R2T

r2t, short for Ready To Transfer, is a protocol data unit (PDU) used in the iSCSI protocol to coordinate data transfer between an initiator (the client) and a target (the storage device). In iSCSI, SCSI commands and data are carried over TCP/IP as PDUs, and the transfer of actual data is governed by R2T messages issued by the target.

The primary role of an R2T is to authorize the initiator to send a specified amount of data for a particular SCSI command. An R2T indicates the maximum data length that may be sent and typically includes identifying information to tie the transfer to a specific command and session. The initiator must not transmit Data PDUs for that command until it has received an R2T.

Once an R2T is received, the initiator can send Data-Out PDUs carrying the payload up to the length declared in the R2T. Transfers can be performed in multiple Data-Out PDUs and, for large operations, the target may issue additional R2Ts to grant further transfer windows. This flow-control mechanism helps manage buffer usage and pacing, allowing reliable delivery of data over IP networks.

R2T is a standard part of iSCSI data transfer and is described in the iSCSI specifications, such as RFC 7143. It is commonly encountered in documentation and implementations related to iSCSI performance tuning and troubleshooting.

See also: iSCSI, SCSI over TCP, RFC 7143.