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Versions: 00 01 02 rfc3783                                 Informational
     IPS                                       Mallikarjun Chadalapaka
     Internet Draft                                        Rob Elliott
     draft-ietf-ips-command-ordering-02.txt       Hewlett-Packard Co.
     Category: Informational-track

             SCSI Command Ordering Considerations with iSCSI

Status of this Memo

     This document is an Internet-Draft and fully conforms to all
     provisions of Section 10 of RFC2026.

     Internet-Drafts are working documents of the Internet
     Engineering Task Force (IETF), its areas, and its working
     groups. Note that other groups may also distribute working
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     The list of Internet-Drafts can be accessed at http://
     The list of Internet-Draft Shadow Directories can be accessed at


     iSCSI is a 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

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     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.


     We are grateful to the IPS working group whose work defined the
     iSCSI protocol.  Thanks also to David Black (EMC) who encouraged
     the publication of this document.  Special thanks to Randy
     Haagens (HP) for his insights on the topic of command ordering.
     Thanks are also due to Elizabeth Rodriguez for carefully
     reviewing this document.

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 Status of this Memo . . . . . . . . . . . . . . . . . . . . . . . . . 1
 Abstract  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions and Acronyms  . . . . . . . . . . . . . . . . . . . . . 5
     2.1 Definitions  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     2.2 Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of the iSCSI Protocol  . . . . . . . . . . . . . . . . . . 7
     3.1 Protocol Mapping Description . . . . . . . . . . . . . . . . . . 7
     3.2 The I_T Nexus Model  . . . . . . . . . . . . . . . . . . . . . . 7
     3.3 Ordered Command Delivery . . . . . . . . . . . . . . . . . . . . 9
       3.3.1 Questions . . . . . . . . . . . . . . . . . . . . . . . . . 9
       3.3.2 The Session Guarantee . . . . . . . . . . . . . . . . . . . 9
       3.3.3 Ordering Onus . . . . . . . . . . . . . . . . . . . . . . .10
       3.3.4 Design Intent . . . . . . . . . . . . . . . . . . . . . . .10
4. The Command Ordering Scenario . . . . . . . . . . . . . . . . . . .11
     4.1 SCSI Layer . . . . . . . . . . . . . . . . . . . . . . . . . . .11
       4.1.1 Command Reference Number (CRN)  . . . . . . . . . . . . . .11
       4.1.2 Task Attributes . . . . . . . . . . . . . . . . . . . . . .11
       4.1.3 Auto Contingent Allegiance (ACA)  . . . . . . . . . . . . .11
       4.1.4 UA Interlock  . . . . . . . . . . . . . . . . . . . . . . .12
     4.2  iSCSI Layer . . . . . . . . . . . . . . . . . . . . . . . . . .12
5. Connection Failure Considerations . . . . . . . . . . . . . . . . .13
6. Command Ordering System Considerations  . . . . . . . . . . . . . .14
7. Reservation Considerations  . . . . . . . . . . . . . . . . . . . .15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . .16
9. Security Considerations . . . . . . . . . . . . . . . . . . . . . .17
10. References and Bibliography  . . . . . . . . . . . . . . . . . . .18
     10.1 Normative References  . . . . . . . . . . . . . . . . . . . . .18
     10.2 Informative References: . . . . . . . . . . . . . . . . . . . .18
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .19
 Full Copyright Statement  . . . . . . . . . . . . . . . . . . . . . .20

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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 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 may not particularly care about
     ordered command execution, the other class does rely on ordered
     command execution (i.e. there is an application-level dependency
     on the ordering among SCSI commands).  As an example, cases b)
     and c) listed earlier clearly require ordered command execution.
     A mirroring application does not want the writes to be committed
     out of order on the remote SCSI target, so as to preserve the
     transactional integrity of the data on that target.  To
     summarize, SCSI command streaming when coupled with the
     guarantee of ordered command execution on the SCSI target is
     extremely valuable for a critical class of applications in long-
     latency networks.

     This document reviews the various protocol considerations in
     designing storage solutions that employ SCSI command ordering.
     This document also analyzes and explains the design intent of
     [iSCSI] with respect to command ordering.

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2. Definitions and Acronyms

2.1  Definitions

     - I_T nexus: [SAM2] defines the I_T nexus as a relationship
     between a SCSI initiator port and a SCSI target port. [iSCSI]
     defines an iSCSI session as the iSCSI representation of an I_T
     nexus. In the iSCSI context, the I_T nexus (i.e. the iSCSI
     session) is a relationship between an iSCSI initiator's end of
     the session (SCSI Initiator Port) and the iSCSI target's Portal
     Group (SCSI Target Port).

     - PDU (Protocol Data Unit): An iSCSI initiator and iSCSI target
     communicate using iSCSI protocol messages. These messages are
     called "iSCSI protocol data units" (iSCSI PDUs).

     - SCSI device: A SCSI device is an entity that contains one or
     more SCSI ports that are connected to a service delivery
     subsystem and supports SCSI application protocols.  In the iSCSI
     context, the SCSI Device is the component within an iSCSI Node
     that provides the SCSI functionality.  The SCSI Device Name is
     defined to be the iSCSI Name of the node.

     - Session: A group of logically related iSCSI connections that
     link an initiator with a target form a session (equivalent to a
     SCSI I-T nexus). The number of participating iSCSI connections
     within an iSCSI session may vary over time. The multiplicity of
     connections at the iSCSI level is completely hidden for the SCSI
     layer - each SCSI port in an I_T nexus sees only one peer SCSI
     port across all the connections of a session.

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2.2  Acronyms

     Acronym                      Definition
     ACA                          Auto Contingent Allegiance
     ASC                          Additional Sense Code
     ASCQ                         Additional Sense Code Qualifier
     CRN                          Command Reference Number
     IETF                         Internet Engineering Task Force
     ISID                         Initiator Session Identifier
     ITT                          Initiator Task Tag
     LU                           Logical Unit
     LUN                          Logical Unit Number
     NIC                          Network Interface Card
     PDU                          Protocol Data Unit
     TMF                          Task Management Function
     TSIH                         Target Session Identifying Handle
     SAM-2                        SCSI Architecture Model - 2
     SAN                          Storage Area Network
     SCSI                         Small Computer Systems Interface
     TCP                          Transmission Control Protocol
     UA                           Unit Attention
     WG                           Working Group

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3. Overview of the iSCSI Protocol

3.1  Protocol Mapping Description

     The iSCSI protocol is a mapping of the SCSI remote procedure
     invocation model (see [SAM2]) over the TCP protocol.

     SCSI's notion of a task maps to an iSCSI task.  Each iSCSI task
     is uniquely identified within that I_T nexus by a 32-bit unique
     identifier called Initiator Task Tag (ITT).  The ITT is both an
     iSCSI identifier of the task and a classic SCSI task tag.

     SCSI commands from the initiator to the target are carried in
     iSCSI requests called SCSI Command PDUs.  SCSI status back to
     the initiator is carried in iSCSI responses called SCSI Response
     PDUs.  SCSI Data-out from the initiator to the target is carried
     in SCSI Data-Out PDUs, and the SCSI Data-in back to the
     initiator is carried in SCSI Data-in PDUs.

3.2  The I_T Nexus Model

     In the iSCSI model, the SCSI I_T nexus maps directly to the
     iSCSI session which is an iSCSI protocol abstraction spanning
     one or more TCP connections.  The iSCSI protocol defines the
     semantics in order to realize one logical flow of bidirectional
     communication on the I_T nexus potentially spanning multiple TCP
     connections (as many as 2^16).  The multiplicity of iSCSI
     connections is thus completely contained at the iSCSI layer,
     while the SCSI layer is presented with a single I_T nexus even
     in a multi-connection session.  A session between a pair of
     given iSCSI nodes is identified by the session identifier (SSID)
     and each connection within a given session is uniquely
     identified by a connection identifier (CID) in iSCSI.  The SSID
     itself has two components - Initiator Session Identifier (ISID)
     and a Target Session Identifying Handler (TSIH) - each
     identifying one end of the same session.

     There are four crucial functional facets of iSCSI that together
     present this single logical flow abstraction to the SCSI layer
     even with an iSCSI session spanning across multiple iSCSI

           a)  Ordered command delivery:  A sequence of SCSI commands
              that is striped across all the connections in the

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             session is "reordered" by the target iSCSI layer into
             an identical sequence based on a Command Sequence
             Number (CmdSN) that is unique across the session.  The
             goal is to achieve bandwidth aggregation from multiple
             TCP connections, but to still make it appear to the
             target SCSI layer as if all the commands had travelled
             in one flow.
           b)  Connection allegiance: All the PDU exchanges for a
             SCSI Command, up to and including the SCSI Response PDU
             for the Command, are required to flow on the same iSCSI
             connection at any given time.  This again is intended
             to hide the multi-connection nature of a session
             because the SCSI layer on either side will never see
             the PDU contents out of order (e.g., status cannot
             bypass read data for an initiator).
           c)  Task set management function handling: [iSCSI]
             specifies an ordered sequence of steps for the iSCSI
             layer on the SCSI target in handling the two SCSI task
             management functions (TMFs) that manage SCSI task sets.
             The two TMFs are ABORT TASK SET that aborts all active
             tasks in a session and CLEAR TASK SET that clears the
             tasks in the task set.  The goal of the sequence of
             steps is to guarantee that the initiator receives the
             SCSI Response PDUs of all unaffected tasks before the
             TMF Response itself arrives, regardless of the number
             of connections in the iSCSI session.  This operational
             model is again intended to preserve the single flow
             abstraction to the SCSI layer.
           d)  Immediate task management function handling: Even when
             a TMF request is marked as "immediate" (i.e. only has a
             position in the command stream, but does not consume a
             CmdSN), [iSCSI] defines semantics that require the
             target iSCSI layer to ensure that the TMF request is
             executed as if the commands and the TMF request were
             all flowing on a single logical channel.  This ensures
             that the TMF request will act on tasks that it was
             meant to manage.

     The following sections will analyze the "Ordered command
     delivery" aspect in more detail, since command ordering is the
     focus of this document.

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3.3  Ordered Command Delivery

3.3.1  Questions

     A couple of important questions related to iSCSI command
     ordering were considered early on in the design of the iSCSI
     protocol.  The questions were:

           a)  What should be the command ordering behavior required
                of iSCSI implementations in the presence of transport
                errors, such as errors that corrupt the data in a
                fashion that is not detected by the TCP checksum (e.g.,
                two offsetting bit flips in the same bit position), but
                is detected by the iSCSI CRC digest?
           b)  Should [iSCSI] require both initiators and targets to
                use ordered command delivery?

     Since the answers to these questions are critical to the
     understanding of the ordering behavior required by the iSCSI
     protocol, the following sub-sections consider them in more

3.3.2  The Session Guarantee

     The final disposition of question a) in section 3.3.1 was
     reflected in [RFC3347], "iSCSI MUST specify strictly ordered
     delivery of SCSI commands over an iSCSI session between an
     initiator/target pair, even in the presence of transport
     errors.".  Stated differently, an iSCSI digest failure, or an
     iSCSI connection termination must not cause the iSCSI layer on a
     target to allow executing the commands in an order different
     from that intended (as indicated by the CmdSN order) by the
     initiator.  This design choice is enormously helpful in building
     storage systems and solutions that can now always assume command
     ordering to be a service characteristic of an iSCSI substrate.

     Note that by taking the position that an iSCSI session always
     guarantees command ordering, [iSCSI] was indirectly implying
     that the principal reason for the multi-connection iSCSI session
     abstraction was to allow ordered bandwidth aggregation for an
     I_T nexus.  In deployment models where this cross-connection
     ordering mandated by [iSCSI] is deemed expensive, a serious
     consideration should be given to deploying multiple single-
     connection sessions in stead.

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3.3.3  Ordering Onus

     The final resolution of b) in section 3.3.1 by the iSCSI
     protocol designers was in favor of not requiring the initiators
     to use command ordering always.  This resolution is reflected in
     dropping the mandatory ACA usage requirement on the initiators,
     and allowing an ABORT TASK TMF to plug a command hole etc.,
     since these are conscious choices an initiator makes in favor of
     not using ordered command delivery.  The net result can be
     discerned by a careful reader of [iSCSI] - the onus of ensuring
     ordered command delivery is always on the iSCSI targets, while
     the initiators may or may not utilize command ordering.  iSCSI
     targets being the servers in the client-server model do not
     really attempt to establish whether or not a client (initiator)
     intends to take advantage of command ordering service, but
     instead simply always provide the guaranteed delivery service.
     The rationale here is that there are inherent SCSI and
     application-level dependencies as we shall see in building a
     command ordered solution that are beyond the scope of [iSCSI],
     to mandate or even discern the intent with respect to the usage
     of command ordering.

3.3.4  Design Intent

     To summarize the design intent of [iSCSI] -

          The service delivery subsystem (see [SAM2]) abstraction
     provided by an iSCSI session is guaranteed to have the intrinsic
     property of ordered delivery of commands to the target SCSI
     layer under all conditions.  Consequently, the guarantee of the
     ordered command delivery is across the entire I_T nexus spanning
     all the LUs that the nexus is authorized to access. It is the
     initiator's discretion to whether or not make use of this

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4. The Command Ordering Scenario

     A storage systems designer working with SCSI and iSCSI has to
     consider the following protocol features in SCSI and iSCSI
     layers, each of which has a role to play in realizing the
     command ordering goal.

4.1  SCSI Layer

     The SCSI application layer has several tools to enforce

4.1.1  Command Reference Number (CRN)

     CRN is an ordered sequence number which when enabled for a
     device server, increments by one for each I_T_L nexus (see
     [SAM2]).  The one notable drawback with CRN is that there is no
     SCSI-generic way (such as through mode pages) to enable or
     disable the CRN feature.  [SAM2] also leaves the usage semantics
     of CRN for the SCSI transport protocol, such as iSCSI, to
     specify.  [iSCSI] chose not to support the CRN feature for
     various reasons.

4.1.2  Task Attributes

     SAM-2 defines the following four task attributes - SIMPLE,
     ORDERED, HEAD OF QUEUE, and ACA. Each task to an LU may be
     assigned an attribute.  [SAM2] defines the ordering constraints
     that each of these attributes conveys to the device server that
     is servicing the task.  In particular, judicious use of ORDERED
     and SIMPLE attributes applied to a stream of pipelined commands
     could convey the precise execution schema for the commands that
     the initiator issues, provided the commands are received in the
     same order on the target.

4.1.3  Auto Contingent Allegiance (ACA)

     ACA is an LU-level condition that is triggered when a command
     (with the NACA bit set to 1) completes with CHECK CONDITION.
     When ACA is triggered, it prevents all commands other than those
     with the ACA attribute from executing until the CLEAR ACA task
     management function is executed, while blocking all the other
     tasks already in the task set.  See [SAM2] for the detailed
     semantics of ACA.  Since ACA is closely tied to the notion of a

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     task set, one would ideally have to select the scope of the task
     set (by setting the TST bit to 1 in the control mode page of the
     LU) to be per-initiator in order to prevent command failures in
     one I_T_L nexus from impacting other I_T_L nexuses through ACA.

4.1.4  UA Interlock

     When UA interlock is enabled, the logical unit does not clear
     any standard Unit Attention condition reported with autosense
     and in addition, establishes a Unit Attention condition when a
     task is terminated with one of BUSY, TASK SET FULL, or
     RESERVATION CONFLICT statuses.  This so-called "interlocked UA"
     is cleared only when the device server executes an explicit
     REQUEST SENSE ([SPC3]) command from the same initiator.  From a
     functionality perspective, the scope of UA interlock today is
     slightly different from ACA's because it enforces ordering
     behavior for completion statuses other than CHECK CONDITION, but
     otherwise conceptually has the same design intent as ACA.  On
     the other hand, ACA is somewhat more sophisticated because it
     allows special "cleanup" tasks (ones with ACA attribute) to
     execute when ACA is active.  One of the principal reasons UA
     interlock came into being was that SCSI designers wanted a
     command ordering feature without the side effects of using the
     aforementioned TST bit in the control mode page.

4.2   iSCSI Layer

     As noted in section 3.2 and section 3.3, the iSCSI protocol
     enforces and guarantees ordered command delivery per iSCSI
     session using the CmdSN, and this is an attribute of the SCSI
     transport layer.  Note further that any command ordering
     solution that seeks to realize ordering from the initiator SCSI
     layer to the target SCSI layer would be of practical value only
     when the command ordering is guaranteed by the SCSI transport
     layer.  In other words, the related SCSI application layer
     protocol features such as ACA etc. are based on the premise of
     an ordered SCSI transport.  Thus iSCSI's command ordering is the
     last piece in completing the puzzle of building solutions that
     rely on ordered command execution, by providing the crucial
     guarantee that all the commands handed to the initiator iSCSI
     layer will be transported and handed to the target SCSI layer in
     the same order.

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5. Connection Failure Considerations

     [iSCSI] mandates that when an iSCSI connection fails, the active
     tasks on that connection must be terminated if not recovered
     within a certain negotiated time limit. When an iSCSI target
     does terminate some subset of tasks due to iSCSI connection
     dynamics, there is a danger that the SCSI layer would simply
     move on to the next tasks waiting to be processed and execute
     them out-of-order unbeknownst to the initiator SCSI layer.  To
     preclude this danger, [iSCSI] further mandates the following -

        a)  The tasks terminated due to the connection failure must
        be internally terminated by the iSCSI target "as if" due to a
        CHECK CONDITION.  While this particular completion status is
        never communicated back to the initiator, the "as if" is
        still meaningful and required because if the initiator were
        using ACA as the command ordering mechanism of choice, a
        SCSI-level ACA will be triggered due to this mandatory CHECK
        CONDITION.  This addresses the aforementioned danger.
        b)  After the tasks are terminated due to the connection
        failure, the iSCSI target must report a Unit Attention
        condition on the next command processed on any connection for
        each affected I_T_L nexus of that session.  This is required
        because if the initiator were using UA interlock as the
        command ordering mechanism of choice, a SCSI-level UA will
        trigger a UA-interlock.  This again addresses the
        aforementioned danger. iSCSI targets must report this UA with
        the status of CHECK CONDITION, and the ASC/ASCQ value of 47h/

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6. Command Ordering System Considerations

     In general, command ordering is automatically enforced if
     targets and initiators comply with the iSCSI specification.
     However, listed below are certain additional related
     implementation considerations for the iSCSI initiators and
     targets to take note of.

        a)   Even when all iSCSI and SCSI command ordering
        considerations earlier noted in this draft were applied, it
        is beneficial for iSCSI initiators to proactively avoid
        scenarios that would otherwise lead to out-of-order command
        execution.  This is simply because the SCSI command ordering
        features such as UA interlock are likely to be costlier in
        performance when they are allowed to be triggered. [iSCSI]
        provides enough guidance on how to implement this proactive
        detection of PDU ordering errors.
        b)  The whole notion of command streaming does of course
        assume that the target in question supports command queueing.
        An iSCSI target desirous of supporting command ordering
        solutions should ensure that the SCSI layer on the target
        supports command queuing.  Especially the remote backup (tape
        vaulting) applications that iSCSI enables make a compelling
        case that tape devices should give a very serious
        consideration to supporting command queuing, at least when
        used in conjunction with iSCSI.
        c)  An iSCSI target desirous of supporting high-performance
        command ordering solutions that involve specifying a
        description of execution schema should ensure that the SCSI
        layer on the target in fact does support the ORDERED and
        SIMPLE task attributes.
        d)  There is some consideration of expanding the scope of UA
        interlock to encompass CHECK CONDITION status and thus make
        it the only required command ordering functionality of
        implementations to build command ordering solutions.  Until
        this is resolved in T10, the currently defined semantics of
        UA interlock and ACA warrant implementing both features by
        iSCSI targets desirous of supporting command ordering

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7. Reservation Considerations

     [iSCSI] describes a "principle of conservative reuse" that
     encourages iSCSI initiators to reuse the same ISIDs (see section
     3.2) to various SCSI target ports, in order to present the same
     SCSI initiator port name to those target ports.  This is in fact
     a very crucial implementation consideration that must be
     complied with.  [SPC3] mandates the SCSI targets to associate
     persistent reservations and the related registrations with the
     SCSI initiator port names whenever they are required by the SCSI
     transport protocol.  Since [iSCSI] requires the mandatory SCSI
     initiator port names based on ISIDs, iSCSI targets are required
     to work off the SCSI initiator port names and thus indirectly
     the ISIDs in enforcing the persistent reservations.

     This fact has the following implications for the

        a)  If a persistent reservation/registration is intended to
        be used across multiple SCSI ports of a SCSI device, the
        initiator iSCSI implementation must use the same ISID across
        associated iSCSI sessions connecting to different iSCSI
        target portal groups of the SCSI device.
        b)  If a persistent reservation/registration is intended to
        be used across the power loss of a SCSI target, the initiator
        iSCSI implementation must use the same ISIDs as before in re-
        establishing the associated iSCSI sessions upon subsequent
        reboot in order to rely on the persist through power loss

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8. IANA Considerations

     This document does not have any IANA considerations.

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9. Security Considerations

     For security considerations in using the iSCSI protocol, refer
     to the Security Considerations section in [iSCSI].  This
     document does not introduce any additional secuirty
     considerations other than those already discussed in [iSCSI].

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10. References and Bibliography

10.1  Normative References

     [iSCSI] J. Satran et. al. draft-ietf-ips-iscsi-20.txt
       (work in progress)
     [SAM] ANSI X3.270-1998, SCSI-3 Architecture Model (SAM).
     [SAM2] T10/1157D, SCSI Architecture Model - 2 (SAM-2).
     [SBC] NCITS.306-1998, SCSI-3 Block Commands (SBC).

10.2  Informative References:

     [RFC2119] Bradner, S. "Key Words for use in RFCs to Indi-
       cate Requirement Levels", BCP 14, RFC 2119, March 1997.
     [RFC3347] M. Krueger et. al., "iSCSI Requirements and
       Design Considerations"
     [SPC3]T10/1416-D, SCSI Primary Commands-3.

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11. Authors' Addresses

       Mallikarjun Chadalapaka
       Hewlett-Packard Company
       8000 Foothills Blvd.
       Roseville, CA 95747-5668, USA
       Phone: +1.916.785.5621
       E-mail: cbm@rose.hp.com

       Rob Elliott
       Hewlett-Packard Company
       MC 150801
       PO Box 692000
       Houston, TX 77269-2000  USA
       Phone: +1.281.518.5037
       E-mail: elliott@hp.com

     Comments may be sent to Mallikarjun Chadalapaka.

Mallikarjun Chadalapaka Expires July 2004                           19

                             Command Ordering        20-January-04

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