Subject: Re: where is ifp->if_baudrate used?
To: None <tech-net@NetBSD.org>
From: David Young <email@example.com>
Date: 03/18/2007 05:41:52
On Sun, Mar 18, 2007 at 11:41:12AM +0900, Izumi Tsutsui wrote:
> struct ifnet has if_baudrate (which is actually if_data.ifi_baudrate),
> member, but where is the member used?
Perhaps ifi_baudrate was intended to inform a routing protocol about the
link speed, so that it could synthesize a link metric (10^6/ifi_baudrate).
I question whether any single rate suffices to describe many links.
Consider that commonplace link technologies use multirate PHYs (56k
modems, WLAN, powerline). Also, a single ethernet/WLAN/powerline link
may connect multiple stations. The NIC may use a different rate to
transmit to each station. How can a single rate characterize such a link?
Now, suppose we treat ifi_baudrate as a maximum PHY bit rate. I feel
dissatisfied comparing a 108 Mb/s WiFi link with a 100 Mb/s ethernet link
by their ifi_baudrate, if the former ifi_baudrate is set to 108 * 10^6,
and the latter to 100 * 10^6. To make an "apples with apples" comparison
of the two links, I need more information than the ifi_baudrate.
A slightly more practical measure of link speed tells how many
ifi_mtu-sized link-layer payloads you can send using the top PHY bitrate,
figuring in all of the overheads; call this the "speed limit" on the link.
David Young OJC Technologies
firstname.lastname@example.org Urbana, IL * (217) 278-3933