Function discover_nat64 

pub async fn discover_nat64<A, E>(
    lookup: Option<impl Future<Output = Result<A, E>>>,
) -> Vec<(Ipv6Addr, u8)>
where
    A: IntoIterator<Item = SocketAddr>,

The prefixes in a NAT64_DISCOVERY_NAME answer. No lookup, a failed one, or one slower than 5 seconds adds no prefix.