The overall structure of an iSCSI PDU 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/ Basic Header Segment (BHS) / +/ / +---------------+---------------+---------------+---------------+ 48/ Additional Header Segment 1 (AHS) (optional) / +/ / +---------------+---------------+---------------+---------------+ / Additional Header Segment 2 (AHS) (optional) / +/ / +---------------+---------------+---------------+---------------+
+---------------+---------------+---------------+---------------+
/ Additional Header Segment n (AHS) (optional) /
+/ /
+---------------+---------------+---------------+---------------+
+---------------+---------------+---------------+---------------+
k/ Header-Digest (optional) /
+/ /
+---------------+---------------+---------------+---------------+
l/ Data Segment(optional) /
+/ /
+---------------+---------------+---------------+---------------+
m/ Data-Digest (optional) /
+/ /
+---------------+---------------+---------------+---------------+
The format of the BHS is:
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| Opcode |F| Opcode-specific fields | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Opcode-specific fields | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20/ Opcode-specific fields / +/ / +---------------+---------------+---------------+---------------+ 48
10.2.1.1 I
For request PDUs, the I bit set to 1 is an immediate delivery marker.
10.2.1.2. Opcode
The Opcode indicates the type of iSCSI PDU the header encapsulates.
The Opcodes are divided into two categories: initiator opcodes and target opcodes. Initiator opcodes are in PDUs sent by the initiator (request PDUs). Target opcodes are in PDUs sent by the target (response PDUs).
Initiators MUST NOT use target opcodes and targets MUST NOT use initiator opcodes.
Initiator opcodes defined in this specification are:
0x00 NOP-Out
0x01 SCSI Command (encapsulates a SCSI Command Descriptor Block)
0x02 SCSI Task Management function request
0x03 Login Request
0x04 Text Request
0x05 SCSI Data-Out (for WRITE operations)
0x06 Logout Request
0x10 SNACK Request
0x1c-0x1e Vendor specific codes
Satran, et al. Standards Track [Page 115] RFC 3720 iSCSI April 2004
Target opcodes are:
0x20 NOP-In
0x21 SCSI Response - contains SCSI status and possibly sense
information or other response information.
0x22 SCSI Task Management function response
0x23 Login Response
0x24 Text Response
0x25 SCSI Data-In - for READ operations.
0x26 Logout Response
0x31 Ready To Transfer (R2T) - sent by target when it is ready
to receive data.
0x32 Asynchronous Message - sent by target to indicate certain
special conditions.
0x3c-0x3e Vendor specific codes
0x3f Reject
All other opcodes are reserved.
10.2.1.3. Final (F) bit
When set to 1 it indicates the final (or only) PDU of a sequence.
10.2.1.4. Opcode-specific Fields
These fields have different meanings for different opcode types.
10.2.1.5. TotalAHSLength
Total length of all AHS header segments in units of four byte words including padding, if any.
The TotalAHSLength is only used in PDUs that have an AHS and MUST be 0 in all other PDUs.
10.2.1.6. DataSegmentLength
This is the data segment payload length in bytes (excluding padding). The DataSegmentLength MUST be 0 whenever the PDU has no data segment.
10.2.1.8. Initiator Task Tag: Initiator分配！identify the task session-wide.
10.2.2. Additional Header Segment (AHS)
The general format of an AHS is:
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| AHSLength | AHSType | AHS-Specific | +---------------+---------------+---------------+---------------+ 4/ AHS-Specific / +/ / +---------------+---------------+---------------+---------------+ x
10.2.2.1. AHSType
The AHSType field is coded as follows:
bit 0-1 - Reserved
bit 2-7 - AHS code
0 - Reserved
1 - Extended CDB
2 - Expected Bidirectional Read Data Length
3 - 63 Reserved
10.2.2.3. Extended CDB AHS
The format of the Extended CDB AHS is:
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| AHSLength (CDBLength-15) | 0x01 | Reserved | +---------------+---------------+---------------+---------------+ 4/ ExtendedCDB...+padding / +/ / +---------------+---------------+---------------+---------------+ x
This type of AHS MUST NOT be used if the CDBLength is less than 17. The length includes the reserved byte 3.
10.2.2.4. Bidirectional Expected Read-Data Length AHS
The format of the Bidirectional Read Expected Data Transfer Length AHS is:
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| AHSLength (0x0005) | 0x02 | Reserved | +---------------+---------------+---------------+---------------+ 4| Expected Read-Data Length | +---------------+---------------+---------------+---------------+ 8 10.3. SCSI Command
The format of the SCSI Command PDU is:
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| 0x01 |F|R|W|. .|ATTR | Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| Logical Unit Number (LUN) | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Expected Data Transfer Length | +---------------+---------------+---------------+---------------+ 24| CmdSN | +---------------+---------------+---------------+---------------+ 28| ExpStatSN | +---------------+---------------+---------------+---------------+ 32/ SCSI Command Descriptor Block (CDB) / +/ / +---------------+---------------+---------------+---------------+ 48/ AHS (Optional) / +---------------+---------------+---------------+---------------+ x/ Header Digest (Optional) / +---------------+---------------+---------------+---------------+ y/ (DataSegment, Command Data) (Optional) / +/ / +---------------+---------------+---------------+---------------+ z/ Data Digest (Optional) / +---------------+---------------+---------------+---------------+
10.3.1. Flags and Task Attributes (byte 1)
The flags for a SCSI Command are:
bit 0 (F) is set to 1 when no unsolicited SCSI Data-Out PDUs follow this PDU. When F=1 for a write and if Expected Data Transfer Length is larger than the DataSegmentLength, the target may solicit additional data through R2T.
bit 1 (R) is set to 1 when the command is expected to input data.
bit 2 (W) is set to 1 when the command is expected to output data.
bit 3-4 Reserved.
bit 5-7 contains Task Attributes.
Task Attributes (ATTR) have one of the following integer values (see [SAM2] for details):
0 - Untagged
1 - Simple
2 - Ordered
3 - Head of Queue
4 - ACA
5-7 - Reserved
10.3.2. CmdSN - Command Sequence Number
Enables ordered delivery across multiple connections in a single session.
10.3.3. ExpStatSN
Command responses up to ExpStatSN-1 (mod 2**32) have been received (acknowledges status) on the connection. 10.4. SCSI Response
The format of the SCSI Response PDU is:
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|.|.| 0x21 |1|. .|o|u|O|U|.| Response | Status | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| SNACK Tag or Reserved | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36| ExpDataSN or Reserved | +---------------+---------------+---------------+---------------+ 40| Bidirectional Read Residual Count or Reserved | +---------------+---------------+---------------+---------------+ 44| Residual Count or Reserved | +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / Data Segment (Optional) / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+ Some of the status codes defined in [SAM2] are:
0x00 GOOD
0x02 CHECK CONDITION
0x08 BUSY
0x18 RESERVATION CONFLICT
0x28 TASK SET FULL
0x30 ACA ACTIVE
0x40 TASK ABORTED
If a SCSI device error is detected while data from the initiator is still expected (the command PDU did not contain all the data and the target has not received a Data PDU with the final bit Set), the target MUST wait until it receives a Data PDU with the F bit set in the last expected sequence before sending the Response PDU.
10.4.10. ExpCmdSN - Next Expected CmdSN from this Initiator
ExpCmdSN is a Sequence Number that the target iSCSI returns to the initiator to acknowledge command reception. It is used to update a local variable with the same name. An ExpCmdSN equal to MaxCmdSN+1 indicates that the target cannot accept new commands.
10.4.11. MaxCmdSN - Maximum CmdSN from this Initiator
MaxCmdSN is a Sequence Number that the target iSCSI returns to the initiator to indicate the maximum CmdSN the initiator can send. It is used to update a local variable with the same name. If MaxCmdSN is equal to ExpCmdSN-1, this indicates to the initiator that the target cannot receive any additional commands. When MaxCmdSN changes at the target while the target has no pending PDUs to convey this information to the initiator, it MUST generate a NOP-IN to carry the new MaxCmdSN. 10.4.8. ExpDataSN
The number of R2T and Data-In (read) PDUs the target has sent for the command.
This field MUST be 0 if the response code is not Command Completed at Target or the target sent no Data-In PDUs for the command.
10.4.9. StatSN - Status Sequence Number
StatSN is a Sequence Number that the target iSCSI layer generates per connection and that in turn, enables the initiator to acknowledge status reception. StatSN is incremented by 1 for every response/status sent on a connection except for responses sent as a result of a retry or SNACK. In the case of responses sent due to a retransmission request, the StatSN MUST be the same as the first time the PDU was sent unless the connection has since been restarted. 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) | +---------------+---------------+---------------+---------------+
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) | +---------------+---------------+---------------+---------------+
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) | +---------------+---------------+---------------+---------------+
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) | +---------------+---------------+---------------+---------------+
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).
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)
10.12.5. 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:
10.13. Login Response
The Login Response indicates the progress and/or end of the Login Phase.
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|.|.| 0x23 |T|C|.|.|CSG|NSG| Version-max | Version-active| +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| ISID | + +---------------+---------------+ 12| | TSIH | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag | +---------------+---------------+---------------+---------------+ 20| Reserved | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36| Status-Class | Status-Detail | Reserved | +---------------+---------------+---------------+---------------+ 40/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48/ DataSegment - Login Parameters in Text request Format / +/ / +---------------+---------------+---------------+---------------+
10.13.3. TSIH
The TSIH is the target assigned session identifying handle. Its internal format and content are not defined by this protocol except for the value 0 that is reserved. With the exception of the Login Final-Response in a new session, this field should be set to the TSIH provided by the initiator in the Login Request. For a new session, the target MUST generate a non-zero TSIH and ONLY return it in the Login Final-Response (see Section 5.3 Login Phase).
10.18. NOP-Out
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| 0x00 |1| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 20| Target Transfer Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 24| CmdSN | +---------------+---------------+---------------+---------------+ 28| ExpStatSN | +---------------+---------------+---------------+---------------+ 32/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / DataSegment - Ping Data (optional) / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+
10.19. NOP-In
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|.|.| 0x20 |1| Reserved | +---------------+---------------+---------------+---------------+ 4|TotalAHSLength | DataSegmentLength | +---------------+---------------+---------------+---------------+ 8| LUN or Reserved | + + 12| | +---------------+---------------+---------------+---------------+ 16| Initiator Task Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 20| Target Transfer Tag or 0xffffffff | +---------------+---------------+---------------+---------------+ 24| StatSN | +---------------+---------------+---------------+---------------+ 28| ExpCmdSN | +---------------+---------------+---------------+---------------+ 32| MaxCmdSN | +---------------+---------------+---------------+---------------+ 36/ Reserved / +/ / +---------------+---------------+---------------+---------------+ 48| Header-Digest (Optional) | +---------------+---------------+---------------+---------------+ / DataSegment - Return Ping Data / +/ / +---------------+---------------+---------------+---------------+ | Data-Digest (Optional) | +---------------+---------------+---------------+---------------+