RE: iSCSI draft-07 Padding

[Robert Snively <rsnively@Brocade.COM>]

The burst of data (sequence in FCP-2, I assume a PDU in iSCSI) 
carrying the last byte (as defined by the highest displacement of
the SCSI data pointer) may be padded by 0,1,2,or 3 bytes to bring
the last PDU to a 4-byte transfer boundary.  The number of bytes
padded is included in the last transfer header, so they can be
ignored and not passed to memory in an automated manner.

Bob

-----Original Message-----
From: Eddy Quicksall [mailto:ESQuicksall@hotmail.com]
Sent: Tuesday, July 24, 2001 10:30 AM
To: Michael Schoberg; ips@ece.cmu.edu
Subject: Re: iSCSI draft-07 Padding


What does a Fibre Channel driver do when the ULP wants to receive data into
memory which is not a multiple of 4 bytes long?

Eddy

----- Original Message -----
From: "Michael Schoberg" <michael_schoberg@cnt.com>
To: <ips@ece.cmu.edu>
Sent: Tuesday, July 24, 2001 11:13 AM
Subject: RE: iSCSI draft-07 Padding


> Yes, at the very least the spec should not allow padding on intermediate
> ReadData/WriteData bursts.  I can't think of a reason why one would want
to
> use odd length intermediate bursts, so hopefully it's safe to restrict it.
> Padding the last burst still seems reasonable for easy CRC calculation.
>
> Setting a fixed size is optional.  A minimum would avoid memory
> fragmentation when working with memory descriptors.
>
> : -----Original Message-----
> : From: Robert Snively [mailto:rsnively@Brocade.COM]
> : Sent: Tuesday, July 24, 2001 8:34 AM
> : To: ips@ece.cmu.edu
> : Subject: RE: iSCSI draft-07 Padding
> :
> :
> : Fibre Channel's approach to this problem is to prohibit padding
> : for all transfers except the transfer that moves the last bytes
> : in the last (or highest pointer value) burst of data.  Intermediate
> : bursts are required to end on 4-byte boundaries, regardless of
> : tranfer order as allowed by EMDP.
> :
> : Bob
> :
> : ----- Original Message -----
> : From: "Michael Schoberg" <michael_schoberg@cnt.com>
> : To: <ips@ece.cmu.edu>
> : Sent: Friday, July 20, 2001 4:27 PM
> : Subject: iSCSI draft-07 Padding
> :
> :
> : > I couldn't find anything in the latest iSCSI spec
> : (draft-07) that prevents
> : > someone from issuing multiple padded PDU's for iSCSI Data
> : In/Out segments.
> : > I guess I'm worried that someone could issue padded
> : odd-length write/read
> : > data when it wasn't necessary.  Example: When moving a 512 byte disk
> : sector
> : > an initiator/target could legally issue 512 1-byte DataSegmentLength
> : > messages (each padded up to a 32-bit boundary).  This isn't
> : the sort of
> : data
> : > stream one would expect, but it's allowed in the draft.
> : >
> : > This also leads into whether fixed or minimum length data
> : segments (for
> : all
> : > but the last) would be nice to include as part of the spec.
> :  It may result
> : > in a simpler software/hardware design.
> : >
> :
>