/* $NetBSD: tcp_output.c,v 1.47 1999/01/20 03:39:54 thorpej Exp $ */ /*- * Copyright (c) 1997, 1998 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. * * 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) 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. * 3. All advertising materials mentioning features or use of this software * must display the following acknowledgement: * This product includes software developed by the University of * California, Berkeley and its contributors. * 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_output.c 8.4 (Berkeley) 5/24/95 */ #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #define TCPOUTFLAGS #include #include #include #include #include #include #ifdef notyet extern struct mbuf *m_copypack(); #endif #define MAX_TCPOPTLEN 32 /* max # bytes that go in options */ /* * Knob to enable Congestion Window Monitoring, and control the * the burst size it allows. Default burst is 4 packets, per * the Internet draft. */ int tcp_cwm = 0; int tcp_cwm_burstsize = 4; static __inline void tcp_segsize __P((struct tcpcb *, int *, int *)); static __inline void tcp_segsize(tp, txsegsizep, rxsegsizep) struct tcpcb *tp; int *txsegsizep, *rxsegsizep; { struct inpcb *inp = tp->t_inpcb; struct rtentry *rt; struct ifnet *ifp; int size; if ((rt = in_pcbrtentry(inp)) == NULL) { size = tcp_mssdflt; goto out; } ifp = rt->rt_ifp; if (rt->rt_rmx.rmx_mtu != 0) size = rt->rt_rmx.rmx_mtu - sizeof(struct tcpiphdr); else if (ip_mtudisc || in_localaddr(inp->inp_faddr) || ifp->if_flags & IFF_LOOPBACK) size = ifp->if_mtu - sizeof(struct tcpiphdr); else size = tcp_mssdflt; size -= (tcp_optlen(tp) + ip_optlen(tp->t_inpcb)); out: *txsegsizep = min(tp->t_peermss, size); *rxsegsizep = min(tp->t_ourmss, size); if (*txsegsizep != tp->t_segsz) { /* * If the new segment size is larger, we don't want to * mess up the congestion window, but if it is smaller * we'll have to reduce the congestion window to ensure * that we don't get into trouble with initial windows * and the rest. In any case, if the segment size * has changed, chances are the path has, too, and * our congestion window will be different. */ if (*txsegsizep < tp->t_segsz) { tp->snd_cwnd = max((tp->snd_cwnd / tp->t_segsz) * *txsegsizep, *txsegsizep); tp->snd_ssthresh = max((tp->snd_ssthresh / tp->t_segsz) * *txsegsizep, *txsegsizep); } tp->t_segsz = *txsegsizep; } } /* * Tcp output routine: figure out what should be sent and send it. */ int tcp_output(tp) register struct tcpcb *tp; { struct socket *so = tp->t_inpcb->inp_socket; struct route *ro = &tp->t_inpcb->inp_route; struct rtentry *rt; struct sockaddr_in *dst; long len, win; int off, flags, error; register struct mbuf *m; register struct tcpiphdr *ti; u_char opt[MAX_TCPOPTLEN]; unsigned optlen, hdrlen; int idle, sendalot, txsegsize, rxsegsize; int maxburst = TCP_MAXBURST; tcp_segsize(tp, &txsegsize, &rxsegsize); idle = (tp->snd_max == tp->snd_una); /* * Restart Window computation. From draft-floyd-incr-init-win-03: * * Optionally, a TCP MAY set the restart window to the * minimum of the value used for the initial window and * the current value of cwnd (in other words, using a * larger value for the restart window should never increase * the size of cwnd). */ if (tcp_cwm) { /* * Hughes/Touch/Heidemann Congestion Window Monitoring. * Count the number of packets currently pending * acknowledgement, and limit our congestion window * to a pre-determined allowed burst size plus that count. * This prevents bursting once all pending packets have * been acknowledged (i.e. transmission is idle). * * XXX Link this to Initial Window? */ tp->snd_cwnd = min(tp->snd_cwnd, (tcp_cwm_burstsize * txsegsize) + (tp->snd_nxt - tp->snd_una)); } else { if (idle && tp->t_idle >= tp->t_rxtcur) { /* * We have been idle for "a while" and no acks are * expected to clock out any data we send -- * slow start to get ack "clock" running again. */ tp->snd_cwnd = min(tp->snd_cwnd, TCP_INITIAL_WINDOW(tcp_init_win, txsegsize)); } } again: /* * Determine length of data that should be transmitted, and * flags that should be used. If there is some data or critical * controls (SYN, RST) to send, then transmit; otherwise, * investigate further. */ sendalot = 0; off = tp->snd_nxt - tp->snd_una; win = min(tp->snd_wnd, tp->snd_cwnd); flags = tcp_outflags[tp->t_state]; /* * If in persist timeout with window of 0, send 1 byte. * Otherwise, if window is small but nonzero * and timer expired, we will send what we can * and go to transmit state. */ if (tp->t_force) { if (win == 0) { /* * If we still have some data to send, then * clear the FIN bit. Usually this would * happen below when it realizes that we * aren't sending all the data. However, * if we have exactly 1 byte of unset data, * then it won't clear the FIN bit below, * and if we are in persist state, we wind * up sending the packet without recording * that we sent the FIN bit. * * We can't just blindly clear the FIN bit, * because if we don't have any more data * to send then the probe will be the FIN * itself. */ if (off < so->so_snd.sb_cc) flags &= ~TH_FIN; win = 1; } else { TCP_TIMER_DISARM(tp, TCPT_PERSIST); tp->t_rxtshift = 0; } } if (win < so->so_snd.sb_cc) { len = win - off; flags &= ~TH_FIN; } else len = so->so_snd.sb_cc - off; if (len < 0) { /* * If FIN has been sent but not acked, * but we haven't been called to retransmit, * len will be -1. Otherwise, window shrank * after we sent into it. If window shrank to 0, * cancel pending retransmit, pull snd_nxt back * to (closed) window, and set the persist timer * if it isn't already going. If the window didn't * close completely, just wait for an ACK. * * If we have a pending FIN, either it has already been * transmitted or it is outside the window, so drop it. * If the FIN has been transmitted, but this is not a * retransmission, then len must be -1. Therefore we also * prevent here the sending of `gratuitous FINs'. This * eliminates the need to check for that case below (e.g. * to back up snd_nxt before the FIN so that the sequence * number is correct). */ len = 0; flags &= ~TH_FIN; if (win == 0) { TCP_TIMER_DISARM(tp, TCPT_REXMT); tp->t_rxtshift = 0; tp->snd_nxt = tp->snd_una; if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) tcp_setpersist(tp); } } if (len > txsegsize) { len = txsegsize; flags &= ~TH_FIN; sendalot = 1; } win = sbspace(&so->so_rcv); /* * Sender silly window avoidance. If connection is idle * and can send all data, a maximum segment, * at least a maximum default-size segment do it, * or are forced, do it; otherwise don't bother. * If peer's buffer is tiny, then send * when window is at least half open. * If retransmitting (possibly after persist timer forced us * to send into a small window), then must resend. */ if (len) { if (len == txsegsize) goto send; if ((so->so_state & SS_MORETOCOME) == 0 && ((idle || tp->t_flags & TF_NODELAY) && len + off >= so->so_snd.sb_cc)) goto send; if (tp->t_force) goto send; if (len >= tp->max_sndwnd / 2) goto send; if (SEQ_LT(tp->snd_nxt, tp->snd_max)) goto send; } /* * Compare available window to amount of window known to peer * (as advertised window less next expected input). If the * difference is at least twice the size of the largest segment * we expect to receive (i.e. two segments) or at least 50% of * the maximum possible window, then want to send a window update * to peer. */ if (win > 0) { /* * "adv" is the amount we can increase the window, * taking into account that we are limited by * TCP_MAXWIN << tp->rcv_scale. */ long adv = min(win, (long)TCP_MAXWIN << tp->rcv_scale) - (tp->rcv_adv - tp->rcv_nxt); if (adv >= (long) (2 * rxsegsize)) goto send; if (2 * adv >= (long) so->so_rcv.sb_hiwat) goto send; } /* * Send if we owe peer an ACK. */ if (tp->t_flags & TF_ACKNOW) goto send; if (flags & (TH_SYN|TH_FIN|TH_RST)) goto send; if (SEQ_GT(tp->snd_up, tp->snd_una)) goto send; /* * TCP window updates are not reliable, rather a polling protocol * using ``persist'' packets is used to insure receipt of window * updates. The three ``states'' for the output side are: * idle not doing retransmits or persists * persisting to move a small or zero window * (re)transmitting and thereby not persisting * * tp->t_timer[TCPT_PERSIST] * is set when we are in persist state. * tp->t_force * is set when we are called to send a persist packet. * tp->t_timer[TCPT_REXMT] * is set when we are retransmitting * The output side is idle when both timers are zero. * * If send window is too small, there is data to transmit, and no * retransmit or persist is pending, then go to persist state. * If nothing happens soon, send when timer expires: * if window is nonzero, transmit what we can, * otherwise force out a byte. */ if (so->so_snd.sb_cc && TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 && TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) { tp->t_rxtshift = 0; tcp_setpersist(tp); } /* * No reason to send a segment, just return. */ return (0); send: /* * Before ESTABLISHED, force sending of initial options * unless TCP set not to do any options. * NOTE: we assume that the IP/TCP header plus TCP options * always fit in a single mbuf, leaving room for a maximum * link header, i.e. * max_linkhdr + sizeof (struct tcpiphdr) + optlen <= MHLEN */ optlen = 0; hdrlen = sizeof (struct tcpiphdr); if (flags & TH_SYN) { struct rtentry *rt = in_pcbrtentry(tp->t_inpcb); tp->snd_nxt = tp->iss; tp->t_ourmss = tcp_mss_to_advertise(rt != NULL ? rt->rt_ifp : NULL); if ((tp->t_flags & TF_NOOPT) == 0) { opt[0] = TCPOPT_MAXSEG; opt[1] = 4; opt[2] = (tp->t_ourmss >> 8) & 0xff; opt[3] = tp->t_ourmss & 0xff; optlen = 4; if ((tp->t_flags & TF_REQ_SCALE) && ((flags & TH_ACK) == 0 || (tp->t_flags & TF_RCVD_SCALE))) { *((u_int32_t *) (opt + optlen)) = htonl( TCPOPT_NOP << 24 | TCPOPT_WINDOW << 16 | TCPOLEN_WINDOW << 8 | tp->request_r_scale); optlen += 4; } } } /* * Send a timestamp and echo-reply if this is a SYN and our side * wants to use timestamps (TF_REQ_TSTMP is set) or both our side * and our peer have sent timestamps in our SYN's. */ if ((tp->t_flags & (TF_REQ_TSTMP|TF_NOOPT)) == TF_REQ_TSTMP && (flags & TH_RST) == 0 && ((flags & (TH_SYN|TH_ACK)) == TH_SYN || (tp->t_flags & TF_RCVD_TSTMP))) { u_int32_t *lp = (u_int32_t *)(opt + optlen); /* Form timestamp option as shown in appendix A of RFC 1323. */ *lp++ = htonl(TCPOPT_TSTAMP_HDR); *lp++ = htonl(tcp_now); *lp = htonl(tp->ts_recent); optlen += TCPOLEN_TSTAMP_APPA; } hdrlen += optlen; #ifdef DIAGNOSTIC if (len > txsegsize) panic("tcp data to be sent is larger than segment"); if (max_linkhdr + hdrlen > MHLEN) panic("tcphdr too big"); #endif /* * Grab a header mbuf, attaching a copy of data to * be transmitted, and initialize the header from * the template for sends on this connection. */ if (len) { if (tp->t_force && len == 1) tcpstat.tcps_sndprobe++; else if (SEQ_LT(tp->snd_nxt, tp->snd_max)) { tcpstat.tcps_sndrexmitpack++; tcpstat.tcps_sndrexmitbyte += len; } else { tcpstat.tcps_sndpack++; tcpstat.tcps_sndbyte += len; } #ifdef notyet if ((m = m_copypack(so->so_snd.sb_mb, off, (int)len, max_linkhdr + hdrlen)) == 0) { error = ENOBUFS; goto out; } /* * m_copypack left space for our hdr; use it. */ m->m_len += hdrlen; m->m_data -= hdrlen; #else MGETHDR(m, M_DONTWAIT, MT_HEADER); if (m == NULL) { error = ENOBUFS; goto out; } m->m_data += max_linkhdr; m->m_len = hdrlen; if (len <= MHLEN - hdrlen - max_linkhdr) { m_copydata(so->so_snd.sb_mb, off, (int) len, mtod(m, caddr_t) + hdrlen); m->m_len += len; } else { m->m_next = m_copy(so->so_snd.sb_mb, off, (int) len); if (m->m_next == 0) { m_freem(m); error = ENOBUFS; goto out; } } #endif /* * If we're sending everything we've got, set PUSH. * (This will keep happy those implementations which only * give data to the user when a buffer fills or * a PUSH comes in.) */ if (off + len == so->so_snd.sb_cc) flags |= TH_PUSH; } else { if (tp->t_flags & TF_ACKNOW) tcpstat.tcps_sndacks++; else if (flags & (TH_SYN|TH_FIN|TH_RST)) tcpstat.tcps_sndctrl++; else if (SEQ_GT(tp->snd_up, tp->snd_una)) tcpstat.tcps_sndurg++; else tcpstat.tcps_sndwinup++; MGETHDR(m, M_DONTWAIT, MT_HEADER); if (m == NULL) { error = ENOBUFS; goto out; } m->m_data += max_linkhdr; m->m_len = hdrlen; } m->m_pkthdr.rcvif = (struct ifnet *)0; ti = mtod(m, struct tcpiphdr *); if (tp->t_template == 0) panic("tcp_output"); bcopy((caddr_t)tp->t_template, (caddr_t)ti, sizeof (struct tcpiphdr)); /* * If we are doing retransmissions, then snd_nxt will * not reflect the first unsent octet. For ACK only * packets, we do not want the sequence number of the * retransmitted packet, we want the sequence number * of the next unsent octet. So, if there is no data * (and no SYN or FIN), use snd_max instead of snd_nxt * when filling in ti_seq. But if we are in persist * state, snd_max might reflect one byte beyond the * right edge of the window, so use snd_nxt in that * case, since we know we aren't doing a retransmission. * (retransmit and persist are mutually exclusive...) */ if (len || (flags & (TH_SYN|TH_FIN)) || TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) ti->ti_seq = htonl(tp->snd_nxt); else ti->ti_seq = htonl(tp->snd_max); ti->ti_ack = htonl(tp->rcv_nxt); if (optlen) { bcopy((caddr_t)opt, (caddr_t)(ti + 1), optlen); ti->ti_off = (sizeof (struct tcphdr) + optlen) >> 2; } ti->ti_flags = flags; /* * Calculate receive window. Don't shrink window, * but avoid silly window syndrome. */ if (win < (long)(so->so_rcv.sb_hiwat / 4) && win < (long)rxsegsize) win = 0; if (win > (long)TCP_MAXWIN << tp->rcv_scale) win = (long)TCP_MAXWIN << tp->rcv_scale; if (win < (long)(tp->rcv_adv - tp->rcv_nxt)) win = (long)(tp->rcv_adv - tp->rcv_nxt); ti->ti_win = htons((u_int16_t) (win>>tp->rcv_scale)); if (SEQ_GT(tp->snd_up, tp->snd_nxt)) { u_int32_t urp = tp->snd_up - tp->snd_nxt; if (urp > IP_MAXPACKET) urp = IP_MAXPACKET; ti->ti_urp = htons((u_int16_t)urp); ti->ti_flags |= TH_URG; } else /* * If no urgent pointer to send, then we pull * the urgent pointer to the left edge of the send window * so that it doesn't drift into the send window on sequence * number wraparound. */ tp->snd_up = tp->snd_una; /* drag it along */ /* * Put TCP length in extended header, and then * checksum extended header and data. */ if (len + optlen) ti->ti_len = htons((u_int16_t)(sizeof (struct tcphdr) + optlen + len)); ti->ti_sum = in_cksum(m, (int)(hdrlen + len)); /* * In transmit state, time the transmission and arrange for * the retransmit. In persist state, just set snd_max. */ if (tp->t_force == 0 || TCP_TIMER_ISARMED(tp, TCPT_PERSIST) == 0) { tcp_seq startseq = tp->snd_nxt; /* * Advance snd_nxt over sequence space of this segment. * There are no states in which we send both a SYN and a FIN, * so we collapse the tests for these flags. */ if (flags & (TH_SYN|TH_FIN)) tp->snd_nxt++; tp->snd_nxt += len; if (SEQ_GT(tp->snd_nxt, tp->snd_max)) { tp->snd_max = tp->snd_nxt; /* * Time this transmission if not a retransmission and * not currently timing anything. */ if (tp->t_rtt == 0) { tp->t_rtt = 1; tp->t_rtseq = startseq; tcpstat.tcps_segstimed++; } } /* * Set retransmit timer if not currently set, * and not doing an ack or a keep-alive probe. * Initial value for retransmit timer is smoothed * round-trip time + 2 * round-trip time variance. * Initialize shift counter which is used for backoff * of retransmit time. */ if (TCP_TIMER_ISARMED(tp, TCPT_REXMT) == 0 && tp->snd_nxt != tp->snd_una) { TCP_TIMER_ARM(tp, TCPT_REXMT, tp->t_rxtcur); if (TCP_TIMER_ISARMED(tp, TCPT_PERSIST)) { TCP_TIMER_DISARM(tp, TCPT_PERSIST); tp->t_rxtshift = 0; } } } else if (SEQ_GT(tp->snd_nxt + len, tp->snd_max)) tp->snd_max = tp->snd_nxt + len; /* * Trace. */ if (so->so_options & SO_DEBUG) tcp_trace(TA_OUTPUT, tp->t_state, tp, ti, 0); /* * Fill in IP length and desired time to live and * send to IP level. There should be a better way * to handle ttl and tos; we could keep them in * the template, but need a way to checksum without them. */ m->m_pkthdr.len = hdrlen + len; ((struct ip *)ti)->ip_len = m->m_pkthdr.len; ((struct ip *)ti)->ip_ttl = tp->t_inpcb->inp_ip.ip_ttl; /* XXX */ ((struct ip *)ti)->ip_tos = tp->t_inpcb->inp_ip.ip_tos; /* XXX */ /* * If we're doing Path MTU discovery, we need to set DF unless * the route's MTU is locked. If we lack a route, we need to * look it up now. * * ip_output() could do this for us, but it's convenient to just * do it here unconditionally. */ if ((rt = ro->ro_rt) == NULL || (rt->rt_flags & RTF_UP) == 0) { if (ro->ro_rt != NULL) { RTFREE(ro->ro_rt); ro->ro_rt = NULL; } dst = satosin(&ro->ro_dst); dst->sin_family = AF_INET; dst->sin_len = sizeof(*dst); dst->sin_addr = tp->t_inpcb->inp_faddr; rtalloc(ro); if ((rt = ro->ro_rt) == NULL) { m_freem(m); ipstat.ips_noroute++; error = EHOSTUNREACH; goto out; } } if (ip_mtudisc != 0 && (rt->rt_rmx.rmx_locks & RTV_MTU) == 0) ((struct ip *)ti)->ip_off |= IP_DF; #if BSD >= 43 error = ip_output(m, tp->t_inpcb->inp_options, ro, so->so_options & SO_DONTROUTE, 0); #else error = ip_output(m, (struct mbuf *)0, ro, so->so_options & SO_DONTROUTE); #endif if (error) { out: if (error == ENOBUFS) { tcp_quench(tp->t_inpcb, 0); return (0); } if ((error == EHOSTUNREACH || error == ENETDOWN) && TCPS_HAVERCVDSYN(tp->t_state)) { tp->t_softerror = error; return (0); } return (error); } tcpstat.tcps_sndtotal++; if (tp->t_flags & TF_DELACK) tcpstat.tcps_delack++; /* * Data sent (as far as we can tell). * If this advertises a larger window than any other segment, * then remember the size of the advertised window. * Any pending ACK has now been sent. */ if (win > 0 && SEQ_GT(tp->rcv_nxt+win, tp->rcv_adv)) tp->rcv_adv = tp->rcv_nxt + win; tp->last_ack_sent = tp->rcv_nxt; tp->t_flags &= ~TF_ACKNOW; TCP_CLEAR_DELACK(tp); #ifdef DIAGNOSTIC if (maxburst < 0) printf("tcp_output: maxburst exceeded by %d\n", -maxburst); #endif if (sendalot && (!tcp_do_newreno || --maxburst)) goto again; return (0); } void tcp_setpersist(tp) register struct tcpcb *tp; { register int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> (1 + 2); int nticks; if (TCP_TIMER_ISARMED(tp, TCPT_REXMT)) panic("tcp_output REXMT"); /* * Start/restart persistance timer. */ if (t < tp->t_rttmin) t = tp->t_rttmin; TCPT_RANGESET(nticks, t * tcp_backoff[tp->t_rxtshift], TCPTV_PERSMIN, TCPTV_PERSMAX); TCP_TIMER_ARM(tp, TCPT_PERSIST, nticks); if (tp->t_rxtshift < TCP_MAXRXTSHIFT) tp->t_rxtshift++; }