[doc] remove non-signaling mentions of BIP125

Our RBF policy is different from the rules specified in BIP125. For
example, the BIP does not mention Rule 6, and our Rule 4 uses the
(configurable) incremental relay feerate (distinct from the
minimum relay feerate). Those interested in our policy should refer to
doc/policy/mempool-replacements.md instead. These rules may also
continue to diverge with package RBF and other RBF improvements. Keep
references to the BIP125 signaling wrt sequence numbers, since that is
still correct and widely used. It is helpful to refer to this as "BIP125
signaling" since it is unambiguous and succint, especially if we have
multiple ways to signal replaceability in the future.

The rule numbers in doc/policy/mempool-replacements.md correspond
largely to those of BIP 125, so we can still refer to them like "Rule 5."

Author: glozow

doc/policy/packages.md

@@ -33,7 +33,7 @@ The following rules are enforced for all packages:
 * Packages cannot have conflicting transactions, i.e. no two transactions in a package can spend
    the same inputs. Packages cannot have duplicate transactions. (#20833)
 
-* No transaction in a package can conflict with a mempool transaction. BIP125 Replace By Fee is
+* No transaction in a package can conflict with a mempool transaction. Replace By Fee is
   currently disabled for packages. (#20833)
 
    - Package RBF may be enabled in the future.

src/init.cpp

@@ -556,7 +556,7 @@ void SetupServerArgs(ArgsManager& argsman)
     SetupChainParamsBaseOptions(argsman);
 
     argsman.AddArg("-acceptnonstdtxn", strprintf("Relay and mine \"non-standard\" transactions (%sdefault: %u)", "testnet/regtest only; ", !testnetChainParams->RequireStandard()), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::NODE_RELAY);
-    argsman.AddArg("-incrementalrelayfee=<amt>", strprintf("Fee rate (in %s/kvB) used to define cost of relay, used for mempool limiting and BIP 125 replacement. (default: %s)", CURRENCY_UNIT, FormatMoney(DEFAULT_INCREMENTAL_RELAY_FEE)), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::NODE_RELAY);
+    argsman.AddArg("-incrementalrelayfee=<amt>", strprintf("Fee rate (in %s/kvB) used to define cost of relay, used for mempool limiting and replacement policy. (default: %s)", CURRENCY_UNIT, FormatMoney(DEFAULT_INCREMENTAL_RELAY_FEE)), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-dustrelayfee=<amt>", strprintf("Fee rate (in %s/kvB) used to define dust, the value of an output such that it will cost more than its value in fees at this fee rate to spend it. (default: %s)", CURRENCY_UNIT, FormatMoney(DUST_RELAY_TX_FEE)), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-bytespersigop", strprintf("Equivalent bytes per sigop in transactions for relay and mining (default: %u)", DEFAULT_BYTES_PER_SIGOP), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);
     argsman.AddArg("-datacarrier", strprintf("Relay and mine data carrier transactions (default: %u)", DEFAULT_ACCEPT_DATACARRIER), ArgsManager::ALLOW_ANY, OptionsCategory::NODE_RELAY);

src/node/mempool_args.cpp

@@ -45,7 +45,7 @@ std::optional<bilingual_str> ApplyArgsManOptions(const ArgsManager& argsman, con
 
     if (auto hours = argsman.GetIntArg("-mempoolexpiry")) mempool_opts.expiry = std::chrono::hours{*hours};
 
-    // incremental relay fee sets the minimum feerate increase necessary for BIP 125 replacement in the mempool
+    // incremental relay fee sets the minimum feerate increase necessary for replacement in the mempool
     // and the amount the mempool min fee increases above the feerate of txs evicted due to mempool limiting.
     if (argsman.IsArgSet("-incrementalrelayfee")) {
         if (std::optional<CAmount> inc_relay_fee = ParseMoney(argsman.GetArg("-incrementalrelayfee", ""))) {

src/policy/policy.h

@@ -31,7 +31,7 @@ static constexpr unsigned int MIN_STANDARD_TX_NONWITNESS_SIZE{82};
 static constexpr unsigned int MAX_P2SH_SIGOPS{15};
 /** The maximum number of sigops we're willing to relay/mine in a single tx */
 static constexpr unsigned int MAX_STANDARD_TX_SIGOPS_COST{MAX_BLOCK_SIGOPS_COST/5};
-/** Default for -incrementalrelayfee, which sets the minimum feerate increase for mempool limiting or BIP 125 replacement **/
+/** Default for -incrementalrelayfee, which sets the minimum feerate increase for mempool limiting or replacement **/
 static constexpr unsigned int DEFAULT_INCREMENTAL_RELAY_FEE{1000};
 /** Default for -bytespersigop */
 static constexpr unsigned int DEFAULT_BYTES_PER_SIGOP{20};

src/policy/rbf.cpp

@@ -65,7 +65,7 @@ std::optional<std::string> GetEntriesForConflicts(const CTransaction& tx,
     uint64_t nConflictingCount = 0;
     for (const auto& mi : iters_conflicting) {
         nConflictingCount += mi->GetCountWithDescendants();
-        // BIP125 Rule #5: don't consider replacing more than MAX_REPLACEMENT_CANDIDATES
+        // Rule #5: don't consider replacing more than MAX_REPLACEMENT_CANDIDATES
         // entries from the mempool. This potentially overestimates the number of actual
         // descendants (i.e. if multiple conflicts share a descendant, it will be counted multiple
         // times), but we just want to be conservative to avoid doing too much work.
@@ -96,7 +96,7 @@ std::optional<std::string> HasNoNewUnconfirmed(const CTransaction& tx,
     }
 
     for (unsigned int j = 0; j < tx.vin.size(); j++) {
-        // BIP125 Rule #2: We don't want to accept replacements that require low feerate junk to be
+        // Rule #2: We don't want to accept replacements that require low feerate junk to be
         // mined first.  Ideally we'd keep track of the ancestor feerates and make the decision
         // based on that, but for now requiring all new inputs to be confirmed works.
         //
@@ -162,15 +162,15 @@ std::optional<std::string> PaysForRBF(CAmount original_fees,
                                       CFeeRate relay_fee,
                                       const uint256& txid)
 {
-    // BIP125 Rule #3: The replacement fees must be greater than or equal to fees of the
+    // Rule #3: The replacement fees must be greater than or equal to fees of the
     // transactions it replaces, otherwise the bandwidth used by those conflicting transactions
     // would not be paid for.
     if (replacement_fees < original_fees) {
         return strprintf("rejecting replacement %s, less fees than conflicting txs; %s < %s",
                          txid.ToString(), FormatMoney(replacement_fees), FormatMoney(original_fees));
     }
 
-    // BIP125 Rule #4: The new transaction must pay for its own bandwidth. Otherwise, we have a DoS
+    // Rule #4: The new transaction must pay for its own bandwidth. Otherwise, we have a DoS
     // vector where attackers can cause a transaction to be replaced (and relayed) repeatedly by
     // increasing the fee by tiny amounts.
     CAmount additional_fees = replacement_fees - original_fees;

src/policy/rbf.h

@@ -19,7 +19,7 @@
 class CFeeRate;
 class uint256;
 
-/** Maximum number of transactions that can be replaced by BIP125 RBF (Rule #5). This includes all
+/** Maximum number of transactions that can be replaced by RBF Rule #5. This includes all
  * mempool conflicts and their descendants. */
 static constexpr uint32_t MAX_REPLACEMENT_CANDIDATES{100};
 
@@ -47,24 +47,25 @@ enum class RBFTransactionState {
 RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool) EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
 RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx);
 
-/** Get all descendants of iters_conflicting. Also enforce BIP125 Rule #5, "The number of original
- * transactions to be replaced and their descendant transactions which will be evicted from the
- * mempool must not exceed a total of 100 transactions." Quit as early as possible. There cannot be
- * more than MAX_REPLACEMENT_CANDIDATES potential entries.
+/** Get all descendants of iters_conflicting. Checks that there are no more than
+ * MAX_REPLACEMENT_CANDIDATES potential entries. May overestimate if the entries in
+ * iters_conflicting have overlapping descendants.
  * @param[in]   iters_conflicting   The set of iterators to mempool entries.
  * @param[out]  all_conflicts       Populated with all the mempool entries that would be replaced,
- *                                  which includes descendants of iters_conflicting. Not cleared at
- *                                  the start; any existing mempool entries will remain in the set.
- * @returns an error message if Rule #5 is broken, otherwise a std::nullopt.
+ *                                  which includes iters_conflicting and all entries' descendants.
+ *                                  Not cleared at the start; any existing mempool entries will
+ *                                  remain in the set.
+ * @returns an error message if MAX_REPLACEMENT_CANDIDATES may be exceeded, otherwise a std::nullopt.
  */
 std::optional<std::string> GetEntriesForConflicts(const CTransaction& tx, CTxMemPool& pool,
                                                   const CTxMemPool::setEntries& iters_conflicting,
                                                   CTxMemPool::setEntries& all_conflicts)
     EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
 
-/** BIP125 Rule #2: "The replacement transaction may only include an unconfirmed input if that input
- * was included in one of the original transactions."
- * @returns error message if Rule #2 is broken, otherwise std::nullopt. */
+/** The replacement transaction may only include an unconfirmed input if that input was included in
+ * one of the original transactions.
+ * @returns error message if tx spends unconfirmed inputs not also spent by iters_conflicting,
+ * otherwise std::nullopt. */
 std::optional<std::string> HasNoNewUnconfirmed(const CTransaction& tx, const CTxMemPool& pool,
                                                const CTxMemPool::setEntries& iters_conflicting)
     EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
@@ -90,9 +91,8 @@ std::optional<std::string> EntriesAndTxidsDisjoint(const CTxMemPool::setEntries&
 std::optional<std::string> PaysMoreThanConflicts(const CTxMemPool::setEntries& iters_conflicting,
                                                  CFeeRate replacement_feerate, const uint256& txid);
 
-/** Enforce BIP125 Rule #3 "The replacement transaction pays an absolute fee of at least the sum
- * paid by the original transactions." Enforce BIP125 Rule #4 "The replacement transaction must also
- * pay for its own bandwidth at or above the rate set by the node's minimum relay fee setting."
+/** The replacement transaction must pay more fees than the original transactions. The additional
+ * fees must pay for the replacement's bandwidth at or above the incremental relay feerate.
  * @param[in]   original_fees       Total modified fees of original transaction(s).
  * @param[in]   replacement_fees    Total modified fees of replacement transaction(s).
  * @param[in]   replacement_vsize   Total virtual size of replacement transaction(s).

src/rpc/mempool.cpp

@@ -690,7 +690,7 @@ static RPCHelpMan getmempoolinfo()
                 {RPCResult::Type::NUM, "maxmempool", "Maximum memory usage for the mempool"},
                 {RPCResult::Type::STR_AMOUNT, "mempoolminfee", "Minimum fee rate in " + CURRENCY_UNIT + "/kvB for tx to be accepted. Is the maximum of minrelaytxfee and minimum mempool fee"},
                 {RPCResult::Type::STR_AMOUNT, "minrelaytxfee", "Current minimum relay fee for transactions"},
-                {RPCResult::Type::NUM, "incrementalrelayfee", "minimum fee rate increment for mempool limiting or BIP 125 replacement in " + CURRENCY_UNIT + "/kvB"},
+                {RPCResult::Type::NUM, "incrementalrelayfee", "minimum fee rate increment for mempool limiting or replacement in " + CURRENCY_UNIT + "/kvB"},
                 {RPCResult::Type::NUM, "unbroadcastcount", "Current number of transactions that haven't passed initial broadcast yet"},
                 {RPCResult::Type::BOOL, "fullrbf", "True if the mempool accepts RBF without replaceability signaling inspection"},
             }},

src/rpc/net.cpp

@@ -604,7 +604,7 @@ static RPCHelpMan getnetworkinfo()
                             }},
                         }},
                         {RPCResult::Type::NUM, "relayfee", "minimum relay fee rate for transactions in " + CURRENCY_UNIT + "/kvB"},
-                        {RPCResult::Type::NUM, "incrementalfee", "minimum fee rate increment for mempool limiting or BIP 125 replacement in " + CURRENCY_UNIT + "/kvB"},
+                        {RPCResult::Type::NUM, "incrementalfee", "minimum fee rate increment for mempool limiting or replacement in " + CURRENCY_UNIT + "/kvB"},
                         {RPCResult::Type::ARR, "localaddresses", "list of local addresses",
                         {
                             {RPCResult::Type::OBJ, "", "",

src/txmempool.h

@@ -365,7 +365,7 @@ enum class MemPoolRemovalReason {
  * - a transaction which doesn't meet the minimum fee requirements.
  * - a new transaction that double-spends an input of a transaction already in
  * the pool where the new transaction does not meet the Replace-By-Fee
- * requirements as defined in BIP 125.
+ * requirements as defined in doc/policy/mempool-replacements.md.
  * - a non-standard transaction.
  *
  * CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping:

src/validation.cpp

@@ -862,8 +862,8 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         // Specifically, the subset of RBF transactions which we allow despite chain limits are those which
         // conflict directly with exactly one other transaction (but may evict children of said transaction),
         // and which are not adding any new mempool dependencies. Note that the "no new mempool dependencies"
-        // check is accomplished later, so we don't bother doing anything about it here, but if BIP 125 is
-        // amended, we may need to move that check to here instead of removing it wholesale.
+        // check is accomplished later, so we don't bother doing anything about it here, but if our
+        // policy changes, we may need to move that check to here instead of removing it wholesale.
         //
         // Such transactions are clearly not merging any existing packages, so we are only concerned with
         // ensuring that (a) no package is growing past the package size (not count) limits and (b) we are
@@ -930,7 +930,7 @@ bool MemPoolAccept::ReplacementChecks(Workspace& ws)
     TxValidationState& state = ws.m_state;
 
     CFeeRate newFeeRate(ws.m_modified_fees, ws.m_vsize);
-    // The replacement transaction must have a higher feerate than its direct conflicts.
+    // Enforce Rule #6. The replacement transaction must have a higher feerate than its direct conflicts.
     // - The motivation for this check is to ensure that the replacement transaction is preferable for
     //   block-inclusion, compared to what would be removed from the mempool.
     // - This logic predates ancestor feerate-based transaction selection, which is why it doesn't
@@ -943,18 +943,18 @@ bool MemPoolAccept::ReplacementChecks(Workspace& ws)
         return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "insufficient fee", *err_string);
     }
 
-    // Calculate all conflicting entries and enforce BIP125 Rule #5.
+    // Calculate all conflicting entries and enforce Rule #5.
     if (const auto err_string{GetEntriesForConflicts(tx, m_pool, ws.m_iters_conflicting, ws.m_all_conflicting)}) {
         return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY,
                              "too many potential replacements", *err_string);
     }
-    // Enforce BIP125 Rule #2.
+    // Enforce Rule #2.
     if (const auto err_string{HasNoNewUnconfirmed(tx, m_pool, ws.m_iters_conflicting)}) {
         return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY,
                              "replacement-adds-unconfirmed", *err_string);
     }
     // Check if it's economically rational to mine this transaction rather than the ones it
-    // replaces and pays for its own relay fees. Enforce BIP125 Rules #3 and #4.
+    // replaces and pays for its own relay fees. Enforce Rules #3 and #4.
     for (CTxMemPool::txiter it : ws.m_all_conflicting) {
         ws.m_conflicting_fees += it->GetModifiedFee();
         ws.m_conflicting_size += it->GetTxSize();