freebsd-dev/sys/net/if_sl.c

/*-
 * Copyright (c) 1987, 1989, 1992, 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.
 * 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.
 *
 *	@(#)if_sl.c	8.6 (Berkeley) 2/1/94
 * $FreeBSD$
 */

/*
 * Serial Line interface
 *
 * Rick Adams
 * Center for Seismic Studies
 * 1300 N 17th Street, Suite 1450
 * Arlington, Virginia 22209
 * (703)276-7900
 * rick@seismo.ARPA
 * seismo!rick
 *
 * Pounded on heavily by Chris Torek (chris@mimsy.umd.edu, umcp-cs!chris).
 * N.B.: this belongs in netinet, not net, the way it stands now.
 * Should have a link-layer type designation, but wouldn't be
 * backwards-compatible.
 *
 * Converted to 4.3BSD Beta by Chris Torek.
 * Other changes made at Berkeley, based in part on code by Kirk Smith.
 * W. Jolitz added slip abort.
 *
 * Hacked almost beyond recognition by Van Jacobson (van@helios.ee.lbl.gov).
 * Added priority queuing for "interactive" traffic; hooks for TCP
 * header compression; ICMP filtering (at 2400 baud, some cretin
 * pinging you can use up all your bandwidth).  Made low clist behavior
 * more robust and slightly less likely to hang serial line.
 * Sped up a bunch of things.
 *
 * Note that splimp() is used throughout to block both (tty) input
 * interrupts and network activity; thus, splimp must be >= spltty.
 */

#include "opt_inet.h"
#include "opt_slip.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/proc.h>
#include <sys/socket.h>
#include <sys/sockio.h>
#include <sys/fcntl.h>
#include <sys/signalvar.h>
#include <sys/tty.h>
#include <sys/clist.h>
#include <sys/kernel.h>
#include <sys/conf.h>
#include <sys/module.h>
#include <sys/proc.h>

#include <net/if.h>
#include <net/if_types.h>
#include <net/netisr.h>

#if INET
#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/in_var.h>
#include <netinet/ip.h>
#else
#error "Huh? Slip without inet?"
#endif

#include <net/slcompress.h>
#include <net/if_slvar.h>
#include <net/slip.h>

#include <net/bpf.h>

static MALLOC_DEFINE(M_SL, "sl", "SLIP Interface");

/*
 * SLRMAX is a hard limit on input packet size.  To simplify the code
 * and improve performance, we require that packets fit in an mbuf
 * cluster, and if we get a compressed packet, there's enough extra
 * room to expand the header into a max length tcp/ip header (128
 * bytes).  So, SLRMAX can be at most
 *	MCLBYTES - 128
 *
 * SLMTU is the default transmit MTU. The transmit MTU should be kept
 * small enough so that interactive use doesn't suffer, but large
 * enough to provide good performance. 552 is a good choice for SLMTU
 * because it is high enough to not fragment TCP packets being routed
 * through this host. Packet fragmentation is bad with SLIP because
 * fragment headers aren't compressed. The previous assumptions about
 * the best MTU value don't really hold when using modern modems with
 * BTLZ data compression because the modem buffers play a much larger
 * role in interactive performance than the MTU. The MTU can be changed
 * at any time to suit the specific environment with ifconfig(8), and
 * its maximum value is defined as SLTMAX. SLTMAX must not be so large
 * that it would overflow the stack if BPF is configured (XXX; if_ppp.c
 * handles this better).
 *
 * SLIP_HIWAT is the amount of data that will be queued 'downstream'
 * of us (i.e., in clists waiting to be picked up by the tty output
 * interrupt).  If we queue a lot of data downstream, it's immune to
 * our t.o.s. queuing.
 * E.g., if SLIP_HIWAT is 1024, the interactive traffic in mixed
 * telnet/ftp will see a 1 sec wait, independent of the mtu (the
 * wait is dependent on the ftp window size but that's typically
 * 1k - 4k).  So, we want SLIP_HIWAT just big enough to amortize
 * the cost (in idle time on the wire) of the tty driver running
 * off the end of its clists & having to call back slstart for a
 * new packet.  For a tty interface with any buffering at all, this
 * cost will be zero.  Even with a totally brain dead interface (like
 * the one on a typical workstation), the cost will be <= 1 character
 * time.  So, setting SLIP_HIWAT to ~100 guarantees that we'll lose
 * at most 1% while maintaining good interactive response.
 */
#define	BUFOFFSET	(128+sizeof(struct ifnet **)+SLIP_HDRLEN)
#define	SLRMAX		(MCLBYTES - BUFOFFSET)
#define	SLBUFSIZE	(SLRMAX + BUFOFFSET)
#ifndef SLMTU
#define	SLMTU		552		/* default MTU */
#endif
#define	SLTMAX		1500		/* maximum MTU */
#define	SLIP_HIWAT	roundup(50,CBSIZE)
#define	CLISTRESERVE	1024		/* Can't let clists get too low */

/*
 * SLIP ABORT ESCAPE MECHANISM:
 *	(inspired by HAYES modem escape arrangement)
 *	1sec escape 1sec escape 1sec escape { 1sec escape 1sec escape }
 *	within window time signals a "soft" exit from slip mode by remote end
 *	if the IFF_DEBUG flag is on.
 */
#define	ABT_ESC		'\033'	/* can't be t_intr - distant host must know it*/
#define	ABT_IDLE	1	/* in seconds - idle before an escape */
#define	ABT_COUNT	3	/* count of escapes for abort */
#define	ABT_WINDOW	(ABT_COUNT*2+2)	/* in seconds - time to count */

static LIST_HEAD(sl_list, sl_softc) sl_list;

#define FRAME_END	 	0xc0		/* Frame End */
#define FRAME_ESCAPE		0xdb		/* Frame Esc */
#define TRANS_FRAME_END	 	0xdc		/* transposed frame end */
#define TRANS_FRAME_ESCAPE 	0xdd		/* transposed frame esc */

static int slisstatic(int);
static void slmarkstatic(int);
static struct sl_softc *slcreate(void);
static void sldestroy(struct sl_softc *sc);
static struct mbuf *sl_btom(struct sl_softc *, int);
static timeout_t sl_keepalive;
static timeout_t sl_outfill;
static l_close_t	slclose;
static l_rint_t		slinput;
static l_ioctl_t	sltioctl;
static l_start_t	sltstart;
static int	slioctl(struct ifnet *, u_long, caddr_t);
static int	slopen(struct cdev *, struct tty *);
static int	sloutput(struct ifnet *,
	    struct mbuf *, struct sockaddr *, struct rtentry *);
static void	slstart(struct ifnet *);

static struct linesw slipdisc = {
	.l_open =	slopen,
	.l_close =	slclose,
	.l_read =	l_noread,
	.l_write =	l_nowrite,
	.l_ioctl =	sltioctl,
	.l_rint =	slinput,
	.l_start =	sltstart,
	.l_modem =	ttymodem
};

/*
 * Called from boot code to establish sl interfaces.
 */
static int
sl_modevent(module_t mod, int type, void *data) 
{ 
	switch (type) { 
	case MOD_LOAD: 
		ldisc_register(SLIPDISC, &slipdisc);
		LIST_INIT(&sl_list);
		break; 
	case MOD_UNLOAD: 
		ldisc_deregister(SLIPDISC);
		printf("if_sl module unload - not possible for this module type\n"); 
		return EINVAL; 
	default:
		return EOPNOTSUPP;
	} 
	return 0; 
} 

static moduledata_t sl_mod = { 
	"if_sl", 
	sl_modevent, 
	0
}; 

DECLARE_MODULE(if_sl, sl_mod, SI_SUB_PSEUDO, SI_ORDER_ANY);

static int *st_unit_list;
static size_t st_unit_max = 0;

static int
slisunitfree(int unit)
{
	struct sl_softc *sc;

	LIST_FOREACH(sc, &sl_list, sl_next) {
		if (SL2IFP(sc)->if_dunit == unit)
			return (0);
	}
	return (1);
}

static struct sl_softc *
sl_for_tty(struct tty *tp)
{
	struct sl_softc *nc;

	LIST_FOREACH(nc, &sl_list, sl_next) {
		if (nc->sc_ttyp == tp)
			return (nc);
	}
	return (NULL);
}
static int
slisstatic(int unit)
{
	size_t i;

	for (i = 0; i < st_unit_max; i++)
		if (st_unit_list[i] == unit)
			return 1;
	return 0;
}

static void
slmarkstatic(int unit)
{
	int *t;

	if (slisstatic(unit))
		return;

	MALLOC(t, int *, sizeof(int) * (st_unit_max+1), M_SL, M_NOWAIT);
	if (t == NULL)
		return;

	if (st_unit_list) {
		bcopy(st_unit_list, t, sizeof(int) * st_unit_max);
		free(st_unit_list, M_SL);
	}
	st_unit_list = t;
	st_unit_list[st_unit_max] = unit;
	st_unit_max++;
}

static struct sl_softc *
slcreate(void)
{
	struct sl_softc *sc;
	int unit;
	struct mbuf *m;

	MALLOC(sc, struct sl_softc *, sizeof(*sc), M_SL, M_WAITOK | M_ZERO);
	sc->sc_ifp = if_alloc(IFT_SLIP);
	if (sc->sc_ifp == NULL) {
		free(sc, M_SL);
		return (NULL);
	}

	m = m_gethdr(M_TRYWAIT, MT_DATA);
	if (m != NULL) {
		MCLGET(m, M_TRYWAIT);
		if ((m->m_flags & M_EXT) == 0) {
			m_free(m);
			m = NULL;
		}
	}

	if (m == NULL) {
		printf("sl: can't allocate buffer\n");
		free(sc, M_SL);
		return (NULL);
	}

	sc->sc_ep = mtod(m, u_char *) + SLBUFSIZE;
	sc->sc_mbuf = m;
	sc->sc_buf = sc->sc_ep - SLRMAX;
	sc->sc_mp = sc->sc_buf;
	sl_compress_init(&sc->sc_comp, -1);

	SL2IFP(sc)->if_softc = sc;
	SL2IFP(sc)->if_mtu = SLMTU;
	SL2IFP(sc)->if_flags =
#ifdef SLIP_IFF_OPTS
	    SLIP_IFF_OPTS;
#else
	    IFF_POINTOPOINT | SC_AUTOCOMP | IFF_MULTICAST | IFF_NEEDSGIANT;
#endif
	SL2IFP(sc)->if_ioctl = slioctl;
	SL2IFP(sc)->if_output = sloutput;
	SL2IFP(sc)->if_start = slstart;
	SL2IFP(sc)->if_snd.ifq_maxlen = 50;
	sc->sc_fastq.ifq_maxlen = 32;
	SL2IFP(sc)->if_linkmib = sc;
	SL2IFP(sc)->if_linkmiblen = sizeof *sc;
	mtx_init(&sc->sc_fastq.ifq_mtx, "sl_fastq", NULL, MTX_DEF);

	/*
	 * Find a suitable unit number.
	 */
	for (unit=0; ; unit++) {
		if (slisstatic(unit))
			continue;
		if (!slisunitfree(unit))
			continue;
		break;
	}
	if_initname(SL2IFP(sc), "sl", unit);
	LIST_INSERT_HEAD(&sl_list, sc, sl_next);

	if_attach(SL2IFP(sc));
	bpfattach(SL2IFP(sc), DLT_SLIP, SLIP_HDRLEN);

	return sc;
}


/*
 * Line specific open routine.
 * Attach the given tty to the first available sl unit.
 */
/* ARGSUSED */
static int
slopen(struct cdev *dev, register struct tty *tp)
{
	register struct sl_softc *sc;
	int s, error;

	error = suser(curthread);
	if (error)
		return (error);

	if ((sc = slcreate()) == NULL)
		return (ENOBUFS);

	tp->t_hotchar = FRAME_END;
	sc->sc_ttyp = tp;
	SL2IFP(sc)->if_baudrate = tp->t_ospeed;
	ttyflush(tp, FREAD | FWRITE);

	/*
	 * We don't use t_canq or t_rawq, so reduce their
	 * cblock resources to 0.  Reserve enough cblocks
	 * for t_outq to guarantee that we can fit a full
	 * packet if the SLIP_HIWAT check allows slstart()
	 * to loop.  Use the same value for the cblock
	 * limit since the reserved blocks should always
	 * be enough.  Reserving cblocks probably makes
	 * the CLISTRESERVE check unnecessary and wasteful.
	 */
	clist_alloc_cblocks(&tp->t_canq, 0, 0);
	clist_alloc_cblocks(&tp->t_outq,
	    SLIP_HIWAT + 2 * SL2IFP(sc)->if_mtu + 1,
	    SLIP_HIWAT + 2 * SL2IFP(sc)->if_mtu + 1);
	clist_alloc_cblocks(&tp->t_rawq, 0, 0);

	s = splnet();
	if_up(SL2IFP(sc));
	splx(s);
	return (0);
}

static void
sldestroy(struct sl_softc *sc)
{
	bpfdetach(SL2IFP(sc));
	if_detach(SL2IFP(sc));
	if_free(SL2IFP(sc));
	LIST_REMOVE(sc, sl_next);
	m_free(sc->sc_mbuf);
	mtx_destroy(&sc->sc_fastq.ifq_mtx);
	if (sc->bpfbuf)
		free(sc->bpfbuf, M_SL);
	free(sc, M_SL);
}

/*
 * Line specific close routine.
 * Detach the tty from the sl unit.
 */
static int
slclose(struct tty *tp, int flag)
{
	register struct sl_softc *sc;
	int s;

	ttyflush(tp, FREAD | FWRITE);
	/*
	 * XXX the placement of the following spl is misleading.  tty
	 * interrupts must be blocked across line discipline switches
	 * and throughout closes to avoid races.
	 */
	s = splimp();		/* actually, max(spltty, splnet) */
	clist_free_cblocks(&tp->t_outq);
	sc = sl_for_tty(tp);
	if (sc != NULL) {
		if (sc->sc_outfill) {
			sc->sc_outfill = 0;
			untimeout(sl_outfill, sc, sc->sc_ofhandle);
		}
		if (sc->sc_keepalive) {
			sc->sc_keepalive = 0;
			untimeout(sl_keepalive, sc, sc->sc_kahandle);
		}
		if_down(SL2IFP(sc));
		sc->sc_ttyp = NULL;
		sldestroy(sc);
	}
	splx(s);
	return 0;
}

/*
 * Line specific (tty) ioctl routine.
 * Provide a way to get the sl unit number.
 */
/* ARGSUSED */
static int
sltioctl(struct tty *tp, u_long cmd, caddr_t data, int flag,
    struct thread *td)
{
	struct sl_softc *sc = sl_for_tty(tp);
	int s, unit, wasup;

	s = splimp();
	switch (cmd) {
	case SLIOCGUNIT:
		*(int *)data = SL2IFP(sc)->if_dunit;
		break;

	case SLIOCSUNIT:
		unit = *(u_int *)data;
		if (unit < 0) {
			splx(s);
			return (ENXIO);
		}
		if (SL2IFP(sc)->if_dunit != unit) {
			if (!slisunitfree(unit)) {
				splx(s);
				return (ENXIO);
			}

			wasup = SL2IFP(sc)->if_flags & IFF_UP;
			bpfdetach(SL2IFP(sc));
			if_detach(SL2IFP(sc));
			LIST_REMOVE(sc, sl_next);
			if_initname(SL2IFP(sc), "sl", unit);
			LIST_INSERT_HEAD(&sl_list, sc, sl_next);
			if_attach(SL2IFP(sc));
			bpfattach(SL2IFP(sc), DLT_SLIP, SLIP_HDRLEN);
			if (wasup)
				if_up(SL2IFP(sc));
			else
				if_down(SL2IFP(sc));
			clist_alloc_cblocks(&tp->t_outq,
			    SLIP_HIWAT + 2 * SL2IFP(sc)->if_mtu + 1,
			    SLIP_HIWAT + 2 * SL2IFP(sc)->if_mtu + 1);
		}
		slmarkstatic(unit);
		break;

	case SLIOCSKEEPAL:
		sc->sc_keepalive = *(u_int *)data * hz;
		if (sc->sc_keepalive) {
			sc->sc_flags |= SC_KEEPALIVE;
			sc->sc_kahandle = timeout(sl_keepalive, sc,
						  sc->sc_keepalive);
		} else {
			if ((sc->sc_flags & SC_KEEPALIVE) != 0) {
				untimeout(sl_keepalive, sc, sc->sc_kahandle);
				sc->sc_flags &= ~SC_KEEPALIVE;
			}
		}
		break;

	case SLIOCGKEEPAL:
		*(int *)data = sc->sc_keepalive / hz;
		break;

	case SLIOCSOUTFILL:
		sc->sc_outfill = *(u_int *)data * hz;
		if (sc->sc_outfill) {
			sc->sc_flags |= SC_OUTWAIT;
			sc->sc_ofhandle = timeout(sl_outfill, sc,
						  sc->sc_outfill);
		} else {
			if ((sc->sc_flags & SC_OUTWAIT) != 0) {
				untimeout(sl_outfill, sc, sc->sc_ofhandle);
				sc->sc_flags &= ~SC_OUTWAIT;
			}
		}
		break;

	case SLIOCGOUTFILL:
		*(int *)data = sc->sc_outfill / hz;
		break;

	default:
		splx(s);
		return (ENOIOCTL);
	}
	splx(s);
	return (0);
}

/*
 * Queue a packet.  Start transmission if not active.
 * Compression happens in slstart; if we do it here, IP TOS
 * will cause us to not compress "background" packets, because
 * ordering gets trashed.  It can be done for all packets in slstart.
 */
static int
sloutput(struct ifnet *ifp, register struct mbuf *m, struct sockaddr *dst,
    struct rtentry *rtp)
{
	register struct sl_softc *sc = ifp->if_softc;
	register struct ip *ip;
	int error;

	/*
	 * `Cannot happen' (see slioctl).  Someday we will extend
	 * the line protocol to support other address families.
	 */
	if (dst->sa_family != AF_INET) {
		if_printf(ifp, "af%d not supported\n", dst->sa_family);
		m_freem(m);
		SL2IFP(sc)->if_noproto++;
		return (EAFNOSUPPORT);
	}

	if (sc->sc_ttyp == NULL || !(ifp->if_flags & IFF_UP)) {
		m_freem(m);
		return (ENETDOWN);
	}
	if ((sc->sc_ttyp->t_state & TS_CONNECTED) == 0) {
		m_freem(m);
		return (EHOSTUNREACH);
	}
	ip = mtod(m, struct ip *);
	if (SL2IFP(sc)->if_flags & SC_NOICMP && ip->ip_p == IPPROTO_ICMP) {
		m_freem(m);
		return (ENETRESET);		/* XXX ? */
	}
	if (ip->ip_tos & IPTOS_LOWDELAY &&
	    !ALTQ_IS_ENABLED(&SL2IFP(sc)->if_snd))
		error = !(IF_HANDOFF(&sc->sc_fastq, m, SL2IFP(sc)));
	else
		IFQ_HANDOFF(SL2IFP(sc), m, error);
	if (error) {
		SL2IFP(sc)->if_oerrors++;
		return (ENOBUFS);
	}
	return (0);
}

static void
slstart(struct ifnet *ifp)
{
	struct sl_softc *sc = ifp->if_softc;
	int s;

	s = splimp();
	if (sc->sc_ttyp->t_outq.c_cc == 0)
		sltstart(sc->sc_ttyp);
	splx(s);
}

/*
 * Start output on interface.  Get another datagram
 * to send from the interface queue and map it to
 * the interface before starting output.
 */
static int
sltstart(struct tty *tp)
{
	register struct sl_softc *sc = sl_for_tty(tp);
	register struct mbuf *m;
	register u_char *cp;
	register struct ip *ip;
	int s;
	register int len = 0;

	GIANT_REQUIRED;		/* tty */

	for (;;) {
		/*
		 * Call output process whether or not there is more in the
		 * output queue.  We are being called in lieu of ttstart
		 * and must do what it would.
		 */
		tt_oproc(tp);

		if (tp->t_outq.c_cc != 0) {
			if (sc != NULL)
				sc->sc_flags &= ~SC_OUTWAIT;
			if (tp->t_outq.c_cc > SLIP_HIWAT)
				return 0;
		}

		/*
		 * This happens briefly when the line shuts down.
		 */
		if (sc == NULL)
			return 0;

		/*
		 * Get a packet and send it to the interface.
		 */
		s = splimp();
		IF_DEQUEUE(&sc->sc_fastq, m);
		if (m)
			SL2IFP(sc)->if_omcasts++;		/* XXX */
		else
			IF_DEQUEUE(&SL2IFP(sc)->if_snd, m);
		splx(s);
		if (m == NULL)
			return 0;

		/*
		 * We do the header compression here rather than in sloutput
		 * because the packets will be out of order if we are using TOS
		 * queueing, and the connection id compression will get
		 * munged when this happens.
		 */
		if (bpf_peers_present(SL2IFP(sc)->if_bpf)) {
			/*
			 * We need to save the TCP/IP header before it's
			 * compressed.  To avoid complicated code, we just
			 * copy the entire packet into a stack buffer (since
			 * this is a serial line, packets should be short
			 * and/or the copy should be negligible cost compared
			 * to the packet transmission time).
			 */
			register struct mbuf *m1 = m;
			register u_char *cp;

			if (sc->bpfbuf == NULL)
				MALLOC(sc->bpfbuf, u_char *,
				    SLTMAX + SLIP_HDRLEN, M_SL, M_NOWAIT);

			if (sc->bpfbuf) {
				cp = sc->bpfbuf + SLIP_HDRLEN;
				len = 0;
				do {
					register int mlen = m1->m_len;

					bcopy(mtod(m1, caddr_t), cp, mlen);
					cp += mlen;
					len += mlen;
				} while ((m1 = m1->m_next) != NULL);
			}
		}
		ip = mtod(m, struct ip *);
		if (ip->ip_v == IPVERSION && ip->ip_p == IPPROTO_TCP) {
			if (SL2IFP(sc)->if_flags & SC_COMPRESS)
				*mtod(m, u_char *) |= sl_compress_tcp(m, ip,
				    &sc->sc_comp, 1);
		}
		if (bpf_peers_present(SL2IFP(sc)->if_bpf) && sc->bpfbuf) {
			/*
			 * Put the SLIP pseudo-"link header" in place.  The
			 * compressed header is now at the beginning of the
			 * mbuf.
			 */
			sc->bpfbuf[SLX_DIR] = SLIPDIR_OUT;
			bcopy(mtod(m, caddr_t), &sc->bpfbuf[SLX_CHDR], CHDR_LEN);
			BPF_TAP(SL2IFP(sc), sc->bpfbuf, len + SLIP_HDRLEN);
		}

		/*
		 * If system is getting low on clists, just flush our
		 * output queue (if the stuff was important, it'll get
		 * retransmitted). Note that SLTMAX is used instead of
		 * the current if_mtu setting because connections that
		 * have already been established still use the original
		 * (possibly larger) mss.
		 */
		if (cfreecount < CLISTRESERVE + SLTMAX) {
			m_freem(m);
			SL2IFP(sc)->if_collisions++;
			continue;
		}

		sc->sc_flags &= ~SC_OUTWAIT;
		/*
		 * The extra FRAME_END will start up a new packet, and thus
		 * will flush any accumulated garbage.  We do this whenever
		 * the line may have been idle for some time.
		 */
		if (tp->t_outq.c_cc == 0) {
			++SL2IFP(sc)->if_obytes;
			(void) putc(FRAME_END, &tp->t_outq);
		}

		while (m) {
			register u_char *ep;

			cp = mtod(m, u_char *); ep = cp + m->m_len;
			while (cp < ep) {
				/*
				 * Find out how many bytes in the string we can
				 * handle without doing something special.
				 */
				register u_char *bp = cp;

				while (cp < ep) {
					switch (*cp++) {
					case FRAME_ESCAPE:
					case FRAME_END:
						--cp;
						goto out;
					}
				}
				out:
				if (cp > bp) {
					/*
					 * Put n characters at once
					 * into the tty output queue.
					 */
					if (b_to_q((char *)bp, cp - bp,
					    &tp->t_outq))
						break;
					SL2IFP(sc)->if_obytes += cp - bp;
				}
				/*
				 * If there are characters left in the mbuf,
				 * the first one must be special..
				 * Put it out in a different form.
				 */
				if (cp < ep) {
					if (putc(FRAME_ESCAPE, &tp->t_outq))
						break;
					if (putc(*cp++ == FRAME_ESCAPE ?
					   TRANS_FRAME_ESCAPE : TRANS_FRAME_END,
					   &tp->t_outq)) {
						(void) unputc(&tp->t_outq);
						break;
					}
					SL2IFP(sc)->if_obytes += 2;
				}
			}
			m = m_free(m);
		}

		if (putc(FRAME_END, &tp->t_outq)) {
			/*
			 * Not enough room.  Remove a char to make room
			 * and end the packet normally.
			 * If you get many collisions (more than one or two
			 * a day) you probably do not have enough clists
			 * and you should increase "nclist" in param.c.
			 */
			(void) unputc(&tp->t_outq);
			(void) putc(FRAME_END, &tp->t_outq);
			SL2IFP(sc)->if_collisions++;
		} else {
			++SL2IFP(sc)->if_obytes;
			SL2IFP(sc)->if_opackets++;
		}
	}
	return 0;
}

/*
 * Copy data buffer to mbuf chain; add ifnet pointer.
 */
static struct mbuf *
sl_btom(struct sl_softc *sc, register int len)
{
	struct mbuf *m, *newm;

	MGETHDR(m, M_DONTWAIT, MT_DATA);
	if (m == NULL)
		return (NULL);

	/*
	 * If we have more than MHLEN bytes, it's cheaper to
	 * queue the cluster we just filled & allocate a new one
	 * for the input buffer.  Otherwise, fill the mbuf we
	 * allocated above.  Note that code in the input routine
	 * guarantees that packet will fit in a cluster.
	 */
	if (len >= MHLEN) {
		MCLGET(m, M_DONTWAIT);
		if ((m->m_flags & M_EXT) == 0) {
			/*
			 * we couldn't get a cluster - if memory's this
			 * low, it's time to start dropping packets.
			 */
			(void) m_free(m);
			return (NULL);
		}
		/* Swap the new and old clusters */
		newm = m;
		m = sc->sc_mbuf;
		sc->sc_mbuf = newm;
		sc->sc_ep = mtod(newm, u_char *) + SLBUFSIZE;
		
		m->m_data = (caddr_t)sc->sc_buf;
	} else
		bcopy((caddr_t)sc->sc_buf, mtod(m, caddr_t), len);

	m->m_len = len;
	m->m_pkthdr.len = len;
	m->m_pkthdr.rcvif = SL2IFP(sc);
	return (m);
}

/*
 * tty interface receiver interrupt.
 */
static int
slinput(int c, struct tty *tp)
{
	register struct sl_softc *sc;
	register struct mbuf *m;
	register int len;
	u_char chdr[CHDR_LEN];

	tk_nin++;
	sc = sl_for_tty(tp);
	if (sc == NULL)
		return 0;
	if (c & TTY_ERRORMASK || (tp->t_state & TS_CONNECTED) == 0) {
		sc->sc_flags |= SC_ERROR;
		return 0;
	}
	c &= TTY_CHARMASK;

	++SL2IFP(sc)->if_ibytes;

	if (SL2IFP(sc)->if_flags & IFF_DEBUG) {
		if (c == ABT_ESC) {
			/*
			 * If we have a previous abort, see whether
			 * this one is within the time limit.
			 */
			if (sc->sc_abortcount &&
			    time_uptime >= sc->sc_starttime + ABT_WINDOW)
				sc->sc_abortcount = 0;
			/*
			 * If we see an abort after "idle" time, count it;
			 * record when the first abort escape arrived.
			 */
			if (time_uptime >= sc->sc_lasttime + ABT_IDLE) {
				if (++sc->sc_abortcount == 1)
					sc->sc_starttime = time_uptime;
				if (sc->sc_abortcount >= ABT_COUNT) {
					slclose(tp,0);
					return 0;
				}
			}
		} else
			sc->sc_abortcount = 0;
		sc->sc_lasttime = time_uptime;
	}

	switch (c) {

	case TRANS_FRAME_ESCAPE:
		if (sc->sc_escape)
			c = FRAME_ESCAPE;
		break;

	case TRANS_FRAME_END:
		if (sc->sc_escape)
			c = FRAME_END;
		break;

	case FRAME_ESCAPE:
		sc->sc_escape = 1;
		return 0;

	case FRAME_END:
		sc->sc_flags &= ~SC_KEEPALIVE;
		if(sc->sc_flags & SC_ERROR) {
			sc->sc_flags &= ~SC_ERROR;
			goto newpack;
		}
		len = sc->sc_mp - sc->sc_buf;
		if (len < 3)
			/* less than min length packet - ignore */
			goto newpack;

		if (bpf_peers_present(SL2IFP(sc)->if_bpf)) {
			/*
			 * Save the compressed header, so we
			 * can tack it on later.  Note that we
			 * will end up copying garbage in some
			 * cases but this is okay.  We remember
			 * where the buffer started so we can
			 * compute the new header length.
			 */
			bcopy(sc->sc_buf, chdr, CHDR_LEN);
		}

		if ((c = (*sc->sc_buf & 0xf0)) != (IPVERSION << 4)) {
			if (c & 0x80)
				c = TYPE_COMPRESSED_TCP;
			else if (c == TYPE_UNCOMPRESSED_TCP)
				*sc->sc_buf &= 0x4f; /* XXX */
			/*
			 * We've got something that's not an IP packet.
			 * If compression is enabled, try to decompress it.
			 * Otherwise, if `auto-enable' compression is on and
			 * it's a reasonable packet, decompress it and then
			 * enable compression.  Otherwise, drop it.
			 */
			if (SL2IFP(sc)->if_flags & SC_COMPRESS) {
				len = sl_uncompress_tcp(&sc->sc_buf, len,
							(u_int)c, &sc->sc_comp);
				if (len <= 0)
					goto error;
			} else if ((SL2IFP(sc)->if_flags & SC_AUTOCOMP) &&
			    c == TYPE_UNCOMPRESSED_TCP && len >= 40) {
				len = sl_uncompress_tcp(&sc->sc_buf, len,
							(u_int)c, &sc->sc_comp);
				if (len <= 0)
					goto error;
				SL2IFP(sc)->if_flags |= SC_COMPRESS;
			} else
				goto error;
		}
		if (bpf_peers_present(SL2IFP(sc)->if_bpf)) {
			/*
			 * Put the SLIP pseudo-"link header" in place.
			 * We couldn't do this any earlier since
			 * decompression probably moved the buffer
			 * pointer.  Then, invoke BPF.
			 */
			register u_char *hp = sc->sc_buf - SLIP_HDRLEN;

			hp[SLX_DIR] = SLIPDIR_IN;
			bcopy(chdr, &hp[SLX_CHDR], CHDR_LEN);
			BPF_TAP(SL2IFP(sc), hp, len + SLIP_HDRLEN);
		}
		m = sl_btom(sc, len);
		if (m == NULL)
			goto error;

		SL2IFP(sc)->if_ipackets++;

		if ((SL2IFP(sc)->if_flags & IFF_UP) == 0) {
			m_freem(m);
			goto newpack;
		}
		if (netisr_queue(NETISR_IP, m)) {	/* (0) on success. */
			SL2IFP(sc)->if_ierrors++;
			SL2IFP(sc)->if_iqdrops++;
		}
		goto newpack;
	}
	if (sc->sc_mp < sc->sc_ep) {
		*sc->sc_mp++ = c;
		sc->sc_escape = 0;
		return 0;
	}

	/* can't put lower; would miss an extra frame */
	sc->sc_flags |= SC_ERROR;

error:
	SL2IFP(sc)->if_ierrors++;
newpack:
	sc->sc_mp = sc->sc_buf = sc->sc_ep - SLRMAX;
	sc->sc_escape = 0;
	return 0;
}

/*
 * Process an ioctl request.
 */
static int
slioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
{
	register struct ifaddr *ifa = (struct ifaddr *)data;
	register struct ifreq *ifr = (struct ifreq *)data;
	register int s, error = 0;
	struct sl_softc *sc = ifp->if_softc;

	s = splimp();

	switch (cmd) {

	case SIOCSIFFLAGS:
		/*
		 * if.c will set the interface up even if we
		 * don't want it to.
		 */
		if (sc->sc_ttyp == NULL) {
			ifp->if_flags &= ~IFF_UP;
		}
		break;
	case SIOCSIFADDR:
		/*
		 * This is "historical" - set the interface up when
		 * setting the address.
		 */
		if (ifa->ifa_addr->sa_family == AF_INET) {
			if (sc->sc_ttyp != NULL)
				ifp->if_flags |= IFF_UP;
		} else {
			error = EAFNOSUPPORT;
		}
		break;

	case SIOCSIFDSTADDR:
		if (ifa->ifa_addr->sa_family != AF_INET)
			error = EAFNOSUPPORT;
		break;

	case SIOCADDMULTI:
	case SIOCDELMULTI:
		break;

	case SIOCSIFMTU:
		/*
		 * Set the interface MTU.
		 */
		if (ifr->ifr_mtu > SLTMAX)
			error = EINVAL;
		else {
			struct tty *tp;

			ifp->if_mtu = ifr->ifr_mtu;
			tp = sc->sc_ttyp;
			if (tp != NULL)
				clist_alloc_cblocks(&tp->t_outq,
				    SLIP_HIWAT + 2 * ifp->if_mtu + 1,
				    SLIP_HIWAT + 2 * ifp->if_mtu + 1);
		}
		break;

	default:
		error = EINVAL;
	}
	splx(s);
	return (error);
}

static void
sl_keepalive(void *chan)
{
	struct sl_softc *sc = chan;

	if (sc->sc_keepalive) {
		if (sc->sc_flags & SC_KEEPALIVE) {
			if (sc->sc_ttyp->t_pgrp != NULL) {
				PGRP_LOCK(sc->sc_ttyp->t_pgrp);
				pgsignal (sc->sc_ttyp->t_pgrp, SIGURG, 1);
				PGRP_UNLOCK(sc->sc_ttyp->t_pgrp);
			}
		} else
			sc->sc_flags |= SC_KEEPALIVE;
		sc->sc_kahandle = timeout(sl_keepalive, sc, sc->sc_keepalive);
	} else {
		sc->sc_flags &= ~SC_KEEPALIVE;
	}
}

static void
sl_outfill(void *chan)
{
	struct sl_softc *sc = chan;
	register struct tty *tp = sc->sc_ttyp;
	int s;

	if (sc->sc_outfill && tp != NULL) {
		if (sc->sc_flags & SC_OUTWAIT) {
			s = splimp ();
			++SL2IFP(sc)->if_obytes;
			(void) putc(FRAME_END, &tp->t_outq);
			tt_oproc(tp);
			splx (s);
		} else
			sc->sc_flags |= SC_OUTWAIT;
		sc->sc_ofhandle = timeout(sl_outfill, sc, sc->sc_outfill);
	} else {
		sc->sc_flags &= ~SC_OUTWAIT;
	}
}