3.5. Request/Response Summary
This section lists and briefly describes all the iSCSI PDU types
(request 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.
3.5.1. Request/Response Types Carrying SCSI Payload
This request carries the SCSI CDB and all the other SCSI execute
command procedure call (see [SAM2]) IN arguments such as task
attributes, Expected Data Transfer Length for one or both transfer
directions (the latter for bidirectional commands), and 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) for the session and the expected status number
(ExpStatSN) for the connection.
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
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:
- 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
188.8.131.52 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 (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.
184.108.40.206. Task Management Function Response
The Task Management function response carries an indication of
function completion for a Task Management function request including
how it was completed (response and qualifier) and additional
information for failure responses.
After the Task Management response indicates Task Management function
completion, the initiator will not receive any additional responses
from the affected tasks.
220.127.116.11. 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 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 call.
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 are built to enable the direction switching for
For input, the target may request positive acknowledgement 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, 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).
18.104.22.168. 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)
3.5.2. Requests/Responses carrying SCSI and iSCSI Payload
22.214.171.124. Asynchronous Message
Asynchronous Messages are used to carry SCSI asynchronous events
(AEN) and iSCSI asynchronous messages.
When carrying an AEN, the event details are reported as sense data in
the data segment.
3.5.3. Requests/Responses Carrying iSCSI Only Payload
126.96.36.199. Text Request and Text Response
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 Request/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 (Final) flag bit in the text
response header to indicate its consent to sequence termination.
Text Request 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) 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
188.8.131.52. Login Request and Login Response
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.
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.
184.108.40.206. Logout Request and Response
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 initiators 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 text key Time2Retain and how long the
initiator must wait before proceeding with recovery in the text key
220.127.116.11. 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 acknowledgement 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 acknowledgement.
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.
18.104.22.168. 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 "unidirectional" 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 initiator/
4. SCSI Mode Parameters for iSCSI
There are no iSCSI specific mode pages.
5. Login and Full Feature Phase Negotiation
iSCSI parameters are negotiated at session or connection
establishment by using Login Requests and Responses (see Section
3.2.3 iSCSI Login) and during the Full Feature Phase (Section 3.2.4
iSCSI Full Feature Phase) 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 if the key is declarative or requires
For the declarative keys, the declaring party sets a value for the
key. The key specification indicates if the key can be declared by
the initiator, 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
PDUs. 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 one of the following Login
or Text Response or Request PDUs. For most of the keys both the
initiator and target can be proposing parties.
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 the
setting of 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 (Transition)
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 (continuation) bit is used (both in Login and Text) to indicate
that "more follows".
The text negotiation uses an additional mechanism by which a target
may deliver larger amounts of data to an enquiring 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 chapter details types of keys and values used, the syntax rules
for parameter formation, and the negotiation schemes to be used with
different types of parameters.
5.1. 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 to be presented and interpreted in the case in which
they appear in this document. They are case sensitive.
The following character symbols are used in this document for text
items (the hexadecimal values represent Unicode code points):
(a-z, A-Z) - letters
(0-9) - 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 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 have the C bit set to 0, or
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 consist 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 consist 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 consist
of minus, dot, colon, or any character allowed by the output of
the iSCSI string-prep template as specified in [RFC3722] (see
also Section 22.214.171.124 iSCSI Name Encoding).
iSCSI-local-name-value: A UTF-8 string; no null characters are
allowed in the string. This encoding is to be used for localized
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 start 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-constant MUST
NOT be used for parameter values if the values can be equal 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 or letters or plus
or slash or equal. The encoding is done according to [RFC2045]
and each character, except equal, represents a base64 digit or a
6-bit binary string. Base64-constants are used to encode
numerical-values or binary strings. When used to encode
numerical values, the excessive use of leading 0 digits (encoded
as A) is discouraged. The string following 0B (or 0b) represents
a base64 number that starts with the most significant base64
digit, followed by all other digits in decreasing order of
significance and ending with the least-significant base64 digit;
the least significant base64 digit may be optionally followed by
pad digits (encoded as equal) that are not considered as part of
the number. When used to encode binary strings, base64-constants
have an implicit
byte-length that includes six bits for every character of the
constant, excluding trailing equals (i.e., a base64-constant of n
base64 characters excluding the trailing equals has a byte-length
of ((the integer part of) (n*3/4)). Correctly encoded base64
strings cannot have n values of 1, 5 ... k*4+1.
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
numeric-range: Two numerical-values separated by a tilde where the
value to the right of tilde must not be lower than the value to
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,
numeric-value, a numeric-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 of at least 64 kilobytes of key=value data (see Appendix
11.1.2 - Simple Public-Key Mechanism (SPKM) - that require support
for public key certificates).
5.2. 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 to 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 Text or Login Response PDU
that has the C bit set to 1 MUST NOT have the F/T bit set to 1.
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 Text
or Login Request with no data segment (DataSegmentLength 0) unless
explicitly required by a general or a key-specific negotiation rule.
The format of a declaration is:
The general format of text negotiation is:
Thus a declaration is a one-way textual exchange while a negotiation
is a two-way exchange.
The proposer or declarer can either be the initiator or the target,
and the acceptor can either be 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 use of some other value has serious
The value proposed or declared can be a numerical-value, a
numerical-range defined by lower and upper values 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 it is 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
negotiation. However the negotiation is not considered as 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
Any key not understood by the acceptor may be ignored by the acceptor
without affecting the basic function. However, the answer for a key
not understood MUST be key=NotUnderstood.
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 re-negotiation and is forbidden for many keys.
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 it is sent, the proposer MAY choose to terminate the
connection or session.
All keys in this document, except for the X extension formats, MUST
be supported by iSCSI initiators and targets when used as specified
here. If used as specified, these keys MUST NOT be answered with
Implementers may introduce new keys by prefixing them with
"X-", followed by their (reversed) domain name, or with new keys
registered with IANA prefixing them with X#. For example, the entity
owning the domain example.com can issue:
or a new registered key may be used as in:
Implementers MAY also introduce new values, but ONLY for new keys or
authentication methods (see Section 11 iSCSI Security Text Keys and
Authentication Methods), or digests (see Section 12.1 HeaderDigest
Whenever parameter action or acceptance is dependent on other
parameters, the dependency rules and parameter sequence must be
specified with the parameters.
In the Login Phase (see Section 5.3 Login Phase), every stage is a
separate negotiation. In the FullFeaturePhase, 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 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 end within a reasonable time or number of exchanges.
5.2.1. List negotiations
In list negotiation, the originator sends a list of values (which may
include "None") in its 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
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 as a protocol error.
5.2.2. 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" or the acceptor
MAY 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).
In Boolean negotiations (i.e., those that result in 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 to answer
with 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.
5.3. 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, security parameters, and authenticates the
initiator and target to each other.
The Login Phase is only implemented via Login Request 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).
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 or both, but MUST include at least
one of them. The included stage MAY be empty except for the
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.
Security MUST be completely negotiated within the Login Phase. The
use of underlying IPsec security is specified in Chapter 8 and in
[RFC3723]. 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 SecurityNegotiation keys appear in Chapter 11 and the
LoginOperationalNegotiation keys appear in Chapter 12. Only a
limited set of keys (marked as Any-Stage in Chapter 12) may be used
in any of the two stages.
Any given Login Request or Response belongs to a specific stage; this
determines the negotiation keys allowed with the request or response.
It is considered to be a protocol error to send a key that is not
allowed in the current stage.
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 through 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 pair of Login
Request-Responses have no bearing on the T bit settings of the next
pair. An initiator that has a 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 Login reject (initiator
error); if detected by the initiator, the initiator MUST drop the
5.3.1. 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 5.3.5) |
|existing | non-zero | new | add a new connection to |
| | existing | | the session |
|existing | non-zero |existing| do connection reinstatement|
| | existing | | (see section 5.3.4) |
|existing | non-zero | any | fail the login |
| | new | | ("session does not exist") |
Determination of "existing" or "new" are made by the target.
Optionally, the Login Request may include:
- Security parameters
- iSCSI operational parameters
- 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 and the CSG and NSG fields are
- Login Response with Login Accept as a final response (T bit set
to 1 and the NSG in both request and response are 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 10.13 Login Response)
without SecurityNegotiation and will terminate the connection with a
response of Authentication failure (see Section 10.13.5 Status-Class
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 12.9 TargetPortalGroupTag), the T bit set to 1, the
CSG set to SecurityNegotiation, and the NSG set to
An initiator that chooses to operate without iSCSI security, 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 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 when TargetName is given and the response is not a redirection.
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.
5.3.2. iSCSI Security Negotiation
The security exchange sets the security mechanism and authenticates
the initiator user and the target to each other. The exchange
proceeds according to the authentication method chosen in the
negotiation phase and is conducted using the Login Requests' and
responses' key=value parameters.
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 5.2 Text Mode Negotiation). The parameters
are encoded in UTF8 as key=value. For security parameters, see
- When the initiator considers that it is 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 respond 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).
5.3.3. Operational Parameter Negotiation During the Login Phase
Operational parameter negotiation during the login 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-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
If the target responds to a Login Request that has the T bit set to 1
with a Login Response that has the T bit set to 0, the initiator
should keep sending the Login Request (even empty) with the T bit set
to 1, while it still wants to switch stage, until it receives the
Login Response that has the T bit set to 1 or it receives a key that
requires it 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
FullFeaturePhase. New or replacement connections can only be added
to a session after the session is operational.
For operational parameters, see Chapter 12.
5.3.4. 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 reinstating a new Full
Feature Phase iSCSI connection in its place (with the same CID).
Thus, the TSIH in the Login 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 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 7.1.3)
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.
5.3.5. Session Reinstatement, Closure, and Timeout
Session reinstatement is the process of the initiator logging in with
an ISID that is possibly active from the target's perspective. 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 initiator.
The initiator session state MUST be FAILED (Section 7.3 Session State
Diagrams) 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 7.2 Connection Cleanup State Diagram
for Initiators and Targets) 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 (N6 transition in the
session state diagram).
126.96.36.199. 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:
a) Successful completion of session reinstatement.
b) Session closure event.
c) Session timeout event.
Certain SCSI object clearing actions may result due to the
notification in the SCSI end nodes, as documented in Appendix F.
- Clearing Effects of Various Events on Targets -.
5.3.6. 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 5.3.4 Connection Reinstatement), or by adding a connection
with a new CID. Either of these actions associates the new transport
connection with the session state.
Session failure is an event where the last Full Feature Phase
connection reaches the CLEANUP_WAIT state (Section 7.2 Connection
Cleanup State Diagram for Initiators and Targets), or completes a
successful recovery logout, thus causing all active tasks (that are
formerly allegiant to the connection) to start waiting for task
5.4. Operational Parameter Negotiation Outside the Login Phase
Some operational parameters MAY be negotiated outside (after) the
Parameter negotiation in Full Feature Phase is done through Text
requests and responses. Operational parameter negotiation MAY
involve several Text request-response exchanges, which the initiator
always starts and terminates using the same Initiator Task Tag. The
initiator MUST indicate its intent to terminate the negotiation by
setting the F bit to 1; the target sets the F bit to 1 on the last
If the target responds to a Text request with the F bit set to 1 and
with a Text response with the F bit set to 0, the initiator should
keep sending the Text request (even empty) 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.
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 pair of Text
request-responses 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
without an intervening operational parameter negotiation reset,
except for responses to specific keys that explicitly allow repeated
key declarations (e.g., TargetAddress). If detected by the target,
this MUST result in a Reject PDU with a reason of "protocol error".
The initiator MUST reset the negotiation as outlined above.