[e2e] purpose of pseudo header in TCP checksum

Lars-Åke Larzon lln at csee.ltu.se
Tue Feb 15 11:24:51 PST 2005

Wesley Eddy wrote:
> Actually, with UDP-Lite, you'd be asking the link layers to parse up
> through the transport layer and check as much of the data as the
> UDP-Lite header designates as sensitive.  That's slightly more complex
> than just checking the protocol field in IP header, which already
> assumes that the link device can parse IP, can see through whatever
> levels of tunnels are present, etc.  

We're getting slightly off topic here, but just to clarify about UDP-Lite:

UDP-Lite gives you *the option* to use the coverage field as an 
indication of how much data to checksum in the link layer if you have a 
link layer supporting that kind of behavior. As it turns out, many of 
the scenarios where this might be desirable have a significantly higher 
cost for retransmission compared to the extra parsing that needs to be 

The recommended default behavior of UDP-Lite, though, is to work exactly 
as UDP, i.e., to checksum the entire datagram. The partial checksum is 
entirely optional and it is up to application and system designers to 
decide whether it is worth it or not.


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