Subject: [rddp] MPA draft serious issues

From: [email-masked]
Date: Fri, 6 Dec 2002 02:35:55 -0500

Some suggestions about the serious technical
issues I pointed the WG at yesterday.  These are with my WG
Chair hat off - take them or leave them as you see fit.

(1) Do middleboxes justify the marker impact on the design?

Assertions of what hypothetical middleboxes could do 
don't seem particularly helpful, as the real issues are
what middleboxes do in practice and how common that sort of
(mis)behavior is likely to be - are these going to be more like
NATs, whose widespread deployment has forced protocols such
as IPsec to cope with them despite the ugliness of doing so, or
more like a hypothetical router that aggressively and arbitrarily
reorders traffic passing through it?  The latter would never be
deployed to any serious extent due to its awful interactions with
TCP, and hence isn't worth doing serious protocol design for.

While I'm not thrilled about it, following SCTP's approach
of using IP fragmentation when forced to retransmit a framed
ULPDU that won't fit into a reduced PMTU is not out of the
question if maintaining alignment for the benefit of receivers
is really important.

(2) Marker impact on software - 512-byte interval issue

The assertion of no software impacts here doesn't strike
me as credible, as among the reasons iSCSI markers survived
was that both the use of markers and the marker interval
were negotiable in a fashion that enabled implementation
control or elimination of undesirable impacts.  I sincerely
doubt that mandatory to use 512-byte interval markers would
have made it through the IP Storage WG, and I think it would
be closer to the truth to characterize iSCSI markers as
having made it, but not by much - I definitely would not
look to them as a ringing endorsement of the marker approach.

Understanding the implications of the Tunable Marker
Intervals ideas would be a good next step here, as they
at least control receiver impacts.

(3) Packing prohibition.

I don't see any defenders of the current prohibition (yet), but
it's too early to think about calling consensus.

(4) Assurance of no confusion about presence of framing.

The "gold standard" here is the SCTP adaption indication -
if the two ends of the connection don't agree on the usage
of DDP or RDMA, the result is no connection.  At the moment,
MPA depends on the ULP to get this right, including when to
turn MPA on after a connection has been established, so every
time a new ULP is developed on MPA, there's a new opportunity
to get this wrong.  Asserting that MPA is a ULP to TCP (and hence
nothing needs to be done here) is unlikely to fly unless one
is prepared to abandon the current link between markers and
PDU alignment at TCP senders.

Thanks,
--David

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