text: /**
   * @file
   *
   * IPv6 fragmentation and reassembly.
   */
  
  /*
   * Copyright (c) 2010 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>
   */
  
  #include "lwip/opt.h"
  #include "lwip/ip6_frag.h"
  #include "lwip/ip6.h"
  #include "lwip/icmp6.h"
  #include "lwip/nd6.h"
  #include "lwip/ip.h"
  
  #include "lwip/pbuf.h"
  #include "lwip/memp.h"
  #include "lwip/stats.h"
  
  #include <string.h>
  
  #if LWIP_IPV6 && LWIP_IPV6_REASS  /* don't build if not configured for use in lwipopts.h */
  
  
  /** Setting this to 0, you can turn off checking the fragments for overlapping
   * regions. The code gets a little smaller. Only use this if you know that
   * overlapping won't occur on your network! */
  #ifndef IP_REASS_CHECK_OVERLAP
  #define IP_REASS_CHECK_OVERLAP 1
  #endif /* IP_REASS_CHECK_OVERLAP */
  
  /** Set to 0 to prevent freeing the oldest datagram when the reassembly buffer is
   * full (IP_REASS_MAX_PBUFS pbufs are enqueued). The code gets a little smaller.
   * Datagrams will be freed by timeout only. Especially useful when MEMP_NUM_REASSDATA
   * is set to 1, so one datagram can be reassembled at a time, only. */
  #ifndef IP_REASS_FREE_OLDEST
  #define IP_REASS_FREE_OLDEST 1
  #endif /* IP_REASS_FREE_OLDEST */
  
  #if IPV6_FRAG_COPYHEADER
  /* The number of bytes we need to "borrow" from (i.e., overwrite in) the header
   * that precedes the fragment header for reassembly pruposes. */
  #define IPV6_FRAG_REQROOM ((s16_t)(sizeof(struct ip6_reass_helper) - IP6_FRAG_HLEN))
  #endif
  
  #define IP_REASS_FLAG_LASTFRAG 0x01
  
  /** This is a helper struct which holds the starting
   * offset and the ending offset of this fragment to
   * easily chain the fragments.
   * It has the same packing requirements as the IPv6 header, since it replaces
   * the Fragment Header in memory in incoming fragments to keep
   * track of the various fragments.
   */
  #ifdef PACK_STRUCT_USE_INCLUDES
  #  include "arch/bpstruct.h"
  #endif
  PACK_STRUCT_BEGIN
  struct ip6_reass_helper {
    PACK_STRUCT_FIELD(struct pbuf *next_pbuf);
    PACK_STRUCT_FIELD(u16_t start);
    PACK_STRUCT_FIELD(u16_t end);
  } PACK_STRUCT_STRUCT;
  PACK_STRUCT_END
  #ifdef PACK_STRUCT_USE_INCLUDES
  #  include "arch/epstruct.h"
  #endif
  
  /* static variables */
  static struct ip6_reassdata *reassdatagrams;
  static u16_t ip6_reass_pbufcount;
  
  /* Forward declarations. */
  static void ip6_reass_free_complete_datagram(struct ip6_reassdata *ipr);
  #if IP_REASS_FREE_OLDEST
  static void ip6_reass_remove_oldest_datagram(struct ip6_reassdata *ipr, int pbufs_needed);
  #endif /* IP_REASS_FREE_OLDEST */
  
  void
  ip6_reass_tmr(void)
  {
    struct ip6_reassdata *r, *tmp;
  
  #if !IPV6_FRAG_COPYHEADER
    LWIP_ASSERT("sizeof(struct ip6_reass_helper) <= IP6_FRAG_HLEN, set IPV6_FRAG_COPYHEADER to 1",
      sizeof(struct ip6_reass_helper) <= IP6_FRAG_HLEN);
  #endif /* !IPV6_FRAG_COPYHEADER */
  
    r = reassdatagrams;
    while (r != NULL) {
      /* Decrement the timer. Once it reaches 0,
       * clean up the incomplete fragment assembly */
      if (r->timer > 0) {
        r->timer--;
        r = r->next;
      } else {
        /* reassembly timed out */
        tmp = r;
        /* get the next pointer before freeing */
        r = r->next;
        /* free the helper struct and all enqueued pbufs */
        ip6_reass_free_complete_datagram(tmp);
       }
     }
  }
  
  /**
   * Free a datagram (struct ip6_reassdata) and all its pbufs.
   * Updates the total count of enqueued pbufs (ip6_reass_pbufcount),
   * sends an ICMP time exceeded packet.
   *
   * @param ipr datagram to free
   */
  static void
  ip6_reass_free_complete_datagram(struct ip6_reassdata *ipr)
  {
    struct ip6_reassdata *prev;
    u16_t pbufs_freed = 0;
    u16_t clen;
    struct pbuf *p;
    struct ip6_reass_helper *iprh;
  
  #if LWIP_ICMP6
    iprh = (struct ip6_reass_helper *)ipr->p->payload;
    if (iprh->start == 0) {
      /* The first fragment was received, send ICMP time exceeded. */
      /* First, de-queue the first pbuf from r->p. */
      p = ipr->p;
      ipr->p = iprh->next_pbuf;
      /* Restore the part that we've overwritten with our helper structure, or we
       * might send garbage (and disclose a pointer) in the ICMPv6 reply. */
      MEMCPY(p->payload, ipr->orig_hdr, sizeof(iprh));
      /* Then, move back to the original ipv6 header (we are now pointing to Fragment header).
         This cannot fail since we already checked when receiving this fragment. */
      if (pbuf_header_force(p, (s16_t)((u8_t*)p->payload - (u8_t*)ipr->iphdr))) {
        LWIP_ASSERT("ip6_reass_free: moving p->payload to ip6 header failed\n", 0);
      }
      else {
        /* Reconstruct the zoned source and destination addresses, so that we do
         * not end up sending the ICMP response over the wrong link. */
        ip6_addr_t src_addr, dest_addr;
        ip6_addr_copy_from_packed(src_addr, IPV6_FRAG_SRC(ipr));
        ip6_addr_set_zone(&src_addr, ipr->src_zone);
        ip6_addr_copy_from_packed(dest_addr, IPV6_FRAG_DEST(ipr));
        ip6_addr_set_zone(&dest_addr, ipr->dest_zone);
        /* Send the actual ICMP response. */
        icmp6_time_exceeded_with_addrs(p, ICMP6_TE_FRAG, &src_addr, &dest_addr);
      }
      clen = pbuf_clen(p);
      LWIP_ASSERT("pbufs_freed + clen <= 0xffff", pbufs_freed + clen <= 0xffff);
      pbufs_freed = (u16_t)(pbufs_freed + clen);
      pbuf_free(p);
    }
  #endif /* LWIP_ICMP6 */
  
    /* First, free all received pbufs.  The individual pbufs need to be released
       separately as they have not yet been chained */
    p = ipr->p;
    while (p != NULL) {
      struct pbuf *pcur;
      iprh = (struct ip6_reass_helper *)p->payload;
      pcur = p;
      /* get the next pointer before freeing */
      p = iprh->next_pbuf;
      clen = pbuf_clen(pcur);
      LWIP_ASSERT("pbufs_freed + clen <= 0xffff", pbufs_freed + clen <= 0xffff);
      pbufs_freed = (u16_t)(pbufs_freed + clen);
      pbuf_free(pcur);
    }
  
    /* Then, unchain the struct ip6_reassdata from the list and free it. */
    if (ipr == reassdatagrams) {
      reassdatagrams = ipr->next;
    } else {
      prev = reassdatagrams;
      while (prev != NULL) {
        if (prev->next == ipr) {
          break;
        }
        prev = prev->next;
      }
      if (prev != NULL) {
        prev->next = ipr->next;
      }
    }
    memp_free(MEMP_IP6_REASSDATA, ipr);
  
    /* Finally, update number of pbufs in reassembly queue */
    LWIP_ASSERT("ip_reass_pbufcount >= clen", ip6_reass_pbufcount >= pbufs_freed);
    ip6_reass_pbufcount = (u16_t)(ip6_reass_pbufcount - pbufs_freed);
  }
  
  #if IP_REASS_FREE_OLDEST
  /**
   * Free the oldest datagram to make room for enqueueing new fragments.
   * The datagram ipr is not freed!
   *
   * @param ipr ip6_reassdata for the current fragment
   * @param pbufs_needed number of pbufs needed to enqueue
   *        (used for freeing other datagrams if not enough space)
   */
  static void
  ip6_reass_remove_oldest_datagram(struct ip6_reassdata *ipr, int pbufs_needed)
  {
    struct ip6_reassdata *r, *oldest;
  
    /* Free datagrams until being allowed to enqueue 'pbufs_needed' pbufs,
     * but don't free the current datagram! */
    do {
      r = oldest = reassdatagrams;
      while (r != NULL) {
        if (r != ipr) {
          if (r->timer <= oldest->timer) {
            /* older than the previous oldest */
            oldest = r;
          }
        }
        r = r->next;
      }
      if (oldest == ipr) {
        /* nothing to free, ipr is the only element on the list */
        return;
      }
      if (oldest != NULL) {
        ip6_reass_free_complete_datagram(oldest);
      }
    } while (((ip6_reass_pbufcount + pbufs_needed) > IP_REASS_MAX_PBUFS) && (reassdatagrams != NULL));
  }
  #endif /* IP_REASS_FREE_OLDEST */
  
  /**
   * Reassembles incoming IPv6 fragments into an IPv6 datagram.
   *
   * @param p points to the IPv6 Fragment Header
   * @return NULL if reassembly is incomplete, pbuf pointing to
   *         IPv6 Header if reassembly is complete
   */
  struct pbuf *
  ip6_reass(struct pbuf *p)
  {
    struct ip6_reassdata *ipr, *ipr_prev;
    struct ip6_reass_helper *iprh, *iprh_tmp, *iprh_prev=NULL;
    struct ip6_frag_hdr *frag_hdr;
    u16_t offset, len, start, end;
    ptrdiff_t hdrdiff;
    u16_t clen;
    u8_t valid = 1;
    struct pbuf *q, *next_pbuf;
  
    IP6_FRAG_STATS_INC(ip6_frag.recv);
  
    /* ip6_frag_hdr must be in the first pbuf, not chained. Checked by caller. */
    LWIP_ASSERT("IPv6 fragment header does not fit in first pbuf",
      p->len >= sizeof(struct ip6_frag_hdr));
  
    frag_hdr = (struct ip6_frag_hdr *) p->payload;
  
    clen = pbuf_clen(p);
  
    offset = lwip_ntohs(frag_hdr->_fragment_offset);
  
    /* Calculate fragment length from IPv6 payload length.
     * Adjust for headers before Fragment Header.
     * And finally adjust by Fragment Header length. */
    len = lwip_ntohs(ip6_current_header()->_plen);
    hdrdiff = (u8_t*)p->payload - (const u8_t*)ip6_current_header();
    LWIP_ASSERT("not a valid pbuf (ip6_input check missing?)", hdrdiff <= 0xFFFF);
    LWIP_ASSERT("not a valid pbuf (ip6_input check missing?)", hdrdiff >= IP6_HLEN);
    hdrdiff -= IP6_HLEN;
    hdrdiff += IP6_FRAG_HLEN;
    if (hdrdiff > len) {
      IP6_FRAG_STATS_INC(ip6_frag.proterr);
      goto nullreturn;
    }
    len = (u16_t)(len - hdrdiff);
    start = (offset & IP6_FRAG_OFFSET_MASK);
    if (start > (0xFFFF - len)) {
      /* u16_t overflow, cannot handle this */
      IP6_FRAG_STATS_INC(ip6_frag.proterr);
      goto nullreturn;
    }
  
    /* Look for the datagram the fragment belongs to in the current datagram queue,
     * remembering the previous in the queue for later dequeueing. */
    for (ipr = reassdatagrams, ipr_prev = NULL; ipr != NULL; ipr = ipr->next) {
      /* Check if the incoming fragment matches the one currently present
         in the reassembly buffer. If so, we proceed with copying the
         fragment into the buffer. */
      if ((frag_hdr->_identification == ipr->identification) &&
          ip6_addr_cmp_packed(ip6_current_src_addr(), &(IPV6_FRAG_SRC(ipr)), ipr->src_zone) &&
          ip6_addr_cmp_packed(ip6_current_dest_addr(), &(IPV6_FRAG_DEST(ipr)), ipr->dest_zone)) {
        IP6_FRAG_STATS_INC(ip6_frag.cachehit);
        break;
      }
      ipr_prev = ipr;
    }
  
    if (ipr == NULL) {
    /* Enqueue a new datagram into the datagram queue */
      ipr = (struct ip6_reassdata *)memp_malloc(MEMP_IP6_REASSDATA);
      if (ipr == NULL) {
  #if IP_REASS_FREE_OLDEST
        /* Make room and try again. */
        ip6_reass_remove_oldest_datagram(ipr, clen);
        ipr = (struct ip6_reassdata *)memp_malloc(MEMP_IP6_REASSDATA);
        if (ipr != NULL) {
          /* re-search ipr_prev since it might have been removed */
          for (ipr_prev = reassdatagrams; ipr_prev != NULL; ipr_prev = ipr_prev->next) {
            if (ipr_prev->next == ipr) {
              break;
            }
          }
        } else
  #endif /* IP_REASS_FREE_OLDEST */
        {
          IP6_FRAG_STATS_INC(ip6_frag.memerr);
          goto nullreturn;
        }
      }
  
      memset(ipr, 0, sizeof(struct ip6_reassdata));
      ipr->timer = IPV6_REASS_MAXAGE;
  
      /* enqueue the new structure to the front of the list */
      ipr->next = reassdatagrams;
      reassdatagrams = ipr;
  
      /* Use the current IPv6 header for src/dest address reference.
       * Eventually, we will replace it when we get the first fragment
       * (it might be this one, in any case, it is done later). */
      /* need to use the none-const pointer here: */
      ipr->iphdr = ip_data.current_ip6_header;
  #if IPV6_FRAG_COPYHEADER
      MEMCPY(&ipr->src, &ip6_current_header()->src, sizeof(ipr->src));
      MEMCPY(&ipr->dest, &ip6_current_header()->dest, sizeof(ipr->dest));
  #endif /* IPV6_FRAG_COPYHEADER */
  #if LWIP_IPV6_SCOPES
      /* Also store the address zone information.
       * @todo It is possible that due to netif destruction and recreation, the
       * stored zones end up resolving to a different interface. In that case, we
       * risk sending a "time exceeded" ICMP response over the wrong link.
       * Ideally, netif destruction would clean up matching pending reassembly
       * structures, but custom zone mappings would make that non-trivial. */
      ipr->src_zone = ip6_addr_zone(ip6_current_src_addr());
      ipr->dest_zone = ip6_addr_zone(ip6_current_dest_addr());
  #endif /* LWIP_IPV6_SCOPES */
      /* copy the fragmented packet id. */
      ipr->identification = frag_hdr->_identification;
  
      /* copy the nexth field */
      ipr->nexth = frag_hdr->_nexth;
    }
  
    /* Check if we are allowed to enqueue more datagrams. */
    if ((ip6_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS) {
  #if IP_REASS_FREE_OLDEST
      ip6_reass_remove_oldest_datagram(ipr, clen);
      if ((ip6_reass_pbufcount + clen) <= IP_REASS_MAX_PBUFS) {
        /* re-search ipr_prev since it might have been removed */
        for (ipr_prev = reassdatagrams; ipr_prev != NULL; ipr_prev = ipr_prev->next) {
          if (ipr_prev->next == ipr) {
            break;
          }
        }
      } else
  #endif /* IP_REASS_FREE_OLDEST */
      {
        /* @todo: send ICMPv6 time exceeded here? */
        /* drop this pbuf */
        IP6_FRAG_STATS_INC(ip6_frag.memerr);
        goto nullreturn;
      }
    }
  
    /* Overwrite Fragment Header with our own helper struct. */
  #if IPV6_FRAG_COPYHEADER
    if (IPV6_FRAG_REQROOM > 0) {
      /* Make room for struct ip6_reass_helper (only required if sizeof(void*) > 4).
         This cannot fail since we already checked when receiving this fragment. */
      u8_t hdrerr = pbuf_header_force(p, IPV6_FRAG_REQROOM);
      LWIP_UNUSED_ARG(hdrerr); /* in case of LWIP_NOASSERT */
      LWIP_ASSERT("no room for struct ip6_reass_helper", hdrerr == 0);
    }
  #else /* IPV6_FRAG_COPYHEADER */
    LWIP_ASSERT("sizeof(struct ip6_reass_helper) <= IP6_FRAG_HLEN, set IPV6_FRAG_COPYHEADER to 1",
      sizeof(struct ip6_reass_helper) <= IP6_FRAG_HLEN);
  #endif /* IPV6_FRAG_COPYHEADER */
  
    /* Prepare the pointer to the helper structure, and its initial values.
     * Do not yet write to the structure itself, as we still have to make a
     * backup of the original data, and we should not do that until we know for
     * sure that we are going to add this packet to the list. */
    iprh = (struct ip6_reass_helper *)p->payload;
    next_pbuf = NULL;
    end = (u16_t)(start + len);
  
    /* find the right place to insert this pbuf */
    /* Iterate through until we either get to the end of the list (append),
     * or we find on with a larger offset (insert). */
    for (q = ipr->p; q != NULL;) {
      iprh_tmp = (struct ip6_reass_helper*)q->payload;
      if (start < iprh_tmp->start) {
  #if IP_REASS_CHECK_OVERLAP
        if (end > iprh_tmp->start) {
          /* fragment overlaps with following, throw away */
          IP6_FRAG_STATS_INC(ip6_frag.proterr);
          goto nullreturn;
        }
        if (iprh_prev != NULL) {
          if (start < iprh_prev->end) {
            /* fragment overlaps with previous, throw away */
            IP6_FRAG_STATS_INC(ip6_frag.proterr);
            goto nullreturn;
          }
        }
  #endif /* IP_REASS_CHECK_OVERLAP */
        /* the new pbuf should be inserted before this */
        next_pbuf = q;
        if (iprh_prev != NULL) {
          /* not the fragment with the lowest offset */
          iprh_prev->next_pbuf = p;
        } else {
          /* fragment with the lowest offset */
          ipr->p = p;
        }
        break;
      } else if (start == iprh_tmp->start) {
        /* received the same datagram twice: no need to keep the datagram */
        goto nullreturn;
  #if IP_REASS_CHECK_OVERLAP
      } else if (start < iprh_tmp->end) {
        /* overlap: no need to keep the new datagram */
        IP6_FRAG_STATS_INC(ip6_frag.proterr);
        goto nullreturn;
  #endif /* IP_REASS_CHECK_OVERLAP */
      } else {
        /* Check if the fragments received so far have no gaps. */
        if (iprh_prev != NULL) {
          if (iprh_prev->end != iprh_tmp->start) {
            /* There is a fragment missing between the current
             * and the previous fragment */
            valid = 0;
          }
        }
      }
      q = iprh_tmp->next_pbuf;
      iprh_prev = iprh_tmp;
    }
  
    /* If q is NULL, then we made it to the end of the list. Determine what to do now */
    if (q == NULL) {
      if (iprh_prev != NULL) {
        /* this is (for now), the fragment with the highest offset:
         * chain it to the last fragment */
  #if IP_REASS_CHECK_OVERLAP
        LWIP_ASSERT("check fragments don't overlap", iprh_prev->end <= start);
  #endif /* IP_REASS_CHECK_OVERLAP */
        iprh_prev->next_pbuf = p;
        if (iprh_prev->end != start) {
          valid = 0;
        }
      } else {
  #if IP_REASS_CHECK_OVERLAP
        LWIP_ASSERT("no previous fragment, this must be the first fragment!",
          ipr->p == NULL);
  #endif /* IP_REASS_CHECK_OVERLAP */
        /* this is the first fragment we ever received for this ip datagram */
        ipr->p = p;
      }
    }
  
    /* Track the current number of pbufs current 'in-flight', in order to limit
    the number of fragments that may be enqueued at any one time */
    ip6_reass_pbufcount = (u16_t)(ip6_reass_pbufcount + clen);
  
    /* Remember IPv6 header if this is the first fragment. */
    if (start == 0) {
      /* need to use the none-const pointer here: */
      ipr->iphdr = ip_data.current_ip6_header;
      /* Make a backup of the part of the packet data that we are about to
       * overwrite, so that we can restore the original later. */
      MEMCPY(ipr->orig_hdr, p->payload, sizeof(*iprh));
      /* For IPV6_FRAG_COPYHEADER there is no need to copy src/dst again, as they
       * will be the same as they were. With LWIP_IPV6_SCOPES, the same applies
       * to the source/destination zones. */
    }
    /* Only after the backup do we get to fill in the actual helper structure. */
    iprh->next_pbuf = next_pbuf;
    iprh->start = start;
    iprh->end = end;
  
    /* If this is the last fragment, calculate total packet length. */
    if ((offset & IP6_FRAG_MORE_FLAG) == 0) {
      ipr->datagram_len = iprh->end;
    }
  
    /* Additional validity tests: we have received first and last fragment. */
    iprh_tmp = (struct ip6_reass_helper*)ipr->p->payload;
    if (iprh_tmp->start != 0) {
      valid = 0;
    }
    if (ipr->datagram_len == 0) {
      valid = 0;
    }
  
    /* Final validity test: no gaps between current and last fragment. */
    iprh_prev = iprh;
    q = iprh->next_pbuf;
    while ((q != NULL) && valid) {
      iprh = (struct ip6_reass_helper*)q->payload;
      if (iprh_prev->end != iprh->start) {
        valid = 0;
        break;
      }
      iprh_prev = iprh;
      q = iprh->next_pbuf;
    }
  
    if (valid) {
      /* All fragments have been received */
      struct ip6_hdr* iphdr_ptr;
  
      /* chain together the pbufs contained within the ip6_reassdata list. */
      iprh = (struct ip6_reass_helper*) ipr->p->payload;
      while (iprh != NULL) {
        next_pbuf = iprh->next_pbuf;
        if (next_pbuf != NULL) {
          /* Save next helper struct (will be hidden in next step). */
          iprh_tmp = (struct ip6_reass_helper*)next_pbuf->payload;
  
          /* hide the fragment header for every succeeding fragment */
          pbuf_remove_header(next_pbuf, IP6_FRAG_HLEN);
  #if IPV6_FRAG_COPYHEADER
          if (IPV6_FRAG_REQROOM > 0) {
            /* hide the extra bytes borrowed from ip6_hdr for struct ip6_reass_helper */
            u8_t hdrerr = pbuf_remove_header(next_pbuf, IPV6_FRAG_REQROOM);
            LWIP_UNUSED_ARG(hdrerr); /* in case of LWIP_NOASSERT */
            LWIP_ASSERT("no room for struct ip6_reass_helper", hdrerr == 0);
          }
  #endif
          pbuf_cat(ipr->p, next_pbuf);
        }
        else {
          iprh_tmp = NULL;
        }
  
        iprh = iprh_tmp;
      }
  
      /* Get the first pbuf. */
      p = ipr->p;
  
  #if IPV6_FRAG_COPYHEADER
      if (IPV6_FRAG_REQROOM > 0) {
        u8_t hdrerr;
        /* Restore (only) the bytes that we overwrote beyond the fragment header.
         * Those bytes may belong to either the IPv6 header or an extension
         * header placed before the fragment header. */
        MEMCPY(p->payload, ipr->orig_hdr, IPV6_FRAG_REQROOM);
        /* get back room for struct ip6_reass_helper (only required if sizeof(void*) > 4) */
        hdrerr = pbuf_remove_header(p, IPV6_FRAG_REQROOM);
        LWIP_UNUSED_ARG(hdrerr); /* in case of LWIP_NOASSERT */
        LWIP_ASSERT("no room for struct ip6_reass_helper", hdrerr == 0);
      }
  #endif
  
      /* We need to get rid of the fragment header itself, which is somewhere in
       * the middle of the packet (but still in the first pbuf of the chain).
       * Getting rid of the header is required by RFC 2460 Sec. 4.5 and necessary
       * in order to be able to reassemble packets that are close to full size
       * (i.e., around 65535 bytes). We simply move up all the headers before the
       * fragment header, including the IPv6 header, and adjust the payload start
       * accordingly. This works because all these headers are in the first pbuf
       * of the chain, and because the caller adjusts all its pointers on
       * successful reassembly. */
      MEMMOVE((u8_t*)ipr->iphdr + sizeof(struct ip6_frag_hdr), ipr->iphdr,
        (size_t)((u8_t*)p->payload - (u8_t*)ipr->iphdr));
  
      /* This is where the IPv6 header is now. */
      iphdr_ptr = (struct ip6_hdr*)((u8_t*)ipr->iphdr +
        sizeof(struct ip6_frag_hdr));
  
      /* Adjust datagram length by adding header lengths. */
      ipr->datagram_len = (u16_t)(ipr->datagram_len + ((u8_t*)p->payload - (u8_t*)iphdr_ptr)
                           - IP6_HLEN);
  
      /* Set payload length in ip header. */
      iphdr_ptr->_plen = lwip_htons(ipr->datagram_len);
  
      /* With the fragment header gone, we now need to adjust the next-header
       * field of whatever header was originally before it. Since the packet made
       * it through the original header processing routines at least up to the
       * fragment header, we do not need any further sanity checks here. */
      if (IP6H_NEXTH(iphdr_ptr) == IP6_NEXTH_FRAGMENT) {
        iphdr_ptr->_nexth = ipr->nexth;
      } else {
        u8_t *ptr = (u8_t *)iphdr_ptr + IP6_HLEN;
        while (*ptr != IP6_NEXTH_FRAGMENT) {
          ptr += 8 * (1 + ptr[1]);
        }
        *ptr = ipr->nexth;
      }
  
      /* release the resources allocated for the fragment queue entry */
      if (reassdatagrams == ipr) {
        /* it was the first in the list */
        reassdatagrams = ipr->next;
      } else {
        /* it wasn't the first, so it must have a valid 'prev' */
        LWIP_ASSERT("sanity check linked list", ipr_prev != NULL);
        ipr_prev->next = ipr->next;
      }
      memp_free(MEMP_IP6_REASSDATA, ipr);
  
      /* adjust the number of pbufs currently queued for reassembly. */
      clen = pbuf_clen(p);
      LWIP_ASSERT("ip6_reass_pbufcount >= clen", ip6_reass_pbufcount >= clen);
      ip6_reass_pbufcount = (u16_t)(ip6_reass_pbufcount - clen);
  
      /* Move pbuf back to IPv6 header. This should never fail. */
      if (pbuf_header_force(p, (s16_t)((u8_t*)p->payload - (u8_t*)iphdr_ptr))) {
        LWIP_ASSERT("ip6_reass: moving p->payload to ip6 header failed\n", 0);
        pbuf_free(p);
        return NULL;
      }
  
      /* Return the pbuf chain */
      return p;
    }
    /* the datagram is not (yet?) reassembled completely */
    return NULL;
  
  nullreturn:
    IP6_FRAG_STATS_INC(ip6_frag.drop);
    pbuf_free(p);
    return NULL;
  }
  
  #endif /* LWIP_IPV6 && LWIP_IPV6_REASS */
  
  #if LWIP_IPV6 && LWIP_IPV6_FRAG
  
  #if !LWIP_NETIF_TX_SINGLE_PBUF
  /** Allocate a new struct pbuf_custom_ref */
  static struct pbuf_custom_ref*
  ip6_frag_alloc_pbuf_custom_ref(void)
  {
    return (struct pbuf_custom_ref*)memp_malloc(MEMP_FRAG_PBUF);
  }
  
  /** Free a struct pbuf_custom_ref */
  static void
  ip6_frag_free_pbuf_custom_ref(struct pbuf_custom_ref* p)
  {
    LWIP_ASSERT("p != NULL", p != NULL);
    memp_free(MEMP_FRAG_PBUF, p);
  }
  
  /** Free-callback function to free a 'struct pbuf_custom_ref', called by
   * pbuf_free. */
  static void
  ip6_frag_free_pbuf_custom(struct pbuf *p)
  {
    struct pbuf_custom_ref *pcr = (struct pbuf_custom_ref*)p;
    LWIP_ASSERT("pcr != NULL", pcr != NULL);
    LWIP_ASSERT("pcr == p", (void*)pcr == (void*)p);
    if (pcr->original != NULL) {
      pbuf_free(pcr->original);
    }
    ip6_frag_free_pbuf_custom_ref(pcr);
  }
  #endif /* !LWIP_NETIF_TX_SINGLE_PBUF */
  
  /**
   * Fragment an IPv6 datagram if too large for the netif or path MTU.
   *
   * Chop the datagram in MTU sized chunks and send them in order
   * by pointing PBUF_REFs into p
   *
   * @param p ipv6 packet to send
   * @param netif the netif on which to send
   * @param dest destination ipv6 address to which to send
   *
   * @return ERR_OK if sent successfully, err_t otherwise
   */
  err_t
  ip6_frag(struct pbuf *p, struct netif *netif, const ip6_addr_t *dest)
  {
    struct ip6_hdr *original_ip6hdr;
    struct ip6_hdr *ip6hdr;
    struct ip6_frag_hdr *frag_hdr;
    struct pbuf *rambuf;
  #if !LWIP_NETIF_TX_SINGLE_PBUF
    struct pbuf *newpbuf;
    u16_t newpbuflen = 0;
    u16_t left_to_copy;
  #endif
    static u32_t identification;
    u16_t left, cop;
    const u16_t mtu = nd6_get_destination_mtu(dest, netif);
    const u16_t nfb = (u16_t)((mtu - (IP6_HLEN + IP6_FRAG_HLEN)) & IP6_FRAG_OFFSET_MASK);
    u16_t fragment_offset = 0;
    u16_t last;
    u16_t poff = IP6_HLEN;
  
    identification++;
  
    original_ip6hdr = (struct ip6_hdr *)p->payload;
  
    /* @todo we assume there are no options in the unfragmentable part (IPv6 header). */
    LWIP_ASSERT("p->tot_len >= IP6_HLEN", p->tot_len >= IP6_HLEN);
    left = (u16_t)(p->tot_len - IP6_HLEN);
  
    while (left) {
      last = (left <= nfb);
  
      /* Fill this fragment */
      cop = last ? left : nfb;
  
  #if LWIP_NETIF_TX_SINGLE_PBUF
      rambuf = pbuf_alloc(PBUF_IP, cop + IP6_FRAG_HLEN, PBUF_RAM);
      if (rambuf == NULL) {
        IP6_FRAG_STATS_INC(ip6_frag.memerr);
        return ERR_MEM;
      }
      LWIP_ASSERT("this needs a pbuf in one piece!",
        (rambuf->len == rambuf->tot_len) && (rambuf->next == NULL));
      poff += pbuf_copy_partial(p, (u8_t*)rambuf->payload + IP6_FRAG_HLEN, cop, poff);
      /* make room for the IP header */
      if (pbuf_add_header(rambuf, IP6_HLEN)) {
        pbuf_free(rambuf);
        IP6_FRAG_STATS_INC(ip6_frag.memerr);
        return ERR_MEM;
      }
      /* fill in the IP header */
      SMEMCPY(rambuf->payload, original_ip6hdr, IP6_HLEN);
      ip6hdr = (struct ip6_hdr *)rambuf->payload;
      frag_hdr = (struct ip6_frag_hdr *)((u8_t*)rambuf->payload + IP6_HLEN);
  #else
      /* When not using a static buffer, create a chain of pbufs.
       * The first will be a PBUF_RAM holding the link, IPv6, and Fragment header.
       * The rest will be PBUF_REFs mirroring the pbuf chain to be fragged,
       * but limited to the size of an mtu.
       */
      rambuf = pbuf_alloc(PBUF_LINK, IP6_HLEN + IP6_FRAG_HLEN, PBUF_RAM);
      if (rambuf == NULL) {
        IP6_FRAG_STATS_INC(ip6_frag.memerr);
        return ERR_MEM;
      }
      LWIP_ASSERT("this needs a pbuf in one piece!",
                  (p->len >= (IP6_HLEN)));
      SMEMCPY(rambuf->payload, original_ip6hdr, IP6_HLEN);
      ip6hdr = (struct ip6_hdr *)rambuf->payload;
      frag_hdr = (struct ip6_frag_hdr *)((u8_t*)rambuf->payload + IP6_HLEN);
  
      /* Can just adjust p directly for needed offset. */
      p->payload = (u8_t *)p->payload + poff;
      p->len = (u16_t)(p->len - poff);
      p->tot_len = (u16_t)(p->tot_len - poff);
  
      left_to_copy = cop;
      while (left_to_copy) {
        struct pbuf_custom_ref *pcr;
        newpbuflen = (left_to_copy < p->len) ? left_to_copy : p->len;
        /* Is this pbuf already empty? */
        if (!newpbuflen) {
          p = p->next;
          continue;
        }
        pcr = ip6_frag_alloc_pbuf_custom_ref();
        if (pcr == NULL) {
          pbuf_free(rambuf);
          IP6_FRAG_STATS_INC(ip6_frag.memerr);
          return ERR_MEM;
        }
        /* Mirror this pbuf, although we might not need all of it. */
        newpbuf = pbuf_alloced_custom(PBUF_RAW, newpbuflen, PBUF_REF, &pcr->pc, p->payload, newpbuflen);
        if (newpbuf == NULL) {
          ip6_frag_free_pbuf_custom_ref(pcr);
          pbuf_free(rambuf);
          IP6_FRAG_STATS_INC(ip6_frag.memerr);
          return ERR_MEM;
        }
        pbuf_ref(p);
        pcr->original = p;
        pcr->pc.custom_free_function = ip6_frag_free_pbuf_custom;
  
        /* Add it to end of rambuf's chain, but using pbuf_cat, not pbuf_chain
         * so that it is removed when pbuf_dechain is later called on rambuf.
         */
        pbuf_cat(rambuf, newpbuf);
        left_to_copy = (u16_t)(left_to_copy - newpbuflen);
        if (left_to_copy) {
          p = p->next;
        }
      }
      poff = newpbuflen;
  #endif /* LWIP_NETIF_TX_SINGLE_PBUF */
  
      /* Set headers */
      frag_hdr->_nexth = original_ip6hdr->_nexth;
      frag_hdr->reserved = 0;
      frag_hdr->_fragment_offset = lwip_htons((u16_t)((fragment_offset & IP6_FRAG_OFFSET_MASK) | (last ? 0 : IP6_FRAG_MORE_FLAG)));
      frag_hdr->_identification = lwip_htonl(identification);
  
      IP6H_NEXTH_SET(ip6hdr, IP6_NEXTH_FRAGMENT);
      IP6H_PLEN_SET(ip6hdr, (u16_t)(cop + IP6_FRAG_HLEN));
  
      /* No need for separate header pbuf - we allowed room for it in rambuf
       * when allocated.
       */
      IP6_FRAG_STATS_INC(ip6_frag.xmit);
      netif->output_ip6(netif, rambuf, dest);
  
      /* Unfortunately we can't reuse rambuf - the hardware may still be
       * using the buffer. Instead we free it (and the ensuing chain) and
       * recreate it next time round the loop. If we're lucky the hardware
       * will have already sent the packet, the free will really free, and
       * there will be zero memory penalty.
       */
  
      pbuf_free(rambuf);
      left = (u16_t)(left - cop);
      fragment_offset = (u16_t)(fragment_offset + cop);
    }
    return ERR_OK;
  }
  
  #endif /* LWIP_IPV6 && LWIP_IPV6_FRAG */
  