| Internet-Draft | DRIP Arch | June 2020 |
| Card, et al. | Expires 25 December 2020 | [Page] |
This document defines an architecture for protocols and services to support Unmanned Aircraft System Remote Identification and tracking (UAS RID), plus RID-related communications, including required architectural building blocks and their interfaces.¶
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This document describes a natural Internet based architecture for Unmanned Aircraft System Remote Identification and tracking (UAS RID), conforming to proposed regulations and external technical standards, satisfying the requirements listed in the companion requirements document [I-D.ietf-drip-reqs]. The requirements document also provides an extended introduction to the problem space, use cases, etc. Only a brief summary of that introduction will be restated here as context, with reference to the general architecture shown in Figure 1 below.¶
General x x Public
Public xxxxx xxxxx Safety
Observer x x Observer
x x
x x ---------+ +---------- x x
x x | | x x
| |
+ +
xxxxxxxxxx
x x
+----------+x Internet x+------------+
| x x |
UA1 x | xxxxxxxxxx | x UA2
Pilot xxxxx + + + xxxxx Pilot
Operator x | | | x Operator
x | | | x
x x | | | x x
x x | | | x x
| | |
+----------+ | | | +----------+
| |------+ | +-------| |
| Public | | | Private |
| Registry | +-----+ | Registry |
| | | DNS | | |
+----------+ +-----+ +----------+
Many considerations (especially safety) dictate that UAS be remotely identifiable. Civil Aviation Authorities (CAAs) worldwide are mandating Unmanned Aircraft Systems (UAS) Remote Identification (RID). CAAs currently (2020) promulgate performance-based regulations that do not specify techniques, but rather cite industry consensus technical standards as acceptable means of compliance.¶
ASTM International, Technical Committee F38 (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item WK65041, developed the new ASTM [F3411-19] Standard Specification for Remote ID and Tracking. It defines one set of RID information and two means of communicating it. If a UAS uses both communication methods, generally the same information must provided via both means. While hybrids are possible (and indeed one is proposed as an optional DRIP service), the two basic methods are defined as follows:¶
The less constrained but more complex case of Network RID is illustrated in Figure 2 below.¶
x x UA
xxxxx ********************
| * ------*---+------------+
| * / * | NET_Rid_DP |
| * ------------/ +---*--+------------+
| RF */ | *
| * INTERNET | * +------------+
| /* +---*--| NET_Rid_SP |
| / * +----*--+------------+
+ / * | *
x / ****************|*** x
xxxxx | xxxxx
x +------- x
x x
x x Operator (GCS) Observer x x
x x x x
Via the direct Radio Frequency (RF) link between the UA and GCS: Command and Control (C2) flows from the GCS to the UA; for all but the simplest hobby aircraft, position and status flow from the UA to the GCS. Via the Internet, through three distinct segments, Network RID information flows from the UAS (comprising the UA and its GCS) to the Observer.¶
Other Standards Development Organizations (SDOs, e.g., 3GPP, Appendix A.4) may define their own communication methods for both Network and Broadcast RID. The CAAs expect any additional methods to maintain consistency of the RID messages.¶
DRIP will enable leveraging existing Internet resources (standard protocols, services, infrastructure and business models) to meet UAS RID and closely related needs. DRIP will specify how to apply IETF standards, complementing [F3411-19] and other external standards, to satisfy UAS RID requirements. DRIP will update existing and develop new protocol standards as needed to accomplish the foregoing.¶
This document will outline the UAS RID architecture into which DRIP must fit, and an architecture for DRIP itself. This includes presenting the gaps between the CAAs' Concepts of Operations and [F3411-19] as it relates to use of Internet technologies and UA direct RF communications. Issues include, but are not limited to:¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document uses terms defined in [I-D.ietf-drip-reqs].¶
Any DRIP solutions for UAS RID must fit into the UTM (or U-space) system. This implies interaction with entities including UA, GCS, USS, Net-RID SP, Net-RID DP, Observers, Operators, Pilots In Command, Remote Pilots, possibly SDSP, etc. The only additional entities introduced in this document are registries, required but not specified by the regulations and [RFC7401], and optionally CS-RID SDSP and Finder nodes.¶
UAS registries hold both public and private UAS information. The public information is primarily pointers to the repositories of, and keys for looking up, the private information. Given these different uses, and to improve scalability, security and simplicity of administration, the public and private information can be stored in different registries, indeed different types of registry.¶
The private information required for UAS RID is similar to that required for Internet domain name registration. Thus a DRIP RID solution can leverage existing Internet resources: registration protocols, infrastructure and business models, by fitting into an ID structure compatible with DNS names. This implies some sort of hierarchy, for scalability, and management of this hierarchy. It is expected that the private registry function will be provided by the same organizations that run USS, and likely integrated with USS.¶
A DRIP UAS ID MUST be amenable to handling as an Internet domain name (at an arbitrary level in the hierarchy), MUST be registered in at least a pseudo-domain (e.g. .ip6 for reverse lookup), and MAY be registered as a sub-domain (for forward lookup).¶
A DRIP private information registry MUST support essential Internet domain name registry operations (e.g. add, delete, update, query) using interoperable open standard protocols. It SHOULD support the Extensible Provisioning Protocol (EPP) and the Registry Data Access Protocol (RDAP) with access controls. It MAY use XACML to specify those access controls. It MUST be listed in a DNS: that DNS MAY be private; but absent any compelling reasons for use of private DNS, SHOULD be the definitive public Internet DNS hierarchy. The DRIP private information registry in which a given UAS is registered MUST be locatable, starting from the UAS ID, using the methods specified in [RFC7484].¶
The public information required to be made available by UAS RID is transmitted as cleartext to local observers in Broadcast RID and is served to a client by a Net-RID DP in Network RID. Therefore, while IETF can offer e.g. [RFC6280] as one way to implement Network RID, the only public information required to support essential DRIP functions for UAS RID is that required to look up Internet domain hosts, services, etc.¶
A DRIP public information registry MUST be a standard DNS server, in the definitive public Internet DNS hierarchy. It MUST support NS, MX, SRV, TXT, AAAA, PTR, CNAME and HIP RR (the last per [RFC8005]) types.¶
ASTM anticipated that regulators would require both Broadcast RID and Network RID for large UAS, but allow RID requirements for small UAS to be satisfied with the operator's choice of either Broadcast RID or Network RID. The EASA initially specified Broadcast RID for UAS of essentially all UAS and is now considering Network RID also. The FAA NPRM requires both for Standard RID and specifies Broadcast RID only for Limited RID. One obvious opportunity is to enhance the architecture with gateways from Broadcast RID to Network RID. This provides the best of both and gives regulators and operators flexibility. Such gateways could be pre-positioned (e.g. around airports and other sensitive areas) and/or crowdsourced (as nothing more than a smartphone with a suitable app is needed). Gateways can also perform multilateration to provide independent measurements of UA position, which is otherwise entirely operator self-reported in UAS RID and UTM. CS-RID would be an option, beyond baseline DRIP functionality; if implemented, it adds 2 more entity types.¶
A CS-RID SDSP MUST appear (i.e. present the same interface) to a Net-RID SP as a Net-RID DP. A CS-RID SDSP MUST appear to a Net-RID DP as a Net-RID SP. A CS-RID SDSP MUST NOT present a standard GCS-facing interface as if it were a Net-RID SP. A CS-RID SDSP MUST NOT present a standard client-facing interface as if it were a Net-RID DP. A CS-RID SDSP MUST present a TBD interface to a CS-RID Finder; this interface SHOULD be based upon but readily distinguishable from that between a GCS and a Net-RID SP.¶
A CS-RID Finder MUST present a TBD interface to a CS-RID SDSP; this interface SHOULD be based upon but readily distinguishable from that between a GCS and a Net-RID SP. A CS-RID Finder must implement, integrate, or accept outputs from, a Broadcast RID receiver. A CS-RID Finder MUST NOT interface directly with a GCS, Net-RID SP, Net-RID DP or Network RID client.¶
A DRIP UA ID needs to be "Trustworthy". This means that within the framework of the RID messages, an observer can establish that the RID used does uniquely belong to the UA. That the only way for any other UA to assert this RID would be to steal something from within the UA. The RID is self-generated by the UAS (either UA or GCS) and registered with the USS.¶
Within the limitations of Broadcast RID, this is extremely challenging as:¶
Standard approaches like X.509 and PKI will not fit these constraints, even using the new EdDSA algorithm. An example of a technology that will fit within these limitations is an enhancement of the Host Identity Tag (HIT) of HIPv2 [RFC7401] introducing hierarchy as defined in HHIT [I-D.moskowitz-hip-hierarchical-hit]; using Hierarchical HITs for UAS RID is outlined in HHIT based UAS RID [I-D.moskowitz-drip-uas-rid]. As PKI with X.509 is being used in other systems with which UAS RID must interoperate (e.g. the UTM Discovery and Synchronization Service and the UTM InterUSS protocol) mappings between the more flexible but larger X.509 certificates and the HHIT based structures must be devised.¶
By using the EdDSA HHIT suite, self-assertions of the RID can be done in as little as 84 bytes. Third-party assertions can be done in 200 bytes. An observer would need Internet access to validate a self-assertion claim. A third-party assertion can be validated via a small credential cache in a disconnected environment. This third-party assertion is possible when the third-party also uses HHITs for its identity and the UA has the public key for that HHIT.¶
A DRIP UAS ID MUST be a HHIT. It SHOULD be self-generated by the UAS (either UA or GCS) and MUST be registered with the Private Information Registry identified in its hierarchy fields. Each UAS ID HHIT MUST NOT be used more than once, with one exception as follows.¶
Each UA MAY be assigned, by its manufacturer, a single HI and derived HHIT encoded as a hardware serial number per [CTA2063A]. Such a static HHIT SHOULD be used only to bind one-time use UAS IDs (other HHITs) to the unique UA. Depending upon implementation, this may leave a HI private key in the possession of the manufacturer (see Security Considerations).¶
Each UA equipped for Broadcast RID MUST be provisioned not only with its HHIT but also with the HI public key from which the HHIT was derived and the corresponding private key, to enable message signature. Each UAS equipped for Network RID MUST be provisioned likewise; the private key SHOULD reside only in the ultimate source of Network RID messages (i.e. on the UA itself if the GCS is merely relaying rather than sourcing Network RID messages). Each observer device MUST be provisioned with public keys of the UAS RID root registries and MAY be provisioned with public keys or certificates for subordinate registries.¶
Operators and Private Information Registries MUST possess and other UTM entities MAY possess UAS ID style HHITs. When present, such HHITs SHOULD be used with HIP to strongly mutually authenticate and optionally encrypt communications.¶
Each Operator MUST generate a "HIo" and derived "HHITo", register them with a Private Information Registry along with whatever Operator data (inc. PII) is required by the cognizant CAA and the registry, and obtain a certificate "Cro" signed with "HIr(priv)" proving such registration.¶
To add an UA, an Operator MUST generate a "HIa" and derived "HHITa", create a certificate "Coa" signed with "HIo(priv)" to associate the UA with its Operator, register them with a Private Information Registry along with whatever UAS data is required by the cognizant CAA and the registry, obtain a certificate "Croa" signed with "HIr(priv)" proving such registration, and obtain a certificate "Cra" signed with "HIr(priv)" proving UA registration in that specific registry while preserving Operator privacy. The operator then MUST provision the UA with "HIa", "HIa(priv)", "HHITa" and "Cra".¶
UA engaging in Broadcast RID MUST use "HIa(priv)" to sign Auth Messages and MUST periodically broadcast "Cra". UAS engaging in Network RID MUST use "HIa(priv)" to sign Auth Messages. Observers MUST use "HIa" from received "Cra" to verify received Broadcast RID Auth messages. Observers without Internet connectivity MAY use "Cra" to identify the trust class of the UAS based on known registry vetting. Observers with Internet connectivity MAY use "HHITa" to perform lookups in the Public Information Registry and MAY then query the Private Information Registry, which MUST enforce AAA policy on Operator PII and other sensitive information.¶
Broadcast RID messages may contain PII. This may be information about the UA such as its destination or Operator information such as GCS location. There is no absolute "right" in hiding PII, as there will be times (e.g., disasters) and places (buffer zones around airports and sensitive facilities) where policy may mandate all information be sent as cleartext. Otherwise, the modern general position (consistent with, e.g., the EU General Data Protection Regulation) is to hide PII unless otherwise instructed. While some have argued that a system like that of automobile registration plates should suffice for UAS, others have argued persuasively that each generation of new identifiers should take advantage of advancing technology to protect privacy, to the extent compatible with the transparency needed to protect safety.¶
A viable architecture for PII protection would be symmetric encryption of the PII using a key known to the UAS and a USS service. An authorized Observer may send the encrypted PII along with the Remote ID (to their UAS display service) to get the plaintext. The authorized Observer may send the Remote ID (to their UAS display service) and receive the key to directly decrypt all PII content from the UA.¶
PII is protected unless the UAS is informed otherwise. This may come from operational instructions to even permit flying in a space/time. It may be special instructions at the start or during a mission. PII protection should not be used if the UAS loses connectivity to the USS. The USS always has the option to abort the mission if PII protection is disallowed.¶
An authorized Observer may instruct a UAS via the USS that conditions have changed mandating no PII protection or land the UA.¶
According to EASA, in EU broadcasting drone identification will be mandatory from July 2020. Following info should be sent in plaintext over Wifi or Bluetooth. In real time during the whole duration of the flight, the direct periodic broadcast from the UA using an open and documented transmission protocol, of the following data, in a way that they can be received directly by existing mobile devices within the broadcasting range:¶
i) the UAS operator registration number;¶
ii) the unique physical serial number of the UA compliant with standard ANSI/CTA2063;¶
iii) the geographical position of the UA and its height above the surface or take-off point;¶
iv) the route course measured clockwise from true north and ground speed of the UA; and¶
v) the geographical position of the remote pilot or, if not available, the take-off point;¶
The architecture proposed in this document partially satisfies EASA requirements. In particular, i) is included to Operator-ID Message as optional. ii) cannot be directly supported due to its heavy privacy implications. A cryptographic identifier that needs to be resolved is proposed instead. iii) and iv) are included into Location/Vector Message. v) is included into a System Message (optional).¶
This document does not make any request to IANA.¶
DRIP is all about safety and security, so content pertaining to such is not limited to this section. The security provided by asymmetric cryptographic techniques depends upon protection of the private keys. A manufacturer that embeds a private key in an UA may have retained a copy. A manufacturer whose UA are configured by a closed source application on the GCS which communicates over the Internet with the factory may be sending a copy of a UA or GCS self-generated key back to the factory. Compromise of a registry private key could do widespread harm. Key revocation procedures are as yet to be determined. These risks are in addition to those involving Operator key management practices.¶
The National Aeronautics and Space Administration (NASA) and FAAs' effort of integrating UAS's operation into the national airspace system (NAS) leads to the development of the concept of UTM and the ecosystem around it. The UTM concept was initially presented in 2013. The eventual development and implementation are conducted by the UTM research transition team which is the joint workforce by FAA and NASA. World efforts took place afterward. The Single European Sky ATM Research (SESAR) started the CORUS project to research its UTM counterpart concept, namely [U-Space]. This effort is led by the European Organization for the Safety of Air Navigation (Eurocontrol).¶
Both NASA and SESAR have published the UTM concept of operations to guide the development of their future air traffic management (ATM) system and make sure safe and efficient integrations of manned and unmanned aircraft into the national airspace.¶
The UTM composes of UAS operation infrastructure, procedures and local regulation compliance policies to guarantee UAS's safe integration and operation. The main functionality of a UTM includes, but is not limited to, providing means of communication between UAS operators and service providers and a platform to facilitate communication among UAS service providers.¶
A USS plays an important role to fulfill the key performance indicators (KPIs) that a UTM has to offer. Such Entity acts as a proxy between UAS operators and UTM service providers. It provides services like real-time UAS traffic monitor and planning, aeronautical data archiving, airspace and violation control, interacting with other third-party control entities, etc. A USS can coexist with other USS(s) to build a large service coverage map which can load-balance, relay and share UAS traffic information.¶
The FAA works with UAS industry shareholders and promotes the Low Altitude Authorization and Notification Capability [LANNC] program which is the first implementation to realize UTM's functionality. The LAANC program can automate the UAS's fly plan application and approval process for airspace authorization in real-time by checking against multiple aeronautical databases such as airspace classification and fly rules associated with it, FAA UAS facility map, special use airspace, Notice to airman (NOTAM) and Temporary flight rule (TFR).¶
This section illustrates a couple of use case scenarios where UAS participation in UTM has significant safety improvement.¶
A RID is an application enabler for a UAS to be identified by a UTM/USS or third parties entities such as law enforcement. Many safety and other considerations dictate that UAS be remotely identifiable. CAAs worldwide are mandating UAS RID. The European Union Aviation Safety Agency (EASA) has published [Delegated] and [Implementing] Regulations. The FAA has published a Notice of Proposed Rule Making [NPRM]. CAAs currently promulgate performance-based regulations that do not specify techniques, but rather cite industry consensus technical standards as acceptable means of compliance.¶
3GPP provides UA service in the LTE network since release 15 in published technical specification [TS-36.777]. Start from its release 16, it completed the UAS RID requirement study in [TS-22.825] and proposed use cases in the mobile network and the services that can be offered based on RID and ongoing release 17 specification works on enhanced UAS service requirement and provides the protocol and application architecture support which is applicable for both 4G and 5G network. ATIS's recent report [ATIS-I-0000074] proposes architecture approaches for the 3GPP network to support UAS and one of which is put RID in higher 3GPP protocol stack such as using ASTM remote ID [F3411-19].¶
The work of the FAA's UAS Identification and Tracking (UAS ID) Aviation Rulemaking Committee (ARC) is the foundation of later ASTM and proposed IETF DRIP WG efforts. The work of ASTM F38.02 in balancing the interests of diverse stakeholders is essential to the necessary rapid and widespread deployment of UAS RID. IETF volunteers who have contributed to this draft include Amelia Andersdotter and Mohamed Boucadair.¶