IBM TotalStorage DS300, Best Practices Manual, Page: 32 / 680

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IBM TotalStorage DS300 and DS400 Best Practices Guide
(NAS). The following sections take a little closer look at some technical aspects
of iSCSI.
iSCSI technical introduction
As mentioned previously, iSCSI is the mapping of a SCSI remote procedure
invocation model on top of the TCP protocol. A SCSI message is encapsulated
inside the TCP frame and is called an “iSCSI protocol data unit”, or iSCSI PDU.
iSCSI protocol is based on version three of the SCSI (SAM-2) protocol standard.
How does iSCSI keep track?
iSCSI uses multiple number techniques to keep track of commands and data.
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Command numbering is session wide and provides the means for ordered
commands.
򐂰 Status numbering is per connection and aids recovery from connection
failures.
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Data sequencing is per command and detects missing data packets.
Fields in the iSCSI PDU contain numbering information. With unidirectional
traffic, special NOP-message PDUs may be utilized to synchronize
communications between client and server. The status number is started after
login. Data sequencing is required since multiple client-server paths may be
used.
iSCSI discovery
Discovery allows an initiator to find the target(s) to which it has access. This
requires a minimum of user configuration. Several methods of discovery may be
used:
A list of targets at the initiator
An administrator can define the iSCSI targets to the host system initiator. This
process allows the administrator to specify the iSCSI target node name and IP
address:port to the host system initiator or its host bus adapter (HBA). See 2.6.3,
“Hardware initiator” on page 26 for more information about HBAs. iSCSI HBAs
should support an administrator defining this information. This type of discovery
is useful in small installations.
Queries to known iSCSI servers
An iSCSI initiator can probe its environment and, when a possible iSCSI target is
found, start a
discovery session with the target by issuing a SendTargets
command. The target can reply to a SendTargets command by returning a list of
all iSCSI target nodes it knows about.