Subject: [rddp] Alignment Consensus attempt

From: [email-masked]
Date: Wed, 22 Jan 2003 17:29:08 -0500

With my WG chair hat (and Kevlar[tm] suit) on, here's
an attempt to call consensus on the alignment issue.

My original post on January 2nd phrased the primary issue as:

  So, I believe the primary issue before the WG is whether to accept:
	- The RDDP protocols must allow implementations over
		unmodified TCP/IP stacks.
  as a requirement.

Based on discussion since then, I believe the rough consensus
of the RDDP WG is to accept this as a requirement.

Restating the implications from that original message, (I've rephrased
the second bullet to discuss RDDP [DDP/RDMA] operations rather than
placement), some important things that this implies are:
	- Unmodified means 100% unmodified, not even "minor" changes.
	- An RDDP sender using an unmodified TCP stack can send RDDP
		payloads that result in the correct execution of RDDP
		operations by an RDDP receiver even if the sends are
		not aligned.
	- There don't appear to be any receiver implications, as
		nothing has been proposed that would prevent a receiver
		from having TCP buffer as usual and then run RDDP
		functionality on the resulting in-order TCP bytestream.
	- The practical meaning of "allow" is that a sender
		implementation is possible without violating any
		MUST, MUST NOT, SHOULD, or SHOULD NOT requirement.

OTOH, conditional requirements for stack modification are not ruled
out.  For example: "If a sender notifies a receiver that the sender is
operating in alignment-modified-mode by <means TBD>, then the sender
SHOULD ..., MUST ..., etc."  If this sort of approach is followed,
the second bullet still applies at the receiver, e.g., an unmodified
sender would not use <means TBD>, but would still be able to send
RDDP payloads that invoke RDDP operations at the receiver.

Beyond this, I also believe there is rough consensus for the RDDP WG
to *not* set requirements (MUST/SHOULD) for receiver performance
when the sender does not align traffic.  It is apparent to me from
list discussion that different implementers favor rather different
implementation approaches that have different performance consequences
for this scenario.

Repeating Jim Williams's spectrum of possibilities:

>     1.  Only functional correctness matters, performance is not an issue.
> 
>     2.  Performance must be as good as conventional (non-RDMA) TCP.
>
>     3.  Performance must be significantly better than conventional
>         (non-RDMA) TCP otherwise RDMA should not be used in this case.

If this second consensus holds, the WG chair intends to hold the WG to
an approach of avoiding design approaches and decisions that have the
effect of ruling out any of the three alternative views listed above.
This should have the effect of broadening the applicability of the
WG's protocols.

Nonetheless, I think the WG's results should capture some of the
list discussion about scenarios that involve unmodified stacks,
including when loss of alignment in such stacks is unavoidable
despite reasonable efforts by an application/ULP (not including
stop-and-wait), as it'll provide some useful insight to implementers.

Anyone who disagrees with either of these consensus calls
should explain why on the list.

Thanks,
--David

----------------------------------------------------
David L. Black, Senior Technologist
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953             FAX: +1 (508) 293-7786
[email-masked]        Mobile: +1 (978) 394-7754
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---

From: "Somesh Gupta" <[email-masked]>
Date: Thu, 23 Jan 2003 10:55:02 -0800

David,

There are many standard track protocols over TCP that
have framing/record delineation. NFS and iSCSI probably
form the two ends in terms of complexity/features.

I am sure many of us would like to see a standards track
TCP framing.

Is there consensus that the TCP framing protocol be such
that it can be standards track?

OR

Can we have two TCP framing protocols, one standards
track and one informational?

Is there a process for requesting a standards track
framing if MPA remains as an informational RFC?

Thanks,
Somesh

---

From: Caitlin Bestler <[email-masked]>
Date: Fri, 24 Jan 2003 12:04:40 -0600

On 1/22/03, [email-masked] wrote:

>
>Beyond this, I also believe there is rough consensus for
>the RDDP WG to *not* set requirements (MUST/SHOULD) for
>receiver performance when the sender does not align
>traffic.  It is apparent to me from list discussion that
>different implementers favor rather different
>implementation approaches that have different performance
>consequences for this scenario.
>
I believe it would be appropriate for the WG to document an
expectation that use of RDDP over unaligned MPA/TCP will not
result in a signifigant performance penalty when compared to
the ULP simply using TCP itself.

My understanding of "SHOULD" is compatible with that type of
goal, but ultimately I do not care if it is a MAY/SHOULD or
a "hey guys, its a real cool idea if".

The point is not to define an IETF approved performance test
suite, but rather to provide some simple guidance to
application developers and those deploying networks.

Caitlin Bestler
http://asomi.com/CaitlinBestler/
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