inet: add rfc 3168 extract in front of INET_ECN_encapsulate()

INET_ECN_encapsulate() is better understood if we can read the official
statement.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/inet_ecn.h

@@ -30,6 +30,14 @@ static inline int INET_ECN_is_capable(__u8 dsfield)
 	return dsfield & INET_ECN_ECT_0;
 }
 
+/*
+ * RFC 3168 9.1.1
+ *  The full-functionality option for ECN encapsulation is to copy the
+ *  ECN codepoint of the inside header to the outside header on
+ *  encapsulation if the inside header is not-ECT or ECT, and to set the
+ *  ECN codepoint of the outside header to ECT(0) if the ECN codepoint of
+ *  the inside header is CE.
+ */
 static inline __u8 INET_ECN_encapsulate(__u8 outer, __u8 inner)
 {
 	outer &= ~INET_ECN_MASK;