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Small Computer Systems Interface (SCSI) Command Ordering Considerations with iSCSI
RFC 3783

Network Working Group                                     M. Chadalapaka
Request for Comments: 3783                                    R. Elliott
Category: Informational                              Hewlett-Packard Co.
                                                                May 2004

                Small Computer Systems Interface (SCSI)
               Command Ordering Considerations with iSCSI

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   Internet Small Computer Systems Interface (iSCSI) is a Small Computer
   Systems Interface (SCSI) transport protocol designed to run on top of
   TCP.  The iSCSI session abstraction is equivalent to the classic SCSI
   "I_T nexus", which represents the logical relationship between an
   Initiator and a Target (I and T) required in order to communicate via
   the SCSI family of protocols.  The iSCSI session provides an ordered
   command delivery from the SCSI initiator to the SCSI target.  This
   document goes into the design considerations that led to the iSCSI
   session model as it is defined today, relates the SCSI command
   ordering features defined in T10 specifications to the iSCSI
   concepts, and finally provides guidance to system designers on how
   true command ordering solutions can be built based on iSCSI.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Definitions and Acronyms . . . . . . . . . . . . . . . . . . .  3
       2.1.  Definitions. . . . . . . . . . . . . . . . . . . . . . .  3
       2.2.  Acronyms . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview of the iSCSI Protocol . . . . . . . . . . . . . . . .  4
       3.1.  Protocol Mapping Description . . . . . . . . . . . . . .  4
       3.2.  The I_T Nexus Model. . . . . . . . . . . . . . . . . . .  5
       3.3.  Ordered Command Delivery . . . . . . . . . . . . . . . .  6
             3.3.1.  Questions. . . . . . . . . . . . . . . . . . . .  6
             3.3.2.  The Session Guarantee. . . . . . . . . . . . . .  6
             3.3.3.  Ordering Onus. . . . . . . . . . . . . . . . . .  7
             3.3.4.  Design Intent. . . . . . . . . . . . . . . . . .  7

Chadalapaka & Elliott        Informational                      [Page 1]
RFC 3783                    Command Ordering                    May 2004

   4.  The Command Ordering Scenario. . . . . . . . . . . . . . . . .  8
       4.1.  SCSI Layer . . . . . . . . . . . . . . . . . . . . . . .  8
             4.1.1.  Command Reference Number (CRN) . . . . . . . . .  8
             4.1.2.  Task Attributes. . . . . . . . . . . . . . . . .  8
             4.1.3.  Auto Contingent Allegiance (ACA) . . . . . . . .  8
             4.1.4.  UA Interlock . . . . . . . . . . . . . . . . . .  9
       4.2.  iSCSI Layer. . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Connection Failure Considerations. . . . . . . . . . . . . . .  9
   6.  Command Ordering System Considerations . . . . . . . . . . . . 10
   7.  Reservation Considerations . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations. . . . . . . . . . . . . . . . . . . . 12
   9.  References and Bibliography. . . . . . . . . . . . . . . . . . 12
       9.1.  Normative References.. . . . . . . . . . . . . . . . . . 12
       9.2.  Informative References . . . . . . . . . . . . . . . . . 12
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
   12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   iSCSI is a SCSI transport protocol ([iSCSI]) designed to enable
   running SCSI application protocols on TCP/IP networks, including
   potentially the Internet.  Given the size and scope of the Internet,
   iSCSI thus enables some exciting new SCSI applications.  Potential
   new application areas for exploiting iSCSI's value include the
   following:

      a) Larger (diameter) Storage Area Networks (SANs) than had been
         possible until now
      b) Asynchronous remote mirroring
      c) Remote tape vaulting

   Each of these applications takes advantage of the practically
   unlimited geographical distance that iSCSI enables between a SCSI
   initiator and a SCSI target.  In each of these cases, because of the
   long delays involved, there is a very high incentive for the
   initiator to stream SCSI commands back-to-back without waiting for
   the SCSI status of previous commands.  Command streaming may be
   employed primarily by two classes of applications - while one class

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