Drone Remote Identification Protocol (DRIP) Requirements
draft-ietf-drip-reqs-09

Document Type Active Internet-Draft (drip WG)
Authors Stuart Card  , Adam Wiethuechter  , Robert Moskowitz  , Andrei Gurtov 
Last updated 2021-03-08 (latest revision 2021-02-17)
Replaces draft-card-drip-reqs
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Andrei Gurtov  for 43 days
Robert Moskowitz  for 43 days
Stuart Card  for 43 days
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DRIP                                                        S. Card, Ed.
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 21 August 2021                                     R. Moskowitz
                                                          HTT Consulting
                                                               A. Gurtov
                                                    Linköping University
                                                        17 February 2021

        Drone Remote Identification Protocol (DRIP) Requirements
                        draft-ietf-drip-reqs-09

Abstract

   This document defines terminology and requirements for Drone Remote
   Identification Protocol (DRIP) Working Group solutions to support
   Unmanned Aircraft System Remote Identification and tracking (UAS RID)
   for security, safety, and other purposes.  Complementing external
   technical standards as regulator-accepted means of compliance with
   UAS RID regulations, DRIP will facilitate use of existing Internet
   resources to support RID and to enable enhanced related services, and
   will enable online and offline verification that RID information is
   trustworthy.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 21 August 2021.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation and External Influences  . . . . . . . . . . .   2
     1.2.  Concerns and Constraints  . . . . . . . . . . . . . . . .   7
     1.3.  DRIP Scope  . . . . . . . . . . . . . . . . . . . . . . .   9
     1.4.  Document Scope  . . . . . . . . . . . . . . . . . . . . .  10
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .  10
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .  10
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .  11
   3.  UAS RID Problem Space . . . . . . . . . . . . . . . . . . . .  18
     3.1.  Network RID . . . . . . . . . . . . . . . . . . . . . . .  20
     3.2.  Broadcast RID . . . . . . . . . . . . . . . . . . . . . .  23
     3.3.  USS in UTM and RID  . . . . . . . . . . . . . . . . . . .  26
     3.4.  DRIP Focus  . . . . . . . . . . . . . . . . . . . . . . .  27
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  28
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  28
     4.2.  Identifier  . . . . . . . . . . . . . . . . . . . . . . .  30
     4.3.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  31
     4.4.  Registries  . . . . . . . . . . . . . . . . . . . . . . .  33
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
   7.  Privacy and Transparency Considerations . . . . . . . . . . .  35
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  36
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  36
   Appendix A.  Discussion and Limitations . . . . . . . . . . . . .  39
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

1.1.  Motivation and External Influences

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

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   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, for
   multicopters, of advanced flight stability algorithms, enabling even
   inexperienced pilots to take off, fly to a location of interest,
   hover, and return to the take-off 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 quite noticeable at short range but difficult to detect
      at distances they can quickly close (500 meters in under 17
      seconds at 60 knots);

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

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

   UA can carry payloads including sensors, cyber 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.
   Consider the importance of identifiers in many Internet protocols and
   services, e.g., Fully Qualified Domain Names (FQDNs), transport
   protocol identifiers, UDP and TCP ports, Uniform Resource Identifiers
   (URIs), X.509 public key identifiers, E.164 numbers, Network Access
   Identifiers (NAIs), email addresses, Digital Object Identifiers
   (DOIs), and pretty much anything for which IANA is responsible.

   The general UAS RID usage scenario is illustrated in Figure 1.

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                        UA1               UA2
                        x x               x x
                       xxxxx             xxxxx

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

                 Figure 1: "General UAS RID Usage Scenario"

   Figure 1 illustrates a typical case where there may be: multiple
   Observers, some of them members of the general public, others
   government officers with public safety/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 as described
   below.

   An Observer of UA may need to classify them, as illustrated
   notionally in Figure 2, for basic airspace Situational Awareness
   (SA).  An Observer who classifies a UAS: as Taskable, can ask it to

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   do something useful; as Low Concern, can reasonably assume it is not
   malicious and would cooperate with requests to modify its flight
   plans for safety concerns that arise; as 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"

   ASTM International, Technical Committee F38 (UAS), Subcommittee
   F38.02 (Aircraft Operations), Work Item WK65041, developed the widely
   cited Standard Specification for Remote ID and Tracking [F3411-19]:
   the published standard is available for purchase from ASTM and as an
   ASTM membership premium; early drafts are freely available as
   [OpenDroneID] specifications.  [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.

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   Using UAS RID to facilitate vehicular (V2X) communications and
   applications such as Detect And Avoid (DAA), which would impose
   tighter latency bounds than RID itself, is an obvious possibility,
   explicitly contemplated in the United States (US) Federal Aviation
   Administration (FAA) Remote Identification of Unmanned Aircraft rule
   [FRUR].  However, applications of RID beyond RID itself, including
   DAA, have been declared out of scope in ASTM F38.02 WK65041, based on
   a distinction between RID as a security standard vs DAA as a safety
   application.

   [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 one-way to Observers, they will
   use 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 is publishing an EU
   standard "Aerospace series - Unmanned Aircraft Systems - Part 002:
   Direct Remote Identification; English version prEN 4709-002:2020" for
   which a current (early 2021) informal overview is freely available in
   [ASDRI].  It will provide compliance to cover the identical DRI
   requirements applicable to drones of classes C1 - [Delegated] Part 2,
   C2 - [Delegated] Part 3, C3 - [Delegated] Part 4, C5 - [Amended] Part
   16, and 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 NAN and/or Wi-Fi Beacon modes.
   The EU standard emphasis was compatibility with [F3411-19], although
   there are differences in mandatory and optional message types and
   fields.

   The DRI system will broadcast locally:

   1.  the UAS operator registration number;

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   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 take-off 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 take-off point;
       and

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

   The data will be sent in plain text and the UAS operator registration
   number will be represented as a 16-byte string including the state
   code.  The private id part will contain 3 characters which are not
   broadcast but used by authorities to access regional registration
   databases for verification.

   ASD-STAN also contemplates corresponding Network Remote
   Identification (NRI) functionality.  The ASD-STAN RID target is to
   revise their current standard with additional functionality (e.g.,
   DRIP) to be published before 2022 [ASDRI].

   Security oriented UAS RID essentially has two goals: enable the
   general public to obtain and record an opaque ID for any observed UA,
   which they can then report to authorities; enable authorities, from
   such an ID, to look up information about the UAS and its operator.
   Safety oriented UAS RID has stronger requirements.  Aviation
   community Standards Development Organizations (SDOs) set a higher bar
   for safety than for security, especially with respect to reliability.

   Although 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 their
   [Recommendations], it is not addressed in any of the subsequent
   regulations or technical specifications.

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 all information in UAS RID is operator self-reports.
   Reports may be initiated by the remote pilot at the Ground Control
   Station (GCS) console, by a software process on the GCS, or by a

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   process on the UA.  Data in the reports may come from the UA (e.g.,
   an on-board GNSS receiver), the GCS (e.g., dead reckoning UA location
   based on takeoff location and piloting commands given since takeoff),
   and/or sensors available to the operator (e.g., radar or cameras).
   Whether information comes proximately from the operator, or from
   automated systems configured by the operator, there are possibilities
   not only of unintentional error in, but also of intentional
   falsification of, this data.

   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
   unspecified 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
   Civil Aviation Authority (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 implies extremely short "advertisement" (beacon)
   frames.

   Wireless data links on the UA are challenging due to low altitude
   flight amidst structures and foliage over terrain, as well as the
   severe Cost, Size, Weight, and Power (CSWaP) constraints of devices
   onboard UA.  CSWaP is a burden not only on the designers of new UA
   for production and sale, but also on owners of existing UA 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, sensitive to both generic CSWaP and application-specific
   considerations.

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   To accommodate the most severely constrained cases, all these
   conspire to motivate system design decisions that complicate the
   protocol design problem.  All UA are constrained by their batteries
   (both instantaneous power and total energy) and small UA imply small
   antennas, so wireless air to ground links will generally be slow and
   unreliable.  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.  Broadcast RID uses one-way data links.
   Bluetooth 4 restricts broadcast messages to fit in extremely short
   "advertisement" packets.  UA onboard devices 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 too infrequent to address, but for some users is
   important and presents further challenges.

   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 being ubiquitous, thus popular targets for malware
   or other compromise, 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 goal is to make RID immediately actionable, in both
   Internet and local-only connected scenarios (especially emergencies),
   in severely constrained UAS environments, balancing legitimate (e.g.,
   public safety) authorities' Need To Know trustworthy information with
   UAS operators' privacy.  By "immediately actionable" is meant
   information of sufficient precision, accuracy, timeliness, etc. for
   an Observer to use it as the basis for immediate decisive action,
   whether that be to trigger a defensive counter-UAS system, to attempt
   to initiate communications with the UAS operator, to accept the
   presence of the UAS in the airspace where/when observed as not
   requiring further action, or whatever, with potentially severe
   consequences of any action or inaction chosen based on that

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   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 higher
   than generally mandated RID requirement for flight in such volumes.
   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) potentially could be applied to verifiably identify other types
   of registered things reported to be in specified physical locations,
   and providing timely trustworthy identification data is also
   prerequisite to identity-oriented networking, but the urgent
   motivation and clear initial focus is UAS.  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 (IANA, security, privacy and
   transparency), and discusses limitations.

   A natural Internet based approach to meet these requirements is
   described in a companion architecture document [drip-architecture]
   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.

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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.

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

   In the UAS community, the plural form of acronyms generally is the
   same as the singular form, e.g., Unmanned Aircraft System (singular)
   and Unmanned Aircraft Systems (plural) are both represented as UAS.
   On this and other terminological issues, to encourage comprehension
   necessary for adoption of DRIP by the intended user community, that
   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 listed terms are from that community (even if specific source
   documents are not cited); any that are DRIP-specific or invented by
   the authors of this document are marked "(DRIP)".

   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.

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   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.  "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" [ICAOATM].

   Authentication Message
      [F3411-19] Message Type 2.  Provides framing for authentication
      data, only.

   Basic ID Message
      [F3411-19] Message Type 0.  Provides UA Type, UAS ID Type, and UAS
      ID, only.

   BVLOS
      Beyond Visual Line Of Sight.  See VLOS.

   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, 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.  "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" [ICAOUAS].

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   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] that presumably can be satisfied with appropriately
      configured [F3411-19] Broadcast RID.

   DSS
      Discovery and Synchronization Service.  Formerly Inter-USS.  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.  [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
      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, dedicated to aviation
      (but not just aircraft).  Currently (2021) in design,
      accommodating the proposed DRIP identifier.

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   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 United Nations
      specialized agency that develops and harmonizes international
      standards relating to aviation.

   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.  US partial
      stopgap 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

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      but also information retrieved from UAS RID registries or
      information beyond UAS RID.  Under superseded [NPRM], not
      recognized as a distinct entity, but 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 NetRID-DP queries
      for information on UAS of which it is aware.  Under superseded
      [NPRM], the USS to which the UAS is subscribed ("Remote ID USS").

   Network Identification Service
      EU regulatory requirement in [Opinion1] and [WG105] that
      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.

   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, Pilot In Command (PIC), or Remote
      Pilot, but possibly of an Observer or other party.

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   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.  Adjective, e.g., "RF link", or 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.

   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.  ASTM, IETF, et al.

   SDSP
      Supplemental Data Service Provider.  An entity that participates
      in the UTM system, but provides services beyond those specified as
      basic UTM system functions (e.g., weather data).  [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
      classes of airspace, specifically including high density urban
      areas, sharing airspace with manned aircraft [InitialView].

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

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   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", 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].

   UAS ID Type
      UAS Identifier type index. 4 bits, see Section 3, Paragraph 6 for
      currently defined values 0-3.  [F3411-19]

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

   UAS RID Verifier Service
      System component designed to handle the authentication
      requirements of RID by offloading verification to a web hosted
      service [F3411-19].

   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" and "... 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" low altitude UAS operations, separate from, but
      complementary to, the FAA's ATC system for "controlled" operations
      of manned aircraft.

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

   VLOS
      Visual Line Of Sight.  Typically used in describing operation of a
      UA by a "remote" pilot who can clearly 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

   Civil Aviation Authorities (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).  CAAs currently (2021) 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.

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   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 for UAS registry
   information lookup using the directly locally received UAS Identifier
   (UAS ID) as a key.  Broadcast RID does not assume IP connectivity of
   UAS; messages are encapsulated by the UA without IP, directly in
   Bluetooth or Wi-Fi Neighbor Awareness Networking [WiFiNAN] link layer
   frames.

   [F3411-19] specifies three UAS ID types:

   TYPE-1  A static, manufacturer assigned, hardware serial number as
           defined in ANSI/CTA-2063-A "Small Unmanned Aerial System
           Serial Numbers" [CTA2063A].

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

   TYPE-3  A UTM system assigned UUID [RFC4122], which can but need not
           be dynamic.

   Per [Delegated], the EU allows only Type 1.  Under [FRUR], the US
   allows Types 1 and 3.  [NPRM] proposed that a Type 3 "Session ID"
   would be "e.g., a randomly-generated alphanumeric code assigned by a
   Remote ID 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 apparently there are no CAA public proposals to use 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.

   Under [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 by either the UA itself,
   which is then designated a "standard remote identification unmanned
   aircraft", or by an add-on "remote identification broadcast module".
   Relative to a standard RID UA, the different requirements for a

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   module are that the latter: 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 counter-intuitively named "FAA-recognized
   identification areas" (FRIA); 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

             x x    UA
            xxxxxxx
             |    \
             |     \
             |      \
             |       \  ********************
             |        \*              ------*---+------------+
             |        *\             /       *  | NET_Rid_SP |
             |        * ------------/    +---*--+------------+
             | RF     */                 |   *
             |        /      INTERNET    |   *  +------------+
             |       /*                  +---*--| NET_Rid_DP |
             |      / *                 +----*--+------------+
             +     /   *                |   *
              x   /     ****************|***      x
            xxxxx                       |       xxxxx
              x                         +-------  x
              x                                   x
             x x   Operator's GCS     Observer   x x
            x   x                               x   x

                  Figure 3: "Network RID Information Flow"

   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.  All three may exist, at the
   same or different times, especially in Beyond Visual Line Of Sight
   (BVLOS) operations.  There must be some 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

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   (and position, if suitably equipped, e.g., with GNSS) to the GCS.
   There also must be some 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).

   Direct UA-Internet wireless links are expected to become more common,
   especially on larger UAS, but currently (2021) are rare.  Instead,
   the RID data flow typically originates on the UA and passes through
   the GCS, or 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) 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 DP) for that operation.

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   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 unspecified (generally presumed to be web
       browser based) means.

   Network RID has several variants.  The UA may have persistent onboard
   Internet connectivity, in which case it can consistently source RID
   information directly over the Internet.  The UA may have intermittent
   onboard Internet connectivity, in which case the GCS must source RID
   information whenever the UA itself is offline.  The UA may not have
   Internet connectivity of its own, but have instead some other form of
   communications to another node that can relay RID information to the
   Internet.  In this last case, the relay would typically be the GCS
   (which to perform its function must know where the UA is, although C2
   link outages do occur).

   The UA may 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 using a web
   browser/application to designate, to a UAS Service Supplier (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".  This may impede disambiguation of ID if
   multiple UAS operate in the same or overlapping 4-D volumes.

   In most cases in the near term (2021), the Network RID first hop data
   link is likely to be cellular Long Term Evolution (LTE), 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 Network RID
   Service Provider (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 Network RID Display Provider (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.

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   Network RID is the less constrained of the defined UAS RID means.
   [F3411-19] specifies only Net-RID SP to Net-RID DP information
   exchanges.  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

             x x  UA
            xxxxx
             |
             |
             | app messages directly over one-way RF data link
             |
             |
             +
              x
            xxxxx
              x
              x
             x x   Observer's device (e.g., smartphone)
            x   x

                 Figure 4: "Broadcast RID Information Flow"

   Figure 4 illustrates 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
   a RF data link (without IP or other middle layer protocols) 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 verifier service and look
   up information in registries using the UAS ID as the primary unique
   key.

   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 with Neighbor Awareness Networking (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 also required

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   concurrently to do so on 4.x (referred to in [F3411-19] as Bluetooth
   Legacy); current (2021) discussions in ASTM F38.02 on revising the
   standard include reversing this.  If broadcasting Wi-Fi NAN at 5 GHz,
   it is also required concurrently to do so at 2.4 GHz; current
   discussions in 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 the Broadcast 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 Application Programming Interface (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 25 byte limit of the Bluetooth 4.x "Broadcast
   Frame" transmitted on beacon channels.  After nominal overheads, this
   limits the UAS ID string to a maximum length of 20 bytes, and
   precludes the same frame carrying position, velocity, and other
   information that should be bound to the UAS ID, much less strong
   authentication data.  This requires segmentation ("paging") of longer
   messages and correlation of short messages (anticipated by ASTM to be
   done on the basis of MAC address, which is weak and unverifiable) on
   Bluetooth 4.x; data elements are not so detached on other media (see
   Message Pack below).

   [F3411-19] Broadcast RID specifies several message types.  The 4 bit
   message type field in the header can index up to 16 types.  Only 7
   are currently defined.  Only 2 are mandatory.  All others are
   optional, unless required by a jurisdictional authority, e.g., a CAA.
   To satisfy EASA and FAA rules, all types are needed, except Self-ID
   and Authentication.  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; it is the
   sole frame type on links that can encapsulate it (Bluetooth 5.x and
   Wi-Fi).

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    +-----------------------+-----------------+-----------+-----------+
    | 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        | Optional  | -         |
    +-----------------------+-----------------+-----------+-----------+
    | 0xF                   | Message Pack    | -         | BT5 and   |
    |                       |                 |           | Wi-Fi     |
    +-----------------------+-----------------+-----------+-----------+
    | See Section 5.4.5 and | -               | -         | -         |
    | Table 3 of [F3411-19] |                 |           |           |
    +-----------------------+-----------------+-----------+-----------+

                      Table 1: F3411-19 Message Types

   [F3411-19] Broadcast RID specifies very few quantitative performance
   requirements: static information must be transmitted at least once
   per 3 seconds; 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, 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 Position/
   Vector message likewise lacks provisions for verification, and does
   not contain the ID, so 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

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   conventional digital signatures and far too short for conventional
   certificates.  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).  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.

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; some limited to a single jurisdiction,
   others spanning 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 by different UAS and
   other (e.g., manned aircraft, rocket launch) operations.  Most
   deconfliction involving a given operation is hoped to be completed
   prior to commencing that operation, and is called "strategic
   deconfliction".  If that fails, "tactical deconfliction" comes into
   play; ABDAA may not involve USS, but GBDAA likely will.  Dynamic
   constraints, formerly called UAS Volume Restrictions (UVR), can be
   necessitated by local emergencies, extreme weather, etc., specified
   by authorities on the ground, and propagated in UTM.

   No role for USS in Broadcast RID is currently specified by regulators
   or [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
   (SDSP) are also likely to find roles, not only in UTM as such but
   also in enhancing UAS RID and related services.  Whether USS, SDSP,
   etc. are involved or not, RID services, narrowly defined, provide
   regulator specified identification information; more broadly defined,
   RID services may leverage identification to facilitate related
   services or functions, likely beginning with V2X.

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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, and 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 at best slow 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, then call a phone number in hopes someone who can
   immediately influence the UAS operation will answer promptly during
   that operation; this is at best unreliable and may not be prudent.
   Should pilots be encouraged to answer phone calls while flying?
   Internet technologies can do much better than this.

   Thus complementing [F3411-19] with protocols enabling strong
   authentication, preserving operator privacy while enabling immediate
   use of information by authorized parties, is critical to achieve
   widespread adoption of a RID system supporting safe and secure
   operation of UAS.

   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.

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

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 DRIP protocol alone satisfy
   each and every requirement.

4.1.  General

   GEN-1   Provable Ownership: DRIP MUST enable verification that the
           UAS ID asserted in the Basic ID message is that of the actual
           current sender of the message (i.e., the message is not a
           replay attack or other spoof, authenticating, e.g., by
           verifying an asymmetric cryptographic signature using a
           sender provided public key from which the asserted ID can be
           at least partially derived), even on an Observer device
           lacking Internet connectivity at the time of observation.

   GEN-2   Provable Binding: DRIP MUST enable binding 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 verification that the
           UAS ID is in a registry and identification of which one, even
           on an Observer device lacking Internet connectivity at the
           time of observation; with UAS ID Type 3, 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 Broadcast RID to Network RID
           application layer gateways to stamp messages with precise
           date/time received and receiver location, then relay them to
           a network service (e.g., SDSP or distributed ledger).

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   GEN-6   Finger: DRIP MUST enable dynamically establishing, with AAA,
           per policy, end-to-end strongly encrypted communications with
           the UAS RID sender and entities looked up from the UAS ID,
           including at least the remote pilot and USS.

   GEN-7   QoS: DRIP MUST enable policy based specification of
           performance and reliability parameters, such as maximum
           message transmission intervals and delivery latencies.

   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 Registry, 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/
           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.

   Requirements imposed either by regulation or [F3411-19] are not
   reiterated here, but drive many of the numbered requirements listed
   here.  The [FRUR] regulatory QoS requirement 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.  So
   instead the DRIP QoS requirement is that performance, reliability,
   etc. parameters be user policy specifiable, which does not imply
   satisfiable in all cases, but (especially together with the
   management requirement) implies that when specifications are not met,
   appropriate parties are notified.

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   The "provable ownership" requirement addresses the possibility that
   the actual sender is not the claimed sender (i.e., is a spoofer).
   The "provable binding" requirement addresses the MAC address
   correlation problem of [F3411-19] noted above.  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 may
   involve machine assisted format conversions, e.g., from binary
   encodings.

   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.

4.2.  Identifier

   ID-1  Length: The DRIP entity identifier MUST NOT be longer than 20
         bytes, to fit in the UAS ID field of the Basic ID message of
         [F3411-19].

   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 de-registered
         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).

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   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.

   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 (have the exclusive capability to use, such that
   receivers can verify its ownership of) the identifier.

   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 (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 HIP or DTLS).

   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 thereamong, is unspecified; however,
   there must be agreement on the UAS ID among these entities.

4.3.  Privacy

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

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   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,
           which information is public and which private.  By default,
           all information required to be transmitted via Broadcast RID,
           even when actually sent via Network RID, is assumed to be
           public; all other information contained 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.

   In some jurisdictions, the configurable enabling and disabling of
   encryption may need to be outside the control of the operator.
   [FRUR] mandates manufacturers design RID equipment with some degree
   of tamper resistance; the preamble 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 requires obfuscation of location to any Network RID subscriber
   engaging in wide area surveillance, limits data retention periods,

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   etc., in the interests of privacy), 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

   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.

   Registries are fundamental to RID.  Only very limited information can
   be Broadcast, 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.  Beyond
   designating the UAS ID as that unique key, the registry information
   model is not specified herein, in part because regulatory
   requirements for different registries (UAS operators and their UA,
   each narrowly for UAS RID and broadly for U-space/UTM) and business
   models for meeting those requirements are in flux.  However those may
   evolve, the essential registry functions remain the same, so are
   specified herein.

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5.  IANA Considerations

   This document does not make any IANA request.

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 of (e.g., 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

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   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.  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, e.g.,
   firefighting in a national forest, are important.

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."

   Privacy and transparency (where essential to security or safety) 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
   [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; herein it will be regarded
   as a matter for CAA, registry, and operator policies, for which
   enforcement mechanisms will be defined within the scope of DRIP WG
   and offered.  Details of the protection mechanisms will be provided
   in other DRIP documents.  Mitigation of adversarial correlation will
   also be addressed.

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8.  References

8.1.  Normative References

   [F3411-19] ASTM International, "Standard Specification for Remote ID
              and Tracking", 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 Union Aviation Safety Agency (EASA), "Commission
              Delegated Regulation (EU) 2020/1058 of 27 April 2020
              amending Delegated Regulation (EU) 2019/945", April 2020,
              <https://eur-lex.europa.eu/legal-content/EN/
              TXT/?uri=CELEX%3A32020R1058>.

   [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>.

   [CPDLC]    Gurtov, A., Polishchuk, T., and M. Wernberg, "Controller-
              Pilot Data Link Communication Security", MDPI
              Sensors 18(5), 1636, 2018,
              <https://www.mdpi.com/1424-8220/18/5/1636>.

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

   [Delegated]
              European Union Aviation Safety Agency (EASA), "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>.

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   [drip-architecture]
              Card, S., Wiethuechter, A., Moskowitz, R., Zhao, S., and
              A. Gurtov, "Drone Remote Identification Protocol (DRIP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-drip-arch-08, 21 January 2021,
              <https://tools.ietf.org/html/draft-ietf-drip-arch-08>.

   [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>.

   [EU2018]   European Parliament and Council, "2015/0277 (COD) PE-CONS
              2/18", February 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>.

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

   [I-D.maeurer-raw-ldacs]
              Maeurer, N., Graeupl, T., and C. Schmitt, "L-band Digital
              Aeronautical Communications System (LDACS)", Work in
              Progress, Internet-Draft, draft-maeurer-raw-ldacs-06, 2
              October 2020,
              <https://tools.ietf.org/html/draft-maeurer-raw-ldacs-06>.

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

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

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   [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 Union Aviation Safety Agency (EASA), "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>.

   [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]
              Intel Corp., "Open Drone ID", March 2019,
              <https://github.com/opendroneid/specs>.

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

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

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

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   [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>.

   [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>.

   [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>.

   [Roadmap]  American National Standards Institute (ANSI) Unmanned
              Aircraft Systems Standardization Collaborative (UASSC),
              "Standardization Roadmap for Unmanned Aircraft Systems
              draft v2.0", April 2020, <https://share.ansi.org/Shared
              Documents/Standards Activities/UASSC/
              UASSC_20-001_WORKING_DRAFT_ANSI_UASSC_Roadmap_v2.pdf>.

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

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

   [WiFiNAN]  Wi-Fi Alliance, "Wi-Fi Aware™ Specification Version 3.2",
              October 2020, <https://www.wi-fi.org/downloads-registered-
              guest/Wi-Fi_Aware_Specification_v3.2.pdf/29731>.

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 this
   standard.  Other organizations, for example in EU, do not necessary
   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

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   industries also have a tradition of broadcasting plane or ship ID,
   coordinates, and even flight plans in plain text.  Community networks
   such as OpenSky and Flightradar 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-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 which
   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 plain text 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.

   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 (WiMAX / IEEE 802.16, 5G) or
   sensor networks (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
   [I-D.maeurer-raw-ldacs].  It provides secure communication,

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   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 a
   global airspace use awareness.

Acknowledgments

   The work of the FAA's UAS Identification and Tracking (UAS ID)
   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, Mohamed Boucadair,
   Toerless Eckert, Susan Hares, Mika Jarvenpaa, Daniel Migault,
   Alexandre Petrescu, Saulo Da Silva and Shuai Zhao.

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

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   Andrei Gurtov
   Linköping University
   IDA
   SE-58183 Linköping
   Sweden

   Email: gurtov@acm.org

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