Function tcp_mss

freebsd/netinet/tcp_input.c:3805–3877

}

void
tcp_mss(struct tcpcb *tp, int offer)
{
	int mss;
	uint32_t bufsize;
	struct inpcb *inp;
	struct socket *so;
	struct hc_metrics_lite metrics;
	struct tcp_ifcap cap;

	KASSERT(tp != NULL, ("%s: tp == NULL", __func__));

	bzero(&cap, sizeof(cap));
	tcp_mss_update(tp, offer, -1, &metrics, &cap);

	mss = tp->t_maxseg;
	inp = tp->t_inpcb;

	/*
	 * If there's a pipesize, change the socket buffer to that size,
	 * don't change if sb_hiwat is different than default (then it
	 * has been changed on purpose with setsockopt).
	 * Make the socket buffers an integral number of mss units;
	 * if the mss is larger than the socket buffer, decrease the mss.
	 */
	so = inp->inp_socket;
	SOCKBUF_LOCK(&so->so_snd);
	if ((so->so_snd.sb_hiwat == V_tcp_sendspace) && metrics.rmx_sendpipe)
		bufsize = metrics.rmx_sendpipe;
	else
		bufsize = so->so_snd.sb_hiwat;
	if (bufsize < mss)
		mss = bufsize;
	else {
		bufsize = roundup(bufsize, mss);
		if (bufsize > sb_max)
			bufsize = sb_max;
		if (bufsize > so->so_snd.sb_hiwat)
			(void)sbreserve_locked(&so->so_snd, bufsize, so, NULL);
	}
	SOCKBUF_UNLOCK(&so->so_snd);
	/*
	 * Sanity check: make sure that maxseg will be large
	 * enough to allow some data on segments even if the
	 * all the option space is used (40bytes).  Otherwise
	 * funny things may happen in tcp_output.
	 *
	 * XXXGL: shouldn't we reserve space for IP/IPv6 options?
	 */
	tp->t_maxseg = max(mss, 64);

	SOCKBUF_LOCK(&so->so_rcv);
	if ((so->so_rcv.sb_hiwat == V_tcp_recvspace) && metrics.rmx_recvpipe)
		bufsize = metrics.rmx_recvpipe;
	else
		bufsize = so->so_rcv.sb_hiwat;
	if (bufsize > mss) {
		bufsize = roundup(bufsize, mss);