diff --git a/synapse/federation/units.py b/synapse/federation/units.py
new file mode 100644
index 0000000000..0efea7b768
--- /dev/null
+++ b/synapse/federation/units.py
@@ -0,0 +1,236 @@
+# -*- coding: utf-8 -*-
+# Copyright 2014 matrix.org
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+""" Defines the JSON structure of the protocol units used by the server to
+server protocol.
+"""
+
+from synapse.util.jsonobject import JsonEncodedObject
+
+import logging
+import json
+import copy
+
+
+logger = logging.getLogger(__name__)
+
+
+class Pdu(JsonEncodedObject):
+ """ A Pdu represents a piece of data sent from a server and is associated
+ with a context.
+
+ A Pdu can be classified as "state". For a given context, we can efficiently
+ retrieve all state pdu's that haven't been clobbered. Clobbering is done
+ via a unique constraint on the tuple (context, pdu_type, state_key). A pdu
+ is a state pdu if `is_state` is True.
+
+ Example pdu::
+
+ {
+ "pdu_id": "78c",
+ "ts": 1404835423000,
+ "origin": "bar",
+ "prev_ids": [
+ ["23b", "foo"],
+ ["56a", "bar"],
+ ],
+ "content": { ... },
+ }
+
+ """
+
+ valid_keys = [
+ "pdu_id",
+ "context",
+ "origin",
+ "ts",
+ "pdu_type",
+ "destinations",
+ "transaction_id",
+ "prev_pdus",
+ "depth",
+ "content",
+ "outlier",
+ "is_state", # Below this are keys valid only for State Pdus.
+ "state_key",
+ "power_level",
+ "prev_state_id",
+ "prev_state_origin",
+ ]
+
+ internal_keys = [
+ "destinations",
+ "transaction_id",
+ "outlier",
+ ]
+
+ required_keys = [
+ "pdu_id",
+ "context",
+ "origin",
+ "ts",
+ "pdu_type",
+ "content",
+ ]
+
+ # TODO: We need to make this properly load content rather than
+ # just leaving it as a dict. (OR DO WE?!)
+
+ def __init__(self, destinations=[], is_state=False, prev_pdus=[],
+ outlier=False, **kwargs):
+ if is_state:
+ for required_key in ["state_key"]:
+ if required_key not in kwargs:
+ raise RuntimeError("Key %s is required" % required_key)
+
+ super(Pdu, self).__init__(
+ destinations=destinations,
+ is_state=is_state,
+ prev_pdus=prev_pdus,
+ outlier=outlier,
+ **kwargs
+ )
+
+ @classmethod
+ def from_pdu_tuple(cls, pdu_tuple):
+ """ Converts a PduTuple to a Pdu
+
+ Args:
+ pdu_tuple (synapse.persistence.transactions.PduTuple): The tuple to
+ convert
+
+ Returns:
+ Pdu
+ """
+ if pdu_tuple:
+ d = copy.copy(pdu_tuple.pdu_entry._asdict())
+
+ d["content"] = json.loads(d["content_json"])
+ del d["content_json"]
+
+ args = {f: d[f] for f in cls.valid_keys if f in d}
+ if "unrecognized_keys" in d and d["unrecognized_keys"]:
+ args.update(json.loads(d["unrecognized_keys"]))
+
+ return Pdu(
+ prev_pdus=pdu_tuple.prev_pdu_list,
+ **args
+ )
+ else:
+ return None
+
+ def __str__(self):
+ return "(%s, %s)" % (self.__class__.__name__, repr(self.__dict__))
+
+ def __repr__(self):
+ return "<%s, %s>" % (self.__class__.__name__, repr(self.__dict__))
+
+
+class Edu(JsonEncodedObject):
+ """ An Edu represents a piece of data sent from one homeserver to another.
+
+ In comparison to Pdus, Edus are not persisted for a long time on disk, are
+ not meaningful beyond a given pair of homeservers, and don't have an
+ internal ID or previous references graph.
+ """
+
+ valid_keys = [
+ "origin",
+ "destination",
+ "edu_type",
+ "content",
+ ]
+
+ required_keys = [
+ "origin",
+ "destination",
+ "edu_type",
+ ]
+
+
+class Transaction(JsonEncodedObject):
+ """ A transaction is a list of Pdus and Edus to be sent to a remote home
+ server with some extra metadata.
+
+ Example transaction::
+
+ {
+ "origin": "foo",
+ "prev_ids": ["abc", "def"],
+ "pdus": [
+ ...
+ ],
+ }
+
+ """
+
+ valid_keys = [
+ "transaction_id",
+ "origin",
+ "destination",
+ "ts",
+ "previous_ids",
+ "pdus",
+ "edus",
+ ]
+
+ internal_keys = [
+ "transaction_id",
+ "destination",
+ ]
+
+ required_keys = [
+ "transaction_id",
+ "origin",
+ "destination",
+ "ts",
+ "pdus",
+ ]
+
+ def __init__(self, transaction_id=None, pdus=[], **kwargs):
+ """ If we include a list of pdus then we decode then as PDU's
+ automatically.
+ """
+
+ # If there's no EDUs then remove the arg
+ if "edus" in kwargs and not kwargs["edus"]:
+ del kwargs["edus"]
+
+ super(Transaction, self).__init__(
+ transaction_id=transaction_id,
+ pdus=pdus,
+ **kwargs
+ )
+
+ @staticmethod
+ def create_new(pdus, **kwargs):
+ """ Used to create a new transaction. Will auto fill out
+ transaction_id and ts keys.
+ """
+ if "ts" not in kwargs:
+ raise KeyError("Require 'ts' to construct a Transaction")
+ if "transaction_id" not in kwargs:
+ raise KeyError(
+ "Require 'transaction_id' to construct a Transaction"
+ )
+
+ for p in pdus:
+ p.transaction_id = kwargs["transaction_id"]
+
+ kwargs["pdus"] = [p.get_dict() for p in pdus]
+
+ return Transaction(**kwargs)
+
+
+
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