Calculating payload in a tcp packet structure

Question

When i want to split a tcp packet and print every part (eth/ip/tcp/payload) separately, how do i calculate the end of the payload?

PrintData(Buffer, iphdrlen);
PrintData(Buffer + iphdrlen, tcpheader->data_offset*4);
PrintData(Buffer + iphdrlen + tcpheader->data_offset*4, ??? );

I tried

PrintData( Buffer + iphdrlen + tcpheader->data_offset*4, ( Size - tcpheader->data_offset*4 - iphdr->ip_header_len*4 ) );

but all the packets was truncated. Which value have i to pass here?

Greetings

Answer 1

IP Header contains the length with the size of the IP header and payload. Also IP header contains IP header size field which equal the number of 32-bit words :

PrintData(Buffer + (iphdrlen * 4) + tcpheader->data_offset * 4, iplen - (iphdrlen * 4 + tcpheader->data_offset * 4));

As I undestand, you forgot to multiply iphdrlen by 4 in offset

Answer 2

TCP format gives the meaning of each field in the TCP header, including details like where the packet data starts and the packet data length.

Note that items in the TCP packet header use 'network order' for each of the fields