[Click] About checksum computation ?

[yacine.djemaiel]

Hi, 
i would like to compute checksum after modifying an ip header
field. I have followed set_cksum method (fromtcpdump.cc), but
i get all the time an incorrect ip header checksum. Lines of
code that i am using are :   

assert(wp_iph == wp->ip_header());   
wp_iph->ip_len = htons(wp->length() - wp->ip_header_offset());
wp_iph->ip_sum = 0;
wp_iph->ip_sum = click_in_cksum((uint8_t *)wp_iph,
(ntohs(wp_iph->ip_len)) << 2);  

I have modified the identification field and the reserved flag.

Thanks for any help
Yacine

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