Subject: Previous discussions

From: "Hattig, Myron" <[email-masked]>
Date: Wed, 28 Feb 2001 09:50:07 -0800

Is there a published analysis of a the interrupts with a direct memory model
data
transfer?

During the first half of 1997 (yes 4 years ago) we (Intel, Microsoft, Peter
J, Kenji) looked at this early in the IP/1394 definition. As Kenji points
out this was discussed on the IP/1394 list and my recollection matches
Kenji's conclusion - if extensions to IP/1394 need to be written for large
file transfers, then go ahead. However, this does not obviate the current
method of transferring IP packets over 1394. Also, I am highly suspicious of
the need for this.

(This is a repeat of the conclusions from 1997)

Facts:
1) Memory mapped model has more interrupts per IP packet due to the need to
lock and unlock memory packets necessary before and after transfers. There
are 3 interrupts per IP packet. In addition, there must be some mechanism to
agree on whether the memory transfer uses reads or writes.
2) Msg passing has fewer interrupts per IP packet because there is only a
single interrupt per IP packet.

The memory mapped model is better for larger packets because the ratio of
number of bytes transferred per interrupts improves. However all IP packets
are 1500 bytes or less due to the MTU size. Previously, we went as far as
attempting to get sample packet sizes from real networks. There were no
conclusive trends in terms of actual packets sizes on network. There were
groupings of 100 bytes or less and up around 1500 bytes. So taking the worse
case of 5 packet of 1500 bytes, the memory mapped model has 15 interrupts
and the current IP/1394 has 5 interrupts. This clearly shows the flaws in
the arguements posted. This is what drove us to the current conclusion 4
years ago.

Changing the MTU size has huge ramifications that we were not aware of when
we chose 1500 bytes. Luckily it was a good decision. Specifically it
improves the ability to bridge to other important link layer technologies
that were not available 4 years ago that also have an MTU of 1500 bytes.
These include  802.11b, HomeRF, HomePNA 1.0, and HomePNA 2.0.

Before any more time is wasted on this issue, the proponents of delaying
IPv6 over 1934 and of changing RFC 2734 should publish a paper (not just
fire away with email) and show us why things are so bad. Afterall it has
been 4 years.

-myron