drip                                                             S. Card
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 8 22 September 2022                                  R. Moskowitz
                                                          HTT Consulting
                                                        S. Zhao (Editor)
                                                                 Tencent
                                                               A. Gurtov
                                                    Linköping University
                                                            7
                                                           21 March 2022

        Drone Remote Identification Protocol (DRIP) Architecture
                        draft-ietf-drip-arch-21
                        draft-ietf-drip-arch-22

Abstract

   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System (UAS) Remote Identification (RID)
   and tracking, plus UAS RID-related communications.  This architecture
   adheres to the requirements listed in the DRIP Requirements document
   (RFC9153).

Status of This Memo

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   This Internet-Draft will expire on 8 22 September 2022.

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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  . . . . . . . . . . . .   7
     1.4.  Overview of DRIP Architecture . . . . . . . . . . . . . .   8
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .  10
     2.1.  Additional Abbreviations  . . . . . . . . . . . . . . . .  10
     2.2.  Additional Definitions  . . . . . . . . . . . . . . . . .  11
   3.  HHIT as the DRIP Entity Identifier  . . . . . . . . . . . . .  11
     3.1.  UAS Remote Identifiers Problem Space  . . . . . . . . . .  12
     3.2.  HHIT as A Trustworthy DRIP Entity Identifier  . . . . . .  12
     3.3.  HHIT for DRIP Identifier Registration and Lookup  . . . .  14
     3.4.  HHIT as a Cryptographic Identifier  . . . . . . . . . . .  14
   4.  DRIP Identifier Registration and Registries . . . . . . . . .  14
     4.1.  Public Information Registry . . . . . . . . . . . . . . .  15
       4.1.1.  Background  . . . . . . . . . . . . . . . . . . . . .  15
       4.1.2.  DNS as the Public DRIP Identifier Registry  . . . . .  15
     4.2.  Private Information Registry  . . . . . . . . . . . . . .  15
       4.2.1.  Background  . . . . . . . . . . . . . . . . . . . . .  15
       4.2.2.  EPP and RDAP as the Private DRIP Identifier
               Registry  . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.3.  Alternative Private DRIP Registry methods . . . . . .  16
   5.  DRIP Identifier Trust . . . . . . . . . . . . . . . . . . . .  16
   6.  Harvesting Broadcast Remote ID messages for UTM Inclusion . .  17
     6.1.  The CS-RID Finder . . . . . . . . . . . . . . . . . . . .  18
     6.2.  The CS-RID SDSP . . . . . . . . . . . . . . . . . . . . .  18
   7.  DRIP Contact  . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. Privacy & Transparency Considerations . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Overview of Unmanned Aircraft Systems (UAS) Traffic
           Management (UTM)  . . . . . . . . . . . . . . . . . . . .  23  24
     A.1.  Operation Concept . . . . . . . . . . . . . . . . . . . .  24
     A.2.  UAS Service Supplier (USS)  . . . . . . . . . . . . . . .  24
     A.3.  UTM Use Cases for UAS Operations  . . . . . . . . . . . .  25
   Appendix B.  Automatic Dependent Surveillance Broadcast
           (ADS-B) . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System (UAS) Remote Identification (RID)
   and tracking, 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 [RFC9153].  The requirements document provides
   an extended introduction to the problem space and use cases.

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

   UAS Remote Identification (RID) is an application that enables a UAS
   to be identified by Unmanned Aircraft Systems Traffic Management
   (UTM) and UAS Service Supplier (USS) (Appendix A) or third party
   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.
   CAAs currently promulgate performance-based regulations that do not
   specify techniques, but rather cite industry consensus technical
   standards as acceptable means of compliance.

   Federal Aviation Administration (FAA)

      The FAA published a Notice of Proposed Rule Making [NPRM] in 2019
      and thereafter published a "Final Rule" in 2021 [FAA_RID],
      imposing requirements on UAS manufacturers and operators, both
      commercial and recreational.  The rule clearly states that
      Automatic Dependent Surveillance Broadcast (ADS-B) Out and
      transponders cannot be used to satisfy the UAS RID requirements on
      UAS to which the rule applies (see Appendix B).

   European Union Aviation Safety Agency (EASA)
      The EASA published a [Delegated] regulation in 2019 imposing
      requirements on UAS manufacturers and third-country operators,
      including but not limited to UAS RID requirements.  The EASA also
      published in 2019 an [Implementing] regulation laying down
      detailed rules and procedures for UAS operations and operating
      personnel.

   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] Standard Specification for Remote ID and Tracking.

      ASTM defines one set of UAS 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
      provided via both means.  [F3411] is cited by the FAA in its UAS
      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 services that can be offered based on UAS 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.  The study of
      Further Architecture Enhancement for Uncrewed Aerial Vehicles
      (UAV) and Urban Air Mobility (UAM) [FS_AEUA] in release 18 further
      enhances the communication mechanism between UAS and USS/UTM.  The
      UAS RID discussed in Section 3 may be used as the 3GPP CAA-level
      UAS ID for Remote Identification purposes.

1.2.  Overview of Types of UAS Remote ID

   This specification introduces two types UAS Remote ID defined in ASTM
   [F3411].

1.2.1.  Broadcast RID

   [F3411] defines a set of UAS RID messages 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.

               +------------------------+
               | Unmanned Aircraft (UA) |
               +-----------o------------+
                           |
                           |
                           |
                           | app messages directly over
                           | one-way RF data link (no IP)
                           |
                           |
                           v
         +------------------o-------------------+
         | Observer's device (e.g., smartphone) |
         +--------------------------------------+

                                  Figure 1

   Broadcast RID provides information only about unmanned aircraft (UA)
   within direct Radio Frequency (RF) Line-Of-Sight (LOS), typically
   similar to Visual LOS (VLOS), with a range up to approximately 1 km.
   This information may be 'harvested' from received broadcasts and made
   available via the Internet, enabling surveillance of areas too large
   for local direct visual observation or direct RF link-based ID (see
   Section 6).

1.2.2.  Network RID

   [F3411], using the same data dictionary that is the basis of
   Broadcast RID messages, defines a Network Remote Identification (Net-
   RID) data flow as follows.

   *  The information to be reported via UAS RID is generated by the
      UAS.  Typically some of this data is generated by the UA and some
      by the GCS (Ground Control Station), e.g., their respective Global
      Navigation Satellite System (GNSS) derived locations.

   *  The information is sent by the UAS (UA or GCS) via unspecified
      means to the cognizant Network Remote Identification Service
      Provider (Net-RID SP), typically the USS under which the UAS is
      operating if participating in UTM.

   *  The Net-RID SP publishes via the Discovery and Synchronization
      Service (DSS) over the Internet that it has operations in various
      4-D airspace volumes (Section 2.2 of [RFC9153]), describing the
      volumes but not the operations.

   *  An Observer's device, which is expected, but not specified, to be
      web-based, queries a Network Remote Identification Display
      Provider (Net-RID DP), typically also a USS, about any operations
      in a specific 4-D airspace volume.

   *  Using fully specified web-based methods over the Internet, the
      Net-RID DP queries all Net-RID SP that have operations in volumes
      intersecting that of the Observer's query for details on all such
      operations.

   *  The Net-RID DP aggregates information received from all such Net-
      RID SP and responds to the Observer's query.

   The minimum Net-RID data flow is illustrated in Figure 2:

    +-------------+     ******************
    |     UA      |     *    Internet    *
    +--o-------o--+     *                *
       |       |        *                *
       |       |        *                *     +------------+
       |       '--------*--(+)-----------*-----o            |
       |                *   |            *     |            |
       |       .--------*--(+)-----------*-----o Net-RID SP |
       |       |        *                *     |            |
       |       |        *         .------*-----o            |
       |       |        *         |      *     +------------+
       |       |        *         |      *
       |       |        *         |      *     +------------+
       |       |        *         '------*-----o            |
       |       |        *                *     | Net-RID DP |
       |       |        *         .------*-----o            |
       |       |        *         |      *     +------------+
       |       |        *         |      *
       |       |        *         |      *     +------------+
    +--o-------o--+     *         '------*-----o Observer's |
    |     GCS     |     *                *     | Device     |
    +-------------+     ******************     +------------+

                                  Figure 2

   Command and Control (C2) must flow from the GCS to the UA via some
   path.  Currently (in the year 2022) this is typically a direct RF
   link; however, with increasing Beyond Visual Line of Sight (BVLOS)
   operations, it is expected often to be a wireless link at either end
   with the Internet between.

   Telemetry (at least UA's position and heading) flows from the UA to
   the GCS via some path, typically the reverse of the C2 path.  Thus,
   UAS 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 operation.

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

   [F3411] prescribes the protocols between the Net-RID SP, Net-RID DP,
   and the DSS.  It also prescribes data elements (in JSON) between the
   Observer and the Net-RID DP.  DRIP could address standardization of
   secure protocols between the UA and GCS (over direct wireless and
   Internet connection), between the UAS and the Net-RID SP, and/or
   between the Net-RID DP and Observer devices.

      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 UAS RID information
         is in the scope of [F3411] Network RID.

1.3.  Overview of USS Interoperability

   With Net-RID, there is direct communication between each UAS and its
   USS.  Multiple USS exchange information with the assistance of a DSS
   so all USS collectively have knowledge about all activities in a 4D
   airspace.  The interactions among an Observer, multiple UAS, and
   their USS are shown in Figure 3.

                   +------+    +----------+    +------+
                   | UAS1 |    | Observer |    | UAS2 |
                   +---o--+    +-----o----+    +--o---+
                       |             |            |
                 ******|*************|************|******
                 *     |             |            |     *
                 *     |         +---o--+         |     *
                 *     |  .------o USS3 o------.  |     *
                 *     |  |      +--o---+      |  |     *
                 *     |  |         |          |  |     *
                 *   +-o--o-+    +--o--+     +-o--o-+   *
                 *   |      o----o DSS o-----o      |   *
                 *   | USS1 |    +-----+     | USS2 |   *
                 *   |      o----------------o      |   *
                 *   +------+                +------+   *
                 *                                      *
                 *               Internet               *
                 ****************************************

                                  Figure 3

1.4.  Overview of DRIP Architecture

   Figure 4 illustrates a global UAS RID usage scenario.  Broadcast RID
   links are not shown as they reach from any UA to any listening
   receiver in range and thus would obscure the intent of the figure.
   Figure 4 shows, as context, some entities and interfaces beyond the
   scope of DRIP (as currently (2022) chartered).

  ***************                                        ***************
  *    UAS1     *                                        *     UAS2    *
  *             *                                        *             *
  * +--------+  *                 DAA/V2V                *  +--------+ *
  * |   UA   o--*----------------------------------------*--o   UA   | *
  * +--o--o--+  *                                        *  +--o--o--+ *
  *    |  |     *   +------+      Lookups     +------+   *     |  |    *
  *    |  |     *   | GPOD o------.    .------o PSOD |   *     |  |    *
  *    |  |     *   +------+      |    |      +------+   *     |  |    *
  *    |  |     *                 |    |                 *     |  |    *
  * C2 |  |     *     V2I      ************     V2I      *     |  | C2 *
  *    |  '-----*--------------*          *--------------*-----'  |    *
  *    |        *              *          *              *        |    *
  *    |        o====Net-RID===*          *====Net-RID===o        |    *
  * +--o--+     *              * Internet *              *     +--o--+ *
  * | GCS o-----*--------------*          *--------------*-----o GCS | *
  * +-----+     * Registration *          * Registration *     +-----+ *
  *             * (and UTM)    *          * (and UTM)    *             *
  ***************              ************              ***************
                                 |  |  |
                  +----------+   |  |  |   +----------+
                  | Public   o---'  |  '---o Private  |
                  | Registry |      |      | Registry |
                  +----------+      |      +----------+
                                 +--o--+
                                 | DNS |
                                 +-----+

  DAA:  Detect And Avoid
  GPOD: General Public Observer Device
  PSOD: Public Safety Observer Device
  V2I:  Vehicle-to-Infrastructure
  V2V:  Vehicle-to-Vehicle

                                 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] and other external standards,
   to satisfy UAS RID requirements.

   This document outlines the DRIP architecture in the context of the
   UAS RID architecture.  This includes presenting the gaps between the
   CAAs' Concepts of Operations and [F3411] 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 identifiers (Section 3).

      -  Mechanisms to leverage Domain Name System (DNS [RFC1034]),
         Extensible Provisioning Protocol (EPP [RFC5731]) and
         Registration Data Access Protocol (RDAP) ([RFC9082]) for
         publishing public and private information (see Section 4.1 and
         Section 4.2).

      -  Specific authentication methods and message payload formats to
         enable verification that Broadcast RID messages were sent by
         the claimed sender (Section 5) and that sender is in the
         claimed registry (Section 4 and Section 5).

      -  Harvesting Broadcast RID messages for UTM inclusion, with the
         optional DRIP extension of Crowd Sourced Remote ID (CS-RID,
         Section 6), using the DRIP support for gateways required by
         GEN-5 [RFC9153].

      -  Methods for instantly establishing secure communications
         between an Observer and the pilot of an observed UAS
         (Section 7), using the DRIP support for dynamic contact
         required by GEN-4 [RFC9153].

      -  Privacy in UAS RID messages (PII protection) (Section 10).

2.  Terms and Definitions

   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.

   To encourage comprehension necessary for adoption of DRIP by the
   intended user community, the UAS community's norms are respected
   herein.

   This document uses terms defined in [RFC9153].

2.1.  Additional Abbreviations

   DET:        DRIP Entity Tag

   EdDSA:      Edwards-Curve Digital Signature Algorithm

   HHIT:       Hierarchical HIT

   HI:         Host Identity
   HIP:        Host Identity Protocol

   HIT:        Host Identity Tag

2.2.  Additional Definitions

   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.

   Claims:

      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").

   Assertions:

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

   Attestations:

      An attestation in DRIP is a signed assertion.  The signer may be
      the claimant or a related party with stake in the assertion(s).
      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.

   Certificates:

      A certificate in DRIP is an attestation, strictly over identity
      information, signed by a third party.  This third party should be
      one with no stake in the attestation(s) over which it is signing.

3.  HHIT as the 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] and regulatory constraints.  It
   justifies and explains the use of Hierarchical Host Identity Tags
   (HHITs) [RFC7401] as self-asserting IPv6 addresses suitable as a UAS
   ID type and, more generally, as trustworthy multipurpose remote
   identifiers.

   Self-asserting in this usage means that, given the Host Identity
   (HI), the HHIT ORCHID construction and a signature of the registry on
   the HHIT, the HHIT can be verified by the receiver.  The explicit
   registration hierarchy within the HHIT provides registry discovery
   (managed by a Registrar) to either yield the HI for a 3rd-party
   (seeking UAS ID attestation) validation or prove that the HHIT and HI
   have been registered uniquely.

3.1.  UAS Remote Identifiers Problem Space

   A DRIP entity identifier needs to be "Trustworthy" (See DRIP
   Requirement GEN-1, ID-4 and ID-5 in [RFC9153]).  This means that
   given a sufficient collection of UAS RID messages, an Observer can
   establish that the identifier claimed therein uniquely belongs to the
   claimant.  To satisfy DRIP requirements and maintain important
   security properties, the DRIP identifier should be self-generated by
   the entity it names (e.g., a UAS) and registered (e.g., with a USS,
   see Requirements GEN-3 and ID-2).

   Broadcast RID, especially its support for Bluetooth 4, imposes severe
   constraints.  ASTM UAS RID [F3411] allows a UAS ID of types 1, 2 and
   3 of 20 bytes; a revision to [F3411], currently in balloting (as of
   Oct 2021), adds type 4, Specific Session ID, to be standardized by
   IETF and other standards development organizations (SDOs) as
   extensions to ASTM UAS RID, consumes one of those bytes to index the
   sub-type, leaving only 19 for the identifier (see DRIP Requirement
   ID-1).

   Likewise, the maximum ASTM UAS RID [F3411] Authentication Message
   payload is 201 bytes for most authentication types.  A type 5 is also
   added in this revision for IETF and other SDOs to develop Specific
   Authentication Methods as extensions to ASTM UAS RID.  One byte out
   of 201 bytes is consumed to index the sub-type which leaves only 200
   for DRIP authentication payloads, including one or more DRIP entity
   identifiers and associated authentication data.

3.2.  HHIT as A Trustworthy DRIP Entity Identifier

   A Remote UAS ID that can be trustworthy for use in Broadcast RID can
   be built from an asymmetric keypair.  In this method, the UAS ID is
   cryptographically derived directly from the public key.  The proof of
   UAS ID ownership (verifiable attestation, versus mere claim) is
   guaranteed by signing this cryptographic UAS ID with the associated
   private key.  The association between the UAS ID and the private key
   is ensured by cryptographically binding the public key with the UAS
   ID; more specifically, the UAS ID results from the hash of the public
   key.  The public key is designated as the HI while the UAS ID is
   designated as the HIT.

   By construction, the HIT is statistically unique through the
   cryptographic hash feature of second-preimage resistance.  The
   cryptographically-bound addition of the Hierarchy and an HHIT
   registration process 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 certificate.

   A UA equipped for Broadcast RID SHOULD 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 SHOULD 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 SHOULD be provisioned either with public keys of the DRIP
   identifier root registries or certificates for subordinate
   registries.

   HHITs can also be used throughout the USS/UTM system.  Operators and
   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.

   A self-attestation of a HHIT used as a UAS ID can be done in as
   little as 84 bytes when Ed25519 [RFC8032] is used, by avoiding an
   explicit encoding technology like ASN.1 or Concise Binary Object
   Representation (CBOR [RFC8949]).  This attestation consists of only
   the HHIT, a timestamp, and the EdDSA signature on them.

   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 SHOULD
   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 9).

   In general, Internet access may be needed to validate Attestations or
   Certificates.  This may be obviated in the most common cases (e.g.,
   attestation of the UAS ID), even in disconnected environments, by
   prepopulating small caches on Observer devices with Registry public
   keys and a chain of Attestations or Certificates (tracing a path
   through the Registry tree).  This is assuming all parties on the
   trust path also use HHITs for their identities.

3.3.  HHIT for DRIP Identifier Registration and Lookup

   UAS RID 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 UAS ID
   itself needs to be a non-spoofable inquiry input into the lookup.

   A DRIP registration process based on the explicit hierarchy within a
   HHIT provides manageable uniqueness of the HI for the HHIT.  This is
   the defense against a cryptographic hash second pre-image attack on
   the HHIT (e.g., multiple HIs yielding the same HHIT, see Requirement
   ID-3).  A lookup of the HHIT into this registration data provides the
   registered HI for HHIT proof of ownership.  A first-come-first-served
   registration for a HHIT provides deterministic access to any other
   needed actionable information based on inquiry access authority (more
   details in Section 4.2).

3.4.  HHIT as a Cryptographic Identifier

   The only (known to the authors at the time of this writing) existing
   types of IP address compatible identifiers cryptographically derived
   from the public keys of the identified entities are Cryptographically
   Generated Addresses (CGAs) [RFC3972] and Host Identity Tags (HITs)
   [RFC7401].  CGAs and HITs lack registration/retrieval capability.  To
   provide this, each HHIT embeds plaintext information designating the
   hierarchy within which it is registered and a cryptographic hash of
   that information concatenated with the entity's public key, etc.
   Although hash collisions may occur, the registrar can detect them and
   reject registration requests rather than issue credentials, e.g., by
   enforcing a first-claimed, first-attested policy.  Pre-image hash
   attacks are also mitigated through this registration process, locking
   the HHIT to a specific HI

4.  DRIP Identifier Registration and Registries

   DRIP registries hold both public and private UAS information (See
   PRIV-1 in [RFC9153]) 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.  This section
   introduces the public and private information registries for DRIP
   identifiers.  This DRIP Identifier registration process satisfies the
   following DRIP requirements defined in [RFC9153]: GEN-3, GEN-4, ID-2,
   ID-4, ID-6, PRIV-3, PRIV-4, REG-1, REG-2, REG-3 and REG-4.

4.1.  Public Information Registry

4.1.1.  Background

   The public information registry provides trustable information such
   as attestations of UAS RID ownership and registration with the HDA
   (Hierarchical HIT Domain Authority).  Optionally, pointers to the
   registries for the HDA and RAA (Registered Assigning Authority)
   implicit in the UAS 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 available via an
   Observer's Net-RID DP that would directly provide all public
   information registry information.  The Net-RID DP is the only source
   of information for a query on an airspace volume.

4.1.2.  DNS as the Public DRIP Identifier Registry

   A DRIP identifier SHOULD be registered as an Internet domain name (at
   an arbitrary level in the hierarchy, e.g., in .ip6.arpa).  Thus DNS
   can provide all the needed public DRIP information.  A standardized
   HHIT FQDN (Fully Qualified Domain Name) can deliver the HI via a HIP
   RR (Resource Record) [RFC8005] and other public information (e.g.,
   RRA and HDA PTRs, and HIP RVS (Rendezvous Servers) [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).

4.2.  Private Information Registry

4.2.1.  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 UAS ID structure compatible with DNS names.  The HHIT
   hierarchy can provide the needed scalability and management
   structure.  It is expected that the private information registry
   function will be provided by the same organizations that run a USS,
   and likely integrated with a USS.  The lookup function may be
   implemented by the Net-RID DPs.

4.2.2.  EPP and RDAP as the Private DRIP Identifier Registry

   A DRIP private information registry supports essential registry
   operations (e.g., add, delete, update, query) using interoperable
   open standard protocols.  It can accomplish this by using the
   Extensible Provisioning Protocol (EPP [RFC5730]) and the Registry
   Data Access Protocol (RDAP [RFC7480] [RFC9082] [RFC9083]).  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].

4.2.3.  Alternative Private DRIP Registry methods

   A DRIP private information registry might be an access-controlled DNS
   (e.g., via DNS over TLS).  Additionally, WebFinger [RFC7033] can be
   deployed.  These alternative methods may be used by Net-RID DP with
   specific customers.

5.  DRIP Identifier Trust

   While the DRIP entity identifier is self-asserting, it alone does not
   provide the trustworthiness (non-repudiability, protection vs.
   spoofing, message integrity protection, scalability, etc.) essential
   to UAS RID, as justified in [RFC9153].  For that it MUST be
   registered (under DRIP Registries) and be actively used by the party
   (in most cases the UA).  A sender's identity can not be approved by
   only possessing a DRIP Entity Tag (DET), which is an HHIT-based UA ID
   and broadcasting a claim that it belongs to that sender.  Even the
   sender using that HI's private key to sign static data proves nothing
   as well, as it is subject to trivial replay attacks.  Only sending
   the DET and a signature on frequently changing data that can be
   sanity-checked by the Observer (such as a Location/Vector message)
   proves that the observed UA possesses the claimed UAS ID.

   For Broadcast RID, it is a challenge to balance the original
   requirements of Broadcast RID and the efforts needed to satisfy the
   DRIP requirements all under severe constraints.  From received
   Broadcast RID messages and information that can be looked up using
   the received UAS ID in online registries or local caches, it is
   possible to establish levels of trust in the asserted information and
   the Operator.

   Optimization of different DRIP Authentication Messages allows an
   Observer, without Internet connection (offline) or with (online), to
   be able to validate a UAS DRIP ID in real-time.  First is the sending
   of Broadcast Attestations (over DRIP Link Authentication Messages)
   [I-D.ietf-drip-auth] containing the relevant registration of the UA's
   DRIP ID in the claimed Registry.  Next is sending DRIP Wrapper
   Authentication Messages that sign over both static (e.g., above
   registration) and dynamically changing data (such as UA location
   data).  Combining these two sets of information, an Observer can
   piece together a chain of trust and real-time evidence to make their
   determination of the UA's claims.

   This process (combining the DRIP entity identifier, Registries and
   Authentication Formats for Broadcast RID) can satisfy the following
   DRIP requirement defined in [RFC9153]: GEN-1, GEN-2, GEN-3, ID-2, ID-
   3, ID-4 and ID-5.

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 UAS 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
   essentially all UAS, and is now also considering Network RID.  The
   FAA UAS RID Final Rules [FAA_RID] permit only Broadcast RID for rule
   compliance, but still encourage 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 advantages
   over either form of UAS RID alone: greater fidelity than Network RID
   reporting of planned area operations; surveillance of areas too large
   for local direct visual observation and direct RF-LOS link based
   Broadcast RID (e.g., a city or a national forest).

   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 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 deception.

   Multilateration technologies use physical layer information, such as
   precise Time Of Arrival (TOA) of transmissions from mobile
   transmitters at receivers with a priori precisely known locations, to
   estimate the locations of the mobile transmitters.

   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 UAS RID messages.
   This

   Section 6.1 and Section 6.2 define two additional entities that are
   required to provide this Crowd Sourced Remote ID (CS-RID) would be a significant
   enhancement, beyond baseline DRIP functionality; if implemented, it
   adds two more entity types. (CS-RID).

   This approach satisfies the following DRIP requirements defined in
   [RFC9153]: GEN-5, GEN-11, and REG-1.

6.1.  The CS-RID Finder

   A CS-RID Finder is the gateway for Broadcast Remote ID Messages into
   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 new interface to a CS-RID SDSP, similar to but readily
   distinguishable from that between a GCS and a Net-RID SP.

6.2.  The CS-RID SDSP

   A CS-RID SDSP aggregates and processes (e.g., estimates UA location
   using multilateration when possible) information collected by CS-RID
   Finders.  A CS-RID SDSP should appear (i.e., present the same
   interface) to a Net-RID SP as a Net-RID DP.

7.  DRIP Contact

   One of the ways in which DRIP can enhance [F3411] with immediately
   actionable information is by enabling an Observer to instantly
   initiate secure communications with the UAS remote pilot, Pilot In
   Command, operator, USS under which the operation is being flown, or
   other entity potentially able to furnish further information
   regarding the operation and its intent and/or to immediately
   influence further conduct or termination of the operation (e.g., land
   or otherwise exit an airspace volume).  Such potentially distracting
   communications demand strong "AAA" (Authentication, Attestation,
   Authorization, Access Control, Accounting, Attribution, Audit) per
   applicable policies (e.g., of the cognizant CAA).

   A DRIP entity identifier based on a HHIT as outlined in Section 3
   embeds an identifier of the registry in which it can be found
   (expected typically to be the USS under which the UAS is flying) and
   the procedures outlined in Section 5 enable Observer verification of
   that relationship.  A DRIP entity identifier with suitable records in
   public and private registries as outlined in Section 5 can enable
   lookup not only of information regarding the UAS, but also identities
   of and pointers to information regarding the various associated
   entities (e.g., the USS under which the UAS is flying an operation),
   including means of contacting those associated entities (i.e.,
   locators, typically IP addresses).

   A suitably equipped Observer could initiate a cryptographic handshake
   to a similarly equipped and identified entity: the UA itself, if
   operating autonomously; the GCS, if the UA is remotely piloted and
   the necessary records have been populated in DNS; the USS, etc.
   Assuming mutual authentication is successful, keys can then be
   negotiated for an IPsec Encapsulating Security Payload (ESP) tunnel,
   over which arbitrary standard higher layer protocols can then be used
   for Observer to Pilot (O2P)communications (e.g., SIP [RFC3261] et
   seq), V2X communications (e.g., [MAVLink]), etc.  Certain
   preconditions are necessary: each party needs a currently usable
   means (typically DNS) of resolving the other party's DRIP entity
   identifier to a currently usable locator (IP address); and there must
   be currently usable bidirectional IP (not necessarily Internet)
   connectivity between the parties.  One method directly supported by
   the use of HHITs as DRIP entity identifiers is initiation of a HIP
   Base Exchange (BEX) and Bound End-to-End Tunnel (BEET).

   This approach satisfies DRIP requirement GEN-6 Contact, supports
   satisfaction of requirements [RFC9153] GEN-8, GEN-9, PRIV-2, PRIV-5
   and REG-3, and is compatible with all other DRIP requirements.

8.  IANA Considerations

   This document does not make any IANA request.

9.  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  It may have retained a copy.  A manufacturer whose
   UA are configured by a closed source application on be necessary for the GCS that
   communicates over
   to have the Internet with key pair to register the HHIT to the factory USS.  Thus it may be sending
   the GCS that generates the key pair and delivers it to the UA, making
   the GCS a copy part of a the key security boundary.  Leakage of the private
   key either from the UA or GCS self-generated key back to the factory.  Keys may component manufacturer is a
   valid concern and steps need to be
   extracted from in place to ensure safe keeping of
   the private key.

   The size of the public key hash in the HHIT is also of concern.  It
   is well within current server array technology to compute another key
   pair that hashes to the same HHIT.  Thus an adversary could
   impersonate a GCS or validly registered UA.  The  This attack would only be
   exposed when the HI in DRIP authentication message is checked back to
   the USS and found not to match.

   Finally, the UAS 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.

10.  Privacy & Transparency Considerations

   Broadcast RID messages can contain Personally Identifiable
   Information (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.  Authorized Observers could obtain plaintext in either
   of two ways.  An Observer can send the UAS ID and the cyphertext to a
   server that offers decryption as a service.  An Observer can send the
   UAS ID only to a server that returns the session key, so that
   Observer can directly locally decrypt all cyphertext sent by that UA
   during that session (UAS operation).  In either case, the server can
   be: a Public Safety USS, the Observer's own USS, or the UA's USS if
   the latter can be determined (which under DRIP it can be, from the
   UAS ID itself).  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 MUST 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 disallowed.

11.  References

11.1.  Normative References

   [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>.

   [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>.

   [RFC9153]  Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements and Terminology", RFC 9153,
              DOI 10.17487/RFC9153, February 2022,
              <https://www.rfc-editor.org/info/rfc9153>.

11.2.  Informative References

   [CTA2063A] ANSI, "Small Unmanned Aerial Systems Serial Numbers",
              2019.

   [Delegated]
              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]    ASTM International, "Standard Specification for Remote ID
              and Tracking", February 2020,
              <http://www.astm.org/cgi-bin/resolver.cgi?F3411>.

   [FAA_RID]  United States Federal Aviation Administration (FAA),
              "Remote Identification of Unmanned Aircraft", 2021,
              <https://www.govinfo.gov/content/pkg/FR-2021-01-15/
              pdf/2020-28948.pdf>.

   [FAA_UAS_Concept_Of_Ops]
              United States Federal Aviation Administration (FAA),
              "Unmanned Aircraft System (UAS) Traffic Management (UTM)
              Concept of Operations (V2.0)", 2020,
              <https://www.faa.gov/uas/research_development/
              traffic_management/media/UTM_ConOps_v2.pdf>.

   [FS_AEUA]  "Study of Further Architecture Enhancement for UAV and
              UAM", 2021, <https://www.3gpp.org/ftp/tsg_sa/WG2_Arch/
              TSGS2_147E_Electronic_2021-10/Docs/S2-2107092.zip>.

   [I-D.ietf-drip-auth]
              Wiethuechter, A., Card, S., and R. Moskowitz, "DRIP
              Authentication Formats & Protocols for Broadcast Remote
              ID", Work in Progress, Internet-Draft, draft-ietf-drip-
              auth-05, 7 March 2022, <https://www.ietf.org/archive/id/
              draft-ietf-drip-auth-05.txt>.

   [Implementing]
              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.,
              <https://www.faa.gov/uas/programs_partnerships/
              data_exchange/>.

   [MAVLink]  "Micro Air Vehicle Communication Protocol", 2021,
              <http://mavlink.io/>.

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

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
              <https://www.rfc-editor.org/info/rfc5730>.

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,
              <https://www.rfc-editor.org/info/rfc5731>.

   [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,
              <https://www.rfc-editor.org/info/rfc7401>.

   [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 7480, DOI 10.17487/RFC7480, March 2015,
              <https://www.rfc-editor.org/info/rfc7480>.

   [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,
              <https://www.rfc-editor.org/info/rfc7519>.

   [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>.

   [RFC8005]  Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
              October 2016, <https://www.rfc-editor.org/info/rfc8005>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [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,
              <https://www.rfc-editor.org/info/rfc8949>.

   [RFC9082]  Hollenbeck, S. and A. Newton, "Registration Data Access
              Protocol (RDAP) Query Format", STD 95, RFC 9082,
              DOI 10.17487/RFC9082, June 2021,
              <https://www.rfc-editor.org/info/rfc9082>.

   [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 9083, DOI 10.17487/RFC9083, June 2021,
              <https://www.rfc-editor.org/info/rfc9083>.

   [TS-22.825]
              3GPP, "Study on Remote Identification of Unmanned Aerial
              Systems (UAS)", n.d.,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3527>.

   [U-Space]  European Organization for the Safety of Air Navigation
              (EUROCONTROL), "U-space Concept of Operations", 2019,
              <https://www.sesarju.eu/sites/default/files/documents/u-
              space/CORUS%20ConOps%20vol2.pdf>.

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 to integrate UAS operations 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 joint FAA and NASA UTM research
   transition team.  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 their UTM concepts of operations
   to guide the development of their future air traffic management (ATM)
   system and ensure safe and efficient integration of manned and
   unmanned aircraft into the national airspace.

   UTM comprises UAS operations infrastructure, procedures and local
   regulation compliance policies to guarantee safe UAS integration and
   operation.  The main functionality of 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 UTM has to offer.  Such an 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 that 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 envisioned
   functionality of UTM.  The LAANC program can automate UAS operational
   intent (flight plan) submission and application for airspace
   authorization in real-time by checking against multiple aeronautical
   databases such as airspace classification and operating rules
   associated with it, FAA UAS facility map, special use airspace,
   Notice to Airmen (NOTAM), and Temporary Flight Restriction (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
       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 facility map database, obtaining the air traffic
       control (ATC) authorization, and monitoring 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
       uncontrolled airspace (e.g., 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 the received operational intent (flight plan) from the
       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 UAS.  Technical reasons include but not limited to the
   following:

   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.

Acknowledgements

   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.  Thanks to
   Alexandre Petrescu and Stephan Wenger for the helpful and positive
   comments.  Thanks to chairs Daniel Migault and Mohamed Boucadair for
   direction of our team of authors and editor, some of whom are
   newcomers to writing IETF documents.  Laura Welch is also thanked for
   her valuable review comments that led to great improvements of this
   memo.  Thanks especially to Internet Area Director Eric Vyncke for
   guidance and support.

Authors' Addresses

   Stuart W. Card
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY,  13495
   United States of America
   Email: stu.card@axenterprize.com

   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
   Tencent
   2747 Park Blvd
   Palo Alto,  94588
   United States of America
   Email: shuai.zhao@ieee.org

   Andrei Gurtov
   Linköping University
   IDA
   SE-58183 Linköping Linköping
   Sweden
   Email: gurtov@acm.org