tcp_output.c

/* SPDX-License-Identifier: BSD-3-Clause */
/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993
 *	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_output.c	8.3 (Berkeley) 12/30/93
 * tcp_output.c,v 1.3 1994/09/15 10:36:55 davidg Exp
 */

/*
 * Changes and additions relating to SLiRP
 * Copyright (c) 1995 Danny Gasparovski.
 */

#include "slirp.h"

static const uint8_t tcp_outflags[TCP_NSTATES] = {
    TH_RST | TH_ACK, 0,      TH_SYN,          TH_SYN | TH_ACK,
    TH_ACK,          TH_ACK, TH_FIN | TH_ACK, TH_FIN | TH_ACK,
    TH_FIN | TH_ACK, TH_ACK, TH_ACK,
};


#undef MAX_TCPOPTLEN
#define MAX_TCPOPTLEN 32 /* max # bytes that go in options */

/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int tcp_output(struct tcpcb *tp)
{
    register struct socket *so = tp->t_socket;
    register long len, win;
    int off, flags, error;
    register struct mbuf *m;
    register struct tcpiphdr *ti, tcpiph_save;
    struct ip *ip;
    struct ip6 *ip6;
    uint8_t opt[MAX_TCPOPTLEN];
    unsigned optlen, hdrlen;
    int idle, sendalot;

    DEBUG_CALL("tcp_output");
    DEBUG_ARG("tp = %p", tp);

    /*
     * Determine length of data that should be transmitted,
     * and flags that will be used.
     * If there is some data or critical controls (SYN, RST)
     * to send, then transmit; otherwise, investigate further.
     */
    idle = (tp->snd_max == tp->snd_una);
    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 = tp->t_maxseg;
again:
    sendalot = 0;
    off = tp->snd_nxt - tp->snd_una;
    win = MIN(tp->snd_wnd, tp->snd_cwnd);

    flags = tcp_outflags[tp->t_state];

    DEBUG_MISC(" --- tcp_output flags = 0x%x", flags);

    /*
     * 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 {
            tp->t_timer[TCPT_PERSIST] = 0;
            tp->t_rxtshift = 0;
        }
    }

    len = MIN(so->so_snd.sb_cc, win) - 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 and pull snd_nxt
         * back to (closed) window.  We will enter persist
         * state below.  If the window didn't close completely,
         * just wait for an ACK.
         */
        len = 0;
        if (win == 0) {
            tp->t_timer[TCPT_REXMT] = 0;
            tp->snd_nxt = tp->snd_una;
        }
    }

    if (len > tp->t_maxseg) {
        len = tp->t_maxseg;
        sendalot = 1;
    }
    if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
        flags &= ~TH_FIN;

    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 == tp->t_maxseg)
            goto send;
        if ((1 || 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 && tp->max_sndwnd > 0)
            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 two
     * max size 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 * tp->t_maxseg))
            goto send;
        if (2 * adv >= (long)so->so_rcv.sb_datalen)
            goto send;
    }

    /*
     * Send if we owe peer an ACK.
     */
    if (tp->t_flags & TF_ACKNOW)
        goto send;
    if (flags & (TH_SYN | TH_RST))
        goto send;
    if (SEQ_GT(tp->snd_up, tp->snd_una))
        goto send;
    /*
     * If our state indicates that FIN should be sent
     * and we have not yet done so, or we're retransmitting the FIN,
     * then we need to send.
     */
    if (flags & TH_FIN &&
        ((tp->t_flags & TF_SENTFIN) == 0 || tp->snd_nxt == 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 && tp->t_timer[TCPT_REXMT] == 0 &&
        tp->t_timer[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) {
        tp->snd_nxt = tp->iss;
        if ((tp->t_flags & TF_NOOPT) == 0) {
            uint16_t mss;

            opt[0] = TCPOPT_MAXSEG;
            opt[1] = 4;
            mss = htons((uint16_t)tcp_mss(tp, 0));
            memcpy((char *)(opt + 2), (char *)&mss, sizeof(mss));
            optlen = 4;
        }
    }

    hdrlen += optlen;

    /*
     * Adjust data length if insertion of options will
     * bump the packet length beyond the t_maxseg length.
     */
    if (len > tp->t_maxseg - optlen) {
        len = tp->t_maxseg - optlen;
        sendalot = 1;
    }

    /*
     * 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) {
        m = m_get(so->slirp);
        if (m == NULL) {
            error = 1;
            goto out;
        }
        m->m_data += IF_MAXLINKHDR;
        m->m_len = hdrlen;

        sbcopy(&so->so_snd, off, (int)len, mtod(m, char *) + hdrlen);
        m->m_len += len;

        /*
         * 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 {
        m = m_get(so->slirp);
        if (m == NULL) {
            error = 1;
            goto out;
        }
        m->m_data += IF_MAXLINKHDR;
        m->m_len = hdrlen;
    }

    ti = mtod(m, struct tcpiphdr *);

    memcpy((char *)ti, &tp->t_template, sizeof(struct tcpiphdr));

    /*
     * Fill in fields, remembering maximum advertised
     * window for use in delaying messages about window sizes.
     * If resending a FIN, be sure not to use a new sequence number.
     */
    if (flags & TH_FIN && tp->t_flags & TF_SENTFIN &&
        tp->snd_nxt == tp->snd_max)
        tp->snd_nxt--;
    /*
     * 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)) || tp->t_timer[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) {
        memcpy((char *)(ti + 1), (char *)opt, 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_datalen / 4) && win < (long)tp->t_maxseg)
        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((uint16_t)(win >> tp->rcv_scale));

    if (SEQ_GT(tp->snd_up, tp->snd_una)) {
        ti->ti_urp = htons((uint16_t)(tp->snd_up - ntohl(ti->ti_seq)));
        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((uint16_t)(sizeof(struct tcphdr) + optlen + len));
    ti->ti_sum = 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 || tp->t_timer[TCPT_PERSIST] == 0) {
        tcp_seq startseq = tp->snd_nxt;

        /*
         * Advance snd_nxt over sequence space of this segment.
         */
        if (flags & (TH_SYN | TH_FIN)) {
            if (flags & TH_SYN)
                tp->snd_nxt++;
            if (flags & TH_FIN) {
                tp->snd_nxt++;
                tp->t_flags |= TF_SENTFIN;
            }
        }
        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;
            }
        }

        /*
         * 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 (tp->t_timer[TCPT_REXMT] == 0 && tp->snd_nxt != tp->snd_una) {
            tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
            if (tp->t_timer[TCPT_PERSIST]) {
                tp->t_timer[TCPT_PERSIST] = 0;
                tp->t_rxtshift = 0;
            }
        }
    } else if (SEQ_GT(tp->snd_nxt + len, tp->snd_max))
        tp->snd_max = tp->snd_nxt + len;

    /*
     * 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_len = hdrlen + len; /* XXX Needed? m_len should be correct */
    tcpiph_save = *mtod(m, struct tcpiphdr *);

    switch (so->so_ffamily) {
    case AF_INET:
        m->m_data +=
            sizeof(struct tcpiphdr) - sizeof(struct tcphdr) - sizeof(struct ip);
        m->m_len -=
            sizeof(struct tcpiphdr) - sizeof(struct tcphdr) - sizeof(struct ip);
        ip = mtod(m, struct ip *);

        ip->ip_len = m->m_len;
        ip->ip_dst = tcpiph_save.ti_dst;
        ip->ip_src = tcpiph_save.ti_src;
        ip->ip_p = tcpiph_save.ti_pr;

        ip->ip_ttl = IPDEFTTL;
        ip->ip_tos = so->so_iptos;
        error = ip_output(so, m);
        break;

    case AF_INET6:
        m->m_data += sizeof(struct tcpiphdr) - sizeof(struct tcphdr) -
                     sizeof(struct ip6);
        m->m_len -= sizeof(struct tcpiphdr) - sizeof(struct tcphdr) -
                    sizeof(struct ip6);
        ip6 = mtod(m, struct ip6 *);

        ip6->ip_pl = tcpiph_save.ti_len;
        ip6->ip_dst = tcpiph_save.ti_dst6;
        ip6->ip_src = tcpiph_save.ti_src6;
        ip6->ip_nh = tcpiph_save.ti_nh6;

        error = ip6_output(so, m, 0);
        break;

    default:
        g_assert_not_reached();
    }

    if (error) {
    out:
        return (error);
    }

    /*
     * 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 | TF_DELACK);
    if (sendalot)
        goto again;

    return (0);
}

void tcp_setpersist(struct tcpcb *tp)
{
    int t = ((tp->t_srtt >> 2) + tp->t_rttvar) >> 1;

    TCPT_RANGESET(tp->t_timer[TCPT_PERSIST], t * tcp_backoff[tp->t_rxtshift],
                  TCPTV_PERSMIN, TCPTV_PERSMAX);
    if (tp->t_rxtshift < TCP_MAXRXTSHIFT)
        tp->t_rxtshift++;
}