Introduce
iSCSI（发音为 /аɪskʌzi/）（iSCSI = internet Small Computer System Interface ）
又稱為IP-SAN，是一种基于因特网及SCSI-3协议下的存储技术，
由IETF提出，並於2003年2月11日成為正式的標準。
与传统的SCSI技术比较起来，iSCSI技术有以下三个革命性的变化：
1. 把原來只用於本機的SCSI協同透過TCP/IP网络傳送，使连接距离可作无限的地域延伸
2. 连接的服务器数量无限（原來的SCSI-3的上限是15）
3. 由于是服务器架构，因此也可以实现在线扩容以至动态部署
iSCSI利用了TCP/IP的port 860 和 3260 作为沟通的渠道。透过两部计算机之间利用iSCSI
的协议来交换SCSI命令，让计算机可以透过高速的局域网集线来把SAN模拟成为本地的储存装置。
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(fixed size, 48 bytes ) 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
For request PDUs, the I bit set to 1 is an immediate delivery marker.
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
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
When set to 1 it indicates the final (or only) PDU of a sequence.
Some opcodes operate on a specific Logical Unit. The Logical Unit
Number (LUN) field identifies which Logical Unit. If the opcode does
not relate to a Logical Unit, this field is either ignored or may be
used in an opcode specific way. The LUN field is 64-bits and should
be formatted in accordance with [SAM2]. For example, LUN[0] from
[SAM2] is BHS byte 8 and so on up to LUN[7] from [SAM2], which is BHS
byte 15.
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
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) /
+---------------+---------------+---------------+---------------+