“SCSI target”. iSCSI uses the phrases “iSCSI initiator” and “iSCSI target” as well. The term initiator or target is used without the protocol identifier when the protocol is not an issue.

I/O operations from user or kernel applications are translated to the SCSI command set within the SCSI protocol layer. A SCSI mass storage transport maps the SCSI client-server protocol to a specific physical transport. For example, iSCSI is a SCSI mass storage transport that maps the SCSI protocol to TCP/IP (see Figure 1). Other examples of SCSI mass storage transports are Fibre Channel and Parallel SCSI. The functionality of the traditional network architecture is not modified by iSCSI.

2.Protocol Layering

SCSI commands and data are sent from the initiator to the target, and SCSI data and responses are sent from the target to the initiator. The SCSI data, commands, and responses are moved between the SCSI initiator and the SCSI target by the mass storage transport protocols. Figure 2 depicts the protocol layering when iSCSI is used as a transport.

Figure 2. Protocol Layering

 

 

 

 

 

 

Host A

 

 

 

Host B

 

 

(Initiator)

 

 

 

(Target)

 

 

 

 

 

 

 

 

 

Application

 

Data

 

SCSI Logical

 

 

 

 

 

 

Units

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SCSI Protocol

 

CDBs, Data,

 

SCSI Protocol

 

 

(Client/Initiator)

 

Responses

 

(Server/Target)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

iSCSI Protocol

 

PDUs

iSCSI Protocol

 

 

(Initiator)

 

 

 

(Target)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TCP/IP

 

Segments,

TCP/IP

 

 

Protocols

 

Datagrams

Protocols

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Physical and

 

Frames

Physical and

 

 

Data Link Layer

 

 

 

Data Link

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Networking is defined between hosts. The networking components of the TCP, IP, and the physical/data link layers define connectivity between two hosts on a network. It is not possible to determine which host is an iSCSI initiator and which host is an iSCSI target based solely on standard-conformant network stack implementations. Communication between the layers of the network stack is not changed when iSCSI is implemented.

In Figure 2, Host A uses the SCSI Protocol as a SCSI initiator and the iSCSI Protocol as an iSCSI initiator, and Host B uses the SCSI Protocol as a SCSI target and the iSCSI Protocol as an iSCSI target. In other words, Host A is a mass storage initiator, and Host B is a mass storage target.

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