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<?rfc sortrefs="yes"?> -ietf-dnsop-nsec3-guidance-10" number="9276" submissionType="IETF" category="bcp
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<rfc ipr="trust200902" docName="draft-ietf-dnsop-nsec3-guidance-10" category="bc
p" consensus="true" updates="5155">
<front> <front>
<title abbrev="title">Guidance for NSEC3 parameter settings</title> <title abbrev="NSEC3 Parameter Settings">Guidance for NSEC3 Parameter Setting
s</title>
<seriesInfo name="RFC" value="9276"/>
<author initials="W." surname="Hardaker" fullname="Wes Hardaker"> <seriesInfo name="BCP" value="236"/>
<organization>USC/ISI</organization> <author initials="W." surname="Hardaker" fullname="Wes Hardaker">
<address> <organization>USC/ISI</organization>
<email>ietf@hardakers.net</email> <address>
</address> <email>ietf@hardakers.net</email>
</author> </address>
<author initials="V." surname="Dukhovni" fullname="Viktor Dukhovni"> </author>
<organization>Bloomberg, L.P.</organization> <author initials="V." surname="Dukhovni" fullname="Viktor Dukhovni">
<address> <organization>Bloomberg, L.P.</organization>
<email>ietf-dane@dukhovni.org</email> <address>
</address> <email>ietf-dane@dukhovni.org</email>
</author> </address>
</author>
<date year="2022" month="August"/>
<date year="2022" month="May" day="25"/> <area>ops</area>
<workgroup>dnsop</workgroup>
<abstract> <keyword>DNSSEC</keyword>
<keyword>DNS</keyword>
<keyword>NSEC3</keyword>
<keyword>NSEC</keyword>
<keyword>Denial of Existence</keyword>
<t>NSEC3 is a DNSSEC mechanism providing proof of non-existence by <abstract>
<t>NSEC3 is a DNSSEC mechanism providing proof of nonexistence by
asserting that there are no names that exist between two domain names asserting that there are no names that exist between two domain names
within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly within a zone. Unlike its counterpart NSEC, NSEC3 avoids directly
disclosing the bounding domain name pairs. This document provides disclosing the bounding domain name pairs. This document provides
guidance on setting NSEC3 parameters based on recent operational guidance on setting NSEC3 parameters based on recent operational
deployment experience. This document updates <xref target="RFC5155"/> with deployment experience. This document updates RFC 5155 with
guidance about selecting NSEC3 iteration and salt parameters.</t> guidance about selecting NSEC3 iteration and salt parameters.</t>
</abstract>
</abstract> </front>
<middle>
</front> <section anchor="introduction">
<name>Introduction</name>
<middle> <t>As with NSEC <xref target="RFC4035"/>, NSEC3
<xref target="RFC5155"/> provides proof of
<section anchor="introduction" title="Introduction"> nonexistence that consists of signed DNS records establishing the
nonexistence of a given name or associated Resource Record Type
<t>As with NSEC <xref target="RFC4035"/>, NSEC3 <xref target="RFC5155"/> provide (RRTYPE) in a DNSSEC-signed zone <xref target="RFC4035"/>. However, in the case
s proof of of NSEC3, the names of valid nodes in the zone are obfuscated through
non-existence that consists of signed DNS records establishing the
non-existence of a given name or associated Resource Record Type
(RRTYPE) in a DNSSEC <xref target="RFC4035"/> signed zone. In the case of NSEC3
,
however, the names of valid nodes in the zone are obfuscated through
(possibly multiple iterations of) hashing (currently only (possibly multiple iterations of) hashing (currently only
SHA-1 is in use on the Internet).</t> SHA-1 is in use on the Internet).</t>
<t>
NSEC3 also provides "opt-out support", allowing for blocks of
unsigned delegations to be covered by a single NSEC3 record. Use of
the opt-out feature allows large registries to only sign as many
NSEC3 records as there are signed DS or other Resource Record sets
(RRsets) in the zone; with opt-out, unsigned delegations don't
require additional NSEC3 records. This sacrifices the tamper-
resistance of the proof of nonexistence offered by NSEC3 in order to
reduce memory and CPU overheads.
</t>
<t>NSEC3 also provides “opt-out support”, allowing for blocks of unsigned <t>NSEC3 records have a number of tunable parameters that are specified
delegations to be covered by a single NSEC3 record. Use of the
opt-out feature allows large registries to only sign as many NSEC3
records as there are signed DS or other RRsets in the zone; with
opt-out, unsigned delegations don’t require additional NSEC3 records.
This sacrifices the tamper-resistance proof of non-existence
offered by NSEC3 in order to reduce memory and CPU overheads.</t>
<t>NSEC3 records have a number of tunable parameters that are specified
via an NSEC3PARAM record at the zone apex. These parameters are the via an NSEC3PARAM record at the zone apex. These parameters are the
hash algorithm, processing flags, the number of hash iterations and hash algorithm, the processing flags, the number of hash iterations, and
the salt. Each of these has security and operational considerations the salt. Each of these has security and operational considerations
that impact both zone owners and validating resolvers. This document that impact both zone owners and validating resolvers. This document
provides some best-practice recommendations for setting the NSEC3 provides some best-practice recommendations for setting the NSEC3
parameters.</t> parameters.</t>
<section anchor="requirements-notation">
<name>Requirements Notation</name>
<t>
The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUI
RED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>R
ECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to b
e interpreted as
described in BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
</section>
</section>
<section anchor="nsec3-parameter-value-discussions">
<name>NSEC3 Parameter Value Discussions</name>
<section anchor="requirements-notation" title="Requirements Notation"> <t>The following sections describe the background of the parameters for
the NSEC3 and NSEC3PARAM RRTYPEs.</t>
<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, <section anchor="algorithms">
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, <name>Algorithms</name>
and “OPTIONAL” in this document are to be interpreted as described <t>The algorithm field is not discussed by this document. Readers are
in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only wh
en, they appear
in all capitals, as shown here.</t>
</section>
</section>
<section anchor="nsec3-parameter-value-discussions" title="NSEC3 Parameter Value
Discussions">
<t>The following sections describes the background of the parameters for
the NSEC3 and NSEC3PARAM resource record types.</t>
<section anchor="algorithms" title="Algorithms">
<t>The algorithm field is not discussed by this document. Readers are
encouraged to read <xref target="RFC8624"/> for guidance about DNSSEC algorithm encouraged to read <xref target="RFC8624"/> for guidance about DNSSEC algorithm
usage.</t> usage.</t>
</section>
</section> <section anchor="flags">
<section anchor="flags" title="Flags"> <name>Flags</name>
<t>The NSEC3PARAM flags field currently contains only reserved and
<t>The NSEC3PARAM flags field currently contains only reserved and unassigned flags. However, individual NSEC3 records contain the
unassigned flags. Individual NSEC3 records, however, contain the "Opt-Out" flag <xref target="RFC5155"/> that specifies whether that NSEC3 record
“Opt-Out” flag <xref target="RFC5155"/>, which specifies whether that NSEC3 reco provides proof of nonexistence. In general, NSEC3 with the Opt-Out
rd
provides proof of non-existence. In general, NSEC3 with the Opt-Out
flag enabled should only be used in large, highly dynamic zones with a flag enabled should only be used in large, highly dynamic zones with a
small percentage of signed delegations. Operationally, this allows small percentage of signed delegations. Operationally, this allows
for fewer signature creations when new delegations are inserted into a for fewer signature creations when new delegations are inserted into a
zone. This is typically only necessary for extremely large zone. This is typically only necessary for extremely large
registration points providing zone updates faster than real-time registration points providing zone updates faster than real-time
signing allows or when using memory-constrained hardware. signing allows or when using memory-constrained hardware.
Operators considering the use of NSEC3 are advised to fully test their
zones deployment architectures and authoritative servers under both
regular operational loads to determine the tradeoffs using NSEC3
instead of NSEC. Smaller zones, or large but relatively static zones,
are encouraged to not use a the opt-opt flag and to take advantage of
DNSSEC’s proof-of-non-existence support.</t>
</section> Operators considering the use of NSEC3 are advised to carefully
<section anchor="iterations" title="Iterations"> weigh the costs and benefits of choosing NSEC3 over NSEC. Smaller
zones, or large but relatively static zones, are encouraged to not
use the opt-opt flag and to take advantage of DNSSEC's
authenticated denial of existence.
</t>
</section>
<section anchor="iterations">
<name>Iterations</name>
<t>NSEC3 records are created by first hashing the input domain and then <t>NSEC3 records are created by first hashing the input domain and then
repeating that hashing using the same algorithm a number of times based on the repeating that hashing using the same algorithm a number of times based on the
iteration parameter in the NSEC3PARM and NSEC3 records. iteration parameter in the NSEC3PARAM and NSEC3 records.
The first hash with NSEC3 is typically sufficient to discourage zone
enumeration performed by “zone walking” an unhashed NSEC chain.</t>
<t>Note that <xref target="RFC5155"/> describes the Iterations field to be “The The first hash with NSEC3 is typically sufficient to discourage zone
enumeration performed by "zone walking" an unhashed NSEC chain.</t>
<t>Note that <xref target="RFC5155"/> describes the Iterations field as f
ollows</t>
<blockquote>The
Iterations field defines the number of additional times the hash Iterations field defines the number of additional times the hash
function has been performed.” This means that an NSEC3 record with an function has been performed.</blockquote>
<t>This means that an NSEC3 record with an
Iterations field of 0 actually requires one hash iteration.</t> Iterations field of 0 actually requires one hash iteration.</t>
<t>Only determined parties
<t>Only determined parties
with significant resources are likely to try and uncover hashed with significant resources are likely to try and uncover hashed
values, regardless of the number of additional iterations performed. values, regardless of the number of additional iterations performed.
If an adversary really wants to expend significant CPU resources to If an adversary really wants to expend significant CPU resources to
mount an offline dictionary attack on a zone’s NSEC3 chain, they’ll mount an offline dictionary attack on a zone's NSEC3 chain, they'll
likely be able to find most of the “guessable” names despite any likely be able to find most of the "guessable" names despite any
level of additional hashing iterations.</t> level of additional hashing iterations.</t>
<t>Most names published in the DNS are rarely secret or unpredictable.
<t>Most names published in the DNS are rarely secret or unpredictable.
They are published to be memorable, used and consumed by humans. They They are published to be memorable, used and consumed by humans. They
are often recorded in many other network logs such as email logs, are often recorded in many other network logs such as email logs,
certificate transparency logs, web page links, intrusion detection certificate transparency logs, web page links, intrusion-detection
systems, malware scanners, email archives, etc. Many times a simple systems, malware scanners, email archives, etc. Many times a simple
dictionary of commonly used domain names prefixes (www, mail, dictionary of commonly used domain names prefixes (www, mail,
imap, login, database, etc.) can be used to quickly reveal a large imap, login, database, etc.) can be used to quickly reveal a large
number of labels within a zone. Because of this, there are increasing number of labels within a zone. Because of this, there are increasing
performance costs yet diminishing returns associated with applying performance costs yet diminishing returns associated with applying
additional hash iterations beyond the first.</t> additional hash iterations beyond the first.</t>
<t>Although <xref target="RFC5155" sectionFormat="of" section="10.3"/> sp
<t>Although Section 10.3 of <xref target="RFC5155"/> specifies upper bounds for ecifies the upper bounds for the
the
number of hash iterations to use, there is no published guidance for number of hash iterations to use, there is no published guidance for
zone owners about good values to select. Recent academic studies zone owners about good values to select. Recent academic studies
have shown that NSEC3 hashing provides only moderate have shown that NSEC3 hashing provides only moderate
protection <xref target="GPUNSEC3"/><xref target="ZONEENUM"/>.</t> protection <xref target="GPUNSEC3"/> <xref target="ZONEENUM"/>.</t>
</section>
</section> <section anchor="salt">
<section anchor="salt" title="Salt"> <name>Salt</name>
<t>NSEC3 records provide an additional salt value, which can be
<t>NSEC3 records provide an additional salt value, which can be combined with a Fully Qualified Domain Name (FQDN) to influence the resulting ha
combined with an FQDN to influence the resulting hash, but properties sh, but properties
of this extra salt are complicated.</t> of this extra salt are complicated.</t>
<t>In cryptography, salts generally add a layer of protection against
<t>In cryptography, salts generally add a layer of protection against
offline, stored dictionary attacks by combining the value to be hashed offline, stored dictionary attacks by combining the value to be hashed
with a unique “salt” value. This prevents adversaries from building up with a unique "salt" value. This prevents adversaries from building up
and remembering a single dictionary of values that can translate a and remembering a single dictionary of values that can translate a
hash output back to the value that it derived from.</t> hash output back to the value that it was derived from.</t>
<t>In the case of DNS, the situation is different because the hashed
<t>In the case of DNS, the situation is different because the hashed names placed in NSEC3 records are always implicitly "salted" by
names placed in NSEC3 records are always implicitly “salted” by hashing the FQDN from each zone. Thus, no
hashing the fully-qualified domain name from each zone. Thus, no
single pre-computed table works to speed up dictionary attacks single pre-computed table works to speed up dictionary attacks
against multiple target zones. An attacker is always required to against multiple target zones. An attacker is always required to
compute a complete dictionary per zone, which is expensive in both compute a complete dictionary per zone, which is expensive in both
storage and CPU time.</t> storage and CPU time.</t>
<t>To understand the role of the additional NSEC3 salt field, we have to
<t>To understand the role of the additional NSEC3 salt field, we have to
consider how a typical zone walking attack works. Typically, the attack consider how a typical zone walking attack works. Typically, the attack
has two phases - online and offline. In the online phase, an attacker has two phases: online and offline. In the online phase, an attacker
“walks the zone” by enumerating (almost) all hashes listed in NSEC3 "walks the zone" by enumerating (almost) all hashes listed in NSEC3
records and storing them for the offline phase. Then, in the offline records and storing them for the offline phase. Then, in the offline
cracking phase, the attacker attempts to crack the underlying hash. In cracking phase, the attacker attempts to crack the underlying hash. In
this phase, the additional salt value raises the cost of the attack this phase, the additional salt value raises the cost of the attack
only if the salt value changes during the online phase of the only if the salt value changes during the online phase of the
attack. In other words, an additional, constant salt value does not attack. In other words, an additional, constant salt value does not
change the cost of the attack.</t> change the cost of the attack.</t>
<t>Changing a zone's salt value requires the construction of a complete
<t>Changing a zone’s salt value requires the construction of a complete
new NSEC3 chain. This is true both when re-signing the entire zone at new NSEC3 chain. This is true both when re-signing the entire zone at
once, and when incrementally signing it in the background where the new once and when incrementally signing it in the background where the new
salt is only activated once every name in the chain has been salt is only activated once every name in the chain has been
completed. As a result, re-salting is a very complex operation, with completed. As a result, re-salting is a very complex operation, with
significant CPU time, memory, and bandwidth consumption. This makes significant CPU time, memory, and bandwidth consumption. This makes
very frequent re-salting impractical, and renders the additional salt very frequent re-salting impractical and renders the additional salt
field functionally useless.</t> field functionally useless.</t>
</section>
</section> </section>
</section> <section anchor="recommendations-for-deploying-and-validating-nsec3-records">
<section anchor="recommendations-for-deploying-and-validating-nsec3-records" tit <name>Recommendations for Deploying and Validating NSEC3 Records</name>
le="Recommendations for Deploying and Validating NSEC3 Records"> <t>The following subsections describe recommendations for the different
<t>The following subsections describe recommendations for the different
operating realms within the DNS.</t> operating realms within the DNS.</t>
<section anchor="best-practice-for-zone-publishers">
<section anchor="best-practice-for-zone-publishers" title="Best-practice for Zon <name>Best Practice for Zone Publishers</name>
e Publishers"> <t>First, if the operational or security features of NSEC3 are not
needed, then NSEC <bcp14>SHOULD</bcp14> be used in preference to NSEC3. NSEC3
<t>First, if the operational or security features of NSEC3 are not
needed, then NSEC SHOULD be used in preference to NSEC3. NSEC3
requires greater computational power (see <xref target="computationalburdens"/>) requires greater computational power (see <xref target="computationalburdens"/>)
for both authoritative servers and validating clients. Specifically, for both authoritative servers and validating clients. Specifically,
there is a nontrivial complexity in finding matching NSEC3 records to there is a nontrivial complexity in finding matching NSEC3 records to
randomly generated prefixes within a DNS zone. NSEC mitigates this randomly generated prefixes within a DNS zone. NSEC mitigates this
concern. If NSEC3 must be used, then an iterations count of 0 MUST be concern. If NSEC3 must be used, then an iterations count of 0 <bcp14>MUST</bcp1 4> be
used to alleviate computational burdens. Note that extra iteration used to alleviate computational burdens. Note that extra iteration
counts other than 0 increase the impact of CPU-exhausting DoS attacks, counts other than 0 increase the impact of CPU-exhausting DoS attacks,
and also increase the risk of interoperability problems.</t> and also increase the risk of interoperability problems.</t>
<t>Note that deploying NSEC with minimally covering NSEC records
<t>Note that deploying NSEC with minimally covering NSEC records <xref target="RFC4470"/> also incurs a cost, and zone owners should measure the
<xref target="RFC4470"></xref> also incurs a cost, and zone owners should measur computational difference in deploying either <xref target="RFC4470"/> or NSEC3.<
e the /t>
computational difference in deploying either RFC4470 or NSEC3.</t> <t>In short, for all zones, the recommended NSEC3 parameters are as shown
<t>In short, for all zones, the recommended NSEC3 parameters are as shown
below:</t> below:</t>
<figure><artwork><![CDATA[ <artwork><![CDATA[
; SHA-1, no extra iterations, empty salt: ; SHA-1, no extra iterations, empty salt:
; ;
bcp.example. IN NSEC3PARAM 1 0 0 - bcp.example. IN NSEC3PARAM 1 0 0 -
]]></artwork></figure> ]]></artwork>
<t>For small zones, the use of opt-out-based NSEC3 records is <bcp14>NOT
<t>For small zones, the use of opt-out based NSEC3 records is NOT RECOMMENDED</bcp14>.</t>
RECOMMENDED.</t> <t>For very large and sparsely signed zones, where the majority of the
records are insecure delegations, opt-out <bcp14>MAY</bcp14> be used.</t>
<t>For very large and sparsely signed zones, where the majority of the <t>Operators <bcp14>SHOULD NOT</bcp14> use a salt by indicating a zero-le
records are insecure delegations, opt-out MAY be used.</t> ngth salt value
instead (represented as a "-" in the presentation format).</t>
<t>Operators SHOULD NOT use a salt by indicating a zero-length salt value <t>If salts are used, note that since the NSEC3PARAM RR is not used by
instead (represented as a “-“ in the presentation format).</t> validating resolvers (see <xref target="RFC5155" sectionFormat="of" section="4"/
>), the iterations and
<t>If salts are used, note that since the NSEC3PARAM RR is not used by
validating resolvers (see <xref target="RFC5155"></xref> section 4), the iterati
ons and
salt parameters can be changed without the need to wait for RRsets to salt parameters can be changed without the need to wait for RRsets to
expire from caches. A complete new NSEC3 chain needs to be expire from caches. A complete new NSEC3 chain needs to be
constructed and the full zone needs to be re-signed.</t> constructed and the full zone needs to be re-signed.</t>
</section>
</section> <section anchor="recommendation-for-validating-resolvers">
<section anchor="recommendation-for-validating-resolvers" title="Recommendation <name>Recommendation for Validating Resolvers</name>
for Validating Resolvers"> <t>Because there has been a large growth of open (public) DNSSEC
<t>Because there has been a large growth of open (public) DNSSEC
validating resolvers that are subject to compute resource constraints validating resolvers that are subject to compute resource constraints
when handling requests from anonymous clients, this document when handling requests from anonymous clients, this document
recommends that validating resolvers change their behavior with recommends that validating resolvers reduce their iteration count
respect to large iteration values. Specifically, validating limits over time.
Specifically, validating
resolver operators and validating resolver software implementers are resolver operators and validating resolver software implementers are
encouraged to continue evaluating NSEC3 iteration count deployments encouraged to continue evaluating NSEC3 iteration count deployment
but lower their default acceptable limits over time. Similarly, because trends and lower their acceptable iteration limits over time.
Because
treating a high iterations count as insecure leaves zones subject to treating a high iterations count as insecure leaves zones subject to
attack, validating resolver operators and validating resolver software attack, validating resolver operators and validating resolver software
implementers are further encouraged to lower their default and acceptable implementers are further encouraged to lower their default
limit for returning SERVFAIL when processing NSEC3 parameters limit for returning SERVFAIL when processing NSEC3 parameters
containing large iteration count values. See containing large iteration count values.
See
<xref target="deploymentmeasurements"/> for measurements taken near the time of <xref target="deploymentmeasurements"/> for measurements taken near the time of
publication of this document and potential starting points.</t> publication of this document and potential starting points.</t>
<t>Validating resolvers <bcp14>MAY</bcp14> return an insecure response to
<t>Validating resolvers MAY return an insecure response to their clients their clients
when processing NSEC3 records with iterations larger when processing NSEC3 records with iterations larger
than 0. than 0.
Note also that a validating resolver returning an insecure response Note also that a validating resolver returning an insecure response
MUST still validate the signature over the NSEC3 record to ensure <bcp14>MUST</bcp14> still validate the signature over the NSEC3 record to ensure
the iteration count was not altered since record publication (see the iteration count was not altered since record publication (see
<xref target="RFC5155"/> section 10.3).</t> <xref target="RFC5155" sectionFormat="of" section="10.3"/>).</t>
<t>Validating resolvers <bcp14>MAY</bcp14> also return a SERVFAIL respons
<t>Validating resolvers MAY also return a SERVFAIL response when e when
processing NSEC3 records with iterations larger than 0. Validating processing NSEC3 records with iterations larger than 0. Validating
resolvers MAY choose to ignore authoritative server responses with resolvers <bcp14>MAY</bcp14> choose to ignore authoritative server responses wit h
iteration counts greater than 0, which will likely result in iteration counts greater than 0, which will likely result in
returning a SERVFAIL to the client when no acceptable responses are returning a SERVFAIL to the client when no acceptable responses are
received from authoritative servers.</t> received from authoritative servers.</t>
<t>Validating resolvers returning an insecure or SERVFAIL answer to their
<t>Validating resolvers returning an insecure or SERVFAIL answer to their
client after receiving and validating an unsupported NSEC3 parameter client after receiving and validating an unsupported NSEC3 parameter
from the authoritative server(s) SHOULD return an Extended DNS from the authoritative server(s) <bcp14>SHOULD</bcp14> return an Extended DNS
Error (EDE) <xref target="RFC8914"/> EDNS0 option of value (RFC EDITOR: TBD). Error (EDE) <xref target="RFC8914"/> EDNS0 option of value 27.
Validating resolvers that choose to ignore a response with an Validating resolvers that choose to ignore a response with an
unsupported iteration count (and do not validate the signature) MUST unsupported iteration count (and that do not validate the signature) <bcp14>MUST
NOT return this EDE option.</t> NOT</bcp14> return this EDE option.</t>
<t>Note that this specification updates <xref target="RFC5155"></xref> by signif <t>Note that this specification updates <xref target="RFC5155"/> by signi
icantly ficantly
decreasing the requirements originally specified in Section 10.3 of decreasing the requirements originally specified in <xref
<xref target="RFC5155"></xref>. See the Security Considerations for arguments on target="RFC5155" sectionFormat="of" section="10.3"/>. See the Security
how to Considerations (<xref target="security-considerations"/>) for arguments on how t
o
handle responses with non-zero iteration count.</t> handle responses with non-zero iteration count.</t>
</section>
<section anchor="recommendation-for-primary-secondary-relationships">
<name>Recommendation for Primary and Secondary Relationships</name>
</section> <t>Primary and secondary authoritative servers for a zone that are not
<section anchor="recommendation-for-primary-secondary-relationships" title="Reco
mmendation for Primary / Secondary Relationships">
<t>Primary and secondary authoritative servers for a zone that are not
being run by the same operational staff and/or using the same software being run by the same operational staff and/or using the same software
and configuration must take into account the potential differences in and configuration must take into account the potential differences in
NSEC3 iteration support.</t> NSEC3 iteration support.</t>
<t>Operators of secondary services should advertise the parameter limits
<t>Operators of secondary services should advertise the parameter limits
that their servers support. Correspondingly, operators of primary that their servers support. Correspondingly, operators of primary
servers need to ensure that their secondaries support the NSEC3 servers need to ensure that their secondaries support the NSEC3
parameters they expect to use in their zones. To ensure reliability, parameters they expect to use in their zones. To ensure reliability,
after primaries change their iteration counts, they should query their after primaries change their iteration counts, they should query their
secondaries with known non-existent labels to verify the secondary secondaries with known nonexistent labels to verify the secondary
servers are responding as expected.</t> servers are responding as expected.</t>
</section>
</section>
<section anchor="security-considerations">
<name>Security Considerations</name>
</section> <t>This entire document discusses security considerations with various
</section> parameter selections of NSEC3 and NSEC3PARAM fields.</t>
<section anchor="security-considerations" title="Security Considerations"> <t>The point where a validating resolver returns insecure versus the point
<t>This entire document discusses security considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields.</t>
<t>The point where a validating resolver returns insecure vs the point
where it returns SERVFAIL must be considered carefully. Specifically, where it returns SERVFAIL must be considered carefully. Specifically,
when a validating resolver treats a zone as insecure above a when a validating resolver treats a zone as insecure above a
particular value (say 100) and returns SERVFAIL above a higher point particular value (say 100) and returns SERVFAIL above a higher point
(say 500), it leaves the zone subject to attacker-in-the-middle (say 500), it leaves the zone subject to attacker-in-the-middle
attacks as if it was unsigned between these values. Thus, validating attacks as if it were unsigned between these values.
resolver operators and software implementers SHOULD set the point
above which a zone is treated as insecure for certain values of NSEC3
iterations counts to the same as the point where a validating resolver
begins returning SERVFAIL.</t>
</section>
<section anchor="operational-considerations" title="Operational Considerations">
<t>This entire document discusses operational considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields.</t>
</section>
<section anchor="iana-considerations" title="IANA Considerations">
<t>This document requests a new allocation in the First Come First Served
range of the “Extended DNS Error Codes” of the “Domain Name System
(DNS) Parameters” registration table with the following
characteristics:</t>
<t><list style="symbols">
<t>INFO-CODE: (RFC EDITOR: TBD)</t>
<t>Purpose: Unsupported NSEC3 iterations value</t>
<t>Reference: (RFC EDITOR: this document)</t>
</list></t>
</section>
</middle>
<back>
<references title='Normative References'>
<reference anchor="RFC2119" target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner'><organization /></
author>
<date year='1997' month='March' />
<abstract><t>In many standards track documents several words are used to signify
the requirements in the specification. These words are often capitalized. This
document defines these words as they should be interpreted in IETF documents.
This document specifies an Internet Best Current Practices for the Internet Comm
unity, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
</reference>
<reference anchor="RFC5155" target='https://www.rfc-editor.org/info/rfc5155'>
<front>
<title>DNS Security (DNSSEC) Hashed Authenticated Denial of Existence</title>
<author initials='B.' surname='Laurie' fullname='B. Laurie'><organization /></au
thor>
<author initials='G.' surname='Sisson' fullname='G. Sisson'><organization /></au
thor>
<author initials='R.' surname='Arends' fullname='R. Arends'><organization /></au
thor>
<author initials='D.' surname='Blacka' fullname='D. Blacka'><organization /></au
thor>
<date year='2008' month='March' />
<abstract><t>The Domain Name System Security (DNSSEC) Extensions introduced the
NSEC resource record (RR) for authenticated denial of existence. This document i
ntroduces an alternative resource record, NSEC3, which similarly provides authen
ticated denial of existence. However, it also provides measures against zone en
umeration and permits gradual expansion of delegation-centric zones. [STANDARDS
-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='5155'/>
<seriesInfo name='DOI' value='10.17487/RFC5155'/>
</reference>
<reference anchor="RFC4035" target='https://www.rfc-editor.org/info/rfc4035'>
<front>
<title>Protocol Modifications for the DNS Security Extensions</title>
<author initials='R.' surname='Arends' fullname='R. Arends'><organization /></au
thor>
<author initials='R.' surname='Austein' fullname='R. Austein'><organization /></
author>
<author initials='M.' surname='Larson' fullname='M. Larson'><organization /></au
thor>
<author initials='D.' surname='Massey' fullname='D. Massey'><organization /></au
thor>
<author initials='S.' surname='Rose' fullname='S. Rose'><organization /></author
>
<date year='2005' month='March' />
<abstract><t>This document is part of a family of documents that describe the DN
S Security Extensions (DNSSEC). The DNS Security Extensions are a collection of
new resource records and protocol modifications that add data origin authentica
tion and data integrity to the DNS. This document describes the DNSSEC protocol
modifications. This document defines the concept of a signed zone, along with
the requirements for serving and resolving by using DNSSEC. These techniques al
low a security-aware resolver to authenticate both DNS resource records and auth
oritative DNS error indications. </t><t> This document obsoletes RFC 2535 and in
corporates changes from all updates to RFC 2535. [STANDARDS-TRACK]</t></abstrac
t>
</front>
<seriesInfo name='RFC' value='4035'/>
<seriesInfo name='DOI' value='10.17487/RFC4035'/>
</reference>
<reference anchor="RFC4470" target='https://www.rfc-editor.org/info/rfc4470'>
<front>
<title>Minimally Covering NSEC Records and DNSSEC On-line Signing</title>
<author initials='S.' surname='Weiler' fullname='S. Weiler'><organization /></au
thor>
<author initials='J.' surname='Ihren' fullname='J. Ihren'><organization /></auth
or>
<date year='2006' month='April' />
<abstract><t>This document describes how to construct DNSSEC NSEC resource recor
ds that cover a smaller range of names than called for by RFC 4034. By generati
ng and signing these records on demand, authoritative name servers can effective
ly stop the disclosure of zone contents otherwise made possible by walking the c
hain of NSEC records in a signed zone. [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='4470'/>
<seriesInfo name='DOI' value='10.17487/RFC4470'/>
</reference>
<reference anchor="RFC8174" target='https://www.rfc-editor.org/info/rfc8174'>
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></auth
or>
<date year='2017' month='May' />
<abstract><t>RFC 2119 specifies common key words that may be used in protocol s
pecifications. This document aims to reduce the ambiguity by clarifying that on
ly UPPERCASE usage of the key words have the defined special meanings.</t></abs
tract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/>
</reference>
<reference anchor="RFC8914" target='https://www.rfc-editor.org/info/rfc8914'>
<front>
<title>Extended DNS Errors</title>
<author initials='W.' surname='Kumari' fullname='W. Kumari'><organization /></au
thor>
<author initials='E.' surname='Hunt' fullname='E. Hunt'><organization /></author
>
<author initials='R.' surname='Arends' fullname='R. Arends'><organization /></au
thor>
<author initials='W.' surname='Hardaker' fullname='W. Hardaker'><organization />
</author>
<author initials='D.' surname='Lawrence' fullname='D. Lawrence'><organization />
</author>
<date year='2020' month='October' />
<abstract><t>This document defines an extensible method to return additional inf
ormation about the cause of DNS errors. Though created primarily to extend SERVF
AIL to provide additional information about the cause of DNS and DNSSEC failures
, the Extended DNS Errors option defined in this document allows all response ty
pes to contain extended error information. Extended DNS Error information does n
ot change the processing of RCODEs.</t></abstract>
</front>
<seriesInfo name='RFC' value='8914'/>
<seriesInfo name='DOI' value='10.17487/RFC8914'/>
</reference>
</references> Thus, validating resolver operators and software implementers
<bcp14>SHOULD</bcp14> set the point above which a zone is treated as
insecure for certain values of NSEC3 iterations to the same as the
point where a validating resolver begins returning SERVFAIL.
<references title='Informative References'> </t>
</section>
<section anchor="operational-considerations">
<name>Operational Considerations</name>
<t>This entire document discusses operational considerations with various
parameter selections of NSEC3 and NSEC3PARAM fields.</t>
</section>
<section anchor="iana-considerations">
<name>IANA Considerations</name>
<reference anchor="GPUNSEC3" > <t>IANA has allocated the following code in the First Come First Served
<front> range <xref target="RFC8126"/> of the "Extended DNS Error Codes" registry w
<title>GPU-Based NSEC3 Hash Breaking</title> ithin the "Domain Name System
<author initials="M." surname="Wander" fullname="M. Wander"> (DNS) Parameters" registry:</t>
<organization></organization> <dl spacing="compact">
</author> <dt>INFO-CODE:</dt> <dd>27</dd>
<author initials="L." surname="Schwittmann" fullname="L. Schwittmann"> <dt>Purpose:</dt> <dd>Unsupported NSEC3 iterations value</dd>
<organization></organization> <dt>Reference:</dt> <dd>RFC 9276</dd>
</author> </dl>
<author initials="C." surname="Boelmann" fullname="C. Boelmann"> </section>
<organization></organization> </middle>
</author> <back>
<author initials="T." surname="Weis" fullname="T. Weis"> <references>
<organization></organization> <name>References</name>
</author> <references>
<date year="2014"/> <name>Normative References</name>
</front> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<seriesInfo name="DOI" value="10.1109/NCA.2014.27"/> C.2119.xml"/>
</reference> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<reference anchor="ZONEENUM" > C.5155.xml"/>
<front> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<title>An efficient DNSSEC zone enumeration algorithm</title> C.4035.xml"/>
<author initials="Z." surname="Wang" fullname="Zheng Wang"> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<organization></organization> C.4470.xml"/>
</author> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<author initials="L." surname="Xiao" fullname="Liyuan Xiao"> C.8174.xml"/>
<organization></organization> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
</author> C.8914.xml"/>
<author initials="R." surname="Wang" fullname="Rui Wang"> </references>
<organization></organization> <references>
</author> <name>Informative References</name>
<date year="n.d."/>
</front>
</reference>
<reference anchor="RFC8624" target='https://www.rfc-editor.org/info/rfc8624'> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<front> C.8126.xml"/>
<title>Algorithm Implementation Requirements and Usage Guidance for DNSSEC</titl
e>
<author initials='P.' surname='Wouters' fullname='P. Wouters'><organization /></
author>
<author initials='O.' surname='Sury' fullname='O. Sury'><organization /></author
>
<date year='2019' month='June' />
<abstract><t>The DNSSEC protocol makes use of various cryptographic algorithms i
n order to provide authentication of DNS data and proof of nonexistence. To ens
ure interoperability between DNS resolvers and DNS authoritative servers, it is
necessary to specify a set of algorithm implementation requirements and usage gu
idelines to ensure that there is at least one algorithm that all implementations
support. This document defines the current algorithm implementation requiremen
ts and usage guidance for DNSSEC. This document obsoletes RFC 6944.</t></abstra
ct>
</front>
<seriesInfo name='RFC' value='8624'/>
<seriesInfo name='DOI' value='10.17487/RFC8624'/>
</reference>
</references> <reference anchor="GPUNSEC3">
<front>
<title>GPU-Based NSEC3 Hash Breaking</title>
<author initials="M." surname="Wander" fullname="Mätthaus Wander">
<organization/>
</author>
<author initials="L." surname="Schwittmann" fullname="Lorenz Schwittm
ann">
<organization/>
</author>
<author initials="C." surname="Boelmann" fullname="Christopher Boelma
nn">
<organization/>
</author>
<author initials="T." surname="Weis" fullname="Torben Weis">
<organization/>
</author>
<date month="August" year="2014"/>
</front>
<seriesInfo name="DOI" value="10.1109/NCA.2014.27"/>
</reference>
<section anchor="deploymentmeasurements" title="Deployment measurements at time <reference anchor="ZONEENUM">
of publication"> <front>
<title>An efficient DNSSEC zone enumeration algorithm</title>
<author initials="Z." surname="Wang" fullname="Zheng Wang">
<organization/>
</author>
<author initials="L." surname="Xiao" fullname="Liyuan Xiao">
<organization/>
</author>
<author initials="R." surname="Wang" fullname="Rui Wang">
<organization/>
</author>
<date month="April" year="2014"/>
</front>
<seriesInfo name="DOI" value="10.2495/MIIT130591"/>
</reference>
<t>At the time of publication, setting an upper limit of 100 iterations <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
C.8624.xml"/>
</references>
</references>
<section anchor="deploymentmeasurements" numbered="true" toc="default">
<name>Deployment Measurements at Time of Publication</name>
<t>At the time of publication, setting an upper limit of 100 iterations
for treating a zone as insecure is interoperable without significant for treating a zone as insecure is interoperable without significant
problems, but at the same time still enables CPU-exhausting DoS problems, but at the same time still enables CPU-exhausting DoS
attacks.</t> attacks.</t>
<t>At the time of publication, returning SERVFAIL beyond 500 iterations
<t>At the time of publication, returning SERVFAIL beyond 500 iterations
appears to be interoperable without significant problems.</t> appears to be interoperable without significant problems.</t>
</section>
</section> <section anchor="computationalburdens" numbered="true" toc="default">
<section anchor="computationalburdens" title="Computational burdens of processin <name>Computational Burdens of Processing NSEC3 Iterations</name>
g NSEC3 iterations"> <t>The queries per second (QPS) of authoritative servers will decrease due
<t>The queries per second (QPS) of authoritative servers will decrease due
to computational overhead when processing DNS requests for zones to computational overhead when processing DNS requests for zones
containing higher NSEC3 iteration counts. The table below containing higher NSEC3 iteration counts. The table below
shows the drop in QPS for various iteration counts.</t> shows the drop in QPS for various iteration counts.</t>
<figure><artwork><![CDATA[ <table anchor="iteration-counts-table" align="center">
| Iterations | QPS [% of 0 iterations QPS] | <name>Drop in QPS for Various Iteration Counts</name>
|------------+-----------------------------| <thead>
| 0 | 100 % | <tr>
| 10 | 89 % | <th rowspan="1" colspan="1">Iterations</th>
| 20 | 82 % | <th rowspan="1" colspan="1">QPS [% of 0 Iterations QPS]</th>
| 50 | 64 % | </tr>
| 100 | 47 % | </thead>
| 150 | 38 % | <tbody>
]]></artwork></figure> <tr>
<td>0</td>
</section> <td>100%</td>
<section title="Acknowledgments" anchor="qps"> </tr>
<t>The authors would like to thank the dns-operations discussion
participants, which took place on mattermost hosted by DNS-OARC.</t>
<t>Additionally, the following people contributed text or review comments
to the draft:</t>
<t><list style="symbols">
<t>Vladimir Cunat</t>
<t>Tony Finch</t>
<t>Paul Hoffman</t>
<t>Warren Kumari</t>
<t>Alexander Mayrhofer</t>
<t>Matthijs Mekking</t>
<t>Florian Obser</t>
<t>Petr Spacek</t>
<t>Paul Vixie</t>
<t>Tim Wicinski</t>
</list></t>
</section>
<section anchor="github-version-of-this-document" title="GitHub Version of This
Document">
<t>(RFCEditor: remove this section)</t>
<t>While this document is under development, it can be viewed, tracked,
issued, pushed with PRs, … here:</t>
<t>https://github.com/hardaker/draft-hardaker-dnsop-nsec3-guidance</t>
</section>
<section anchor="implementation-notes" title="Implementation Notes">
<t>(RFCEditor: remove this section)</t>
<t>The following implementations have implemented the guidance in this
document. They have graciously provided notes about the details of
their implementation below.</t>
<section anchor="opendnssec" title="OpenDNSSEC">
<t>The OpenDNSSEC configuration checking utility will alert the user
about nsec3 iteration values larger than 100.</t>
</section>
<section anchor="powerdns" title="PowerDNS">
<t>PowerDNS 4.5.2 changed the default value of nsec3-max-iterations to 150.</t>
</section>
<section anchor="knot-dns-and-knot-resolver" title="Knot DNS and Knot Resolver">
<t>Knot DNS 3.0.6 warns when signing with more than 20 NSEC3 iterations.
Knot Resolver 5.3.1 treats NSEC3 iterations above 150 as insecure.</t>
</section> <tr>
<section anchor="google-public-dns-resolver" title="Google Public DNS Resolver"> <td>10</td>
<td>89%</td>
</tr>
<t>Google Public DNS treats NSEC3 iterations above 100 as insecure since <tr>
September 2021.</t> <td>20</td>
<td>82%</td>
</tr>
</section> <tr>
<section anchor="google-cloud-dns" title="Google Cloud DNS"> <td>50</td>
<td>64%</td>
</tr>
<t>Google Cloud DNS uses 1 iteration and 64-bits of fixed random salt for <tr>
all zones using NSEC3. These parameters cannot be adjusted by users.</t> <td>100</td>
<td>47%</td>
</tr>
</section> <tr>
</section> <td>150</td>
<td>38%</td>
</tr>
</back> </tbody>
</table>
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