RE: iSCSI error recovery

["Y P Cheng" <ycheng@advansys.com>]

> > With a single CRC covering both the frame header and payload, when a bad
> > frame is thrown away which is the only one in a sequence, the
> > only way for detecting the missing frame is timeout.
>
> Not true for a Data PDU. An I/O timeout only occurs when the Status PDU is
> discarded due to a CRC error [or format error]. Discarding a Data
> PDU [due to a CRC error or format error] results in I/O underrun,
> [a terminology used to describe initiator receiving less data than
> sent by target] and can be detected by a count check .
> [using a DataSN based count as described by Julian in his
> last clarification on this issue.]
>
> If the Data PDU also contained the Status within, then discarding
> such a PDU does cause an I/O timeout.
> Santosh

Lets try a little more specific:

1. Bad command PDU    Thrown away by target     Initiator Timeout
2. Bad write data     Thrown away by target     Target Timeout
3. Bad read data      Thrown away by initiator  Initiator data underrun
4. Bad status PDU     Thrown away by initiator  Initiator Timeout

In case 1, in receiving another command frame, the target detects a missing
CmdSN and decides to wait for a while and then asks initiator to resend.

In case 2, the target can detect a missing DataSN except the very last one.

Case 3 is the specific case you are refrering to.

In case 4, in receiving another status frame, the initiator detects a
missing StatSN and decides to wait for a while and then asks target to
resend.

Y.P.