net-tcp_bbr: v2: Fix missing ECT markings on retransmits for BBRv2

Adds a new flag TCP_ECN_ECT_PERMANENT that is used by CCAs to
indicate that retransmitted packets and pure ACKs must have the
ECT bit set. This is a necessary fix for BBRv2, which when using
ECN expects ECT to be set even on retransmitted packets and ACKs.
Currently CCAs like BBRv2 which can use ECN but don't "need" it
do not have a way to indicate that ECT should be set on
retransmissions/ACKs.

Signed-off-by: Adithya Abraham Philip <abrahamphilip@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>

include/net/tcp.h

@@ -368,6 +368,7 @@ static inline void tcp_dec_quickack_mode(struct sock *sk,
 #define	TCP_ECN_QUEUE_CWR	2
 #define	TCP_ECN_DEMAND_CWR	4
 #define	TCP_ECN_SEEN		8
+#define	TCP_ECN_ECT_PERMANENT	16
 
 enum tcp_tw_status {
 	TCP_TW_SUCCESS = 0,

net/ipv4/tcp_bbr2.c

@@ -2471,6 +2471,9 @@ static void bbr2_init(struct sock *sk)
 	bbr->alpha_last_delivered_ce = 0;
 
 	tp->fast_ack_mode = min_t(u32, 0x2U, bbr_fast_ack_mode);
+
+	if ((tp->ecn_flags & TCP_ECN_OK) && bbr_ecn_enable)
+		tp->ecn_flags |= TCP_ECN_ECT_PERMANENT;
 }
 
 /* Core TCP stack informs us that the given skb was just marked lost. */

net/ipv4/tcp_output.c

@@ -377,7 +377,8 @@ static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
 				th->cwr = 1;
 				skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
 			}
-		} else if (!tcp_ca_needs_ecn(sk)) {
+		} else if (!(tp->ecn_flags & TCP_ECN_ECT_PERMANENT) &&
+			!tcp_ca_needs_ecn(sk)) {
 			/* ACK or retransmitted segment: clear ECT|CE */
 			INET_ECN_dontxmit(sk);
 		}