xref: /freebsd/sys/netgraph/ng_pptpgre.c (revision e962a823087f27141470dd856cdd84e1d28cc557)

/*
 * ng_pptpgre.c
 *
 * Copyright (c) 1996-1999 Whistle Communications, Inc.
 * All rights reserved.
 *
 * Subject to the following obligations and disclaimer of warranty, use and
 * redistribution of this software, in source or object code forms, with or
 * without modifications are expressly permitted by Whistle Communications;
 * provided, however, that:
 * 1. Any and all reproductions of the source or object code must include the
 *    copyright notice above and the following disclaimer of warranties; and
 * 2. No rights are granted, in any manner or form, to use Whistle
 *    Communications, Inc. trademarks, including the mark "WHISTLE
 *    COMMUNICATIONS" on advertising, endorsements, or otherwise except as
 *    such appears in the above copyright notice or in the software.
 *
 * THIS SOFTWARE IS BEING PROVIDED BY WHISTLE COMMUNICATIONS "AS IS", AND
 * TO THE MAXIMUM EXTENT PERMITTED BY LAW, WHISTLE COMMUNICATIONS MAKES NO
 * REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED, REGARDING THIS SOFTWARE,
 * INCLUDING WITHOUT LIMITATION, ANY AND ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT.
 * WHISTLE COMMUNICATIONS DOES NOT WARRANT, GUARANTEE, OR MAKE ANY
 * REPRESENTATIONS REGARDING THE USE OF, OR THE RESULTS OF THE USE OF THIS
 * SOFTWARE IN TERMS OF ITS CORRECTNESS, ACCURACY, RELIABILITY OR OTHERWISE.
 * IN NO EVENT SHALL WHISTLE COMMUNICATIONS BE LIABLE FOR ANY DAMAGES
 * RESULTING FROM OR ARISING OUT OF ANY USE OF THIS SOFTWARE, INCLUDING
 * WITHOUT LIMITATION, ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
 * PUNITIVE, OR CONSEQUENTIAL DAMAGES, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES, LOSS OF USE, DATA OR PROFITS, HOWEVER CAUSED AND UNDER 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 WHISTLE COMMUNICATIONS IS ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * Author: Archie Cobbs <archie@whistle.com>
 *
 * $FreeBSD$
 * $Whistle: ng_pptpgre.c,v 1.7 1999/12/08 00:10:06 archie Exp $
 */

/*
 * PPTP/GRE netgraph node type.
 *
 * This node type does the GRE encapsulation as specified for the PPTP
 * protocol (RFC 2637, section 4).  This includes sequencing and
 * retransmission of frames, but not the actual packet delivery nor
 * any of the TCP control stream protocol.
 *
 * The "upper" hook of this node is suitable for attaching to a "ppp"
 * node link hook.  The "lower" hook of this node is suitable for attaching
 * to a "ksocket" node on hook "inet/raw/gre".
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/time.h>
#include <sys/mbuf.h>
#include <sys/malloc.h>
#include <sys/errno.h>
#include <sys/socket.h>
#include <sys/syslog.h>
#include <sys/ctype.h>

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>

#include <netgraph/ng_message.h>
#include <netgraph/netgraph.h>
#include <netgraph/ng_parse.h>
#include <netgraph/ng_pptpgre.h>

/* GRE packet format, as used by PPTP */
struct greheader {
#if BYTE_ORDER == LITTLE_ENDIAN
	u_char		recursion:3;		/* recursion control */
	u_char		ssr:1;			/* strict source route */
	u_char		hasSeq:1;		/* sequence number present */
	u_char		hasKey:1;		/* key present */
	u_char		hasRoute:1;		/* routing present */
	u_char		hasSum:1;		/* checksum present */
	u_char		vers:3;			/* version */
	u_char		flags:4;		/* flags */
	u_char		hasAck:1;		/* acknowlege number present */
#elif BYTE_ORDER == BIG_ENDIAN
	u_char		hasSum:1;		/* checksum present */
	u_char		hasRoute:1;		/* routing present */
	u_char		hasKey:1;		/* key present */
	u_char		hasSeq:1;		/* sequence number present */
	u_char		ssr:1;			/* strict source route */
	u_char		recursion:3;		/* recursion control */
	u_char		hasAck:1;		/* acknowlege number present */
	u_char		flags:4;		/* flags */
	u_char		vers:3;			/* version */
#else
#error BYTE_ORDER is not defined properly
#endif
	u_int16_t	proto;			/* protocol (ethertype) */
	u_int16_t	length;			/* payload length */
	u_int16_t	cid;			/* call id */
	u_int32_t	data[0];		/* opt. seq, ack, then data */
};

/* The PPTP protocol ID used in the GRE 'proto' field */
#define PPTP_GRE_PROTO		0x880b

/* Bits that must be set a certain way in all PPTP/GRE packets */
#define PPTP_INIT_VALUE		((0x2001 << 16) | PPTP_GRE_PROTO)
#define PPTP_INIT_MASK		0xef7fffff

/* Min and max packet length */
#define PPTP_MAX_PAYLOAD	(0xffff - sizeof(struct greheader) - 8)

/* All times are scaled by this (PPTP_TIME_SCALE time units = 1 sec.) */
#define PPTP_TIME_SCALE		1000			/* milliseconds */
typedef u_int32_t		pptptime_t;

/* Acknowledgment timeout parameters and functions */
#define PPTP_XMIT_WIN		16			/* max xmit window */
#define PPTP_MIN_RTT		(PPTP_TIME_SCALE / 10)	/* 100 milliseconds */
#define PPTP_MIN_TIMEOUT	(PPTP_TIME_SCALE / 100)	/* 10 milliseconds */
#define PPTP_MAX_TIMEOUT	(10 * PPTP_TIME_SCALE)	/* 10 seconds */

/* See RFC 2637 section 4.4 */
#define PPTP_ACK_ALPHA(x)	((x) >> 3)	/* alpha = 0.125 */
#define PPTP_ACK_BETA(x)	((x) >> 2)	/* beta = 0.25 */
#define PPTP_ACK_CHI(x) 	((x) << 2)	/* chi = 4 */
#define PPTP_ACK_DELTA(x) 	((x) << 1)	/* delta = 2 */

#define PPTP_SEQ_DIFF(x,y)	((int32_t)(x) - (int32_t)(y))

/* We keep packet retransmit and acknowlegement state in this struct */
struct ng_pptpgre_ackp {
	int32_t			ato;		/* adaptive time-out value */
	int32_t			rtt;		/* round trip time estimate */
	int32_t			dev;		/* deviation estimate */
	u_int16_t		xmitWin;	/* size of xmit window */
	struct callout_handle	sackTimer;	/* send ack timer */
	struct callout_handle	rackTimer;	/* recv ack timer */
	node_p			*sackTimerPtr;	/* send ack timer pointer */
	node_p			*rackTimerPtr;	/* recv ack timer pointer */
	u_int32_t		winAck;		/* seq when xmitWin will grow */
	pptptime_t		timeSent[PPTP_XMIT_WIN];
};

/* When we recieve a packet, we wait to see if there's an outgoing packet
   we can piggy-back the ACK off of.  These parameters determine the mimimum
   and maxmimum length of time we're willing to wait in order to do that. */
#define PPTP_MAX_ACK_DELAY	((int) (0.25 * PPTP_TIME_SCALE))

/* Node private data */
struct ng_pptpgre_private {
	hook_p			upper;		/* hook to upper layers */
	hook_p			lower;		/* hook to lower layers */
	struct ng_pptpgre_conf	conf;		/* configuration info */
	struct ng_pptpgre_ackp	ackp;		/* packet transmit ack state */
	u_int32_t		recvSeq;	/* last seq # we rcv'd */
	u_int32_t		xmitSeq;	/* last seq # we sent */
	u_int32_t		recvAck;	/* last seq # peer ack'd */
	u_int32_t		xmitAck;	/* last seq # we ack'd */
	struct timeval		startTime;	/* time node was created */
	struct ng_pptpgre_stats	stats;		/* node statistics */
};
typedef struct ng_pptpgre_private *priv_p;

/* Netgraph node methods */
static ng_constructor_t	ng_pptpgre_constructor;
static ng_rcvmsg_t	ng_pptpgre_rcvmsg;
static ng_shutdown_t	ng_pptpgre_rmnode;
static ng_newhook_t	ng_pptpgre_newhook;
static ng_rcvdata_t	ng_pptpgre_rcvdata;
static ng_disconnect_t	ng_pptpgre_disconnect;

/* Helper functions */
static int	ng_pptpgre_xmit(node_p node, struct mbuf *m, meta_p meta);
static int	ng_pptpgre_recv(node_p node, struct mbuf *m, meta_p meta);
static void	ng_pptpgre_start_send_ack_timer(node_p node, long ackTimeout);
static void	ng_pptpgre_start_recv_ack_timer(node_p node);
static void	ng_pptpgre_recv_ack_timeout(void *arg);
static void	ng_pptpgre_send_ack_timeout(void *arg);
static void	ng_pptpgre_reset(node_p node);
static pptptime_t ng_pptpgre_time(node_p node);

/* Parse type for struct ng_pptpgre_conf */
static const struct ng_parse_struct_info
	ng_pptpgre_conf_type_info = NG_PPTPGRE_CONF_TYPE_INFO;
static const struct ng_parse_type ng_pptpgre_conf_type = {
	&ng_parse_struct_type,
	&ng_pptpgre_conf_type_info,
};

/* Parse type for struct ng_pptpgre_stats */
static const struct ng_parse_struct_info
	ng_pptpgre_stats_type_info = NG_PPTPGRE_STATS_TYPE_INFO;
static const struct ng_parse_type ng_pptp_stats_type = {
	&ng_parse_struct_type,
	&ng_pptpgre_stats_type_info
};

/* List of commands and how to convert arguments to/from ASCII */
static const struct ng_cmdlist ng_pptpgre_cmdlist[] = {
	{
	  NGM_PPTPGRE_COOKIE,
	  NGM_PPTPGRE_SET_CONFIG,
	  "setconfig",
	  &ng_pptpgre_conf_type,
	  NULL
	},
	{
	  NGM_PPTPGRE_COOKIE,
	  NGM_PPTPGRE_GET_CONFIG,
	  "getconfig",
	  NULL,
	  &ng_pptpgre_conf_type
	},
	{
	  NGM_PPTPGRE_COOKIE,
	  NGM_PPTPGRE_GET_STATS,
	  "getstats",
	  NULL,
	  &ng_pptp_stats_type
	},
	{
	  NGM_PPTPGRE_COOKIE,
	  NGM_PPTPGRE_CLR_STATS,
	  "clrstats",
	  NULL,
	  NULL
	},
	{
	  NGM_PPTPGRE_COOKIE,
	  NGM_PPTPGRE_GETCLR_STATS,
	  "getclrstats",
	  NULL,
	  &ng_pptp_stats_type
	},
	{ 0 }
};

/* Node type descriptor */
static struct ng_type ng_pptpgre_typestruct = {
	NG_VERSION,
	NG_PPTPGRE_NODE_TYPE,
	NULL,
	ng_pptpgre_constructor,
	ng_pptpgre_rcvmsg,
	ng_pptpgre_rmnode,
	ng_pptpgre_newhook,
	NULL,
	NULL,
	ng_pptpgre_rcvdata,
	ng_pptpgre_rcvdata,
	ng_pptpgre_disconnect,
	ng_pptpgre_cmdlist
};
NETGRAPH_INIT(pptpgre, &ng_pptpgre_typestruct);

#define ERROUT(x)	do { error = (x); goto done; } while (0)

/************************************************************************
			NETGRAPH NODE STUFF
 ************************************************************************/

/*
 * Node type constructor
 */
static int
ng_pptpgre_constructor(node_p *nodep)
{
	priv_p priv;
	int error;

	/* Allocate private structure */
	MALLOC(priv, priv_p, sizeof(*priv), M_NETGRAPH, M_WAITOK);
	if (priv == NULL)
		return (ENOMEM);
	bzero(priv, sizeof(*priv));

	/* Call generic node constructor */
	if ((error = ng_make_node_common(&ng_pptpgre_typestruct, nodep))) {
		FREE(priv, M_NETGRAPH);
		return (error);
	}
	(*nodep)->private = priv;

	/* Initialize state */
	callout_handle_init(&priv->ackp.sackTimer);
	callout_handle_init(&priv->ackp.rackTimer);

	/* Done */
	return (0);
}

/*
 * Give our OK for a hook to be added.
 */
static int
ng_pptpgre_newhook(node_p node, hook_p hook, const char *name)
{
	const priv_p priv = node->private;
	hook_p *hookPtr;

	/* Check hook name */
	if (strcmp(name, NG_PPTPGRE_HOOK_UPPER) == 0)
		hookPtr = &priv->upper;
	else if (strcmp(name, NG_PPTPGRE_HOOK_LOWER) == 0)
		hookPtr = &priv->lower;
	else
		return (EINVAL);

	/* See if already connected */
	if (*hookPtr != NULL)
		return (EISCONN);

	/* OK */
	*hookPtr = hook;
	return (0);
}

/*
 * Receive a control message.
 */
static int
ng_pptpgre_rcvmsg(node_p node, struct ng_mesg *msg,
	      const char *raddr, struct ng_mesg **rptr, hook_p lasthook)
{
	const priv_p priv = node->private;
	struct ng_mesg *resp = NULL;
	int error = 0;

	switch (msg->header.typecookie) {
	case NGM_PPTPGRE_COOKIE:
		switch (msg->header.cmd) {
		case NGM_PPTPGRE_SET_CONFIG:
		    {
			struct ng_pptpgre_conf *const newConf =
				(struct ng_pptpgre_conf *) msg->data;

			/* Check for invalid or illegal config */
			if (msg->header.arglen != sizeof(*newConf))
				ERROUT(EINVAL);
			ng_pptpgre_reset(node);		/* reset on configure */
			priv->conf = *newConf;
			break;
		    }
		case NGM_PPTPGRE_GET_CONFIG:
			NG_MKRESPONSE(resp, msg, sizeof(priv->conf), M_NOWAIT);
			if (resp == NULL)
				ERROUT(ENOMEM);
			bcopy(&priv->conf, resp->data, sizeof(priv->conf));
			break;
		case NGM_PPTPGRE_GET_STATS:
		case NGM_PPTPGRE_CLR_STATS:
		case NGM_PPTPGRE_GETCLR_STATS:
		    {
			if (msg->header.cmd != NGM_PPTPGRE_CLR_STATS) {
				NG_MKRESPONSE(resp, msg,
				    sizeof(priv->stats), M_NOWAIT);
				if (resp == NULL)
					ERROUT(ENOMEM);
				bcopy(&priv->stats,
				    resp->data, sizeof(priv->stats));
			}
			if (msg->header.cmd != NGM_PPTPGRE_GET_STATS)
				bzero(&priv->stats, sizeof(priv->stats));
			break;
		    }
		default:
			error = EINVAL;
			break;
		}
		break;
	default:
		error = EINVAL;
		break;
	}
	if (rptr)
		*rptr = resp;
	else if (resp)
		FREE(resp, M_NETGRAPH);

done:
	FREE(msg, M_NETGRAPH);
	return (error);
}

/*
 * Receive incoming data on a hook.
 */
static int
ng_pptpgre_rcvdata(hook_p hook, struct mbuf *m, meta_p meta,
		struct mbuf **ret_m, meta_p *ret_meta)
{
	const node_p node = hook->node;
	const priv_p priv = node->private;

	/* If not configured, reject */
	if (!priv->conf.enabled) {
		NG_FREE_DATA(m, meta);
		return (ENXIO);
	}

	/* Treat as xmit or recv data */
	if (hook == priv->upper)
		return ng_pptpgre_xmit(node, m, meta);
	if (hook == priv->lower)
		return ng_pptpgre_recv(node, m, meta);
	panic("%s: weird hook", __FUNCTION__);
}

/*
 * Destroy node
 */
static int
ng_pptpgre_rmnode(node_p node)
{
	const priv_p priv = node->private;

	/* Reset node */
	ng_pptpgre_reset(node);

	/* Take down netgraph node */
	node->flags |= NG_INVALID;
	ng_cutlinks(node);
	ng_unname(node);
	bzero(priv, sizeof(*priv));
	FREE(priv, M_NETGRAPH);
	node->private = NULL;
	ng_unref(node);
	return (0);
}

/*
 * Hook disconnection
 */
static int
ng_pptpgre_disconnect(hook_p hook)
{
	const node_p node = hook->node;
	const priv_p priv = node->private;

	/* Zero out hook pointer */
	if (hook == priv->upper)
		priv->upper = NULL;
	else if (hook == priv->lower)
		priv->lower = NULL;
	else
		panic("%s: unknown hook", __FUNCTION__);

	/* Go away if no longer connected to anything */
	if (node->numhooks == 0)
		ng_rmnode(node);
	return (0);
}

/*************************************************************************
		    TRANSMIT AND RECEIVE FUNCTIONS
*************************************************************************/

/*
 * Transmit an outgoing frame, or just an ack if m is NULL.
 */
static int
ng_pptpgre_xmit(node_p node, struct mbuf *m, meta_p meta)
{
	const priv_p priv = node->private;
	struct ng_pptpgre_ackp *const a = &priv->ackp;
	u_char buf[sizeof(struct greheader) + 2 * sizeof(u_int32_t)];
	struct greheader *const gre = (struct greheader *)buf;
	int grelen, error;

	/* Check if there's data */
	if (m != NULL) {

		/* Is our transmit window full? */
		if ((u_int32_t)PPTP_SEQ_DIFF(priv->xmitSeq, priv->recvAck)
		      >= a->xmitWin) {
			priv->stats.xmitDrops++;
			NG_FREE_DATA(m, meta);
			return (ENOBUFS);
		}

		/* Sanity check frame length */
		if (m != NULL && m->m_pkthdr.len > PPTP_MAX_PAYLOAD) {
			priv->stats.xmitTooBig++;
			NG_FREE_DATA(m, meta);
			return (EMSGSIZE);
		}
	} else
		priv->stats.xmitLoneAcks++;

	/* Build GRE header */
	((u_int32_t *) gre)[0] = htonl(PPTP_INIT_VALUE);
	gre->length = (m != NULL) ? htons((u_short)m->m_pkthdr.len) : 0;
	gre->cid = htons(priv->conf.peerCid);

	/* Include sequence number if packet contains any data */
	if (m != NULL) {
		gre->hasSeq = 1;
		a->timeSent[priv->xmitSeq - priv->recvAck]
		    = ng_pptpgre_time(node);
		priv->xmitSeq++;
		gre->data[0] = htonl(priv->xmitSeq);
		if (priv->xmitSeq == priv->recvAck + 1)
			ng_pptpgre_start_recv_ack_timer(node);
	}

	/* Include acknowledgement (and stop send ack timer) if needed */
	if (PPTP_SEQ_DIFF(priv->xmitAck, priv->recvSeq) < 0) {
		gre->hasAck = 1;
		priv->xmitAck = priv->recvSeq;
		gre->data[gre->hasSeq] = htonl(priv->xmitAck);
		a->sackTimerPtr = NULL;
	}

	/* Prepend GRE header to outgoing frame */
	grelen = sizeof(*gre) + sizeof(u_int32_t) * (gre->hasSeq + gre->hasAck);
	if (m == NULL) {
		MGETHDR(m, M_DONTWAIT, MT_DATA);
		if (m == NULL) {
			NG_FREE_META(meta);
			return (ENOBUFS);
		}
		m->m_len = m->m_pkthdr.len = grelen;
		m->m_pkthdr.rcvif = NULL;
	} else {
		M_PREPEND(m, grelen, M_NOWAIT);
		if (m == NULL || (m->m_len < grelen
		    && (m = m_pullup(m, grelen)) == NULL)) {
			NG_FREE_META(meta);
			return (ENOBUFS);
		}
	}
	bcopy(gre, mtod(m, u_char *), grelen);

	/* Update stats */
	priv->stats.xmitPackets++;
	priv->stats.xmitOctets += m->m_pkthdr.len;

	/* Deliver packet */
	NG_SEND_DATA(error, priv->lower, m, meta);
	return (error);
}

/*
 * Handle an incoming packet.  The packet includes the IP header.
 */
static int
ng_pptpgre_recv(node_p node, struct mbuf *m, meta_p meta)
{
	const priv_p priv = node->private;
	int iphlen, grelen, extralen;
	struct greheader *gre;
	struct ip *ip;
	int error = 0;

	/* Update stats */
	priv->stats.recvPackets++;
	priv->stats.recvOctets += m->m_pkthdr.len;

	/* Sanity check packet length */
	if (m->m_pkthdr.len < sizeof(*ip) + sizeof(*gre)) {
		priv->stats.recvRunts++;
bad:
		NG_FREE_DATA(m, meta);
		return (EINVAL);
	}

	/* Safely pull up the complete IP+GRE headers */
	if (m->m_len < sizeof(*ip) + sizeof(*gre)
	    && (m = m_pullup(m, sizeof(*ip) + sizeof(*gre))) == NULL) {
		NG_FREE_META(meta);
		return (ENOBUFS);
	}
	ip = mtod(m, struct ip *);
	iphlen = ip->ip_hl << 2;
	if (m->m_len < iphlen + sizeof(*gre)) {
		if ((m = m_pullup(m, iphlen + sizeof(*gre))) == NULL) {
			NG_FREE_META(meta);
			return (ENOBUFS);
		}
		ip = mtod(m, struct ip *);
	}
	gre = (struct greheader *)((u_char *)ip + iphlen);
	grelen = sizeof(*gre) + sizeof(u_int32_t) * (gre->hasSeq + gre->hasAck);
	if (m->m_pkthdr.len < iphlen + grelen) {
		priv->stats.recvRunts++;
		goto bad;
	}
	if (m->m_len < iphlen + grelen) {
		if ((m = m_pullup(m, iphlen + grelen)) == NULL) {
			NG_FREE_META(meta);
			return (ENOBUFS);
		}
		ip = mtod(m, struct ip *);
		gre = (struct greheader *)((u_char *)ip + iphlen);
	}

	/* Sanity check packet length and GRE header bits */
	extralen = m->m_pkthdr.len
	    - (iphlen + grelen + (u_int16_t)ntohs(gre->length));
	if (extralen < 0) {
		priv->stats.recvBadGRE++;
		goto bad;
	}
	if ((ntohl(*((u_int32_t *)gre)) & PPTP_INIT_MASK) != PPTP_INIT_VALUE) {
		priv->stats.recvBadGRE++;
		goto bad;
	}
	if (ntohs(gre->cid) != priv->conf.cid) {
		priv->stats.recvBadCID++;
		goto bad;
	}

	/* Look for peer ack */
	if (gre->hasAck) {
		struct ng_pptpgre_ackp *const a = &priv->ackp;
		const u_int32_t	ack = ntohl(gre->data[gre->hasSeq]);
		const int index = ack - priv->recvAck - 1;
		const long sample = ng_pptpgre_time(node) - a->timeSent[index];
		long diff;

		/* Sanity check ack value */
		if (PPTP_SEQ_DIFF(ack, priv->xmitSeq) > 0) {
			priv->stats.recvBadAcks++;
			goto badAck;		/* we never sent it! */
		}
		if (PPTP_SEQ_DIFF(ack, priv->recvAck) <= 0)
			goto badAck;		/* ack already timed out */
		priv->recvAck = ack;

		/* Update adaptive timeout stuff */
		diff = sample - a->rtt;
		a->rtt += PPTP_ACK_ALPHA(diff);
		if (diff < 0)
			diff = -diff;
		a->dev += PPTP_ACK_BETA(diff - a->dev);
		a->ato = a->rtt + PPTP_ACK_CHI(a->dev);
		if (a->ato > PPTP_MAX_TIMEOUT)
			a->ato = PPTP_MAX_TIMEOUT;
		if (a->ato < PPTP_MIN_TIMEOUT)
			a->ato = PPTP_MIN_TIMEOUT;

		/* Shift packet transmit times in our transmit window */
		ovbcopy(a->timeSent + index + 1, a->timeSent,
		    sizeof(*a->timeSent) * (PPTP_XMIT_WIN - (index + 1)));

		/* If we sent an entire window, increase window size by one */
		if (PPTP_SEQ_DIFF(ack, a->winAck) >= 0
		    && a->xmitWin < PPTP_XMIT_WIN) {
			a->xmitWin++;
			a->winAck = ack + a->xmitWin;
		}

		/* Stop/(re)start receive ACK timer as necessary */
		ng_pptpgre_start_recv_ack_timer(node);
	}
badAck:

	/* See if frame contains any data */
	if (gre->hasSeq) {
		struct ng_pptpgre_ackp *const a = &priv->ackp;
		const u_int32_t seq = ntohl(gre->data[0]);

		/* Sanity check sequence number */
		if (PPTP_SEQ_DIFF(seq, priv->recvSeq) <= 0) {
			if (seq == priv->recvSeq)
				priv->stats.recvDuplicates++;
			else
				priv->stats.recvOutOfOrder++;
			goto bad;		/* out-of-order or dup */
		}
		priv->recvSeq = seq;

		/* We need to acknowledge this packet; do it soon... */
		if (a->sackTimerPtr == NULL) {
			long maxWait;

			/* Take half of the estimated round trip time */
			maxWait = (a->rtt >> 1);

			/* If too soon, just send one right now */
			if (!priv->conf.enableDelayedAck)
				ng_pptpgre_xmit(node, NULL, NULL);
			else {			/* send the ack later */
				if (maxWait > PPTP_MAX_ACK_DELAY)
					maxWait = PPTP_MAX_ACK_DELAY;
				ng_pptpgre_start_send_ack_timer(node, maxWait);
			}
		}

		/* Trim mbuf down to internal payload */
		m_adj(m, iphlen + grelen);
		if (extralen > 0)
			m_adj(m, -extralen);

		/* Deliver frame to upper layers */
		NG_SEND_DATA(error, priv->upper, m, meta);
	} else {
		priv->stats.recvLoneAcks++;
		NG_FREE_DATA(m, meta);		/* no data to deliver */
	}
	return (error);
}

/*************************************************************************
		    TIMER RELATED FUNCTIONS
*************************************************************************/

/*
 * Start a timer for the peer's acknowledging our oldest unacknowledged
 * sequence number.  If we get an ack for this sequence number before
 * the timer goes off, we cancel the timer.  Resets currently running
 * recv ack timer, if any.
 */
static void
ng_pptpgre_start_recv_ack_timer(node_p node)
{
	const priv_p priv = node->private;
	struct ng_pptpgre_ackp *const a = &priv->ackp;
	int remain;

	/* "Stop" current recv ack timer, if any */
	a->rackTimerPtr = NULL;

	/* Are we waiting for an acknowlegement? */
	if (priv->recvAck == priv->xmitSeq)
		return;

	/* Compute how long until oldest unack'd packet times out,
	   and reset the timer to that time. */
	remain = (a->timeSent[0] + a->ato) - ng_pptpgre_time(node);
	if (remain < 0)
		remain = 0;

	/* Start new timer */
	MALLOC(a->rackTimerPtr, node_p *, sizeof(node_p), M_NETGRAPH, M_NOWAIT);
	if (a->rackTimerPtr == NULL)
		return;			/* XXX potential hang here */
	*a->rackTimerPtr = node;	/* insures the correct timeout event */
	node->refs++;
	a->rackTimer = timeout(ng_pptpgre_recv_ack_timeout,
	    a->rackTimerPtr, remain * hz / PPTP_TIME_SCALE);
}

/*
 * The peer has failed to acknowledge the oldest unacknowledged sequence
 * number within the time allotted.  Update our adaptive timeout parameters
 * and reset/restart the recv ack timer.
 */
static void
ng_pptpgre_recv_ack_timeout(void *arg)
{
	int s = splnet();
	const node_p node = *((node_p *)arg);
	const priv_p priv = node->private;
	struct ng_pptpgre_ackp *const a = &priv->ackp;

	/* This complicated stuff is needed to avoid race conditions */
	FREE(arg, M_NETGRAPH);
	KASSERT(node->refs > 0, ("%s: no refs", __FUNCTION__));
	if ((node->flags & NG_INVALID) != 0) {	/* shutdown race condition */
		ng_unref(node);
		splx(s);
		return;
	}
	ng_unref(node);
	if (arg != a->rackTimerPtr) {	/* timer stopped race condition */
		splx(s);
		return;
	}
	a->rackTimerPtr = NULL;

	/* Update adaptive timeout stuff */
	priv->stats.recvAckTimeouts++;
	a->rtt = PPTP_ACK_DELTA(a->rtt);
	a->ato = a->rtt + PPTP_ACK_CHI(a->dev);
	if (a->ato > PPTP_MAX_TIMEOUT)
		a->ato = PPTP_MAX_TIMEOUT;
	if (a->ato < PPTP_MIN_TIMEOUT)
		a->ato = PPTP_MIN_TIMEOUT;

	/* Reset ack and sliding window */
	priv->recvAck = priv->xmitSeq;		/* pretend we got the ack */
	a->xmitWin = (a->xmitWin + 1) / 2;	/* shrink transmit window */
	a->winAck = priv->recvAck + a->xmitWin;	/* reset win expand time */
	splx(s);
}

/*
 * Start the send ack timer. This assumes the timer is not
 * already running.
 */
static void
ng_pptpgre_start_send_ack_timer(node_p node, long ackTimeout)
{
	const priv_p priv = node->private;
	struct ng_pptpgre_ackp *const a = &priv->ackp;

	/* Start new timer */
	KASSERT(a->sackTimerPtr == NULL, ("%s: sackTimer", __FUNCTION__));
	MALLOC(a->sackTimerPtr, node_p *, sizeof(node_p), M_NETGRAPH, M_NOWAIT);
	if (a->sackTimerPtr == NULL)
		return;			/* XXX potential hang here */
	*a->sackTimerPtr = node;
	node->refs++;
	a->sackTimer = timeout(ng_pptpgre_send_ack_timeout,
	    a->sackTimerPtr, ackTimeout * hz / PPTP_TIME_SCALE);
}

/*
 * We've waited as long as we're willing to wait before sending an
 * acknowledgement to the peer for received frames. We had hoped to
 * be able to piggy back our acknowledgement on an outgoing data frame,
 * but apparently there haven't been any since. So send the ack now.
 */
static void
ng_pptpgre_send_ack_timeout(void *arg)
{
	int s = splnet();
	const node_p node = *((node_p *)arg);
	const priv_p priv = node->private;
	struct ng_pptpgre_ackp *const a = &priv->ackp;

	/* This complicated stuff is needed to avoid race conditions */
	FREE(arg, M_NETGRAPH);
	KASSERT(node->refs > 0, ("%s: no refs", __FUNCTION__));
	if ((node->flags & NG_INVALID) != 0) {	/* shutdown race condition */
		ng_unref(node);
		splx(s);
		return;
	}
	ng_unref(node);
	if (a->sackTimerPtr != arg) {	/* timer stopped race condition */
		splx(s);
		return;
	}
	a->sackTimerPtr = NULL;

	/* Send a frame with an ack but no payload */
  	ng_pptpgre_xmit(node, NULL, NULL);
	splx(s);
}

/*************************************************************************
		    MISC FUNCTIONS
*************************************************************************/

/*
 * Reset state
 */
static void
ng_pptpgre_reset(node_p node)
{
	const priv_p priv = node->private;
	struct ng_pptpgre_ackp *const a = &priv->ackp;

	/* Reset adaptive timeout state */
	a->ato = PPTP_MAX_TIMEOUT;
	a->rtt = priv->conf.peerPpd * PPTP_TIME_SCALE / 10;  /* ppd in 10ths */
	if (a->rtt < PPTP_MIN_RTT)
		a->rtt = PPTP_MIN_RTT;
	a->dev = 0;
	a->xmitWin = (priv->conf.recvWin + 1) / 2;
	if (a->xmitWin < 2)		/* often the first packet is lost */
		a->xmitWin = 2;		/*   because the peer isn't ready */
	if (a->xmitWin > PPTP_XMIT_WIN)
		a->xmitWin = PPTP_XMIT_WIN;
	a->winAck = a->xmitWin;

	/* Reset sequence numbers */
	priv->recvSeq = ~0;
	priv->recvAck = ~0;
	priv->xmitSeq = ~0;
	priv->xmitAck = ~0;

	/* Reset start time */
	getmicrouptime(&priv->startTime);

	/* Reset stats */
	bzero(&priv->stats, sizeof(priv->stats));

	/* "Stop" timers */
	a->sackTimerPtr = NULL;
	a->rackTimerPtr = NULL;
}

/*
 * Return the current time scaled & translated to our internally used format.
 */
static pptptime_t
ng_pptpgre_time(node_p node)
{
	const priv_p priv = node->private;
	struct timeval tv;

	getmicrouptime(&tv);
	if (tv.tv_sec < priv->startTime.tv_sec
	    || (tv.tv_sec == priv->startTime.tv_sec
	      && tv.tv_usec < priv->startTime.tv_usec))
		return (0);
	timevalsub(&tv, &priv->startTime);
	tv.tv_sec *= PPTP_TIME_SCALE;
	tv.tv_usec /= 1000000 / PPTP_TIME_SCALE;
	return(tv.tv_sec + tv.tv_usec);
}