len = (BUF->len[0] << 8) + BUF->len[1] - (BUF->vhl & 0x0f) * 4;
    offset = (((BUF->ipoffset[0] & 0x3f) << 8) + BUF->ipoffset[1]) * 8;

    /* If the offset or the offset + fragment length overflows the
       reassembly buffer, we discard the entire packet. */
    if(offset > UIP_REASS_BUFSIZE ||
       offset + len > UIP_REASS_BUFSIZE) {
      uip_reasstmr = 0;
      goto nullreturn;
    }

    /* Copy the fragment into the reassembly buffer, at the right
       offset. */
    memcpy(&uip_reassbuf[IP_HLEN + offset],
	   (char *)BUF + (int)((BUF->vhl & 0x0f) * 4),
	   len);
      
    /* Update the bitmap. */
    if(offset / (8 * 8) == (offset + len) / (8 * 8)) {
      /* If the two endpoints are in the same byte, we only update
	 that byte. */
	     
      uip_reassbitmap[offset / (8 * 8)] |=
	     bitmap_bits[(offset / 8 ) & 7] &
	     ~bitmap_bits[((offset + len) / 8 ) & 7];
    } else {
      /* If the two endpoints are in different bytes, we update the
	 bytes in the endpoints and fill the stuff inbetween with
	 0xff. */
      uip_reassbitmap[offset / (8 * 8)] |=
	bitmap_bits[(offset / 8 ) & 7];
      for(i = 1 + offset / (8 * 8); i < (offset + len) / (8 * 8); ++i) {
	uip_reassbitmap[i] = 0xff;
      }      
      uip_reassbitmap[(offset + len) / (8 * 8)] |=
	~bitmap_bits[((offset + len) / 8 ) & 7];
    }
    
    /* If this fragment has the More Fragments flag set to zero, we
       know that this is the last fragment, so we can calculate the
       size of the entire packet. We also set the
       IP_REASS_FLAG_LASTFRAG flag to indicate that we have received
       the final fragment. */

    if((BUF->ipoffset[0] & IP_MF) == 0) {
      uip_reassflags |= UIP_REASS_FLAG_LASTFRAG;
      uip_reasslen = offset + len;
    }
    
    /* Finally, we check if we have a full packet in the buffer. We do
       this by checking if we have the last fragment and if all bits
       in the bitmap are set. */
    if(uip_reassflags & UIP_REASS_FLAG_LASTFRAG) {
      /* Check all bytes up to and including all but the last byte in
	 the bitmap. */
      for(i = 0; i < uip_reasslen / (8 * 8) - 1; ++i) {
	if(uip_reassbitmap[i] != 0xff) {
	  goto nullreturn;
	}
      }
      /* Check the last byte in the bitmap. It should contain just the
	 right amount of bits. */
      if(uip_reassbitmap[uip_reasslen / (8 * 8)] !=
	 (u8_t)~bitmap_bits[uip_reasslen / 8 & 7]) {
	goto nullreturn;
      }

      /* If we have come this far, we have a full packet in the
	 buffer, so we allocate a pbuf and copy the packet into it. We
	 also reset the timer. */
      uip_reasstmr = 0;
      memcpy(BUF, FBUF, uip_reasslen);

      /* Pretend to be a "normal" (i.e., not fragmented) IP packet
	 from now on. */
      BUF->ipoffset[0] = BUF->ipoffset[1] = 0;
      BUF->len[0] = uip_reasslen >> 8;
      BUF->len[1] = uip_reasslen & 0xff;
      BUF->ipchksum = 0;
      BUF->ipchksum = ~(uip_ipchksum());

      return uip_reasslen;
    }
  }

 nullreturn:
  return 0;
}
#endif /* UIP_REASSEMBL */
/*-----------------------------------------------------------------------------------*/
static void
uip_add_rcv_nxt(u16_t n)
{
  uip_add32(uip_conn->rcv_nxt, n);
  uip_conn->rcv_nxt[0] = uip_acc32[0];
  uip_conn->rcv_nxt[1] = uip_acc32[1];
  uip_conn->rcv_nxt[2] = uip_acc32[2];
  uip_conn->rcv_nxt[3] = uip_acc32[3];
}
/*-----------------------------------------------------------------------------------*/
void
uip_process(u8_t flag)
{
  register struct uip_conn *uip_connr = uip_conn;
  
  uip_appdata = &uip_buf[40 + UIP_LLH_LEN];

  
  /* Check if we were invoked because of the perodic timer fireing. */
  if(flag == UIP_TIMER) {
#if UIP_REASSEMBLY
    if(uip_reasstmr != 0) {
      --uip_reasstmr;
    }
#endif /* UIP_REASSEMBLY */
    /* Increase the initial sequence number. */
    if(++iss[3] == 0) {
      if(++iss[2] == 0) {
	if(++iss[1] == 0) {
	  ++iss[0];
	}
      }
    }    
    uip_len = 0;
    if(uip_connr->tcpstateflags == TIME_WAIT ||
       uip_connr->tcpstateflags == FIN_WAIT_2) {
      ++(uip_connr->timer);
      if(uip_connr->timer == UIP_TIME_WAIT_TIMEOUT) {
	uip_connr->tcpstateflags = CLOSED;
      }
    } else if(uip_connr->tcpstateflags != CLOSED) {
      /* If the connection has outstanding data, we increase the
	 connection's timer and see if it has reached the RTO value
	 in which case we retransmit. */
      if(uip_outstanding(uip_connr)) {
	if(uip_connr->timer-- == 0) {
	  if(uip_connr->nrtx == UIP_MAXRTX ||
	     ((uip_connr->tcpstateflags == SYN_SENT ||
	       uip_connr->tcpstateflags == SYN_RCVD) &&
	      uip_connr->nrtx == UIP_MAXSYNRTX)) {
	    uip_connr->tcpstateflags = CLOSED;

	    /* We call UIP_APPCALL() with uip_flags set to
	       UIP_TIMEDOUT to inform the application that the
	       connection has timed out. */
	    uip_flags = UIP_TIMEDOUT;
	    UIP_APPCALL();

	    /* We also send a reset packet to the remote host. */
	    BUF->flags = TCP_RST | TCP_ACK;
	    goto tcp_send_nodata;
	  }

	  /* Exponential backoff. */
	  uip_connr->timer = UIP_RTO << (uip_connr->nrtx > 4?
					 4:
					 uip_connr->nrtx);
	  ++(uip_connr->nrtx);
	  
	  /* Ok, so we need to retransmit. We do this differently
	     depending on which state we are in. In ESTABLISHED, we
	     call upon the application so that it may prepare the
	     data for the retransmit. In SYN_RCVD, we resend the
	     SYNACK that we sent earlier and in LAST_ACK we have to
	     retransmit our FINACK. */
	  UIP_STAT(++uip_stat.tcp.rexmit);
	  switch(uip_connr->tcpstateflags & TS_MASK) {
	  case SYN_RCVD:
	    /* In the SYN_RCVD state, we should retransmit our
               SYNACK. */
	    goto tcp_send_synack;
	    
#if UIP_ACTIVE_OPEN
	  case SYN_SENT:
	    /* In the SYN_SENT state, we retransmit out SYN. */
	    BUF->flags = 0;
	    goto tcp_send_syn;
#endif /* UIP_ACTIVE_OPEN */
	    
	  case ESTABLISHED:
	    /* In the ESTABLISHED state, we call upon the application
               to do the actual retransmit after which we jump into
               the code for sending out the packet (the apprexmit
               label). */
	    uip_len = 0;
	    uip_slen = 0;
	    uip_flags = UIP_REXMIT;
	    UIP_APPCALL();
	    goto apprexmit;
	    
	  case FIN_WAIT_1:
	  case CLOSING:
	  case LAST_ACK:
	    /* In all these states we should retransmit a FINACK. */
	    goto tcp_send_finack;
	    
	  }
	}
      } else if((uip_connr->tcpstateflags & TS_MASK) == ESTABLISHED) {
	/* If there was no need for a retransmission, we poll the
           application for new data. */
	uip_len = 0;
	uip_slen = 0;
	uip_flags = UIP_POLL;
	UIP_APPCALL();
	goto appsend;
      }
    }
    goto drop;
  }
#if UIP_UDP 
  if(flag == UIP_UDP_TIMER) {
    if(uip_udp_conn->lport != 0) {
      uip_appdata = &uip_buf[UIP_LLH_LEN + 28];
      uip_len = uip_slen = 0;
      uip_flags = UIP_POLL;
      UIP_UDP_APPCALL();
      goto udp_send;
    } else {
      goto drop;
    }
  }
#endif

  /* This is where the input processing starts. */
  UIP_STAT(++uip_stat.ip.recv);


  /* Start of IPv4 input header processing code. */
  
  /* Check validity of the IP header. */  
  if(BUF->vhl != 0x45)  { /* IP version and header length. */
    UIP_STAT(++uip_stat.ip.drop);
    UIP_STAT(++uip_stat.ip.vhlerr);
    UIP_LOG("ip: invalid version or header length.");
    goto drop;
  }
  
  /* Check the size of the packet. If the size reported to us in
     uip_len doesn't match the size reported in the IP header, there
     has been a transmission error and we drop the packet. */
  
  if(BUF->len[0] != (uip_len >> 8)) { /* IP length, high byte. */
    uip_len = (uip_len & 0xff) | (BUF->len[0] << 8);
  }
  if(BUF->len[1] != (uip_len & 0xff)) { /* IP length, low byte. */
    uip_len = (uip_len & 0xff00) | BUF->len[1];
  }

  /* Check the fragment flag. */
  if((BUF->ipoffset[0] & 0x3f) != 0 ||
     BUF->ipoffset[1] != 0) { 
#if UIP_REASSEMBLY
    uip_len = uip_reass();
    if(uip_len == 0) {
      goto drop;
    }
#else
    UIP_STAT(++uip_stat.ip.drop);
    UIP_STAT(++uip_stat.ip.fragerr);
    UIP_LOG("ip: fragment dropped.");    
    goto drop;
#endif /* UIP_REASSEMBLY */
  }

  /* If we are configured to use ping IP address configuration and
     hasn't been assigned an IP address yet, we accept all ICMP
     packets. */
#if UIP_PINGADDRCONF
  if((uip_hostaddr[0] | uip_hostaddr[1]) == 0) {
    if(BUF->proto == UIP_PROTO_ICMP) {
      UIP_LOG("ip: possible ping config packet received.");
      goto icmp_input;
    } else {
      UIP_LOG("ip: packet dropped since no address assigned.");
      goto drop;
    }
  }
#endif /* UIP_PINGADDRCONF */
  
  /* Check if the packet is destined for our IP address. */  
  if(BUF->destipaddr[0] != uip_hostaddr[0]) {
    UIP_STAT(++uip_stat.ip.drop);
    UIP_LOG("ip: packet not for us.");        
    goto drop;
  }
  if(BUF->destipaddr[1] != uip_hostaddr[1]) {
    UIP_STAT(++uip_stat.ip.drop);
    UIP_LOG("ip: packet not for us.");        
    goto drop;
  }

  if(uip_ipchksum() != 0xffff) { /* Compute and check the IP header
				    checksum. */
    UIP_STAT(++uip_stat.ip.drop);
    UIP_STAT(++uip_stat.ip.chkerr);
    UIP_LOG("ip: bad checksum.");    
    goto drop;
  }

  if(BUF->proto == UIP_PROTO_TCP)  /* Check for TCP packet. If so, jump
                                     to the tcp_input label. */
    goto tcp_input;

#if UIP_UDP
  if(BUF->proto == UIP_PROTO_UDP)
    goto udp_input;
#endif /* UIP_UDP */

  if(BUF->proto != UIP_PROTO_ICMP) { /* We only allow ICMP packets from
					here. */
    UIP_STAT(++uip_stat.ip.drop);
    UIP_STAT(++uip_stat.ip.protoerr);
    UIP_LOG("ip: neither tcp nor icmp.");        
    goto drop;
  }
  
 icmp_input:
  UIP_STAT(++uip_stat.icmp.recv);
  
  /* ICMP echo (i.e., ping) processing. This is simple, we only change
     the ICMP type from ECHO to ECHO_REPLY and adjust the ICMP
     checksum before we return the packet. */
  if(ICMPBUF->type != ICMP_ECHO) {
    UIP_STAT(++uip_stat.icmp.drop);
    UIP_STAT(++uip_stat.icmp.typeerr);
    UIP_LOG("icmp: not icmp echo.");
    goto drop;
  }

  /* If we are configured to use ping IP address assignment, we use
     the destination IP address of this ping packet and assign it to
     ourself. */
#if UIP_PINGADDRCONF
  if((uip_hostaddr[0] | uip_hostaddr[1]) == 0) {
    uip_hostaddr[0] = BUF->destipaddr[0];
    uip_hostaddr[1] = BUF->destipaddr[1];
  }
#endif /* UIP_PINGADDRCONF */  
  
  ICMPBUF->type = ICMP_ECHO_REPLY;
  
  if(ICMPBUF->icmpchksum >= HTONS(0xffff - (ICMP_ECHO << 8))) {
    ICMPBUF->icmpchksum += HTONS(ICMP_ECHO << 8) + 1;
  } else {
    ICMPBUF->icmpchksum += HTONS(ICMP_ECHO << 8);
  }
  
  /* Swap IP addresses. */
  tmp16 = BUF->destipaddr[0];
  BUF->destipaddr[0] = BUF->srcipaddr[0];
  BUF->srcipaddr[0] = tmp16;
  tmp16 = BUF->destipaddr[1];
  BUF->destipaddr[1] = BUF->srcipaddr[1];
  BUF->srcipaddr[1] = tmp16;

  UIP_STAT(++uip_stat.icmp.sent);
  goto send;

  /* End of IPv4 input header processing code. */
  

#if UIP_UDP
  /* UDP input processing. */
 udp_input:
  /* UDP processing is really just a hack. We don't do anything to the
     UDP/IP headers, but let the UDP application do all the hard
     work. If the application sets uip_slen, it has a packet to
     send. */
#if UIP_UDP_CHECKSUMS
  if(uip_udpchksum() != 0xffff) { 
    UIP_STAT(++uip_stat.udp.drop);
    UIP_STAT(++uip_stat.udp.chkerr);
    UIP_LOG("udp: bad checksum.");    
    goto drop;
  }