diff --git a/TransportApp.markdown b/TransportApp.markdown
index c3c2d06..ecd5af4 100644
--- a/TransportApp.markdown
+++ b/TransportApp.markdown
@@ -1,15 +1,19 @@
-TransportApp is an application that uses [BSP] to synchronise information about how to connect to devices over various transports.
+TransportApp is an application that uses [BSP](BSP) to synchronise information about how to connect to devices over various transports.
 
 ### Message types
 
 TransportApp uses a single message type:
 
-**0: PROPERTIES** - The content is a [BDF] list with three elements: transport identifier (string), version (int), and transport properties (dictionary). The identifier and properties are supplied by the transport plugin. The version is incremented whenever the properties change.
+**0: PROPERTIES** - The content is a [BDF](BDF) list with three elements: transport identifier (string), version (int), and transport properties (dictionary). The identifier and properties are supplied by the transport plugin. The version is incremented whenever the properties change.
+
+### Validation
+
+* A message of type PROPERTIES is valid if it is well-formed as described above
 
 ### Storage policy
 
-* The local message with the highest version for each transport is stored and any other versions are deleted
-* For each remote device, the remote message with the highest version for each transport is stored and any other versions are deleted
+* The local message with the highest version for each transport is stored, and any other versions are deleted
+* For each remote device, the remote message with the highest version for each transport is stored, and any other versions are deleted
 
 ### Sharing policy