Re: Speed specific Link-Layer Header Types for USB 2.0

[Tomasz Moń via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Tomasz Moń <desowin () gmail com>
>
> Date: Mon, 9 May 2022 10:58:51 +0200
>
> On Mon, May 9, 2022 at 9:17 AM Guy Harris <gharris () sonic net> wrote:
> > On May 8, 2022, at 10:47 PM, Tomasz Moń <desowin () gmail com> wrote:
> > > On Sun, May 8, 2022 at 8:53 PM Guy Harris <gharris () 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.
>
> --- End Message ---
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