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TCP,UDP checksums

bpmf
bpmf asked
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Last Modified: 2013-12-23
Hello,
Why do TCP and UDP have their own checksums?Is the IP checksum not enough?
and Why lost TCP acknowledgements do not necessarily force retransmissions?
Thanks.
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Hey,

TCP and UDP are protocols at the similar OSI-Layer, but UDP is no secure end-to-end-protocol, because no acknoledgements are sent from the destination and a checksum has not to be calculated  (checksum=0 means, no checksum will be calculated). So transmission errors can only be checked by IP checksum.

The TCP-checksum is calculated to secure the transmission of the data from Layer 4 to Layer 3 (IP). The original data will be checked here. The IP checksum will vary on every hop, because the IP header has the source adress and the destination adress of the next hop inside. So with every router on the run the checksum will be a different. The TCP-checksum can controll changings of the original data.

The retransmissions are not builded bye the TCP protocol but bye the application. The RFCs for TCP only describe the TCP-packet but not the applications they are created by. So I think, the ability  to force retransmission depends on the application.

I hope this will answer your questions.

Yours Josh

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