DNSOP Working Group Paul Vixie, ISC (Ed.) INTERNET-DRAFT Akira Kato, WIDE <draft-ietf-dnsop-respsize-00.txt> June, 2003 DNS Response Size Issues Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 (2003). 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. 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. Expires December 2003 [Page 1]
INTERNET-DRAFT June 2003 RESPSIZE 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 or positive 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 was 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 present and what was omitted. If a retry using TCP is needed, the total cost of the transaction is much higher. 2.3. RRsets are never sent partially, so if truncation occurs, entire RRsets are omitted. Note that the authority section consists of a single RRset. It is absolutely essential that truncation not occur in the authority section. 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 Expires December 2003 [Page 2]
INTERNET-DRAFT June 2003 RESPSIZE 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 glue is two address records. (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, without considering whether the omitted data was actually necessary.) Expires December 2003 [Page 3]
INTERNET-DRAFT June 2003 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, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 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 December 2003 [Page 4]
INTERNET-DRAFT June 2003 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 13 13 0 common name, average case: msg:303 glue#13 (green) common name, worst case: msg:495 glue# 1 (red) uncommon name, average case: msg:457 glue# 3 (orange) uncommon name, worst case: msg:649(*) glue# 0 (red) % perl respsize.pl 13 13 2 common name, average case: msg:303 glue#11 (orange) common name, worst case: msg:495 glue# 1 (red) uncommon name, average case: msg:457 glue# 2 (orange) uncommon name, worst case: msg:649(*) glue# 0 (red) (Note: The response simulator's source code is contained in the appendix.) Here we use the term "green" if all address records could fit, or "orange" if two or more could fit, or "red" if fewer than two could fit. It's clear that without a common parent for nameserver names, much space would be lost. 4 - Further Work 4.1. Traces from one or more root name servers and at least a dozen diverse TLD name servers should be analyzed to measure the actual minimum, maximum, average, and standard deviation in query name sizes. 4.2. Current delegation response sizes from the root server system for all TLDs should be measured in light of the known query name sizes found to be in use. 4.3. A policy should be created for the addition of AAAA RR's for existing name servers, in both TLD delegations under the root zone, and SLD delegations under interested TLDs. 4.4. Participants in the Internationalized Domain Names (IDN) effort should take careful note of the performance effects of larger query names on root name server system delegation sizes. Expires December 2003 [Page 5]
INTERNET-DRAFT June 2003 RESPSIZE 5 - Source Code #!/usr/bin/perl -w $asize = 2+2+2+4+2+4; $aaaasize = 2+2+2+4+2+16; ($nns, $na, $naaaa) = @ARGV; test("common", "average", common_name_average($nns), $na, $naaaa); test("common", "worst", common_name_worst($nns), $na, $naaaa); test("uncommon", "average", uncommon_name_average($nns), $na, $naaaa); test("uncommon", "worst", uncommon_name_worst($nns), $na, $naaaa); exit 0; sub test { my ($namekind, $casekind, $msg, $na, $naaaa) = @_; my $nglue = numglue($msg, $na, $naaaa); printf "%8s name, %7s case: msg:%3d%s glue#%2d (%s)\n", $namekind, $casekind, $msg, ($msg > 512) ? "(*)" : " ", $nglue, ($nglue == $na + $naaaa) ? "green" : ($nglue >= 2) ? "orange" : "red"; } sub pnum { my ($num, $tot) = @_; return sprintf "%3d%s", } sub numglue { my ($msg, $na, $naaaa) = @_; my $space = ($msg > 512) ? 0 : (512 - $msg); my $num = 0; while ($space && ($na || $naaaa )) { if ($na) { if ($space >= $asize) { $space -= $asize; $num++; } $na--; } if ($naaaa) { if ($space >= $aaaasize) { $space -= $aaaasize; $num++; } $naaaa--; Expires December 2003 [Page 6]
INTERNET-DRAFT June 2003 RESPSIZE } } return $num; } sub msgsize { my ($qname, $nns, $nsns) = @_; return 12 + # header $qname+2+2 + # query 0 + # answer $nns * (4+2+2+4+2+$nsns); # authority } sub average_case { my ($nns, $nsns) = @_; return msgsize(64, $nns, $nsns); } sub worst_case { my ($nns, $nsns) = @_; return msgsize(256, $nns, $nsns); } sub common_name_average { my ($nns) = @_; return 15 + average_case($nns, 2); } sub common_name_worst { my ($nns) = @_; return 15 + worst_case($nns, 2); } sub uncommon_name_average { my ($nns) = @_; return average_case($nns, 15); } sub uncommon_name_worst { my ($nns) = @_; return worst_case($nns, 15); } Expires December 2003 [Page 7]
INTERNET-DRAFT June 2003 RESPSIZE 5 - Author's Address Paul Vixie 950 Charter Street Redwood City, CA 94063 +1 650 779 7000 paul@vix.com 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 December 2003 [Page 8]
