Re: TCP limitations (was Re: ISCSI: Urgent Flag requirement violates TCP.)

[Vern Paxson <vern@ee.lbl.gov>]

> Interesting argument. How would you end up paying the extra copy but not
> paying the extra memory?

I think you have to pay both, unfortunately.

If you had perfect framing, then you could avoid the extra copy.  But with
the Urgent pointer approach, you don't have perfect framing for packets
that don't begin new ISCSI messages (or if there's coalescence).  So you
need the extra memory to keep these around somewhere, and a copy to move
them to where you want them once you know where that is.  I'd argue that
this copy isn't all that expensive, because it only happens when you have
a sequence hole, which likely means that the associated stream is about
to slow down considerably anyway.

With a different approach to framing, such as a TCP option that tells
you where each payload is with respect to its parent PDU, or if you used
SCTP streams, then I think you can avoid the extra copy, and with it the
extra memory.  But the first of these is outside the scope of the WG's
charter, other than making a recommendation about them.

		Vern