Return-Path: Message-ID: <5318E560.6050200@openbossa.org> Date: Thu, 06 Mar 2014 18:15:12 -0300 From: Andre Guedes MIME-Version: 1.0 To: Marcel Holtmann , Johan Hedberg CC: linux-bluetooth@vger.kernel.org Subject: Re: Background scanning and white list usage References: <20140228064422.GA28466@localhost.P-661HNU-F1> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed List-ID: Hi Johan/Marcel, On 02/28/2014 03:51 AM, Marcel Holtmann wrote: > Hi Johan, > >>> The complicated part comes into play when we have devices with >>> LE Privacy enabled and when they are using resolvable private >>> addresses. Meaning when our IRK list is populated with identity >>> addresses and their IRKs. The only way to make this work with the >>> current available controller features is if we program the RPA >>> into the white list. Since that RPA is going to change over time, >>> we need to stop scanning with the white list filter every now and >>> then, scan for all devices and resolve the RPA. If we see a new >>> RPA for a know IRK, we have to replace the old RPA in the white >>> list with the new RPA. And then we go back to scanning with the >>> white list filter policy. >>> >>> Now the important question is what are good enough intervals to >>> make this work smoothly. Devices using LE Privacy will take a hit >>> in their re-connection time, but that is what we have to trade in >>> for compared to waking up the host for every single advertising >>> packet. >>> >>> My initial idea is to scan 5 minutes using the white list, then >>> scan 10 seconds without the white list, then back to 5 minutes >>> using the white list and so on. >>> >>> The default value for the PRA lifetime according to the >>> specification is 15 minutes. I timed recent iOS devices which >>> seem to be using 9 minutes intervals. So we have to play a little >>> bit with this and see what are good values. >>> >>> Maybe 3 minutes white list scan and 5 seconds without white list >>> is better. Things to try out. >> >> I don't think this is a good idea at all. With LE starting >> advertising is typically seen as the initiating action of >> connection creation (unlike with BR/EDR where HCI_Create_Connection >> is the initiating action). Typically peripherals mean "connect to >> me now!" when they start connectable advertising. >> >> Let's stay you switch on your peripheral device, or it comes into >> range you haven't used it for some time (hours or even days). If >> it's using RPAs it's pretty much guaranteed to have a different one >> than what we know of and even if we're using 3 minutes white list >> scanning the user is on average going to have to wait for 1.5 >> minutes for the device to get connected which is not acceptable >> behavior (the extreme example would be if this is a keyboard or a >> mouse which you start using for the first time in the morning - >> moving the mouse or pressing a key on the keyboard should certainly >> get you a connection in less than 1 second). > > HID devices would suffer the most here. Fully agree here. > > However since neither Microsoft Windows nor OS X can deal with RPAs > at the moment, I do not think we are entering a dangerous zone here > from an interoperability point of view. Actually Windows 8.1 is not > able to connect to any random address for that matter. > > My point is that we certainly not make it worse. HoG is not the only problem. I'm afraid delaying re-connection 3-5 minutes we might become others profiles impractical (e.g. Fob keeps beeping even if it is beside the cellphone). >> I fully understand the desire to use the white list as it's a very >> nice power saving feature, but I don't think we can win here as >> long as we don't have a way to have the controller do the resolving >> for us. >> >> One thing we could potentially try to do (but which I doubt is >> really worth it in the end) is to track the age of resolved RPAs. >> If we have an RPA which was resolved say less than 5 minutes ago we >> consider it appropriate to place into the white list. Otherwise we >> skip using the white list. > > I was thinking about this as well. Over time we could learn the age > of a RPA and thus schedule the scanning without white list times most > efficient. Frankly, I want to get the white list usage going first. > As long as you only have public or static addresses, it is the way to > go. And then we optimize it when we have to deal with RPAs. I agree with Johan about this aging approach, I seems not worthy. Honestly, I don't see how we would properly deal with white list and LE Privacy issue by now. > We can not close our eyes to iOS devices using RPAs and I am not > willing to take the power hit of getting flooded with advertising > reports. To fix this other issue (advertising flooding), filtering duplicates is just fine. The problem with enabling filter duplicates in background scan happens in the following scenario: * Background scan is running. * A device disconnects and starts advertising. * Before host gets the disconnect event, the advertising is reported to host. Since there is no pending LE connection at that time, nothing happens. * Host gets the disconnection event and adds a pending connection. * No advertising is reported (since controller is filtering) and the connection is never established. To address this scenario, all we have to do is: always restart background scan when a new LE pending connection is added. This way, we unsure that we don't miss the advertising report. If you guys agree with this approach I can write the patches. Regards, Andre