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- DNSOP Working Group Paul Vixie, ISC
- INTERNET-DRAFT Akira Kato, WIDE
- <draft-ietf-dnsop-respsize-05.txt> August 2006
-
- DNS Referral Response Size Issues
-
- Status of this Memo
- By submitting this Internet-Draft, each author represents that any
- applicable patent or other IPR claims of which he or she is aware
- have been or will be disclosed, and any of which he or she becomes
- aware will be disclosed, in accordance with Section 6 of BCP 79.
-
- Internet-Drafts are working documents of the Internet Engineering
- Task Force (IETF), its areas, and its working groups. Note that
- other groups may also distribute working documents as Internet-
- Drafts.
-
- Internet-Drafts are draft documents valid for a maximum of six months
- and may be updated, replaced, or obsoleted by other documents at any
- time. It is inappropriate to use Internet-Drafts as reference
- material or to cite them other than as "work in progress."
-
- The list of current Internet-Drafts can be accessed at
- http://www.ietf.org/ietf/1id-abstracts.txt
-
- The list of Internet-Draft Shadow Directories can be accessed at
- http://www.ietf.org/shadow.html.
-
- Copyright Notice
-
- 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, and suggests ways to optimize the use of
- this limited space. Guidance is offered to DNS server implementors
- and to DNS zone operators.
-
-
-
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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 IP
- reassembly limit for IPv4, it 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 requester and responder.
- The 512 octet message size limit will remain in practical effect until
- there is widespread deployment of EDNS0 in DNS resolvers on the
- Internet.
-
- 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. RELEVANT PROTOCOL ELEMENTS
-
- 2.1.1. 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)
-
- 2.1.2. If the total response size exceeds 512 octets, and if the data
- that does not fit was "required", then the TC bit will be set
- (indicating truncation). This will usually cause the requester 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 [RFC1123 6.1.3.2] for details on
- the requirement that UDP be attempted before falling back to TCP.
-
- 2.1.3. RRsets are never sent partially unless TC bit set to indicate
- truncation. When TC bit is set, the final apparent RRset in the final
- non-empty section must be considered "possibly damaged" (see [RFC1035
- 6.2], [RFC2181 9]).
-
-
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- 2.1.4. With or without truncation, the glue present in the additional
- data section should be considered "possibly incomplete", and requesters
- should be prepared to re-query for any damaged or missing RRsets. Note
- that truncation of the additional data section might not be signalled
- via the TC bit since additional data is often optional.
-
- 2.1.5. 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 (see [RFC1035
- 4.1.4]). If all nameserver names in a message share a common parent
- (for example, all ending in ".ROOT-SERVERS.NET"), then more space will
- be available for incompressable data (such as nameserver addresses).
-
- 2.1.6. 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.)
-
- 2.2. ADVICE TO ZONE OWNERS
-
- 2.2.1. 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. Note that if the zone
- contains any wildcards, it is possible for maximum length queries to
- require positive responses, but that it is reasonable to expect
- truncation and TCP retry in that case. For cost and performance
- reasons, the majority of requests should be satisfied without truncation
- or TCP retry.
-
- 2.2.2. Some queries to non-existing names can be large, but this is not
- a problem because negative responses need not contain any answer,
- authority or additional records. See [RFC2308 2.1] for more information
- about the format of negative responses.
-
- 2.2.3. The minimum useful number of name servers is two, for redundancy
- (see [RFC1034 4.1]). A zone's name servers should be reachable by all
- IP transport protocols (e.g., IPv4 and IPv6) in common use.
-
- 2.2.4. The best case is no truncation at all. This is because many
- requesters 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.
-
-
-
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- 2.3. ADVICE TO SERVER IMPLEMENTORS
-
- 2.3.1. In case of multi-homed name servers, it is advantageous to
- include an address record from each of several name servers before
- including several address records 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 message is full.
-
- 2.3.2. Each added NS RR for a zone will add between 16 and 44 octets to
- every non-truncated 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).
-
- 2.3.3. 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.
-
- 2.3.4. 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.
-
- 2.3.5. A 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 preferably 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.
-
-
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- Whenever there are multiple candidates for a position in this priority
- scheme, one should be chosen on a round-robin or fully random basis.
-
- The goal of this priority scheme is to offer "necessary" glue first,
- avoiding silent truncation for this glue if possible.
-
- 2.3.6. 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.
-
- 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
-
-
-
-
-
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- ;; 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
-
- 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, due to the use of DNS compression pointers in the last 12
- occurances of the parent domain name. 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)
-
-
-
-
-
-
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-
- % 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.
-
- We're assuming a medium query name size of 64 since that is the typical
- 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 thus 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, and
-
-
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-
- will be able to respond with answers rather than referrals if a
- requester who wants that glue comes back asking for it. In this case
- the name server will likely be a "stealth server" -- authoritative but
- unadvertised in the glue zone's NS RRset. See [RFC1996 2] for more
- information about stealth servers.
-
- 4.3. Thirteen (13) is the effective maximum number of nameserver names
- usable traditional (non-extended) DNS, assuming a common parent domain
- name, and given that implicit referral response truncation is
- undesirable in the average case.
-
- 4.4. Multi-homing of name servers within a protocol family is
- inadvisable since the necessary glue RRsets (A or AAAA) are atomically
- indivisible, and will be larger than a single resource record. Larger
- RRsets are more likely to lead to or encounter truncation.
-
- 4.5. Multi-homing of name servers across protocol families is less
- likely to lead to or encounter truncation, partly because multiprotocol
- clients are more likely to speak EDNS which can use a larger response
- size limit, and partly because the resource records (A and AAAA) are in
- different RRsets and are therefore divisible from each other.
-
- 4.6. Name server names which are at or below the zone they serve are
- more sensitive to referral response truncation, and glue records for
- them should be considered "less optional" than other glue records, in
- the assembly of referral responses.
-
- 4.7. If a zone is served by thirteen (13) name servers having a common
- parent name (such as ?.ROOT-SERVERS.NET) and each such name server has a
- single address record in some protocol family (e.g., an A RR), then all
- thirteen name servers or any subset thereof could multi-home in a second
- protocol family by adding a second address record (e.g., an AAAA RR)
- without reducing the reachability of the zone thus served.
-
- 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 $n_ns = 0;
-
- getopt('z', %opts);
- if (defined($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: ";
- 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, Rob Austein, and Joe Abley for their
- valuable comments and suggestions.
-
-
-
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-
- This work was supported by the US National Science Foundation (research
- grant SCI-0427144) and DNS-OARC.
-
- 9 - References
-
- [RFC1034] Mockapetris, P.V., "Domain names - Concepts and Facilities",
- RFC1034, November 1987.
-
- [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.
-
- [RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
- Changes (DNS NOTIFY)", RFC1996, August 1996.
-
- [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.
-
- 10 - Authors' Addresses
-
- Paul Vixie
- Internet Systems Consortium, Inc.
- 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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-
- Full Copyright Statement
-
- Copyright (C) The Internet Society (2006).
-
- 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
- "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR
- IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
- ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
- INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
- INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
- WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
-
- Intellectual Property
-
- The IETF takes no position regarding the validity or scope of any
- Intellectual Property Rights or other rights that might be claimed to
- pertain to the implementation or use of the technology described in this
- document or the extent to which any license under such rights might or
- might not be available; nor does it represent that it has made any
- independent effort to identify any such rights. Information on the
- procedures with respect to rights in RFC documents can be found in BCP
- 78 and BCP 79.
-
- Copies of IPR disclosures made to the IETF Secretariat and any
- assurances of licenses to be made available, or the result of an attempt
- made to obtain a general license or permission for the use of such
- proprietary rights by implementers or users of this specification can be
- obtained from the IETF on-line IPR repository at
- http://www.ietf.org/ipr.
-
- The IETF invites any interested party to bring to its attention any
- copyrights, patents or patent applications, or other proprietary rights
- that may cover technology that may be required to implement this
- standard. Please address the information to the IETF at
- ietf-ipr@ietf.org.
-
- Acknowledgement
-
- Funding for the RFC Editor function is provided by the IETF
- Administrative Support Activity (IASA).
-
-
-
-
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