1. Mobile Plus: Multi-device Mobile Platform for Cross-device Functionality Sharing
Sangeun Oh, Hyuck Yoo, Dae R. Jeong, Duc Hoang Bui, and Insik Shin
School of Computing, KAIST
Daejeon, Republic of Korea

2. Recent Mobile Trends 1. Multiple devices owned by users
2. Various functionalities from devices
System func: sensor, camera
APP func: 支付宝，社交软件，login, payment
Average 3.64 devices per person
System func、APP func

3. Potential for Multi-device Usage
Q. What if devices can share their functionalities each other?
“Many opportunities to provide new UX!”

4. User Cases 1: Remote Sensor

5. User Cases 2: Secure Login

6. Existing Solutions: System-level
Develop cross-device platforms enabling unmodified apps to use functionalities of other devices
Focus on utilizing system functionalities related to I/O resources
e.g., Rio [Mobisys’14], Flux [Eurosys’15], remote file systems, etc.
Limitation
Not supporting app functionalities (e.g., login, payment, etc.) and non-I/O system functionalities (e.g., clipboard, notification, etc.)

7. Our Goal
Design a multi-device mobile platform that Enables 1) unmodified legacy applications to share 2) a broad range of functionalities across devices

8. Background: RPC Scheme
Designed to allow apps (processes) to share their functionalities using Remote Procedure Call (RPC)
RPC: 远程过程调用

9. Background: Binder IPC
Internally utilizes Binder Inter-Process Communication (IPC) to transfer RPC messages (called parcels)
Client APP
Within-device RPC
Server APP
User Space
Parcel
1、Forwarding a parcel for RPC function call
2、Executing the corresponding RPC function
Kernel Space
Binder Channel
Binder IPC Driver

10. System Design
Extend the existing RPC scheme to multi-device environments

11. RPC function execution
Problems
P1. The extension of the Binder IPC
P2. Support for RPC function execution
Client Device
Server Device
RPC function execution
Client APP
Server APP
Network
Binder IPC Driver
Binder IPC Driver

12. P1. The extension of the Binder IPC
Binder IPC interception point
2 Binder channels + 1 Network channel

13. P2. Support for RPC function execution
Binder IPC extension cannot guarantee successful execution of RPC functions
RPC functions may fail due to
1) Invalid RPC arguments
2) Not maintaining some execution context
3) Not acquiring metadata about counterpart apps
Server Device
RPC function execution
Server APP
Binder IPC Driver

14. Cross-device Argument Passing
Detect reference-type arguments
Copy the corresponding resources to other device
Change original arguments to new values
Client Device
FD = 7
Client APP
FD = 5
Server APP
Shared memory
Shared memory
Network
Mobile Plus
Mobile Plus

15. Cannot maintain Execution Context
Caller-callee relationships
Activity states

16. Evaluation Questions Environments
Functionality coverage: “How many functionalities can M+ support?”
Compatibility study: “How compatible is M+ with different Android versions?”
Performance test: “How much overhead does M+ impose?”
Environments
Two Nexus 6 devices (Android 5.0) with M+ (Client & Server devices)
On the same WiFi network

17. Functionality Coverage
Using 22 use cases for functionality sharing
9 App functionalities
Login, payment, viewer, photo sharing, etc.
13 System functionalities
Sensor, camera, notification, contact, etc.

18. Compatibility Study
Different Android versions may have different RPC functions
Cannot support functionality sharing between them
We compared the five latest Android versions for RPC functions
JellyBean (4.3)
KitKat (4.4)
Lollipop (5.0)
Marshamallow (6.0)
Nougat (7.0)

19. Compatibility Study Comparisons with Lollipop (5.0)
Same: 50 – 79 %
Translatable: 10 – 13 %
Incompatible: 11 – 37 %
Possible to mitigate the differences via translations for RPC messages (i.e., parcels)

20. Conclusion
We designed a multi-device mobile platform, Mobile Plus (M+), by providing the cross-device RPC scheme
Enable to support a broad range of functionalities
Without any modifications of legacy apps
We evaluated our system with 22 use cases
We expect M+ to accelerate development of creative and useful apps to provide novel UX