Internet DRAFT - draft-wkumari-dnsop-hammer
draft-wkumari-dnsop-hammer
Network Working Group W. Kumari
Internet-Draft Google
Intended status: Informational R. Arends
Expires: December 29, 2017 ICANN
S. Woolf
D. Migault
Ericsson
June 27, 2017
Highly Automated Method for Maintaining Expiring Records
draft-wkumari-dnsop-hammer-03
Abstract
This document describes a simple DNS cache optimization which keeps
the most popular Resource Records set (RRset) in the DNS cache:
Highly Automated Method for Maintaining Expiring Records (HAMMER).
The principle is that popular RRset in the cache are fetched, that is
to say resolved before their TTL expires and flushed. By fetching
RRset before they are being queried by an end user, that is to say
prefetched, HAMMER is expected to improve the quality of experience
of the end users as well as to optimize the resources involved in
large DNSSEC resolving platforms.
Status of This Memo
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This Internet-Draft will expire on December 29, 2017.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Improving browsing Quality of Experience by reducing
response time . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Optimize the resources involved in large DNSSEC resolving
platforms . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol Description . . . . . . . . . . . . . . . . . . . . 5
5. Configuration Variables . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Known implementations . . . . . . . . . . . . . . . 8
A.1. Unbound (NLNet Labs) . . . . . . . . . . . . . . . . . . 9
A.2. OpenDNS . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.3. ISC BIND . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Changes / Author Notes. . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
A recursive DNS resolver may cache a Resource Record set (RRset) for,
at most, the Time To Live (TTL) associated with that RRset. While
the TTL is greater than zero, the resolver may respond to queries
from its cache; but once the TTL has reached zero, the resolver
flushes the RRset. When the resolver gets another query for that
RRset, the RRset is not anymore in the cache, thus the resolver need
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to proceed to a new resolution for that RRset with its associated
latency and processing. The resolved RRset are then cached and
returned to the original querying client. This document discusses an
optimization (Highly Automated Method for Maintaining Expiring
Records -- (HAMMER), also known as "prefetch") to help keep popular
responses in the cache, by fetching (or resolving) resources before
their TTL expires.
In that document, a resolver implementing HAMMER (HAMMER resolver)
prefetches a RRset candidate to HAMMER (HAMMER RRset) when it
receives a query and its TTL is lower than HAMMER TIME.
Note that [RFC4035] assumes that all RR of a RRset have the same TTL,
while [RFC2181] allows the TTL of the RR of a RRset to be different.
When the RRset does not follow [RFC4035], the TTL of the RRset that
is considered is the minimum value of the TTL.
1.1. Requirements notation
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 [RFC2119].
2. Terminology
HAMMER resolver: A DNS resolver that implements HAMMER mechanism.
HAMMER RRset: A RRset that is a candidate for the HAMMER process.
HAMMER TIME: a number of seconds that indicates the period preceding
the TTL expiration time. When a query for a HAMMER RRset is
received during that period, the HAMMER resolver prefetches the
HAMMER RRset by initiating a resolution.
3. Motivations
When a recursive resolver responds to a client, it either responds
from cache, or it initiates an iterative query to resolve the answer,
caches the answer and then responds with that answer.
3.1. Improving browsing Quality of Experience by reducing response time
Any end user querying a fetched RRset will get the response from the
cache of the resolver. This provides faster responses, thus
improving the end user experience for browsing and other
applications/activities.
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Popular RRsets are highly queried, and end users have high
expectations in terms of application responsiveness for these RRsets.
With regular DNS rules, once the RRset has been flushed from the
cache, it waits for the next end user to request the RRset before
initiating a resolution for this given RRset with iterative queries.
This results in at least one end user waiting for this resolution to
be performed over the Internet before the response is sent to them.
This may provide a poor user experience since DNS response times over
the Internet are unpredictable at best and it provides a response
time longer then usual. Note that the response time may also be
increased by the use of DNSSEC since DNSSEC may involve additional
resolutions, larger payloads, and signature checks.
In addition to that first end user querying RRset after it has been
flushed, end users querying the RRset during its resolution or
fetching phase are also impacted. As a result, especially for
popular RRsets, multiple end users are likely to be impacted and be
provided a poor user experience.
The impact on users also depends on the architecture of resolving
platform architecture. In some case, a centralized resolver is
implemented as a farm of independent resolving nodes and the traffic
is split between the nodes according to the IP addresses and ports.
In such architectures, the number of affected end users is
proportional to the number of resolving nodes as each query is
pseudo-randomly associated to one of the resolving node. Similarly,
some global resolving platform uses anycast to steer the queries to
the resolving node associated with the shortest path. Unless all
queries comes from a single region, such architecture are also
expected to impact a number of user proportional to the number of
resolving nodes.
3.2. Optimize the resources involved in large DNSSEC resolving
platforms
As mentioned in Section 3.1, large resolving platforms are often
composed of a set of independent resolving nodes in order to
distribute the traffic between these nodes. Traffic can be
distributed using various forms of load balancing between the
resolving nodes. This includes, for example, a pseudo-random
distribution when load balancing is based on the hash of the IP
addresses and ports or shortest path when anycast is deployed. Such
distributions split the traffic independently of the queried RRset.
Ignoring the coordination of the resolutions implicitly assumes the
resource to perform the resolutions is negligible compared to those
necessary to handle the queries of the end users.
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As a result, such platforms perform multiple parallel resolutions on
their various nodes. With DNS, the necessary resource associated to
the resolution were in fact minimal so little effort have been
considered to synchronize these nodes in order to reduce the number
of resolutions. On the other hand DNSSEC resolutions involve
additional resolutions, larger payloads and signature checks. The
consequent increase of resource to perform DNSSEC resolutions versus
DNS resolution makes parallel resolutions a non negligible lost of
resource and leave place for synchronization mechanisms.
One way to reduce the number of DNSSEC resolutions is to prefetch (or
provision) the nodes with the most popular RRsets before their TTL
expire. Note that in this case, the resolution is not performed by
the resolving node. At a node level, prefetching increases the nodes
availability. At the platform level, synchronizing the resolving
nodes' resolution globally reduce the number of resolution and so the
overall resource of the platform.
Synchronization of the resolution may be performed by configuring
each node as a forwarder for these RRsets. This avoids parallel
resolutions and overall reduces cost, because signature checks are
not performed by each resolving node. In this case prefetching
enables to still benefit from the already existing load balancing
architecture that split the load of the end users' queries traffic
between the nodes. Note that the advantages of synchronizing the
resolutions between the resolving nodes may depend on the popularity
of the RRsets. This architecture takes advantage of the Zipf [ZIPF]
distribution of the RRsets' popularity. In fact, a few number of
RRsets need to be cached (a few thousands) to address most of the
traffic (up to 70%) [PREFETCH].
4. Protocol Description
This section describes HAMMER. This section is not normative and
implementation may implement this mechanism with their own flavor.
When a recursive resolver that implements HAMMER receives a query for
a HAMMER RRset that it has in the cache, it responds from the cache.
If the queried RRset is a HAMMER RRset, the HAMMER resolver compares
the TTL value to the HAMMER TIME, as well as if the RRset is being
(pre)fetched.
If the HAMMER RRset has a TTL greater then the HAMMER TIME, nothing
is done.
If the HAMMER RRset has a TTL less than the HAMMER TIME, the HAMMER
resolver starts a resolution for the RRset in order to fill the
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cache, just as if the TTL had expired. The HAMMER RRset is
prefetched. Note that during the resolution, the HAMMER RRset is
still cached, and queries are responded form the cache until the TTL
expires. Once the resolution is performed, the freshly resolved
RRset replace the existing cached RRset. This ensures the cache has
fresh data for subsequent queries.
Since prefetching is initiated before the existing cached entry
expires (and is flushed), responses will come from the cache more
often. This decreases the client resolution latency and improves the
user experience.
Prefetching is triggered by an incoming query (and only if that query
arrives shortly before the record would expire anyway). This
effectively keeps the most popular RRsets uniformly queried in the
cache, without having to maintain counters in the cache or
proactively resolve responses that are not likely to be needed as
often. This is purely an implementation optimization - resolvers
always have the option to cache records for less than the TTL (for
example, when running low on cache space, etc), this simply triggers
a refresh of the RRset before it expires.
Note that non-uniformly queried RRsets may be popular and may not
benefit from the HAMMER mechanism. For example, a RRset MAY be
heavily queried the first 10 minutes of every hour with a 30 minute
TTL. In that case DNS queries are not expected to come between TTL -
HAMMER TIME and TTL.
HAMMER RRset with small TTL may generate a prefetching process even
though they are not so popular. Suppose an end user is setting a
specific session which requires multiple DNS resolutions on a given
FQDN. These resolutions are necessary for a short period of time,
i.e. the necessary time to establish the session. If these RRset
have been set with a small TTL - in the order of the time session
establishment - the multiple queries to a HAMMER resolver may trigger
an unnecessary resolution. As a result HAMMER would not scale
thousands of these RRsets. As a result, if the original TTL of the
RRset is less than (or close to HAMMER TIME), the described method
could cause excessive prefetching queries to occur. In order to
prevent this an additional variable named STOP (described below) is
introduced. If the original TTL of the RRset is less than STOP *
HAMMER TIME then the cache entry should be marked with a "Can't touch
this" flag, and the described method should not be used.
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5. Configuration Variables
These are the mandatory variables:
HAMMER TIME: a number of seconds that indicates the period preceding
the TTL expiration time. When a query for a HAMMER RRset is
received during that period, the HAMMER resolver prefetches the
HAMMER RRset by initiating a resolution. A default of 2
seconds is RECOMMENDED.
STOP: should be a user configurable variable. A default of 3 is
recommended.
Implementations may consider additional variables. These are not
mandatory but would address specific use of the HAMMER.
HAMMER MATCH: should be a user configurable variable. It defines
RRsets that are expected to implement HAMMER. This rule can be
expressed in different ways. It can be a list of RRsets, or a
number indicating the number of most popular RRsets that needs
to be considered. How HAMMER MATCH is expressed is
implementation dependent. Implementations can use a list of
FQDNs, others can use a matching rule on the RRsets, or define
the HAMMER RRsets as the X most popular RRsets.
HAMMER FORWARDER: should be a user configurable variable. It is
optional and designates the DNS server the resolver forwards
the request to.
6. IANA Considerations
This document makes no request of the IANA.
7. Security Considerations
This technique leverages existing protocols, and should not introduce
any new risks, other than a slight increase in traffic.
By initiating cache fill entries before the existing RR has expired
this technique will slightly increase the number of queries seen by
authoritative servers. This increase will be inversely proportional
to the average TTL of the records that they serve.
It is unlikely, but possible, that this increase could cause a denial
of service condition.
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8. Acknowledgements
The authors wish to thank Tony Finch and MC Hammer. We also wish to
thank Brian Somers and Wouter Wijngaards for telling us that they
already do this :-) (They should probably be co-authors, but I left
this too close to the draft cutoff time to confirm with them that
they are willing to have their names on this).
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<http://www.rfc-editor.org/info/rfc2181>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
9.2. Informative References
[PREFETCH]
Migault, D., Francfort, S., Senecal, S., Herbert, E., and
M. Laurent, "PREFETCHing to optimize DNSSEC deployment
over large Resolving Platforms", Jul 2013,
<https://www.researchgate.net/publication/270571591_PREFET
CHing_to_optimize_DNSSEC_deployment_over_large_Resolving_P
latforms>.
[ZIPF] Powers, D., "Applications and Explanations of Zipf's Law",
Jan 1998, <http://aclweb.org/anthology/W98-1218>.
Appendix A. Known implementations
[RFC Editor: Please remove this section before publication ]
[Ed: Well, this is kinda embarrassing. This idea occurred to us one
day while sitting around a pool in New Hampshire. It then took a
while before I wrote it down, mostly because I *really* wanted to get
"Stop! Hammer Time!" into a draft. Anyway, we presented it in
Berlin, and Wouter Wijngaards stood up and mentioned that Unbound
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already does this (they use a percentage of TTL, instead of a number
of seconds). Then we heard from OpenDNS that they *also* implement
something similar. Then we had a number of discussions, then got
sidetracked into other things. Anyway, BIND as of 9.10, around Feb
2014 now implements something like this
(https://deepthought.isc.org/article/AA-01122/0/Early-refresh-of-
cache-records-cache-prefetch-in-BIND-9.10.html), and enables it by
default. Unfortunately, while BIND uses the times based approach,
they named their parameters "trigger" and "eligibility" - and
shouting "Eligibility! Trigger time!" simply isn't funny (unless you
have a very odd sense of humor... So, we are now documenting
implementations that existed before this was published and an
impl,entation that we think was based on this. We think that this
has value to the community. I'm also leaving in the HAMMER TIME bit,
because it makes me giggle. This below section should be filled out
with more detail, in collaboration with the implementors, but this is
being written *just* before the draft cutoff.].
A number of recursive resolvers implement techniques similar to the
techniques described in this document. This section documents some
of these and tradeoffs they make in picking their techniques.
A.1. Unbound (NLNet Labs)
The Unbound validating, recursive, and caching DNS resolver
implements a HAMMER type feature, called "prefetch". This feature
can be enabled or disabled though the configuration option "prefetch:
<yes or no>". When enabled, Unbound will fetch expiring records when
their remaining TTL is less than 10% of their original TTL.
[Ed: Unbound's "prefetch" function was developed independently,
before this draft was written. The authors were unaware of it when
writing the document.]
A.2. OpenDNS
The public DNS resolver, OpenDNS implements a prefetch like solution.
[Ed: Will work with OpenDNS to get more details.]
A.3. ISC BIND
As of version 9.10, Internet Systems Consortium's BIND implements the
HAMMER functionality. This feature is enabled by default.
The functionality is configured using the "prefetch" options
statement, with two parameters:
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Trigger This is equivalent to the HAMMER_TIME parameter described
below.
Eligibility This is equivalent to the STOP parameter described
below.
Appendix B. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ]
From -01 to -02:
o Readbility / cleanup.
o Tried to make it more clear that most implementations now support
this (although they call it "prefetch" )
From -00 to 01:
o Fairly large rewrite.
o Added text on the fact that there are implmentations that do this.
o Added the "prefetch" name, cleaned up some readability.
o Daniel's test (Section 3.2) added.
From -template to -00.
o Wrote some text.
o Changed the name.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
Roy Arends
ICANN
Email: roy.arends@icann.org
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Suzanne Woolf
39 Dodge St. #317
Beverly, MA 01915
US
Email: suzworldwide@gmail.com
Daniel Migault
Ericsson
2039 Rue Cohen
Saint-Laurent H4R 2A4
Canada
Email: daniel.migaultf@ericsson.com
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