iSCSI: Iterating long text responses

[Mark Bakke <mbakke@cisco.com>]

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