Subject: [rddp] Alignment considerations

From: [email-masked]
Date: Mon, 25 Nov 2002 18:30:55 -0500

Some comments on the alignment issue that seems to underly
a significant piece of the CRC discussion:

Somesh Gupta wrote:
> The question about whether alignment can be guaranteed or not
> is of course very important, and if alignment was guaranteed
> there would perhaps be not too much debate.

Alignment can not be *guaranteed* - all it takes is a resegmenting
middlebox to completely ruin one's day.  The actual question is
whether to design for unaligned reception as a common case (has
to go fast) or an exceptional case (just has to work).  For example,
Mike Krause clearly views this as an exceptional case:

> Endnode memory can be used as an intermediate 
> buffer to mitigate on-chip buffer requirements which is reasonable
> for this exception case [... snip ...]

Let me remind folks of what our AD said in Atlanta about convincing
the ADs that it's acceptable to assume alignment as the common case
(this is more of an issue about the MPA additions to the bytestream,
some of which [markers] are alignment-related) -- 
between the design of what is sent and how it is sent and analysis
of what happens when modified and unmodified senders communicate,
it must be all but impossible for an endpoint to get confused about
what's in the bytestream and do the wrong thing (unmodified endpoint
passes the stuff added to the bytestream upwards, modified endpoint
attempts to strip the added stuff when nothing was added).

SCTP will address this by exchanging an adaption indication as part
of connection setup - if the adaption indication isn't passed, DDP
(and hence RDMA) isn't supported and both ends know it.  MPA
currently brings up a TCP connection in unmodified mode and makes
synchronization of transition to modified mode wrt the bytestream
the ULP's responsibility.  That's somewhat more fragile and provides
opportunities for the ULP to get it wrong ... some of which may be
ok provided that it's easy for a ULP to get it right and the ULP
that gets it wrong can only shoot itself in the foot (as opposed to
shooting some other ULP's feet out from under it).

Thanks,
--David

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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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