Re: iSCSI: Iterating long text responses

[Mark Bakke <mbakke@cisco.com>]

The "bookmark" solution that Julian had mentioned is
currently in the -98 version of the iSCSI draft at:

http://www.haifa.il.ibm.com/satran/ips/draft-ietf-ips-iSCSI-06-98.txt

This solution is more akin to #3, since the target does need
to keep track of bookmarks, and the initiator requests each
piece.

This doesn't seem quite as simple as #5, but it will work.  Simple
targets that will not return enough addresses to fill a 4k text
response PDU would not have to implement the bookmarks at all.

Anyway, this is a change from what we had been discussing on the
list, so it's worth looking at, and would perhaps be a good thing
to do a consensus call on so we can put this thing to rest either
way.

--
Mark

Julian Satran wrote:
> 
> I think that both 4 and 5 involve in fact some state to be kept at the
> target between PDUs sent in something related to
> to a "task control block" if we assume that all the text commands carry the
> same ITT.
> 4 enables the initiator to reuse its parse buffer while 5 requires the
> initiator to allocate a buffer for all the text responses (or keep the pipe
> closed).
> 4 is simpler than 5.  If you add to 4 a handle that the initiator has to
> give back the target next time (the bookmark)
> then the target does not have to keep state.
> A 0 bookmark says start from the top.  It is very much like an offset (that
> was mentioned) but it is generic and opaque.
> 
> Regards,
> Julo
> 
> Mark Bakke <mbakke@cisco.com> on 29-06-2001 23:18:20
> 
> Please respond to Mark Bakke <mbakke@cisco.com>
> 
> To:   IPS <ips@ece.cmu.edu>
> cc:
> Subject:  iSCSI: Iterating long text responses
> 
> Initiator developers-
> 
>    Please respond to the questions at the end.
> 
> Thanks,
> 
> Mark
> 
> The current iSCSI draft allows text command and response
> PDUs of up to 4096 bytes.  While we don't see any real
> problems for the command PDU size, commands such as SendTargets
> can easily exceed the response size.
> 
> There are several ways we can fix this.  The first two solutions
> require no differences in the current iSCSI text command and
> response; the latter three involve the use of the F bit.
> 
> 1. Assume that such commands are done on a "special" connection
>    or are handled completely in software, and allow its response
>    PDU to be as large as it needs to be.
> 
>    This one appears too restrictive to be a solid solution.  It
>    will also weaken any data digests done on the longer PDU.
> 
> 2. Create an iterative SendTargets key (and do the same for any
>    other text commands that need this), that would allow the
>    initiator to request the "next target" or "next address".
> 
>    This would work, but would require any new command that needed
>    a large response to implement an iterator.  It also means that
>    each part of the response from the iterator would have to fit
>    in the 4k PDU.
> 
> The remainder of these require that only one text command sequence
> be outstanding on a connection at a given time.  They use the F
> bit to indicate the last PDU of the sequence.  Note that connection
> allegiance is assumed for the entire sequence, and text commands
> are never retried on another connection; a new command is issued
> instead.
> 
> 3. Do a text-response R2T, where the initiator controls when the
>    next partial response is sent.  This would be more generic:
> 
>    I->T  Text Command (F bit set)
>    T->I  Text Response (first PDU, F bit cleared)
>    I->T  Text Command (with some indicator that we want more)
>    T->I  Text Response (next PDU, F bit cleared)
> 
>    ...
> 
>    I->T  Text Command (with indicator that we want more)
>    T->I  Text Response (last PDU, F bit set)
> 
>    The main problem with this is that the target must keep track
>    of the state of its responses; if the initiator just stops asking,
>    it may have to keep a buffered response around until it times out,
>    the connection is dropped, or another text command comes along.
> 
> 4. Allow multiple response PDUs to a single text command:
> 
>    I->T  Text Command (F bit set)
>    T->I  Text Response (first PDU, F bit cleared)
>    T->I  Text Response (next PDU, F bit cleared)
> 
>    ...
> 
>    T->I  Text Response (last PDU, F bit set)
> 
>    Basically, we are doing (3) without the R2T.  The initiator,
>    upon sending the text command, must be prepared either to
>    accept as many PDUs as come back, or throw them away if it
>    can't handle them.  This solution is a lot like #1, but with
>    the response spread across 4k PDUs.
> 
>    Also note that this (and the following scheme) avoid the problem
>    of the target keeping state; it sends ALL of the response PDUs
>    without the initiator asking for them.
> 
> 5. Do #4, but allow the initiator to specify the amount of data
>    it is willing to accept:
> 
>    I->T  Text Command (F bit set, AcceptResponseLength=4096)
>    T->I  Text Response (first PDU, F bit set, TotalResponseLength=12288)
> 
>    In the above example, we have created a new text command field:
> 
>       AcceptResponseLength
> 
>    And in the text response PDU:
> 
>       TotalResponseLength
> 
>    The initiator indicated it was willing to accept a 4k response.
>    The target returned the first 4k in the text response, but set
>    the F bit since it was at its limit.  It also returned a
>    TotalResponseLength field.  Since this was greater than the
>    AcceptResponseLength, the initiator knows the amount of buffer
>    space it will need to get the full response.  It can then choose
>    whether it will re-send the command, and if so, can give it a
>    large enough response length:
> 
>    I->T  Text Command (F bit set, AcceptResponseLength=12288)
>    T->I  Text Response (first PDU, F bit clear)
>    T->I  Text Response (next PDU, F bit clear)
>    T->I  Text Response (last PDU, F bit set, TotalResponseLength=12288)
> 
>    Note that most initiators will probably send an AcceptResponseLength
>    of the largest response they would normally accept, and that most
>    target responses will fit in one or a few PDUs anyway.
> 
>    #5 is really a compromise between #3 and #4; the target has the
>    benefit of being less statefull, and the initiator has the benefit
>    of controlling the amount of data it receives.
> 
> I would like to recommend either #4 or #5.  I think that #5 is
> probably the safest solution, but #4 may not be a problem for anyone.
> 
> Assuming that none of the implementors of initiators have a problem
> with #4, I would pick that.
> 
> If they do have a problem with it, we should go with #5.
> 
> Targets probably don't care much whether we do #4 or #5.
> 
> Initiator developers-
> 
>   Please indicate which solution (#4 or #5) appeals to you.
> 
> --
> Mark A. Bakke
> Cisco Systems
> mbakke@cisco.com
> 763.398.1054

-- 
Mark A. Bakke
Cisco Systems
mbakke@cisco.com
763.398.1054