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

<david.black@emc.com> Tue, 12 June 2012 01:44 UTC

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From: <david.black@emc.com>
To: <cbm@chadalapaka.com>, <Paul_Koning@Dell.com>
Date: Mon, 11 Jun 2012 21:44:24 -0400
Thread-Topic: [storm] iSER - problem with unsolicited NOP-IN right after final Login Response
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Subject: Re: [storm] iSER - problem with unsolicited NOP-IN right after final Login Response
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> FWIW, I did not believe both sides were compliant in this case.

There's an additional consideration here - one of the goals of the iSER
update is to match "running code", and we have already made a number of
changes that diverge from RFC 5046 for this reason, including in this
area of iSER start-up (see Appendix A of the iSER draft for details).
As a result, we are  already well into non-compliance territory, as
omission of the Hello messages does not comply with RFC 5046, but those
messages are simply not implemented in practice.

The implementation under discussion is (I believe) the predominant iSER
implementation and hence I think the new iSER RFC needs to explain what
that implementation has done (even if it may not be what should have been
done), and (more importantly) explain how to interoperate.  The key
requirement is the following (IMHO, a fine example of "be conservative
in what you send"):

> > 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.

Not putting this into the iSER update requires either that Hello messages
be implemented (not done, primarily for optimization reasons), or that the
initiator effectively commit the buffer resources when starting negotiation.

Thanks,
--David

> -----Original Message-----
> From: Mallikarjun Chadalapaka [mailto:cbm@chadalapaka.com]
> Sent: Monday, June 11, 2012 2:57 PM
> To: Paul_Koning@Dell.com
> Cc: Black, David; 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
> 
> 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<mailto: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<mailto: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<mailto: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<mailto: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
> >
> >
> >
> 
>