RE: iSCSI: current UNH Plugfest

["Barry Reinhold" <bbrtrebia@mediaone.net>]

Using features such as out of order command delivery on a connection tend to
be the sort of things that lead to interoperability problems. It is
unexpected and probably going to hit poorly tested code paths even if the
standard is written to allow it.

>-----Original Message-----
>From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
>Dave Sheehy
>Sent: Friday, November 02, 2001 4:19 PM
>To: IETF IP SAN Reflector
>Subject: Re: iSCSI: current UNH Plugfest
>
>
>
>> 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
>