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

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

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>
>
> From: Guy Harris <gharris () sonic net>
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> Date: Mon, 9 May 2022 13:17:25 -0700
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> 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?
>
> 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.
>
> What *is* the correct term to use for a single cable, the traffic on which one might be sniffing?
>
> > 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.
>
> --- End Message ---
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