X3T9.2/90-041






March 1, 1990

Mr. Eric Tausheck
Hewlett Packard Company
8000 Foothills Blvd.   
Roseville, CA 95678    

Subject: Response to BSR X3.131-198x (SCSI-2) Public Review Comment #9

Dear Mr. Tausheck,

Thank you for your interest in the SCSI-2 draft proposed standard.  The X3T9.2 
committee has reviewed your public review comment (X3T9.2/90-030) requesting 
that the Asynchronous Event Notification (AEN) protocol be changed to an 
unsolicited reselection method.  

As you noted, the committee had considered your proposed method and rejected 
it in favor of the present method.  The committee was aware that this could 
pose a problem for implementations which did not include hardware to support 
the initiator role.  A poll of committee members at the time (late '86 or 
early '87) indicated that new SCSI protocol chip implementations were 
routinely including both target and initiator role.  The committee did not 
expect this to be a significant problem by the time SCSI-2 would be approved.

Some of the advantages of the present AEN method are:

  1. The AEN overhead is low.  AEN-capable devices conduct a survey at power-
  on time to establish which SCSI addresses contain processor devices.  While 
  this survey may take up to 1.75 seconds to complete, thereafter, AENs are 
  only sent to valid addresses.  (Note that a late-arriving processor device 
  will be detected the first time the target receives an I/O process from it.)

  2. Only processor devices that are AEN-capable are notified.  A bit in the 
  processor device INQUIRY data identifies whether it can accept AENs.  This 
  improves compatibility with older devices which may not support AEN 
  protocol.  Also, it avoids notifying devices such as COPY managers which 
  have no interest in the target except during the actual execution of the 
  COPY command.  Initiators (processor devices) which employ the COPY command 
  are expected to deal with any AENs they receive between the execution of 
  COPY commands.  If an event occurs during the execution of a COPY command, 
  the target may notify the COPY manager via ordinary CHECK CONDITION status 
  mechanisms.

Response to BSR X3.131-198x (SCSI-2) Public Review Comment #9    X3T9.2/90-041

The X3T9.2 committee selected the present method of doing AENs 3 years ago, 
with active participation from several Hewlett Packard representatives.  There 
is no doubt that your proposed method of doing asynchronous event notification 
is a viable alternative protocol.  However, your proposal cannot be accepted 
without creating potential harm to other implementors who, acting on good 
faith, have begun implementing the current AEN method.  Accepting both methods 
is contrary to the purpose of standards which is to limit options -- not 
expand them.  Consequently, your comment cannot be accepted.

Please send a written statement to the X3 Secretariat (Attn: Lynn Barra) 
within fifteen working days saying whether you accept or reject this response.  
If you do not respond within fifteen working days, the Secretariat will assume 
that you have withdrawn the comment.

Sincerely Yours,


Del Shoemaker, X3T9 Chairman

cc: L. Barra, X3 Secretariat
    J. Lohmeyer, X3T9.2 Chairman
    J. Ryland, SPARC Liaison






























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