policy: introduce p2p constants from the reference implementation

This introduces some constants defined by Bitcoin Core which as a
consequence define some network rules in a new 'policy' module.

Only some were picked, which are very unlikely to change. Nonetheless a
Warning has been put in the module documentation.

Script-level constants are left into rust-miniscript where they are
already defined (src/miniscript/limits.rs).

src/blockdata/constants.rs

@@ -46,7 +46,8 @@ pub const MAX_BLOCK_WEIGHT: u32 = 4_000_000;
 pub const MIN_TRANSACTION_WEIGHT: u32 = 4 * 60;
 /// The factor that non-witness serialization data is multiplied by during weight calculation
 pub const WITNESS_SCALE_FACTOR: usize = 4;
-
+/// The maximum allowed number of signature check operations in a block
+pub const MAX_BLOCK_SIGOPS_COST: i64 = 80_000;
 
 /// In Bitcoind this is insanely described as ~((u256)0 >> 32)
 pub fn max_target(_: Network) -> Uint256 {

src/lib.rs

@@ -68,6 +68,7 @@ pub mod blockdata;
 pub mod util;
 pub mod consensus;
 pub mod hash_types;
+pub mod policy;
 
 pub use hash_types::*;
 pub use blockdata::block::Block;

src/policy/mod.rs

@@ -0,0 +1,54 @@
+// Rust Bitcoin Library
+// Written in 2014 by
+//   Andrew Poelstra <apoelstra@wpsoftware.net>
+//
+// To the extent possible under law, the author(s) have dedicated all
+// copyright and related and neighboring rights to this software to
+// the public domain worldwide. This software is distributed without
+// any warranty.
+//
+// You should have received a copy of the CC0 Public Domain Dedication
+// along with this software.
+// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
+//
+
+//! Policy
+//!
+//! This module exposes some constants and functions used in the reference implementation and which
+//! as a consequence defines some network rules.
+//!
+//! # *Warning*
+//! While the constants present in this module are very unlikely to change, they do not define
+//! Bitcoin. As such they must not be relied upon as if they were consensus rules.
+//!
+//! These values were taken from bitcoind v0.21.1 (194b9b8792d9b0798fdb570b79fa51f1d1f5ebaf).
+
+use super::blockdata::constants::MAX_BLOCK_SIGOPS_COST;
+
+/// Maximum weight of a transaction for it to be relayed by most nodes on the network
+pub const MAX_STANDARD_TX_WEIGHT: u32 = 400_000;
+
+/// Minimum non-witness size for a standard transaction (1 segwit input + 1 P2WPKH output = 82 bytes)
+pub const MIN_STANDARD_TX_NONWITNESS_SIZE: u32 = 82;
+
+/// Maximum number of sigops in a standard tx.
+pub const MAX_STANDARD_TX_SIGOPS_COST: u32 = MAX_BLOCK_SIGOPS_COST as u32 / 5;
+
+/// The minimum incremental *feerate* (despite the name), in sats per virtual kilobyte for RBF.
+pub const DEFAULT_INCREMENTAL_RELAY_FEE: u32 = 1_000;
+
+/// The number of bytes equivalent per signature operation. Affects transaction relay through the
+/// virtual size computation.
+pub const DEFAULT_BYTES_PER_SIGOP: u32 = 20;
+
+/// The minimum feerate, in sats per kilo-virtualbyte, for defining dust. An output is considered
+/// dust if spending it under this feerate would cost more in fee.
+pub const DUST_RELAY_TX_FEE: u32 = 3_000;
+
+/// Minimum feerate, in sats per virtual kilobyte, for a transaction to be relayed by most nodes on
+/// the network.
+pub const DEFAULT_MIN_RELAY_TX_FEE: u32 = 1_000;
+
+/// Default number of hours for an unconfirmed transaction to expire in most of the network nodes'
+/// mempools.
+pub const DEFAULT_MEMPOOL_EXPIRY: u32 = 336;