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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?
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