Internet Engineering Task Force (IETF)
Request for Comments: 8572
Category: Standards Track
ISSN: 2070-1721

K. Watsen
Watsen Networks
I. Farrer
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.

Copyright Notice

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.

   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
   
     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

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

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.

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.
   
   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.
   
   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.

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.

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.

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

(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

(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.

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):

   +---------------------+---------------+--------------+--------------+
   | 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.

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).

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)

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].
      
      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

[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.

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.

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.

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

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.

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.

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.

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.

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.

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

(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.

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.

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.

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

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.

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 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.

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

(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 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.

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.

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,

           "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=="
     }
   }

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.

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)
         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.

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;
              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 {
    
                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.

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>

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

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>

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>

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

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)').

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)";
     }
     
     // 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
              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.
       
              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
     
               "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 {
               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 {
               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
                  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
                 "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.";
             }
       
           }
           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

"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 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

   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].

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.

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].

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.

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.

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 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.

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.

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.

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 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.

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.
   
   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

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

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>.
   
   [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>.
   
   [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.
   
   [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>.
   
   [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.

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.

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 {
       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.";
     }
     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;
   
             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.";
         }
       }
     }
   }

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.

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.

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.

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).

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.

                                  +-----------+
   +------------+                 |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.

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 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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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,

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

Secure Zero Touch Provisioning (SZTP)

Abstract

Status of This Memo

Copyright Notice

Table of Contents

1. Introduction

1.1. Use Cases

1.2. Terminology

1.3. Requirements Language

1.4. Tree Diagrams

2. Types of Conveyed Information

2.1. Redirect Information

2.2. Onboarding Information

3. Artifacts

3.1. Conveyed Information

3.2. Owner Certificate

3.3. Ownership Voucher

3.4. Artifact Encryption

3.5. Artifact Groupings

4. Sources of Bootstrapping Data

4.1. Removable Storage

4.2. DNS Server

4.2.1. DNS Queries

4.2.2. DNS Response for Device-Specific Queries

4.2.3. DNS Response for Device-Independent Queries

4.2.4. Size of Signed Data

4.3. DHCP Server

4.4. Bootstrap Server

5. Device Details

5.1. Initial State

5.2. Boot Sequence

5.3. Processing a Source of Bootstrapping Data

5.4. Validating Signed Data

5.5. Processing Redirect Information

5.6. Processing Onboarding Information

6. The Conveyed Information Data Model

6.1. Data Model Overview

6.2. Example Usage

6.3. YANG Module

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.

7. The SZTP Bootstrap Server API

7.1. API Overview

7.2. Example Usage

7.3. YANG Module

8. DHCP Options

8.1. DHCPv4 SZTP Redirect Option

DHCPv4 SZTP Redirect Option

8.2. DHCPv6 SZTP Redirect Option

DHCPv6 SZTP Redirect Option

8.3. Common Field Encoding

9. Security Considerations

9.1. Clock Sensitivity

9.2. Use of IDevID Certificates

9.3. Immutable Storage for Trust Anchors

9.4. Secure Storage for Long-Lived Private Keys

9.5. Blindly Authenticating a Bootstrap Server

9.6. Disclosing Information to Untrusted Servers

9.7. Sequencing Sources of Bootstrapping Data

9.8. Safety of Private Keys Used for Trust

9.9. Increased Reliance on Manufacturers

9.10. Concerns with Trusted Bootstrap Servers

9.11. Validity Period for Conveyed Information

9.12. Cascading Trust via Redirects

9.13. Possible Reuse of Private Keys

9.14. Non-issue with Encrypting Signed Artifacts

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

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

10. IANA Considerations

10.1. The IETF XML Registry

10.2. The YANG Module Names Registry

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

10.4. The BOOTP Vendor Extensions and DHCP Options Registry

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

Registry

10.6. The Service Name and Transport Protocol Port Number Registry

10.7. The Underscored and Globally Scoped DNS Node Names Registry

11. References

11.1. Normative References