/*-
 * 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>
 */

/*
 * 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/epoch.h>
#include <sys/libkern.h>

#include <net/vnet.h>

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

#ifdef NG_SEPARATE_MALLOC
static 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           128                     /* 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)     ((int16_t)((x) - (y)))

#define SESSHASHSIZE            0x0020
#define SESSHASH(x)             (((x) ^ ((x) >> 8)) & (SESSHASHSIZE - 1))

/* Hook private data (data session hooks only) */
struct ng_l2tp_hook_private {
        struct ng_l2tp_sess_config      conf;   /* hook/session config */
        struct ng_l2tp_session_stats    stats;  /* per sessions statistics */
        hook_p                          hook;   /* hook reference */
        u_int16_t                       ns;     /* data ns sequence number */
        u_int16_t                       nr;     /* data nr sequence number */
        LIST_ENTRY(ng_l2tp_hook_private) sessions;
};
typedef struct ng_l2tp_hook_private *hookpriv_p;

/*
 * 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               inproc;         /* packet is in processing */
        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 */
        struct callout          rack_timer;     /* retransmit timer */
        struct callout          xack_timer;     /* delayed ack timer */
        struct mbuf             *xwin[L2TP_MAX_XWIN];   /* transmit window */
        struct mtx              mtx;                    /* seq mutex */
};

/* 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 */
        ng_ID_t                 ftarget;        /* failure message target */
        LIST_HEAD(, ng_l2tp_hook_private) sesshash[SESSHASHSIZE];
};
typedef struct ng_l2tp_private *priv_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_rcvdata_t     ng_l2tp_rcvdata_lower;
static ng_rcvdata_t     ng_l2tp_rcvdata_ctrl;
static ng_disconnect_t  ng_l2tp_disconnect;

/* Internal functions */
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_set(priv_p priv,
                        const struct ng_l2tp_seq_config *conf);
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 void     ng_l2tp_seq_xack_timeout(void *);
static void     ng_l2tp_seq_rack_timeout(void *);

static hookpriv_p       ng_l2tp_find_session(priv_p privp, u_int16_t sid);
static ng_fn_eachhook   ng_l2tp_reset_session;

/* Parse type for struct ng_l2tp_seq_config. */
static const struct ng_parse_struct_field
        ng_l2tp_seq_config_fields[] = NG_L2TP_SEQ_CONFIG_TYPE_INFO;
static const struct ng_parse_type ng_l2tp_seq_config_type = {
        &ng_parse_struct_type,
        &ng_l2tp_seq_config_fields
};

/* 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_l2tp_stats_type = {
        &ng_parse_struct_type,
        &ng_l2tp_stats_type_fields
};

/* Parse type for struct ng_l2tp_session_stats. */
static const struct ng_parse_struct_field
        ng_l2tp_session_stats_type_fields[] = NG_L2TP_SESSION_STATS_TYPE_INFO;
static const struct ng_parse_type ng_l2tp_session_stats_type = {
        &ng_parse_struct_type,
        &ng_l2tp_session_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_l2tp_stats_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_CLR_STATS,
          "clrstats",
          NULL,
          NULL
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GETCLR_STATS,
          "getclrstats",
          NULL,
          &ng_l2tp_stats_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GET_SESSION_STATS,
          "getsessstats",
          &ng_parse_int16_type,
          &ng_l2tp_session_stats_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_CLR_SESSION_STATS,
          "clrsessstats",
          &ng_parse_int16_type,
          NULL
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_GETCLR_SESSION_STATS,
          "getclrsessstats",
          &ng_parse_int16_type,
          &ng_l2tp_session_stats_type
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_ACK_FAILURE,
          "ackfailure",
          NULL,
          NULL
        },
        {
          NGM_L2TP_COOKIE,
          NGM_L2TP_SET_SEQ,
          "setsequence",
          &ng_l2tp_seq_config_type,
          NULL
        },
        { 0 }
};

/* Node type descriptor */
static struct ng_type ng_l2tp_typestruct = {
        .version =      NG_ABI_VERSION,
        .name =         NG_L2TP_NODE_TYPE,
        .constructor =  ng_l2tp_constructor,
        .rcvmsg =       ng_l2tp_rcvmsg,
        .shutdown =     ng_l2tp_shutdown,
        .newhook =      ng_l2tp_newhook,
        .rcvdata =      ng_l2tp_rcvdata,
        .disconnect =   ng_l2tp_disconnect,
        .cmdlist =      ng_l2tp_cmdlist,
};
NETGRAPH_INIT(l2tp, &ng_l2tp_typestruct);

/* Sequence number state locking & sanity checking */
#ifdef INVARIANTS
static void     ng_l2tp_seq_check(struct l2tp_seq *seq);
#define SEQ_LOCK(seq)   do {                                    \
                                mtx_lock(&(seq)->mtx);          \
                                ng_l2tp_seq_check(seq);         \
} while (0)
#define SEQ_UNLOCK(seq) do {                                    \
                                ng_l2tp_seq_check(seq);         \
                                mtx_unlock(&(seq)->mtx);        \
} while (0)
#else
#define SEQ_LOCK(seq)           mtx_lock(&(seq)->mtx)
#define SEQ_UNLOCK(seq)         mtx_unlock(&(seq)->mtx)
#endif
#define SEQ_LOCK_ASSERT(seq)    mtx_assert(&(seq)->mtx, MA_OWNED)

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

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

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

/*
 * Node type constructor
 */
static int
ng_l2tp_constructor(node_p node)
{
        priv_p priv;
        int     i;

        /* Allocate private structure */
        priv = malloc(sizeof(*priv), M_NETGRAPH_L2TP, M_WAITOK | M_ZERO);
        NG_NODE_SET_PRIVATE(node, priv);
        priv->node = node;

        /* 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);

        for (i = 0; i < SESSHASHSIZE; i++)
            LIST_INIT(&priv->sesshash[i]);

        /* 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;
                NG_HOOK_SET_RCVDATA(hook, ng_l2tp_rcvdata_ctrl);
        } else if (strcmp(name, NG_L2TP_HOOK_LOWER) == 0) {
                if (priv->lower != NULL)
                        return (EISCONN);
                priv->lower = hook;
                NG_HOOK_SET_RCVDATA(hook, ng_l2tp_rcvdata_lower);
        } else {
                static const char hexdig[16] = "0123456789abcdef";
                u_int16_t session_id;
                hookpriv_p hpriv;
                uint16_t hash;
                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 = malloc(sizeof(*hpriv),
                    M_NETGRAPH_L2TP, M_NOWAIT | M_ZERO);
                if (hpriv == NULL)
                        return (ENOMEM);
                hpriv->conf.session_id = session_id;
                hpriv->conf.control_dseq = L2TP_CONTROL_DSEQ;
                hpriv->conf.enable_dseq = L2TP_ENABLE_DSEQ;
                hpriv->hook = hook;
                NG_HOOK_SET_PRIVATE(hook, hpriv);
                hash = SESSHASH(hpriv->conf.session_id);
                LIST_INSERT_HEAD(&priv->sesshash[hash], hpriv, sessions);
        }

        /* Done */
        return (0);
}

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

        NGI_GET_MSG(item, msg);
        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;
                        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 */
                        priv->ftarget = NGI_RETADDR(item);

                        /* 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;
                        break;
                    }
                case NGM_L2TP_SET_SESS_CONFIG:
                    {
                        struct ng_l2tp_sess_config *const conf =
                            (struct ng_l2tp_sess_config *)msg->data;
                        hookpriv_p hpriv;

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

                        /* Find matching hook */
                        hpriv = ng_l2tp_find_session(priv, conf->session_id);
                        if (hpriv == NULL) {
                                error = ENOENT;
                                break;
                        }

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

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

                        /* Find matching hook */
                        hpriv = ng_l2tp_find_session(priv, session_id);
                        if (hpriv == 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;
                        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;
                    }
                case NGM_L2TP_GET_SESSION_STATS:
                case NGM_L2TP_CLR_SESSION_STATS:
                case NGM_L2TP_GETCLR_SESSION_STATS:
                    {
                        uint16_t session_id;
                        hookpriv_p hpriv;

                        /* Get session ID. */
                        if (msg->header.arglen != sizeof(session_id)) {
                                error = EINVAL;
                                break;
                        }
                        bcopy(msg->data, &session_id, sizeof(uint16_t));

                        /* Find matching hook. */
                        hpriv = ng_l2tp_find_session(priv, session_id);
                        if (hpriv == NULL) {
                                error = ENOENT;
                                break;
                        }

                        if (msg->header.cmd != NGM_L2TP_CLR_SESSION_STATS) {
                                NG_MKRESPONSE(resp, msg,
                                    sizeof(hpriv->stats), M_NOWAIT);
                                if (resp == NULL) {
                                        error = ENOMEM;
                                        break;
                                }
                                bcopy(&hpriv->stats, resp->data,
                                        sizeof(hpriv->stats));
                        }
                        if (msg->header.cmd != NGM_L2TP_GET_SESSION_STATS)
                                bzero(&hpriv->stats, sizeof(hpriv->stats));
                        break;
                    }
                case NGM_L2TP_SET_SEQ:
                    {
                        struct ng_l2tp_seq_config *const conf =
                                (struct ng_l2tp_seq_config *)msg->data;

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

                        /* Set sequence numbers. */
                        error = ng_l2tp_seq_set(priv, conf);
                        break;
                    }
                default:
                        error = EINVAL;
                        break;
                }
                break;
        default:
                error = EINVAL;
                break;
        }

        /* Done */
        NG_RESPOND_MSG(error, node, item, resp);
        NG_FREE_MSG(msg);
        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;

        /* Reset sequence number state */
        SEQ_LOCK(seq);
        ng_l2tp_seq_reset(priv);
        SEQ_UNLOCK(seq);

        /* Free private data if neither timer is running */
        callout_drain(&seq->rack_timer);
        callout_drain(&seq->xack_timer);

        mtx_destroy(&seq->mtx);

        free(priv, M_NETGRAPH_L2TP);

        /* 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 {
                const hookpriv_p hpriv = NG_HOOK_PRIVATE(hook);
                LIST_REMOVE(hpriv, sessions);
                free(hpriv, M_NETGRAPH_L2TP);
                NG_HOOK_SET_PRIVATE(hook, NULL);
        }

        /* Go away if no longer connected to anything */
        if (NG_NODE_NUMHOOKS(node) == 0 && NG_NODE_IS_VALID(node))
                ng_rmnode_self(node);
        return (0);
}

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

/*
 * Find the hook with a given session ID.
 */
static hookpriv_p
ng_l2tp_find_session(priv_p privp, u_int16_t sid)
{
        uint16_t        hash = SESSHASH(sid);
        hookpriv_p      hpriv = NULL;

        LIST_FOREACH(hpriv, &privp->sesshash[hash], sessions) {
                if (hpriv->conf.session_id == sid)
                        break;
        }

        return (hpriv);
}

/*
 * Reset a hook's session state.
 */
static int
ng_l2tp_reset_session(hook_p hook, void *arg)
{
        const hookpriv_p hpriv = NG_HOOK_PRIVATE(hook);

        if (hpriv != NULL) {
                hpriv->conf.control_dseq = 0;
                hpriv->conf.enable_dseq = 0;
                bzero(&hpriv->stats, sizeof(struct ng_l2tp_session_stats));
                hpriv->nr = 0;
                hpriv->ns = 0;
        }
        return (-1);
}

/*
 * Handle an incoming frame from below.
 */
static int
ng_l2tp_rcvdata_lower(hook_p h, item_p item)
{
        static const u_int16_t req_bits[2][2] = {
                { L2TP_DATA_0BITS, L2TP_DATA_1BITS },
                { L2TP_CTRL_0BITS, L2TP_CTRL_1BITS },
        };
        const node_p node = NG_HOOK_NODE(h);
        const priv_p priv = NG_NODE_PRIVATE(node);
        hookpriv_p hpriv = NULL;
        hook_p hook = NULL;
        struct mbuf *m;
        u_int16_t tid, sid;
        u_int16_t hdr;
        u_int16_t ns, nr;
        int is_ctrl;
        int error;
        int len, plen;

        /* If not configured, reject */
        if (!priv->conf.enabled) {
                NG_FREE_ITEM(item);
                ERROUT(ENXIO);
        }

        /* Grab mbuf */
        NGI_GET_M(item, m);

        /* Remember full packet length; needed for per session accounting. */
        plen = m->m_pkthdr.len;

        /* Update stats */
        priv->stats.recvPackets++;
        priv->stats.recvOctets += plen;

        /* Get initial header */
        if (m->m_pkthdr.len < 6) {
                priv->stats.recvRunts++;
                NG_FREE_ITEM(item);
                NG_FREE_M(m);
                ERROUT(EINVAL);
        }
        if (m->m_len < 2 && (m = m_pullup(m, 2)) == NULL) {
                priv->stats.memoryFailures++;
                NG_FREE_ITEM(item);
                ERROUT(EINVAL);
        }
        hdr = (mtod(m, uint8_t *)[0] << 8) + mtod(m, uint8_t *)[1];
        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_ITEM(item);
                NG_FREE_M(m);
                ERROUT(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_ITEM(item);
                NG_FREE_M(m);
                ERROUT(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_ITEM(item);
                        ERROUT(EINVAL);
                }
                len = (mtod(m, uint8_t *)[0] << 8) + mtod(m, uint8_t *)[1] - 4;
                m_adj(m, 2);
                if (len < 0 || len > m->m_pkthdr.len) {
                        priv->stats.recvInvalid++;
                        NG_FREE_ITEM(item);
                        NG_FREE_M(m);
                        ERROUT(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_ITEM(item);
                ERROUT(EINVAL);
        }
        tid = (mtod(m, u_int8_t *)[0] << 8) + mtod(m, u_int8_t *)[1];
        sid = (mtod(m, u_int8_t *)[2] << 8) + mtod(m, u_int8_t *)[3];
        m_adj(m, 4);

        /* Check tunnel ID */
        if (tid != priv->conf.tunnel_id &&
            (priv->conf.match_id || tid != 0)) {
                priv->stats.recvWrongTunnel++;
                NG_FREE_ITEM(item);
                NG_FREE_M(m);
                ERROUT(EADDRNOTAVAIL);
        }

        /* Check session ID (for data packets only) */
        if ((hdr & L2TP_HDR_CTRL) == 0) {
                hpriv = ng_l2tp_find_session(priv, sid);
                if (hpriv == NULL) {
                        priv->stats.recvUnknownSID++;
                        NG_FREE_ITEM(item);
                        NG_FREE_M(m);
                        ERROUT(ENOTCONN);
                }
                hook = hpriv->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_ITEM(item);
                        ERROUT(EINVAL);
                }
                ns = (mtod(m, u_int8_t *)[0] << 8) + mtod(m, u_int8_t *)[1];
                nr = (mtod(m, u_int8_t *)[2] << 8) + mtod(m, u_int8_t *)[3];
                m_adj(m, 4);
        } else
                ns = nr = 0;

        /* 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_ITEM(item);
                        ERROUT(EINVAL);
                }
                offset = (mtod(m, u_int8_t *)[0] << 8) + mtod(m, u_int8_t *)[1];

                /* Trim offset padding */
                if ((2+offset) > m->m_pkthdr.len) {
                        priv->stats.recvInvalid++;
                        NG_FREE_ITEM(item);
                        NG_FREE_M(m);
                        ERROUT(EINVAL);
                }
                m_adj(m, 2+offset);
        }

        /* Handle control packets */
        if ((hdr & L2TP_HDR_CTRL) != 0) {
                struct l2tp_seq *const seq = &priv->seq;

                SEQ_LOCK(seq);

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

                /* Discard ZLB packets */
                if (m->m_pkthdr.len == 0) {
                        SEQ_UNLOCK(seq);
                        priv->stats.recvZLBs++;
                        NG_FREE_ITEM(item);
                        NG_FREE_M(m);
                        ERROUT(0);
                }

                /*
                 * If not what we expect or we are busy, drop packet and
                 * send an immediate ZLB ack.
                 */
                if (ns != seq->nr || seq->inproc) {
                        if (L2TP_SEQ_DIFF(ns, seq->nr) <= 0)
                                priv->stats.recvDuplicates++;
                        else
                                priv->stats.recvOutOfOrder++;
                        ng_l2tp_xmit_ctrl(priv, NULL, seq->ns);
                        NG_FREE_ITEM(item);
                        NG_FREE_M(m);
                        ERROUT(0);
                }

                /* Prepend session ID to packet. */
                M_PREPEND(m, 2, M_NOWAIT);
                if (m == NULL) {
                        SEQ_UNLOCK(seq);
                        priv->stats.memoryFailures++;
                        NG_FREE_ITEM(item);
                        ERROUT(ENOBUFS);
                }
                mtod(m, u_int8_t *)[0] = sid >> 8;
                mtod(m, u_int8_t *)[1] = sid & 0xff;

                /*
                 * Until we deliver this packet we can't receive next one as
                 * we have no information for sending ack.
                 */
                seq->inproc = 1;
                SEQ_UNLOCK(seq);

                /* Deliver packet to upper layers */
                NG_FWD_NEW_DATA(error, item, priv->ctrl, m);
                
                SEQ_LOCK(seq);
                /* Ready to process next packet. */
                seq->inproc = 0;

                /* If packet was successfully delivered send ack. */
                if (error == 0) {
                        /* Update recv sequence number */
                        seq->nr++;
                        /* Start receive ack timer, if not already running */
                        if (!callout_active(&seq->xack_timer)) {
                                callout_reset(&seq->xack_timer,
                                    L2TP_DELAYED_ACK, ng_l2tp_seq_xack_timeout,
                                    node);
                        }
                }
                SEQ_UNLOCK(seq);

                ERROUT(error);
        }

        /* Per session packet, account it. */
        hpriv->stats.recvPackets++;
        hpriv->stats.recvOctets += plen;

        /* 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_ITEM(item);     /* duplicate or out of order */
                        NG_FREE_M(m);
                        priv->stats.recvDataDrops++;
                        ERROUT(0);
                }
                hpriv->nr = ns + 1;
        }

        /* Drop empty data packets */
        if (m->m_pkthdr.len == 0) {
                NG_FREE_ITEM(item);
                NG_FREE_M(m);
                ERROUT(0);
        }

        /* Deliver data */
        NG_FWD_NEW_DATA(error, item, hook, m);
done:
        return (error);
}

/*
 * Handle an outgoing control frame.
 */
static int
ng_l2tp_rcvdata_ctrl(hook_p hook, item_p item)
{
        const node_p node = NG_HOOK_NODE(hook);
        const priv_p priv = NG_NODE_PRIVATE(node);
        struct l2tp_seq *const seq = &priv->seq;
        struct mbuf *m;
        int error;
        int i;
        u_int16_t       ns;

        /* If not configured, reject */
        if (!priv->conf.enabled) {
                NG_FREE_ITEM(item);
                ERROUT(ENXIO);
        }

        /* Grab mbuf and discard other stuff XXX */
        NGI_GET_M(item, m);
        NG_FREE_ITEM(item);

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

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

        SEQ_LOCK(seq);

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

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

        /* Start retransmit timer if not already running */
        if (!callout_active(&seq->rack_timer))
                callout_reset(&seq->rack_timer, hz, ng_l2tp_seq_rack_timeout,
                    node);

        ns = seq->ns++;

        /* Copy packet */
        if ((m = L2TP_COPY_MBUF(m, M_NOWAIT)) == NULL) {
                SEQ_UNLOCK(seq);
                priv->stats.memoryFailures++;
                ERROUT(ENOBUFS);
        }

        /* Send packet and increment xmit sequence number */
        error = ng_l2tp_xmit_ctrl(priv, m, ns);
done:
        return (error);
}

/*
 * Handle an outgoing data frame.
 */
static int
ng_l2tp_rcvdata(hook_p hook, item_p item)
{
        const priv_p priv = NG_NODE_PRIVATE(NG_HOOK_NODE(hook));
        const hookpriv_p hpriv = NG_HOOK_PRIVATE(hook);
        struct mbuf *m;
        uint8_t *p;
        u_int16_t hdr;
        int error;
        int i = 2;

        /* If not configured, reject */
        if (!priv->conf.enabled) {
                NG_FREE_ITEM(item);
                ERROUT(ENXIO);
        }

        /* Get mbuf */
        NGI_GET_M(item, m);

        /* Check max length */
        if (m->m_pkthdr.len >= 0x10000 - 12) {
                priv->stats.xmitDataTooBig++;
                NG_FREE_ITEM(item);
                NG_FREE_M(m);
                ERROUT(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_ITEM(item);
                ERROUT(ENOBUFS);
        }
        p = mtod(m, uint8_t *);
        hdr = L2TP_DATA_HDR;
        if (hpriv->conf.include_length) {
                hdr |= L2TP_HDR_LEN;
                p[i++] = m->m_pkthdr.len >> 8;
                p[i++] = m->m_pkthdr.len & 0xff;
        }
        p[i++] = priv->conf.peer_id >> 8;
        p[i++] = priv->conf.peer_id & 0xff;
        p[i++] = hpriv->conf.peer_id >> 8;
        p[i++] = hpriv->conf.peer_id & 0xff;
        if (hpriv->conf.enable_dseq) {
                hdr |= L2TP_HDR_SEQ;
                p[i++] = hpriv->ns >> 8;
                p[i++] = hpriv->ns & 0xff;
                p[i++] = hpriv->nr >> 8;
                p[i++] = hpriv->nr & 0xff;
                hpriv->ns++;
        }
        p[0] = hdr >> 8;
        p[1] = hdr & 0xff;

        /* Update per session stats. */
        hpriv->stats.xmitPackets++;
        hpriv->stats.xmitOctets += m->m_pkthdr.len;

        /* And the global one. */
        priv->stats.xmitPackets++;
        priv->stats.xmitOctets += m->m_pkthdr.len;

        /* Send packet */
        NG_FWD_NEW_DATA(error, item, priv->lower, m);
done:
        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;
        int error;

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

/************************************************************************
                        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;
        mtx_init(&seq->mtx, "ng_l2tp", NULL, MTX_DEF);
        callout_init_mtx(&seq->rack_timer, &seq->mtx, CALLOUT_RETURNUNLOCKED);
        callout_init_mtx(&seq->xack_timer, &seq->mtx, CALLOUT_RETURNUNLOCKED);
}

/*
 * Set sequence number state as given from user.
 */
static int
ng_l2tp_seq_set(priv_p priv, const struct ng_l2tp_seq_config *conf)
{
        struct l2tp_seq *const seq = &priv->seq;

        /* If node is enabled, deny update to sequence numbers. */
        if (priv->conf.enabled)
                return (EBUSY);

        /* We only can handle the simple cases. */
        if (conf->xack != conf->nr || conf->ns != conf->rack)
                return (EINVAL);

        /* Set ns,nr,rack,xack parameters. */
        seq->ns = conf->ns;
        seq->nr = conf->nr;
        seq->rack = conf->rack;
        seq->xack = conf->xack;

        return (0);
}

/*
 * 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;
        int error = 0;

        SEQ_LOCK(seq);
        /* If disabling node, reset state sequence number */
        if (!conf->enabled) {
                ng_l2tp_seq_reset(priv);
                SEQ_UNLOCK(seq);
                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)
                ERROUT(EINVAL);
        if (new_wmax < seq->wmax)
                ERROUT(EBUSY);
        seq->wmax = new_wmax;

done:
        SEQ_UNLOCK(seq);
        return (error);
}

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

        SEQ_LOCK_ASSERT(seq);

        /* Stop timers */
        (void )callout_stop(&seq->rack_timer);
        (void )callout_stop(&seq->xack_timer);

        /* 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 */
        NG_NODE_FOREACH_HOOK(priv->node, ng_l2tp_reset_session, NULL);

        /* Reset node's sequence number state */
        seq->ns = 0;
        seq->nr = 0;
        seq->rack = 0;
        seq->xack = 0;
        seq->wmax = L2TP_MAX_XWIN;
        seq->cwnd = 1;
        seq->ssth = seq->wmax;
        seq->acks = 0;
        seq->rexmits = 0;
        bzero(seq->xwin, sizeof(seq->xwin));
}

/*
 * 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     *xwin[L2TP_MAX_XWIN];   /* partial local copy */
        int             nack;
        int             i, j;
        uint16_t        ns;

        SEQ_LOCK_ASSERT(seq);

        /* 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 (callout_active(&seq->rack_timer))
                (void )callout_stop(&seq->rack_timer);

        /* 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);

        /*
         * Send more packets, trying to keep peer's receive window full.
         * Make copy of everything we need before lock release.
         */
        ns = seq->ns;
        j = 0;
        while ((i = L2TP_SEQ_DIFF(seq->ns, seq->rack)) < seq->cwnd
            && seq->xwin[i] != NULL) {
                xwin[j++] = seq->xwin[i];
                seq->ns++;
        }

        /*
         * Send prepared.
         * If there is a memory error, pretend packet was sent, as it
         * will get retransmitted later anyway.
         */
        for (i = 0; i < j; i++) {
                struct mbuf     *m;
                if ((m = L2TP_COPY_MBUF(xwin[i], M_NOWAIT)) == NULL)
                        priv->stats.memoryFailures++;
                else {
                        ng_l2tp_xmit_ctrl(priv, m, ns);
                        SEQ_LOCK(seq);
                }
                ns++;
        }
}

/*
 * 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 epoch_tracker et;
        struct l2tp_seq *const seq = &priv->seq;

        SEQ_LOCK_ASSERT(seq);
        MPASS(!callout_pending(&seq->xack_timer));
        MPASS(callout_active(&seq->xack_timer));

        NET_EPOCH_ENTER(et);
        CURVNET_SET(node->nd_vnet);
        /* Send a ZLB */
        ng_l2tp_xmit_ctrl(priv, NULL, seq->ns);
        CURVNET_RESTORE();
        NET_EPOCH_EXIT(et);

        /* callout_deactivate() is not needed here 
            as callout_stop() was called by ng_l2tp_xmit_ctrl() */
}

/* 
 * 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 epoch_tracker et;
        struct l2tp_seq *const seq = &priv->seq;
        struct mbuf *m;
        u_int delay;

        SEQ_LOCK_ASSERT(seq);
        MPASS(seq->xwin[0]);
        MPASS(!callout_pending(&seq->rack_timer));
        MPASS(callout_active(&seq->rack_timer));

        NET_EPOCH_ENTER(et);
        CURVNET_SET(node->nd_vnet);

        priv->stats.xmitRetransmits++;

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

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

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

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

        CURVNET_RESTORE();
        NET_EPOCH_EXIT(et);

        /* callout_deactivate() is not needed here 
            as ng_callout() is getting called each time */
}

/*
 * Transmit a control stream packet, payload optional.
 * The transmit sequence number is not incremented.
 * Requires seq lock, returns unlocked.
 */
static int
ng_l2tp_xmit_ctrl(priv_p priv, struct mbuf *m, u_int16_t ns)
{
        struct l2tp_seq *const seq = &priv->seq;
        uint8_t *p;
        uint16_t nr, session_id = 0;
        int error;

        SEQ_LOCK_ASSERT(seq);

        /* Stop ack timer: we're sending an ack with this packet.
           Doing this before to keep state predictable after error. */
        if (callout_active(&seq->xack_timer))
                (void )callout_stop(&seq->xack_timer);

        nr = seq->xack = seq->nr;

        SEQ_UNLOCK(seq);

        /* 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);
                }
                session_id = (mtod(m, u_int8_t *)[0] << 8) + mtod(m, u_int8_t *)[1];

                /* Make room for L2TP header */
                M_PREPEND(m, 10, M_NOWAIT);     /* - 2 + 12 = 10 */
                if (m == NULL) {
                        priv->stats.memoryFailures++;
                        return (ENOBUFS);
                }

                /*
                 * The below requires 12 contiguous bytes for the L2TP header
                 * to be written into.
                 */
                m = m_pullup(m, 12);
                if (m == NULL) {
                        priv->stats.memoryFailures++;
                        return (ENOBUFS);
                }
        }

        /* Fill in L2TP header */
        p = mtod(m, u_int8_t *);
        p[0] = L2TP_CTRL_HDR >> 8;
        p[1] = L2TP_CTRL_HDR & 0xff;
        p[2] = m->m_pkthdr.len >> 8;
        p[3] = m->m_pkthdr.len & 0xff;
        p[4] = priv->conf.peer_id >> 8;
        p[5] = priv->conf.peer_id & 0xff;
        p[6] = session_id >> 8;
        p[7] = session_id & 0xff;
        p[8] = ns >> 8;
        p[9] = ns & 0xff;
        p[10] = nr >> 8;
        p[11] = nr & 0xff;

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

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

#ifdef INVARIANTS
/*
 * Sanity check sequence number state.
 */
static void
ng_l2tp_seq_check(struct l2tp_seq *seq)
{
        int self_unack, peer_unack;
        int i;

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

        SEQ_LOCK_ASSERT(seq);

        self_unack = L2TP_SEQ_DIFF(seq->nr, seq->xack);
        peer_unack = L2TP_SEQ_DIFF(seq->ns, seq->rack);
        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) ^ callout_active(&seq->xack_timer));
        CHECK((peer_unack == 0) ^ callout_active(&seq->rack_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 */