template                                                       W. Kumari
Internet-Draft                                                    Google
Intended status: Informational                                 R. Arends
Expires: January 5, 2015                                         Nominet
                                                                S. Woolf

                                                              D. Migault
                                                            July 4, 2014

        Highly Automated Method for Maintaining Expiring Records


   This document describes a simple DNS cache optimization which keeps
   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

Status of This Memo

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   This Internet-Draft will expire on January 5, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   5
   5.  Known implementations . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Unbound (NLNet Labs)  . . . . . . . . . . . . . . . . . .   6
     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

   A recursive DNS resolver may cache a Resource Record (RR) for, at
   most, the Time To Live (TTL) associated with that record.  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 RR.  When the resolver gets another query for that resource, it
   needs to initiate a new query.  This is then cached and returned to
   the 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 new responses before the TTL expires.  This behavior is

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   triggered by an incoming query, and only shortly before the cache
   entry was due to expire.

1.1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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

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 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/

   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

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   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

   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

   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%).

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   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/
   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.

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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

   Eligibility  This is equivalent to the STOP parameter described

6.  An example / reference implementation

   When a recursive resolver that implements HAMMER receives a query for
   information that it has in the cache, it responds from the cache.

   If the queried FQDN is a HAMMER FQDN, the HAMMER resolver compares
   the TTL value to the HAMMER TIME, as well as if the FQDN has already
   been fetched.

   If the HAMMER FQDN has already been fetched or provisioned) then
   nothing is done.

   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

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   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

   Since the cache fill query 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.

   The cache fill resolution 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 data 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 RR before it expires.

   Note that non-uniformly queried FQDNs may be popular and may not
   benefit from the HAMMER mechanism.  For example, an FQDNs 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 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.

6.1.  Variables

   These are the mandatory variables:

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   - 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

   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.

8.  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.

9.  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 thier names on this).

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10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

10.2.  Informative References

              Manderson, T., Vegoda, L., and S. Kent, "RPKI Objects
              issued by IANA", draft-ietf-sidr-iana-objects-03 (work in
              progress), May 2011.

Appendix A.  Changes / Author Notes.

   [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.

   o  Wrote some text.

   o  Changed the name.

Authors' Addresses

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: warren@kumari.net

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   Roy Arends
   Edmund Halley Road
   Oxford  OX4 4DQ
   United Kingdom

   Email: roy@nominet.org.uk

   Suzanne Woolf
   39 Dodge St. #317
   Beverly, MA  01915

   Email: suzworldwide@gmail.com

   Daniel Migault
   38 rue du General Leclerc
   92794 Issy-les-Moulineaux Cedex 9

   Phone: +33 1 45 29 60 52
   Email: daniel.migaultf@orange.com

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