tcp: introduce per-route feature RTAX_FEATURE_ECN_LOW

Define and implement a new per-route feature, RTAX_FEATURE_ECN_LOW. This feature indicates that the given destination network is a low-latency ECN environment, meaning both that ECN CE marks are applied by the network using a low-latency marking threshold and also that TCP endpoints provide precise per-data-segment ECN feedback in ACKs (where the ACK ECE flag echoes the received CE status of all newly-acknowledged data segments). This feature indication can be used by congestion control algorithms to decide how to interpret ECN signals over the given destination network. This feature is appropriate for datacenter-style ECN marking, such as the ECN marking approach expected by DCTCP or BBR congestion control modules. Signed-off-by: David Morley <morleyd@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Yuchung Cheng <ycheng@google.com> Tested-by: David Morley <morleyd@google.com> Change-Id: I6bc06e9c6cb426fbae7243fc71c9a8c18175f5d3 Signed-off-by: Alexandre Frade <kernel@xanmod.org>
xanmod · Aug 16, 2023 · 3f63752 · 3f63752
1 parent 9bc8bc1
commit 3f63752
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Showing 4 changed files with 19 additions and 3 deletions.
diff --git a/include/net/tcp.h b/include/net/tcp.h
@@ -375,6 +375,7 @@ 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_LOW		16
 
 enum tcp_tw_status {
 	TCP_TW_SUCCESS = 0,
@@ -728,6 +729,15 @@ static inline void tcp_fast_path_check(struct sock *sk)
 		tcp_fast_path_on(tp);
 }
 
+static inline void tcp_set_ecn_low_from_dst(struct sock *sk,
+					    const struct dst_entry *dst)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	if (dst_feature(dst, RTAX_FEATURE_ECN_LOW))
+		tp->ecn_flags |= TCP_ECN_LOW;
+}
+
 /* Compute the actual rto_min value */
 static inline u32 tcp_rto_min(struct sock *sk)
 {

diff --git a/include/uapi/linux/rtnetlink.h b/include/uapi/linux/rtnetlink.h
@@ -506,9 +506,11 @@ enum {
 #define RTAX_FEATURE_SACK	(1 << 1)
 #define RTAX_FEATURE_TIMESTAMP	(1 << 2)
 #define RTAX_FEATURE_ALLFRAG	(1 << 3)
+#define RTAX_FEATURE_ECN_LOW	(1 << 4)
 
 #define RTAX_FEATURE_MASK	(RTAX_FEATURE_ECN | RTAX_FEATURE_SACK | \
-				 RTAX_FEATURE_TIMESTAMP | RTAX_FEATURE_ALLFRAG)
+				 RTAX_FEATURE_TIMESTAMP | RTAX_FEATURE_ALLFRAG \
+				 | RTAX_FEATURE_ECN_LOW)
 
 struct rta_session {
 	__u8	proto;

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
@@ -423,6 +423,8 @@ void tcp_ca_openreq_child(struct sock *sk, const struct dst_entry *dst)
 	u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
 	bool ca_got_dst = false;
 
+	tcp_set_ecn_low_from_dst(sk, dst);
+
 	if (ca_key != TCP_CA_UNSPEC) {
 		const struct tcp_congestion_ops *ca;
 

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
@@ -327,10 +327,9 @@ static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
 	bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
 	bool use_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn) == 1 ||
 		tcp_ca_needs_ecn(sk) || bpf_needs_ecn;
+	const struct dst_entry *dst = __sk_dst_get(sk);
 
 	if (!use_ecn) {
-		const struct dst_entry *dst = __sk_dst_get(sk);
-
 		if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
 			use_ecn = true;
 	}
@@ -342,6 +341,9 @@ static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
 		tp->ecn_flags = TCP_ECN_OK;
 		if (tcp_ca_needs_ecn(sk) || bpf_needs_ecn)
 			INET_ECN_xmit(sk);
+
+		if (dst)
+			tcp_set_ecn_low_from_dst(sk, dst);
 	}
 }