RFC 3720
Internet Small Computer Systems Interface (iSCSI)
10.5. Task Management Function Request
Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|I| 0x02 |1| Function | Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| Logical Unit Number (LUN) or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Referenced Task Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 24| CmdSN | +---------------+---------------+---------------+---------------+ 28| ExpStatSN | +---------------+---------------+---------------+---------------+ 32| RefCmdSN or Reserved | +---------------+---------------+---------------+---------------+ 36| ExpDataSN or Reserved | +---------------+---------------+---------------+---------------+ 40/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+10.5.1. Function
The Task Management functions provide an initiator with a way to explicitly control the execution of one or more Tasks (SCSI and iSCSI tasks). The Task Management function codes are listed below. For a more detailed description of SCSI task management, see [SAM2]. 1 - ABORT TASK - aborts the task identified by the Referenced Task Tag field. 2 - ABORT TASK SET - aborts all Tasks issued via this session on the logical unit. 3 - CLEAR ACA - clears the Auto Contingent Allegiance condition.
4 - CLEAR TASK SET - aborts all Tasks in the appropriate task set as
defined by the TST field in the Control mode page (see [SPC3]).
5 - LOGICAL UNIT RESET
6 - TARGET WARM RESET
7 - TARGET COLD RESET
8 - TASK REASSIGN - reassigns connection allegiance for the task
identified by the Referenced Task Tag field to this connection,
thus resuming the iSCSI exchanges for the task.
For all these functions, the Task Management function response MUST
be returned as detailed in Section 10.6 Task Management Function
Response. All these functions apply to the referenced tasks
regardless of whether they are proper SCSI tasks or tagged iSCSI
operations. Task management requests must act on all the commands
from the same session having a CmdSN lower than the task management
CmdSN. LOGICAL UNIT RESET, TARGET WARM RESET and TARGET COLD RESET
may affect commands from other sessions or commands from the same
session with CmdSN equal or exceeding CmdSN.
If the task management request is marked for immediate delivery, it
must be considered immediately for execution, but the operations
involved (all or part of them) may be postponed to allow the target
to receive all relevant tasks. According to [SAM2], for all the
tasks covered by the Task Management response (i.e., with CmdSN lower
than the task management command CmdSN) but except the Task
Management response to a TASK REASSIGN, additional responses MUST NOT
be delivered to the SCSI layer after the Task Management response.
The iSCSI initiator MAY deliver to the SCSI layer all responses
received before the Task Management response (i.e., it is a matter of
implementation if the SCSI responses, received before the Task
Management response but after the task management request was issued,
are delivered to the SCSI layer by the iSCSI layer in the initiator).
The iSCSI target MUST ensure that no responses for the tasks covered
by a task management function are delivered to the iSCSI initiator
after the Task Management response except for a task covered by a
TASK REASSIGN.
For ABORT TASK SET and CLEAR TASK SET, the issuing initiator MUST
continue to respond to all valid target transfer tags (received via
R2T, Text Response, NOP-In, or SCSI Data-In PDUs) related to the
affected task set, even after issuing the task management request.
The issuing initiator SHOULD however terminate (i.e., by setting the
F-bit to 1) these response sequences as quickly as possible. The
target on its part MUST wait for responses on all affected target
transfer tags before acting on either of these two task management requests. In case all or part of the response sequence is not received (due to digest errors) for a valid TTT, the target MAY treat it as a case of within-command error recovery class (see Section 6.1.4.1 Recovery Within-command) if it is supporting ErrorRecoveryLevel >= 1, or alternatively may drop the connection to complete the requested task set function. If an ABORT TASK is issued for a task created by an immediate command then RefCmdSN MUST be that of the Task Management request itself (i.e., CmdSN and RefCmdSN are equal); otherwise RefCmdSN MUST be set to the CmdSN of the task to be aborted (lower than CmdSN). If the connection is still active (it is not undergoing an implicit or explicit logout), ABORT TASK MUST be issued on the same connection to which the task to be aborted is allegiant at the time the Task Management Request is issued. If the connection is implicitly or explicitly logged out (i.e., no other request will be issued on the failing connection and no other response will be received on the failing connection), then an ABORT TASK function request may be issued on another connection. This Task Management request will then establish a new allegiance for the command to be aborted as well as abort it (i.e., the task to be aborted will not have to be retried or reassigned, and its status, if issued but not acknowledged, will be reissued followed by the Task Management response). At the target an ABORT TASK function MUST NOT be executed on a Task Management request; such a request MUST result in Task Management response of "Function rejected". For the LOGICAL UNIT RESET function, the target MUST behave as dictated by the Logical Unit Reset function in [SAM2]. The implementation of the TARGET WARM RESET function and the TARGET COLD RESET function is OPTIONAL and when implemented, should act as described below. The TARGET WARM RESET is also subject to SCSI access controls on the requesting initiator as defined in [SPC3]. When authorization fails at the target, the appropriate response as described in Section 10.6 Task Management Function Response MUST be returned by the target. The TARGET COLD RESET function is not subject to SCSI access controls, but its execution privileges may be managed by iSCSI mechanisms such as login authentication. When executing the TARGET WARM RESET and TARGET COLD RESET functions, the target cancels all pending operations on all Logical Units known by the issuing initiator. Both functions are equivalent to the Target Reset function specified by [SAM2]. They can affect many other initiators logged in with the servicing SCSI target port.
The target MUST treat the TARGET COLD RESET function additionally as a power on event, thus terminating all of its TCP connections to all initiators (all sessions are terminated). For this reason, the Service Response (defined by [SAM2]) for this SCSI task management function may not be reliably delivered to the issuing initiator port. For the TASK REASSIGN function, the target should reassign the connection allegiance to this new connection (and thus resume iSCSI exchanges for the task). TASK REASSIGN MUST ONLY be received by the target after the connection on which the command was previously executing has been successfully logged-out. The Task Management response MUST be issued before the reassignment becomes effective. For additional usage semantics see Section 6.2 Retry and Reassign in Recovery. At the target a TASK REASSIGN function request MUST NOT be executed to reassign the connection allegiance of a Task Management function request, an active text negotiation task, or a Logout task; such a request MUST result in Task Management response of "Function rejected". TASK REASSIGN MUST be issued as an immediate command.10.5.2. TotalAHSLength and DataSegmentLength
For this PDU TotalAHSLength and DataSegmentLength MUST be 0.10.5.3. LUN
This field is required for functions that address a specific LU (ABORT TASK, CLEAR TASK SET, ABORT TASK SET, CLEAR ACA, LOGICAL UNIT RESET) and is reserved in all others.10.5.4. Referenced Task Tag
The Initiator Task Tag of the task to be aborted for the ABORT TASK function or reassigned for the TASK REASSIGN function. For all the other functions this field MUST be set to the reserved value 0xffffffff.10.5.5. RefCmdSN
If an ABORT TASK is issued for a task created by an immediate command then RefCmdSN MUST be that of the Task Management request itself (i.e., CmdSN and RefCmdSN are equal).
For an ABORT TASK of a task created by non-immediate command RefCmdSN MUST be set to the CmdSN of the task identified by the Referenced Task Tag field. Targets must use this field as described in section 10.6.1 when the task identified by the Referenced Task Tag field is not with the target. Otherwise, this field is reserved.10.5.6. ExpDataSN
For recovery purposes, the iSCSI target and initiator maintain a data acknowledgement reference number - the first input DataSN number unacknowledged by the initiator. When issuing a new command, this number is set to 0. If the function is TASK REASSIGN, which establishes a new connection allegiance for a previously issued Read or Bidirectional command, ExpDataSN will contain an updated data acknowledgement reference number or the value 0; the latter indicating that the data acknowledgement reference number is unchanged. The initiator MUST discard any data PDUs from the previous execution that it did not acknowledge and the target MUST transmit all Data-In PDUs (if any) starting with the data acknowledgement reference number. The number of retransmitted PDUs may or may not be the same as the original transmission depending on if there was a change in MaxRecvDataSegmentLength in the reassignment. The target MAY also send no more Data-In PDUs if all data has been acknowledged. The value of ExpDataSN MUST be 0 or higher than the DataSN of the last acknowledged Data-In PDU, but not larger than DataSN+1 of the last Data-In PDU sent by the target. Any other value MUST be ignored by the target. For other functions this field is reserved.
10.6. Task Management Function Response
Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|.| 0x22 |1| Reserved | Response | Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------------------------------------------------------+ 8/ Reserved / / / +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Reserved | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ For the functions ABORT TASK, ABORT TASK SET, CLEAR ACA, CLEAR TASK SET, LOGICAL UNIT RESET, TARGET COLD RESET, TARGET WARM RESET and TASK REASSIGN, the target performs the requested Task Management function and sends a Task Management response back to the initiator. For TASK REASSIGN, the new connection allegiance MUST ONLY become effective at the target after the target issues the Task Management Response.10.6.1. Response
The target provides a Response, which may take on the following values: a) 0 - Function complete. b) 1 - Task does not exist. c) 2 - LUN does not exist. d) 3 - Task still allegiant. e) 4 - Task allegiance reassignment not supported.
f) 5 - Task management function not supported.
g) 6 - Function authorization failed.
h) 255 - Function rejected.
All other values are reserved.
For a discussion on usage of response codes 3 and 4, see Section
6.2.2 Allegiance Reassignment.
For the TARGET COLD RESET and TARGET WARM RESET functions, the target
cancels all pending operations across all Logical Units known to the
issuing initiator. For the TARGET COLD RESET function, the target
MUST then close all of its TCP connections to all initiators
(terminates all sessions).
The mapping of the response code into a SCSI service response code
value, if needed, is outside the scope of this document. However, in
symbolic terms Response values 0 and 1 map to the SCSI service
response of FUNCTION COMPLETE. All other Response values map to the
SCSI service response of FUNCTION REJECTED. If a Task Management
function response PDU does not arrive before the session is
terminated, the SCSI service response is SERVICE DELIVERY OR TARGET
FAILURE.
The response to ABORT TASK SET and CLEAR TASK SET MUST only be issued
by the target after all of the commands affected have been received
by the target, the corresponding task management functions have been
executed by the SCSI target, and the delivery of all responses
delivered until the task management function completion have been
confirmed (acknowledged through ExpStatSN) by the initiator on all
connections of this session. For the exact timeline of events, refer
to Section 10.6.2 Task Management Actions on Task Sets.
For the ABORT TASK function,
a) If the Referenced Task Tag identifies a valid task leading to
a successful termination, then targets must return the
"Function complete" response.
b) If the Referenced Task Tag does not identify an existing task,
but if the CmdSN indicated by the RefCmdSN field in the Task
Management function request is within the valid CmdSN window
and less than the CmdSN of the Task Management function
request itself, then targets must consider the CmdSN received
and return the "Function complete" response.
c) If the Referenced Task Tag does not identify an existing task
and if the CmdSN indicated by the RefCmdSN field in the Task
Management function request is outside the valid CmdSN window,
then targets must return the "Task does not exist" response.
10.6.2. Task Management Actions on Task Sets
The execution of ABORT TASK SET and CLEAR TASK SET Task Management
function requests consists of the following sequence of events in the
specified order on each of the entities.
The initiator:
a) Issues ABORT TASK SET/CLEAR TASK SET request.
b) Continues to respond to each target transfer tag received
for the affected task set.
c) Receives any responses for the tasks in the affected task
set (may process them as usual because they are guaranteed
to be valid).
d) Receives the task set management response, thus concluding
all the tasks in the affected task set.
The target:
a) Receives the ABORT TASK SET/CLEAR TASK SET request.
b) Waits for all target transfer tags to be responded to and
for all affected tasks in the task set to be received.
c) Propagates the command to and receives the response from the
target SCSI layer.
d) Takes note of last-sent StatSN on each of the connections in
the iSCSI sessions (one or more) sharing the affected task
set, and waits for acknowledgement of each StatSN (may
solicit for acknowledgement by way of a NOP-In). If some
tasks originate from non-iSCSI I_T_L nexi then the means by
which the target insures that all affected tasks have
returned their status to the initiator are defined by the
specific protocol.
e) Sends the task set management response to the issuing
initiator.
10.6.3. TotalAHSLength and DataSegmentLength
For this PDU TotalAHSLength and DataSegmentLength MUST be 0.10.7. SCSI Data-Out & SCSI Data-In
The SCSI Data-Out PDU for WRITE operations has the following format: Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|.| 0x05 |F| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Target Transfer Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 24| Reserved | +---------------+---------------+---------------+---------------+ 28| ExpStatSN | +---------------+---------------+---------------+---------------+ 32| Reserved | +---------------+---------------+---------------+---------------+ 36| DataSN | +---------------+---------------+---------------+---------------+ 40| Buffer Offset | +---------------+---------------+---------------+---------------+ 44| Reserved | +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / DataSegment / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+
The SCSI Data-In PDU for READ operations has the following format:
Byte/ 0 | 1 | 2 | 3 |
/ | | | |
|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
+---------------+---------------+---------------+---------------+
0|.|.| 0x25 |F|A|0 0 0|O|U|S| Reserved |Status or Rsvd |
+---------------+---------------+---------------+---------------+
4|TotalAHSLength | DataSegmentLength |
+---------------+---------------+---------------+---------------+
8| LUN or Reserved |
+ +
12| |
+---------------+---------------+---------------+---------------+
16| Initiator Task Tag |
+---------------+---------------+---------------+---------------+
20| Target Transfer Tag or 0xffffffff |
+---------------+---------------+---------------+---------------+
24| StatSN or Reserved |
+---------------+---------------+---------------+---------------+
28| ExpCmdSN |
+---------------+---------------+---------------+---------------+
32| MaxCmdSN |
+---------------+---------------+---------------+---------------+
36| DataSN |
+---------------+---------------+---------------+---------------+
40| Buffer Offset |
+---------------+---------------+---------------+---------------+
44| Residual Count |
+---------------+---------------+---------------+---------------+
48| Header-Digest (Optional) |
+---------------+---------------+---------------+---------------+
/ DataSegment /
+/ /
+---------------+---------------+---------------+---------------+
| Data-Digest (Optional) |
+---------------+---------------+---------------+---------------+
Status can accompany the last Data-In PDU if the command did not end
with an exception (i.e., the status is "good status" - GOOD,
CONDITION MET or INTERMEDIATE CONDITION MET). The presence of status
(and of a residual count) is signaled though the S flag bit.
Although targets MAY choose to send even non-exception status in
separate responses, initiators MUST support non-exception status in
Data-In PDUs.
10.7.1. F (Final) Bit
For outgoing data, this bit is 1 for the last PDU of unsolicited data or the last PDU of a sequence that answers an R2T. For incoming data, this bit is 1 for the last input (read) data PDU of a sequence. Input can be split into several sequences, each having its own F bit. Splitting the data stream into sequences does not affect DataSN counting on Data-In PDUs. It MAY be used as a "change direction" indication for Bidirectional operations that need such a change. DataSegmentLength MUST not exceed MaxRecvDataSegmentLength for the direction it is sent and the total of all the DataSegmentLength of all PDUs in a sequence MUST not exceed MaxBurstLength (or FirstBurstLength for unsolicited data). However the number of individual PDUs in a sequence (or in total) may be higher than the MaxBurstLength (or FirstBurstLength) to MaxRecvDataSegmentLength ratio (as PDUs may be limited in length by the sender capabilities). Using DataSegmentLength of 0 may increase beyond what is reasonable for the number of PDUs and should therefore be avoided. For Bidirectional operations, the F bit is 1 for both the end of the input sequences and the end of the output sequences.10.7.2. A (Acknowledge) Bit
For sessions with ErrorRecoveryLevel 1 or higher, the target sets this bit to 1 to indicate that it requests a positive acknowledgement from the initiator for the data received. The target should use the A bit moderately; it MAY only set the A bit to 1 once every MaxBurstLength bytes, or on the last Data-In PDU that concludes the entire requested read data transfer for the task from the target's perspective, and it MUST NOT do so more frequently. The target MUST NOT set to 1 the A bit for sessions with ErrorRecoveryLevel=0. The initiator MUST ignore the A bit set to 1 for sessions with ErrorRecoveryLevel=0. On receiving a Data-In PDU with the A bit set to 1 on a session with ErrorRecoveryLevel greater than 0, if there are no holes in the read data until that Data-In PDU, the initiator MUST issue a SNACK of type DataACK except when it is able to acknowledge the status for the task immediately via ExpStatSN on other outbound PDUs if the status for the task is also received. In the latter case (acknowledgement through ExpStatSN), sending a SNACK of type DataACK in response to the A bit is OPTIONAL, but if it is done, it must not be sent after the status acknowledgement through ExpStatSN. If the initiator has detected holes in the read data prior to that Data-In PDU, it MUST
postpone issuing the SNACK of type DataACK until the holes are filled. An initiator also MUST NOT acknowledge the status for the task before those holes are filled. A status acknowledgement for a task that generated the Data-In PDUs is considered by the target as an implicit acknowledgement of the Data-In PDUs if such an acknowledgement was requested by the target.10.7.3. Flags (byte 1)
The last SCSI Data packet sent from a target to an initiator for a SCSI command that completed successfully (with a status of GOOD, CONDITION MET, INTERMEDIATE or INTERMEDIATE CONDITION MET) may also optionally contain the Status for the data transfer. As Sense Data cannot be sent together with the Command Status, if the command is completed with an error, then the response and sense data MUST be sent in a SCSI Response PDU (i.e., MUST NOT be sent in a SCSI Data packet). If Status is sent with the data, then a SCSI Response PDU MUST NOT be sent as this would violate SCSI rules (a single status). For Bidirectional commands, the status MUST be sent in a SCSI Response PDU. bit 2-4 - Reserved. bit 5-6 - used the same as in a SCSI Response. These bits are only valid when S is set to 1. For details see Section 10.4.1 Flags (byte 1). bit 7 S (status)- set to indicate that the Command Status field contains status. If this bit is set to 1, the F bit MUST also be set to 1. The fields StatSN, Status, and Residual Count only have meaningful content if the S bit is set to 1 and their values are defined in Section 10.4 SCSI Response.10.7.4. Target Transfer Tag and LUN
On outgoing data, the Target Transfer Tag is provided to the target if the transfer is honoring an R2T. In this case, the Target Transfer Tag field is a replica of the Target Transfer Tag provided with the R2T. On incoming data, the Target Transfer Tag and LUN MUST be provided by the target if the A bit is set to 1; otherwise they are reserved. The Target Transfer Tag and LUN are copied by the initiator into the SNACK of type DataACK that it issues as a result of receiving a SCSI Data-In PDU with the A bit set to 1.
The Target Transfer Tag values are not specified by this protocol except that the value 0xffffffff is reserved and means that the Target Transfer Tag is not supplied. If the Target Transfer Tag is provided, then the LUN field MUST hold a valid value and be consistent with whatever was specified with the command; otherwise, the LUN field is reserved.10.7.5. DataSN
For input (read) or bidirectional Data-In PDUs, the DataSN is the input PDU number within the data transfer for the command identified by the Initiator Task Tag. R2T and Data-In PDUs, in the context of bidirectional commands, share the numbering sequence (see Section 3.2.2.3 Data Sequencing). For output (write) data PDUs, the DataSN is the Data-Out PDU number within the current output sequence. The current output sequence is either identified by the Initiator Task Tag (for unsolicited data) or is a data sequence generated for one R2T (for data solicited through R2T).10.7.6. Buffer Offset
The Buffer Offset field contains the offset of this PDU payload data within the complete data transfer. The sum of the buffer offset and length should not exceed the expected transfer length for the command. The order of data PDUs within a sequence is determined by DataPDUInOrder. When set to Yes, it means that PDUs have to be in increasing Buffer Offset order and overlays are forbidden. The ordering between sequences is determined by DataSequenceInOrder. When set to Yes, it means that sequences have to be in increasing Buffer Offset order and overlays are forbidden.10.7.7. DataSegmentLength
This is the data payload length of a SCSI Data-In or SCSI Data-Out PDU. The sending of 0 length data segments should be avoided, but initiators and targets MUST be able to properly receive 0 length data segments. The Data Segments of Data-In and Data-Out PDUs SHOULD be filled to the integer number of 4 byte words (real payload) unless the F bit is set to 1.
10.8. Ready To Transfer (R2T)
Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|.| 0x31 |1| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Target Transfer Tag | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36| R2TSN | +---------------+---------------+---------------+---------------+ 40| Buffer Offset | +---------------+---------------+---------------+---------------+ 44| Desired Data Transfer Length | +---------------------------------------------------------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ When an initiator has submitted a SCSI Command with data that passes from the initiator to the target (WRITE), the target may specify which blocks of data it is ready to receive. The target may request that the data blocks be delivered in whichever order is convenient for the target at that particular instant. This information is passed from the target to the initiator in the Ready To Transfer (R2T) PDU. In order to allow write operations without an explicit initial R2T, the initiator and target MUST have negotiated the key InitialR2T to No during Login. An R2T MAY be answered with one or more SCSI Data-Out PDUs with a matching Target Transfer Tag. If an R2T is answered with a single Data-Out PDU, the Buffer Offset in the Data PDU MUST be the same as
the one specified by the R2T, and the data length of the Data PDU
MUST be the same as the Desired Data Transfer Length specified in the
R2T. If the R2T is answered with a sequence of Data PDUs, the Buffer
Offset and Length MUST be within the range of those specified by R2T,
and the last PDU MUST have the F bit set to 1. If the last PDU
(marked with the F bit) is received before the Desired Data Transfer
Length is transferred, a target MAY choose to Reject that
PDU with "Protocol error" reason code. DataPDUInOrder governs the
Data-Out PDU ordering. If DataPDUInOrder is set to Yes, the Buffer
Offsets and Lengths for consecutive PDUs MUST form a continuous
non-overlapping range and the PDUs MUST be sent in increasing offset
order.
The target may send several R2T PDUs. It, therefore, can have a
number of pending data transfers. The number of outstanding R2T PDUs
are limited by the value of the negotiated key MaxOutstandingR2T.
Within a connection, outstanding R2Ts MUST be fulfilled by the
initiator in the order in which they were received.
R2T PDUs MAY also be used to recover Data Out PDUs. Such an R2T
(Recovery-R2T) is generated by a target upon detecting the loss of
one or more Data-Out PDUs due to:
- Digest error
- Sequence error
- Sequence reception timeout
A Recovery-R2T carries the next unused R2TSN, but requests part of or
the entire data burst that an earlier R2T (with a lower R2TSN) had
already requested.
DataSequenceInOrder governs the buffer offset ordering in consecutive
R2Ts. If DataSequenceInOrder is Yes, then consecutive R2Ts MUST
refer to continuous non-overlapping ranges except for Recovery-R2Ts.
10.8.1. TotalAHSLength and DataSegmentLength
For this PDU TotalAHSLength and DataSegmentLength MUST be 0.
10.8.2. R2TSN
R2TSN is the R2T PDU input PDU number within the command identified
by the Initiator Task Tag.
For bidirectional commands R2T and Data-In PDUs share the input PDU
numbering sequence (see Section 3.2.2.3 Data Sequencing).
10.8.3. StatSN
The StatSN field will contain the next StatSN. The StatSN for this connection is not advanced after this PDU is sent.10.8.4. Desired Data Transfer Length and Buffer Offset
The target specifies how many bytes it wants the initiator to send because of this R2T PDU. The target may request the data from the initiator in several chunks, not necessarily in the original order of the data. The target, therefore, also specifies a Buffer Offset that indicates the point at which the data transfer should begin, relative to the beginning of the total data transfer. The Desired Data Transfer Length MUST NOT be 0 and MUST not exceed MaxBurstLength.10.8.5. Target Transfer Tag
The target assigns its own tag to each R2T request that it sends to the initiator. This tag can be used by the target to easily identify the data it receives. The Target Transfer Tag and LUN are copied in the outgoing data PDUs and are only used by the target. There is no protocol rule about the Target Transfer Tag except that the value 0xffffffff is reserved and MUST NOT be sent by a target in an R2T.
10.9. Asynchronous Message
An Asynchronous Message may be sent from the target to the initiator without correspondence to a particular command. The target specifies the reason for the event and sense data. Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|.| 0x32 |1| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| 0xffffffff | +---------------+---------------+---------------+---------------+ 20| Reserved | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36| AsyncEvent | AsyncVCode | Parameter1 or Reserved | +---------------+---------------+---------------+---------------+ 40| Parameter2 or Reserved | Parameter3 or Reserved | +---------------+---------------+---------------+---------------+ 44| Reserved | +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / DataSegment - Sense Data and iSCSI Event Data / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+ Some Asynchronous Messages are strictly related to iSCSI while others are related to SCSI [SAM2]. StatSN counts this PDU as an acknowledgeable event (StatSN is advanced), which allows for initiator and target state synchronization.
10.9.1. AsyncEvent
The codes used for iSCSI Asynchronous Messages (events) are: 0 - a SCSI Asynchronous Event is reported in the sense data. Sense Data that accompanies the report, in the data segment, identifies the condition. The sending of a SCSI Event (Asynchronous Event Reporting in SCSI terminology) is dependent on the target support for SCSI asynchronous event reporting (see [SAM2]) as indicated in the standard INQUIRY data (see [SPC3]). Its use may be enabled by parameters in the SCSI Control mode page (see [SPC3]). 1 - target requests Logout. This Async Message MUST be sent on the same connection as the one requesting to be logged out. The initiator MUST honor this request by issuing a Logout as early as possible, but no later than Parameter3 seconds. Initiator MUST send a Logout with a reason code of "Close the connection" OR "Close the session" to close all the connections. Once this message is received, the initiator SHOULD NOT issue new iSCSI commands on the connection to be logged out. The target MAY reject any new I/O requests that it receives after this Message with the reason code "Waiting for Logout". If the initiator does not Logout in Parameter3 seconds, the target should send an Async PDU with iSCSI event code "Dropped the connection" if possible, or simply terminate the transport connection. Parameter1 and Parameter2 are reserved. 2 - target indicates it will drop the connection. The Parameter1 field indicates the CID of the connection that is going to be dropped. The Parameter2 field (Time2Wait) indicates, in seconds, the minimum time to wait before attempting to reconnect or reassign. The Parameter3 field (Time2Retain) indicates the maximum time allowed to reassign commands after the initial wait (in Parameter2). If the initiator does not attempt to reconnect and/or reassign the outstanding commands within the time specified by Parameter3, or if Parameter3 is 0, the target will terminate all outstanding commands on this connection. In this case, no other responses should be expected from the target for the outstanding commands on this connection.
A value of 0 for Parameter2 indicates that reconnect can be
attempted immediately.
3 - target indicates it will drop all the connections of this
session.
Parameter1 field is reserved.
The Parameter2 field (Time2Wait) indicates, in seconds, the
minimum time to wait before attempting to reconnect. The
Parameter3 field (Time2Retain) indicates the maximum time
allowed to reassign commands after the initial wait (in
Parameter2).
If the initiator does not attempt to reconnect and/or reassign
the outstanding commands within the time specified by
Parameter3, or if Parameter3 is 0, the session is terminated.
In this case, the target will terminate all outstanding
commands in this session; no other responses should be
expected from the target for the outstanding commands in this
session. A value of 0 for Parameter2 indicates that reconnect
can be attempted immediately.
4 - target requests parameter negotiation on this connection. The
initiator MUST honor this request by issuing a Text Request
(that can be empty) on the same connection as early as
possible, but no later than Parameter3 seconds, unless a Text
Request is already pending on the connection, or by issuing a
Logout Request. If the initiator does not issue a Text
Request the target may reissue the Asynchronous Message
requesting parameter negotiation.
255 - vendor specific iSCSI Event. The AsyncVCode details the
vendor code, and data MAY accompany the report.
All other event codes are reserved.
10.9.2. AsyncVCode
AsyncVCode is a vendor specific detail code that is only valid if the
AsyncEvent field indicates a vendor specific event. Otherwise, it is
reserved.
10.9.3. LUN
The LUN field MUST be valid if AsyncEvent is 0. Otherwise, this
field is reserved.
10.9.4. Sense Data and iSCSI Event Data
For a SCSI event, this data accompanies the report in the data segment and identifies the condition. For an iSCSI event, additional vendor-unique data MAY accompany the Async event. Initiators MAY ignore the data when not understood while processing the rest of the PDU. If the DataSegmentLength is not 0, the format of the DataSegment is as follows: Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|SenseLength | Sense Data | +---------------+---------------+---------------+---------------+ x/ Sense Data / +---------------+---------------+---------------+---------------+ y/ iSCSI Event Data / / / +---------------+---------------+---------------+---------------+ z|10.9.4.1. SenseLength
This is the length of Sense Data. When the Sense Data field is empty (e.g., the event is not a SCSI event) SenseLength is 0.
10.10. Text Request
The Text Request is provided to allow for the exchange of information and for future extensions. It permits the initiator to inform a target of its capabilities or to request some special operations. Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|I| 0x04 |F|C| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Target Transfer Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 24| CmdSN | +---------------+---------------+---------------+---------------+ 28| ExpStatSN | +---------------+---------------+---------------+---------------+ 32/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / DataSegment (Text) / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+ An initiator MUST have at most one outstanding Text Request on a connection at any given time. On a connection failure, an initiator must either explicitly abort any active allegiant text negotiation task or must cause such a task to be implicitly terminated by the target.
10.10.1. F (Final) Bit
When set to 1, indicates that this is the last or only text request in a sequence of Text Requests; otherwise, it indicates that more Text Requests will follow.10.10.2. C (Continue) Bit
When set to 1, indicates that the text (set of key=value pairs) in this Text Request is not complete (it will be continued on subsequent Text Requests); otherwise, it indicates that this Text Request ends a set of key=value pairs. A Text Request with the C bit set to 1 MUST have the F bit set to 0.10.10.3. Initiator Task Tag
The initiator assigned identifier for this Text Request. If the command is sent as part of a sequence of text requests and responses, the Initiator Task Tag MUST be the same for all the requests within the sequence (similar to linked SCSI commands). The I bit for all requests in a sequence also MUST be the same.10.10.4. Target Transfer Tag
When the Target Transfer Tag is set to the reserved value 0xffffffff, it tells the target that this is a new request and the target resets any internal state associated with the Initiator Task Tag (resets the current negotiation state). The target sets the Target Transfer Tag in a text response to a value other than the reserved value 0xffffffff whenever it indicates that it has more data to send or more operations to perform that are associated with the specified Initiator Task Tag. It MUST do so whenever it sets the F bit to 0 in the response. By copying the Target Transfer Tag from the response to the next Text Request, the initiator tells the target to continue the operation for the specific Initiator Task Tag. The initiator MUST ignore the Target Transfer Tag in the Text Response when the F bit is set to 1. This mechanism allows the initiator and target to transfer a large amount of textual data over a sequence of text-command/text-response exchanges, or to perform extended negotiation sequences. If the Target Transfer Tag is not 0xffffffff, the LUN field MUST be sent by the target in the Text Response.
A target MAY reset its internal negotiation state if an exchange is
stalled by the initiator for a long time or if it is running out of
resources.
Long text responses are handled as in the following example:
I->T Text SendTargets=All (F=1,TTT=0xffffffff)
T->I Text <part 1> (F=0,TTT=0x12345678)
I->T Text <empty> (F=1, TTT=0x12345678)
T->I Text <part 2> (F=0, TTT=0x12345678)
I->T Text <empty> (F=1, TTT=0x12345678)
...
T->I Text <part n> (F=1, TTT=0xffffffff)
10.10.5. Text
The data lengths of a text request MUST NOT exceed the iSCSI target
MaxRecvDataSegmentLength (a per connection and per direction
negotiated parameter). The text format is specified in Section 5.2
Text Mode Negotiation.
Chapter 11 and Chapter 12 list some basic Text key=value pairs, some
of which can be used in Login Request/Response and some in Text
Request/Response.
A key=value pair can span Text request or response boundaries. A
key=value pair can start in one PDU and continue on the next. In
other words the end of a PDU does not necessarily signal the end of a
key=value pair.
The target responds by sending its response back to the initiator.
The response text format is similar to the request text format. The
text response MAY refer to key=value pairs presented in an earlier
text request and the text in the request may refer to earlier
responses.
Chapter 5 details the rules for the Text Requests and Responses.
Text operations are usually meant for parameter setting/
negotiations, but can also be used to perform some long lasting
operations.
Text operations that take a long time should be placed in their own
Text request.
10.11. Text Response
The Text Response PDU contains the target's responses to the initiator's Text request. The format of the Text field matches that of the Text request. Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 0|.|.| 0x24 |F|C| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Target Transfer Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / DataSegment (Text) / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+10.11.1. F (Final) Bit
When set to 1, in response to a Text Request with the Final bit set to 1, the F bit indicates that the target has finished the whole operation. Otherwise, if set to 0 in response to a Text Request with the Final Bit set to 1, it indicates that the target has more work to do (invites a follow-on text request). A Text Response with the F bit set to 1 in response to a Text Request with the F bit set to 0 is a protocol error.
A Text Response with the F bit set to 1 MUST NOT contain key=value pairs that may require additional answers from the initiator. A Text Response with the F bit set to 1 MUST have a Target Transfer Tag field set to the reserved value of 0xffffffff. A Text Response with the F bit set to 0 MUST have a Target Transfer Tag field set to a value other than the reserved 0xffffffff.10.11.2. C (Continue) Bit
When set to 1, indicates that the text (set of key=value pairs) in this Text Response is not complete (it will be continued on subsequent Text Responses); otherwise, it indicates that this Text Response ends a set of key=value pairs. A Text Response with the C bit set to 1 MUST have the F bit set to 0.10.11.3. Initiator Task Tag
The Initiator Task Tag matches the tag used in the initial Text Request.10.11.4. Target Transfer Tag
When a target has more work to do (e.g., cannot transfer all the remaining text data in a single Text Response or has to continue the negotiation) and has enough resources to proceed, it MUST set the Target Transfer Tag to a value other than the reserved value of 0xffffffff. Otherwise, the Target Transfer Tag MUST be set to 0xffffffff. When the Target Transfer Tag is not 0xffffffff, the LUN field may be significant. The initiator MUST copy the Target Transfer Tag and LUN in its next request to indicate that it wants the rest of the data. When the target receives a Text Request with the Target Transfer Tag set to the reserved value of 0xffffffff, it resets its internal information (resets state) associated with the given Initiator Task Tag (restarts the negotiation). When a target cannot finish the operation in a single Text Response, and does not have enough resources to continue, it rejects the Text Request with the appropriate Reject code.
A target may reset its internal state associated with an Initiator Task Tag (the current negotiation state), state expressed through the Target Transfer Tag if the initiator fails to continue the exchange for some time. The target may reject subsequent Text Requests with the Target Transfer Tag set to the "stale" value.10.11.5. StatSN
The target StatSN variable is advanced by each Text Response sent.10.11.6. Text Response Data
The data lengths of a text response MUST NOT exceed the iSCSI initiator MaxRecvDataSegmentLength (a per connection and per direction negotiated parameter). The text in the Text Response Data is governed by the same rules as the text in the Text Request Data (see Section 10.10.5 Text). Although the initiator is the requesting party and controls the request-response initiation and termination, the target can offer key=value pairs of its own as part of a sequence and not only in response to the initiator.10.12. Login Request
After establishing a TCP connection between an initiator and a target, the initiator MUST start a Login Phase to gain further access to the target's resources. The Login Phase (see Chapter 5) consists of a sequence of Login Requests and Responses that carry the same Initiator Task Tag. Login Requests are always considered as immediate.
Byte/ 0 | 1 | 2 | 3 |
/ | | | |
|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
+---------------+---------------+---------------+---------------+
0|.|1| 0x03 |T|C|.|.|CSG|NSG| Version-max | Version-min |
+---------------+---------------+---------------+---------------+
4|TotalAHSLength | DataSegmentLength |
+---------------+---------------+---------------+---------------+
8| ISID |
+ +---------------+---------------+
12| | TSIH |
+---------------+---------------+---------------+---------------+
16| Initiator Task Tag |
+---------------+---------------+---------------+---------------+
20| CID | Reserved |
+---------------+---------------+---------------+---------------+
24| CmdSN |
+---------------+---------------+---------------+---------------+
28| ExpStatSN or Reserved |
+---------------+---------------+---------------+---------------+
32| Reserved |
+---------------+---------------+---------------+---------------+
36| Reserved |
+---------------+---------------+---------------+---------------+
40/ Reserved /
+/ /
+---------------+---------------+---------------+---------------+
48/ DataSegment - Login Parameters in Text request Format /
+/ /
+---------------+---------------+---------------+---------------+
10.12.1. T (Transit) Bit
If set to 1, indicates that the initiator is ready to transit to the
next stage.
If the T bit is set to 1 and NSG is FullFeaturePhase, then this also
indicates that the initiator is ready for the Final Login Response
(see Chapter 5).
10.12.2. C (Continue) Bit
When set to 1, indicates that the text (set of key=value pairs) in
this Login Request is not complete (it will be continued on
subsequent Login Requests); otherwise, it indicates that this Login
Request ends a set of key=value pairs. A Login Request with the C
bit set to 1 MUST have the T bit set to 0.
10.12.3. CSG and NSG
Through these fields, Current Stage (CSG) and Next Stage (NSG), the Login negotiation requests and responses are associated with a specific stage in the session (SecurityNegotiation, LoginOperationalNegotiation, FullFeaturePhase) and may indicate the next stage to which they want to move (see Chapter 5). The next stage value is only valid when the T bit is 1; otherwise, it is reserved. The stage codes are: - 0 - SecurityNegotiation - 1 - LoginOperationalNegotiation - 3 - FullFeaturePhase All other codes are reserved.10.12.4. Version
The version number of the current draft is 0x00. As such, all devices MUST carry version 0x00 for both Version-min and Version-max.10.12.4.1. Version-max
Maximum Version number supported. All Login Requests within the Login Phase MUST carry the same Version-max. The target MUST use the value presented with the first Login Request.10.12.4.2. Version-min
All Login Requests within the Login Phase MUST carry the same Version-min. The target MUST use the value presented with the first Login Request.
10.12.5. ISID
This is an initiator-defined component of the session identifier and is structured as follows (see [RFC3721] and Section 9.1.1 Conservative Reuse of ISIDs for details): Byte/ 0 | 1 | 2 | 3 | / | | | | |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| +---------------+---------------+---------------+---------------+ 8| T | A | B | C | +---------------+---------------+---------------+---------------+ 12| D | +---------------+---------------+ The T field identifies the format and usage of A, B, C, and D as indicated below: T 00b OUI-Format A&B are a 22 bit OUI (the I/G & U/L bits are omitted) C&D 24 bit qualifier 01b EN - Format (IANA Enterprise Number) A - Reserved B&C EN (IANA Enterprise Number) D - Qualifier 10b "Random" A - Reserved B&C Random D - Qualifier 11b A,B,C&D Reserved For the T field values 00b and 01b, a combination of A and B (for 00b) or B and C (for 01b) identifies the vendor or organization whose component (software or hardware) generates this ISID. A vendor or organization with one or more OUIs, or one or more Enterprise Numbers, MUST use at least one of these numbers and select the appropriate value for the T field when its components generate ISIDs. An OUI or EN MUST be set in the corresponding fields in network byte order (byte big-endian). If the T field is 10b, B and C are set to a random 24-bit unsigned integer value in network byte order (byte big-endian). See [RFC3721] for how this affects the principle of "conservative reuse".
The Qualifier field is a 16 or 24-bit unsigned integer value that provides a range of possible values for the ISID within the selected namespace. It may be set to any value within the constraints specified in the iSCSI protocol (see Section 3.4.3 Consequences of the Model and Section 9.1.1 Conservative Reuse of ISIDs). The T field value of 11b is reserved. If the ISID is derived from something assigned to a hardware adapter or interface by a vendor, as a preset default value, it MUST be configurable to a value assigned according to the SCSI port behavior desired by the system in which it is installed (see Section 9.1.1 Conservative Reuse of ISIDs and Section 9.1.2 iSCSI Name, ISID, and TPGT Use). The resultant ISID MUST also be persistent over power cycles, reboot, card swap, etc.10.12.6. TSIH
TSIH must be set in the first Login Request. The reserved value 0 MUST be used on the first connection for a new session. Otherwise, the TSIH sent by the target at the conclusion of the successful login of the first connection for this session MUST be used. The TSIH identifies to the target the associated existing session for this new connection. All Login Requests within a Login Phase MUST carry the same TSIH. The target MUST check the value presented with the first Login Request and act as specified in Section 5.3.1 Login Phase Start.10.12.7. Connection ID - CID
A unique ID for this connection within the session. All Login Requests within the Login Phase MUST carry the same CID. The target MUST use the value presented with the first Login Request. A Login Request with a non-zero TSIH and a CID equal to that of an existing connection implies a logout of the connection followed by a Login (see Section 5.3.4 Connection Reinstatement). For the details of the implicit Logout Request, see Section 10.14 Logout Request.
10.12.8. CmdSN
CmdSN is either the initial command sequence number of a session (for the first Login Request of a session - the "leading" login), or the command sequence number in the command stream if the login is for a new connection in an existing session. Examples: - Login on a leading connection - if the leading login carries the CmdSN 123, all other Login Requests in the same Login Phase carry the CmdSN 123 and the first non-immediate command in FullFeaturePhase also carries the CmdSN 123. - Login on other than a leading connection - if the current CmdSN at the time the first login on the connection is issued is 500, then that PDU carries CmdSN=500. Subsequent Login Requests that are needed to complete this Login Phase may carry a CmdSN higher than 500 if non-immediate requests that were issued on other connections in the same session advance CmdSN. If the Login Request is a leading Login Request, the target MUST use the value presented in CmdSN as the target value for ExpCmdSN.10.12.9. ExpStatSN
For the first Login Request on a connection this is ExpStatSN for the old connection and this field is only valid if the Login Request restarts a connection (see Section 5.3.4 Connection Reinstatement). For subsequent Login Requests it is used to acknowledge the Login Responses with their increasing StatSN values.10.12.10. Login Parameters
The initiator MUST provide some basic parameters in order to enable the target to determine if the initiator may use the target's resources and the initial text parameters for the security exchange. All the rules specified in Section 10.10.5 Text for text requests also hold for Login Requests. Keys and their explanations are listed in Chapter 11 (security negotiation keys) and Chapter 12 (operational parameter negotiation keys). All keys in Chapter 12, except for the X extension formats, MUST be supported by iSCSI initiators and targets. Keys in Chapter 11 only need to be supported when the function to which they refer is mandatory to implement.