AX Enterprize

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AX Enterprize

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HTT Consulting

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Linköping University

IDA Linköping 58183 Sweden gurtov@acm.org

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

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

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

• typically have small radar cross sections;
• make noise quite noticeable at short 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., for those to which RID applies in the US, under 400 feet Above Ground Level (AGL) as per );
• are highly maneuverable so can fly under trees and between buildings.

UA can carry payloads including sensors, cyber and kinetic weapons, or can be used themselves as weapons by flying them into targets. They can be flown by clueless, careless, or criminal operators. Thus the most basic function of UAS RID is "Identification Friend or Foe" (IFF) to mitigate the significant threat they present. Diverse other applications can be enabled or facilitated by RID. Internet protocols typically start out with at least one entity already knowing an identifier or locator of another; but an entity (e.g., UAS or Observer device) encountering an a priori unknown UA in physical space has no identifier or logical space locator for that UA, unless and until one is provided somehow. RID provides an identifier, which, if well chosen, can facilitate use of a variety of Internet family protocols and services to support arbitrary applications, beyond the basic security functions of RID. For most of these, some type of identifier is essential, e.g., Network Access Identifier (NAI), Digital Object Identifier (DOI), Uniform Resource Identifier (URI), domain name, or public key. DRIP motivations include both the basic security and the broader application support functions of RID. The general scenario is illustrated in .

"General UAS RID Usage Scenario" *************** *************** * UAS1 * * UAS2 * * * * * * +--------+ * DAA/V2V * +--------+ * * | UA o--*----------------------------------------*--o UA | * * +--o--o--+ * * +--o--o--+ * * | | * +------+ Lookups +------+ * | | * * | | * | GPOD o------. .------o PSOD | * | | * * | | * +------+ | | +------+ * | | * * | | * | | * | | * * C2 | | * V2I ************ V2I * | | C2 * * | '-----*--------------* *--------------*-----' | * * | * * * * | * * | o====NetRID====* *====NetRID====o | * * +--o--+ * * Internet * * +--o--+ * * | GCS o-----*--------------* *--------------*-----o GCS | * * +-----+ * Registration * * Registration * +-----+ * * * (and UTM) * * (and UTM) * * *************** ************ *************** | | | +----------+ | | | +----------+ | Public o---' | '---o Private | | Registry | | | Registry | +----------+ | +----------+ +--o--+ | DNS | +-----+ GPOD: General Public Observer Device (used only to fit this figure) PSOD: Public Safety Observer Device (used only to fit this figure)

illustrates a typical case where there may be: multiple Observers, some of them members of the general public, others government officers with public safety/security responsibilities; multiple UA in flight within observation range, each with its own pilot/operator; at least one registry each for lookup of public and (by authorized parties only) private information regarding the UAS and their pilots/operators; and in the DRIP vision, DNS resolving various identifiers and locators of the entities involved. Note that Broadcast RID direct RF links are not shown, as they are indeed broadcast, so reach anywhere within range; they do not connect specific entity pairings, as edges do vertices in a graph. Further, RID and other connectivity involving the UA varies, as described subsequently herein under . Not all the links shown in above necessarily exist in all scenarios (e.g., UA support for direct connectivity to the Internet is very rare as of 2021), and even those links that exist sometimes in a given scenario are not necessarily up at all times in that same scenario (e.g., remote Observer connectivity to the Internet may be very intermittent). An Observer of UA may need to classify them, as illustrated notionally in , for basic airspace Situational Awareness (SA). An Observer who classifies a UAS: as Taskable, can ask it to do something useful; as Low Concern, can reasonably assume it is not malicious and would cooperate with requests to modify its flight plans for safety concerns that arise; as High Concern or Unidentified, can focus surveillance on it.

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

ASTM International, Technical Committee F38 (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item WK65041, developed the widely cited Standard Specification for Remote ID and Tracking : the published standard is available for purchase from ASTM and as an ASTM membership premium; early drafts are freely available as specifications. 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 (V2X) communications and applications such as Detect And Avoid (DAA), which would impose tighter latency bounds than RID itself, is an obvious possibility, explicitly contemplated in the United States (US) Federal Aviation Administration (FAA) Remote Identification of Unmanned Aircraft rule . However, usage of RID systems and information beyond mere identification (primarily to hold operators accountable after the fact), including DAA, have been declared out of scope in ASTM F38.02 WK65041, based on a distinction between RID as a security standard vs DAA as a safety application. Aviation community Standards Development Organizations (SDOs) 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 vs security; the denotative definitions of the International Civil Aviation Organization (ICAO) are cited in . and cite the Direct Remote Identification (DRI) previously required and specified, explicitly stating that whereas DRI is primarily for security purposes, the "Network Identification Service" (in the context of U-space ) or "Electronic Identification" is primarily for safety purposes (e.g., Air Traffic Management, especially hazards deconfliction) and also is allowed to be used for other purposes such as support of efficient operations. These emerging standards allow the security and safety oriented systems to be separate or merged. In addition to mandating both Broadcast and Network one-way to Observers, they will use V2V to other UAS (also likely to and/or from some manned aircraft). These reflect the broad scope of the European Union (EU) U-space concept, as being developed in the Single European Sky ATM Research (SESAR) Joint Undertaking, the U-space architectural principles of which are outlined in . ASD-STAN is an Associated Body to CEN (European Committee for Standardization) for Aerospace Standards. It is publishing an EU standard "Aerospace series - Unmanned Aircraft Systems - Part 002: Direct Remote Identification; English version prEN 4709-002:2020" for which a current (early 2021) informal overview is freely available in . It will provide compliance to cover the identical DRI requirements applicable to drones of classes C1 - Part 2, C2 - Part 3, C3 - Part 4, C5 - Part 16, and C6 - Part 17. The standard contemplated in 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, ) and/or IEEE 802.11 Beacon modes. The EU standard emphasis was compatibility with , although there are differences in mandatory and optional message types and fields. The contemplated DRI system will broadcast locally:

1. the UAS operator registration number;
2. the compliant unique serial number of the UA;
3. a time stamp, the geographical position of the UA, and its height AGL or above its take-off point;
4. the UA ground speed and route course measured clockwise from true north;
5. the geographical position of the remote pilot, or if that is not available, the geographical position of the UA take-off point; and
6. for Classes C1, C2, C3, the UAS emergency status.

Under the contemplated standard, data will be sent in plain text 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 3 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 to revise their current standard with additional functionality (e.g., DRIP) to be published before 2022 . Security oriented UAS RID essentially has two goals: enable the general public to obtain and record an opaque ID for any observed UA, which they can then report to authorities; enable authorities, from such an ID, to look up information about the UAS and its operator. Safety oriented UAS RID has stronger requirements. Although dynamic establishment of secure communications between the Observer and the UAS pilot seems to have been contemplated by the FAA UAS ID and Tracking Aviation Rulemaking Committee (ARC) in their , it is not addressed in any of the subsequent regulations or international SDO technical specifications, other than DRIP, known to the authors as of early 2021.

Concerns and Constraints Disambiguation of multiple UA flying in close proximity may be very challenging, even if each is reporting its identity, position, and velocity as accurately as it can. The origin of information in UAS RID and UAS Traffic Management (UTM) generally is the UAS or its operator. Self-reports may be initiated by the remote pilot at the console of the Ground Control Station (GCS, the UAS subsystem used to remotely operate the UA), or automatically by GCS software; in Broadcast RID, they typically would be 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, and most of the data comes ultimately from, the UA itself. Whether information comes proximately from the operator, or from automated systems configured by the operator, there are possibilities not only of unintentional error in but also of intentional falsification of this data. Mandating UAS RID, specifying data elements required to be sent, monitoring compliance and enforcing it (or penalizing non-compliance) are matters for Civil Aviation Authorities (CAAs) et al; specifying message formats, etc. 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, are opportunities 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. , the basis for most current (2021) thinking about and efforts to provide UAS RID, specifies only how to get the UAS ID to the Observer: how the Observer can perform these lookups and how the registries first can be populated with information are unspecified therein. The need for nearly universal deployment of UAS RID is pressing: consider how negligible the value of an automobile license plate system would be if only 90% of the cars displayed plates. This implies the need to support use by Observers of already ubiquitous mobile devices (typically smartphones and tablets). Anticipating CAA requirements to support legacy devices, especially in light of , specifies that any UAS sending Broadcast RID over Bluetooth must do so over Bluetooth 4, regardless of whether it also does so over newer versions; as UAS sender devices and Observer receiver devices are unpaired, this implies extremely short "advertisement" (beacon) frames. Wireless data links 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, sensitive to both generic CSWaP and application-specific considerations. To accommodate the most severely constrained cases, all these 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 too infrequent to address. However, the preamble to cites "remote and rural areas that do not have reliable Internet access" as a major reason for requiring Broadcast rather than Network RID, and states that "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 web based Broadcast Authentication Verifier Service or 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 , even the most basic datum, the UAS ID itself, can be merely an unsubstantiated claim. Observer devices being ubiquitous, thus popular targets for malware or other compromise, cannot be generally trusted (although the user of each device is compelled to trust that device, to some extent); a "fair witness" functionality (inspired by ) is desirable. Despite work by regulators and SDOs, there are substantial gaps in UAS standards generally and UAS RID specifically. 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.

DRIP Scope DRIP’s initial charter is to make RID immediately actionable, in both Internet and local-only connected scenarios (especially emergencies), in severely constrained UAS environments, balancing legitimate (e.g., public safety) authorities’ Need To Know trustworthy information with UAS operators’ privacy. By "immediately actionable" is meant information of sufficient precision, accuracy, timeliness, etc. for an Observer to use it as the basis for immediate decisive action, whether that be to trigger a defensive counter-UAS system, to attempt to initiate communications with the UAS operator, to accept the presence of the UAS in the airspace where/when observed as not requiring further action, or whatever, with potentially severe consequences of any action or inaction chosen based on that information. For further explanation of the concept of immediate actionability, see . Note that UAS RID must achieve nearly universal adoption, but DRIP can add value even if only selectively deployed. Authorities with jurisdiction over more sensitive airspace volumes may set a higher than generally mandated RID requirement for flight in such volumes. Those with a greater need for high-confidence IFF can equip with DRIP, enabling strong authentication of their own aircraft and allied operators without regard for the weaker (if any) authentication of others. DRIP (originally Trustworthy Multipurpose Remote Identification, TM-RID) potentially could be applied to verifiably identify other types of registered things reported to be in specified physical locations, and providing timely trustworthy identification data is also prerequisite to identity-oriented networking, but the urgent motivation and clear initial focus is UAS. Existing Internet resources (protocol standards, services, infrastructure, and business models) should be leveraged.

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 and elaborated in other DRIP documents.

Terms and Definitions

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 when, and only when, they appear in all capitals, as shown here.

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. provides a glossary of Internet security terms that should be used where applicable. In the UAS community, the plural form of acronyms generally is the same as the singular form, e.g., Unmanned Aircraft System (singular) and Unmanned Aircraft Systems (plural) are both represented as UAS. On this and other terminological issues, to encourage comprehension necessary for adoption of DRIP by the intended user community, that community's norms are respected herein, and definitions are quoted in cases where they have been found in that community's documents. Most of the listed terms are from that community (even if specific source documents are not cited); any that are DRIP-specific or invented by the authors of this document are marked "(DRIP)".

4-D

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

AAA

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

ABDAA

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

ADS-B

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

AGL

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

ATC

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

ATM

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

Authentication Message

Message Type 2. Provides framing for authentication data, only; the only message that can be extended in length by segmenting it across more than one page.

Basic ID Message

Message Type 0. Provides UA Type, UAS ID Type, 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 .

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

CAA

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

CSWaP

Cost, Size, Weight, and Power.

C2

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

DAA

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

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". that presumably can be satisfied with appropriately configured Broadcast RID.

DSS

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

EUROCAE

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

GBDAA

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

GCS

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

GNSS

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

GPS

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

GRAIN

Global Resilient Aviation Interoperable Network. ICAO managed IPv6 overlay internetwork based on IATF, dedicated to aviation (but not just aircraft). Currently (2021) in design, accommodating the proposed DRIP identifier.

IATF

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

ICAO

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

IFF

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

LAANC

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

Location/Vector Message

Message Type 1. Provides UA location, altitude, heading, speed, and status.

LOS

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

Message Pack

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

MSL

Mean Sea Level. Shorthand for relative altitude, above the variously defined mean sea level, typically of a UA (but in 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 that aggregates data from Net-RID SPs as needed in response to user queries regarding UAS operating within specified airspace volumes, to enable display by a user application on a user device. Potentially could provide not only information sent via UAS RID but also information retrieved from UAS RID registries or information beyond UAS RID. Under superseded , not recognized as a distinct entity, but a service provided by USS, including Public Safety USS that may exist primarily for this purpose rather than to manage any subscribed UAS.

Net-RID SP

Network RID Service Provider. logical entity that collects RID messages from UAS and responds to NetRID-DP queries for information on UAS of which it is aware. Under superseded , the USS to which the UAS is subscribed ("Remote ID USS").

Network Identification Service

EU regulatory requirement in and that presumably can be satisfied with appropriately configured Network RID.

Observer

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

Operation

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

Operator

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

Operator ID Message

Message Type 5. Provides CAA issued Operator ID, only. Operator ID is distinct from UAS ID.

page

Payload of a frame, containing a chunk of a message that has been segmented, to allow transport of a message longer than can be encapsulated in a single frame.

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" .

PII

Personally Identifiable Information. In the UAS RID context, typically of the UAS Operator, Pilot In Command (PIC), or Remote Pilot, but possibly of an Observer or other party. 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, which the person might wish to keep private, or may have a statutorily recognized right to keep private (e.g., under the EU ), 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" .

RF

Radio Frequency. Adjective, e.g., "RF link", or noun.

RF LOS

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

RTCA

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

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

Security

"Safeguarding civil aviation against acts of unlawful interference." From Annex 17 of the Chicago Convention, quoted in

Self-ID Message

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

SDO

Standards Development Organization. ASTM, IETF, et al.

SDSP

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

System Message

Message Type 4. Provides general UAS information, including remote pilot location, multiple UA group operational area, etc.

U-space

EU concept and emerging framework for integration of UAS into all classes of airspace, specifically including high density urban areas, sharing airspace with manned aircraft .

UA

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

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 .

UAS ID

UAS identifier. Although called "UAS ID", it is actually unique to the UA, neither to the operator (as some UAS registration numbers have been and for exclusively recreational purposes are continuing to be assigned), nor to the combination of GCS and UA that comprise the UAS. Maximum length of 20 bytes .

UAS ID Type

UAS Identifier type index. 4 bits, see for currently defined values 0-3.

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" and "... provide services to support the UAS community, to connect Operators and other entities to enable information flow across the USS Network, and to promote shared situational awareness among UTM participants" .

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" . In the US, according to the FAA, a "traffic management" ecosystem for "uncontrolled" low altitude UAS operations, separate from, but complementary to, the FAA's ATC system for "controlled" operations of manned aircraft.

V2V

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

VLOS

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

UAS RID Problem Space CAAs worldwide are mandating UAS RID. The European Union Aviation Safety Agency (EASA) has published and Regulations. The US FAA has published a "final" rule and has described the key role that UAS RID plays in UAS Traffic Management (UTM) in (especially Section 2.6). CAAs currently (2021) promulgate performance-based regulations that do not specify techniques, but rather cite industry consensus technical standards as acceptable means of compliance. The most widely cited such industry consensus technical standard for UAS RID is , 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 . 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 Identifier (UAS ID) as the primary unique key for database lookup. 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 in the future perhaps others). specifies three UAS ID Type values:

1. A static, manufacturer assigned, hardware serial number as defined in ANSI/CTA-2063-A "Small Unmanned Aerial System Serial Numbers" .
2. A CAA assigned (generally static) ID, like the registration number of a manned aircraft.
3. A UTM system assigned UUID , which can but need not be dynamic.

Per , the EU allows only UAS ID Type 1. Under , the US allows types 1 and 3. 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 , how this is to be assigned in the US is still to be determined. As yet apparently there are no CAA public proposals to use UAS ID Type 2. In the preamble of , the FAA argues that registration numbers should not be sent in RID, insists that the capability of looking up registration numbers from information contained in RID should be restricted to FAA and other Government agencies, and implies that Session ID would be linked to the registration number only indirectly via the serial number in the registration database. The possibility of cryptographically blinding registration numbers, such that they can be revealed under specified circumstances, does not appear to be mentioned in applicable regulations or external technical standards. Under , the EU also requires an operator registration number (an additional identifier distinct from the UAS ID) that can be carried in an optional Operator ID message. allows RID requirements to be met by either the UA itself, which is then designated a "standard remote identification unmanned aircraft", or by an add-on "remote identification broadcast module". Relative to a standard RID UA, the different requirements for a module are that the latter: must transmit its own serial number (neither the serial number of the UA to which it is attached, nor a Session ID); must transmit takeoff location as a proxy for the location of the pilot/GCS; need not transmit UA emergency status; and is allowed to be used only for operations within VLOS of the remote pilot. Jurisdictions may relax or waive RID requirements for certain operators and/or under certain conditions. For example, allows operators with UAS not equipped for RID to conduct VLOS operations at counter-intuitively named "FAA-recognized identification areas" (FRIA); radio controlled model aircraft flying clubs and other eligible organizations can apply to the FAA for such recognition of their operating areas.

Network RID

"Network RID Information Flow" +-------------+ ****************** | UA | * Internet * +--o-------o--+ * * | | * * | | * * +------------+ | '--------*--(+)-----------*-----o | | * | * | | | .--------*--(+)-----------*-----o NET-Rid SP | | | * * | | | | * .------*-----o | | | * | * +------------+ | | * | * | | * | * +------------+ | | * '------*-----o | | | * * | NET-Rid DP | | | * .------*-----o | | | * | * +------------+ | | * | * | | * | * +------------+ +--o-------o--+ * '------*-----o Observer's | | GCS | * * | Device | +-------------+ ****************** +------------+

illustrates Network RID information flows. Only two of the three typically wireless links shown involving the UAS (UA-GCS, UA-Internet, and GCS-Internet) need exist. All three may exist, at the same or different times, especially in Beyond Visual Line Of Sight (BVLOS) operations. There must be some information flow path (direct or indirect) between the GCS and the UA, for the former to exercise C2 over the latter. If this path is two-way (as increasingly it is, even for inexpensive small UAS), the UA will also send its status (and position, if suitably equipped, e.g., with GNSS) to the GCS. There also must be some path between at least one subsystem of the UAS (UA or GCS) and the Internet, for the former to send status and position updates to its USS (serving inter alia as a Net-RID SP). Direct UA-Internet wireless links are expected to become more common, especially on larger UAS, but currently (2021) are rare. Instead, the RID data flow typically originates on the UA and passes through the GCS, or originates on the GCS. Network RID data makes three trips through the Internet (GCS-SP, SP-DP, DP-Observer, unless any of them are colocated), implying use of IP (and other middle layer protocols, e.g., TLS/TCP or DTLS/UDP) on those trips. IP is not necessarily used or supported on the UA-GCS link (if indeed that direct link exists, as it typically does now, but in BVLOS operations often will not). Network RID is publish-subscribe-query. In the UTM context:

1. The UAS operator pushes an "operational intent" (the current term in UTM corresponding to a flight plan in manned aviation) to the USS (call it USS#1) that will serve that UAS (call it UAS#1) for that operation, primarily to enable deconfliction with other operations potentially impinging upon that operation's 4-D airspace volume (call it Volume#1).
2. Assuming the operation is approved and commences, UAS#1 periodically pushes location/status updates to USS#1, which serves inter alia as the Network RID Service Provider (Net-RID SP) for that operation.
3. When users of any other USS (whether they be other UAS operators or Observers) develop an interest in any 4-D airspace volume (e.g., because they wish to submit an operational intent or because they have observed a UA), they query their own USS on the volumes in which they are interested.
4. Their USS query, via the UTM Discovery and Synchronization Service (DSS), all other USS in the UTM system, and learn of any USS that have operations in those volumes (including any volumes intersecting them); thus those USS whose query volumes intersect Volume#1 (call them USS#2 through USS#n) learn that USS#1 has such operations.
5. Interested parties can then subscribe to track updates on that operation of UAS#1, via their own USS, which serve as Network RID Display Providers (Net-RID DP) 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 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/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 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 UAS RID means. specifies only Net-RID SP to Net-RID DP information exchanges. It is presumed that IETF efforts supporting the more constrained Broadcast RID (see next section) can be generalized for Network RID and potentially also for UAS to USS or other UTM communications.

Broadcast RID

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

illustrates Broadcast RID information flow. Note the absence of the Internet from the figure. This is because Broadcast RID is one-way direct transmission of application layer messages over a RF data link (without IP) 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. Broadcast RID is conceptually similar to Automatic Dependent Surveillance - Broadcast (ADS-B). However, for various technical and other reasons, regulators including the EASA have not indicated intent to allow, and FAA has explicitly prohibited, use of ADS-B for UAS RID. 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 also required concurrently to do so on 4.x (referred to in as Bluetooth Legacy); current (2021) discussions in ASTM F38.02 on revising the standard, motivated by European standards drafts, suggest that both Bluetooth versions will be required. If broadcasting Wi-Fi NAN at 5 GHz, it is also required concurrently to do so at 2.4 GHz; current discussions in F38.02 include relaxing this. Wi-Fi Beacons are also under consideration. Future revisions of may allow other data links. The selection of the Broadcast media was driven by research into what is commonly available on 'ground' units (smartphones and tablets) and what was found as prevalent or 'affordable' in UA. Further, there must be an API for the Observer's receiving application to have access to these messages. As yet only Bluetooth 4.x support is readily available, thus the current focus is on working within the 31 byte payload limit of the Bluetooth 4.x "Broadcast Frame" transmitted as an "advertisement" on beacon channels. After overheads, this limits the RID message to 25 bytes and UAS ID string to a maximum length of 20 bytes. Length constraints also preclude a single Bluetooth 4.x frame carrying not only the UAS ID but also position, velocity, and other information that should be bound to the UAS ID, much less strong authentication data. 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 ). Message pages must somehow be correlated as belonging to the same message. Messages carrying position, velocity and other data must somehow be correlated with the Basic ID message that carries the UAS ID. This correlation is expected to be done on the basis of MAC address: this may be complicated by MAC address randomization; not all the common devices expected to be used by Observers have APIs that make sender MAC addresses available to user space receiver applications; and MAC addresses are easily spoofed. Data elements are not so detached on other media (see Message Pack in the paragraph after next). Broadcast RID specifies several message types. The 4 bit message type field in the header can index up to 16 types. Only 7 are currently defined. Only 2 are mandatory. All others are optional, unless required by a jurisdictional authority, e.g., a CAA. To satisfy 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 optional. The Message Pack itself is mandatory on data links that can encapsulate it in a single frame (Bluetooth 5.x and Wi-Fi). F3411-19 Message Types

Index	Name	Req	Notes
0x0	Basic ID	Mandatory	-
0x1	Location/Vector	Mandatory	-
0x2	Authentication	Optional	paged
0x3	Self-ID	Optional	free text
0x4	System	Optional	-
0x5	Operator	Optional	-
0xF	Message Pack	-	BT5 and Wi-Fi
See Section 5.4.5 and Table 3 of	-	-	-

Broadcast RID specifies very few quantitative performance requirements: static information must be transmitted at least once per 3 seconds; dynamic information (the Location/Vector message) must be transmitted at least once per second and be no older than one second when sent. requires all information be sent at least once per second. Broadcast RID transmits all information as cleartext (ASCII or binary), so static IDs enable trivial correlation of patterns of use, unacceptable in many applications, e.g., package delivery routes of competitors. Any UA can assert any ID using the required Basic ID message, which lacks any provisions for verification. The Position/Vector message likewise lacks provisions for verification, and does not contain the ID, so must be correlated somehow with a Basic ID message: the developers of 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 optional Authentication Message specifies framing for authentication data, but does not specify any authentication method, and the maximum length of the specified framing is too short for conventional digital signatures and far too short for conventional certificates (e.g., X.509). Fetching certificates via the Internet is not always possible (e.g., Observers working in remote areas, such as national forests), so devising a scheme whereby certificates can be transported over Broadcast RID is necessary. The one-way nature of Broadcast RID precludes challenge-response security protocols (e.g., Observers sending nonces to UA, to be returned in signed messages). Without DRIP extensions to , 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.

USS in UTM and RID UAS RID and UTM are complementary; Network RID is a UTM service. The backbone of the UTM system is comprised of multiple USS: one or several per jurisdiction; some limited to a single jurisdiction, others spanning multiple jurisdictions. USS also serve as the principal or perhaps the sole interface for operators and UAS into the UTM environment. Each operator subscribes to at least one USS. Each UAS is registered by its operator in at least one USS. Each operational intent is submitted to one USS; if approved, that UAS and operator can commence that operation. During the operation, status and location of that UAS must be reported to that USS, which in turn provides information as needed about that operator, UAS, and operation into the UTM system and to Observers via Network RID. USS provide services not limited to Network RID; indeed, the primary USS function is deconfliction of airspace usage by different UAS and other (e.g., manned aircraft, rocket launch) operations. Most deconfliction involving a given operation is hoped to be completed prior to commencing that operation, and is called "strategic deconfliction". If that fails, "tactical deconfliction" comes into play; ABDAA may not involve USS, but GBDAA likely will. Dynamic constraints, formerly called UAS Volume Restrictions (UVR), can be necessitated by local emergencies, extreme weather, etc., specified by authorities on the ground, and propagated in UTM. No role for USS in Broadcast RID is currently specified by regulators or . However, USS are likely to serve as registries (or perhaps registrars) for UAS (and perhaps operators); if so, USS will have a role in all forms of RID. Supplemental Data Service Providers (SDSP) are also likely to find roles, not only in UTM as such but also in enhancing UAS RID and related services. Whether USS, SDSP, etc. are involved or not, RID services, narrowly defined, provide regulator specified identification information; more broadly defined, RID services may leverage identification to facilitate related services or functions, likely beginning with V2X.

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 etc. provide limited information potentially enabling, and no technical means for, an Observer to communicate with the pilot, e.g., to request further information on the UAS operation or exit from an airspace volume in an emergency. The System Message provides the location of the pilot/GCS, so an Observer could physically go to the asserted location to look for the remote pilot; this is at best slow and may not be feasible. What if the pilot is on the opposite rim of a canyon, or there are multiple UAS operators to contact, whose GCS all lie in different directions from the Observer? An Observer with Internet connectivity and access privileges could look up operator PII in a registry, then call a phone number in hopes someone who can immediately influence the UAS operation will answer promptly during that operation; this is at best unreliable and may not be prudent. Should pilots be encouraged to answer phone calls while flying? Internet technologies can do much better than this. Thus complementing with protocols enabling strong authentication, preserving operator privacy while enabling immediate use of information by authorized parties, is critical to achieve widespread adoption of a RID system supporting safe and secure operation of UAS. DRIP will focus on making information obtained via UAS RID immediately usable:

1. by making it trustworthy (despite the severe constraints of Broadcast RID);
2. by enabling verification that a UAS is registered for RID, and if so, in which registry (for classification of trusted operators on the basis of known registry vetting, even by Observers lacking Internet connectivity at observation time);
3. by facilitating independent reports of UA aeronautical data (location, velocity, etc.) to confirm or refute the operator self-reports upon which UAS RID and UTM tracking are based;
4. by enabling instant establishment, by authorized parties, of secure communications with the remote pilot.

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

Requirements The following requirements apply to DRIP as a set of related protocols, various subsets of which, in conjunction with other IETF and external technical standards, may suffice to comply with the regulations in any given jurisdiction or meet any given user need. It is not intended that each and every DRIP protocol alone satisfy each and every requirement.

General

Normative Requirements

1. Provable Ownership: DRIP MUST enable verification that the UAS ID asserted in the Basic ID message is that of the actual current sender of the message (i.e., the message is not a replay attack or other spoof, authenticating, e.g., by verifying an asymmetric cryptographic signature using a sender provided public key from which the asserted ID can be at least partially derived), even on an Observer device lacking Internet connectivity at the time of observation.
2. Provable Binding: DRIP MUST enable binding all other messages from the same actual current sender to the UAS ID asserted in the Basic ID message.
3. Provable Registration: DRIP MUST enable 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; with UAS ID Type 3, the same sender may have multiple IDs, potentially in different registries, but each ID must clearly indicate in which registry it can be found.
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.
5. Gateway: DRIP MUST enable Broadcast RID to Network RID application layer gateways to stamp messages with precise date/time received and receiver location, then relay them to a network service (e.g., SDSP or distributed ledger).
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 pilot (remote pilot or Pilot In Command), the USS (if any) under which the operation is being conducted, and registries in which data on the UA and pilot are held.
7. QoS: DRIP MUST enable policy based specification of performance and reliability parameters.
8. Mobility: DRIP MUST support physical and logical mobility of UA, GCS and Observers. DRIP SHOULD support mobility of essentially all participating nodes (UA, GCS, Observers, Net-RID SP, Net-RID DP, Private Registry, SDSP, and potentially others as RID and UTM evolve).
9. Multihoming: DRIP MUST support multihoming of UA and GCS, for make-before-break smooth handoff and resiliency against path/link failure. DRIP SHOULD support multihoming of essentially all participating nodes.
10. Multicast: DRIP SHOULD support multicast for efficient and flexible publish-subscribe notifications, e.g., of UAS reporting positions in designated airspace volumes.
11. Management: DRIP SHOULD support monitoring of the health and coverage of Broadcast and Network RID services.

Rationale Requirements imposed either by regulation or are not reiterated here, but drive many of the numbered requirements listed here. The regulatory QoS requirement currently would be satisfied by ensuring information refresh rates of at least 1 Hertz, with latencies no greater than 1 second, at least 80% of the time, but these numbers may vary between jurisdictions and over time. So instead the DRIP QoS requirement is that performance, reliability, etc. parameters be user policy specifiable, which does not imply satisfiable in all cases, but (especially together with the management requirement) implies that when specifications are not met, appropriate parties are notified. The "provable ownership" requirement addresses the possibility that the actual sender is not the claimed sender (i.e., is a spoofer). The "provable binding" requirement addresses the MAC address correlation problem of noted above. The "provable registration" requirement may impose burdens not only on the UAS sender and the Observer's receiver, but also on the registry; yet it cannot depend upon the Observer being able to contact the registry at the time of observing the UA. The "readability" requirement pertains to the structure and format of information at endpoints rather than its encoding in transit, so may involve machine assisted format conversions, e.g., from binary encodings, and/or decryption (see ). The "gateway" requirement is in pursuit of three objectives: (1) mark up a RID message with where and when it was actually received, which may agree or disagree with the self-report in the set of messages; (2) defend against replay attacks; and (3) support optional SDSP services such as multilateration, to complement UAS position self-reports with independent measurements. This is the only instance in which DRIP transports messages; most of DRIP pertains to the authentication of such messages and identifiers carried in them. The "QoS" requirement is only that performance and reliability parameters can be specified by policy, not that any such specifications must be guaranteed to be met; any failure to meet such would be reported under the "management" requirement. Examples of such parameters are the maximum time interval at which messages carrying required data elements may be transmitted, the maximum tolerable rate of loss of such messages, and the maximum tolerable latency between a dynamic data element (e.g., GNSS position of UA) being provided to the DRIP sender and that element being delivered by the DRIP receiver to an application. The "mobility" requirement refers to rapid geographic mobility of nodes, changes of their points of attachment to networks, and changes to their IP addresses; it is not limited to micro-mobility within a small geographic area or single Internet access provider.

Identifier

Normative Requirements

1. Length: The DRIP entity identifier MUST NOT be longer than 20 bytes, to fit in the UAS ID field of the Basic ID message of .
2. Registry ID: The DRIP identifier MUST be sufficient to identify a registry in which the entity identified therewith is listed.
3. Entity ID: The DRIP identifier MUST be sufficient to enable lookups of other data associated with the entity identified therewith in that registry.
4. Uniqueness: The DRIP identifier MUST be unique within the applicable global identifier space from when it is first registered therein until it is explicitly de-registered therefrom (due to, e.g., expiration after a specified lifetime, revocation by the registry, or surrender by the operator).
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).
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.

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 (have the exclusive capability to use, such that receivers can verify its ownership of) the identifier. The DRIP identifier can be used at various layers. In Broadcast RID, it would be used by the application running directly over the data link. In Network RID, it would be used by the application running over HTTPS (and possibly other protocols). In RID initiated V2X applications such as DAA and C2, it could be used between the network and transport layers, e.g., with the Host Identity Protocol (HIP, , , etc.), or between the transport and application layers, e.g., with Datagram Transport Layer Security (DTLS, ). Registry ID (which registry the entity is in) and Entity ID (which entity it is, within that registry) are requirements on a single DRIP entity identifier, not separate (types of) ID. In the most common use case, the entity will be the UA, and the DRIP identifier will be the UAS ID; however, other entities may also benefit from having DRIP identifiers, so the entity type is not prescribed here. Whether a UAS ID is generated by the operator, GCS, UA, USS, registry, or some collaboration thereamong, is unspecified; however, there must be agreement on the UAS ID among these entities. Management of DRIP identifiers is the primary function of their registration hierarchies, from the root (presumably IANA), through sector-specific and regional authorities (presumably ICAO and CAAs), to the identified entities themselves. While "uniqueness" might be considered an implicit requirement for any identifier, here the point of the explicit requirement is not just that it should be unique, but also where and when it should be unique: global scope within a specified space, from registration to deregistration. While "non-spoofability" imposes requirements for and on a DRIP authentication protocol, it also imposes requirements on the properties of the identifier itself. An example of how the nature of the identifier can support non-spoofability is embedding a hash of both the registry ID and a public key of the entity in the entity identifier, thus making it self-authenticating any time the entity's corresponding private key is used to sign a message. While "unlinkability" is a privacy desideratum (see next section), 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).

Privacy

Normative Requirements

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

Rationale Most data to be sent via Broadcast RID or Network RID is public, thus the "encrypted transport" requirement for private data is selective, e.g., for the entire payload of the Operator ID Message, but only the pilot/GCS location fields of the System Message. In some jurisdictions, the configurable enabling and disabling of encryption may need to be outside the control of the operator. mandates manufacturers design RID equipment with some degree of tamper resistance; the preamble and other FAA commentary suggest this is to reduce the likelihood that an operator, intentionally or unintentionally, might alter the values of the required data elements or disable their transmission in the required manner (e.g., as cleartext). How information is stored on end systems is out of scope for DRIP. Encouraging privacy best practices, including end system storage encryption, by facilitating it with protocol design reflecting such considerations, is in scope. Similar logic applies to methods for designating information as public or private. The privacy requirements above are for DRIP, neither for (which requires obfuscation of location to any Network RID subscriber engaging in wide area surveillance, limits data retention periods, etc., in the interests of privacy), nor for UAS RID in any specific jurisdiction (which may have its own regulatory requirements). The requirements above are also in a sense parameterized: who are the "authorized actors", how are they designated, how are they authenticated, etc.?

Registries

Normative Requirements

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.
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.
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.
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.

Rationale Registries are fundamental to RID. Only very limited information can be Broadcast, but extended information is sometimes needed. The most essential element of information sent is the UAS ID itself, the unique key for lookup of extended information in registries. Beyond designating the UAS ID as that unique key, the registry information model is not specified herein, in part because regulatory requirements for different registries (UAS operators and their UA, each narrowly for UAS RID and broadly for U-space/UTM) and business models for meeting those requirements are in flux. While it is expected that registry functions will be integrated with USS, who will provide them is not yet determined in most, and is expected to vary between, jurisdictions. However this evolves, the essential registry functions, starting with management of identifiers, are expected to remain the same, so 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. As specific AAA requirements will vary by jurisdictional regulation, provider philosophy, customer demand, etc., they are left to specification in policies, which should be human readable to facilitate analysis and discussion, and machine readable to enable automated enforcement, using a language amenable to both, e.g., XACML.

IANA Considerations This document does not make any IANA request.

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

• Sybil attacks
• confusion created by many spoofed unsigned messages
• processing overload induced by attempting to verify many spoofed signed messages (where verification will fail but still consume cycles)
• malicious or malfunctioning registries
• interception by on-path attacker of (i.e., Man In The Middle attacks on) registration messages
• UA impersonation through private key extraction, improper key sharing, or carriage of a small (presumably harmless) UA, i.e., as a "false flag", by a larger (malicious) UA

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

• message integrity
• non-repudiation
• defense against replay attacks
• defense against spoofing

One approach to so doing involves verifiably binding the DRIP identifier to a public key. Providing these security features, whether via this approach or another, is likely to be especially challenging for Observers without Internet connectivity at the time of observation. For example, checking the signature of a registry on a public key certificate received via Broadcast RID in a remote area presumably would require that the registry’s public key had been previously installed on the Observer’s device, yet there may be many registries and the Observer’s device may be storage constrained, and new registries may come on-line subsequent to installation of DRIP software on the Observer’s device. Thus there may be caveats on the extent to which requirements can be satisfied in such cases, yet strenuous effort should be made to satisfy them, as such cases, e.g., firefighting in a national forest, are important.

Privacy and Transparency Considerations Privacy and transparency are important for legal reasons including regulatory consistency. states "harmonised and interoperable national registration systems... should comply with the applicable Union and national law on privacy and processing of personal data, and the information stored in those registration systems should be easily accessible.” Privacy and transparency (where essential to security or safety) are also ethical and moral imperatives. Even in cases where old practices (e.g., automobile registration plates) could be imitated, when new applications involving PII (such as UAS RID) are addressed and newer technologies could enable improving privacy, such opportunities should not be squandered. Thus it is recommended that all DRIP work give due regard to and more broadly . However, privacy and transparency are often conflicting goals, demanding careful attention to their balance. DRIP information falls into two classes: that which, to achieve the purpose, must be published openly as cleartext, for the benefit of any Observer (e.g., the basic UAS ID itself); and that which must be protected (e.g., PII of pilots) but made available to properly authorized parties (e.g., public safety personnel who urgently need to contact pilots in emergencies). How properly authorized parties are authorized, authenticated, etc. are questions that extend beyond the scope of DRIP, but DRIP may be able to provide support for such processes. Classification of information as public or private must be made explicit and reflected with markings, design, etc. Classifying the information will be addressed primarily in external standards; herein it will be regarded as a matter for CAA, registry, and operator policies, for which enforcement mechanisms will be defined within the scope of DRIP WG and offered. Details of the protection mechanisms will be provided in other DRIP documents. Mitigation of adversarial correlation will also be addressed.

References Normative References ASTM International Informative References European Union Aviation Safety Agency (EASA) ASD-STAN ANSI European Union Aviation Safety Agency (EASA) European Union Agency for Cybersecurity (ENISA) European Parliament and Council FAA Office of NextGen Flightradar24 AB Federal Aviation Administration European Parliament and Council International Civil Aviation Organization International Civil Aviation Organization International Civil Aviation Organization International Civil Aviation Organization European Union Aviation Safety Agency (EASA) SESAR Joint Undertaking United States Federal Aviation Administration (FAA) Intel Corp. OpenSky Network non-profit association European Union Aviation Safety Agency (EASA) United States Federal Aviation Administration FAA UAS Identification and Tracking Aviation Rulemaking Committee American National Standards Institute (ANSI) Unmanned Aircraft Systems Standardization Collaborative (UASSC) EUROCAE Wi-Fi Alliance

Discussion and Limitations This document is largely based on the process of one SDO, ASTM. Therefore, it is tailored to specific needs and data formats of this standard. Other organizations, for example in EU, do not necessary follow the same architecture. The need for drone ID and operator privacy is an open discussion topic. For instance, in the ground vehicular domain each car carries a publicly visible plate number. In some countries, for nominal cost or even for free, anyone can resolve the identity and contact information of the owner. Civil commercial aviation and maritime industries also have a tradition of broadcasting plane or ship ID, coordinates, and even flight plans in plain text. Community networks such as OpenSky and Flightradar24 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 which indicate radio communication failure. The codes are unique in each traffic area and can be re-assigned when entering another control area. The code is transmitted in plain text by the transponder and also used for collision avoidance by a system known as Traffic alert and Collision Avoidance System (TCAS). The system could be used for UAS as well initially, but the code space is quite limited and likely to be exhausted soon. The number of UAS far exceeds the number of civil airplanes in operation. The ADS-B system is utilized in civil aviation for each “ADS-B Out” equipped airplane to broadcast its ID, coordinates, and altitude for other airplanes and ground control stations. If this system is adopted for drone IDs, it has additional benefit with backward compatibility with civil aviation infrastructure; then, pilots and dispatchers will be able to see UA on their control screens and take those into account. If not, a gateway translation system between the proposed drone ID and civil aviation system should be implemented. Again, system saturation due to large numbers of UAS is a concern. 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 / IEEE 802.16, 5G) or sensor networks (Sigfox, LoRa). Such transmission technologies can impose additional restrictions on packet sizes and frequency of transmissions, but could provide better energy efficiency and range. In civil aviation, Controller-Pilot Data Link Communications (CPDLC) is used to transmit command and control between the pilots and ATC. It could be considered for UAS as well due to long range and proven use despite its lack of security . L-band Digital Aeronautical Communications System (LDACS) is being standardized by ICAO and IETF for use in future civil aviation . It provides secure communication, positioning, and control for aircraft using a dedicated radio band. It should be analyzed as a potential provider for UAS RID as well. This will bring the benefit of a global integrated system creating a global airspace use awareness.

Acknowledgments The work of the FAA's UAS Identification and Tracking (UAS ID) Aviation Rulemaking Committee (ARC) is the foundation of later ASTM and IETF DRIP efforts. The work of Gabriel Cox, Intel Corp., and their Open Drone ID collaborators opened UAS RID to a wider community. The work of ASTM F38.02 in balancing the interests of diverse stakeholders is essential to the necessary rapid and widespread deployment of UAS RID. IETF volunteers who have extensively reviewed or otherwise contributed to this document include Amelia Andersdotter, Carsten Bormann, Mohamed Boucadair, Toerless Eckert, Susan Hares, Mika Jarvenpaa, Daniel Migault, Alexandre Petrescu, Saulo Da Silva and Shuai Zhao. Thanks to Linda Dunbar for the Secdir review and Nagendra Nainar for the Opsdir review.