Adi,
Thank you for the response.
Yeah, I'm fully aware of the WEP problems with 802.11 (my former life was
developing the first 802.11b chipsets... which ended up being used to crack
WEP).
Is the SATA 32bit CRC also transmitted in the AoE packet over the Ethernet?
Or are they just used for the immediate SATA cable error detection? Looking
at vblade code makes me think that there is no additional SATA CRC being
sent over the Ethernet.
To be honest, I'm not worried about the errors that the Ethernet card
detects. It is the errors that the card doesn't detect. I agree that CRC32
is fairly powerful but there are classes of errors that it won't be able to
detect or will have a reduced chance of detecting and this is where a
protocol level protection comes into play. Remember that TCP even has a
simple checksum which in the past folks tried to turn off but quickly
discovered that it was a bad idea.
Let me clarify my question a bit. There are points on the network where
errors can be introduced to the packet stream. These includes switches and
routers and such and may be simply hardware errors that introduce the errors
to the packet. If the error gets introduced at this point (not maliciously
but by a hardware error) then it can be outside of the application of the
Ethernet CRC and thus not caught by the Ethernet hardware. This is where the
underlying protocol being carried should include some sort of error
detection. The current definition of AoE does not include any error
detection so I'm trying to understand how this protocol can be used in an
enterprise style environment if there is a chance of corrupted data being
written to the target?
See the paper I referenced earlier about various sources of network errors:
http://portal.acm.org/citation.cfm?id=347561&dl=ACM&coll=DL&CFID=110304004&CFTOKEN=23505194
.
I'm hoping maybe someone from Coraid could respond to this query since they
seem to be the only company selling enterprise quality AoE gear.
David
On Mon, Nov 29, 2010 at 4:19 AM, Adi Kriegisch <a...@cg.tuwien.ac.at> wrote:
> David,
>
> > I've been looking at AoE and I'm trying to understand what affect the
> Ethernet
> > CRC-32 data integrity checking has on the AoE communications?
> Particularly when
> > going to GbE and jumbo frames support there seems to be some data out
> there
> > that there is a chance that the CRC32 won't detect the error in the frame
> and
> > with a protocol like AoE that error would most probably end up being
> written to
> > the target disk.
> First of, it is important to state that checksums like CRC32 are only
> helpful in detecting "wire" errors -- they do not protect against
> intentional modification (this was one of the design problems with WEP --
> for such purposes cryptographic hash functions are required).
>
> Now to the powers of CRC32: a 32bit CRC is able to detect any single error
> burst that is no longer than 32bit. The length of the datagram does not
> matter in this case. For all error bursts longer than 32bit it will only
> detect a fraction of them: 99.999999976716936% (1-2^-32).
>
> The ATA protocol uses checksums as well: Ultra-ATA introduced a checksum on
> the data transmission and SATA introduced a 32bit CRC for all bits
> transmitted over the wire. Besides that, for every block written on a disk,
> a parity will be stored as well.
>
> To sum it up: check your network cards for frame errors and your switches
> as well. When you see errors, act. The risk is IMHO low.
>
> > I think that there is a further problem to understand and that is with
> network
> > connection points. AoE is not routable but that doesn't mean you can't
> use
> > network switches to interconnect initiators with their AoE targets and at
> these
> > switches there seems to be a possible error point introduced which AoE
> isn't
> > protecting against? Are there best practices for AoE installations to
> protect
> > against these error points?
> I think I did not get the question. What error point do you introduce using
> a switch?
>
> -- Adi
>
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