UDP checksum : does it include the length?

This question does not fit in any forum section because it is on general networking theory.
The UDP checksum covers the UDP/IP pseudo-header, the UDP header, and the user data padded to a 16-bit boundary. The UDP/IP pseudo-header includes the UDP length. The UDP header also includes the UDP length. The checksum is a 16-bit 1's complement.
Does that mean that the UDP length is effectively not included in the checksum calculation?
The other strange thing about the UDP checksum is that according to the RFC, it is optional. Is this so in real life?
Kevin Dorrell
Luxembourg

Harold,
At first sight, I didn't agree with you about " ... If the UDP length was changed between the source and destination, the checksum discrenpency would just be double what it would be if we only included the length value once. .. ". I reasoned like this: the checksum is a 1's complement, so if you include a field twice in the calculation, the net effect of the field is zero. So you can change the field as much as you like, and it wouldn't make a ha'p'orth of difference.
This is what I think now: as far as the transmitter is concerned, the two fields (real and pseudo) come from the same place: the length of the user datagram data. But as far as the receiver is concerned, they come from different places: the real field from the UDP header is explicit, and the pseudo field is calculated from the IP packet length (IP octets 2+3), minus 4 * IP-HLEN. So if you wanted to fiddle the UDP length, you would also have to fiddle the IP header octets 2+3, and therefore also IP header octets 10+11, and therefore also ....
I suppose this hinges on how the UDP header checksum algorithm is implemented in the receiver. If it does it the lazy way, and constructs the pseudo-header using the length explicitly declared in the UDP header, then there is no check on the length at all. If it calculates it from the IP header, then the UDP length is validated. So I can only speculate whether this is a potential problem.
Thanks for your thoughts.
Kevin Dorrell
Luxembourg

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