[TCP]: TCP Compound congestion control

	loss packets.

	See http://www.ntu.edu.sg/home5/ZHOU0022/papers/CPFu03a.pdf

config TCP_CONG_COMPOUND

	tristate "TCP Compound"

	depends on EXPERIMENTAL

	default n

	---help---

	TCP Compound is a sender-side only change to TCP that uses

	a mixed Reno/Vegas approach to calculate the cwnd.

	For further details look here:

	  ftp://ftp.research.microsoft.com/pub/tr/TR-2005-86.pdf

endmenu

config TCP_CONG_BIC

obj-$(CONFIG_TCP_CONG_VENO) += tcp_veno.o

obj-$(CONFIG_TCP_CONG_SCALABLE) += tcp_scalable.o

obj-$(CONFIG_TCP_CONG_LP) += tcp_lp.o

obj-$(CONFIG_TCP_CONG_COMPOUND) += tcp_compound.o

obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \

		      xfrm4_output.o

/*

 * TCP Vegas congestion control

 *

 * This is based on the congestion detection/avoidance scheme described in

 *    Lawrence S. Brakmo and Larry L. Peterson.

 *    "TCP Vegas: End to end congestion avoidance on a global internet."

 *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,

 *    October 1995. Available from:

 *	ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps

 *

 * See http://www.cs.arizona.edu/xkernel/ for their implementation.

 * The main aspects that distinguish this implementation from the

 * Arizona Vegas implementation are:

 *   o We do not change the loss detection or recovery mechanisms of

 *     Linux in any way. Linux already recovers from losses quite well,

 *     using fine-grained timers, NewReno, and FACK.

 *   o To avoid the performance penalty imposed by increasing cwnd

 *     only every-other RTT during slow start, we increase during

 *     every RTT during slow start, just like Reno.

 *   o Largely to allow continuous cwnd growth during slow start,

 *     we use the rate at which ACKs come back as the "actual"

 *     rate, rather than the rate at which data is sent.

 *   o To speed convergence to the right rate, we set the cwnd

 *     to achieve the right ("actual") rate when we exit slow start.

 *   o To filter out the noise caused by delayed ACKs, we use the

 *     minimum RTT sample observed during the last RTT to calculate

 *     the actual rate.

 *   o When the sender re-starts from idle, it waits until it has

 *     received ACKs for an entire flight of new data before making

 *     a cwnd adjustment decision. The original Vegas implementation

 *     assumed senders never went idle.

 *

 *

 *   TCP Compound based on TCP Vegas

 *

 *   further details can be found here:

 *      ftp://ftp.research.microsoft.com/pub/tr/TR-2005-86.pdf

 */

#include <linux/config.h>

#include <linux/mm.h>

#include <linux/module.h>

#include <linux/skbuff.h>

#include <linux/inet_diag.h>

#include <net/tcp.h>

/* Default values of the Vegas variables, in fixed-point representation

 * with V_PARAM_SHIFT bits to the right of the binary point.

 */

#define V_PARAM_SHIFT 1

#define TCP_COMPOUND_ALPHA          3U

#define TCP_COMPOUND_BETA           1U

#define TCP_COMPOUND_KAPPA_POW      3

#define TCP_COMPOUND_KAPPA_NSQRT    2

#define TCP_COMPOUND_GAMMA         30

#define TCP_COMPOUND_ZETA           1

/* TCP compound variables */

struct compound {

	u32 beg_snd_nxt;	/* right edge during last RTT */

	u32 beg_snd_una;	/* left edge  during last RTT */

	u32 beg_snd_cwnd;	/* saves the size of the cwnd */

	u8 doing_vegas_now;	/* if true, do vegas for this RTT */

	u16 cntRTT;		/* # of RTTs measured within last RTT */

	u32 minRTT;		/* min of RTTs measured within last RTT (in usec) */

	u32 baseRTT;		/* the min of all Vegas RTT measurements seen (in usec) */

	u32 cwnd;

	u32 dwnd;

};

/* There are several situations when we must "re-start" Vegas:

 *

 *  o when a connection is established

 *  o after an RTO

 *  o after fast recovery

 *  o when we send a packet and there is no outstanding

 *    unacknowledged data (restarting an idle connection)

 *

 * In these circumstances we cannot do a Vegas calculation at the

 * end of the first RTT, because any calculation we do is using

 * stale info -- both the saved cwnd and congestion feedback are

 * stale.

 *

 * Instead we must wait until the completion of an RTT during

 * which we actually receive ACKs.

 */

static inline void vegas_enable(struct sock *sk)

{

	const struct tcp_sock *tp = tcp_sk(sk);

	struct compound *vegas = inet_csk_ca(sk);

	/* Begin taking Vegas samples next time we send something. */

	vegas->doing_vegas_now = 1;

	/* Set the beginning of the next send window. */

	vegas->beg_snd_nxt = tp->snd_nxt;

	vegas->cntRTT = 0;

	vegas->minRTT = 0x7fffffff;

}

/* Stop taking Vegas samples for now. */

static inline void vegas_disable(struct sock *sk)

{

	struct compound *vegas = inet_csk_ca(sk);

	vegas->doing_vegas_now = 0;

}

static void tcp_compound_init(struct sock *sk)

{

	struct compound *vegas = inet_csk_ca(sk);

	const struct tcp_sock *tp = tcp_sk(sk);

	vegas->baseRTT = 0x7fffffff;

	vegas_enable(sk);

	vegas->dwnd = 0;

	vegas->cwnd = tp->snd_cwnd;

}

/* Do RTT sampling needed for Vegas.

 * Basically we:

 *   o min-filter RTT samples from within an RTT to get the current

 *     propagation delay + queuing delay (we are min-filtering to try to

 *     avoid the effects of delayed ACKs)

 *   o min-filter RTT samples from a much longer window (forever for now)

 *     to find the propagation delay (baseRTT)

 */

static void tcp_compound_rtt_calc(struct sock *sk, u32 usrtt)

{

	struct compound *vegas = inet_csk_ca(sk);

	u32 vrtt = usrtt + 1;	/* Never allow zero rtt or baseRTT */

	/* Filter to find propagation delay: */

	if (vrtt < vegas->baseRTT)

		vegas->baseRTT = vrtt;

	/* Find the min RTT during the last RTT to find

	 * the current prop. delay + queuing delay:

	 */

	vegas->minRTT = min(vegas->minRTT, vrtt);

	vegas->cntRTT++;

}

static void tcp_compound_state(struct sock *sk, u8 ca_state)

{

	if (ca_state == TCP_CA_Open)

		vegas_enable(sk);

	else

		vegas_disable(sk);

}

/*

 * If the connection is idle and we are restarting,

 * then we don't want to do any Vegas calculations

 * until we get fresh RTT samples.  So when we

 * restart, we reset our Vegas state to a clean

 * slate. After we get acks for this flight of

 * packets, _then_ we can make Vegas calculations

 * again.

 */

static void tcp_compound_cwnd_event(struct sock *sk, enum tcp_ca_event event)

{

	if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)

		tcp_compound_init(sk);

}

static void tcp_compound_cong_avoid(struct sock *sk, u32 ack,

				    u32 seq_rtt, u32 in_flight, int flag)

{

	struct tcp_sock *tp = tcp_sk(sk);

	struct compound *vegas = inet_csk_ca(sk);

	u8 inc = 0;

	if (vegas->cwnd + vegas->dwnd > tp->snd_cwnd) {

		if (vegas->cwnd > tp->snd_cwnd || vegas->dwnd > tp->snd_cwnd) {

			vegas->cwnd = tp->snd_cwnd;

			vegas->dwnd = 0;

		} else

			vegas->cwnd = tp->snd_cwnd - vegas->dwnd;

	}

	if (!tcp_is_cwnd_limited(sk, in_flight))

		return;

	if (vegas->cwnd <= tp->snd_ssthresh)

		inc = 1;

	else if (tp->snd_cwnd_cnt < tp->snd_cwnd)

		tp->snd_cwnd_cnt++;

	if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {

		inc = 1;

		tp->snd_cwnd_cnt = 0;

	}

	if (inc && tp->snd_cwnd < tp->snd_cwnd_clamp)

		vegas->cwnd++;

	/* The key players are v_beg_snd_una and v_beg_snd_nxt.

	 *

	 * These are so named because they represent the approximate values

	 * of snd_una and snd_nxt at the beginning of the current RTT. More

	 * precisely, they represent the amount of data sent during the RTT.

	 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,

	 * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding

	 * bytes of data have been ACKed during the course of the RTT, giving

	 * an "actual" rate of:

	 *

	 *     (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)

	 *

	 * Unfortunately, v_beg_snd_una is not exactly equal to snd_una,

	 * because delayed ACKs can cover more than one segment, so they

	 * don't line up nicely with the boundaries of RTTs.

	 *

	 * Another unfortunate fact of life is that delayed ACKs delay the

	 * advance of the left edge of our send window, so that the number

	 * of bytes we send in an RTT is often less than our cwnd will allow.

	 * So we keep track of our cwnd separately, in v_beg_snd_cwnd.

	 */

	if (after(ack, vegas->beg_snd_nxt)) {

		/* Do the Vegas once-per-RTT cwnd adjustment. */

		u32 old_wnd, old_snd_cwnd;

		/* Here old_wnd is essentially the window of data that was

		 * sent during the previous RTT, and has all

		 * been acknowledged in the course of the RTT that ended

		 * with the ACK we just received. Likewise, old_snd_cwnd

		 * is the cwnd during the previous RTT.

		 */

		if (!tp->mss_cache)

			return;

		old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) /

		    tp->mss_cache;

		old_snd_cwnd = vegas->beg_snd_cwnd;

		/* Save the extent of the current window so we can use this

		 * at the end of the next RTT.

		 */

		vegas->beg_snd_una = vegas->beg_snd_nxt;

		vegas->beg_snd_nxt = tp->snd_nxt;

		vegas->beg_snd_cwnd = tp->snd_cwnd;

		/* We do the Vegas calculations only if we got enough RTT

		 * samples that we can be reasonably sure that we got

		 * at least one RTT sample that wasn't from a delayed ACK.

		 * If we only had 2 samples total,

		 * then that means we're getting only 1 ACK per RTT, which

		 * means they're almost certainly delayed ACKs.

		 * If  we have 3 samples, we should be OK.

		 */

		if (vegas->cntRTT > 2) {

			u32 rtt, target_cwnd, diff;

			u32 brtt, dwnd;

			/* We have enough RTT samples, so, using the Vegas

			 * algorithm, we determine if we should increase or

			 * decrease cwnd, and by how much.

			 */

			/* Pluck out the RTT we are using for the Vegas

			 * calculations. This is the min RTT seen during the

			 * last RTT. Taking the min filters out the effects

			 * of delayed ACKs, at the cost of noticing congestion

			 * a bit later.

			 */

			rtt = vegas->minRTT;

			/* Calculate the cwnd we should have, if we weren't

			 * going too fast.

			 *

			 * This is:

			 *     (actual rate in segments) * baseRTT

			 * We keep it as a fixed point number with

			 * V_PARAM_SHIFT bits to the right of the binary point.

			 */

			if (!rtt)

				return;

			brtt = vegas->baseRTT;

			target_cwnd = ((old_wnd * brtt)

				       << V_PARAM_SHIFT) / rtt;

			/* Calculate the difference between the window we had,

			 * and the window we would like to have. This quantity

			 * is the "Diff" from the Arizona Vegas papers.

			 *

			 * Again, this is a fixed point number with

			 * V_PARAM_SHIFT bits to the right of the binary

			 * point.

			 */

			diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;

			dwnd = vegas->dwnd;

			if (diff < (TCP_COMPOUND_GAMMA << V_PARAM_SHIFT)) {

				u32 i, j, x, x2;

				u64 v;

				v = 1;

				for (i = 0; i < TCP_COMPOUND_KAPPA_POW; i++)

					v *= old_wnd;

				for (i = 0; i < TCP_COMPOUND_KAPPA_NSQRT; i++) {

					x = 1;

					for (j = 0; j < 200; j++) {

						x2 = (x + v / x) / 2;

						if (x2 == x || !x2)

							break;

						x = x2;

					}

					v = x;

				}

				x = (u32) v >> TCP_COMPOUND_ALPHA;

				if (x > 1)

					dwnd = x - 1;

				else

					dwnd = 0;

				dwnd += vegas->dwnd;

			} else if ((dwnd << V_PARAM_SHIFT) <

				   (diff * TCP_COMPOUND_BETA))

				dwnd = 0;

			else

				dwnd =

				    ((dwnd << V_PARAM_SHIFT) -

				     (diff *

				      TCP_COMPOUND_BETA)) >> V_PARAM_SHIFT;

			vegas->dwnd = dwnd;

		}

		/* Wipe the slate clean for the next RTT. */

		vegas->cntRTT = 0;

		vegas->minRTT = 0x7fffffff;

	}

	tp->snd_cwnd = vegas->cwnd + vegas->dwnd;

}

/* Extract info for Tcp socket info provided via netlink. */

static void tcp_compound_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)

{

	const struct compound *ca = inet_csk_ca(sk);

	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {

		struct tcpvegas_info *info;

		info = RTA_DATA(__RTA_PUT(skb, INET_DIAG_VEGASINFO,

					  sizeof(*info)));

		info->tcpv_enabled = ca->doing_vegas_now;

		info->tcpv_rttcnt = ca->cntRTT;

		info->tcpv_rtt = ca->baseRTT;

		info->tcpv_minrtt = ca->minRTT;

	rtattr_failure:;

	}

}

static struct tcp_congestion_ops tcp_compound = {

	.init		= tcp_compound_init,

	.ssthresh	= tcp_reno_ssthresh,

	.cong_avoid	= tcp_compound_cong_avoid,

	.min_cwnd	= tcp_reno_min_cwnd,

	.rtt_sample	= tcp_compound_rtt_calc,

	.set_state	= tcp_compound_state,

	.cwnd_event	= tcp_compound_cwnd_event,

	.get_info	= tcp_compound_get_info,

	.owner		= THIS_MODULE,

	.name		= "compound",

};

static int __init tcp_compound_register(void)

{

	BUG_ON(sizeof(struct compound) > ICSK_CA_PRIV_SIZE);

	tcp_register_congestion_control(&tcp_compound);

	return 0;

}

static void __exit tcp_compound_unregister(void)

{

	tcp_unregister_congestion_control(&tcp_compound);

}

module_init(tcp_compound_register);

module_exit(tcp_compound_unregister);

MODULE_AUTHOR("Angelo P. Castellani, Stephen Hemminger");

MODULE_LICENSE("GPL");

MODULE_DESCRIPTION("TCP Compound");