Subject: RE: iSCSI: definition of TSID (2.11.4 of draft 07)

From: "Jim Hafner" <[email-masked]>
Date: Wed, 29 Aug 2001 08:16:20 -0700

Mallikarjun,

There are two assumptions that go into the model and requirements (as I see
it).  One is currently implicit in draft-07 and is an iSCSI issue, the
other is a bit more explicit and is a SCSI issue with iSCSI implications.
These are

A) (Uniqueness of SIDs) Between a given iSCSI Initiator Node and iSCSI
Target Node there can be only one session with a given SID.  [This is
implied by the language that says that a login attempt that uses an
existing SID=ISID||TSID is an attempt to *add* a connection to the existing
session. (This is the response I got from Julian when I suggested making
this explicit.)

B) (No parallel I_T nexus) Between a given SCSI Initiator Port and SCSI
Target Port there can be only one nexus.  [With the definition of SCSI
Initiator Port as the endpoint of a session identified by iSCSI Initiator
Name + ISID, and with the definition of the SCSI Target Port as the target
portal group identified by the iSCSI Target Name + portal group tag, we get
the ISID RULE as stated (or intended) in the draft.]

The minimalist requirement is to meet the these two rules (assumptions).

There is a typo in the text you quote.  It should read "The TSID name space
of the iSCSI Target should be partitioned among the target portal groups".
But as we've discussed this really less of a requirement than it is a
desirable "note to implementers" (and I'm going to propose that this sort
of text move to that section).

In what you say below, the "no parallel nexus" (assumption B) is enforced
by the ISID RULE.  Assumption A (uniquely identified sessions) is enforced
if the target does the right thing when selecting a TSID *to pair with* an
ISID.  My text for the definition of TSID simply says that the target
should find a new TSID that enforces Assumption A (no more and no less).
All the stuff about partitioning name space belongs in "notes to
implementers".  So, between my proposed definition of TSID and the ISID
RULE, both assumptions are satisfied and the model works.  No finer
restrictions are required *by the model*.

Note that in all this description, we haven't had to provide a formal
definition of "nexus identifier".
The reason is two fold.  First, independent of what we might use to
identify them, the I_T nexus is defined as a relationship between a SCSI
Initiator Port and a SCSI Target Port.  Assumption B says we can't have two
such relationships active at one time.  As I've noted above, the ISID RULE
enforces that.  Second, the nexus identifier doesn't get used in SAM-2
explicitly anyway and in SAM-3 it will probably be "up to the transport to
define AND relative to the viewpoint of the initiator or target".  That
last part means that the nexus identifier may not need a globally unique
value, but may depend on whose end of the nexus you're looking from.  E.g.,
the target side need only track the "relative target port" and need not
care about the external address/name/identifier of that port when defining
its value for the nexus identifier.  In short, the nexus identifier issue
is tangential (and we can safely skip it formally).

Jim Hafner

[P.S. See also the proposed rewrite of the relevant draft sections in the
stuff I sent you off-line, as well, and see if that doesn't help clarify.]

"Mallikarjun C." <[email-masked]>@ece.cmu.edu on 08/28/2001 06:20:16 pm

Please respond to [email-masked]

Sent by:  [email-masked]

To:   [email-masked]
cc:
Subject:  RE: iSCSI: definition of TSID (2.11.4 of draft 07)

Jim,

Thanks for the explanation.

>The model says that the *combination of ISID || TSID* has to be unique
>between a given initiator and a given target.

Actually, I do not find this formulation stated anywhere
in rev07.

The closest about TSID assignment policy is this sentence
in section 1.5.3: "The TSID name space of the iSCSI Initiator
should be partitioned among the target portal groups."
My earlier comments were based on this quote.

More importantly, let me try to understand the reasoning
behind this formulation.  The only reason that I could come
up with is to preclude a "parallel nexus".  In our previous
conversation, we identified two types of nexus ids for this
enforcement - a)InitiatorName-ISID-TargetName-TargetPortalGroupTag
b)SSID-InitiatorName-TargetName.  By choosing the stated
formulation (in conjunction with the ISID RULE), are you
attempting to ensure that a parallel nexus cannot be created
if one goes with either definition?

Thanks.
--
Mallikarjun

Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions Organization
MS 5668   Hewlett-Packard, Roseville.
[email-masked]

>Mallikarjun,
>
>What you suggest is *more* than is required for uniqueness!  Again, that's
>an implementation choice that goes beyond the model.
>
>The model says that the *combination of ISID || TSID* has to be unique
>between a given initiator and a given target.
>When you say " If there are multiple sessions with an initiator, each
would
>anyway be assigned a different TSID"  you are assuming that the TSID alone
>provide the uniqueness (between a pair).   It can, but it's more than is
>necessary.
>
>In short, you're still selecting an implementation.   Here's two other
>implementations.  Suppose I have exactly one Target Portal Group.  Then I
>can meet the uniqueness requirements by (a) enforcing the ISID RULE and
(b)
>using TSID=1 *for every session*!  This one is "minimalist" with respect
to
>the model.  (Because there is only one portal group, I can leverage the
>uniqueness of the ISID to meet the general uniqueness rule -- it's very
>different if I have more portal groups (see note below)) The "maximalist"
>implementation with respect to the model is (a) enforce the ISID RULE and
>(b') use a unique TSID for each session I have regardless of initiator
>name!
>
>The spec should say what is required to implement the model.  In my mind,
>that's the rule that says the combination of ISID and TSID (the SID) must
>be unique between a pair (initiator and target).   And that's all I'm
>asking for in the definition of TSID.
>
>Jim Hafner
>
>Note: If I had more than one portal group, I might legitimately see the
>same ISID (from the same initiator) come in to each portal group.  In that
>case, I need to use a different TSID for each such occurrence.  If I say,
>for example, that each portal group always uses it's portal group tag as
>it's TSID (not required, just a choice), then I get uniqueness of SID's
>again.  This is again, a minimalist implementation with respect to the
>model.   This also gives an example of why the TSID namespace "should" be
>partitioned between the portal groups, so they can always be sure that
they
>won't accidentally create an SID that is the same as an existing one.  As
>long as the "half" of the SID one portal group generates is different from
>the half generated by another portal group, there will be no chance of
>collision.  [This is delegation of naming authority!!!]  The same
principle
>holds on the initiator side in how it generates ISIDs.
>
>Are we getting closer?
>
>"Mallikarjun C." <[email-masked]>@ece.cmu.edu on 08/28/2001 04:10:24 pm
>
>Please respond to [email-masked]
>
>Sent by:  [email-masked]
>
>
>To:   Jim Hafner/Almaden/IBM@IBMUS
>cc:   [email-masked]
>Subject:  RE: iSCSI: definition of TSID (2.11.4 of draft 07)
>
>
>
>Jim,
>
>I see your point about the current wording possibly suggesting
>an implementation option.
>
>Can I suggest the following text:
>
>  The TSID is a target-assigned tag which together with the InitiatorName
>  uniquely identifies the session within the target.
>
>It appears to me that it's all that's required for uniqueness, I
>don't think we need to mention ISID here.  If there are multiple
>sessions with an initiator, each would anyway be assigned a different
>TSID.  What am I missing?
>--
>Mallikarjun
>
>
>Mallikarjun Chadalapaka
>Networked Storage Architecture
>Network Storage Solutions Organization
>MS 5668   Hewlett-Packard, Roseville.
>[email-masked]
>
>
>>Marj,
>>
>>Comments in line (as usual...).  But for the person of few words, I'd
>agree
>>with just this text (your first sentence only):
>>
>>"The TSID is the target assigned tag for a session with a specific named
>>initiator
>>that, together with the ISID uniquely identifies a session with that
>>initiator."
>>
>>(I moved the word "name" around because the session is with the initiator
>>not with the name.)
>>
>>Jim Hafner
>>
>>
>>"KRUEGER,MARJORIE (HP-Roseville,ex1)" <[email-masked]>
>@ece.cmu.edu
>>on 08/28/2001 02:46:45 pm
>>
>>Sent by:  [email-masked]
>>
>>
>>To:   [email-masked]
>>cc:
>>Subject:  RE: iSCSI: definition of TSID (2.11.4 of draft 07)
>>
>>
>>
>>I'm curious why you've phrased it this way.  I think of it more like;
>>
>>"The TSID is the target assigned tag for a session with a specific
>>initiator
>>name that, together with the ISID uniquely identifies a session with that
>>initiator.
>><JLH>
>>I don't really see the subtle difference between this text and mine, so I
>>guess I don't have any problem with it.  At least it doesn't bind the
>>definition to anything SCSI'ish.
>></JLH>
>>
>>However, the target MUST enforce uniqueness of the SSID for a given I_T
>>combination (in order for login and recovery rules to make sense).
>><JLH>
>>I think I see what you're getting at here, but I don't think the words
>>capture it (it looks like an incorrect use of "I_T nexus" (a SCSI thing).
>>Do you mean:
>>
>>  "However, the target MUST enforce the uniqueness of the SSID [aside:
>>isn't this 'SID'?] for sessions with a given initiator."
>>
>>If so, I can go with this text, but I think this is stated in other
places
>>and need not be here.  But I won't argue if it is also here.
>></JLH>
>>
>>The TSID MAY be a unique tag for a session with a specific initiator
>name."
>><JLH>
>>I don't think this belongs at all.  It is an implementation statement and
>>so outside the scope.  It's a true statement, but I don't think it should
>>be made explicit (especially in this part of the document -- I'd have
less
>>problem with this in the "Notes to Implementers" but even then it is
>>suggesting a prefered(?) implementation).
>></JLH>
>>
>>I know the last "rule" is more for convenience than to follow the
>"modeling
>>rules" (but that's the most straight forward way to implement TSID
>>assignment?).  From the target viewpoint, it must first set the context
of
>>the initiator name, secondly the ISID.
>><JLH>
>>First, it's not necessarily the most straightforward way. It could
>generate
>>a unique TSID for every session regardless of initiator name.  That gives
>>it a twobyte pointer to the session context.  That's a lot more efficient
>>(perhaps) than having to also have an initiator name in that "pointer".
>>So, if you go with your implementation, you have to index on the
>name+TSID,
>>and that's too big, so you need an indirect pointer to the context and at
>>that point it doesn't really matter what the TSID is relative to other
>>TSIDs; you can use the name + joint SID=ISID||TSID as your index (it's
>only
>>2 bytes more out of a possible 258!).
>>
>>So, I argue for no statement about implementation options, leave that to
>>the clever programmers!
>></JLH>
>>
>>Marjorie Krueger
>>Networked Storage Architecture
>>Networked Storage Solutions Org.
>>Hewlett-Packard
>>tel: +1 916 785 2656
>>fax: +1 916 785 0391
>>email: [email-masked]
>>
>>> -----Original Message-----
>>> From: Jim Hafner [mailto:[email-masked]]
>>> Sent: Tuesday, August 28, 2001 12:59 PM
>>> To: [email-masked]
>>> Subject: iSCSI: definition of TSID (2.11.4 of draft 07)
>>>
>>>
>>> Folks,
>>>
>>> The following text defines TSID in referenced document (and with the
>>> changes proposed by Mallikarjun):
>>>
>>> "The TSID is a tag that identifies the SCSI initiator port.
>>> TSID is set by
>>> the target.  It MUST be valid only in the final response."
>>>
>>> I'm trying to figure out what the TSID has to do with the
>>> SCSI initiator
>>> port.  As I understand it, they are completely unrelated (at an
>>> architecture level).  In implementation may choose to use the
>>> TSID as a
>>> pointer to the session in a unique way across all sessions in
>>> the target or
>>> in a unique way across all sessions with a given named iSCSI
>>> initiator or
>>> it can do any number of other things.  It is, in my opinion,
>>> unrelated to
>>> the SCSI initiator port.
>>>
>>> I would propose the following alternative text:
>>>
>>> "The TSID is a tag set by the target that, together with the ISID,
>>> identifies a unique session with the initiator."
>>>
>>> Comments?
>>>
>>> Jim Hafner
>>>
>>
>>
>>
>>
>
>
>
>
>
>

---

From: "Mallikarjun C." <[email-masked]>
Date: Thu, 30 Aug 2001 21:57:09 -0700

Jim,

Sorry, I was travelling, so couldn't get back sooner.  Let me say first 
off that I am not disagreeing with you, only suggesting more explicit
wording.

I see that the text you sent offline does contain "assumption A" stated
explicitly - thanks.  I believe that's necessary since one could very
well
attach new connections to an existing SSID when its uniqueness is
maintained
between just the target portal group and the initiator. 

I am still though missing the rationale for mandating uniqueness of SSID
between the target (across all portals) and initiator.  I assume target 
knows the portal group based on which IP address the login arrives on - 
eventhough not explicitly carried in the payload.

Coming to my harping on the nexus identifier, there are two reasons -
    -  It seems to provide a nice rationale for the above issue (refer
       to my previous mail at the bottom on how).  Please set me
straight
       if I am totally ignoring something I shouldn't.
    -  You stated that iSCSI is the first transport which opens the 
       possibility of "parallel nexus" and so iSCSI mandates
implementations
       not to allow it - but appears to stop short of specifying an
       identifier/means to enforce this mandatory requirement.  May be
       it's just me who is troubled by this...

Regards.
-- 
Mallikarjun 

Mallikarjun Chadalapaka