#endif /* UIP_UDP_CHECKSUMS */

  /* Demultiplex this UDP packet between the UDP "connections". */
  for(uip_udp_conn = &uip_udp_conns[0];
      uip_udp_conn < &uip_udp_conns[UIP_UDP_CONNS];
      ++uip_udp_conn) {
    if(uip_udp_conn->lport != 0 &&
       UDPBUF->destport == uip_udp_conn->lport &&
       (uip_udp_conn->rport == 0 ||
        UDPBUF->srcport == uip_udp_conn->rport) &&
       BUF->srcipaddr[0] == uip_udp_conn->ripaddr[0] &&
       BUF->srcipaddr[1] == uip_udp_conn->ripaddr[1]) {
      goto udp_found; 
    }
  }
  goto drop;
  
 udp_found:
  uip_len = uip_len - 28;
  uip_appdata = &uip_buf[UIP_LLH_LEN + 28];
  uip_flags = UIP_NEWDATA;
  uip_slen = 0;
  UIP_UDP_APPCALL();
 udp_send:
  if(uip_slen == 0) {
    goto drop;      
  }
  uip_len = uip_slen + 28;

  BUF->len[0] = (uip_len >> 8);
  BUF->len[1] = (uip_len & 0xff);
  
  BUF->proto = UIP_PROTO_UDP;

  UDPBUF->udplen = HTONS(uip_slen + 8);
  UDPBUF->udpchksum = 0;
#if UIP_UDP_CHECKSUMS 
  /* Calculate UDP checksum. */
  UDPBUF->udpchksum = ~(uip_udpchksum());
  if(UDPBUF->udpchksum == 0) {
    UDPBUF->udpchksum = 0xffff;
  }
#endif /* UIP_UDP_CHECKSUMS */

  BUF->srcport  = uip_udp_conn->lport;
  BUF->destport = uip_udp_conn->rport;

  BUF->srcipaddr[0] = uip_hostaddr[0];
  BUF->srcipaddr[1] = uip_hostaddr[1];
  BUF->destipaddr[0] = uip_udp_conn->ripaddr[0];
  BUF->destipaddr[1] = uip_udp_conn->ripaddr[1];
 
  uip_appdata = &uip_buf[UIP_LLH_LEN + 40];
  goto ip_send_nolen;
#endif /* UIP_UDP */
  
  /* TCP input processing. */  
 tcp_input:
  UIP_STAT(++uip_stat.tcp.recv);

  /* Start of TCP input header processing code. */
  
  if(uip_tcpchksum() != 0xffff) {   /* Compute and check the TCP
				       checksum. */
    UIP_STAT(++uip_stat.tcp.drop);
    UIP_STAT(++uip_stat.tcp.chkerr);
    UIP_LOG("tcp: bad checksum.");    
    goto drop;
  }
  
  /* Demultiplex this segment. */
  /* First check any active connections. */
  for(uip_connr = &uip_conns[0]; uip_connr < &uip_conns[UIP_CONNS]; ++uip_connr) {
    if(uip_connr->tcpstateflags != CLOSED &&
       BUF->destport == uip_connr->lport &&
       BUF->srcport == uip_connr->rport &&
       BUF->srcipaddr[0] == uip_connr->ripaddr[0] &&
       BUF->srcipaddr[1] == uip_connr->ripaddr[1]) {
      goto found;    
    }
  }

  /* If we didn't find and active connection that expected the packet,
     either this packet is an old duplicate, or this is a SYN packet
     destined for a connection in LISTEN. If the SYN flag isn't set,
     it is an old packet and we send a RST. */
  if((BUF->flags & TCP_CTL) != TCP_SYN)
    goto reset;
  
  tmp16 = BUF->destport;
  /* Next, check listening connections. */  
  for(c = 0; c < UIP_LISTENPORTS; ++c) {
    if(tmp16 == uip_listenports[c])
      goto found_listen;
  }
  
  /* No matching connection found, so we send a RST packet. */
  UIP_STAT(++uip_stat.tcp.synrst);
 reset:

  /* We do not send resets in response to resets. */
  if(BUF->flags & TCP_RST) 
    goto drop;

  UIP_STAT(++uip_stat.tcp.rst);
  
  BUF->flags = TCP_RST | TCP_ACK;
  uip_len = 40;
  BUF->tcpoffset = 5 << 4;

  /* Flip the seqno and ackno fields in the TCP header. */
  c = BUF->seqno[3];
  BUF->seqno[3] = BUF->ackno[3];  
  BUF->ackno[3] = c;
  
  c = BUF->seqno[2];
  BUF->seqno[2] = BUF->ackno[2];  
  BUF->ackno[2] = c;
  
  c = BUF->seqno[1];
  BUF->seqno[1] = BUF->ackno[1];
  BUF->ackno[1] = c;
  
  c = BUF->seqno[0];
  BUF->seqno[0] = BUF->ackno[0];  
  BUF->ackno[0] = c;

  /* We also have to increase the sequence number we are
     acknowledging. If the least significant byte overflowed, we need
     to propagate the carry to the other bytes as well. */
  if(++BUF->ackno[3] == 0) {
    if(++BUF->ackno[2] == 0) {
      if(++BUF->ackno[1] == 0) {
	++BUF->ackno[0];
      }
    }
  }
 
  /* Swap port numbers. */
  tmp16 = BUF->srcport;
  BUF->srcport = BUF->destport;
  BUF->destport = tmp16;
  
  /* 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;

  
  /* And send out the RST packet! */
  goto tcp_send_noconn;

  /* This label will be jumped to if we matched the incoming packet
     with a connection in LISTEN. In that case, we should create a new
     connection and send a SYNACK in return. */
 found_listen:
  /* First we check if there are any connections avaliable. Unused
     connections are kept in the same table as used connections, but
     unused ones have the tcpstate set to CLOSED. Also, connections in
     TIME_WAIT are kept track of and we'll use the oldest one if no
     CLOSED connections are found. Thanks to Eddie C. Dost for a very
     nice algorithm for the TIME_WAIT search. */
  uip_connr = 0;
  for(c = 0; c < UIP_CONNS; ++c) {
    if(uip_conns[c].tcpstateflags == CLOSED) {
      uip_connr = &uip_conns[c];
      break;
    }
    if(uip_conns[c].tcpstateflags == TIME_WAIT) {
      if(uip_connr == 0 ||
	 uip_conns[c].timer > uip_connr->timer) {
	uip_connr = &uip_conns[c];
      }
    }
  }

  if(uip_connr == 0) {
    /* All connections are used already, we drop packet and hope that
       the remote end will retransmit the packet at a time when we
       have more spare connections. */
    UIP_STAT(++uip_stat.tcp.syndrop);
    UIP_LOG("tcp: found no unused connections.");
    goto drop;
  }
  uip_conn = uip_connr;
  
  /* Fill in the necessary fields for the new connection. */
  uip_connr->rto = uip_connr->timer = UIP_RTO;
  uip_connr->sa = 0;
  uip_connr->sv = 4;  
  uip_connr->nrtx = 0;
  uip_connr->lport = BUF->destport;
  uip_connr->rport = BUF->srcport;
  uip_connr->ripaddr[0] = BUF->srcipaddr[0];
  uip_connr->ripaddr[1] = BUF->srcipaddr[1];
  uip_connr->tcpstateflags = SYN_RCVD;

  uip_connr->snd_nxt[0] = iss[0];
  uip_connr->snd_nxt[1] = iss[1];
  uip_connr->snd_nxt[2] = iss[2];
  uip_connr->snd_nxt[3] = iss[3];
  uip_connr->len = 1;

  /* rcv_nxt should be the seqno from the incoming packet + 1. */
  uip_connr->rcv_nxt[3] = BUF->seqno[3];
  uip_connr->rcv_nxt[2] = BUF->seqno[2];
  uip_connr->rcv_nxt[1] = BUF->seqno[1];
  uip_connr->rcv_nxt[0] = BUF->seqno[0];
  uip_add_rcv_nxt(1);

  /* Parse the TCP MSS option, if present. */
  if((BUF->tcpoffset & 0xf0) > 0x50) {
    for(c = 0; c < ((BUF->tcpoffset >> 4) - 5) << 2 ;) {
      opt = uip_buf[UIP_TCPIP_HLEN + UIP_LLH_LEN + c];
      if(opt == 0x00) {
	/* End of options. */	
	break;
      } else if(opt == 0x01) {
	++c;
	/* NOP option. */
      } else if(opt == 0x02 &&
		uip_buf[UIP_TCPIP_HLEN + UIP_LLH_LEN + 1 + c] == 0x04) {
	/* An MSS option with the right option length. */	
	tmp16 = ((u16_t)uip_buf[UIP_TCPIP_HLEN + UIP_LLH_LEN + 2 + c] << 8) |
	  (u16_t)uip_buf[40 + UIP_LLH_LEN + 3 + c];
	uip_connr->initialmss = uip_connr->mss =
	  tmp16 > UIP_TCP_MSS? UIP_TCP_MSS: tmp16;
	
	/* And we are done processing options. */
	break;
      } else {
	/* All other options have a length field, so that we easily
	   can skip past them. */
	if(uip_buf[UIP_TCPIP_HLEN + UIP_LLH_LEN + 1 + c] == 0) {
	  /* If the length field is zero, the options are malformed
	     and we don't process them further. */
	  break;
	}
	c += uip_buf[UIP_TCPIP_HLEN + UIP_LLH_LEN + 1 + c];
      }      
    }
  }
  
  /* Our response will be a SYNACK. */
#if UIP_ACTIVE_OPEN
 tcp_send_synack:
  BUF->flags = TCP_ACK;    
  
 tcp_send_syn:
  BUF->flags |= TCP_SYN;    
#else /* UIP_ACTIVE_OPEN */
 tcp_send_synack:
  BUF->flags = TCP_SYN | TCP_ACK;    
#endif /* UIP_ACTIVE_OPEN */
  
  /* We send out the TCP Maximum Segment Size option with our
     SYNACK. */
  BUF->optdata[0] = 2;
  BUF->optdata[1] = 4;
  BUF->optdata[2] = (UIP_TCP_MSS) / 256;
  BUF->optdata[3] = (UIP_TCP_MSS) & 255;
  uip_len = 44;
  BUF->tcpoffset = 6 << 4;
  goto tcp_send;

  /* This label will be jumped to if we found an active connection. */
 found:
  uip_conn = uip_connr;
  uip_flags = 0;

  /* We do a very naive form of TCP reset processing; we just accept
     any RST and kill our connection. We should in fact check if the
     sequence number of this reset is wihtin our advertised window
     before we accept the reset. */
  if(BUF->flags & TCP_RST) {
    uip_connr->tcpstateflags = CLOSED;
    UIP_LOG("tcp: got reset, aborting connection.");
    uip_flags = UIP_ABORT;
    UIP_APPCALL();
    goto drop;
  }      
  /* Calculated the length of the data, if the application has sent
     any data to us. */
  c = (BUF->tcpoffset >> 4) << 2;
  /* uip_len will contain the length of the actual TCP data. This is
     calculated by subtracing the length of the TCP header (in
     c) and the length of the IP header (20 bytes). */
  uip_len = uip_len - c - 20;

  /* First, check if the sequence number of the incoming packet is
     what we're expecting next. If not, we send out an ACK with the
     correct numbers in. */
  if(uip_len > 0 &&
     (BUF->seqno[0] != uip_connr->rcv_nxt[0] ||
      BUF->seqno[1] != uip_connr->rcv_nxt[1] ||
      BUF->seqno[2] != uip_connr->rcv_nxt[2] ||
      BUF->seqno[3] != uip_connr->rcv_nxt[3])) {
    goto tcp_send_ack;
  }

  /* Next, check if the incoming segment acknowledges any outstanding
     data. If so, we update the sequence number, reset the length of
     the outstanding data, calculate RTT estimations, and reset the
     retransmission timer. */
  if((BUF->flags & TCP_ACK) && uip_outstanding(uip_connr)) {
    uip_add32(uip_connr->snd_nxt, uip_connr->len);
    if(BUF->ackno[0] == uip_acc32[0] &&
       BUF->ackno[1] == uip_acc32[1] &&
       BUF->ackno[2] == uip_acc32[2] &&
       BUF->ackno[3] == uip_acc32[3]) {
      /* Update sequence number. */
      uip_connr->snd_nxt[0] = uip_acc32[0];
      uip_connr->snd_nxt[1] = uip_acc32[1];
      uip_connr->snd_nxt[2] = uip_acc32[2];
      uip_connr->snd_nxt[3] = uip_acc32[3];
	

      /* Do RTT estimation, unless we have done retransmissions. */
      if(uip_connr->nrtx == 0) {
	signed char m;
	m = uip_connr->rto - uip_connr->timer;
	/* This is taken directly from VJs original code in his paper */
	m = m - (uip_connr->sa >> 3);
	uip_connr->sa += m;
	if(m < 0) {
	  m = -m;
	}
	m = m - (uip_connr->sv >> 2);
	uip_connr->sv += m;
	uip_connr->rto = (uip_connr->sa >> 3) + uip_connr->sv;

      }
      /* Set the acknowledged flag. */
      uip_flags = UIP_ACKDATA;
      /* Reset the retransmission timer. */
      uip_connr->timer = uip_connr->rto;
    }
    
  }

  /* Do different things depending on in what state the connection is. */
  switch(uip_connr->tcpstateflags & TS_MASK) {
    /* CLOSED and LISTEN are not handled here. CLOSE_WAIT is not
	implemented, since we force the application to close when the
	peer sends a FIN (hence the application goes directly from
	ESTABLISHED to LAST_ACK). */
  case SYN_RCVD:
    /* In SYN_RCVD we have sent out a SYNACK in response to a SYN, and
       we are waiting for an ACK that acknowledges the data we sent
       out the last time. Therefore, we want to have the UIP_ACKDATA
       flag set. If so, we enter the ESTABLISHED state. */
    if(uip_flags & UIP_ACKDATA) {
      uip_connr->tcpstateflags = ESTABLISHED;
      uip_flags = UIP_CONNECTED;
      uip_connr->len = 0;
      if(uip_len > 0) {
        uip_flags |= UIP_NEWDATA;
        uip_add_rcv_nxt(uip_len);
      }
      uip_slen = 0;
      UIP_APPCALL();
      goto appsend;
    }
    goto drop;
#if UIP_ACTIVE_OPEN
  case SYN_SENT:
    /* In SYN_SENT, we wait for a SYNACK that is sent in response to
       our SYN. The rcv_nxt is set to sequence number in the SYNACK
       plus one, and we send an ACK. We move into the ESTABLISHED
       state. */
    if((uip_flags & UIP_ACKDATA) &&
       BUF->flags == (TCP_SYN | TCP_ACK)) {

      /* Parse the TCP MSS option, if present. */