Re: rationale to send PDUs in increasing CmdSn on single connection

[Julian Satran <Julian_Satran@xxxxxxxxxx>]

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

> Regards,
> Parav Pandit
> 
> 
> 
>        
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