--- Begin Message ---On May 8, 2022, at 10:47 PM, Tomasz Moń <deso...@gmail.com> wrote: > On Sun, May 8, 2022 at 8:53 PM Guy Harris <ghar...@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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