< draft-wkumari-dnsop-hammer-00.txt   draft-wkumari-dnsop-hammer-01.txt >
template W. Kumari template W. Kumari
Internet-Draft Google Internet-Draft Google
Intended status: Informational R. Arends Intended status: Informational R. Arends
Expires: January 02, 2014 Nominet Expires: January 5, 2015 Nominet
S. Woolf S. Woolf
Internet Systems Consortium, Inc.
July 01, 2013 D. Migault
Orange
July 4, 2014
Highly Automated Method for Maintaining Expiring Records Highly Automated Method for Maintaining Expiring Records
draft-wkumari-dnsop-hammer-00 draft-wkumari-dnsop-hammer-01
Abstract Abstract
This document describes a simple DNS cache optimization which keeps This document describes a simple DNS cache optimization which keeps
the most popular records in the DNS cache. the most popular records in the DNS cache: Highly Automated Method
for Maintaining Expiring Records (HAMMER). The principle is that
records in the cache are fetched, that is to say resolved before
their TTL expires and the record is flushed from the cache. By
fetching Records before they are being queried by an end user, 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 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.
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This Internet-Draft will expire on January 02, 2014. This Internet-Draft will expire on January 5, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 2 1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 3.1. Improving browsing Quality of Experience by reducing
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3 response time . . . . . . . . . . . . . . . . . . . . . . 3
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Optimize the resources involved in large DNSSEC resolving
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 platforms . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4 4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 4 5. Known implementations . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 4 5.1. Unbound (NLNet Labs) . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 4 5.2. OpenDNS . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. ISC BIND . . . . . . . . . . . . . . . . . . . . . . . . 6
6. An example / reference implementation . . . . . . . . . . . 6
6.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Changes / Author Notes. . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction 1. Introduction
A recursive DNS resolver may cache a Resource Record (RR) for, at A recursive DNS resolver may cache a Resource Record (RR) for, at
most, the Time To Live (TTL) associated with that record. While the most, the Time To Live (TTL) associated with that record. While the
TTL is greater than zero, the resolver may respond to queries from TTL is greater than zero, the resolver may respond to queries from
its cache, but once the TTL has reached zero, the resolver flushes its cache, but once the TTL has reached zero, the resolver flushes
the RR. When the resolver gets another query for that resource, it the RR. When the resolver gets another query for that resource, it
needs to initiate a new query. This is then cached and returned to needs to initiate a new query. This is then cached and returned to
the querying client. This document proposes an optimization (Highly the querying client. This document discusses an optimization (Highly
Automated Method for Maintaining Expiring Records -- (HAMMER)) to Automated Method for Maintaining Expiring Records -- (HAMMER), also
help keep popular responses in the cache, by fetching new responses known as "prefetch") to help keep popular responses in the cache, by
before the TTL expires. It is designed to not require much fetching new responses before the TTL expires. This behavior is
additional state. triggered by an incoming query, and only shortly before the cache
entry was due to expire.
[I-D.ietf-sidr-iana-objects] and this is a reference to a draft.
1.1. Requirements notation 1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Details 2. Terminology
- HAMMER resolver: A DNS resolver that implements HAMMER mechanism.
- HAMMER FQDN: A FQDN that is a candidate for the HAMMER process.
- HAMMER TIME: TTL Time to consider before triggering the HAMMER
mechanism.
3. Motivations
When a recursive resolver responds to a client, it either responds When a recursive resolver responds to a client, it either responds
from cache, or it initiates an iterative query to resolve the answer, from cache, or it initiates an iterative query to resolve the answer,
caches the answer and then respond with that answer. Performing caches the answer and then responds with that answer.
iterative queries to resolve the answers takes significant time, and
so responding from cache is preferable. 3.1. Improving browsing Quality of Experience by reducing response time
Any end user querying a fetched FQDN will get the response from the
cache of the resolver. This provides faster responses, and thus
improves end user experience for browsing and other applications/
activities.
Popular FQDNs are highly queried, and end users have high
expectations in terms of application response for these FQDNs. With
regular DNS rules, once the FQDN has been flushed from the cache, it
waits for the next end user to request the FQDN before initiating a
resolution for this given FQDN 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 him. 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.
In some cases, not only the first end user querying that FQDN may be
impacted, but also other end users that request the FQDN between the
time the FQDN TTL expires and the time the cache is again filled. In
this case, the result is impact on multiple end users and possible
unnecessary load on the platform. Note that this load is increased
by the use of DNSSEC since DNSSEC may involve additional resolutions,
larger payloads, and signature checks.
DNS response time for a resolution over the Internet is highly
unpredictable as it depends on network congestion and servers'
availability. Links share their bandwidth, so heavily loaded links
result in higher response time, regardless of whether the congestion
occurs close to the resolver, close to the client, or close to the
authoritative servers. Loaded switches or routers may result in
packet drop, which requires the resolver to notice the packet has
been dropped (usually with a time out) and restart the iterative
resolution. These issues are increased by the use of DNSSEC which
makes DNS packets larger. Similarly, loaded servers have longer
response times.
3.2. Optimize the resources involved in large DNSSEC resolving
platforms
Large resolving platforms are often composed of a set of independent
resolving nodes. The traffic is usually load balanced based on the
query source IP addresses. This results in most popular FQDNs being
resolved independently by all nodes. First this increases the number
of end users who may experience unnecessary latency. Also, when
DNSSEC is used, all nodes independently perform signature check
operations, possibly resulting in high loads on the authoritative
server.
The challenge these large DNSSEC resolving platforms have to overcome
is to provide a uniform distribution of the nodes given that end user
and FQDNs do not have a uniform distribution of the resources. More
specifically, FQDNs and end users usually present Zipf popularity
distributions, which means that most of the traffic is performed by a
small set of end users and by a small set of FQDNs.
DNS and large resolving DNS platforms have resulted in uniformly
balanced traffic among the nodes. In fact the resolving traffic on
the Internet interface was rather small (at least in term of CPU)
compared to traffic received from the end users. DNSSEC changed
this, as CPU are involved in performing signature checks. One way to
reduce the number of DNSSEC resolutions is to fetch the nodes with
the most popular FQDNs. This avoids parallel resolutions and overall
reduces cost, because signature checks are not performed, while
benefiting from the already existing load balancing architecture.
This architecture takes advantage of the Zipf distribution of the
FQDNs' popularity. In fact, a few number of FQDNs can be cached (a
few thousands) to address most of the traffic (up to 70%).
Note that to perform a single resolution for the global platform,
nodes may be configured as forwarders for the most popular FQDNs
4. Overview of Operation
When an incoming query is received, and the result is in the cache,
the query is answered from the cache. If the remaining TTL of the
record is below some threshold, the recursive server will also
initiate a cache fill operation in the background to refresh the
cache entry.
The fact that the behavior is triggered by an incoming query (and not
by periodically scanning the cache and refreshing all entries that
are about to expire) allows unpopular names to age out of the cache
naturally, while keeping popular entries in the cache.
5. Known implementations
[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
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.
5.1. 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.]
5.2. OpenDNS
The public DNS resolver, OpenDNS implements a prefetch like solution.
[Ed: Will work with OpenDNS to get more details.]
5.3. 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:
Trigger This is equivalent to the HAMMER_TIME parameter described
below.
Eligibility This is equivalent to the STOP parameter described
below.
6. An example / reference implementation
When a recursive resolver that implements HAMMER receives a query for When a recursive resolver that implements HAMMER receives a query for
information that it has in the cache, it responds from the cache. It information that it has in the cache, it responds from the cache.
also checks to see if the remaining TTL on the cached data is less
than HAMMER_TIME (described below). If it is, the resolver will If the queried FQDN is a HAMMER FQDN, the HAMMER resolver compares
initiate a "cache fill" iterative query, just like it would have if the TTL value to the HAMMER TIME, as well as if the FQDN has already
the cached data had expired. During this cache fill operation the been fetched.
resolver continues to respond from cache (until the TTL expires).
When the cache fill query completes, the new response replaces the If the HAMMER FQDN has already been fetched or provisioned) then
existing cached information. This ensure that the cache has fresh nothing is done.
data for subsequent queries.
If the HAMMER FQDN has not yet been fetched and the TTL is less then
the HAMMER_TIME, the HAMMER resolver starts a resolution for the
queried FQDN in order to fill the cache, just as if the TTL had
expired. During this cache fill operation the resolver continues to
respond from cache (until the TTL expires). When the cache fill
query completes, the new response replaces the existing cached
information. This ensures the cache has fresh data for subsequent
queries.
Since the cache fill query is initiated before the existing cached Since the cache fill query is initiated before the existing cached
entry expires (and is flushed), responses will come from the cache entry expires (and is flushed), responses will come from the cache
more often. This decreases the client resolution latency and more often. This decreases the client resolution latency and
improves the user experience. improves the user experience.
As the cache fill resolution is triggered by an incoming query (and The cache fill resolution is triggered by an incoming query (and only
only if that query arrives shortly before the record would expire if that query arrives shortly before the record would expire anyway).
anyway), this effectively keeps the most popular data in the cache, This effectively keeps the most popular data uniformly queried in the
without having to maintain counters in the cache, or proactively cache, without having to maintain counters in the cache or
resolve responses that are not likely to be needed as often. This is proactively resolve responses that are not likely to be needed as
purely an implementation optimization - resolvers always have the often. This is purely an implementation optimization - resolvers
option to cache records for less than the TTL (for example, when always have the option to cache records for less than the TTL (for
running low on cache space, etc), this simply triggers a refresh of example, when running low on cache space, etc), this simply triggers
the RR before it expires. a refresh of the RR before it expires.
If the original TTL of the RR is less than (or close to HAMMER_TIME), Note that non-uniformly queried FQDNs may be popular and may not
the described method could cause excessive cache fill queries to benefit from the HAMMER mechanism. For example, an FQDNs MAY be
occur. In order to prevent this an additional variable named STOP heavily queried the first 10 minutes of every hour with a 30 minute
(described below) is introduced. If the original TTL of the RR is TTL. In that case DNS queries are not expected to come between TTL -
less than STOP * HAMMER_TIME then the cache entry should be marked HAMMER_TIME and TTL.
with a "Can't touch this" flag, and the described method should not
be used.
2.1. Variables HAMMER FQDNs with small TTL may generate a cache fill 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 FQDNs
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 FQDNs. As a result, if the original TTL of the RR
is less than (or close to HAMMER_TIME), the described method could
cause excessive cache fill queries to occur. In order to prevent
this an additional variable named STOP (described below) is
introduced. If the original TTL of the RR 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.
HAMMER_TIME is the number of seconds before TTL expiration that a 6.1. Variables
cache fill query should be initiated. This should be a user
configurable value. A default of 2 seconds is RECOMMENDED.
STOP should be a user configurable variable. A default of 3 is These are the mandatory variables:
recommended.
3. IANA Considerations - HAMMER_TIME: is the number of seconds before TTL expiration that a
cache fill query should be initiated. This should be a user
configurable value. 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
FQDNs that are expected to implement HAMMER. This rule can be
expressed in different ways. It can be a list of FQDNs, or a
number indicating the number of most popular FQDNs 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 FQDNs, or define
the HAMMER_FQDNs as the X most popular FQDNs.
- HAMMER_FORWARDER: should be a user configurable variable. It is
optional and designates the DNS server the resolver forwards
the request to.
7. IANA Considerations
This document makes no request of the IANA. This document makes no request of the IANA.
4. Security Considerations 8. Security Considerations
This technique leverages existing protocols, and should not introduce This technique leverages existing protocols, and should not introduce
any new risks, other than a slight increase in traffic. any new risks, other than a slight increase in traffic.
By initiating cache fill entries before the existing RR has expired By initiating cache fill entries before the existing RR has expired
this technique will slightly increase the number of queries seen by this technique will slightly increase the number of queries seen by
authoritative servers. This increase will be inversely proportional authoritative servers. This increase will be inversely proportional
to the average TTL of the records that they serve. to the average TTL of the records that they serve.
It is unlikely, but possible that this increase could cause a denial It is unlikely, but possible that this increase could cause a denial
of service condition. of service condition.
5. Acknowledgements 9. Acknowledgements
The authors wish to thank Tony Finch and MC Hammer.
6. References 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 thier names on this).
6.1. Normative References 10. References
[IANA.AS_Numbers] 10.1. Normative References
IANA, "Autonomous System (AS) Numbers", ,
<http://www.iana.org/assignments/as-numbers>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
6.2. Informative References 10.2. Informative References
[I-D.ietf-sidr-iana-objects] [I-D.ietf-sidr-iana-objects]
Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects
issued by IANA", draft-ietf-sidr-iana-objects-03 (work in issued by IANA", draft-ietf-sidr-iana-objects-03 (work in
progress), May 2011. progress), May 2011.
Appendix A. Changes / Author Notes. Appendix A. Changes / Author Notes.
[RFC Editor: Please remove this section before publication ] [RFC Editor: Please remove this section before publication ]
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. From -template to -00.
o Wrote some text. o Wrote some text.
o Changed the name. o Changed the name.
Authors' Addresses Authors' Addresses
Warren Kumari Warren Kumari
Google Google
1600 Amphitheatre Parkway 1600 Amphitheatre Parkway
Mountain View, CA 94043 Mountain View, CA 94043
US US
Email: warren@kumari.net Email: warren@kumari.net
Roy Arends Roy Arends
Nominet Nominet
Edmund Halley Road Edmund Halley Road
Oxford OX4 4DQ Oxford OX4 4DQ
United Kingdom United Kingdom
Email: roy@nominet.org.uk Email: roy@nominet.org.uk
Suzanne Woolf Suzanne Woolf
Internet Systems Consortium, Inc. 39 Dodge St. #317
950 Charter Street Beverly, MA 01915
Redwood City, CA 94063
US US
Email: woolf@isc.org Email: suzworldwide@gmail.com
Daniel Migault
Orange
38 rue du General Leclerc
92794 Issy-les-Moulineaux Cedex 9
France
Phone: +33 1 45 29 60 52
Email: daniel.migaultf@orange.com
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