Re: rationale to send PDUs in increasing CmdSn onsingle connection

[Julian Satran <Julian_Satran@xxxxxxxxxx>]

yes it is must and besides being pointless
sending out of order would break some assumptions about detecting missing
items (as Pat has nicely outlined in her note).

Julo

From:	William Studenmund <wrstuden@xxxxxxx>
To:	Eddy Quicksall <Quicksall_iSCSI@xxxxxxxxxxxxx>
Cc:	Parav Pandit <paravpandit@xxxxxxxxx>, Julian Satran/Haifa/IBM@IBMIL, Ips <ips@xxxxxxxx>
Date:	19/10/07 19:07
Subject:	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: ips@xxxxxxxx
Sent: Friday, August 31, 2007 11:53 AM
Subject: Re:  rationale to send PDUs in increasing
CmdSn onsingle connection



Parav Pandit <paravpandit@xxxxxxxxx>
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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