FCP Flow Control and QoS

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

FCP Flow Control

The primary flow-control mechanism employed by FCP is the FCP_XFER_RDY IU. FCP targets use the FCP_XFER_RDY IU to control the flow of SCSI data during write commands. The FCP_BURST_LEN field in the FCP_XFER_RDY IU header controls how much data may be transferred per FCP_DATA IU. The FCP_XFER_RDY IU is complemented by a variety of other mechanisms. The Class 3 Service Parameters field in the PLOGI ELS header determines how many FCP_XFER_RDY IUs may be outstanding simultaneously. This is negotiated indirectly via the maximum number of concurrent sequences within each Exchange. The use of implicit FCP_XFER_RDY IUs (unsolicited data) is negotiated via the WRITE FCP_XFER_RDY DISABLED field in the PRLI Service Parameter Page.

When unsolicited data is supported, the First Burst Size parameter in the SCSI Disconnect-Reconnect mode page controls how much data may be transferred in the unsolicited FCP_DATA IU, thus performing an equivalent function to the FCP_BURST_LEN field. The Maximum Burst Size parameter in the SCSI Disconnect-Reconnect mode page controls how much data may be transferred in a single FCP_DATA IU (solicited or unsolicited). Thus, the First Burst Size value must be equal to or less than the Maximum Burst Size value. FCP does not support negotiation of the maximum number of SCSI commands that may be outstanding simultaneously because the architectural limit imposed by the size of the CRN field in the FCP_CMND IU header is 255 (versus 4,294,967,296 for iSCSI). For more information about FCP flow control, readers are encouraged to consult the ANSI T10 FCP-3 and ANSI T11 FC-LS specifications.

FCP QoS

FCP depends primarily on lower-layer protocols to provide QoS. However, FCP provides support for expedited command processing via the Priority field in the FCP_CMND IU header. For more information about FCP QoS, readers are encouraged to consult the ANSI T10 FCP-3 specification.