ethernet_misc.c

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/**
 * @file ethernet_misc.c
 * @brief Helper functions for Ethernet
 *
 * @section License
 *
 * SPDX-License-Identifier: GPL-2.0-or-later
 *
 * Copyright (C) 2010-2023 Oryx Embedded SARL. All rights reserved.
 *
 * This file is part of CycloneTCP Open.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 *
 * @author Oryx Embedded SARL (www.oryx-embedded.com)
 * @version 2.2.4
 **/
 
//Switch to the appropriate trace level
#define TRACE_LEVEL ETH_TRACE_LEVEL
 
//Dependencies
#include <stdlib.h>
#include <string.h>
#include "core/net.h"
#include "core/nic.h"
#include "core/ethernet.h"
#include "core/ethernet_misc.h"
#include "core/socket.h"
#include "core/raw_socket.h"
#include "core/tcp_timer.h"
#include "ipv4/arp.h"
#include "ipv4/ipv4.h"
#include "ipv4/ipv4_misc.h"
#include "ipv6/ipv6.h"
#include "mibs/mib2_module.h"
#include "mibs/if_mib_module.h"
#include "debug.h"
 
//Check TCP/IP stack configuration
#if (ETH_SUPPORT == ENABLED)
 
//Padding bytes
const uint8_t ethPadding[64] =
{
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
 
//A lookup table can be used to speed up CRC calculation
#if (ETH_FAST_CRC_SUPPORT == ENABLED)
 
static const uint32_t crc32Table[256] =
{
   0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA,
   0x076DC419, 0x706AF48F, 0xE963A535, 0x9E6495A3,
   0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988,
   0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91,
   0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE,
   0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
   0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC,
   0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5,
   0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172,
   0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B,
   0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940,
   0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
   0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116,
   0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F,
   0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
   0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D,
   0x76DC4190, 0x01DB7106, 0x98D220BC, 0xEFD5102A,
   0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
   0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818,
   0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01,
   0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E,
   0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457,
   0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C,
   0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
   0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2,
   0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB,
   0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0,
   0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9,
   0x5005713C, 0x270241AA, 0xBE0B1010, 0xC90C2086,
   0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
   0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4,
   0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD,
   0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A,
   0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683,
   0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8,
   0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
   0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE,
   0xF762575D, 0x806567CB, 0x196C3671, 0x6E6B06E7,
   0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC,
   0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5,
   0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252,
   0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
   0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60,
   0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79,
   0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
   0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F,
   0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92, 0x5CB36A04,
   0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
   0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A,
   0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713,
   0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38,
   0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21,
   0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E,
   0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
   0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C,
   0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45,
   0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2,
   0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB,
   0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0,
   0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
   0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6,
   0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF,
   0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94,
   0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D
};
 
#endif
 
 
/**
 * @brief Ethernet frame padding
 * @param[in] buffer Multi-part buffer containing the Ethernet frame
 * @param[in,out] length Length of the Ethernet frame, in bytes
 * @return Error code
 **/
 
error_t ethPadFrame(NetBuffer *buffer, size_t *length)
{
   error_t error;
   size_t n;
 
   //Ethernet frames have a minimum length of 64 byte
   if(*length < (ETH_MIN_FRAME_SIZE - ETH_CRC_SIZE))
   {
      //Add padding as necessary
      n = (ETH_MIN_FRAME_SIZE - ETH_CRC_SIZE) - *length;
 
      //Append padding bytes
      error = netBufferAppend(buffer, ethPadding, n);
 
      //Check status code
      if(!error)
      {
         //Adjust frame length
         *length += n;
      }
   }
   else
   {
      //No padding needed
      error = NO_ERROR;
   }
 
   //Return status code
   return error;
}
 
 
/**
 * @brief VLAN tag encoding
 * @param[in] buffer Multi-part buffer containing the payload
 * @param[in,out] offset Offset to the first payload byte
 * @param[in] vlanId VLAN identifier
 * @param[in] vlanPcp VLAN priority
 * @param[in] vlanDei Drop eligible indicator
 * @param[in] type Ethernet type
 * @return Error code
 **/
 
error_t ethEncodeVlanTag(NetBuffer *buffer, size_t *offset, uint16_t vlanId,
   int8_t vlanPcp, int8_t vlanDei, uint16_t type)
{
   VlanTag *vlanTag;
 
   //Sanity check
   if(*offset < sizeof(VlanTag))
      return ERROR_INVALID_PARAMETER;
 
   //Valid PCP value?
   if(vlanPcp >= 0)
   {
      //The PCP field specifies the frame priority level. Different PCP values
      //can be used to prioritize different classes of traffic
      vlanId = (((uint16_t) vlanPcp << VLAN_PCP_POS) & VLAN_PCP_MASK) |
         (vlanId & ~VLAN_PCP_MASK);
   }
 
   //Valid DEI value?
   if(vlanDei >= 0)
   {
      //The DEI flag may be used to indicate frames eligible to be dropped in
      //the presence of congestion
      vlanId = (((uint16_t) vlanDei << VLAN_DEI_POS) & VLAN_DEI_MASK) |
         (vlanId & ~VLAN_DEI_MASK);
   }
 
   //Make room for the VLAN tag
   *offset -= sizeof(VlanTag);
   //Point to the VLAN tag
   vlanTag = netBufferAt(buffer, *offset);
 
   //The TCI field is divided into PCP, DEI, and VID
   vlanTag->tci = htons(vlanId);
 
   //The EtherType field indicates which protocol is encapsulated in the
   //payload
   vlanTag->type = htons(type);
 
   //Successful processing
   return NO_ERROR;
}
 
 
/**
 * @brief VLAN tag decoding
 * @param[in] frame Pointer to the received Ethernet frame
 * @param[in] length Length of the frame, in bytes
 * @param[out] vlanId VLAN identifier
 * @param[out] type Ethernet type
 * @return Error code
 **/
 
error_t ethDecodeVlanTag(const uint8_t *frame, size_t length, uint16_t *vlanId,
   uint16_t *type)
{
   VlanTag *vlanTag;
 
   //Malformed Ethernet frame?
   if(length < sizeof(VlanTag))
   {
      //Drop the received frame
      return ERROR_INVALID_LENGTH;
   }
 
   //Point to the VLAN tag
   vlanTag = (VlanTag *) frame;
 
   //The VID is encoded in a 12-bit field. The null VID indicates that the
   //tag header contains only priority information (refer to IEEE 802.1Q,
   //section 9.6)
   *vlanId = ntohs(vlanTag->tci) & VLAN_VID_MASK;
 
   //The EtherType field indicates which protocol is encapsulated in the
   //payload
   *type = ntohs(vlanTag->type);
 
   //Successful processing
   return NO_ERROR;
}
 
 
/**
 * @brief Destination MAC address filtering
 * @param[in] interface Underlying network interface
 * @param[in] macAddr Destination MAC address to be checked
 * @return Error code
 **/
 
error_t ethCheckDestAddr(NetInterface *interface, const MacAddr *macAddr)
{
   error_t error;
   uint_t i;
   MacFilterEntry *entry;
   NetInterface *logicalInterface;
 
   //Filter out any invalid addresses
   error = ERROR_INVALID_ADDRESS;
 
   //Point to the logical interface
   logicalInterface = nicGetLogicalInterface(interface);
 
   //Interface MAC address?
   if(macCompAddr(macAddr, &logicalInterface->macAddr))
   {
      error = NO_ERROR;
   }
   //Broadcast address?
   else if(macCompAddr(macAddr, &MAC_BROADCAST_ADDR))
   {
      error = NO_ERROR;
   }
   //Multicast address?
   else if(macIsMulticastAddr(macAddr))
   {
#if (IPV4_SUPPORT == ENABLED && IGMP_ROUTER_SUPPORT == ENABLED)
      //Trap multicast packets when IGMP router is enabled
      if(interface->igmpRouterContext != NULL &&
         interface->igmpRouterContext->running)
      {
         error = NO_ERROR;
      }
      else
#endif
      {
         //Go through the MAC filter table
         for(i = 0; i < MAC_ADDR_FILTER_SIZE; i++)
         {
            //Point to the current entry
            entry = &interface->macAddrFilter[i];
 
            //Valid entry?
            if(entry->refCount > 0)
            {
               //Check whether the destination MAC address matches
               //a relevant multicast address
               if(macCompAddr(&entry->addr, macAddr))
               {
                  //The MAC address is acceptable
                  error = NO_ERROR;
                  //Stop immediately
                  break;
               }
            }
         }
      }
   }
 
   //Return status code
   return error;
}
 
 
/**
 * @brief Trap IGMP packets
 * @param[in] header Pointer to the Ethernet header
 * @param[in] data Pointer to the payload data
 * @param[in] length Length of the payload data, in bytes
 * @return TRUE if the Ethernet frame contains an IGMP message, else FALSE
 **/
 
bool_t ethTrapIgmpPacket(EthHeader *header, uint8_t *data, size_t length)
{
   bool_t flag;
 
   //Initialize flag
   flag = FALSE;
 
#if (IPV4_SUPPORT == ENABLED)
   //Multicast IPv4 packet?
   if(macIsMulticastAddr(&header->destAddr) &&
      ntohs(header->type) == ETH_TYPE_IPV4)
   {
      //Check the length of the payload data
      if(length >= sizeof(Ipv4Header))
      {
         //Check whether the IPv4 packet contains an IGMP message
         flag = ipv4TrapIgmpPacket((Ipv4Header *) data);
      }
   }
#endif
 
   //Return TRUE if the Ethernet frame contains an IGMP message
   return flag;
}
 
 
/**
 * @brief Update Ethernet input statistics
 * @param[in] interface Underlying network interface
 * @param[in] destMacAddr Destination MAC address
 **/
 
void ethUpdateInStats(NetInterface *interface, const MacAddr *destMacAddr)
{
   //Check whether the destination address is a unicast, broadcast or multicast address
   if(macCompAddr(destMacAddr, &MAC_BROADCAST_ADDR))
   {
      //Number of non-unicast packets delivered to a higher-layer protocol
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifInNUcastPkts, 1);
 
      //Number of broadcast packets delivered to a higher-layer protocol
      IF_MIB_INC_COUNTER32(ifXTable[interface->index].ifInBroadcastPkts, 1);
      IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCInBroadcastPkts, 1);
   }
   else if(macIsMulticastAddr(destMacAddr))
   {
      //Number of non-unicast packets delivered to a higher-layer protocol
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifInNUcastPkts, 1);
 
      //Number of multicast packets delivered to a higher-layer protocol
      IF_MIB_INC_COUNTER32(ifXTable[interface->index].ifInMulticastPkts, 1);
      IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCInMulticastPkts, 1);
   }
   else
   {
      //Number of unicast packets delivered to a higher-layer protocol
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifInUcastPkts, 1);
      IF_MIB_INC_COUNTER32(ifTable[interface->index].ifInUcastPkts, 1);
      IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCInUcastPkts, 1);
   }
}
 
 
/**
 * @brief Update Ethernet output statistics
 * @param[in] interface Underlying network interface
 * @param[in] destMacAddr Destination MAC address
 * @param[in] length Length of the Ethernet frame, in bytes
 **/
 
void ethUpdateOutStats(NetInterface *interface, const MacAddr *destMacAddr,
   size_t length)
{
   //Total number of octets transmitted out of the interface
   MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifOutOctets, length);
   IF_MIB_INC_COUNTER32(ifTable[interface->index].ifOutOctets, length);
   IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCOutOctets, length);
 
   //Check whether the destination address is a unicast, broadcast or multicast address
   if(macCompAddr(destMacAddr, &MAC_BROADCAST_ADDR))
   {
      //Number of non-unicast packets that higher-level protocols requested be transmitted
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifOutNUcastPkts, 1);
 
      //Number of broadcast packets that higher-level protocols requested be transmitted
      IF_MIB_INC_COUNTER32(ifXTable[interface->index].ifOutBroadcastPkts, 1);
      IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCOutBroadcastPkts, 1);
   }
   else if(macIsMulticastAddr(destMacAddr))
   {
      //Number of non-unicast packets that higher-level protocols requested be transmitted
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifOutNUcastPkts, 1);
 
      //Number of multicast packets that higher-level protocols requested be transmitted
      IF_MIB_INC_COUNTER32(ifXTable[interface->index].ifOutMulticastPkts, 1);
      IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCOutMulticastPkts, 1);
   }
   else
   {
      //Number of unicast packets that higher-level protocols requested be transmitted
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifOutUcastPkts, 1);
      IF_MIB_INC_COUNTER32(ifTable[interface->index].ifOutUcastPkts, 1);
      IF_MIB_INC_COUNTER64(ifXTable[interface->index].ifHCOutUcastPkts, 1);
   }
}
 
 
/**
 * @brief Update Ethernet error statistics
 * @param[in] interface Underlying network interface
 * @param[in] error Status code describing the error
 **/
 
void ethUpdateErrorStats(NetInterface *interface, error_t error)
{
   //Check error code
   switch(error)
   {
   case ERROR_INVALID_ADDRESS:
   case ERROR_WRONG_IDENTIFIER:
      //Number of inbound packets which were chosen to be discarded even
      //though no errors had been detected
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifInDiscards, 1);
      IF_MIB_INC_COUNTER32(ifTable[interface->index].ifInDiscards, 1);
      break;
   case ERROR_INVALID_LENGTH:
   case ERROR_WRONG_CHECKSUM:
      //Number of inbound packets that contained errors
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifInErrors, 1);
      IF_MIB_INC_COUNTER32(ifTable[interface->index].ifInErrors, 1);
      break;
   case ERROR_INVALID_PROTOCOL:
      //Number of packets received via the interface which were discarded
      //because of an unknown or unsupported protocol
      MIB2_IF_INC_COUNTER32(ifTable[interface->index].ifInUnknownProtos, 1);
      IF_MIB_INC_COUNTER32(ifTable[interface->index].ifInUnknownProtos, 1);
      break;
   default:
      //Just for sanity
      break;
   }
}
 
 
/**
 * @brief Ethernet CRC calculation
 * @param[in] data Pointer to the data over which to calculate the CRC
 * @param[in] length Number of bytes to process
 * @return Resulting CRC value
 **/
 
uint32_t ethCalcCrc(const void *data, size_t length)
{
//A lookup table can be used to speed up CRC calculation
#if (ETH_FAST_CRC_SUPPORT == ENABLED)
   uint_t i;
 
   //Point to the data over which to calculate the CRC
   const uint8_t *p = (uint8_t *) data;
   //CRC preset value
   uint32_t crc = 0xFFFFFFFF;
 
   //Loop through data
   for(i = 0; i < length; i++)
   {
      //The message is processed byte by byte
      crc = (crc >> 8) ^ crc32Table[(crc & 0xFF) ^ p[i]];
   }
 
   //Return 1's complement value
   return ~crc;
 
//Bit by bit CRC calculation
#else
   uint_t i;
   uint_t j;
 
   //Point to the data over which to calculate the CRC
   const uint8_t *p = (uint8_t *) data;
   //CRC preset value
   uint32_t crc = 0xFFFFFFFF;
 
   //Loop through data
   for(i = 0; i < length; i++)
   {
      //Update CRC value
      crc ^= p[i];
 
      //The message is processed bit by bit
      for(j = 0; j < 8; j++)
      {
         if(crc & 0x00000001)
            crc = (crc >> 1) ^ 0xEDB88320;
         else
            crc = crc >> 1;
      }
   }
 
   //Return 1's complement value
   return ~crc;
#endif
}
 
 
/**
 * @brief Calculate CRC over a multi-part buffer
 * @param[in] buffer Pointer to the multi-part buffer
 * @param[in] offset Offset from the beginning of the buffer
 * @param[in] length Number of bytes to process
 * @return Resulting CRC value
 **/
 
uint32_t ethCalcCrcEx(const NetBuffer *buffer, size_t offset, size_t length)
{
   uint_t i;
   uint_t n;
   uint32_t crc;
   uint8_t *p;
#if (ETH_FAST_CRC_SUPPORT == DISABLED)
   uint_t k;
#endif
 
   //CRC preset value
   crc = 0xFFFFFFFF;
 
   //Loop through data chunks
   for(i = 0; i < buffer->chunkCount && length > 0; i++)
   {
      //Is there any data to process in the current chunk?
      if(offset < buffer->chunk[i].length)
      {
         //Point to the first data byte
         p = (uint8_t *) buffer->chunk[i].address + offset;
         //Compute the number of bytes to process
         n = MIN(buffer->chunk[i].length - offset, length);
         //Adjust byte counter
         length -= n;
 
         //Process current chunk
         while(n > 0)
         {
#if (ETH_FAST_CRC_SUPPORT == ENABLED)
            //The message is processed byte by byte
            crc = (crc >> 8) ^ crc32Table[(crc & 0xFF) ^ *p];
#else
            //Update CRC value
            crc ^= *p;
 
            //The message is processed bit by bit
            for(k = 0; k < 8; k++)
            {
               if(crc & 0x00000001)
                  crc = (crc >> 1) ^ 0xEDB88320;
               else
                  crc = crc >> 1;
            }
#endif
            //Next byte
            p++;
            n--;
         }
 
         //Process the next block from the start
         offset = 0;
      }
      else
      {
         //Skip the current chunk
         offset -= buffer->chunk[i].length;
      }
   }
 
   //Return 1's complement value
   return ~crc;
}
 
 
/**
 * @brief Ethernet CRC verification
 * @param[in] interface Underlying network interface
 * @param[in] frame Pointer to the received Ethernet frame
 * @param[in] length Length of the frame, in bytes
 * @return Error code
 **/
 
error_t ethCheckCrc(NetInterface *interface, const uint8_t *frame,
   size_t length)
{
   uint32_t crc;
 
   //Malformed Ethernet frame?
   if(length < (sizeof(EthHeader) + ETH_CRC_SIZE))
   {
      //Drop the received frame
      return ERROR_INVALID_LENGTH;
   }
 
   //CRC verification not supported by hardware?
   if(!interface->nicDriver->autoCrcVerif)
   {
      //The value of the residue is 0x2144DF1C when no CRC errors
      //are detected
      if(ethCalcCrc(frame, length) != 0x2144DF1C)
      {
         //Drop the received frame
         return ERROR_WRONG_CHECKSUM;
      }
   }
 
   //Retrieve CRC value
   crc = LOAD32BE(frame + length - ETH_CRC_SIZE);
 
   //Gather entropy
   netContext.entropy += crc;
 
   //Successful CRC verification
   return NO_ERROR;
}
 
#endif