[doc] CPFP carve out and single-conflict RBF exemption

doc/policy/README.md

@@ -9,7 +9,7 @@ contents. Policy is *not* applied to transactions in blocks.
 
 This documentation is not an exhaustive list of all policy rules.
 
+- [Mempool Limits](mempool-limits.md)
 - [Mempool Replacements](mempool-replacements.md)
 - [Packages](packages.md)
 
-

doc/policy/mempool-limits.md

@@ -0,0 +1,65 @@
+# Mempool Limits
+
+## Definitions
+
+Given any two transactions Tx0 and Tx1 where Tx1 spends an output of Tx0,
+Tx0 is a *parent* of Tx1 and Tx1 is a *child* of Tx0.
+
+A transaction's *ancestors* include, recursively, its parents, the parents of its parents, etc.
+A transaction's *descendants* include, recursively, its children, the children of its children, etc.
+
+A mempool entry's *ancestor count* is the total number of in-mempool (unconfirmed) transactions in
+its ancestor set, including itself.
+A mempool entry's *descendant count* is the total number of in-mempool (unconfirmed) transactions in
+its descendant set, including itself.
+
+A mempool entry's *ancestor size* is the aggregated virtual size of in-mempool (unconfirmed)
+transactions in its ancestor set, including itself.
+A mempool entry's *descendant size* is the aggregated virtual size of in-mempool (unconfirmed)
+transactions in its descendant set, including itself.
+
+Transactions submitted to the mempool must not exceed the ancestor and descendant limits (aka
+mempool *package limits*) set by the node (see `-limitancestorcount`, `-limitancestorsize`,
+`-limitdescendantcount`, `-limitdescendantsize`).
+
+## Exemptions
+
+### CPFP Carve Out
+
+**CPFP Carve Out** if a transaction candidate for submission to the
+mempool would cause some mempool entry to exceed its descendant limits, an exemption is made if all
+of the following conditions are met:
+
+1. The candidate transaction is no more than 10,000 virtual bytes.
+
+2. The candidate transaction has an ancestor count of 2 (itself and exactly 1 ancestor).
+
+3. The in-mempool transaction's descendant count, including the candidate transaction, would only
+   exceed the limit by 1.
+
+*Rationale*: this rule was introduced to prevent pinning by domination of a transaction's descendant
+limits in two-party contract protocols such as LN.  Also see the [mailing list
+post](https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-November/016518.html).
+
+This rule was introduced in [PR #15681](https://github.com/bitcoin/bitcoin/pull/15681).
+
+### Single-Conflict RBF Carve Out
+
+When a candidate transaction for submission to the mempool would replace mempool entries, it may
+also decrease the descendant count of other mempool entries. Since ancestor/descendant limits are
+calculated prior to removing the would-be-replaced transactions, they may be overestimated.
+
+An exemption is given for a candidate transaction that would replace mempool transactions and meets
+all of the following conditions:
+
+1. The candidate transaction has exactly 1 directly conflicting transaction.
+
+2. The candidate transaction does not spend any unconfirmed inputs that are not also spent by the
+   directly conflicting transaction.
+
+The following discounts are given to account for the would-be-replaced transaction(s):
+
+1. The descendant count limit is temporarily increased by 1.
+
+2. The descendant size limit temporarily is increased by the virtual size of the to-be-replaced
+   directly conflicting transaction.