text: /**
   * @file
   * This is the IPv4 layer implementation for incoming and outgoing IP traffic.
   *
   * @see ip_frag.c
   *
   */
  
  /*
   * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
   * Copyright (c) 2019 Kai-Björn Gemlau, Institute of Computer and Network Engineering, TU Braunschweig
   *
   * 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: Adam Dunkels <adam@sics.se>
   *
   */
  
  #include "lwip/opt.h"
  
  #if LWIP_IPV4
  
  #include "lwip/ip.h"
  #include "lwip/def.h"
  #include "lwip/mem.h"
  //#include "lwip/ip4_frag.h"
  #include "lwip/inet_chksum.h"
  #include "lwip/netif.h"
  //#include "lwip/icmp.h"
  //#include "lwip/igmp.h"
  //#include "lwip/priv/raw_priv.h"
  #include "lwip/udp.h"
  //#include "lwip/priv/tcp_priv.h"
  //#include "lwip/autoip.h"
  #include "lwip/stats.h"
  #include "lwip/prot/iana.h"
  #include "ida-lwip/ida_lwip_igmp.h"
  
  #include <string.h>
  
  #if !LWIP_SINGLE_NETIF
  #error "IDA-Lwip only supports one network interface"
  #endif
  
  #if IP_REASSEMBLY
  #error "IDA-Lwip does not support packet reassembly"
  #endif
  
  #ifdef LWIP_HOOK_FILENAME
  #include LWIP_HOOK_FILENAME
  #endif
  
  /** Set this to 0 in the rare case of wanting to call an extra function to
   * generate the IP checksum (in contrast to calculating it on-the-fly). */
  #ifndef LWIP_INLINE_IP_CHKSUM
  #if LWIP_CHECKSUM_CTRL_PER_NETIF
  #define LWIP_INLINE_IP_CHKSUM   0
  #else /* LWIP_CHECKSUM_CTRL_PER_NETIF */
  #define LWIP_INLINE_IP_CHKSUM   1
  #endif /* LWIP_CHECKSUM_CTRL_PER_NETIF */
  #endif
  
  #if LWIP_INLINE_IP_CHKSUM && CHECKSUM_GEN_IP
  #define CHECKSUM_GEN_IP_INLINE  1
  #else
  #define CHECKSUM_GEN_IP_INLINE  0
  #endif
  
  #if LWIP_DHCP
  #error "IDA-Lwip does not support DHCP"
  #endif
  
  #if LWIP_AUTOIP
  #error "IDA-Lwip does not support AUTOIP"
  #endif
  
  #if IP_ACCEPT_LINK_LAYER_ADDRESSING == 1
  #error "IDA-Lwip does not support IP-Link-layer-Addressing"
  #endif
  
  #if IP_FORWARD
  #error "IDA-Lwip does not support IP forwarding"
  #endif
  
  #if LWIP_NETIF_USE_HINTS
  #error "IDA-Lwip does not support LWIP_NETIF_USE_HINTS"
  #endif
  
  /** The IP header ID of the next outgoing IP packet */
  static u16_t ip_id;
  
  #if LWIP_MULTICAST_TX_OPTIONS
  /** The default netif used for multicast */
  static struct netif *ip4_default_multicast_netif;
  
  /**
   * @ingroup ip4
   * Set a default netif for IPv4 multicast. */
  void
  ip4_set_default_multicast_netif(struct netif *default_multicast_netif)
  {
    ip4_default_multicast_netif = default_multicast_netif;
  }
  #endif /* LWIP_MULTICAST_TX_OPTIONS */
  
  #ifdef LWIP_HOOK_IP4_ROUTE_SRC
  #error "Source based IPv4 Routing not part of ida-lwip"
  #endif /* LWIP_HOOK_IP4_ROUTE_SRC */
  
  /**
   * Finds the appropriate network interface for a given IP address. It
   * searches the list of network interfaces linearly. A match is found
   * if the masked IP address of the network interface equals the masked
   * IP address given to the function.
   *
   * @param dest the destination IP address for which to find the route
   * @return the netif on which to send to reach dest
   */
  struct netif *
  ip4_route(const ip4_addr_t *dest)
  {
    if ((netif_default == NULL) || !netif_is_up(netif_default) || !netif_is_link_up(netif_default) ||
        ip4_addr_isany_val(*netif_ip4_addr(netif_default)) || ip4_addr_isloopback(dest)) {
      /* No matching netif found and default netif is not usable.
         If this is not good enough for you, use LWIP_HOOK_IP4_ROUTE() */
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_route: No route to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
                  ip4_addr1_16(dest), ip4_addr2_16(dest), ip4_addr3_16(dest), ip4_addr4_16(dest)));
      IP_STATS_INC(ip.rterr);
      MIB2_STATS_INC(mib2.ipoutnoroutes);
      return NULL;
    }
  
    return netif_default;
  }
  
  /** Return true if the current input packet should be accepted on this netif */
  static int
  ip4_input_accept(struct netif *netif)
  {
    LWIP_DEBUGF(IP_DEBUG, ("ip_input: iphdr->dest 0x%"X32_F" netif->ip_addr 0x%"X32_F" (0x%"X32_F", 0x%"X32_F", 0x%"X32_F")\n",
                           ip4_addr_get_u32(ip4_current_dest_addr()), ip4_addr_get_u32(netif_ip4_addr(netif)),
                           ip4_addr_get_u32(ip4_current_dest_addr()) & ip4_addr_get_u32(netif_ip4_netmask(netif)),
                           ip4_addr_get_u32(netif_ip4_addr(netif)) & ip4_addr_get_u32(netif_ip4_netmask(netif)),
                           ip4_addr_get_u32(ip4_current_dest_addr()) & ~ip4_addr_get_u32(netif_ip4_netmask(netif))));
  
    /* interface is up and configured? */
    if ((netif_is_up(netif)) && (!ip4_addr_isany_val(*netif_ip4_addr(netif)))) {
      /* unicast to this interface address? */
      if (ip4_addr_cmp(ip4_current_dest_addr(), netif_ip4_addr(netif)) ||
          /* or broadcast on this interface network address? */
          ip4_addr_isbroadcast(ip4_current_dest_addr(), netif)
  #if LWIP_NETIF_LOOPBACK && !LWIP_HAVE_LOOPIF
          || (ip4_addr_get_u32(ip4_current_dest_addr()) == PP_HTONL(IPADDR_LOOPBACK))
  #endif /* LWIP_NETIF_LOOPBACK && !LWIP_HAVE_LOOPIF */
         ) {
        LWIP_DEBUGF(IP_DEBUG, ("ip4_input: packet accepted on interface %c%c\n",
                               netif->name[0], netif->name[1]));
        /* accept on this netif */
        return 1;
      }
    }
    return 0;
  }
  
  /**
   * This function is called by the network interface device driver when
   * an IP packet is received. The function does the basic checks of the
   * IP header such as packet size being at least larger than the header
   * size etc. If the packet was not destined for us, the packet is
   * forwarded (using ip_forward). The IP checksum is always checked.
   *
   * Finally, the packet is sent to the upper layer protocol input function.
   *
   * @param p the received IP packet (p->payload points to IP header)
   * @param inp the netif on which this packet was received
   * @return ERR_OK if the packet was processed (could return ERR_* if it wasn't
   *         processed, but currently always returns ERR_OK)
   */
  err_t
  ip4_input(struct pbuf *p, struct netif *inp)
  {
    const struct ip_hdr *iphdr;
    struct netif *netif;
    u16_t iphdr_hlen;
    u16_t iphdr_len;
    int check_ip_src = 1;
  
    IP_STATS_INC(ip.recv);
    MIB2_STATS_INC(mib2.ipinreceives);
  
    /* identify the IP header */
    iphdr = (struct ip_hdr *)p->payload;
    if (IPH_V(iphdr) != 4) {
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_WARNING, ("IP packet dropped due to bad version number %"U16_F"\n", (u16_t)IPH_V(iphdr)));
      ip4_debug_print(p);
      pbuf_free(p);
      IP_STATS_INC(ip.err);
      IP_STATS_INC(ip.drop);
      MIB2_STATS_INC(mib2.ipinhdrerrors);
      return ERR_OK;
    }
  
    /* obtain IP header length in bytes */
    iphdr_hlen = IPH_HL_BYTES(iphdr);
    /* obtain ip length in bytes */
    iphdr_len = lwip_ntohs(IPH_LEN(iphdr));
  
    /* Trim pbuf. This is especially required for packets < 60 bytes. */
    if (iphdr_len < p->tot_len) {
      pbuf_realloc(p, iphdr_len);
    }
  
    /* header length exceeds first pbuf length, or ip length exceeds total pbuf length? */
    if ((iphdr_hlen > p->len) || (iphdr_len > p->tot_len) || (iphdr_hlen < IP_HLEN)) {
      if (iphdr_hlen < IP_HLEN) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
                    ("ip4_input: short IP header (%"U16_F" bytes) received, IP packet dropped\n", iphdr_hlen));
      }
      if (iphdr_hlen > p->len) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
                    ("IP header (len %"U16_F") does not fit in first pbuf (len %"U16_F"), IP packet dropped.\n",
                     iphdr_hlen, p->len));
      }
      if (iphdr_len > p->tot_len) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
                    ("IP (len %"U16_F") is longer than pbuf (len %"U16_F"), IP packet dropped.\n",
                     iphdr_len, p->tot_len));
      }
      /* free (drop) packet pbufs */
      pbuf_free(p);
      IP_STATS_INC(ip.lenerr);
      IP_STATS_INC(ip.drop);
      MIB2_STATS_INC(mib2.ipindiscards);
      return ERR_OK;
    }
  
    /* verify checksum */
  #if CHECKSUM_CHECK_IP
    IF__NETIF_CHECKSUM_ENABLED(inp, NETIF_CHECKSUM_CHECK_IP) {
      if (inet_chksum(iphdr, iphdr_hlen) != 0) {
  
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
                    ("Checksum (0x%"X16_F") failed, IP packet dropped.\n", inet_chksum(iphdr, iphdr_hlen)));
        ip4_debug_print(p);
        pbuf_free(p);
        IP_STATS_INC(ip.chkerr);
        IP_STATS_INC(ip.drop);
        MIB2_STATS_INC(mib2.ipinhdrerrors);
        return ERR_OK;
      }
    }
  #endif
  
    /* copy IP addresses to aligned ip_addr_t */
    ip_addr_copy_from_ip4(ip_data.current_iphdr_dest, iphdr->dest);
    ip_addr_copy_from_ip4(ip_data.current_iphdr_src, iphdr->src);
  
    /* match packet against an interface, i.e. is this packet for us? */
    if (ip4_addr_ismulticast(ip4_current_dest_addr())) {
      if ((inp->flags & NETIF_FLAG_IGMP) && (ida_lwip_igmp_is_member(ip4_current_dest_addr()) >= 0)) {
        /* IGMP snooping switches need 0.0.0.0 to be allowed as source address (RFC 4541) */
        ip4_addr_t allsystems;
        IP4_ADDR(&allsystems, 224, 0, 0, 1);
        if (ip4_addr_cmp(ip4_current_dest_addr(), &allsystems) &&
            ip4_addr_isany(ip4_current_src_addr())) {
          check_ip_src = 0;
        }
        netif = inp;
      } else {
        netif = NULL;
      }
    } else {
      /* start trying with inp. if that's not acceptable, start walking the
         list of configured netifs. */
      if (ip4_input_accept(inp)) {
        netif = inp;
      } else {
        netif = NULL;
      }
    }
  
    /* broadcast or multicast packet source address? Compliant with RFC 1122: 3.2.1.3 */
    if (check_ip_src)
    {
      if ((ip4_addr_isbroadcast(ip4_current_src_addr(), inp)) ||
          (ip4_addr_ismulticast(ip4_current_src_addr()))) {
        /* packet source is not valid */
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_WARNING, ("ip4_input: packet source is not valid.\n"));
        /* free (drop) packet pbufs */
        pbuf_free(p);
        IP_STATS_INC(ip.drop);
        MIB2_STATS_INC(mib2.ipinaddrerrors);
        MIB2_STATS_INC(mib2.ipindiscards);
        return ERR_OK;
      }
    }
  
    /* packet not for us? */
    if (netif == NULL) {
      /* packet not for us, route or discard */
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_TRACE, ("ip4_input: packet not for us.\n"));
      return ERR_NOTUS;
    }
    /* packet consists of multiple fragments? */
    if ((IPH_OFFSET(iphdr) & PP_HTONS(IP_OFFMASK | IP_MF)) != 0) {
      pbuf_free(p);
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("IP packet dropped since it was fragmented (0x%"X16_F") (while IP_REASSEMBLY == 0).\n",
                  lwip_ntohs(IPH_OFFSET(iphdr))));
      IP_STATS_INC(ip.opterr);
      IP_STATS_INC(ip.drop);
      /* unsupported protocol feature */
      MIB2_STATS_INC(mib2.ipinunknownprotos);
      return ERR_OK;
    }
  
  #if IP_OPTIONS_ALLOWED == 0 /* no support for IP options in the IP header? */
  
    /* there is an extra "router alert" option in IGMP messages which we allow for but do not police */
    if ((iphdr_hlen > IP_HLEN) &&  (IPH_PROTO(iphdr) != IP_PROTO_IGMP)) {
      LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("IP packet dropped since there were IP options (while IP_OPTIONS_ALLOWED == 0).\n"));
      pbuf_free(p);
      IP_STATS_INC(ip.opterr);
      IP_STATS_INC(ip.drop);
      /* unsupported protocol feature */
      MIB2_STATS_INC(mib2.ipinunknownprotos);
      return ERR_OK;
    }
  #endif /* IP_OPTIONS_ALLOWED == 0 */
  
    /* send to upper layers */
    LWIP_DEBUGF(IP_DEBUG, ("ip4_input: \n"));
    ip4_debug_print(p);
    LWIP_DEBUGF(IP_DEBUG, ("ip4_input: p->len %"U16_F" p->tot_len %"U16_F"\n", p->len, p->tot_len));
  
    ip_data.current_netif = netif;
    ip_data.current_input_netif = inp;							// | todo: thread safe
    ip_data.current_ip4_header = iphdr;							// | ip_data is global variable
    ip_data.current_ip_header_tot_len = IPH_HL_BYTES(iphdr);		// |
  
      pbuf_remove_header(p, iphdr_hlen); /* Move to payload, no check necessary. */
  
      switch (IPH_PROTO(iphdr)) {
  #if LWIP_UDP
        case IP_PROTO_UDP:
          MIB2_STATS_INC(mib2.ipindelivers);
          return udp_input(p, inp);
          break;
  #endif /* LWIP_UDP */
  
        default:
  		LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("Unsupported transport protocol %"U16_F"\n", (u16_t)IPH_PROTO(iphdr)));
  		IP_STATS_INC(ip.proterr);
  		IP_STATS_INC(ip.drop);
  		MIB2_STATS_INC(mib2.ipinunknownprotos);
          LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("DEBUG: ip4_input: ERR_NOTUS weil DEFAULT\n"));
          return ERR_NOTUS;
  
          break;
      }
  
    return ERR_OK;
  }
  
  /**
   * Sends an IP packet on a network interface. This function constructs
   * the IP header and calculates the IP header checksum. If the source
   * IP address is NULL, the IP address of the outgoing network
   * interface is filled in as source address.
   * If the destination IP address is LWIP_IP_HDRINCL, p is assumed to already
   * include an IP header and p->payload points to it instead of the data.
   *
   * @param p the packet to send (p->payload points to the data, e.g. next
              protocol header; if dest == LWIP_IP_HDRINCL, p already includes an
              IP header and p->payload points to that IP header)
   * @param src the source IP address to send from (if src == IP4_ADDR_ANY, the
   *         IP  address of the netif used to send is used as source address)
   * @param dest the destination IP address to send the packet to
   * @param ttl the TTL value to be set in the IP header
   * @param tos the TOS value to be set in the IP header
   * @param proto the PROTOCOL to be set in the IP header
   * @param netif the netif on which to send this packet
   * @return ERR_OK if the packet was sent OK
   *         ERR_BUF if p doesn't have enough space for IP/LINK headers
   *         returns errors returned by netif->output
   *
   * @note ip_id: RFC791 "some host may be able to simply use
   *  unique identifiers independent of destination"
   */
  err_t
  ip4_output_if(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
                u8_t ttl, u8_t tos,
                u8_t proto, struct netif *netif)
  {
    return ip4_output_if_opt(p, src, dest, ttl, tos, proto, netif, NULL, 0);
  }
  
  /**
   * Same as ip_output_if() but with the possibility to include IP options:
   *
   * @ param ip_options pointer to the IP options, copied into the IP header
   * @ param optlen length of ip_options
   */
  err_t
  ip4_output_if_opt(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
                    u8_t ttl, u8_t tos, u8_t proto, struct netif *netif, void *ip_options,
                    u16_t optlen)
  {
    const ip4_addr_t *src_used = src;
    if (dest != LWIP_IP_HDRINCL) {
      if (ip4_addr_isany(src)) {
        src_used = netif_ip4_addr(netif);
      }
    }
  
    return ip4_output_if_opt_src(p, src_used, dest, ttl, tos, proto, netif,
                                 ip_options, optlen);
  }
  
  /**
   * Same as ip_output_if() but 'src' address is not replaced by netif address
   * when it is 'any'.
   */
  err_t
  ip4_output_if_src(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
                    u8_t ttl, u8_t tos,
                    u8_t proto, struct netif *netif)
  {
    return ip4_output_if_opt_src(p, src, dest, ttl, tos, proto, netif, NULL, 0);
  }
  
  /**
   * Same as ip_output_if_opt() but 'src' address is not replaced by netif address
   * when it is 'any'.
   */
  err_t
  ip4_output_if_opt_src(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
                        u8_t ttl, u8_t tos, u8_t proto, struct netif *netif, void *ip_options,
                        u16_t optlen)
  {
    struct ip_hdr *iphdr;
    ip4_addr_t dest_addr;
  #if CHECKSUM_GEN_IP_INLINE
    u32_t chk_sum = 0;
  #endif /* CHECKSUM_GEN_IP_INLINE */
  
    LWIP_ASSERT_CORE_LOCKED();
    LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
  
    MIB2_STATS_INC(mib2.ipoutrequests);
  
    /* Should the IP header be generated or is it already included in p? */
    if (dest != LWIP_IP_HDRINCL) {
      u16_t ip_hlen = IP_HLEN;
  #if IP_OPTIONS_SEND
      u16_t optlen_aligned = 0;
      if (optlen != 0) {
  #if CHECKSUM_GEN_IP_INLINE
        int i;
  #endif /* CHECKSUM_GEN_IP_INLINE */
        if (optlen > (IP_HLEN_MAX - IP_HLEN)) {
          /* optlen too long */
          LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output_if_opt: optlen too long\n"));
          IP_STATS_INC(ip.err);
          MIB2_STATS_INC(mib2.ipoutdiscards);
          return ERR_VAL;
        }
        /* round up to a multiple of 4 */
        optlen_aligned = (u16_t)((optlen + 3) & ~3);
        ip_hlen = (u16_t)(ip_hlen + optlen_aligned);
        /* First write in the IP options */
        if (pbuf_add_header(p, optlen_aligned)) {
          LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output_if_opt: not enough room for IP options in pbuf\n"));
          IP_STATS_INC(ip.err);
          MIB2_STATS_INC(mib2.ipoutdiscards);
          return ERR_BUF;
        }
        MEMCPY(p->payload, ip_options, optlen);
        if (optlen < optlen_aligned) {
          /* zero the remaining bytes */
          memset(((char *)p->payload) + optlen, 0, (size_t)(optlen_aligned - optlen));
        }
  #if CHECKSUM_GEN_IP_INLINE
        for (i = 0; i < optlen_aligned / 2; i++) {
          chk_sum += ((u16_t *)p->payload)[i];
        }
  #endif /* CHECKSUM_GEN_IP_INLINE */
      }
  #endif /* IP_OPTIONS_SEND */
      /* generate IP header */
      if (pbuf_add_header(p, IP_HLEN)) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output: not enough room for IP header in pbuf\n"));
  
        IP_STATS_INC(ip.err);
        MIB2_STATS_INC(mib2.ipoutdiscards);
        return ERR_BUF;
      }
  
      iphdr = (struct ip_hdr *)p->payload;
      LWIP_ASSERT("check that first pbuf can hold struct ip_hdr",
                  (p->len >= sizeof(struct ip_hdr)));
  
      IPH_TTL_SET(iphdr, ttl);
      IPH_PROTO_SET(iphdr, proto);
  #if CHECKSUM_GEN_IP_INLINE
      chk_sum += PP_NTOHS(proto | (ttl << 8));
  #endif /* CHECKSUM_GEN_IP_INLINE */
  
      /* dest cannot be NULL here */
      ip4_addr_copy(iphdr->dest, *dest);
  #if CHECKSUM_GEN_IP_INLINE
      chk_sum += ip4_addr_get_u32(&iphdr->dest) & 0xFFFF;
      chk_sum += ip4_addr_get_u32(&iphdr->dest) >> 16;
  #endif /* CHECKSUM_GEN_IP_INLINE */
  
      IPH_VHL_SET(iphdr, 4, ip_hlen / 4);
      IPH_TOS_SET(iphdr, tos);
  #if CHECKSUM_GEN_IP_INLINE
      chk_sum += PP_NTOHS(tos | (iphdr->_v_hl << 8));
  #endif /* CHECKSUM_GEN_IP_INLINE */
      IPH_LEN_SET(iphdr, lwip_htons(p->tot_len));
  #if CHECKSUM_GEN_IP_INLINE
      chk_sum += iphdr->_len;
  #endif /* CHECKSUM_GEN_IP_INLINE */
      IPH_OFFSET_SET(iphdr, 0);
      IPH_ID_SET(iphdr, lwip_htons(ip_id));
  #if CHECKSUM_GEN_IP_INLINE
      chk_sum += iphdr->_id;
  #endif /* CHECKSUM_GEN_IP_INLINE */
      ++ip_id;
  
      if (src == NULL) {
        ip4_addr_copy(iphdr->src, *IP4_ADDR_ANY4);
      } else {
        /* src cannot be NULL here */
        ip4_addr_copy(iphdr->src, *src);
      }
  
  #if CHECKSUM_GEN_IP_INLINE
      chk_sum += ip4_addr_get_u32(&iphdr->src) & 0xFFFF;
      chk_sum += ip4_addr_get_u32(&iphdr->src) >> 16;
      chk_sum = (chk_sum >> 16) + (chk_sum & 0xFFFF);
      chk_sum = (chk_sum >> 16) + chk_sum;
      chk_sum = ~chk_sum;
      IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_IP) {
        iphdr->_chksum = (u16_t)chk_sum; /* network order */
      }
  #if LWIP_CHECKSUM_CTRL_PER_NETIF
      else {
        IPH_CHKSUM_SET(iphdr, 0);
      }
  #endif /* LWIP_CHECKSUM_CTRL_PER_NETIF*/
  #else /* CHECKSUM_GEN_IP_INLINE */
      IPH_CHKSUM_SET(iphdr, 0);
  #if CHECKSUM_GEN_IP
      IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_IP) {
        IPH_CHKSUM_SET(iphdr, inet_chksum(iphdr, ip_hlen));
      }
  #endif /* CHECKSUM_GEN_IP */
  #endif /* CHECKSUM_GEN_IP_INLINE */
    } else {
      /* IP header already included in p */
      if (p->len < IP_HLEN) {
        LWIP_DEBUGF(IP_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("ip4_output: LWIP_IP_HDRINCL but pbuf is too short\n"));
        IP_STATS_INC(ip.err);
        MIB2_STATS_INC(mib2.ipoutdiscards);
        return ERR_BUF;
      }
      iphdr = (struct ip_hdr *)p->payload;
      ip4_addr_copy(dest_addr, iphdr->dest);
      dest = &dest_addr;
    }
  
    IP_STATS_INC(ip.xmit);
  
    LWIP_DEBUGF(IP_DEBUG, ("ip4_output_if: %c%c%"U16_F"\n", netif->name[0], netif->name[1], (u16_t)netif->num));
    ip4_debug_print(p);
  
  #if ENABLE_LOOPBACK
    if (ip4_addr_cmp(dest, netif_ip4_addr(netif))
  #if !LWIP_HAVE_LOOPIF
        || ip4_addr_isloopback(dest)
  #endif /* !LWIP_HAVE_LOOPIF */
       ) {
      /* Packet to self, enqueue it for loopback */
      LWIP_DEBUGF(IP_DEBUG, ("netif_loop_output()"));
      return netif_loop_output(netif, p);
    }
  #if LWIP_MULTICAST_TX_OPTIONS
    if ((p->flags & PBUF_FLAG_MCASTLOOP) != 0) {
      netif_loop_output(netif, p);
    }
  #endif /* LWIP_MULTICAST_TX_OPTIONS */
  #endif /* ENABLE_LOOPBACK */
  
    LWIP_DEBUGF(IP_DEBUG, ("ip4_output_if: call netif->output()\n"));
    return netif->output(netif, p, dest);
  }
  
  /**
   * Simple interface to ip_output_if. It finds the outgoing network
   * interface and calls upon ip_output_if to do the actual work.
   *
   * @param p the packet to send (p->payload points to the data, e.g. next
              protocol header; if dest == LWIP_IP_HDRINCL, p already includes an
              IP header and p->payload points to that IP header)
   * @param src the source IP address to send from (if src == IP4_ADDR_ANY, the
   *         IP  address of the netif used to send is used as source address)
   * @param dest the destination IP address to send the packet to
   * @param ttl the TTL value to be set in the IP header
   * @param tos the TOS value to be set in the IP header
   * @param proto the PROTOCOL to be set in the IP header
   *
   * @return ERR_RTE if no route is found
   *         see ip_output_if() for more return values
   */
  err_t
  ip4_output(struct pbuf *p, const ip4_addr_t *src, const ip4_addr_t *dest,
             u8_t ttl, u8_t tos, u8_t proto)
  {
    struct netif *netif;
  
    LWIP_IP_CHECK_PBUF_REF_COUNT_FOR_TX(p);
  
    if ((netif = ip4_route_src(src, dest)) == NULL) {
      LWIP_DEBUGF(IP_DEBUG, ("ip4_output: No route to %"U16_F".%"U16_F".%"U16_F".%"U16_F"\n",
                             ip4_addr1_16(dest), ip4_addr2_16(dest), ip4_addr3_16(dest), ip4_addr4_16(dest)));
      IP_STATS_INC(ip.rterr);
      return ERR_RTE;
    }
  
    return ip4_output_if(p, src, dest, ttl, tos, proto, netif);
  }
  
  #if IP_DEBUG
  /* Print an IP header by using LWIP_DEBUGF
   * @param p an IP packet, p->payload pointing to the IP header
   */
  void
  ip4_debug_print(struct pbuf *p)
  {
    struct ip_hdr *iphdr = (struct ip_hdr *)p->payload;
  
    LWIP_DEBUGF(IP_DEBUG, ("IP header:\n"));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|%2"S16_F" |%2"S16_F" |  0x%02"X16_F" |     %5"U16_F"     | (v, hl, tos, len)\n",
                           (u16_t)IPH_V(iphdr),
                           (u16_t)IPH_HL(iphdr),
                           (u16_t)IPH_TOS(iphdr),
                           lwip_ntohs(IPH_LEN(iphdr))));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|    %5"U16_F"      |%"U16_F"%"U16_F"%"U16_F"|    %4"U16_F"   | (id, flags, offset)\n",
                           lwip_ntohs(IPH_ID(iphdr)),
                           (u16_t)(lwip_ntohs(IPH_OFFSET(iphdr)) >> 15 & 1),
                           (u16_t)(lwip_ntohs(IPH_OFFSET(iphdr)) >> 14 & 1),
                           (u16_t)(lwip_ntohs(IPH_OFFSET(iphdr)) >> 13 & 1),
                           (u16_t)(lwip_ntohs(IPH_OFFSET(iphdr)) & IP_OFFMASK)));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|  %3"U16_F"  |  %3"U16_F"  |    0x%04"X16_F"     | (ttl, proto, chksum)\n",
                           (u16_t)IPH_TTL(iphdr),
                           (u16_t)IPH_PROTO(iphdr),
                           lwip_ntohs(IPH_CHKSUM(iphdr))));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  | (src)\n",
                           ip4_addr1_16_val(iphdr->src),
                           ip4_addr2_16_val(iphdr->src),
                           ip4_addr3_16_val(iphdr->src),
                           ip4_addr4_16_val(iphdr->src)));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
    LWIP_DEBUGF(IP_DEBUG, ("|  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  |  %3"U16_F"  | (dest)\n",
                           ip4_addr1_16_val(iphdr->dest),
                           ip4_addr2_16_val(iphdr->dest),
                           ip4_addr3_16_val(iphdr->dest),
                           ip4_addr4_16_val(iphdr->dest)));
    LWIP_DEBUGF(IP_DEBUG, ("+-------------------------------+\n"));
  }
  #endif /* IP_DEBUG */
  
  #endif /* LWIP_IPV4 */
  