4.6. Request/Response Summary
This section lists and briefly describes all the iSCSI PDU types
(requests and responses).
All iSCSI PDUs are built as a set of one or more header segments
(basic and auxiliary) and zero or one data segments. The header
group and the data segment may each be followed by a CRC (digest).
The basic header segment has a fixed length of 48 bytes.
4.6.1. Request/Response Types Carrying SCSI Payload
126.96.36.199. SCSI Command
This request carries the SCSI CDB and all the other SCSI Execute
Command [SAM2] procedure call IN arguments, such as task attributes,
Expected Data Transfer Length for one or both transfer directions
(the latter for bidirectional commands), and a task tag (as part of
the I_T_L_x nexus). The I_T_L nexus is derived by the initiator and
target from the LUN field in the request, and the I_T nexus is
implicit in the session identification.
In addition, the SCSI Command PDU carries information required for
the proper operation of the iSCSI protocol -- the command sequence
number (CmdSN) and the expected status sequence number (ExpStatSN) on
the connection it is issued.
All or part of the SCSI output (write) data associated with the SCSI
command may be sent as part of the SCSI Command PDU as a data
188.8.131.52. SCSI Response
The SCSI Response carries all the SCSI Execute Command procedure call
(see [SAM2]) OUT arguments and the SCSI Execute Command procedure
call return value.
The SCSI Response contains the residual counts from the operation, if
any; an indication of whether the counts represent an overflow or an
underflow; and the SCSI status if the status is valid or a response
code (a non-zero return value for the Execute Command procedure call)
if the status is not valid.
For a valid status that indicates that the command has been processed
but resulted in an exception (e.g., a SCSI CHECK CONDITION), the PDU
data segment contains the associated sense data. The use of
Autosense ([SAM2]) is REQUIRED by iSCSI.
Some data segment content may also be associated (in the data
segment) with a non-zero response code.
In addition, the SCSI Response PDU carries information required for
the proper operation of the iSCSI protocol:
- ExpDataSN - the number of Data-In PDUs that a target has sent
(to enable the initiator to check that all have arrived)
- StatSN - the status sequence number on this connection
- ExpCmdSN - the next expected command sequence number at the
- MaxCmdSN - the maximum CmdSN acceptable at the target from this
184.108.40.206. Task Management Function Request
The Task Management Function Request provides an initiator with a way
to explicitly control the execution of one or more SCSI tasks or
iSCSI functions. The PDU carries a function identifier (i.e., which
task management function to perform) and enough information to
unequivocally identify the task or task set on which to perform the
action, even if the task(s) to act upon has not yet arrived or has
been discarded due to an error.
The referenced tag identifies an individual task if the function
refers to an individual task.
The I_T_L nexus identifies task sets. In iSCSI, the I_T_L nexus is
identified by the LUN and the session identification (the session
identifies an I_T nexus).
For task sets, the CmdSN of the Task Management Function Request
helps identify the tasks upon which to act, namely all tasks
associated with a LUN and having a CmdSN preceding the Task
Management Function Request CmdSN.
For a task management function, the coordination between responses to
the tasks affected and the Task Management Function Response is done
by the target.
220.127.116.11. Task Management Function Response
The Task Management Function Response carries an indication of
function completion for a Task Management Function Request, including
how it completed (response and qualifier) and additional information
for failure responses.
After the Task Management Function Response indicates task management
function completion, the initiator will not receive any additional
responses from the affected tasks.
18.104.22.168. SCSI Data-Out and SCSI Data-In
SCSI Data-Out and SCSI Data-In are the main vehicles by which SCSI
data payload is carried between the initiator and target. Data
payload is associated with a specific SCSI command through the
Initiator Task Tag. For target convenience, outgoing solicited data
also carries a Target Transfer Tag (copied from R2T) and the LUN.
Each PDU contains the payload length and the data offset relative to
the buffer address contained in the SCSI Execute Command procedure
In each direction, the data transfer is split into "sequences". An
end-of-sequence is indicated by the F bit.
An outgoing sequence is either unsolicited (only the first sequence
can be unsolicited) or consists of all the Data-Out PDUs sent in
response to an R2T.
Input sequences enable the switching of direction for bidirectional
commands as required.
For input, the target may request positive acknowledgment of input
data. This is limited to sessions that support error recovery and is
implemented through the A bit in the SCSI Data-In PDU header.
Data-In and Data-Out PDUs also carry the DataSN to enable the
initiator and target to detect missing PDUs (discarded due to an
In addition, the StatSN is carried by the Data-In PDUs.
To enable a SCSI command to be processed while involving a minimum
number of messages, the last SCSI Data-In PDU passed for a command
may also contain the status if the status indicates termination with
no exceptions (no sense or response involved).
22.214.171.124. Ready To Transfer (R2T)
R2T is the mechanism by which the SCSI target "requests" the
initiator for output data. R2T specifies to the initiator the offset
of the requested data relative to the buffer address from the Execute
Command procedure call and the length of the solicited data.
To help the SCSI target associate the resulting Data-Out with an R2T,
the R2T carries a Target Transfer Tag that will be copied by the
initiator in the solicited SCSI Data-Out PDUs. There are no
protocol-specific requirements with regard to the value of these
tags, but it is assumed that together with the LUN, they will enable
the target to associate data with an R2T.
R2T also carries information required for proper operation of the
iSCSI protocol, such as:
- R2TSN (to enable an initiator to detect a missing R2T)
4.6.2. Requests/Responses Carrying SCSI and iSCSI Payload
126.96.36.199. Asynchronous Message
Asynchronous Message PDUs are used to carry SCSI asynchronous event
notifications (AENs) and iSCSI asynchronous messages.
When carrying an AEN, the event details are reported as sense data in
the data segment.
4.6.3. Requests/Responses Carrying iSCSI-Only Payload
188.8.131.52. Text Requests and Text Responses
Text Requests and Responses are designed as a parameter negotiation
vehicle and as a vehicle for future extension.
In the data segment, Text Requests/Responses carry text information
using a simple "key=value" syntax.
Text Requests/Responses may form extended sequences using the same
Initiator Task Tag. The initiator uses the F (Final) flag bit in the
Text Request header to indicate its readiness to terminate a
sequence. The target uses the F bit in the Text Response header to
indicate its consent to sequence termination.
Text Requests and Responses also use the Target Transfer Tag to
indicate continuation of an operation or a new beginning. A target
that wishes to continue an operation will set the Target Transfer Tag
in a Text Response to a value different from the default 0xffffffff.
An initiator willing to continue will copy this value into the Target
Transfer Tag of the next Text Request. If the initiator wants to
restart the current target negotiation (start fresh), it will set the
Target Transfer Tag to 0xffffffff.
Although a complete exchange is always started by the initiator,
specific parameter negotiations may be initiated by the initiator or
184.108.40.206. Login Requests and Login Responses
Login Requests and Responses are used exclusively during the Login
Phase of each connection to set up the session and connection
parameters. (The Login Phase consists of a sequence of Login
Requests and Responses carrying the same Initiator Task Tag.)
A connection is identified by an arbitrarily selected connection ID
(CID) that is unique within a session.
Similar to the Text Requests and Responses, Login Requests/Responses
carry key=value text information with a simple syntax in the data
The Login Phase proceeds through several stages (security
negotiation, operational parameter negotiation) that are selected
with two binary coded fields in the header -- the Current Stage (CSG)
and the Next Stage (NSG) -- with the appearance of the latter being
signaled by the "Transit" flag (T).
The first Login Phase of a session plays a special role, called the
leading login, which determines some header fields (e.g., the version
number, the maximum number of connections, and the session
The CmdSN initial value is also set by the leading login.
The StatSN for each connection is initiated by the connection login.
A Login Request may indicate an implied logout (cleanup) of the
connection to be logged in (a connection restart) by using the same
connection ID (CID) as an existing connection as well as the same
session-identifying elements of the session to which the old
connection was associated.
220.127.116.11. Logout Requests and Logout Responses
Logout Requests and Responses are used for the orderly closing of
connections for recovery or maintenance. The Logout Request may be
issued following a target prompt (through an Asynchronous Message) or
at an initiator's initiative. When issued on the connection to be
logged out, no other request may follow it.
The Logout Response indicates that the connection or session cleanup
is completed and no other responses will arrive on the connection (if
received on the logging-out connection). In addition, the Logout
Response indicates how long the target will continue to hold
resources for recovery (e.g., command execution that continues on a
new connection) in the Time2Retain field and how long the initiator
must wait before proceeding with recovery in the Time2Wait field.
18.104.22.168. SNACK Request
With the SNACK Request, the initiator requests retransmission of
numbered responses or data from the target. A single SNACK Request
covers a contiguous set of missing items, called a run, of a given
type of items. The type is indicated in a type field in the PDU
header. The run is composed of an initial item (StatSN, DataSN,
R2TSN) and the number of missed Status, Data, or R2T PDUs. For long
Data-In sequences, the target may request (at predefined minimum
intervals) a positive acknowledgment for the data sent. A SNACK
Request with a type field that indicates ACK and the number of
Data-In PDUs acknowledged conveys this positive acknowledgment.
Reject enables the target to report an iSCSI error condition (e.g.,
protocol, unsupported option) that uses a Reason field in the PDU
header and includes the complete header of the bad PDU in the Reject
PDU data segment.
22.214.171.124. NOP-Out Request and NOP-In Response
This request/response pair may be used by an initiator and target as
a "ping" mechanism to verify that a connection/session is still
active and all of its components are operational. Such a ping may be
triggered by the initiator or target. The triggering party indicates
that it wants a reply by setting a value different from the default
0xffffffff in the corresponding Initiator/Target Transfer Tag.
NOP-In/NOP-Out may also be used in "unidirectional" fashion to convey
to the initiator/target command, status, or data counter values when
there is no other "carrier" and there is a need to update the
5. SCSI Mode Parameters for iSCSI
There are no iSCSI-specific mode pages.
6. Login and Full Feature Phase Negotiation
iSCSI parameters are negotiated at session or connection
establishment by using Login Requests and Responses (see
Section 4.2.4) and during the Full Feature Phase (Section 4.2.5) by
using Text Requests and Responses. In both cases, the mechanism used
is an exchange of iSCSI-text-key=value pairs. For brevity,
iSCSI-text-keys are called just "keys" in the rest of this document.
Keys are either declarative or require negotiation, and the key
description indicates whether the key is declarative or requires
For the declarative keys, the declaring party sets a value for the
key. The key specification indicates whether the key can be declared
by the initiator, the target, or both.
For the keys that require negotiation, one of the parties (the
proposing party) proposes a value or set of values by including the
key=value in the data part of a Login or Text Request or Response.
The other party (the accepting party) makes a selection based on the
value or list of values proposed and includes the selected value in a
key=value in the data part of the following Login or Text Response or
Request. For most of the keys, both the initiator and target can be
The login process proceeds in two stages -- the security negotiation
stage and the operational parameter negotiation stage. Both stages
are optional, but at least one of them has to be present to enable
setting some mandatory parameters.
If present, the security negotiation stage precedes the operational
parameter negotiation stage.
Progression from stage to stage is controlled by the T (Transit) bit
in the Login Request/Response PDU header. Through the T bit set
to 1, the initiator indicates that it would like to transition. The
target agrees to the transition (and selects the next stage) when
ready. A field in the Login PDU header indicates the current stage
(CSG), and during transition, another field indicates the next stage
(NSG) proposed (initiator) and selected (target).
The text negotiation process is used to negotiate or declare
operational parameters. The negotiation process is controlled by the
F (Final) bit in the PDU header. During text negotiations, the F bit
is used by the initiator to indicate that it is ready to finish the
negotiation and by the target to acquiesce the end of negotiation.
Since some key=value pairs may not fit entirely in a single PDU, the
C (Continue) bit is used (both in Login and Text) to indicate that
The text negotiation uses an additional mechanism by which a target
may deliver larger amounts of data to an inquiring initiator. The
target sets a Target Task Tag to be used as a bookmark that, when
returned by the initiator, means "go on". If reset to a "neutral
value", it means "forget about the rest".
This section details the types of keys and values used, the syntax
rules for parameter formation, and the negotiation schemes to be used
with different types of parameters.
6.1. Text Format
The initiator and target send a set of key=value pairs encoded in
UTF-8 Unicode. All the text keys and text values specified in this
document are case sensitive; they are to be presented and interpreted
as they appear in this document without change of case.
The following character symbols are used in this document for text
items (the hexadecimal values represent Unicode code points):
(a-z, A-Z) (0x61-0x7a, 0x41-0x5a) - letters
(0-9) (0x30-0x39) - digits
" " (0x20) - space
"." (0x2e) - dot
"-" (0x2d) - minus
"+" (0x2b) - plus
"@" (0x40) - commercial at
"_" (0x5f) - underscore
"=" (0x3d) - equal
":" (0x3a) - colon
"/" (0x2f) - solidus or slash
"[" (0x5b) - left bracket
"]" (0x5d) - right bracket
null (0x00) - null separator
"," (0x2c) - comma
"~" (0x7e) - tilde
Key=value pairs may span PDU boundaries. An initiator or target that
sends partial key=value text within a PDU indicates that more text
follows by setting the C bit in the Text or Login Request or the Text
or Login Response to 1. Data segments in a series of PDUs that have
the C bit set to 1 and end with a PDU that has the C bit set to 0, or
that include a single PDU that has the C bit set to 0, have to be
considered as forming a single logical-text-data-segment (LTDS).
Every key=value pair, including the last or only pair in a LTDS, MUST
be followed by one null (0x00) delimiter.
A key-name is whatever precedes the first "=" in the key=value pair.
The term "key" is used frequently in this document in place of
A value is whatever follows the first "=" in the key=value pair up to
the end of the key=value pair, but not including the null delimiter.
The following definitions will be used in the rest of this document:
- standard-label: A string of one or more characters that consists
of letters, digits, dot, minus, plus, commercial at, or
underscore. A standard-label MUST begin with a capital letter
and must not exceed 63 characters.
- key-name: A standard-label.
- text-value: A string of zero or more characters that consists of
letters, digits, dot, minus, plus, commercial at, underscore,
slash, left bracket, right bracket, or colon.
- iSCSI-name-value: A string of one or more characters that
consists of minus, dot, colon, or any character allowed by the
output of the iSCSI stringprep template as specified in
[RFC3722] (see also Section 126.96.36.199).
- iSCSI-local-name-value: A UTF-8 string; no null characters are
allowed in the string. This encoding is to be used for
localized (internationalized) aliases.
- boolean-value: The string "Yes" or "No".
- hex-constant: A hexadecimal constant encoded as a string that
starts with "0x" or "0X" followed by one or more digits or the
letters a, b, c, d, e, f, A, B, C, D, E, or F. Hex-constants
are used to encode numerical values or binary strings. When
used to encode numerical values, the excessive use of leading 0
digits is discouraged. The string following 0X (or 0x)
represents a base16 number that starts with the most significant
base16 digit, followed by all other digits in decreasing order
of significance and ending with the least significant base16
digit. When used to encode binary strings, hexadecimal
constants have an implicit byte-length that includes four bits
for every hexadecimal digit of the constant, including leading
zeroes. For example, a hex-constant of n hexadecimal digits has
a byte-length of (the integer part of) (n + 1)/2.
- decimal-constant: An unsigned decimal number with the digit 0 or
a string of one or more digits that starts with a non-zero
digit. Decimal-constants are used to encode numerical values or
binary strings. Decimal-constants can only be used to encode
binary strings if the string length is explicitly specified.
There is no implicit length for decimal strings.
Decimal-constants MUST NOT be used for parameter values if the
values can be equal to or greater than 2**64 (numerical) or for
binary strings that can be longer than 64 bits.
- base64-constant: Base64 constant encoded as a string that starts
with "0b" or "0B" followed by 1 or more digits, letters, plus
sign, slash, or equals sign. The encoding is done according to
- numerical-value: An unsigned integer always less than 2**64
encoded as a decimal-constant or a hex-constant. Unsigned
integer arithmetic applies to numerical-values.
- large-numerical-value: An unsigned integer that can be larger
than or equal to 2**64 encoded as a hex-constant or
base64-constant. Unsigned integer arithmetic applies to large-
- numerical-range: Two numerical-values separated by a tilde,
where the value to the right of the tilde must not be lower than
the value to the left.
- regular-binary-value: A binary string not longer than 64 bits
encoded as a decimal-constant, hex-constant, or base64-constant.
The length of the string is either specified by the key
definition or is the implicit byte-length of the encoded string.
- large-binary-value: A binary string longer than 64 bits encoded
as a hex-constant or base64-constant. The length of the string
is either specified by the key definition or is the implicit
byte-length of the encoded string.
- binary-value: A regular-binary-value or a large-binary-value.
Operations on binary values are key-specific.
- simple-value: Text-value, iSCSI-name-value, boolean-value,
numerical-value, a numerical-range, or a binary-value.
- list-of-values: A sequence of text-values separated by a comma.
If not otherwise specified, the maximum length of a simple-value (not
its encoded representation) is 255 bytes, not including the delimiter
(comma or zero byte).
Any iSCSI target or initiator MUST support receiving at least
8192 bytes of key=value data in a negotiation sequence. When
proposing or accepting authentication methods that explicitly require
support for very long authentication items, the initiator and target
MUST support receiving at least 64 kilobytes of key=value data.
6.2. Text Mode Negotiation
During login, and thereafter, some session or connection parameters
are either declared or negotiated through an exchange of textual
The initiator starts the negotiation and/or declaration through a
Text or Login Request and indicates when it is ready for completion
(by setting the F bit to 1 and keeping it at 1 in a Text Request, or
the T bit in the Login Request). As negotiation text may span PDU
boundaries, a Text or Login Request or a Text or Login Response PDU
that has the C bit set to 1 MUST NOT have the F bit or T bit set
A target receiving a Text or Login Request with the C bit set to 1
MUST answer with a Text or Login Response with no data segment
(DataSegmentLength 0). An initiator receiving a Text or Login
Response with the C bit set to 1 MUST answer with a Text or Login
Request with no data segment (DataSegmentLength 0).
A target or initiator SHOULD NOT use a Text or Login Response or a
Text or Login Request with no data segment (DataSegmentLength 0)
unless explicitly required by a general or a key-specific negotiation
There MUST NOT be more than one outstanding Text Request, or Text
Response PDU on an iSCSI connection. An outstanding PDU in this
context is one that has not been acknowledged by the remote iSCSI
The format of a declaration is:
The general format of text negotiation is:
Thus, a declaration is a one-way textual exchange (unless the key is
not understood by the receiver), while a negotiation is a two-way
The proposer or declarer can be either the initiator or the target,
and the acceptor can be either the target or initiator, respectively.
Targets are not limited to respond to key=value pairs as proposed by
the initiator. The target may propose key=value pairs of its own.
All negotiations are explicit (i.e., the result MUST only be based on
newly exchanged or declared values). There are no implicit
proposals. If a proposal is not made, then a reply cannot be
expected. Conservative design also requires that default values
should not be relied upon when the use of some other value has
The value proposed or declared can be a numerical-value, a numerical-
range defined by the lower and upper value with both integers
separated by a tilde, a binary value, a text-value, an iSCSI-name-
value, an iSCSI-local-name-value, a boolean-value (Yes or No), or a
list of comma-separated text-values. A range, a large-numerical-
value, an iSCSI-name-value, and an iSCSI-local-name-value MAY ONLY be
used if explicitly allowed. An accepted value can be a numerical-
value, a large-numerical-value, a text-value, or a boolean-value.
If a specific key is not relevant for the current negotiation, the
acceptor may answer with the constant "Irrelevant" for all types of
negotiations. However, the negotiation is not considered to have
failed if the answer is "Irrelevant". The "Irrelevant" answer is
meant for those cases in which several keys are presented by a
proposing party but the selection made by the acceptor for one of the
keys makes other keys irrelevant. The following example illustrates
the use of "Irrelevant":
I->T X-rdname-vkey1=(bla,alb,None), X-rdname-vkey2=(bla,alb)
T->I X-rdname-vkey1=None, X-rdname-vkey2=Irrelevant
Any key not understood by the acceptor may be ignored by the acceptor
without affecting the basic function. However, the answer for a key
that is not understood MUST be key=NotUnderstood. Note that
NotUnderstood is a valid answer for both declarative and negotiated
keys. The general iSCSI philosophy is that comprehension precedes
processing for any iSCSI key. A proposer of an iSCSI key, negotiated
or declarative, in a text key exchange MUST thus be able to properly
handle a NotUnderstood response.
The proper way to handle a NotUnderstood response depends on where
the key is specified and whether the key is declarative or
negotiated. An iSCSI implementation MUST comprehend all text keys
defined in this document. Returning a NotUnderstood response on any
of these text keys therefore MUST be considered a protocol error and
handled accordingly. For all other "later" keys, i.e., text keys
defined in later specifications, a NotUnderstood answer concludes the
negotiation for a negotiated key, whereas for a declarative key a
NotUnderstood answer simply informs the declarer of a lack of
comprehension by the receiver.
In either case, a NotUnderstood answer always requires that the
protocol behavior associated with that key not be used within the
scope of the key (connection/session) by either side.
The constants "None", "Reject", "Irrelevant", and "NotUnderstood" are
reserved and MUST ONLY be used as described here. Violation of this
rule is a protocol error (in particular, the use of "Reject",
"Irrelevant", and "NotUnderstood" as proposed values).
"Reject" or "Irrelevant" are legitimate negotiation options where
allowed, but their excessive use is discouraged. A negotiation is
considered complete when the acceptor has sent the key value pair
even if the value is "Reject", "Irrelevant", or "NotUnderstood".
Sending the key again would be a renegotiation and is forbidden for
If the acceptor sends "Reject" as an answer, the negotiated key is
left at its current value (or default if no value was set). If the
current value is not acceptable to the proposer on the connection or
to the session in which it is sent, the proposer MAY choose to
terminate the connection or session.
All keys in this document MUST be supported by iSCSI initiators and
targets when used as specified here. If used as specified, these
keys MUST NOT be answered with NotUnderstood.
Implementers may introduce new private keys by prefixing them with X-
followed by their (reverse) domain name, or with new public keys
registered with IANA. For example, the entity owning the domain
example.com can issue:
Each new public key in the course of standardization MUST define the
acceptable responses to the key, including NotUnderstood as
appropriate. Unlike [RFC3720], note that this document prohibits the
X# prefix for new public keys. Based on iSCSI implementation
experience, we know that there is no longer a need for a standard
name prefix for keys that allow a NotUnderstood response. Note that
NotUnderstood will generally have to be allowed for new public keys
for backwards compatibility, as well as for private X- keys. Thus,
the name prefix "X#" in new public key-names does not carry any
significance. To avoid confusion, new public key-names MUST NOT
begin with an "X#" prefix.
Implementers MAY also introduce new values, but ONLY for new keys or
authentication methods (see Section 12) or digests (see
Whenever parameter actions or acceptance are dependent on other
parameters, the dependency rules and parameter sequence must be
specified with the parameters.
In the Login Phase (see Section 6.3), every stage is a separate
negotiation. In the Full Feature Phase, a Text Request/Response
sequence is a negotiation. Negotiations MUST be handled as atomic
operations. For example, all negotiated values go into effect after
the negotiation concludes in agreement or are ignored if the
Some parameters may be subject to integrity rules (e.g., parameter-x
must not exceed parameter-y, or parameter-u not 1 implies that
parameter-v be Yes). Whenever required, integrity rules are
specified with the keys. Checking for compliance with the integrity
rule must only be performed after all the parameters are available
(the existent and the newly negotiated). An iSCSI target MUST
perform integrity checking before the new parameters take effect. An
initiator MAY perform integrity checking.
An iSCSI initiator or target MAY terminate a negotiation that does
not terminate within an implementation-specific reasonable time or
number of exchanges but SHOULD allow at least six (6) exchanges.
6.2.1. List Negotiations
In list negotiation, the originator sends a list of values (which may
include "None"), in order of preference.
The responding party MUST respond with the same key and the first
value that it supports (and is allowed to use for the specific
originator) selected from the originator list.
The constant "None" MUST always be used to indicate a missing
function. However, "None" is only a valid selection if it is
explicitly proposed. When "None" is proposed as a selection item in
a negotiation for a key, it indicates to the responder that not
supporting any functionality related to that key is legal, and if
"None" is the negotiation result for such a key, it means that key-
specific semantics are not operational for the negotiation scope
(connection or session) of that key.
If an acceptor does not understand any particular value in a list, it
MUST ignore it. If an acceptor does not support, does not
understand, or is not allowed to use any of the proposed options with
a specific originator, it may use the constant "Reject" or terminate
the negotiation. The selection of a value not proposed MUST be
handled by the originator as a protocol error.
6.2.2. Simple-Value Negotiations
For simple-value negotiations, the accepting party MUST answer with
the same key. The value it selects becomes the negotiation result.
Proposing a value not admissible (e.g., not within the specified
bounds) MAY be answered with the constant "Reject"; otherwise, the
acceptor MUST select an admissible value.
The selection, by the acceptor, of a value not admissible under the
selection rules is considered a protocol error. The selection rules
For a numerical range, the value selected MUST be an integer within
the proposed range or "Reject" (if the range is unacceptable).
For Boolean negotiations (i.e., keys taking the values "Yes" or
"No"), the accepting party MUST answer with the same key and the
result of the negotiation when the received value does not determine
that result by itself. The last value transmitted becomes the
negotiation result. The rules for selecting the value with which to
answer are expressed as Boolean functions of the value received, and
the value that the accepting party would have selected if given a
Specifically, the two cases in which answers are OPTIONAL are:
- The Boolean function is "AND" and the value "No" is received.
The outcome of the negotiation is "No".
- The Boolean function is "OR" and the value "Yes" is received.
The outcome of the negotiation is "Yes".
Responses are REQUIRED in all other cases, and the value chosen and
sent by the acceptor becomes the outcome of the negotiation.
6.3. Login Phase
The Login Phase establishes an iSCSI connection between an initiator
and a target; it also creates a new session or associates the
connection to an existing session. The Login Phase sets the iSCSI
protocol parameters and security parameters, and authenticates the
initiator and target to each other.
The Login Phase is only implemented via Login Requests and Responses.
The whole Login Phase is considered as a single task and has a single
Initiator Task Tag (similar to the linked SCSI commands).
There MUST NOT be more than one outstanding Login Request or Login
Response on an iSCSI connection. An outstanding PDU in this context
is one that has not been acknowledged by the remote iSCSI side.
The default MaxRecvDataSegmentLength is used during login.
The Login Phase sequence of requests and responses proceeds as
- Login initial request
- Login partial response (optional)
- More Login Requests and Responses (optional)
- Login Final-Response (mandatory)
The initial Login Request of any connection MUST include the
InitiatorName key=value pair. The initial Login Request of the first
connection of a session MAY also include the SessionType key=value
pair. For any connection within a session whose type is not
"Discovery", the first Login Request MUST also include the TargetName
The Login Final-Response accepts or rejects the Login Request.
The Login Phase MAY include a SecurityNegotiation stage and a
LoginOperationalNegotiation stage and MUST include at least one of
them, but the included stage MAY be empty except for the mandatory
The Login Requests and Responses contain a field (CSG) that indicates
the current negotiation stage (SecurityNegotiation or
LoginOperationalNegotiation). If both stages are used, the
SecurityNegotiation MUST precede the LoginOperationalNegotiation.
Some operational parameters can be negotiated outside the login
through Text Requests and Responses.
Authentication-related security keys (Section 12) MUST be completely
negotiated within the Login Phase. The use of underlying IPsec
security is specified in Section 9.3, in [RFC3723], and in [RFC7146].
iSCSI support for security within the protocol only consists of
authentication in the Login Phase.
In some environments, a target or an initiator is not interested in
authenticating its counterpart. It is possible to bypass
authentication through the Login Request and Response.
The initiator and target MAY want to negotiate iSCSI authentication
parameters. Once this negotiation is completed, the channel is
Most of the negotiation keys are only allowed in a specific stage.
The keys used during the SecurityNegotiation stage are listed in
Section 12, and the keys used during the LoginOperationalNegotiation
stage are discussed in Section 13. Only a limited set of keys
(marked as Any-Stage in Section 13) may be used in either of the two
Any given Login Request or Response belongs to a specific stage; this
determines the negotiation keys allowed with the request or response.
Sending a key that is not allowed in the current stage is considered
a protocol error.
Stage transition is performed through a command exchange
(request/response) that carries the T bit and the same CSG code.
During this exchange, the next stage is selected by the target via
the Next Stage code (NSG). The selected NSG MUST NOT exceed the
value stated by the initiator. The initiator can request a
transition whenever it is ready, but a target can only respond with a
transition after one is proposed by the initiator.
In a negotiation sequence, the T bit settings in one Login Request-
Login Response pair have no bearing on the T bit settings of the next
pair. An initiator that has the T bit set to 1 in one pair and is
answered with a T bit setting of 0 may issue the next request with
the T bit set to 0.
When a transition is requested by the initiator and acknowledged by
the target, both the initiator and target switch to the selected
Targets MUST NOT submit parameters that require an additional
initiator Login Request in a Login Response with the T bit set to 1.
Stage transitions during login (including entering and exit) are only
possible as outlined in the following table:
|From To -> | Security | Operational | FullFeature |
| | | | | |
| V | | | |
| (start) | yes | yes | no |
| Security | no | yes | yes |
| Operational | no | no | yes |
The Login Final-Response that accepts a Login Request can only come
as a response to a Login Request with the T bit set to 1, and both
the request and response MUST indicate FullFeaturePhase as the next
phase via the NSG field.
Neither the initiator nor the target should attempt to declare or
negotiate a parameter more than once during login, except for
responses to specific keys that explicitly allow repeated key
declarations (e.g., TargetAddress). An attempt to
renegotiate/redeclare parameters not specifically allowed MUST be
detected by the initiator and target. If such an attempt is detected
by the target, the target MUST respond with a Login reject (initiator
error); if detected by the initiator, the initiator MUST drop the
6.3.1. Login Phase Start
The Login Phase starts with a Login Request from the initiator to the
target. The initial Login Request includes:
- Protocol version supported by the initiator
- iSCSI Initiator Name and iSCSI Target Name
- ISID, TSIH, and connection IDs
- Negotiation stage that the initiator is ready to enter
A login may create a new session, or it may add a connection to an
existing session. Between a given iSCSI initiator node (selected
only by an InitiatorName) and a given iSCSI target defined by an
iSCSI TargetName and a Target Portal Group Tag, the login results are
defined by the following table:
|ISID | TSIH | CID | Target Action |
|new | non-zero | any | fail the login |
| | | | ("session does not exist") |
|new | zero | any | instantiate a new session |
|existing| zero | any | do session reinstatement |
| | | | (see Section 6.3.5) |
|existing| non-zero | new | add a new connection to |
| | existing | | the session |
|existing| non-zero |existing| do connection reinstatement |
| | existing | | (see Section 188.8.131.52) |
|existing| non-zero | any | fail the login |
| | new | | ("session does not exist") |
The determination of "existing" or "new" is made by the target.
Optionally, the Login Request may include:
- Security parameters OR
- iSCSI operational parameters AND/OR
- The next negotiation stage that the initiator is ready to
The target can answer the login in the following ways:
- Login Response with Login reject. This is an immediate
rejection from the target that causes the connection to
terminate and the session to terminate if this is the first (or
only) connection of a new session. The T bit, the CSG field,
and the NSG field are reserved.
- Login Response with Login accept as the Final-Response (T bit
set to 1 and the NSG in both request and response is set to
FullFeaturePhase). The response includes the protocol version
supported by the target and the session ID and may include iSCSI
operational or security parameters (that depend on the current
- Login Response with Login accept as a partial response (NSG not
set to FullFeaturePhase in both request and response) that
indicates the start of a negotiation sequence. The response
includes the protocol version supported by the target and either
security or iSCSI parameters (when no security mechanism is
chosen) supported by the target.
If the initiator decides to forego the SecurityNegotiation stage, it
issues the Login with the CSG set to LoginOperationalNegotiation, and
the target may reply with a Login Response that indicates that it is
unwilling to accept the connection (see Section 11.13) without
SecurityNegotiation and will terminate the connection with a response
of Authentication failure (see Section 11.13.5).
If the initiator is willing to negotiate iSCSI security, but is
unwilling to make the initial parameter proposal and may accept a
connection without iSCSI security, it issues the Login with the T bit
set to 1, the CSG set to SecurityNegotiation, and the NSG set to
LoginOperationalNegotiation. If the target is also ready to skip
security, the Login Response only contains the TargetPortalGroupTag
key (see Section 13.9), the T bit set to 1, the CSG set to
SecurityNegotiation, and the NSG set to LoginOperationalNegotiation.
An initiator that chooses to operate without iSCSI security and with
all the operational parameters taking the default values issues the
Login with the T bit set to 1, the CSG set to
LoginOperationalNegotiation, and the NSG set to FullFeaturePhase. If
the target is also ready to forego security and can finish its
LoginOperationalNegotiation, the Login Response has the T bit set to
1, the CSG set to LoginOperationalNegotiation, and the NSG set to
FullFeaturePhase in the next stage.
During the Login Phase, the iSCSI target MUST return the
TargetPortalGroupTag key with the first Login Response PDU with which
it is allowed to do so (i.e., the first Login Response issued after
the first Login Request with the C bit set to 0) for all session
types. The TargetPortalGroupTag key value indicates the iSCSI portal
group servicing the Login Request PDU. If the reconfiguration of
iSCSI portal groups is a concern in a given environment, the iSCSI
initiator should use this key to ascertain that it had indeed
initiated the Login Phase with the intended target portal group.
6.3.2. iSCSI Security Negotiation
The security exchange sets the security mechanism and authenticates
the initiator and the target to each other. The exchange proceeds
according to the authentication method chosen in the negotiation
phase and is conducted using the key=value parameters carried in the
Login Requests and Responses.
An initiator-directed negotiation proceeds as follows:
- The initiator sends a Login Request with an ordered list of the
options it supports (authentication algorithm). The options are
listed in the initiator's order of preference. The initiator
MAY also send private or public extension options.
- The target MUST reply with the first option in the list it
supports and is allowed to use for the specific initiator,
unless it does not support any, in which case it MUST answer
with "Reject" (see Section 6.2). The parameters are encoded in
UTF-8 as key=value. For security parameters, see Section 12.
- When the initiator considers itself ready to conclude the
SecurityNegotiation stage, it sets the T bit to 1 and the NSG to
what it would like the next stage to be. The target will then
set the T bit to 1 and set the NSG to the next stage in the
Login Response when it finishes sending its security keys. The
next stage selected will be the one the target selected. If the
next stage is FullFeaturePhase, the target MUST reply with a
Login Response with the TSIH value.
If the security negotiation fails at the target, then the target MUST
send the appropriate Login Response PDU. If the security negotiation
fails at the initiator, the initiator SHOULD close the connection.
It should be noted that the negotiation might also be directed by the
target if the initiator does support security but is not ready to
direct the negotiation (propose options); see Appendix B for an
6.3.3. Operational Parameter Negotiation during the Login Phase
Operational parameter negotiation during the Login Phase MAY be done:
- starting with the first Login Request if the initiator does not
propose any security/integrity option.
- starting immediately after the security negotiation if the
initiator and target perform such a negotiation.
Operational parameter negotiation MAY involve several Login Request-
Login Response exchanges started and terminated by the initiator.
The initiator MUST indicate its intent to terminate the negotiation
by setting the T bit to 1; the target sets the T bit to 1 on the last
Even when the initiator indicates its intent to switch stages by
setting the T bit to 1 in a Login Request, the target MAY respond
with a Login Response with the T bit set to 0. In that case, the
initiator SHOULD continue to set the T bit to 1 in subsequent Login
Requests (even empty requests) that it sends, until the target sends
a Login Response with the T bit set to 1 or sends a key that requires
the initiator to set the T bit to 0.
Some session-specific parameters can only be specified during the
Login Phase of the first connection of a session (i.e., begun by a
Login Request that contains a zero-valued TSIH) -- the leading Login
Phase (e.g., the maximum number of connections that can be used for
A session is operational once it has at least one connection in the
Full Feature Phase. New or replacement connections can only be added
to a session after the session is operational.
For operational parameters, see Section 13.
6.3.4. Connection Reinstatement
Connection reinstatement is the process of an initiator logging in
with an ISID-TSIH-CID combination that is possibly active from the
target's perspective, which causes the implicit logging out of the
connection corresponding to the CID and reinstatement of a new Full
Feature Phase iSCSI connection in its place (with the same CID).
Thus, the TSIH in the Login Request PDU MUST be non-zero, and the CID
does not change during a connection reinstatement. The Login Request
performs the logout function of the old connection if an explicit
logout was not performed earlier. In sessions with a single
connection, this may imply the opening of a second connection with
the sole purpose of cleaning up the first. Targets MUST support
opening a second connection even when they do not support multiple
connections in the Full Feature Phase if ErrorRecoveryLevel is 2 and
SHOULD support opening a second connection if ErrorRecoveryLevel is
less than 2.
If the operational ErrorRecoveryLevel is 2, connection reinstatement
enables future task reassignment. If the operational
ErrorRecoveryLevel is less than 2, connection reinstatement is the
replacement of the old CID without enabling task reassignment. In
this case, all the tasks that were active on the old CID must be
immediately terminated without further notice to the initiator.
The initiator connection state MUST be CLEANUP_WAIT (Section 8.1.3)
when the initiator attempts a connection reinstatement.
In practical terms, in addition to the implicit logout of the old
connection, reinstatement is equivalent to a new connection login.
6.3.5. Session Reinstatement, Closure, and Timeout
Session reinstatement is the process of an initiator logging in with
an ISID that is possibly active from the target's perspective for
that initiator, thus implicitly logging out the session that
corresponds to the ISID and reinstating a new iSCSI session in its
place (with the same ISID). Therefore, the TSIH in the Login PDU
MUST be zero to signal session reinstatement. Session reinstatement
causes all the tasks that were active on the old session to be
immediately terminated by the target without further notice to the
The initiator session state MUST be FAILED (Section 8.3) when the
initiator attempts a session reinstatement.
Session closure is an event defined to be one of the following:
- a successful "session close" logout.
- a successful "connection close" logout for the last Full Feature
Phase connection when no other connection in the session is
waiting for cleanup (Section 8.2) and no tasks in the session
are waiting for reassignment.
Session timeout is an event defined to occur when the last connection
state timeout expires and no tasks are waiting for reassignment.
This takes the session to the FREE state (see the session state
diagrams in Section 8.3).
184.108.40.206. Loss of Nexus Notification
The iSCSI layer provides the SCSI layer with the "I_T nexus loss"
notification when any one of the following events happens:
- successful completion of session reinstatement
- session closure event
- session timeout event
Certain SCSI object clearing actions may result due to the
notification in the SCSI end nodes, as documented in Appendix E.
6.3.6. Session Continuation and Failure
Session continuation is the process by which the state of a
preexisting session continues to be used by connection reinstatement
(Section 6.3.4) or by adding a connection with a new CID. Either of
these actions associates the new transport connection with the
Session failure is an event where the last Full Feature Phase
connection reaches the CLEANUP_WAIT state (Section 8.2) or completes
a successful recovery logout, thus causing all active tasks (that are
formerly allegiant to the connection) to start waiting for task
6.4. Operational Parameter Negotiation outside the Login Phase
Some operational parameters MAY be negotiated outside (after) the
Parameter negotiation in the Full Feature Phase is done through Text
Requests and Responses. Operational parameter negotiation MAY
involve several Text Request-Text Response exchanges, all of which
use the same Initiator Task Tag; the initiator always starts and
terminates each of these exchanges. The initiator MUST indicate its
intent to finish the negotiation by setting the F bit to 1; the
target sets the F bit to 1 on the last response.
If the target responds to a Text Request with the F bit set to 1 with
a Text Response with the F bit set to 0, the initiator should keep
sending the Text Request (even empty requests) with the F bit set to
1 while it still wants to finish the negotiation, until it receives
the Text Response with the F bit set to 1. Responding to a Text
Request with the F bit set to 1 with an empty (no key=value pairs)
response with the F bit set to 0 is discouraged.
Even when the initiator indicates its intent to finish the
negotiation by setting the F bit to 1 in a Text Request, the target
MAY respond with a Text Response with the F bit set to 0. In that
case, the initiator SHOULD continue to set the F bit to 1 in
subsequent Text Requests (even empty requests) that it sends, until
the target sends the final Text Response with the F bit set to 1.
Note that in the same case of a Text Request with the F bit set to 1,
the target SHOULD NOT respond with an empty (no key=value pairs) Text
Response with the F bit set to 0, because such a response may cause
the initiator to abandon the negotiation.
Targets MUST NOT submit parameters that require an additional
initiator Text Request in a Text Response with the F bit set to 1.
In a negotiation sequence, the F bit settings in one Text Request-
Text Response pair have no bearing on the F bit settings of the next
pair. An initiator that has the F bit set to 1 in a request and is
being answered with an F bit setting of 0 may issue the next request
with the F bit set to 0.
Whenever the target responds with the F bit set to 0, it MUST set the
Target Transfer Tag to a value other than the default 0xffffffff.
An initiator MAY reset an operational parameter negotiation by
issuing a Text Request with the Target Transfer Tag set to the value
0xffffffff after receiving a response with the Target Transfer Tag
set to a value other than 0xffffffff. A target may reset an
operational parameter negotiation by answering a Text Request with a
Neither the initiator nor the target should attempt to declare or
negotiate a parameter more than once during any negotiation sequence,
except for responses to specific keys that explicitly allow repeated
key declarations (e.g., TargetAddress). If such an attempt is
detected by the target, the target MUST respond with a Reject PDU
with a reason of "Protocol Error". The initiator MUST reset the
negotiation as outlined above.
Parameters negotiated by a text exchange negotiation sequence only
become effective after the negotiation sequence is completed.