mptcp: Use 32-bit DATA_ACK when possible

RFC8684 allows to send 32-bit DATA_ACKs as long as the peer is not
sending 64-bit data-sequence numbers. The 64-bit DSN is only there for
extreme scenarios when a very high throughput subflow is combined with a
long-RTT subflow such that the high-throughput subflow wraps around the
32-bit sequence number space within an RTT of the high-RTT subflow.

It is thus a rare scenario and we should try to use the 32-bit DATA_ACK
instead as long as possible. It allows to reduce the TCP-option overhead
by 4 bytes, thus makes space for an additional SACK-block. It also makes
tcpdumps much easier to read when the DSN and DATA_ACK are both either
32 or 64-bit.

Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/mptcp.h

@@ -16,7 +16,10 @@ struct seq_file;
 
 /* MPTCP sk_buff extension data */
 struct mptcp_ext {
-	u64		data_ack;
+	union {
+		u64	data_ack;
+		u32	data_ack32;
+	};
 	u64		data_seq;
 	u32		subflow_seq;
 	u16		data_len;