source: rtems/cpukit/libnetworking/netinet/tcp_subr.c @ 657e6c93

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993, 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.
 * 4. 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_subr.c  8.2 (Berkeley) 5/24/95
 * $FreeBSD: src/sys/netinet/tcp_subr.c,v 1.226 2005/05/07 00:41:36 cperciva Exp $
 */
 

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "opt_tcpdebug.h"

#include <sys/param.h>
#include <sys/queue.h>
#include <sys/proc.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/protosw.h>
#include <errno.h>

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

#define _IP_VHL
#include <netinet/in.h>
#include <rtems/rtems_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>
#include <netinet/ip_icmp.h>
#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>
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif

int     tcp_mssdflt = TCP_MSS;
SYSCTL_INT(_net_inet_tcp, TCPCTL_MSSDFLT, mssdflt, CTLFLAG_RW,
    &tcp_mssdflt , 0, "Default TCP Maximum Segment Size");

static int      tcp_do_rfc1323 = 1;
#if !defined(__rtems__)
static int      tcp_rttdflt = TCPTV_SRTTDFLT / PR_SLOWHZ;
SYSCTL_INT(_net_inet_tcp, TCPCTL_RTTDFLT, rttdflt,
        CTLFLAG_RW, &tcp_rttdflt , 0, "");

SYSCTL_INT(_net_inet_tcp, TCPCTL_DO_RFC1323, rfc1323,
        CTLFLAG_RW, &tcp_do_rfc1323 , 0, "");
#endif

static void     tcp_notify(struct inpcb *, int);

/*
 * Target size of TCP PCB hash table. Will be rounded down to a prime
 * number.
 */
#ifndef TCBHASHSIZE
#define TCBHASHSIZE     128
#endif

/*
 * Tcp initialization
 */
void
tcp_init(void)
{

        tcp_iss = random();     /* wrong, but better than a constant */
        tcp_ccgen = 1;
        LIST_INIT(&tcb);
        tcbinfo.listhead = &tcb;
        tcbinfo.hashbase = hashinit(TCBHASHSIZE, M_PCB, &tcbinfo.hashmask);
        if (max_protohdr < sizeof(struct tcpiphdr))
                max_protohdr = sizeof(struct tcpiphdr);
        if (max_linkhdr + sizeof(struct tcpiphdr) > MHLEN)
                panic("tcp_init");
}

/*
 * Create template to be used to send tcp packets on a connection.
 * Call after host entry created, allocates an mbuf and fills
 * in a skeletal tcp/ip header, minimizing the amount of work
 * necessary when the connection is used.
 */
struct tcpiphdr *
tcp_template(struct tcpcb *tp)
{
        register struct inpcb *inp = tp->t_inpcb;
        register struct mbuf *m;
        register struct tcpiphdr *n;

        if ((n = tp->t_template) == 0) {
                m = m_get(M_DONTWAIT, MT_HEADER);
                if (m == NULL)
                        return (0);
                m->m_len = sizeof (struct tcpiphdr);
                n = mtod(m, struct tcpiphdr *);
        }
        n->ti_next = n->ti_prev = 0;
        n->ti_x1 = 0;
        n->ti_pr = IPPROTO_TCP;
        n->ti_len = htons(sizeof (struct tcpiphdr) - sizeof (struct ip));
        n->ti_src = inp->inp_laddr;
        n->ti_dst = inp->inp_faddr;
        n->ti_sport = inp->inp_lport;
        n->ti_dport = inp->inp_fport;
        n->ti_seq = 0;
        n->ti_ack = 0;
        n->ti_x2 = 0;
        n->ti_off = 5;
        n->ti_flags = 0;
        n->ti_win = 0;
        n->ti_sum = 0;
        n->ti_urp = 0;
        return (n);
}

/*
 * Send a single message to the TCP at address specified by
 * the given TCP/IP header.  If m == 0, then we make a copy
 * of the tcpiphdr at ti and send directly to the addressed host.
 * This is used to force keep alive messages out using the TCP
 * template for a connection tp->t_template.  If flags are given
 * then we send a message back to the TCP which originated the
 * segment ti, and discard the mbuf containing it and any other
 * attached mbufs.
 *
 * In any case the ack and sequence number of the transmitted
 * segment are as specified by the parameters.
 *
 * NOTE: If m != NULL, then ti must point to *inside* the mbuf.
 */
void
tcp_respond(struct tcpcb *tp, struct tcpiphdr *ti, struct mbuf *m,
    tcp_seq ack, tcp_seq seq, int flags)
{
        register int tlen;
        int win = 0;
        struct route *ro = 0;
        struct route sro;

        if (tp) {
                win = sbspace(&tp->t_inpcb->inp_socket->so_rcv);
                ro = &tp->t_inpcb->inp_route;
        } else {
                ro = &sro;
                bzero(ro, sizeof *ro);
        }
        if (m == NULL) {
                m = m_gethdr(M_DONTWAIT, MT_HEADER);
                if (m == NULL)
                        return;
#ifdef TCP_COMPAT_42
                tlen = 1;
#else
                tlen = 0;
#endif
                m->m_data += max_linkhdr;
                *mtod(m, struct tcpiphdr *) = *ti;
                ti = mtod(m, struct tcpiphdr *);
                flags = TH_ACK;
        } else {
                m_freem(m->m_next);
                m->m_next = NULL;
                m->m_data = (caddr_t)ti;
                m->m_len = sizeof (struct tcpiphdr);
                tlen = 0;
#define xchg(a,b,type) { type t; t=a; a=b; b=t; }
                xchg(ti->ti_dst.s_addr, ti->ti_src.s_addr, u_long);
                xchg(ti->ti_dport, ti->ti_sport, u_short);
#undef xchg
        }
        ti->ti_len = htons((u_short)(sizeof (struct tcphdr) + tlen));
        tlen += sizeof (struct tcpiphdr);
        m->m_len = tlen;
        m->m_pkthdr.len = tlen;
        m->m_pkthdr.rcvif = (struct ifnet *) 0;
        ti->ti_next = ti->ti_prev = 0;
        ti->ti_x1 = 0;
        ti->ti_seq = htonl(seq);
        ti->ti_ack = htonl(ack);
        ti->ti_x2 = 0;
        ti->ti_off = sizeof (struct tcphdr) >> 2;
        ti->ti_flags = flags;
        if (tp)
                ti->ti_win = htons((u_short) (win >> tp->rcv_scale));
        else
                ti->ti_win = htons((u_short)win);
        ti->ti_urp = 0;
        ti->ti_sum = 0;
        ti->ti_sum = in_cksum(m, tlen);
        ((struct ip *)ti)->ip_len = tlen;
        ((struct ip *)ti)->ip_ttl = ip_defttl;
#ifdef TCPDEBUG
        if (tp == NULL || (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
                tcp_trace(TA_OUTPUT, 0, tp, ti, 0);
#endif
        (void) ip_output(m, NULL, ro, 0, NULL);
        if (ro == &sro && ro->ro_rt) {
                RTFREE(ro->ro_rt);
        }
}

/*
 * Create a new TCP control block, making an
 * empty reassembly queue and hooking it to the argument
 * protocol control block.
 */
struct tcpcb *
tcp_newtcpcb(struct inpcb *inp)
{
        struct tcpcb *tp;

        tp = malloc(sizeof(*tp), M_PCB, M_NOWAIT);
        if (tp == NULL)
                return ((struct tcpcb *)0);
        bzero((char *) tp, sizeof(struct tcpcb));
        tp->seg_next = tp->seg_prev = (struct tcpiphdr *)tp;
        tp->t_maxseg = tp->t_maxopd = tcp_mssdflt;

        if (tcp_do_rfc1323)
                tp->t_flags = (TF_REQ_SCALE|TF_REQ_TSTMP);
        tp->t_inpcb = inp;
        /*
         * Init srtt to TCPTV_SRTTBASE (0), so we can tell that we have no
         * rtt estimate.  Set rttvar so that srtt + 4 * rttvar gives
         * reasonable initial retransmit time.
         */
        tp->t_srtt = TCPTV_SRTTBASE;
        tp->t_rttvar = ((TCPTV_RTOBASE - TCPTV_SRTTBASE) << TCP_RTTVAR_SHIFT) / 4;
        tp->t_rttmin = TCPTV_MIN;
        tp->t_rxtcur = TCPTV_RTOBASE;
        tp->snd_cwnd = TCP_MAXWIN << TCP_MAX_WINSHIFT;
        tp->snd_ssthresh = TCP_MAXWIN << TCP_MAX_WINSHIFT;
        inp->inp_ip_ttl = ip_defttl;
        inp->inp_ppcb = (caddr_t)tp;
        return (tp);
}

/*
 * Drop a TCP connection, reporting
 * the specified error.  If connection is synchronized,
 * then send a RST to peer.
 */
struct tcpcb *
tcp_drop(struct tcpcb *tp, int errnum)
{
        struct socket *so = tp->t_inpcb->inp_socket;

        if (TCPS_HAVERCVDSYN(tp->t_state)) {
                tp->t_state = TCPS_CLOSED;
                (void) tcp_output(tp);
                tcpstat.tcps_drops++;
        } else
                tcpstat.tcps_conndrops++;
        if (errnum == ETIMEDOUT && tp->t_softerror)
                errnum = tp->t_softerror;
        so->so_error = errnum;
        return (tcp_close(tp));
}

/*
 * Close a TCP control block:
 *      discard all space held by the tcp
 *      discard internet protocol block
 *      wake up any sleepers
 */
struct tcpcb *
tcp_close(struct tcpcb *tp)
{
        register struct tcpiphdr *t;
        struct inpcb *inp = tp->t_inpcb;
        struct socket *so = inp->inp_socket;
        register struct mbuf *m;
        register struct rtentry *rt;

        /*
         * If we got enough samples through the srtt filter,
         * save the rtt and rttvar in the routing entry.
         * 'Enough' is arbitrarily defined as the 16 samples.
         * 16 samples is enough for the srtt filter to converge
         * to within 5% of the correct value; fewer samples and
         * we could save a very bogus rtt.
         *
         * Don't update the default route's characteristics and don't
         * update anything that the user "locked".
         */
        if (tp->t_rttupdated >= 16 &&
            (rt = inp->inp_route.ro_rt) &&
            ((struct sockaddr_in *)rt_key(rt))->sin_addr.s_addr != INADDR_ANY) {
                register u_long i = 0;

                if ((rt->rt_rmx.rmx_locks & RTV_RTT) == 0) {
                        i = tp->t_srtt *
                            (RTM_RTTUNIT / (PR_SLOWHZ * TCP_RTT_SCALE));
                        if (rt->rt_rmx.rmx_rtt && i)
                                /*
                                 * filter this update to half the old & half
                                 * the new values, converting scale.
                                 * See route.h and tcp_var.h for a
                                 * description of the scaling constants.
                                 */
                                rt->rt_rmx.rmx_rtt =
                                    (rt->rt_rmx.rmx_rtt + i) / 2;
                        else
                                rt->rt_rmx.rmx_rtt = i;
                        tcpstat.tcps_cachedrtt++;
                }
                if ((rt->rt_rmx.rmx_locks & RTV_RTTVAR) == 0) {
                        i = tp->t_rttvar *
                            (RTM_RTTUNIT / (PR_SLOWHZ * TCP_RTTVAR_SCALE));
                        if (rt->rt_rmx.rmx_rttvar && i)
                                rt->rt_rmx.rmx_rttvar =
                                    (rt->rt_rmx.rmx_rttvar + i) / 2;
                        else
                                rt->rt_rmx.rmx_rttvar = i;
                        tcpstat.tcps_cachedrttvar++;
                }
                /*
                 * update the pipelimit (ssthresh) if it has been updated
                 * already or if a pipesize was specified & the threshhold
                 * got below half the pipesize.  I.e., wait for bad news
                 * before we start updating, then update on both good
                 * and bad news.
                 */
                if (((rt->rt_rmx.rmx_locks & RTV_SSTHRESH) == 0 &&
                    ((i = tp->snd_ssthresh) != 0) && rt->rt_rmx.rmx_ssthresh) ||
                    i < (rt->rt_rmx.rmx_sendpipe / 2)) {
                        /*
                         * convert the limit from user data bytes to
                         * packets then to packet data bytes.
                         */
                        i = (i + tp->t_maxseg / 2) / tp->t_maxseg;
                        if (i < 2)
                                i = 2;
                        i *= (u_long)(tp->t_maxseg + sizeof (struct tcpiphdr));
                        if (rt->rt_rmx.rmx_ssthresh)
                                rt->rt_rmx.rmx_ssthresh =
                                    (rt->rt_rmx.rmx_ssthresh + i) / 2;
                        else
                                rt->rt_rmx.rmx_ssthresh = i;
                        tcpstat.tcps_cachedssthresh++;
                }
        }
        /* free the reassembly queue, if any */
        t = tp->seg_next;
        while (t != (struct tcpiphdr *)tp) {
                t = (struct tcpiphdr *)t->ti_next;
#if (defined(__GNUC__) && (defined(__arm__) || defined(__mips__)))
        LD32_UNALGN((struct tcpiphdr *)t->ti_prev,m);
#else
                m = REASS_MBUF((struct tcpiphdr *)t->ti_prev);
#endif
                remque(t->ti_prev);
                m_freem(m);
        }
        if (tp->t_template)
                (void) m_free(dtom(tp->t_template));
        free(tp, M_PCB);
        inp->inp_ppcb = 0;
        soisdisconnected(so);
        in_pcbdetach(inp);
        tcpstat.tcps_closed++;
        return ((struct tcpcb *)0);
}

void
tcp_drain(void)
{

}

/*
 * Notify a tcp user of an asynchronous error;
 * store error as soft error, but wake up user
 * (for now, won't do anything until can select for soft error).
 */
static void
tcp_notify(struct inpcb *inp, int error)
{
        struct tcpcb *tp = (struct tcpcb *)inp->inp_ppcb;
        struct socket *so = inp->inp_socket;

        /*
         * Ignore some errors if we are hooked up.
         * If connection hasn't completed, has retransmitted several times,
         * and receives a second error, give up now.  This is better
         * than waiting a long time to establish a connection that
         * can never complete.
         */
        if (tp->t_state == TCPS_ESTABLISHED &&
            (error == EHOSTUNREACH || error == ENETUNREACH ||
             error == EHOSTDOWN)) {
                return;
        } else if (tp->t_state < TCPS_ESTABLISHED && tp->t_rxtshift > 3 &&
            tp->t_softerror)
                so->so_error = error;
        else
                tp->t_softerror = error;
        soconnwakeup (so);
        sorwakeup(so);
        sowwakeup(so);
}

#ifdef __rtems__
#define INP_INFO_RLOCK(a)
#define INP_INFO_RUNLOCK(a)
#define INP_LOCK(a)
#define INP_UNLOCK(a)
#endif

static int
tcp_pcblist(SYSCTL_HANDLER_ARGS)
{
        int error, i, n, s;
        struct inpcb *inp, **inp_list;
        inp_gen_t gencnt;
        struct xinpgen xig;

        /*
         * The process of preparing the TCB list is too time-consuming and
         * resource-intensive to repeat twice on every request.
         */
        if (req->oldptr == NULL) {
                n = tcbinfo.ipi_count;
                req->oldidx = 2 * (sizeof xig)
                        + (n + n/8) * sizeof(struct xtcpcb);
                return (0);
        }

        if (req->newptr != NULL)
                return (EPERM);

        /*
         * OK, now we're committed to doing something.
         */
        s = splnet();
        INP_INFO_RLOCK(&tcbinfo);
        gencnt = tcbinfo.ipi_gencnt;
        n = tcbinfo.ipi_count;
        INP_INFO_RUNLOCK(&tcbinfo);
        splx(s);

        sysctl_wire_old_buffer(req, 2 * (sizeof xig)
                + n * sizeof(struct xtcpcb));

        xig.xig_len = sizeof xig;
        xig.xig_count = n;
        xig.xig_gen = gencnt;
/*      xig.xig_sogen = so_gencnt; remove by ccj */
        error = SYSCTL_OUT(req, &xig, sizeof xig);
        if (error)
                return error;

        /* ccj add exit if the count is 0 */
        if (!n)
                return error;
  
        inp_list = malloc(n * sizeof *inp_list, M_TEMP, M_WAITOK);
        if (inp_list == 0)
                return ENOMEM;
        
        s = splnet();
        INP_INFO_RLOCK(&tcbinfo);
        for (inp = LIST_FIRST(tcbinfo.listhead), i = 0; inp && i < n;
             inp = LIST_NEXT(inp, inp_list)) {
                INP_LOCK(inp);
                if (inp->inp_gencnt <= gencnt)
#if 0
      &&
                    cr_canseesocket(req->td->td_ucred, inp->inp_socket) == 0)
#endif
                        inp_list[i++] = inp;
                INP_UNLOCK(inp);
        }
        INP_INFO_RUNLOCK(&tcbinfo);
        splx(s);
        n = i;

        error = 0;
        for (i = 0; i < n; i++) {
                inp = inp_list[i];
                INP_LOCK(inp);
                if (inp->inp_gencnt <= gencnt) {
                        struct xtcpcb xt;
                        caddr_t inp_ppcb;
                        xt.xt_len = sizeof xt;
                        /* XXX should avoid extra copy */
                        bcopy(inp, &xt.xt_inp, sizeof *inp);
                        inp_ppcb = inp->inp_ppcb;
                        if (inp_ppcb != NULL)
                                bcopy(inp_ppcb, &xt.xt_tp, sizeof xt.xt_tp);
                        else
                                bzero((char *) &xt.xt_tp, sizeof xt.xt_tp);
#if 0      
                        if (inp->inp_socket)
                                sotoxsocket(inp->inp_socket, &xt.xt_socket);
#endif
                        error = SYSCTL_OUT(req, &xt, sizeof xt);
                }
                INP_UNLOCK(inp);
        }
        if (!error) {
                /*
                 * Give the user an updated idea of our state.
                 * If the generation differs from what we told
                 * her before, she knows that something happened
                 * while we were processing this request, and it
                 * might be necessary to retry.
                 */
                s = splnet();
                INP_INFO_RLOCK(&tcbinfo);
                xig.xig_gen = tcbinfo.ipi_gencnt;
#if 0    
                xig.xig_sogen = so_gencnt;
#endif
                xig.xig_count = tcbinfo.ipi_count;
                INP_INFO_RUNLOCK(&tcbinfo);
                splx(s);
                error = SYSCTL_OUT(req, &xig, sizeof xig);
        }
        free(inp_list, M_TEMP);
        return error;
}

SYSCTL_PROC(_net_inet_tcp, TCPCTL_PCBLIST, pcblist, CTLFLAG_RD, 0, 0,
            tcp_pcblist, "S,xtcpcb", "List of active TCP connections");

void
tcp_ctlinput(int cmd, struct sockaddr *sa, void *vip)
{
        struct ip *ip = vip;
        struct tcphdr *th;
        void (*notify)(struct inpcb *, int) = tcp_notify;

        if (cmd == PRC_QUENCH)
                notify = tcp_quench;
#if 1
        else if (cmd == PRC_MSGSIZE)
                notify = tcp_mtudisc;
#endif
        else if (!PRC_IS_REDIRECT(cmd) &&
                 ((unsigned)cmd > PRC_NCMDS || inetctlerrmap[cmd] == 0))
                return;
        if (ip != NULL) {
#ifdef _IP_VHL
                th = (struct tcphdr *)((caddr_t)ip 
                                       + (IP_VHL_HL(ip->ip_vhl) << 2));
#else
                th = (struct tcphdr *)((caddr_t)ip
                                       + (ip->ip_hl << 2));
#endif
                in_pcbnotify(&tcb, sa, th->th_dport, ip->ip_src, th->th_sport,
                        cmd, notify);
        } else
                in_pcbnotify(&tcb, sa, 0, zeroin_addr, 0, cmd, notify);
}

/*
 * When a source quench is received, close congestion window
 * to one segment.  We will gradually open it again as we proceed.
 */
void
tcp_quench( struct inpcb *inp, int errnum)
{
        struct tcpcb *tp = intotcpcb(inp);

        if (tp)
                tp->snd_cwnd = tp->t_maxseg;
}

/*
 * When `need fragmentation' ICMP is received, update our idea of the MSS
 * based on the new value in the route.  Also nudge TCP to send something,
 * since we know the packet we just sent was dropped.
 * This duplicates some code in the tcp_mss() function in tcp_input.c.
 */
void
tcp_mtudisc(struct inpcb *inp, int errnum)
{
        struct tcpcb *tp = intotcpcb(inp);
        struct rtentry *rt;
        struct rmxp_tao *taop;
        struct socket *so = inp->inp_socket;
        int offered;
        int mss;

        if (tp) {
                rt = tcp_rtlookup(inp);
                if (!rt || !rt->rt_rmx.rmx_mtu) {
                        tp->t_maxopd = tp->t_maxseg = tcp_mssdflt;
                        return;
                }
                taop = rmx_taop(rt->rt_rmx);
                offered = taop->tao_mssopt;
                mss = rt->rt_rmx.rmx_mtu - sizeof(struct tcpiphdr);
                if (offered)
                        mss = min(mss, offered);
                /*
                 * XXX - The above conditional probably violates the TCP
                 * spec.  The problem is that, since we don't know the
                 * other end's MSS, we are supposed to use a conservative
                 * default.  But, if we do that, then MTU discovery will
                 * never actually take place, because the conservative
                 * default is much less than the MTUs typically seen
                 * on the Internet today.  For the moment, we'll sweep
                 * this under the carpet.
                 *
                 * The conservative default might not actually be a problem
                 * if the only case this occurs is when sending an initial
                 * SYN with options and data to a host we've never talked
                 * to before.  Then, they will reply with an MSS value which
                 * will get recorded and the new parameters should get
                 * recomputed.  For Further Study.
                 */
                if (tp->t_maxopd <= mss)
                        return;
                tp->t_maxopd = mss;

                if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP &&
                    (tp->t_flags & TF_RCVD_TSTMP) == TF_RCVD_TSTMP)
                        mss -= TCPOLEN_TSTAMP_APPA;
                if ((tp->t_flags & (TF_REQ_CC|TF_NOOPT)) == TF_REQ_CC &&
                    (tp->t_flags & TF_RCVD_CC) == TF_RCVD_CC)
                        mss -= TCPOLEN_CC_APPA;
#if     (MCLBYTES & (MCLBYTES - 1)) == 0
                if (mss > MCLBYTES)
                        mss &= ~(MCLBYTES-1);
#else
                if (mss > MCLBYTES)
                        mss = mss / MCLBYTES * MCLBYTES;
#endif
                if (so->so_snd.sb_hiwat < mss)
                        mss = so->so_snd.sb_hiwat;

                tp->t_maxseg = mss;

                tcpstat.tcps_mturesent++;
                tp->t_rtt = 0;
                tp->snd_nxt = tp->snd_una;
                tcp_output(tp);
        }
}

/*
 * Look-up the routing entry to the peer of this inpcb.  If no route
 * is found and it cannot be allocated, then return NULL.  This routine
 * is called by TCP routines that access the rmx structure and by tcp_mss
 * to get the interface MTU.
 */
struct rtentry *
tcp_rtlookup(struct inpcb *inp)
{
        struct route *ro;
        struct rtentry *rt;

        ro = &inp->inp_route;
        rt = ro->ro_rt;
        if (rt == NULL || !(rt->rt_flags & RTF_UP)) {
                /* No route yet, so try to acquire one */
                if (inp->inp_faddr.s_addr != INADDR_ANY) {
                        ro->ro_dst.sa_family = AF_INET;
                        ro->ro_dst.sa_len = sizeof(ro->ro_dst);
                        ((struct sockaddr_in *) &ro->ro_dst)->sin_addr =
                                inp->inp_faddr;
                        rtalloc(ro);
                        rt = ro->ro_rt;
                }
        }
        return rt;
}

/*
 * Return a pointer to the cached information about the remote host.
 * The cached information is stored in the protocol specific part of
 * the route metrics.
 */
struct rmxp_tao *
tcp_gettaocache(struct inpcb *inp)
{
        struct rtentry *rt = tcp_rtlookup(inp);

        /* Make sure this is a host route and is up. */
        if (rt == NULL ||
            (rt->rt_flags & (RTF_UP|RTF_HOST)) != (RTF_UP|RTF_HOST))
                return NULL;

        return rmx_taop(rt->rt_rmx);
}