Protocol of a packet

Question

I'm programming an offline packets decoding program in C under Windows 7 x86.

I wonder how it is possible to know packet protocol, either if it is UDP or TCP?

Answer 1

You can know by checking the IP packet header, there is a Protocol field in the packet header that is used to indicate the type of the packet according to its value :

• 1 is ICMP
• 6 is TCP
• 17 is UDP

and so on. More information on this is available on Wikipedia

Edit: Here's the list of all the possible values for that field.

P.S: I'm assuming IPv4 here, I don't know if things are the same with IPv6