| Internet-Draft | DRIP Auth Formats | October 2022 |
| Wiethuechter (Editor), et al. | Expires 17 April 2023 | [Page] |
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. It 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.¶
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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/.¶
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This Internet-Draft will expire on 17 April 2023.¶
Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.¶
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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. This is a requirement that will need 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 charter is:¶
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]).¶
Without authentication, an Observer has no basis for trust. As the messages are sent via wireless broadcast, they may be transmitted anywhere within wireless range and making any claims desired by the sender.¶
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.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document makes use of the terms (Observer, USS, UTM, etc.) defined in [RFC9153]. Other terms (such as DIME) are from [drip-arch], while others (RAA, HDA, etc.) are from [drip-rid].¶
In addition, the following terms are defined for this document:¶
Legacy Transports:¶
Extended Transports:¶
[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. [F3411] Annex A1 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 [drip-arch] in Section 5, 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.¶
When an Observer receives a DRIP-based Authentication Message (Section 4.3, Section 4.4, Section 4.5) containing UA signed Evidence it SHOULD validate the signature using the HI corresponding to the UA's DET.¶
The UA's HI, SHOULD be retrieved from DNS (Section 5, [drip-registries]). 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 SHOULD be 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.¶
When an Observer receives a DRIP Link Authentication Message (Section 4.2) containing an Endorsement by the DIME of the UA DET/HI registration (Appendix B), 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.¶
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.¶
Section 3.1.1, Section 3.1.2 and Section 3.1.3 above 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).¶
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.¶
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.
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 4.1.1).¶
The Authentication Message is structured as a set of pages. 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 3.2.3 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).¶
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.¶
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.
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.¶
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.¶
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:¶
Authentication Data / Signature data MUST fit in the first 9 pages (Page Numbers 0 through 8).¶
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.¶
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.¶
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 C shows an example of using the formats defined in this section.¶
ASTM Message (25-bytes):¶
ASTM Message Hash (8-bytes):¶
Broadcast Endorsement (136-bytes):¶
Current Manifest Hash (12-bytes):¶
Evidence (0 to 112 bytes):¶
Frame Type (1-byte):¶
Previous Manifest Hash (12-bytes):¶
UA DRIP Entity Tag (DET) (16-bytes):¶
UA Signature (64-bytes):¶
Valid Not After (VNA) Timestamp by UA (4-bytes):¶
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):¶
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.¶
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).
The SAM Type field is maintained by a TBD registrar and for DRIP four are planned to be allocated:¶
| SAM Type | Description |
|---|---|
| To Be Assigned | DRIP Link (Section 4.2) |
| To Be Assigned | DRIP Wrapper (Section 4.3) |
| To Be Assigned | DRIP Manifest (Section 4.4) |
| To Be Assigned | DRIP Frame (Section 4.5) |
This field has a maximum size of 200-bytes, as defined by Section 3.2.3. The Broadcast Attestation Structure (Section 4.1.2) MUST be used in this space.¶
The DRIP Endorsement Structure (DES) [drip-registries] is used to created Signed Evidence by the UA during flight. It is encapsulated by the SAM Authentication Data field of Figure 3.¶
The DES MUST be 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 the DES as it will not fit in the ASTM Authentication Message.¶
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 +---------------+---------------+---------------+---------------+ | | | UA | | DRIP Entity Tag | | | +---------------+---------------+---------------+---------------+ | | . . . Evidence . . . | | +---------------+---------------+---------------+---------------+ | VNA Timestamp by UA | +---------------+---------------+---------------+---------------+ | VNB Timestamp by UA | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | UA Signature | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+
UA DRIP Entity Tag:¶
identity section of the DES and MUST be set to the UA DET (hhit).¶
Evidence:¶
evidence section MUST be filled in with data in the form of an opaque object specified in the DRIP Wrapper, Manifest or Frame sections.¶
UA Signature:¶
sig_b16) found in the signature section.¶
The DES MUST be encoded in the binary form (as defined in [drip-registries]) to create the UA Signed Evidence. The general structure of the binary form can be seen in Figure 4.¶
When using the DES, 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).¶
The DRIP Link SAM Type is used to transmit Broadcast Endorsements. For example, the Broadcast Endorsement: DIME, UA is sent (see Section 6.3) as a DRIP Link message. The structure is defined in [drip-registries] and an example of it can be found in Appendix B.¶
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 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 +---------------+---------------+---------------+---------------+ | | . . . Broadcast Endorsement . . . | | +---------------+---------------+---------------+---------------+
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.¶
This SAM Type is used to wrap and sign over a list of other [F3411] Broadcast RID messages.¶
The evidence section of the DES (Section 4.1.2) is populated with full (25-byte) [F3411] Broadcast RID messages. The ASTM Messages can be concatenated together to form a contiguous byte sequence (like in Figure 6) or be set in the evidence section as individual Claims.¶
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) . . . | | +---------------+---------------+---------------+---------------+
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;
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 DES 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 +---------------+---------------+---------------+---------------+ | | | UA | | DRIP Entity Tag | | | +---------------+---------------+---------------+---------------+ | Not Before Timestamp by UA | +---------------+---------------+---------------+---------------+ | Not After Timestamp by UA | +---------------+---------------+---------------+---------------+ | | | | | | | | | | | | | | | UA Signature | | | | | | | | | | | | | | | | | +---------------+---------------+---------------+---------------+
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 DES evidence section before performing signature verification.¶
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.¶
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.¶
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 E).¶
The evidence section of the DES (Section 4.1.2) 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 or be set in the evidence section individually. The Previous Manifest Hash and Current Manifest Hash MUST always come before the ASTM Message Hashes as seen in Figure 9.¶
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 E.¶
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 . . . | | +---------------+---------------+---------------+---------------+
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;
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. This gives a pseudo-blockchain provenance to the Manifest that could be traced back if the Observer was present for extended periods of time.¶
The hash algorithm used for the Manifest is the same hash algorithm used in creation of the DET [drip-rid] 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")¶
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.¶
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.¶
Under this transport DRIP hashes the full ASTM Message Pack (Message Type 0xF) - regardless of its content.¶
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 DES (Section 4.1.2) is not defined in this document and MUST be openly specified by the implementation (or specification) using it.¶
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 . . . | | +---------------+---------------+---------------+---------------+
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 |
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.¶
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.¶
When encoding two things are REQUIRED:¶
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.¶
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.¶
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 | | | | | | | +---------------+---------------+---------------+---------------+
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.¶
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.¶
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
}
Implementations MAY also implement an heuristic extension (Appendix D) to decode if both the first page (Page 0) and last page (Last Page Index) are missing.¶
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.¶
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.¶
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.¶
It is REQUIRED that a UA send the following DRIP Authentication Formats to fulfill the requirements in [RFC9153]:¶
Broadcast Endorsement: DIME:Apex, DIME:RAA (satisfying GEN-3); at least once per 5 minutes¶
Broadcast Endorsement: DIME:RAA, DIME:HDA (satisfying GEN-3); at least once per 5 minutes¶
Broadcast Endorsement: DIME:HDA, UA (satisfying ID-5, GEN-1 and GEN-3); at least once per minute¶
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: DIME, 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):¶
Broadcast Endorsement: DIME:HDA, UA every minute, Broadcast Endorsement: DIME:RAA, DIME:HDA every 5 minutes, Broadcast Endorsement: DIME:Apex, DIME:RAA every 5 minutes.¶
The reasoning and math behind this recommendation can be found in Appendix E.¶
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.¶
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.¶
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.¶
The following [RFC9153] are addressed in this document:¶
ID-5: Non-spoofability¶
GEN-1: Provable Ownership¶
GEN-2: Provable Binding¶
GEN-3: Provable Registration¶
This document requests a new subregistry for Frame Type under the DRIP registry.¶
| Frame Type | Name | Description | | ---------- | ------------ | ---------------- | | 0x00 | Reserved | Reserved | | 0xC0-0xFF | Experimental | Experimental Use |¶
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.¶
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.¶
ASTM Authentication has only 3 states: None, Invalid or Valid. This is because under ASTM the idea is that 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. With DRIP this classification becomes more complex with the support of "offline" scenarios where the receiver does not have Internet connectivity. With the use of asymmetric keys this means 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 gives a clear way 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 you may have part of the key-chain 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.¶
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 |
This section gives some RECOMMENDED state flows that DRIP should follow. Note that the state diagrams do not have all error conditions mapped.¶
o--------------o | PROCESS | o--------------o +--------------+ | STATE | +--------------+ ooooo o N o Transition N ooooo +-----> Transition Option False/No -----> Transition Option True/Yes
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
| Transition | Transition Query | Next State/Process/Transition (Yes, No) |
|---|---|---|
| 1 | Receiving Authentication Pages? | 2, None |
| 2 | Authentication Type Supported? | 3, Unsupported |
| 3 | All Pages of Authentication Message Received? | 4, Partial |
| 4 | Is Authentication Type received 5? | 5, AuthType Decoder |
| 5 | Is SAM Type Supported? | SAM Decoder, Unsupported |
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
| Transition | Transition Query | Next State/Process/Transition (Yes, No) |
|---|---|---|
| 6 | Is SAM Type DRIP Link? | DRIP Link, DRIP Wrapper/Manifest/Frame |
| 7 | Messages in Verification Queue? | Extract Message from Verification Queue, NOP / Return |
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
| Transition | Transition Query | Next State/Process/Transition (Yes, No) |
|---|---|---|
| 8 | DIME DET/PK in Key Cache? | 10, 9 |
| 9 | DIME PK found Online? | 10, Unverifiable |
| 10 | DIME Signature Verified? | Add UA DET/PK to Key Cache, Unverified |
| 11 | DIME DET/PK marked as Trusted in Key Cache? | Mark UA DET/PK as Trusted in Key Cache, Verified |
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 |
+---------+
| 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 type been marked as Trusted? | Conflicting, 16 |
| 16 | Has past Messages of this type been marked as Questionable or Verified? | Questionable, Unverified |
| 17 | Has past Messages of this type been marked as Conflicting? | Conflicting, 18 |
| 18 | Has past Messages of this type been marked as Questionable or Unverified? | Questionable, 19 |
| 19 | Is UA DET/PK marked as Trusted in Key Cache? | Trusted, Verified |
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
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of DIME |
| |
+---------------+---------------+---------------+---------------+
| |
| DRIP |
| Entity Tag of UA |
| |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| Host Identity of UA |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
| VNB Timestamp by DIME |
+---------------+---------------+---------------+---------------+
| VNA Timestamp by DIME |
+---------------+---------------+---------------+---------------+
| |
| |
| |
| |
| |
| |
| |
| Signature by DIME |
| |
| |
| |
| |
| |
| |
| |
| |
+---------------+---------------+---------------+---------------+
DRIP Entity Tag of DIME: (16-bytes)
DET of DIME.
DRIP Entity Tag of UA: (16-bytes)
DET of UA.
Host Identity of UA: (32-bytes)
HI of UA.
VNB Timestamp by DIME (4-bytes):
Current time at signing.
VNA Timestamp by DIME (4-bytes):
Timestamp denoting recommended time to trust DIME Endorsement
of UA DET and HI (may be minutes to months in the future).
DIME Signature (64-bytes):
Signature over preceding fields using the keypair of
the DIME.
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).¶
+-------------------+
.-----| 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.¶
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 20) 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.¶
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
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.¶
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.¶
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.¶
To calculate the frame count of a given DRIP Authentication Message the following formula is used:¶
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.¶
Comparing DRIP Wrapper and Manifest Authentication Message frame counts we have the following:¶
| Authenticated Frames | Wrapper Frames | Manifest Frames | Total Wrapper Frames | Total 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 |
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.¶
For this example we will assume the following ASTM Messages are in play:¶
This message set is 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 ID's 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.¶
+-----------------------------------------------------------+ | 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
Manifest messages in the schedule are filled with unique messages from previously transmitted messages before the new Manifest is sent. In Figure 21 this is denoted by the * symbol as being part of the Manifest. For the figure 24 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.¶
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: ??? on 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. In the US only one Basic ID (0x0) is selected at any given time, opening up 3 more slots.¶