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Internet DRIP Working Group Internet-Draft 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.

Introduction The initial regulations (e.g. ) and standards (e.g. ) 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 System's (NAS). DRIP's goal as stated in the 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 ). 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 in 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 () as mandated by the United States (US) Federal Aviation Administration (FAA) , which is missing from over Legacy Transports (Bluetooth 4.x). These DRIP enhancements to further support the important use case of Observers who are sometimes offline at the time of observation. A summary of DRIP requirements addressed herein is provided in .

Terminology

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 when, and only when, they appear in all capitals, as shown here.

Definitions This document makes use of the terms defined in . In addition, the following terms are defined: DRIP Entity Tag (DET):

• An HHIT that is used as an identifier in DRIP as specified in .

DRIP Identity Management Entity:

• Registry service for DETs and other information in DRIP as specified in .

Legacy Transports:

• use of broadcast frames (Bluetooth 4.x) as specified in .

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 . Must use ASTM Message Pack (Message Type 0xF).

Hierarchial Host Identity Tag (HHIT):

• A special-use, non-routable, IPv6 address constructed as specified in .

HHIT Domain Authority (HDA):

• A class of DIME usually associated with a USS in UTM.

Hierarchial ID (HID):

• Encoding of the RAA and HDA into the HHIT structure as defined in .

Host Identity (HI):

• Public key have of an asymmetric keypair used in generating a HHIT as specified in .

Registered Assigning Authority (RAA):

• A class of DIME usually associated with a CAA such as the US FAA.

Background

Reasoning for IETF DRIP Authentication defines Authentication Message framing only. It does not define authentication formats or methods. It explicitly anticipates several signature options, but does not fully define even those. Annex A1 defines a Broadcast Authentication Verifier Service, which has a heavy reliance on Observer real-time connectivity to the Internet (specifically into UTM) that is not always guaranteed. Fortunately, 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 in Section 5, there is a need to have Authentication formats to relay information for Observers to determine trust. No existing formats (defined in or other organizations leveraging this feature) provide the functionality to satisfy this goal resulting in the work reflected in this document.

UA Signed Evidence When an Observer receives a DRIP-based Authentication Message (, , ) 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, ). 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 (). 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 () are possible.

DIME Endorsement of UA DET/HI When an Observer receives a DRIP Link Authentication Message () containing an Endorsement by the DIME of the UA DET/HI 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, ), when available. It MAY be cached from a prior DNS lookup or it may be stored in a distinct local store.

DIME Hierarchy Endorsements An Observer can receive a series of DRIP Link Authentication Messages () each one pertaining to a DIME's registration in the DIME above it in the hierarchy. Similar to , each link in this chain SHOULD be validated.

UAS RID Trust , and above complete the trust chain but the chain cannot yet be trusted as having any relevance to the observed UA because reply 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 DRIP Wrapper or Manifest (, ) 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 ().

ASTM Authentication Message The ASTM Authentication Message (Message Type 0x2) is a unique message in the Broadcast 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.

Authentication Page

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 . This is denoted in every Authentication Page in the Page Header. The SAM Type is denoted as a field in the Authentication Payload (see ). 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 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).

Authentication Payload Field is the source data view of the data fields found in the Authentication Message as defined by . This data is placed into 's Authentication Payload, spanning multiple pages.

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 .

ASTM Broadcast RID Constraints

Wireless Frame Constraints A UA has the option of broadcasting using Bluetooth (4.x and 5.x) or Wi-Fi (BEACON or NAN), see . 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 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 messaging framing dictated by Bluetooth 4.x constraints is inherited by over other media.

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.

DRIP Authentication Formats All formats defined in this section are the content for the Authentication Data / Signature field in and use the Specific Authentication Method (SAM, Authentication Type 0x5). The first byte of the Authentication Data / Signature of , 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 MUST be used. gives a high-level overview of a state machine for decoding and determining a trustworthiness state. shows an example of using the formats defined in this section.

DRIP Authentication Field Definitions ASTM Message (25-bytes):

• Full ASTM Message as defined in ; 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 (). Multiple hashes MUST be in Message Type order.

Broadcast Endorsement (136-bytes):

• DIME HI over UA DET/HI. Generated by a DIME during a UA DET, being used as a Session ID, registration. Used in .

Current Manifest Hash (12-bytes):

• Hash of the current Manifest Message (). See .

Evidence (0 to 112 bytes):

• Opaque evidence data that the UA is endorsing during its flight in .

Frame Type (1-byte):

• Sub-type for future different DRIP Frame formats. See .

Previous Manifest Hash (12-bytes):

• Hash of the previously sent Manifest Message (). See .

UA DRIP Entity Tag (DET) (16-bytes):

• The UA DET in byte form (network byte order) and is part of .

UA Signature (64-bytes):

• Signature over all 4 preceding fields of using the HI of the UA.

Valid Not After (VNA) Timestamp by UA (4-bytes):

• Timestamp denoting recommended time to stop trusting data in . MUST follow the format defined in . 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 . MUST follow the format defined in . 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.

SAM Data Format is the general format to hold authentication data when using SAM and is placed inside the Authentication Data / Signature field in .

SAM Data Format

SAM Type The SAM Type field is maintained by the International Civil Aviation Organization (ICAO) and for DRIP four are planned to be allocated:

SAM Type	Description
0x01	DRIP Link ()
0x02	DRIP Wrapper ()
0x03	DRIP Manifest ()
0x04	DRIP Frame ()

SAM Authentication Data This field has a maximum size of 200-bytes, as defined by . The Broadcast Attestation Structure () MUST be used in this space.

UA Signed Evidence The DRIP Endorsement Structure (DES) is used to created Signed Evidence by the UA during flight. It is encapsulated by the SAM Authentication Data field of . The DES MUST be used by DRIP Wrapper (), Manifest and Frame (). DRIP Link () MUST NOT use the DES as it will not fit in the ASTM Authentication Message.

Binary Encoded DRIP Endorsement Structure

UA DRIP Entity Tag:

• This is the identity section of the DES and MUST be set to the UA DET (hhit).

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.

UA Signature:

• The UA private key MUST be used to generate the signature (sig_b16) found in the signature section.

The DES MUST be encoded in the binary form (as defined in ) to create the UA Signed Evidence. The general structure of the binary form can be seen in . 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 or by extracting it from a Broadcast Endorsement (see and ).

DRIP Link The DRIP Link SAM Type is used to transmit Broadcast Endorsements. For example, the Broadcast Endorsement: DIME, UA is sent (see ) as a DRIP Link message. The structure is defined in and an example of it can be found in . 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.

DRIP Link

This DRIP Authentication Message is used in conjunction with other DRIP SAM Types (such as Manifest or 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.

DRIP Wrapper This SAM Type is used to wrap and sign over a list of other Broadcast RID messages. The evidence section of the DES () is populated with full (25-byte) Broadcast RID messages. The ASTM Messages can be concatenated together into a single byte object (like in ) 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 . 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 DRIP Manifest instead.

DRIP Wrapper Evidence

Message Count When decoding a DRIP Wrapper on a receiver, the number of messages wrapped can be determined by checking the length between the UA DET and the VNB Timestamp by UA is a multiple of 25-bytes.

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 DES is cleared after signing leaving the following binary structure that is placed into the SAM Authentication Data of and sent in the same Message Pack.

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 DES evidence section before performing signature verification. The functionality of Wrapper in this form is identical to Message Set Signature (Authentication Type 0x3) when running over Extended Transports. What 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 Wrapper, and all DRIP Authentication Formats, avoid when the UA key is obtained via a DRIP Link Authentication Message.

Wrapper Limitations The primary limitation of the Wrapper format 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 Wrapper is being used the wrapper data must effectively be sent twice, once as a single framed message (as specified in ) and then again wrapped within the Wrapper format.

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 Format () and greatly reduce overhead. Judicious use of 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 ). The evidence section of the DES () is populated with 8-byte hashes of Broadcast RID messages and two special hashes (). All these hashes can be concatenated together into a single byte object 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 . 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 and .

DRIP Manifest Evidence Structure

Hash Count The number of hashes in the Manifest can be variable (2-12). An easy way to determine the number of hashes is to take the length of the data between the end of the UA DET and VNB Timestamp by UA and divide it by the hash length (8). If this value is not an integer, the message is invalid.

Pseudo-Blockchain Hashes Two special hashes are included in all Manifest messages; the Previous Manifest Hash, which links to the previous manifest message, as well as the Current Manifest Hash. This gives a pseudo-blockchain provenance to the manifest message that could be traced back if the Observer was present for extended periods of time.

Hash Algorithms and Operation The hash algorithm used for the Manifest Message is the same hash algorithm used in creation of a DET that is signing the Manifest. An DET using cSHAKE128 computes the hash as follows:

• Informative Note: specifies cSHAKE128 but is open for the expansion of other OGAs.

When building the manifest of hashes the Previous Manifest Hash is known from the previous Manifest message. 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.

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.

Extended Transport Hashing Under this transport DRIP hashes the full ASTM Message Pack (Message Type 0xF) - regardless of its content.

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 DES () is not defined in this document and MUST be openly specified by the implementation (or specification) using it.

DRIP Frame

Frame Type Byte to sub-type for future different DRIP Frame formats. It takes the first byte in leaving 111-bytes available for Frame Evidence Data.

Frame Type	Name	Description
0x00	Reserved	Reserved
0xC0-0xFF	Experimental	Experimental Use

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. DRIP supports 2 different FEC encodings. Single Page FEC is the simpler scheme, but can correct for only a single erased page. Multiple Page FEC is more complex, but can correct for multiple erased pages. The data added during FEC is not included in the Authentication Data / Signature but instead in the Additional Data field of . This may cause the Authentication Message to exceed 9-pages, up to a maximum of 16-pages.

General Encoding Rules 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 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.

General Decoding Rules To determine if Single Page FEC or Multiple Page 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 Single Page FEC has been used otherwise Multiple Page 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 either Single Page FEC or Multiple Page FEC. 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 can be used for both checks.

Pseudo-code for Decode Checks = 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 () to decode if both the first page (Page 0) and last page (Last Page Index) are missing.

Single Page

Encoding 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 is used in the XOR operations. For Page 0, a 23-byte null pad is used for the previous parity page. 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.

Example Single Page FEC Encoding

Decoding To decode Single Page 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 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.

Multiple Page

Encoding For Multiple Page FEC there are two variations: Frame Recovery and Page Recovery. Both follow a similar process, but are offset at what data is protected. For DRIP it is REQUIRED to use a Reed Solomon codes over a Galois Field of 2^8. The Galois Field is constructed using the primitive polynomial of 1 + x^2 + x^3 + x^4 + x^8. These parameters were selected as it commonly used in Reed Solomon implementations. A form of it was deployed by the National Aeronautics and Space Administration (NASA) for the Voyager probes ; a problem space (pun intended) with far tighter constraints than RID. is a generic example of Multiple Page FEC Authentication Message where 3-pages out of N are used for Reed Solomon FEC. The Additional Data Length is set to 72 as there are 10-bytes of padding and 62-bytes of parity from Reed Solomon.

Example Multiple Page FEC Encoding

• Informative Note: the last page in the example is padded to fill the full page as specified by .

Page Recovery Page Recovery in Multiple Page FEC protects the same content, just the Authentication Message, as Single Page FEC. The benefit is increased protection from a maximum of one page, up to the page maximum (minus pages being used for authentication). In Page Recovery, the Authentication Payload field of for each page is used. Reed Solomon is performed in a byte-wise fashion across each Authentication Page to generate a number of parity bytes. The number of these parity bytes directly corresponds to the number of pages of FEC to be append to the Authentication Message. The resulting parity bytes are placed to the corresponding bytes in the FEC pages. This can be considered a form of interleaving that takes advantage of the fixed page length. See for a detailed example of encoding for Page Recovery.

Frame Recovery Frame Recovery in Multiple Page FEC protects not just the Authentication Message it is carried in but also other ASTM Messages being sent. This is at the cost of much longer Authentication Messages. Up to 240 messages (255 minus 15 pages maximum for FEC) can be protected using Frame Recovery. For Frame Recovery both transmitter and receiver need to agree on what messages are being Reed Solomon'd over. It is RECOMMENDED that the data is limited to a full transmission set of ASTM Messages. For example in the European Union (EU) a full set of ASTM Messages would include: 1x Basic ID, 1x Location/Vector, 1x System and 1x Operator ID. With DRIP this would also included 1x Authentication that would be carrying the FEC (along with DRIP Authentication). Similar to , Reed Solomon is performed in a bytes-wise fashion across messages to generate the desired number of parity bytes. These messages MUST be in Message Type order when performing the Reed Solomon operation. All 25-bytes of the ASTM Message are used during this operation for Frame Recovery. After the computation the new pseudo-frames formed by the parity are concatenated together. The length of this data is used in the calculation of the Additional Data Length with the amount of padding needed to align to a new Authentication Page. The padding and parity data are then placed in the Additional Data field. See for a detailed example of encoding for Frame Recovery.

Decoding To determine if Page Recovery or Frame Recovery is used two modulo checks with the ADL after the length of the null-pad is removed are needed. One against the value of 23, and the other against the value of 25. If 23 comes back with a value of 0 then Page Recovery is being used. If 25 comes back with 0 then Frame Recovery is used. Any other combination indicates an error. As it is known which pages were not received in an Authentication Message (or were erased by Bluetooth due to detected errors), the Reed Solomon capacity can be dedicated exclusively to correction of erasures, rather than to detection and correction of errors, thereby doubling its effective capacity. This is accomplished by marking the erasures, i.e., filling the dummy page(s) or frames with nulls. For either Page Recovery or Frame recovery the first step on the receiver is to create empty (or dummy) Authentication Pages for any pages missing in the Authentication Message. Then the Additional Data can be extracted from the Authentication Message, have its null-padding removed and further processed.

Page Recovery To decode Page Recovery, the received FEC data along with the Authentication Payload of each Authentication Page has Reed Solomon performed using erasures byte-wise across the pages. The results should have all pages of the Authentication Message recovered. The receiver SHOULD validate the rebuilt message before decoding the actual authentication.

Frame Recovery To decode Frame Recovery, the receiver breaks the Additional Data into 25-byte chunks. This will produce the pseudo-frames of parity bytes from the Authentication Message. To build the rest of the message set, static messages such as Basic ID and Operator ID are constant and can be filled in using any received copy that is cache by the receiver for that UA. Dynamic messages in the set, such as Location/Vector or System can be nulled out to have Reed Solomon alway recover them and the results checked against cached copies from recent transmissions of the UA. With all the frames in the set, Reed Solomon can be used in an erasure mode to decode byte-wise across the frames to fill in the erasures and rebuild the entire set of messages. Validation of the Authentication Message SHOULD be performed before further processing of authentication data.

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.

Requirements & Recommendations

Legacy Transports With Legacy Advertisements the goal is to attempt to bring reliable receipt of the paged Authentication Message. FEC () MUST be used, per mandated RID rules (for example the US FAA RID Rule ), 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.

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). 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 () MUST NOT be used as it is impractical.

Authentication It is REQUIRED that a UA send the following DRIP Authentication Formats to fulfill the requirements in :

1. SHOULD: send DRIP Link () using the Broadcast Endorsement: DIME:Apex, DIME:RAA (satisfying GEN-3); at least once per 5 minutes
2. MUST: send DRIP Link () using the Broadcast Endorsement: DIME:RAA, DIME:HDA (satisfying GEN-3); at least once per 5 minutes
3. MUST: send DRIP Link () using the Broadcast Endorsement: DIME: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 () or DRIP Wrapper ()) 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

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: DIME, UA. A separate issue is the frequency of transmitting the DRIP Link Authentication Message for the Broadcast Endorsement: DIME, UA when using a Manifest Message. 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 ):

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 .
3. Under Legacy Transports, generate and transmit every 5 seconds a DRIP Manifest () hashing as many sets of recent CAA required ASTM Messages. The system MAY periodically replace the DRIP Manifest with a DRIP Wrapper () containing at least a Location Message (Message Type 0x2).
4. Under both Legacy or Extended Transports, generate and transmit a DRIP Link's () containing; 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 .

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 . 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 format 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.

UAS RID Trust Assessment As described in , 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 ) 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.

Summary of Addressed DRIP Requirements The following are addressed in this document: ID-5: Non-spoofability

• Addressed using the DRIP Wrapper (), DRIP Manifest () or DRIP Frame ().

GEN-1: Provable Ownership

• Addressed using the DRIP Link () and DRIP Wrapper (), DRIP Manifest () or DRIP Frame ().

GEN-2: Provable Binding

• Addressed using the DRIP Wrapper (), DRIP Manifest () or DRIP Frame ().

GEN-3: Provable Registration

• Addressed using the DRIP Link ().

ICAO Considerations DRIP requests the following SAM Types to be allocated:

1. DRIP Link
2. DRIP Wrapper
3. DRIP Manifest
4. DRIP Frame

IANA Considerations

IANA DRIP Registry This document requests a new subregistry for Frame Type under the DRIP registry.

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 ). The following values are defined:

Security Considerations

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 message 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 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.

VNA Timestamp Offsets for DRIP Authentication Formats Note the discussion of VNA Timestamp offsets here is in context of the DRIP Wrapper (), DRIP Manifest () and DRIP Frame (). For DRIP Link () these offsets are set by the DIME and have their own set of considerations as seen in . 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.

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).
• Many thanks to Rick Salz for the secdir review.

References Normative References This document defines terminology and requirements for solutions produced by the Drone Remote Identification Protocol (DRIP) Working Group. These solutions will support Unmanned Aircraft System Remote Identification and tracking (UAS RID) for security, safety, and other purposes (e.g., initiation of identity-based network sessions supporting UAS applications). DRIP will facilitate use of existing Internet resources to support RID and to enable enhanced related services, and it will enable online and offline verification that RID information is trustworthy. AX Enterprize AX Enterprize HTT Consulting Intel Linköping University This document describes an architecture for protocols and services to support Unmanned Aircraft System (UAS) Remote Identification (RID) and tracking, plus UAS RID-related communications. This architecture adheres to the requirements listed in the DRIP Requirements document (RFC9153). In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. National Institute of Standards and Technology National Institute of Standards and Technology National Institute of Standards and Technology Informative References Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. HTT Consulting AX Enterprize AX Enterprize Linköping University This document describes the use of Hierarchical Host Identity Tags (HHITs) as a self-asserting and thereby trustable Identifier for use as the UAS Remote ID. HHITs include explicit hierarchy to provide Registrar discovery for 3rd-party ID attestation. AX Enterprize, LLC AX Enterprize, LLC HTT Consulting RTFM llp This document details the required mechanisms for the registration and discovery of DRIP Entity Tags (DETs). The registration process relies upon the Extensible Provisioning Protocol (EPP). The discovery process leverages DNS, DNSSEC, and related technology. The lookup process relies upon the Registration Data Access Protocol (RDAP). The DETs can be registered with as their "raw public keys" or in X.509 certificates.

Authentication State Diagrams & Color Scheme 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 - 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 and ) 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.

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

State Diagrams This section gives some RECOMMENDED state flows that DRIP should follow. Note that the state diagrams do not have all error conditions mapped.

Notations

Diagram Notations Transition Option False/No -----> Transition Option True/Yes ]]>

General

Standard Authentication Colors/State 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

DRIP SAM

DRIP 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

DRIP Link

DRIP Link State Decoder 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

DRIP Wrapper/Manifest/Frame

DRIP Wrapper/Manifest/Frame State Decoder 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

Broadcast Endorsement: DIME, UA

Example DRIP Broadcast Endorsement: DIME, UA

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 ). As the above example shows to properly authenticate both a DRIP Link and a DRIP Wrapper or DRIP Manifest are required.

Additional FEC Information

Optional Decoding Heuristic With , 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 () of any DRIP Authentication Format the payload data is always a DET. For DRIP Link () this DET is of the DIME while for DRIP Wrapper (), Manifest () and Frame () it is the DET of the DIME.

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 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, 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.

Example: Page Recovery The following example is an Authentication Message with 7 pages that 3 pages of parity are to be generated for. The first column is just the Page Header with a visual space here to show the boundary. For Page Recovery the first column is ignored and the last 23-bytes of each page are extracted to have Reed Solomon performed on them in a column wise fashion to produce parity bytes. This can be considered a form of interleaving that takes advantage of the fixed page length. For the example the following 3-bytes of parity are generated with the first byte of each page: Each set of parity is the placed into a pseudo-frame as follows (each byte in its own message in the same column). Below is an example of the full parity generated and each 23-bytes of parity added into the additional pages as Additional Data:

Example: Frame Recovery Below is an example of a number of messages. The first column is an additional ASTM Header that contain the Message Type; with a visual space for clarity. The last 24-bytes are the actual message contents; be it location information or an Authentication Page. A similar process is followed as in . Here every column of bytes has parity generated for it (even the ASTM Header). In the below example 5-bytes of parity are generated using the ASTM Header column: After doing this to all columns the following pseudo-frames would have been generated: These 25-byte chunks are now concatenated together and are placed in Authentication Pages, using the Additional Data, 23-bytes at a time. In the below figure the first column is the ASTM Header as before, the second column is the Page Header for each Authentication Page and then last column is the 23-bytes of data for each page.

Operational Recommendation Proof The recommendations found in () 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 ().

Definitions Frame:

• A single Bluetooth 4.x frame containing an ASTM Message or ASTM Authentication Page.

Cycle:

• A set of frames transmitted in a 1 second window.

Methodology In the US, the required ASTM Messages to be transmitted every cycle 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.

• Information Note: in Europe the Operator ID Message (0x5) is also included; pushing the frame count to 8 per cycle.

To calculate the frame count of a given DRIP Authentication Message the following formula is used:

• 1 + ceiling(((88 + (Item Size * Item Count)) - 16) / 23) + 1

The leading 1 is counting for the Page 0 which is always present. 88 is the number of bytes for the DRIP framing in every DRIP Authentication Format, this is the DET (16-bytes), timestamps (8-bytes) and signature (64-bytes). Item Size (in bytes) is size of each item in a given format; for Wrapper it is 25 (a full ASTM Message), while for Manifest it is 8 (A single hash). 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 all DRIP Authentication Message frame counts we have the following: Frame Counts

Item Count	Wrapper Frames	Manifest Frames	Total Frames (Wrapper, Manifest)
1	7	6	8, 7
2	8	6	10, 8
3	9	7	12, 10
4	10	7	14, 11
5	N/A	7	N/A, 12
6	N/A	8	N/A, 14
7	N/A	8	N/A, 15
8	N/A	8	N/A, 16
9	N/A	9	N/A, 18
10	N/A	9	N/A, 19
11	N/A	9	N/A, 20
12	N/A	10	N/A, 22

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.

US Examples Each row represents a second and single cycle consisting of two sets of required messages (2x Basic ID, Location, System). Each DRIP Manifest contains the previous 2 seconds worth of required messages. Every required message is manifested. Needs 2 prime seconds, could be filled with a link or a wrapper? 1x link every 8 seconds Manifest every 2 seconds (half coverage of stream - can get full by only doing 1/2 of each second?), Wrapper every 2 seconds, 1x Link every 6 seconds, 1x Link every 16 seconds. This gives 1 long running link, 1 short running link, 1 wrapper every 2 seconds and 1 manifest every 2 seconds covering last 4 seconds (1/2 of each second) Either link could be chosen to be replaced by another link or not sent at all Manifest every 2 seconds (full stream coverage), Link every 2 seconds, could send 3x more links (2 pages of one in second 4, 2 pages each of 2 in second 2).