iSCSI - ExpDataSN

["Julian Satran" <Julian_Satran@il.ibm.com>]

Mallikarjun has expressed a lingering concern that we should allow the value 0
for the field ExpDataSN in a TASK REASSIGN TM function to say "give me all unacked data" (as we do for SNACK).
Please observe that we already allow the target to do this on its own and this change does not affect
any recovery mechanism. To do it I am suggesting the following rephrasing of 9.5.6

For recovery purposes the iSCSI target and initiator maintain a data acknowledgement reference number - the first input DataSN number unacknowledged by the initiator. When issuing a new command this num-ber is set to 0. If the function is TASK REASSIGN, which establishes a new connection allegiance for a previously issued Read or Bidirec-tional command ExpDataSN will contain either a new data acknowldge-ment reference number or the value 0 indicating that the data acknowledgement reference number is unchanged. The initiator MUST discard any data PDUs from the previous execution that it did not acknowledge and the target MUST transmit all Data-in PDUs (if any) starting with the data acknowledgement reference number.  The number of retransmitted PDUs, may or may not be the same as the original transmission depending on if there was a change in MaxRecvDataSeg-mentLength in the reassignment. The target MAY also  send no more Data-In !
PDUs if it sent all its data in PDUs with DataSN less than ExpDataSN.

The value of ExpDataSN  MUST be either 0 or higher than the DataSN of the last acknowledged Data-In PDU but not larger than DataSN+1 of the last Data-IN PDU sent by the target. Any other value MUST be ignored by the target. 

For other functions this field is reserved.

Please let me know what you think.

Julo