text: /**
   * @file
   *
   * 6LowPAN output for IPv6. Uses ND tables for link-layer addressing. Fragments packets to 6LowPAN units.
   *
   * This implementation aims to conform to IEEE 802.15.4(-2015), RFC 4944 and RFC 6282.
   * @todo: RFC 6775.
   */
  
  /*
   * Copyright (c) 2015 Inico Technologies Ltd.
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without modification,
   * are permitted provided that the following conditions are met:
   *
   * 1. Redistributions of source code must retain the above copyright notice,
   *    this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   * 3. The name of the author may not be used to endorse or promote products
   *    derived from this software without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
   * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
   * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
   * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
   * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
   * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
   * OF SUCH DAMAGE.
   *
   * This file is part of the lwIP TCP/IP stack.
   *
   * Author: Ivan Delamer <delamer@inicotech.com>
   *
   *
   * Please coordinate changes and requests with Ivan Delamer
   * <delamer@inicotech.com>
   */
  
  /**
   * @defgroup sixlowpan 6LoWPAN (RFC4944)
   * @ingroup netifs
   * 6LowPAN netif implementation
   */
  
  #include "netif/lowpan6.h"
  
  #if LWIP_IPV6
  
  #include "lwip/ip.h"
  #include "lwip/pbuf.h"
  #include "lwip/ip_addr.h"
  #include "lwip/netif.h"
  #include "lwip/nd6.h"
  #include "lwip/mem.h"
  #include "lwip/udp.h"
  #include "lwip/tcpip.h"
  #include "lwip/snmp.h"
  #include "netif/ieee802154.h"
  
  #include <string.h>
  
  #if LWIP_6LOWPAN_802154_HW_CRC
  #define LWIP_6LOWPAN_DO_CALC_CRC(buf, len) 0
  #else
  #define LWIP_6LOWPAN_DO_CALC_CRC(buf, len) LWIP_6LOWPAN_CALC_CRC(buf, len)
  #endif
  
  /** This is a helper struct for reassembly of fragments
   * (IEEE 802.15.4 limits to 127 bytes)
   */
  struct lowpan6_reass_helper {
    struct lowpan6_reass_helper *next_packet;
    struct pbuf *reass;
    struct pbuf *frags;
    u8_t timer;
    struct lowpan6_link_addr sender_addr;
    u16_t datagram_size;
    u16_t datagram_tag;
  };
  
  /** This struct keeps track of per-netif state */
  struct lowpan6_ieee802154_data {
    /** fragment reassembly list */
    struct lowpan6_reass_helper *reass_list;
  #if LWIP_6LOWPAN_NUM_CONTEXTS > 0
    /** address context for compression */
    ip6_addr_t lowpan6_context[LWIP_6LOWPAN_NUM_CONTEXTS];
  #endif
    /** Datagram Tag for fragmentation */
    u16_t tx_datagram_tag;
    /** local PAN ID for IEEE 802.15.4 header */
    u16_t ieee_802154_pan_id;
    /** Sequence Number for IEEE 802.15.4 transmission */
    u8_t tx_frame_seq_num;
  };
  
  /* Maximum frame size is 127 bytes minus CRC size */
  #define LOWPAN6_MAX_PAYLOAD (127 - 2)
  
  /** Currently, this state is global, since there's only one 6LoWPAN netif */
  static struct lowpan6_ieee802154_data lowpan6_data;
  
  #if LWIP_6LOWPAN_NUM_CONTEXTS > 0
  #define LWIP_6LOWPAN_CONTEXTS(netif) lowpan6_data.lowpan6_context
  #else
  #define LWIP_6LOWPAN_CONTEXTS(netif) NULL
  #endif
  
  static const struct lowpan6_link_addr ieee_802154_broadcast = {2, {0xff, 0xff}};
  
  #if LWIP_6LOWPAN_INFER_SHORT_ADDRESS
  static struct lowpan6_link_addr short_mac_addr = {2, {0, 0}};
  #endif /* LWIP_6LOWPAN_INFER_SHORT_ADDRESS */
  
  /* IEEE 802.15.4 specific functions: */
  
  /** Write the IEEE 802.15.4 header that encapsulates the 6LoWPAN frame.
   * Src and dst PAN IDs are filled with the ID set by @ref lowpan6_set_pan_id.
   *
   * Since the length is variable:
   * @returns the header length
   */
  static u8_t
  lowpan6_write_iee802154_header(struct ieee_802154_hdr *hdr, const struct lowpan6_link_addr *src,
                                 const struct lowpan6_link_addr *dst)
  {
    u8_t ieee_header_len;
    u8_t *buffer;
    u8_t i;
    u16_t fc;
  
    fc = IEEE_802154_FC_FT_DATA; /* send data packet (2003 frame version) */
    fc |= IEEE_802154_FC_PANID_COMPR; /* set PAN ID compression, for now src and dst PANs are equal */
    if (dst != &ieee_802154_broadcast) {
      fc |= IEEE_802154_FC_ACK_REQ; /* data packet, no broadcast: ack required. */
    }
    if (dst->addr_len == 2) {
      fc |= IEEE_802154_FC_DST_ADDR_MODE_SHORT;
    } else {
      LWIP_ASSERT("invalid dst address length", dst->addr_len == 8);
      fc |= IEEE_802154_FC_DST_ADDR_MODE_EXT;
    }
    if (src->addr_len == 2) {
      fc |= IEEE_802154_FC_SRC_ADDR_MODE_SHORT;
    } else {
      LWIP_ASSERT("invalid src address length", src->addr_len == 8);
      fc |= IEEE_802154_FC_SRC_ADDR_MODE_EXT;
    }
    hdr->frame_control = fc;
    hdr->sequence_number = lowpan6_data.tx_frame_seq_num++;
    hdr->destination_pan_id = lowpan6_data.ieee_802154_pan_id; /* pan id */
  
    buffer = (u8_t *)hdr;
    ieee_header_len = 5;
    i = dst->addr_len;
    /* reverse memcpy of dst addr */
    while (i-- > 0) {
      buffer[ieee_header_len++] = dst->addr[i];
    }
    /* Source PAN ID skipped due to PAN ID Compression */
    i = src->addr_len;
    /* reverse memcpy of src addr */
    while (i-- > 0) {
      buffer[ieee_header_len++] = src->addr[i];
    }
    return ieee_header_len;
  }
  
  /** Parse the IEEE 802.15.4 header from a pbuf.
   * If successful, the header is hidden from the pbuf.
   *
   * PAN IDs and seuqence number are not checked
   *
   * @param p input pbuf, p->payload pointing at the IEEE 802.15.4 header
   * @param src pointer to source address filled from the header
   * @param dest pointer to destination address filled from the header
   * @returns ERR_OK if successful
   */
  static err_t
  lowpan6_parse_iee802154_header(struct pbuf *p, struct lowpan6_link_addr *src,
                                 struct lowpan6_link_addr *dest)
  {
    u8_t *puc;
    s8_t i;
    u16_t frame_control, addr_mode;
    u16_t datagram_offset;
  
    /* Parse IEEE 802.15.4 header */
    puc = (u8_t *)p->payload;
    frame_control = puc[0] | (puc[1] << 8);
    datagram_offset = 2;
    if (frame_control & IEEE_802154_FC_SEQNO_SUPPR) {
      if (IEEE_802154_FC_FRAME_VERSION_GET(frame_control) <= 1) {
        /* sequence number suppressed, this is not valid for versions 0/1 */
        return ERR_VAL;
      }
    } else {
      datagram_offset++;
    }
    datagram_offset += 2; /* Skip destination PAN ID */
    addr_mode = frame_control & IEEE_802154_FC_DST_ADDR_MODE_MASK;
    if (addr_mode == IEEE_802154_FC_DST_ADDR_MODE_EXT) {
      /* extended address (64 bit) */
      dest->addr_len = 8;
      /* reverse memcpy: */
      for (i = 0; i < 8; i++) {
        dest->addr[i] = puc[datagram_offset + 7 - i];
      }
      datagram_offset += 8;
    } else if (addr_mode == IEEE_802154_FC_DST_ADDR_MODE_SHORT) {
      /* short address (16 bit) */
      dest->addr_len = 2;
      /* reverse memcpy: */
      dest->addr[0] = puc[datagram_offset + 1];
      dest->addr[1] = puc[datagram_offset];
      datagram_offset += 2;
    } else {
      /* unsupported address mode (do we need "no address"?) */
      return ERR_VAL;
    }
  
    if (!(frame_control & IEEE_802154_FC_PANID_COMPR)) {
      /* No PAN ID compression, skip source PAN ID */
      datagram_offset += 2;
    }
  
    addr_mode = frame_control & IEEE_802154_FC_SRC_ADDR_MODE_MASK;
    if (addr_mode == IEEE_802154_FC_SRC_ADDR_MODE_EXT) {
      /* extended address (64 bit) */
      src->addr_len = 8;
      /* reverse memcpy: */
      for (i = 0; i < 8; i++) {
        src->addr[i] = puc[datagram_offset + 7 - i];
      }
      datagram_offset += 8;
    } else if (addr_mode == IEEE_802154_FC_DST_ADDR_MODE_SHORT) {
      /* short address (16 bit) */
      src->addr_len = 2;
      src->addr[0] = puc[datagram_offset + 1];
      src->addr[1] = puc[datagram_offset];
      datagram_offset += 2;
    } else {
      /* unsupported address mode (do we need "no address"?) */
      return ERR_VAL;
    }
  
    /* hide IEEE802.15.4 header. */
    if (pbuf_remove_header(p, datagram_offset)) {
      return ERR_VAL;
    }
    return ERR_OK;
  }
  
  /** Calculate the 16-bit CRC as required by IEEE 802.15.4 */
  u16_t
  lowpan6_calc_crc(const void* buf, u16_t len)
  {
  #define CCITT_POLY_16 0x8408U
    u16_t i;
    u8_t b;
    u16_t crc = 0;
    const u8_t* p = (const u8_t*)buf;
  
    for (i = 0; i < len; i++) {
      u8_t data = *p;
      for (b = 0U; b < 8U; b++) {
        if (((data ^ crc) & 1) != 0) {
          crc = (u16_t)((crc >> 1) ^ CCITT_POLY_16);
        } else {
          crc = (u16_t)(crc >> 1);
        }
        data = (u8_t)(data >> 1);
      }
      p++;
    }
    return crc;
  }
  
  /* Fragmentation specific functions: */
  
  static void
  free_reass_datagram(struct lowpan6_reass_helper *lrh)
  {
    if (lrh->reass) {
      pbuf_free(lrh->reass);
    }
    if (lrh->frags) {
      pbuf_free(lrh->frags);
    }
    mem_free(lrh);
  }
  
  /**
   * Removes a datagram from the reassembly queue.
   **/
  static void
  dequeue_datagram(struct lowpan6_reass_helper *lrh, struct lowpan6_reass_helper *prev)
  {
    if (lowpan6_data.reass_list == lrh) {
      lowpan6_data.reass_list = lowpan6_data.reass_list->next_packet;
    } else {
      /* it wasn't the first, so it must have a valid 'prev' */
      LWIP_ASSERT("sanity check linked list", prev != NULL);
      prev->next_packet = lrh->next_packet;
    }
  }
  
  /**
   * Periodic timer for 6LowPAN functions:
   *
   * - Remove incomplete/old packets
   */
  void
  lowpan6_tmr(void)
  {
    struct lowpan6_reass_helper *lrh, *lrh_next, *lrh_prev = NULL;
  
    lrh = lowpan6_data.reass_list;
    while (lrh != NULL) {
      lrh_next = lrh->next_packet;
      if ((--lrh->timer) == 0) {
        dequeue_datagram(lrh, lrh_prev);
        free_reass_datagram(lrh);
      } else {
        lrh_prev = lrh;
      }
      lrh = lrh_next;
    }
  }
  
  /*
   * Encapsulates data into IEEE 802.15.4 frames.
   * Fragments an IPv6 datagram into 6LowPAN units, which fit into IEEE 802.15.4 frames.
   * If configured, will compress IPv6 and or UDP headers.
   * */
  static err_t
  lowpan6_frag(struct netif *netif, struct pbuf *p, const struct lowpan6_link_addr *src, const struct lowpan6_link_addr *dst)
  {
    struct pbuf *p_frag;
    u16_t frag_len, remaining_len, max_data_len;
    u8_t *buffer;
    u8_t ieee_header_len;
    u8_t lowpan6_header_len;
    u8_t hidden_header_len;
    u16_t crc;
    u16_t datagram_offset;
    err_t err = ERR_IF;
  
    LWIP_ASSERT("lowpan6_frag: netif->linkoutput not set", netif->linkoutput != NULL);
  
    /* We'll use a dedicated pbuf for building 6LowPAN fragments. */
    p_frag = pbuf_alloc(PBUF_RAW, 127, PBUF_RAM);
    if (p_frag == NULL) {
      MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
      return ERR_MEM;
    }
    LWIP_ASSERT("this needs a pbuf in one piece", p_frag->len == p_frag->tot_len);
  
    /* Write IEEE 802.15.4 header. */
    buffer = (u8_t *)p_frag->payload;
    ieee_header_len = lowpan6_write_iee802154_header((struct ieee_802154_hdr *)buffer, src, dst);
    LWIP_ASSERT("ieee_header_len < p_frag->len", ieee_header_len < p_frag->len);
  
  #if LWIP_6LOWPAN_IPHC
    /* Perform 6LowPAN IPv6 header compression according to RFC 6282 */
    /* do the header compression (this does NOT copy any non-compressed data) */
    err = lowpan6_compress_headers(netif, (u8_t *)p->payload, p->len,
      &buffer[ieee_header_len], p_frag->len - ieee_header_len, &lowpan6_header_len,
      &hidden_header_len, LWIP_6LOWPAN_CONTEXTS(netif), src, dst);
    if (err != ERR_OK) {
      MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
      pbuf_free(p_frag);
      return err;
    }
    pbuf_remove_header(p, hidden_header_len);
  
  #else /* LWIP_6LOWPAN_IPHC */
    /* Send uncompressed IPv6 header with appropriate dispatch byte. */
    lowpan6_header_len = 1;
    hidden_header_len = 0;
    buffer[ieee_header_len] = 0x41; /* IPv6 dispatch */
  #endif /* LWIP_6LOWPAN_IPHC */
  
    /* Calculate remaining packet length */
    remaining_len = p->tot_len;
  
    if (remaining_len > 0x7FF) {
      MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
      /* datagram_size must fit into 11 bit */
      pbuf_free(p_frag);
      return ERR_VAL;
    }
  
    /* Fragment, or 1 packet? */
    max_data_len = LOWPAN6_MAX_PAYLOAD - ieee_header_len - lowpan6_header_len;
    if (remaining_len > max_data_len) {
      u16_t data_len;
      /* We must move the 6LowPAN header to make room for the FRAG header. */
      memmove(&buffer[ieee_header_len + 4], &buffer[ieee_header_len], lowpan6_header_len);
  
      /* Now we need to fragment the packet. FRAG1 header first */
      buffer[ieee_header_len] = 0xc0 | (((p->tot_len + hidden_header_len) >> 8) & 0x7);
      buffer[ieee_header_len + 1] = (p->tot_len + hidden_header_len) & 0xff;
  
      lowpan6_data.tx_datagram_tag++;
      buffer[ieee_header_len + 2] = (lowpan6_data.tx_datagram_tag >> 8) & 0xff;
      buffer[ieee_header_len + 3] = lowpan6_data.tx_datagram_tag & 0xff;
  
      /* Fragment follows. */
      data_len = (max_data_len - 4) & 0xf8;
      frag_len = data_len + lowpan6_header_len;
  
      pbuf_copy_partial(p, buffer + ieee_header_len + lowpan6_header_len + 4, frag_len - lowpan6_header_len, 0);
      remaining_len -= frag_len - lowpan6_header_len;
      /* datagram offset holds the offset before compression */
      datagram_offset = frag_len - lowpan6_header_len + hidden_header_len;
      LWIP_ASSERT("datagram offset must be a multiple of 8", (datagram_offset & 7) == 0);
  
      /* Calculate frame length */
      p_frag->len = p_frag->tot_len = ieee_header_len + 4 + frag_len + 2; /* add 2 bytes for crc*/
  
      /* 2 bytes CRC */
      crc = LWIP_6LOWPAN_DO_CALC_CRC(p_frag->payload, p_frag->len - 2);
      pbuf_take_at(p_frag, &crc, 2, p_frag->len - 2);
  
      /* send the packet */
      MIB2_STATS_NETIF_ADD(netif, ifoutoctets, p_frag->tot_len);
      LWIP_DEBUGF(LWIP_LOWPAN6_DEBUG | LWIP_DBG_TRACE, ("lowpan6_send: sending packet %p\n", (void *)p));
      err = netif->linkoutput(netif, p_frag);
  
      while ((remaining_len > 0) && (err == ERR_OK)) {
        struct ieee_802154_hdr *hdr = (struct ieee_802154_hdr *)buffer;
        /* new frame, new seq num for ACK */
        hdr->sequence_number = lowpan6_data.tx_frame_seq_num++;
  
        buffer[ieee_header_len] |= 0x20; /* Change FRAG1 to FRAGN */
  
        LWIP_ASSERT("datagram offset must be a multiple of 8", (datagram_offset & 7) == 0);
        buffer[ieee_header_len + 4] = (u8_t)(datagram_offset >> 3); /* datagram offset in FRAGN header (datagram_offset is max. 11 bit) */
  
        frag_len = (127 - ieee_header_len - 5 - 2) & 0xf8;
        if (frag_len > remaining_len) {
          frag_len = remaining_len;
        }
  
        pbuf_copy_partial(p, buffer + ieee_header_len + 5, frag_len, p->tot_len - remaining_len);
        remaining_len -= frag_len;
        datagram_offset += frag_len;
  
        /* Calculate frame length */
        p_frag->len = p_frag->tot_len = frag_len + 5 + ieee_header_len + 2;
  
        /* 2 bytes CRC */
        crc = LWIP_6LOWPAN_DO_CALC_CRC(p_frag->payload, p_frag->len - 2);
        pbuf_take_at(p_frag, &crc, 2, p_frag->len - 2);
  
        /* send the packet */
        MIB2_STATS_NETIF_ADD(netif, ifoutoctets, p_frag->tot_len);
        LWIP_DEBUGF(LWIP_LOWPAN6_DEBUG | LWIP_DBG_TRACE, ("lowpan6_send: sending packet %p\n", (void *)p));
        err = netif->linkoutput(netif, p_frag);
      }
    } else {
      /* It fits in one frame. */
      frag_len = remaining_len;
  
      /* Copy IPv6 packet */
      pbuf_copy_partial(p, buffer + ieee_header_len + lowpan6_header_len, frag_len, 0);
      remaining_len = 0;
  
      /* Calculate frame length */
      p_frag->len = p_frag->tot_len = frag_len + lowpan6_header_len + ieee_header_len + 2;
      LWIP_ASSERT("", p_frag->len <= 127);
  
      /* 2 bytes CRC */
      crc = LWIP_6LOWPAN_DO_CALC_CRC(p_frag->payload, p_frag->len - 2);
      pbuf_take_at(p_frag, &crc, 2, p_frag->len - 2);
  
      /* send the packet */
      MIB2_STATS_NETIF_ADD(netif, ifoutoctets, p_frag->tot_len);
      LWIP_DEBUGF(LWIP_LOWPAN6_DEBUG | LWIP_DBG_TRACE, ("lowpan6_send: sending packet %p\n", (void *)p));
      err = netif->linkoutput(netif, p_frag);
    }
  
    pbuf_free(p_frag);
  
    return err;
  }
  
  /**
   * @ingroup sixlowpan
   * Set context
   */
  err_t
  lowpan6_set_context(u8_t idx, const ip6_addr_t *context)
  {
  #if LWIP_6LOWPAN_NUM_CONTEXTS > 0
    if (idx >= LWIP_6LOWPAN_NUM_CONTEXTS) {
      return ERR_ARG;
    }
  
    IP6_ADDR_ZONECHECK(context);
  
    ip6_addr_set(&lowpan6_data.lowpan6_context[idx], context);
  
    return ERR_OK;
  #else
    LWIP_UNUSED_ARG(idx);
    LWIP_UNUSED_ARG(context);
    return ERR_ARG;
  #endif
  }
  
  #if LWIP_6LOWPAN_INFER_SHORT_ADDRESS
  /**
   * @ingroup sixlowpan
   * Set short address
   */
  err_t
  lowpan6_set_short_addr(u8_t addr_high, u8_t addr_low)
  {
    short_mac_addr.addr[0] = addr_high;
    short_mac_addr.addr[1] = addr_low;
  
    return ERR_OK;
  }
  #endif /* LWIP_6LOWPAN_INFER_SHORT_ADDRESS */
  
  /* Create IEEE 802.15.4 address from netif address */
  static err_t
  lowpan6_hwaddr_to_addr(struct netif *netif, struct lowpan6_link_addr *addr)
  {
    addr->addr_len = 8;
    if (netif->hwaddr_len == 8) {
      LWIP_ERROR("NETIF_MAX_HWADDR_LEN >= 8 required", sizeof(netif->hwaddr) >= 8, return ERR_VAL;);
      SMEMCPY(addr->addr, netif->hwaddr, 8);
    } else if (netif->hwaddr_len == 6) {
      /* Copy from MAC-48 */
      SMEMCPY(addr->addr, netif->hwaddr, 3);
      addr->addr[3] = addr->addr[4] = 0xff;
      SMEMCPY(&addr->addr[5], &netif->hwaddr[3], 3);
    } else {
      /* Invalid address length, don't know how to convert this */
      return ERR_VAL;
    }
    return ERR_OK;
  }
  
  /**
   * @ingroup sixlowpan
   * Resolve and fill-in IEEE 802.15.4 address header for outgoing IPv6 packet.
   *
   * Perform Header Compression and fragment if necessary.
   *
   * @param netif The lwIP network interface which the IP packet will be sent on.
   * @param q The pbuf(s) containing the IP packet to be sent.
   * @param ip6addr The IP address of the packet destination.
   *
   * @return err_t
   */
  err_t
  lowpan6_output(struct netif *netif, struct pbuf *q, const ip6_addr_t *ip6addr)
  {
    err_t result;
    const u8_t *hwaddr;
    struct lowpan6_link_addr src, dest;
  #if LWIP_6LOWPAN_INFER_SHORT_ADDRESS
    ip6_addr_t ip6_src;
    struct ip6_hdr *ip6_hdr;
  #endif /* LWIP_6LOWPAN_INFER_SHORT_ADDRESS */
  
  #if LWIP_6LOWPAN_INFER_SHORT_ADDRESS
    /* Check if we can compress source address (use aligned copy) */
    ip6_hdr = (struct ip6_hdr *)q->payload;
    ip6_addr_copy_from_packed(ip6_src, ip6_hdr->src);
    ip6_addr_assign_zone(&ip6_src, IP6_UNICAST, netif);
    if (lowpan6_get_address_mode(&ip6_src, &short_mac_addr) == 3) {
      src.addr_len = 2;
      src.addr[0] = short_mac_addr.addr[0];
      src.addr[1] = short_mac_addr.addr[1];
    } else
  #endif /* LWIP_6LOWPAN_INFER_SHORT_ADDRESS */
    {
      result = lowpan6_hwaddr_to_addr(netif, &src);
      if (result != ERR_OK) {
        MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
        return result;
      }
    }
  
    /* multicast destination IP address? */
    if (ip6_addr_ismulticast(ip6addr)) {
      MIB2_STATS_NETIF_INC(netif, ifoutnucastpkts);
      /* We need to send to the broadcast address.*/
      return lowpan6_frag(netif, q, &src, &ieee_802154_broadcast);
    }
  
    /* We have a unicast destination IP address */
    /* @todo anycast? */
  
  #if LWIP_6LOWPAN_INFER_SHORT_ADDRESS
    if (src.addr_len == 2) {
      /* If source address was compressible to short_mac_addr, and dest has same subnet and
       * is also compressible to 2-bytes, assume we can infer dest as a short address too. */
      dest.addr_len = 2;
      dest.addr[0] = ((u8_t *)q->payload)[38];
      dest.addr[1] = ((u8_t *)q->payload)[39];
      if ((src.addr_len == 2) && (ip6_addr_netcmp_zoneless(&ip6_hdr->src, &ip6_hdr->dest)) &&
          (lowpan6_get_address_mode(ip6addr, &dest) == 3)) {
        MIB2_STATS_NETIF_INC(netif, ifoutucastpkts);
        return lowpan6_frag(netif, q, &src, &dest);
      }
    }
  #endif /* LWIP_6LOWPAN_INFER_SHORT_ADDRESS */
  
    /* Ask ND6 what to do with the packet. */
    result = nd6_get_next_hop_addr_or_queue(netif, q, ip6addr, &hwaddr);
    if (result != ERR_OK) {
      MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
      return result;
    }
  
    /* If no hardware address is returned, nd6 has queued the packet for later. */
    if (hwaddr == NULL) {
      return ERR_OK;
    }
  
    /* Send out the packet using the returned hardware address. */
    dest.addr_len = netif->hwaddr_len;
    /* XXX: Inferring the length of the source address from the destination address
     * is not correct for IEEE 802.15.4, but currently we don't get this information
     * from the neighbor cache */
    SMEMCPY(dest.addr, hwaddr, netif->hwaddr_len);
    MIB2_STATS_NETIF_INC(netif, ifoutucastpkts);
    return lowpan6_frag(netif, q, &src, &dest);
  }
  /**
   * @ingroup sixlowpan
   * NETIF input function: don't free the input pbuf when returning != ERR_OK!
   */
  err_t
  lowpan6_input(struct pbuf *p, struct netif *netif)
  {
    u8_t *puc, b;
    s8_t i;
    struct lowpan6_link_addr src, dest;
    u16_t datagram_size = 0;
    u16_t datagram_offset, datagram_tag;
    struct lowpan6_reass_helper *lrh, *lrh_next, *lrh_prev = NULL;
  
    if (p == NULL) {
      return ERR_OK;
    }
  
    MIB2_STATS_NETIF_ADD(netif, ifinoctets, p->tot_len);
  
    if (p->len != p->tot_len) {
      /* for now, this needs a pbuf in one piece */
      goto lowpan6_input_discard;
    }
  
    if (lowpan6_parse_iee802154_header(p, &src, &dest) != ERR_OK) {
      goto lowpan6_input_discard;
    }
  
    /* Check dispatch. */
    puc = (u8_t *)p->payload;
  
    b = *puc;
    if ((b & 0xf8) == 0xc0) {
      /* FRAG1 dispatch. add this packet to reassembly list. */
      datagram_size = ((u16_t)(puc[0] & 0x07) << 8) | (u16_t)puc[1];
      datagram_tag = ((u16_t)puc[2] << 8) | (u16_t)puc[3];
  
      /* check for duplicate */
      lrh = lowpan6_data.reass_list;
      while (lrh != NULL) {
        u8_t discard = 0;
        lrh_next = lrh->next_packet;
        if ((lrh->sender_addr.addr_len == src.addr_len) &&
            (memcmp(lrh->sender_addr.addr, src.addr, src.addr_len) == 0)) {
          /* address match with packet in reassembly. */
          if ((datagram_tag == lrh->datagram_tag) && (datagram_size == lrh->datagram_size)) {
            /* duplicate fragment. */
            goto lowpan6_input_discard;
          } else {
            /* We are receiving the start of a new datagram. Discard old one (incomplete). */
            discard = 1;
          }
        }
        if (discard) {
          dequeue_datagram(lrh, lrh_prev);
          free_reass_datagram(lrh);
        } else {
          lrh_prev = lrh;
        }
        /* Check next datagram in queue. */
        lrh = lrh_next;
      }
  
      pbuf_remove_header(p, 4); /* hide frag1 dispatch */
  
      lrh = (struct lowpan6_reass_helper *) mem_malloc(sizeof(struct lowpan6_reass_helper));
      if (lrh == NULL) {
        goto lowpan6_input_discard;
      }
  
      lrh->sender_addr.addr_len = src.addr_len;
      for (i = 0; i < src.addr_len; i++) {
        lrh->sender_addr.addr[i] = src.addr[i];
      }
      lrh->datagram_size = datagram_size;
      lrh->datagram_tag = datagram_tag;
      lrh->frags = NULL;
      if (*(u8_t *)p->payload == 0x41) {
        /* This is a complete IPv6 packet, just skip dispatch byte. */
        pbuf_remove_header(p, 1); /* hide dispatch byte. */
        lrh->reass = p;
      } else if ((*(u8_t *)p->payload & 0xe0 ) == 0x60) {
        lrh->reass = lowpan6_decompress(p, datagram_size, LWIP_6LOWPAN_CONTEXTS(netif), &src, &dest);
        if (lrh->reass == NULL) {
          /* decompression failed */
          mem_free(lrh);
          goto lowpan6_input_discard;
        }
      }
      /* TODO: handle the case where we already have FRAGN received */
      lrh->next_packet = lowpan6_data.reass_list;
      lrh->timer = 2;
      lowpan6_data.reass_list = lrh;
  
      return ERR_OK;
    } else if ((b & 0xf8) == 0xe0) {
      /* FRAGN dispatch, find packet being reassembled. */
      datagram_size = ((u16_t)(puc[0] & 0x07) << 8) | (u16_t)puc[1];
      datagram_tag = ((u16_t)puc[2] << 8) | (u16_t)puc[3];
      datagram_offset = (u16_t)puc[4] << 3;
      pbuf_remove_header(p, 4); /* hide frag1 dispatch but keep datagram offset for reassembly */
  
      for (lrh = lowpan6_data.reass_list; lrh != NULL; lrh_prev = lrh, lrh = lrh->next_packet) {
        if ((lrh->sender_addr.addr_len == src.addr_len) &&
            (memcmp(lrh->sender_addr.addr, src.addr, src.addr_len) == 0) &&
            (datagram_tag == lrh->datagram_tag) &&
            (datagram_size == lrh->datagram_size)) {
          break;
        }
      }
      if (lrh == NULL) {
        /* rogue fragment */
        goto lowpan6_input_discard;
      }
      /* Insert new pbuf into list of fragments. Each fragment is a pbuf,
         this only works for unchained pbufs. */
      LWIP_ASSERT("p->next == NULL", p->next == NULL);
      if (lrh->reass != NULL) {
        /* FRAG1 already received, check this offset against first len */
        if (datagram_offset < lrh->reass->len) {
          /* fragment overlap, discard old fragments */
          dequeue_datagram(lrh, lrh_prev);
          free_reass_datagram(lrh);
          goto lowpan6_input_discard;
        }
      }
      if (lrh->frags == NULL) {
        /* first FRAGN */
        lrh->frags = p;
      } else {
        /* find the correct place to insert */
        struct pbuf *q, *last;
        u16_t new_frag_len = p->len - 1; /* p->len includes datagram_offset byte */
        for (q = lrh->frags, last = NULL; q != NULL; last = q, q = q->next) {
          u16_t q_datagram_offset = ((u8_t *)q->payload)[0] << 3;
          u16_t q_frag_len = q->len - 1;
          if (datagram_offset < q_datagram_offset) {
            if (datagram_offset + new_frag_len > q_datagram_offset) {
              /* overlap, discard old fragments */
              dequeue_datagram(lrh, lrh_prev);
              free_reass_datagram(lrh);
              goto lowpan6_input_discard;
            }
            /* insert here */
            break;
          } else if (datagram_offset == q_datagram_offset) {
            if (q_frag_len != new_frag_len) {
              /* fragment mismatch, discard old fragments */
              dequeue_datagram(lrh, lrh_prev);
              free_reass_datagram(lrh);
              goto lowpan6_input_discard;
            }
            /* duplicate, ignore */
            pbuf_free(p);
            return ERR_OK;
          }
        }
        /* insert fragment */
        if (last == NULL) {
          lrh->frags = p;
        } else {
          last->next = p;
          p->next = q;
        }
      }
      /* check if all fragments were received */
      if (lrh->reass) {
        u16_t offset = lrh->reass->len;
        struct pbuf *q;
        for (q = lrh->frags; q != NULL; q = q->next) {
          u16_t q_datagram_offset = ((u8_t *)q->payload)[0] << 3;
          if (q_datagram_offset != offset) {
            /* not complete, wait for more fragments */
            return ERR_OK;
          }
          offset += q->len - 1;
        }
        if (offset == datagram_size) {
          /* all fragments received, combine pbufs */
          u16_t datagram_left = datagram_size - lrh->reass->len;
          for (q = lrh->frags; q != NULL; q = q->next) {
            /* hide datagram_offset byte now */
            pbuf_remove_header(q, 1);
            q->tot_len = datagram_left;
            datagram_left -= q->len;
          }
          LWIP_ASSERT("datagram_left == 0", datagram_left == 0);
          q = lrh->reass;
          q->tot_len = datagram_size;
          q->next = lrh->frags;
          lrh->frags = NULL;
          lrh->reass = NULL;
          dequeue_datagram(lrh, lrh_prev);
          mem_free(lrh);
  
          /* @todo: distinguish unicast/multicast */
          MIB2_STATS_NETIF_INC(netif, ifinucastpkts);
          return ip6_input(q, netif);
        }
      }
      /* pbuf enqueued, waiting for more fragments */
      return ERR_OK;
    } else {
      if (b == 0x41) {
        /* This is a complete IPv6 packet, just skip dispatch byte. */
        pbuf_remove_header(p, 1); /* hide dispatch byte. */
      } else if ((b & 0xe0 ) == 0x60) {
        /* IPv6 headers are compressed using IPHC. */
        p = lowpan6_decompress(p, datagram_size, LWIP_6LOWPAN_CONTEXTS(netif), &src, &dest);
        if (p == NULL) {
          MIB2_STATS_NETIF_INC(netif, ifindiscards);
          return ERR_OK;
        }
      } else {
        goto lowpan6_input_discard;
      }
  
      /* @todo: distinguish unicast/multicast */
      MIB2_STATS_NETIF_INC(netif, ifinucastpkts);
  
      return ip6_input(p, netif);
    }
  lowpan6_input_discard:
    MIB2_STATS_NETIF_INC(netif, ifindiscards);
    pbuf_free(p);
    /* always return ERR_OK here to prevent the caller freeing the pbuf */
    return ERR_OK;
  }
  
  /**
   * @ingroup sixlowpan
   */
  err_t
  lowpan6_if_init(struct netif *netif)
  {
    netif->name[0] = 'L';
    netif->name[1] = '6';
    netif->output_ip6 = lowpan6_output;
  
    MIB2_INIT_NETIF(netif, snmp_ifType_other, 0);
  
    /* maximum transfer unit */
    netif->mtu = IP6_MIN_MTU_LENGTH;
  
    /* broadcast capability */
    netif->flags = NETIF_FLAG_BROADCAST /* | NETIF_FLAG_LOWPAN6 */;
  
    return ERR_OK;
  }
  
  /**
   * @ingroup sixlowpan
   * Set PAN ID
   */
  err_t
  lowpan6_set_pan_id(u16_t pan_id)
  {
    lowpan6_data.ieee_802154_pan_id = pan_id;
  
    return ERR_OK;
  }
  
  #if !NO_SYS
  /**
   * @ingroup sixlowpan
   * Pass a received packet to tcpip_thread for input processing
   *
   * @param p the received packet, p->payload pointing to the
   *          IEEE 802.15.4 header.
   * @param inp the network interface on which the packet was received
   */
  err_t
  tcpip_6lowpan_input(struct pbuf *p, struct netif *inp)
  {
    return tcpip_inpkt(p, inp, lowpan6_input);
  }
  #endif /* !NO_SYS */
  
  #endif /* LWIP_IPV6 */
  