diff --git a/.gitignore b/.gitignore
index 99f905526..93802b22b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,6 +9,7 @@ venv*
 *~
 *.DS_Store
 .*.kate-swp
+*.bak
 
 /build/
 /support/
diff --git a/newsfragments/3374.minor b/newsfragments/3374.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/codec.py b/src/allmydata/codec.py
index 1ceb146e4..a4baab4b6 100644
--- a/src/allmydata/codec.py
+++ b/src/allmydata/codec.py
@@ -1,4 +1,16 @@
-# -*- test-case-name: allmydata.test.test_encode_share -*-
+"""
+CRS encoding and decoding.
+
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 
 from zope.interface import implementer
 from twisted.internet import defer
@@ -9,7 +21,7 @@ import zfec
 
 @implementer(ICodecEncoder)
 class CRSEncoder(object):
-    ENCODER_TYPE = "crs"
+    ENCODER_TYPE = b"crs"
 
     def set_params(self, data_size, required_shares, max_shares):
         assert required_shares <= max_shares
@@ -27,8 +39,8 @@ class CRSEncoder(object):
         return (self.data_size, self.required_shares, self.max_shares)
 
     def get_serialized_params(self):
-        return "%d-%d-%d" % (self.data_size, self.required_shares,
-                             self.max_shares)
+        return b"%d-%d-%d" % (self.data_size, self.required_shares,
+                              self.max_shares)
 
     def get_block_size(self):
         return self.share_size
@@ -37,7 +49,7 @@ class CRSEncoder(object):
         precondition(desired_share_ids is None or len(desired_share_ids) <= self.max_shares, desired_share_ids, self.max_shares)
 
         if desired_share_ids is None:
-            desired_share_ids = range(self.max_shares)
+            desired_share_ids = list(range(self.max_shares))
 
         for inshare in inshares:
             assert len(inshare) == self.share_size, (len(inshare), self.share_size, self.data_size, self.required_shares)
@@ -71,5 +83,5 @@ class CRSDecoder(object):
         return defer.succeed(data)
 
 def parse_params(serializedparams):
-    pieces = serializedparams.split("-")
+    pieces = serializedparams.split(b"-")
     return int(pieces[0]), int(pieces[1]), int(pieces[2])
diff --git a/src/allmydata/interfaces.py b/src/allmydata/interfaces.py
index c93c1d81d..43565dc4f 100644
--- a/src/allmydata/interfaces.py
+++ b/src/allmydata/interfaces.py
@@ -1,3 +1,19 @@
+"""
+Interfaces for Tahoe-LAFS.
+
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    # Don't import object/str/dict/etc. types, so we don't break any
+    # interfaces. Not importing open() because it triggers bogus flake8 error.
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, pow, round, super, range, max, min  # noqa: F401
+
 from past.builtins import long
 
 from zope.interface import Interface, Attribute
@@ -58,7 +74,7 @@ class RIBucketReader(RemoteInterface):
     def read(offset=Offset, length=ReadSize):
         return ShareData
 
-    def advise_corrupt_share(reason=str):
+    def advise_corrupt_share(reason=bytes):
         """Clients who discover hash failures in shares that they have
         downloaded from me will use this method to inform me about the
         failures. I will record their concern so that my operator can
@@ -71,7 +87,7 @@ class RIBucketReader(RemoteInterface):
         """
 
 
-TestVector = ListOf(TupleOf(Offset, ReadSize, str, str))
+TestVector = ListOf(TupleOf(Offset, ReadSize, bytes, bytes))
 # elements are (offset, length, operator, specimen)
 # operator is one of "lt, le, eq, ne, ge, gt"
 # nop always passes and is used to fetch data while writing.
@@ -89,13 +105,13 @@ ReadData = ListOf(ShareData)
 
 
 class RIStorageServer(RemoteInterface):
-    __remote_name__ = "RIStorageServer.tahoe.allmydata.com"
+    __remote_name__ = b"RIStorageServer.tahoe.allmydata.com"
 
     def get_version():
         """
         Return a dictionary of version information.
         """
-        return DictOf(str, Any())
+        return DictOf(bytes, Any())
 
     def allocate_buckets(storage_index=StorageIndex,
                          renew_secret=LeaseRenewSecret,
@@ -277,8 +293,8 @@ class RIStorageServer(RemoteInterface):
         """
         return TupleOf(bool, DictOf(int, ReadData))
 
-    def advise_corrupt_share(share_type=str, storage_index=StorageIndex,
-                             shnum=int, reason=str):
+    def advise_corrupt_share(share_type=bytes, storage_index=StorageIndex,
+                             shnum=int, reason=bytes):
         """Clients who discover hash failures in shares that they have
         downloaded from me will use this method to inform me about the
         failures. I will record their concern so that my operator can
@@ -2859,7 +2875,7 @@ UploadResults = Any() #DictOf(str, str)
 
 
 class RIEncryptedUploadable(RemoteInterface):
-    __remote_name__ = "RIEncryptedUploadable.tahoe.allmydata.com"
+    __remote_name__ = b"RIEncryptedUploadable.tahoe.allmydata.com"
 
     def get_size():
         return Offset
@@ -2875,7 +2891,7 @@ class RIEncryptedUploadable(RemoteInterface):
 
 
 class RICHKUploadHelper(RemoteInterface):
-    __remote_name__ = "RIUploadHelper.tahoe.allmydata.com"
+    __remote_name__ = b"RIUploadHelper.tahoe.allmydata.com"
 
     def get_version():
         """
@@ -2888,7 +2904,7 @@ class RICHKUploadHelper(RemoteInterface):
 
 
 class RIHelper(RemoteInterface):
-    __remote_name__ = "RIHelper.tahoe.allmydata.com"
+    __remote_name__ = b"RIHelper.tahoe.allmydata.com"
 
     def get_version():
         """
@@ -2915,7 +2931,7 @@ class RIHelper(RemoteInterface):
 
 
 class RIStatsProvider(RemoteInterface):
-    __remote_name__ = "RIStatsProvider.tahoe.allmydata.com"
+    __remote_name__ = b"RIStatsProvider.tahoe.allmydata.com"
     """
     Provides access to statistics and monitoring information.
     """
@@ -2932,7 +2948,7 @@ class RIStatsProvider(RemoteInterface):
 
 
 class RIStatsGatherer(RemoteInterface):
-    __remote_name__ = "RIStatsGatherer.tahoe.allmydata.com"
+    __remote_name__ = b"RIStatsGatherer.tahoe.allmydata.com"
     """
     Provides a monitoring service for centralised collection of stats
     """
diff --git a/src/allmydata/monitor.py b/src/allmydata/monitor.py
index 98c463014..1559a30d9 100644
--- a/src/allmydata/monitor.py
+++ b/src/allmydata/monitor.py
@@ -1,7 +1,21 @@
+"""
+Manage status of long-running operations.
+
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 
 from zope.interface import Interface, implementer
 from allmydata.util import observer
 
+
 class IMonitor(Interface):
     """I manage status, progress, and cancellation for long-running operations.
 
diff --git a/src/allmydata/test/test_codec.py b/src/allmydata/test/test_codec.py
index 706e62dff..ee64e2bf2 100644
--- a/src/allmydata/test/test_codec.py
+++ b/src/allmydata/test/test_codec.py
@@ -1,8 +1,21 @@
+"""
+Tests for allmydata.codec.
+
+Ported to Python 3.
+"""
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
+
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 
 import os
 from twisted.trial import unittest
 from twisted.python import log
-from allmydata.codec import CRSEncoder, CRSDecoder
+from allmydata.codec import CRSEncoder, CRSDecoder, parse_params
 import random
 from allmydata.util import mathutil
 
@@ -13,6 +26,8 @@ class T(unittest.TestCase):
         enc.set_params(size, required_shares, max_shares)
         params = enc.get_params()
         assert params == (size, required_shares, max_shares)
+        serialized_params = enc.get_serialized_params()
+        self.assertEqual(parse_params(serialized_params), params)
         log.msg("params: %s" % (params,))
         d = enc.encode(data0s)
         def _done_encoding_all(shares_and_shareids):
@@ -23,7 +38,7 @@ class T(unittest.TestCase):
         d.addCallback(_done_encoding_all)
         if fewer_shares is not None:
             # also validate that the desired_shareids= parameter works
-            desired_shareids = random.sample(range(max_shares), fewer_shares)
+            desired_shareids = random.sample(list(range(max_shares)), fewer_shares)
             d.addCallback(lambda res: enc.encode(data0s, desired_shareids))
             def _check_fewer_shares(some_shares_and_their_shareids):
                 (some_shares, their_shareids) = some_shares_and_their_shareids
@@ -38,11 +53,11 @@ class T(unittest.TestCase):
             return d1
 
         def _check_data(decoded_shares):
-            self.failUnlessEqual(len(''.join(decoded_shares)), len(''.join(data0s)))
+            self.failUnlessEqual(len(b''.join(decoded_shares)), len(b''.join(data0s)))
             self.failUnlessEqual(len(decoded_shares), len(data0s))
             for (i, (x, y)) in enumerate(zip(data0s, decoded_shares)):
                 self.failUnlessEqual(x, y, "%s: %r != %r....  first share was %r" % (str(i), x, y, data0s[0],))
-            self.failUnless(''.join(decoded_shares) == ''.join(data0s), "%s" % ("???",))
+            self.failUnless(b''.join(decoded_shares) == b''.join(data0s), "%s" % ("???",))
             # 0data0sclipped = tuple(data0s)
             # data0sclipped[-1] =
             # self.failUnless(tuple(decoded_shares) == tuple(data0s))
@@ -59,7 +74,7 @@ class T(unittest.TestCase):
         def _decode_some_random(res):
             log.msg("_decode_some_random")
             # use a randomly-selected minimal subset
-            l = random.sample(zip(self.shares, self.shareids), required_shares)
+            l = random.sample(list(zip(self.shares, self.shareids)), required_shares)
             some_shares = [ x[0] for x in l ]
             some_shareids = [ x[1] for x in l ]
             return _decode((some_shares, some_shareids))
@@ -70,10 +85,10 @@ class T(unittest.TestCase):
             log.msg("_decode_multiple")
             # make sure we can re-use the decoder object
             shares1 = random.sample(self.shares, required_shares)
-            sharesl1 = random.sample(zip(self.shares, self.shareids), required_shares)
+            sharesl1 = random.sample(list(zip(self.shares, self.shareids)), required_shares)
             shares1 = [ x[0] for x in sharesl1 ]
             shareids1 = [ x[1] for x in sharesl1 ]
-            sharesl2 = random.sample(zip(self.shares, self.shareids), required_shares)
+            sharesl2 = random.sample(list(zip(self.shares, self.shareids)), required_shares)
             shares2 = [ x[0] for x in sharesl2 ]
             shareids2 = [ x[1] for x in sharesl2 ]
             dec = CRSDecoder()
diff --git a/src/allmydata/test/test_monitor.py b/src/allmydata/test/test_monitor.py
new file mode 100644
index 000000000..7010da73a
--- /dev/null
+++ b/src/allmydata/test/test_monitor.py
@@ -0,0 +1,52 @@
+"""
+Tests for allmydata.monitor.
+"""
+
+from __future__ import unicode_literals
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+
+from twisted.trial import unittest
+
+from allmydata.monitor import Monitor, OperationCancelledError
+
+
+class MonitorTests(unittest.TestCase):
+    """Tests for the Monitor class."""
+
+    def test_cancellation(self):
+        """The monitor can be cancelled."""
+        m = Monitor()
+        self.assertFalse(m.is_cancelled())
+        m.raise_if_cancelled()
+        m.cancel()
+        self.assertTrue(m.is_cancelled())
+        with self.assertRaises(OperationCancelledError):
+            m.raise_if_cancelled()
+
+    def test_status(self):
+        """The monitor can have its status set."""
+        m = Monitor()
+        self.assertEqual(m.get_status(), None)
+        m.set_status("discombobulated")
+        self.assertEqual(m.get_status(), "discombobulated")
+
+    def test_finish(self):
+        """The monitor can finish."""
+        m = Monitor()
+        self.assertFalse(m.is_finished())
+        d = m.when_done()
+        self.assertNoResult(d)
+
+        result = m.finish(300)
+        self.assertEqual(result, 300)
+        self.assertEqual(m.get_status(), 300)
+        self.assertTrue(m.is_finished())
+
+        d.addBoth(self.assertEqual, 300)
+        return d
diff --git a/src/allmydata/util/_python3.py b/src/allmydata/util/_python3.py
index 2ec91ed96..683834673 100644
--- a/src/allmydata/util/_python3.py
+++ b/src/allmydata/util/_python3.py
@@ -24,6 +24,7 @@ if PY2:
 
 # Keep these sorted alphabetically, to reduce merge conflicts:
 PORTED_MODULES = [
+    "allmydata.codec",
     "allmydata.crypto",
     "allmydata.crypto.aes",
     "allmydata.crypto.ed25519",
@@ -32,6 +33,8 @@ PORTED_MODULES = [
     "allmydata.crypto.util",
     "allmydata.hashtree",
     "allmydata.immutable.happiness_upload",
+    "allmydata.interfaces",
+    "allmydata.monitor",
     "allmydata.storage.crawler",
     "allmydata.storage.expirer",
     "allmydata.test.common_py3",
@@ -68,6 +71,7 @@ PORTED_TEST_MODULES = [
     "allmydata.test.test_abbreviate",
     "allmydata.test.test_base32",
     "allmydata.test.test_base62",
+    "allmydata.test.test_codec",
     "allmydata.test.test_configutil",
     "allmydata.test.test_connection_status",
     "allmydata.test.test_crawler",
@@ -81,6 +85,7 @@ PORTED_TEST_MODULES = [
     "allmydata.test.test_humanreadable",
     "allmydata.test.test_iputil",
     "allmydata.test.test_log",
+    "allmydata.test.test_monitor",
     "allmydata.test.test_netstring",
     "allmydata.test.test_observer",
     "allmydata.test.test_pipeline",