diff --git a/synapse/federation/replication.py b/synapse/federation/replication.py
index c79ce44688..a48a7ac15f 100644
--- a/synapse/federation/replication.py
+++ b/synapse/federation/replication.py
@@ -292,8 +292,8 @@ class ReplicationLayer(object):
         transaction = Transaction(**transaction_data)
 
         for p in transaction.pdus:
-            if "age" in p:
-                p["age_ts"] = int(self.clock.time_msec()) - int(p["age"])
+            if "age_ts" in p:
+                p["age"] = int(self._clock.time_msec()) - int(p["age_ts"])
 
         pdu_list = [Pdu(**p) for p in transaction.pdus]
 
@@ -602,8 +602,21 @@ class _TransactionQueue(object):
             logger.debug("TX [%s] Sending transaction...", destination)
 
             # Actually send the transaction
+
+            # FIXME (erikj): This is a bit of a hack to make the Pdu age
+            # keys work
+            def cb(transaction):
+                now = int(self._clock.time_msec())
+                if "pdus" in transaction:
+                    for p in transaction["pdus"]:
+                        if "age_ts" in p:
+                            p["age"] = now - int(p["age_ts"])
+
+                return transaction
+
             code, response = yield self.transport_layer.send_transaction(
-                transaction
+                transaction,
+                on_send_callback=cb,
             )
 
             logger.debug("TX [%s] Sent transaction", destination)
diff --git a/synapse/federation/transport.py b/synapse/federation/transport.py
index 6e62ae7c74..afc777ec9e 100644
--- a/synapse/federation/transport.py
+++ b/synapse/federation/transport.py
@@ -144,7 +144,7 @@ class TransportLayer(object):
 
     @defer.inlineCallbacks
     @log_function
-    def send_transaction(self, transaction):
+    def send_transaction(self, transaction, on_send_callback=None):
         """ Sends the given Transaction to it's destination
 
         Args:
@@ -165,10 +165,23 @@ class TransportLayer(object):
 
         data = transaction.get_dict()
 
+        # FIXME (erikj): This is a bit of a hack to make the Pdu age
+        # keys work
+        def cb(destination, method, path_bytes, producer):
+            if not on_send_callback:
+                return
+
+            transaction = json.loads(producer.body)
+
+            new_transaction = on_send_callback(transaction)
+
+            producer.reset(new_transaction)
+
         code, response = yield self.client.put_json(
             transaction.destination,
             path=PREFIX + "/send/%s/" % transaction.transaction_id,
-            data=data
+            data=data,
+            on_send_callback=cb,
         )
 
         logger.debug(
diff --git a/synapse/http/client.py b/synapse/http/client.py
index ece6318e00..eb11bfd4d5 100644
--- a/synapse/http/client.py
+++ b/synapse/http/client.py
@@ -122,7 +122,7 @@ class TwistedHttpClient(HttpClient):
         self.hs = hs
 
     @defer.inlineCallbacks
-    def put_json(self, destination, path, data):
+    def put_json(self, destination, path, data, on_send_callback=None):
         if destination in _destination_mappings:
             destination = _destination_mappings[destination]
 
@@ -131,7 +131,8 @@ class TwistedHttpClient(HttpClient):
             "PUT",
             path.encode("ascii"),
             producer=_JsonProducer(data),
-            headers_dict={"Content-Type": ["application/json"]}
+            headers_dict={"Content-Type": ["application/json"]},
+            on_send_callback=on_send_callback,
         )
 
         logger.debug("Getting resp body")
@@ -218,7 +219,7 @@ class TwistedHttpClient(HttpClient):
     @defer.inlineCallbacks
     def _create_request(self, destination, method, path_bytes, param_bytes=b"",
                         query_bytes=b"", producer=None, headers_dict={},
-                        retry_on_dns_fail=True):
+                        retry_on_dns_fail=True, on_send_callback=None):
         """ Creates and sends a request to the given url
         """
         headers_dict[b"User-Agent"] = [b"Synapse"]
@@ -242,6 +243,9 @@ class TwistedHttpClient(HttpClient):
         endpoint = self._getEndpoint(reactor, destination);
 
         while True:
+            if on_send_callback:
+                on_send_callback(destination, method, path_bytes, producer)
+
             try:
                 response = yield self.agent.request(
                     destination,
@@ -310,6 +314,9 @@ class _JsonProducer(object):
     """ Used by the twisted http client to create the HTTP body from json
     """
     def __init__(self, jsn):
+        self.reset(jsn)
+
+    def reset(self, jsn):
         self.body = encode_canonical_json(jsn)
         self.length = len(self.body)