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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: iSCSI: Partial Session ConsensusDavid, Counters as an option is quite ugly and violates some of our design rules - that said that a protocol analyzer capturing the traffic on the wire will be able to read headers starting from any arbitrary point. We can add a next header thing or a flag - but I did not hear explicit objections to the counters. And if you come to think about it most implementations will want to support everything so they will have the counters implemented and we will have the added complexity of a next header field or 2 lengths for headers - none too appealing. I suggest we wait a couple of days? Julo Black_David@emc.com on 12/09/2000 19:24:25 Please respond to Black_David@emc.com To: ips@ece.cmu.edu cc: (bcc: Julian Satran/Haifa/IBM) Subject: iSCSI: Partial Session Consensus In reading the mailing list traffic on Asymmetric vs. Symmetric models for multiple TCP connections in an iSCSI session, I don't believe that there is consensus on the issue phrased in that fashion. I believe that there is consensus on a couple of underlying issues: (1) An iSCSI session containing a single TCP connection should not be required to use the currently specified iSCSI command reference numbers and sliding window mechanism because TCP will deliver commands in order. (2) Use of more than one TCP connection per iSCSI session is OPTIONAL. The consensus on (1) is rough; if anyone disagrees with this for a reason other than wanting to use the Symmetric model when an iSCSI session contains multiple TCP connections, please say so on the list. My reading of consensus is based on the fact that much of the recent discussion of the Asymmetric model has been motivated by single connection sessions, whereas essentially all of the recent discussion of the Symmetric model seems to have been focussed on multiple connection sessions. The consensus on (2) is sufficiently long standing to be a closed issue. The resolution to the "deadlock" scenarios posted by Costa is that the target must not stop reading from the TCP connection - SCSI provides means for a target to throw away things it can't deal with (e.g., TASK SET FULL), and with regard to the ordered command issues, I believe Steve Byan is correct when he says: I think this is a T10 bug, and iSCSI should defer to T10 to fix it. In addition to Steve noting that disks tend not to use ordered commands, I would note that a single tape device/drive tends not to have multiple simultaneous initiators sending commands to it, both of which limit the practical impact of this potential problem. I don't see consensus on the model for iSCSI sessions that contain multiple TCP connections. The consensus on (1) above for single TCP connection sessions does not take the Symmetric model out of consideration for multiple TCP connection sessions. If multiple sessions are negotiated on connection establishment, command reference numbers could be added to subsequent headers as a result of that successful negotiation. In order to make progress, this additional complexity should not be used as an argument against the consensus in (1) for single connection sessions. I hope the consensus on (1), or something close to it holds, as if it does not, we may have to form an offline design team or teams to work on this set of session issues, and that could take some time ... meanwhile, discussion of Asymmetric vs. Symmetric for multiple connection sessions should continue. --David --------------------------------------------------- David L. Black, Senior Technologist EMC Corporation, 42 South St., Hopkinton, MA 01748 +1 (508) 435-1000 x75140 FAX: +1 (508) 497-8500 black_david@emc.com Mobile: +1 (978) 394-7754 ---------------------------------------------------
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