sys/netgraph/l2tp/ng_l2tp.c

root/sys/netgraph/l2tp/ng_l2tp.c

/*
 * Copyright (c) 2001-2002 Packet Design, LLC.
 * 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
 * Packet Design; provided, however, that:
 * 
 *    (i)  Any and all reproductions of the source or object code
 *         must include the copyright notice above and the following
 *         disclaimer of warranties; and
 *    (ii) No rights are granted, in any manner or form, to use
 *         Packet Design trademarks, including the mark "PACKET DESIGN"
 *         on advertising, endorsements, or otherwise except as such
 *         appears in the above copyright notice or in the software.
 * 
 * THIS SOFTWARE IS BEING PROVIDED BY PACKET DESIGN "AS IS", AND
 * TO THE MAXIMUM EXTENT PERMITTED BY LAW, PACKET DESIGN 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.  PACKET DESIGN 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 PACKET DESIGN 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 PACKET DESIGN IS ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 * 
 * Author: Archie Cobbs <archie@freebsd.org>
 *
 * $FreeBSD: src/sys/netgraph/ng_l2tp.c,v 1.1.2.1 2002/08/20 23:48:15 archie Exp $
 */

/*
 * L2TP netgraph node type.
 *
 * This node type implements the lower layer of the
 * L2TP protocol as specified in RFC 2661.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/time.h>
#include <sys/conf.h>
#include <sys/mbuf.h>
#include <sys/malloc.h>
#include <sys/errno.h>
#include <sys/libkern.h>
#include <sys/thread2.h>

#include <netgraph/ng_message.h>
#include <netgraph/netgraph.h>
#include <netgraph/ng_parse.h>
#include "ng_l2tp.h"

#ifdef NG_SEPARATE_MALLOC
MALLOC_DEFINE(M_NETGRAPH_L2TP, "netgraph_l2tp", "netgraph l2tp node");
#else
#define M_NETGRAPH_L2TP M_NETGRAPH
#endif

/* L2TP header format (first 2 bytes only) */
#define L2TP_HDR_CTRL           0x8000                  /* control packet */
#define L2TP_HDR_LEN            0x4000                  /* has length field */
#define L2TP_HDR_SEQ            0x0800                  /* has ns, nr fields */
#define L2TP_HDR_OFF            0x0200                  /* has offset field */
#define L2TP_HDR_PRIO           0x0100                  /* give priority */
#define L2TP_HDR_VERS_MASK      0x000f                  /* version field mask */
#define L2TP_HDR_VERSION        0x0002                  /* version field */

/* Bits that must be zero or one in first two bytes of header */
#define L2TP_CTRL_0BITS         0x030d                  /* ctrl: must be 0 */
#define L2TP_CTRL_1BITS         0xc802                  /* ctrl: must be 1 */
#define L2TP_DATA_0BITS         0x800d                  /* data: must be 0 */
#define L2TP_DATA_1BITS         0x0002                  /* data: must be 1 */

/* Standard xmit ctrl and data header bits */
#define L2TP_CTRL_HDR           (L2TP_HDR_CTRL | L2TP_HDR_LEN \
                                    | L2TP_HDR_SEQ | L2TP_HDR_VERSION)
#define L2TP_DATA_HDR           (L2TP_HDR_VERSION)      /* optional: len, seq */

/* Some hard coded values */
#define L2TP_MAX_XWIN           16                      /* my max xmit window */
#define L2TP_MAX_REXMIT         5                       /* default max rexmit */
#define L2TP_MAX_REXMIT_TO      30                      /* default rexmit to */
#define L2TP_DELAYED_ACK        ((hz + 19) / 20)        /* delayed ack: 50 ms */

/* Default data sequence number configuration for new sessions */
#define L2TP_CONTROL_DSEQ       1                       /* we are the lns */
#define L2TP_ENABLE_DSEQ        1                       /* enable data seq # */

/* Compare sequence numbers using circular math */
#define L2TP_SEQ_DIFF(x, y)     ((int)((int16_t)(x) - (int16_t)(y)))

/*
 * Sequence number state
 *
 * Invariants:
 *    - If cwnd < ssth, we're doing slow start, otherwise congestion avoidance
 *    - The number of unacknowledged xmit packets is (ns - rack) <= seq->wmax
 *    - The first (ns - rack) mbuf's in xwin[] array are copies of these
 *      unacknowledged packets; the remainder of xwin[] consists first of
 *      zero or more further untransmitted packets in the transmit queue
 *    - We try to keep the peer's receive window as full as possible.
 *      Therefore, (i < cwnd && xwin[i] != NULL) implies (ns - rack) > i.
 *    - rack_timer is running iff (ns - rack) > 0 (unack'd xmit'd pkts)
 *    - If xack != nr, there are unacknowledged recv packet(s) (delayed ack)
 *    - xack_timer is running iff xack != nr (unack'd rec'd pkts)
 */
struct l2tp_seq {
        u_int16_t               ns;             /* next xmit seq we send */
        u_int16_t               nr;             /* next recv seq we expect */
        u_int16_t               rack;           /* last 'nr' we rec'd */
        u_int16_t               xack;           /* last 'nr' we sent */
        u_int16_t               wmax;           /* peer's max recv window */
        u_int16_t               cwnd;           /* current congestion window */
        u_int16_t               ssth;           /* slow start threshold */
        u_int16_t               acks;           /* # consecutive acks rec'd */
        u_int16_t               rexmits;        /* # retransmits sent */
        u_int16_t               max_rexmits;    /* max # retransmits sent */
        u_int16_t               max_rexmit_to;  /* max retransmit timeout */
        struct callout          rack_timer;     /* retransmit timer */
        struct callout          xack_timer;     /* delayed ack timer */
        u_char                  rack_timer_running;     /* xmit timer running */
        u_char                  xack_timer_running;     /* ack timer running */
        struct mbuf             *xwin[L2TP_MAX_XWIN];   /* transmit window */
};

/* Node private data */
struct ng_l2tp_private {
        node_p                  node;           /* back pointer to node */
        hook_p                  ctrl;           /* hook to upper layers */
        hook_p                  lower;          /* hook to lower layers */
        struct ng_l2tp_config   conf;           /* node configuration */
        struct ng_l2tp_stats    stats;          /* node statistics */
        struct l2tp_seq         seq;            /* ctrl sequence number state */
        char                    *ftarget;       /* failure message target */
};
typedef struct ng_l2tp_private *priv_p;

/* Hook private data (data session hooks only) */
struct ng_l2tp_hook_private {
        struct ng_l2tp_sess_config      conf;   /* hook/session config */
        u_int16_t                       ns;     /* data ns sequence number */
        u_int16_t                       nr;     /* data nr sequence number */
};
typedef struct ng_l2tp_hook_private *hookpriv_p;

/* Netgraph node methods */
static ng_constructor_t ng_l2tp_constructor;
static ng_rcvmsg_t      ng_l2tp_rcvmsg;
static ng_shutdown_t    ng_l2tp_shutdown;
static ng_newhook_t     ng_l2tp_newhook;
static ng_rcvdata_t     ng_l2tp_rcvdata;
static ng_disconnect_t  ng_l2tp_disconnect;

/* Internal functions */
static int      ng_l2tp_recv_lower(node_p node, struct mbuf *m, meta_p meta);
static int      ng_l2tp_recv_ctrl(node_p node, struct mbuf *m, meta_p meta);
static int      ng_l2tp_recv_data(node_p node,
                        struct mbuf *m, meta_p meta, hookpriv_p hpriv);

static int      ng_l2tp_xmit_ctrl(priv_p priv, struct mbuf *m, u_int16_t ns);

static void     ng_l2tp_seq_init(priv_p priv);
static int      ng_l2tp_seq_adjust(priv_p priv,
                        const struct ng_l2tp_config *conf);
static void     ng_l2tp_seq_reset(priv_p priv);
static void     ng_l2tp_seq_failure(priv_p priv);
static void     ng_l2tp_seq_recv_nr(priv_p priv, u_int16_t nr);
static int      ng_l2tp_seq_recv_ns(priv_p priv, u_int16_t ns);
static void     ng_l2tp_seq_xack_timeout(void *arg);
static void     ng_l2tp_seq_rack_timeout(void *arg);

#ifdef INVARIANTS
static void     ng_l2tp_seq_check(struct l2tp_seq *seq);
#endif

/* Parse type for struct ng_l2tp_config */
static const struct ng_parse_struct_field
        ng_l2tp_config_type_fields[] = NG_L2TP_CONFIG_TYPE_INFO;
static const struct ng_parse_type ng_l2tp_config_type = {
        &ng_parse_struct_type,
        &ng_l2tp_config_type_fields,
};

/* Parse type for struct ng_l2tp_sess_config */
static const struct ng_parse_struct_field
        ng_l2tp_sess_config_type_fields[] = NG_L2TP_SESS_CONFIG_TYPE_INFO;
static const struct ng_parse_type ng_l2tp_sess_config_type = {
        &ng_parse_struct_type,
        &ng_l2tp_sess_config_type_fields,
};

/* Parse type for struct ng_l2tp_stats */
static const struct ng_parse_struct_field
        ng_l2tp_stats_type_fields[] = NG_L2TP_STATS_TYPE_INFO;
static const struct ng_parse_type ng_pptp_stats_type = {
        &ng_parse_struct_type,
        &ng_l2tp_stats_type_fields
};

/* List of commands and how to convert arguments to/from ASCII */
static const struct ng_cmdlist ng_l2tp_cmdlist[] = {
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_SET_CONFIG,
          "setconfig",
          &ng_l2tp_config_type,
          NULL
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GET_CONFIG,
          "getconfig",
          NULL,
          &ng_l2tp_config_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_SET_SESS_CONFIG,
          "setsessconfig",
          &ng_l2tp_sess_config_type,
          NULL
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GET_SESS_CONFIG,
          "getsessconfig",
          &ng_parse_hint16_type,
          &ng_l2tp_sess_config_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GET_STATS,
          "getstats",
          NULL,
          &ng_pptp_stats_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_CLR_STATS,
          "clrstats",
          NULL,
          NULL
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GETCLR_STATS,
          "getclrstats",
          NULL,
          &ng_pptp_stats_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_ACK_FAILURE,
          "ackfailure",
          NULL,
          NULL
        },
        { 0 }
};

/* Node type descriptor */
static struct ng_type ng_l2tp_typestruct = {
        NG_VERSION,
        NG_L2TP_NODE_TYPE,
        NULL,
        ng_l2tp_constructor,
        ng_l2tp_rcvmsg,
        ng_l2tp_shutdown,
        ng_l2tp_newhook,
        NULL,
        NULL,
        ng_l2tp_rcvdata,
        ng_l2tp_rcvdata,
        ng_l2tp_disconnect,
        ng_l2tp_cmdlist
};
NETGRAPH_INIT(l2tp, &ng_l2tp_typestruct);

/* Sequence number state sanity checking */
#ifdef INVARIANTS
#define L2TP_SEQ_CHECK(seq)     ng_l2tp_seq_check(seq)
#else
#define L2TP_SEQ_CHECK(x)       do { } while (0)
#endif

/* Whether to use m_copypacket() or m_dup() */
#define L2TP_COPY_MBUF          m_copypacket

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

/*
 * Node type constructor
 */
static int
ng_l2tp_constructor(node_p *nodep)
{
        priv_p priv;
        int error;

        /* Allocate private structure */
        priv = kmalloc(sizeof(*priv), M_NETGRAPH_L2TP, M_NOWAIT | M_ZERO);
        if (priv == NULL)
                return (ENOMEM);

        /* Apply a semi-reasonable default configuration */
        priv->conf.peer_win = 1;
        priv->conf.rexmit_max = L2TP_MAX_REXMIT;
        priv->conf.rexmit_max_to = L2TP_MAX_REXMIT_TO;

        /* Initialize sequence number state */
        ng_l2tp_seq_init(priv);

        /* Call generic node constructor */
        if ((error = ng_make_node_common(&ng_l2tp_typestruct, nodep))) {
                kfree(priv, M_NETGRAPH_L2TP);
                return (error);
        }
        NG_NODE_SET_PRIVATE(*nodep, priv);
        priv->node = *nodep;

        /* Done */
        return (0);
}

/*
 * Give our OK for a hook to be added.
 */
static int
ng_l2tp_newhook(node_p node, hook_p hook, const char *name)
{
        const priv_p priv = NG_NODE_PRIVATE(node);

        /* Check hook name */
        if (strcmp(name, NG_L2TP_HOOK_CTRL) == 0) {
                if (priv->ctrl != NULL)
                        return (EISCONN);
                priv->ctrl = hook;
        } else if (strcmp(name, NG_L2TP_HOOK_LOWER) == 0) {
                if (priv->lower != NULL)
                        return (EISCONN);
                priv->lower = hook;
        } else {
                static const char hexdig[16] = "0123456789abcdef";
                u_int16_t session_id;
                hookpriv_p hpriv;
                const char *hex;
                int i;
                int j;

                /* Parse hook name to get session ID */
                if (strncmp(name, NG_L2TP_HOOK_SESSION_P,
                    sizeof(NG_L2TP_HOOK_SESSION_P) - 1) != 0)
                        return (EINVAL);
                hex = name + sizeof(NG_L2TP_HOOK_SESSION_P) - 1;
                for (session_id = i = 0; i < 4; i++) {
                        for (j = 0; j < 16 && hex[i] != hexdig[j]; j++);
                        if (j == 16)
                                return (EINVAL);
                        session_id = (session_id << 4) | j;
                }
                if (hex[i] != '\0')
                        return (EINVAL);

                /* Create hook private structure */
                hpriv = kmalloc(sizeof(*hpriv), M_NETGRAPH_L2TP,
                                M_NOWAIT | M_ZERO);
                if (hpriv == NULL)
                        return (ENOMEM);
                hpriv->conf.session_id = htons(session_id);
                hpriv->conf.control_dseq = L2TP_CONTROL_DSEQ;
                hpriv->conf.enable_dseq = L2TP_ENABLE_DSEQ;
                NG_HOOK_SET_PRIVATE(hook, hpriv);
        }

        /* Done */
        return (0);
}

/*
 * Receive a control message.
 */
static int
ng_l2tp_rcvmsg(node_p node, struct ng_mesg *msg,
        const char *raddr, struct ng_mesg **rptr)
{
        const priv_p priv = NG_NODE_PRIVATE(node);
        struct ng_mesg *resp = NULL;
        int error = 0;

        switch (msg->header.typecookie) {
        case NGM_L2TP_COOKIE:
                switch (msg->header.cmd) {
                case NGM_L2TP_SET_CONFIG:
                    {
                        struct ng_l2tp_config *const conf =
                                (struct ng_l2tp_config *)msg->data;

                        /* Check for invalid or illegal config */
                        if (msg->header.arglen != sizeof(*conf)) {
                                error = EINVAL;
                                break;
                        }
                        conf->enabled = !!conf->enabled;
                        conf->match_id = !!conf->match_id;
                        conf->tunnel_id = htons(conf->tunnel_id);
                        conf->peer_id = htons(conf->peer_id);
                        if (priv->conf.enabled
                            && ((priv->conf.tunnel_id != 0
                               && conf->tunnel_id != priv->conf.tunnel_id)
                              || ((priv->conf.peer_id != 0
                               && conf->peer_id != priv->conf.peer_id)))) {
                                error = EBUSY;
                                break;
                        }

                        /* Save calling node as failure target */
                        if (priv->ftarget != NULL)
                                kfree(priv->ftarget, M_NETGRAPH_L2TP);
                        priv->ftarget = kmalloc(strlen(raddr) + 1,
                                                M_NETGRAPH_L2TP, M_NOWAIT);
                        if (priv->ftarget == NULL) {
                                error = ENOMEM;
                                break;
                        }
                        strcpy(priv->ftarget, raddr);

                        /* Adjust sequence number state */
                        if ((error = ng_l2tp_seq_adjust(priv, conf)) != 0)
                                break;

                        /* Update node's config */
                        priv->conf = *conf;
                        break;
                    }
                case NGM_L2TP_GET_CONFIG:
                    {
                        struct ng_l2tp_config *conf;

                        NG_MKRESPONSE(resp, msg, sizeof(*conf), M_NOWAIT);
                        if (resp == NULL) {
                                error = ENOMEM;
                                break;
                        }
                        conf = (struct ng_l2tp_config *)resp->data;
                        *conf = priv->conf;

                        /* Put ID's in host order */
                        conf->tunnel_id = ntohs(conf->tunnel_id);
                        conf->peer_id = ntohs(conf->peer_id);
                        break;
                    }
                case NGM_L2TP_SET_SESS_CONFIG:
                    {
                        struct ng_l2tp_sess_config *const conf =
                            (struct ng_l2tp_sess_config *)msg->data;
                        hookpriv_p hpriv = NULL;
                        hook_p hook;

                        /* Check for invalid or illegal config */
                        if (msg->header.arglen != sizeof(*conf)) {
                                error = EINVAL;
                                break;
                        }

                        /* Put ID's in network order */
                        conf->session_id = htons(conf->session_id);
                        conf->peer_id = htons(conf->peer_id);

                        /* Find matching hook */
                        LIST_FOREACH(hook, &node->hooks, hooks) {
                                if ((hpriv = NG_HOOK_PRIVATE(hook)) == NULL)
                                        continue;
                                if (hpriv->conf.session_id == conf->session_id)
                                        break;
                        }
                        if (hook == NULL) {
                                error = ENOENT;
                                break;
                        }

                        /* Update hook's config */
                        hpriv->conf = *conf;
                        break;
                    }
                case NGM_L2TP_GET_SESS_CONFIG:
                    {
                        struct ng_l2tp_sess_config *conf;
                        hookpriv_p hpriv = NULL;
                        u_int16_t session_id;
                        hook_p hook;

                        /* Get session ID */
                        if (msg->header.arglen != sizeof(session_id)) {
                                error = EINVAL;
                                break;
                        }
                        memcpy(&session_id, msg->data, 2);
                        session_id = htons(session_id);

                        /* Find matching hook */
                        LIST_FOREACH(hook, &node->hooks, hooks) {
                                if ((hpriv = NG_HOOK_PRIVATE(hook)) == NULL)
                                        continue;
                                if (hpriv->conf.session_id == session_id)
                                        break;
                        }
                        if (hook == NULL) {
                                error = ENOENT;
                                break;
                        }

                        /* Send response */
                        NG_MKRESPONSE(resp, msg, sizeof(hpriv->conf), M_NOWAIT);
                        if (resp == NULL) {
                                error = ENOMEM;
                                break;
                        }
                        conf = (struct ng_l2tp_sess_config *)resp->data;
                        *conf = hpriv->conf;

                        /* Put ID's in host order */
                        conf->session_id = ntohs(conf->session_id);
                        conf->peer_id = ntohs(conf->peer_id);
                        break;
                    }
                case NGM_L2TP_GET_STATS:
                case NGM_L2TP_CLR_STATS:
                case NGM_L2TP_GETCLR_STATS:
                    {
                        if (msg->header.cmd != NGM_L2TP_CLR_STATS) {
                                NG_MKRESPONSE(resp, msg,
                                    sizeof(priv->stats), M_NOWAIT);
                                if (resp == NULL) {
                                        error = ENOMEM;
                                        break;
                                }
                                memcpy(resp->data,
                                    &priv->stats, sizeof(priv->stats));
                        }
                        if (msg->header.cmd != NGM_L2TP_GET_STATS)
                                memset(&priv->stats, 0, sizeof(priv->stats));
                        break;
                    }
                default:
                        error = EINVAL;
                        break;
                }
                break;
        default:
                error = EINVAL;
                break;
        }
        if (rptr)
                *rptr = resp;
        else if (resp)
                kfree(resp, M_NETGRAPH);
        kfree(msg, M_NETGRAPH);
        return (error);
}

/*
 * Receive incoming data on a hook.
 */
static int
ng_l2tp_rcvdata(hook_p hook, struct mbuf *m, meta_p meta)
{
        const node_p node = NG_HOOK_NODE(hook);
        const priv_p priv = NG_NODE_PRIVATE(node);
        int error;

        /* Sanity check */
        L2TP_SEQ_CHECK(&priv->seq);

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

        /* Handle incoming frame from below */
        if (hook == priv->lower) {
                error = ng_l2tp_recv_lower(node, m, meta);
                goto done;
        }

        /* Handle outgoing control frame */
        if (hook == priv->ctrl) {
                error = ng_l2tp_recv_ctrl(node, m, meta);
                goto done;
        }

        /* Handle outgoing data frame */
        error = ng_l2tp_recv_data(node, m, meta, NG_HOOK_PRIVATE(hook));

done:
        /* Done */
        L2TP_SEQ_CHECK(&priv->seq);
        return (error);
}

/*
 * Destroy node
 */
static int
ng_l2tp_shutdown(node_p node)
{
        const priv_p priv = NG_NODE_PRIVATE(node);
        struct l2tp_seq *const seq = &priv->seq;

        /* Sanity check */
        L2TP_SEQ_CHECK(seq);

        /* Reset sequence number state */
        node->flags |= NG_INVALID;
        ng_l2tp_seq_reset(priv);
        ng_cutlinks(node);
        ng_unname(node);

        /* Free private data if neither timer is running */
        if (!seq->rack_timer_running && !seq->xack_timer_running) {
                if (priv->ftarget != NULL)
                        kfree(priv->ftarget, M_NETGRAPH_L2TP);
                kfree(priv, M_NETGRAPH_L2TP);
                NG_NODE_SET_PRIVATE(node, NULL);
        }

        /* Unref node */
        NG_NODE_UNREF(node);
        return (0);
}

/*
 * Hook disconnection
 */
static int
ng_l2tp_disconnect(hook_p hook)
{
        const node_p node = NG_HOOK_NODE(hook);
        const priv_p priv = NG_NODE_PRIVATE(node);

        /* Zero out hook pointer */
        if (hook == priv->ctrl)
                priv->ctrl = NULL;
        else if (hook == priv->lower)
                priv->lower = NULL;
        else {
                kfree(NG_HOOK_PRIVATE(hook), M_NETGRAPH_L2TP);
                NG_HOOK_SET_PRIVATE(hook, NULL);
        }

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

/*************************************************************************
                        INTERNAL FUNCTIONS
*************************************************************************/

/*
 * Handle an incoming frame from below.
 */
static int
ng_l2tp_recv_lower(node_p node, struct mbuf *m, meta_p meta)
{
        static const u_int16_t req_bits[2][2] = {
                { L2TP_DATA_0BITS, L2TP_DATA_1BITS },
                { L2TP_CTRL_0BITS, L2TP_CTRL_1BITS },
        };
        const priv_p priv = NG_NODE_PRIVATE(node);
        hookpriv_p hpriv = NULL;
        hook_p hook = NULL;
        u_int16_t ids[2];
        u_int16_t hdr;
        u_int16_t ns;
        u_int16_t nr;
        int is_ctrl;
        int error;
        int len;

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

        /* Get initial header */
        if (m->m_pkthdr.len < 6) {
                priv->stats.recvRunts++;
                NG_FREE_DATA(m, meta);
                return (EINVAL);
        }
        if (m->m_len < 2 && (m = m_pullup(m, 2)) == NULL) {
                priv->stats.memoryFailures++;
                NG_FREE_META(meta);
                return (EINVAL);
        }
        hdr = ntohs(*mtod(m, u_int16_t *));
        m_adj(m, 2);

        /* Check required header bits and minimum length */
        is_ctrl = (hdr & L2TP_HDR_CTRL) != 0;
        if ((hdr & req_bits[is_ctrl][0]) != 0
            || (~hdr & req_bits[is_ctrl][1]) != 0) {
                priv->stats.recvInvalid++;
                NG_FREE_DATA(m, meta);
                return (EINVAL);
        }
        if (m->m_pkthdr.len < 4                         /* tunnel, session id */
            + (2 * ((hdr & L2TP_HDR_LEN) != 0))         /* length field */
            + (4 * ((hdr & L2TP_HDR_SEQ) != 0))         /* seq # fields */
            + (2 * ((hdr & L2TP_HDR_OFF) != 0))) {      /* offset field */
                priv->stats.recvRunts++;
                NG_FREE_DATA(m, meta);
                return (EINVAL);
        }

        /* Get and validate length field if present */
        if ((hdr & L2TP_HDR_LEN) != 0) {
                if (m->m_len < 2 && (m = m_pullup(m, 2)) == NULL) {
                        priv->stats.memoryFailures++;
                        NG_FREE_META(meta);
                        return (EINVAL);
                }
                len = (u_int16_t)ntohs(*mtod(m, u_int16_t *)) - 4;
                m_adj(m, 2);
                if (len < 0 || len > m->m_pkthdr.len) {
                        priv->stats.recvInvalid++;
                        NG_FREE_DATA(m, meta);
                        return (EINVAL);
                }
                if (len < m->m_pkthdr.len)              /* trim extra bytes */
                        m_adj(m, -(m->m_pkthdr.len - len));
        }

        /* Get tunnel ID and session ID */
        if (m->m_len < 4 && (m = m_pullup(m, 4)) == NULL) {
                priv->stats.memoryFailures++;
                NG_FREE_META(meta);
                return (EINVAL);
        }
        memcpy(ids, mtod(m, u_int16_t *), 4);
        m_adj(m, 4);

        /* Check tunnel ID */
        if (ids[0] != priv->conf.tunnel_id
            && (priv->conf.match_id || ids[0] != 0)) {
                priv->stats.recvWrongTunnel++;
                NG_FREE_DATA(m, meta);
                return (EADDRNOTAVAIL);
        }

        /* Check session ID (for data packets only) */
        if ((hdr & L2TP_HDR_CTRL) == 0) {
                LIST_FOREACH(hook, &node->hooks, hooks) {
                        if ((hpriv = NG_HOOK_PRIVATE(hook)) == NULL)
                                continue;
                        if (hpriv->conf.session_id == ids[1])
                                break;
                }
                if (hook == NULL) {
                        priv->stats.recvUnknownSID++;
                        NG_FREE_DATA(m, meta);
                        return (ENOTCONN);
                }
                hpriv = NG_HOOK_PRIVATE(hook);
        }

        /* Get Ns, Nr fields if present */
        if ((hdr & L2TP_HDR_SEQ) != 0) {
                if (m->m_len < 4 && (m = m_pullup(m, 4)) == NULL) {
                        priv->stats.memoryFailures++;
                        NG_FREE_META(meta);
                        return (EINVAL);
                }
                memcpy(&ns, &mtod(m, u_int16_t *)[0], 2);
                ns = ntohs(ns);
                memcpy(&nr, &mtod(m, u_int16_t *)[1], 2);
                nr = ntohs(nr);
                m_adj(m, 4);
        } else {
                nr = 0; /* avoid gcc complaint */
                ns = 0; /* avoid gcc complaint */
        }

        /* Strip offset padding if present */
        if ((hdr & L2TP_HDR_OFF) != 0) {
                u_int16_t offset;

                /* Get length of offset padding */
                if (m->m_len < 2 && (m = m_pullup(m, 2)) == NULL) {
                        priv->stats.memoryFailures++;
                        NG_FREE_META(meta);
                        return (EINVAL);
                }
                memcpy(&offset, mtod(m, u_int16_t *), 2);
                offset = ntohs(offset);

                /* Trim offset padding */
                if (offset <= 2 || offset > m->m_pkthdr.len) {
                        priv->stats.recvInvalid++;
                        NG_FREE_DATA(m, meta);
                        return (EINVAL);
                }
                m_adj(m, offset);
        }

        /* Handle control packets */
        if ((hdr & L2TP_HDR_CTRL) != 0) {

                /* Handle receive ack sequence number Nr */
                ng_l2tp_seq_recv_nr(priv, nr);

                /* Discard ZLB packets */
                if (m->m_pkthdr.len == 0) {
                        priv->stats.recvZLBs++;
                        NG_FREE_DATA(m, meta);
                        return (0);
                }

                /*
                 * Prepend session ID to packet here: we don't want to accept
                 * the send sequence number Ns if we have to drop the packet
                 * later because of a memory error, because then the upper
                 * layer would never get the packet.
                 */
                M_PREPEND(m, 2, M_NOWAIT);
                if (m == NULL) {
                        priv->stats.memoryFailures++;
                        NG_FREE_META(meta);
                        return (ENOBUFS);
                }
                memcpy(mtod(m, u_int16_t *), &ids[1], 2);

                /* Now handle send sequence number */
                if (ng_l2tp_seq_recv_ns(priv, ns) == -1) {
                        NG_FREE_DATA(m, meta);
                        return (0);
                }

                /* Deliver packet to upper layers */
                NG_SEND_DATA(error, priv->ctrl, m, meta);
                return (error);
        }

        /* Follow peer's lead in data sequencing, if configured to do so */
        if (!hpriv->conf.control_dseq)
                hpriv->conf.enable_dseq = ((hdr & L2TP_HDR_SEQ) != 0);

        /* Handle data sequence numbers if present and enabled */
        if ((hdr & L2TP_HDR_SEQ) != 0) {
                if (hpriv->conf.enable_dseq
                    && L2TP_SEQ_DIFF(ns, hpriv->nr) < 0) {
                        NG_FREE_DATA(m, meta);  /* duplicate or out of order */
                        priv->stats.recvDataDrops++;
                        return (0);
                }
                hpriv->nr = ns + 1;
        }

        /* Drop empty data packets */
        if (m->m_pkthdr.len == 0) {
                NG_FREE_DATA(m, meta);
                return (0);
        }

        /* Deliver data */
        NG_SEND_DATA(error, hook, m, meta);
        return (error);
}

/*
 * Handle an outgoing control frame.
 */
static int
ng_l2tp_recv_ctrl(node_p node, struct mbuf *m, meta_p meta)
{
        const priv_p priv = NG_NODE_PRIVATE(node);
        struct l2tp_seq *const seq = &priv->seq;
        int i;

        /* Discard meta XXX should queue meta's along with packet */
        NG_FREE_META(meta);

        /* Packet should have session ID prepended */
        if (m->m_pkthdr.len < 2) {
                priv->stats.xmitInvalid++;
                m_freem(m);
                return (EINVAL);
        }

        /* Check max length */
        if (m->m_pkthdr.len >= 0x10000 - 14) {
                priv->stats.xmitTooBig++;
                m_freem(m);
                return (EOVERFLOW);
        }

        /* Find next empty slot in transmit queue */
        for (i = 0; i < L2TP_MAX_XWIN && seq->xwin[i] != NULL; i++);
        if (i == L2TP_MAX_XWIN) {
                priv->stats.xmitDrops++;
                m_freem(m);
                return (ENOBUFS);
        }
        seq->xwin[i] = m;

        /* Sanity check receive ack timer state */
        KASSERT((i == 0) ^ seq->rack_timer_running,
            ("%s: xwin %d full but rack timer %srunning",
            __func__, i, seq->rack_timer_running ? "" : "not "));

        /* If peer's receive window is already full, nothing else to do */
        if (i >= seq->cwnd)
                return (0);

        /* Start retransmit timer if not already running */
        if (!seq->rack_timer_running) {
                callout_reset(&seq->rack_timer,
                    hz, ng_l2tp_seq_rack_timeout, node);
                seq->rack_timer_running = 1;
                NG_NODE_REF(node);
        }

        /* Copy packet */
        if ((m = L2TP_COPY_MBUF(seq->xwin[i], M_NOWAIT)) == NULL) {
                priv->stats.memoryFailures++;
                return (ENOBUFS);
        }

        /* Send packet and increment xmit sequence number */
        return (ng_l2tp_xmit_ctrl(priv, m, seq->ns++));
}

/*
 * Handle an outgoing data frame.
 */
static int
ng_l2tp_recv_data(node_p node, struct mbuf *m, meta_p meta, hookpriv_p hpriv)
{
        const priv_p priv = NG_NODE_PRIVATE(node);
        u_int16_t hdr;
        int error;
        int i = 1;

        /* Check max length */
        if (m->m_pkthdr.len >= 0x10000 - 12) {
                priv->stats.xmitDataTooBig++;
                NG_FREE_META(meta);
                m_freem(m);
                return (EOVERFLOW);
        }

        /* Prepend L2TP header */
        M_PREPEND(m, 6
            + (2 * (hpriv->conf.include_length != 0))
            + (4 * (hpriv->conf.enable_dseq != 0)),
            M_NOWAIT);
        if (m == NULL) {
                priv->stats.memoryFailures++;
                NG_FREE_META(meta);
                return (ENOBUFS);
        }
        hdr = L2TP_DATA_HDR;
        if (hpriv->conf.include_length) {
                hdr |= L2TP_HDR_LEN;
                mtod(m, u_int16_t *)[i++] = htons(m->m_pkthdr.len);
        }
        mtod(m, u_int16_t *)[i++] = priv->conf.peer_id;
        mtod(m, u_int16_t *)[i++] = hpriv->conf.peer_id;
        if (hpriv->conf.enable_dseq) {
                hdr |= L2TP_HDR_SEQ;
                mtod(m, u_int16_t *)[i++] = htons(hpriv->ns);
                mtod(m, u_int16_t *)[i++] = htons(hpriv->nr);
                hpriv->ns++;
        }
        mtod(m, u_int16_t *)[0] = htons(hdr);

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

/*
 * Send a message to our controlling node that we've failed.
 */
static void
ng_l2tp_seq_failure(priv_p priv)
{
        struct ng_mesg *msg;

        NG_MKMESSAGE(msg, NGM_L2TP_COOKIE, NGM_L2TP_ACK_FAILURE, 0, M_NOWAIT);
        if (msg == NULL)
                return;
        ng_send_msg(priv->node, msg, priv->ftarget, NULL);
}

/************************************************************************
                        SEQUENCE NUMBER HANDLING
************************************************************************/

/*
 * Initialize sequence number state.
 */
static void
ng_l2tp_seq_init(priv_p priv)
{
        struct l2tp_seq *const seq = &priv->seq;

        KASSERT(priv->conf.peer_win >= 1,
            ("%s: peer_win is zero", __func__));
        memset(seq, 0, sizeof(*seq));
        seq->cwnd = 1;
        seq->wmax = priv->conf.peer_win;
        if (seq->wmax > L2TP_MAX_XWIN)
                seq->wmax = L2TP_MAX_XWIN;
        seq->ssth = seq->wmax;
        seq->max_rexmits = priv->conf.rexmit_max;
        seq->max_rexmit_to = priv->conf.rexmit_max_to;
        callout_init(&seq->rack_timer);
        callout_init(&seq->xack_timer);
        L2TP_SEQ_CHECK(seq);
}

/*
 * Adjust sequence number state accordingly after reconfiguration.
 */
static int
ng_l2tp_seq_adjust(priv_p priv, const struct ng_l2tp_config *conf)
{
        struct l2tp_seq *const seq = &priv->seq;
        u_int16_t new_wmax;

        /* If disabling node, reset state sequence number */
        if (!conf->enabled) {
                ng_l2tp_seq_reset(priv);
                return (0);
        }

        /* Adjust peer's max recv window; it can only increase */
        new_wmax = conf->peer_win;
        if (new_wmax > L2TP_MAX_XWIN)
                new_wmax = L2TP_MAX_XWIN;
        if (new_wmax == 0)
                return (EINVAL);
        if (new_wmax < seq->wmax)
                return (EBUSY);
        seq->wmax = new_wmax;

        /* Update retransmit parameters */
        seq->max_rexmits = conf->rexmit_max;
        seq->max_rexmit_to = conf->rexmit_max_to;

        /* Done */
        return (0);
}

/*
 * Reset sequence number state.
 */
static void
ng_l2tp_seq_reset(priv_p priv)
{
        struct l2tp_seq *const seq = &priv->seq;
        hookpriv_p hpriv = NULL;
        hook_p hook;
        int i;

        /* Sanity check */
        L2TP_SEQ_CHECK(seq);

        /* Stop timers */
        if (seq->rack_timer_running && callout_stop(&seq->rack_timer) == 1) {
                seq->rack_timer_running = 0;
                NG_NODE_UNREF(priv->node);
        }
        if (seq->xack_timer_running && callout_stop(&seq->xack_timer) == 1) {
                seq->xack_timer_running = 0;
                NG_NODE_UNREF(priv->node);
        }

        /* Free retransmit queue */
        for (i = 0; i < L2TP_MAX_XWIN; i++) {
                if (seq->xwin[i] == NULL)
                        break;
                m_freem(seq->xwin[i]);
        }

        /* Reset session hooks' sequence number states */
        LIST_FOREACH(hook, &priv->node->hooks, hooks) {
                if ((hpriv = NG_HOOK_PRIVATE(hook)) == NULL)
                        continue;
                hpriv->conf.control_dseq = 0;
                hpriv->conf.enable_dseq = 0;
                hpriv->nr = 0;
                hpriv->ns = 0;
        }

        /* Reset node's sequence number state */
        memset(seq, 0, sizeof(*seq));
        seq->cwnd = 1;
        seq->wmax = L2TP_MAX_XWIN;
        seq->ssth = seq->wmax;

        /* Done */
        L2TP_SEQ_CHECK(seq);
}

/*
 * Handle receipt of an acknowledgement value (Nr) from peer.
 */
static void
ng_l2tp_seq_recv_nr(priv_p priv, u_int16_t nr)
{
        struct l2tp_seq *const seq = &priv->seq;
        struct mbuf *m;
        int nack;
        int i;

        /* Verify peer's ACK is in range */
        if ((nack = L2TP_SEQ_DIFF(nr, seq->rack)) <= 0)
                return;                         /* duplicate ack */
        if (L2TP_SEQ_DIFF(nr, seq->ns) > 0) {
                priv->stats.recvBadAcks++;      /* ack for packet not sent */
                return;
        }
        KASSERT(nack <= L2TP_MAX_XWIN,
            ("%s: nack=%d > %d", __func__, nack, L2TP_MAX_XWIN));

        /* Update receive ack stats */
        seq->rack = nr;
        seq->rexmits = 0;

        /* Free acknowledged packets and shift up packets in the xmit queue */
        for (i = 0; i < nack; i++)
                m_freem(seq->xwin[i]);
        memmove(seq->xwin, seq->xwin + nack,
            (L2TP_MAX_XWIN - nack) * sizeof(*seq->xwin));
        memset(seq->xwin + (L2TP_MAX_XWIN - nack), 0,
            nack * sizeof(*seq->xwin));

        /*
         * Do slow-start/congestion avoidance windowing algorithm described
         * in RFC 2661, Appendix A. Here we handle a multiple ACK as if each
         * ACK had arrived separately.
         */
        if (seq->cwnd < seq->wmax) {

                /* Handle slow start phase */
                if (seq->cwnd < seq->ssth) {
                        seq->cwnd += nack;
                        nack = 0;
                        if (seq->cwnd > seq->ssth) {    /* into cg.av. phase */
                                nack = seq->cwnd - seq->ssth;
                                seq->cwnd = seq->ssth;
                        }
                }

                /* Handle congestion avoidance phase */
                if (seq->cwnd >= seq->ssth) {
                        seq->acks += nack;
                        while (seq->acks >= seq->cwnd) {
                                seq->acks -= seq->cwnd;
                                if (seq->cwnd < seq->wmax)
                                        seq->cwnd++;
                        }
                }
        }

        /* Stop xmit timer */
        if (seq->rack_timer_running && callout_stop(&seq->rack_timer) == 1) {
                seq->rack_timer_running = 0;
                NG_NODE_UNREF(priv->node);
        }

        /* If transmit queue is empty, we're done for now */
        if (seq->xwin[0] == NULL)
                return;

        /* Start restransmit timer again */
        callout_reset(&seq->rack_timer,
            hz, ng_l2tp_seq_rack_timeout, priv->node);
        seq->rack_timer_running = 1;
        NG_NODE_REF(priv->node);

        /*
         * Send more packets, trying to keep peer's receive window full.
         * If there is a memory error, pretend packet was sent, as it
         * will get retransmitted later anyway.
         */
        while ((i = L2TP_SEQ_DIFF(seq->ns, seq->rack)) < seq->cwnd
            && seq->xwin[i] != NULL) {
                if ((m = L2TP_COPY_MBUF(seq->xwin[i], M_NOWAIT)) == NULL)
                        priv->stats.memoryFailures++;
                else
                        ng_l2tp_xmit_ctrl(priv, m, seq->ns);
                seq->ns++;
        }
}

/*
 * Handle receipt of a sequence number value (Ns) from peer.
 * We make no attempt to re-order out of order packets.
 *
 * This function should only be called for non-ZLB packets.
 *
 * Returns:
 *       0      Accept packet
 *      -1      Drop packet
 */
static int
ng_l2tp_seq_recv_ns(priv_p priv, u_int16_t ns)
{
        struct l2tp_seq *const seq = &priv->seq;

        /* If not what we expect, drop packet and send an immediate ZLB ack */
        if (ns != seq->nr) {
                if (L2TP_SEQ_DIFF(ns, seq->nr) < 0)
                        priv->stats.recvDuplicates++;
                else
                        priv->stats.recvOutOfOrder++;
                ng_l2tp_xmit_ctrl(priv, NULL, seq->ns);
                return (-1);
        }

        /* Update recv sequence number */
        seq->nr++;

        /* Start receive ack timer, if not already running */
        if (!seq->xack_timer_running) {
                callout_reset(&seq->xack_timer,
                    L2TP_DELAYED_ACK, ng_l2tp_seq_xack_timeout, priv->node);
                seq->xack_timer_running = 1;
                NG_NODE_REF(priv->node);
        }

        /* Accept packet */
        return (0);
}

/*
 * Handle an ack timeout. We have an outstanding ack that we
 * were hoping to piggy-back, but haven't, so send a ZLB.
 */
static void
ng_l2tp_seq_xack_timeout(void *arg)
{
        const node_p node = arg;
        const priv_p priv = NG_NODE_PRIVATE(node);
        struct l2tp_seq *const seq = &priv->seq;

        /* Check if node is going away */
        crit_enter();
        if (NG_NODE_NOT_VALID(node)) {
                seq->xack_timer_running = 0;
                if (!seq->rack_timer_running) {
                        if (priv->ftarget != NULL)
                                kfree(priv->ftarget, M_NETGRAPH_L2TP);
                        kfree(priv, M_NETGRAPH_L2TP);
                        NG_NODE_SET_PRIVATE(node, NULL);
                }
                NG_NODE_UNREF(node);
                crit_exit();
                return;
        }

        /* Sanity check */
        L2TP_SEQ_CHECK(seq);

        /* If ack is still outstanding, send a ZLB */
        if (seq->xack != seq->nr)
                ng_l2tp_xmit_ctrl(priv, NULL, seq->ns);

        /* Done */
        seq->xack_timer_running = 0;
        NG_NODE_UNREF(node);
        L2TP_SEQ_CHECK(seq);
        crit_exit();
}

/* 
 * Handle a transmit timeout. The peer has failed to respond
 * with an ack for our packet, so retransmit it.
 */
static void
ng_l2tp_seq_rack_timeout(void *arg)
{
        const node_p node = arg;
        const priv_p priv = NG_NODE_PRIVATE(node);
        struct l2tp_seq *const seq = &priv->seq;
        struct mbuf *m;
        u_int delay;

        /* Check if node is going away */
        crit_enter();
        if (NG_NODE_NOT_VALID(node)) {
                seq->rack_timer_running = 0;
                if (!seq->xack_timer_running) {
                        if (priv->ftarget != NULL)
                                kfree(priv->ftarget, M_NETGRAPH_L2TP);
                        kfree(priv, M_NETGRAPH_L2TP);
                        NG_NODE_SET_PRIVATE(node, NULL);
                }
                NG_NODE_UNREF(node);
                crit_exit();
                return;
        }

        /* Sanity check */
        L2TP_SEQ_CHECK(seq);

        /* Make sure peer's ack is still outstanding before doing anything */
        if (seq->rack == seq->ns) {
                seq->rack_timer_running = 0;
                NG_NODE_UNREF(node);
                goto done;
        }
        priv->stats.xmitRetransmits++;

        /* Have we reached the retransmit limit? If so, notify owner. */
        if (seq->rexmits++ >= seq->max_rexmits)
                ng_l2tp_seq_failure(priv);

        /* Restart timer, this time with an increased delay */
        delay = (seq->rexmits > 12) ? (1 << 12) : (1 << seq->rexmits);
        if (delay > seq->max_rexmit_to)
                delay = seq->max_rexmit_to;
        callout_reset(&seq->rack_timer,
            hz * delay, ng_l2tp_seq_rack_timeout, node);

        /* Do slow-start/congestion algorithm windowing algorithm */
        seq->ssth = (seq->cwnd + 1) / 2;
        seq->cwnd = 1;
        seq->acks = 0;

        /* Retransmit oldest unack'd packet */
        if ((m = L2TP_COPY_MBUF(seq->xwin[0], M_NOWAIT)) == NULL)
                priv->stats.memoryFailures++;
        else
                ng_l2tp_xmit_ctrl(priv, m, seq->rack);

done:
        /* Done */
        L2TP_SEQ_CHECK(seq);
        crit_exit();
}

/*
 * Transmit a control stream packet, payload optional.
 * The transmit sequence number is not incremented.
 */
static int
ng_l2tp_xmit_ctrl(priv_p priv, struct mbuf *m, u_int16_t ns)
{
        struct l2tp_seq *const seq = &priv->seq;
        u_int16_t session_id = 0;
        meta_p meta = NULL;
        int error;

        /* If no mbuf passed, send an empty packet (ZLB) */
        if (m == NULL) {

                /* Create a new mbuf for ZLB packet */
                MGETHDR(m, M_NOWAIT, MT_DATA);
                if (m == NULL) {
                        priv->stats.memoryFailures++;
                        return (ENOBUFS);
                }
                m->m_len = m->m_pkthdr.len = 12;
                m->m_pkthdr.rcvif = NULL;
                priv->stats.xmitZLBs++;
        } else {

                /* Strip off session ID */
                if (m->m_len < 2 && (m = m_pullup(m, 2)) == NULL) {
                        priv->stats.memoryFailures++;
                        return (ENOBUFS);
                }
                memcpy(&session_id, mtod(m, u_int16_t *), 2);
                m_adj(m, 2);

                /* Make room for L2TP header */
                M_PREPEND(m, 12, M_NOWAIT);
                if (m == NULL) {
                        priv->stats.memoryFailures++;
                        return (ENOBUFS);
                }
        }

        /* Fill in L2TP header */
        mtod(m, u_int16_t *)[0] = htons(L2TP_CTRL_HDR);
        mtod(m, u_int16_t *)[1] = htons(m->m_pkthdr.len);
        mtod(m, u_int16_t *)[2] = priv->conf.peer_id;
        mtod(m, u_int16_t *)[3] = session_id;
        mtod(m, u_int16_t *)[4] = htons(ns);
        mtod(m, u_int16_t *)[5] = htons(seq->nr);

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

        /* Stop ack timer: we're sending an ack with this packet */
        if (seq->xack_timer_running && callout_stop(&seq->xack_timer) == 1) {
                seq->xack_timer_running = 0;
                NG_NODE_UNREF(priv->node);
        }
        seq->xack = seq->nr;

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

#ifdef INVARIANTS
/*
 * Sanity check sequence number state.
 */
static void
ng_l2tp_seq_check(struct l2tp_seq *seq)
{
        const int self_unack = L2TP_SEQ_DIFF(seq->nr, seq->xack);
        const int peer_unack = L2TP_SEQ_DIFF(seq->ns, seq->rack);
        int i;

#define CHECK(p)        KASSERT((p), ("%s: not: %s", __func__, #p))

        CHECK(seq->wmax <= L2TP_MAX_XWIN);
        CHECK(seq->cwnd >= 1);
        CHECK(seq->cwnd <= seq->wmax);
        CHECK(seq->ssth >= 1);
        CHECK(seq->ssth <= seq->wmax);
        if (seq->cwnd < seq->ssth)
                CHECK(seq->acks == 0);
        else
                CHECK(seq->acks <= seq->cwnd);
        CHECK(self_unack >= 0);
        CHECK(peer_unack >= 0);
        CHECK(peer_unack <= seq->wmax);
        CHECK((self_unack == 0) ^ seq->xack_timer_running);
        CHECK((peer_unack == 0) ^ seq->rack_timer_running);
        CHECK(seq->rack_timer_running || !callout_pending(&seq->rack_timer));
        CHECK(seq->xack_timer_running || !callout_pending(&seq->xack_timer));
        for (i = 0; i < peer_unack; i++)
                CHECK(seq->xwin[i] != NULL);
        for ( ; i < seq->cwnd; i++)         /* verify peer's recv window full */
                CHECK(seq->xwin[i] == NULL);

#undef CHECK
}
#endif  /* INVARIANTS */
DragonflyBSD
