Status summary on multiple connections

[julian_satran@il.ibm.com]

Dear colleagues,

I am attempting to summarize where we stand with regard to the multiple
connection issue
and to the two possible models - Symmetric (S) and Asymmetric (A).

Many of us feel strongly that the multiple connection issue is central to
the whole design
and cannot be added as an afterthought.  Moreover designing the hooks to
allow it later
will certainly already force us to make a decision. And both the hardware
and the software designers will be ill-served if we hand them a half-backed
solution.

However this is not an invitation to reiterate positions that where already
stated.
If you feel that I have grossly misstated anything in this note please
write me and use
the mailing list only if my answer is not satisfying.

And yes - like the chairman - I would like to make progress but I don't see
any way to
do it without satisfactorily closing this issue.

The reasons for multiple connections where discussed at some length and
where very nicely summarized in a series of notes by Michael Krause
(beginning of August).

The core reasons for having multiple connections where the need for more
bandwidth and availability than a single link can supply with a level of
complexity affordable for simple
installations and with a traffic engineering and management clearly
separated from the transport users (SCSI).

The session is embodying this requirement.

The only major objections I have heard against this where those requiring
to go all
the way in having one TCP connection/LU - and after a short debate this
objection
was practically removed.

The other objections we heard where that this is basically a transport
issue and it should be
solved at transport level.
That might be true - but since many, if not most, of TCP applications do
not have this
requirement it is highly unlikely that TCP is going to do connection
trunking in the foreseeable
future.

iSCSI can be designed to use multiple TCP connections in one of two ways:

-Asymmetric - one TCP flow only carries commands the others carry only data

-Symmetric - every flow carries both commands and their associated data

The S version is designed in the I-D version 01
The S version requires an command ordering scheme and that is provided by a
command counter and a sliding window scheme. It was argued that ordering
needs might be more prevalent than usually thought and a good conservative
design should preserve ordering.
Ordering-per-LU (as it is designed in FCP-2 version 4)  was considered
impractical as it
required initiators to maintain state for each LU - while the rest of the
design required initiators only to maintain state for outstanding commands.
Several comments on this list suggested that this windowing mechanism could
also be used as a command-flow-control mechanism and that is a "bonus" of
the scheme.

The A version - comes in two flawors:

- pure A (PA) in which ONLY commands  flow on one TCP connection while data
flow
   on DIFFERENT connections (with only one data connection being selected
for a
   command);   this scheme requires a minimum of 2 TCP connections although
not
   necessarily on  different physical links.

- collapsed A (CA) in which commands flow one a single TCP connection while
data
   flow on ANY connection; this scheme requires as a minimum 1 TCP
connection.

Here is first attempt to list the benefits and drawbacks of all of them:


- S - benefits
        - well understood
        - simple hardware setup and/or TCP API activation
        - window mechanism can be also used for flow control
        - the minimum required is a single TCP connection

     - drawbacks
       - need to maintain a window mechanism
       - a multiplexing mechanism has to be carefully crafted to avoid
          closing TCP windows to severely affect command flow and
performance


   - PA - benefits
          - TCP will both order the commands and provide for flow control
             through the TCP window mechanism
          - the multiplexing mechanism is simpler as closing TCP windows
will
             never affect command flow
          - data flow can use a streamlined header (and processing)

        - drawbacks
           - more complex hardware and/or software API activation
           - need for a minimum of two TCP connections (not links)

    - CA - benefits
            - TCP will order the commands
            - needs a single TCP connection as a minimum

       - drawbacks
           - more complex hardware and/or software API activation
           - need for a minimum of two TCP connections (not links)
           - if command flow control is required counters and the sliding
window mechanism
              are required
           - a multiplexing mechanism has to be carefully crafted to avoid
              closing TCP windows to severely affect command flow and
performance


From the above it should be apparent - as was already pointed out by Matt
Wakeley -
that the CA inherits the drawbacks of S and PA and as such the choice is
really
between S and PA.

Julo