tcp: remove Appropriate Byte Count support

TCP Appropriate Byte Count was added by me, but later disabled.
There is no point in maintaining it since it is a potential source
of bugs and Linux already implements other better window protection
heuristics.

Signed-off-by: Stephen Hemminger <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/ip-sysctl.txt

···
 	Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
 	for TCP sockets.
 
-tcp_abc - INTEGER
-	Controls Appropriate Byte Count (ABC) defined in RFC3465.
-	ABC is a way of increasing congestion window (cwnd) more slowly
-	in response to partial acknowledgments.
-	Possible values are:
-		0 increase cwnd once per acknowledgment (no ABC)
-		1 increase cwnd once per acknowledgment of full sized segment
-		2 allow increase cwnd by two if acknowledgment is
-		  of two segments to compensate for delayed acknowledgments.
-	Default: 0 (off)
-
 tcp_abort_on_overflow - BOOLEAN
 	If listening service is too slow to accept new connections,
 	reset them. Default state is FALSE. It means that if overflow

include/linux/tcp.h

···
 	u32	sacked_out;	/* SACK'd packets			*/
 	u32	fackets_out;	/* FACK'd packets			*/
 	u32	tso_deferred;
-	u32	bytes_acked;	/* Appropriate Byte Counting - RFC3465 */
 
 	/* from STCP, retrans queue hinting */
 	struct sk_buff* lost_skb_hint;

include/net/tcp.h

···
 extern int sysctl_tcp_nometrics_save;
 extern int sysctl_tcp_moderate_rcvbuf;
 extern int sysctl_tcp_tso_win_divisor;
-extern int sysctl_tcp_abc;
 extern int sysctl_tcp_mtu_probing;
 extern int sysctl_tcp_base_mss;
 extern int sysctl_tcp_workaround_signed_windows;

kernel/sysctl_binary.c

···
 	{ CTL_INT,	NET_TCP_MODERATE_RCVBUF,		"tcp_moderate_rcvbuf" },
 	{ CTL_INT,	NET_TCP_TSO_WIN_DIVISOR,		"tcp_tso_win_divisor" },
 	{ CTL_STR,	NET_TCP_CONG_CONTROL,			"tcp_congestion_control" },
-	{ CTL_INT,	NET_TCP_ABC,				"tcp_abc" },
 	{ CTL_INT,	NET_TCP_MTU_PROBING,			"tcp_mtu_probing" },
 	{ CTL_INT,	NET_TCP_BASE_MSS,			"tcp_base_mss" },
 	{ CTL_INT,	NET_IPV4_TCP_WORKAROUND_SIGNED_WINDOWS,	"tcp_workaround_signed_windows" },

net/ipv4/sysctl_net_ipv4.c

···
 		.proc_handler	= proc_tcp_congestion_control,
 	},
 	{
-		.procname	= "tcp_abc",
-		.data		= &sysctl_tcp_abc,
-		.maxlen		= sizeof(int),
-		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
-	},
-	{
 		.procname	= "tcp_mtu_probing",
 		.data		= &sysctl_tcp_mtu_probing,
 		.maxlen		= sizeof(int),

net/ipv4/tcp.c

···
 	tp->packets_out = 0;
 	tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	tp->snd_cwnd_cnt = 0;
-	tp->bytes_acked = 0;
 	tp->window_clamp = 0;
 	tcp_set_ca_state(sk, TCP_CA_Open);
 	tcp_clear_retrans(tp);

net/ipv4/tcp_cong.c

···
 		snd_cwnd = 1U;
 	}
 
-	/* RFC3465: ABC Slow start
-	 * Increase only after a full MSS of bytes is acked
-	 *
-	 * TCP sender SHOULD increase cwnd by the number of
-	 * previously unacknowledged bytes ACKed by each incoming
-	 * acknowledgment, provided the increase is not more than L
-	 */
-	if (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
-		return;
-
 	if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
 		cnt = sysctl_tcp_max_ssthresh >> 1;	/* limited slow start */
 	else
 		cnt = snd_cwnd;				/* exponential increase */
-
-	/* RFC3465: ABC
-	 * We MAY increase by 2 if discovered delayed ack
-	 */
-	if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
-		cnt <<= 1;
-	tp->bytes_acked = 0;
 
 	tp->snd_cwnd_cnt += cnt;
 	while (tp->snd_cwnd_cnt >= snd_cwnd) {
···
 	/* In "safe" area, increase. */
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
 		tcp_slow_start(tp);
-
 	/* In dangerous area, increase slowly. */
-	else if (sysctl_tcp_abc) {
-		/* RFC3465: Appropriate Byte Count
-		 * increase once for each full cwnd acked
-		 */
-		if (tp->bytes_acked >= tp->snd_cwnd*tp->mss_cache) {
-			tp->bytes_acked -= tp->snd_cwnd*tp->mss_cache;
-			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
-				tp->snd_cwnd++;
-		}
-	} else {
+	else
 		tcp_cong_avoid_ai(tp, tp->snd_cwnd);
-	}
 }
 EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);
 

net/ipv4/tcp_input.c

···
 int sysctl_tcp_thin_dupack __read_mostly;
 
 int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
-int sysctl_tcp_abc __read_mostly;
 int sysctl_tcp_early_retrans __read_mostly = 2;
 
 #define FLAG_DATA		0x01 /* Incoming frame contained data.		*/
···
 	tp->snd_cwnd_cnt = 0;
 	tp->snd_cwnd_stamp = tcp_time_stamp;
 	tp->frto_counter = 0;
-	tp->bytes_acked = 0;
 
 	tp->reordering = min_t(unsigned int, tp->reordering,
 			       sysctl_tcp_reordering);
···
 	tp->snd_cwnd_cnt   = 0;
 	tp->snd_cwnd_stamp = tcp_time_stamp;
 
-	tp->bytes_acked = 0;
 	tcp_clear_retrans_partial(tp);
 
 	if (tcp_is_reno(tp))
···
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	tp->high_seq = tp->snd_nxt;
-	tp->bytes_acked = 0;
 	tp->snd_cwnd_cnt = 0;
 	tp->prior_cwnd = tp->snd_cwnd;
 	tp->prr_delivered = 0;
···
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	tp->prior_ssthresh = 0;
-	tp->bytes_acked = 0;
 	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
 		tp->undo_marker = 0;
 		tcp_init_cwnd_reduction(sk, set_ssthresh);
···
 {
 	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
 	tp->snd_cwnd_cnt = 0;
-	tp->bytes_acked = 0;
 	TCP_ECN_queue_cwr(tp);
 	tcp_moderate_cwnd(tp);
 }
···
 
 	if (after(ack, prior_snd_una))
 		flag |= FLAG_SND_UNA_ADVANCED;
-
-	if (sysctl_tcp_abc) {
-		if (icsk->icsk_ca_state < TCP_CA_CWR)
-			tp->bytes_acked += ack - prior_snd_una;
-		else if (icsk->icsk_ca_state == TCP_CA_Loss)
-			/* we assume just one segment left network */
-			tp->bytes_acked += min(ack - prior_snd_una,
-					       tp->mss_cache);
-	}
 
 	prior_fackets = tp->fackets_out;
 	prior_in_flight = tcp_packets_in_flight(tp);

net/ipv4/tcp_minisocks.c

···
 		 */
 		newtp->snd_cwnd = TCP_INIT_CWND;
 		newtp->snd_cwnd_cnt = 0;
-		newtp->bytes_acked = 0;
 
 		newtp->frto_counter = 0;
 		newtp->frto_highmark = 0;