Edit File: draft-ietf-dnsop-respsize-06.txt
DNSOP Working Group Paul Vixie, ISC INTERNET-DRAFT Akira Kato, WIDE <draft-ietf-dnsop-respsize-06.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. Expires January 2007 [Page 1] INTERNET-DRAFT August 2006 RESPSIZE 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, or a CNAME/DNAME chain 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]). Expires January 2007 [Page 2] INTERNET-DRAFT August 2006 RESPSIZE 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 (see discussion in [RFC4472 B]). 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 octets of network data. In this worst case scenario, the question section will be 259 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 immediately, or will automatically re- query for RRsets that are possibly truncated, without considering whether the omitted data was actually necessary. Expires January 2007 [Page 3] INTERNET-DRAFT August 2006 RESPSIZE 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 12 fixed octets (name, type, class, ttl, and rdlen) plus 2 to 255 variable octets (for the NSDNAME). Each A RR will require 16 octets, and each AAAA RR will require 28 octets. 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. Expires January 2007 [Page 4] INTERNET-DRAFT August 2006 RESPSIZE 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 Expires January 2007 [Page 5] INTERNET-DRAFT August 2006 RESPSIZE ;; 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) Expires January 2007 [Page 6] INTERNET-DRAFT August 2006 RESPSIZE % 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 Expires January 2007 [Page 7] INTERNET-DRAFT August 2006 RESPSIZE 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; Expires January 2007 [Page 8] INTERNET-DRAFT August 2006 RESPSIZE 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); Expires January 2007 [Page 9] INTERNET-DRAFT August 2006 RESPSIZE $n_a_aaaa = atmost(int($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, Joe Abley, and Mark Andrews for their valuable comments and suggestions. Expires January 2007 [Page 10] INTERNET-DRAFT August 2006 RESPSIZE 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. [RFC2181] Elz, R., Bush, R., "Clarifications to the DNS Specification", RFC2181, July 1997. [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC2308, March 1998. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC2671, August 1999. [RFC4472] Durand, A., Ihren, J., Savola, P., "Operational Consideration and Issues with IPV6 DNS", April 2006. 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 Expires January 2007 [Page 11] INTERNET-DRAFT August 2006 RESPSIZE 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. 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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). Expires January 2007 [Page 12]
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