diff --git a/nano/core_test/CMakeLists.txt b/nano/core_test/CMakeLists.txt
index a1dd3ffcf6..3d8418e2eb 100644
--- a/nano/core_test/CMakeLists.txt
+++ b/nano/core_test/CMakeLists.txt
@@ -23,6 +23,7 @@ add_executable (core_test
 	message_parser.cpp
 	memory_pool.cpp
 	network.cpp
+	network_filter.cpp
 	node.cpp
 	node_telemetry.cpp
 	processor_service.cpp
diff --git a/nano/core_test/network_filter.cpp b/nano/core_test/network_filter.cpp
new file mode 100644
index 0000000000..a0df312df0
--- /dev/null
+++ b/nano/core_test/network_filter.cpp
@@ -0,0 +1,111 @@
+#include <nano/core_test/testutil.hpp>
+#include <nano/node/common.hpp>
+#include <nano/secure/buffer.hpp>
+#include <nano/secure/common.hpp>
+#include <nano/secure/network_filter.hpp>
+
+#include <gtest/gtest.h>
+
+TEST (network_filter, unit)
+{
+	nano::genesis genesis;
+	nano::network_filter filter (1);
+	auto one_block = [&genesis, &filter](std::shared_ptr<nano::block> const & block_a, bool expect_duplicate_a) {
+		nano::publish message (block_a);
+		auto bytes (message.to_bytes ());
+		nano::bufferstream stream (bytes->data (), bytes->size ());
+
+		// First read the header
+		bool error{ false };
+		nano::message_header header (error, stream);
+		ASSERT_FALSE (error);
+
+		// This validates nano::message_header::size
+		ASSERT_EQ (bytes->size (), block_a->size (block_a->type ()) + header.size);
+
+		// Now filter the rest of the stream
+		bool duplicate (filter.apply (bytes->data (), bytes->size () - header.size));
+		ASSERT_EQ (expect_duplicate_a, duplicate);
+
+		// Make sure the stream was rewinded correctly
+		auto block (nano::deserialize_block (stream, header.block_type ()));
+		ASSERT_NE (nullptr, block);
+		ASSERT_EQ (*block, *block_a);
+	};
+	one_block (genesis.open, false);
+	for (int i = 0; i < 10; ++i)
+	{
+		one_block (genesis.open, true);
+	}
+	auto new_block (std::make_shared<nano::state_block> (nano::test_genesis_key.pub, genesis.open->hash (), nano::test_genesis_key.pub, nano::genesis_amount - 10 * nano::xrb_ratio, nano::public_key (), nano::test_genesis_key.prv, nano::test_genesis_key.pub, 0));
+	one_block (new_block, false);
+	for (int i = 0; i < 10; ++i)
+	{
+		one_block (new_block, true);
+	}
+	for (int i = 0; i < 100; ++i)
+	{
+		one_block (genesis.open, false);
+		one_block (new_block, false);
+	}
+}
+
+TEST (network_filter, many)
+{
+	nano::genesis genesis;
+	nano::network_filter filter (4);
+	nano::keypair key1;
+	for (int i = 0; i < 100; ++i)
+	{
+		auto block (std::make_shared<nano::state_block> (nano::test_genesis_key.pub, genesis.open->hash (), nano::test_genesis_key.pub, nano::genesis_amount - i * 10 * nano::xrb_ratio, key1.pub, nano::test_genesis_key.prv, nano::test_genesis_key.pub, 0));
+
+		nano::publish message (block);
+		auto bytes (message.to_bytes ());
+		nano::bufferstream stream (bytes->data (), bytes->size ());
+
+		// First read the header
+		bool error{ false };
+		nano::message_header header (error, stream);
+		ASSERT_FALSE (error);
+
+		// This validates nano::message_header::size
+		ASSERT_EQ (bytes->size (), block->size + header.size);
+
+		// Now filter the rest of the stream
+		// All blocks should pass through
+		ASSERT_FALSE (filter.apply (bytes->data (), block->size));
+		ASSERT_FALSE (error);
+
+		// Make sure the stream was rewinded correctly
+		auto deserialized_block (nano::deserialize_block (stream, header.block_type ()));
+		ASSERT_NE (nullptr, deserialized_block);
+		ASSERT_EQ (*block, *deserialized_block);
+	}
+}
+
+TEST (network_filter, clear)
+{
+	nano::network_filter filter (1);
+	std::vector<uint8_t> bytes1{ 1, 2, 3 };
+	std::vector<uint8_t> bytes2{ 1 };
+	ASSERT_FALSE (filter.apply (bytes1.data (), bytes1.size ()));
+	ASSERT_TRUE (filter.apply (bytes1.data (), bytes1.size ()));
+	filter.clear (bytes1.data (), bytes1.size ());
+	ASSERT_FALSE (filter.apply (bytes1.data (), bytes1.size ()));
+	ASSERT_TRUE (filter.apply (bytes1.data (), bytes1.size ()));
+	filter.clear (bytes2.data (), bytes2.size ());
+	ASSERT_TRUE (filter.apply (bytes1.data (), bytes1.size ()));
+	ASSERT_FALSE (filter.apply (bytes2.data (), bytes2.size ()));
+}
+
+TEST (network_filter, optional_digest)
+{
+	nano::network_filter filter (1);
+	std::vector<uint8_t> bytes1{ 1, 2, 3 };
+	nano::uint128_t digest{ 0 };
+	ASSERT_FALSE (filter.apply (bytes1.data (), bytes1.size (), &digest));
+	ASSERT_NE (0, digest);
+	ASSERT_TRUE (filter.apply (bytes1.data (), bytes1.size ()));
+	filter.clear (digest);
+	ASSERT_FALSE (filter.apply (bytes1.data (), bytes1.size ()));
+}
diff --git a/nano/lib/CMakeLists.txt b/nano/lib/CMakeLists.txt
index b8468a6bcf..ad1b531d01 100644
--- a/nano/lib/CMakeLists.txt
+++ b/nano/lib/CMakeLists.txt
@@ -50,6 +50,7 @@ add_library (nano_lib
 	rpcconfig.cpp
 	stats.hpp
 	stats.cpp
+	stream.hpp
 	threading.hpp
 	threading.cpp
 	timer.hpp
diff --git a/nano/lib/blocks.hpp b/nano/lib/blocks.hpp
index a05aa59406..d355ea9030 100644
--- a/nano/lib/blocks.hpp
+++ b/nano/lib/blocks.hpp
@@ -3,45 +3,15 @@
 #include <nano/crypto/blake2/blake2.h>
 #include <nano/lib/errors.hpp>
 #include <nano/lib/numbers.hpp>
+#include <nano/lib/stream.hpp>
 #include <nano/lib/utility.hpp>
 
 #include <boost/property_tree/ptree_fwd.hpp>
 
-#include <cassert>
-#include <streambuf>
 #include <unordered_map>
 
 namespace nano
 {
-// We operate on streams of uint8_t by convention
-using stream = std::basic_streambuf<uint8_t>;
-// Read a raw byte stream the size of `T' and fill value. Returns true if there was an error, false otherwise
-template <typename T>
-bool try_read (nano::stream & stream_a, T & value)
-{
-	static_assert (std::is_standard_layout<T>::value, "Can't stream read non-standard layout types");
-	auto amount_read (stream_a.sgetn (reinterpret_cast<uint8_t *> (&value), sizeof (value)));
-	return amount_read != sizeof (value);
-}
-// A wrapper of try_read which throws if there is an error
-template <typename T>
-void read (nano::stream & stream_a, T & value)
-{
-	auto error = try_read (stream_a, value);
-	if (error)
-	{
-		throw std::runtime_error ("Failed to read type");
-	}
-}
-
-template <typename T>
-void write (nano::stream & stream_a, T const & value)
-{
-	static_assert (std::is_standard_layout<T>::value, "Can't stream write non-standard layout types");
-	auto amount_written (stream_a.sputn (reinterpret_cast<uint8_t const *> (&value), sizeof (value)));
-	(void)amount_written;
-	assert (amount_written == sizeof (value));
-}
 class block_visitor;
 enum class block_type : uint8_t
 {
diff --git a/nano/lib/stream.hpp b/nano/lib/stream.hpp
new file mode 100644
index 0000000000..190fc0f178
--- /dev/null
+++ b/nano/lib/stream.hpp
@@ -0,0 +1,38 @@
+
+#pragma once
+
+#include <cassert>
+#include <streambuf>
+
+namespace nano
+{
+// We operate on streams of uint8_t by convention
+using stream = std::basic_streambuf<uint8_t>;
+// Read a raw byte stream the size of `T' and fill value. Returns true if there was an error, false otherwise
+template <typename T>
+bool try_read (nano::stream & stream_a, T & value)
+{
+	static_assert (std::is_standard_layout<T>::value, "Can't stream read non-standard layout types");
+	auto amount_read (stream_a.sgetn (reinterpret_cast<uint8_t *> (&value), sizeof (value)));
+	return amount_read != sizeof (value);
+}
+// A wrapper of try_read which throws if there is an error
+template <typename T>
+void read (nano::stream & stream_a, T & value)
+{
+	auto error = try_read (stream_a, value);
+	if (error)
+	{
+		throw std::runtime_error ("Failed to read type");
+	}
+}
+
+template <typename T>
+void write (nano::stream & stream_a, T const & value)
+{
+	static_assert (std::is_standard_layout<T>::value, "Can't stream write non-standard layout types");
+	auto amount_written (stream_a.sputn (reinterpret_cast<uint8_t const *> (&value), sizeof (value)));
+	(void)amount_written;
+	assert (amount_written == sizeof (value));
+}
+}
diff --git a/nano/node/common.hpp b/nano/node/common.hpp
index 44d037ba4c..4d3a3174c6 100644
--- a/nano/node/common.hpp
+++ b/nano/node/common.hpp
@@ -199,6 +199,7 @@ class message_header final
 	uint8_t version_min;
 	nano::message_type type;
 	std::bitset<16> extensions;
+	static size_t constexpr size = sizeof (network_params::header_magic_number) + sizeof (version_max) + sizeof (version_using) + sizeof (version_min) + sizeof (type) + sizeof (/* extensions */ uint16_t);
 
 	void flag_set (uint8_t);
 	static uint8_t constexpr bulk_pull_count_present_flag = 0;
diff --git a/nano/secure/CMakeLists.txt b/nano/secure/CMakeLists.txt
index 87c8cc3407..cfe874160b 100644
--- a/nano/secure/CMakeLists.txt
+++ b/nano/secure/CMakeLists.txt
@@ -45,6 +45,8 @@ add_library (secure
 	epoch.cpp
 	ledger.hpp
 	ledger.cpp
+	network_filter.hpp
+	network_filter.cpp
 	utility.hpp
 	utility.cpp
 	versioning.hpp
diff --git a/nano/secure/network_filter.cpp b/nano/secure/network_filter.cpp
new file mode 100644
index 0000000000..572d2e57d1
--- /dev/null
+++ b/nano/secure/network_filter.cpp
@@ -0,0 +1,79 @@
+#include <nano/crypto_lib/random_pool.hpp>
+#include <nano/lib/locks.hpp>
+#include <nano/secure/buffer.hpp>
+#include <nano/secure/common.hpp>
+#include <nano/secure/network_filter.hpp>
+
+nano::network_filter::network_filter (size_t size_a) :
+items (size_a, nano::uint128_t{ 0 })
+{
+	nano::random_pool::generate_block (key, key.size ());
+}
+
+bool nano::network_filter::apply (uint8_t const * bytes_a, size_t count_a, nano::uint128_t * digest_a)
+{
+	// Get hash before locking
+	auto digest (hash (bytes_a, count_a));
+
+	nano::lock_guard<std::mutex> lock (mutex);
+	auto & element (get_element (digest));
+	bool existed (element == digest);
+	if (!existed)
+	{
+		// Replace likely old element with a new one
+		element = digest;
+	}
+	if (digest_a)
+	{
+		*digest_a = digest;
+	}
+	return existed;
+}
+
+void nano::network_filter::clear (nano::uint128_t const & digest_a)
+{
+	nano::lock_guard<std::mutex> lock (mutex);
+	auto & element (get_element (digest_a));
+	if (element == digest_a)
+	{
+		element = nano::uint128_t{ 0 };
+	}
+}
+
+void nano::network_filter::clear (uint8_t const * bytes_a, size_t count_a)
+{
+	clear (hash (bytes_a, count_a));
+}
+
+template <typename OBJECT>
+void nano::network_filter::clear (OBJECT const & object_a)
+{
+	std::vector<uint8_t> bytes;
+	{
+		nano::vectorstream stream (bytes);
+		object_a->serialize (stream);
+	}
+	clear (bytes.data (), bytes.size ());
+}
+
+void nano::network_filter::clear ()
+{
+	nano::lock_guard<std::mutex> lock (mutex);
+	items.assign (items.size (), nano::uint128_t{ 0 });
+}
+
+nano::uint128_t & nano::network_filter::get_element (nano::uint128_t const & hash_a)
+{
+	assert (!mutex.try_lock ());
+	assert (items.size () > 0);
+	size_t index (hash_a % items.size ());
+	return items[index];
+}
+
+nano::uint128_t nano::network_filter::hash (uint8_t const * bytes_a, size_t count_a) const
+{
+	nano::uint128_union digest{ 0 };
+	siphash_t siphash (key, static_cast<unsigned int> (key.size ()));
+	siphash.CalculateDigest (digest.bytes.data (), bytes_a, count_a);
+	return digest.number ();
+}
diff --git a/nano/secure/network_filter.hpp b/nano/secure/network_filter.hpp
new file mode 100644
index 0000000000..60b99dbd08
--- /dev/null
+++ b/nano/secure/network_filter.hpp
@@ -0,0 +1,74 @@
+
+#pragma once
+
+#include <nano/lib/numbers.hpp>
+
+#include <crypto/cryptopp/seckey.h>
+#include <crypto/cryptopp/siphash.h>
+
+#include <mutex>
+
+namespace nano
+{
+/**
+ * A probabilistic duplicate filter based on directed map caches, using SipHash 2/4/128
+ * The probability of false negatives (unique packet marked as duplicate) is the probability of a 128-bit SipHash collision.
+ * The probability of false positives (duplicate packet marked as unique) shrinks with a larger filter.
+ * @note This class is thread-safe.
+ */
+class network_filter final
+{
+public:
+	network_filter () = delete;
+	network_filter (size_t size_a);
+	/**
+	 * Reads \p count_a bytes starting from \p bytes_a and inserts the siphash digest in the filter.
+	 * @param \p digest_a if given, will be set to the resulting siphash digest
+	 * @warning will read out of bounds if [ \p bytes_a, \p bytes_a + \p count_a ] is not a valid range
+	 * @return a boolean representing the previous existence of the hash in the filter.
+	 **/
+	bool apply (uint8_t const * bytes_a, size_t count_a, nano::uint128_t * digest_a = nullptr);
+
+	/**
+	 * Sets the corresponding element in the filter to zero, if it matches \p digest_a exactly.
+	 **/
+	void clear (nano::uint128_t const & digest_a);
+
+	/**
+	 * Reads \p count_a bytes starting from \p bytes_a and digests the contents.
+	 * Then, sets the corresponding element in the filter to zero, if it matches the digest exactly.
+	 * @warning will read out of bounds if [ \p bytes_a, \p bytes_a + \p count_a ] is not a valid range
+	 **/
+	void clear (uint8_t const * bytes_a, size_t count_a);
+
+	/**
+	 * Serializes \p object_a and runs clears the resulting siphash digest.
+	 * @return a boolean representing the previous existence of the hash in the filter.
+	 **/
+	template <typename OBJECT>
+	void clear (OBJECT const & object_a);
+
+	/** Sets every element of the filter to zero, keeping its size and capacity. */
+	void clear ();
+
+private:
+	using siphash_t = CryptoPP::SipHash<2, 4, true>;
+
+	/**
+	 * Get element from digest.
+	 * @note must have a lock on mutex
+	 * @return a reference to the element with key \p hash_a
+	 **/
+	nano::uint128_t & get_element (nano::uint128_t const & hash_a);
+
+	/**
+	 * Hashes \p count_a bytes starting from \p bytes_a .
+	 * @return the siphash digest of the contents in \p bytes_a .
+	 **/
+	nano::uint128_t hash (uint8_t const * bytes_a, size_t count_a) const;
+
+	std::vector<nano::uint128_t> items;
+	CryptoPP::SecByteBlock key{ siphash_t::KEYLENGTH };
+	std::mutex mutex;
+};
+}