HP HP-UX iSCSI User's Guide
A Conceptual Overview of iSCSI
Executive Summary.........................................................................................................................................2
1. Introduction ..............................................................................................................................................2
2. Protocol Layering.....................................................................................................................................3
3. Hardware/Software Separation................................................................................................................4
4. Frame Construction .................................................................................................................................5
5. Network Entities and iSCSI nodes ...........................................................................................................6
6. Sessions and Connectivity.......................................................................................................................7
7. Portal Groups...........................................................................................................................................8
8. Bibliography .............................................................................................................................................9
9. Acronyms................................................................................................................................................. 9
Executive Summary
This White Paper provides a conceptual overview of iSCSI emphasizing the relationship of the
iSCSI protocol to other computer subsystems. A basic understanding of mass storage and
networking technologies is assumed. Concepts presented in this document include:
• iSCSI relationship with SCSI and TCP/IP
• iSCSI co-existence with current network traffic
• iSCSI utilization of mass storage and networking concepts
• iSCSI definition in terms of current network model
• iSCSI definition in terms of current mass storage model
1. Introduction
The Internet Small Computer Systems Interface (iSCSI) is a SCSI mass storage transport that
operates between the Transport Control Protocol (TCP) and the SCSI Protocol Layers. The iSCSI
protocol is defined in RFC 3720 [iSCSI], which was finalized by the Internet Engineering Task Force
(IETF) in April, 2004. A mandate of the iSCSI protocol design required no modifications to the
existing SCSI or TCP/IP protocols.
Figure 1. SCSI Transport Protocols
Applications
File Systems
Volume Management
SCSI Protocol Layer
Parallel SCSI
Transport
FCP and
Fibre Channel
Transport
iSCSI
Transport
Networking
Upper
Level
Protocols
The Small Computer Systems Interface (SCSI) Architectural Model [SAM] is a family of protocols
defining a communication protocol with SCSI I/O devices. SCSI is a client-server architecture. The
phrase “SCSI Client” is synonymous with “SCSI initiator” and “SCSI server” is synonymous with
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“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
(Initiator)
Host B
(Target)
Application
SCSI Protocol
(Client/Initiator)
iSCSI Protocol
(Initiator)
TCP/IP
Protocols
Physical and
Data Link Layer
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.
Data
CDBs, Data,
Responses
PDUs
Segments,
Datagrams
Frames
SCSI Logical
Units
SCSI Protocol
(Server/Target)
iSCSI Protocol
(Target)
TCP/IP
Protocols
Physical and
Data Link
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