DNSOP Working Group Paul Vixie, ISC (Ed.) INTERNET-DRAFT Akira Kato, WIDE <draft-ietf-dnsop-respsize-01.txt> July, 2004 DNS Response Size Issues Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which we are aware have been or will be disclosed, and any of which we become aware will be disclosed, in accordance with RFC 3668. 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 (2003-2004). 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 December 2004 [Page 1]
INTERNET-DRAFT June 2003 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 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 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. Expires December 2004 [Page 2]
INTERNET-DRAFT June 2003 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. 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, without considering whether the omitted data was actually necessary.) Expires December 2004 [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, 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 December 2004 [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 nsaddr#13 (green) common name, worst case: msg:495 nsaddr# 1 (red) uncommon name, average case: msg:457 nsaddr# 3 (orange) uncommon name, worst case: msg:649(*) nsaddr# 0 (red) % perl respsize.pl 13 13 2 common name, average case: msg:303 nsaddr#11 (orange) common name, worst case: msg:495 nsaddr# 1 (red) uncommon name, average case: msg:457 nsaddr# 2 (orange) uncommon name, worst case: msg:649(*) nsaddr# 0 (red) (Note: The response simulator program is shown in Section 5.) 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. 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 more 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. (Note that in this case it is wise to serve the common parent domain's zone from the same servers that are named within it, in order to limit external dependencies when all your eggs are in a single basket.) 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 assuming that additional-data truncation is undesirable in the average case. Expires December 2004 [Page 5]
INTERNET-DRAFT June 2003 RESPSIZE 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 -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 nsaddr#%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; Expires December 2004 [Page 6]
INTERNET-DRAFT June 2003 RESPSIZE $num++; } $na--; } if ($naaaa) { if ($space >= $aaaasize) { $space -= $aaaasize; $num++; } $naaaa--; } } 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 2004 [Page 7]
INTERNET-DRAFT June 2003 RESPSIZE 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. IPR Statement Copyright (C) The Internet Society (2003-2004). 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. 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 Expires December 2004 [Page 8]
