RE: iSCSI: Flow Control

[julian_satran@il.ibm.com]

David,

In your example C1, C2, D1, D2 will always work (as will any ordered
combination).
The troublesome situation is C1, C2+D2 (immediate), C3  (window closed)
followed by an R2T for D1. This is why I think we should either explicitly
forbid immediate data if R2T is enabled or specify that this behavior may
get an initiator into trouble.

Julo

David Robinson <David.Robinson@EBay.Sun.COM> on 21/09/2000 03:13:05

Please respond to David Robinson <David.Robinson@EBay.Sun.COM>

To:   ips@ece.cmu.edu
cc:    (bcc: Julian Satran/Haifa/IBM)
Subject:  RE: iSCSI: Flow Control




> I'm not a s/w engineer, so I could be getting myself into trouble here,
> but I still see an issue....
>
> iSCSI will not know whether the next PDU in the TCP buffer is a command
> or data PDU until it actually reads the data.  iSCSI should NEVER stop
> reading
> data from TCP or else deadlock if the next message is a data PDU (as was
so
> eloquently explained to me in a prior message).  It must continue reading
> data (thus emptying the TCP receive window), and if the SCSI command
queue
> is full, then SCSI will respond with TASK SET FULL.  This works okay
> (depending
> on who you ask) in a parallel SCSI or FC world where latency is measured
in
> us,
> but in iSCSI it could be 100ms and 1000+ commands later before the
initiator
> ever finds out.

The TCP connection can have either a command or data.  In either
case reading a short fixed amount of data can determine if it is
one or the other. With a single connection I can only see deadlock
occuring if the initiator sends Command1, Command2, then Data1 on a single
command/data connection. With a single connection this seems like
a broken initiator as doing the right thing of C1, D1, C2 shouldn't
be any more expensive and it is definately safer. Well one other
case can occur is if the initiator wants the target to issue an RTT
and it interposes another command before the RTT is received, this
too seems like a broken things to do as I don't see much value in doing RTT
on a single connection.

In the multiple connection asymettric case this isn't an issue as data
is never interleved with commands.

So is there a fundamental requirement that the target must handle
the case of C1, C2, then D1? It seems easy to just write the iSCSI
initiator to never issue such a sequence and if the target gets
one return an error when C2 arrives before D1.

It again seems like we are trying to impose or solve datagram problems
in a stream world. Does parallel SCSI have this problem?

     -David