tcp: more accurately detect spurious TLP probes

Previously TLP is considered spurious if the sender receives any
DSACK during a TLP episode. This patch further checks the DSACK
sequences match the TLP's to improve accuracy.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -100,6 +100,7 @@ int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
 #define FLAG_UPDATE_TS_RECENT	0x4000 /* tcp_replace_ts_recent() */
 #define FLAG_NO_CHALLENGE_ACK	0x8000 /* do not call tcp_send_challenge_ack()	*/
 #define FLAG_ACK_MAYBE_DELAYED	0x10000 /* Likely a delayed ACK */
+#define FLAG_DSACK_TLP		0x20000 /* DSACK for tail loss probe */
 
 #define FLAG_ACKED		(FLAG_DATA_ACKED|FLAG_SYN_ACKED)
 #define FLAG_NOT_DUP		(FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
@@ -991,6 +992,8 @@ static u32 tcp_dsack_seen(struct tcp_sock *tp, u32 start_seq,
 		return 0;
 	if (seq_len > tp->mss_cache)
 		dup_segs = DIV_ROUND_UP(seq_len, tp->mss_cache);
+	else if (tp->tlp_high_seq && tp->tlp_high_seq == end_seq)
+		state->flag |= FLAG_DSACK_TLP;
 
 	tp->dsack_dups += dup_segs;
 	/* Skip the DSACK if dup segs weren't retransmitted by sender */
@@ -3650,7 +3653,7 @@ static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
 	if (!tp->tlp_retrans) {
 		/* TLP of new data has been acknowledged */
 		tp->tlp_high_seq = 0;
-	} else if (flag & FLAG_DSACKING_ACK) {
+	} else if (flag & FLAG_DSACK_TLP) {
 		/* This DSACK means original and TLP probe arrived; no loss */
 		tp->tlp_high_seq = 0;
 	} else if (after(ack, tp->tlp_high_seq)) {