-IPv4 should also be able to use Diffserv instead of TOS. I have simply
-not been able to find the API to set IPv4 Diffserv values.
+The TOS-related options have a number of interesting quirks:
+.TP
+1
+It is currently unclear to me whether Linux has an API to set IPv6
+DSCP values, so it is left unimplemented for now.
+.TP
+2
+I am rather sure that Linux lacks an API to set IPv4 DSCP
+values. However, it seems that it is possible to use the TOS API to
+set DSCP values, so it has been implemented as an option (see the
+\fBnet.dscp-tos\fP options above).
+.TP
+3
+Even though Linux lacks an explicit API to set the DSCP field in IPv4,
+the TOS API is "DSCP compliant" in the interesting way that it masks
+away the two least significant bits. Therefore, the minimum cost TOS
+value cannot currently be set on Linux.
+.TP
+4
+I have not examined how these issues compare to other operating
+systems, like FreeBSD.