Re: [storm] iSER - problem with unsolicited NOP-IN right after final Login Response

Michael Ko <mkosjc@gmail.com> Tue, 12 June 2012 02:00 UTC

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Date: Mon, 11 Jun 2012 19:00:35 -0700
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From: Michael Ko <mkosjc@gmail.com>
To: Mallikarjun Chadalapaka <cbm@chadalapaka.com>
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Cc: "michaelc@cs.wisc.edu" <michaelc@cs.wisc.edu>, "storm@ietf.org" <storm@ietf.org>, "ogerlitz@mellanox.com" <ogerlitz@mellanox.com>
Subject: Re: [storm] iSER - problem with unsolicited NOP-IN right after final Login Response
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I agree with Mallikarjun.  In the scenario described by Alex, the initiator
side was not compliant with the standard.

>From section 5.1.1 of the iSER draft on the initiator behavior during
connection setup:

"If the outcome of the iSCSI negotiation is to enable iSER-assisted mode,
then on the initiator side, prior to sending the Login Request with the T
(Transit) bit set to 1 and the NSG (Next Stage) field set to
FullFeaturePhase, the iSCSI Layer MUST request the iSER Layer to allocate
the connection resources necessary to support RCaP by invoking the
Allocate_Connection_Resources Operational Primitive."

It clearly states that the initiator must "allocate the connection
resources necessary to support RCaP" before sending the Login Request to
transition to FullFeaturePhase.  Had the initiator been compliant with this
requirement, it would not run into the insufficient Rx buffer problem.
Negotiating iSERHelloRequired to "Yes" is a way to shortcut this
requirement and buy time for the initiator if both sides support it, but it
should not be the modus operandi, and I don't think clarifying text is
needed to justify its usage to solve this particular problem caused by
non-compliance..

Mike
On Mon, Jun 11, 2012 at 11:56 AM, Mallikarjun Chadalapaka <
cbm@chadalapaka.com> wrote:

> Paul,
>
> I completely agree. This promise is conceptually similar to the CmdSN
> window promise. FWIW, I did not believe both sides were compliant in this
> case.
>
> Mallikarjun
>
>
> -----Original Message-----
> From: Paul_Koning@Dell.com [mailto:Paul_Koning@Dell.com]
> Sent: Monday, June 11, 2012 11:41 AM
> To: Mallikarjun Chadalapaka
> Cc: david.black@emc.com; mkosjc@gmail.com; nezhinsky@gmail.com;
> ogerlitz@mellanox.com; michaelc@cs.wisc.edu; storm@ietf.org
> Subject: Re: [storm] iSER - problem with unsolicited NOP-IN right after
> final Login Response
>
> If the standard says that the expected semantics is that the buffers
> promised are all there at end of negotiation, then yes, it's an
> implementation issue.  In that case, the original premise (both ends
> compliant) would not be true -- one end is in violation.  On the other
> hand, if the specification of the negotiation process doesn't say when the
> promised resources have to be available, that's a hole in the standard.
>
>        paul
>
> On Jun 11, 2012, at 2:32 PM, Mallikarjun Chadalapaka wrote:
>
> > Sorry, if the implementation is promising x number of unsolicited
> buffers but it is has <x buffers ready at the end of negotiation, I would
> consider the implementation to not comply with the standard anymore.
> Standard on its part should clearly define the semantics of the promise -
> which I presume it does in this case.
> >
> > I am all for providing helpful implementation guidance in the standard,
> but we have to be cautious and re-confirm the need for the guidance, when
> we start providing guidance down to latency numbers (see #2) - that's what
> got me concerned when I read the original note.
> >
> > Thanks.
> >
> > Mallikarjun
> >
> >
> > -----Original Message-----
> > From: Paul_Koning@Dell.com [mailto:Paul_Koning@Dell.com]
> > Sent: Monday, June 11, 2012 10:59 AM
> > To: david.black@emc.com
> > Cc: Mallikarjun Chadalapaka; mkosjc@gmail.com; nezhinsky@gmail.com;
> ogerlitz@mellanox.com; michaelc@cs.wisc.edu; storm@ietf.org
> > Subject: Re: [storm] iSER - problem with unsolicited NOP-IN right after
> final Login Response
> >
> > That sounds like the right way to look at it.  The whole point of
> protocol standards is to specify what is needed to interoperate.  Not more,
> but also not less.  Whenever two implementations both comply with a
> standard yet do not interoperate, that's a bug in the standard.
> >
> > paul
> >
> > On Jun 11, 2012, at 11:34 AM, <david.black@emc.com<mailtomailto:
> david.black@emc.com>>
> > <david.black@emc.com<mailto:david.black@emc.com>> wrote:
> >
> > Hi Mallikarjun,
> >
> > IMHO, the reason that this is a protocol issue is that two
> implementations that comply with the RFC can nonetheless reliably and
> repeatedly fail to interoperate because the lack of additional buffers
> causes the RCaP connection to close before full feature phase is reached by
> the initiator.
> >
> > I believe that we have a responsibility to tell implementers what can go
> wrong here and how to avoid it - the technique you describe (post all
> unsolicited buffers before sending final negotiation message) could be
> mentioned as part of this, and we should also describe what's possible with
> use of a single buffer, as that approach is being pursued as Alexander
> describes.
> >
> > Thanks,
> > --David
> >
> > From: Mallikarjun Chadalapaka [mailto:cbm@chadalapaka.com]
> > Sent: Friday, May 25, 2012 9:02 PM
> > To: Black, David; mkosjc@gmail.com<mailto:mkosjc@gmail.com>;
> nezhinsky@gmail.com<mailto:nezhinsky@gmail.com>
> > Cc: ogerlitz@mellanox.com<mailto:ogerlitz@mellanox.com>;
> michaelc@cs.wisc.edu<mailto:michaelc@cs.wisc.edu>; storm@ietf.org<mailtolto:
> storm@ietf.org>
> > Subject: RE: [storm] iSER - problem with unsolicited NOP-IN right after
> final Login Response
> >
> > I am curious to understand a bit more on why this is a protocol issue
> per se.
> >
> > Seems like one way to address this problem is via an implementation
> approach with the initiator posting in advance the negotiated number of
> unsolicited PDU buffers, at the same time it makes the (final) negotiation
> offer. As the in-bound unsolicited PDUs can technically arrive any time
> after the offer, due to standard network latency mechanics that Alexander
> summarized. Has that approach been considered?
> >
> > Mallikarjun
> >
> >
> >
> >
> > From: storm-bounces@ietf.org<mailto:storm-bounces@ietf.org> [mailto:
> storm-bounces@ietf.org] On Behalf Of david.black@emc.com<mailtolto:
> david.black@emc.com>
> > Sent: Thursday, May 24, 2012 12:03 AM
> > To: mkosjc@gmail.com<mailto:mkosjc@gmail.com>; nezhinsky@gmail.com
> <mailto:nezhinsky@gmail.com>
> > Cc: ogerlitz@mellanox.com<mailto:ogerlitz@mellanox.com>;
> michaelc@cs.wisc.edu<mailto:michaelc@cs.wisc.edu>; storm@ietf.org<mailtolto:
> storm@ietf.org>
> > Subject: Re: [storm] iSER - problem with unsolicited NOP-IN right after
> final Login Response
> > Importance: High
> >
> > Mike (Ko) and Alexander,
> >
> > Mike is of course correct that iSER Hello usage can be forced by
> negotiating iSERHelloRequired to "Yes".  However, existing implementations
> are likely to reply with iSERHelloRequired=NotUnderstood, so we do need to
> specify what should be done in order to interoperate with an implementation
> that refuses to deal with the iSER Hello exchange.
> >
> > I think the situation that Alexander described should be documented in a
> new section 5.1.4 of the iSER draft.  My general rule of thumb on this sort
> of surprise found by implementers in the "running code" is that it
> indicates that something is missing in the spec.  I believe that Alexander
> has described the solution - more below.
> >
> > The new section 5.1.4 (suggested section title: Omission of the iSER
> Hello Exchange) should describe default omission of the exchange, use of
> iSERHelloRequired key to omit the iSER Hello exchange, and the consequences
> of target use of unsolicited PDUs after login when the exchange is omitted,
> including IB's use of NOP-IN (as a keep-alive measure, right?)
> >
> > The crucial requirements points that I take away from Alexander's
> description are that if the iSER Hello exchange is omitted, then:
> >
> > 1) The target MAY send *one* unsolicited PDU immediately after sending
> the Login Response.
> >
> > 2) The target MUST wait at least 200ms (use some other number if 200ms
> isn't a good choice) or until it receives a full feature mode PDU from the
> initiator before sending a second unsolicited PDU in order to ensure that
> initiator has sufficient
> >      time to allocate the full feature buffer resources for the
> connection.
> > 3) The initiator SHOULD allocate at least one additional buffer for use
> during login (so that at least two buffers are in use during login) in
> order to receive an unsolicited PDU that may follow login completion.
>  Failure to allocate this second buffer may cause connection termination if
> no buffer is available when an unsolicited PDU arrives.
> >
> > Both Mike and I are on vacation, so it may be a few weeks until we can
> agree on the new text and get a -12 version of the draft with that new text
> submitted.  In the interim, I've asked our AD (Martin Stiemerling) to hold
> off on further processing of the iSER draft until a -12 version with this
> new text is submitted.  I'd prefer to work this text out now rather than
> deal with it as an IETF Last Call comment - as the problem turned up in
> actual implementations, I think it's worth the extra month that it's likely
> to take to get correct text on how to avoid the problem into the draft.
> >
> > I'd suggest that Mike Ko post an initial draft of the text for the new
> section 5.1.4 to the list when he resurfaces ...
> >
> > Thanks,
> > --David
> >
> > From: Michael Ko [mailto:mkosjc@gmail.com]<mailto:[mailto:
> mkosjc@gmail.com]>
> > Sent: Monday, May 21, 2012 10:23 AM
> > To: Alexander Nezhinsky
> > Cc: storm@ietf.org<mailto:storm@ietf.org>; Black, David; Or Gerlitz;
> Mike Christie
> > Subject: Re: iSER - problem with unsolicited NOP-IN right after final
> Login Response
> >
> > Alex,
> >
> > The iSER Hello support has never been removed in the latest spec.  Only
> its use is made optional.  So during login negotiation, just negotiate
> iSERHelloRequired to Yes.
> >
> > Mike
> > On Mon, May 21, 2012 at 6:15 AM, Alexander Nezhinsky <
> nezhinsky@gmail.com<mailto:nezhinsky@gmail.com>> wrote:
> > Hi
> >
> > I understand that it is a bad timing for sending this kind of mail, now
> that iSER draft was submitted, but actually we still have a small problem.
> > It is related to the final Login Response handling and the transition to
> Full-Featured phase on the initiator side in Infiniband setups.
> >
> > When the target receives the final Login Request it send the final Login
> Response and from its perspective the connection is now in Full Featured
> Phase (assuming that it agreed to transition in the Login Response being
> sent).
> >
> > This means that the target is ready to accept SCSI commands, Text
> Requests etc. sent by the initiator.
> > It also means that the target is eligible to send some unsolicited PDUs,
> notably unsolicited NOP-INs.
> >
> > With IB sending NOP-IN periodically is the easiest (an almost only
> feasible) way to determine closed connections reliably, because this kind
> of error is delivered to user only in response to a previously initiated TX
> operation.
> >
> > This leaves the initiator in a dubious position. It posts its RX buffers
> for that connection only when the final Login Response arrives. But during
> that time (after the target had sent the Last Login Response but before the
> Full Featured phase related RX-buffers are posted on the initiator side)
> the target may send the first NOP-IN as it considers the connection in Full
> Featured phase already and NumOfUnsolicited PDUs accounting for NOP-INs has
> been negotiated to a non-zero value.
> >
> > If the initiator works with a single RX-buffer posted during the entire
> login phase (which is a logical thing to do judging by the login exchange
> protocol) then an error occurs, as no buffers are posted when the NOP-IN
> arrives and the connection is shut down.
> >
> > Posting a single extra buffer before sending the last Login Request only
> alleviates the problem. Although this often solves it in practical terms
> (as the target most probably sends the next NOP-IN only after some timeout
> period measuring seconds or hundreds of milliseconds), it does not solves
> it in terms of protocol completeness, as the target MAY theoretically send
> more than one NOP-IN until FF buffers are posted.
> >
> > This issue was encountered recently in linux iscsi/iser initiator and
> the above solution has been applied to solve it against the existing target
> implementation (STGT), but the initiator remains exposed to this kind of
> errors.
> >
> > The solution is actually quite simple (theoretically) - if we bring back
> the requirement for iSER Hello exchange then the iSER assisted Full
> Featured phase does not commence until HelloReply PDU arrives at the target
> and the initiator has a definitive point in time when it can safely post
> its RX buffers - after the final LoginResponse returns but before it sends
> iSER Hello PDU.
> >
> > In practical terms it means that iSER Hello support requirement should
> be brought back to spec, which is a hassle.
> >
> > Should we decide on this now?
> >
> > Alexander
> >
> > P.S. : Thanx to Mike Christie and Or Gerlitz, the maintainers of linux
> iSCSI and iSER initiator for raising the issue.
> >
> > _______________________________________________
> > storm mailing list
> > storm@ietf.org<mailto:storm@ietf.org>
> > https://www.ietf.org/mailman/listinfo/storm
> >
> >
> >
>
>
>
>