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- When the CRC and TCP Checksum Disagree
Jonathan Stone (Stanford University)
Craig Partridge (BBN Technologies)
Traces of Internet packets over the last two years show that 1 in
30,000 packets fails the TCP checksum, even on links where link-level
CRCs should catch all but 1 in 4 billion errors.
Analysis of 100,000 packets which fail the TCP checksum shows the
Internet has a wide variety of error sources which are not detected by
link-level checksums. Analysis tools have identified nearly 100
different patterns, or likely cause, of the observed errors. These
categories explain half our observe errors, and show that errors occur
at all levels of the network.
From the observed rate of incorrect checksums, and excluding errors
that will always be caught, we conclude that the TCP checksum will
fail to detect an error for rougly 1 in 200 milllion packets. From
our analysis of the cause of erros, we propose simple changes to
several protocols which can decrease the rate of undetected error.