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

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

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

User Cases 1: Remote Sensor

User Cases 2: Secure Login

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.)

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

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

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

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

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

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

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

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

Cannot maintain Execution Context Caller-callee relationships Activity states

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

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.

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)

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)

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