The Small Computer System Interface (SCSI) is a popular family of
protocols for communicating with I/O devices, especially storage
devices. 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". A "SCSI transport" maps the client-server SCSI
protocol to a specific interconnect. An initiator is one endpoint of
a SCSI transport, and a target is the other endpoint.
The SCSI protocol has been mapped over various transports, including
Parallel SCSI, Intelligent Peripheral Interface (IPI), IEEE 1394
(FireWire), and Fibre Channel. These transports are I/O-specific and
have limited distance capabilities.
The iSCSI protocol defined in this document describes a means of
transporting SCSI packets over TCP/IP, providing for an interoperable
solution that can take advantage of existing Internet infrastructure,
Internet management facilities, and address distance limitations.
2. Acronyms, Definitions, and Document Summary
3DES Triple Data Encryption Standard
ACA Auto Contingent Allegiance
AEN Asynchronous Event Notification
AES Advanced Encryption Standard
AH Additional Header (not the IPsec AH!)
AHS Additional Header Segment
API Application Programming Interface
ASC Additional Sense Code
ASCII American Standard Code for Information Interchange
ASCQ Additional Sense Code Qualifier
ATA AT Attachment
BHS Basic Header Segment
CBC Cipher Block Chaining
CD Compact Disk
CDB Command Descriptor Block
CHAP Challenge Handshake Authentication Protocol
CID Connection ID
CO Connection Only
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
EDTL Expected Data Transfer Length
ESP Encapsulating Security Payload
EUI Extended Unique Identifier
FFP Full Feature Phase
FFPO Full Feature Phase Only
HBA Host Bus Adapter
HMAC Hashed Message Authentication Code
IANA Internet Assigned Numbers Authority
IDN Internationalized Domain Name
IEEE Institute of Electrical and Electronics Engineers
IETF Internet Engineering Task Force
IKE Internet Key Exchange
IO Initialize Only
IP Internet Protocol
IPsec Internet Protocol Security
IPv4 Internet Protocol Version 4
IPv6 Internet Protocol Version 6
IQN iSCSI Qualified Name
iSCSI Internet SCSI
iSER iSCSI Extensions for RDMA (see [RFC7145])
ISID Initiator Session ID
iSNS Internet Storage Name Service (see [RFC4171])
ITN iSCSI Target Name
ITT Initiator Task Tag
KRB5 Kerberos V5
LFL Lower Functional Layer
LO Leading Only
LU Logical Unit
LUN Logical Unit Number
MAC Message Authentication Code
NA Not Applicable
NAA Network Address Authority
NIC Network Interface Card
NOP No Operation
NSG Next Stage
OCSP Online Certificate Status Protocol
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
SA Security Association
SAM SCSI Architecture Model
SAM-2 SCSI Architecture Model - 2
SAN Storage Area Network
SAS Serial Attached SCSI
SATA Serial AT Attachment
SCSI Small Computer System Interface
SLP Service Location Protocol
SN Sequence Number
SNACK Selective Negative Acknowledgment - also
Sequence Number Acknowledgement for data
SPDTL SCSI-Presented Data Transfer Length
SPKM Simple Public-Key Mechanism
SRP Secure Remote Password
SSID Session ID
TCB Task Control Block
TCP Transmission Control Protocol
TMF Task Management Function
TPGT Target Portal Group Tag
TSIH Target Session Identifying Handle
TTT Target Transfer Tag
UA Unit Attention
UFL Upper Functional Layer
ULP Upper Level Protocol
URN Uniform Resource Name
UTF Universal Transformation Format
WG Working Group
- Alias: An alias string can also be associated with an iSCSI node.
The alias allows an organization to associate a user-friendly
string with the iSCSI name. However, the alias string is not a
substitute for the iSCSI name.
- CID (connection ID): Connections within a session are identified by
a connection ID. It is a unique ID for this connection within the
session for the initiator. It is generated by the initiator and
presented to the target during Login Requests and during logouts
that close connections.
- Connection: A connection is a TCP connection. Communication
between the initiator and target occurs over one or more TCP
connections. The TCP connections carry control messages, SCSI
commands, parameters, and data within iSCSI Protocol Data Units
- I/O Buffer: An I/O Buffer is a buffer that is used in a SCSI read
or write operation so SCSI data may be sent from or received into
that buffer. For a read or write data transfer to take place for a
task, an I/O Buffer is required on the initiator and at least one
is required on the target.
- INCITS: "INCITS" stands for InterNational Committee for Information
Technology Standards. The INCITS has a broad standardization scope
within the field of Information and Communications Technologies
(ICT), encompassing storage, processing, transfer, display,
management, organization, and retrieval of information. INCITS
serves as ANSI's Technical Advisory Group for the ISO/IEC Joint
Technical Committee 1 (JTC 1). See <http://www.incits.org>.
- InfiniBand: InfiniBand is an I/O architecture originally intended
to replace Peripheral Component Interconnect (PCI) and address
high-performance server interconnectivity [IB].
- iSCSI Device: An iSCSI device is a SCSI device using an iSCSI
service delivery subsystem. The Service Delivery Subsystem is
defined by [SAM2] as a transport mechanism for SCSI commands and
- iSCSI Initiator Name: The iSCSI Initiator Name specifies the
worldwide unique name of the initiator.
- iSCSI Initiator Node: An iSCSI initiator node is the "initiator"
device. The word "initiator" has been appropriately qualified as
either a port or a device in the rest of the document when the
context is ambiguous. All unqualified usages of "initiator" refer
to an initiator port (or device), depending on the context.
- iSCSI Layer: This layer builds/receives iSCSI PDUs and
relays/receives them to/from one or more TCP connections that form
an initiator-target "session".
- iSCSI Name: This is the name of an iSCSI initiator or iSCSI target.
- iSCSI Node: The iSCSI node represents a single iSCSI initiator or
iSCSI target, or a single instance of each. There are one or more
iSCSI nodes within a Network Entity. The iSCSI node is accessible
via one or more Network Portals. An iSCSI node is identified by
its iSCSI name. The separation of the iSCSI name from the
addresses used by and for the iSCSI node allows multiple iSCSI
nodes to use the same address and the same iSCSI node to use
- iSCSI Target Name: The iSCSI Target Name specifies the worldwide
unique name of the target.
- iSCSI Target Node: The iSCSI target node is the "target" device.
The word "target" has been appropriately qualified as either a port
or a device in the rest of the document when the context is
ambiguous. All unqualified usages of "target" refer to a target
port (or device), depending on the context.
- iSCSI Task: An iSCSI task is an iSCSI request for which a response
- iSCSI Transfer Direction: The iSCSI transfer direction is defined
with regard to the initiator. Outbound or outgoing transfers are
transfers from the initiator to the target, while inbound or
incoming transfers are from the target to the initiator.
- ISID: The ISID is the initiator part of the session identifier. It
is explicitly specified by the initiator during login.
- I_T Nexus: According to [SAM2], the I_T nexus is a relationship
between a SCSI initiator port and a SCSI target port. For iSCSI,
this relationship is a session, defined as a relationship between
an iSCSI initiator's end of the session (SCSI initiator port) and
the iSCSI target's portal group. The I_T nexus can be identified
by the conjunction of the SCSI port names; that is, the I_T nexus
identifier is the tuple (iSCSI Initiator Name + ',i,' + ISID, iSCSI
Target Name + ',t,' + Target Portal Group Tag).
- I_T_L Nexus: An I_T_L nexus is a SCSI concept and is defined as the
relationship between a SCSI initiator port, a SCSI target port, and
a Logical Unit (LU).
- NAA: "NAA" refers to Network Address Authority, a naming format
defined by the INCITS T11 Fibre Channel protocols [FC-FS3].
- Network Entity: The Network Entity represents a device or gateway
that is accessible from the IP network. A Network Entity must have
one or more Network Portals, each of which can be used to gain
access to the IP network by some iSCSI nodes contained in that
- Network Portal: The Network Portal is a component of a Network
Entity that has a TCP/IP network address and that may be used by an
iSCSI node within that Network Entity for the connection(s) within
one of its iSCSI sessions. A Network Portal in an initiator is
identified by its IP address. A Network Portal in a target is
identified by its IP address and its listening TCP port.
- Originator: In a negotiation or exchange, the originator is the
party that initiates the negotiation or exchange.
- PDU (Protocol Data Unit): The initiator and target divide their
communications into messages. The term "iSCSI Protocol Data Unit"
(iSCSI PDU) is used for these messages.
- Portal Groups: iSCSI supports multiple connections within the same
session; some implementations will have the ability to combine
connections in a session across multiple Network Portals. A portal
group defines a set of Network Portals within an iSCSI Network
Entity that collectively supports the capability of coordinating a
session with connections spanning these portals. Not all Network
Portals within a portal group need participate in every session
connected through that portal group. One or more portal groups may
provide access to an iSCSI node. Each Network Portal, as utilized
by a given iSCSI node, belongs to exactly one portal group within
- Portal Group Tag: This 16-bit quantity identifies a portal group
within an iSCSI node. All Network Portals with the same Portal
Group Tag in the context of a given iSCSI node are in the same
- Recovery R2T: A recovery R2T is an R2T generated by a target upon
detecting the loss of one or more Data-Out PDUs through one of the
following means: a digest error, a sequence error, or a sequence
reception timeout. A recovery R2T carries the next unused R2TSN
but requests all or part of the data burst that an earlier R2T
(with a lower R2TSN) had already requested.
- Responder: In a negotiation or exchange, the responder is the party
that responds to the originator of the negotiation or exchange.
- SAS: The Serial Attached SCSI (SAS) standard contains both a
physical layer compatible with Serial ATA, and protocols for
transporting SCSI commands to SAS devices and ATA commands to SATA
devices [SAS] [SPL].
- SCSI Device: This is the SAM-2 term for an entity that contains one
or more SCSI ports that are connected to a service delivery
subsystem and supports a SCSI application protocol. For example, a
SCSI initiator device contains one or more SCSI initiator ports and
zero or more application clients. A target device contains one or
more SCSI target ports and one or more device servers and
associated LUs. For iSCSI, the SCSI device is the component within
an iSCSI node that provides the SCSI functionality. As such, there
can be at most one SCSI device within a given iSCSI node. Access
to the SCSI device can only be achieved in an iSCSI Normal
operational session. The SCSI device name is defined to be the
iSCSI name of the node.
- SCSI Layer: This builds/receives SCSI CDBs (Command Descriptor
Blocks) and relays/receives them with the remaining Execute Command
[SAM2] parameters to/from the iSCSI Layer.
- Session: The group of TCP connections that link an initiator with a
target form a session (loosely equivalent to a SCSI I_T nexus).
TCP connections can be added and removed from a session. Across
all connections within a session, an initiator sees one and the
- SCSI Port: This is the SAM-2 term for an entity in a SCSI device
that provides the SCSI functionality to interface with a service
delivery subsystem. For iSCSI, the definitions of the SCSI
initiator port and the SCSI target port are different.
- SCSI Initiator Port: This maps to the endpoint of an iSCSI Normal
operational session. An iSCSI Normal operational session is
negotiated through the login process between an iSCSI initiator
node and an iSCSI target node. At successful completion of this
process, a SCSI initiator port is created within the SCSI initiator
device. The SCSI initiator port name and SCSI initiator port
identifier are both defined to be the iSCSI Initiator Name together
with (a) a label that identifies it as an initiator port
name/identifier and (b) the ISID portion of the session identifier.
- SCSI Port Name: This is a name consisting of UTF-8 [RFC3629]
encoding of Unicode [UNICODE] characters and includes the iSCSI
name + 'i' or 't' + ISID or Target Portal Group Tag.
- SCSI-Presented Data Transfer Length (SPDTL): SPDTL is the aggregate
data length of the data that the SCSI layer logically "presents" to
the iSCSI layer for a Data-In or Data-Out transfer in the context
of a SCSI task. For a bidirectional task, there are two SPDTL
values -- one for Data-In and one for Data-Out. Note that the
notion of "presenting" includes immediate data per the data
transfer model in [SAM2] and excludes overlapping data transfers,
if any, requested by the SCSI layer.
- SCSI Target Port: This maps to an iSCSI target portal group.
- SCSI Target Port Name and SCSI Target Port Identifier: These are
both defined to be the iSCSI Target Name together with (a) a label
that identifies it as a target port name/identifier and (b) the
Target Portal Group Tag.
- SSID (Session ID): A session between an iSCSI initiator and an
iSCSI target is defined by a session ID that is a tuple composed of
an initiator part (ISID) and a target part (Target Portal Group
Tag). The ISID is explicitly specified by the initiator at session
establishment. The Target Portal Group Tag is implied by the
initiator through the selection of the TCP endpoint at connection
establishment. The TargetPortalGroupTag key must also be returned
by the target as a confirmation during connection establishment.
- T10: T10 is a technical committee within INCITS that develops
standards and technical reports on I/O interfaces, particularly the
series of SCSI (Small Computer System Interface) standards. See
- T11: T11 is a technical committee within INCITS responsible for
standards development in the areas of Intelligent Peripheral
Interface (IPI), High-Performance Parallel Interface (HIPPI), and
Fibre Channel (FC). See <http://www.t11.org>.
- Target Portal Group Tag: This is a numerical identifier (16-bit)
for an iSCSI target portal group.
- Target Transfer Tag (TTT): The TTT is an iSCSI protocol field used
in a few iSCSI PDUs (e.g., R2T, NOP-In) that is always sent from
the target to the initiator first and then quoted as a reference in
initiator-sent PDUs back to the target relating to the same
task/exchange. Therefore, the TTT effectively acts as an opaque
handle to an existing task/exchange to help the target associate
the incoming PDUs from the initiator to the proper execution
- Third-party: This term is used in this document as a qualifier to
nexus objects (I_T or I_T_L) and iSCSI sessions, to indicate that
these objects and sessions reap the side effects of actions that
take place in the context of a separate iSCSI session. One example
of a third-party session is an iSCSI session discovering that its
I_T_L nexus to a LU got reset due to a LU reset operation
orchestrated via a separate I_T nexus.
- TSIH (Target Session Identifying Handle): This is a target-assigned
tag for a session with a specific named initiator. The target
generates it during session establishment. Other than defining it
as a 16-bit binary string, its internal format and content are not
defined by this protocol but for the value with all bits set to 0
that is reserved and used by the initiator to indicate a new
session. It is given to the target during additional connection
establishment for the same session.
2.3. Summary of Changes
1) Consolidated RFCs 3720, 3980, 4850, and 5048, and made the
necessary editorial changes.
2) Specified iSCSIProtocolLevel as "1" in Section 13.24 and added a
related normative reference to [RFC7144].
3) Removed markers and related keys.
4) Removed SPKM authentication and related keys.
5) Added a new Section 13.25 on responding to obsoleted keys.
6) Have explicitly allowed initiator+target implementations
throughout the text.
7) Clarified in Section 4.2.7 that implementations SHOULD NOT rely
on SLP-based discovery.
8) Added Unified Modeling Language (UML) diagrams and related
conventions in Section 3.
9) Made FastAbort implementation a "SHOULD" requirement in
Section 184.108.40.206, rather than the previous "MUST" requirement.
10) Required in Section 220.127.116.11 that iSCSI Target Name be the same as
iSCSI Initiator Name for SCSI (composite) devices with both
11) Changed the "MUST NOT" to "should be avoided" in Section 18.104.22.168
regarding usage of characters such as punctuation marks in iSCSI
12) Updated Section 9.3 to require the following: MUST implement
IPsec, 2400-series RFCs (IPsec v2, IKEv1); and SHOULD implement
IPsec, 4300-series RFCs (IPsec v3, IKEv2).
13) Clarified in Section 10.2 that ACA is a "SHOULD" only for iSCSI
14) Prohibited usage of X# name prefix for new public keys in
15) Prohibited usage of Y# name prefix for new digest extensions in
Section 13.1 and Z# name prefix for new authentication method
extensions in Section 12.1.
16) Added a "SHOULD" in Section 6.2 that initiators and targets
support at least six (6) exchanges during text negotiation.
17) Added a clarification that Appendix C is normative.
18) Added a normative requirement on [RFC7146] and made a few related
changes in Section 9.3 to align the text in this document with
that of [RFC7146].
19) Added a new Section 9.2.3 covering Kerberos authentication
20) Added text in Section 9.3.3 noting that OCSP is now allowed for
checking certificates used with IPsec in addition to the use
21) Added text in Section 9.3.1 specifying that extended sequence
numbers (ESNs) are now required for ESPv2 (part of IPsec v2).
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 RFC 2119 [RFC2119].
3. UML Conventions
3.1. UML Conventions Overview
The SCSI Architecture Model (SAM) uses class diagrams and object
diagrams with notation that is based on the Unified Modeling Language
[UML]. Therefore, this document also uses UML to model the
relationships for SCSI and iSCSI objects.
A treatise on the graphical notation used in UML is beyond the scope
of this document. However, given the use of ASCII drawing for UML
static class diagrams, a description of the notational conventions
used in this document is included in the remainder of this section.
3.2. Multiplicity Notion
Not specified The number of instances of an attribute is not
1 One instance of the class or attribute exists.
0..* Zero or more instances of the class or attribute
1..* One or more instances of the class or attribute
0..1 Zero or one instance of the class or attribute
n..m n to m instances of the class or attribute exist
x, n..m Multiple disjoint instances of the class or
attribute exist (e.g., 2, 8..15).
3.3. Class Diagram Conventions
+--------------+ +--------------+ +--------------+
| Class Name | | Class Name | | Class Name |
+--------------+ +--------------+ +--------------+
| | | |
The previous three diagrams are examples of a class with no
attributes and with no operations.
| Class Name | | Class Name |
| attribute 01 | | attribute 01 |
| attribute 02 | | attribute 02 |
The preceding two diagrams are examples of a class with attributes
and with no operations.
| Class Name |
| attribute 01[1..*] |
| attribute 02 |
| operation 01() |
| operation 02() |
The preceding diagram is an example of a class with attributes
that have a specified multiplicity and operations.
3.4. Class Diagram Notation for Associations
| Class A |
+-----------------+ association_name +-----------------+
| attribute 01 |<------------------>| Class B |
| attribute 02 | 1..* 0..1 +-----------------+
+-----------------+ | attribute 03 |
| operation 1() | +-----------------+
The preceding diagram is an example where Class A knows about
Class B (i.e., read as "Class A association_name Class B") and
Class B knows about Class A (i.e., read as "Class B
association_name Class A"). The use of association_name is
optional. The multiplicity notation (1..* and 0..1) indicates the
number of instances of the object.
| Class A |
| attribute 01 |<-------------| Class B |
| attribute 02 | 1 0..1 +--------------------+
+--------------------+ | attribute 03 |
| operation 1() | +--------------------+
The preceding diagram is an example where Class B knows about
Class A (i.e., read as "Class B knows about Class A") but Class A
does not know about Class B.
| Class A |
| attribute 01 |----------->| Class B |
| attribute 02 | 0..* 1 +--------------------+
+----------------------+ | attribute 03 |
| operation 1() | +--------------------+
The preceding diagram is an example where Class A knows about
Class B (i.e., read as "Class A knows about Class B") but Class B
does not know about Class A.
3.5. Class Diagram Notation for Aggregations
| Class whole |o------------| Class part |
The preceding diagram is an example where Class whole is an
aggregate that contains Class part and where Class part may
continue to exist even if Class whole is removed (i.e., read as
"the whole contains the part").
| Class whole |@------------| Class part |
The preceding diagram is an example where Class whole is an
aggregate that contains Class part where Class part only belongs
to one Class whole, and the Class part does not continue to exist
if the Class whole is removed (i.e., read as "the whole contains
+ =(a)= +
The preceding diagram is an example where there is a constraint
between the associations, where the (a) footnote describes the
3.6. Class Diagram Notation for Generalizations
| Superclass |
| Subclass |
The preceding diagram is an example where the subclass is a kind
of superclass. A subclass shares all the attributes and
operations of the superclass (i.e., the subclass inherits from the
4.1. 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 -- LUs of a server known as a "target". The
"device server" on the LU accepts SCSI commands and processes them.
A "SCSI transport" maps the client-server SCSI protocol to a specific
interconnect. The initiator is one endpoint of a SCSI transport.
The "target" is the other endpoint. A target can contain multiple
LUs. Each LU has an address within a target called a Logical Unit
A SCSI task is a SCSI command or possibly a linked set of SCSI
commands. Some LUs support multiple pending (queued) tasks, but the
queue of tasks is managed by the LU. 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 the target (e.g., write), from the target to the
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 (CDBs) are the data structures used to
contain the command parameters that an initiator sends to a target.
The CDB content and structure are defined by [SAM2] and device-type
specific SCSI standards.
4.2. iSCSI Concepts and Functional Overview
The iSCSI protocol is a mapping of the SCSI command, event, and task
management 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 iSCSI), unless otherwise qualified.
As its title suggests, Section 4 presents an overview of the iSCSI
concepts, and later sections in the rest of the specification contain
the normative requirements -- in many cases covering the same
concepts discussed in Section 4. Such normative requirements text
overrides the overview text in Section 4 if there is a disagreement
between the two.
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
4.2.1. Layers and Sessions
The following conceptual layering model is used to specify initiator
and target actions and the way in which they relate to transmitted
and received Protocol Data Units:
- The SCSI layer builds/receives SCSI CDBs (Command Descriptor
Blocks) and passes/receives them with the remaining Execute
Command [SAM2] parameters to/from
- the iSCSI layer that builds/receives iSCSI PDUs and
relays/receives them to/from one or more TCP connections; the
group of connections form an initiator-target "session".
Communication between the initiator and target occurs over one or
more TCP connections. The TCP connections carry control messages,
SCSI commands, parameters, and data within iSCSI Protocol Data Units
(iSCSI PDUs). The group of TCP connections that link an initiator
with a target form a session (equivalent to a SCSI I_T nexus; see
Section 4.4.2). A session is defined by a session ID that is
composed of an initiator part and a target part. TCP connections can
be added and removed from a session. Each connection within a
session is identified by a connection ID (CID).
Across all connections within a session, an initiator sees one
"target image". All target-identifying elements, such as a LUN, are
the same. A target also sees one "initiator image" across all
connections within a session. Initiator-identifying elements, such
as the Initiator Task Tag, are global across the session, regardless
of the connection on which they are sent or received.
iSCSI targets and initiators MUST support at least one TCP connection
and MAY support several connections in a session. For error recovery
purposes, targets and initiators that support a single active
connection in a session SHOULD support two connections during
4.2.2. Ordering and iSCSI Numbering
iSCSI uses command and status numbering schemes and a data sequencing
Command numbering is session-wide and is used for ordered command
delivery over multiple connections. It can also be used as a
mechanism for command flow control over a session.
Status numbering is per connection and is used to enable missing
status detection and recovery in the presence of transient or
permanent communication errors.
Data sequencing is per command or part of a command (R2T-triggered
sequence) and is used to detect missing data and/or R2T PDUs due to
header digest errors.
Typically, fields in the iSCSI PDUs communicate the sequence numbers
between the initiator and target. During periods when traffic on a
connection is unidirectional, iSCSI NOP-Out/NOP-In PDUs may be
utilized to synchronize the command and status ordering counters of
the target and initiator.
The iSCSI session abstraction is equivalent to the SCSI I_T nexus,
and the iSCSI session provides an ordered command delivery from the
SCSI initiator to the SCSI target. For detailed design
considerations that led to the iSCSI session model as it is defined
here and how it relates the SCSI command ordering features defined in
SCSI specifications to the iSCSI concepts, see [RFC3783].
22.214.171.124. Command Numbering and Acknowledging
iSCSI performs ordered command delivery within a session. All
commands (initiator-to-target PDUs) in transit from the initiator to
the target are numbered.
iSCSI considers a task to be instantiated on the target in response
to every request issued by the initiator. A set of task management
operations, including abort and reassign (see Section 11.5), may be
performed on an iSCSI task; however, an abort operation cannot be
performed on a task management operation, and usage of reassign
operations has certain constraints. See Section 11.5.1 for details.
Some iSCSI tasks are SCSI tasks, and many SCSI activities are related
to a SCSI task ([SAM2]). In all cases, the task is identified by the
Initiator Task Tag for the life of the task.
The command number is carried by the iSCSI PDU as the CmdSN (command
sequence number). The numbering is session-wide. Outgoing iSCSI
PDUs carry this number. The iSCSI initiator allocates CmdSNs with a
32-bit unsigned counter (modulo 2**32). Comparisons and arithmetic
on CmdSNs use Serial Number Arithmetic as defined in [RFC1982] where
SERIAL_BITS = 32.
Commands meant for immediate delivery are marked with an immediate
delivery flag; they MUST also carry the current CmdSN. The CmdSN
MUST NOT advance after a command marked for immediate delivery is
Command numbering starts with the first Login Request on the first
connection of a session (the leading login on the leading
connection), and the CmdSN MUST be incremented by 1 in a Serial
Number Arithmetic sense, as defined in [RFC1982], for every
non-immediate command issued afterwards.
If immediate delivery is used with task management commands, these
commands may reach the target before the tasks on which they are
supposed to act. However, their CmdSN serves as a marker of their
position in the stream of commands. The initiator and target MUST
ensure that the SCSI task management functions specified in [SAM2]
act in accordance with the [SAM2] specification. For example, both
commands and responses appear as if delivered in order. Whenever the
CmdSN for an outgoing PDU is not specified by an explicit rule, the
CmdSN will carry the current value of the local CmdSN variable (see
later in this section).
The means by which an implementation decides to mark a PDU for
immediate delivery or by which iSCSI decides by itself to mark a PDU
for immediate delivery are beyond the scope of this document.
The number of commands used for immediate delivery is not limited,
and their delivery to execution is not acknowledged through the
numbering scheme. An iSCSI target MAY reject immediate commands,
e.g., due to lack of resources to accommodate additional commands.
An iSCSI target MUST be able to handle at least one immediate task
management command and one immediate non-task-management iSCSI
command per connection at any time.
In this document, delivery for execution means delivery to the SCSI
execution engine or an iSCSI protocol-specific execution engine
(e.g., for Text Requests with public or private extension keys
involving an execution component). With the exception of the
commands marked for immediate delivery, the iSCSI target layer MUST
deliver the commands for execution in the order specified by the
CmdSN. Commands marked for immediate delivery may be delivered by
the iSCSI target layer for execution as soon as detected. iSCSI may
avoid delivering some commands to the SCSI target layer if required
by a prior SCSI or iSCSI action (e.g., a CLEAR TASK SET task
management request received before all the commands on which it was
supposed to act).
On any connection, the iSCSI initiator MUST send the commands in
increasing order of CmdSN, except for commands that are retransmitted
due to digest error recovery and connection recovery.
For the numbering mechanism, the initiator and target maintain the
following three variables for each session:
- CmdSN: the current command sequence number, advanced by 1 on
each command shipped except for commands marked for immediate
delivery as discussed above. The CmdSN always contains the
number to be assigned to the next command PDU.
- ExpCmdSN: the next expected command by the target. The target
acknowledges all commands up to, but not including, this number.
The initiator treats all commands with a CmdSN less than the
ExpCmdSN as acknowledged. The target iSCSI layer sets the
ExpCmdSN to the largest non-immediate CmdSN that it can deliver
for execution "plus 1" per [RFC1982]. There MUST NOT be any
holes in the acknowledged CmdSN sequence.
- MaxCmdSN: the maximum number to be shipped. The queuing
capacity of the receiving iSCSI layer is
MaxCmdSN - ExpCmdSN + 1.
The initiator's ExpCmdSN and MaxCmdSN are derived from target-to-
initiator PDU fields. Comparisons and arithmetic on the ExpCmdSN and
MaxCmdSN MUST use Serial Number Arithmetic as defined in [RFC1982]
where SERIAL_BITS = 32.
The target MUST NOT transmit a MaxCmdSN that is less than
ExpCmdSN - 1. For non-immediate commands, the CmdSN field can take
any value from the ExpCmdSN to the MaxCmdSN inclusive. The target
MUST silently ignore any non-immediate command outside of this range
or non-immediate duplicates within the range. The CmdSN carried by
immediate commands may lie outside the ExpCmdSN-to-MaxCmdSN range.
For example, if the initiator has previously sent a non-immediate
command carrying the CmdSN equal to the MaxCmdSN, the target window
is closed. For group task management commands issued as immediate
commands, the CmdSN indicates the scope of the group action (e.g., an
ABORT TASK SET indicates which commands are to be aborted).
MaxCmdSN and ExpCmdSN fields are processed by the initiator as
- If the PDU MaxCmdSN is less than the PDU ExpCmdSN - 1 (in a
Serial Number Arithmetic sense), they are both ignored.
- If the PDU MaxCmdSN is greater than the local MaxCmdSN (in a
Serial Number Arithmetic sense), it updates the local MaxCmdSN;
otherwise, it is ignored.
- If the PDU ExpCmdSN is greater than the local ExpCmdSN (in a
Serial Number Arithmetic sense), it updates the local ExpCmdSN;
otherwise, it is ignored.
This sequence is required because updates may arrive out of order
(e.g., the updates are sent on different TCP connections).
iSCSI initiators and targets MUST support the command numbering
A numbered iSCSI request will not change its allocated CmdSN,
regardless of the number of times and circumstances in which it is
reissued (see Section 7.2.1). At the target, the CmdSN is only
relevant while the command has not created any state related to its
execution (execution state); afterwards, the CmdSN becomes
irrelevant. Testing for the execution state (represented by
identifying the Initiator Task Tag) MUST precede any other action at
the target. If no execution state is found, it is followed by
ordering and delivery. If an execution state is found, it is
followed by delivery if it has not already been delivered.
If an initiator issues a command retry for a command with CmdSN R on
a connection when the session CmdSN value is Q, it MUST NOT advance
the CmdSN past R + 2**31 - 1 unless
- the connection is no longer operational (i.e., it has returned
to the FREE state; see Section 8.1.3),
- the connection has been reinstated (see Section 6.3.4), or
- a non-immediate command with a CmdSN equal to or greater than Q
was issued subsequent to the command retry on the same
connection and the reception of that command is acknowledged by
the target (see Section 10.4).
A target command response or Data-In PDU with status MUST NOT precede
the command acknowledgment. However, the acknowledgment MAY be
included in the response or the Data-In PDU.
126.96.36.199. Response/Status Numbering and Acknowledging
Responses in transit from the target to the initiator are numbered.
The StatSN (status sequence number) is used for this purpose. The
StatSN is a counter maintained per connection. The ExpStatSN is used
by the initiator to acknowledge status. The status sequence number
space is 32-bit unsigned integers, and the arithmetic operations are
the regular mod(2**32) arithmetic.
Status numbering starts with the Login Response to the first Login
Request of the connection. The Login Response includes an initial
value for status numbering (any initial value is valid).
To enable command recovery, the target MAY maintain enough state
information for data and status recovery after a connection failure.
A target doing so can safely discard all of the state information
maintained for recovery of a command after the delivery of the status
for the command (numbered StatSN) is acknowledged through the
A large absolute difference between the StatSN and the ExpStatSN may
indicate a failed connection. Initiators MUST undertake recovery
actions if the difference is greater than an implementation-defined
constant that MUST NOT exceed 2**31 - 1.
Initiators and targets MUST support the response-numbering scheme.
188.8.131.52. Response Ordering
184.108.40.206.1. Need for Response Ordering
Whenever an iSCSI session is composed of multiple connections, the
Response PDUs (task responses or TMF Responses) originating in the
target SCSI layer are distributed onto the multiple connections by
the target iSCSI layer according to iSCSI connection allegiance
rules. This process generally may not preserve the ordering of the
responses by the time they are delivered to the initiator SCSI layer.
Since ordering is not expected across SCSI Response PDUs anyway, this
approach works fine in the general case. However, to address the
special cases where some ordering is desired by the SCSI layer, we
introduce the notion of a "Response Fence": a Response Fence is
logically the attribute/property of a SCSI response message handed
off to a target iSCSI layer that indicates that there are special
SCSI-level ordering considerations associated with this particular
response message. Whenever a Response Fence is set or required on a
SCSI response message, we define the semantics in Section 220.127.116.11.2
with respect to the target iSCSI layer's handling of such SCSI
18.104.22.168.2. Response Ordering Model Description
The target SCSI protocol layer hands off the SCSI response messages
to the target iSCSI layer by invoking the "Send Command Complete"
protocol data service ([SAM2], Clause 5.4.2) and "Task Management
Function Executed" ([SAM2], Clause 6.9) service. On receiving the
SCSI response message, the iSCSI layer exhibits the Response Fence
behavior for certain SCSI response messages (Section 22.214.171.124.4
describes the specific instances where the semantics must be
Whenever the Response Fence behavior is required for a SCSI response
message, the target iSCSI layer MUST ensure that the following
conditions are met in delivering the response message to the
initiator iSCSI layer:
- A response with a Response Fence MUST be delivered
chronologically after all the "preceding" responses on the I_T_L
nexus, if the preceding responses are delivered at all, to the
initiator iSCSI layer.
- A response with a Response Fence MUST be delivered
chronologically prior to all the "following" responses on the
The notions of "preceding" and "following" refer to the order of
handoff of a response message from the target SCSI protocol layer to
the target iSCSI layer.
126.96.36.199.3. iSCSI Semantics with the Interface Model
Whenever the TaskReporting key (Section 13.23) is negotiated to
ResponseFence or FastAbort for an iSCSI session and the Response
Fence behavior is required for a SCSI response message, the target
iSCSI layer MUST perform the actions described in this section for
a) If it is a single-connection session, no special processing is
required. The standard SCSI Response PDU build and dispatch
b) If it is a multi-connection session, the target iSCSI layer
takes note of the last-sent and unacknowledged StatSN on each
of the connections in the iSCSI session, and waits for an
acknowledgment (NOP-In PDUs MAY be used to solicit
acknowledgments as needed in order to accelerate this process)
of each such StatSN to clear the fence. The SCSI Response PDU
requiring the Response Fence behavior MUST NOT be sent to the
initiator before acknowledgments are received for each of the
c) The target iSCSI layer must wait for an acknowledgment of the
SCSI Response PDU that carried the SCSI response requiring the
Response Fence behavior. The fence MUST be considered cleared
only after receiving the acknowledgment.
d) All further status processing for the LU is resumed only after
clearing the fence. If any new responses for the I_T_L nexus
are received from the SCSI layer before the fence is cleared,
those Response PDUs MUST be held and queued at the iSCSI layer
until the fence is cleared.
188.8.131.52.4. Current List of Fenced Response Use Cases
This section lists the situations in which fenced response behavior
is REQUIRED in iSCSI target implementations. Note that the following
list is an exhaustive enumeration as currently identified -- it is
expected that as SCSI protocol specifications evolve, the
specifications will enumerate when response fencing is required on a
Whenever the TaskReporting key (Section 13.23) is negotiated to
ResponseFence or FastAbort for an iSCSI session, the target iSCSI
layer MUST assume that the Response Fence is required for the
following SCSI completion messages:
a) The first completion message carrying the UA after the multi-
task abort on issuing and third-party sessions. See
Section 184.108.40.206 for related TMF discussion.
b) The TMF Response carrying the multi-task TMF Response on the
c) The completion message indicating ACA establishment on the
d) The first completion message carrying the ACA ACTIVE status
after ACA establishment on issuing and third-party sessions.
e) The TMF Response carrying the CLEAR ACA response on the issuing
f) The response to a PERSISTENT RESERVE OUT/PREEMPT AND ABORT
- Due to the absence of ACA-related fencing requirements in
[RFC3720], initiator implementations SHOULD NOT use ACA on
multi-connection iSCSI sessions with targets complying only with
[RFC3720]. This can be determined via TaskReporting key
(Section 13.23) negotiation -- when the negotiation results in
either "RFC3720" or "NotUnderstood".
- Initiators that want to employ ACA on multi-connection iSCSI
sessions SHOULD first assess response-fencing behavior via
negotiating for the "ResponseFence" or "FastAbort" value for the
TaskReporting (Section 13.23) key.
220.127.116.11. Data Sequencing
Data and R2T PDUs transferred as part of some command execution MUST
be sequenced. The DataSN field is used for data sequencing. For
input (read) data PDUs, the DataSN starts with 0 for the first data
PDU of an input command and advances by 1 for each subsequent data
PDU. For output data PDUs, the DataSN starts with 0 for the first
data PDU of a sequence (the initial unsolicited sequence or any data
PDU sequence issued to satisfy an R2T) and advances by 1 for each
subsequent data PDU. R2Ts are also sequenced per command. For
example, the first R2T has an R2TSN of 0 and advances by 1 for each
subsequent R2T. For bidirectional commands, the target uses the
DataSN/R2TSN to sequence Data-In and R2T PDUs in one continuous
sequence (undifferentiated). Unlike command and status, data PDUs
and R2Ts are not acknowledged by a field in regular outgoing PDUs.
Data-In PDUs can be acknowledged on demand by a special form of the
SNACK PDU. Data and R2T PDUs are implicitly acknowledged by status
for the command. The DataSN/R2TSN field enables the initiator to
detect missing data or R2T PDUs.
For any read or bidirectional command, a target MUST issue less than
2**32 combined R2T and Data-In PDUs. Any output data sequence MUST
contain less than 2**32 Data-Out PDUs.