Subject: [Tsvwg] IP Storage & multiple TCP connections

From: [email-masked]
Date: Thu, 22 Aug 2002 10:49:37 -0400

I thought this response to a private question may be of general interest:

>    Could you elaborate a bit on your statement: "all of the IP Storage 
> protocols can spread their traffic across multiple TCP connections, and 
> hence can keep congestion windows down in that fashion".
> 
> Is the purpose of spreading the data across multiple connections really to

> "keep congestion windows down", and if so, why is that a good thing?

Yes and no, the primary purposes/goals of using multiple TCP connections
are allowing a single IP Storage protocol session to span multiple
network interfaces for performance, load balancing, and (in some cases)
error recovery.  The ability to limit the size of congestion windows
for connections on the same interface is a secondary benefit of this
approach.  I believe there are similar things being done in the grid
community (e.g., GridFTP), but I leave it to someone more familiar with
that work to explain exactly what's happening there.

As for why it's a good thing, I recommend reading Sally's draft - the
essential rationale is that cutting an enormous congestion window in
half in response to a single packet drop is excessive (far more than the
network needs in the way of a congestion response to a single packet
drop) and makes it very difficult to run TCP at multi-gigabit speeds
with good utilization - one has to drop packets essentially never and
obtain link/node BERs (Bit Error Rates) that approach or exceed the
best the hardware engineers believe they can deliver.

Thanks,
--David
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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 249-6449            FAX: +1 (508) 497-8018
[email-masked]       Mobile: +1 (978) 394-7754
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