CRC         Cyclic Redundancy Check
   CRL         Certificate Revocation List
   CSG         Current Stage
   CSM         Connection State Machine
   DES         Data Encryption Standard
   DNS         Domain Name Server
   DOI         Domain of Interpretation
   DVD         Digital Versatile Disk
   ESP         Encapsulating Security Payload
   EUI         Extended Unique Identifier
   FFP         Full Feature Phase
   FFPO        Full Feature Phase Only
   FIM         Fixed Interval Marker
   Gbps        Gigabits per Second
   HBA         Host Bus Adapter
   HMAC        Hashed Message Authentication Code
   I_T         Initiator_Target
   I_T_L       Initiator_Target_LUN
   IANA        Internet Assigned Numbers Authority



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   ID          Identifier
   IDN         Internationalized Domain Name
   IEEE        Institute of Electrical & Electronics Engineers
   IETF        Internet Engineering Task Force
   IKE         Internet Key Exchange
   I/O         Input - Output
   IO          Initialize Only
   IP          Internet Protocol
   IPsec       Internet Protocol Security
   IPv4        Internet Protocol Version 4
   IPv6        Internet Protocol Version 6
   IQN         iSCSI Qualified Name
   ISID        Initiator Session ID
   ITN         iSCSI Target Name
   ITT         Initiator Task Tag
   KRB5        Kerberos V5
   LFL         Lower Functional Layer
   LTDS        Logical-Text-Data-Segment
   LO          Leading Only
   LU          Logical Unit
   LUN         Logical Unit Number
   MAC         Message Authentication Codes
   NA          Not Applicable
   NIC         Network Interface Card
   NOP         No Operation
   NSG         Next Stage
   OS          Operating System
   PDU         Protocol Data Unit
   PKI         Public Key Infrastructure
   R2T         Ready To Transfer
   R2TSN       Ready To Transfer Sequence Number
   RDMA        Remote Direct Memory Access
   RFC         Request For Comments
   SAM         SCSI Architecture Model
   SAM2        SCSI Architecture Model - 2
   SAN         Storage Area Network
   SCSI        Small Computer Systems Interface
   SN          Sequence Number
   SNACK       Selective Negative Acknowledgment - also
               Sequence Number Acknowledgement for data
   SPKM        Simple Public-Key Mechanism
   SRP         Secure Remote Password
   SSID        Session ID
   SW          Session Wide
   TCB         Task Control Block
   TCP         Transmission Control Protocol
   TPGT        Target Portal Group Tag
   TSIH        Target Session Identifying Handle



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   TTT         Target Transfer Tag
   UFL         Upper Functional Layer
   ULP         Upper Level Protocol
   URN         Uniform Resource Names [RFC2396]
   UTF         Universal Transformation Format
   WG          Working Group

2.3.  Conventions

   In examples, "I->" and "T->" show iSCSI PDUs sent by the initiator
   and target respectively.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14 [RFC2119].

   iSCSI messages - PDUs - are represented by diagrams as in the
   following example:

    Byte/     0       |       1       |       2       |       3       |
       /              |               |               |               |
      |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
      +---------------+---------------+---------------+---------------+
     0| Basic Header Segment (BHS)                                    |
      +---------------+---------------+---------------+---------------+
    ----------
     +|                                                               |
      +---------------+---------------+---------------+---------------+

   The diagrams include byte and bit numbering.

   The following representation and ordering rules are observed in this
   document:

     - Word Rule
     - Half-word Rule
     - Byte Rule

2.3.1.  Word Rule

   A word holds four consecutive bytes.  Whenever a word has numeric
   content, it is considered an unsigned number in base 2 positional
   representation with the lowest numbered byte (e.g., byte 0) bit 0
   representing 2**31 and bit 1 representing 2**30 through lowest
   numbered byte + 3 (e.g., byte 3) bit 7 representing 2**0.

   Decimal and hexadecimal representation of word values map this
   representation to decimal or hexadecimal positional notation.



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2.3.2.  Half-Word Rule

   A half-word holds two consecutive bytes.  Whenever a half-word has
   numeric content it is considered an unsigned number in base 2
   positional representation with the lowest numbered byte (e.g., byte
   0), bit 0 representing 2**15 and bit 1 representing 2**14 through
   lowest numbered byte + 1 (e.g., byte 1), bit 7 representing 2**0.

   Decimal and hexadecimal representation of half-word values map this
   representation to decimal or hexadecimal positional notation.

2.3.3.  Byte Rule

   For every PDU, bytes are sent and received in increasing numbered
   order (network order).

   Whenever a byte has numerical content, it is considered an unsigned
   number in base 2 positional representation with bit 0 representing
   2**7 and bit 1 representing 2**6 through bit 7 representing 2**0.

3.  Overview

3.1.  SCSI Concepts

   The SCSI Architecture Model-2 [SAM2] describes in detail the
   architecture of the SCSI family of I/O protocols.  This section
   provides a brief background of the SCSI architecture and is intended
   to familiarize readers with its terminology.

   At the highest level, SCSI is a family of interfaces for requesting
   services from I/O devices, including hard drives, tape drives, CD and
   DVD drives, printers, and scanners.  In SCSI terminology, an
   individual I/O device is called a "logical unit" (LU).

   SCSI is a client-server architecture.  Clients of a SCSI interface
   are called "initiators".  Initiators issue SCSI "commands" to request
   services from components, logical units, of a server known as a
   "target".  The "device server" on the logical unit accepts SCSI
   commands and processes them.

   A "SCSI transport" maps the client-server SCSI protocol to a specific
   interconnect.  Initiators are one endpoint of a SCSI transport.  The
   "target" is the other endpoint.  A target can contain multiple
   Logical Units (LUs).  Each Logical Unit has an address within a
   target called a Logical Unit Number (LUN).

   A SCSI task is a SCSI command or possibly a linked set of SCSI
   commands.  Some LUs support multiple pending (queued) tasks, but the



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   queue of tasks is managed by the logical unit.  The target uses an
   initiator provided "task tag" to distinguish between tasks.  Only one
   command in a task can be outstanding at any given time.

   Each SCSI command results in an optional data phase and a required
   response phase.  In the data phase, information can travel from the
   initiator to target (e.g., WRITE), target to initiator (e.g., READ),
   or in both directions.  In the response phase, the target returns the
   final status of the operation, including any errors.

   Command Descriptor Blocks (CDB) are the data structures used to
   contain the command parameters that an initiator sends to a target.
   The CDB content and structure is defined by [SAM2] and device-type
   specific SCSI standards.

3.2.  iSCSI Concepts and Functional Overview

   The iSCSI protocol is a mapping of the SCSI remote procedure
   invocation model (see [SAM2]) over the TCP protocol.  SCSI commands
   are carried by iSCSI requests and SCSI responses and status are
   carried by iSCSI responses.  iSCSI also uses the request response
   mechanism for iSCSI protocol mechanisms.

   For the remainder of this document, the terms "initiator" and
   "target" refer to "iSCSI initiator node" and "iSCSI target node",
   respectively (see Section 3.4.1 iSCSI Architecture Model) unless
   otherwise qualified.

   In keeping with similar protocols, the initiator and target divide
   their communications into messages.  This document uses the term
   "iSCSI protocol data unit" (iSCSI PDU) for these messages.

   For performance reasons, iSCSI allows a "phase-collapse".  A command
   and its associated data may be shipped together from initiator to
   target, and data and responses may be shipped together from targets.

   The iSCSI transfer direction is defined with respect to the
   initiator.  Outbound or outgoing transfers are transfers from an
   initiator to a target, while inbound or incoming transfers are from a
   target to an initiator.

   An iSCSI task is an iSCSI request for which a response is expected.

   In this document "iSCSI request", "iSCSI command", request, or
   (unqualified) command have the same meaning.  Also, unless otherwise
   specified, status, response, or numbered response have the same
   meaning.




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3.2.1.  Layers and Sessions

   The following conceptual layering model is used to specify initiator