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: Mon, 9 May 2022 00:17:40 -0700
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> On May 8, 2022, at 10:47 PM, Tomasz Moń <desowin () gmail com> wrote:
>
> > 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.
>
> So what happens if you plug a low-speed keyboard or mouse into a host that supports USB 2.0?  Does that link not run at 
> low speed?
>
> > > 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.
>
> Sorry - that's "wire" in the sense of "cable", not in the literal sense.
>
> > > 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.
>
> "super-speed" is USB 3.0, right?  No LINKTYPE_/DLT_ has been proposed for the 3.0 link layer, as far as I know.
>
> But no full-speed or low-speed will go over that connection, either, so it's never the case that, in a capture on a USB 
> cable, there will be both high-speed and full/low-speed traffic, right?
>
> (And presumably this is for captures on a single USB cable; if you're capturing on more than one cable, that's with 
> more than one capture interface, so that's a job for pcapng, with different interfaces having different link-layer 
> types.)
>
> > > 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.
>
> So, as per a few paragraphs above ("If you connect high-speed hub to high-speed host ... the communication on the 
> connecting wires will be at high
> speed (480 Mb/s)."), if you have a high-speed hub connected to a high-speed host, and the high-speed hub has full-speed 
> or low-speed devices downstream, the packets from the host to the hub, ultimately intended for the full-speed or 
> low-speed device, are sent as high-speed traffic, and only the downstream traffic from the host to the full-speed or 
> low-speed device is full-speed or low-speed?
>
> However, if you have a full-speed hub connected to a full-speed or high-speed host, and the full-speed hub has 
> low-speed devices downstream, the packets from the host to the hub, ultimately intended for the low-speed device, are 
> sent as a full-speed PRE packet followed by a transaction sent as low-speed traffic?
>
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