Subject: [rddp] RE: [Tsvwg] Concerns about SCTP

From: Caitlin Bestler <[email-masked]>
Date: Thu, 9 Jan 2003 12:13:47 -0600

On 1/9/03, Williams, Jim wrote:

>
>Interestingly, though, RDDP has the property that all
>received data can be immediately processed.  There is
>never any need to stop the remote sender.  The only
>problem occurs when data comes in faster than it
>can be processed.  In this case the desired behavior
>is to slow down the sender a bit, not stop it.
>Therefore ECN might be a better functional fit
>for this requirement than rwnd.  
>
That property is intrinsic to RDDP no matter what transport
it runs over. The ULP is responsible for pacing untagged
messages, but only the LLP paces tagged messages.

RDDP releases all LLP buffers "immediately", meaning as
quickly as the placement of the tagged/untagged data can be
completed. Given that characteristic it is highly unlikely
that there would be buffering resources dedicated to
specific assocation or connection.

Therefore the challenge, whether over MPA/TCP and/or SCTP,
is to ensure that the internal bandwidth required to support
DDP placement will not be swamped by all incoming data.

Specifically the windows SHOULD be set so that the risk of
exhausting the placement bandwith is as low as possible.

However those windoes CANNOT beset so that said risk is
non-existent. LLP Credits are delivered asynchronously,
uncertainty on placemnt speed because of potential host bus
contention or retries, and more importantly in integral
amounts. Those integral amounts, spread over hundreds of
thousands of connections, either force a severe
under-allocation or an over-allocation.

And certainly the available to-host bandwidth over a
conventional host bus is statistically reliable but
defintely not guaranteed. Limiting your RNIC to the host bus
*guaranteed* rates would be a severe limitation.

I believe most implementers will properly choose to take a
slight risk of over-allocation, knowing that in the worst
case the result is that a packet is dropped. A properly
credited LLP packet arrives and there is no available buffer
for it. No LLP ack is generated, and it is eventually
retransmitted. The same thing can happen at any number of
routers and switches along the route for the very same
reason.

Also consider that in some cases the host memory placement
capacity may actually exceed the capacity of the external
link layer. In that case LLP flow control deals only with
network congestion, RNIC and host buffering is irrelevant.

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