Re: rationale to send PDUs in increasing CmdSn onsingle connection

On Oct 19, 2007, at 8:03 AM, Eddy Quicksall wrote:

> Julian, below you said "no" to #2. But is there a restriction in the  
> RFC (maybe I just don't remember it)? I agree it has no practical  
> value but I have used it to test my re-ordering logic with only one  
> connection.

Parav quoted a MUST from the RFC on this, so I'd call that a  
restriction. :-)

I agree that it's a reasonable thing to do to test re-ordering logic.  
But I think that test environments don't count in that to be a good  
test of error handling, they _must_ break the spec. :-) Otherwise it's  
exceptionally difficult to reproducibly test how devices handle error  
conditions. :-)

Take care,

Bill

> ---- Original Message -----
> From: Julian Satran
> To: Parav Pandit
> Cc: [email protected]
> Sent: Friday, August 31, 2007 11:53 AM
> Subject: Re: [Ips] rationale to send PDUs in increasing CmdSn  
> onsingle connection
>
>
>
> Parav Pandit <[email protected]> wrote on 31/08/2007 12:52:04:
>
> > Hi,
> >
> > RFC 3720, section 3.2.2.1 says
> >
> > "On any connection, the iSCSI initiator MUST send the
> > commands in increasing order of CmdSN, except for
> > commands that are retransmitted due to digest error
> > recovery and connection recovery. "
> >
> > (Assuming Single TCP connection ISCSI session)
> >
> > 1. I interpret above 3.2.2.1 statement as
> > SCSI layer gives SCSI commands to the ISCSI stack in
> > the order of Cmd-1 and Cmd-2.
> > Cmd-1 will have CmdSn = 10.
> > Cmd-2 will have CmdSn = 11.
> > ISCSI stack CAN send PDUs to the TCP layer in
> > following order ONLY.
> > PDU-1 with Cmd-1.
> > PDU-2 with Cmd-2.
> >
> > Is this correct interpretation?
> > Or
> >
> Yes
> > 2. On a SINGLE connection can ISCSI stack send the
> > PDU-1 with Cmd-2 followed by
> > PDU-2 with Cmd-1?
> >
> NO
> > Assuming the answer of the question #2 is No,
> >
> > 3. If there are multiple connections in a session then
> > command MAY any way reach out of order. And targets
> > need to wait for the previous expected commands.
> >
> > So targets will receive out of order ISCSI PDUs from
> > the TCP layer and ISCSI stack handles them.
> >
> > So then why initiators have restriction of sending
> > command in the increasing order of CmdSn on SINGLE TCP
> > connection?
> >
> To simplify recovery and to...
> > Is it to simplify the implementation of targets
> > supporting only single TCP connection?
> >
> >
>
> and there was no visible motivation for out of order commands on a  
> single connection

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