Drone Remote Identification Protocol (DRIP) Requirements

1.1. 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 [Part107]);¶
• are highly maneuverable so can fly under trees and between buildings.¶

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

Diverse other applications can be enabled or facilitated by RID. 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 Figure 1.¶

***************                                        ***************
*    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)

Figure 1 illustrates a typical case where there may be: multiple Observers, some of them members of the general public, others government officers with public safety/security responsibilities; multiple UA in flight within observation range, each with its own pilot/operator; at least one registry each for lookup of public and (by authorized parties only) private information regarding the UAS and their pilots/operators; and in the DRIP vision, DNS resolving various identifiers and locators of the entities involved. Note 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 Figure 3. Not all the links shown in Figure 1 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 Figure 2, 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.¶

                     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 [F3411-19]: the published standard is available for purchase from ASTM and as an ASTM membership premium; early drafts are freely available as [OpenDroneID] specifications. [F3411-19] is frequently referenced in DRIP, where building upon its link layers and both enhancing support for and expanding the scope of its applications are central foci.¶

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

Using UAS RID to facilitate vehicular (V2X) communications and applications such as Detect And Avoid (DAA), which would impose tighter latency bounds than RID itself, is an obvious possibility, explicitly contemplated in the United States (US) Federal Aviation Administration (FAA) Remote Identification of Unmanned Aircraft rule [FRUR]. However, 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 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 one-way to Observers, they will use V2V to other UAS (also likely to and/or from some manned aircraft). These reflect the broad scope of the European Union (EU) U-space concept, as being developed in the Single European Sky ATM Research (SESAR) Joint Undertaking, the U-space architectural principles of which are outlined in [InitialView].¶

ASD-STAN is an Associated Body to CEN (European Committee for Standardization) for Aerospace Standards. It is publishing an EU standard "Aerospace series - Unmanned Aircraft Systems - Part 002: Direct Remote Identification; English version prEN 4709-002:2020" for which a current (early 2021) informal overview is freely available in [ASDRI]. It will provide compliance to cover the identical DRI requirements applicable to drones of classes C1 - [Delegated] Part 2, C2 - [Delegated] Part 3, C3 - [Delegated] Part 4, C5 - [Amended] Part 16, and C6 - [Amended] Part 17.¶

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

The [ASDRI] contemplated DRI system will broadcast locally:¶

1. the UAS operator registration number;¶
2. the [CTA2063A] compliant unique serial number of the UA;¶
3. a time stamp, the geographical position of the UA, and its height AGL or above its take-off point;¶
4. the UA ground speed and route course measured clockwise from true north;¶
5. the geographical position of the remote pilot, or if that is not available, the geographical position of the UA take-off point; and¶
6. for Classes C1, C2, C3, the UAS emergency status.¶

Under the [ASDRI] 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 [ASDRI].¶

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

Although dynamic establishment of secure communications between the Observer and the UAS pilot seems to have been contemplated by the FAA UAS ID and Tracking Aviation Rulemaking Committee (ARC) in their [Recommendations], it is not addressed in any of the subsequent regulations or international SDO technical specifications, other than DRIP, 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 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. [F3411-19], the basis for most current (2021) thinking about and efforts to provide UAS RID, specifies only how to get the UAS ID to the Observer: how the Observer can perform these lookups and how the registries first can be populated with information are unspecified therein.¶

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

Wireless data links 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 [FRUR] 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 [F3411-19] 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 [F3411-19], even the most basic datum, the UAS ID itself, can be merely an unsubstantiated claim.¶

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

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

1.3. DRIP Scope

DRIP's initial 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 [ENISACSIRT].¶

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

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

1.4. Document Scope

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

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

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

3.2. Broadcast RID

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

Figure 4 illustrates Broadcast RID information flow. Note the absence of the Internet from the figure. This is because Broadcast RID is one-way direct transmission of application layer messages over a RF data link (without IP) 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.¶

[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 also required concurrently to do so on 4.x (referred to in [F3411-19] as Bluetooth Legacy); current (2021) discussions in ASTM F38.02 on revising the standard, 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 [F3411-19] may allow other data links.¶

The selection of the Broadcast media was driven by research into what is commonly available on 'ground' units (smartphones and tablets) and what was found as prevalent or 'affordable' in UA. Further, there must be an 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 [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 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).¶

[F3411-19] Broadcast RID specifies several message types. The 4 bit message type field in the header can index up to 16 types. Only 7 are currently defined. Only 2 are mandatory. All others are optional, unless required by a jurisdictional authority, e.g., a CAA. To satisfy 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] Broadcast RID specifies very few quantitative performance requirements: static information must be transmitted at least once per 3 seconds; dynamic information (the Location/Vector message) must be transmitted at least once per second and be no older than one second when sent. [FRUR] requires all information be sent at least once per second.¶

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

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

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

3.3. USS in UTM and RID

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

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

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

3.4. DRIP Focus

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

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

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

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

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

4.1. General

4.2. Identifier

4.4. Registries

8.1. Normative References

[F3411-19]

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

[RFC2119]

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

[RFC8174]

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

8.2. Informative References

[Amended]

European Union Aviation Safety Agency (EASA), "Commission Delegated Regulation (EU) 2020/1058 of 27 April 2020 amending Delegated Regulation (EU) 2019/945", April 2020, <https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32020R1058>.

[ASDRI]

ASD-STAN, "Introduction to the European UAS Digital Remote ID Technical Standard", January 2021, <https://asd-stan.org/wp-content/uploads/ASD-STAN_DRI_Introduction_to_the_European_digital_RID_UAS_Standard.pdf>.

[CPDLC]

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

[CTA2063A]

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

[Delegated]

European Union Aviation Safety Agency (EASA), "Commission Delegated Regulation (EU) 2019/945 of 12 March 2019 on unmanned aircraft systems and on third-country operators of unmanned aircraft systems", March 2019, <https://eur-lex.europa.eu/eli/reg_del/2019/945/oj>.

[drip-architecture]

Card, S. W., Wiethuechter, A., Moskowitz, R., Zhao, S., and A. Gurtov, "Drone Remote Identification Protocol (DRIP) Architecture", Work in Progress, Internet-Draft, draft-ietf-drip-arch-11, 23 February 2021, <https://tools.ietf.org/html/draft-ietf-drip-arch-11>.

[ENISACSIRT]

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

[EU2018]

European Parliament and Council, "2015/0277 (COD) PE-CONS 2/18", February 2018, <https://www.consilium.europa.eu/media/35805/easa-regulation-june-2018.pdf>.

[FAACONOPS]

FAA Office of NextGen, "UTM Concept of Operations v2.0", March 2020, <https://www.faa.gov/uas/research_development/traffic_management/media/UTM_ConOps_v2.pdf>.

[FR24]

Flightradar24 AB, "Flightradar24 Live Air Traffic", May 2021, <https://www.flightradar24.com/about>.

[FRUR]

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

[GDPR]

European Parliament and Council, "General Data Protection Regulation", April 2016, <https://eur-lex.europa.eu/eli/reg/2016/679/oj>.

[I-D.maeurer-raw-ldacs]

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

[ICAOATM]

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

[ICAODEFS]

International Civil Aviation Organization, "Defined terms from the Annexes to the Chicago Convention and ICAO guidance material", July 2017, <https://www.icao.int/safety/cargosafety/Documents/Draft%20Glossary%20of%20terms.docx>.

[ICAOUAS]

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

[ICAOUTM]

International Civil Aviation Organization, "Unmanned Aircraft Systems Traffic Management (UTM) - A Common Framework with Core Principles for Global Harmonization, Edition 3", October 2020, <https://www.icao.int/safety/UA/Documents/UTM%20Framework%20Edition%203.pdf>.

[Implementing]

European Union Aviation Safety Agency (EASA), "Commission Implementing Regulation (EU) 2019/947 of 24 May 2019 on the rules and procedures for the operation of unmanned aircraft", May 2019, <https://eur-lex.europa.eu/eli/reg_impl/2019/947/oj>.

[InitialView]

SESAR Joint Undertaking, "Initial view on Principles for the U-space architecture", July 2019, <https://www.sesarju.eu/sites/default/files/documents/u-space/SESAR%20principles%20for%20U-space%20architecture.pdf>.

[NPRM]

United States Federal Aviation Administration (FAA), "Notice of Proposed Rule Making on Remote Identification of Unmanned Aircraft Systems", December 2019, <https://www.federalregister.gov/documents/2019/12/31/2019-28100/remote-identification-of-unmanned-aircraft-systems>.

[OpenDroneID]

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

[OpenSky]

OpenSky Network non-profit association, "The OpenSky Network", May 2021, <https://opensky-network.org/about/about-us>.

[Opinion1]

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

[Part107]

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

[Recommendations]

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

[RFC4122]

Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, July 2005, <https://www.rfc-editor.org/info/rfc4122>.

[RFC4423]

Moskowitz, R. and P. Nikander, "Host Identity Protocol (HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May 2006, <https://www.rfc-editor.org/info/rfc4423>.

[RFC4949]

Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://www.rfc-editor.org/info/rfc4949>.

[RFC6347]

Rescorla, E. and N. Modadugu, "Datagram Transport Layer 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, DOI 10.17487/RFC6973, July 2013, <https://www.rfc-editor.org/info/rfc6973>.

[RFC7401]

Moskowitz, R., Ed., Heer, T., Jokela, P., and T. Henderson, "Host Identity Protocol Version 2 (HIPv2)", RFC 7401, DOI 10.17487/RFC7401, April 2015, <https://www.rfc-editor.org/info/rfc7401>.

[RFC8280]

ten Oever, N. and C. Cath, "Research into Human Rights Protocol Considerations", RFC 8280, DOI 10.17487/RFC8280, October 2017, <https://www.rfc-editor.org/info/rfc8280>.

[Roadmap]

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

[Stranger]

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

[WG105]

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

[WiFiNAN]

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