-The protocol version selection stuff is not yet implemented: I'm not
-yet convinced it's a good idea. Instead, the initiator could try
-using its preferred protocol (which starts with a different magic
-number) and fall back if there's no reply.
+The optional additional data after the sender's name consists of some
+initial subset of the following list of items:
+ * A 32-bit integer with a set of capability flags, representing the
+ abilities of the sender.
+ * More data which is yet to be defined and which must be ignored
+ by receivers.
+The optional additional data after the receiver's name is not
+currently used. If any is seen, it must be ignored.
+
+Capability flag bits must be in one the following two categories:
+
+1. Early capability flags must be advertised in MSG1 or MSG2, as
+ applicable. If MSG3 or MSG4 advertise any "early" capability bits,
+ MSG1 or MSG3 (as applicable) must have advertised them too. Sadly,
+ advertising an early capability flag will produce MSG1s which are
+ not understood by versions of secnet which predate the capability
+ mechanism.
+
+2. Late capability flags are advertised in MSG2 or MSG3, as
+ applicable. They may also appear in MSG1, but this is not
+ guaranteed. MSG4 must advertise the same set as MSG2.
+
+No capability flags are currently defined. Unknown capability flags
+should be treated as late ones.
+