[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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: January 10, 2013                                         Netnod
                                                            J. Dickinson
                                                                 Sinodun
                                                            July 9, 2012


                    DNSSEC Key Timing Considerations
               draft-ietf-dnsop-dnssec-key-timing-03.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 January 10, 2013.

Copyright Notice

   Copyright (c) 2012 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



Morris, et al.          Expires January 10, 2013                [Page 1]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   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.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Rollover Methods . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  ZSK Rollovers  . . . . . . . . . . . . . . . . . . . . . .  5
     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  . . . . . . . . . . . . . . . 12
       3.2.3.  Double-RRSIG Method  . . . . . . . . . . . . . . . . . 13
     3.3.  Key-Signing Key Rollover Timelines . . . . . . . . . . . . 15
       3.3.1.  Double-Signature Method  . . . . . . . . . . . . . . . 16
       3.3.2.  Double-DS Method . . . . . . . . . . . . . . . . . . . 18
       3.3.3.  Double-RRset Method  . . . . . . . . . . . . . . . . . 21
       3.3.4.  Interaction with Configured Trust Anchors  . . . . . . 23
       3.3.5.  Introduction of First Keys . . . . . . . . . . . . . . 24
   4.  Standby Keys . . . . . . . . . . . . . . . . . . . . . . . . . 25
   5.  Algorithm Considerations . . . . . . . . . . . . . . . . . . . 26
   6.  Limitation of Scope  . . . . . . . . . . . . . . . . . . . . . 26
   7.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   11. Normative References . . . . . . . . . . . . . . . . . . . . . 27
   Appendix A.  List of Symbols . . . . . . . . . . . . . . . . . . . 28
   Appendix B.  Change History (To be removed on publication) . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33













Morris, et al.          Expires January 10, 2013                [Page 2]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


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 associated
      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 "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, 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.




Morris, et al.          Expires January 10, 2013                [Page 3]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   A high-level overview of key rollover can be found in
   [I-D.ietf-dnsop-rfc4641bis].  In contrast, this document focuses on
   the low-level timing detail of two classes of operations described
   there, the rollover of key-signing keys, and the rollover of zone
   signing keys.

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 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 the authoritative server.  The rollover
   process needs to happen in such a way that at all times during 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.

   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.  It is also possible to
   use a single key as both the ZSK and KSK.  However, the rolling of
   this type of key is not treated in this document.

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.

1.4.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].






Morris, et al.          Expires January 10, 2013                [Page 4]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


2.  Rollover Methods

2.1.  ZSK Rollovers

   A ZSK can be rolled in one of three ways:

   o  Pre-Publication: described in [I-D.ietf-dnsop-rfc4641bis], 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, 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 resolver was created with; cached copies of
      the DNSKEY RRset will contain 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 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 [I-D.ietf-dnsop-rfc4641bis],
      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 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 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



Morris, et al.          Expires January 10, 2013                [Page 5]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


      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 conceptually the simplest -
   introduce the new key and new signatures, then approximately one TTL
   later remove the old key and old signatures.  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.
   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.

2.2.  KSK Rollovers

   For ZSKs, 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 and 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 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 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



Morris, et al.          Expires January 10, 2013                [Page 6]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


      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,
      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 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
      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 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.

2.3.  Summary

   The methods can be summarised as follows:

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

                                  Table 1






Morris, et al.          Expires January 10, 2013                [Page 7]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


3.  Key Rollover Timelines

3.1.  Key States

   During the rolling process, a key moves through different states.
   The defined 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 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 data" should be taken to mean "key or associated
               information".

   Ready       The new key data has been published for long enough to
               guarantee that any previous versions of the DNSKEY RRset
               have expired from caches.

   Active      The key has started to be used to sign RRsets.  Note that
               when this state is entered, it may not be possible for
               validating resolvers to use the key 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 key's
               predecessor instead.

   Retired     A successor key has become active and this key is no
               longer being used to generate RRSIGs.  However, as there
               may still be RRSIGs in caches that were generated using
               this key, it is being retained in the zone until they
               have expired.

   Dead        The key is published in the zone but there is no longer
               information anywhere that requires its presence.  Hence
               the key can be removed from the zone at any time.







Morris, et al.          Expires January 10, 2013                [Page 8]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   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 an [RFC5011] trust anchor that it is
               about to be removed from the zone.

3.2.  Zone-Signing Key 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

   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|   |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 its 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



Morris, et al.          Expires January 10, 2013                [Page 9]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   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 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

   Here, Dprp is the propagation delay - the time taken in the worst-
   case situation for a 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 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 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 activation time (Tact) and after this, the
   key is said to be active.

   Event 5: 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.  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



Morris, et al.          Expires January 10, 2013               [Page 10]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   than Lzsk.

   Event 6: 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 7: 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 = Tact + Lzsk

   It is also the activation time of the successor key (TactS).  Note
   that operational considerations may cause key N to remain in use for
   longer than Lzsk; if so, the retirement actually occurs when the
   successor key is made 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 caches.  (It is possible that a validating 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 maximum
   TTL of all the RRSIG records in the zone created with the ZSK.

   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's DNSKEY RRset, which must then be re-signed with the
   current key-signing key.  This time is the removal time (Trem), given
   by:






Morris, et al.          Expires January 10, 2013               [Page 11]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


             Trem >= Tdea

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

3.2.2.  Double-Signature Method

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



                    |1|  |2|           |3|          |4|  |5|
                     |    |             |            |    |
             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 its 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 then used to sign
   the zone; existing signatures in the zone are not removed.  This is
   the activation time (Tact), after which the key is said to be active.

   Event 3: After the current key (key N) has been in use for its
   intended 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 is key is now active and the current key is said to be
   retired.  This time is the retire time of the key (Tret); it is also
   the activation time of the successor key (TactS).

             Tret = Tact + Lzsk

   Event 4: 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



Morris, et al.          Expires January 10, 2013               [Page 12]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   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.  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 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 = Tret + Iret

   Event 5: 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 >= Tdea

3.2.3.  Double-RRSIG Method

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



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

                                ---- Time ---->

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




Morris, et al.          Expires January 10, 2013               [Page 13]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   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 its 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
   publication 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 (still-absent) key N and
   once by its predecessor.

   Event 4: There is now a delay while the old signature information
   expires from caches.  This interval is given by the expression:

             Dprp + TTLsig

   As before, Dprp is the propagation delay and TTLsig is the maximum
   TTL of all the RRSIG records in the zone created with the ZSK.

   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 signed RRs have
   signatures created by the old and new keys, the records can still be
   authenticated.  This time is the activation time (Tact) and the key
   is now said to be active.

   Event 6: At some point thought must be given to rolling the key.  The
   first step is to publish signatures created by the successor key (key



Morris, et al.          Expires January 10, 2013               [Page 14]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   N+1) early enough for key N to 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 7: The signatures have propagated and the new key could be
   added to the zone.  This time is the ready time of the successor key
   (TrdyS).

             TrdyS = TpubS + IpubG

   ... where IpubG is as defined above.

   Event 8: At some later time key N is removed from the zone's DNSKEY
   RRset and the successor key (key N+1) is added to it.  This is the
   retire time of the key (Tret).

   Event 9: 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 as they cannot be validated
   by any key.  In analogy with other rollover methods, 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 = Dprp + TTLkey

   Dprp is as defined earlier and TTLkey is the TTL of the DNSKEY RRset.

   Event 10: At some later time the signatures can be removed from the
   zone.  In analogy with other rollover methods, this time is called
   the remove time (Trem) and is given by:

             Trem >= Tdea

3.3.  Key-Signing Key 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).







Morris, et al.          Expires January 10, 2013               [Page 15]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


3.3.1.  Double-Signature Method

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



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

                            ---- Time ---->

        (continued ...)

                    |6|      |7|   |8|      |9|      |10|    |11|
                     |        |     |        |        |       |
        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 the generate time (Tgen).  Although
   there is no reason why the key cannot be generated immediately prior
   to its 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 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
   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



Morris, et al.          Expires January 10, 2013               [Page 16]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


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

             Ipub = DprpC + TTLkey

   ... 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's ready time, Trdy,
   given by:

             Trdy = Tpub + Ipub

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

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

             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 for the organisational overhead of
   transferring a record.)

   Event 6: 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 both the
   current and successor KSK.)  This time is the publication time of the
   successor key, TpubS, given by:

             TpubS <= Tact + Lksk - Dreg - Ipub

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




Morris, et al.          Expires January 10, 2013               [Page 17]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   Event 7: After an interval Ipub, 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 (TrdyS).

   Event 8: At the submission time of the successor (TsubS), the DS
   record corresponding to key N+1 is submitted to the parent zone.

   Event 9: 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:

             Tret = Tact + Lksk

   Event 10: 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, is said to be dead.  This
   point is the dead time (Tdea), given by:

             Tdea = Tret + Iret

   Event 11: 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 >= Tdea

3.3.2.  Double-DS Method

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















Morris, et al.          Expires January 10, 2013               [Page 18]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


             |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 the generate time (Tgen).  Although
   there is no reason why the key cannot be generated immediately prior
   to its 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: The DS RR is submitted to the parent zone for publication.
   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 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 key's ready time,
   Trdy, and is given by:





Morris, et al.          Expires January 10, 2013               [Page 19]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


             Trdy = Tpub + IpubP

   IpubP is the parent publication interval 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 5: 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
   that.

   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 - the
   validating resolver either has a copy of the DNSKEY RRset
   authenticated by the predecessor key, or it has a copy of the updated
   RRset authenticated with the new key.

   This time is key N's activation 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 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, given by:

             TsubS <= Tact + Lksk - IpubP - Dreg

   ... where Lksk is the lifetime of key N according to policy.

   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.
   This is the ready time of the successor key, TrdyS.

   (The interval between events 6 and 7 for the key N+1 correspond to
   the interval between events 2 and 4 for key N)

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






Morris, et al.          Expires January 10, 2013               [Page 20]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


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

             Tdea = Tret + Iret

   ... where Iret is the retire interval, 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 >= Tdea

3.3.3.  Double-RRset Method

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



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

                                 ---- Time ---->

            Figure 6: Timeline for a Double-RRset KSK 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 its 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



Morris, et al.          Expires January 10, 2013               [Page 21]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   are said to be generated.

   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: At some later time, the DS record is published in the parent
   zone and at some time after that, the updated information has reached
   all caches: any cache that holds a DNSKEY RRset from the child zone
   will have a copy that includes the new KSK, and any cache that has a
   copy of the parent DS RRset will have a copy that includes the new DS
   record.

   The time at which this occurs is called the activation time of the
   new KSK (Tact), given by:

             Tact = Tpub + Ipub

   ... where Ipub is the publication interval, given by:

             Ipub = max(IpubP, IpubC),

   IpubP being the publication interval in the parent zone and IpubC the
   publication interval in the child zone.  The parent zone's
   publication interval is given by:

             IpubP = Dreg + DprpP + TTLds

   where Dreg is the registration delay, the time taken for the DS
   record to be published in the parent zone.  DprpP is the parent
   zone's propagation delay and TTLds is the TTL of the DS record in
   that zone.

   The child'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.

   Event 4: At some point we need to give thought to key replacement.
   The successor key (key N+1) 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:



Morris, et al.          Expires January 10, 2013               [Page 22]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


             TpubS <= Tact + Lksk - Ipub

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

   Event 5: Key N+1's DNSKEY and DS records are in any caches that
   contain the child zone DNSKEY and/or the parent zone DS RR, and so
   the zone can be validated with the new key.  This is the activation
   time of the successor key (TactS) and by analogy with other rollover
   methods, it is also the retire time of the current key.  Since at
   this time the zone can be validated by the successor key, there is no
   reason to keep the current key in the zone and the time can also be
   regarded as the current key's dead time.  Thus:

             Tret = Tdea = TactS = Tact + Lksk

   Event 6: 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 >= Tdea

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 publication interval (Ipub) in
   the Double-Signature and Double-RRset methods should also be subject
   to the condition:

             Ipub >= Dprp + max(Itrp, TTLkey)

   ... where the right hand side of the expression is the time taken for
   the change to propagate to all nameservers for the zone plus the
   "trust point" interval.  This latter term is the interval required to
   guarantee that a resolver configured for the automatic update of keys
   from a particular trust point will see at least two validated DNSKEY
   RRsets containing the new key (a requirement from [RFC5011], section
   2.4.1).  It is defined by the expression:



Morris, et al.          Expires January 10, 2013               [Page 23]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


             Itrp >= (2 * queryInterval) + (n * retryTime)

   ... where queryInterval and retryTime are as defined in section 2.3
   of [RFC5011]. "n" is the total number of retries needed by the
   resolver during the two attempts to get the DNSKEY RRset.

   The first term of the expression (2 * queryInterval) represents the
   time to obtain two validated DNSKEY RRsets.  The second term (n *
   retryTime) is a safety margin, with the value of "n" reflecting the
   degree of confidence in the communication between a resolver and the
   trust point.

   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.

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 >= 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.

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.

   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
   is not 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 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



Morris, et al.          Expires January 10, 2013               [Page 24]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


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



Morris, et al.          Expires January 10, 2013               [Page 25]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   early as possible.


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
   considering differ from this those described in this document.  Of
   particular interest 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 transitions between
   algorithms.  The timing issues associated with algorithm rollovers
   will require a separate document.

   The reader is therefore reminded that DNSSEC is, as of date of
   publication, in the early stages of deployment, and best practices
   may further develop over time.


7.  Summary

   For ZSKs, "Pre-Publication" is generally considered to be the
   preferred way of rolling keys.  As shown in this document, the time



Morris, et al.          Expires January 10, 2013               [Page 26]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   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
   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 standby 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, Matthijs Mekking and Wouter Wijngaards.


11.  Normative References

   [I-D.ietf-dnsop-rfc4641bis]
              Kolkman, O. and M. Mekking, "DNSSEC Operational Practices,
              Version 2", draft-ietf-dnsop-rfc4641bis-11 (work in
              progress), April 2012.

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

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



Morris, et al.          Expires January 10, 2013               [Page 27]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


              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.


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

   T         a point in time

   TTL       TTL of a record

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

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



Morris, et al.          Expires January 10, 2013               [Page 28]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   act       activation

   pub       publication

   ret       retire

   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

   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: 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 organisation to that
             managing the child zone, the delays associated with passing
             data between zones 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 key before it can be assumed
             that any resolvers that have the DNSKEY RRset cached have a
             copy of this key.

   IpubC     Publication interval in the child zone.







Morris, et al.          Expires January 10, 2013               [Page 29]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   IpubG     Publication interval for the signature created by a ZSK:
             the amount of time that must elapse after the signature has
             been created before it can be assumed that any resolves
             that have the RRset and RRSIG cached have a copy of this
             signature.

   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.

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

   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.

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

   TactS     Activation 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      Publication time of a key.  The time that a key appears in
             a zone for the first time.






Morris, et al.          Expires January 10, 2013               [Page 30]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


   TpubS     Publication 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      Submission time - the time at which the DS record of a KSK
             is submitted to the parent.

   TsubS     Submission 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.  (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.


Appendix B.  Change History (To be removed on publication)

   o  draft-ietf-dnsop-dnssec-key-timing-03
      * Clarifications of and corrections to wording (Marc Lampo, Alfred
      Hoenes).
      * Updated timings related to trust anchor interaction (Matthijs
      Mekking).
      * Updated RFC 4641 reference to 4641bis (Alfred Hoenes).
      * Moved change history to end of document (Alfred Hoenes).

   o  draft-ietf-dnsop-dnssec-key-timing-02
      * Significant re-wording of some sections.
      * Removal of events noting change of state of predecessor key from
      ZSK Double-RRSIG and Double-Signature methods.
      * Change order of bullet points (and some wording) in section 1.1.



Morris, et al.          Expires January 10, 2013               [Page 31]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


      * Remove discussion of advantages and disadvantages of key roll
      methods from section 2: draft is informative and does not give
      recommendations.
      * Removal of discussion of upper limit to retire time relationship
      to signature lifetime.
      * Remove timing details of first key in the zone and move
      discussion of first signing of a zone to later in the document).
      (Matthijs Mekking)
      * Removal of redundant symbols from Appendix A.

   o  draft-ietf-dnsop-dnssec-key-timing-01
      * Added section on limitation of scope.

   o  draft-ietf-dnsop-dnssec-key-timing-00
      * Change to author contact details.

   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).

   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.



Morris, et al.          Expires January 10, 2013               [Page 32]


Internet-Draft      DNSSEC Key Timing Considerations           July 2012


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





















Morris, et al.          Expires January 10, 2013               [Page 33]


Html markup produced by rfcmarkup 1.120, available from https://tools.ietf.org/tools/rfcmarkup/