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DRIP                                                        S. Card, Ed.
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 26 November 2020                                   R. Moskowitz
                                                          HTT Consulting
                                                               A. Gurtov
                                               Linköping University
                                                             25 May 2020


        Drone Remote Identification Protocol (DRIP) Requirements
                        draft-ietf-drip-reqs-01

Abstract

   This document defines the requirements for Drone Remote
   Identification Protocol (DRIP) Working Group protocols to support
   Unmanned Aircraft System Remote Identification and tracking (UAS RID)
   for safety, regulatory compliance 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 on-line and off-line verification that UAS 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
   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 26 November 2020.






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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
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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   6
     2.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  UAS RID Problem Space . . . . . . . . . . . . . . . . . . . .  12
     3.1.  Network RID . . . . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Broadcast RID . . . . . . . . . . . . . . . . . . . . . .  14
     3.3.  DRIP Focus  . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.2.  Identifier  . . . . . . . . . . . . . . . . . . . . . . .  17
     4.3.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.4.  Registries  . . . . . . . . . . . . . . . . . . . . . . .  18
   5.  Discussion and Limitations  . . . . . . . . . . . . . . . . .  19
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     Normative References  . . . . . . . . . . . . . . . . . . . . .  21
     Informative References  . . . . . . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Many considerations (especially safety) dictate that UAS be remotely
   identifiable.  Any observer with responsibilities involving aircraft
   inherently must classify them situationally according to basic
   considerations, as illustrated notionally in Figure 1 below.






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                        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 1

   Civil Aviation Authorities (CAAs) worldwide are mandating Unmanned
   Aircraft System Remote Identification and tracking (UAS RID).  The
   European Union Aviation Safety Agency (EASA) has published
   [Delegated] and [Implementing] Regulations.  The United States (US)
   Federal Aviation Administration (FAA) has published a Notice of
   Proposed Rule Making ([NPRM]) and has described the key role that UAS
   RID plays in UAS Traffic Management (UTM [CONOPS] especially
   Section 2.6).  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 International, Technical Committee F38 (UAS), Subcommittee
   F38.02 (Aircraft Operations), Work Item WK65041, developed ASTM
   F3411-19 [F3411-19] Standard Specification for Remote ID and
   Tracking.  It defines two means of UAS RID:

      Network RID defines a set of information for UAS to make available
      globally indirectly via the Internet.

      Broadcast RID defines a set of messages for Unmanned Aircraft (UA)
      to transmit locally directly one-way over Bluetooth or Wi-Fi.

   Generally the same information must provided via both means.  Network
   RID depends upon Internet connectivity in several segments from the
   UAS to the observer.  Broadcast RID should need Internet (or other



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   Wide Area Network) connectivity only for UAS registry information
   lookup using the directly locally received UAS ID as a key.

   [F3411-19] specifies 3 UAS ID types:

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

   TYPE-2  A CAA assigned (presumably static) ID.

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

   The EU allows only Type 1; the US allows Types 1 and 3, but requires
   Type 3 IDs (if used) each to be used only once (for a single UAS
   flight, which in the context of UTM is called an "operation").
   [F3411-19] Broadcast RID transmits all information in the clear as
   plaintext (ASCII or binary), so static IDs enable trivial correlation
   of patterns of use, unacceptable in many applications, e.g., package
   delivery routes of competitors.

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

   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]
   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.

   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 FAA NPRM.  However, applications of
   RID beyond RID itself have been omitted from [F3411-19]; DAA has been
   explicitly declared out of scope in ASTM working group discussions,
   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




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   any of the subsequent proposed regulations or technical
   specifications.

   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.

   UA onboard RID devices are severely constrained in Cost, Size, Weight
   and Power ($SWaP).  Cost is a significant impediment to the necessary
   near-universal adoption of UAS send and observer receive RID
   capabilities. $SWaP 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 $SWaP and application-specific
   considerations.

   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.

   Despite work by regulators and Standards Development Organizations
   (SDOs), there are substantial gaps in UAS standards generally and UAS
   RID specifically.  [Roadmap] especially Section 7.8 catalogs UAS RID
   standards, ongoing standardization activities and gaps.

   Given not only packet payload length and bandwidth, but also
   processing and storage within the $SWaP constraints of very small
   (e.g. consumer toy) UA, heavyweight cryptographic security protocols
   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




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   extent); a "fair witness" functionality (inspired by [Stranger]) may
   be desirable.

   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.  Potential follow-on goals may extend beyond providing
   timely and trustworthy identification data, to using it to enable
   identity-oriented networking of UAS.

   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 [I-D.ietf-drip-arch]; this document describes only relevant
   requirements.

2.  Terms and Definitions

2.1.  Requirements Terminology

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

2.2.  Definitions

   This section defines a set of terms that are 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.

   $SWaP
      Cost, Size, Weight and Power.



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   AAA
      Attestation, Authentication, Authorization, Access Control,
      Accounting, Attribution, Audit.

   ABDAA
      AirBorne DAA.  Also known as "self-separation".

   ADS-B
      Automatic Dependent Surveillance - Broadcast.  "ADS-B Out"
      equipment obtains aircraft position from other on-board systems
      (typically GPS) 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 an UA, typically measured
      in feet.

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

   ATM
      Air Traffic Management.  All systems that assist aircraft from
      departure to landing.  A broader functional and geographic scope
      and/or a higher layer of abstraction than ATC.

   Authentication Message
      F3411 Message Type 2.  Provides framing for authentication data,
      only.

   Basic ID Message
      F3411 Message Type 0.  Provides UA Type, UAS ID Type and UAS ID,
      only.

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

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

   CAA
      Civil Aviation Authority.  Two examples are the United States
      Federal Aviation Administration (FAA) and the European Union
      Aviation Safety Agency (EASA).




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   C2
      Command and Control.  A set of organizational and technical
      attributes and processes that employs human, physical, and
      information resources to solve problems and accomplish missions.
      Previously primarily used in military contexts.  In the UAS
      context, typically refers to the link between GCS and UA over
      which the former controls the latter.

   DAA
      Detect And Avoid, formerly Sense And Avoid (SAA).  A means of
      keeping aircraft "well clear" of each other for safety.

   Direct RID
      Direct Remote Identification.  Per [Delegated], "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".
      Requirement could be met with ASTM Broadcast RID: Basic ID message
      with UAS ID Type 1; Location/Vector message; Operator ID message;
      System Message.  Corresponds roughly to the Broadcast RID portion
      of FAA NPRM Standard RID.

   E2E
      End to End.

   GBDAA
      Ground Based DAA.

   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 GPS waypoints, or
      otherwise directing its flight.

   GPS
      Global Positioning System.  In this context, misused in place of
      Global Navigation Satellite System (GNSS) or more generally SATNAV
      to refer generically to satellite based timing and/or positioning.

   GRAIN
      Global Resilient Aviation Information Network.  An effort to
      develop an international IPv6 overlay network with end-to-end
      security supporting all aspects of aviation.

   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.



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   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.

   Limited RID
      Per the FAA NPRM, 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.

   Location/Vector Message
      F3411 Message Type 1.  Provides UA location, altitude, heading and
      speed, only.

   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 V-LOS.

   MSL
      Mean Sea Level.  Relative altitude, above the variously defined
      mean sea level, typically of an UA (but in FAA NPRM for a GCS),
      typically measured in feet.

   Net-RID DP
      Network RID Display Provider.  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.  Under the FAA 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.  Logical entity that participates in
      Network RID and provides to NetRID-DPs information on UAS it



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      manages.  Under the FAA NPRM, the USS to which the UAS is
      subscribed ("Remote ID USS").

   Network Identification Service
      EU regulatory requirement for Network RID.  Requirement could be
      met with ASTM Network RID: Basic ID message with UAS ID Type 1;
      Location/Vector message; Operator ID message; System Message.
      Corresponds roughly to the Network RID portion of FAA NPRM
      Standard RID.

   Observer
      Referred to in other UAS RID documents as a "user", but there are
      also other classes of UAS RID users, so here "observer" is
      preferred to denote specifically an individual who has observed an
      UA and wishes to know something about it, starting with its ID.

   Operator
      UAS operator.  Typically an organization that owns or leases the
      UAS.

   Operator ID Message
      F3411 Message Type 5.  Provides CAA issued Operator ID, only.

   PII
      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.

   RF
      Radio Frequency.  May be used as an adjective or as a noun; in the
      latter case, typically means Radio Frequency energy.

   RF-LOS
      RF LOS.  Typically used in describing operation of 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.

   Self-ID Message
      F3411 Message Type 3.  Provides a 1 byte descriptor and 23 byte
      ASCII free text field, only.

   Standard RID
      Per the FAA NPRM, 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,



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      range) of Limited RID and for all UAS equipped for Standard RID
      (even if operated within parameters that would otherwise permit
      Limited RID).  The Broadcast RID portion corresponds roughly to EU
      Direct RID; the Network RID portion corresponds roughly to EU
      Network Identification Service.

   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.

   System Message
      F3411 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.

   UA
      Unmanned Aircraft.  An aircraft which is intended to operate with
      no pilot on board.  In popular parlance, "drone".  Plural form of
      UA is UA.

   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.  Plural form of UAS is UAS.

   UAS ID
      UAS identifier.  Although called "UAS ID", unique to the UA:
      neither to the operator (as previous registration numbers have
      been assigned), nor to the combination of GCS and UA that comprise
      the UAS.  Per [F3411-19], maximum length of 20 bytes.

   UAS ID Type
      Identifier type index.  Per [F3411-19], 4 bits, values 0-3 already
      specified.

   UAS RID
      UAS Remote Identification.  System for identifying UA during
      flight by other parties.



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   UAS RID Verification Service
      System component designed to handle the authentication
      requirements of RID by offloading verification to a web hosted
      service.

   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." per
      [CONOPS].

   UTM
      UAS Traffic Management.  Per ICAO, "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."  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.

   V-LOS
      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

   UA may be fixed wing Short Take-Off and Landing (STOL), rotary wing
   (e.g., helicopter) Vertical Take-Off and Landing (VTOL), or hybrid.
   They may be single engine 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
   Positioning System (GPS) waypoint to waypoint in a weak form of
   autonomy; stronger autonomy is coming.  UA are "low observable": they
   typically have a small radar cross section; 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



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   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.

   Network RID from the UA itself (rather than from its GCS) and
   Broadcast RID require one or more wireless data links from the UA,
   but such communications are challenging due to $SWaP constraints and
   low altitude flight amidst structures and foliage over terrain.

   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.

3.1.  Network RID

   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).

   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 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 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



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   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 coverage of an airspace 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/browsers.

   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

   [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 this standard, 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).

   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).

3.3.  DRIP Focus

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

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




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   2.  by enabling verification that an UAS is registered, 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 location 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.

   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, but these may be randomized by the operating system stack
   to avoid the adversarial correlation problems of static identifiers.

   The [F3411-19] optional Authentication Message specifies framing for
   authentication data, but does not specify any authentication method,
   and the maximum length of the specified framing is too short for
   conventional digital signatures and far too short for conventional
   certificates.  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.

   Further, [F3411-19] provides 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 GCS location to look for
   the remote pilot.  An observer with Internet connectivity 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.

   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.

4.  Requirements




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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
           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 -> Network RID
           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 reply attacks;
           and support optional SDSP services such as multilateration
           (to complement UAS position self-reports with independent
           measurements).

   GEN-6   Finger (placeholder name): DRIP MUST enable dynamically
           establishing, with AAA, per policy, E2E 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.




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   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 sensitive airspace volumes.

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

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
         listed.

   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 a to-be-
         defined scope.

   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.

   Whether a 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.




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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.

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

   As satisfying these requirements may require that authorized actors
   have connectivity to third parties, e.g., Internet to a Remote ID
   USS, to enable decryption, and such connectivity cannot be assured,
   DRIP SHOULD provide automatic fallback to plaintext transmission of
   safety-critical information when necessary.

4.4.  Registries

   REG-1  Public Lookup: DRIP MUST enable lookup, from the UAS ID, of
          information designated by cognizant authority as public.

   REG-2  Private Lookup: DRIP MUST enable lookup, with AAA, per policy,
          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).

   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 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.






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5.  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.  IETF traditionally operates assuming
   the source material for the standardization process is publicly
   available.  However, ASTM standards require a fee for download.
   Therefore a double-liaison program at IETF might need to be
   activated, providing free access to ASTM specifications for
   contributors to IETF documents.

   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.




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   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, 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.

6.  IANA Considerations

   This document does not make any IANA request.

7.  Security Considerations

   DRIP is all about safety and security, so content pertaining to such
   is not limited to this section.  DRIP information falls into two
   classes: that which, to achieve the purpose, must be published openly
   in clear plaintext, for the benefit of any observer; 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).  This classification
   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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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 WG efforts.  The work of ASTM F38.02 in
   balancing the interests of diverse stakeholders is essential to the
   necessary rapid and widespread deployment of UAS RID.

References

Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Informative References

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

   [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.

   [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.

   [F3411-19] ASTM, "Standard Specification for Remote ID and Tracking",
              December 2019.

   [I-D.ietf-drip-arch]
              Card, S., Wiethuechter, A., Moskowitz, R., and S. Zhao,
              "Drone Remote Identification Protocol (DRIP)
              Architecture", Work in Progress, Internet-Draft, draft-




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              ietf-drip-arch-00, 18 May 2020,
              <https://tools.ietf.org/html/draft-ietf-drip-arch-00>.

   [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-02, 1
              April 2020,
              <https://tools.ietf.org/html/draft-maeurer-raw-ldacs-02>.

   [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.

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

   [Recommendations]
              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,
              <https://www.rfc-editor.org/info/rfc4122>.

   [Roadmap]  American National Standards Institute (ANSI) Unmanned
              Aircraft Systems Standardization Collaborative (UASSC),
              "Standardization Roadmap for Unmanned Aircraft Systems
              draft v2.0", April 2020.

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

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 ASTM F38.02 in
   balancing the interests of diverse stakeholders is essential to the
   necessary rapid and widespread deployment of UAS RID.  IETF
   volunteers who have contributed to this draft include Amelia
   Andersdotter, Mohamed Boucadair, Toerless Eckert, Susan Hares, Mika
   J&#228;rvenp&#228;&#228;, Daniel Migault, Saulo Da Silva and Shuai
   Zhao.



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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
   Link&#246;ping University
   IDA
   SE-58183 Link&#246;ping
   Sweden

   Email: gurtov@acm.org
















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