draft-ietf-dnsop-qname-minimisation-01.txt   draft-ietf-dnsop-qname-minimisation-02.txt 
Domain Name System Operations (dnsop) Working Group S. Bortzmeyer Domain Name System Operations (dnsop) Working Group S. Bortzmeyer
Internet-Draft AFNIC Internet-Draft AFNIC
Intended status: Experimental February 15, 2015 Intended status: Experimental March 4, 2015
Expires: August 19, 2015 Expires: September 5, 2015
DNS query name minimisation to improve privacy DNS query name minimisation to improve privacy
draft-ietf-dnsop-qname-minimisation-01 draft-ietf-dnsop-qname-minimisation-02
Abstract Abstract
This document describes one of the techniques that could be used to This document describes one of the techniques that could be used to
improve DNS privacy (see [I-D.ietf-dprive-problem-statement]), a improve DNS privacy (see [I-D.ietf-dprive-problem-statement]), a
technique called "qname minimisation". technique called "qname minimisation".
Discussions of the document should take place on the DNSOP working REMOVE BEFORE PUBLICATION Discussions of the document should take
group mailing list [dnsop]. place on the DNSOP working group mailing list [dnsop].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 19, 2015. This Internet-Draft will expire on September 5, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 13 skipping to change at page 2, line 13
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2 1. Introduction and background . . . . . . . . . . . . . . . . . 2
2. Qname minimisation . . . . . . . . . . . . . . . . . . . . . 2 2. Qname minimisation . . . . . . . . . . . . . . . . . . . . . 2
3. Operational considerations . . . . . . . . . . . . . . . . . 3 3. Operational considerations . . . . . . . . . . . . . . . . . 3
4. Performance implications . . . . . . . . . . . . . . . . . . 5 4. Performance implications . . . . . . . . . . . . . . . . . . 5
5. Security considerations . . . . . . . . . . . . . . . . . . . 5 5. Security considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 6. Implementation status - REMOVE BEFORE PUBLICATION . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. An algorithm to find the zone cut . . . . . . . . . 7 8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A. An algorithm to find the zone cut . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction and background 1. Introduction and background
The problem statement is exposed in The problem statement is exposed in
[I-D.ietf-dprive-problem-statement] TODO: add a reference to the [I-D.ietf-dprive-problem-statement] TODO: add a reference to the
specific section when ietf-dprive-problem-statement will be published specific section when ietf-dprive-problem-statement will be published
as RFC. The terminology ("qname", "resolver", etc) is also defined as RFC. The terminology ("qname", "resolver", etc) is also defined
in this companion document. This specific solution is not intended in this companion document. This specific solution is not intended
to fully solve the DNS privacy problem; instead, it should be viewed to fully solve the DNS privacy problem; instead, it should be viewed
as one tool amongst many. as one tool amongst many.
It follows the principle explained in section 6.1 of [RFC6973]: the It follows the principle explained in section 6.1 of [RFC6973]: the
less data you send out, the less privacy problems you'll get. less data you send out, the fewer privacy problems you'll get.
2. Qname minimisation 2. Qname minimisation
The idea is to minimise the amount of data sent from the DNS The idea is to minimise the amount of data sent from the DNS
resolver. When a resolver receives the query "What is the AAAA resolver. Under current practice, when a resolver receives the query
record for www.example.com?", it sends to the root (assuming a cold "What is the AAAA record for www.example.com?", it sends to the root
resolver, whose cache is empty) the very same question. Sending (assuming a cold resolver, whose cache is empty) the very same
"What are the NS records for .com?" would be sufficient (since it question. Sending "What are the NS records for .com?" would be
will be the answer from the root anyway). To do so would be sufficient (since it will be the answer from the root anyway). This
compatible with the current DNS system and therefore could be easily is compatible with the current DNS system and therefore can easily be
deployable, since it is an unilateral change to the resolvers, it deployed; since it is a unilateral change to the resolver, it does
does not change the protocol. Because of that, resolver implementers not change the protocol. Because of that, resolver implementers may
may do qname minmisation in slightly different ways. do qname minmisation in slightly different ways.
If "minimisation" is too long, you can write it "m10n".
To do such minimisation, the resolver needs to know the zone cut To do such minimisation, the resolver needs to know the zone cut
[RFC2181]. Zone cuts do not necessarily exist at every label [RFC2181]. Zone cuts do not necessarily exist at every label
boundary. If we take the name www.foo.bar.example, it is possible boundary. If we take the name www.foo.bar.example, it is possible
that there is a zone cut between "foo" and "bar" but not between that there is a zone cut between "foo" and "bar" but not between
"bar" and "example". So, assuming the resolver already knows the "bar" and "example". So, assuming the resolver already knows the
name servers of .example, when it receives the query "What is the name servers of .example, when it receives the query "What is the
AAAA record of www.foo.bar.example", it does not always know if the AAAA record of www.foo.bar.example", it does not always know whether
request should be sent to the name servers of bar.example or to those the request should be sent to the name servers of bar.example or to
of example. [RFC2181] suggests a method to find the zone cut those of example. [RFC2181] suggests a method to find the zone cut
(section 6), so resolvers may try it. (section 6), so resolvers may try it.
Note that DNSSEC-validating resolvers already have access to this Note that DNSSEC-validating resolvers already have access to this
information, since they have to find the zone cut (the DNSKEY record information, since they have to find the zone cut (the DNSKEY record
set is just below, the DS record set just above). set is just below, the DS record set just above).
Minimising the amount of data sent also, in part, addresses the case
of a wire sniffer as well the case of privacy invasion by the
servers.
One should note that the behaviour suggested here (minimising the One should note that the behaviour suggested here (minimising the
amount of data sent in qnames) is NOT forbidden by the [RFC1034] amount of data sent in qnames from the resolver) is NOT forbidden by
(section 5.3.3) or [RFC1035] (section 7.2). Sending the full qname the [RFC1034] (section 5.3.3) or [RFC1035] (section 7.2). Sending
to the authoritative name server is a tradition, not a protocol the full qname to the authoritative name server is a tradition, not a
requirment. This tradition comes[mockapetris-history] from a desire protocol requirement. This tradition comes[mockapetris-history] from
to optimize the number of requests, when the same name server is a desire to optimize the number of requests, when the same name
authoritative for many zones in a given name (something which was server is authoritative for many zones in a given name (something
more common in the old days, where the same name servers served .com which was more common in the old days, where the same name servers
and the root) or when the same name server is both recursive and served .com and the root) or when the same name server is both
authoritative (something which is strongly discouraged now). recursive and authoritative (something which is strongly discouraged
Whatever the merits of this choice at this time, the DNS is quite now). Whatever the merits of this choice at this time, the DNS is
different now. quite different now.
It may be noticed that many documents explaining the DNS and intended
for a wide audience, incorrectly describe the resolution process as
using qname minimisation, for instance by showing a request going to
the root, with just the TLD in the query. As a result, these
documents may confuse the privacy analysis of the users who see them.
As mentioned before, there are several ways to implement qname As mentioned before, there are several ways to implement qname
minimisation. Two main strategies are the aggressive one and the minimisation. Two main strategies are the aggressive one and the
lazy one. In the aggressive one, the resolver only sends NS queries lazy one. In the aggressive one, the resolver only sends NS queries
as long as it does not know the zone cuts. This is the safest, from as long as it does not know the zone cuts. This is the safest, from
a privacy point of view. The lazy way "piggybacks" on the a privacy point of view. The lazy way "piggybacks" on the
traditional resolution code. It sends traditional full qnames and traditional resolution code. It sends traditional full qnames and
learn the zone cuts from the referrals received, then switching to NS learns the zone cuts from the referrals received, then switches to NS
queries. This leaks more data but probably requires less changes in queries asking only for the minimum domain name. This leaks more
the existing resolver codebase. data but probably requires fewer changes in the existing resolver
codebase.
3. Operational considerations 3. Operational considerations
The administrators of the forwarders, and of the authoritative name The administrators of the forwarders, and of the authoritative name
servers, will get less data, which will reduce the utility of the servers, will get less data, which will reduce the utility of the
statistics they can produce (such as the percentage of the various statistics they can produce (such as the percentage of the various
qtypes). On the other hand, it may decrease their legal qtypes). On the other hand, it may decrease their legal
responsibility. responsibility in some jurisdictions. (TODO: do we keep any mention
of legal issues? We're not lawyers.)
Some broken name servers do not react properly to qtype=NS requests. Some broken name servers do not react properly to qtype=NS requests.
For instance, some authoritative name servers embedded in load For instance, some authoritative name servers embedded in load
balancers reply properly to A queries but send REFUSED to NS queries. balancers reply properly to A queries but send REFUSED to NS queries.
REMOVE THIS SENTENCE BEFORE PUBLICATION: As an example of today, look REMOVE THIS SENTENCE BEFORE PUBLICATION: As an example of today, look
at www.ratp.fr (not ratp.fr). This behaviour is a gross protocol at www.ratp.fr (not ratp.fr). This behaviour is a gross protocol
violation, and there is no need to stop improving the DNS because of violation, and there is no need to stop improving the DNS because of
such brokenness. However, qname minimisation may still work with such brokenness. However, qname minimisation may still work with
such domains since they are only leaf domains (no need to send them such domains since they are only leaf domains (no need to send them
NS requests). Such setup breaks more than just qname minimisation. NS requests). Such setup breaks more than just qname minimisation.
skipping to change at page 4, line 38 skipping to change at page 4, line 40
See also section 3 of [I-D.vixie-dnsext-resimprove] for the other bad See also section 3 of [I-D.vixie-dnsext-resimprove] for the other bad
consequences of this brokenness. consequences of this brokenness.
Another way to deal with such broken name servers would be to try Another way to deal with such broken name servers would be to try
with A requests (A being chosen because it is the most common and with A requests (A being chosen because it is the most common and
hence a qtype which will be always accepted, while a qtype NS may hence a qtype which will be always accepted, while a qtype NS may
ruffle the feathers of some middleboxes). Instead of querying name ruffle the feathers of some middleboxes). Instead of querying name
servers with a query "NS example.com", we could use "A _.example.com" servers with a query "NS example.com", we could use "A _.example.com"
and see if we get a referral. and see if we get a referral.
Other strange and illegal practices may pose a problem: for instance, Other strange and illegal practices may pose a problem: there is a
there is a common DNS anti-pattern used by low-end web hosters that common DNS anti-pattern used by low-end web hosters that also do DNS
also do DNS hosting that exploits the fact that the DNS protocol hosting that exploits the fact that the DNS protocol (pre-DNSSEC)
(pre-DNSSEC) allows certain serious misconfigurations, such as parent allows certain serious misconfigurations, such as parent and child
and child zones disagreeing on the location of a zone cut. zones disagreeing on the location of a zone cut. Basically, they
Basically, they have a single zone with wildcards like: have a single zone with wildcards for each TLD like:
*.example. 60 IN A 192.0.2.6 *.example. 60 IN A 192.0.2.6
(It is not known why they don't just wildcard all of "*." and be done (It is not known why they don't just wildcard all of "*." and be done
with it.) with it.)
This lets them turn up many web hosting customers without having to
This lets them turn up tons of web hosting customers without having configure thousands of individual zones on their nameservers. They
to configure thousands of individual zones on their nameservers. just tell the prospective customer to point their NS records at the
They just tell the prospective customer to point their NS records at hoster's nameservers, and the Web hoster doesn't have to provision
their nameservers, and the Web hoster doesn't have to provision anything in order to make the customer's domain resolve. NS queries
anything in order to make the customer's domain resolve. to the hoster will therefore do not give the right result, which may
endanger qname minimisation (it will be a problem for DNSSEC, too).
Qname minimisation can decrease performance in some cases, for Qname minimisation can decrease performance in some cases, for
instance for a deep domain name (like instance for a deep domain name (like
www.host.group.department.example.com where www.host.group.department.example.com where
host.group.department.example.com is hosted on example.com's name host.group.department.example.com is hosted on example.com's name
servers). For such a name, a cold resolver will, depending how qname servers). For such a name, a cold resolver will, depending how qname
minimisation is implemented, send more queries. Once warm, there minimisation is implemented, send more queries. Once the cache is
will be no difference with a traditional resolver. A possible warm, there will be no difference with a traditional resolver. A
solution is to always use the traditional algorithm when the cache is possible solution is to always use the traditional algorithm when the
cold and then to move to qname minimisation. This will decrease the cache is cold and then to move to qname minimisation. This will
privacy a bit but will guarantee no degradation of performance. decrease the privacy a bit but will guarantee no degradation of
performance.
Another useful optimisation may be, in the spirit of the HAMMER idea Another useful optimisation may be, in the spirit of the HAMMER idea
[I-D.wkumari-dnsop-hammer] to probe in advance for the introduction [I-D.wkumari-dnsop-hammer] to probe in advance for the introduction
of zone cuts where none previously existed (i.e. confirm their of zone cuts where none previously existed (i.e. confirm their
continued absence, or discover them.) continued absence, or discover them.)
4. Performance implications 4. Performance implications
The main goal of qname minimisation is to improve privacy by sending The main goal of qname minimisation is to improve privacy by sending
less data. However, it may have other advantages. For instance, if less data. However, it may have other advantages. For instance, if
a root name server receives a query from some resolver for A.CORP a root name server receives a query from some resolver for A.CORP
followed by B.CORP followed by C.CORP, the result will be three followed by B.CORP followed by C.CORP, the result will be three
NXDOMAINs, since .CORP does not exist in the root zone. Under query NXDOMAINs, since .CORP does not exist in the root zone. Under query
name minimisation, the root name servers would hear only one question name minimisation, the root name servers would hear only one question
(for .CORP itself) to which they could answer NXDOMAIN, thus opening (for .CORP itself) to which they could answer NXDOMAIN, thus opening
up a negative caching opportunity in which the full resolver could up a negative caching opportunity in which the full resolver could
know a priori that neither B.CORP or C.CORP could exist. Thus in know a priori that neither B.CORP or C.CORP could exist. Thus in
this common case the total number of upstream queries under qname this common case the total number of upstream queries under qname
minimisation would be counter-intuitively less than the number of minimisation would be counter-intuitively inferior to the number of
queries under the traditional iteration (as described in the DNS queries under the traditional iteration (as described in the DNS
standard). standard).
Qname minimisation may also improve look-up performance for TLD Qname minimisation may also improve look-up performance for TLD
operators. For a typical TLD, delegation-only, and with delegations operators. For a typical TLD, delegation-only, and with delegations
just under the TLD, a 2-label QNAME query is optimal for finding the just under the TLD, a 2-label QNAME query is optimal for finding the
delegation owner name. delegation owner name.
5. Security considerations 5. Security considerations
Qname minimisation's benefits are clear in the case where you want to
decrease exposure to the authoritative name server. But minimising
the amount of data sent also, in part, addresses the case of a wire
sniffer as well the case of privacy invasion by the servers.
(Encryption is of course a better defense against wire sniffers but,
unlike qname minimisation, it changes the protocol and cannot be
deployed unilaterally.)
Qname minimisation offers zero protection against the recursive
resolver, which still sees the full request coming from the stub
resolver.
At this stage, this document does not recommend one of the two qname
minimisation approaches (aggressive or lazy) against the other.
No security consequence (besides privacy improvment) is known at this No security consequence (besides privacy improvment) is known at this
time. time.
6. Acknowledgments 6. Implementation status - REMOVE BEFORE PUBLICATION
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC6982], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
As of today, no production resolver implements qname minimisation.
For Unbound, see ticket 648 [1].
The algorithm to find the zone cuts described in Appendix A is
implemented with qname minimisation in the sample code zonecut.go
[2]. It is also implemented, for a much longer time, in an option of
dig, "dig +trace", but without qname minimisation.
7. Acknowledgments
Thanks to Olaf Kolkman for the original idea although the concept is Thanks to Olaf Kolkman for the original idea although the concept is
probably much older [1]. Thanks to Mark Andrews and Francis Dupont probably much older [3]. Thanks to Mark Andrews and Francis Dupont
for the interesting discussions. Thanks to Brian Dickson, Warren for the interesting discussions. Thanks to Brian Dickson, Warren
Kumari and David Conrad for remarks and suggestions. Thanks to Kumari, Evan Hunt and David Conrad for remarks and suggestions.
Mohsen Souissi for proofreading. Thanks to Tony Finch for the zone Thanks to Mohsen Souissi for proofreading. Thanks to Tony Finch for
cut algorithm in Appendix A. Thanks to Paul Vixie for pointing out the zone cut algorithm in Appendix A. Thanks to Paul Vixie for
that there are practical advantages (besides privacy) to qname m10n. pointing out that there are practical advantages (besides privacy) to
Thanks to Phillip Hallam-Baker for the fallback on A queries, to deal qname minimisation. Thanks to Phillip Hallam-Baker for the fallback
with broken servers. Thanks to Robert Edmonds for an interesting on A queries, to deal with broken servers. Thanks to Robert Edmonds
anti-pattern. for an interesting anti-pattern.
7. References 8. References
7.1. Normative References 8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July Considerations for Internet Protocols", RFC 6973, July
2013. 2013.
[I-D.ietf-dprive-problem-statement] [I-D.ietf-dprive-problem-statement]
Bortzmeyer, S., "DNS privacy considerations", draft-ietf- Bortzmeyer, S., "DNS privacy considerations", draft-ietf-
dprive-problem-statement-01 (work in progress), January dprive-problem-statement-01 (work in progress), January
2015. 2015.
7.2. Informative References 8.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, July 1997.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, July
2013.
[I-D.wkumari-dnsop-hammer] [I-D.wkumari-dnsop-hammer]
Kumari, W., Arends, R., Woolf, S., and D. Migault, "Highly Kumari, W., Arends, R., Woolf, S., and D. Migault, "Highly
Automated Method for Maintaining Expiring Records", draft- Automated Method for Maintaining Expiring Records", draft-
wkumari-dnsop-hammer-01 (work in progress), July 2014. wkumari-dnsop-hammer-01 (work in progress), July 2014.
[I-D.vixie-dnsext-resimprove] [I-D.vixie-dnsext-resimprove]
Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
Resolvers for Resiliency, Robustness, and Responsiveness", Resolvers for Resiliency, Robustness, and Responsiveness",
draft-vixie-dnsext-resimprove-00 (work in progress), June draft-vixie-dnsext-resimprove-00 (work in progress), June
2010. 2010.
[dnsop] IETF, , "The DNSOP working group of IETF", March 2014, [dnsop] IETF, , "The DNSOP working group of IETF", March 2014,
<https://datatracker.ietf.org/wg/dnsop/charter/>. <https://datatracker.ietf.org/wg/dnsop/charter/>.
[mockapetris-history] [mockapetris-history]
Mockapetris, P., "Private discussion", January 2015. Mockapetris, P., "Private discussion", January 2015.
7.3. URIs [kaliski-minimum]
Kaliski, B., "Minimum Disclosure: What Information Does a
Name Server Need to Do Its Job?", March 2015,
<http://blogs.verisigninc.com/blog/entry/
minimum_disclosure_what_information_does>.
[1] https://lists.dns-oarc.net/pipermail/dns- 8.3. URIs
[1] https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=648
[2] https://github.com/bortzmeyer/my-IETF-work/blob/master/draft-
ietf-dnsop-qname-minimisation/zonecut.go
[3] https://lists.dns-oarc.net/pipermail/dns-
operations/2010-February/005003.html operations/2010-February/005003.html
Appendix A. An algorithm to find the zone cut Appendix A. An algorithm to find the zone cut
Although a validating resolver already has the logic to find the zone Although a validating resolver already has the logic to find the zone
cut, other resolvers may be interested by this algorithm to follow in cut, other resolvers may be interested by this algorithm to follow in
order to locate this cut: order to locate this cut:
(0) If the query can be answered from the cache, do so, otherwise (0) If the query can be answered from the cache, do so, otherwise
iterate as follows: iterate as follows:
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