[e2e] UDP checksum field?
cannara at attglobal.net
Tue Apr 5 15:18:35 PDT 2005
Of course, David, but the opposite is: no checksum = no chance of
correctness. And, the way NAT and other boxes have been intended and
deployed, many people consider them as "ends", making the mythical End-End
Principle even more of a fantasy.
"David P. Reed" wrote:
> When all is said and done, the UDP checksum isn't, and never was, fully
> end-to-end protection, since there are few, if any, applications where
> the correctness of the application data can be *fully assured* by making
> sure that a single datagram gets delivered correctly. It's an optional
> standardized way to help deal with a common risk that can arise due to
> bugs and other issues that show up in engineered systems, nto a
> guarantee of any particular property.
> Since UDP datagrams can still be duplicated and modified by a
> checksum-preserving modification in the network (such modifications are
> now common, given middleboxes that discard the checksum and compute a
> new one in many cases), there is no way to assure by a mere checksum
> field that data has not been corrupted somewhere in the network.
> Assurance is not the benefit, applications still need to do truly
> end-to-end checking - UDP's ability to help in detecting incipient
> problems is very useful, however.
> I won't elaborate here on the more subtle issues of TCP's lack of true
> end-to-end reliability. Suffice it to say that there is a difficult
> issue in a definition of reliability that must depend on the difference
> between "design errors" and "random errors".
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