DNSOP Working Group                               Paul Vixie, ISC (Ed.)
   INTERNET-DRAFT                                         Akira Kato, WIDE
   <draft-ietf-dnsop-respsize-00.txt>                           June, 2003
   <draft-ietf-dnsop-respsize-01.txt>                           July, 2004

                           DNS Response Size Issues

   Status of this Memo
      This document is an Internet-Draft and is in full conformance with subject to all provisions
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   Copyright Notice

      Copyright (C) The Internet Society (2003). (2003-2004).  All Rights Reserved.


      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
   reassembly limit for IPv4, it is a hard DNS protocol limit and is not
   implicitly relaxed by changes in transport, for example to IPv6.

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   1.2. The EDNS0 standard (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.
   For negative or positive responses, there is rarely a space constraint.  For
   positive and delegation responses, though, 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 was in the question, answer, or authority section,
   then the TC bit will be set (indicating truncation) which may cause the
   requestor to retry using TCP, depending on what information was present
   and what was omitted.  If a retry using TCP is needed, the total cost of
   the transaction is much higher.

   2.3. RRsets are never sent partially, so if truncation occurs, entire
   RRsets are omitted.  Note that the authority section consists of a
   single RRset.  It is absolutely essential that truncation not occur in
   the authority section.

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   2.4. 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. 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
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   leave only 240 octets for the authority and additional sections (after
   deducting 12 octets for the fixed length header.)

   2.6. 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.

   2.7. Requestors who deliberately send large queries to force truncation
   are only increasing their own costs, and cannot effectively attack the
   resources 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. The minimum useful glue is two number of address records.  (With records is two, since with
   only one address, the probability that it would refer to an unreachable
   server is too high.) high.  Truncation which occurs after two address records
   have been added to the additional data section is therefore less
   operationally significant than truncation which occurs earlier.

   2.9. The best case is no truncation.  (This is because many requestors
   will retry using TCP by reflex, without considering whether the omitted
   data was actually necessary.)

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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

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

      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

      A.GTLD-SERVERS.NET.  86400 A           ;; @320
      B.GTLD-SERVERS.NET.  86400 A         ;; @336
      C.GTLD-SERVERS.NET.  86400 A         ;; @352
      D.GTLD-SERVERS.NET.  86400 A         ;; @368
      E.GTLD-SERVERS.NET.  86400 A         ;; @384
      F.GTLD-SERVERS.NET.  86400 A         ;; @400
      G.GTLD-SERVERS.NET.  86400 A         ;; @416
      H.GTLD-SERVERS.NET.  86400 A        ;; @432
      I.GTLD-SERVERS.NET.  86400 A        ;; @448
      J.GTLD-SERVERS.NET.  86400 A         ;; @464
      K.GTLD-SERVERS.NET.  86400 A        ;; @480
      L.GTLD-SERVERS.NET.  86400 A        ;; @496
      M.GTLD-SERVERS.NET.  86400 A         ;; @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

      % perl respsize.pl 13 13 0
        common name, average case: msg:303    glue#13    nsaddr#13 (green)
        common name,   worst case: msg:495    glue#    nsaddr# 1 (red)
      uncommon name, average case: msg:457    glue#    nsaddr# 3 (orange)
      uncommon name,   worst case: msg:649(*) glue# nsaddr# 0 (red)
      % perl respsize.pl 13 13 2
        common name, average case: msg:303    glue#11    nsaddr#11 (orange)
        common name,   worst case: msg:495    glue#    nsaddr# 1 (red)
      uncommon name, average case: msg:457    glue#    nsaddr# 2 (orange)
      uncommon name,   worst case: msg:649(*) glue# nsaddr# 0 (red)

   (Note: The response simulator's source code simulator program is contained shown in the
   appendix.) Section 5.)

   Here we use the term "green" if all address records could fit, or
   "orange" if two or more could fit, or "red" if fewer than two could fit.
   It's clear that without a common parent for nameserver names, much space
   would be lost.

   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 more new measurements and new estimates will have to be

   4 - Further Work Conclusions

   4.1. Traces from one The current practice of giving all nameserver names a common parent
   (such as GTLD-SERVERS.NET or more root name servers ROOT-SERVERS.NET) saves space in DNS
   responses and at least a dozen
   diverse TLD name servers should allows for more nameservers to be analyzed enumerated than would
   otherwise be possible.  (Note that in this case it is wise to measure serve the actual
   minimum, maximum, average, and standard deviation in query name sizes.

   4.2. Current delegation response sizes
   common parent domain's zone from the root server system for
   all TLDs should be measured same servers that are named within
   it, in light of the known query name sizes found order to be limit external dependencies when all your eggs are in use.

   4.3. A policy should a
   single basket.)

   4.2. Thirteen (13) seems to be created for the addition effective maximum number of AAAA RR's for
   existing name servers, in both TLD delegations under the root zone,
   nameserver names usable traditional (non-extended) DNS, assuming a
   common parent domain name, and
   SLD delegations under interested TLDs.

   4.4. Participants assuming that additional-data truncation
   is undesirable in the Internationalized Domain Names (IDN) effort
   should take careful note of the performance effects of larger query
   names on root name server system delegation sizes. average case.

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   4.3. Adding two 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 -w

   $asize = 2+2+2+4+2+4;
   $aaaasize = 2+2+2+4+2+16;
   ($nns, $na, $naaaa) = @ARGV;
   test("common", "average", common_name_average($nns),
        $na, $naaaa);
   test("common", "worst", common_name_worst($nns),
        $na, $naaaa);
   test("uncommon", "average", uncommon_name_average($nns),
        $na, $naaaa);
   test("uncommon", "worst", uncommon_name_worst($nns),
        $na, $naaaa);
   exit 0;

   sub test { my ($namekind, $casekind, $msg, $na, $naaaa) = @_;
        my $nglue = numglue($msg, $na, $naaaa);
        printf "%8s name, %7s case: msg:%3d%s glue#%2d nsaddr#%2d (%s)\n",
             $namekind, $casekind,
             $msg, ($msg > 512) ? "(*)" : "   ",
             $nglue, ($nglue == $na + $naaaa) ? "green"
                  : ($nglue >= 2) ? "orange"
                       : "red";

   sub pnum { my ($num, $tot) = @_;
        return sprintf "%3d%s",

   sub numglue { my ($msg, $na, $naaaa) = @_;
        my $space = ($msg > 512) ? 0 : (512 - $msg);
        my $num = 0;

        while ($space && ($na || $naaaa )) {
             if ($na) {
                  if ($space >= $asize) {
                       $space -= $asize;

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             if ($naaaa) {
                  if ($space >= $aaaasize) {
                       $space -= $aaaasize;
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        return $num;

   sub msgsize { my ($qname, $nns, $nsns) = @_;
        return  12 +                            # header
                $qname+2+2 +                    # query
                0 +                             # answer
                $nns * (4+2+2+4+2+$nsns);       # authority

   sub average_case { my ($nns, $nsns) = @_;
        return msgsize(64, $nns, $nsns);

   sub worst_case { my ($nns, $nsns) = @_;
        return msgsize(256, $nns, $nsns);

   sub common_name_average { my ($nns) = @_;
        return 15 + average_case($nns, 2);

   sub common_name_worst { my ($nns) = @_;
        return 15 + worst_case($nns, 2);

   sub uncommon_name_average { my ($nns) = @_;
        return average_case($nns, 15);

   sub uncommon_name_worst { my ($nns) = @_;
        return worst_case($nns, 15);

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   5 - Author's Address

   Security Considerations

   The recommendations contained in this document have no known security

   IANA Considerations

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

   IPR Statement

   Copyright (C) The Internet Society (2003-2004).  This document is
   subject to the rights, licenses and restrictions contained in BCP 78,
   and except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

   Authors' Addresses

   Paul Vixie
      950 Charter Street
      Redwood City, CA 94063
      +1 650 779 7000
      paul@vix.com 423 1301

   Akira Kato
      University of Tokyo, Information Technology Center
      2-11-16 Yayoi Bunkyo
      Tokyo 113-8658, JAPAN
      +81 3 5841 2750