    Internet Engineering Task Force (IETF)                         K. Watsen
    Request for Comments: 8572                               Watsen Networks
    Category: Standards Track                                      I. Farrer
    ISSN: 2070-1721                                      Deutsche Telekom AG
                                                              M. Abrahamsson
                                                                   T-Systems
                                                                  April 2019

                     Secure Zero Touch Provisioning (SZTP)

    Abstract

       This document presents a technique to securely provision a networking
       device when it is booting in a factory-default state.  Variations in
       the solution enable it to be used on both public and private
       networks.  The provisioning steps are able to update the boot image,
       commit an initial configuration, and execute arbitrary scripts to
       address auxiliary needs.  The updated device is subsequently able to
       establish secure connections with other systems.  For instance, a
       device may establish NETCONF (RFC 6241) and/or RESTCONF (RFC 8040)
       connections with deployment-specific network management systems.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8572.

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    Copyright Notice

       Copyright (c) 2019 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
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
         1.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   5
         1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
         1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   8
         1.4.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .   8
       2.  Types of Conveyed Information . . . . . . . . . . . . . . . .   8
         2.1.  Redirect Information  . . . . . . . . . . . . . . . . . .   8
         2.2.  Onboarding Information  . . . . . . . . . . . . . . . . .   9
       3.  Artifacts . . . . . . . . . . . . . . . . . . . . . . . . . .  10
         3.1.  Conveyed Information  . . . . . . . . . . . . . . . . . .  10
         3.2.  Owner Certificate . . . . . . . . . . . . . . . . . . . .  12
         3.3.  Ownership Voucher . . . . . . . . . . . . . . . . . . . .  13
         3.4.  Artifact Encryption . . . . . . . . . . . . . . . . . . .  13
         3.5.  Artifact Groupings  . . . . . . . . . . . . . . . . . . .  14
       4.  Sources of Bootstrapping Data . . . . . . . . . . . . . . . .  15
         4.1.  Removable Storage . . . . . . . . . . . . . . . . . . . .  15
         4.2.  DNS Server  . . . . . . . . . . . . . . . . . . . . . . .  16
         4.3.  DHCP Server . . . . . . . . . . . . . . . . . . . . . . .  20
         4.4.  Bootstrap Server  . . . . . . . . . . . . . . . . . . . .  21
       5.  Device Details  . . . . . . . . . . . . . . . . . . . . . . .  22
         5.1.  Initial State . . . . . . . . . . . . . . . . . . . . . .  22
         5.2.  Boot Sequence . . . . . . . . . . . . . . . . . . . . . .  24
         5.3.  Processing a Source of Bootstrapping Data . . . . . . . .  25
         5.4.  Validating Signed Data  . . . . . . . . . . . . . . . . .  27
         5.5.  Processing Redirect Information . . . . . . . . . . . . .  28
         5.6.  Processing Onboarding Information . . . . . . . . . . . .  28
       6.  The Conveyed Information Data Model . . . . . . . . . . . . .  32
         6.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  32
         6.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  32
         6.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  34
       7.  The SZTP Bootstrap Server API . . . . . . . . . . . . . . . .  41
         7.1.  API Overview  . . . . . . . . . . . . . . . . . . . . . .  41
         7.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  42
         7.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  45
       8.  DHCP Options  . . . . . . . . . . . . . . . . . . . . . . . .  56
         8.1.  DHCPv4 SZTP Redirect Option . . . . . . . . . . . . . . .  56
         8.2.  DHCPv6 SZTP Redirect Option . . . . . . . . . . . . . . .  58
         8.3.  Common Field Encoding . . . . . . . . . . . . . . . . . .  59
       9.  Security Considerations . . . . . . . . . . . . . . . . . . .  59
         9.1.  Clock Sensitivity . . . . . . . . . . . . . . . . . . . .  59
         9.2.  Use of IDevID Certificates  . . . . . . . . . . . . . . .  60
         9.3.  Immutable Storage for Trust Anchors . . . . . . . . . . .  60
         9.4.  Secure Storage for Long-Lived Private Keys  . . . . . . .  60
         9.5.  Blindly Authenticating a Bootstrap Server . . . . . . . .  60
         9.6.  Disclosing Information to Untrusted Servers . . . . . . .  60
         9.7.  Sequencing Sources of Bootstrapping Data  . . . . . . . .  61

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         9.8.  Safety of Private Keys Used for Trust . . . . . . . . . .  62
         9.9.  Increased Reliance on Manufacturers . . . . . . . . . . .  62
         9.10. Concerns with Trusted Bootstrap Servers . . . . . . . . .  63
         9.11. Validity Period for Conveyed Information  . . . . . . . .  63
         9.12. Cascading Trust via Redirects . . . . . . . . . . . . . .  64
         9.13. Possible Reuse of Private Keys  . . . . . . . . . . . . .  65
         9.14. Non-issue with Encrypting Signed Artifacts  . . . . . . .  65
         9.15. The "ietf-sztp-conveyed-info" YANG Module . . . . . . . .  65
         9.16. The "ietf-sztp-bootstrap-server" YANG Module  . . . . . .  66
       10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  67
         10.1.  The IETF XML Registry  . . . . . . . . . . . . . . . . .  67
         10.2.  The YANG Module Names Registry . . . . . . . . . . . . .  67
         10.3.  The SMI Security for S/MIME CMS Content Type Registry  .  68
         10.4.  The BOOTP Vendor Extensions and DHCP Options Registry  .  68
         10.5.  The Dynamic Host Configuration Protocol for IPv6
                (DHCPv6) Registry  . . . . . . . . . . . . . . . . . . .  68
         10.6.  The Service Name and Transport Protocol Port Number
                Registry . . . . . . . . . . . . . . . . . . . . . . . .  69
         10.7.  The Underscored and Globally Scoped DNS Node Names
                Registry . . . . . . . . . . . . . . . . . . . . . . . .  69
       11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  69
         11.1.  Normative References . . . . . . . . . . . . . . . . . .  69
         11.2.  Informative References . . . . . . . . . . . . . . . . .  71
       Appendix A.  Example Device Data Model  . . . . . . . . . . . . .  74
         A.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  74
         A.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  75
         A.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  75
       Appendix B.  Promoting a Connection from Untrusted to Trusted . .  79
       Appendix C.  Workflow Overview  . . . . . . . . . . . . . . . . .  80
         C.1.  Enrollment and Ordering Devices . . . . . . . . . . . . .  80
         C.2.  Owner Stages the Network for Bootstrap  . . . . . . . . .  83
         C.3.  Device Powers On  . . . . . . . . . . . . . . . . . . . .  85
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  87
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  87

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    1.  Introduction

       A fundamental business requirement for any network operator is to
       reduce costs where possible.  For network operators, deploying
       devices to many locations can be a significant cost, as sending
       trained specialists to each site for installations is both cost
       prohibitive and does not scale.

       This document defines Secure Zero Touch Provisioning (SZTP), a
       bootstrapping strategy enabling devices to securely obtain
       bootstrapping data with no installer action beyond physical placement
       and connecting network and power cables.  As such, SZTP enables non-
       technical personnel to bring up devices in remote locations without
       the need for any operator input.

       The SZTP solution includes updating the boot image, committing an
       initial configuration, and executing arbitrary scripts to address
       auxiliary needs.  The updated device is subsequently able to
       establish secure connections with other systems.  For instance, a
       device may establish NETCONF [RFC6241] and/or RESTCONF [RFC8040]
       connections with deployment-specific network management systems.

       This document primarily regards physical devices, where the setting
       of the device's initial state (described in Section 5.1) occurs
       during the device's manufacturing process.  The SZTP solution may be
       extended to support virtual machines or other such logical
       constructs, but details for how this can be accomplished is left for
       future work.

    1.1.  Use Cases

       o  Device connecting to a remotely administered network

             This use case involves scenarios, such as a remote branch
             office or convenience store, whereby a device connects as an
             access gateway to an ISP's network.  Assuming it is not
             possible to customize the ISP's network to provide any
             bootstrapping support, and with no other nearby device to
             leverage, the device has no recourse but to reach out to an
             Internet-based bootstrap server to bootstrap from.

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       o  Device connecting to a locally administered network

             This use case covers all other scenarios and differs only in
             that the device may additionally leverage nearby devices, which
             may direct it to use a local service to bootstrap from.  If no
             such information is available, or the device is unable to use
             the information provided, it can then reach out to the network
             just as it would for the remotely administered network use
             case.

       Conceptual workflows for how SZTP might be deployed are provided in
       Appendix C.

    1.2.  Terminology

       This document uses the following terms (sorted alphabetically):

       Artifact:  The term "artifact" is used throughout this document to
           represent any of the three artifacts defined in Section 3
           (conveyed information, ownership voucher, and owner certificate).
           These artifacts collectively provide all the bootstrapping data a
           device may use.

       Bootstrapping Data:  The term "bootstrapping data" is used throughout
           this document to refer to the collection of data that a device
           may obtain during the bootstrapping process.  Specifically, it
           refers to the three artifacts defined in Section 3 (conveyed
           information, owner certificate, and ownership voucher).

       Bootstrap Server:  The term "bootstrap server" is used within this
           document to mean any RESTCONF server implementing the YANG module
           defined in Section 7.3.

       Conveyed Information:  The term "conveyed information" is used herein
           to refer to either redirect information or onboarding
           information.  Conveyed information is one of the three
           bootstrapping artifacts described in Section 3.

       Device:  The term "device" is used throughout this document to refer
           to a network element that needs to be bootstrapped.  See
           Section 5 for more information about devices.

       Manufacturer:  The term "manufacturer" is used herein to refer to the
           manufacturer of a device or a delegate of the manufacturer.

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       Network Management System (NMS):  The acronym "NMS" is used
           throughout this document to refer to the deployment-specific
           management system that the bootstrapping process is responsible
           for introducing devices to.  From a device's perspective, when
           the bootstrapping process has completed, the NMS is a NETCONF or
           RESTCONF client.

       Onboarding Information:  The term "onboarding information" is used
           herein to refer to one of the two types of "conveyed information"
           defined in this document, the other being "redirect information".
           Onboarding information is formally defined by the "onboarding-
           information" container within the "conveyed-information" yang-
           data structure in Section 6.3.

       Onboarding Server:  The term "onboarding server" is used herein to
           refer to a bootstrap server that only returns onboarding
           information.

       Owner:  The term "owner" is used throughout this document to refer to
           the person or organization that purchased or otherwise owns a
           device.

       Owner Certificate:  The term "owner certificate" is used in this
           document to represent an X.509 certificate that binds an owner
           identity to a public key, which a device can use to validate a
           signature over the conveyed information artifact.  The owner
           certificate may be communicated along with its chain of
           intermediate certificates leading up to a known trust anchor.
           The owner certificate is one of the three bootstrapping artifacts
           described in Section 3.

       Ownership Voucher:  The term "ownership voucher" is used in this
           document to represent the voucher artifact defined in [RFC8366].
           The ownership voucher is used to assign a device to an owner.
           The ownership voucher is one of the three bootstrapping artifacts
           described in Section 3.

       Redirect Information:  The term "redirect information" is used herein
           to refer to one of the two types of "conveyed information"
           defined in this document, the other being "onboarding
           information".  Redirect information is formally defined by the
           "redirect-information" container within the "conveyed-
           information" yang-data structure in Section 6.3.

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       Redirect Server:  The term "redirect server" is used to refer to a
           bootstrap server that only returns redirect information.  A
           redirect server is particularly useful when hosted by a
           manufacturer, as a well-known (e.g., Internet-based) resource to
           redirect devices to deployment-specific bootstrap servers.

       Signed Data:  The term "signed data" is used throughout to mean
           conveyed information that has been signed, specifically by a
           private key possessed by a device's owner.

       Unsigned Data:  The term "unsigned data" is used throughout to mean
           conveyed information that has not been signed.

    1.3.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT 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.

    1.4.  Tree Diagrams

       Tree diagrams used in this document follow the notation defined in
       [RFC8340].

    2.  Types of Conveyed Information

       This document defines two types of conveyed information that devices
       can access during the bootstrapping process.  These conveyed
       information types are described in this section.  Examples are
       provided in Section 6.2.

    2.1.  Redirect Information

       Redirect information redirects a device to another bootstrap server.
       Redirect information encodes a list of bootstrap servers, each
       specifying the bootstrap server's hostname (or IP address), an
       optional port, and an optional trust anchor certificate that the
       device can use to authenticate the bootstrap server with.

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       Redirect information is YANG-modeled data formally defined by the
       "redirect-information" container in the YANG module presented in
       Section 6.3.  This container has the tree diagram shown below.

                   +--:(redirect-information)
                      +-- redirect-information
                         +-- bootstrap-server* [address]
                            +-- address         inet:host
                            +-- port?           inet:port-number
                            +-- trust-anchor?   cms

       Redirect information may be trusted or untrusted.  The redirect
       information is trusted whenever it is obtained via a secure
       connection to a trusted bootstrap server or whenever it is signed by
       the device's owner.  In all other cases, the redirect information is
       untrusted.

       Trusted redirect information is useful for enabling a device to
       establish a secure connection to a specified bootstrap server, which
       is possible when the redirect information includes the bootstrap
       server's trust anchor certificate.

       Untrusted redirect information is useful for directing a device to a
       bootstrap server where signed data has been staged for it to obtain.
       Note that, when the redirect information is untrusted, devices
       discard any potentially included trust anchor certificates.

       How devices process redirect information is described in Section 5.5.

    2.2.  Onboarding Information

       Onboarding information provides data necessary for a device to
       bootstrap itself and establish secure connections with other systems.
       As defined in this document, onboarding information can specify
       details about the boot image a device must be running, an initial
       configuration the device must commit, and scripts that the device
       must successfully execute.

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       Onboarding information is YANG-modeled data formally defined by the
       "onboarding-information" container in the YANG module presented in
       Section 6.3.  This container has the tree diagram shown below.

                +--:(onboarding-information)
                   +-- onboarding-information
                      +-- boot-image
                      |  +-- os-name?              string
                      |  +-- os-version?           string
                      |  +-- download-uri*         inet:uri
                      |  +-- image-verification* [hash-algorithm]
                      |     +-- hash-algorithm    identityref
                      |     +-- hash-value        yang:hex-string
                      +-- configuration-handling?      enumeration
                      +-- pre-configuration-script?    script
                      +-- configuration?               binary
                      +-- post-configuration-script?   script

       Onboarding information must be trusted for it to be of any use to a
       device.  There is no option for a device to process untrusted
       onboarding information.

       Onboarding information is trusted whenever it is obtained via a
       secure connection to a trusted bootstrap server or whenever it is
       signed by the device's owner.  In all other cases, the onboarding
       information is untrusted.

       How devices process onboarding information is described in
       Section 5.6.

    3.  Artifacts

       This document defines three artifacts that can be made available to
       devices while they are bootstrapping.  Each source of bootstrapping
       data specifies how it provides the artifacts defined in this section
       (see Section 4).

    3.1.  Conveyed Information

       The conveyed information artifact encodes the essential bootstrapping
       data for the device.  This artifact is used to encode the redirect
       information and onboarding information types discussed in Section 2.

       The conveyed information artifact is a Cryptographic Message Syntax
       (CMS) structure, as described in [RFC5652], encoded using ASN.1
       distinguished encoding rules (DER), as specified in ITU-T X.690
       [ITU.X690.2015].  The CMS structure MUST contain content conforming
       to the YANG module specified in Section 6.3.

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       The conveyed information CMS structure may encode signed or unsigned
       bootstrapping data.  When the bootstrapping data is signed, it may
       also be encrypted, but from a terminology perspective, it is still
       "signed data"; see Section 1.2.

       When the conveyed information artifact is unsigned and unencrypted,
       as it might be when communicated over trusted channels, the CMS
       structure's topmost content type MUST be one of the OIDs described in
       Section 10.3 (i.e., id-ct-sztpConveyedInfoXML or
       id-ct-sztpConveyedInfoJSON) or the OID id-data
       (1.2.840.113549.1.7.1).  When the OID id-data is used, the encoding
       (JSON, XML, etc.) SHOULD be communicated externally.  In either case,
       the associated content is an octet string containing
       "conveyed-information" data in the expected encoding.

       When the conveyed information artifact is unsigned and encrypted, as
       it might be when communicated over trusted channels but, for some
       reason, the operator wants to ensure that only the device is able to
       see the contents, the CMS structure's topmost content type MUST be
       the OID id-envelopedData (1.2.840.113549.1.7.3).  Furthermore, the
       encryptedContentInfo's content type MUST be one of the OIDs described
       in Section 10.3 (i.e., id-ct-sztpConveyedInfoXML or
       id-ct-sztpConveyedInfoJSON) or the OID id-data
       (1.2.840.113549.1.7.1).  When the OID id-data is used, the encoding
       (JSON, XML, etc.)  SHOULD be communicated externally.  In either
       case, the associated content is an octet string containing
       "conveyed-information" data in the expected encoding.

       When the conveyed information artifact is signed and unencrypted, as
       it might be when communicated over untrusted channels, the CMS
       structure's topmost content type MUST be the OID id-signedData
       (1.2.840.113549.1.7.2).  Furthermore, the inner eContentType MUST be
       one of the OIDs described in Section 10.3 (i.e.,
       id-ct-sztpConveyedInfoXML or id-ct-sztpConveyedInfoJSON) or the OID
       id-data (1.2.840.113549.1.7.1).  When the OID id-data is used, the
       encoding (JSON, XML, etc.)  SHOULD be communicated externally.  In
       either case, the associated content or eContent is an octet string
       containing "conveyed-information" data in the expected encoding.

       When the conveyed information artifact is signed and encrypted, as it
       might be when communicated over untrusted channels and privacy is
       important, the CMS structure's topmost content type MUST be the OID
       id-envelopedData (1.2.840.113549.1.7.3).  Furthermore, the
       encryptedContentInfo's content type MUST be the OID id-signedData
       (1.2.840.113549.1.7.2), whose eContentType MUST be one of the OIDs
       described in Section 10.3 (i.e., id-ct-sztpConveyedInfoXML or
       id-ct-sztpConveyedInfoJSON), or the OID id-data
       (1.2.840.113549.1.7.1).  When the OID id-data is used, the encoding

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       (JSON, XML, etc.) SHOULD be communicated externally.  In either case,
       the associated content or eContent is an octet string containing
       "conveyed-information" data in the expected encoding.

    3.2.  Owner Certificate

       The owner certificate artifact is an X.509 certificate [RFC5280] that
       is used to identify an "owner" (e.g., an organization).  The owner
       certificate can be signed by any certificate authority (CA).  The
       owner certificate MUST have no Key Usage specified, or the Key Usage
       MUST, at a minimum, set the "digitalSignature" bit.  The values for
       the owner certificate's "subject" and/or "subjectAltName" are not
       constrained by this document.

       The owner certificate is used by a device to verify the signature
       over the conveyed information artifact (Section 3.1) that the device
       should have also received, as described in Section 3.5.  In
       particular, the device verifies the signature using the public key in
       the owner certificate over the content contained within the conveyed
       information artifact.

       The owner certificate artifact is formally a CMS structure, as
       specified by [RFC5652], encoded using ASN.1 DER, as specified in
       ITU-T X.690 [ITU.X690.2015].

       The owner certificate CMS structure MUST contain the owner
       certificate itself, as well as all intermediate certificates leading
       to the "pinned-domain-cert" certificate specified in the ownership
       voucher.  The owner certificate artifact MAY optionally include the
       "pinned-domain-cert" as well.

       In order to support devices deployed on private networks, the owner
       certificate CMS structure MAY also contain suitably fresh, as
       determined by local policy, revocation objects (e.g., Certificate
       Revocation Lists (CRLs) [RFC5280] and OCSP Responses [RFC6960]).
       Having these revocation objects stapled to the owner certificate may
       obviate the need for the device to have to download them dynamically
       using the CRL distribution point or an Online Certificate Status
       Protocol (OCSP) responder specified in the associated certificates.

       When unencrypted, the topmost content type of the owner certificate
       artifact's CMS structure MUST be the OID id-signedData
       (1.2.840.113549.1.7.2).  The inner SignedData structure is the
       degenerate form, whereby there are no signers, that is commonly used
       to disseminate certificates and revocation objects.

       When encrypted, the topmost content type of the owner certificate
       artifact's CMS structure MUST be the OID id-envelopedData

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       (1.2.840.113549.1.7.3), and the encryptedContentInfo's content type
       MUST be the OID id-signedData (1.2.840.113549.1.7.2), whereby the
       inner SignedData structure is the degenerate form that has no signers
       commonly used to disseminate certificates and revocation objects.

    3.3.  Ownership Voucher

       The ownership voucher artifact is used to securely identify a
       device's owner, as it is known to the manufacturer.  The ownership
       voucher is signed by the device's manufacturer.

       The ownership voucher is used to verify the owner certificate
       (Section 3.2) that the device should have also received, as described
       in Section 3.5.  In particular, the device verifies that the owner
       certificate has a chain of trust leading to the trusted certificate
       included in the ownership voucher ("pinned-domain-cert").  Note that
       this relationship holds even when the owner certificate is a self-
       signed certificate and hence also the pinned-domain-cert.

       When unencrypted, the ownership voucher artifact is as defined in
       [RFC8366].  As described, it is a CMS structure whose topmost content
       type MUST be the OID id-signedData (1.2.840.113549.1.7.2), whose
       eContentType MUST be OID id-ct-animaJSONVoucher
       (1.2.840.113549.1.9.16.1), or the OID id-data (1.2.840.113549.1.7.1).
       When the OID id-data is used, the encoding (JSON, XML, etc.) SHOULD
       be communicated externally.  In either case, the associated content
       is an octet string containing ietf-voucher data in the expected
       encoding.

       When encrypted, the topmost content type of the ownership voucher
       artifact's CMS structure MUST be the OID id-envelopedData
       (1.2.840.113549.1.7.3), and the encryptedContentInfo's content type
       MUST be the OID id-signedData (1.2.840.113549.1.7.2), whose
       eContentType MUST be OID id-ct-animaJSONVoucher
       (1.2.840.113549.1.9.16.1), or the OID id-data (1.2.840.113549.1.7.1).
       When the OID id-data is used, the encoding (JSON, XML, etc.) SHOULD
       be communicated externally.  In either case, the associated content
       is an octet string containing ietf-voucher data in the expected
       encoding.

    3.4.  Artifact Encryption

       Each of the three artifacts MAY be individually encrypted.
       Encryption may be important in some environments where the content is
       considered sensitive.

       Each of the three artifacts are encrypted in the same way, by the
       unencrypted form being encapsulated inside a CMS EnvelopedData type.

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       As a consequence, both the conveyed information and ownership voucher
       artifacts are signed and then encrypted; they are never encrypted and
       then signed.

       This sequencing has the following advantages: shrouding the signer's
       certificate and ensuring that the owner knows the content being
       signed.  This sequencing further enables the owner to inspect an
       unencrypted voucher obtained from a manufacturer and then encrypt the
       voucher later themselves, perhaps while also stapling in current
       revocation objects, when ready to place the artifact in an unsafe
       location.

       When encrypted, the CMS MUST be encrypted using a secure device
       identity certificate for the device.  This certificate MAY be the
       same as the TLS-level client certificate the device uses when
       connecting to bootstrap servers.  The owner must possess the device's
       identity certificate at the time of encrypting the data.  How the
       owner comes to posses the device's identity certificate for this
       purpose is outside the scope of this document.

    3.5.  Artifact Groupings

       The previous sections discussed the bootstrapping artifacts, but only
       certain groupings of these artifacts make sense to return in the
       various bootstrapping situations described in this document.  These
       groupings are:

          Unsigned Data:  This artifact grouping is useful for cases when
             transport-level security can be used to convey trust (e.g.,
             HTTPS) or when the conveyed information can be processed in a
             provisional manner (i.e., unsigned redirect information).

          Signed Data, without revocations:  This artifact grouping is
             useful when signed data is needed (i.e., because the data is
             obtained from an untrusted source and it cannot be processed
             provisionally) and revocations either are not needed or can be
             obtained dynamically.

          Signed Data, with revocations:  This artifact grouping is useful
             when signed data is needed (i.e., because the data is obtained
             from an untrusted source and it cannot be processed
             provisionally) and when revocations are needed but the
             revocations cannot be obtained dynamically.

       The presence of each artifact and any distinguishing characteristics
       are identified for each artifact grouping in the table below ("yes"
       and "no" indicate whether or not the artifact is present in the
       artifact grouping):

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       +---------------------+---------------+--------------+--------------+
       | Artifact            | Conveyed      | Ownership    | Owner        |
       | Grouping            | Information   | Voucher      | Certificate  |
       +=====================+===============+==============+==============+
       | Unsigned Data       | Yes, no sig   | No           | No           |
       +---------------------+---------------+--------------+--------------+
       | Signed Data,        | Yes, with sig | Yes, without | Yes, without |
       | without revocations |               | revocations  | revocations  |
       +---------------------+---------------+--------------+--------------+
       | Signed Data,        | Yes, with sig | Yes, with    | Yes, with    |
       | with revocations    |               | revocations  | revocations  |
       +---------------------+---------------+--------------+--------------+

    4.  Sources of Bootstrapping Data

       This section defines some sources for bootstrapping data that a
       device can access.  The list of sources defined here is not meant to
       be exhaustive.  It is left to future documents to define additional
       sources for obtaining bootstrapping data.

       For each source of bootstrapping data defined in this section,
       details are given for how the three artifacts listed in Section 3 are
       provided.

    4.1.  Removable Storage

       A directly attached removable storage device (e.g., a USB flash
       drive) MAY be used as a source of SZTP bootstrapping data.

       Use of a removable storage device is compelling, as it does not
       require any external infrastructure to work.  It is notable that the
       raw boot image file can also be located on the removable storage
       device, enabling a removable storage device to be a fully self-
       standing bootstrapping solution.

       To use a removable storage device as a source of bootstrapping data,
       a device need only detect if the removable storage device is plugged
       in and mount its filesystem.

       A removable storage device is an untrusted source of bootstrapping
       data.  This means that the information stored on the removable
       storage device either MUST be signed or MUST be information that can
       be processed provisionally (e.g., unsigned redirect information).

       From an artifact perspective, since a removable storage device
       presents itself as a filesystem, the bootstrapping artifacts need to
       be presented as files.  The three artifacts defined in Section 3 are
       mapped to files below.

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       Artifact to File Mapping:

          Conveyed Information:  Mapped to a file containing the binary
             artifact described in Section 3.1 (e.g., conveyed-
             information.cms).

          Owner Certificate:  Mapped to a file containing the binary
             artifact described in Section 3.2 (e.g., owner-
             certificate.cms).

          Ownership Voucher:  Mapped to a file containing the binary
             artifact described in Section 3.3 (e.g., ownership-voucher.cms
             or ownership-voucher.vcj).

       The format of the removable storage device's filesystem and the
       naming of the files are outside the scope of this document.  However,
       in order to facilitate interoperability, it is RECOMMENDED that
       devices support open and/or standards-based filesystems.  It is also
       RECOMMENDED that devices assume a file naming convention that enables
       more than one instance of bootstrapping data (i.e., for different
       devices) to exist on a removable storage device.  The file naming
       convention SHOULD additionally be unique to the manufacturer, in
       order to enable bootstrapping data from multiple manufacturers to
       exist on a removable storage device.

    4.2.  DNS Server

       A DNS server MAY be used as a source of SZTP bootstrapping data.

       Using a DNS server may be a compelling option for deployments having
       existing DNS infrastructure, as it enables a touchless bootstrapping
       option that does not entail utilizing an Internet-based resource
       hosted by a third party.

       DNS is an untrusted source of bootstrapping data.  Even if DNSSEC
       [RFC6698] is used to authenticate the various DNS resource records
       (e.g., A, AAAA, CERT, TXT, and TLSA), the device cannot be sure that
       the domain returned to it, e.g., from a DHCP server, belongs to its
       rightful owner.  This means that the information stored in the DNS
       records either MUST be signed (per this document, not DNSSEC) or MUST
       be information that can be processed provisionally (e.g., unsigned
       redirect information).

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    4.2.1.  DNS Queries

       Devices claiming to support DNS as a source of bootstrapping data
       MUST first query for device-specific DNS records and then, only if
       doing so does not result in a successful bootstrap, MUST query for
       device-independent DNS records.

       For each of the device-specific and device-independent queries,
       devices MUST first query using multicast DNS [RFC6762] and then, only
       if doing so does not result in a successful bootstrap, MUST query
       again using unicast DNS [RFC1035] [RFC7766].  This assumes the
       address of a DNS server is known, such as it may be using techniques
       similar to those described in Section 11 of [RFC6763].

       When querying for device-specific DNS records, devices MUST query for
       TXT records [RFC1035] under "<serial-number>._sztp", where <serial-
       number> is the device's serial number (the same value as in the
       device's secure device identity certificate), and "_sztp" is the
       globally scoped DNS attribute registered per this document (see
       Section 10.7).

       Example device-specific DNS record queries:

          TXT in <serial-number>._sztp.local.  (multicast)
          TXT in <serial-number>._sztp.<domain>.  (unicast)

       When querying for device-independent DNS records, devices MUST query
       for SRV records [RFC2782] under "_sztp._tcp", where "_sztp" is the
       service name registered per this document (see Section 10.6), and
       "_tcp" is the globally scoped DNS attribute registered per [RFC8552].

       Note that a device-independent response is only able to encode
       unsigned data anyway, since signed data necessitates the use of a
       device-specific ownership voucher.  Use of SRV records maximumly
       leverages existing DNS standards.  A response containing multiple SRV
       records is comparable to an unsigned redirect information's list of
       bootstrap servers.

       Example device-independent DNS record queries:

          SRV in _sztp._tcp.local.  (multicast)
          SRV in _sztp._tcp.<domain>.  (unicast)

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    4.2.2.  DNS Response for Device-Specific Queries

       For device-specific queries, the three bootstrapping artifacts
       defined in Section 3 are encoded into the TXT records using key/value
       pairs, similar to the technique described in Section 6.3 of
       [RFC6763].

       Artifact to TXT Record Mapping:

          Conveyed Information:  Mapped to a TXT record having the key "ci"
             and the value being the binary artifact described in
             Section 3.1.

          Owner Certificate:  Mapped to a TXT record having the key "oc" and
             the value being the binary artifact described in Section 3.2.

          Ownership Voucher:  Mapped to a TXT record having the key "ov" and
             the value being the binary artifact described in Section 3.3.

       Devices MUST ignore any other keys that may be returned.

       Note that, despite the name, TXT records can and SHOULD (per
       Section 6.5 of [RFC6763]) encode binary data.

       Following is an example of a device-specific response, as it might be
       presented by a user agent, containing signed data.  This example
       assumes that the device's serial number is "<serial-number>", the
       domain is "example.com", and "<binary data>" represents the binary
       artifact:

         <serial-number>._sztp.example.com. 3600 IN TXT "ci=<binary data>"
         <serial-number>._sztp.example.com. 3600 IN TXT "oc=<binary data>"
         <serial-number>._sztp.example.com. 3600 IN TXT "ov=<binary data>"

       Note that, in the case that "ci" encodes unsigned data, the "oc" and
       "ov" keys would not be present in the response.

    4.2.3.  DNS Response for Device-Independent Queries

       For device-independent queries, the three bootstrapping artifacts
       defined in Section 3 are encoded into the SVR records as follows.

       Artifact to SRV Record Mapping:

          Conveyed Information:  This artifact is not supported directly.
             Instead, the essence of unsigned redirect information is mapped
             to SVR records per [RFC2782].

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          Owner Certificate:  Not supported.  Device-independent responses
             never encode signed data; hence, there is no need for an owner
             certificate artifact.

          Ownership Voucher:  Not supported.  Device-independent responses
             never encode signed data; hence, there is no need for an
             ownership voucher artifact.

       Following is an example of a device-independent response, as it might
       be presented by a user agent, containing (effectively) unsigned
       redirect information to four bootstrap servers.  This example assumes
       that the domain is "example.com" and that there are four bootstrap
       servers "sztp[1-4]":

          _sztp._tcp.example.com. 1800 IN SRV 0 0 443 sztp1.example.com.
          _sztp._tcp.example.com. 1800 IN SRV 1 0 443 sztp2.example.com.
          _sztp._tcp.example.com. 1800 IN SRV 2 0 443 sztp3.example.com.
          _sztp._tcp.example.com. 1800 IN SRV 2 0 443 sztp4.example.com.

       Note that, in this example, "sztp3" and "sztp4" have equal priority
       and hence effectively represent a clustered pair of bootstrap
       servers.  While "sztp1" and "sztp2" only have a single SRV record
       each, it may be that the record points to a load balancer fronting a
       cluster of bootstrap servers.

       While this document does not use DNS-SD [RFC6763], per Section 12.2
       of that RFC, Multicast DNS (mDNS) responses SHOULD also include all
       address records (type "A" and "AAAA") named in the SRV rdata.

    4.2.4.  Size of Signed Data

       The signed data artifacts are large by DNS conventions.  In the
       smallest-footprint scenario, they are each a few kilobytes in size.
       However, onboarding information can easily be several kilobytes in
       size and has the potential to be many kilobytes in size.

       All resource records, including TXT records, have an upper size limit
       of 65535 bytes, since "RDLENGTH" is a 16-bit field (Section 3.2.1 of
       [RFC1035]).  If it is ever desired to encode onboarding information
       that exceeds this limit, the DNS records returned should instead
       encode redirect information, to direct the device to a bootstrap
       server from which the onboarding information can be obtained.

       Given the expected size of the TXT records, it is unlikely that
       signed data will fit into a UDP-based DNS packet, even with the
       Extension Mechanisms for DNS (EDNS(0)) extensions [RFC6891] enabled.
       Depending on content, signed data may also not fit into a multicast
       DNS packet, which bounds the size to 9000 bytes, per Section 17 of

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       [RFC6762].  Thus, it is expected that DNS Transport over TCP
       [RFC7766] will be required in order to return signed data.

    4.3.  DHCP Server

       A DHCP server MAY be used as a source of SZTP bootstrapping data.

       Using a DHCP server may be a compelling option for deployments having
       existing DHCP infrastructure, as it enables a touchless bootstrapping
       option that does not entail utilizing an Internet-based resource
       hosted by a third party.

       A DHCP server is an untrusted source of bootstrapping data.  Thus,
       the information stored on the DHCP server either MUST be signed or
       MUST be information that can be processed provisionally (e.g.,
       unsigned redirect information).

       However, unlike other sources of bootstrapping data described in this
       document, the DHCP protocol (especially DHCP for IPv4) is very
       limited in the amount of data that can be conveyed, to the extent
       that signed data cannot be communicated.  This means that only
       unsigned redirect information can be conveyed via DHCP.

       Since the redirect information is unsigned, it SHOULD NOT include the
       optional trust anchor certificate, as it takes up space in the DHCP
       message, and the device would have to discard it anyway.  For this
       reason, the DHCP options defined in Section 8 do not enable the trust
       anchor certificate to be encoded.

       From an artifact perspective, the three artifacts defined in
       Section 3 are mapped to the DHCP fields specified in Section 8 as
       follows.

       Artifact to DHCP Option Fields Mapping:

          Conveyed Information:  This artifact is not supported directly.
             Instead, the essence of unsigned redirect information is mapped
             to the DHCP options described in Section 8.

          Owner Certificate:  Not supported.  There is not enough space in
             the DHCP packet to hold an owner certificate artifact.

          Ownership Voucher:  Not supported.  There is not enough space in
             the DHCP packet to hold an ownership voucher artifact.

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    4.4.  Bootstrap Server

       A bootstrap server MAY be used as a source of SZTP bootstrapping
       data.  A bootstrap server is defined as a RESTCONF [RFC8040] server
       implementing the YANG module provided in Section 7.

       Using a bootstrap server as a source of bootstrapping data is a
       compelling option as it MAY use transport-level security, obviating
       the need for signed data, which may be easier to deploy in some
       situations.

       Unlike any other source of bootstrapping data described in this
       document, a bootstrap server is not only a source of data, but it can
       also receive data from devices using the YANG-defined "report-
       progress" RPC defined in the YANG module provided in Section 7.3.
       The "report-progress" RPC enables visibility into the bootstrapping
       process (e.g., warnings and errors) and provides potentially useful
       information upon completion (e.g., the device's Secure Shell (SSH)
       host keys and/or TLS trust anchor certificates).

       A bootstrap server may be a trusted or an untrusted source of
       bootstrapping data, depending on if the device learned about the
       bootstrap server's trust anchor from a trusted source.  When a
       bootstrap server is trusted, the conveyed information returned from
       it MAY be signed.  When the bootstrap server is untrusted, the
       conveyed information either MUST be signed or MUST be information
       that can be processed provisionally (e.g., unsigned redirect
       information).

       From an artifact perspective, since a bootstrap server presents data
       conforming to a YANG data model, the bootstrapping artifacts need to
       be mapped to YANG nodes.  The three artifacts defined in Section 3
       are mapped to "output" nodes of the "get-bootstrapping-data" RPC
       defined in Section 7.3.

       Artifact to Bootstrap Server Mapping:

          Conveyed Information:  Mapped to the "conveyed-information" leaf
             in the output of the "get-bootstrapping-data" RPC.

          Owner Certificate:  Mapped to the "owner-certificate" leaf in the
             output of the "get-bootstrapping-data" RPC.

          Ownership Voucher:  Mapped to the "ownership-voucher" leaf in the
             output of the "get-bootstrapping-data" RPC.

       SZTP bootstrap servers have only two endpoints: one for the
       "get-bootstrapping-data" RPC and one for the "report-progress" RPC.

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       These RPCs use the authenticated RESTCONF username to isolate the
       execution of the RPC from other devices.

    5.  Device Details

       Devices supporting the bootstrapping strategy described in this
       document MUST have the pre-configured state and bootstrapping logic
       described in the following sections.

    5.1.  Initial State

          +-------------------------------------------------------------+
          |                          <device>                           |
          |                                                             |
          | +---------------------------------------------------------+ |
          | |                   <read/write storage>                  | |
          | |                                                         | |
          | | 1.  flag to enable SZTP bootstrapping set to "true"     | |
          | +---------------------------------------------------------+ |
          |                                                             |
          | +---------------------------------------------------------+ |
          | |                   <read-only storage>                   | |
          | |                                                         | |
          | | 2.  TLS client cert & related intermediate certificates | |
          | | 3.  list of trusted well-known bootstrap servers        | |
          | | 4.  list of trust anchor certs for bootstrap servers    | |
          | | 5.  list of trust anchor certs for ownership vouchers   | |
          | +---------------------------------------------------------+ |
          |                                                             |
          |   +-----------------------------------------------------+   |
          |   |                 <secure storage>                    |   |
          |   |                                                     |   |
          |   |  6.  private key for TLS client certificate         |   |
          |   |  7.  private key for decrypting SZTP artifacts      |   |
          |   +-----------------------------------------------------+   |
          |                                                             |
          +-------------------------------------------------------------+

       Each numbered item below corresponds to a numbered item in the
       diagram above.

       1.  Devices MUST have a configurable variable that is used to enable/
           disable SZTP bootstrapping.  This variable MUST be enabled by
           default in order for SZTP bootstrapping to run when the device
           first powers on.  Because it is a goal that the configuration
           installed by the bootstrapping process disables SZTP
           bootstrapping, and because the configuration may be merged into
           the existing configuration, using a configuration node that

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           relies on presence is NOT RECOMMENDED, as it cannot be removed by
           the merging process.

       2.  Devices that support loading bootstrapping data from bootstrap
           servers (see Section 4.4) SHOULD possess a TLS-level client
           certificate and any intermediate certificates leading to the
           certificate's well-known trust anchor.  The well-known trust
           anchor certificate may be an intermediate certificate or a self-
           signed root certificate.  To support devices not having a client
           certificate, devices MAY, alternatively or in addition to,
           identify and authenticate themselves to the bootstrap server
           using an HTTP authentication scheme, as allowed by Section 2.5 of
           [RFC8040]; however, this document does not define a mechanism for
           operator input enabling, for example, the entering of a password.

       3.  Devices that support loading bootstrapping data from well-known
           bootstrap servers MUST possess a list of the well-known bootstrap
           servers.  Consistent with redirect information (Section 2.1),
           each bootstrap server can be identified by its hostname or IP
           address and an optional port.

       4.  Devices that support loading bootstrapping data from well-known
           bootstrap servers MUST also possess a list of trust anchor
           certificates that can be used to authenticate the well-known
           bootstrap servers.  For each trust anchor certificate, if it is
           not itself a self-signed root certificate, the device SHOULD also
           possess the chain of intermediate certificates leading up to and
           including the self-signed root certificate.

       5.  Devices that support loading signed data (see Section 1.2) MUST
           possess the trust anchor certificates for validating ownership
           vouchers.  For each trust anchor certificate, if it is not itself
           a self-signed root certificate, the device SHOULD also possess
           the chain of intermediate certificates leading up to and
           including the self-signed root certificate.

       6.  Devices that support using a TLS-level client certificate to
           identify and authenticate themselves to a bootstrap server MUST
           possess the private key that corresponds to the public key
           encoded in the TLS-level client certificate.  This private key
           SHOULD be securely stored, ideally in a cryptographic processor,
           such as a trusted platform module (TPM) chip.

       7.  Devices that support decrypting SZTP artifacts MUST posses the
           private key that corresponds to the public key encoded in the
           secure device identity certificate used when encrypting the
           artifacts.  This private key SHOULD be securely stored, ideally
           in a cryptographic processor, such as a trusted platform module

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           (TPM) chip.  This private key MAY be the same as the one
           associated to the TLS-level client certificate used when
           connecting to bootstrap servers.

       A YANG module representing this data is provided in Appendix A.

    5.2.  Boot Sequence

       A device claiming to support the bootstrapping strategy defined in
       this document MUST support the boot sequence described in this
       section.

            Power On
                |
                v                           No
        1.  SZTP bootstrapping configured ------> Boot normally
                |
                | Yes
                v
        2.  For each supported source of bootstrapping data,
            try to load bootstrapping data from the source
                |
                |
                v                               Yes
        3.  Able to bootstrap from any source? -----> Run with new config
                |
                | No
                v
        4.  Loop back to Step 1

        Note: At any time, the device MAY be configured via an alternate
              provisioning mechanism (e.g., command-line interface (CLI)).

       Each numbered item below corresponds to a numbered item in the
       diagram above.

       1.  When the device powers on, it first checks to see if SZTP
           bootstrapping is configured, as is expected to be the case for
           the device's pre-configured initial state.  If SZTP bootstrapping
           is not configured, then the device boots normally.

       2.  The device iterates over its list of sources for bootstrapping
           data (Section 4).  Details for how to process a source of
           bootstrapping data are provided in Section 5.3.

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       3.  If the device is able to bootstrap itself from any of the sources
           of bootstrapping data, it runs with the new bootstrapped
           configuration.

       4.  Otherwise, the device MUST loop back through the list of
           bootstrapping sources again.

       This document does not limit the simultaneous use of alternate
       provisioning mechanisms.  Such mechanisms may include, for instance,
       a CLI, a web-based user interface, or even another bootstrapping
       protocol.  Regardless of how it is configured, the configuration
       SHOULD unset the flag enabling SZTP bootstrapping as discussed in
       Section 5.1.

    5.3.  Processing a Source of Bootstrapping Data

       This section describes a recursive algorithm that devices can use to,
       ultimately, obtain onboarding information.  The algorithm is
       recursive because sources of bootstrapping data may return redirect
       information, which causes the algorithm to run again, for the newly
       discovered sources of bootstrapping data.  An expression that
       captures all possible successful sequences of bootstrapping data is:
       zero or more redirect information responses, followed by one
       onboarding information response.

       An important aspect of the algorithm is knowing when data needs to be
       signed or not.  The following figure provides a summary of options:

                                        Untrusted Source  Trusted Source
           Kind of Bootstrapping Data     Can Provide?     Can Provide?

           Unsigned Redirect Info     :       Yes+             Yes
           Signed Redirect Info       :       Yes              Yes*
           Unsigned Onboarding Info   :        No              Yes
           Signed Onboarding Info     :       Yes              Yes*

           The '+' above denotes that the source redirected to MUST
           return signed data or more unsigned redirect information.

           The '*' above denotes that, while possible, it is generally
           unnecessary for a trusted source to return signed data.

       The recursive algorithm uses a conceptual globally scoped variable
       called "trust-state".  The trust-state variable is initialized to
       FALSE.  The ultimate goal of this algorithm is for the device to
       process onboarding information (Section 2.2) while the trust-state
       variable is TRUE.

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       If the source of bootstrapping data (Section 4) is a bootstrap server
       (Section 4.4), and the device is able to authenticate the bootstrap
       server using X.509 certificate path validation ([RFC6125], Section 6)
       to one of the device's pre-configured trust anchors, or to a trust
       anchor that it learned from a previous step, then the device MUST set
       trust-state to TRUE.

       When establishing a connection to a bootstrap server, whether trusted
       or untrusted, the device MUST identify and authenticate itself to the
       bootstrap server using a TLS-level client certificate and/or an HTTP
       authentication scheme, per Section 2.5 of [RFC8040].  If both
       authentication mechanisms are used, they MUST both identify the same
       serial number.

       When sending a client certificate, the device MUST also send all of
       the intermediate certificates leading up to, and optionally
       including, the client certificate's well-known trust anchor
       certificate.

       For any source of bootstrapping data (e.g., Section 4), if any
       artifact obtained is encrypted, the device MUST first decrypt it
       using the private key associated with the device certificate used to
       encrypt the artifact.

       If the conveyed information artifact is signed, and the device is
       able to validate the signed data using the algorithm described in
       Section 5.4, then the device MUST set trust-state to TRUE; otherwise,
       if the device is unable to validate the signed data, the device MUST
       set trust-state to FALSE.  Note that this is worded to cover the
       special case when signed data is returned even from a trusted source
       of bootstrapping data.

       If the conveyed information artifact contains redirect information,
       the device MUST, within limits of how many recursive loops the device
       allows, process the redirect information as described in Section 5.5.
       Implementations MUST limit the maximum number of recursive redirects
       allowed; the maximum number of recursive redirects allowed SHOULD be
       no more than ten.  This is the recursion step; it will cause the
       device to reenter this algorithm, but this time the data source will
       definitely be a bootstrap server, as redirect information is only
       able to redirect devices to bootstrap servers.

       If the conveyed information artifact contains onboarding information,
       and trust-state is FALSE, the device MUST exit the recursive
       algorithm (as this is not allowed; see the figure above), returning
       to the bootstrapping sequence described in Section 5.2.  Otherwise,
       the device MUST attempt to process the onboarding information as
       described in Section 5.6.  Whether the processing of the onboarding

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       information succeeds or fails, the device MUST exit the recursive
       algorithm, returning to the bootstrapping sequence described in
       Section 5.2; the only difference is how it responds to the "Able to
       bootstrap from any source?" conditional described in the figure in
       that section.

    5.4.  Validating Signed Data

       Whenever a device is presented signed data, it MUST validate the
       signed data as described in this section.  This includes the case
       where the signed data is provided by a trusted source.

       Whenever there is signed data, the device MUST also be provided an
       ownership voucher and an owner certificate.  How all the needed
       artifacts are provided for each source of bootstrapping data is
       described in Section 4.

       In order to validate signed data, the device MUST first authenticate
       the ownership voucher by validating its signature to one of its pre-
       configured trust anchors (see Section 5.1), which may entail using
       additional intermediate certificates attached to the ownership
       voucher.  If the device has an accurate clock, it MUST verify that
       the ownership voucher was created in the past (i.e., "created-on" <
       now), and if the "expires-on" leaf is present, the device MUST verify
       that the ownership voucher has not yet expired (i.e., now < "expires-
       on").  The device MUST verify that the ownership voucher's
       "assertion" value is acceptable (e.g., some devices may only accept
       the assertion value "verified").  The device MUST verify that the
       ownership voucher specifies the device's serial number in the
       "serial-number" leaf.  If the "idevid-issuer" leaf is present, the
       device MUST verify that the value is set correctly.  If the
       authentication of the ownership voucher is successful, the device
       extracts the "pinned-domain-cert" node, an X.509 certificate, that is
       needed to verify the owner certificate in the next step.

       The device MUST next authenticate the owner certificate by performing
       X.509 certificate path verification to the trusted certificate
       extracted from the ownership voucher's "pinned-domain-cert" node.
       This verification may entail using additional intermediate
       certificates attached to the owner certificate artifact.  If the
       ownership voucher's "domain-cert-revocation-checks" node's value is
       set to "true", the device MUST verify the revocation status of the
       certificate chain used to sign the owner certificate, and if a
       suitably fresh revocation status is unattainable or if it is
       determined that a certificate has been revoked, the device MUST NOT
       validate the owner certificate.

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       Finally, the device MUST verify that the conveyed information
       artifact was signed by the validated owner certificate.

       If any of these steps fail, the device MUST invalidate the signed
       data and not perform any subsequent steps.

    5.5.  Processing Redirect Information

       In order to process redirect information (Section 2.1), the device
       MUST follow the steps presented in this section.

       Processing redirect information is straightforward; the device
       sequentially steps through the list of provided bootstrap servers
       until it can find one it can bootstrap from.

       If a hostname is provided, and the hostname's DNS resolution is to
       more than one IP address, the device MUST attempt to connect to all
       of the DNS resolved addresses at least once, before moving on to the
       next bootstrap server.  If the device is able to obtain bootstrapping
       data from any of the DNS resolved addresses, it MUST immediately
       process that data, without attempting to connect to any of the other
       DNS resolved addresses.

       If the redirect information is trusted (e.g., trust-state is TRUE),
       and the bootstrap server entry contains a trust anchor certificate,
       then the device MUST authenticate the specified bootstrap server's
       TLS server certificate using X.509 certificate path validation
       ([RFC6125], Section 6) to the specified trust anchor.  If the
       bootstrap server entry does not contain a trust anchor certificate
       device, the device MUST establish a provisional connection to the
       bootstrap server (i.e., by blindly accepting its server certificate)
       and set trust-state to FALSE.

       If the redirect information is untrusted (e.g., trust-state is
       FALSE), the device MUST discard any trust anchors provided by the
       redirect information and establish a provisional connection to the
       bootstrap server (i.e., by blindly accepting its TLS server
       certificate).

    5.6.  Processing Onboarding Information

       In order to process onboarding information (Section 2.2), the device
       MUST follow the steps presented in this section.

       When processing onboarding information, the device MUST first process
       the boot image information (if any), then execute the pre-
       configuration script (if any), then commit the initial configuration

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       (if any), and then execute the post-configuration script (if any), in
       that order.

       When the onboarding information is obtained from a trusted bootstrap
       server, the device MUST send the "bootstrap-initiated" progress
       report and send a terminating "boot-image-installed-rebooting",
       "bootstrap-complete", or error-specific progress report.  If the
       "reporting-level" node of the bootstrap server's "get-bootstrapping-
       data" RPC-reply is the value "verbose", the device MUST additionally
       send all appropriate non-terminating progress reports (e.g.,
       initiated, warning, complete, etc.).  Regardless of the reporting
       level requested by the bootstrap server, the device MAY send progress
       reports beyond those required by the reporting level.

       When the onboarding information is obtained from an untrusted
       bootstrap server, the device MUST NOT send any progress reports to
       the bootstrap server, even though the onboarding information was,
       necessarily, signed and authenticated.  Please be aware that
       bootstrap servers are recommended to promote untrusted connections to
       trusted connections, in the last paragraph of Section 9.6, so as to,
       in part, be able to collect progress reports from devices.

       If the device encounters an error at any step, it MUST stop
       processing the onboarding information and return to the bootstrapping
       sequence described in Section 5.2.  In the context of a recursive
       algorithm, the device MUST return to the enclosing loop, not back to
       the very beginning.  Some state MAY be retained from the
       bootstrapping process (e.g., updated boot image, logs, remnants from
       a script, etc.).  However, the retained state MUST NOT be active in
       any way (e.g., no new configuration or running of software) and MUST
       NOT hinder the ability for the device to continue the bootstrapping
       sequence (i.e., process onboarding information from another bootstrap
       server).

       At this point, the specific ordered sequence of actions the device
       MUST perform is described.

       If the onboarding information is obtained from a trusted bootstrap
       server, the device MUST send a "bootstrap-initiated" progress report.
       It is an error if the device does not receive back the "204 No
       Content" HTTP status line.  If an error occurs, the device MUST try
       to send a "bootstrap-error" progress report before exiting.

       The device MUST parse the provided onboarding information document,
       to extract values used in subsequent steps.  Whether using a stream-
       based parser or not, if there is an error when parsing the onboarding

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       information, and the device is connected to a trusted bootstrap
       server, the device MUST try to send a "parsing-error" progress report
       before exiting.

       If boot image criteria are specified, the device MUST first determine
       if the boot image it is running satisfies the specified boot image
       criteria.  If the device is already running the specified boot image,
       then it skips the remainder of this step.  If the device is not
       running the specified boot image, then it MUST download, verify, and
       install, in that order, the specified boot image, and then reboot.
       If connected to a trusted bootstrap server, the device MAY try to
       send a "boot-image-mismatch" progress report.  To download the boot
       image, the device MUST only use the URIs supplied by the onboarding
       information.  To verify the boot image, the device MUST use either
       one of the verification fingerprints supplied by the onboarding
       information or a cryptographic signature embedded into the boot image
       itself using a mechanism not described by this document.  Before
       rebooting, if connected to a trusted bootstrap server, the device
       MUST try to send a "boot-image-installed-rebooting" progress report.
       Upon rebooting, the bootstrapping process runs again, which will
       eventually come to this step again, but then the device will be
       running the specified boot image and thus will move to processing the
       next step.  If an error occurs at any step while the device is
       connected to a trusted bootstrap server (i.e., before the reboot),
       the device MUST try to send a "boot-image-error" progress report
       before exiting.

       If a pre-configuration script has been specified, the device MUST
       execute the script, capture any output emitted from the script, and
       check if the script had any warnings or errors.  If an error occurs
       while the device is connected to a trusted bootstrap server, the
       device MUST try to send a "pre-script-error" progress report before
       exiting.

       If an initial configuration has been specified, the device MUST
       atomically commit the provided initial configuration, using the
       approach specified by the "configuration-handling" leaf.  If an error
       occurs while the device is connected to a trusted bootstrap server,
       the device MUST try to send a "config-error" progress report before
       exiting.

       If a post-configuration script has been specified, the device MUST
       execute the script, capture any output emitted from the script, and
       check if the script had any warnings or errors.  If an error occurs
       while the device is connected to a trusted bootstrap server, the
       device MUST try to send a "post-script-error" progress report before
       exiting.

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       If the onboarding information was obtained from a trusted bootstrap
       server, and the result of the bootstrapping process did not disable
       the "flag to enable SZTP bootstrapping" described in Section 5.1, the
       device SHOULD send an "bootstrap-warning" progress report.

       If the onboarding information was obtained from a trusted bootstrap
       server, the device MUST send a "bootstrap-complete" progress report.
       It is an error if the device does not receive back the "204 No
       Content" HTTP status line.  If an error occurs, the device MUST try
       to send a "bootstrap-error" progress report before exiting.

       At this point, the device has completely processed the bootstrapping
       data.

       The device is now running its initial configuration.  Notably, if
       NETCONF Call Home or RESTCONF Call Home [RFC8071] is configured, the
       device initiates trying to establish the call home connections at
       this time.

       Implementation Notes:

          Implementations may vary in how to ensure no unwanted state is
          retained when an error occurs.

          If the implementation chooses to undo previous steps, the
          following guidelines apply:

          *  When an error occurs, the device must rollback the current step
             and any previous steps.

          *  Most steps are atomic.  For example, the processing of a
             configuration is atomic (as specified above), and the
             processing of scripts is atomic (as specified in the "ietf-
             sztp-conveyed-info" YANG module).

          *  In case the error occurs after the initial configuration was
             committed, the device must restore the configuration to the
             configuration that existed prior to the configuration being
             committed.

          *  In case the error occurs after a script had executed
             successfully, it may be helpful for the implementation to
             define scripts as being able to take a conceptual input
             parameter indicating that the script should remove its
             previously set state.

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    6.  The Conveyed Information Data Model

       This section defines a YANG 1.1 [RFC7950] module that is used to
       define the data model for the conveyed information artifact described
       in Section 3.1.  This data model uses the "yang-data" extension
       statement defined in [RFC8040].  Examples illustrating this data
       model are provided in Section 6.2.

    6.1.  Data Model Overview

       The following tree diagram provides an overview of the data model for
       the conveyed information artifact.

             module: ietf-sztp-conveyed-info

               yang-data conveyed-information:
                 +-- (information-type)
                    +--:(redirect-information)
                    |  +-- redirect-information
                    |     +-- bootstrap-server* [address]
                    |        +-- address         inet:host
                    |        +-- port?           inet:port-number
                    |        +-- trust-anchor?   cms
                    +--:(onboarding-information)
                       +-- onboarding-information
                          +-- boot-image
                          |  +-- os-name?              string
                          |  +-- os-version?           string
                          |  +-- download-uri*         inet:uri
                          |  +-- image-verification* [hash-algorithm]
                          |     +-- hash-algorithm    identityref
                          |     +-- hash-value        yang:hex-string
                          +-- configuration-handling?      enumeration
                          +-- pre-configuration-script?    script
                          +-- configuration?               binary
                          +-- post-configuration-script?   script

    6.2.  Example Usage

       The following example illustrates how redirect information
       (Section 2.1) can be encoded using JSON [RFC8259].

       {
         "ietf-sztp-conveyed-info:redirect-information" : {
           "bootstrap-server" : [
             {
               "address" : "sztp1.example.com",
               "port" : 8443,

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               "trust-anchor" : "base64encodedvalue=="
             },
             {
               "address" : "sztp2.example.com",
               "port" : 8443,
               "trust-anchor" : "base64encodedvalue=="
             },
             {
               "address" : "sztp3.example.com",
               "port" : 8443,
               "trust-anchor" : "base64encodedvalue=="
             }
           ]
         }
       }

       The following example illustrates how onboarding information
       (Section 2.2) can be encoded using JSON [RFC8259].

       [Note: '\' line wrapping for formatting only]

       {
         "ietf-sztp-conveyed-info:onboarding-information" : {
           "boot-image" : {
             "os-name" : "VendorOS",
             "os-version" : "17.2R1.6",
             "download-uri" : [ "https://example.com/path/to/image/file" ],
             "image-verification" : [
               {
                 "hash-algorithm" : "ietf-sztp-conveyed-info:sha-256",
                 "hash-value" : "ba:ec:cf:a5:67:82:b4:10:77:c6:67:a6:22:ab:\
       7d:50:04:a7:8b:8f:0e:db:02:8b:f4:75:55:fb:c1:13:b2:33"
               }
             ]
           },
           "configuration-handling" : "merge",
           "pre-configuration-script" : "base64encodedvalue==",
           "configuration" : "base64encodedvalue==",
           "post-configuration-script" : "base64encodedvalue=="
         }
       }

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    6.3.  YANG Module

       The conveyed information data model is defined by the YANG module
       presented in this section.

       This module uses data types defined in [RFC5280], [RFC5652],
       [RFC6234], and [RFC6991]; an extension statement from [RFC8040]; and
       an encoding defined in [ITU.X690.2015].

      <CODE BEGINS> file "ietf-sztp-conveyed-info@2019-04-30.yang"
      module ietf-sztp-conveyed-info {
        yang-version 1.1;
        namespace "urn:ietf:params:xml:ns:yang:ietf-sztp-conveyed-info";
        prefix sztp-info;

        import ietf-yang-types {
          prefix yang;
          reference
            "RFC 6991: Common YANG Data Types";
        }
        import ietf-inet-types {
          prefix inet;
          reference
            "RFC 6991: Common YANG Data Types";
        }
        import ietf-restconf {
          prefix rc;
          reference
            "RFC 8040: RESTCONF Protocol";
        }

        organization
          "IETF NETCONF (Network Configuration) Working Group";
        contact
          "WG Web:   <https://datatracker.ietf.org/wg/netconf/>
           WG List:  <mailto:netconf@ietf.org>
           Author:   Kent Watsen <mailto:kent+ietf@watsen.net>";
        description
          "This module defines the data model for the conveyed
           information artifact defined in RFC 8572 ('Secure Zero Touch
           Provisioning (SZTP)').

           The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
           'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
           'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
           are to be interpreted as described in BCP 14 (RFC 2119)
           (RFC 8174) when, and only when, they appear in all
           capitals, as shown here.

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           Copyright (c) 2019 IETF Trust and the persons identified as
           authors of the code.  All rights reserved.

           Redistribution and use in source and binary forms, with or
           without modification, is permitted pursuant to, and subject
           to the license terms contained in, the Simplified BSD License
           set forth in Section 4.c of the IETF Trust's Legal Provisions
           Relating to IETF Documents
           (https://trustee.ietf.org/license-info).

           This version of this YANG module is part of RFC 8572; see the
           RFC itself for full legal notices.";

        revision 2019-04-30 {
          description
            "Initial version";
          reference
            "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
        }

        // identities

        identity hash-algorithm {
          description
            "A base identity for hash algorithm verification.";
        }

        identity sha-256 {
          base hash-algorithm;
          description
            "The SHA-256 algorithm.";
          reference
            "RFC 6234: US Secure Hash Algorithms";
        }

        // typedefs

        typedef cms {
          type binary;
          description
            "A ContentInfo structure, as specified in RFC 5652,
             encoded using ASN.1 distinguished encoding rules (DER),
             as specified in ITU-T X.690.";
          reference
            "RFC 5652:
               Cryptographic Message Syntax (CMS)

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             ITU-T X.690:
               Information technology - ASN.1 encoding rules:
               Specification of Basic Encoding Rules (BER),
               Canonical Encoding Rules (CER) and Distinguished
               Encoding Rules (DER)";
        }

        // yang-data
        rc:yang-data conveyed-information {
          choice information-type {
            mandatory true;
            description
              "This choice statement ensures the response contains
               redirect-information or onboarding-information.";
            container redirect-information {
              description
                "Redirect information is described in Section 2.1 of
                 RFC 8572.  Its purpose is to redirect a device to
                 another bootstrap server.";
              reference
                "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
              list bootstrap-server {
                key "address";
                min-elements 1;
                description
                  "A bootstrap server entry.";
                leaf address {
                  type inet:host;
                  mandatory true;
                  description
                    "The IP address or hostname of the bootstrap server the
                     device should redirect to.";
                }
                leaf port {
                  type inet:port-number;
                  default "443";
                  description
                    "The port number the bootstrap server listens on.  If no
                     port is specified, the IANA-assigned port for 'https'
                     (443) is used.";
                }
                leaf trust-anchor {
                  type cms;
                  description
                    "A CMS structure that MUST contain the chain of
                     X.509 certificates needed to authenticate the TLS
                     certificate presented by this bootstrap server.

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                     The CMS MUST only contain a single chain of
                     certificates.  The bootstrap server MUST only
                     authenticate to last intermediate CA certificate
                     listed in the chain.

                     In all cases, the chain MUST include a self-signed
                     root certificate.  In the case where the root
                     certificate is itself the issuer of the bootstrap
                     server's TLS certificate, only one certificate
                     is present.

                     If needed by the device, this CMS structure MAY
                     also contain suitably fresh revocation objects
                     with which the device can verify the revocation
                     status of the certificates.

                     This CMS encodes the degenerate form of the SignedData
                     structure that is commonly used to disseminate X.509
                     certificates and revocation objects (RFC 5280).";
                  reference
                    "RFC 5280:
                       Internet X.509 Public Key Infrastructure Certificate
                       and Certificate Revocation List (CRL) Profile";
                }
              }
            }
            container onboarding-information {
              description
                "Onboarding information is described in Section 2.2 of
                 RFC 8572.  Its purpose is to provide the device everything
                 it needs to bootstrap itself.";
              reference
                "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
              container boot-image {
                description
                  "Specifies criteria for the boot image the device MUST
                   be running, as well as information enabling the device
                   to install the required boot image.";
                leaf os-name {
                  type string;
                  description
                    "The name of the operating system software the device
                     MUST be running in order to not require a software
                     image upgrade (e.g., VendorOS).";
                }
                leaf os-version {
                  type string;

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                  description
                    "The version of the operating system software the
                     device MUST be running in order to not require a
                     software image upgrade (e.g., 17.3R2.1).";
                }
                leaf-list download-uri {
                  type inet:uri;
                  ordered-by user;
                  description
                    "An ordered list of URIs to where the same boot image
                     file may be obtained.  How the URI schemes (http, ftp,
                     etc.) a device supports are known is vendor specific.
                     If a secure scheme (e.g., https) is provided, a device
                     MAY establish an untrusted connection to the remote
                     server, by blindly accepting the server's end-entity
                     certificate, to obtain the boot image.";
                }
                list image-verification {
                  must '../download-uri' {
                    description
                      "Download URIs must be provided if an image is to
                       be verified.";
                  }
                  key "hash-algorithm";
                  description
                    "A list of hash values that a device can use to verify
                     boot image files with.";
                  leaf hash-algorithm {
                    type identityref {
                      base hash-algorithm;
                    }
                    description
                      "Identifies the hash algorithm used.";
                  }
                  leaf hash-value {
                    type yang:hex-string;
                    mandatory true;
                    description
                      "The hex-encoded value of the specified hash
                       algorithm over the contents of the boot image
                       file.";
                  }
                }
              }
              leaf configuration-handling {
                type enumeration {
                  enum merge {

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                    description
                      "Merge configuration into the running datastore.";
                  }
                  enum replace {
                    description
                      "Replace the existing running datastore with the
                       passed configuration.";
                  }
                }
                must '../configuration';
                description
                  "This enumeration indicates how the server should process
                   the provided configuration.";
              }
              leaf pre-configuration-script {
                type script;
                description
                  "A script that, when present, is executed before the
                   configuration has been processed.";
              }
              leaf configuration {
                type binary;
                must '../configuration-handling';
                description
                  "Any configuration known to the device.  The use of
                   the 'binary' type enables content (e.g., XML) to be
                   embedded into a JSON document.  The exact encoding
                   of the content, as with the scripts, is vendor
                   specific.";
              }
              leaf post-configuration-script {
                type script;
                description
                  "A script that, when present, is executed after the
                   configuration has been processed.";
              }
            }
          }
        }

        typedef script {
          type binary;
          description
            "A device-specific script that enables the execution of
             commands to perform actions not possible thru configuration
             alone.

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             No attempt is made to standardize the contents, running
             context, or programming language of the script, other than
             that it can indicate if any warnings or errors occurred and
             can emit output.  The contents of the script are considered
             specific to the vendor, product line, and/or model of the
             device.

             If the script execution indicates that a warning occurred,
             then the device MUST assume that the script had a soft error
             that the script believes will not affect manageability.

             If the script execution indicates that an error occurred,
             the device MUST assume the script had a hard error that the
             script believes will affect manageability.  In this case,
             the script is required to gracefully exit, removing any
             state that might hinder the device's ability to continue
             the bootstrapping sequence (e.g., process onboarding
             information obtained from another bootstrap server).";
        }
      }
      <CODE ENDS>

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    7.  The SZTP Bootstrap Server API

       This section defines the API for bootstrap servers.  The API is
       defined as that produced by a RESTCONF [RFC8040] server that supports
       the YANG 1.1 [RFC7950] module defined in this section.

    7.1.  API Overview

       The following tree diagram provides an overview for the bootstrap
       server RESTCONF API.

       module: ietf-sztp-bootstrap-server

         rpcs:
           +---x get-bootstrapping-data
           |  +---w input
           |  |  +---w signed-data-preferred?   empty
           |  |  +---w hw-model?                string
           |  |  +---w os-name?                 string
           |  |  +---w os-version?              string
           |  |  +---w nonce?                   binary
           |  +--ro output
           |     +--ro reporting-level?    enumeration {onboarding-server}?
           |     +--ro conveyed-information    cms
           |     +--ro owner-certificate?      cms
           |     +--ro ownership-voucher?      cms
           +---x report-progress {onboarding-server}?
              +---w input
                 +---w progress-type         enumeration
                 +---w message?              string
                 +---w ssh-host-keys
                 |  +---w ssh-host-key* []
                 |     +---w algorithm    string
                 |     +---w key-data     binary
                 +---w trust-anchor-certs
                    +---w trust-anchor-cert*   cms

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    7.2.  Example Usage

       This section presents three examples illustrating the bootstrap
       server's API.  Two examples are provided for the "get-bootstrapping-
       data" RPC (one to an untrusted bootstrap server and the other to a
       trusted bootstrap server), and one example is provided for the
       "report-progress" RPC.

       The following example illustrates a device using the API to fetch its
       bootstrapping data from an untrusted bootstrap server.  In this
       example, the device sends the "signed-data-preferred" input parameter
       and receives signed data in the response.

       REQUEST

       [Note: '\' line wrapping for formatting only]

       POST /restconf/operations/ietf-sztp-bootstrap-server:get-bootstrappi\
       ng-data HTTP/1.1
       HOST: example.com
       Content-Type: application/yang.data+xml

       <input
         xmlns="urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server">
         <signed-data-preferred/>
       </input>

       RESPONSE

       HTTP/1.1 200 OK
       Date: Sat, 31 Oct 2015 17:02:40 GMT
       Server: example-server
       Content-Type: application/yang.data+xml

       <output
         xmlns="urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server">
         <conveyed-information>base64encodedvalue==</conveyed-information>
         <owner-certificate>base64encodedvalue==</owner-certificate>
         <ownership-voucher>base64encodedvalue==</ownership-voucher>
       </output>

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       The following example illustrates a device using the API to fetch its
       bootstrapping data from a trusted bootstrap server.  In this example,
       the device sends additional input parameters to the bootstrap server,
       which it may use when formulating its response to the device.

       REQUEST

       [Note: '\' line wrapping for formatting only]

       POST /restconf/operations/ietf-sztp-bootstrap-server:get-bootstrappi\
       ng-data HTTP/1.1
       HOST: example.com
       Content-Type: application/yang.data+xml

       <input
         xmlns="urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server">
         <hw-model>model-x</hw-model>
         <os-name>vendor-os</os-name>
         <os-version>17.3R2.1</os-version>
         <nonce>extralongbase64encodedvalue=</nonce>
       </input>

       RESPONSE

       HTTP/1.1 200 OK
       Date: Sat, 31 Oct 2015 17:02:40 GMT
       Server: example-server
       Content-Type: application/yang.data+xml

       <output
         xmlns="urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server">
         <reporting-level>verbose</reporting-level>
         <conveyed-information>base64encodedvalue==</conveyed-information>
       </output>

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       The following example illustrates a device using the API to post a
       progress report to a bootstrap server.  Illustrated below is the
       "bootstrap-complete" message, but the device may send other progress
       reports to the server while bootstrapping.  In this example, the
       device is sending both its SSH host keys and a TLS server
       certificate, which the bootstrap server may, for example, pass to an
       NMS, as discussed in Appendix C.3.

       REQUEST

       [Note: '\' line wrapping for formatting only]

       POST /restconf/operations/ietf-sztp-bootstrap-server:report-progress\
        HTTP/1.1
       HOST: example.com
       Content-Type: application/yang.data+xml

       <input
         xmlns="urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server">
         <progress-type>bootstrap-complete</progress-type>
         <message>example message</message>
         <ssh-host-keys>
           <ssh-host-key>
             <algorithm>ssh-rsa</algorithm>
             <key-data>base64encodedvalue==</key-data>
           </ssh-host-key>
           <ssh-host-key>
             <algorithm>rsa-sha2-256</algorithm>
             <key-data>base64encodedvalue==</key-data>
           </ssh-host-key>
         </ssh-host-keys>
         <trust-anchor-certs>
           <trust-anchor-cert>base64encodedvalue==</trust-anchor-cert>
         </trust-anchor-certs>
       </input>

       RESPONSE

       HTTP/1.1 204 No Content
       Date: Sat, 31 Oct 2015 17:02:40 GMT
       Server: example-server

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    7.3.  YANG Module

       The bootstrap server's device-facing API is normatively defined by
       the YANG module defined in this section.

       This module uses data types defined in [RFC4253], [RFC5652],
       [RFC5280], and [RFC8366]; uses an encoding defined in
       [ITU.X690.2015]; and makes a reference to [RFC4250], [RFC6187], and
       [Std-802.1AR].

       <CODE BEGINS> file "ietf-sztp-bootstrap-server@2019-04-30.yang"
       module ietf-sztp-bootstrap-server {
         yang-version 1.1;
         namespace "urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server";
         prefix sztp-svr;

         organization
           "IETF NETCONF (Network Configuration) Working Group";
         contact
           "WG Web:   <https://datatracker.ietf.org/wg/netconf/>
            WG List:  <mailto:netconf@ietf.org>
            Author:   Kent Watsen <mailto:kent+ietf@watsen.net>";
         description
           "This module defines an interface for bootstrap servers, as
            defined by RFC 8572 ('Secure Zero Touch Provisioning (SZTP)').

            The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
            'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
            'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
            are to be interpreted as described in BCP 14 (RFC 2119)
            (RFC 8174) when, and only when, they appear in all
            capitals, as shown here.

            Copyright (c) 2019 IETF Trust and the persons identified as
            authors of the code.  All rights reserved.

            Redistribution and use in source and binary forms, with or
            without modification, is permitted pursuant to, and subject
            to the license terms contained in, the Simplified BSD License
            set forth in Section 4.c of the IETF Trust's Legal Provisions
            Relating to IETF Documents
            (https://trustee.ietf.org/license-info).

            This version of this YANG module is part of RFC 8572; see the
            RFC itself for full legal notices.";

         revision 2019-04-30 {
           description

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             "Initial version";
           reference
             "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
         }

         // features

         feature redirect-server {
           description
             "The server supports being a 'redirect server'.";
         }

         feature onboarding-server {
           description
             "The server supports being an 'onboarding server'.";
         }

         // typedefs

         typedef cms {
           type binary;
           description
             "A CMS structure, as specified in RFC 5652, encoded using
              ASN.1 distinguished encoding rules (DER), as specified in
              ITU-T X.690.";
           reference
             "RFC 5652:
                Cryptographic Message Syntax (CMS)
              ITU-T X.690:
                Information technology - ASN.1 encoding rules:
                Specification of Basic Encoding Rules (BER),
                Canonical Encoding Rules (CER) and Distinguished
                Encoding Rules (DER)";
         }

         // RPCs

         rpc get-bootstrapping-data {
           description
             "This RPC enables a device, as identified by the RESTCONF
              username, to obtain bootstrapping data that has been made
              available for it.";
           input {
             leaf signed-data-preferred {
               type empty;
               description
                 "This optional input parameter enables a device to
                  communicate to the bootstrap server that it prefers

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                  to receive signed data.  Devices SHOULD always send
                  this parameter when the bootstrap server is untrusted.
                  Upon receiving this input parameter, the bootstrap
                  server MUST return either signed data or unsigned
                  redirect information; the bootstrap server MUST NOT
                  return unsigned onboarding information.";
             }
             leaf hw-model {
               type string;
               description
                 "This optional input parameter enables a device to
                  communicate to the bootstrap server its vendor-specific
                  hardware model number.  This parameter may be needed,
                  for instance, when a device's IDevID certificate does
                  not include the 'hardwareModelName' value in its
                  subjectAltName field, as is allowed by 802.1AR.";
               reference
                 "IEEE 802.1AR: IEEE Standard for Local and
                    metropolitan area networks - Secure
                    Device Identity";
             }
             leaf os-name {
               type string;
               description
                 "This optional input parameter enables a device to
                  communicate to the bootstrap server the name of its
                  operating system.  This parameter may be useful if
                  the device, as identified by its serial number, can
                  run more than one type of operating system (e.g.,
                  on a white-box system.";
             }
             leaf os-version {
               type string;
               description
                 "This optional input parameter enables a device to
                  communicate to the bootstrap server the version of its
                  operating system.  This parameter may be used by a
                  bootstrap server to return an operating-system-specific
                  response to the device, thus negating the need for a
                  potentially expensive boot image update.";
             }
             leaf nonce {
               type binary {
                 length "16..32";
               }
               description
                 "This optional input parameter enables a device to
                  communicate to the bootstrap server a nonce value.

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                  This may be especially useful for devices lacking
                  an accurate clock, as then the bootstrap server
                  can dynamically obtain from the manufacturer a
                  voucher with the nonce value in it, as described
                  in RFC 8366.";
               reference
                 "RFC 8366:
                    A Voucher Artifact for Bootstrapping Protocols";
             }
           }
           output {
             leaf reporting-level {
               if-feature "onboarding-server";
               type enumeration {
                 enum minimal {
                   description
                     "Send just the progress reports required by RFC 8572.";
                   reference
                     "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
                 }
                 enum verbose {
                   description
                     "Send additional progress reports that might help
                      troubleshooting an SZTP bootstrapping issue.";
                 }
               }
               default "minimal";
               description
                 "Specifies the reporting level for progress reports the
                  bootstrap server would like to receive when processing
                  onboarding information.  Progress reports are not sent
                  when processing redirect information or when the
                  bootstrap server is untrusted (e.g., device sent the
                  '<signed-data-preferred>' input parameter).";
             }
             leaf conveyed-information {
               type cms;
               mandatory true;
               description
                 "An SZTP conveyed information artifact, as described in
                  Section 3.1 of RFC 8572.";
               reference
                 "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
             }
             leaf owner-certificate {
               type cms;
               must '../ownership-voucher' {
                 description

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                   "An ownership voucher must be present whenever an owner
                    certificate is presented.";
               }
               description
                 "An owner certificate artifact, as described in Section
                  3.2 of RFC 8572.  This leaf is optional because it is
                  only needed when the conveyed information artifact is
                  signed.";
               reference
                 "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
             }
             leaf ownership-voucher {
               type cms;
               must '../owner-certificate' {
                 description
                   "An owner certificate must be present whenever an
                    ownership voucher is presented.";
               }
               description
                 "An ownership voucher artifact, as described by Section
                  3.3 of RFC 8572.  This leaf is optional because it is
                  only needed when the conveyed information artifact is
                  signed.";
               reference
                 "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
             }
           }
         }

         rpc report-progress {
           if-feature "onboarding-server";
           description
             "This RPC enables a device, as identified by the RESTCONF
              username, to report its bootstrapping progress to the
              bootstrap server.  This RPC is expected to be used when
              the device obtains onboarding-information from a trusted
              bootstrap server.";
           input {
             leaf progress-type {
               type enumeration {
                 enum bootstrap-initiated {
                   description
                     "Indicates that the device just used the
                      'get-bootstrapping-data' RPC.  The 'message' node
                      below MAY contain any additional information that
                      the manufacturer thinks might be useful.";
                 }
                 enum parsing-initiated {

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                   description
                     "Indicates that the device is about to start parsing
                      the onboarding information.  This progress type is
                      only for when parsing is implemented as a distinct
                      step.";
                 }
                 enum parsing-warning {
                   description
                     "Indicates that the device had a non-fatal error when
                      parsing the response from the bootstrap server.  The
                      'message' node below SHOULD indicate the specific
                      warning that occurred.";
                 }
                 enum parsing-error {
                   description
                     "Indicates that the device encountered a fatal error
                      when parsing the response from the bootstrap server.
                      For instance, this could be due to malformed encoding,
                      the device expecting signed data when only unsigned
                      data is provided, the ownership voucher not listing
                      the device's serial number, or because the signature
                      didn't match.  The 'message' node below SHOULD
                      indicate the specific error.  This progress type
                      also indicates that the device has abandoned trying
                      to bootstrap off this bootstrap server.";
                 }
                 enum parsing-complete {
                   description
                     "Indicates that the device successfully completed
                      parsing the onboarding information.  This progress
                      type is only for when parsing is implemented as a
                      distinct step.";
                 }
                 enum boot-image-initiated {
                   description
                     "Indicates that the device is about to start
                      processing the boot image information.";
                 }
                 enum boot-image-warning {
                   description
                     "Indicates that the device encountered a non-fatal
                      error condition when trying to install a boot image.
                      A possible reason might include a need to reformat a
                      partition causing loss of data.  The 'message' node
                      below SHOULD indicate any warning messages that were
                      generated.";
                 }
                 enum boot-image-error {

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                   description
                     "Indicates that the device encountered an error when
                      trying to install a boot image, which could be for
                      reasons such as a file server being unreachable,
                      file not found, signature mismatch, etc.  The
                      'message' node SHOULD indicate the specific error
                      that occurred.  This progress type also indicates
                      that the device has abandoned trying to bootstrap
                      off this bootstrap server.";
                 }
                 enum boot-image-mismatch {
                   description
                     "Indicates that the device has determined that
                      it is not running the correct boot image.  This
                      message SHOULD precipitate trying to download
                      a boot image.";
                 }
                 enum boot-image-installed-rebooting {
                   description
                     "Indicates that the device successfully installed
                      a new boot image and is about to reboot.  After
                      sending this progress type, the device is not
                      expected to access the bootstrap server again
                      for this bootstrapping attempt.";
                 }
                 enum boot-image-complete {
                   description
                     "Indicates that the device believes that it is
                      running the correct boot image.";
                 }
                 enum pre-script-initiated {
                   description
                     "Indicates that the device is about to execute the
                      'pre-configuration-script'.";
                 }
                 enum pre-script-warning {
                   description
                     "Indicates that the device obtained a warning from the
                      'pre-configuration-script' when it was executed.  The
                      'message' node below SHOULD capture any output the
                      script produces.";
                 }
                 enum pre-script-error {
                   description
                     "Indicates that the device obtained an error from the
                      'pre-configuration-script' when it was executed.  The
                      'message' node below SHOULD capture any output the
                      script produces.  This progress type also indicates

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                      that the device has abandoned trying to bootstrap
                      off this bootstrap server.";
                 }
                 enum pre-script-complete {
                   description
                     "Indicates that the device successfully executed the
                      'pre-configuration-script'.";
                 }
                 enum config-initiated {
                   description
                     "Indicates that the device is about to commit the
                      initial configuration.";
                 }
                 enum config-warning {
                   description
                     "Indicates that the device obtained warning messages
                      when it committed the initial configuration.  The
                      'message' node below SHOULD indicate any warning
                      messages that were generated.";
                 }
                 enum config-error {
                   description
                     "Indicates that the device obtained error messages
                      when it committed the initial configuration.  The
                      'message' node below SHOULD indicate the error
                      messages that were generated.  This progress type
                      also indicates that the device has abandoned trying
                      to bootstrap off this bootstrap server.";
                 }
                 enum config-complete {
                   description
                     "Indicates that the device successfully committed
                      the initial configuration.";
                 }
                 enum post-script-initiated {
                   description
                     "Indicates that the device is about to execute the
                      'post-configuration-script'.";
                 }
                 enum post-script-warning {
                   description
                     "Indicates that the device obtained a warning from the
                      'post-configuration-script' when it was executed.  The
                      'message' node below SHOULD capture any output the
                      script produces.";
                 }
                 enum post-script-error {
                   description

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                     "Indicates that the device obtained an error from the
                      'post-configuration-script' when it was executed.  The
                      'message' node below SHOULD capture any output the
                      script produces.  This progress type also indicates
                      that the device has abandoned trying to bootstrap
                      off this bootstrap server.";
                 }
                 enum post-script-complete {
                   description
                     "Indicates that the device successfully executed the
                      'post-configuration-script'.";
                 }
                 enum bootstrap-warning {
                   description
                     "Indicates that a warning condition occurred for which
                      no other 'progress-type' enumeration is deemed
                      suitable.  The 'message' node below SHOULD describe
                      the warning.";
                 }
                 enum bootstrap-error {
                   description
                     "Indicates that an error condition occurred for which
                      no other 'progress-type' enumeration is deemed
                      suitable.  The 'message' node below SHOULD describe
                      the error.  This progress type also indicates that
                      the device has abandoned trying to bootstrap off
                      this bootstrap server.";
                 }
                 enum bootstrap-complete {
                   description
                     "Indicates that the device successfully processed
                      all 'onboarding-information' provided and that it
                      is ready to be managed.  The 'message' node below
                      MAY contain any additional information that the
                      manufacturer thinks might be useful.  After sending
                      this progress type, the device is not expected to
                      access the bootstrap server again.";
                 }
                 enum informational {
                   description
                     "Indicates any additional information not captured
                      by any of the other progress types.  For instance,
                      a message indicating that the device is about to
                      reboot after having installed a boot image could
                      be provided.  The 'message' node below SHOULD
                      contain information that the manufacturer thinks
                      might be useful.";
                 }

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               }
               mandatory true;
               description
                 "The type of progress report provided.";
             }
             leaf message {
               type string;
               description
                 "An optional arbitrary value.";
             }
             container ssh-host-keys {
               when "../progress-type = 'bootstrap-complete'" {
                 description
                   "SSH host keys are only sent when the progress type
                    is 'bootstrap-complete'.";
               }
               description
                 "A list of SSH host keys an NMS may use to authenticate
                  subsequent SSH-based connections to this device (e.g.,
                  netconf-ssh, netconf-ch-ssh).";
               list ssh-host-key {
                 description
                   "An SSH host key an NMS may use to authenticate
                    subsequent SSH-based connections to this device
                    (e.g., netconf-ssh and netconf-ch-ssh).";
                 reference
                   "RFC 4253: The Secure Shell (SSH) Transport Layer
                              Protocol";
                 leaf algorithm {
                   type string;
                   mandatory true;
                   description
                     "The public key algorithm name for this SSH key.

                      Valid values are listed in the 'Public Key Algorithm
                      Names' subregistry of the 'Secure Shell (SSH) Protocol
                      Parameters' registry maintained by IANA.";
                   reference
                     "RFC 4250: The Secure Shell (SSH) Protocol Assigned
                                Numbers
                      IANA URL: <https://www.iana.org/assignments/ssh-para\\
                                meters>
                                ('\\' added for formatting reasons)";
                 }
                 leaf key-data {
                   type binary;
                   mandatory true;
                   description

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                     "The binary public key data for this SSH key, as
                      specified by RFC 4253, Section 6.6; that is:

                        string    certificate or public key format
                                  identifier
                        byte[n]   key/certificate data.";
                   reference
                     "RFC 4253: The Secure Shell (SSH) Transport Layer
                                Protocol";
                 }
               }
             }
             container trust-anchor-certs {
               when "../progress-type = 'bootstrap-complete'" {
                 description
                   "Trust anchors are only sent when the progress type
                    is 'bootstrap-complete'.";
               }
               description
                 "A list of trust anchor certificates an NMS may use to
                  authenticate subsequent certificate-based connections
                  to this device (e.g., restconf-tls, netconf-tls, or
                  even netconf-ssh with X.509 support from RFC 6187).
                  In practice, trust anchors for IDevID certificates do
                  not need to be conveyed using this mechanism.";
               reference
                 "RFC 6187: X.509v3 Certificates for Secure Shell
                            Authentication";
               leaf-list trust-anchor-cert {
                 type cms;
                 description
                   "A CMS structure whose topmost content type MUST be the
                    signed-data content type, as described by Section 5 of
                    RFC 5652.

                    The CMS MUST contain the chain of X.509 certificates
                    needed to authenticate the certificate presented by
                    the device.

                    The CMS MUST contain only a single chain of
                    certificates.  The last certificate in the chain
                    MUST be the issuer for the device's end-entity
                    certificate.

                    In all cases, the chain MUST include a self-signed
                    root certificate.  In the case where the root
                    certificate is itself the issuer of the device's
                    end-entity certificate, only one certificate is

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                    present.

                    This CMS encodes the degenerate form of the SignedData
                    structure that is commonly used to disseminate X.509
                    certificates and revocation objects (RFC 5280).";
                 reference
                   "RFC 5280: Internet X.509 Public Key Infrastructure
                              Certificate and Certificate Revocation List
                              (CRL) Profile
                    RFC 5652: Cryptographic Message Syntax (CMS)";
               }
             }
           }
         }
       }
       <CODE ENDS>

    8.  DHCP Options

       This section defines two DHCP options: one for DHCPv4 and one for
       DHCPv6.  These two options are semantically the same, though
       syntactically different.

    8.1.  DHCPv4 SZTP Redirect Option

       The DHCPv4 SZTP Redirect Option is used to provision the client with
       one or more URIs for bootstrap servers that can be contacted to
       attempt further configuration.

                 0                             1
                 0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
                +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                |   option-code (143)   |     option-length     |
                +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
                .                                               .
                .    bootstrap-server-list (variable length)    .
                .                                               .
                +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                * option-code: OPTION_V4_SZTP_REDIRECT (143)
                * option-length: The option length in octets.
                * bootstrap-server-list: A list of servers for the
                   client to attempt contacting, in order to obtain
                   further bootstrapping data, in the format shown
                   in Section 8.3.

                          DHCPv4 SZTP Redirect Option

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       DHCPv4 Client Behavior

       Clients MAY request the OPTION_V4_SZTP_REDIRECT option by including
       its option code in the Parameter Request List (55) in DHCP request
       messages.

       On receipt of a DHCPv4 Reply message that contains the
       OPTION_V4_SZTP_REDIRECT option, the client processes the response
       according to Section 5.5, with the understanding that the "address"
       and "port" values are encoded in the URIs.

       Any invalid URI entries received in the uri-data field are ignored by
       the client.  If the received OPTION_V4_SZTP_REDIRECT option does not
       contain at least one valid URI entry in the uri-data field, then the
       client MUST discard the option.

       As the list of URIs may exceed the maximum allowed length of a single
       DHCPv4 option (255 octets), the client MUST implement the decoding
       agent behavior described in [RFC3396], to correctly process a URI
       list split across a number of received OPTION_V4_SZTP_REDIRECT option
       instances.

       DHCPv4 Server Behavior

       The DHCPv4 server MAY include a single instance of the
       OPTION_V4_SZTP_REDIRECT option in DHCP messages it sends.  Servers
       MUST NOT send more than one instance of the OPTION_V4_SZTP_REDIRECT
       option.

       The server's DHCP message MUST contain only a single instance of the
       OPTION_V4_SZTP_REDIRECT's 'bootstrap-server-list' field.  However,
       the list of URIs in this field may exceed the maximum allowed length
       of a single DHCPv4 option (per [RFC3396]).

       If the length of 'bootstrap-server-list' is small enough to fit into
       a single instance of OPTION_V4_SZTP_REDIRECT, the server MUST NOT
       send more than one instance of this option.

       If the length of the 'bootstrap-server-list' field is too large to
       fit into a single option, then OPTION_V4_SZTP_REDIRECT MUST be split
       into multiple instances of the option according to the process
       described in [RFC3396].

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    8.2.  DHCPv6 SZTP Redirect Option

       The DHCPv6 SZTP Redirect Option is used to provision the client with
       one or more URIs for bootstrap servers that can be contacted to
       attempt further configuration.

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |       option-code (136)       |          option-length        |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         .           bootstrap-server-list (variable length)             .
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         * option-code: OPTION_V6_SZTP_REDIRECT (136)
         * option-length: The option length in octets.
         * bootstrap-server-list: A list of servers for the client to
           attempt contacting, in order to obtain further bootstrapping
           data, in the format shown in Section 8.3.

                          DHCPv6 SZTP Redirect Option

       DHCPv6 Client Behavior

       Clients MAY request OPTION_V6_SZTP_REDIRECT using the process defined
       in [RFC8415], Sections 18.2.1, 18.2.2, 18.2.4, 18.2.5, 18.2.6, and
       21.7.  As a convenience to the reader, we mention here that the
       client includes requested option codes in the Option Request option.

       On receipt of a DHCPv6 Reply message that contains the
       OPTION_V6_SZTP_REDIRECT option, the client processes the response
       according to Section 5.5, with the understanding that the "address"
       and "port" values are encoded in the URIs.

       Any invalid URI entries received in the uri-data field are ignored by
       the client.  If the received OPTION_V6_SZTP_REDIRECT option does not
       contain at least one valid URI entry in the uri-data field, then the
       client MUST discard the option.

       DHCPv6 Server Behavior

       Section 18.3 of [RFC8415] governs server operation in regard to
       option assignment.  As a convenience to the reader, we mention here
       that the server will send a particular option code only if configured
       with specific values for that option code and if the client requested
       it.

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       The OPTION_V6_SZTP_REDIRECT option is a singleton.  Servers MUST NOT
       send more than one instance of this option.

    8.3.  Common Field Encoding

       Both of the DHCPv4 and DHCPv6 options defined in this section encode
       a list of bootstrap server URIs.  The "URI" structure is a DHCP
       option that can contain multiple URIs (see [RFC7227], Section 5.7).
       Each URI entry in the bootstrap-server-list is structured as follows:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
        |       uri-length              |          URI                  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+

        * uri-length: 2 octets long; specifies the length of the URI data.
        * URI: URI of the SZTP bootstrap server.

       The URI of the SZTP bootstrap server MUST use the "https" URI scheme
       defined in Section 2.7.2 of [RFC7230], and it MUST be in form
       "https://<ip-address-or-hostname>[:<port>]".

    9.  Security Considerations

    9.1.  Clock Sensitivity

       The solution in this document relies on TLS certificates, owner
       certificates, and ownership vouchers, all of which require an
       accurate clock in order to be processed correctly (e.g., to test
       validity dates and revocation status).  Implementations SHOULD ensure
       devices have an accurate clock when shipped from manufacturing
       facilities and take steps to prevent clock tampering.

       If it is not possible to ensure clock accuracy, it is RECOMMENDED
       that implementations disable the aspects of the solution having clock
       sensitivity.  In particular, such implementations should assume that
       TLS certificates, ownership vouchers, and owner certificates never
       expire and are not revocable.  From an ownership voucher perspective,
       manufacturers SHOULD issue a single ownership voucher for the
       lifetime of such devices.

       Implementations SHOULD NOT rely on NTP for time, as NTP is not a
       secure protocol at this time.  Note that there is an IETF document
       that focuses on securing NTP [NTS-NTP].

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    9.2.  Use of IDevID Certificates

       IDevID certificates, as defined in [Std-802.1AR], are RECOMMENDED,
       both for the TLS-level client certificate used by devices when
       connecting to a bootstrap server, as well as for the device identity
       certificate used by owners when encrypting the SZTP bootstrapping
       data artifacts.

    9.3.  Immutable Storage for Trust Anchors

       Devices MUST ensure that all their trust anchor certificates,
       including those for connecting to bootstrap servers and verifying
       ownership vouchers, are protected from external modification.

       It may be necessary to update these certificates over time (e.g., the
       manufacturer wants to delegate trust to a new CA).  It is therefore
       expected that devices MAY update these trust anchors when needed
       through a verifiable process, such as a software upgrade using signed
       software images.

    9.4.  Secure Storage for Long-Lived Private Keys

       Manufacturer-generated device identifiers may have very long
       lifetimes.  For instance, [Std-802.1AR] recommends using the
       "notAfter" value 99991231235959Z in IDevID certificates.  Given the
       long-lived nature of these private keys, it is paramount that they
       are stored so as to resist discovery, such as in a secure
       cryptographic processor (e.g., a trusted platform module (TPM) chip).

    9.5.  Blindly Authenticating a Bootstrap Server

       This document allows a device to blindly authenticate a bootstrap
       server's TLS certificate.  It does so to allow for cases where the
       redirect information may be obtained in an unsecured manner, which is
       desirable to support in some cases.

       To compensate for this, this document requires that devices, when
       connected to an untrusted bootstrap server, assert that data
       downloaded from the server is signed.

    9.6.  Disclosing Information to Untrusted Servers

       This document allows devices to establish connections to untrusted
       bootstrap servers.  However, since the bootstrap server is untrusted,
       it may be under the control of an adversary; therefore, devices
       SHOULD be cautious about the data they send to the bootstrap server
       in such cases.

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       Devices send different data to bootstrap servers at each of the
       protocol layers: TCP, TLS, HTTP, and RESTCONF.

       At the TCP protocol layer, devices may relay their IP address,
       subject to network translations.  Disclosure of this information is
       not considered a security risk.

       At the TLS protocol layer, devices may use a client certificate to
       identify and authenticate themselves to untrusted bootstrap servers.
       At a minimum, the client certificate must disclose the device's
       serial number and may disclose additional information such as the
       device's manufacturer, hardware model, public key, etc.  Knowledge of
       this information may provide an adversary with details needed to
       launch an attack.  It is RECOMMENDED that secrecy of the network
       constituency not be relied on for security.

       At the HTTP protocol layer, devices may use an HTTP authentication
       scheme to identify and authenticate themselves to untrusted bootstrap
       servers.  At a minimum, the authentication scheme must disclose the
       device's serial number and, concerningly, may, depending on the
       authentication mechanism used, reveal a secret that is only supposed
       to be known to the device (e.g., a password).  Devices SHOULD NOT use
       an HTTP authentication scheme (e.g., HTTP Basic) with an untrusted
       bootstrap server that reveals a secret that is only supposed to be
       known to the device.

       At the RESTCONF protocol layer, devices use the "get-bootstrapping-
       data" RPC, but not the "report-progress" RPC, when connected to an
       untrusted bootstrap server.  The "get-bootstrapping-data" RPC allows
       additional input parameters to be passed to the bootstrap server
       (e.g., "os-name", "os-version", and "hw-model").  It is RECOMMENDED
       that devices only pass the "signed-data-preferred" input parameter to
       an untrusted bootstrap server.  While it is okay for a bootstrap
       server to immediately return signed onboarding information, it is
       RECOMMENDED that bootstrap servers instead promote the untrusted
       connection to a trusted connection, as described in Appendix B, thus
       enabling the device to use the "report-progress" RPC while processing
       the onboarding information.

    9.7.  Sequencing Sources of Bootstrapping Data

       For devices supporting more than one source for bootstrapping data,
       no particular sequencing order has to be observed for security
       reasons, as the solution for each source is considered equally
       secure.  However, from a privacy perspective, it is RECOMMENDED that
       devices access local sources before accessing remote sources.

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    9.8.  Safety of Private Keys Used for Trust

       The solution presented in this document enables bootstrapping data to
       be trusted in two ways: through either transport-level security or
       the signing of artifacts.

       When transport-level security (i.e., a trusted bootstrap server) is
       used, the private key for the end-entity certificate must be online
       in order to establish the TLS connection.

       When artifacts are signed, the signing key is required to be online
       only when the bootstrap server is returning a dynamically generated
       signed-data response.  For instance, a bootstrap server, upon
       receiving the "signed-data-preferred" input parameter to the
       "get-bootstrapping-data" RPC, may dynamically generate a response
       that is signed.

       Bootstrap server administrators are RECOMMENDED to follow best
       practices to protect the private key used for any online operation.
       For instance, use of a hardware security module (HSM) is RECOMMENDED.
       If an HSM is not used, frequent private key refreshes are
       RECOMMENDED, assuming all bootstrapping devices have an accurate
       clock (see Section 9.1).

       For best security, it is RECOMMENDED that owners only provide
       bootstrapping data that has been signed (using a protected private
       key) and encrypted (using the device's public key from its secure
       device identity certificate).

    9.9.  Increased Reliance on Manufacturers

       The SZTP bootstrapping protocol presented in this document shifts
       some control of initial configuration away from the rightful owner of
       the device and towards the manufacturer and its delegates.

       The manufacturer maintains the list of well-known bootstrap servers
       its devices will trust.  By design, if no bootstrapping data is found
       via other methods first, the device will try to reach out to the
       well-known bootstrap servers.  There is no mechanism to prevent this
       from occurring other than by using an external firewall to block such
       connections.  Concerns related to trusted bootstrap servers are
       discussed in Section 9.10.

       Similarly, the manufacturer maintains the list of voucher-signing
       authorities its devices will trust.  The voucher-signing authorities
       issue the vouchers that enable a device to trust an owner's domain

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       certificate.  It is vital that manufacturers ensure the integrity of
       these voucher-signing authorities, so as to avoid incorrect
       assignments.

       Operators should be aware that this system assumes that they trust
       all the pre-configured bootstrap servers and voucher-signing
       authorities designated by the manufacturers.  While operators may use
       points in the network to block access to the well-known bootstrap
       servers, operators cannot prevent voucher-signing authorities from
       generating vouchers for their devices.

    9.10.  Concerns with Trusted Bootstrap Servers

       Trusted bootstrap servers, whether well-known or discovered, have the
       potential to cause problems, such as the following.

       o  A trusted bootstrap server that has been compromised may be
          modified to return unsigned data of any sort.  For instance, a
          bootstrap server that is only supposed to return redirect
          information might be modified to return onboarding information.
          Similarly, a bootstrap server that is only supposed to return
          signed data may be modified to return unsigned data.  In both
          cases, the device will accept the response, unaware that it wasn't
          supposed to be any different.  It is RECOMMENDED that maintainers
          of trusted bootstrap servers ensure that their systems are not
          easily compromised and, in case of compromise, have mechanisms in
          place to detect and remediate the compromise as expediently as
          possible.

       o  A trusted bootstrap server hosting data that is either unsigned or
          signed but not encrypted may disclose information to unwanted
          parties (e.g., an administrator of the bootstrap server).  This is
          a privacy issue only, but it could reveal information that might
          be used in a subsequent attack.  Disclosure of redirect
          information has limited exposure (it is just a list of bootstrap
          servers), whereas disclosure of onboarding information could be
          highly revealing (e.g., network topology, firewall policies,
          etc.).  It is RECOMMENDED that operators encrypt the bootstrapping
          data when its contents are considered sensitive, even to the point
          of hiding it from the administrators of the bootstrap server,
          which may be maintained by a third party.

    9.11.  Validity Period for Conveyed Information

       The conveyed information artifact does not specify a validity period.
       For instance, neither redirect information nor onboarding information
       enable "not-before" or "not-after" values to be specified, and
       neither artifact alone can be revoked.

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       For unsigned data provided by an untrusted source of bootstrapping
       data, it is not meaningful to discuss its validity period when the
       information itself has no authenticity and may have come from
       anywhere.

       For unsigned data provided by a trusted source of bootstrapping data
       (i.e., a bootstrap server), the availability of the data is the only
       measure of it being current.  Since the untrusted data comes from a
       trusted source, its current availability is meaningful, and since
       bootstrap servers use TLS, the contents of the exchange cannot be
       modified or replayed.

       For signed data, whether provided by an untrusted or trusted source
       of bootstrapping data, the validity is constrained by the validity of
       both the ownership voucher and owner certificate used to authenticate
       it.

       The ownership voucher's validity is primarily constrained by the
       ownership voucher's "created-on" and "expires-on" nodes.  While
       [RFC8366] recommends short-lived vouchers (see Section 6.1), the
       "expires-on" node may be set to any point in the future or omitted
       altogether to indicate that the voucher never expires.  The ownership
       voucher's validity is secondarily constrained by the manufacturer's
       PKI used to sign the voucher; whilst an ownership voucher cannot be
       revoked directly, the PKI used to sign it may be.

       The owner certificate's validity is primarily constrained by the
       X.509's validity field, the "notBefore" and "notAfter" values, as
       specified by the certificate authority that signed it.  The owner
       certificate's validity is secondarily constrained by the validity of
       the PKI used to sign the voucher.  Owner certificates may be revoked
       directly.

       For owners that wish to have maximum flexibility in their ability to
       specify and constrain the validity of signed data, it is RECOMMENDED
       that a unique owner certificate be created for each signed artifact.
       Not only does this enable a validity period to be specified, for each
       artifact, but it also enables the validity of each artifact to be
       revoked.

    9.12.  Cascading Trust via Redirects

       Redirect information (Section 2.1), by design, instructs a
       bootstrapping device to initiate an HTTPS connection to the specified
       bootstrap servers.

       When the redirect information is trusted, the redirect information
       can encode a trust anchor certificate used by the device to

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       authenticate the TLS end-entity certificate presented by each
       bootstrap server.

       As a result, any compromise in an interaction providing redirect
       information may result in compromise of all subsequent interactions.

    9.13.  Possible Reuse of Private Keys

       This document describes two uses for secure device identity
       certificates.

       The primary use is for when the device authenticates itself to a
       bootstrap server, using its private key for TLS-level client-
       certificate-based authentication.

       A secondary use is for when the device needs to decrypt provided
       bootstrapping artifacts, using its private key to decrypt the data
       or, more precisely, per Section 6 of [RFC5652], decrypt a symmetric
       key used to decrypt the data.

       Section 3.4 of this document allows for the possibility that the same
       secure device identity certificate is utilized for both uses, as
       [Std-802.1AR] states that a DevID certificate MAY have the
       "keyEncipherment" KeyUsage bit, in addition to the "digitalSignature"
       KeyUsage bit, set.

       While it is understood that it is generally frowned upon to reuse
       private keys, this document views such reuse acceptable as there are
       not any known ways to cause a signature made in one context to be
       (mis)interpreted as valid in the other context.

    9.14.  Non-issue with Encrypting Signed Artifacts

       This document specifies the encryption of signed objects, as opposed
       to the signing of encrypted objects, as might be expected given well-
       publicized oracle attacks (e.g., the padding oracle attack).

       This document does not view such attacks as feasible in the context
       of the solution because the decrypted text never leaves the device.

    9.15.  The "ietf-sztp-conveyed-info" YANG Module

       The "ietf-sztp-conveyed-info" module defined in this document defines
       a data structure that is always wrapped by a CMS structure.  When
       accessed by a secure mechanism (e.g., protected by TLS), then the CMS
       structure may be unsigned.  However, when accessed by an insecure
       mechanism (e.g., a removable storage device), the CMS structure must
       be signed, in order for the device to trust it.

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       Implementations should be aware that signed bootstrapping data only
       protects the data from modification and that the content is still
       visible to others.  This doesn't affect security so much as privacy.
       That the contents may be read by unintended parties when accessed by
       insecure mechanisms is considered next.

       The "ietf-sztp-conveyed-info" module defines a top-level "choice"
       statement that declares the content is either redirect-information or
       onboarding-information.  Each of these two cases are now considered.

       When the content of the CMS structure is redirect-information, an
       observer can learn about the bootstrap servers the device is being
       directed to, their IP addresses or hostnames, ports, and trust anchor
       certificates.  Knowledge of this information could provide an
       observer some insight into a network's inner structure.

       When the content of the CMS structure is onboarding-information, an
       observer could learn considerable information about how the device is
       to be provisioned.  This information includes the operating system
       version, initial configuration, and script contents.  This
       information should be considered sensitive, and precautions should be
       taken to protect it (e.g., encrypt the artifact using the device's
       public key).

    9.16.  The "ietf-sztp-bootstrap-server" YANG Module

       The "ietf-sztp-bootstrap-server" module defined in this document
       specifies an API for a RESTCONF [RFC8040].  The lowest RESTCONF layer
       is HTTPS, and the mandatory-to-implement secure transport is TLS
       [RFC8446].

       The NETCONF Access Control Model (NACM) [RFC8341] provides the means
       to restrict access for particular users to a pre-configured subset of
       all available protocol operations and content.

       This module presents no data nodes (only RPCs).  There is no need to
       discuss the sensitivity of data nodes.

       This module defines two RPC operations that may be considered
       sensitive in some network environments.  These are the operations and
       their sensitivity/vulnerability:

       get-bootstrapping-data:  This RPC is used by devices to obtain their
           bootstrapping data.  By design, each device, as identified by its
           authentication credentials (e.g., client certificate), can only
           obtain its own data.  NACM is not needed to further constrain
           access to this RPC.

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       report-progress:  This RPC is used by devices to report their
           bootstrapping progress.  By design, each device, as identified by
           its authentication credentials (e.g., client certificate), can
           only report data for itself.  NACM is not needed to further
           constrain access to this RPC.

    10.  IANA Considerations

    10.1.  The IETF XML Registry

       IANA has registered two URIs in the "ns" subregistry of the "IETF XML
       Registry" [RFC3688] maintained at <https://www.iana.org/assignments/
       xml-registry>.  The following registrations have been made per the
       format in [RFC3688]:

          URI: urn:ietf:params:xml:ns:yang:ietf-sztp-conveyed-info
          Registrant Contact: The NETCONF WG of the IETF.
          XML: N/A, the requested URI is an XML namespace.

          URI: urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server
          Registrant Contact: The NETCONF WG of the IETF.
          XML: N/A, the requested URI is an XML namespace.

    10.2.  The YANG Module Names Registry

       IANA has registered two YANG modules in the "YANG Module Names"
       registry [RFC6020] maintained at <https://www.iana.org/assignments/
       yang-parameters>.  The following registrations have been made per the
       format in [RFC6020]:

          name:      ietf-sztp-conveyed-info
          namespace: urn:ietf:params:xml:ns:yang:ietf-sztp-conveyed-info
          prefix:    sztp-info
          reference: RFC 8572

          name:      ietf-sztp-bootstrap-server
          namespace: urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server
          prefix:    sztp-svr
          reference: RFC 8572

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    10.3.  The SMI Security for S/MIME CMS Content Type Registry

       IANA has registered two subordinate object identifiers in the "SMI
       Security for S/MIME CMS Content Type (1.2.840.113549.1.9.16.1)"
       registry maintained at <https://www.iana.org/assignments/
       smi-numbers>.  The following registrations have been made per the
       format in Section 3.4 of [RFC7107]:

          Decimal   Description                  References
          -------   --------------------------   ----------
          42        id-ct-sztpConveyedInfoXML    RFC 8572
          43        id-ct-sztpConveyedInfoJSON   RFC 8572

       id-ct-sztpConveyedInfoXML indicates that the "conveyed-information"
       is encoded using XML.  id-ct-sztpConveyedInfoJSON indicates that the
       "conveyed-information" is encoded using JSON.

    10.4.  The BOOTP Vendor Extensions and DHCP Options Registry

       IANA has registered one DHCP code point in the "BOOTP Vendor
       Extensions and DHCP Options" registry maintained at
       <https://www.iana.org/assignments/bootp-dhcp-parameters>:

          Tag:         143
          Name:        OPTION_V4_SZTP_REDIRECT
          Data Length: N
          Meaning:     This option provides a list of URIs
                       for SZTP bootstrap servers
          Reference:   RFC 8572

    10.5.  The Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
           Registry

       IANA has registered one DHCP code point in the "Option Codes"
       subregistry of the "Dynamic Host Configuration Protocol for IPv6
       (DHCPv6)" registry maintained at <https://www.iana.org/assignments/
       dhcpv6-parameters>:

          Value:            136
          Description:      OPTION_V6_SZTP_REDIRECT
          Client ORO:       Yes
          Singleton Option: Yes
          Reference:        RFC 8572

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    10.6.  The Service Name and Transport Protocol Port Number Registry

       IANA has registered one service name in the "Service Name and
       Transport Protocol Port Number Registry" [RFC6335] maintained at
       <https://www.iana.org/assignments/service-names-port-numbers>.  The
       following registration has been made per the format in Section 8.1.1
       of [RFC6335]:

         Service Name:            sztp
         Transport Protocol(s):   TCP
         Assignee:                IESG <iesg@ietf.org>
         Contact:                 IETF Chair <chair@ietf.org>
         Description:             This service name is used to construct the
                                  SRV service label "_sztp" for discovering
                                  SZTP bootstrap servers.
         Reference:               RFC 8572
         Port Number:             N/A
         Service Code:            N/A
         Known Unauthorized Uses: N/A
         Assignment Notes:        This protocol uses HTTPS as a substrate.

    10.7.  The Underscored and Globally Scoped DNS Node Names Registry

       IANA has registered one service name in the "Underscored and Globally
       Scoped DNS Node Names" subregistry [RFC8552] of the "Domain Name
       System (DNS) Parameters" registry maintained at
       <https://www.iana.org/assignments/dns-parameters>.  The following
       registration has been made per the format in Section 3 of [RFC8552]:

          RR Type:            TXT
          _NODE NAME:         _sztp
          Reference:          RFC 8572

    11.  References

    11.1.  Normative References

       [ITU.X690.2015]
                  International Telecommunication Union, "Information
                  Technology - ASN.1 encoding rules: Specification of Basic
                  Encoding Rules (BER), Canonical Encoding Rules (CER) and
                  Distinguished Encoding Rules (DER)", ITU-T Recommendation
                  X.690, ISO/IEC 8825-1, August 2015,
                  <https://www.itu.int/rec/T-REC-X.690/>.

       [RFC1035]  Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
                  November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                  specifying the location of services (DNS SRV)", RFC 2782,
                  DOI 10.17487/RFC2782, February 2000,
                  <https://www.rfc-editor.org/info/rfc2782>.

       [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
                  Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
                  DOI 10.17487/RFC3396, November 2002,
                  <https://www.rfc-editor.org/info/rfc3396>.

       [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
                  Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
                  January 2006, <https://www.rfc-editor.org/info/rfc4253>.

       [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                  Housley, R., and W. Polk, "Internet X.509 Public Key
                  Infrastructure Certificate and Certificate Revocation List
                  (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
                  <https://www.rfc-editor.org/info/rfc5280>.

       [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
                  RFC 5652, DOI 10.17487/RFC5652, September 2009,
                  <https://www.rfc-editor.org/info/rfc5652>.

       [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
                  the Network Configuration Protocol (NETCONF)", RFC 6020,
                  DOI 10.17487/RFC6020, October 2010,
                  <https://www.rfc-editor.org/info/rfc6020>.

       [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
                  Verification of Domain-Based Application Service Identity
                  within Internet Public Key Infrastructure Using X.509
                  (PKIX) Certificates in the Context of Transport Layer
                  Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
                  2011, <https://www.rfc-editor.org/info/rfc6125>.

       [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
                  DOI 10.17487/RFC6762, February 2013,
                  <https://www.rfc-editor.org/info/rfc6762>.

       [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
                  RFC 6991, DOI 10.17487/RFC6991, July 2013,
                  <https://www.rfc-editor.org/info/rfc6991>.

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       [RFC7227]  Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
                  S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
                  BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
                  <https://www.rfc-editor.org/info/rfc7227>.

       [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
                  Protocol (HTTP/1.1): Message Syntax and Routing",
                  RFC 7230, DOI 10.17487/RFC7230, June 2014,
                  <https://www.rfc-editor.org/info/rfc7230>.

       [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
                  RFC 7950, DOI 10.17487/RFC7950, August 2016,
                  <https://www.rfc-editor.org/info/rfc7950>.

       [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
                  Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
                  <https://www.rfc-editor.org/info/rfc8040>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

       [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
                  "A Voucher Artifact for Bootstrapping Protocols",
                  RFC 8366, DOI 10.17487/RFC8366, May 2018,
                  <https://www.rfc-editor.org/info/rfc8366>.

       [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
                  Richardson, M., Jiang, S., Lemon, T., and T. Winters,
                  "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
                  RFC 8415, DOI 10.17487/RFC8415, November 2018,
                  <https://www.rfc-editor.org/info/rfc8415>.

       [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,
                  <https://www.rfc-editor.org/info/rfc8552>.

       [Std-802.1AR]
                  IEEE, "IEEE Standard for Local and metropolitan area
                  networks - Secure Device Identity", IEEE 802.1AR.

    11.2.  Informative References

       [NTS-NTP]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and
                  R. Sundblad, "Network Time Security for the Network Time
                  Protocol", Work in Progress, draft-ietf-ntp-using-nts-for-
                  ntp-18, April 2019.

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       [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
                  DOI 10.17487/RFC3688, January 2004,
                  <https://www.rfc-editor.org/info/rfc3688>.

       [RFC4250]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
                  Protocol Assigned Numbers", RFC 4250,
                  DOI 10.17487/RFC4250, January 2006,
                  <https://www.rfc-editor.org/info/rfc4250>.

       [RFC6187]  Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure
                  Shell Authentication", RFC 6187, DOI 10.17487/RFC6187,
                  March 2011, <https://www.rfc-editor.org/info/rfc6187>.

       [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
                  (SHA and SHA-based HMAC and HKDF)", RFC 6234,
                  DOI 10.17487/RFC6234, May 2011,
                  <https://www.rfc-editor.org/info/rfc6234>.

       [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
                  and A. Bierman, Ed., "Network Configuration Protocol
                  (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
                  <https://www.rfc-editor.org/info/rfc6241>.

       [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and
                  S. Cheshire, "Internet Assigned Numbers Authority (IANA)
                  Procedures for the Management of the Service Name and
                  Transport Protocol Port Number Registry", BCP 165,
                  RFC 6335, DOI 10.17487/RFC6335, August 2011,
                  <https://www.rfc-editor.org/info/rfc6335>.

       [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
                  of Named Entities (DANE) Transport Layer Security (TLS)
                  Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
                  2012, <https://www.rfc-editor.org/info/rfc6698>.

       [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
                  Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
                  <https://www.rfc-editor.org/info/rfc6763>.

       [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
                  for DNS (EDNS(0))", STD 75, RFC 6891,
                  DOI 10.17487/RFC6891, April 2013,
                  <https://www.rfc-editor.org/info/rfc6891>.

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       [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
                  Galperin, S., and C. Adams, "X.509 Internet Public Key
                  Infrastructure Online Certificate Status Protocol - OCSP",
                  RFC 6960, DOI 10.17487/RFC6960, June 2013,
                  <https://www.rfc-editor.org/info/rfc6960>.

       [RFC7107]  Housley, R., "Object Identifier Registry for the S/MIME
                  Mail Security Working Group", RFC 7107,
                  DOI 10.17487/RFC7107, January 2014,
                  <https://www.rfc-editor.org/info/rfc7107>.

       [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
                  D. Wessels, "DNS Transport over TCP - Implementation
                  Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
                  <https://www.rfc-editor.org/info/rfc7766>.

       [RFC8071]  Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
                  RFC 8071, DOI 10.17487/RFC8071, February 2017,
                  <https://www.rfc-editor.org/info/rfc8071>.

       [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
                  Interchange Format", STD 90, RFC 8259,
                  DOI 10.17487/RFC8259, December 2017,
                  <https://www.rfc-editor.org/info/rfc8259>.

       [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
                  BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
                  <https://www.rfc-editor.org/info/rfc8340>.

       [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
                  Access Control Model", STD 91, RFC 8341,
                  DOI 10.17487/RFC8341, March 2018,
                  <https://www.rfc-editor.org/info/rfc8341>.

       [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
                  Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
                  <https://www.rfc-editor.org/info/rfc8446>.

       [YANG-CRYPTO-TYPES]
                  Watsen, K. and H. Wang, "Common YANG Data Types for
                  Cryptography", Work in Progress, draft-ietf-netconf-
                  crypto-types-05, March 2019.

       [YANG-TRUST-ANCHORS]
                  Watsen, K., "YANG Data Model for Global Trust Anchors",
                  Work in Progress, draft-ietf-netconf-trust-anchors-03,
                  March 2019.

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    Appendix A.  Example Device Data Model

       This section defines a non-normative data model that enables the
       configuration of SZTP bootstrapping and the discovery of what
       parameters are used by a device's bootstrapping logic.

    A.1.  Data Model Overview

       The following tree diagram provides an overview for the SZTP device
       data model.

        module: example-device-data-model
          +--rw sztp
             +--rw enabled?                          boolean
             +--ro idevid-certificate?               ct:end-entity-cert-cms
             |       {bootstrap-servers}?
             +--ro bootstrap-servers {bootstrap-servers}?
             |  +--ro bootstrap-server* [address]
             |     +--ro address    inet:host
             |     +--ro port?      inet:port-number
             +--ro bootstrap-server-trust-anchors {bootstrap-servers}?
             |  +--ro reference*   ta:pinned-certificates-ref
             +--ro voucher-trust-anchors {signed-data}?
                +--ro reference*   ta:pinned-certificates-ref

       In the above diagram, notice that there is only one configurable
       node: "enabled".  The expectation is that this node would be set to
       "true" in the device's factory default configuration and that it
       would be either set to "false" or deleted when the SZTP bootstrapping
       is longer needed.

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    A.2.  Example Usage

       Following is an instance example for this data model.

       <sztp xmlns="https://example.com/sztp-device-data-model">
         <enabled>true</enabled>
         <idevid-certificate>base64encodedvalue==</idevid-certificate>
         <bootstrap-servers>
           <bootstrap-server>
             <address>sztp1.example.com</address>
             <port>8443</port>
           </bootstrap-server>
           <bootstrap-server>
             <address>sztp2.example.com</address>
             <port>8443</port>
           </bootstrap-server>
           <bootstrap-server>
             <address>sztp3.example.com</address>
             <port>8443</port>
           </bootstrap-server>
         </bootstrap-servers>
         <bootstrap-server-trust-anchors>
           <reference>manufacturers-root-ca-certs</reference>
         </bootstrap-server-trust-anchors>
         <voucher-trust-anchors>
           <reference>manufacturers-root-ca-certs</reference>
         </voucher-trust-anchors>
       </sztp>

    A.3.  YANG Module

       The device model is defined by the YANG module defined in this
       section.

       This module references [Std-802.1AR] and uses data types defined in
       [RFC6991], [YANG-CRYPTO-TYPES], and [YANG-TRUST-ANCHORS].

       module example-device-data-model {
         yang-version 1.1;
         namespace "https://example.com/sztp-device-data-model";
         prefix sztp-ddm;

         import ietf-inet-types {
           prefix inet;
           reference "RFC 6991: Common YANG Data Types";
         }

         import ietf-crypto-types {

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           prefix ct;
           revision-date 2019-03-09;
           description
            "ietf-crypto-types is defined in
             draft-ietf-netconf-crypto-types";
           reference
            "draft-ietf-netconf-crypto-types-05:
               Common YANG Data Types for Cryptography";
         }

         import ietf-trust-anchors {
           prefix ta;
           revision-date 2019-03-09;
           description
            "ietf-trust-anchors is defined in
             draft-ietf-netconf-trust-anchors.";
           reference
            "draft-ietf-netconf-trust-anchors-03:
               YANG Data Model for Global Trust Anchors";
         }

         organization
           "Example Corporation";

         contact
           "Author: Bootstrap Admin <mailto:admin@example.com>";

         description
           "This module defines a data model to enable SZTP
            bootstrapping and discover what parameters are used.
            This module assumes the use of an IDevID certificate,
            as opposed to any other client certificate, or the
            use of an HTTP-based client authentication scheme.";

         revision 2019-04-30 {
           description
             "Initial version";
           reference
             "RFC 8572: Secure Zero Touch Provisioning (SZTP)";
         }

         // features

         feature bootstrap-servers {
           description
             "The device supports bootstrapping off bootstrap servers.";
         }

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         feature signed-data {
           description
             "The device supports bootstrapping off signed data.";
         }

         // protocol accessible nodes

         container sztp {
           description
             "Top-level container for the SZTP data model.";
           leaf enabled {
             type boolean;
             default false;
             description
               "The 'enabled' leaf controls if SZTP bootstrapping is
                enabled or disabled.  The default is 'false' so that, when
                not enabled, which is most of the time, no configuration
                is needed.";
           }
           leaf idevid-certificate {
             if-feature bootstrap-servers;
             type ct:end-entity-cert-cms;
             config false;
             description
               "This CMS structure contains the IEEE 802.1AR
                IDevID certificate itself and all intermediate
                certificates leading up to, and optionally including,
                the manufacturer's well-known trust anchor certificate
                for IDevID certificates.  The well-known trust anchor
                does not have to be a self-signed certificate.";
             reference
               "IEEE 802.1AR:
                  IEEE Standard for Local and metropolitan area
                  networks - Secure Device Identity";
           }
           container bootstrap-servers {
             if-feature bootstrap-servers;
             config false;
             description
               "List of bootstrap servers this device will attempt
                to reach out to when bootstrapping.";
             list bootstrap-server {
               key "address";
               description
                 "A bootstrap server entry.";
               leaf address {
                 type inet:host;
                 mandatory true;

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                 description
                   "The IP address or hostname of the bootstrap server the
                    device should redirect to.";
               }
               leaf port {
                 type inet:port-number;
                 default "443";
                 description
                   "The port number the bootstrap server listens on.  If no
                    port is specified, the IANA-assigned port for 'https'
                    (443) is used.";
               }
             }
           }
           container bootstrap-server-trust-anchors {
             if-feature bootstrap-servers;
             config false;
             description "Container for a list of trust anchor references.";
             leaf-list reference {
               type ta:pinned-certificates-ref;
               description
                 "A reference to a list of pinned certificate authority (CA)
                  certificates that the device uses to validate bootstrap
                  servers with.";
             }
           }
           container voucher-trust-anchors {
             if-feature signed-data;
             config false;
             description "Container for a list of trust anchor references.";
             leaf-list reference {
               type ta:pinned-certificates-ref;
               description
                 "A reference to a list of pinned certificate authority (CA)
                  certificates that the device uses to validate ownership
                  vouchers with.";
             }
           }
         }
       }

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    Appendix B.  Promoting a Connection from Untrusted to Trusted

       The following diagram illustrates a sequence of bootstrapping
       activities that promote an untrusted connection to a bootstrap server
       to a trusted connection to the same bootstrap server.  This enables a
       device to limit the amount of information it might disclose to an
       adversary hosting an untrusted bootstrap server.

                                                             +-----------+
                                                             |Deployment-|
                                                             | Specific  |
       +------+                                              | Bootstrap |
       |Device|                                              |  Server   |
       +------+                                              +-----------+
          |                                                        |
          | 1.  "HTTPS" Request ("signed-data-preferred", nonce)   |
          |------------------------------------------------------->|
          | 2.  "HTTPS" Response (signed redirect information)     |
          |<-------------------------------------------------------|
          |                                                        |
          |                                                        |
          | 3.  HTTPS Request (os-name=xyz, os-version=123, etc.)  |
          |------------------------------------------------------->|
          | 4.  HTTPS Response (unsigned onboarding information    |
          |<-------------------------------------------------------|
          |                                                        |

       The interactions in the above diagram are described below.

       1.  The device initiates an untrusted connection to a bootstrap
           server, as is indicated by putting "HTTPS" in double quotes
           above.  It is still an HTTPS connection, but the device is unable
           to authenticate the bootstrap server's TLS certificate.  Because
           the device is unable to trust the bootstrap server, it sends the
           "signed-data-preferred" input parameter, and optionally also the
           "nonce" input parameter, in the "get-bootstrapping-data" RPC.
           The "signed-data-preferred" parameter informs the bootstrap
           server that the device does not trust it and may be holding back
           some additional input parameters from the server (e.g., other
           input parameters, progress reports, etc.).  The "nonce" input
           parameter enables the bootstrap server to dynamically obtain an
           ownership voucher from a Manufacturer Authorized Signing
           Authority (MASA), which may be important for devices that do not
           have a reliable clock.

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       2.  The bootstrap server, seeing the "signed-data-preferred" input
           parameter, knows that it can send either unsigned redirect
           information or signed data of any type.  But, in this case, the
           bootstrap server has the ability to sign data and chooses to
           respond with signed redirect information, not signed onboarding
           information as might be expected, securely redirecting the device
           back to it again.  Not displayed but, if the "nonce" input
           parameter was passed, the bootstrap server could dynamically
           connect to a MASA and download a voucher having the nonce value
           in it.  Details regarding a protocol enabling this integration is
           outside the scope of this document.

       3.  Upon validating the signed redirect information, the device
           establishes a secure connection to the bootstrap server.
           Unbeknownst to the device, it is the same bootstrap server it was
           connected to previously, but because the device is able to
           authenticate the bootstrap server this time, it sends its normal
           "get-bootstrapping-data" request (i.e., with additional input
           parameters) as well as its progress reports (not depicted).

       4.  This time, because the "signed-data-preferred" parameter was not
           passed, having access to all of the device's input parameters,
           the bootstrap server returns, in this example, unsigned
           onboarding information to the device.  Note also that, because
           the bootstrap server is now trusted, the device will send
           progress reports to the server.

    Appendix C.  Workflow Overview

       The solution presented in this document is conceptualized to be
       composed of the non-normative workflows described in this section.
       Implementation details are expected to vary.  Each diagram is
       followed by a detailed description of the steps presented in the
       diagram, with further explanation on how implementations may vary.

    C.1.  Enrollment and Ordering Devices

       The following diagram illustrates key interactions that may occur
       from when a prospective owner enrolls in a manufacturer's SZTP
       program to when the manufacturer ships devices for an order placed by
       the prospective owner.

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                                      +-----------+
       +------------+                 |Prospective|                    +---+
       |Manufacturer|                 |   Owner   |                    |NMS|
       +------------+                 +-----------+                    +---+
             |                              |                            |
             |                              |                            |
             |  1. initiate enrollment      |                            |
             #<-----------------------------|                            |
             #                              |                            |
             #                              |                            |
             #     IDevID trust anchor      |                            |
             #----------------------------->#  set IDevID trust anchor   |
             #                              #--------------------------->|
             #                              |                            |
             #     bootstrap server         |                            |
             #     account credentials      |                            |
             #----------------------------->#  set credentials           |
             |                              #--------------------------->|
             |                              |                            |
             |                              |                            |
             |  2. set owner certificate trust anchor                    |
             |<----------------------------------------------------------|
             |                              |                            |
             |                              |                            |
             |  3. place device order       |                            |
             |<-----------------------------#  model devices             |
             |                              #--------------------------->|
             |                              |                            |
             |  4. ship devices and send    |                            |
             |     device identifiers and   |                            |
             |     ownership vouchers       |                            |
             |----------------------------->#  set device identifiers    |
             |                              #  and ownership vouchers    |
             |                              #--------------------------->|
             |                              |                            |

       Each numbered item below corresponds to a numbered item in the
       diagram above.

       1.  A prospective owner of a manufacturer's devices initiates an
           enrollment process with the manufacturer.  This process includes
           the following:

           *  Regardless of how the prospective owner intends to bootstrap
              their devices, they will always obtain from the manufacturer
              the trust anchor certificate for the IDevID certificates.
              This certificate is installed on the prospective owner's NMS

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              so that the NMS can authenticate the IDevID certificates when
              they are presented to subsequent steps.

           *  If the manufacturer hosts an Internet-based bootstrap server
              (e.g., a redirect server) such as described in Section 4.4,
              then credentials necessary to configure the bootstrap server
              would be provided to the prospective owner.  If the bootstrap
              server is configurable through an API (outside the scope of
              this document), then the credentials might be installed on the
              prospective owner's NMS so that the NMS can subsequently
              configure the manufacturer-hosted bootstrap server directly.

       2.  If the manufacturer's devices are able to validate signed data
           (Section 5.4), and assuming that the prospective owner's NMS is
           able to prepare and sign the bootstrapping data itself, the
           prospective owner's NMS might set a trust anchor certificate onto
           the manufacturer's bootstrap server, using the credentials
           provided in the previous step.  This certificate is the trust
           anchor certificate that the prospective owner would like the
           manufacturer to place into the ownership vouchers it generates,
           thereby enabling devices to trust the owner's owner certificate.
           How this trust anchor certificate is used to enable devices to
           validate signed bootstrapping data is described in Section 5.4.

       3.  Some time later, the prospective owner places an order with the
           manufacturer, perhaps with a special flag checked for SZTP
           handling.  At this time, or perhaps before placing the order, the
           owner may model the devices in their NMS, creating virtual
           objects for the devices with no real-world device associations.
           For instance, the model can be used to simulate the device's
           location in the network and the configuration it should have when
           fully operational.

       4.  When the manufacturer fulfills the order, shipping the devices to
           their intended locations, they may notify the owner of the
           devices' serial numbers and shipping destinations, which the
           owner may use to stage the network for when the devices power on.
           Additionally, the manufacturer may send one or more ownership
           vouchers, cryptographically assigning ownership of those devices
           to the owner.  The owner may set this information on their NMS,
           perhaps binding specific modeled devices to the serial numbers
           and ownership vouchers.

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    C.2.  Owner Stages the Network for Bootstrap

       The following diagram illustrates how an owner might stage the
       network for bootstrapping devices.

                 +-----------+ +-------------+
                 |Deployment-| |Manufacturer-| +------+ +------+
                 | Specific  | |   Hosted    | | Local| | Local| +---------+
           +---+ | Bootstrap | |  Bootstrap  | |  DNS | | DHCP | |Removable|
           |NMS| |  Server   | |   Server    | |Server| |Server| | Storage |
           +---+ +-----------+ +-------------+ +------+ +------+ +---------+
             |        |             |            |        |         |
     1.      |        |             |            |        |         |
     activate|        |             |            |        |         |
     modeled |        |             |            |        |         |
     device  |        |             |            |        |         |
     ------->|        |             |            |        |         |
             | 2. (optional)        |            |        |         |
             |    configure         |            |        |         |
             |    bootstrap         |            |        |         |
             |    server            |            |        |         |
             |------->|             |            |        |         |
             |        |             |            |        |         |
             | 3. (optional) configure           |        |         |
             |    bootstrap server  |            |        |         |
             |--------------------->|            |        |         |
             |        |             |            |        |         |
             |        |             |            |        |         |
             | 4. (optional) configure DNS server|        |         |
             |---------------------------------->|        |         |
             |        |             |            |        |         |
             |        |             |            |        |         |
             | 5. (optional) configure DHCP server        |         |
             |------------------------------------------->|         |
             |        |             |            |        |         |
             |        |             |            |        |         |
             | 6. (optional) store bootstrapping artifacts on media |
             |----------------------------------------------------->|
             |        |             |            |        |         |
             |        |             |            |        |         |

       Each numbered item below corresponds to a numbered item in the
       diagram above.

       1.  Having previously modeled the devices, including setting their
           fully operational configurations and associating device serial
           numbers and (optionally) ownership vouchers, the owner might
           "activate" one or more modeled devices.  That is, the owner tells

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           the NMS to perform the steps necessary to prepare for when the
           real-world devices power up and initiate the bootstrapping
           process.  Note that, in some deployments, this step might be
           combined with the last step from the previous workflow.  Here, it
           is depicted that an NMS performs the steps, but they may be
           performed manually or through some other mechanism.

       2.  If it is desired to use a deployment-specific bootstrap server,
           it must be configured to provide the bootstrapping data for the
           specific devices.  Configuring the bootstrap server may occur via
           a programmatic API not defined by this document.  Illustrated
           here as an external component, the bootstrap server may be
           implemented as an internal component of the NMS itself.

       3.  If it is desired to use a manufacturer-hosted bootstrap server,
           it must be configured to provide the bootstrapping data for the
           specific devices.  The configuration must be either redirect or
           onboarding information.  That is, the manufacturer-hosted
           bootstrap server will either redirect the device to another
           bootstrap server or provide the device with the onboarding
           information itself.  The types of bootstrapping data the
           manufacturer-hosted bootstrap server supports may vary by
           implementation; some implementations may support only redirect
           information or only onboarding information, while others may
           support both redirect and onboarding information.  Configuring
           the bootstrap server may occur via a programmatic API not defined
           by this document.

       4.  If it is desired to use a DNS server to supply bootstrapping
           data, a DNS server needs to be configured.  If multicast DNS is
           desired, then the DNS server must reside on the local network;
           otherwise, the DNS server may reside on a remote network.  Please
           see Section 4.2 for more information about how to configure DNS
           servers.  Configuring the DNS server may occur via a programmatic
           API not defined by this document.

       5.  If it is desired to use a DHCP server to supply bootstrapping
           data, a DHCP server needs to be configured.  The DHCP server may
           be accessed directly or via a DHCP relay.  Please see Section 4.3
           for more information about how to configure DHCP servers.
           Configuring the DHCP server may occur via a programmatic API not
           defined by this document.

       6.  If it is desired to use a removable storage device (e.g., a USB
           flash drive) to supply bootstrapping data, the data would need to
           be placed onto it.  Please see Section 4.1 for more information
           about how to configure a removable storage device.

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    C.3.  Device Powers On

       The following diagram illustrates the sequence of activities that
       occur when a device powers on.

                                                        +-----------+
                                         +-----------+  |Deployment-|
                                         | Source of |  | Specific  |
      +------+                           | Bootstrap |  | Bootstrap |  +---+
      |Device|                           |   Data    |  |  Server   |  |NMS|
      +------+                           +-----------+  +-----------+  +---+
         |                                     |              |          |
         |                                     |              |          |
         | 1. if SZTP bootstrap service        |              |          |
         |    is not enabled, then exit.       |              |          |
         |                                     |              |          |
         | 2. for each source supported, check |              |          |
         |    for bootstrapping data.          |              |          |
         |------------------------------------>|              |          |
         |                                     |              |          |
         | 3. if onboarding information is     |              |          |
         |    found, initialize self and, only |              |          |
         |    if source is a trusted bootstrap |              |          |
         |    server, send progress reports.   |              |          |
         |------------------------------------>#              |          |
         |                                     # webhook      |          |
         |                                     #------------------------>|
         |                                                    |          |
         | 4. else, if redirect information is found, for     |          |
         |    each bootstrap server specified, check for data.|          |
         |-+------------------------------------------------->|          |
         | |                                                  |          |
         | | if more redirect information is found, recurse   |          |
         | | (not depicted); else, if onboarding information  |          |
         | | is found, initialize self and post progress      |          |
         | | reports.                                         |          |
         | +------------------------------------------------->#          |
         |                                                    # webhook  |
         |                                                    #--------->|
         |
         | 5. retry sources and/or wait for manual provisioning.
         |

       The interactions in the above diagram are described below.

       1.  Upon power being applied, the device checks to see if SZTP
           bootstrapping is configured, such as must be the case when
           running its "factory default" configuration.  If SZTP

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           bootstrapping is not configured, then the bootstrapping logic
           exits and none of the following interactions occur.

       2.  For each source of bootstrapping data the device supports,
           preferably in order of closeness to the device (e.g., removable
           storage before Internet-based servers), the device checks to see
           if there is any bootstrapping data for it there.

       3.  If onboarding information is found, the device initializes itself
           accordingly (e.g., installing a boot image and committing an
           initial configuration).  If the source is a bootstrap server, and
           the bootstrap server can be trusted (i.e., TLS-level
           authentication), the device also sends progress reports to the
           bootstrap server.

           *  The contents of the initial configuration should configure an
              administrator account on the device (e.g., username, SSH
              public key, etc.), should configure the device to either
              listen for NETCONF or RESTCONF connections or initiate call
              home connections [RFC8071], and should disable the SZTP
              bootstrapping service (e.g., the "enabled" leaf in data model
              presented in Appendix A).

           *  If the bootstrap server supports forwarding device progress
              reports to external systems (e.g., via a webhook), a
              "bootstrap-complete" progress report (Section 7.3) informs the
              external system to know when it can, for instance, initiate a
              connection to the device.  To support this scenario further,
              the "bootstrap-complete" progress report may also relay the
              device's SSH host keys and/or TLS certificates, which the
              external system can use to authenticate subsequent connections
              to the device.

           If the device successfully completes the bootstrapping process,
           it exits the bootstrapping logic without considering any
           additional sources of bootstrapping data.

       4.  Otherwise, if redirect information is found, the device iterates
           through the list of specified bootstrap servers, checking to see
           if the bootstrap server has bootstrapping data for the device.
           If the bootstrap server returns more redirect information, then
           the device processes it recursively.  Otherwise, if the bootstrap
           server returns onboarding information, the device processes it
           following the description provided in (3) above.

       5.  After having tried all supported sources of bootstrapping data,
           the device may retry again all the sources and/or provide
           manageability interfaces for manual configuration (e.g., CLI,

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           HTTP, NETCONF, etc.).  If manual configuration is allowed, and
           such configuration is provided, the configuration should also
           disable the SZTP bootstrapping service, as the need for
           bootstrapping would no longer be present.

    Acknowledgements

       The authors would like to thank the following for lively discussions
       on list and in the halls (ordered by last name): Michael Behringer,
       Martin Bjorklund, Dean Bogdanovic, Joe Clarke, Dave Crocker, Toerless
       Eckert, Stephen Farrell, Stephen Hanna, Wes Hardaker, David
       Harrington, Benjamin Kaduk, Radek Krejci, Suresh Krishnan, Mirja
       Kuehlewind, David Mandelberg, Alexey Melnikov, Russ Mundy, Reinaldo
       Penno, Randy Presuhn, Max Pritikin, Michael Richardson, Adam Roach,
       Juergen Schoenwaelder, and Phil Shafer.

       Special thanks goes to Steve Hanna, Russ Mundy, and Wes Hardaker for
       brainstorming the original solution during the IETF 87 meeting in
       Berlin.

    Authors' Addresses

       Kent Watsen
       Watsen Networks

       Email: kent+ietf@watsen.net

       Ian Farrer
       Deutsche Telekom AG

       Email: ian.farrer@telekom.de

       Mikael Abrahamsson
       T-Systems

       Email: mikael.abrahamsson@t-systems.se

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