Re: iSCSI: AHS use

[Bill Studenmund <wrstuden@wasabisystems.com>]

On Mon, 10 Jun 2002, Michael Smith wrote:

> I have some questions on the current AHS descriptions in 12-97. I just
> tripped over this, so I think maybe others might too.

[snip]

> 2. However, in the PDU format diagram on p. 131 of 12-97 it is not clear
> that we have the option of zero, one or more Additional Header Segments.
> After I re-read things a few times, and apologies if I have this wrong, I
> think that the use of multiple Additional Header Segments is along these
> lines:
>
> (a) Use one and only one Additional Header Segment for an extended CDB (SPC
> calls this a variable-length CDB). This extended CDB (or variable-length
> CDB) Additional Header Segment must immediately follow the BHS. (A suggested
> exact use definition of this type of Additional Header Segment coming up.)
>
> (b) Use one and only one Additional Header Segment for an Bidirectional
> Expected Read-Data Length. This Bidirectional Expected Read-Data Length
> Additional Header Segment must immediately follow the BHS.
>
> (c) Use of more than one Additional Header Segment is left up to the user.
>
> How many Additional Header Segments were we expecting to be able to be used?
> Would this maximum length of Additional Header Segments be limited only by
> the TotalAHSLength (8-bit field measuring length in four-byte words or
> ((2**8) - 1) * 4 bytes or roughly 1 kbyte)? Can we follow an extended CDB
> Additional Header Segment or a Bidirectional Expected Read-Data Length
> Additional Header Segment by one or more user-defined Additional Header
> Segment?
>
> Did I get this anywhere close to correct?

Not sure about correct, but you got it as I understand it. :-)

The one thing I'm not sure about is the use of, "must immediately follow
the BHS." I agree it must be in the AHS associated with the BHS, but if
you have a variable-length command (with CDB spill) which is also a bidi
command, you will have both an (a) and a (b), and only one can immediately
follow the BHS. :-)

> 3. In one of Bob Russell's emails we have the following, which does seem to
> make the use of multiple Additional Header Segments a little clearer to me:
>
>
> [quote http://www.pdl.cmu.edu/mailinglists/ips/mail/msg03784.html]
>
> [view in fixed-width font on wide window]
>      +------------------------+
>      |     required BHS       | > fixed length of 48 bytes
>      +------------------------+
>      |     optional AHS 1     |\
>      | - - - - - - - - - - -  | \
>      |     optional AHS 2     |  \
>      | - - - - - - - - - - -  |   > total length in AHS_length field in BHS
>      |        . . . .         |  /
>      | - - - - - - - - - - -  | /
>      |     optional AHS n     |/
>      +------------------------+
>      | optional header digest | -- covers preceding (48 + AHS_length) bytes
>      +------------------------+
>      |                        |\
>      |     optional data      | > total length in DATA_length field in BHS
>      |                        |/
>      +------------------------+
>      |  optional data digest  | -- covers preceding (DATA_length) bytes
>      +------------------------+
>
> [end view in fixed-width font on wide window]
>
> [end quote http://www.pdl.cmu.edu/mailinglists/ips/mail/msg03784.html]
>
> Is it possible to add something like this picture to the format diagram on
> p.131. I wouldn't ask if I hadn't tripped myself up on this already.

I think such a diagram would help too.

> 4. On p. 139 of 12-97 we have the following:

[snip text suggestion]

> There are 16 bytes in the CDB field to accommodate bytes 0-15 of the
> commonly used CDBs. An Extended CDB AHS MUST be used to contain bytes 16-259
> of a variable-length CDB [SPC].

Sounds like a good change too.

Take care,

Bill