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Versions: (draft-morris-dnsop-dnssec-key-timing) 00 01 02 03 04 05 06 RFC 7583

Internet Engineering Task Force                                S. Morris
Internet-Draft                                                       ISC
Intended status: Informational                                  J. Ihren
Expires: April 28, 2011                                           Netnod
                                                            J. Dickinson
                                                                 Sinodun
                                                        October 25, 2010


                    DNSSEC Key Timing Considerations
               draft-ietf-dnsop-dnssec-key-timing-01.txt

Abstract

   This document describes the issues surrounding the timing of events
   in the rolling of a key in a DNSSEC-secured zone.  It presents
   timelines for the key rollover and explicitly identifies the
   relationships between the various parameters affecting the process.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 28, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Key Rolling Considerations . . . . . . . . . . . . . . . .  3
     1.2.  Types of Keys  . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Rollover Methods . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  ZSK Rollovers  . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  KSK Rollovers  . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Key Rollover Timelines . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Key States . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Zone-Signing Key Timelines . . . . . . . . . . . . . . . .  9
       3.2.1.  Pre-Publication Method . . . . . . . . . . . . . . . .  9
       3.2.2.  Double-Signature Method  . . . . . . . . . . . . . . . 13
       3.2.3.  Double-RRSIG Method  . . . . . . . . . . . . . . . . . 14
     3.3.  Key-Signing Key Rollover Timelines . . . . . . . . . . . . 17
       3.3.1.  Double-Signature Method  . . . . . . . . . . . . . . . 17
       3.3.2.  Double-DS Method . . . . . . . . . . . . . . . . . . . 20
       3.3.3.  Double-RRset Method  . . . . . . . . . . . . . . . . . 22
       3.3.4.  Interaction with Configured Trust Anchors  . . . . . . 25
         3.3.4.1.  Addition of KSK  . . . . . . . . . . . . . . . . . 25
         3.3.4.2.  Removal of KSK . . . . . . . . . . . . . . . . . . 25
       3.3.5.  Introduction of First KSK  . . . . . . . . . . . . . . 26
   4.  Standby Keys . . . . . . . . . . . . . . . . . . . . . . . . . 27
   5.  Algorithm Considerations . . . . . . . . . . . . . . . . . . . 28
   6.  Limitation of Scope  . . . . . . . . . . . . . . . . . . . . . 28
   7.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 29
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     12.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Appendix A.  List of Symbols . . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35










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1.  Introduction

1.1.  Key Rolling Considerations

   When a zone is secured with DNSSEC, the zone manager must be prepared
   to replace ("roll") the keys used in the signing process.  The
   rolling of keys may be caused by compromise of one or more of the
   existing keys, or it may be due to a management policy that demands
   periodic key replacement for security or operational reasons.  In
   order to implement a key rollover, the keys need to be introduced
   into and removed from the zone at the appropriate times.
   Considerations that must be taken into account are:

   o  DNSKEY records and associated information (such as RRSIG records
      created with the key or the associated DS records) are not only
      held at the authoritative nameserver, they are also cached at
      client resolvers.  The data on these systems can be interlinked,
      e.g. a validating resolver may try to validate a signature
      retrieved from a cache with a key obtained separately.

   o  A query for the key RRset returns all DNSKEY records in the zone.
      As there is limited space in the UDP packet (even with EDNS0
      support), dead key records must be periodically removed.  (For the
      same reason, the number of stand-by keys in the zone should be
      restricted to the minimum required to support the key management
      policy.)

   o  Zone "boot-strapping" events, where a zone is signed for the first
      time, can be common in configurations where a large number of
      zones are being served.  Procedures should be able to cope with
      the introduction of keys into the zone for the first time as well
      as "steady-state", where the records are being replaced as part of
      normal zone maintenance.

   o  To allow for an emergency re-signing of the zone as soon as
      possible after a key compromise has been detected, stand-by keys
      (additional keys over and above those used to sign the zone) need
      to be present.

   Management policy, e.g. how long a key is used for, also needs to be
   considered.  However, the point of key management logic is not to
   ensure that a "rollover" is completed at a certain time but rather to
   ensure that no changes are made to the state of keys published in the
   zone until it is "safe" to do so ("safe" in this context meaning that
   at no time during the rollover process does any part of the zone ever
   go bogus).  In other words, although key management logic enforces
   policy, it may not enforce it strictly.




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1.2.  Types of Keys

   Although DNSSEC validation treats all keys equally, [RFC4033]
   recognises the broad classification of zone-signing keys (ZSK) and
   key-signing keys (KSK).  A ZSK is used to authenticate information
   within the zone; a KSK is used to authenticate the key set in the
   zone.  The main implication for this distinction concerns the
   consistency of information during a rollover.

   During operation, a validating resolver must use separate pieces of
   information to perform an authentication.  At the time of
   authentication, each piece of information may be in the validating
   resolver's cache or may need to be retrieved from the authoritative
   server.  The rollover process needs to happen in such a way that at
   all times through the rollover the information is consistent.  With a
   ZSK, the information is the RRSIG (plus associated RRset) and the
   DNSKEY.  These are both obtained from the same zone.  In the case of
   the KSK, the information is the DNSKEY and DS RRset with the latter
   being obtained from a different zone.

   There are similarities in the rolling of ZSKs and KSKs: replace the
   RRSIG (plus RR) by the DNSKEY and replace the DNSKEY by the DS, and
   the ZSK rolling algorithms are virtually the same as the KSK
   algorithms.  However, there are a number of differences, and for this
   reason the two types of rollovers are described separately in this
   document.

1.3.  Terminology

   The terminology used in this document is as defined in [RFC4033] and
   [RFC5011].

   A large number of symbols are used to identify times, intervals, etc.
   All are listed in Appendix A.


2.  Rollover Methods

2.1.  ZSK Rollovers

   A ZSK can be rolled in one of three ways:

   o  Pre-Publication.  Described in [RFC4641], the new key is
      introduced into the DNSKEY RRset, leaving the existing keys and
      signatures in place.  This state of affairs remains in place for
      long enough to ensure that any DNSKEY RRsets cached in client
      validating resolvers contain both keys.  At that point signatures
      created with the old key can be replaced by those created with the



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      new key, and the old signatures removed.  During the re-signing
      process (which may or may not be atomic depending on how the zone
      is managed), it doesn't matter which key an RRSIG record retrieved
      by a client was created with; clients with a cached copy of the
      DNSKEY RRset will have a copy containing both the old and new
      keys.

      Once the zone contains only signatures created with the new key,
      there is an interval during which RRSIG records created with the
      old key expire from client caches.  After this, there will be no
      signatures anywhere that were created using the old key, and it
      can can be removed from the DNSKEY RRset.

   o  Double-Signature.  Also mentioned in [RFC4641], this involves
      introducing the new key into the zone and using it to create
      additional RRSIG records; the old key and existing RRSIG records
      are retained.  During the period in which the zone is being signed
      (again, the signing process may not be atomic), client resolvers
      are always able to validate RRSIGs: any combination of old and new
      DNSKEY RRset and RRSIG allows at least one signature to be
      validated.

      Once the signing process is complete and enough time has elapsed
      to allow all old information to expire from caches, the old key
      and signatures can be removed from the zone.  As before, during
      this period any combination of DNSKEY RRset and RRSIG will allow
      validation of at least one signature.

   o  Double-RRSIG.  Strictly speaking, the use of the term "Double-
      Signature" above is a misnomer as the method is not only double
      signature, it is also double key as well.  A true Double-Signature
      method (here called the Double-RRSIG method) involves introducing
      new signatures in the zone (while still retaining the old ones)
      but not the new key.

      Once the signing process is complete and enough time has elapsed
      to ensure that all caches that may contain an RR and associated
      RRSIG to have a copy of both signatures, the ZSK is changed.
      After a further interval during which the old DNSKEY RRset expires
      from caches, the old signatures are removed from the zone.

   Of three methods, Double-Signature is the simplest conceptually -
   introduce the new key and new signatures, then approximately one TTL
   later remove the old key and signatures.  The drawback of this method
   is a noticeable increase in the size of the DNSSEC data, affecting
   both the overall size of the zone and the size of the responses.

   Pre-Publication is more complex - introduce the new key,



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   approximately one TTL later sign the records, and approximately one
   TTL after that remove the old key.  However, the amount of DNSSEC
   data is kept to a minimum which reduces the impact on performance.

   The Double-RRSIG combines the increase in data volume of the Double-
   Signature method with the complexity of Pre-Publication.  It has few
   (if any) advantages and a description is only included here for
   completeness.

2.2.  KSK Rollovers

   In the ZSK case the issue for the validating resolver is to ensure
   that it has access to the ZSK that corresponds to a particular
   signature.  In the KSK case this can never be a problem as the KSK is
   only used for one signature (that over the DNSKEY RRset) and both the
   key the signature travel together.  Instead, the issue is to ensure
   that the KSK is trusted.

   Trust in the KSK is either due to the existence of an explicitly
   configured trust anchor in the validating resolver or DS record in
   the parent zone (which is itself trusted).  If the former, [RFC5011]
   timings will be needed to roll the keys.  If the latter, the rollover
   algorithm will need to involve the parent zone in the addition and
   removal of DS records, so timings are not wholly under the control of
   the zone manager.  (The zone manager may elect to include [RFC5011]
   timings in the key rolling process so as to cope with the possibility
   that the key has also been explicitly configured as a trust anchor.)

   It is important to note that this does not preclude the development
   of key rollover logic; in accordance with the goal of the rollover
   logic being able to determine when a state change is "safe", the only
   effect of being dependent on the parent is that there may be a period
   of waiting for the parent to respond in addition to any delay the key
   rollover logic requires.  Although this introduces additional delays,
   even with a parent that is less than ideally responsive the only
   effect will be a slowdown in the rollover state transitions.  This
   may cause a policy violation, but will not cause any operational
   problems.

   Like the ZSK case, there are three methods for rolling a KSK:

   o  Double-Signature: Also known as Double-DNSKEY, the new KSK is
      added to the DNSKEY RRset which is then signed with both the old
      and new key.  After waiting for the old RRset to expire from
      caches, the DS record in the parent zone is changed.  After
      waiting a further interval for this change to be reflected in
      caches, the old key is removed from the RRset.  (The name "Double-
      Signature" is used because, like the ZSK method of the same name,



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      the new key is introduced and immediately used for signing.)

   o  Double-DS: the new DS record is published.  After waiting for this
      change to propagate into the caches of all validating resolvers,
      the KSK is changed.  After a further interval during which the old
      DNSKEY RRset expires from caches, the old DS record is removed.

   o  Double-RRset: the new KSK is added to the DNSKEY RRset which is
      then signed with both the old and new key, and the new DS record
      added to the parent zone.  After waiting a suitable interval for
      the old DS and DNSKEY RRsets to expire from validating resolver
      caches, the old DNSKEY and DS record are removed.

   In essence, "Double-Signature" means that the new KSK is introduced
   first and used to sign the DNSKEY RRset.  The DS record is changed,
   and finally the old KSK removed.  With "Double-DS" it is the other
   way around.  Finally, Double-RRset does both updates more or less in
   parallel.

   The strategies have different advantages and disadvantages:

   o  Double-Signature minimizes the number of interactions with the
      parent zone.  However, for the period of the rollover the DNSKEY
      RRset is signed with two KSKs, so increasing the size of the
      response to a query for this data.

   o  Double-DS keeps the size of the DNSKEY RRset to a minimum, but
      does require the additional administrative overhead of two
      interactions with the parent to roll a KSK.  (Although this can be
      mitigated if the parent has the ability for a child zone to
      schedule the withdrawal of the old key at the same time as the
      introduction of the new one.)

   o  Finally, Double-RRset allows the rollover to be done in roughly
      half the time of the other two methods; it also supports policies
      that require a period of running with old and new KSKs
      simultaneously.  However, it does have the disadvantages of both
      the other two methods - it requires two signatures during the
      period of the rollover and two interactions with the parent.

2.3.  Summary

   The methods can be summarised as follows:








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   +------------------+------------------+-----------------------------+
   | ZSK Method       | KSK Method       | Description                 |
   +------------------+------------------+-----------------------------+
   | Pre-Publication  | (not applicable) | Publish the DNSKEY before   |
   |                  |                  | the RRSIG.                  |
   | Double-Signature | Double-Signature | Publish the DNSKEY and      |
   |                  |                  | RRSIG at same time.  (For a |
   |                  |                  | KSK, this happens before    |
   |                  |                  | the DS is published.)       |
   | Double-RRSIG     | (not applicable) | Publish RRSIG before the    |
   |                  |                  | DNSKEY.                     |
   | (not applicable) | Double-DS        | Publish DS before the       |
   |                  |                  | DNSKEY.                     |
   | (not applicable) | Double-RRset     | Publish DNSKEY and DS in    |
   |                  |                  | parallel.                   |
   +------------------+------------------+-----------------------------+

                                  Table 1


3.  Key Rollover Timelines

3.1.  Key States

   During the rolling process, a key moves through different states.
   These states are:

   Generated   The key has been created, but has not yet been used for
               anything.

   Published   The DNSKEY record - or information associated with it -
               is published in the zone, but predecessors of the key (or
               associated information) may be held in resolver caches.

               The idea of "associated information" is used in rollover
               methods where RRSIG or DS records are published first and
               the DNSKEY is changed in an atomic operation.  It allows
               the rollover still to be thought of as moving through a
               set of states.  In the rest of this section, the term
               "key" should be taken to mean "key or associated
               information".

   Ready       The key has been published for long enough to guarantee
               that all caches that might contain a copy of the key
               RRset have a copy that includes this key.






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   Active      The key is in the zone and has started to be used to sign
               RRsets or authenticate the DNSKEY RRset.  Note that when
               this state is entered, it might not be possible for every
               validating resolver to use the key for validation: the
               zone signing may not have finished, or the data might not
               have reached the resolver because of propagation delays
               and/or caching issues.  If this is the case, the resolver
               will have to rely on the key's predecessor instead.

   Retired     The key is in the zone but a successor key has become
               active.  As there may still be information in caches that
               that require use of the key, it is being retained until
               this information expires.

   Dead        The key is published in the zone but there is no
               information anywhere that requires its presence.

   Removed     The key has been removed from the zone.

   There is one additional state, used where [RFC5011] considerations
   are in effect (see Section 3.3.4):

   Revoked     The key is published for a period with the "revoke" bit
               set as a way of notifying validating resolvers that have
               configured it as a trust anchor that it is about to be
               removed from the zone.

3.2.  Zone-Signing Key Timelines

3.2.1.  Pre-Publication Method

   The following diagram shows the time line of a particular ZSK and its
   replacement by its successor using the Pre-Publication method.  Time
   increases along the horizontal scale from left to right and the
   vertical lines indicate events in the life of the key.  The events
   are numbered; significant times and time intervals are marked.















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            |1|  |2|      |3|   |4|   |5|      |6| |7|      |8|   |9|
             |    |        |     |     |        |   |        |     |
     Key N   |    |<-Ipub->|<--->|<-------Lzsk----->|<-Iret->|<--->|
             |    |        |     |     |        |   |        |     |
     Key N+1 |    |        |     |     |<-Ipub->|<->|<---Lzsk-- - -
             |    |        |     |     |        |   |        |     |
            Tgen Tpub     Trdy  Tact  TpubS        Tret     Tdea  Trem

                             ---- Time ---->


          Figure 1: Timeline for a Pre-Publication ZSK rollover.

   Event 1: key N is generated at the generate time (Tgen).  Although
   there is no reason why the key cannot be generated immediately prior
   to publication in the zone (Event 2), some implementations may find
   it convenient to create a pool of keys in one operation and draw from
   that pool as required.  For this reason, it is shown as a separate
   event.  Keys that are available for use but not published are said to
   be generated.

   Event 2: key N's DNSKEY record is put into the zone, i.e. it is added
   to the DNSKEY RRset which is then re-signed with the current key-
   signing key.  The time at which this occurs is the key's publication
   time (Tpub), and the key is now said to be published.  Note that the
   key is not yet used to sign records.

   Event 3: before it can be used, the key must be published for long
   enough to guarantee that any resolver that has a copy of the DNSKEY
   RRset from the zone in its cache will have a copy of the RRset that
   includes this key: in other words, that any prior cached information
   about the DNSKEY RRset has expired.

   This interval is the publication interval (Ipub) and, for the second
   or subsequent keys in the zone, is given by:

             Ipub = Dprp + TTLkey

   Here, Dprp is the propagation delay - the time taken for any change
   introduced at the master to replicate to all slave servers - which
   depends on the depth of the master-slave hierarchy.  TTLkey is the
   time-to-live (TTL) for the DNSKEY records in the zone.  The sum is
   therefore the time taken for existing DNSKEY records to expire from
   the caches of downstream validating resolvers, regardless of the
   nameserver from which they were retrieved.

   In the case of the first key in the zone, Ipub is slightly different
   because it is not information about a DNSKEY RRset that may be



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   cached, it is information about its absence.  In this case:

             Ipub = Dprp + Ingc

   where Ingc is the negative cache interval from the zone's SOA record,
   calculated according to [RFC2308] as the minimum of the TTL of the
   SOA record itself (TTLsoa), and the "minimum" field in the record's
   parameters (SOAmin), i.e.

             Ingc = min(TTLsoa, SOAmin)

   After a delay of Ipub, the key is said to be ready and could be used
   to sign records.  The time at which this event occurs is the key's
   ready time (Trdy), which is given by:

             Trdy = Tpub + Ipub

   Event 4: at some later time, the key starts being used to sign
   RRsets.  This point is the active time (Tact) and after this, the key
   is said to be active.

   Event 5: while this key is active, thought must be given to its
   successor (key N+1).  As with the introduction of the currently
   active key into the zone, the successor key will need to be published
   at least Ipub before it is used.  Denoting the publication time of
   the successor key by TpubS, then:

             TpubS <= Tact + Lzsk - Ipub

   Here, Lzsk is the length of time for which a ZSK will be used (the
   ZSK lifetime).  It should be noted that unlike the publication
   interval, Lzsk is not determined by timing logic, but by key
   management policy.  Lzsk will be set by the operator according to
   their assessment of the risks posed by continuing to use a key and
   the risks associated with key rollover.  However, operational
   considerations may mean a key is active for slightly more or less
   than Lzsk.

   Event 6: while the key N is still active, its successor becomes
   ready.  From this time onwards, it could be used to sign the zone.

   Event 7: at some point the decision is made to start signing the zone
   using the successor key.  This will be when the current key has been
   in use for an interval equal to the ZSK lifetime, hence:

             Tret = Tact + Lzsk

   This point in time is the retire time (Tret) of key N; after this the



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   key is said to be retired.  (This time is also the point at which the
   successor key becomes active.)

   Event 8: the retired key needs to be retained in the zone whilst any
   RRSIG records created using this key are still published in the zone
   or held in resolver caches.  (It is possible that a resolver could
   have an unexpired RRSIG record and an expired DNSKEY RRset in the
   cache when it is asked to provide both to a client.  In this case the
   DNSKEY RRset would need to be looked up again.)  This means that once
   the key is no longer used to sign records, it should be retained in
   the zone for at least the retire interval (Iret) given by:

             Iret = Dsgn + Dprp + TTLsig

   Dsgn is the delay needed to ensure that all existing RRsets have been
   re-signed with the new key.  Dprp is (as described above) the
   propagation delay, required to guarantee that the updated zone
   information has reached all slave servers, and TTLsig is the TTL of
   the RRSIG records.

   (It should be noted that an upper limit on the retire interval is
   given by:

             Iret = Lsig + Dskw

   where Lsig is the lifetime of a signature (i.e. the interval between
   the time the signature was created and the signature end time), and
   Dskw is the clock skew - the maximum difference in time between the
   server and a validating resolver.  The reasoning here is that
   whatever happens, a key only has to be available while any signatures
   created with it are valid.  Wherever a signature record is held -
   published in the zone and/or held in a resolver cache - it won't be
   valid for longer than Lsig after it was created.  The Dskw term is
   present to account for the fact that the signature end time is an
   absolute time rather than interval, and systems may not agree exactly
   about the time.)

   The time at which all RRSIG records created with this key have
   expired from resolver caches is the dead time (Tdea), given by:

             Tdea = Tret + Iret

   ...at which point the key is said to be dead.

   Event 9: at any time after the key becomes dead, it can be removed
   from the zone and the DNSKEY RRset re-signed with the current key-
   signing key.  This time is the removal time (Trem), given by:




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             Trem >= Tdea

   ...at which time the key is said to be removed.

3.2.2.  Double-Signature Method

   In the Double-Signature method, both the new key and signatures
   created using it are introduced at the same time.  After a period
   during which this information propagates to validating resolver
   caches, the old key and signature are removed.  The time line for the
   method is shown below:



               |1|  |2|            |3|  |4|              |5|  |6|
                |    |              |    |                |    |
        Key N   |    |<-------Lzsk------>|<-----Iret----->|    |
                |    |              |    |                |    |
        Key N+1 |    |              |    |<----------Lzsk------- - -
                |    |              |    |                |    |
               Tgen Tact               Tret             Tdea  Trem

                              ---- Time ---->


          Figure 2: Timeline for a Double-Signature ZSK rollover.

   Event 1: key N is generated at the generate time (Tgen).  Although
   there is no reason why the key cannot be generated immediately prior
   to publication in the zone (Event 2), some implementations may find
   it convenient to create a pool of keys in one operation and draw from
   that pool as required.  For this reason, it is shown as a separate
   event.  Keys that are available for use but not published are said to
   be generated.

   Event 2: key N is added to the DNSKEY RRset and is immediately used
   to sign the zone; existing signatures in the zone are not removed.
   This is the active time (Tact) and the key is said to be active.

   Event 3: at some time later, the predecessor key (key N-1) and its
   signatures can be withdrawn from the zone.  (The timing of key
   removal is discussed further in the description of event 5.)

   Event 4: the successor key (key N+1) is introduced into the zone and
   starts being used to sign RRsets.  The successor is key is now active
   and the current key (key N) is said to be retired.  This time is the
   retire time of the key (Tret); it is also the active time of the
   successor key (TactS).



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             Tret = Tact + Lzsk

   Event 5: before key N can be withdrawn from the zone, all RRsets that
   need to be signed must have been signed by the successor key (as a
   result, all these RRsets are now signed twice, once by key N and once
   by its successor) and the information must have reached all
   validating resolvers that may have RRsets from this zone cached.

   This takes Iret, the retire interval, given by the expression:

             Iret = Dsgn + Dprp + max(TTLkey, TTLsig)

   As before, Dsgn is the time taken to sign the zone with the new key
   and Dprp is the propagation delay.  The final term is the period to
   wait for old key and signature data to expire from caches.  After the
   end of this interval, key N is said to be dead.  This occurs at the
   dead time (Tdea) so:

             Tdea = Tret + Iret

   Event 6: at some later time key N and its signatures can be removed
   from the zone.  This is the removal time Trem, given by:

             Trem >= Tdea

3.2.3.  Double-RRSIG Method

   As noted above, "Double-Signature" is simultaneous rollover of both
   signature and key.  The time line for a pure Double-Signature ZSK
   rollover (the Double-RRSIG method) - where new signatures are
   introduced, the key changed, and finally the old signatures removed -
   is shown below:



          |1||2|      |3| |4||5|       |6||7|       |8||9|     |10| |11|
           |  |        |   |  |         |  |         |  |        |   |
   Key N   |  |<-Dsgn->|   |  |<-----------Lzsk-------->|<-Iret->|   |
           |  |<---IpubG-->|  |<-IpubK->|  |         |  |        |   |
           |  |        |   |  |         |  |         |  |        |   |
   Key N+1 |  |        |   |  |         |  |<-IpubG->|  |        |   |
           |  |        |   |  |         |  |         |  |        |   |
        Tgen Tpub        Trdy Tact        TpubS   TrdyS Tret   Tdea Trem

                                ---- Time ---->


          Figure 3: Timeline for a Double-Signature ZSK rollover.



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   Event 1: key N is generated at the generate time (Tgen).  Although
   there is no reason why the key cannot be generated immediately prior
   to publication in the zone (Event 2), some implementations may find
   it convenient to create a pool of keys in one operation and draw from
   that pool as required.  For this reason, it is shown as a separate
   event.  Keys that are available for use but not published are said to
   be generated.

   Event 2: key N is used to sign the zone but existing signatures are
   retained.  Although the new ZSK is not published in the zone at this
   point, for analogy with the other ZSK rollover methods and because
   this is the first time that key information is visible (albeit
   indirectly through the created signatures) this time is called the
   publish time (Tpub).

   Event 3: after the signing interval, Dsgn, all RRsets that need to be
   signed have been signed by the new key.  (As a result, all these
   RRsets are now signed twice, once by the existing key and once by the
   (still-absent) key N.

   Event 4: there is now a delay while the this information reaches all
   validating resolvers that may have RRsets from the zone cached.  This
   interval is given by the expression:

             Dprp + TTLsig

   ...comprising the delay for the information to propagate through the
   nameserver infrastructure plus the time taken for signature
   information to expire from caches.

   Again in analogy with other key rollover methods, this is defined as
   key N's ready time (Trdy) and the key is said to be in the ready
   state.  (Although the key is not in the zone, it is ready to be
   used.)  The interval between the publication and ready times is the
   publication interval of the signature, IpubG, i.e.

             Trdy = Tpub + IpubG

   where

             IpubG = Dsgn + Dprp + TTLsig

   Event 5: at some later time the predecessor key is removed and the
   key N added to the DNSKEY RRset.  As all the RRs have signatures
   created by the old and new keys, the records can still be
   authenticated.  This time is the active time (Tact) and the key is
   now said to be active.




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   Event 6: After IpubK - the publication interval of the key - the
   newly added DNSKEY RRset has propagated through to all validating
   resolvers.  At this point the old signatures can be removed from the
   zone.  IpubK is given by:

             IpubK = Dprp + TTLkey

   Event 7: as before, at some later time thought must be given to
   rolling the key.  The first step is to publish signatures created by
   the successor key (key N+1) early enough so that key N can be
   replaced after it has been active for its scheduled lifetime.  This
   occurs at TpubS (the publication time of the successor), given by:

             TpubS <= Tact + Lzsk - IpubG

   Event 8: the signatures have propagated through the zone and the new
   key could be added to the zone.  This time is the ready time of the
   successor (TrdyS).

             TrdyS = TpubS + IpubG

   ... where IpubG is as defined above.

   Event 9: at some later time key N is removed from the zone and the
   successor key added.  This is the retire time of the key (Tret).

   Event 10: The signatures must remain in the zone for long enough that
   the new DNSKEY RRset has had enough time to propagate to all caches.
   Once caches contain the new DNSKEY, the old signatures are no longer
   of use and can be considered to be dead.  The time at which this
   occurs is the dead time (Tdea), given by:

             Tdea = Tret + Iret

   ... where Iret is the retire interval, given by:

             Iret = IpubK

   Event 11: At some later time the signatures can be removed from the
   zone.  Although the key has is not longer in the zone, this is
   information associated with it and so the time can be regarded as the
   key's remove time (Trem), given by:

             Trem >= Tdea







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3.3.  Key-Signing Key Rollover Timelines

3.3.1.  Double-Signature Method

   The Double-Signature method (also knows as the double-DNSKEY method)
   involves introducing the new KSK to the zone and waiting until its
   presence has been registered by all validating resolvers.  At this
   point, the DS record in the parent is changed.  Once that change has
   propagated to all validating resolvers, the old KSK is removed.

   The timing diagram for such a rollover is:



               |1|  |2|      |3|   |4|      |5|      |6|
                |    |        |     |        |        |
        Key N   |    |<-Ipub->|<--->|<-Dreg->|<---------Lksk--- - -
                |    |        |     |        |        |
        Key N+1 |    |        |     |        |        |
                |    |        |     |        |        |
               Tgen Tpub     Trdy  Tsub    Tact

                            ---- Time ---->

        (continued...)

                    |7|      |8|   |9|      |10|     |11|    |12|
                     |        |     |        |        |       |
        Key N   - - -------------Lksk------->|<-Iret->|       |
                     |        |     |        |        |       |
        Key N+1      |<-Ipub->|<--->|<-Dreg->|<--------Lksk----- - -
                     |        |     |        |        |       |
                   TpubS    TrdyS  TsubS   Tret      Tdea     Trem

                        ---- Time (cont) ---->


          Figure 4: Timeline for a Double-Signature KSK rollover.

   Event 1: key N is generated at time Tgen.  As before, although there
   is no reason why the key cannot be generated immediately prior to
   publication, some implementations may find it convenient to create a
   central pool of keys and draw from it.  For this reason, it is again
   shown as a separate event.

   Event 2: key N is introduced into the zone; it is added to the DNSKEY
   RRset, which is then signed by key N and all currently active KSKs.
   (So at this point, the DNSKEY RRset is signed by both key N and its



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   predecessor KSK.  If other KSKs were active, it is signed by these as
   well.)  This is the publication time (Tpub); after this the key is
   said to be published.

   Event 3: before it can be used, the key must be published for long
   enough to guarantee that any validating resolver that has a copy of
   the DNSKEY RRset from the zone in its cache will have a copy of the
   RRset that includes this key: in other words, that any prior cached
   information about the DNSKEY RRset has expired.

   The interval is the publication interval (Ipub) and, for the second
   or subsequent KSKs in the zone, is given by:

             Ipub = Dprp + TTLkey

   ... where Dprp is the propagation delay for the zone and TTLkey the
   TTL for the DNSKEY RRset.  The time at which this occurs is the key's
   ready time, Trdy, given by:

             Trdy = Tpub + Ipub

   Event 4: at some later time, the DS RR corresponding to the new KSK
   is submitted to the parent zone for publication.  This time is the
   submission time, Tsub.

   Event 5: the DS record is published in the parent zone.  As this is
   the point at which all information for authentication - both DNSKEY
   and DS record - is available in the two zones, it is the active time
   of the key:

             Tact = Tsub + Dreg

   ... where Dreg is the registration delay, the time taken after the DS
   record has been received by the parent zone manager for it to be
   placed in the zone.  (Parent zones are often managed by different
   entities, and this term accounts of the organisational overhead of
   transferring a record.)

   Event 6: at some time later, all validating resolvers that have the
   DS RRset cached will have a copy that includes the new DS record.
   For the second or subsequent DS records, this interval is given by
   the expression:

             DprpP + TTLds

   ... where DprpP is the propagation delay in the parent zone and TTLds
   the TTL assigned to DS records in that zone.




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   In the case of the first DS record for the zone in question, the
   expression is slightly different because it is not information about
   a DS RRset that may be cached, it is information about its absence.
   In this case, the interval is:

             DprpP + IngcP

   where IngcP is the negative cache interval from the zone's SOA
   record, calculated according to [RFC2308] as the minimum of the TTL
   of the parent SOA record itself (TTLsoaP), and the "minimum" field in
   the record's parameters (SOAminP), i.e.

             IngcP = min(TTLsoaP, SOAminP)

   Event 7: while key N is active, thought needs to be given to its
   successor (key N+1).  At some time before the scheduled end of the
   KSK lifetime, the successor KSK is introduced into the zone and is
   used to sign the DNSKEY RRset.  (As before, this means that the
   DNSKEY RRset is signed by both the current and successor KSK.)  This
   is the publication time of the successor key, TpubS.

   Event 8: after an interval Ipub, the successor key becomes ready (in
   that all validating resolvers that have a copy of the DNSKEY RRset
   have a copy of this key).  This is the successor ready time, TrdyS.

   Event 9: at the successor submission time (TsubS), the DS record
   corresponding to the successor key is submitted to the parent zone.

   Event 10: the successor DS record is published in the parent zone and
   the current DS record withdrawn.  The current key is said to be
   retired and the time at which this occurs is Tret, given by:

   The relationships between these times are:

             TpubS <= Tact + Lksk - Dreg - Ipub

             Tret = Tact + Lksk

   ... where Lksk is the scheduled lifetime of the KSK.

   Event 11: key N must remain in the zone until any validators that
   have the DS RRset cached have a copy of the DS RRset containing the
   new DS record.  This interval is the retire interval, given by:

             Iret = DprpP + TTLds

   ... where DprpP is the propagation delay in the parent zone and TTLds
   the TTL of a DS record.



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   As the key is no longer used for anything, it can also be said to be
   dead, in which case:

             Tdea = Tret + Iret

   Event 12: at some later time, key N is removed from the zone (at the
   remove time Trem); the key is now said to be removed.

             Trem >= Tdea

3.3.2.  Double-DS Method

   The Double-DS method is the reverse of the Double-Signature method is
   that it is the DS record that is pre-published (in the parent), and
   not the DNSKEY.

   The timeline for the key rollover is shown below:



             |1|  |2|      |3|       |4|  |5|    |6|
              |    |        |         |    |      |
      Key N   |    |<-Dreg->|<-IpubP->|<-->|<---------Lksk------- - -
              |    |        |         |    |      |
      Key N+1 |    |        |         |    |<---->|<--Dreg+IpubP- - -
              |    |        |         |    |      |
             Tgen Tsub     Tpub      Trdy Tact  TsubS

                              ---- Time ---->

       (continued...)

                               |7|   |8|      |9|    |10|
                                |     |        |      |
      Key N   - - -----Lksk---------->|<-Iret->|      |
                                |     |        |      |
      Key N+1 - - --Dreg+IpubP->|<--->|<------Lksk------ - -
                                |     |        |      |
                              TrdyS  Tret    Tdea    Trem

                              ---- Time ---->


             Figure 5: Timeline for a Double-DS KSK rollover.

   Event 1: key N is generated at time Tgen.  As before, although there
   is no reason why the key cannot be generated immediately prior to
   publication, some implementations may find it convenient to create a



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   central pool of keys and draw from it.  For this reason, it is again
   shown as a separate event.

   Event 2: the DS record corresponding to key N is submitted for
   publication in the parent zone.  This time is the submission time
   (Tsub).

   Event 3: after the registration delay, Dreg, the DS record is
   published in the parent zone.  This is the publication time Tpub,
   given by:

             Tpub = Tsub + Dreg

   Event 4: at some later time, any validating resolver that has copies
   of the DS RRset in its cache will have a copy of the DS record for
   key N. At this point, key N, if introduced into the DNSKEY RRset,
   could be used to validate the zone.  For this reason, this time is
   known as the key's ready time, Trdy, and is given by:

             Trdy = Tpub + IpubP

   IpubP is the parent publication interval and is given by the
   expression:

             IpubP = DprpP + TTLds

   ... where DprpP is the propagation delay in the parent zone and TTLds
   the TTL assigned to DS records in that zone.

   Event 5: at some later time, the key rollover takes place.  The
   predecessor key is withdrawn from the DNSKEY RRset and the new key
   (key N) introduced and used to sign the RRset.

   As both DS records have been in the parent zone long enough to ensure
   that they are in the cache of any validating resolvers that have the
   DS RRset cached, the zone can be authenticated throughout the
   rollover - either the resolver has a copy of the DNSKEY RRset (and
   associated RRSIGs) authenticated by the predecessor key, or it has a
   copy of the updated RRset authenticated with the new key.

   This time is the key's active time (Tact) and at this point the key
   is said to be active.

   Event 6: at some point thought must be given to key replacement.  The
   DS record for the successor key must be submitted to the parent zone
   at a time such that when the current key is withdrawn, any validating
   resolver that has DS records in its cache will have data about the DS
   record of the successor key.  The time at which this occurs is the



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   submission time of the successor, given by:

             TsubS <= Tact + Lksk - IpubP - Dreg

   ... where Lksk is the lifetime of the KSK.

   Event 7: the successor key (key N+1) enters the ready state i.e. its
   DS record is now in the caches of all validating resolvers that have
   the parent DS RRset cached.  (This is the ready time of the
   successor, TrdyS.)

   Event 8: when the current key has been active for its lifetime
   (Lksk), the current key is removed from the DNSKEY RRset and the
   successor key added; the RRset is then signed with the successor key.
   This point is the retire time of the key, Tret, given by:

             Tret = Tact + Lksk

   Event 9: at some later time, all copies of the old DNSKEY RRset have
   expired from caches and the old DS record is no longer needed.  This
   is called the dead time, Tdea, and is given by:

             Tdea = Tret + Iret

   ... where Iret is the retire interval, given by:

             Iret = Dprp + TTLkey

   As before, this term includes the time taken to propagate the RRset
   change through the master-slave hierarchy and the time take for the
   DNSKEY RRset to expire from caches.

   Event 10: at some later time, the DS record is removed from the
   parent zone.  This is the removal time (Trem), given by:

             Trem >= Tdea

3.3.3.  Double-RRset Method

   In the Double-RRset method, both the DS and DNSKEY records are
   changed at the same time, so for a period the zone can be
   authenticated with either key.  The advantage of this method is its
   applicability in cases where zone management policy requires overlap
   of authentication keys during a roll.

   The timeline for this rollover is shown below:





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                |1|  |2|      |3|     |4|      |5|      |6|   |7|
                 |    |        |       |        |        |     |
         Key N   |    |<-Dreg->|<-----Lksk----->|<-Iret->|     |
                 |    |        |       |        |        |     |
         Key N+1 |    |        |       |<-Dreg->|<-----Lksk-- - -
                 |    |        |       |        |        |     |
               Tgen  Tpub    Tact    TpubS     Tret    Tdea   Trem

                                 ---- Time ---->


            Figure 6: Timeline for a Double-RRset KSK rollover.

   Event 1: key N is created at time Tgen and thereby immediately
   becomes generated.  As before, although there is no reason why the
   key cannot be generated immediately prior to publication, some
   implementations may find it convenient to create a central pool of
   keys and draw from it.  For this reason, it is again shown as a
   separate event.

   Event 2: the key is added to and used for signing the DNSKEY RRset
   and is thereby published in the zone.  At the same time the
   corresponding DS record is submitted to the parent zone for
   publication.  This time is the publish time (Tpub) and the key is now
   said to be published.

   Event 3: after Dreg, the registration delay, the DS record is
   published in the parent zone.  At this point, the zones have all the
   information needed for a validating resolver to authenticate the
   zone, although the information may not yet have reached all
   validating resolver caches.  This time is the active time (Tact) and
   the key is said to be active.

             Tact = Tpub + Dreg

   Event 4: at some point we need to give thought to key replacement.
   The successor key must be introduced into the zone (and its DS record
   submitted to the parent) at a time such that it becomes active when
   the current key has been active for its lifetime, Lksk.  This time is
   TpubS, the publication time of the successor key, and is given by:

             TpubS <= Tact + Lksk - Dreg

   ... where Lksk is the lifetime of the KSK.

   Event 5: the successor key's DS record appears in the parent zone and
   the successor key becomes active.  At this point, the current key
   becomes retired.  This occurs at Tret, given by:



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             Tret = Tact + Lksk

   Event 6: the current DNSKEY and DS record must be retained in the
   zones until any any validating resolver that has cached the DNSKEY
   and/or DS RRsets will have a copy of the data for the successor key.
   At this point the current key information is dead, as any validating
   resolver can perform authentication using the successor key.  This is
   the dead time, Tdea, given by:

             Tdea = Tret + Iret

   ... where Iret is the retire interval.  This depends on how long both
   the successor DNSKEY and DS records take to propagate through the
   nameserver infrastructure and thence into validator caches.  These
   delays are the publication intervals of the child and parent zones
   respectively, so a suitable expression for Iret is:

             Iret = max(IpubP, IpubC)

   IpubC is the publication interval of the DNSKEY in the child zone,
   IpubP that of the DS record in the parent.

   The child term comprises two parts - the time taken for the
   introduction of the DNSKEY record to be propagated to the downstream
   secondary servers (= DprpC, the child propagation delay) and the time
   taken for information about the DNSKEY RRset to expire from the
   validating resolver cache, i.e.

             IpubC = DprpC + TTLkey

   TTLkey is the TTL for a DNSKEY record in the child zone.  The parent
   term is similar:

             IpubP = DprpP + TTLds

   DprpP the propagation delay in the parent zone and TTLds the TTL for
   a DS record in the parent zone.

   Event 7: at some later time, the DNSKEY record can be removed from
   the child zone and a request can be made to remove the DS record from
   the parent zone.  This is the removal time, Trem and is given by:

             Trem >= Tdea








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3.3.4.  Interaction with Configured Trust Anchors

   Although the preceding sections have been concerned with rolling KSKs
   where the trust anchor is a DS record in the parent zone, zone
   managers may want to take account of the possibility that some
   validating resolvers may have configured trust anchors directly.

   Rolling a configured trust anchor is dealt with in [RFC5011].  It
   requires introducing the KSK to be used as the trust anchor into the
   zone for a period of time before use, and retaining it (with the
   "revoke" bit set) for some time after use.  The Double-Signature and
   Double-RRset methods can be adapted to include [RFC5011]
   recommendations so that the rollover will also be signalled to
   validating resolvers with configured trust anchors.  (The
   recommendations are not suitable for the Double-DS method.
   Introducing the new key early and retaining the old key after use
   effectively converts it into a form of Double-RRset.)

3.3.4.1.  Addition of KSK

   When the new key is introduced, the publication interval (Ipub) in
   the Double-Signature method should also be subject to the condition:

             Ipub >= max(30 days, TTLkey)

   ... where the right had side of the expression is the add hold-down
   time defined in section 2.4.1 of [RFC5011].

   In the Double-RRSIG method, the key should not be regarded as being
   active until the add hold-down time has passed.  In other words, the
   following condition should be enforced:

             Tact >= Tpub + max(30 days, TTLkey)

   (Effectively, this means extending the lifetime of the key by an
   appropriate amount.)

3.3.4.2.  Removal of KSK

   The timeline for the removal of the key in both methods is modified
   by introducing a new state, "revoked".  When the key reaches the end
   of the retire period, instead of being declared "dead", it is
   revoked; the "revoke" bit is set on the DNSKEY RR and is published in
   (and used to sign) the DNSKEY RRset.  The key is maintained in this
   state for the "revoke" interval, Irev, given by:






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             Irev >= 30 days

   ... 30 days being the [RFC5011] remove hold-down time.  After this
   time, the key is dead and can be removed from the zone when
   convenient.

3.3.5.  Introduction of First KSK

   There is an additional consideration when introducing a KSK into a
   zone for the first time, and that is that no validating resolver
   should be in a position where it can access the trust anchor before
   the KSK appears in the zone.  To do so will cause the validating
   resolver to declare the zone to be bogus.

   This is important: in the case of a secure parent, it means ensuring
   that the DS record is not published in the parent zone until there is
   no possibility that a validating resolver can obtain the record yet
   not be able to obtain the corresponding DNSKEY.  In the case of an
   insecure parent, i.e. the initial creation of a new security apex, it
   is important to not configure trust anchors in validating resolvers
   until the DNSKEY RRset has had sufficient time to propagate.  In both
   cases, this time is the trust anchor availability time (Ttaa) given
   by:

             Ttaa >= Tpub + IpubC

   where

             IpubC = DprpC + TTLkeyC

   or

             IpubC = DprpC + IngcC

   The first expression applies if there was previously a DNSKEY RRset
   in the child zone, the expression for IpubC including the TTLkeyC
   term to account for the time taken for that RRset to expire from
   caches.  (It is possible that the zone was signed but that the trust
   anchor had not been submitted to the parent.)

   If the introduction of the KSK caused the appearance of the first
   DNSKEY RRset in the child zone, the second expression applies in
   which the TTLkeyC term is replaced by Ingc to allow for the effect of
   negative caching.

   As before, IngcC is the negative cache interval from the child zone's
   SOA record, calculated according to [RFC2308] as the minimum of the
   TTL of the SOA record itself (TTLsoaC), and the "minimum" field in



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   the record's parameters (SOAminC), i.e.

             IngcC = min(TTLsoaC, SOAminC)


4.  Standby Keys

   Although keys will usually be rolled according to some regular
   schedule, there may be occasions when an emergency rollover is
   required, e.g. if the active key is suspected of being compromised.
   The aim of the emergency rollover is to allow the zone to be re-
   signed with a new key as soon as possible.  As a key must be in the
   ready state to sign the zone, having at least one additional key (a
   standby key) in this state at all times will minimise delay.

   In the case of a ZSK, a standby key only really makes sense with the
   Pre-Publication method.  A permanent standby DNSKEY RR should be
   included in zone or successor keys could be introduced as soon as
   possible after a key becomes active.  Either way results in an
   additional ZSK in the DNSKEY RRset that can immediately be used to
   sign the zone if the current key is compromised.

   (Although in theory the mechanism could be used with both the Double-
   Signature and Double-RRSIG methods, it would require Pre-Publication
   of the signatures.  Essentially, the standby key would be permanently
   active, as it would have to be periodically used to renew signatures.
   Zones would also permanently require two sets of signatures,
   something that could have a performance impact in large zones.)

   A standby key can also be used with the Double-Signature and
   Double-DS methods of rolling a KSK.  (The idea of a standby key in
   the Double-RRset effectively means having two active keys.)  The
   Double-Signature method requires that the standby KSK be included in
   the DNSKEY RRset; rolling the key then requires just the introduction
   of the DS record in the parent.  (Note that the DNSKEY should also be
   used to sign the DNSKEY RRset.  As the RRset and its signatures
   travel together, merely adding the DNSKEY does not provide the
   desired time saving; to be used in a rollover requires that the
   DNSKEY RRset be signed with the standby key, and this introduces a
   delay whilst the RRset and its signatures propagate to the caches of
   validating resolvers.  There is no time advantage over introducing a
   new DNSKEY and signing the RRset with it at the same time.)

   In the Double-DS method of rolling a KSK, it is not a standby key
   that is present, it is a standby DS record in the parent zone.
   Whatever algorithm is used, the standby item of data can be
   introduced as a permanent standby, or be a successor introduced as
   early as possible.



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5.  Algorithm Considerations

   The preceding sections have implicitly assumed that all keys and
   signatures are created using a single algorithm.  However, [RFC4035]
   (section 2.4) states that "There MUST be an RRSIG for each RRset
   using at least one DNSKEY of each algorithm in the zone apex DNSKEY
   RRset".

   Except in the case of an algorithm rollover - where the algorithms
   used to create the signatures are being changed - there is no
   relationship between the keys of different algorithms.  This means
   that they can be rolled independently of one another.  In other
   words, the key rollover logic described above should be run
   separately for each algorithm; the union of the results is included
   in the zone, which is signed using the active key for each algorithm.


6.  Limitation of Scope

   This document represents current thinking at the time of publication.
   However, the subject matter is evolving and it is more than likely
   that this document will need to be revised in the future.

   Some of the techniques and ideas that DNSSEC operators are thinking
   about differ from this those described in this document.  Of note are
   alternatives to the strict split into KSK and ZSK key roles and the
   consequences for rollover logic from partial signing (i.e. when the
   new key initially only signs a fraction of the zone while leaving
   other signatures generated by the old key in place).

   Furthermore, as noted in section 5, this document covers only rolling
   keys of the same algorithm, it does not cover transition to/from/
   addition/deletion of different algorithm.  Algorithm rollovers will
   require a separate document.

   The reader is therefore reminded that DNSSEC is as of publication in
   early stages of deployment, and best practices will likely change in
   the near term.


7.  Summary

   For ZSKs, "Pre-Publication" is generally considered to be the
   preferred way of rolling keys.  As shown in this document, the time
   taken to roll is wholly dependent on parameters under the control of
   the zone manager.

   In contrast, "Double-RRset" is the most efficient method for KSK



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   rollover due to the ability to have new DS records and DNSKEY RRsets
   propagate in parallel.  The time taken to roll KSKs may depend on
   factors related to the parent zone if the parent is signed.  For
   zones that intend to comply with the recommendations of [RFC5011], in
   virtually all cases the rollover time will be determined by the
   RFC5011 "add hold-down" and "remove hold-down" times.  It should be
   emphasized that this delay is a policy choice and not a function of
   timing values and that it also requires changes to the rollover
   process due to the need to manage revocation of trust anchors.

   Finally, the treatment of emergency key rollover is significantly
   simplified by the introduction of stand-by keys as standard practice
   during all types of rollovers.


8.  IANA Considerations

   This memo includes no request to IANA.


9.  Security Considerations

   This document does not introduce any new security issues beyond those
   already discussed in [RFC4033], [RFC4034], [RFC4035] and [RFC5011].


10.  Acknowledgements

   The authors gratefully acknowledge help and contributions from Roy
   Arends and Wouter Wijngaards.


11.  Change History

   o  draft-ietf-dnsop-dnssec-key-timing-00
      * Added section on limitation of scope.

   o  draft-morris-dnsop-dnssec-key-timing-02
      * General restructuring.
      * Added descriptions of more rollovers (IETF-76 meeting).
      * Improved description of key states and removed diagram.
      * Provided simpler description of standby keys.
      * Added section concerning first key in a zone.
      * Moved [RFC5011] to a separate section.
      * Various nits fixed (Alfred Hoenes, Jeremy Reed, Scott Rose, Sion
      Lloyd, Tony Finch).





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   o  draft-morris-dnsop-dnssec-key-timing-01
      * Use latest boilerplate for IPR text.
      * List different ways to roll a KSK (acknowledgements to Mark
      Andrews).
      * Restructure to concentrate on key timing, not management
      procedures.
      * Change symbol notation (Diane Davidowicz and others).
      * Added key state transition diagram (Diane Davidowicz).
      * Corrected spelling, formatting, grammatical and style errors
      (Diane Davidowicz, Alfred Hoenes and Jinmei Tatuya).
      * Added note that in the case of multiple algorithms, the
      signatures and rollovers for each algorithm can be considered as
      more or less independent (Alfred Hoenes).
      * Take account of the fact that signing a zone is not atomic
      (Chris Thompson).
      * Add section contrasting pre-publication rollover with double
      signature rollover (Matthijs Mekking).
      * Retained distinction between first and subsequent keys in
      definition of initial publication interval (Matthijs Mekking).

   o  draft-morris-dnsop-dnssec-key-timing-00
      Initial draft.


12.  References

12.1.  Normative References

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, March 1998.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", RFC 5011, September 2007.






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12.2.  Informative References

   [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
              RFC 4641, September 2006.


Appendix A.  List of Symbols

   The document defines a number of symbols, all of which are listed
   here.  All are of the form:

   All symbols used in the text are of the form:

             <TYPE><id><INST>

   where:

   <TYPE> is an upper-case character indicating what type the symbol is.
   Defined types are:

   D         delay: interval that is a feature of the process

   I         interval between two events

   L         lifetime: interval set by the zone manager

   SOA       parameter related to SOA RR

   T         a point in time

   TTL       TTL of a record

   T and I are self-explanatory.  D, and L are also time periods, but
   whereas I values are intervals between two events (even if the events
   are defined in terms of the interval, e.g. the dead time occurs
   "retire interval" after the retire time), D, and L are fixed
   intervals.  An "L" interval (lifetime) is chosen by the zone manager
   and is a feature of policy.  A "D" interval (delay) is a feature of
   the process, probably outside control of the zone manager.  SOA and
   TTL are used just because they are common terms.

   <id> is lower-case and defines what object or event the variable is
   related to, e.g.








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   act       active

   ngc       negative cache

   pub       publication

   Finally, <INST> is a capital letter that distinguishes between the
   same variable applying to different instances of an object and is one
   of:

   C         child

   G         signature

   K         key

   P         parent

   S         successor

   The list of variables used in the text is:

   Dprp      Propagation delay.  The amount of time for a change made at
             a master nameserver to propagate to all the slave
             nameservers.

   DprpC     Propagation delay in the child zone.

   DprpP     Propagation delay in the parent zone.

   Dreg      Registration delay.  As a parent zone is often managed by a
             different organisation to that managing the child zone, the
             delays associated with passing data between zones is
             captured by this term.

   Dskw      Clock skew.  The maximum difference in time between the
             signing system and the resolver.

   Dsgn      Signing delay.  After the introduction of a new ZSK, the
             amount of time taken for all the RRs in the zone to be
             signed with it.

   Ingc      Negative cache interval.

   IngcP     Negative cache interval of the child zone.






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   IngcP     Negative cache interval of the parent zone.

   Ipub      Publication interval.  The amount of time that must elapse
             after the publication of a key before it can be considered
             to have entered the ready state.

   IpubC     Publication interval in the child zone.

   IpubG     Publication interval for the signature.

   IpubK     Publication interval for the key.

   IpubP     Publication interval in the parent zone.

   Iret      Retire interval.  The amount of time that must elapse after
             a key enters the retire state for any signatures created
             with it to be purged from validating resolver caches.

   Irev      Revoke interval.  The amount of time that a KSK must remain
             published with the revoke bit set to satisfy [RFC5011]
             considerations.

   Lksk      Lifetime of a key-signing key.  This is the intended amount
             of time for which this particular KSK is regarded as the
             active KSK.  Depending on when the key is rolled-over, the
             actual lifetime may be longer or shorter than this.

   Lzsk      Lifetime of a zone-signing key.  This is the intended
             amount of time for which the ZSK is used to sign the zone.
             Depending on when the key is rolled-over, the actual
             lifetime may be longer or shorter than this.

   Lsig      Lifetime of a signature: the difference in time between the
             signature's expiration time and the time at which the
             signature was created.  (Note that this is not the
             difference between the signature's expiration and inception
             times: the latter is usually set a small amount of time
             before the signature is created to allow for clock skew
             between the signing system and the validating resolver.)

   SOAmin    Value of the "minimum" field from an SOA record.

   SOAminC   Value of the "minimum" field from an SOA record in the
             child zone.







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   SOAminP   Value of the "minimum" field from an SOA record in the
             parent zone.

   Tact      Active time of the key; the time at which the key is
             regarded as the principal key for the zone.

   TactS     Active time of the successor key.

   Tdea      Dead time of a key.  Applicable only to ZSKs, this is the
             time at which any record signatures held in validating
             resolver caches are guaranteed to be created with the
             successor key.

   Tgen      Generate time of a key.  The time that a key is created.

   Tpub      Publish time of a key.  The time that a key appears in a
             zone for the first time.

   TpubS     Publish time of the successor key.

   Trem      Removal time of a key.  The time at which a key is removed
             from the zone.

   Tret      Retire time of a key.  The time at which a successor key
             starts being used to sign the zone.

   Trdy      Ready time of a key.  The time at which it can be
             guaranteed that validating resolvers that have key
             information from this zone cached have a copy of this key
             in their cache.  (In the case of KSKs, should the
             validating resolvers also have DS information from the
             parent zone cached, the cache must include information
             about the DS record corresponding to the key.)

   TrdyS     Ready time of a successor key.

   Tsub      Submit time - the time at which the DS record of a KSK is
             submitted to the parent.

   TsubS     Submit time of the successor key.

   TTLds     Time to live of a DS record (in the parent zone).

   TTLkey    Time to live of a DNSKEY record.







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   TTLkeyC   Time to live of a DNSKEY record in the child zone.

   TTLsoa    Time to live of a SOA record.

   TTLsoaC   Time to live of a SOA record in the child zone.

   TTLsoaP   Time to live of a SOA record in the parent zone.

   TTLsig    Time to live of an RRSIG record.

   Ttaa      Trust anchor availability time.  The time at which a trust
             anchor record can be made available when a KSK is first
             introduced into a zone.


Authors' Addresses

   Stephen Morris
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063
   USA

   Phone: +1 650 423 1300
   Email: stephen@isc.org


   Johan Ihren
   Netnod
   Franzengatan 5
   Stockholm,   SE-112 51
   Sweden

   Phone: +46 8615 8573
   Email: johani@autonomica.se


   John Dickinson
   Sinodun Internet Technologies Ltd
   Stables 4 Suite 11, Howbery Park
   Wallingford, Oxfordshire  OX10 8BA
   UK

   Phone: +44 1491 818120
   Email: jad@sinodun.com






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