net/ipv4/tcp_vegas.c - linux - Git at Google

 /*
  * 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.
  */

 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/inet_diag.h>

 #include <net/tcp.h>

 #include "tcp_vegas.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
 static int alpha = 2<<V_PARAM_SHIFT;
 static int beta  = 4<<V_PARAM_SHIFT;
 static int gamma = 1<<V_PARAM_SHIFT;

 module_param(alpha, int, 0644);
 MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)");
 module_param(beta, int, 0644);
 MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)");
 module_param(gamma, int, 0644);
 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");


 /* 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 void vegas_enable(struct sock *sk)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct vegas *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 vegas *vegas = inet_csk_ca(sk);

 	vegas->doing_vegas_now = 0;
 }

 void tcp_vegas_init(struct sock *sk)
 {
 	struct vegas *vegas = inet_csk_ca(sk);

 	vegas->baseRTT = 0x7fffffff;
 	vegas_enable(sk);
 }
 EXPORT_SYMBOL_GPL(tcp_vegas_init);

 /* 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)
  */
 void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 {
 	struct vegas *vegas = inet_csk_ca(sk);
 	u32 vrtt;

 	if (rtt_us < 0)
 		return;

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

 	/* 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++;
 }
 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);

 void tcp_vegas_state(struct sock *sk, u8 ca_state)
 {

 	if (ca_state == TCP_CA_Open)
 		vegas_enable(sk);
 	else
 		vegas_disable(sk);
 }
 EXPORT_SYMBOL_GPL(tcp_vegas_state);

 /*
  * 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.
  */
 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 {
 	if (event == CA_EVENT_CWND_RESTART ||
 	    event == CA_EVENT_TX_START)
 		tcp_vegas_init(sk);
 }
 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);

 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct vegas *vegas = inet_csk_ca(sk);

 	if (!vegas->doing_vegas_now)
 		return tcp_reno_cong_avoid(sk, ack, in_flight);

 	/* 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.
 		 */
 		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) {
 			/* We don't have enough RTT samples to do the Vegas
 			 * calculation, so we'll behave like Reno.
 			 */
 			tcp_reno_cong_avoid(sk, ack, in_flight);
 		} else {
 			u32 rtt, target_cwnd, diff;

 			/* 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.
 			 */
 			target_cwnd = ((old_wnd * vegas->baseRTT)
 				       << 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;

 			if (diff > gamma && tp->snd_ssthresh > 2 ) {
 				/* Going too fast. Time to slow down
 				 * and switch to congestion avoidance.
 				 */
 				tp->snd_ssthresh = 2;

 				/* Set cwnd to match the actual rate
 				 * exactly:
 				 *   cwnd = (actual rate) * baseRTT
 				 * Then we add 1 because the integer
 				 * truncation robs us of full link
 				 * utilization.
 				 */
 				tp->snd_cwnd = min(tp->snd_cwnd,
 						   (target_cwnd >>
 						    V_PARAM_SHIFT)+1);

 			} else if (tp->snd_cwnd <= tp->snd_ssthresh) {
 				/* Slow start.  */
 				tcp_slow_start(tp);
 			} else {
 				/* Congestion avoidance. */
 				u32 next_snd_cwnd;

 				/* Figure out where we would like cwnd
 				 * to be.
 				 */
 				if (diff > beta) {
 					/* The old window was too fast, so
 					 * we slow down.
 					 */
 					next_snd_cwnd = old_snd_cwnd - 1;
 				} else if (diff < alpha) {
 					/* We don't have enough extra packets
 					 * in the network, so speed up.
 					 */
 					next_snd_cwnd = old_snd_cwnd + 1;
 				} else {
 					/* Sending just as fast as we
 					 * should be.
 					 */
 					next_snd_cwnd = old_snd_cwnd;
 				}

 				/* Adjust cwnd upward or downward, toward the
 				 * desired value.
 				 */
 				if (next_snd_cwnd > tp->snd_cwnd)
 					tp->snd_cwnd++;
 				else if (next_snd_cwnd < tp->snd_cwnd)
 					tp->snd_cwnd--;
 			}

 			if (tp->snd_cwnd < 2)
 				tp->snd_cwnd = 2;
 			else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
 				tp->snd_cwnd = tp->snd_cwnd_clamp;
 		}

 		/* Wipe the slate clean for the next RTT. */
 		vegas->cntRTT = 0;
 		vegas->minRTT = 0x7fffffff;
 	}
 	/* Use normal slow start */
 	else if (tp->snd_cwnd <= tp->snd_ssthresh)
 		tcp_slow_start(tp);

 }

 /* Extract info for Tcp socket info provided via netlink. */
 void tcp_vegas_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
 {
 	const struct vegas *ca = inet_csk_ca(sk);
 	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 		struct tcpvegas_info info = {
 			.tcpv_enabled = ca->doing_vegas_now,
 			.tcpv_rttcnt = ca->cntRTT,
 			.tcpv_rtt = ca->baseRTT,
 			.tcpv_minrtt = ca->minRTT,
 		};

 		nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
 	}
 }
 EXPORT_SYMBOL_GPL(tcp_vegas_get_info);

 static struct tcp_congestion_ops tcp_vegas = {
 	.flags		= TCP_CONG_RTT_STAMP,
 	.init		= tcp_vegas_init,
 	.ssthresh	= tcp_reno_ssthresh,
 	.cong_avoid	= tcp_vegas_cong_avoid,
 	.min_cwnd	= tcp_reno_min_cwnd,
 	.pkts_acked	= tcp_vegas_pkts_acked,
 	.set_state	= tcp_vegas_state,
 	.cwnd_event	= tcp_vegas_cwnd_event,
 	.get_info	= tcp_vegas_get_info,

 	.owner		= THIS_MODULE,
 	.name		= "vegas",
 };

 static int __init tcp_vegas_register(void)
 {
 	BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
 	tcp_register_congestion_control(&tcp_vegas);
 	return 0;
 }

 static void __exit tcp_vegas_unregister(void)
 {
 	tcp_unregister_congestion_control(&tcp_vegas);
 }

 module_init(tcp_vegas_register);
 module_exit(tcp_vegas_unregister);

 MODULE_AUTHOR("Stephen Hemminger");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("TCP Vegas");
	/*
	* 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.
	*/

	#include <linux/mm.h>
	#include <linux/module.h>
	#include <linux/skbuff.h>
	#include <linux/inet_diag.h>

	#include <net/tcp.h>

	#include "tcp_vegas.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
	static int alpha = 2<<V_PARAM_SHIFT;
	static int beta = 4<<V_PARAM_SHIFT;
	static int gamma = 1<<V_PARAM_SHIFT;

	module_param(alpha, int, 0644);
	MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)");
	module_param(beta, int, 0644);
	MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)");
	module_param(gamma, int, 0644);
	MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");


	/* 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 void vegas_enable(struct sock *sk)
	{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct vegas *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 vegas *vegas = inet_csk_ca(sk);

	vegas->doing_vegas_now = 0;
	}

	void tcp_vegas_init(struct sock *sk)
	{
	struct vegas *vegas = inet_csk_ca(sk);

	vegas->baseRTT = 0x7fffffff;
	vegas_enable(sk);
	}
	EXPORT_SYMBOL_GPL(tcp_vegas_init);

	/* 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)
	*/
	void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us)
	{
	struct vegas *vegas = inet_csk_ca(sk);
	u32 vrtt;

	if (rtt_us < 0)
	return;

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

	/* 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++;
	}
	EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);

	void tcp_vegas_state(struct sock *sk, u8 ca_state)
	{

	if (ca_state == TCP_CA_Open)
	vegas_enable(sk);
	else
	vegas_disable(sk);
	}
	EXPORT_SYMBOL_GPL(tcp_vegas_state);

	/*
	* 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.
	*/
	void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
	{
	if (event == CA_EVENT_CWND_RESTART \|\|
	event == CA_EVENT_TX_START)
	tcp_vegas_init(sk);
	}
	EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);

	static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
	{
	struct tcp_sock *tp = tcp_sk(sk);
	struct vegas *vegas = inet_csk_ca(sk);

	if (!vegas->doing_vegas_now)
	return tcp_reno_cong_avoid(sk, ack, in_flight);

	/* 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.
	*/
	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) {
	/* We don't have enough RTT samples to do the Vegas
	* calculation, so we'll behave like Reno.
	*/
	tcp_reno_cong_avoid(sk, ack, in_flight);
	} else {
	u32 rtt, target_cwnd, diff;

	/* 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.
	*/
	target_cwnd = ((old_wnd * vegas->baseRTT)
	<< 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;

	if (diff > gamma && tp->snd_ssthresh > 2 ) {
	/* Going too fast. Time to slow down
	* and switch to congestion avoidance.
	*/
	tp->snd_ssthresh = 2;

	/* Set cwnd to match the actual rate
	* exactly:
	* cwnd = (actual rate) * baseRTT
	* Then we add 1 because the integer
	* truncation robs us of full link
	* utilization.
	*/
	tp->snd_cwnd = min(tp->snd_cwnd,
	(target_cwnd >>
	V_PARAM_SHIFT)+1);

	} else if (tp->snd_cwnd <= tp->snd_ssthresh) {
	/* Slow start. */
	tcp_slow_start(tp);
	} else {
	/* Congestion avoidance. */
	u32 next_snd_cwnd;

	/* Figure out where we would like cwnd
	* to be.
	*/
	if (diff > beta) {
	/* The old window was too fast, so
	* we slow down.
	*/
	next_snd_cwnd = old_snd_cwnd - 1;
	} else if (diff < alpha) {
	/* We don't have enough extra packets
	* in the network, so speed up.
	*/
	next_snd_cwnd = old_snd_cwnd + 1;
	} else {
	/* Sending just as fast as we
	* should be.
	*/
	next_snd_cwnd = old_snd_cwnd;
	}

	/* Adjust cwnd upward or downward, toward the
	* desired value.
	*/
	if (next_snd_cwnd > tp->snd_cwnd)
	tp->snd_cwnd++;
	else if (next_snd_cwnd < tp->snd_cwnd)
	tp->snd_cwnd--;
	}

	if (tp->snd_cwnd < 2)
	tp->snd_cwnd = 2;
	else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
	tp->snd_cwnd = tp->snd_cwnd_clamp;
	}

	/* Wipe the slate clean for the next RTT. */
	vegas->cntRTT = 0;
	vegas->minRTT = 0x7fffffff;
	}
	/* Use normal slow start */
	else if (tp->snd_cwnd <= tp->snd_ssthresh)
	tcp_slow_start(tp);

	}

	/* Extract info for Tcp socket info provided via netlink. */
	void tcp_vegas_get_info(struct sock sk, u32 ext, struct sk_buff skb)
	{
	const struct vegas *ca = inet_csk_ca(sk);
	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
	struct tcpvegas_info info = {
	.tcpv_enabled = ca->doing_vegas_now,
	.tcpv_rttcnt = ca->cntRTT,
	.tcpv_rtt = ca->baseRTT,
	.tcpv_minrtt = ca->minRTT,
	};

	nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
	}
	}
	EXPORT_SYMBOL_GPL(tcp_vegas_get_info);

	static struct tcp_congestion_ops tcp_vegas = {
	.flags = TCP_CONG_RTT_STAMP,
	.init = tcp_vegas_init,
	.ssthresh = tcp_reno_ssthresh,
	.cong_avoid = tcp_vegas_cong_avoid,
	.min_cwnd = tcp_reno_min_cwnd,
	.pkts_acked = tcp_vegas_pkts_acked,
	.set_state = tcp_vegas_state,
	.cwnd_event = tcp_vegas_cwnd_event,
	.get_info = tcp_vegas_get_info,

	.owner = THIS_MODULE,
	.name = "vegas",
	};

	static int __init tcp_vegas_register(void)
	{
	BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
	tcp_register_congestion_control(&tcp_vegas);
	return 0;
	}

	static void __exit tcp_vegas_unregister(void)
	{
	tcp_unregister_congestion_control(&tcp_vegas);
	}

	module_init(tcp_vegas_register);
	module_exit(tcp_vegas_unregister);

	MODULE_AUTHOR("Stephen Hemminger");
	MODULE_LICENSE("GPL");
	MODULE_DESCRIPTION("TCP Vegas");