spec: Note the potential for all-zero source MAC addresses

Signed-off-by: Michael Brown <[email protected]>

spec/cx.lyx

@@ -1492,6 +1492,15 @@ Such techniques MAY include the use of single-use or short-lived values
  to avoid linkage attacks.
 \end_layout
 
+\begin_layout Standard
+Note that when the logical broadcast is implemented as a single physical
+ broadcast, there is no need for any device to be able to respond to the
+ physical broadcast or to be able to determine its origin.
+ Under these circumstances, the Broadcast Layer MAY use fixed and non-identifyin
+g values for fields that would normally contain identifying information
+ (such as using an all-zero source MAC address).
+\end_layout
+
 \begin_layout Standard
 An Observer MAY record additional relevant information provided by the Broadcast
  Layer at the time of receiving the Contact Identifier, such as received