draft-ietf-dnsop-respsize-02.txt

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   DNSOP Working Group                                     Paul Vixie, ISC
   INTERNET-DRAFT                                         Akira Kato, WIDE
   <draft-ietf-dnsop-respsize-02.txt>                            July 2005

                           DNS Response Size Issues

   Status of this Memo
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   Copyright Notice

      Copyright (C) The Internet Society (2005).  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
   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 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 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 belonged 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 desired and what information was omitted.  If a retry using TCP is
   needed, the total cost of the transaction is much higher.  (See [RFC1123
   6.1.3.2] for details on the protocol requirement that UDP be attempted
   before falling back to TCP.)

   2.3. RRsets are never sent partially unless truncation occurs, in which
   case the final apparent RRset in the final nonempty section must be
   considered "possibly damaged".  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,



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   this can mean querying for an IPv6 (AAAA) RRset even when an IPv4 (A)
   RRset is present.

   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.

   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 with
   only one address, the probability that it would refer to an unreachable
   server is too 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, 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 and A/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
   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):
              if only A is considered:     # of A is 4 (green)
              if A and AAAA are condered:  # of A+AAAA is 3 (yellow)
              if prefer_glue A is assumed: # of A is 4, # of AAAA is 3 (yellow)
      For average size query (64 byte):
              if only A is considered:     # of A is 4 (green)
              if A and AAAA are condered:  # of A+AAAA is 4 (green)
              if prefer_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