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

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

> --- Begin Message ---
>
>
> From: Tomasz Moń <desowin () gmail com>
>
> Date: Mon, 9 May 2022 08:09:59 +0200
>
> On Sun, May 8, 2022 at 11:15 PM Guy Harris <gharris () sonic net> wrote:
> > On May 8, 2022, at 1:30 PM, Michael Richardson <mcr () sandelman ca> wrote:
> > > I guess I would have thought that a physical bus could have a mix of
> > > different devices which operate at different speeds.  As such, I wondered if
> > > you really needed pcapng to be able to mix LINKTYPES in the same file, or
> > > a different bit of meta-data to indicate bus speed for each frame captured.
>
> In most cases all the packets will be at the same speed. The only
> corner case is how the full/low speed is being handled. This dates
> back to the original USB 1.0 specification and is kept for
> compatibility reasons.
>
> > > But, maybe I'm wrong and that actually requires there to be a USB hub out there.
>
> The hub is the complex part. Between host and device, with no
> intermediate hubs in the path, all packets are always sent at the same
> speed.
>
> > I think a point-to-point connection between the host and another entity may always run at a single speed, as well as 
> > a connection between a hub and a function.
> >
> > It might also be the case that a hub-to-hub connection also runs at a single speed.  Section 11.14 "Transaction 
> > Translator" says:
> >
> >         A hub has a special responsibility when it is operating in high-speed and has full-/low-speed devices 
> > connected on downstream facing ports. In this case, the hub must isolate the high-speed signaling environment from 
> > the full-/low-speed signaling environment. This function is performed by the Transaction Translator (TT) portion of 
> > the hub.
> >
> > so if you have a full-speed or low-speed device plugged into a USB 2.0 hub, and that hub is connected to a host, the 
> > host-to-hub link is high-speed, and the hub-to-device link is full-speed or low-speed, and the hub does the 
> > translation.  That way, you can plug a high-speed device and a full-speed or low-speed device into the hub, and the 
> > host will be able to talk at high speed to the high-speed device.
>
> All packets between high speed host and high speed hub are at high
> speed (including the packets that are intended for a full or low speed
> devices). Transaction Translator basically buffers the data and
> sends/receives packets to/from downstream devices at the correct speed
> (full/low). Determining the target device speed during (the high speed
> traffic from host to hub) dissection is simple as the speed is
> indicated in the SPLIT packet.
>
> > USB isn't a shared bus like non-switched Ethernet; it's more like switched Ethernet or point-to-point Ethernet, with 
> > links being point-to-point, either a direct connection between end nodes or connections to a switching device that 
> > handles speed translation if two end nodes of different speed capabilities are communicating.
>
> Note that end nodes cannot directly communicate with each other. The
> communication is always between host and a device. Device to device
> communication is not possible. If Transaction-Translator is involved,
> it essentially fulfills the speed translation from host (when reading
> from downstream device Transaction Translator does keep issuing IN
> until the device responds with data or stalls the endpoint; as long as
> NAKs are received Transaction Translator will keep responding to the
> host SPLIT COMPLETE with NYET).
>
> --- End Message ---
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