Re: iSCSI: header digest error at initiator

[Santosh Rao <santoshr@cup.hp.com>]

Pierre Labat wrote:

> Julian,
>
> In the section "4.5 Digest errors"
> you say that if the initiator detects  an header digest error
> in an incoming iSCSI PDU, the TCP connection must be
> restarted.
>
> Is it to address the following case or for something else?
>
> 1) the initiator issues a READ
>
> 2) one of the inbound data PDU has a header digest error
>
> 3) the initiator (as specified in "1.2.5 iSCSI Full Feature Phase":
>    "Initiators MUST NOT perform any score boarding for data
>    and the residual count calculation is to be performed by the
> targets".)
>     doesn't check the total length of the data received when the
> completion
>    comes. Because it trusts the target and TCP.
>
> 4) the initiator thinks the READ is ok and in fact it is not.

This sounds like  extreme behaviour to me. Why is iSCSI attempting to
prevent initiators from score-boarding data ? This is a standard
practice most parallel scsi and FC initiators would follow to detect
cases of I/O underrun. Not doing this will cause the initiator to trust
the target's resid value and if there was an underrun, reporting success
back to upper layers can result in data corruption.

What is the justification for NOT allowing initiators to score-board ?


>
> To solve that, you propose the drop of  the connection in 2) ?
>

If this is the intention of the recommended error recovery, it is the
result of not allowing score-boarding. By score-boarding an initiator
would detect an underrun and would just error the affected I/O back. In
this case, all outstanding tasks on that connection are being affected.

Both the Format Error and Digest Error handling seem too extreme. A
format error or digest error recovery should only involve the affected
task and none others.

Thanks,
Santosh

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