How would I go about calculating the size and starting byte of the data in a TCP packet (excluding the header information)?
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How would I go about calculating the size and starting byte of the data in a TCP packet (excluding the header information)?
I am going to assume that you are dealing with a TCP/IP packet. You will need to calculate this size yourself.
The IP header has a ‘Total Length’ field that gives you the length of the entire IP packet in bytes. If you subtract the number of 32-bit words that make up the header (given by the Header Length field in the IP header) you will know the size of the TCP packet. Usually, the header is 20 bytes for the IP packet, unless Options are present.
In the TCP header, the Data Offset field specifies the size of the TCP header in 32-bit words. Again, you can subtract the number (multiplied with 4 to give you the number of bytes in the header) from the size of the TCP packet you calculated earlier to get you the size of the data in the TCP packet.
Given the Header Length in the IP header and the Data Offset in the TCP header, you can add those two and multiply by 4 to give you the byte offset till the data in the TCP packet starts.