Re: iscsi : DataPDULength can differ in each direction.

[Santosh Rao <santoshr@cup.hp.com>]

Rod,

The issue came up as a result of Deva pointing out that an initiator may
not be able to function with the minimal of the 2 DataPDULengths
(initiator's & target's) in the case where the value chosen was not its
value.

In order to allow for such implementations that had issues with the
DataPDULength, I raised this question. 

The benefit of both sides being allowed different DataPDULengths is :

a) Each side can specify its max supported value which the other side
would not exceed. The sender & receiver would be able to function with
different DataPDULengths, with the guarantee that their advertised
receive pdu length will not be exceeded, thus, allowing more flexible
interoperability of implementations. (btw, the proposal should have read
: Each side is allowed to advertise its maximum receive pdu length.)

b) It may also allow 2 parties with differing PDU lengths to send larger
sized PDUs in one direction to the party that advertised a higher
receive pdu length.

However, I do admit there's a cost to making this change, given we are
so late in the draft rev and several implementations have already been
made. If this is not an issue of concern to the majority of
implementations, I am ok with the current definition, although it is
more limiting on implementations.


Thanks,
Santosh


Rod Harrison wrote:
> 
>         Can someone give a tangible benefit to this that can outweigh the
> spec and implementation churn at this late stage of the game?
> 
>         From my point of view the benefit of asymmetric PDU sizes would have
> to be very large to make it worth the extra complexity in buffer
> management code alone.
> 
>         - Rod
> 
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Ayman Ghanem
> Sent: Wednesday, October 03, 2001 8:29 PM
> To: ips@ece.cmu.edu
> Subject: RE: iscsi : DataPDULength can differ in each direction.
> 
> Well, the proposal says:
> 
>         Each side should be allowed to specify the DataPDULength it will be
>         using and there should be no attempt to negotiate this value. Rather,
>         the key is exchanged in either direction to inform the other side as
> to
>         what sized PDUs it should expect.
> 
> So, I understand that as if the initiator sends
> DataPDULength=reallyBigNumber, then it is informing the target that
> this is
> what it will be using, but the target should not attempt to negotiate
> it.
> 
> -Ayman
> 
> > -----Original Message-----
> > From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf
> Of
> > Buck Landry
> > Sent: Wednesday, October 03, 2001 2:13 PM
> > To: ips@ece.cmu.edu
> > Subject: RE: iscsi : DataPDULength can differ in each direction.
> >
> >
> > It may add complexity, but, since DataPDULength is only a boundary
> on
> > the maximum number of bytes transmitted in a Data PDU (not the
> minimum),
> > "the first" *does* have a say about it.  Right?
> >
> > -----Original Message-----
> > From: Ayman Ghanem [mailto:aghanem@cisco.com]
> > Sent: Wednesday, October 03, 2001 1:38 PM
> > To: ips@ece.cmu.edu
> > Subject: RE: iscsi : DataPDULength can differ in each direction.
> >
> >
> > I think this will add unnecessary complexity, specially for data
> CRCs
> > having
> > to be calculated based on two different PDU sizes for Reads and
> Writes.
> > Also, one end may choose to do data digests in software. If the
> other
> > end
> > decides to use a really large PDU length (and the first doesn't have
> a
> > say
> > about it), this could be a problem.
> >
> > -Ayman
> >
> > <snip>
> >

-- 
##################################
Santosh Rao
Software Design Engineer,
HP-UX iSCSI Driver Team,
Hewlett Packard, Cupertino.
email : santoshr@cup.hp.com
Phone : 408-447-3751
##################################