Re: How many bytes should a CRC-32 protect?
Matt Mathis <[email protected]>
| Newsgroups | gmane.ietf.pilc |
|---|---|
| Message-ID | <[email protected]> |
The standard answer is 12KB, but I believe that this statement is based on an engineering requirement that the maximum allowed probability that the CRC fails to detect a corrupted packet is below some threshold. (I would greatly appreciate it if somebody can provide a specific reference for this calculation.) The logic behind the calculation seems to be flawed in the following way: If you have a large quantity of data to move (say a Peta Byte, 1e+15 Bytes) then the total undetected error rate is independent of the packet size across a huge range of sizes. This is because changes in per-packet exposure to undetected errors are exactly offset by the change in the number of packets. For example if I send 1e+15 bytes over a link that has a raw error rate of 1E-12 then I compute: Using 1000 byte packets: Raw bit error rate: 1.0e-12 Per packet error rate: 1.0e-09 Undetected packet errors: 2.5e-19 Packets per data set: 1.0e+12 Total undetected errors per data set: 2.5e-07 Using 10000000 byte packets: Raw bit error rate: 1.0e-12 Per packet error rate: 1.0e-05 Undetected packet errors: 2.5e-15 Packets per data set: 1.0e+08 Total undetected errors per data set: 2.5e-07 You can find some of my musing about the Internet cell size at http://www.psc.edu/~mathis/MTU/ Comments and feedback appreciated, --MM-- On Mon, 6 May 2002, 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. > -- --MM-- ---------------------------------------------- Matt Mathis <[email protected]> W:412.268.3319 http://www.psc.edu/~mathis H:412.654.7529 ---------------------------------------------- _______________________________________________ pilc mailing list [email protected] https://www1.ietf.org/mailman/listinfo/pilc