Internet-Draft CS-RID March 2020
Moskowitz, et al. Expires 20 September 2020 [Page]
Intended Status:
Standards Track
R. Moskowitz
HTT Consulting
S. Card
AX Enterprize
A. Wiethuechter
AX Enterprize

Crowd Sourced Remote ID


This document describes using the ASTM Broadcast Remote ID (B-RID) specification in a "crowd sourced" smart phone environment to provide much of the FAA mandated Network Remote ID (N-RID) functionality. This crowd sourced B-RID data will use multi-lateration to add a level of reliability in the location data on the Unmanned Aircraft (UA). The crowd sourced environment will also provide a monitoring coverage map to authorized observers.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 20 September 2020.

Table of Contents

1. Introduction

This document defines a mechanism to capture the ASTM Broadcast Remote ID messages (B-RID) [F3411-19] on any Internet connected device that receives them and can forward them to the SDSP(s) responsible for the geographic area the UA and receivers are in. This will create a ecosystem that will meet most if not all data collection requriments that CAAs are placing on Network Remote ID (N-RID).

These Internet connected devices are herein called "Finders", as they find UAs by listening for B-RID messages. The Finders are B-RID forwarding proxies. Their potentially limited spacial view of RID messages could result in bad decisions on what messages to send to the SDSP and which to drop. The SDSP will make any filtering decisions in what it forwards to the UTM(s).

Finders can be smartphones, tablets, connected cars, or any computing platform with Internet connectivity that can meet the requirements defined in this document. It is not expected, nor necessary, that Finders have any information about a UAS beyond the content in the B-RID messages.

Finders MAY only need a loose association with the SDSP(s). They may only have the SDSP's Public Key and FQDN. It would use these, along with the Finder's Public Key to use ECIES, or other security methods, to send the messages in a secure manner to the SDSP. The SDSP MAY require a stronger relationship to the Finders. This may range from the Finder's Public Key being registered to the SDSP with other information so that the SDSP has some level of trust in the Finders to requiring transmissions be sent over long-lived transport connections like ESP or DTLS.

This document has minimal information about the actions of SDSPs. In general the SDSP is out of scope of this document. That said, the SDSPs should not simply proxy B-RID messages to the UTM(s). They should perform some minimal level of filtering and content checking before forwarding those messages that pass these tests in a secure manner to the UTM(s).

The SDSPs are also capable of maintaining a monitoring map, based on location of active Finders. UTMs may use this information to notify authorized observers of where this is and there is not monitoring coverage. They may also use there information of where to place pro-active monitoring coverage.

An SDSP SHOULD only forward Authenticated B-RID messages like those defined in [tmrid-auth] to the UTM(s). Further, the SDSP SHOULD validate the Remote ID (RID) and the Authentication signature before forwarding anything from the UA.

When 3 or more Finders are reporting to an SDSP on a specific UA, the SDSP is in a unique position to perform multilateration on these messages and compute the Finder's view of the UA location to compare with the UA Location/Vector messages. This check against the UA's location claims is both a validation on the UA's reliability as well as the trustworthiness of the Finders. Other than providing data to allow for multilateration, this SDSP feature is out of scope of this document.

1.1. Draft Status

This draft is still incomplete. New features are being added as capabilities are researched. The actual message formats also still need work.

2. Terms and Definitions

2.1. Requirements Terminology

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

2.2. Definitions

Broadcast Remote ID. A method of sending RID messages as 1-way transmissions from the UA to any Observers within radio range.
Civil Aeronautics Administration. An example is the Federal Aviation Administration (FAA) in the United States of America.
Elliptic Curve Integrated Encryption Scheme. A hybrid encryption scheme which provides semantic security against an adversary who is allowed to use chosen-plaintext and chosen-ciphertext attacks.
Ground Control Station. The part of the UAS that the remote pilot uses to exercise C2 over the UA, whether by remotely exercising UA flight controls to fly the UA, by setting GPS waypoints, or otherwise directing its flight.
In Internet connected device that can receive B-RID messages and forward them to a UTM.
Referred to in other UAS documents as a "user", but there are also other classes of RID users, so we prefer "observer" to denote an individual who has observed an UA and wishes to know something about it, starting with its RID.
Multilateration (more completely, pseudo range multilateration) is a navigation and surveillance technique based on measurement of the times of arrival (TOAs) of energy waves (radio, acoustic, seismic, etc.) having a known propagation speed.
Network RID Service Provider. USS receiving Network RID messages from UAS (UA or GCS), storing for a short specified time, making available to NETDP.
Network RID Display Provider. Entity (might be USS) aggregating data from multiple NETSPs to answer query from observer (or other party) desiring Situational Awareness of UAS operating in a specific airspace volume.
Network Remote ID. A method of sending RID messages via the Internet connection of the UAS directly to the UTM.
Remote ID. A unique identifier found on all UA to be used in communication and in regulation of UA operation.
Supplemental Data Service Provider. Entity providing information that is allowed, but not required to be present in the UTM system.
Unmanned Aircraft. In this document UA's are typically though of as drones of commerical or military variety. This is a very strict definition which can be relaxed to include any and all aircraft that are unmanned.
Unmanned Aircraft System. Composed of Unmanned Aircraft and 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 the control station.
UAS Traffic Management. A "traffic management" ecosystem for uncontrolled operations that is separate from, but complementary to, the FAA's Air Traffic Management (ATM) system.
UAS Service Supplier. Provide UTM 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. (From FAA UTM ConOps V1, May 2018).

3. Problem Space

3.1. Meeting the needs of Network ID

The Federal (US) Aviation Authority (FAA), in the December 31, 2019 Remote ID Notice of Proposed Rulemaking [FAA-NPRM], is requiring "Standard" and "Limited" Remote ID. Standard is when the UAS provides both Network and Broadcast RID. Limited is when the UAS provides only Network RID. The FAA has dropped their previous position on allowing for only Broadcast RID. We can guess as to their reasons; they are not spelled out in the NPRM. It may be that just B-RID does not meet the FAA's statutory UA tracking responsibility.

The UAS vendors have commented that N-RID places considerable demands on currently used UAS. For some UAS like RC planes, meaningful N-RID (via the Pilot's smartphone) are of limited value. A mechanism that can augment B-RID to provide N-RID would help all members of the UAS environment to provide safe operation and allow for new applications.

3.2. Trustworthiness of Proxied Data

When a proxy is introduced in any communication protocol, there is a risk of corrupted data and DOS attacks.

3.3. Defense against fraudulent RID Messages



4. The Finder - SDSP Security Relationship

The SDSP(s) and Finders SHOULD use EDDSA [RFC8032] keys as their trusted Identities. The public keys SHOULD be registered Hierarchical HITS, [hierarchical-hit] and [hhit-registries].

The SDSP uses some process (out of scope here) to register the Finders and their EDDSA Public Key. During this registration, the Finder gets the SDSP's EDDSA Public Key. These Public Keys allow for the following options for authenticated messaging from the Finder to the SDSP.

  1. ECIES can be used with a unique nonce to authenticate each message sent from a Finder to the SDSP.
  2. ECIES can be used at the start of some period (e.g. day) to establish a shared secret that is then used to authenticate each message sent from a Finder to the SDSP sent during that period.
  3. HIPv2 [RFC7401] can be used to establish a session secret that is then used with ESP [RFC4303] to authenticate each message sent from a Finder to the SDSP.
  4. DTLS [RFC5238] can be used to establish a secure connection that is then used to authenticate each message sent from a Finder to the SDSP.

4.1. The Finder Map

The Finders are regularly providing their SDSP with their location. This is through the B-RID Proxy Messages and Finder Location Update Messages. With this information, the SDSP can maintain a monitoring map. That is a map of where there Finder coverage.

4.2. Managing Finders

Finder density will vary over time and space. For example, sidewalks outside an urban train station can be packed with pedestrians at rush hour, either coming or going to their commute trains. An SDSP may want to proactively limit the number of active Finders in such situations.

Using the Finder mapping feature, the SDSP can instruct Finders to NOT proxy B-RID messages. These Finders will continue to report their location and through that reporting, the SDSP can instruct them to again take on the proxying role. For example a Finder moving slowly along with dozens of other slow-moving Finders may be instructed to suspend proxying. Whereas a fast-moving Finder at the same location (perhaps a connected car or a pedestrian on a bus) would not be asked to suspend proxying as it will soon be out of the congested area.

5. The CS-RID Messages

The CS-RID messages between the Finders and the SDSPs primarily support the proxy role of the Finders in forwarding the B-RID messages. There are also Finder registration and status messages.

CS-RID information is represented in CBOR [RFC7049]. COSE [RFC8152] may be used for CS-RID MAC and COAP [RFC7252] for the CS-RID protocol.

The following is a general representation of the content in the CS-RID messages.

         (   CS-RID MESSAGE TYPE,
             CS-RID MAC)




Number   CS-RID Message Type
------   -----------------
0        Reserved
1        B-RID Forwarding
2        Finder Registration
3        SDSP Response
4        Finder Location

5.2. The CS-RID B-RID Proxy Message

The Finders add their own information to the B-RID messages, permitting the SDSP(s) to gain additional knowledge about the UA(s). The RID information is the B-RID message content plus the MAC address. The MAC address is critical, as it is the only field that links a UA's B-RID messages together. Only the ASTM Basic ID Message and possibly the Authentication Message contain the UAS ID field.

The Finders add an SDSP assigned ID, a 64 bit timestamp, GPS information, and type of B-RID media to the B-RID message. Both the timestamp and GPS information are for when the B-RID message(s) were received, not forwarded to the SDSP. All this content is MACed using a key shared between the Finder and SDSP.

The following is a representation of the content in the CS-RID messages.

         (   CS-RID MESSAGE TYPE,
             CS-RID ID,
             RECEIVE GPS,
             RECEIVE RADIO TYPE,
             B-RID MAC ADDRESS,
             B-RID MESSAGE,
             CS-RID MAC)

5.2.1. CS-RID ID

The CS-RID ID is the ID recognized by the SDSP. This may be an HHIT Hierarchical HITs [hierarchical-hit], or any ID used by the SDSP.

5.3. CS-RID Finder Registration

The CS-RID Finder MAY use HIPv2 [RFC7401] with the SDSP to establish a Security Association and a shared secret to use for the CS-RID MAC generation. In this approach, the HIPv2 mobility functionality and ESP [RFC4303] support are not used.

When HIPv2 is used as above, the Finder Registration is a SDSP "wake up". It is sent prior to the Finder sending any proxied B-RID messages to ensure that the SDSP is able to receive and process the messages.

In this usage, the CS-RID is the Finder HIT. If the SDSP has lost state with the Finder, it initiates the HIP exchange with the Finder to reestablish HIP state and a new shared secret for the CS-RID B-RID Proxy Messages. In this case the Finder Registration Message is:

         (   CS-RID MESSAGE TYPE,
             CS-RID ID,
             CS-RID TIMESTAMP,
             CS-RID GPS,
             CS-RID MAC)

5.4. CS-RID SDSP Response

The SDSP MAY respond to any Finder messages to instruct the Finder on its behavior.

         (   CS-RID MESSAGE TYPE,
             SDSP ID,
             CS-RID ID,
             CS-RID PROXY STATUS,
             CS-RID MAC)

The Proxy Status instructs the Finder if it should actively proxy B-RID messages, or suspend proxying and only report its location.

The Update Interval is the frequency that the Finder SHOULD notify the SDSP of its current location using the Location Update message.

5.5. CS-RID Location Update

The Finder SHOULD provide regular location updates to the SDSP. The interval is based on the Update Interval from Section 5.4 plus a random slew less than 1 second. The Location Update message is only sent when no other CS-RID messages, containing the Finder's GPS location, have been sent since the Update Interval.

If the Finder has not recieved a SDSP Registration Response, a default of 5 minutes is used for the Update Interval.

         (   CS-RID MESSAGE TYPE,
             CS-RID ID,
             CS-RID TIMESTAMP,
             CS-RID GPS,
             CS-RID MAC)

6. IANA Considerations


7. Security Considerations


7.1. Privacy Concerns


8. Acknowledgments

The Crowd Sourcing idea in this document came from the Apple "Find My Device" presentation at the International Association for Cryptographic Research's Real World Crypto 2020 conference.

9. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

10. Informative References

ASTM International, "Standard Specification for Remote ID and Tracking", , <>.
Federal (US) Aviation Authority, "FAA Remote ID Notice of Proposed Rule Making", , <>.
Moskowitz, R., Card, S., and A. Wiethuechter, "Hierarchical HIT Registries", Work in Progress, Internet-Draft, draft-moskowitz-hip-hhit-registries-02, , <>.
Moskowitz, R., Card, S., and A. Wiethuechter, "Hierarchical HITs for HIPv2", Work in Progress, Internet-Draft, draft-moskowitz-hip-hierarchical-hit-04, , <>.
Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, , <>.
Phelan, T., "Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)", RFC 5238, DOI 10.17487/RFC5238, , <>.
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, , <>.
Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, , <>.
Moskowitz, R., Ed., Heer, T., Jokela, P., and T. Henderson, "Host Identity Protocol Version 2 (HIPv2)", RFC 7401, DOI 10.17487/RFC7401, , <>.
Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, , <>.
Wiethuechter, A., Card, S., and R. Moskowitz, "TM-RID Authentication Formats", Work in Progress, Internet-Draft, draft-wiethuechter-tmrid-auth-05, , <>.

Appendix A. Using LIDAR for UA location

If the Finder has LIDAR or similar detection equipment (e.g. on a connected car) that has full sky coverage, the Finder can use this equipment to locate UAs in its airspace. The Finder would then be able to detect non-participating UAs.

This would provide valuable information to SDSPs to forward to UTMs on potential at-risk situations.

At this time, research on LIDAR and other detection technology is needed. Would more than UA location information be available? What information can be sent in a CS-RID message for such "unmarked" UAs?

Authors' Addresses

Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Stuart W. Card
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Adam Wiethuechter
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America