diff --git a/blip-tap.md b/blip-tap.md index 941b216..099a63e 100644 --- a/blip-tap.md +++ b/blip-tap.md @@ -919,149 +919,316 @@ order to clamp down on price volatility exposure. TODO(roasbeef): explain AMP/keyspend, basically unsolicited quote req, can accept if favorable - #### RFQ Negotiation -##### Request For Quote (`tap_rfq`) +This section describes the Request-For-Quote (RFQ) negotiation process. A +successful outcome of the RFQ process between two peers is an agreed currency +exchange rate. This rate can be used to convert a TAP asset amount into BTC, or +vice versa. -When a receiver wishes to receive `N` units of TAP asset ID `asset_id`, a new -p2p message `tap_rfq` is sent with the following structure: +##### Request For Quote (`tap_rfq`) -1. type: ?? (`tap_rfq`) -2. data: - * [`32*byte`:`rfq_id`] - * [`32*byte`:`asset_id`] - * [`BigSize`:`asset_amt`] - * [`BigSize`:`suggested_rate_tick`] +An RFQ negotiation is initiated when a requesting peer sends a new quote request +p2p message called `tap_rfq` to their channel counterparty. + +In this message, the peer includes the asset they wish to obtain (identified by +`in_asset_id`/`in_asset_group_key`) and the asset they wish to divest +(identified by `out_asset_id`/`out_asset_group_key`). + +The `tap_rfq` message has a BOLT 01 wire message type integer of 52884, falling +within the custom message range. Its TLV stream consists of the following TLV +fields: + +| TLV Type | Field Name | Field Type | +|----------|---------------------------|------------| +| 0 | version | 1*byte | +| 2 | rfq_id | 32*byte | +| 4 | transfer_type | 1*byte | +| 6 | expiry | 8*byte | +| 9 | in_asset_id | 32*byte | +| 11 | in_asset_group_key | 33*byte | +| 13 | out_asset_id | 32*byte | +| 15 | out_asset_group_key | 33*byte | +| 16 | max_transferable_in_asset | BigSize | +| 19 | in_asset_to_btc | FixedPoint | +| 21 | out_asset_to_btc | FixedPoint | where: -* `rfq_id` is a randomly generate 32-byte value to uniquely identify this RFQ - request -* `asset_id` is the asset ID of the asset the receiver intends to receive -* `asset_amt` is the amount of units of said asset -* `suggested_rate_tick` is the internal unit used for asset conversions. A tick - is 1/10000th of a currency unit. It gives us up to 4 decimal places of - precision (0.0001 or 0.01% or 1 bps). As an example, if the BTC/USD rate was - $61,234.95, then we multiply that by 10,000 to arrive at the `usd_rate_tick`: - `$61,234.95 * 10000 = 612,349,500`. To convert back to our normal rate, we - decide by `10,000` to arrive back at `$61,234.95`. - -Given valid `rfq_id`, we then define an `tap_rfq_scid` by taking the last `8` -bytes of the `rfq_id` and interpreting them as a 64-bit integer. +* `version` is the version of the quote request message; the current version + is `1`. + +* `rfq_id` is a randomly generated 32-byte value to uniquely identify this RFQ + request. + +* `transfer_type` (`uint8`): Specifies the type of transaction that will take + place if the quote request results in an accepted agreement. It is set to: + * `0` when the transfer type is unspecified. + * `1` when the requesting peer is attempting to pay a lightning invoice. + * `2` when the requesting peer is seeking to receive funds corresponding to + a lightning invoice. + +* `expiry` is the Unix timestamp in seconds representing the exact time after + which the quote request and rate information are no longer valid. + +* `in_asset_id` represents the identifier of the asset which will be inbound + to the requesting peer (therefore outbound to the counterparty peer). The + values of `in_asset_id` and `in_asset_group_key` must both be set to all + zeros to indicate that the in-asset is BTC milli-satoshis. This field is + optional; however, either `in_asset_id` or `in_asset_group_key` must be set. + +* `in_asset_group_key` is the serialized compressed public group key of the + asset which will be inbound to the requesting peer (therefore outbound to + the counterparty). The values of `in_asset_id` and `in_asset_group_key` must + both be set to all zeros to indicate that the in-asset is BTC milli-satoshis. + This field is optional; however, either `in_asset_id` or `in_asset_group_key` + must be set. + +* `out_asset_id` represents the identifier of the asset which will be outbound + to the requesting peer (and therefore inbound to the counterparty). The values + of `out_asset_id` and `out_asset_group_key` must both be set to all zeros to + indicate that the out-asset is BTC milli-satoshis. This field is optional; + however, either `out_asset_id` or `out_asset_group_key` must be set. + +* `out_asset_group_key` is the serialized compressed public group key of the + asset which will be outbound to the requesting peer (and therefore inbound to + the counterparty). The values of `out_asset_id` and `out_asset_group_key` must + both be set to all zeros to indicate that the out-asset is BTC milli-satoshis. + This field is optional; however, either `out_asset_id` or + `out_asset_group_key` must be set. + +* `max_transferable_in_asset` represents the maximum quantity of in-asset that + the counterparty is expected to divest, whether the asset involved is BTC or + otherwise. + +* `in_asset_to_btc` is an optional proposed in-asset to BTC conversion rate + represented as a [fixed-point number](#fixed-point-number-type). When the + in-asset is BTC milli-satoshis (msats), this field should be set to 100 + billion since there are 100 billion msats in a BTC. + +* `out_asset_to_btc` is an optional proposed out-asset to BTC conversion rate + represented as a [fixed-point number](#fixed-point-number-type). When the + out-asset is BTC milli-satoshis (msats), this field should be set to 100 + billion since there are 100 billion msats in a BTC. + +Note that all numeric types, except for `BigSize` types, are encoded in +big-endian byte order. + +Given a valid `rfq_id`, the RFQ specific SCID `tap_rfq_scid` is defined by +taking the last `8` bytes of the `rfq_id` and interpreting them as a 64-bit +integer. ###### Requirements The sender: - - MUST ensure that the `tap_rfq_scid` mapping of an `rfq_id` doesn't collide - with the `scid` of any active channels. +- MUST ensure that the `tap_rfq_scid` mapping of an `rfq_id` doesn't collide + with the `scid` of any active channels. - - MUST ensure that an `rfq_id` is never repeated for the lifetime of a - connection +- MUST ensure that an `rfq_id` is never repeated for the lifetime of a + connection. - It is recommended that the value be generated using a CSPRNG. Otherwise, a simple counter system from a starting value can be used, with the nonce offer incrementing by one each time. - - MUST set `asset_id` to the ID of an asset contained in the backing channel. +- MUST set either `in_asset_id` or `in_asset_group_key`, but not both. If set + to all zeros, `in_asset_id` represents BTC. + +- MUST set either `out_asset_id` or `out_asset_group_key`, but not both. If + set to all zeros, `out_asset_id` represents BTC. + +- MUST ensure that the specified TAP assets are present in the backing + channel. - - SHOULD specify reasonable values for `suggested_rate_tick` +- SHOULD specify reasonable values for `expiry`, `in_asset_to_btc`, + `out_asset_to_btc`, and `max_transferable_in_asset`. The recipient: - - SHOULD send a `tap_rfq_reject` message if `rfq_id` has been used before +- SHOULD send a `tap_rfq_reject` message if `rfq_id` has been used before. - - MUST send an `tap_rfq_reject` message if `asset_id` is not committed to in - the open channel +- MUST send a `tap_rfq_reject` message if `in_asset_id` is not committed to + in the open channel, unless `in_asset_id` is all zeros, representing BTC. - - MUST send an `tap_rfq_reject` message if the requested `asset_amt` is - greater than the settled remote balance of that asset +- MUST send a `tap_rfq_reject` message if `out_asset_id` is not committed to + in the open channel, unless `out_asset_id` is all zeros, representing BTC. - - SHOULD take the `suggested_rate_tick` values into account when deciding - whether to accept or reject the quote +- MUST send a `tap_rfq_reject` message if both `in_asset_id` and + `out_asset_id` are all zeros, representing BTC, unless either + `in_asset_group_key` or `out_asset_group_key` is set. + +- MUST send a `tap_rfq_reject` message if the requested + `max_transferable_in_asset` is greater than the settled remote balance of + that asset. + +- SHOULD take the values of `expiry`, `in_asset_to_btc`, `out_asset_to_btc`, and + `max_transferable_in_asset` into account when deciding whether to accept or + reject the quote request. #### Request For Quote Response (`tap_rfq_accept`) + (`tap_rfq_reject`) Once the edge node has received, the `tap_rfq` message, then it should decide if it's able to accommodate the quote or not. -#### Accepting Quotes (`tap_rfq_accept`) - -If it can, then it should send `tap_rfq_accept` that returns the quote amount -the edge node is willing to observe to move `N` units of asset `asset_id`: +#### Accepting Quotes (`tap_rfq_accept`) -1. type: ?? (`tap_req_accept`) -2. data: - * [`32*byte`:`rfq_id`] - * [`BigSize`:`accepted_rate_tick] - * [`BigSize`:`expiry`] - * [`64*byte`:`rfq_sig`] +The edge node may optionally respond to the quote request with a +`tap_rfq_accept` message, indicating the acceptance of the quote request. This +message has a BOLT 01 wire message type integer of 52885, which falls within the +custom message range. The associated TLV stream includes the following TLV +fields: -TODO(roasbeef): tlv err where? +| TLV Type | Field Name | Field Type | +|----------|------------------|------------| +| 0 | version | 1*byte | +| 2 | rfq_id | 32*byte | +| 4 | expiry | 8*byte | +| 6 | rfq_sig | 64*byte | +| 8 | in_asset_to_btc | FixedPoint | +| 10 | out_asset_to_btc | FixedPoint | where: -* `rfq_id` matches the existing `rfq_id` of a set `tap_rfq` +* `version` is the version of the quote accept message. The current version is + `1`. -* `accepted_rate_tick` is the proposed rate for the volume unit expressed in - the internal unit of a `tick`. +* `rfq_id` matches the existing `rfq_id` of a set `tap_rfq`. * `expiry` is the quote expiry lifetime unix timestamp. The quote is only valid until this time. -* `rfq_sig` is a signature over the serialized contents of the message +* `rfq_sig` is a signature over the serialized contents of the message. + +* `in_asset_to_btc` is the in-asset to BTC conversion rate represented as a + [fixed-point number](#fixed-point-number-type). When the in-asset is BTC milli-satoshis (msats), + this field should be set to 100 billion since there are 100 billion msats in a + BTC. + +* `out_asset_to_btc` is the out-asset to BTC conversion rate represented as a + [fixed-point number](#fixed-point-number-type). When the out-asset is BTC milli-satoshis (msats), + this field should be set to 100 billion since there are 100 billion msats in a + BTC. + +Note that all numeric types, except for `BigSize` types, are encoded in +big-endian byte order. ##### Requirements The sender: - - MUST set `rfq_id` to the matching `rfq_id` sent in a prior `tap_rfq` message +- MUST set `rfq_id` to the matching `rfq_id` sent in a prior `tap_rfq` message. - - MUST set `accepted_rate_tick` to a value they deem to be an acceptable - exchange rate +- MUST set `expiry` to a future unix timestamp after which the quote is no + longer valid. - - MUST set `expiry` to a future unix timestamp after which the quote is no - longer valid +- MUST set `rfq_sig` to be a BIP-340 schnorr signature over the serialized + contents of the message without the `rfq_sig` serialized, using their node + public key. - - MUST set `rfq_sig` to be a BIP-340 schnorr signature over the serialized - contents of the message without the `rfq_sig` serialized, using their node - public key +- MUST set the fields `in_asset_to_btc` and `out_asset_to_btc` to appropriate + values based on the asset exchange rates. The recipient: - - MUST abandon the attempt if `rfq_sig` is invalid +- MUST abandon the RFQ session if `version` corresponds to an unsupported + version. + +- MUST abandon the RFQ session if `rfq_sig` is invalid. - - MUST abandon the attempt if they deem that `accepted_rate_tick` is - unreasonable +- MUST abandon the attempt if they deem that `in_asset_to_btc` + or `out_asset_to_btc` is unreasonable. - - SHOULD no longer attempt to utilize the cleared quote after unix timestamp - `expiry` +- SHOULD no longer attempt to utilize the cleared quote after unix timestamp + `expiry`. #### Rejecting Quotes (`tap_rfq_reject`) In the event that an edge node is unable to satisfy a quote request, then they should send `tap_rfq_reject`, identifying the rejected quote ID. A quote might be rejected if the channel cannot accommodate the proposed volume, or if the -edge node is unwilling to carry any HTLCs for that `asset_id`. +edge node is unwilling to carry any HTLCs for the subject TAP asset(s). -1. type: ?? (`tap_req_accept`) -2. data: - * [`32*byte`:`rfq_id`] +The `tap_rfq_reject` message has a BOLT 01 wire message type integer of 52886, +falling within the custom message range. Its TLV stream consists of the +following TLV fields: + +| TLV Type | Field Name | Field Type | +|----------|------------|------------| +| 0 | version | 1*byte | +| 2 | rfq_id | 32*byte | +| 5 | err | RejectErr | where: -* `rfq_id` is the quote ID that they wish to reject +* `version` is the version of the quote reject message. The current version is + `1`. + +* `rfq_id` is the quote ID that they wish to reject. + +* `err` is an optional error field with TLV type `RejectErr`. It specifies the + reason for the rejection. `RejectErr` includes the following TLV fields: + + | TLV Type | Field Name | Field Type | + |----------|------------|------------| + | 0 | code | 1*byte | + | 2 | message | VarByte | + + where: + * `code` is an error code. + + * `message` is a variable length error message. + + Existing error codes and messages are as follows: + + | Error Code | Error Message | + |------------|--------------------------| + | 0 | unknown error | + | 1 | price oracle unavailable | + + The error message corresponding to code `0` is customizable. + +Note that all numeric types, except for `BigSize` types, are encoded in +big-endian byte order. ##### Requirements The sender: - - MUST set `rfq_id` to the matching `rfq_id` sent in a prior `tap_rfq` message +- MUST set `rfq_id` to the matching `rfq_id` sent in a prior `tap_rfq` + message. The recipient: - - MUST not attempt to send/receive using the rejected `rfq_id` +- MUST not attempt to send/receive using the rejected `rfq_id`. +#### Fixed-Point Number Type + +The fixed-point number type represents a scaled integer representation of a +fractional number. It is used to represent asset-to-BTC conversion rates in the +`tap_rfq` and `tap_rfq_accept` messages. + +This number type consists of two integer fields: a coefficient and a scale. +Using this format enables precise and consistent representation of rates while +avoiding floating-point data types, which are prone to precision errors. + +The relationship between the fractional representation of a rate and its +fixed-point representation is expressed as: +``` +R = F_c / (10^F_s) +``` +where: + +* `R` is the rate as a fraction. + +* `F_c` is the coefficient component of the fixed-point representation. It is + the scaled-up conversion rate represented as an integer. + +* `F_s` is the fractional scale component. It is an integer specifying how + many decimal places `F_c` should be divided by to obtain the fractional + representation of the conversion rate. + +When serialized into a wire message, the fixed-point number is represented as a +concatenation of its coefficient and scale fields: ``. #### First Hop TAP HTLC Onion Processing @@ -1076,9 +1243,10 @@ the payload for the first hop (the liquidity provider): The `tap_onion_rfq_id` MUST be present in the onion to the outgoing peer if the HTLC has a TAP asset origin. -In addition to the normal hop payload checks, the forwarding node MUST also -verify that the conversion rate of the outgoing HTLC as specified in the onion -matches the `accepted_rate_tick` of the corresponding `tap_rfq_id`. +In addition to performing the standard hop payload checks, the forwarding node +MUST verify that the outgoing HTLC amount, as specified in the onion, aligns +with the asset-to-BTC rates provided in the corresponding accepted RFQ quote +(identified by the `tap_rfq_id` field). When receiving an incoming onion TLV payload sourced from a TAP channel, the receiving node: @@ -1091,72 +1259,109 @@ receiving node: - MUST reject the payment if the `tap_rfq_id` has expired based on the posted `expiry` value. -- MUST reject the payment the `amt_to_forward != (amt_asset_incoming * tick * - msat_multiplier) / accepted_rate_tick - -where: - - * `amt_asset_incoming` is the HTLC value of the `asset_id` quote accepted in - the earlier RFQ round - - * `msat_multiplier` is a factor used to scale from `BTC/asset` to - `msat/asset_id`, this value is constant within the protocol and is derived - by multiplying the number of sats per Bitcoin, by the amount of sats in an - msat: `msat_multiplier = 100_000_000 * 1000 = 100_000_000_000` (100 - billion) - - * As an example: - * If the incoming channel is a USD backed asset, and wishes to send $1000 - outbound, with a rate of $61234.95 (`accepted_rate_tick = 61234.95 * - tick = 612_349_500` then the forwarding node expects to be instructed - to send `1,633,054,326 mSAT` over the outgoing link: - * `amt_to_forward = ($1000 * tick * msat_multiplier) // 612_349_500` - * `amt_to_forward = (1_000 * 10_000 * 100_000_000_000) // 612_349_500` - * `amt_to_forward = (1000000000000000000) // 612_349_500` - * `amt_to_forward = 1_633_054_326` +- MUST reject the payment if + ``` + amt_msat_to_forward > (amt_asset_incoming / incoming_asset_to_btc) * btc_to_msat_multiplier + ``` + This equation stipulates an upper bound on the HTLC msat value that the + liquidity provider should forward to the next hop. + + In this equation, the right-hand side converts the incoming TAP asset amount + to BTC by dividing it by the asset-to-BTC conversion rate + (`incoming_asset_to_btc`). The resulting BTC value is then converted to + milli-satoshis (msats). + + The terms in this equation are defined as follows: + + * `amt_msat_to_forward` is the HTLC msat value that the liquidity provider + should forward to the next hop. + + * `amt_asset_incoming` is the HTLC value of the incoming asset amount. + + * `btc_to_msat_multiplier` is a constant used to convert BTC to + milli-satoshis (msats). It is defined as: + ``` + btc_to_msat_multiplier = 100_000_000 * 1000 = 100_000_000_000` + ``` + + This constant combines two conversions: + 1. BTC to Satoshis: 1 BTC = 100,000,000 satoshis. + 2. Satoshis to msats: 1 satoshi = 1,000 msats. + + * `incoming_asset_to_btc` represents the fractional conversion rate between + the incoming asset and BTC. This rate is derived from the + [fixed-point](#fixed-point-number-type) conversion rate, which was agreed + upon during the RFQ negotiation between the sender and the liquidity + provider. #### Last Hop TAP HTLC Onion Processing -In the event that the last hop in a route receive a payload that has an +In the event that the last hop in a route receive a payload that has a `short_channel_id` value that matches a prior accepted `rfq_scid` value, then this indicates that the sender is attempting to pay a receiver in the asset bound by the `rfq_id` and `rfq_scid`. -Note that we don't require that the sender use any special values other than -what is already known in the existing protocol. This enables _unupgraded_ -senders to send BTC, with the receiver obtaining their asset of choice, without -the sender needing to worry about exchange rates at all. +Note that no special values are required beyond what is already known in the +existing protocol. This enables _un-upgraded_ senders to send BTC, allowing the +receiver to obtain their asset of choice without the sender needing to worry +about exchange rates at all. -When receieving an incoming onion payload with a known `rfq_scid` value, the +When receiving an incoming onion payload with a known `rfq_scid` value, the receiver: - MUST reject the HTLC is `tap_scid` is expired based on the posted `expiry` value -- MUST reject the entire HTLC set if at anytime, the sum of HTLCs (the - `amt_to_forward` field) targetting `tap_rfq_scid` eceeds the negotiated +- MUST reject the entire HTLC set if at any time, the sum of HTLCs (the + `amt_to_forward` field) targeting `tap_rfq_scid` exceeds the negotiated `asset_amt` field (volume for quote exhausted) - MUST extend a TAP HTLC with an `asset_id` corresponding to the accepted - `tap_rfq_scid` with an asset value of: `((amt_asset_incoming * - accepted_rate_tick) // msat_multiplier) / tick` - - * As an example: - * If the outgoing channel (last hop receiver) is a USD backed asset, and - requested an invoice `1_633_054_326 mSAT`, but wants USD with an accepted - tick rate of `612_349_500` ($61,234.95), then the penultimate node should - send $1000 to the last hop: - * `amt_to_forward = ((incoming_amt_msat * accepted_rate_tick) // msat_multiplier) / tick` - * `amt_to_forward = ((1_633_054_326 * 612_349_500) // 100_000_000_000) / 10_000` - * `amt_to_forward = ((999999999998937000) // 100_000_000_000) / 10_000` - * `amt_to_forward = (9999999 / 10_000)` - * `amt_to_forward = (9999999 / 10_000)` - * `amt_to_forward = 999.9` - * Note that all assets internally are accounted in a unit of a `tick` - (1/1000th) of an asset. When convering back to the main asset, the value - should be rounded up, giving us the original value of `$1000`. + `tap_rfq_scid` with an asset value of: + ``` + amt_asset_to_forward = incoming_amt_msat * (asset_to_btc_rate / btc_to_msat_multiplier) + ``` + where: + * `amt_asset_to_forward` is the amount of the TAP asset that the receiver + should forward to the next hop. + + * `incoming_amt_msat` is the amount of the incoming HTLC in milli-satoshis. + + * `asset_to_btc_rate` refers to the asset-to-BTC conversion rate established + during the RFQ negotiation between the sender and the liquidity provider. + This fractional rate is derived from the + [fixed-point](#fixed-point-number-type) representation of the conversion + rate found in the RFQ wire messages. + + * `btc_to_msat_multiplier` is a constant used to convert BTC to milli-satoshis + (msats). It is defined as: + ``` + btc_to_msat_multiplier = 100_000_000 * 1000 = 100_000_000_000` + ``` + + This constant combines two conversions: + 1. BTC to Satoshis: 1 BTC = 100,000,000 satoshis. + 2. Satoshis to msats: 1 satoshi = 1,000 msats. + + Here's an example of how the `amt_asset_to_forward` value is calculated: + + Consider an outgoing channel which uses a USD-backed asset. The last hop + receiver issues an invoice for `1_633_054_326 mSAT` but expects USD. The + accepted asset-to-BTC conversion rate expressed as a + [fixed-point](#fixed-point-number-type) has a coefficient of `612_349_500` and + a scale of `4`. The fractional conversion rate is therefore `61_234.95` + (implying `$61,234.95 = 1BTC`). As a result, the penultimate node should send + $1,000 to the last hop: + + * `amt_asset_to_forward = incoming_amt_msat * (asset_to_btc_rate / btc_to_msat_multiplier)` + * `amt_asset_to_forward = 1_633_054_326 * (61_234.95 / 100_000_000_000)` + * `amt_asset_to_forward = 999.9999999989369` + * `⌊amt_asset_to_forward⌋ = 999` + +Integer division is used in this formulation (rounding down). Invoices may be +settled with an underpayment of up to 1 asset unit. #### Invoice Format @@ -1175,24 +1380,32 @@ When creating an invoice, the creator MUST ensure that the invoice expiry value is set exactly to the lifetime of the accepted RFQ based on the RFQ `expiry` unix timestamp. -When creating an invoice from an `accepted_rate_tick`, with a base currency of -`asset_id`, a conversion MUST be carried out to express the desired amount in -terms of _BTC_ rather than the asset tick. As an example: - - * A user wants to receive $100 over their USD backed channel. The - `accepted_rate_tick` that can satisfy that volume, and hasn't expired yet - is `612_349_500` or `$61,234.95`. To arrive at the `mSAT` value they should - put into the invoice: - * `invoice_amt = (asset_amt * tick * msat_multiplier) / accepted_rate_tick` - * `invoice_amt = (100 * 10_000 * 100_000_000_000) / 612_349_500` - * `invoice_amt = (100000000000000000) / 612_349_500` - * `invoice_amt = (100000000000000000) / 612_349_500` - * `invoice_amt = 163305432 mSAT` - * `invoice_amt = 163305 SAT` - -Always expressing the invoice amount in BTC/mSAT ensures that unpugraded -senders will be able to send over these asset channels. +When creating an invoice, the asset-to-BTC conversion rate is used to express +the invoice amount in milli-satoshis (msats), not in the TAP asset units that +the invoice creator wants to receive. This conversion rate is agreed upon during +the RFQ process. + +For example, consider the case where a user wants to receive $100 over their USD +backed channel. An RFQ quote was agreed with a fractional conversion rate +`asset_to_btc_rate`. The agreed quote was sufficient to support a volume of +$100. + +The conversion rate as a [fixed-point](#fixed-point-number-type) found in the +RFQ wire messages has a coefficient of `612_349_500` and a scale of `4`. The +fractional conversion rate is therefore `61_234.95` (implying +`$61,234.95 = 1BTC`). + +The user calculates the invoice amount as follows: +* `invoice_amt = (asset_amt / asset_to_btc_rate) * btc_to_msat_multiplier` +* `invoice_amt = (100 / 61_234.95) * 100_000_000_000` +* `invoice_amt = 163_305_432.60017` +* `⌊invoice_amt⌋ = 163_305_432 mSAT` + +Integer division is used in this formulation (rounding down). Invoices may be +settled with an underpayment of up to 1 asset unit. +Always expressing the invoice amount in BTC/mSAT ensures that senders who have +not upgraded can still send payments over these asset channels. ## Universality