DRIP                                                        S. Card, Ed.
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 19 April 5 May 2021                                         R. Moskowitz
                                                          HTT Consulting
                                                               A. Gurtov
                                                    Linköping University
                                                         16 October
                                                         1 November 2020

        Drone Remote Identification Protocol (DRIP) Requirements


   This document defines terminology and requirements for Drone Remote
   Identification Protocol (DRIP) Working Group protocols 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 UAS RID
      and to enable enhanced related services;

      enable online and offline verification that UAS RID information is

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 19 April 5 May 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction (Informative)  . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Concerns and Constraints  . . . . . . . . . . . . . . . .   6
     1.3.  DRIP Scope  . . . . . . . . . . . . . . . . . . . . . . .   7   8
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   8
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   8
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   8   9
   3.  UAS RID Problem Space . . . . . . . . . . . . . . . . . . . .  16
     3.1.  Network RID . . . . . . . . . . . . . . . . . . . . . . .  17  18
     3.2.  Broadcast RID . . . . . . . . . . . . . . . . . . . . . .  19  20
     3.3.  USS in UTM and RID  . . . . . . . . . . . . . . . . . . .  22
     3.4.  DRIP Focus  . . . . . . . . . . . . . . . . . . . . . . .  22  23
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  22  24
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  22  24
     4.2.  Identifier  . . . . . . . . . . . . . . . . . . . . . . .  24  26
     4.3.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  25  27
     4.4.  Registries  . . . . . . . . . . . . . . . . . . . . . . .  26  28
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27  29
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  27  29
   7.  Privacy and Transparency Considerations . . . . . . . . . . .  29  30
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29  30
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  29  31
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  30  31
   Appendix A.  Discussion and Limitations . . . . . . . . . . . . .  32  33
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  33  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34  35

1.  Introduction (Informative)
1.1.  Motivation

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

   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), (STOL) capability; rotary
   wing (e.g., helicopter) and hybrid UA typically have Vertical Take-Off and Landing
   (VTOL), or hybrid.  They
   (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 GNSS waypoint to waypoint in a weak form
   of autonomy; stronger autonomy is coming.  UA are "low observable":
   they typically have small radar cross sections; they 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); they
   typically fly at low altitudes (for the small UAS to which RID
   applies in the US, under 400 feet AGL); they 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.  Numerous
   other applications can be enabled or facilitated by RID: consider the
   importance of identifiers in many Internet protocols and services.
   The general scenario is illustrated in Figure 1.

                        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
                               |  |
                               +  +
                           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 Scenario"

   Note the absence of any links to/from the UA in Figure 1.  This is
   because UAS RID and other connectivity involving the UA varies as
   described below.

   Inherently, any responsible Observer of UA must classify them, as
   illustrated notionally in Figure 2.  For basic airspace Situational
   Awareness (SA), an Observer who classifies an UAS: as Taskable, can
   ask it to 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.  These classes are not
   standard, but derive from first principles.

                        xxxxxxx        +--------------+
                       x       x  No   |              |
                      x   ID?   x+---->| UNIDENTIFIED |
                       x       x       |              |
                        xxxxxxx        +--------------+
                           | Yes
                       x       x
           +---------+x  TYPE?  x+----------+
           |           x       x            |
           |            xxxxxxx             |
           |               +                |
           v               v                v
   +--------------+ +--------------+ +--------------+
   |              | |              | |              |
   |              | |              | |              |
   +--------------+ +--------------+ +--------------+

                  Figure 2: "Notional UAS Classification"

   An ID is not an end in itself; it exists to enable lookups and
   provision of services complementing mere identification.

   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) Notice of Proposed Rule Making [NPRM].  However,
   applications of RID beyond RID itself, including DAA, have been
   declared out of scope in ASTM International, Technical Committee F38
   (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item WK65041, working group discussions, WK65041
   (source of the widely cited [F3411-19]), based on a distinction
   between RID as a security standard vs DAA as a safety application.
   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
   proposed regulations or technical specifications.

   [Opinion1] and [WG105] cite the Direct Remote Identification
   previously required and specified, explicitly stating that whereas
   Direct RID is primarily for security purposes, "Electronic
   Identification" (or the "Network Identification Service" in the
   context of U-space) 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 EU U-space concept, as being developed
   in the Single European Sky ATM Research (SESAR) Joint Undertaking,
   whose U-space architectural principles are outlined in [InitialView].

   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 SDOs set a higher bar for safety than for security,
   especially with respect to reliability.

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.  As the

   The origin of all information in UAS RID is self-reports 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
   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 operators,
   automated systems configured by the operator, there are possibilities
   not only of unintentional error, error in, but also of intentional
   falsification, of
   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 per 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]  [F3411-19], the
   basis for most current 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, is unspecified. unspecified therein.

   The need for near-universal deployment of UAS RID is pressing.  This
   implies the need to support use by Observers of already ubiquitous
   mobile devices (typically smartphones and tablets).  Anticipating
   likely 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

   To accommodate the most severely constrained cases, all these
   conspire to motivate system design decisions, especially for the
   Broadcast RID data link, which complicate the protocol design
   problem: one-way links; extremely short packets; and Internet-
   disconnected operation of UA onboard devices.  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 with limited bandwidth, short packet
   payload length limits, and one-way links, 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 string
   (typically number) 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 Standards Development Organizations
   (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 early 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
   information.  For further explanation of the concept of immediate
   actionability, see [ENISACSIRT].  Note that UAS RID must achieve near
   universal adoption, but DRIP can add value even if only selectively
   deployed, as those with jurisdiction over more sensitive airspace
   volumes may set a higher than generally mandated RID bar for flight
   in those volumes.  Providing timely trustworthy identification data
   is also prerequisite to identity-oriented networking.

   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,
   but the urgent motivation and clear initial focus is UAS.  Existing
   Internet resources (protocol standards, services, infrastructure, and
   business models) should be leveraged.  A natural Internet based
   architecture for UAS RID conforming to proposed regulations and
   external technical standards is described in a companion architecture
   document [drip-architecture] and elaborated in other DRIP documents;
   this document describes only relevant requirements and defines
   terminology for the set of DRIP documents.

2.  Terms and Definitions

2.1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  Definitions

   This section defines a set of terms expected to be used in DRIP
   documents.  This list is meant to be the DRIP terminology reference.
   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)".

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

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

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

      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.

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

      Air Traffic Control.  Explicit flight direction to pilots from
      ground controllers.  Contrast with 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.  Optional per [F3411-19] but may be required by

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

      Beyond Line Of Sight (LOS).  Term to be avoided due to ambiguity.
      See LOS.

      Beyond Visual Line Of Sight (V-LOS).  See V-LOS.

      Civil Aviation Authority.  Two examples are the United States
      Federal Aviation Administration (FAA) and the Japan Civil Aviation

      Cost, Size, Weight and Power.

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

      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]

   Direct RID
      Direct Remote Identification. "a system that ensures the local
      broadcast of information about a an UA in operation, including the
      marking of the UA, so that this information can be obtained
      without physical access to the UA".  [Delegated] Corresponds
      roughly to the Broadcast RID portion of [NPRM] Standard RID.

      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 deconfliction of planned and Network RID
      surveillance of active operations.  [F3411-19]

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

      Ground Based DAA.  Accomplished with the aid of ground based

      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 GPS waypoints, or
      otherwise directing its flight.

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

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

      Global Resilient Aviation Interoperable Network.  ICAO managed
      IPv6 overlay internetwork per IATF, dedicated to aviation (but not
      just aircraft).  Currently in design.

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

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

      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.

   Limited RID
      A mode of operation that must use Network RID, must not use
      Broadcast RID, and must provide pilot/GCS location only (not UA
      location).  This mode is only allowed for UA that neither require
      (due to e.g. size) nor are equipped for Standard RID, operated
      within V-LOS and within 400 feet of the pilot, below 400 feet AGL,
      etc.  [NPRM]

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

      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 V-LOS.

      Mean Sea Level.  Relative altitude, above the variously defined
      mean sea level, typically of an UA (but in [NPRM] also for a GCS),
      measured in feet or meters.  Should be explicitly specified as
      either barometric (pressure) or geodetic (GNSS).

   Net-RID DP
      Network RID Display Provider.  [F3411-19] logical entity that
      aggregates data from Net-RID SPs as needed in response to user
      queries regarding UAS operating within specified airspace volumes,
      to enable display by a user application on a user device.
      Potentially could provide not only information sent via UAS RID
      but also information retrieved from UAS RID registries, or
      information beyond UAS RID.  Under [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 [NPRM], the
      USS to which the UAS is subscribed ("Remote ID USS").

   Network Identification Service
      EU regulatory requirement for Network RID.  [Opinion1] and [WG105]
      Corresponds roughly to the Network RID portion of [NPRM] Standard

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

      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)

      "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.  Optional per [F3411-19] but
      may be required by regulations.

      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]

      Personally Identifiable Information.  In this context, typically
      of the UAS Operator, Pilot In Command (PIC) or Remote Pilot, but
      possibly of an Observer or other party.

   Remote Pilot
      A pilot using a GCS to exercise proximate control of an 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]

      Radio Frequency.  Noun or adjective, e.g.  "RF link."

      RF LOS.  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 V-LOS.

      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.  Optional per
      [F3411-19] but may be required by regulations.

   Standard RID
      A mode of operation that must use both Network RID (if Internet
      connectivity is available at the time in the operating area) and
      Broadcast RID (always and everywhere), and must provide both
      pilot/GCS location and UA location.  This mode is required for UAS
      that exceed the allowed envelope (e.g. size, range) of Limited RID
      and for all UAS equipped for Standard RID (even if operated within
      parameters that would otherwise permit Limited RID).  [NPRM] The
      Broadcast RID portion corresponds roughly to EU Direct RID; the
      Network RID portion corresponds roughly to EU Network
      Identification Service.

      Standards Development Organization.  ASTM, IETF, et al.

      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., provides weather data.

   System Message
      [F3411-19] Message Type 4.  Provides general UAS information,
      including remote pilot location, multiple UA group operational
      area, etc.  Optional per [F3411-19] but may be required by

      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]

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

      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 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 5 for
      currently defined values 0-3.  [F3411-19]

      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]

      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" per

      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, per
      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.

      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.

      Visual LOS.  Typically used in describing operation of an UA by a
      "remote" pilot who can clearly directly (without video cameras or
      any other 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 described
   the key role that UAS RID plays in UAS Traffic Management (UTM) in
   [NPRM] and [FAACONOPS] (especially Section 2.6 of the latter).  CAAs
   currently (2020) promulgate performance-based regulations that do not
   specify techniques, but rather cite industry consensus technical
   standards as acceptable means of compliance.

   ASTM developed a widely cited Standard Specification for Remote ID
   and Tracking [F3411-19] (early drafts are freely available as
   [OpenDroneID] specifications).  It 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 layer), to be
      received in real time by local Observers.

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

   [F3411-19] specifies three UAS ID types:

   TYPE-1  A static, manufacturer assigned, hardware serial number per
           ANSI/CTA-2063-A "Small Unmanned Aerial System Serial Numbers"

   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.  Per [NPRM], the US
   allows Types 1 and 3, but requires Type 3 IDs (if used) each to be
   used only once as a "Session ID" (for a single UAS flight, which in
   the context of UTM is called an "operation").  Per [Delegated], the
   EU also requires an operator registration number (an additional
   identifier distinct from the UAS ID) that can be carried in an
   [F3411-19] optional Operator ID message.  Per [NPRM], the US allows
   but does not require that operator registration numbers be sent.  As
   yet apparently there are no CAA public proposals to use Type 2.

3.1.  Network RID

             x x    UA
             |    \
             |     \
             |      \
             |       \  ********************
             |         *        \*              ------*---+------------+
             |        *        *\             /       *  | 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"


   Only two of the data flow typically originates on three links UA-GCS, UA-Internet and GCS-Internet need
   exist, although all three may.  There must be some path (direct or at least passes through
   the Ground Control Station (GCS), rather than comes direct from
   indirect) between the
   UA as in Broadcast RID (below), GCS and the UA, for the former to exercise C2
   over the latter; if this path is two-way (as increasingly it is, even
   for inexpensive small UAS), the UA will also send its status (and
   position, if suitably equipped) information to the GCS.  There 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 Net-RID SP.

   Currently, the RID data flow typically originates on the UA and
   passes through the GCS, or originates on the GCS, rather than comes
   direct from the UA as in Broadcast RID (below), and makes up to 3 three
   trips through the Internet, implying use of IP (and other middle
   layer protocols) on those trips, but not necessarily on the an UA-GCS
   link (if indeed that direct even exists and further the Network RID
   data even flows across that link). it).

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

   1.  The UAS operator pushes an operation "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, for primarily to enable deconfliction with other operations.
       operations potentially impinging upon that operation's 4-D
       airspace volume (call it Volume#1).

   2.  Assuming the plan receives approval and the operation is approved and commences, that UAS #1
       periodically pushes location/status updates to that USS (call it USS#1), USS#1, which
       serves _inter alia_ as the Network RID Service Provider (Net-RID
       SP) for that operation.  If

   3.  When users of any other USS (whether they be other UAS operators
       or Observers) develop an interest in any 4-D airspace volume intersecting the 4-D volume
   containing that UAS operation,
       (e.g. because they wish to submit an operational intent or
       because they have observed an UA), they query their own USS (call them
   USS#2 through USS#n). 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.  Observers or other interested

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

   6.  USS#1 (as Net-RID SP (USS#1) SP) will then publish updates of the UAS position/status 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
   (USS#2 through USS#n), which in turn subscribed to that operation of UAS#1 will deliver the
       its track information to their users who subscribed to that
       operation of UAS#1, via unspecified (but expected (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; this would typically be the GCS (which to perform its
   function must know where the UA is, although C2 link outages do

   The UA may have no means of sourcing RID information, in which case
   the GCS must source it; this is typical under FAA NPRM Limited RID
   proposed rules, which require providing the location of the GCS (not
   that of the UA).  In the extreme case, this could be the pilot using
   a web browser/application to designate, to an UAS Service Supplier
   (USS) or other UTM entity, a time-bounded airspace volume in which an
   operation will be conducted; this may impede disambiguation of ID if
   multiple UAS operate in the same or overlapping spatio-temporal 4-D volumes.

   In most cases in the near term, if the RID information is fed to the
   Internet directly by the UA or GCS, the first hop data links will be
   cellular Long Term Evolution (LTE) or Wi-Fi, but provided the data
   link can support at least UDP/IP and ideally also TCP/IP, its type is
   generally immaterial to the higher layer protocols.  An UAS as the
   ultimate source of Network RID information feeds an USS acting as a
   Network RID Service Provider (Net-RID SP), which essentially proxies
   for that and other sources; an observer or other ultimate consumer of
   Network RID information obtains it from a Network RID Display
   Provider (Net-RID DP), 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, accessible via typical means such as web APIs/

   Network RID is the more flexible and less constrained of the defined
   UAS RID means, but is only partially specified in [F3411-19].  It is
   presumed that IETF efforts supporting Broadcast RID (see next
   section) can be easily generalized for Network RID.

3.2.  Broadcast RID

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

                 Figure 4: "Broadcast RID Information Flow"

   Note the absence of the Internet from this information flow sketch.
   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 and FAA have not
   indicated intent to allow, and FAA has proposed explicitly to
   prohibit, use of ADS-B for UAS RID.

   [F3411-19] specifies three Broadcast RID data links: Bluetooth 4.X;
   Bluetooth 5.X Long Range; and Wi-Fi with Neighbor Awareness
   Networking (NAN).  For compliance with [F3411-19], an UA must
   broadcast (using advertisement mechanisms where no other option
   supports broadcast) on at least one of these; if broadcasting on
   Bluetooth 5.x, it is also required concurrently to do so on 4.x
   (referred to in [F3411-19] as Bluetooth Legacy).  Future revisions
   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 26 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 or message bundles ("Message Pack"), and/or correlation of
   short messages (anticipated by ASTM to be done on the basis of
   Bluetooth 4 MAC address, which is weak and unverifiable).

   [F3411-19] Broadcast RID specifies several message types: Basic,
   Location, Authentication, Self-ID, System and Operator ID.  To
   satisfy EASA and FAA proposed rules, all types are needed, except
   Authentication and Self-ID.

   [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 message) must be
   transmitted at least once per second and be no older than one second
   when sent.  [NPRM] proposes all information be sent at least once per

   [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 operating system stack to avoid the adversarial correlation
   problems of static identifiers.

   The [F3411-19] optional Authentication Message specifies framing for
   authentication data, but does not specify any authentication method,
   and the maximum length of the specified framing is too short for
   conventional digital signatures and far too short for conventional
   certificates.  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; from this point until the end
   of 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 stack and to avoid the adversarial correlation
   problems of static identifiers.

   The [F3411-19] optional Authentication Message specifies framing for
   authentication data, but does Observers via
   Network RID.

   USS provide services not specify any authentication method,
   and limited to Network RID; indeed, the maximum length primary
   USS function is deconfliction of the specified framing airspace usage by different UAS and
   other (e.g. manned aircraft, rocket launch) operations.  Most
   deconfliction involving a given operation is too short for
   conventional digital signatures hoped to be completed
   prior to commencing that operation, and far too short is called "strategic
   deconfliction."  If that fails, "tactical deconfliction" comes into
   play; ABDAA may not involve USS, but GBDAA likely will.  Also,
   dynamic constraints (formerly 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 conventional
   certificates.  The one-way nature of USS in Broadcast RID precludes
   challenge-response security protocols (e.g., observers sending nonces
   to UA, is currently specified by regulators
   or [F3411-19].  However, USS are likely to be returned serve as registries (or
   perhaps registrars) for UAS (and perhaps operators); if so, USS will
   have a role in signed messages).  An observer would be
   seriously challenged all forms of RID.  Supplemental Data Service Providers
   (SDSP) are also likely to validate the asserted find roles, not only in UTM as such but
   also in enhancing UAS ID RID and related services.  Whether USS, SDSP,
   etc. are involved or any other
   information about the UAS not, RID services, narrowly defined, provide
   regulator specified identification information; more broadly defined,
   RID services may leverage identification to facilitate related
   services or its operator looked up therefrom.

3.3. functions, likely beginning with V2X.

3.4.  DRIP Focus

   In addition to the gaps described above, there is a fundamental gap
   in almost all current or proposed regulations and technical standards
   for UAS RID.  As noted above, ID is not an end in itself, but a
   means.  [F3411-19] etc.  provide very limited choices 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 be contacted 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
   unreliable.  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 an 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.

4.  Requirements

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

   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

   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), to
           support three objectives: 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); defend
           against replay attacks; and support optional SDSP services
           such as multilateration (to complement UAS position self-
           reports with independent measurements).

   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).

   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 [NPRM] regulatory QoS requirement currently would be
   satisfied generally by ensuring information refresh rates of at least 1 Hertz,
   with latencies no greater than 1 second, at least 80% of the time; time,
   but these numbers may change, so vary between jurisdictions and over time.  So
   instead the DRIP QoS requirement is that
   they performance, reliability,
   etc. parameters be user policy specifiable (which specifiable, which does not imply
   satisfiable in all cases, but (especially together with the
   management requirement) implies that when the specs specifications are not met,
   appropriate parties are notified). notified.  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 the only instance in which DRIP transports [F3411-19]
   messages; most of DRIP pertains to the authentication of such
   messages and the identifier carried within them.

4.2.  Identifier

   ID-1  Length: The DRIP (UAS) entity (remote) identifier must be no
         longer than 20 bytes (per [F3411-19] to fit in a Bluetooth 4
         advertisement payload).

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

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

   ID-4  Uniqueness: The DRIP identifier MUST be unique within the
         global UAS RID 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
         such as the FAA's proposed 6 months RID data retention period,
         revocation by the registry, or surrender by the operator).

   ID-5  Non-spoofability: The DRIP identifier MUST be non-spoofable
         within the context of Remote ID broadcast messages (some
         collection of messages provides proof of UA ownership of ID).

   ID-6  Unlinkability: A DRIP UAS ID MUST NOT facilitate adversarial
         correlation over multiple UAS operations; this may be
         accomplished e.g. by limiting each identifier to a single use,
         but if so, the UAS ID MUST support well-defined scalable timely
         registration methods.

   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); and in RID initiated V2X
   applications such as DAA and C2, it could be used between the network
   and transport layers (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 an UAS ID is generated by the operator, GCS, UA, USS or
   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).

   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.

   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,
   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, per 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 MUST enable closing the AAA-policy registry
          loop by governing AAA per registered policies and
          administering policies only via 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.

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.  Potential vulnerabilities of DRIP
   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

   *  Malicious or malfunctioning registries

   *  Interception of (e.g.  Man In The Middle attacks on) registration

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

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

   *  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.  E.g. checking the signature of a registry on a
   public key certificate received via Broadcast RID in a remote area
   presumably would require that the registry's public key had been
   previously installed on the Observer's device, yet there may be many
   registries and the Observer's device may be storage constrained, and
   new registries may come on-line subsequent to installation of DRIP
   software on the Observer's device.  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 is closely related to but not synonymous with security, and
   conflicts with transparency.  Privacy and transparency are important
   for legal reasons including regulatory consistency.  [EU2018]
   [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 documents give due regard to [RFC6973] and more broadly

   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.

8.  References
8.1.  Normative References

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

   [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

   [cpdlc]    Gurtov, A., Polishchuk, T., and M. Wernberg, "Controller-
              Pilot Data Link Communication Security", MDPI
              Sensors 18(5), 1636, 2018,

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

              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.

              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-03, 13 July
              ietf-drip-arch-04, 28 October 2020,

              European Union Agency for Cybersecurity (ENISA),
              "Actionable information for Security Incident Response",
              November 2014, <https://www.enisa.europa.eu/topics/csirt-

   [EU2018]   European Parliament and Council, "2015/0277 (COD) PE-CONS
              2/18", February 2018.

   [F3411-19] ASTM International, "Standard Specification for Remote ID
              and Tracking", February 2020,

              FAA Office of NextGen, "UTM Concept of Operations v2.0",
              March 2020.

              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,

   [ICAOATM]  International Civil Aviation Organization, "Doc 4444:
              Procedures for Air Navigation Services: Air Traffic
              Management", November 2016.

   [ICAOUAS]  International Civil Aviation Organization, "Circular 328:
              Unmanned Aircraft Systems", February 2011.

   [ICAOUTM]  International Civil Aviation Organization, "Unmanned
              Aircraft Systems Traffic Management (UTM) - A Common
              Framework with Core Principles for Global Harmonization,
              Edition 2", November 2019.

              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.

              SESAR Joint Undertaking, "Initial view on Principles for
              the U-space architecture", July 2019.

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

              Intel Corp., "Open Drone ID", March 2019,

   [Opinion1] European Union Aviation Safety Agency (EASA), "Opinion No
              01/2020: High-level regulatory framework for the U-space",
              March 2020.

              FAA UAS Identification and Tracking Aviation Rulemaking
              Committee, "UAS ID and Tracking ARC Recommendations Final
              Report", September 2017.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [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,

   [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/

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

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

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
   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, (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,
   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.


   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
   Andrei Gurtov
   Linköping University
   SE-58183 Linköping

   Email: gurtov@acm.org