No empty .Rs/.Re

[netbsd-mini2440.git] / sys / netinet / tcp_congctl.c

/*      $NetBSD: tcp_congctl.c,v 1.14 2008/02/29 07:39:17 matt Exp $    */

/*-
 * Copyright (c) 1997, 1998, 1999, 2001, 2005, 2006 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Jason R. Thorpe and Kevin M. Lahey of the Numerical Aerospace Simulation
 * Facility, NASA Ames Research Center.
 * This code is derived from software contributed to The NetBSD Foundation
 * by Charles M. Hannum.
 * This code is derived from software contributed to The NetBSD Foundation
 * by Rui Paulo.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the project nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 *      @(#)COPYRIGHT   1.1 (NRL) 17 January 1995
 *
 * NRL grants permission for redistribution and use in source and binary
 * forms, with or without modification, of the software and documentation
 * created at NRL provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgements:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 *      This product includes software developed at the Information
 *      Technology Division, US Naval Research Laboratory.
 * 4. Neither the name of the NRL nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THE SOFTWARE PROVIDED BY NRL IS PROVIDED BY NRL AND CONTRIBUTORS ``AS
 * IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
 * PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL NRL OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * The views and conclusions contained in the software and documentation
 * are those of the authors and should not be interpreted as representing
 * official policies, either expressed or implied, of the US Naval
 * Research Laboratory (NRL).
 */

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994, 1995
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *      @(#)tcp_input.c 8.12 (Berkeley) 5/24/95
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: tcp_congctl.c,v 1.14 2008/02/29 07:39:17 matt Exp $");

#include "opt_inet.h"
#include "opt_tcp_debug.h"
#include "opt_tcp_congctl.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/errno.h>
#include <sys/syslog.h>
#include <sys/pool.h>
#include <sys/domain.h>
#include <sys/kernel.h>
#include <sys/mutex.h>

#include <net/if.h>
#include <net/route.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_pcb.h>
#include <netinet/in_var.h>
#include <netinet/ip_var.h>

#ifdef INET6
#ifndef INET
#include <netinet/in.h>
#endif
#include <netinet/ip6.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_var.h>
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#endif

#include <netinet/tcp.h>
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcpip.h>
#include <netinet/tcp_congctl.h>
#ifdef TCP_DEBUG
#include <netinet/tcp_debug.h>
#endif

/*
 * TODO:
 *   consider separating the actual implementations in another file.
 */

static int  tcp_reno_fast_retransmit(struct tcpcb *, const struct tcphdr *);
static void tcp_reno_slow_retransmit(struct tcpcb *);
static void tcp_reno_fast_retransmit_newack(struct tcpcb *,
    const struct tcphdr *);
static void tcp_reno_newack(struct tcpcb *, const struct tcphdr *);
static void tcp_reno_congestion_exp(struct tcpcb *tp);

static int  tcp_newreno_fast_retransmit(struct tcpcb *, const struct tcphdr *);
static void tcp_newreno_fast_retransmit_newack(struct tcpcb *,
        const struct tcphdr *);
static void tcp_newreno_newack(struct tcpcb *, const struct tcphdr *);


static void tcp_congctl_fillnames(void);

extern int tcprexmtthresh;

MALLOC_DEFINE(M_TCPCONGCTL, "tcpcongctl", "TCP congestion control structures");

/* currently selected global congestion control */
char tcp_congctl_global_name[TCPCC_MAXLEN];

/* available global congestion control algorithms */
char tcp_congctl_avail[10 * TCPCC_MAXLEN];

/*
 * Used to list the available congestion control algorithms.
 */
TAILQ_HEAD(, tcp_congctlent) tcp_congctlhd =
    TAILQ_HEAD_INITIALIZER(tcp_congctlhd);

static struct tcp_congctlent * tcp_congctl_global;

static kmutex_t tcp_congctl_mtx;

void
tcp_congctl_init(void)
{
        int r;
        
        mutex_init(&tcp_congctl_mtx, MUTEX_DEFAULT, IPL_NONE);

        /* Base algorithms. */
        r = tcp_congctl_register("reno", &tcp_reno_ctl);
        KASSERT(r == 0);
        r = tcp_congctl_register("newreno", &tcp_newreno_ctl);
        KASSERT(r == 0);

        /* NewReno is the default. */
#ifndef TCP_CONGCTL_DEFAULT
#define TCP_CONGCTL_DEFAULT "newreno"
#endif

        r = tcp_congctl_select(NULL, TCP_CONGCTL_DEFAULT);
        KASSERT(r == 0);
}

/*
 * Register a congestion algorithm and select it if we have none.
 */
int
tcp_congctl_register(const char *name, const struct tcp_congctl *tcc)
{
        struct tcp_congctlent *ntcc, *tccp;

        TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent) 
                if (!strcmp(name, tccp->congctl_name)) {
                        /* name already registered */
                        return EEXIST;
                }

        ntcc = malloc(sizeof(*ntcc), M_TCPCONGCTL, M_WAITOK|M_ZERO);

        strlcpy(ntcc->congctl_name, name, sizeof(ntcc->congctl_name) - 1);
        ntcc->congctl_ctl = tcc;

        TAILQ_INSERT_TAIL(&tcp_congctlhd, ntcc, congctl_ent);
        tcp_congctl_fillnames();

        if (TAILQ_FIRST(&tcp_congctlhd) == ntcc)
                tcp_congctl_select(NULL, name);
                
        return 0;
}

int
tcp_congctl_unregister(const char *name)
{
        struct tcp_congctlent *tccp, *rtccp;
        unsigned int size;
        
        rtccp = NULL;
        size = 0;
        TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent) {
                if (!strcmp(name, tccp->congctl_name))
                        rtccp = tccp;
                size++;
        }
        
        if (!rtccp)
                return ENOENT;

        if (size <= 1 || tcp_congctl_global == rtccp || rtccp->congctl_refcnt)
                return EBUSY;

        TAILQ_REMOVE(&tcp_congctlhd, rtccp, congctl_ent);
        free(rtccp, M_TCPCONGCTL);
        tcp_congctl_fillnames();

        return 0;
}

/*
 * Select a congestion algorithm by name.
 */
int
tcp_congctl_select(struct tcpcb *tp, const char *name)
{
        struct tcp_congctlent *tccp, *old_tccp, *new_tccp;
        bool old_found, new_found;

        KASSERT(name);

        old_found = (tp == NULL || tp->t_congctl == NULL);
        old_tccp = NULL;
        new_found = false;
        new_tccp = NULL;

        TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent) {
                if (!old_found && tccp->congctl_ctl == tp->t_congctl) {
                        old_tccp = tccp;
                        old_found = true;
                }

                if (!new_found && !strcmp(name, tccp->congctl_name)) {
                        new_tccp = tccp;
                        new_found = true;
                }

                if (new_found && old_found) {
                        if (tp) {
                                mutex_enter(&tcp_congctl_mtx);
                                if (old_tccp)
                                        old_tccp->congctl_refcnt--;
                                tp->t_congctl = new_tccp->congctl_ctl;
                                new_tccp->congctl_refcnt++;
                                mutex_exit(&tcp_congctl_mtx);
                        } else {
                                tcp_congctl_global = new_tccp;
                                strlcpy(tcp_congctl_global_name,
                                    new_tccp->congctl_name,
                                    sizeof(tcp_congctl_global_name) - 1);
                        }
                        return 0;
                }
        }

        return EINVAL;
}

void
tcp_congctl_release(struct tcpcb *tp)
{
        struct tcp_congctlent *tccp;

        KASSERT(tp->t_congctl);
        
        TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent) {
                if (tccp->congctl_ctl == tp->t_congctl) {
                        tccp->congctl_refcnt--;
                        return;
                }
        }
}

/*
 * Returns the name of a congestion algorithm.
 */
const char *
tcp_congctl_bystruct(const struct tcp_congctl *tcc)
{
        struct tcp_congctlent *tccp;
        
        KASSERT(tcc);
        
        TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent)
                if (tccp->congctl_ctl == tcc)
                        return tccp->congctl_name;

        return NULL;
}

static void
tcp_congctl_fillnames(void)
{
        struct tcp_congctlent *tccp;
        const char *delim = " ";
        
        tcp_congctl_avail[0] = '\0';
        TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent) {
                strlcat(tcp_congctl_avail, tccp->congctl_name,
                    sizeof(tcp_congctl_avail) - 1);
                if (TAILQ_NEXT(tccp, congctl_ent))
                        strlcat(tcp_congctl_avail, delim, 
                            sizeof(tcp_congctl_avail) - 1);
        }       
        
}

/* ------------------------------------------------------------------------ */

/*
 * TCP/Reno congestion control.
 */
static void
tcp_reno_congestion_exp(struct tcpcb *tp)
{
        u_int win;

        /* 
         * Halve the congestion window and reduce the
         * slow start threshold.
         */
        win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_segsz;
        if (win < 2)
                win = 2;

        tp->snd_ssthresh = win * tp->t_segsz;
        tp->snd_recover = tp->snd_max;
        tp->snd_cwnd = tp->snd_ssthresh;

        /*
         * When using TCP ECN, notify the peer that
         * we reduced the cwnd.
         */
        if (TCP_ECN_ALLOWED(tp))
                tp->t_flags |= TF_ECN_SND_CWR;
}



static int
tcp_reno_fast_retransmit(struct tcpcb *tp, const struct tcphdr *th)
{
        /*
         * We know we're losing at the current
         * window size so do congestion avoidance
         * (set ssthresh to half the current window
         * and pull our congestion window back to
         * the new ssthresh).
         *
         * Dup acks mean that packets have left the
         * network (they're now cached at the receiver)
         * so bump cwnd by the amount in the receiver
         * to keep a constant cwnd packets in the
         * network.
         *
         * If we are using TCP/SACK, then enter
         * Fast Recovery if the receiver SACKs
         * data that is tcprexmtthresh * MSS
         * bytes past the last ACKed segment,
         * irrespective of the number of DupAcks.
         */
        
        tcp_seq onxt;
        
        onxt = tp->snd_nxt;
        tcp_reno_congestion_exp(tp);
        tp->t_partialacks = 0;
        TCP_TIMER_DISARM(tp, TCPT_REXMT);
        tp->t_rtttime = 0;
        if (TCP_SACK_ENABLED(tp)) {
                tp->t_dupacks = tcprexmtthresh;
                tp->sack_newdata = tp->snd_nxt;
                tp->snd_cwnd = tp->t_segsz;
                (void) tcp_output(tp);
                return 0;
        }
        tp->snd_nxt = th->th_ack;
        tp->snd_cwnd = tp->t_segsz;
        (void) tcp_output(tp);
        tp->snd_cwnd = tp->snd_ssthresh + tp->t_segsz * tp->t_dupacks;
        if (SEQ_GT(onxt, tp->snd_nxt))
                tp->snd_nxt = onxt;
        
        return 0;
}

static void
tcp_reno_slow_retransmit(struct tcpcb *tp)
{
        u_int win;

        /*
         * Close the congestion window down to one segment
         * (we'll open it by one segment for each ack we get).
         * Since we probably have a window's worth of unacked
         * data accumulated, this "slow start" keeps us from
         * dumping all that data as back-to-back packets (which
         * might overwhelm an intermediate gateway).
         *
         * There are two phases to the opening: Initially we
         * open by one mss on each ack.  This makes the window
         * size increase exponentially with time.  If the
         * window is larger than the path can handle, this
         * exponential growth results in dropped packet(s)
         * almost immediately.  To get more time between
         * drops but still "push" the network to take advantage
         * of improving conditions, we switch from exponential
         * to linear window opening at some threshhold size.
         * For a threshhold, we use half the current window
         * size, truncated to a multiple of the mss.
         *
         * (the minimum cwnd that will give us exponential
         * growth is 2 mss.  We don't allow the threshhold
         * to go below this.)
         */

        win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_segsz;
        if (win < 2)
                win = 2;
        /* Loss Window MUST be one segment. */
        tp->snd_cwnd = tp->t_segsz;
        tp->snd_ssthresh = win * tp->t_segsz;
        tp->t_partialacks = -1;
        tp->t_dupacks = 0;
        tp->t_bytes_acked = 0;
}

static void
tcp_reno_fast_retransmit_newack(struct tcpcb *tp,
    const struct tcphdr *th)
{
        if (tp->t_partialacks < 0) {
                /*
                 * We were not in fast recovery.  Reset the duplicate ack
                 * counter.
                 */
                tp->t_dupacks = 0;
        } else {
                /*
                 * Clamp the congestion window to the crossover point and
                 * exit fast recovery.
                 */
                if (tp->snd_cwnd > tp->snd_ssthresh)
                        tp->snd_cwnd = tp->snd_ssthresh;
                tp->t_partialacks = -1;
                tp->t_dupacks = 0;
                tp->t_bytes_acked = 0;
        }
}

static void
tcp_reno_newack(struct tcpcb *tp, const struct tcphdr *th)
{
        /*
         * When new data is acked, open the congestion window.
         */

        u_int cw = tp->snd_cwnd;
        u_int incr = tp->t_segsz;

        if (tcp_do_abc) {

                /*
                 * RFC 3465 Appropriate Byte Counting (ABC)
                 */

                int acked = th->th_ack - tp->snd_una;

                if (cw >= tp->snd_ssthresh) {
                        tp->t_bytes_acked += acked;
                        if (tp->t_bytes_acked >= cw) {
                                /* Time to increase the window. */
                                tp->t_bytes_acked -= cw;
                        } else {
                                /* No need to increase yet. */
                                incr = 0;
                        }
                } else {
                        /*
                         * use 2*SMSS or 1*SMSS for the "L" param,
                         * depending on sysctl setting.
                         *
                         * (See RFC 3465 2.3 Choosing the Limit)
                         */
                        u_int abc_lim;

                        abc_lim = (tcp_abc_aggressive == 0 ||
                            tp->snd_nxt != tp->snd_max) ? incr : incr * 2;
                        incr = min(acked, abc_lim);
                }
        } else {

                /*
                 * If the window gives us less than ssthresh packets
                 * in flight, open exponentially (segsz per packet).
                 * Otherwise open linearly: segsz per window
                 * (segsz^2 / cwnd per packet).
                 */

                if (cw >= tp->snd_ssthresh) {
                        incr = incr * incr / cw;
                }
        }

        tp->snd_cwnd = min(cw + incr, TCP_MAXWIN << tp->snd_scale);
}

const struct tcp_congctl tcp_reno_ctl = {
        .fast_retransmit = tcp_reno_fast_retransmit,
        .slow_retransmit = tcp_reno_slow_retransmit,
        .fast_retransmit_newack = tcp_reno_fast_retransmit_newack,
        .newack = tcp_reno_newack,
        .cong_exp = tcp_reno_congestion_exp,
};

/*
 * TCP/NewReno Congestion control.
 */
static int
tcp_newreno_fast_retransmit(struct tcpcb *tp, const struct tcphdr *th)
{
        if (SEQ_LT(th->th_ack, tp->snd_high)) {
                /*
                 * False fast retransmit after timeout.
                 * Do not enter fast recovery
                 */
                tp->t_dupacks = 0;
                return 1;
        } else {
                /*
                 * Fast retransmit is same as reno.
                 */
                return tcp_reno_fast_retransmit(tp, th);
        }

        return 0;
}

/*
 * Implement the NewReno response to a new ack, checking for partial acks in
 * fast recovery.
 */
static void
tcp_newreno_fast_retransmit_newack(struct tcpcb *tp, const struct tcphdr *th)
{
        if (tp->t_partialacks < 0) {
                /*
                 * We were not in fast recovery.  Reset the duplicate ack
                 * counter.
                 */
                tp->t_dupacks = 0;
        } else if (SEQ_LT(th->th_ack, tp->snd_recover)) {
                /*
                 * This is a partial ack.  Retransmit the first unacknowledged
                 * segment and deflate the congestion window by the amount of
                 * acknowledged data.  Do not exit fast recovery.
                 */
                tcp_seq onxt = tp->snd_nxt;
                u_long ocwnd = tp->snd_cwnd;

                /*
                 * snd_una has not yet been updated and the socket's send
                 * buffer has not yet drained off the ACK'd data, so we
                 * have to leave snd_una as it was to get the correct data
                 * offset in tcp_output().
                 */
                if (++tp->t_partialacks == 1)
                        TCP_TIMER_DISARM(tp, TCPT_REXMT);
                tp->t_rtttime = 0;
                tp->snd_nxt = th->th_ack;
                /*
                 * Set snd_cwnd to one segment beyond ACK'd offset.  snd_una
                 * is not yet updated when we're called.
                 */
                tp->snd_cwnd = tp->t_segsz + (th->th_ack - tp->snd_una);
                (void) tcp_output(tp);
                tp->snd_cwnd = ocwnd;
                if (SEQ_GT(onxt, tp->snd_nxt))
                        tp->snd_nxt = onxt;
                /*
                 * Partial window deflation.  Relies on fact that tp->snd_una
                 * not updated yet.
                 */
                tp->snd_cwnd -= (th->th_ack - tp->snd_una - tp->t_segsz);
        } else {
                /*
                 * Complete ack.  Inflate the congestion window to ssthresh
                 * and exit fast recovery.
                 *
                 * Window inflation should have left us with approx.
                 * snd_ssthresh outstanding data.  But in case we
                 * would be inclined to send a burst, better to do
                 * it via the slow start mechanism.
                 */
                if (SEQ_SUB(tp->snd_max, th->th_ack) < tp->snd_ssthresh)
                        tp->snd_cwnd = SEQ_SUB(tp->snd_max, th->th_ack)
                            + tp->t_segsz;
                else
                        tp->snd_cwnd = tp->snd_ssthresh;
                tp->t_partialacks = -1;
                tp->t_dupacks = 0;
                tp->t_bytes_acked = 0;
        }
}

static void
tcp_newreno_newack(struct tcpcb *tp, const struct tcphdr *th)
{
        /*
         * If we are still in fast recovery (meaning we are using
         * NewReno and we have only received partial acks), do not
         * inflate the window yet.
         */
        if (tp->t_partialacks < 0)
                tcp_reno_newack(tp, th);
}


const struct tcp_congctl tcp_newreno_ctl = {
        .fast_retransmit = tcp_newreno_fast_retransmit,
        .slow_retransmit = tcp_reno_slow_retransmit,
        .fast_retransmit_newack = tcp_newreno_fast_retransmit_newack,
        .newack = tcp_newreno_newack,
        .cong_exp = tcp_reno_congestion_exp,
};