diff --git a/electrum/blockchain.py b/electrum/blockchain.py
index c4c557826..e0ab5d162 100644
--- a/electrum/blockchain.py
+++ b/electrum/blockchain.py
@@ -308,7 +308,7 @@ class Blockchain(util.PrintError):
         elif height == 0:
             return constants.net.GENESIS
         elif height < len(self.checkpoints) * 2016:
-            assert (height+1) % 2016 == 0, height
+            assert (height+1) % 2016 == 0, (height, len(self.checkpoints), (height+1) % 2016)
             index = height // 2016
             h, t = self.checkpoints[index]
             return h
diff --git a/electrum/interface.py b/electrum/interface.py
index 1db430fe7..23119333b 100644
--- a/electrum/interface.py
+++ b/electrum/interface.py
@@ -87,7 +87,7 @@ class Interface(PrintError):
             return False
         return True
 
-    @util.aiosafe
+    @aiosafe
     async def run(self):
         if self.protocol != 's':
             await self.open_session(None, exit_early=False)
@@ -183,11 +183,15 @@ class Interface(PrintError):
             self.tip = subscription_res['height']
             self.mark_ready()
             self.session = session
+            copy_header_queue = asyncio.Queue()
+            conniface = Conn(self.server, session, lambda idx, tip: self.network.request_chunk(idx, tip, session))
+            block_retriever = asyncio.get_event_loop().create_task(self.run_fetch_blocks(subscription_res, copy_header_queue, conniface))
             while True:
                 try:
                     new_header = await asyncio.wait_for(header_queue.get(), 300)
                     self.tip_header = new_header
                     self.tip = new_header['block_height']
+                    await copy_header_queue.put(new_header)
                 except concurrent.futures.TimeoutError:
                     await asyncio.wait_for(session.send_request('server.ping'), 5)
 
@@ -197,6 +201,212 @@ class Interface(PrintError):
     def close(self):
         self.fut.cancel()
 
+    @aiosafe
+    async def run_fetch_blocks(self, sub_reply, replies, conniface):
+        async with self.network.bhi_lock:
+            bhi = BlockHeaderInterface(conniface, sub_reply['height'], self.blockchain.height()+1, self)
+            await replies.put(blockchain.deserialize_header(bfh(sub_reply['hex']), sub_reply['height']))
+            self.print_error("checking if catched up {}-1 > {}, tip {}".format(sub_reply['height'], self.blockchain.height(), bhi.tip))
+            if sub_reply['height']-1 > self.blockchain.height():
+                last_status = await bhi.sync_until()
+                assert last_status == 'catchup', last_status
+                assert self.blockchain.height()+1 == bhi.height, (self.blockchain.height(), bhi.height)
+
+        while True:
+            self.network.notify('updated')
+            item = await replies.get()
+            async with self.network.bhi_lock:
+                if self.blockchain.height() >= bhi.height and self.blockchain.check_header(item):
+                    # another interface amended the blockchain
+                    self.print_error("SKIPPING HEADER", bhi.height)
+                    continue
+                if bhi.tip < bhi.height:
+                    bhi.height = bhi.tip
+                await bhi.step(item)
+                bhi.tip = max(bhi.height, bhi.tip)
+
+class BlockHeaderInterface(PrintError):
+    def __init__(self, conn, tip, height, iface):
+        self.tip = tip
+        self.height = height
+        self.conn = conn
+        self.iface = iface
+
+    def diagnostic_name(self):
+        return self.conn.server
+
+    async def sync_until(self, next_height=None):
+        if next_height is None:
+            next_height = self.tip
+        last = None
+        while last is None or self.height < next_height:
+            if next_height > self.height + 10:
+                could_connect, num_headers = await self.conn.request_chunk(self.height, next_height)
+                self.tip = max(self.height + num_headers, self.tip)
+                if not could_connect:
+                    last = await self.step()
+                    self.tip = max(self.height, self.tip)
+                    continue
+                self.height = (self.height // 2016 * 2016) + num_headers
+                if self.height > next_height:
+                    assert False, (self.height, self.tip)
+                last = 'catchup'
+            else:
+                last = await self.step()
+                self.tip = max(self.height, self.tip)
+        return last
+
+    async def step(self, header=None):
+        assert self.height != 0
+        if header is None:
+            header = await self.conn.get_block_header(self.height, 'catchup')
+        can_connect = blockchain.can_connect(header) if 'mock' not in header else header['mock']['connect'](self)
+
+        bad_header = None
+        if not can_connect:
+            self.print_error("can't connect", self.height)
+            #backward
+            bad = self.height
+            bad_header = header
+            self.height -= 1
+            checkp = False
+            if self.height <= self.iface.network.max_checkpoint():
+                self.height = self.iface.network.max_checkpoint() + 1
+                checkp = True
+
+            header = await self.conn.get_block_header(self.height, 'backward')
+            chain = blockchain.check_header(header) if 'mock' not in header else header['mock']['check'](header)
+            can_connect = blockchain.can_connect(header) if 'mock' not in header else header['mock']['connect'](self)
+            if checkp:
+                assert can_connect or chain, (can_connect, chain)
+            while not chain and not can_connect:
+                bad = self.height
+                bad_header = header
+                delta = self.tip - self.height
+                next_height = self.tip - 2 * delta
+                checkp = False
+                if next_height <= self.iface.network.max_checkpoint():
+                    next_height = self.iface.network.max_checkpoint() + 1
+                    checkp = True
+                self.height = next_height
+
+                header = await self.conn.get_block_header(self.height, 'backward')
+                chain = blockchain.check_header(header) if 'mock' not in header else header['mock']['check'](header)
+                can_connect = blockchain.can_connect(header) if 'mock' not in header else header['mock']['connect'](self)
+                if checkp:
+                    assert can_connect or chain, (can_connect, chain)
+            self.print_error("exiting backward mode at", self.height)
+        if can_connect:
+            self.print_error("could connect", self.height)
+            chain = blockchain.check_header(header) if 'mock' not in header else header['mock']['check'](header)
+
+            if type(can_connect) is bool:
+                # mock
+                self.height += 1
+                if self.height > self.tip:
+                    assert False
+                return 'catchup'
+            self.iface.blockchain = can_connect
+            self.height += 1
+            self.iface.blockchain.save_header(header)
+            return 'catchup'
+
+        if not chain:
+            raise Exception("not chain") # line 931 in 8e69174374aee87d73cd2f8005fbbe87c93eee9c's network.py
+
+        # binary
+        if type(chain) in [int, bool]:
+            pass # mock
+        else:
+            self.iface.blockchain = chain
+        good = self.height
+        self.height = (bad + good) // 2
+        header = await self.conn.get_block_header(self.height, 'binary')
+        while True:
+            self.print_error("binary step")
+            chain = blockchain.check_header(header) if 'mock' not in header else header['mock']['check'](header)
+            if chain:
+                assert bad != self.height, (bad, self.height)
+                good = self.height
+                self.iface.blockchain = self.iface.blockchain if type(chain) in [bool, int] else chain
+            else:
+                bad = self.height
+                assert good != self.height
+                bad_header = header
+            if bad != good + 1:
+                self.height = (bad + good) // 2
+                header = await self.conn.get_block_header(self.height, 'binary')
+                continue
+            mock = bad_header and 'mock' in bad_header and bad_header['mock']['connect'](self)
+            real = not mock and self.iface.blockchain.can_connect(bad_header, check_height=False)
+            if not real and not mock:
+                raise Exception('unexpected bad header during binary' + str(bad_header)) # line 948 in 8e69174374aee87d73cd2f8005fbbe87c93eee9c's network.py
+            branch = blockchain.blockchains.get(bad)
+            if branch is not None:
+                ismocking = False
+                if type(branch) is dict:
+                    ismocking = True
+                # FIXME: it does not seem sufficient to check that the branch
+                # contains the bad_header. what if self.blockchain doesn't?
+                # the chains shouldn't be joined then. observe the incorrect
+                # joining on regtest with a server that has a fork of height
+                # one. the problem is observed only if forking is not during
+                # electrum runtime
+                if ismocking and branch['check'](bad_header) or not ismocking and branch.check_header(bad_header):
+                    self.print_error('joining chain', bad)
+                    self.height += 1
+                    return 'join'
+                else:
+                    if ismocking and branch['parent']['check'](header) or not ismocking and branch.parent().check_header(header):
+                        self.print_error('reorg', bad, self.tip)
+                        self.iface.blockchain = branch.parent() if not ismocking else branch['parent']
+                        self.height = bad
+                        header = await self.conn.get_block_header(self.height, 'binary')
+                    else:
+                        if ismocking:
+                            self.height = bad + 1
+                            self.print_error("TODO replace blockchain")
+                            return 'conflict'
+                        self.print_error('forkpoint conflicts with existing fork', branch.path())
+                        branch.write(b'', 0)
+                        branch.save_header(bad_header)
+                        self.iface.blockchain = branch
+                        self.height = bad + 1
+                        return 'conflict'
+            else:
+                bh = self.iface.blockchain.height()
+                if bh > good:
+                    forkfun = self.iface.blockchain.fork
+                    if 'mock' in bad_header:
+                        chain = bad_header['mock']['check'](bad_header)
+                        forkfun = bad_header['mock']['fork'] if 'fork' in bad_header['mock'] else forkfun
+                    else:
+                        chain = self.iface.blockchain.check_header(bad_header)
+                    if not chain:
+                        b = forkfun(bad_header)
+                        assert bad not in blockchain.blockchains, (bad, list(blockchain.blockchains.keys()))
+                        blockchain.blockchains[bad] = b
+                        self.iface.blockchain = b
+                        self.height = b.forkpoint + 1
+                        assert b.forkpoint == bad
+                    return 'fork'
+                else:
+                    assert bh == good
+                    if bh < self.tip:
+                        self.print_error("catching up from %d"% (bh + 1))
+                        self.height = bh + 1
+                    return 'no_fork'
+
+class Conn:
+    def __init__(self, server, session, get_chunk):
+        self.server = server
+        self.session = session # type: aiorpcx.ClientSession
+        self.request_chunk = get_chunk
+    async def get_block_header(self, height, assert_mode):
+        res = await asyncio.wait_for(self.session.send_request('blockchain.block.header', [height]), 1)
+        return blockchain.deserialize_header(bytes.fromhex(res), height)
+
+
 def check_cert(host, cert):
     try:
         b = pem.dePem(cert, 'CERTIFICATE')
diff --git a/electrum/network.py b/electrum/network.py
index 1d5055f69..92378d9fa 100644
--- a/electrum/network.py
+++ b/electrum/network.py
@@ -37,7 +37,7 @@ import dns
 import dns.resolver
 
 from . import util
-from .util import PrintError, print_error, bfh
+from .util import PrintError, print_error, aiosafe, bfh
 from .bitcoin import COIN
 from . import constants
 from . import blockchain
@@ -533,7 +533,12 @@ class Network(PrintError):
         if self.server_is_lagging() and self.auto_connect:
             # switch to one that has the correct header (not height)
             header = self.blockchain().read_header(self.get_local_height())
-            filtered = list(map(lambda x: x[0], filter(lambda x: x[1].tip_header == header, self.interfaces.items())))
+            def filt(x):
+                a = x[1].tip_header
+                b = header
+                assert type(a) is type(b)
+                return a == b
+            filtered = list(map(lambda x: x[0], filter(filt, self.interfaces.items())))
             if filtered:
                 choice = random.choice(filtered)
                 self.switch_to_interface(choice)
@@ -668,7 +673,7 @@ class Network(PrintError):
                 if b.catch_up == server:
                     b.catch_up = None
 
-    @util.aiosafe
+    @aiosafe
     async def new_interface(self, server):
         # todo: get tip first, then decide which checkpoint to use.
         self.add_recent_server(server)
@@ -677,8 +682,8 @@ class Network(PrintError):
         try:
             await asyncio.wait_for(interface.ready, 5)
         except BaseException as e:
-            import traceback
-            traceback.print_exc()
+            #import traceback
+            #traceback.print_exc()
             self.print_error(interface.server, "couldn't launch because", str(e), str(type(e)))
             self.connection_down(interface.server)
             return
@@ -713,6 +718,29 @@ class Network(PrintError):
         except concurrent.futures.CancelledError:
             pass
 
+    async def get_merkle_for_transaction(self, tx_hash, tx_height):
+        print("getting merkle for transaction", tx_hash, tx_height)
+        return await self.interface.session.send_request('blockchain.transaction.get_merkle', [tx_hash, tx_height])
+
+    def broadcast_transaction(self, tx):
+        fut = asyncio.run_coroutine_threadsafe(self.interface.session.send_request('blockchain.transaction.broadcast', [str(tx)]), self.asyncio_loop)
+        return True, fut.result(1)
+
+    async def request_chunk(self, height, tip, session=None):
+        if session is None: session = self.interface.session
+        index = height // 2016
+        size = 2016
+        if tip is not None and height + 2016 >= tip:
+            size = tip - height
+        #if index * 2016 < height:
+        #    size = height - index * 2016
+        res = await session.send_request('blockchain.block.headers', [index * 2016, size])
+        conn = self.blockchain().connect_chunk(index, res['hex'])
+        if not conn:
+            return conn, 0
+        self.blockchain().save_chunk(index, bfh(res['hex']))
+        return conn, res['count']
+
     @with_interface_lock
     def blockchain(self):
         if self.interface and self.interface.blockchain is not None:
diff --git a/electrum/tests/test_interface.py b/electrum/tests/test_interface.py
deleted file mode 100644
index 402588ca8..000000000
--- a/electrum/tests/test_interface.py
+++ /dev/null
@@ -1,28 +0,0 @@
-import unittest
-
-from electrum import interface
-
-from . import SequentialTestCase
-
-
-class TestInterface(SequentialTestCase):
-
-    def test_match_host_name(self):
-        self.assertTrue(interface._match_hostname('asd.fgh.com', 'asd.fgh.com'))
-        self.assertFalse(interface._match_hostname('asd.fgh.com', 'asd.zxc.com'))
-        self.assertTrue(interface._match_hostname('asd.fgh.com', '*.fgh.com'))
-        self.assertFalse(interface._match_hostname('asd.fgh.com', '*fgh.com'))
-        self.assertFalse(interface._match_hostname('asd.fgh.com', '*.zxc.com'))
-
-    def test_check_host_name(self):
-        i = interface.TcpConnection(server=':1:', queue=None, config_path=None)
-
-        self.assertFalse(i.check_host_name(None, None))
-        self.assertFalse(i.check_host_name(
-            peercert={'subjectAltName': []}, name=''))
-        self.assertTrue(i.check_host_name(
-            peercert={'subjectAltName': [('DNS', 'foo.bar.com')]},
-            name='foo.bar.com'))
-        self.assertTrue(i.check_host_name(
-            peercert={'subject': [('commonName', 'foo.bar.com')]},
-            name='foo.bar.com'))
diff --git a/electrum/tests/test_network.py b/electrum/tests/test_network.py
new file mode 100644
index 000000000..8181951de
--- /dev/null
+++ b/electrum/tests/test_network.py
@@ -0,0 +1,118 @@
+import asyncio
+import tempfile
+import unittest
+
+from electrum.constants import set_regtest
+from electrum.simple_config import SimpleConfig
+from electrum import blockchain
+from electrum.interface import BlockHeaderInterface
+
+class MockConnection:
+    def __init__(self):
+        self.q = asyncio.Queue()
+        self.server = 'mock-server'
+    async def get_block_header(self, height, assert_mode):
+        assert self.q.qsize() > 0, (height, assert_mode)
+        item = await self.q.get()
+        print("step with height", height, item)
+        assert item['block_height'] == height, (item['block_height'], height)
+        assert assert_mode in item['mock'], (assert_mode, item)
+        return item
+
+class TestNetwork(unittest.TestCase):
+    def setUp(self):
+        self.config = SimpleConfig({'electrum_path': tempfile.mkdtemp(prefix="test_network")})
+
+    def blockchain_iface_pair(self, forkpoint=2002):
+        b = blockchain.Blockchain(self.config, forkpoint, None)
+        class FakeNetwork:
+            max_checkpoint = lambda: 0
+        class FakeIface:
+            blockchain = b
+            network = FakeNetwork
+        return FakeIface
+
+    def test_new_fork(self):
+        blockchain.blockchains = {}
+        conn = MockConnection()
+        conn.q.put_nowait({'block_height': 8, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: False}})
+        def mock_connect(block_header_iface):
+            return block_header_iface.height == 6
+        conn.q.put_nowait({'block_height': 7, 'mock': {'backward':1,'check': lambda x: False, 'connect': mock_connect, 'fork': self.mock_fork}})
+        conn.q.put_nowait({'block_height': 2, 'mock': {'backward':1,'check':lambda x: True, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 4, 'mock': {'binary':1,'check':lambda x: True, 'connect': lambda x: True}})
+        conn.q.put_nowait({'block_height': 5, 'mock': {'binary':1,'check':lambda x: True, 'connect': lambda x: True}})
+        conn.q.put_nowait({'block_height': 6, 'mock': {'binary':1,'check':lambda x: True, 'connect': lambda x: True}})
+        ifa = BlockHeaderInterface(conn, 12, 8, self.blockchain_iface_pair())
+        self.assertEqual('fork', asyncio.get_event_loop().run_until_complete(ifa.sync_until(next_height=8)))
+        self.assertEqual(conn.q.qsize(), 0)
+
+    def test_new_can_connect_during_backward(self):
+        blockchain.blockchains = {}
+        conn = MockConnection()
+        conn.q.put_nowait({'block_height': 8, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: False}})
+        def mock_connect(block_header_iface):
+            return block_header_iface.height == 2
+        conn.q.put_nowait({'block_height': 7, 'mock': {'backward':1, 'check': lambda x: False, 'connect': mock_connect, 'fork': self.mock_fork}})
+        conn.q.put_nowait({'block_height': 2, 'mock': {'backward':1, 'check': lambda x: False, 'connect': mock_connect, 'fork': self.mock_fork}})
+        conn.q.put_nowait({'block_height': 3, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: True}})
+        conn.q.put_nowait({'block_height': 4, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: True}})
+        ifa = BlockHeaderInterface(conn, 12, 8, self.blockchain_iface_pair())
+        self.assertEqual('catchup', asyncio.get_event_loop().run_until_complete(ifa.sync_until(next_height=5)))
+        self.assertEqual(conn.q.qsize(), 0)
+
+    def mock_fork(self, bad_header):
+        return blockchain.Blockchain(self.config, bad_header['block_height'], None)
+
+    def test_new_chain_false_during_binary(self):
+        blockchain.blockchains = {}
+        conn = MockConnection()
+        conn.q.put_nowait({'block_height': 8, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: False}})
+        mock_connect = lambda bhi: bhi.height == 3
+        conn.q.put_nowait({'block_height': 7, 'mock': {'backward':1, 'check': lambda x: False, 'connect': mock_connect}})
+        conn.q.put_nowait({'block_height': 2, 'mock': {'backward':1, 'check': lambda x: True,  'connect': mock_connect}})
+        conn.q.put_nowait({'block_height': 4, 'mock': {'binary':1, 'check': lambda x: False, 'fork': self.mock_fork, 'connect': mock_connect}})
+        conn.q.put_nowait({'block_height': 3, 'mock': {'binary':1, 'check': lambda x: True, 'connect': lambda x: True}})
+        conn.q.put_nowait({'block_height': 5, 'mock': {'catchup':1, 'check': lambda x: True, 'connect': lambda x: True}})
+        conn.q.put_nowait({'block_height': 6, 'mock': {'catchup':1, 'check': lambda x: True, 'connect': lambda x: True}})
+        ifa = BlockHeaderInterface(conn, 12, 8, self.blockchain_iface_pair(1000))
+        self.assertEqual('catchup', asyncio.get_event_loop().run_until_complete(ifa.sync_until(next_height=7)))
+        self.assertEqual(conn.q.qsize(), 0)
+
+    def test_new_join(self):
+        blockchain.blockchains = {7: {'check': lambda bad_header: True}}
+        conn = MockConnection()
+        conn.q.put_nowait({'block_height': 8, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 7, 'mock': {'backward':1, 'check': lambda x: False, 'connect': lambda x: x.height == 6}})
+        conn.q.put_nowait({'block_height': 2, 'mock': {'backward':1, 'check': lambda x: True,  'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 4, 'mock': {'binary':1, 'check': lambda x: True, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 5, 'mock': {'binary':1, 'check': lambda x: True, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 6, 'mock': {'binary':1, 'check': lambda x: True, 'connect': lambda x: True}})
+        ifa = BlockHeaderInterface(conn, 12, 8, self.blockchain_iface_pair())
+        self.assertEqual('join', asyncio.get_event_loop().run_until_complete(ifa.sync_until(next_height=7)))
+        self.assertEqual(conn.q.qsize(), 0)
+
+    def test_new_reorg(self):
+        times = 0
+        def check(header):
+            nonlocal times
+            self.assertEqual(header['block_height'], 7)
+            times += 1
+            return times != 1
+        blockchain.blockchains = {7: {'check': check, 'parent': {'check': lambda x: True}}}
+        conn = MockConnection()
+        conn.q.put_nowait({'block_height': 8, 'mock': {'catchup':1, 'check': lambda x: False, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 7, 'mock': {'backward':1, 'check': lambda x: False, 'connect': lambda x: x.height == 6}})
+        conn.q.put_nowait({'block_height': 2, 'mock': {'backward':1, 'check': lambda x: 1,  'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 4, 'mock': {'binary':1, 'check': lambda x: 1, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 5, 'mock': {'binary':1, 'check': lambda x: 1, 'connect': lambda x: False}})
+        conn.q.put_nowait({'block_height': 6, 'mock': {'binary':1, 'check': lambda x: 1, 'connect': lambda x: True}})
+        conn.q.put_nowait({'block_height': 7, 'mock': {'binary':1, 'check': lambda x: False, 'connect': lambda x: True}})
+        ifa = BlockHeaderInterface(conn, 12, 8, self.blockchain_iface_pair())
+        self.assertEqual('join', asyncio.get_event_loop().run_until_complete(ifa.sync_until(next_height=8)))
+        self.assertEqual(conn.q.qsize(), 0)
+        self.assertEqual(times, 2)
+
+if __name__=="__main__":
+    set_regtest()
+    unittest.main()
diff --git a/electrum/verifier.py b/electrum/verifier.py
index f64e7fa47..61642364e 100644
--- a/electrum/verifier.py
+++ b/electrum/verifier.py
@@ -70,8 +70,7 @@ class SPV(ThreadJob):
             if header is None:
                 index = tx_height // 2016
                 if index < len(blockchain.checkpoints):
-                    # FIXME disabled until async block header download has been merged
-                    pass #await self.network.request_chunk(tx_height, None)
+                    await self.network.request_chunk(tx_height, None)
             elif (tx_hash not in self.requested_merkle
                     and tx_hash not in self.merkle_roots):
                 self.print_error('requested merkle', tx_hash)