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: Tue, 10 May 2022 07:28:41 +0200
>
> On Mon, May 9, 2022 at 10:17 PM Guy Harris <gharris () sonic net> wrote:
> > On May 9, 2022, at 12:31 PM, Tomasz Moń <desowin () gmail com> wrote:
> > > There is no such thing as "low-speed bus" because low-speed is only
> > > allowed for non-hub devices. USB hosts and hubs *must* support atleast
> > > full and high speed. USB devices are allowed to be low-speed (such
> > > devices can operate *only* at low speed).
> >
> > So what is the term used for a cable between a low-speed-only device and either a host or a hub?
>
> Universal Serial Bus Specification Revision 2.0:
>     6.6 Cable Mechanical Con figuration and Material Requirements does specify:
>     High-/full-speed and low-speed cables differ in data conductor
> arrangement and shielding.
> and then it specifies:
>     7.1.1.2 Low-speed (1.5 Mb/s) Driver Characteristics
>     A low-speed device must have a captive cable with the Series A
> connector on the plug end.
>
> So the term is "low-speed cable" and that cable is not removable
> (captive cable is attached to the circuit board).
>
> > The USB 2.0 spec appears to use "bus" for an "edge", in the graph-theory sense:
> >
> >         https://en.wikipedia.org/wiki/Glossary_of_graph_theory#edge
> >
> > rather than for the entire tree.
>
> In USB 1.x the "bus" did stand for the entire tree. Then it diverged
> in subsequent revisions :-)
>
> > What *is* the correct term to use for a single cable, the traffic on which one might be sniffing?
>
> "Low-speed cable" and "High-/full-speed cable" (hopefully this
> discussion does not cover later revisions where the cabling is way
> more complicated).
>
> > > It is important that the analysis engine know whether the packets were
> > > full or low-speed as there are slightly different rules. There is not
> > > so clear distinction between layers as USB does not really use ISO/OSI
> > > model.
> > >
> > > So I think it definitely makes sense to have separate link types for
> > > full-speed and low-speed.
> >
> > It makes sense to indicate whether packets are full-speed or low-speed; nobody is arguing otherwise.
> >
> > The question is whether the right way to do that is to have separate link types, so that you can't have a mix of 
> > full-speed and low-speed packets in a single pcap capture or on a single interface in a pcapng capture, or to have a 
> > single link-layer type with a per-packet full-speed/low-speed indicator.
>
> I think the separate link types are the way to go.
>
> The full to low speed transition is not technically a packet, but
> rather a "Preamble". While the transceivers, e.g. the one used in
> OpenVizsla, have special mode to send that preamble together with
> low-speed packet, they do not have a mode to capture low-speed
> transactions. The handling logic has to be bundled in specialized hub
> chip (and only there, the non-hub full speed devices will happily
> discard what they see as garbage).
>
> --- End Message ---
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