BIP434: FEATURE message support

diff --git a/doc/bips.md b/doc/bips.md
index ebf6b8fc..6f9b322e 100644
--- a/doc/bips.md
+++ b/doc/bips.md
@@ -79,3 +79,4 @@ BIPs that are implemented by Bitcoin Core:
 * [`BIP 390`](https://github.com/bitcoin/bips/blob/master/bip-0390.mediawiki): MuSig2 Descriptor parsing is implemented in **v30.0** ([PR 31244](https://github.com/bitcoin/bitcoin/pull/31244)) and signing in **v31.0** ([PR 29675](https://github.com/bitcoin/bitcoin/pull/29675))
 * [`BIP 431`](https://github.com/bitcoin/bips/blob/master/bip-0431.mediawiki): transactions with nVersion=3 are standard and treated as Topologically Restricted Until Confirmation as of **v28.0** ([PR 29496](https://github.com/bitcoin/bitcoin/pull/29496)).
 * [`BIP 433`](https://github.com/bitcoin/bips/blob/master/bip-0433.mediawiki): Spending of Pay to Anchor (P2A) outputs is standard as of **v28.0** ([PR 30352](https://github.com/bitcoin/bitcoin/pull/30352)).
+* [`BIP 434`](https://github.com/bitcoin/bips/blob/master/bip-0434.md): Peer Feature Negotiation as of **v32.0** ([PR 35221](https://github.com/bitcoin/bitcoin/pull/35221)).
diff --git a/src/bip324.h b/src/bip324.h
index 396a28a4..821cc3f7 100644
--- a/src/bip324.h
+++ b/src/bip324.h
@@ -15,6 +15,8 @@
 #include <pubkey.h>
 #include <span.h>
 
+static constexpr unsigned BIP324_SHORTIDS_IMPLEMENTED{38};
+
 /** The BIP324 packet cipher, encapsulating its key derivation, stream cipher, and AEAD. */
 class BIP324Cipher
 {
diff --git a/src/net.cpp b/src/net.cpp
index 6fb54118..df5848a6 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -921,7 +921,7 @@ namespace {
  * Only message types that are actually implemented in this codebase need to be listed, as other
  * messages get ignored anyway - whether we know how to decode them or not.
  */
-const std::array<std::string, 33> V2_MESSAGE_IDS = {
+const std::array<std::string, BIP324_SHORTIDS_IMPLEMENTED> V2_MESSAGE_IDS = {
     "", // 12 bytes follow encoding the message type like in V1
     NetMsgType::ADDR,
     NetMsgType::BLOCK,
@@ -951,11 +951,10 @@ const std::array<std::string, 33> V2_MESSAGE_IDS = {
     NetMsgType::GETCFCHECKPT,
     NetMsgType::CFCHECKPT,
     NetMsgType::ADDRV2,
-    // Unimplemented message types that are assigned in BIP324:
-    "",
-    "",
-    "",
-    ""
+    "", "", "", // Unimplemented message types 29-31
+    "", "", "", "", // Unimplemented message types 32-35
+    "",  // Unimplemented message type 36
+    NetMsgType::FEATURE,
 };
 
 class V2MessageMap
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index def72704..2e82d7f8 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -722,6 +722,15 @@ private:
     {
         m_connman.PushMessage(&node, NetMsg::Make(std::move(msg_type), std::forward<Args>(args)...));
     }
+    template <typename... Args>
+    void MakeAndPushFeature(CNode& node, std::string_view feature_id, Args&&... args) const
+    {
+        if (!Assume(feature_id.size() >= 4 && feature_id.size() <= MAX_FEATUREID_LENGTH)) return;
+        std::vector<unsigned char> feature_data;
+        VectorWriter{feature_data, 0, std::forward<Args>(args)...};
+        if (!Assume(feature_data.size() <= MAX_FEATUREDATA_LENGTH)) return;
+        MakeAndPushMessage(node, NetMsgType::FEATURE, feature_id, std::move(feature_data));
+    }
 
     /** Send a version message to a peer */
     void PushNodeVersion(CNode& pnode, const Peer& peer);
@@ -3738,6 +3747,11 @@ void PeerManagerImpl::ProcessMessage(Peer& peer, CNode& pfrom, const std::string
             }
         }
 
+        if (greatest_common_version >= FEATURE_VERSION) {
+            // announce supported features
+            // MakeAndPushFeature(pfrom, NetMsgFeature::FOO, uint32_t{1});
+        }
+
         MakeAndPushMessage(pfrom, NetMsgType::VERACK);
 
         // Potentially mark this peer as a preferred download peer.
@@ -3953,6 +3967,45 @@ void PeerManagerImpl::ProcessMessage(Peer& peer, CNode& pfrom, const std::string
         return;
     }
 
+    if (msg_type == NetMsgType::FEATURE) {
+        if (pfrom.fSuccessfullyConnected) {
+            // Disconnect peers that send a FEATURE message after VERACK.
+            LogDebug(BCLog::NET, "feature received after verack, %s", pfrom.DisconnectMsg());
+            pfrom.fDisconnect = true;
+            return;
+        } else if (pfrom.GetCommonVersion() < FEATURE_VERSION) {
+            // Disconnect peers that send a FEATURE message without valid version negotiation.
+            LogDebug(BCLog::NET, "feature received with incompatible version %d, %s", pfrom.GetCommonVersion(), pfrom.DisconnectMsg());
+            pfrom.fDisconnect = true;
+            return;
+        }
+
+        std::string feature_id;
+        DataStream feature_data;
+        try {
+            vRecv >> LIMITED_STRING(feature_id, MAX_FEATUREID_LENGTH);
+            std::vector<unsigned char> feature_data_vec;
+            vRecv >> LIMITED_VECTOR(feature_data_vec, MAX_FEATUREDATA_LENGTH);
+            feature_data = DataStream(feature_data_vec);
+        } catch (const std::exception&) {
+            feature_id.clear(); // use empty feature_id as error indicator
+        }
+        if (feature_id.size() < 4 || !vRecv.empty()) {
+            LogDebug(BCLog::NET, "invalid feature payload, %s", pfrom.DisconnectMsg());
+            pfrom.fDisconnect = true;
+            return;
+        }
+
+        // if (feature_id == NetMsgFeature::FOO) {
+        //     ...
+        //     return;
+        // }
+
+        // ignore unknown feature_id
+        LogDebug(BCLog::NET, "unknown feature advertised: %s", SanitizeString(feature_id));
+        return;
+    }
+
     // Received from a peer demonstrating readiness to announce transactions via reconciliations.
     // This feature negotiation must happen between VERSION and VERACK to avoid relay problems
     // from switching announcement protocols after the connection is up.
diff --git a/src/node/protocol_version.h b/src/node/protocol_version.h
index 7904086f..a72ac777 100644
--- a/src/node/protocol_version.h
+++ b/src/node/protocol_version.h
@@ -9,7 +9,7 @@
  * network protocol versioning
  */
 
-static const int PROTOCOL_VERSION = 70016;
+static const int PROTOCOL_VERSION = 70017;
 
 //! initial proto version, to be increased after version/verack negotiation
 static const int INIT_PROTO_VERSION = 209;
@@ -35,4 +35,7 @@ static const int INVALID_CB_NO_BAN_VERSION = 70015;
 //! "wtxidrelay" message type for wtxid-based relay starts with this version
 static const int WTXID_RELAY_VERSION = 70016;
 
+//! "feature" message type for feature negotiation starts with this version
+static const int FEATURE_VERSION = 70017;
+
 #endif // BITCOIN_NODE_PROTOCOL_VERSION_H
diff --git a/src/protocol.h b/src/protocol.h
index 8ed90dd3..24851e2f 100644
--- a/src/protocol.h
+++ b/src/protocol.h
@@ -264,6 +264,10 @@ inline constexpr const char* WTXIDRELAY{"wtxidrelay"};
  * txreconciliation, as described by BIP 330.
  */
 inline constexpr const char* SENDTXRCNCL{"sendtxrcncl"};
+/**
+ * BIP 434 Peer feature negotiation
+ */
+inline constexpr const char* FEATURE{"feature"};
 }; // namespace NetMsgType
 
 /** All known message types (see above). Keep this in the same order as the list of messages above. */
@@ -303,8 +307,16 @@ inline const std::array ALL_NET_MESSAGE_TYPES{std::to_array<std::string>({
     NetMsgType::CFCHECKPT,
     NetMsgType::WTXIDRELAY,
     NetMsgType::SENDTXRCNCL,
+    NetMsgType::FEATURE,
 })};
 
+static constexpr size_t MAX_FEATUREID_LENGTH{80};
+static constexpr size_t MAX_FEATUREDATA_LENGTH{512};
+
+namespace NetMsgFeature {
+//inline constexpr std::string_view FOO{"BIP-FOO"};
+}
+
 /** nServices flags */
 enum ServiceFlags : uint64_t {
     // NOTE: When adding here, be sure to update serviceFlagToStr too
diff --git a/src/test/net_tests.cpp b/src/test/net_tests.cpp
index 32801d97..77d7b31b 100644
--- a/src/test/net_tests.cpp
+++ b/src/test/net_tests.cpp
@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <addrman.h>
+#include <bip324.h>
 #include <chainparams.h>
 #include <clientversion.h>
 #include <common/args.h>
@@ -1533,7 +1534,7 @@ BOOST_AUTO_TEST_CASE(v2transport_test)
         tester.CompareSessionIDs();
         auto msg_data_1 = m_rng.randbytes<uint8_t>(4000000); // test that receiving 4M payload works
         auto msg_data_2 = m_rng.randbytes<uint8_t>(4000000); // test that sending 4M payload works
-        tester.SendMessage(uint8_t(m_rng.randrange(223) + 33), {}); // unknown short id
+        tester.SendMessage(uint8_t(m_rng.randrange(256 - BIP324_SHORTIDS_IMPLEMENTED) + BIP324_SHORTIDS_IMPLEMENTED), {}); // unknown short id
         tester.SendMessage(uint8_t(2), msg_data_1); // "block" short id
         tester.AddMessage("blocktxn", msg_data_2); // schedule blocktxn to be sent to us
         ret = tester.Interact();
diff --git a/test/functional/test_framework/messages.py b/test/functional/test_framework/messages.py
index a0f2a174..07abd5b6 100755
--- a/test/functional/test_framework/messages.py
+++ b/test/functional/test_framework/messages.py
@@ -1928,6 +1928,28 @@ class msg_sendtxrcncl:
         return "msg_sendtxrcncl(version=%lu, salt=%lu)" %\
             (self.version, self.salt)
 
+class msg_feature:
+    """FEATURE message for negotiating optional features."""
+    __slots__ = ("feature_id", "feature_data")
+    msgtype = b"feature"
+
+    def __init__(self, feature_id="", feature_data=b""):
+        self.feature_id = feature_id
+        self.feature_data = feature_data
+
+    def deserialize(self, f):
+        self.feature_id = deser_string(f).decode()
+        self.feature_data = deser_string(f)
+
+    def serialize(self):
+        r = ser_string(self.feature_id.encode())
+        r += ser_string(self.feature_data)
+        return r
+
+    def __repr__(self):
+        return f"msg_feature(feature_id={self.feature_id}, data={self.feature_data.hex()})"
+
+
 class TestFrameworkScript(unittest.TestCase):
     def test_addrv2_encode_decode(self):
         def check_addrv2(ip, net):
diff --git a/test/functional/test_framework/p2p.py b/test/functional/test_framework/p2p.py
index 4d812ce3..c6e3280d 100755
--- a/test/functional/test_framework/p2p.py
+++ b/test/functional/test_framework/p2p.py
@@ -43,6 +43,7 @@ from test_framework.messages import (
     msg_cfheaders,
     msg_cfilter,
     msg_cmpctblock,
+    msg_feature,
     msg_feefilter,
     msg_filteradd,
     msg_filterclear,
@@ -124,6 +125,7 @@ MESSAGEMAP = {
     b"cfheaders": msg_cfheaders,
     b"cfilter": msg_cfilter,
     b"cmpctblock": msg_cmpctblock,
+    b"feature": msg_feature,
     b"feefilter": msg_feefilter,
     b"filteradd": msg_filteradd,
     b"filterclear": msg_filterclear,
@@ -543,6 +545,7 @@ class P2PInterface(P2PConnection):
     def on_cfheaders(self, message): pass
     def on_cfilter(self, message): pass
     def on_cmpctblock(self, message): pass
+    def on_feature(self, message): pass
     def on_feefilter(self, message): pass
     def on_filteradd(self, message): pass
     def on_filterclear(self, message): pass
diff --git a/test/functional/test_framework/v2_p2p.py b/test/functional/test_framework/v2_p2p.py
index 087b885a..4057fd9a 100644
--- a/test/functional/test_framework/v2_p2p.py
+++ b/test/functional/test_framework/v2_p2p.py
@@ -50,6 +50,7 @@ SHORTID = {
     26: b"getcfcheckpt",
     27: b"cfcheckpt",
     28: b"addrv2",
+    37: b"feature",
 }
 
 # Dictionary which contains short message type ID for the P2P message