tcp: retire FACK loss detection

FACK loss detection has been disabled by default and the
successor RACK subsumed FACK and can handle reordering better.
This patch removes FACK to simplify TCP loss recovery.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -289,8 +289,7 @@ tcp_ecn_fallback - BOOLEAN
 	Default: 1 (fallback enabled)
 
 tcp_fack - BOOLEAN
-	Enable FACK congestion avoidance and fast retransmission.
-	The value is not used, if tcp_sack is not enabled.
+	This is a legacy option, it has no effect anymore.
 
 tcp_fin_timeout - INTEGER
 	The length of time an orphaned (no longer referenced by any

include/linux/tcp.h

@@ -85,7 +85,6 @@ struct tcp_sack_block {
 
 /*These are used to set the sack_ok field in struct tcp_options_received */
 #define TCP_SACK_SEEN     (1 << 0)   /*1 = peer is SACK capable, */
-#define TCP_FACK_ENABLED  (1 << 1)   /*1 = FACK is enabled locally*/
 #define TCP_DSACK_SEEN    (1 << 2)   /*1 = DSACK was received from peer*/
 
 struct tcp_options_received {

include/net/tcp.h

@@ -384,7 +384,6 @@ void tcp_update_metrics(struct sock *sk);
 void tcp_init_metrics(struct sock *sk);
 void tcp_metrics_init(void);
 bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst);
-void tcp_disable_fack(struct tcp_sock *tp);
 void tcp_close(struct sock *sk, long timeout);
 void tcp_init_sock(struct sock *sk);
 void tcp_init_transfer(struct sock *sk, int bpf_op);
@@ -776,7 +775,7 @@ struct tcp_skb_cb {
 	};
 	__u8		tcp_flags;	/* TCP header flags. (tcp[13])	*/
 
-	__u8		sacked;		/* State flags for SACK/FACK.	*/
+	__u8		sacked;		/* State flags for SACK.	*/
 #define TCPCB_SACKED_ACKED	0x01	/* SKB ACK'd by a SACK block	*/
 #define TCPCB_SACKED_RETRANS	0x02	/* SKB retransmitted		*/
 #define TCPCB_LOST		0x04	/* SKB is lost			*/
@@ -1066,7 +1065,6 @@ void tcp_rate_check_app_limited(struct sock *sk);
  *
  * tcp_is_sack - SACK enabled
  * tcp_is_reno - No SACK
- * tcp_is_fack - FACK enabled, implies SACK enabled
  */
 static inline int tcp_is_sack(const struct tcp_sock *tp)
 {
@@ -1078,16 +1076,6 @@ static inline bool tcp_is_reno(const struct tcp_sock *tp)
 	return !tcp_is_sack(tp);
 }
 
-static inline bool tcp_is_fack(const struct tcp_sock *tp)
-{
-	return tp->rx_opt.sack_ok & TCP_FACK_ENABLED;
-}
-
-static inline void tcp_enable_fack(struct tcp_sock *tp)
-{
-	tp->rx_opt.sack_ok |= TCP_FACK_ENABLED;
-}
-
 static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
 {
 	return tp->sacked_out + tp->lost_out;

include/uapi/linux/snmp.h

@@ -191,7 +191,6 @@ enum
 	LINUX_MIB_TCPRENORECOVERY,		/* TCPRenoRecovery */
 	LINUX_MIB_TCPSACKRECOVERY,		/* TCPSackRecovery */
 	LINUX_MIB_TCPSACKRENEGING,		/* TCPSACKReneging */
-	LINUX_MIB_TCPFACKREORDER,		/* TCPFACKReorder */
 	LINUX_MIB_TCPSACKREORDER,		/* TCPSACKReorder */
 	LINUX_MIB_TCPRENOREORDER,		/* TCPRenoReorder */
 	LINUX_MIB_TCPTSREORDER,			/* TCPTSReorder */

net/ipv4/proc.c

@@ -212,7 +212,6 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPRenoRecovery", LINUX_MIB_TCPRENORECOVERY),
 	SNMP_MIB_ITEM("TCPSackRecovery", LINUX_MIB_TCPSACKRECOVERY),
 	SNMP_MIB_ITEM("TCPSACKReneging", LINUX_MIB_TCPSACKRENEGING),
-	SNMP_MIB_ITEM("TCPFACKReorder", LINUX_MIB_TCPFACKREORDER),
 	SNMP_MIB_ITEM("TCPSACKReorder", LINUX_MIB_TCPSACKREORDER),
 	SNMP_MIB_ITEM("TCPRenoReorder", LINUX_MIB_TCPRENOREORDER),
 	SNMP_MIB_ITEM("TCPTSReorder", LINUX_MIB_TCPTSREORDER),

net/ipv4/tcp.c

@@ -2509,8 +2509,6 @@ static int tcp_repair_options_est(struct sock *sk,
 				return -EINVAL;
 
 			tp->rx_opt.sack_ok |= TCP_SACK_SEEN;
-			if (sock_net(sk)->ipv4.sysctl_tcp_fack)
-				tcp_enable_fack(tp);
 			break;
 		case TCPOPT_TIMESTAMP:
 			if (opt.opt_val != 0)

net/ipv4/tcp_input.c

@@ -842,18 +842,6 @@ __u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
 	return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
 }
 
-/*
- * Packet counting of FACK is based on in-order assumptions, therefore TCP
- * disables it when reordering is detected
- */
-void tcp_disable_fack(struct tcp_sock *tp)
-{
-	/* RFC3517 uses different metric in lost marker => reset on change */
-	if (tcp_is_fack(tp))
-		tp->lost_skb_hint = NULL;
-	tp->rx_opt.sack_ok &= ~TCP_FACK_ENABLED;
-}
-
 /* Take a notice that peer is sending D-SACKs */
 static void tcp_dsack_seen(struct tcp_sock *tp)
 {
@@ -881,7 +869,6 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
 			 tp->sacked_out,
 			 tp->undo_marker ? tp->undo_retrans : 0);
 #endif
-		tcp_disable_fack(tp);
 	}
 
 	tp->rack.reord = 1;
@@ -891,8 +878,6 @@ static void tcp_update_reordering(struct sock *sk, const int metric,
 		mib_idx = LINUX_MIB_TCPTSREORDER;
 	else if (tcp_is_reno(tp))
 		mib_idx = LINUX_MIB_TCPRENOREORDER;
-	else if (tcp_is_fack(tp))
-		mib_idx = LINUX_MIB_TCPFACKREORDER;
 	else
 		mib_idx = LINUX_MIB_TCPSACKREORDER;
 
@@ -970,7 +955,6 @@ void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb)
  * 3. Loss detection event of two flavors:
  *	A. Scoreboard estimator decided the packet is lost.
  *	   A'. Reno "three dupacks" marks head of queue lost.
- *	   A''. Its FACK modification, head until snd.fack is lost.
  *	B. SACK arrives sacking SND.NXT at the moment, when the
  *	   segment was retransmitted.
  * 4. D-SACK added new rule: D-SACK changes any tag to S.
@@ -1248,7 +1232,7 @@ static u8 tcp_sacktag_one(struct sock *sk,
 		fack_count += pcount;
 
 		/* Lost marker hint past SACKed? Tweak RFC3517 cnt */
-		if (!tcp_is_fack(tp) && tp->lost_skb_hint &&
+		if (tp->lost_skb_hint &&
 		    before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
 			tp->lost_cnt_hint += pcount;
 
@@ -2051,10 +2035,6 @@ static inline int tcp_fackets_out(const struct tcp_sock *tp)
  * counter when SACK is enabled (without SACK, sacked_out is used for
  * that purpose).
  *
- * Instead, with FACK TCP uses fackets_out that includes both SACKed
- * segments up to the highest received SACK block so far and holes in
- * between them.
- *
  * With reordering, holes may still be in flight, so RFC3517 recovery
  * uses pure sacked_out (total number of SACKed segments) even though
  * it violates the RFC that uses duplicate ACKs, often these are equal
@@ -2064,10 +2044,10 @@ static inline int tcp_fackets_out(const struct tcp_sock *tp)
  */
 static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
 {
-	return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
+	return tp->sacked_out + 1;
 }
 
-/* Linux NewReno/SACK/FACK/ECN state machine.
+/* Linux NewReno/SACK/ECN state machine.
  * --------------------------------------
  *
  * "Open"	Normal state, no dubious events, fast path.
@@ -2132,16 +2112,6 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
  *		dynamically measured and adjusted. This is implemented in
  *		tcp_rack_mark_lost.
  *
- *		FACK (Disabled by default. Subsumbed by RACK):
- *		It is the simplest heuristics. As soon as we decided
- *		that something is lost, we decide that _all_ not SACKed
- *		packets until the most forward SACK are lost. I.e.
- *		lost_out = fackets_out - sacked_out and left_out = fackets_out.
- *		It is absolutely correct estimate, if network does not reorder
- *		packets. And it loses any connection to reality when reordering
- *		takes place. We use FACK by default until reordering
- *		is suspected on the path to this destination.
- *
  *		If the receiver does not support SACK:
  *
  *		NewReno (RFC6582): in Recovery we assume that one segment
@@ -2190,7 +2160,7 @@ static bool tcp_time_to_recover(struct sock *sk, int flag)
 }
 
 /* Detect loss in event "A" above by marking head of queue up as lost.
- * For FACK or non-SACK(Reno) senders, the first "packets" number of segments
+ * For non-SACK(Reno) senders, the first "packets" number of segments
  * are considered lost. For RFC3517 SACK, a segment is considered lost if it
  * has at least tp->reordering SACKed seqments above it; "packets" refers to
  * the maximum SACKed segments to pass before reaching this limit.
@@ -2226,12 +2196,12 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
 			break;
 
 		oldcnt = cnt;
-		if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
+		if (tcp_is_reno(tp) ||
 		    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
 			cnt += tcp_skb_pcount(skb);
 
 		if (cnt > packets) {
-			if ((tcp_is_sack(tp) && !tcp_is_fack(tp)) ||
+			if (tcp_is_sack(tp) ||
 			    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
 			    (oldcnt >= packets))
 				break;
@@ -2262,11 +2232,6 @@ static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
 
 	if (tcp_is_reno(tp)) {
 		tcp_mark_head_lost(sk, 1, 1);
-	} else if (tcp_is_fack(tp)) {
-		int lost = tp->fackets_out - tp->reordering;
-		if (lost <= 0)
-			lost = 1;
-		tcp_mark_head_lost(sk, lost, 0);
 	} else {
 		int sacked_upto = tp->sacked_out - tp->reordering;
 		if (sacked_upto >= 0)
@@ -3199,8 +3164,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
 			if (reord < prior_fackets && reord <= tp->fackets_out)
 				tcp_update_reordering(sk, tp->fackets_out - reord, 0);
 
-			delta = tcp_is_fack(tp) ? pkts_acked :
-						  prior_sacked - tp->sacked_out;
+			delta = prior_sacked - tp->sacked_out;
 			tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
 		}
 
@@ -5708,9 +5672,6 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 			tp->tcp_header_len = sizeof(struct tcphdr);
 		}
 
-		if (tcp_is_sack(tp) && sock_net(sk)->ipv4.sysctl_tcp_fack)
-			tcp_enable_fack(tp);
-
 		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		tcp_initialize_rcv_mss(sk);
 

net/ipv4/tcp_metrics.c

@@ -470,10 +470,8 @@ void tcp_init_metrics(struct sock *sk)
 		tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	}
 	val = tcp_metric_get(tm, TCP_METRIC_REORDERING);
-	if (val && tp->reordering != val) {
-		tcp_disable_fack(tp);
+	if (val && tp->reordering != val)
 		tp->reordering = val;
-	}
 
 	crtt = tcp_metric_get(tm, TCP_METRIC_RTT);
 	rcu_read_unlock();

net/ipv4/tcp_minisocks.c

@@ -509,10 +509,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 						       keepalive_time_when(newtp));
 
 		newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
-		if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
-			if (sock_net(sk)->ipv4.sysctl_tcp_fack)
-				tcp_enable_fack(newtp);
-		}
+		newtp->rx_opt.sack_ok = ireq->sack_ok;
 		newtp->window_clamp = req->rsk_window_clamp;
 		newtp->rcv_ssthresh = req->rsk_rcv_wnd;
 		newtp->rcv_wnd = req->rsk_rcv_wnd;

net/ipv4/tcp_output.c

@@ -1257,7 +1257,7 @@ static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int de
 
 	if (tp->lost_skb_hint &&
 	    before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
-	    (tcp_is_fack(tp) || (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)))
+	    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
 		tp->lost_cnt_hint -= decr;
 
 	tcp_verify_left_out(tp);
@@ -2961,9 +2961,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
  * retransmitted data is acknowledged.  It tries to continue
  * resending the rest of the retransmit queue, until either
  * we've sent it all or the congestion window limit is reached.
- * If doing SACK, the first ACK which comes back for a timeout
- * based retransmit packet might feed us FACK information again.
- * If so, we use it to avoid unnecessarily retransmissions.
  */
 void tcp_xmit_retransmit_queue(struct sock *sk)
 {