iSCSI Flow Control and QoS

This section summarizes the flow-control and QoS mechanisms supported by iSCSI.

iSCSI Flow Control

The primary flow-control mechanism employed by iSCSI is the Ready To Transfer (R2T) Protocol Data Unit (PDU). iSCSI targets use the R2T PDU to control the flow of SCSI data during write commands. The Desired Data Transfer Length field in the R2T PDU header controls how much data may be transferred per Data-Out PDU sequence. The R2T PDU is complemented by several other mechanisms. The MaxOutstandingR2T text key controls how many R2T PDUs may be outstanding simultaneously. The use of implicit R2T PDUs (unsolicited data) is negotiated via the InitialR2T and ImmediateData text keys. When unsolicited data is supported, the FirstBurstLength text key controls how much data may be transferred in or with the SCSI Command PDU, thus performing an equivalent function to the Desired Data Transfer Length field. The MaxRecvDataSegmentLength text key controls how much data may be transferred in a single Data-Out or Data-In PDU. The MaxBurstLength text key controls how much data may be transferred in a single PDU sequence (solicited or unsolicited). Thus, the FirstBurstLength value must be equal to or less than the MaxBurstLength value. The MaxConnections text key controls how many TCP connections may be aggregated into a single iSCSI session, thus controlling the aggregate TCP window size available to a session. The MaxCmdSN field in the Login Response BHS and SCSI Response BHS controls how many SCSI commands may be outstanding simultaneously. For more information about iSCSI flow control, readers are encouraged to consult IETF RFC 3720.

iSCSI QoS

iSCSI depends primarily on lower-layer protocols to provide QoS. However, iSCSI provides support for expedited command processing via the I bit in the BHS of the Login Request PDU, the SCSI Command PDU, and the TMF Request PDU. For more information about iSCSI QoS, readers are encouraged to consult IETF RFC 3720.