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On Mon, May 9, 2022 at 9:17 AM Guy Harris <ghar...@sonic.net> wrote:
> On May 8, 2022, at 10:47 PM, Tomasz Moń <deso...@gmail.com> wrote:
> > On Sun, May 8, 2022 at 8:53 PM Guy Harris <ghar...@sonic.net> wrote:
> >> At least from a quick look at section 5.2.3 "Physical Bus Topology" of the
> >> USB 2.0 spec, a given bus can either be a high-speed bus or a
> >> full/low-speed bus.
> >
> > The full/low-speed bus applies only to upstream link from full speed hub.
>
> So what happens if you plug a low-speed keyboard or mouse into a host that
> supports USB 2.0? Does that link not run at low speed?
The link will run at low speed.
> "super-speed" is USB 3.0, right? No LINKTYPE_/DLT_ has been proposed for the
> 3.0 link layer, as far as I know.
Yes, "super-speed" is USB 3.0. I don't know of any open source sniffer
nor any tools that would really want to export the packets to pcap
format. Hopefully USB 3.0 is a completely separate matter.
> But no full-speed or low-speed will go over that connection, either, so it's
> never the case that, in a capture on a USB cable, there will be both
> high-speed and full/low-speed traffic, right?
Yes. You either get solely high-speed traffic or full/low-speed traffic.
> (And presumably this is for captures on a single USB cable; if you're
> capturing on more than one cable, that's with more than one capture
> interface, so that's a job for pcapng, with different interfaces having
> different link-layer types.)
Yes, combining multiple capture interfaces should be handled by pcapng.
> So, as per a few paragraphs above ("If you connect high-speed hub to
> high-speed host ... the communication on the connecting wires will be at high
> speed (480 Mb/s)."), if you have a high-speed hub connected to a high-speed
> host, and the high-speed hub has full-speed or low-speed devices downstream,
> the packets from the host to the hub, ultimately intended for the full-speed
> or low-speed device, are sent as high-speed traffic, and only the downstream
> traffic from the host to the full-speed or low-speed device is full-speed or
> low-speed?
If by "downstream traffic from the host" you mean "downstream traffic
from the hub", then this is correct.
> However, if you have a full-speed hub connected to a full-speed or high-speed
> host, and the full-speed hub has low-speed devices downstream, the packets
> from the host to the hub, ultimately intended for the low-speed device, are
> sent as a full-speed PRE packet followed by a transaction sent as low-speed
> traffic?
Yes, and only the low-speed traffic will get passed through to the
downstream low speed devices. That is, on the cable connecting to the
low speed device there is only low speed traffic.
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