Subject: [Tsvwg] RE: [rddp] I-D ACTION:draft-williams-iwarp-ift-00.txt

From: "Williams, Jim" <[email-masked]>
Date: Tue, 22 Oct 2002 14:48:43 -0400

Regarding separation of header and payload CRCs:

Paul Culley wrote:

> It was observed that a NIC must buffer the entire packet before it 
> can be used in any way if the Ethernet CRC and IP/TCP checksums are 
> checked before use.  Most NICs have macros that check this stuff 
> before any layer 4 or higher processing is started. The NIC 
> vendors found that adding CRC on the whole packet to this scheme 
> offered no disadvantage over separate CRCs.  They would not be 
> able to do a "flow-through" design anyway.

Thanks for your response, Paul.

I agree that if DDP PDUs are always aligned with TCP PDUs, then
the CRC can be checked before any layer 4 processing is done.
If, however, DDP PDUs are not necessarily aligned, then layer 4
processing must be done first to determine where the CRC computation
begins and ends, and fetch the starting seed if a partial CRC has
already been computed.  When the payload is split between two
TCP segments, placement of the beginning of the payload will
ideally begin before the end of the payload and 
associated CRC has been received.

So the critical question is should we rely on the remote end
to always align DDP PDUs to TCP PDUs?  TCP does not by default
do this.  Certainly TCP implementation can be easily modified
to do this.  

I am not arguing that TCP implementations should or should not
be so modified.  However the addition of a second CRC is 
relatively low incremental cost even if the remote TCP does
alignment, and it is a big win if the remote TCP does not
do alignment.  It seems desirable not to be dependent on
the remote end supporting a modified TCP implementation,
especially when the cost of avoiding this dependency is low.

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---

From: Caitlin Bestler <[email-masked]>
Date: Tue, 22 Oct 2002 13:59:57 -0500

On 10/22/02, Williams, Jim wrote:

>
>I am not arguing that TCP implementations should or should
> not be so modified.  However the addition of a second CRC
> is relatively low incremental cost even if the remote TCP
> does alignment, and it is a big win if the remote TCP
> does not do alignment.  It seems desirable not to be
> dependent on the remote end supporting a modified TCP
> implementation, especially when the cost of avoiding this
> dependency is low.
>

If the remote TCP does not do alignment, OR an intermediate
middle-box breaks that alignment. The latter problem is what
cannot be avoided.

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