Re: Concensus Call on Urgent Pointer.

[julian_satran@il.ibm.com]

Clive,

Interesting.  It may keep you from having to add memory to the card but
then you have to put the data in temporary memory on the host and, if it is
a client, move it to where it belongs by copy.
You can't claim that you avoid the copy - or at least not for general
purpose applications.

Julo


Clive Philbrick <clive@alacritech.com> on 21/11/2000 00:55:29

Please respond to Clive Philbrick <clive@alacritech.com>

To:
cc:   ips@ece.cmu.edu
Subject:  Re: Concensus Call on Urgent Pointer.




The only useful point I can add to this discussion is that as long as there
are
alternatives to the URGENT mechanism, it seems to be unnecessary or even
unfair,
to make it mandatory. Using DRAM on the card with or without
overcommittment is
at least one alternative.
The solution used by an Alacritech card, which may be proprietary, involves
a
mechanism whereby control of the offloaded TCP connection can be flipped
between
the card and the host stack. Who ever has control at any given time does
all the
TCP processing for it while it is under their control. The huge majority of
the
time the connection is under control of the card. If a dropped frame causes
an
out-of-order frame to arrive on an offloaded connection, the card handles
the
situation by holding the frame, flipping control of the connection to the
host
and forwarding the frame to the host stack as a normal dumb NIC. Then the
host
stack handles that frame and all subsequent frames on that connection  (TCP
reasembly) until the retrans occurs and the connection is back in sync.
Then he
flips the connection back to the card. The net result is that the card will
not
need to buffer much data on that connection - the frames are forwarded to
the
host. It also means the host will need to copy a usually-small number of
frames'
data to user buffers during sync-up. The point is the reassembly queue is
in
host memory.

I just want to reiterate: regardless of what criticisms may be raised
against
this mechanism, it is an alternative to using the URGENT flag. This is not
to
say a subsequent modification would not be to use the URGENT flag and
handle the
OOO frame entirely on the card - if it became clear it was needed for
performance.

Clive Philbrick