Re: iSCSI:flow control, acknowledgement, and a deterministic recovery

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

Douglas Otis wrote:
> 
> Santosh said all commands could use a null CmdSN in his first
> statement.  Perhaps iSCSI should explicitly exclude this use.  This does
> imply there is no acknowledgements, no flow control, and no sequential
> delivery within iSCSI.

Doug,

What you state above is no different than a traditional SCSI transport
implementation. The acknowledgements, flow control and sequential
delivery properties are dervied from TCP. iSCSI behaves as an
encapsulation only. Most host O.S. stacks and data applications have no
expectations of strict ordering from the scsi transport. The QUEUE FULL
has served as a flow control mechanism in the past. 

IOW, simple implementations may choose to derive benefits from existing
mature TCP and SCSI algorithms rather than re-invent & re-implement all
of the transport capabilities within iSCSI.

There is no need to preclude implementations from sending all commands
with a 0 CmdSN. 

As for your second conern regarding I/O timeouts, there is no need for
any timestamp. An I/O timeout is dealt with by an Abort Task. The abort
task response guarantees that the abort reached the target and pushed
all intermediate stale frames. Failure to complete Abort Task leads to
higher level error recovery (ex : Logout, or some higher form of task
mgmt).

- Santosh
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