Internet Engineering Task Force P. Vixie
Internet-Draft Internet Systems Consortium
Intended status: Informational A. Kato
Expires: June 20, 2008 The University of Tokyo/WIDE
Project
December 18, 2007
DNS Referral Response Size Issues
draft-ietf-dnsop-respsize-09
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
With a mandated default minimum maximum UDP 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
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implementors and to DNS zone operators.
1. Introduction
1.1. Introduction and Overview
The DNS standard limits UDP message size to 512 octets (see [RFC1035]
4.2.1). 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 a network layer protocol, for
example to IPv6.
The EDNS protocol extension starting with version 0 permits larger
responses by mutual agreement of the requester and responder (see
[RFC2671] 2.3, 4.5), and it is recommended to support EDNS. The 512
octets UDP message size limit will remain in practical effect until
virtually all DNS servers and resolvers support EDNS.
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
referral responses, every octet must be carefully and sparingly
allocated. While the response size of positive responses is also a
concern[RFC3226], this document specifically addresses referral
response size.
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Delegation Details
2.1. Relevant Protocol Elements
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)
If the total size of the UDP response exceeds 512 octets or
advertised size in EDNS, and if the data that does not fit was
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"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.
RRsets are never sent partially unless the TC bit is set to indicate
truncation. When the 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).
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
signaled via the TC bit since additional data is often optional (see
discussion in [RFC4472] B).
DNS label compression allows the component labels of a domain name to
be instantiated exactly 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 incompressible
data (such as nameserver addresses).
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) in a
referral.
2.2. Advice to Zone Owners
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.
Some queries to non-existing names can be large, but this is not a
problem because negative responses need not contain any answer,
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authority or additional records. See [RFC2308] 2.1 for more
information about the format of negative responses.
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
protocols versions (e.g., IPv4 and IPv6) in common use. As long as
the servers are well managed, the server serving IPv6 might be
different from the server serving IPv4 sharing the same server name.
It is important to ensure that a zone should have servers reachable
by all IP protocol in common use (e.g., IPv4 and IPv6).
The best case is no truncation at all. This is because many
requesters will retry using TCP immediately, or will automatically
requery for RRsets that are possibly truncated, without considering
whether the omitted data was actually necessary.
Anycasting[RFC3258] is a useful tool for performance and reliability
without increasing the size of referral response.
While it is irrelevant to the response size issue, all zones have to
be served via IPv4 as well to avoid name space
fragmentation[RFC3901].
2.3. Advice to Server Implementors
Each 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.
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.
Recall that the TC bit is only set when the required RRset can not be
included in its entirety (see [RFC2181] 9). Even when some of the
RRsets to be included in the additional section are not fit in the
response size, the TC bit isn't set. These RRsets may be important
for a referral. Some DNS implementations try to resolve these
missing glue records separately which will introduce extra queries
and extra time to resolve a given name.
A delegation response should prioritize glue records as follows.
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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.
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.
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
An instrumented protocol trace of a best case delegation response is
shown in Figure 1. Note that 13 servers are named, and 13 addresses
are given. This query was artificially designed to exactly reach the
512 octets limit.
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;; 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
Figure 1
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 occurrences of the parent domain name. The following outputs
shown in Figure 2 and Figure 3 from a response simulator in
Appendix A written in perl[PERL] demonstrate these properties.
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% 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)
Figure 2
% 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)
Figure 3
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
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EDNS, then new measurements and new estimates will have to be made.
4. Conclusions
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.
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 zone. This is to ensure
that the zone's servers always have the zone's nameservers' glue
available when delegating, and 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.
Thirteen (13) is the effective maximum number of nameserver names
usable with traditional (non-extended) DNS, assuming a common parent
domain name, and given that implicit referral response truncation is
undesirable in the average case.
More than one address records in a protocol family per a server 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.
More than one address records across protocol families is less likely
to lead to or encounter truncation, partly because multiprotocol
clients, which are required to handle larger RRsets such as AAAA RRs,
are more likely to speak EDNS which can use a larger UDP response
size limit, and partly because the resource records (A and AAAA) are
in different RRsets and are therefore divisible from each other.
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 "more important" than other glue records, in the
assembly of referral responses.
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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 have
address records 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. Security Considerations
The recommendations contained in this document have no known security
implications.
6. IANA Considerations
This document does not call for changes or additions to any IANA
registry.
7. Acknowledgement
The authors thank Peter Koch, Rob Austein, Joe Abley, Mark Andrews,
Kenji Rikitake, Stephane Bortzmeyerand, Olafur Gudmundsson, and
Alfred Hines for their valuable comments and suggestions.
This work was supported by the US National Science Foundation
(research grant SCI-0427144) and DNS-OARC.
8. Normative References
[PERL] Wall, L., Christiansen, T., and j. Orwant, "Programing
Perl", July 2000.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, August 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001.
[RFC3258] Hardie, T., "Distributing Authoritative Name Servers via
Shared Unicast Addresses", RFC 3258, April 2002.
[RFC3901] Durand, A. and J. Ihren, "DNS IPv6 Transport Operational
Guidelines", BCP 91, RFC 3901, September 2004.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472,
April 2006.
Appendix A. The response simulator program
#!/usr/bin/perl
#
# SYNOPSIS
# respsize.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 $n_ns = 0;
getopt('z', %opts);
if (defined($opts{'z'})) {
server_name_len($opts{'z'}); # just register it
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}
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 + $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, $n_ns);
printf " preferred-glue A is assumed: ";
printf "# of A is %d, # of AAAA is %d (%s)\n",
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$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;
}
Authors' Addresses
Paul Vixie
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Phone: +1 650 423 1300
Email: paul@vix.com
Akira Kato
The University of Tokyo/WIDE Project
Information Technology Center, 2-11-16 Yayoi
Bunkyo, Tokyo 113-8658
JP
Phone: +81 3 5841 2750
Email: kato@wide.ad.jp
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