Add feefilter documentation

_includes/devdoc/ref_p2p_networking.md

@@ -792,6 +792,46 @@ alert.cpp] source code for the parameters of this message.
 
 {% endautocrossref %}
 
+#### FeeFilter
+{% include helpers/subhead-links.md %}
+
+{% autocrossref %}
+
+*Added in protocol version 70013 as described by BIP133.*
+
+The `feefilter` message is a request to the receiving peer to not relay any
+transaction inv messages to the sending peer where the fee rate for the
+transaction is below the fee rate specified in the feefilter message.
+
+`feefilter` was introduced in V0.13.0 following the introduction
+of mempool limiting in V0.12.0. Mempool limiting provides protection against
+attacks and spam transactions that have low fee rates and are unlikely to be
+included in mined blocks. The `feefilter` messages allows a node to inform its
+peers that it will not accept transactions below a specified fee rate into
+its mempool, and therefore that the peers can skip relaying inv messages for
+transactions below that fee rate to that node.
+
+| Bytes | Name    | Data Type | Description
+|-------|---------|-----------|---------------
+| 8     | feerate | uint64_t  | The fee rate (in Satoshis per kilobyte) under which transactions should not be relayed to this peer.
+
+The receiving peer may choose to ignore the message and not filter transaction
+inv messages.
+
+The fee filter is additive with bloom filters. If an SPV client loads a bloom
+filter and sends a feefilter message, transactions should only be relayed if
+they pass both filters.
+
+Note however that feefilter has no effect on block propagation or responses to
+getdata messages. For example, if a node requests a merkleblock from its peer
+by sending a getdata message with inv type MSG_FILTERED_BLOCK and it has
+previously sent a feefilter to that peer, the peer should respond with a
+merkleblock containing *all* the transactions matching the bloom filter, even
+if they are below the feefilter fee rate.
+
+inv messages generated from a mempool message are subject to a fee filter if it exists.
+
+{% endautocrossref %}
 
 #### FilterAdd
 {% include helpers/subhead-links.md %}