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   DNSOP Working Group                                     Paul Vixie, ISC
   INTERNET-DRAFT                                         Akira Kato, WIDE
   <draft-ietf-dnsop-respsize-03.txt>                            June 2006
                           DNS Response Size Issues
   Status of this Memo
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   Copyright Notice
      Copyright (C) The Internet Society (2006).  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.
   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, then the
   TC bit will be set (indicating truncation) which may cause the requestor
   to retry using TCP, depending on what information was desired and what
   information was omitted.  If a retry using TCP is needed, the total cost
   of the transaction is much higher.  (See [RFC1123] for details
   on the requirement that UDP be attempted before falling back to 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 [RFC2181
   9]).  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 is present.
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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
   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.  Some queries to non-existing names can be large, however,
   this is not a problem because the responses include a SOA record in the
   authority section.
   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 number of address records is two, since giving
   only one address undermines the redundancy requirement.  Implicit
   truncation (truncation without setting TC bit) which occurs after two
   address records have been added to the additional data section is
   therefore less operationally significant than truncation which occurs
   2.9. 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. 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
   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).
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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, ANS: 0, AUTH: 13, 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 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"
   4.2. 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. 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
   #    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;
   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;
        $len = server_name_len($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);
       $n_a_aaaa = atmost(int($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;
   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
   Acknowledgement The authors thank Peter Koch and Rob Austein for their
   valuable comments and suggestions.
   [RFC1035] Mockapetris, P.V., "Domain names - implementation and
      specification", RFC1035, November 1987.
   [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
      Application and Support", RFC1123, October 1989.
   [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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   Authors' Addresses
   Paul Vixie
      950 Charter Street
      Redwood City, CA 94063
      +1 650 423 1301
   Akira Kato
      University of Tokyo, Information Technology Center
      2-11-16 Yayoi Bunkyo
      Tokyo 113-8658, JAPAN
      +81 3 5841 2750
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