RE: iSCSI: [Fwd: Crc-32c example in iSCSI spec]

["Wheat, Stephen R" <stephen.r.wheat@intel.com>]

Quoting from Richard Black,
www.cl.cam.ac.uk/Research/SRG/bluebook/21/crc/node2.html
the inversion of the CRC before sending it precludes erroneous addition of
trailing zeros.

Without complementing the remainder, the result of a checked message would
be 0x00000000.  If the message, for some reason, had any number of zero bits
appended to it (after the crc word), the result would still be 0x00000000.

Thus, it seems that complementing the remainder is a relic from the bit
serial days where the end of the message was not determined beforehand, as
is the case for iSCSI PDUs.

As Mark originally opined, keeping with the reflected/complemented algorithm
has a certain consistency to it.

So, it appears that complementing the result for iSCSI CRC is an arbitrary
decision, that consistency seems to have won out.

Does anyone have an opinion as to whether checking for a zero result, versus
a non-zero result, has a performance implication?

Stephen

-----Original Message-----
From: Douglas Otis [mailto:dotis@sanlight.net]
Sent: Friday, August 17, 2001 11:54 AM
To: Mark Bakke; Sanjay Goyal
Cc: Ips (E-mail)
Subject: RE: iSCSI: [Fwd: Crc-32c example in iSCSI spec]


Mark,

The reason for the bit swap within the table is to allow a serial hardware
scheme as CRC is processed ms to ls over the entire stream as if a single
number.  Ethernet sends the byte out least significant bit first.  The
entire table is also swapped ls to ms and actually reduces the operations
needed within the table calculations.  This reflects the method used for
Ethernet as it is being stored inverted and the initial value is started as
all ones.  The reason for the initial value being set to all ones is clear
for leading zero interaction, but I do not understand the value in storing
the CRC inverted.  I thought to include my ignorance to the mix.

Doug

> Sanjay Goyal wrote:
> >
> > Hi
> >  I get the remainder for iSCSI as 0x1c2d19ed if I complement my
> CRC and then
> > pass it through the CRC engine.
> >
> >  As per CRC generation for data: the thing which is not clear to me is
> >    why do we need to bit-swap the CRC reminader as is done in all your
> > examples.
>
> As far as I can tell, bit-swapping does nothing to help or hurt
> the effectiveness of the CRC.  When I ran my examples, I thought
> that it would be the simplest for our CRC to different from the
> Ethernet CRC only in the polynomial.  Ethernet, FDDI, FC, and SCSI
> all do this, so I figured it would cause the least confusion if
> we did the same.
>
> > We can just complement it and append it after the DATA bytes.
> >  The other side also can just pass it through the CRC engine to
> check it.
> >
> > Regards
> > Sanjay Goyal
> >
> > -----Original Message-----
> > From: Mark Bakke [mailto:mbakke@cisco.com]
> > Sent: Thursday, August 16, 2001 1:43 PM
> > To: Steve Blightman; ips@ece.cmu.edu
> > Subject: Re: [Fwd: Crc-32c example in iSCSI spec]
> >
> > One more thing that might be helpful.  When the Ethernet
> > polynomial is used in SCSI to generate its CRC, the T10
> > doc specifies the remainder polynomial that one should see
> > after running the data with a valid CRC through.  For
> > the Ethernet CRC, this was specified as 0xc704dd7b.  This
> > remainder polynomial is taken before the CRC is complemented
> > and bit-reflected.  For iSCSI, I came up with a remainder of
> > 0x1c2d19ed.  Can anyone verify this result?
> >
> > --
> > Mark
> >
> > Mark Bakke wrote:
> > >
> > > Steve-
> > >
> > > I just ran some Ethernet packets with known CRCs through
> > > my iSCSI/Ethernet CRC generator, and found the same thing
> > > as you did.
> > >
> > > All of my examples need to be byte-swapped, along with the
> > > fix I already posted for the all-ones example.  Here is a
> > > new set of examples, which will be in -08.  I also ran
> > > 64 bytes of zeroes and ones, which now agree with your
> > > numbers as well.
> > >
> > > Thanks again for bringing this up.
> > >
> > > --
> > > Mark
> > >
> > >      07 CRC Examples
> > >
> > >         N.B. all Values are Hexadecimal
> > >
> > >           Byte:        0  1  2  3
> > >
> > >              0:       01 a0 00 00
> > >              4:       00 00 00 00
> > >              8:       00 00 00 00
> > >             12:       00 00 00 00
> > >             16:       04 05 00 00
> > >             20:       00 01 00 00
> > >             24:       00 00 00 05
> > >             28:       00 00 00 04
> > >             32:       2a 00 00 00
> > >             36:       00 00 00 00
> > >             40:       80 00 00 00
> > >             44:       00 00 00 00
> > >
> > >            CRC:       93 70 51 db
> > >
> > >         32 bytes of zeroes:
> > >
> > >           Byte:        0  1  2  3
> > >
> > >              0:       00 00 00 00
> > >            ...
> > >             28:       00 00 00 00
> > >
> > >            CRC:       aa 36 91 8a
> > >
> > >         32 bytes of ones:
> > >
> > >           Byte:        0  1  2  3
> > >
> > >              0:       ff ff ff ff
> > >            ...
> > >             28:       ff ff ff ff
> > >
> > >            CRC:       43 ab a8 62
> > >
> > >         32 bytes of incrementing 00..1f:
> > >
> > >           Byte:        0  1  2  3
> > >
> > >              0:       00 01 02 03
> > >            ...
> > >             28:       1c 1d 1e 1f
> > >
> > >            CRC:       4e 79 dd 46
> > >
> > >
> > > Steve Blightman wrote:
> > > >
> > > > I believe the examples for the ISCSI CRC  have the wrong endianness.
> > > >
> > > > As yiou suugested over the phone I ran some Ethernet frames
> through a
> > > > simulation. I have some difficulty running the exact simulations you
> > > > wanted becuase the minimum size Ethernet frame is 64 bytes.
> > > >
> > > > However using the Ethernet CRC polynomial,
> > > > Running 64 bytes of 0 onto the wire, we append "36 63 8d
> 75" onto the
> > > > wire for the CRC - "36" goes out first.
> > > > Running 64 bytes of all 1, we append "ba 87 61 0f" onto the
> wire - "ba"
> > > > goes out first.
> > > >
> > > > Using the same logic for the ISCSI polynomial
> > > > Running 64 bytes of 0 I think we should append "67 eb c8 03" - "67"
> > > > going out first
> > > > and running 64 bytes of all 1 we should append "66 4e cd 2f" - "66"
> > > > going out first
> > > >
> > > > And now for 32 bytes with the ISCSI polynomial
> > > >
> > > > Running 32 bytes of 0 we should append "aa 36 91 8a" - "aa"
> going out
> > > > first
> > > > Running 32 bytes of all 1 we should append "43 ab a8 62" -
> "43" going
> > > > out first
> > > >
> > > > I don't want to get into an endless endian debate, but I
> believe it is
> > > > important to get the order of these bytes in the right
> order, so that we
> > > > can use the same hardware to check as well as to generate CRCs.
> > > >
> > > > Thanks for your help on this,
> > > > Steve Blightman
> > >
> > > --
> > > Mark A. Bakke
> > > Cisco Systems
> > > mbakke@cisco.com
> > > 763.398.1054
> >
> > --
> > Mark A. Bakke
> > Cisco Systems
> > mbakke@cisco.com
> > 763.398.1054
> >
> >
> ------------------------------------------------------------------
> ----------------------------------
> >
> >    Part 1.2    Type: application/ms-tnef
> >            Encoding: base64
>
> --
> Mark A. Bakke
> Cisco Systems
> mbakke@cisco.com
> 763.398.1054
>