wdiff rfc9153v1.txt rfc9153.txt

Internet Engineering Task Force (IETF) S. Card, Ed.
Request for Comments: 9153 A. Wiethuechter
Category: Informational AX Enterprize
ISSN: 2070-1721 R. Moskowitz
HTT Consulting
A. Gurtov
Linköping University
November 2021
January 2022

Drone Remote Identification Protocol (DRIP) Requirements and Terminology

Abstract

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

Status of This Memo

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

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

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

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Table of Contents

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

1. Introduction

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

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

1.1. Motivation and External Influences

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

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

Small UA are "low observable" as they:

* typically have small radar cross sections;

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

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

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

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

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

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

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

Figure 1: General UAS RID Usage Scenario

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

* multiple Observers, some of them members of the general public and
others government officers with public safety and security
responsibilities, (2)

* multiple UA in flight within observation range, each with its own
pilot/operator, (3)

* at least one registry each for lookup of public and (by authorized
parties only) private information regarding the UAS and their
pilots/operators, and (4)

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

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

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

* Taskable: can ask it to do something useful; as useful.

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

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

Figure 2: Notional UAS Classification

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

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

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

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

ASD-STAN is an Associated Body to CEN (European Committee for
Standardization) for Aerospace Standards. It is publishing has published an EU
standard titled "Aerospace series - Unmanned Aircraft Systems - Part
002: Direct Remote Identification; English version prEN
4709-002:2020"; Identification" [ASDSTAN4709-002]; a current
(early 2021) informal overview is freely available in [ASDRI]. [ASDRI] (note
that [ASDRI] may not precisely reflect the final standard as it was
published before [ASDSTAN4709-002]). It will provide compliance to
cover the identical DRI requirements applicable to drones of the
following classes:

* C1 ([Delegated], Part 2), 2)

* C2 ([Delegated], Part 3), 3)

* C3 ([Delegated], Part 4), 4)

* C5 ([Amended], Part 16), and 16)

* C6 ([Amended], Part 17). 17)

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

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

1. the UAS operator registration number;

2. the [CTA2063A]-compliant unique serial number of the UA, which is compliant with
[CTA2063A]; UA;

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

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

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

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

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

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

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

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

1.2. Concerns and Constraints

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

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

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

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

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

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

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

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

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

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

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

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

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

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

1.3. DRIP Scope

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

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

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

1.4. Document Scope

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

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

2. Terms and Definitions

2.1. Requirements Terminology

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

2.2. Definitions

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

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

In the UAS community, the plural form of abbreviations is generally
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

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

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

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

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

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

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

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

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

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

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

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

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

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

BVLOS
Beyond Visual Line Of Sight. See VLOS.

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

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

CSWaP
Cost, Size, Weight, and Power

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

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

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

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

EUROCAE
European Organisation for Civil Aviation Equipment. Aviation SDO,
originally European, now with broader membership. Cooperates
extensively with the Radio Technical Commission for Aeronautics
(RTCA). RTCA.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

UAS ID
UAS identifier. Although called "UAS ID", it is actually unique
to the UA, not neither to the operator (as some UAS registration
numbers have been and for exclusively recreational purposes are
continuing to be assigned) or assigned), nor to the combination of GCS and UA
that comprise the UAS. _Maximum length of 20 bytes_ [F3411-19].
If the UAS ID Type is 4, the proposed Specific Session ID, then the 20
bytes includes the subtype index, leaving only 19 bytes for the
actual identifier.

UAS

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

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

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

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

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

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

3. UAS RID Problem Space

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* need not transmit UA emergency status; status, and

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

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

3.1. Network RID

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

Figure 3: Network RID Information Flow

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

Figure 3: Network RID Information Flow

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

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

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

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

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

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

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

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

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

Network RID has several connectivity scenarios:

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

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

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

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

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

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

3.2. Broadcast RID

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

Figure 4: Broadcast RID Information Flow

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

Figure 4: Broadcast RID Information Flow

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

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

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

Length constraints also preclude a single Bluetooth 4.x frame
carrying not only
carrying, along with the UAS ID but also position, velocity, and ID, other information that should somehow must
be bound to the UAS ID, much less thereto, such as position and velocity, which are strong
authentication data. data elements that are much less lengthy. Messages
that cannot be encapsulated in a single frame (thus far, only the
Authentication Message) must be segmented into message "pages" (in
the terminology of [F3411-19]). Message pages must somehow be
correlated as belonging to the same message. Messages carrying
position, velocity and other data must somehow be correlated with the
Basic ID message Message that carries the UAS ID. This correlation is
expected to be done on the basis of Media Access Control (MAC) address: this
address. This may be complicated by MAC address
randomization; not randomization. Not
all the common devices expected to be used by Observers have APIs
that make sender MAC addresses available to user space receiver applications; and
applications. MAC addresses are easily spoofed. Data elements are
not so detached on other media (see Message Pack in the paragraph
after next).

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

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

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

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

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

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

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

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

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

3.3. USS in UTM and RID

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

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

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

3.4. DRIP Focus

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

Thus, 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 that supports supporting safe
and secure operation of UAS. UAS, protocols must do the following (despite
the intrinsic tension among these objectives):

* preserve operator privacy,

* enable strong authentication, and

* enable the immediate use of information by authorized parties.

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

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

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

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

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

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

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

4. Requirements

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

4.1. General

4.1.1. Normative Requirements

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

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

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

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

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

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

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

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

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

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

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

4.1.2. Rationale

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

The "provable ownership" Provable Ownership requirement addresses the possibility that the
actual sender is not the claimed sender (i.e., is a spoofer). DRIP
could meet this requirement by, for example, verifying an asymmetric
cryptographic signature using a sender-provided public key from which
the asserted UAS ID can be at least partially derived. The "provable binding" Provable
Binding requirement addresses the problem with MAC address
correlation [F3411-19] noted above. in Section 3.2. The "provable
registration" 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" Readability requirement pertains
to the structure and format of information at endpoints rather than
its encoding in transit, so it may involve machine-assisted format
conversions, e.g.,
conversions (e.g., from binary encodings, encodings) and/or decryption (see
Section 4.3).

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

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

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

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

4.2. Identifier

4.2.1. Normative Requirements

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

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

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

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

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

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

4.2.2. Rationale

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

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

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

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

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

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

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

4.3. Privacy

4.3.1. Normative Requirements

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

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

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

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

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

4.3.2. Rationale

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

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

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

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

The privacy Privacy requirements above are for DRIP, not neither for [F3411-19]
(which, in the interest of privacy, requires obfuscation of location
to any Network RID subscriber engaging in wide area surveillance,
limits data retention periods, etc.) or etc.), nor for UAS RID in any specific
jurisdiction (which may have its own regulatory requirements). The
requirements above are also in a sense parameterized: who are the
"authorized actors", how are they designated, how are they
authenticated, etc.?

4.4. Registries

4.4.1. Normative Requirements

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

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

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

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

4.4.2. Rationale

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

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

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

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

5. IANA Considerations

This document has no IANA actions.

6. Security Considerations

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

* Sybil attacks

* confusion created by many spoofed unsigned messages

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

* malicious or malfunctioning registries

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

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

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

* message integrity

* non-repudiation

* defense against replay attacks

* defense against spoofing

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

7. Privacy and Transparency Considerations

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

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

Privacy and transparency

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

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

DRIP information falls into two classes:

* that which, to achieve the purpose, must be published openly as
cleartext, for the benefit of any Observer (e.g., the basic UAS ID
itself)
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) 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, in this document, it will
be regarded as a matter for CAA, registry, and operator policies, for
which enforcement mechanisms will be defined within the scope of the
DRIP WG and offered. Details of the protection mechanisms will be
provided in other DRIP documents. Mitigation of adversarial
correlation will also be addressed.

8. References

8.1. Normative References

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

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

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

8.2. Informative References

[Amended] European Union Aviation Safety Agency (EASA), Parliament and Council, "Commission Delegated
Regulation (EU) 2020/1058 of 27 April 2020 amending
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Card, S., Wiethuechter, A., Moskowitz, R., Zhao, S., Ed.,
and A. Gurtov, "Drone Remote Identification Protocol
(DRIP) Architecture", Work in Progress, Internet-Draft,
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Regulation (EU) 2019/947 of 24 May 2019 on the rules and
procedures for the operation of unmanned aircraft", May
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the U-space architecture", July 2019,
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(LDACS)", Work in Progress, Internet-Draft, draft-ietf-
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ldacs-09>.

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Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
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[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
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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.
ASTM's "Standard Specification for Remote ID and Tracking"
[F3411-19]. Other organizations (for example, in the EU) do not
necessarily follow the same architecture.

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

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

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

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

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

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

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

Acknowledgments

The work of the FAA's UAS Identification and Tracking Aviation
Rulemaking Committee (ARC) is the foundation of later ASTM [F3411-19]
and IETF DRIP efforts. The work of Gabriel Cox, Intel Corp., and
their Open Drone ID collaborators opened UAS RID to a wider
community. The work of ASTM F38.02 in balancing the interests of
diverse stakeholders is essential to the necessary rapid and
widespread deployment of UAS RID. IETF volunteers who have
extensively reviewed or otherwise contributed to this document
include Amelia Andersdotter, Carsten Bormann, Toerless Eckert, Susan
Hares, Mika Jarvenpaa, Alexandre Petrescu, Saulo Da Silva, and Shuai
Zhao. Thanks to Linda Dunbar for the SECDIR review, Nagendra Nainar
for the OPSDIR review, and Suresh Krishnan for the Gen-ART review.
Thanks to IESG members Roman Danyliw, Erik Kline, Murray Kucherawy,
and Robert Wilton for helpful and positive comments. Thanks to
chairs Daniel Migault and Mohamed Boucadair for direction of our team
of authors and editor, some of whom are newcomers to writing IETF
documents. Thanks especially to Internet Area Director Éric Vyncke
for guidance and support.

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

Authors' Addresses

Stuart W. Card (editor)
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America

Email: stu.card@axenterprize.com

Adam Wiethuechter
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America

Email: adam.wiethuechter@axenterprize.com

Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America

Email: rgm@labs.htt-consult.com

Andrei Gurtov
Linköping University
IDA
SE-58183 Linköping
Sweden

Email: gurtov@acm.org