Reply To: Using WinpkFilter to modify the payload of TCP/IP packets

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#6211

Daneel
Participant

Hi SerpentFly

@SerpentFly wrote:

Well, yes, IP packet can be fragmented and you may have to gather all fragments before processing the packet. However, fragmented IP packets are rather rare in modern networks, also these packets are recommended to be blocked by firewalls. Basically, I don’t think you will ever meet fragmented TCP packet, but it is still possible to generate fragmented ICMP or UDP packets.

Many thanks. I’m downloading content from one node in my LAN to another node and my experimentation with WinpkFilter showed me that there are many ethernet headers for each IP header. From this I (wrongly) concluded that each IP packet consists of many ethernet frames with the IP header appearing only in the first of these ethernet frames.

I’ve now read some of the relevant literature and I think this was a misconception. Actually, each ethernet frame also includes the IP header and consequently the TCP header, i.e., each ethernet packet looks like this:

payload

Is this ethernet checksum (http://folk.uio.no/od/tcp-ip-intro/subsection3_2_3.html) something which I need to care about (in addition to the length / checksum fields in the other headers)? Or is this computed automatically?

This definitely makes my life easier … I think 😉

Many thanks,
Michael