Date:   Wed, 18 Mar 2020 11:43:16 +0200
From:   Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
To:     netdev@...r.kernel.org
Cc:     Yuchung Cheng <ycheng@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Olivier Tilmans <olivier.tilmans@...ia-bell-labs.com>
Subject: [RFC PATCH 12/28] tcp: AccECN core

From: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>

Accurate ECN (without negotiation and AccECN Option).

Signed-off-by: Olivier Tilmans <olivier.tilmans@...ia-bell-labs.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...helsinki.fi>
---
 include/linux/tcp.h   |   2 +
 include/net/tcp.h     |  29 ++++++++++++
 net/ipv4/tcp.c        |   1 +
 net/ipv4/tcp_input.c  | 108 +++++++++++++++++++++++++++++++++++-------
 net/ipv4/tcp_output.c |  22 ++++++++-
 5 files changed, 144 insertions(+), 18 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 3dc964010fef..9bdf67dd0d1d 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -315,6 +315,8 @@ struct tcp_sock {
 	u32	prr_out;	/* Total number of pkts sent during Recovery. */
 	u32	delivered;	/* Total data packets delivered incl. rexmits */
 	u32	delivered_ce;	/* Like the above but only ECE marked packets */
+	u32	received_ce;	/* Like the above but for received CE marked packets */
+	u32	received_ce_tx; /* Like the above but max transmitted value */
 	u32	lost;		/* Total data packets lost incl. rexmits */
 	u32	app_limited;	/* limited until "delivered" reaches this val */
 	u64	first_tx_mstamp;  /* start of window send phase */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index f4ac4c029215..ee938066fd8c 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -404,6 +404,16 @@ static inline void tcp_ecn_mode_set(struct tcp_sock *tp, u8 mode)
 	tp->ecn_flags |= mode;
 }
 
+static inline u8 tcp_accecn_ace(const struct tcphdr *th)
+{
+	return (th->ae << 2) | (th->cwr << 1) | th->ece;
+}
+
+static inline u32 tcp_accecn_ace_deficit(const struct tcp_sock *tp)
+{
+	return tp->received_ce - tp->received_ce_tx;
+}
+
 enum tcp_tw_status {
 	TCP_TW_SUCCESS = 0,
 	TCP_TW_RST = 1,
@@ -833,6 +843,20 @@ static inline u64 tcp_skb_timestamp_us(const struct sk_buff *skb)
 #define TCPHDR_ACE (TCPHDR_ECE | TCPHDR_CWR | TCPHDR_AE)
 #define TCPHDR_SYN_ECN	(TCPHDR_SYN | TCPHDR_ECE | TCPHDR_CWR)
 
+#define TCP_ACCECN_CEP_ACE_MASK 0x7
+#define TCP_ACCECN_ACE_MAX_DELTA 6
+
+/* To avoid/detect middlebox interference, not all counters start at 0.
+ * See draft-ietf-tcpm-accurate-ecn for the latest values.
+ */
+#define TCP_ACCECN_CEP_INIT_OFFSET 5
+
+static inline void tcp_accecn_init_counters(struct tcp_sock *tp)
+{
+	tp->received_ce = 0;
+	tp->received_ce_tx = 0;
+}
+
 /* This is what the send packet queuing engine uses to pass
  * TCP per-packet control information to the transmission code.
  * We also store the host-order sequence numbers in here too.
@@ -1528,7 +1552,12 @@ static inline bool tcp_paws_reject(const struct tcp_options_received *rx_opt,
 
 static inline void __tcp_fast_path_on(struct tcp_sock *tp, u32 snd_wnd)
 {
+	u32 ace = tcp_ecn_mode_accecn(tp) ?
+		  ((tp->delivered_ce + TCP_ACCECN_CEP_INIT_OFFSET) &
+		   TCP_ACCECN_CEP_ACE_MASK) : 0;
+
 	tp->pred_flags = htonl((tp->tcp_header_len << 26) |
+			       (ace << 22) |
 			       ntohl(TCP_FLAG_ACK) |
 			       snd_wnd);
 }
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index fbf365dd51e4..2ee1e4794c7d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2624,6 +2624,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	tp->window_clamp = 0;
 	tp->delivered = 0;
 	tp->delivered_ce = 0;
+	tcp_accecn_init_counters(tp);
 	tcp_set_ca_state(sk, TCP_CA_Open);
 	tp->is_sack_reneg = 0;
 	tcp_clear_retrans(tp);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 59078fa2240d..65bbfadbee67 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -249,6 +249,7 @@ static bool tcp_in_quickack_mode(struct sock *sk)
 
 static void tcp_ecn_queue_cwr(struct tcp_sock *tp)
 {
+	/* Do not set CWR if in AccECN mode! */
 	if (tcp_ecn_mode_rfc3168(tp))
 		tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
 }
@@ -257,7 +258,7 @@ static void tcp_ecn_accept_cwr(struct sock *sk, const struct sk_buff *skb)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	if (tcp_hdr(skb)->cwr) {
+	if (tcp_ecn_mode_rfc3168(tp) && tcp_hdr(skb)->cwr) {
 		tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
 
 		/* If the sender is telling us it has entered CWR, then its
@@ -293,17 +294,16 @@ static void tcp_data_ecn_check(struct sock *sk, const struct sk_buff *skb)
 		if (tcp_ca_needs_ecn(sk))
 			tcp_ca_event(sk, CA_EVENT_ECN_IS_CE);
 
-		if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
+		if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR) &&
+		    tcp_ecn_mode_rfc3168(tp)) {
 			/* Better not delay acks, sender can have a very low cwnd */
 			tcp_enter_quickack_mode(sk, 2);
 			tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
 		}
-		tp->ecn_flags |= TCP_ECN_SEEN;
 		break;
 	default:
 		if (tcp_ca_needs_ecn(sk))
 			tcp_ca_event(sk, CA_EVENT_ECN_NO_CE);
-		tp->ecn_flags |= TCP_ECN_SEEN;
 		break;
 	}
 }
@@ -320,11 +320,43 @@ static void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th)
 		tcp_ecn_mode_set(tp, TCP_ECN_DISABLED);
 }
 
-static bool tcp_ecn_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
+static u32 tcp_ecn_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
 {
 	if (th->ece && !th->syn && tcp_ecn_mode_rfc3168(tp))
-		return true;
-	return false;
+		return 1;
+	return 0;
+}
+
+/* Returns the ECN CE delta */
+static u32 tcp_accecn_process(struct tcp_sock *tp, const struct sk_buff *skb,
+			      u32 delivered_pkts, int flag)
+{
+	u32 delta, safe_delta;
+	u32 corrected_ace;
+
+	/* Reordered ACK? (...or uncertain due to lack of data to send and ts) */
+	if (!(flag & (FLAG_FORWARD_PROGRESS|FLAG_TS_PROGRESS)))
+		return 0;
+
+	if (!(flag & FLAG_SLOWPATH)) {
+		/* AccECN counter might overflow on large ACKs */
+		if (delivered_pkts <= TCP_ACCECN_CEP_ACE_MASK)
+			return 0;
+	}
+
+	/* ACE field is not available during handshake */
+	if (flag & FLAG_SYN_ACKED)
+		return 0;
+
+	corrected_ace = tcp_accecn_ace(tcp_hdr(skb)) - TCP_ACCECN_CEP_INIT_OFFSET;
+	delta = (corrected_ace - tp->delivered_ce) & TCP_ACCECN_CEP_ACE_MASK;
+	if (delivered_pkts < TCP_ACCECN_CEP_ACE_MASK)
+		return delta;
+
+	safe_delta = delivered_pkts -
+		     ((delivered_pkts - delta) & TCP_ACCECN_CEP_ACE_MASK);
+
+	return safe_delta;
 }
 
 /* Buffer size and advertised window tuning.
@@ -3590,7 +3622,8 @@ static void tcp_xmit_recovery(struct sock *sk, int rexmit)
 }
 
 /* Returns the number of packets newly acked or sacked by the current ACK */
-static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
+static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered,
+			       u32 ecn_count)
 {
 	const struct net *net = sock_net(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -3598,10 +3631,18 @@ static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
 
 	delivered = tp->delivered - prior_delivered;
 	NET_ADD_STATS(net, LINUX_MIB_TCPDELIVERED, delivered);
-	if (flag & FLAG_ECE) {
-		tp->delivered_ce += delivered;
-		NET_ADD_STATS(net, LINUX_MIB_TCPDELIVEREDCE, delivered);
+
+	if (ecn_count) {
+		if (tcp_ecn_mode_rfc3168(tp))
+			ecn_count = delivered;
+
+		tp->delivered_ce += ecn_count;
+		if (tcp_ecn_mode_accecn(tp) &&
+		    tcp_accecn_ace_deficit(tp) >= TCP_ACCECN_ACE_MAX_DELTA)
+			inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
+		NET_ADD_STATS(net, LINUX_MIB_TCPDELIVEREDCE, ecn_count);
 	}
+
 	return delivered;
 }
 
@@ -3621,6 +3662,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	u32 delivered = tp->delivered;
 	u32 lost = tp->lost;
 	int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
+	u32 ecn_count = 0; /* Did we receive ECE/an AccECN ACE update? */
 	u32 prior_fack;
 
 	sack_state.first_sackt = 0;
@@ -3690,8 +3732,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 		if (TCP_SKB_CB(skb)->sacked)
 			flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
 							&sack_state);
-
-		if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb)))
+		ecn_count = tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb));
+		if (ecn_count > 0)
 			flag |= FLAG_ECE;
 	}
 
@@ -3709,6 +3751,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 
 	tcp_rack_update_reo_wnd(sk, &rs);
 
+	if (tcp_ecn_mode_accecn(tp)) {
+		ecn_count = tcp_accecn_process(tp, skb,
+					       tp->delivered - delivered, flag);
+		if (ecn_count > 0)
+			flag |= FLAG_ECE;
+	}
+
 	tcp_in_ack_event(sk, flag);
 
 	if (tp->tlp_high_seq)
@@ -3731,7 +3780,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
 		sk_dst_confirm(sk);
 
-	delivered = tcp_newly_delivered(sk, delivered, flag);
+	delivered = tcp_newly_delivered(sk, delivered, ecn_count);
+
 	lost = tp->lost - lost;			/* freshly marked lost */
 	rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
 	tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
@@ -3740,12 +3790,18 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	return 1;
 
 no_queue:
+	if (tcp_ecn_mode_accecn(tp)) {
+		ecn_count = tcp_accecn_process(tp, skb,
+					       tp->delivered - delivered, flag);
+		if (ecn_count > 0)
+			flag |= FLAG_ECE;
+	}
 	tcp_in_ack_event(sk, flag);
 	/* If data was DSACKed, see if we can undo a cwnd reduction. */
 	if (flag & FLAG_DSACKING_ACK) {
 		tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
 				      &rexmit);
-		tcp_newly_delivered(sk, delivered, flag);
+		tcp_newly_delivered(sk, delivered, ecn_count);
 	}
 	/* If this ack opens up a zero window, clear backoff.  It was
 	 * being used to time the probes, and is probably far higher than
@@ -3766,7 +3822,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 						&sack_state);
 		tcp_fastretrans_alert(sk, prior_snd_una, num_dupack, &flag,
 				      &rexmit);
-		tcp_newly_delivered(sk, delivered, flag);
+		tcp_newly_delivered(sk, delivered, ecn_count);
 		tcp_xmit_recovery(sk, rexmit);
 	}
 
@@ -5425,6 +5481,21 @@ static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *t
 	}
 }
 
+/* Updates Accurate ECN received counters from the received IP ECN field */
+static void tcp_ecn_received_counters(struct sock *sk, const struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u8 ecnfield = TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK;
+	u8 is_ce = INET_ECN_is_ce(ecnfield);
+
+	if (!INET_ECN_is_not_ect(ecnfield)) {
+		tp->ecn_flags |= TCP_ECN_SEEN;
+
+		/* ACE counter tracks *all* segments including pure acks */
+		tp->received_ce += is_ce * max_t(u16, 1, skb_shinfo(skb)->gso_segs);
+	}
+}
+
 /* Accept RST for rcv_nxt - 1 after a FIN.
  * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
  * FIN is sent followed by a RST packet. The RST is sent with the same
@@ -5656,6 +5727,8 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 				    tp->rcv_nxt == tp->rcv_wup)
 					flag |= __tcp_replace_ts_recent(tp, tstamp_delta);
 
+				tcp_ecn_received_counters(sk, skb);
+
 				/* We know that such packets are checksummed
 				 * on entry.
 				 */
@@ -5697,6 +5770,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 
 			/* Bulk data transfer: receiver */
 			__skb_pull(skb, tcp_header_len);
+			tcp_ecn_received_counters(sk, skb);
 			eaten = tcp_queue_rcv(sk, skb, &fragstolen);
 
 			tcp_event_data_recv(sk, skb);
@@ -5733,6 +5807,8 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 		return;
 
 step5:
+	tcp_ecn_received_counters(sk, skb);
+
 	if (tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT) < 0)
 		goto discard;
 
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 71a96983987d..a1414d1a8ef1 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -360,6 +360,18 @@ tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
 		th->ece = 1;
 }
 
+static void tcp_accecn_set_ace(struct tcphdr *th, struct tcp_sock *tp)
+{
+	u32 wire_ace;
+
+	tp->received_ce_tx += min_t(u32, tcp_accecn_ace_deficit(tp),
+				    TCP_ACCECN_ACE_MAX_DELTA);
+	wire_ace = tp->received_ce_tx + TCP_ACCECN_CEP_INIT_OFFSET;
+	th->ece = !!(wire_ace & 0x1);
+	th->cwr = !!(wire_ace & 0x2);
+	th->ae = !!(wire_ace & 0x4);
+}
+
 /* Set up ECN state for a packet on a ESTABLISHED socket that is about to
  * be sent.
  */
@@ -368,11 +380,17 @@ static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
-	if (tcp_ecn_mode_rfc3168(tp)) {
+	if (!tcp_ecn_mode_any(tp))
+		return;
+
+	INET_ECN_xmit(sk);
+	if (tcp_ecn_mode_accecn(tp)) {
+		tcp_accecn_set_ace(th, tp);
+		skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ACCECN;
+	} else {
 		/* Not-retransmitted data segment: set ECT and inject CWR. */
 		if (skb->len != tcp_header_len &&
 		    !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
-			INET_ECN_xmit(sk);
 			if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
 				tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
 				th->cwr = 1;
-- 
2.20.1

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