FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_congctl.c

     /*      $NetBSD: tcp_congctl.c,v 1.12 2006/11/16 01:33:45 christos 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.
     * 3. All advertising materials mentioning features or use of this software
     *    must display the following acknowledgement:
     *      This product includes software developed by the NetBSD
     *      Foundation, Inc. and its contributors.
     * 4. Neither the name of The NetBSD Foundation 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 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 1.12 2006/11/16 01:33:45 christos 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/lock.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");
   
   /*
    * Used to list the available congestion control algorithms.
    */
   struct tcp_congctlent {
           TAILQ_ENTRY(tcp_congctlent) congctl_ent;
           char               congctl_name[TCPCC_MAXLEN];
           struct tcp_congctl *congctl_ctl;
   };
   TAILQ_HEAD(, tcp_congctlent) tcp_congctlhd;
   
   struct simplelock tcp_congctl_slock;
   
   void
   tcp_congctl_init(void)
   {
           int r;
           
           TAILQ_INIT(&tcp_congctlhd);
           simple_lock_init(&tcp_congctl_slock);
   
           /* 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, 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);
   
           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->congctl_ctl ||
               rtccp->congctl_ctl->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;
   
           KASSERT(name);
   
           TAILQ_FOREACH(tccp, &tcp_congctlhd, congctl_ent)
                   if (!strcmp(name, tccp->congctl_name)) {
                           if (tp) {
                                   simple_lock(&tcp_congctl_slock);
                                   tp->t_congctl->refcnt--;
                                   tp->t_congctl = tccp->congctl_ctl;
                                   tp->t_congctl->refcnt++;
                                   simple_unlock(&tcp_congctl_slock);
                           } else {
                                   tcp_congctl_global = tccp->congctl_ctl;
                                   strlcpy(tcp_congctl_global_name,
                                       tccp->congctl_name,
                                       sizeof(tcp_congctl_global_name) - 1);
                           }
                           return 0;
                   }
           
           return EINVAL;
   }
   
   /*
    * 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);
   }
   
   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);
   }
   
   
   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,
   };
   
   

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