DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote ID
draft-ietf-drip-auth-30
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| Authors | Adam Wiethuechter , Stuart W. Card , Robert Moskowitz | ||
| Last updated | 2023-08-29 (Latest revision 2023-03-27) | ||
| Replaces | draft-wiethuechter-drip-auth | ||
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draft-ietf-drip-auth-30
DRIP Working Group A. Wiethuechter (Editor)
Internet-Draft S. Card
Intended status: Standards Track AX Enterprize, LLC
Expires: 29 September 2023 R. Moskowitz
HTT Consulting
28 March 2023
DRIP Entity Tag Authentication Formats & Protocols for Broadcast Remote
ID
draft-ietf-drip-auth-30
Abstract
This document describes how to add trust into the Broadcast Remote ID
(RID) specification discussed in the DRIP Architecture; first trust
in the RID ownership and second in the source of the RID messages.
The document defines message types and associated formats (sent
within the Authentication Message) that can be used to authenticate
past messages sent by an unmanned aircraft (UA) and provide proof of
UA trustworthiness even in the absence of Internet connectivity at
the receiving node.
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 https://datatracker.ietf.org/drafts/current/.
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 29 September 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. DET Authentication Goals for Broadcast RID . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Required Terminology . . . . . . . . . . . . . . . . . . 4
2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 5
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Reasoning for IETF DRIP Authentication . . . . . . . . . 7
3.1.1. UA Signed Evidence . . . . . . . . . . . . . . . . . 7
3.1.2. DIME Endorsements of Subordinate DET . . . . . . . . 8
3.1.3. DIME Hierarchy Endorsements . . . . . . . . . . . . . 8
3.1.4. UAS RID Trust . . . . . . . . . . . . . . . . . . . . 8
3.2. ASTM Authentication Message . . . . . . . . . . . . . . . 8
3.2.1. Authentication Page . . . . . . . . . . . . . . . . . 8
3.2.2. Authentication Payload Field . . . . . . . . . . . . 9
3.2.3. Specific Authentication Method . . . . . . . . . . . 10
3.2.4. ASTM Broadcast RID Constraints . . . . . . . . . . . 12
4. DRIP Authentication Formats . . . . . . . . . . . . . . . . . 12
4.1. Endorsement Structure for UA Signed Evidence . . . . . . 13
4.2. DRIP Link . . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . . . 17
4.3.1. Wrapped Count & Sanity Check . . . . . . . . . . . . 17
4.3.2. Wrapper over Extended Transports . . . . . . . . . . 18
4.3.3. Wrapper Limitations . . . . . . . . . . . . . . . . . 19
4.4. DRIP Manifest . . . . . . . . . . . . . . . . . . . . . . 19
4.4.1. Hash Count & Sanity Check . . . . . . . . . . . . . . 20
4.4.2. Manifest Ledger Hashes . . . . . . . . . . . . . . . 20
4.4.3. Hash Algorithms and Operation . . . . . . . . . . . . 21
4.5. DRIP Frame . . . . . . . . . . . . . . . . . . . . . . . 21
4.5.1. Frame Type . . . . . . . . . . . . . . . . . . . . . 22
5. Forward Error Correction . . . . . . . . . . . . . . . . . . 22
5.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 23
5.2. Decoding . . . . . . . . . . . . . . . . . . . . . . . . 24
5.3. FEC Limitations . . . . . . . . . . . . . . . . . . . . . 27
6. Requirements & Recommendations . . . . . . . . . . . . . . . 27
6.1. Legacy Transports . . . . . . . . . . . . . . . . . . . . 27
6.2. Extended Transports . . . . . . . . . . . . . . . . . . . 27
6.3. Authentication . . . . . . . . . . . . . . . . . . . . . 27
6.4. Operational . . . . . . . . . . . . . . . . . . . . . . . 28
6.4.1. DRIP Wrapper . . . . . . . . . . . . . . . . . . . . 29
6.4.2. UAS RID Trust Assessment . . . . . . . . . . . . . . 29
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7. Summary of Addressed DRIP Requirements . . . . . . . . . . . 30
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
8.1. IANA DRIP Registry . . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9.1. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 31
9.2. VNA Timestamp Offsets for DRIP Authentication Formats . . 31
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.1. Normative References . . . . . . . . . . . . . . . . . . 32
11.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Authentication State Diagrams & Color Scheme . . . . 33
A.1. State Colors . . . . . . . . . . . . . . . . . . . . . . 34
A.2. State Diagrams . . . . . . . . . . . . . . . . . . . . . 34
A.2.1. Notations . . . . . . . . . . . . . . . . . . . . . . 34
A.2.2. General . . . . . . . . . . . . . . . . . . . . . . . 35
A.2.3. DRIP SAM . . . . . . . . . . . . . . . . . . . . . . 36
A.2.4. DRIP Link . . . . . . . . . . . . . . . . . . . . . . 37
A.2.5. DRIP Wrapper/Manifest/Frame . . . . . . . . . . . . . 38
Appendix B. Example TX/RX Flow . . . . . . . . . . . . . . . . . 40
Appendix C. Additional FEC Decoding Heuristic . . . . . . . . . 41
Appendix D. Operational Recommendation Analysis . . . . . . . . 43
D.1. Methodology . . . . . . . . . . . . . . . . . . . . . . . 43
D.2. ASTM Maximum Schedule Example . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
1. Introduction
The initial regulations (e.g., [FAA-14CFR]) and standards (e.g.,
[F3411]) for Unmanned Aircraft (UA) Systems (UAS) Remote
Identification and tracking (RID) do not address trust. However,
this is a requirement that needs to be addressed for various
different parties that have a stake in the safe operation of National
Airspace Systems (NAS). DRIP's goal as stated in the WG charter is:
to specify how RID can be made trustworthy and available in both
Internet and local-only connected scenarios, especially in
emergency situations.
UAS often operate in a volatile environment. Small UA offer little
capacity for computation and communication. UAS RID must also be
accessible with ubiquitous and inexpensive devices without
modification. This limits options.
Generally two communication schemes for UAS RID are considered:
Broadcast and Network. This document focuses on adding trust to
Broadcast RID (Section 3.2 of [RFC9153]).
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Without authentication, an Observer has no basis for trust. As the
messages are sent via wireless broadcast, they may be transmitted
anywhere within the wireless range and making any claims desired by
the sender.
1.1. DET Authentication Goals for Broadcast RID
DRIP Specific Authentication Methods, carried in ASTM Authentication
Messages (Message Type 0x2) are defined herein. These methods, when
properly used, enable a high level of trust that the content of other
ASTM Messages was generated by their claimed registered source.
These messages are designed to provide the Observers with immediately
actionable information.
This authentication approach also provides some error correction
(Section 5) as mandated by the United States (US) Federal Aviation
Administration (FAA) [FAA-14CFR], which is missing from [F3411] over
Legacy Transports (Bluetooth 4.x).
These DRIP enhancements to [F3411] further support the important use
case of Observers who are sometimes offline at the time of
observation.
A summary of DRIP requirements [RFC9153] addressed herein is provided
in Section 7.
Note: The Endorsement (used in Section 4.2) that proves that a DET
is registered MUST come from its immediate parent in the
registration hierarchy, e.g. a DRIP Identity Management Entity
(DIME) [drip-registries]. In the definitive hierarchy the parent
of the UA is its HDA, the parent of an HDA is its RAA, etc. It is
also assumed that DRIP aware entities all use a DET as their
identifier during interactions with other DRIP aware entities.
2. Terminology
2.1. Required 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.
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2.2. Definitions
This document makes use of the terms (Observer, USS, UTM, etc.)
defined in [RFC9153]. Other terms (such as DIME) are from
[drip-arch], while others (DET, RAA, HDA, etc.) are from [RFC9374].
In addition, the following terms are defined for this document:
ASTM Message (25-bytes):
Full ASTM Message as defined in [F3411]; specifically Message
Types 0x0, 0x1, 0x3, 0x4, and 0x5
ASTM Message Hash (8-bytes):
Hash of a single full ASTM Message using hash operations described
in (Section 4.4.3). Multiple hashes MUST be in Message Type
order.
Broadcast Endorsement (136-bytes):
A class of Endorsement under DRIP which is carried by the Link
Message Section 4.2. They are generated by a DIME during the
registration of a subordinate DET-based entity.
Current Manifest Hash (8-bytes):
Hash of the current Manifest Message (Section 4.4). See
Section 4.4.2.
Evidence (0 to 112 bytes):
Opaque evidence data that the UA is endorsing during its flight in
Figure 4.
Extended Transports:
use of extended advertisements (Bluetooth 5.x), service info (Wi-
Fi NAN) or vendor specific element information (Wi-Fi BEACON) in
broadcast frames as specified in [F3411]. Must use ASTM Message
Pack (Message Type 0xF).
Frame Type (1-byte):
Sub-type for future different DRIP Frame formats. See
Section 4.5.1.
Legacy Transports:
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use of broadcast frames (Bluetooth 4.x) as specified in [F3411].
Previous Manifest Hash (8-bytes):
Hash of the previously sent Manifest Message (Section 4.4). See
Section 4.4.2.
UA DRIP Entity Tag (DET) (16-bytes):
The UA DET [RFC9374] in byte form (network byte order) and is part
of Figure 4.
UA Signature (64-bytes):
Signature over all 4 preceding fields of Figure 4 using the HI of
the UA.
Valid Not After (VNA) Timestamp by UA (4-bytes):
Timestamp denoting recommended time to stop trusting data in
Figure 4. MUST follow the format defined in [F3411]. That is a
Unix-style timestamp but with an epoch of 01/01/2019 00:00:00 with
an additional offset is then added to push a short time into the
future (relative to Not Before Timestamp) to avoid replay attacks.
The offset used against the Unix-style timestamp is not defined in
this document. Best practice identifying an acceptable offset
should be used taking into consideration the UA environment, and
propagation characteristics of the messages being sent and clock
differences between the UA and Observers. A reasonable time would
be to set Not After Timestamp 2 minutes after Not Before
Timestamp.
Valid Not Before (VNB) Timestamp by UA (4-bytes):
Timestamp denoting recommended time to start trusting data in
Figure 4. MUST follow the format defined in [F3411]. That is a
Unix-style timestamp but with an epoch of 01/01/2019 00:00:00.
MUST be set no earlier than the time the signature is generated.
3. Background
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3.1. Reasoning for IETF DRIP Authentication
[F3411] defines Authentication Message framing only. It does not
define authentication formats or methods. It explicitly anticipates
several signature options, but does not fully define those. Annex A1
of [F3411] defines a Broadcast Authentication Verifier Service, which
has a heavy reliance on Observer real-time connectivity to the
Internet. Fortunately, [F3411] also allows third party standard
Authentication Types, several of which DRIP defines herein.
The standardization of specific formats to support the DRIP
requirements in UAS RID for trustworthy communications over Broadcast
RID is an important part of the chain of trust for a UAS ID. Per
Section 5 of [drip-arch], there is a need to have Authentication
formats to relay information for Observers to determine trust. No
existing formats (defined in [F3411] or other organizations
leveraging this feature) provide the functionality to satisfy this
goal resulting in the work reflected in this document.
3.1.1. UA Signed Evidence
When an Observer receives a DRIP-based Authentication Message
(Section 4.3, Section 4.4, or Section 4.5) containing UA-signed
Evidence (in an Endorsement structure Section 4.1) it SHOULD validate
the signature using the HI corresponding to the UA's DRIP Entity Tag
(DET).
The UA's HI SHOULD be retrieved from DNS. If not available it may
have been revoked. Note that accurate revocation status is a DIME
inquiry; DNS non-response is a hint to the DET being expired or
revoked. It MAY be retrieved from a local cache, if present. The
local cache is typically populated by DNS lookups and/or by received
Broadcast Endorsements (Section 3.1.2).
Once the Observer has the registered UA's DET and HI, all further (or
cached previous) DRIP-based Authentication Messages using the UA DET
can be validated. Signed content, tied to the DET, can now be
trusted to have been signed by the holder of the private key
corresponding to the DET.
Whether the content is true is a separate question which DRIP cannot
address but sanity checks (Section 6) are possible.
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3.1.2. DIME Endorsements of Subordinate DET
When an Observer receives a DRIP Link Authentication Message
(Section 4.2) containing an Endorsement by the DIME of the a child
DET registration, it SHOULD validate the signature using the HI
corresponding to the DIME's DET.
The DIME's HI, SHOULD be retrieved from from DNS (Section 5,
[drip-registries]), when available. It MAY be cached from a prior
DNS lookup or it may be stored in a distinct local store.
3.1.3. DIME Hierarchy Endorsements
An Observer can receive a series of DRIP Link Authentication Messages
(Section 4.2), each one pertaining to a DIME's registration in the
DIME above it in the hierarchy. Similar to Section 3.1.2, each link
in this chain SHOULD be validated.
3.1.4. UAS RID Trust
Section 3.1.1, Section 3.1.2, and Section 3.1.3 complete the trust
chain but the chain cannot yet be trusted as having any relevance to
the observed UA because replay attacks are trivial. At this point
the key nominally possessed by the UA is trusted but the UA has not
yet been proven to possess that private key.
It is necessary for the UA to prove possession by dynamically signing
data that is unique and unpredictable but easily verified by the
Observer. This can be in the form of a DRIP Wrapper or Manifest
(Section 4.3, Section 4.4) containing at least one ASTM Vector/
Location Message and/or System Message (which contains a timestamp).
Verification of this signed data MUST be performed by the Observer as
part of the received UAS RID information trust assessment
(Section 6.4.2).
3.2. ASTM Authentication Message
The ASTM Authentication Message (Message Type 0x2) is a unique
message in the Broadcast [F3411] standard as it is the only one that
is larger than the Bluetooth 4.x frame size. To address this, it is
defined as a set of "pages" that each fits into a single Bluetooth
4.x broadcast frame. For other media these pages are still used but
all in a single frame.
3.2.1. Authentication Page
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| |
| |
| Authentication Payload |
| |
| |
+---------------+---------------+---------------+---------------+
Page Header: (1 byte)
Authentication Type (4 bits)
Page Number (4 bits)
Authentication Payload: (23 bytes per page)
Authentication Payload, including headers. Null padded.
Figure 1: Standard ASTM Authentication Message Page
This document leverages Authentication Type 0x5, Specific
Authentication Method (SAM), as the principal authentication
container, defining a set of SAM Types in Section 4. This is denoted
in every Authentication Page in the Page Header. The SAM Type is
denoted as a field in the Authentication Payload (see
Section 3.2.3.1).
The Authentication Message is structured as a set of pages Figure 1.
There is a technical maximum of 16 pages (indexed 0 to 15 in the Page
Header) that can be sent for a single Authentication Message, with
each page carrying a maximum 23-byte Authentication Payload. See
Section 3.2.4 for more details. Over Bluetooth 4.x, these messages
are "fragmented", with each page sent in a separate Bluetooth 4.x
broadcast frame.
Either as a single Authentication Message or a set of fragmented
Authentication Message Pages the structure is further wrapped by
outer ASTM framing and the specific link framing (Bluetooth or Wi-
Fi).
3.2.2. Authentication Payload Field
Figure 2 is the source data view of the data fields found in the
Authentication Message as defined by [F3411]. This data is placed
into Figure 1's Authentication Payload, spanning multiple pages.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Authentication Headers |
| +---------------+---------------+
| | |
+---------------+---------------+ |
. .
. Authentication Data / Signature .
. .
| |
+---------------+---------------+---------------+---------------+
| ADL | |
+---------------+ |
. .
. Additional Data .
. .
| |
+---------------+---------------+---------------+---------------+
Authentication Headers: (6-bytes)
As defined in F3411.
Authentication Data / Signature: (255-bytes max)
Opaque authentication data.
Additional Data Length (ADL): (1-byte - unsigned)
Length in bytes of Additional Data.
Additional Data: (255-bytes max):
Data that follows the Authentication Data / Signature but
is not considered part of the Authentication Data.
Figure 2: ASTM Authentication Message Fields
When Additional Data is being sent, a single unsigned byte
(Additional Data Length) directly follows the Authentication Data /
Signature and has the length, in bytes, of the following Additional
Data. For DRIP, this field is used to carry Forward Error Correction
as defined in Section 5.
3.2.3. Specific Authentication Method
3.2.3.1. SAM Data Format
Figure 3 is the general format to hold authentication data when using
SAM and is placed inside the Authentication Data/Signature field in
Figure 2.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| SAM Type | |
+---------------+ |
. .
. SAM Authentication Data .
. .
| |
+---------------+---------------+---------------+---------------+
SAM Type (1 byte):
Byte defined by F3411 to multiplex SAMs
SAM Authentication Data (0 to 200 bytes):
Authentication data (opaque to baseline F3411 but parsed by DRIP).
Figure 3: SAM Data Format
3.2.3.1.1. SAM Type
For DRIP the following SAM Types are allocated:
+==========+=============================+
| SAM Type | Description |
+==========+=============================+
| TBD1 | DRIP Link (Section 4.2) |
+----------+-----------------------------+
| TBD2 | DRIP Wrapper (Section 4.3) |
+----------+-----------------------------+
| TBD3 | DRIP Manifest (Section 4.4) |
+----------+-----------------------------+
| TBD4 | DRIP Frame (Section 4.5) |
+----------+-----------------------------+
Table 1
Note: ASTM International is the owner of these code points as they
are defined in [F3411]. In [F3411], ASTM is responsible for
selecting a registrar to manage allocations of these code points.
At the time of publication this registrar is still TBD.
3.2.3.1.2. SAM Authentication Data
This field has a maximum size of 200-bytes, as defined by
Section 3.2.4.
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3.2.4. ASTM Broadcast RID Constraints
3.2.4.1. Wireless Frame Constraints
A UA has the option of broadcasting using Bluetooth (4.x and 5.x) or
Wi-Fi (BEACON or NAN), see Section 6. With Bluetooth, FAA and other
Civil Aviation Authorities (CAA) mandate transmitting simultaneously
over both 4.x and 5.x. With Wi-Fi, use of BEACON is recommended.
Wi-Fi NAN is another option, depending on the CAA. The same
application layer information defined in [F3411] MUST be transmitted
over all the physical layer interfaces performing the function of
RID.
Bluetooth 4.x presents a payload size challenge in that it can only
transmit 25-bytes of payload per frame where the others all can
support larger payloads per frame. However, the [F3411] messaging
framing dictated by Bluetooth 4.x constraints is inherited by [F3411]
over other media.
3.2.4.2. Paged Authentication Message Constraints
To keep consistent formatting across the different transports (Legacy
and Extended) and their independent restrictions, the authentication
data being sent is REQUIRED to fit within the page limit that the
most constrained existing transport can support. Under Broadcast RID
the Extended Transport that can hold the least amount of
authentication data is Bluetooth 5.x at 9 pages.
As such DRIP transmitters are REQUIRED to adhere to the following
when using the Authentication Message:
1. Authentication Data / Signature data MUST fit in the first 9
pages (Page Numbers 0 through 8).
2. The Length field in the Authentication Headers (which denotes the
length in bytes of Authentication Data / Signature only) MUST NOT
exceed the value of 201. This includes the SAM Type but excludes
Additional Data such as FEC.
4. DRIP Authentication Formats
All formats defined in this section are the content for the
Authentication Data/Signature field in Figure 2 and use the Specific
Authentication Method (SAM, Authentication Type 0x5). The first byte
of the Authentication Data / Signature of Figure 2, is used to
multiplex between these various formats.
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When sending data over a medium that does not have underlying Forward
Error Correction (FEC), for example Bluetooth 4.x, then Section 5
MUST be used. Appendix A gives a high-level overview of a state
machine for decoding and determining a trustworthiness state.
Appendix B shows an example of using the formats defined in this
section.
4.1. Endorsement Structure for UA Signed Evidence
The Endorsement Structure for UA Signed Evidence (Figure 4) is used
by the UA during flight to sign over information elements using the
private key associated with the current UA DET. It is encapsulated
by the SAM Authentication Data field of Figure 3.
This structure is used by the DRIP Wrapper (Section 4.3), Manifest
Section 4.4, and Frame (Section 4.5). DRIP Link (Section 4.2) MUST
NOT use it as it will not fit in the ASTM Authentication Message with
its intended content (i.e. a Broadcast Endorsement).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| VNB Timestamp by UA |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
. .
. Evidence .
. .
| |
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 4: Endorsement Structure for UA Signed Evidence
UA DRIP Entity Tag:
This is the current DET [RFC9374] being used by the UA.
Evidence:
The evidence section MUST be filled in with data in the form of an
opaque object specified in the DRIP Wrapper, Manifest, or Frame
sections.
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UA Signature:
The UA private key MUST be used over all preceding fields to
generate the signature.
When using this structure, the UA is minimally self-endorsing its
DET. The HI of the UA DET can be looked up by mechanisms described
in [drip-registries] or by extracting it from a Broadcast Endorsement
(see Section 4.2 and Section 6.3).
4.2. DRIP Link
This SAM Type is used to transmit Broadcast Endorsements. For
example, the Broadcast Endorsement: HDA, UA is sent (see Section 6.3)
as a DRIP Link message.
DRIP Link is important as its contents are used to provide trust in
the DET/HI pair that the UA is currently broadcasting. This message
does not require Internet connectivity to perform signature
validations of the contents when the DIME DET/HI is in the receiver's
cache. It also provides the UA HI, when it is a Broadcast
Endorsement: HDA, UA, so that connectivity is not required when
performing validation of other DRIP Authentication Messages.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| VNB Timestamp by Parent |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by Parent |
+---------------+---------------+---------------+---------------+
| |
| DET |
| of Child |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| HI of Child |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| |
| DET |
| of Parent |
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| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Signature by Parent |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
VNB Timestamp by Parent (4-bytes):
Current time at signing, set by Parent Entity.
VNA Timestamp by Parent (4-bytes):
Timestamp denoting recommended time to trust Endorsement.
DET of Child: (16-bytes)
DRIP Entity Tag of Child Entity.
HI of Child: (32-bytes)
Host Identity of Child Entity.
DET of Parent: (16-bytes)
DRIP Entity Tag of Parent Entity.
Signature by Parent(64-bytes):
Signature over preceding fields using the keypair of
the Parent DET.
Figure 5: Broadcast Endorsement / DRIP Link
This DRIP Authentication Message is used in conjunction with other
DRIP SAM Types (such as the Manifest or the Wrapper) that contain
data (e.g., the ASTM Location/Vector Message, Message Type 0x2) that
is guaranteed to be unique, unpredictable and easily cross checked by
the receiving device. The hash of such a message SHOULD merely be
included in a DRIP Manifest, but an entire such message MAY be
encapsulated in a DRIP Wrapper periodically for stronger security.
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4.3. DRIP Wrapper
This SAM Type is used to wrap and sign over a list of other [F3411]
Broadcast RID messages.
The evidence section of the Endorsement Structure for UA Signed
Evidence (Section 4.1) is populated with full (25-byte) [F3411]
Broadcast RID messages. The ASTM Messages can be concatenated
together to form a contiguous byte sequence as shown in Figure 6.
The minimum number of messages support is 1 and the maximum supported
is 4. The messages MUST be in Message Type order as defined by
[F3411]. All message types except Authentication (Message Type 0x2)
and Message Pack (Message Type 0xF) are allowed. Thus it may be
preferred in some operation modes to use a DRIP Manifest Section 4.4
instead.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| |
. .
. ASTM Message(s) .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 6: DRIP Wrapper Evidence
4.3.1. Wrapped Count & Sanity Check
When decoding a DRIP Wrapper on a receiver, a calculation of the
number of messages wrapped and a sanity check can be performed by
using the number of bytes (defined as wrapperLength) between the UA
DET and the VNB Timestamp by UA such as in Figure 7.
if (wrapperLength MOD 25) != 0 {
return DECODE_FAILURE
}
wrappedCount = wrapperLength / 25;
Figure 7: Pseudo-code for Wrapper sanity check and number of messages
calculation
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4.3.2. Wrapper over Extended Transports
To send the DRIP Wrapper over Extended Transports the messages being
wrapped are co-located with the Authentication Message in a ATM
Message Pack (Message Type 0xF). The evidence section of the
Endorsement Structure for UA Signed Evidence is cleared after signing
leaving the following binary structure that is placed into the SAM
Authentication Data of Figure 3 and sent in the same Message Pack.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| VNB Timestamp by UA |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by UA |
+---------------+---------------+---------------+---------------+
| |
| UA |
| DRIP Entity Tag |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| UA Signature |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 8: DRIP Wrapper over Extended Transports
To verify the signature the receiver must concatenate all the
messages in the Message Pack (excluding Authentication Message found
in the same Message Pack) in Message Type order and set the evidence
section of the Endorsement Structure for UA Signed Evidence before
performing signature verification.
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The functionality of a Wrapper in this form is identical to Message
Set Signature (Authentication Type 0x3) when running over Extended
Transports. What the Wrapper provides is the same format but over
both Extended and Legacy Transports allowing the transports to be
similar. Message Set Signature also implies using the ASTM validator
system architecture which relies on Internet connectivity for
verification which the receiver may not have at the time of receipt
of an Authentication Message. This is something the Wrapper, and all
DRIP Authentication Formats, avoid when the UA key is obtained via a
DRIP Link Authentication Message.
4.3.3. Wrapper Limitations
The primary limitation of the Wrapper is the bounding of up to 4 ASTM
Messages that can be sent within it. Another limitation is that the
format can not be used as a surrogate for messages it is wrapping.
This is due to high potential a receiver on the ground does not
support DRIP. Thus, when a Wrapper is being used the wrapped data
must effectively be sent twice, once as a single framed message (as
specified in [F3411]) and then again within the Wrapper.
4.4. DRIP Manifest
This SAM Type is used to create message manifests that contain hashes
of previously sent ASTM Messages.
By hashing previously sent messages and signing them we gain trust in
a UA's previous reports without retransmitting them. An Observer who
has been listening for any length of time SHOULD hash received
messages and cross-check them against the Manifest hashes. This is a
way to evade the limitation of a maximum of 4 messages in the Wrapper
(Section 4.3.3) and greatly reduce overhead.
Judicious use of a Manifest enables an entire Broadcast RID message
stream to be strongly authenticated with less than 100% overhead
relative to a completely unauthenticated message stream (see
Appendix D).
The evidence section of the Endorsement Structure for UA Signed
Evidence (Section 4.1) is populated with 8-byte hashes of [F3411]
Broadcast RID messages (from 2 to 11) and two special hashes
(Section 4.4.2). All these hashes can be concatenated to form a
contiguous byte sequence in the evidence section. The Previous
Manifest Hash and Current Manifest Hash MUST always come before the
ASTM Message Hashes as seen in Figure 9.
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A receiver SHOULD use the Manifest to verify each ASTM Message hashed
therein that it has previously received. It can do this without
having received them all. A Manifest SHOULD typically encompass a
single transmission cycle of messages being sent, see Section 6.4 and
Appendix D.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Previous Manifest |
| Hash |
+---------------+---------------+---------------+---------------+
| Current Manifest |
| Hash |
+---------------+---------------+---------------+---------------+
| |
. .
. ASTM Message Hashes .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 9: DRIP Manifest Evidence Structure
4.4.1. Hash Count & Sanity Check
When decoding a DRIP Manifest on a receiver, a calculation of the
number of hashes and a sanity check can be performed by using the
number of bytes (defined as manifestLength) between the UA DET and
the VNB Timestamp by UA such as in Figure 10.
hashLength = 8
if (manifestLength MOD hashLength) != 0 {
return DECODE_FAILURE
}
hashCount = (manifestLength / hashLength) - 2;
Figure 10: Pseudo-code for Manifest sanity check and number of hashes
calculation
4.4.2. Manifest Ledger Hashes
Two special hashes are included in all Manifests; the Previous
Manifest Hash, which links to the previous Manifest, as well as the
Current Manifest Hash. These hashes act as a ledger to provenance to
the Manifest that could be traced back if the Observer was present
for extended periods of time.
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4.4.3. Hash Algorithms and Operation
The hash algorithm used for the Manifest is the same hash algorithm
used in creation of the DET [RFC9374] that is signing the Manifest.
DET's using cSHAKE128 [NIST.SP.800-185] compute the hash as follows:
cSHAKE128(ASTM Message, 8, "", "Remote ID Auth Hash")
Informative Note: [RFC9374] specifies cSHAKE128 but is open for
the expansion of other OGAs.
When building the list of hashes the Previous Manifest Hash is known
from the previous Manifest. For the first built Manifest this value
is null filled. The Current Manifest Hash is null filled while ASTM
Messages are hashed and fill the ASTM Messages Hashes section. When
all messages are hashed the Current Manifest Hash is computed over
the Previous Manifest Hash, Current Manifest Hash (null filled) and
ASTM Messages Hashes. This hash value replaces the null filled
Current Manifest Hash and becomes the Previous Manifest Hash for the
next Manifest.
4.4.3.1. Legacy Transport Hashing
Under this transport DRIP hashes the full ASTM Message being sent
over the Bluetooth Advertising frame. For paged ASTM Messages
(currently only Authentication Messages) all the pages are
concatenated together and hashed as one object. For all other
Message Types each individual 25-byte message is hashed.
4.4.3.2. Extended Transport Hashing
Under this transport DRIP hashes the full ASTM Message Pack (Message
Type 0xF) - regardless of its content.
4.5. DRIP Frame
This SAM Type is for when the authentication data does not fit in
other defined formats under DRIP and is reserved for future expansion
under DRIP if required.
The population of the evidence section of the Endorsement Structure
for UA Signed Evidence (Section 4.1) is not defined in this document
and MUST be openly specified by the implementation (or specification)
using it.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Frame Type | |
+---------------+ .
. Frame Evidence Data .
. .
| |
+---------------+---------------+---------------+---------------+
Figure 11: DRIP Frame
4.5.1. Frame Type
Byte to sub-type for future different DRIP Frame formats. It takes
the first byte in Figure 11 leaving 111-bytes available for Frame
Evidence Data.
+============+==============+==================+
| Frame Type | Name | Description |
+============+==============+==================+
| 0x00 | Reserved | Reserved |
+------------+--------------+------------------+
| 0xC0-0xFF | Experimental | Experimental Use |
+------------+--------------+------------------+
Table 2
5. Forward Error Correction
For Broadcast RID, Forward Error Correction (FEC) is provided by the
lower layers in Extended Transports (Bluetooth 5.x, Wi-Fi NAN, and
Wi-Fi BEACON). The Bluetooth 4.x Legacy Transport does not have
supporting FEC so with DRIP Authentication the following application
level FEC scheme is used to add FEC. When sending data over a medium
that does not have underlying FEC, for example Bluetooth 4.x, then
this section MUST be used.
The Bluetooth 4.x lower layers have error detection but not
correction. Any frame in which Bluetooth detects an error is dropped
and not delivered to higher layers (in our case, DRIP). Thus it can
be treated as an erasure.
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DRIP standardizes a single page FEC scheme using XOR parity across
all page data of an Authentication Message. This allows the
correction of single erased page in an Authentication Message. Other
FEC schemes, to protect more than a single page of an Authentication
Message or multiple [F3411] Messages, is left for future
standardization if operational experience proves it necessary and/or
practical.
The data added during FEC is not included in the Authentication Data
/ Signature but instead in the Additional Data field of Figure 2.
This may cause the Authentication Message to exceed 9-pages, up to a
maximum of 16-pages.
5.1. Encoding
When encoding two things are REQUIRED:
1. The FEC data MUST start on a new Authentication Page. To do this
the results of parity encoding MUST be placed in the Additional
Data field of Figure 2 with null padding before it to line up
with the next page. The Additional Data Length field MUST be set
to number of padding bytes + number of parity bytes.
2. The Last Page Index field (in Page 0) MUST be incremented from
what it would have been without FEC by the number of pages
required for the Additional Data Length field, null padding and
FEC.
To generate the parity a simple XOR operation using the previous
parity page and current page is used. Only the 23-byte
Authentication Payload field of Figure 1 is used in the XOR
operations. For Page 0, a 23-byte null pad is used for the previous
parity page.
Figure 12 shows an example of the last two pages (out of N) of an
Authentication Message using DRIP Single Page FEC. The Additional
Data Length is set to 33 as there are always 23-bytes of FEC data and
in this example 10-bytes of padding to line it up into Page N.
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Page N-1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| Authentication Data / Signature |
| |
| +---------------+---------------+---------------+
| | ADL=33 | |
+---------------+---------------+ |
| Null Padding |
| |
+---------------+---------------+---------------+---------------+
Page N:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ |
| |
| Forward Error Correction |
| |
| |
| |
+---------------+---------------+---------------+---------------+
Figure 12: Example Single Page FEC Encoding
5.2. Decoding
To determine if FEC has been used a simple check of the Last Page
Index can be used. In general if the Last Page Index field is one
greater than that necessary to hold Length bytes of Authentication
Data then FEC has been used. Note however that if Length bytes was
exhausted exactly at the end of an Authentication Page then the
Additional Data Length will occupy the first byte of the following
page the remainder of which under DRIP will be null padded: in this
case the Last Page Index will have been incremented by one more for
FEC.
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To decode FEC in DRIP a rolling XOR is used on each Authentication
Page received in the current Authentication Message. A Message
Counter, outside of the ASTM Message but specified in [F3411] is used
to signal a different Authentication Message and to correlate pages
to messages. This Message Counter is only 1-byte in length, so it
will roll over (to 0x00) after reaching its maximum value (0xFF). If
only 1-page is missing in the Authentication Message the resulting
parity bytes should be the data of the erased page.
Authentication Page 0 contains various important fields, only located
on that page, that help decode the full ASTM Authentication Message.
If Page 0 has been reconstructed the Last Page Index and Length
fields are REQUIRED to be sanity checked by DRIP. The pseudo-code in
Figure 13 can be used for both checks.
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function decode_check(auth_pages[], decoded_lpi, decoded_length) {
// check decoded Last Page Index (LPI) does not exceed maximum LPI
if (decoded_lpi >= 16) {
return DECODE_FAILURE
}
// check that decoded length does not exceed DRIP maximum
if (decoded_length > 201) {
return DECODE_FAILURE
}
// grab the page at index where length ends and extract its data
auth_data = auth_pages[(decoded_length - 17) / 23].data
// find the index of last auth byte
last_auth_byte = (17 + (23 * last_auth_page)) - decoded_length
// look for non-nulls after the last auth byte
if (auth_data[(last_auth_byte + 2):] has non-nulls) {
return DECODE_FAILURE
}
// check that byte directly after last auth byte is null
if (auth_data[last_auth_byte + 1] equals null) {
return DECODE_FAILURE
}
// we set our presumed Additional Data Length (ADL)
presumed_adl = auth_data[last_auth_byte + 1]
// use the presumed ADL to calculate a presumed LPI
presumed_lpi = (presumed_adl + decoded_length - 17) / 23
// check that presumed LPI and decoded LPI match
if (presumed_lpi not equal decoded_lpi) {
return DECODE_FAILURE
}
return DECODE_SUCCESS
}
Figure 13: Pseudo-code for Decode Checks
Implementations MAY also implement an heuristic extension
(Appendix C) to decode if both the first page (Page 0) and last page
(Last Page Index) are missing.
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5.3. FEC Limitations
The worst case scenario is when the Authentication Data / Signature
ends perfectly on a page (Page N-1). This means the Additional Data
Length would start the next page (Page N) and have 22-bytes worth of
null padding to align the FEC to begin at the start of the next page
(Page N+1). In this scenario an entire page (Page N) is being wasted
just to carry the Additional Data Length. This should be avoided
where possible in an effort to maintain efficiency.
6. Requirements & Recommendations
6.1. Legacy Transports
With Legacy Advertisements the goal is to attempt to bring reliable
receipt of the paged Authentication Message. FEC (Section 5) MUST be
used, per mandated RID rules (for example the US FAA RID Rule
[FAA-14CFR]), when using Legacy Advertising methods (such as
Bluetooth 4.x).
Under ASTM Bluetooth 4.x rules, transmission of dynamic messages is
at least every 1 second. DRIP Authentication Messages typically
contain dynamic data (such as the DRIP Manifest or DRIP Wrapper) and
should be sent at the dynamic rate of 1 per second.
6.2. Extended Transports
Under the ASTM specification, Bluetooth 5.x, Wi-Fi NAN, and Wi-Fi
BEACON transport of RID is to use the Message Pack (Message Type 0xF)
format for all transmissions. Under Message Pack messages are sent
together (in Message Type order) in a single Bluetooth 5.x extended
frame (up to 9 single frame equivalent messages under Bluetooth 4.x).
Message Packs are required by ASTM to be sent at a rate of 1 per
second (like dynamic messages).
Without any fragmentation or loss of pages with transmission FEC
(Section 5) MUST NOT be used as it is impractical.
6.3. Authentication
It is REQUIRED that a UA send the following DRIP Authentication
Formats to fulfill the requirements in [RFC9153]:
1. SHOULD: send DRIP Link (Section 4.2) using the Broadcast
Endorsement: Apex, RAA (satisfying GEN-3); at least once per 5
minutes
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2. MUST: send DRIP Link (Section 4.2) using the Broadcast
Endorsement: RAA, HDA (satisfying GEN-3); at least once per 5
minutes
3. MUST: send DRIP Link (Section 4.2) using the Broadcast
Endorsement: HDA, UA (satisfying ID-5, GEN-1 and GEN-3); at least
once per minute
4. MUST: send any other DRIP Authentication Format (RECOMMENDED:
DRIP Manifest (Section 4.4) or DRIP Wrapper (Section 4.3)) where
the UA is dynamically signing data that is guaranteed to be
unique, unpredictable and easily cross checked by the receiving
device (satisfying ID-5, GEN-1 and GEN-2); at least once per 5
seconds
6.4. Operational
UAS operation may impact the frequency of sending DRIP Authentication
messages. Where a UA is dwelling in one location, and the channel is
heavily used by other devices, "occasional" message authentication
may be sufficient for an Observer. Contrast this with a UA
traversing an area, and then every message should be authenticated as
soon as possible for greatest success as viewed by the receiver.
Thus how/when these DRIP Authentication Messages are sent is up to
each implementation. Further complication comes in contrasting
Legacy and Extended Transports. In Legacy, each message is a
separate hash within the Manifest. So, again in dwelling, may lean
toward occasional message authentication. In Extended Transports,
the hash is over the Message Pack so only few hashes need to be in a
Manifest. A single Manifest can handle a potential two Message Packs
(for a full set of messages) and a DRIP Link Authentication Message
for the Broadcast Endorsement: HDA, UA.
A separate issue is the frequency of transmitting the DRIP Link
Authentication Message for the Broadcast Endorsement: DIME, UA when
using the Manifest. This message content is static; its hash never
changes radically. The only change is the 4-byte timestamp in the
Authentication Message headers. Thus, potentially, in a dwelling
operation it can be sent once per minute, where its hash is in every
Manifest. A receiver can cache all DRIP Link Authentication Message
for the Broadcast Endorsement: DIME, UA to mitigate potential packet
loss.
The following operational configuration is RECOMMENDED (in alignment
with Section 6.3):
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1. Per CAA requirements, generate and transmit a set of ASTM
Messages (example; Basic ID, Location and System).
2. Under Extended Transports, generate and include in the same
Message Pack as the CAA required ASTM Messages a DRIP Wrapper as
specified in Section 4.3.2.
3. Under Legacy Transports, generate and transmit every 5 seconds a
DRIP Manifest (Section 4.4) hashing as many sets of recent CAA
required ASTM Messages. The system MAY periodically replace the
DRIP Manifest with a DRIP Wrapper (Section 4.3) containing at
least a Location Message (Message Type 0x2).
4. Under both Legacy or Extended Transports, generate and transmit a
DRIP Link's (Section 4.2) containing; Broadcast Endorsement: HDA,
UA every minute, Broadcast Endorsement: RAA, HDA every 5 minutes,
Broadcast Endorsement: Apex, RAA every 5 minutes.
The reasoning and math behind this recommendation can be found in
Appendix D.
6.4.1. DRIP Wrapper
The DRIP Wrapper MUST NOT be used in place of sending the ASTM
messages as is. All receivers MUST be able to process all the
messages specified in [F3411]. Sending them within the DRIP Wrapper
makes them opaque to receivers lacking support for DRIP
Authentication Messages. Thus, messages within a Wrapper are sent
twice: in the clear and authenticated within the Wrapper. The DRIP
Manifest would seem to be a more efficient use of the transport
channel.
The DRIP Wrapper has a specific use case for DRIP aware receivers.
For receiver plotting Location Messages (Message Type 0x2) on a map
display an embedded Location Message in a DRIP Wrapper can be marked
differently (e.g. via color) to signify trust in the Location data.
6.4.2. UAS RID Trust Assessment
As described in Section 3.1.4, the receiver MUST perform verification
of the data being received in Broadcast RID.
After signature validation of any DRIP Authentication Message
containing UAS RID information elements (e.g. DRIP Wrapper
Section 4.3) the Observer MUST use other sources of information to
correlate against and perform verification. An example of another
source of information is a visual confirmation of the UA position.
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When correlation of these different data streams do not match in
acceptable thresholds the data SHOULD be rejected as if the signature
failed to validate. Acceptable thresholds limits and what happens
after such a rejection are out of scope for this document.
7. Summary of Addressed DRIP Requirements
The following [RFC9153] requirements are addressed in this document:
ID-5: Non-spoofability
Addressed using the DRIP Wrapper (Section 4.3), DRIP Manifest
(Section 4.4) or DRIP Frame (Section 4.5).
GEN-1: Provable Ownership
Addressed using the DRIP Link (Section 4.2) and DRIP Wrapper
(Section 4.3), DRIP Manifest (Section 4.4) or DRIP Frame
(Section 4.5).
GEN-2: Provable Binding
Addressed using the DRIP Wrapper (Section 4.3), DRIP Manifest
(Section 4.4) or DRIP Frame (Section 4.5).
GEN-3: Provable Registration
Addressed using the DRIP Link (Section 4.2).
8. IANA Considerations
8.1. IANA DRIP Registry
This document requests a new subregistry for Frame Type under the
DRIP registry (https://www.iana.org/assignments/drip/drip.xhtml).
DRIP Frame Type: This 8-bit valued subregistry is for Frame Types in
DRIP Frame Authentication Messages. Future additions to this
subregistry are to be made through Expert Review (Section 4.5 of
[RFC8126]). The following values are defined:
| Frame Type | Name | Description |
| ---------- | ------------ | ---------------- |
| 0x00 | Reserved | Reserved |
| 0xC0-0xFF | Experimental | Experimental Use |
9. Security Considerations
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9.1. Replay Attacks
The astute reader may note that the DRIP Link messages, which are
recommended to be sent, are static in nature and contain various
timestamps. These DRIP Link messages can easily be replayed by an
attacker who has copied them from previous broadcasts.
If an attacker (who is smart and spoofs more than just the UAS ID/
data payloads) willing replays an DRIP Link message they have in
principle actually helped by ensuring the DRIP Link is sent more
frequently and be received by potential Observers.
The primary mitigation is the UA is REQUIRED to send more than DRIP
Link messages, specifically the Manifest and/or Wrapper messages that
sign over changing data ASTM Messages (e.g. Location/Vector
Messages) using the DET private key. An UA sending these messages
then actually signing these and other messages using the DET key
provides the Observer with data that proves realtime signing. An UA
who does not either run DRIP themselves or does not have possession
of the same private key, would be clearly exposed upon signature
verification.
9.2. VNA Timestamp Offsets for DRIP Authentication Formats
Note the discussion of VNA Timestamp offsets here is in context of
the DRIP Wrapper (Section 4.3), DRIP Manifest (Section 4.4) and DRIP
Frame (Section 4.5). For DRIP Link (Section 4.2) these offsets are
set by the DIME and have their own set of considerations in
[drip-registries].
The offset of the VNA Timestamp by UA is one that needs careful
consideration for any implementation. The offset should be shorter
than any given flight duration (typically less than an hour) but be
long enough to be received and processed by Observers (larger than a
few seconds). It recommended that 3-5 minutes should be sufficient
to serve this purpose in any scenario, but is not limited by design.
10. Acknowledgments
* Ryan Quigley and James Mussi of AX Enterprize, LLC for early
prototyping to find holes in the draft specifications.
* Soren Friis for pointing out that Wi-Fi implementations would not
always give access to the MAC Address, originally used in
calculation of the hashes for DRIP Manifest. Also, for confirming
that Message Packs (0xF) can only carry up to 9 ASTM frames worth
of data (9 Authentication pages).
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* Many thanks to Rick Salz for the secdir review.
* Thanks to Matt Joras for a genart review.
11. References
11.1. Normative References
[drip-arch]
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-31, 6 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-drip-
arch-31>.
[F3411] "F3411-22a: Standard Specification for Remote ID and
Tracking", July 2022.
[NIST.SP.800-185]
Kelsey, J., Change, S., Perlner, R., and NIST, "SHA-3
derived functions: cSHAKE, KMAC, TupleHash and
ParallelHash", NIST Special Publications
(General) 800-185, DOI 10.6028/NIST.SP.800-185, December
2016,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-185.pdf>.
[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>.
[RFC9153] Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
Gurtov, "Drone Remote Identification Protocol (DRIP)
Requirements and Terminology", RFC 9153,
DOI 10.17487/RFC9153, February 2022,
<https://www.rfc-editor.org/info/rfc9153>.
[RFC9374] Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
"DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,
<https://www.rfc-editor.org/info/rfc9374>.
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11.2. Informative References
[drip-registries]
Wiethuechter, A. and J. Reid, "DRIP Entity Tag (DET)
Identity Management Architecture", Work in Progress,
Internet-Draft, draft-ietf-drip-registries-07, 5 December
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
drip-registries-07>.
[FAA-14CFR]
"Remote Identification of Unmanned Aircraft", January
2021, <https://www.govinfo.gov/content/pkg/FR-2021-01-15/
pdf/2020-28948.pdf>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Authentication State Diagrams & Color Scheme
ASTM Authentication has only three states: None, Invalid, and Valid.
This is because, under ASTM, the authentication is done by an
external service hosted somewhere on the Internet so it is assumed
you will always get some sort of answer back. This classification
becomes more complex in DRIP with the support of "offline" scenarios
where a receiver does not have Internet connectivity. With the use
of asymmetric keys this means that the public key (PK) must somehow
be obtained. [drip-registries] gets more into detail how these keys
are stored on DNS and one reason for DRIP Authentication is to send
PK's over Broadcast RID.
There are two keys of interest: the PK of the UA and the PK of the
DIME. This document describes how to send the PK of the UA over the
Broadcast RID messages. The key of the DIME can be sent over
Broadcast RID using the same mechanisms (see Section 4.2 and
Section 6.3) but is not required due to potential operational
constraints of sending multiple DRIP Link messages. As such, there
are scenarios where part of the key-chain is available, but not all
of it.
The intent of this appendix is to give some kind of recommended way
to classify these various states and convey it to the user through
colors and state names/text.
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A.1. State Colors
The table below lays out the RECOMMENDED colors to associate with
state.
+==============+========+===================================+
| State | Color | Details |
+==============+========+===================================+
| None | Black | No Authentication being received |
+--------------+--------+-----------------------------------+
| Partial | Gray | Authentication being received but |
| | | missing pages |
+--------------+--------+-----------------------------------+
| Unsupported | Brown | Authentication Type/SAM Type of |
| | | received message not supported |
+--------------+--------+-----------------------------------+
| Unverifiable | Yellow | Data needed for verification |
| | | missing |
+--------------+--------+-----------------------------------+
| Verified | Green | Valid verification results |
+--------------+--------+-----------------------------------+
| Trusted | Blue | Valid verification results and |
| | | DIME is marked as trusted |
+--------------+--------+-----------------------------------+
| Questionable | Orange | Inconsistent verification results |
+--------------+--------+-----------------------------------+
| Unverified | Red | Invalid verification results |
+--------------+--------+-----------------------------------+
| Conflicting | Purple | Inconsistent verification results |
| | | and DIME is marked as trusted |
+--------------+--------+-----------------------------------+
Table 3
A.2. State Diagrams
This section gives some RECOMMENDED state flows that DRIP
implementations should follow. Note that the state diagrams do not
have all error conditions mapped.
A.2.1. Notations
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o--------------o
| PROCESS |
o--------------o
+--------------+
| STATE |
+--------------+
ooooo
o N o Transition N
ooooo
+-----> Transition Option False/No
-----> Transition Option True/Yes
Figure 14: Diagram Notations
A.2.2. General
o---------------------o ooooo +------+
| Start |---->o 1 o+----->| None |
o---------------------o ooooo +------+
|
v
ooooo +-------------+
o 2 o+----->| Unsupported |
ooooo +-------------+
| ^
v |
+---------+ ooooo |
| Partial |<-----+o 3 o |
+---------+ ooooo |
| |
v +
ooooo ooooo o-------------o
o 4 o------>o 5 o------>| SAM Decoder |
ooooo ooooo o-------------o
+
|
v
o------------------o
| AuthType Decoder |
o------------------o
Figure 15: Standard Authentication Colors/State
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+============+=============================+======================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+=============================+======================+
| 1 | Receiving Authentication | 2, None |
| | Pages? | |
+------------+-----------------------------+----------------------+
| 2 | Authentication Type | 3, Unsupported |
| | Supported? | |
+------------+-----------------------------+----------------------+
| 3 | All Pages of Authentication | 4, Partial |
| | Message Received? | |
+------------+-----------------------------+----------------------+
| 4 | Is Authentication Type | 5, AuthType Decoder |
| | received 5? | |
+------------+-----------------------------+----------------------+
| 5 | Is SAM Type Supported? | SAM Decoder, |
| | | Unsupported |
+------------+-----------------------------+----------------------+
Table 4
A.2.3. DRIP SAM
o-------------o ooooo o-----------------------------o
| SAM Decoder |---->o 6 o------>| DRIP Wrapper/Manifest/Frame |
o-------------o ooooo o-----------------------------o
+ | ^
| | |
v v |
o-----------o o--------------------o |
| DRIP Link |--->| Update State Cache | |
o-----------o o--------------------o |
| |
v |
o--------------o ooooo o----------------------o
| NOP / Return |<------+o 7 o----->| Extract Message from |
o--------------o ooooo | Verification Queue |
o----------------------o
Figure 16: DRIP SAM Decoder
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+============+=====================+========================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+=====================+========================+
| 6 | Is SAM Type DRIP | DRIP Link, DRIP |
| | Link? | Wrapper/Manifest/Frame |
+------------+---------------------+------------------------+
| 7 | Messages in | Extract Message from |
| | Verification Queue? | Verification Queue, |
| | | NOP / Return |
+------------+---------------------+------------------------+
Table 5
A.2.4. DRIP Link
o-----------o ooooo ooooo +--------------+
| DRIP Link |----->o 8 o+----->o 9 o+----->| Unverifiable |
o-----------o ooooo ooooo +--------------+
| |
|-------------'
v
ooooo +------------+
o 10 o+----->| Unverified |
ooooo +------------+
|
v
o---------------------o
| Add UA DET/PK |
| to Key Cache |
o---------------------o
|
v
ooooo +----------+
o 11 o+------>| Verified |
ooooo +----------+
| ^
v |
o-------------------------o
| Mark UA DET/PK |
| as Trusted in Key Cache |
o-------------------------o
Figure 17: DRIP Link State Decoder
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+============+=======================+===========================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+=======================+===========================+
| 8 | DIME DET/PK in Key | 10, 9 |
| | Cache? | |
+------------+-----------------------+---------------------------+
| 9 | DIME PK found Online? | 10, Unverifiable |
+------------+-----------------------+---------------------------+
| 10 | DIME Signature | Add UA DET/PK to Key |
| | Verified? | Cache, Unverified |
+------------+-----------------------+---------------------------+
| 11 | DIME DET/PK marked as | Mark UA DET/PK as Trusted |
| | Trusted in Key Cache? | in Key Cache, Verified |
+------------+-----------------------+---------------------------+
Table 6
A.2.5. DRIP Wrapper/Manifest/Frame
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o-----------------------------o +--------------+
| DRIP Wrapper/Manifest/Frame | | Unverifiable |
o-----------------------------o +--------------+
| ^
v |
ooooo ooooo o--------------------o
o 12 o+----->o 13 o+----->| Add Message to |
ooooo ooooo | Verification Queue |
| | o--------------------o
| |
|-------------'
v
ooooo ooooo ooooo +------------+
o 14 o+----->o 15 o+----->o 16 o+----->| Unverified |
ooooo ooooo ooooo +------------+
| | |
v v |
ooooo +-------------+ |
o 17 o+----->| Conflicting | |
ooooo +-------------+ |
| |
v v
ooooo +--------------+
o 18 o---------------->| Questionable |
ooooo +--------------+
+
|
v
ooooo +----------+
o 19 o+----->| Verified |
ooooo +----------+
|
v
+---------+
| Trusted |
+---------+
Figure 18: DRIP Wrapper/Manifest/Frame State Decoder
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+============+==============================+======================+
| Transition | Transition Query | Next State/Process/ |
| | | Transition (Yes, No) |
+============+==============================+======================+
| 12 | UA DET/PK in Key Cache? | 14, 13 |
+------------+------------------------------+----------------------+
| 13 | UA PK found Online? | 14, Add Message to |
| | | Verification Queue |
+------------+------------------------------+----------------------+
| 14 | UA Signature Verified? | 17, 15 |
+------------+------------------------------+----------------------+
| 15 | Has past Messages of this | Conflicting, 16 |
| | type been marked as Trusted? | |
+------------+------------------------------+----------------------+
| 16 | Has past Messages of this | Questionable, |
| | type been marked as | Unverified |
| | Questionable or Verified? | |
+------------+------------------------------+----------------------+
| 17 | Has past Messages of this | Conflicting, 18 |
| | type been marked as | |
| | Conflicting? | |
+------------+------------------------------+----------------------+
| 18 | Has past Messages of this | Questionable, 19 |
| | type been marked as | |
| | Questionable or Unverified? | |
+------------+------------------------------+----------------------+
| 19 | Is UA DET/PK marked as | Trusted, Verified |
| | Trusted in Key Cache? | |
+------------+------------------------------+----------------------+
Table 7
Appendix B. Example TX/RX Flow
In this example, the UA is sending all DRIP Authentication Message
formats (DRIP Link, DRIP Wrapper, and DRIP Manifest) during flight,
along with standard ASTM Messages. The objective is to show the
combinations of messages that must be received to properly validate a
DRIP-equipped UA and examples of their various states (as described
in Appendix A).
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+-------------------+
.-----| Unmanned Aircraft |-----.
| +-------------------+ |
| 0 | 1 | 2
| | |
O O O
--|-- --|-- --|--
/ \ / \ / \
A B C
Broadcast Paths: Messages Received
0: None
1: DRIP Link or DRIP Wrapper or DRIP Manifest
2: DRIP Link and DRIP Wrapper or DRIP Manifest
Observers: Authentication State
A: None
B: Unverifiable
C: Verified, Trusted, Unverified, Questionable, or Conflicting
As the above example shows to properly authenticate both a DRIP Link
and a DRIP Wrapper or DRIP Manifest are required.
Appendix C. Additional FEC Decoding Heuristic
With Section 5, if Page 0 and the FEC page are missing from the
Authentication Message there is a heuristic that can be applied
instead of FEC decoding to obtain the Authentication Data. This is
based on the structure of the DRIP Authentication Messages and
additional information sent over the broadcast or via lookup in DNS.
Looking at Page 0 (Figure 19) of any DRIP Authentication Format the
payload data is always a DET. For DRIP Link (Section 4.2) this DET
is of the DIME while for DRIP Wrapper (Section 4.3), Manifest
(Section 4.4) and Frame (Section 4.5) it is the DET of the UA.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Page Header | |
+---------------+ Authentication Headers +---------------+
| | SAM Type |
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag |
| |
+---------------+---------------+---------------+---------------+
Page Header: (1-byte)
Authentication Type (4-bits)
Page Number (4 bits)
Authentication Headers: (6-bytes)
As defined in F3411
SAM Type (1-byte):
Byte defined by F3411 to multiplex SAMs
DRIP Entity Tag: (16-bytes)
DET of an entity in network byte order
Figure 19: Example Page 0 from DRIP Authentication Message
Under DRIP, the Basic ID Message (Message Type 0x1) SHOULD be using
Specific Session ID (ID Type 4) subtype IETF DRIP Entity ID (Type 1).
This DET of the UA can be used in place of the missing DET in a DRIP
Wrapper, Manifest and Frame. For DRIP Link, which is missing the DET
of the DIME, the lookup properties of the DET enables the discovery,
via DNS, the DIME's DET.
These DETs obtained via other means can replace the missing payload
of Authentication Page 0 and enable the full decoding and
verification of the DRIP Authentication Message.
When the missing DET is supposed to be of the UA the DET MAY be
sourced from the Basic ID Message (Message Type 0x1). Under DRIP,
this SHOULD be set to the DET missing in the Authentication Data.
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Appendix D. Operational Recommendation Analysis
The recommendations found in (Section 6.4) may seem heavy handed and
specific. This appendix lays out the math and assumptions made to
come to the recommendations listed there. This section is solely
based on operations using Bluetooth 4.x; as such, all calculations of
frame counts for DRIP included FEC using Section 5.
D.1. Methodology
In the US, the required ASTM Messages to be transmitted every second
are: Basic ID (0x1), Location (0x2), and System (0x4). Typical
implementations will most likely send at a higher rate (2x sets per
cycle) resulting in 6 frames sent per cycle.
Informational Note: In Europe, the Operator ID Message (0x5) is
also included; pushing the frame count to 8 per cycle. In Japan,
two Basic ID (0x0), Location (0x1), and Authentication (0x2) are
required.
To calculate the frame count of a given DRIP Authentication Message
the following formula is used:
1 + ceiling((((16 + 8 + 64) + (Item Size * Item Count) + 2) - 16)
/ 23) + 1
The leading 1 is counting for the Page 0 which is always present.
The DET (16-bytes), timestamps (8-bytes) and signature (64-bytes) all
make up the required fields for DRIP. Item Size (in bytes) is size
of each item in a given format; for a Wrapper it is 25 (a full ASTM
Message), while for a Manifest it is 8 (a single hash). 2 more is
added to account for the SAM Type and the ADL byte. The value 16 is
the number of bytes not counted (as they are part of Page 0 which is
already counted for). 23 is the number of bytes per Authentication
Page (pages 1 - 15). After dividing by 23 the value is raised to the
nearest whole value as we can only send full frames, not partial.
The final 1 is counting for a single page of FEC applied in DRIP
under Bluetooth 4.x.
Informational Note: for DRIP Link the Item Size is 48 and Item
Count is 1; resulting in a frame count of 8
Comparing DRIP Wrapper and Manifest Authentication Message frame
counts we have the following:
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+===============+=========+==========+===============+==========+
| Authenticated | Wrapper | Manifest | Total Wrapper | Total |
| Frames | Frames | Frames | Frames | Manifest |
| | | | | Frames |
+===============+=========+==========+===============+==========+
| 1 | 7 | 7 | 8 | 8 |
+---------------+---------+----------+---------------+----------+
| 2 | 8 | 7 | 10 | 9 |
+---------------+---------+----------+---------------+----------+
| 3 | 9 | 7 | 12 | 10 |
+---------------+---------+----------+---------------+----------+
| 4 | 10 | 8 | 14 | 12 |
+---------------+---------+----------+---------------+----------+
| 5 | N/A | 8 | N/A | 13 |
+---------------+---------+----------+---------------+----------+
| 6 | N/A | 8 | N/A | 14 |
+---------------+---------+----------+---------------+----------+
| 7 | N/A | 9 | N/A | 16 |
+---------------+---------+----------+---------------+----------+
| 8 | N/A | 9 | N/A | 17 |
+---------------+---------+----------+---------------+----------+
| 9 | N/A | 10 | N/A | 19 |
+---------------+---------+----------+---------------+----------+
| 10 | N/A | 10 | N/A | 20 |
+---------------+---------+----------+---------------+----------+
| 11 | N/A | 10 | N/A | 21 |
+---------------+---------+----------+---------------+----------+
Table 8: Frame Counts
Note that for Manifest Frames the calculations use an Item Count that
is 2 + Authentication Frames. This is to account for the two special
hashes.
The values in Total Frames is calculated by adding in the Item Count
(to either the Wrapper Frames or Manifest Frames column) to account
for the ASTM Messages being sent outside the Authentication Message.
D.2. ASTM Maximum Schedule Example
For this example we will assume the following ASTM Messages are in
play:
* 1x Basic ID (0x0) set as ID Type for Serial Number (0x1)
* 1x Basic ID (0x0) set as ID Type for CAA Assigned ID (0x2)
* 1x Basic ID (0x0) set as ID Type for UTM Assigned ID (0x3)
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* 1x Basic ID (0x0) set as ID Type for Specific Session ID (0x4)
* 2x Location (0x1)
* 1x Self ID (0x3)
* 2x System (0x4)
* 2x Operator ID (0x5)
This message set uses all single frame ASTM Messages, sending a set
of them (Location, System and Operator ID) at a rate of 2 per second.
Two Basic IDs are sent in a single second and rotate between the 4
defined (1x per type). A single Self ID is sent every second. All
messages in a given second, if appear more than once, are exact
duplicates.
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+-----------------------------------------------------------+
| Frame Slots |
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A* | V* | S | O | B | V | S* | O | I | L/W[0,2] |
+----+----+----+----+----+----+----+----+----+----+----+----+
| C* | V | S* | O | D* | V* | S | O | I* | L/W[3,5] |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A | V* | S | O* | B* | V | S | O | I |L/W[6,7] | ## |
+----+----+----+----+----+----+----+----+----+----+----+----+
| C | V | S | O | D | V | S | O | I | M[0,2] |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A | V | S | O | B | V | S | O | I | M[3,5] |
+----+----+----+----+----+----+----+----+----+----+----+----+
| C | V | S | O | D | V | S | O | I | M[6,8] |
+----+----+----+----+----+----+----+----+----+----+----+----+
| A | V | S | O | B | V | S | O | I |M[9]| ## | ## |
+----+----+----+----+----+----+----+----+----+----+----+----+
# = Empty Frame Slot
A = Basic ID Message (0x0) ID Type 1
B = Basic ID Message (0x0) ID Type 2
C = Basic ID Message (0x0) ID Type 3
D = Basic ID Message (0x0) ID Type 4
V = Location/Vector Message (0x1)
I = Self ID Message (0x3)
S = System Message (0x4)
O = Operator ID Message (0x5)
L[y,z] = DRIP Link Authentication Message (0x2)
W[y,z] = DRIP Wrapper Authentication Message (0x2)
Wrapping Location (0x1) and System (0x4)
M(x)[y,z] = DRIP Manifest Authentication Message (0x2)
x = Number Hashes
y = Start Page
z = End Page
* = Message in DRIP Manifest Authentication Message
Figure 20: Example Transmit Schedule
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Manifest messages in the schedule are filled with unique messages
from previously transmitted messages before the new Manifest is sent.
In Figure 20, this is denoted by the * symbol as being part of the
Manifest. In Figure 20, messages are eligible for the Manifest in
the very first cycle of transmission. In future iterations, 56
messages are eligible across the 7 seconds it takes to send the
previous Manifest and the next Link/Wrapper. Care should be given
into the selection of messages for a Manifest as there is a limit of
11 hashes.
Informational Note: the term "unique message" above is used as in
the example schedule the 2nd Location and System messages MAY be
exact copies of the previous Location and System messages sent in
the same second. Duplicates of this kind SHOULD NOT be included
in a Manifest.
In the schedule the Wrapper and the Link messages switch back and
forth the contents of them are changing in the following order:
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Link: Apex on RAA
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Wrapper: Location (0x1), System (0x4)
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Link: Apex on RAA
Link: HDA on UA
Link: RAA on HDA
Link: HDA on UA
Wrapper: Location (0x1), System (0x4)
Link: IANA on UAS RID Apex
Any messages not required for a local jurisdiction can be removed
from the schedule. It is RECOMMENDED this empty frame slot is left
empty to help with timing due to RF constraints/concerns. For
example, in the US the Self ID (0x3) and Operator ID (0x5) are not
required and can be ignored in the above figures. Only one Basic ID
(0x0) is selected in the US at any given time, opening up three (3)
more slots.
Authors' Addresses
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Adam Wiethuechter
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: adam.wiethuechter@axenterprize.com
Stuart Card
AX Enterprize, LLC
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: stu.card@axenterprize.com
Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Email: rgm@labs.htt-consult.com
Wiethuechter (Editor), eExpires 29 September 2023 [Page 48]