drip                                                             S. Card
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 10 January 2022                                    R. Moskowitz
                                                          HTT Consulting
                                                        S. Zhao (Editor)
                                                               A. Gurtov
                                                    Linköping University
                                                             9 July 2021

        Drone Remote Identification Protocol (DRIP) Architecture


   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus RID-related communications.  This architecture
   adheres to the requirements listed in the DRIP Requirements document.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 10 January 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview of Unmanned Aircraft System (UAS) Remote ID (RID)
           and Standardization . . . . . . . . . . . . . . . . . . .   3
     1.2.  Overview of Types of UAS Remote ID  . . . . . . . . . . .   4
       1.2.1.  Broadcast RID . . . . . . . . . . . . . . . . . . . .   4
       1.2.2.  Network RID . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Overview of USS Interoperability  . . . . . . . . . . . .   6
     1.4.  Overview of DRIP Architecture . . . . . . . . . . . . . .   7
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  Definitions and Abbreviations . . . . . . . . . . . . . . . .   9
     3.1.  Additional Definitions  . . . . . . . . . . . . . . . . .   9
     3.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Claims, Assertions, Attestations, and Certificates  . . .  10
   4.  HHIT as the Primary DRIP Entity Identifier  . . . . . . . . .  11
     4.1.  UAS Remote Identifiers Problem Space  . . . . . . . . . .  11
     4.2.  HIT as A Trustworthy DRIP Entity Identifier . . . . . . .  12
     4.3.  HHIT for DRIP Identifier Registration and Lookup  . . . .  14
     4.4.  HHIT for DRIP Identifier Cryptographic  . . . . . . . . .  14
   5.  DRIP Identifier Registration and Registries . . . . . . . . .  14
     5.1.  Public Information Registry . . . . . . . . . . . . . . .  15
       5.1.1.  Background  . . . . . . . . . . . . . . . . . . . . .  15
       5.1.2.  Proposed Approach . . . . . . . . . . . . . . . . . .  15
     5.2.  Private Information Registry  . . . . . . . . . . . . . .  15
       5.2.1.  Background  . . . . . . . . . . . . . . . . . . . . .  15
       5.2.2.  Proposed Approach . . . . . . . . . . . . . . . . . .  16
   6.  Harvesting Broadcast Remote ID messages for UTM Inclusion . .  16
     6.1.  The CS-RID Finder . . . . . . . . . . . . . . . . . . . .  17
     6.2.  The CS-RID SDSP . . . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Consideration  . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  Privacy & Transparency Considerations . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Overview of Unmanned Aircraft Systems (UAS) Traffic
           Management (UTM)  . . . . . . . . . . . . . . . . . . . .  21
     A.1.  Operation Concept . . . . . . . . . . . . . . . . . . . .  21
     A.2.  UAS Service Supplier (USS)  . . . . . . . . . . . . . . .  22
     A.3.  UTM Use Cases for UAS Operations  . . . . . . . . . . . .  22
   Appendix B.  Automatic Dependent Surveillance Broadcast
           (ADS-B) . . . . . . . . . . . . . . . . . . . . . . . . .  23

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   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus RID-related communications.  The architecture takes
   into account both current (including proposed) regulations and non-
   IETF technical standards.

   The architecture adheres to the requirements listed in the DRIP
   Requirements document [I-D.ietf-drip-reqs].

1.1.  Overview of Unmanned Aircraft System (UAS) Remote ID (RID) and

   CAAs currently promulgate performance-based regulations that do not
   specify techniques, but rather cite industry consensus technical
   standards as acceptable means of compliance.

   UAS Remote Identification (RID) is an application enabler for a UAS
   to be identified by Unmanned Aircraft Systems Traffic Management
   (UTM) and UAS Service Supplier (USS) (Appendix A) or third parties
   entities such as law enforcement.  Many considerations (e.g., safety)
   dictate that UAS be remotely identifiable.  Civil Aviation
   Authorities (CAAs) worldwide are mandating UAS RID.  For example, the
   European Union Aviation Safety Agency (EASA) has published
   [Delegated] and [Implementing] Regulations.

   Federal Aviation Administration (FAA)

      The FAA published a Notice of Proposed Rule Making [NPRM] in 2019
      and whereafter published the "Final Rule" in 2021 [FAA_RID].  In
      FAA's final rule, it is clearly stated that Automatic Dependent
      Surveillance Broadcast (ADS-B) Out and transponders can not be
      used to serve the purpose of an remote identification.  More
      details about ADS-B can be found in Appendix B.

   American Society for Testing and Materials (ASTM)

      ASTM International, Technical Committee F38 (UAS), Subcommittee
      F38.02 (Aircraft Operations), Work Item WK65041, developed the
      ASTM [F3411-19] Standard Specification for Remote ID and Tracking.

      ASTM defines one set of RID information and two means, MAC-layer
      broadcast and IP-layer network, of communicating it.  If an UAS
      uses both communication methods, the same information must be

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      provided via both means.  [F3411-19] is cited by FAA in its RID
      final rule [FAA_RID] as "a potential means of compliance" to a
      Remote ID rule.

   The 3rd Generation Partnership Project (3GPP)

      With release 16, the 3GPP completed the UAS RID requirement study
      [TS-22.825] and proposed a set of use cases in the mobile network
      and the services that can be offered based on RID.  Release 17
      specification focuses on enhanced UAS service requirements and
      provides the protocol and application architecture support that
      will be applicable for both 4G and 5G networks.

1.2.  Overview of Types of UAS Remote ID

1.2.1.  Broadcast RID

   A set of RID messages are defined for direct, one-way, broadcast
   transmissions from the UA over Bluetooth or Wi-Fi.  These are
   currently defined as MAC-Layer messages.  Internet (or other Wide
   Area Network) connectivity is only needed for UAS registry
   information lookup by Observers using the directly received UAS ID.
   Broadcast RID should be functionally usable in situations with no
   Internet connectivity.

   The minimum Broadcast RID data flow is illustrated in Figure 1.

                  x x  UA
                   |     app messages directly over
                   |     one-way RF data link (no IP)
                   x x   Observer's device (e.g. smartphone)
                 x   x

                                  Figure 1

   With queries sent over the Internet using harvested RID (see
   Section 6), the Observer may gain more information about those
   visible UAS" is only true if the locally observed UAS is (or very

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   recently was) observed somewhere else; harvesting RID is not so much
   about learning more about directly observed nearby UAS as it is about
   surveillance of areas too large for local direct visual observation &
   direct RF link based ID (e.g., an entire air force base, or even
   larger, a national forest)

1.2.2.  Network RID

   A RID data dictionary and data flow for Network RID are defined in
   [F3411-19].  This data flow is emitted from an UAS via unspecified
   means (but at least in part over the Internet) to a Network Remote ID
   Service Provider (Net-RID SP).  A Net-RID SP provides the RID data to
   Network Remote ID Display Providers (Net-RID DP).  It is the Net-RID
   DP that responds to queries from Network Remote ID Observers
   (expected typically, but not specified exclusively, to be web-based)
   specifying airspace volumes of interest.  Network RID depends upon
   connectivity, in several segments, via the Internet, from the UAS to
   the Observer.

      Editor-note 1: + list all the segments mentioned above + specify
      how DRIP provide solutions for each segment

   The mimunum Network RID data flow is illustrated in Figure 2:

               x x  UA
               xxxxx       ********************
                |   \    *                ------*---+------------+
                |    \   *              /       *  | NET_RID_SP |
                |     \  * ------------/    +---*--+------------+
                | RF   \ */                 |   *
                |        *      INTERNET    |   *  +------------+
                |       /*                  +---*--| NET_RID_DP |
                |      / *                  +---*--+------------+
                +     /   *                 |   *
                 x   /     *****************|***      x
               xxxxx                        |       xxxxx
                 x                          +-------  x
                 x                                    x
                x x   Operator (GCS)      Observer   x x
               x   x                                x   x

                                  Figure 2

   Command and Control (C2) must flow from the GCS to the UA via some
   path, currently (in the year of 2021) typically a direct RF link, but
   with increasing beyond Visual Line of Sight (BVLOS) operations
   expected often to be wireless links at either end with the Internet

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      Editor-note 2: Explain the difference with wireless and RF link
      includes what are the end entities, usages for each transport

   For all but the simplest hobby aircraft, telemetry (at least position
   and heading) flows from the UA to the GCS via some path, typically
   the reverse of the C2 path.  Thus, RID information pertaining to both
   the GCS and the UA can be sent, by whichever has Internet
   connectivity, to the Net-RID SP, typically the USS managing the UAS

      Editor-note 3: Does all UAS support telemetry? explain what are
      simplsest hobby aircraft vs UAS in general.  Is it necessary to
      keep "For all but the simplest hobby aircraft"?

   The Net-RID SP forwards RID information via the Internet to
   subscribed Net-RID DP, typically USS.  Subscribed Net-RID DP forward
   RID information via the Internet to subscribed Observer devices.
   Regulations require and [F3411-19] describes RID data elements that
   must be transported end-to-end from the UAS to the subscribed
   Observer devices.

   [F3411-19] prescribes the protocols only between the Net-RID SP, Net-
   RID DP, and the Discovery and Synchronization Service (DSS).  DRIP
   may also address standardization of protocols between the UA and GCS,
   between the UAS and the Net-RID SP, and/or between the Net-RID DP and
   Observer devices.

      Editor-note 4: "DRIP may also..." Specify what protocol DRIP can
      or will standardize.

         Informative note: Neither link layer protocols nor the use of
         links (e.g., the link often existing between the GCS and the
         UA) for any purpose other than carriage of RID information is
         in the scope of [F3411-19] Network RID.

1.3.  Overview of USS Interoperability

   With Net-RID, there is direct communication between the UAS and its
   USS.  With Broadcast-RID, the UAS Operator has either pre-filed a 4D
   space volume for USS operational knowledge and/or Observers can be
   providing information about observed UA to a USS.  USS exchange
   information via a Discovery and Synchronization Service (DSS) so all
   USS collectively have knowledge about all activities in a 4D

   The interactions among Observer, UA, and USS are shown in Figure 3.

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                               | Observer |
                              /            \
                             /              \
                      +-----+                +-----+
                      | UAS1 |                | UAS2 |
                      +-----+                +-----+
                             \              /
                              \            /
                               | Internet |
                              /            \
                             /              \
                       +-------+           +-------+
                       | USS1 | <-------> | USS2 |
                       +-------+           +-------+
                                \         /
                                 \       /
                                 |  DSS |

                                  Figure 3

1.4.  Overview of DRIP Architecture

   The requirements document [I-D.ietf-drip-reqs] provides an extended
   introduction to the problem space and use cases.  Only a brief
   summary of that introduction is restated here as context, with
   reference to the general UAS RID usage scenarios shown in Figure 4.

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         General      x                           x     Public
         Public     xxxxx                       xxxxx   Safety
         Observer     x                           x     Observer
                      x                           x
                     x x ---------+  +---------- x x
                    x   x         |  |          x   x
                                  |  |
            UA1 x x               |  |  +------------ x x UA2
               xxxxx              |  |  |            xxxxx
                  |               +  +  +              |
                  |            xxxxxxxxxx              |
                  |           x          x             |
                  +----------+x Internet x+------------+
       UA1        |           x          x             |       UA1
      Pilot     x |            xxxxxxxxxx              | x    Pilot
     Operator  xxxxx              + + +                xxxxx Operator
      GCS1      x                 | | |                  x    GCS2
                x                 | | |                  x
               x x                | | |                 x x
              x   x               | | |                x   x
                                  | | |
                +----------+      | | |       +----------+
                |          |------+ | +-------|          |
                | Public   |        |         | Private  |
                | Registry |     +-----+      | Registry |
                |          |     | DNS |      |          |
                +----------+     +-----+      +----------+

                                  Figure 4

   DRIP is meant to leverage existing Internet resources (standard
   protocols, services, infrastructures, and business models) to meet
   UAS RID and closely related needs.  DRIP will specify how to apply
   IETF standards, complementing [F3411-19] and other external
   standards, to satisfy UAS RID requirements.

   This document outlines the UAS RID architecture.  This includes
   presenting the gaps between the CAAs' Concepts of Operations and
   [F3411-19] as it relates to the use of Internet technologies and UA
   direct RF communications.  Issues include, but are not limited to:

      -  Design of trustworthy remote ID and trust in RID messages
         (Section 4)

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      -  Mechanisms to leverage Domain Name System (including DNS:
         [RFC1034]), Extensible Provisioning Protocol (EPP [RFC5731])
         and Registration Data Access Protocol (RDAP) ([RFC7482]) for
         publishing public and private information (see Section 5.1 and
         Section 5.2).

      -  Harvesting broadcast RID messages for UTM inclusion
         (Section 6).

      -  Privacy in RID messages (PII protection) (Section 9).

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 above.

3.  Definitions and Abbreviations

      Editor-note 13: 1) should we merge Section 2 and Section 3 2) how
      should we list abbr in the Arch?  Previous WG agreement is that
      all the DRIP terms shall be defined in -reqs, which may or may not
      be used in -reqs itself, but other documents such as Arch-. And
      arch- can list terms when they are used in the arch- only.  So
      which is which ?

3.1.  Additional Definitions

   This document uses terms defined in [I-D.ietf-drip-reqs].

3.2.  Abbreviations

   ADS-B:      Automatic Dependent Surveillance Broadcast

   DSS:        Discovery & Synchronization Service

   EdDSA:      Edwards-Curve Digital Signature Algorithm

   GCS:        Ground Control Station

   HHIT:       Hierarchical HIT Registries

   HIP:        Host Identity Protocol

   HIT:        Host Identity Tag

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   RID:        Remote ID

   Net-RID SP: Network RID Service Provider

   Net-RID DP: Network RID Display Provider.

   PII:        Personally Identifiable Information

   RF:         Radio Frequency

   SDSP:       Supplemental Data Service Provider

   UA:         Unmanned Aircraft

   UAS:        Unmanned Aircraft System

   USS:        UAS Service Supplier

   UTM:        UAS Traffic Management

3.3.  Claims, Assertions, Attestations, and Certificates

   This section introduces the terms "Claims", "Assertions",
   "Attestations", and "Certificates" as used in DRIP.  DRIP certificate
   has a different context compared with security certificates and
   Public Key Infrastructure used in X.509.

   Editor-note 5: To be confirmed


      A claim in DRIP is a predicate (e.g., "X is Y", "X has property
      Y", and most importantly "X owns Y" or "X is owned by Y").


      An assertion in DRIP is a set of claims.  This definition is
      borrowed from JWT [RFC7519] and CWT [RFC8392].


      An attestation in DRIP is a signed assertion.  The signer may be a
      claimant or a third party.  Under DRIP this is normally used when
      an entity asserts a relationship with another entity, along with
      other information, and the asserting entity signs the assertion,
      thereby making it an attestation.


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      A certificate in DRIP is an attestation, strictly over identity
      information, signed by a third party.

4.  HHIT as the Primary DRIP Entity Identifier

   This section describes the DRIP architectural approach to meeting the
   basic requirements of a DRIP entity identifier within external
   technical standard ASTM [F3411-19] and regulatory constraints.  It
   justifies and explains the use of Hierarchical Host Identity Tags
   (HHITs) as self-asserting IPv6 addresses suitable as a UAS ID type
   and more generally as trustworthy multipurpose remote identifiers.

   A HHIT, together with the Host Identity (HI) from which it is partly
   derived, self-attests to its included explicit registration
   hierarchy, providing Registrar discovery for a 3rd-party who is
   looking for ID attestation retrieves the necessary information to the
   registrar via a DNS request HHIT.

      Editor-note 6: Is there a need to specify how self-attest works?
      if yes then where? possible a new section under Section 4}

4.1.  UAS Remote Identifiers Problem Space

      Editor-note 14: Good to have: adding match requirment numbering
      from requirement document

   A DRIP entity identifier needs to be "Trustworthy".  This means that
   within the framework of the RID messages, an Observer can establish
   that the DRIP identifier uniquely belong to the UAS.  That the only
   way for any other UAS to assert this DRIP identifier would be to
   steal something from within the UAS.  The DRIP identifier is self-
   generated by the UAS (either UA or GCS) and registered with the USS.

   The Broadcast RID data exchange faces extreme challenges due to the
   limitation of the demanding support for Bluetooth.  The ASTM
   [F3411-19] defines the basic RID message which is expected to contain
   certain RID data and the Authentication message.  The Basic RID
   message has a maximum payload of 25 bytes and the maximum size
   allocated by ASTM for the RID is 20 bytes and only 3 bytes are left
   unused. currently, the authentication maximum payload is defined to
   be 201 bytes.

      Editor-note 7: To be more specific about the RID message header
      and payload structure, such as 1) list different type of BRID
      messages defined in ASTM F3411, 2) how many bytes for each filed.

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   Standard approaches like X.509 and PKI will not fit these
   constraints, even using the new EdDSA [RFC8032] algorithm cannot fit
   within the maximum 201 byte limit, due in large measure to ASN.1
   encoding format overhead.

   An example of a technology that will fit within these limitations is
   an enhancement of the Host Identity Tag (HIT) of HIPv2 [RFC7401]
   using Hierarchical HITs (HHITs) for UAS RID [I-D.ietf-drip-rid].  As
   PKI with X.509 is being used in other systems with which UAS RID must
   interoperate (e.g.  Discovery and Synchronization Service and any
   other communications involving USS) mappings between the more
   flexible but larger X.509 certificates and the HHIT-based structures
   can must be devised.  This could be as in [RFC8002] or simply the
   HHIT as Subject Alternative Name (SAN) and no Distinguished Name

      Editor-note 8: is there a need to explain the how binding/proxy/
      translation between the HHIT and X509?  Should this be addressed
      in Arch- or solution?

   A self-attestation of the HHIT RID can be done in as little as 84
   bytes, by avoiding an explicit encoding technology like ASN.1 or
   Concise Binary Object Representation (CBOR [RFC8949]).  This
   compressed attestation consists of only the HHIT, a timestamp, and
   the EdDSA signature on them.

      Editor-note 9: to be more specific regarding how HHIT can only use
      as little as 84 bytes to address the crypto concern.

   The HHIT prefix and suiteID provide crypto agility and implicit
   encoding rules.  Similarly, a self-attestation of the Hierarchical
   registration of the RID (an attestation of a RID third-party
   registration "certificate") can be done in 200 bytes.  Both these are
   detailed in UAS RID [I-D.ietf-drip-rid].

      Editor-note 10: to be more specific why 200 bytes is sufficient.

   An Observer would need Internet access to validate a self-
   attestations claim.  A third-party Certificate can be validated via a
   small credential cache in a disconnected environment.  This third-
   party Certificate is possible when the third-party also uses HHITs
   for its identity and the UA has the public key and the Certificate
   for that HHIT.

4.2.  HIT as A Trustworthy DRIP Entity Identifier

      Editor-note 15: general comments about rewrite of this section due
      to lack of coherence.

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   A Remote ID that can be trustworthily used in the RID Broadcast mode
   can be built from an asymmetric keypair.  Rather than using a key
   signing operation to claim ownership of an ID that does not guarantee
   name uniqueness, in this method the ID is cryptographically derived
   directly from the public key.  The proof of ID ownership (verifiable
   attestation, versus mere claim) is guaranteed by signing this
   cryptographic ID with the associated private key.  The association
   between the ID and the private key is ensured by cryptographically
   binding the public key with the ID, more specifically the ID results
   from the hash of the public key.  It is statistically hard for
   another entity to create a public key that would generate (spoof) the

   The HITs is designed statistically unique through the cryptographic
   hash feature of second-preimage resistance.  The cryptographically-
   bound addition of the Hierarchy and an HHIT registration process
   (e.g. based on Extensible Provisioning Protocol, [RFC5730]) provide
   complete, global HHIT uniqueness.  This registration forces the
   attacker to generate the same public key rather than a public key
   that generates the same HHIT.  This is in contrast to general IDs
   (e.g. a UUID or device serial number) as the subject in an X.509

      Editor-note 11: Explain how HIT itself and HHIT registry address
      naming collision.

   A DRIP identifier can be assigned to a UAS as a static HHIT by its
   manufacturer, such as a single HI and derived HHIT encoded as a
   hardware serial number per [CTA2063A].  Such a static HHIT can only
   be used to bind one-time use DRIP identifiers to the unique UA.
   Depending upon implementation, this may leave a HI private key in the
   possession of the manufacturer (more details in Section 8).

   In another case, a UAS equipped for Broadcast RID can be provisioned
   not only with its HHIT but also with the HI public key from which the
   HHIT was derived and the corresponding private key, to enable message
   signature.  A UAS equipped for Network RID can be provisioned
   likewise; the private key resides only in the ultimate source of
   Network RID messages (i.e. on the UA itself if the GCS is merely
   relaying rather than sourcing Network RID messages).  Each Observer
   device can be provisioned either with public keys of the DRIP
   identifier root registries or certificates for subordinate

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   HHITs can also be used throughout the USS/UTM system.  The Operators,
   Private Information Registries, as well as other UTM entities, can
   use HHITs for their IDs.  Such HHITs can facilitate DRIP security
   functions such as used with HIP to strongly mutually authenticate and
   encrypt communications.

4.3.  HHIT for DRIP Identifier Registration and Lookup

   Remote ID needs a deterministic lookup mechanism that rapidly
   provides actionable information about the identified UA.  Given the
   size constraints imposed by the Bluetooth 4 broadcast media, the
   Remote ID itself needs to be the inquiry input into the lookup.  An
   HHIT DRIP identifier contains cryptographically embedded registration
   information.  This HHIT registration hierarchy, along with the IPv6
   prefix, is trustable and sufficient information that can be used to
   perform such a lookup.  Additionally, the IPv6 prefix can enhance the
   HHITs use beyond the basic Remote ID function (e.g use in HIP,

      Editor-note 12: more description regarding 1) Is that something we
      should address in the Arch- 2) if yes, then adding more text about
      how a trustable lookup is performed

   Therefore, a DRIP identifier can be represented as a HHIT.  It can be
   self-generated by a UAS (either UA or GCS) and registered with the
   Private Information Registry (More details in Section 5.2) identified
   in its hierarchy fields.  Each DRIP identifier represented as an HHIT
   can not be used more than once.

4.4.  HHIT for DRIP Identifier Cryptographic

   The only (known to the authors of this document at the time of its
   writing) extant fixed-length ID cryptographically derived from a
   public key are the Host Identity Tag [RFC7401], HITs, and
   Cryptographically Generated Addresses [RFC3972], CGAs.  However, both
   HITs and CGAs lack registration/retrieval capability.  HHIT, on the
   other hand, is capable of providing a cryptographic hashing function,
   along with a registration process to mitigate the probability of a
   hash collision (first registered, first allowed).

5.  DRIP Identifier Registration and Registries

      Editor-note 16: Fundamentally disagree with not actually
      specifying an architecture in the DRIP Architecture document (From
      Stuart Card)

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   UAS registries can hold both public and private UAS information
   resulting from the DRIP identifier registration process.  Given these
   different uses, and to improve scalability, security, and simplicity
   of administration, the public and private information can be stored
   in different registries.  A DRIP identifier is amenable to handling
   as an Internet domain name (at an arbitrary level in the hierarchy).
   It also can be registered in at least a pseudo-domain (e.g. .ip6.arpa
   for reverse lookup), or as a sub-domain (for forward lookup).  This
   section introduces the public and private information registries for
   DRIP identifiers.

5.1.  Public Information Registry

5.1.1.  Background

   The public registry provides trustable information such as
   attestations of RID ownership and HDA registration.  Optionally,
   pointers to the repositories for the HDA and RAA implicit in the RID
   can be included (e.g. for HDA and RAA HHIT|HI used in attestation
   signing operations).  This public information will be principally
   used by Observers of Broadcast RID messages.  Data on UAS that only
   use Network RID, is only available via an Observer's Net-RID DP that
   would tend to provide all public registry information directly.  The
   Observer can visually "see" these UAS, but they are silent to the
   Observer; the Net-RID DP is the only source of information based on a
   query for an airspace volume.

5.1.2.  Proposed Approach

   A DRIP public information registry can respond to standard DNS
   queries, in the definitive public Internet DNS hierarchy.  If a DRIP
   public information registry lists, in a HIP RR, any HIP RVS servers
   for a given DRIP identifier, those RVS servers can restrict relay
   services per AAA policy; this requires extensions to [RFC8004].
   These public information registries can use secure DNS transport
   (e.g.  DNS over TLS) to deliver public information that is not
   inherently trustable (e.g. everything other than attestations).

5.2.  Private Information Registry

5.2.1.  Background

   The private information required for DRIP identifiers is similar to
   that required for Internet domain name registration.  A DRIP
   identifier solution can leverage existing Internet resources:
   registration protocols, infrastructure and business models, by
   fitting into an ID structure compatible with DNS names.  This implies
   some sort of hierarchy, for scalability, and management of this

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   hierarchy.  It is expected that the private registry function will be
   provided by the same organizations that run a USS, and likely
   integrated with a USS.

5.2.2.  Proposed Approach

   A DRIP private information registry can support essential Internet
   domain name registry operations (e.g. add, delete, update, query)
   using interoperable open standard protocols.  It can also support the
   Extensible Provisioning Protocol (EPP) and the Registry Data Access
   Protocol (RDAP) with access controls.  It might be listed in a DNS:
   that DNS could be private; but absent any compelling reasons for use
   of private DNS, a public DNS hierarchy needs to be in place.  The
   DRIP private information registry in which a given UAS is registered
   needs to be findable, starting from the UAS ID, using the methods
   specified in [RFC7484].  A DRIP private information registry can also
   support WebFinger as specified in [RFC7033].

6.  Harvesting Broadcast Remote ID messages for UTM Inclusion

   ASTM anticipated that regulators would require both Broadcast RID and
   Network RID for large UAS, but allow RID requirements for small UAS
   to be satisfied with the operator's choice of either Broadcast RID or
   Network RID.  The EASA initially specified Broadcast RID for UAS of
   essentially all UAS and is now also considering Network RID.  The FAA
   RID Final Rules [FAA_RID] only specify Broadcast RID for UAS,
   however, still encourages Network RID for complementary
   functionality, especially in support of UTM.

   One obvious opportunity is to enhance the architecture with gateways
   from Broadcast RID to Network RID.  This provides the best of both
   and gives regulators and operators flexibility.  It offers
   considerable enhancement over some Network RID options such as only
   reporting planned 4D operation space by the operator.

   These gateways could be pre-positioned (e.g. around airports, public
   gatherings, and other sensitive areas) and/or crowd-sourced (as
   nothing more than a smartphone with a suitable app is needed).  As
   Broadcast RID media have limited range, gateways receiving messages
   claiming locations far from the gateway can alert authorities or a
   SDSP to the failed sanity check possibly indicating intent to
   deceive.  Surveillance SDSPs can use messages with precise date/time/
   position stamps from the gateways to multilaterate UA location,
   independent of the locations claimed in the messages (which may have
   a natural time lag as it is), which are entirely operator self-
   reported in UAS RID and UTM, and thus are subject not only to natural
   time lag and error but also operator misconfiguration or intentional

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   Further, gateways with additional sensors (e.g. smartphones with
   cameras) can provide independent information on the UA type and size,
   confirming or refuting those claims made in the RID messages.  This
   Crowd Sourced Remote ID (CS-RID) would be a significant enhancement,
   beyond baseline DRIP functionality; if implemented, it adds two more
   entity types.

6.1.  The CS-RID Finder

   A CS-RID Finder is the gateway for Broadcast Remote ID Messages into
   the UTM.  It performs this gateway function via a CS-RID SDSP.  A CS-
   RID Finder could implement, integrate, or accept outputs from, a
   Broadcast RID receiver.  However, it should not depend upon a direct
   interface with a GCS, Net-RID SP, Net-RID DP or Network RID client.
   It would present a TBD interface to a CS-RID SDSP; this interface
   should be based upon but readily distinguishable from that between a
   GCS and a Net-RID SP.

6.2.  The CS-RID SDSP

   A CS-RID SDSP would present a TBD interface to a CS-RID Finder; this
   interface should be based upon but readily distinguishable from that
   between a GCS and a Net-RID SP.  A CS-RID SDSP should appear (i.e.
   present the same interface) to a Net-RID SP as a Net-RID DP.

7.  IANA Consideration

   This document does not make any IANA request.

8.  Security Considerations

   The security provided by asymmetric cryptographic techniques depends
   upon protection of the private keys.  A manufacturer that embeds a
   private key in an UA may have retained a copy.  A manufacturer whose
   UA are configured by a closed source application on the GCS which
   communicates over the Internet with the factory may be sending a copy
   of a UA or GCS self-generated key back to the factory.  Keys may be
   extracted from a GCS or UA.  The RID sender of a small harmless UA
   (or the entire UA) could be carried by a larger dangerous UA as a
   "false flag."  Compromise of a registry private key could do
   widespread harm.  Key revocation procedures are as yet to be
   determined.  These risks are in addition to those involving Operator
   key management practices.

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9.  Privacy & Transparency Considerations

   Broadcast RID messages can contain PII.  A viable architecture for
   PII protection would be symmetric encryption of the PII using a
   session key known to the UAS and its USS.  An authorized Observer
   could send the encrypted PII along with the UAS ID (to the USS in
   which the UAS ID is registered if that can be determined, e.g., from
   received Broadcast RID information such as the UAS ID itself, or to
   the Observer's USS, or to a Public Safety USS) to get the plaintext.
   Alternatively, the authorized Observer can receive the key to
   directly decrypt all PII content sent by that UA during that session
   (UAS operation).

   An authorized Observer can instruct a UAS via the USS that conditions
   have changed mandating no PII protection or land the UA (abort the

   PII can be protected unless the UAS is informed otherwise.  This
   could come as part of UTM operation authorization.  It can be special
   instructions at the start or during an operation.  PII protection can
   not be used if the UAS loses connectivity to the USS.  The UAS always
   has the option to abort the operation if PII protection is

10.  References

10.1.  Normative References

              Card, S. W., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-17, 7 July 2021,

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

   [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>.

10.2.  Informative References

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   [CTA2063A] ANSI, "Small Unmanned Aerial Systems Serial Numbers",

              European Union Aviation Safety Agency (EASA), "EU
              Commission Delegated Regulation 2019/945 of 12 March 2019
              on unmanned aircraft systems and on third-country
              operators of unmanned aircraft systems", 2019.

   [F3411-19] ASTM, "Standard Specification for Remote ID and Tracking",

   [FAA_RID]  United States Federal Aviation Administration (FAA),
              "Remote Identification of Unmanned Aircraft", 2021,

              United States Federal Aviation Administration (FAA),
              "Unmanned Aircraft System (UAS) Traffic Management (UTM)
              Concept of Operations (V2.0)", 2020,

              Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
              Gurtov, "UAS Remote ID", Work in Progress, Internet-Draft,
              draft-ietf-drip-rid-07, 28 January 2021,

              European Union Aviation Safety Agency (EASA), "EU
              Commission Implementing Regulation 2019/947 of 24 May 2019
              on the rules and procedures for the operation of unmanned
              aircraft", 2019.

   [LAANC]    United States Federal Aviation Administration (FAA), "Low
              Altitude Authorization and Notification Capability", n.d.,

   [NPRM]     United States Federal Aviation Administration (FAA),
              "Notice of Proposed Rule Making on Remote Identification
              of Unmanned Aircraft Systems", 2019.

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   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,

   [RFC7033]  Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
              "WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
              2013, <https://www.rfc-editor.org/info/rfc7033>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,

   [RFC7482]  Newton, A. and S. Hollenbeck, "Registration Data Access
              Protocol (RDAP) Query Format", RFC 7482,
              DOI 10.17487/RFC7482, March 2015,

   [RFC7484]  Blanchet, M., "Finding the Authoritative Registration Data
              (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
              2015, <https://www.rfc-editor.org/info/rfc7484>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [RFC8002]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 8002, DOI 10.17487/RFC8002, October
              2016, <https://www.rfc-editor.org/info/rfc8002>.

   [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
              October 2016, <https://www.rfc-editor.org/info/rfc8004>.

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   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,

              3GPP, "Study on Remote Identification of Unmanned Aerial
              Systems (UAS)", n.d.,

   [U-Space]  European Organization for the Safety of Air Navigation
              (EUROCONTROL), "U-space Concept of Operations", 2019,

Appendix A.  Overview of Unmanned Aircraft Systems (UAS) Traffic
             Management (UTM)

A.1.  Operation Concept

   The National Aeronautics and Space Administration (NASA) and FAA's
   effort of integrating UAS's operation into the national airspace
   system (NAS) led to the development of the concept of UTM and the
   ecosystem around it.  The UTM concept was initially presented in 2013
   and version 2.0 was published in 2020 [FAA_UAS_Concept_Of_Ops].

   The eventual concept refinement, initial prototype implementation and
   testing were conducted by the UTM research transition team which is
   the joint workforce by FAA and NASA.  World efforts took place
   afterward.  The Single European Sky ATM Research (SESAR) started the
   CORUS project to research its UTM counterpart concept, namely
   [U-Space].  This effort is led by the European Organization for the
   Safety of Air Navigation (Eurocontrol).

   Both NASA and SESAR have published the UTM concept of operations to
   guide the development of their future air traffic management (ATM)
   system and ensure safe and efficient integrations of manned and
   unmanned aircraft into the national airspace.

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   The UTM comprises UAS operation infrastructure, procedures and local
   regulation compliance policies to guarantee safe UAS integration and
   operation.  The main functionality of a UTM includes, but is not
   limited to, providing means of communication between UAS operators
   and service providers and a platform to facilitate communication
   among UAS service providers.

A.2.  UAS Service Supplier (USS)

   A USS plays an important role to fulfill the key performance
   indicators (KPIs) that a UTM has to offer.  Such Entity acts as a
   proxy between UAS operators and UTM service providers.  It provides
   services like real-time UAS traffic monitoring and planning,
   aeronautical data archiving, airspace and violation control,
   interacting with other third-party control entities, etc.  A USS can
   coexist with other USS to build a large service coverage map which
   can load-balance, relay and share UAS traffic information.

   The FAA works with UAS industry shareholders and promotes the Low
   Altitude Authorization and Notification Capability [LAANC] program
   which is the first system to realize some of the UTM envisioned
   functionality.  The LAANC program can automate the UAS's flight plan
   application and approval process for airspace authorization in real-
   time by checking against multiple aeronautical databases such as
   airspace classification and fly rules associated with it, FAA UAS
   facility map, special use airspace, Notice to Airman (NOTAM), and
   Temporary Flight Rule (TFR).

A.3.  UTM Use Cases for UAS Operations

   This section illustrates a couple of use case scenarios where UAS
   participation in UTM has significant safety improvement.

   1.  For a UAS participating in UTM and taking off or landing in a
       controlled airspace (e.g., Class Bravo, Charlie, Delta and Echo
       in the United States), the USS under which the UAS is operating
       is responsible for verifying UA registration, authenticating the
       UAS operational intent (flight plan) by checking against
       designated UAS fly map database, obtaining the air traffic
       control (ATC) authorization and monitor the UAS flight path in
       order to maintain safe margins and follow the pre-authorized
       sequence of authorized 4-D volumes (route).

   2.  For a UAS participating in UTM and taking off or landing in an
       uncontrolled airspace (ex.  Class Golf in the United States),
       pre-flight authorization must be obtained from a USS when
       operating beyond-visual-of-sight (BVLOS).  The USS either accepts
       or rejects received operational intent (flight plan) from the

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       UAS.  Accepted UAS operation may share its current flight data
       such as GPS position and altitude to USS.  The USS may keep the
       UAS operation status near real-time and may keep it as a record
       for overall airspace air traffic monitoring.

Appendix B.  Automatic Dependent Surveillance Broadcast (ADS-B)

   The ADS-B is the de jure technology used in manned aviation for
   sharing location information, from the aircraft to ground and
   satellite-based systems, designed in the early 2000s.  Broadcast RID
   is conceptually similar to ADS-B, but with the receiver target being
   the general public on generally available devices (e.g. smartphones).

   For numerous technical reasons, ADS-B itself is not suitable for low-
   flying small UA.  Technical reasons include but not limited to the

   1.  Lack of support for the 1090 MHz ADS-B channel on any consumer
       handheld devices

   2.  Weight and cost of ADS-B transponders on CSWaP constrained UA

   3.  Limited bandwidth of both uplink and downlink, which would likely
       be saturated by large numbers of UAS, endangering manned aviation

   Understanding these technical shortcomings, regulators worldwide have
   ruled out the use of ADS-B for the small UAS for which UAS RID and
   DRIP are intended.


   The work of the FAA's UAS Identification and Tracking (UAS ID)
   Aviation Rulemaking Committee (ARC) is the foundation of later ASTM
   and proposed IETF DRIP WG efforts.  The work of ASTM F38.02 in
   balancing the interests of diverse stakeholders is essential to the
   necessary rapid and widespread deployment of UAS RID.  IETF
   volunteers who have contributed to this draft include Amelia
   Andersdotter and Mohamed Boucadair.

Authors' Addresses

   Stuart W. Card
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY,  13495
   United States of America

   Email: stu.card@axenterprize.com

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   Adam Wiethuechter
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY,  13495
   United States of America

   Email: adam.wiethuechter@axenterprize.com

   Robert Moskowitz
   HTT Consulting
   Oak Park, MI,  48237
   United States of America

   Email: rgm@labs.htt-consult.com

   Shuai Zhao
   2747 Park Blvd
   Palo Alto,  94588
   United States of America

   Email: shuai.zhao@ieee.org

   Andrei Gurtov
   Linköping University
   SE-58183 Linköping Linköping

   Email: gurtov@acm.org

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