iprh_prev->next_pbuf = new_p;
      if (iprh_prev->end != iprh->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 = new_p;
    }
  }

  /* At this point, the validation part begins: */
  /* If we already received the last fragment */
  if ((ipr->flags & IP_REASS_FLAG_LASTFRAG) != 0) {
    /* and had no wholes so far */
    if (valid) {
      /* then check if the rest of the fragments is here */
      /* Check if the queue starts with the first datagram */
      if (((struct ip_reass_helper*)ipr->p->payload)->start != 0) {
        valid = 0;
      } else {
        /* and check that there are no wholes after this datagram */
        iprh_prev = iprh;
        q = iprh->next_pbuf;
        while (q != NULL) {
          iprh = (struct ip_reass_helper*)q->payload;
          if (iprh_prev->end != iprh->start) {
            valid = 0;
            break;
          }
          iprh_prev = iprh;
          q = iprh->next_pbuf;
        }
        /* if still valid, all fragments are received
         * (because to the MF==0 already arrived */
        if (valid) {
          LWIP_ASSERT("sanity check", ipr->p != NULL);
          LWIP_ASSERT("sanity check",
            ((struct ip_reass_helper*)ipr->p->payload) != iprh);
          LWIP_ASSERT("validate_datagram:next_pbuf!=NULL",
            iprh->next_pbuf == NULL);
          LWIP_ASSERT("validate_datagram:datagram end!=datagram len",
            iprh->end == ipr->datagram_len);
        }
      }
    }
    /* If valid is 0 here, there are some fragments missing in the middle
     * (since MF == 0 has already arrived). Such datagrams simply time out if
     * no more fragments are received... */
    return valid;
  }
  /* If we come here, not all fragments were received, yet! */
  return 0; /* not yet valid! */
#if IP_REASS_CHECK_OVERLAP
freepbuf:
  ip_reass_pbufcount -= pbuf_clen(new_p);
  pbuf_free(new_p);
  return 0;
#endif /* IP_REASS_CHECK_OVERLAP */
}

/**
 * Reassembles incoming IP fragments into an IP datagram.
 *
 * @param p points to a pbuf chain of the fragment
 * @return NULL if reassembly is incomplete, ? otherwise
 */
struct pbuf *
ip_reass(struct pbuf *p)
{
  struct pbuf *r;
  struct ip_hdr *fraghdr;
  struct ip_reassdata *ipr;
  struct ip_reass_helper *iprh;
  u16_t offset, len;
  u8_t clen;
  struct ip_reassdata *ipr_prev = NULL;

  IPFRAG_STATS_INC(ip_frag.recv);
  snmp_inc_ipreasmreqds();

  fraghdr = (struct ip_hdr*)p->payload;

  if ((IPH_HL(fraghdr) * 4) != IP_HLEN) {
    LWIP_DEBUGF(IP_REASS_DEBUG,("ip_reass: IP options currently not supported!\n"));
    IPFRAG_STATS_INC(ip_frag.err);
    goto nullreturn;
  }

  offset = (ntohs(IPH_OFFSET(fraghdr)) & IP_OFFMASK) * 8;
  len = ntohs(IPH_LEN(fraghdr)) - IPH_HL(fraghdr) * 4;

  /* Check if we are allowed to enqueue more datagrams. */
  clen = pbuf_clen(p);
  if ((ip_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS) {
#if IP_REASS_FREE_OLDEST
    if (!ip_reass_remove_oldest_datagram(fraghdr, clen) ||
        ((ip_reass_pbufcount + clen) > IP_REASS_MAX_PBUFS))
#endif /* IP_REASS_FREE_OLDEST */
    {
      /* No datagram could be freed and still too many pbufs enqueued */
      LWIP_DEBUGF(IP_REASS_DEBUG,("ip_reass: Overflow condition: pbufct=%d, clen=%d, MAX=%d\n",
        ip_reass_pbufcount, clen, IP_REASS_MAX_PBUFS));
      IPFRAG_STATS_INC(ip_frag.memerr);
      /* @todo: send ICMP time exceeded here? */
      /* drop this pbuf */
      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 != 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 (IP_ADDRESSES_AND_ID_MATCH(&ipr->iphdr, fraghdr)) {
      LWIP_DEBUGF(IP_REASS_DEBUG, ("ip_reass: matching previous fragment ID=%"X16_F"\n",
        ntohs(IPH_ID(fraghdr))));
      IPFRAG_STATS_INC(ip_frag.cachehit);
      break;
    }
    ipr_prev = ipr;
  }

  if (ipr == NULL) {
  /* Enqueue a new datagram into the datagram queue */
    ipr = ip_reass_enqueue_new_datagram(fraghdr, clen);
    /* Bail if unable to enqueue */
    if(ipr == NULL) {
      goto nullreturn;
    }
  } else {
    if (((ntohs(IPH_OFFSET(fraghdr)) & IP_OFFMASK) == 0) && 
      ((ntohs(IPH_OFFSET(&ipr->iphdr)) & IP_OFFMASK) != 0)) {
      /* ipr->iphdr is not the header from the first fragment, but fraghdr is
       * -> copy fraghdr into ipr->iphdr since we want to have the header
       * of the first fragment (for ICMP time exceeded and later, for copying
       * all options, if supported)*/
      SMEMCPY(&ipr->iphdr, fraghdr, IP_HLEN);
    }
  }
  /* 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 */
  ip_reass_pbufcount += clen;

  /* At this point, we have either created a new entry or pointing 
   * to an existing one */

  /* check for 'no more fragments', and update queue entry*/
  if ((ntohs(IPH_OFFSET(fraghdr)) & IP_MF) == 0) {
    ipr->flags |= IP_REASS_FLAG_LASTFRAG;
    ipr->datagram_len = offset + len;
    LWIP_DEBUGF(IP_REASS_DEBUG,
     ("ip_reass: last fragment seen, total len %"S16_F"\n",
      ipr->datagram_len));
  }
  /* find the right place to insert this pbuf */
  /* @todo: trim pbufs if fragments are overlapping */
  if (ip_reass_chain_frag_into_datagram_and_validate(ipr, p)) {
    /* the totally last fragment (flag more fragments = 0) was received at least
     * once AND all fragments are received */
    ipr->datagram_len += IP_HLEN;

    /* save the second pbuf before copying the header over the pointer */
    r = ((struct ip_reass_helper*)ipr->p->payload)->next_pbuf;

    /* copy the original ip header back to the first pbuf */
    fraghdr = (struct ip_hdr*)(ipr->p->payload);
    SMEMCPY(fraghdr, &ipr->iphdr, IP_HLEN);
    IPH_LEN_SET(fraghdr, htons(ipr->datagram_len));
    IPH_OFFSET_SET(fraghdr, 0);
    IPH_CHKSUM_SET(fraghdr, 0);
    /* @todo: do we need to set calculate the correct checksum? */
    IPH_CHKSUM_SET(fraghdr, inet_chksum(fraghdr, IP_HLEN));

    p = ipr->p;

    /* chain together the pbufs contained within the reass_data list. */
    while(r != NULL) {
      iprh = (struct ip_reass_helper*)r->payload;

      /* hide the ip header for every succeding fragment */
      pbuf_header(r, -IP_HLEN);
      pbuf_cat(p, r);
      r = iprh->next_pbuf;
    }
    /* release the sources allocate for the fragment queue entry */
    ip_reass_dequeue_datagram(ipr, ipr_prev);

    /* and adjust the number of pbufs currently queued for reassembly. */
    ip_reass_pbufcount -= pbuf_clen(p);

    /* Return the pbuf chain */
    return p;
  }
  /* the datagram is not (yet?) reassembled completely */
  LWIP_DEBUGF(IP_REASS_DEBUG,("ip_reass_pbufcount: %d out\n", ip_reass_pbufcount));
  return NULL;

nullreturn:
  LWIP_DEBUGF(IP_REASS_DEBUG,("ip_reass: nullreturn\n"));
  IPFRAG_STATS_INC(ip_frag.drop);
  pbuf_free(p);
  return NULL;
}
#endif /* IP_REASSEMBLY */

#if IP_FRAG
#if IP_FRAG_USES_STATIC_BUF
static u8_t buf[LWIP_MEM_ALIGN_SIZE(IP_FRAG_MAX_MTU + MEM_ALIGNMENT - 1)];
#endif /* IP_FRAG_USES_STATIC_BUF */

/**
 * Fragment an IP datagram if too large for the netif.
 *
 * Chop the datagram in MTU sized chunks and send them in order
 * by using a fixed size static memory buffer (PBUF_REF) or
 * point PBUF_REFs into p (depending on IP_FRAG_USES_STATIC_BUF).
 *
 * @param p ip packet to send
 * @param netif the netif on which to send
 * @param dest destination ip address to which to send
 *
 * @return ERR_OK if sent successfully, err_t otherwise
 */
err_t 
ip_frag(struct pbuf *p, struct netif *netif, struct ip_addr *dest)
{
  struct pbuf *rambuf;
#if IP_FRAG_USES_STATIC_BUF
  struct pbuf *header;
#else
  struct pbuf *newpbuf;
  struct ip_hdr *original_iphdr;
#endif
  struct ip_hdr *iphdr;
  u16_t nfb;
  u16_t left, cop;
  u16_t mtu = netif->mtu;
  u16_t ofo, omf;
  u16_t last;
  u16_t poff = IP_HLEN;
  u16_t tmp;
#if !IP_FRAG_USES_STATIC_BUF
  u16_t newpbuflen = 0;
  u16_t left_to_copy;
#endif

  /* Get a RAM based MTU sized pbuf */
#if IP_FRAG_USES_STATIC_BUF
  /* When using a static buffer, we use a PBUF_REF, which we will
   * use to reference the packet (without link header).
   * Layer and length is irrelevant.
   */
  rambuf = pbuf_alloc(PBUF_LINK, 0, PBUF_REF);
  if (rambuf == NULL) {
    LWIP_DEBUGF(IP_REASS_DEBUG, ("ip_frag: pbuf_alloc(PBUF_LINK, 0, PBUF_REF) failed\n"));
    return ERR_MEM;
  }
  rambuf->tot_len = rambuf->len = mtu;
  rambuf->payload = LWIP_MEM_ALIGN((void *)buf);

  /* Copy the IP header in it */
  iphdr = rambuf->payload;
  SMEMCPY(iphdr, p->payload, IP_HLEN);
#else /* IP_FRAG_USES_STATIC_BUF */
  original_iphdr = p->payload;
  iphdr = original_iphdr;
#endif /* IP_FRAG_USES_STATIC_BUF */

  /* Save original offset */
  tmp = ntohs(IPH_OFFSET(iphdr));
  ofo = tmp & IP_OFFMASK;
  omf = tmp & IP_MF;

  left = p->tot_len - IP_HLEN;

  nfb = (mtu - IP_HLEN) / 8;

  while (left) {
    last = (left <= mtu - IP_HLEN);

    /* Set new offset and MF flag */
    tmp = omf | (IP_OFFMASK & (ofo));
    if (!last)
      tmp = tmp | IP_MF;

    /* Fill this fragment */
    cop = last ? left : nfb * 8;

#if IP_FRAG_USES_STATIC_BUF
    poff += pbuf_copy_partial(p, (u8_t*)iphdr + IP_HLEN, cop, poff);
#else /* IP_FRAG_USES_STATIC_BUF */
    /* When not using a static buffer, create a chain of pbufs.
     * The first will be a PBUF_RAM holding the link and IP 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, IP_HLEN, PBUF_RAM);
    if (rambuf == NULL) {
      return ERR_MEM;
    }
    LWIP_ASSERT("this needs a pbuf in one piece!",
                (p->len >= (IP_HLEN)));
    SMEMCPY(rambuf->payload, original_iphdr, IP_HLEN);
    iphdr = rambuf->payload;

    /* Can just adjust p directly for needed offset. */
    p->payload = (u8_t *)p->payload + poff;
    p->len -= poff;

    left_to_copy = cop;
    while (left_to_copy) {
      newpbuflen = (left_to_copy < p->len) ? left_to_copy : p->len;
      /* Is this pbuf already empty? */
      if (!newpbuflen) {
        p = p->next;
        continue;
      }
      newpbuf = pbuf_alloc(PBUF_RAW, 0, PBUF_REF);
      if (newpbuf == NULL) {
        pbuf_free(rambuf);
        return ERR_MEM;
      }
      /* Mirror this pbuf, although we might not need all of it. */
      newpbuf->payload = p->payload;
      newpbuf->len = newpbuf->tot_len = newpbuflen;
      /* 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 -= newpbuflen;
      if (left_to_copy)
        p = p->next;
    }
    poff = newpbuflen;
#endif /* IP_FRAG_USES_STATIC_BUF */

    /* Correct header */
    IPH_OFFSET_SET(iphdr, htons(tmp));
    IPH_LEN_SET(iphdr, htons(cop + IP_HLEN));
    IPH_CHKSUM_SET(iphdr, 0);
    IPH_CHKSUM_SET(iphdr, inet_chksum(iphdr, IP_HLEN));

#if IP_FRAG_USES_STATIC_BUF
    if (last)
      pbuf_realloc(rambuf, left + IP_HLEN);

    /* This part is ugly: we alloc a RAM based pbuf for 
     * the link level header for each chunk and then 
     * free it.A PBUF_ROM style pbuf for which pbuf_header
     * worked would make things simpler.
     */
    header = pbuf_alloc(PBUF_LINK, 0, PBUF_RAM);
    if (header != NULL) {
      pbuf_chain(header, rambuf);
      netif->output(netif, header, dest);
      IPFRAG_STATS_INC(ip_frag.xmit);
      snmp_inc_ipfragcreates();
      pbuf_free(header);
    } else {
      LWIP_DEBUGF(IP_REASS_DEBUG, ("ip_frag: pbuf_alloc() for header failed\n"));
      pbuf_free(rambuf);
      return ERR_MEM;
    }
#else /* IP_FRAG_USES_STATIC_BUF */
    /* No need for separate header pbuf - we allowed room for it in rambuf
     * when allocated.
     */
    netif->output(netif, rambuf, dest);
    IPFRAG_STATS_INC(ip_frag.xmit);

    /* 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);
#endif /* IP_FRAG_USES_STATIC_BUF */
    left -= cop;
    ofo += nfb;
  }
#if IP_FRAG_USES_STATIC_BUF
  pbuf_free(rambuf);
#endif /* IP_FRAG_USES_STATIC_BUF */
  snmp_inc_ipfragoks();
  return ERR_OK;
}
#endif /* IP_FRAG */