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RFC 7583

Informational
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DNSSEC Key Rollover Timing Considerations

 


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Internet Engineering Task Force (IETF)                         S. Morris
Request for Comments: 7583                                           ISC
Category: Informational                                         J. Ihren
ISSN: 2070-1721                                                   Netnod
                                                            J. Dickinson
                                                                 Sinodun
                                                              W. Mekking
                                                                     Dyn
                                                            October 2015


               DNSSEC Key Rollover Timing Considerations

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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7583.

Page 2 
Copyright Notice

   Copyright (c) 2015 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
   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
      1.4. Limitation of Scope ........................................5
   2. Rollover Methods ................................................5
      2.1. ZSK Rollovers ..............................................5
      2.2. KSK Rollovers ..............................................7
   3. Key Rollover Timelines ..........................................8
      3.1. Key States .................................................8
      3.2. ZSK Rollover Timelines ....................................10
           3.2.1. Pre-Publication Method .............................10
           3.2.2. Double-Signature Method ............................12
      3.3. KSK Rollover Timelines ....................................14
           3.3.1. Double-KSK Method ..................................14
           3.3.2. Double-DS Method ...................................17
           3.3.3. Double-RRset Method ................................20
           3.3.4. Interaction with Configured Trust Anchors ..........22
           3.3.5. Introduction of First Keys .........................24
   4. Standby Keys ...................................................24
   5. Algorithm Considerations .......................................25
   6. Summary ........................................................26
   7. Security Considerations ........................................26
   8. Normative References ...........................................26
   Appendix A.  List of Symbols ......................................28
   Acknowledgements ..................................................31
   Authors' Addresses ................................................31

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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 the DS records
      or RRSIG records created with the key) are not only held at the
      authoritative nameserver, they are also cached by 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  Zone "bootstrapping" 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, standby keys
      (additional keys over and above those used to sign the zone) need
      to be present.

   o  A query for the DNSKEY RRset returns all DNSKEY records in the
      zone.  As there is limited space in the UDP packet (even with
      EDNS0 support), key records no longer needed must be periodically
      removed.  (For the same reason, the number of standby keys in the
      zone should be restricted to the minimum required to support the
      key management policy.)

   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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   A high-level overview of key rollover can be found in [RFC6781].  In
   contrast, this document focuses on the low-level timing detail of two
   classes of operations described there, the rollover of Zone Signing
   Keys (ZSKs), and the rollover of Key Signing Keys (KSKs).

   Note that the process for the introduction of keys into a zone is
   different from that of rolling a key; see Section 3.3.5 for more
   information.

1.2.  Types of Keys

   Although DNSSEC validation treats all keys equally, [RFC4033]
   recognizes the broad classification of ZSKs and KSKs.  A ZSK is used
   to authenticate information within the zone; a KSK is used to
   authenticate the zone's DNSKEY RRset.  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 its cache or may
   need to be retrieved from an authoritative server.  The rollover
   process needs to happen in such a way that the information is
   consistent at all times during the rollover.  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.

   Although there are similarities in the algorithms to roll ZSKs and
   KSKs, there are a number of differences.  For this reason, the two
   types of rollovers are described separately.

1.3.  Terminology

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

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

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1.4.  Limitation of Scope

   This document represents current thinking at the time of publication.
   However, the subject matter is evolving and it is not possible for
   the document to be comprehensive.  In particular, it does not cover:

   o  Rolling a key that is used as both a ZSK and KSK.

   o  Algorithm rollovers.  Only the rolling of keys of the same
      algorithm is described here: not transitions between algorithms.

   o  Changing TTLs.

   Algorithm rollover is excluded from the document owing to the need
   for there to be an RRSIG for at least one DNSKEY of each algorithm in
   the DNSKEY RRset [RFC4035].  This introduces additional constraints
   on rollovers that are not considered here.  Such considerations do
   not apply where other properties of the key, such as key length, are
   changed during the rollover: the DNSSEC protocol does not impose any
   restrictions in these cases.

   Also excluded from consideration is the effect of changing the Time
   to Live (TTL) of records in a zone.  TTLs can be changed at any time,
   but doing so around the time of a key rollover may have an impact on
   event timings.  In the timelines below, it is assumed that TTLs are
   constant.

2.  Rollover Methods

2.1.  ZSK Rollovers

   For ZSKs, the issue for the zone operator/signer is to ensure that
   any caching validator that has access to a particular signature also
   has access to the corresponding valid ZSK.

   A ZSK can be rolled in one of three ways:

   o  Pre-Publication: described in [RFC6781], the new key is introduced
      into the DNSKEY RRset, which is then re-signed.  This state of
      affairs remains in place for long enough to ensure that any cached
      DNSKEY RRsets contain both keys.  At that point, signatures
      created with the old key can be replaced by those created with the
      new key.  During the re-signing process (which may or may not be
      atomic depending on how the zone is managed), it doesn't matter
      with which key an RRSIG record retrieved by a resolver was
      created; cached copies of the DNSKEY RRset will contain both the
      old and new keys.

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      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 caches.  After this, there will be no
      signatures anywhere that were created using the old key, and it
      can be removed from the DNSKEY RRset.

   o  Double-Signature: also mentioned in [RFC6781], 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), validating
      resolvers are always able to validate RRSIGs: any combination of
      old and new DNSKEY RRset and RRSIGs allows at least one signature
      to be validated.

      Once the signing process is complete and enough time has elapsed
      to make sure that all validators that have the DNSKEY and
      signatures in cache have both the old and new information, the old
      key and signatures can be removed from the zone.  As before,
      during this period any combination of DNSKEY RRset and RRSIGs 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 introducing 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 have a copy of both signatures, the key is changed.  After a
      further interval during which the old DNSKEY RRset expires from
      caches, the old signatures are removed from the zone.

   Of the three methods, Double-Signature is conceptually the simplest:
   introduce the new key and new signatures, then approximately one TTL
   later remove the old key and old signatures.  It is also the fastest,
   but suffers from increasing the size of the zone and the size of
   responses.

   Pre-Publication is more complex: introduce the new key, approximately
   one TTL later sign the records, and approximately one TTL after that
   remove the old key.  It does however keep the zone and response sizes
   to a minimum.

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   Double-RRSIG is essentially the reverse of Pre-Publication: introduce
   the new signatures, approximately one TTL later change the key, and
   approximately one TTL after that remove the old signatures.  However,
   it has the disadvantage of the Pre-Publication method in terms of
   time taken to perform the rollover, the disadvantage of the Double-
   Signature rollover in terms of zone and response sizes, and none of
   the advantages of either.  For these reasons, it is unlikely to be
   used in any real-world situations and so will not be considered
   further in this document.

2.2.  KSK Rollovers

   In the KSK case, there should be no problem with a caching validator
   not having access to a signature created with a valid KSK.  The KSK
   is only used for one signature (that over the DNSKEY RRset) and both
   the key and the signature travel together.  Instead, the issue is to
   ensure that the KSK is trusted.

   Trust in the KSK is due to either the existence of a signed and
   validated DS record in the parent zone or an explicitly configured
   trust anchor.  If the former, 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.  If the
   latter, [RFC5011] timings will be needed to roll the keys.  (Even in
   the case where authentication is via a DS record, 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 the need to interact with the parent
   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-KSK: 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.

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   o  Double-DS: the new DS record is published.  After waiting for this
      change to propagate into caches, 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
      is added to the parent zone.  After waiting a suitable interval
      for the old DS and DNSKEY RRsets to expire from caches, the old
      DNSKEY and DS records are removed.

   In essence, Double-KSK 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 is removed.  It limits interactions with the parent to a
   minimum but, for the duration of the rollover, the size of the DNSKEY
   RRset is increased.

   With Double-DS, the order of operations is the other way around:
   introduce the new DS, change the DNSKEY, then remove the old DS.  The
   size of the DNSKEY RRset is kept to a minimum, but two interactions
   are required with the parent.

   Finally, Double-RRset is the fastest way to roll the KSK, but has the
   drawbacks of both of the other methods: a larger DNSKEY RRset and two
   interactions with the parent.

3.  Key Rollover Timelines

3.1.  Key States

   DNSSEC validation requires both the DNSKEY and information created
   from it (referred to as "associated data" in this section).  In the
   case of validation of an RR, the data associated with the key is the
   corresponding RRSIG.  Where there is a need to validate a chain of
   trust, the associated data is the DS record.

   During the rolling process, keys move through different states.  The
   defined states are:

   Generated   Although keys may be created immediately prior to first
               use, some implementations may find it convenient to
               create a pool of keys in one operation and draw from it
               as required.  (Note: such a pre-generated pool must be
               secured against surreptitious use.)  In the timelines
               below, before the first event, the keys are considered to
               be created but not yet used: they are said to be in the
               "Generated" state.

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   Published   A key enters the published state when either it or its
               associated data first appears in the appropriate zone.

   Ready       The DNSKEY or its associated data have been published for
               long enough to guarantee that copies of the key(s) it is
               replacing (or associated data related to that key) have
               expired from caches.

   Active      The data is starting to be used for validation.  In the
               case of a ZSK, it means that the key is now being used to
               sign RRsets and that both it and the created RRSIGs
               appear in the zone.  In the case of a KSK, it means that
               it is possible to use it to validate a DNSKEY RRset as
               both the DNSKEY and DS records are present in their
               respective zones.  Note that when this state is entered,
               it may not be possible for validating resolvers to use
               the data for validation in all cases: 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 predecessor data instead.

   Retired     The data has ceased to be used for validation.  In the
               case of a ZSK, it means that the key is no longer used to
               sign RRsets.  In the case of a KSK, it means that the
               successor DNSKEY and DS records are in place.  In both
               cases, the key (and its associated data) can be removed
               as soon as it is safe to do so, i.e., when all validating
               resolvers are able to use the new key and associated data
               to validate the zone.  However, until this happens, the
               current key and associated data must remain in their
               respective zones.

   Dead        The key and its associated data are present in their
               respective zones, but there is no longer information
               anywhere that requires their presence for use in
               validation.  Hence, they can be removed at any time.

   Removed     Both the DNSKEY and its associated data have been removed
               from their respective zones.

   Revoked     The DNSKEY 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, as used in
               [RFC5011], that it is about to be removed from the zone.
               This state is used when [RFC5011] considerations are in
               effect (see Section 3.3.4).

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3.2.  ZSK Rollover Timelines

   The following sections describe the rolling of a ZSK.  They show the
   events in the lifetime of a key (referred to as "key N") and cover
   its replacement by its successor (key N+1).

3.2.1.  Pre-Publication Method

   In this method, the new key is introduced into the DNSKEY RRset.
   After enough time to ensure that any cached DNSKEY RRsets contain
   both keys, the zone is signed using the new key and the old
   signatures are removed.  Finally, when all signatures created with
   the old key have expired from caches, the old key is removed.

   The following diagram shows the timeline of a Pre-Publication
   rollover.  Time increases along the horizontal scale from left to
   right and the vertical lines indicate events in the process.
   Significant times and time intervals are marked.

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

                                ---- Time ---->

           Figure 1: Timeline for a Pre-Publication ZSK Rollover

   Event 1: 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 currently
   active KSKs.  The time at which this occurs is the publication time
   (Tpub), and the key is now said to be published.  Note that the key
   is not yet used to sign records.

   Event 2: Before it can be used, the key must be published for long
   enough to guarantee that any cached version of the zone's DNSKEY
   RRset includes this key.

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

      Ipub = Dprp + TTLkey

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   Here, Dprp is the propagation delay -- the time taken for a change
   introduced at the master to replicate to all nameservers.  TTLkey is
   the TTL for the DNSKEY records in the zone.  The sum is therefore the
   maximum time taken for existing DNSKEY records to expire from caches,
   regardless of the nameserver from which they were retrieved.

   (The case of introducing the first ZSK into the zone is discussed in
   Section 3.3.5.)

   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 key N's
   ready time (Trdy), which is given by:

      Trdy(N) = Tpub(N) + Ipub

   Event 3: At some later time, the key starts being used to sign
   RRsets.  This point is the activation time (Tact) and after this, key
   N is said to be active.

      Tact(N) >= Trdy(N)

   Event 4: At some point 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 activated.  The publication time of key N+1 depends on
   the activation time of key N:

      Tpub(N+1) <= Tact(N) + Lzsk - Ipub

   Here, Lzsk is the length of time for which a ZSK will be used (the
   ZSK lifetime).  It should be noted that in the diagrams, the actual
   key lifetime is represented; this may differ slightly from the
   intended lifetime set by key management policy.

   Event 5: While key N is still active, its successor becomes ready.
   From this time onwards, key N+1 could be used to sign the zone.

   Event 6: When key N has been in use for an interval equal to the ZSK
   lifetime, it is retired (i.e., it will never again be used to
   generate new signatures) and key N+1 activated and used to sign the
   zone.  This is the retire time of key N (Tret), and is given by:

      Tret(N) = Tact(N) + Lzsk

   It is also the activation time of the successor key N+1.  Note that
   operational considerations may cause key N to remain in use for a
   longer (or shorter) time than the lifetime set by the key management
   policy.

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   Event 7: 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 caches.  (It is possible that a validating resolver could
   have an old RRSIG record in the cache, but the old DNSKEY RRset has
   expired 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 the propagation delay, required
   to guarantee that the updated zone information has reached all slave
   servers, and TTLsig is the maximum TTL of all the RRSIG records in
   the zone created with the retiring key.

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

      Tdea(N) = Tret(N) + Iret

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

   Event 8: At any time after the key becomes dead, it can be removed
   from the zone's DNSKEY RRset, which must then be re-signed with the
   current KSK.  This time is the removal time (Trem), given by:

      Trem(N) >= Tdea(N)

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

3.2.2.  Double-Signature Method

   In this rollover, a new key is introduced and used to sign the zone;
   the old key and signatures are retained.  Once all cached DNSKEY and/
   or RRSIG information contains copies of the new DNSKEY and RRSIGs
   created with it, the old DNSKEY and RRSIGs can be removed from the
   zone.

   The timeline for a Double-Signature rollover is shown below.  The
   diagram follows the convention described in Section 3.2.1.

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

                                  ---- Time ---->

          Figure 2: Timeline for a Double-Signature ZSK Rollover

   Event 1: Key N is added to the DNSKEY RRset and is then used to sign
   the zone; existing signatures in the zone are not removed.  The key
   is published and active: this is key N's activation time (Tact),
   after which the key is said to be active.

   Event 2: As the current key (key N) approaches the end of its actual
   lifetime (Lzsk), the successor key (key N+1) is introduced into the
   zone and starts being used to sign RRsets: neither the current key
   nor the signatures created with it are removed.  The successor key is
   now also active.

      Tact(N+1) = Tact(N) + Lzsk - Iret

   Event 3: Before key N can be withdrawn from the zone, all RRsets that
   need to be signed must have been signed by the successor key (key
   N+1) and any old RRsets that do not include the new key or new RRSIGs
   must have expired from caches.  Note that the signatures are not
   replaced: each RRset is signed by both the old and new key.

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

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

   As before, Dsgn is the delay needed to ensure that all existing
   RRsets have been signed with the new key and Dprp is the propagation
   delay, required to guarantee that the updated zone information has
   reached all slave servers.  The final term (the maximum of TTLkey and
   TTLsig) is the period to wait for key and signature data associated
   with key N to expire from caches.  (TTLkey is the TTL of the DNSKEY
   RRset and TTLsig is the maximum TTL of all the RRSIG records in the
   zone created with the ZSK.  The two may be different: although the

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   TTL of an RRSIG is equal to the TTL of the RRs in the associated
   RRset [RFC4034], the DNSKEY RRset only needs to be signed with the
   KSK.)

   At the end of this interval, key N is said to be dead.  This occurs
   at the dead time (Tdea) so:

      Tdea(N) = Tact(N+1) + Iret

   Event 4: At some later time, key N and the signatures generated with
   it can be removed from the zone.  This is the removal time (Trem),
   given by:

      Trem(N) >= Tdea(N)

3.3.  KSK Rollover Timelines

   The following sections describe the rolling of a KSK.  They show the
   events in the lifetime of a key (referred to as "key N") and cover it
   replacement by its successor (key N+1).  (The case of introducing the
   first KSK into the zone is discussed in Section 3.3.5.)

3.3.1.  Double-KSK Method

   In this rollover, the new DNSKEY is added to the zone.  After an
   interval long enough to guarantee that any cached DNSKEY RRsets
   contain the new DNSKEY, the DS record in the parent zone is changed.
   After a further interval to allow the old DS record to expire from
   caches, the old DNSKEY is removed from the zone.

   The timeline for a Double-KSK rollover is shown below.  The diagram
   follows the convention described in Section 3.2.1.

Top      ToC       Page 15 
                       |1|       |2|   |3|      |4|
                        |         |     |        |
       Key N            |<-IpubC->|<--->|<-Dreg->|<-----Lksk--- - -
                        |         |     |        |
       Key N+1          |         |     |        |
                        |         |     |        |
       Key N           Tpub      Trdy  Tsbm     Tact
       Key N+1

                                   ---- Time ---->

               (continued ...)

                   |5|       |6|   |7|      |8|      |9|    |10|
                    |         |     |        |        |       |
       Key N   - - --------------Lksk------->|<-Iret->|<----->|
                    |         |     |        |        |       |
       Key N+1      |<-IpubC->|<--->|<-Dreg->|<--------Lksk----- - -
                    |         |     |        |        |       |
       Key N                                Tret     Tdea    Trem
       Key N+1     Tpub      Trdy  Tsbm     Tact

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

               Figure 3: Timeline for a Double-KSK Rollover

   Event 1: Key N is introduced into the zone; it is added to the DNSKEY
   RRset, which is then signed by all currently active KSKs.  (So at
   this point, the DNSKEY RRset is signed by both key N and its
   predecessor KSK.  If other KSKs were active, it is signed by these as
   well.)  This is the publication time of key N (Tpub); after this, the
   key is said to be published.

   Event 2: 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 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 in the child zone (IpubC)
   and is given by:

      IpubC = DprpC + TTLkey

Top      ToC       Page 16 
   ... where DprpC is the propagation delay for the child zone (the zone
   containing the KSK being rolled) and TTLkey the TTL for the DNSKEY
   RRset.  The time at which this occurs is the key N's ready time,
   Trdy, given by:

      Trdy(N) = Tpub(N) + IpubC

   Event 3: At some later time, the DS record corresponding to the new
   KSK is submitted to the parent zone for publication.  This time is
   the submission time, Tsbm:

      Tsbm(N) >= Trdy(N)

   Event 4: 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, in analogy with other
   rollover methods, this is called the activation time of key N (Tact):

      Tact(N) = Tsbm(N) + Dreg

   ... where Dreg is the registration delay, the time taken after the DS
   record has been submitted to the parent zone manager for it to be
   placed in the zone.  (Parent zones are often managed by different
   entities, and this term accounts for the organizational overhead of
   transferring a record.  In practice, Dreg will not be a fixed time:
   instead, the end of Dreg will be signaled by the appearance of the DS
   record in the parent zone.)

   Event 5: 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 published in the zone.  (As
   before, this means that the DNSKEY RRset is signed by all KSKs.)
   This time is the publication time of the successor key N+1, given by:

      Tpub(N+1) <= Tact(N) + Lksk - Dreg - IpubC

   ... where Lksk is the actual lifetime of the KSK, and Dreg the
   registration delay.

   Event 6: After an interval IpubC, key N+1 becomes ready (in that all
   caches that have a copy of the DNSKEY RRset have a copy of this key).
   This time is the ready time of the successor key N+1 (Trdy).

   Event 7: At the submission time of the successor key N+1, Tsbm(N+1),
   the DS record corresponding to key N+1 is submitted to the parent
   zone.

Top      ToC       Page 17 
   Event 8: The successor DS record is published in the parent zone and
   the current DS record withdrawn.  Key N is said to be retired and the
   time at which this occurs is Tret(N), given by:

      Tret(N) = Tsbm(N+1) + Dreg

   Event 9: Key N must remain in the zone until any caches that contain
   a copy of the DS RRset have a copy 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 in the parent zone.

   As the key is no longer used for anything, it is said to be dead.
   This point is the dead time (Tdea), given by:

      Tdea(N) = Tret(N) + Iret

   Event 10: At some later time, key N is removed from the zone's DNSKEY
   RRset (at the remove time Trem); the key is now said to be removed.

      Trem(N) >= Tdea(N)

3.3.2.  Double-DS Method

   In this rollover, the new DS record is published in the parent zone.
   When any caches that contain the DS RRset contain a copy of the new
   record, the KSK in the zone is changed.  After a further interval for
   the old DNSKEY RRset to expire from caches, the old DS record is
   removed from the parent.

   The timeline for a Double-DS rollover is shown below.  The diagram
   follows the convention described in Section 3.2.1.

Top      ToC       Page 18 
                     |1|      |2|       |3|  |4|    |5|
                      |        |         |    |      |
        Key N         |<-Dreg->|<-IpubP->|<-->|<-------Lksk---- - -
                      |        |         |    |      |
        Key N+1       |        |         |    |      |<--Dreg-- - -
                      |        |         |    |      |
        Key N        Tsbm     Tpub      Trdy Tact
        Key N+1                                     Tsbm
                                ---- Time ---->

               (continued ...)

                          |6|       |7|  |8|      |9|    |10|
                           |         |    |        |      |
        Key N   - ----------Lksk--------->|<-Iret->|<---->|
                           |         |    |        |      |
        Key N+1 - --Dreg-->|<-IpubP->|<-->|<---Lksk------- - -
                           |         |    |        |      |
        Key N                            Tret     Tdea   Trem
        Key N+1           Tpub      Trdy Tact

                                      ---- Time ---->

              Figure 4: Timeline for a Double-DS KSK Rollover

   Event 1: The DS RR is submitted to the parent zone for publication.
   This time is the submission time, Tsbm.

   Event 2: After the registration delay, Dreg, the DS record is
   published in the parent zone.  This is the publication time (Tpub) of
   key N, given by:

      Tpub(N) = Tsbm(N) + Dreg

   As before, in practice, Dreg will not be a fixed time.  Instead, the
   end of Dreg will be signaled by the appearance of the DS record in
   the parent zone.

   Event 3: At some later time, any cache that has a copy of the DS
   RRset 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 ready time,
   Trdy, and is given by:

      Trdy(N) = Tpub(N) + IpubP

Top      ToC       Page 19 
   IpubP is the publication interval of the DS record (in the parent
   zone) and is given by the expression:

      IpubP = DprpP + TTLds

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

   Event 4: At some later time, the key rollover takes place and the new
   key (key N) is introduced into the DNSKEY RRset and used to sign it.
   This time is key N's activation time (Tact) and at this point key N
   is said to be active:

      Tact(N) >= Trdy(N)

   Event 5: 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 cache
   that contains the zone's DS records has data about the DS record of
   the successor key.  The time at which this occurs is the submission
   time of the successor key N+1, given by:

      Tsbm(N+1) <= Tact(N) + Lksk - IpubP - Dreg

   ... where Lksk is the actual lifetime of key N (which may differ
   slightly from the lifetime set in the key management policy) and Dreg
   is the registration delay.

   Event 6.  After an interval Dreg, the successor DS record is
   published in the zone.

   Event 7: The successor key (key N+1) enters the ready state, i.e.,
   its DS record is now in caches that contain the parent DS RRset.

   Event 8: When key N has been active for its lifetime (Lksk), it is
   replaced in the DNSKEY RRset by key N+1; the RRset is then signed
   with the new key.  At this point, as both the old and new DS records
   have been in the parent zone long enough to ensure that they are in
   caches that contain the DS RRset, the zone can be authenticated
   throughout the rollover.  A validating resolver can authenticate
   either the old or new KSK.

   This time is the retire time (Tret) of key N, given by:

      Tret(N) = Tact(N) + Lksk

   This is also the activation time of the successor key N+1.

Top      ToC       Page 20 
   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.  In
   analogy with other rollover methods, this is called the dead time,
   Tdea, and is given by:

      Tdea(N) = Tret(N) + Iret

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

      Iret = DprpC + TTLkey

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

   Event 10: At some later time, the DS record is removed from the
   parent zone.  In analogy with other rollover methods, this is the
   removal time (Trem), given by:

      Trem(N) >= Tdea(N)

3.3.3.  Double-RRset Method

   In the Double-RRset rollover, the new DNSKEY and DS records are
   published simultaneously in the appropriate zones.  Once enough time
   has elapsed for the old DNSKEY and DS RRsets to expire from caches,
   the old DNSKEY and DS records are removed from their respective
   zones.

   The timeline for this rollover is shown below.  The diagram follows
   the convention described in Section 3.2.1.

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

                           ---- Time ---->

            Figure 5: Timeline for a Double-RRset KSK Rollover

Top      ToC       Page 21 
   Event 1: The DS and DNSKEY records have appeared in their respective
   zones and the latter has been used to sign the DNSKEY RRset.  The key
   is published and active: this is key N's activation time (Tact).

   Event 2: As the current key (key N) approaches the end of its actual
   lifetime (Lksk), the successor key (key N+1) is introduced into the
   zone and is used to sign the DNSKEY RRset.  At the same time, the
   successor DS record is submitted to the parent zone.  This is the
   publication time of the successor key (Tpub):

      Tpub(N+1) <= Tact(N) + Lksk - Ipub

   ... where Ipub is defined below.

   Event 3: After the registration delay (Dreg), the DS record appears
   in the parent zone.  The DNSKEY record is already in the child zone,
   so with both the new key and its associated data now visible, this is
   the key's activation time (Tact) and the key is now said to be
   active.

      Tact(N+1) = Tpub(N+1) + Dreg

   Event 4: Before key N and its associated data can be withdrawn, all
   RRsets in the caches of validating resolvers must contain the new DS
   and/or DNSKEY.  The time at which this occurs is the dead time of key
   N (Tdea), given by:

      Tdea(N) = Tpub(N+1) + Ipub

   Ipub is the time it takes to guarantee that any prior cached
   information about the DNSKEY and the DS RRsets have expired.  For the
   DNSKEY, this is the publication interval of the child (IpubC).  For
   the DS, the publication interval (IpubP) starts once the record
   appears in the parent zone, which is Dreg after it has been
   submitted.  Hence:

      Ipub = max(Dreg + IpubP, IpubC)

   The parent zone's publication interval is given by:

      IpubP = DprpP + TTLds

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

Top      ToC       Page 22 
   The child zone's publication interval is given by a similar equation:

      IpubC = DprpC + TTLkey

   where DprpC is the propagation delay in the child zone and TTLkey the
   TTL of a DNSKEY record.

   In analogy with other rollovers, we can also define a retire interval
   -- the interval between a key becoming active and the time at which
   its predecessor is considered dead.  In this case, Iret is given by:

      Iret = Ipub - Dreg

   In other words, the retire interval of the predecessor key is the
   greater of the publication interval of the parent, or the publication
   interval of the child minus the registration delay.

   Event 5: At some later time, the key N's DS and DNSKEY records are
   removed from their respective zones.  In analogy with other rollover
   methods, this is the removal time (Trem), given by:

      Trem(N) >= Tdea(N)

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.

3.3.4.1.  Addition of KSK

   When the new key is introduced, the expression for the publication
   interval of the DNSKEY (IpubC) in the Double-KSK and Double-RRset
   methods is modified to:

      IpubC >= DprpC + max(Itrp, TTLkey)

Top      ToC       Page 23 
   ... where the right-hand side of the expression now includes the
   "trust point" interval.  This term is the interval required to
   guarantee that a resolver configured for the automatic update of keys
   according to [RFC5011] will accept the new key as a new trust point.
   That interval is given by:

      Itrp >= queryInterval + AddHoldDownTime + queryInterval

   ... where queryInterval is as defined in Section 2.3 of [RFC5011] and
   AddHoldDownTime is the Add Hold-Down Time defined in Section 2.4.1 of
   the same document.

   The first term of the expression (queryInterval) represents the time
   after which all validating resolvers can be guaranteed to have
   obtained a copy of the DNSKEY RRset containing the new key.  Once
   retrieved, a validating resolver needs to wait for AddHoldDownTime.
   Providing it does not see a validly signed DNSKEY RRset without the
   new key in that period, it will treat it as a trust anchor the next
   time it retrieves the RRset, a process that can take up to another
   queryInterval (the third term).

   However, the expression for queryInterval given in [RFC5011] contains
   the DNSKEY's RRSIG expiration interval, a parameter that only the
   validating resolver can really calculate.  In practice, a modified
   query interval that depends only on TTLkey can be used:

      modifiedQueryInterval = MAX(1hr, MIN(15 days, TTLkey / 2))

   (This is obtained by taking the expression for queryInterval in
   [RFC5011] and assuming a worst case for RRsigExpirationInterval.  It
   is greater than or equal to queryInterval for all values of the
   expiration time.)  The expression above then becomes (after
   collecting terms):

      Itrp >= AddHoldDownTime + 2 * modifiedQueryInterval

   In the Double-DS method, instead of swapping the KSK RRs in a single
   step, there must now be a period of overlap.  In other words, the new
   KSK must be introduced into the zone at least:

      DprpC + max(Itrp, TTLkey)

   ... before the switch is made.

Top      ToC       Page 24 
3.3.4.2.  Removal of KSK

   The timeline for the removal of the key in all methods is modified by
   introducing a new state, "revoked".  When the key reaches its dead
   time, instead of being declared "dead", it is revoked; the "revoke"
   bit is set in the published DNSKEY RR, and the DNSKEY RRset re-signed
   with the current and revoked keys.  The key is maintained in this
   state for the revoke interval, Irev, given by:

      Irev >= DprpC + modifiedQueryInterval

   As before, DprpC is the time taken for the revoked DNSKEY to
   propagate to all slave zones, and modifiedQueryInterval is the time
   after which it can be guaranteed that all validating resolvers that
   adhere to RFC 5011 have retrieved a copy of the DNSKEY RRset
   containing the revoked key.

   After this time, the key is dead and can be removed from the zone.

3.3.5.  Introduction of First Keys

   There are no timing considerations associated with the introduction
   of the first keys into a zone other that they must be introduced and
   the zone validly signed before a chain of trust to the zone is
   created.

   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 is not able to
   obtain the corresponding DNSKEY.  In the case of an insecure parent,
   i.e., the initial creation of a chain of trust or "security apex", it
   is not possible to guarantee this.  It is up to the operator of the
   validating resolver to wait for the new KSK to appear at all servers
   for the zone before configuring the trust anchor.

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 minimize 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 the zone or successor keys could be introduced as soon as

Top      ToC       Page 25 
   possible after a key becomes active.  Either way results in one or
   more additional ZSKs 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.)

   It is also possible to have a standby KSK.  The Double-KSK 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 standby KSK should also be used to sign
   the DNSKEY RRset.  As the RRset and its signatures travel together,
   merely adding the KSK without using it to sign the DNSKEY RRset does
   not provide the desired time saving: for a KSK to be used in a
   rollover, the DNSKEY RRset must be signed with it, and this would
   introduce a delay while the old RRset (not signed with the new key)
   expires from caches.

   The idea of a standby KSK in the Double-RRset rollover method
   effectively means having two active keys (as the standby KSK and
   associated DS record would both be published at the same time in
   their respective zones).

   Finally, 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 included
   in the zone on a permanent basis, or be a successor introduced as
   early as possible.

5.  Algorithm Considerations

   The preceding sections have implicitly assumed that all keys and
   signatures are created using a single algorithm.  However,
   Section 2.2 of [RFC4035] requires that there 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

Top      ToC       Page 26 
   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.  Summary

   For ZSKs, the Pre-Publication method 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, the Double-RRset method is the most efficient for KSK
   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
   many cases, the rollover time will be determined by the times defined
   by RFC 5011.  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 standby keys as standard practice
   during all types of rollovers.

7.  Security Considerations

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

8.  Normative References

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <http://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <http://www.rfc-editor.org/info/rfc4035>.

Top      ToC       Page 27 
   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <http://www.rfc-editor.org/info/rfc5011>.

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012,
              <http://www.rfc-editor.org/info/rfc6781>.

Top      ToC       Page 28 
Appendix A.  List of Symbols

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

      <TYPE><id><ZONE>

   where:

   <TYPE> is an uppercase 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

   T         a point in time

   TTL       TTL of a record

   I, T, and TTL are self-explanatory.  Like I, both D and L are time
   periods, but whereas I values are intervals between two events, a "D"
   interval (delay) is a feature of the process, probably outside
   control of the zone manager, and an "L" interval (lifetime) is chosen
   by the zone manager and is a feature of policy.

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

   act       activation

   pub       publication

   ret       retire

   <ZONE> is an optional uppercase letter that distinguishes between the
   same variable applied to different zones and is one of:

   C         child

   P         parent

   Within the rollover descriptions, times may have a number in
   parentheses affixed to their end indicating the instance of the key
   to which they apply, e.g., Tact(N) is the activation time of key N,
   Tpub(N+1) the publication time of key N+1 etc.

Top      ToC       Page 29 
   The list of variables used in the text given below.

   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: the time taken for a DS record
             submitted to a parent zone to appear in it.  As a parent
             zone is often managed by a different organization than that
             managing the child zone, the delays associated with passing
             data between organizations is captured by this term.

   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.

   Ipub      Publication interval.  The amount of time that must elapse
             after the publication of a DNSKEY and/or its associated
             data before it can be assumed that any resolvers that have
             the relevant RRset cached have a copy of the new
             information.

   IpubC     Publication interval in the child zone.

   IpubP     Publication interval in the parent zone.

   Iret      Retire interval.  The amount of time that must elapse after
             a DNSKEY or associated data enters the retire state for any
             dependent information (e.g., RRSIG for a ZSK) 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
             considerations of [RFC5011].

   Itrp      Trust-point interval.  The amount of time that a trust
             anchor must be published for in order to guarantee that a
             resolver configured for an automatic update of keys will
             see the new key at least twice.

Top      ToC       Page 30 
   Lksk      Lifetime of a KSK.  This is the actual 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 the intended key
             lifetime indicated by management policy.

   Lzsk      Lifetime of a ZSK.  This is the actual 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 the intended key lifetime indicated by
             management policy.

   Tact      Activation time.  The time at which the key is regarded as
             the principal key for the zone.

   Tdea      Dead time.  The time at which any information held in
             validating resolver caches is guaranteed to contain
             information related to the successor key.  At this point,
             the current key and its associated information are not
             longed required for validation purposes.

   Tpub      Publication time.  The time that the key or associated data
             appears in the zone for the first time.

   Trem      Removal time.  The time at which the key and its associated
             information starts being removed from their respective
             zones.

   Tret      Retire time.  The time at which successor information
             starts being used.

   Trdy      Ready time.  The time at which it can be guaranteed that
             validating resolvers that have information about the key
             and/or associated data cached have a copy of the new
             information.

   Tsbm      Submission time.  The time at which the DS record of a KSK
             is submitted to the parent zone.

   TTLds     Time to live of a DS record.

   TTLkey    Time to live of a DNSKEY record.  (By implication, this is
             also the time to live of the signatures on the DNSKEY
             RRset.)

   TTLsig    The maximum time to live of all the RRSIG records in the
             zone that were created with the ZSK.

Top      ToC       Page 31 
Acknowledgements

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

Authors' Addresses

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

   Email: stephen@isc.org
   URI:   http://www.isc.org


   Johan Ihren
   Netnod
   Franzengatan 5
   Stockholm  SE-112 51
   Sweden

   Email: johani@netnod.se
   URI:   http://www.netnod.se


   John Dickinson
   Sinodun Internet Technologies Ltd
   Magdalen Centre
   Oxford Science Park
   Robert Robertson Avenue
   Oxford, Oxfordshire  OX4 4GA
   United Kingdom

   Email: jad@sinodun.com
   URI:   http://www.sinodun.com


   W. (Matthijs) Mekking
   Dyn, Inc.
   150 Dow St
   Manchester  NH 03101
   United States

   Email: mmekking@dyn.com
   URI:   https://www.dyn.com