diff --git a/src/allmydata/mutable/filenode.py b/src/allmydata/mutable/filenode.py
index 02f99b6..cafe20f 100644
--- a/src/allmydata/mutable/filenode.py
+++ b/src/allmydata/mutable/filenode.py
@@ -118,17 +118,6 @@ class MutableFileNode:
         self._privkey = None
         self._encprivkey = None
 
-        # Starting with MDMF caps, we allowed arbitrary extensions in
-        # caps. If we were initialized with a cap that had extensions,
-        # we want to remember them so we can tell MutableFileVersions
-        # about them.
-        extensions = self._uri.get_extension_params()
-        if extensions:
-            extensions = map(int, extensions)
-            suspected_k, suspected_segsize = extensions
-            self._downloader_hints['k'] = suspected_k
-            self._downloader_hints['segsize'] = suspected_segsize
-
         return self
 
     def create_with_keys(self, (pubkey, privkey), contents,
@@ -701,9 +690,6 @@ class MutableFileNode:
 
     def set_downloader_hints(self, hints):
         self._downloader_hints = hints
-        extensions = [ hints["k"], hints["segsize"] ]
-        self._uri.set_extension_params(extensions)
-
 
     def _did_upload(self, res, size):
         self._most_recent_size = size
diff --git a/src/allmydata/scripts/debug.py b/src/allmydata/scripts/debug.py
index 16e57b3..b482fb9 100644
--- a/src/allmydata/scripts/debug.py
+++ b/src/allmydata/scripts/debug.py
@@ -369,8 +369,7 @@ def dump_MDMF_share(m, length, options):
         if base32.could_be_base32_encoded(piece):
             storage_index = base32.a2b(piece)
             fingerprint = hashutil.ssk_pubkey_fingerprint_hash(pubkey)
-            hints = [str(k), str(segsize)]
-            u = MDMFVerifierURI(storage_index, fingerprint, hints)
+            u = MDMFVerifierURI(storage_index, fingerprint)
             verify_cap = u.to_string()
             print >>out, "  verify-cap:", quote_output(verify_cap, quotemarks=False)
 
diff --git a/src/allmydata/test/common.py b/src/allmydata/test/common.py
index 1e29f0d..42890d8 100644
--- a/src/allmydata/test/common.py
+++ b/src/allmydata/test/common.py
@@ -208,7 +208,6 @@ class FakeMutableFileNode:
         data = initial_contents.read(initial_contents.get_size())
         data = "".join(data)
         self.all_contents[self.storage_index] = data
-        self.my_uri.set_extension_params([self._k, self._segsize])
         return defer.succeed(self)
     def _get_initial_contents(self, contents):
         if contents is None:
@@ -358,7 +357,6 @@ class FakeMutableFileNode:
         new_data = new_contents.read(new_contents.get_size())
         new_data = "".join(new_data)
         self.all_contents[self.storage_index] = new_data
-        self.my_uri.set_extension_params([self._k, self._segsize])
         return defer.succeed(None)
     def modify(self, modifier):
         # this does not implement FileTooLargeError, but the real one does
@@ -368,7 +366,6 @@ class FakeMutableFileNode:
         old_contents = self.all_contents[self.storage_index]
         new_data = modifier(old_contents, None, True)
         self.all_contents[self.storage_index] = new_data
-        self.my_uri.set_extension_params([self._k, self._segsize])
         return None
 
     # As actually implemented, MutableFilenode and MutableFileVersion
diff --git a/src/allmydata/test/test_cli.py b/src/allmydata/test/test_cli.py
index 5d6b728..fb6f143 100644
--- a/src/allmydata/test/test_cli.py
+++ b/src/allmydata/test/test_cli.py
@@ -1229,7 +1229,7 @@ class Put(GridTestMixin, CLITestMixin, unittest.TestCase):
         d = self.do_cli("put", "--mutable", "--mutable-type=mdmf", fn1)
         def _got_cap((rc, out, err)):
             self.failUnlessEqual(rc, 0)
-            self.cap = out
+            self.cap = out.strip()
         d.addCallback(_got_cap)
         # Now try to write something to the cap using put.
         data2 = "data2" * 100000
@@ -1248,13 +1248,11 @@ class Put(GridTestMixin, CLITestMixin, unittest.TestCase):
             self.failUnlessEqual(rc, 0)
             self.failUnlessEqual(out, data2)
         d.addCallback(_got_data)
-        # Now strip the extension information off of the cap and try
-        # to put something to it.
-        def _make_bare_cap(ignored):
-            cap = self.cap.split(":")
-            cap = ":".join(cap[:len(cap) - 2])
-            self.cap = cap
-        d.addCallback(_make_bare_cap)
+        # add some extension information to the cap and try to put something
+        # to it.
+        def _make_extended_cap(ignored):
+            self.cap = self.cap + ":Extension-Stuff"
+        d.addCallback(_make_extended_cap)
         data3 = "data3" * 100000
         fn3 = os.path.join(self.basedir, "data3")
         fileutil.write(fn3, data3)
@@ -1277,7 +1275,7 @@ class Put(GridTestMixin, CLITestMixin, unittest.TestCase):
         d = self.do_cli("put", "--mutable", "--mutable-type=sdmf", fn1)
         def _got_cap((rc, out, err)):
             self.failUnlessEqual(rc, 0)
-            self.cap = out
+            self.cap = out.strip()
         d.addCallback(_got_cap)
         # Now try to write something to the cap using put.
         data2 = "data2" * 100000
diff --git a/src/allmydata/test/test_dirnode.py b/src/allmydata/test/test_dirnode.py
index 9f9a5ad..145419b 100644
--- a/src/allmydata/test/test_dirnode.py
+++ b/src/allmydata/test/test_dirnode.py
@@ -41,11 +41,11 @@ class MemAccum:
 setup_py_uri = "URI:CHK:n7r3m6wmomelk4sep3kw5cvduq:os7ijw5c3maek7pg65e5254k2fzjflavtpejjyhshpsxuqzhcwwq:3:20:14861"
 one_uri = "URI:LIT:n5xgk" # LIT for "one"
 mut_write_uri = "URI:SSK:vfvcbdfbszyrsaxchgevhmmlii:euw4iw7bbnkrrwpzuburbhppuxhc3gwxv26f6imekhz7zyw2ojnq"
-mdmf_write_uri = "URI:MDMF:x533rhbm6kiehzl5kj3s44n5ie:4gif5rhneyd763ouo5qjrgnsoa3bg43xycy4robj2rf3tvmhdl3a:1:131072"
+mdmf_write_uri = "URI:MDMF:x533rhbm6kiehzl5kj3s44n5ie:4gif5rhneyd763ouo5qjrgnsoa3bg43xycy4robj2rf3tvmhdl3a"
 empty_litdir_uri = "URI:DIR2-LIT:"
 tiny_litdir_uri = "URI:DIR2-LIT:gqytunj2onug64tufqzdcosvkjetutcjkq5gw4tvm5vwszdgnz5hgyzufqydulbshj5x2lbm" # contains one child which is itself also LIT
 mut_read_uri = "URI:SSK-RO:jf6wkflosyvntwxqcdo7a54jvm:euw4iw7bbnkrrwpzuburbhppuxhc3gwxv26f6imekhz7zyw2ojnq"
-mdmf_read_uri = "URI:MDMF-RO:d4cydxselputycfzkw6qgz4zv4:4gif5rhneyd763ouo5qjrgnsoa3bg43xycy4robj2rf3tvmhdl3a:1:131072"
+mdmf_read_uri = "URI:MDMF-RO:d4cydxselputycfzkw6qgz4zv4:4gif5rhneyd763ouo5qjrgnsoa3bg43xycy4robj2rf3tvmhdl3a"
 future_write_uri = "x-tahoe-crazy://I_am_from_the_future."
 future_read_uri = "x-tahoe-crazy-readonly://I_am_from_the_future."
 future_nonascii_write_uri = u"x-tahoe-even-more-crazy://I_am_from_the_future_rw_\u263A".encode('utf-8')
diff --git a/src/allmydata/test/test_mutable.py b/src/allmydata/test/test_mutable.py
index f88cb90..d42a9ab 100644
--- a/src/allmydata/test/test_mutable.py
+++ b/src/allmydata/test/test_mutable.py
@@ -375,28 +375,6 @@ class Filenode(unittest.TestCase, testutil.ShouldFailMixin):
         return d
 
 
-    def test_create_from_mdmf_writecap_with_extensions(self):
-        # Test that the nodemaker is capable of creating an MDMF
-        # filenode when given a writecap with extension parameters in
-        # them.
-        d = self.nodemaker.create_mutable_file(version=MDMF_VERSION)
-        def _created(n):
-            self.failUnless(isinstance(n, MutableFileNode))
-            s = n.get_uri()
-            # We need to cheat a little and delete the nodemaker's
-            # cache, otherwise we'll get the same node instance back.
-            self.failUnlessIn(":3:131073", s)
-            n2 = self.nodemaker.create_from_cap(s)
-
-            self.failUnlessEqual(n2.get_storage_index(), n.get_storage_index())
-            self.failUnlessEqual(n.get_writekey(), n2.get_writekey())
-            hints = n2._downloader_hints
-            self.failUnlessEqual(hints['k'], 3)
-            self.failUnlessEqual(hints['segsize'], 131073)
-        d.addCallback(_created)
-        return d
-
-
     def test_create_from_mdmf_readcap(self):
         d = self.nodemaker.create_mutable_file(version=MDMF_VERSION)
         def _created(n):
@@ -411,26 +389,6 @@ class Filenode(unittest.TestCase, testutil.ShouldFailMixin):
         return d
 
 
-    def test_create_from_mdmf_readcap_with_extensions(self):
-        # We should be able to create an MDMF filenode with the
-        # extension parameters without it breaking.
-        d = self.nodemaker.create_mutable_file(version=MDMF_VERSION)
-        def _created(n):
-            self.failUnless(isinstance(n, MutableFileNode))
-            s = n.get_readonly_uri()
-            self.failUnlessIn(":3:131073", s)
-
-            n2 = self.nodemaker.create_from_cap(s)
-            self.failUnless(isinstance(n2, MutableFileNode))
-            self.failUnless(n2.is_readonly())
-            self.failUnlessEqual(n.get_storage_index(), n2.get_storage_index())
-            hints = n2._downloader_hints
-            self.failUnlessEqual(hints["k"], 3)
-            self.failUnlessEqual(hints["segsize"], 131073)
-        d.addCallback(_created)
-        return d
-
-
     def test_internal_version_from_cap(self):
         # MutableFileNodes and MutableFileVersions have an internal
         # switch that tells them whether they're dealing with an SDMF or
@@ -606,6 +564,9 @@ class Filenode(unittest.TestCase, testutil.ShouldFailMixin):
         d = self.nodemaker.create_mutable_file(version=MDMF_VERSION)
         def _created(node):
             self.uri = node.get_uri()
+            # also confirm that the cap has no extension fields
+            pieces = self.uri.split(":")
+            self.failUnlessEqual(len(pieces), 4)
 
             return node.overwrite(MutableData("contents1" * 100000))
         def _then(ignored):
@@ -619,37 +580,6 @@ class Filenode(unittest.TestCase, testutil.ShouldFailMixin):
         return d
 
 
-    def test_create_and_download_from_bare_mdmf_cap(self):
-        # MDMF caps have extension parameters on them by default. We
-        # need to make sure that they work without extension parameters.
-        contents = MutableData("contents" * 100000)
-        d = self.nodemaker.create_mutable_file(version=MDMF_VERSION,
-                                               contents=contents)
-        def _created(node):
-            uri = node.get_uri()
-            self._created = node
-            self.failUnlessIn(":3:131073", uri)
-            # Now strip that off the end of the uri, then try creating
-            # and downloading the node again.
-            bare_uri = uri.replace(":3:131073", "")
-            assert ":3:131073" not in bare_uri
-
-            return self.nodemaker.create_from_cap(bare_uri)
-        d.addCallback(_created)
-        def _created_bare(node):
-            self.failUnlessEqual(node.get_writekey(),
-                                 self._created.get_writekey())
-            self.failUnlessEqual(node.get_readkey(),
-                                 self._created.get_readkey())
-            self.failUnlessEqual(node.get_storage_index(),
-                                 self._created.get_storage_index())
-            return node.download_best_version()
-        d.addCallback(_created_bare)
-        d.addCallback(lambda data:
-            self.failUnlessEqual(data, "contents" * 100000))
-        return d
-
-
     def test_mdmf_write_count(self):
         # Publishing an MDMF file should only cause one write for each
         # share that is to be published. Otherwise, we introduce
@@ -3068,62 +2998,6 @@ class Version(GridTestMixin, unittest.TestCase, testutil.ShouldFailMixin, \
         return d
 
 
-    def test_version_extension_api(self):
-        # We need to define an API by which an uploader can set the
-        # extension parameters, and by which a downloader can retrieve
-        # extensions.
-        d = self.do_upload_mdmf()
-        d.addCallback(lambda ign: self.mdmf_node.get_best_mutable_version())
-        def _got_version(version):
-            hints = version.get_downloader_hints()
-            # Should be empty at this point.
-            self.failUnlessIn("k", hints)
-            self.failUnlessEqual(hints['k'], 3)
-            self.failUnlessIn('segsize', hints)
-            self.failUnlessEqual(hints['segsize'], 131073)
-        d.addCallback(_got_version)
-        return d
-
-
-    def test_extensions_from_cap(self):
-        # If we initialize a mutable file with a cap that has extension
-        # parameters in it and then grab the extension parameters using
-        # our API, we should see that they're set correctly.
-        d = self.do_upload_mdmf()
-        def _then(ign):
-            mdmf_uri = self.mdmf_node.get_uri()
-            new_node = self.nm.create_from_cap(mdmf_uri)
-            return new_node.get_best_mutable_version()
-        d.addCallback(_then)
-        def _got_version(version):
-            hints = version.get_downloader_hints()
-            self.failUnlessIn("k", hints)
-            self.failUnlessEqual(hints["k"], 3)
-            self.failUnlessIn("segsize", hints)
-            self.failUnlessEqual(hints["segsize"], 131073)
-        d.addCallback(_got_version)
-        return d
-
-
-    def test_extensions_from_upload(self):
-        # If we create a new mutable file with some contents, we should
-        # get back an MDMF cap with the right hints in place.
-        contents = "foo bar baz" * 100000
-        d = self.nm.create_mutable_file(contents, version=MDMF_VERSION)
-        def _got_mutable_file(n):
-            rw_uri = n.get_uri()
-            expected_k = str(self.c.DEFAULT_ENCODING_PARAMETERS['k'])
-            self.failUnlessIn(expected_k, rw_uri)
-            # XXX: Get this more intelligently.
-            self.failUnlessIn("131073", rw_uri)
-
-            ro_uri = n.get_readonly_uri()
-            self.failUnlessIn(expected_k, ro_uri)
-            self.failUnlessIn("131073", ro_uri)
-        d.addCallback(_got_mutable_file)
-        return d
-
-
     def test_cap_after_upload(self):
         # If we create a new mutable file and upload things to it, and
         # it's an MDMF file, we should get an MDMF cap back from that
diff --git a/src/allmydata/test/test_uri.py b/src/allmydata/test/test_uri.py
index 7662c7f..c6fb5bc 100644
--- a/src/allmydata/test/test_uri.py
+++ b/src/allmydata/test/test_uri.py
@@ -1,5 +1,5 @@
 
-import re
+import os, re
 from twisted.trial import unittest
 from allmydata import uri
 from allmydata.util import hashutil, base32
@@ -430,84 +430,40 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         u4 = u2.get_verify_cap()
         self.failUnlessReallyEqual(u4, u2)
 
-    def test_mdmf_cap_extra_information(self):
-        # MDMF caps can be arbitrarily extended after the fingerprint
-        # and key/storage index fields. 
+    def test_mdmf_cap_ignore_extensions(self):
+        # MDMF caps can be arbitrarily extended after the fingerprint and
+        # key/storage index fields. tahoe-1.9 is supposed to ignore any
+        # extensions, and not add any itself.
         u1 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint)
-        self.failUnlessEqual([], u1.get_extension_params())
-
-        cap = u1.to_string()
-        # Now let's append some fields. Say, 131073 (the segment size)
-        # and 3 (the "k" encoding parameter).
-        expected_extensions = []
-        for e in ('131073', '3'):
-            cap += (":%s" % e)
-            expected_extensions.append(e)
-
-            u2 = uri.WriteableMDMFFileURI.init_from_string(cap)
-            self.failUnlessReallyEqual(self.writekey, u2.writekey)
-            self.failUnlessReallyEqual(self.fingerprint, u2.fingerprint)
-            self.failIf(u2.is_readonly())
-            self.failUnless(u2.is_mutable())
-
-            c2 = u2.to_string()
-            u2n = uri.WriteableMDMFFileURI.init_from_string(c2)
-            self.failUnlessReallyEqual(u2, u2n)
-
-            # We should get the extra back when we ask for it.
-            self.failUnlessEqual(expected_extensions, u2.get_extension_params())
-
-            # These should be preserved through cap attenuation, too.
-            u3 = u2.get_readonly()
-            self.failUnlessReallyEqual(self.readkey, u3.readkey)
-            self.failUnlessReallyEqual(self.fingerprint, u3.fingerprint)
-            self.failUnless(u3.is_readonly())
-            self.failUnless(u3.is_mutable())
-            self.failUnlessEqual(expected_extensions, u3.get_extension_params())
-
-            c3 = u3.to_string()
-            u3n = uri.ReadonlyMDMFFileURI.init_from_string(c3)
-            self.failUnlessReallyEqual(u3, u3n)
-
-            u4 = u3.get_verify_cap()
-            self.failUnlessReallyEqual(self.storage_index, u4.storage_index)
-            self.failUnlessReallyEqual(self.fingerprint, u4.fingerprint)
-            self.failUnless(u4.is_readonly())
-            self.failIf(u4.is_mutable())
-
-            c4 = u4.to_string()
-            u4n = uri.MDMFVerifierURI.init_from_string(c4)
-            self.failUnlessReallyEqual(u4n, u4)
-
-            self.failUnlessEqual(expected_extensions, u4.get_extension_params())
-
-
-    def test_sdmf_cap_extra_information(self):
-        # For interface consistency, we define a method to get
-        # extensions for SDMF files as well. This method must always
-        # return no extensions, since SDMF files were not created with
-        # extensions and cannot be modified to include extensions
-        # without breaking older clients.
-        u1 = uri.WriteableSSKFileURI(self.writekey, self.fingerprint)
         cap = u1.to_string()
-        u2 = uri.WriteableSSKFileURI.init_from_string(cap)
-        self.failUnlessEqual([], u2.get_extension_params())
-
-    def test_extension_character_range(self):
-        # As written now, we shouldn't put things other than numbers in
-        # the extension fields.
-        writecap = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint).to_string()
-        readcap  = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint).to_string()
-        vcap     = uri.MDMFVerifierURI(self.storage_index, self.fingerprint).to_string()
-        self.failUnlessRaises(uri.BadURIError,
-                              uri.WriteableMDMFFileURI.init_from_string,
-                              ("%s:invalid" % writecap))
-        self.failUnlessRaises(uri.BadURIError,
-                              uri.ReadonlyMDMFFileURI.init_from_string,
-                              ("%s:invalid" % readcap))
-        self.failUnlessRaises(uri.BadURIError,
-                              uri.MDMFVerifierURI.init_from_string,
-                              ("%s:invalid" % vcap))
+
+        cap2 = cap+":I COME FROM THE FUTURE"
+        u2 = uri.WriteableMDMFFileURI.init_from_string(cap2)
+        self.failUnlessReallyEqual(self.writekey, u2.writekey)
+        self.failUnlessReallyEqual(self.fingerprint, u2.fingerprint)
+        self.failIf(u2.is_readonly())
+        self.failUnless(u2.is_mutable())
+
+        cap3 = cap+":"+os.urandom(40) # parse *that*!
+        u3 = uri.WriteableMDMFFileURI.init_from_string(cap3)
+        self.failUnlessReallyEqual(self.writekey, u3.writekey)
+        self.failUnlessReallyEqual(self.fingerprint, u3.fingerprint)
+        self.failIf(u3.is_readonly())
+        self.failUnless(u3.is_mutable())
+
+        cap4 = u1.get_readonly().to_string()+":ooh scary future stuff"
+        u4 = uri.from_string_mutable_filenode(cap4)
+        self.failUnlessReallyEqual(self.readkey, u4.readkey)
+        self.failUnlessReallyEqual(self.fingerprint, u4.fingerprint)
+        self.failUnless(u4.is_readonly())
+        self.failUnless(u4.is_mutable())
+
+        cap5 = u1.get_verify_cap().to_string()+":spoilers!"
+        u5 = uri.from_string(cap5)
+        self.failUnlessReallyEqual(self.storage_index, u5.storage_index)
+        self.failUnlessReallyEqual(self.fingerprint, u5.fingerprint)
+        self.failUnless(u5.is_readonly())
+        self.failIf(u5.is_mutable())
 
 
     def test_mdmf_valid_human_encoding(self):
@@ -517,22 +473,16 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         # test that a valid cap (with and without the traditional
         # separators) is recognized and accepted by the classes.
         w1 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint)
-        w2 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint,
-                                     ['131073', '3'])
         r1 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint)
-        r2 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint,
-                                     ['131073', '3'])
         v1 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint)
-        v2 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint,
-                                 ['131073', '3'])
 
-        # These will yield six different caps.
-        for o in (w1, w2, r1 , r2, v1, v2):
+        # These will yield three different caps.
+        for o in (w1, r1, v1):
             url = base + o.to_string()
             o1 = o.__class__.init_from_human_encoding(url)
             self.failUnlessReallyEqual(o1, o)
 
-            # Note that our cap will, by default, have : as separators. 
+            # Note that our cap will, by default, have : as separators.
             # But it's expected that users from, e.g., the WUI, will
             # have %3A as a separator. We need to make sure that the
             # initialization routine handles that, too.
@@ -550,17 +500,11 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         # test that a valid cap (with and without the traditional
         # separators) is recognized and accepted by the classes.
         w1 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint)
-        w2 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint,
-                                     ['131073', '3'])
         r1 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint)
-        r2 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint,
-                                     ['131073', '3'])
         v1 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint)
-        v2 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint,
-                                 ['131073', '3'])
 
-        # These will yield six different caps.
-        for o in (w1, w2, r1 , r2, v1, v2):
+        # These will yield three different caps.
+        for o in (w1, r1, v1):
             url = base + o.to_string()
             self.failUnlessRaises(uri.BadURIError,
                                   o.__class__.init_from_human_encoding,
@@ -572,17 +516,11 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         # test that a valid cap (with and without the traditional
         # separators) is recognized and accepted by the classes.
         w1 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint)
-        w2 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint,
-                                     ['131073', '3'])
         r1 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint)
-        r2 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint,
-                                     ['131073', '3'])
         v1 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint)
-        v2 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint,
-                                 ['131073', '3'])
 
-        # These will yield six different caps.
-        for o in (w1, w2, r1 , r2, v1, v2):
+        # These will yield three different caps.
+        for o in (w1, r1, v1):
             # not exhaustive, obviously...
             url = base + o.to_string() + "foobarbaz"
             url2 = base + "foobarbaz" + o.to_string()
@@ -603,16 +541,6 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         u3 = uri.from_string_mutable_filenode(cap)
         self.failUnlessEqual(u3, u1)
 
-        # XXX: We should refactor the extension field into setUp
-        u1 = uri.WriteableMDMFFileURI(self.writekey, self.fingerprint,
-                                     ['131073', '3'])
-        cap = u1.to_string()
-        self.failUnless(uri.is_uri(cap))
-        u2 = uri.from_string(cap)
-        self.failUnlessReallyEqual(u1, u2)
-        u3 = uri.from_string_mutable_filenode(cap)
-        self.failUnlessEqual(u3, u1)
-
         u1 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint)
         cap = u1.to_string()
         self.failUnless(uri.is_uri(cap))
@@ -621,15 +549,6 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         u3 = uri.from_string_mutable_filenode(cap)
         self.failUnlessEqual(u3, u1)
 
-        u1 = uri.ReadonlyMDMFFileURI(self.readkey, self.fingerprint,
-                                     ['131073', '3'])
-        cap = u1.to_string()
-        self.failUnless(uri.is_uri(cap))
-        u2 = uri.from_string(cap)
-        self.failUnlessReallyEqual(u1, u2)
-        u3 = uri.from_string_mutable_filenode(cap)
-        self.failUnlessEqual(u3, u1)
-
         u1 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint)
         cap = u1.to_string()
         self.failUnless(uri.is_uri(cap))
@@ -638,15 +557,6 @@ class Mutable(testutil.ReallyEqualMixin, unittest.TestCase):
         u3 = uri.from_string_verifier(cap)
         self.failUnlessEqual(u3, u1)
 
-        u1 = uri.MDMFVerifierURI(self.storage_index, self.fingerprint,
-                                 ['131073', '3'])
-        cap = u1.to_string()
-        self.failUnless(uri.is_uri(cap))
-        u2 = uri.from_string(cap)
-        self.failUnlessReallyEqual(u1, u2)
-        u3 = uri.from_string_verifier(cap)
-        self.failUnlessEqual(u3, u1)
-
 
 class Dirnode(testutil.ReallyEqualMixin, unittest.TestCase):
     def test_pack(self):
@@ -816,35 +726,6 @@ class Dirnode(testutil.ReallyEqualMixin, unittest.TestCase):
         d3 = uri.from_string(d2.to_string(), deep_immutable=True)
         self.failUnlessIsInstance(d3, uri.UnknownURI)
 
-    def test_mdmf_with_extensions(self):
-        writekey = "\x01" * 16
-        fingerprint = "\x02" * 32
-        uri1 = uri.WriteableMDMFFileURI(writekey, fingerprint)
-        d1 = uri.MDMFDirectoryURI(uri1)
-        d1_uri = d1.to_string()
-        # Add some extensions, verify that the URI is interpreted
-        # correctly.
-        d1_uri += ":3:131073"
-        uri2 = uri.from_string(d1_uri)
-        self.failUnlessIsInstance(uri2, uri.MDMFDirectoryURI)
-        self.failUnless(IURI.providedBy(uri2))
-        self.failUnless(IDirnodeURI.providedBy(uri2))
-        self.failUnless(uri1.is_mutable())
-        self.failIf(uri1.is_readonly())
-
-        d2_uri = uri2.to_string()
-        self.failUnlessIn(":3:131073", d2_uri)
-
-        # Now attenuate, verify that the extensions persist
-        ro_uri = uri2.get_readonly()
-        self.failUnlessIsInstance(ro_uri, uri.ReadonlyMDMFDirectoryURI)
-        self.failUnless(ro_uri.is_mutable())
-        self.failUnless(ro_uri.is_readonly())
-        self.failUnless(IURI.providedBy(ro_uri))
-        self.failUnless(IDirnodeURI.providedBy(ro_uri))
-        ro_uri_str = ro_uri.to_string()
-        self.failUnlessIn(":3:131073", ro_uri_str)
-
     def test_mdmf_attenuation(self):
         writekey = "\x01" * 16
         fingerprint = "\x02" * 32
diff --git a/src/allmydata/test/test_web.py b/src/allmydata/test/test_web.py
index 370f09a..adf4f71 100644
--- a/src/allmydata/test/test_web.py
+++ b/src/allmydata/test/test_web.py
@@ -892,22 +892,7 @@ class Web(WebMixin, WebErrorMixin, testutil.StallMixin, testutil.ReallyEqualMixi
         return d
 
     def test_GET_FILE_URI_mdmf_extensions(self):
-        base = "/uri/%s" % urllib.quote("%s:3:131073" % self._quux_txt_uri)
-        d = self.GET(base)
-        d.addCallback(self.failUnlessIsQuuxDotTxt)
-        return d
-
-    def test_GET_FILE_URI_mdmf_bare_cap(self):
-        cap_elements = self._quux_txt_uri.split(":")
-        # 6 == expected cap length with two extensions.
-        self.failUnlessEqual(len(cap_elements), 6)
-
-        # Now lop off the extension parameters and stitch everything
-        # back together
-        quux_uri = ":".join(cap_elements[:len(cap_elements) - 2])
-
-        # Now GET that. We should get back quux.
-        base = "/uri/%s" % urllib.quote(quux_uri)
+        base = "/uri/%s" % urllib.quote("%s:RANDOMSTUFF" % self._quux_txt_uri)
         d = self.GET(base)
         d.addCallback(self.failUnlessIsQuuxDotTxt)
         return d
@@ -949,7 +934,7 @@ class Web(WebMixin, WebErrorMixin, testutil.StallMixin, testutil.ReallyEqualMixi
         return d
 
     def test_PUT_FILE_URI_mdmf_extensions(self):
-        base = "/uri/%s" % urllib.quote("%s:3:131073" % self._quux_txt_uri)
+        base = "/uri/%s" % urllib.quote("%s:EXTENSIONSTUFF" % self._quux_txt_uri)
         self._quux_new_contents = "new_contents"
         d = self.GET(base)
         d.addCallback(lambda res: self.failUnlessIsQuuxDotTxt(res))
@@ -959,22 +944,6 @@ class Web(WebMixin, WebErrorMixin, testutil.StallMixin, testutil.ReallyEqualMixi
                                                        res))
         return d
 
-    def test_PUT_FILE_URI_mdmf_bare_cap(self):
-        elements = self._quux_txt_uri.split(":")
-        self.failUnlessEqual(len(elements), 6)
-
-        quux_uri = ":".join(elements[:len(elements) - 2])
-        base = "/uri/%s" % urllib.quote(quux_uri)
-        self._quux_new_contents = "new_contents" * 50000
-
-        d = self.GET(base)
-        d.addCallback(self.failUnlessIsQuuxDotTxt)
-        d.addCallback(lambda ignored: self.PUT(base, self._quux_new_contents))
-        d.addCallback(lambda ignored: self.GET(base))
-        d.addCallback(lambda res:
-            self.failUnlessEqual(res, self._quux_new_contents))
-        return d
-
     def test_PUT_FILE_URI_mdmf_readonly(self):
         # We're not allowed to PUT things to a readonly cap.
         base = "/uri/%s" % self._quux_txt_readonly_uri
@@ -1043,7 +1012,7 @@ class Web(WebMixin, WebErrorMixin, testutil.StallMixin, testutil.ReallyEqualMixi
         return d
 
     def test_GET_FILEURL_info_mdmf_extensions(self):
-        d = self.GET("/uri/%s:3:131073?t=info" % self._quux_txt_uri)
+        d = self.GET("/uri/%s:STUFF?t=info" % self._quux_txt_uri)
         def _got(res):
             self.failUnlessIn("mutable file (mdmf)", res)
             self.failUnlessIn(self._quux_txt_uri, res)
@@ -1051,19 +1020,6 @@ class Web(WebMixin, WebErrorMixin, testutil.StallMixin, testutil.ReallyEqualMixi
         d.addCallback(_got)
         return d
 
-    def test_GET_FILEURL_info_mdmf_bare_cap(self):
-        elements = self._quux_txt_uri.split(":")
-        self.failUnlessEqual(len(elements), 6)
-
-        quux_uri = ":".join(elements[:len(elements) - 2])
-        base = "/uri/%s?t=info" % urllib.quote(quux_uri)
-        d = self.GET(base)
-        def _got(res):
-            self.failUnlessIn("mutable file (mdmf)", res)
-            self.failUnlessIn(quux_uri, res)
-        d.addCallback(_got)
-        return d
-
     def test_PUT_overwrite_only_files(self):
         # create a directory, put a file in that directory.
         contents, n, filecap = self.makefile(8)
@@ -1288,51 +1244,6 @@ class Web(WebMixin, WebErrorMixin, testutil.StallMixin, testutil.ReallyEqualMixi
         d.addCallback(_got_json, "sdmf")
         return d
 
-    def test_GET_FILEURL_json_mdmf_extensions(self):
-        # A GET invoked against a URL that includes an MDMF cap with
-        # extensions should fetch the same JSON information as a GET
-        # invoked against a bare cap.
-        self._quux_txt_uri = "%s:3:131073" % self._quux_txt_uri
-        self._quux_txt_readonly_uri = "%s:3:131073" % self._quux_txt_readonly_uri
-        d = self.GET("/uri/%s?t=json" % urllib.quote(self._quux_txt_uri))
-        d.addCallback(self.failUnlessIsQuuxJSON)
-        return d
-
-    def test_GET_FILEURL_json_mdmf_bare_cap(self):
-        elements = self._quux_txt_uri.split(":")
-        self.failUnlessEqual(len(elements), 6)
-
-        quux_uri = ":".join(elements[:len(elements) - 2])
-        # so failUnlessIsQuuxJSON will work.
-        self._quux_txt_uri = quux_uri
-
-        # we need to alter the readonly URI in the same way, again so
-        # failUnlessIsQuuxJSON will work
-        elements = self._quux_txt_readonly_uri.split(":")
-        self.failUnlessEqual(len(elements), 6)
-        quux_ro_uri = ":".join(elements[:len(elements) - 2])
-        self._quux_txt_readonly_uri = quux_ro_uri
-
-        base = "/uri/%s?t=json" % urllib.quote(quux_uri)
-        d = self.GET(base)
-        d.addCallback(self.failUnlessIsQuuxJSON)
-        return d
-
-    def test_GET_FILEURL_json_mdmf_bare_readonly_cap(self):
-        elements = self._quux_txt_readonly_uri.split(":")
-        self.failUnlessEqual(len(elements), 6)
-
-        quux_readonly_uri = ":".join(elements[:len(elements) - 2])
-        # so failUnlessIsQuuxJSON will work
-        self._quux_txt_readonly_uri = quux_readonly_uri
-        base = "/uri/%s?t=json" % quux_readonly_uri
-        d = self.GET(base)
-        # XXX: We may need to make a method that knows how to check for
-        # readonly JSON, or else alter that one so that it knows how to
-        # do that.
-        d.addCallback(self.failUnlessIsQuuxJSON, readonly=True)
-        return d
-
     def test_GET_FILEURL_json_mdmf(self):
         d = self.GET("/uri/%s?t=json" % urllib.quote(self._quux_txt_uri))
         d.addCallback(self.failUnlessIsQuuxJSON)
diff --git a/src/allmydata/uri.py b/src/allmydata/uri.py
index 62bf9b9..c500c7b 100644
--- a/src/allmydata/uri.py
+++ b/src/allmydata/uri.py
@@ -28,7 +28,6 @@ BASE32STR_256bits = '(%s{51}%s)' % (base32.BASE32CHAR, base32.BASE32CHAR_1bits)
 SEP='(?::|%3A)'
 NUMBER='([0-9]+)'
 NUMBER_IGNORE='(?:[0-9]+)'
-OPTIONAL_EXTENSION_FIELD = '(' + SEP + '[0-9' + SEP + ']+|)'
 
 # "human-encoded" URIs are allowed to come with a leading
 # 'http://127.0.0.1:(8123|3456)/uri/' that will be ignored.
@@ -294,12 +293,6 @@ class WriteableSSKFileURI(_BaseURI):
     def get_verify_cap(self):
         return SSKVerifierURI(self.storage_index, self.fingerprint)
 
-    def get_extension_params(self):
-        return []
-
-    def set_extension_params(self, params):
-        pass
-
 class ReadonlySSKFileURI(_BaseURI):
     implements(IURI, IMutableFileURI)
 
@@ -354,12 +347,6 @@ class ReadonlySSKFileURI(_BaseURI):
     def get_verify_cap(self):
         return SSKVerifierURI(self.storage_index, self.fingerprint)
 
-    def get_extension_params(self):
-        return []
-
-    def set_extension_params(self, params):
-        pass
-
 class SSKVerifierURI(_BaseURI):
     implements(IVerifierURI)
 
@@ -404,53 +391,39 @@ class SSKVerifierURI(_BaseURI):
     def get_verify_cap(self):
         return self
 
-    def get_extension_params(self):
-        return []
-
-    def set_extension_params(self, params):
-        pass
-
 class WriteableMDMFFileURI(_BaseURI):
     implements(IURI, IMutableFileURI)
 
     BASE_STRING='URI:MDMF:'
-    STRING_RE=re.compile('^'+BASE_STRING+BASE32STR_128bits+':'+BASE32STR_256bits+OPTIONAL_EXTENSION_FIELD+'$')
-    HUMAN_RE=re.compile('^'+OPTIONALHTTPLEAD+'URI'+SEP+'MDMF'+SEP+BASE32STR_128bits+SEP+BASE32STR_256bits+OPTIONAL_EXTENSION_FIELD+'$')
+    STRING_RE=re.compile('^'+BASE_STRING+BASE32STR_128bits+':'+BASE32STR_256bits+'(:|$)')
+    HUMAN_RE=re.compile('^'+OPTIONALHTTPLEAD+'URI'+SEP+'MDMF'+SEP+BASE32STR_128bits+SEP+BASE32STR_256bits+'(:|$)')
 
-    def __init__(self, writekey, fingerprint, params=[]):
+    def __init__(self, writekey, fingerprint):
         self.writekey = writekey
         self.readkey = hashutil.ssk_readkey_hash(writekey)
         self.storage_index = hashutil.ssk_storage_index_hash(self.readkey)
         assert len(self.storage_index) == 16
         self.fingerprint = fingerprint
-        self.extension = params
 
     @classmethod
     def init_from_human_encoding(cls, uri):
         mo = cls.HUMAN_RE.search(uri)
         if not mo:
             raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
-        params = filter(lambda x: x != '', re.split(SEP, mo.group(3)))
-        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)), params)
+        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     @classmethod
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
             raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
-        params = mo.group(3)
-        params = filter(lambda x: x != '', params.split(":"))
-        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)), params)
+        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
         assert isinstance(self.writekey, str)
         assert isinstance(self.fingerprint, str)
         ret = 'URI:MDMF:%s:%s' % (base32.b2a(self.writekey),
                                   base32.b2a(self.fingerprint))
-        if self.extension:
-            ret += ":"
-            ret += ":".join(self.extension)
-
         return ret
 
     def __repr__(self):
@@ -469,40 +442,30 @@ class WriteableMDMFFileURI(_BaseURI):
         return True
 
     def get_readonly(self):
-        return ReadonlyMDMFFileURI(self.readkey, self.fingerprint, self.extension)
+        return ReadonlyMDMFFileURI(self.readkey, self.fingerprint)
 
     def get_verify_cap(self):
-        return MDMFVerifierURI(self.storage_index, self.fingerprint, self.extension)
-
-    def get_extension_params(self):
-        return self.extension
-
-    def set_extension_params(self, params):
-        params = map(str, params)
-        self.extension = params
+        return MDMFVerifierURI(self.storage_index, self.fingerprint)
 
 class ReadonlyMDMFFileURI(_BaseURI):
     implements(IURI, IMutableFileURI)
 
     BASE_STRING='URI:MDMF-RO:'
-    STRING_RE=re.compile('^' +BASE_STRING+BASE32STR_128bits+':'+BASE32STR_256bits+OPTIONAL_EXTENSION_FIELD+'$')
-    HUMAN_RE=re.compile('^'+OPTIONALHTTPLEAD+'URI'+SEP+'MDMF-RO'+SEP+BASE32STR_128bits+SEP+BASE32STR_256bits+OPTIONAL_EXTENSION_FIELD+'$')
+    STRING_RE=re.compile('^' +BASE_STRING+BASE32STR_128bits+':'+BASE32STR_256bits+'(:|$)')
+    HUMAN_RE=re.compile('^'+OPTIONALHTTPLEAD+'URI'+SEP+'MDMF-RO'+SEP+BASE32STR_128bits+SEP+BASE32STR_256bits+'(:|$)')
 
-    def __init__(self, readkey, fingerprint, params=[]):
+    def __init__(self, readkey, fingerprint):
         self.readkey = readkey
         self.storage_index = hashutil.ssk_storage_index_hash(self.readkey)
         assert len(self.storage_index) == 16
         self.fingerprint = fingerprint
-        self.extension = params
 
     @classmethod
     def init_from_human_encoding(cls, uri):
         mo = cls.HUMAN_RE.search(uri)
         if not mo:
             raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
-        params = mo.group(3)
-        params = filter(lambda x: x!= '', re.split(SEP, params))
-        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)), params)
+        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     @classmethod
     def init_from_string(cls, uri):
@@ -510,19 +473,13 @@ class ReadonlyMDMFFileURI(_BaseURI):
         if not mo:
             raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
 
-        params = mo.group(3)
-        params = filter(lambda x: x != '', params.split(":"))
-        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)), params)
+        return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
         assert isinstance(self.readkey, str)
         assert isinstance(self.fingerprint, str)
         ret = 'URI:MDMF-RO:%s:%s' % (base32.b2a(self.readkey),
                                      base32.b2a(self.fingerprint))
-        if self.extension:
-            ret += ":"
-            ret += ":".join(self.extension)
-
         return ret
 
     def __repr__(self):
@@ -544,55 +501,39 @@ class ReadonlyMDMFFileURI(_BaseURI):
         return self
 
     def get_verify_cap(self):
-        return MDMFVerifierURI(self.storage_index, self.fingerprint, self.extension)
-
-    def get_extension_params(self):
-        return self.extension
-
-    def set_extension_params(self, params):
-        params = map(str, params)
-        self.extension = params
+        return MDMFVerifierURI(self.storage_index, self.fingerprint)
 
 class MDMFVerifierURI(_BaseURI):
     implements(IVerifierURI)
 
     BASE_STRING='URI:MDMF-Verifier:'
-    STRING_RE=re.compile('^'+BASE_STRING+BASE32STR_128bits+':'+BASE32STR_256bits+OPTIONAL_EXTENSION_FIELD+'$')
-    HUMAN_RE=re.compile('^'+OPTIONALHTTPLEAD+'URI'+SEP+'MDMF-Verifier'+SEP+BASE32STR_128bits+SEP+BASE32STR_256bits+OPTIONAL_EXTENSION_FIELD+'$')
+    STRING_RE=re.compile('^'+BASE_STRING+BASE32STR_128bits+':'+BASE32STR_256bits+'(:|$)')
+    HUMAN_RE=re.compile('^'+OPTIONALHTTPLEAD+'URI'+SEP+'MDMF-Verifier'+SEP+BASE32STR_128bits+SEP+BASE32STR_256bits+'(:|$)')
 
-    def __init__(self, storage_index, fingerprint, params=[]):
+    def __init__(self, storage_index, fingerprint):
         assert len(storage_index) == 16
         self.storage_index = storage_index
         self.fingerprint = fingerprint
-        self.extension = params
 
     @classmethod
     def init_from_human_encoding(cls, uri):
         mo = cls.HUMAN_RE.search(uri)
         if not mo:
             raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
-        params = mo.group(3)
-        params = filter(lambda x: x != '', re.split(SEP, params))
-        return cls(si_a2b(mo.group(1)), base32.a2b(mo.group(2)), params)
+        return cls(si_a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     @classmethod
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
             raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
-        params = mo.group(3)
-        params = filter(lambda x: x != '', params.split(":"))
-        return cls(si_a2b(mo.group(1)), base32.a2b(mo.group(2)), params)
+        return cls(si_a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
         assert isinstance(self.storage_index, str)
         assert isinstance(self.fingerprint, str)
         ret = 'URI:MDMF-Verifier:%s:%s' % (si_b2a(self.storage_index),
                                            base32.b2a(self.fingerprint))
-        if self.extension:
-            ret += ':'
-            ret += ":".join(self.extension)
-
         return ret
 
     def is_readonly(self):
@@ -607,9 +548,6 @@ class MDMFVerifierURI(_BaseURI):
     def get_verify_cap(self):
         return self
 
-    def get_extension_params(self):
-        return self.extension
-
 class _DirectoryBaseURI(_BaseURI):
     implements(IURI, IDirnodeURI)
     def __init__(self, filenode_uri=None):