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Before getting into iSCSI, few words about Storage in general: Several years of storage evolution has resulted in many choices which has made it more and more confusing for the users to decide what they really need. We hope this section helps in clarifying some of this....

  • Lets start with some basics. Storage can either be Internal or External.

  • External storage can either be Direct Attached Storage (DAS) or Network Storage.

  • Anytime there is a network between the computers and storage, I call that Network Storage. That includes, NAS, SAN (iSCSI, Fibre Channel), Infiniband, even a file server that is shared among several computers is a Network Storage.

  • The main reason Network Storage was created was to share the same storage among several computers. In old days without Network storage, there was always the problem that one server had too much storage and the other one did not. Network storage solves that.  

  • A Network storage is basically a computer with bunch of hard drive, and most common types of hard drives are SATA, SAS, and SSD these days.

    • SATA disks are the lowest cost of the three

    • SAS drives have two ports but that will only be useful if they are connected to a dual port controller. SAS also provides higher IO per second if that is what you are looking for. 

    • There are no mechanical components and spinning platters inside the SSD drives and solid state memory is used instead, which results in much higher speeds. That is what makes them to be the highest cost of the three. SSD is relatively a new technology, therefore as any other new technologies it is going to go through some rapid change.

  • Storage products combine these hard drives into storage pools. RAID is the predominant technology to pool hard drives and in some cases it provides some level of protection against drive failures. There are many different RAID levels, 0, 1, 10, 3, 5, 6, ...

  • There are other technologies to pool drives, and InteliSAN provides one of them. 

  • You must have heard about SAN and NAS which are the two most common external storage technologies. To put it simply, SAN or NAS is made of bunch of disks along with an operating system and a storage software in a computer.

  • There are two different SAN technologies, Fibre Channel SAN and IP SAN (or iSCSI SAN). We use  IP SAN for the rest of this discussion.   

  • So what is the difference between NAS and IP SAN? They both are external storage technologies, they both use ethernet for their data transfer. One Stands for Network Attached Storage and one stands for Storage Area Network....This sounds the same to me..... These few points help understanding their difference:

    • NAS data transfer uses files, and SAN data transfer uses blocks of data.

    • If multiple computers need to access the same storage space/volume at the same time, you have to use NAS.

    • If your application requires its own raw disk and not a file system underneath the volume, to operate, then you have to use SAN.

    • If you do not need to share a storage space/volume (each computer can have its own storage volume), you are better off using iSCSI SAN. iSCSI transfers data in blocks that result in higher speeds

iSCSI Technology


In short, computers and external storage get connected with iSCSI using ethernet cables, and transfer data in blocks of data and not files. iSCSI volumes given from an iSCSI Target to a  computer will look like blank local disks that can be formatted, partitioned and used for anything. Block data transfer provides a higher speed than file data transfer.




  • iSCSI stands for internet SCSI

  • iSCSI is really SCSI protocol which is a standard storage protocol running over Ethernet cables

  • iSCSI is a protocol defined for data transfer between computers/servers and external storage

  • iSCSI Storage is also called iSCSI Target, or IP SAN

  • SAN stands for "Storage Area Network" which is basically an external storage that provides storage for several computers from a central location/device. SAN started with Fibre Channel, but because of high cost of Fibre Channel, iSCSI SAN was created.

  • iSCSI transfers data in blocks and not file. NAS transfers data by file

  • Servers install an iSCSI initiator to connect to iSCSI storage

  • iSCSI Initiators are mostly a piece of software that get loaded on the computer operating systems. Most new operating systems like vista, windows 7, windows 2008, etc come with a pre-installed iSCSI initiator. Most iSCSI initiator software's are free. There are however some iSCSI HBAs, but cost of them are high and in most cases not needed. They do provide for lower CPU utilization, but with the performance of CPUs doubling each year, value of paying for an iSCSI HBA is debatable. 

  • Severs with iSCSI Initiators are often simply called "Initiators"

  • iSCSI Storage carves up the storage capacity to smaller chucks called volumes. Volumes are allocated to different computers. Computers use iSCSI Initiator GUI to log into the iSCSI targets, and any volume allocated to them will be seen as a blank unformatted disk, and can be used for any purpose just like a new hard disk. 

So, why do I need iSCSI?

  • Higher data transfer speed than regular file transfer protocol (NAS)

  • You can boot from iSCSI which you can not do with NAS. What it means is that you can allocate a piece of your iSCSI storage to an external computer, load an operating system there and boot your computer from that iSCSI storage instead of an internal hard drive. You can even have multiple different OSes for the same computer and depending on what your needs are, you select which OS to boot from. 

  • Content of the iSCSI volumes that you allocate to a computer can not be seen or accessed from any other computer. That is a requirement for some environments. NAS shares are visible to all computers on the same network. 

  • You can have SAN (Storage Area Network) without the high costs of Fibre Channel SAN equipments.

  • When you give an iSCSI volume to a computer, that volume looks like a local disk in that computer. It kind of looks like you added a hard drive to that computer. So when you run out of room to add hard drives to your computers you can get an external iSCSI storage and provide additional storage for your computers.

  • You can use Apple's Time Machine to backup your apple computers into an iSCSI storage

  • You can have your storage in one building and computers in another building

  • You can back up your computers into an iSCSI storage in a different building to protect your data against disasters

  • iSCSI Storage provides for a central storage for multiple computers which make it much easier to manage.

  • There are some environments that require iSCSI storage and do not run on NAS shares. Examples of these environments are some virtualization environments, some particular applications, or data bases, and several more.

  • Need to know more reasons, do a little Google search……..

Now if you want to learn more, you can continue reading the section below which is part of a document generated by Microsoft long time ago......



iSCSI Basics

Introduction. 2

Storage Attachments and Transfer Protocols. 3

Embedded Storage. 3

ATA. 3

Direct Attached Storage. 3


Networked Storage. 4

Fibre Channel 4


iSCSI Advantages and Deployment Scenarios. 7

Enable Storage Area Network Capabilities. 7

Reduce TCO.. 7

Simplify Implementation. 7

Enable Remote Capabilities. 8

Simplify Clustering. 8

iSCSI Basics. 9

iSCSI Protocol 9

Naming and Addressing. 10

Discovery. 10

Session Management 11

Error Handling. 11

Security. 11

Performance. 12

iSCSI for Windows. 13

iSCSI Initiator (Data Transfer) 13

Software iSCSI 13

Hardware iSCSI 13

iSCSI Initiator Service (Management) 14

Security. 15

Summary. 16




Transfer protocols enable data transfer between host systems and their targets, such as storage devices. Transfer protocols find the appropriate target device, translate application commands into commands understood by the storage device, and move the data between the device and the computer system’s memory.

Networking protocols define the way that information is transferred among computers and target devices on the network[1]. Depending on the user application, data is transported either as files or as low-level blocks. File-based data on networked Windows platforms is sent using SMB/CIFS (Server Messenger Block/Common Internet File System) transfer protocols, and on networked UNIX/Linux platforms using the NFS (Network File System) transfer protocols. The family of SCSI (Small Computer System Interface) protocols is used to transfer block level data for most storage interconnects.

While file transport can occur over existing company intranets (Local Area Networks, or LANs), as well as over Metropolitan and Wide Area Networks (MANs and WANs), transport of block data was initially limited to short distances within the same building or, in many cases, the same physical enclosure. This changed with the development of serial SCSI architecture, which enabled rapid transfer of block data across distances as great as metropolitan areas, making storage area networking (SAN) technology an effective (although costly) solution for enterprise-sized businesses. For smaller businesses—perhaps as much as 85% of the storage market[2], Fibre Channel’s dedicated hardware and transfer protocol technologies have made the solution too costly to implement.

The development of iSCSI and the February 2003 ratification of iSCSI transport protocol standards by the Internet Engineering Task Force (IETF)[3] stands as the first challenge to the dominance of Fibre Channel. The iSCSI protocol, which unifies the well established Transmission Control Protocol/Internet Protocol (TCP/IP) networking protocol and the SCSI storage protocol, defines the rules and processes for transmitting and receiving block storage data over TCP/IP networks. Block transport of data will be enabled without requiring expensive installation of proprietary wiring technologies. Storage area networking (SAN)—a technology which solves many problems of scalability, disaster protection, efficient backup, and data protection—is enabled by iSCSI, making it available to midsize and small businesses as well enterprise businesses. Moreover, using existing IP networks, iSCSI enables the networking of storage over metropolitan or wide areas. Given these and other benefits, iSCSI is now viewed as an enabling technology that can help bring the benefits of sophisticated storage solutions to a broader market.

To use the iSCSI protocols and standards, drivers and other components specific to each operating system had to be developed. Microsoft began support for iSCSI on the Windows platform in 2000, and has since focused on developing an integrated architecture to capitalize on existing Windows functionality, ease of management, and strong security. In partnership with the IETF, Microsoft has made contributions to the iSCSI security draft (2001), and co-contributed to the Internet Storage Name Service (iSNS) protocol, the mechanism by which the host system discovers storage devices on a network.

Storage Attachments and Transfer Protocols

All data, whether sent from the application in file format or as blocks, is ultimately stored in block format. Block-data transfer between computers and storage devices is under control of a hardware device inside the computer system known variously as the host bus adapter (HBA)[4], storage controller, or network adapter. Transfer occurs across a single cable (bus), as in the case of direct attached storage, or across a network.

There are a number of dominant interconnect wiring technologies, each with its associated transfer protocol, including Advanced Technology Attachment (ATA), SCSI, Fibre Channel, and the newly ratified iSCSI technology[5]. The type of interconnect used in a particular configuration depends on how storage is attached to computers (whether internally or externally), whether or not the storage devices are to be shared, and the distances over which the sharing must reliably occur. The following sections briefly outline each of the major interconnect technologies and explores the limitations of each.

Embedded Storage


Physical Components

Most desktops and laptops store data on internal disk drives, using either parallel or serial ATA as the internal storage interconnect. Parallel[6] ATA (also sometimes referred to as integrated drive electronics (IDE)) is the dominant interconnect technology for internal storage, largely because of its simplicity and ease of implementation. Despite these advantages, parallel ATA has performance and distance limitations, and does not provide a standardized mechanism for hot-plugging devices. Serial ATA promises to address these by improving bandwidth and providing more reliable command queuing, while doubling the length of the interconnect.

Transfer Protocol

Direct Memory Access (DMA) allows transfer of data from hard drive to memory without using the host CPU, and is the most commonly used data transfer mode. Because the distance traveled is so short, transfer of data is relatively fast (up to 133 MB/S).


Even with the improvements promised by serial ATA, and its low cost, embedded storage has two disadvantages: it scales poorly with storage growth, and it does not allow device sharing.

Direct Attached Storage


Physical Components

The SCSI bus is comprised of the cabling and connectors that directly attach a computer (usually a server) to multiple devices, whether for storage or other functions. Storage devices include hard disks and tape drives, as well as storage subsystems such as a redundant array of inexpensive disks (RAID).

Parallel SCSI is the dominant wiring technology over which the SCSI commands are transmitted, although the SCSI-3 standard has been further developed to include serial SCSI (see Fibre Channel and iSCSI).

Devices are identified by a SCSI target ID or address. Narrow SCSI has seven addresses available for use by devices; wide SCSI doubles the width of the bus, and allows 15 addresses. Each SCSI address can itself be subdivided, as might be necessary when attaching a RAID storage subsystem. The storage subsystem sits at a single SCSI address, and the disks inside the subsystem each receive a unique address by logically dividing each physical SCSI address into sub-units, known as logical units, and identified by logical unit numbers (LUN). Windows supports up to 256 LUNs at each SCSI target ID.

Because there are multiple devices on the SCSI bus, a process is required to ensure that the devices can obtain access to the cable to transfer commands and data. Each SCSI ID is assigned a priority. The device with the highest priority takes control of the bus when it needs to transfer data, a strategy that can result in contention and a “lack of fairness” to lower priority devices.

Transfer Protocols

The SCSI protocol was designed as the communication language between the SCSI controller and the storage devices. The original SCSI bus was limited to 8-bit transfers (referred to as Narrow SCSI), and transferred data at a rate of up to 5 megabytes/second (MB/s). The SCSI-2 protocol standard introduced Fast SCSI (8 bits, 10 MB/s) and Wide SCSI (16 bits, 20 MB/s), as well as the intelligence to support command queuing (up to 256 commands per logical unit). The SCSI Parallel Interface version 5 (one of many SCSI-3 specifications) is the current version of the protocol, supporting a throughput of 320 MB/s. (The next speed increase to 640 MB/s has now been dropped in favor of using a new form of serialized SCSI known as Serial Attached SCSI (SAS); the allowable distances for using parallel signals at this higher data rate would have been too short to be useful.)


The two most critically limiting factors of parallel SCSI are the restricted distance the device can be from the host, and the number of devices that can be attached. In the first case, the copper SCSI cabling cannot be more than 12 or 25 meters from the host (depending on the signal level) without experiencing degradation in the quality of the signal. In the second case, as more devices are added to the bus, the total distance supported declines and the large pin count connectors become more prone to failure.

Networked Storage

Networking two or more computers facilitates the sharing of both data and storage devices, and it does so over longer distances than are enabled with the SCSI interconnect. NFS and SMB/CIFS were developed to facilitate transmission of files across IP networks[7]. Transfer of block data across networks was not initially enabled using IP network technologies. Instead, Fibre Channel was developed as the physical medium, and the SCSI-3 protocols were adapted for use with storage devices on Fibre Channel.

Fibre Channel

Physical Components

Fibre Channel is a network solution that eliminates the need for direct cabling between the host computer and the storage device, thus allowing multiple computers to access and share the same storage devices. Fibre Channel networks use dedicated HBAs to provide high performance block IO transfers.

Fibre Channel is based on serial wiring technology (and therefore does not have the electrical limitations associated with parallel technologies). Despite the name, Fibre Channel wires can consist of either copper or fiber optic cabling. Since copper interconnects are limited in the distances they can extend (signal degradation and interference occurs over long distances), they are best used for departmental configurations. Fiber optic cabling and connectors allow transmission distances up to 10 km or more, enabling data replication over a metropolitan area for the purposes of disaster recovery.

In Fibre Channel loop networks, all the cables directly connect to a hub, so devices must arbitrate for transmission control. Fibre Channel fabrics are networks connected by switches, making arbitration unnecessary. Both the computer and storage device can have one or more ports to connect them to the network. Each port is assigned a unique port address, and millions of such addresses are possible in a Fibre Channel fabric (126 in a loop).

Transfer Protocols

A number of protocols can be implemented over Fibre Channel, including IP and SCSI-3. The Fibre Channel Protocol (FCP or the later FCP-2) maps the SCSI-3 protocol standard to implement serial SCSI over Fibre Channel networks.

Fibre Channel supports 1 gigabit/second and 2 gigabit/second speeds, although 4 gigabit/second and 10 gigabit/second devices are being developed. Fibre Channel transmission is highly reliable, with error rates roughly corresponding to one error per 1-2 terabytes of data (these errors are recovered as part of the protocol).

Security is not generally considered an issue with Fibre Channel, since the storage networks are by nature sealed off from outside access. Shared storage is kept “secure” internally by implementing zoning and LUN access controls. Nevertheless, as bridging protocols allow access to Fibre Channel networks and as Fibre Channel devices are shared across departments or separate companies, security will become a greater concern.


Fibre Channel networks typically use dedicated fiber optic cables, which, although they can extend up to 10 km (or farther, if using specialized electro-optical components), remain a limitation for businesses dispersed over geographic expanses wider than metropolitan areas. Several protocols that allow Fibre Channel traffic to be transported over IP networks will soon be standardized by the IETF and should help alleviate this problem.

Although Fibre Channel networks are essentially unlimited in the number of possible ports, the high cost of switch ports ($500-$4000/port) and the need for HBAs ($500-$2000/adapter) raises network costs considerably. Additionally, Fibre Channel networks require considerable expertise to configure correctly. Moreover, interoperability is elusive, and training is scarce and expensive.


Physical Components

iSCSI uses company IP networks to transfer block-level data between computer systems and storage devices. Unlike Fibre Channel, iSCSI uses existing network infrastructure: network adapters, network cabling, hubs, switches, routers, and supporting software. The use of network adapters, rather than HBAs, allows transfer of both SCSI block commands and normal messaging traffic. (This gives iSCSI an advantage over Fibre Channel network configurations, which require use of both HBAs and network adapters to accommodate both types of traffic. While this is not a problem for large servers, thin servers can accommodate only a limited number of interconnects.)

iSCSI is based on the serial SCSI standards, and can operate over existing Category 5 (or higher) copper Ethernet cable or fiber optic wiring.

Transfer Protocols

iSCSI describes the transport protocol for carrying SCSI commands over TCP/IP. TCP handles flow control and facilitates reliable transmission of data to the recipient by providing guaranteed in-order delivery of the data stream. IP is responsible for routing packets to the destination network. Together these protocols ensure that data is correctly transferred from requesting applications (initiators) to target devices.

Transmissions across Category 5 network cabling are at speeds up to 1 gigabit or 10 gigabit per second. Error rates on gigabit Ethernet are in the same low range as Fibre Channel.


The amount of time it takes to queue, transfer, and process data across the network is referred to as latency. One of the drawbacks of transmitting SCSI commands across a TCP/IP network is that latency is higher than it is on Fibre Channel networks, in part because of the overhead associated with TCP/IP protocols. Additionally, many currently deployed Ethernet switches were not designed with the low latency specifications associated with Fibre Channel. Thus, although Ethernet cabling is capable of high speeds, the actual speed of transmission may be lower than expected, particularly during periods of network congestion.

The second concern about iSCSI transmission is data integrity, both in terms of errors and security. Error handling is addressed at each protocol level. Security such risks as tampering or snooping as the data passes over networks can be handled by implementing the IP security protocol (IPsec). Both of these measures are detailed in the iSCSI Basics section later in this document.

iSCSI Advantages and Deployment Scenarios

Block-storage over IP provides businesses with new flexibility in their storage solutions. iSCSI technologies:

Enable Storage Area Network Capabilities

iSCSI enables SANs by connecting storage to existing company network infrastructure. Small and midsize organizations, previously priced out of the Fibre Channel solution, can now afford SAN technologies. iSCSI or “SAN over IP” makes the following SAN solutions widely available:

·         Pooled Storage. Multiple computer systems can access shared storage resources, enabling highly effective storage use, maximum scalability, and equipment consolidation.

·         Highly Available Storage. With multiple connectors and the appropriate software supporting multiple I/O paths between the SAN and servers, multiple failover paths are supported if some aspect of the hardware fails.

·         Data Redundancy. Better data protection is achieved through disk mirroring to a second storage box for fault tolerance, and replication to a remote device for disaster recovery.

·         Network Attached Storage (NAS) with Backend SAN. NAS-SAN convergence is enabled using a NAS box with an iSCSI device driver initiator that is attached to a backend iSCSI SAN.

Reduce TCO

Using three business scenarios, one study[8] compared the total cost of ownership (TCO) to build Fibre Channel and iSCSI storage networks from the ground up. In all cases, the TCO for iSCSI was lower than for Fibre Channel. For companies that are considering expansion into storage area networking and have a suitable pre-existing network infrastructure (gigabit network adapters, cables, gigabit switches), the TCO is even lower for iSCSI in comparison with Fibre Channel.

Moreover, iSCSI-based SANs capitalize on an existing staff knowledge base of TCP/IP infrastructure and existing network management tools, making it likely that they will place fewer demands on staff, especially as compared to Fibre Channel networks.

Simplify Implementation

iSCSI allows storage networks to be created from existing network components, rather than having to add a second network fabric type. This simplifies not only hardware configurations (since Ethernet switches can be used), but also allows the use of existing security methods, such as firewalls and IP security (which includes encryption, authentication, and data integrity measures).

Sharing storage among multiple systems requires a method for managing storage access so that systems access only the storage that is assigned to them. In Fibre Channel networks this is done by assigning systems to zones. In iSCSI, this can be done by using virtual LAN (VLAN) techniques. In Fibre Channel, LUN masking must be used to provide finer granularity of storage access; for iSCSI this is handled as part of the design by allowing targets to be specific to individual hosts.

The use of IP traffic prioritization or Quality of Service (QoS) can help ensure that storage traffic has the highest priority on the network, which helps to alleviate latency issues.

Enable Remote Capabilities

iSCSI is not limited to the metropolitan-wide areas to which Fibre Channel is limited. iSCSI storage networks can be LANs, MANs, or WANs, essentially allowing global distribution. iSCSI has the ability to eliminate the conventional boundaries of storage networking, enabling businesses to access data world-wide, and ensuring the most robust disaster protection possible. To do this for Fibre Channel based SANs, it is necessary to introduce additional protocol translations (such as Fibre Channel IP) and devices that provide this capability on each end of the SAN links.

Simplify Clustering

When multiple servers share access to the same storage, as is done with Microsoft Cluster Service (MSCS) clusters, configuration of Fibre Channel SANs can be very difficult—one improperly configured system impacts the entire SAN. iSCSI clusters, unlike Fibre Channel clusters, do not require complex configurations. Instead, iSCSI configuration is easily accomplished as part of the iSCSI protocol, with little need for intervention by system administrators. Changes introduced by hardware replacement are largely transparent on iSCSI but are a major source of errors on Fibre Channel implementations.

iSCSI Basics

An iSCSI-based network consists of 1) the server and storage device end nodes, 2) either network interface cards or HBAs with iSCSI over TCP/IP capability on the server, 3) storage devices with iSCSI-enabled Ethernet connections, and in some cases, 4) iSCSI storage switches and routers. Since most current SANs use Fibre Channel technology, multi-protocol switches (or storage routers capable of translating iSCSI to Fibre Channel) must be used so that iSCSI connected hosts can communicate with existing Fibre Channel devices.

Storage traffic is commonly initiated by a host computer—the initiator—and received by the target storage device. Since target devices can have multiple storage devices associated with them (each one being a logical unit), the final destination of the data is not the target per se, but specific logical units within the target[9].

iSCSI Protocol

The iSCSI protocol stack links SCSI commands for storage and IP protocols for networking to provide an end-to-end protocol for transporting commands and block-level data down through the host initiator layers and up through the stack layers of the target storage devices. This communication is fully bidirectional, as shown in Figure 1, where the arrows indicate the communication path between the initiator and the target by means of the network.


Initiator                               Target

Application                             Application

SCSI                                     SCSI

iSCSI Protocol                        iSCSI Protocol

TCP/IP                                  TCP/IP

Ethernet <<<<<<<>>>>>>>Ethernet


Figure 1. iSCSI Protocol Stack Layers


The initiator (usually a server) makes the application requests. These are converted (by the SCSI class driver) to SCSI commands, which are transported in command description blocks (CDBs). At the iSCSI protocol layer, the SCSI CDBs (under control of the iSCSI device driver) are packaged in a protocol data unit (PDU) which now carries additional information, including the logical unit number of the destination device. The PDU is passed on to TCP/IP. TCP encapsulates the PDU and passes it to IP, which adds the routing address of the final device destination. Finally, the network layer (typically Ethernet) adds information and sends the packet across the physical network to the target storage device.

Additional PDUs are used for target responses and for the actual data flow. Write requests are sent from the initiator to the target, and are encapsulated by the initiator. Read requests are sent from the target to the initiator, and the target does the encapsulation.

Naming and Addressing

All initiator and target devices on the network must be named with a unique identifier and assigned an address for access. iSCSI initiators and target nodes can either use an iSCSI qualified name (IQN) or an enterprise unique identifier (EUI). Both types of identifiers confer names that are permanent and globally unique.

Each node has an address consisting of the IP address, the TCP port number, and either the IQN or EUI name. The IP address can be assigned by using the same methods commonly employed on networks, such as Dynamic Host Control Protocol (DHCP) or manual configuration.


Storage area networks can become very large and complex. While pooled storage resources is a desirable configuration, initiators must be able to determine both the storage resources available on the network, and whether or not access to that storage is permitted. A number of discovery methods are possible, and to some degree the method used depends on the size and complexity of the SAN configuration.

·         Administrator Control. For simple SAN configurations, the administrator can manually specify the target node name, IP address, and port to the initiator and target devices. If any changes occur on the SAN, the administrator must upgrade these names as well.

·         SendTargets. A second small storage network solution is for the initiator to use the SendTargets operation to discover targets. The address of a target portal is manually configured and the initiator establishes a discovery session to perform the SendTargets command. The target device responds by sending a complete list of additional targets that are available to the initiator. This method is semi-automated, which means that the administrator might still be required to enter a range of target addresses.

·         SLP. A third method is to use the Service Location Protocol (SLP). Early versions of this protocol did not scale well to large networks. In the attempt to rectify this limitation, a number of “agents” were developed to help discover targets, making discovery management administratively complex.

·         iSNS. The Internet Storage Name Service (iSNS) is a relatively new device discovery protocol (ratified by the IETF) that provides both naming and resource discovery services for storage devices on the IP network. iSNS builds upon both IP and Fibre Channel technologies.[10]

The protocol uses an iSNS server as the central location for tracking information about targets and initiators. The server can run on any host, target, or initiator on the network. iSNS client software is required in each host initiator or storage target device to enable communication with the server. In the initiator, the iSNS client registers the initiator and queries the list of targets. In the target, the iSNS client registers the target with the server.

iSNS provides the following capabilities:

·         Name Registration Service: allows initiators and targets to register and query the iSNS server directory for information regarding initiator and target ID and addresses.

·         Network Zoning and Logon Control Service: iSNS initiators can be restricted to zones so that they are prevented from discovering target devices outside their discovery domains. This prevents initiators from accessing storage devices that are not intended for their use. Logon control allows targets to determine which initiators can access them.

·         State Change Notification Service: This service allows iSNS to notify clients of changes in the network, such as the addition or removal of targets, or changes in zoning membership. Only initiators that are registered to receive notifications will get these packets, reducing random broadcast traffic on the network.

From its inception, iSNS was designed to be scaleable, working effectively in both centralized and distributed environments. Since iSNS also supports Fibre Channel IP, configurations that link Fibre Channel and iSCSI can use iSNS to get information from Fibre Channel networks as well. Hence, iSNS can act as a unifying protocol for discovery.

Session Management

For the initiator to transmit information to the target, the initiator must first establish a session with the target through an iSCSI logon process. This process starts the TCP/IP connection, verifies that the initiator has access to the target (authentication), and allows negotiation of various parameters including the type of security protocol to be used, and the maximum data packet size. If the logon is successful, an ID is assigned to both initiator (an initiator session ID, or ISID) and target (a target session ID, or TSID). Thereafter, the full feature phase—which allows for reading and writing of data—can begin. Multiple TCP connections can be established between each initiator target pair, allowing unrelated transactions during one session. Sessions between the initiator and its storage devices generally remain open, but logging out is available as an option.

Error Handling

While iSCSI can be deployed over gigabit Ethernets, which have low error rates, it is also designed to run over both standard IP networks and WANs, which have higher error rates. WANs ,are particularly error-prone since the possibility of errors increases with distance and the number of devices the information must travel across. Errors can occur at a number of levels, including the iSCSI session level (connection to host lost), the TCP connection level (TCP connection lost), and the SCSI level (loss or damage to PDU).

Error recovery is enabled through initiator and target buffering of commands and responses. If the target does not acknowledge receipt of the data because it was lost or corrupted, the buffered data can be resent by the initiator, a target, or a switch.

iSCSI session recovery—necessary if the connection to the target is lost due to network problems or protocol errors—can be reestablished by the iSCSI initiator. The initiator attempts to reconnect to the target, continuing until the connection is reestablished.


Since iSCSI operates in the Internet environment, security is critically important. The IP protocol itself does not authenticate legitimacy of the data source (sender), and it does not protect the transferred data. ISCSI, therefore, requires strict measures to ensure security across IP networks.

The iSCSI protocol specifies the use of IP security (IPsec) to ensure that:

·         The communicating end points (initiator and target) are authentic.

·         The transferred data has been secured through encryption and is thus kept confidential.

·         Data integrity is maintained without modification by a third party.

·         Data is not processed more than once, even if it has been received multiple times.

(The Internet Key Exchange (IKE) protocol can assist with key exchanges, a necessary part of the IPsec implementation.)

iSCSI also requires that the Challenge Handshake Authentication Protocol (CHAP) be implemented to further authenticate end node identities. Other optional authentication protocols include Kerberos (such as the Windows implementation), which is a highly scalable option.

Even though the standard requires that these protocols be implemented, there is no such requirement to use them in an iSCSI network. Before implementing iSCSI, a network administrator should review the security measures to make sure that they are appropriate for the intended use and configuration of the iSCSI storage network.


iSCSI will always have a higher performance than NAS purely because NAS is a file transfer protocol and alway has the burden of a file system in between, but iSCSI transfers data blocks to raw disks. But both NAS and iSCSI can suffer some performance degradation comparing to Fibre Channel because of TCP/IP overhead. This problem can be solved by offloading the TCP/IP processing to a TCP/IP offload engine (TOE) on a chip or HBA. These solutions are currently supported by a number of vendors.

iSCSI for Windows

The Microsoft iSCSI initiator[11] package is designed to run on Windows 2000, Windows XP, and Windows Server 2003. The package consists of several software components:

·         Initiator. The optional iSCSI device driver component that is responsible for moving data from the storage stack over to the standard network stack. This initiator is only used when iSCSI traffic goes over standard network adapters, not when specialized iSCSI adapters are used.

·         Initiator service. A service that manages all iSCSI initiators (including network adapters and HBAs) on behalf of the OS. Its functions include aggregating discovery information and managing security. It includes an iSNS client, the code required for device discovery.

·         Management applications. The iSCSI command line interface (iSCSICLI), property pages for device management, and a control panel application.

iSCSI Initiator (Data Transfer)

Rather than simply adding an iSCSI driver onto the existing operating system architecture, iSCSI support was fully integrated with the OS. This integrated design facilitates two types of driver/hardware implementations, either in the host software, or in hardware. By providing support for multiple iSCSI implementations, Microsoft provides businesses with maximum flexibility in choosing the solutions that fit not only their pricing requirements, but also their network bandwidth needs and CPU overhead constraints.

Software iSCSI

With this implementation, the complete iSCSI initiator (the driver) is built into the operating system software and functions with the network stack (iSCSI over TCP/IP). The Microsoft software initiator supports both standard Ethernet network adapters and TCP/IP offloaded network adapters.

Microsoft support for standard Ethernet network adapters enables businesses to take advantage of iSCSI technologies by using standard off-the-shelf network adapter technology, without the need to purchase specialized hardware. For businesses where high performance solutions are critical, the Microsoft iSCSI initiator also supports the use of accelerated network adapters to offload TCP overhead from the host processor to the network adapter.

Hardware iSCSI

The iSCSI initiator or driver can be implemented in the hardware adapter card, rather than in the host software. This can be done by using an iSCSI HBA or a multifunction offload device. In either case, the iSCSI processing is offloaded to the hardware adapter card instead of processing the iSCSI protocol in the host software. With both TCP and iSCSI processing on the adapter card, high-speed transport of block data with minimal CPU overhead is possible[12]. (The drawbacks to this approach are that these high-performance solutions are more costly.)


iSCSI HBA drivers are designed by third-party vendors to function with the Windows operating system.[13] Because this solution is dedicated to block transport only, additional adapters are required for network traffic messaging. While this is not a problem for large chassis, it can be a problem for thin servers.

iSCSI HBAs must support iSNS discovery (and they can support any other discovery mechanism as well.) The tightly integrated design of the initiator service with iSNS is especially advantageous in HBA configurations (or with multifunction offload devices) that do device discovery. Prior to the development of the iSCSI initiator service, these adapters were able to discover targets but were unable—without additional management application code—to report those targets to Windows. With the Windows initiator service, all targets, irrespective of their method of discovery, are consistently made known to the Windows OS. The service ensures that multiple sessions to the same target, using different HBAs, are not allowed, unless the session has a flag indicating that the proper multipathing software is installed. The service also prevents unintended logging off of initiators when a target is shared.

Security demands are somewhat less stringent for iSCSI HBAs: they must support CHAP, but are not required to support IPsec in the HBA network stack[14].

Multifunction Offload Devices

Multifunction adapter cards can support more than one function. In this case, these devices support both network adapter and iSCSI HBA functions[15]. Of all the options, this provides the most flexibility because it accelerates all IP traffic, offloads TCP/IP traffic, and may offload iSCSI traffic as well. Again, the drawback with this approach is the higher cost associated with the more complicated hardware and software implementations.

As with iSCSI HBAs, multifunction offload devices must support iSNS for discovery and are required to use the initiator service (by using the WMI interface), ensuring that Windows maintains a centralized system of discovery.

Security requirements are the same as for iSCSI HBAs.

iSCSI Initiator Service (Management)

The iSCSI initiator service was designed to enable uniform storage management regardless of whether the iSCSI driver is implemented in hardware or software. The initiator service provides streamlined storage management for all aspects of the iSCSI service, including:

·         Discovery. Allows aggregation of multiple discovery mechanisms (iSNS, SLP, SendTarget, and manual configuration by an administrator).

·         Security. Provides iSNS server and client support for security credentials.

·         Session initiation and termination. Provides parameter settings for iSCSI sessions.

·         Device management. Provides HBA or network adapter-based initiators with the necessary parameters.

Additionally, a number of mechanisms can be used to communicate with the initiator service, including the iSCSI Control Panel applet, the iSCSICLI, iSCSI Property Pages, and Windows Management Instrumentation (WMI). The control panel applet allows the administrator to do the most common iSCSI operations by using a graphical user interface (GUI); full functionality is provided by the scriptable command line tool. Property pages provide a mechanism for accessing common information and configuring common implementations, such as configuring IP addresses for HBAs. The WMI interfaces enable management applications to obtain information about the iSCSI initiator and to control the initiator. They also allow scripting of management operations.

The initiator service enables the host computer system to discover target storage devices on the storage area network and to determine whether or not it has access to those devices. iSNS client—the code required to interact with the iSNS server—is built directly into the initiator service, enabling the initiator service to maintain a list of targets reported via the iSNS server as changes are made.

The Windows iSCSI initiator service supports all four storage target discovery mechanisms listed in the iSCSI Basics section of this paper, including iSNS support for discovery domains, state change notification, and IPsec information. [16]

Additionally, the initiator service interfaces with Plug and Play, allowing dynamic discovery of available storage targets. To prevent unauthorized initiators, an administrator must authorize the use of a target; enforcement of such authorization is under initiator service control. The service also allows persistent logons, thus ensuring that targets are logged on every time the operating system boots.


iSCSI initiator security was designed for integration with the Windows security infrastructure. This design enables customers to capitalize on both the core functionality of IPsec and Windows security features, which include advanced support for IKE features and Active Directory distribution of security information to each machine.

In accordance with iSCSI standards, IPsec is used for encryption and CHAP for authentication. Key exchange for encrypted communication is provided with the Windows Internet Key Exchange Security features. The initiator service has a common API that can be used for configuring both the software initiator and the iSCSI HBAs.




iSCSI, or Internet SCSI, is an IP-based storage networking standard developed by the IETF. By enabling block-based storage over IP networks, iSCSI enables storage management over greater distances than Fibre Channel technologies, and at lower cost. The Microsoft iSCSI initiator service helps to bring the advantages of high-end SAN solutions to small and midsized businesses. The service enables a full range of solutions, from low-end network, adapter-only implementations to high-end offloaded configurations that can rival Fibre Channel solutions.

[1] The Open Systems Interconnection (OSI) reference model provides a hierarchical framework for conceptualizing the critical tasks involved in network communications. Of the seven OSI layers, the transport layer (layer four) handles the transmission, receipt, and error checking of the data.


[2] SNIA, 2001.

[4] If the intelligent hardware (silicon chips plus software) resides on the host, it is known as the HBA. If it resides on the target, it is known as the controller. In either case, it controls transfer of data.

[5] The Universal Serial Bus (USB) is an important technology for low and medium speed devices. Another interconnect technology, IEE 1394, runs at considerably faster speeds.

[6] Parallel transmission occurs across multiple wires. Serial transmission passes across a single wire.

[7] This is the file server technology employed by network attached storage (NAS) boxes. Transfer of NAS data to storage occurs from the back-end using parallel SCSI or Fibre Channel.

[8] Bruce Makin, Adaptec. A Study of iSCSI Total Cost of Ownership (TCO) vs. Fibre Channel and SCSI. October 2001.


[9] This is true for all SCSI architectures (iSCSI, SCSI, Fibre Channel).

[10] Specifically, iSNS uses IP’s Domain Name Server (DNS) to resolve numerical addresses into host names. It also uses Fibre Channel’s methods of zoning and state change notification, as well as the Simple Name Server (SNS), a database which registers device ids, addresses, and other information.

[11] The term “initiator” refers both to the host computer that initiates the discussion in order to send commands and data, and to the part of the host—the network adapter or HBA—that actually does the work. If there is only a single adapter on the system, the two can be considered identical.

[12] Hardware-based iSCSI initiators have a few other advantages, including support for system boot using a volume accessible to an iSCSI HBA.


[13] Only vendors participating in the Logo program are approved for use with the Windows OS.

[14] This is a requirement of the Microsoft Logo program.

[15] It is able to do this because it has more than one driver stack.

[16] A beta software version of iSNS server is included in the iSCSI initiator package, in case the operating environment lacks one.






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