net-tcp_bbr: v3: ensure ECN-enabled BBR flows set ECT on retransmits

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 necessary for BBR, which when using
ECN expects ECT to be set even on retransmitted packets and ACKs.

Previous to this addition of TCP_ECN_ECT_PERMANENT, CCAs which can use
ECN but don't "need" it did 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>
Change-Id: I8b048eaab35e136fe6501ef6cd89fd9faa15e6d2

include/net/tcp.h

@@ -376,6 +376,7 @@ static inline void tcp_dec_quickack_mode(struct sock *sk,
 #define	TCP_ECN_DEMAND_CWR	4
 #define	TCP_ECN_SEEN		8
 #define	TCP_ECN_LOW		16
+#define	TCP_ECN_ECT_PERMANENT	32
 
 enum tcp_tw_status {
 	TCP_TW_SUCCESS = 0,

net/ipv4/tcp_bbr.c

@@ -2151,6 +2151,9 @@ __bpf_kfunc static void bbr_init(struct sock *sk)
 	bbr->plb.pause_until = 0;
 
 	tp->fast_ack_mode = bbr_fast_ack_mode ? 1 : 0;
+
+	if (bbr_can_use_ecn(sk))
+		tp->ecn_flags |= TCP_ECN_ECT_PERMANENT;
 }
 
 /* BBR marks the current round trip as a loss round. */

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);
 		}