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

[Guy Harris via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Guy Harris <gharris () sonic net>
>
> Date: Sun, 8 May 2022 14:15:38 -0700
>
> 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.
> >
> > But, maybe I'm wrong and that actually requires there to be a USB hub out there.
>
> "Bus" is a bit weird here.
>
> To quote section 4.1.1 "Bus Topology" of the USB 2.0 pec:
>
>         The USB connects USB devices with the USB host. The USB physical interconnect is a tiered star topology. A hub 
> is at the center of each star. Each wire segment is a point-to-point connection between the host and a hub or function, 
> or a hub connected to another hub or function. Figure 4-1 illustrates the topology of the USB.
>
> and Figure 5-6 "Multiple Full-speed Buses in a High-speed System" seems to use the term "bus" to refer to wire segments.
>
> 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.
>
> 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.
>
> --- End Message ---
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