RE: iSCSI: reusing ISID for recovery

[Venkat Rangan <vrangan@rhapsodynetworks.com>]

Steph,

This is a long thread, almost as long as the thread "iSCSI: response to
second login (with same ISID)" we had a few months ago. One aspect from the
other thread that may not have been mentioned is the fact that you could
have a situation where a rogue software on the initiator running in user
mode which simply issues ISID=existing, TSID=0 to the target. (This may be
what was meant by "wild closing of connections" in this thread.) Choosing
option A) would destroy an otherwise well protected ISID managed in the
kernel space. This could also occur when you accidentally start another
application that does not coordinate its ISID selection.

The earlier thread was leaning towards "Reject the second login attempt with
In-Use error code" but the issue of targets cleaning up inactive and failed
sessions still lingers on.

Some observations:

1. There is no way to protect against rogue software at the intiator; even
the best IPSec policies and key management can not prevent such software
from disturbing other well-behaved components. So trying to design
mechanisms to prevent this at the target would be fruitless.

2. There is some value in protecting an accidental use of an existing ISID
by a second client (ISID=<existing>, TSID=0). This can be done with a Reject
of the login attempt, even without verification of login credentials - (less
of a DOS attack). You also do not need to analyze connection states in this
case.

3. From the perspective of the Target, case 2) above is no different from an
initiator rebooting and accidentally choosing an existing ISID with TSID=0.
To distinguish 2) from 3), we can make use of the Clear bit (or the C-bit)
and avoid many unnecessary trips at reboot time, trying to probe for an
unused ISID. The usage is that if C-bit is present, any previous sessions
are also closed by the target. I think this was Option B) in the original
email.

4. If initiators always set the C-bit, then they can't benefit from the
duplicate session identification capability of 2). It is a bad initiator in
that it doesn't respect the presence of other sessions at the initiator
system, but the protocol can't bloat handling these cases of bad initiators.

5. If an initiator only implements mandatory session recovery, it can help a
target recover faster by explicitly logging out the previous session and use
same or different ISID with TSID=0 and an C-bit to really ensure that it can
recover on a new session. This is a fast operation because we are not
waiting for lengthy round trips or timeouts, and the presence of the C-bit
makes the intent clear to the target that it must wipe off the existing
session. The perceived delay noted in Option C) does not arise.

6. If a target determines that all connections in a session are dead (TCP
keepalives can help here), and there is no activity on a session, it can
simply close the session and release the ISID and the TSID and recover
target side resources. This is not driven be explicit protocol action from
initiator, but simply target timer based, and is designed for initiators
that disappear without cleanup of sessions and don't reappear (no reboot for
a long time).

With this limited understanding, my preference would be Option B). One could
argue that B) offers protection only against an accidental use of the same
ISID, but if (as was pointed out earlier and in the other thread by Nick),
there are multiple ways in which iSCSI applications are packaged, so that
there is no shared visibility into ISID space, the protection offered by B)
can be useful.

Venkat Rangan
Rhapsody Networks Inc.
http://www.rhapsodynetworks.com


-----Original Message-----
From: Stephen Bailey [mailto:steph@cs.uchicago.edu]
Sent: Wednesday, August 29, 2001 1:53 PM
To: Santosh Rao
Cc: ips@ece.cmu.edu
Subject: Re: iSCSI: reusing ISID for recovery 


> The same was earlier proposed by myself in a previous thread on this
> issue a few months ago.

You're not the only one.  I'm also in favor of (A).  I have also
carefully laid out the chain that Marjorie's following on at least one
(and probably several) past occasions.  It seems sound to me.  An
initiator that has lost its mind (this might be an OS reboot, or
`simply' a fat adapter crash & restart) is going to want to stomp out
any existing sessions in its name.  It's not going to want to pick up
old context unless it's aware that there IS old context, in which case
it hasn't completely lost its mind.

I've been keeping my mouth shut in hopes that it would actually pass,
but I guess I don't have to articulate a position so much as just hold
it to ensure it gets killed.  Given my track record, I'm now for sale
to the highest bidder.

Steph