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>
>
> Date: Sun, 8 May 2022 11:53:52 -0700
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> On May 8, 2022, at 4:48 AM, Tomasz Moń via tcpdump-workers <tcpdump-workers () lists tcpdump org> wrote:
>
> > I would like to remedy the situation by requesting additional speed
> > specific link layer header types, for example:
> >  * LINKTYPE_USB_2_0_LOW_SPEED
> >  * LINKTYPE_USB_2_0_FULL_SPEED
> >  * LINKTYPE_USB_2_0_HIGH_SPEED
> >
> > The description for existing LINKTYPE_USB_2_0 could be updated to
> > mention that for new captures, the speed specific link layer header
> > types should be used to enable better dissection.
>
> To quote a comment of yours in the Wireshark issue:
>
> > I should have gone for three separate link-layer header types for "USB 1.0/1.1/2.0 packets" each at different capture 
> > speed (low/full/high). I think technically we can still add these alongside the current "unknown speed" one. The 
> > reason behind having separate link-layer header types is that the capture tool must know the capture link speed 
> > (agreed speed does not change during the transmission, and the handshaking is not on packet level) and the capture 
> > link speed is useful when analyzing packets.
>
> 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 idea, then, is presumably that a capture tool is capturing on a single bus (single wire), so it's either capturing 
> on a high-speed bus or a full/low-speed bus.
>
> It looks as if a high-speed bus will always run at 480 Mb/s, so that capture would be a LINKTYPE_USB_2_0_HIGH_SPEED 
> capture.  Is that correct?
>
> For full/low-speed buses, will those also always run at full peed or low speed, so that there would never be a mixture 
> of full-speed and low-speed transactions?
>
> --- End Message ---
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