Re: iSCSI: current UNH Plugfest

[Dave Sheehy <dbs@acropora.rose.agilent.com>]

> 3. Can commands be sent out of order on the same connection?
> 
>    The behavior of targets is clearly specified in Section 2.2.2.3 on
>    page 25 of draft 8, which says:
>      "Except for the commands marked for immediate delivery the iSCSI
>      target layer MUST eliver the commands for execution in the order
>      specified by CmdSN."
> 
>    Section 2.2.2.3 on page 26 of draft 8 also says:
>      "- CmdSN - the current command Sequence Number advanced by 1 on
>      each command shipped except for commands marked for immediate
>      delivery."
>    but the meaning of the term "shipped" is vague, and does not 
> necessarily
>    require that the PDUs arrive on the other end of a TCP connection
>    in the same order that the CmdSN values were assigned to these PDUs.
> 
>    Some initiators have been designed to send commands out of CmdSN
>    order on one connection.  Consider the situation where there is only
>    one connection and a high-level dispatcher creates a PDU for a SCSI
>    command that involves writing immediate data to the target.  This PDU
>    is enqueued to a lower-level layer which has to setup, start, and
>    wait-for a DMA operation to move the immediate data into an onboard
>    buffer before the PDU can be put onto the wire.  While this is
>    happening, the dispatcher creates another unrelated PDU for a SCSI
>    read command (for example), and when this PDU is passed to the
>    lower-level layer it can be sent immediately, ahead of the previous
>    write PDU and therefore out of order on this connection.
> 
>    The standard clearly allows this to happen if the two PDUs were sent
>    on different connections, and seems to imply that this can also happen
>    when the two PDUs are sent on the same connection.
> 
>    The suggestion is to put in the standard an explicit statement that
>    this is allowed or not allowed, as appropriate.
>  
>    If this is allowed, such a statement would avoid the erroneous
>    assumption being made by some target implementers that within a single
>    connection, commands will arrive in order.
> 
>    If this is not allowed, such a statement would avoid the erroneous
>    assumption being made by some initiator implementers that within a
>    single connection, commands can be put on the wire out of order.
> 
> +++ 
> 
> will add an explicit statement saying that this behaviour is forbidden.
> 2.2.2.1 will contain:
> 
> On any given connection, the iSCSI initiator MUST send the commands in the 
> order specified by CmdSN.
> 
> +++

Why do you feel this behavior should be forbidden? Targets already have to
order commands across the session. I don't see why it's a problem to extend
that to the connection as well. I, for one, believe we should take a liberal
stance on this.

Dave Sheehy