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Generalized DNS Notifications

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Authors Johan Stenstam , Peter Thomassen
Last updated 2022-11-28
Replaced by draft-ietf-dnsop-generalized-notify
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DNSOP Working Group                                          J. Stenstam
Internet-Draft                           The Swedish Internet Foundation
Intended status: Standards Track                            P. Thomassen
Expires: 1 June 2023                   deSEC, Secure Systems Engineering
                                                        28 November 2022

                     Generalized DNS Notifications


   Changes in CDS/CDNSKEY, CSYNC, and other records related to
   delegation maintenance are usually detected through scheduled scans
   run by the consuming party (e.g. top-level domain registry),
   incurring an uncomfortable trade-off between scanning cost and update

   A similar problem exists when scheduling zone transfers, and has been
   solved using the well-known DNS NOTIFY mechanism ([RFC1996]).  This
   mechanism enables a primary nameserver to proactively inform
   secondaries about zone changes, allowing the secondary to initiate an
   ad-hoc transfer independently of when the next SOA check would be

   This document extends the use of DNS NOTIFY beyond conventional zone
   transfer hints, bringing the benefits of ad-hoc notifications to DNS
   delegation maintenance in general.  Use cases include DNSSEC key
   rollovers hints via NOTIFY(CDS) and NOTIFY(DNSKEY), and quicker
   changes to a delegation's NS record set via NOTIFY(CSYNC).

   TO BE REMOVED: This document is being collaborated on in Github at:
   dns-notify (
   generalized-dns-notify).  The most recent working version of the
   document, open issues, etc. should all be available there.  The
   authors (gratefully) accept pull requests.

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

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   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 1 June 2023.

Copyright Notice

   Copyright (c) 2022 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 (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   3
   2.  Costs and Dangers of Slow Convergence . . . . . . . . . . . .   3
   3.  Efficiency Issues with DNS Scanning vs. Notification  . . . .   4
     3.1.  CDS and CDNSKEY Scanners  . . . . . . . . . . . . . . . .   5
     3.2.  CSYNC Scanners  . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Multi-Signer Setups . . . . . . . . . . . . . . . . . . .   5
   4.  CDS/CDNSKEY and CSYNC Notifications . . . . . . . . . . . . .   6
     4.1.  Where to send CDS and CSYNC Notifications . . . . . . . .   6
     4.2.  How to Interpret CDS and CSYNC Notifications  . . . . . .   7
   5.  DNSKEY Notifications  . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Where to send DNSKEY Notifications  . . . . . . . . . . .   8
     5.2.  How to Interpret DNSKEY Notifications . . . . . . . . . .   8
   6.  Who Should Send the Notifications?  . . . . . . . . . . . . .   8
   7.  Timing Considerations . . . . . . . . . . . . . . . . . . . .   9
   8.  Open Questions  . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  SRV vs new record type  . . . . . . . . . . . . . . . . .   9
     8.2.  Open Question For DNSKEY Notifications  . . . . . . . . .  10
   9.  Out of Scope  . . . . . . . . . . . . . . . . . . . . . . . .  10
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Change History (to be removed before publication)  .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   This document introduces a generalization of the DNS NOTIFY mechanism
   described in [RFC1996].

   Traditional DNS notifications, which are here referred to as
   "NOTIFY(SOA)", are sent from a primary server to a secondary server
   to minimize the latter's convergence time to a new version of the
   zone.  This mechanism successfully addresses a significant
   inefficiency in the original protocol.

   Today there are several new cases where similar inefficiencies cause
   significant problems.  However, just as in the NOTIFY(SOA) case, a
   new set of notification types will have a major positive benefit by
   allowing the DNS infrastructure to completely sidestep these

   There are no DNS protocol changes introduced by this document.

   There are, however, proposals for how to interpret a wider range of
   DNS messages that are already allowed (but not used) by the protocol.

   Readers are expected to be familiar with DNSSEC, including [RFC4033],
   [RFC4034], [RFC4035], [RFC6781], [RFC7344], [RFC7477], [RFC7583], and

1.1.  Requirements Notation

   The key words "*MUST*", "*MUST NOT*", "*REQUIRED*", "*SHALL*",
   RECOMMENDED*", "*MAY*", and "*OPTIONAL*" in this document are to be
   interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only
   when, they appear in all capitals, as shown here.

2.  Costs and Dangers of Slow Convergence

   [RFC1996] introduced the concept of a DNS Notify message which was
   used to improve the convergence time for secondary servers when a DNS
   zone had been updated in the primary.  The basic idea was to augment
   the traditional "pull" mechanism (a periodic SOA query) with a "push"
   mechanism (a Notify) for a common case that was otherwise very
   inefficient (due to either slow convergence or wasteful overly
   frequent scanning of the primary for changes).

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   Today we have similar issues with slow updates of DNS data in spite
   of the data having been published.  The two most obvious cases are
   the scalability issues of modern CDS and CSYNC scanners that are
   beginning to be deployed in a growing number of TLD registries.
   Because of the large number of child delegations, scanning for CDS
   and CSYNC records is rather slow (as in infrequent).

   Another use case is for so-called "multi-signer" setups where there
   are multiple, semi-independent, "signers" that each sign the same
   zone, but with individual sets of keys.  One requirement for multi-
   signer setups to work is the ability to insert additional DNSKEY
   records in each signer's version of the zone.  This is not the only
   multi-signer requirement, but it is the one that matters here.  The
   problem here is that modern signers will commonly roll DNSSEC Zone
   Signing Keys automatically and without informing anyone, assuming a
   single-signer setup where ZSK rollovers are a local matter (as
   opposed to KSK rollovers that involve the parent).

   However, when the ZSKs of one signer are rolled the other signers
   will be unaware of this event.  Having the individual signers sign
   DNSKEY RRsets where the ZSKs are not the same create a possibly
   broken signature chain and is therefore a dangerous situation.
   Eventually the signers will catch up, as multi-signer operators
   import each other's DNSKEY public key(s) to be able to compute the
   combined RRsets that should be published, but the period between the
   ZSK rollover in one signer and the other signers becoming aware
   constitutes a period of potential brokenness.

3.  Efficiency Issues with DNS Scanning vs. Notification

   The original problem that [RFC1996] addressed was the problem of
   optimization between frequent checking for new versions of a zone by
   a secondary and infrequent checking.  While it is possible to
   indicate how frequently checks should occur (via the SOA Refresh
   parameter), these checks did not allow catching zone changes that
   fall between checkpoints.  NOTIFY replaces scheduled scanning with a
   more efficient mechanism.

   In modern DNS infrastructure there are several new scanning based
   inefficiencies that did not exist at the time of the writing of

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3.1.  CDS and CDNSKEY Scanners

   An increasing number of TLD registries are implementing periodic
   scanning for CDS and CDNSKEY records in child zones.  Because of the
   large number of delegations this is rather costly, especially as it
   is only a very small number of the signed delegations that will have
   updated the CDS or CDNSKEY record between two scanning runs.

   Frequent scanning is costly.  Infrequent scanning causes slower
   convergence (i.e. delay until the DS in the parent is updated).

   Not every zone runs a CDS or CSYNC scanner (today).  However, for
   those that do, the problem is actually worse than in the traditional
   primary to secondary case.  The reason is that in the original case
   the primary is able to indicate how frequently changes are to be
   expected (via the SOA Refresh parameter).  No equivalent mechanism
   exist for the new scanning services.

3.2.  CSYNC Scanners

   This is basically the same problem as for the CDS / CDNSKEY scanners:
   large number of delegations vs infrequent updates to the CSYNC record
   in the child zones.

   Frequent scanning is costly.  Infrequent scanning causes slower
   convergence (i.e. updated delegation information in the parent).

3.3.  Multi-Signer Setups

   [I-D.wisser-dnssec-automation] describes processes for managing
   signed zones using multiple semi-independent "signers" (i.e. services
   that take an unsigned zone, sign it using unique DNSKEYs and publish
   the zone on the public Internet).  The setup most commonly discussed
   for multi-signer uses a "multi-signer controller", which is a
   separate service responsible for keeping the signing and delegation
   information in sync across multiple signers.

   To keep information in sync, the signers need to be scanned for
   current information about the DNSKEY RRset in each signer.  The
   problem is that modern "signers" will typically do DNSKEY rollovers
   (especially ZSK rollovers) automatically without informing anyone
   else, because a single-signer setup is often assume (in which case
   the ZSK rollover is a matter completely internal to the signer).

   In the multi-signer case, this is not a correct assumption.  It is
   therefore necessary to run frequent polls frequently to minimize the
   time window between one signer changing its version of the DNSKEY
   RRset and the controller noticing and re-synchronizing all signers.

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   Frequent scanning: costly.  Infrequent scanning: slower convergence
   (i.e. longer potential inconsistency across signers).

4.  CDS/CDNSKEY and CSYNC Notifications

   The [RFC1996] NOTIFY message sends a SOA record in the Query Section.
   We refer to this as a NOTIFY(SOA).  By generalizing the concept of
   DNS NOTIFY it is possible to address not only the original
   inefficiency (primary name server to secondary nameserver
   convergence) but also the problems with CDS/CDNSKEY, CSYNC and Multi-
   Signer scanning for zone updates.

   The CDS/CDNSKEY inefficiency may be addressed by the child sending a
   NOTIFY(CDS) to an address where the parent listens for such
   notifications.  To address the CDS/CDNSKEY dichotomy, NOTIFY(CDS) is
   defined to indicate any child-side changes pertaining to a upcoming
   update of DS records.  Upon receipt of NOTIFY(CDS), the parent SHOULD
   initiate the same scan that would other be triggered based on a

   The CSYNC inefficiency may similarly be addressed by the child
   sinding a NOTIFY(CSYNC) to an address where the parent is listening
   to CSYNC notifications.

   In both cases the notification will speed up the CDS and CSYNC
   scanning by providing the parent with a hint that a particular child
   zone has published new CDS, CDNSKEY and/or CSYNC records.

   The DNS protocol already allows these new notification types, so no
   protocol change is required, the only thing needed is specification
   of where to send the new types of Notifies and how to interpret them
   in the receiving end.

4.1.  Where to send CDS and CSYNC Notifications

   In the case of NOTIFY(CDS) and NOTIFY(CSYNC) the ultimate recipient
   of the notification is the parent, to improve the speed and
   efficiency of the parent's CDS/CSYNC scanning.  Because the name of
   the parent zone is known, and because the parent obviously controls
   the contents of its own zone the simplest solution is for the parent
   to publish the address where it prefers to have notifications sent.
   To be slightly more general than just using an address record we
   propose to use SRV records, as they are well suited to this.

   In the zone parent.:

   _cds-notifications.parent.   IN SRV n m  53 scanner.parent.
   _csync-notifications.parent. IN SRV n m  53 scanner.parent.

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4.2.  How to Interpret CDS and CSYNC Notifications

   Upon receipt of a NOTIFY(CDS) for a particular child zone at the
   published address for CDS notifications, the parent has two options:

   1.  Schedule an immediate check of the CDS and CDNSKEY RRsets as
       published by that particular child zone.

       The parent MAY reset the scanning timer for children for which
       NOTIFY(CDS) is received, or reduce the periodic scanning
       frequency accordingly (e.g. to every two weeks).  This will
       decrease the scanning effort for the parent.  If a CDS/CDNSKEY
       change is then detected (without having received a notification),
       the parent SHOULD clear that state and revert to the default
       scanning schedule.

       Parents introducing CDS/CDNSKEY scanning support at the same time
       as NOTIFY(CDS) support are not in danger of breaking children's
       scanning assumption, and MAY therefore use a low-frequency
       scanning schedule in default mode.

   2.  Ignore the notification, in which case the system works exactly
       as before.

   If the parent implements the first option, the convergence time (time
   between publication of a new CDS and propagation of the resulting DS)
   will decrease significantly, thereby providing improved service to
   the child zone.

   If the parent, in addition to scheduling an immediate check for the
   child zone of the notification, also choses to modify the scanning
   schedule (to be less frequent), the cost of providing the scanning
   service will be reduced.

   Upon receipt of a NOTIFY(CSYNC) to the published address for CSYNC
   notifications, the parent has exactly the same options to choose
   among as for the NOTIFY(CDS).

5.  DNSKEY Notifications

   In the multi-signer case the problem is slightly different, because
   it is not the parent that should be notified, but rather a third

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5.1.  Where to send DNSKEY Notifications

   The question is how the multi-signer controller should inform the
   signer about where to send the notification.  In the NOTIFY(CDS) and
   NOTIFY(CSYNC) cases the child (the service that signs the child zone)
   knows the name of the parent zone and can look up the notification
   address there.  In the NOTIFY(DNSKEY) case this is not possible.
   However, what is possible is to look up the notification address in
   the child zone itself.  This works out as a requirement for multi-
   signer setups to work is that at least one external party (the multi-
   signer controller) has the ability to insert or modify RRsets in the
   child zone.  Therefore the controller should be able to insert a
   record that documents the wanted address for DNSKEY notifications.

   In analogy with the other cases, but here in the zone child.parent.:

   _dnskey-notification.child.parent. IN SRV n m 53 scanner.signerA.
   _dnskey-notification.child.parent. IN SRV n m 53 scanner.signerB.

   should act as a trigger for the signer that whenever there are
   changes to the DNSKEY RRset it should send a corresponding
   NOTIFY(DNSKEY) to all signers that have signaled interest via
   insertion of SRV records in the zone.

5.2.  How to Interpret DNSKEY Notifications

   The receipt of a NOTIFY(DNSKEY) is a hint that the DNSKEY RRset at
   the sending signer has been updated.  If the child zone is not part
   of a multi-signer setup this should be a no-op that does not cause
   any action.  In a multi-signer setup, though, this hint provides the
   recipient of the notification with the same three options as in the
   NOTIFY(CDS) and NOTIFY(CSYNC) cases: schedule an immediate check,
   delay regular checks and ignore.

6.  Who Should Send the Notifications?

   Because of the security model where a notification by itself never
   causes a change (it can only speed up the time until the next
   scheduled check for the same thing) who the sender is is not crucial.

   This opens up the possibility of having the controller in a multi-
   signer setup send the CDS and CSYNC notifications to the parent,
   thereby enabling this new functionality even before the emergence of
   support for generalized DNS notifications in the name servers used
   for signing.

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   However, as multi-signer is strongly dependent on DNSKEY
   notifications (which, in case of an automatic ZSK rollover, can only
   be generated by the signing software), this does not alleviate the
   need for modifications also to signing software to support

7.  Timing Considerations

   When a primary name server publishes a new CDS RRset there will be a
   time delay until all copies of the zone world-wide will have been
   updated.  Usually this delay is short (on the order of seconds), but
   it is larger than zero.  If the primary sends a NOTIFY(CDS) at the
   exact time of publication of the new zone there is a potential for
   the parent to schedule the CDS check that the notification triggers
   faster than the zone propagates to all secondaries.  In this case the
   parent may draw the wrong conclusion ("the CDS RRset has not been

   Having a delay between the publication of the new data and the check
   for the new data would alleviate this issue.  However, as the parent
   has no way of knowing how quickly the child zone propagates, any such
   intentional delay should be implemented in the sending end, i.e. at
   the server generating the NOTIFY(CDS), not in the receiving end.

   NOTIFY(CSYNC) has the same timing consideration as NOTIFY(CDS) while
   NOTIFY(DNSKEY) does not.

8.  Open Questions

8.1.  SRV vs new record type

   In the case of the "vertical" Notify(CDS) and NOTIFY(CSYNC) nothing
   needs to be done by the nameservers serving the parent zone.  So
   therefore the SRV records appear to be a near perfect match.  In the
   case of the more "horizontal" NOTIFY(DNSKEY), however, the nameserver
   will have to act on the insertion of a NOTIFY(DNSKEY) instruction.

   It may therefore be preferable to use a new record type, to make it
   possible to more easily associate the special processing needed with
   the new type rather than the standard SRV RRtype that occurs in
   completely other scenarios than are described here.  If that is the
   case it could be that a new record type would provide a cleaner
   solution to all the new types of notification signaling.  Eg.:

   parent.         IN NOTIFY  CDS     53   scanner.parent.
   parent.         IN NOTIFY  CSYNC   53   scanner.parent.
   child.parent.   IN NOTIFY  DNSKEY  5300 music.service.provider.

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8.2.  Open Question For DNSKEY Notifications

   In a multi-signer setup there are multiple signers.  How will the
   multi-signer controller know which signer sent the notification?  As
   the number of signers for a particular zone is low (typically 2 or
   possibly 3), there is no major problem caused by not knowing which
   signer sent the notification and instead always check all the signers
   for updates to the DNSKEY RRset.

9.  Out of Scope

   In the multi-signer case, where the same zone is signed by multiple
   semi-independent "signers" it is obviously necessary to devise means
   of keeping the non-DNSSEC contents of the zone in sync.  That is out
   of scope of the multi-signer work and also out of scope of this

   A corner case of this scoping regards making changes to the NS RRset.
   Individual signers (as in DNS service providers) may want to be able
   to make such changes independently of the other contents of the zone.
   See previous section.

10.  Security Considerations

   The original NOTIFY specification ([RFC1996]) sidesteps most security
   issues by not relying on the information in the NOTIFY message in any
   way but instead only use it to set the timer until the next scheduled
   check (for the SOA record) to zero.  Therefore it is impossible to
   affect the behaviour of the recipient of the NOTIFY other than by
   changing the timing for when different checks are initiated.

   This mechanism and security model is reused for all the generalized
   NOTIFY messages.

   Another consideration is whether generalized DNS Notifications can be
   used as an amplification attack.  The answer seems to be "NO",
   because the size of the generalized NOTIFY messages is mostly equal
   to the size of the response and it is also mostly equal to the size
   of the resulting outbound query (for the child zone's CDS, CSYNC or
   DNSKEY RRset).

   Hence the amplification attack potential of generalized Notifications
   is the same as for the original NOTIFY(SOA), which has never been
   found to be useful for amplification attacks.

   In any case, NOTIFY consumers MAY configure rate limits to address
   concerns about the impact of unsolicited NOTIFY messages.

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11.  IANA Considerations

   Per [RFC8552], IANA is requested to create the "Generalized DNS
   Notifications" registry with the following columns and initial

     | NOTIFY type | Location             | relative to | Reference |
     | CDS         | _cds-notifications   | parent      | [this     |
     |             |                      |             | document] |
     | CSYNC       | _csync-notifications | parent      | [this     |
     |             |                      |             | document] |
     | DNSKEY      | TODO                 | TODO        | [this     |
     |             |                      |             | document] |

                                 Table 1

   [TODO: (1) Detail out use cases? (2) Add entries via RFC or web

12.  Acknowledgements

   Christian Elmerot

13.  Normative References

              Wisser, U. and S. Huque, "DNSSEC automation", Work in
              Progress, Internet-Draft, draft-wisser-dnssec-automation-
              03, 6 March 2022, <

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [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,

   [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,

   [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,

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012,

   [RFC7344]  Kumari, W., Gudmundsson, O., and G. Barwood, "Automating
              DNSSEC Delegation Trust Maintenance", RFC 7344,
              DOI 10.17487/RFC7344, September 2014,

   [RFC7477]  Hardaker, W., "Child-to-Parent Synchronization in DNS",
              RFC 7477, DOI 10.17487/RFC7477, March 2015,

   [RFC7583]  Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
              "DNSSEC Key Rollover Timing Considerations", RFC 7583,
              DOI 10.17487/RFC7583, October 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8552]  Crocker, D., "Scoped Interpretation of DNS Resource
              Records through "Underscored" Naming of Attribute Leaves",
              BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,

   [RFC8901]  Huque, S., Aras, P., Dickinson, J., Vcelak, J., and D.
              Blacka, "Multi-Signer DNSSEC Models", RFC 8901,
              DOI 10.17487/RFC8901, September 2020,

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Appendix A.  Change History (to be removed before publication)

   *  draft-thomassen-dnsop-generalized-dns-notify-00

      Initial public draft.

Authors' Addresses

   Johan Stenstam
   The Swedish Internet Foundation

   Peter Thomassen
   deSEC, Secure Systems Engineering

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