X3T9.2/92-75
Date:     March, 18, 1992

To:       X3T9.2 Membership

From:     John A. Gallant
          Software Engineer, USSG, Digital Equipment Corp. 

Subject:  Creating a TIB for SCSI-2 Tag Queuing working model.


    I would like to setup a group to produce a Technical Information Bulletin,
(TIB), to help system, adapter, and peripheral vendors implement a model
for tag queuing that will allow them to work together.

  I propose the following two goals for the group:

    1)  To provide a model for Tag Queuing to handle error conditions using
        existing SCSI-2 tools.  This model must work in single and multi-
        initiator environments.

    2)  To get this model as a TIB out to the real world be for too many
        vendors, (Peripheral and Host), implement Tag Queuing with varied
        and unique error handling implementations.


  The model has the following functionality:

    Targets and hosts that use Tag Queuing must also use Extended Contingent
    Allegiance, (ECA).

    ECA provides MOST(!) of the tools to effectively allow the host to deal
    properly with error conditions in a Tag Queuing environment.

    There are CAM, and "Packet-ized" issues with Tag Queuing and Auto-sense.

  There are some more "tools" that are needed:

    The remaining tools for this model work with ECA and modify it's behavior
    to allow Tag Queuing error recovery.

    A)  When a Target is in ECA, "Reservation Conflict" status is used
        instead of "Busy" status.
        (Futures: localization of error condition)

    B)  The Target enters a "Reserved like" state.  To allow Inquiry and
        Request Sense from other Initiators.

    C)  Any Tagged Request from the same Initiator will also get Reservation
        Conflict.

    D)  The Initiator must explicitly send a Release Recovery Message to 
        remove the ECA condition.