Re: Performance of iSCSI, FCIP and iFCP

[Stephen Bailey <steph@cs.uchicago.edu>]

Victor,

> I have to admit that we did not have the model to support
> numerically our contention that multiple links will behave better in
> the presence of errors and performance will degrade more gracefully
> even on congestion.

The last time we had this discussion (multiple links between the same
endpoints enabling higher performance) on this list, it was my opinion
that doing this is not `TCP friendly', and so should not be allowed in
the general, Internet context, which is one of the design points for
iSCSI.

Clearly, when slow starting, and recovering from congestion, n
connections can effectively open the window n times as fast as 1
connection, but doing so will potentially be at the expense of other
competing flows which don't use multiple connections.  If ALL flows
use multiple connections, you simply end up hitting the congestion
wall harder, which, in an abstract sense, makes the system more likely
to oscillate (or collapse?).

I don't know if it WILL create oscillation or collapse (I'm just a
hammer mechanic), but it seems to me that if hitting the congestion
wall harder in this way were acceptable to overall network behavior,
the single connection TCP congestion avoidance could be adjusted to
create this behavior (and capture this benefit) without requiring
multiple TCP connections.

Your refereed, published work on this seems suggests that I am holding
on to `myths' about the need to play fair with TCP.  If so, I would
greatly appreciate your debunking my myths here on this list, in the
clearest way.  Particularly, we need to know if the IETF (or IESG or
IAB, or whomever it is....), will accept this type of behavior out of
iSCSI.

While I am opposed to iSCSI's multiconnection session, I do admit its
benefits.  My objections are that the feature will not be widely used,
is difficult to implement correctly, AND the same benefits are already
available through long-standing upper layer mechanisms.  That aside, I
accept the WG consensus on multiconnection sessions, but what the
iSCSI spec still needs to do, in no uncertain terms, is indicate
whether multiple connections per endpoint pair is something which
implementations SHOULD or SHOULD NOT allow for use in a general,
Internet context, or at all.

Even if iSCSI specifies that multiple connections per endpoint pair
SHOULD NOT be used in a general context, we know that implementations
are not going to prohibit it (it's primarily a configuration
decision), and that still allows the feature to be used in
specifically engineered applications, such as those built with
dedicated storage fabrics.

Thanks,
  Steph