net/ipv4/tcp_compound.c - kernel/msm - Gitiles

 /*
  * 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_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);
 }


 /* 64bit divisor, dividend and result. dynamic precision */
 static inline u64 div64_64(u64 dividend, u64 divisor)
 {
 	u32 d = divisor;

 	if (divisor > 0xffffffffULL) {
 		unsigned int shift = fls(divisor >> 32);

 		d = divisor >> shift;
 		dividend >>= shift;
 	}

 	/* avoid 64 bit division if possible */
 	if (dividend >> 32)
 		do_div(dividend, d);
 	else
 		dividend = (u32) dividend / d;

 	return dividend;
 }

 /* calculate the quartic root of "a" using Newton-Raphson */
 static u32 qroot(u64 a)
 {
 	u32 x, x1;

 	/* Initial estimate is based on:
 	 * qrt(x) = exp(log(x) / 4)
 	 */
 	x = 1u << (fls64(a) >> 2);

 	/*
 	 * Iteration based on:
 	 *                         3
 	 * x    = ( 3 * x  +  a / x  ) / 4
 	 *  k+1          k         k
 	 */
 	do {
 		u64 x3 = x;

 		x1 = x;
 		x3 *= x;
 		x3 *= x;

 		x = (3 * x + (u32) div64_64(a, x3)) / 4;
 	} while (abs(x1 - x) > 1);

 	return x;
 }


 /*
  * 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)) {
 				u64 v;
 				u32 x;

 				/*
 				 * The TCP Compound paper describes the choice
 				 * of "k" determines the agressiveness,
 				 * ie. slope of the response function.
 				 *
 				 * For same value as HSTCP would be 0.8
 				 * but for computaional reasons, both the
 				 * original authors and this implementation
 				 * use 0.75.
 				 */
 				v = old_wnd;
 				x = qroot(v * v * 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,
 	.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");
	/*
	* 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_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);
	}


	/* 64bit divisor, dividend and result. dynamic precision */
	static inline u64 div64_64(u64 dividend, u64 divisor)
	{
	u32 d = divisor;

	if (divisor > 0xffffffffULL) {
	unsigned int shift = fls(divisor >> 32);

	d = divisor >> shift;
	dividend >>= shift;
	}

	/* avoid 64 bit division if possible */
	if (dividend >> 32)
	do_div(dividend, d);
	else
	dividend = (u32) dividend / d;

	return dividend;
	}

	/* calculate the quartic root of "a" using Newton-Raphson */
	static u32 qroot(u64 a)
	{
	u32 x, x1;

	/* Initial estimate is based on:
	* qrt(x) = exp(log(x) / 4)
	*/
	x = 1u << (fls64(a) >> 2);

	/*
	* Iteration based on:
	* 3
	* x = ( 3 * x + a / x ) / 4
	* k+1 k k
	*/
	do {
	u64 x3 = x;

	x1 = x;
	x3 *= x;
	x3 *= x;

	x = (3 * x + (u32) div64_64(a, x3)) / 4;
	} while (abs(x1 - x) > 1);

	return x;
	}


	/*
	* 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)) {
	u64 v;
	u32 x;

	/*
	* The TCP Compound paper describes the choice
	* of "k" determines the agressiveness,
	* ie. slope of the response function.
	*
	* For same value as HSTCP would be 0.8
	* but for computaional reasons, both the
	* original authors and this implementation
	* use 0.75.
	*/
	v = old_wnd;
	x = qroot(v * v * 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,
	.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");