RE: iSCSI variable CDB length

["Pat LaVarre" <LAVARRE@iomega.com>]

> restricting the maximum CDB length to 260 bytes.
 
Max allowed is 260 = 8 + 252, yes.
 
Max expressible is 263 = 8 + 255.  We can only hope nobody visits the land between max allowed and max expressible.
 
> That field must be a multiple of 4
 
The day's soon coming when people will prefer multiples of 8 ...
 
Pat LaVarre

	-----Original Message----- 
	From: Elliott, Robert (Server Storage) [mailto:Elliott@hp.com] 
	Sent: Mon 6/10/2002 8:01 AM 
	To: ips@ece.cmu.edu 
	Cc: 
	Subject: RE: iSCSI variable CDB length
	
	


	> -----Original Message-----
	> From: Michael J. S. Smith (PacBell) [mailto:smithmjs@pacbell.net]
	> Sent: Sunday, June 09, 2002 2:24 PM
	> Subject: variable CDB length
	>
	> I'm trying to bound some of the iSCSI structures. This email
	> is regarding the AHS for extended CDBs.
	>
	> The following (page 37 of the PDF, page 7 in the draft page
	> numbering) is from ftp://ftp.t10.org/t10/drafts/spc3/spc3r07.pdf.
	> [quote]
	> Command descriptor block (CDB): The structure used to communicate
	> commands from an application client to a device server. A CDB may
	> have a fixed length of up to 16 bytes or a variable length of
	> between 12 and 260 bytes.
	> [end quote]
	>
	> I can't find another reference within the 442 pages of the 07
	> draft of SPC-3 to the 260-byte maximum length for a
	> variable-length CDB.
	>
	> 1. Is the 260-byte figure in the definitions section of the
	> 07 draft of SPC-3 correct?
	
	The variable length CDB structure (spc3r07 table 7) has an 8 byte
	header, with a one byte ADDITIONAL CDB LENGTH field indicating the
	additional size in bytes.  That field must be a multiple of 4, so
	the maximum value is 252, restricting the maximum CDB length
	to 260 bytes.
	
	> 2. Is it intended that the maximum length of ExtendedCDB
	> field in the iSCSI Extended CDB AHS comes from SPC-3?
	
	The iSCSI AHS carries bytes 16-259 of a variable length CDB.
	The 2-byte AHSLength field needs to contain a value big enough
	to hold the CDB. It should be 8 less than the ADDITIONAL CDB
	LENGTH field in the CDB itself.
	
	> ...
	> Mike Smith
	> CTO, iReady
	
	--
	Rob Elliott, elliott@hp.com
	Industry Standard Server Storage Advanced Technology
	Hewlett-Packard