ndisc: add router advertisement validity check

Author: bergzand

src/wire/ndiscoption.rs

@@ -402,6 +402,16 @@ pub struct PrefixInformation {
     pub prefix: Ipv6Address,
 }
 
+impl PrefixInformation {
+    /// Validates the prefix information option against check a, b, c in
+    /// https://www.rfc-editor.org/rfc/rfc4862#section-5.5.3
+    pub fn valid_prefix_info(&self) -> bool {
+        self.flags.contains(PrefixInfoFlags::ADDRCONF)
+            && !self.prefix.is_link_local()
+            && self.preferred_lifetime <= self.valid_lifetime
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
 pub struct RedirectedHeader<'a> {