Internet Engineering Task Force (IETF)                      S. Card, Ed.
Request for Comments: 9153                               A. Wiethuechter
Category: Informational                                    AX Enterprize
ISSN: 2070-1721                                             R. Moskowitz
                                                          HTT Consulting
                                                               A. Gurtov
                                                    Linköping University
                                                           February 2022


Drone Remote Identification Protocol (DRIP) Requirements and Terminology

Abstract

   This document defines terminology and requirements for solutions
   produced by the Drone Remote Identification Protocol (DRIP) Working
   Group.  These solutions will support Unmanned Aircraft System Remote
   Identification and tracking (UAS RID) for security, safety, and other
   purposes (e.g., initiation of identity-based network sessions
   supporting UAS applications).  DRIP will facilitate use of existing
   Internet resources to support RID and to enable enhanced related
   services, and it will enable online and offline verification that RID
   information is trustworthy.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9153.

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Table of Contents

   1.  Introduction
     1.1.  Motivation and External Influences
     1.2.  Concerns and Constraints
     1.3.  DRIP Scope
     1.4.  Document Scope
   2.  Terms and Definitions
     2.1.  Requirements Terminology
     2.2.  Definitions
   3.  UAS RID Problem Space
     3.1.  Network RID
     3.2.  Broadcast RID
     3.3.  USS in UTM and RID
     3.4.  DRIP Focus
   4.  Requirements
     4.1.  General
       4.1.1.  Normative Requirements
       4.1.2.  Rationale
     4.2.  Identifier
       4.2.1.  Normative Requirements
       4.2.2.  Rationale
     4.3.  Privacy
       4.3.1.  Normative Requirements
       4.3.2.  Rationale
     4.4.  Registries
       4.4.1.  Normative Requirements
       4.4.2.  Rationale
   5.  IANA Considerations
   6.  Security Considerations
   7.  Privacy and Transparency Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Discussion and Limitations
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document defines terminology and requirements for solutions
   produced by the Drone Remote Identification Protocol (DRIP) Working
   Group.  These solutions will support Unmanned Aircraft System Remote
   Identification and tracking (UAS RID) for security, safety, and other
   purposes (e.g., initiation of identity-based network sessions
   supporting UAS applications).  DRIP will facilitate use of existing
   Internet resources to support RID and to enable enhanced related
   services, and it will enable online and offline verification that RID
   information is trustworthy.

   For any unfamiliar or a priori ambiguous terminology herein, see
   Section 2.

1.1.  Motivation and External Influences

   Many considerations (especially safety and security) necessitate
   Unmanned Aircraft System Remote Identification and tracking (UAS
   RID).

   Unmanned Aircraft (UA) may be fixed-wing, rotary-wing (e.g.,
   helicopter), hybrid, balloon, rocket, etc.  Small fixed-wing UA
   typically have Short Take-Off and Landing (STOL) capability; rotary-
   wing and hybrid UA typically have Vertical Take-Off and Landing
   (VTOL) capability.  UA may be single- or multi-engine.  The most
   common today are multicopters (rotary-wing, multi-engine).  The
   explosion in UAS was enabled by hobbyist development of advanced
   flight stability algorithms for multicopters that enabled even
   inexperienced pilots to take off, fly to a location of interest,
   hover, and return to the takeoff location or land at a distance.  UAS
   can be remotely piloted by a human (e.g., with a joystick) or
   programmed to proceed from Global Navigation Satellite System (GNSS)
   waypoint to waypoint in a weak form of autonomy; stronger autonomy is
   coming.

   Small UA are "low observable" as they:

   *  typically have small radar cross sections;

   *  make noise that is quite noticeable at short ranges but difficult
      to detect at distances they can quickly close (500 meters in under
      13 seconds by the fastest consumer mass-market drones available in
      early 2021);

   *  typically fly at low altitudes (e.g., under 400 feet Above Ground
      Level (AGL) for UA to which RID applies in the US, as per
      [Part107]); and

   *  are highly maneuverable and thus can fly under trees and between
      buildings.

   UA can carry payloads (including sensors, cyber weapons, and kinetic
   weapons) or can be used themselves as weapons by flying them into
   targets.  They can be flown by clueless, careless, or criminal
   operators.  Thus, the most basic function of UAS RID is
   "Identification Friend or Foe (IFF)" to mitigate the significant
   threat they present.

   Diverse other applications can be enabled or facilitated by RID.
   Internet protocols typically start out with at least one entity
   already knowing an identifier or locator of another; but an entity
   (e.g., UAS or Observer device) encountering an a priori unknown UA in
   physical space has no identifier or logical space locator for that
   UA, unless and until one is provided somehow.  RID provides an
   identifier, which, if well chosen, can facilitate use of a variety of
   Internet family protocols and services to support arbitrary
   applications beyond the basic security functions of RID.  For most of
   these, some type of identifier is essential, e.g., Network Access
   Identifier (NAI), Digital Object Identifier (DOI), Uniform Resource
   Identifier (URI), domain name, or public key.  DRIP motivations
   include both the basic security and the broader application support
   functions of RID.  The general scenario is illustrated in Figure 1.

                  +-----+    +-----+
                  | UA1 |    | UA2 |
                  +-----+    +-----+

      +----------+                   +----------+
      | General  |                   | Public   |
      | Public   |                   | Safety   |
      | Observer o------\     /------o Observer |
      +----------+      |     |      +----------+
                        |     |
                     *************
   +----------+      *           *      +----------+
   | UA1      |      *           *      | UA2      |
   | Pilot/   o------*  Internet *------o Pilot/   |
   | Operator |      *           *      | Operator |
   +----------+      *           *      +----------+
                     *************
                       |   |   |
        +----------+   |   |   |   +----------+
        | Public   o---/   |   \---o Private  |
        | Registry |       |       | Registry |
        +----------+       |       +----------+
                        +--o--+
                        | DNS |
                        +-----+

                  Figure 1: General UAS RID Usage Scenario

   Figure 1 illustrates a typical case where there may be the following:

   *  multiple Observers, some of them members of the general public and
      others government officers with public safety and security
      responsibilities,

   *  multiple UA in flight within observation range, each with its own
      pilot/operator,

   *  at least one registry each for lookup of public and (by authorized
      parties only) private information regarding the UAS and their
      pilots/operators, and

   *  in the DRIP vision, DNS resolving various identifiers and locators
      of the entities involved.

   Note the absence of any links to/from the UA in the figure; this is
   because UAS RID and other connectivity involving the UA varies.  Some
   connectivity paths do or do not exist depending upon the scenario.
   Command and Control (C2) from the Ground Control Station (GCS) to the
   UA via the Internet (e.g., using LTE cellular) is expected to become
   much more common as Beyond Visual Line Of Sight (BVLOS) operations
   increase; in such a case, there is typically not also a direct
   wireless link between the GCS and UA.  Conversely, if C2 is running
   over a direct wireless link, then the GCS typically has Internet
   connectivity, but the UA does not.  Further, paths that nominally
   exist, such as between an Observer device and the Internet, may be
   severely intermittent.  These connectivity constraints are likely to
   have an impact, e.g., on how reliably DRIP requirements can be
   satisfied.

   An Observer of UA may need to classify them, as illustrated
   notionally in Figure 2, for basic airspace Situational Awareness
   (SA).  An Observer can classify a UAS as one of the following and
   treat as:

   *  Taskable: can ask it to do something useful.

   *  Low Concern: can reasonably assume it is not malicious and would
      cooperate with requests to modify its flight plans for safety
      concerns that arise.

   *  High Concern or Unidentified: can focus surveillance on it.

                        xxxxxxx
                       x       x   No  +--------------+
                      x   ID?   x+---->| Unidentified |
                       x       x       +--------------+
                        xxxxxxx
                           +
                           | Yes
                           v
                        xxxxxxx
                       x       x
           .---------+x  Type?  x+----------.
           |           x       x            |
           |            xxxxxxx             |
           |               +                |
           v               v                v
   +--------------+ +--------------+ +--------------+
   |  Taskable    | | Low Concern  | | High Concern |
   +--------------+ +--------------+ +--------------+

                   Figure 2: Notional UAS Classification

   The widely cited "Standard Specification for Remote ID and Tracking"
   [F3411-19] was developed by ASTM International, Technical Committee
   F38 (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item
   WK65041.  The published standard is available for purchase from ASTM
   and is also available as an ASTM membership premium; early draft
   versions are freely available as Open Drone ID specifications
   [OpenDroneID].  [F3411-19] is frequently referenced in DRIP, where
   building upon its link layers and both enhancing support for and
   expanding the scope of its applications are central foci.

   In many applications, including UAS RID, identification and
   identifiers are not ends in themselves; they exist to enable lookups
   and provision of other services.

   Using UAS RID to facilitate vehicular (i.e., Vehicle-to-Everything
   (V2X)) communications and applications such as Detect And Avoid
   (DAA), which would impose tighter latency bounds than RID itself, is
   an obvious possibility; this is explicitly contemplated in the
   "Remote Identification of Unmanned Aircraft" rule of the US Federal
   Aviation Administration (FAA) [FRUR].  However, usage of RID systems
   and information beyond mere identification (primarily to hold
   operators accountable after the fact), including DAA, were declared
   out of scope in ASTM F38.02 WK65041, based on a distinction between
   RID as a security standard versus DAA as a safety application.
   Standards Development Organizations (SDOs) in the aviation community
   generally set a higher bar for safety than for security, especially
   with respect to reliability.  Each SDO has its own cultural set of
   connotations of safety versus security; the denotative definitions of
   the International Civil Aviation Organization (ICAO) are cited in
   Section 2.

   [Opinion1] and [WG105] cite the Direct Remote Identification (DRI)
   previously required and specified, explicitly stating that whereas
   DRI is primarily for security purposes, the "Network Identification
   Service" [Opinion1] (in the context of U-space [InitialView]) or
   "Electronic Identification" [WG105] is primarily for safety purposes
   (e.g., Air Traffic Management, especially hazards deconfliction) and
   also is allowed to be used for other purposes such as support of
   efficient operations.  These emerging standards allow the security-
   and safety-oriented systems to be separate or merged.  In addition to
   mandating both Broadcast and Network RID one-way to Observers, they
   will use Vehicle-to-Vehicle (V2V) to other UAS (also likely to and/or
   from some manned aircraft).  These reflect the broad scope of the
   European Union (EU) U-space concept, as being developed in the Single
   European Sky ATM Research (SESAR) Joint Undertaking, the U-space
   architectural principles of which are outlined in [InitialView].

   ASD-STAN is an Associated Body to CEN (European Committee for
   Standardization) for Aerospace Standards.  It has published an EU
   standard titled "Aerospace series - Unmanned Aircraft Systems - Part
   002: Direct Remote Identification" [ASDSTAN4709-002]; a current
   (early 2021) informal overview is freely available in [ASDRI] (note
   that [ASDRI] may not precisely reflect the final standard as it was
   published before [ASDSTAN4709-002]).  It will provide compliance to
   cover the identical DRI requirements applicable to drones of the
   following classes:

   *  C1 ([Delegated], Part 2)

   *  C2 ([Delegated], Part 3)

   *  C3 ([Delegated], Part 4)

   *  C5 ([Amended], Part 16)

   *  C6 ([Amended], Part 17)

   The standard contemplated in [ASDRI] will provide UA capability to be
   identified in real time during the whole duration of the flight,
   without specific connectivity or ground infrastructure link,
   utilizing existing mobile devices within broadcast range.  It will
   use Bluetooth 4, Bluetooth 5, Wi-Fi Neighbor Awareness Networking
   (NAN) (also known as "Wi-Fi Aware" [WiFiNAN]), and/or IEEE 802.11
   Beacon modes.  The emphasis of the EU standard is compatibility with
   [F3411-19], although there are differences in mandatory and optional
   message types and fields.

   The DRI system contemplated in [ASDRI] will broadcast the following
   locally:

   1.  the UAS operator registration number;

   2.  the [CTA2063A]-compliant unique serial number of the UA;

   3.  a time stamp, the geographical position of the UA, and its height
       AGL or above its takeoff point;

   4.  the UA ground speed and route course measured clockwise from true
       north;

   5.  the geographical position of the Remote Pilot, or if that is not
       available, the geographical position of the UA takeoff point; and

   6.  for classes C1, C2, C3, the UAS emergency status.

   Under the standard contemplated in [ASDRI], data will be sent in
   plaintext, and the UAS operator registration number will be
   represented as a 16-byte string including the (European) state code.
   The corresponding private ID part will contain three characters that
   are not broadcast but used by authorities to access regional
   registration databases for verification.

   ASD-STAN also contemplates corresponding Network Remote
   Identification (NRI) functionality.  ASD-STAN plans to revise their
   current standard with additional functionality (e.g., DRIP) to be
   published no later than 2022 [ASDRI].

   Security-oriented UAS RID essentially has two goals: 1) enable the
   general public to obtain and record an opaque ID for any observed UA,
   which they can then report to authorities and 2) enable authorities,
   from such an ID, to look up information about the UAS and its
   operator.  Safety-oriented UAS RID has stronger requirements.

   Dynamic establishment of secure communications between the Observer
   and the UAS pilot seems to have been contemplated by the FAA UAS ID
   and Tracking Aviation Rulemaking Committee (ARC) in
   [Recommendations]; however, aside from DRIP, it is not addressed in
   any of the subsequent regulations or international SDO technical
   specifications known to the authors as of early 2021.

1.2.  Concerns and Constraints

   Disambiguation of multiple UA flying in close proximity may be very
   challenging, even if each is reporting its identity, position, and
   velocity as accurately as it can.

   The origin of information in UAS RID and UAS Traffic Management (UTM)
   generally is the UAS or its operator.  Self-reports may be initiated
   by the Remote Pilot at the console of the GCS (the UAS subsystem used
   to remotely operate the UA) or automatically by GCS software; in
   Broadcast RID, they are typically initiated automatically by a
   process on the UA.  Data in the reports may come from sensors
   available to the operator (e.g., radar or cameras), the GCS (e.g.,
   "dead reckoning" UA location, starting from the takeoff location and
   estimating the displacements due to subsequent piloting commands,
   wind, etc.), or the UA itself (e.g., an on-board GNSS receiver).  In
   Broadcast RID, all the data must be sent proximately by the UA, and
   most of the data ultimately comes from the UA.  Whether information
   comes proximately from the operator or from automated systems
   configured by the operator, there are possibilities of unintentional
   error in and intentional falsification of this data.  Mandating UAS
   RID, specifying data elements required to be sent, monitoring
   compliance, and enforcing compliance (or penalizing non-compliance)
   are matters for Civil Aviation Authorities (CAAs) and potentially
   other authorities.  Specifying message formats and supporting
   technologies to carry those data elements has been addressed by other
   SDOs.  Offering technical means, as extensions to external standards,
   to facilitate verifiable compliance and enforcement/monitoring is an
   opportunity for DRIP.

   Minimal specified information must be made available to the public.
   Access to other data, e.g., UAS operator Personally Identifiable
   Information (PII), must be limited to strongly authenticated
   personnel, properly authorized in accordance with applicable policy.
   The balance between privacy and transparency remains a subject for
   public debate and regulatory action; DRIP can only offer tools to
   expand the achievable trade space and enable trade-offs within that
   space.  [F3411-19], the basis for most current (2021) thinking about
   and efforts to provide UAS RID, specifies only how to get the UAS ID
   to the Observer: how the Observer can perform these lookups and how
   the registries first can be populated with information are not
   specified therein.

   The need for nearly universal deployment of UAS RID is pressing:
   consider how negligible the value of an automobile license plate
   system would be if only 90% of the cars displayed plates.  This
   implies the need to support use by Observers of already-ubiquitous
   mobile devices (typically smartphones and tablets).  Anticipating CAA
   requirements to support legacy devices, especially in light of
   [Recommendations], [F3411-19] specifies that any UAS sending
   Broadcast RID over Bluetooth must do so over Bluetooth 4, regardless
   of whether it also does so over newer versions.  As UAS sender
   devices and Observer receiver devices are unpaired, this unpaired
   state requires use of the extremely short BT4 "advertisement"
   (beacon) frames.

   Wireless data links to or from UA are challenging.  Flight is often
   amidst structures and foliage at low altitudes over varied terrain.
   UA are constrained in both total energy and instantaneous power by
   their batteries.  Small UA imply small antennas.  Densely populated
   volumes will suffer from link congestion: even if UA in an airspace
   volume are few, other transmitters nearby on the ground, sharing the
   same license free spectral band, may be many.  Thus, air-to-air and
   air-to-ground links will generally be slow and unreliable.

   UAS Cost, Size, Weight, and Power (CSWaP) constraints are severe.
   CSWaP is a burden not only on the designers of new UAS for sale but
   also on owners of existing UAS that must be retrofit.  Radio
   Controlled (RC) aircraft modelers, "hams" who use licensed amateur
   radio frequencies to control UAS, drone hobbyists, and others who
   custom build UAS all need means of participating in UAS RID that are
   sensitive to both generic CSWaP and application-specific
   considerations.

   To accommodate the most severely constrained cases, all of the
   concerns described above conspire to motivate system design decisions
   that complicate the protocol design problem.

   Broadcast RID uses one-way local data links.  UAS may have Internet
   connectivity only intermittently, or not at all, during flight.

   Internet-disconnected operation of Observer devices has been deemed
   by ASTM F38.02 as too infrequent to address.  However, the preamble
   to [FRUR] cites "remote and rural areas that do not have reliable
   Internet access" as a major reason for requiring Broadcast rather
   than Network RID.  [FRUR] also states:

   |  Personal wireless devices that are capable of receiving 47 CFR
   |  part 15 frequencies, such as smart phones, tablets, or other
   |  similar commercially available devices, will be able to receive
   |  broadcast remote identification information directly without
   |  reliance on an Internet connection.

   Internet-disconnected operation presents challenges, e.g., for
   Observers needing access to the [F3411-19] web-based Broadcast
   Authentication Verifier Service or needing to do external lookups.

   As RID must often operate within these constraints, heavyweight
   cryptographic security protocols or even simple cryptographic
   handshakes are infeasible, yet trustworthiness of UAS RID information
   is essential.  Under [F3411-19], _even the most basic datum, the UAS
   ID itself, can be merely an unsubstantiated claim_.

   Observer devices are ubiquitous; thus, they are popular targets for
   malware or other compromise, so they cannot be generally trusted
   (although the user of each device is compelled to trust that device,
   to some extent).  A "fair witness" functionality (inspired by
   [Stranger]) is desirable.

   Despite work by regulators and SDOs, there are substantial gaps in
   UAS standards generally and UAS RID specifically.  [Roadmap] catalogs
   UAS-related standards, ongoing standardization activities, and gaps
   (as of 2020); Section 7.8 catalogs those related specifically to UAS
   RID.  DRIP will address the most fundamental of these gaps, as
   foreshadowed above.

1.3.  DRIP Scope

   DRIP's initial objective is to make RID immediately actionable,
   especially in emergencies, in severely constrained UAS environments
   (both Internet and local-only connected scenarios), balancing
   legitimate (e.g., public safety) authorities' Need To Know
   trustworthy information with UAS operators' privacy.  The phrase
   "immediately actionable" means information of sufficient precision,
   accuracy, and timeliness for an Observer to use it as the basis for
   immediate decisive action (e.g., triggering a defensive counter-UAS
   system, attempting to initiate communications with the UAS operator,
   accepting the presence of the UAS in the airspace where/when observed
   as not requiring further action, etc.) with potentially severe
   consequences of any action or inaction chosen based on that
   information.  For further explanation of the concept of immediate
   actionability, see [ENISACSIRT].

   Note that UAS RID must achieve nearly universal adoption, but DRIP
   can add value even if only selectively deployed.  Authorities with
   jurisdiction over more sensitive airspace volumes may set a RID
   requirement, for flight in such volumes, that is higher than
   generally mandated.  Those with a greater need for high-confidence
   IFF can equip with DRIP, enabling strong authentication of their own
   aircraft and allied operators without regard for the weaker (if any)
   authentication of others.

   DRIP (originally "Trustworthy Multipurpose Remote Identification (TM-
   RID)") could be applied to verifiably identify other types of
   registered things reported to be in specified physical locations.
   Providing timely trustworthy identification data is also prerequisite
   to identity-oriented networking.  Despite the value of DRIP to these
   and other potential applications, UAS RID is the urgent motivation
   and clear initial focus of DRIP.  Existing Internet resources
   (protocol standards, services, infrastructure, and business models)
   should be leveraged.

1.4.  Document Scope

   This document describes the problem space for UAS RID conforming to
   proposed regulations and external technical standards, defines common
   terminology, specifies numbered requirements for DRIP, identifies
   some important considerations (security, privacy, and transparency),
   and discusses limitations.

   A natural Internet-based approach to meet these requirements is
   described in a companion architecture document [DRIP-ARCH] and
   elaborated in other DRIP documents.

2.  Terms and Definitions

2.1.  Requirements Terminology

   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.

2.2.  Definitions

   This section defines a non-comprehensive set of terms expected to be
   used in DRIP documents.  This list is meant to be the DRIP
   terminology reference; as such, some of the terms listed below are
   not used in this document.

   To encourage comprehension necessary for adoption of DRIP by the
   intended user community, the UAS community's norms are respected
   herein, and definitions are quoted in cases where they have been
   found in that community's documents.  Most of the terms listed below
   are from that community (even if specific source documents are not
   cited); any terms that are DRIP-specific or defined by this document
   are marked "(DRIP)".

   Note that, in the UAS community, the plural form of an acronym,
   generally, is the same as the singular form, e.g., Unmanned Aircraft
   System (singular) and Unmanned Aircraft Systems (plural) are both
   represented as UAS.

   [RFC4949] provides a glossary of Internet security terms that should
   be used where applicable.

   4-D
      Four-dimensional.  Latitude, Longitude, Altitude, Time.  Used
      especially to delineate an airspace volume in which an operation
      is being or will be conducted.

   AAA
      Attestation, Authentication, Authorization, Access Control,
      Accounting, Attribution, Audit, or any subset thereof (uses differ
      by application, author, and context).  (DRIP)

   ABDAA
      AirBorne DAA.  Accomplished using systems onboard the aircraft
      involved.  Supports "self-separation" (remaining "well clear" of
      other aircraft) and collision avoidance.

   ADS-B
      Automatic Dependent Surveillance - Broadcast.  "ADS-B Out"
      equipment obtains aircraft position from other on-board systems
      (typically GNSS) and periodically broadcasts it to "ADS-B In"
      equipped entities, including other aircraft, ground stations, and
      satellite-based monitoring systems.

   AGL
      Above Ground Level.  Relative altitude, above the variously
      defined local ground level, typically of a UA, measured in feet or
      meters.  Should be explicitly specified as either barometric
      (pressure) or geodetic (GNSS) altitude.

   ATC
      Air Traffic Control.  Explicit flight direction to pilots from
      ground controllers.  Contrast with ATM.

   ATM
      Air Traffic Management.  A broader functional and geographic scope
      and/or a higher layer of abstraction than ATC.  [ICAOATM] defines
      ATM as the following: "The dynamic, integrated management of air
      traffic and airspace including air traffic services, airspace
      management and air traffic flow management -- safely, economically
      and efficiently -- through the provision of facilities and
      seamless services in collaboration with all parties and involving
      airborne and ground-based functions".

   Authentication Message
      [F3411-19] Message Type 2.  Provides framing for authentication
      data only; the only message that can be extended in length by
      segmenting it across more than one page.

   Basic ID Message
      [F3411-19] Message Type 0.  Provides UA Type, ID Type (and
      Specific Session ID subtype if applicable), and UAS ID only.

   Broadcast Authentication Verifier Service
      System component designed to handle any authentication of
      Broadcast RID by offloading signature verification to a web
      service [F3411-19].

   BVLOS
      Beyond Visual Line Of Sight.  See VLOS.

   byte
      Used here in its now-customary sense as a synonym for "octet", as
      "byte" is used exclusively in definitions of data structures
      specified in [F3411-19].

   CAA
      Civil Aviation Authority of a regulatory jurisdiction.  Often so
      named, but other examples include the United States Federal
      Aviation Administration (FAA) and the Japan Civil Aviation Bureau.

   CSWaP
      Cost, Size, Weight, and Power

   C2
      Command and Control.  Previously mostly used in military contexts.
      Properly refers to a function that is exercisable over arbitrary
      communications, but in the small UAS context, often refers to the
      communications (typically RF data link) over which the GCS
      controls the UA.

   DAA
      Detect And Avoid, formerly "Sense And Avoid (SAA)".  A means of
      keeping aircraft "well clear" of each other and obstacles for
      safety.  [ICAOUAS] defines DAA as the following: "The capability
      to see, sense or detect conflicting traffic or other hazards and
      take the appropriate action to comply with the applicable rules of
      flight".

   DRI (not to be confused with DRIP)
      Direct Remote Identification.  EU regulatory requirement for "a
      system that ensures the local broadcast of information about a UA
      in operation, including the marking of the UA, so that this
      information can be obtained without physical access to the UA"
      [Delegated].  This requirement can presumably be satisfied with
      appropriately configured [F3411-19] Broadcast RID.

   DSS
      Discovery and Synchronization Service.  The UTM system overlay
      network backbone.  Most importantly, it enables one USS to learn
      which other USS have UAS operating in a given 4-D airspace volume,
      for strategic deconfliction of planned operations and Network RID
      surveillance of active operations.  See [F3411-19].

   EUROCAE
      European Organisation for Civil Aviation Equipment.  Aviation SDO,
      originally European, now with broader membership.  Cooperates
      extensively with RTCA.

   GBDAA
      Ground-Based DAA.  Accomplished with the aid of ground-based
      functions.

   GCS
      Ground Control Station.  The part of the UAS that the Remote Pilot
      uses to exercise C2 over the UA, whether by remotely exercising UA
      flight controls to fly the UA, by setting GNSS waypoints, or by
      otherwise directing its flight.

   GNSS
      Global Navigation Satellite System.  Satellite-based timing and/or
      positioning with global coverage, often used to support
      navigation.

   GPS
      Global Positioning System.  A specific GNSS, but in the UAS
      context, the term is typically misused in place of the more
      generic term "GNSS".

   GRAIN
      Global Resilient Aviation Interoperable Network.  ICAO-managed
      IPv6 overlay internetwork based on IATF that is dedicated to
      aviation (but not just aircraft).  As currently (2021) designed,
      it accommodates the proposed DRIP identifier.

   IATF
      International Aviation Trust Framework.  ICAO effort to develop a
      resilient and secure by design framework for networking in support
      of all aspects of aviation.

   ICAO
      International Civil Aviation Organization.  A specialized agency
      of the United Nations that develops and harmonizes international
      standards relating to aviation.

   IFF
      Identification Friend or Foe. Originally, and in its narrow sense
      still, a self-identification broadcast in response to
      interrogation via radar to reduce friendly fire incidents, which
      led to military and commercial transponder systems such as ADS-B.
      In the broader sense used here, any process intended to
      distinguish friendly from potentially hostile UA or other entities
      encountered.

   LAANC
      Low Altitude Authorization and Notification Capability.  Supports
      ATC authorization requirements for UAS operations: Remote Pilots
      can apply to receive a near real-time authorization for operations
      under 400 feet in controlled airspace near airports.  FAA-
      authorized partial stopgap in the US until UTM comes.

   Location/Vector Message
      [F3411-19] Message Type 1.  Provides UA location, altitude,
      heading, speed, and status.

   LOS
      Line Of Sight.  An adjectival phrase describing any information
      transfer that travels in a nearly straight line (e.g.,
      electromagnetic energy, whether in the visual light, RF, or other
      frequency range) and is subject to blockage.  A term to be avoided
      due to ambiguity, in this context, between RF LOS and VLOS.

   Message Pack
      [F3411-19] Message Type 15.  The framed concatenation, in message
      type index order, of at most one message of each type of any
      subset of the other types.  Required to be sent in Wi-Fi NAN and
      in Bluetooth 5 Extended Advertisements, if those media are used;
      cannot be sent in Bluetooth 4.

   MSL
      Mean Sea Level.  Shorthand for relative altitude, above the
      variously defined mean sea level, typically of a UA (but in
      [FRUR], also for a GCS), measured in feet or meters.  Should be
      explicitly specified as either barometric (pressure) or geodetic
      (e.g., as indicated by GNSS, referenced to the WGS84 ellipsoid).

   Net-RID DP
      Network RID Display Provider.  [F3411-19] logical entity that
      aggregates data from Net-RID SPs as needed in response to user
      queries regarding UAS operating within specified airspace volumes
      to enable display by a user application on a user device.
      Potentially could provide not only information sent via UAS RID
      but also information retrieved from UAS RID registries or
      information beyond UAS RID.  Under superseded [NPRM], not
      recognized as a distinct entity, but as a service provided by USS,
      including public safety USS that may exist primarily for this
      purpose rather than to manage any subscribed UAS.

   Net-RID SP
      Network RID Service Provider.  [F3411-19] logical entity that
      collects RID messages from UAS and responds to Net-RID DP queries
      for information on UAS of which it is aware.  Under superseded
      [NPRM], the USS to which the UAS is subscribed (i.e., the "Remote
      ID USS").

   Network Identification Service
      EU regulatory requirement in [Opinion1], corresponding to the
      Electronic Identification for which Minimum Operational
      Performance Standards are specified in [WG105], which presumably
      can be satisfied with appropriately configured [F3411-19] Network
      RID.

   Observer
      An entity (typically, but not necessarily, an individual human)
      who has directly or indirectly observed a UA and wishes to know
      something about it, starting with its ID.  An Observer typically
      is on the ground and local (within VLOS of an observed UA), but
      could be remote (observing via Network RID or other surveillance),
      operating another UA, aboard another aircraft, etc.  (DRIP)

   Operation
      A flight, or series of flights of the same mission, by the same
      UAS, separated by, at most, brief ground intervals.  (Inferred
      from UTM usage; no formal definition found.)

   Operator
      "A person, organization or enterprise engaged in or offering to
      engage in an aircraft operation" [ICAOUAS].

   Operator ID Message
      [F3411-19] Message Type 5.  Provides CAA-issued Operator ID only.
      Operator ID is distinct from UAS ID.

   page
      Payload of a frame, containing a chunk of a message that has been
      segmented, that allows transport of a message longer than can be
      encapsulated in a single frame.  See [F3411-19].

   PIC
      Pilot In Command.  "The pilot designated by the operator, or in
      the case of general aviation, the owner, as being in command and
      charged with the safe conduct of a flight" [ICAOUAS].

   PII
      Personally Identifiable Information.  In the UAS RID context,
      typically of the UAS Operator, PIC, or Remote Pilot, but possibly
      of an Observer or other party.  This specific term is used
      primarily in the US; other terms with essentially the same meaning
      are more common in other jurisdictions (e.g., "personal data" in
      the EU).  Used herein generically to refer to personal information
      that the person might wish to keep private or may have a
      statutorily recognized right to keep private (e.g., under the EU
      [GDPR]), potentially imposing (legally or ethically) a
      confidentiality requirement on protocols/systems.

   Remote Pilot
      A pilot using a GCS to exercise proximate control of a UA.  Either
      the PIC or under the supervision of the PIC.  "The person who
      manipulates the flight controls of a remotely-piloted aircraft
      during flight time" [ICAOUAS].

   RF
      Radio Frequency.  Can be used as an adjective (e.g., "RF link") or
      as a noun.

   RF LOS
      RF Line Of Sight.  Typically used in describing a direct radio
      link between a GCS and the UA under its control, potentially
      subject to blockage by foliage, structures, terrain, or other
      vehicles, but less so than VLOS.

   RTCA
      Radio Technical Commission for Aeronautics.  US aviation SDO.
      Cooperates extensively with EUROCAE.

   Safety
      "The state in which risks associated with aviation activities,
      related to, or in direct support of the operation of aircraft, are
      reduced and controlled to an acceptable level" (from Annex 19 of
      the Chicago Convention, quoted in [ICAODEFS]).

   Security
      "Safeguarding civil aviation against acts of unlawful
      interference" (from Annex 17 of the Chicago Convention, quoted in
      [ICAODEFS]).

   Self-ID Message
      [F3411-19] Message Type 3.  Provides a 1-byte descriptor and
      23-byte ASCII free text field, only.  Expected to be used to
      provide context on the operation, e.g., mission intent.

   SDO
      Standards Development Organization, such as ASTM, IETF, etc.

   SDSP
      Supplemental Data Service Provider.  An entity that participates
      in the UTM system but provides services (e.g., weather data)
      beyond those specified as basic UTM system functions.  See
      [FAACONOPS].

   System Message
      [F3411-19] Message Type 4.  Provides general UAS information,
      including Remote Pilot location, multiple UA group operational
      area, etc.

   U-space
      EU concept and emerging framework for integration of UAS into all
      types of airspace, including but not limited to volumes that are
      in high-density urban areas and/or shared with manned aircraft
      [InitialView].

   UA
      Unmanned Aircraft.  In popular parlance, "drone".  "An aircraft
      which is intended to operate with no pilot on board" [ICAOUAS].

   UAS
      Unmanned Aircraft System.  Composed of UA, all required on-board
      subsystems, payload, control station, other required off-board
      subsystems, any required launch and recovery equipment, all
      required crew members, and C2 links between UA and control station
      [F3411-19].

   UAS ID
      UAS identifier.  Although called "UAS ID", it is actually unique
      to the UA, neither to the operator (as some UAS registration
      numbers have been and for exclusively recreational purposes are
      continuing to be assigned), nor to the combination of GCS and UA
      that comprise the UAS.  _Maximum length of 20 bytes_ [F3411-19].
      If the ID Type is 4, the proposed Specific Session ID, then the 20
      bytes includes the subtype index, leaving only 19 bytes for the
      actual identifier.

   ID Type
      UAS identifier type index. 4 bits.  See Section 3, Paragraph 6 for
      current standard values 0-3 and currently proposed additional
      value 4.  See also [F3411-19].

   UAS RID
      UAS Remote Identification and tracking.  System to enable
      arbitrary Observers to identify UA during flight.

   USS
      UAS Service Supplier.  "A USS is an entity that assists UAS
      Operators with meeting UTM operational requirements that enable
      safe and efficient use of airspace" [FAACONOPS].  In addition,
      "USSs provide services to support the UAS community, to connect
      Operators and other entities to enable information flow across the
      USS Network, and to promote shared situational awareness among UTM
      participants" [FAACONOPS].

   UTM
      UAS Traffic Management.  "A specific aspect of air traffic
      management which manages UAS operations safely, economically and
      efficiently through the provision of facilities and a seamless set
      of services in collaboration with all parties and involving
      airborne and ground-based functions" [ICAOUTM].  In the US,
      according to the FAA, a "traffic management" ecosystem for
      "uncontrolled" UAS operations at low altitudes, separate from, but
      complementary to, the FAA's ATC system for "controlled" operations
      of manned aircraft.

   V2V
      Vehicle-to-Vehicle.  Originally communications between
      automobiles, now extended to apply to communications between
      vehicles generally.  Often, together with Vehicle-to-
      Infrastructure (V2I) and similar functions, generalized to V2X.

   VLOS
      Visual Line Of Sight.  Typically used in describing operation of a
      UA by a "remote" pilot who can clearly and directly (without video
      cameras or any aids other than glasses or, under some rules,
      binoculars) see the UA and its immediate flight environment.
      Potentially subject to blockage by foliage, structures, terrain,
      or other vehicles, more so than RF LOS.

3.  UAS RID Problem Space

   CAAs worldwide are mandating UAS RID.  The European Union Aviation
   Safety Agency (EASA) has published [Delegated] and [Implementing]
   regulations.  The US FAA has published a "final" rule [FRUR] and has
   described the key role that UAS RID plays in UAS Traffic Management
   (UTM) in [FAACONOPS] (especially Section 2.6).  At the time of
   writing, CAAs promulgate performance-based regulations that do not
   specify techniques but rather cite industry consensus technical
   standards as acceptable means of compliance.

   The most widely cited such industry consensus technical standard for
   UAS RID is [F3411-19], which defines two means of UAS RID:

   *  Network RID defines a set of information for UAS to make available
      globally indirectly via the Internet, through servers that can be
      queried by Observers.

   *  Broadcast RID defines a set of messages for UA to transmit locally
      directly one-way over Bluetooth or Wi-Fi (without IP or any other
      protocols between the data link and application layers), to be
      received in real time by local Observers.

   UAS using both means must send the same UAS RID application-layer
   information via each [F3411-19].  The presentation may differ, as
   Network RID defines a data dictionary, whereas Broadcast RID defines
   message formats (which carry items from that same data dictionary).
   The interval (or rate) at which it is sent may differ, as Network RID
   can accommodate Observer queries asynchronous to UAS updates (which
   generally need be sent only when information, such as location,
   changes), whereas Broadcast RID depends upon Observers receiving UA
   messages at the time they are transmitted.

   Network RID depends upon Internet connectivity in several segments
   from the UAS to each Observer.  Broadcast RID should need Internet
   (or other Wide Area Network) connectivity only to retrieve registry
   information, using, as the primary unique key for database lookup,
   the UAS Identifier (UAS ID) that was directly locally received.
   Broadcast RID does not assume IP connectivity of UAS; messages are
   encapsulated by the UA _without IP_, directly in link-layer frames
   (Bluetooth 4, Bluetooth 5, Wi-Fi NAN, IEEE 802.11 Beacon, or perhaps
   others in the future).

   [F3411-19] specifies three ID Type values, and its proposed revision
   (at the time of writing) adds a fourth:

   1  A static, manufacturer-assigned, hardware serial number as defined
      in "Small Unmanned Aerial Systems Serial Numbers" [CTA2063A].

   2  A CAA-assigned (generally static) ID, like the registration number
      of a manned aircraft.

   3  A UTM-system-assigned Universally Unique Identifier (UUID)
      [RFC4122], which can but need not be dynamic.

   4  A Specific Session ID, of any of an 8-bit range of subtypes
      defined external to ASTM and registered with ICAO, for which
      subtype 1 has been reserved by ASTM for the DRIP entity ID.

   Per [Delegated], the EU allows only ID Type 1.  Under [FRUR], the US
   allows ID Type 1 and ID Type 3.  [NPRM] proposed that a "Session ID"
   would be "e.g., a randomly-generated alphanumeric code assigned by a
   Remote ID UAS Service Supplier (USS) on a per-flight basis designed
   to provide additional privacy to the operator", but given the
   omission of Network RID from [FRUR], how this is to be assigned in
   the US is still to be determined.

   As yet, there are apparently no CAA public proposals to use ID Type
   2.  In the preamble of [FRUR], the FAA argues that registration
   numbers should not be sent in RID, insists that the capability of
   looking up registration numbers from information contained in RID
   should be restricted to FAA and other Government agencies, and
   implies that Session ID would be linked to the registration number
   only indirectly via the serial number in the registration database.
   The possibility of cryptographically blinding registration numbers,
   such that they can be revealed under specified circumstances, does
   not appear to be mentioned in applicable regulations or external
   technical standards.

   Per [Delegated], the EU also requires an operator registration number
   (an additional identifier distinct from the UAS ID) that can be
   carried in an [F3411-19] optional Operator ID Message.

   [FRUR] allows RID requirements to be met either by the UA itself,
   which is then designated a "standard remote identification unmanned
   aircraft", or by an add-on "remote identification broadcast module".
   The requirements for a module are different than for a standard RID
   UA.  The module:

   *  must transmit its own serial number (neither the serial number of
      the UA to which it is attached, nor a Session ID),

   *  must transmit takeoff location as a proxy for the location of the
      pilot/GCS,

   *  need not transmit UA emergency status, and

   *  is allowed to be used only for operations within VLOS of the
      Remote Pilot.

   Jurisdictions may relax or waive RID requirements for certain
   operators and/or under certain conditions.  For example, [FRUR]
   allows operators with UAS not equipped for RID to conduct VLOS
   operations at counterintuitively named "FAA-Recognized Identification
   Areas (FRIAs)"; radio-controlled model aircraft flying clubs and
   other eligible organizations can apply to the FAA for such
   recognition of their operating areas.

3.1.  Network RID

   Figure 3 illustrates Network RID information flows.  Only two of the
   three typically wireless links shown involving the UAS (UA-GCS, UA-
   Internet, and GCS-Internet) need exist to support C2 and Network RID.
   All three may exist, at the same or different times, especially in
   BVLOS operations.  There must be at least one information flow path
   (direct or indirect) between the GCS and the UA, for the former to
   exercise C2 over the latter.  If this path is two-way (as
   increasingly it is, even for inexpensive small UAS), the UA will also
   send its status (and position, if suitably equipped, e.g., with GNSS)
   to the GCS.  There also must be a path between at least one subsystem
   of the UAS (UA or GCS) and the Internet, for the former to send
   status and position updates to its USS (serving inter alia as a Net-
   RID SP).

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

                   Figure 3: Network RID Information Flow

   Direct UA-Internet wireless links are expected to become more common,
   especially on larger UAS, but, at the time of writing, they are rare.
   Instead, the RID data flow typically originates on the UA and passes
   through the GCS, or it originates on the GCS.  Network RID data makes
   three trips through the Internet (GCS-SP, SP-DP, DP-Observer, unless
   any of them are colocated), implying use of IP (and other middle-
   layer protocols, e.g., TLS/TCP or DTLS/UDP) on those trips.  IP is
   not necessarily used or supported on the UA-GCS link (if indeed that
   direct link exists, as it typically does now, but in BVLOS operations
   often will not).

   Network RID is publish-subscribe-query.  In the UTM context:

   1.  The UAS operator pushes an "operational intent" (the current term
       in UTM corresponding to a flight plan in manned aviation) to the
       USS (call it USS#1) that will serve that UAS (call it UAS#1) for
       that operation, primarily to enable deconfliction with other
       operations potentially impinging upon that operation's 4-D
       airspace volume (call it Volume#1).

   2.  Assuming the operation is approved and commences, UAS#1
       periodically pushes location/status updates to USS#1, which
       serves inter alia as the Network RID Service Provider (Net-RID
       SP) for that operation.

   3.  When users of any other USS (whether they be other UAS operators
       or Observers) develop an interest in any 4-D airspace volume
       (e.g., because they wish to submit an operational intent or
       because they have observed a UA), they query their own USS on the
       volumes in which they are interested.

   4.  Their USS query, via the UTM Discovery and Synchronization
       Service (DSS), all other USS in the UTM system and learn of any
       USS that have operations in those volumes (including any volumes
       intersecting them); thus, those USS whose query volumes intersect
       Volume#1 (call them USS#2 through USS#n) learn that USS#1 has
       such operations.

   5.  Interested parties can then subscribe to track updates on that
       operation of UAS#1, via their own USS, which serve as Network RID
       Display Providers (Net-RID DPs) for that operation.

   6.  USS#1 (as Net-RID SP) will then publish updates of UAS#1 status
       and position to all other subscribed USS in USS#2 through USS#n
       (as Net-RID DP).

   7.  All Net-RID DP subscribed to that operation of UAS#1 will deliver
       its track information to their users who subscribed to that
       operation of UAS#1 (via means unspecified by [F3411-19], etc.,
       but generally presumed to be web browser based).

   Network RID has several connectivity scenarios:

   *  _Persistently Internet-connected UA_ can consistently directly
      source RID information; this requires wireless coverage throughout
      the intended operational airspace volume, plus a buffer (e.g.,
      winds may drive the UA out of the volume).

   *  _Intermittently Internet-connected UA_, can usually directly
      source RID information, but when offline (e.g., due to signal
      blockage by a large structure being inspected using the UAS), need
      the GCS to proxy source RID information.

   *  _Indirectly connected UA_ lack the ability to send IP packets that
      will be forwarded into and across the Internet but instead have
      some other form of communications to another node that can relay
      or proxy RID information to the Internet; typically, this node
      would be the GCS (which to perform its function must know where
      the UA is, although C2 link outages do occur).

   *  _Non-connected UA_ have no means of sourcing RID information, in
      which case the GCS or some other interface available to the
      operator must source it.  In the extreme case, this could be the
      pilot or other agent of the operator using a web browser or
      application to designate, to a USS or other UTM entity, a time-
      bounded airspace volume in which an operation will be conducted.
      This is referred to as a "non-equipped network participant"
      engaging in "area operations".  This may impede disambiguation of
      ID if multiple UAS operate in the same or overlapping 4-D volumes.
      In most airspace volumes, most classes of UA will not be permitted
      to fly if non-connected.

   In most cases in the near term (2021), the Network RID first-hop data
   link is likely to be either cellular (which can also support BVLOS C2
   over existing large coverage areas) or Wi-Fi (which can also support
   Broadcast RID).  However, provided the data link can support at least
   UDP/IP and ideally also TCP/IP, its type is generally immaterial to
   higher-layer protocols.  The UAS, as the ultimate source of Network
   RID information, feeds a Net-RID SP (typically the USS to which the
   UAS operator subscribes), which proxies for the UAS and other data
   sources.  An Observer or other ultimate consumer of Network RID
   information obtains it from a Net-RID DP (also typically a USS),
   which aggregates information from multiple Net-RID SPs to offer
   airspace Situational Awareness (SA) coverage of a volume of interest.
   Network RID Service and Display Providers are expected to be
   implemented as servers in well-connected infrastructure,
   communicating with each other via the Internet and accessible by
   Observers via means such as web Application Programming Interfaces
   (APIs) and browsers.

   Network RID is the less constrained of the defined means of UAS RID.
   [F3411-19] only specifies information exchanges from Net-RID SP to
   Net-RID DP.  It is presumed that IETF efforts supporting the more
   constrained Broadcast RID (see next section) can be generalized for
   Network RID and potentially also for UAS-to-USS or other UTM
   communications.

3.2.  Broadcast RID

   Figure 4 illustrates the Broadcast RID information flow.  Note the
   absence of the Internet from the figure.  This is because Broadcast
   RID is one-way direct transmission of application-layer messages over
   an RF data link (without IP) from the UA to local Observer devices.
   Internet connectivity is involved only in what the Observer chooses
   to do with the information received, such as verify signatures using
   a web-based Broadcast Authentication Verifier Service and look up
   information in registries using the UAS ID as the primary unique key.

            +-------------------+
            | Unmanned Aircraft |
            +---------o---------+
                      |
                      |
                      |
                      | app messages directly over one-way RF data link
                      |
                      |
                      v
   +------------------o-------------------+
   | Observer's device (e.g., smartphone) |
   +--------------------------------------+

                  Figure 4: Broadcast RID Information Flow

   Broadcast RID is conceptually similar to Automatic Dependent
   Surveillance - Broadcast (ADS-B).  However, for various technical and
   other reasons, regulators including the EASA have not indicated
   intent to allow, and FAA has explicitly prohibited, use of ADS-B for
   UAS RID.

   [F3411-19] specifies four Broadcast RID data links: Bluetooth 4.x,
   Bluetooth 5.x with Extended Advertisements and Long-Range Coded PHY
   (S=8), Wi-Fi NAN at 2.4 GHz, and Wi-Fi NAN at 5 GHz.  A UA must
   broadcast (using advertisement mechanisms where no other option
   supports broadcast) on at least one of these.  If sending on
   Bluetooth 5.x, it is required to do so concurrently on 4.x (referred
   to in [F3411-19] as "Bluetooth Legacy"); current (2021) discussions
   in ASTM F38.02 on revising [F3411-19], motivated by drafts of
   European standards, suggest that both Bluetooth versions will be
   required.  If broadcasting Wi-Fi NAN at 5 GHz, it is required to do
   so concurrently at 2.4 GHz; current discussions in ASTM F38.02
   include relaxing this.  Wi-Fi Beacons are also under consideration.
   Future revisions of [F3411-19] may allow other data links.

   The selection of Broadcast RID media was driven by research into what
   is commonly available on "ground" units (smartphones and tablets) and
   what was found as prevalent or "affordable" in UA.  Further, there
   must be an API for the Observer's receiving application to have
   access to these messages.  As yet, only Bluetooth 4.x support is
   readily available; thus, the current focus is on working within the
   31-byte payload limit of the Bluetooth 4.x "Broadcast Frame"
   transmitted as an "advertisement" on beacon channels.  After
   overheads, this limits the RID message to 25 bytes and the UAS ID
   string to a maximum length of 20 bytes.

   A single Bluetooth 4.x advertisement frame can just barely fit any
   UAS ID long enough to be sufficiently unique for its purpose.

   There is related information, which especially includes, but is not
   limited to, the UA position and velocity, which must be represented
   by data elements long enough to provide precision sufficient for
   their purpose while remaining unambiguous with respect to their
   reference frame.

   In order to enable Observer devices to verify that 1) the claimed UAS
   ID is indeed owned by the sender and 2) the related information was
   indeed sent by the owner of that same UAS ID, authentication data
   elements would typically be lengthy with conventional cryptographic
   signature schemes.  They would be too long to fit in a single frame,
   even with the latest schemes currently being standardized.

   Thus, it is infeasible to bundle information related to the UAS ID
   and corresponding authentication data elements in a single Bluetooth
   4.x frame; yet, somehow all these must be securely bound together.

   Messages that cannot be encapsulated in a single frame (thus far,
   only the Authentication Message) must be segmented into message
   "pages" (in the terminology of [F3411-19]).  Message pages must
   somehow be correlated as belonging to the same message.  Messages
   carrying position, velocity and other data must somehow be correlated
   with the Basic ID Message that carries the UAS ID.  This correlation
   is expected to be done on the basis of Media Access Control (MAC)
   address.  This may be complicated by MAC address randomization.  Not
   all the common devices expected to be used by Observers have APIs
   that make sender MAC addresses available to user space receiver
   applications.  MAC addresses are easily spoofed.  Data elements are
   not so detached on other media (see Message Pack in the paragraph
   after next).

   [F3411-19] Broadcast RID specifies several message types (see
   Section 5.4.5 and Table 3 of [F3411-19]).  The table below lists
   these message types.  The 4-bit Message Type field in the header can
   index up to 16 types.  Only seven are defined at the time of writing.
   Only two are mandatory.  All others are optional, unless required by
   a jurisdictional authority, e.g., a CAA.  To satisfy both EASA and
   FAA rules, all types are needed, except Self-ID and Authentication,
   as the data elements required by the rules are scattered across
   several message types (along with some data elements not required by
   the rules).

   The Message Pack (type 0xF) is not actually a message but the framed
   concatenation of at most one message of each type of any subset of
   the other types, in type index order.  Some of the messages that it
   can encapsulate are mandatory; others are optional.  The Message Pack
   itself is mandatory on data links that can encapsulate it in a single
   frame (Bluetooth 5.x and Wi-Fi).

          +-------+-----------------+-----------+---------------+
          | Index | Name            | Req       | Notes         |
          +-------+-----------------+-----------+---------------+
          | 0x0   | Basic ID        | Mandatory | -             |
          +-------+-----------------+-----------+---------------+
          | 0x1   | Location/Vector | Mandatory | -             |
          +-------+-----------------+-----------+---------------+
          | 0x2   | Authentication  | Optional  | paged         |
          +-------+-----------------+-----------+---------------+
          | 0x3   | Self-ID         | Optional  | free text     |
          +-------+-----------------+-----------+---------------+
          | 0x4   | System          | Optional  | -             |
          +-------+-----------------+-----------+---------------+
          | 0x5   | Operator ID     | Optional  | -             |
          +-------+-----------------+-----------+---------------+
          | 0xF   | Message Pack    | -         | BT5 and Wi-Fi |
          +-------+-----------------+-----------+---------------+

                Table 1: Message Types Defined in [F3411-19]

   [F3411-19] Broadcast RID specifies very few quantitative performance
   requirements: static information must be transmitted at least once
   per three seconds, and dynamic information (the Location/Vector
   Message) must be transmitted at least once per second and be no older
   than one second when sent.  [FRUR] requires all information be sent
   at least once per second.

   [F3411-19] Broadcast RID transmits all information as cleartext
   (ASCII or binary), so static IDs enable trivial correlation of
   patterns of use, which is unacceptable in many applications, e.g.,
   package delivery routes of competitors.

   Any UA can assert any ID using the [F3411-19] required Basic ID
   Message, which lacks any provisions for verification.  The Location/
   Vector Message likewise lacks provisions for verification and does
   not contain the ID, so it must be correlated somehow with a Basic ID
   Message: the developers of [F3411-19] have suggested using the MAC
   addresses on the Broadcast RID data link, but these may be randomized
   by the operating system stack to avoid the adversarial correlation
   problems of static identifiers.

   The [F3411-19] optional Authentication Message specifies framing for
   authentication data but does not specify any authentication method,
   and the maximum length of the specified framing is too short for
   conventional digital signatures and far too short for conventional
   certificates (e.g., X.509).  Fetching certificates via the Internet
   is not always possible (e.g., Observers working in remote areas, such
   as national forests), so devising a scheme whereby certificates can
   be transported over Broadcast RID is necessary.  The one-way nature
   of Broadcast RID precludes challenge-response security protocols
   (e.g., Observers sending nonces to UA, to be returned in signed
   messages).  Without DRIP extensions to [F3411-19], an Observer would
   be seriously challenged to validate the asserted UAS ID or any other
   information about the UAS or its operator looked up therefrom.

   At the time of writing, the proposed revision of [F3411-19] defines a
   new Authentication Type 5 ("Specific Authentication Method (SAM)") to
   enable SDOs other than ASTM to define authentication payload formats.
   The first byte of the payload is the SAM Type, used to demultiplex
   such variant formats.  All formats (aside from those for private
   experimental use) must be registered with ICAO, which assigns the SAM
   Type.  Any Authentication Message payload that is to be sent in
   exactly the same form over all currently specified Broadcast RID
   media is limited by lower-layer constraints to a total length of 201
   bytes.  For Authentication Type 5, which is expected to be used by
   DRIP, the SAM Type byte consumes the first of these, limiting DRIP
   authentication payload formats to a maximum of 200 bytes.

3.3.  USS in UTM and RID

   UAS RID and UTM are complementary; Network RID is a UTM service.  The
   backbone of the UTM system is comprised of multiple USS: one or
   several per jurisdiction with some being limited to a single
   jurisdiction while others span multiple jurisdictions.  USS also
   serve as the principal, or perhaps the sole, interface for operators
   and UAS into the UTM environment.  Each operator subscribes to at
   least one USS.  Each UAS is registered by its operator in at least
   one USS.  Each operational intent is submitted to one USS; if
   approved, that UAS and operator can commence that operation.  During
   the operation, status and location of that UAS must be reported to
   that USS, which, in turn, provides information as needed about that
   operator, UAS, and operation into the UTM system and to Observers via
   Network RID.

   USS provide services not limited to Network RID; indeed, the primary
   USS function is deconfliction of airspace usage between different UAS
   (and their operators).  It will occasionally deconflict UAS from non-
   UAS operations, such as manned aircraft and rocket launch.  Most
   deconfliction involving a given operation is hoped to be completed
   prior to commencing that operation; this is called "strategic
   deconfliction".  If that fails, "tactical deconfliction" comes into
   play; AirBorne DAA (ABDAA) may not involve USS, but Ground-Based DAA
   (GBDAA) likely will.  Dynamic constraints, formerly called "UAS
   Volume Restrictions (UVRs)", can be necessitated by circumstances
   such as local emergencies and extreme weather, specified by
   authorities on the ground, and propagated in UTM.

   No role for USS in Broadcast RID is currently specified by regulators
   or by [F3411-19].  However, USS are likely to serve as registries (or
   perhaps registrars) for UAS (and perhaps operators); if so, USS will
   have a role in all forms of RID.  Supplemental Data Service Providers
   (SDSPs) are also likely to find roles, not only in UTM as such but
   also in enhancing UAS RID and related services.  RID services are
   used in concert with USS, SDSP, or other UTM entities (if and as
   needed and available).  Narrowly defined, RID services provide
   regulator-specified identification information; more broadly defined,
   RID services may leverage identification to facilitate related
   services or functions, likely beginning with V2X.

3.4.  DRIP Focus

   In addition to the gaps described above, there is a fundamental gap
   in almost all current or proposed regulations and technical standards
   for UAS RID.  As noted above, ID is not an end in itself, but a
   means.  Protocols specified in [F3411-19] etc. provide limited
   information potentially enabling (but no technical means for) an
   Observer to communicate with the pilot, e.g., to request further
   information on the UAS operation or exit from an airspace volume in
   an emergency.  The System Message provides the location of the pilot/
   GCS, so an Observer could physically go to the asserted location to
   look for the Remote Pilot; this is slow, at best, and may not be
   feasible.  What if the pilot is on the opposite rim of a canyon, or
   there are multiple UAS operators to contact whose GCS all lie in
   different directions from the Observer?  An Observer with Internet
   connectivity and access privileges could look up operator PII in a
   registry and then call a phone number in hopes that someone who can
   immediately influence the UAS operation will answer promptly during
   that operation; this is unreliable, at best, and may not be prudent.
   Should pilots be encouraged to answer phone calls while flying?
   Internet technologies can do much better than this.

   Thus, to achieve widespread adoption of a RID system supporting safe
   and secure operation of UAS, protocols must do the following (despite
   the intrinsic tension among these objectives):

   *  preserve operator privacy,

   *  enable strong authentication, and

   *  enable the immediate use of information by authorized parties.

   Just as [F3411-19] is expected to be approved by regulators as a
   basic means of compliance with UAS RID regulations, DRIP is likewise
   expected to be approved to address further issues, starting with the
   creation and registration of Session IDs.

   DRIP will focus on making information obtained via UAS RID
   immediately usable:

   1.  by making it trustworthy (despite the severe constraints of
       Broadcast RID);

   2.  by enabling verification that a UAS is registered for RID, and,
       if so, in which registry (for classification of trusted operators
       on the basis of known registry vetting, even by Observers lacking
       Internet connectivity at observation time);

   3.  by facilitating independent reports of UA aeronautical data
       (location, velocity, etc.) to confirm or refute the operator
       self-reports upon which UAS RID and UTM tracking are based;

   4.  by enabling instant establishment, by authorized parties, of
       secure communications with the Remote Pilot.

   The foregoing considerations, beyond those addressed by baseline UAS
   RID standards such as [F3411-19], imply the requirements for DRIP
   detailed in Section 4.

4.  Requirements

   The following requirements apply to DRIP as a set of related
   protocols, various subsets of which, in conjunction with other IETF
   and external technical standards, may suffice to comply with the
   regulations in any given jurisdiction or meet any given user need.
   It is not intended that each and every protocol of the DRIP set,
   alone, satisfy each and every requirement.  To satisfy these
   requirements, Internet connectivity is required some of the time
   (e.g., to support DRIP Entity Identifier creation/registration) but
   not all of the time (e.g., authentication of an asserted DRIP Entity
   Identifier can be achieved by a fully working and provisioned
   Observer device even when that device is off-line so is required at
   all times).

4.1.  General

4.1.1.  Normative Requirements

   GEN-1    Provable Ownership: DRIP MUST enable verification that the
            asserted entity (typically UAS) ID is that of the actual
            current sender (i.e., the Entity ID in the DRIP
            authenticated message set is not a replay attack or other
            spoof), even on an Observer device lacking Internet
            connectivity at the time of observation.

   GEN-2    Provable Binding: DRIP MUST enable the cryptographic binding
            of all other [F3411-19] messages from the same actual
            current sender to the UAS ID asserted in the Basic ID
            Message.

   GEN-3    Provable Registration: DRIP MUST enable cryptographically
            secure verification that the UAS ID is in a registry and
            identification of that registry, even on an Observer device
            lacking Internet connectivity at the time of observation;
            the same sender may have multiple IDs, potentially in
            different registries, but each ID must clearly indicate in
            which registry it can be found.

   GEN-4    Readability: DRIP MUST enable information (regulation
            required elements, whether sent via UAS RID or looked up in
            registries) to be read and utilized by both humans and
            software.

   GEN-5    Gateway: DRIP MUST enable application-layer gateways from
            Broadcast RID to Network RID to stamp messages with precise
            date/time received and receiver location, then relay them to
            a network service (e.g., SDSP or distributed ledger)
            whenever the gateway has Internet connectivity.

   GEN-6    Contact: DRIP MUST enable dynamically establishing, with
            AAA, per policy, strongly mutually authenticated, end-to-end
            strongly encrypted communications with the UAS RID sender
            and entities looked up from the UAS ID, including at least
            the (1) pilot (Remote Pilot or Pilot In Command), (2) the
            USS (if any) under which the operation is being conducted,
            and (3) registries in which data on the UA and pilot are
            held.  This requirement applies whenever each party to such
            desired communications has a currently usable means of
            resolving the other party's DRIP Entity Identifier to a
            locator (IP address) and currently usable bidirectional IP
            (not necessarily Internet) connectivity with the other
            party.

   GEN-7    QoS: DRIP MUST enable policy-based specification of
            performance and reliability parameters.

   GEN-8    Mobility: DRIP MUST support physical and logical mobility of
            UA, GCS, and Observers.  DRIP SHOULD support mobility of
            essentially all participating nodes (UA, GCS, Observers,
            Net-RID SP, Net-RID DP, Private Registries, SDSP, and
            potentially others as RID and UTM evolve).

   GEN-9    Multihoming: DRIP MUST support multihoming of UA and GCS,
            for make-before-break smooth handoff and resiliency against
            path or link failure.  DRIP SHOULD support multihoming of
            essentially all participating nodes.

   GEN-10   Multicast: DRIP SHOULD support multicast for efficient and
            flexible publish-subscribe notifications, e.g., of UAS
            reporting positions in designated airspace volumes.

   GEN-11   Management: DRIP SHOULD support monitoring of the health and
            coverage of Broadcast and Network RID services.

4.1.2.  Rationale

   Requirements imposed either by regulation or by [F3411-19] are not
   reiterated in this document, but they drive many of the numbered
   requirements listed here.  The regulatory performance requirement in
   [FRUR] currently would be satisfied by ensuring information refresh
   rates of at least 1 Hertz, with latencies no greater than 1 second,
   at least 80% of the time, but these numbers may vary between
   jurisdictions and over time.  Instead, the DRIP QoS requirement is
   that parameters such as performance and reliability be specifiable by
   user policy, which does not imply satisfiable in all cases but does
   imply (especially together with the Management requirement) that when
   specifications are not met, appropriate parties are notified.

   The Provable Ownership requirement addresses the possibility that the
   actual sender is not the claimed sender (i.e., is a spoofer).  DRIP
   could meet this requirement by, for example, verifying an asymmetric
   cryptographic signature using a sender-provided public key from which
   the asserted UAS ID can be at least partially derived.  The Provable
   Binding requirement addresses the problem with MAC address
   correlation [F3411-19] noted in Section 3.2.  The Provable
   Registration requirement may impose burdens not only on the UAS
   sender and the Observer's receiver, but also on the registry; yet, it
   cannot depend upon the Observer being able to contact the registry at
   the time of observing the UA.  The Readability requirement pertains
   to the structure and format of information at endpoints rather than
   its encoding in transit, so it may involve machine-assisted format
   conversions (e.g., from binary encodings) and/or decryption (see
   Section 4.3).

   The Gateway requirement is in pursuit of three objectives: (1) mark
   up a RID message with where and when it was actually received, which
   may agree or disagree with the self-report in the set of messages;
   (2) defend against replay attacks; and (3) support optional SDSP
   services such as multilateration, to complement UAS position self-
   reports with independent measurements.  This is the only instance in
   which DRIP transports [F3411-19] messages; most of DRIP pertains to
   the authentication of such messages and identifiers carried in them.

   The Contact requirement allows any party that learns a UAS ID (that
   is a DRIP Entity Identifier rather than another ID Type) to request
   establishment of a communications session with the corresponding UAS
   RID sender and certain entities associated with that UAS, but AAA and
   policy restrictions, inter alia on resolving the identifier to any
   locators (typically IP addresses), should prevent unauthorized
   parties from distracting or harassing pilots.  Thus, some but not all
   Observers of UA, receivers of Broadcast RID, clients of Network RID,
   and other parties can become successfully initiating endpoints for
   these sessions.

   The QoS requirement is only that performance and reliability
   parameters can be _specified_ by policy, not that any such
   specifications must be guaranteed to be met; any failure to meet such
   would be reported under the Management requirement.  Examples of such
   parameters are the maximum time interval at which messages carrying
   required data elements may be transmitted, the maximum tolerable rate
   of loss of such messages, and the maximum tolerable latency between a
   dynamic data element (e.g., GNSS position of UA) being provided to
   the DRIP sender and that element being delivered by the DRIP receiver
   to an application.

   The Mobility requirement refers to rapid geographic mobility of
   nodes, changes of their points of attachment to networks, and changes
   to their IP addresses; it is not limited to micro-mobility within a
   small geographic area or single Internet access provider.

4.2.  Identifier

4.2.1.  Normative Requirements

   ID-1     Length: The DRIP Entity Identifier MUST NOT be longer than
            19 bytes, to fit in the Specific Session ID subfield of the
            UAS ID field of the Basic ID Message of the proposed
            revision of [F3411-19] (at the time of writing).

   ID-2     Registry ID: The DRIP identifier MUST be sufficient to
            identify a registry in which the entity identified therewith
            is listed.

   ID-3     Entity ID: The DRIP identifier MUST be sufficient to enable
            lookups of other data associated with the entity identified
            therewith in that registry.

   ID-4     Uniqueness: The DRIP identifier MUST be unique within the
            applicable global identifier space from when it is first
            registered therein until it is explicitly deregistered
            therefrom (due to, e.g., expiration after a specified
            lifetime, revocation by the registry, or surrender by the
            operator).

   ID-5     Non-spoofability: The DRIP identifier MUST NOT be spoofable
            within the context of a minimal Remote ID broadcast message
            set (to be specified within DRIP to be sufficient
            collectively to prove sender ownership of the claimed
            identifier).

   ID-6     Unlinkability: The DRIP identifier MUST NOT facilitate
            adversarial correlation over multiple operations.  If this
            is accomplished by limiting each identifier to a single use
            or brief period of usage, the DRIP identifier MUST support
            well-defined, scalable, timely registration methods.

4.2.2.  Rationale

   The DRIP identifier can refer to various entities.  In the primary
   initial use case, the entity to be identified is the UA.  Entities to
   be identified in other likely use cases include, but are not limited
   to, the operator, USS, and Observer.  In all cases, the entity
   identified must own the identifier (i.e., have the exclusive
   capability to use the identifier, such that receivers can verify the
   entity's ownership of it).

   The DRIP identifier can be used at various layers.  In Broadcast RID,
   it would be used by the application running directly over the data
   link.  In Network RID, it would be used by the application running
   over HTTPS (not required by DRIP but generally used by Network RID
   implementations) and possibly other protocols.  In RID-initiated V2X
   applications such as DAA and C2, it could be used between the network
   and transport layers (e.g., with the Host Identity Protocol (HIP)
   [RFC9063] [RFC7401]) or between the transport and application layers
   (e.g., with DTLS [RFC6347]).

   Registry ID (which registry the entity is in) and Entity ID (which
   entity it is, within that registry) are requirements on a single DRIP
   Entity Identifier, not separate (types of) ID.  In the most common
   use case, the entity will be the UA, and the DRIP identifier will be
   the UAS ID; however, other entities may also benefit from having DRIP
   identifiers, so the entity type is not prescribed here.

   Whether a UAS ID is generated by the operator, GCS, UA, USS,
   registry, or some collaboration among them is unspecified; however,
   there must be agreement on the UAS ID among these entities.
   Management of DRIP identifiers is the primary function of their
   registration hierarchies, from the root (presumably IANA), through
   sector-specific and regional authorities (presumably ICAO and CAAs),
   to the identified entities themselves.

   While Uniqueness might be considered an implicit requirement for any
   identifier, here the point of the explicit requirement is not just
   that it should be unique, but also where and when it should be
   unique: global scope within a specified space, from registration to
   deregistration.

   While Non-spoofability imposes requirements for and on a DRIP
   authentication protocol, it also imposes requirements on the
   properties of the identifier itself.  An example of how the nature of
   the identifier can support non-spoofability is embedding a hash of
   both the Registry ID and a public key of the entity in the entity
   identifier, thus making it self-authenticating any time the entity's
   corresponding private key is used to sign a message.

   While Unlinkability is a privacy desideratum (see Section 4.3), it
   imposes requirements on the DRIP identifier itself, as distinct from
   other currently permitted choices for the UAS ID (including primarily
   the static serial number of the UA or RID module).

4.3.  Privacy

4.3.1.  Normative Requirements

   PRIV-1   Confidential Handling: DRIP MUST enable confidential
            handling of private information (i.e., any and all
            information that neither the cognizant authority nor the
            information owner has designated as public, e.g., personal
            data).

   PRIV-2   Encrypted Transport: DRIP MUST enable selective strong
            encryption of private data in motion in such a manner that
            only authorized actors can recover it.  If transport is via
            IP, then encryption MUST be end-to-end, at or above the IP
            layer.  DRIP MUST NOT encrypt safety critical data to be
            transmitted over Broadcast RID in any situation where it is
            unlikely that local Observers authorized to access the
            plaintext will be able to decrypt it or obtain it from a
            service able to decrypt it.  DRIP MUST NOT encrypt data
            when/where doing so would conflict with applicable
            regulations or CAA policies/procedures, i.e., DRIP MUST
            support configurable disabling of encryption.

   PRIV-3   Encrypted Storage: DRIP SHOULD facilitate selective strong
            encryption of private data at rest in such a manner that
            only authorized actors can recover it.

   PRIV-4   Public/Private Designation: DRIP SHOULD facilitate
            designation, by cognizant authorities and information
            owners, of which information is public and which is private.
            By default, all information required to be transmitted via
            Broadcast RID, even when actually sent via Network RID or
            stored in registries, is assumed to be public; all other
            information held in registries for lookup using the UAS ID
            is assumed to be private.

   PRIV-5   Pseudonymous Rendezvous: DRIP MAY enable mutual discovery of
            and communications among participating UAS operators whose
            UA are in 4-D proximity, using the UAS ID without revealing
            pilot/operator identity or physical location.

4.3.2.  Rationale

   Most data to be sent via Broadcast RID or Network RID is public;
   thus, the Encrypted Transport requirement for private data is
   selective, e.g., for the entire payload of the Operator ID Message,
   but only the pilot/GCS location fields of the System Message.  Safety
   critical data includes at least the UA location.  Other data also may
   be deemed safety critical, e.g., in some jurisdictions the pilot/GCS
   location is implied to be safety critical.

   UAS have several potential means of assessing the likelihood that
   local Observers authorized to access the plaintext will be able to
   decrypt it or obtain it from a service able to decrypt it.  If the
   UAS is not participating in UTM, an Observer would have no means of
   obtaining a decryption key or decryption services from a cognizant
   USS.  If the UAS is participating in UTM but has lost connectivity
   with its USS, then an Observer within visual LOS of the UA is also
   unlikely to be able to communicate with that USS (whether due to the
   USS being offline or the UAS and Observer being in an area with poor
   Internet connectivity).  Either of these conditions (UTM non-
   participation or USS unreachability) would be known to the UAS.

   In some jurisdictions, the configurable enabling and disabling of
   encryption may need to be outside the control of the operator.
   [FRUR] mandates that manufacturers design RID equipment with some
   degree of tamper resistance; the preamble of [FRUR] and other FAA
   commentary suggest this is to reduce the likelihood that an operator,
   intentionally or unintentionally, might alter the values of the
   required data elements or disable their transmission in the required
   manner (e.g., as cleartext).

   How information is stored on end systems is out of scope for DRIP.
   Encouraging privacy best practices, including end system storage
   encryption, by facilitating it with protocol design reflecting such
   considerations is in scope.  Similar logic applies to methods for
   designating information as public or private.

   The Privacy requirements above are for DRIP, neither for [F3411-19]
   (which, in the interest of privacy, requires obfuscation of location
   to any Network RID subscriber engaging in wide area surveillance,
   limits data retention periods, etc.), nor for UAS RID in any specific
   jurisdiction (which may have its own regulatory requirements).  The
   requirements above are also in a sense parameterized: who are the
   "authorized actors", how are they designated, how are they
   authenticated, etc.?

4.4.  Registries

4.4.1.  Normative Requirements

   REG-1    Public Lookup: DRIP MUST enable lookup, from the UAS ID, of
            information designated by cognizant authority as public and
            MUST NOT restrict access to this information based on
            identity or role of the party submitting the query.

   REG-2    Private Lookup: DRIP MUST enable lookup of private
            information (i.e., any and all information in a registry,
            associated with the UAS ID, that is designated by neither
            cognizant authority nor the information owner as public),
            and MUST, according to applicable policy, enforce AAA,
            including restriction of access to this information based on
            identity or role of the party submitting the query.

   REG-3    Provisioning: DRIP MUST enable provisioning registries with
            static information on the UAS and its operator, dynamic
            information on its current operation within the U-space/UTM
            (including means by which the USS under which the UAS is
            operating may be contacted for further, typically even more
            dynamic, information), and Internet direct contact
            information for services related to the foregoing.

   REG-4    AAA Policy: DRIP AAA MUST be specifiable by policies; the
            definitive copies of those policies must be accessible in
            registries; administration of those policies and all DRIP
            registries must be protected by AAA.

4.4.2.  Rationale

   Registries are fundamental to RID.  Only very limited information can
   be transmitted via Broadcast RID, but extended information is
   sometimes needed.  The most essential element of information sent is
   the UAS ID itself, the unique key for lookup of extended information
   in registries.  The regulatory requirements for the registry
   information models for UAS and their operators for RID and, more
   broadly, for U-space/UTM needs are in flux.  Thus, beyond designating
   the UAS ID as that unique key, the registry information model is not
   specified in this document.  While it is expected that registry
   functions will be integrated with USS, who will provide them is
   expected to vary between jurisdictions and has not yet been
   determined in most jurisdictions.  However this evolves, the
   essential registry functions, starting with management of
   identifiers, are expected to remain the same, so those are specified
   herein.

   While most data to be sent via Broadcast or Network RID is public,
   much of the extended information in registries will be private.
   Thus, AAA for registries is essential, not just to ensure that access
   is granted only to strongly authenticated, duly authorized parties,
   but also to support subsequent attribution of any leaks, audit of who
   accessed information when and for what purpose, etc.  Specific AAA
   requirements will vary by jurisdictional regulation, provider
   philosophy, customer demand, etc., so they are left to specification
   in policies.  Such policies should be human readable to facilitate
   analysis and discussion, be machine readable to enable automated
   enforcement, and use a language amenable to both, e.g., eXtensible
   Access Control Markup Language (XACML).

   The intent of the negative and positive access control requirements
   on registries is to ensure that no member of the public would be
   hindered from accessing public information, while only duly
   authorized parties would be enabled to access private information.
   Mitigation of denial-of-service attacks and refusal to allow database
   mass scraping would be based on those behaviors, not on identity or
   role of the party submitting the query per se; however, information
   on the identity of the party submitting the query might be gathered
   on such misbehavior by security systems protecting DRIP
   implementations.

   "Internet direct contact information" means a locator (e.g., IP
   address), or identifier (e.g., FQDN) that can be resolved to a
   locator, which enables initiation of an end-to-end communication
   session using a well-known protocol (e.g., SIP).

5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   DRIP is all about safety and security, so content pertaining to such
   is not limited to this section.  This document does not define any
   protocols, so security considerations of such are speculative.
   Potential vulnerabilities of DRIP solutions to these requirements
   include but are not limited to:

   *  Sybil attacks

   *  confusion created by many spoofed unsigned messages

   *  processing overload induced by attempting to verify many spoofed
      signed messages (where verification will fail but still consume
      cycles)

   *  malicious or malfunctioning registries

   *  interception by on-path attacker of (i.e., man-in-the-middle
      attacks on) registration messages

   *  UA impersonation through private key extraction, improper key
      sharing, or carriage of a small (presumably harmless) UA, i.e., as
      a "false flag", by a larger (malicious) UA

   It may be inferred from the General requirements (Section 4.1) for
   Provable Ownership, Provable Binding, and Provable Registration,
   together with the Identifier requirements (Section 4.2), that DRIP
   must provide:

   *  message integrity

   *  non-repudiation

   *  defense against replay attacks

   *  defense against spoofing

   One approach to so doing involves verifiably binding the DRIP
   identifier to a public key.  Providing these security features,
   whether via this approach or another, is likely to be especially
   challenging for Observers without Internet connectivity at the time
   of observation.  For example, checking the signature of a registry on
   a public key certificate received via Broadcast RID in a remote area
   presumably would require that the registry's public key had been
   previously installed on the Observer's device, yet there may be many
   registries and the Observer's device may be storage constrained, and
   new registries may come on-line subsequent to installation of DRIP
   software on the Observer's device.  See also Figure 1 and the
   associated explanatory text, especially the second paragraph after
   the figure.  Thus, there may be caveats on the extent to which
   requirements can be satisfied in such cases, yet strenuous effort
   should be made to satisfy them, as such cases are important, e.g.,
   firefighting in a national forest.  Each numbered requirement a
   priori expected to suffer from such limitations (General requirements
   for Gateway and Contact functionality) contains language stating when
   it applies.

7.  Privacy and Transparency Considerations

   Privacy and transparency are important for legal reasons including
   regulatory consistency.  [EU2018] states:

   |  harmonised and interoperable national registration systems ...
   |  should comply with the applicable Union and national law on
   |  privacy and processing of personal data, and the information
   |  stored in those registration systems should be easily accessible.

   Transparency (where essential to security or safety) and privacy are
   also ethical and moral imperatives.  Even in cases where old
   practices (e.g., automobile registration plates) could be imitated,
   when new applications involving PII (such as UAS RID) are addressed
   and newer technologies could enable improving privacy, such
   opportunities should not be squandered.  Thus, it is recommended that
   all DRIP work give due regard to [RFC6973] and, more broadly, to
   [RFC8280].

   However, privacy and transparency are often conflicting goals,
   demanding careful attention to their balance.

   DRIP information falls into two classes:

   *  that which, to achieve the purpose, must be published openly as
      cleartext, for the benefit of any Observer (e.g., the basic UAS ID
      itself); and

   *  that which must be protected (e.g., PII of pilots) but made
      available to properly authorized parties (e.g., public safety
      personnel who urgently need to contact pilots in emergencies).

   How properly authorized parties are authorized, authenticated, etc.
   are questions that extend beyond the scope of DRIP, but DRIP may be
   able to provide support for such processes.  Classification of
   information as public or private must be made explicit and reflected
   with markings, design, etc.  Classifying the information will be
   addressed primarily in external standards; in this document, it will
   be regarded as a matter for CAA, registry, and operator policies, for
   which enforcement mechanisms will be defined within the scope of the
   DRIP WG and offered.  Details of the protection mechanisms will be
   provided in other DRIP documents.  Mitigation of adversarial
   correlation will also be addressed.

8.  References

8.1.  Normative References

   [F3411-19] ASTM International, "Standard Specification for Remote ID
              and Tracking", ASTM F3411-19, DOI 10.1520/F3411-19,
              February 2020,
              <http://www.astm.org/cgi-bin/resolver.cgi?F3411>.

   [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>.

8.2.  Informative References

   [Amended]  European Parliament and Council, "Commission Delegated
              Regulation (EU) 2020/1058 of 27 April 2020 amending
              Delegated Regulation (EU) 2019/945 as regards the
              introduction of two new unmanned aircraft systems
              classes", April 2020,
              <https://eur-lex.europa.eu/eli/reg_del/2020/1058/oj>.

   [ASDRI]    ASD-STAN, "Introduction to the European UAS Digital Remote
              ID Technical Standard", January 2021, <https://asd-
              stan.org/wp-content/uploads/ASD-STAN_DRI_Introduction_to_t
              he_European_digital_RID_UAS_Standard.pdf>.

   [ASDSTAN4709-002]
              ASD-STAN, "Aerospace series - Unmanned Aircraft Systems -
              Part 002: Direct Remote Identification", ASD-STAN
              prEN 4709-002 P1, October 2021, <https://asd-
              stan.org/downloads/asd-stan-pren-4709-002-p1/>.

   [CPDLC]    Gurtov, A., Polishchuk, T., and M. Wernberg, "Controller-
              Pilot Data Link Communication Security", Sensors 18, no.
              5: 1636, DOI 10.3390/s18051636, 2018,
              <https://www.mdpi.com/1424-8220/18/5/1636>.

   [CTA2063A] ANSI, "Small Unmanned Aerial Systems Serial Numbers",
              ANSI/CTA 2063-A, September 2019,
              <https://shop.cta.tech/products/small-unmanned-aerial-
              systems-serial-numbers>.

   [Delegated]
              European Parliament and Council, "Commission Delegated
              Regulation (EU) 2019/945 of 12 March 2019 on unmanned
              aircraft systems and on third-country operators of
              unmanned aircraft systems", March 2019,
              <https://eur-lex.europa.eu/eli/reg_del/2019/945/oj>.

   [DRIP-ARCH]
              Card, S., Wiethuechter, A., Moskowitz, R., Zhao, S., Ed.,
              and A. Gurtov, "Drone Remote Identification Protocol
              (DRIP) Architecture", Work in Progress, Internet-Draft,
              draft-ietf-drip-arch-20, 28 January 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-drip-
              arch-20>.

   [ENISACSIRT]
              European Union Agency for Cybersecurity (ENISA),
              "Actionable information for Security Incident Response",
              November 2014, <https://www.enisa.europa.eu/topics/csirt-
              cert-services/reactive-services/copy_of_actionable-
              information/actionable-information>.

   [EU2018]   European Parliament and Council, "2015/0277 (COD) PE-CONS
              2/18", June 2018,
              <https://www.consilium.europa.eu/media/35805/easa-
              regulation-june-2018.pdf>.

   [FAACONOPS]
              FAA Office of NextGen, "UTM Concept of Operations v2.0",
              March 2020, <https://www.faa.gov/uas/research_development/
              traffic_management/media/UTM_ConOps_v2.pdf>.

   [FR24]     Flightradar24, "About Flightradar24",
              <https://www.flightradar24.com/about>.

   [FRUR]     Federal Aviation Administration (FAA), "Remote
              Identification of Unmanned Aircraft", January 2021,
              <https://www.federalregister.gov/
              documents/2021/01/15/2020-28948/remote-identification-of-
              unmanned-aircraft>.

   [GDPR]     European Parliament and Council, "Regulation (EU) 2016/679
              of the European Parliament and of the Council of 27 April
              2016 on the protection of natural persons with regard to
              the processing of personal data and on the free movement
              of such data, and repealing Directive 95/46/EC (General
              Data Protection Regulation)", April 2016,
              <https://eur-lex.europa.eu/eli/reg/2016/679/oj>.

   [ICAOATM]  International Civil Aviation Organization, "Procedures for
              Air Navigation Services: Air Traffic Management",
              Doc 4444, November 2016, <https://store.icao.int/en/
              procedures-for-air-navigation-services-air-traffic-
              management-doc-4444>.

   [ICAODEFS] International Civil Aviation Organization, "Defined terms
              from the Annexes to the Chicago Convention and ICAO
              guidance material", July 2017,
              <https://www.icao.int/safety/cargosafety/Documents/
              Draft%20Glossary%20of%20terms.docx>.

   [ICAOUAS]  International Civil Aviation Organization, "Unmanned
              Aircraft Systems", Circular 328, 2011,
              <https://www.icao.int/meetings/uas/documents/
              circular%20328_en.pdf>.

   [ICAOUTM]  International Civil Aviation Organization, "Unmanned
              Aircraft Systems Traffic Management (UTM) - A Common
              Framework with Core Principles for Global Harmonization,
              Edition 3", October 2020,
              <https://www.icao.int/safety/UA/Documents/
              UTM%20Framework%20Edition%203.pdf>.

   [Implementing]
              European Parliament and Council, "Commission Implementing
              Regulation (EU) 2019/947 of 24 May 2019 on the rules and
              procedures for the operation of unmanned aircraft", May
              2019,
              <https://eur-lex.europa.eu/eli/reg_impl/2019/947/oj>.

   [InitialView]
              SESAR Joint Undertaking, "Initial view on Principles for
              the U-space architecture", July 2019,
              <https://www.sesarju.eu/sites/default/files/documents/u-
              space/SESAR%20principles%20for%20U-
              space%20architecture.pdf>.

   [LDACS]    Maeurer, N., Ed., Graeupl, T., Ed., and C. Schmitt, Ed.,
              "L-band Digital Aeronautical Communications System
              (LDACS)", Work in Progress, Internet-Draft, draft-ietf-
              raw-ldacs-09, 22 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-raw-
              ldacs-09>.

   [NPRM]     United States Federal Aviation Administration (FAA),
              "Notice of Proposed Rule Making on Remote Identification
              of Unmanned Aircraft Systems", December 2019,
              <https://www.federalregister.gov/
              documents/2019/12/31/2019-28100/remote-identification-of-
              unmanned-aircraft-systems>.

   [OpenDroneID]
              "The Open Drone ID specification", commit c4c8bb8, March
              2020, <https://github.com/opendroneid/specs>.

   [OpenSky]  OpenSky Network, "About the OpenSky Network",
              <https://opensky-network.org/about/about-us>.

   [Opinion1] European Union Aviation Safety Agency (EASA), "High-level
              regulatory framework for the U-space", Opinion No 01/2020,
              March 2020, <https://www.easa.europa.eu/document-
              library/opinions/opinion-012020>.

   [Part107]  Code of Federal Regulations, "Part 107 - SMALL UNMANNED
              AIRCRAFT SYSTEMS", June 2016,
              <https://www.ecfr.gov/cgi-bin/text-idx?node=pt14.2.107>.

   [Recommendations]
              FAA UAS Identification and Tracking (UAS ID) Aviation
              Rulemaking Committee (ARC), "UAS Identification and
              Tracking (UAS ID) Aviation Rulemaking Committee (ARC): ARC
              Recommendations Final Report", September 2017, <https://ww
              w.faa.gov/regulations_policies/rulemaking/committees/
              documents/media/
              UAS%20ID%20ARC%20Final%20Report%20with%20Appendices.pdf>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [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>.

   [RFC8280]  ten Oever, N. and C. Cath, "Research into Human Rights
              Protocol Considerations", RFC 8280, DOI 10.17487/RFC8280,
              October 2017, <https://www.rfc-editor.org/info/rfc8280>.

   [RFC9063]  Moskowitz, R., Ed. and M. Komu, "Host Identity Protocol
              Architecture", RFC 9063, DOI 10.17487/RFC9063, July 2021,
              <https://www.rfc-editor.org/info/rfc9063>.

   [Roadmap]  ANSI Unmanned Aircraft Systems Standardization
              Collaborative (UASSC), "Standardization Roadmap for
              Unmanned Aircraft Systems", Working Draft, Version 2.0,
              April 2020, <https://share.ansi.org/Shared Documents/
              Standards Activities/UASSC/
              UASSC_20-001_WORKING_DRAFT_ANSI_UASSC_Roadmap_v2.pdf>.

   [Stranger] Heinlein, R., "Stranger in a Strange Land", June 1961.

   [WG105]    EUROCAE, "Minimum Operational Performance Standards (MOPS)
              for Unmanned Aircraft System (UAS) Electronic
              Identification", WG-105 SG-32 draft ED-282, June 2020.

   [WiFiNAN]  Wi-Fi Alliance, "Wi-Fi Aware", October 2020,
              <https://www.wi-fi.org/discover-wi-fi/wi-fi-aware>.

Appendix A.  Discussion and Limitations

   This document is largely based on the process of one SDO -- ASTM.
   Therefore, it is tailored to specific needs and data formats of
   ASTM's "Standard Specification for Remote ID and Tracking"
   [F3411-19].  Other organizations (for example, in the EU) do not
   necessarily follow the same architecture.

   The need for drone ID and operator privacy is an open discussion
   topic.  For instance, in the ground vehicular domain, each car
   carries a publicly visible plate number.  In some countries, for
   nominal cost or even for free, anyone can resolve the identity and
   contact information of the owner.  Civil commercial aviation and
   maritime industries also have a tradition of broadcasting plane or
   ship ID, coordinates, and even flight plans in plaintext.  Community
   networks such as OpenSky [OpenSky] and Flightradar24 [FR24] use this
   open information through ADS-B to deploy public services of flight
   tracking.  Many researchers also use these data to perform
   optimization of routes and airport operations.  Such ID information
   should be integrity protected, but not necessarily confidential.

   In civil aviation, aircraft identity is broadcast by a device known
   as transponder.  It transmits a four-octal digit squawk code, which
   is assigned by a traffic controller to an airplane after approving a
   flight plan.  There are several reserved codes, such as 7600, that
   indicate radio communication failure.  The codes are unique in each
   traffic area and can be re-assigned when entering another control
   area.  The code is transmitted in plaintext by the transponder and
   also used for collision avoidance by a system known as Traffic alert
   and Collision Avoidance System (TCAS).  The system could be used for
   UAS as well initially, but the code space is quite limited and likely
   to be exhausted soon.  The number of UAS far exceeds the number of
   civil airplanes in operation.

   The ADS-B system is utilized in civil aviation for each "ADS-B Out"
   equipped airplane to broadcast its ID, coordinates, and altitude for
   other airplanes and ground control stations.  If this system is
   adopted for drone IDs, it has additional benefit with backward
   compatibility with civil aviation infrastructure; then, pilots and
   dispatchers will be able to see UA on their control screens and take
   those into account.  If not, a gateway translation system between the
   proposed drone ID and civil aviation system should be implemented.
   Again, system saturation due to large numbers of UAS is a concern.

   The Mode S transponders used in all TCAS and most "ADS-B Out"
   installations are assigned an ICAO 24-bit "address" (arguably really
   an identifier rather than a locator) that is associated with the
   aircraft as part of its registration.  In the US alone, well over
   2^20 UAS are already flying; thus, a 24-bit space likely would be
   rapidly exhausted if used for UAS (other than large UAS flying in
   controlled airspace, especially internationally, under rules other
   than those governing small UAS at low altitudes).

   Wi-Fi and Bluetooth are two wireless technologies currently
   recommended by ASTM specifications due to their widespread use and
   broadcast nature.  However, those have limited range (max 100s of
   meters) and may not reliably deliver UAS ID at high altitude or
   distance.  Therefore, a study should be made of alternative
   technologies from the telecom domain (e.g., WiMAX / IEEE 802.16, 5G)
   or sensor networks (e.g., Sigfox, LoRa).  Such transmission
   technologies can impose additional restrictions on packet sizes and
   frequency of transmissions but could provide better energy efficiency
   and range.

   In civil aviation, Controller-Pilot Data Link Communications (CPDLC)
   is used to transmit command and control between the pilots and ATC.
   It could be considered for UAS as well due to long-range and proven
   use despite its lack of security [CPDLC].

   L-band Digital Aeronautical Communications System (LDACS) is being
   standardized by ICAO and IETF for use in future civil aviation
   [LDACS].  LDACS provides secure communication, positioning, and
   control for aircraft using a dedicated radio band.  It should be
   analyzed as a potential provider for UAS RID as well.  This will
   bring the benefit of a global integrated system creating awareness of
   global airspace use.

Acknowledgments

   The work of the FAA's UAS Identification and Tracking Aviation
   Rulemaking Committee (ARC) is the foundation of later ASTM [F3411-19]
   and IETF DRIP efforts.  The work of Gabriel Cox, Intel Corp., and
   their Open Drone ID collaborators opened UAS RID to a wider
   community.  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
   extensively reviewed or otherwise contributed to this document
   include Amelia Andersdotter, Carsten Bormann, Toerless Eckert, Susan
   Hares, Mika Jarvenpaa, Alexandre Petrescu, Saulo Da Silva, and Shuai
   Zhao.  Thanks to Linda Dunbar for the SECDIR review, Nagendra Nainar
   for the OPSDIR review, and Suresh Krishnan for the Gen-ART review.
   Thanks to IESG members Roman Danyliw, Erik Kline, Murray Kucherawy,
   and Robert Wilton for 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.  Thanks especially to Internet Area Director Éric Vyncke
   for guidance and support.

   This work was partly supported by the EU project AiRMOUR (enabling
   sustainable air mobility in urban contexts via emergency and medical
   services) under grant agreement no. 101006601.

Authors' Addresses

   Stuart W. Card (editor)
   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


   Andrei Gurtov
   Linköping University
   IDA
   SE-58183 Linköping
   Sweden

   Email: gurtov@acm.org