X-Git-Url: http://git.bitcoin.ninja/index.cgi?a=blobdiff_plain;f=lightning%2Fsrc%2Fevents%2Fbump_transaction.rs;h=1b019ba349e0ae8687a190b92ebd144fe164f6f9;hb=7c9463668a4f71663746e57bdb09ee6b91797d5a;hp=55c12d23e749cf020a7308f998ab686f28ea22d3;hpb=f534ce26c90db49cc2eb7b42e8cae49c109758d4;p=rust-lightning diff --git a/lightning/src/events/bump_transaction.rs b/lightning/src/events/bump_transaction.rs index 55c12d23..1b019ba3 100644 --- a/lightning/src/events/bump_transaction.rs +++ b/lightning/src/events/bump_transaction.rs @@ -21,20 +21,23 @@ use crate::ln::channel::ANCHOR_OUTPUT_VALUE_SATOSHI; use crate::ln::chan_utils; use crate::ln::chan_utils::{ ANCHOR_INPUT_WITNESS_WEIGHT, HTLC_SUCCESS_INPUT_ANCHOR_WITNESS_WEIGHT, - HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT, ChannelTransactionParameters, HTLCOutputInCommitment + HTLC_TIMEOUT_INPUT_ANCHOR_WITNESS_WEIGHT, HTLCOutputInCommitment }; -use crate::ln::features::ChannelTypeFeatures; -use crate::ln::PaymentPreimage; use crate::prelude::*; -use crate::sign::{EcdsaChannelSigner, SignerProvider, WriteableEcdsaChannelSigner, P2WPKH_WITNESS_WEIGHT}; +use crate::sign::{ + ChannelDerivationParameters, HTLCDescriptor, SignerProvider, P2WPKH_WITNESS_WEIGHT +}; +use crate::sign::ecdsa::{EcdsaChannelSigner, WriteableEcdsaChannelSigner}; use crate::sync::Mutex; use crate::util::logger::Logger; -use bitcoin::{OutPoint, PackedLockTime, PubkeyHash, Sequence, Script, Transaction, Txid, TxIn, TxOut, Witness, WPubkeyHash}; +use bitcoin::{OutPoint, PubkeyHash, Sequence, ScriptBuf, Transaction, TxIn, TxOut, Witness, WPubkeyHash}; use bitcoin::blockdata::constants::WITNESS_SCALE_FACTOR; +use bitcoin::blockdata::locktime::absolute::LockTime; use bitcoin::consensus::Encodable; +use bitcoin::psbt::PartiallySignedTransaction; use bitcoin::secp256k1; -use bitcoin::secp256k1::{PublicKey, Secp256k1}; +use bitcoin::secp256k1::Secp256k1; use bitcoin::secp256k1::ecdsa::Signature; const EMPTY_SCRIPT_SIG_WEIGHT: u64 = 1 /* empty script_sig */ * WITNESS_SCALE_FACTOR as u64; @@ -43,26 +46,6 @@ const BASE_INPUT_SIZE: u64 = 32 /* txid */ + 4 /* vout */ + 4 /* sequence */; const BASE_INPUT_WEIGHT: u64 = BASE_INPUT_SIZE * WITNESS_SCALE_FACTOR as u64; -/// The parameters required to derive a channel signer via [`SignerProvider`]. -#[derive(Clone, Debug, PartialEq, Eq)] -pub struct ChannelDerivationParameters { - /// The value in satoshis of the channel we're attempting to spend the anchor output of. - pub value_satoshis: u64, - /// The unique identifier to re-derive the signer for the associated channel. - pub keys_id: [u8; 32], - /// The necessary channel parameters that need to be provided to the re-derived signer through - /// [`ChannelSigner::provide_channel_parameters`]. - /// - /// [`ChannelSigner::provide_channel_parameters`]: crate::sign::ChannelSigner::provide_channel_parameters - pub transaction_parameters: ChannelTransactionParameters, -} - -impl_writeable_tlv_based!(ChannelDerivationParameters, { - (0, value_satoshis, required), - (2, keys_id, required), - (4, transaction_parameters, required), -}); - /// A descriptor used to sign for a commitment transaction's anchor output. #[derive(Clone, Debug, PartialEq, Eq)] pub struct AnchorDescriptor { @@ -88,14 +71,14 @@ impl AnchorDescriptor { pub fn unsigned_tx_input(&self) -> TxIn { TxIn { previous_output: self.outpoint.clone(), - script_sig: Script::new(), + script_sig: ScriptBuf::new(), sequence: Sequence::ENABLE_RBF_NO_LOCKTIME, witness: Witness::new(), } } /// Returns the witness script of the anchor output in the commitment transaction. - pub fn witness_script(&self) -> Script { + pub fn witness_script(&self) -> ScriptBuf { let channel_params = self.channel_derivation_parameters.transaction_parameters.as_holder_broadcastable(); chan_utils::get_anchor_redeemscript(&channel_params.broadcaster_pubkeys().funding_pubkey) } @@ -110,126 +93,7 @@ impl AnchorDescriptor { /// Derives the channel signer required to sign the anchor input. pub fn derive_channel_signer(&self, signer_provider: &SP) -> S where - SP::Target: SignerProvider - { - let mut signer = signer_provider.derive_channel_signer( - self.channel_derivation_parameters.value_satoshis, - self.channel_derivation_parameters.keys_id, - ); - signer.provide_channel_parameters(&self.channel_derivation_parameters.transaction_parameters); - signer - } -} - -/// A descriptor used to sign for a commitment transaction's HTLC output. -#[derive(Clone, Debug, PartialEq, Eq)] -pub struct HTLCDescriptor { - /// The parameters required to derive the signer for the HTLC input. - pub channel_derivation_parameters: ChannelDerivationParameters, - /// The txid of the commitment transaction in which the HTLC output lives. - pub commitment_txid: Txid, - /// The number of the commitment transaction in which the HTLC output lives. - pub per_commitment_number: u64, - /// The key tweak corresponding to the number of the commitment transaction in which the HTLC - /// output lives. This tweak is applied to all the basepoints for both parties in the channel to - /// arrive at unique keys per commitment. - /// - /// See for more info. - pub per_commitment_point: PublicKey, - /// The details of the HTLC as it appears in the commitment transaction. - pub htlc: HTLCOutputInCommitment, - /// The preimage, if `Some`, to claim the HTLC output with. If `None`, the timeout path must be - /// taken. - pub preimage: Option, - /// The counterparty's signature required to spend the HTLC output. - pub counterparty_sig: Signature -} - -impl_writeable_tlv_based!(HTLCDescriptor, { - (0, channel_derivation_parameters, required), - (2, commitment_txid, required), - (4, per_commitment_number, required), - (6, per_commitment_point, required), - (8, htlc, required), - (10, preimage, option), - (12, counterparty_sig, required), -}); - -impl HTLCDescriptor { - /// Returns the outpoint of the HTLC output in the commitment transaction. This is the outpoint - /// being spent by the HTLC input in the HTLC transaction. - pub fn outpoint(&self) -> OutPoint { - OutPoint { - txid: self.commitment_txid, - vout: self.htlc.transaction_output_index.unwrap(), - } - } - - /// Returns the UTXO to be spent by the HTLC input, which can be obtained via - /// [`Self::unsigned_tx_input`]. - pub fn previous_utxo(&self, secp: &Secp256k1) -> TxOut { - TxOut { - script_pubkey: self.witness_script(secp).to_v0_p2wsh(), - value: self.htlc.amount_msat / 1000, - } - } - - /// Returns the unsigned transaction input spending the HTLC output in the commitment - /// transaction. - pub fn unsigned_tx_input(&self) -> TxIn { - chan_utils::build_htlc_input(&self.commitment_txid, &self.htlc, &ChannelTypeFeatures::anchors_zero_htlc_fee_and_dependencies()) - } - - /// Returns the delayed output created as a result of spending the HTLC output in the commitment - /// transaction. - pub fn tx_output(&self, secp: &Secp256k1) -> TxOut { - let channel_params = self.channel_derivation_parameters.transaction_parameters.as_holder_broadcastable(); - let broadcaster_keys = channel_params.broadcaster_pubkeys(); - let counterparty_keys = channel_params.countersignatory_pubkeys(); - let broadcaster_delayed_key = chan_utils::derive_public_key( - secp, &self.per_commitment_point, &broadcaster_keys.delayed_payment_basepoint - ); - let counterparty_revocation_key = chan_utils::derive_public_revocation_key( - secp, &self.per_commitment_point, &counterparty_keys.revocation_basepoint - ); - chan_utils::build_htlc_output( - 0 /* feerate_per_kw */, channel_params.contest_delay(), &self.htlc, - &ChannelTypeFeatures::anchors_zero_htlc_fee_and_dependencies(), &broadcaster_delayed_key, &counterparty_revocation_key - ) - } - - /// Returns the witness script of the HTLC output in the commitment transaction. - pub fn witness_script(&self, secp: &Secp256k1) -> Script { - let channel_params = self.channel_derivation_parameters.transaction_parameters.as_holder_broadcastable(); - let broadcaster_keys = channel_params.broadcaster_pubkeys(); - let counterparty_keys = channel_params.countersignatory_pubkeys(); - let broadcaster_htlc_key = chan_utils::derive_public_key( - secp, &self.per_commitment_point, &broadcaster_keys.htlc_basepoint - ); - let counterparty_htlc_key = chan_utils::derive_public_key( - secp, &self.per_commitment_point, &counterparty_keys.htlc_basepoint - ); - let counterparty_revocation_key = chan_utils::derive_public_revocation_key( - secp, &self.per_commitment_point, &counterparty_keys.revocation_basepoint - ); - chan_utils::get_htlc_redeemscript_with_explicit_keys( - &self.htlc, &ChannelTypeFeatures::anchors_zero_htlc_fee_and_dependencies(), &broadcaster_htlc_key, &counterparty_htlc_key, - &counterparty_revocation_key, - ) - } - - /// Returns the fully signed witness required to spend the HTLC output in the commitment - /// transaction. - pub fn tx_input_witness(&self, signature: &Signature, witness_script: &Script) -> Witness { - chan_utils::build_htlc_input_witness( - signature, &self.counterparty_sig, &self.preimage, witness_script, &ChannelTypeFeatures::anchors_zero_htlc_fee_and_dependencies() /* opt_anchors */ - ) - } - - /// Derives the channel signer required to sign the HTLC input. - pub fn derive_channel_signer(&self, signer_provider: &SP) -> S - where - SP::Target: SignerProvider + SP::Target: SignerProvider { let mut signer = signer_provider.derive_channel_signer( self.channel_derivation_parameters.value_satoshis, @@ -279,8 +143,8 @@ pub enum BumpTransactionEvent { /// an empty `pending_htlcs`), confirmation of the commitment transaction can be considered to /// be not urgent. /// - /// [`EcdsaChannelSigner`]: crate::sign::EcdsaChannelSigner - /// [`EcdsaChannelSigner::sign_holder_anchor_input`]: crate::sign::EcdsaChannelSigner::sign_holder_anchor_input + /// [`EcdsaChannelSigner`]: crate::sign::ecdsa::EcdsaChannelSigner + /// [`EcdsaChannelSigner::sign_holder_anchor_input`]: crate::sign::ecdsa::EcdsaChannelSigner::sign_holder_anchor_input /// [`build_anchor_input_witness`]: crate::ln::chan_utils::build_anchor_input_witness ChannelClose { /// The unique identifier for the claim of the anchor output in the commitment transaction. @@ -333,9 +197,8 @@ pub enum BumpTransactionEvent { /// longer able to commit external confirmed funds to the HTLC transaction or the fee committed /// to the HTLC transaction is greater in value than the HTLCs being claimed. /// - /// [`EcdsaChannelSigner`]: crate::sign::EcdsaChannelSigner - /// [`EcdsaChannelSigner::sign_holder_htlc_transaction`]: crate::sign::EcdsaChannelSigner::sign_holder_htlc_transaction - /// [`HTLCDescriptor::tx_input_witness`]: HTLCDescriptor::tx_input_witness + /// [`EcdsaChannelSigner`]: crate::sign::ecdsa::EcdsaChannelSigner + /// [`EcdsaChannelSigner::sign_holder_htlc_transaction`]: crate::sign::ecdsa::EcdsaChannelSigner::sign_holder_htlc_transaction HTLCResolution { /// The unique identifier for the claim of the HTLCs in the confirmed commitment /// transaction. @@ -350,7 +213,7 @@ pub enum BumpTransactionEvent { /// by the same transaction. htlc_descriptors: Vec, /// The locktime required for the resulting HTLC transaction. - tx_lock_time: PackedLockTime, + tx_lock_time: LockTime, }, } @@ -395,7 +258,7 @@ impl Utxo { outpoint, output: TxOut { value, - script_pubkey: Script::new_p2pkh(pubkey_hash), + script_pubkey: ScriptBuf::new_p2pkh(pubkey_hash), }, satisfaction_weight: script_sig_size * WITNESS_SCALE_FACTOR as u64 + 1 /* empty witness */, } @@ -411,7 +274,7 @@ impl Utxo { outpoint, output: TxOut { value, - script_pubkey: Script::new_p2sh(&Script::new_v0_p2wpkh(pubkey_hash).script_hash()), + script_pubkey: ScriptBuf::new_p2sh(&ScriptBuf::new_v0_p2wpkh(pubkey_hash).script_hash()), }, satisfaction_weight: script_sig_size * WITNESS_SCALE_FACTOR as u64 + P2WPKH_WITNESS_WEIGHT, } @@ -423,7 +286,7 @@ impl Utxo { outpoint, output: TxOut { value, - script_pubkey: Script::new_v0_p2wpkh(pubkey_hash), + script_pubkey: ScriptBuf::new_v0_p2wpkh(pubkey_hash), }, satisfaction_weight: EMPTY_SCRIPT_SIG_WEIGHT + P2WPKH_WITNESS_WEIGHT, } @@ -481,7 +344,10 @@ pub trait CoinSelectionSource { ) -> Result; /// Signs and provides the full witness for all inputs within the transaction known to the /// trait (i.e., any provided via [`CoinSelectionSource::select_confirmed_utxos`]). - fn sign_tx(&self, tx: Transaction) -> Result; + /// + /// If your wallet does not support signing PSBTs you can call `psbt.extract_tx()` to get the + /// unsigned transaction and then sign it with your wallet. + fn sign_psbt(&self, psbt: PartiallySignedTransaction) -> Result; } /// An alternative to [`CoinSelectionSource`] that can be implemented and used along [`Wallet`] to @@ -491,11 +357,14 @@ pub trait WalletSource { fn list_confirmed_utxos(&self) -> Result, ()>; /// Returns a script to use for change above dust resulting from a successful coin selection /// attempt. - fn get_change_script(&self) -> Result; + fn get_change_script(&self) -> Result; /// Signs and provides the full [`TxIn::script_sig`] and [`TxIn::witness`] for all inputs within /// the transaction known to the wallet (i.e., any provided via /// [`WalletSource::list_confirmed_utxos`]). - fn sign_tx(&self, tx: Transaction) -> Result; + /// + /// If your wallet does not support signing PSBTs you can call `psbt.extract_tx()` to get the + /// unsigned transaction and then sign it with your wallet. + fn sign_psbt(&self, psbt: PartiallySignedTransaction) -> Result; } /// A wrapper over [`WalletSource`] that implements [`CoinSelection`] by preferring UTXOs that would @@ -547,7 +416,7 @@ where } } let fee_to_spend_utxo = fee_for_weight( - target_feerate_sat_per_1000_weight, BASE_INPUT_WEIGHT as u64 + utxo.satisfaction_weight, + target_feerate_sat_per_1000_weight, BASE_INPUT_WEIGHT + utxo.satisfaction_weight, ); let should_spend = if tolerate_high_network_feerates { utxo.output.value > fee_to_spend_utxo @@ -642,8 +511,8 @@ where .or_else(|_| do_coin_selection(true, true)) } - fn sign_tx(&self, tx: Transaction) -> Result { - self.source.sign_tx(tx) + fn sign_psbt(&self, psbt: PartiallySignedTransaction) -> Result { + self.source.sign_psbt(psbt) } } @@ -687,16 +556,16 @@ where } /// Updates a transaction with the result of a successful coin selection attempt. - fn process_coin_selection(&self, tx: &mut Transaction, mut coin_selection: CoinSelection) { - for utxo in coin_selection.confirmed_utxos.drain(..) { + fn process_coin_selection(&self, tx: &mut Transaction, coin_selection: &CoinSelection) { + for utxo in coin_selection.confirmed_utxos.iter() { tx.input.push(TxIn { previous_output: utxo.outpoint, - script_sig: Script::new(), + script_sig: ScriptBuf::new(), sequence: Sequence::ZERO, witness: Witness::new(), }); } - if let Some(change_output) = coin_selection.change_output.take() { + if let Some(change_output) = coin_selection.change_output.clone() { tx.output.push(change_output); } else if tx.output.is_empty() { // We weren't provided a change output, likely because the input set was a perfect @@ -706,7 +575,7 @@ where log_debug!(self.logger, "Including dummy OP_RETURN output since an output is needed and a change output was not provided"); tx.output.push(TxOut { value: 0, - script_pubkey: Script::new_op_return(&[]), + script_pubkey: ScriptBuf::new_op_return(&[]), }); } } @@ -726,20 +595,20 @@ where let must_spend = vec![Input { outpoint: anchor_descriptor.outpoint, previous_utxo: anchor_utxo, - satisfaction_weight: commitment_tx.weight() as u64 + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT, + satisfaction_weight: commitment_tx.weight().to_wu() + ANCHOR_INPUT_WITNESS_WEIGHT + EMPTY_SCRIPT_SIG_WEIGHT, }]; #[cfg(debug_assertions)] let must_spend_amount = must_spend.iter().map(|input| input.previous_utxo.value).sum::(); log_debug!(self.logger, "Peforming coin selection for commitment package (commitment and anchor transaction) targeting {} sat/kW", package_target_feerate_sat_per_1000_weight); - let coin_selection = self.utxo_source.select_confirmed_utxos( + let coin_selection: CoinSelection = self.utxo_source.select_confirmed_utxos( claim_id, must_spend, &[], package_target_feerate_sat_per_1000_weight, )?; let mut anchor_tx = Transaction { version: 2, - lock_time: PackedLockTime::ZERO, // TODO: Use next best height. + lock_time: LockTime::ZERO, // TODO: Use next best height. input: vec![anchor_descriptor.unsigned_tx_input()], output: vec![], }; @@ -751,22 +620,36 @@ where let total_input_amount = must_spend_amount + coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum::(); - self.process_coin_selection(&mut anchor_tx, coin_selection); + self.process_coin_selection(&mut anchor_tx, &coin_selection); let anchor_txid = anchor_tx.txid(); - debug_assert_eq!(anchor_tx.output.len(), 1); + // construct psbt + let mut anchor_psbt = PartiallySignedTransaction::from_unsigned_tx(anchor_tx).unwrap(); + // add witness_utxo to anchor input + anchor_psbt.inputs[0].witness_utxo = Some(anchor_descriptor.previous_utxo()); + // add witness_utxo to remaining inputs + for (idx, utxo) in coin_selection.confirmed_utxos.into_iter().enumerate() { + // add 1 to skip the anchor input + let index = idx + 1; + debug_assert_eq!(anchor_psbt.unsigned_tx.input[index].previous_output, utxo.outpoint); + if utxo.output.script_pubkey.is_witness_program() { + anchor_psbt.inputs[index].witness_utxo = Some(utxo.output); + } + } + + debug_assert_eq!(anchor_psbt.unsigned_tx.output.len(), 1); #[cfg(debug_assertions)] - let unsigned_tx_weight = anchor_tx.weight() as u64 - (anchor_tx.input.len() as u64 * EMPTY_SCRIPT_SIG_WEIGHT); + let unsigned_tx_weight = anchor_psbt.unsigned_tx.weight().to_wu() - (anchor_psbt.unsigned_tx.input.len() as u64 * EMPTY_SCRIPT_SIG_WEIGHT); log_debug!(self.logger, "Signing anchor transaction {}", anchor_txid); - anchor_tx = self.utxo_source.sign_tx(anchor_tx)?; + anchor_tx = self.utxo_source.sign_psbt(anchor_psbt)?; let signer = anchor_descriptor.derive_channel_signer(&self.signer_provider); let anchor_sig = signer.sign_holder_anchor_input(&anchor_tx, 0, &self.secp)?; anchor_tx.input[0].witness = anchor_descriptor.tx_input_witness(&anchor_sig); #[cfg(debug_assertions)] { - let signed_tx_weight = anchor_tx.weight() as u64; + let signed_tx_weight = anchor_tx.weight().to_wu(); let expected_signed_tx_weight = unsigned_tx_weight + total_satisfaction_weight; // Our estimate should be within a 1% error margin of the actual weight and we should // never underestimate. @@ -774,7 +657,7 @@ where expected_signed_tx_weight - (expected_signed_tx_weight / 100) <= signed_tx_weight); let expected_package_fee = fee_for_weight(package_target_feerate_sat_per_1000_weight, - signed_tx_weight + commitment_tx.weight() as u64); + signed_tx_weight + commitment_tx.weight().to_wu()); let package_fee = total_input_amount - anchor_tx.output.iter().map(|output| output.value).sum::(); // Our fee should be within a 5% error margin of the expected fee based on the @@ -794,7 +677,7 @@ where /// fully-signed, fee-bumped HTLC transaction that is broadcast to the network. fn handle_htlc_resolution( &self, claim_id: ClaimId, target_feerate_sat_per_1000_weight: u32, - htlc_descriptors: &[HTLCDescriptor], tx_lock_time: PackedLockTime, + htlc_descriptors: &[HTLCDescriptor], tx_lock_time: LockTime, ) -> Result<(), ()> { let mut htlc_tx = Transaction { version: 2, @@ -828,7 +711,7 @@ where #[cfg(debug_assertions)] let must_spend_amount = must_spend.iter().map(|input| input.previous_utxo.value).sum::(); - let coin_selection = self.utxo_source.select_confirmed_utxos( + let coin_selection: CoinSelection = self.utxo_source.select_confirmed_utxos( claim_id, must_spend, &htlc_tx.output, target_feerate_sat_per_1000_weight, )?; @@ -839,13 +722,30 @@ where let total_input_amount = must_spend_amount + coin_selection.confirmed_utxos.iter().map(|utxo| utxo.output.value).sum::(); - self.process_coin_selection(&mut htlc_tx, coin_selection); + self.process_coin_selection(&mut htlc_tx, &coin_selection); + + // construct psbt + let mut htlc_psbt = PartiallySignedTransaction::from_unsigned_tx(htlc_tx).unwrap(); + // add witness_utxo to htlc inputs + for (i, htlc_descriptor) in htlc_descriptors.iter().enumerate() { + debug_assert_eq!(htlc_psbt.unsigned_tx.input[i].previous_output, htlc_descriptor.outpoint()); + htlc_psbt.inputs[i].witness_utxo = Some(htlc_descriptor.previous_utxo(&self.secp)); + } + // add witness_utxo to remaining inputs + for (idx, utxo) in coin_selection.confirmed_utxos.into_iter().enumerate() { + // offset to skip the htlc inputs + let index = idx + htlc_descriptors.len(); + debug_assert_eq!(htlc_psbt.unsigned_tx.input[index].previous_output, utxo.outpoint); + if utxo.output.script_pubkey.is_witness_program() { + htlc_psbt.inputs[index].witness_utxo = Some(utxo.output); + } + } #[cfg(debug_assertions)] - let unsigned_tx_weight = htlc_tx.weight() as u64 - (htlc_tx.input.len() as u64 * EMPTY_SCRIPT_SIG_WEIGHT); + let unsigned_tx_weight = htlc_psbt.unsigned_tx.weight().to_wu() - (htlc_psbt.unsigned_tx.input.len() as u64 * EMPTY_SCRIPT_SIG_WEIGHT); - log_debug!(self.logger, "Signing HTLC transaction {}", htlc_tx.txid()); - htlc_tx = self.utxo_source.sign_tx(htlc_tx)?; + log_debug!(self.logger, "Signing HTLC transaction {}", htlc_psbt.unsigned_tx.txid()); + htlc_tx = self.utxo_source.sign_psbt(htlc_psbt)?; let mut signers = BTreeMap::new(); for (idx, htlc_descriptor) in htlc_descriptors.iter().enumerate() { @@ -857,7 +757,7 @@ where } #[cfg(debug_assertions)] { - let signed_tx_weight = htlc_tx.weight() as u64; + let signed_tx_weight = htlc_tx.weight().to_wu(); let expected_signed_tx_weight = unsigned_tx_weight + total_satisfaction_weight; // Our estimate should be within a 1% error margin of the actual weight and we should // never underestimate.