Examples of org.jscsi.parser.snack.SNACKRequestParser

• org.jscsi.parser.snack.SNACKRequestParser

  SNACKRequestParser

  This class parses a SNACK Request message defined in the iSCSI Standard (RFC3720).

  If the implementation supports ErrorRecoveryLevel greater than zero, it MUST support all SNACK types.

  The SNACK is used by the initiator to request the retransmission of numbered-responses, data, or R2T PDUs from the target. The SNACK request indicates the numbered-responses or data "runs" whose retransmission is requested by the target, where the run starts with the first StatSN, DataSN, or R2TSN whose retransmission is requested and indicates the number of Status, Data, or R2T PDUs requested including the first. 0 has special meaning when used as a starting number and length:

  • When used in RunLength, it means all PDUs starting with the initial.
  • When used in both BegRun and RunLength, it means all unacknowledged PDUs.

  The numbered-response(s) or R2T(s), requested by a SNACK, MUST be delivered as exact replicas of the ones that the target transmitted originally except for the fields ExpCmdSN, MaxCmdSN, and ExpDataSN, which MUST carry the current values. R2T(s)requested by SNACK MUST also carry the current value of StatSN.

  The numbered Data-In PDUs, requested by a Data SNACK MUST be delivered as exact replicas of the ones that the target transmitted originally except for the fields ExpCmdSN and MaxCmdSN, which MUST carry the current values and except for resegmentation (see Section 10.16.3 Resegmentation).

  Any SNACK that requests a numbered-response, Data, or R2T that was not sent by the target or was already acknowledged by the initiator, MUST be rejected with a reason code of "Protocol error".

  Data Acknowledgement

  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 A bit set to 1. However, if the initiator has detected holes in the input sequence, it MUST postpone issuing the SNACK of type DataACK until the holes are filled. An initiator MAY ignore the A bit if it deems that the bit is being set aggressively by the target (i.e., before the MaxBurstLength limit is reached).

  The DataACK is used to free resources at the target and not to request or imply data retransmission.

  An initiator MUST NOT request retransmission for any data it had already acknowledged.

  Resegmentation

  If the initiator MaxRecvDataSegmentLength changed between the original transmission and the time the initiator requests retransmission, the initiator MUST issue a R-Data SNACK (see Section 10.16.1 Type). With R-Data SNACK, the initiator indicates that it discards all the unacknowledged data and expects the target to resend it. It also expects resegmentation. In this case, the retransmitted Data-In PDUs MAY be different from the ones originally sent in order to reflect changes in MaxRecvDataSegmentLength. Their DataSN starts with the BegRun of the last DataACK received by the target if any was received; otherwise it starts with 0 and is increased by 1 for each resent Data-In PDU.

  A target that has received a R-Data SNACK MUST return a SCSI Response that contains a copy of the SNACK Tag field from the R-Data SNACK in the SCSI Response SNACK Tag field as its last or only Response. For example, if it has already sent a response containing another value in the SNACK Tag field or had the status included in the last Data-In PDU, it must send a new SCSI Response PDU. If a target sends more than one SCSI Response PDU due to this rule, all SCSI responses must carry the same StatSN (see Section 10.4.4 SNACK Tag). If an initiator attempts to recover a lost SCSI Response (with a Status SNACK, see Section 10.16.1 Type) when more than one response has been sent, the target will send the SCSI Response with the latest content known to the target, including the last SNACK Tag for the command.

  For considerations in allegiance reassignment of a task to a connection with a different MaxRecvDataSegmentLength, refer to Section 6.2.2 Allegiance Reassignment.

  Initiator Task Tag

  For Status SNACK and DataACK, the Initiator Task Tag MUST be set to the reserved value 0xffffffff. In all other cases, the Initiator Task Tag field MUST be set to the Initiator Task Tag of the referenced command.

  @author Volker Wildi

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