What is iSCSI Connection and Session

Here is a listing of technical definitions for iSCSI Connection & iSCSI Session.

Connection: A connection is a TCP connection. Communication between the initiator and target occurs over one or more TCP connections. The TCP connections carry control messages, SCSI commands, parameters, and data within iSCSI Protocol Data Units (iSCSI PDUs).

Session: The group of TCP connections that link an initiator with a target form a session (loosely equivalent to a SCSI I-T nexus). TCP connections can be added and removed from a session. Across all connections within a session, an initiator sees one and the same target.

SSID (Session ID): A session between an iSCSI initiator and an iSCSI target is defined by a session ID that is a tuple composed of an initiator part (ISID) and a target part (Target Portal Group Tag). The ISID is explicitly specified by the initiator at session establishment. The Target Portal Group Tag is implied by the
initiator through the selection of the TCP endpoint at connection establishment. The TargetPortalGroupTag key must also be returned by the target as a confirmation during connection establishment when TargetName is given.

CID (Connection ID): Connections within a session are identified by a connection ID. It is a unique ID for this connection within the session for the initiator. It is generated by the initiator and presented to the target during login requests and during logouts that close connections.

ISID: The initiator part of the Session Identifier. It is explicitly specified by the initiator during Login.


TSIH (Target Session Identifying Handle): A target assigned tag for a session with a specific named initiator. The target generates it during session establishment. Its internal format and content are not defined by iSCSI protocol, except for the value 0 that is reserved and used by the initiator to indicate a new session. It is given to the target during additional connection establishment for the same session.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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