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 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. Expires November 2006 [Page 1]
INTERNET-DRAFT June 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 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 6.1.3.2] 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. Expires November 2006 [Page 2]
INTERNET-DRAFT June 2006 RESPSIZE 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 earlier. 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). Expires November 2006 [Page 3]
INTERNET-DRAFT June 2006 RESPSIZE 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 Expires November 2006 [Page 4]
INTERNET-DRAFT June 2006 RESPSIZE 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): 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. Expires November 2006 [Page 5]
INTERNET-DRAFT June 2006 RESPSIZE 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" thereafter. 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 #!/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; 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 $n_ns = 0; getopt('z', %opts); Expires November 2006 [Page 6]
INTERNET-DRAFT June 2006 RESPSIZE if (defined($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); $n_a_aaaa = atmost(int($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: "; Expires November 2006 [Page 7]
INTERNET-DRAFT June 2006 RESPSIZE 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 implications. IANA Considerations This document does not call for changes or additions to any IANA registry. Acknowledgement The authors thank Peter Koch and Rob Austein for their valuable comments and suggestions. Refrenaces [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. Expires November 2006 [Page 8]
INTERNET-DRAFT June 2006 RESPSIZE Authors' Addresses Paul Vixie 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 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 Expires November 2006 [Page 9]
INTERNET-DRAFT June 2006 RESPSIZE 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 November 2006 [Page 10]
