Re: iSCSI - positive data ack - change proposal

[Santosh Rao <santoshr@cup.hp.com>]

Julian,

I'm opposed to any use of out-of-band techniques by initiators to perform acks of data-in pdu's.

If at all this level of data ack is required, I would like to suggest a simpler in-band ack mechanism which is that targets number all PDUs that are transmitted from target to initiator with a sequence number. (say, InboundSN) on a per connection basis. Initiators can then acknowledge inbound PDUs as they are received by sending ExpInboundSN updates on outbound PDUs.

Such a technique is simpler to implement for initiators, less expensive in terms of resources and does not impact performance.

Specific to your proposal below, I am concerned about the data ack's being based at sequence boundaries (thru the use of F bit in data-in PDU to indicate "Data ACK" required). This may result in data ack's being generated too frequently. The data ack boundaries should be signalled through a seperate bit rather than over-riding the semantics of the F bit.

That said, I am opposed to the below proposal for all the reasons outlined earlier + the above. IF a data ack scheme is required (I did'nt hear enough "yes" calls to over-ride the Orlando WG consensus (?) ), then, I would favor an in-band technique such as the one suggested in this mail, wherein no extra PDUs are required from initiator to target for the sole purpose of acknowledging another inbound PDU or set of inbound PDUs.

Thanks,
Santosh


Julian Satran wrote:

> Here is updated version (in the previous I had excluded the sequences ended
> with Status with no good reason.
>
> Julo
>
> (See attached file: ack.txt)
>
> ----- Forwarded by Julian Satran/Haifa/IBM on 23-09-01 04:41 -----
>
>
>                     Julian Satran
>                                          To:     ips@ece.cmu.edu
>                     22-09-2001           cc:
>                     14:04                From:   Julian Satran/Haifa/IBM@IBMIL
>                                          Subject:     iSCSI - positive data ack - change
>                                           proposal
>
>
>
>
> Dear colleagues,
>
> As  I mentioned earlier all the elements needed for positive data-ack are
> already in place.
>
> I am suggesting the following changes to the document to reintroduce the
> data-ACK.
>
> Comments?
>
> Julo
>
> **** Attachment ack.txt has been removed from this note on 23 September
> 2001 by Julian Satran ****
>
>   ------------------------------------------------------------------------
> Within 2.2.2.3
> ---------------
>
> Initiators MAY also acknowledge received data and reduce by this the resources a target needs to dedicate to data recovery if target and initiator support this type of recovery.
>
> ------
> 3.7.1 F (Final) Bit
>
> For outgoing data, this bit is 1 for the last PDU of unsolicited data or the last PDU of a sequence answering an R2T.
>
> For incoming data, this bit is 1 for the last input (read) data PDU of a sequence.  Input can be split in several sequences each one having it's own F bit. Splitting in sequences does not affect DataSN counting on Data-In PDUs but MAY be used as a "change direction" indication for Bidirectional operations that need such a change and/or end of recoverable sequences by targets with a limited retransmission buffer.  Upon receiving an Data-In PDU with the F set to 1 in a session with ErrorRecoveryLevel 1 or higher the initiator MUST issue a DataACK type of SNACK indicating the next expected DataSN for this task.
>
> For Bidirectional operations, the F bit is 1 both for the end of the input sequences as well as the end of the output sequences.
>
>
> -----------
>
> 3.16  SNACK Request
>
> Byte /    0       |       1       |       2       |       3       |
>    /              |               |               |               |
>   |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
>   +---------------+---------------+---------------+---------------+
>  0|0|1| 0x10      |1|Rsrvd| Type  | Reserved                      |
>   +---------------+---------------+---------------+---------------+
>  4/ Reserved                                                      /
>  +/                                                               /
>   +---------------+---------------+---------------+---------------+
> 16| Initiator Task Tag or 0xffffffff                              |
>   +---------------+---------------+---------------+---------------+
> 20| BegRun                                                        |
>   +---------------+---------------+---------------+---------------+
> 24| RunLength                                                     |
>   +---------------+---------------+---------------+---------------+
> 28| ExpStatSN                                                     |
>   +---------------+---------------+---------------+---------------+
> 32| Reserved                                                      |
>   +---------------+---------------+---------------+---------------+
> 36| ExpDataSN or Reserved                                         |
>   +---------------+---------------+---------------+---------------+
> 32/ Reserved                                                      /
>  +/                                                               /
>   +---------------+---------------+---------------+---------------+
> 48
>
> Support for SNACK is optional.
>
> SNACK request is used to request retransmission of numbered-responses, data or R2T PDUs from the target.  The SNACK request indicates to the target the missed numbered-response or data run, where the run is composed of an initial missed StatSN, DataSN or R2TSN and the number of additional missed Status, Data or R2T PDUs (0 means only the initial).
>
> The numbered-response, Data or R2T PDUs requested by a SNACK have to be delivered as exact replicas of the ones the initiator missed including all its flags.
>
> Any SNACK requesting a numbered-response, Data or R2T that was not sent by the target MUST be rejected with a reason code of "Invalid SNACK".
>
> SNACK is also used to positively acknowledge Data-In PDUs.
>
> 3.16.1 Type
>
> This field encodes the SNACK function as follows:
>
> 0-Data/R2T SNACK - requesting retransmission of a Data-In or R2T PDU
> 1-Status SNACK - requesting retransmission of a numbered response
> 2-DataACK - positively acknowledges Data-In PDUs
>
> All other values are reserved.
>
> Data/R2T SNACK for a command MUST precede status acknowledgement for the given command.
>
> For a Data/R2T SNACK or a DataACK, the Initiator Task Tag MUST be set to the Initiator Task Tag of the referenced Command. Otherwise, it is reserved.
>
> For a Status SNACK the ExpDataSN field is reserved.
>
> An iSCSI target that does not support recovery within connection MAY discard status SNACK. If the target supports command recovery within session it MAY discard the SNACK after which it MUST issue an Asynchronous Message PDU with an iSCSI event indicating "Request Logout".
>
> If an initiator operates at ErrorRecoveryLevel 1 or higher it MUST issue a SNACK of type DataACK after receiving a Data-In PDU with the F bit set to 1.
>
> 3.16.2 BegRun
>
> First missed DataSN, R2TSN or StatSN
>
> 3.16.3 RunLength
>
> RunLength is the number of sequential missed DataSN, R2TSN or StatSN. RunLength 0 signals that all Data-In, R2T or Response PDUs carrying numbers equal or greater to BegRun have to be resent.
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