Add info about fees for mempool inclusion

docs/architecture/mempool.md

@@ -21,11 +21,55 @@ The transaction is also checked against other transactions in the mempool, to en
 
 ## Fee Required for Inclusion
 
-If the mempool is not full, all transactions regardless of fee are accepted into the mempool. The maximum mempool size can vary by version, and transactions have a large variance in size, but it is generally 10-100 blocks.
+When you submit a transaction, one of several possible scenarios will play out, depending on how full the mempool is, how large of a fee was included, the CLVM cost of the transaction, and other factors. Keep in mind that each farmer has its own copy of the mempool, with its own set of rules. The default mempool behavior discussed in this section will apply to most, but not all, nodes. It is up to each individual farmer to decide which transactions to include upon creating a block.
 
-Transactions with 0 fee are accepted into the mempool as of chia-blockchain version 1.2.12. Fees that are very close to zero are considered equivalent to zero. The threshold is set at 5 mojo per cost, but this can vary by implementation, version, and settings, so it's not guaranteed by the protocol.
+To view the current status of the mempool, see the dashboard for [mainnet](https://dashboard.chia.net/d/46EAA05E/mempool-transactions-and-fees?orgId=1&var-network=mainnet) and [testnet10](https://dashboard.chia.net/d/46EAA05E/mempool-transactions-and-fees?orgId=1&var-network=testnet10).
 
-When the mempool gets full, nodes will start rejecting transactions that don't meet a minimum fee. The full node sorts the transactions by fee-per-cost, and kicks out the least valuable transactions first, when including new ones.
+:::info
+
+Currently, the block size is artificially limited to 50% of its capacity. Eventually this limitation will be lifted, but the numbers discussed in this section assume it is being enforced.
+
+:::
+
+:::info
+
+The total size of the mempool differs by network.
+* Mainnet: 550 billion cost, or 100 blocks
+* Testnet10: 110 billion cost, or 20 blocks
+
+:::
+
+### Scenario 1: Mempool Not Busy
+
+If the transaction you just submitted -- plus the entire contents of the mempool -- can fit into one block, then your transaction will be added to the next block. This is true even if you don't include a transaction fee.
+
+The reason for this is straightforward -- the farmer has nothing to gain by excluding certain transactions, so it will include everything. Note that some proprietary software takes the opposite approach: the farmer will _only_ include transactions that pay a fee, regardless of mempool size.
+
+The mempool for Chia's mainnet is often in this state. This does not mean that no transactions are being submitted. It simply means that the network's speed of around 20 transactions per second is sufficient to keep up with demand.
+
+### Scenario 2: Mempool Busy But Not Full
+
+If the mempool's contents will occupy more than one block, but the mempool is not full, then it is considered _busy_. In this case:
+* Transactions that don't include fees will be added to the mempool, but they won't make it into the next block. Instead, they will have to "wait in line" for higher-priority transactions to be cleared. They likely will eventually be included in a block, but this is not guaranteed.
+* Transactions with fees will be added to the mempool and prioritized according to the size of their fee-per-cost. For example, a transaction with a 1-mojo fee will enter the queue ahead of zero-fee transactions. 
+
+:::info Testnet10 info
+
+Testnet10 is constantly being "dusted" (thousands of small transactions are being included) in order to simulate a busy network, which can be useful for testing. The dust transactions do not include any fees, so in order for your transaction to be prioritized ahead of the dust, you simply have to include a 1-mojo fee. In this case, your transaction will likely be included in the next transaction block. However, if you don't include a fee, it will likely need to wait ~40-60 minutes before being included.
+
+:::
+
+### Scenario 3: Mempool Full
+
+If the mempool is completely full, then in order for your transaction to be added, it will need to kick out one or more transactions. In this scenario:
+* Transactions with no fee will not be added to the mempool.
+* Transactions with a fee of less than five mojos per cost (~100 million mojos for 2-input, 2-output transactions) will be treated as zero-fee transactions, i.e. they will not be added to the mempool.
+* Transactions with a fee of at least five mojos per cost will be added to the mempool, prioritized by fee-per-cost, _if_ they are not the lowest priority transactions. In this case, one or more of the lowest-priority transactions will be removed.
+* If the lowest-cost transaction in the mempool is higher than than the new transaction, then the new transaction will not be added. For example, if the lowest priority transaction in the mempool has a fee of 100 mojos-per-cost (as might be the case in a very busy network), then a new transaction will have to include a higher fee in order to be added to the mempool.
+
+This scenario often occurs on testnet10. When the mempool is completely full, the dusters stop submitting transactions until some of the dust has been cleared. This scenario might occasionally happen on mainnet as well, in which case a minimum fee would be required.
+
+If you see `INVALID_FEE_TOO_CLOSE_TO_ZERO` in your log file, the mempool was likely full when you submitted your transaction, and you did not include a sufficient fee to kick out an existing transaction. Try resubmitting your transaction with a higher fee.
 
 ## Replace by Fee
 

docs/resources/faq.md

@@ -927,6 +927,10 @@ Regarding feature 1, the only ephemeral spends that are currently supported must
 
 Chia version 1.8.2 introduces identical spend aggregation, which addresses some of the use cases of Feature 2. Spends that make use of this feature are still restricted by the lack of `AGG_SIG_*` conditions, which means that anyone who needs to assert an announcement from an aggregated spend could also have spent the coin. However, this version brings Chia one step closer to offering full support for inter-transaction announcements.
 
+### Why do I see `INVALID_FEE_TOO_CLOSE_TO_ZERO` in my log file?
+
+This error message occurs when you submit a transaction that does not include a high enough fee, for example when the network is quite busy. Try submitting the transaction again with a higher fee (100 million mojos will generally suffice for standard transactions). For more details, see our [mempool documentation](/mempool#fee-required-for-inclusion).
+
 ---
 
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