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

[Sanjay Goyal <sanjay_goyal@ivivity.com>]

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. 

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

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