diff --git a/changelog.d/4514.misc b/changelog.d/4514.misc
new file mode 100644
index 000000000..43f896361
--- /dev/null
+++ b/changelog.d/4514.misc
@@ -0,0 +1 @@
+Add infrastructure to support different event formats
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index df30c2cea..8f9e330da 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -22,6 +22,7 @@ from unpaddedbase64 import decode_base64
 
 from synapse.api.constants import (
     KNOWN_ROOM_VERSIONS,
+    EventFormatVersions,
     EventTypes,
     JoinRules,
     Membership,
@@ -55,7 +56,6 @@ def check(room_version, event, auth_events, do_sig_check=True, do_size_check=Tru
 
     if do_sig_check:
         sender_domain = get_domain_from_id(event.sender)
-        event_id_domain = get_domain_from_id(event.event_id)
 
         is_invite_via_3pid = (
             event.type == EventTypes.Member
@@ -72,9 +72,13 @@ def check(room_version, event, auth_events, do_sig_check=True, do_size_check=Tru
             if not is_invite_via_3pid:
                 raise AuthError(403, "Event not signed by sender's server")
 
-        # Check the event_id's domain has signed the event
-        if not event.signatures.get(event_id_domain):
-            raise AuthError(403, "Event not signed by sending server")
+        if event.format_version in (EventFormatVersions.V1,):
+            # Only older room versions have event IDs to check.
+            event_id_domain = get_domain_from_id(event.event_id)
+
+            # Check the origin domain has signed the event
+            if not event.signatures.get(event_id_domain):
+                raise AuthError(403, "Event not signed by sending server")
 
     if auth_events is None:
         # Oh, we don't know what the state of the room was, so we
diff --git a/synapse/events/validator.py b/synapse/events/validator.py
index c53bf44e5..a072674b0 100644
--- a/synapse/events/validator.py
+++ b/synapse/events/validator.py
@@ -15,7 +15,7 @@
 
 from six import string_types
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import EventFormatVersions, EventTypes, Membership
 from synapse.api.errors import SynapseError
 from synapse.types import EventID, RoomID, UserID
 
@@ -29,7 +29,8 @@ class EventValidator(object):
         """
         self.validate_builder(event)
 
-        EventID.from_string(event.event_id)
+        if event.format_version == EventFormatVersions.V1:
+            EventID.from_string(event.event_id)
 
         required = [
             "auth_events",
diff --git a/synapse/federation/federation_base.py b/synapse/federation/federation_base.py
index 5c31e5f85..a400091db 100644
--- a/synapse/federation/federation_base.py
+++ b/synapse/federation/federation_base.py
@@ -20,7 +20,7 @@ import six
 from twisted.internet import defer
 from twisted.internet.defer import DeferredList
 
-from synapse.api.constants import MAX_DEPTH, EventTypes, Membership
+from synapse.api.constants import KNOWN_ROOM_VERSIONS, MAX_DEPTH, EventTypes, Membership
 from synapse.api.errors import Codes, SynapseError
 from synapse.crypto.event_signing import check_event_content_hash
 from synapse.events import event_type_from_format_version
@@ -66,7 +66,7 @@ class FederationBase(object):
         Returns:
             Deferred : A list of PDUs that have valid signatures and hashes.
         """
-        deferreds = self._check_sigs_and_hashes(pdus)
+        deferreds = self._check_sigs_and_hashes(room_version, pdus)
 
         @defer.inlineCallbacks
         def handle_check_result(pdu, deferred):
@@ -121,16 +121,17 @@ class FederationBase(object):
         else:
             defer.returnValue([p for p in valid_pdus if p])
 
-    def _check_sigs_and_hash(self, pdu):
+    def _check_sigs_and_hash(self, room_version, pdu):
         return logcontext.make_deferred_yieldable(
-            self._check_sigs_and_hashes([pdu])[0],
+            self._check_sigs_and_hashes(room_version, [pdu])[0],
         )
 
-    def _check_sigs_and_hashes(self, pdus):
+    def _check_sigs_and_hashes(self, room_version, pdus):
         """Checks that each of the received events is correctly signed by the
         sending server.
 
         Args:
+            room_version (str): The room version of the PDUs
             pdus (list[FrozenEvent]): the events to be checked
 
         Returns:
@@ -141,7 +142,7 @@ class FederationBase(object):
               * throws a SynapseError if the signature check failed.
             The deferreds run their callbacks in the sentinel logcontext.
         """
-        deferreds = _check_sigs_on_pdus(self.keyring, pdus)
+        deferreds = _check_sigs_on_pdus(self.keyring, room_version, pdus)
 
         ctx = logcontext.LoggingContext.current_context()
 
@@ -203,16 +204,17 @@ class FederationBase(object):
 
 
 class PduToCheckSig(namedtuple("PduToCheckSig", [
-    "pdu", "redacted_pdu_json", "event_id_domain", "sender_domain", "deferreds",
+    "pdu", "redacted_pdu_json", "sender_domain", "deferreds",
 ])):
     pass
 
 
-def _check_sigs_on_pdus(keyring, pdus):
+def _check_sigs_on_pdus(keyring, room_version, pdus):
     """Check that the given events are correctly signed
 
     Args:
         keyring (synapse.crypto.Keyring): keyring object to do the checks
+        room_version (str): the room version of the PDUs
         pdus (Collection[EventBase]): the events to be checked
 
     Returns:
@@ -225,9 +227,7 @@ def _check_sigs_on_pdus(keyring, pdus):
 
     # we want to check that the event is signed by:
     #
-    # (a) the server which created the event_id
-    #
-    # (b) the sender's server.
+    # (a) the sender's server
     #
     #     - except in the case of invites created from a 3pid invite, which are exempt
     #     from this check, because the sender has to match that of the original 3pid
@@ -241,34 +241,26 @@ def _check_sigs_on_pdus(keyring, pdus):
     #     and signatures are *supposed* to be valid whether or not an event has been
     #     redacted. But this isn't the worst of the ways that 3pid invites are broken.
     #
+    # (b) for V1 and V2 rooms, the server which created the event_id
+    #
     # let's start by getting the domain for each pdu, and flattening the event back
     # to JSON.
+
     pdus_to_check = [
         PduToCheckSig(
             pdu=p,
             redacted_pdu_json=prune_event(p).get_pdu_json(),
-            event_id_domain=get_domain_from_id(p.event_id),
             sender_domain=get_domain_from_id(p.sender),
             deferreds=[],
         )
         for p in pdus
     ]
 
-    # first make sure that the event is signed by the event_id's domain
-    deferreds = keyring.verify_json_objects_for_server([
-        (p.event_id_domain, p.redacted_pdu_json)
-        for p in pdus_to_check
-    ])
-
-    for p, d in zip(pdus_to_check, deferreds):
-        p.deferreds.append(d)
-
-    # now let's look for events where the sender's domain is different to the
-    # event id's domain (normally only the case for joins/leaves), and add additional
-    # checks.
+    # First we check that the sender event is signed by the sender's domain
+    # (except if its a 3pid invite, in which case it may be sent by any server)
     pdus_to_check_sender = [
         p for p in pdus_to_check
-        if p.sender_domain != p.event_id_domain and not _is_invite_via_3pid(p.pdu)
+        if not _is_invite_via_3pid(p.pdu)
     ]
 
     more_deferreds = keyring.verify_json_objects_for_server([
@@ -279,19 +271,37 @@ def _check_sigs_on_pdus(keyring, pdus):
     for p, d in zip(pdus_to_check_sender, more_deferreds):
         p.deferreds.append(d)
 
+    # now let's look for events where the sender's domain is different to the
+    # event id's domain (normally only the case for joins/leaves), and add additional
+    # checks. Only do this if the room version has a concept of event ID domain
+    if room_version in KNOWN_ROOM_VERSIONS:
+        pdus_to_check_event_id = [
+            p for p in pdus_to_check
+            if p.sender_domain != get_domain_from_id(p.pdu.event_id)
+        ]
+
+        more_deferreds = keyring.verify_json_objects_for_server([
+            (get_domain_from_id(p.pdu.event_id), p.redacted_pdu_json)
+            for p in pdus_to_check_event_id
+        ])
+
+        for p, d in zip(pdus_to_check_event_id, more_deferreds):
+            p.deferreds.append(d)
+
     # replace lists of deferreds with single Deferreds
     return [_flatten_deferred_list(p.deferreds) for p in pdus_to_check]
 
 
 def _flatten_deferred_list(deferreds):
-    """Given a list of one or more deferreds, either return the single deferred, or
-    combine into a DeferredList.
+    """Given a list of deferreds, either return the single deferred,
+    combine into a DeferredList, or return an already resolved deferred.
     """
     if len(deferreds) > 1:
         return DeferredList(deferreds, fireOnOneErrback=True, consumeErrors=True)
-    else:
-        assert len(deferreds) == 1
+    elif len(deferreds) == 1:
         return deferreds[0]
+    else:
+        return defer.succeed(None)
 
 
 def _is_invite_via_3pid(event):
@@ -319,7 +329,7 @@ def event_from_pdu_json(pdu_json, event_format_version, outlier=False):
     """
     # we could probably enforce a bunch of other fields here (room_id, sender,
     # origin, etc etc)
-    assert_params_in_dict(pdu_json, ('event_id', 'type', 'depth'))
+    assert_params_in_dict(pdu_json, ('type', 'depth'))
 
     depth = pdu_json['depth']
     if not isinstance(depth, six.integer_types):
diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index 9b4acd2ed..4e4f58b41 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -205,7 +205,7 @@ class FederationClient(FederationBase):
 
         # FIXME: We should handle signature failures more gracefully.
         pdus[:] = yield logcontext.make_deferred_yieldable(defer.gatherResults(
-            self._check_sigs_and_hashes(pdus),
+            self._check_sigs_and_hashes(room_version, pdus),
             consumeErrors=True,
         ).addErrback(unwrapFirstError))
 
@@ -268,7 +268,7 @@ class FederationClient(FederationBase):
                     pdu = pdu_list[0]
 
                     # Check signatures are correct.
-                    signed_pdu = yield self._check_sigs_and_hash(pdu)
+                    signed_pdu = yield self._check_sigs_and_hash(room_version, pdu)
 
                     break
 
@@ -757,7 +757,7 @@ class FederationClient(FederationBase):
         pdu = event_from_pdu_json(pdu_dict, format_ver)
 
         # Check signatures are correct.
-        pdu = yield self._check_sigs_and_hash(pdu)
+        pdu = yield self._check_sigs_and_hash(room_version, pdu)
 
         # FIXME: We should handle signature failures more gracefully.
 
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 668161423..aeadc9c56 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -25,7 +25,7 @@ from twisted.internet import defer
 from twisted.internet.abstract import isIPAddress
 from twisted.python import failure
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import EventTypes, Membership
 from synapse.api.errors import (
     AuthError,
     FederationError,
@@ -620,16 +620,19 @@ class FederationServer(FederationBase):
         """
         # check that it's actually being sent from a valid destination to
         # workaround bug #1753 in 0.18.5 and 0.18.6
-        if origin != get_domain_from_id(pdu.event_id):
+        if origin != get_domain_from_id(pdu.sender):
             # We continue to accept join events from any server; this is
             # necessary for the federation join dance to work correctly.
             # (When we join over federation, the "helper" server is
             # responsible for sending out the join event, rather than the
-            # origin. See bug #1893).
+            # origin. See bug #1893. This is also true for some third party
+            # invites).
             if not (
                 pdu.type == 'm.room.member' and
                 pdu.content and
-                pdu.content.get("membership", None) == 'join'
+                pdu.content.get("membership", None) in (
+                    Membership.JOIN, Membership.INVITE,
+                )
             ):
                 logger.info(
                     "Discarding PDU %s from invalid origin %s",
@@ -642,9 +645,12 @@ class FederationServer(FederationBase):
                     pdu.event_id, origin
                 )
 
+        # We've already checked that we know the room version by this point
+        room_version = yield self.store.get_room_version(pdu.room_id)
+
         # Check signature.
         try:
-            pdu = yield self._check_sigs_and_hash(pdu)
+            pdu = yield self._check_sigs_and_hash(room_version, pdu)
         except SynapseError as e:
             raise FederationError(
                 "ERROR",
diff --git a/synapse/federation/transaction_queue.py b/synapse/federation/transaction_queue.py
index fe787abae..1f0b67f5f 100644
--- a/synapse/federation/transaction_queue.py
+++ b/synapse/federation/transaction_queue.py
@@ -175,7 +175,7 @@ class TransactionQueue(object):
                 def handle_event(event):
                     # Only send events for this server.
                     send_on_behalf_of = event.internal_metadata.get_send_on_behalf_of()
-                    is_mine = self.is_mine_id(event.event_id)
+                    is_mine = self.is_mine_id(event.sender)
                     if not is_mine and send_on_behalf_of is None:
                         return
 
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index fcaf7530b..f89dabb9e 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -2293,6 +2293,10 @@ class FederationHandler(BaseHandler):
 
             EventValidator().validate_new(event)
 
+            # We need to tell the transaction queue to send this out, even
+            # though the sender isn't a local user.
+            event.internal_metadata.send_on_behalf_of = self.hs.hostname
+
             try:
                 yield self.auth.check_from_context(room_version, event, context)
             except AuthError as e:
@@ -2342,6 +2346,10 @@ class FederationHandler(BaseHandler):
             raise e
         yield self._check_signature(event, context)
 
+        # We need to tell the transaction queue to send this out, even
+        # though the sender isn't a local user.
+        event.internal_metadata.send_on_behalf_of = get_domain_from_id(event.sender)
+
         # XXX we send the invite here, but send_membership_event also sends it,
         # so we end up making two requests. I think this is redundant.
         returned_invite = yield self.send_invite(origin, event)
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index 9ed5a05cc..2beffdf41 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -940,7 +940,8 @@ class RoomMemberHandler(object):
         # first member event?
         create_event_id = current_state_ids.get(("m.room.create", ""))
         if len(current_state_ids) == 1 and create_event_id:
-            defer.returnValue(self.hs.is_mine_id(create_event_id))
+            # We can only get here if we're in the process of creating the room
+            defer.returnValue(True)
 
         for etype, state_key in current_state_ids:
             if etype != EventTypes.Member or not self.hs.is_mine_id(state_key):