[RFC] LE: Low Latency GATT Write-Sign-Cmd

[Brian Gix <bgix@codeaurora.org>]

Hi All,

One of the useful features of Low Energy is the ability to rapidly 
establish a connection, send a responseless Signed Write Command, and 
tear the connection down.  The obvious use case for this would be a 
standard remote control, where extremely low latency is expected.

The difference between this use case, and something like one of the 
Health profiles, is that a Health device (Heart rate Monitor, or 
Thermometer) is that where the Health device would have a persistent 
connection with high latency (less frequent Beacons), the Remote Control 
use case would rarely be connected, but when it is it will be as low 
latency as possible.

Currently we do not support Write Signed Commands at all, although I 
hope to have that addressed in the not too distant future.  However the 
nature of it's usage model is not very well established here 
(BlueZ/Linux) so I would like to float a few ideas.


This is what should happen when the user hits a button on his remote 
control (or in his remote control linux APP):

1. Request LE Connection with Low Latency Parameters
2. Compose & sign the GATT Write Cmd (in parallel with Connect Req)
3. When Successful Connect Cmplt Evt occurs, send the Write Cmd Pkt
4. WAIT for the Num Completed Pkts Evt (VERY IMPORTANT)
5. As soon as you have it, Disconnect

This use case is only valid for LE (GATT connections must be Secure on 
BR/EDR, and therefore will never be as fast, and will never in fact even 
need to be signed).  If done properly, the entire sequence can be 
completed in the 10's of milliseconds, from button press to Disconnect.

This calls for a different thread of operation from current GATT 
operation for a few reasons:

1. Standard GATT connections will have different Connect Parameters

2. Standard GATT connection establishment requires on a round trip from
User space--> Kernel --> Baseband and back before the ACL packet is even 
written to the Socket in User Space.

3. The signing crypto code is in the Kernel, and must be applied to each 
signed write command in a way that suggests some LE-GATT-Write-Cmd 
knowledge must exist in the kernel, as opposed to being a simple ACL pipe.

4. There is no current mechanism for the Kernel to inform the User space 
code that the ACL packet has finished transmission, and that it is OK to 
disconnect.

5. We already special case the handling of the LE-L2Cap CIDs for 
Signaling and SMP.


So I would like to suggest the following straw-man:

A new LE only MGMT command, which accepts as parameters the Remote 
Address, a length and a buffer.  The buffer would be reasonable sized (I 
would recommend 27 bytes, which is the default LE MTU+HDR). The byte 
stream itself could either be composed by the User space GATT code 
(L2CAP header, Attribute Handle, Write Data) or they could be structure 
fields in the MGMT packet. On the Kernel side, the MGMT code would 
shepherd the packet through the connection, signing, and send process. 
If the connection to the remote device was already established, the 
existing connection would be used, the signing would only be done if the 
link was not encrypted, and the connection would be left up. When the 
Num-Completed-Pkts evt arrives, send the MGMT cmd_status response.

I think this would be way cleaner than trying to wedge this use case 
into our current GATT usage model.


Thoughts?



-- 
Brian Gix
bgix@codeaurora.org
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum