Re: How many bytes should a CRC-32 protect?
Dan Grossman <[email protected]>
| Newsgroups | gmane.ietf.pilc |
|---|---|
| Message-ID | <[email protected]> |
Some properties of CRCs are slowly bubbling up from the recesses of ancient memory. This with some uncertainty as to the accuracy of this memory, and which are general properties of CRCs and which are properties of the CRC-32 polynomial. Corrections are welcome. A CRC of length N bits produces a unique codeword for packets of length 2^N-1 bits. It will therefore detect any single bit error in a packet of that length or less. The particular polynomial (actually, all polynomials that either have or lack a particular factor -- I think it was lacking an N(N+1) factor, but not sure) will detect all odd numbers of errors. It will detect all single bursts of less than 32 bits, and all 2 bit errors that occur within 2048 bits. For all other even number of bit errors, the probability of detection is 1-2^-32. In practice, I recollect that the probability of undetected errors in a burst error environment is difficult to solve analytically, and that people have tended to do it by simulation. Dr G Fairhurst wrote: > I was asked by someone how big a frame size can a CRC-32 reasonably protect? > > One of my students came across: > > http://sd.wareonearth.com/~phil/jumbo.html > > which seems to suggest that the CRC-32 is good for 10-12KB or so of > data, and that > this could be one reason why we don't see many subnetworks supporting > MTUs > 12KB. > > Is there more wisdom out there on the use of CRCs over large subnetwork PDUs? > - perhaps the ATM or gigEthernet people have thought this one through? > > Gorry Fairhurst. _______________________________________________ pilc mailing list [email protected] https://www1.ietf.org/mailman/listinfo/pilc http://www.ietf.org/html.charters/pilc-charter.html http://pilc.grc.nasa.gov/