uip.c 56.8 KB
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#define DEBUG_PRINTF(...) /*printf(__VA_ARGS__)*/

/**
 * \defgroup uip The uIP TCP/IP stack
 * @{
 *
 * uIP is an implementation of the TCP/IP protocol stack intended for
 * small 8-bit and 16-bit microcontrollers.
 *
 * uIP provides the necessary protocols for Internet communication,
 * with a very small code footprint and RAM requirements - the uIP
 * code size is on the order of a few kilobytes and RAM usage is on
 * the order of a few hundred bytes.
 */

/**
 * \file
 * The uIP TCP/IP stack code.
 * \author Adam Dunkels <adam@dunkels.com>
 */

/*
 * Copyright (c) 2001-2003, Adam Dunkels.
 * 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 uIP TCP/IP stack.
 *
 * $Id: uip.c,v 1.65 2006/06/11 21:46:39 adam Exp $
 *
 */

/*
 * uIP is a small implementation of the IP, UDP and TCP protocols (as
 * well as some basic ICMP stuff). The implementation couples the IP,
 * UDP, TCP and the application layers very tightly. To keep the size
 * of the compiled code down, this code frequently uses the goto
 * statement. While it would be possible to break the uip_process()
 * function into many smaller functions, this would increase the code
 * size because of the overhead of parameter passing and the fact that
 * the optimier would not be as efficient.
 *
 * The principle is that we have a small buffer, called the uip_buf,
 * in which the device driver puts an incoming packet. The TCP/IP
 * stack parses the headers in the packet, and calls the
 * application. If the remote host has sent data to the application,
 * this data is present in the uip_buf and the application read the
 * data from there. It is up to the application to put this data into
 * a byte stream if needed. The application will not be fed with data
 * that is out of sequence.
 *
 * If the application whishes to send data to the peer, it should put
 * its data into the uip_buf. The uip_appdata pointer points to the
 * first available byte. The TCP/IP stack will calculate the
 * checksums, and fill in the necessary header fields and finally send
 * the packet back to the peer.
*/

#include "uip.h"
#include "uipopt.h"
#include "uip_arch.h"

#if UIP_CONF_IPV6
#include "uip-neighbor.h"
#endif /* UIP_CONF_IPV6 */

#include <string.h>

/*---------------------------------------------------------------------------*/
/* Variable definitions. */


/* The IP address of this host. If it is defined to be fixed (by
   setting UIP_FIXEDADDR to 1 in uipopt.h), the address is set
   here. Otherwise, the address */
#if UIP_FIXEDADDR > 0
const uip_ipaddr_t uip_hostaddr =
  {HTONS((UIP_IPADDR0 << 8) | UIP_IPADDR1),
   HTONS((UIP_IPADDR2 << 8) | UIP_IPADDR3)};
const uip_ipaddr_t uip_draddr =
  {HTONS((UIP_DRIPADDR0 << 8) | UIP_DRIPADDR1),
   HTONS((UIP_DRIPADDR2 << 8) | UIP_DRIPADDR3)};
const uip_ipaddr_t uip_netmask =
  {HTONS((UIP_NETMASK0 << 8) | UIP_NETMASK1),
   HTONS((UIP_NETMASK2 << 8) | UIP_NETMASK3)};
#else
uip_ipaddr_t uip_hostaddr, uip_draddr, uip_netmask;
#endif /* UIP_FIXEDADDR */

static const uip_ipaddr_t all_ones_addr =
#if UIP_CONF_IPV6
  {0xffff,0xffff,0xffff,0xffff,0xffff,0xffff,0xffff,0xffff};
#else /* UIP_CONF_IPV6 */
  {0xffff,0xffff};
#endif /* UIP_CONF_IPV6 */
static const uip_ipaddr_t all_zeroes_addr =
#if UIP_CONF_IPV6
  {0x0000,0x0000,0x0000,0x0000,0x0000,0x0000,0x0000,0x0000};
#else /* UIP_CONF_IPV6 */
  {0x0000,0x0000};
#endif /* UIP_CONF_IPV6 */


#if UIP_FIXEDETHADDR
const struct uip_eth_addr uip_ethaddr = {{UIP_ETHADDR0,
					  UIP_ETHADDR1,
					  UIP_ETHADDR2,
					  UIP_ETHADDR3,
					  UIP_ETHADDR4,
					  UIP_ETHADDR5}};
#else
struct uip_eth_addr uip_ethaddr = {{0,0,0,0,0,0}};
#endif

#ifndef UIP_CONF_EXTERNAL_BUFFER
u8_t uip_buf[UIP_BUFSIZE + 2];   /* The packet buffer that contains
				    incoming packets. */
#endif /* UIP_CONF_EXTERNAL_BUFFER */

void *uip_appdata;               /* The uip_appdata pointer points to
				    application data. */
void *uip_sappdata;              /* The uip_appdata pointer points to
				    the application data which is to
				    be sent. */
#if UIP_URGDATA > 0
void *uip_urgdata;               /* The uip_urgdata pointer points to
   				    urgent data (out-of-band data), if
   				    present. */
u16_t uip_urglen, uip_surglen;
#endif /* UIP_URGDATA > 0 */

u16_t uip_len, uip_slen;
                             /* The uip_len is either 8 or 16 bits,
				depending on the maximum packet
				size. */

u8_t uip_flags;     /* The uip_flags variable is used for
				communication between the TCP/IP stack
				and the application program. */
struct uip_conn *uip_conn;   /* uip_conn always points to the current
				connection. */

struct uip_conn uip_conns[UIP_CONNS];
                             /* The uip_conns array holds all TCP
				connections. */
u16_t uip_listenports[UIP_LISTENPORTS];
                             /* The uip_listenports list all currently
				listning ports. */
#if UIP_UDP
struct uip_udp_conn *uip_udp_conn;
struct uip_udp_conn uip_udp_conns[UIP_UDP_CONNS];
#endif /* UIP_UDP */

static u16_t ipid;           /* Ths ipid variable is an increasing
				number that is used for the IP ID
				field. */

void uip_setipid(u16_t id) { ipid = id; }

static u8_t iss[4];          /* The iss variable is used for the TCP
				initial sequence number. */

#if UIP_ACTIVE_OPEN
static u16_t lastport;       /* Keeps track of the last port used for
				a new connection. */
#endif /* UIP_ACTIVE_OPEN */

/* Temporary variables. */
u8_t uip_acc32[4];
static u8_t c, opt;
static u16_t tmp16;

/* Structures and definitions. */
#define TCP_FIN 0x01
#define TCP_SYN 0x02
#define TCP_RST 0x04
#define TCP_PSH 0x08
#define TCP_ACK 0x10
#define TCP_URG 0x20
#define TCP_CTL 0x3f

#define TCP_OPT_END     0   /* End of TCP options list */
#define TCP_OPT_NOOP    1   /* "No-operation" TCP option */
#define TCP_OPT_MSS     2   /* Maximum segment size TCP option */

#define TCP_OPT_MSS_LEN 4   /* Length of TCP MSS option. */

#define ICMP_ECHO_REPLY 0
#define ICMP_ECHO       8

#define ICMP6_ECHO_REPLY             129
#define ICMP6_ECHO                   128
#define ICMP6_NEIGHBOR_SOLICITATION  135
#define ICMP6_NEIGHBOR_ADVERTISEMENT 136

#define ICMP6_FLAG_S (1 << 6)

#define ICMP6_OPTION_SOURCE_LINK_ADDRESS 1
#define ICMP6_OPTION_TARGET_LINK_ADDRESS 2


/* Macros. */
#define BUF ((struct uip_tcpip_hdr *)&uip_buf[UIP_LLH_LEN])
#define FBUF ((struct uip_tcpip_hdr *)&uip_reassbuf[0])
#define ICMPBUF ((struct uip_icmpip_hdr *)&uip_buf[UIP_LLH_LEN])
#define UDPBUF ((struct uip_udpip_hdr *)&uip_buf[UIP_LLH_LEN])


#if UIP_STATISTICS == 1
struct uip_stats uip_stat;
#define UIP_STAT(s) s
#else
#define UIP_STAT(s)
#endif /* UIP_STATISTICS == 1 */

#if UIP_LOGGING == 1
#include <stdio.h>
void uip_log(char *msg);
#define UIP_LOG(m) uip_log(m)
#else
#define UIP_LOG(m)
#endif /* UIP_LOGGING == 1 */

#if ! UIP_ARCH_ADD32
void
uip_add32(u8_t *op32, u16_t op16)
{
  uip_acc32[3] = op32[3] + (op16 & 0xff);
  uip_acc32[2] = op32[2] + (op16 >> 8);
  uip_acc32[1] = op32[1];
  uip_acc32[0] = op32[0];
  
  if(uip_acc32[2] < (op16 >> 8)) {
    ++uip_acc32[1];
    if(uip_acc32[1] == 0) {
      ++uip_acc32[0];
    }
  }
  
  
  if(uip_acc32[3] < (op16 & 0xff)) {
    ++uip_acc32[2];
    if(uip_acc32[2] == 0) {
      ++uip_acc32[1];
      if(uip_acc32[1] == 0) {
	++uip_acc32[0];
      }
    }
  }
}

#endif /* UIP_ARCH_ADD32 */

#if ! UIP_ARCH_CHKSUM
/*---------------------------------------------------------------------------*/
static u16_t
chksum(u16_t sum, const u8_t *data, u16_t len)
{
  u16_t t;
  const u8_t *dataptr;
  const u8_t *last_byte;

  dataptr = data;
  last_byte = data + len - 1;
  
  while(dataptr < last_byte) {	/* At least two more bytes */
    t = (dataptr[0] << 8) + dataptr[1];
    sum += t;
    if(sum < t) {
      sum++;		/* carry */
    }
    dataptr += 2;
  }
  
  if(dataptr == last_byte) {
    t = (dataptr[0] << 8) + 0;
    sum += t;
    if(sum < t) {
      sum++;		/* carry */
    }
  }

  /* Return sum in host byte order. */
  return sum;
}
/*---------------------------------------------------------------------------*/
u16_t
uip_chksum(u16_t *data, u16_t len)
{
  return htons(chksum(0, (u8_t *)data, len));
}
/*---------------------------------------------------------------------------*/
#ifndef UIP_ARCH_IPCHKSUM
u16_t
uip_ipchksum(void)
{
  u16_t sum;

  sum = chksum(0, &uip_buf[UIP_LLH_LEN], UIP_IPH_LEN);
  DEBUG_PRINTF("uip_ipchksum: sum 0x%04x\n", sum);
  return (sum == 0) ? 0xffff : htons(sum);
}
#endif
/*---------------------------------------------------------------------------*/
static u16_t
upper_layer_chksum(u8_t proto)
{
  u16_t upper_layer_len;
  u16_t sum;
  
#if UIP_CONF_IPV6
  upper_layer_len = (((u16_t)(BUF->len[0]) << 8) + BUF->len[1]);
#else /* UIP_CONF_IPV6 */
  upper_layer_len = (((u16_t)(BUF->len[0]) << 8) + BUF->len[1]) - UIP_IPH_LEN;
#endif /* UIP_CONF_IPV6 */
  
  /* First sum pseudoheader. */
  
  /* IP protocol and length fields. This addition cannot carry. */
  sum = upper_layer_len + proto;
  /* Sum IP source and destination addresses. */
  sum = chksum(sum, (u8_t *)&BUF->srcipaddr[0], 2 * sizeof(uip_ipaddr_t));

  /* Sum TCP header and data. */
  sum = chksum(sum, &uip_buf[UIP_IPH_LEN + UIP_LLH_LEN],
	       upper_layer_len);
    
  return (sum == 0) ? 0xffff : htons(sum);
}
/*---------------------------------------------------------------------------*/
#if UIP_CONF_IPV6
u16_t
uip_icmp6chksum(void)
{
  return upper_layer_chksum(UIP_PROTO_ICMP6);
  
}
#endif /* UIP_CONF_IPV6 */
/*---------------------------------------------------------------------------*/
u16_t
uip_tcpchksum(void)
{
  return upper_layer_chksum(UIP_PROTO_TCP);
}
/*---------------------------------------------------------------------------*/
#if UIP_UDP_CHECKSUMS
u16_t
uip_udpchksum(void)
{
  return upper_layer_chksum(UIP_PROTO_UDP);
}
#endif /* UIP_UDP_CHECKSUMS */
#endif /* UIP_ARCH_CHKSUM */
/*---------------------------------------------------------------------------*/
void
uip_init(void)
{
  for(c = 0; c < UIP_LISTENPORTS; ++c) {
    uip_listenports[c] = 0;
  }
  for(c = 0; c < UIP_CONNS; ++c) {
    uip_conns[c].tcpstateflags = UIP_CLOSED;
  }
#if UIP_ACTIVE_OPEN
  lastport = 1024;
#endif /* UIP_ACTIVE_OPEN */

#if UIP_UDP
  for(c = 0; c < UIP_UDP_CONNS; ++c) {
    uip_udp_conns[c].lport = 0;
  }
#endif /* UIP_UDP */
  

  /* IPv4 initialization. */
#if UIP_FIXEDADDR == 0
  /*  uip_hostaddr[0] = uip_hostaddr[1] = 0;*/
#endif /* UIP_FIXEDADDR */

}
/*---------------------------------------------------------------------------*/
#if UIP_ACTIVE_OPEN
struct uip_conn *
uip_connect(uip_ipaddr_t *ripaddr, u16_t rport)
{
  register struct uip_conn *conn, *cconn;
  
  /* Find an unused local port. */
 again:
  ++lastport;

  if(lastport >= 32000) {
    lastport = 4096;
  }

  /* Check if this port is already in use, and if so try to find
     another one. */
  for(c = 0; c < UIP_CONNS; ++c) {
    conn = &uip_conns[c];
    if(conn->tcpstateflags != UIP_CLOSED &&
       conn->lport == htons(lastport)) {
      goto again;
    }
  }

  conn = 0;
  for(c = 0; c < UIP_CONNS; ++c) {
    cconn = &uip_conns[c];
    if(cconn->tcpstateflags == UIP_CLOSED) {
      conn = cconn;
      break;
    }
    if(cconn->tcpstateflags == UIP_TIME_WAIT) {
      if(conn == 0 ||
	 cconn->timer > conn->timer) {
	conn = cconn;
      }
    }
  }

  if(conn == 0) {
    return 0;
  }
  
  conn->tcpstateflags = UIP_SYN_SENT;

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

  conn->initialmss = conn->mss = UIP_TCP_MSS;
  
  conn->len = 1;   /* TCP length of the SYN is one. */
  conn->nrtx = 0;
  conn->timer = 1; /* Send the SYN next time around. */
  conn->rto = UIP_RTO;
  conn->sa = 0;
  conn->sv = 16;   /* Initial value of the RTT variance. */
  conn->lport = htons(lastport);
  conn->rport = rport;
  uip_ipaddr_copy(&conn->ripaddr, ripaddr);
  
  return conn;
}
#endif /* UIP_ACTIVE_OPEN */
/*---------------------------------------------------------------------------*/
#if UIP_UDP
struct uip_udp_conn *
uip_udp_new(uip_ipaddr_t *ripaddr, u16_t rport)
{
  register struct uip_udp_conn *conn;
  
  /* Find an unused local port. */
 again:
  ++lastport;

  if(lastport >= 32000) {
    lastport = 4096;
  }
  
  for(c = 0; c < UIP_UDP_CONNS; ++c) {
    if(uip_udp_conns[c].lport == htons(lastport)) {
      goto again;
    }
  }


  conn = 0;
  for(c = 0; c < UIP_UDP_CONNS; ++c) {
    if(uip_udp_conns[c].lport == 0) {
      conn = &uip_udp_conns[c];
      break;
    }
  }

  if(conn == 0) {
    return 0;
  }
  
  conn->lport = HTONS(lastport);
  conn->rport = rport;
  if(ripaddr == NULL) {
    memset(conn->ripaddr, 0, sizeof(uip_ipaddr_t));
  } else {
    uip_ipaddr_copy(&conn->ripaddr, ripaddr);
  }
  conn->ttl = UIP_TTL;
  
  return conn;
}
#endif /* UIP_UDP */
/*---------------------------------------------------------------------------*/
void
uip_unlisten(u16_t port)
{
  for(c = 0; c < UIP_LISTENPORTS; ++c) {
    if(uip_listenports[c] == port) {
      uip_listenports[c] = 0;
      return;
    }
  }
}
/*---------------------------------------------------------------------------*/
void
uip_listen(u16_t port)
{
  for(c = 0; c < UIP_LISTENPORTS; ++c) {
    if(uip_listenports[c] == 0) {
      uip_listenports[c] = port;
      return;
    }
  }
}
/*---------------------------------------------------------------------------*/
/* XXX: IP fragment reassembly: not well-tested. */

#if UIP_REASSEMBLY && !UIP_CONF_IPV6
#define UIP_REASS_BUFSIZE (UIP_BUFSIZE - UIP_LLH_LEN)
static u8_t uip_reassbuf[UIP_REASS_BUFSIZE];
static u8_t uip_reassbitmap[UIP_REASS_BUFSIZE / (8 * 8)];
static const u8_t bitmap_bits[8] = {0xff, 0x7f, 0x3f, 0x1f,
				    0x0f, 0x07, 0x03, 0x01};
static u16_t uip_reasslen;
static u8_t uip_reassflags;
#define UIP_REASS_FLAG_LASTFRAG 0x01
static u8_t uip_reasstmr;

#define IP_MF   0x20

static u8_t
uip_reass(void)
{
  u16_t offset, len;
  u16_t i;

  /* If ip_reasstmr is zero, no packet is present in the buffer, so we
     write the IP header of the fragment into the reassembly
     buffer. The timer is updated with the maximum age. */
  if(uip_reasstmr == 0) {
    memcpy(uip_reassbuf, &BUF->vhl, UIP_IPH_LEN);
    uip_reasstmr = UIP_REASS_MAXAGE;
    uip_reassflags = 0;
    /* Clear the bitmap. */
    memset(uip_reassbitmap, 0, sizeof(uip_reassbitmap));
  }

  /* Check if the incoming fragment matches the one currently present
     in the reasembly buffer. If so, we proceed with copying the
     fragment into the buffer. */
  if(BUF->srcipaddr[0] == FBUF->srcipaddr[0] &&
     BUF->srcipaddr[1] == FBUF->srcipaddr[1] &&
     BUF->destipaddr[0] == FBUF->destipaddr[0] &&
     BUF->destipaddr[1] == FBUF->destipaddr[1] &&
     BUF->ipid[0] == FBUF->ipid[0] &&
     BUF->ipid[1] == FBUF->ipid[1]) {

    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[UIP_IPH_LEN + 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_REASSEMBLY */
/*---------------------------------------------------------------------------*/
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;

#if UIP_UDP
  if(flag == UIP_UDP_SEND_CONN) {
    goto udp_send;
  }
#endif /* UIP_UDP */
  
  uip_sappdata = uip_appdata = &uip_buf[UIP_IPTCPH_LEN + UIP_LLH_LEN];

  /* Check if we were invoked because of a poll request for a
     particular connection. */
  if(flag == UIP_POLL_REQUEST) {
    if((uip_connr->tcpstateflags & UIP_TS_MASK) == UIP_ESTABLISHED &&
       !uip_outstanding(uip_connr)) {
	uip_flags = UIP_POLL;
	UIP_APPCALL();
	goto appsend;
    }
    goto drop;
    
    /* Check if we were invoked because of the perodic timer fireing. */
  } else 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];
	}
      }
    }

    /* Reset the length variables. */
    uip_len = 0;
    uip_slen = 0;

    /* Check if the connection is in a state in which we simply wait
       for the connection to time out. If so, we increase the
       connection's timer and remove the connection if it times
       out. */
    if(uip_connr->tcpstateflags == UIP_TIME_WAIT ||
       uip_connr->tcpstateflags == UIP_FIN_WAIT_2) {
      ++(uip_connr->timer);
      if(uip_connr->timer == UIP_TIME_WAIT_TIMEOUT) {
	uip_connr->tcpstateflags = UIP_CLOSED;
      }
    } else if(uip_connr->tcpstateflags != UIP_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 == UIP_SYN_SENT ||
	       uip_connr->tcpstateflags == UIP_SYN_RCVD) &&
	      uip_connr->nrtx == UIP_MAXSYNRTX)) {
	    uip_connr->tcpstateflags = UIP_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 & UIP_TS_MASK) {
	  case UIP_SYN_RCVD:
	    /* In the SYN_RCVD state, we should retransmit our
               SYNACK. */
	    goto tcp_send_synack;
	    
#if UIP_ACTIVE_OPEN
	  case UIP_SYN_SENT:
	    /* In the SYN_SENT state, we retransmit out SYN. */
	    BUF->flags = 0;
	    goto tcp_send_syn;
#endif /* UIP_ACTIVE_OPEN */
	    
	  case UIP_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_flags = UIP_REXMIT;
	    UIP_APPCALL();
	    goto apprexmit;
	    
	  case UIP_FIN_WAIT_1:
	  case UIP_CLOSING:
	  case UIP_LAST_ACK:
	    /* In all these states we should retransmit a FINACK. */
	    goto tcp_send_finack;
	    
	  }
	}
      } else if((uip_connr->tcpstateflags & UIP_TS_MASK) == UIP_ESTABLISHED) {
	/* If there was no need for a retransmission, we poll the
           application for new data. */
	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_conn = NULL;
      uip_sappdata = uip_appdata = &uip_buf[UIP_LLH_LEN + UIP_IPUDPH_LEN];
      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 IP input header processing code. */
  
#if UIP_CONF_IPV6
  /* Check validity of the IP header. */
  if((BUF->vtc & 0xf0) != 0x60)  { /* IP version and header length. */
    UIP_STAT(++uip_stat.ip.drop);
    UIP_STAT(++uip_stat.ip.vhlerr);
    UIP_LOG("ipv6: invalid version.");
    goto drop;
  }
#else /* UIP_CONF_IPV6 */
  /* 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;
  }
#endif /* UIP_CONF_IPV6 */
  
  /* Check the size of the packet. If the size reported to us in
     uip_len is smaller the size reported in the IP header, we assume
     that the packet has been corrupted in transit. If the size of
     uip_len is larger than the size reported in the IP packet header,
     the packet has been padded and we set uip_len to the correct
     value.. */

  if((BUF->len[0] << 8) + BUF->len[1] <= uip_len) {
    uip_len = (BUF->len[0] << 8) + BUF->len[1];
#if UIP_CONF_IPV6
    uip_len += 40; /* The length reported in the IPv6 header is the
		      length of the payload that follows the
		      header. However, uIP uses the uip_len variable
		      for holding the size of the entire packet,
		      including the IP header. For IPv4 this is not a
		      problem as the length field in the IPv4 header
		      contains the length of the entire packet. But
		      for IPv6 we need to add the size of the IPv6
		      header (40 bytes). */
#endif /* UIP_CONF_IPV6 */
  } else {
    UIP_LOG("ip: packet shorter than reported in IP header.");
    goto drop;
  }

#if !UIP_CONF_IPV6
  /* 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_REASSEMBLY */
    UIP_STAT(++uip_stat.ip.drop);
    UIP_STAT(++uip_stat.ip.fragerr);
    UIP_LOG("ip: fragment dropped.");
    goto drop;
#endif /* UIP_REASSEMBLY */
  }
#endif /* UIP_CONF_IPV6 */

  if(uip_ipaddr_cmp(uip_hostaddr, all_zeroes_addr)) {
    /* 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 && !UIP_CONF_IPV6
    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 */

  } else {
    /* If IP broadcast support is configured, we check for a broadcast
       UDP packet, which may be destined to us. */
#if UIP_BROADCAST
    DEBUG_PRINTF("UDP IP checksum 0x%04x\n", uip_ipchksum());
    if(BUF->proto == UIP_PROTO_UDP &&
       uip_ipaddr_cmp(BUF->destipaddr, all_ones_addr)
       /*&&
	 uip_ipchksum() == 0xffff*/) {
      goto udp_input;
    }
#endif /* UIP_BROADCAST */
    
    /* Check if the packet is destined for our IP address. */
#if !UIP_CONF_IPV6
    if(!uip_ipaddr_cmp(BUF->destipaddr, uip_hostaddr)) {
      UIP_STAT(++uip_stat.ip.drop);
      goto drop;
    }
#else /* UIP_CONF_IPV6 */
    /* For IPv6, packet reception is a little trickier as we need to
       make sure that we listen to certain multicast addresses (all
       hosts multicast address, and the solicited-node multicast
       address) as well. However, we will cheat here and accept all
       multicast packets that are sent to the ff02::/16 addresses. */
    if(!uip_ipaddr_cmp(BUF->destipaddr, uip_hostaddr) &&
       BUF->destipaddr[0] != HTONS(0xff02)) {
      UIP_STAT(++uip_stat.ip.drop);
      goto drop;
    }
#endif /* UIP_CONF_IPV6 */
  }

#if !UIP_CONF_IPV6
  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;
  }
#endif /* UIP_CONF_IPV6 */

  if(BUF->proto == UIP_PROTO_TCP) { /* Check for TCP packet. If so,
				       proceed with TCP input
				       processing. */
    goto tcp_input;
  }

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

#if !UIP_CONF_IPV6
  /* ICMPv4 processing code follows. */
  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;
  }

#if UIP_PINGADDRCONF
 icmp_input:
#endif /* UIP_PINGADDRCONF */
  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 {
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