tcp: accecn: detect loss ACK w/ AccECN option and add TCP_ACCECN_OPTION_PERSIST

Detect spurious retransmission of a previously sent ACK carrying the AccECN option after the second retransmission. Since this might be caused by the middlebox dropping ACK with options it does not recognize, disable the sending of the AccECN option in all subsequent ACKs. This patch follows Section 3.2.3.2.2 of AccECN spec (RFC9768), and a new field (accecn_opt_sent_w_dsack) is added to indicate that an AccECN option was sent with duplicate SACK info. Also, a new AccECN option sending mode is added to tcp_ecn_option sysctl: (TCP_ECN_OPTION_PERSIST), which ignores the AccECN fallback policy and persistently sends AccECN option once it fits into TCP option space.

Chia-Yu Chang committed 9 months ago

Commit 1247fb11 parent 2ed66126 files · +26 −5Browse files

Unified · Split

6 files

Documentation/networking/ip-sysctl.rst +3 −1
include/linux/tcp.h +2 −1
include/net/tcp_ecn.h +2 −0
net/ipv4/sysctl_net_ipv4.c +1 −1
net/ipv4/tcp_input.c +12 −1
net/ipv4/tcp_output.c +6 −1

Documentation/networking/ip-sysctl.rst+3−1

 	1 Send AccECN option sparingly according to the minimum option
 	  rules outlined in draft-ietf-tcpm-accurate-ecn.
 	2 Send AccECN option on every packet whenever it fits into TCP
-	  option space.
+	  option space except when AccECN fallback is triggered.
+	3 Send AccECN option on every packet whenever it fits into TCP
+	  option space even when AccECN fallback is triggered.
 	= ============================================================
 	Default: 2

include/linux/tcp.h+2−1

 	u8	nonagle     : 4,/* Disable Nagle algorithm?             */
 		rate_app_limited:1;  /* rate_{delivered,interval_us} limited? */
 	u8	received_ce_pending:4, /* Not yet transmit cnt of received_ce */
-		unused2:4;
+		accecn_opt_sent_w_dsack:1,/* Sent ACCECN opt in previous ACK w/ D-SACK */
+		unused2:3;
 	u8	accecn_minlen:2,/* Minimum length of AccECN option sent */
 		est_ecnfield:2,/* ECN field for AccECN delivered estimates */
 		accecn_opt_demand:2,/* Demand AccECN option for n next ACKs */

include/net/tcp_ecn.h+2−0

 	TCP_ACCECN_OPTION_DISABLED = 0,
 	TCP_ACCECN_OPTION_MINIMUM = 1,
 	TCP_ACCECN_OPTION_FULL = 2,
+	TCP_ACCECN_OPTION_PERSIST = 3,
 };
 /* Apply either ECT(0) or ECT(1) based on TCP_CONG_ECT_1_NEGOTIATION flag */
 	tp->received_ce_pending = 0;
 	__tcp_accecn_init_bytes_counters(tp->received_ecn_bytes);
 	__tcp_accecn_init_bytes_counters(tp->delivered_ecn_bytes);
+	tp->accecn_opt_sent_w_dsack = 0;
 	tp->accecn_minlen = 0;
 	tp->accecn_opt_demand = 0;
 	tp->est_ecnfield = 0;

net/ipv4/sysctl_net_ipv4.c+1−1

@@ -749,7 +749,7 @@ static struct ctl_table ipv4_net_table[] = { · 748 unmodified lines
749	.mode = 0644,	749	.mode = 0644,
750	.proc_handler = proc_dou8vec_minmax,	750	.proc_handler = proc_dou8vec_minmax,
751	.extra1 = SYSCTL_ZERO,	751	.extra1 = SYSCTL_ZERO,
752	.extra2 = SYSCTL_TWO,	752	.extra2 = SYSCTL_THREE,
753	},	753	},
754	{	754	{
755	.procname = "tcp_ecn_option_beacon",	755	.procname = "tcp_ecn_option_beacon",

net/ipv4/tcp_input.c+12−1

 		tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
 }
-static void tcp_rcv_spurious_retrans(struct sock *sk, const struct sk_buff *skb)
+static void tcp_rcv_spurious_retrans(struct sock *sk,
+				     const struct sk_buff *skb)
 {
+	struct tcp_sock *tp = tcp_sk(sk);
 	/* When the ACK path fails or drops most ACKs, the sender would
 	 * timeout and spuriously retransmit the same segment repeatedly.
 	 * If it seems our ACKs are not reaching the other side,
 	/* Save last flowlabel after a spurious retrans. */
 	tcp_save_lrcv_flowlabel(sk, skb);
 #endif
+	/* Check DSACK info to detect that the previous ACK carrying the
+	 * AccECN option was lost after the second retransmision, and then
+	 * stop sending AccECN option in all subsequent ACKs.
+	 */
+	if (tcp_ecn_mode_accecn(tp) &&
+	    tp->accecn_opt_sent_w_dsack &&
+	    TCP_SKB_CB(skb)->seq == tp->duplicate_sack[0].start_seq)
+		tcp_accecn_fail_mode_set(tp, TCP_ACCECN_OPT_FAIL_SEND);
 }
 static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)

net/ipv4/tcp_output.c+6−1

 		if (tp) {
 			tp->accecn_minlen = 0;
 			tp->accecn_opt_tstamp = tp->tcp_mstamp;
+			tp->accecn_opt_sent_w_dsack = tp->rx_opt.dsack;
 			if (tp->accecn_opt_demand)
 				tp->accecn_opt_demand--;
 		}
+	} else if (tp) {
+		tp->accecn_opt_sent_w_dsack = 0;
 	}
 	if (unlikely(OPTION_SACK_ADVERTISE & options)) {
 	if (tcp_ecn_mode_accecn(tp)) {
 		int ecn_opt = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_option);
-		if (ecn_opt && tp->saw_accecn_opt && !tcp_accecn_opt_fail_send(tp) &&
+		if (ecn_opt && tp->saw_accecn_opt &&
+		    (ecn_opt >= TCP_ACCECN_OPTION_PERSIST ||
+		     !tcp_accecn_opt_fail_send(tp)) &&
 		    (ecn_opt >= TCP_ACCECN_OPTION_FULL || tp->accecn_opt_demand ||
 		     tcp_accecn_option_beacon_check(sk))) {
 			opts->use_synack_ecn_bytes = 0;