checksum.c

/*
    EtherEncLib - Ethernet ENC28J60 Library for Arduino
    Copyright (C) 2015  Renato Aloi
    renato.aloi@gmail.com -- http://www.seriallink.com.br

    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 3 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, see <http://www.gnu.org/licenses/>.

*/

#include "checksum.h"

// The Ip checksum is calculated over the ip header only starting
// with the header length field and a total length of 20 bytes
// unitl ip.dst
// You must set the IP checksum field to zero before you start
// the calculation.
// len for ip is 20.
//
// For UDP/TCP we do not make up the required pseudo header. Instead we 
// use the ip.src and ip.dst fields of the real packet:
// The udp checksum calculation starts with the ip.src field
// Ip.src=4bytes,Ip.dst=4 bytes,Udp header=8bytes + data length=16+len
// In other words the len here is 8 + length over which you actually
// want to calculate the checksum.
// You must set the checksum field to zero before you start
// the calculation.
// len for udp is: 8 + 8 + data length
// len for tcp is: 4+4 + 20 + option len + data length
//
// For more information on how this algorithm works see:
// http://www.netfor2.com/checksum.html
// http://www.msc.uky.edu/ken/cs471/notes/chap3.htm
// The RFC has also a C code example: http://www.faqs.org/rfcs/rfc1071.html


uint16_t checksum(uint8_t *buf, uint16_t len, uint8_t type)
{
        // type 0=ip 
        //      1=udp
        //      2=tcp
        uint32_t sum = 0;

        //if(type==0){
        //        // do not add anything
        //}
        if(type==1){
                sum+=IP_PROTO_UDP_V; // protocol udp
                // the length here is the length of udp (data+header len)
                // =length given to this function - (IP.scr+IP.dst length)
                sum+=len-8; // = real tcp len
        }
        if(type==2){
                sum+=IP_PROTO_TCP_V; 
                // the length here is the length of tcp (data+header len)
                // =length given to this function - (IP.scr+IP.dst length)
                sum+=len-8; // = real tcp len
        }
        // build the sum of 16bit words
        while(len >1){
                sum += 0xFFFF & (*buf<<8|*(buf+1));
                buf+=2;
                len-=2;
        }
        // if there is a byte left then add it (padded with zero)
        if (len){
//--- made by SKA ---                sum += (0xFF & *buf)<<8;
                sum += 0xFFFF & (*buf<<8|0x00);
        }
        // now calculate the sum over the bytes in the sum
        // until the result is only 16bit long
        while (sum>>16){
                sum = (sum & 0xFFFF)+(sum >> 16);
        }
        // build 1's complement:
        return( (uint16_t) sum ^ 0xFFFF);
}

void fillChecksum(uint8_t *buf)
{
        uint16_t ck;
        // clear the 2 byte checksum
        buf[IP_CHECKSUM_P] = 0;
        buf[IP_CHECKSUM_P+1] = 0;
        buf[IP_FLAGS_P] = 0x40; // don't fragment
        buf[IP_FLAGS_P + 1] = 0;  // fragement offset
        buf[IP_TTL_P] = 64; // ttl
        
        // calculate the checksum:
        ck = checksum(&buf[IP_P], IP_HEADER_LEN_V,0);
        buf[IP_CHECKSUM_P] = ck >> 8;
        buf[IP_CHECKSUM_P+1] = ck & 0xff;
}