+ * Make sure that only what we expect to change changed. The first
+ * line of the `if' checks the IP protocol version, header length &
+ * type of service. The 2nd line checks the "Don't fragment" bit.
+ * The 3rd line checks the time-to-live and protocol (the protocol
+ * check is unnecessary but costless). The 4th line checks the TCP
+ * header length. The 5th line checks IP options, if any. The 6th
+ * line checks TCP options, if any. If any of these things are
+ * different between the previous & current datagram, we send the
+ * current datagram `uncompressed'.