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

[Tomasz Moń via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

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>
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> From: Tomasz Moń <desowin () gmail com>
>
> Date: Mon, 9 May 2022 07:47:45 +0200
>
> 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.
>
> > 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.
>
> I assume that by single wire you meant "single wire pair"
> (differential pair). USB 2.0 has only single differential pair, formed
> by D+ and D- signal wires, so the high/full/low speed communication
> always occurs on the same wire pair.
>
> > 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?
>
> Yes. If you connect high-speed hub to high-speed host (or super-speed
> host, but super-speed host essentially contains high-speed host, aka
> dual-bus) the communication on the connecting wires will be at high
> speed (480 Mb/s). Similarly if high-speed device is connected to
> high-speed host (or hub) then the communication will be at high speed.
>
> > 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?
>
> 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.
>
> Hopefully I have recently found full-speed only hub (thanks to Linux
> kernel bug report [1]), so I have all the equipment necessary to check
> how the OpenVizsla deals with the problematic case.
>
> [1] https://bugzilla.kernel.org/show_bug.cgi?id=213839
>
> --- End Message ---
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