requires 4 bytes
      c.dns.br requires 4 bytes
      d.dns.br requires 4 bytes
      # of NS: 4
      For maximum size query (255 byte):
          only A is considered:        # of A is 4 (green)
          A and AAAA are considered:   # of A+AAAA is 3 (yellow)
          preferred-glue A is assumed: # of A is 4, # of AAAA is 3 (yellow)
      For average size query (64 byte):
          only A is considered:        # of A is 4 (green)
          A and AAAA are considered:   # of A+AAAA is 4 (green)
          preferred-glue A is assumed: # of A is 4, # of AAAA is 4 (green)

      % perl respsize.pl ns-ext.isc.org ns.psg.com ns.ripe.net ns.eu.int
      ns-ext.isc.org requires 16 bytes
      ns.psg.com requires 12 bytes
      ns.ripe.net requires 13 bytes
      ns.eu.int requires 11 bytes
      # of NS: 4
      For maximum size query (255 byte):
          only A is considered:        # of A is 4 (green)
          A and AAAA are considered:   # of A+AAAA is 3 (yellow)
          preferred-glue A is assumed: # of A is 4, # of AAAA is 2 (yellow)
      For average size query (64 byte):
          only A is considered:        # of A is 4 (green)
          A and AAAA are considered:   # of A+AAAA is 4 (green)
          preferred-glue A is assumed: # of A is 4, # of AAAA is 4 (green)

   (Note: The response simulator program is shown in Section 5.)

   Here we use the term "green" if all address records could fit, or
   "yellow" if two or more could fit, or "orange" if only one could fit, or
   "red" if no address record could fit.  It's clear that without a common
   parent for nameserver names, much space would be lost.  For these
   examples we use an average/common name size of 15 octets, befitting our
   assumption of GTLD-SERVERS.NET as our common parent name.

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   We're assuming an average query name size of 64 since that is the
   typical average maximum size seen in trace data at the time of this
   writing.  If Internationalized Domain Name (IDN) or any other technology
   which results in larger query names be deployed significantly in advance
   of EDNS, then new measurements and new estimates will have to be made.

   4 - Conclusions

   4.1. The current practice of giving all nameserver names a common parent
   (such as GTLD-SERVERS.NET or ROOT-SERVERS.NET) saves space in DNS
   responses and allows for more nameservers to be enumerated than would
   otherwise be possible, since the common parent domain name only appears
   once in a DNS message and is referred to via "compression pointers"
   thereafter.

   4.2. If all nameserver names for a zone share a common parent, then it
   is operationally advisable to make all servers for the zone so served
   also be authoritative for the zone of that common parent.  For example,
   the root name servers (?.ROOT-SERVERS.NET) can answer authoritatively
   for the ROOT-SERVERS.NET.  This is to ensure that the zone's servers
   always have the zone's nameservers' glue available when delegating.

   4.3. Thirteen (13) seems to be the effective maximum number of
   nameserver names usable traditional (non-extended) DNS, assuming a
   common parent domain name, and given that response truncation is
   undesirable as an average case, and assuming mostly IPv4-only
   reachability (only A RRs exist, not AAAA RRs).

   4.3.

   XXX 4.4. Adding two up to five IPv6 nameserver address records (AAAA RRs) to
   a prototypical delegation that currently contains thirteen (13) IPv4
   nameserver addresses (A RRs) for thirteen (13) nameserver names under a
   common parent, would not have a significant negative operational impact
   on the domain name system.

   5 - Source Code

   #!/usr/bin/perl
   #
   # SYNOPSIS
   #    repsize.pl [ -z zone ] fqdn_ns1 fqdn_ns2 ...
   #        if all queries are assumed to have a same zone suffix,
   #     such as "jp" in JP TLD servers, specify it in -z option
   #
   use strict;
   use Getopt::Std;
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   my ($sz_msg) = (512);
   my ($sz_header, $sz_ptr, $sz_rr_a, $sz_rr_aaaa) = (12, 2, 16, 28);
   my ($sz_type, $sz_class, $sz_ttl, $sz_rdlen) = (2, 2, 4, 2);
   my (%namedb, $name, $nssect, %opts, $optz);
   my $n_ns = 0;

   getopt('z', %opts);
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   if (defined($opts{'z'})) {
       server_name_len($opts{'z'}); # just register it
   }

   foreach $name (@ARGV) {
       my $len;
       $n_ns++;
       $len = server_name_len($name);
       print "$name requires $len bytes\n";
       $nssect += $sz_ptr + $sz_type + $sz_class + $sz_ttl
               +  $sz_rdlen + $len;
   }
   print "# of NS: $n_ns\n";
   arsect(255, $nssect, $n_ns, "maximum");
   arsect(64, $nssect, $n_ns, "average");

   sub server_name_len {
       my ($name) = @_;
       my (@labels, $len, $n, $suffix);

       $name =~ tr/A-Z/a-z/;
       @labels = split(/\./, $name);
       $len = length(join('.', @labels)) + 2;
       for ($n = 0; $#labels >= 0; $n++, shift @labels) {
           $suffix = join('.', @labels);
           return length($name) - length($suffix) + $sz_ptr
               if (defined($namedb{$suffix}));
           $namedb{$suffix} = 1;
       }
       return $len;
   }

   sub arsect {
       my ($sz_query, $nssect, $n_ns, $cond) = @_;
       my ($space, $n_a, $n_a_aaaa, $n_p_aaaa, $ansect);
       $ansect = $sz_query + 1 + $sz_type + $sz_class;
       $space = $sz_msg - $sz_header - $ansect - $nssect;
       $n_a = atmost(int($space / $sz_rr_a), $n_ns);
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       $n_a_aaaa = atmost(int($space
                              / ($sz_rr_a + $sz_rr_aaaa)), $n_ns);
       $n_p_aaaa = atmost(int(($space - $sz_rr_a * $n_ns)
                              / $sz_rr_aaaa), $n_ns);
       printf "For %s size query (%d byte):\n", $cond, $sz_query;
       printf "    only A is considered:        ";
       printf "# of A is %d (%s)\n", $n_a, &judge($n_a, $n_ns);
       printf "    A and AAAA are considered:   ";
       printf "# of A+AAAA is %d (%s)\n",
              $n_a_aaaa, &judge($n_a_aaaa, $n_ns);
       printf "    preferred-glue A is assumed: ";
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       printf "# of A is %d, # of AAAA is %d (%s)\n",
           $n_a, $n_p_aaaa, &judge($n_p_aaaa, $n_ns);
   }

   sub judge {
       my ($n, $n_ns) = @_;
       return "green" if ($n >= $n_ns);
       return "yellow" if ($n >= 2);
       return "orange" if ($n == 1);
       return "red";
   }

   sub atmost {
       my ($a, $b) = @_;
       return 0 if ($a < 0);
       return $b if ($a > $b);
       return $a;
   }

   6 - Security Considerations

   The recommendations contained in this document have no known security
   implications.

   7 - IANA Considerations

   This document does not call for changes or additions to any IANA
   registry.

   8 - Acknowledgement The authors thank Peter Koch and Rob Austein for
   their valuable comments and suggestions.

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   9 - Refrenaces

   [RFC1034] Mockapetris, P.V., "Domain names - Concepts and Facilities",
      RFC1034, November 1987.

   [RFC1035] Mockapetris, P.V., "Domain names - implementation Implementation and
      specification",
      Specification", RFC1035, November 1987.

   [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
      Application and Support", RFC1123, October 1989.

   [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)",
      RFC2308, March 1998.

   [RFC2181] Elz, R., Bush, R., "Clarifications to the DNS Specification",
      RFC2181, July 1997.

   [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC2671,
      August 1999.

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   10 - Authors' Addresses

   Paul Vixie
      950 Charter Street
      Redwood City, CA 94063
      +1 650 423 1301
      vixie@isc.org

   Akira Kato
      University of Tokyo, Information Technology Center
      2-11-16 Yayoi Bunkyo
      Tokyo 113-8658, JAPAN
      +81 3 5841 2750
      kato@wide.ad.jp

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