2. Christopher Bednarz Justin Jones Prof. Xiang ECE 4986 Fall 2011 http://code.google.com/p/alt-hs/ Department of Electrical and Computer Engineering University of Michigan Dearborn

3. Overview/Features  USB Tethering System: Android Phone & Linux Computer  Unique: Scalable for a network configuration Support for multiple machines  Permanent Network Solution Easy on the phone Customizable Security Features  No modifications to Android Phone & Linux Computer  Android 1.5+ Achievements  Fully functional under moderately heavy loads  Network Scalable Limitations  Currently Limited to 256 Open Connection: Limited up to four devices  Maximum Cellular throughput ~ 50%

4. Technical Problem Provide a flexible, secure, and reliable system that enables an Android device to send and receive network traffic for a small LAN Design Requirements 1. Unmodified Android Device and Linux Computer 2. Routing capable 3. USB Interface (Phone/Router) 4. Support the major protocols: TCP, UDP, ICMP 5. Scalable customizable security features 6. FAST

5. Minimum System Specifications  Routing Computer: 1GHz, 64 MB Ram, 64 MB Space, Linux 2.6, NICs, USB 2.0, switch (optional)  Android Device: 64MB Ram, 20 MB Storage, Android 1.5+, Data / Tethering Services/Subscription Performance Specifications  Maximum cellular throughput: 95%  Maintain enough connectivity for up to 4 machines  Fully Functional, Stable System Standards  Router Development: POSIX C API: Threading, Sockets, Portability  Phone Development: Java/Dalvik JVM, Android API  Network Protocols: IPv4, TCP, UDP, ICMP  Network Inter: IEEE 802.11, IEEE 802.3 (Ethernet), USB 2.0

6.  Interface Tasks (Chris) Tunnel to Router Interface: Hook Traffic into router program Phone to Router USB Interface: Controlled Socket link between phone and router  Router Tasks (Chris) TCP/UDP/ICMP Implementation: TCP/UDP/ICMP to Application Converter  Phone Tasks (Chris & Justin) TCP/UDP/ICMP Implementation: Socket/Datagram Channel Connection  Integration (Chris) Debugging, Validation & Verification Optimization

7. Internet/Transport Features Protocol Selection: IPv4, TCP, UDP, ICMP ○ Alternatives: IPv6, DCCP, SCTP, RSVP, ECN, etc. Constraints: ○ NO IP Fragmentation ○ Client connections only TCP Features TCP Options: MSS Support ~ 1460 byes vs. 536 ○ Alternatives: Timestamp, SACK, Window Scaling

8. System Optimization (Phone) Open Connections Maintained Single Connection Manager Thread ○ Alternatives: Dynamic Thread Allocation, Thread Pool Resource Limitation: limited open connections to 256 Idle Connection lifetime, TCP: 20 seconds, UDP 5 seconds (Limited Connections) ○ Alternatives: Unlimited lifetime, Very Low lifetime

9. Network Request User Mode Program Network to Application Conversion Phone Socket/Datagram Time  Application to Network Conversion

10. Tunnel Interface  Tunnel Driver (Linux 2.6+) Intercept Internet Traffic Must be: started, setup/configured, obtained  Dedicated Thread: Non-blocking Read/Writes  Raw Buffers stored into a BufferQueue Phone/Router USB Interface  Android Debugging Bridge Features: Converts USB signals into a network socket  Phone/Router: Dedicated threads: read into a BufferQueue, processed in respective programs

11. General  Convert Packets to Simple Commands for phone to process  Convert Simple Commands to Packets for Network Consumption TCP Emulation  Reliability tradeoff Complex  Must maintain State diagram: 3 – Way Handshake (Connection Establishment) Data Transfer 4 – Way Handshake (Connection Termination)  Flow Control Track Sequence Numbers Sliding Window Sized limited to 40 MSS ~ 57KB UDP/ICMP Emulation  No Reliability, Simple Design  Stateless: Send or Receive Data, phone handles connectivity

12. Network Traffic TCP Header IPv4 Header Data Lookup Table Android Device Command Data

13. General  Pre-allocated Thread Pool: limit allocation overhead  Maintain a table that stores state/connection information  Timers maintained to eliminate idle connections Emulation  TCP  Non-blocking Socket Channel  Data had to wait until ACK command  Persist timer held, in case of potential deadlock  UDP: Non-blocking Datagram Channel  ICMP: isReachable  Received data appended the data received command  If no command ready or data not on channel, thread sleeps to allow other threads to be processed  Phone notifies router of any errors, and flushes commands from the failed connection (sync to router)

14. Router  Abstract Tester: Phone command emulator, allowed testing of the router implementation ~ 98% of Maximum Data Throughput (Fast Computer) Phone  Thread Pool Model maximum throughput ~ 50%  Limited throughput utilizing Java/Dalvik sockets/datagram Overall  System stable even under heavy loads  No port-mapping correlation, indirectly very secure form of NAT  Very Difficult to debug  Some redundancy between the phone and router

15. Conclusions  Functional system, cellular optimization could lead to great potential  Performance limited by Java socket/datagram implementation  Security Improvements, Network flexibility of the Linux OS Improvements:  Possibly port to Windows Server for those interested  Provide an interactive GUI

16. Faculty Advisor: Professor Xiang Coordinator: Professor Miller

19. Router Abstract Tester

20. Simplified VHL Design

21. Tunnel Interface Structure Source/Destination Computer WLAN / ETH Router, Kernel Mode User Mode Program TUN Router, User Mode IPTables (Firewall)

23. Phone/Router Interface User Mode Program ADB/USB Android Program Internet Cell Phone