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

[Guy Harris via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Guy Harris <gharris () sonic net>
>
> Date: Mon, 9 May 2022 11:52:54 -0700
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> On May 9, 2022, at 1:58 AM, Tomasz Moń <desowin () gmail com> wrote:
>
> > 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.
>
> So what kind of bus is that link?  High-speed, full/low-speed, or 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.
>
> And, if there ever *are* (I see no reason to rule it out), they can ask for another link-layer type when they need it.
>
> > > 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.
>
> OK, so it makes sense to have a separate link-layer type for high-speed traffic, rather than a single link-layer type 
> for "USB link-layer with metadata header, with the per-packet metadata header indicating the speed".
>
> But, if, as you said earlier:
>
> > If you capture at the connection between low speed device and
> > host/hub, there will only ever be low speed packets. It would be a
> > LINKTYPE_USB_2_0_LOW_SPEED capture.
> >
> > The problematic case (and the reason why full/low-speed bus is
> > mentioned) is the LINKTYPE_USB_2_0_FULL_SPEED. It is the case when you
> > capture at the connection between full speed hub and the host (and
> > possibly full speed device connected to a full speed hub if there are
> > low speed devices connected to the full speed hub). If there is low
> > speed device connected to downstream hub port, then when the host
> > wants to send packets to the low speed device, these will be sent at
> > low speed to the hub. However, there will be PRE packet (sent at full
> > speed) before every low speed transaction.
>
> can there be separate link-layer types for full-speed and low-speed traffic, or does there need to be a single type for 
> full/low-speed traffic, with a per-packet metadata header indicating the speed"?
>
> --- End Message ---
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