Re: The case for SLIP CRCs

Paul Milazzo (
Fri, 8 Apr 88 09:30:50 CDT

Larry Swift asks:

   "Are you talking about checksums or CRC's? They're not the same."

I'm not sure to which mention you refer, but I'll try to clear up any
ambiguities. My original message discussed four distinct data
integrity tests, in the following order:

1) I used the phrase "link-level checking" to mean any data integrity
   test applied at the Data Link layer.

2) I used the phrase "UDP checksumming" to mean the IP header checksum
   algorithm as applied to UDP datagrams, with 0 meaning "no checksum".

3) I used the phrase "IP checksum algorithm" to mean the checksum
   algorithm described on p. 14 of RFC 791, applied to IP headers and
   TCP segments.

4) Finally, I used the phrase "link-level CRC" to mean any of the class
   of polynomial error-detecting codes known as Cyclic Redundancy
   Codes, applied at the Data Link layer.

Previous messages dismissed the need for Data Link layer integrity
tests with the argument that the existing TCP and UDP checksums provide
an end-to-end integrity test, and that with such a test, hop-by-hop
tests are unnecessary. The point of my message was to question
RFC 791's description of the effectiveness of the IP header checksum
algorithm: "experimental evidence indicates it is adequate". MY
experimental evidence indicates otherwise.

I hope these clarifications answer your question.

                                Paul G. Milazzo <milazzo@rice.EDU>
                                Dept. of Computer Science
                                Rice University, Houston, TX

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